Id Proteins Regulate Capillary Repair and Perivascular Cell Proliferation following Ischemia-Reperfusion Injury

David Lee,Shantheri Shenoy,Yezina Nigatu,Matt Plotkin,David Long

doi:10.1371/journal.pone.0088417

Abstract

Acute kidney injury (AKI) results in microvascular damage that if not normally repaired, may lead to fibrosis. The Id1 and 3 proteins have a critical role in promoting angiogenesis during development, tumor growth and wound repair by functioning as dominant negative regulators of bHLH transcription factors. The goal of this study was to determine if Id proteins regulate microvascular repair and remodeling and if increased Id1 expression results in decreased capillary loss following AKI. The effect of changes in Id expression in vivo was examined using Id1−/−, Id3RFP/+ (Id1/Id3 KO) and Tek (Tie2)-rtTA, TRE-lacz/TRE Id1 (TRE Id1) mice with doxycycline inducible endothelial Id1 and β-galactosidase expression. Id1 and 3 were co-localized in endothelial cells in normal adult kidneys and protein levels were increased at day 3 following ischemia-reperfusion injury (IRI) and contralateral nephrectomy. Id1/Id3 KO mice had decreased baseline capillary density and pericyte coverage and increased tubular damage following IRI but decreased interstitial cell proliferation and fibrosis compared with WT littermates. No compensatory increase in kidney size occurred in KO mice resulting in increased creatinine compared with WT and TRE Id1 mice. TRE Id1 mice had no capillary rarefaction within 1 week following IRI in comparison with WT littermates. TRE Id1 mice had increased proliferation of PDGFRβ positive interstitial cells and medullary collagen deposition and developed capillary rarefaction and albuminuria at later time points. These differences were associated with increased Angiopoietin 1 (Ang1) and decreased Ang2 expression in TRE Id1 mice. Examination of gene expression in microvascular cells isolated from WT, Id1/Id3 KO and TRE Id1 mice showed increased Ang1 and αSMA in Id1 overexpressing cells and decreased pericyte markers in cells from KO mice. These results suggest that increased Id levels following AKI result in microvascular remodeling associated with increased fibrosis.

Highlights

Following acute kidney injury, kidney interstitial cells become activated in response to cytokines and growth factors secreted by injured epithelial and endothelial cells and infiltrating inflammatory cells
In the normal adult kidney, Id1 was expressed at variable levels in CD31 positive peritubular endothelial cells in the cortex and medulla (Fig. 1A) and was absent from pericytes and other PDGF receptor beta (PDGFRb) positive interstitial cells (Fig. 1B)
Expression levels are generally decreased in adult organs and are increased with inflammation and tumor formation where Id1 and Id3 have been shown to have a key role in promoting angiogenesis

Summary

Introduction

Kidney interstitial cells become activated in response to cytokines and growth factors secreted by injured epithelial and endothelial cells and infiltrating inflammatory cells. Fibroblast activation results in remodeling of the extracellular matrix that promotes repair of damaged tubules and peritubular capillaries. With severe or irreversible injury, this process is persistently activated, resulting in tissue fibrosis, capillary rarefaction and chronic renal failure [1]. Recent studies have demonstrated that endothelial cells and pericytes that form the peritubular microvasculature are a source of injury induced fibroblasts and myofibroblasts that produce extracellular matrix [2]. During the normal adaptive process for repairing tissue damage, TGFb and BMP signals regulate cell proliferation and differentiation. BMP signal transduction is mediated by nuclear effector R-Smads, with downstream activation of regulatory factors including Id proteins [2,3].

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