Neural control of phenotypic expression in mammalian muscle fibers

Plasticity and structural alterations of mitochondria and sarcoplasmic organelles in muscles of mice deficient in α-dystrobrevin, a component of the dystrophin-glycoprotein complex

The dystrophin-glycoprotein complex (DGC) plays a crucial role in maintaining the structural integrity of the plasma membrane and the neuromuscular junction. In this study, we investigated the impact of the deficiency of α-dystrobrevin (αdbn), a component of the DGC, on the homeostasis of intracellular organelles, specifically mitochondria and the sarcoplasmic reticulum (SR). In αdbn deficient muscles, we observed a significant increase in the membrane-bound ATP synthase complex levels, a marker for mitochondria in oxidative muscle fiber types compared to wild-type. Furthermore, examination of muscle fibers deficient in αdbn using electron microscopy revealed profound alterations in the organization of mitochondria and the SR within certain myofibrils of muscle fibers. This included the formation of hyper-branched intermyofibrillar mitochondria with extended connections, an extensive network spanning several myofibrils, and a substantial increase in the number/density of subsarcolemmal mitochondria. Concurrently, in some cases, we observed significant structural alterations in mitochondria, such as cristae loss, fragmentation, swelling, and the formation of vacuoles and inclusions within the mitochondrial matrix cristae. Muscles deficient in αdbn also displayed notable alterations in the morphology of the SR, along with the formation of distinct anomalous concentric SR structures known as whorls. These whorls were prevalent in αdbn-deficient mice but were absent in wild-type muscles. These results suggest a crucial role of the DGC αdbn in regulating intracellular organelles, particularly mitochondria and the SR, within muscle cells. The remodeling of the SR and the formation of whorls may represent a novel mechanism of the unfolded protein response (UPR) in muscle cells.

Dirk Pette, remembered for his pioneering muscle research

It is with great sadness that we learned of the passing of Prof. Dr. Dr. h.c. Dirk Pette. He passed away suddenly and unexpectedly on June 4, 2022. Dirk was an outstanding professor of biochemistry at the University of Konstanz, Germany and an internationally renowned researcher in the field of skeletal muscle biology. His research on electrical stimulation has had a profound impact on our understanding of myofiber type specification and the enormous adaptive potential of skeletal muscle. Under Dirk's leadership, new biological questions in the field of neuromuscular biology have developed into multidisciplinary approaches using advanced physiological, cell biological, and biochemical techniques. Dirk's research laboratory was frequently visited by a large number of national and international collaborators who familiarized themselves with the technically demanding stimulation protocols and bioanalytical techniques to study the intricate details of the highly complex process of fast-to-slow muscle transitions. Importantly, fundamental studies on the physiological effects of changes in innervation patterns on muscle phenotype have provided the scientific evidence base for a variety of innovative clinical applications. The skeletal muscle research community has lost one of its leading figures and an outstanding teacher of protein biochemistry. He leaves an inspiring legacy in the field of basic and applied myology. Dirk will be missed by his colleagues and by many students of neuromuscular biology and beyond.

Volume loss during muscle reinnervation surgery is correlated with reduced CMAP amplitude but not reduced force output in a rat hindlimb model

Introduction: Muscle reinnervation (MR) surgery offers rehabilitative benefits to amputees by taking severely damaged nerves and providing them with new denervated muscle targets (DMTs). However, the influence of physical changes to muscle tissue during MR surgery on long-term functional outcomes remains understudied. Methods: Our rat hindlimb model of MR surgery utilizes vascularized, directly neurotized DMTs made from the lateral gastrocnemius (LG), which we employed to assess the impact of muscle tissue size on reinnervation outcomes, specifically pairing the DMT with the transected peroneal nerve. We conducted MR surgery with both DMTs at full volume and DMTs with partial volume loss of 500 mg at the time of surgery (n = 6 per group) and measured functional outcomes after 100 days of reinnervation. Compound motor action potentials (CMAPs) and isometric tetanic force production was recorded from reinnervated DMTs and compared to contralateral naïve LG muscles as positive controls. Results: Reinnervated DMTs consistently exhibited lower mass than positive controls, while DMTs with partial volume loss showed no significant mass reduction compared to full volume DMTs (p = 0.872). CMAP amplitudes were lower on average in reinnervated DMTs, but a broad linear correlation also exists between muscle mass and maximum CMAP amplitude irrespective of surgical group (R2 = 0.495). Surprisingly, neither MR group, with or without volume loss, demonstrated decreased force compared to positive controls. The average force output of reinnervated DMTs, as a fraction of the contralateral LG's force output, approached 100% for both MR groups, a notable deviation from the 9.6% (±6.3%) force output observed in our negative control group at 7 days post-surgery. Tissue histology analysis revealed few significant differences except for a marked decrease in average muscle fiber area of reinnervated DMTs with volume loss compared to positive controls (p = 0.001). Discussion: The results from our rat model of MR suggests that tissue electrophysiology (CMAPs) and kinesiology (force production) may recover on different time scales, with volumetric muscle loss at the time of MR surgery not significantly reducing functional outcome measurements for the DMTs after 100 days of reinnervation.

Nicotinamide riboside kinases regulate skeletal muscle fiber-type specification and are rate-limiting for metabolic adaptations during regeneration

Nicotinamide riboside kinases (NRKs) control the conversion of dietary Nicotinamide Riboside (NR) to NAD+, but little is known about their contribution to endogenous NAD+ turnover and muscle plasticity during skeletal muscle growth and remodeling. Using NRK1/2 double KO (NRKdKO) mice, we investigated the influence of NRKs on NAD+ metabolism and muscle homeostasis, and on the response to neurogenic muscle atrophy and regeneration following muscle injury. Muscles from NRKdKO animals have altered nicotinamide (NAM) salvage and a decrease in mitochondrial content. In single myonuclei RNAseq of skeletal muscle, NRK2 mRNA expression is restricted to type IIx muscle fibers, and perturbed NAD+ turnover and mitochondrial metabolism shifts the fiber type composition of NRKdKO muscle to fast glycolytic IIB fibers. NRKdKO does not influence muscle atrophy during denervation but alters muscle repair after myofiber injury. During regeneration, muscle stem cells (MuSCs) from NRKdKO animals hyper-proliferate but fail to differentiate. NRKdKO also alters the recovery of NAD+ during muscle regeneration as well as mitochondrial adaptations and extracellular matrix remodeling required for tissue repair. These metabolic perturbations result in a transient delay of muscle regeneration which normalizes during myofiber maturation at late stages of regeneration via over-compensation of anabolic IGF1-Akt signaling. Altogether, we demonstrate that NAD+ synthesis controls mitochondrial metabolism and fiber type composition via NRK1/2 and is rate-limiting for myogenic commitment and mitochondrial maturation during skeletal muscle repair.

