Understanding the renal pathogenesis of diarrhoea-associated haemolytic uraemic syndrome (D+HUS): the role of the podocyte

Abstract

<h3>Background</h3> Diarrhoea-associated haemolytic uraemic syndrome (D+HUS) is the leading cause of paediatric acute renal failure. It occurs after infection by shiga toxin (stx) producing bacteria, usually E.coli O157. There is no direct treatment. HUS was considered an endothelial disease until an inducible podocyte-specific VEGFA knockdown mouse also developed the pathological signature of glomerular thrombotic microangiopathy (TMA). Stx binds to cellular Gb3 receptors. Human podocytes express Gb3 and show reduced VEGFA production following stx2 treatment. Mouse podocytes lack Gb3 and mice do not develop TMA if exposed to stx. The atypical form of HUS is caused by mutations in alternative complement pathway proteins, particularly Factor H. Eculizumab, a drug that blocks the complement cascade, has been used in atypical HUS and experimentally in D+HUS. <h3>Aims</h3> We hypothesise that podocytes, VEGFA and complement regulators play a co-ordinated role in the renal pathogenesis of D+HUS. Understanding how stx causes kidney damage will help devise treatment options for D+HUS. <h3>Methods</h3> Expression of Gb3 on human and murine glomerular cells was investigated using immunofluorescence. Cellular Stx1 sensitivity was determined using a radio-labelled protein synthesis assay. Glomerular endothelial cells (GEnC) were treated to remove surface bound Factor H and then exposed to serum free media with or without VEGFA for 24 hours. Factor H was detected by immunofluorescence and images quantified. Secreted Factor H was detected using SDS PAGE gels. <h3>Results</h3> Human glomerular cells express Gb3 and are stx1 sensitive whilst mouse podocytes lack Gb3 and are stx1 insensitive. Podocytes are 2.6-fold more sensitive to stx1 than glomerular endothelial cells (p&lt;0.002). VEGF-A up regulates human GEnC surface factor H (p&lt;0.0001). Factor H was detected in GEnC conditioned media. <h3>Conclusion</h3> Podocytes are exquisitely sensitive stx1 targets. Reduced podocyte VEGFA down-regulates glomerular endothelial cell factor H expression and production making the glomerular endothelium potentially vulnerable to complement-mediated attack. This evidence supports a central role for podocyte injury and complement mediated endothelial cell attack in the pathogenesis of D+HUS; suggesting a potential role for complement blockade in the treatment of D+HUS.