Regulation of connexin gene expression during skeletal muscle regeneration in the adult rat

Empowering engineered muscle in biohybrid pump by extending connexin 43 duration with reduced graphene oxides

Engineered skeletal muscle act as therapeutics invaluable to treat injured or diseased muscle and a "living" material essential to assemble biological machinery. For normal development, skeletal myoblasts should express connexin 43, one of the gap junction proteins that promote myoblast fusion and myogenesis, during the early differentiation stage. However, myoblasts cultured in vitro often down-regulate connexin 43 before differentiation, limiting myogenesis and muscle contraction. This study demonstrates that tethering myoblasts with reduced graphene oxide (rGO) slows connexin 43 regression during early differentiation and increases myogenic mRNA synthesis. The whole RNA sequencing also confirms that the rGO on cells increases regulator genes for myogenesis, including troponin, while decreasing negative regulator genes. The resulting myotubes generated a three-fold larger contraction force than the rGO-free myotubes. Accordingly, a valveless biohybrid pump assembled with the rGO-tethered muscle increased the fluid velocity and flow rate considerably. The results of this study would provide an important foundation for developing physiologically relevant muscle and powering up biomachines that will be used for various bioscience studies and unexplored applications.

Structural alterations and inflammation in the heart after multiple trauma followed by reamed versus non-reamed femoral nailing

BACKGROUND

Approximately 30,000 patients with blunt cardiac trauma are recorded each year in the United States. Blunt cardiac injuries after trauma are associated with a longer hospital stay and a poor overall outcome. Organ damage after trauma is linked to increased systemic release of pro-inflammatory cytokines and damage-associated molecular patterns. However, the interplay between polytrauma and local cardiac injury is unclear. Additionally, the impact of surgical intervention on this process is currently unknown. This study aimed to determine local cardiac immunological and structural alterations after multiple trauma. Furthermore, the impact of the chosen fracture stabilization strategy (reamed versus non-reamed femoral nailing) on cardiac alterations was studied.

EXPERIMENTAL APPROACH

15 male pigs were either exposed to multiple trauma (blunt chest trauma, laparotomy, liver laceration, femur fracture and haemorrhagic shock) or sham conditions. Blood samples as well as cardiac tissue were analysed 4 h and 6 h after trauma. Additionally, murine HL-1 cells were exposed to a defined polytrauma-cocktail, mimicking the pro-inflammatory conditions after multiple trauma in vitro.

RESULTS

After multiple trauma, cardiac structural changes were observed in the left ventricle. More specifically, alterations in the alpha-actinin and desmin protein expression were found. Cardiac structural alterations were accompanied by enhanced local nitrosative stress, increased local inflammation and elevated systemic levels of the high-mobility group box 1 protein. Furthermore, cardiac alterations were observed predominantly in pigs that were treated by non-reamed intramedullary reaming. The polytrauma-cocktail impaired the viability of HL-1 cells in vitro, which was accompanied by a release of troponin I and HFABP.

DISCUSSION

Multiple trauma induced cardiac structural alterations in vivo, which might contribute to the development of early myocardial damage (EMD). This study also revealed that reamed femoral nailing (reamed) is associated with more prominent immunological cardiac alterations compared to nailing without reaming (non-reamed). This suggests that the choice of the initial fracture treatment strategy might be crucial for the overall outcome as well as for any post-traumatic cardiac consequences.

Integrin-α7 signaling regulates connexin 43, M-cadherin, and myoblast fusion

Regenerative medicine treatments for severe skeletal muscle injuries are limited, resulting in persistent functional deficits. Clinical options include neglecting the wound with the expectation that fibrosis will develop or using an autologous muscle graft with minimal functional improvement. A regenerative matrix can be used, but muscle fiber development on these matrices remains a challenge in vivo. Here, we explored the fundamental mechanisms that mediate cell-substrate signaling and its effect on cell-cell communication during myoblast fusion and tube formation to improve outcomes following implantation of matrices used to stimulate muscle regeneration. We previously reported that integrin-α7 was increased on anisotropic biomaterials, suggesting a role for α7β1 signaling in myoblast communication via connexin 43 and M-cadherin. Our results demonstrated that α7 silencing blocked expression of myogenic differentiation factor 1 (Myod), myogenin (Myog), myogenic factor 6 (Myf6), myosin heavy chain type 1 (Myh1), and transmembrane protein 8c (Tmem8c), indicating that myoblast fusion was inhibited. Expression of α5 and M-cadherin decreased but β1 and connexin 43 increased. We examined protein production and observed reduced extracellular-signal regulated kinase 1/2 (ERK) in α7-silenced cells that correlated with upregulation of connexin 43 and M-cadherin, suggesting a compensatory pathway. These results indicate that α7 signaling plays a critical role in ex vivo fusion and implicates a relationship with connexin 43 and M-cadherin.

