Intramuscular force transmission

SERCA2a overexpression improves muscle function in a canine Duchenne muscular dystrophy model

Excessive cytosolic calcium accumulation contributes to muscle degeneration in Duchenne muscular dystrophy (DMD). Sarco/endoplasmic reticulum calcium ATPase (SERCA) is a sarcoplasmic reticulum (SR) calcium pump that actively transports calcium from the cytosol into the SR. We previously showed that adeno-associated virus (AAV)-mediated SERCA2a therapy reduced cytosolic calcium overload and improved muscle and heart function in the murine DMD model. Here, we tested whether AAV SERCA2a therapy could ameliorate muscle disease in the canine DMD model. 7.83 × 1013 vector genome particles of the AAV vector were injected into the extensor carpi ulnaris (ECU) muscles of four juvenile affected dogs. Contralateral ECU muscles received excipient. Three months later, we observed widespread transgene expression and significantly increased SERCA2a levels in the AAV-injected muscles. Treatment improved SR calcium uptake, significantly reduced calpain activity, significantly improved contractile kinetics, and significantly enhanced resistance to eccentric contraction-induced force loss. Nonetheless, muscle histology was not improved. To evaluate the safety of AAV SERCA2a therapy, we delivered the vector to the ECU muscle of adult normal dogs. We achieved strong transgene expression without altering muscle histology and function. Our results suggest that AAV SERCA2a therapy has the potential to improve muscle performance in a dystrophic large mammal.

Physiological Assessment of Muscle, Heart, and Whole Body Function in the Canine Model of Duchenne Muscular Dystrophy

Duchenne muscular dystrophy (DMD) is a lethal muscle disease caused by dystrophin deficiency. Patients gradually lose motor function, become wheelchair-bound, and die from respiratory and/or cardiac muscle failure. Dystrophin-null dogs have been used as a large animal model for DMD since 1988 and are considered an excellent bridge between rodent models and human patients. While numerous protocols have been published for studying muscle and heart physiology in mice, few such protocols exist for studying skeletal muscle contractility, heart function, and whole-body activity in dogs. Over the last 20 years, we have developed and adapted an array of assays to evaluate whole-body movement, gait, single muscle force, whole limb torque, cardiac electrophysiology, and hemodynamic function in normal and dystrophic dogs. In this chapter, we present detailed working protocols for these assays and lessons we learned during the development and use of these protocols.

Extensor carpi ulnaris muscle shows unexpected slow-to-fast fiber type switch in Duchenne muscular dystrophy dogs

Aged dystrophin-null canines are excellent models for studying experimental therapies for Duchenne muscular dystrophy, a lethal muscle disease caused by dystrophin deficiency. To establish the baseline, we studied the extensor carpi ulnaris (ECU) muscle in 15 terminal age (3-year-old) male affected dogs and 15 age/sex-matched normal dogs. Affected dogs showed histological and anatomical hallmarks of dystrophy, including muscle inflammation and fibrosis, myofiber size variation and centralized myonuclei, as well as a significant reduction of muscle weight, muscle-to-body weight ratio and muscle cross-sectional area. To rigorously characterize the contractile properties of the ECU muscle, we developed a novel in situ assay. Twitch and tetanic force, contraction and relaxation rate, and resistance to eccentric contraction-induced force loss were significantly decreased in affected dogs. Intriguingly, the time-to-peak tension and half-relaxation time were significantly shortened in affected dogs. Contractile kinetics predicted an unforeseen slow-to-fast myofiber-type switch, which we confirmed at the protein and transcript level. Our study establishes a foundation for studying long-term and late-stage therapeutic interventions in dystrophic canines. The unexpected myofiber-type switch highlights the complexity of muscle remodeling in dystrophic large mammals.

This article has an associated First Person interview with the first author of the paper.

Summary: A slow-to-fast fiber-type switch in dystrophic canine ECU muscle is revealed by contraction kinetics and myosin protein and transcript expression. This highlights the complexity of muscle remodeling in Duchenne muscular dystrophy.

