Calcium signaling in neurons: molecular mechanisms and cellular consequences

Calcium influx induces neurite growth through a Src-Ras signaling cassette

We find that calcium influx through voltage-dependent calcium channels causes extensive neurite outgrowth in PC12 cells. The calcium signal transduction pathway promoting neurite outgrowth causes the rapid activation of protein tyrosine kinases, which include Src. Protein tyrosine phosphorylation results in the formation of an Shc/Grb2 complex, leading to Ras activation, MAP kinase activation, and the subsequent induction of the immediate early gene NGFI-A. Protein tyrosine phosphorylation, gene induction, and neurite outgrowth are inhibited by the expression of dominant negative forms of both Src and Ras, indicating a requirement for both proto-oncoproteins in calcium signaling. Our results suggest that a signaling cassette which includes Src and Ras is likely to underlie a broad range of calcium of actions in the nervous system.

Characterization of the signaling interactions that promote the survival and growth of developing retinal ganglion cells in culture

The signaling mechanisms that control the survival of CNS neurons are poorly understood. Here we show that, in contrast to PNS neurons, the survival of purified postnatal rat retinal ganglion cells (RGCs) in vitro is not promoted by peptide trophic factors unless their intracellular cAMP is increased pharmacologically or they are depolarized by K+ or glutamate agonists. Long-term survival of most RGCs in culture can be promoted by a combination of trophic factors normally produced along the visual pathway, including BDNF, CNTF, IGF1, an oligodendrocyte-derived protein, and forskolin. These results suggest that neurotransmitter stimulation and electrical activity enhance the survival of developing RGCs and raise the question of whether the survival control mechanisms of PNS and CNS neurons are different.

alpha-Amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid, but not N-methyl-D-aspartate, activates mitogen-activated protein kinase through G-protein beta gamma subunits in rat cortical neurons

alpha-Amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor channels play important roles in plasticity, neurotransmission, and neurotoxicity in the central nervous system. AMPA, but not N-methyl-D-aspartate (NMDA), receptor signaling in rat cortical neurons was found to involve a G-protein coupled to a protein kinase cascade. While both NMDA and AMPA activated p42 mitogen-activated protein kinase (MAPK) in neurons, only AMPA-induced MAPK was inhibited by pertussis toxin. AMPA, but not NMDA, caused an association of a G-protein beta subunit with a Ras, Raf kinase, and MAP/ERK kinase (MEK)-1 complex. The evidence indicates that AMPA triggers MAPK activation via a novel mechanism in which G-protein beta gamma dimers released from G alpha bind to a Ras protein complex causing the activation of Ras, Raf kinase, MEK-1, and finally MAPK.

Calcium dynamics in the central nervous system

Calcium ions are critically important in many functions of the nervous system from neurotransmitter release to intracellular signal transduction. The large difference between intracellular and extracellular calcium ion concentration ([Ca2+]) highlights the importance of the mechanisms controlling influx and efflux of this ion. Loss of the regulatory ability of these mechanisms and the subsequent increased intracellular calcium levels may be involved in pathological events of brain trauma, stroke, epilepsy and other diseases. Ca2+ dynamics in the CNS ranging from 'waves' to 'spirals' are being studied because of the availability of fluorescent indicators of Ca2+ combined with confocal microscopy. Cellular mechanisms of Ca2+ signal transduction have been extensively reviewed (Tsien and Tsein, 1990; Carafoli, 1992; Berridge, 1993; Berridge and Dupont, 1994; Pozzan et al., 1994; Clapham, 1995; Ghosh and Greenberg, 1995). The aim of this review is to present the types of Ca2+ dynamics observed in the CNS thus far, both in normal brain function as well as in response after injury.