Beneficial impacts of neuromuscular electrical stimulation on muscle structure and function in the zebrafish model of Duchenne muscular dystrophy

Neuromuscular electrical stimulation (NMES) allows activation of muscle fibers in the absence of voluntary force generation. NMES could have the potential to promote muscle homeostasis in the context of muscle disease, but the impacts of NMES on diseased muscle are not well understood. We used the zebrafish Duchenne muscular dystrophy (dmd) mutant and a longitudinal design to elucidate the consequences of NMES on muscle health. We designed four neuromuscular stimulation paradigms loosely based on weightlifting regimens. Each paradigm differentially affected neuromuscular structure, function, and survival. Only endurance neuromuscular stimulation (eNMES) improved all outcome measures. We found that eNMES improves muscle and neuromuscular junction morphology, swimming, and survival. Heme oxygenase and integrin alpha7 are required for eNMES-mediated improvement. Our data indicate that neuromuscular stimulation can be beneficial, suggesting that the right type of activity may benefit patients with muscle disease.

Age-Related Alterations at Neuromuscular Junction: Role of Oxidative Stress and Epigenetic Modifications

With advancing aging, a decline in physical abilities occurs, leading to reduced mobility and loss of independence. Although many factors contribute to the physio-pathological effects of aging, an important event seems to be related to the compromised integrity of the neuromuscular system, which connects the brain and skeletal muscles via motoneurons and the neuromuscular junctions (NMJs). NMJs undergo severe functional, morphological, and molecular alterations during aging and ultimately degenerate. The effect of this decline is an inexorable decrease in skeletal muscle mass and strength, a condition generally known as sarcopenia. Moreover, several studies have highlighted how the age-related alteration of reactive oxygen species (ROS) homeostasis can contribute to changes in the neuromuscular junction morphology and stability, leading to the reduction in fiber number and innervation. Increasing evidence supports the involvement of epigenetic modifications in age-dependent alterations of the NMJ. In particular, DNA methylation, histone modifications, and miRNA-dependent gene expression represent the major epigenetic mechanisms that play a crucial role in NMJ remodeling. It is established that environmental and lifestyle factors, such as physical exercise and nutrition that are susceptible to change during aging, can modulate epigenetic phenomena and attenuate the age-related NMJs changes. This review aims to highlight the recent epigenetic findings related to the NMJ dysregulation during aging and the role of physical activity and nutrition as possible interventions to attenuate or delay the age-related decline in the neuromuscular system.

Gerta Vrbová, a guide and a friend for a generation of neuro-myologists - Her scientific legacies and relations with colleagues

Gerta Sidonová - Vrbová, (Trnava, Slovakia, November 28, 1926 - London, UK, October 2, 2020) has been a key neuroscientist, who for almost half a century has contributed important findings and hypotheses on the relationships between motoneurons and skeletal muscle fibers, in particular on the differentiation and extent of plasticity of the peculiar characteristics of the different types of fibers present in mammalian muscles. This issue, Ejtm 31 (1), 2021, opens with the personal obituary authored by Dirk Pette, who remember his lifelong collaboration with Gerta, describing the many molecular and metabolic events that occur by changing the pattern of activation of adult muscle fibers through neuromuscular low frequency electrical stimulation. To honor the many scientific legacies of Gerta Vrbová and her impact on a generation of researchers studying myology and managements of neuromuscular disorders I add here additional examples of Gerta's scientific heritage and of her relations with colleagues.

Gerta Vrbová, a guide and a friend for a generation of neuro-myologists – Her scientific legacies and relations with colleagues

Gerta Sidonová - Vrbová, (Trnava, Slovakia, November 28, 1926 - London, UK, October 2, 2020) has been a key neuroscientist, who for almost half a century has contributed important findings and hypotheses on the relationships between motoneurons and skeletal muscle fibers, in particular on the differentiation and extent of plasticity of the peculiar characteristics of the different types of fibers present in mammalian muscles. This issue, Ejtm 31 (1), 2021, opens with the personal obituary authored by Dirk Pette, who remember his lifelong collaboration with Gerta, describing the many molecular and metabolic events that occur by changing the pattern of activation of adult muscle fibers through neuromuscular low frequency electrical stimulation. To honor the many scientific legacies of Gerta Vrbová and her impact on a generation of researchers studying myology and managements of neuromuscular disorders I add here additional examples of Gerta’s scientific heritage and of her relations with colleagues.

Myosin heavy chains in extraocular muscle fibres: Distribution, regulation and function

This review examines kinetic properties and distribution of the 11 isoforms of myosin heavy chain (MyHC) expressed in extraocular muscle (EOM) fibre types and the regulation and function of these MyHCs. Although recruitment and discharge characteristics of ocular motoneurons during fixation and eye movements are well documented, work directly linking these properties with motor unit contractile speed and MyHC composition is lacking. Recruitment of motor units according to Henneman's size principle has some support in EOMs but needs consolidation. Both neurogenic and myogenic mechanisms regulate MyHC expression as in other muscle allotypes. Developmentally, multiply-innervated (MIFs) and singly-innervated fibres (SIFs) are derived presumably from distinct myoblast lineages, ending up expressing MyHCs in the slow and fast ends of the kinetic spectrum respectively. They modulate the synaptic inputs of their motoneurons through different retrogradely transported neurotrophins, thereby specifying their tonic and phasic impulse patterns. Immunohistochemical analyses of EOMs regenerating in situ and in limb muscle beds suggest that the very impulse patterns driving various ocular movements equip effectors with appropriate MyHC compositions and speeds to accomplish their tasks. These experiments also suggest that satellite cells of SIFs and MIFs are distinct lineages expressing different MyHCs during regeneration. MyHC compositions and functional characteristics of orbital fibres show longitudinal variations that facilitate linear ocular rotation during saccades. Palisade endings on global MIFs are postulated to respond to active and passive tensions by triggering axon reflexes that play important roles during fixation, saccades and vergence. How EOMs implement Listings law during ocular rotation is discussed.

Morphological variability is greater at developing than mature mouse neuromuscular junctions

The neuromuscular junction (NMJ) is the highly specialised peripheral synapse formed between lower motor neuron terminals and muscle fibres. Post‐synaptic acetylcholine receptors (AChRs), which are found in high density in the muscle membrane, bind to acetylcholine released into the synaptic cleft of the NMJ, thereby enabling the conversion of motor action potentials to muscle contractions. NMJs have been studied for many years as a general model for synapse formation, development and function, and are known to be early sites of pathological changes in many neuromuscular diseases. However, information is limited on the diversity of NMJs in different muscles, how synaptic morphology changes during development, and the relevance of these parameters to neuropathology. Here, this crucial gap was addressed using a robust and standardised semi‐automated workflow called NMJ‐morph to quantify features of pre‐ and post‐synaptic NMJ architecture in an unbiased manner. Five wholemount muscles from wild‐type mice were dissected and compared at immature (post‐natal day, P7) and early adult (P31−32) timepoints. The inter‐muscular variability was greater in mature post‐synaptic AChR morphology than that of the pre‐synaptic motor neuron terminal. Moreover, the developing NMJ showed greater differences across muscles than the mature synapse, perhaps due to the observed distinctions in synaptic growth between muscles. Nevertheless, the amount of nerve to muscle contact was consistent, suggesting that pathological denervation can be reliably compared across different muscles in mouse models of neurodegeneration. Additionally, mature post‐synaptic endplate diameters correlated with fibre type, independently of muscle fibre diameter. Altogether, this work provides detailed information on healthy pre‐ and post‐synaptic NMJ morphology from five anatomically and functionally distinct mouse muscles, delivering useful reference data for future comparison with neuromuscular disease models.

