Study of Short Forms of P/Q-Type Voltage-Gated Calcium Channels

P/Q-type voltage-gated calcium channels (CaV2.1) are expressed in both central and peripheral nervous systems, where they play a critical role in neurotransmitter release. Mutations in the pore-forming α1 subunit of CaV2.1 can cause neurological disorders such as episodic ataxia type 2, familial hemiplegic migraine type 1 and spinocerebellar ataxia type 6. Interestingly, a 190-kDa fragment of CaV2.1 was found in mouse brain tissue and cultured mouse cortical neurons, but not in heterologous systems expressing full-length CaV2.1. In the brain, the 190-kDa species is the predominant form of CaV2.1, while in cultured cortical neurons the amount of the 190-kDa species is comparable to that of the full-length channel. The 190-kDa fragment contains part of the II-III loop, repeat III, repeat IV and the C-terminal tail. A putative complementary fragment of 80-90 kDa was found along with the 190-kDa form. Moreover, preliminary data show that the abundance of the 190-kDa species and the 80-90-kDa species relative to the full-length channel is upregulated by increased intracellular Ca²⁺ concentration. Truncation mutations in the P/Q-type calcium channel have been found to cause the neurological disease episodic ataxia type 2. Some of the disease-causing truncations resemble the 190-kDa truncated channel that we found. Three pairs of truncated versions of CaV2.1 were engineered to resemble putative products of proteolytic cleavage in the three intracellular loops. Electrophysiological properties of these truncated channels were studied. The truncated channel corresponding to the N-terminal fragment produced by cleavage in the II-III loop has a suppressive effect on full-length P/Q channel currents, resembling the effects of several truncation mutants that cause episodic ataxia type 2. The complementary pair of truncated channels created by a truncation site in the I-II loop forms a functional channel when coexpressed. These results shed light on the functional effects of proteolytic cleavage in the intracellular loops of the P/Q channel.