|Nanolitre liquid patterning in aqueous environments for spatially defined reagent delivery to mammalian cells.|
|Jump to Full Text|
|PMID: 19684584 Owner: NLM Status: MEDLINE|
|Microscale biopatterning enables regulation of cell-material interactions and cell shape, and enables multiplexed high-throughput studies in a cell- and reagent-efficient manner. The majority of available techniques rely on physical contact of a stamp, pin, or mask with mainly a dry surface. Inkjet and piezoelectric printing is carried out in a non-contact manner but still requires a substantially dry substrate to ensure fidelity of printed patterns. These existing methods, therefore, are limited for patterning onto delicate surfaces of living cells because physical contact or substantially dry conditions are damaging to them. Microfluidic patterning with laminar streams does enable non-contact patterning in fully aqueous environments but with limited throughput and reagent diffusion across interfacial flows. Here, we describe a polymeric aqueous two-phase system that enables patterning nanolitres of a reagent-containing aqueous phase, in arbitrary shapes, within a second aqueous phase covering a cell monolayer. With the appropriate medium formulation, reagents of interest remain confined to the patterned phase without significant diffusion. The fully aqueous environment ensures high reagent activity and cell viability. The utility of this strategy is demonstrated with patterned delivery of genetic materials to mammalian cells for phenotypic screening of gene expression and gene silencing.|
|H Tavana; A Jovic; B Mosadegh; Q Y Lee; X Liu; K E Luker; G D Luker; S J Weiss; S Takayama|
Related Documents :
|22770404 - Role of adam10 and adam17 in cd16b shedding mediated by different stimulators.
6686824 - Synchronization effect of cis-dichlorodiammineplatinum on a human neuroblastoma in vitr...
22335784 - The expression of calcitonin receptor detected in malignant cells of the brain tumour g...
6979774 - Characterization of normal human mononuclear blood cells during diffusion chamber (dc) ...
11105924 - Toxic effects of polychlorinated biphenyls on cultured rat sertoli cells.
7887124 - Blood cell-producing site in the land slug, incilaria fruhstorferi.
|Type: Journal Article; Research Support, N.I.H., Extramural; Research Support, Non-U.S. Gov't Date: 2009-08-16|
|Title: Nature materials Volume: 8 ISSN: 1476-1122 ISO Abbreviation: Nat Mater Publication Date: 2009 Sep|
|Created Date: 2009-08-24 Completed Date: 2009-09-10 Revised Date: 2013-06-02|
Medline Journal Info:
|Nlm Unique ID: 101155473 Medline TA: Nat Mater Country: England|
|Languages: eng Pagination: 736-41 Citation Subset: IM|
|Department of Biomedical Engineering, University of Michigan, Ann Arbor, Michigan 48109, USA.|
|APA/MLA Format Download EndNote Download BibTex|
Cells / cytology, metabolism*
Drug Delivery Systems*
Gene Expression Profiling / methods*
Indicators and Reagents / metabolism
Oligonucleotide Array Sequence Analysis
Water / chemistry*
|P50 CA093990-07A1/CA/NCI NIH HHS; P50CA093990/CA/NCI NIH HHS; R01 CA136553-01/CA/NCI NIH HHS; R01 CA136829-01A1/CA/NCI NIH HHS; R01CA136553/CA/NCI NIH HHS|
|0/Indicators and Reagents; 7732-18-5/Water|
|Nat Mater. 2009 Sep;8(9):700-2
Journal ID (nlm-journal-id): 101155473
Journal ID (pubmed-jr-id): 30248
Journal ID (nlm-ta): Nat Mater
License:Users may view, print, copy, download and text and data- mine the content in such documents, for the purposes of academic research, subject always to the full Conditions of use: http://www.nature.com/authors/editorial_policies/license.html#terms
nihms-submitted publication date: Day: 23 Month: 7 Year: 2009
Electronic publication date: Day: 16 Month: 8 Year: 2009
Print publication date: Month: 9 Year: 2009
pmc-release publication date: Day: 1 Month: 3 Year: 2010
Volume: 8 Issue: 9
First Page: 736 Last Page: 741
PubMed Id: 19684584
R01 CA136829-01A1 ||CA
R01 CA136553-01 ||CA
