What are the mechanisms by which neurons differentiate to achieve the spectacular complexity of the brain? Remarkably, voltage-dependent ion channels and neurotransmitter receptors are expressed at early stages of development, substantially before synapse formation, and ion channel activity participates in signal transduction that directs subsequent steps of development. We have discovered that spontaneous transient elevations of intracellular calcium, generated by ion channels and receptors, control several aspects of differentiation during an early period in embryonic development. Our work is aimed at understanding the roles of electrical activity in assembly of the nervous system, by analyzing the effects of calcium transients on neuronal differentiation and determining the molecular mechanisms by which they exert these effects.
Specification of neurotransmitters and selection of transmitter receptors are processes that depend on patterned spontaneous embryonic calcium-dependent electrical activity. We are investigating the triggers of this spontaneous activity to understand its origins. We are studying activity-dependent regulation of expression of serotonin and dopamine in the embryonic brain, because these transmitters have broad impact on cognitive states and on behavior. We have begun analyzing the signaling mechanisms mediating activity-dependent transmitter specification, generating transgenic lines expressing fluorescent reporters of neurotransmitter synthesis to enable mutant screens. We are determining the extent to which there is environmental regulation of activity-dependent differentiation at early stages of development, revealing a partnership of electrical activity and genetic programs in the assembly of the nervous system.
The frequency of transient elevations of intracellular calcium in growth cones regulates axon extension in identified neurons in situ. The image shows the preparation used to study this phenomenon - exposed nerve projections of an embryonic Xenopus spinal cord and notochord, double-labeled for β-tubulin (red) and actin (green) and visualized by confocal microscopy.
The image shows sibling tadpoles, one adapted to a dark background and the other to a white background. Natural light increases the number of dopaminergic neurons in the hypothalamus where camouflage behavior is controlled; dark exposure causes a decrease. Like endogenously dopaminergic neurons, neurons newly expressing dopamine drive this simple behavior. Such activity-dependent plasticity in the developing nervous system may be relevant to changes in cognitive states regulated by biogenic amines.
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Plazas, P.V., Nicol, X. and Spitzer, N.C. (2013) Activity-dependent competition regulates motor neuron axon pathfinding via PlexinA3. Proc. Nat. Acad. Sci. USA. 110: 1524-1529.
Spitzer, N.C. (2012) Activity-dependent neurotransmitter respecification. Nature Reviews Neurosci. 13: 94-106.
Nicol, X., Hong, K.P. and Spitzer, N.C. (2011) Spatial and temporal second messenger codes for growth cone turning. Proc. Nat. Acad. Sci. USA. 108: 13776-13781.
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Demarque, M. and Spitzer, N.C. (2010) Activity-dependent expression of Lmx1b regulates specification of serotonergic neurons modulating swimming behavior. Neuron 67: 321-334.
Marek, K.W., Kurtz, L.M. and Spitzer, N.C. (2010) cJun integrates calcium spike activity and tlx3 expression to regulate neurotransmitter specification. Nature Neurosci. 13: 944-950.
Xiao, Q., Xu, L. and Spitzer, N.C. (2010) Muscle-dependent regulation of neurotransmitter specification and embryonic neuronal calcium spike activity. J. Neurosci. 30: 5792-5801.
Chang, L.W. and Spitzer, N.C. (2009) Spontaneous calcium spike activity in embryonic spinal neurons is regulated by developmental expression of the Na+, K+-ATPase β3 subunit. J. Neurosci. 29: 7877-7885.
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Root, C.M., Velazquez-Ulloa, N.A., Monsalve, G.C., Minakova, E. and Spitzer, N.C. (2008). Embryonically expressed GABA and glutamate drive electrical activity regulating neurotransmitter specification. J. Neurosci. 28:4777-4784.
Sann, S.B., Xu, L., Nishimune, H., Sanes, J.R. and Spitzer, N.C. (2008). Neurite outgrowth and in vivo sensory innervation mediated by a CaV2.2-laminin beta-2 stop signal. J. Neurosci. 28:2366-2374.
Borodinsky, L.N. and Spitzer, N.C. (2007). Activity-dependent neurotransmitter-receptor matching at the neuromuscular junction. Proc. Natl. Acad. Sci. 104:335-340.
Spitzer, N.C. (2006). Electrical activity in early neuronal development. Nature 444:707-712.
Conklin, M.W., Lin, M.S. and Spitzer, N.C. (2005). Local calcium trnasients contribute to disappearance of pFAK, focal complex removal and deadhesion of neuronal growth cones and fibroblasts. Dev. Biol. 287:201-212.
Borodinsky, L.N., Root, C.M., Cronin, J.A., Sann, S.B., Gu, X. and Spitzer, N.C. (2004). Actiity-dependent homeostatic specification of transmitter expression in embryonic neurons. Nature 429:523-530.
Gorbunova, Y.V. and Spitzer, N.C. (2002). Dynamic interactions of cAMP transients and spontaneous calcium spikes. Nature 418:93-96.
Gomez, T.M., Robles, E., Poo, M. and Spitzer, N.C. (2001). Filipodial calcium transients promote substrate-dependent growth cone turning. Science 291:1983-1987.