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

The Unexplored Role of Connexin Hemichannels in Promoting Facioscapulohumeral Muscular Dystrophy Progression

DUX4 is typically a repressed transcription factor, but its aberrant activation in Facioscapulohumeral Muscular Dystrophy (FSHD) leads to cell death by disrupting muscle homeostasis. This disruption affects crucial processes such as myogenesis, sarcolemma integrity, gene regulation, oxidative stress, immune response, and many other biological pathways. Notably, these disrupted processes have been associated, in other pathological contexts, with the presence of connexin (Cx) hemichannels-transmembrane structures that mediate communication between the intracellular and extracellular environments. Thus, hemichannels have been implicated in skeletal muscle atrophy, as observed in human biopsies and animal models of Duchenne Muscular Dystrophy, Becker Muscular Dystrophy, and Dysferlinopathies, suggesting a potentially shared mechanism of muscle atrophy that has not yet been explored in FSHD. Despite various therapeutic strategies proposed to manage FSHD, no treatment or cure is currently available. This review summarizes the current understanding of the mechanisms underlying FSHD progression, with a focus on hormones, inflammation, reactive oxygen species (ROS), and mitochondrial function. Additionally, it explores the potential of targeting hemichannels as a therapeutic strategy to slow disease progression by preventing the spread of pathogenic factors between muscle cells.

Skeletal muscle atrophy after sciatic nerve damage: Mechanistic insights

Sciatic nerve injury leads to molecular events that cause muscular dysfunction advancement in atrophic conditions. Nerve damage renders muscles permanently relaxed which elevates intracellular resting Ca2+ levels. Increased Ca2+ levels are associated with several cellular signaling pathways including AMPK, cGMP, PLC-β, CERB, and calcineurin. Also, multiple enzymes involved in the tricarboxylic acid cycle and oxidative phosphorylation are activated by Ca2+ influx into mitochondria during muscle contraction, to meet increased ATP demand. Nerve damage induces mitophagy and skeletal muscle atrophy through increased sensitivity to Ca2+-induced opening of the permeability transition pore (PTP) in mitochondria attributed to Ca2+, ROS, and AMPK overload in muscle. Activated AMPK interacts negatively with Akt/mTOR is a highly prevalent and well-described central pathway for anabolic processes. Over the decade several reports indicate abnormal behavior of signaling machinery involved in denervation-induced muscle loss but end up with some controversial outcomes. Therefore, understanding how the synthesis and inhibitory stimuli interact with cellular signaling to control muscle mass and morphology may lead to new pharmacological insights toward understanding the underlying mechanism of muscle loss after sciatic nerve damage. Hence, the present review summarizes the existing literature on denervation-induced muscle atrophy to evaluate the regulation and expression of differential regulators during sciatic damage.

Orally administered boldine reduces muscle atrophy and promotes neuromuscular recovery in a rodent model of delayed nerve repair

Peripheral nerve injury often results in poor functional recovery due to a prolonged period of muscle denervation. In particular, absent axonal contact, denervated muscle can undergo irrevocable atrophy and diminished receptiveness for reinnervation over time, ultimately reducing the likelihood for meaningful neuromuscular recovery. While innovative surgical approaches can minimize the harmful effects of denervation by re-routing neighboring-otherwise uninjured-axons, there are no clinically-available approaches to preserve the reinnervation capacity of denervated muscles. Blocking intramuscular connexin hemichannel formation has been reported to improve muscle innervation in vitro and prevent atrophy in vivo. Therefore, the current study investigated the effects of orally administered boldine, a connexin hemichannel inhibitor, on denervated-related muscle changes and nerve regeneration in a rat model of delayed peripheral nerve repair. We found that daily boldine administration significantly enhanced an evoked response in the tibialis anterior muscle at 2 weeks after common peroneal nerve transection, and decreased intramuscular connexin 43 and 45 expression, intraneural Schwann cell expression of connexin 43, and muscle fiber atrophy up to 4 weeks post transection. Additional animals underwent a cross nerve repair procedure (tibial to common peroneal neurorrhaphy) at 4 weeks following the initial transection injury. Here, we found elevated nerve electrophysiological activity and greater muscle fiber maturation at 6 weeks post repair in boldine treated animals. These findings suggest that boldine may be a promising pharmacological approach to minimize the deleterious effects of prolonged denervation and, with further optimization, may improve levels of functional recovery following nerve repair.

Assessing the impact of boldine on the gastrocnemius using multiomics profiling at 7 and 28 days post-complete spinal cord injury in young male mice

Spinal cord injury (SCI) results in rapid muscle loss. Exogenous molecular interventions to slow muscle atrophy after SCI have been relatively ineffective and require the search for novel therapeutic targets. Connexin hemichannels (CxHCs) allow nonselective passage of small molecules into and out of the cell. Boldine, a CxHC-inhibiting aporphine found in the boldo tree (Peumus boldus), has shown promising preclinical results in slowing atrophy during sepsis and restoring muscle function in dysferlinopathy. We administered 50 mg/kg/day of boldine to spinal cord transected mice beginning 3 days post-injury. Tissue was collected 7 and 28 days post-SCI and the gastrocnemius was used for multiomics profiling. Boldine did not prevent body or muscle mass loss but attenuated SCI-induced changes in the abundance of the amino acids proline, phenylalanine, leucine and isoleucine, as well as glucose, 7 days post-SCI. SCI resulted in the differential expression of ∼7,700 and ∼2,000 genes at 7 and 28 days, respectively, compared with Sham controls. Pathway enrichment of these genes highlighted ribosome biogenesis at 7 days and translation and oxidative phosphorylation at both timepoints. Boldine altered the expression of ∼150 genes at 7 days and ∼110 genes at 28 days post-SCI. Pathway enrichment of these genes indicated a potential role for boldine in suppressing protein ubiquitination and degradation at the 7-day timepoint. Methylation analyses showed minimal differences between groups. Taken together, boldine is not an efficacious therapy to preserve body and muscle mass after complete SCI, though it attenuated some SCI-induced changes across the metabolome and transcriptome.NEW & NOTEWORTHY This is the first study to describe the multiome of skeletal muscle paralyzed by a spinal cord injury (SCI) in mice across the acute and subacute timeframe after injury. We show large-scale changes in the metabolome and transcriptome at 7 days post-injury compared with 28 days. Furthermore, we show that the alkaloid boldine was able to prevent SCI-induced changes in muscle glucose and free amino acid levels at 7 days, but not 28 days, after SCI.

