Dominant-negative inhibition of Ca2+ influx via TRPV2 ameliorates muscular dystrophy in animal models

TCAF1 promotes TRPV2-mediated Ca2+ release in response to cytosolic DNA to protect stressed replication forks

The protection of the replication fork structure under stress conditions is essential for genome maintenance and cancer prevention. A key signaling pathway for fork protection involves TRPV2-mediated Ca2+ release from the ER, which is triggered after the generation of cytosolic DNA and the activation of cGAS/STING. This results in CaMKK2/AMPK activation and subsequent Exo1 phosphorylation, which prevent aberrant fork processing, thereby ensuring genome stability. However, it remains poorly understood how the TRPV2 channel is activated by the presence of cytosolic DNA. Here, through a genome-wide CRISPR-based screen, we identify TRPM8 channel-associated factor 1 (TCAF1) as a key factor promoting TRPV2-mediated Ca2+ release under replication stress or other conditions that activate cGAS/STING. Mechanistically, TCAF1 assists Ca2+ release by facilitating the dissociation of STING from TRPV2, thereby relieving TRPV2 repression. Consistent with this function, TCAF1 is required for fork protection, chromosomal stability, and cell survival after replication stress.

Ion channel TRPV2 is critical in enhancing B cell activation and function

The function of transient receptor potential vanilloid (TRPV) cation channels governing B cell activation remains to be explored. We present evidence that TRPV2 is highly expressed in B cells and plays a crucial role in the formation of the B cell immunological synapse and B cell activation. Physiologically, TRPV2 expression level is positively correlated to influenza-specific antibody production and is low in newborns and seniors. Pathologically, a positive correlation is established between TRPV2 expression and the clinical manifestations of systemic lupus erythematosus (SLE) in adult and child SLE patients. Correspondingly, mice with deficient TRPV2 in B cells display impaired antibody responses following immunization. Mechanistically, the pore and N-terminal domains of TRPV2 are crucial for gating cation permeation and executing mechanosensation in B cells upon antigen stimulation. These processes synergistically contribute to membrane potential depolarization and cytoskeleton remodeling within the B cell immunological synapse, fostering efficient B cell activation. Thus, TRPV2 is critical in augmenting B cell activation and function.

Hypochlorous acid exposure impairs skeletal muscle function and Ca2+ signalling: implications for Duchenne muscular dystrophy pathology

Duchenne muscular dystrophy (DMD) is a fatal X-linked disease characterised by severe muscle wasting. The mechanisms underlying the DMD pathology likely involve the interaction between inflammation, oxidative stress and impaired Ca2+ signalling. Hypochlorous acid (HOCl) is a highly reactive oxidant produced endogenously via myeloperoxidase; an enzyme secreted by neutrophils that is significantly elevated in dystrophic muscle. Oxidation of Ca2+ -handling proteins by HOCl may impair Ca2+ signalling. This study aimed to determine the effects of HOCl on skeletal muscle function and its potential contribution to the dystrophic pathology. Extensor digitorum longus (EDL), soleus and interosseous muscles were surgically isolated from anaesthetised C57 (wild-type) and mdx (dystrophic) mice for measurement of ex vivo force production and intracellular Ca2+ concentration. In whole EDL muscle, HOCl (200 μM) significantly decreased maximal force and increased resting muscle tension which was only partially reversible by dithiothreitol. The effects of HOCl (200 μM) on maximal force in slow-twitch soleus were lower than found in the fast-twitch EDL muscle. In single interosseous myofibres, HOCl (10 μM) significantly increased resting intracellular Ca2+ concentration and decreased Ca2+ transient amplitude. These effects of HOCl were reduced by the application of tetracaine, Gd3+ or streptomycin, implicating involvement of ryanodine receptors and transient receptor potential channels. These results demonstrate the potent effects of HOCl on skeletal muscle function potentially mediated by HOCl-induced oxidation to Ca2+ signalling proteins. Hence, HOCl may provide a link between chronic inflammation, oxidative stress and impaired Ca2+ handling that is characteristic of DMD and presents a potential therapeutic target for DMD. KEY POINTS: Duchenne muscular dystrophy is a fatal genetic disease with pathological mechanisms which involve the complex interaction of chronic inflammation, increased reactive oxygen species production and increased cytosolic Ca2+ concentrations. Hypochlorous acid can be endogenously produced by neutrophils via the enzyme myeloperoxidase. Both neutrophil and myeloperoxidase activity are increased in dystrophic mice. This study found that hypochlorous acid decreased muscle force production and increased cytosolic Ca2+ concentrations in isolated muscles from wild-type and dystrophic mice at relatively low concentrations of hypochlorous acid. These results indicate that hypochlorous acid may be key in the Duchenne muscular dystrophy disease pathology and may provide a unifying link between the chronic inflammation, increased reactive oxygen species production and increased cytosolic Ca2+ concentrations observed in Duchenne muscular dystrophy. Hypochlorous acid production may be a potential target for therapeutic treatments of Duchenne muscular dystrophy.

Calcium handling dysfunction and cardiac damage following acute ventricular preload challenge in the dystrophin-deficient mouse heart

Duchenne muscular dystrophy (DMD) is the most common muscular dystrophy and is caused by mutations in the dystrophin gene. Dystrophin deficiency is associated with structural and functional changes of the muscle cell sarcolemma and/or stretch-induced ion channel activation. In this investigation, we use mice with transgenic cardiomyocyte-specific expression of the GCaMP6f Ca2+ indicator to test the hypothesis that dystrophin deficiency leads to cardiomyocyte Ca2+ handling abnormalities following preload challenge. α-MHC-MerCreMer-GCaMP6f transgenic mice were developed on both a wild-type (WT) or dystrophic (Dmdmdx-4Cv) background. Isolated hearts of 3-7-mo male mice were perfused in unloaded Langendorff mode (0 mmHg) and working heart mode (preload = 20 mmHg). Following a 30-min preload challenge, hearts were perfused in unloaded Langendorff mode with 40 μM blebbistatin, and GCaMP6f was imaged using confocal fluorescence microscopy. Incidence of premature ventricular complexes (PVCs) was monitored before and following preload elevation at 20 mmHg. Hearts of both wild-type and dystrophic mice exhibited similar left ventricular contractile function. Following preload challenge, dystrophic hearts exhibited a reduction in GCaMP6f-positive cardiomyocytes and an increase in number of cardiomyocytes exhibiting Ca2+ waves/overload. Incidence of cardiac arrhythmias was low in both wild-type and dystrophic hearts during unloaded Langendorff mode. However, after preload elevation to 20-mmHg hearts of dystrophic mice exhibited an increased incidence of PVCs compared with hearts of wild-type mice. In conclusion, these data indicate susceptibility to preload-induced Ca2+ overload, ventricular damage, and ventricular dysfunction in male Dmdmdx-4Cv hearts. Our data support the hypothesis that cardiomyocyte Ca2+ overload underlies cardiac dysfunction in muscular dystrophy.NEW & NOTEWORTHY The mechanisms of cardiac disease progression in muscular dystrophy are complex and poorly understood. Using a transgenic mouse model with cardiomyocyte-specific expression of the GCaMP6f Ca2+ indicator, the present study provides further support for the Ca2+-overload hypothesis of disease progression and ventricular arrhythmogenesis in muscular dystrophy.