Effect of neuromuscular electrical stimulation on skeletal muscle size and function in patients with breast cancer receiving chemotherapy

Exercise has numerous benefits for patients with cancer, but implementation is challenging because of practical and logistical hurdles. This study examined whether neuromuscular electrical stimulation (NMES) can serve as a surrogate for classic exercise by eliciting an exercise training response in skeletal muscle of women diagnosed with breast cancer undergoing chemotherapy. Patients (n = 22) with histologically confirmed stage I, II, or III breast cancer scheduled to receive neoadjuvant or adjuvant chemotherapy were randomized to 8 wk of bilateral neuromuscular electrical stimulation (NMES; 5 days/wk) to their quadriceps muscles or control. Biopsy of the vastus lateralis was performed at baseline and after 8 wk of intervention to assess muscle fiber size, contractility, and mitochondrial content. Seventeen patients (8 control/9 NMES) completed the trial and were included in analyses. NMES promoted muscle fiber hypertrophy (P < 0.001), particularly in fast-twitch, myosin heavy chain (MHC) IIA fibers (P < 0.05) and tended to induce fiber type shifts in MHC II fibers. The effects of NMES on single-muscle fiber contractility were modest, and it was unable to prevent declines in the function in MHC IIA fibers. NMES did not alter intermyofibrillar mitochondrial content/structure but was associated with reductions in subsarcolemmal mitochondria. Our results demonstrate that NMES induces muscle fiber hypertrophy and fiber type shifts in MHC II fibers but had minimal effects on fiber contractility and promoted reductions in subsarcolemmal mitochondria. Further studies are warranted to evaluate the utility of NMES as an exercise surrogate in cancer patients and other conditions.NEW & NOTEWORTHY This is the first study to evaluate whether neuromuscular electrical stimulation (NMES) can be used as an exercise surrogate to improve skeletal muscle fiber size or function in cancer patients receiving treatment. We show that NMES promoted muscle fiber hypertrophy and fiber type shifts but had minimal effects on single-fiber contractility and reduced subsarcolemmal mitochondria.

Medial gastrocnemius muscles fatigue but do not atrophy in paralyzed cat hindlimb after long-term spinal cord hemisection and unilateral deafferentation

This study of medial gastrocnemius (MG) muscle and motor units (MUs) after spinal cord hemisection and deafferentation (HSDA) in adult cats, asked 1) whether the absence of muscle atrophy and unaltered contractile speed demonstrated previously in HSDA-paralyzed peroneus longus (PerL) muscles, was apparent in the unloaded HSDA-paralyzed MG muscle, and 2) how ankle unloading impacts MG muscle and MUs after dorsal root sparing (HSDA-SP) with foot placement during standing and locomotion. Chronic isometric contractile forces and speeds were maintained for up to 12 months in all conditions, but fatigability increased exponentially. MU recordings at 8-11½ months corroborated the unchanged muscle force and speed with significantly increased fatigability; normal weights of MG muscle confirmed the lack of disuse atrophy. Fast MUs transitioned from fatigue resistant and intermediate to fatigable accompanied by corresponding fiber type conversion to fast oxidative (FOG) and fast glycolytic (FG) accompanied by increased GAPDH enzyme activity in absolute terms and relative to oxidative citrate synthase enzyme activity. Myosin heavy chain composition, however, was unaffected. MG muscle behaved like the PerL muscle after HSDA with maintained muscle and MU contractile force and speed but with a dramatic increase in fatigability, irrespective of whether all the dorsal roots were transected. We conclude that reduced neuromuscular activity accounts for increased fatigability but is not, in of itself, sufficient to promote atrophy and slow to fast conversion. Position and relative movements of hindlimb muscles are likely contributors to sustained MG muscle and MU contractile force and speed after HSDA and HSDA-SP surgeries.

Neuromuscular Junction as an Entity of Nerve-Muscle Communication

One of the crucial systems severely affected in several neuromuscular diseases is the loss of effective connection between muscle and nerve, leading to a pathological non-communication between the two tissues. The neuromuscular junction (NMJ) represents the critical region at the level of which muscle and nerve communicate. Defects in signal transmission between terminal nerve endings and muscle membrane is a common feature of several physio-pathologic conditions including aging and Amyotrophic Lateral Sclerosis (ALS). Nevertheless, controversy exists on whether pathological events beginning at the NMJ precede or follow loss of motor units. In this review, the role of NMJ in the physio-pathologic interplay between muscle and nerve is discussed.

Tooth extraction and subsequent dental implant placement in Sprague-Dawley rats induce differential changes in anterior digastric myofibre size and myosin heavy chain isoform expression

OBJECTIVE

to determine if tooth loss and dental implant placement in rats induce changes in the morphological and histochemical features of the Anterior Digastric muscle.

DESIGN

Adult male Sprague-Dawley rats had their right maxillary molar teeth extracted. 'Extraction-1' and 'Extraction-2 groups were sacrificed, respectively, 4 or 8 weeks later, and an Implant group had an implant placement 2 weeks after the molar extraction, and rats were sacrificed 3 weeks later (n = 4/group). Naive rats (n = 3) had no treatment. Morphometric and immunohistochemical techniques quantified Anterior Digastric muscle myofibres' cross-sectional area (CSA) and myosin heavy chain (MyHC) isoform proportions. Significant ANOVAs were followed by post-hoc tests; p < 0.05 and 0.1 were considered to reflect levels of statistical significance.

RESULTS

In naïve rats, the peripheral regions of the Anterior Digastric muscle was dominated by MyHC-IIx/b isoform and there were no MyHC-I isoforms; the central regions dominated by MyHC-IIx/b and MyHC-IIa isoforms. Compared with naive rats, tooth extraction produced, 8 (but not 4) weeks later, a decreased proportion of fast-contracting fatigue-resistant MyHC-IIa isoform (p = 0.08), and increased proportion of fast and intermediate fatigue-resistance MyHC-IIa/x/b isoform (p = 0.03). Dental implant placement following tooth extraction attenuated the extraction effects but produced a decreased proportion of fast-contracting fatiguable MyHC-llx/b isoform (p = 0.03) in the peripheral region, and increased inter-animal variability in myofibre-CSAs.

CONCLUSIONS

Given the crucial role that the Anterior Digastric muscle plays in many vital oral functions (e.g., chewing, swallowing), these changes may contribute to the changes in oral sensorimotor functions that occur in humans following such treatments.

