Myostatin regulates energy homeostasis in the heart and prevents heart failure.

RATIONALE: Myostatin is a major negative regulator of skeletal muscle mass and initiates multiple metabolic changes, including enhanced insulin sensitivity. However, the function of myostatin in the heart is barely understood, although it is upregulated in the myocardium under several pathological conditions. OBJECTIVE: Here, we aimed to decipher the role of myostatin and myostatin-dependent signaling pathways for cardiac function and cardiac metabolism in adult mice. To avoid potential counterregulatory mechanisms occurring in constitutive and germ-line-based myostatin mutants, we generated a mouse model that allows myostatin inactivation in adult cardiomyocytes. METHODS AND RESULTS: Cardiac MRI revealed that genetic inactivation of myostatin signaling in the adult murine heart caused cardiac hypertrophy and heart failure, partially recapitulating effects of the age-dependent decline of the myostatin paralog growth and differentiation factor 11. We found that myostatin represses AMP-activated kinase activation in the heart via transforming growth factor-ß-activated kinase 1, thereby preventing a metabolic switch toward glycolysis and glycogen accumulation. Furthermore, myostatin stimulated expression of regulator of G-protein signaling 2, a GTPase-activating protein that restricts Gaq and Gas signaling and thereby protects against cardiac failure. Inhibition of AMP-activated kinase in vivo rescued cardiac hypertrophy and prevented enhanced glycolytic flow and glycogen accumulation after inactivation of myostatin in cardiomyocytes. CONCLUSIONS: Our results uncover an important role of myostatin in the heart for maintaining cardiac energy homeostasis and preventing cardiac hypertrophy.

Myostatin induces interstitial fibrosis in the heart via TAK1 and p38.

Myostatin, a member of the TGF-ß superfamily of secreted growth factors, is a negative regulator of skeletal muscle growth. In the heart, it is expressed at lower levels compared to skeletal muscle but up-regulated under disease conditions. Cre recombinase-mediated inactivation of myostatin in adult cardiomyocytes leads to heart failure and increased mortality but cardiac function of surviving mice is restored after several weeks probably due to compensatory expression in non-cardiomyocytes. To study long-term effects of increased myostatin expression in the heart and to analyze the putative crosstalk between cardiomyocytes and fibroblasts, we overexpressed myostatin in cardiomyocytes. Increased expression of myostatin in heart muscle cells caused interstitial fibrosis via activation of the TAK-1-MKK3/6-p38 signaling pathway, compromising cardiac function in older mice. Our results uncover a novel role of myostatin in the heart and highlight the necessity for tight regulation of myostatin to maintain normal heart function.

Newt-omics: a comprehensive repository for omics data from the newt Notophthalmus viridescens.

Notophthalmus viridescens, a member of the salamander family is an excellent model organism to study regenerative processes due to its unique ability to replace lost appendages and to repair internal organs. Molecular insights into regenerative events have been severely hampered by the lack of genomic, transcriptomic and proteomic data, as well as an appropriate database to store such novel information. Here, we describe 'Newt-omics' (http://newt-omics.mpi-bn.mpg.de), a database, which enables researchers to locate, retrieve and store data sets dedicated to the molecular characterization of newts. Newt-omics is a transcript-centred database, based on an Expressed Sequence Tag (EST) data set from the newt, covering ~50,000 Sanger sequenced transcripts and a set of high-density microarray data, generated from regenerating hearts. Newt-omics also contains a large set of peptides identified by mass spectrometry, which was used to validate 13,810 ESTs as true protein coding. Newt-omics is open to implement additional high-throughput data sets without changing the database structure. Via a user-friendly interface Newt-omics allows access to a huge set of molecular data without the need for prior bioinformatical expertise.

A microarray analysis of gene expression patterns during early phases of newt lens regeneration.

