Assessing the Impact of Calmodulinopathic Mutations on Neuronal Ion Channel Regulation

Abstract

Calmodulin (CaM) represents an essential component of the cellular response to calcium influx within multiple excitable cell types, including neurons and cardiac myocytes. Calcium binding to CaM is known to mediate the opposing phenomena of calcium-dependent inactivation (CDI) and calcium-dependent facilitation (CDF), in different voltage-gated calcium channels (VGCCs). In humans, mutations within CaM can lead to clinical disorders known as calmodulinopathies, in which patients suffer from severe cardiac symptoms such as long-QT syndrome (LQTS). LQTS in calmodulinopathy patients can be explained by a loss of calcium binding to mutant CaM, which in turn causes a deficit in CDI of the cardiac CaV1.2 L-type calcium channel. However, CaM-mediated regulation is a hallmark of numerous VGCCs, and disruption of these processes in channels outside the heart may explain why calmodulinopathy patients exhibit a high incidence of neurological impairments including developmental delay, seizures, and autism. We therefore probed the effect of pathogenic CaM mutants on VGCCs which are critical to normal neurological function. To begin, we examined the effect of CaM mutants on the robust CDI of CaV1.3 L-type calcium channels, identifying CDI deficits similar to what has been seen in CaV1.2. We next examined the effect of the CaM mutants on the regulation of CaV2.1 channels, which display both CDI and CDF. The CaM mutants caused a significant disruption of CDF in CaV2.1, however the pattern of severity differed from that of the two L-type channels. Thus, the variability of neurologic phenotypes across calmodulinopathy patients may be due, in part, to a differential effect of CaM mutations on the myriad of VGCCs in the brain. Overall, these results suggest a multifaceted role for calmodulinopathic mutations in neuronal dysfunction through disrupted calcium dependent feedback of VGCCs.