MMPs in the central nervous system: where the good guys go bad.

Matrix metalloproteinases (MMPs) are expressed in the developing, healthy adult and diseased CNS. We emphasize the regulation of neurogenesis and oligodendrogenesis by MMPs during CNS development, and highlight physiological roles of MMPs in the healthy adult CNS, such as in synaptic plasticity, learning and memory. Nonetheless, MMPs as "the good guys" go bad in neurological conditions, likely aided by the sudden and massive upregulation of several MMP members. We stress the necessity of drawing a fine balance in the treatment of neurological diseases, and we suggest that MMP inhibitors do have therapeutic potential early after CNS injury.