Elucidation of the Regulatory Role of Osteopontin on the Pathology of Duchenne Muscular Dystrophy
- Author(s): Capote Zulueta, Joana Carolina
- Advisor(s): Spencer, Melissa J
- et al.
Osteopontin (OPN) is one of the most highly up-regulated genes in Duchenne Muscular Dystrophy (DMD) patients and also in the mdx mouse. Single nucleotide polymorphisms in the SPP1 gene (OPN gene) are associated with changes in muscle strength and age of loss of ambulation in DMD patients. Previous work in the lab has shown that ablation of OPN in mdx mice significantly ameliorates their dystrophic phenotype by decreasing muscle fibrosis and increasing muscle regeneration and strength in young mice. These changes in the pathology of mdx mice were associated with a decrease in NKT and Gr-1+ cell populations, and decreased intramuscular TGF-β.
In this project, we expanded on these previous observations and further explored the ways in which OPN exerts its regulatory role in the dystrophic pathology. Our current study shows that OPN ablation in mdx mice induces long term benefits on dystrophic muscles. OPN-/-mdx (up to more than one year old) display increased muscle mass and myofiber size, which translates to an improved performance in wire, grip and pulmonary function tests in dystrophic mice.
We show here that OPN expression in dystrophic muscle impairs muscle regeneration indirectly, by skewing the macrophage population toward a pro-inflammatory/pro-fibrotic phenotype. OPN ablation in mdx mice leads to reductions in M1 and M2a macrophages and increases in M2c. These changes were also associated with increased macrophage expression of pro-regenerative factors, without significantly changing pro-fibrotic factor expression. Moreover, OPN may impact regeneration directly by impairing terminal differentiation of myoblasts. In addition, we showed that OPN may promote muscle fibrosis in dystrophic muscle by directly interacting with fibroblasts and increasing collagen expression. This effect of OPN in fibroblast cultures is likely dependent in cell-type specific post-translational modifications.
This study validates OPN as a therapeutic target in DMD and lays a foundation for therapies involving pharmacological targeting of OPN, by providing a mechanistic understanding of OPN’s relationship to the processes of immune response, regeneration and fibrosis in dystrophic muscle.