Neural stem cells at the crossroads: MMPs may tell the way.

Matrix metalloproteinases (MMP) constitute a family of more than 25 enzymes which process a large number of pericellular substrates. Even though initially reported to have an ability to degrade almost all of the extracellular components, MMP are now known to play roles which are not limited to the breakdown of extracellular barriers. In fact, MMPs regulate many biological processes, being involved not only in physiological events, but also in pathological processes. Strikingly, MMPs have been found to be involved in the physiology of the Central Nervous System (CNS), taking part and playing important roles in several processes such as repair and ontogeny, as well as in pathological conditions of the CNS. Initially considered to be a static structure, lacking regenerative capability, the CNS has been considered for a long time to be a system without renewal capabilities. Recently, the discovery of constant neural replacement has changed our way of considering the adult brain, and the finding of the existence of neural stem cells has opened the way to exciting and fascinating perspectives of the CNS. So, could MMPs, originally found during metamorphosis in tadpoles, and now amazingly identified in the CNS, have something to do in neuronal function? In this review we take into consideration the possible roles of two metalloproteinases, MMP-2 and MMP-9, also called gelatinases, in controlling several aspects of CNS organization, including the modulation of neural stem cell properties and the differentiation of their progeny, both under normal and pathophysiological conditions.