Published online before print June 14, 2007

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(American Journal of Pathology. 2007;171:415-427.)
© 2007 American Society for Investigative Pathology
DOI: 10.2353/ajpath.2007.070075

Synaptopodin Protects Against Proteinuria by Disrupting Cdc42:IRSp53:Mena Signaling Complexes in Kidney Podocytes

Etsuko Yanagida-Asanuma^*, Katsuhiko Asanuma^*, Kwanghee Kim^*, Mary Donnelly^*, Hoon Young Choi^*, Jae Hyung Chang^*, Shiro Suetsugu, Yasuhiko Tomino, Tadaomi Takenawa, Christian Faul^* and Peter Mundel^*

From the Department of Medicine,^* Mount Sinai School of Medicine, New York, New York; the Department of Medicine, Jacobi Medical Center, Albert Einstein College of Medicine, Bronx, New York; the Division of Nephrology, Juntendo University School of Medicine, Tokyo, Japan; and the Department of Biochemistry, Institute of Medical Science, University of Tokyo, Tokyo, Japan

The actin-based foot processes of kidney podocytes and the interposed slit diaphragm form the final barrier to proteinuria. Mutations affecting several podocyte proteins cause disruption of the filtration barrier and rearrangement of the highly dynamic podocyte actin cytoskeleton. Proteins regulating the plasticity of the podocyte actin cytoskeleton are therefore of critical importance for sustained kidney barrier function. Synaptopodin is an actin-associated protein essential for the integrity of the podocyte actin cytoskeleton because synaptopodin-deficient mice display impaired recovery from protamine sulfate-induced foot process effacement and lipopolysaccharide-induced nephrotic syndrome. Moreover, bigenic heterozygosity for synaptopodin and CD2AP is sufficient to induce spontaneous proteinuria and focal segmental glomerulosclerosis-like glomerular damage in mice. Mechanistically, synaptopodin induces stress fibers by blocking the proteasomal degradation of RhoA. Here we show that synaptopodin directly binds to IRSp53 and suppresses Cdc42:IRSp53:Mena-initiated filopodia formation by blocking the binding of Cdc42 and Mena to IRSp53. The Mena inhibitor FP₄-Mito suppresses aberrant filopodia formation in synaptopodin knockdown podocytes, and when delivered into mice protects against lipopolysaccharide-induced proteinuria. The identification of synaptopodin as an inhibitor of Cdc42:IRSp53:Mena signaling defines a novel antiproteinuric signaling pathway and offers new targets for the development of antiproteinuric therapeutic modalities.

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