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

The atypical protein kinase C (aPKC) isotypes PKC lambda/iota and PKC zeta 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 PKC lambda/iota Leads to a severe glomerular phenotype, whereas mice deficient in PKC develop no renal phenotype. We analyzed various effects caused by PKC lambda/iota and PKC zeta deficiency in cultured murine podocytes. In contrast to PKC zeta-deficient podocytes, PKC lambda/iota-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 PKC zeta-deficient podocytes. In vivo Def-6 expression is significantly increased in podocytes of PKC lambda/iota-deficient mice. Cultured PKC zeta-deficient podocytes exhibited an enhanced membrane association of Def-6, indicating enhanced activation. Overexpression of aPKC lambda/iota in PKC lambda/iota-deficient podocytes could reduce the membrane-associated expression of Def-6 and rescue the actin phenotype. In the present study, PKC lambda/iota was identified as an important factor for actin cytoskeletal regulation in podocytes and Def-6 as a specific downstream target of PKC lambda/iota that regulates the activity of small GTPases and subsequently the actin cytoskeleton of podocytes.