TRP channels in skeletal muscle: gene expression, function and implications for disease

Is it time for genetic modifiers to predict prognosis in Duchenne muscular dystrophy?

Patients with Duchenne muscular dystrophy (DMD) show clinically relevant phenotypic variability, despite sharing the same primary biochemical defect (dystrophin deficiency). Factors contributing to this clinical variability include allelic heterogeneity (specific DMD mutations), genetic modifiers (trans-acting genetic polymorphisms) and variations in clinical care. Recently, a series of genetic modifiers have been identified, mostly involving genes and/or proteins that regulate inflammation and fibrosis - processes increasingly recognized as being causally linked with physical disability. This article reviews genetic modifier studies in DMD to date and discusses the effect of genetic modifiers on predicting disease trajectories (prognosis), clinical trial design and interpretation (inclusion of genotype-stratified subgroup analyses) and therapeutic approaches. The genetic modifiers identified to date underscore the importance of progressive fibrosis, downstream of dystrophin deficiency, in driving the disease process. As such, genetic modifiers have shown the importance of therapies aimed at slowing this fibrotic process and might point to key drug targets.

Polycystin-2 (PC2) is a key determinant of in vitro myogenesis

The development of skeletal muscle (myogenesis) is a well-orchestrated process where myoblasts withdraw from the cell cycle and differentiate into myotubes. Signaling by fluxes in intracellular calcium (Ca²⁺) is known to contribute to myogenesis, and increased mitochondrial biogenesis is required to meet the metabolic demand of mature myotubes. However, gaps remain in the understanding of how intracellular Ca²⁺ signals can govern myogenesis. Polycystin-2 (PC2 or TRPP1) is a nonselective cation channel permeable to Ca²⁺. It can interact with intracellular calcium channels to control Ca²⁺ release and concurrently modulates mitochondrial function and remodeling. Due to these features, we hypothesized that PC2 is a central protein in mediating both the intracellular Ca²⁺ responses and mitochondrial changes seen in myogenesis. To test this hypothesis, we created CRISPR/Cas9 knockout (KO) C2C12 murine myoblast cell lines. PC2 KO cells were unable to differentiate into myotubes, had impaired spontaneous Ca²⁺ oscillations, and did not develop depolarization-evoked Ca²⁺ transients. The autophagic-associated pathway beclin-1 was downregulated in PC2 KO cells, and direct activation of the autophagic pathway resulted in decreased mitochondrial remodeling. Re-expression of full-length PC2, but not a calcium channel dead pathologic mutant, restored the differentiation phenotype and increased the expression of mitochondrial proteins. Our results establish that PC2 is a novel regulator of in vitro myogenesis by integrating PC2-dependent Ca²⁺ signals and metabolic pathways.

Calcium selective channel TRPV6: Structure, function, and implications in health and disease

Calcium-selective channel TRPV6 (Transient Receptor Potential channel family, Vanilloid subfamily member 6) belongs to the TRP family of cation channels and plays critical roles in transcellular calcium (Ca2+) transport, reuptake of Ca2+ into cells, and maintaining a local low Ca2+ environment for certain biological processes. Recent crystal and cryo-electron microscopy-based structures of TRPV6 have revealed mechanistic insights on how the protein achieves Ca2+ selectivity, permeation, and inactivation by calmodulin. The TRPV6 protein is expressed in a range of epithelial tissues such as the intestine, kidney, placenta, epididymis, and exocrine glands such as the pancreas, prostate and salivary, sweat, and mammary glands. The TRPV6 gene is a direct transcriptional target of the active form of vitamin D and is efficiently regulated to meet the body's need for Ca2+ demand. In addition, TRPV6 is also regulated by the level of dietary Ca2+ and under physiological conditions such as pregnancy and lactation. Genetic models of loss of function in TRPV6 display hypercalciuria, decreased bone marrow density, deficient weight gain, reduced fertility, and in some cases alopecia. The models also reveal that the channel plays an indispensable role in maintaining maternal-fetal Ca2+ transport and low Ca2+ environment in the epididymal lumen that is critical for male fertility. Most recently, loss of function mutations in TRPV6 gene is linked to transient neonatal hyperparathyroidism and early onset chronic pancreatitis. TRPV6 is overexpressed in a wide range of human malignancies and its upregulation is strongly correlated to tumor aggressiveness, metastasis, and poor survival in selected cancers. This review summarizes the current state of knowledge on the expression, structure, biophysical properties, function, polymorphisms, and regulation of TRPV6. The aberrant expression, polymorphisms, and dysfunction of this protein linked to human diseases are also discussed.

Pharmacological Modulation and (Patho)Physiological Roles of TRPM4 Channel-Part 2: TRPM4 in Health and Disease

Transient receptor potential melastatin 4 (TRPM4) is a unique member of the TRPM protein family and, similarly to TRPM5, is Ca²⁺ sensitive and permeable for monovalent but not divalent cations. It is widely expressed in many organs and is involved in several functions; it regulates membrane potential and Ca²⁺ homeostasis in both excitable and non-excitable cells. This part of the review discusses the currently available knowledge about the physiological and pathophysiological roles of TRPM4 in various tissues. These include the physiological functions of TRPM4 in the cells of the Langerhans islets of the pancreas, in various immune functions, in the regulation of vascular tone, in respiratory and other neuronal activities, in chemosensation, and in renal and cardiac physiology. TRPM4 contributes to pathological conditions such as overactive bladder, endothelial dysfunction, various types of malignant diseases and central nervous system conditions including stroke and injuries as well as in cardiac conditions such as arrhythmias, hypertrophy, and ischemia-reperfusion injuries. TRPM4 claims more and more attention and is likely to be the topic of research in the future.