Mini-review: Local and downstream responses to the neuromuscular junction: Potential roles for integrins, connexins and ephrins in altering muscle characteristics and function

Skeletal muscle develops in a manner directly related to its innervating motor neuron. The formation of the neuromuscular junction (NMJ) is a well-described process that is coordinated to allow for efficient communication between the central nervous system and muscle for muscle contraction and movement. Some of the major mediators of NMJ formation, like muscle-specific kinase, agrin and laminin, have been thoroughly described but there are other important proteins that have an integral role in muscle health that have also been associated with proper NMJ integrity and fiber health and function. This mini-review focuses on integrins, connexin hemichannels and ephrins and their relationship with the NMJin regulating muscle health.

Pannexin 1 Regulates Skeletal Muscle Regeneration by Promoting Bleb-Based Myoblast Migration and Fusion Through a Novel Lipid Based Signaling Mechanism

Adult skeletal muscle has robust regenerative capabilities due to the presence of a resident stem cell population called satellite cells. Muscle injury leads to these normally quiescent cells becoming molecularly and metabolically activated and embarking on a program of proliferation, migration, differentiation, and fusion culminating in the repair of damaged tissue. These processes are highly coordinated by paracrine signaling events that drive cytoskeletal rearrangement and cell-cell communication. Pannexins are a family of transmembrane channel proteins that mediate paracrine signaling by ATP release. It is known that Pannexin1 (Panx1) is expressed in skeletal muscle, however, the role of Panx1 during skeletal muscle development and regeneration remains poorly understood. Here we show that Panx1 is expressed on the surface of myoblasts and its expression is rapidly increased upon induction of differentiation and that Panx1^-/- mice exhibit impaired muscle regeneration after injury. Panx1^-/- myoblasts activate the myogenic differentiation program normally, but display marked deficits in migration and fusion. Mechanistically, we show that Panx1 activates P2 class purinergic receptors, which in turn mediate a lipid signaling cascade in myoblasts. This signaling induces bleb-driven amoeboid movement that in turn supports myoblast migration and fusion. Finally, we show that Panx1 is involved in the regulation of cell-matrix interaction through the induction of ADAMTS (Disintegrin-like and Metalloprotease domain with Thrombospondin-type 5) proteins that help remodel the extracellular matrix. These studies reveal a novel role for lipid-based signaling pathways activated by Panx1 in the coordination of myoblast activities essential for skeletal muscle regeneration.

Ca2+-mediated coupling between neuromuscular junction and mitochondria in skeletal muscle

The volitional movement of skeletal is controlled by the motor neuron at the site of neuromuscular junction (NMJ) where the retrograde signals are also passed back from muscle to the motor neuron. As the normal function of muscle largely depends on mitochondria that determine the fate of a skeletal muscle myofiber, there must exist a fine-controlled functional coupling between NMJ and mitochondria in myofibers. This mini-review discusses recent publications that reveal how spatiotemporal profiles of intracellular free Ca²⁺ could couple mitochondrial function with the activity of NMJ in skeletal muscle myofibers.

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.

eATP/P2X7R Axis: An Orchestrated Pathway Triggering Inflammasome Activation in Muscle Diseases

In muscle ATP is primarily known for its function as an energy source and as a mediator of the "excitation-transcription" process, which guarantees muscle plasticity in response to environmental stimuli. When quickly released in massive concentrations in the extracellular space as in presence of muscle membrane damage, ATP acts as a damage-associated molecular pattern molecule (DAMP). In experimental murine models of muscular dystrophies characterized by membrane instability, blockade of eATP/P2X7 receptor (R) purinergic signaling delayed the progression of the dystrophic phenotype dampening the local inflammatory response and inducing Foxp3⁺ T Regulatory lymphocytes. These discoveries highlighted the relevance of ATP as a harbinger of immune-tissue damage in muscular genetic diseases. Given the interactions between the immune system and muscle regeneration, the comprehension of ATP/purinerigic pathway articulated organization in muscle cells has become of extreme interest. This review explores ATP release, metabolism, feedback control and cross-talk with members of muscle inflammasome in the context of muscular dystrophies.

