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Deletions of chromosome 22q13.3 cause Phelan–McDermid syndrome (PMDS), a neurodevelopmental disorder associated with autism; here induced pluripotent stem cells from PMDS patients with autism are used to produce neurons, they are shown to have reduced SHANK3 expression and a defect in excitatory synaptic transmission which can be restored either by increasing SHANK3 or with insulin-like growth factor 1. The nature of Phelan–McDermid syndrome Deletions of chromosome 22q13.3 cause Phelan–McDermid syndrome (PMDS), a neurodevelopmental disorder associated with autism. Ricardo Dolmetsch and colleagues generated induced pluripotent stem (iPS) cells from PMDS patients with autism and used them to produce neurons. PMDS neurons have reduced expression of the SHANK3 gene, which encodes a protein found in a structure known as the postsynaptic density, and a defect in excitatory synaptic transmission that can be restored either by increasing SHANK3 or with insulin-like growth factor 1. These findings add to the picture of synaptic deficits observed in autism spectrum disorders, and point to potential mechanisms for restoring them. Phelan–McDermid syndrome (PMDS) is a complex neurodevelopmental disorder characterized by global developmental delay, severely impaired speech, intellectual disability, and an increased risk of autism spectrum disorders (ASDs) 1 . PMDS is caused by heterozygous deletions of chromosome 22q13.3. Among the genes in the deleted region is SHANK3 , which encodes a protein in the postsynaptic density (PSD) 2 , 3 . Rare mutations in SHANK3 have been associated with idiopathic ASDs 4 , 5 , 6 , 7 , non-syndromic intellectual disability 8 , and schizophrenia 9 . Although SHANK3 is considered to be the most likely candidate gene for the neurological abnormalities in PMDS patients 10 , the cellular and molecular phenotypes associated with this syndrome in human neurons are unknown. We generated induced pluripotent stem (iPS) cells from individuals with PMDS and autism and used them to produce functional neurons. We show that PMDS neurons have reduced SHANK3 expression and major defects in excitatory, but not inhibitory, synaptic transmission. Excitatory synaptic transmission in PMDS neurons can be corrected by restoring SHANK3 expression or by treating neurons with insulin-like growth factor 1 (IGF1). IGF1 treatment promotes formation of mature excitatory synapses that lack SHANK3 but contain PSD95 and N -methyl- d -aspartate (NMDA) receptors with fast deactivation kinetics. Our findings provide direct evidence for a disruption in the ratio of cellular excitation and inhibition in PMDS neurons, and point to a molecular pathway that can be recruited to restore it.

Neurons from fibroblasts Three papers in this issue demonstrate the production of functional induced neuronal (iN) cells from human fibroblasts, a procedure that holds great promise for regenerative medicine. Pang et al . show that a combination of the three transcription factors Ascl1 (also known as Mash1 ), Brn2 (or Pou3f2 ) and Myt1l greatly enhances the neuronal differentiation of human embryonic stem cells. When combined with the basic helix–loop–helix transcription factor NeuroD1, these factors can also convert fetal and postnatal human fibroblasts into iN cells. Caiazzo et al . use a cocktail of three transcription factors to convert prenatal and adult mouse and human fibroblasts into functional dopaminergic neurons. The three are Mash1 , Nurr1 (or Nr4a2 ) and Lmx1a . Conversion is direct with no reversion to a progenitor cell stage, and it occurs in cells from Parkinson's disease patients as well as from healthy donors. Yoo et al . use an alternative approach. They show that microRNAs can have an instructive role in neural fate determination. Expression of miR-9/9* and miR-124 in human fibroblasts induces their conversion into functional neurons, and the process is facilitated by the addition of some neurogenic transcription factors. Neurogenic transcription factors and evolutionarily conserved signalling pathways have been found to be instrumental in the formation of neurons 1 , 2 . However, the instructive role of microRNAs (miRNAs) in neurogenesis remains unexplored. We recently discovered that miR-9* and miR-124 instruct compositional changes of SWI/SNF-like BAF chromatin-remodelling complexes, a process important for neuronal differentiation and function 3 , 4 , 5 , 6 . Nearing mitotic exit of neural progenitors, miR-9* and miR-124 repress the BAF53a subunit of the neural-progenitor (np)BAF chromatin-remodelling complex. After mitotic exit, BAF53a is replaced by BAF53b, and BAF45a by BAF45b and BAF45c, which are then incorporated into neuron-specific (n)BAF complexes essential for post-mitotic functions 4 . Because miR-9/9* and miR-124 also control multiple genes regulating neuronal differentiation and function 5 , 7 , 8 , 9 , 10 , 11 , 12 , 13 , we proposed that these miRNAs might contribute to neuronal fates. Here we show that expression of miR-9/9* and miR-124 (miR-9/9*-124) in human fibroblasts induces their conversion into neurons, a process facilitated by NEUROD2 . Further addition of neurogenic transcription factors ASCL1 and MYT1L enhances the rate of conversion and the maturation of the converted neurons, whereas expression of these transcription factors alone without miR-9/9*-124 was ineffective. These studies indicate that the genetic circuitry involving miR-9/9*-124 can have an instructive role in neural fate determination.