P50 CA093990-07A1 ||CA
National Cancer Institute : NCI
|Nanoliter Liquid Patterning in Aqueous Environments for Spatially-Defined Reagent Delivery to Mammalian Cells|
|Q. Y. Lee1|
|S. J. Weiss2|
1 Department of Biomedical Engineering, University of Michigan, Ann Arbor, MI 48109
2 Division of Molecular Medicine and Genetics, Department of Internal Medicine, The Life Sciences Institute, University of Michigan, Ann Arbor, MI, 48109
3 Department of Radiology, University of Michigan, Ann Arbor, MI 48109
4 Department of Microbiology and Immunology, University of Michigan, Ann Arbor, MI 48109
|*Corresponding Author: Prof. S. Takayama, Address: 2200 Bonisteel Blvd., Ann Arbor, MI 48109-2099, USA, Email: email@example.com, Tel.: (734) 615-5539
We selected polyethylene glycol (PEG) and dextran (DEX) as the phase-forming polymers because, first, these polymers form stable ATPSs in a wide range of temperatures14. This enhances convenience and stability of experiments because the phases maintain segregation under refrigeration at low temperatures as well as during incubation at higher temperatures. Second, high molecular weights of PEG and DEX form ATPSs at low polymer concentrations and ensure that the bulks of both phases remain highly aqueous and non-toxic to cells (Supplementary Fig. 1). Third, due to density differences, DEX always forms the bottom phase and PEG the top phase of the two-phase system14 (Supplementary Fig. 2).
To generate user-defined shapes of the reagent phase, we load a pipette tip with the DEX phase and lower it into the PEG phase in close proximity (typically <500 μm) to the cell monolayer. Moving the pipette tip horizontally results in the formation of a continuous pattern of the dispensing DEX phase on cells (Fig. 1a). This is demonstrated by patterning “UMICH” on a monolayer of HEK293H cells (Fig. 1b). The resolution of patterns can significantly be improved by using dispensing mechanisms with finer tips (Supplementary Fig. 3). Other shapes, such as triangles and squares, can also be created; the patterns are quite stable over long incubation time periods and are not disturbed by carefully moving the cell culture system (Supplementary Fig. 4). The key to the stability of patterns is an extremely low interfacial energy between the two immiscible phases (γ12~0.003 mJ/m2)14 and roughness of the cell monolayer surface and associated cell surface-DEX phase interactions (Supplementary Fig. 5). Thermodynamically speaking, cell monolayer surface gives rise to free energy barriers that prevent the PEG-DEX interfacial tension, γ12, from retracting the three-phase contact line (PEG-DEX-cell surface) of the patterns to a lower energetic state15, and thus, patterns retain their shapes.
Following, we show utility of this patterning technology for microarray format multiplexed cell-based studies of gene expression and gene silencing. First, we studied partitioning of cell transfection materials in the PEG-DEX ATPS. Complexes of a lipid transfection reagent, Lipofectamine 2000, and 50 nM Alexa fluor-labeled RNA were prepared and suspended in the DEX phase. A droplet of this solution was dispensed into a bath of the PEG phase and imaged every 15 min. The results show that over a period of 4hrs, the fluorescent signal from transfection materials remains quite confined to the DEX droplet (Fig. 1c). To form a microarray, an ATPS is prepared using cell culture media as the solvent. The DEX phase containing the reagent is transferred into the wells of a 1536-well plate. An array of slot pins resting on a commercially-available fixture is dipped into the wells to load. The pins are then lowered close to the cell monolayer in culture in the PEG phase. The DEX phase dispenses and forms droplets over distinct populations of cells (Fig. 2a). This is demonstrated by a microarray of 96 droplets of FITC-labeled DEX solution formed within a lawn of PEG solution covering HEK293H cells (Fig. 2b).