Myofibers deficient in connexins 43 and 45 expression protect mice from skeletal muscle and systemic dysfunction promoted by a dysferlin mutation

Dysferlinopathy is a genetic human disease caused by mutations in the gene that encodes the dysferlin protein (DYSF). Dysferlin is believed to play a relevant role in cell membrane repair. However, in dysferlin-deficient (blAJ) mice (a model of dysferlinopathies) the recovery of the membrane resealing function by means of the expression of a mini-dysferlin does not arrest progressive muscular damage, suggesting the participation of other unknown pathogenic mechanisms. Here, we show that proteins called connexins 39, 43 and 45 (Cx39, Cx43 and Cx45, respectively) are expressed by blAJ myofibers and form functional hemichannels (Cx HCs) in the sarcolemma. At rest, Cx HCs increased the sarcolemma permeability to small molecules and the intracellular Ca²⁺ signal. In addition, skeletal muscles of blAJ mice showed lipid accumulation and lack of dysferlin immunoreactivity. As sign of extensive damage and atrophy, muscles of blAJ mice presented elevated numbers of myofibers with internal nuclei, increased number of myofibers with reduced cross-sectional area and elevated creatine kinase activity in serum. In agreement with the extense muscle damage, mice also showed significantly low motor performance. We generated blAJ mice with myofibers deficient in Cx43 and Cx45 expression and found that all above muscle and systemic alterations were absent, indicating that these two Cxs play a critical role in a novel pathogenic mechanism of dysfernolophaties, which is discussed herein. Therefore, Cx HCs could constitute an attractive target for pharmacologic treatment of dyferlinopathies.

Sepsis-Induced Channelopathy in Skeletal Muscles is Associated with Expression of Non-Selective Channels

Skeletal muscles (∼50% of the body weight) are affected during acute and late sepsis and represent one sepsis associate organ dysfunction. Cell membrane changes have been proposed to result from a channelopathy of yet unknown cause associated with mitochondrial dysfunction and muscle atrophy. We hypothesize that the channelopathy might be explained at least in part by the expression of non-selective channels. Here, this possibility was studied in a characterized mice model of late sepsis with evident skeletal muscle atrophy induced by cecal ligation and puncture (CLP). At day seven after CLP, skeletal myofibers were found to present de novo expression (immunofluorescence) of connexins 39, 43, and 45 and P2X7 receptor whereas pannexin1 did not show significant changes. These changes were associated with increased sarcolemma permeability (∼4 fold higher dye uptake assay), ∼25% elevated in intracellular free-Ca concentration (FURA-2), activation of protein degradation via ubiquitin proteasome pathway (Murf and Atrogin 1 reactivity), moderate reduction in oxygen consumption not explained by changes in levels of relevant respiratory proteins, ∼3 fold decreased mitochondrial membrane potential (MitoTracker Red CMXRos) and ∼4 fold increased mitochondrial superoxide production (MitoSox). Since connexin hemichannels and P2X7 receptors are permeable to ions and small molecules, it is likely that they are main protagonists in the channelopathy by reducing the electrochemical gradient across the cell membrane resulting in detrimental metabolic changes and muscular atrophy.

Connexin 43 Hemichannel Activity Promoted by Pro-Inflammatory Cytokines and High Glucose Alters Endothelial Cell Function

The present work was done to elucidate whether hemichannels of a cell line derived from endothelial cells are affected by pro-inflammatory conditions (high glucose and IL-1β/TNF-α) known to lead to vascular dysfunction. We used EAhy 926 cells treated with high glucose and IL-1β/TNF-α. The hemichannel activity was evaluated with the dye uptake method and was abrogated with selective inhibitors or knocking down of hemichannel protein subunits with siRNA. Western blot analysis, cell surface biotinylation, and confocal microscopy were used to evaluate total and plasma membrane amounts of specific proteins and their cellular distribution, respectively. Changes in intracellular Ca²⁺ and nitric oxide (NO) signals were estimated by measuring FURA-2 and DAF-FM probes, respectively. High glucose concentration was found to elevate dye uptake, a response that was enhanced by IL-1β/TNF-α. High glucose plus IL-1β/TNF-α-induced dye uptake was abrogated by connexin 43 (Cx43) but not pannexin1 knockdown. Furthermore, Cx43 hemichannel activity was associated with enhanced ATP release and activation of p38 MAPK, inducible NO synthase, COX₂, PGE₂ receptor EP₁, and P2X₇/P2Y₁ receptors. Inhibition of the above pathways prevented completely the increase in Cx43 hemichannel activity of cells treated high glucose and IL-1β/TNF-α. Both synthetic and endogenous cannabinoids (CBs) also prevented the increment in Cx43 hemichannel opening, as well as the subsequent generation and release of ATP and NO induced by pro-inflammatory conditions. The counteracting action of CBs also was extended to other endothelial alterations evoked by IL-1β/TNF-α and high glucose, including increased ATP-dependent Ca²⁺ dynamics and insulin-induced NO production. Finally, inhibition of Cx43 hemichannels also prevented the ATP release from endothelial cells treated with IL-1β/TNF-α and high glucose. Therefore, we propose that reduction of hemichannel activity could represent a strategy against the activation of deleterious pathways that lead to endothelial dysfunction and possibly cell damage evoked by high glucose and pro-inflammatory conditions during cardiovascular diseases.

The two faces of pannexins: new roles in inflammation and repair

Pannexins belong to a family of ATP-release channels expressed in almost all cell types. An increasing body of literature on pannexins suggests that these channels play dual and sometimes contradictory roles, contributing to normal cell function, as well as to the pathological progression of disease. In this review, we summarize our understanding of pannexin "protective" and "harmful" functions in inflammation, regeneration and mechanical signaling. We also suggest a possible basis for pannexin's dual roles, related to extracellular ATP and K⁺ levels and the activation of various types of P2 receptors that are associated with pannexin. Finally, we speculate upon therapeutic strategies related to pannexin using eyes, lacrimal glands, and peripheral nerves as examples of interesting therapeutic targets.

