Podocytes are highly specialized cells that play a central role in the physiology and pathology of the kidney glomerulus. Podocytes consist of cell body, major processes and foot processes. Podocyte foot processes contain a complete actin-based contractile apparatus. Synaptopodin is a proline-rich, actin-associated protein of podocyte foot processes and telencephalic dendrites without significant homology to any known protein. In brain and kidney, in vivo and in vitro, synaptopodin gene expression is differentiation-dependent; in podocytes, synaptopodin expression coincides with growth arrest and process formation. The C-terminal half of synaptopodin associates with actin filaments and shows about 45 percent homology with myopodin, the second gene family member that we have cloned from skeletal and heart muscle. Myopodin colocalizes with a-actinin at the Zdisc of skeletal and heart muscle sarcomer. Similarly, synaptopodin colocalizes with a-actinin cultured podocytes and in transfected NIH3T3 fibroblasts. Therefore synaptopodin may be involved in the organization or regifiation of a Z-disc equivalent in podocytes. Mice lacking synaptopodin are viable and do not show an overt phenotype in a mixed 1 29/C57 background. However they show a down regulation of paxillin and a-actinin in podocytes and desmin, an early marker of podocyte injury, is induced in podocytes of synpo-/- mice. Moreover, the adaptation of synaptopodin deficient mice to glomerular stress is reduced resulting in increased levels of proteinuria and albuminuria. Based upon these findings we hypothesize that synaptopodin functions as a key protein in the organization/regulation of the podocyte foot processes actin cytoskeleton and in modulating the severity or progression of glomerular disease. To test this hypothesis, I propose the following three Specific Aims: 1.Elucidate the functional roles of synaptopodin in podocytes in the animal. 2. Generate synaptopodin-deficient podocyte cell lines and test whether the lack of synaptopodin affects the structure, the adhesive properties and the mechanical stability of these cells. 3. Identify and characterize functional domains of synaptopodin and interacting proteins. These studies should reveal whether synaptopodin functions in stabilization of the actin cytoskeleton and in providing podocyte resistance to strain and whether synpo-!- mice may be prone to the development of glomerular injury.
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