Denervated frog skeletal muscle: calcium content and kinetics of exchange

Pathophysiological role of connexin and pannexin hemichannels in neuromuscular disorders

A growing body of research has provided evidence that de novo expression of connexin hemichannels and upregulation of pannexin hemichannels (Cx HCs and Panx HCs, respectively) in the cytoplasmic membrane of skeletal muscle (sarcolemma) are critical steps in the pathogenesis of muscle dysfunction of many genetic and acquired muscle diseases. This review provides an overview of the current understanding of the molecular mechanisms regulating the expression of Cx and Panx HCs in skeletal muscle, as well as their roles in both muscle physiology and pathologies. Additionally, it addresses existing gaps in knowledge and outlines future challenges in the field.

Advancing Nerve Regeneration: Translational Perspectives of Tacrolimus (FK506)

Peripheral nerve injuries have far-reaching implications for individuals and society, leading to functional impairments, prolonged rehabilitation, and substantial socioeconomic burdens. Tacrolimus, a potent immunosuppressive drug known for its neuroregenerative properties, has emerged in experimental studies as a promising candidate to accelerate nerve fiber regeneration. This review investigates the therapeutic potential of tacrolimus by exploring the postulated mechanisms of action in relation to biological barriers to nerve injury recovery. By mapping both the preclinical and clinical evidence, the benefits and drawbacks of systemic tacrolimus administration and novel delivery systems for localized tacrolimus delivery after nerve injury are elucidated. Through synthesizing the current evidence, identifying practical barriers for clinical translation, and discussing potential strategies to overcome the translational gap, this review provides insights into the translational perspectives of tacrolimus as an adjunct therapy for nerve regeneration.

Strategies to Improve Cross-Face Nerve Grafting in Facial Paralysis

Cross-face nerve grafting enables the reanimation of the contralateral hemiface in unilateral facial palsy and may recover a spontaneous smile. This chapter discusses various clinically applicable strategies to increase the chances for good functional outcomes by maintaining the viability of the neural pathway and target muscle, increasing the number of reinnervating nerve fibers and selecting functionally compatible donor nerve branches. Adopting those strategies may help to further improve patient outcomes in facial reanimation surgery.

Old and new biomarkers for volumetric muscle loss

Volumetric muscle loss (VML) impacts skeletal muscles and causes damage to associated tissues such as blood vessels and other structural tissues. Despite progress in the VML field, current preclinical approaches are often ineffective at restoring muscle volume. Additional research is paramount to develop strategies that improve muscle mass and function, while restoring supporting tissues. We highlight mechanisms that govern normal muscle function that are also key players for VML, including intracellular calcium signaling/homeostasis, mitochondria signaling (calcium, reactiove oxidative species (ROS)/oxidative stress), and angiogenesis. We propose an integration of these processes within the context of emerging biomaterials that provide structural support for muscle regeneration. We posit that new biomarkers (i.e. myokines and lipid signaling mediators) may serve as sentinels of early muscle injury and regeneration. We conclude that as new ideas, approaches, and models come together, new treatments will emerge to allow the full rebuilding of skeletal muscles and functional recovery of skeletal muscles after VML.

Cellular and molecular features of neurogenic skeletal muscle atrophy

Neurogenic atrophy refers to the loss of muscle mass and function that results directly from injury or disease of the peripheral nervous system. Individuals with neurogenic atrophy may experience reduced functional status and quality of life and, in some circumstances, reduced survival. Distinct pathological findings on muscle histology can aid in diagnosis of a neurogenic cause for muscle dysfunction, and provide indicators for the chronicity of denervation. Denervation induces pleiotypic responses in skeletal muscle, and the molecular mechanisms underlying neurogenic muscle atrophy appear to share common features with other causes of muscle atrophy, including activation of FOXO transcription factors and corresponding induction of ubiquitin-proteasomal and lysosomal degradation. In this review, we provide an overview of histologic features of neurogenic atrophy and a summary of current understanding of underlying mechanisms.

