“The FSGS Factor”: Enrichment and in Vivo Effect of Activity from Focal Segmental Glomerulosclerosis Plasma

Abstract

Abstract.: A circulating causative factor has been postulated in focal segmental glomerulosclerosis (FSGS). It has been shown that serum or plasma from some FSGS increases glomerular albumin permeability (P alb) in vitro. P alb greater than 0.5 (i.e., FS activity) is associated with recurrence after transplantation. Specimens from 15 FSGS patients were studied to document the presence of a permeability factor, to isolate this factor, to characterize its biochemical properties, and to show its effect in vivo. Total lipids were extracted by chloroform/methanol (2:1); FS activity was absent from total lipid extract. Chylomicrons and lipoproteins were removed from the plasma with dextran sulfate, followed by sequential precipitation of proteins at 50 and 70% ammonium sulfate saturation. FS activity was retained in the 70% ammonium sulfate supernatant and exhibited a 100-fold purification. FS activity was lost after heating at 100°C for 10 min or after protease digestion. Under nondenaturing conditions, electrophoresis of the FSGS 70% supernatant showed a prominent low molecular weight band that was not evident in the 70% supernatant from normal plasma. Dialysis and centrifugation-based membrane ultrafiltration of the FSGS factor indicated a molecular size between 30 and 50 kD. Injection of the 70% FSGS supernatant into rats caused a threefold increase in urine protein in collections from 6 to 24 h after injection. No increase in proteinuria occurred in rats injected with 70% supernatant from normal individuals. It is concluded that the FSGS factor is a low molecular weight protein with the potential to increase P alb in vitro and to cause proteinuria in vivo.