A bovine homolog to the human myogenic determination factor myf-5: sequence conservation and 3' processing of transcripts

Linkage mapping and comparative analysis of bovine expressed sequence tags (ESTs)

Bovine expressed sequence tags (ESTs) containing microsatellites are suitable markers for both linkage and comparative maps. We isolated clones from a bovine fetal thigh skeletal muscle cDNA library that were positive for a (CA)10 probe. Thirty individual clones were isolated and characterised by sequencing. Sequences from the 5' and 3' ends of a clone were considered as separate ESTs until a contiguous sequence was identified. A total of 47 ESTs were sequenced from the 5' and/or 3' ends and full sequence was obtained for the 30 clones. BLAST nucleotide analysis identified significant homology to known mammalian coding regions for 31 of the bovine ESTs, 30 of which also matched human ESTs or sequence-tagged sites (STS). The remaining 16 bovine ESTs represented novel transcripts. Microsatellites were isolated in 27 of the ESTs, 11 of which were developed into markers and placed on the MARC bovine linkage map. Human cytogenetic map positions were available for 20 of the 30 human EST orthologs, and a putative bovine map position for 17 of the sequences could be inferred using comparative mapping data. These results demonstrated that mapping bovine ESTs containing microsatellites is a plausible strategy to increase the density of gene markers on the bovine linkage and comparative maps.

A genetic linkage map for cattle

We report the most extensive physically anchored linkage map for cattle produced to date. Three-hundred thirteen genetic markers ordered in 30 linkage groups, anchored to 24 autosomal chromosomes (n = 29), the X and Y chromosomes, four unanchored syntenic groups and two unassigned linkage groups spanning 2464 cM of the bovine genome are summarized. The map also assigns 19 type I loci to specific chromosomes and/or syntenic groups and four cosmid clones containing informative microsatellites to chromosomes 13, 25 and 29 anchoring syntenic groups U11, U7 and U8, respectively. This map provides the skeletal framework prerequisite to development of a comprehensive genetic map for cattle and analysis of economic trait loci (ETL).

Expression of bovine myf5 induces ectopic skeletal muscle formation in transgenic mice

myf5 is one of a family of four myogenic determination genes that control skeletal muscle differentiation. To study the role of myf5 in vivo, we generated transgenic mice harboring the bovine homolog, bmyf, under control of the murine sarcoma virus promoter. Ectopic expression of the full-length bmyf transgene was detected in brain and heart tissue samples of F1 progeny from transgenic founder mice. Ectopic bmyf expression activated endogenous skeletal myogenic determination genes in the hearts and brains of transgenic animals. Incomplete skeletal myogenesis in most hearts gave rise to cardiomegaly and focal areas of cardiomyopathy. In brains in which ectopic expression led to a more complete myogenesis, focal areas of multinucleated, striated myotubes containing actin, desmin, and myosin were observed. These unexpected results show that myf5 can initiate myogenic differentiation in vivo, supporting the hypothesis that myf5 is responsible for determination of cells to the myogenic lineage in normal embryogenesis.

Expression of bovine myf5 induces ectopic skeletal muscle formation in transgenic mice

myf5 is one of a family of four myogenic determination genes that control skeletal muscle differentiation. To study the role of myf5 in vivo, we generated transgenic mice harboring the bovine homolog, bmyf, under control of the murine sarcoma virus promoter. Ectopic expression of the full-length bmyf transgene was detected in brain and heart tissue samples of F1 progeny from transgenic founder mice. Ectopic bmyf expression activated endogenous skeletal myogenic determination genes in the hearts and brains of transgenic animals. Incomplete skeletal myogenesis in most hearts gave rise to cardiomegaly and focal areas of cardiomyopathy. In brains in which ectopic expression led to a more complete myogenesis, focal areas of multinucleated, striated myotubes containing actin, desmin, and myosin were observed. These unexpected results show that myf5 can initiate myogenic differentiation in vivo, supporting the hypothesis that myf5 is responsible for determination of cells to the myogenic lineage in normal embryogenesis.

