UCLA

Duchenne muscular dystrophy (DMD) is one of the most common inherited, lethal childhood diseases. This X-linked recessive disorder is often caused by mutations in the DMD gene that leads to loss of functional dystrophin protein and results in myofibers that are susceptible to membrane rupture. Sarcolemmal fragility leads to chronic cycles of muscle degeneration and regeneration and a subsequent reprogramming of the muscle niche that includes unresolved inflammation, defective muscle stem cell activity, aberrant pathological fibrosis, intramuscular adipose tissue (IMAT) accumulation and ultimately failed regeneration. Our lab has shown that osteopontin (SPP1) is a critical regulator of DMD disease progression that links many of these processes through its complex biology. SPP1 is multifunctional matricelluar protein, encoded by the Spp1 gene, that is post-translationally modified in many ways, expressed by a host of different cells in the muscle niche and can bind to a plethora of different receptors. This complexity can greatly impact how SPP1 acts on target cells and the signaling cascades that result in DMD disease progression. In this dissertation, we utilized single cell RNA sequencing to unbiasedly explore how two cell-specific sources of SPP1, macrophage-derived and muscle stem cell-derived, affect cell-cell interactions in dystrophic muscle.Here we show that macrophage-derived SPP1 is an autocrine regulator of macrophage TGFβ1. We identified two novel adipogenically primed stromal cell populations that are regulated by macrophage-derived TGFβ and contribute to IMAT. Our work established a link between macrophage-derived SPP1, reduced macrophage-derived TGFβ and ectopic fatty infiltration that is a hallmark of progressive DMD. Additionally, we showed that muscle stem cell (MuSC)-specific SPP1 has an autocrine inhibitory effect on MuSC stemness and positive effect on the fibrotic and inflammatory phenotype of MuSCs. Endothelial cells greatly expand in the MuSC Spp1 cKO showing a pro-angiogenic, anti-inflammatory state. We hypothesize that MuSC-derived SPP1 crosstalks with endothelial cells (ECs), leading to reduced EC expansion and an anti-angiogenic, pro-inflammatory, activated state. Furthermore, we provide evidence that this MuSC-EC regulatory axis affects macrophage phenotype. Altogether, this work advances knowledge of cell-specific SPP1 biology that drives DMD disease progression.