Idiopathic nephrotic syndrome

Idiopathic nephrotic syndrome is defined by the combination of massive proteinuria, hypoalbuminaemia, hyperlipidaemia, and oedema, and of non-specific histological abnormalities of the glomeruli. Light microscopy may disclose minimal change disease, diffuse mesangial proliferation, or focal segmental glomerular sclerosis (FSGS). The two main causes of idiopathic nephrotic syndrome are characterized histologically. On electron microscopy the glomerular capillaries show a fusion of visceral epithelial cell (podocyte) foot processes and with the exception of some variants no significant deposits of immunoglobulins or complement by immunofluorescence. In a majority of children only minimal changes are seen on light microscopy. These children are referred to as having ‘minimal change disease’. In adults with idiopathic nephrotic syndrome, lesions of FSGS are more frequent.

The Glomerular Filtration Barrier: Components and Crosstalk

The glomerular filtration barrier is a highly specialized blood filtration interface that displays a high conductance to small and midsized solutes in plasma but retains relative impermeability to macromolecules. Its integrity is maintained by physicochemical and signalling interplay among its three core constituents—the glomerular endothelial cell, the basement membrane and visceral epithelial cell (podocyte). Understanding the pathomechanisms of inherited and acquired human diseases as well as experimental injury models of this barrier have helped to unravel this interdependence. Key among the consequences of interference with the integrity of the glomerular filtration barrier is the appearance of significant amounts of proteins in the urine. Proteinuria correlates with kidney disease progression and cardiovascular mortality. With specific reference to proteinuria in human and animal disease phenotypes, the following review explores the roles of the endothelial cell, glomerular basement membrane, and the podocyte and attempts to highlight examples of essential crosstalk within this barrier.

Podocyte Injury Associated with Mutant α-Actinin-4

Focal segmental glomerulosclerosis (FSGS) is an important cause of proteinuria and nephrotic syndrome in humans. The pathogenesis of FSGS may be associated with glomerular visceral epithelial cell (GEC; podocyte) injury, leading to apoptosis, detachment, and “podocytopenia”, followed by glomerulosclerosis. Mutations in α-actinin-4 are associated with FSGS in humans. In cultured GECs, α-actinin-4 mediates adhesion and cytoskeletal dynamics. FSGS-associated α-actinin-4 mutants show increased binding to actin filaments, compared with the wild-type protein. Expression of an α-actinin-4 mutant in mouse podocytes in vivo resulted in proteinuric FSGS. GECs that express mutant α-actinin-4 show defective spreading and motility, and such abnormalities could alter the mechanical properties of the podocyte, contribute to cytoskeletal disruption, and lead to injury. The potential for mutant α-actinin-4 to injure podocytes is also suggested by the characteristics of this mutant protein to form microaggregates, undergo ubiquitination, impair the ubiquitin-proteasome system, enhance endoplasmic reticulum stress, and exacerbate apoptosis.

Complement C5b-9 complex activates phospholipases in glomerular epithelial cells

In rat membranous nephropathy, formation of the C5b-9 membrane attack complex (MAC) leads to proteinuria in association with glomerular visceral epithelial cell (GEC) injury. These alterations in GEC function and morphology might result from changes in intracellular free Ca2+ concentration [( Ca2+]i) and activation of phospholipases. We demonstrate that in cultured rat GEC, antibody-directed formation of noncytolytic amounts of the MAC induced a rapid and sustained increase in [Ca2+]i that was partly inhibited by ethylene glycol-bis(beta-aminoethyl ether)-N,N,N',N'-tetraacetic acid (EGTA). The MAC elevated levels of inositol bis- (IP2) and trisphosphate (IP3), as well as 1,2-diacylglycerol (DAG) and phosphatidic acid (PA). In permeabilized GEC, IP3 released Ca2+ from intracellular stores. Cellular 45Ca2+ uptake was also increased by the MAC. Thus, in GEC, the MAC induced Ca2+ mobilization from intracellular stores secondary to activation of phospholipase C and production of IP3, as well as enhanced Ca2+ influx. In addition, C5b-9 stimulated release of arachidonic acid (AA), prostaglandin F2 alpha, and thromboxane A2. Indomethacin partially inhibited the increase in DAG levels observed with the MAC, whereas the prostaglandin H2/thromboxane A2 analogue U46619 elevated DAG, suggesting that an eicosanoid product of MAC-induced AA release may enhance the activation of phospholipase C. Activation of phospholipases by the MAC may lead to altered GEC function and thereby contribute to the pathophysiological changes that characterize complement-dependent rat membranous nephropathy.

Inhibition of rat glomerular visceral epithelial cell growth by heparin

The effect of several glycosaminoglycans and sulfated polysaccharides on the growth of cultured rat glomerular visceral epithelial cells (GEC) was studied in vitro. Heparin, one preparation of heparan sulfate proteoglycan, dextran sulfate, and pentosan polysulfate significantly inhibited the growth of several GEC clones studied (36.0-77.1% inhibition at 100 micrograms/ml). Other glycosaminoglycans studied did not affect GEC growth. Growth inhibition by heparin was dose related and did not appear to reflect cytotoxicity. Heparins with high or low affinity for antithrombin inhibited growth to similar degrees. When heparin was fractionated into high- and low-anticoagulant activity fractions by physicochemical means the high activity fraction displayed significantly greater growth inhibition. The degree of growth inhibition significantly correlated with serum concentration in the media (r = 0.64; P less than 0.001). Removal of heparin binding factors from serum resulted in a loss of this correlation as well as less overall growth inhibition. These experiments suggest that interactions of GEC with heparan sulfates and other heparin-like molecules in the extracellular matrix may be important in the control of GEC growth.