Cav2.2 channels are presynaptic voltage-gated Ca2+ channels that regulate neurotransmitter release. In addition, they are major therapeutic targets from neuropathic pain, a chronic pain disorder caused by injury to the nerve. Pain-relieving drugs such as opioids and ziconotide block Cav2.2 channels. Unfortunately, these drugs are associated with severe adverse side effects. Therefore, there is a need to understand the factors that regulate Cav2.2 channels to design more effective therapies. My dissertation uses electrophysiological techniques to understand the factors that regulate Cav2.2 channel function. My research will provide insights into how Cav2.2 channels integrate diverse cellular signals to shape neurotransmission. This knowledge can be used to treat neurological disorders, such as chronic pain and Myoclonus- Dystonia syndrome, a movement disorder associated with a mutation in the gene that encodes Cav2.2.
A variety of regulatory mechanisms modulate Ca2+ entry through Cav2.2 channels. One prominent from of regulation is Ca2+-dependent inactivation, a negative feedback mechanism. Incoming Ca2+ ions bind to the Ca2+ sensor calmodulin, which is tethered to the channel. The interaction between Ca2+ and calmodulin is thought to induce a conformational change in the structure of Cav2.2 to reduce further Ca2+ entry. The related voltage-gated Ca2+ channel Cav2.1 undergoes an additional and opposing form of regulation, Ca2+-dependent facilitation, which enhances Ca2+ entry. Ca2+-dependent inactivation and facilitation of Cav2.1 can adjust the amount of neurotransmitter released at a synapse in ways that modify information processing in the nervous system. Unlike Cav2.1, Cav2.2 does not undergo Ca2+-dependent facilitation, but the mechanism underlying this difference is unknown.
One possibility is that Cav2.2 channels do not contain the molecular components necessary to support Ca2+-dependent facilitation, which have been identified in Cav2.1 in previous studies. I hypothesized that the analogous regions of Cav2.2 contain slight modifications, which prevents Ca2+-dependent facilitation. In support of this hypothesis, I found that Cav2.2 channels can undergo Ca2+-dependent facilitation upon transferring portions of the C-terminal domain of Cav2.1 to Cav2.2. A second possibility is that Cav2.2 undergoes other forms of regulation that oppose Ca2+-dependent facilitation. Cav2.2 is strongly inhibited by ligands for some G protein-coupled receptors, which helps prevent excess release of neurotransmitters in the nervous system. I hypothesized that strong G protein modulation of Cav2.2 opposes Ca2+-dependent facilitation. I found that Cav2.2 channels could undergo a form of Ca2+-dependent facilitation upon inhibiting G-protein signaling, which supported my hypothesis.
Taken together, my results demonstrate that multiple factors contribute the lack of Ca2+-dependent facilitation observed for Cav2.2 channels. My results provide new insights into the intrinsic and extrinsic forces that regulate Cav2.2 function, which expands our understanding of how Cav2.2-mediated Ca2+ signals can modified by normal patterns of neuronal activity. This knowledge will aid our understanding of the pathogenic mechanisms underlying neurological conditions associated with Cav2.2 dysfunction and how to treat them.