Podocin, a raft-associated component of the glomerular slit diaphragm, interacts with CD2AP and nephrin

Abstract

Glomerular podocytes are highly specialized cells consisting of a cell body, major processes, and foot processes with their interconnecting slit diaphragms (SDs) (1).As part of the glomerular filtration barrier, the SD is thought to function as a size-selective filter, whereas the charge selectivity is thought to be located in the glomerular basement membrane (2, 3).Under normal conditions, the filtration barrier is freely permeable to water, ions, and proteins smaller than albumin.In the nephrotic syndrome, the normal podocyte substructure is lost, with effacement of podocyte foot processes and massive proteinuria (4, 5).Although relatively little is known about the cellular or molecular changes that occur within podocytes during the development of nephrotic syndrome, cytoskeletal proteins likely play a central role in these changes (4).Indeed, mutations in ACTN4, encoding α-actinin-4, cause familial focal segmental glomerulosclerosis (FSGS) (6).Due to the central role of podocytes in glomerular physiology and pathology, various groups have investigated podocyte proteins including CD2AP and nephrin.Mutations in nephrin lead to congenital nephrotic syndrome in humans (7-11), and deletion of CD2AP leads to experimental congenital nephrotic syndrome (12, 13).Recently, NPHS2 was identified by positional cloning to be the target gene of autosomal recessive steroidresistant nephrotic syndrome ( 14).The NPHS2 gene product, podocin, is a new member of the band-7stomatin protein family of lipid raft-associated proteins (15).Comparison of amino acid sequence demonstrates that podocin is 47% identical to human stomatin and 44% to Caenorhabditis elegans Mec-2 (14, 16).Due to its structural similarity to stomatin, podocin is predicted to be an integral membrane protein with both NH 2 -and COOH-terminal intracellular domains that form a hairpin-like structure (14, 17, 18).Podocin expression is restricted to podocytes as shown by in situ RNA hybridization ( 14), but its subcellular distribution is unknown.Mutations in NPHS2 are associated with familial steroid-resistant nephrotic syndrome manifest as early childhood onset of proteinuria, rapid progression to end-stage renal disease, and FSGS.This suggests a regulatory role for podocin in determining glomerular permeability.