Prostaglandins and cyclic AMP stimulate creatine kinase synthesis but not fusion in cultured embryonic chick muscle cells

Stem cell transplantation for muscular dystrophy: the challenge of immune response

Treating muscle disorders poses several challenges to the rapidly evolving field of regenerative medicine. Considerable progress has been made in isolating, characterizing, and expanding myogenic stem cells and, although we are now envisaging strategies to generate very large numbers of transplantable cells (e.g., by differentiating induced pluripotent stem cells), limitations directly linked to the interaction between transplanted cells and the host will continue to hamper a successful outcome. Among these limitations, host inflammatory and immune responses challenge the critical phases after cell delivery, including engraftment, migration, and differentiation. Therefore, it is key to study the mechanisms and dynamics that impair the efficacy of cell transplants in order to develop strategies that can ultimately improve the outcome of allogeneic and autologous stem cell therapies, in particular for severe disease such as muscular dystrophies. In this review we provide an overview of the main players and issues involved in this process and discuss potential approaches that might be beneficial for future regenerative therapies of skeletal muscle.

Cytokines in muscle damage

Multiple cellular and molecular processes are rapidly activated following skeletal muscle damage to restore normal muscle structure and function. These processes typically involve an inflammatory response and potentially the consequent occurrence of secondary damage before their resolution and the completion of muscle repair or regeneration. The overall outcome of the inflammatory process is potentially divergent, with the induction of prolonged inflammation and further muscle damage, or its active termination and the promotion of muscle repair and regeneration. The final, detrimental, or beneficial effect of the inflammatory response on muscle repair is influenced by specific interactions between inflammatory and muscle cell-derived cytokines that act as positive and/or negative regulators to coordinate local and systemic inflammatory-related events and modulate muscle repair process. A crucial balance between proinflammatory and anti-inflammatory cytokines appears to attenuate an excessive inflammatory reaction, prevent the development of muscle fibrosis, and adequately promote the regenerative process. In this review, we address the interactive cytokine responses following muscle damage, in the context of induction and progression, or resolution of muscle inflammation and the promotion of muscle repair.

Regulatory interactions between muscle and the immune system during muscle regeneration

Recent discoveries reveal complex interactions between skeletal muscle and the immune system that regulate muscle regeneration. In this review, we evaluate evidence that indicates that the response of myeloid cells to muscle injury promotes muscle regeneration and growth. Acute perturbations of muscle activate a sequence of interactions between muscle and inflammatory cells. The initial inflammatory response is a characteristic Th1 inflammatory response, first dominated by neutrophils and subsequently by CD68(+) M1 macrophages. M1 macrophages can propagate the Th1 response by releasing proinflammatory cytokines and cause further tissue damage through the release of nitric oxide. Myeloid cells in the early Th1 response stimulate the proliferative phase of myogenesis through mechanisms mediated by TNF-alpha and IL-6; experimental prolongation of their presence is associated with delayed transition to the early differentiation stage of myogenesis. Subsequent invasion by CD163(+)/CD206(+) M2 macrophages attenuates M1 populations through the release of anti-inflammatory cytokines, including IL-10. M2 macrophages play a major role in promoting growth and regeneration; their absence greatly slows muscle growth following injury or modified use and inhibits muscle differentiation and regeneration. Chronic muscle injury leads to profiles of macrophage invasion and function that differ from acute injuries. For example, mdx muscular dystrophy yields invasion of muscle by M1 macrophages, but their early invasion is accompanied by a subpopulation of M2a macrophages. M2a macrophages are IL-4 receptor(+)/CD206(+) cells that reduce cytotoxicity of M1 macrophages. Subsequent invasion of dystrophic muscle by M2c macrophages is associated with progression of the regenerative phase in pathophysiology. Together, these findings show that transitions in macrophage phenotype are an essential component of muscle regeneration in vivo following acute or chronic muscle damage.

