Duchenne Muscular Dystrophy (DMD) is a degenerative muscle disorder characterized by a lack of dystrophin expression that ultimately results in cardiac or respiratory failure. DMD patients also acquire osteopenia, fragility fractures, and scoliosis indicating that a deficiency in skeletal system homeostasis also occurs in DMD patients. It is speculated that these skeletal abnormalities are likely a secondary consequence to muscle loss (sarcopenia);however, it remains unclear if they could be due to a direct intrinsic skeletal defect. Recent evidence has emerged implicating adult stem cell dysfunction in the histopathogenesis of DMD. Muscle derived progenitor cells (MPCs) isolated from dystrophin/utrophin double knock-out (dKO) mice (a severe animal model of DMD) have been found to be defective in their proliferation and differentiation capacities. We, and others, have reported that these dKO mice exhibit a spectrum of degenerative changes in their bone, articular cartilage, and intervertebral discs and experience spinal deformities, heterotopic ossification, cardiomyopathy and a decreased lifespan, all of which support a premature musculoskeletal aging phenotype in this mouse model. A defect in bone healing was also observed in these mice;however, it is still unclear whether this defect is an intrinsic bone healing problem or associated with the secondary effects of sarcopenia (Aim 1). Preliminary evidence supports the existence of an adult stem cell defect in both MPCs and mesenchymal stem cells (MSCs) in these mice, supporting the theory that abnormal bone healing could be the consequence of an autonomous defect in the adult stem cell compartment. Thus the second aim of this project will be to further validate whether the MPCs and MSCs in these mice, analyzed at different ages, are defective in their proliferation and osteogenic differentiation capacities compared to MPCs and MSCs isolated from mdx and wild type (WT) mice. It has recently been shown that reducing fibroblast growth factor-2 (FGF2) activity prevents stem cell depletion/exhaustion;therefore, we also propose to determine whether FGF2 inhibitor-loaded biomimetic coacervate could rescue this autonomous adult stem cell defect and delay the onset of bone related histopathologies in dKO mice (Aim 2). Since there is also evidence that the stem cell niche may also negatively impact adult stem cell function, via a non-autonomous mechanism, we propose experiments to determine if the bone defect observed in dKO mice can be rescued through parabiotic pairing which will rejuvenate the dystrophic microenvironment by creating a shared circulation between a dKO and a young WT animal (Aim 3). We have preliminary data that supports the fact that circulating factors from young animals have a beneficial effect on the bone morphologies and healing capacity of dKO mice. In summary, this innovative grant application will: 1) determine whether the bone abnormalities and healing in dKO mice represent an intrinsic bone defect and 2) characterize whether the progressive bone histopathology observed in the dKO mice, is primarily driven by cell autonomous and/or non-autonomous mechanisms.
Duchenne muscular dystrophy (DMD) is a lethal genetic disease characterized by a lack of dystrophin protein that causes skeletal muscle degeneration and leads to the patient becoming wheelchair bound due to severe muscle weakness in their early teens and ultimately death by their mid-twenties due to cardiac or respiratory failure. Besides skeletal muscle abnormalities DMD patients also acquire osteopenia, fragility fractures, and scoliosis indicating that a deficiency in their skeletal system which is speculated to be a secondary consequence to muscle loss (sarcopenia);however, it remains unclear if they could be due to a direct intrinsic skeletal defect. The goal of the current proposal is to determine the nature of these skeletal system defects and characterize whether the progressive bone histopathology observed in a mouse model of DMD, known as the dKO mouse, is primarily driven by abnormalities in the stem cell pool or via other systemic circulating factors.