Deletion of Von Hippel-Lindau in Glomerular Podocytes Results in Glomerular Basement Membrane Thickening, Ectopic Subepithelial Deposition of Collagen α1α2α1(IV), Expression of Neuroglobin, and Proteinuria

Abstract

Vascular endothelial growth factor, which is critical for blood vessel formation, is regulated by hypoxia inducible transcription factors (HIFs). A component of the E3 ubiquitin ligase complex, von Hippel-Lindau (VHL) facilitates oxygen-dependent polyubiquitination and proteasomal degradation of HIF␣ subunits. Hypothesizing that deletion of podocyte VHL would result in HIF␣ hyperstabilization, we crossed podocin promoter-Cre transgenic mice, which express Cre recombinase in podocytes beginning at the capillary loop stage of glomerular development, with floxed VHL mice. Vascular patterning and glomerular development appeared unaltered in progeny lacking podocyte VHL. However, urinalysis showed increased albumin excretion by 4 weeks when compared with wild-type littermates with several sever cases (>1000 g/ml). Many glomerular ultrastructural changes were seen in mutants, including focal subendothelial delamination and widespread podocyte foot process broadening, and glomerular basement membranes (GBMs) were significantly thicker in 16-week-old mutants compared with controls. Moreover, immunoelectron microscopy showed ectopic deposition of collagen ␣1␣2␣1(IV) in GBM humps beneath podocytes. Significant increases in the number of Ki-67-positive mesangial cells were also found, but glomerular WT1 expression was significantly decreased, signifying podocyte death and/or de-differentiation. Indeed, expression profiling of mutant glomeruli suggested a negative regulatory feedback loop involving the HIF␣ prolyl hydroxylase, Egln3. In addition, the brain oxygen-binding protein, Neuroglobin, was induced in mutant podocytes. We conclude that podocyte VHL is required for normal maintenance of podocytes, GBM composition and ultrastructure, and glomerular barrier properties.

Conditional inactivation of the mouse von Hippel–Lindau tumor suppressor gene results in wide-spread hyperplastic, inflammatory and fibrotic lesions in the kidney

“…For example, Vhlh inactivation in the podocytes resulted in fibrotic glomerular defects. 35,36 The Pepck-Cre-driven conditional knockout mice also developed fibrosis in 418-monthold mice after partial nephrectomy of one kidney and complete removal of the other. 37 Inactivation of Hif-1α in the proximal tubular epithelium was protective against unilateral ureteral obstruction-induced kidney injury.…”

Hypoxia-inducible factor-1α promotes glomerulosclerosis and regulates COL1A2 expression through interactions with Smad3

“…However, others have shown that deletion of von Hippel-Lindau protein, a HIF ubiquitin ligase, in podocytes promotes apoptosis and proteinuria. 54, 55 Podocyte-specific ND HIF-1α-expressing mice are reported to lack a disease phenotype, although only a low expression level could be achieved. 56 It is possible that we did not observe an effect of HIF-1α on proteinuria because direct podocyte damage by LMB2 bypasses any potential role for HIF-1α in these cells.…”

The role of TRPC6 in oxidative stress-induced podocyte ischemic injury

“…After the stated perfusion times, blood samples were collected, and both kidneys were removed for mRNA, protein and histological analyses. Glomeruli were isolated by graded sieving as previously described [14].…”