Fundamental constraints in synchronous muscle limit superfast motor control in vertebrates

Superfast muscles (SFMs) are extremely fast synchronous muscles capable of contraction rates up to 250 Hz, enabling precise motor execution at the millisecond time scale. SFM phenotypes have been discovered in most major vertebrate lineages, but it remains unknown whether all SFMs share excitation-contraction coupling pathway adaptations for speed, and if SFMs arose once, or from independent evolutionary events. Here, we demonstrate that to achieve rapid actomyosin crossbridge kinetics bat and songbird SFM express myosin heavy chain genes that are evolutionarily and ontologically distinct. Furthermore, we show that all known SFMs share multiple functional adaptations that minimize excitation-contraction coupling transduction times. Our results suggest that SFM evolved independently in sound-producing organs in ray-finned fish, birds, and mammals, and that SFM phenotypes operate at a maximum operational speed set by fundamental constraints in synchronous muscle. Consequentially, these constraints set a fundamental limit to the maximum speed of fine motor control.

Across animals, different muscle types have evolved to perform vastly different tasks at different speeds. For example, tortoise leg muscles move slowly over several seconds, while the flight muscles of a hummingbird move quickly dozens of times per second. The speed record holders among vertebrates are the so-called superfast muscles, which can move up to 250 times per second. Superfast muscles power the alarming rattle of rattlesnakes, courtship calls in fish, rapid echolocation calls in bats and the elaborate vocal gymnastics of songbirds. Thus these extreme muscles are all around us and are always involved in sound production.

Did superfast muscles evolve from a common ancestor? And how do different superfast muscles achieve their extreme behavior? To answer these questions, Mead et al. studied the systems known to limit contraction speed in all currently known superfast muscles found in rattlesnakes, toadfish, bats and songbirds. This revealed that all the muscles share certain specific adaptations that allow superfast contractions. Furthermore, the three fastest examples – toadfish, songbird and bat – have nearly identical maximum speeds. Although this appears to support the idea that the adaptations all evolved from a shared ancestor, Mead et al. found evidence that suggests otherwise. Each of the three superfast muscles are powered by a different motor protein, which argues strongly in favor of the muscles evolving independently. The existence of such similar mechanisms and performance in independently evolved muscles raises the possibility that the fastest contraction rates measured by Mead et al. represent a maximum speed limit for all vertebrate muscles.

Any technical failure in a racecar most likely will slow it down, while the same failure in a robustly engineered family car may not be so noticeable. Similarly in superfast muscle many cellular and molecular systems need to perform maximally. Therefore by understanding how these extreme muscles work, we also gain a better understanding of how normal muscles contract.

(How) do animals know how much they weigh?

Animal species varying in size and musculoskeletal design all support and move their body weight. This implies the existence of evolutionarily conserved feedback between sensors that produce quantitative signals encoding body weight and proximate determinants of musculoskeletal designs. Although studies at the level of whole organisms and tissue morphology and function clearly indicate that musculoskeletal designs are constrained by body weight variation, the corollary to this - i.e. that the molecular-level composition of musculoskeletal designs is sensitive to body weight variation - has been the subject of only minimal investigation. The main objective of this Commentary is to briefly summarize the former area of study but, in particular, to highlight the latter hypothesis and the relevance of understanding the mechanisms that control musculoskeletal function at the molecular level. Thus, I present a non-exhaustive overview of the evidence - drawn from different fields of study and different levels of biological organization - for the existence of body weight sensing mechanism(s).

Effect of Locomotor Training on Exhaustion of Leg Muscle Activity in Chronic Complete Spinal Cord Injury

The aim of this study was to evaluate the effect of a continuous locomotor training on leg muscle electromyographic (EMG) exhaustion during assisted stepping movements in a patient with motor complete spinal cord injury (SCI). EMG exhaustion and loss of potentials starts to develop in untrained patients at ∼6 months after injury. In the trained patient examined in this study, exhaustion was also observed but occurred with a delay of several months. In contrast to an untrained patient, no more EMG exhaustion was observed in the very chronic stage. At this time (12 years after injury) a basic locomotor pattern of leg muscle activity of reduced amplitude could still be elicited, but it was resistant to exhaustion and unchanged in amplitude after 12 min of assisted stepping. It is suggested that fatigue-resistant motor units prevail at this stage and can still be activated during stepping as a result of the training.

The Contribution of Neuromuscular Stimulation in Elucidating Muscle Plasticity Revisited

Studies carried out during the past 45 years on the effects of chronic low-frequency stimulation on skeletal muscle have revealed a multiplicity of adaptive changes of muscle fibres in response to increased activity. As reflected by induced changes in the metabolic properties, protein profiles of the contractile machinery and elements of the Ca²⁺-regulatory system, all essential components of the muscle fibre undergo pronounced changes in their properties that ultimately lead to their reversible transformation from fast-to-slow phenotype. The chronic low-frequency stimulation experiment thus allows exploring many aspects of the plasticity of mammalian skeletal muscle. Moreover it offers the possibility of elucidating molecular mechanisms that remodel phenotypic properties of a differentiated post-mitotic cell during adaptation to altered functional demands. The understanding of the adaptive potential of muscle can be taken advantage of for repairing muscle damage in various muscle diseases. In addition it can be used to prevent muscle wasting during inactivity and aging. Indeed, pioneering studies are still the sound grounds for the many current applications of Functional Electrical Stimulation and for the related research activities that are still proposed and funded.

A novel triple immunoenzyme staining enables simultaneous identification of all muscle fiber types on a single skeletal muscle cryosection from normal, denervated or reinnervated rats

Triple immunofluorescence staining has recently been developed to simultaneously identify all muscle fibers on a single cryosection which is helpful for clinical and basic research, but it has disadvantages such as fast photobleaching and unclear outlines of muscle fibers. Triple immunoenzyme staining (TIE) is likely to avoid these disadvantages. In this study, we aimed to establish a sensitive and specific TIE technique to identify fiber types in normal, denervated, and reinnervated rat muscles, and to develop a systematic sampling method for muscle fiber quantification. Tibialis anterior and soleus from normal, denervated, and reinnervated Lewis rat hind limbs were used. Five consecutive cryosections were cut from each muscle, including one for TIE and four for single immunoenzyme staining (SIE). The TIE was performed using the polymerized reporter enzyme staining system for the first two antigens (A4.74 for MyHC-IIA, BA-F8 for MyHC-I) and alkaline phosphatase staining system for the third antigen (BF-F3 for MyHC-IIB), followed by corresponding detective systems and respective chromogens. The type of muscle fibers was quantified by systematic sampling at 12.5%, 25%, 33% and 50% of all muscle fibers, and was compared with that acquired from counting all the fibers (100%). All muscle fiber phenotypes, including pure and hybrid, could be simultaneously identified on a single TIE cryosection with clear outlines. The fiber types on TIE slides matched well with their respective counterpart on the consecutive SIE slides with a 95% match rate. Systematic sampling of 12.5% fibers could represent the true fiber type distribution of the entire muscle section. Our results suggest that novel TIE can effectively visualize fiber types in normal, denervated or reinnervated rat muscles.

