Alpha 2-macroglobulin synthesis in an astrocyte subpopulation.

The proteinase inhibitor alpha 2-macroglobulin (alpha 2-M) is an acute phase protein in the adult rat. During inflammatory events, it is synthesized in the liver and secreted into the bloodstream to remove proteases that are released on injury. Recently, its occurrence in fetal rat brain has been reported. Its cellular origin and biological function in the developing brain, however, remained obscure. In this article, it is shown that astroglial cells cultured from newborn rat brain synthesize and secrete alpha 2-M. Its synthesis markedly increases with time in culture. Immunocytochemical studies reveal that only a subpopulation of astrocytes is alpha 2-M positive, alpha 2-M synthesis in the developing brain by neuroectoderm-derived cells asks for a broader definition of its function in the body. Since interactions of proteases and protease inhibitors appear to play a crucial role in cell migration and neurite outgrowth, alpha 2-M expression in astrocytes is discussed not only in relation to its potential role in the acute phase response to injury in the adult brain but also in regard to its possible involvement in brain development.