Downregulated expression in high IgA (HIGA) mice and the renal protective role of meprinβ

Abstract

This study discusses the critical role of the metalloproteinase meprinβ in the progression of glomerulonephritis. Using a microarray technique, the gene expression profiles in glomeruli isolated from high serum IgA (HIGA) mice with a purity of 97% or greater were examined. HIGA mice are a valid model of human IgA nephropathy (IgAN), with the typical pathological features of this condition, including a consistently high serum IgA level as well as dominant mesangial IgA deposition and mesangial enlargement. Among the many upregulated/downregulated genes after the development of IgAN, the downregulation of meprinβ was intriguing. The expression level of the meprinβ gene at 40 weeks of age was 52% of that observed at 8 weeks of age (prior to the development of IgAN), although in the control BALB/c mice, a 2.19-fold elevation was seen. These results were also confirmed by semi-quantitative RT-PCR and immunostaining analyses. As meprinβ is a subunit of metalloproteinase meprins (meprin A, meprin B) and meprins are capable of proteolytically degrading extracellular matrix (ECM) components and proteolytically processing bioactive peptides, the downregulation of meprinβ may contribute to the progression of glomerulonephritis and the eventual glomerular scarring. This working hypothesis was examined using an in vivo meprinβ inhibition study. The inhibition of meprins by actinonin exacerbated some parameters of renal injury in mice afflicted with anti-glomerular basement membrane (anti-GBM) antibody-associated nephritis. These in vitro and in vivo results suggest that meprinβ may play a protective role against the progression of renal injury through the degradation of ECM and bioactive peptides.

Introduction

In current medical treatments of renal diseases like chronic glomerulonephritis and diabetic nephropathy, careful control of blood pressure using antihypertensive agents has been shown to be effective in controlling the progression of renal injury. However, the effects of the currently available treatments are limited, and the renal function in many patients with renal diseases eventually deteriorates, thereby requiring that patients begin receiving hemodialysis. One of the major reasons for the lack of consistently effective treatments in the clinical setting is the fact that the molecular mechanisms underlying the development of renal diseases are not fully understood, and almost no treatments with a molecular basis are available yet.

Under these circumstances, we performed a microarray analysis using highly pure glomeruli isolated from a murine model of IgAN, namely, HIGA mice, to gain some insight into the molecular mechanisms underlying the pathogenesis and progression of renal diseases. HIGA mice were established from ddY mice and exhibit features typical of IgAN, namely, high serum IgA levels, IgA deposition in the glomerular mesangium and mesangial expansion (Muso et al., 1996, Miyawaki et al., 1997). These HIGA mice do not exhibit any renal abnormalities until about 10 weeks of age, however, from that time-point onwards, the serum IgA levels begin to increase dramatically, becoming markedly elevated by the age of 25 weeks. Along with this development, glomerular IgA deposition and mesangial expansion are also observed, with these histological alterations becoming marked by 40 weeks of age. As IgAN is the most commonly encountered type of primary glomerulonephritis worldwide (Levy and Berger, 1988), and mesangial expansion as a result of the proliferation of the glomerular mesangial cells and increased mesangial matrix is a pathological feature seen in common to many chronic renal diseases, HIGA mice seem to be an appropriate animal model in which to conduct a gene expression analysis to identify genes involved in the progression of renal diseases. In addition, although some interesting microarray analyses using mouse kidneys have been reported (Katsuma et al., 2001, Kieran et al., 2003, Sadlier et al., 2004), most of these studies have utilized whole kidneys. Use of only glomeruli for the microarray analysis may reveal the changes in the expressions of minor, but important, genes associated with glomerular injury more clearly. Consequently, we used highly pure glomeruli for the microarray analysis in the present study.

Several upregulated/downregulated genes were identified in tissues obtained after the development of renal injury. Among these, we found the downregulation of the meprinβ gene intriguing, because meprinβ is a subunit of the metalloproteinase meprins which have been reported to be expressed in the brush-border membranes of both the kidneys and the intestines (Bond and Beynon, 1995, Walker et al., 1998, Carmago et al., 2002). Meprins (meprin A: EC3.4.24.18 and meprin B: EC3.4.24.63) are the major proteases in the renal cortex that are capable of degrading components of the ECM proteins, including fibronectin, collagens, laminin and nidogen (Kaushal et al., 1994, Walker et al., 1998, Kruse et al., 2004). Meprins are also capable of proteolytically processing bioactive peptides, cytokines and growth factors (Chestukhin et al., 1997, Bertenshaw et al., 2001). As accumulation of ECM in the glomeruli is a common histological alteration in renal diseases, reduction of meprinβ gene expression in renal tissues may contribute to progression of the fibrotic process through the reduced ECM degradation. This hypothesis prompted us to investigate the association between meprinβ and the progression of renal injury in vivo using a mouse model of anti-GBM antibody-associated nephritis in the present study. The results of these in vitro and in vivo studies are expected to provide some insight into the role of meprinβ in the progression of chronic renal injury.