Podocyte-Specific Expression of Angiopoietin-2 Causes Proteinuria and Apoptosis of Glomerular Endothelia

Abstract

Angiopoietin-2 (Ang-2) modulates embryonic vascular differentiation primarily by inhibiting the antiapoptotic effects of Ang-1 on endothelia that express the Tie-2 receptor. Ang-2 is transiently expressed by developing glomeruli but is downregulated with normal maturation. Glomerular Ang-2 expression is, however, markedly upregulated in animal models of diabetic nephropathy and glomerulonephritis, both leading causes of human chronic renal disease, affecting 10% of the world population. It was hypothesized that Ang-2 might have significant roles in the pathobiology of glomerular disease. Mice with inducible podocyte-specific Ang-2 overexpression were generated. When the transgene was induced in adults for up to 10 wk, mice had significant increases in both albuminuria and glomerular endothelial apoptosis, with significant decreases of both vascular endothelial growth factor-A and nephrin proteins, critical for maintenance of glomerular endothelia and filtration barrier functional integrity, respectively. There was, however, no significant change of systemic BP, creatinine clearance, or markers of renal fibrosis, and podocytes appeared structurally intact. In kidneys of young animals in which Ang-2 had been upregulated during organogenesis, increased apoptosis occurred in just-formed glomeruli. In vitro, short-term exposure of isolated wild-type murine glomeruli to exogenous Ang-2 led to decreased levels of vascular endothelial growth factor-A protein. These novel results provide insight into molecular mechanisms underlying proteinuric disorders, highlight potentially complex interactions between subsets of glomerular cells, and emphasize how a vascular growth factor that has critical roles in normal development may be harmful when re-expressed in the context of adult disease.

Roles of Angiopoietins in Kidney Development and Disease

“…In vitro, short-term exposure of isolated wild-type murine glomeruli to exogenous Ang-2 also led to decreased protein levels of VEGF-A. 43 The increased in vivo apoptosis of glomerular endothelia was not an unexpected effect, consistent with the overexpressed Ang-2 accessing and directly altering the biology of these cells, antagonizing signaling of endogenous Ang-1; in fact, endothelial death may have been enhanced by VEGF-A downregulation in transgenic Ang-2-overexpressing glomeruli. 43 The changes in podocyte gene expression might have resulted from altered signaling between endothelia and podocytes.…”

“…43 We used a reverse tetracycline-controlled transcriptional activator (rtTA) activated by doxycycline and elected to drive expression of rtTA using the Podocin promoter, resulting in podocyte-specific activation. We considered that the adult podocyte was an appropriate glomerular cell type to use as a source for the angiopoietins because they normally express Ang-1.…”

“…48 In human and animal diabetic nephropathy, glomerular expression of nephrin is downregulated, 49 and a similar change occurred when Ang-2 overexpression was induced in transgenic (nondiabetic) mice. 43 Experimental models of type 1 diabetes are associated with altered renal expression of angiopoietins. Rizkalla et al 50 reported that administration of streptozotocin, an islet toxin, to adult rats led to hyperglycemia within 1 wk, with increased albumin excretion, systemic hypertension, and nephromegaly at 4 and 8 wk.…”

“…The effects of downregulating endogenous, glomerular-derived Ang-1 have yet to be published, although, on the basis of the idea that Ang-2 can act as an Ang-1 antagonist, 2 such animals may be found to share several features (e.g., glomerular endothelial apoptosis, albuminuria) of those that overexpress Ang-2 in glomeruli. 43 The observation that Ang-2 is highly expressed in tubules that surround mature vasa rectae, 35 which themselves express Tie-2, 62 suggests that the factor has a special role in patterning and/or maintaining these vessels; however, Ang-2 null mice die neonatally, 42 too soon to be informative for study of mature vasa rectae. Thus, proof of these ideas would require site-specific downregulation of Ang-2.…”

“…If such expression could be established, then it may help to explain some nonendothelial effects observed when angiopoietins are overexpressed. 43 Along the same lines, Ang-1 is implicated in branching morphogenesis of the developing lung, 67 although whether this is a direct effect or is an indirect one mediated by enhanced angiogenesis is not known.…”