The origin and selective innervation of early muscle fiber types in the rat

DNA Methylation in Skeletal Muscle Stem Cell Specification, Proliferation, and Differentiation

An unresolved and critically important question in skeletal muscle biology is how muscle stem cells initiate and regulate the genetic program during muscle development. Epigenetic dynamics are essential for cellular development and organogenesis in early life and it is becoming increasingly clear that epigenetic remodeling may also be responsible for the cellular adaptations that occur in later life. DNA methylation of cytosine bases within CpG dinucleotide pairs is an important epigenetic modification that reduces gene expression when located within a promoter or enhancer region. Recent advances in the field suggest that epigenetic regulation is essential for skeletal muscle stem cell identity and subsequent cell development. This review summarizes what is currently known about how skeletal muscle stem cells regulate the myogenic program through DNA methylation, discusses a novel role for metabolism in this process, and addresses DNA methylation dynamics in adult skeletal muscle in response to physical activity.

Stage of perinatal development regulates skeletal muscle mitochondrial biogenesis and myogenic regulatory factor genes with little impact of growth restriction or cross-fostering

Foetal growth restriction impairs skeletal muscle development and adult muscle mitochondrial biogenesis. We hypothesized that key genes involved in muscle development and mitochondrial biogenesis would be altered following uteroplacental insufficiency in rat pups, and improving postnatal nutrition by cross-fostering would ameliorate these deficits. Bilateral uterine vessel ligation (Restricted) or sham (Control) surgery was performed on day 18 of gestation. Males and females were investigated at day 20 of gestation (E20), 1 (PN1), 7 (PN7) and 35 (PN35) days postnatally. A separate cohort of Control and Restricted pups were cross-fostered onto a different Control or Restricted mother and examined at PN7. In both sexes, peroxisome proliferator-activated receptor (PPAR)-γ coactivator-1α (PGC-1α), cytochrome c oxidase subunits 3 and 4 (COX III and IV) and myogenic regulatory factor 4 expression increased from late gestation to postnatal life, whereas mitochondrial transcription factor A, myogenic differentiation 1 (MyoD), myogenin and insulin-like growth factor I (IGF-I) decreased. Foetal growth restriction increased MyoD mRNA in females at PN7, whereas in males IGF-I mRNA was higher at E20 and PN1. Cross-fostering Restricted pups onto a Control mother significantly increased COX III mRNA in males and COX IV mRNA in both sexes above controls with little effect on other genes. Developmental age appears to be a major factor regulating skeletal muscle mitochondrial and developmental genes, with growth restriction and cross-fostering having only subtle effects. It therefore appears that reductions in adult mitochondrial biogenesis markers likely develop after weaning.

The ups and downs of gene regulation by electrical activity in skeletal muscles

Adult skeletal muscles retain an adaptive capacity to switch between slow- and fast-twitch properties that are largely dependent on motoneuron activity. Our studies on the transcriptional regulation of the Troponin I slow (TnIs) and fast (TnIf) genes uncovered a dual mechanism of transcriptional enhancement and repression by a single activity pattern, that promotes the phenotypic differences among myofibers while preserving their adaptive capacity. Using the Tnf Fast Intronic Regulatory Element (FIRE), we initially demonstrated that fast-patterned activity (infrequent, high frequency depolarization) is necessary to up-regulate FIRE-dependent transcription and that its effect differs dramatically from muscle denervation. Hence, the "fast muscle program" is not a default state mimicked simply by denervation or muscle inactivity. Next, we found that slow-patterned activity (tonic, slow frequency stimulation) selectively represses FIRE-dependent transcription while enhancing transcription from the TnIs Slow Upstream Regulatory Element. Unexpectedly, repression of the TnIf FIRE by slow-patterned activity is mediated by an NFAT element that directly binds NFATc1, a transcription factor that translocates to the nucleus selectively by slow-pattern depolarization and has been implicated in the up-regulation of the slow muscle program. Transfection of siRNAs targeting NFATc1 or mutation of the TnIFIRE NFAT site result in the upregulation of FIRE-dependent transcription in slow muscle, but have no effect in fast muscle. These findings demonstrate a novel function of NFAT as a repressor of transcription of fast contractile genes in slow muscles and, more importantly, they illustrate how specific activity patterns can enhance the phenotypic differences among fibre-types by differentially regulating transcription in a use-dependent manner while retaining the adaptive properties of adult muscles.