Fresh whole blood resuscitation does not exacerbate skeletal muscle edema and long-term functional deficit after ischemic injury and hemorrhagic shock

BACKGROUND

Hemorrhagic shock caused by extremity vascular injuries is common in combat injuries. Fluid resuscitation is the standard treatment for severe hemorrhage (HEM). Tourniquets (TKs) used for HEM control cause ischemia-reperfusion (I/R) injury that induces edema formation in the injured muscle. Resuscitation fluids affect edema formation; however, its effect on long-term functional response remains unknown. The objectives of this study are to (1) compare acute muscle damage; (2) determine long-term functional recovery of ischemic muscle; and (3) compare local and systemic inflammatory response including the expression of junctional proteins following early resuscitation with Hextend and fresh whole blood using a rodent model of combined HEM and TK-induced limb I/R.

METHODS

Anesthetized Sprague-Dawley rats underwent 42.5% arterial HEM, followed by 3 hours of TK application. Animals were either not resuscitated or resuscitated with Hextend or fresh whole blood. Two time points were evaluated, 2 and 28 days. Plasma cytokine concentrations were determined at baseline and end resuscitation. At 2 days, edema formation, expression of junctional proteins, and tissue level cytokines concentrations were evaluated. At 28 days, in vivo muscle contractile properties were determined. At both time points, routine histology was performed and graded using a semiquantitative grading system.

RESULTS

All animals developed hemorrhagic hypovolemia; the mortality rate was 100% in nonresuscitated rats. Hextend resuscitation exacerbated muscle edema (~11%) and muscle strength deficit (~20%). Fresh whole blood resuscitation presented edema and muscle strength akin to TK only. Fresh whole blood resuscitation upregulated expression of junctional proteins including proangiogenic factors and dampened the inflammatory response.

CONCLUSION

Fresh whole blood resuscitation does not exacerbate either TK-induced edema or muscle strength deficit. Fresh whole blood resuscitation may reduce both acute and long-term morbidity associated with extremity trauma. To our knowledge, this is the first study to demonstrate the nature of the resuscitation fluid administered following HEM impacts short- and long-term indices of I/R in skeletal muscle.

Characteristics of the Localization of Connexin 43 in Satellite Cells during Skeletal Muscle Regeneration In Vivo

For myogenesis, new myotubes are formed by the fusion of differentiated myoblasts. In the sequence of events for myotube formation, intercellular communication through gap junctions composed of connexin 43 (Cx43) plays critical roles in regulating the alignment and fusion of myoblasts in advances of myotube formation in vitro. On the other hand, the relationship between the expression patterns of Cx43 and the process of myotube formation in satellite cells during muscle regeneration in vivo remains poorly understood. The present study investigated the relationship between Cx43 and satellite cells in muscle regeneration in vivo. The expression of Cx43 was detected in skeletal muscles on day 1 post-muscle injury, but not in control muscles. Interestingly, the expression of Cx43 was not localized on the inside of the basement membrane of myofibers in the regenerating muscles. Moreover, although the clusters of differentiated satellite cells, which represent a more advanced stage of myotube formation, were observed on the inside of the basement membrane of myofibers in regenerating muscles, the expression of Cx43 was not localized in the clusters of these satellite cells. Therefore, in the present study, it was suggested that Cx43 may not directly contribute to muscle regeneration via satellite cells.

Hyperthermia differently affects connexin43 expression and gap junction permeability in skeletal myoblasts and HeLa cells

Stress kinases can be activated by hyperthermia and modify the expression level and properties of membranous and intercellular channels. We examined the role of c-Jun NH2-terminal kinase (JNK) in hyperthermia-induced changes of connexin43 (Cx43) expression and permeability of Cx43 gap junctions (GJs) in the rabbit skeletal myoblasts (SkMs) and Cx43-EGFP transfected HeLa cells. Hyperthermia (42°C for 6 h) enhanced the activity of JNK and its target, the transcription factor c-Jun, in both SkMs and HeLa cells. In SkMs, hyperthermia caused a 3.2-fold increase in the total Cx43 protein level and enhanced the efficacy of GJ intercellular communication (GJIC). In striking contrast, hyperthermia reduced the total amount of Cx43 protein, the number of Cx43 channels in GJ plaques, the density of hemichannels in the cell membranes, and the efficiency of GJIC in HeLa cells. Both in SkMs and HeLa cells, these changes could be prevented by XG-102, a JNK inhibitor. In HeLa cells, the changes in Cx43 expression and GJIC under hyperthermic conditions were accompanied by JNK-dependent disorganization of actin cytoskeleton stress fibers while in SkMs, the actin cytoskeleton remained intact. These findings provide an attractive model to identify the regulatory players within signalosomes, which determine the cell-dependent outcomes of hyperthermia.

