Ablation of the fetal mouse spinal cord: the effect on soleus muscle cytoarchitecture

The role of semaphorin3A in myogenic regeneration and the formation of functional neuromuscular junctions on new fibres

Current research on skeletal muscle injury and regeneration highlights the crucial role of nerve-muscle interaction in the restoration of innervation during that process. Activities of muscle satellite or stem cells, recognized as the 'currency' of myogenic repair, have a pivotal role in these events, as shown by ongoing research. More recent investigation of myogenic signalling events reveals intriguing roles for semaphorin3A (Sema3A), secreted by activated satellite cells, in the muscle environment during development and regeneration. For example, Sema3A makes important contributions to regulating the formation of blood vessels, balancing bone formation and bone remodelling, and inflammation, and was recently implicated in the establishment of fibre-type distribution through effects on myosin heavy chain gene expression. This review highlights the active or potential contributions of satellite-cell-derived Sema3A to regulation of the processes of motor neurite ingrowth into a regenerating muscle bed. Successful restoration of functional innervation during muscle repair is essential; this review emphasizes the integrative role of satellite-cell biology in the progressive coordination of adaptive cellular and tissue responses during the injury-repair process in voluntary muscle.

First Neuromuscular Contact Correlates with Onset of Primary Myogenesis in Rat and Mouse Limb Muscles

Skeletal muscle development has been the focus of intensive study for many decades. Recent advances in genetic manipulation of the mouse have increased our understanding of the cell signalling involved in the development of muscle progenitors which give rise to adult skeletal muscles and their stem cell populations. However, the influence of a vital tissue type – the peripheral nerve—has largely been ignored since its earliest descriptions. Here we carefully describe the timing in which myogenic progenitors expressing Pax3 and Pax7 (the earliest markers of myogenic cells) enter the limb buds of rat and mouse embryos, as well as the spatiotemporal relationship between these progenitors and the ingrowing peripheral nerve. We show that progenitors expressing Pax3 enter the limb bud one full day ahead of the first neurites and that Pax7-expressing progenitors (associated with secondary myogenesis in the limb) are first seen in the limb bud at the time of nerve entry and in close proximity to the nerve. The initial entry of the nerve also coincides with the first expression of myosin heavy chain showing that the first contact between nerves and myogenic cells correlates with the onset of myogenic differentiation. Furthermore, as the nerve grows into the limb, Pax3 expression is progressively replaced by Pax7 expression in myogenic progenitors. These findings indicate that the ingrowing nerve enters the limb presumptive muscle masses earlier than what was generally described and raises the possibility that nerve may influence the differentiation of muscle progenitors in rodent limbs.

Muscle development and obesity: Is there a relationship?

The formation of skeletal muscle from the epithelial somites involves a series of events triggered by temporally and spatially discrete signals resulting in the generation of muscle fibers which vary in their contractile and metabolic nature. The fiber type composition of muscles varies between individuals and it has now been found that there are differences in fiber type proportions between lean and obese animals and humans. Amongst the possible causes of obesity, it has been suggested that inappropriate prenatal environments may 'program' the fetus and may lead to increased risks for disease in adult life. The characteristics of muscle are both heritable and plastic, giving the tissue some ability to adapt to signals and stimuli both pre and postnatally. Given that muscle is a site of fatty acid oxidation and carbohydrate metabolism and that its development can be changed by prenatal events, it is interesting to examine the possible relationship between muscle development and the risk of obesity.

