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

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.