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With human median lifespan extending into the 80s in many developed countries, the societal burden of age-related muscle loss (sarcopenia) is increasing. mTORC1 promotes skeletal muscle hypertrophy, but also drives organismal aging. Here, we address the question of whether mTORC1 activation or suppression is beneficial for skeletal muscle aging. We demonstrate that chronic mTORC1 inhibition with rapamycin is overwhelmingly, but not entirely, positive for aging mouse skeletal muscle, while genetic, muscle fiber-specific activation of mTORC1 is sufficient to induce molecular signatures of sarcopenia. Through integration of comprehensive physiological and extensive gene expression profiling in young and old mice, and following genetic activation or pharmacological inhibition of mTORC1, we establish the phenotypically-backed, mTORC1-focused, multi-muscle gene expression atlas, SarcoAtlas (https://sarcoatlas.scicore.unibas.ch/), as a user-friendly gene discovery tool. We uncover inter-muscle divergence in the primary drivers of sarcopenia and identify the neuromuscular junction as a focal point of mTORC1-driven muscle aging. mTORC1 expression is increased during ageing of muscle, and on the other hand, its activation promotes muscle hypertrophy. Here, the authors assess whether mTORC1 has positive or negative effects on ageing, and show that its long-term inhibition preserves muscle mass and function and neuromuscular junction integrity, whereas muscle-specific activation is associated with sarcopenia.

Profound muscle weakness during and after critical illness is termed intensive care unit-acquired weakness (ICUAW). To develop diagnostic recommendations for ICUAW. A multidisciplinary expert committee generated diagnostic questions. A systematic review was performed, and recommendations were developed using the Grading, Recommendations, Assessment, Development, and Evaluation (GRADE) approach. Severe sepsis, difficult ventilator liberation, and prolonged mechanical ventilation are associated with ICUAW. Physical rehabilitation improves outcomes in heterogeneous populations of ICU patients. Because it may not be feasible to provide universal physical rehabilitation, an alternative approach is to identify patients most likely to benefit. Patients with ICUAW may be such a group. Our review identified only one case series of patients with ICUAW who received physical therapy. When compared with a case series of patients with ICUAW who did not receive structured physical therapy, evidence suggested those who receive physical rehabilitation were more frequently discharged home rather than to a rehabilitative facility, although confidence intervals included no difference. Other interventions show promise, but fewer data proving patient benefit existed, thus precluding specific comment. Additionally, prior comorbidity was insufficiently defined to determine its influence on outcome, treatment response, or patient preferences for diagnostic efforts. We recommend controlled clinical trials in patients with ICUAW that compare physical rehabilitation with usual care and further research in understanding risk and patient preferences. Research that identifies treatments that benefit patients with ICUAW is necessary to determine whether the benefits of diagnostic testing for ICUAW outweigh its burdens.

Following prolonged activity blockade, amplitudes of miniature excitatory postsynaptic currents (mEPSCs) increase, a form of plasticity termed ‘homeostatic synaptic plasticity’. We previously showed that a presynaptic protein, the small GTPase Rab3a , is required for full expression of the increase in miniature endplate current amplitudes following prolonged blockade of action potential activity at the mouse neuromuscular junction (NMJ) in vivo, where an increase in postsynaptic receptors does not contribute. It is unknown whether this form of Rab3a -dependent homeostatic plasticity at the NMJ shares any characteristics with central synapses. We show here that homeostatic synaptic plasticity of mEPSCs is impaired in mouse cortical neuron cultures prepared from Rab3a −/− and mutant mice expressing a single-point mutation of Rab3a , Rab3a Earlybird mice. To determine if Rab3a is involved in the well-established homeostatic increase in postsynaptic AMPA-type receptors (AMPARs), we performed a series of experiments in which electrophysiological recordings of mEPSCs and confocal imaging of synaptic AMPAR immunofluorescence were assessed within the same cultures. We found that the increase in postsynaptic AMPAR levels in wild-type cultures was more variable than that of mEPSC amplitudes, which might be explained by a presynaptic contribution, but we cannot rule out variability in the measurement. Finally, we demonstrate that Rab3a is acting in neurons because only selective loss of Rab3a in neurons, not glia, disrupted the homeostatic increase in mEPSC amplitudes. This is the first demonstration that a protein thought to function presynaptically is required for homeostatic synaptic plasticity of quantal size in central neurons.