Investigating regeneration and functional integration of CNS neurons: lessons from zebrafish genetics and other fish species

Zebrafish possess a robust, innate CNS regenerative ability. Combined with their genetic tractability and vertebrate CNS architecture, this ability makes zebrafish an attractive model to gain requisite knowledge for clinical CNS regeneration. In treatment of neurological disorders, one can envisage replacing lost neurons through stem cell therapy or through activation of latent stem cells in the CNS. Here we review the evidence that radial glia are a major source of CNS stem cells in zebrafish and thus activation of radial glia is an attractive therapeutic target. We discuss the regenerative potential and the molecular mechanisms thereof, in the zebrafish spinal cord, retina, optic nerve and higher brain centres. We evaluate various cell ablation paradigms developed to induce regeneration, with particular emphasis on the need for (high throughput) indicators that neuronal regeneration has restored sensory or motor function. We also examine the potential confound that regeneration imposes as the community develops zebrafish models of neurodegeneration. We conclude that zebrafish combine several characters that make them a potent resource for testing hypotheses and discovering therapeutic targets in functional CNS regeneration. This article is part of a Special Issue entitled Zebrafish Models of Neurological Diseases.

Neuronal control of myogenic regulatory factor accumulation in fetal muscle

The lumbosacral spinal cords of 14.5-day gestation mice (E14.5) were ablated. The number of molecules of each of the four myogenic regulatory factor (MRF) mRNAs per nanogram of total RNA were evaluated in innervated and aneural fetal crural muscles. Accumulation of all four MRF mRNAs was affected in aneural muscle, but was never more than threefold different than in innervated muscles, considerably less than after adult denervation. The effect of the nerve varied with the MRF, the fetal age, and with the muscle (extensor digitorum longus muscle [EDL] vs. soleus muscle), with the nerve having multiple effects including down-regulation of certain MRF genes at specific periods (e.g., myoD and myogenin [E16.5-E18.5] and MRF4 in the EDL only [E18.5-E19.5]); limiting the up-regulation of certain genes, which occurred in the absence of innervation (e.g., myf-5 [E18.5-E19.5] and myogenin [E14.5-E16.5]); and even enhancing the accumulation of MRF4 mRNA (E14.5-E16.5). We hypothesize that factors other than nerve contribute to the down-regulation of myf-5 and myogenin mRNAs to adult levels. Innervation was required for the emergence of the slow, but not the fast, MRF mRNA profile at birth. MyoD, found in both the nuclear and cytoplasmic protein extracts of innervated fetal muscle, increased by approximately 5-fold in the nuclear extracts (approximately 2.5-fold in the cytoplasmic) of E19.5 aneural muscles, significantly less than the 12-fold increase found in the nuclear extract of 4-day denervated adult muscle. This increase in aneural fetal muscle was due primarily to an increased concentration of myoD in muscle lineage nuclei, rather than to the presence of additional myoD(+) muscle lineage nuclei.

Development of the neuromuscular junction

The differentiation of the neuromuscular junction is a multistep process requiring coordinated interactions between nerve terminals and muscle. Although innervation is not needed for muscle production, the formation of nerve-muscle contacts, intramuscular nerve branching, and neuronal survival require reciprocal signals from nerve and muscle to regulate the formation of synapses. Following the production of muscle fibers, clusters of acetylcholine receptors (AChRs) are concentrated in the central regions of the myofibers via a process termed "prepatterning". The postsynaptic protein MuSK is essential for this process activating possibly its own expression, in addition to the expression of AChR. AChR complexes (aggregated and stabilized by rapsyn) are thus prepatterned independently of neuronal signals in developing myofibers. ACh released by branching motor nerves causes AChR-induced postsynaptic potentials and positively regulates the localization and stabilization of developing synaptic contacts. These "active" contact sites may prevent AChRs clustering in non-contacted regions and counteract the establishment of additional contacts. ACh-induced signals also cause the dispersion of non-synaptic AChR clusters and possibly the removal of excess AChR. A further neuronal factor, agrin, stabilizes the accumulation of AChR at synaptic sites. Agrin released from the branching motor nerve may form a structural link specifically to the ACh-activated endplates, thereby enhancing MuSK kinase activity and AChR accumulation and preventing dispersion of postsynaptic specializations. The successful stabilization of prepatterned AChR clusters by agrin and the generation of singly innervated myofibers appear to require AChR-mediated postsynaptic potentials indicating that the differentiation of the nerve terminal proceeds only after postsynaptic specializations have formed.