Localized cyclic AMP-dependent protein kinase activity is required for myogenic cell fusion

Multinucleated myotubes are formed by fusion of mononucleated myogenic progenitor cells (myoblasts) during terminal skeletal muscle differentiation. In addition, myoblasts fuse with myotubes, but terminally differentiated myotubes have not been shown to fuse with each other. We show here that an adenylate cyclase activator, forskolin, and other reagents that elevate intracellular cyclic AMP (cAMP) levels induced cell fusion between small bipolar myotubes in vitro. Then an extra-large myotube, designated a "myosheet," was produced by both primary and established mouse myogenic cells. Myotube-to-myotube fusion always occurred between the leading edge of lamellipodia at the polar end of one myotube and the lateral plasma membrane of the other. Forskolin enhanced the formation of lamellipodia where cAMP-dependent protein kinase (PKA) was accumulated. Blocking enzymatic activity or anchoring of PKA suppressed forskolin-enhanced lamellipodium formation and prevented fusion of multinucleated myotubes. Localized PKA activity was also required for fusion of mononucleated myoblasts. The present results suggest that localized PKA plays a pivotal role in the early steps of myogenic cell fusion, such as cell-to-cell contact/recognition through lamellipodium formation. Furthermore, the localized cAMP-PKA pathway might be involved in the specification of the fusion-competent areas of the plasma membrane in lamellipodia of myogenic cells.

Inhibition of myoblast migration by prostacyclin is associated with enhanced cell fusion

Satellite cells are stem cells that are critical for the formation and growth of skeletal muscle during myogenesis. To differentiate and fuse, proliferating satellite cells or myoblasts must migrate and establish stable cell-cell contacts. However, the factors that regulate myoblast migration and fusion are not understood completely. We have identified PGI2 as a novel regulator of myogenesis in vitro. PGI2 is a member of the family of prostaglandins (PG), autocrine/paracrine signaling molecules synthesized via the cyclooxygenase-1 and -2 pathways. Primary mouse muscle cells both secrete PGI2 and express the PGI2 receptor, IP, at various stages of myogenesis. Using genetic and pharmacological approaches, we show that PGI2 is a negative regulator of myoblast migration that also enhances cell fusion. Thus, PGI2 may act as a "brake" on migrating cells to facilitate cell-cell contact and fusion. Together, our results highlight the importance of the balance between positive and negative regulators in cell migration and myogenesis. This work may have implications for migration of other populations of adult stem cells and/or cells that undergo fusion.

The COX-2 pathway regulates growth of atrophied muscle via multiple mechanisms

Loss of muscle mass occurs with disease, injury, aging, and inactivity. Restoration of normal muscle mass depends on myofiber growth, the regulation of which is incompletely understood. Cyclooxygenase (COX)-2 is one of two isoforms of COX that catalyzes the synthesis of prostaglandins, paracrine hormones that regulate diverse physiological and pathophysiological processes. Previously, we demonstrated that the COX-2 pathway regulates early stages of myofiber growth during muscle regeneration. However, whether the COX-2 pathway plays a common role in adult myofiber growth or functions specifically during muscle regeneration is unknown. Therefore, we examined the role of COX-2 during myofiber growth following atrophy in mice. Muscle atrophy was induced by hindlimb suspension (HS) for 2 wk, followed by a reloading period, during which mice were treated with either the COX-2-selective inhibitor SC-236 (6 mg x kg(-1) x day(-1)) or vehicle. COX-2 protein was expressed and SC-236 attenuated myofiber growth during reloading in both soleus and plantaris muscles. Attenuated myofiber growth in the soleus was associated with both decreased myonuclear addition and decreased inflammation, whereas neither of these processes mediated the effects of SC-236 on plantaris growth. In addition, COX-2(-/-) satellite cells exhibited impaired activation/proliferation in vitro, suggesting direct regulation of muscle cell activity by COX-2. Together, these data suggest that the COX-2 pathway plays a common regulatory role during various types of muscle growth via multiple mechanisms.

