Shank2 Regulates Renal Albumin Endocytosis.

Evgenia Dobrinskikh,Judith Blaine,R Brian Doctor,Linda Lewis,Sarah Faubel,Min Goo Lee,Kayo Okamura,Jeffrey B Kopp,Christopher Altmann

doi:10.14814/phy2.12510

Abstract

Albuminuria is a strong and independent predictor of kidney disease progression but the mechanisms of albumin handling by the kidney remain to be fully defined. Previous studies have shown that podocytes endocytose albumin. Here we demonstrate that Shank2, a large scaffolding protein originally identified at the neuronal postsynaptic density, is expressed in podocytes in vivo and in vitro and plays an important role in albumin endocytosis in podocytes. Knockdown of Shank2 in cultured human podocytes decreased albumin uptake, but the decrease was not statistically significant likely due to residual Shank2 still present in the knockdown podocytes. Complete knockout of Shank2 in podocytes significantly diminished albumin uptake in vitro. Shank2 knockout mice develop proteinuria by 8 weeks of age. To examine albumin handling in vivo in wild-type and Shank2 knockout mice we used multiphoton intravital imaging. While FITC-labeled albumin was rapidly seen in the renal tubules of wild-type mice after injection, little albumin was seen in the tubules of Shank2 knockout mice indicating dysregulated renal albumin trafficking in the Shank2 knockouts. We have previously found that caveolin-1 is required for albumin endocytosis in cultured podocytes. Shank2 knockout mice had significantly decreased expression and altered localization of caveolin-1 in podocytes suggesting that disruption of albumin endocytosis in Shank2 knockouts is mediated via caveolin-1. In summary, we have identified Shank2 as another component of the albumin endocytic pathway in podocytes.

Highlights

Albuminuria is strongly associated with kidney disease progression (Hemmelgarn et al 2010; Hallan et al 2012)
Processes that ramify from the podocyte cell body divide into progressively smaller processes to form foot processes which abut the glomerular basement membrane and interdigitate with neighboring podocytes
A first step in determining whether Shank2 is involved in albumin endocytosis in the kidney is evaluating Shank2

Summary

Introduction

Albuminuria is strongly associated with kidney disease progression (Hemmelgarn et al 2010; Hallan et al 2012). Renal handling of albumin is complex and both the glomerulus and the proximal tubule have been shown to be involved in albumin handling (Eyre et al 2007; Russo et al 2007; Sandoval et al 2012; Dobrinskikh et al 2014). The GFB consists of three adjacent layers – fenestrated vascular endothelial cells, the glomerular basement membrane, and podocytes. Podocytes are specialized epithelial cells which have a large cell body and multiple processes that extend to cover the glomerular capillaries. Processes that ramify from the podocyte cell body divide into progressively smaller processes to form foot processes which abut the glomerular basement membrane and interdigitate with neighboring podocytes. Paracellular passage of molecules between podocytes is restricted by the slit diaphragm which contains integral membrane proteins that extend between foot processes of adjacent podocytes.

Methods

Results

Conclusion