Functional and molecular aspects of the glomerular barrier

Abstract

The kidneys are crucial for maintaining a normal salt-water balance in the body, which is a prerequisite for life. They do that by filtering 180 liters of plasma per day across the highly permselective glomerular barrier and reabsorbing 99 % of the filtrate. Proteinuria is a hallmark of renal disease and reflects damage to one or more of the components of the barrier, namely fenestrated endothelial cells, basement membrane, and podocytes. For several years, the basement membrane was considered to be the main barrier. Recently, research has focused on the podocytes, while the endothelium has received much less attention. The glomerular endothelial cells are covered however by a thick negatively charged cell surface coat, a glycocalyx, which most likely has selective properties. The glycocalyx is composed of plasma proteins, glycoproteins and proteoglycans. Proteoglycans (PG) are negatively charged molecules with a protein-core to which glycosaminoglycan (GAG) chains are attached. In this thesis, we have made a detailed analysis of the biosynthesis of PG and GAG by human and bovine glomerular endothelial cells, by human podocytes and by rat glomeruli in vivo. These cells were shown to express the following core proteins: syndecan, glypican, versican, perlecan, biglycan and decorin. When treating the cells with puromycin amino nucleoside (PAN), a drug known to induce nephrotic syndrome, we could see dramatic down-regulation of PGs, mainly versican, and of enzymes involved in the synthesis and modification of the glycosaminoglycan chains. Indeed, when analyzing the cell media after PAN treatment, a significant decrease in the amount of sulfate groups and the chain length was detected. These effects of PAN on glomerular cells were confirmed in a study on rats, in which molecular and physiological measurements were used to study the course of proteinuria for seven days. In these animals, PAN decreased the expression of nephrin, podocin and VEGF as well as with certain PGs and GAG enzymes. The marked changes of PG and GAG induced by PAN are likely to reduce both size and charge selectivity of the glomerular barrier.