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Sickling cells, cyclic nucleotides, and protein kinases: the pathophysiology of urogenital disorders in sickle cell anemia.
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Sickle cell anemia is one of the best studied inherited diseases, and despite being caused by a single point mutation in the HBB gene, multiple pleiotropic effects of the abnormal hemoglobin S production range from vaso-occlusive crisis, stroke, and pulmonary hypertension to osteonecrosis and leg ulcers. Urogenital function is not spared, and although priapism is most frequently remembered, other related clinical manifestations have been described, such as nocturia, enuresis, increased frequence of lower urinary tract infections, urinary incontinence, hypogonadism, and testicular infarction. Studies on sickle cell vaso-occlusion and priapism using both in vitro and in vivo models have shed light on the pathogenesis of some of these events. The authors review what is known about the deleterious effects of sickling on the genitourinary tract and how the role of cyclic nucleotides signaling and protein kinases may help understand the pathophysiology underlying these manifestations and develop novel therapies in the setting of urogenital disorders in sickle cell disease.
Mário Angelo Claudino; Kleber Yotsumoto Fertrin
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Type:  Journal Article     Date:  2012-06-13
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Title:  Anemia     Volume:  2012     ISSN:  2090-1275     ISO Abbreviation:  Anemia     Publication Date:  2012  
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Created Date:  2012-06-29     Completed Date:  2012-08-23     Revised Date:  2013-03-07    
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Nlm Unique ID:  101536021     Medline TA:  Anemia     Country:  Egypt    
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Languages:  eng     Pagination:  723520     Citation Subset:  -    
Laboratory of Multidisciplinary Research, São Francisco University (USF), 12916-900 Bragança Paulista, SP, Brazil.
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Journal ID (nlm-ta): Anemia
Journal ID (iso-abbrev): Anemia
Journal ID (publisher-id): ANE
ISSN: 2090-1267
ISSN: 2090-1275
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Copyright © 2012 M. A. Claudino and K. Y. Fertrin.
Received Day: 23 Month: 1 Year: 2012
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Accepted Day: 22 Month: 4 Year: 2012
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Electronic publication date: Day: 13 Month: 6 Year: 2012
Volume: 2012E-location ID: 723520
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PubMed Id: 22745902
DOI: 10.1155/2012/723520

Sickling Cells, Cyclic Nucleotides, and Protein Kinases: The Pathophysiology of Urogenital Disorders in Sickle Cell Anemia
Mário Angelo Claudino1*
Kleber Yotsumoto Fertrin2
1Laboratory of Multidisciplinary Research, São Francisco University (USF), 12916-900 Bragança Paulista, SP, Brazil
2Hematology and Hemotherapy Center, University of Campinas (UNICAMP), 13083-970 Campinas, SP, Brazil
Correspondence: *Mário Angelo Claudino:
[other] Academic Editor: Solomon F. Ofori-Acquah

1. Introduction

Sickle cell anemia (SCA) has been first described over a century ago [1] and has become one of the best studied inherited human diseases. Despite being caused by a single point mutation in the HBB gene, multiple pleiotropic effects of the abnormal hemoglobin S production range from vaso-occlusive crisis, stroke, and pulmonary hypertension to osteonecrosis and leg ulcers [24].

Genitourinary tract function is also affected in SCA, and although priapism is most frequently remembered, other related clinical manifestations have been described, such as nocturia, enuresis, increased frequency of lower urinary tract infections, urinary incontinence, hypogonadism, and testicular infarction. Sickle hemoglobin S (HbS) polymerizes when deoxygenated, resulting in a series of cellular alterations in red cell morphology and function that shorten the red cell life span and lead to vascular occlusion. Sickle cell disease (SCD) vaso-occlusion constitutes a complex multifactorial process characterized by oxidative stress and recurrent ischemia-reperfusion injury in a vicious circle contributing to reduced blood flow and results, eventually, in complete obstruction of the microcirculation and organic dysfunction [36]. The exact pathogenetic mechanisms that tie genitourinary complications to the fundamental event of HbS polymerization and hemolytic anemia in SCA have just about started to be unraveled.

This paper focuses on how previous, sometimes poorly explained, clinical observations of urogenital disorders in patients with SCD relate to more recent discoveries on the role of cyclic nucleotides and protein kinases in the pathophysiology of sickle vaso-occlusion.

2. Priapism

Priapism is defined as a prolonged and persistent penile erection, unassociated with sexual interest or stimulation, and is one of the complications associated with sickle cell anemia (SCA) since early in 1934 [7]. Priapism reaches a frequency of up to 45% in male patients with SCA, and the rate of resulting erectile dysfunction (ED) exceeds 30% [810]. Although this complication has been previously reviewed in depth in this journal [11], the main concepts behind its pathophysiology will be summarized here for better understanding of the mechanisms discussed throughout the paper, but readers are encouraged to read the previous review.

According to the American Urological Association Guidelines on the Management of Priapism, priapism can be subdivided into three categories: ischemic, stuttering, and nonischemic. Ischemic priapism (veno-occlusive, low flow) is a persistent erection marked by rigidity of the corpora cavernosa (CC) and little or no cavernous arterial inflow. In ischemic priapism, there are time-dependent changes in the corporal metabolic environment with progressive hypoxia, hypercarbia, and acidosis that typically generate penile pain. Penile sinusoids are regions prone to red blood cell sickling in SCD men because of blood stasis and slow flow rates, and ischemic priapism is thought to result from prolonged blockage of venous outflow by the vaso-occlusive process. Clinically, there is congestion and tenderness in the CC, sparing the glans and corpus spongiosum, usually with a prolonged course of over 3 hours, and frequently resulting in fibrosis and erectile dysfunction. Stuttering priapism (acute, intermittent, recurrent ischemic priapism) is characterized by a pattern of recurrence, but an increasing frequency or duration of stuttering episodes may herald a major ischemic priapism. Nonischemic priapism (arterial, high flow) is a persistent erection caused by unregulated cavernous arterial inflow. Typically, the corpora are tumescent but not rigid, the penis is not painful and is most frequently associated with trauma [1216].

Conventional treatments are largely symptomatic, usually administered after the episode of priapism has already occurred, because the etiology and mechanisms involved in the development of priapism are poorly characterized [17, 18]. Preventive interventions have been proposed but, without a clear idea of the molecular mechanisms involved, they remain largely impractical to be applied in a regular basis in the clinic [17]. Due to the difficulty in exploring these mechanisms in patients, the use of animal models of priapism has become of utmost importance to decipher this devastating clinical challenge [19]. Animal models for priapism include dogs [20, 21], rabbits [22], rats [2327], and mice [2841].

Molecular biology and genetic engineering have been widely used in animal models to explore gene function in both human physiology and in the study of pathology of human priapism. Four major priapism animal models have been developed and have yielded greater knowledge on the intrinsic mechanisms underlying priapism: the intracorporal opiorphins gene transfer rat model [4245], the endothelial nitric oxide synthase (eNOS) with or without neuronal NOS (nNOS) knock-out (eNOS−/− ± nNOS−/−) mouse models [28, 29, 3133], the adenosine deaminase knock-out (Ada−/−) mouse model [35, 36, 40, 41] and the transgenic sickle cell Berkeley mouse model [30, 33, 34, 3739]. However, the Berkeley mouse is the only well-accepted animal model that displays clinical manifestations similar to those seen in humans with severe forms of SCD, including priapism [30, 34].

Priapism is essentially a derangement of normal erection. Penile erection is a hemodynamic event that is regulated by smooth muscle relaxation/contraction of corpora cavernosa and associated arterioles during sexual stimulation. The penile flaccidity (detumescence state) is mainly maintained by tonic release of norepinephrine through the sympathetic innervations of vascular and cavernosal smooth muscle cells [46]. During penile erection (tumescence state), vascular smooth muscle relaxation decreases vascular resistance, thereby increasing blood flow through cavernous and helicine arteries and filling sinusoids, which are expanded due to the relaxation of smooth muscle cells in the CC [47]. This physiological relaxation of penile smooth muscle is mainly, although not solely, mediated by the neurotransmitter nitric oxide (NO) that is produced by enzymes called NO synthases (NOS). NOSs are subdivided into three isoforms, endothelial NOS (eNOS or NOS3), neural NOS (nNOS or NOS1), and inducible NOS (iNOS or NOS2) [48, 49]. In the penile smooth muscle, NO is released from both nitrergic nerves and the sinusoidal endothelium [46, 5052]. NO stimulates the soluble guanylyl cyclase (sGC) in the cavernosal smooth muscle, triggering increased synthesis of cyclic GMP (cGMP) that provides the main signal for smooth muscle relaxation [53]. cGMP levels in the CC are regulated by the rate of synthesis determined by sGC and the rate of cGMP hydrolysis mediated by phosphodiesterase type 5 (PDE5) [54, 55]. It has been reported that plasma hemoglobin released by intravascularly hemolysed sickle erythrocytes consumes NO, reducing its bioavailability in the erectile tissue, skewing the normal balance of smooth muscle tone towards vasoconstriction [17, 56, 57]. Champion and collaborators [33] showed that the penile smooth muscle of SCD transgenic mice presents with dysregulated PDE5A expression activity. Moreover, these mice had spontaneous priapism, amplified CC relaxation response mediated by the NO-cGMP signaling pathway, and increased intracavernosal pressure in vivo [37, 38].