Active acetylcholine receptors prevent the atrophy of skeletal muscles and favor reinnervation

Denervation of skeletal muscles induces severe muscle atrophy, which is preceded by cellular alterations such as increased plasma membrane permeability, reduced resting membrane potential and accelerated protein catabolism. The factors that induce these changes remain unknown. Conversely, functional recovery following denervation depends on successful reinnervation. Here, we show that activation of nicotinic acetylcholine receptors (nAChRs) by quantal release of acetylcholine (ACh) from motoneurons is sufficient to prevent changes induced by denervation. Using in vitro assays, ACh and non-hydrolysable ACh analogs repressed the expression of connexin43 and connexin45 hemichannels, which promote muscle atrophy. In co-culture studies, connexin43/45 hemichannel knockout or knockdown increased innervation of muscle fibers by dorsal root ganglion neurons. Our results show that ACh released by motoneurons exerts a hitherto unknown function independent of myofiber contraction. nAChRs and connexin hemichannels are potential molecular targets for therapeutic intervention in a variety of pathological conditions with reduced synaptic neuromuscular transmission.

Denervation of muscle fibres induces muscle atrophy, via mechanisms that remain unclear. Here, the authors show that binding of acetylcoline to its receptor at the neuromuscular junction represses the expression of connexins 43 and 45, which promote atrophy, and is sufficient to prevent denervation-induced loss of myofibre mass.

Connexin hemichannels explain the ionic imbalance and lead to atrophy in denervated skeletal muscles

Denervated fast skeletal muscles undergo atrophy, which is associated with an increase in sarcolemma permeability and protein imbalance. However, the mechanisms responsible for these alterations remain largely unknown. Recently, a close association between de novo expression of hemichannels formed by connexins 43 and 45 and increase in sarcolemma permeability of denervated fast skeletal myofibers was demonstrated. However, it remains unknown whether these connexins cause the ionic imbalance of denervates fast myofibers. To elucidate the latter and the role of hemichannels formed by connexins (Cx HCs) in denervation-induced atrophy, skeletal myofibers deficient in Cx43 and Cx45 expression (Cx43^fl/flCx45^fl/fl:Myo-Cre mice) and control (Cx43^fl/flCx45^fl/fl mice) were denervated and several muscle features were systematically analyzed at different post-denervation (PD) times (1, 3, 5, 7 and 14days). The following sequence of events was found in denervated myofibers of Cx43^fl/flCx45^fl/fl mice: 1) from day 3 PD, increase in sarcolemmal permeability, 2) from day 5 PD, increases of intracellular Ca²⁺ and Na⁺ signals as well as a significant increase in protein synthesis and degradation, yielding a negative protein balance and 3) from day 7 PD, a fall in myofibers cross-section area. All the above alterations were either absent or drastically reduced in denervated myofibers of Cx43^fl/flCx45^fl/fl:Myo-Cre mice. Thus, the denervation-induced Cx HCs expression is an early event that precedes the electrochemical gradient dysregulation across the sarcolemma and critically contributes to the progression of skeletal muscle atrophy. Consequently, Cx HCs could be a therapeutic target to drastically prevent the denervation-induced atrophy of fast skeletal muscles.

Neuronal involvement in muscular atrophy

The innervation of skeletal myofibers exerts a crucial influence on the maintenance of muscle tone and normal operation. Consequently, denervated myofibers manifest atrophy, which is preceded by an increase in sarcolemma permeability. Recently, de novo expression of hemichannels (HCs) formed by connexins (Cxs) and other none selective channels, including P2X7 receptors (P2X7Rs), and transient receptor potential, sub-family V, member 2 (TRPV2) channels was demonstrated in denervated fast skeletal muscles. The denervation-induced atrophy was drastically reduced in denervated muscles deficient in Cxs 43 and 45. Nonetheless, the transduction mechanism by which the nerve represses the expression of the above mentioned non-selective channels remains unknown. The paracrine action of extracellular signaling molecules including ATP, neurotrophic factors (i.e., brain-derived neurotrophic factor (BDNF)), agrin/LDL receptor-related protein 4 (Lrp4)/muscle-specific receptor kinase (MuSK) and acetylcholine (Ach) are among the possible signals for repression for connexin expression. This review discusses the possible role of relevant factors in maintaining the normal functioning of fast skeletal muscles and suppression of connexin hemichannel expression.