Gene transfer into intact fetal skeletal muscle grown in vitro

The development of an organ culture system for growing prenatal intercostal muscle in vitro and its use to study gene function is described. Fetal skeletal muscle is relatively inaccessible during the key stages of its development, and this method enables DNA transfections and other manipulations to be carried out. The system allows cell proliferation and differentiation to continue and also maintains the morphology and fiber types of developing muscle. Gene transfer into cultured embryonic intercostal muscle was achieved by square-pulse electroporation of intact pieces of tissue. Expression of a marker gene (GFP) was found within 5 h and maintained for 2 days in muscle fibers and cells. The technique should enable the function of genes implicated in muscle development and disease to be studied at stages when access is difficult and in a controlled environment.

Molecular dissection of DNA sequences and factors involved in slow muscle-specific transcription

Transcription is a major regulatory mechanism for the generation of slow- and fast-twitch myofibers. We previously identified an upstream region of the slow TnI gene (slow upstream regulatory element [SURE]) and an intronic region of the fast TnI gene (fast intronic regulatory element [FIRE]) that are sufficient to direct fiber type-specific transcription in transgenic mice. Here we demonstrate that the downstream half of TnI SURE, containing E box, NFAT, MEF-2, and CACC motifs, is sufficient to confer pan-skeletal muscle-specific expression in transgenic mice. However, upstream regions of SURE and FIRE are required for slow and fast fiber type specificity, respectively. By adding back upstream SURE sequences to the pan-muscle-specific enhancer, we delineated a 15-bp region necessary for slow muscle specificity. Using this sequence in a yeast one-hybrid screen, we isolated cDNAs for general transcription factor 3 (GTF3)/muscle TFII-I repeat domain-containing protein 1 (MusTRD1). GTF3 is a multidomain nuclear protein related to initiator element-binding transcription factor TF II-I; the genes for both proteins are deleted in persons with Williams-Beuren syndrome, who often manifest muscle weakness. Gel retardation assays revealed that full-length GTF3, as well as its carboxy-terminal half, specifically bind the bicoid-like motif of SURE (GTTAATCCG). GTF3 expression is neither muscle nor fiber type specific. Its levels are highest during a period of fetal development that coincides with the emergence of specific fiber types and transiently increases in regenerating muscles damaged by bupivacaine. We further show that transcription from TnI SURE is repressed by GTF3 when overexpressed in electroporated adult soleus muscles. These results suggest a role for GTF3 as a regulator of slow TnI expression during early stages of muscle development and suggest how it could contribute to Williams-Beuren syndrome.

Interrelations of myogenic response, progressive atrophy of muscle fibers, and cell death in denervated skeletal muscle