The mechanism underlying transient weakness in myotonia congenita

In addition to the hallmark muscle stiffness, patients with recessive myotonia congenita (Becker disease) experience debilitating bouts of transient weakness that remain poorly understood despite years of study. We performed intracellular recordings from muscle of both genetic and pharmacologic mouse models of Becker disease to identify the mechanism underlying transient weakness. Our recordings reveal transient depolarizations (plateau potentials) of the membrane potential to -25 to -35 mV in the genetic and pharmacologic models of Becker disease. Both Na+ and Ca2+ currents contribute to plateau potentials. Na+ persistent inward current (NaPIC) through NaV1.4 channels is the key trigger of plateau potentials and current through CaV1.1 Ca2+ channels contributes to the duration of the plateau. Inhibiting NaPIC with ranolazine prevents the development of plateau potentials and eliminates transient weakness in vivo. These data suggest that targeting NaPIC may be an effective treatment to prevent transient weakness in myotonia congenita.

Specialized, pro-resolving mediators as potential therapeutic agents for alleviating fibromyalgia symptomatology

OBJECTIVE

To present a hypothesis on a novel strategy in the treatment of fibromyalgia (FM).

DESIGN

A narrative review.

SETTING

FM as a disease remains a challenging concept for numerous reasons, including undefined etiopathogenesis, unclear triggers and unsuccessful treatment modalities. We hypothesize that the inflammatome, the entire set of molecules involved in inflammation, acting as a common pathophysiological instrument of gut dysbiosis, sarcopenia, and neuroinflammation, is one of the major mechanisms underlying FM pathogenesis. In this setup, dysbiosis is proposed as the primary trigger of the inflammatome, sarcopenia as the peripheral nociceptive source, and neuroinflammation as the central mechanism of pain sensitization, transmission and symptomatology of FM. Whereas neuroinflammation is highly-considered as a critical deleterious element in FM pathogenesis, the presumed pathogenic roles of sarcopenia and systemic inflammation remain controversial. Nevertheless, sarcopenia-associated processes and dysbiosis have been recently detected in FM individuals. The prevalence of pro-inflammatory factors in the cerebrospinal fluid and blood has been repeatedly observed in FM individuals, supporting an idea on the role of inflammatome in FM pathogenesis. As such, failed inflammation resolution might be one of the underlying pathogenic mechanisms. In accordance, the application of specialized, inflammation pro-resolving mediators (SPMs) seems most suitable for this goal.

CONCLUSIONS

The capability of various SPMs to prevent and attenuate pain has been repeatedly demonstrated in laboratory animal experiments. Since SPMs suppress inflammation in a manner that does not compromise host defense, they could be attractive and safe candidates for the alleviation of FM symptomatology, probably in combination with anti-dysbiotic medicine.

TRPV4 Antagonism Prevents Mechanically Induced Myotonia

OBJECTIVE

Myotonia is caused by involuntary firing of skeletal muscle action potentials and causes debilitating stiffness. Current treatments are insufficiently efficacious and associated with side effects. Myotonia can be triggered by voluntary movement (electrically induced myotonia) or percussion (mechanically induced myotonia). Whether distinct molecular mechanisms underlie these triggers is unknown. Our goal was to identify ion channels involved in mechanically induced myotonia and to evaluate block of the channels involved as a novel approach to therapy.

METHODS

We developed a novel system to enable study of mechanically induced myotonia using both genetic and pharmacologic mouse models of myotonia congenita. We extended ex vivo studies of excitability to in vivo studies of muscle stiffness.

RESULTS

As previous work suggests activation of transient receptor potential vanilloid 4 (TRPV4) channels by mechanical stimuli in muscle, we examined the role of this cation channel. Mechanically induced myotonia was markedly suppressed in TRPV4-null muscles and in muscles treated with TRPV4 small molecule antagonists. The suppression of mechanically induced myotonia occurred without altering intrinsic muscle excitability, such that myotonia triggered by firing of action potentials (electrically induced myotonia) was unaffected. When injected intraperitoneally, TRPV4 antagonists lessened the severity of myotonia in vivo by approximately 80%.

INTERPRETATION

These data demonstrate that there are distinct molecular mechanisms triggering electrically induced and mechanically induced myotonia. Our data indicates that activation of TRPV4 during muscle contraction plays an important role in triggering myotonia in vivo. Elimination of mechanically induced myotonia by TRPV4 inhibition offers a new approach to treating myotonia. ANN NEUROL 2020;88:297-308.

TRPCs: Influential Mediators in Skeletal Muscle

Ca²⁺ itself or Ca²⁺-dependent signaling pathways play fundamental roles in various cellular processes from cell growth to death. The most representative example can be found in skeletal muscle cells where a well-timed and adequate supply of Ca²⁺ is required for coordinated Ca²⁺-dependent skeletal muscle functions, such as the interactions of contractile proteins during contraction. Intracellular Ca²⁺ movements between the cytosol and sarcoplasmic reticulum (SR) are strictly regulated to maintain the appropriate Ca²⁺ supply in skeletal muscle cells. Added to intracellular Ca²⁺ movements, the contribution of extracellular Ca²⁺ entry to skeletal muscle functions and its significance have been continuously studied since the early 1990s. Here, studies on the roles of channel proteins that mediate extracellular Ca²⁺ entry into skeletal muscle cells using skeletal myoblasts, myotubes, fibers, tissue, or skeletal muscle-originated cell lines are reviewed with special attention to the proposed functions of transient receptor potential canonical proteins (TRPCs) as store-operated Ca²⁺ entry (SOCE) channels under normal conditions and the potential abnormal properties of TRPCs in muscle diseases such as Duchenne muscular dystrophy (DMD).