Gamma and alpha motor neurons distinguished by expression of transcription factor Err3

Spinal motor neurons are specified to innervate different muscle targets through combinatorial programs of transcription factor expression. Whether transcriptional programs also establish finer aspects of motor neuron subtype identity, notably the prominent functional distinction between alpha and gamma motor neurons, remains unclear. In this study, we identify DNA binding proteins with complementary expression profiles in alpha and gamma motor neurons, providing evidence for molecular distinctions in these two motor neuron subtypes. The transcription factor Err3 is expressed at high levels in gamma but not alpha motor neurons, whereas the neuronal DNA binding protein NeuN marks alpha but not gamma motor neurons. Signals from muscle spindles are needed to support the differentiation of Err3(on)/NeuN(off) presumptive gamma motor neurons, whereas direct proprioceptive sensory input to a motor neuron pool is apparently dispensable. Together, these findings provide evidence that transcriptional programs define functionally distinct motor neuron subpopulations, even within anatomically defined motor pools.

Activity-dependent repression of muscle genes by NFAT

Adult skeletal muscles retain an adaptive capacity to switch between slow- and fast-twitch properties that largely depend on motoneuron activity. The NFAT (nuclear factor of activated T cells) family of calcium-dependent transcription factors has been implicated in the up-regulation of genes encoding slow contractile proteins in response to slow-patterned motoneuron depolarization. Here, we demonstrate an unexpected, novel function of NFATc1 in slow-twitch muscles. Using the troponin I fast (TnIf) intronic regulatory element (FIRE), we identified sequences that down-regulate its function selectively in response to patterns of electrical activity that mimic slow motoneuron firing. A bona fide NFAT binding site in the TnIf FIRE was identified by site-directed mutations and by electrophoretic mobility and supershift assays. The activity-dependent transcriptional repression of FIRE is mediated through this NFAT site and, importantly, its mutation did not alter the up-regulation of TnIf transcription by fast-patterned activity. siRNA-mediated knockdown of NFATc1 in adult muscles resulted in ectopic activation of the FIRE in the slow soleus, without affecting enhancer activity in the fast extensor digitorum longus muscle. These findings demonstrate that NFAT can function as a repressor of fast contractile genes in slow muscles and they exemplify how an activity pattern can increase or decrease the expression of distinct contractile genes in a use-dependent manner as to enhance phenotypic differences among fiber types or induce adaptive changes in adult muscles.

Postsynaptic contributions to hippocampal network hyperexcitability induced by chronic activity blockade in vivo

Neuronal activity is thought to play an important role in refining patterns of synaptic connectivity during development and in the molecular maturation of synapses. In experiments reported here, a 2-week infusion of tetrodotoxin (TTX) into rat hippocampus beginning on postnatal day 12 produced abnormal synchronized network discharges in in vitro slices. Discharges recorded upon TTX washout were called 'minibursts', owing to their small amplitude. They were routinely recorded in area CA3 and abolished by CNQX, an AMPA receptor antagonist. Because recurrent excitatory axon collaterals remodel and glutamate receptor subunit composition changes after postnatal day 12, experiments examined possible TTX-induced alterations in recurrent excitation that could be responsible for network hyperexcitability. In biocytin-labelled pyramidal cells, recurrent axon arbors were neither longer nor more highly branched in the TTX infusion site compared with saline-infused controls. However, varicosity size and density were increased. Whereas most varicosities contained synaptophysin and synaptic vesicles, many were not adjacent to postsynaptic specializations, and thus failed to form anatomically identifiable synapses. An increased pattern of excitatory connectivity does not appear to explain network hyperexcitability. Quantitative immunoblots also indicated that presynaptic markers were unaltered in the TTX infusion site. However, the postsynaptic AMPA and NMDA receptor subunits, GluR1, NR1 and NR2B, were increased. In electrophysiological studies EPSPs recorded in slices from TTX-infused hippocampus had an enhanced sensitivity to the NR2B containing NMDA receptor antagonist, ifenprodil. Thus, increases in subunit protein result in alterations in the composition of synaptic NMDA receptors. Postsynaptic changes are likely to be the major contributors to the hippocampal network hyperexcitability and should enhance both excitatory synaptic efficacy and plasticity.

