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Targeted inactivation of myogenic factor genes reveals their role during mouse myogenesis: a review
1Department of Cell and Molecular Biology, Technical University of Braunschweig, Germany.
Abstract:
The role of the four myogenic regulating genes Myf-5, myogenin, MyoD, and MRF4 (herculin, Myf-6) during mouse embryogenesis has been investigated by targeted gene inactivation. Null mutations for the MyoD gene generate no skeletal muscle phenotype due to a compensatory activation of the Myf-5 gene. Mice carrying a homozygous Myf-5 mutation exert considerably delayed myotome formation with unexpected consequences. While skeletal myogenesis in these mutant mice resumes normally at the onset of MyoD expression, a skeletal defect of the ribs persists. Apparently, Myf-5 and MyoD individually are not absolutely essential for skeletal muscle development, most likely because they have overlapping or redundant functions. In fact, double mutants lacking both, MyoD and Myf-5, fail to develop skeletal musculature and the muscle forming regions seem to be devoid of myoblasts. Homozygous inactivation of the myogenin gene leads to drastically reduced myofiber formation. These mice accumulate apparently normal numbers of myoblasts which are arrested in their terminal differentiation program. Myf-6 null mutant mice exhibit drastically reduced expression of Myf-5 for reasons presently unknown. The phenotype is very similar to Myf-5 mutants with an additional reduction of deep back muscles and minor alterations in sarcomeric protein isoforms. Based on the phenotypes obtained from these various gene "knock-out" mice, we now begin to understand the regulatory network and the homostatic relationship of genes which are critically involved in myogenesis of vertebrates.
Insights
Investigating myogenic regulatory genes in mice reveals functional redundancy between Myf-5 and MyoD. Gene inactivation studies show Myf-5 and MyoD are not essential individually but crucial together for skeletal muscle development.
Area of Science:
- Developmental Biology
- Genetics
- Molecular Biology
Background:
- Myogenic regulatory factors (MRFs) are transcription factors essential for skeletal muscle development.
- Four key MRFs, Myf-5, myogenin, MyoD, and MRF4, orchestrate myogenesis.
- Understanding their precise roles and interactions is crucial for comprehending vertebrate muscle formation.
Purpose of the Study:
- To investigate the specific roles of Myf-5, myogenin, MyoD, and MRF4 in mouse embryogenesis.
- To elucidate the functional redundancy and regulatory network among these myogenic genes.
- To determine the consequences of targeted gene inactivation on skeletal muscle development.
Main Methods:
- Targeted gene inactivation (gene knockout) in mice.
- Analysis of skeletal muscle development and myoblast differentiation in mutant embryos.
- Phenotypic characterization of homozygous and double mutant mice.
Main Results:
- MyoD null mutants show no phenotype due to compensatory Myf-5 activation.
- Myf-5 mutants exhibit delayed myotome formation and persistent rib defects.
- Double MyoD/Myf-5 mutants lack skeletal musculature, indicating essential combined function.
- Myogenin inactivation arrests myoblasts, preventing myofiber formation.
- Myf-6 inactivation affects Myf-5 expression and causes muscle defects.
Conclusions:
- Myf-5 and MyoD possess overlapping functions, making them individually non-essential but collectively critical for myogenesis.
- Myogenin is indispensable for terminal differentiation of myoblasts into myofibers.
- The study reveals a complex regulatory network and homeostatic relationships among myogenic genes in vertebrates.