Little is known concerning the time-course and structural dynamics of reactivation of compensatory myogenesis in denervated muscle, its initiating cellular mechanisms, and the relationship between this process and the progression of postdenervation atrophy. The purpose of this study was to investigate the interrelations between temporal and spatial patterns of the myogenic response in denervated muscle and progressive atrophy of muscle fibers. Another objective was to study whether reactivation of myogenesis correlates with destabilization of the differentiated state and death of denervated muscle cells. It has remained unclear whether muscle fiber atrophy was the primary factor activating the myogenic response, what levels of cellular atrophy were associated with its activation, and whether the initiation and intensity of myogenesis depended on the local and individual heterogeneity of atrophic changes among fibers. For this reason, our objective was also to identify the levels of atrophic and degenerative changes in denervated muscle fibers that are correlated with activation of the myogenic response. We found that the reactivation of myogenesis in the tibialis anterior and extensor digitorum longus muscles of the rat starts between days 10-21 following nerve transection, before atrophy has attained advanced level, long before dead cells are found in the tissue. Formation of new muscle fibers reaches its maximum between 2 and 4 months following denervation and gradually decreases with progressive postdenervation atrophy. The myogenic response is biphasic and includes two distinct processes. The first process resembles the formation of secondary and tertiary generations of myotubes during normal muscle development and dominates during the first 2 months of denervation. During this period, activated satellite cells form new myotubes on live differentiated muscle fibers. Most of the daughter myotubes in 1- and 2-month denervated muscle develop on the surface of fast type parent muscle fibers, and some of the newly formed muscle fibers express slow myosin. Some fast type parent fibers are weakly or, more rarely, moderately immunopositive for embryonic isomyosin. This indicates that reactivation of myogenesis may also depend on the fiber type. The level of atrophy, destabilization of the differentiated myofiber phenotype, and degenerative changes of individual fibers in denervated muscle are very heterogeneous. The myogenic response of the first type is associated predominantly with fibers of average and higher than average levels of atrophy. Muscle cells that undergo a lesser degree of atrophy also form daughter fibers, although with a lower incidence. We did not find any correlation between the size of newly formed fibers and the level of atrophy of parent fibers. The topographical distribution of new myotubes both in the peripheral and central areas of the mid-belly equatorial sections at the early stages following nerve transection indicates that myogenesis of the first type represents a systemic reaction of muscle to the loss of neural control. These data indicate that activation of the myogenic response does not depend on cell death and degenerative processes per se. The second type of myogenesis is a typical regenerative reaction that occurs mainly within the spaces surrounded by the basal laminae of dead muscle fibers. Myocytes of different sizes are susceptible to degeneration and death, which indicates that cell death in denervated muscle does not correlate with levels of muscle cell atrophy. The regenerative process frequently results in development of abnormal muscle cells that branch or form small clusters. Replacement of lost fibers becomes activated between 2 and 4 months following nerve transection, i.e., mainly at advanced stages of postdenervation atrophy, when cell death becomes a contributing factor of the atrophic process. In long-term denervated muscle, the first and second types of myogenesisoccur concurrently, and the topographical distribution of the myogenic response becomes more heterogeneous than during the first weeks following denervation. Thus, our data demonstrate differential temporal and spatial expression of two patterns of myogenesis in denervated muscle that appear to be controlled by different regulatory mechanisms during the postdenervation period. (c) 2001 Wiley-Liss, Inc.

Effect of chronic denervation and denervation-reinnervation on cytoplasmic creatine kinase transcript accumulation

The extensor digitorum longus (EDL) and soleus muscles of adult mice were chronically denervated or denervated and allowed to reinnervate. Muscles were evaluated 1, 5, 14, 21, and 52 days after sciaticectomy. In terms of weight loss, myofiber atrophy, degeneration, and fibrosis, the soleus muscle was more affected than the EDL by chronic denervation. Fifty-two days after chronic denervation, the number of molecules of MCK/ng total RNA in both muscles (determined with competitive PCR) decreased, with the soleus muscle being more affected. At that stage, BCK mRNA levels in the denervated soleus were unchanged, but they were increased (>50%) in the EDL. Reinnervation restored MCK transcript accumulation in the EDL, whereas, in the soleus MCK, transcripts exceeded control values by 57%, approaching levels in the reinnervated EDL. Despite restoration of MCK mRNA levels, the number of molecules of BCK mRNA/ng total RNA was four- to fivefold higher in reinnervated versus control muscles, suggesting that the genes encoding the CK mRNAs are not coordinately regulated in adult muscle. The role of denervation induced, fiber type changes in regulating CK mRNA accumulation has been evaluated. Electron microscopic analyses have established that fibrosis is not a factor that determines BCK mRNA levels in the chronically denervated or denervated-reinnervated muscles. CK isozyme analyses support the hypothesis that a greater proportion of BCK mRNA found in 52 day chronically denervated and denervated-reinnervated muscles is produced in myofibers vs. nonmuscle cells than in control muscles.

