Cofilin-1 Inactivation Leads to Proteinuria – Studies in Zebrafish, Mice and Humans

Abstract

BackgroundPodocytes are highly specialized epithelial cells on the visceral side of the glomerulus. Their interdigitating primary and secondary foot processes contain an actin based contractile apparatus that can adjust to changes in the glomerular perfusion pressure. Thus, the dynamic regulation of actin bundles in the foot processes is critical for maintenance of a well functioning glomerular filtration barrier. Since the actin binding protein, cofilin-1, plays a significant role in the regulation of actin dynamics, we examined its role in podocytes to determine the impact of cofilin-1 dysfunction on glomerular filtration.Methods and FindingsWe evaluated zebrafish pronephros function by dextran clearance and structure by TEM in cofilin-1 morphant and mutant zebrafish and we found that cofilin-1 deficiency led to foot process effacement and proteinuria. In vitro studies in murine and human podocytes revealed that PMA stimulation induced activation of cofilin-1, whereas treatment with TGF-β resulted in cofilin-1 inactivation. Silencing of cofilin-1 led to an accumulation of F-actin fibers and significantly decreased podocyte migration ability. When we analyzed normal and diseased murine and human glomerular tissues to determine cofilin-1 localization and activity in podocytes, we found that in normal kidney tissues unphosphorylated, active cofilin-1 was distributed throughout the cell. However, in glomerular diseases that affect podocytes, cofilin-1 was inactivated by phosphorylation and observed in the nucleus.ConclusionsBased on these in vitro and in vivo studies we concluded cofilin-1 is an essential regulator for actin filament recycling that is required for the dynamic nature of podocyte foot processes. Therefore, we describe a novel pathomechanism of proteinuria development.

Direct Regulation of Nephrin Tyrosine Phosphorylation by Nck Adaptor Proteins

“…Decreased Akt phosphorylation on Ser 473 has been reported during the onset of proteinuria in the rat puromycin aminonucleoside model (9). Furthermore, mice with a podocyte-specific knock-out of cofilin developed renal disease (10), and phosphorylation of cofilin, corresponding with inactivation, was increased in some forms of human glomerular disease including minimal change disease (39). Interestingly, decreased phosphorylation on the Nck binding sites Tyr 1193 and Tyr 1217 has been observed in the same animal models (19,20) and human diseases (20) where decreased p85-based signaling has been reported (9, 39).…”

Focus on ADF/Cofilin: Beyond Actin Cytoskeletal Regulation

“…Also, the actin cytoskeleton is critical for all three functional domains [16,67]. Inactivation of ADF/cofilin by serine-3 phosphorylation is reported to be correlated with proteinuria in zebra fish, mice, and humans [70]. Proteinuria is a type of nephropathy caused by the injury of three cell types including mesangial cells, podocytes, and endothelial cells in glomerular units.…”

“…Degeneration of podocytes is a primary event in the development of chronic renal failure and proteinuria. Activated ADF/cofilin is well distributed in normal podocytes, but inactivated (phosphorylated) ADF/cofilin can be found throughout misfunctional podocyte cells [70]. Of interest, ADF/cofilin can be activated for actin filamental reorganization through receptor tyrosine kinase activation, the so-called NephrinNeph1 receptor complex on the membranes of podocytes [64].…”

Nephrin Tyrosine Phosphorylation Is Required to Stabilize and Restore Podocyte Foot Process Architecture

“…19 Similarly, reduced phosphorylation of the downstream survival factor Akt on serine (S) 473 can be observed in MCD, 20,21 and inactivation of the actin binding protein cofilin is detected in patients with MCD, membranous nephropathy, and FSGS. 22 These observations raise the intriguing possibility that uncoupling of phosphonephrin signaling from actin leads to pathologic disruptions in foot process morphology. However, additional investigation is required to determine whether perturbations in nephrin tyrosine phosphorylation directly contribute to the development of disease as a result of altered downstream signaling or rather, if decreases in nephrin tyrosine phosphorylation occur during the progression of disease simply as a consequence of changes in other signaling pathways.…”