Transient involvement of gap junctional communication before fusion of newborn rat myoblasts

Bioelectric Properties of Myogenic Progenitor Cells

Modern stem cell research has mainly focused on protein expression and transcriptional networks. However, transmembrane voltage gradients generated by ion channels and transporters have demonstrated to be powerful regulators of cellular processes. These physiological cues exert influence on cell behaviors ranging from differentiation and proliferation to migration and polarity. Bioelectric signaling is a fundamental element of living systems and an untapped reservoir for new discoveries. Dissecting these mechanisms will allow for novel methods of controlling cell fate and open up new opportunities in biomedicine. This review focuses on the role of ion channels and the resting membrane potential in the proliferation and differentiation of skeletal muscle progenitor cells. In addition, findings relevant to this topic are presented and potential implications for tissue engineering and regenerative medicine are discussed.

Extremely Low-Frequency Electromagnetic Fields Affect Myogenic Processes in C2C12 Myoblasts: Role of Gap-Junction-Mediated Intercellular Communication

Extremely low-frequency electromagnetic fields (ELF-EMFs) can interact with biological systems. Although they are successfully used as therapeutic agents in physiatrics and rehabilitative practice, they might represent environmental pollutants and pose a risk to human health. Due to the lack of evidence of their mechanism of action, the effects of ELF-EMFs on differentiation processes in skeletal muscle were investigated. C2C12 myoblasts were exposed to ELF-EMFs generated by a solenoid. The effects of ELF-EMFs on cell viability and on growth and differentiation rates were studied using colorimetric and vital dye assays, cytomorphology, and molecular analysis of MyoD and myogenin expression, respectively. The establishment of functional gap junctions was investigated analyzing connexin 43 expression levels and measuring cell permeability, using microinjection/dye-transfer assays. The ELF-EMFs did not affect C2C12 myoblast viability or proliferation rate. Conversely, at ELF-EMF intensity in the mT range, the myogenic process was accelerated, through increased expression of MyoD, myogenin, and connexin 43. The increase in gap-junction function suggests promoting cell fusion and myotube differentiation. These data provide the first evidence of the mechanism through which ELF-EMFs may provide therapeutic benefits and can resolve, at least in part, some conditions of muscle dysfunction.

Gap junctional intercellular communication in hypoxia-ischemia-induced neuronal injury

Brain hypoxia-ischemia is a relatively common and serious problem in neonates and in adults. Its consequences include long-term histological and behavioral changes and reduction in seizure threshold. Gap junction intercellular communication is pivotal in the spread of hypoxia-ischemia related injury and in mediating its long-term effects. This review provides a comprehensive and critical review of hypoxia-ischemia and hypoxia in the brain and the potential role of gap junctions in the spread of the neuronal injury induced by these insults. It also presents the effects of hypoxia-ischemia and of hypoxia on the state of gap junctions in vitro and in vivo. Understanding the mechanisms involved in gap junction-mediated neuronal injury due to hypoxia will lead to the development of novel therapeutic strategies.

Myoblast proliferation and syncytial fusion both depend on connexin43 function in transfected skeletal muscle primary cultures

Muscles are formed by fusion of individual postmitotic myoblasts to form multinucleated syncytial myotubes. The process requires a well-coordinated transition from proliferation, through migratory alignment and cycle exit, to breakdown of apposed membranes. Connexin43 protein and cell-cycle inhibitor levels are correlated, and gap junction blockers can delay muscle regeneration, so a coordinating role for gap junctions has been proposed. Here, wild-type and dominant-negative connexin43 variants (wtCx43, dnCx43) were introduced into rat myoblasts in primary culture through pIRES-eGFP constructs that made transfected cells fluoresce. GFP-positive cells and vitally-stained nuclei were counted on successive days to reveal differences in proliferation, and myotubes were counted to reveal differences in fusion. Individual transfected cells were injected with Cascade Blue, which permeates gap junctions, mixed with FITC-dextran, which requires cytoplasmic continuity to enter neighbouring cells. Myoblasts transfected with wtCx43 showed more gap-junctional coupling than GFP-only controls, began fusion sooner as judged by the incidence of cytoplasmic coupling, and formed more myotubes. Myoblasts transfected with dnCx43 remained proliferative for longer than either GFP-only or wtCx43 myoblasts, showed less coupling, and underwent little fusion into myotubes. These results highlight the critical role of gap-junctional coupling in myotube formation.

Novel Electrical Stimulation Sets the Cultured Myoblast Contractile Function to ‘On’

OBJECTIVE

In the present study, the effect of electrical stimulation was examined for the ability to induce morphological, physiological, and molecular biological effects on myoblasts during cell differentiation.