Development of the vertebrate neuromuscular junction

We describe the formation, maturation, elimination, maintenance, and regeneration of vertebrate neuromuscular junctions (NMJs), the best studied of all synapses. The NMJ forms in a series of steps that involve the exchange of signals among its three cellular components--nerve terminal, muscle fiber, and Schwann cell. Although essentially any motor axon can form NMJs with any muscle fiber, an additional set of cues biases synapse formation in favor of appropriate partners. The NMJ is functional at birth but undergoes numerous alterations postnatally. One step in maturation is the elimination of excess inputs, a competitive process in which the muscle is an intermediary. Once elimination is complete, the NMJ is maintained stably in a dynamic equilibrium that can be perturbed to initiate remodeling. NMJs regenerate following damage to nerve or muscle, but this process differs in fundamental ways from embryonic synaptogenesis. Finally, we consider the extent to which the NMJ is a suitable model for development of neuron-neuron synapses.

Control of muscle fibre and motoneuron diversification

Recent studies have elucidated both the mechanism of early formation of diverse muscle fibre types and the matching of diverse populations of motoneurons to their appropriate muscle targets. Highlights include the demonstration that distinct signals are necessary for the formation of several distinct myoblast populations in the vertebrate somite, the identification of motoneuron subtypes, studies of how motoneurons target appropriate muscles, and rapid progress on the Drosophila neuromuscular system. We propose a model in which four classes of decision control the patterning of both motoneurons and muscles.

Primary myotubes preferentially mature into either the fastest or slowest muscle fibers

Myoblasts and myotubes are heterogeneous, but what is the significance of this heterogeneity? Is it a vital component of the mechanism by which a muscle develops or is it part of the process that generates mature fibers with diverse sizes, speeds of contracture, and metabolisms? We have begun to explore these questions by using BrdU to selectively label rat primary myotubes, thus enabling their mature characteristics to be defined for the first time. In the soleus, the type I fibers of primary myotube origin were 21% larger than those of secondary myotube origin, indicating that the origin of a fiber can affect its mature force production. In the extensor digitorum longus (EDL), the primary myotubes differentiated into all known fibers types, but with marked variation in frequency. In the superficial portion of the EDL, 97% of primary myotubes became IIB fibers, even though approximately 41% of the fibers in this region are IIA or IIX. In the deep portion, primary myotubes preferentially developed into type I fibers. Thus, primary myotubes in the EDL predominantly differentiate into the two most dissimilar fiber types: the slowest, smallest, most oxidative, type I fibers and the largest, fastest, most glycolytic, type IIB fibers. Each of the subtypes of primary myotubes had a different fate. In the EDL, the slow and fast primary myotubes appeared to differentiate into type I and IIB fibers, respectively. This implies that spatial and temporal signals operating in the limb are major determinants of the mature pattern of fiber types and that innervation of a muscle involves a selective matching between the various types of motoneurons and muscle fibers.

Selective fasciculation and divergent pathfinding decisions of embryonic chick motor axons projecting to fast and slow muscle regions

Proper motor function requires the precise matching of motoneuron and muscle fiber properties. The lack of distinguishing markers for early motoneurons has made it difficult to determine whether this matching is established by selective innervation during development or later via motoneuron-muscle fiber interactions. To examine whether chick motoneurons selectively innervate regions of their target containing either fast or slow muscle fibers, we backlabeled neurons from each of these regions with lipophilic dyes. We found that motor axons projecting to fast and slow muscle regions sorted into separate but adjacent fascicles proximally in the limb, long before they reached the muscle. More distally, these fascicles made divergent pathfinding decisions to course directly to the appropriate muscle fiber region. In contrast, axons projecting to different areas of an all-fast muscle did not fasciculate separately and became more intermingled as they coursed through the limb. Selective fasciculation of fast- and slow-projecting motoneurons was similar both before and after motoneuron cell death, suggesting that motoneurons specifically recognized and fasciculated with axons growing to muscle regions containing the appropriate muscle fiber type. Taken together, these results strongly support the hypothesis that "fast" and "slow" motoneurons are molecularly distinct before target innervation and that they use these differences to selectively fasciculate, pathfind to, and branch within the correct muscle fiber region from the outset of neuromuscular development.

Specificity and development of innervation pattern of Xenopus pectoralis muscle

Multielectrode recordings were used to identify and measure the axonal inputs to each end plate on contiguous surface fibers covering about 25% of the Xenopus pectoralis muscle in mature and developing animals. The mature innervation pattern was remarkably precise. Individual axons tended to innervate fibers of similar input resistance (R(in)) in compact motor units restricted to only a portion of the region studied. Motor units comprising fibers of similar R(in) overlapped mainly near their borders. Most fibers had two end plates. In more than 80% of these fibers, both end plates received input from the same axon. In 57%, this was the only input to both end plates. This implies a powerful mechanism for excluding or eliminating inputs from other axons. About 16% of the mature junctions showed focal polyneuronal innervation, with the weaker end plate potential component often less than 1 mV in noncurarized preparation. However, we have no evidence that the weaker inputs were being eliminated. During development, motor units became more compact, which was associated with synapse elimination; but from the earliest times studied, soon after metamorphosis when many fibers were adding second end plates, a majority of those that had two end plates were innervated at both sites by the same axon.