Isolation, sequence, and characterization of the bovine myogenic factor-encoding gene myf-5

We report here the isolation, sequencing, and characterization of the bovine homologue of the myogenic factor-encoding gene myf-5 (termed bmyf). The transcription unit of the gene is 4.2 kb in length and contains two introns. Primer extension experiments mapped the transcription start point at 158 bp upstream from the predicted start codon and 29 bp downstream from a canonical TATA box. The 3' untranslated region contains four AATAAA sites, three of which are used to produce transcripts of 1.5, 2, and 3 kb in bovine fetal muscle RNA.

Delayed somite formation in a quail line exhibiting myofiber hyperplasia is accompanied by delayed expression of myogenic regulatory factors and myosin heavy chain

A myofiber hyperplastic quail line P has been developed through selection for heavy body weight. Since the number of muscle fibers is determined early in development and skeletal muscle originates from somites, we compared somite formation and muscle-specific gene expression in P- and control C-line quail embryos. At 47 hours of incubation, C embryos had 18 somite pairs and P embryos had 14.3. By 72 and 120 hours, both lines appeared to be at the same stage of somite development. To determine whether the delay in the formation of the brachial somites was accompanied by alterations in muscle-specific gene expression, we conducted whole-mount in situ hybridization and immunofluorescence studies. At 47 hours of incubation, C embryos were expressing qmf1 in the first 12 somites, while in P embryos only the first 7 somites showed qmf1 activation. Delays in expression were also observed for qmf3 at 43 hours and for all three myogenic factors (qmf1, qmf2 and qmf3) at 60 hours. At 65 hours, C embryos expressed myosin heavy chain in the first 15 somite pairs and P embryos in the first 7. At 72 hours, the transient delay in somite formation had disappeared and there was no lag in myosin heavy chain expression between the lines. The phase delay in brachial somite formation, myogenic factors and myosin heavy chain expression may be associated with the observed myofiber hyperplasia in P-line quail by allowing an increase in the muscle stem cell population.

The MyoD family of myogenic factors is regulated by electrical activity: isolation and characterization of a mouse Myf-5 cDNA

A full-length cDNA coding for a homolog of the human Myf-5 was isolated from a BC3H-1 mouse library and characterized. The clone codes for a protein of 255 amino acids that is 89%, 88% and 68% identical to the human, bovine and Xenopus myf-5, respectively. The mouse Myf-5 cDNA (mmyf-5), as well as sequences coding for MyoD, myogenin and Mrf-4, were used to probe Northern blots to analyze the effects of innervation on the expression of the MyoD family of myogenic factors. Mouse myf-5, MyoD and myogenin mRNAs levels were found to decline in hind limb muscles of mice between embryonic day 15 (E15) and the first postnatal week, a period that coincides with innervation. In contrast, Mrf-4 transcripts increase during this period and reach steady-state levels by 1-week after birth. To distinguish if the changes in myogenic factor expression are due to a developmental program or to innervation, mRNA levels were analyzed at different times after muscle denervation. Mmyf-5 transcripts begin to accumulate 2 days postdenervation; after 1 week levels are 7-fold higher than in innervated muscle. Mrf-4, MyoD and myogenin transcripts begin to accumulate as soon as 8h after denervation, and attain levels that are 8-, 15- and 40-fold higher than found in innervated skeletal muscle, respectively. The accumulation of these three mRNAs precedes the increase of nicotinic acetylcholine receptor alpha subunit transcripts, a gene that is transcriptionally regulated by MyoD-related factors in vitro. Using extracellular electrodes to directly stimulate in situ the soleus muscle of rats, we found that 'electrical activity' per se, in absence of the nerve, represses the increases of myogenic factor mRNAs associated with denervation.

Regulation of muscle cell growth and differentiation by the MyoD family of helix-loop-helix proteins

The skeletal muscle cell system provides a powerful model for exploring the mechanistic basis for the antagonism between cell growth and differentiation. The recent identification of the MyoD family of muscle-specific transcription factors now offers opportunities to dissect at the molecular level of the mechanisms through which defined cell type-specific transcription factors can activate an entire differentiation program as well as to unravel the mechanisms through which growth factor and oncogenic signals can disrupt cellular differentiation. Because the mechanisms for growth factor signaling and induction of cell proliferation are conserved in diverse cell types, it is tempting to speculate that the molecular mechanisms responsible for the antagonism between cell proliferation and differentiation in muscle cells are also operative in other cell types. Resolution of this question, however, must await identification of the regulatory factors that specify cell fate in other lineages.