PURPOSE: Notophthalmus viridescens, the red-spotted newt, possesses tremendous regenerative capabilities. Among the tissues and organs newts can regenerate, the lens is regenerated via transdifferentiation of the pigment epithelial cells of the dorsal iris, following complete removal (lentectomy). Under normal conditions, the same cells from the ventral iris are not capable of regenerating. This study aims to further understand the initial signals of lens regeneration. METHODS: We performed microarray analysis using RNA from a dorsal or ventral iris isolated 1, 3, and 5 days after lentectomy and compared to RNA isolated from an intact iris. This analysis was supported with quantitative real-time polymerase chain reaction (qRT-PCR) of selected genes. RESULTS: Microarrays showed 804 spots were differentially regulated 1, 3, and 5 days post-lentectomy in the dorsal and ventral iris. Functional annotation using Gene Ontology revealed interesting terms. Among them, factors related to cell cycle and DNA repair were mostly upregulated, in the microarray, 3 and 5 days post-lentectomy. qRT-PCR for rad1 and vascular endothelial growth factor receptor 1 showed upregulation for the dorsal iris 3 and 5 days post- lentectomy and for the ventral iris 5 days post-lentectomy. Rad1 was also upregulated twofold more in the dorsal iris than in the ventral iris 5 days post-lentectomy (p<0.001). Factors related to redox homeostasis were mostly upregulated in the microarray in all time points and samples. qRT-PCR for glutathione peroxidase 1 also showed upregulation in all time points for the ventral and dorsal iris. For the most part, mitochondrial enzymes were downregulated with the notable exception of cytochrome c-related oxidases that were mostly upregulated at all time points. qRT-PCR for cytochrome c oxidase subunit 2 showed upregulation especially 3 days post-lentectomy for the dorsal and ventral iris (p<0.001). Factors related to extracellular matrix and tissue remodeling showed mostly upregulation (except collagen I) for all time points and samples. qRT-PCR for stromelysin 1/2 alpha and avidin showed upregulation in all the time points for the dorsal and ventral iris. CONCLUSIONS: The results show that the dorsal iris and the ventral iris follow the same general pattern with some distinct differences especially 5 days after lentectomy. In addition, while the expression of genes involved in DNA repair, redox homeostasis, and tissue remodeling in preparation for proliferation and transdifferentiation is altered in the entire iris, the response is more prominent in the dorsal iris following lentectomy.

Distinct effects of AMPAR subunit depletion on spatial memory.

Pharmacological studies established a role for AMPARs in the mammalian forebrain in spatial memory performance. Here we generated global GluA1/3 double knockout mice (Gria1/3-/-) and conditional knockouts lacking GluA1 and GluA3 AMPAR subunits specifically from principal cells across the forebrain (Gria1/3ΔFb). In both models, loss of GluA1 and GluA3 resulted in reduced hippocampal GluA2 and increased levels of the NMDAR subunit GluN2A. Electrically-evoked AMPAR-mediated EPSPs were greatly diminished, and there was an absence of tetanus-induced LTP. Gria1/3-/- mice showed premature mortality. Gria1/3ΔFb mice were viable, and their memory performance could be analyzed. In the Morris water maze (MWM), Gria1/3ΔFb mice showed profound long-term memory deficits, in marked contrast to the normal MWM learning previously seen in single Gria1-/- and Gria3-/- knockout mice. Our results suggest a redundancy of function within the pool of available ionotropic glutamate receptors for long-term spatial memory performance.

Spiked-in pulsed in vivo labeling identifies a new member of the CCN family in regenerating newt hearts.

The newt Notophthalmus viridescens , which belongs to the family of salamanders (Urodela), owns remarkable regenerative capacities allowing efficient scar-free repair of various organs including the heart. Salamanders can regrow large parts of the myocardium unlike mammals, which cannot replace lost cardiomyocytes efficiently. Unfortunately, very little is known about the molecules and the regulatory circuits facilitating efficient heart regeneration in newts or salamanders. To identify proteins that are involved in heart regeneration, we have developed a pulsed SILAC-based mass spectrometry method based on the detection of paired peptide peaks after ¹³C6-lysine incorporation into proteins in vivo. Proteins were identified by matching mass spectrometry derived peptide sequences to a recently established normalized newt EST library. Our approach enabled us to identify more than 2200 nonredundant proteins in the regenerating newt heart. Because of the pulsed in vivo labeling approach, accurate quantification was achieved for 1353 proteins, of which 72 were up- and 31 down-regulated with a (|log 2 ratio| > 1) during heart regeneration. One deregulated member was identified as a new member of the CCN protein family, showing a wound specific activation. We reason that the detection of such deregulated newt-specific proteins in regenerating hearts supports the idea of a local evolution of tissue regeneration in salamanders. Our results significantly improve understanding of dynamic changes in the complex protein network that underlies heart regeneration and provides a basis for further mechanistic studies.

