Osteopontin is extensively expressed by macrophages following CNS demyelination but has a redundant role in remyelination

Abstract

Osteopontin (OPN) is a key immunoregulator in the autoimmune-mediated demyelinating disease multiple sclerosis. OPN may also play a role in the remyelination since it is 1) a ligand for αV integrins, several of which regulate the properties of the oligodendrocyte precursor cells (OPCs) primarily responsible for remyelination, and 2) enhances myelin membrane formation in OPC lines. Here we show that OPN is expressed at high levels during remyelination of toxin-induced demyelination. The increased expression is due to mRNA expression in macrophages and follows differences in macrophage responses to demyelination in young and old adult animals. To identify the role of OPN in remyelination focal demyelination was induced in wild-type and OPN^−/− mice. There was no difference in the rate of remyelination between the two groups indicating that OPN is not a critical component of remyelination.

Introduction

Remyelination, a regenerative event in which demyelinated axons are reinvested with new myelin sheaths, is a complex process involving interactions between oligodendrocyte precursor cells (OPCs), demyelinated axons and a wide spectrum of secreted or membrane bound proteins (Franklin and Goldman, 2004). There is strong evidence based on in vitro and developmental studies that extracellular matrix (ECM)-integrin interactions are potent regulators of OPC biology (ffrench-Constant and Colognato, 2004). It might therefore be expected that similar interactions will be involved in the regulation of remyelination. In this study we focus on osteopontin (OPN), an Arg-Gly-Asp (RGD) integrin-binding motif-containing phosphoglycoprotein component of the ECM that has many developmental, physiological and immunological roles in a wide range of tissue (Ashkar et al., 2000, Rangaswami et al., 2006, Reinholt et al., 1990, Sodek et al., 2000).

In the normal adult CNS OPN is expressed in neuronal cell bodies in several brain regions. The levels of expression are substantially increased following a wide range of CNS injuries including ischemia (Ellison et al., 1998, Lee et al., 1999, Wang et al., 1998), toxic injury (Choi et al., 2003, Kim et al., 2002), and virus induced encephalomyelitis (Shin and Koh, 2004). In the context of CNS pathology it has been most extensively studied in the primary human demyelinating disease of the CNS, multiple sclerosis (MS) and its animal model experimental autoimmune encephalomyelitis (EAE), where it is strongly implicated in the exacerbation of disease activity through the promotion of T-cell survival (Chabas et al., 2001, Hur et al., 2007, Sinclair et al., 2005). OPN has also emerged as a significantly upregulated gene during the remyelination of demyelination induced in the mouse brain by the dietary toxin cuprizone (Selvaraju et al., 2004). A possible role in remyelination is suggested by in vitro studies in which recombinant OPN was shown to increase myelin basic protein (MBP) synthesis and enhance myelin membrane formation in oligodendrocyte precursor cell (OPC) lines (Selvaraju et al., 2004).

In this study we use toxin-models of demyelination, characterised by an inflammatory response predominantly mediated by cells of the innate immune response and in which there is a clear temporal separation between the acute demyelination and subsequent remyelination, to describe the expression pattern of OPN during remyelination and to assess its functional role in remyelination using OPN^−/− mice.