Cell-to-cell communication and myogenesis

Bioelectrical domain walls in homogeneous tissues

Electrical signaling in biology is typically associated with action potentials, transient spikes in membrane voltage that return to baseline. Hodgkin-Huxley and related conductance-based models of electrophysiology belong to a more general class of reaction-diffusion equations which could, in principle, support spontaneous emergence of patterns of membrane voltage which are stable in time but structured in space. Here we show theoretically and experimentally that homogeneous or nearly homogeneous tissues can undergo spontaneous spatial symmetry breaking through a purely electrophysiological mechanism, leading to formation of domains with different resting potentials separated by stable bioelectrical domain walls. Transitions from one resting potential to another can occur through long-range migration of these domain walls. We map bioelectrical domain wall motion using all-optical electrophysiology in an engineered cell line and in human induced pluripotent stem cell (iPSC)-derived myoblasts. Bioelectrical domain wall migration may occur during embryonic development and during physiological signaling processes in polarized tissues. These results demonstrate that nominally homogeneous tissues can undergo spontaneous bioelectrical symmetry breaking.

Arterial Delivery of Genetically Labelled Skeletal Myoblasts to the Murine Heart: Long-Term Survival and Phenotypic Modification of Implanted Myoblasts

The ability to replace damaged myocardial tissue with new striated muscle would constitute a major advance in the treatment of diseases that irreversibly injure cardiac muscle cells. The creation of focal grafts of skeletal muscle has been reported following the intramural injection of skeletal myoblasts into both normal and injured myocardium. The goals of this study were to determine whether skeletal myoblast-derived cells can be engrafted into the murine heart following arterial delivery. The murine heart was seeded with genetically labeled C2C12 myoblasts introduced into the arterial circulation of the heart via a transventricular injection. A transventricular injection provided access to the coronary and systemic circulations. Implanted cells were characterized using histochemical staining for β-galactosidase, immunofluorescent staining for muscle-specific antigens, and electron microscopy. Initially the injected cells were observed entrapped in myocardial capillaries. One week after injection myoblasts were present in the myocardial interstitium and were largely absent from the myocardial capillary bed. Implanted cells underwent myogenic development, characterized by the expression of a fast-twitch skeletal muscle sarco-endoplasmic reticulum calcium ATPase (SERCA1) and formation of myofilaments. Four months following injection myoblast-derived cells began to express a slow-twitch/cardiac protein, phospholamban, that is normally not expressed by C2C12 cells in vitro. Most surprisingly, regions of close apposition between LacZ labeled cells and native cardiomyocytes contained structures that resembled desmosomes, fascia adherens junctions, and gap junctions. The cardiac gap junction protein, connexin43, was localized to some of the interfaces between implanted cells and cardiomyocytes. Collectively, these findings suggest that arterially delivered myoblasts can be engrafted into the heart, and that prolonged residence in the myocardium may alter the phenotype of these skeletal muscle-derived cells. Further studies are necessary to determine whether arterial delivery of skeletal myoblasts can be developed as treatment for myocardial dysfunction.

MyoD transcription factor induces myogenesis by inhibiting Twist-1 through miR-206

Twist-1 is mostly expressed during development and has been previously shown to control myogenesis. Because its regulation in muscle has not been fully exploited, the aim of this project was to identify micro (mi)RNAs in muscle that regulate Twist-1. miR-206, one of the most important muscle-specific miRNAs (myomiRs), was identified as a possible regulator of Twist-1 mRNA. Luciferase assays and transfections in human foetal myoblasts showed that Twist-1 is a direct target of miR-206 and that through this pathway muscle cell differentiation is promoted. We next investigated whether MyoD, a major myogenic transcription factor, regulates Twist-1 because it is known that MyoD induces expression of the miR-206 gene. We found that forced MyoD expression induced miR-206 upregulation and Twist-1 downregulation through binding to the miR-206 promoter, followed by increased muscle cell differentiation. Finally, experiments were performed in muscle cells from subjects with congenital myotonic dystrophy type 1, in which myoblasts fail to differentiate into myotubes. MyoD overexpression inhibited Twist-1 through miR-206 induction, which was followed by an increase in muscle cell differentiation. These results reveal a previously unidentified mechanism of myogenesis that might also play an important role in muscle 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.

Neuronal involvement in muscular atrophy

The innervation of skeletal myofibers exerts a crucial influence on the maintenance of muscle tone and normal operation. Consequently, denervated myofibers manifest atrophy, which is preceded by an increase in sarcolemma permeability. Recently, de novo expression of hemichannels (HCs) formed by connexins (Cxs) and other none selective channels, including P2X7 receptors (P2X7Rs), and transient receptor potential, sub-family V, member 2 (TRPV2) channels was demonstrated in denervated fast skeletal muscles. The denervation-induced atrophy was drastically reduced in denervated muscles deficient in Cxs 43 and 45. Nonetheless, the transduction mechanism by which the nerve represses the expression of the above mentioned non-selective channels remains unknown. The paracrine action of extracellular signaling molecules including ATP, neurotrophic factors (i.e., brain-derived neurotrophic factor (BDNF)), agrin/LDL receptor-related protein 4 (Lrp4)/muscle-specific receptor kinase (MuSK) and acetylcholine (Ach) are among the possible signals for repression for connexin expression. This review discusses the possible role of relevant factors in maintaining the normal functioning of fast skeletal muscles and suppression of connexin hemichannel expression.