We examined the efficacy of this strategy for patterned transfection of mammalian cells. Liposomal complexes of an expression construct for eGFP were suspended in the DEX phase and arrayed on discrete groups of HEK293H cells. Imaging cells with a fluorescence microscope 48hrs post-transfection yielded an array of 24 spatially distinct cell clusters expressing eGFP in a lawn of non-transfected cells (Fig. 2c,d). We tested different ratios of the plasmid DNA to the transfection reagent and found that a 1/1 ratio yields optimum eGFP expression (Fig. 2e). The level of protein expression at various concentrations of plasmid DNA was also examined. Similar to conventional well-based transfections, fluorescent signal intensity proportionally increased with the concentration of the plasmid up to a certain point and saturated thereafter (Fig. 2f). The post-transfection cellular viability was also similar to conventional methods (Supplementary Fig. 6). Next, we showed the possibility of patterning more than one reagent on a single monolayer of cells. Transfection of HEK293H cells with plasmid DNAs for eGFP and dsRed resulted in spatially-distinct groups of cells fluorescing green and red, respectively (Fig. 2g). Due to the fundamental importance of simultaneous expression of multiple genes in a cell to many biological events16, we induced cells in the same assay to coexpress eGFP and dsRed proteins as well (yellow spots in Fig. 2g).
Each cluster of transfected cells in the above microarrays was exposed to a 500 nl droplet containing only ~10 ng plasmid, which is significantly less than that typically used in microwell-based transfections and is similar to reverse transfection protocols8. Choosing dispensing pins with smaller volumes will reduce the size of fluorescent spots and the amount of plasmid. For example, 20 nl pins give spots of ~340 μm. Since the diameter of the DEX droplet approximately varies linearly with the square root of the drop volume, the area of transfected cell clusters can be pre-determined from the pin volume for the range of volumes studied (Fig. 2h). In principle, the resolution of circular patterns can be enhanced by using smaller volumes of the DEX phase or an ATPS containing higher concentrations of phase-forming polymers (Supplementary Fig. 7,8).
Lipid-mediated transfection is a straightforward method to induce transient effects of gene overexpression/knockdown in cells. However, many cell lines and primary cells do not transfect efficiently with lipofection and require infection with viral vectors containing cDNA- or short hairpin RNA (shRNA)-expressing cassettes. We showed that the two-phase patterning approach facilitates lentiviral-mediated transduction of MDA-MB-231 human breast cancer cells. Virus particles encoding eGFP remained confined to patterned DEX droplets and resulted in the localized infection of subpopulations of cells (Fig. 3a,b). The titer of the lentiviral solution in the DEX phase was 1.3×107 infectious units (IFU)/ml that corresponded to 10 viral particles per cell. We prepared serial dilutions of the eGFP-encoding lentivirus and arrayed them on the MDA-MB-231 cells at 4, 2, and 1 viral particles per each cell. Subsequent analysis of fluorescent intensity of infected cells showed that similar to conventional well-based lentiviral infections, the level of eGFP expression increases proportionally with the amount of lentivirus in the patterned DEX phase (Fig. 3c). Importantly, the lentiviral solution titer for efficient two-phase patterned infection of cells is 2–3 orders of magnitude less than that required with the reverse transfection approach where the viruses are printed onto a solid substrate and subsequently overlaid with cells17. This is a major advantage that significantly reduces toxicity to cells and eliminates the need for hard-to-obtain highly concentrated viral solutions.
