CLIC5A, a component of the ezrin-podocalyxin complex in glomeruli, is a determinant of podocyte integrity

Abstract

The chloride intracellular channel 5A (CLIC5A) protein, one of two isoforms produced by the CLIC5 gene, was isolated originally as part of a cytoskeletal protein complex containing ezrin from placental microvilli. Whether CLIC5A functions as a bona fide ion channel is controversial. We reported previously that a CLIC5 transcript is enriched approximately 800-fold in human renal glomeruli relative to most other tissues. Therefore, this study sought to explore CLIC5 expression and function in glomeruli. RT-PCR and Western blots show that CLIC5A is the predominant CLIC5 isoform expressed in glomeruli. Confocal immunofluorescence and immunogold electron microscopy reveal high levels of CLIC5A protein in glomerular endothelial cells and podocytes. In podocytes, CLIC5A localizes to the apical plasma membrane of foot processes, similar to the known distribution of podocalyxin and ezrin. Ezrin and podocalyxin colocalize with CLIC5A in glomeruli, and podocalyxin coimmunoprecipitates with CLIC5A from glomerular lysates. In glomeruli of jitterbug (jbg/jbg) mice, which lack the CLIC5A protein, ezrin and phospho-ERM levels in podocytes are markedly lower than in wild-type mice. Transmission electron microscopy reveals patchy broadening and effacement of podocyte foot processes as well as vacuolization of glomerular endothelial cells. These ultrastructural changes are associated with microalbuminuria at baseline and increased susceptibility to adriamycin-induced glomerular injury compared with wild-type mice. Together, the data suggest that CLIC5A is required for the development and/or maintenance of the proper glomerular endothelial cell and podocyte architecture. We postulate that the interaction between podocalyxin and subjacent filamentous actin, which requires ezrin, is compromised in podocytes of CLIC5A-deficient mice, leading to dysfunction under unfavorable genetic or environmental conditions.

Progressive hearing loss and vestibular dysfunction caused by a homozygous nonsense mutation in CLIC5

“…The jbg mice have abnormalities in the foot processes of the kidney podocytes leading to proteinuria. 17,18 The elevated albumin/creatinine ratio and pre-hypertension in affected individual II.3 indicate mild renal dysfunction and may well be the first signs of a nephropathy. Therefore, follow-up of renal function is indicated for individual II.3 but also for his hearing-impaired sister.…”

“…9 Secondly, Clic5 functions as an adapter between the plasma membrane of podocytes and the actin cytoskeleton by facilitating the interaction between ezrin and podocalyxin. 17,18,23 Thirdly, a recent study proposes that Clic5 functions as part of a multiprotein linker complex in companion with radixin, erzin and taperin. 24 Protein tyrosine phosphatase receptor Q (Ptprq), which is mislocalized as radixin in the jbg mice, and Myosin VI, key regulator of the proper localization of Ptprq, 25 might well participate in this complex too.…”

Clustered phosphatidylinositol 4,5 bisphosphate accumulation and ezrin phosphorylation in response to CLIC5A

“…In glomeruli, CLIC5A colocalizes with ezrin and podocalyxin in a highly polarized fashion at the apical plasma membrane of podocyte foot processes. In mice lacking CLIC5A, transmission electron microscopy shows broadening and patchy fusion of podocyte foot processes, and ezrin as well as phosphoezrin levels are reduced relative to those of wild-type mice (Pierchala et al, 2010;Wegner et al, 2010). These findings suggest that CLIC5A is required for the development and maintenance of the ezrin-dependent ultrastructural organization of glomerular podocyte foot processes, akin to the dependence of stereocilia integrity on CLIC5A in cochlear and vestibular hair cells.…”

Tips, Stalks, Tubes: Notch-Mediated Cell Fate Determination and Mechanisms of Tubulogenesis during Angiogenesis

“…They exist as both soluble cytoplasmic proteins and membrane bound channel proteins with a putative transmembrane domain near the amino terminus, making them unique as mammalian ion channels (Redhead et al 1997;Berry and Hobert 2006). Of the six known mammalian CLICs, only CLIC1 and CLIC4 are expressed highly in endothelial cells Money et al 2007;Suh et al 2007;Ulmasov et al 2007;Tung et al 2009;Tung and Kitajewski 2010;Wegner et al 2010). In addition, CLIC5 levels are found to be low in placental endothelial cells (Money et al 2007) and high in glomerular endothelial cells (Wegner et al 2010).…”

“…Of the six known mammalian CLICs, only CLIC1 and CLIC4 are expressed highly in endothelial cells Money et al 2007;Suh et al 2007;Ulmasov et al 2007;Tung et al 2009;Tung and Kitajewski 2010;Wegner et al 2010). In addition, CLIC5 levels are found to be low in placental endothelial cells (Money et al 2007) and high in glomerular endothelial cells (Wegner et al 2010). CLIC1, CLIC4, and CLIC5 are also regulated by F-actin when reconstituted in planar lipid bilayers (Singh et al 2007), suggesting a potential role in cytoskeletal reorganization.…”