Fibronectin and laminin regulate the in vitro differentiation of microglial cells

During development, the differentiation of ameboid microglia (brain macrophages) into ramified microglia is marked by a loss of macrophage-like properties and the extension of thin cytoplasmic projections. We have studied the influence of two extracellular matrix proteins, laminin and fibronectin, on microglia differentiation, using cell cultures. Brain macrophages were isolated from primary glial layers derived from embryonic rat brain and further cultured in serum-free medium. The addition of fibronectin induced the transformation of round or spindle-shaped brain macrophages into cells displaying a reduced cell body and extending thin and long processes. This morphological transformation was associated with a reorganization of the vimentin network, including a condensation of dispersed filaments into thick bundles and a modification of the phosphorylation state of vimentin monomers. In addition, compared to brain macrophages, the process-bearing microglia lost the ability to engulf zymosan particles, and showed reduction in non-specific esterase activity and superoxide anion generation. In contrast, laminin reduced the spontaneous transformation of brain macrophages into process-bearing cells. Moreover, laminin and serum induced a reverse transformation of process-bearing cells when added to cultures pretreated with fibronectin. Altogether these results demonstrate antagonist effects of fibronectin and laminin on the in vitro differentiation of brain macrophages towards a "resting" phenotype, which shares several properties with the ramified microglia present in the adult brain. We suggest that fibronectin and laminin regulate the differentiation of microglial cells, which takes place during development or following various types of lesions in the adult brain.