Date of Completion

2-3-2014

Embargo Period

2-2-2014

Keywords

MyoD, myogenesis, core enhancer, distal regulatory region, embryo, determination

Major Advisor

David J. Goldhamer

Co-Major Advisor

Michael O'Neill

Associate Advisor

Charles Giardina

Associate Advisor

David Knecht

Associate Advisor

Joseph LoTurco

Field of Study

Genetics and Genomics

Degree

Doctor of Philosophy

Open Access

Open Access

Abstract

In the developing vertebrate embryo, multipotent cells of the paraxial mesoderm form epithelial spheres called somites that physically define the segmentation pattern of the embryo. MyoD is transcribed in the myogenic precursor cells of the developing somite and is essential for proper hypaxial muscle formation. Whether MyoD is a determination factor or downstream member of the myogenic program had yet to be shown and the cis-transcriptional control of this important gene was incompletely defined.

By permanently labeling cells that have transcribed the MyoD locus in MyoD-/-/Myf-5-/- embryos, where myogenesis does not occur, the determination state of presumptive myoblasts has been revealed. Previous studies used a temporary cell labeling system and suggested that in MyoD-/-/Myf-5-/- embryos, cells of the MyoD lineage apoptose and do not contribute to other cell types. This suggests that the MyoD lineage is committed to myogenesis before myoblasts form. In the results presented herein, cells that have activated the MyoD locus persist until birth and contribute to bone, brown adipose tissue and connective tissue demonstrating the multipotent nature of premyogenic cells prior to MyoD activation.

The genetic regulatory elements controlling MyoD expression were thought to be the core enhancer (CE) and distal regulatory region (DRR), where the CE initiates MyoD expression and the DRR maintains it. Deletion of either the CE or DRR from the mouse genome resulted in only mild phenotypes and suggested more complexity in MyoD regulation. Here, we deleted both the CE and DRR to determine if the enhancers have compensatory abilities or if other unknown regulatory elements exist. In situ hybridization for MyoD mRNA in our new line of mouse embryos shows that removal of both enhancers does not seriously alter the MyoD expression profile. The phenotypes seen in the individual knockout embryos are both present when the CE and DRR are removed. Genomic database analysis implicates the introns of MyoD as the uncharacterized enhancers.

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