Recent evidence has shown that another signaling pathway that may also contribute to the pathophysiology of priapism in SCD involves adenosine regulation. Similarly to NO, adenosine is a potent vasodilator produced by adenine nucleotide degradation. Adenosine is predominantly generated by adenosine monophosphate (AMP) dephosphorylation catalyzed by intracellular 5′-nucleotidase. Hydrolysis of s-adenosyl-homocysteine also contributes to intracellular adenosine formation [58, 59]. Extracellular adenosine may be generated by both adenine nucleotide degradation and dephosphorylation by ectonucleotidases [60]. Adenosine is then catabolized by two enzymes: adenosine kinase (ADK), which phosphorylates adenosine to AMP and is an important regulator of intracellular adenosine levels; and adenosine deaminase (ADA), which catalyzes the irreversible conversion of adenosine to inosine [58].

Several physiological processes may be affected by extracellular adenosine and this is mediated by four different receptors, referred to as A1, A2A, A2B, and A3. All four subtypes are members of the G protein-coupled receptor (GPCR) superfamily. The activation of the A1 and A3 adenosine receptors inhibits adenylyl cyclase activity and also results in increased activity of phospholipase C, while activation of the A2A and A2B subtypes increases adenylyl cyclase activity [58, 61]. Adenosine-induced vasodilation is mediated by increasing intracellular cyclic adenosine monophosphate (cAMP) levels in vascular smooth muscle cells via A2 receptor signaling [62, 63]. cAMP activates protein kinase A (PKA) resulting in decreased calcium-calmodulin-dependent MLC phosphorylation and enhanced smooth muscle relaxation [64]. Its role in penile erection has been investigated in studies showing that intracavernous injection of adenosine resulted in tumescence and penile erection [36, 61, 65]. In addition, adenosine induces NO synthesis in endothelial cells through A2 receptor signaling, and adenosine-mediated CC relaxation is partially dependent on endothelium-derived NO [36, 6670].

A priapic phenotype in Ada−/− mice was identified and led to further investigation of the impact of adenosine in the pathophysiology of priapism [59]. Previous reports showed that high levels of adenosine caused prolonged corporal smooth muscle relaxation in vitro. However, this effect was quickly corrected by intraperitoneal injection of a high dose of polyethylene glycol-ADA (PEG-ADA), which effectively reduces adenosine levels systemically [36, 71]. Moreover, adenosine induced significant increases in cavernosal cAMP levels via A2B receptor activation. This demonstrated that A2B receptor signaling is required for adenosine-mediated stimulation of cAMP production in CC smooth muscle cells [36, 71]. Mi and collaborators [36] have studied adenosine levels in the penis of sickle cell mice and have found a significant increase in adenosine levels, suggesting that overproduction of adenosine may contribute to priapic activity in SCD [71, 72]. Sickle cell mice submitted to PEG-ADA treatment suffered significant reduction of force and duration of relaxation when compared with untreated mice [71]. In addition, increased adenosine levels contributed to the development of penile fibrosis in Ada−/− mice as well as in transgenic sickle cell mice [72]. These findings suggest a general contributory role of elevated adenosine in the pathophysiology of priapism associated with SCD.

Although the penile vascular endothelium and smooth muscle cells are sources of vasodilation factors such as NO and adenosine, there are vasoconstriction pathways important to the penile hemodynamics, such as the Rho-kinase (ROCK) pathway. The RhoA/ROCK signal transduction pathway has been shown to influence erectile function in vivo through an array of mechanisms, including vasoconstriction of the penile vasculature via smooth muscle contraction and regulation of eNOS [7376]. This pathway is involved in the regulation of smooth muscle tone by modulating the sensitivity of contractile proteins to Ca2+ [77]. RhoA regulates smooth muscle contraction by cycling between a GDP-bound inactive form (coupled to a guanine dissociation inhibitor, RhoGDI) and a GTP-bound active form [7880]. Upstream activation of heterotrimeric G proteins leads to the exchange of GDP for GTP, an event carried out by the guanine exchange factors (GEFs) p115RhoGEF [81], PDZ-RhoGEF [82], and LARG (Leukemia-associated RhoGEF) [83], which are able to transduce signals from G protein-coupled receptors to RhoA [8486]. ROCK is activated by RhoA and inhibits myosin phosphatase through the phosphorylation of its myosin-binding subunit, leading to an increase in Ca2+ sensitivity. The RhoA/ROCK Ca2+ sensitization pathway has been implicated in the regulation of penile smooth muscle contraction and tone both in humans and animals [77, 87]. ROCK exerts contractile effects in the penis by Ca2+-independent promotion of myosin light chain (MLC) kinase or the attenuation of MLC phosphatase activity and reduction in endothelial-derived NO production [88]. RhoA activation, ROCK2 protein expression, as well as total ROCK activity decline in penile of SCD transgenic mice, highlighting that the molecular mechanism of priapism in SCD is associated with decreased vasoconstrictor activity in the penis [39]. Therefore, should impaired RhoA/ROCK-mediated vasoconstriction contribute to SCD-associated priapism, this pathway may become a novel therapeutic target in the management of this complication.

There has been no definite advance in the management of sickle cell-associated acute, severe priapism. Penile aspiration with or without saline intracavernosal injection and eventually performing surgical shunts remains mainstays of care, with no evident benefit of more common approaches, such as intravenous hydration, blood transfusions, and urinary alkalinization [89, 90]. Pharmacological interventions in such cases have been limited to intracavernosal use of sympathomimetic drugs, such as epinephrine, norepinephrine, and etilefrine, but there are anecdotal reports of acute use of PDE5 inhibitor sildenafil [91].

Nonetheless, most attempts to control SCD priapism have focused on its recurrent, stuttering form. Small case series of hormonal manipulation with diethylstilbestrol [92], gonadotropin-releasing hormone (GnRH) analogues [93], and finasteride [94] have been reported to successfully manage recurrent priapism. Increasing smooth muscle tone with oral α-agonist etilefrine has also yielded only anecdotal evidence of benefit [95]. Unfortunately, a prospective study comparing etilefrine and ephedrine failed to demonstrate superiority or equivalence of both drugs in preventing recurrent priapism due to poor compliance and low recruitment reducing statistical power, but some evidence was obtained reassuring safety of the use of such strategies, and possibly indicating a lower severity of priapism attacks among compliant patients [96]. This favors off-label use of pseudoephedrine at bedtime advocated by some experts [57, 90]. Hydroxyurea has also been effective in preventing priapism recurrence in SCD in a small number of cases [97, 98]. Based on current knowledge of NO-dependent pathways, the use of PDE5 inhibitors has been studied. One clinical trial testing tadalafil in SCD patients has been terminated, but no outcome data have yet been published ( NCT00538564), and one ongoing trial aims at the effect of sildenafil in the same setting ( NCT00940901). Despite these efforts, scientists have become less optimistic concerning the tolerability of this approach, ever since the premature termination of the sildenafil trial for pulmonary hypertension in SCD patients, in which subjects on PDE5 inhibitor were more likely to have severe pain crises requiring hospitalization [99]. Therefore, novel therapies for preventing and treating priapism in SCD are still warranted if the incidence of impotence among these patients is expected to be reduced in the long term.

3. Infertility

Progress in the therapy of SCD, particularly the use of hydroxyurea, has considerably improved the prognosis of patients with SCD [100, 101], with their mean life expectancy reaching much over 40 years [102104], rendering infertility an important issue. Nevertheless, long before hydroxyurea became a standard of care in SCD, seminal fluid parameters of SCD males had been reported to fall within the subfertile range due to decreased sperm concentration, total count, motility, and altered morphology [105107], and a more recent study reported over 90% of patients had at least one abnormal sperm parameter [108].

Hydroxyurea (HU) has been reported to impair spermatogenesis, causing testicular atrophy, reversible decrease in sperm count, as well as abnormal sperm morphology and motility [108114], and its current or previous use should be among the first probable causes to be considered in SCD patients complaining of infertility. Moreover, sperm abnormalities prior to HU have been attributed to variable effects of hypogonadism induced by SCD itself, and lack of appropriate testosterone production seems to be exacerbated by HU use in a mouse SCD model [115].

Considering that male fertility does not rely solely on the quality of the seminal fluid, other causes that may also render male patients with SCD prone to suffer from infertility include sexual problems, such as loss of libido, premature ejaculation, frequent priapism, and priapism-related impotence [105107, 116121].

Finding a single main cause for male infertility in a particular SCD patient is highly unlikely and probably will involve some degree of endocrinological impairment. A broader understanding of how hypogonadism takes place in SCD is necessary to explain fertility problems and requires knowledge of the complexity of sex hormone production regulation.

4. Hypogonadism

The etiology of hypogonadism in SCD patients is multifactorial, as several mechanisms have been suggested to contribute to its occurrence, such as primary gonadal failure [117, 122, 123], associated with or caused by repeated testicular infarction [124], zinc deficiency [125, 126], and partial hypothalamic hypogonadism [127].

Physical and sexual development are affected in both male and female SCD patients, with onset of puberty (menarche) and appearance of secondary sexual characteristics (pubic and axillary hair and beard) being usually delayed. The delay is greater in homozygous SCA and S-β0-thalassemia than in SC disease and S-β+-thalassemia [128130]. Moreover, studies in male patients with SCD reported reduction of ejaculate volume, spermatozoa count, motility, and abnormal sperm morphology [106, 116].