Drug repurposing for cardiovascular diseases: New targets and indications for probenecid

The available pharmacological options in the management of cardiovascular diseases such as ischaemic heart disease and subsequent heart failure are effective in slowing the progression of this condition. However, the long-term prognosis is still poor, raising the demand for new therapeutic strategies. Drug repurposing is a time- and cost-effective drug development strategy that offers approved and abandoned drugs a new chance for new indications. Recently, drugs used for the management of gout-related inflammation such as canakinumab or colchicine have been considered for drug repurposing in cardiovascular indications. The old uricosuric drug, probenecid, has been identified as a novel therapeutic option in the management of specific cardiac diseases as well. Probenecid can modulate myocardial contractility and vascular tone and exerts anti-inflammatory properties. The mechanisms behind these beneficial effects might be related inhibition of inflammasomes, and to modulation purinergic-pannexin-1 signalling and TRPV2 channels, which are recently identified molecular targets of probenecid. In this review, we provide an overview on repurposing probenecid for ischaemic heart disease and subsequent heart failure by summarizing the related experimental and clinical data and propose its potential repurposing to treat cardiovascular diseases.

Cytosolic DNA sensing by cGAS/STING promotes TRPV2-mediated Ca2+ release to protect stressed replication forks

The protection of DNA replication forks under stress is essential for genome maintenance and cancer suppression. One mechanism of fork protection involves an elevation in intracellular Ca2+ ([Ca2+]i), which in turn activates CaMKK2 and AMPK to prevent uncontrolled fork processing by Exo1. How replication stress triggers [Ca2+]i elevation is unclear. Here, we report a role of cytosolic self-DNA (cytosDNA) and the ion channel TRPV2 in [Ca2+]i induction and fork protection. Replication stress leads to the generation of ssDNA and dsDNA species that, upon translocation into cytoplasm, trigger the activation of the sensor protein cGAS and the production of cGAMP. The subsequent binding of cGAMP to STING causes its dissociation from TRPV2, leading to TRPV2 derepression and Ca2+ release from the ER, which in turn activates the downstream signaling cascade to prevent fork degradation. This Ca2+-dependent genome protection pathway is also activated in response to replication stress caused by oncogene activation.

Ion Channels of the Sarcolemma and Intracellular Organelles in Duchenne Muscular Dystrophy: A Role in the Dysregulation of Ion Homeostasis and a Possible Target for Therapy

Duchenne muscular dystrophy (DMD) is caused by the absence of the dystrophin protein and a properly functioning dystrophin-associated protein complex (DAPC) in muscle cells. DAPC components act as molecular scaffolds coordinating the assembly of various signaling molecules including ion channels. DMD shows a significant change in the functioning of the ion channels of the sarcolemma and intracellular organelles and, above all, the sarcoplasmic reticulum and mitochondria regulating ion homeostasis, which is necessary for the correct excitation and relaxation of muscles. This review is devoted to the analysis of current data on changes in the structure, functioning, and regulation of the activity of ion channels in striated muscles in DMD and their contribution to the disruption of muscle function and the development of pathology. We note the prospects of therapy based on targeting the channels of the sarcolemma and organelles for the correction and alleviation of pathology, and the problems that arise in the interpretation of data obtained on model dystrophin-deficient objects.

Impact of the TRPV2 Inhibitor on Advanced Heart Failure in Patients with Muscular Dystrophy: Exploratory Study of Biomarkers Related to the Efficacy of Tranilast

Cardiomyopathy is the leading cause of death in patients with muscular dystrophy (MD). Tranilast, a widely used anti-allergic drug, has displayed inhibitory activity against the transient receptor potential cation channel subfamily V member 2 and improved cardiac function in MD patients. To identify urinary biomarkers that assess improved cardiac function after tranilast administration, we performed a urinary metabolomic study focused on oxidative fatty acids. Accompanying the clinical trial of tranilast, urine specimens were collected over 24 weeks from MD patients with advanced heart failure. Urinary levels of tetranor-PGDM (tetranor-prostaglandin D metabolite), a metabolite of prostaglandin D₂, significantly decreased 12 weeks after tranilast administration and were correlated with BNP. These results suggest that prostaglandin-mediated inflammation, which increases with the pathological progression of heart failure in MD patients, was attenuated. Urinary prostaglandin E₃ (PGE₃) levels significantly increased 4 weeks after tranilast administration. There were positive correlations between the urinary levels of PGE₃ and 8-hydroxy-2'-deoxyguanosine, an oxidative stress marker. High PGE₃ levels may have a protective effect against cardiomyopathy in MD patients with high oxidative stress. Although further validation studies are necessary, urinary tetranor-PGDM and PGE₃ levels may help the current understanding of the extent of advanced heart failure in patients with MD after tranilast administration.

Childhood muscular dystrophies

Infancy- and childhood-onset muscular dystrophies are associated with a characteristic distribution and progression of motor dysfunction. The underlying causes of progressive childhood muscular dystrophies are heterogeneous involving diverse genetic pathways and genes that encode proteins of the plasma membrane, extracellular matrix, sarcomere, and nuclear membrane components. The prototypical clinicopathological features in an affected child may be adequate to fully distinguish it from other likely diagnoses based on four common features: (1) weakness and wasting of pelvic-femoral and scapular muscles with involvement of heart muscle; (2) elevation of serum muscle enzymes in particular serum creatine kinase; (3) necrosis and regeneration of myofibers; and (4) molecular neurogenetic assessment particularly utilizing next-generation sequencing of the genome of the likeliest candidates genes in an index case or family proband. A number of different animal models of therapeutic strategies have been developed for gene transfer therapy, but so far these techniques have not yet entered clinical practice. Treatment remains for the most part symptomatic with the goal of ameliorating locomotor and cardiorespiratory manifestations of the disease.

The Transient Receptor Potential Vanilloid 2 (TRPV2) Channel Facilitates Virus Infection Through the Ca2+ -LRMDA Axis in Myeloid Cells

The transient receptor potential vanilloid 2 (TRPV2) channel is a nonselective cation channel that has been implicated in multiple sensory processes in the nervous system. Here, it is shown that TRPV2 in myeloid cells facilitates virus penetration by promoting the tension and mobility of cell membrane through the Ca²⁺ -LRMDA axis. Knockout of TRPV2 in myeloid cells or inhibition of TRPV2 channel activity suppresses viral infection and protects mice from herpes simplex virus 1 (HSV-1) and vesicular stomatitis virus (VSV) infection. Reconstitution of TRPV2 but not the Ca²⁺ -impermeable mutant TRPV2^E572Q into LyZ2-Cre;Trpv2^fl/fl bone marrow-derived dendritic cells (BMDCs) restores viral infection. Mechanistically, knockout of TRPV2 in myeloid cells inhibits the tension and mobility of cell membrane and the penetration of viruses, which is restored by reconstitution of TRPV2 but not TRPV2^E572Q . In addition, knockout of TRPV2 leads to downregulation of Lrmda in BMDCs and BMDMs, and knockdown of Lrmda significantly downregulates the mobility and tension of cell membrane and inhibits viral infections in Trpv2^fl/fl but not LyZ2-Cre;Trpv2^fl/fl BMDCs. Consistently, complement of LRMDA into LyZ2-Cre;Trpv2^fl/fl BMDCs partially restores the tension and mobility of cell membrane and promotes viral penetration and infection. These findings characterize a previously unknown function of myeloid TRPV2 in facilitating viral infection though the Ca2⁺ -LRMDA axis.

Tranilast for advanced heart failure in patients with muscular dystrophy: a single-arm, open-label, multicenter study

Background

The transient receptor potential cation channel subfamily V member 2 (TRPV2) is a stretch-sensitive calcium channel. TRPV2 overexpression in the sarcolemma of skeletal and cardiac myocytes causes calcium influx into the cytoplasm, which triggers myocyte degeneration. In animal models of cardiomyopathy and muscular dystrophy (MD), TRPV2 inhibition was effective against heart failure and motor function. Our previous pilot study showed that tranilast, a TRPV2 inhibitor, reduced brain natriuretic peptide (BNP) levels in two MD patients with advanced heart failure. Thus, this single-arm, open-label, multicenter study aimed to evaluate the safety and efficacy of tranilast for heart failure.