Morphology and Physiology of Masticatory Muscle Motor Units

Motor unit territories in masticatory muscles appear to be smaller than territories in limb muscles, and this would suggest a more localized organization of motor control in masticatory muscles. Motor unit cross-sectional areas show a wide range of values, which explains the large variability of motor unit force output. The proportion of motor unit muscle fibers containing more than one myosin heavy-chain (MHC) isoform is considerably larger in masticatory muscles than in limb and trunk muscles. This explains the continuous range of contraction speeds found in masticatory muscle motor units. Hence, in masticatory muscles, a finer gradation of force and contraction speeds is possible than in limb and in trunk muscles. The proportion of slow-type motor units is relatively large in deep and anterior masticatory muscle regions, whereas more fast-type units are more common in the superficial and posterior muscle regions. Muscle portions with a high proportion of slow-type motor units are better equipped for a finer control of muscle force and a larger resistance to fatigue during chewing and biting than muscle portions with a high proportion of fast units. For the force modulation, masticatory muscles rely mostly on recruitment gradation at low force levels and on rate gradation at high force levels. Henneman's principle of an orderly recruitment of motor units has also been reported for various masticatory muscles. The presence of localized motor unit territories and task-specific motor unit activity facilitates differential control of separate muscle portions. This gives the masticatory muscles the capacity of producing a large diversity of mechanical actions. In this review, the properties of masticatory muscle motor units are discussed.

Loss of JAM-C leads to impaired esophageal innervations and megaesophagus in mice

Megaesophagus is a disease where peristalsis fails to occur properly and esophagus is enlarged. The etiology and mechanism of megaesophagus are not well understood. In this study, we reported that junctional adhesion molecule C (JAM-C) knockout mice on a C57/B6 background developed progressive megaesophagus from embryonic day (E) 15.5 onward with complete penetrance. JAM-C knockout mice exhibited a significant reduction in the number of nerve fibers/ganglia in the wall of the esophagus. However, histological analysis revealed that the esophageal wall thickness and structure of JAM-C knockout mice at embryonic stages and young adult were comparable to that of control littermates. Thus, megaesophagus observed in JAM-C knockout mice could be attributed, at least in part, to impaired esophageal innervations. Our data suggest JAM-C as a potential candidate gene for human megaesophagus, and JAM-C knockout mice might serve as a model for the study of human megaesophagus.

Influence of Botulinumtoxin A on the Expression of Adult MyHC Isoforms in the Masticatory Muscles in Dystrophin-Deficient Mice (Mdx-Mice)

The most widespread animal model to investigate Duchenne muscular dystrophy is the mdx-mouse. In contrast to humans, phases of muscle degeneration are replaced by regeneration processes; hence there is only a restricted time slot for research. The aim of the study was to investigate if an intramuscular injection of BTX-A is able to break down muscle regeneration and has direct implications on the gene expression of myosin heavy chains in the corresponding treated and untreated muscles. Therefore, paralysis of the right masseter muscle was induced in adult healthy and dystrophic mice by a specific intramuscular injection of BTX-A. After 21 days the mRNA expression and protein content of MyHC isoforms of the right and left masseter, temporal, and the tongue muscle were determined using quantitative RT-PCR and Western blot technique. MyHC-IIa and MyHC-I-mRNA expression significantly increased in the paralyzed masseter muscle of control-mice, whereas MyHC-IIb and MyHC-IIx/d-mRNA were decreased. In dystrophic muscles no effect of BTX-A could be detected at the level of MyHC. This study suggests that BTX-A injection is a suitable method to simulate DMD-pathogenesis in healthy mice but further investigations are necessary to fully analyse the BTX-A effect and to generate sustained muscular atrophy in mdx-mice.

Mesenchymal Stem Cell Treatment of Intervertebral Disc Lesion Prevents Fatty Infiltration and Fibrosis of the Multifidus Muscle, but not Cytokine and Muscle Fiber Changes

STUDY DESIGN

Longitudinal case-control animal model.

OBJECTIVE

To investigate effects of mesenchymal stem cell (MSC) treatment on multifidus muscle remodeling after intervertebral disc (IVD) lesion.

SUMMARY OF BACKGROUND DATA

Lesion and degeneration of IVDs cause structural remodeling of the multifidus muscle. Proinflammatory cytokines are thought to contribute. MSC treatment restores IVD health after lesion but its effects on surrounding tissues remains unknown. Using an animal model of IVD degeneration, we assessed the effects of MSC treatment of IVDs on the structural remodeling and cytokine expression within the multifidus muscle.

METHODS

An anterolateral lesion was performed on the L1-2, L3-4, and L5-6 IVDs in sheep. At either 4 (early treatment) or 12 (late treatment) weeks after IVD lesion, MSCs were injected into the lesioned IVD. Multifidus muscle was harvested from L2 (gene expression analysis) and L4 (histological analysis) at 3 or 6 months after IVD lesion and naïve controls for histological analysis of muscle, adipose, and connective tissue cross-sectional areas, and immunohistochemistry to study muscle fiber types. Real-time polymerase chain reactions quantified expression of tumor necrosis factor, interleukin-1β, and transforming growth factor-β1.

RESULTS

MSC treatment of IVD lesion prevented the increased adipose and connective tissue cross-sectional area expected after IVD lesion. MSC treatment did not prevent slow-to-fast muscle fiber type transformation. Gene expression of proinflammatory cytokines within the muscle was altered by the MSC treatment of IVD. Increased interleukin-1β expression was prevented in the early treatment group and tumor necrosis factor and transforming growth factor-β1 expression was upregulated at 6 months.

CONCLUSION

Results show that although MSC treatment prevents fatty infiltration and fibrosis of the multifidus muscle after IVD lesion, it cannot prevent a muscle inflammatory response and muscle fiber transformation. These findings highlight the potential role of MSC therapy after IVD injury, but reveals that other interventions may also be necessary to optimize recovery of muscle.

LEVEL OF EVIDENCE

4.

Understanding the Role of ECM Protein Composition and Geometric Micropatterning for Engineering Human Skeletal Muscle

Skeletal muscle lost through trauma or disease has proven difficult to regenerate due to the challenge of differentiating human myoblasts into aligned, contractile tissue. To address this, we investigated microenvironmental cues that drive myoblast differentiation into aligned myotubes for potential applications in skeletal muscle repair, organ-on-chip disease models and actuators for soft robotics. We used a 2D in vitro system to systematically evaluate the role of extracellular matrix (ECM) protein composition and geometric patterning for controlling the formation of highly aligned myotubes. Specifically, we analyzed myotubes differentiated from murine C2C12 cells and human skeletal muscle derived cells (SkMDCs) on micropatterned lines of laminin compared to fibronectin, collagen type I, and collagen type IV. Results showed that laminin supported significantly greater myotube formation from both cells types, resulting in greater than twofold increase in myotube area on these surfaces compared to the other ECM proteins. Species specific differences revealed that human SkMDCs uniaxially aligned over a wide range of micropatterned line dimensions, while C2C12s required specific line widths and spacings to do the same. Future work will incorporate these results to engineer aligned human skeletal muscle tissue in 2D for in vitro applications in disease modeling, drug discovery and toxicity screening.

