UC Davis

Muscle regeneration, an essential homoeostatic phenomenon, entails the process of quiescent muscle stem cells (MuSCs) undergoing activation, proliferation, migration, differentiation, and fusion to myotubes [1]–[3]. MuSCs can sense and respond to the mechanical microenvironment of the tissue while migrating through the interstitial matrix [4], [5]. The regeneration of muscle fibers is impaired in fibrotic diseases where the interstitial matrix volume increases, such as Duchenne muscular dystrophy (DMD). In fibrosis the mechanical microenvironment of the tissue is modified [6]–[9]. The extracellular matrix (ECM) has an increased stiffness and decreased porosity [10]–[12]. This constrained environment has been found to modulate the differentiation ability as well as nuclear integrity of MuSCs [13]. The migration patterns and frequency of nuclear rupture/DNA damage of MuSCs through a constrained environment are not well understood. I propose that satellite cells are blocked by the constrictions made by fibrotic tissue. In this thesis, we demonstrate that the capacity of human myoblasts to undergo constricted migration through small rigid pores is hindered and only those cells that are highly migratory can cross them.In normal tissue, myoblasts undergo morphological changes and may increase collagen cross- links and ECM remodeling to propel themselves forward. However, in fibrotic conditions, the ECM architecture is already modified, and it is characterized by excessive collagen production and an increase in cross-linking [11]. ECM architecture modulates stem cell differentiation and proliferation, but the effect of migration is unknown [14]. I hypothesized that in ECM architecture mimicking fibrosis myoblast migration will be compromised, leading to reduced speed. In this thesis, we demonstrate the differential effects of different 2D and 3D matrices on myoblast migration. From our results, we can look at the constrictive environment with modified ECM architecture presented by muscle fibrosis, as a potential biomechanical target, and a critical aspect in defining treatment approaches for fibrotic skeletal muscle diseases such as DMD.