Methods

The study enrolled MD patients with advanced heart failure whose serum BNP levels were > 100 pg/mL despite receiving standard cardioprotective therapy. Tranilast was administered orally at 100 mg, thrice daily. The primary endpoint was the change in log (BNP) (Δlog [BNP]) at 6 months from baseline. The null hypothesis was determined based on a previous multicenter study of carvedilol results in a mean population Δlog (BNP) of 0.18. TRPV2 expression on peripheral blood mononuclear cell surface, cardiac events, total mortality, left ventricular fractional shortening, human atrial natriuretic peptide, cardiac troponin T, and creatine kinase, and pinch strength were also assessed.

Results

Because of the poor general condition of many patients, only 18 of 34 patients were included and 13 patients could be treated according to the protocol throughout the 6-month period. However, there were no serious adverse events related to tranilast except diarrhea, a known adverse effect, and the drug was administered safely. TRPV2 expression on the mononuclear cell surface was elevated at baseline and reduced after treatment. Cardiac biomarkers such as BNP, human atrial natriuretic peptide, and fractional shortening remained stable, suggesting a protective effect against the progression of heart failure. In the per protocol set group, Δlog [BNP] was − 0.2 and significantly lower than that in the null hypothesis.

Conclusions

Tranilast is safe and effective in inhibiting TRPV2 expression, even in MD patients with advanced heart failure. Further trials are needed to evaluate the efficacy of tranilast in preventing myocardial damage, heart failure, motor impairment, and respiratory failure.

Clinical trial registration The study was registered in the UMIN Clinical Trials Registry (UMIN-CTR: UMIN000031965, URL: http://www.umin.ac.jp/ctr/) [March 30, 2018] and the Japan Registry of Clinical Trials (jRCT, registration number: jRCTs031180038, URL: https://jrct.niph.go.jp/) [November 12, 2021]. Patient registration was started in December 19, 2018.

Reprint of: Mechanosensitive ion channels in cell migration

Cellular processes are initiated and regulated by different stimuli, including mechanical forces. Cell membrane mechanosensors represent the first step towards the conversion of mechanical stimuli to a biochemical or electrical response. Mechanosensitive (MS) ion channels form a growing family of ion gating channels that respond to direct physical force or plasma membrane deformations. A number of calcium (Ca²⁺) permeable MS channels are known to regulate the initiation, direction, and persistence of cell migration during development and tumour progression. While the evidence that links individual MS ion channels to cell migration is growing, a unified analysis of the molecular mechanisms regulated downstream of MS ion channel activation is lacking. In this review, we describe the MS ion channel families known to regulate cell migration. We discuss the molecular mechanisms that act downstream of MS ion channels with an emphasis on Ca²⁺ mediated processes. Finally, we propose the future directions and impact of MS ion channel activity in the field of cell migration.

Involvement of TRP Channels in Adipocyte Thermogenesis: An Update

Obesity prevalence became a severe global health problem and it is caused by an imbalance between energy intake and expenditure. Brown adipose tissue (BAT) is a major site of mammalian non-shivering thermogenesis or energy dissipation. Thus, modulation of BAT thermogenesis might be a promising application for body weight control and obesity prevention. TRP channels are non-selective calcium-permeable cation channels mainly located on the plasma membrane. As a research focus, TRP channels have been reported to be involved in the thermogenesis of adipose tissue, energy metabolism and body weight regulation. In this review, we will summarize and update the recent progress of the pathological/physiological involvement of TRP channels in adipocyte thermogenesis. Moreover, we will discuss the potential of TRP channels as future therapeutic targets for preventing and combating human obesity and related-metabolic disorders.

Study Protocol for a Multicenter, Open-Label, Single-Arm Study of Tranilast for Cardiomyopathy of Muscular Dystrophy

Duchenne (DMD) and other forms of muscular dystrophy (MD) are collectively rare and affect approx imately 20 per 100,000 people. The on-going development of exon skipping and other novel therapies for DMD is expected to lead to improvements in motor function prognosis. However, improvements in motor dysfunction with these novel therapies are associated with the risk of increase in cardiac burden. Development of therapies to improve cardiac function, therefore, is an urgent issue. This single-arm, open-label, multicenter study will include 20 patients with MD aged 13 years or older. Tranilast, a transient receptor potential cation channel subfamily V member 2 (TRPV2) inhibitor, will be administered orally for a period of 28 weeks at a dose of 300 mg/day divided into three daily doses. If consent to continue administration is obtained at 28 weeks, the drug will be administered for an additional 116 weeks. The primary outcome will be the change in brain natriuretic peptide (BNP) at 6 months after the start of administration compared to baseline. Tranilast is an anti-allergy agent that was developed in Japan. It has been used in a large number of clinical cases, including pediatric cases, and has been shown to be safe. We expect this study to provide basic data for developing new treatment method in cardiomyopathy/skeletal myopathy using TRPV2 inhibitors. Moreover, such therapies may also be effective in treating general heart failure without MD. Therefore, if the effectiveness of TRPV2 inhibitors could be confirmed in this study, great social and economic benefits could be achieved.

Mechanosensitive ion channels in cell migration

Cellular processes are initiated and regulated by different stimuli, including mechanical forces. Cell membrane mechanosensors represent the first step towards the conversion of mechanical stimuli to a biochemical or electrical response. Mechanosensitive (MS) ion channels form a growing family of ion gating channels that respond to direct physical force or plasma membrane deformations. A number of calcium (Ca2+) permeable MS channels are known to regulate the initiation, direction, and persistence of cell migration during development and tumour progression. While the evidence that links individual MS ion channels to cell migration is growing, a unified analysis of the molecular mechanisms regulated downstream of MS ion channel activation is lacking. In this review, we describe the MS ion channel families known to regulate cell migration. We discuss the molecular mechanisms that act downstream of MS ion channels with an emphasis on Ca2+ mediated processes. Finally, we propose the future directions and impact of MS ion channel activity in the field of cell migration.

Stiffness of the extracellular matrix affects apoptosis of nucleus pulposus cells by regulating the cytoskeleton and activating the TRPV2 channel protein

It is known that nucleus pulposus cells (NPs) play an important role in intervertebral disc degeneration (IVDD), and a previous study indicated that the stiffness of NP tissue changes during the degeneration process. However, the mechanism underlying the cellular response to ECM stiffness is still unclear. To analyze the effects of extracellular matrix (ECM) with different degrees of stiffness on NPs, we prepared polyacrylamide (PA) gels with different elastic moduli, and cells grown under different stiffness conditions were obtained and analyzed. The results showed that the spreading morphology of NPs changed significantly under increased ECM elastic modulus conditions and that TRPV2 and the PI3K / AKT signaling pathway were activated by stiffer ECM. At the same time, mitochondria released cytochrome c (Cyt c) and activated caspase proteins to promote the apoptosis of NPs. After TRPV2 was specifically knocked out, the activation of the PI3K / AKT signaling pathway decreased, and the release of Cyt c and NP apoptosis were reduced. These results indicate that TRPV2 is closely linked to the detection of extracellular mechanical signals, and that conversion of mechanical and biological signals plays an important role in regulating the biological behavior of cells. This study offers a new perspective on the cellular and biochemical events underlying IVDD which could result in novel treatments.