Metabolic capacity of muscle fibers from high-altitude natives

We evaluate the effects of chronic hypoxia on the metabolic phenotype of the muscle fiber types of humans. The subjects were three Quechua natives residing in the Peruvian Andes at an altitude greater than 3300 m, and three lowlanders from below 700 m. Biopsy specimens were obtained from the vastus lateralis muscles of volunteers. Muscle fibers were identified histochemically as type 1 (oxidative), 2a (oxidative-glycolytic) or 2b (glycolytic). The relative contribution of each fiber type to the total cross-sectional area of each biopsy sample was determined. In individual fibers, the activities of malate dehydrogenase (MDH, citric acid cycle), lactate dehydrogenase (LDH, glycolysis) and adenylokinase (high-energy phosphate) were quantified. The cross-sectional area of the muscle occupied by each fiber type is comparable between Quechuas and lowlanders. Type 1 fibers are the only fiber type to demonstrate statistically significant (P < or = 0.05) differences in enzyme activities between Quechaus and lowlanders. MDH activity is, on average, 19.6% less (P < or = 0.0001) and LDH activity 28.1% more (P < or = 0.0001) in the type 1 fibers of the Quechuas. Chronic hypoxia appears to produce a shift from oxidative to glycolytic metabolism in those fibers which are typically the most aerobic in human muscle.

Mammalian muscle cells bear a cell-autonomous, heritable memory of their rostrocaudal position

We previously documented a greater than 100-fold rostrocaudal gradient of chloramphenicol acetyltransferase (CAT) expression in the muscles of adult mice that bear a myosin light chain-CAT transgene: successively more caudal muscles express successively higher levels of CAT. Here we studied the development and maintenance of this positional information in vitro. CAT levels reflect the rostrocaudal positions of the muscles from which the cells are derived in cultures established from adult muscles, in clones derived from individual adult myogenic (satellite) cells, in cultures prepared from embryonic myoblasts, and in cell lines derived by retrovirus-mediated transfer of an oncogene to satellite cells. Our results suggest that myoblasts bear a positional memory that is established in embryos, retained in adults, cell autonomous, heritable, stable to transformation, and accessible to study in clonal cell lines.

Metabolic response to a high-fat diet in neonatal and adult rat muscle

Neonatal rats were exposed to a high-fat low-carbohydrate diet to determine how substrate availability might affect the metabolic phenotype of muscle. Mixed-fiber homogenates of extensor digitorum longus, soleus, and diaphragm muscles were assayed for beta-hydroxyacyl-CoA dehydrogenase (beta-OAC), succinate dehydrogenase, malate dehydrogenase, lactate dehydrogenase, phosphofructokinase (PFK), adenylokinase, and creatine kinase. The three muscles showed significant increases in enzyme activity for fatty acid oxidation (beta-OAC) in weaned neonatal rats maintained on the high-fat diet compared with normal weaned controls. This effect persisted for 6 wk of the diet. The other consistent metabolic change was a decrease in PFK. Adult animals subjected to the same diet had similar increases in fatty acid oxidation and a fall in PFK after 1 wk, with most of these changes persisting for the 4 wk of the diet. Examination of individual fibers revealed enzyme changes in fibers of all types, but with the largest effect in type IIb fibers. The data indicate that both adult and neonatal muscles are similarly capable of adjusting their energy metabolism in response to dietary factors.

Fiber type- and position-dependent expression of a myosin light chain-CAT transgene detected with a novel histochemical stain for CAT