Advanced identification of proteins in uncharacterized proteomes by pulsed in vivo stable isotope labeling-based mass spectrometry.

Despite progress in the characterization of their genomes, proteomes of several model organisms are often only poorly characterized. This problem is aggravated by the presence of large numbers of expressed sequence tag clones that lack homologues in other species, which makes it difficult to identify new proteins irrespective of whether such molecules are involved in species-specific biological processes. We have used a pulsed stable isotope labeling with amino acids in cell culture (SILAC)-based mass spectrometry method, which is based on the detection of paired peptides after [(13)C(6)]lysine incorporation into proteins in vivo, to greatly increase the confidence of protein identification in cross-species database searches. The method was applied to identify nearly 3000 proteins in regenerating tails of the urodele amphibian Notophthalmus viridescens, which possesses outstanding capabilities in the regeneration of complex tissues. We reason that pulsed in vivo SILAC represents a versatile tool to identify new proteins in species for which only limited sequence information exists.

Comparative transcriptional profiling of regenerating damaged knee joints in two animal models of the newt Notophthalmus viridescens strengthens the role of candidate genes involved in osteoarthritis.

Objectives: To compare joint regeneration in adult newts (N. viridescens) upon both newly established surgical removal and previously reported enzymatic destruction of articular cartilage to identify molecular factors and functionally analyze potentially important regulators involved in osteoarthritis (OA). Methods: Damage of knee cartilage was induced by either intra-articular injection of collagenase or by surgical removal of articular cartilage as a novel additional approach. Changes over time were clinically and histologically analyzed and studied by cDNA microarray analysis, real-time quantitative PCR, immunohistochemistry and functional assays to identify relevant candidate genes and determine their impact on regeneration. Results: Several genes were found to be up-regulated during regeneration, including extracellular matrix components and mediators of cell-matrix interactions, genes encoding for cellular components, for cell and tissue homeostasis and tissue remodelling, for cellular processes as well as signalling molecules. A high activity and diversity of transcription was detected on days 10 and 20, especially in the surgical model. 10 candidate genes were further analyzed. The matricellular protein tenascin C (TN-C) attracted our particular attention due to its prominent up-regulation during regeneration in both models and at different time points. Conclusions: Newts are able to regenerate OA-like articular cartilage damage ad integrum both after enzymatic and mechanical injury. Most of the genes involved in amphibians are also known to be operative in humans and other mammals, especially matricellular factors interfering with optimized matrix remodelling. Our results stress the necessity to elucidate mechanistic differences in different species potentially using identical molecules but with different functional results.

Endogenous regeneration after collagenase-induced knee joint damage in the adult newt Notophthalmus viridescens.

OBJECTIVES: To determine whether adult newts (Notophthalmus viridescens) are able to repair experimentally-induced joint damage in order to generate a model system for the study of endogenous joint regeneration. METHODS: Joint instability and articular cartilage lesions of the knee joint of adult newts (N viridescens) were induced by intra-articular injection of collagenase. The changes over time were analysed clinically, by MRI, histologically and by reverse transcription PCR to detect selected relevant markers. RESULTS: After rapid onset of disease with joint luxation, loss of proteoglycans and cartilage volume, the signs ameliorated continuously by regeneration of the affected joint compartments. The majority of joints were morphologically intact and functionally operative after 10 weeks. Upregulation of chondrogenic key genes, homogenous expression levels of factors implicated in cartilage homeostasis and limb regeneration as well as the distribution of the blastemal marker 22/18 in both treated and untreated knees suggest that joint regeneration in adult newts only partially invokes pathways of embryological organogenesis. CONCLUSIONS: Newts are able to regenerate articular cartilage injuries and to restore tissue integrity and function after induction of damage using a procedure known to induce experimental osteoarthritis in murine models. Further analysis of the underlying molecular mechanisms may contribute to the development of novel treatment approaches in joint failure.

Analysis of newly established EST databases reveals similarities between heart regeneration in newt and fish.

