The familial hemiplegic migraine mutation R192q reduces G-protein-mediated inhibition of p/q-type (Ca_v2.1) calcium channels expressed in human embryonic kidney cells

Abstract

Familial hemiplegic migraine is associated with at least 13 different missense mutations in the α1A Ca²⁺ channel subunit. Some of these mutations have been shown to affect the biophysical properties of α1A currents. To date, no study has examined the influence of such mutations on the G-protein regulation of channel function. Because G-proteins inhibit movement of the voltage sensor, we examined the effects of the R192Q mutation, which neutralizes a positive charge in the first S4 segment. Human wild-type (WT) or R192Q mutant channels were expressed in human embryonic kidney tsA-201 cells along with dopamine D2 receptors. Application of quinpirole induced fast (≈1 s), pertussis toxin-sensitive inhibition of α1A_WT and α1A_R192Q Ca²⁺ currents, consistent with the activation of a membrane-delimited pathway. α1A_WT Ca²⁺ currents were inhibited by 62.9 ± 0.9 % (n = 27), whereas α1A_R192Q Ca²⁺ currents were inhibited by only 47.9 ± 1.8 % (n = 35; P < 0.001). Concentration-response analysis showed that only the extent of inhibition was affected, with no change in agonist potency (EC₅₀ = 1 nm). Prepulse facilitation, which is a characteristic of voltage-dependent inhibition, was also reduced by the R192Q mutation. However, the kinetics of facilitation and slow activation were not affected, suggesting that G-protein-Ca²⁺ channel affinity was unchanged. These results show that the R192Q mutation reduces the G-protein inhibition of P/Q-type Ca²⁺ channels, probably by altering mechanisms by which Gβγ subunit binding induces a change in channel gating. Altered G-protein modulation and the consequent reduced presynaptic inhibition may contribute to migraine attacks by favouring a persistent state of hyperexcitability.