Time course of COX-1 and COX-2 expression during ischemia-reperfusion in rat skeletal muscle

The aim of this study was to assess cyclooxygenase (COX)-1 and COX-2 expression in skeletal muscle after an ischemia-reperfusion (I/R). Male Sprague-Dawley rats were subjected to unilateral hindlimb ischemia for 2 h and then euthanized after 0, 1, 2, 4, 6, 10, 24, and 72 h of reperfusion. The COX protein and mRNA were assessed in control and injured gastrocnemius muscle. Muscle damage was indirectly determined by plasma creatine kinase activity and edema by weighing wet muscle. Creatine kinase activity in plasma increased as early as 1 h after reperfusion and returned to control levels by 72 h of reperfusion. Edema was observed at 6 and 10 h of reperfusion, but histological investigations showed an absence of tissular inflammatory cell infiltration. COX-1 mRNA was expressed in control muscle and was increased at 72 h of reperfusion, but the levels of associated COX-1 protein detected in control and injured gastrocnemius muscle were similar. COX-2 mRNA was not, or only slightly, detectable in control muscle and after I/R. In contrast, I/R induced major overexpression of COX-2 immunoreactivity at 6 and 10 h of reperfusion with a maximum at 10 h, whereas COX-2 protein was undetectable in control muscle. In conclusion, hindlimb I/R induced a large overexpression of COX-2 but not COX-1 protein between 6 and 10 h after injury. These results suggest a role for COX-2 enzyme in such pathophysiological conditions of the skeletal muscle.

Stretch-induced myoblast proliferation is dependent on the COX2 pathway

Skeletal muscle increases in size due to weight bearing loads or passive stretch. This growth response is dependent in part upon myoblast proliferation. Although skeletal muscles are responsive to mechanical forces, the effect on myoblast proliferation remains unknown. To investigate the effects of mechanical stretch on myoblast proliferation, primary myoblasts isolated from Balb/c mice were subjected to 25% cyclical uniaxial stretch for 5 h at 0.5 Hz. Stretch stimulated myoblast proliferation by 32% and increased cell number by 41% 24 and 48 h after stretch, respectively. COX2 mRNA increased 3.5-fold immediately poststretch. Prostaglandin E2 and F2alpha increased 2.4- and 1.6-fold 6 h after stretch, respectively. Because COX2 has been implicated in regulating muscle growth and regeneration, we hypothesized that stretched myoblasts may proliferate via a COX2-dependent mechanism. We employed two different models to disrupt COX2 activity: (1) treatment with a COX2-selective drug, and (2) transgenic mice null for COX2. Treating myoblasts with a COX2-specific inhibitor blocked stretch-induced proliferation. Likewise, stretched COX2-/- myoblasts failed to proliferate compared to controls. However, supplementing stretched, COX2-/- myoblasts with prostaglandin E2 or fluprostenol increased proliferation. These data suggest that the COX2 pathway is critical for myoblast proliferation in response to stretch.

The COX-2 pathway is essential during early stages of skeletal muscle regeneration

Skeletal muscle regeneration comprises several overlapping cellular processes, including inflammation and myogenesis. Prostaglandins (PGs) may regulate muscle regeneration, because they modulate inflammation and are involved in various stages of myogenesis in vitro. PG synthesis is catalyzed by different isoforms of cyclooxygenase (COX), which are inhibited by nonsteroidal anti-inflammatory drugs. Although experiments employing nonsteroidal anti-inflammatory drugs have implicated PGs in tissue repair, how PGs regulate muscle regeneration remains unclear, and the potentially distinct roles of different COX isoforms have not been investigated. To address these questions, a localized freeze injury was induced in the tibialis anterior muscles of mice chronically treated with either a COX-1- or COX-2-selective inhibitor (SC-560 and SC-236, respectively), starting before injury. The size of regenerating myofibers was analyzed at time points up to 5 wk after injury and found to be decreased by SC-236 and in COX-2−/−muscles, but unaffected by SC-560. In contrast, SC-236 had no effect on myofiber growth when administered starting 7 days after injury. The attenuation of myofiber growth by SC-236 treatment and in COX-2−/−muscles is associated with decreases in the number of myoblasts and intramuscular inflammatory cells at early times after injury. Together, these data suggest that COX-2-dependent PG synthesis is required during early stages of muscle regeneration and thus raise caution about the use of COX-2-selective inhibitors in patients with muscle injury or disease.

