Hyperfunction of the mTORC1 pathway has been associated with idiopathic and syndromic forms of autism spectrum disorder (ASD), including tuberous sclerosis, caused by loss of either TSC1 or TSC2. It remains largely unknown how developmental processes and biochemical signaling affected by mTORC1 dysregulation contribute to human neuronal dysfunction. Here, we have characterized multiple stages of neurogenesis and synapse formation in human neurons derived from TSC2-deleted pluripotent stem cells. Homozygous TSC2 deletion causes severe developmental abnormalities that recapitulate pathological hallmarks of cortical malformations in patients. Both TSC2(+/-) and TSC2(-/-) neurons display altered synaptic transmission paralleled by molecular changes in pathways associated with autism, suggesting the convergence of pathological mechanisms in ASD. Pharmacological inhibition of mTORC1 corrects developmental abnormalities and synaptic dysfunction during independent developmental stages. Our results uncouple stage-specific roles of mTORC1 in human neuronal development and contribute to a better understanding of the onset of neuronal pathophysiology in tuberous sclerosis.

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In striated muscle, activation of contraction is initiated by membrane depolarisation caused by an action potential, which triggers the release of Ca 2+ stored in the sarcoplasmic reticulum by a process called excitation-contraction coupling. Excitation-contraction coupling occurs via a highly sophisticated supramolecular signalling complex at the junction between the sarcoplasmic reticulum and the transverse tubules. It is generally accepted that the core components of the excitation-contraction coupling machinery are the dihydropyridine receptors, ryanodine receptors and calsequestrin, which serve as voltage sensor, Ca 2+ release channel, and Ca 2+ storage protein, respectively. Nevertheless, a number of additional proteins have been shown to be essential both for the structural formation of the machinery involved in excitation-contraction coupling and for its fine tuning. In this review we discuss the functional role of minor sarcoplasmic reticulum protein components. The definition of their roles in excitation-contraction coupling is important in order to understand how mutations in genes involved in Ca 2+ signalling cause neuromuscular disorders.

Malignant hyperthermia (MH) is an autosomal dominant disorder characterized by abnormal calcium homeostasis in skeletal muscle in response to triggering agents. Today, genetic investigations on ryanodine receptor type 1 (RYR1) gene and alpha1 subunit of the dihydropyridine receptor (DHPR) (CACNA1S) gene have improved the procedures associated with MH diagnosis. In approximately 50% of MH cases a causative RYR1 mutation was found. Molecular genetic testing based on RYR1 mutations for MH diagnosis is challenging, because the causative mutations, most of which are private, are distributed throughout the RYR1 gene. A more comprehensive genetic testing procedure is needed. Therefore, we aim to expand the genetic information related to MH and to evaluate the effect of mutations on the MH phenotype. Performing an in-depth mutation screening of the RYR1 transcript sequence in 36 unrelated MH susceptible (MHS) patients, we identified 17 novel, five rare, and eight non-disease-causing variants in 23 patients. The 13 remaining MHS patients presented no known variants, neither in RYR1 nor in the CACNA1S binding regions to RYR1. The 17 novel variants were found to affect highly conserved amino acids and were absent in 100 controls. Excellent genotype-phenotype correlations were found by investigating 21 MHS families-a total of 186 individuals. Epstein-Barr virus (EBV) lymphoblastoid cells carrying four of these novel mutations showed abnormal calcium homeostasis. The results of this study contribute to the establishment of a robust genetic testing procedure for MH diagnosis.

Muscle strength declines with age in part due to a decline of Ca2+ release from sarcoplasmic reticulum calcium stores. Skeletal muscle dihydropyridine receptors (Cav1.1) initiate muscle contraction by activating ryanodine receptors in the sarcoplasmic reticulum. Cav1.1 channel activity is enhanced by a retrograde stimulatory signal delivered by the ryanodine receptor. JP45 is a membrane protein interacting with Cav1.1 and the sarcoplasmic reticulum Ca2+ storage protein calsequestrin (CASQ1). Here we show that JP45 and CASQ1 strengthen skeletal muscle contraction by modulating Cav1.1 channel activity. Using muscle fibres from JP45 and CASQ1 double knockout mice, we demonstrate that Ca2+ transients evoked by tetanic stimulation are the result of massive Ca2+ influx due to enhanced Cav1.1 channel activity, which restores muscle strength in JP45/CASQ1 double knockout mice. We envision that JP45 and CASQ1 may be candidate targets for the development of new therapeutic strategies against decay of skeletal muscle strength caused by a decrease in sarcoplasmic reticulum Ca2+ content.