Connexin- and pannexin-based channels in normal skeletal muscles and their possible role in muscle atrophy

Precursor cells of skeletal muscles express connexins 39, 43 and 45 and pannexin1. In these cells, most connexins form two types of membrane channels, gap junction channels and hemichannels, whereas pannexin1 forms only hemichannels. All these channels are low-resistance pathways permeable to ions and small molecules that coordinate developmental events. During late stages of skeletal muscle differentiation, myofibers become innervated and stop expressing connexins but still express pannexin1 hemichannels that are potential pathways for the ATP release required for potentiation of the contraction response. Adult injured muscles undergo regeneration, and connexins are reexpressed and form membrane channels. In vivo, connexin reexpression occurs in undifferentiated cells that form new myofibers, favoring the healing process of injured muscle. However, differentiated myofibers maintained in culture for 48 h or treated with proinflammatory cytokines for less than 3 h also reexpress connexins and only form functional hemichannels at the cell surface. We propose that opening of these hemichannels contributes to drastic changes in electrochemical gradients, including reduction of membrane potential, increases in intracellular free Ca(2+) concentration and release of diverse metabolites (e.g., NAD(+) and ATP) to the extracellular milieu, contributing to multiple metabolic and physiologic alterations that characterize muscles undergoing atrophy in several acquired and genetic human diseases. Consequently, inhibition of connexin hemichannels expressed by injured or denervated skeletal muscles might reduce or prevent deleterious changes triggered by conditions that promote muscle atrophy.

Succinylcholine-induced fasciculations in denervated rat muscles as measured using 31P-NMR spectroscopy: the effect of pretreatment with dantrolene or vecuronium

BACKGROUND

We have previously demonstrated by 31P nuclear magnetic resonance (NMR) that succinylcholine (SCh) induces metabolic changes in denervated muscle. To specify those changes, we attempted to inhibit them using two different kinds of drugs, dantrolene and vecuronium.

METHODS

Three weeks after unilateral sciatic nerve section, 75 male Wistar rats were randomly assigned to one of the following 5 groups: (1) non-pretreated normal muscle group; (2) non-pretreated denervated muscle group; (3) denervated muscle group pretreated with a low dose of vecuronium (0.02 mg.kg(-1)); (4) denervated muscle group pretreated with a high dose of vecuronium (0.2 mg.kg(-1)); (5) denervated muscle group pretreated with dantrolene (2 mg.kg(-1)). The change of the inorganic phosphate/phosphocreatine (Pi/PCr) ratio of each muscle was measured by 31P-NMR before and after SCh (1 mg.kg(-1)) administration and the corresponding peak amplitude of the electromyograms (EMG) was determined.

RESULTS

The high dose of vecuronium totally inhibited SCh-induced fasciculation on EMG (100%-->2%). In this group, though the Pi/PCr ratio significantly increased 10 min after SCh, the peak after 5 min disappeared. The inhibition with dantrolene was about the same order of magnitude as with the low dose of vecuronium (35%:21%). However, the increase in the Pi/PCr only lasted about 10 min, in contrast to the other drugs.

CONCLUSION

Our findings indicate that the Pi/PCr increases 5 and 10 min after SCh, respectively, as a result of two different processes. The first peak is caused by an excessive energy consumption in response to excessive muscle contraction. This in turn triggers the second peak, caused by breakdown of glycogen, initiated by an increased Ca2+ concentration.