Biochemical analyses have demonstrated low levels of testosterone and dihydrotestosterone and variable levels of follicle-stimulating hormone (FSH) and luteinizing hormone (LH) in patients with SCD [105107, 118, 119, 121, 131]. The comparison between patients and controls matched according to stage of development of secondary sexual characteristics showed higher levels of LH in sickle cell disease, favoring some role for hypergonadotropic hypogonadism.

Leydig cells of the testes and other steroidogenic tissues produce hormones by a multienzymatic process, in which free cholesterol from intracellular stores is transferred to the outer and then to the inner mitochondrial membrane. Leydig cells produce androgens under the control of LH or its placental counterpart human chorionic gonadotropin (hCG), as well as in response to numerous intratesticular factors [114, 132]. LH/hCG receptors belong to the sGC-coupled seven-transmembrane-domain receptor family, whose activation leads to stimulation of adenylyl cyclase [133]. The resulting accumulation of intracellular cyclic adenosine monophosphate (cAMP) levels and the concomitant activation of the cAMP-dependent protein kinase (PKA) lead to the phosphorylation of numerous proteins, including the steroidogenic acute regulatory (StAR) protein [134, 135]. StAR localizes predominantly to steroid hormone-producing tissues and consists of a 37 kDa precursor containing an NH2-terminal mitochondrial targeting sequence and several isoelectric 30 kDa mature protein forms [136138]. Steroid production in gonadal and adrenal cells requires both de novo synthesis and PKA-dependent phosphorylation of StAR-37 protein [139]. The newly synthesized StAR is functional and plays a critical role in the transfer of cholesterol from the outer to the inner mitochondrial membrane, whereas mitochondrial import and processing to 30 kDa StAR protein terminate this action [140142].

HbS polymerization is mediated by upstream activation of adenosine receptor A2BR by hypoxia, and hemolysis of irreversibly sickled red blood cells increases adenosine bioavailability through conversion of ATP by ectonucleotidases CD39 and CD73, thus predisposing patients with SCD to sustained high levels of cAMP [143, 144]. From this point of view, steroidogenesis could be expected to be increased in these patients.

Although Leydig cell steroidogenesis is predominantly regulated by cAMP/PKA, other pathways also influence this process [145], including the NO-cGMP signaling pathway [146]. NO promotes a biphasic modulation in the androgen production, stimulatory at low concentrations, and inhibitory at high concentrations [49, 147, 148]. SCA causes NO depletion, and in low levels, NO stimulates Leydig cell steroidogenesis by activating sGC [48, 49, 149] and promotes the formation of low levels of cGMP, albeit enough to activate the cGMP-dependent protein kinase (PKG) and phosphorylate StAR [49, 150]. This signaling is controlled by phosphodiesterases (PDEs) [151] and active transport systems that export cyclic nucleotides (multidrug-resistance proteins) from the cell [152]. In zona glomerulosa cells, activation of PKG II by cGMP regulates basal levels of aldosterone production and phosphorylation of StAR protein [150], but whether there is a role for cGMP in the zona reticularis, where adrenal androgenesis takes place, is unknown.

Hypogonadism observed in patients with SCD with lower circulating testosterone and higher LH levels suggests that, at least in this setting, despite the reduced cGMP- and elevated cAMP-mediated stimuli on androgen production, gonadal failure with Leydig cell impairment predominates in sex hormone production dysfunction (Figure 1). This further highlights that primary hypogonadism is possibly largely underdiagnosed and elicits more studies on the pathogenesis of testicular infarction.

5. Testicular Infarction

Segmental testicular infarction is an infrequent cause of acute scrotum and is rarely reported, with fewer than 40 cases published at the time of this paper. Its etiology is not always well defined, and it may be, at first, clinically mistaken for a testicular tumour [153, 154]. Common causes for testicular infarction are torsion of the spermatic cord, incarcerated hernia, infection, trauma, and vasculitis [131]. The usual presentation is a painful testicular mass unresponsive to antibiotics [155]. This testicular disorder has been associated with epididymitis, hypersensitivity angiitis, intimal fibroplasia of the spermatic cord arteries, polycythemia, anticoagulant use, benign testicular tumors and, in the interest of this review, sickle cell trait and sickle cell disease [124, 131, 155158].

Testicular infarction related to sickling has been very rarely reported with only five individual cases found retrospectively, three associated with sickle cell disease and two with sickle cell trait [124, 155157, 159]. Holmes and Kane reported the first testicular infarction in a patient with SCD who presented with testicular swelling unresponsive to antibiotics. Physical examination revealed that a lesion suspicious for malignancy and ultrasonography demonstrated a hyperechoic mass with an anechoid rim and normal blood flow in the surrounding parenchyma. Radical orchiectomy revealed hemorrhagic infarction with sickle blood red cells. In another case report, SCA patient presented with acute scrotum and history of acute chest syndrome, splenic infarction, osteomyelitis, and hemolysis. Physical examination demonstrated an erythematous, tender, swollen testicle and ultrasound once again revealed normal echotexture and blood flow. Surgical exploration and pathological examination diagnosed segmental testicular infarction with vascular congestion and sickled red blood cells [124]. In the last testicular infarction case report in a patient with SCD presented with increased testicular volume, scrotal ultrasonography showed both echogenic and hypoechogenic regions and Doppler ultrasonography revealed vascular changes compatible with testicular infarction. Radical orchiectomy was performed 10 days after the initial presentation and microscopic evaluation showed necrotic seminiferous tubules devoid of nuclear debris, congestion, or acute inflammatory infiltrate, consistent with coagulative necrosis of ischemic origin [131].

Testicular blood flow is dependent on the internal spermatic, cremasteric, and deferential arteries. Obstruction of venous outflow may create venous thrombosis, testicular engorgement, and subsequent hemorrhagic infarction. In SCD, low oxygen tensions in erythrocytes lead to sickling cells that lose pliability in the microcirculation. Consequently, capillary flow becomes obstructed, worsening local tissue hypoxia, perpetuating the cycle of sickling, and promoting testicular infarction [124, 131, 157].

The cyclic nucleotides and protein kinases may play an important role in the pathophysiology of testicular infarction in SCD. Enhanced hemolysis and oxidative stress contribute to a reduction in nitric oxide (NO) bioavailability due to NO scavenging by free hemoglobin and reactive oxygen species (ROS) generation [160, 161]. As mentioned before, testicular NO signaling pathway is involved in the regulation of Leydig cell steroidogenesis [48, 49, 147149, 162164] but may also influence testicular circulation. We suggest that the reduction of NO bioavailability and consequent reduction of GMPc levels and of activity of PKG may decrease the vasodilation process in the testes. Moreover, reduced NO levels in patients with sickle cell disease contribute to the development of thrombus formation in the vascular system and could further enhance local ischemia [165, 166]. Furthermore, the cGMP-dependent protein kinase signaling pathway would normally inhibit RhoA-induced Ca2+ sensitization, RhoA/ROCK signaling, and protein kinase C (PKC) activity that mediate contraction in vascular smooth muscle [167171]. Thus, reduced NO levels may decrease cGMP-dependent protein kinase activity and promote increasing RhoA-induced Ca2+ sensitization and PKC activity, favoring vasoconstriction in the testes. Therefore, tissue hypoxia, sickling of red blood cells, reduced levels of NO, possible thrombus formation, increased RhoA-induced Ca2+ sensitization, and PKC activity may all lead to capillary and venous flow obstruction promoting testicular infarction (Figure 1).

Although testicular infarction in SCD has been very rarely reported, it has been speculated that silent testicular infarctions are much more common but generally overlooked clinically. Testicular biopsy in patients is rarely performed and additional studies are necessary to establish the true incidence of testicular infarction in patients with SCD or even sickle cell trait.

6. Urinary Bladder Dysfunction

The urinary bladder has two important functions: urine storage and emptying. Urine storage occurs at low pressure, implying that the bladder relaxes during the filling phase. Disturbances of the storage function may result in lower urinary tract symptoms (LUTSs), such as urgency, increased frequency, and urge incontinence, the components of the hypoactive or overactive bladder syndromes [172, 173]. The passive phase of bladder filling allows an increase in volume at a low intravesical pressure. The bladder neck and urethra remain in a tonic state to prevent leakage, thus maintaining urinary continence. Bladder emptying is accompanied by a reversal of function in which detrusor smooth muscle (DSM) contraction predominates in the bladder body that is accompanied by a concomitant reduction in outlet resistance of the bladder neck and urethra [174176]. The bladder filling and emptying are regulated by interactions of norepinephrine (sympathetic component released by hypogastric nerve stimulation), acetylcholine and ATP (parasympathetic components released by pelvic nerve stimulation) with activation of adrenergic, muscarinic, and purinergic receptors, respectively [175].

Urinary bladder dysfunction is rarely spontaneously reported by SCD patients to their caregivers. With increasing survival of these patients, physicians may expect that urinary complaints increase in association with classical urological disorders associated with advanced age, such as urinary stress incontinence in multiparous women and benign prostatic hyperplasia in men. Nonetheless, clinical observations of medical complaints involving the urinary bladder start as early as childhood, with enuresis, and continue onto adulthood with nocturia and urinary tract infections, to name a few, although frequently neglected.