Endogenous purines modulate K+ -evoked ACh secretion at the mouse neuromuscular junction

At the mouse neuromuscular junction, adenosine triphosphate (ATP) is co-released with the neurotransmitter acetylcholine (ACh), and once in the synaptic cleft, it is hydrolyzed to adenosine. Both ATP/adenosine diphosphate (ADP) and adenosine modulate ACh secretion by activating presynaptic P2Y₁₃ and A₁ , A_2A , and A₃ receptors, respectively. To elucidate the action of endogenous purines on K⁺ -dependent ACh release, we studied the effect of purinergic receptor antagonists on miniature end-plate potential (MEPP) frequency in phrenic diaphragm preparations. At 10 mM K⁺ , the P2Y₁₃ antagonist N-[2-(methylthio)ethyl]-2-[3,3,3-trifluoropropyl]thio-5'-adenylic acid, monoanhydride with (dichloromethylene)bis[phosphonic acid], tetrasodium salt (AR-C69931MX) increased asynchronous ACh secretion while the A₁ , A₃ , and A_2A antagonists 8-cyclopentyl-1,3-dipropylxanthine (DPCPX), (3-Ethyl-5-benzyl-2-methyl-4-phenylethynyl-6-phenyl-1, 4-(±)-dihydropyridine-3,5-, dicarboxylate (MRS-1191), and 2-(2-Furanyl)-7-(2-phenylethyl)-7H-pyrazolo[4,3-e][1,2,4]triazolo[1,5-c]pyrimidin-5-amine (SCH-58261) did not modify neurosecretion. The inhibition of equilibrative adenosine transporters by S-(p-nitrobenzyl)-6-thioinosine provoked a reduction of 10 mM K⁺ -evoked ACh release, suggesting that the adenosine generated from ATP is being removed from the synaptic space by the transporters. At 15 and 20 mM K⁺ , endogenous ATP/ADP and adenosine bind to inhibitory P2Y₁₃ and A₁ and A₃ receptors since AR-C69931MX, DPCPX, and MRS-1191 increased MEPP frequency. Similar results were obtained when the generation of adenosine was prevented by using the ecto-5'-nucleotidase inhibitor α,β-methyleneadenosine 5'-diphosphate sodium salt. SCH-58261 only reduced neurosecretion at 20 mM K⁺ , suggesting that more adenosine is needed to activate excitatory A_2A receptors. At high K⁺ concentration, the equilibrative transporters appear to be saturated allowing the accumulation of adenosine in the synaptic cleft. In conclusion, when motor nerve terminals are depolarized by increasing K⁺ concentrations, the ATP/ADP and adenosine endogenously generated are able to modulate ACh secretion by sequential activation of different purinergic receptors.

Pannexin 1 Modulates Axonal Growth in Mouse Peripheral Nerves

The pannexin family of channels consists of three members—pannexin-1 (Panx1), pannexin-2 (Panx2), and pannexin-3 (Panx3) that enable the exchange of metabolites and signaling molecules between intracellular and extracellular compartments. Pannexin-mediated release of intracellular ATP into the extracellular space has been tied to a number of cellular activities, primarily through the activity of type P2 purinergic receptors. Previous work indicates that the opening of Panx1 channels and activation of purinergic receptors by extracellular ATP may cause inflammation and apoptosis. In the CNS (central nervous system) and PNS (peripheral nervous system), coupled pannexin, and P2 functions have been linked to peripheral sensitization (pain) pathways. Purinergic pathways are also essential for other critical processes in the PNS, including myelination and neurite outgrowth. However, whether such pathways are pannexin-dependent remains to be determined. In this study, we use a Panx1 knockout mouse model and pharmacological inhibitors of the Panx1 and the ATP-mediated signaling pathway to fill gaps in our understanding of Panx1 localization in peripheral nerves, roles for Panx1 in axonal outgrowth and myelination, and neurite extension. Our data show that Panx1 is localized to axonal, myelin, and vascular compartments of the peripheral nerves. Knockout of Panx1 gene significantly increased axonal caliber in vivo and axonal growth rate in cultured dorsal root ganglia (DRG) neurons. Furthermore, genetic knockout of Panx1 or inhibition of components of purinergic signaling, by treatment with probenecid and apyrase, resulted in denser axonal outgrowth from cultured DRG explants compared to untreated wild-types. Our findings suggest that Panx1 regulates axonal growth in the peripheral nervous system.

Connexins and Pannexins in Bone and Skeletal Muscle

PURPOSE OF REVIEW

To discuss current knowledge on the role of connexins and pannexins in the musculoskeletal system.

RECENT FINDINGS

Connexins and pannexins are crucial for the development and maintenance of both bone and skeletal muscle. In bone, the presence of connexin and more recently of pannexin channels in osteoblasts, osteoclasts, and osteocytes has been described and shown to be essential for normal skeletal development and bone adaptation. In skeletal muscles, connexins and pannexins play important roles during development and regeneration through coordinated regulation of metabolic functions via cell-to-cell communication. Further, under pathological conditions, altered expression of these proteins can promote muscle atrophy and degeneration by stimulating inflammasome activity. In this review, we highlight the important roles of connexins and pannexins in the development, maintenance, and regeneration of musculoskeletal tissues, with emphasis on the mechanisms by which these molecules mediate chemical (e.g., ATP and prostaglandin E2) and physical (e.g., mechanical stimulation) stimuli that target the musculoskeletal system and their involvement in the pathophysiological changes in both genetic and acquired diseases.