Our objective was to evaluate an ex vivo muscle–nerve preparation used to study mechanosensory signalling by low threshold mechanosensory receptors (LTMRs). Specifically, we aimed to assess how well the ex vivo preparation represents in vivo firing behaviours of the three major LTMR subtypes of muscle primary sensory afferents, namely type Ia and II muscle spindle (MS) afferents and type Ib tendon organ afferents. Using published procedures for ex vivo study of LTMRs in mouse hindlimb muscles, we replicated earlier reports on afferent firing in response to conventional stretch paradigms applied to non‐contracting, that is passive, muscle. Relative to in vivo studies, stretch‐evoked firing for confirmed MS afferents in the ex vivo preparation was markedly reduced in firing rate and deficient in encoding dynamic features of muscle stretch. These deficiencies precluded conventional means of discriminating type Ia and II afferents. Muscle afferents, including confirmed Ib afferents were often indistinguishable based on their similar firing responses to the same physiologically relevant stretch paradigms. These observations raise uncertainty about conclusions drawn from earlier ex vivo studies that either attribute findings to specific afferent types or suggest an absence of treatment effects on dynamic firing. However, we found that replacing the recording solution with bicarbonate buffer resulted in afferent firing rates and profiles more like those seen in vivo. Improving representation of the distinctive sensory encoding properties in ex vivo muscle–nerve preparations will promote accuracy in assigning molecular markers and mechanisms to heterogeneous types of muscle mechanosensory neurons. What is the central question of this study? How well have studies using ex vivo  muscle‐ nerve preparations represented in vivo features of sensory encoding by low threshold mechanoreceptors in muscle? What is the main finding and its importance? We find experimental adjustments to the ex vivo approach that improve representation of mechanosensory encoding observed in vivo. These adjustments will enhance accuracy in the search for molecular identity and encoding mechanisms of heterogeneous muscle mechanosensory neurons.

Objective Hyperkalemic periodic paralysis (hyperKPP) is characterized by attacks of transient weakness. A subset of hyperKPP patients suffers from transient involuntary contraction of muscle (myotonia). The goal of this study was to determine mechanisms causing myotonia in hyperKPP. Methods Intracellular electrophysiology, single‐fiber Ca2+ imaging, and whole muscle contractility studies were performed in a mouse model of hyperKPP. Results Myotonia in hyperkPP was caused by both involuntary myogenic action potentials (AP myotonia) lasting less than 5 min and action potential‐independent myotonia (non‐AP myotonia) lasting over 1 h. Non‐AP myotonia was caused by prolonged subthreshold depolarization and elevated intracellular Ca2+ in the absence of action potentials. Treatment with dantrolene effectively mitigated non‐AP myotonia, suggesting that the source of Ca2+ was the sarcoplasmic reticulum. Although non‐AP myotonia occurred in the absence of action potentials, Na+ channel blockers were effective as therapy. Discussion We propose myotonia in hyperKPP occurs via two mechanisms: (1) suprathreshold depolarization triggering action potentials that are detectable with EMG and (2) sustained subthreshold depolarization resulting in Na+ overload and Ca2+ leak from the sarcoplasmic reticulum. Notably, clinical diagnostics such as EMG cannot detect the second mechanism as it occurs in the absence of action potentials. Currently, only a minority of patients with hyperKPP are treated with Na+ channel blockers and none are treated with dantrolene. Our data suggest hyperKPP patients, as well as patients with a number of other neuromuscular disorders, may benefit from trials of these therapies, even if they do not have myotonia detectable clinically or by EMG.