BACKGROUND: The newt Notophthalmus viridescens possesses the remarkable ability to respond to cardiac damage by formation of new myocardial tissue. Surprisingly little is known about changes in gene activities that occur during the course of regeneration. To begin to decipher the molecular processes, that underlie restoration of functional cardiac tissue, we generated an EST database from regenerating newt hearts and compared the transcriptional profile of selected candidates with genes deregulated during zebrafish heart regeneration. RESULTS: A cDNA library of 100,000 cDNA clones was generated from newt hearts 14 days after ventricular injury. Sequencing of 11520 cDNA clones resulted in 2894 assembled contigs. BLAST searches revealed 1695 sequences with potential homology to sequences from the NCBI database. BLAST searches to TrEMBL and Swiss-Prot databases assigned 1116 proteins to Gene Ontology terms. We also identified a relatively large set of 174 ORFs, which are likely to be unique for urodele amphibians. Expression analysis of newt-zebrafish homologues confirmed the deregulation of selected genes during heart regeneration. Sequences, BLAST results and GO annotations were visualized in a relational web based database followed by grouping of identified proteins into clusters of GO Terms. Comparison of data from regenerating zebrafish hearts identified biological processes, which were uniformly overrepresented during cardiac regeneration in newt and zebrafish. CONCLUSION: We concluded that heart regeneration in newts and zebrafish led to the activation of similar sets of genes, which suggests that heart regeneration in both species might follow similar principles. The design of the newly established newt EST database allows identification of molecular pathways important for heart regeneration.

The AMPA receptor subunits GluR-A and GluR-B reciprocally modulate spinal synaptic plasticity and inflammatory pain.

Ca(2+)-permeable AMPA receptors are densely expressed in the spinal dorsal horn, but their functional significance in pain processing is not understood. By disrupting the genes encoding GluR-A or GluR-B, we generated mice exhibiting increased or decreased numbers of Ca(2+)-permeable AMPA receptors, respectively. Here, we demonstrate that AMPA receptors are critical determinants of nociceptive plasticity and inflammatory pain. A reduction in the number of Ca(2+)-permeable AMPA receptors and density of AMPA channel currents in spinal neurons of GluR-A-deficient mice is accompanied by a loss of nociceptive plasticity in vitro and a reduction in acute inflammatory hyperalgesia in vivo. In contrast, an increase in spinal Ca(2+)-permeable AMPA receptors in GluR-B-deficient mice facilitated nociceptive plasticity and enhanced long-lasting inflammatory hyperalgesia. Thus, AMPA receptors are not mere determinants of fast synaptic transmission underlying basal pain sensitivity as previously thought, but are critically involved in activity-dependent changes in synaptic processing of nociceptive inputs.

Glutamatergic plasticity by synaptic delivery of GluR-B(long)-containing AMPA receptors.

Activity-driven delivery of AMPA receptors is proposed to mediate glutamatergic synaptic plasticity, both during development and learning. In hippocampal CA1 principal neurons, such trafficking is primarily mediated by the abundant GluR-A subunit. We now report a study of GluR-B(long), a C-terminal splice variant of the GluR-B subunit. GluR-B(long) synaptic delivery is regulated by two forms of activity. Spontaneous synaptic activity-driven GluR-B(long) transport maintains one-third of the steady-state AMPA receptor-mediated responses, while GluR-B(long) delivery following the induction of LTP is responsible for approximately 50% of the resulting potentiation at the hippocampal CA3 to CA1 synapses at the time of GluR-B(long) peak expression-the second postnatal week. Trafficking of GluR-B(long)-containing receptors thus mediates a GluR-A-independent form of glutamatergic synaptic plasticity in the juvenile hippocampus.

A pathway-specific function for different AMPA receptor subunits in amygdala long-term potentiation and fear conditioning.

The AMPA receptor subunit glutamate receptor 1 (GluR1 or GluR-A) contributes to amygdala-dependent emotional learning. It remains unclear, however, to what extent different amygdala pathways depend on GluR1, or other AMPA receptor subunits, for proper synaptic transmission and plasticity, and whether GluR1-dependent long-term potentiation (LTP) is necessary for auditory and contextual fear conditioning. Here, we dissected the role of GluR1 and GluR3 (GluR-C) subunits in AMPA receptor-dependent amygdala LTP and fear conditioning using knock-out mice (GluR1-/- and GluR3-/-). We found that, whereas LTP at thalamic inputs to lateral amygdala (LA) projection neurons and at glutamatergic synapses in the basal amygdala was completely absent in GluR1-/- mice, both GluR1 and GluR3 contributed to LTP in the cortico-LA pathway. Because both auditory and contextual fear conditioning were selectively impaired in GluR1-/- but not GluR3-/- mice, we conclude that GluR1-dependent synaptic plasticity is the dominant form of LTP underlying the acquisition of auditory and contextual fear conditioning, and that plasticity in distinct amygdala pathways differentially contributes to aversive conditioning.