Terminal cisternae of denervated rabbit skeletal muscle: alterations of functional properties of Ca2+ release channels

Terminal cisternae (TC) of skeletal muscle represent the specialized compartment from which Ca2+ is released into the myoplasm after a propagated action potential. In this study we have investigated the morphology, protein composition, and Ca2+ release properties of TC isolated from rabbit gastrocnemius muscle 2 wk after nerve sectioning. Thin-section electron microscopy showed that TC vesicles from denervated muscle were enriched in calsequestrin (CS) and contained a larger fraction of the junctional sarcoplasmic reticulum (SR), as judged by membrane profiles with morphologically intact feet structures. Accordingly, the yield of junctional SR from denervated muscle was twice that of control muscle, and the protein pattern of TC vesicles exhibited an increase in junctional protein components, e.g., CS and the 350-kDa protein. The larger content of the 350-kDa protein, or ryanodine receptor (F.A. Lai, H. Erickson, E. Rousseau, Q.-Y. Liu, and G. Meissner, Nature Lond. 331: 315-319, 1988; T. Imagawa, J. S. Smith, R. Coronado, and K. P. Campbell. J. Biol. Chem. 262: 10636-10643, 1987; L. Hymel, M. Inui, S. Fleischer, and H. Schindler, Proc. Natl. Acad. Sci. USA 85:441-445, 1988) was paralleled by an increased binding site density (Bmax) for ryanodine binding in denervated muscle TC. The effects of ruthenium red, a Ca2+ release blocker, on Ca2+ loading rate and Ca2+-ATPase activity suggested that TC from denervated muscle were less permeable to Ca2+. After active Ca2+ loading, both doxorubicin and caffeine induced Ca2+ release from isolated TC, yet Ca2+ release rates were reduced in denervated muscle TC.(ABSTRACT TRUNCATED AT 250 WORDS)

Halothane-caffeine contracture testing in neuromuscular diseases

The association of malignant hyperthermia (MH) with neuromuscular disorders has been recognized since 1970. These disorders include central core disease, Duchenne muscular dystrophy, myotonia congenita, myotonic dystrophy, nonspecific myopathies, and King-Denborough syndrome. In order to assess the anesthetic risk of MH in the neuromuscular population, we performed halothane and caffeine contracture testing for MH susceptibility on biopsied muscle removed from 25 consecutive neuromuscular patients during diagnostic evaluation. Positive contracture tests were found in 7 of 18 patients with myopathic disorders and 3 of 7 patients with neurogenic disorders. Two of our patients had anesthetic events suggesting MH. These findings suggest that myopathic and neuropathic disorders share pathogenic mechanisms with MH, resulting in positive contracture tests and possibly leading to clinical events during anesthesia. Although there is controversy regarding the interpretation of a positive contracture test, contracture testing remains the most widely accepted test for MH susceptibility. Thus, a variety of neuromuscular disorders may be associated with MH susceptibility, and caution should be exercised during anesthesia in this group of patients.

Denervated frog skeletal muscle. Some electrical and mechanical properties

The effects of denervation on several mechanical and electrical parameters of frog sartorius muscle have been investigated. In denervated muscles, there is no change in the resting potential and a relatively small change in the action potential. The first alteration in the action potential is a reduction of about 30% in the maximum rate of repolarization in muscles that have been denervated for 40 days or longer. Later, the overshoot and maximum rate of depolarization also decline. No tetrodotoxin resistant action potentials could be detected. Fibrillatory potentials were observed infrequently and in most cases in depolarized fibers. Twitch tension is significantly reduced by denervation while the tetanus tension is practically unaffected by denervation. The experiments suggest that the decline in twitch tension produced by denervation reflect a defect in some step of the excitation contraction coupling sequence. On the other hand, post-tetanic potentiation of the twitch is much larger in denervated than in control muscles. This potentiation in denervated muscles is paralleled by an increased action potential duration which returns to its pretetanic duration with a time course indistinguishable from that of the twitch potentiation.