Central Role of Subthreshold Currents in Myotonia

It is generally thought that muscle excitability is almost exclusively controlled by currents responsible for generation of action potentials. We propose that smaller ion channel currents that contribute to setting the resting potential and to subthreshold fluctuations in membrane potential can also modulate excitability in important ways. These channels open at voltages more negative than the action potential threshold and are thus termed subthreshold currents. As subthreshold currents are orders of magnitude smaller than the currents responsible for the action potential, they are hard to identify and easily overlooked. Discovery of their importance in regulation of excitability opens new avenues for improved therapy for muscle channelopathies and diseases of the neuromuscular junction. ANN NEUROL 2020;87:175-183.

Overactivation of the sodium-calcium exchanger and transient receptor potential in anesthesia-induced malignant hyperthermia

Malignant hyperthermia is a pharmacogenetic disorder, which is an uncommon but frequently fatal intricacy of inhalation anesthesia in man. It causes a quick rise in body temperature to highly irreversible levels, which causes death in around three of four cases. The trigger anesthetics cause an anomalous, continued ascent in myoplasmic calcium levels. Possible mechanisms by which continuous release of sodium, calcium from skeletal muscle plasma membrane and sarcoplasmic reticulum stores respectively can produce the profound hyperthermia are discussed.

Regional differences in Ca2+ entry along the proximal-middle-distal muscle axis during eccentric contractions in rat skeletal muscle

Eccentric (ECC) contraction-induced muscle damage is associated with calcium ion (Ca²⁺) influx from the extracellular milieu through stretch-activated channels. It remains unknown whether Ca²⁺ influx consequent to repetitive ECC contractions is nonuniform across different muscle regions. We tested the hypothesis that there are regional differences in Ca²⁺ entry along the proximal-middle-distal muscle axis. Tibialis anterior (TA) muscles of adult male Wistar rats were exposed by reflecting the overlying skin and fasciae and ECC contractions evoked by peroneal nerve stimulation paired with simultaneous ankle extension (50 times/set, 2 protocols: 1 set and 10 sets). During ECC in the proximal, middle, and distal TA, we determined 1) muscle fiber extension by high-speed camera (200 frames/s) and 2) Ca²⁺ accumulation by in vivo bioimaging (Ca²⁺-sensitive probe Fura-2-acetoxymethyl ester). Muscle fiber extension from resting was significantly different among regions (i.e., proximal, 4.0%: < middle, 11.2%: < distal, 17.0%; ECC phase length at 500th contraction). Intracellular Ca²⁺ accumulation after 1 set of ECC was higher in the distal (1.46 ± 0.04, P < 0.05) than the proximal (1.27 ± 0.04) or middle (1.26 ± 0.05) regions. However, this regional Ca²⁺ accumulation difference disappeared by 32.5 min after the 1 set protocol when the muscle was quiescent and by contraction set 5 for the 10-set protocol. The initial preferential ECC-induced Ca²⁺ accumulation observed distally was associated spatially with the greater muscle extension compared with that of the proximal and middle regions. Disappearance of the regional Ca²⁺ accumulation disparity in quiescent and ECC-contracting muscle might be explained, in part, by axial Ca²⁺ propagation and account for the uniformity of muscle damage across regions evident 3 days post-ECC.NEW & NOTEWORTHY After 1 set of 50 eccentric (ECC) contractions in the anterior tibialis muscle, intracellular Ca²⁺ ([Ca²⁺]i) accumulation evinces substantial regional heterogeneity that is spatially coherent with muscle length changes (i.e., distal [Ca²⁺]i > middle, proximal). However, irrespective of whether 50 or 500 ECC contractions are performed, this heterogeneity is subsequently abolished, at least in part, by axial intracellular Ca²⁺ propagation. This Ca²⁺ homogenization across regions is consistent with the absence of any interregional difference in muscle damage 3 days post-ECC.

Human induced pluripotent stem cell models for the study and treatment of Duchenne and Becker muscular dystrophies

Duchenne and Becker muscular dystrophies are the most common muscle diseases and are both currently incurable. They are caused by mutations in the dystrophin gene, which lead to the absence or reduction/truncation of the encoded protein, with progressive muscle degeneration that clinically manifests in muscle weakness, cardiac and respiratory involvement and early death. The limits of animal models to exactly reproduce human muscle disease and to predict clinically relevant treatment effects has prompted the development of more accurate in vitro skeletal muscle models. However, the challenge of effectively obtaining mature skeletal muscle cells or satellite stem cells as primary cultures has hampered the development of in vitro models. Here, we discuss the recently developed technologies that enable the differentiation of skeletal muscle from human induced pluripotent stem cells (iPSCs) of Duchenne and Becker patients. These systems recapitulate key disease features including inflammation and scarce regenerative myogenic capacity that are partially rescued by genetic and pharmacological therapies and can provide a useful platform to study and realize future therapeutic treatments. Implementation of this model also takes advantage of the developing genome editing field, which is a promising approach not only for correcting dystrophin, but also for modulating the underlying mechanisms of skeletal muscle development, regeneration and disease. These data prove the possibility of creating an accurate Duchenne and Becker in vitro model starting from iPSCs, to be used for pathogenetic studies and for drug screening to identify strategies capable of stopping or reversing muscular dystrophinopathies and other muscle diseases.