Nocturia has long been attributed to constant increased urinary volumes in SCD. As part of the renal complications of sickling, renal medullary infarcts lead to decreased ability to concentrate urine, yielding higher daily urinary volumes [177], compensatory polydipsia, and eventually, the need for nocturnal bladder voiding.

For comparison, the effects of polyuria on bladder function have been better characterized in diabetic bladder dysfunction (DBD). Both SCD and diabetes mellitus cause increased urinary volume and, to some extent, the two diseases involve cellular damage by oxidative stress mediators; so data from previous studies on DBD may help shed some light on preliminary data on bladder function in SCD animal models by understanding a known model of bladder dysfunction.

It has been suggested that DBD comprehends so-called early and late phases of the disease, owing to cumulative effects of initial polyuria secondary to hyperglycemia, complicated by oxidative stress influence on the urothelium and nervous damage in the long term of the natural history of diabetes mellitus. In the early phase of DBD, the bladder is hyperactive, leading to LUTS comprised mainly by nocturia and urge incontinence. Later in the course of the disease, the detrusor smooth muscle becomes atonic, abnormally distended, and incontinence is mainly by overflow associated with a poor control of urethral sphincters, and voiding problems take over [178].

DSM physiology also involves cyclic nucleotides and activation of protein kinases. DSM contractions are a consequence of cholinergic-mediated contractions and decreased β-adrenoceptor-mediated relaxations [179]. DSM contains a heterogeneous population of muscarinic receptor subtypes [180, 181], with a predominance of the M2 subtype and a smaller population of M3 receptors. However, functional studies showed that M3 receptors are responsible for promotion of contraction in the DSM of several animal models [182185] and in humans [186, 187]. Activation of M3 muscarinic receptors in the DSM promotes stimulation of phospholipase C, activates PKC, and increases formation of inositol trisphosphate (IP3) and diacylglycerol (DAG) to release calcium from intracellular stores, leading to DSM contraction [87]. Moreover, activation of M2 receptors also induces a DSM contraction indirectly by inhibiting the production of cAMP, reducing PKA activity, and reversing the relaxation induced by β-adrenoceptors [179]. Hence, both mechanisms promote urinary bladder emptying.

There is evidence that the Ca2+-independent RhoA/ROCK pathway is involved in the regulation of smooth muscle tone by altering the sensitivity of contractile proteins to Ca2+ [77]. This pathway has been shown to influence erectile function in vivo through an array of mechanisms, including phosphorylation of the myosin-binding subunit of MLC phosphatase, resulting in increased myosin phosphorylation. RhoA, a member of the Ras (Rat Sarcoma) low molecular weight of GTP-binding proteins, mediates agonist-induced activation of ROCK. The exchange of GDP for GTP on RhoA and translocation of RhoA from the cytosol to the membrane are markers of its activation and enable the downstream stimulation of various effectors such as ROCK, protein kinase N, phosphatidylinositol 3-kinase, and tyrosine phosphorylation [77]. The RhoA/ROCK Ca2+ sensitization pathway has been implicated in the regulation of bladder smooth muscle contraction and tone in humans and animals [77, 188191]. Thus, alterations in the contraction or relaxation mechanisms of DSM during the filling and emptying phases may contribute to urinary bladder dysfunction. Patients with SCD have not been evaluated for bladder dysfunction in a systematic manner, but preliminary data have shown that Berkeley mice (homozygous SS) exhibit hypocontractile DSM ex vivo, due to a significant decrease of contractile responses to muscarinic agonist carbachol and electrical field stimulation [192]. This bladder dysfunction may contribute to the increased risk of urinary tract infections observed in SCD patients.

In an epidemiological study of 321 children with SCD, 7% had a documented urinary tract infection (UTI), one-third had recurrent infections, and two-thirds had had a febrile UTI [193]. As in normal children, there was a strong predominance of females, and gram-negative organisms, particularly Escherichia coli, were usually cultured. Most episodes of gram-negative septicemia in SCD are secondary to UTI [194]. Moreover, UTIs are more frequent during pregnancy in women with SCA or sickle cell trait [195197]. The prevalence of UTI in women with SCA is nearly twofold that of unaffected black American women. This association appears to be directly related to HbS levels, since patients with sickle trait have an increased prevalence of bacteriuria, but to a lesser degree than those with SCA. More recently, a study detected that a group of SCD children and adolescents had more symptoms of overactive bladder than a control group [198]. This could be a first documentation of a clinically evident of an early phase of sickle cell bladder dysfunction, but whether there is a late, hypotonic bladder phase in older sickle cell adults remains to be demonstrated.

The presence of increased intracavernosal pressure associated with the amplified corpus cavernosum relaxation response (priapism) mediated by NO-cGMP signaling pathway, the lack of RhoA/ROCK-mediated vasoconstriction in sickle cell transgenic Berkeley mice, and the association of priapism with genitourinary infections and urinary retention further suggest the possibility that changes in the DSM reactivity may contribute to urogenital complications in SCD [36, 3840, 192]. Despite advances in the understanding of urogenital disorders in the SCD, further studies should clarify the pathophysiological mechanisms that underlie genitourinary manifestations of SCD.

7. Conclusions

Urogenital disorders in SCD are the result of pleotropic effects of the production of the abnormal sickling hemoglobin S. While priapism still stands out as the most frequently encountered, current knowledge of the effects of cyclic nucleotide production and activation of protein kinases allows to suspect underdiagnosis of bladder dysfunction and hypogonadism secondary to testicular failure. Moreover, despite our growing understanding of these complications, adequate, efficacious, and well-tolerated treatments are still unavailable, and male patients continue to suffer from infertility and erectile dysfunction. Further work in, both clinical assessments and experimental studies in this field are promising and should help increase physicians' awareness of the importance of more accurate diagnoses, design improved therapeutic strategies, and eventually, achieve better quality of life for SCD patients.

ROS: Reactive oxygen species
NO: Nitric oxide
cAMP: Cyclic adenosine monophosphate
PKA: Cyclic adenosine monophosphate-dependent protein kinase
cGMP: Cyclic Guanosine monophosphate;
PKG: Cyclic Guanosine monophosphate protein kinase;
PKC: Protein kinase C.

