Characterizing Heterogeneity in the Regenerative Capacity of Human Muscle Progenitor Cells

Abstract

Skeletal muscle regeneration following muscle damage is imperative to maintain skeletal muscle structure and function throughout the lifespan. Regeneration requires a complex series of events including activation of skeletal muscle specific stem cells (satellite cells) followed by proliferation and differentiation of committed myoblasts [muscle progenitor cells (MPCs)], and formation/repair of functional multinucleated muscle cells. Activation of satellite cells and expansion of MPCs is essential to generate a sufficient number of viable cells to repair damaged skeletal muscle. In animal models, age is associated with reduced MPC expansion capacity in vitro. Additionally, male mouse-MPCs (mMPCs) exhibit higher proliferation rates than female mMPCs. However, the impact of age and sex on expansion of human MPCs (hMPCs) remains unknown. To evaluate the age- and sex-related differences in expansion capacity of primary hMPCs, we compared markers of expansion between young and old, male and female primary hMPC cultures. hMPCs from older males have reduced expansion capacity compared to their younger counterparts, but hMPCs from females are largely unaffected by age. Proliferating MPCs have high energetic and biosynthetic material requirements, and the ability to utilize oxidative phosphorylation (OXPHOS) and/or glycolysis may affect the expansion capacity of MPCs. Results from our study suggest that reduced expansion capacity in OM-hMPCs is accompanied by alterations in measurements of OXPHOS while glycolysis is maintained. However, we and others have demonstrated that not all old hMPCs have impaired expansion capacity and similarly, not all young hMPCs expand better than old hMPCs. Using K-means cluster analysis and measurements of culture population doubling time and saturation density, we are the first to unbiasedly cluster cultures with similar growth parameters and to identify drivers of expansion capacity that weren’t related to categorical age or sex. Our primary findings demonstrate that cultures with enhanced expansion capacity have DEgenes enriched in functional classifications, pathways and networks that suggest promotion of the cell cycle, reduced apoptosis and cellular senescence, and enhanced DNA replication. Acute inflammation following muscle injury is essential to activate satellite cells. However, chronic muscle inflammation likely contributes to impaired regenerative capacity in skeletal muscle. Previous research has identified muscle inflammatory susceptibility, or the ability to manage and respond to inflammation, as a predictor of failed muscle regeneration and regrowth following surgery. We determined a transcript profile that distinguishes human muscle progenitor cell (hMPC) cultures with high and low inflammatory susceptibility. DEgene enrichment suggested that MuIS+ cells had promotion of inflammatory pathways and inhibition of muscle differentiation pathways. Novel (KISS1) and known (SMARCA4, MYOD1, IL1β) genes emerged as regulators for identified functional pathways. When compared to other transcriptomics datasets, MuIS+ cultures share overlap in transcript profiles to both individuals who do not respond to progressive resistance exercise training and to older adults with diminished muscle strength. This dissertation highlights the heterogeneity that exists between humans throughout the muscle regenerative process. Uncovering the molecular underpinnings of this heterogeneity is essential as the field of medicine moves towards a personalized approach.