A mutation in Ca2.1 linked to a severe neurodevelopmental disorder impairs channel gating.

Ca flux into axon terminals via P-/Q-type Ca2.1 channels is the trigger for neurotransmitter vesicle release at neuromuscular junctions (NMJs) and many central synapses. Recently, an arginine to proline substitution (R1673P) in the S4 voltage-sensing helix of the fourth membrane-bound repeat of Ca2.1 was linked to a severe neurological disorder characterized by generalized hypotonia, ataxia, cerebellar atrophy, and global developmental delay. The R1673P mutation was proposed to cause a gain of function in Ca2.1 leading to neuronal Ca toxicity based on the ability of the mutant channel to rescue the photoreceptor response in Ca2.1-deficient larvae. Here, we show that the corresponding mutation in rat Ca2.1 (R1624P) causes a profound loss of channel function; voltage-clamp analysis of tsA-201 cells expressing this mutant channel revealed an ∼25-mV depolarizing shift in the voltage dependence of activation. This alteration in activation implies that a significant fraction of Ca2.1 channels resident in presynaptic terminals are unlikely to open in response to an action potential, thereby increasing the probability of synaptic failure at both NMJs and central synapses. Indeed, the mutant channel supported only minimal Ca flux in response to an action potential-like waveform. Application of GV-58, a compound previously shown to stabilize the open state of wild-type Ca2.1 channels, partially restored Ca current by shifting mutant activation to more hyperpolarizing potentials and slowing deactivation. Consequently, GV-58 also rescued a portion of Ca flux during action potential-like stimuli. Thus, our data raise the possibility that therapeutic agents that increase channel open probability or prolong action potential duration may be effective in combatting this and other severe neurodevelopmental disorders caused by loss-of-function mutations in Ca2.1.