Traumatic muscle injury promotes aberrant fibro-adipogenic progenitor cell population, leading to fibrosis and impaired muscle stem cell function
Authors: Morgan Nyssa , Zhang Hongmanlin , Anderson Shannon , Jang Young 
Published: May 2023

Skeletal muscle has an exceptional regenerative capacity that relies on the temporal activation and regulation of immune cells, fibro-adipogenic progenitors (FAPs), and muscle stem cells (MuSCs). However, following traumatic injury to skeletal muscle, such as in extensive muscle deficits caused by injury or surgery, also known as volumetric muscle loss (VML), this coordinated regenerative response is diminished, leading to fibrosis, inflammation, and chronic functional deficiencies. The goal of this study was to compare the temporal response and phenotype of key cell populations, including macrophages, FAPs, and MuSCs following critical and sub-critical VML injuries in the mouse quadriceps. We hypothesize that traumatic injury, specifically VML, drives cell phenotypic variability amongst these cell populations. Unilateral VML injuries were used to quantify cell type distribution via flow cytometry at 1, 3, and 7 days post-VML (pVML, n=4 per injury size and time point). At day 7 pVML, anti-inflammatory M2 macrophages, FAPs, and MuSCs were all present in significantly elevated numbers per tissue volume in critical (3-mm) injuries compared to sub-critical (2-mm) injuries (p<0.05). Using multi-dimensional flow cytometry analysis, a distinct sub-population of FAPs was identified in critically sized VML injuries which had larger cell content and highly expressed the β1-integrin cell-surface marker. FAPs from quadriceps at day 7 pVML (VML-FAPs) were purified using fluorescence-activated cell sorting and cultured in vitro. VML-FAPs exhibited increased proliferation, increased cell area, and showed elevated levels of β1-integrin expression. In co-culture experiments, VML-FAPs were also shown to significantly decrease the myogenic capacity of primary MuSCs by quantification of myotube formation (p<0.05). When cultured with the anti-inflammatory cytokine TGF-β1, VML-FAPs more readily differentiated down a fibroblastic lineage. Collectively, these results indicate that following traumatic injury, namely VML, there is an aberrant shift in the FAPs cell population, possibly driven by the accumulation of M2 macrophages, towards a lineage which is persistent, pro-fibrotic, and anti-myogenic. The identification of this shift in VML-FAPs will be key to the development of pro-regenerative therapeutics for VML, as success of these VML-FAP interventions will rely on the ability to downregulate the fibrotic lineage of FAPs while maintaining their traditional pro-myogenic qualities during regeneration. Furthermore, this study also provokes investigation of differing FAP phenotypes in other traumatic muscle injuries to identify possible trends or incongruencies that may exist. Funding: This work was supported by the National Institute of Health under award number R01AG072309, R01AR071708, and Department of Defense under award number W81XWH-20-1-0336. This is the full abstract presented at the American Physiology Summit 2023 meeting and is only available in HTML format. There are no additional versions or additional content available for this abstract. Physiology was not involved in the peer review process.