Ricardo E. Dolmetsch

The C-terminus of the voltage-gated calcium channel Ca v 1.2 encodes a transcription factor, the calcium channel associated transcriptional regulator (CCAT), that regulates neurite extension and inhibits Ca v 1.2 expression. The mechanisms by which CCAT is generated in neurons and myocytes are poorly understood. Here we show that CCAT is produced by activation of a cryptic promoter in exon 46 of CACNA1C, the gene that encodes Ca V 1.2. Expression of CCAT is independent of Ca v 1.2 expression in neuroblastoma cells, in mice, and in human neurons derived from induced pluripotent stem cells (iPSCs), providing strong evidence that CCAT is not generated by cleavage of Ca V 1.2. Analysis of the transcriptional start sites in CACNA1C and immune-blotting for channel proteins indicate that multiple proteins are generated from the 3′ end of the CACNA1C gene. This study provides new insights into the regulation of CACNA1C, and provides an example of how exonic promoters contribute to the complexity of mammalian genomes.

The authors report that calcium channels with a mutation associated with Timothy syndrome cause activity-dependent dendrite retraction in rodent neurons and in induced pluripotent stem cell–derived neurons from individuals with Timothy syndrome. This retraction was independent of Ca permeation but was associated with activation of RhoA signaling. L-type voltage gated calcium channels have an important role in neuronal development by promoting dendritic growth and arborization. A point mutation in the gene encoding CaV1.2 causes Timothy syndrome, a neurodevelopmental disorder associated with autism spectrum disorders (ASDs). We report that channels with the Timothy syndrome alteration cause activity-dependent dendrite retraction in rat and mouse neurons and in induced pluripotent stem cell (iPSC)-derived neurons from individuals with Timothy syndrome. Dendrite retraction was independent of calcium permeation through the mutant channel, was associated with ectopic activation of RhoA and was inhibited by overexpression of the channel-associated GTPase Gem. These results suggest that CaV1.2 can activate RhoA signaling independently of Ca2+ and provide insights into the cellular basis of Timothy syndrome and other ASDs.

Genetic mutations in ion channel genes that are associated with cardiac arrhythmias have been identified over the past several decades. However, little is known about the pathophysiological processes. An important limitation has been the difficulty of using human cardiomyocytes to study arrhythmias and identify drugs. To circumvent this issue, we have developed a method using human-induced pluripotent stem cells to generate cardiomyocytes from individuals with Timothy syndrome (TS), a genetic disorder characterized by QT prolongation, ventricular tachycardia, and autism. The TS ventricular-like cardiomyocytes exhibit deficits in contraction, electrical signaling, and calcium handling, as revealed by live cell imaging and electrophysiological studies. We tested candidate drugs in TS cardiomyocytes and found that roscovitine could successfully rescue these cellular phenotypes. The use of a human cellular model of cardiac arrhythmias provides a useful new platform not only to study disease mechanisms but also to develop new therapies to treat cardiac arrhythmias.

Timothy syndrome is a neurodevelopmental disease that includes autism-like features. Using iPS-derived neurons from individuals with Timothy syndrome, Ricardo Dolmetsch and his colleagues identify changes in cortical neuron fate and neurotransmitter expression that may begin to explain the neural mechanisms that underlie this disorder. Monogenic neurodevelopmental disorders provide key insights into the pathogenesis of disease and help us understand how specific genes control the development of the human brain. Timothy syndrome is caused by a missense mutation in the L-type calcium channel Cav1.2 that is associated with developmental delay and autism1. We generated cortical neuronal precursor cells and neurons from induced pluripotent stem cells derived from individuals with Timothy syndrome. Cells from these individuals have defects in calcium (Ca2+) signaling and activity-dependent gene expression. They also show abnormalities in differentiation, including decreased expression of genes that are expressed in lower cortical layers and in callosal projection neurons. In addition, neurons derived from individuals with Timothy syndrome show abnormal expression of tyrosine hydroxylase and increased production of norepinephrine and dopamine. This phenotype can be reversed by treatment with roscovitine, a cyclin-dependent kinase inhibitor and atypical L-type–channel blocker2,3,4. These findings provide strong evidence that Cav1.2 regulates the differentiation of cortical neurons in humans and offer new insights into the causes of autism in individuals with Timothy syndrome.