1. Herrick CJ. The evolution of intelligence and its organsScienceYear: 19103178471817758520
2. Steinberg MH. Management of sickle cell diseaseThe New England Journal of MedicineYear: 1999340131021103010099145
3. Kato GJ,Gladwin MT. Evolution of novel small-molecule therapeutics targeting sickle cell vasculopathyJournal of the American Medical AssociationYear: 2008300222638264619066384
4. Conran N,Franco-Penteado CF,Costa FF. Newer aspects of the pathophysiology of sickle cell disease vaso-occlusionHemoglobinYear: 200933111619205968
5. Hebbel RP,Boogaerts MAB,Eaton JW,Steinberg MH. Erythrocyte adherence to endothelium in sickle-cell anemia. A possible determinant of disease severityThe New England Journal of MedicineYear: 1980302189929957366623
6. Francis RB Jr.,Johnson CS. Vascular occlusion in sickle cell disease: current concepts and unanswered questionsBloodYear: 1991777140514142009364
7. Diggs LW,Ching RE. Pathology of sickle cell anemiaSouthern Medical JournalYear: 193427839845
8. Adeyoju AB,Olujohungbe ABK,Morris J,et al. Priapism in sickle-cell disease; incidence, risk factors and complications—an international multicentre studyBJU InternationalYear: 200290989890212460353
9. Nolan VG,Wyszynski DF,Farrer LA,Steinberg MH. Hemolysis-associated priapism in sickle cell diseaseBloodYear: 200510693264326715985542
10. Bivalacqua TJ,Burnett AL. Priapism: new concepts in the pathophysiology and new treatment strategiesCurrent Urology ReportsYear: 20067649750217052448
11. Crane GM,Bennett NE Jr.. Priapism in sickle cell anemia: emerging mechanistic understanding and better preventative strategiesAnemiaYear: 201120116 pages Article ID 297364..
12. American Foundation for Urologic DiseaseThought leader panel on evaluation and treatment of priapism. Report of the American Foundation for Urologic Disease (AFUD) thought leader panel for evaluation and treatment of priapismInternational Journal of Impotence ResearchYear: 200115supplementS39S43
13. Numan F,Cantasdemir M,Ozbayrak M,et al. Posttraumatic nonischemic priapism treated with autologous blood clot embolizationJournal of Sexual MedicineYear: 20085117317918173765
14. Burnett AL,Bivalacqua TJ. Glucose-6-phosphate dehydrogenase deficiency: an etiology for idiopathic priapism?Journal of Sexual MedicineYear: 20085123724017971109
15. Finley DS. Glucose-6-phosphate dehydrogenase deficiency associated stuttering priapism: report of a caseJournal of Sexual MedicineYear: 20085122963296618823322
16. Jin YC,Gam SC,Jung JH,Hyun JS,Chang KC,Hyun JS. Expression and activity of heme oxygenase-1 in artificially induced low-flow priapism in rat penile tissuesJournal of Sexual MedicineYear: 2008581876188218554260
17. Burnett AL. Pathophysiology of priapism: dysregulatory erection physiology thesisJournal of UrologyYear: 20031701263412796638
18. Bivalacqua TJ,Musicki B,Kutlu O,Burnett AL. New insights into the pathophysiology of sickle cell disease-associated priapismJournal of Sexual MedicineYear: 20119798721554553
19. Dong Q,Deng S,Wang R,Yuan J. In vitro and in vivo animal models in priapism researchJournal of Sexual MedicineYear: 20118234735920946160
20. Chen KK,Chan JY,Chang LS,Chen MT,Chan SH. Intracavernous pressure as an experimental index in a rat model for the evaluation of penile erectionJournal of UrologyYear: 19921474112411281552608
21. Ul-Hasan M,El-Sakka AI,Lee C,Yen TS,Dahiya R,Lue TF. Expression of TGF-beta-1 mRNA and ultrastructural alterations in pharmacologically induced prolonged penile erection in a canine modelThe Journal of UrologyYear: 19981606226322669817381
22. Munarriz R,Park K,Huang YH,et al. Reperfusion of ischemic corporal tissue: physiologic and biochemical changes in an animal model of ischemic priapismUrologyYear: 200362476076414550468
23. Evliyaoglu Y,Kayrin L,Kaya B. Effect of allopurinol on lipid peroxidation induced in corporeal tissue by veno-occlusive priapism in a rat modelBritish Journal of UrologyYear: 19978034764799313672
24. Evliyaoğlu Y,Kayrin L,Kaya B. Effect of pentoxifylline on veno-occlusive priapism-induced corporeal tissule lipid peroxidation in a rat modelUrological ResearchYear: 19972521431479144883
25. Sanli O,Armagan A,Kandirali E,et al. TGF-β1 neutralizing antibodies decrease the fibrotic effects of ischemic priapismInternational Journal of Impotence ResearchYear: 200416649249715284835
26. Jin YC,Gam SC,Jung JH,Hyun JS,Chang KC,Hyun JS. Expression and activity of heme oxygenase-1 in artificially induced low-flow priapism in rat penile tissuesJournal of Sexual MedicineYear: 2008581876188218554260
27. Uluocak N,AtIlgan D,Erdemir F,et al. An animal model of ischemic priapism and the effects of melatonin on antioxidant enzymes and oxidative injury parameters in rat penisInternational Urology and NephrologyYear: 201042488989520112132
28. Huang PL,Dawson TM,Bredt DS,Snyder SH,Fishman MC. Targeted disruption of the neuronal nitric oxide synthase geneCellYear: 1993757127312867505721
29. Huang PL,Huang Z,Mashimo H,et al. Hypertension in mice lacking the gene for endothelial nitric oxide synthaseNatureYear: 199537765462392427545787
30. Pászty C,Brion CM,Manci E,et al. Transgenic knockout mice with exclusively human sickle hemoglobin and sickle cell diseaseScienceYear: 199727853398768789346488
31. Huang PL. Lessons learned from nitric oxide synthase knockout animalsSeminars in PerinatologyYear: 2000241879010709868
32. Barouch LA,Harrison RW,Skaf MW,et al. Nitric oxide regulates the heart by spatial confinement of nitric oxide synthase isoformsNatureYear: 2002416687833734011907582
33. Champion HC,Bivalacqua TJ,Takimoto E,Kass DA,Burnett AL. Phosphodiesterase-5A dysregulation in penile erectile tissue is a mechanism of priapismProceedings of the National Academy of Sciences of the United States of AmericaYear: 200510251661166615668387
34. HsuHemolysis in sickle cell mice causes pulmonary hypertension due to global impairment in nitric oxide bioavailabilityBloodYear: 200710973088309817158223
35. Yuan JH,Chunn JL,Mi TJ,et al. Adenosine deaminase knockout in mice induces priapism via A2b receptorJournal of UrologyYear: 2007177, supplementp. 227
36. Mi T,Abbasi S,Zhang H,et al. Excess adenosine in murine penile erectile tissues contributes to priapism via A2B adenosine receptor signalingJournal of Clinical InvestigationYear: 200811841491150118340377
37. Bivalacqua TJ,Musicki B,Hsu LL,Gladwin MT,Burnett AL,Champion HC. Establishment of a transgenic sickle-cell mouse model to study the pathophysiology of priapismJournal of Sexual MedicineYear: 2009692494250419523035
38. Claudino MA,Franco-penteado CF,Corat MAF,et al. Increased cavernosal relaxations in sickle cell mice priapism are associated with alterations in the NO-cGMP signaling pathwayJournal of Sexual MedicineYear: 2009682187219619493282
39. Bivalacqua TJ,Ross AE,Strong TD,et al. Attenuated rhoA/rho-kinase signaling in penis of transgenic sickle cell miceUrologyYear: 2010762510.e7510.e1220538321
40. Wen J,Jiang X,Dai Y,et al. Adenosine deaminase enzyme therapy prevents and reverses the heightened cavernosal relaxation in priapismJournal of Sexual MedicineYear: 2010793011302219845544
41. Wen J,Jiang X,Dai Y,et al. Increased adenosine contributes to penile fibrosis, a dangerous feature of priapism, via A2B adenosine receptor signalingThe FASEB JournalYear: 2010243740749
42. Tong Y,Tar M,Davelman F,Christ G,Melman A,Davies KP. Variable coding sequence protein A1 as a marker for erectile dysfunctionBJU InternationalYear: 200698239640116879685
43. Tong Y,Tar M,Monrose V,DiSanto M,Melman A,Davies KP. hSMR3A as a marker for patients with erectile dysfunctionJournal of UrologyYear: 2007178133834317512016
44. Tong Y,Tar M,Melman A,Davies K. The opiorphin gene (ProL1) and its homologues function in erectile physiologyBJU InternationalYear: 2008102673674018410445
45. Kanika ND,Tar M,Tong Y,Kuppam DSR,Melman A,Davies KP. The mechanism of opiorphin-induced experimental priapism in rats involves activation of the polyamine synthetic pathwayAmerican Journal of PhysiologyYear: 20092974C916C92719657052
46. Andersson KE. Pharmacology of penile erectionPharmacological ReviewsYear: 200153341745011546836
47. Phatarpekar PV,Wen J,Xia Y. Role of adenosine signaling in penile erection and erectile disordersJournal of Sexual MedicineYear: 20107113553356419889148
48. Davidoff MS,Middendorff R,Mayer B,DeVente J,Koesling D,Holstein AF. Nitric oxide/cGMP pathway components in the Leydig cells of the human testisCell and Tissue ResearchYear: 199728711611709011392
49. Andric SA,Janjic MM,Stojkov NJ,Kostic TS. Protein kinase G-mediated stimulation of basal Leydig cell steroidogenesisAmerican Journal of PhysiologyYear: 20072935E1399E140817848628
50. Burnett AL,Lowenstein CJ,Bredt DS,Chang TSK,Snyder SH. Nitric oxide: a physiologic mediator of penile erectionScienceYear: 199225750684014031378650
51. Andersson KE,Wagner G. Physiology of penile erectionPhysiological ReviewsYear: 19957511912367831397
52. Lue TF. Erectile dysfunctionThe New England Journal of MedicineYear: 20003421802181310853004
53. Lucas KA,Pitari GM,Kazerounian S,et al. Guanylyl cyclases and signaling by cyclic GMPPharmacological ReviewsYear: 200052337541410977868
