Regulating the regulators of skeletal muscle development
We have had a long-standing interest in understanding the transcriptional control mechanisms that regulate expression of the muscle lineage-determining gene, MyoD. These studies, which utilized transgenic and knockout strategies in the mouse to interrogate regulatory sequences that control MyoD expression in the embryo, have provided key insights into how MyoD transcriptional activation functions as a developmental switch to commit multipotent somite cells to the skeletal muscle lineage. More recently, we have used gene targeting to knock out the two known enhancers that regulate MyoD, the core enhancer and DRR. Surprisingly, these studies showed that both enhancers are dispensable for muscle specific expression of MyoD in the embryo and in muscle stem cells. PRO-seq analyses (in collaboration with our colleague in MCB, Dr. Leighton Core) identified novel putative regulatory sequences that we are now evaluating in transgenic mice. Future studies will address potential genetic interactions with the core enhancer and DRR and mechanisms of activation of these novel regulatory sequences.
The cellular and molecular basis of heterotopic ossification and muscle degeneration in fibrodysplasia ossificans progressiva (FOP)
Heterotopic ossification (HO) is a debilitating condition that is characterized by the growth of bone at extraskeletal soft tissue sites. The most extreme example of HO is manifested in FOP, a rare, autosomal-dominant genetic disorder in which heterotopic bone forms progressively throughout the life of the individual, typically beginning in early childhood. The cumulative effects of HO result in muscle loss, ankylosis of major joints, and other complications that cause pain, severely restrict mobility, and reduced life expectancy. We developed an accurate genetic mouse model of FOP to discover the causative cells of HO and to investigate pathophysiological mechanisms of the disease at the tissue, cellular, and molecular mechanisms. Our work showed that fibro-adipogenic progenitors (FAPs) – multipotent cells resident in the muscle interstitium – are key causal cells of HO. Current projects include, but are not limited to, genomics studies of FAP reprogramming, mechanisms of muscle degeneration, the roles of activin A in disease progression, and development and evaluation of therapeutic strategies to mitigate disease progression. These projects include both fundamental investigations of basic mechanisms and pre-clinical studies, sometimes in collaboration with industry partners.
Mechanisms of muscle stem cell programming
Muscle stem cells (satellite cells), which reside beneath the basal lamina of muscle fibers, are essential for regeneration of injured skeletal muscle. Satellite cells are uniquely endowed with the capacity to regenerate muscle and many aspects of regeneration are related to muscle development during embryogenesis. We showed that the muscle determining genes, MyoD and Myf5, which play essential roles in embryonic and fetal myogenesis, are also required for muscle regeneration. Using a conditional knockout of MyoD that we developed, we showed that muscle loses all regenerative capacity when MyoD and Myf5 are knocked out in satellite cells. Lineage tracing showed that satellite cell descendants persist in the injured muscle but cannot execute the myogenic program. We are currently using genomics approaches to identify the transcriptional targets of these myogenic transcription factors and to molecularly define alterations in satellite cell programming that lead to stem cell dysfunction and failed regeneration.
The origin and regulation of non-myogenic accumulations in muscle degenerative diseases
Accumulations of fibrotic and adipose tissue are histopathological hallmarks of myopathies of diverse etiologies, including most of the studied muscular dystrophies. Because productive muscle contraction and consequent force generation require precise cell- and tissue-level organization, fibrotic and adipose infiltrations impair skeletal muscle function. These accumulations of non-myogenic cells are a near-universal feature of mouse models of muscular dystrophies and other experimental models of impaired muscle regeneration. We are using several models of impaired muscle regeneration to determine the extent to which there is a common cellular origin of fat and fibrotic accumulations across myopathies of distinct causes. We have shown that muscle stem cells (satellite cells) can contribute to fat accumulations in injured, non-regenerating, muscle when their programming is genetically manipulated, but whether satellite cells are a causal cell in other myopathies is unknown. In some experimental settings, fibro-adipogenic progenitors (FAPs) represent a cell-of-origin of these infiltrates. However, the extent to which FAPs (or FAP sub-populations) are the primary or exclusive offending cell type in myopathies of distinct origins is unclear. In addition to addressing these questions, we are also interested in understanding how the myopathic signaling environment alters progenitor cell programming such that their normally supportive regenerative function is co-opted toward pathological differentiation.
