An Essential Role of the Universal Polarity Protein, aPKCλ, on the Maintenance of Podocyte Slit Diaphragms

Abstract

Glomerular visceral epithelial cells (podocytes) contain interdigitated processes that form specialized intercellular junctions, termed slit diaphragms, which provide a selective filtration barrier in the renal glomerulus. Analyses of disease-causing mutations in familial nephrotic syndromes and targeted mutagenesis in mice have revealed critical roles of several proteins in the assembly of slit diaphragms. The nephrin–podocin complex is the main constituent of slit diaphragms. However, the molecular mechanisms regulating these proteins to maintain the slit diaphragms are still largely unknown. Here, we demonstrate that the PAR3–atypical protein kinase C (aPKC)–PAR6β cell polarity proteins co-localize to the slit diaphragms with nephrin. Furthermore, selective depletion of aPKCλ in mouse podocytes results in the disassembly of slit diaphragms, a disturbance in apico-basal cell polarity, and focal segmental glomerulosclerosis (FSGS). The aPKC–PAR3 complex associates with the nephrin–podocin complex in podocytes through direct interaction between PAR3 and nephrin, and the kinase activity of aPKC is required for the appropriate distribution of nephrin and podocin in podocytes. These observations not only establish a critical function of the polarity proteins in the maintenance of slit diaphragms, but also imply their potential involvement in renal failure in FSGS.

Recent advances of animal model of focal segmental glomerulosclerosis

“…Myosin 1e (Myo1e) is one of the two Src homology 3 domain-containing "long-tailed" type I myosins in Actin filament cross-linking protein/Interacts with integrins and strengthens the podocyte-GBM interaction Atypical protein kinase C [68][69][70][71] Tight junctions/Formation of Par complex and interacts with slit diaphragm Rhophilin-1 [80] Rho GTPase activating protein 24 [80] Cytoplasm/Rho GTPase-interacting protein, integrity of glomerular filtration barrier Angiotensin II receptor [55][56][57] Angiotensin-converting enzyme [55][56][57] Membrane/pseudocyst formation at podocyte CD2-associated protein [65][66][67] CD2-associated protein [64] Insertion site of the slit diaphragm/Formation SD complex with podocin and nephrin Laminin subunit beta-2 [81][82][83][84] Laminin subunit beta-2 [81][82][83][84] Podocyte anchoring and differentiation in GBM microRNA 193α [74,75] Cytoplasm/Inhibition of expression of WT-1 Myosin 1e [49,50] Myosin 1e [49,50] Actin binding long-tailed motor protein/Regulation of actin cytoskeleton Nuclear factor of activated T cells [76,77] Transient receptor potential 6 [53,54] Membrane/the activation of calcineurin-NFAT/Wnt signaling via the increased calcium influx Podocin [46] Podocin [58,59] Insertion site of the SD/SD assembly and maintaining the signaling of nephrin Shroom family member 3 …”

“…aPKC isoforms orchestrate formation of the podocyte processes essential for normal glomerular development and kidney function. Defective aPKC signaling resulted in dramatically simplified glomerular architecture, leading to severe proteinuria and perinatal death [69,70]. The regulation of SD turnover by aPKC is crucial for maintenance of SD integrity, and defects in aPKC signaling can cause proteinuria [71] (Table 2; Fig.…”

Regulation of epithelial cell polarity by PAR-3 depends on Girdin transcription and Girdin–Gαi3 signaling

“…During epithelial morphogenesis, epithelial cells are arranged into cysts or tubular structures enclosing a central lumen (Bryant and Mostov, 2008;Lubarsky and Krasnow, 2003), which forms cell layers with apicobasal polarity, resulting in functionally distinct apical and basolateral membrane domains separated by junctional complexes such as tight junctions (Farquhar and Palade, 1963;Shin et al, 2006). Apicobasal polarity is crucial for a variety of biological processes, including asymmetric cell division and tissue morphogenesis, and its dysfunction contributes to various diseases including cancer (Feigin and Muthuswamy, 2009;Hirose et al, 2009;Knoblich, 2010). Thus, uncovering the regulatory mechanisms of apicobasal polarity should provide greater insight into fundamental aspects of cell and developmental biology and tumorigenesis.…”

Podocyte-Specific Loss of Cdc42 Leads to Congenital Nephropathy

“…For example, mice deficient in aPKCl/i showed polarization defect in podocytes, loss of nephrin expression, and death from renal failure by 6 weeks of age. 23,24 The similar appearance of irregular cell adhesions and loss of cell polarity proteins ZO-1, Par3, and aPKCl/i within Cdc42-deficient glomeruli strongly suggest that both the actin-regulatory and polarity functions of the Cdc42 GTPase are required for proper podocyte architecture and function.…”