Abundant expression and functional participation of TRPV1 at Zusanli acupoint (ST36) in mice: mechanosensitive TRPV1 as an "acupuncture-responding channel"

Background

Acupuncture is a therapy that involves applying mechanical stimulation to acupoints using needles. Although acupuncture is believed to trigger neural regulation by opioids or adenosine, still little is known about how physical stimulation is turned into neurological signaling. The transient receptor potential vanilloid receptors 1 and 4 (TRPV1 and TRPV4) and the acid-sensing ion channel 3 (ASIC3) are regarded as mechanosensitive channels. This study aimed to clarify their role at the Zusanli acupoint (ST36) and propose possible sensing pathways linking channel activation to neurological signaling.

Methods

First, tissues from different anatomical layers of ST36 and the sham point were sampled, and channel expressions between the two points were compared using western blotting. Second, immunofluorescence was performed at ST36 to reveal distribution pattern of the channels. Third, agonist of the channels were injected into ST36 and tested in a mouse inflammatory pain model to seek if agonist injection could replicate acupuncture-like analgesic effect. Last, the components of proposed downstream sensing pathway were tested with western blotting to determine if they were expressed in tissues with positive mechanosensitive channel expression.

Results

The results from western blotting demonstrated an abundance of TRPV1, TRPV4, and ASIC3 in anatomical layers of ST36. Furthermore, immunofluorescence showed these channels were expressed in both neural and non-neural cells at ST36. However, only capsaicin, a TRPV1 agonist, replicated the analgesic effect of acupuncture when injected into ST36. Components of calcium wave propagation (CWP, the proposed downstream sensing pathway) were also expressed in tissues with abundant TRPV1 expression, the muscle and epimysium layers.

Conclusions

The results demonstrated mechanosensitive channel TRPV1 is highly expressed at ST36 and possibly participated in acupuncture related analgesia. Since CWP was reported by other to occur during acupuncture and its components were shown here to express in tissues with positive TRPV1 expression. These findings suggest TRPV1 might act as acupuncture-responding channel by sensing physical stimulation from acupuncture and conducting the signaling via CWP to nerve terminals. This study provided a better understanding between physical stimulation from acupuncture to neurological signaling.

Connexin36 (Cx36) expression and protein detection in the mouse carotid body and myenteric plexus

Although connexin36 (Cx36) has been studied in several tissues, it is notable that no data are available on Cx36 expression in the carotid body and the intestine. The present study was undertaken to evaluate using immunohistochemistry, PCR and Western blotting procedures, whether Cx36 was expressed in the mouse carotid body and in the intestine at ileum and colon level. In the carotid body, Cx36 was detected as diffuse punctate immunostaining and as protein by Western blotting and mRNA by RT-PCR. Cx36 punctate immunostaining was also evident in the intestine with localization restricted to the myenteric plexus of both the ileum and the colon, and this detection was also confirmed by Western blotting and RT-PCR. All the data obtained were validated using Cx36 knockout mice. Taken together the present data on localization of Cx36 gap-junctions in two tissues of neural crest-derived neuroendocrine organs may provide an anatomical basis for future functional investigations.

Evidence for cell density affecting C2C12 myogenesis: possible regulation of myogenesis by cell-cell communication

INTRODUCTION

Community effect is a phenomenon caused by cell-cell communication during myogenesis. In myogenic C2C12 cells in vitro, the confluent phase is needed for myogenesis induction.

METHODS

To examine the cell-density effect, growth kinetics and myogenic differentiation were investigated in cells plated at four different cell densities.

RESULTS

We found that expression of a myogenic differentiation marker was high in a density-dependent manner. At high density, where cell-cell contact was obvious, contact inhibition after the proliferation stage was accompanied by microarray findings demonstrating upregulation of negative regulating cell-cycle markers, including CDKI p21 and the muscle differentiation markers MyoD and myogenin. Interestingly, developmentally regulated protein expression (drebrin) protein expression was also upregulated in a density-dependent manner.

CONCLUSIONS

These results suggest that contact inhibition after the proliferation stage may induce growth arrest via cell-cell communication through the expression of CDKI p21 and may be responsible for progressing cell fusion.