Caffeine contractures in denervated frog muscle

Caffeine contracture tension, effect of caffeine on the resting membrane potential, and caffeine influx in normal and denervated frog sartorius muscle have been investigated. Peak caffeine contracture tension is increased after denervation at all caffeine concentrations. The percentage increases in tension are highest for lower caffeine concentrations. The caffeine concentration required for half maximum tension is decreased from about 3.6 mM in control muscles to 2.6 mM in denervated muscles. Caffeine at 3.5 mM produces a depolarization of about 6 mV in control muscles and 16mV in denervated muscles. The large contracture tensions observed in denervated muscles are not due to the greater depolarization produced by the drug in denervated muscles since innervated muscles depolarized to the same level by external K+ do not enhance caffeine contracture tension. Both control and denervated muscles are highly permeable to caffeine. The increases in sarcoplasmic reticulum development ( Moscatello et al. 1965) and calcium content ( Picken and Kirby 1976) promoted by denervation may explain the larger tension elicited by caffeine in denervated muscles.

Calcium binding capacity and calcium sensitive protein content in denervated frog muscles

Total and ionic calcium content, calcium binding capacity of sarcoplasmic proteins and calcium insensitive proteins were examined in atrophying leg muscles of frog after 1-5 months period of denervation. Different muscles showed different levels of atrophy and the total calcium content varied with reference to the type of muscle. Ionic calcium levels doubled in the gastrocnemius muscle after three months denervation. Calcium binding capacity of proteins and calcium insensitive proteins decreased rapidly up to four months after denervation in the gastrocnemius muscle. However no significant changes in the levels of calcium binding capacity and calcium insensitive proteins were found with reference to the type of muscle. Since total calcium content remains constant and wet muscle mass (expressed as atrophy) decreased markedly, an apparent increase in calcium concentration occurs in each muscle on denervation.

Alteration of sarcoplasmic reticulum after denervation of chicken pectoralis muscle

To determine the neural influence on the function of the sarcoplasmic reticulum (SR) of fast-twitch skeletal muscle, the superior pectoralis muscle of adult chicken was denervated, and the SR was isolated at 20 days post-denervation. The isolated SR was probably derived from the longitudinal SR and was relatively free of contaminants. The protein profile of the SR was quantitatively changed after denervation with an increase in the M55 and 30000-mol.wt. proteins relative to the Ca2+-ATPase. Ca2+-dependent ATPase activity and phosphoenzyme formation were lower in the denervated-muscle SR; however, the enzyme catalytic-centre activity was similar to the control value. The decrease in Ca2+-ATPase activity in denervated-muscle SR was accompanied by a lower Ca2+ accumulation so that the relationship between Ca2+ accumulation and Ca2+-dependent ATPase activity was well maintained in the SR from denervated muscle. The data imply that denervation may result in a diminution of functional Ca2+ pump sites. Evidence is presented, though, which suggests that denervation affects a single class of Ca2+-binding sites of the Ca2+-ATPase, resulting in a lower affinity for Ca2+.

Changes in thromboxane B2 and 6-keto-PGF1 alpha after direct electrical stimulation of the denervated rat gastrocnemius1,2

Radio-immunoassay was used to measure the amounts of Thromboxane B2 (TXB2) and 6-keto-PGF1 alpha produced in the gastrocnemius muscles of rats after denervation and direct electrical stimulation (2-4 mA intensity and 4 msec duration pulses given at 10 Hz continuously, 8 hours each day for 14 days). Compared to the normal control side, the normal-stimulated, the denervated control and the denervated-stimulated muscles, all indicated increases in the content (ng/g muscle) of TXB2 (120-155%, P less than 0.01) and 6-keto-PGF1 alpha (90-10 7%). This study suggests that electrical stimulation as well as denervation may lead to an elevation of these prostanoids.