CGRP-CLR mediated regulation of bone healing

Periosteum, a primary site of the fracture healing response, is highly vascularized and densely innervated. Studies on bone fracture have primarily focused on the role of growth factors and vascularization in the healing process, but we postulate that sensory nerve signals are a critical part of the regulatory mechanism that initiate the stem/progenitor cell responses required for fracture callus formation. A number of lines of evidence point to sensory innervation, or signals associated with sensory nerves as promoters of bone accrual and healing. For example, fracture healing is impaired following chemical sensory denervation, but the sensory nerve-derived signals that promote healing as not yet defined. We hypothesize that damage to sensory nerves in the periosteum orchestrates the bone-healing cascade through calcitonin gene-related peptide (CGRP) – calcitonin like receptor (CLR) signaling. This is important given that multiple CGRP inhibitors were recently approved by the FDA for prevention and treatment of migraines. In Aim 1 we will evaluate the effects of CGRP inhibitors on bone healing. Given that CGRP plays a role in bone turnover and potentially healing, it is important to understand the impact of CGRP inhibition on fracture healing. In the Aim 2, we will determine which cell lineage or lineages responsive to CGRP signals during healing using targeted deletion of the CLR receptor. CLR deletion in early fracture healing will be targeted to the following lineages using inducible Cre’s: MPCs (αSMA-CreER), chondrocyte (Acan)-CreER, osteoblasts using Col2.3CreER and in endothelial (Cdh5-CreER) during fracture healing. The effects on callus formation and strength, as well as differentiation in the callus will be determined. We will examine cellular mechanisms of CGRP/CLR action utilizing in vivo approaches to study MPC expansion, differentiation and vascularization. We propose to dissect CLR signaling by distinguishing effects of the ligands. The main ligands are CGRP and Adrenomedullin that act via CLR and RAMP, of which CLR-RAMP1 is main complex for CGRP signaling while CLR-RAMP2 and RAMP3 is responsible for ADM signal activation. In Aim 3 we will evaluate effects of ADM deletion in mesenchymal population using ADMfl/fl mice. We will also define downstream signaling mechanism of CLR deletion in MSCs and endothelial cells using 10x genomics. Our study will also provide critical information on the effects that newly approved inhibitors of CGRP signaling exhibit on bone healing and what cellular mechanisms affect healing via CLR receptor Finally, testing approaches to modulate CGRP/CLR sensory singling may lead to a therapeutic strategy to enhance bone healing.