Relationship between dorsal muscle progenitors and adult musculature

Abstract

The determination and specification of skeletal muscle in vertebrates is orchestrated by the myogenic regulatory factors (MRFs): Myf5, Mrf4, MyoD and Myogenin. Myf5 is the first to be expressed in the embryo, initiating and co-ordinating the myogenic cascade. In absence of Myf5 progenitors fail to be specified at the correct developmental stage. Activation of MyoD rescues the phenotype and myogenesis progresses. In Myf5/MyoD KO animals rescue does not take place and animals lack all skeletal muscle. We and others have shown that Myf5 transcription is controlled by over 25 regulatory elements, most of them drive expression in a specific spatiotemporal context. The Early Epaxial Enhancer (EEE) operates in the dermomyotomal dorsomedial lip (DML) and is the first enhancer to activate Myf5 expression at E8.5 Although the contribution of the different regulatory elements to the expression pattern is well defined, we still lack an understanding on the contribution of the different subpopulations of muscle progenitor cells to adult musculature. Furthermore, there is still no connection between the spatiotemporal activation of Myf5 and its function within the particular set of myogenic precursors in which it is active. In order to address these questions we are following two strategies: 1/ Generation of transgenic strains to analyse the function of the EEE by means of lineage tracing and cell ablation experiments and 2/ Generation of a new mouse knockout strain in which the EEE has been targeted. Comparison of Evolutionary Conserved Regions between different species shows two highly conserved peaks within the experimentally defined EEE element (EEE1 and EEE2). As these could represent different functions within the enhancer, we have cloned each peak in our standard vector and used to generate transgenic animals. Preliminary data show that EEE2 drives reporter gene expression at E8.5 and is turned off at E9.5, while EEE1 activates gene expression at E9.5. We are now transferring these refined enhancers into cell ablation and lineage tracing constructs. We are in the process of analyzing the data obtained by RNAseq from wild type and Myf5EEE-/EEE- embryos in order to determine the downstream targets of Myf5 in the DML. Crucially, we have crossed this new allele into the MyoD KO and show that in the absence of MyoD rescue, some muscles are lost (or severely reduced), linking for the first time a particular set of progenitors to specific adult muscles.