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ORIGINAL RESEARCH ARTICLE

The Focal Segmental Glomerulosclerosis Permeability Factor: Biochemical Characteristics and Biological Effects

Nephrology Division, Department of Medicine, Medical College of Wisconsin, Milwaukee, Wisconsin 53226

To whom requests for reprints should be addressed at ¹ Medical College of Wisconsin, Room M4040, MEB/CVC/Nephrology, 8701 Watertown Plank Road, Milwaukee, WI 53226. E-mail: msharma{at}mcw.edu

Focal segmental glomerulosclerosis (FSGS) is characterized by steroid resistant nephrotic syndrome and progression to end-stage renal disease. Proteinuria in certain patients with FSGS may be caused by a circulating factor (FSGS permeability factor [FSPF]). The current report documents the biochemical characteristics and the biological and molecular effects of 70% ammonium sulfate supernatant of plasma from patients with recurrence of FSGS after transplantation (FSGS 70% supernatant). FS permeability activity, defined as the capacity of plasma from patients with FSGS to increase albumin permeability (P_alb) of isolated glomeruli, was assessed in vitro. Permeability activity was not affected by lyophilization. FSPF bound strongly to matrices containing Mono-Q anion exchanger or protein A. It eluted from matrix-bound Cibacron blue F3GA over a wide range of salt concentrations, indicating a potential binding with other proteins, such as albumin. FSPF caused a maximal increase in P_alb within 2 mins of incubation in vitro. Cellular proteins isolated from glomeruli with increased P_alb showed decreased tyrosine phosphorylation of focal adhesion kinase, paxillin, and other proteins. Tyrosine phosphatase ]inhibition prevented the increase in P_alb. Intravenous administration of as little as 3 mg protein in FSGS 70% supernatant increased P_alb, while 9 mg or more were required to produce proteinuria. We conclude that FSPF is a low-molecular-weight protein, carries an anionic charge, and binds to protein A. Effects of FSPF on the glomerular permeability barrier are rapid and dose dependent and involve signaling through altered phosphorylation of cellular proteins. Identification of these biochemical and biological characteristics may be used to design strategies for removing FSPF from circulation and for purification and identification of this factor.

Key Words: recurrent FSGS • FSGS factor • FSPF • proteinuria • glomerular albumin permeability • plasmapheresis • protein tyrosine phosphorylation • FAK • paxillin

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