Kazunobu Togashi

Membrane potential shifts caused by diffusible guidance signals direct growth-cone turning

Plasma membrane potentials gate the ion channel conductance that controls external signal–induced neuronal functions. We found that diffusible guidance molecules caused membrane potential shifts that resulted in repulsion or attraction of Xenopus laevis spinal neuron growth cones. The repellents Sema3A and Slit2 caused hyperpolarization, and the attractants netrin-1 and BDNF caused depolarization. Clamping the growth-cone potential at the resting state prevented Sema3A-induced repulsion; depolarizing potentials converted the repulsion to attraction, whereas hyperpolarizing potentials had no effect. Sema3A increased the intracellular concentration of guanosine 3′,5′-cyclic monophosphate ([cGMP]i) by soluble guanylyl cyclase, resulting in fast onset and long-lasting hyperpolarization. Pharmacological increase of [cGMP]i caused protein kinase G (PKG)-mediated depolarization, switching Sema3A-induced repulsion to attraction. This bimodal switch required activation of either Cl− or Na+ channels, which, in turn, regulated the differential intracellular Ca2+ concentration increase across the growth cone. Thus, the polarity of growth-cone potential shifts imposes either attraction or repulsion, and Sema3A achieves this through cGMP signaling.

Cyclic GMP-Gated CNG Channels Function in Sema3A-Induced Growth Cone Repulsion

Cyclic nucleotide-gated channels (CNGCs) transduce external signals required for sensory processes, e.g., photoreception, olfaction, and taste. Nerve growth cone guidance by diffusible attractive and repulsive molecules is regulated by differential growth cone Ca2+ signaling. However, the Ca2+-conducting ion channels that transduce guidance molecule signals are largely unknown. We show that rod-type CNGC-like channels function in the repulsion of cultured Xenopus spinal neuron growth cones by Sema3A, which triggers the production of the cGMP that activates the Xenopus CNGA1 (xCNGA1) subunit-containing channels in interneurons. Downregulation of xCNGA1 or overexpression of a mutant xCNGA1 incapable of binding cGMP abolished CNG currents and converted growth cone repulsion to attraction in response to Sema3A. We also show that Ca2+ entry through xCNGCs is required to mediate the repulsive Sema3A signal. These studies extend our knowledge of the function of CNGCs by demonstrating their requirement for signal transduction in growth cone guidance.

Semaphorin 3A induces CaV2.3 channel-dependent conversion of axons to dendrites.

Polarized neurites (axons and dendrites) form the functional circuitry of the nervous system. Secreted guidance cues often control the polarity of neuron migration and neurite outgrowth by regulating ion channels. Here, we show that secreted semaphorin 3A (Sema3A) induces the neurite identity of Xenopus spinal commissural interneurons (xSCINs) by activating CaV2.3 channels (CaV2.3). Sema3A treatment converted the identity of axons of cultured xSCINs to that of dendrites by recruiting functional CaV2.3. Inhibition of Sema3A signalling prevented both the expression of CaV2.3 and acquisition of the dendrite identity, and inhibition of CaV2.3 function resulted in multiple axon-like neurites of xSCINs in the spinal cord. Furthermore, Sema3A-triggered cGMP production and PKG activity induced, respectively, the expression of functional CaV2.3 and the dendrite identity. These results reveal a mechanism by which a guidance cue controls the identity of neurites during nervous system development.

GABAergic activities control spike timing- and frequency-dependent long-term depression at hippocampal excitatory synapses.

GABAergic interneuronal network activities in the hippocampus control a variety of neural functions, including learning and memory, by regulating θ and γ oscillations. How these GABAergic activities at pre- and postsynaptic sites of hippocampal CA1 pyramidal cells differentially contribute to synaptic function and plasticity during their repetitive pre- and postsynaptic spiking at θ and γ oscillations is largely unknown. We show here that activities mediated by postsynaptic GABAARs and presynaptic GABABRs determine, respectively, the spike timing- and frequency-dependence of activity-induced synaptic modifications at Schaffer collateral-CA1 excitatory synapses. We demonstrate that both feedforward and feedback GABAAR-mediated inhibition in the postsynaptic cell controls the spike timing-dependent long-term depression of excitatory inputs (“e-LTD”) at the θ frequency. We also show that feedback postsynaptic inhibition specifically causes e-LTD of inputs that induce small postsynaptic currents (<70 pA) with LTP-timing, thus enforcing the requirement of cooperativity for induction of long-term potentiation at excitatory inputs (“e-LTP”). Furthermore, under spike-timing protocols that induce e-LTP and e-LTD at excitatory synapses, we observed parallel induction of LTP and LTD at inhibitory inputs (“i-LTP” and “i-LTD”) to the same postsynaptic cells. Finally, we show that presynaptic GABABR-mediated inhibition plays a major role in the induction of frequency-dependent e-LTD at α and β frequencies. These observations demonstrate the critical influence of GABAergic interneuronal network activities in regulating the spike timing- and frequency-dependences of long-term synaptic modifications in the hippocampus.

Multi-phasic bi-directional chemotactic responses of the growth cone

The nerve growth cone is bi-directionally attracted and repelled by the same cue molecules depending on the situations, while other non-neural chemotactic cells usually show uni-directional attraction or repulsion toward their specific cue molecules. However, how the growth cone differs from other non-neural cells remains unclear. Toward this question, we developed a theory for describing chemotactic response based on a mathematical model of intracellular signaling of activator and inhibitor. Our theory was first able to clarify the conditions of attraction and repulsion, which are determined by balance between activator and inhibitor, and the conditions of uni- and bi-directional responses, which are determined by dose-response profiles of activator and inhibitor to the guidance cue. With biologically realistic sigmoidal dose-responses, our model predicted tri-phasic turning response depending on intracellular Ca2+ level, which was then experimentally confirmed by growth cone turning assays and Ca2+ imaging. Furthermore, we took a reverse-engineering analysis to identify balanced regulation between CaMKII (activator) and PP1 (inhibitor) and then the model performance was validated by reproducing turning assays with inhibitions of CaMKII and PP1. Thus, our study implies that the balance between activator and inhibitor underlies the multi-phasic bi-directional turning response of the growth cone.

GABAergic regulation of spike-timing-dependent synaptic plasticity

Synaptic plasticity is important for the formation and stability of neuronal circuits and provides substrates for learning and memory. Large number of neuropsychiatric disorders and neurodegenerative diseases are associated with impaired synaptic plasticity. We have recently shown that Semaphorin3A (Sema3A), a repulsive axonal guidance molecule, regulates dendritic spine morphology. However, its role on synaptic plasticity is still unknown. To clarify whether Sema3A-induced signaling regulates synaptic transmission, we performed electrophysiological recording from the CA1 region of acute hippocampus slices expressing PlexinA mutants. We found that PlexinAcyto, a dominant negative mutant of semaphorin signaling, decreased the AMPA-type glutamate receptors (AMPARs)-mediated currents. In contrast, PlexinAect, a constitutive active mutant of semaphoring signaling, enhanced it. Furthermore, PlexinAect enhanced the AMPARsmediated inward rectification. In addition, PlexinA interacted with GluR1 AMPARs subunit in hippocampal lysate. These results suggest that PlexinA signaling regulates synaptic plasticity through interacting with GluR1.