Pannexin hemichannels: A novel promising therapy target for oxidative stress related diseases

Pannexins, which contain three subtypes: pannexin-1, -2, and -3, are vertebrate glycoproteins that form non-junctional plasma membrane intracellular hemichannels via oligomerization. Oxidative stress refers to an imbalance of the generation and elimination of reactive oxygen species (ROS). Studies have shown that elevated ROS levels are pivotal in the development of a variety of diseases. Recent studies indicate that the occurrence of these oxidative stress related diseases is associated with pannexin hemichannels. It is also reported that pannexins regulate the production of ROS which in turn may increase the opening of pannexin hemichannels. In this paper, we review recent researches about the important role of pannexin hemichannels in oxidative stress related diseases. Thus, pannexin hemichannels, novel therapeutic targets, hold promise in managing oxidative stress related diseases such as the tumor, inflammatory bowel diseases (IBD), pulmonary fibrosis, chronic obstructive pulmonary disease (COPD), cardiovascular disease, insulin resistance (IR), and neural degeneration diseases.

Single-cell Microinjection for Cell Communication Analysis

Gap junctions are intercellular channels that allow the communication of neighboring cells. This communication depends on the contribution of a hemichannel by each neighboring cell to form the gap junction. In mammalian cells, the hemichannel is formed by six connexins, monomers with four transmembrane domains and a C and N terminal within the cytoplasm. Gap junctions permit the exchange of ions, second messengers, and small metabolites. In addition, they have important roles in many forms of cellular communication within physiological processes such as synaptic transmission, heart contraction, cell growth and differentiation. We detail how to perform a single-cell microinjection of Lucifer Yellow to visualize cellular communication via gap-junctions in living cells. It is expected that in functional gap junctions, the dye will diffuse from the loaded cell to the connected cells. It is a very useful technique to study gap junctions since you can evaluate the diffusion of the fluorescence in real time. We discuss how to prepare the cells and the micropipette, how to use a micromanipulator and inject a low molecular weight fluorescent dye in an epithelial cell line.

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.

Dysferlin function in skeletal muscle: Possible pathological mechanisms and therapeutical targets in dysferlinopathies

Mutations in the dysferlin gene are linked to a group of muscular dystrophies known as dysferlinopathies. These myopathies are characterized by progressive atrophy. Studies in muscle tissue from dysferlinopathy patients or dysferlin-deficient mice point out its importance in membrane repair. However, expression of dysferlin homologous proteins that restore sarcolemma repair function in dysferlinopathy animal models fail to arrest muscle wasting, therefore suggesting that dysferlin plays other critical roles in muscle function. In the present review, we discuss dysferlin functions in the skeletal muscle, as well as pathological mechanisms related to dysferlin mutations. Particular focus is presented related the effect of dysferlin on cell membrane related function, which affect its repair, vesicle trafficking, as well as Ca(2+) homeostasis. Such mechanisms could provide accessible targets for pharmacological therapies.

The absence of dysferlin induces the expression of functional connexin-based hemichannels in human myotubes

Background

Mutations in the gene encoding for dysferlin cause recessive autosomal muscular dystrophies called dysferlinopathies. These mutations induce several alterations in skeletal muscles, including, inflammation, increased membrane permeability and cell death. Despite the fact that the etiology of dysferlinopathies is known, the mechanism that explains the aforementioned alterations is still elusive. Therefore, we have now evaluated the potential involvement of connexin based hemichannels in the pathophysiology of dysferlinopathies.

Results

Human deltoid muscle biopsies of 5 Chilean dysferlinopathy patients exhibited the presence of muscular connexins (Cx40.1, Cx43 and Cx45). The presence of these connexins was also observed in human myotubes derived from immortalized myoblasts derived from other patients with mutated forms of dysferlin. In addition to the aforementioned connexins, these myotubes expressed functional connexin based hemichannels, evaluated by ethidium uptake assays, as opposed to myotubes obtained from a normal human muscle cell line, RCMH. This response was reproduced in a knock-down model of dysferlin, by treating RCMH cell line with small hairpin RNA specific for dysferlin (RCMH-sh Dysferlin). Also, the presence of P2X₇ receptor and the transient receptor potential channel, TRPV2, another Ca²⁺ permeable channels, was detected in the myotubes expressing mutated dysferlin, and an elevated resting intracellular Ca²⁺ level was found in the latter myotubes, which was in turn reduced to control levels in the presence of the molecule D4, a selective Cx HCs inhibitor.

Conclusions

The data suggests that dysferlin deficiency, caused by mutation or downregulation of dysferlin, promotes the expression of Cx HCs. Then, the de novo expression Cx HC causes a dysregulation of intracellular free Ca²⁺ levels, which could underlie muscular damage associated to dysferlin mutations. This mechanism could constitute a potential therapeutical target in dysferlinopathies.

Electronic supplementary material

The online version of this article (doi:10.1186/s12860-016-0096-6) contains supplementary material, which is available to authorized users.

Role of connexins and pannexins during ontogeny, regeneration, and pathologies of bone

Electron micrographs revealed the presence of gap junctions in osteoblastic cells over 40 years ago. These intercellular channels formed from connexins are present in bone forming osteoblasts, bone resorbing osteoclasts, and osteocytes (mature osteoblasts embedded in the mineralized bone matrix). More recently, genetic and pharmacologic studies revealed the role of connexins, and in particular Cx43, in the differentiation and function of all bone types. Furthermore, mutations in the gene encoding Cx43 were found to be causally linked to oculodentodigital dysplasia, a condition that results in an abnormal skeleton. Pannexins, molecules with similar structure and single-membrane channel forming potential as connexins when organized as hemichannels, are also expressed in osteoblastic cells. The function of pannexins in bone and cartilage is beginning to be uncovered, but more research is needed to determine the role of pannexins in bone development, adult bone mass and skeletal homeostasis. We describe here the current knowledge on the role of connexins and pannexins on skeletal health and disease.

