Signaling at the slit diaphragm.

Diseases of the glomerular filter of the kidney are a leading cause of end-stage renal failure. Recent studies have emphasized the critical role of the slit diaphragm of podocytes for the size-selective filtration barrier of the kidney and revealed novel aspects of the mechanisms that lead to proteinuria, in both inherited and acquired diseases (1–4). Several critical structural protein components of the slit diaphragm have been identified. Recently, it has been speculated that these slit diaphragm proteins, in addition to their structural functions, participate in common signaling pathways. This review focuses on what is known about signaling at the slit diaphragm. It provides a snapshot of our current understanding of the signaling properties of slit diaphragm proteins and projects a framework for further studies necessary to delineate the function and dynamics of the slit diaphragm protein complex and the pathogenesis of nephrotic syndrome. Ultrafiltration of plasma in the renal glomeruli is a major function of the kidney. The glomerular filter through which the ultrafiltrate has to pass consists of three layers: the fenestrated endothelium, the intervening glomerular basement membrane, and the epithelial podocyte foot processes. This filtration barrier behaves as a size-selective sieve restricting the passage of macromolecules on the basis of their size, shape, and charge (5–7). Although the glomerular filter is a primary target of a large number of progressive disorders that lead to chronic renal insufficiency, until recently, little was known about the importance of podocytes for establishing the size-selective filtration barrier of the kidney. The recent description of gene defects in hereditary nephrotic syndrome resulting in the loss of podocyte proteins has dramatically changed this situation. Together with data from various animal models, these studies have unraveled important aspects of the pathogenesis of proteinuric kidney diseases and challenged our understanding of the glomerular filter (1,8–10). Podocytes are the visceral epithelial cells of the kidney glomerulus (11–13). They elaborate long, regularly spaced, interdigitated foot processes that completely enwrap the glomerular capillaries. Interdigitating podocyte foot processes form an ~40-nm-wide filtration slit and are connected by a continuous membrane-like structure called the slit diaphragm. When podocytes are injured, the intercellular junctions and cytoskeletal structure of the foot processes are altered and the cell takes on an “effaced” phenotype. Typical slit diaphragm structures disappear and proteinuria develops (10). Because there is no case of nephrotic syndrome without major changes in podocyte morphology, it has been speculated for many years that the slit diaphragm is an important component of the glomerular filter (14). A milestone in glomerular research was the cloning of nephrin by the Tryggvason group in 1998 together with its localization to the slit diaphragm of podocytes (15–18). This work suggested that nephrin is a critical structural component of the slit membrane and bridges the distance between interdigitating podocyte foot processes (3,9,19). Nephrin, encoded by NPHS1, the gene mutated in congenital nephrotic syndrome of the Finnish type, is a transmembrane adhesion protein of the Ig superfamily (Figure 1). Both humans and mice lacking nephrin are born without typical slit diaphragms and exhibit severe podocyte abnormalities and massive proteinuria already in utero (20,21). Recently, several additional components of the slit diaphragm have been identified (22–25). Mutations in the genes encoding for these proteins cause severe podocyte changes and nephrotic syndrome. NPHS2, the gene responsible for a steroid-resistant form of nephrotic syndrome, was cloned by the Antignac group and encodes for podocin, a stomatin family membrane protein (23). Podocin almost exclusively localizes to the slit diaphragm of podocytes and interacts with the cytoplasmic tail of nephrin (26–28). Gene disruption in mice results in a severe congenital nephrotic syndrome (29). The cytoplasmic multi-adaptor protein CD2AP was cloned by the Shaw laboratory as a CD2-interacting protein in lymphocytes (30). CD2AP also interacts with nephrin and localizes to the cytoplasmic face of the slit diaphragm (31). It is interesting that mice completely lacking CD2AP die of massive proteinuria 6 wk after birth, suggesting a critical role for CD2AP in slit diaphragm function (22). Moreover, CD2AP haploinsufficiency seems to be linked to glomerular disease susceptibility both in mice and in humans, further supporting the critical role of CD2AP for the integrity of the glomerular filter (32,33). Donoviel et al. (24) identified neph1, another Ig superfamily protein (Figure 1) that localizes to the slit and causes congenital nephrotic syndrome in knockout mice. Neph1 is a member of a family of Ig adhesion molecules that are expressed in podocytes and interact with podocin (34). Recently, the adhesion protein and member of the protocadherin superfamily of proteins FAT1 was shown to localize to the slit membrane of podocytes (35). Targeted deletion of the fat1 gene in mice results in nephrotic syndrome and podocyte changes that resemble nephrin mutation (25). Correspondence to Thomas Benzing, Renal Division, University Hospital Freiburg, Hugstetterstrasse 55, 79106 Freiburg, Germany. Phone: 49-761-2703559; Fax: 49-761-270-6362; E-mail: [email protected]