METHODS

L6 rat myoblasts were electrically stimulated by newly developed methods on culture days 6, 8, 10 and 12.

RESULTS

This electrical stimulation accelerated the appearance of myotubes, and subsequently produced spontaneously contracting muscle fibers. Measurement of membrane potential showed that the contracting cell had functional ion channels and gap junctional intercellular communication. In the electrically stimulated cells, an enhanced expression of MyoD family and M-cadherin was also observed. Expression of connexin 43 was increased and maintained at a high level in the electrically stimulated cells.

CONCLUSION

This is the first demonstration of in vitro induction of myoblasts in spontaneously contractile muscle fibers by intermittent stimulation. This novel method for induction of myoblast differentiation represents an important advance in cell therapy.

MIR-206 regulates connexin43 expression during skeletal muscle development

Skeletal myoblast fusion in vitro requires the expression of connexin43 (Cx43) gap junction channels. However, gap junctions are rapidly downregulated after the initiation of myoblast fusion in vitro and in vivo. In this study we show that this downregulation is accomplished by two related microRNAs, miR-206 and miR-1, that inhibit the expression of Cx43 protein during myoblast differentiation without altering Cx43 mRNA levels. Cx43 mRNA contains two binding sites for miR-206/miR-1 in its 3′-untranslated region, both of which are required for efficient downregulation. While it has been demonstrated before that miR-1 is involved in myogenesis, in this work we show that miR-206 is also upregulated during perinatal skeletal muscle development in mice in vivo and that both miR-1 and miR-206 downregulate Cx43 expression during myoblast fusion in vitro. Proper development of singly innervated muscle fibers requires muscle contraction and NMJ terminal selection and it is hypothesized that prolonged electrical coupling via gap junctions may be detrimental to this process. This work details the mechanism by which initial downregulation of Cx43 occurs during myogenesis and highlights the tight control mechanisms that are utilized for the regulation of gap junctions during differentiation and development.

In differentiating prefusion myoblasts connexin43 gap junction coupling is upregulated before myoblast alignment then reduced in post-mitotic cells

Previously we have shown that during in vivo muscle regeneration differentiating rat primary myoblasts transiently upregulate connexin43 (Cx43) gap junctions and leave cell cycle synchronously. Here, we studied the temporal regulation of Cx expression in relation to functional dye coupling in allogenic primary myoblast cultures using western blotting, immuno-confocal microscopy and dye transfer assays. As in vivo, Cx43 was the only Cx isotype out of Cx26, 32, 37, 40, 43 and 45 found in cultured rat myoblasts by immunostaining. Cultured myoblasts showed similar temporal regulation of Cx43 expression and phenotypic maturation to those regenerating in vivo. Cx43 protein was progressively upregulated in prefusion myoblasts, first by the cytoplasmic assembly in sparse myoblast meshworks and then in cell membrane particles in aligned cells. Dye injection using either Lucifer Yellow alone, Cascade Blue with a non-junction permeant FITC-dextran revealed an extensive gap junction coupling between the sparse interacting myoblasts and a reduced communication between the aligned, but still prefused cells. The aligned myoblasts, uniformly upregulate p21(waf1/cip1) and p27(kip1) cell cycle control proteins. Taken together, in prefusion myoblasts less membrane-bound Cx43 was found to mediate substantially more efficient dye coupling in the growing cell fraction than those in the aligned post-mitotic myoblasts. These and our in vivo results in early muscle differentiation are consistent with the role of Cx43 gap junctions in synchronizing cell cycle control of myoblasts to make them competent for a coordinated syncytial fusion.

Transient upregulation of connexin43 gap junctions and synchronized cell cycle control precede myoblast fusion in regenerating skeletal muscle in vivo

The spatio-temporal expression of gap junction connexins (Cx) was investigated and correlated with the progression of cell cycle control in regenerating soleus muscle of Wistar rats. Notexin caused a selective myonecrosis followed by the complete recapitulation of muscle differentiation in vivo, including the activation, commitment, proliferation, differentiation and fusion of myogenic cells. In regenerating skeletal muscle, only Cx43 protein, out of Cx-s 26, -32, -37, -40, -43 and -45, was detected in desmin positive cells. Early expression of Cx43 in the proliferating single myogenic progenitors was followed by a progressive upregulation in interacting myoblasts until syncytial fusion, and then by a rapid decline in multinucleate myotubes. The significant upregulation of Cx43 gap junctions in aligned myoblasts preceding fusion was accompanied by the widespread nuclear expression of cyclin-dependent kinase inhibitors p21(waf1/Cip1) and p27(kip1) and the complete loss of Ki67 protein. The synchronized exit of myoblasts from the cell cycle following extensive gap junction formation suggests a role for Cx43 channels in the regulation of cell cycle control. The potential of Cx43 channels to stimulate p21(waf1/Cip1) and p27(kip1) is known. In the muscle, proving the involvement of Cx43 in either a direct or a bystander cell cycle regulation requires functional investigations.