Regulation of pannexin and connexin channels and their functional role in skeletal muscles

Myogenic precursor cells express connexins (Cx) and pannexins (Panx), proteins that form different membrane channels involved in cell-cell communication. Cx channels connect either the cytoplasm of adjacent cells, called gap junction channels (GJC), or link the cytoplasm with the extracellular space, termed hemichannels (HC), while Panx channels only support the latter. In myoblasts, Panx1 HCs play a critical role in myogenic differentiation, and Cx GJCs and possibly Cx HCs coordinate metabolic responses during later steps of myogenesis. After innervation, myofibers do not express Cxs, but still express Panx1. In myotubes and innervated myofibers, Panx1 HCs allow release of adenosine triphosphate and thus they might be involved in skeletal muscle plasticity. In addition, Panx1 HCs present in adult myofibers mediate adenosine triphosphate release and glucose uptake required for potentiation of muscle contraction. Under pathological conditions, such as upon denervation and spinal cord injury, levels of Panx1 are upregulated. However, Panx1(-/-) mice show similar degree of atrophy as denervated wild-type muscles. Skeletal muscles also express Cx HCs in the sarcolemma after denervation or spinal cord injury, plus other non-selective membrane channels, including purinergic P2X7 receptors and transient receptor potential type V2 channels. The absence of Cx43 and Cx45 is sufficient to drastically reduce denervation atrophy. Moreover, inflammatory cytokines also induce the expression of Cxs in myofibers, suggesting the expression of these Cxs as a common factor for myofiber degeneration under diverse pathological conditions. Inhibitors of skeletal muscle Cx HCs could be promising tools to prevent muscle wasting induced by conditions associated with synaptic dysfunction and inflammation.

Pannexin 1 channels in skeletal muscles

Normal myotubes and adult innervated skeletal myofibers express the glycoprotein pannexin1 (Panx1). Six of them form a “gap junction hemichannel-like” structure that connects the cytoplasm with the extracellular space; here they will be called Panx1 channels. These are poorly selective channels permeable to ions, small metabolic substrate, and signaling molecules. So far little is known about the role of Panx1 channels in muscles but skeletal muscles of Panx1^−/− mice do not show an evident phenotype. Innervated adult fast and slow skeletal myofibers show Panx1 reactivity in close proximity to dihydropyridine receptors in the sarcolemma of T-tubules. These Panx1 channels are activated by electrical stimulation and extracellular ATP. Panx1 channels play a relevant role in potentiation of muscle contraction because they allow release of ATP and uptake of glucose, two molecules required for this response. In support of this notion, the absence of Panx1 abrogates the potentiation of muscle contraction elicited by repetitive electrical stimulation, which is reversed by exogenously applied ATP. Phosphorylation of Panx1 Thr and Ser residues might be involved in Panx1 channel activation since it is enhanced during potentiation of muscle contraction. Under denervation, Panx1 levels are upregulated and this partially explains the reduction in electrochemical gradient, however its absence does not prevent denervation-induced atrophy but prevents the higher oxidative state. Panx1 also forms functional channels at the cell surface of myotubes and their functional state has been associated with intracellular Ca²⁺ signals and regulation of myotube plasticity evoked by electrical stimulation. We proposed that Panx1 channels participate as ATP channels and help to keep a normal oxidative state in skeletal muscles.

The role of pannexin hemichannels in inflammation and regeneration

Tissue injury involves coordinated systemic responses including inflammatory response, targeted cell migration, cell-cell communication, stem cell activation and proliferation, and tissue inflammation and regeneration. The inflammatory response is an important prerequisite for regeneration. Multiple studies suggest that extensive cell-cell communication during tissue regeneration is coordinated by purinergic signaling via extracellular adenosine triphosphate (ATP). Most recent data indicates that ATP release for such communication is mediated by hemichannels formed by connexins and pannexins. The Pannexin family consists of three vertebrate proteins (Panx 1, 2, and 3) that have low sequence homology with other gap junction proteins and were shown to form predominantly non-junctional plasma membrane hemichannels. Pannexin-1 (Panx1) channels function as an integral component of the P2X/P2Y purinergic signaling pathway and is arguably the major contributor to pathophysiological ATP release. Panx1 is expressed in many tissues, with highest levels detected in developing brain, retina and skeletal muscles. Panx1 channel expression and activity is reported to increase significantly following injury/inflammation and during regeneration and differentiation. Recent studies also report that pharmacological blockade of the Panx1 channel or genetic ablation of the Panx1 gene cause significant disruption of progenitor cell migration, proliferation, and tissue regeneration. These findings suggest that pannexins play important roles in activation of both post-injury inflammatory response and the subsequent process of tissue regeneration. Due to wide expression in multiple tissues and involvement in diverse signaling pathways, pannexins and connexins are currently being considered as therapeutic targets for traumatic brain or spinal cord injuries, ischemic stroke and cancer. The precise role of pannexins and connexins in the balance between tissue inflammation and regeneration needs to be further understood.

The pannexins: past and present

The pannexins (Panxs) are a family of chordate proteins homologous to the invertebrate gap junction forming proteins named innexins. Three distinct Panx paralogs (Panx1, Panx2, and Panx3) are shared among the major vertebrate phyla, but they appear to have suppressed (or even lost) their ability to directly couple adjacent cells. Connecting the intracellular and extracellular compartments is now widely accepted as Panx's primary function, facilitating the passive movement of ions and small molecules along electrochemical gradients. The tissue distribution of the Panxs ranges from pervasive to very restricted, depending on the paralog, and are often cell type-specific and/or developmentally regulated within any given tissue. In recent years, Panxs have been implicated in an assortment of physiological and pathophysiological processes, particularly with respect to ATP signaling and inflammation, and they are now considered to be a major player in extracellular purinergic communication. The following is a comprehensive review of the Panx literature, exploring the historical events leading up to their discovery, outlining our current understanding of their biochemistry, and describing the importance of these proteins in health and disease.