Controlling protein-protein interaction with high temporal and spatial resolution is essential for understanding many cellular processes. Yazawa et al . present genetically encoded tags that can induce protein dimer formation upon stimulation with blue light. Protein-protein interactions are essential for many cellular processes. We have developed a technology called light-activated dimerization (LAD) to artificially induce protein hetero- and homodimerization in live cells using light. Using the FKF1 and GIGANTEA (GI) proteins of Arabidopsis thaliana , we have generated protein tags whose interaction is controlled by blue light. We demonstrated the utility of this system with LAD constructs that can recruit the small G-protein Rac1 to the plasma membrane and induce the local formation of lamellipodia in response to focal illumination. We also generated a light-activated transcription factor by fusing domains of GI and FKF1 to the DNA binding domain of Gal4 and the transactivation domain of VP16, respectively, showing that this technology is easily adapted to other systems. These studies set the stage for the development of light-regulated signaling molecules for controlling receptor activation, synapse formation and other signaling events in organisms.

New model for arrhythmias 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 Ca V 1.2, a calcium channel that is defective in patients with Timothy syndrome. A mutation in the gene CACNA1C , encoding the L-type calcium channel Ca V 1.2 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 Ca V 1.2. Individuals with congenital or acquired prolongation of the QT interval, or long QT syndrome (LQTS), are at risk of life-threatening ventricular arrhythmia 1 , 2 . LQTS is commonly genetic in origin but can also be caused or exacerbated by environmental factors 1 , 3 . A missense mutation in the L-type calcium channel Ca V 1.2 leads to LQTS in patients with Timothy syndrome 4 , 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 (Ca 2+ ) imaging studies of these cells revealed irregular contraction, excess Ca 2+ 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 Ca V 1.2 (refs 6–8 ), restored the electrical and Ca 2+ 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.

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.

Ca²⁺-dependent inactivation (CDI) is a key regulator and hallmark of the Ca²⁺ release-activated Ca²⁺ (CRAC) channel, a prototypic store-operated Ca²⁺ channel. Although the roles of the endoplasmic reticulum Ca²⁺ sensor STIM1 and the channel subunit Orai1 in CRAC channel activation are becoming well understood, the molecular basis of CDI remains unclear. Recently, we defined a minimal CRAC activation domain (CAD; residues 342-448) that binds directly to Orai1 to activate the channel. Surprisingly, CAD-induced CRAC currents lack fast inactivation, revealing a critical role for STIM1 in this gating process. Through truncations of full-length STIM1, we identified a short domain (residues 470-491) C-terminal to CAD that is required for CDI. This domain contains a cluster of 7 acidic amino acids between residues 475 and 483. Neutralization of aspartate or glutamate pairs in this region either reduced or enhanced CDI, whereas the combined neutralization of six acidic residues eliminated inactivation entirely. Based on bioinformatics predictions of a calmodulin (CaM) binding site on Orai1, we also investigated a role for CaM in CDI. We identified a membrane-proximal N-terminal domain of Orai1 (residues 68-91) that binds CaM in a Ca²⁺-dependent manner and mutations that eliminate CaM binding abrogate CDI. These studies identify novel structural elements of STIM1 and Orai1 that are required for CDI and support a model in which CaM acts in concert with STIM1 and the N terminus of Orai1 to evoke rapid CRAC channel inactivation.

