NGF-TrkA Signaling in Load-Induced Bone Formation

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Project Summary/Abstract Nearly all of the sensory nerves that innervate bone express neurotrophic tyrosine kinase receptor type 1 (TrkA), the high affinity receptor for nerve growth factor (NGF). In mature bone, these specialized sensory nerves blanket the periosteum in a dense mesh-like network, occupying a preferential location for biomechanical signaling. However, the mechanism by which NGF is regulated in the osteoblast and the signals transmitted to bone by sensory nerves remain unknown. Our main objective in this project is to determine the upstream mediators and downstream effectors of NGF-TrkA signaling during strain adaptive bone remodeling. Our central hypothesis is that activation of NF-κB signaling in mature osteoblasts is required for the expression of NGF in response to mechanical load, which in turn activates TrKA sensory nerves to provide osteogenic cues that support load- induced bone formation. In Specific Aim 1, we will determine the regulation of NGF expression in osteoblasts using in vitro pulsatile fluid flow and in vivo forelimb axial compression of conditional knockout mice. In Specific Aim 2, we will identify nerve-derived signals that promote load-induced bone formation by analyzing mRNA and protein from loaded limbs in mouse models of diminished NGF-TrkA signaling, with validation using an in vitro microfluidic platform. In Specific Aim 3, we will assess a potential therapeutic application of this signaling pathway to increase bone accrual following osteogenic mechanical loading without hyperalgesia. In total, the results from this study will define the upstream mediators and downstream effectors of NGF-TrkA signaling during strain adaptive bone remodeling and provide a potential therapeutic tool for leveraging this system to improve skeletal health. Moreover, the proposed studies are highly aligned with the NIAMS Long-Range Plan, and our interdisciplinary team is uniquely positioned to advance these specific aims.