3D-printed wearable backpack stimulator for chronic in vivo aquatic stimulation

The neural mechanisms underlying changes in gene expression in the interconversion between skeletal muscle and the non-contractile electrogenic cells of the electric organ in electric fishes require several days to be manifested. It is extremely challenging to study these non-immediate forms of plasticity in reduced preparations in cell culture due to the time requirements. To address this experimental obstacle we developed a 3D-printed wearable backpack that allows chronic electrical stimulation of aquatic teleost fish. The backpack holds a biphasic simulator using a full H-bridge driver structure. Stimulation amplitude is adjusted with a current source controlled by a micro potentiometer whereas the stimulation waveform is reconfigurable through a micro-controller. A 3.7 V Lithium Ion Polymer battery powers the entire circuit. This backpack system will allow underwater chronic stimulation experiments aimed to study the role that neuronal input exerts on cell phenotypes in a vertebrate species with high tissue regeneration and cell trans-differentiation capabilities.

Ephrin-A3 promotes and maintains slow muscle fiber identity during postnatal development and reinnervation

Each adult mammalian skeletal muscle has a unique complement of fast and slow myofibers, reflecting patterns established during development and reinforced via their innervation by fast and slow motor neurons. Existing data support a model of postnatal "matching" whereby predetermined myofiber type identity promotes pruning of inappropriate motor axons, but no molecular mechanism has yet been identified. We present evidence that fiber type-specific repulsive interactions inhibit innervation of slow myofibers by fast motor axons during both postnatal maturation of the neuromuscular junction and myofiber reinnervation after injury. The repulsive guidance ligand ephrin-A3 is expressed only on slow myofibers, whereas its candidate receptor, EphA8, localizes exclusively to fast motor endplates. Adult mice lacking ephrin-A3 have dramatically fewer slow myofibers in fast and mixed muscles, and misexpression of ephrin-A3 on fast myofibers followed by denervation/reinnervation promotes their respecification to a slow phenotype. We therefore conclude that Eph/ephrin interactions guide the fiber type specificity of neuromuscular interactions during development and adult life.

Multifidus Muscle Changes After Back Injury Are Characterized by Structural Remodeling of Muscle, Adipose and Connective Tissue, but Not Muscle Atrophy: Molecular and Morphological Evidence

STUDY DESIGN

Longitudinal case-controlled animal study.

OBJECTIVE

To investigate putative cellular mechanisms to explain structural changes in muscle and adipose and connective tissues of the back muscles after intervertebral disc (IVD) injury.

SUMMARY OF BACKGROUND DATA

Structural back muscle changes are ubiquitous with back pain/injury and considered relevant for outcome, but their exact nature, time course, and cellular mechanisms remain elusive. We used an animal model that produces phenotypic back muscle changes after IVD injury to study these issues at the cellular/molecular level.

METHODS

Multifidus muscle was harvested from both sides of the spine at L1-L2 and L3-L4 IVDs in 27 castrated male sheep at 3 (n = 10) or 6 (n = 17) months after a surgical anterolateral IVD injury at both levels. Ten control sheep underwent no surgery (3 mo, n = 4; 6 mo, n = 6). Tissue was harvested at L4 for histological analysis of cross-sectional area of muscle and adipose and connective tissue (whole muscle), plus immunohistochemistry to identify proportion and cross-sectional area of individual muscle fiber types in the deepest fascicle. Quantitative polymerase chain reaction measured gene expression of typical cytokines/signaling molecules at L2.

RESULTS

Contrary to predictions, there was no multifidus muscle atrophy (whole muscle or individual fiber). There was increased adipose and connective tissue (fibrotic proliferation) cross-sectional area and slow-to-fast muscle fiber transition at 6 but not 3 months. Within the multifidus muscle, increases in the expression of several cytokines (tumor necrosis factor α and interleukin-1β) and molecules that signal trophic/atrophic processes for the 3 tissue types (e.g., growth factor pathway [IGF-1, PI3k, Akt1, mTOR], potent tissue modifiers [calcineurin, PCG-1α, and myostatin]) were present.

CONCLUSION

This study provides cellular evidence that refutes the presence of multifidus muscle atrophy accompanying IVD degeneration at this intermediate time point. Instead, adipose/connective tissue increased in parallel with the expression of the genes that provide putative mechanisms for multifidus structural remodeling. This provides novel targets for pharmacological and physical interventions.

LEVEL OF EVIDENCE

N/A.

Unilateral muscle overuse causes bilateral changes in muscle fiber composition and vascular supply

Unilateral strength training can cause cross-transfer strength effects to the homologous contralateral muscles. However, the impact of the cross-over effects on the muscle tissue is unclear. To test the hypothesis that unilateral muscle overuse causes bilateral alterations in muscle fiber composition and vascular supply, we have used an experimental rabbit model with unilateral unloaded overstrain exercise via electrical muscle stimulation (E/EMS). The soleus (SOL) and gastrocnemius (GA) muscles of both exercised (E) and contralateral non-exercised (NE) legs (n = 24) were morphologically analyzed after 1 w, 3 w and 6 w of EMS. Non-exercised rabbits served as controls (n = 6). After unilateral intervention the muscles of both E and NE legs showed myositis and structural and molecular tissue changes that to various degrees mirrored each other. The fiber area was bilaterally smaller than in controls after 3 w of E/EMS in both SOL (E 4420 and NE 4333 µm2 vs. 5183 µm2, p<0.05) and GA (E 3572 and NE 2983 µm2 vs. 4697 µm2, p<0.02) muscles. After 6 w of E/EMS, the percentage of slow MyHCI fibers was lower than in controls in the NE legs of SOL (88.1% vs. 98.1%, p<0.009), while the percentage of fast MyHCIIa fibers was higher in the NE legs of GA (25.7% vs. 15.8%, p = 0.02). The number of capillaries around fibers in the E and NE legs was lower (SOL 13% and 15%, respectively, GA 25% and 23%, respectively, p<0.05) than in controls. The overall alterations were more marked in the fast GA muscle than in the slow SOL muscle, which on the other hand showed more histopathological muscle changes. We conclude that unilateral repetitive unloaded overuse exercise via EMS causes myositis and muscle changes in fiber type proportions, fiber area and fiber capillarization not only in the exercised leg, but also in the homologous muscles in the non-exercised leg.

Key changes in denervated muscles and their impact on regeneration and reinnervation

The neuromuscular junction becomes progressively less receptive to regenerating axons if nerve repair is delayed for a long period of time. It is difficult to ascertain the denervated muscle's residual receptivity by time alone. Other sensitive markers that closely correlate with the extent of denervation should be found. After a denervated muscle develops a fibrillation potential, muscle fiber conduction velocity, muscle fiber diameter, muscle wet weight, and maximal isometric force all decrease; remodeling increases neuromuscular junction fragmentation and plantar area, and expression of myogenesis-related genes is initially up-regulated and then down-regulated. All these changes correlate with both the time course and degree of denervation. The nature and time course of these denervation changes in muscle are reviewed from the literature to explore their roles in assessing both the degree of detrimental changes and the potential success of a nerve repair. Fibrillation potential amplitude, muscle fiber conduction velocity, muscle fiber diameter, mRNA expression levels of myogenic regulatory factors and nicotinic acetylcholine receptor could all reflect the severity and length of denervation and the receptiveness of denervated muscle to regenerating axons, which could possibly offer an important clue for surgical choices and predict the outcomes of delayed nerve repair.