Excitation-contraction coupling (ECC) is the process by which electrical excitation of muscle is converted into force generation. Depolarization of skeletal muscle resting potential contributes to failure of ECC in diseases such as periodic paralysis, intensive care unit acquired weakness and possibly fatigue of muscle during vigorous exercise. When extracellular K + is raised to depolarize the resting potential, failure of ECC occurs suddenly, over a narrow range of resting potentials. Simultaneous imaging of Ca 2+ transients and recording of action potentials (APs) demonstrated failure to generate Ca 2+ transients when APs peaked at potentials more negative than –30mV. An AP property that closely correlated with failure of the Ca 2+ transient was the integral of AP voltage with respect to time. Simultaneous recording of Ca 2+ transients and APs with electrodes separated by 1.6mm revealed AP conduction fails when APs peak below –21mV. We hypothesize propagation of APs and generation of Ca 2+ transients are governed by distinct AP properties: AP conduction is governed by AP peak, whereas Ca 2+ release from the sarcoplasmic reticulum is governed by AP integral. The reason distinct AP properties may govern distinct steps of ECC is the kinetics of the ion channels involved. Na channels, which govern propagation, have rapid kinetics and are insensitive to AP width (and thus AP integral) whereas Ca 2+ release is governed by gating charge movement of Cav1.1 channels, which have slower kinetics such that Ca 2+ release is sensitive to AP integral. The quantitative relationships established between resting potential, AP properties, AP conduction and Ca 2+ transients provide the foundation for future studies of failure of ECC induced by depolarization of the resting potential.

In addition to the hallmark muscle stiffness, patients with recessive myotonia congenita (Becker disease) experience debilitating bouts of transient weakness that remain poorly understood despite years of study. We performed intracellular recordings from muscle of both genetic and pharmacologic mouse models of Becker disease to identify the mechanism underlying transient weakness. Our recordings reveal transient depolarizations (plateau potentials) of the membrane potential to −25 to −35 mV in the genetic and pharmacologic models of Becker disease. Both Na + and Ca 2+ currents contribute to plateau potentials. Na + persistent inward current (NaPIC) through Na V 1.4 channels is the key trigger of plateau potentials and current through Ca V 1.1 Ca 2+ channels contributes to the duration of the plateau. Inhibiting NaPIC with ranolazine prevents the development of plateau potentials and eliminates transient weakness in vivo. These data suggest that targeting NaPIC may be an effective treatment to prevent transient weakness in myotonia congenita. Myotonia is a neuromuscular condition that causes problems with the relaxation of muscles following voluntary movements. One type of myotonia is Becker disease, also called recessive myotonia congenita. This is a genetic condition that causes muscle stiffness as a result of involuntary muscle activity. Patients may also suffer transient weakness for a few seconds or as long as several minutes after initiating a movement. The cause of these bouts of temporary weakness is still unclear, but there are hints that it could be linked to the muscle losing its excitability, the ability to respond to the stimuli that make it contract. However, this is at odds with findings that show that muscles in Becker disease are hyperexcitable. Muscle excitability depends on the presence of different concentrations of charged ions (positively charged sodium, calcium and potassium ions and negatively charged chloride ions) inside and outside of each muscle cells. These different concentrations of ions create an electric potential across the cell membrane, also called the ‘membrane potential’. When a muscle cell gets stimulated, proteins on the cell membrane known as ion channels open. This allows the flow of ions between the inside and the outside of the cell, which causes an electrical current that triggers muscle contraction. To better understand the causes behind this muscle weakness, Myers et al. used mice that had either been genetically manipulated or given drugs to mimic Becker disease. By measuring both muscle force and the electrical currents that drive contraction, Myers et al. found that the mechanism underlying post-movement weakness involved a transient change in the concentrations of positively charged ions inside and outside the cells. Further experiments showed that proteins that regulate the passage of both sodium and calcium in and out of the cell – called sodium and calcium channels – contributed to this change in concentration. In addition, Myers et al. discovered that using a drug called ranolazine to stop sodium ions from entering the cell eliminated transient weakness in live mice. These findings suggest that in Becker disease, muscles cycle rapidly between being hyperexcited or not able to be excited, and that targeting the flow of sodium ions into the cell could be an effective treatment to prevent transient weakness in myotonia congenita. This study paves the way towards the development of new therapies to treat Becker disease as well as other muscle ion channel diseases with transient weakness such as periodic paralysis.

