Spatial transcriptomics reveals a role for sensory nerves in preserving cranial suture patency through modulation of BMP/TGFβ signaling

The patterning and ossification of the mammalian skeleton requires the coordinated actions of both intrinsic bone morphogens and extrinsic neurovascular signals, which function in a temporal and spatial fashion to control mesenchymal progenitor cell (MPC) fate. Here we show genetic inhibition of Tropomyosin receptor kinase A (TrkA) sensory nerve innervation of the developing cranium results in premature calvarial suture closure, associated with a decrease in suture MPC proliferation. In vitro, axons from peripheral afferent neurons derived from DRGs of wild type mice induce MPC proliferation in a spatially-restricted manner via a soluble factor when co-cultured in microfluidic chambers. Comparative spatial transcriptomic analysis of the cranial sutures in vivo confirmed a positive association between sensory axons and proliferative MPCs. SpatialTime analysis across the developing suture revealed regional-specific alterations in BMP and TGFβ signaling pathway transcripts in response to TrkA inhibition. RNA sequencing of DRG cell bodies following direct axonal co-culture with MPCs confirmed alterations in BMP/TGFβ signaling pathway transcripts. Among these, the BMP inhibitor FSTL1 (Follistatin-like 1) replicated key features of the neural-to-bone influence, including mitogenic and anti-osteogenic effects via inhibition of BMP/TGFβ signaling. Taken together, our results demonstrate that sensory nerve-derived signals, including FSTL1, function to coordinate cranial bone patterning by regulating MPC proliferation and differentiation in the suture mesenchyme.