STIM2β is a novel STIM2 splice isoform that inhibits Orai channels.

The syndromic autism spectrum disorder (ASD) Timothy syndrome (TS) is caused by a point mutation in the alternatively spliced exon 8A of the calcium channel Cav1.2. Using mouse brain and human induced pluripotent stem cells (iPSCs), we provide evidence that the TS mutation prevents a normal developmental switch in Cav1.2 exon utilization, resulting in persistent expression of gain-of-function mutant channels during neuronal differentiation. In iPSC models, the TS mutation reduces the abundance of SATB2-expressing cortical projection neurons, leading to excess CTIP2+ neurons. We show that expression of TS-Cav1.2 channels in the embryonic mouse cortex recapitulates these differentiation defects in a calcium-dependent manner and that in utero Cav1.2 gain-and-loss of function reciprocally regulates the abundance of these neuronal populations. Our findings support the idea that disruption of developmentally regulated calcium channel splicing patterns instructively alters differentiation in the developing cortex, providing important in vivo insights into the pathophysiology of a syndromic ASD.

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The signals regulating stem cell activation during tissue regeneration remain poorly understood. We investigated the baldness associated with mutations in the voltagegated calcium channel (VGCC) Ca v 1.2 underlying Timothy syndrome (TS). While hair follicle stem cells express Ca v 1.2, they lack detectable voltage-dependent calcium currents. Ca v 1.2 TS acts in a dominant-negative manner to markedly delay anagen, while L-type channel blockers act through Ca v 1.2 to induce anagen and overcome the TS phenotype. Ca v 1.2 regulates production of the bulgederived BMP inhibitor follistatin-like1 (Fstl1), derepressing stem cell quiescence. Our findings show how channels act in nonexcitable tissues to regulate stem cells and may lead to novel therapeutics for tissue regeneration.

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Store-operated Ca 2+ entry (SOCE), the fundamental Ca 2+ signaling mechanism in myogenesis, is mediated by stromal interaction molecule (STIM), which senses the depletion of endoplasmic reticulum Ca 2+ stores and induces Ca 2+ influx by activating Orai channels in the plasma membrane. Recently, STIM2β, an eight-residue-inserted splice variant of STIM2, was found to act as an inhibitor of SOCE. Although a previous study demonstrated an increase in STIM2β splicing during in vitro differentiation of skeletal muscle, the underlying mechanism and detailed function of STIM2β in myogenesis remain unclear. In this study, we investigated the function of STIM2β in myogenesis using the C2C12 cell line with RNA interference-mediated knockdown and CRISPR-Cas-mediated knockout approaches. Deletion of STIM2β delayed myogenic differentiation through the MEF2C and NFAT4 pathway in C2C12 cells. Further, loss of STIM2β increased cell proliferation by altering Ca 2+ homeostasis and inhibited cell cycle arrest mediated by the cyclin D1-CDK4 degradation pathway. Thus, this study identified a previously unknown function of STIM2β in myogenesis and improves the understanding of how cells effectively regulate the development process via alternative splicing.

Understanding the complexity of neuronal biology requires the manipulation of cellular processes with high specificity and spatio-temporal precision. The recent development of synthetic photoactivatable proteins designed using the light-oxygen-voltage and phytochrome domains provide a new set of tools for genetically targeted optical control of cell signaling. Their modular design, functional diversity, precisely controlled activity and in-vivo applicability offer many advantages for investigating neuronal function. Although designing these proteins is still a considerable challenge, future advances in rational protein design and a deeper understanding of their photoactivation mechanisms will allow the development of the next generation of optogenetic techniques.