Nicotinic Acetylcholine Receptors on Osteocytes Impact Bone Mechanoadaptation in a Sexually Dimorphic Manner

Bone’s ability to adapt to mechanical loads is attributed the osteocyte. Osteocytes reside embedded in calcified bone matrix, where they function as the resident tissue mechanosensor. Ca2+ signaling is a key second messenger in mechanotransduction in osteocytes. It is used to encode mechanical load magnitude and has been linked to downstream signaling pathways which regulate bone resorbing and bone formation cell activity, however the details regarding Ca2+ signaling regulation in osteocytes in vivo are not well stratified. Osteocytes express components for nicotinic acetylcholine receptors (nAChRs) are known for calcium signaling at the neuromuscular junction. Indeed, cholinergic signaling is known to impact bone mass and fracture risk from studies in both humans and rodents. Despite this convincing evidence of the role of ACh, the details regarding cholinergic signaling in bone mechanotransduction remain largely unexplored. Here, we determine osteocytes as a functional target of cholinergic signaling for bone mechanoadaptation. We generated osteocyte-targeted conditional knockout mice for Chrna1 or Rapsn using the 10kb DMP1 promotor to delete the cholinergic receptor subunit α1 and the channel grouping protein raspyn, respectively. We then performed longitudinal studies to assess skeletal morphology, tissue material make-up, and dynamic responses to anabolic challenge. Our results show sexually dimorphic differences in bone formation rates and bone structure between Cre-negative controls and conditional knockout mice. In females, the reductions to bone geometry were rescued with anabolic loading, but not in males. Our results confirm osteocytes as direct cholinergic targets with an impact on bone mechanoadaptation and suggest a direct link between bone mechanobiology and the central nervous system. ### Competing Interest Statement The authors have declared no competing interest.