Isolation of Human Myoblasts, Assessment of Myogenic Differentiation, and Store-operated Calcium Entry Measurement

Satellite cells (SC) are muscle stem cells located between the plasma membrane of muscle fibers and the surrounding basal lamina. They are essential for muscle regeneration. Upon injury, which occurs frequently in skeletal muscles, SCs are activated. They proliferate as myoblasts and differentiate to repair muscle lesions. Among many events that take place during muscle differentiation, cytosolic Ca²⁺ signals are of great importance. These Ca²⁺ signals arise from Ca²⁺ release from internal Ca²⁺ stores, as well as from Ca²⁺ entry from the extracellular space, particularly the store-operated Ca²⁺ entry (SOCE). This paper describes a methodology used to obtain a pure population of human myoblasts from muscle samples collected after orthopedic surgery. The tissue is mechanically and enzymatically digested, and the cells are amplified and then sorted by flow cytometry according to the presence of specific membrane markers. Once obtained, human myoblasts are expanded and committed to differentiate by removing growth factors from the culture medium. The expression levels of specific transcription factors and in vitro immunofluorescence are used to assess the myogenic differentiation process in control conditions and after silencing proteins involved in Ca²⁺ signaling. Finally, we detail the use of Fura-2 as a ratiometric Ca²⁺ probe that provides reliable and reproducible measurements of SOCE.

TRPC1 and TRPC4 channels functionally interact with STIM1L to promote myogenesis and maintain fast repetitive Ca2+ release in human myotubes

STIM1 and Orai1 are essential players of store-operated Ca²⁺ entry (SOCE) in human skeletal muscle cells and are required for adult muscle differentiation. Besides these two proteins, TRPC (transient receptor potential canonical) channels and STIM1L (a longer STIM1 isoform) are also present on muscle cells. In the present study, we assessed the role of TRPC1, TRPC4 and STIM1L in SOCE, in the maintenance of repetitive Ca²⁺ transients and in muscle differentiation. Knockdown of TRPC1 and TRPC4 reduced SOCE by about 50% and significantly delayed the onset of Ca²⁺ entry, both effects similar to STIM1L invalidation. Upon store depletion, TRPC1 and TRPC4 appeared to interact preferentially with STIM1L compared to STIM1. STIM1L invalidation affected myoblast differentiation, with the formation of smaller myotubes, an effect similar to what we reported for TRPC1 and TRPC4 knockdown. On the contrary, the overexpression of STIM1L leads to the formation of larger myotubes. All together, these data strongly suggest that STIM1L and TRPC1/4 are working together in myotubes to ensure efficient store refilling and a proper differentiation program.

Differential Expression of NADPH Oxidases Depends on Skeletal Muscle Fiber Type in Rats

NADPH oxidases (NOX) are important sources of reactive oxygen species (ROS) in skeletal muscle, being involved in excitation-contraction coupling. Thus, we aimed to investigate if NOX activity and expression in skeletal muscle are fiber type specific and the possible contribution of this difference to cellular oxidative stress. Oxygen consumption rate, NOX activity and mRNA levels, and the activity of catalase (CAT), glutathione peroxidase (GPX), and superoxide dismutase (SOD), as well as the reactive protein thiol levels, were measured in the soleus (SOL), red gastrocnemius (RG), and white gastrocnemius (WG) muscles of rats. RG showed higher oxygen consumption flow than SOL and WG, while SOL had higher oxygen consumption than WG. SOL showed higher NOX activity, as well as NOX2 and NOX4 mRNA levels, antioxidant enzymatic activities, and reactive protein thiol contents when compared to WG and RG. NOX activity and NOX4 mRNA levels as well as antioxidant enzymatic activities were higher in RG than in WG. Physical exercise increased NOX activity in SOL and RG, specifically NOX2 mRNA levels in RG and NOX4 mRNA levels in SOL. In conclusion, we demonstrated that NOX activity and expression differ according to the skeletal muscle fiber type, as well as antioxidant defense.

TRP channels in schistosomes

Praziquantel (PZQ) is effectively the only drug currently available for treatment and control of schistosomiasis, a disease affecting hundreds of millions of people worldwide. Many anthelmintics, likely including PZQ, target ion channels, membrane protein complexes essential for normal functioning of the neuromusculature and other tissues. Despite this fact, only a few classes of parasitic helminth ion channels have been assessed for their pharmacological properties or for their roles in parasite physiology. One such overlooked group of ion channels is the transient receptor potential (TRP) channel superfamily. TRP channels share a common core structure, but are widely diverse in their activation mechanisms and ion selectivity. They are critical to transducing sensory signals, responding to a wide range of external stimuli. They are also involved in other functions, such as regulating intracellular calcium and organellar ion homeostasis and trafficking. Here, we review current literature on parasitic helminth TRP channels, focusing on those in schistosomes. We discuss the likely roles of these channels in sensory and locomotor activity, including the possible significance of a class of TRP channels (TRPV) that is absent in schistosomes. We also focus on evidence indicating that at least one schistosome TRP channel (SmTRPA) has atypical, TRPV1-like pharmacological sensitivities that could potentially be exploited for future therapeutic targeting.

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We provide an overview of transient receptor potential (TRP) channels in schistosomes and other parasitic helminths.

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TRP channels are important for sensory signaling, ion homeostasis, organellar trafficking, and a host of other functions.

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Very little work has been done on TRP channels in parasitic helminths.

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TRPV channels, found throughout the Metazoa, appear not to be present in parasitic platyhelminths.

•

TRP channels in schistosomes appear to have atypical pharmacology, perhaps an entrée for therapeutic targeting.