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.

MicroRNAs involved in skeletal muscle differentiation

MicroRNAs (miRNAs) negatively regulate gene expression by promoting degradation of target mRNAs or inhibiting their translation. Previous studies have expanded our understanding that miRNAs play an important role in myogenesis and have a big impact on muscle mass, muscle fiber type and muscle-related diseases. The muscle-specific miRNAs, miR-206, miR-1 and miR-133, are among the most studied and best characterized miRNAs in skeletal muscle differentiation. They have a profound influence on multiple muscle differentiation processes, such as alternative splicing, DNA synthesis, and cell apoptosis. Many non-muscle-specific miRNAs are also required for the differentiation of muscle through interaction with myogenic factors. Studying the regulatory mechanisms of these miRNAs in muscle differentiation will extend our knowledge of miRNAs in muscle biology and will improve our understanding of the myogenesis regulation.

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

Precursor cells of skeletal muscles express connexins 39, 43 and 45 and pannexin1. In these cells, most connexins form two types of membrane channels, gap junction channels and hemichannels, whereas pannexin1 forms only hemichannels. All these channels are low-resistance pathways permeable to ions and small molecules that coordinate developmental events. During late stages of skeletal muscle differentiation, myofibers become innervated and stop expressing connexins but still express pannexin1 hemichannels that are potential pathways for the ATP release required for potentiation of the contraction response. Adult injured muscles undergo regeneration, and connexins are reexpressed and form membrane channels. In vivo, connexin reexpression occurs in undifferentiated cells that form new myofibers, favoring the healing process of injured muscle. However, differentiated myofibers maintained in culture for 48 h or treated with proinflammatory cytokines for less than 3 h also reexpress connexins and only form functional hemichannels at the cell surface. We propose that opening of these hemichannels contributes to drastic changes in electrochemical gradients, including reduction of membrane potential, increases in intracellular free Ca(2+) concentration and release of diverse metabolites (e.g., NAD(+) and ATP) to the extracellular milieu, contributing to multiple metabolic and physiologic alterations that characterize muscles undergoing atrophy in several acquired and genetic human diseases. Consequently, inhibition of connexin hemichannels expressed by injured or denervated skeletal muscles might reduce or prevent deleterious changes triggered by conditions that promote muscle atrophy.

Connexin 39.9 protein is necessary for coordinated activation of slow-twitch muscle and normal behavior in zebrafish

In many tissues and organs, connexin proteins assemble between neighboring cells to form gap junctions. These gap junctions facilitate direct intercellular communication between adjoining cells, allowing for the transmission of both chemical and electrical signals. In rodents, gap junctions are found in differentiating myoblasts and are important for myogenesis. Although gap junctions were once believed to be absent from differentiated skeletal muscle in mammals, recent studies in teleosts revealed that differentiated muscle does express connexins and is electrically coupled, at least at the larval stage. These findings raised questions regarding the functional significance of gap junctions in differentiated muscle. Our analysis of gap junctions in muscle began with the isolation of a zebrafish motor mutant that displayed weak coiling at day 1 of development, a behavior known to be driven by slow-twitch muscle (slow muscle). We identified a missense mutation in the gene encoding Connexin 39.9. In situ hybridization found connexin 39.9 to be expressed by slow muscle. Paired muscle recordings uncovered that wild-type slow muscles are electrically coupled, whereas mutant slow muscles are not. The further examination of cellular activity revealed aberrant, arrhythmic touch-evoked Ca(2+) transients in mutant slow muscle and a reduction in the number of muscle fibers contracting in response to touch in mutants. These results indicate that Connexin 39.9 facilitates the spreading of neuronal inputs, which is irregular during motor development, beyond the muscle cells and that gap junctions play an essential role in the efficient recruitment of slow muscle fibers.

