Background & Aims
Low back pain (LBP) is one of the most prevalent chronic pain conditions [1] and the leading global cause of years lived with disability [2].Understanding the complex nature of low back pain, which arises from various factors, poses a challenge in accurately replicating these processes in animals. This replication is crucial for gaining insights into chronic pain. Other research groups genome-wide association studies (GWAS) and our team’s research indicate the potential implication of axonal guidance pathways in chronic pain [3-6]. Semaphorin-3A (sema-3A) and their co-receptors, plexins and neuropilins (Nrp), are canonically recognized for their roles in guiding axonal growth [7]. Sema-3A also has a role in bone remodeling [8] and its reduced expression in the human intervertebral disc has been linked to neuronal ingrowth and pain [9]. We thus abolished sema-3A signalling in mice and hypothesized that these mice would be more susceptible t low back pain.
Methods
To abolish the Sema-3A pathway in nociceptors, we crossed mice with floxed semaphorin co-receptor, neuropilin-1(Nrp1Fl/Fl) with mice that express the Cre recombinase specifically in their DRG nociceptors (PirtCre). () To assess low back pain sensitivity, Nrp1Fl/Fl;Pirtcre and wild-type (WT) littermates were subjected to the tail suspension test. To fully assess pain sensitivity, we also conducted inflammatory and thermal pain assays in the hind paw of Nrp1Fl/Fl;Pirtcre and WT mice. (2) To examine the distribution of peptidergic and non-peptidergic sensory nerve fibers of Nrp1Fl/Fl;Pirtcre mice, we stained the dorsal horn with calcitonin gene-related peptide (CGRP) and isolectin B4 (IB4), respectively, and compared it to that of WT mice. (3) To investigate the effects on bone structure and quality, we compared L3-L5 of Nrp1Fl/Fl;Pirtcre and WT mice using microcomputed tomography (Micro-CT).
Results
(1) We observed heightened low back pain sensitivity in Nrp1Fl/Fl;Pirtcre mice compared to WT mice when subjected to the tail suspension assay. Additionally, these mice exhibited increased sensitivity to heat and cold stimuli but not inflammatory hypersensitivity. (2) There were no distinctions in the distribution area of peptidergic and non-peptidergic sensory nerve fibers in the dorsal horn between Nrp1Fl/Fl;Pirtcre and WT mice. (3) Nrp1Fl/Fl;Pirtcre mice displayed a reduction in both bone density and volume compared to WT mice.
Conclusions
Our results shows that Nrp1Fl/Fl;Pirtcre mice demonstrate elevated low back pain sensitivity and increased sensitivity to thermal but not inflammatory stimuli. The increased sensitivity to low back pain could be explained by the bone alterations seen in Nrp1Fl/Fl;Pirtcre mice, but the link between increased sensitivity to thermal stimuli and the abolishment of sema-3A signalling remains to be clarified.
References
1.Maher, C., M. Underwood, and R. Buchbinder, Non-specific low back pain. The Lancet, 2017. 389(10070): p. 736-747.
2.Global, regional, and national incidence, prevalence, and years lived with disability for 310 diseases and injuries, 1990-2015: a systematic analysis for the Global Burden of Disease Study 2015. Lancet, 2016. 388(10053): p. 1545-1602.
3.Bortsov, A.V., et al., Brain-specific genes contribute to chronic but not to acute back pain. Pain Rep, 2022. 7(5): p. e1018.
4.Khoury, S., et al., Genome-wide analysis identifies impaired axonogenesis in chronic overlapping pain conditions. Brain, 2021. 145(3): p. 1111-1123.
5.Johnston, K.J.A., et al., Genome-wide association study of multisite chronic pain in UK Biobank. PLoS Genet, 2019. 15(6): p. e1008164.
6.Freidin, M.B., et al., Insight into the genetic architecture of back pain and its risk factors from a study of 509,000 individuals. Pain, 2019. 160(6): p. 1361-1373.
7.Nakamura, F., R.G. Kalb, and S.M. Strittmatter, Molecular basis of semaphorin?mediated axon guidance. Journal of neurobiology, 2000. 44(2): p. 219-229.
8.Li, Z., et al., The Role of Semaphorin 3A in Bone Remodeling. Front Cell Neurosci, 2017. 11: p. 40.
9.Tolofari, S.K., et al., Expression of semaphorin 3A and its receptors in the human intervertebral disc: potential role in regulating neural ingrowth in the degenerate intervertebral disc. Arthritis Res Ther, 2010. 12(1): p. R1.