Towards a better understanding of Twist2+ myogenic progenitors within human skeletal muscle

Human skeletal muscle is the engine of locomotion and serves as an integral regulator of metabolic and endocrine actions. Muscle tissue has a remarkable repair capacity following injury owing to the activity of its endogenous stem cell population termed satellite cells. Since skeletal muscle is post-mitotic, satellite cells serve as a reservoir of new myonuclei to regenerate muscle fibers following damage and facilitate muscle growth in response to increase load (i.e., resistance exercise). While the functional importance of satellite cells is well appreciated, recent data in rodents has identified a separate muscle progenitor cell pool that are anatomically and transcriptionally distinct from satellite cells and are identified by the expression of the transcription factor Twist2. Twist2+ cells specifically contribute to type IIb/x muscle fibers during maintenance and repair following damage. However, little is known of the presence or function of Twist2+ cells in humans. To this end, using a combination of single-cell RNA sequencing (scRNAseq) and immunofluorescence, we have recently described a population of Twist2 expressing cells in humans that are transcriptional and anatomically similar to mouse Twist2+ cells. Human Twist2+ cells reside in the interstitium and do not express satellite cell markers but do express the mesenchymal progenitor cell marker PDGFRa. We also described that Twist2+ cells are more abundant in the skeletal muscles of older compared to younger individuals. To date, however, it is unknown if human Twist2+ cells respond to muscle damage, have myogenic capacity or are functionally influenced by age. The current proposal, which will serve as the basis of my research program for the next five years, where we will perform a series of studies that will utilize vitro and in vivo approaches to interrogate the function, activity and abundance of Twist2+ cells in human skeletal muscle. Specific aim 1 will utilize immunofluorescence and single-nucleus RNA sequencing (snRNAseq) to quantify and examine the transcriptional profile to Twist2+ cells following muscle damage in young and older individuals. Specific aim 2 will utilize cell sorting to isolate an enriched population of Twist2+ cells and test their in vitro myogenic capacity. Further, we will also probe the function to Twist2, acting as a transcription factor to regulate myogenesis, by using knock down and overexpression approaches. Taken together, these experiments will significantly expand our knowledge of the mechanisms governing human skeletal muscle repair and the impact of aging. Importantly, this work will provide meaningful and exciting research opportunities to enrich the training of students and HQP.