Cytokine-induced changes in the ability of astrocytes to support migration of oligodendrocyte precursors and axon growth

The glial scar and central nervous system repair

Damage to the central nervous system (CNS) results in a glial reaction, leading eventually to the formation of a glial scar. In this environment, axon regeneration fails, and remyelination may also be unsuccessful. The glial reaction to injury recruits microglia, oligodendrocyte precursors, meningeal cells, astrocytes and stem cells. Damaged CNS also contains oligodendrocytes and myelin debris. Most of these cell types produce molecules that have been shown to be inhibitory to axon regeneration. Oligodendrocytes produce NI250, myelin-associated glycoprotein (MAG), and tenascin-R, oligodendrocyte precursors produce NG2 DSD-1/phosphacan and versican, astrocytes produce tenascin, brevican, and neurocan, and can be stimulated to produce NG2, meningeal cells produce NG2 and other proteoglycans, and activated microglia produce free radicals, nitric oxide, and arachidonic acid derivatives. Many of these molecules must participate in rendering the damaged CNS inhibitory for axon regeneration. Demyelinated plaques in multiple sclerosis consists mostly of scar-type astrocytes and naked axons. The extent to which the astrocytosis is responsible for blocking remyelination is not established, but astrocytes inhibit the migration of both oligodendrocyte precursors and Schwann cells which must restrict their access to demyelinated axons.

N-Cadherin inhibits Schwann cell migration on astrocytes

Astrocytes exclude Schwann cells (SCs) from the central nervous system (CNS) at peripheral nerve entry zones and restrict their migration after transplantation into the CNS. We have modeled the interactions between SCs, astrocytes, and fibroblasts in vitro. Astrocytes and SCs in vitro form separate territories, with sharp boundaries between them. SCs migrate poorly when placed on astrocyte monolayers, but migrate well on various other surfaces such as laminin (LN) and skin fibroblasts. Interactions between individual SCs and astrocytes result in long-lasting adhesive contacts during which the SC is unable to migrate away from the astrocyte. In contrast, SC interactions with fibroblasts are much shorter with less arrest of migration. SCs adhere strongly to astrocytes and other SCs, but less well to substrates that promote migration, such as LN and fibroblasts. SC-astrocyte and SC-SC adhesion is mediated by the calcium-dependent cell adhesion molecule N-cadherin. Inhibition of N-cadherin function by calcium withdrawal, peptides containing the classical cadherin cell adhesion recognition sequence His-Ala-Val, or antibodies directed against this sequence inhibit SC adhesion and increase SC migration on astrocytes. We suggest that N-cadherin-mediated adhesion to astrocytes inhibits the widespread migration of SCs in CNS tissue.

Inflammatory cytokines interact to modulate extracellular matrix and astrocytic support of neurite outgrowth

Following injury to the central nervous system, an astroglial scar forms that is thought to impede neuronal regeneration and recovery of function. It is our hypothesis that inflammatory cytokines act upon astrocytes to alter their biochemical and physical properties, which may in turn be responsible for failed neuronal regeneration. We have therefore examined the interactions of two cytokines with prominent actions following injury, interferon-gamma (IFN-gamma) and basic fibroblast growth factor (FGF2), in modulating the extracellular matrix and proliferation of astrocytes in culture. We also evaluated the effects of these cytokines on the ability of astrocytes to support the growth of neurites. IFN-gamma significantly inhibited the proliferation of rat cortical astrocytes both in serum-free and serum-containing media as measured by [3H]thymidine incorporation. Furthermore, IFN-gamma also antagonized FGF2-induced proliferation. In parallel, IFN-gamma reduced the levels of the ECM molecules tenascin, laminin, and fibronectin as evaluated by Western blot analysis and immunocytochemistry. Similarly, IFN-gamma also antagonized FGF2-induced tenascin formation. While IFN-gamma-pretreated astrocyte monolayers did not differ from control in their ability to support neurite outgrowth of cortical neurons, it antagonized the enhancement of neurite outgrowth on FGF2-treated monolayers. We demonstrate that IFN-gamma did not alter signal transduction through the FGF2 receptor down to the phosphorylation of mitogen-activated protein kinase, suggesting that the interaction is at the level of transcriptional regulation or that an alternate pathway is involved. These results support the hypothesis that inflammatory cytokines interact to modulate several facets of the gliotic response and such interactions may be important in creating the biochemical and physical properties of the glial scar.