Calcium sensor regulation of the CaV2.1 Ca2+ channel contributes to short-term synaptic plasticity in hippocampal neurons |
| |
Authors: | Evanthia Nanou Jane M Sullivan Todd Scheuer William A Catterall |
| |
Institution: | aDepartment of Pharmacology, University of Washington, Seattle, WA, 98195-7280;;bDepartment of Physiology & Biophysics, University of Washington, Seattle, WA, 98195-7290 |
| |
Abstract: | Short-term synaptic plasticity is induced by calcium (Ca2+) accumulating in presynaptic nerve terminals during repetitive action potentials. Regulation of voltage-gated CaV2.1 Ca2+ channels by Ca2+ sensor proteins induces facilitation of Ca2+ currents and synaptic facilitation in cultured neurons expressing exogenous CaV2.1 channels. However, it is unknown whether this mechanism contributes to facilitation in native synapses. We introduced the IM-AA mutation into the IQ-like motif (IM) of the Ca2+ sensor binding site. This mutation does not alter voltage dependence or kinetics of CaV2.1 currents, or frequency or amplitude of spontaneous miniature excitatory postsynaptic currents (mEPSCs); however, synaptic facilitation is completely blocked in excitatory glutamatergic synapses in hippocampal autaptic cultures. In acutely prepared hippocampal slices, frequency and amplitude of mEPSCs and amplitudes of evoked EPSCs are unaltered. In contrast, short-term synaptic facilitation in response to paired stimuli is reduced by ∼50%. In the presence of EGTA-AM to prevent global increases in free Ca2+, the IM-AA mutation completely blocks short-term synaptic facilitation, indicating that synaptic facilitation by brief, local increases in Ca2+ is dependent upon regulation of CaV2.1 channels by Ca2+ sensor proteins. In response to trains of action potentials, synaptic facilitation is reduced in IM-AA synapses in initial stimuli, consistent with results of paired-pulse experiments; however, synaptic depression is also delayed, resulting in sustained increases in amplitudes of later EPSCs during trains of 10 stimuli at 10–20 Hz. Evidently, regulation of CaV2.1 channels by CaS proteins is required for normal short-term plasticity and normal encoding of information in native hippocampal synapses.Modification of synaptic strength in central synapses is highly dependent upon presynaptic activity. The frequency and pattern of presynaptic action potentials regulates the postsynaptic response through diverse forms of short- and long-term plasticity that are specific to individual synapses and depend upon accumulation of intracellular Ca2+ (1–4). Presynaptic plasticity regulates neurotransmission by varying the amount of neurotransmitter released by each presynaptic action potential (1–5). P/Q-type Ca2+ currents conducted by voltage-gated CaV2.1 Ca2+ channels initiate neurotransmitter release at fast excitatory glutamatergic synapses in the brain (6–9) and regulate short-term presynaptic plasticity (3, 10). These channels exhibit Ca2+-dependent facilitation and inactivation that is mediated by the Ca2+ sensor (CaS) protein calmodulin (CaM) bound to a bipartite site in their C-terminal domain composed of an IQ-like motif (IM) and a CaM binding domain (CBD) (11–14). Ca2+-dependent facilitation and inactivation of P/Q-type Ca2+ currents correlate with facilitation and rapid depression of synaptic transmission at the Calyx of Held (15–18). Elimination of CaV2.1 channels by gene deletion prevents facilitation of synaptic transmission at the Calyx of Held (19, 20). Cultured sympathetic ganglion neurons with presynaptic expression of exogenous CaV2.1 channels harboring mutations in their CaS regulatory site have reduced facilitation and slowed depression of postsynaptic responses because of reduced Ca2+-dependent facilitation and Ca2+-dependent inactivation of CaV2.1 currents (21). The CaS proteins Ca2+-binding protein 1 (CaBP-1), visinin-like protein-2 (VILIP-2), and neuronal Ca2+ sensor-1 (NCS-1) induce different degrees of Ca2+-dependent facilitation and inactivation of channel activity (22–26). Expression of these different CaS proteins with CaV2.1 channels in cultured sympathetic ganglion neurons results in corresponding bidirectional changes in facilitation and depression of the postsynaptic response (25, 26). Therefore, binding of CaS proteins to CaV2.1 channels at specific synapses can change the balance of CaS-dependent facilitation and inactivation of CaV2.1 channels, and determine the outcome of synaptic plasticity (27). Currently, it is not known whether such molecular regulation of CaV2.1 by CaS proteins induces or modulates synaptic plasticity in native hippocampal synapses.To understand the functional role of regulation of CaV2.1 channels by CaS proteins in synaptic plasticity in vivo, we generated knock-in mice with paired alanine substitutions for the isoleucine and methionine residues in the IM motif (IM-AA) in their C-terminal domain. Here we investigated the effects of mutating this CaS regulatory site on hippocampal neurotransmission and synaptic plasticity. This mutation had no effect on basal Ca2+ channel function or on basal synaptic transmission. However, we found reduced short-term facilitation in response to paired stimuli in autaptic synapses in hippocampal cultures and in Schaffer collateral (SC)-CA1 synapses in acutely prepared hippocampal slices. Moreover, synaptic facilitation in mutant SC-CA1 synapses developed and decayed more slowly during trains of stimuli. These results identify a critical role for modulation of CaV2.1 channels by CaS proteins in short-term synaptic plasticity, which is likely to have important consequences for encoding and transmitting information in the hippocampus. |
| |
Keywords: | calcium channel calcium sensor proteins synaptic facilitation calmodulin hippocampus |
|
|