The role of podocytes in glomerular pathobiology

Podocytes are unique cells with a complex cellular organization. With respect to their cytoarchitecture, podocytes may be divided into three structurally and functionally different segments: cell body, major processes, and foot processes (FPs). The FPs of neighboring podocytes regularly interdigitate, leaving between them the filtration slits that are bridged by an extracellular structure, known as the slit diaphragm (SD). Podocytes cover the outer aspect of the glomerular basement membrane (GBM). They therefore form the final barrier to protein loss, which explains why podocyte injury is typically associated with marked proteinuria. Chronic podocyte injury may lead to podocyte detachment from the GBM. Our knowledge of the molecular structure of the SD has been remarkably improved in the past few years. Several molecules, including nephrin, CD2AP, FAT, ZO-1, P-cadherin, Podocin, and Neph 1-3 have all been shown to be associated with the SD complex, and some of these molecules are critical for its integrity. Podocytes are injured in many forms of human and experimental glomerular disease. The early events are characterized either by alterations in the molecular composition of the SD without visible changes in morphology or, more obviously, by a reorganization of FP structure with the fusion of filtration slits and the apical displacement of the SD. Based on recent insights into the molecular pathology of podocyte injury, at least four major causes have been identified that lead to the uniform reaction of FP effacement and proteinuria: (1) interference with the SD complex and its lipid rafts; (2) direct interference with the actin cytoskeleton; (3) interference with the GBM or with podocyte-GBM interaction; and (4) interference with the negative surface charge of podocytes. There is also evidence, in focal segmental glomerular sclerosis (FSGS) and in idiopathic nephrotic syndrome in humans and rats, that podocyte damage may be caused by circulating albuminuric factors. Ongoing studies in many laboratories are aiming at an understanding of the dynamic relationship between SD proteins, the actin cytoskeleton, and the dynamics of FP structure in nephrotic syndrome and FSGS. These studies should provide us with a better understanding of the biological mechanism underlying the podocyte response to injury. Such studies will potentially translate into more refined treatment and the prevention of proteinuria and progressive glomerular disease.