Data from our mouse model of rapid bone loss following Botulinum Toxin A (BTxA) induced muscle paralysis has revealed that neuromuscular function, outside the axis of mechanical loading deficits, is a critical modulator of bone homeostasis. Consistent with this thesis, we have observed that transient muscle paralysis triggers acute inflammatory signaling within bone marrow that precedes the onset of focal RANKL mediated osteoclastogenesis, which results in profound cortical and trabecular bone resorption. However, the intercellular signaling responsible for initiating acute bone marrow inflammation and subsequent bone resorption has not been elucidated and is therefore a barrier to identifying strategies that would decouple neuromuscular dysfunction from bone loss. One potential initiator of this pathway is neurogenic inflammation, which is triggered by the rapid release of the neuropeptides from sensory nerves and is amplified by mast cell mediated histamine release. We therefore pursued a series of preliminary studies to assess the potential activation of this pathway following muscle paralysis and found that: 1) Substance P, a classic initiator of neurogenic inflammation, is upregulated in tibia bone marrow within 1 d of calf paralysis, 2) genes associated with connective tissue mast cell presence and activation were elevated within 3 d following muscle paralysis, and 3) muscle paralysis induced bone resorption was significantly diminished in mast cell deficient KitW-sh/W-sh mice. We therefore hypothesize that: Bone resorption following muscle paralysis is initiated by neuropeptide signaling and is amplified by mast cell dependent histamine release. We will pursue this thesis through four complementary Specific Aims (SA), each with a corresponding sub-hypothesis. We anticipate that neuropeptides within bone marrow will be elevated by BTxA induced muscle paralysis prior to evidence of mast cell activation or bone resorption (SA#1). SA#2 will demonstrate that simultaneous antagonism of primary neurogenic inflammatory neuropeptides will be required to successfully inhibit bone resorption induced by muscle paralysis. In SA#3, we will leverage a cKit independent, connective tissue mast cell deficient mouse to demonstrate that mast cell mediated histamine signaling is responsible for the profound osteoclastogenesis induced by muscle paralysis. SA#4 will then provide proof of concept that treatment with histamine receptor antagonists will significantly attenuate bone resorption caused by muscle paralysis. Each aspect of the proposed signaling pathway (neurogenic inflammation, neuropeptide signaling, mast cell activation, paralysis induced bone resorption) has been explored in other contexts, but not integrated into a cellular signaling cascade that couples muscle, nerve, and bone physiology. Importantly, if our thesis is supported, the broad clinical experience with histamine antagonists will enable repurposing of approved drugs toward the goal of ameliorating acute bone resorption precipitated by paralysis or neuromuscular impairment.
This project will determine whether mast cell mediated histamine release triggered by focal neurogenic inflammation is a fundamental mediator of the acute bone resorption precipitated by muscle paralysis. While this intercellular signaling cascade has not previously been implicated in bone, its individual components are well explored in other contexts. Critically, this pathway holds substantial potential to be targeted by antihistamine therapy as a novel strategy to prevent bone loss caused by acute neuromuscular dysfunction.
