Three Distinct Cell Populations Express Extracellular Matrix Proteins

21 Tissue extracellular matrix provides structural support and creates unique environments 22 for resident cells (4, 27). However, the identities of cells responsible for creating specific ECM 23 components have not been determined. In striated muscle, the identity of these cells becomes 24 important in disease when ECM changes result in fibrosis and subsequent increased tissue 25 stiffness and dysfunction. Here we describe a novel approach to isolate and identify cells that 26 maintain the ECM in both healthy and fibrotic muscle. Using a collagen I reporter mouse, we 27 show that there are three distinct cell populations that express collagen I in both healthy and 28 fibrotic skeletal muscle. Interestingly, the number of collagen I expressing cells in all three cell 29 populations increase proportionally in fibrotic muscle indicating that all cell types participate in 30 the fibrosis process. Furthermore, while some profibrotic ECM and ECM-associated genes are 31 significantly upregulated in fibrotic muscle, the fibrillar collagen gene expression profile is not 32 qualitatively altered. This suggests that muscle fibrosis in this model results from an increased 33 number of collagen I expressing cells and not the initiation of a specific fibrotic collagen gene 34 expression program. Finally, in fibrotic muscle, we show that these collagen I expressing cell 35 populations differentially express distinct ECM proteins – fibroblasts express the fibrillar 36 components of ECM, fibro/adipogenic progenitors cells differentially express basal laminar 37 proteins and skeletal muscle progenitor cells differentially express genes important for the 38 satellite cell. 39 40