Next we demonstrated the utility of the two-phase patterning for lentiviral-mediated RNA interference (RNAi)18. MDA-MB-231 cells permanently expressing CXCR4-eGFP (target gene) and mPlum (red reporter gene) constructs were seeded at a density of 37500 cells/cm2. Lentiviruses encoding a shRNA that specifically targets eGFP mRNA were resuspended in the DEX phase and patterned on the cells. Analysis of cells 72hrs post-infection showed a significant reduction in eGFP expression levels only within targeted cell clusters (Fig. 3d). The fact that cells within the spots actively express the reporter red protein ensures the specificity of the shRNA for the target gene (Fig. 3e).
Cells inside the body reside in a tissue specific environment where cell-extracellular matrix (ECM) interactions are a major regulator of cellular behavior19. Therefore, screening functions of genes in cells cultured on ECM substrates such as soft proteineous gels, rather than solid substrates, may elicit more physiological cellular responses20,21. Direct patterning of reagents on soft aqueous gels is another area where conventional contact-mediated patterning methods would face challenges but the non-contact, aqueous nature of the ATPS patterning technique should be ideal. To establish this point, we showed patterned degradation of collagen I fibrils by matrix metalloproteinase (MMP)-expressing cells. Degradation of collagen is implicated in the physiological remodeling of connective tissue during growth and development22 as well as in cancer invasion and metastasis23,24 where malignant cells cleave their subjacent matrix proteins and initiate invasiveness. We cultured HEK293H cells on Alexa fluor-labeled collagen I substrates and patterned transfection complexes of full-length membrane-type1 matrix metalloproteinase (MT1-MMP) and MMP2 cDNAs and eGFP plasmid DNA as a control. Imaging 72hrs post-transfection showed degradation of type I collagen only by MT1-MMP-expressing cells (Fig. 4a,b). The loss of collagen appears as black pits under the fluorescent light (Fig. 4b) and immunostaining of cells with anti-HA.11 antibody shows that degradation of the matrix correlates well with the expression of this epitope-tagged MT1-MMP protein (Fig. 4c). This observation is consistent with recent findings that show the pro-form of MT1-MMP undergoes intracellular processing to its active form prior to its display on the cell surface and highlight MT1-MMP as the major regulator of the collagenolytic activity of normal and neoplastic cells24,25,26. On the other hand, the MMP-2 zymogen is unable to degrade type I collagen directly under these conditions. Two-phase reagent micropatterning on soft substrates enabled detection of the mechanistic role of MT1-MMP in ECM invasion. This type of strategy may generally be useful to study how expression or knockdown of different genes affects proteolytic activities of cells during events such as tissue remodeling and cancer metastasis.
In summary, we described a new approach for spatially-defined delivery and retention of nanoliters of biological reagents over living cells. We demonstrated broad utility of this strategy with patterned gene expression/knockdown in mammalian cells cultured on soft and solid substrates using both lipid- and lentiviral-mediated gene delivery techniques. The method enables delivery of reagents to cells in arbitrary geometrical shapes as well as in standard high-throughput 1536 array formats using only nanoliters of reagents. Although reagents of interest are confined over cells within only nanoliter volumes of the reagent phase –a volume which would typically compromise cell viability over substantial period of culture– the reagent-excluding phase is also aqueous and abundant in nutrients supporting high cellular viability. With slight media adjustments, patterning other reagents including proteins, antibodies, and small molecule drugs on cells is also envisioned. This reagent patterning strategy is straightforward to implement, economically sound requiring only off-the-shelf equipment, and conveniently accessible to researchers without a need for fabrication expertise or complicated equipment.
