Heparan sulfate (HS) within the glomerular basement membrane (GBM) is thought to play a major role in the charge-selective properties of the glomerular capillary wall. Recent data, however, raise questions regarding the direct role of HS in glomerular filtration. For example, in situ studies suggest that HS may prevent plasma macromolecules from clogging the GBM, keeping it in an "open" state. We evaluated this potential role of HS in vivo by studying the passage of protein through the glomerular capillary wall in the presence and absence of HS. Intravenous administration of neuraminidase removed neuraminic acid-but not HS-from the GBM, and this led to albuminuria. Concomitant removal of HS with heparinase III, confirmed by ultrastructural imaging, prevented the development of albuminuria in response to neuraminidase treatment. Taken together, these results suggest that HS keeps the GBM in an open state, facilitating passage of proteins through the glomerular capillary wall. The glomerular capillary wall, which consists of fenestrated endothelial cells, the glomerular basement membrane (GBM), and podocytes with foot processes interconnected by slit diaphragms, forms the major filtration barrier of the glomerulus. It excludes plasma proteins with the size of albumin (69 kD, 3.6 nm) and larger from its filtrate. 1,2 Next to size, permeability of the glomerular capillary wall is controlled by charge interactions. Neutral and cationic molecules can more easily penetrate the GBM than anionic molecules, which encounter electrostatic repulsion by HS. In addition, the negative charges of the cell coat (glycocalyx) of the endothelial cells and podocytes may play a role in the charge-dependent glomerular filtration. 3,4 HS contains multiple carboxylic groups and negatively charged N-, 2-O-, 6-O-, and 3-O-sulfate groups. 5,6 Because the majority of HS resides in the GBM, it is assumed that GBM HS is responsible for the charge-selective properties of the glomerular capillary wall. 3,7,8 This is supported by a number of studies. First, a reduction in glomerular HS has been described in a number of human kidney diseases 8 -11 and experimental animal models 8,12-14 characterized by proteinuria. Second, intravenous injection of rats with anti-HS antibody JM403 resulted in acute albuminuria. 15 Third, perfusion of isolated rat kidneys or glomeruli with the HS-degrading enzyme heparinase III resulted in penetration of ferritin and bovine serum albumin into the GBM and passage to the urinary space. 16,17 However, recent observations question the direct role of