Neuroprotection mediated by glial cell line-derived neurotrophic factor: Involvement of a reduction of NMDA-Induced calcium influx by the mitogen-activated protein kinase pathway

The glial cell line-derived neurotrophic factor (GDNF) is first characteriz ed for its trophic activity on dopaminergic neurons. Recent data suggested that GDNF could modulate the neuronal death induced by ischemia. The purpos e of this study was to characterize the influence of GDNF on cultured corti cal neurons subjected to two paradigms of injury (necrosis and apoptosis) t hat have been identified during cerebral ischemia and to determine the mole cular mechanisms involved. First, we demonstrated that both neurons and ast rocytes express the mRNA and the protein for GDNF and its receptor complex (GFR alpha -1 and c-Ret). Next, we showed that the application of recombina nt human GDNF to cortical neurons and astrocytes induces the activation of the MAP kinase (MAPK) pathway, as visualized by an increase in the phosphor ylated forms of extracellular signal-regulated kinases (ERKs). Thereafter, we demonstrated that GDNF fails to prevent apoptotic neuronal death but sel ectively attenuates slowly triggered NMDA-induced excitotoxic neuronal deat h via a direct effect on cortical neurons. To further characterize the neur oprotective mechanisms of GDNF against NMDA-mediated neuronal death, we sho wed that a pretreatment with GDNF reduces NMDA-induced calcium influx. This effect likely results from a reduction of NMDA receptor activity rather th an an enhanced buffering or extrusion capacity for calcium. Finally, we als o demonstrated that an ERKs activation pathway is necessary for GDNF-mediat ed reduction of the NMDA-induced calcium response. Together, these results describe a novel mechanism by which the activation of MAPK induced by GDNF modulates NMDA receptor activity, a mechanism that could be responsible for the neuroprotective effect of GDNF in acute brain injury.