Ferlin proteins in myoblast fusion and muscle growth

Myoblast fusion contributes to muscle growth in development and during regeneration of mature muscle. Myoblasts fuse to each other as well as to multinucleate myotubes to enlarge the myofiber. The molecular mechanisms of myoblast fusion are incompletely understood. Adhesion, apposition, and membrane fusion are accompanied by cytoskeletal rearrangements. The ferlin family of proteins is implicated in human muscle disease and has been implicated in fusion events in muscle, including myoblast fusion, vesicle trafficking and membrane repair. Dysferlin was the first mammalian ferlin identified and it is now known that there are six different ferlins. Loss-of-function mutations in the dysferlin gene lead to limb girdle muscular dystrophy and the milder disorder Miyoshi Myopathy. Dysferlin is a membrane-associated protein that has been implicated in resealing disruptions in the muscle plasma membrane. Newer data supports a broader role for dysferlin in intracellular vesicular movement, a process also important for resealing. Myoferlin is highly expressed in myoblasts that undergoing fusion, and the absence of myoferlin leads to impaired myoblast fusion. Myoferlin also regulates intracellular trafficking events, including endocytic recycling, a process where internalized vesicles are returned to the plasma membrane. The trafficking role of ferlin proteins is reviewed herein with a specific focus as to how this machinery alters myogenesis and muscle growth.

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.

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.

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.

Sphingosine 1-phosphate induces myoblast differentiation through Cx43 protein expression: a role for a gap junction-dependent and -independent function

Although sphingosine 1-phosphate (S1P) has been considered a potent regulator of skeletal muscle biology, acting as a physiological anti-mitogenic and prodifferentiating agent, its downstream effectors are poorly known. In the present study, we provide experimental evidence for a novel mechanism by which S1P regulates skeletal muscle differentiation through the regulation of gap junctional protein connexin (Cx) 43. Indeed, the treatment with S1P greatly enhanced Cx43 expression and gap junctional intercellular communication during the early phases of myoblast differentiation, whereas the down-regulation of Cx43 by transfection with short interfering RNA blocked myogenesis elicited by S1P. Moreover, calcium and p38 MAPK-dependent pathways were required for S1P-induced increase in Cx43 expression. Interestingly, enforced expression of mutated Cx43(Delta130-136) reduced gap junction communication and totally inhibited S1P-induced expression of the myogenic markers, myogenin, myosin heavy chain, caveolin-3, and myotube formation. Notably, in S1P-stimulated myoblasts, endogenous or wild-type Cx43 protein, but not the mutated form, coimmunoprecipitated and colocalized with F-actin and cortactin in a p38 MAPK-dependent manner. These data, together with the known role of actin remodeling in cell differentiation, strongly support the important contribution of gap junctional communication, Cx43 expression and Cx43/cytoskeleton interaction in skeletal myogenesis elicited by S1P.

Electrical synapses--gap junctions in the brain

In the nervous system, interneuronal communication can occur via indirect or direct transmission. The mode of indirect communication involves chemical synapses, in which transmitters are released into the extracellular space to subsequently bind to the postsynaptic cell membrane. Direct communication is mediated by electrical synapses, and will be the focus of this review. The most prevalent group of electrical synapses are neuronal gap junctions (both terms are used interchangeably in this article), which directly connect the intracellular space of two cells by gap junction channels. The structural components of gap junction channels in the nervous system are connexin proteins, and, as recently identified, pannexin proteins. Connexin gap junction channels enable the intercellular, bidirectional transport of ions, metabolites, second messengers and other molecules smaller than 1 kD. More than 20 connexin genes have been found in the mouse and human genome. With the cloning of connexin36 (Cx36), a connexin protein with predominantly neuronal expression, the biochemical correlate of electrotonic transmission between neurons was identified. We outline the distribution of Cx36 as well as two other neuronal connexins (Cx57 and Cx45) in the nervous system, describing their spatial and temporal expression patterns. One focus in this review was the retina, as it shows many and diverse electrical synapses whose connexin components have been identified in fish and mammals. In view of the function of neuronal gap junctions, the network of inhibitory interneurons will be reviewed in detail, focussing on the hippocampus. Although in vivo data on pannexin proteins are still restricted to information on mRNA expression, electrophysiological data and the expression pattern in the nervous system have been included.

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 formation of skeletal muscle myotubes requires functional membrane receptors activated by extracellular ATP

Skeletal muscle differentiation follows an organized sequence of events including commitment, cell cycle withdrawal, and cell fusion to form multinucleated myotubes. The role of adenosine 5'-triphosphate (ATP)-mediated signaling in differentiation of skeletal muscle myoblasts was evaluated in C(2)C(12) cells, a myoblast cell line. Cell differentiation was inhibited by P2X receptor blockers or by degradation of endogenous ATP with apyrase. However, pertussis toxin, known to block only a group of P2Y receptors, did not alter the differentiation process. Cells were heterogeneous in their expression of functional P2X receptors, evaluated by the uptake of fluorescent permeability tracers (Lucifer yellow and ethidium bromide), and by immunofluorescence of P2X(7) receptors. Moreover, xestospongin C, a selective and membrane-permeable inhibitor of IP(3) receptors, inhibited both myotube formation and myogenin expression. Based on these results, we suggest that the known increase in intracellular Ca(2+) concentration required for differentiation is due at least in part to Ca(2+) influx through P2X receptors and Ca(2+) release from intracellular stores. The possible involvement of P2X receptors and other pathways that might set the intracellular Ca(2+) at the level required for myoblast differentiation as well as the possible involvement of gap junction channels in the intercellular transfer of second messengers involved in coordinating myogenesis is proposed.

