Examining the roles of resident non-myogenic mesenchymal cell populations in skeletal muscle development and injury

Abstract

The success of muscle development and regeneration requires cooperation from both myogenic and their supportive niche cells. The muscular niche is complex. At the cellular level it is composed of a broad number of cell types including: endothelial vessels, nerve and nerve-supporting cells, resident immune populations, and a heterogenous group of non-myogenic mesenchymal cells. The non-myogenic mesenchymal cells include pericytes, vascular smooth muscle cells, interstitial tenocyte-like cells, and fibro-adipogenic progenitors (FAPs). Like all members of the muscular niche, this fraction is vital to muscle development and regeneration. Despite their importance to muscle development, regeneration, and homeostasis, detailed identities within non-myogenic mesenchymal cells remain elusive. By understanding the distinct makeup of this population, we can provide a foundation to examine their important regulatory roles in the processes of muscle development, homeostasis, injury and disease. This thesis utilizes single cell RNA sequencing to establish the populations of non-myogenic mesenchymal cells in developing muscle. Our analysis identified pericytes, vascular smooth muscle cells, and tenocyte-like cell populations while uncovering a new level of heterogeneity in FAPs that not previously appreciated. Despite classical understanding of FAPs as one group, this work found that FAPs were sub-divided into five distinct populations, which compose two trajectories spawning from a common progenitor. This thesis defines the functional differences of each FAP population through a series of experiments including: fluorescence activated cell sorting of various FAP groups, studying their spatial localization on immunofluorescence, and testing the response of different FAPs to multiple injury and disease models. Separate preliminary work examines the impact of NOTCH signaling in FAPs and the broader non-myogenic mesenchymal cell groups. These studies discovered that NOTCH signaling in the non-myogenic mesenchymal group, but not FAPs specifically, regulates muscular growth and intramuscular adipogenesis. Altogether this thesis advances our understanding of the identity, and role of, non-myogenic mesenchymal cells, in muscle development and regeneration.