Use of a conditional MyoD transcription factor in studies of MyoD trans-activation and muscle determination

Myogenin is required for late but not early aspects of myogenesis during mouse development

Mice with a targeted mutation in the myogenic basic helix-loop-helix regulatory protein myogenin have severe muscle defects resulting in perinatal death. In this report, the effect of myogenin's absence on embryonic and fetal development is investigated. The initial events of somite differentiation occurred normally in the myogenin-mutant embryos. During primary myogenesis, muscle masses in mutant embryos developed simultaneously with control siblings, although muscle differentiation within the mutant muscle masses was delayed. More dramatic effects were observed when secondary myofibers form. During this time, very little muscle formation took place in the mutants, suggesting that the absence of myogenin affected secondary myogenesis more severely than primary myogenesis. Monitoring mutant neonates with fiber type-specific myosin isoforms indicated that different fiber types were present in the residual muscle. No evidence was found to indicate that myogenin was required for the formation of muscle in one region of the embryo and not another. The expression patterns of a MyoD-lacZ transgene in myogenin-mutant embryos demonstrated that myogenin was not essential for the activation of the MyoD gene. Together, these results indicate that late stages of embryogenesis are more dependent on myogenin than early stages, and that myogenin is not required for the initial aspects of myogenesis, including myotome formation and the appearance of myoblasts.

Role of tyrosine kinase in the regulation of myogenin expression

Using an affinity-purified anti-myogenin antibody, three stages of mouse myoblast C2C12 cells during myogenesis could be identified: proliferating myoblasts as myogenin-negative mononucleated cells, differentiating myoblasts as myogenin-positive mononucleated cells, and myotubes as myogenin-positive multinucleated cells. We found differential effects of genistein, an inhibitor of protein-tyrosine kinase, on myogenic cells during these three stages. Genistein severely inhibited myotube formation and myogenin production in differentiating myoblasts by inhibiting the transcription of the myogenin gene in a dose-dependent manner. We also found that genistein inactivated mitogen-activated protein kinase (MAP kinase) accompanied by suppression of myogenin expression. In contrast, genistein failed to inactivate MAP kinase and eliminate myogenin from myotubes. The results suggest that protein-tyrosine kinase plays a role in the transcriptional regulation of myogenin through the MAP kinase cascade during myogenesis. Furthermore, genistein inhibited the transactivation of the myosin heavy chain gene by constitutively expressed myogenin. Therefore, it is suggested that protein-tyrosine kinase is involved in the post-translational regulation of myogenin as well as in transcriptional regulation during myogenesis.

Epidermal cell fate determination in Arabidopsis: patterns defined by a steroid-inducible regulator

The Arabidopsis mutant ttg lacks both trichomes (epidermal hairs) and anthocyanin pigments. Trichomes and anthocyanins are restored by the constitutive expression of the maize transcriptional regulator (R). The expression of an R-glucocorticoid receptor chimeric protein results in a steroid hormone-dependent, conditional allele of R that functions in whole Arabidopsis plants. The response of the chimeric protein to pulses of hormone was used to define the pattern and timing of trichome formation on the developing leaf epidermis. Each adaxial epidermal leaf cell appears to have an equal probability of differentiating into a trichome; there is a temporal zone of decision for trichome cell fate that proceeds as a wave from the tip to the base of developing leaves.

Myogenic specification in somites: induction by axial structures

Specification of the myogenic phenotype in somites was examined in the early chick embryo using organotypic explant cultures stained with monoclonal antibodies to myosin heavy chain. It was found that myogenic specification (formation of muscle fibers in explants of somites or segmental plates cultured alone) does not occur until Hamburger and Hamilton stage 11 (12-14 somites). At this stage, only the somites in the rostral half of the embryo are myogenically specified. By Hamburger and Hamilton stage 12 (15-17 somites), the three most caudal somites were not specified to be myogenic while most or all of the more rostral somites are specified to myogenesis. Somites from older embryos (stage 13–15, 18–26 somites) showed the same pattern of myogenic specification--all but the three most caudal somites were specified. We investigated the effects of the axial structures, the notochord and neural tube, on myogenic specification. Both the notochord and neural tube were able to induce myogenesis in unspecified somites. In contrast, the neural tube, but not the notochord, was able to induce myogenesis in explants of segmental plate, a structure which is not myogenic when cultured alone. When explants of specified somites were stained with antibodies to slow or fast MyHC, it was found that myofiber diversity (fast and fast slow fibers) was established very early in development (as early as Hamburger and Hamilton stage 11). We also found fiber diversity in explants of unspecified somites (the three most caudal somites from stage 11 to 15) when they were recombined with notochord or neural tube. We conclude that myogenic specification in the embryo results in diverse fiber types and is an inductive process which is mediated by factors produced by the neural tube and notochord.

bHLH factors in muscle development: dead lines and commitments, what to leave in and what to leave out

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