Preparation of cells cultured on silicon wafers for mass spectrometry analysis

The distribution of specific atoms and molecules within living cells is of high interest in bio-medical research. Laser secondary neutral mass spectrometry (laser-SNMS) and time-of-flight secondary ion mass spectrometry (TOF-SIMS) detect atoms with high sensitivity and spatial resolution. The application of these methods to cultured cells requires special preparation techniques preserving morphological and chemical integrity of the living cells. The cells should, therefore, be grown on a conducting material preventing charging of the sample during ion bombardment. Silicon is currently used as the preferred support material for non-biological samples in mass spectrometry. This study investigates (1) the influence of silicon surfaces on cell growth and (2) the suitability of a sandwiched, rapid freezing method to analyse transmembrane ion gradients. Human melanoma cells were grown on silicon with polished or etched surfaces. Growth kinetics were studied using the Sulforhodamine-B assay. Number, shape, and morphology of the cells were assessed by epifluorescence microscopy of calcein AM- and DAPI-stained cells. Cells were subjected to rapid freezing, freeze-fracturing, and freeze-drying prior to analysis by TOF-SIMS and laser-SNMS. While cell numbers and morphology on the rough silicon wafers were impaired, morphology and growth kinetics of cells on polished silicon were identical to control cells on cell culture tested polystyrene. TOF-SIMS and laser-SNMS resulted in high-resolution elemental images and mass spectra. Measurement of the intracellular Na+ and K+ concentrations revealed a ratio as observed in living cells. In conclusion, culturing cells on polished silicon wafers followed by sandwiched, rapid freezing is an adequate preparation method to study intracellular ion distribution with mass spectrometry.

Expression of connexins during differentiation and regeneration of skeletal muscle: functional relevance of connexin43

The molecular mechanisms regulating skeletal muscle regeneration and differentiation are not well understood. We analyzed the expression of connexins (Cxs) 40, 43 and 45 in normal and regenerating tibialis anterior muscle and in primary cultures of differentiating myoblasts in adult and newborn mice, respectively. Cxs 45 and 43, but not 40, were strongly expressed in normal muscle and their expression was upregulated during regeneration. Furthermore, the functional role of Cx43 during differentiation and regeneration was examined after induced deletion of Cx43 in transgenic mice. In vivo, the inducible deletion of Cx43 delayed the formation of myofibers and prolonged the expression of myogenin during regeneration. In primary cultures of satellite cell-derived myoblasts, induced deletion of Cx43 led to decreased expression of myogenin and MyoD, dye coupling, creatine kinase activity and myoblast fusion. Thus, the expression of Cx45 and Cx43 is upregulated during skeletal muscle regeneration and Cx43 is required for normal myogenesis in vitro and adult muscle regeneration in vivo.

Presence and importance of connexin43 during myogenesis

We analyzed the expression of connexin(Cx)43 in proliferating and differentiating C(2)C(12) cells and in myoblasts obtained from newborn mice. Cx43 was present in both cell types and under both conditions. The functional role of gap junctional communication (GJC) during terminal differentiation was evaluated in C(2)C(12) myoblasts in the presence or absence of the gap junction blocker 18beta-glycyrrhetinic acid (beta-GA). Differentiation was temporally analyzed through myogenin expression, activity of creatine kinase (CK), and yield of multinucleated cells. In cells treated with beta-GA, the CK activity and myotube formation were reversibly blocked. While in control cultures positive myogenin expression was seen in cell clusters, in beta-GA treated cultures the myogenin immunoreactivity was detected in few, preferentially sparse cells. The role of Cx43 during terminal differentiation was evaluated in cultures of myoblasts obtained from Cx43(Cre-ER(T)/fl) transgenic mice. Inducible deletion of Cx43 was obtained upon activation of Cre-ER(T) via 4-OH-tamoxifen applications. Cx43 deletion led to a drastic decrease in myogenin expression at 24 h of differentiation as compared to myoblasts from control mice. Our results indicate that Cx43-containing gap junctions are required for normal skeletal muscle terminal differentiation. These channels might provide a pathway for the intercellular transfer of signals involved in myogenesis.