Expression of the rat connexin 39 (rCx39) gene in myoblasts and myotubes in developing and regenerating skeletal muscles: an in situ hybridization study

We report a detailed analysis of the expression pattern of the recently identified rat connexin gene, named rat connexin 39 (rCx39), both during embryonic development and in adult life. Qualitative and quantitative reverse transcription/polymerase chain reaction analysis showed intense expression of rCx39 restricted to differentiating skeletal muscles, with a peak of expression detected at 18 days of embryonic life, followed by a rapid decline to undetectable levels within the first week of postnatal life. A combination of the in situ hybridization technique for the detection of rCx39 mRNA and immunohistochemistry for myogenin, a myoblast-specific marker, allowed us to establish that the mRNA for this connexin was expressed in myogenin-positive myoblasts and early myotubes but disappeared in mature myotubes. Moreover, in adult animals, rCx39 mRNA was expressed in myogenic cells involved in skeletal myofiber regeneration following a crush injury. This is the first case of a connexin being mainly expressed in the myogenic cell lineage. The information presented should pave the way to novel molecular approaches in studies on the role of connexin-based gap-junctional communication in skeletal muscle differentiation and regeneration.

The novel mouse connexin39 gene is expressed in developing striated muscle fibers

The recently identified mouse connexin39 (mCx39) gene encodes a peptide of 364 amino acids that shows only 61% sequence similarity to its putative human orthologue connexin40.1 (hCx40.1). The coding regions of mCx39 and hCx40.1 are located on two different exons as described for murine and human connexin36. Northern blot and RT-PCR analyses revealed that mCx39 is expressed after embryonic day (ED) 13.5 up to birth and is absent from the adult stage. Polyclonal antibodies raised to a peptide corresponding to the 16 C-terminal amino acid residues detected a protein band of about 40 kDa apparent molecular mass in lysates of several embryonic tissues. In sections of ED14.5, ED16.5 and neonatal (P0) tissues, immunofluorescent signals were prominent between myotubes in the developing diaphragm, within the intercostal muscle, in the region around the occipital bone, as well as in muscles of the limb, tongue and connective tissue around the eye. These antibodies yielded punctate signals on apposed plasma membranes of HeLa cells transfected with Cx39 cDNA but did not react with wild-type cells. Furthermore, no intercellular permeation of microinjected neurobiotin and other tracers could be detected in Cx39 transfected HeLa cells. However, after microinjection of Alexa488 into myotubes of dissected neonatal diaphragm, we found spreading of this dye into neighbouring cells. As expression of no other known connexin could be verified in these cells, intercellular dye transfer might result from functional expression of Cx39 in developing striated muscle fibers.

Fusion-independent expression of functional ACh receptors in mouse mesoangioblast stem cells contacting muscle cells

Mesoangioblasts are vessel-associated fetal stem cells that can be induced to differentiate into skeletal muscle, both in vitro and in vivo. Whether this is due to fusion or to transdifferentiation into bona fide satellite cells is still an open question, for mesoangioblasts as well as for other types of stem cells. The early steps of satellite cell myogenic differentiation involve MyoD activation, membrane hyperpolarization and the appearance of ACh sensitivity and gap junctional communication. If mesoangioblasts differentiate into satellite cells, these characteristics should be observed in stem cells prior to fusion into multinucleated myotubes. We have investigated the functional properties acquired by mononucleated green fluorescent protein (GFP)-positive mesoangioblasts co-cultured with differentiating C2C12 myogenic cells, using the patch-clamp technique. Mesoangioblasts whose membrane contacted myogenic cells developed a hyperpolarized membrane resting potential and ACh-evoked current responses. Dye and electrical coupling was observed among mesoangioblasts but not between mesoangioblasts and myotubes. Mouse MyoD was detected by RT-PCR both in single, mononucleated mesoangioblasts co-cultured with C2C12 myotubes and in the total mRNA from mouse mesoangioblasts co-cultured with human myotubes, but not in human myotubes or stem cells cultured in isolation. In conclusion, when co-cultured with muscle cells, mesoangioblasts acquire many of the functional characteristics of differentiating satellite cells in the absence of cell fusion, strongly indicating that these stem cells undergo transdifferentiation into satellite cells, when exposed to a myogenic environment.

