Extensive extracellular matrix depositions in active multiple sclerosis lesions

Abstract

In the central nervous system, basement membrane (BM) constituents are predominantly associated with the vasculature. However, under inflammatory conditions, the expression of BM components may alter. Here, we investigated the distribution of several BM components, including laminin, collagen type IV and heparan sulfate proteoglycans in various multiple sclerosis (MS) lesions. We observed irregular and discontinuous BMs in active lesions. Throughout active MS lesions, we found dense networks of BM proteins, which were surprisingly not associated with the cerebrovasculature. These striking parenchymal networks were not observed in chronic inactive MS lesions and brains of non-neurological controls. In addition, we studied the distribution of transforming growth factor-β1 (TGF-β1), since it is known as a major modulator of ECM production. Leukocytes, in particular CD68-positive macrophages, expressed high levels of TGF-β1 and were located in close proximity to parenchymal BM deposits in the MS lesions. We postulate that these BM networks may play a role in the further recruitment of inflammatory cells and form a barrier for axonal regeneration.

Introduction

Neuropathologically, multiple sclerosis (MS) lesions are characterized by infiltration of leukocytes, particularly monocyte-derived macrophages (Hafler, 2004). Leukocyte trafficking into the brain is normally restricted by the presence of the blood–brain barrier (BBB) (de Vries et al., 1997); however, recent studies have shown that BBB breakdown is an early phenomenon in the formation of new MS lesions (Vos et al., 2005). After migration across the BBB, inflammatory cells contribute to the demyelination process and axonal damage, as observed in brain parenchyma of MS lesions. Moreover, it is suggested that under inflammatory conditions the extracellular matrix (ECM) of the central nervous system (CNS) may also be affected (Sobel, 1998).

The ECM of the CNS white matter is primarily composed of aggregates and polymers of macromolecules, particularly glycosaminoglycans, either bound to proteins forming proteoglycans, such as chondroitin sulfate proteoglycans and dermatan sulfate proteoglycans, or unbound in the form of hyaluronan. Dispersed in this network of proteoglycans are glycoproteins, like tenascins, thrombospondin and osteonectin (Yamaguchi, 2000, Bandtlow and Zimmermann, 2000). Immunohistochemical studies demonstrated alterations in the composition of the ECM in various types of MS plaques. In active and chronic active MS lesions that are characterized by massive influx of inflammatory cells, a decreased immunoreactivity of chondroitin and dermatan sulfate proteoglycans was observed. Conversely, the expression of these proteoglycans was increased in the hypercellular edge of chronic active lesions. In active lesions, white matter-associated proteoglycans accumulate in foamy macrophages, suggesting that chondroitin and dermatan sulfate proteoglycans are phagocytosed together with myelin or myelin breakdown products (Sobel and Ahmed, 2001, Sobel, 2001, Gutowski et al., 1999). Recently, Back et al. (2005) observed accumulation of hyaluronan in MS lesions. Finally, decreased expression of the glycoproteins tenascin-C and tenascin-R was observed in active lesions, whereas their expression profile was unaltered in chronic lesions (Gutowski et al., 1999).

In contrast to chondroitin sulfate proteoglycans and dermatan sulfate proteoglycans, ECM constituents like laminins, heparan sulfate proteoglycans (HSPGs), fibronectin and collagens predominantly reside in basement membranes (BMs) that surround the cerebrovasculature and are rarely found in the brain parenchyma (Colognato and Yurchenco, 2000). The BM is not only a structural component of cerebral blood vessels, but it is also involved the regulation of cell behavior (Kalluri, 2003). We previously showed the presence of inflammatory cuffs in various MS lesion stages (Vos et al., 2005). Within such perivascular infiltrates, we demonstrated organized deposition of BM proteins, such as laminin, HSPGs and collagen type IV (van Horssen et al., 2005). Collectively, these data emphasize that dynamic ECM changes occur in brain tissue of MS patients.

One of the major regulators of ECM production is transforming growth factor-β (TGF-β). The TGF-β superfamily consists of a wide array of proteins that regulate numerous physiological processes, including development, wound healing, cellular differentiation and chemotaxis (Massague, 1998). Enhanced expression of various TGF-β isoforms has been observed in various types of MS lesions (De Groot et al., 1999). TGF-β causes matrix deposition by promoting transcription of genes that regulate ECM production and suppress the activity of ECM-degrading enzymes (Overall et al., 1989) and may be a key factor in the observed alterations of the ECM in MS lesions.

It is suggested that perivascular BM structures may play a role in leukocyte adhesion and cellular migration and transport; however, little is known about the expression and regulation of BM proteins in the parenchyma of MS lesions. Hence, we studied the distribution of several BM components and TGF-β1 within various well-characterized MS lesions identified by post-mortem magnetic resonance imaging (MRI).