Abnormal Calcium Handling in Duchenne Muscular Dystrophy: Mechanisms and Potential Therapies

Duchenne muscular dystrophy (DMD) is an X-linked muscle-wasting disease caused by the loss of dystrophin. DMD is associated with muscle degeneration, necrosis, inflammation, fatty replacement, and fibrosis, resulting in muscle weakness, respiratory and cardiac failure, and premature death. There is no curative treatment. Investigations on disease-causing mechanisms offer an opportunity to identify new therapeutic targets to treat DMD. An abnormal elevation of the intracellular calcium (Cai2+) concentration in the dystrophin-deficient muscle is a major secondary event, which contributes to disease progression in DMD. Emerging studies have suggested that targeting Ca²⁺-handling proteins and/or mechanisms could be a promising therapeutic strategy for DMD. Here, we provide an updated overview of the mechanistic roles the sarcolemma, sarcoplasmic/endoplasmic reticulum, and mitochondria play in the abnormal and sustained elevation of Cai2+ levels and their involvement in DMD pathogenesis. We also discuss current approaches aimed at restoring Ca²⁺ homeostasis as potential therapies for DMD.

Cannabis sativa: Interdisciplinary Strategies and Avenues for Medical and Commercial Progression Outside of CBD and THC

Cannabis sativa (Cannabis) is one of the world’s most well-known, yet maligned plant species. However, significant recent research is starting to unveil the potential of Cannabis to produce secondary compounds that may offer a suite of medical benefits, elevating this unique plant species from its illicit narcotic status into a genuine biopharmaceutical. This review summarises the lengthy history of Cannabis and details the molecular pathways that underpin the production of key secondary metabolites that may confer medical efficacy. We also provide an up-to-date summary of the molecular targets and potential of the relatively unknown minor compounds offered by the Cannabis plant. Furthermore, we detail the recent advances in plant science, as well as synthetic biology, and the pharmacology surrounding Cannabis. Given the relative infancy of Cannabis research, we go on to highlight the parallels to previous research conducted in another medically relevant and versatile plant, Papaver somniferum (opium poppy), as an indicator of the possible future direction of Cannabis plant biology. Overall, this review highlights the future directions of cannabis research outside of the medical biology aspects of its well-characterised constituents and explores additional avenues for the potential improvement of the medical potential of the Cannabis plant.

Involvement of calcium channels in the regulation of adipogenesis

As an important second messenger in adipocytes, calcium ions (Ca²⁺) are essential in regulating various intracellular signalling pathways that control critical cellular functions. Calcium channels show selective permeability to Ca²⁺ and facilitate Ca²⁺ entry into the cytoplasm, which are normally located in the plasmatic and intracellular membranes. The increase of cytosolic Ca²⁺ modulates a variety of signalling pathways and results in the transcription of target genes that contribute to adipogenesis, a key cellular event includes proliferation and differentiation of adipocyte. In the past decades, the involvement of some Ca²⁺-permeable ion channels, such as Ca²⁺ release-activated Ca²⁺ channels, transient receptor potential channels, voltage-gated calcium channels and others, in adipogenesis has been extensively explored. In the present review, we provided a summary of the expression and contributions of these Ca²⁺-permeable channels in mediating Ca²⁺ influxes that drive adipogenesis. Moreover, we discussed their potentials as future therapeutic targets.

Probenecid Improves Cardiac Function in Subjects with a Fontan Circulation and Augments Cardiomyocyte Calcium Homeostasis

Subjects with functionally univentricular circulation who have completed staged single ventricle palliation, with the final stage culminating in the Fontan procedure, are often living into adulthood. However, high morbidity and mortality remain prevalent in these patients, as diastolic and systolic dysfunction of the single systemic ventricle are linked to Fontan circulatory failure. We presently investigated the effects of probenecid in post-Fontan patients. Used for decades for the treatment of gout, probenecid has been shown in recent years to positively influence cardiac function via effects on the Transient Receptor Potential Vanilloid 2 (TRPV2) channel in cardiomyocytes. Indeed, we observed that probenecid improved cardiac function and exercise performance in patients with a functionally univentricular circulation. This was consistent with our findings from a retrospective cohort of patients with single ventricle physiology where TRPV2 expression was increased. Experiments in isolated cardiomyocytes associated these positive actions to augmentation of diastolic calcium homeostasis.

The nSMase2/Smpd3 gene modulates the severity of muscular dystrophy and the emotional stress response in mdx mice

BACKGROUND

Duchenne muscular dystrophy (DMD) is a progressive, degenerative muscular disorder and cognitive dysfunction caused by mutations in the dystrophin gene. It is characterized by excess inflammatory responses in the muscle and repeated degeneration and regeneration cycles. Neutral sphingomyelinase 2/sphingomyelin phosphodiesterase 3 (nSMase2/Smpd3) hydrolyzes sphingomyelin in lipid rafts. This protein thus modulates inflammatory responses, cell survival or apoptosis pathways, and the secretion of extracellular vesicles in a Ca²⁺-dependent manner. However, its roles in dystrophic pathology have not yet been clarified.

METHODS

To investigate the effects of the loss of nSMase2/Smpd3 on dystrophic muscles and its role in the abnormal behavior observed in DMD patients, we generated mdx mice lacking the nSMase2/Smpd3 gene (mdx:Smpd3 double knockout [DKO] mice).

RESULTS

Young mdx:Smpd3 DKO mice exhibited reduced muscular degeneration and decreased inflammation responses, but later on they showed exacerbated muscular necrosis. In addition, the abnormal stress response displayed by mdx mice was improved in the mdx:Smpd3 DKO mice, with the recovery of brain-derived neurotrophic factor (Bdnf) expression in the hippocampus.

CONCLUSIONS

nSMase2/Smpd3-modulated lipid raft integrity is a potential therapeutic target for DMD.

Identification of two natural coumarin enantiomers for selective inhibition of TRPV2 channels

Thermosensitive transient receptor potential vanilloid 2 (thermoTRPV2) is a nonselective Ca²⁺ -permeable cation channel broadly expressed, and is implicated in the pathology of diseases such as diabetes and pancreatitis. However, the physiological and pharmacological functions of TRPV2 channels have not been extensively investigated because of the absence of specific modulators. In this study, we report a pair of natural coumarin derivative enantiomers (-)-murraxocin (B304-1) and (+)-murraxocin (B304-2) from Murraya exotica for their selective inhibition of TRPV2 channels expressed in HEK293 cells and native TRPV2 currents in differentiated brown adipocytes. Whole-cell patch clamp recordings confirmed the enantiomers B304-1 and B304-2 could selectively inhibit the agonist mediated activation of TRPV2 current with IC₅₀ values of 22.2 ± 7.8 μM and 3.7 ± 0.7 μM, respectively. Molecular docking and site-directed mutagenesis revealed a key residue I600 of TRPV2 critical for the binding of the enantiomers. Furthermore, B304-1 and B304-2 significantly reversed TRPV2 agonist-induced inhibition of mouse brown adipocyte differentiation. Taken together, our identification of two natural coumarin enantiomers provides valuable tools and chemical leads for further elucidation of TRPV2 channel function, and pharmacological modulation of thermoTRPV2 in brown adipocytes may represent a new therapeutic strategy for treatment of energy imbalance or metabolic disorders.

Fine Tuning of Calcium Constitutive Entry by Optogenetically-Controlled Membrane Polarization: Impact on Cell Migration

Anomalies in constitutive calcium entry (CCE) have been commonly attributed to cell dysfunction in pathological conditions such as cancer. Calcium influxes of this type rely on channels, such as transient receptor potential (TRP) channels, to be constitutively opened and strongly depend on membrane potential and a calcium driving force. We developed an optogenetic approach based on the expression of the halorhodopsin chloride pump to study CCE in non-excitable cells. Using C2C12 cells, we found that halorhodopsin can be used to achieve a finely tuned control of membrane polarization. Escalating the membrane polarization by incremental changes in light led to a concomitant increase in CCE through transient receptor potential vanilloid 2 (TRPV2) channels. Moreover, light-induced calcium entry through TRPV2 channels promoted cell migration. Our study shows for the first time that by modulating CCE and related physiological responses, such as cell motility, halorhodopsin serves as a potentially powerful tool that could open new avenues for the study of CCE and associated cellular behaviors.