Spatial variation and inconsistency between estimates of onset of muscle activation from EMG and ultrasound

Delayed onset of muscle activation can be a descriptor of impaired motor control. Activation onset can be estimated from electromyography (EMG)-registered muscle excitation and from ultrasound-registered muscle motion, which enables non-invasive measurements in deep muscles. However, in voluntary activation, EMG- and ultrasound-detected activation onsets may not correspond. To evaluate this, ten healthy men performed isometric elbow flexion at 20% to 70% of their maximal force. Utilising a multi-channel electrode transparent to ultrasound, EMG and M(otion)-mode ultrasound were recorded simultaneously over the biceps brachii muscle. The time intervals between automated and visually estimated activation onsets were correlated with the regional variation of EMG and muscle motion onset, contraction level and speed. Automated and visual onsets indicated variable time intervals between EMG- and motion onset, median (interquartile range) 96 (121) ms and 48 (72) ms, respectively. In 17% (computed analysis) or 23% (visual analysis) of trials, motion onset was detected before local EMG onset. Multi-channel EMG and M-mode ultrasound revealed regional differences in activation onset, which decreased with higher contraction speed (Spearman ρ ≥ 0.45, P < 0.001). In voluntary activation the heterogeneous motor unit recruitment together with immediate motion transmission may explain the high variation of the time intervals between local EMG- and ultrasound-detected activation onset.

Investigation of neuromuscular abnormalities in neurotrophin-3-deficient mice

Neurotrophin-3 (NT-3) is a trophic factor that is essential for the normal development and maintenance of proprioceptive sensory neurons and is widely implicated as an important modulator of synaptic function and development. We have previously found that animals lacking NT-3 have a number of structural abnormalities in peripheral nerves and skeletal muscles. Here we investigated whether haploinsufficiency-induced reduction in NT-3 resulted in impaired neuromuscular performance and synaptic function. Motor nerve terminal function was tested by monitoring the uptake/release of the fluorescent membrane dye FM1-43 by the electrophysiological examination of synaptic transmission and electron microscopic determination of synaptic vesicle density at the presynaptic active zone. We investigated skeletal muscle form and function by measuring force in response to both nerve-mediated and direct muscle stimulation and by quantification of fiber number and area from transverse sections. Synaptic transmission was not markedly different between the two groups, although the uptake and release of FM1-43 were impaired in mature NT-3-deficient mice but not in immature mice. The electron microscopic examination of mature nerve terminals showed no genotype-dependent variation in the number of synaptic vesicles near the active zone. NT-3(+/-) mice had normal soleus muscle fiber numbers but their fibers had smaller cross-sectional areas and were more densely-packed than wild-type littermates. Moreover, the muscles of adult NT-3-deficient animals were weaker than those of wild-type animals to both nerve and direct muscle stimulation. The results indicate that a reduction in NT-3 availability during development impairs motor nerve terminal maturation and synaptic vesicle recycling and leads to a reduction in muscle fiber diameter.

Molecular and cellular adaptations to chronic myotendinous strain injury in mdx mice expressing a truncated dystrophin

Myotendinous strain injury is the most common injury of human skeletal muscles because the majority of muscle forces are transmitted through this region. Although the immediate response to strain injury is well characterized, the chronic response to myotendinous strain injury is less clear. Here we examined the molecular and cellular adaptations to chronic myotendinous strain injury in mdx mice expressing a microdystrophin transgene (microdystrophin(DeltaR4-R23)). We found that muscles with myotendinous strain injury had an increased expression of utrophin and alpha7-integrin together with the dramatic restructuring of peripheral myofibrils into concentric rings. The sarcolemma of the microdystrophin(DeltaR4-R23)/mdx gastrocnemius muscles was highly protected from experimental lengthening contractions, better than wild-type muscles. We also found a positive correlation between myotendinous strain injury and ringed fibers in the HSA(LR) (human skeletal actin, long repeat) mouse model of myotonic dystrophy. We suggest that changes in protein expression and the formation of rings are adaptations to myotendinous strain injury that help to prevent muscle necrosis and retain the function of necessary muscles during injury, ageing and disease.

In vivo behaviour of human muscle architecture and mechanomyographic response using the interpolated twitch technique

This study investigated the origin of curvilinear change in the superimposed mechanomyogram (MMG) amplitude of the human medial gastrocnemius muscle (MG) with increasing contraction intensity. The superimposed twitch amplitude, the superimposed MMG amplitude and the extent of fascicle shortening were measured using ultrasonic images of electrical stimulation during isometric plantar flexions at levels 20%, 40%, 60%, 80%, and 100% of the maximal voluntary contraction (MVC). The superimposed twitch amplitude, the superimposed MMG amplitude and the extent of fascicle shortening decreased with increasing contraction intensity. The superimposed MMG amplitude and the extent of fascicle shortening showed a curvilinear decrease, while the superimposed twitch amplitude showed a linear decrease at levels up to 80% of the MVC. There was a linear relationship between the superimposed MMG amplitude and the extent of fascicle shortening at different contraction intensities. These results indicate that the superimposed MMG amplitude reflects changes in the extent of fascicle shortening at different contraction intensities better than the superimposed twitch amplitude. Our study suggests that the origin of the curvilinear decrease of superimposed MMG amplitude is associated with a curvilinear decrease of the extent of fascicle shortening with increasing contraction intensity in the human MG.