Cell and molecular biology of myoblast fusion

In organisms from Drosophila to mammals, the musculature is comprised of an elaborate array of distinct fibers that are generated by the fusion of committed myoblasts. These muscle fibers differ from each other in features that include location, pattern of innervation, site of attachment, and size. The sizes of the newly formed muscles of an embryo are controlled in large part by the number of cells that form the syncitial fiber. Over the past few decades, an extensive body of literature has described the process of myoblast fusion in vertebrates, relying primarily on the strengths of tissue culture model systems. More recently, genetic studies in Drosophila embryos have provided new insights into the process. Together, these studies define the steps necessary for myoblast differentiation, the acquisition of fusion competence, the recognition and adhesion between myoblasts, and the fusion of two lipid bilayers into one. In this review, we have attempted to combine insights from both Drosophila and vertebrate studies to trace the processes and molecules involved in myoblast fusion. Implicit in this approach is the assumption that fundamental aspects of myoblast fusion will be similar, independent of the organism in which it is occurring.

Prostaglandin F2(alpha) stimulates growth of skeletal muscle cells via an NFATC2-dependent pathway

Skeletal muscle growth requires multiple steps to form large multinucleated muscle cells. Molecules that stimulate muscle growth may be therapeutic for muscle loss associated with aging, injury, or disease. However, few factors are known to increase muscle cell size. We demonstrate that prostaglandin F2alpha (PGF2alpha) as well as two analogues augment muscle cell size in vitro. This increased myotube size is not due to PGF2alpha-enhancing cell fusion that initially forms myotubes, but rather to PGF2alpha recruiting the fusion of cells with preexisting multinucleated cells. This growth is mediated through the PGF2alpha receptor (FP receptor). As the FP receptor can increase levels of intracellular calcium, the involvement of the calcium-regulated transcription factor nuclear factor of activated T cells (NFAT) in mediating PGF2alpha-enhanced cell growth was examined. We show that NFAT is activated by PGF2alpha, and the isoform NFATC2 is required for PGF2alpha-induced muscle cell growth and nuclear accretion, demonstrating the first intersection between prostaglandin receptor activation and NFAT signaling. Given this novel role for PGF2alpha in skeletal muscle cell growth, these studies raise caution that extended use of drugs that inhibit PG production, such as nonsteroidal antiinflammatory drugs, may be deleterious for muscle growth.

The absence of MyoD in regenerating skeletal muscle affects the expression pattern of basement membrane, interstitial matrix and integrin molecules that is consistent with delayed myotube formation

MyoD is a member of a skeletal muscle specific family of transcription factors which directs the events of myogenesis during development and regeneration. Muscle cells that lack MyoD show delayed fusion in vivo and in vitro and defects have been observed in vitro in the attachment of MyoD(-/-) myoblasts to complex substrates such as Matrigel. Since interactions with the extracellular matrix (ECM) are important during myoblast fusion (i. e. myotube formation), it was hypothesised that expression of ECM molecules or their receptors may be altered in MyoD(-/-) muscle. The production of basement membrane molecules such as collagen type IV and several laminins, the interstitial molecules fibronectin and tenascin-C, and the cell surface molecules integrin alpha5 and alpha6 were quantitated in vitro using ELISA on cultured cells from MyoD(-/-) and wild type mice. Differences were observed in the production of fibronectin, tenascin-C, collagen type IV, laminin-1 and integrin alpha5 between control and MyoD(-/-) myotubes in vitro. This corresponded with delayed fusion of myoblasts in MyoD(-/-) cultures. On the basis of these findings with respect to matrix expression in vitro, fluorescent immunohistochemistry was carried out on adult whole muscle autografts to examine whether the expression of these molecules, as well as integrin alpha7, was altered in the complex in vivo environment. Some minor differences in expression patterns were observed in MyoD(-/-) as compared to normal BALB/c autografts. The overall expression of matrix components was consistent with the delayed onset of myotube formation. These results suggest that the delay in myotube formation in MyoD(-/-) muscle is not a direct result of altered expression of the matrix molecules collagen type IV, laminins, fibronectin, tenascin-C, and integrins alpha5, alpha6 or alpha7.

