Mechanisms defining functional heterogeneity of anatomically distinct myogenic populations – MUSE

Dynamic (or live), confocal, and lightsheet imaging; transcriptional analyses at the single-cell or single-nucleus level (scRNA-seq, snRNA-seq); and genetically modified mouse models.

MUSE represented the first global analysis of gene regulatory networks operating in anatomically distinct muscles. These results allowed us to provide innovative insights into the molecular bases that govern the specialization of stem cells and muscle fibers in craniofacial muscles. Among other findings, we were able to show how extraocular muscle stem cells become specified during embryonic and postnatal development, and to uncover the cellular and molecular mechanisms that give them a greater capacity for proliferation ex vivo.

In addition, we characterized the regulatory program heterogeneity of muscle fibers across different craniofacial muscles compared to trunk muscles using single-nucleus sequencing, and we validated the evolutionary conservation of these molecular programs.

This knowledge may pave the way for new strategies to protect or repair muscles that are more vulnerable in various pathologies.

Skeletal muscle constitutes a major portion of the body mass and has emerged as a paradigm to study diverse aspects of stem cell biology, tissuegenesis, and regeneration. Muscle stem cells (MuSCs) are normally quiescent, but in response to injury, they proliferate, differentiate and fuse into myofibers to restore muscle function. An unexpected heterogeneity between head and trunk muscles exists in view of their embryological origins, regenerative capacity and variable sensitivity to different myopathies. However, the mechanisms conferring distinct properties to stem and committed muscle cells depending on their anatomical location remains unknown.
Cranial muscles, and specifically the extraocular muscles (EOM) and their resident MuSCs, have unique properties that distinguish them from other muscle groups. EOMs are spared in Duchenne muscular dystrophy, the most frequent neuromuscular disorder of childhood that causes progressive muscle wasting. Remarkably, some cranial and EOM MuSCs, outperform the limb counterparts in terms of expansion capacity and regenerative potential. EOM MuSCs and other cranial MuSC populations constantly proliferate and fuse with myofibers under basal conditions, in contrast to the quiescent state of limb MuSCs in uninjured muscles.

The global objective of MUSE is to provide an integrated view of muscle heterogeneity. Over the past years, I established an independent research program to identify intrinsic and extrinsic regulators that confer robust features to cranial muscles. With expertise in developmental, and more recently adult muscle biology, here I propose to test two main hypotheses that could explain the phenotypical differences between cranial and trunk muscles:
1) The embryological origins and molecular signatures of MuSCs and their progenitors define and impact the life-long performance of the respective myofibers.
2) Cells and signals from the local milieu modulate the properties of cranial muscles and MuSCs.
To do so, I will exploit a novel mouse reporter line and use complementary forefront technologies including live-imaging pipelines and unbiased genomic approaches (sc-RNAseq, sn-RNAseq). I will characterize MuSC lineage progression at the anatomical level and the crosstalk between MuSCs, myo?bers and their own local environment, using cranial (EOM and Esophagus) and limb (TA) muscles as comparative points. To our knowledge, this will constitute the first comprehensive analysis of the gene regulatory networks operating in anatomically distinct muscles. With these analyses, I aim to provide insights into the molecular underpinnings that control the differential susceptibility of muscles to disease and provide critical insights into the development of rational strategies for cell-based therapies.