Engineered miniaturized models of musculoskeletal diseases

The musculoskeletal system is an incredible machine that protects, supports and moves the human body. However, several diseases can limit its functionality, compromising patient quality of life. Designing novel pathological models would help to clarify the mechanisms driving such diseases, identify new biomarkers and screen potential drug candidates. Miniaturized models in particular can mimic the structure and function of basic tissue units within highly controlled microenvironments, overcoming the limitations of traditional macroscale models and complementing animal studies, which despite being closer to the in vivo situation, are affected by species-specific differences. Here, we discuss the miniaturized models engineered over the past few years to analyze osteochondral and skeletal muscle pathologies, demonstrating how the rationale design of novel systems could provide key insights into the pathological mechanisms behind diseases of the musculoskeletal system.

Early pathogenesis of Duchenne muscular dystrophy modelled in patient-derived human induced pluripotent stem cells

Duchenne muscular dystrophy (DMD) is a progressive and fatal muscle degenerating disease caused by a dystrophin deficiency. Effective suppression of the primary pathology observed in DMD is critical for treatment. Patient-derived human induced pluripotent stem cells (hiPSCs) are a promising tool for drug discovery. Here, we report an in vitro evaluation system for a DMD therapy using hiPSCs that recapitulate the primary pathology and can be used for DMD drug screening. Skeletal myotubes generated from hiPSCs are intact, which allows them to be used to model the initial pathology of DMD in vitro. Induced control and DMD myotubes were morphologically and physiologically comparable. However, electric stimulation of these myotubes for in vitro contraction caused pronounced calcium ion (Ca²⁺) influx only in DMD myocytes. Restoration of dystrophin by the exon-skipping technique suppressed this Ca²⁺ overflow and reduced the secretion of creatine kinase (CK) in DMD myotubes. These results suggest that the early pathogenesis of DMD can be effectively modelled in skeletal myotubes induced from patient-derived iPSCs, thereby enabling the development and evaluation of novel drugs.

Osmosensation in TRPV2 dominant negative expressing skeletal muscle fibres

Increased plasma osmolarity induces intracellular water depletion and cell shrinkage (CS) followed by activation of a regulatory volume increase (RVI). In skeletal muscle, the hyperosmotic shock-induced CS is accompanied by a small membrane depolarization responsible for a release of Ca(2+) from intracellular pools. Hyperosmotic shock also induces phosphorylation of STE20/SPS1-related proline/alanine-rich kinase (SPAK). TRPV2 dominant negative expressing fibres challenged with hyperosmotic shock present a slower membrane depolarization, a diminished Ca(2+) response, a smaller RVI response, a decrease in SPAK phosphorylation and defective muscle function. We suggest that hyperosmotic shock induces TRPV2 activation, which accelerates muscle cell depolarization and allows the subsequent Ca(2+) release from the sarcoplasmic reticulum, activation of the Na(+) -K(+) -Cl(-) cotransporter by SPAK, and the RVI response. Increased plasma osmolarity induces intracellular water depletion and cell shrinkage followed by activation of a regulatory volume increase (RVI). In skeletal muscle, this is accompanied by transverse tubule (TT) dilatation and by a membrane depolarization responsible for a release of Ca(2+) from intracellular pools. We observed that both hyperosmotic shock-induced Ca(2+) transients and RVI were inhibited by Gd(3+) , ruthenium red and GsMTx4 toxin, three inhibitors of mechanosensitive ion channels. The response was also completely absent in muscle fibres overexpressing a non-permeant, dominant negative (DN) mutant of the transient receptor potential, V2 isoform (TRPV2) ion channel, suggesting the involvement of TRPV2 or of a TRP isoform susceptible to heterotetramerization with TRPV2. The release of Ca(2+) induced by hyperosmotic shock was increased by cannabidiol, an activator of TRPV2, and decreased by tranilast, an inhibitor of TRPV2, suggesting a role for the TRPV2 channel itself. Hyperosmotic shock-induced membrane depolarization was impaired in TRPV2-DN fibres, suggesting that TRPV2 activation triggers the release of Ca(2+) from the sarcoplasmic reticulum by depolarizing TTs. RVI requires the sequential activation of STE20/SPS1-related proline/alanine-rich kinase (SPAK) and NKCC1, a Na(+) -K(+) -Cl(-) cotransporter, allowing ion entry and driving osmotic water flow. In fibres overexpressing TRPV2-DN as well as in fibres in which Ca(2+) transients were abolished by the Ca(2+) chelator BAPTA, the level of P-SPAK(Ser373) in response to hyperosmotic shock was reduced, suggesting a modulation of SPAK phosphorylation by intracellular Ca(2+) . We conclude that TRPV2 is involved in osmosensation in skeletal muscle fibres, acting in concert with P-SPAK-activated NKCC1.

Store-operated Ca2+ entry in muscle physiology and diseases

Ca(2+) release from intracellular stores and influx from extracellular reservoir regulate a wide range of physiological functions including muscle contraction and rhythmic heartbeat. One of the most ubiquitous pathways involved in controlled Ca(2+) influx into cells is store-operated Ca(2+) entry (SOCE), which is activated by the reduction of Ca(2+) concentration in the lumen of endoplasmic or sarcoplasmic reticulum (ER/SR). Although SOCE is pronounced in non-excitable cells, accumulating evidences highlight its presence and important roles in skeletal muscle and heart. Recent discovery of STIM proteins as ER/SR Ca(2+) sensors and Orai proteins as Ca(2+) channel pore forming unit expedited the mechanistic understanding of this pathway. This review focuses on current advances of SOCE components, regulation and physiologic and pathophysiologic roles in muscles. The specific property and the dysfunction of this pathway in muscle diseases, and new directions for future research in this rapidly growing field are discussed.

