QS45. Parallels And Deviations in Mesenchymal Progenitor Cell Activation in Regenerative and Fibrotic Injuries

Purpose: The formation of ectopic bone during heterotopic ossification (HO) and formation of new tissues after digit tip amputation (DTA) have been shown to progress through a Pdgfra+ mesenchymal progenitor cell (MPC). Both processes result in the formation of de novo bone at the injury site. Here, we sought to compare gene expression and differentiation trajectories of the MPCs in HO formation and DTA repair in order to identify similarities and differences in these processes that may be beneficial for future wound regeneration strategies. Methods: Single cell RNA sequencing (scRNAseq) datasets from multiple time points in the burn/tenotomy model of achilles tendon HO, amputation of the P3 digit (regenerative) and P2 digit (fibrotic) were analyzed. MPCs were identified using known markers (Pdgfra, Prrx1), subsetted and re-analyzed for more in-depth analysis. Computational assessments were carried out using the R packages Seurat and Monocle2. Results: Our analyses point to a common response to injury, including expansion of the progenitor pool, deposition and interaction with rebuilding matrix components, and regulation of major morphogen pathways such as TGFβ and WNT within Pdgfra+ MPCs. However, these common injury responses were regulated via unique mechanisms. Surprisingly, MPCs of the non-fibrotic P2 DTA showed more transcriptional similarity with injury-induced tendon HO than with the MPCs present within the blastema of the regenerative P3 DTA. In addition to divergent regulations of the TGFβ and WNT pathways, analyses show a unique fingerprint of inflammatory signaling activation within the MPCs of P2 and P3 DTA, as well as HO. This included preferential activation of the IL1 and IL6/10 signaling in regenerative P3 DTA, Toll-like receptor signaling in the non-regenerative P2 DTA, NFκB in both DTA models and the classical complement pathway in HO. Conclusion: Combined, this data suggests that while similar mechanisms are employed to overcome injury, these pathways are alternatively regulated to meet their individual injury requirements. Modulating these pathways may prove valuable tools to shift the balance between regenerative and fibrotic healing response.