Cell Differentiation Section
Current Research
Section on Cellular Differentiation
The Section on Cell Differentiation (SCD), directed by Dr. Peggy Zelenka, studies signaling mechanisms that regulate adhesion, migration, and differentiation of epithelial cells of the lens and cornea. Currently these studies focus on determining the function of two signaling proteins involved in regulating these processes: the protein kinase, Cdk5, and the cell surface receptor, Notch. The SCD explores the role of these proteins using cultured cells and animal models, including transgenic mice and mice with homozygous gene deletions.
Previous studies from this laboratory have demonstrated that Cdk5 activity regulates both cell-cell and cell-matrix adhesion in lens and corneal epithelial cells. Moreover, since cell migration depends on coordinated formation and release of cell adhesions, Cdk5 activity also affects the rate of cell migration. Inhibiting Cdk5 activity increases the rate of corneal cell migration, both in vitro and in vivo using an epithelial debridement wound healing model. A major goal of this section is to determine the regulatory pathways that mediate these Cdk5-dependent effects.
We have recently shown that Cdk5 activity is a major regulator of Rho-dependent myosin regulatory light chain (MRLC) phosphorylation. Cdk5 and its activator, p35, also co-localize with phosphorylated MRLC on contracting stress fibers. Inhibiting Cdk5 activity leads to dephosphorylation of MRLC, dissolution of stress fibers, reduced cell adhesion, and elevated levels of Src activity. The observed effects of Cdk5 inhibition on stress fibers can be completely reversed by inhibiting the activity of cSrc or its substrate, p190RhoGAP, indicating that Cdk5-dependent regulation of Src is a major determinant of Rho-dependent MRLC phosphorylation. This regulatory pathway is especially important for maintaining the stability of central stress fibers, which firmly anchor the cell to the extracellular matrix. Current studies focus on the mechanism by which Cdk5 activity regulates Src activity.
Interestingly, we have found that Cdk5 also regulates Src in ways that seem to depend on Cdk5’s structure rather than its kinase activity. When Cdk5 protein is absent or is mutated in ways that prevent it from being phosphorylated on tyrosine, Src fails to be activated in spreading cells. Consequently, no active Src binds to FAK at nascent focal adhesions, focal adhesion formation is aborted, and stress fibers do not form. These effects require only the presence of Cdk5 protein, not its kinase activity, indicating that this protein functions as an adaptor as well as a kinase. The molecular interactions that underlie this function are under active investigation.
Members of this section have also initiated studies of the Notch signaling pathway and its possible role in lens differentiation. In many systems, Notch suppresses differentiation of progenitor cells to maintain a population of cells with the capacity to differentiate. Investigators in other laboratories have shown that Notch signaling from fibers to epithelial cells in the germinative zone functions in this way. We have recently found that Notch signaling is also activated in the differentiating cells. Moreover, in the presence of factors such as FGF, which induce differentiation, Notch signaling promotes-- rather than suppresses--differentiation. Notch signaling exerts its effects by regulating expression of genes such as p57Kip2 and N-cadherin, which are required for normal fiber cell integrity and lens clarity. Current work seeks to understand how signaling pathways downstream of FGF may interact with Notch to alter Notch functions.
Staff
| Name | Title | Phone | |
|---|---|---|---|
| Peggy S. Zelenka, Ph.D. | Section Head | zelenkap@nei.nih.gov | (301) 496-7490 |
| Chun Gao, M.D., Ph.D. | Staff Scientist | gaoc@nei.nih.gov | (301) 496-3053 |
| Brajendra Tripathi Ph.D. | Visiting Fellow | tripathib@nei.nih.gov | (301) 402-4659 |
| Arpitha Parthasarathy | Visiting Fellow | saravanana@nei.nih.gov | (301) 402-4733 |
| Senthil Saravanamuthu Ph.D. | Visiting Fellow | saravanamuthus@nei.nih.gov | (301) 402-4577 |
| Pan Qing | Visiting Fellow | panqi@nei.nih.gov | (301) 451-1986 |
Selected Publications
- Ledee, D.R., Tripathi, B.K., and Zelenka, P.S. The CDK5 activtor, p39, binds specifically to myosin essential light chain. Biophys. Res. Commun., 354(4), 1034-9, 2007.
- Zelenka, P.S. and Arpitha, P. Coordinating Proliferation and Migration in the Lens and Cornea. Seminars in Cell and Develop. Biol. 19(2), 113-124, 2007.
- Qiao, F.Y., Tripathi, B.K., Gao, C.Y. and Zelenka, P.S. Distinct functions of Cdk5(Y15) phosphorylation and Cdk5 activity in stress fiber formation and organization. Exp Cell Res. 314(19):3542-50, 2008.
- Tripathi, B.K., Stepp, M.A., Gao, C.Y., and Zelenka, P.S., The Cdk5 Inhibitor, Olomoucine Promotes Corneal Debridement Wound Closure In Vivo. Mol Vis. 14: 542–549, 2008.
- Golestaneh, N., Fan, J., Zelenka, P.S., and Chepelinsky, A. PKC putative phosphorylation site Ser235 is required for MIP/AQP0 translocation to the plasma membrane. Mol Vis. 14:1006–1014, 2008.
- Saravanamuthu, S.S. and Zelenka, P.S. Jagged 1-Notch signaling is required for Jagged 1 induction during FGF-dependent differentiation of lens fiber cells. Develop. Biol. 332:166-176, 2009.