Absence of γ-sarcoglycan alters the response of p70S6 kinase to mechanical perturbation in murine skeletal muscle

Background

The dystrophin glycoprotein complex (DGC) is located at the sarcolemma of muscle fibers, providing structural integrity. Mutations in and loss of DGC proteins cause a spectrum of muscular dystrophies. When only the sarcoglycan subcomplex is absent, muscles display severe myofiber degeneration, but little susceptibility to contractile damage, suggesting that disease occurs not by structural deficits but through aberrant signaling, namely, loss of normal mechanotransduction signaling through the sarcoglycan complex. We extended our previous studies on mechanosensitive, γ-sarcoglycan-dependent ERK1/2 phosphorylation, to determine whether additional pathways are altered with the loss of γ-sarcoglycan.

Methods

We examined mechanotransduction in the presence and absence of γ-sarcoglycan, using C2C12 myotubes, and primary cultures and isolated muscles from C57Bl/6 (C57) and γ-sarcoglycan-null (γ-SG^-/-) mice. All were subjected to cyclic passive stretch. Signaling protein phosphorylation was determined by immunoblotting of lysates from stretched and non-stretched samples. Calcium dependence was assessed by maintaining muscles in calcium-free or tetracaine-supplemented Ringer’s solution. Dependence on mTOR was determined by stretching isolated muscles in the presence or absence of rapamycin.

Results

C2C12 myotube stretch caused a robust increase in P-p70S6K, but decreased P-FAK and P-ERK2. Neither Akt nor ERK1 were responsive to passive stretch. Similar but non-significant trends were observed in C57 primary cultures in response to stretch, and γ-SG^-/- cultures displayed no p70S6K response. In contrast, in isolated muscles, p70S6K was mechanically responsive. Basal p70S6K activation was elevated in muscles of γ-SG^-/- mice, in a calcium-independent manner. p70S6K activation increased with stretch in both C57 and γ-SG^-/- isolated muscles, and was sustained in γ-SG^-/- muscles, unlike the transient response in C57 muscles. Rapamycin treatment blocked all of p70S6K activation in stretched C57 muscles, and reduced downstream S6RP phosphorylation. However, even though rapamycin treatment decreased p70S6K activation in stretched γ-SG^-/- muscles, S6RP phosphorylation remained elevated.

Conclusions

p70S6K is an important component of γ-sarcoglycan-dependent mechanotransduction in skeletal muscle. Our results suggest that loss of γ-sarcoglycan uncouples the response of p70S6K to stretch and implies that γ-sarcoglycan is important for inactivation of this pathway. Overall, we assert that altered load-sensing mechanisms exist in muscular dystrophies where the sarcoglycans are absent.

Adaptive responses of TRPC1 and TRPC3 during skeletal muscle atrophy and regrowth

INTRODUCTION

We assessed the time-dependent changes of transient receptor potential canonical type 1 (TRPC1) and TRPC3 expression and localization associated with muscle atrophy and regrowth in vivo.

METHODS

Mice were subjected to hindlimb unloading for 7 or 14 days (7U, 14U) followed by 3, 7, or 14 days of reloading (3R, 7R, 14R).

RESULTS

Soleus muscle mass and tetanic force were reduced significantly at 7U and 14U and recovered by 14R. Recovery of muscle fiber cross-sectional area was observed by 28R. TRPC1 mRNA was unaltered during the unloading-reloading period. However, protein expression remained depressed through 14R. Decreased localization of TRPC1 to the sarcolemma was observed. TRPC3 mRNA and protein expression levels were decreased significantly during the early phase of reloading.

CONCLUSIONS

Given the known role of these channels in muscle development, changes observed in TRPC1 and TRPC3 may relate closely to muscle atrophy and remodeling processes.

Mechanical stretch-induced activation of ROS/RNS signaling in striated muscle

SIGNIFICANCE

Mechanical activation of reactive oxygen species (ROS) and reactive nitrogen species (RNS) occurs in striated muscle and affects Ca(2+) signaling and contractile function. ROS/RNS signaling is tightly controlled, spatially compartmentalized, and source specific.

RECENT ADVANCES

Here, we review the evidence that within the contracting myocyte, the trans-membrane protein NADPH oxidase 2 (Nox2) is the primary source of ROS generated during contraction. We also review a newly characterized signaling cascade in cardiac and skeletal muscle in which the microtubule network acts as a mechanotransduction element that activates Nox2-dependent ROS generation during mechanical stretch, a pathway termed X-ROS signaling.

CRITICAL ISSUES

In the heart, X-ROS acts locally and affects the sarcoplasmic reticulum (SR) Ca(2+) release channels (ryanodine receptors) and tunes Ca(2+) signaling during physiological behavior, but excessive X-ROS can promote Ca(2+)-dependent arrhythmias in pathology. In skeletal muscle, X-ROS sensitizes Ca(2+)-permeable sarcolemmal "transient receptor potential" channels, a pathway that is critical for sustaining SR load during repetitive contractions, but when in excess, it is maladaptive in diseases such as Duchenne Musclar dystrophy.

FUTURE DIRECTIONS

New advances in ROS/RNS detection as well as molecular manipulation of signaling pathways will provide critical new mechanistic insights into the details of X-ROS signaling. These efforts will undoubtedly reveal new avenues for therapeutic intervention in the numerous diseases of striated muscle in which altered mechanoactivation of ROS/RNS production has been identified.