Creatine kinase transcript accumulation: effect of nerve during muscle development

To determine the role of the nerve in regulating the accumulation of cytoplasmic creatine kinase (CK) mRNAs in hindleg muscles of the developing mouse, the lumbosacral spinal cords of 14-day gestation mice (E14) were laser ablated, and the accumulation of muscle CK (MCK) and brain CK (BCK) mRNAs was evaluated just prior to birth with in situ hybridization. Numbers of molecules of each of these transcripts/ng total RNA in the soleus and extensor digitorum longus (EDL) muscles were determined with competitive PCR and compared to transcripts found in innervated crural muscles. Data suggest that: 1) the level of BCK mRNA accumulation in innervated hindlimb muscles peaks at E16.5 and remains at fetal levels until the second month postnatal, when it falls to the level found in the adult. Given that MCK transcripts meet or exceed adult levels by day 28 postnatal, the "down-regulation" of the BCK gene and the "up-regulation" of the MCK gene are not tightly coupled; 2) the developmental switch from BCK to MCK, as the dominant cytoplasmic CK mRNA, occurs in innervated and aneural leg muscles between E14 and E16.5, indicating this switch is not nerve dependent; 3) the absence of innervation has no effect on BCK mRNA accumulation. MCK transcripts/ng total RNA continue to increase in aneural muscle throughout the late fetal period, but from E16.5-E19.5 the MCK transcript levels in aneural muscles become progressively lower than in age-matched innervated muscles. Thus, the accumulation of the muscle specific cytoplasmic CK, but not BCK, transcripts is affected by the absence of innervation during the fetal period. Dev Dyn 1999;215:285-296.

Initial studies of holmium:YAG laser creation of spinal defects in fetal rabbits: model for urologic effects of myelomeningocele

Myelomeningocele (MMC) is characterized by paraplegia and incontinence, often necessitating surgery. Current models of MMC in sheep and primates create a spinal defect long after anomalous neural tube closure ordinarily occurs. An ideal model of MMC would allow creation of the defect at the earliest age in a low-cost species with a short gestation. We present a method utilizing the holmium laser to create spinal defects in rabbits in utero for the study of the pathophysiology and repair of MMC. Pregnant rabbits of 22 to 23 days' gestational age were prepared and draped in sterile fashion for laparotomy under general anesthesia. The abdomen was opened, and both uterine horns were inspected. Double opposing pursestring sutures were placed to secure the chorioamniotic membranes over the fetal lumbar spine. Amniotic fluid was removed with a needle and saved. Electrocautery was used to open the uterus within the pursestring suture, exposing the fetal dorsum. The spine was exposed by laser dissection of the fetal dorsal musculature. Posterior laminectomy was accomplished with laser incisions of each side of the spinous process, leaving the underlying dura and cord exposed. The pursestring was then cinched, amniotic fluid was returned, and the uterus and trocar sites were closed. Cesarian section was performed at 30 to 31 gestational days, and the pups were examined and then humanely sacrificed for histologic evaluation of the lesion. The rabbit is an inexpensive species with a short gestation (33-35 days), and four or more fetuses may be operated on per litter, with the remainder serving as controls. Utilization of minimally invasive techniques including holmium:YAG laser dissection facilitates creation of spinal defects at an early age in this small-animal model.

Role of the nerve in determining fetal skeletal muscle phenotype

To determine the role of the nerve on the establishment of myofiber diversity in skeletal muscles, the lumbosacral spinal cord of 14-day gestation mice (E14) was laser ablated, and the accumulation of the myosin alkali light chains (MLC) mRNAs in crural (hindleg) muscles was evaluated just prior to birth with in situ hybridization. Numbers of molecules of each alkali MLC/ng total RNA in the extensor digitorum longus (EDL) and soleus muscles were determined with competitive polymerase chain reaction. Transcripts for all four alkali MLCs accumulate in aneural muscles. Data suggest that: (1) the absence of the nerve to either future fast or slow muscles results in less accumulation of MLC1V transcript. Moreover, the presence of the nerve is required for the enhanced accumulation of this transcript in future slow muscles; (2) the absence of innervation of future slow, but not fast, muscles decreases the accumulation of MLC1A transcript. Since increased accumulation of MLC1A and MLC1V transcripts are found in future slow muscles at birth, the nerve is necessary for the development of the slow phenotype during myogenesis; (3) MLC1F and MLC3F transcripts do not display any preferential accumulation in future fast muscles during the fetal period. Therefore, the establishment of the differential distribution of these mRNAs, based on fiber type, is a postnatal phenomenon. The nerve is required during the fetal period to allow accumulation of MLC3F messages above a basal level in future fast as well as slow muscles; whereas, the absence of the innervation to future fast, but not slow, muscles reduces the accumulation of MLC1F. Thus, the accumulation of the various alkali MLC mRNAs shows a differential, rather than coordinate, response to the absence of the nerve, and this response may vary depending on the future fiber type of the muscles.

