Revisiting nephrin signaling and its specialized effects on the uniquely adaptable podocyte

Abstract

Nephrin is a transmembrane Ig-like domain-containing protein that serves as a central structural and signaling scaffold in kidney filtration. First identified in 1998 as mutated in congenital nephrotic syndrome, the recent identification of nephrin autoantibodies in acquired kidney diseases has sparked renewed interest in nephrin biology. In specialized cells known as podocytes, nephrin helps establish and maintain the slit diaphragm (SD), a unique cell-cell junction formed between interdigitating cell projections known as foot processes (FPs). Together, the SD and FP are among the first stages of renal filtration, where they are subject to numerous biochemical and mechanical stressors. Although podocytes are highly adapted to this environment, over time and with injury, this elevated strain can lead to pathological structural changes, detachment, and proteinuria. As such, the complex set of signaling mechanisms provided by nephrin are essential for controlling podocyte adaptability. Herein, we provide a thorough and up-to-date review on nephrin signaling, including a focus on cross-talk between nephrin interactors and signaling regions across podocytes. We first highlight new findings regarding podocyte structure and function, followed by an emphasis on why nephrin is among the most critical proteins for maintaining these features. We then detail a comprehensive list of known nephrin interactors and describe several of their effects, including calcium regulation, cell survival, cell polarity, phase separation-mediated actin reorganization, and SD-focal adhesion dynamics. Collectively, our emerging understanding of the broader cellular context of nephrin signaling provides important insight for clinical strategies to mitigate podocyte injury and kidney disease progression.

Keywords: cell signaling; chronic kidney disease; focal adhesion; nephrin; podocytes.

Figure 1. Podocyte localization, structure, and microenvironment.

Figure 2. Nephrin structure at the slit diaphragm (SD) and its interactors.

(A) The extracellular region of nephrin projects into the SD space. This region contains eight C2-type immunoglobulin-like domains (Ig-L), one fibronectin type-III domain (FNIII), and ten residues that are glycosylated (G). Extracellular regions of nephrin can form homophilic cis- and trans-interactions with nephrin on the same or opposing foot processes (FPs), respectively. Nephrin’s extracellular region can also be bound by secreted factors (e.g., HGF) and other SD membrane proteins (e.g., NEPH1/2/3 and Crumbs2). (B) The cytoplasmic tail of nephrin is situated at the basolateral domain of FPs and co-ordinates a large network of signaling processes. Conserved phosphorylated (P) residues and domain-binding motifs for signaling are shown with their residue number and adjacent amino acids (human sequence). Nck1/2 adaptor-binding partners are bolded as an example of diverse downstream signal transduction from nephrin. Inhibition of β-arrestin2-nephrin binding by pY1193 is indicated via dashed red line, and p130Cas contribution to Crk1/2-nephrin binding is shown with black dashed arrow. Asterisk indicates this interaction site has not been consistently observed across studies. These interactions are not comprehensive, and an extended list of interactions is given in Table 1.

Figure 3. Mechanisms of nephrin–focal adhesion cross-talk.

Illustration of four confirmed or proposed signaling axes downstream of nephrin that regulate integrin-containing focal adhesions. Direct signaling is indicated with arrows with a solid border, while indirect signals are shown with dashed border arrows with a lighter colour. Asterisk (*) indicates that calcium and Rap1 activation downstream of a nephrin-Nck2-PLCε1 axis has not yet been confirmed experimentally. Octothorpe (#) indicates that signaling between nephrin and ROBO2 has only been confirmed with Nck1 but potentially could be carried out via Nck2 as well. FP, foot process; GBM, glomerular basement membrane; pY, phosphotyrosine; SD, slit diaphragm.