CD2AP Regulates SUMOylation of CIN85 in Podocytes

Podocytes are highly differentiated and polarized epithelial cells located on the visceral side of the glomerulus. They form an indispensable component of the glomerular filter, the slit diaphragm, formed by several transmembrane proteins and adaptor molecules. Disruption of the slit diaphragm can lead to massive proteinuria and nephrotic syndrome in mice and humans. CD2AP is an adaptor protein that is important for the maintenance of the slit diaphragm. Together with its paralogue, CIN85, CD2AP belongs to a family of adaptor proteins that are primarily described as being involved in endocytosis and downregulation of receptor tyrosine kinase activity. We have shown that full-length CIN85 is upregulated in podocytes in the absence of CD2AP, whereas in wild-type cells, full-length CIN85 is not detectable. In this study, we show that full-length CIN85 is postranslationally modified by SUMOylation in wild-type podocytes. We can demonstrate that CIN85 is SUMOylated by SUMO-1, -2, and -3 and that SUMOylation is enhanced in the presence of CD2AP. Conversion of lysine 598 to arginine completely abolishes SUMOylation and leads to increased binding of CIN85 to nephrin. Our results indicate a novel role for CD2AP in regulating posttranslational modification of CIN85

Def-6, a Novel Regulator of Small GTPases in Podocytes, Acts Downstream of Atypical Protein Kinase C (aPKC) λ/ι

Supplemental Data Supplemental Figure S1 Characterization of WT and aPKC-deficient podocytes. A–C: Genomic DNA isolated from deficient and control cell lines was tested for the presence of Cre recombinase (A), floxed and WT alleles of PKCλ/ι (B), or WT and knockout alleles of PKCζ (C). As controls, genomic DNA samples of tail biopsies were used. D: Differentiated deficient or control cells were stained with antibodies against synaptopodin or WT-1. All used cell lines were positive for the tested podocyte markers. Scale bars = 50 μm. Download Supplemental Figure S2 Relative mRNA and protein expression of PKCλ/ι, PKCζ, and Def-6 in deficient and control podocytes. A–C: Real-time PCR measurements and Western blot analysis of PKCλ/ι- and PKCζ-deficient cells in comparison with control cells. A: PKCλ/ι mRNA and protein are reduced in the PKCλ/ι −/− cells. B: PKCζ mRNA and protein are reduced in the PKCζ −/− cells. C: Def-6 mRNA is up-regulated in the PKCλ/ι −/− cells but not in PKCζ −/− cells. mRNA level is normalized for HPRT-1. Def-6 protein expression is not changed. ∗∗P < 0.01. Download Supplemental Table S1 Download Supplemental Table S2 Download Supplemental Table S3 Download Supplemental Table S4 Download Supplemental Table S5 Download Supplemental Table S6 Download Supplemental Table S7 Download Supplemental Table S8 Download Supplemental Data Supplemental material for this article can be found at . The atypical protein kinase C (aPKC) isotypes PKCλ/ι and PKCζ are both expressed in podocytes; however, little is known about differences in their function. Previous studies in mice have demonstrated that podocyte-specific loss of PKCλ/ι leads to a severe glomerular phenotype, whereas mice deficient in PKCζ develop no renal phenotype. We analyzed various effects caused by PKCλ/ι and PKCζ deficiency in cultured murine podocytes. In contrast to PKCζ-deficient podocytes, PKCλ/ι-deficient podocytes exhibited a severe actin cytoskeletal phenotype, reduced cell size, decreased number of focal adhesions, and increased activation of small GTPases. Comparative microarray analysis revealed that the guanine nucleotide exchange factor Def-6 was specifically up-regulated in PKCλ/ι-deficient podocytes. In vivo Def-6 expression is significantly increased in podocytes of PKCλ/ι-deficient mice. Cultured PKCλ/ι-deficient podocytes exhibited an enhanced membrane association of Def-6, indicating enhanced activation. Overexpression of aPKCλ/ι in PKCλ/ι-deficient podocytes could reduce the membrane-associated expression of Def-6 and rescue the actin phenotype. In the present study, PKCλ/ι was identified as an important factor for actin cytoskeletal regulation in podocytes and Def-6 as a specific downstream target of PKCλ/ι that regulates the activity of small GTPases and subsequently the actin cytoskeleton of podocytes

Die Rolle der atypischen Protein Kinase C Isoformen PKC[lambda]/[iota] und PKC[zeta] in Podozyten

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