Nephrin Signaling in the Podocyte: An Updated View of Signal Regulation at the Slit Diaphragm and Beyond

Abstract

Podocytes are a major component of the glomerular blood filtration barrier, and alterations to the morphology of their unique actin-based foot processes (FP) are a common feature of kidney disease. Adjacent FP are connected by a specialized intercellular junction known as the slit diaphragm (SD), which serves as the ultimate barrier to regulate passage of macromolecules from the blood. While the link between SD dysfunction and reduced filtration selectivity has been recognized for nearly 50 years, our understanding of the underlying molecular circuitry began only 20 years ago, sparked by the identification of NPHS1, encoding the transmembrane protein nephrin. Nephrin not only functions as the core component of the extracellular SD filtration network but also as a signaling scaffold via interactions at its short intracellular region. Phospho-regulation of several conserved tyrosine residues in this region influences signal transduction pathways which control podocyte cell adhesion, shape, and survival, and emerging studies highlight roles for nephrin phospho-dynamics in mechanotransduction and endocytosis. The following review aims to summarize the last 5 years of advancement in our knowledge of how signaling centered at nephrin directs SD barrier formation and function. We further provide insight on promising frontiers in podocyte biology, which have implications for SD signaling in the healthy and diseased kidney.

HGF-induced activation of NEPHRIN and NEPH1 serves as a novel mechanism for recovery of podocytes from injury

“…Several proteins are known to participate in maintaining the structural integrity and composition of slit diaphragm including NEPHRIN and NEPH1, whose genetic defects in humans and mice lead to FSGS, which is characterized by podocyte dysfunction and nephrotic range proteinuria [19,27,[34][35][36][37][38]. The prevailing hypotheses suggest that the extracellular domains of NEPHRIN and NEPH1 constitute the structural framework of slit diaphragm, thus, maintaining its structural integrity [17,37,39,40].…”

“…We now present compelling evidences that NEPHRIN and NEPH1 have receptor like properties, where they can be activated in a ligand-based fashion by HGF and are deactivated by the phosphatase SHP-2. Previous biochemical and genetic studies have shown that Src family kinases (SFK's) mediated tyrosine phosphorylations of NEPHRIN and NEPH1 cytoplasmic domains subsequently induce signal transduction events leading to podocyte actin cytoskeletal reorganization [16,18,36,41]. However, the activating and deactivating mechanisms that may regulate the phosphorylation of these proteins remain unknown [17,19,39].…”

Phosphorylation of Nephrin induces phase separated domains that move through actomyosin contraction

“…Notably, out of approximately 150 missense mutation of Nephrin annotated in the HGMD, nine mutations were reported in the cytoplasmic domain and an additional 25 mutations would produce truncations lacking all phosphorylation sites. Further, in multiple disease models, reduced Nephrin phosphorylation levels have been reported (Martin & Jones, 2018). However, it is still largely unknown how Nephrin signaling can result in the formation of the complicated structures seen in the slit diaphragm.…”

“…Nephrin is an essential structural component of the kidney glomerulus, a specialized structure for ultrafiltration of plasma, consisting of eight extracellular IgG repeats, a transmembrane domain, and an unstructured tail of ~160 amino acids (Martin & Jones, 2018, Ruotsalainen, Ljungberg et al, 1999. Nephrin maintains extracellular junctions between foot processes of podocyte cells in the kidney, while also regulating actin cytoskeletal support within the cells (Martin & Jones, 2018, Welsh & Saleem, 2010. At these interfaces, the eight extracellular IgG domains of Nephrin interact in-trans between cells to create an inter-digitating pattern of tight junctions that act as a biological filter (Gerke, 2003).…”

TRPC Channels in Proteinuric Kidney Diseases

“…glomerular ultrafiltrate [7,8]. It is composed of proteins commonly found in tight and adherens junctions such as P-cadherin, zonula occludens-1 (ZO-1), and catenin family members, as well as proteins predominantly expressed in podocytes including nephrin, neph1, and podocin [7,8]. Both nephrin and neph1 are members of the immunoglobulin (Ig) superfamily [7].…”