TRPV2 channel-based therapies in the cardiovascular field. Molecular underpinnings of clinically relevant therapies

The transient receptor potential (TRP) ion channel family is composed of twenty-seven channel proteins that are ubiquitously expressed in the human body. The TRPV (vanilloid) subfamily has been a recent target of investigation within the cardiovascular field. TRPV1, which is sensitive to heat as well as vanilloids, is the best characterized TRPV channel and is the namesake for the subfamily that includes six members. Research into the function of TRPV2 has suggested that it plays an important role in cardiovascular function. Over the last twenty years a greater understanding of the differences among the TRPV channels has allowed for more precise experimentation and has opened various translational opportunities. TRPV2 has been found to be a both a mechanosensor and a mediator of calcium handling and has been found to play important roles in healthy and diseased cardiomyocytes. These roles have been translated into clinical studies in patients with muscular dystrophy (both agonism and antagonism) as well as in patients with cardiomyopathy and heart failure with reduced ejection fraction. Its role as a structural protein has also been elucidated, though the clinical significance of this finding has yet to be established. Despite the clinical progress that has been made there is still a need for large, prospective randomized studies with TRPV2 channel agonists and antagonists in order to bring these basic and translational science findings to the bedside.

Production of TRPV2-targeting functional antibody ameliorating dilated cardiomyopathy and muscular dystrophy in animal models

Abnormal Ca²⁺ handling is essential in the pathophysiology of degenerative muscle disorders, such as dilated cardiomyopathy (DCM) and muscular dystrophy (MD). Transient receptor potential cation channel, subfamily V, member 2 (TRPV2) is a candidate for Ca²⁺ entry and a potential therapeutic target for degenerative muscle disorders, there are few specific inhibitors for TRPV2. In this study, we produced a monoclonal antibody (designated mAb88-2) and two polyclonal antibodies (pAb591 and pAb592) that selectively recognize TRPV2 from the outside of cells and interact with the turret region of the pore-forming outer gate. These antibodies inhibited Ca²⁺ influx via TRPV2 in cultured cells and substantially reduced TRPV2 in the plasma membrane via cellular internalization. We evaluated the therapeutic efficacy of the functional antibody in δ-sarcoglycan-deficient hamster (J2N-k) models of DCM and MD and in the 4C30 DCM model of murine heart failure. The intraperitoneal administration of the functional antibody (0.5 mg/kg) for 2 weeks (once a week) prevented the progression of cardiac dysfunction, as evaluated by echocardiography and histological staining, and improved the abnormal Ca²⁺ handling (high diastolic Ca²⁺ level and small Ca²⁺ transient peak) in cardiomyocytes isolated from J2N-k hamsters and prevented skeletal muscle damage. Further, the antibody effectively prevented heart failure in the 4C30 mouse model with end-stage DCM. Interestingly, endogenous TRPV2 that accumulated in the cardiac and skeletal muscle sarcolemma disappeared upon antibody administration. Thus, the newly produced antibodies are capable of ameliorating DCM and MD by promoting the cellular internalization of TRPV2; antibodies specific to human TRPV2 may substantially improve the treatment of patients with degenerative muscle diseases.

Potential roles in cardiac physiology and pathology of the cation channel TRPV2 expressed in cardiac cells and cardiac macrophages: a mini-review

TRPV2 is a well-conserved channel protein expressed in almost all tissues. Cardiomyocyte TRPV2 is expressed in the intercalated disks of the cardiac sarcomeres, where it is involved in maintaining the proper mechanoelectric coupling and structure. It is also abundantly expressed in the intracellular pools, mainly the endoplasmic reticulum. Under pathological conditions, TRPV2 is translocated to the sarcolemma, where it mediates an abnormal [Ca]²⁺ entry that may contribute to disease progression. In addition, an intracellularly diffused TRPV2 expression is present in resident cardiac macrophages. Upon infection or inflammation, TRPV2 is engaged in early phagosomes and is, therefore, potentially involved in protecting the cardiac tissue. Following acute myocardial infarction, a profound elevated expression of TRPV2 is observed on the cell membrane of the peri-infarct macrophages. The macrophage TRPV2 may harbor a detrimental effect in cardiac recovery by increasing unfavorable migration and phagocytosis processes in the injured heart. Most reports suggest that while cardiac TRPV2 activation may be beneficial under specific physiological conditions, both cardiac- and macrophage-related TRPV2 blocking can significantly ameliorate disease progression in various pathological states. To verify this possibility, the time frame of TRPV2 overexpression and its mediated signaling need to be fully characterized in both cardiomyocyte and cardiac macrophage populations.

Blockade of TRPV2 is a Novel Therapy for Cardiomyopathy in Muscular Dystrophy

Muscular dystrophy and dilated cardiomyopathy are intractable diseases and their treatment options are very limited. Transient receptor potential cation channel subfamily V, member 2 (TRPV2), is a stretch-sensitive Ca²⁺-permeable channel that causes sustained intracellular Ca²⁺ increase in muscular cells, which is a pathophysiological feature of degenerative muscular disease. Recent reports have clarified that TRPV2 is concentrated and activated in the sarcolemma of cardiomyocytes/myocytes during cardiomyopathy/heart failure and muscular dystrophy. Furthermore, these reports showed that inactivation of TRPV2 ameliorates muscle dysgenesis to improve cardiac function and survival prognosis. Although TRPV2 is a potential therapeutic target for cardiomyopathy, there were no TRPV2 inhibitors available until recently. In this review, we introduce our recent findings and discuss the current progress in the development of TRPV2 inhibitors and their therapeutic applications for cardiomyopathy associated with muscular dystrophy.

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.

Role of STIM1/ORAI1-mediated store-operated Ca2+ entry in skeletal muscle physiology and disease

Store-operated Ca²⁺ entry (SOCE) is a Ca²⁺ entry mechanism activated by depletion of intracellular Ca²⁺ stores. In skeletal muscle, SOCE is mediated by an interaction between stromal-interacting molecule-1 (STIM1), the Ca²⁺ sensor of the sarcoplasmic reticulum, and ORAI1, the Ca²⁺-release-activated-Ca²⁺ (CRAC) channel located in the transverse tubule membrane. This review focuses on the molecular mechanisms and physiological role of SOCE in skeletal muscle, as well as how alterations in STIM1/ORAI1-mediated SOCE contribute to muscle disease. Recent evidence indicates that SOCE plays an important role in both muscle development/growth and fatigue. The importance of SOCE in muscle is further underscored by the discovery that loss- and gain-of-function mutations in STIM1 and ORAI1 result in an eclectic array of disorders with clinical myopathy as central defining component. Despite differences in clinical phenotype, all STIM1/ORAI1 gain-of-function mutations-linked myopathies are characterized by the abnormal accumulation of intracellular membranes, known as tubular aggregates. Finally, dysfunctional STIM1/ORAI1-mediated SOCE also contributes to the pathogenesis of muscular dystrophy, malignant hyperthermia, and sarcopenia. The picture to emerge is that tight regulation of STIM1/ORAI1-dependent Ca²⁺ signaling is critical for optimal skeletal muscle development/function such that either aberrant increases or decreases in SOCE activity result in muscle dysfunction.

Role of Thermo-Sensitive Transient Receptor Potential Channels in Brown Adipose Tissue

Brown and beige adipocytes are a major site of mammalian non-shivering thermogenesis and energy dissipation. Obesity is caused by an imbalance between energy intake and expenditure and has become a worldwide health problem. Therefore modulation of thermogenesis in brown and beige adipocytes could be an important application for body weight control and obesity prevention. Over the last few decades, the involvement of thermo-sensitive transient receptor potential (TRP) channels (including TRPV1, TRPV2, TRPV3, TRPV4, TRPM4, TRPM8, TRPC5, and TRPA1) in energy metabolism and adipogenesis in adipocytes has been extensively explored. In this review, we summarize the expression, function, and pathological/physiological contributions of these TRP channels and discuss their potential as future therapeutic targets for preventing and combating human obesity and obesity-related metabolic disorders.