During post-natal human myogenesis, normal myotube size requires TRPC1- and TRPC4-mediated Ca²⁺ entry

Myogenesis involves expression of muscle-specific transcription factors such as myogenin and myocyte enhancer factor 2 (MEF2), and is essentially regulated by fluctuations of cytosolic Ca(2+) concentration. Recently we demonstrated that molecular players of store-operated Ca(2+) entry (SOCE), stromal interacting molecule (STIM) and Orai, were fundamental in the differentiation process of post-natal human myoblasts. Besides STIM and Orai proteins, the family of transient receptor potential canonical (TRPC) channels was shown to be part of SOCE in several cellular systems. In the present study, we investigated the role of TRPC channels in the human myogenesis process. We demonstrate, using an siRNA strategy or dominant negative TRPC overexpression, that TRPC1 and TRPC4 participate in SOCE, are necessary for MEF2 expression, and allow the fusion process to generate myotubes of normal size. Conversely, the overexpression of STIM1 with TRPC4 or TRPC1 increased SOCE, accelerated myoblast fusion, and produced hypertrophic myotubes. Interestingly, in cells depleted of TRPC1 or TRPC4, the normalization of SOCE by increasing the extracellular calcium concentration or by overexpressing STIM1 or Orai1 was not sufficient to restore normal fusion process. A normal differentiation occurred only when TRPC channel was re-expressed. These findings indicate that Ca(2+) entry mediated specifically by TRPC1 and TRPC4 allow the formation of normal-sized myotubes.

Characterization of functional TRPV1 channels in the sarcoplasmic reticulum of mouse skeletal muscle

TRPV1 represents a non-selective cation channel activated by capsaicin, acidosis and high temperature. In the central nervous system where TRPV1 is highly expressed, its physiological role in nociception is clearly identified. In skeletal muscle, TRPV1 appears implicated in energy metabolism and exercise endurance. However, how as a Ca(2+) channel, it contributes to intracellular calcium concentration ([Ca(2+)]i) maintenance and muscle contraction remains unknown. Here, as in rats, we report that TRPV1 is functionally expressed in mouse skeletal muscle. In contrast to earlier reports, our analysis show TRPV1 presence only at the sarcoplasmic reticulum (SR) membrane (preferably at the longitudinal part) in the proximity of SERCA1 pumps. Using intracellular Ca(2+) imaging, we directly accessed to the channel functionality in intact FDB mouse fibers. Capsaicin and resiniferatoxin, both agonists as well as high temperature (45°C) elicited an increase in [Ca(2+)]i. TRPV1-inhibition by capsazepine resulted in a strong inhibition of TRPV1-mediated functional responses and abolished channel activation. Blocking the SR release (with ryanodine or dantrolene) led to a reduced capsaicin-induced Ca(2+) elevation suggesting that TRPV1 may participate to a secondary SR Ca(2+) liberation of greater amplitude. In conclusion, our experiments point out that TRPV1 is a functional SR Ca(2+) leak channel and may crosstalk with RyR1 in adult mouse muscle fibers.

Partial opening and subconductance gating of mechanosensitive ion channels in dystrophic skeletal muscle

We recorded the activity of single mechanosensitive (MS) ion channels in skeletal muscle from the mdx mouse, a deletion mutant that lacks the cytoskeletal protein, dystrophin. Experiments were designed to examine the influence of dystrophin, a major component of skeletal muscle costameres, on the behaviour of single MS channels. In the majority of recordings from cell-attached patches, MS channels have a conductance of ∼23 pS. Recordings from some patches, however, showed a smaller conductance channel of ∼7-14 pS. Large and small conductance channels were detected in a single patch and showed serial, non-random gating, suggesting different opening levels of a single channel. Analysis of the distribution of current amplitudes within the open channel showed MS channels fluctuate between subconductance levels. MS channels in dystrophic muscle spend ∼60% of the time at smaller subconductance levels, often failing to reach the fully open level. Applying pressure to the membrane of mdx fibres increases in a graded manner occupancy of the fully open state, while reducing occupancy of subconductance levels. Recordings also show partial openings of MS channels in both wild-type and mdx muscle that fail to reach the fully open state. Partial openings occur at a higher frequency in mdx muscle and reflect occupancy of subconductance levels seen during complete activations. In muscle from mdx/utrn(-/-) double knockout mice, MS channels also spend more time at subconductance levels than the fully open state. Conductance variability of MS channels may represent gating of a heteromeric protein composed of different channel subunits. The results also show that partial opening and prolonged burst duration are distinct mechanisms that contribute to excess Ca(2+) entry in dystrophic muscle.

Uncoupling of sarcoplasmic reticulum Ca²⁺-ATPase by N-arachidonoyl dopamine. Members of the endocannabinoid family as thermogenic drugs

BACKGROUND AND PURPOSE

The sarcoplasmic reticulum Ca²⁺-ATPase (SERCA) plays a role in thermogenesis. The exogenous compound capsaicin increased SERCA-mediated ATP hydrolysis not coupled to Ca²⁺ transport. Here, we have sought to identify endogenous compounds that may function as SERCA uncoupling agents.

EXPERIMENTAL APPROACH

Using isolated SR vesicles from rabbits, we have screened for endogenous compounds that uncouple SERCA. We have also studied their ability to deplete cytoplasmic ATP from human skeletal muscle cells in culture.

KEY RESULTS

Studies on SR vesicles showed that the endogenous lipid metabolite N-arachidonoyl dopamine (NADA) was a potent stimulator of SERCA uncoupling. NADA stabilized an E₁-like pump conformation that had a lower dephosphorylation rate, low affinity for Ca²⁺ at the luminal sites and a specific proteinase K cleavage pattern involving protection of the C-terminal p83C fragment from further cleavage. Moreover, we found a significantly decreased cytoplasmic ATP levels following treatment of skeletal muscle cells with 100 nM NADA. This effect was dependent on the presence of glucose and abolished by pretreatment with the specific SERCA inhibitor thapsigargin, regardless of the presence of glucose.

