The expression of the structural proteins dendrin and Neph1 in the glomerular filtration barrier in proteinuria

Abstract

Background: In the normal kidney, the glomerular filtration barrier successfully clears about one litre blood per minute. Damage to the filtration barrier might lead to protein leakage in the urine – proteinuria. The major ultra-structural finding in glomerular diseases with proteinuria is foot process effacement (FPE). Despite these being the most common signs of glomerular dysfunction, the underlying pathophysiological mechanisms are still not fully understood. However, a number of proteins identified in the slit diaphragm (SD) of the podocytes, including Dendrin and Neph1, are believed to be significant. Dendrin is a protein that has previously been described in mouse podocytes, associated to the actin cytoskeleton. Neph1 is a transmembrane protein that forms a complex with Nephrin in the SD. Recent studies have indicated the complex involvement in polymerization of the actin cytoskeleton and proteinuria.

Aim: We aimed to study the expression of Dendrin in normal human kidney and in the glomerular disease Minimal Change Nephrotic Syndrome (MCNS) with proteinuria and foot process effacement (FPE). In the second study, we wanted to investigate the subcellular localization of Neph1 in normal human kidney and the expression in Focal Segmental Glomerulosclerosis (FSGS), MCNS and in the corresponding experimental models Adriamycin nephropathy (ADR) and puromycin aminonucleoside (PAN). All characterized by substantial FPE and proteinuria.

Methods: The localization of Dendrin and Neph1 was first studied in normal kidney tissue and then compared to the expression in biopsy specimens of the above mentioned diseases, using light and electron microscopy. The expression was semiquantified by immuno- electron microscopy (iEM).

Results: Dendrin was localized solely in the podocytes close to the SD. There was no significant change in the total amount of Dendrin in MCNS compared to controls by immunofluorescence and iEM. Neph1 was also localized mainly to the SD. Double staining of Neph-1 and Nephrin showed the proteins in close connection in the SD. The total amount of Neph1 was significantly reduced in the glomerular diseases FSGS, MCNS and in ADR and PAN. The reduction of Neph1 was also seen in areas without FPE. Nephrin was reduced in MCNS and PAN but unchanged in FSGS.

Conclusion: In preserved slits and in areas without FPE in MCNS, the amounts of Dendrin were unchanged compared to controls. The redistribution might therefore be secondary to FPE. Neph1 co-localize with Nephrin in the SD and was reduced in FSGS, MCNS, ADR and PAN. Nephrin was however, unchanged in FSGS which could indicate a disruption of the Neph1-Nephrin complex and an involvement of Neph1 in the pathogenesis of this disease.