Highly restricted pattern of connexin36 expression in chick somite development

The gap junction protein connexin36 (CX36) has been well studied in the mature central nervous system, but there has been little information regarding its possible roles in embryonic development. We report here the isolation of the full-length chick CX36 coding sequence (predicted M(r) 35.1 kDa) and its strikingly restricted pattern of gene expression in the mesoderm of the chick embryo. In situ hybridization experiments demonstrated CX36 expression in somites by embryonic day 2. The transcripts first appeared dorsomedially within the somite and expanded ventrolaterally to form stripes in the middle of each somite. The CX36 stripes fell within somitic territories enriched in MYOD and FGF8 expression and impoverished in PAX3 transcripts, establishing that CX36 mRNA is expressed in the myotome. We compared the somitic expression pattern of CX36 with those of three other connexins, CX42, CX43, and CX45. At embryonic day 4, CX42 transcripts were localized to the myotome in a pattern resembling that of CX36. In contrast, CX43 was enriched in the dermomyotome, and CX45 was detected in both the myotome and the dermomyotome. Immunoblotting using Cx36 antibodies demonstrated bands of identical electrophoretic mobilities in trunk and retinal homogenates, and Cx36 immunostaining detected punctate immunoreactivity in the myotome. These results demonstrate that some connexins in the developing mesoderm are broadly expressed whereas others are highly localized, and suggest that CX36, CX42, and CX45 are involved in intercellular communication among developing muscle cells.

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.

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.

Cytochemical localization of calcium in prefusion myoblasts from the chick embryo myotome

Myoblast fusion is a Ca2+-dependent process. The aim of this report was to study the localization of Ca2+ in prefusion myoblasts from the brachial somites of chick embryos (51-108 h of incubation), using the potassium pyroantimonate cytochemical method. When observed under a transmission electron microscope, electron-dense precipitates of Ca2+-antimonate were found in the basement membrane of the myotome, which separates the myotome from the adjacent mesenchyma. Within myoblasts, triads and sarcoplasmic reticulum associated with the first newly formed sarcomeres were observed, but a T-tubule network was not found. Moreover, Ca2+-antimonate precipitates were not observed in structures resembling T-tubules or sarcoplasmic reticulum. The results suggest that sarcomerogenesis and sarcoplasmic reticulum development occur simultaneously and that prefusion myoblasts have neither a T-tubule network nor Ca2+ deposits on sarcoplasmic reticulum. Small Ca2+ pools were found in the myoblast nuclei, cytoplasmic vesicles and mitochondrias. Ca2+-antimonate precipitates periodically distributed at the cell periphery, close to the cell membrane, were observed. These precipitates could represent internal Ca2+ stores located in the peripheral couplings and it is proposed that these pools of Ca2+ could be mobilized before fusion, leading to the increase in free intracellular Ca2+ that precedes myoblast fusion.

Myoblast cell grafting into heart muscle: cellular biology and potential applications

This review surveys a wide range of cellular and molecular approaches to strengthening the injured or weakened heart, focusing on strategies to replace dysfunctional, necrotic, or apoptotic cardiomyocytes with new cells of mesodermal origin. A variety of cell types, including myogenic cell lines, adult skeletal myoblasts, immoratalized atrial cells, embryonic and adult cardiomyocytes, embryonic stem cells, tetratoma cells, genetically altered fibroblasts, smooth muscle cells, and bone marrow-derived cells have all been proposed as useful cells in cardiac repair and may have the capacity to perform cardiac work. We focus on the implantation of mesodermally derived cells, the best developed of the options. We review the developmental and cell biology that have stimulated these studies, examine the limitations of current knowledge, and identify challenges for the future, which we believe are considerable.

Radiation-induced micronuclei formation in human breast cancer cells: dependence on serum and cell cycle distribution

Micronuclei (MN) formation was defined as a form of radiation-induced damage in MCF-7 cells. MN appeared post-mitosis and were scored in bi-nucleated cells of cytochalasin B treated cultures. MN were surrounded by an envelope composed of inner and outer membranes, and contained fragmented chromosomes. However, typical features of apoptosis, such as chromatin margination or condensation were not observed. Reducing serum concentration resulted in a decreased MN formation, suggesting that serum factors directly affected MN formation and/or that serum depletion decreased the availability of radiation sensitive MN-forming cells for mitosis. Irradiation of G1 and S phase enriched populations revealed that S phase cells were more prone to MN formation than G1 cells. Radiation-induced chromosomal aberration can therefore be modulated by altering serum level and cell cycle distribution.

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

Heptanol-sensitive gap junction communication was characterized by the gap-FRAP method (fluorescence recovery after photobleaching) in confluent rat myoblasts developing in primary culture. Cell to cell dye diffusion was mainly restricted to a short period of the perfusion lag period and disappeared during fusion promotion except between some myoblasts and myotubes. This short period of occurrence of gap junction communication might be transiently and partially involved during the first steps preparing the subsequent fusion, since treatment with an uncoupler (heptanol) reduced the formation of multinucleated myotubes. During subsequent steps, functional gap junctions are not involved between myoblasts in the process of fusing, but a possible secondary involvement for fusion of remaining myoblasts to newly-formed myotubes is discussed. These data, together with results from other authors, suggest a regulatory role of gap junction communication in development and fusion of skeletal muscle cells, by providing a pathway for exchanging small molecules from one myoblast to another.

