Scientific Report 2007

Molecular and Experimental Medicine

Division of Blood Cell and Vascular Biology

Regulation of Microglial Activation by Extracellular Matrix Proteins

R. Milner, S.J. Crocker, G.J. del Zoppo, J.C. LaManna*

* Case Western Reserve University, Cleveland, Ohio

Microglia are the primary immune effector cells resident within the CNS. In addition to playing a protective role in host defense, microglia may also be involved in the initiation and maintenance of demyelination in multiple sclerosis. Upon activation, microglia become highly aggressive, migratory phagocytic cells that secrete cytokines, chemokines, and proteases of the matrix metalloproteinase (MMP) family, including MMP-9, which has been implicated in the pathogenesis of multiple sclerosis.

An early event in multiple sclerosis is breakdown of the blood-brain barrier, which leads to deposition of the plasma proteins fibrinogen, fibronectin, and vitronectin within the cerebral parenchyma. Because microglial activation is promoted by fibronectin and vitronectin in vitro, we tested the hypothesis that deposition of fibronectin and vitronectin during demyelinating disease promotes activation of microglia and expression of MMP-9, thus leading to death of oligodendrocytes and demyelination.

Using experimental autoimmune encephalomyelitis in mice as a model of multiple sclerosis, we found that levels of fibronectin and vitronectin were strongly increased and that a close spatial relationship existed between fibronectin/vitronectin deposition and microglial activation and MMP-9 expression. In vitro studies indicated that microglial activation and MMP-9 expression were directly promoted by fibronectin and vitronectin and that these effects were mediated by the integrins α₅β₁ and α_vβ₅, respectively. Currently, using microglia from mice that lack the gene for the β₅ subunit of α_vβ₅, we are evaluating the requirement for this integrin in microglial activation and phagocytosis. Early experiments suggest that microglial activation is reduced in the absence of α_vβ₅.

In another project, we are investigating the potential role of α₅β₁ in cerebral angiogenesis. Because proteins in the extracellular matrix play an important angiogenic role during development and tumor formation, we examined expression of extracellular matrix proteins and β₁ integrins during CNS development. We found that cerebral blood vessels make a switch in expression, from fibronectin and the α₄β₁/α₅β₁ integrins during angiogenesis to laminin and the α₁β₁/α₆β₁ integrins in adults. In vitro studies showed that fibronectin promotes survival and proliferation of brain endothelial cells and that this effect is mediated via the α₅β₁ and α_vβ₃ integrins. Because of the suggested angiogenic role for α₅β₁ during CNS development, we recently examined whether cerebral hypoxia promotes induction of this integrin on angiogenic vessels in the CNS in adults. Early results indicate that cerebral hypoxia promotes a strong induction of α₅β₁ on cerebral endothelial cells.

Publications

Crocker, S.J., Milner, R., Pham-Mitchell, N., Campbell, I.L. Cell and agonist-specific regulation of genes for matrix metalloproteinases and their tissue inhibitors by primary glial cells. J. Neurochem. 98:812, 2006.

del Zoppo, G.J., Milner, R. Integrin-matrix interactions in the cerebral microvasculature. Arterioscler. Thromb. Vasc. Biol. 26:1966, 2006.

del Zoppo, G.J., Milner, R., Mabuchi, T., Hung, S., Wang, X., Berg, G.I., Koziol, J.A. Microglial activation and matrix protease generation during focal cerebral ischemia. Stroke. 38(2 Suppl.):646, 2007.

Milner, R. A novel three-dimensional system to study interactions between endothelial cells and neural cells of the developing central nervous system. BMC Neurosci. 8:3, 2007.

Milner, R., Campbell, I.L. Increased expression of the β₄ and α₅ integrin subunits in cerebral blood vessels of transgenic mice chronically producing the pro-inflammatory cytokines IL-6 or IFN-α in the central nervous system. Mol. Cell. Neurosci. 33:429, 2006.

Milner, R., Crocker S.J., Hung, S., Wang, X., Frausto, R.F., del Zoppo G.J. Fibronectin- and vitronectin-induced microglial activation and matrix metalloproteinase-9 expression is mediated by integrins α₅β₁ and α_vβ₅. J. Immunol. 178:8158, 2007.

Wang, J., Milner, R. Fibronectin promotes brain capillary endothelial cell survival and proliferation through α₅β₁ and α_vβ₃ integrins via MAP kinase signaling. J. Neurochem. 96:148, 2006.