Agenda

Skeletal muscles of the head and trunk arise from distinct lineages with different upstream regulatory programs converging on activation of myogenic determination factors of the MyoD family. Clonal analysis has shown that branchiomeric head and neck muscles share a common origin with second heart field cardiac progenitor cells in cardiopharyngeal mesoderm (CPM) associated with the pharyngeal arches of the early embryo. Thus, the first pharyngeal arches give rise to mastication muscles and right ventricle myocardium, the second pharyngeal arches form facial expression muscles as well as outflow tract myocardium and the posterior pharyngeal arches give rise to neck muscles including the trapezius muscle as well as myocardium of the atria and part of the outflow tract. While the trapezius muscle originates in CPM it extends deep into the trunk territory to coordinate head and trunk movements as well as shoulder mobility. The retinoic acid (RA) signalling pathway is required for normal deployment of cardiac progenitor cells from posterior CPM and blocking RA signalling during a defined early time window leads to conotruncal and atrial septation defects. Here I investigated the role of RA signalling during development of the trapezius neck muscle derived from posterior CPM. Blocking RA signalling pathway with a pan-RA receptor synthetic retinoic antagonist during an early developmental time-window around E8 results in strikingly selective loss of the trapezius muscle, without affecting other branchiomeric or somitic muscles. This reveals differences in the regulatory program driving muscle development in posterior and anterior pharyngeal arches. RA signalling acts downstream of CPM regulators such as Tbx1 but upstream of the myogenic determination factor MyoD. Despite being derived from CPM, RA signal reception is not required in the CPM lineage for trapezius muscle development. Firstly, cells responding to RA signalling around E8 give rise to non-myogenic cells intercalated between trapezius muscle fibres, including connective tissue and endothelial cells. The distribution of these cells resembles that of the Pax3 genetic lineage. Secondly, a genetic approach, using the truncated dominant negative RA receptor RARα403, revealed that trapezius muscle development requires RA signal reception not in CPM but in the Pax3 lineage, that includes somitic and neural crest cells. However, activation of the dominant negative RA receptor in neural crest cells using Wnt1-Cre does not lead to trapezius muscle defects, suggesting that RA signalling is required in the somitic lineage for trapezius muscle development.Pax3-Cre;RARα403 embryos also display defective occipital somites, and RNAscope in situ hybridisation reveals that anteriormost occipital somites and posterior CPM are juxtaposed at the time of trapezius muscle RA-sensitivity. In addition, I showed that Hoxa1 and Hoxb1 are not individually required for trapezius muscle development and that chromatin remodeller genes Chd7, associated with CHARGE syndrome, and Chd8, play a role in posterior pharyngeal arches myogenesis. This work uncovers how cross talk between different mesodermal cell populations at the head trunk interface orchestrates development of the trapezius neck muscle, with implications for muscle pathology and evolution of the vertebrate neck.