Expression of gap junction genes, connexin40 and connexin43, during fetal mouse development

The expression patterns of the gap junction genes connexin40 and connexin43 have been analyzed during late mouse fetal development, i.e., at embryonic days 14.5 and 16.5, by in situ hybridization and immunofluorescence. Connexin40 was found in endothelial cells of vessels, cardiomyocytes and in developing myoblasts and myotubes. Expression of connexin40 in developing muscle fibers was strong in the back muscles and weaker in the muscles of the limbs. The number of labeled cells in the back muscle decreased with ongoing differentiation of myoblasts, in accordance with the idea that connexin40 is only expressed in the early stages of muscle cell differentiation. Within a muscle bundle, connexin40 expression was predominantly found at the outermost side where myoblasts fuse to multinucleated myotubes. In contrast, connexin43 exhibits a wide and complex pattern of expression in fetal mouse development. It is found in organs originating from all three germ layers, such as epidermis, heart, lung, muscle, kidney and gut. Connexin43 transcript and protein were very abundant in tissues that had been undergoing inductive interactions, e.g., the inner enamel epithelium of the teeth, the glomeruli of the kidneys and the infundibulum forming the neural part of the pituitary gland. Very high connexin43 expression was found in the embryonic meninges (dura mater) and in the fetal adrenal cortex. During keratinocyte differentiation connexin43 mRNA expression decreased, being much stronger in the stratum basale than in stratum granulosum. No obvious discrepancy between the amount of mRNA and protein of either connexin was noticed, suggesting that there is no specific translational regulation at these developmental stages.

Is intercellular communication via gap junctions required for myoblast fusion?

Fusion of myoblasts to form syncitial muscle cells results from a complex series of sequential events including cell alignment, cell adhesion and cell communication. The aim of the present investigation was to assess whether intercellular communication through gap junctions would be required for subsequent membrane fusion. The presence of the gap junction protein connexin 43 at areas of contact between prefusing rat L6 myoblasts was established by immunofluorescent staining. These myoblasts were dye-coupled, as demonstrated by the use of the scrape-loading/dye transfer technique. L6 myoblast dye coupling was reversibly blocked by heptanol in short term experiments as well as after chronic treatment. After a single addition of 3.5 mM heptanol, gap junctions remained blocked for up to 8 hours, then this inhibitory effect decreased gradually, likely because the alcohol was evaporated. Changing heptanol solutions every 8 hours during the time course of L6 differentiation resulted in a lasting drastic inhibition of myoblast fusion. We further investigated the effect of heptanol and of other uncoupling agents on the differentiation of primary cultures of embryonic chicken myoblasts. These cells are transiently coupled by gap junctions before myoblast fusion and prolonged application of heptanol, octanol and 18-beta-glycyrrhetinic acid also inhibited their fusion. The effect of heptanol and octanol was neither due to a cytotoxic effect nor to a modification of cell proliferation. Moreover, heptanol treatment did not alter myoblast alignment and adhesion. Taken together these observations suggest that intercellular communication might be a necessary step for myoblast fusion.

Differences in the histogenesis of EDL and diaphragm in rat

We have examined the histogenesis of the diaphragm and extensor digitorum muscle in rat embryos, with the aim of defining differences in developmental patterns that can be related to the functional requirements of these muscles during and after development. Patterns of interactions between myotubes and other cells, and frequency of gap junctions are quite different in the two muscles. In diaphragm, primary myotubes (at day 16 in utero) are closely associated with each other, forming parallel sheets or palisades and communicating by gap junctions. Secondary myotubes have formed by day 18, but are immature, and the frequency of gap junctions is lower. The arrangement in palisades is maintained even after fibers are separated from each other by their individual basal lamina. In EDL primary fibers at day 16 have fewer gap junctions, and the peak in communication occurs after the appearance of secondary myotubes (day 18 and 21). Secondary myotubes are more mature than in diaphragm at day 18.

Fusion between myogenic cells in vivo: an ultrastructural study in regenerating murine skeletal muscle

Fusion of myogenic cells in adult murine skeletal muscle regenerating in vivo was examined at the ultrastructural level. Fusion of myoblast to myoblast, myoblast to myotube, and myotube to myotube was observed by 4 to 5 days after injury. Fusion between myogenic cells (myoblasts or myotubes) lacking a definitive glycocalyx or external lamina (basal lamina) occurred at multiple sites. It was defined by zones of cytoplasmic confluence between apposed cells at sites where contiguous segments of the cell membranes were interrupted while their edges had united resulting in linear continuity; vesicles of varying dimensions were frequent in these areas of fusion. Myoblasts were seen invaginating the surface of myofibres and again vesicles were seen in abundance in such regions. Cilia were often observed at this junctional zone suggesting that they might play a role in fusion. In the one example of probable fusion between a myotube and a myofibre, only a single area of cytoplasmic continuity was apparent.

