SUMMARY Modulation of voltage-gated Ca2+ channels controls activities of excitable cells. We show that high-voltage activated Ca2+ channels are regulated by membrane phosphatidylinositol 4,5-bisphosphate (PIP2) with different sensitivities. Plasma membrane PIP2 depletion by rapamycin-induced translocation of an inositol lipid 5-phosphatase or by a voltage-sensitive 5-phosphatase (VSP) suppresses CaV1.2 and CaV1.3 channel currents by ~35%, and CaV2.1 and CaV2.2 currents by 29 and 55%, respectively. Other CaV channels are less sensitive. Inhibition is not relieved by strong depolarizing prepulses. It changes the voltage dependence of channel gating little. Recovery of currents from inhibition needs intracellular hydrolysable ATP, presumably for PIP2 resynthesis. When PIP2 is increased by overexpressing PIP 5-kinase, activation and inactivation of CaV2.2 current slow and voltage-dependent gating shifts to slightly higher voltages. Thus, endogenous membrane PIP2 supports high-voltage activated L-, N-, and P/Q- type Ca2+ channels, and stimuli that activate phospholipase C deplete PIP2 and reduce those Ca2+ channel currents.

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Voltage-gated Ca2؉ channels in presynaptic nerve terminals initiate neurotransmitter release in response to depolarization by action potentials from the nerve axon. The strength of synaptic transmission is dependent on the third to fourth power of Ca 2؉ entry, placing the Ca 2؉ channels in a unique position for regulation of synaptic strength. Short-term synaptic plasticity regulates the strength of neurotransmission through facilitation and depression on the millisecond time scale and plays a key role in encoding information in the nervous system. Ca V 2.1 channels are the major source of Ca 2؉ entry for neurotransmission in the central nervous system. They are tightly regulated by Ca 2؉ , calmodulin, and related Ca 2؉ sensor proteins, which cause facilitation and inactivation of channel activity. Emerging evidence reviewed here points to this mode of regulation of Ca V 2.1 channels as a major contributor to short-term synaptic plasticity of neurotransmission and its diversity among synapses. Ca2ϩ influx through voltage-gated Ca 2ϩ (termed Ca V ) channels at presynaptic nerve terminals is an essential step in neurotransmission and plays a crucial role in short-term synaptic plasticity. The Ca V 2 subfamily is predominant in initiating synaptic transmission at fast conventional synapses (1-3). Multiple mechanisms modulate the function of presynaptic Ca V 2 channels and thereby regulate synaptic transmission (2, 4 -6). Ca V 2 channels bind the ubiquitous Ca 2ϩ sensor protein calmodulin (CaM) 2 to a site in their C-terminal domain, which induces Ca 2ϩ -dependent facilitation and inactivation of Ca V 2.1 channel activity in response to repetitive stimuli (7-10). Facilitation and inactivation of Ca V 2 channel activity can cause facilitation and depression of synaptic transmission (11,12). In this minireview, we focus on regulation of the presynaptic Ca V 2 channels by different calcium sensor proteins and the role of this mechanism in short-term synaptic plasticity. Presynaptic Calcium ChannelThe Ca 2ϩ channels in the Ca V 1 and Ca V 2 subfamilies are composed of an ␣ 1 subunit and auxiliary subunits ␤, ␣ 2 ␦, and sometimes ␥ (Fig. 1) (4, 13). The ␣ 1 subunit of 190 -250 kDa includes the pore, voltage sensors, gating apparatus, and most sites of channel regulation. The auxiliary subunits have an important influence on Ca 2ϩ channel function (4, 13). The intracellular ␤ subunit is a hydrophilic protein of 50 -65 kDa (4, 13). Its structure is composed of an SH3 (Src homology-3) domain and a guanylate kinase domain, both well known protein interaction motifs (14, 15). The guanylate kinase domain binds to an ␣-helical segment in the intracellular loop connecting domains I and II (Fig. 1) Presynaptic Ca 2؉ Current and NeurotransmissionCa 2ϩ influx through Ca V 2 channels is the predominant source of Ca 2ϩ for initiation of exocytosis of neurotransmitters (2, 3). Ca V 2.1 channels play a major role in neurotransmission at the neuromuscular junction and most synapses in the central nervous system (2). In contrast, Ca V...