HEK293H cell line was obtained from Invitrogen (11913-019). Cells were maintained in DMEM (Gibco) supplemented with 10% FBS (Gibco) at 37°C in a humid incubator with 5% CO2 and passaged every 3–4 days. For transfection experiments, we used cells with passage number between 5 and 30. MDA-MB-231 breast cancer cell line was obtained from ATCC (HTB-26) and maintained in DMEM supplemented with 10% heat-inactivated FBS (Hyclone), 1% glutamax (Gibco), and 0.5% penicillin/streptomycin (Gibco). MDA-MB-231 cells stably expressing CXCR4-eGFP were previously described27.
eGFP (PT3148-5) and dsRed (6924-1) plasmid DNAs were obtained from Clonetech. mPlum plasmid DNA was provided by R.Y. Tsien and described before28. Full-length MT1-MMP cDNA was prepared as described before25. Full-length MMP2 cDNA was obtained from Origene (SC117323). The lentiviral vector for eGFP (pSico) has been described before29. The eGFP shRNA Control Vector (SHC005) was obtained from Sigma. Lentiviruses were generated and titered according to the protocol described previously30.
Solutions of 4% (w/w) PEG (Mw:8000, Sigma) and 8% (w/w) DEX (Mw:500000, Pharmacosmos) were prepared in Optimem (Gibco) and mixed. After adding 5 mM KH2PO4 salt to adjust the media composition, the mixture was shaken thoroughly and kept at 4°C overnight to equilibrate and form an ATPS. The two phases were carefully separated and centrifuged at 3500 rpm for 45 min to remove counter polymers excessively dissolved in each phase. The stock solutions of PEG and DEX phases were stored at 4°C.
Dilutions of 2.5 μg plasmid DNA in 12.5 μl Optimem and 2.5 μl Lipofectamine 2000 (Invitrogen) in 12.5 μl Optimem were prepared and incubated for 5 min at room temperature. The solutions were mixed and incubated for 5–20 min at room temperature. The resulting solution was mixed with 90 μl of the DEX phase stock solution, incubated for 5 min at room temperature, and transferred to a 1536-well plate (Corning).
Prior to experiments, slot pins (V&P Scientific, FP3S500H) were mounted on a pin tool fixture (V&P Scientific, AFIX1536FP3), which itself was assembled with a micromanipulator. Pins were dipped into a cleaning solution (V&P Scientific, VP110), DI water, and isopropyl alcohol, as indicated by the manufacturer. Clean pins were dipped three times into the wells and filled with the solution containing transfection complexes. Final retraction of pins from the solution was done slowly to minimize residue on the outer surface of the pins. Next, pins were lowered into the close vicinity of the cell monolayer covered with the PEG-Optimem solution and were allowed to dispense the transfection complexes-containing DEX-Optimem solution. After formation of droplets on cells, pins were slowly retracted and moved out of the culture dish.
For viral transduction experiments, solutions of 4% (w/w) PEG and 15% (w/w) DEX were prepared separately in MDA-MB-231 culture medium. 40 μl lentiviral solution was suspended in 20 μl of the DEX solution to a final titer of 1.3×107 (IFU)/ml. To enhance viral infection of cells, a cationic agent, polybrene (Sigma), was added to the resulting solution at a final concentration of 10 μg/ml. Droplets of this solution were arrayed on cells as described above.
Type I collagen was prepared from rat tail tendons and dissolved in 0.2% acetic acid to a final concentration of 2.7 mg/ml. To induce gelling, collagenwas mixed with 10× MEM (Gibco) and 0.34 N NaOH in an 8:1:1 ratio at 4°C and 2 ml of this mixture was added to each chamber of a 2-well chambered slide (VWR). To obtain a thin film, collagen was immediately removed and the slide was kept at 37°C for 45 min to allow gelling to complete. The collagen film was then labeled with Alexa fluor 594 carboxylic acid, succinimidyl ester (Molecular probes) for 1h at room temperature. After removing the dye, the film was incubated three times with PBS PH7.4 (Gibco) at room temperature for a total of 30 min. Then, 2 ml PBS was added to each chamber and the slide was wrapped in aluminum foil and stored at 4°C.
