Impact of the Sensory and Sympathetic Nervous System on Fracture Healing in Ovariectomized Mice

Abstract

The peripheral nervous system modulates bone repair under physiological and pathophysiological conditions. Previously, we reported an essential role for sensory neuropeptide substance P (SP) and sympathetic nerve fibers (SNF) for proper fracture healing and bone structure in a murine tibial fracture model. A similar distortion of bone microarchitecture has been described for mice lacking the sensory neuropeptide α-calcitonin gene-related peptide (α-CGRP). Here, we hypothesize that loss of SP, α-CGRP, and SNF modulates inflammatory and pain-related processes and also affects bone regeneration during fracture healing under postmenopausal conditions. Intramedullary fixed femoral fractures were set to 28 days after bilateral ovariectomy (OVX) in female wild type (WT), SP-, α-CGRP-deficient, and sympathectomized (SYX) mice. Locomotion, paw withdrawal threshold, fracture callus maturation and numbers of TRAP-, CD4-, CD8-, F4/80-, iNos-, and Arg1-positive cells within the callus were analyzed. Nightly locomotion was reduced in unfractured SP-deficient and SYX mice after fracture. Resistance to pressure was increased for the fractured leg in SP-deficient mice during the later stages of fracture healing, but was decreased in α-CGRP-deficient mice. Hypertrophic cartilage area was increased nine days after fracture in SP-deficient mice. Bony callus maturation was delayed in SYX mice during the later healing stages. In addition, the number of CD 4-positive cells was reduced after five days and the number of CD 8-positive cells was additionally reduced after 21 days in SYX mice. The number of Arg1-positive M2 macrophages was higher in α-CGRP-deficient mice five days after fracture. The alkaline phosphatase level was increased in SYX mice 16 days after fracture. Absence of α-CGRP appears to promote M2 macrophage polarization and reduces the pain threshold, but has no effect on callus tissue maturation. Absence of SP reduces locomotion, increases the pain-threshold, and accelerates hypertrophic callus tissue remodeling. Destruction of SNF reduces locomotion after fracture and influences bony callus tissue remodeling during the later stages of fracture repair, whereas pain-related processes are not affected.

The neuropeptide calcitonin gene-related peptide alpha is essential for bone healing

“…These findings clearly point towards a neuronal origin of aCGRP-induction during bone regeneration and highlight the importance of sensory innervation of fracture tissue during bone healing. In a recent study by Niedermair et al, it was suggested that aCGRP-deficiency alters M2 macrophage polarization without affecting callus maturation [21]. These divergent findings may be explained by the differences in the fracture model used.…”

“…Furthermore, increased in-growth of new nerve fibres containing CGRP at the fracture site have been reported in rats [20] . And finally, although inactivation of CGRP was suggested to alter M2 macrophage polarization without affecting callus maturation in ovariectomized mice [21] , another study demonstrated that magnesium implants promote bone regeneration in rats through CGRP receptor-dependent, osteogenic differentiation of periosteal stem cells [22] .…”

Effect and mechanism of propranolol on promoting osteogenic differentiation and early implant osseointegration

“…As an integrated part of the autonomic nervous system of the human body, the sympathetic nervous system serves a key role in regulating homeostasis. Growing evidence has shown that the sympathetic nervous system is involved in bone remodeling (6,35). In 1977, Duncan and Shim (36) demonstrated that the surface of intraosseous vessels is covered with rich adrenergic nerve fibers by observing the bone tissue of rabbits using histochemistry and fluorescence electron microscopy.…”

“…As the effects of the sympathetic nervous system on bone remodeling and healing are complex and extensive, the underlying mechanism has not yet been fully recognized (6). Numerous experiments have investigated the effect of propranolol, a sympathetic blocker drug, on bone metabolism (7)(8)(9).…”

Clinically relevant preclinical animal models for testing novel cranio‐maxillofacial bone 3D‐printed biomaterials

“…As previously mentioned, the periosteum is a critical tissue for bone repair, being a valuable source for regenerative cells, blood vessels for nutrient supply, 81 but also contains sensory neurons. As a source of the neuropeptides substance P and calcitonin gene‐related peptide, sensory neurons appear to play important roles in the bone healing process, 82 and fracture repair in long bones has recently been shown to require nerve growth factor expression in periosteal cells and tropomyosin receptor kinase A signalling in skeletal sensory nerve fibres 83 . Therefore, damage to the periosteum may limit bone healing by interfering with these reparative processes and/or cell populations and should therefore be avoided where at all possible.…”