Modulation of P/Q-type Ca 2+ currents through presynaptic voltage-gated calcium channels (Ca V 2.1) by binding of Ca 2+ /calmodulin contributes to short-term synaptic plasticity. Ca 2+ -binding protein-1 (CaBP1) and Visinin-like protein-2 (VILIP-2) are neurospecific calmodulin-like Ca 2+ sensor proteins that differentially modulate Ca V 2.1 channels, but how they contribute to short-term synaptic plasticity is unknown. Here, we show that activity-dependent modulation of presynaptic Ca V 2.1 channels by CaBP1 and VILIP-2 has opposing effects on short-term synaptic plasticity in superior cervical ganglion neurons. Expression of CaBP1, which blocks Ca 2+ -dependent facilitation of P/Q-type Ca 2+ current, markedly reduced facilitation of synaptic transmission. VILIP-2, which blocks Ca 2+ -dependent inactivation of P/Q-type Ca 2+ current, reduced synaptic depression and increased facilitation under conditions of high release probability. These results demonstrate that activity-dependent regulation of presynaptic Ca V 2.1 channels by differentially expressed Ca 2+ sensor proteins can fine-tune synaptic responses to trains of action potentials and thereby contribute to the diversity of short-term synaptic plasticity. N eurons fire repetitively in different frequencies and patterns, and activity-dependent alterations in synaptic strength result in diverse forms of short-term synaptic plasticity that are crucial for information processing in the nervous system (1-3). Shortterm synaptic plasticity on the time scale of milliseconds to seconds leads to facilitation or depression of synaptic transmission through changes in neurotransmitter release. This form of plasticity is thought to result from residual Ca 2+ that builds up in synapses during repetitive action potentials and binds to a Ca 2+ sensor distinct from the one that evokes neurotransmitter release (1, 2, 4, 5). However, it remains unclear how changes in residual Ca 2+ cause short-term synaptic plasticity and how neurotransmitter release is regulated to generate distinct patterns of shortterm plasticity.In central neurons, voltage-gated calcium (Ca V 2.1) channels are localized in high density in presynaptic active zones where their P/Q-type Ca 2+ current triggers neurotransmitter release (6-11). Because synaptic transmission is proportional to the third or fourth power of Ca 2+ entry through presynaptic Ca V 2.1 channels, small changes in Ca 2+ current have profound effects on synaptic transmission (2, 12). Studies at the calyx of Held synapse have provided important insights into the contribution of presynaptic Ca 2+ current to short-term synaptic plasticity (13-17). Ca V 2.1 channels are required for synaptic facilitation, and Ca 2+ -dependent facilitation and inactivation of the P/Q-type Ca 2+ currents are correlated temporally with synaptic facilitation and rapid synaptic depression (13-17).Molecular interactions between Ca 2+ /calmodulin (CaM) and Ca V 2.1 channels induce sequential Ca 2+ -dependent facilitation and inactivation of P/Q-type Ca 2+ currents in...

Facilitation and inactivation of P/Q-type Ca2+ currents mediated by Ca2+/calmodulin binding to CaV2.1 channels contribute to facilitation and rapid depression of synaptic transmission, respectively. Other calcium sensor proteins displace calmodulin from its binding site and differentially modulate P/Q-type Ca2+ currents, resulting in diverse patterns of short-term synaptic plasticity. Neuronal calcium sensor-1 (NCS-1, frequenin) has been shown to enhance synaptic facilitation, but the underlying mechanism is unclear. We report here that NCS-1 directly interacts with IQ-like motif and calmodulin-binding domain in the C-terminal domain of CaV2.1 channel. NCS-1 reduces Ca2+-dependent inactivation of P/Q-type Ca2+ current through interaction with the IQ-like motif and calmodulin-binding domain without affecting peak current or activation kinetics. Expression of NCS-1 in presynaptic superior cervical ganglion neurons has no effect on synaptic transmission, eliminating effects of this calcium sensor protein on endogenous N-type Ca2+ currents and the endogenous neurotransmitter release machinery. However, in superior cervical ganglion neurons expressing wild-type CaV2.1 channels, co-expression of NCS-1 induces facilitation of synaptic transmission in response to paired pulses and trains of depolarizing stimuli, and this effect is lost in CaV2.1 channels with mutations in the IQ-like motif and calmodulin-binding domain. These results reveal that NCS-1 directly modulates CaV2.1 channels to induce short-term synaptic facilitation and further demonstrate that CaS proteins are crucial in fine-tuning short-term synaptic plasticity.