Biochemical and Functional Interplay Between Ion Channels and the Components of the Dystrophin-Associated Glycoprotein Complex

Dystrophin is a cytoskeleton-linked membrane protein that binds to a larger multiprotein assembly called the dystrophin-associated glycoprotein complex (DGC). The deficiency of dystrophin or the components of the DGC results in the loss of connection between the cytoskeleton and the extracellular matrix with significant pathophysiological implications in skeletal and cardiac muscle as well as in the nervous system. Although the DGC plays an important role in maintaining membrane stability, it can also be considered as a versatile and flexible molecular complex that contribute to the cellular organization and dynamics of a variety of proteins at specific locations in the plasma membrane. This review deals with the role of the DGC in transmembrane signaling by forming supramolecular assemblies for regulating ion channel localization and activity. These interactions are relevant for cell homeostasis, and its alterations may play a significant role in the etiology and pathogenesis of various disorders affecting muscle and nerve function.

Novel inhibitor candidates of TRPV2 prevent damage of dystrophic myocytes and ameliorate against dilated cardiomyopathy in a hamster model

Transient receptor potential cation channel, subfamily V, member 2 (TRPV2) is a principal candidate for abnormal Ca²⁺-entry pathways, which is a potential target for therapy of muscular dystrophy and cardiomyopathy. Here, an in silico drug screening and the following cell-based screening to measure the TRPV2 activation were carried out in HEK293 cells expressing TRPV2 using lead compounds (tranilast or SKF96365) and off-patent drug stocks. We identified 4 chemical compounds containing amino-benzoyl groups and 1 compound (lumin) containing an ethylquinolinium group as candidate TRPV2 inhibitors. Three of these compounds inhibited Ca²⁺ entry through both mouse and human TRPV2, with IC₅₀ of less than 10 μM, but had no apparent effect on other members of TRP family such as TRPV1 and TRPC1. Particularly, lumin inhibited agonist-induced TRPV2 channel activity at a low dose. These compounds inhibited abnormally increased Ca²⁺ influx and prevented stretch-induced skeletal muscle damage in cultured myocytes from dystrophic hamsters (J2N-k). Further, they ameliorated cardiac dysfunction, and prevented disease progression in vivo in the same J2N-k hamsters developing dilated cardiomyopathy as well as muscular dystrophy. The identified compounds described here are available as experimental tools and represent potential treatments for patients with cardiomyopathy and muscular dystrophy.

A Pilot Study of Tranilast for Cardiomyopathy of Muscular Dystrophy

Objective Heart failure is currently the most serious complication of muscular dystrophy. The transient receptor potential cation channel, subfamily V, member 2 (TRPV2) is a stretch-sensitive Ca channel. In damaged myocytes or cardiomyocytes, TRPV2 translocates to the cytoplasmic membrane and enhances Ca influx, triggering cell damage. Evidence suggests that the inhibition of TRPV2 may be a new therapeutic target in heart failure. We found that tranilast, which is widely used as an anti-allergic drug, inhibits TRPV2. A pilot study was conducted to assess the safety and efficacy of tranilast in muscular dystrophy patients with cardiomyopathy. Methods After obtaining informed consent, two muscular dystrophy patients with advanced heart failure took tranilast (300 mg/day) for three months. Blood tests, echocardiography, electrocardiography (ECG), Holter ECG, analyses of the TRPV2 expression in peripheral mononuclear cells, and circulating micro ribonucleic acid profiling were performed to assess the safety and efficacy of tranilast. Results The brain natriuretic peptide levels decreased after treatment. The expression of TRPV2 on the cytoplasmic membrane of peripheral mononuclear cells was enhanced before treatment and was decreased after treatment. Some heart-related micro ribonucleic acids (miR-208a-5p, miR-223-3p) were elevated and then decreased after treatment. Some adverse events, including the potentiation of warfarin, the worsening of renal dysfunction, an increased heart rate and premature ventricular contractions, were observed. Conclusion Tranilast can inhibit TRPV2 and can be effective for treating heart failure, even in patients with muscular dystrophy. Although careful attention is needed, the inhibition of TRPV2 can be a new treatment target for cardiomyopathy. A multi-center trial is planned.

Transient receptor potential channel 6 regulates abnormal cardiac S-nitrosylation in Duchenne muscular dystrophy

Duchenne muscular dystrophy (DMD) is an X-linked disorder with dystrophin loss that results in skeletal and cardiac muscle weakening and early death. Loss of the dystrophin-sarcoglycan complex delocalizes nitric oxide synthase (NOS) to alter its signaling, and augments mechanosensitive intracellular Ca²⁺ influx. The latter has been coupled to hyperactivation of the nonselective cation channel, transient receptor potential canonical channel 6 (Trpc6), in isolated myocytes. As Ca²⁺ also activates NOS, we hypothesized that Trpc6 would help to mediate nitric oxide (NO) dysregulation and that this would be manifest in increased myocardial S-nitrosylation, a posttranslational modification increasingly implicated in neurodegenerative, inflammatory, and muscle disease. Using a recently developed dual-labeling proteomic strategy, we identified 1,276 S-nitrosylated cysteine residues [S-nitrosothiol (SNO)] on 491 proteins in resting hearts from a mouse model of DMD (dmd^mdx:utrn^+/-). These largely consisted of mitochondrial proteins, metabolic regulators, and sarcomeric proteins, with 80% of them also modified in wild type (WT). S-nitrosylation levels, however, were increased in DMD. Genetic deletion of Trpc6 in this model (dmd^mdx:utrn^+/-:trpc6^-/-) reversed ∼70% of these changes. Trpc6 deletion also ameliorated left ventricular dilation, improved cardiac function, and tended to reduce fibrosis. Furthermore, under catecholamine stimulation, which also increases NO synthesis and intracellular Ca²⁺ along with cardiac workload, the hypernitrosylated state remained as it did at baseline. However, the impact of Trpc6 deletion on the SNO proteome became less marked. These findings reveal a role for Trpc6-mediated hypernitrosylation in dmd^mdx:utrn^+/- mice and support accumulating evidence that implicates nitrosative stress in cardiac and muscle disease.

Reducing sarcolipin expression mitigates Duchenne muscular dystrophy and associated cardiomyopathy in mice

Sarcolipin (SLN) is an inhibitor of the sarco/endoplasmic reticulum (SR) Ca²⁺ ATPase (SERCA) and is abnormally elevated in the muscle of Duchenne muscular dystrophy (DMD) patients and animal models. Here we show that reducing SLN levels ameliorates dystrophic pathology in the severe dystrophin/utrophin double mutant (mdx:utr ^-/-) mouse model of DMD. Germline inactivation of one allele of the SLN gene normalizes SLN expression, restores SERCA function, mitigates skeletal muscle and cardiac pathology, improves muscle regeneration, and extends the lifespan. To translate our findings into a therapeutic strategy, we knock down SLN expression in 1-month old mdx:utr ^-/- mice via adeno-associated virus (AAV) 9-mediated RNA interference. The AAV treatment markedly reduces SLN expression, attenuates muscle pathology and improves diaphragm, skeletal muscle and cardiac function. Taken together, our findings suggest that SLN reduction is a promising therapeutic approach for DMD.

The role of transient receptor potential vanilloid 2 channel in cardiac aging

BACKGROUND

The aging heart is characterized by cellular and molecular changes leading to a decline in physiologic function and cardiac remodeling, specifically the development of myocyte hypertrophy and fibrosis. Transient receptor potential vanilloid 2 (TRPV2), a stretch-mediated channel and regulator of calcium homeostasis, plays a key role in the function and structure of the heart. TRPV2 also plays an important role in the adaptive and maladaptive compensatory mechanisms of the heart in response to pathologic and exercise-induced stress. Our current study seeks to elucidate the potential role of TRPV2 channels in the regulation of cardiac function in aging.