Neuropathy and myopathy can cause weakness during critical illness. To determine whether reduced excitability of peripheral nerves, rather than degeneration, is the mechanism underlying acute neuropathy in critically ill patients, we prospectively followed patients during the acute phase of critical illness and early recovery and assessed nerve conduction. During the period of early recovery from critical illness, patients recovered from neuropathy within days. This rapidly reversible neuropathy has not to our knowledge been previously described in critically ill patients and may be a novel type of neuropathy. In vivo intracellular recordings from dorsal root axons in septic rats revealed reduced action potential amplitude, demonstrating that reduced excitability of nerve was the mechanism underlying neuropathy. When action potentials were triggered by hyperpolarizing pulses, their amplitudes largely recovered, indicating that inactivation of sodium channels was an important contributor to reduced excitability. There was no depolarization of axon resting potential in septic rats, which ruled out a contribution of resting potential to the increased inactivation of sodium channels. Our data suggest that a hyperpolarized shift in the voltage dependence of sodium channel inactivation causes increased sodium inactivation and reduced excitability. Acquired sodium channelopathy may be the mechanism underlying acute neuropathy in critically ill patients.

Patients with myotonia congenita suffer from slowed relaxation of muscle (myotonia), due to hyperexcitability caused by loss-of-function mutations in the ClC-1 chloride channel. A recent study suggested that block of large-conductance voltage- and Ca2+- activated K+ channels (BK) may be effective as therapy. The mechanism underlying efficacy was suggested to be lessening of the depolarizing effect of build-up of K+ in t-tubules of muscle during repetitive firing. BK channels are widely expressed in the nervous system and have been shown to play a central role in regulation of excitability, but their contribution to muscle excitability has not been determined. We performed intracellular recordings as well as force measurements in both wild type and BK−/− mouse extensor digitorum longus muscles. Action potential width was increased in BK−/− muscle due to slowing of repolarization, consistent with the possibility K+ build-up in t-tubules is lessened by block of BK channels in myotonic muscle. However, there was no difference in the severity of myotonia triggered by block of muscle Cl− channels with 9-anthracenecarboxylic acid (9AC) in wild type and BK−/− muscle fibers. Further study revealed no difference in the interspike membrane potential during repetitive firing suggesting there was no reduction in K+ build-up in t-tubules of BK−/− muscle. Force recordings following block of muscle Cl− channels demonstrated little reduction in myotonia in BK−/− muscle. In contrast, the current standard of care, mexiletine, significantly reduced myotonia. Our data suggest BK channels regulate muscle excitability, but are not an attractive target for therapy of myotonia.

When neuronal activity is reduced over a period of days, compensatory changes in synaptic strength and/or cellular excitability are triggered, which are thought to act in a manner to homeostatically recover normal activity levels. The time course over which changes in homeostatic synaptic strength and cellular excitability occur are not clear. Although many studies show that 1-2 days of activity block are necessary to trigger increases in excitatory quantal strength, few studies have been able to examine whether these mechanisms actually underlie recovery of network activity. Here, we examine the mechanisms underlying recovery of embryonic motor activity following block of either excitatory GABAergic or glutamatergic inputs in vivo. We find that GABAA receptor blockade triggers fast changes in cellular excitability that occur during the recovery of activity but before changes in synaptic scaling. This increase in cellular excitability is mediated in part by an increase in sodium currents and a reduction in the fast-inactivating and calcium-activated potassium currents. These findings suggest that compensatory changes in cellular excitability, rather than synaptic scaling, contribute to activity recovery. Further, we find a special role for the GABAA receptor in triggering several homeostatic mechanisms after activity perturbations, including changes in cellular excitability and GABAergic and AMPAergic synaptic strength. The temporal difference in expression of homeostatic changes in cellular excitability and synaptic strength suggests that there are multiple mechanisms and pathways engaged to regulate network activity, and that each may have temporally distinct functions.