Structural and functional insights of the podocyte slit diaphragm complex.

Abstract

Glomerular podocytes, specialized epithelial cells, are central to the filtration function of vertebrate kidneys. Through their interdigitating foot processes, podocytes provide epithelial coverage to capillaries. They maintain selective filtration by allowing water, ions, and small solutes to filter while retaining proteins and larger molecules in the blood. The slit diaphragm (SD), a specialized junction between podocyte foot processes, along with glomerular basement membrane (GBM) and fenestrated endothelium, serves as a glomerular filtration barrier (GFB). Injury to GFB, such as loss of SD integrity and foot process effacement, compromises permselectivity and results in proteinuria. The SD consists of junctional proteins (nephrin, Neph1), adaptors (podocin, CD2AP), and channels (e.g. TRPC6), which assemble into a molecular sieve and a dynamic signaling hub. Monogenic mutations and resultant structural defects in SD components perturb podocyte filtration function, leading to proteinuria, nephrotic syndrome, and focal segmental glomerulosclerosis. This review summarizes structural and functional insights into SD architecture and emphasizes advances from biochemical, biophysical, and high-resolution imaging approaches. We particularly discuss the role of intrinsically disordered regions in mediating oligomerization and protein - protein networks within the SD. Emerging Cryo-EM studies further provide new perspectives on the stoichiometry of Nephrin - Neph1 complexes and their implications for SD ultrastructure. Finally, we outline unresolved questions regarding SD composition, assembly, and signaling, proposing how integrative structural biology may illuminate mechanisms underlying proteinuric kidney diseases.