Correlation of development stage and gap junction formation between chick embryo neurons and cloned skeletal muscle myoblasts

The frequency of gap junction formation between neurons and myoblasts of chick embryo leg skeletal muscle changes as a function of the developmental stage of the muscle. Cells from leg muscle of various ages and states of innervation were first cloned in vitro and then co-cultured with ciliary ganglion, spinal cord, or dorsal root ganglion neurons. The presence of gap junctions between cells was identified by the passage of fluorescent dyes or electric currents from one cell to another. The clones examined were fusing muscle clones (myoblasts) as well as nonfusing clones. Mononuclear cells of either kind of clone derived from legs younger than stage 24 (E4) dye-couple with other clone cells but do not dye-couple with neurons. Myoblasts of fusing muscle clones derived from stage 24 through stage 29 (E5) legs dye-couple with neurons at high frequency; nonfusing clones from these same embryos do not contain cells that dye-couple with neurons. Mononuclear cells of both fusing and nonfusing clones from normally innervated stage 30 (E6) through 38 (E12) legs do not dye-couple with neurons at significant frequencies. Additionally, aneural legs of denervated E10-E12 embryos yield muscle clones in which the myoblasts again dye-couple with neurons at high frequency. The ability to form communicating junctions between muscle cells and neurons in culture is restricted to myoblasts cloned from legs of those stages of development and conditions of innervation that define and limit neuron-dependent alteration of myoblast populations in vivo.

The roles of haemocytes during degeneration and regeneration of crayfish muscle fibres

Crayfish haemolymph contains three types of haemocytes with cytoplasmic granules: coagulocytes, granulocytes and amoebocytes. Muscle degeneration was induced by either a gross mechanical injury or a mild puncture injury of m. extensor carpopoditi. Granulocytes and amoebocytes were involved in the phagocytosis of disintegrating muscle fibres. Within three weeks after the gross injury the first myotubes were found. The formation of regenerated fibres started before the degenerating material was removed completely. Mild injury resulted in the formation of contraction clots, localized at the ends of a fibre and connected to a persistent external lamina in the form of an empty sheath. The external lamina sheaths were invaded by amoebocytes. They arranged themselves into a superficial layer similar to an epithelium, formed gap junctions and zonulae adherentes, and showed an increase in the number of cytoplasmic microtubules. These transformed haemocytes retained their ability to engulf material of the disintegrating fibre. In about three weeks the number of microtubules in the transformed haemocytes decreased, and newly formed contractile filaments appeared. Satellite cells are present along the normal crayfish muscle fibres. Following their activation in degenerated material, they might conceivably induce the transformation of haemocytes into myogenic cells.

Differentiating astroglia in nervous tissue histogenesis/regeneration: studies in a model system of regenerating peripheral nerve

The role of astroglia in nervous system histogenesis/regeneration (morphogenesis) was studied as a function of cell age. The effect of inoculated astroglia at different cell maturation stages on axonal growth was examined in a peripheral nerve regenerating model system. This model system consists of rat sciatic nerve stumps that regenerate through an empty silicone chamber (Lundborg et al.: Journal of Neuropathology and Experimental Neurology 41:412-422, 1982). Rat astroglial cell populations grown in cell culture were suspended either in a liquid (physiological solution) or in a solid (isotonic collagen gel) medium and inoculated within the silicone chamber at the time of surgery. Immature and mature cell populations, at ages corresponding to 9-69 postnatal days (P9-P69), were inoculated, and their effect on neural growth was analyzed by histological, immunocytochemical, and ultrastructural techniques, 2-6 weeks later. Astroglial cells differentially modulated the process of nerve regeneration, an effect that is a function of the cells' developmental stage. P19 astroglia and older cells inhibited the regeneration process, encapsulating the axons in a glia-limitans-like structure. Immature astrocytes (P9) did not seem to interfere with the regeneration process; nerve outgrowth in their presence resembled and was comparable to the ones obtained in the presence of inoculated Schwann cells. The differential effects of the developing astroglia were not significantly changed by the inoculation media, e.g., liquid or solid, and were already pronounced 2 weeks after neural transection. It is suggested by the results of the study that the role and function of astroglia in nervous system morphogenesis are changing with cell differentiation. Adult astrocytes seem to downregulate axonal growth; presumably, their function is to confine the neurites within designated structural and functional boundaries.

Gap junctions form in culture between chick embryo neurons and skeletal muscle myoblasts

Possible mechanisms by which neurons can affect the differentiated states of mononucleated cells in the early chick embryo leg skeletal myogenic lineage are being investigated. Under conditions of co-culture known to promote inductive interactions between embryonic neurons and leg cells, spinal cord neurons and mononucleated leg cells are coupled by gap junctions as shown by cell-to-cell passage of Lucifer yellow and hyperpolarizing or depolarizing currents. Co-cultures composed of ciliary ganglion neurons and clones of leg cells showed that the ability to form gap junctions with neurons is a differentiated function of myoblasts. Dye-coupling with neurons was found at high frequency only when the partner cell was a myoblast of a differentiating muscle clone. Myoblasts in undifferentiated clones formed gap junctions with each other but not with neurons. The nature of the junction-forming neuron is not restricted since neurons from spinal cord, ciliary ganglion and dorsal root ganglion all readily form junctions with myoblasts.

