Nerve growth factor activation of the extracellular signal-regulated kinase pathway is modulated by Ca2+ and calmodulin

Nerve growth factor is a member of the neurotrophin family of trophic facto rs that have been reported to be essential for the survival and development of sympathetic neurons and a subset of sensory neurons. Nerve growth facto r exerts its effects mainly by interaction with the specific receptor TrkA, which leads to the activation of several intracellular signaling pathways. Once activated, TrkA also allows for a rapid and moderate increase in intr acellular calcium levels, which would contribute to the effects triggered b y nerve growth factor in neurons. In this report, we analyzed the relations hip of calcium to the activation of the Ras/extracellular signal-regulated kinase pathway in PC12 cells. We observed that calcium and calmodulin are b oth necessary for the acute activation of extracellular signal-regulated ki nases after TrkA stimulation. We analyzed the elements of the pathway that lead to this activation, and we observed that calmodulin antagonists comple tely block the initial Raf-1 activation without affecting the function of u pstream elements, such as Ras, Grb2, Shc, and Trk We have broadened our stu dy to other stimuli that activate extracellular signal-regulated kinases th rough tyrosine kinase receptors, and we have observed that calmodulin also modulates the activation of such kinases after epidermal growth factor rece ptor stimulation in PC12 cells and after TrkB stimulation in cultured chick en embryo motoneurons. Calmodulin seems to regulate the full activation of Raf-1 after Ras activation, since functional Ras is necessary for Raf-1 act ivation after nerve growth factor stimulation and calmodulin-Sepharose is a ble to precipitate Raf-1 in a calcium-dependent manner.