The mannose receptor in the brain.

The mannose receptor is a transmembrane glycoprotein mainly expressed by macrophages, that specifically binds to mannosylated molecules and mediates their endocytosis. Known ligands of the receptor are lysosomases and various pathogens. Ligand specificity and cellular distribution provide the mannose receptor with a very important role in homeostasis and in the immune response. Expression of the mannose receptor has recently been demonstrated in the brain. Astrocytes and microglia, two types of glial cells that can be turned into immune-competent cells, are the main site of expression in vivo and in vitro. The mannose receptor mediates in vitro pinocytosis by astrocytes and microglia and phagocytosis by microglia. Expression and endocytic activities of the mannose receptor in these cells are regulated by various cytokines. Based on our current knowledge on mannose receptor activities in brain cells, on its regional and temporal expression in that organ, and on its putative ligands therein, the possible involvement of the mannose receptor in brain homeostasis, neuronal functions, and brain defense is discussed.