METHODS

Wild-type (WT) and TRPV2 functional knockout (FKO) mice were aged out to various time points, and their cardiac function was measured using advanced echocardiography. Furthermore, we histologically analyzed the heart morphology to determine myocyte hypertrophy, the development of fibrosis and the relative expression of TRPV2.

RESULTS

Our results demonstrate that even though TRPV2-FKO mice have impaired function at baseline, their cardiac function as measured via standard and advanced echocardiographic parameters (ejection fraction, cardiac output and circumferential strain) decreased less with aging in comparison with the WT group. Furthermore, there was less fibrosis and hypertrophy in the TRPV2-FKO group with aging in comparison with the WT. The expression of TRPV2 in the WT group did not significantly change with aging.

CONCLUSIONS

TRPV2 functional deletion is compatible with aging and associated with a decreased development of myocyte hypertrophy and fibrosis. It may be an important target for prevention of age-induced cardiac remodeling.

Involvement of thermosensitive TRP channels in energy metabolism

To date, 11 thermosensitive transient receptor potential (thermo-TRP) channels have been identified. Recent studies have characterized the mechanism of thermosensing by thermo-TRPs and the physiological role of thermo-TRPs in energy metabolism. In this review, we highlight the role of various thermo-TRPs in energy metabolism and hormone secretion. In the pancreas, TRPM2 and other TRPs regulate insulin secretion. TRPV2 expressed in brown adipocytes contributes to differentiation and/or thermogenesis. Sensory nerves that express TRPV1 promote increased energy expenditure by activating sympathetic nerves and adrenaline secretion. Here, we first show that capsaicin-induced adrenaline secretion is completely impaired in TRPV1 knockout mice. The thermogenic effects of TRPV1 agonists are attributable to brown adipose tissue (BAT) activation in mice and humans. Moreover, TRPA1- and TRPM8-expressing sensory nerves also contribute to potentiation of BAT thermogenesis and energy expenditure in mice. Together, thermo-TRPs are promising targets for combating obesity and metabolic disorders.

Haplotypes for Type, Degree, and Rate of Marbling in Cattle Are Syntenic with Human Muscular Dystrophy

Traditional analyses of a QTL on Bota 19 implicate a surfeit of candidates, but each is of marginal significance in explaining the deposition of healthy, low melting temperature fat within marbled muscle of Wagyu cattle. As an alternative approach, we have used genomic, multigenerational segregation to identify 14 conserved, ancestral 20 Mb haplotypes. These determine the degree and rate of marbling in Wagyu and other breeds of cattle. The melting temperature of intramuscular fat is highly heritable and traceable by haplotyping. Fortunately, for the production of healthy beef, some of these haplotypes are sufficiently penetrant to be expressed in heterozygous crossbreds, thereby allowing selection of sires which will improve the healthiness of beef produced under even harsh climatic conditions. The region of Bota 19 is syntenic to a region of Hosa 17 known to be important in muscle metabolism and in determining susceptibility to a form of human muscular dystrophy.

Transient receptor potential vanilloid 2 function regulates cardiac hypertrophy via stretch-induced activation

OBJECTIVE

Hypertension (increased afterload) results in cardiomyocyte hypertrophy leading to left ventricular hypertrophy and subsequently, heart failure with preserved ejection fraction. This study was performed to test the hypothesis that transient receptor potential vanilloid 2 subtype (TRPV2) function regulates hypertrophy under increased afterload conditions.

METHODS

We used functional (pore specific) TRPV2 knockout mice to evaluate the effects of increased afterload-induced stretch on cardiac size and function via transverse aortic constriction (TAC) as well as hypertrophic stimuli including adrenergic and angiotensin stimulation via subcutaneous pumps. Wild-type animals served as control for all experiments. Expression and localization of TRPV2 was investigated in wild-type cardiac samples. Changes in cardiac function were measured in vivo via echocardiography and invasive catheterization. Molecular changes, including protein and real-time PCR markers of hypertrophy, were measured in addition to myocyte size.

RESULTS

TRPV2 is significantly upregulated in wild-type mice after TAC, though not in response to beta-adrenergic or angiotensin stimulation. TAC-induced stretch stimulus caused an upregulation of TRPV2 in the sarcolemmal membrane. The absence of functional TRPV2 resulted in significantly reduced left ventricular hypertrophy after TAC, though not in response to beta-adrenergic or angiotensin stimulation. The decreased development of hypertrophy was not associated with significant deterioration of cardiac function.

CONCLUSION

We conclude that TRPV2 function, as a stretch-activated channel, regulates the development of cardiomyocyte hypertrophy in response to increased afterload.

Stretch-activated TRPV2 channels: Role in mediating cardiopathies

Transient receptor potential vanilloid type 2, TRPV2, is a calcium-permeable cation channel belonging to the TRPV channel family. Although this channel has been first characterized as a noxious heat sensor, its mechanosensor property recently gained importance in various physiological functions. TRPV2 has been described as a stretch-mediated channel and a regulator of calcium homeostasis in several cell types and has been shown to be involved in the stretch-dependent responses in cardiomyocytes. Hence, several studies in the last years support the idea that TRPV2 play a key role in the function and structure of the heart, being involved in the cardiac compensatory mechanisms in response to pathologic or exercise-induced stress. We present here an overview of the current literature and concepts of TRPV2 channels involvement (i) in the mechanical coupling mechanisms in heart and (ii) in the mechanisms that lead to cardiomyopathies. All these studies lead us to think that TRPV2 may also be an important cardiac drug target based on its major physiological roles in heart.

Heat induces interleukin-6 in skeletal muscle cells via TRPV1/PKC/CREB pathways

Interleukin-6 (IL-6) is released from skeletal muscle cells and induced by exercise, heat, catecholamine, glucose, lipopolysaccharide, reactive oxygen species, and inflammation. However, the mechanism that induces release of IL-6 from skeletal muscle cells remains unknown. Thermosensitive transient receptor potential (TRP) proteins such as TRPV1-4 play vital roles in cellular functions. In this study we hypothesized that TRPV1 senses heat, transmits a signal into the nucleus, and produces IL-6. The purpose of the present study is to investigate the underlying mechanisms whereby skeletal muscle cells sense and respond to heat. When mouse myoblast cells were exposed to 37-42°C for 2 h, mRNA expression of IL-6 increased in a temperature-dependent manner. Heat also increased IL-6 secretion in myoblast cells. A fura 2 fluorescence dual-wavelength excitation method showed that heat increased intracellular calcium flux in a temperature-dependent manner. Intracellular calcium flux and IL-6 mRNA expression were increased by the TRPV1 agonists capsaicin and N-arachidonoyldopamine and decreased by the TRPV1 antagonists AMG9810 and SB366791 and siRNA-mediated knockdown of TRPV1. TRPV2, 3, and 4 agonists did not change intracellular calcium flux. Western blotting with inhibitors demonstrated that heat increased phosphorylation levels of TRPV1, followed by PKC and cAMP response element-binding protein (CREB). PKC inhibitors, Gö6983 and staurosporine, CREB inhibitors, curcumin and naphthol AS-E, and knockdown of CREB suppressed the heat-induced increases in IL-6. These results indicate that heat increases IL-6 in skeletal muscle cells through the TRPV1, PKC, and CREB signal transduction pathway.NEW & NOTEWORTHY Heat increases the release of interleukin-6 (IL-6) from skeletal muscle cells. IL-6 has been shown to serve immune responses and metabolic functions in muscle. It can be anti-inflammatory as well as proinflammatory. However, the mechanism that induces release of IL-6 from skeletal muscle cells remains unknown. Here we show that heat increases IL-6 in skeletal muscle cells through the transient receptor potential vannilloid 1, PKC, and cAMP response element-binding protein signal transduction pathway.