54. Boolell M,Allen MJ,Ballard SA,et al. Sildenafil: an orally active type 5 cyclic GMP-specific phosphodiesterase inhibitor for the treatment of penile erectile dysfunctionInternational Journal of Impotence ResearchYear: 19968247528858389
55. Gopal VK,Francis SH,Corbin JD. Allosteric sites of phosphodiesterase-5 (PDE5). A potential role in negative feedback regulation of cGMP signaling in corpus cavernosumEuropean Journal of BiochemistryYear: 2001268113304331211389733
56. Ohene-Frempong K,Steinberg MH. Steinberg MH,Forget BG,Higgs DR,Nagel RLClinical aspects of sickle cell anemia in adults and childrenDisorders of Hemoglobin: Genetics, Pathophysiology and Clinical ManagementYear: 2001Cambridge, UKCambridge University Press611670
57. Rogers ZR. Priapism in sickle cell diseaseHematology/Oncology Clinics of North AmericaYear: 20051991792816214652
58. Fredholm BB,Ijzerman AP,Jacobson KA,Klotz KN,Linden J. International Union of Pharmacology. XXV. Nomenclature and classification of adenosine receptorsPharmacological ReviewsYear: 200153452755211734617
59. Phatarpekar PV,Wen J,Xia Y. Role of adenosine signaling in penile erection and erectile disordersJournal of Sexual MedicineYear: 20107113553356419889148
60. Colgan SP,Eltzschig HK,Eckle T,Thompson LF. Physiological roles for ecto-5′-nucleotidase (CD73)Purinergic SignallingYear: 20062235136018404475
61. Tostes RC,Giachini FRC,Carneiro FS,Leite R,Inscho EW,Webb RC. Determination of adenosine effects and adenosine receptors in murine corpus cavernosumJournal of Pharmacology and Experimental TherapeuticsYear: 2007322267868517494861
62. Olsson RA,Pearson JD. Cardiovascular purinoceptorsPhysiological ReviewsYear: 19907037618452194223
63. Tager AM,LaCamera P,Shea BS,et al. The lysophosphatidic acid receptor LPA1 links pulmonary fibrosis to lung injury by mediating fibroblast recruitment and vascular leakNature MedicineYear: 20081414554
64. Lin CS,Lin G,Lue TF. Cyclic nucleotide signaling in cavernous smooth muscleJournal of Sexual MedicineYear: 20052447849116422842
65. Prieto D. Physiological regulation of penile arteries and veinsInternational Journal of Impotence ResearchYear: 2008201172917637789
66. Vials A,Burnstock G. A2-purinoceptor-mediated relaxation in the guinea-pig coronary vasculature: a role for nitric oxideBritish Journal of PharmacologyYear: 199310924244298358543
67. Sobrevia L,Yudilevich DL,Mann GE. Activation of A2-purinoceptors by adenosine stimulates L-arginine transport (system y+) and nitric oxide synthesis in human fetal endothelial cellsJournal of PhysiologyYear: 199749911351409061645
68. Li JM,Fenton RA,Wheeler HB,et al. Adenosine A2a receptors increase arterial endothelial cell nritric oxideJournal of Surgical ResearchYear: 19988023573649878338
69. Chiang PH,Wu SN,Tsai EM,et al. Adenosine modulation of neurotransmission in penile erectionBritish Journal of Clinical PharmacologyYear: 19943843573627833226
70. Faria M,Magalhães-Cardoso T,Lafuente-De-Carvalho JM,Correia-De-Sá P. Corpus cavernosum from men with vasculogenic impotence is partially resistant to adenosine relaxation due to endothelial A2B receptor dysfunctionJournal of Pharmacology and Experimental TherapeuticsYear: 2006319140541316837560
71. Dai Y,Zhang Y,Phatarpekar P,et al. Adenosine signaling, priapism and novel therapiesJournal of Sexual MedicineYear: 200963, supplement29230119267852
72. Wen J,Jiang X,Dai Y,et al. Increased adenosine contributes to penile fibrosis, a dangerous feature of priapism, via A2B adenosine receptor signalingThe FASEB JournalYear: 2010243740749
73. Chitaley K,Wingard CJ,Clinton Webb R,et al. Antagonism of Rho-kinase stimulates rat penile erection via a nitric oxide-independent pathwayNature MedicineYear: 200171119122
74. Mills TM,Chitaley K,Wingard CJ,Lewis RW,Webb RC. Effect of rho-kinase inhibition on vasoconstriction in the penile circulationJournal of Applied PhysiologyYear: 20019131269127311509525
75. Bivalacqua TJ,Champion HC,Usta MF,et al. RhoA/Rho-kinase suppresses endothelial nitric oxide synthase in the penis: a mechanism for diabetes-associated erectile dysfunctionProceedings of the National Academy of Sciences of the United States of AmericaYear: 2004101249121912615184671
76. Musicki B,Ross AE,Champion HC,Burnett AL,Bivalacqua TJ. Posttranslational modification of constitutive nitric oxide synthase in the penisJournal of AndrologyYear: 200930435236219342700
77. Somlyo AP,Somlyo AV. Ca2+ sensitivity of smooth muscle and nonmuscle myosin II: modulated by G proteins, kinases, and myosin phosphatasePhysiological ReviewsYear: 20038341325135814506307
78. Wettschureck N,Offermanns S. Rho/Rho-kinase mediated signaling in physiology and pathophysiologyJournal of Molecular MedicineYear: 2002801062963812395147
79. Riento K,Ridley AJ. Rocks: multifunctional kinases in cell behaviourNature Reviews Molecular Cell BiologyYear: 200346446456
80. Bhattacharya M,Babwah AV,Ferguson SSG. Small GTP-binding protein-coupled receptorsBiochemical Society TransactionsYear: 20043261040104415506958
81. Hart MJ,Sharma S,Elmasry N,et al. Identification of a novel guanine nucleotide exchange factor for the Rho GTPaseJournal of Biological ChemistryYear: 19962714125452254588810315
82. Fukuhara S,Murga C,Zohar M,Igishi T,Gutkind JS. A novel PDZ domain containing guanine nucleotide exchange factor links heterotrimeric G proteins to RhoJournal of Biological ChemistryYear: 199927495868587910026210
83. Kourlas PJ,Strout MP,Becknell B,et al. Identification of a gene at 11q23 encoding a guanine nucleotide exchange factor: evidence for its fusion with MLL in acute myeloid leukemiaProceedings of the National Academy of Sciences of the United States of AmericaYear: 20009752145215010681437
84. Ross EM,Wilkie TM. GTPase-activating proteins for heterotrimeric G proteins: regulators of G Protein Signaling (RGS) and RGS-like proteinsAnnual Review of BiochemistryYear: 200069795827
85. Fukuharaa S,Chikumi H,Silvio Gutkind J. RGS-containing RhoGEFs: the missing link between transforming G proteins and Rho?OncogeneYear: 200120131661166811313914
86. Schmidt A,Hall A. Guanine nucleotide exchange factors for Rho GTPases: turning on the switchGenes and DevelopmentYear: 200216131587160912101119
87. Teixeira CE,Priviero FBM,Webb RC. Effects of 5-cyclopropyl-2-[1-(2-fluoro-benzyl)-1H-pyrazolo[3,4-b]pyridine- 3-yl]pyrimidin-4-ylamine (BAY 41-2272) on smooth muscle tone, soluble guanylyl cyclase activity, and NADPH oxidase activity/expression in corpus cavernosum from wild-type, neuronal, and endothelial nitric-oxide synthase null miceJournal of Pharmacology and Experimental TherapeuticsYear: 200732231093110217596536
88. Somlyo AV. New roads leading to Ca2+ sensitizationCirculation ResearchYear: 2002912838412142337
89. Mantadakis E,Ewalt DH,Cavender JD,Rogers ZR,Buchanan GR. Outpatient penile aspiration and epinephrine irrigation for young patients with sickle cell anemia and prolonged priapismBloodYear: 2000951788210607688
90. Kato GJ. Priapism in sickle-cell disease: a hematologist's perspectiveThe Journal of Sexual MedicineYear: 201291707821554552
91. Bialecki ES,Bridges KR. Sildenafil relieves priapism in patients with sickle cell diseaseAmerican Journal of MedicineYear: 20021133p. 252
92. Serjeant GR,de Ceulaer K,Maude GH. Stilboestrol and stuttering priapism in homozygous sickle-cell diseaseThe LancetYear: 19852846712741276
93. Levine LA,Guss SP. Gonadotropin-releasing hormone analogues in the treatment of sickle cell anemia-associated priapismJournal of UrologyYear: 199315024754778326584
94. Rachid-Filho D,Cavalcanti AG,Favorito LA,Costa WS,Sampaio FJB. Treatment of recurrent priapism in sickle cell anemia with finasteride: a new approachUrologyYear: 20097451054105719616292
95. Okpala I,Westerdale N,Jegede T,Cheung B. Etilefrine for the prevention of priapism in adult sickle cell diseaseBritish Journal of HaematologyYear: 2002118391892112181066
96. Seftel AD. A prospective diary study of stuttering priapism in adolescents and young men with sickle cell anemia: report of an international randomized control trial; The priapism in sickle cell study (PISCES study)Journal of UrologyYear: 201118551837183822088730
97. Saad STO,Lajolo C,Gilli S,et al. Follow-up of sickle cell disease patients with priapism treated by hydroxyureaAmerican Journal of HematologyYear: 2004771454915307105
98. Hassan A,Jam’a A,Al Dabbous IA. Hydroxyurea in the treatment of sickle cell associated priapismJournal of UrologyYear: 19981595p. 1642
99. Machado RF,Barst RJ,Yovetich NA,et al. Hospitalization for pain in patients with sickle cell disease treated with sildenafil for elevated TRV and low exercise capacityBloodYear: 2011118485586421527519
100. Charache S,Terrin ML,Moore RD,et al. Effect of hydroxyurea on the frequency of painful crises in Sickle cell anemiaThe New England Journal of MedicineYear: 199533220131713227715639
101. Bakanay SM,Dainer E,Clair B,et al. Mortality in sickle cell patients on hydroxyurea therapyBloodYear: 2005105254554715454485