E and F alpha series prostaglandins in developing muscles

Prostaglandins are known to affect myoblast proliferation and fusion in vitro and are putative regulators of in vivo myogenesis. The levels of E and F alpha series prostaglandins in the thigh muscles of chicken embryos were measured by radioimmunoassays and correlated with indicators of muscle development. Just prior to the onset of secondary myogenesis, the amounts of PGE1, PGE2 and PGF1 alpha plus PGF2 alpha per mg of protein were high. In temporal association with myotube formation, the amount of PGE1 and PGE2 per mg of protein decreased. PGF alpha levels also fell, but at a slower rate than observed with the E series prostaglandins. The decreases in the amounts of prostaglandins per mg protein appeared to be due to a decline in the total amount of prostaglandin within each muscle. These observations are consistent with prostaglandins being one of the factors that controls in vivo muscle formation.

Effects of prostaglandin E2 and cyclooxygenase inhibitors on clustering and level of nicotinic acetylcholine receptor in mouse myotubes co-cultured with spinal cord explant

The clustering and level of nicotinic acetylcholine receptor (n-AChR) in cultured mouse myotubes are negatively controlled by endogenous phospholipase A2 (PLA2) (Kimura et al., Int. J. Devl. Neurosci. 5, 127-133, 1987). The effects of PLA2-related metabolites, prostaglandins, leukotrienes and platelet-activating factor (PAF) were investigated using fluorescein isothiocyanate-alpha-bungarotoxin. Peak and total fluorescence within a cluster were used as indices of clustering and level of n-AChR, respectively. Prostaglandin E2 (PGE2, 1-10 microM) decreased both indices in a concentration-dependent manner. Aspirin and indomethacin, cyclooxygenase inhibitors, increased the indices at 1.0 microM and 10-30 nM, and decreased them at higher concentrations of 10-30 microM and 0.1-1 microM, respectively. Prostaglandin F2 alpha (PGF2 alpha, 1-10 microM), nordihydroguaiaretic acid (30 microM), a lipoxygenase inhibitor, and PAF (10 microM) had no effect. These results suggest that the control of endogenous PLA2 on the clustering and level of n-AChR is due to PGE2, but not to PGF2 alpha, leukotrienes or PAF.

Towards understanding skeletal muscle regeneration

Factors which effect proliferation and fusion of muscle precursor cells have been studied extensively in tissue culture, although little is known about these events in vivo. This review assesses the tissue culture derived data with a view to understanding factors which may control the regeneration of mature skeletal muscle in vivo. The following topics are discussed in the light of recent developments in cell and molecular biology: 1) Injury and necrosis of mature skeletal muscle fibres 2) Phagocytosis of myofibre debris 3) Revascularisation of injured muscle 4) Activation and proliferation of muscle precursor cells (mpc) in vivo Identification of mpcs; Satellite cell relationships; Extracellular matrix; Growth factors; Hormones; Replication. 5) Differentiation and fusion of muscle precursor cells in vivo Differentiation; Fusion; Extracellular matrix; Cell surface molecules: Growth factors and prostaglandins 6) Myotubes and innervation.

Possible role of prostaglandins in the regulation of mouse myoblasts

A differentiation-defective mouse myoblast subclone (DD-1), cells of which do not fuse into myotubes nor synthesize muscle-specific proteins, was employed to help define the role of eicosanoids in mouse myoblast differentiation. We observed by hplc, tlc, and radioimmunoassay that the DD-1 cells release strikingly higher levels of cyclooxygenase pathway products prostaglandin E2 and F2 alpha into the culture medium than the parental non-differentiation-defective cells (DZ). In contrast, the levels of 15-hydroxyeicosatetraenoic acid (15-HETE), a lipoxygenase product, and a putatively identified second lipoxygenase product (LLP) did not differ greatly in the two cell types. The DD-1 cells also have strikingly higher levels of cyclooxygenase activity than the parental cells as determined by intact and broken cell assays. Additional fusion-defective clones were isolated on the basis of their flattened appearance and ability to grow in "mitogen-poor" medium and these cells also released strikingly higher levels of prostaglandins E2 and F2 alpha into the growth medium. The "turn on" of the cyclooxygenase pathway in the DD-1 cells and other fusion-defective cells is consistent with the hypothesis that the products of this pathway contribute to the inability of myoblasts to fuse with one another. This hypothesis is supported by the observation that there is a dose-dependent decrease in fusion of DZ cells when PGE2 is added to commitment medium.