Purinergic signalling in the musculoskeletal system

It is now widely recognised that extracellular nucleotides, signalling via purinergic receptors, participate in numerous biological processes in most tissues. It has become evident that extracellular nucleotides have significant regulatory effects in the musculoskeletal system. In early development, ATP released from motor nerves along with acetylcholine acts as a cotransmitter in neuromuscular transmission; in mature animals, ATP functions as a neuromodulator. Purinergic receptors expressed by skeletal muscle and satellite cells play important pathophysiological roles in their development or repair. In many cell types, expression of purinergic receptors is often dependent on differentiation. For example, sequential expression of P2X5, P2Y1 and P2X2 receptors occurs during muscle regeneration in the mdx model of muscular dystrophy. In bone and cartilage cells, the functional effects of purinergic signalling appear to be largely negative. ATP stimulates the formation and activation of osteoclasts, the bone-destroying cells. Another role appears to be as a potent local inhibitor of mineralisation. In osteoblasts, the bone-forming cells, ATP acts via P2 receptors to limit bone mineralisation by inhibiting alkaline phosphatase expression and activity. Extracellular ATP additionally exerts significant effects on mineralisation via its hydrolysis product, pyrophosphate. Evidence now suggests that purinergic signalling is potentially important in several bone and joint disorders including osteoporosis, rheumatoid arthritis and cancers. Strategies for future musculoskeletal therapies might involve modulation of purinergic receptor function or of the ecto-nucleotidases responsible for ATP breakdown or ATP transport inhibitors.

Electrical stimuli are anti-apoptotic in skeletal muscle via extracellular ATP. Alteration of this signal in Mdx mice is a likely cause of dystrophy

ATP signaling has been shown to regulate gene expression in skeletal muscle and to be altered in models of muscular dystrophy. We have previously shown that in normal muscle fibers, ATP released through Pannexin1 (Panx1) channels after electrical stimulation plays a role in activating some signaling pathways related to gene expression. We searched for a possible role of ATP signaling in the dystrophy phenotype. We used muscle fibers from flexor digitorum brevis isolated from normal and mdx mice. We demonstrated that low frequency electrical stimulation has an anti-apoptotic effect in normal muscle fibers repressing the expression of Bax, Bim and PUMA. Addition of exogenous ATP to the medium has a similar effect. In dystrophic fibers, the basal levels of extracellular ATP were higher compared to normal fibers, but unlike control fibers, they do not present any ATP release after low frequency electrical stimulation, suggesting an uncoupling between electrical stimulation and ATP release in this condition. Elevated levels of Panx1 and decreased levels of Cav1.1 (dihydropyridine receptors) were found in triads fractions prepared from mdx muscles. Moreover, decreased immunoprecipitation of Cav1.1 and Panx1, suggest uncoupling of the signaling machinery. Importantly, in dystrophic fibers, exogenous ATP was pro-apoptotic, inducing the transcription of Bax, Bim and PUMA and increasing the levels of activated Bax and cytosolic cytochrome c. These evidence points to an involvement of the ATP pathway in the activation of mechanisms related with cell death in muscular dystrophy, opening new perspectives towards possible targets for pharmacological therapies.

Role of gap junctions and hemichannels in parasitic infections

In vertebrates, connexins (Cxs) and pannexins (Panxs) are proteins that form gap junction channels and/or hemichannels located at cell-cell interfaces and cell surface, respectively. Similar channel types are formed by innexins in invertebrate cells. These channels serve as pathways for cellular communication that coordinate diverse physiologic processes. However, it is known that many acquired and inherited diseases deregulate Cx and/or Panx channels, condition that frequently worsens the pathological state of vertebrates. Recent evidences suggest that Cx and/or Panx hemichannels play a relevant role in bacterial and viral infections. Nonetheless, little is known about the role of Cx- and Panx-based channels in parasitic infections of vertebrates. In this review, available data on changes in Cx and gap junction channel changes induced by parasitic infections are summarized. Additionally, we describe recent findings that suggest possible roles of hemichannels in parasitic infections. Finally, the possibility of new therapeutic designs based on hemichannel blokers is presented.

De novo expression of connexin hemichannels in denervated fast skeletal muscles leads to atrophy

Denervation of skeletal muscles induces atrophy, preceded by changes in sarcolemma permeability of causes not yet completely understood. Here, we show that denervation-induced Evans blue dye uptake in vivo of fast, but not slow, myofibers was acutely inhibited by connexin (Cx) hemichannel/pannexin1 (Panx1) channel and purinergic ionotropic P2X7 receptor (P2X7R) blockers. Denervated myofibers showed up-regulation of Panx1 and de novo expression of Cx39, Cx43, and Cx45 hemichannels as well as P2X7Rs and transient receptor potential subfamily V, member 2, channels, all of which are permeable to small molecules. The sarcolemma of freshly isolated WT myofibers from denervated muscles also showed high hemichannel-mediated permeability that was slightly reduced by blockade of Panx1 channels or the lack of Panx1 expression, but was completely inhibited by Cx hemichannel or P2X7R blockers, as well as by degradation of extracellular ATP. However, inhibition of transient receptor potential subfamily V, member 2, channels had no significant effect on membrane permeability. Moreover, activation of the transcription factor NFκB and higher mRNA levels of proinflammatory cytokines (TNF-α and IL-1β) were found in denervated WT but not Cx43/Cx45-deficient muscles. The atrophy observed after 7 d of denervation was drastically reduced in Cx43/Cx45-deficient but not Panx1-deficient muscles. Therefore, expression of Cx hemichannels and P2X7R promotes a feed-forward mechanism activated by extracellular ATP, most likely released through hemichannels, that activates the inflammasome. Consequently, Cx hemichannels are potential targets for new therapeutic agents to prevent or reduce muscle atrophy induced by denervation of diverse etiologies.

