The expression of the structural proteins dendrin, Plekhh2 and Neph1 in the glomerular filtration barrier

Abstract

Background: The podocytes are injured in many acquired glomerular diseases. This may present as proteinuria or morphological changes such as foot process effacement (FPE) or loss of podocytes in the urine. The exact mechanisms are still mainly unknown. However, a number of proteins identified in the slit diaphragm (SD) of the podocytes, including Dendrin and Neph1, are believed to be of significance in the podocytes’ response to injury and in the resulting pathophysiological development. Dendrin was primarily found to be associated to the actin cytoskeleton in mouse podocytes. Neph1 is a transmembrane protein that together with Nephrin forms a complex in the SD, involved in polymerization of the actin cytoskeleton and proteinuria. Plekhh2 is an uncharacterized protein that we localized to the podocyte cytoplasm.

Aim: The principal aim was to study the expression and role of novel structural glomerular proteins in acquired human glomerular disease. In the first study, focusing on the expression of Dendrin in normal human kidney and in the glomerular disease Minimal Change Nephrotic Syndrome (MCNS), light and immune electron microscopy (iEM) was used. In the second study, the subcellular localization of the uncharacterized protein Plekhh2 in normal human kidney and Focal Segmental Glomerulosclerosis (FSGS) was investigated by immunofluorescence (IFL) and iEM. Neph1 and Nephrin were studied in FSGS, MCNS and in the corresponding experimental models Adriamycin nephropathy (ADR) and puromycin aminonucleoside nephrosis (PAN). Lastly, we returned to Dendrin, and studied the expression in the podocyte nuclei in IgA Nephropathy (IgAN) and Membranous Nephropathy (MN) with iEM and analyzed the gene expression with microarray.

Results: Dendrin was localized to the glomerular slit diaphragm (SD). There was no significant change in the amount of Dendrin in MCNS compared to controls by IFL and iEM. In areas of FPE Dendrin was redistributed from the SD to the podocyte cytoplasm. Plekhh2 was localized mainly to the podocyte cytoplasm. The expression was reduced in FSGS. Double staining of Neph-1 and Nephrin showed the proteins in close connection in the SD. Neph1 was significantly reduced in FSGS, MCNS, ADR and PAN. The reduction of Neph1 was also seen in areas without FPE. Nephrin was unchanged in FSGS, but reduced in MCNS and PAN. Dendrin was increased in the podocyte nuclei in IgAN, with a corresponding two fold increase of the gene expression. Both protein and gene expression of Dendrin was unchanged in MN.

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, per se. However, the increase of nuclear Dendrin in IgAN suggests an upregulated apoptotic pathway and a possible role for Dendrin in the pathogenesis of this disease. Plekhh2 was reduced and relocated from the plasma membrane to centrally in the foot processes in FSGS. Neph1 was reduced in FSGS, MCNS, ADR and PAN, in contrast to Nephrin which was unchanged in FSGS. This could indicate a disruption of the Neph1-Nephrin complex and an involvement of Neph1 in the pathogenesis of this disease.