Coordinate interactions of cyclic nucleotide and phospholipid metabolizing pathways in calcium-dependent cellular processes

It is hoped that his review enables the reader to appreciate the complexities implicit in the interactions among Ca2+, cyclic nucleotides, and phospholipid-metabolizing pathways in cell signal transduction. The interactions are varied and intricate, often involving several levels of cell amplification mechanisms. Upsetting the balance of fatty acids in membrane phospholipids can have detrimental effects on adenylate cyclase. Thus, n - 3 fatty acid enrichment of phospholipids suppresses adenylate cyclase activity. The effects of significant alterations in dietary fatty acids, such as might occur with the current vogue for n - 3 eicosapentaenoic acid and docosahexaenoic acid (fish oil) dietary enrichment regimens, will need to be assessed more fully with regard to stimulus-induced changes in cyclic nucleotide production in various tissues. Since the n - 3 fatty acids have not been demonstrated to affect guanylate cyclase activity, dietary changes in certain of these fatty acids would not be expected to contribute to changes in cGMP generation as much as in cAMP production. Moreover, the ingestion of large quantities of these n - 3 fatty acids can alter the profile of cyclooxygenase and lipoxygenase products produced in cells. According to the paradigm developed in this article, changes in the metabolism of fatty acids are amplified by alterations in cyclic nucleotide production and phospholipase activities, with the eventual physiological impact predicated on the tissue type and the specific stimulus response. There appears to be a rather clear distinction between the regulatory properties of eicosanoids regarding adenylate and guanylate cyclase activities. Whereas prostaglandins often stimulate adenylate cyclase activity, they have little effect on guanylate cyclase activity. On the other hand, the HETE compounds seem to play an important role in guanylate cyclase regulation in certain cells. Moreover, arachidonic acid affects adenylate cyclase activity without prior peroxidation, whereas endoperoxides and hydroperoxides are more effective than arachidonic acid with regard to guanylate cyclase stimulation. However, in the intact cell there is a strong implication that the dual stimulation of guanylate cyclase by Ca2+ and fatty acid evokes optimal enzyme activity. An advantage of multidimensional response mechanisms in cells includes the ability to recognize different stimuli and to respond with specific, coordinated responses modulated in their intensity and/or duration by messenger interaction. Few cell types respond to receptor stimulation in an all-or-none fashion, and the "milieu interior" depends on specific, graded responses to the autonomic nervous system and endocrine stimuli.(ABSTRACT TRUNCATED AT 400 WORDS)

Growth factors, signaling pathways, and the regulation of proliferation and differentiation in BC3H1 muscle cells. I. A pertussis toxin-sensitive pathway is involved

Cells of the nonfusing muscle cell line BC3H1 stop proliferating and express a family of muscle-specific proteins when the FBS concentration is reduced from 20 to 0.5% (Munson, R., K.L. Caldwell, and L. Glaser. 1982. J. Cell Biol. 92:350-356). Several growth factors have been shown to block differentiation in this cell line. To begin to investigate the potential role of G proteins in signal transducing pathways from these receptors, we have examined the effects of cholera toxin (CT) and pertussis toxin (PT) on proliferation and differentiation in BC3H1 cells. PT specifically ADP ribosylates a protein with an apparent molecular mass of 40 kD in BC3H1 cell membranes, whereas CT specifically ADP ribosylates three proteins of 35-43 kD. When added to exponentially growing cells in 20% FBS, CT and PT inhibited [3H]thymidine incorporation by up to 75% in a dose-dependent fashion. We found the synthesis of creatine kinase (CK) and skeletal muscle myosin light chain was reversibly induced in cells in 20% FBS treated with PT, but no increased synthesis was seen in cells treated with CT or in control cells; Northern analysis indicated this induction was at the level of mRNA. In cells shifted to 0.5% FBS, CT inhibited the normally induced synthesis of CK whereas PT potentiated it by approximately 50%. Forskolin also inhibited growth in 20% FBS and differentiation in 0.5% FBS medium in a dose-dependent fashion. both forskolin and CT elevated cAMP levels compared with control or PT-treated cells, suggesting that CT is blocking proliferation and differentiation by elevating cAMP levels. These results establish that a PT-sensitive pathway is involved in regulating proliferation and differentiation in BC3H1 cells, and we postulate that PT functions by ADP ribosylating a G protein that transduces signals from growth factor receptors in these cells.