Anti-HA.11 mAb was used to detect epitope-tagged MT1-MMP protein expression in transfected HEK293H cells. After fixing in −20°C methanol for 6 min, cells were washed three times with PBS and twice with PBS containing 5% BSA. The mouse anti-HA.11 mAb (Covance, MMS-101P) in PBS/5% BSA at a concentration of 1 μg/ml was added to fixed cells for 1h. After washing, the primary antibody was visualized with fluorescently labeled goat anti-mouse IgG (Molecular Probes, A-11001) at a concentration of 1 μg/ml.
We imaged transfected microarrays section by section using an inverted fluorescence microscope (Nikon, TE300). After removing the background intensity of images and uniformly enhancing brightness and contrast in Matlab R2007a (MathWorks), images were pseudocolored, merged, and superimposed in Photoshop 10.0 CS3 (Adobe). We used SimplePCI (Compix) for fluorescence intensity measurements.
H.T. designed and performed experiments, analyzed the data, and wrote the manuscript. A.J. helped optimize transfection experiments. B.M. helped with patterning experiments and imaging microarrays. Q.Y.L. performed image processing of transfected cell microarrays. X.L. prepared collagen and helped with collagen degradation assay. K.E.L. prepared lentiviruses. G.D.L. helped design viral transduction experiments and optimize them. S.J.W. helped design collagen degradation assay. S.T. designed the project and edited the manuscript. All authors commented on the manuscript.
FN3Competing interests statement
The authors declare no competing financial interests.
The financial support for this research was provided by a gift from Jacque Passino and NIH grants P50CA093990 and R01CA136553. H. Tavana acknowledges a postdoctoral fellowship (NSERC PDF-329449-2006). We thank Dr. M. El-Sayed and Yen-Ling Lin for spectrophotometry measurements.
|1.||Chen CS,Mrksich M,Huang S,Whitesides GM,Ingber DE. Geometric control of cell life and deathScience 276:1425–1428.1997; [pmid: 9162012]|
|2.||Khetani SR,Bhatia SN. Microscale culture of human liver cells for drug developmentNature Biotech 26:120–126.2008;|
|3.||Singhvi R,et al. Engineering cell shape and functionScience 264:696–698.1994; [pmid: 8171320]|
|4.||Anderson DG,Levenberg S,Langer R. Nanoliter-scale synthesis of arrayed biomaterials and application to human embryonic stem cellsNature Biotech 22:863–866.2004;|
|5.||Anderson DG,Putnam D,Lavik E,Mahmood T,Langer R. Biomaterial microarrays: Rapid, microscale screening of polymer-cell interactionBiomater 26:4892–4897.2005;|
|6.||Bailey SN,Sabatini DM,Stockwell BR. Microarrays of small molecules embedded in biodegradable polymers for use in mammalian cell-based screensProc Natl Acad Sci USA 101:16144–16149.2004; [pmid: 15534212]|
|7.||Flaim CJ,Chien S,Bhatia SN. An extracellular matrix microarray for probing cellular differentiationNature Methods 2:119–125.2005; [pmid: 15782209]|
|8.||Ziauddin J,Sabatini DM. Microarrays of cells expressing defined cDNAsNature 411:107–110.2001; [pmid: 11333987]|
|9.||Delamarche E,Bernard A,Schmid H,Michel B,Biebuyck H. Patterned delivery of immunoglobulins to surfaces using microfluidic networksScience 276:779–781.1997; [pmid: 9115199]|
|10.||Folch A,Jo BH,Hurtado O,Beebe DJ,Toner M. Microfabricated elastomeric stencils for micropatterning cell culturesJ Biomed Mater Res 52:346–353.2000; [pmid: 10951374]|