Somatic Accumulation of GluA1-AMPA Receptors Leads to Selective Cognitive Impairments in Mice.

The GluA1 subunit of the L-α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor (AMPAR) plays a crucial, but highly selective, role in cognitive function. Here we analyzed AMPAR expression, AMPAR distribution and spatial learning in mice (Gria1R/R ), expressing the "trafficking compromised" GluA1(Q600R) point mutation. Our analysis revealed somatic accumulation and reduction of GluA1(Q600R) and GluA2, but only slightly reduced CA1 synaptic localization in hippocampi of adult Gria1R/R mice. These immunohistological changes were accompanied by a strong reduction of somatic AMPAR currents in CA1, and a reduction of plasticity (short-term and long-term potentiation, STP and LTP, respectively) in the CA1 subfield following tetanic and theta-burst stimulation. Nevertheless, spatial reference memory acquisition in the Morris water-maze and on an appetitive Y-maze task was unaffected in Gria1R/R mice. In contrast, spatial working/short-term memory during both spontaneous and rewarded alternation tasks was dramatically impaired. These findings identify the GluA1(Q600R) mutation as a loss of function mutation that provides independent evidence for the selective role of GluA1 in the expression of short-term memory.

Involvement of the AMPA receptor GluR-C subunit in alcohol-seeking behavior and relapse.

Craving and relapse are core symptoms of drug addiction and alcoholism. It is suggested that, after chronic drug consumption, long-lasting neuroplastic changes within the glutamatergic system are important determinants of addictive behavior. Here, we show that the AMPA type glutamate receptor plays a crucial role in alcohol craving and relapse. We observed, in two animal models of alcohol craving and relapse, that the AMPA antagonist GYKI 52466 [1-(4-aminophenyl)-4-methyl-7, 8-methylenedioxy-5H-2, 3-benzodiazepine] dose-dependently reduced cue-induced reinstatement of alcohol-seeking behavior and the alcohol deprivation effect. The involvement of the AMPA receptor in these phenomena was further studied using mice deficient for the GluR-C AMPA subunit [GluR-C knock-out (KO)]. GluR-C KOs displayed a blunted, cue-induced reinstatement response and alcohol deprivation effect, when compared with wild-type controls; however, no differences between genotypes could be observed regarding ethanol self-administration under operant or home cage drinking conditions. These results imply a role for GluR-C in alcohol relapse, although this phenotype could also be attributable to a reduction in the total number of AMPA receptors in specific brain areas. In conclusion, AMPA receptors seem to be involved in the neuroplastic changes underlying alcohol seeking behavior and relapse. Thus, AMPA receptors represent a novel therapeutic target in preventing relapse.

Cardiovascular regeneration in non-mammalian model systems: what are the differences between newts and man?

The mammalian heart cannot regenerate substantial cardiac injuries, while certain non-mammalian vertebrates such as certain fish (Danio rerio) and amphibiae (Notophthalmus viridescens) are able to repair the heart without functional impairment. In mammalians, the prevailing repair process is accompanied by fibrosis and scarring, while zebrafish and newts can replace lost contractile tissue by newly formed cardiac muscle with only little or no scar formation. A better understanding of cardiac regeneration in non-mammalian vertebrates might provide new insights for the manipulation of regenerative pathways in the human heart. Here, we summarize the current knowledge in cardiac regeneration of newts and the principal differences to repair processes in mammalian hearts.

NMDA receptor 2 (NR2) C-terminal control of NR open probability regulates synaptic transmission and plasticity at a cerebellar synapse.