102. Platt OS,Brambilla DJ,Rosse WF,et al. Mortality in sickle cell disease—life expectancy and risk factors for early deathThe New England Journal of MedicineYear: 199433023163916447993409
103. Powars DR,Chan LS,Hiti A,Ramicone E,Johnson C. Outcome of sickle cell anemia: a 4-decade observational study of 1056 patientsMedicineYear: 200584636337616267411
104. Fitzhugh CD,Lauder N,Jonassaint JC,et al. Cardiopulmonary complications leading to premature deaths in adult patients with sickle cell diseaseAmerican Journal of HematologyYear: 2010851364020029950
105. Nahoum CRD,Fontes EA,Freire FR. Semen analysis in sickle cell diseaseAndrologiaYear: 19801265425457469046
106. Osegbe DN,Akinyanju O,Amaku EO. Fertility in males with sickle cell diseaseThe LancetYear: 198128241275276
107. Agbaraji VO,Scott RB,Leto S,Kingslow LW. Fertility studies in sickle cell disease: semen analysis in adult male patientsInternational Journal of FertilityYear: 19883353473522904423
108. Berthaut I,Guignedoux G,Kirsch-Noir F,et al. Influence of sickle cell disease and treatment with hydroxyurea on sperm parameters and fertility of human malesHaematologicaYear: 200893798899318508803
109. Lu CC,Meistrich ML. Cytotoxic effects of chemotherapeutic drugs on mouse testis cellsCancer ResearchYear: 197939935753582476683
110. Ficsor G,Ginsberg LC. The effect of hydroxyurea and mitomycin C on sperm motility in miceMutation ResearchYear: 19807033833877383042
111. Singh H,Taylor C. Effects of Thio-TEPA and hydroxyurea on sperm production in Lakeview hamstersJournal of Toxicology and Environmental HealthYear: 198181-23073166799658
112. Evenson DP,Jost LK. Hydroxyurea exposure alters mouse testicular kinetics and sperm chromatin structureCell ProliferationYear: 19932621471598471672
113. Wiger R,Hongslo JK,Evenson DP,De Angelis P,Schwarze PE,Holme JA. Effects of acetaminophen and hydroxyurea on spermatogenesis and sperm chromatin structure in laboratory miceReproductive ToxicologyYear: 19959121338520128
114. Saez JM. Leydig cells: endocrine, paracrine, and autocrine regulationEndocrine ReviewsYear: 19941555746267843069
115. Jones KM,Niaz MS,Brooks CM,et al. Adverse effects of a clinically relevant dose of hydroxyurea used for the treatment of sickle cell disease on male fertility endpointsInternational Journal of Environmental Research and Public HealthYear: 2009631124114419440437
116. Friedman G,Freeman R,Bookchin R. Testicular function in sickel cell diseaseFertility and SterilityYear: 19742512101810214430384
117. Abbasi AA,Prasad AS,Ortega J. Gonadal function abnormalities in sickle cell anemia; studies in male adult patientsAnnals of Internal MedicineYear: 1976855601605984611
118. Modebe O,Ezeh UO. Effect of age on testicular function in adult males with sickle cell anemiaFertility and SterilityYear: 19956349079127890081
119. Dada OA,Nduka EU. Endocrine function and hemoglobinopathies: relation between the sickle cell gene and circulating plasma levels of testosterone, luteinizing hormone (LH) and follicle stimulating hormone (FSH) in adult malesClinica Chimica ActaYear: 19801052269273
120. El-Hazmi MAF,Bahakim HM,Al-Fawaz I. Endocrine functions in sickle cell anaemia patientsJournal of Tropical PediatricsYear: 19923863073131844090
121. Abudu EK,Akanmu SA,Soriyan OO,et al. Serum testosterone levels of HbSS (sickle cell disease) male subjects in Lagos, NigeriaBMC Research NotesYear: 2011174p. 298
122. Osegbe DN,Akinyanju OO. Testicular dysfunction in men with sickle cell diseasePostgraduate Medical JournalYear: 19876373695983118348
123. Abdulwaheed OO,Abdulrasaq AA,Sulaiman AK,et al. The hormonal assessment of the infertile male in Ilorin, NigeriaAfrican Journal of Clinical Endocrinology & MetabolismYear: 200236264
124. Gofrit ON,Rund D,Shapiro A,Pappo O,Landau EH,Pode D. Segmental testicular infarction due to sickle cell diseaseJournal of UrologyYear: 19981603, part 18358369720564
125. Prasad AS,Schoomaker EB,Ortega J. Zinc deficiency in sickle cell diseaseClinical ChemistryYear: 19752145825871116294
126. Prasad AS,Cossack ZT. Zinc supplementation and growth in sickle cell diseaseAnnals of Internal MedicineYear: 198410033673716696358
127. Landefeld CS,Schambelan M,Kaplan SL,Embury SH. Clomiphene-responsive hypogonadism in sickle cell anemiaAnnals of Internal MedicineYear: 19839944804836414355
128. Jiminez CT,Scott RB,Henry WL,et al. Studies in sickle cell anemia. XXVI. The effect of homozygous sickle cell disease on the onset of menarche, pregnancy, fertility, pubescent changes and body growth in Negro subjectsAmerican Journal Of Diseases Of ChildrenYear: 1966111497503
129. Platt OS,Rosenstock W,Espeland MA. Influence of sickle hemoglobinopathies on growth and developmentThe New England Journal of MedicineYear: 198431117126727978
130. Zago MA,Kerbauy J,Souza HM,et al. Growth and sexual maturation of Brazilian patients with sickle cell diseasesTropical and Geographical MedicineYear: 19924443173211295140
131. Li M,Fogarty J,Whitney KD,Stone P. Repeated testicular infarction in a patient with sickle cell disease: a possible mechanism for testicular failureUrologyYear: 2003623p. 551
132. Dufau ML. The luteinizing hormone receptorAnnual Review of PhysiologyYear: 199860461496
133. Ascoli M,Fanelli F,Segaloff DL. The lutropin/choriogonadotropin receptor, a 2002 perspectiveEndocrine ReviewsYear: 200223214117411943741
134. Stocco DM. StAR protein and the regulation of steroid hormone biosynthesisAnnual Review of PhysiologyYear: 200163193213
135. Tremblay JJ,Hamel F,Viger RS. Protein kinase A-dependent cooperation between GATA and CCAAT/enhancer-binding protein transcription factors regulates steroidogenic acute regulatory protein promoter activityEndocrinologyYear: 2002143103935394512239105
136. Epstein LF,Orme-Johnson NR. Acute action of luteinizing hormone on mouse Leydig cells: accumulation of mitochondrial phosphoproteins and stimulation of testosterone synthesisMolecular and Cellular EndocrinologyYear: 1991811–31131261797581
137. Epstein LF,Orme-Johnson NR. Regulation of steroid hormone biosynthesis: identification of precursors of a phosphoprotein targeted to the mitochondrion in stimulated rat adrenal cortex cellsJournal of Biological ChemistryYear: 19912662919739197451655794
138. Seebacher T,Beitz E,Kumagami H,Wild K,Ruppersberg JP,Schultz JE. Expression of membrane-bound and cytosolic guanylyl cyclases in the rat inner earHearing ResearchYear: 19991271-2951029925020
139. Arakane F,King SR,Du Y,et al. Phosphorylation of steroidogenic acute regulatory protein (StAR) modulates its steroidogenic activityJournal of Biological ChemistryYear: 19972725132656326629405483
140. Artemenko IP,Zhao D,Hales DB,Hales KH,Jefcoate CR. Mitochondrial processing of newly synthesized steroidogenic acute regulatory protein (StAR), but not total StAR, mediates cholesterol transfer to cytochrome P450 side chain cleavage enzyme in adrenal cellsJournal of Biological ChemistryYear: 200127649465834659611579102
141. Jefcoate C. High-flux mitochondrial cholesterol trafficking, a specialized function of the adrenal cortexJournal of Clinical InvestigationYear: 2002110788189012370263
142. Liu J,Rone MB,Papadopoulos V. Protein-protein interactions mediate mitochondrial cholesterol transport and steroid biosynthesisJournal of Biological ChemistryYear: 200628150388793889317050526
143. Eltzschig HK,Ibla JC,Furuta GT,et al. Coordinated adenine nucleotide phosphohydrolysis and nucleoside signaling in posthypoxic endothelium: role of ectonucleotidases and adenosine A2B receptorsJournal of Experimental MedicineYear: 2003198578379612939345
144. Zhang Y,Dai Y,Wen J,et al. Detrimental effects of adenosine signaling in sickle cell diseaseNature MedicineYear: 20111717986
145. Stocco DM,Wang X,Jo Y,Manna PR. Multiple signaling pathways regulating steroidogenesis and steroidogenic acute regulatory protein expression: more complicated than we thoughtMolecular EndocrinologyYear: 200519112647265915831519
146. Khurana ML,Pandey KN. Receptor-mediated stimulatory effect of atrial natriuretic factor, brain natriuretic peptide, and C- type natriuretic peptide on testosterone production in purified mouse Leydig cells: activation of cholesterol side- chain cleavage enzymeEndocrinologyYear: 19931335214121498404664
147. Del Punta K,Charreau EH,Pignataro OP. Nitric oxide inhibits leydig cell steroidogenesisEndocrinologyYear: 199613712533753438940355
148. Drewett JG,Adams-Hays RL,Ho BY,Hegge DJ. Nitric oxide potently inhibits the rate-limiting enzymatic step in steroidogenesisMolecular and Cellular EndocrinologyYear: 20021941-2394512242026
149. Valenti S,Cuttica CM,Fazzuoli L,Giordano G,Giusti M. Biphasic effect of nitric oxide on testosterone and cyclic GMP production by purified rat Leydig cells cultured in vitroInternational Journal of AndrologyYear: 199922533634110509235
150. Gambaryan S,Butt E,Marcus K,et al. cGMP-dependent protein kinase type II regulates basal level of aldosterone production by zona glomerulosa cells without increasing expression of the steroidogenic acute regulatory protein geneJournal of Biological ChemistryYear: 200327832296402964812775716
151. Kass DA,Champion HC,Beavo JA. Phosphodiesterase type 5: expanding roles in cardiovascular regulationCirculation ResearchYear: 2007101111084109518040025
