Spinal cord injury reprograms muscle fibroadipogenic progenitors to form heterotopic bones within muscles

Hsu-Wen Tseng,Beulah Jose,François Genêt,Adrienne Anginot,Dorothée Girard,Frédéric Torossian,Marjorie Salga,Denis Clay,Susan M Millard,Bianca Nowlan,Kylie A Alexander,Jean-Pierre Lévesque,Jules Gueguen,Marie-Caroline Le Bousse-Kerdilès,Marie-Emmanuelle Goriot,Sébastien Banzet,Allison R Pettit,Whitney Fleming

doi:10.1038/s41413-022-00188-y

Abstract

The cells of origin of neurogenic heterotopic ossifications (NHOs), which develop frequently in the periarticular muscles following spinal cord injuries (SCIs) and traumatic brain injuries, remain unclear because skeletal muscle harbors two progenitor cell populations: satellite cells (SCs), which are myogenic, and fibroadipogenic progenitors (FAPs), which are mesenchymal. Lineage-tracing experiments using the Cre recombinase/LoxP system were performed in two mouse strains with the fluorescent protein ZsGreen specifically expressed in either SCs or FAPs in skeletal muscles under the control of the Pax7 or Prrx1 gene promoter, respectively. These experiments demonstrate that following muscle injury, SCI causes the upregulation of PDGFRα expression on FAPs but not SCs and the failure of SCs to regenerate myofibers in the injured muscle, with reduced apoptosis and continued proliferation of muscle resident FAPs enabling their osteogenic differentiation into NHOs. No cells expressing ZsGreen under the Prrx1 promoter were detected in the blood after injury, suggesting that the cells of origin of NHOs are locally derived from the injured muscle. We validated these findings using human NHO biopsies. PDGFRα+ mesenchymal cells isolated from the muscle surrounding NHO biopsies could develop ectopic human bones when transplanted into immunocompromised mice, whereas CD56+ myogenic cells had a much lower potential. Therefore, NHO is a pathology of the injured muscle in which SCI reprograms FAPs to undergo uncontrolled proliferation and differentiation into osteoblasts.

Highlights

Neurogenic heterotopic ossifications (NHOs) are pathological heterotopic bones that develop in peri-articular muscles[1,2] following severe lesions of the central nervous system (CNS), such as spinal cord injuries (SCIs), traumatic brain injuries, strokes and cerebral anoxia
Zoanthus green (ZsGreen)+ cells identify satellite cells (SCs) and fibroadipogenic progenitors (FAPs) in the muscles of Pax7CreERT2 and Prrx1-Cre mice are frequent after severe CNS injury, fibrodysplasia ossificans progressiva (FOP) is an extremely rare To determine whether NHOs are derived from muscle SCs and disorder caused by a gain-of-function missense single nucleotide FAPs, we first assessed the specificity and efficiency of ZsGreen mutation in the bone morphogenetic protein (BMP) type I reporter expression in the Pax7-CreERT2; Rosa26-LoxP-STOP-LoxPreceptor gene ACVR1,30 resulting in a switch in ligand preference ZsGreen mice and the with aberrant activation of BMP signaling by activin-A instead of BMPs.[31]
Based on Cre recombinase driven by the Pax[7] (Paired Box 7) or Cdh[5] (Cadherin 5) gene promoters, which are specific to SCs and endothelial cells, respectively, it was concluded that neither SCs nor endothelial cells were the cells of origin of heterotopic ossifications in FOP.[32]

Summary

Introduction

Neurogenic heterotopic ossifications (NHOs) are pathological heterotopic bones that develop in peri-articular muscles[1,2] following severe lesions of the central nervous system (CNS), such as spinal cord injuries (SCIs), traumatic brain injuries, strokes and cerebral anoxia. As parabiosis experiments are forbidden for ethical reasons in Australia, we investigated whether ZsGreen+ cells were detectable in the circulation of Prrx1ZsG mice 1, 2, 3 and 7 days following SCI and CDTX-mediated muscle injury (Fig. 4b, c).

Results

Conclusion