The C-terminal domain of NMDA receptor 2 (NR2) subunits has been proposed to play a critical role in regulating NMDA receptor localization and function in postsynaptic densities. However, the mechanism of this regulation is not completely understood. In this paper we show that C-terminal truncation of NR2A and NR2C subunits in mice (NR2A/C(DeltaC/DeltaC)) impairs synaptic transmission and plasticity at the cerebellar mossy fiber-granule cell relay. Activation of synaptic NMDA receptors could be distinguished from that of extrasynaptic receptors by using the glutamate scavenger glutamate pyruvate transaminase and the open channel blocker MK801. NR2A/C(DeltaC/DeltaC) mice exhibited a specific reduction in synaptic NMDA receptor activation attributable to a severalfold decrease in channel open probability but not channel conductance. Immunodetection revealed normal developmental expression of NR subunit proteins. Quantitative immunogold analyses with an antibody to NR1 indicated that the reduction in receptor activation is not attributed to a reduced number of NR1-containing receptors in postsynaptic densities. Thus, NR2A/NR2C subunits and particularly their C termini regulate synaptic NMDA receptor activation and function by enhancing channel open probability, which is critical for long-term potentiation induction.

Reconstitution of the myocardium in regenerating newt hearts is preceded by transient deposition of extracellular matrix components.

Adult newts efficiently regenerate the heart after injury in a process that involves proliferation of cardiac muscle and nonmuscle cells and repatterning of the myocardium. To analyze the processes that underlie heart regeneration in newts, we characterized the structural changes in the myocardium that allow regeneration after mechanical injury. We found that cardiomyocytes in the damaged ventricle mainly die by necrosis and are removed during the first week after injury, paving the way for the extension of thin myocardial trabeculae, which initially contain only very few cardiomyocytes. During the following 200 days, these thin trabeculae fill up with new cardiomyocytes until the myocardium is fully reconstituted. Interestingly, reconstruction of the newly formed trabeculated network is accompanied by transient deposition of extracellular matrix (ECM) components such as collagen III. We conclude that the ECM is a critical guidance cue for outgrowing and branching trabeculae to reconstruct the trabeculated network, which represents a hallmark of uninjured cardiac tissue in newts.

A de novo assembly of the newt transcriptome combined with proteomic validation identifies new protein families expressed during tissue regeneration.

BACKGROUND: Notophthalmus viridescens, an urodelian amphibian, represents an excellent model organism to study regenerative processes, but mechanistic insights into molecular processes driving regeneration have been hindered by a paucity and poor annotation of coding nucleotide sequences. The enormous genome size and the lack of a closely related reference genome have so far prevented assembly of the urodelian genome. RESULTS: We describe the de novo assembly of the transcriptome of the newt Notophthalmus viridescens and its experimental validation. RNA pools covering embryonic and larval development, different stages of heart, appendage and lens regeneration, as well as a collection of different undamaged tissues were used to generate sequencing datasets on Sanger, Illumina and 454 platforms. Through a sequential de novo assembly strategy, hybrid datasets were converged into one comprehensive transcriptome comprising 120,922 non-redundant transcripts with a N50 of 975. From this, 38,384 putative transcripts were annotated and around 15,000 transcripts were experimentally validated as protein coding by mass spectrometry-based proteomics. Bioinformatical analysis of coding transcripts identified 826 proteins specific for urodeles. Several newly identified proteins establish novel protein families based on the presence of new sequence motifs without counterparts in public databases, while others containing known protein domains extend already existing families and also constitute new ones. CONCLUSIONS: We demonstrate that our multistep assembly approach allows de novo assembly of the newt transcriptome with an annotation grade comparable to well characterized organisms. Our data provide the groundwork for mechanistic experiments to answer the question whether urodeles utilize proprietary sets of genes for tissue regeneration.

Different autonomous myogenic cell populations revealed by ablation of Myf5-expressing cells during mouse embryogenesis.

The development of myogenic cells is mainly determined by expression of two myogenic factors, Myf5 and Myod1 (MyoD), which genetically compensate for each other during embryogenesis. Here, we demonstrate by conditional cell ablation in mice that Myf5 determines a distinct myogenic cell population, which also contains some Myod1-positive cells. Ablation of this lineage uncovers the presence of a second autonomous myogenic lineage, which superseded Myf5-dependent myogenic cells and expressed Myod1. By contrast, ablation of myogenin-expressing cells erased virtually all differentiated muscle cells, indicating that some aspects of the myogenic program are shared by most skeletal muscle cells. We conclude that Myf5 and Myod1 define different cell lineages with distinct contributions to muscle precursor cells and differentiated myotubes. Individual myogenic cell lineages seem to substitute for each other within the developing embryo.