Roles of cell junctions in gametogenesis and in early embryonic development

Recent reviews of the role of cell junctions in development have focused primarily upon functions related to the relatively subtle physiological modulation of their subunits in relation to fundamental developmental processes in a wide variety of organisms. There is, however, considerable support from numerous laboratories that the more radical modulation of the presence and number of junctional subunits in many diverse tissues may play a pivotal role in a wide spectrum of developmental phenomena ranging from gametogenesis to organogenesis. Since a great deal of recent interest in this latter subject has concentrated upon vertebrate systems including mammals, this review will examine the functional significance of the modulation of gap junctions, tight junctions and desmosomes in a developing idealized mammalian system from gamete formation to tissue and organ differentiation during embryo-genesis.

Functional assembly of gap junction conductance in lipid bilayers: demonstration that the major 27 kd protein forms the junctional channel

Gap junctions isolated from rat liver were incorporated into planar lipid bilayers. A channel activity that was directly dependent on voltage was recorded. Changes of pH and (Ca2+) had no direct effect on channel activity; however, they modulated the voltage-dependent gating of the gap junction channels differently. Single-channel fluctuations showed large scatter with peak amplitudes of 140 and 280 picoSiemmens in 0.1 M NaCl. The major protein of gap junctions (Mr of 27 kd) was also reconstituted into bilayers, giving channel properties similar to those of intact gap junctions. Polyclonal antibodies specific for this protein caused inhibition of the junctional conductance in bilayers. These data provide direct evidence that the 27 kd protein is the molecular species responsible for gap junction communication between cells.

Chicken lactose lectin: cell-to-cell or cell-to-matrix adhesion molecule?

Endogenous chicken muscle lectin isolated by lactose affinity chromatography inhibits myoblast fusion. Similar lectins isolated from embryonic brain, heart, and liver and from adult intestine exhibit the same ability. Elevated levels of any of these lectins canceled the inhibitory effect. Peanut agglutinin isolated by the same procedure had no effect at any concentration tested. Concanavalin A affected fusion only at high concentrations. Muscle lectin was shown to agglutinate myoblasts in microtiter plates, whereas exogenous addition in culture inhibited alignment as seen by time lapse microcinematography. Cell-to-cell communication between lectin-treated cells was shown by nucleotide exchange, and lectin-coated culture dishes did not affect cell attachment. Our evidence shows a lack of specificity to muscle, but suggests an aggregating capacity between cells, or possibly an interaction between the cell membrane and the extracellular matrix.

Imaging intracellular elemental distribution and ion fluxes in cultured cells using ion microscopy: a freeze-fracture methodology

A freeze-fracture methodology was standardized for tissue culture cells to study intracellular distribution of diffusible elements with ion microscopy. Chinese hamster ovary (CHO) and normal rat kidney (NRK) cells grown on a silicon substrate were sandwiched using another smooth surface (silicon, glass, mica) in the presence of spacers and fast frozen in liquid nitrogen slush. The sandwich was fractured by prying the two halves apart under liquid nitrogen. This procedure produced large areas on the silicon substrate containing hundreds of cells grouped together and fractured at the apical cell surface. After freeze-drying, these cells revealed a subcellular distribution of Na, K, Ca, Mg, P, Cl and S with the approximately 0.5 micron lateral resolution of the ion microscope. Between the nuclei and the cytoplasm of cells, no major differences were observed for Na, K, Mg, P, Cl and S intensities. Calcium alone, however, exhibited a remarkable distribution. Calcium accumulated more in the cytoplasm than in the nuclei of cells. Even within the cytoplasm its distribution was heterogeneous, suggesting Ca binding sites. The fractured cells consistently exhibited high K-low Na intensities. The injured or dead cells were easily recognized among the healthy ones due to their abnormal ion composition. This simple freeze-fracture methodology allowed fracturing of cells without removing the cells from the substrate. In addition, it eliminated the need for washing the nutrient media away and cryo-sectioning before ion microanalysis. The methodology was successfully extended to 3T3 mouse fibroblast, PtK2 rat kangaroo and L5 rat myoblast cultures.

Filament-directed intercellular contacts during differentiation of cultured chick myoblasts

Detergent-extracted, critical point dried chicken myoblasts at early stages of development in tissue culture were observed by electron microscopy. It was found that the organization of filaments within these cells changes significantly during development. A particular specialization of the cellular filament framework is the formation of microprocesses; long extensions of the cellular filament system. These microprocesses appear to be involved in cell-to-cell contact. The filaments of these processes appear to integrate with the filament system of a contacted cell, and possibly transmit tension from one cell to another. The role of these structures in effecting muscle differentiation by forming cytoplasmic connections and the implications for muscle gene expression are discussed.