Reduced ACh release at neuromuscular synapses of heterozygous leaner Ca(v)2.1-mutant mice

Episodic ataxia type 2 (EA2) is an autosomal dominantly inherited neurological disorder. Patients have CACNA1A gene mutations resulting in truncation or single amino acid changes in the pore-forming subunit of Ca(v)2.1 (P/Q-type) Ca(2+) channels. These neuronal channels mediate synaptic neurotransmitter release. EA2 symptoms are thought to result from disturbed neurotransmission at cerebellar and neuromuscular synapses, caused by loss-of-function of Ca(v)2.1 channels. Heterozygous leaner (Ln/wt) mice, carrying a Cacna1a truncation mutation, as well as heterozygous Ca(v)2.1 null-mutant (KO/wt) mice may model synaptic aspects of EA2. We studied Ca(v)2.1-mediated acetylcholine (ACh) release at their neuromuscular junctions (NMJs) ex vivo. KO/wt mice did not show any ACh release abnormalities, not even at older age. However, Ln/wt mice had approximately 25% reduced spontaneous uniquantal ACh release and approximately 10% reduced nerve-stimulation evoked release, compared with wild-type. EA2 is treated with acetazolamide (AZA), but the pharmacotherapeutic mechanism is unknown. We tested the possibility of a direct influence on (mutant) presynaptic Ca(v)2.1 channel function by studying the acute effect of 50 muM AZA on ACh release at ex vivo NMJs of wild-type, KO/wt, and Ln/wt mice. No changes were found in any of the release parameters. Our results indicate that Ln-mutated Ca(v)2.1 channels at Ln/wt NMJs are either normally inserted in the presynaptic membrane but have reduced function, or that they inhibit wild-type channels by hampering their expression, trafficking, membrane insertion and/or function. In this respect Ln/wt NMJs may model EA2 synapses. Furthermore, AZA does not exert an acute, direct influence on the function of presynaptic (mutant) Ca(v)2.1 channels.