A Rodent Model to Advance the Field Treatment of Crush Muscle Injury During Earthquakes and Other Natural Disasters

Approximately 170 earthquakes of 6.0 or higher magnitude occur annually worldwide. Victims often suffer crush muscle injuries involving impaired blood flow to the affected muscle and damage to the muscle fiber membrane. Current rescue efforts are directed toward preventing acute kidney injury (AKI), which develops upon extrication and muscle reperfusion. But field-usable, muscle-specific interventions may promote muscle regeneration and prevent or minimize the pathologic changes of reperfusion. Although current rodent crush injury models involve reperfusion upon removal of the crush stimulus, an analysis of their methodological aspects is needed to ensure adequate simulation of the earthquake-related crush injury. The objectives of this systematic review are to (a) describe rodent crush muscle injury models, (b) discuss the benefits and limitations of these models, and (c) offer a recommendation for animal models that would increase our understanding of muscle recovery processes after an earthquake-induced crush muscle injury. The most commonly used rodent model uses a clamping or pressing crush stimulus directly applied to murine hindlimb muscle. This model has increased our understanding of muscle regeneration but its open approach does not adequately represent the earthquake-related crush injury. The model we recommend for developing field-usable, muscle-specific interventions is a closed approach that involves a nonclamping crush stimulus. Findings from studies employing this recommended model may have greater relevance for developing interventions that lessen the earthquake’s devastating impact on individual and community health and quality of life, especially in developing countries.

Connexin39 deficient mice display accelerated myogenesis and regeneration of skeletal muscle

During muscle development and regeneration of skeletal muscle in mice connexin43 (Cx43) and connexin39 (Cx39) are specifically expressed: Cx43 in satellite cells and myoblasts, whereas Cx39 is exclusively expressed in myogenin-positive cells. We generated Cx39 deficient mice by replacing the coding region of the Gjd4 gene by DNA coding for the enhanced green fluorescent protein eGFP. Adult Cx39 deficient mice exhibit no obvious phenotypic alterations of skeletal muscle compared to wild type mice in the resting state. However, myogenesis in Cx39 deficient embryos is accelerated as indicated by increased myogenin expression on ED13.5 and ED16.5 and increased expression of Cx43 in developing skeletal muscle. In addition, the regeneration process of skeletal muscle in Cx39 deficient mice is accelerated as shown by a 2day earlier onset of MyoD and myogenin expression, relative to wild type littermates. Interestingly, Cx43 expression was also upregulated in Cx39 deficient mice during regeneration of skeletal muscle. We hypothesize that Cx43 may compensate for the loss of Cx39 during myogenesis and regeneration.

Functional interaction between TRPC1 channel and connexin-43 protein: a novel pathway underlying S1P action on skeletal myogenesis

We recently demonstrated that skeletal muscle differentiation induced by sphingosine 1-phosphate (S1P) requires gap junctions and transient receptor potential canonical 1 (TRPC1) channels. Here, we searched for the signaling pathway linking the channel activity with Cx43 expression/function, investigating the involvement of the Ca(2+)-sensitive protease, m-calpain, and its targets in S1P-induced C2C12 myoblast differentiation. Gene silencing and pharmacological inhibition of TRPC1 significantly reduced Cx43 up-regulation and Cx43/cytoskeletal interaction elicited by S1P. TRPC1-dependent functions were also required for the transient increase of m-calpain activity/expression and the subsequent decrease of PKCα levels. Remarkably, Cx43 expression in S1P-treated myoblasts was reduced by m-calpain-siRNA and enhanced by pharmacological inhibition of classical PKCs, stressing the relevance for calpain/PKCα axis in Cx43 protein remodeling. The contribution of this pathway in myogenesis was also investigated. In conclusion, these findings provide novel mechanisms by which S1P regulates myoblast differentiation and offer interesting therapeutic options to improve skeletal muscle regeneration.

Connexins: sensors and regulators of cell cycling

It is nowadays well established that gap junctions are critical gatekeepers of cell proliferation, by controlling the intercellular exchange of essential growth regulators. In recent years, however, it has become clear that the picture is not as simple as originally anticipated, as structural precursors of gap junctions can affect cell cycling by performing actions not related to gap junctional intercellular communication. Indeed, connexin hemichannels also foresee a pathway for cell growth communication, albeit between the intracellular compartment and the extracellular environment, while connexin proteins as such can directly or indirectly influence the production of cell cycle regulators independently of their channel activities. Furthermore, a novel set of connexin-like proteins, the pannexins, have lately joined in as regulators of the cell proliferation process, which they can affect as either single units or as channel entities. In the current paper, these multifaceted aspects of connexin-related signalling in cell cycling are reviewed.