152. Andric SA,Kostic TS,Stojilkovic SS. Contribution of multidrug resistance protein MRP5 in control of cyclic guanosine 5′-monophosphate intracellular signaling in anterior pituitary cellsEndocrinologyYear: 200614773435344516614078
153. Fernández-Pérez GC,Tardáguila FM,Velasco M,et al. Radiologic findings of segmental testicular infarctionAmerican Journal of RoentgenologyYear: 200518451587159315855121
154. Madaan S,Joniau S,Klockaerts K,et al. Segmental testicular infarction: conservative management is feasible and safeEuropean UrologyYear: 200853244144517408849
155. Han DP,Dmochowski RR,Blasser MH,Auman JR. Segmental infarction of the testicle: atypical presentation of a testicular massJournal of UrologyYear: 199415111591608254802
156. Urwin GH,Kehoe N,Dundas S,Fox M. Testicular infarction in a patient with sickle cell traitBritish Journal of UrologyYear: 19865833403413719262
157. Holmes NM,Kane CJ. Testicular infarction associated with sickle cell diseaseJournal of UrologyYear: 19981601p. 130
158. Bruno D,Wigfall DR,Zimmerman SA,Rosoff PM,Wiener JS. Genitourinary complications of sickle cell diseaseJournal of UrologyYear: 2001166380381111490223
159. Sarma PS. Testis involvement in sickle cell traitThe Journal of the Association of Physicians of IndiaYear: 1987354p. 321
160. Hurt KJ,Musicki B,Palese MA,et al. Akt-dependent phosphorylation of endothelial nitric-oxide synthase mediates penile erectionProceedings of the National Academy of Sciences of the United States of AmericaYear: 20029964061406611904450
161. Wood KC,Hsu LL,Gladwin MT. Sickle cell disease vasculopathy: a state of nitric oxide resistanceFree Radical Biology and MedicineYear: 20084481506152818261470
162. Adams ML,Meyer ER,Sewing BN,Cicero TJ. Effects of nitric oxide-related agents on rat testicular functionJournal of Pharmacology and Experimental TherapeuticsYear: 199426912302377513358
163. Hales DB. Testicular macrophage modulation of Leydig cell steroidogenesisJournal of Reproductive ImmunologyYear: 2002571-231812385830
164. Mondillo C,Pagotto RM,Piotrkowski B,et al. Involvement of nitric oxide synthase in the mechanism of histamine-induced inhibition of leydig cell steroidogenesis via histamine receptor subtypes in sprague-dawley ratsBiology of ReproductionYear: 200980114415218768916
165. Solovey A,Kollander R,Milbauer LC,et al. Endothelial nitric oxide synthase and nitric oxide regulate endothelial tissue factor expression in vivo in the sickle transgenic mouseAmerican Journal of HematologyYear: 2010851414520029945
166. De Franceschi L,Cappellini MD,Olivieri O. Thrombosis and sickle cell diseaseSeminars in Thrombosis and HemostasisYear: 201137322623621455857
167. Gopalakrishna R,Zhen Hai Chen,Gundimeda U. Nitric oxide and nitric oxide-generating agents induce a reversible inactivation of protein kinase C activity and phorbol ester bindingJournal of Biological ChemistryYear: 19932683627180271858262958
168. Sauzeau V,Le Jeune H,Cario-Toumaniantz C,et al. Cyclic GMP-dependent protein kinase signaling pathway inhibits RhoA- induced Ca2+ sensitization of contraction in vascular smooth muscleJournal of Biological ChemistryYear: 200027528217229172910783386
169. Sawada N,Itoh H,Yamashita J,et al. cGMP-dependent protein kinase phosphorylates and inactivates RhoABiochemical and Biophysical Research CommunicationsYear: 2001280379880511162591
170. Jernigan NL,Walker BR,Resta TC. Chronic hypoxia augments protein kinase G-mediated Ca2+ desensitization in pulmonary vascular smooth muscle through inhibition of RhoA/Rho kinase signalingAmerican Journal of PhysiologyYear: 20042876L1220L122915310556
171. Priviero FBM,Jin LM,Ying Z,Teixeira CE,Webb RC. Up-regulation of the RhoA/Rho-kinase signaling pathway in corpus cavernosum from endothelial Nitric-Oxide Synthase (NOS), but not neuronal NOS, null miceJournal of Pharmacology and Experimental TherapeuticsYear: 2010333218419220093396
172. Abrams P. Describing bladder storage function: overactive bladder syndrome and detrusor overactivityUrologyYear: 2003625, supplement 2283714662404
173. Michel MC,Barendrecht MM. Physiological and pathological regulation of the autonomic control of urinary bladder contractilityPharmacology and TherapeuticsYear: 2008117329731218221785
174. Andersson KE,Hedlund P,Wein AJ,Dmochowski RR,Staskin DR. Pharmacologic perspective on the physiology of the lower urinary tractUrologyYear: 2002605132012493344
175. Andersson KE,Arner A. Urinary bladder contraction and relaxation: physiology and pathophysiologyPhysiological ReviewsYear: 200484393598615269341
176. Peters SLM,Schmidt M,Michel MC. Rho kinase: a target for treating urinary bladder dysfunction?Trends in Pharmacological SciencesYear: 200627949249716870270
177. Ataga KI,Orringer EP. Renal abnormalities in sickle cell diseaseAmerican Journal of HematologyYear: 200063420521110706765
178. Daneshgari F,Liu G,Birder L,Hanna-Mitchell AT,Chacko S. Diabetic bladder dysfunction: current translational knowledgeJournal of UrologyYear: 20091826S18S2619846137
179. Chapple CR,Yamanishi T,Chess-Williams R,Ouslander JG,Weiss JP,Andersson KE. Muscarinic receptor subtypes and management of the overactive bladderUrologyYear: 2002605828912493364
180. Hegde SS,Eglen RM. Muscarinic receptor subtypes modulating smooth muscle contractility in the urinary bladderLife SciencesYear: 1999646-741942810069505
181. Hegde SS,Choppin A,Bonhaus D,et al. Functional role of M2 and M3 muscarinic receptors in the urinary bladder of rats in vitro and in vivoBritish Journal of PharmacologyYear: 19971208140914189113359
182. Longhurst PA,Leggett RE,Briscoe JAK. Characterization of the functional muscarinic receptors in the rat urinary bladderBritish Journal of PharmacologyYear: 19951164227922858564260
183. Choppin A,Eglen RM,Hegde SS. Pharmacological characterization of muscarinic receptors in rabbit isolated iris sphincter muscle and urinary bladder smooth muscleBritish Journal of PharmacologyYear: 199812458838889692772
184. Mutoh S,Latifpour J,Saito M,Weiss RM. Evidence for the presence of regional differences in the subtype specificity of muscarinic receptors in rabbit lower urinary tractJournal of UrologyYear: 199715727177218996405
185. Sellers DJ,Yamanishi T,Chapple CR,Couldwell C,Yasuda K,Chess-Williams R. M3 muscarinic receptors but not M2 mediate contraction of the porcine detrusor muscle in vitroJournal of Autonomic PharmacologyYear: 200020317117611193006
186. D’Agostino G,Bolognesi ML,Lucchelli A,et al. Prejunctional muscarinic inhibitory control of acetylcholine release in the human isolated detrusor: involvement of the M4 receptor subtypeBritish Journal of PharmacologyYear: 2000129349350010711347
187. Chess-Williams R,Chapple CR,Yamanishi T,Yasuda K,Sellers DJ. The minor population of M3-receptors mediate contraction of human detrusor muscle in vitroJournal of Autonomic PharmacologyYear: 200121524324812123469
188. Leiria LOS,Mõnica FZT,Carvalho FDGF,et al. Functional, morphological and molecular characterization of bladder dysfunction in streptozotocin-induced diabetic mice: eidence of a role for L-type voltage-operated Ca2+ channelsBritish Journal of PharmacologyYear: 201116361276128821391978
189. Ramos-Filho AC,Mónica FZ,Franco-Penteado CF,et al. Characterization of the urinary bladder dysfunction in renovascular hypertensive ratsNeurourology and UrodynamicsYear: 2011307139240221661033
190. Nakanishi K,Kamai T,Mizuno T,Arai K,Yamanishi T. Expression of RhoA mRNA and activated RhoA in urothelium and smooth muscle, and effects of a Rho-kinase inhibitor on contraction of the porcine urinary bladderNeurourology and UrodynamicsYear: 200928652152819214992
191. Boberg L,Poljakovic M,Rahman A,Eccles R,Arner A. Role of Rho-kinase and protein kinase C during contraction of hypertrophic detrusor in mice with partial urinary bladder outlet obstructionBJU InternationalYear: 2012109113214021883831
192. Claudino MA,Franco-Penteado CF,Corat MAF,et al. Reduction of urinary bladder activity in transgenic sickle cell disease miceBloodYear: 2009114, abstract 2580 (ASH Annual Meeting Abstracts).
193. Tarry WF,Duckett JW,Snyder MIH. Urological complications of sickle cell disease in a pediatric populationJournal of UrologyYear: 198713835925943625863
194. Zarkowsky HS,Gallagher D,Gill FM. Bacteremia in sickle hemoglobinopathiesJournal of PediatricsYear: 198610945795853531449
195. Miller JM Jr.. Sickle cell trait in pregnancySouthern Medical JournalYear: 19837689629636879289
196. Baill IC,Witter FR. Sickle trait and its association with birthweight and urinary tract infections in pregnancyInternational Journal of Gynecology and ObstetricsYear: 199033119211974527
197. Pasture LM,Savitz DA,Thorp JM. Predictors of urinary tract infection at the first prenatal visitEpidemiologyYear: 199910328228710230839
198. Portocarrero ML,Portocarrero ML,Sobral MM,Lyra I,Lordêlo P,Barroso U Jr.. Prevalence of enuresis and daytime urinary incontinence in children and adolescents with sickle cell diseaseJournal of UrologyYear: 201218731037104022264459

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