CONCLUSIONS AND IMPLICATIONS

NADA is an endogenous molecule that may function as SERCA uncoupling agent in vivo. Members of the endocannabinoid family exert concerted actions on several Ca²⁺-handling proteins. Uncoupling of SERCA by exogenous compounds could be a novel post-mitochondrial strategy for reduction of cellular ATP levels. In addition, signalling networks leading to SERCA uncoupling can be explored to study the importance of this ion pump in pathophysiological conditions related to metabolism.

Orai1 mediates exacerbated Ca(2+) entry in dystrophic skeletal muscle

There is substantial evidence indicating that disruption of Ca²⁺ homeostasis and activation of cytosolic proteases play a key role in the pathogenesis and progression of Duchenne Muscular Dystrophy (DMD). However, the exact nature of the Ca²⁺ deregulation and the Ca²⁺ signaling pathways that are altered in dystrophic muscles have not yet been resolved. Here we examined the contribution of the store-operated Ca²⁺ entry (SOCE) for the pathogenesis of DMD. RT-PCR and Western blot found that the expression level of Orai1, the pore-forming unit of SOCE, was significantly elevated in the dystrophic muscles, while parallel increases in SOCE activity and SR Ca²⁺ storage were detected in adult mdx muscles using Fura-2 fluorescence measurements. High-efficient shRNA probes against Orai1 were delivered into the flexor digitorum brevis muscle in live mice and knockdown of Orai1 eliminated the differences in SOCE activity and SR Ca²⁺ storage between the mdx and wild type muscle fibers. SOCE activity was repressed by intraperitoneal injection of BTP-2, an Orai1 inhibitor, and cytosolic calpain1 activity in single muscle fibers was measured by a membrane-permeable calpain substrate. We found that BTP-2 injection for 2 weeks significantly reduced the cytosolic calpain1 activity in mdx muscle fibers. Additionally, ultrastructural changes were observed by EM as an increase in the number of triad junctions was identified in dystrophic muscles. Compensatory changes in protein levels of SERCA1, TRP and NCX3 appeared in the mdx muscles, suggesting that comprehensive adaptations occur following altered Ca²⁺ homeostasis in mdx muscles. Our data indicates that upregulation of the Orai1-mediated SOCE pathway and an overloaded SR Ca²⁺ store contributes to the disrupted Ca²⁺ homeostasis in mdx muscles and is linked to elevated proteolytic activity, suggesting that targeting Orai1 activity may be a promising therapeutic approach for the prevention and treatment of muscular dystrophy.

Evidence TRPV4 contributes to mechanosensitive ion channels in mouse skeletal muscle fibers

We recorded the activity of single mechanosensitive (MS) ion channels from membrane patches on single muscle fibers isolated from mice. We investigated the actions of various TRP (transient receptor potential) channel blockers on MS channel activity. 2-aminoethoxydiphenyl borate (2-APB) neither inhibited nor facilitated single channel activity at submillimolar concentrations. The absence of an effect of 2-APB indicates MS channels are not composed purely of TRPC or TRPV1, 2 or 3 proteins. Exposing patches to 1-oleolyl-2-acetyl-sn-glycerol (OAG), a potent activator of TRPC channels, also had no effect on MS channel activity. In addition, flufenamic acid and spermidine had no effect on the activity of single MS channels. By contrast, SKF-96365 and ruthenium red blocked single-channel currents at micromolar concentrations. SKF-96365 produced a rapid block of the open channel current. The blocking rate depended linearly on blocker concentration, while the unblocking rate was independent of concentration, consistent with a simple model of open channel block. A fit to the concentration-dependence of block gave k(on) = 13 x 10 ( 6) M (-1) s (-1) and k(off) = 1609 sec (-1) with K(D) = ~124 µM. Block by ruthenium red was complex, involving both reduction of the amplitude of the single-channel current and increased occupancy of subconductance levels. The reduction in current amplitude with increasing concentration of ruthenium red gave a K(D) = ~49 µM. The high sensitivity of MS channels to block by ruthenium red suggests MS channels in skeletal muscle contain TRPV subunits. Recordings from skeletal muscle isolated from TRPV4 knockout mice failed to show MS channel activity, consistent with a contribution of TRPV4. In addition, exposure to hypo-osmotic solutions increases opening of MS channels in muscle. Our results provide evidence TRPV4 contributes to MS channels in skeletal muscle.

TRP channels in normal and dystrophic skeletal muscle

TRP proteins constitute non-selective cation-permeable ion channels, most of which are permeable to Ca²⁺. In skeletal muscle, several isoforms of the TRPC (Canonical), TRPV (Vanilloid) and TRPM (Melastatin) subfamilies are expressed. In particular, TRPC1, C3 and C6, TRPV2 and V4, TRPM4 and TRPM7 have been consistently found in cultured myoblasts or in adult muscles. These channels seem to directly or indirectly respond to membrane stretch or to Ca²⁺ stores depletion; some isoforms might also constitute unregulated Ca²⁺ leak channels. Their function is largely unknown. TRPC1 and C3 have been involved in muscle development, in particular in myoblasts migration and differentiation. TRPC1 and V4 might allow a basal influx of Ca²⁺ at rest. Their lack has consequences on muscle fatigue. TRPV2 seems to be stretch-sensitive. It localizes mainly in intracellular pools at rest, and translocates to the plasma membrane upon IGF-1 stimulation. TRP channels seem to be involved in the pathophysiology of muscle disorders. In particular in Duchenne muscular dystrophy, the lack of the cytoskeletal protein dystrophin induces a disregulation of several ion channels leading to an abnormal influx of Ca²⁺. We discuss here, the possible involvement of TRP channels in this abnormal influx of Ca²⁺.