Plasma membrane channels formed by connexins: their regulation and functions

Members of the connexin gene family are integral membrane proteins that form hexamers called connexons. Most cells express two or more connexins. Open connexons found at the nonjunctional plasma membrane connect the cell interior with the extracellular milieu. They have been implicated in physiological functions including paracrine intercellular signaling and in induction of cell death under pathological conditions. Gap junction channels are formed by docking of two connexons and are found at cell-cell appositions. Gap junction channels are responsible for direct intercellular transfer of ions and small molecules including propagation of inositol trisphosphate-dependent calcium waves. They are involved in coordinating the electrical and metabolic responses of heterogeneous cells. New approaches have expanded our knowledge of channel structure and connexin biochemistry (e.g., protein trafficking/assembly, phosphorylation, and interactions with other connexins or other proteins). The physiological role of gap junctions in several tissues has been elucidated by the discovery of mutant connexins associated with genetic diseases and by the generation of mice with targeted ablation of specific connexin genes. The observed phenotypes range from specific tissue dysfunction to embryonic lethality.

Apoptosis and syncytial fusion in human placental trophoblast and skeletal muscle

Skeletal muscle fibers and placental villous trophoblast are the main representatives of syncytia in the human. Both syncytia are derived from fusion of mononucleated stem cells, show a high degree of differentiation, and have lost their generative potency. Consequently, for their growth both depend on fusion of additional stem cells. There is evidence that syncytial fusion is directly or indirectly related to apoptotic events: As early as in the differentiated stages of the mononucleated stem cells, initiation stages of the apoptosis cascade have been observed. After syncytial fusion progression of the cascade is retarded or blocked by a variety of mechanisms. In this review we emphasize the links between apoptosis cascade, differentiation pathways and syncytial fusion. It needs to be elucidated whether these processes simply take place in parallel, both temporally and spatially, or whether there are causal connections between apoptosis cascade and syncytial fusion. Based on recent data obtained for placental villous trophoblast, it is tempting to speculate that early molecular mechanisms of the apoptosis cascade are involved in differentiation and syncytial fusion. Data obtained in skeletal muscles support this assumption and reveal a considerable degree of homology in genesis, maintenance and turnover of both tissues.

Involvement of gap junctional communication in myogenesis

Cell-to-cell communication plays important roles in development and in tissue morphogenesis. Gap junctional intercellular communication (GJIC) has been implicated in embryonic development of various tissues and provides a pathway to exchange ions, secondary messengers, and metabolites through the intercellular gap junction channels. Although GJIC is absent in adult skeletal muscles, the formation of skeletal muscles involves a sequence of complex events including cell-cell interaction processes where myogenic cells closely adhere to each other. Much experimental evidence has shown that myogenic precursors and developing muscle fibers can directly communicate through junctional channels. This review summarizes current knowledge on the GJIC and developmental events involved in the formation of skeletal muscle fibers and describes recent progress in the investigation of the role of GJIC in myogenesis: evidence of gap junctions in somitic and myotomal tissue as well as in developing muscle fibers in situ, GJIC between perfusion myoblasts in culture, and involvement of GJIC in cytodifferentiation of skeletal muscle cells and in myoblast fusion. A model of intercellular signaling is proposed where GJIC participates to coordinate a multicellular population of interacting myogenic precursors to allow commitment to the skeletal muscle fate.

Normal differentiation of cultured lens cells after inhibition of gap junction-mediated intercellular communication

The cells of the vertebrate lens are linked to each other by gap junctions, clusters of intercellular channels that mediate the direct transfer of low-molecular-weight substances between the cytosols of adjoining cells. Although gap junctions are detectable in the unspecialized epithelial cells that comprise the anterior face of the organ, both their number and size are greatly increased in the secondary fiber cells that differentiate from them at the lens equator. In other organs, gap junctions have been shown to play an important role in tissue development and differentiation. It has been proposed, although not experimentally tested, that this may be true in the lens as well. To investigate the function of gap junctions in the development of the lens, we have examined the effect of the gap junction blocker 18beta-glycyrrhetinic acid (betaGA) on the differentiation of primary cultures (both dissociated cell-derived monolayers and central epithelium explants) of embryonic chick lens epithelial cells. We found that betaGA greatly reduced gap junction-mediated intercellular transfer of Lucifer yellow and biocytin throughout the 8-day culture period. betaGA did not, however, affect the differentiation of these cells into MP28-expressing secondary fibers. Furthermore, inhibition of gap junctions had no apparent effect on either of the two other types of intercellular (adherens and tight) junctions present in the lens. We conclude that the high level of gap junctional intercellular communication characteristic of the lens equator in vivo is not required for secondary fiber formation as assayed in culture. Up-regulation of gap junctions is therefore likely to be a consequence rather than a cause of lens fiber differentiation and may primarily play a role in lens physiology.