It is difficult to model cardiac arrhythmias in mice and other genetically tractable animals because the mechanisms of cardiomyocyte contraction in these animals are unlike those in humans. A new model for studying these conditions is reported, in the form of cardiomyocytes produced from induced pluripotent stem cells derived by reprogramming fibroblasts from two patients with Timothy syndrome, a disorder characterized by autism, immune deficiency and cardiac arrhythmias. The abnormal electrical and calcium-signalling properties of these patients' cells were restored by a drug, roscovitine, known to increase voltage-dependent inactivation of CaV1.2, a calcium channel that is defective in patients with Timothy syndrome. A mutation in the gene CACNA1C, encoding the L-type calcium channel CaV1.2 in humans, causes Timothy syndrome, a disorder characterized by autism, syndactyly, immune deficiency and cardiac arrhythmias. This study generated induced pluripotent stem cells from the fibroblasts of two patients with Timothy syndrome and converted them into cardiac cells. The patient cells displayed abnormal electrical and calcium signalling properties, which were restored by a drug, roscovitine, known to increase the voltage-dependent inactivation of CaV1.2. Individuals with congenital or acquired prolongation of the QT interval, or long QT syndrome (LQTS), are at risk of life-threatening ventricular arrhythmia1,2. LQTS is commonly genetic in origin but can also be caused or exacerbated by environmental factors1,3. A missense mutation in the L-type calcium channel CaV1.2 leads to LQTS in patients with Timothy syndrome4,5. To explore the effect of the Timothy syndrome mutation on the electrical activity and contraction of human cardiomyocytes, we reprogrammed human skin cells from Timothy syndrome patients to generate induced pluripotent stem cells, and differentiated these cells into cardiomyocytes. Electrophysiological recording and calcium (Ca2+) imaging studies of these cells revealed irregular contraction, excess Ca2+ influx, prolonged action potentials, irregular electrical activity and abnormal calcium transients in ventricular-like cells. We found that roscovitine, a compound that increases the voltage-dependent inactivation of CaV1.2 (refs 6–8), restored the electrical and Ca2+ signalling properties of cardiomyocytes from Timothy syndrome patients. This study provides new opportunities for studying the molecular and cellular mechanisms of cardiac arrhythmias in humans, and provides a robust assay for developing new drugs to treat these diseases.

Channel STIMulation: The STIM1 protein functions as a calcium sensor and regulates entry of calcium into cells across the plasma membrane. When cell surface receptors are stimulated and cause release of calcium from internal stores in the endoplasmic reticulum (ER), STIM proteins in the ER membrane interact with the Orai channel pore protein in the plasma membrane to allow calcium entry from the outside of the cell (see the Perspective by Cahalan ). Park et al. (p. 101 ) and Wang et al. (p. 105 ) now show that STIM also acts to suppress conductance by another calcium channel—the voltage-operated Ca _V 1.2 channel. STIM1 appeared to interact directly with Ca _V 1.2 channels in multiple cell types, including vascular smooth muscle cells, neurons, and cultured cells derived from T lymphocytes. The interaction inhibited opening of the Ca _V 1.2 channels and caused depletion of the channel from the cell surface.

Voltage-gated Ca2+ channels (VGCCs) play a key role in neuronal signaling but can also contribute to cellular dysfunction and death under pathological conditions such as stroke and neurodegenerative diseases. We report that activation of N-methyl-d-aspartic acid receptors causes internalization and degradation of CaV1.2 channels, resulting in decreased Ca2+ entry and reduced toxicity. CaV1.2 internalization and degradation requires binding to phosphatidylinositol 3-phosphate 5-kinase (PIKfyve), a lipid kinase which generates phosphatidylinositol (3,5)-bisphosphate (PtdIns(3,5)P2) and regulates endosome and lysosome function. Sustained activation of glutamate receptors recruits PIKfyve to CaV1.2 channels, increases cellular levels of PtdIns(3,5)P2, and promotes targeting of CaV1.2 to lysosomes. Knockdown of PIKfyve prevents CaV1.2 degradation and increases neuronal susceptibility to excitotoxicity. These experiments identify a novel mechanism by which neurons are protected from excitotoxicity and provide a possible explanation for neuronal death in diseases caused by mutations that affect PtdIns(3,5)P2 regulation.