Structural and functional features of human muscle-tendon unit

This paper reviews the architectural details and the in vivo behavior of the human muscle-tendon unit with the focus on the triceps surae and quadriceps femoris muscles. Recent advances in experimental techniques allow in vivo measurements of muscle-tendon architecture and function. In particular, the use of ultrasonography for measurement of tendon and muscle has expanded our knowledge in the last decade. Furthermore, the nuclear magnetic resonance imaging is opening up new insights not only for three-dimensional anatomical information but also for examining musculo-skeletal motion in vivo. While these two completely non-invasive methods provide kinematic data, in vivo force measurements still require somewhat invasive procedures and are scarce. Thus, muscle forces are frequently calculated using both simple and complex models. These models can give us suggestions for further experimental work. There is a need to examine the experimental data ranging from single-fiber experiments to the muscle function in human movement in order to understand the muscle-tendon function in vivo fully. Furthermore, appreciation of the structure-function relationships may help us to understand the entity of muscle-tendon function both from the perspective of mechanical behavior and neural control.

Strength at the extracellular matrix-muscle interface

Mechanical force is generated within skeletal muscle cells by contraction of specialized myofibrillar proteins. This paper explores how the contractile force generated at the sarcomeres within an individual muscle fiber is transferred through the connective tissue to move the bones. The initial key point for transfer of the contractile force is the muscle cell membrane (sarcolemma) where force is transferred laterally to the basement membrane (specialized extracellular matrix rich in laminins) to be integrated within the connective tissue (rich in collagens) before transmission to the tendons. Connections between (1) key molecules outside the myofiber in the basement membrane to (2) molecules within the sarcolemma of the myofiber and (3) the internal cytoplasmic structures of the cytoskeleton and sarcomeres are evaluated. Disturbances to many components of this complex interactive system adversely affect skeletal muscle strength and integrity, and can result in severe muscle diseases. The mechanical aspects of these crucial linkages are discussed, with particular reference to defects in laminin-alpha2 and integrin-alpha7. Novel interventions to potentially increase muscle strength and reduce myofiber damage are mentioned, and these are also highly relevant to muscle diseases and aging muscle.

Structural organization of the perimysium in bovine skeletal muscle: Junctional plates and associated intracellular subdomains

We analyzed the structural features of the perimysium collagen network in bovine Flexor carpi radialis muscle using various sample preparation methods and microscopy techniques. We first observed by scanning electron microscopy that perimysium formed a regular network of collagen fibers with three hierarchical levels including (i) a loose lattice of large interwoven fibers ramified in (ii) numerous collagen plexi attaching together adjacent myofibers at the level of (iii) specific structures that we call perimysial junctional plates. Second, we looked more closely at the intracellular organization underneath each plate using transmission electron microscopy, immunohistochemistry, and a three-dimensional reconstruction from serial sections. We observed the accumulation of myonuclei arranged in clusters surrounded by a high density of subsarcolemmal mitochondria and the proximity of capillary branches. Third, we analyzed the distribution of these perimysial junctional plates, subsarcolemmal mitochondria, and myonuclei clusters along the myofibers using a statistical analysis of the distances between these structures. This revealed a global colocalization and the existence of adhesion domains between endomysium and perimysium. Taken together, our observations give a better description of the perimysium organization in skeletal muscle, and provide evidence that perimysial junctional plates with associated intracellular subdomains may participate in the lateral transmission of contractile forces as well as mechanosensing.

High frequency oscillations in respiratory networks: functionally significant or phenomenological?

Inspiratory activities, whether recorded from medullary neurons, motoneurons or motor nerves, feature prominent oscillations in high (50-120 Hz) and medium (15-50 Hz) frequency ranges. These oscillations have been extensively characterized and are considered signatures of respiratory network activity. Their functional significance, however, if any, remains unknown. Here we review the literature describing the nature and origin of these oscillations as well as their modulation during development and by mechanoreceptive and chemoreceptive feedback, respiratory- and non-respiratory-related behaviors, temperature and anesthesia. We then consider the potential significance of these oscillations for respiratory network function by drawing on analyses of distributed motor and sensory networks of the cortex where current interest in oscillatory activity, and the synchronization of neural discharge that can result, is based on the increased efficacy with which synchronous inputs influence neuronal output, and the role that synchronous activity may play in information coding. We speculate that synchronized oscillations at the network level help coordinate activity in distributed rhythm and pattern generating systems and at the muscle level enhance force development. Data most strongly support that oscillatory synaptic inputs play an important role in controlling timing and pattern of action potential output.