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Disruption of the mouse MRF4 gene identifies multiple waves of myogenesis in the myotome
A Patapoutian1, J K Yoon, J H Miner
1Division of Biology 156-29, California Institute of Technology, Pasadena 91125, USA.
Abstract:
MRF4 (herculin/Myf-6) is one of the four member MyoD family of transcription factors identified by their ability to enforce skeletal muscle differentiation upon a wide variety of nonmuscle cell types. In this study the mouse germline MRF4 gene was disrupted by targeted recombination. Animals homozygous for the MRF4bh1 allele, a deletion of the functionally essential bHLH domain, displayed defective axial myogenesis and rib pattern formation, and they died at birth. Differences in somitogenesis between homozygous MRF4bh1 embryos and their wild-type littermates provided evidence for three distinct myogenic regulatory programs (My1-My3) in the somite, which correlate temporally and spatially with three waves of cellular recruitment to the expanding myotome. The first program (My1), marked initially by Myf-5 expression and followed by myogenin, began on schedule in the MRF4bh1/bh1 embryos at day 8 post coitum (E8). A second program (My2) was highly deficient in homozygous mutant MRF4 embryos, and normal expansion of the myotome failed. Moreover, expression of downstream muscle-specific genes, including FGF-6, which is a candidate regulator of inductive interactions, did not occur normally. The onset of MyoD expression around E10.5 in wild-type embryos marks a third myotomal program (My3), the execution of which was somewhat delayed in MRF4 mutant embryos but ultimately led to extensive myogenesis in the trunk. By E15 it appeared to have largely compensated for the defective My2 program in MRF4 mutants. Homozygous MRF4bh1 animals also showed improper rib pattern formation perhaps due to the absence of signals from cells expressing the My2 program. Finally, a later and relatively mild phenotype was detected in intercostal muscles of newborn animals.
Insights
Disrupting the MRF4 gene in mice caused severe defects in skeletal muscle development and rib formation, leading to embryonic lethality. This highlights MRF4
Area of Science:
- Developmental Biology
- Molecular Biology
- Genetics
Background:
- MRF4 (herculin/Myf-6) is a key transcription factor in the MyoD family, crucial for skeletal muscle differentiation.
- Understanding MRF4's role is vital for comprehending muscle development and associated congenital defects.
Purpose of the Study:
- To investigate the function of the mouse germline MRF4 gene by creating a targeted knockout.
- To elucidate the specific myogenic regulatory programs affected by MRF4 deficiency.
Main Methods:
- Targeted recombination was used to disrupt the mouse germline MRF4 gene, creating the MRF4bh1 allele.
- Embryonic development and gene expression patterns (Myf-5, myogenin, MyoD, FGF-6) were analyzed in homozygous MRF4bh1 mutants and wild-type littermates.
Main Results:
- Homozygous MRF4bh1 mutant embryos exhibited defective axial myogenesis and rib patterning, resulting in embryonic lethality at birth.
- A deficiency in the second myogenic program (My2) was observed, impairing normal myotome expansion and downstream gene expression.
- While the third program (My3) showed delayed onset, it eventually compensated for the My2 defect in trunk myogenesis.
Conclusions:
- MRF4 is essential for proper axial myogenesis and rib formation, likely through its role in regulating distinct myogenic programs.
- The study reveals three sequential myogenic programs (My1-My3) within the somite, with MRF4 critically regulating the My2 program.
- MRF4 deficiency leads to embryonic lethality due to severe developmental defects, underscoring its indispensable role in skeletal muscle development.