Charge selective function in childhood glomerular diseases

The glomerular basement membrane as a model system to study the bioactivity of heparan sulfate glycosaminoglycans

The glomerular basement membrane and its associated cells are critical elements in the renal ultrafiltration process. Traditionally the anionic charge associated with several carbohydrate moieties in the glomerular basement membrane are thought to form a charge selective barrier that restricts the transmembrane flux of anionic proteins across the glomerular basement membrane into the urinary space. The charge selective function, along with the size selective component of the basement membrane, serves to limit the efflux of plasma proteins from the capillary lumen. Heparan sulfate glycosaminoglycans are anionically charged carbohydrate structures attached to proteoglycan core proteins and have a role in establishing the charge selective function of the glomerular basement membrane. Although there are a large number of studies in the literature that support this concept, the results of several recent studies using molecular genetic approaches to minimize the anionic charge of the glomerular basement membrane would suggest that the role of heparan sulfate glycosaminoglycans in the glomerular capillary wall are still not yet entirely resolved, suggesting that this research area still requires new and novel exploration.

Reevaluation of glomerular charge selective protein-sieving function

Recently, disorders of the slit diaphragm have been considered as major causes of proteinuria in renal disease and the charge barrier function of the glomerular capillary wall has been given less attention. We evaluated the charge selectivity index (CSI) in 40 patients with podocyte disease (PD), 75 with chronic glomerulonephritis (CGN), and 8 with Dent disease, to reexamine the charge barrier function. We evaluated CSI in Dent disease because the urinary protein profile in Dent disease was assumed to be a concentrate of a normal glomerular filtrate. CSI was defined as the renal clearance ratio between IgA and IgG. CSI values (mean +/- SD) in the CGN and PD groups and in Dent disease were 1.12 +/- 0.25, 0.42 +/- 0.31, and 0.16 +/- 0.06, respectively, suggesting that the charge barrier function was defective in the CGN group and of reduced capacity in the PD group. The results suggest that functional interactions between the slit diaphragm and the glomerular basement membrane exist, and that a slit diaphragm disorder is accompanied by a decrease in the charge barrier function in PD, as argued by the conventional hypothesis.

Revisiting the glomerular charge barrier in the molecular era

PURPOSE OF REVIEW

The glomerular filtration barrier consists of fenestrated glomerular endothelium, podocyte foot processes/slit diaphragms, and intervening glomerular basement membrane. Its characterization as both a size and charge-selective barrier emerged from studies conducted decades ago. The charge selectivity phenomenon is receiving renewed attention now that the identities and mechanisms of synthesis of relevant molecules are known. Here we summarize studies employing genetic or other in-vivo strategies to investigate glomerular charge.

RECENT FINDINGS

Attention has focused on glomerular basement membrane heparan sulfate proteoglycans, long considered primary charge barrier components. Agrin contributes significantly to glomerular basement membrane charge but, like perlecan and collagen XVIII, is dispensable for glomerular structure and function. Disruption of glomerular heparan sulfate through transgenic methods or administration of heparanase in vivo provides further evidence against a role for heparan sulfate in glomerular function. Disruption of glomerular sialoproteins, however, causes proteinuria and indicates a critical role for these cell-associated glycoproteins in glomerular filtration.

SUMMARY

Recent in-vivo manipulations of glomerular heparan sulfate proteoglycans fail to reveal a crucial role for either them or their anionic charge in glomerular filtration. In contrast, cell-associated sialoproteins are clearly important, but whether their functions actually involve contributions to the charge barrier is unknown.