We recently generated and characterized transgenic mice in which regulatory sequences from a myosin light chain gene (MLC1f/3f) are linked to the chloramphenicol acetyltransferase (CAT) gene. Transgene expression in these mice is specific to skeletal muscle and graded along the rostrocaudal axis: adult muscles derived from successively more caudal somites express successively higher levels of CAT. To investigate the cellular basis of these patterns of expression, we developed and used a histochemical stain that allows detection of CAT in individual cells. Our main results are as follows: (a) Within muscles, CAT is detected only in muscle fibers and not in associated connective tissue, blood vessels, or nerves. Thus, the tissue specificity of transgene expression observed by biochemical assay reflects a cell-type specificity demonstrable histochemically. (b) Within individual muscles, CAT levels vary with fiber type. Like the endogenous MLC1f/3f gene, the transgene is expressed at higher levels in fast-twitch (type II) than in slow-twitch (type I) muscle fibers. In addition, CAT levels vary among type II fiber subtypes, in the order IIB greater than IIX greater than IIA. (c) Among muscles that are similar in fiber type composition, the average level of CAT per fiber varies with rostrocaudal position. This position-dependent variation in CAT level is apparent even when fibers of a single type are compared. From these results, we conclude that fiber type and position affect CAT expression independently. We therefore infer the existence of separate fiber type-specific and positionally graded transcriptional regulators that act together to determine levels of transgene expression.

Metabolic and contractile uniformity of isolated motor unit fibres of snake muscle

1. Motor units in the thin transversus abdominis muscle of the garter snake were identified and physiologically characterized in the living state. Motor unit fibres, and fibres chosen randomly to serve as controls, were subsequently excised and subjected to biochemical analyses. 2. The metabolic capacity of fibres was assessed by measuring activities of three enzymes, each representing a different metabolic pathway. The microchemical enzyme assays were performed using enzyme extraction preparations of whole single fibres. 3. Metabolic capacity ranged widely among the muscle's entire fibre population, even among fibres of the same type. In contrast, enzyme activities of twitch fibres belonging to individual motor units were, within analytical error, identical. 4. Twitch contraction times of individual fibres within one motor unit were similar, compared to a wide range of contraction times observed among fibres of the same type but belonging to different motor units. 5. When several motor units were studied in one muscle, a systematic relationship was observed among motor unit tension, enzymatic profile and contraction time. As motor unit tension increased, fibres exhibited greater capacities for glycolytic and high-energy phosphate metabolism, diminished capacity for oxidative metabolism, and faster twitch contraction times. 6. Given the great diversity of metabolic and contractile properties exhibited within the fibre population, the uniformity of such properties within motor units indicates that neural influence dominates over other extrinsic factors present in the microenvironment of the muscle fibres.

Metabolic capacity and myosin expression in single muscle fibres of the garter snake

1. The transversus abdominis muscle of the garter snake contains fibres of three types: tonic (T), slower twitch (S) and faster twitch (F). Fibre types can be determined by anatomical criteria in living preparations. Individual fibres identified as T, S or F were excised from the muscle and subdivided for two types of biochemical examination. Enzymes of energy metabolism were assayed using quantitative microfluorometric methods. Myosin heavy chain composition was determined by gel electrophoresis. In separate experiments, twitch time-to-peaks of F and S fibres were measured to assess the range of contraction times present within the muscle's twitch fibre population. 2. Metabolic subgroups of fibres were delineated by the relative activities of adenylokinase (AK), lactate dehydrogenase (LDH) and beta-hydroxyacyl-CoA-dehydrogenase (beta OAC). The metabolic subgroups corresponded to the anatomical fibre types. Type F fibres had high levels of enzymes associated with glycolytic (LDH) and high-energy phosphate (AK) metabolism. Type T fibres had high levels of the oxidative enzyme beta OAC. Type S fibres had both types of enzyme activity in intermediate and variable amounts. 3. Three myosin heavy chain isoforms were present in the muscle. Type F and type T fibres each expressed a single isoform, denoted F and T respectively. Type S fibres expressed significant quantities of two isoforms: an isoform unique to this fibre type (denoted S) and the F isoform. 4. Electrophoretic mobility and antibody reactivity of the F myosin heavy chain isoform resembled that of mammalian fast-twitch myosin. By the same criteria, the T isoform resembled mammalian slow-twitch myosin. The S isoform exhibited intermediate characteristics: its antibody reactivity was similar to mammalian fast-twitch myosin, but its electrophoretic mobility was that of mammalian slow-twitch myosin. 5. Based on whole-muscle analysis, two myosin alkali light chains, denoted ALC1 and ALC2, and one myosin regulatory light chain were present. Gel patterns suggested that ALC1 and ALC2 exist as both homodimers and heterodimers. 6. The population of type S fibres within a given muscle exhibited a much wider range of twitch contraction times than did the population of type F fibres. Diversity of contractile properties among type S fibres may result, in part, from differential co-expression of two myosin heavy chain isoforms, together with highly variable ratios of enzymes from two major metabolic pathways. 7. The clear biochemical distinction among fibre types indicates that each type possesses a unique and limited range of physiological properties.(ABSTRACT TRUNCATED AT 400 WORDS)