NG2 expressed by macrophages and oligodendrocyte precursor cells is dispensable in experimental autoimmune encephalomyelitis.

Increased expression of the chondroitin proteoglycan NG2 is a prominent feature in central nervous system injury with unknown cellular source and biological relevance. Here, we describe the first detailed analysis of experimental autoimmune encephalomyelitis in NG2 knockout mice and NG2 knockout bone marrow chimeras. We show that both macrophages and oligodendrocyte progenitor cells express and secrete NG2 in response to transforming growth factor-ß. A subpopulation of macrophages expresses NG2 within leucocyte infiltrates in the central nervous system, but only oligodendrocyte progenitor cells contribute to NG2 accumulation. Notably, NG2 plays no role in experimental autoimmune encephalomyelitis initiation, progression or recuperation. In concurrence, the immune response is unaltered in NG2-deficient mice as are the extent of central nervous system damage and degree of remyelination.

Expression of the HGF receptor c-met by macrophages in experimental autoimmune encephalomyelitis.

Hepatocyte growth factor (HGF) is a pleiotropic cytokine able to evoke a wide array of cellular responses including proliferation, migration, and survival through activation of its receptor c-met. Various types of leukocytes have been described to express c-met suggesting that HGF/c-met signaling may directly influence leukocyte responses in inflammation. We have investigated the HGF/c-met pathway in experimental autoimmune encephalomyelitis (EAE), a common mouse model of multiple sclerosis (MS), in which macrophages play a dual role, contributing directly to CNS damage at disease onset but promoting recovery during remission by removing myelin debris. Here we show that during EAE both HGF and c-met are expressed in the CNS and that c-met is activated. We subsequently demonstrate that c-met is primarily expressed in inflammatory lesions by macrophages and a small number of dendritic cells (DCs) and oligodendrocyte progenitor cells (OPCs) but not by microglia or T cells. Complementary in vitro experiments show that only LPS and TNFalpha, but not IL-6, IL-10, or IL-13, are able to induce c-met expression in macrophages. In addition, using TNF signaling deficient macrophages we demonstrate that LPS and TNFalpha induce c-met through distinct pathways. Furthermore, TNFalpha- and LPS-induced c-met is functional because treatment of macrophages with recombinant HGF results in rapid phosphorylation of c-met. Interestingly, HGF/c-met signaling does not modulate cytokine expression, phagocytosis, or antigen presentation but promotes proliferation of activated macrophages. Taken together, our data indicate a pro-inflammatory role for the HGF/c-met pathway in EAE rather than a role in the initiation of repair mechanisms.

Gelsolin is required for macrophage recruitment during remyelination of the peripheral nervous system.

Reorganization of the actin cytoskeleton is necessary for Schwann cell proliferation, migration and for the morphological changes associated with sorting, ensheathing and myelination of axons. Such reorganization requires regulated severing and depolymerization of actin filaments. Gelsolin is an actin filament severing protein expressed in many cell types including Schwann cells. Using Gelsolin knockout mice, we investigated the role of this protein in the myelination and remyelination of the peripheral nervous system. Our results show that although gelsolin is not necessary for developmental myelination, it is required for timely remyelination of the sciatic nerve following crush injury. Gelsolin is necessary for macrophage motility in culture, and its absence is likely to impair the recruitment of macrophages to the injury site.

TGF-beta-treated microglia induce oligodendrocyte precursor cell chemotaxis through the HGF-c-Met pathway.

In acute experimental autoimmune encephalomyelitis (EAE), demyelination is induced by myelin-specific CD4(+) T lymphocytes and myelin-specific antibodies. Recovery from the disease is initiated by cytokines which suppress T cell expansion and the production of myelin-toxic molecules by macrophages. Th2/3 cell-derived signals may also be involved in central nervous system (CNS) repair. Remyelination is thought to be initiated by the recruitment and differentiation of oligodendrocyte precursor cells (OPC) in demyelinated CNS lesions. Here, we report that unlike Th1 cytokines (TNF-alpha, IFN-gamma), the Th2/3 cytokine TGF-beta induces primary microglia from C57BL/6 mice to secrete a chemotactic factor for primary OPC. We identified this factor to be the hepatocyte growth factor (HGF). Our studies show that TGF-beta-1-2-3 as well as IFN-beta induce HGF secretion by microglia and that antibodies to the HGF receptor c-Met abrogate OPC chemotaxis induced by TGF-beta2-treated microglia. In addition we show spinal cord lesions in EAE induced in SJL/J mice to contain both OPC and HGF producing macrophages in the recovery phase, but not in the acute stage of disease. Taken these findings, TGF-beta may play a pivotal role in remyelination by inducing microglia to release HGF which is both a chemotactic and differentiation factor for OPC.