Mitochondria mediate cell membrane repair and contribute to Duchenne muscular dystrophy

Dystrophin deficiency is the genetic basis for Duchenne muscular dystrophy (DMD), but the cellular basis of progressive myofiber death in DMD is not fully understood. Using two dystrophin-deficient mdx mouse models, we find that the mitochondrial dysfunction is among the earliest cellular deficits of mdx muscles. Mitochondria in dystrophic myofibers also respond poorly to sarcolemmal injury. These mitochondrial deficits reduce the ability of dystrophic muscle cell membranes to repair and are associated with a compensatory increase in dysferlin-mediated membrane repair proteins. Dysferlin deficit in mdx mice further compromises myofiber cell membrane repair and enhances the muscle pathology at an asymptomatic age for dysferlin-deficient mice. Restoring partial dystrophin expression by exon skipping improves mitochondrial function and offers potential to improve myofiber repair. These findings identify that mitochondrial deficit in muscular dystrophy compromises the repair of injured myofibers and show that this repair mechanism is distinct from and complimentary to the dysferlin-mediated repair of injured myofibers.

Anti-inflammatory drugs for Duchenne muscular dystrophy: focus on skeletal muscle-releasing factors

Duchenne muscular dystrophy (DMD), an incurable and a progressive muscle wasting disease, is caused by the absence of dystrophin protein, leading to recurrent muscle fiber damage during contraction. The inflammatory response to fiber damage is a compelling candidate mechanism for disease exacerbation. The only established pharmacological treatment for DMD is corticosteroids to suppress muscle inflammation, however this treatment is limited by its insufficient therapeutic efficacy and considerable side effects. Recent reports show the therapeutic potential of inhibiting or enhancing pro- or anti-inflammatory factors released from DMD skeletal muscles, resulting in significant recovery from muscle atrophy and dysfunction. We discuss and review the recent findings of DMD inflammation and opportunities for drug development targeting specific releasing factors from skeletal muscles. It has been speculated that nonsteroidal anti-inflammatory drugs targeting specific inflammatory factors are more effective and have less side effects for DMD compared with steroidal drugs. For example, calcium channels, reactive oxygen species, and nuclear factor-κB signaling factors are the most promising targets as master regulators of inflammatory response in DMD skeletal muscles. If they are combined with an oligonucleotide-based exon skipping therapy to restore dystrophin expression, the anti-inflammatory drug therapies may address the present therapeutic limitation of low efficiency for DMD.

Fast skeletal myofibers of mdx mouse, model of Duchenne muscular dystrophy, express connexin hemichannels that lead to apoptosis

Skeletal muscles of patients with Duchenne muscular dystrophy (DMD) show numerous alterations including inflammation, apoptosis, and necrosis of myofibers. However, the molecular mechanism that explains these changes remains largely unknown. Here, the involvement of hemichannels formed by connexins (Cx HCs) was evaluated in skeletal muscle of mdx mouse model of DMD. Fast myofibers of mdx mice were found to express three connexins (39, 43 and 45) and high sarcolemma permeability, which was absent in myofibers of mdx Cx43(fl/fl)Cx45(fl/fl):Myo-Cre mice (deficient in skeletal muscle Cx43/Cx45 expression). These myofibers did not show elevated basal intracellular free Ca(2+) levels, immunoreactivity to phosphorylated p65 (active NF-κB), eNOS and annexin V/active Caspase 3 (marker of apoptosis) but presented dystrophin immunoreactivity. Moreover, muscles of mdx Cx43(fl/fl)Cx45(fl/fl):Myo-Cre mice exhibited partial decrease of necrotic features (big cells and high creatine kinase levels). Accordingly, these muscles showed similar macrophage infiltration as control mdx muscles. Nonetheless, the hanging test performance of mdx Cx43(fl/fl)Cx45(fl/fl):Myo-Cre mice was significantly better than that of control mdx Cx43(fl/fl)Cx45(fl/fl) mice. All three Cxs found in skeletal muscles of mdx mice were also detected in fast myofibers of biopsy specimens from patients with muscular dystrophy. Thus, reduction of Cx expression and/or function of Cx HCs may be potential therapeutic approaches to abrogate myofiber apoptosis in DMD.

Activation of TRPV2 negatively regulates the differentiation of mouse brown adipocytes

Transient receptor potential vanilloid 2 (TRPV2) acts as a Ca(2+)-permeable non-selective cation channel that has been reported to be sensitive to temperature, mechanical force, and some chemicals. We recently showed that TRPV2 is critical for maintenance of the thermogenic function of brown adipose tissue in mice. However, the involvement of TRPV2 in the differentiation of brown adipocytes remains unexplored. We found that the expression of TRPV2 was dramatically increased during the differentiation of brown adipocytes. Non-selective TRPV2 agonists (2-aminoethoxydiphenyl borate and lysophosphatidylcholine) inhibited the differentiation of brown adipocytes in a dose-dependent manner during the early stage of differentiation of brown adipocytes. The inhibition was rescued by a TRPV2-selective antagonist, SKF96365 (SKF). Mechanical force, which activates TRPV2, also inhibited the differentiation of brown adipocytes in a strength-dependent manner, and the effect was reversed by SKF. In addition, the inhibition of adipocyte differentiation by either TRPV2 ligand or mechanical stimulation was significantly smaller in the cells from TRPV2KO mice. Moreover, calcineurin inhibitors, cyclosporine A and FK506, partially reversed TRPV2 activation-induced inhibition of brown adipocyte differentiation. Thus, we conclude that TRPV2 might be involved in the modulation of brown adipocyte differentiation partially via a calcineurin pathway.

Cancer cachexia causes skeletal muscle damage via transient receptor potential vanilloid 2-independent mechanisms, unlike muscular dystrophy

BACKGROUND

Muscle wasting during cancer cachexia contributes to patient morbidity. Cachexia-induced muscle damage may be understood by comparing its symptoms with those of other skeletal muscle diseases, but currently available data are limited.

METHODS

We modelled cancer cachexia in mice bearing Lewis lung carcinoma/colon adenocarcinoma and compared the associated muscle damage with that in a murine muscular dystrophy model (mdx mice). We measured biochemical and immunochemical parameters: amounts/localization of cytoskeletal proteins and/or Ca(2+) signalling proteins related to muscle function and abnormality. We analysed intracellular Ca(2+) mobilization and compared results between the two models. Involvement of Ca(2+)-permeable channel transient receptor potential vanilloid 2 (TRPV2) was examined by inoculating Lewis lung carcinoma cells into transgenic mice expressing dominant-negative TRPV2.

RESULTS

Tumourigenesis caused loss of body and skeletal muscle weight and reduced muscle force and locomotor activity. Similar to mdx mice, cachexia muscles exhibited myolysis, reduced sarcolemmal sialic acid content, and enhanced lysosomal exocytosis and sarcolemmal localization of phosphorylated Ca(2+)/CaMKII. Abnormal autophagy and degradation of dystrophin also occurred. Unlike mdx muscles, cachexia muscles did not exhibit regeneration markers (centrally nucleated fibres), and levels of autophagic proteolytic pathway markers increased. While a slight accumulation of TRPV2 was observed in cachexia muscles, Ca(2+) influx via TRPV2 was not elevated in cachexia-associated myotubes, and the course of cachexia pathology was not ameliorated by dominant-negative inhibition of TRPV2.

CONCLUSIONS

Thus, cancer cachexia may induce muscle damage through TRPV2-independent mechanisms distinct from those in muscular dystrophy; this may help treat patients with tumour-induced muscle wasting.