Molecular composition and function of the slit diaphragm: nephrin, the molecule responsible for proteinuria

Abstract

Although the role of the glomerular basement membrane has been emphasized as the barrier for retaining plasma proteins, some recent studies have clearly shown that the slit diaphragm is the structure most likely to be the main barrier in the glomerular capillary wall. Nephrin, ZO-1, P-cadherin and FAT are reported to be components of the slit diaphragm. Of these molecules, the molecular natures of ZO-1 and nephrin are relatively well investigated. ZO-1 was originally identified as a component of the tight junction. Nephrin was identified as a gene product mutated in congenital nephrotic syndrome. ZO-1 is localized at the cytoplasmic surface just at the insertion point of the slit diaphragm, whereas nephrin is located at the outer leaflet of plasma membranes. The expression of nephrin is decreased in several kinds of proteinuric states, and anti-nephrin monoclonal antibody (mAb) 5-1-6 is capable of inducing massive proteinuria, which indicates that nephrin is a functional protein in the slit diaphragm. Nephrin is expected to be a target of therapy for proteinuria. A model of nephrin assembly that forms the structure of the slit diaphragm has been proposed. However, we observed that the slit diaphragm structure was maintained in mAb 5-1-6 nephropathy, although nephrin expression was dramatically decreased. These findings suggest that nephrin is not essential for maintaining the structural integrity of the slit diaphragm. Although it has been proposed that P-cadherin is a core protein in the slit diaphragm, this hypothesis is still controversial. To better understand the structure of the slit diaphragm, it is particularly important to identify other components that build up the structure of the slit diaphragm together with nephrin and ZO-1.