Glial cell line-derived neurotrophic factor increases intracellular calcium concentration -: Role of calcium/calmodulin in the activation of the phosphatidylinositol 3-kinase pathway

Moderate increases of intracellular Ca2+ concentration ([Ca2+](i)), induced by either the activation of tropomyosin receptor kinase (Trk) receptors for neurotrophins or by neuronal activity, regulate different intracellular pathways and neuronal survival. In the present report we demonstrate that glial cell line-derived neurotrophic factor (GDNF) treatment also induces [Ca2+](i) elevation by mobilizing this cation from internal stores. The effects of [Ca2+](i) increase after membrane depolarization are mainly mediated by calmodulin (CaM). However, the way in which CaM exerts its effects after tyrosine kinase receptor activation remains poorly characterized. It has been reported that phosphatidylinositol 3-kinase (PI 3-kinase) and its downstream target protein kinase B (PKB) play a central role in cell survival induced by neurotrophic factors; in fact, GDNF promotes neuronal survival through the activation of the PI 3-kinase/PKB pathway. We show that CaM antagonists inhibit PI 3-kinase and PKB activation as well as motoneuron survival induced by GDNF. We also demonstrate that endogenous Ca2+/CaM associates with the 85-kDa regulatory subunit of PI 3-kinase (p85). We conclude that changes of [Ca2+](i), induced by GDNF, promote neuronal survival through a mechanism that involves a direct regulation of PI 3-kinase activation by CaM thus suggesting a central role for Ca2+ and CaM in the signaling cascade for neuronal survival mediated by neurotrophic factors.