Microdeletions and microduplications of the 16p11.2 chromosomal locus are associated with syndromic neurodevelopmental disorders and reciprocal physiological conditions such as macro/microcephaly and high/low body mass index. To facilitate cellular and molecular investigations into these phenotypes, 65 clones of human induced pluripotent stem cells (hiPSCs) were generated from 13 individuals with 16p11.2 copy number variations (CNVs). To ensure these cell lines were suitable for downstream mechanistic investigations, a customizable bioinformatic strategy for the detection of random integration and expression of reprogramming vectors was developed and leveraged towards identifying a subset of ‘footprint’-free hiPSC clones. Transcriptomic profiling of cortical neural progenitor cells derived from these hiPSCs identified alterations in gene expression patterns which precede morphological abnormalities reported at later neurodevelopmental stages. Interpreting clinical information—available with the cell lines by request from the Simons Foundation Autism Research Initiative—with this transcriptional data revealed disruptions in gene programs related to both nervous system function and cellular metabolism. As demonstrated by these analyses, this publicly available resource has the potential to serve as a powerful medium for probing the etiology of developmental disorders associated with 16p11.2 CNVs.

Dravet Syndrome is an intractable form of childhood epilepsy associated with deleterious mutations in SCN1A, the gene encoding neuronal sodium channel Nav1.1. Earlier studies using human induced pluripotent stem cells (iPSCs) have produced mixed results regarding the importance of Nav1.1 in human inhibitory versus excitatory neurons. We studied a Nav1.1 mutation (p.S1328P) identified in a pair of twins with Dravet Syndrome and generated iPSC-derived neurons from these patients. Characterization of the mutant channel revealed a decrease in current amplitude and hypersensitivity to steady-state inactivation. We then differentiated Dravet-Syndrome and control iPSCs into telencephalic excitatory neurons or medial ganglionic eminence (MGE)-like inhibitory neurons. Dravet inhibitory neurons showed deficits in sodium currents and action potential firing, which were rescued by a Nav1.1 transgene, whereas Dravet excitatory neurons were normal. Our study identifies biophysical impairments underlying a deleterious Nav1.1 mutation and supports the hypothesis that Dravet Syndrome arises from defective inhibitory neurons.

Increases in the intracellular concentration of calcium ([ Ca2+]i) activate various signaling pathways that lead to the expression of genes that are essential for dendritic development, neuronal survival, and synaptic plasticity. The mode of Ca2+entry into a neuron plays a key role in determining which signaling pathways are activated and thus specifies the cellular response to Ca2+. Ca2+influx through L-type voltage-activated channels (LTCs) is particularly effective at activating transcription factors such as CREB and MEF-2. We developed a functional knock-in technique to investigate the features of LTCs that specifically couple them to the signaling pathways that regulate gene expression. We found that an isoleucine-glutamine (\"IQ\") motif in the carboxyl terminus of the LTC that binds Ca2+-calmodulin (CaM) is critical for conveying the Ca2+signal to the nucleus. Ca2+-CaM binding to the LTC was necessary for activation of the Ras/mitogen-activated protein kinase (MAPK) pathway, which conveys local Ca2+signals from the mouth of the LTC to the nucleus. CaM functions as a local Ca2+sensor at the mouth of the LTC that activates the MAPK pathway and leads to the stimulation of genes that are essential for neuronal survival and plasticity.

Plasticity is a remarkable feature of the brain, allowing neuronal structure and function to accommodate to patterns of electrical activity. One component of these long-term changes is the activity-driven induction of new gene expression, which is required for both the long-lasting long-term potentiation of synaptic transmission associated with learning and memory, and the activity-dependent survival events that help to shape and wire the brain during development. We have characterized molecular mechanisms by which neuronal membrane depolarization and subsequent calcium influx into the cytoplasm lead to the induction of new gene transcription. We have identified three points within this cascade of events where the specificity of genes induced by different types of stimuli can be regulated. By using the induction of the gene that encodes brain-derived neurotrophic factor (BDNF) as a model, we have found that the ability of a calcium influx to induce transcription of this gene is regulated by the route of calcium entry into the cell, by the pattern of phosphorylation induced on the transcription factor cAMP-response element (CRE) binding protein (CREB), and by the complement of active transcription factors recruited to the BDNF promoter. These results refine and expand the working model of activity-induced gene induction in the brain, and help to explain how different types of neuronal stimuli can activate distinct transcriptional responses.