Muscle disuse alters skeletal muscle contractile function at the molecular and cellular levels in older adult humans in a sex-specific manner

Physical inactivity that accompanies ageing and disease may hasten disability by reducing skeletal muscle contractility. To characterize skeletal muscle functional adaptations to muscle disuse, we compared contractile performance at the molecular, cellular and whole‐muscle levels in healthy active older men and women (n = 15) and inactive older men and women with advanced‐stage, symptomatic knee osteoarthritis (OA) (n = 16). OA patients showed reduced (P < 0.01) knee extensor function. At the cellular level, single muscle fibre force production was reduced in OA patients in myosin heavy chain (MHC) I and IIA fibres (both P < 0.05) and differences in IIA fibres persisted after adjustments for fibre cross‐sectional area (P < 0.05). Although no group differences in contractile velocity or power output were found for any fibre type, sex was found to modify the effect of OA, with a reduction in MHC IIA power output and a trend towards reduced shortening velocity in women, but increases in both variables in men (P < 0.05 and P = 0.07, respectively). At the molecular level, these adaptations in MHC IIA fibre function were explained by sex‐specific differences (P ≤ 0.05) in myosin–actin cross‐bridge kinetics. Additionally, cross‐bridge kinetics were slowed in MHC I fibres in OA patients (P < 0.01), attributable entirely to reductions in women with knee OA (P < 0.05), a phenotype that could be reproduced in vitro by chemical modification of protein thiol residues. Our results identify molecular and cellular functional adaptations in skeletal muscle that may contribute to reduced physical function with knee OA‐associated muscle disuse, with sex‐specific differences that may explain a greater disposition towards disability in women.

Fibre-specific responses to endurance and low volume high intensity interval training: striking similarities in acute and chronic adaptation

The current study involved the completion of two distinct experiments. Experiment 1 compared fibre specific and whole muscle responses to acute bouts of either low-volume high-intensity interval training (LV-HIT) or moderate-intensity continuous endurance exercise (END) in a randomized crossover design. Experiment 2 examined the impact of a six-week training intervention (END or LV-HIT; 4 days/week), on whole body and skeletal muscle fibre specific markers of aerobic and anaerobic capacity. Six recreationally active men (Age: 20.7±3.8 yrs; VO₂peak: 51.9±5.1 mL/kg/min) reported to the lab on two separate occasions for experiment 1. Following a muscle biopsy taken in a fasted state, participants completed an acute bout of each exercise protocol (LV-HIT: 8, 20-second intervals at ∼170% of VO₂peak separated by 10 seconds of rest; END: 30 minutes at ∼65% of VO₂peak), immediately followed by a muscle biopsy. Glycogen content of type I and IIA fibres was significantly (p<0.05) reduced, while p-ACC was significantly increased (p<0.05) following both protocols. Nineteen recreationally active males (n = 16) and females (n = 3) were VO₂peak-matched and assigned to either the LV-HIT (n = 10; 21±2 yrs) or END (n = 9; 20.7±3.8 yrs) group for experiment 2. After 6 weeks, both training protocols induced comparable increases in aerobic capacity (END: Pre: 48.3±6.0, Mid: 51.8±6.0, Post: 55.0±6.3 mL/kg/min LV-HIT: Pre: 47.9±8.1, Mid: 50.4±7.4, Post: 54.7±7.6 mL/kg/min), fibre-type specific oxidative and glycolytic capacity, glycogen and IMTG stores, and whole-muscle capillary density. Interestingly, only LV-HIT induced greater improvements in anaerobic performance and estimated whole-muscle glycolytic capacity. These results suggest that 30 minutes of END exercise at ∼65% VO₂peak or 4 minutes of LV-HIT at ∼170% VO₂peak induce comparable changes in the intra-myocellular environment (glycogen content and signaling activation); correspondingly, training-induced adaptations resulting for these protocols, and other HIT and END protocols are strikingly similar.

Engineered skeletal muscle tissue for soft robotics: fabrication strategies, current applications, and future challenges

Skeletal muscle is a scalable actuator system used throughout nature from the millimeter to meter length scales and over a wide range of frequencies and force regimes. This adaptability has spurred interest in using engineered skeletal muscle to power soft robotics devices and in biotechnology and medical applications. However, the challenges to doing this are similar to those facing the tissue engineering and regenerative medicine fields; specifically, how do we translate our understanding of myogenesis in vivo to the engineering of muscle constructs in vitro to achieve functional integration with devices. To do this researchers are developing a number of ways to engineer the cellular microenvironment to guide skeletal muscle tissue formation. This includes understanding the role of substrate stiffness and the mechanical environment, engineering the spatial organization of biochemical and physical cues to guide muscle alignment, and developing bioreactors for mechanical and electrical conditioning. Examples of engineered skeletal muscle that can potentially be used in soft robotics include 2D cantilever-based skeletal muscle actuators and 3D skeletal muscle tissues engineered using scaffolds or directed self-organization. Integration into devices has led to basic muscle-powered devices such as grippers and pumps as well as more sophisticated muscle-powered soft robots that walk and swim. Looking forward, current, and future challenges include identifying the best source of muscle precursor cells to expand and differentiate into myotubes, replacing cardiomyocytes with skeletal muscle tissue as the bio-actuator of choice for soft robots, and vascularization and innervation to enable control and nourishment of larger muscle tissue constructs.

Early postnatal motor experience shapes the motor properties of C57BL/6J adult mice

This study aimed to evaluate the long-term consequences of early motor training on the muscle phenotype and motor output of middle-aged C57BL/6J mice. Neonatal mice were subjected to a variety of motor training procedures, for 3 weeks during the period of acquisition of locomotion. These procedures are widely used for motor training in adults; they include enriched environment, forced treadmill, chronic centrifugation, and hindlimb suspension. At 9 months, the mice reared in the enriched environment showed a slower type of fibre in slow muscles and a faster type in fast muscles, improved performance in motor tests, and a modified gait and body posture while walking. The proportion of fibres in the postural muscles of centrifuged mice did not change, but these mice showed improved resistance to fatigue. The suspended mice showed increased persistence of immature hybrid fibres in the tibialis, with a slower shift in the load-bearing soleus, without any behavioural changes. The forced treadmill was very stressful for the mice, but had limited effects on motor output, although a slower profile was observed in the tibialis. These results support the hypothesis that motor experience during a critical period of motor development shapes muscle phenotype and motor output. The different impacts of the various training procedures suggest that motor performance in adults can be optimized by appropriate training during a defined period of motor development.