ATP is required and advances cytokine-induced gap junction formation in microglia in vitro

Microglia are the immune cells in the central nervous system. After injury microglia release bioactive molecules, including cytokines and ATP, which modify the functional state of hemichannels (HCs) and gap junction channels (GJCs), affecting the intercellular communication via extracellular and intracellular compartments, respectively. Here, we studied the role of extracellular ATP and several cytokines as modulators of the functional state of microglial HCs and GJCs using dye uptake and dye coupling techniques, respectively. In microglia and the microglia cell line EOC20, ATP advanced the TNF-α/IFN-γ-induced dye coupling, probably through the induction of IL-1β release. Moreover, TNF-α/IFN-γ, but not TNF-α plus ATP, increased dye uptake in EOC20 cells. Blockade of Cx43 and Panx1 HCs prevented dye coupling induced by TNF-α/IFN-γ, but not TNF-α plus ATP. In addition, IL-6 prevented the induction of dye coupling and HC activity induced by TNF-α/IFN-γ in EOC20 cells. Our data support the notion that extracellular ATP affects the cellular communication between microglia through autocrine and paracrine mechanisms, which might affect the timing of immune response under neuroinflammatory conditions.

Antibodies targeting extracellular domain of connexins for studies of hemichannels

Hemichannels are transmembrane channels composed of either a connexin or pannexin hexamer. The docking of the extracellular domains of connexin hemichannels contributed by neighboring cells forms a gap junction channel that joins the cytoplasm of adjacent cells. Connexins are expressed ubiquitously in different organs, but some subtypes are expressed exclusively in certain tissues and tumors. Both gap junction channels and hemichannels participate in diverse physiological and pathological responses. However, the lack of specific reagents that inhibit only gap junction channels or hemichannels is a challenge that makes it different to discern the specific roles of either channel. Fortunately, the available information regarding the connexin sequence, secondary and tertiary structure, and their biochemical and physiological properties permits the development of strategies to block exclusively the hemichannel activity exclusively, with no effect on gap junction activity. This task is accomplished through the use of specifics antibodies that target the extracellular sites of desired connexin subtype. However, the underlying mechanism of how antibodies targeting extracellular connexin epitopes actually inhibit hemichannels remains unknown. Although these antibodies are being used for detecting and blocking of hemichannels in normal and tumor cells, they can also be potentially used for tissue-specific treatment and drug delivery in clinical applications. In this article, we will first review the literature concerning the structure of connexins and the unique properties of extracellular loop domains of the connexins. Furthermore, we will discuss briefly the development of connexin (Cx) 43(E2) antibody, a specific antibody which detects the second extracellular loop of Cx43 and specifically prevents the opening of Cx43 hemichannels. We will then summarize the reported studies of specific reagents used for the inhibition of connexin hemichannels including antibodies developed against extracellular loop domains. This article is part of the Special Issue Section entitled 'Current Pharmacology of Gap Junction Channels and Hemichannels'.

Cross-talk between skeletal muscle and immune cells: muscle-derived mediators and metabolic implications

Skeletal muscles contain resident immune cell populations and their abundance and type is altered in inflammatory myopathies, endotoxemia or different types of muscle injury/insult. Within tissues, monocytes differentiate into macrophages and polarize to acquire pro- or anti-inflammatory phenotypes. Skeletal muscle macrophages play a fundamental role in repair and pathogen clearance. These events require a precisely regulated cross-talk between myofibers and immune cells, involving paracrine/autocrine and contact interactions. Skeletal muscle also undergoes continuous repair as a result of contractile activity that involves participation of myokines and anti-inflammatory input. Finally, skeletal muscle is the major site of dietary glucose disposal; therefore, muscle insulin resistance is essential to the development of whole body insulin resistance. Notably, muscle inflammation is emerging as a potential contributor to insulin resistance. Recent reports show that inflammatory macrophage numbers within muscle are elevated during obesity and that muscle cells in vitro can mount autonomous inflammatory responses under metabolic challenge. Here, we review the nature of skeletal muscle inflammation associated with muscle exercise, damage, and regeneration, endotoxin presence, and myopathies, as well as the new evidence of local inflammation arising with obesity that potentially contributes to insulin resistance.

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

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

Beyond gap junctions: Connexin43 and bone cell signaling

Connexin43 (Cx43) is the most abundant gap junction protein expressed in bone cells and plays a central role in cell-to-cell communication in the skeleton. Findings of the last decade uncovered functions of Cx43 hemichannels expressed on unopposed plasma cell membranes as mediators of the communication between bone cells and their extracellular milieu. Additionally, through its cytoplasmic C-terminus domain, Cx43 serves as a scaffolding protein that associates with structural and signaling molecules leading to regulation of intracellular signaling, independent of channel activity. This perspective discusses the evidence demonstrating that via these diverse mechanisms Cx43 is a key component of the intracellular machinery responsible for signal transduction in bone in response to pharmacologic, hormonal and mechanical stimuli. This advance in the knowledge of the role of connexins increases our understanding of the pathophysiological mechanisms that regulate bone cell function and provides new opportunities to treat bone diseases.