Muscle-specific gene expression during myogenesis in the mouse

Over the past decade, significant advances in molecular biological techniques have substantially increased our understanding of in vivo myogenesis, supplementing the information that previously had been obtained from classical embryological and morphological studies of muscle development. In this review, we have attempted to correlate morphogenetic events in developing murine muscle with the expression of genes encoding the MyoD family of myogenic regulatory factors and the contractile proteins. Differences in the pattern of expression of these genes in murine myotomal and limb muscle are discussed in the context of muscle cell lineage and environmental factors. The differences in gene expression in these two types of muscle suggest that no single coordinated pattern of gene activation is required during the initial formation of the muscles of the mouse.

Modulation of contractile protein gene expression in fetal murine crural muscles: emergence of muscle diversity

The modulation of contractile protein gene expression in mouse crural muscles (i.e., muscles located in the region between the knee and ankle) during the fetal period (defined as 15 days gestation to birth), resulting in diversity among and within these muscles, has been evaluated with in situ hybridization and correlated with morphogenetic events in the extensor digitorum longus and soleus muscles. During the fetal period extensive secondary myotube formation occurs in the crural muscles, and the myotubes become innervated (Ontell and Kozeka [1984a,b] Am. J. Anat. 171:133-148, 149-161; Ontell et al. [1988a,b] Am. J. Anat. 181:267-278, 181:278-288). At 15 days gestation, hybridization with 35S-labeled antisense cRNA probes demonstrates the accumulation of transcripts for alpha-cardiac and alpha-skeletal actin; MLC 1A, MLC 1F, and MLC 3F; and MHC emb, MHC pn, and MHC beta/slow. At 16 days gestation, accumulation of MHC emb transcripts is reduced (as compared with earlier developmental stages); intensity of signal following hybridization with the probe for alpha-skeletal actin is, for the first time, equal to that for the cardiac isoform; and MLC 1V mRNA accumulation is discernible. At this stage, variation in transcript accumulation for some mRNAs among and within crural muscles becomes evident. Two factors may play a role in the selective distribution of these transcripts: 1) the stage of muscle maturation; and 2) the future myofiber type. At 16 days gestation anterior crural muscles (which mature approximately 2 days before posterior crural muscles; Ontell and Kozeka [1984a,b], ibid., Ontell et al. [1988a,b], ibid.) exhibit a greater accumulation of transcripts for alpha-skeletal actin and for MLC 3F than is found in posterior crural muscles. In muscles that in the neonate are composed, in large part, of slow myofibers, MHC beta/slow and MLC 1V mRNAs accumulate in greater amounts, whereas MHC pn transcripts are less abundant in the soleus muscle than in other crural muscles. By 19 days gestation regionalization of transcript accumulation is more pronounced. The soleus muscle, a predominantly slow twitch muscle in the newborn mouse (Wirtz et al. [1983] J. Anat. 137:109-126) exhibits strong signal after hybridization with probes specific for MHC beta/slow and MLC 1V. While the level of transcript accumulation for the development isoforms, MHC emb, MLC 1A, and alpha-cardiac actin, is greatly reduced in most crural muscles at 19 days gestation, these transcripts persist in the soleus muscle at levels equal ot or exceeding their amount in limb muscles of 13 day gestation mouse embryos.(ABSTRACT TRUNCATED AT 400 WORDS)