Aspects of the glomerular barrier in healthy and diabetic kidneys

Abstract

Each day 180 liters of plasma is filtered in the kidneys. Under normal conditions, the glomerular barrier restricts the passage of macromolecules such as albumin while it is highly permeable to water and small solutes. Proteinuria is a hallmark of renal disease and reflects impairment of the glomerular barrier. The glomerular barrier has size and charge selective properties and consists of the fenestrated endothelium covered with a glycocalyx, the glomerular basement membrane, and the podocytes.In this thesis, the somewhat controversial issue of involvement of the endothelial cell glycocalyx in glomerular charge selectivity is investigated. The glomerular barrier has been studied with respect to function, morphology and gene expression in healthy, enzyme treated, and diabetic kidneys. Experiments were performed using the isolated perfused kidneys at 8C in order to investigate the glomerular barrier without the tubular modifications of primary urine that occurs in vivo. Kidneys in which the endothelial cell glycocalyx was altered with glucose/galactoseaminoglycan (GAG) degrading enzymes showed an up to 5-fold increase in fractional clearance for albumin. This is due to an alteration in glomerular charge selectivity since the fractional clearance for Ficoll 35.5 Å, the neutral counterpart of albumin, was unaltered. The enzyme action on the glycocalyx was confirmed morphologically in electron microscopy where Intralipid® droplets were used as indirect markers of the glycocalyx. To clarify if long term diabetes alters glomerular size or charge selectivity or both, we studied non obese diabetic mice for 10 and 40 weeks. Diabetes for 40 weeks resulted in altered glomerular charge selectivity as shown by a 3-fold increase in the fractional clearance for albumin, without any change of the neutral counterpart Ficoll 35.5 Å. Real-time PCR with the low density arrays revealed a down-regulation of cortex mRNA expression for versican, decorin, biglycan, matrix metalloprotease-9, and podocin after 40 weeks of diabetes. In summary, this thesis describes the importance of the glomerular endothelial cell glycocalyx in charge selectivity. In addition, albuminuria in long term diabetes originates from an alteration in charge selectivity which is coupled to down-regulation of the glycocalyx component, versican.