Skeletal muscle development in the mouse embryo

n this review we discuss the recent findings concerning the mechanisms that restrict somitic cells to the skeletal muscle fate, the myogenic regulatory factors controlling skeletal muscle differentiation and specification of myogenic cell lineages, the nature of inductive signals and the role of secreted proteins in embryonic patterning of the myotome. More specifically, we review data which strongly support the hypothesis that Myf-5 plays a unique role in development of epaxial muscle, that MyoD plays a unique role in development of hypaxial muscles derived from migratory myogenic precursor cells, and that both genes are responsible for development of intercostal and abdominal muscles (hypaxial muscles that develop from the dermatomal epithelia). In addition, while discussing upstream and post-translational regulation of myogenic regulatory factors (MRFs), we suggest that correct formation of the myotome requires a complex cooperation of DNA binding proteins and cofactors, as well as inhibitory function of non-muscle cells of the forming somite, whose proteins would sequester and suppress the transcription of MRFs. Moreover, in the third part of our review, we discuss embryonic structures, secreted proteins and myogenic induction. However, although different signaling molecules with activity in the process of somite patterning have been identified, not many of them are found to be necessary during in vivo embryonic development. To understand their functions, generation of multiple mutants or conditional/tissue-specific mutants will be necessary

Use of SNP-arrays for ChIP assays: computational aspects

The simultaneous genotyping of thousands of single nucleotide polymorphisms (SNPs) in a genome using SNP-Arrays is a very important tool that is revolutionizing genetics and molecular biology. We expanded the utility of this technique by using it following chromatin immunoprecipitation (ChIP) to assess the multiple genomic locations protected by a protein complex recognized by an antibody. The power of this technique is illustrated through an analysis of the changes in histone H4 acetylation, a marker of open chromatin and transcriptionally active genomic regions, which occur during differentiation of human myoblasts into myotubes. The findings have been validated by the observation of a significant correlation between the detected histone modifications and the expression of the nearby genes, as measured by DNA expression microarrays. This chapter focuses on the computational analysis of the data

Developmental biology: One source for muscle

Producing muscle as an embryo, and making or repairing it as an adult, could be considered to be quite different processes. But it seems that cells that share a common origin carry out both of these tasks

Muscle satellite cell and atypical myogenic progenitor response following exercise

Skeletal muscle satellite cells play an essential role in muscle regeneration and exercise adaptation. In recent years atypical myogenic progenitors (non-satellite-cell muscle stem cells) have been identified in skeletal muscle and have been hypothesized to play an important role in the process of muscle regeneration. It remains unknown, however, whether any populations other than satellite cells play a significant role in repair and adaptation following exercise-induced damage. We assessed the response of the satellite cell population and the CD45+:Sca-1+ cell population, previously shown to support muscle regeneration following cardiotoxin-induced injury, after acute eccentrically biased exercise in wild-type mice. We observed evidence of focal muscle damage and repair following the exercise protocol using electron microscopy, hematoxylin-eosin staining, and single-fiber analysis. In addition, we observed an approximately sixfold increase in the number of Myf5-expressing cells by 48 h, which remained elevated until at least 96 h following exercise. We did not, however, observe any significant expansion of the CD45+:Sca-1+ cell population or commitment of resident CD45+:Sca-1+ cells to the myogenic lineage. Furthermore, expression of Wnt gene family members, previously associated with myogenic specification of CD45+:Sca-1+ cells, did not differ following exercise. Therefore, we conclude that muscle satellite cells are the primary responders to exercise-induced stress and that the CD45+:Sca-1+ myogenic progenitors do not contribute to muscle repair/adaptation following exercise

The molecular regulation of muscle stem cell function

Muscle satellite cells are responsible for the postnatal growth and robust regeneration capacity of adult skeletal muscle. A subset of satellite cells purified from adult skeletal muscle is capable of repopulating the satellite cell pool, suggesting that it has direct therapeutic potential for treating degenerative muscle disease. Satellite cells uniformly express the transcription factor Pax7, and Pax7 is required for satellite cell viability and to give rise to myogenic precursors that express the basic helix-loophelic (bHLH) transcription factors Myf5 and MyoD. Pax7 activates expression of target genes such as Myf5 and MyoD through recruitment of the Wdr5/Ash2L/MLL2 histone methyltransferase complex. Extensive genetic analysis has revealed that Myf5 and MyoD are required for myogenic determination, whereas myogenin and MRF4 have roles in terminal differentiation. Using a Myf5-Cre knockin allele and an R26R-YFP Cre reporter, we observed that in vivo about 10% of satellite cells only express Pax7 and have never expressed Myf5. Moreover, we found that Pax7+/Myf5- satellite cells give rise to Pax7 +/Myf5+ satellite cells through basal-apical asymmetric cell divisions. Therefore, satellite cells in skeletal muscle are a heterogeneous population composed of satellite stem cells (Pax7 +/Myf5-) and satellite myogenic cells (Pax7 +/Myf5+). Evidence is accumulating that indicates that satellite stem cells represent a true stem cell reservoir, and targeting mechanisms that regulate their function represents an important therapeutic strategy for the treatment of neuromuscular disease

Paired sage-microarray expression data sets reveal antisense transcripts differentially expressed in embryonic stem cell differentiation

Serial Analysis of Gene Expression (SAGE) is a sequence-based measure of gene expression that provides quantitative information on the population of transcripts through the generation and counting of specific sequence tags. Many SAGE datasets are publicly available for analysis, constituting a valuable resource for the study of gene expression. These datasets contain tags that are not obviously derived from known transcripts and thus hint at the existence of a large number of novel transcripts; however, the prioritization of candidates for further experimental verification is difficult. Here we demonstrate a method to identify non-coding antisense transcripts which may be implicated in stem cell differentiation by combining SAGE data with gene expression data derived by a complementary method. We produced SAGE libraries and paired microarray gene expression data pre- and post-differentiation of three mouse stem cell types (embryonic, mammary and neural). We found 1,674 SAGE tags antisense to 1,351 protein coding genes. A majority of these antisense tags overlap the 3’UTRs of sense genes; their abundance correlates with the expression of the corresponding sense genes and appears to be tissue specific. We did not find significant association between the expression of these tags and alternative splicing. We measured the expression of three genes expressed in the mouse embryo (Zfp42/Rex1, Ywhag/14- 3-3g and Pspr1) and corresponding putative antisense transcripts by qPCR before and after differentiation of mESC. We conclude that it is possible to identify putative novel antisense transcripts with a potential role in ES cell differentiation by integrating data from existing SAGE libraries with expression data derived by a complementary method. All data used in this work are available from the Gene Expression Omnibus (GEO) and StemBase databases