Muscles within muscles: a tensiomyographic and histochemical analysis of the normal human vastus medialis longus and vastus medialis obliquus muscles

The aim of this study was to show the connection between structure (anatomical and histochemical) and function (muscle contraction properties) of vastus medialis obliquus (VMO) and vastus medialis longus (VML). The non-invasive tensiomyography (TMG) method was used to determine the contractile properties (contraction time; T(c)) of VML and VMO muscle, as a reflection of the ratio between the slow and fast fibers in two groups of nine young men. VML and VMO significantly (P < 0.01) differ in the proportion of type 1 (59.6: 44%) and type 2b (6.3: 15%) fibers. The VML muscle is almost entirely composed of type 1 and type 2a fibers. In many samples of this muscle no type 2b fibers were found. The proportion of slow-twitch type 1 fibers is nearly twice as high as the proportion of fast-twitch type 2a fibers. These observations indicate that VML is a slower and more fatigue-resistant muscle than VMO muscle. These characteristics correspond to the different functions of the VML, which is an extensor of the knee, and to the VMO, which maintains the stable position of the patella in the femoral groove. Our results obtained by TMG provided additional evidence that muscle fibers within the segments of VM muscle were not homogenous with regard to their contractile properties, thereby confirming the histochemical results. T(c) can be attributed to the higher percentage of slow-twitch fibers - type 1. The statistically shorter T(c) (P ≤ 0.001) of VMO (22.8 ± 4.0 ms) compared with VML (26.7 ± 4.0 ms) in our study is consistent with previously found differences in histochemical, morphological and electrophysiological data. In conclusion, the results of this study provide evidence that the VML and VMO muscles are not only anatomically and histochemically different muscles, but also functionally different biological structures.

Cav1.1 controls frequency-dependent events regulating adult skeletal muscle plasticity

An important pending question in neuromuscular biology is how skeletal muscle cells decipher the stimulation pattern coming from motoneurons to define their phenotype as slow or fast twitch muscle fibers. We have previously shown that voltage-gated L-type calcium channel (Cav1.1) acts as a voltage sensor for activation of inositol (1,4,5)-trisphosphate [Ins(1,4,5)P₃]-dependent Ca(2+) signals that regulates gene expression. ATP released by muscle cells after electrical stimulation through pannexin-1 channels plays a key role in this process. We show now that stimulation frequency determines both ATP release and Ins(1,4,5)P₃ production in adult skeletal muscle and that Cav1.1 and pannexin-1 colocalize in the transverse tubules. Both ATP release and increased Ins(1,4,5)P₃ was seen in flexor digitorum brevis fibers stimulated with 270 pulses at 20 Hz, but not at 90 Hz. 20 Hz stimulation induced transcriptional changes related to fast-to-slow muscle fiber phenotype transition that required ATP release. Addition of 30 µM ATP to fibers induced the same transcriptional changes observed after 20 Hz stimulation. Myotubes lacking the Cav1.1-α1 subunit released almost no ATP after electrical stimulation, showing that Cav1.1 has a central role in this process. In adult muscle fibers, ATP release and the transcriptional changes produced by 20 Hz stimulation were blocked by both the Cav1.1 antagonist nifedipine (25 µM) and by the Cav1.1 agonist (-)S-BayK 8644 (10 µM). We propose a new role for Cav1.1, independent of its calcium channel activity, in the activation of signaling pathways allowing muscle fibers to decipher the frequency of electrical stimulation and to activate specific transcriptional programs that define their phenotype.

Tongue anatomy and physiology, the scientific basis for a novel targeted neurostimulation system designed for the treatment of obstructive sleep apnea

INTRODUCTION

 Obstructive sleep apnea (OSA) is a chronic condition that affects millions adults. The effective standard treatment is positive airway pressure (PAP). However, approximately half of the patients that are prescribed PAP are unable or unwilling to comply with this therapy. Untreated OSA ultimately leads to very serious comorbidities. An alternative therapy for this patient population, therefore, is desirable. Hypoglossal nerve (HGN) stimulation is under investigation by multiple groups as a possible alternative therapy for OSA.

OBJECTIVE

To understand the underlying mechanisms of actions related to HGN stimulation, and the implication of this knowledge for specifying and designing a neurostimulation system for the treatment of OSA.

RESULTS

 Loss of lingual and pharyngeal tone within a narrow airway is the primary mechanism for OSA. Posterior and anterior tongues are different in their anatomy and physiology. Muscle fibers in the posterior tongue are predominantly fatigue resistant that are responsible for the long sustained tonic activities required for maintaining the tongue's position and preventing its mass from falling into the retroglossal airway. The human tongue is a muscular hydrostat and hence would benefit from a sophisticated HGN stimulation system that is capable of achieving a concerted spatio-temporal interplay of multiple lingual muscles, including retrusors.

CONCLUSION

 Targeted neurostimulation of the proximal HGN presents as a viable system approach that is far more versatile and physiologic and quite different than prior systems.

Characteristics of muscle fibers in rats with limb movements during sleep after spinal cord injury

BACKGROUND/AIMS

Previous studies have demonstrated that spinal cord injury (SCI) results in changes in sleep patterns through increased arousals and limb movements during sleep. Dramatic changes in muscle myosin heavy-chain isoforms have also been reported. The aim of this study was to investigate the characteristics of muscle fibers after SCI in rats with limb movements during sleep.

METHODS

Forty male Wistar rats were divided into four groups: SHAM, SCI 3, 7 and 15 days. Animals were subjected to electrode insertion surgery, 24-hour baseline sleep recording, SCI, and subsequent sleep recording for 3, 7, or 15 consecutive days. In addition, the gastrocnemius muscle and spinal cord were collected for histopathological/histochemical analyses.

RESULTS

Our results indicate a rapid and progressive decrease in the cross-sectional area of type I fibers in the gastrocnemius muscle (35.76-24.74 μm(2)) after SCI. Additionally, we found SCI-induced changes in sleep patterns. Following SCI, we also observed limb movements in sleeping rats, as well as significant negative moderate correlations between type I fibers and limb movement.

CONCLUSION

Our study strengthened the hypothesis by correlation between changes in types of muscle fiber (decline in type I fibers) and an increase in limb movements during sleep after SCI.

Changes in muscle composition during the development of diving ability in the Australian fur seal

During development the Australian fur seal transitions from a terrestrial, maternally dependent pup to an adult marine predator. Adult seals have adaptations that allow them to voluntarily dive at depth for long periods, including increased bradycardic control, increased myoglobin levels and haematocrit. To establish whether the profile of skeletal muscle also changes in line with the development of diving ability, biopsy samples were collected from the trapezius muscle of pups, juveniles and adults. The proportions of different fibre types and their oxidative capacity were determined. Only oxidative fibre types (Type I and IIa) were identified, with a significant change in proportions from pup to adult. There was no change in oxidative capacity of Type I and IIa fibres between pups and juveniles but there was a two-fold increase between juveniles and adults. Myoglobin expression increased between pups and juveniles, suggesting improved oxygen delivery, but with no increase in oxidative capacity, oxygen utilisation within the muscle may still be limited. Adult muscle had the highest oxidative capacity, suggesting that fibres are able to effectively utilise available oxygen during prolonged dives. Elevated levels of total creatine in the muscles of juveniles may act as an energy buffer when fibres are transitioning from a fast to slow fibre type.