Elimination of Heparan Sulfate Glycosaminoglycans from the Glomerular Basement Membrane has Profound Effects on Podocyte Ultrastructure and Function

Abstract

Heparan sulfate (HS) proteoglycans (PG) are considered important in GBM ultrafiltration and are synthesized primarily by podocytes. The EXT1 gene encodes for the glycosyltransferase that is the limiting step in HS‐GAG polymerization on PG core proteins. PEXTKO (podocyte EXT1 knockout) mice were developed by breeding EXT1fl/fl with mice transgenic for CRE‐recombinase expression driven by the podocin promoter. Podocytes from PEXTKO mice lack HS‐GAG, yet renal development and maturation occurs. H&E stained sections from kidneys of PEXTKO animals were similar to control animals. Electron microscopy (EM) studies showed podocyte foot process effacement occurred PEXTKO mice at all ages. GBM ultrastructure was compromised in 1 month old animals, with GBM splitting in some areas. In adult animals there was evidence of enhanced GBM synthesis yet overall GBM thickness appeared normal. Immunostaining kidney sections from PEXTKO mice with core protein specific antibodies against perlecan showed no difference in the pattern of staining between control animals and PEXTKO animals, however agrin core protein staining was increased. Albuminuria was not seen in 3 month old PEXTKO animals but mild microalbuminuria was seen in 8 month old animals. The data indicate that the presence of HS chains on GBM proteoglycan core proteins may not exclusively function as a passive sieve, but rather modulate podocyte behavior.