Palovarotene action against heterotopic ossification includes the inhibition of local participating activin-A expressing cell populations
Heterotopic ossification (HO) consists of extraskeletal bone formation. One form of HO is acquired and instigated by traumas or surgery, and another form is genetic and characterizes Fibrodysplasia Ossificans Progressiva (FOP). We and others showed that activin A promotes both acquired and genetic HO and found that the retinoid agonist Palovarotene inhibits both HO forms in mice.  We asked whether Palovarotene's action against HO may include an interference with endogenous activin A expression and/or function. Using a mouse model of acquired HO, we found that activin A and its encoding RNA (Inhba) were prominent in chondrogenic cells within developing HO masses in untreated mice. Single cell RNAseq (scRNAseq) assays verified that  Inhba expression characterized chondroprogenitors and chondrocytes in untreated HO, in addition to its expected expression in inflammatory cells and macrophages. Palovarotene administration caused a sharp inhibition of both HO and amounts of activin A and Inhba transcripts. Bioinformatic analyses of scRNAseq datasets indicated that the drug had reduced interactions and crosstalk amongst cell populations. Our data reveal that Palovarotene markedly reduces the number of local Inhba-expressing HO-forming populations. Overall design: Female mice were implanted subcutaneously with rhBMP2/Matrigel mixture and received daily doses of vehicle or Palovarotene. As an additional control, companion mice were implanted with Matrigel without rhBMP2 and did not receive Palovatorene. On day 5, ectopic tissue masses were harvested from each mouse group, and their cells were liberated by enzymatic digestion and directly processed for scRNA-seq.