Recovery of free muscle grafts in rat: improvement is associated with an increase in cyclic adenosine monophosphate concentration or use of the condition/test paradigm

The muscle fibers in freely grafted skeletal muscles degenerate and are replaced by new fibers which develop within the graft. Myogenesis in regenerating muscle recapitulates, to a large extent, developmental myogenesis and may depend on similar modulating influences. In addition to the generation of new fibers, functional recovery of free muscle grafts also requires reinnervation and revascularization of the new fibers. Recovery of function should be improved by enhancing either myogenesis or reinnervation and revascularization. We have used two procedures, shown previously to stimulate peripheral nerve regeneration, to improve the morphologic and functional recovery of free, orthotopic grafts of rat extensor digitorum longus muscle. Each of the procedures was effective, but had potentially different sites of action. The first procedure, the condition/test paradigm, presumably increases the rate and extent of graft reinnervation. The second procedure, continuous infusion of the adenylate cyclase activator forskolin during the first 21 days after grafting, may influence both myogenesis and nerve regeneration. Each procedure increased regenerating muscle fiber size and functional capacity, and forskolin also significantly increased capillary density and fatigue resistance.

The role of hormones and prostanoids in the in vitro proliferation and differentiation of human myoblasts

Fetal human myoblasts have been employed to examine the role of hormonal factors in human myogenesis. The results show that human myoblast proliferation is stimulated by insulin, hydrocortisone, and prostaglandin F2 alpha (PGF2 alpha). Exposure of human myoblasts preparing to differentiate to either PGE2 or isoproterenol results in the precocious initiation of differentiation (i.e., cell fusion and increase in creatine kinase activity). Three antagonists of prostanoid synthesis, indomethacin, aspirin, and DL-6-chloro-alpha-methylcarbozole-2-acetic acid, inhibit cell number increase with complete inhibitions of proliferation at 5 X 10(-5) M indomethacin and 6 X 10(-4) M aspirin. Reversal of the indomethacin-imposed block is achieved by prostaglandin F2 alpha. The same antagonists of prostanoid synthesis, when added to older cultures, depress prostaglandin E (PGE) levels and inhibit human myoblast differentiation. During differentiation, PGE is present in both the intracellular compartment (0.47 to 0.66 pmol/microgram DNA) and the culture medium (1.83 to 4.53 nmol PGE). The results suggest a role for prostanoids in the regulation of both human myoblast proliferation and differentiation. They also demonstrate that the active cyclooxygenase products are produced endogenously by the in vitro myogenic population. The findings are discussed within the context of what is known of the relationship between growth factor and prostanoid actions and the roles of these two categories of hormones in the regulation of myogenesis.

Phorbol ester inhibits myoblast fusion and activates beta-adrenergic receptor coupled adenylate cyclase

Primary cultures of myoblasts, derived from embryonic chick pectoral muscle, were treated with phorbol ester (TPA) for 8-96 h. TPA treatment blocked the fusion of myoblasts along with the expression of the MM form of creatine kinase. Interestingly, TPA treatment markedly increased the activity of beta-adrenergic receptor coupled adenylate cyclase (AC) activity. The study suggests that TPA treatment augments the functional interaction between a coupling Ns protein and catalytic unit of AC. The likely significance of these results is briefly presented.

The fusion of myoblasts

Images