|11.||Roth EA,et al. Inkjet printing for high-throughput cell patterningBiomater 25:3707–3715.2004;|
|12.||Juncker D,Schmid H,Delamarche E. Multipurpose microfluidic probeNature Mater 4:622–628.2005; [pmid: 16041377]|
|13.||Takayama S,et al. Subcellular positioning of small moleculesNature 411:1016.2001; [pmid: 11429594]|
|14.||Albertsson P-A. Partition of cell particles and macromolecules (3). 3. John Wiley & Sons, Inc; 1986|
|15.||Eick JD,Good RJ,Neumann AW. Thermodynamics of contact angles. II Rough solid surfacesJ Colloid Interface Sci 53:235–238.1975;|
|16.||Stuart JM,Segal E,Koller D,Kim SK. A gene-coexpression network for global discovery of conserved genetic modulesScience 302:249–255.2003; [pmid: 12934013]|
|17.||Bailey SN,Ali SM,Carpenter AE,Higgins CO,Sabatini DM. Microarrays of lentiviruses for gene function screens in immortalized and primary cellsNature Methods 3:117–122.2006; [pmid: 16432521]|
|18.||Hannon GJ. RNA interferenceNature 418:244–251.2002; [pmid: 12110901]|
|19.||Nelson CM,Bissell MJ. Of extracellular matrix, scaffolds, and signaling: tissue architecture regulates development, homeostasis, and cancerAnn Rev Cell Dev Biol 22:287–309.2006; [pmid: 16824016]|
|20.||Streuli CH,Schmidhauser C,Kobrin M,Bissell MJ,Derynck R. Extracellular matrix regulates expression of the TGF-beta 1 geneJ Cell Biol 120:253–260.1993; [pmid: 8416992]|
|21.||Chen SS,Fitzgerald W,Zimmerberg J,Kleinmanc HK,Margolis L. Cell-cell and cell-extracellular matrix interactions regulate embryonic stem cell differentiationStem Cells 25:553 –561.2008; [pmid: 17332514]|
|22.||Liu X,et al. A targeted mutation at the known collagenase cleavage site in mouse type I collagen impairs tissue remodelingJ Cell Biol 130:227–237.1995; [pmid: 7790374]|
|23.||Chun T,et al. A pericellular collagenase directs the 3-dimensional development of white adipose tissueCell 125:577 – 591.2006; [pmid: 16678100]|
|24.||Sabeh F,et al. Tumor cell traffic through the extracellular matrix is controlled by the membrane-anchored collagenase MT1-MMPJ Cell Biol 167:769–781.2004; [pmid: 15557125]|
|25.||Hotary K,Allen E,Punturieri A,Yana I,Weiss SJ. Regulation of cell invasion and morphogenesis in a three-dimensional type I collagen matrix by membrane-type matrix metalloproteinases 1, 2, and 3J Cell Biol 149:1309–1323.2000; [pmid: 10851027]|
|26.||Li XY,Ota I,Yana I,Sabeh F,Weiss SJ. Molecular dissection of the structural machinery underlying the tissue-invasive activity of membrane type-1 matrix metalloproteinaseMol Biol Cell 19:3221–3233.2008; [pmid: 18495869]|
|27.||Song JW,et al. Microfluidic endothelium for studying the intravascular adhesion of metastatic breast cancer cellsPLoS One 4:e5756.2009; [pmid: 19484126]|
|28.||Wang L,Jackson WC,Steinbach PA,Tsien RY. Evolution of new nonantibody proteins via iterative somatic hypermutationProc Natl Acad Sci USA 101:16745–16749.2004; [pmid: 15556995]|
|29.||Ventura A,et al. Cre-lox-regulated conditional RNA interference from transgenesProc Natl Acad Sci USA 101:10380–10385.2004; [pmid: 15240889]|
|30.||Smith MC,et al. CXCR4 regulates growth of both primary and metastatic breast cancerCancer Res 64:8604–8612.2004; [pmid: 15574767]|
Previous Document: A rapid and efficient single-cell manipulation method for screening antigen-specific antibody-secret...
Next Document: Nitrogen interaction with hydrogen-terminated silicon surfaces at the atomic scale.