Murine Tissue-Resident PDGFRα+ Fibro-Adipogenic Progenitors Spontaneously Acquire Osteogenic Phenotype in an Altered Inflammatory Environment

Abstract

Acquired heterotopic ossifications (HO) arising as a result of various traumas, including injury or surgical interventions, often result in pain and loss of motion. Though triggers for HO have been identified, the cellular source of these heterotopic lesions as well as the underlying mechanisms that drive the formation of acquired HO remain poorly understood, and treatment options, including preventative treatments, remain limited. Here, we explore the cellular source of HO and a possible underlying mechanism for their spontaneous osteogenic differentiation. We demonstrate that HO lesions arise from tissue-resident PDGFRα+ fibro/adipogenic progenitors (FAPs) in skeletal muscle and not from circulating bone marrow-derived progenitors. Further, we show that accumulation of these cells in the tissue after damage due to alterations in the inflammatory environment can result in activation of their inherent osteogenic potential. This work suggests a mechanism by which an altered inflammatory cell and FAP interactions can lead to the formation of HO after injury and presents potential targets for therapeutics in acquired HO.

Evolving Roles of Muscle-Resident Fibro-Adipogenic Progenitors in Health, Regeneration, Neuromuscular Disorders, and Aging

“…Remarkably, the cells responsible for the rapid expansion of ectopic bone in fibrodysplasia ossificans progressiva (FOP), a genetic condition caused by mutations in ACTVR1 that alter the intracellular signaling pathways triggered by ACTIVIN-A, have been identified as FAPs (Dey et al, 2016;Upadhyay et al, 2017;Lees-Shepard et al, 2018a,b). By utilizing lineage-tracing experiments and a BMP2-induced HO, we recently showed that muscle-resident PDGFRα+ FAPs is the predominant cellular source of muscle ectopic ossification (Eisner et al, 2020). Using a parabiosis mice model, we also showed that muscle-resident PDGFRα+ FAPs, and not bone-marrow-derived PDGFRα+ cells, are the cellular source of heterotopic ossicles.…”

“…Myonecrosis, sarcolemmal disorganization, inflammation, and the accumulation of fibro-fatty scar tissue are observed early in DMD and increase with age (Cáceres et al, 2000;Porter et al, 2002;Reed and Bloch, 2005;Serrano et al, 2011;Grounds et al, 2020). Remarkably, FAPs generate activated fibroblasts, myofibroblasts, adipocytes, and bone-like cell progeny in muscular dystrophy, albeit to different extents depending on the degree of damage, inflammation and degeneration (Uezumi et al, 2011(Uezumi et al, , 2014Lemos et al, 2015;Contreras et al, 2016Contreras et al, , 2019aIeronimakis et al, 2016;Kopinke et al, 2017;Eisner et al, 2020;Mázala et al, 2020). As known for other muscle-resident cells, the exit of FAPs from quiescence and their entry into proliferative and differentiation programs are not only finely tuned by microenvironmental cues (Uezumi et al, 2010;Heredia et al, 2013;Moratal et al, 2018Moratal et al, , 2019 but also through intrinsic molecular mechanisms like epigenetic HDAC-dependent pathways (Mozzetta et al, 2013;Saccone et al, 2014;Sandonà et al, 2020) or Hic1-dependent quiescence maintenance (Scott et al, 2019;Contreras, 2020).…”

“…When stimulated, PDGFRα+ FAPs efficiently differentiate into the osteogenic lineage in vitro (Uezumi et al, 2010;Oishi et al, 2013). An increasing body of evidence suggests that murine and human stromal cells with osteogenic properties accumulate in muscles following injury, BMP2 intramuscular injections, and transplantation (Leblanc et al, 2011;Wosczyna et al, 2012;Oishi et al, 2013;Eisner et al, 2020). These muscle-resident cells also contribute to bone fracture repair in mice (Glass et al, 2011).…”

Mechanism of traumatic heterotopic ossification: In search of injury‐induced osteogenic factors

“…85,86 While a number of previous studies have demonstrated that depletion of macrophages prevents HO formation, 20,21,40 some recent studies have reported that depletion of macrophages or inhibition of macrophage infiltration into damaged tissue can promote HO. 73,87,88 The reported discrepancy may be due to altered macrophage phenotype resulting from non-identical clodronate liposome-based macrophage depletion scheme being used. Indeed, monocyte depletion by clodronate liposome increases local proliferation of macrophage subsets after skeletal muscle injury.…”

“…Macrophages represent such a target, given that both glucocorticoids and TGF‐β family proteins can modulate their phenotype (Figure 4), which has been reported to affect stem cell osteogenesis 85,86 . While a number of previous studies have demonstrated that depletion of macrophages prevents HO formation, 20,21,40 some recent studies have reported that depletion of macrophages or inhibition of macrophage infiltration into damaged tissue can promote HO 73,87,88 . The reported discrepancy may be due to altered macrophage phenotype resulting from non‐identical clodronate liposome‐based macrophage depletion scheme being used.…”