TRPV1 Channel Activated by the PGE2/EP4 Pathway Mediates Spinal Hypersensitivity in a Mouse Model of Vertebral Endplate Degeneration

Abstract

Low back pain (LBP) is the primary cause of disability globally. There is a close relationship between Modic changes or endplate defects and LBP. Endplates undergo ossification and become highly porous during intervertebral disc (IVD) degeneration. In our study, we used a mouse model of vertebral endplate degeneration by lumbar spine instability (LSI) surgery. Safranin O and fast green staining and μCT scan showed that LSI surgery led to endplate ossification and porosity, but the endplates in the sham group were cartilaginous and homogenous. Immunofluorescent staining demonstrated the innervation of calcitonin gene-related peptide- (CGRP-) positive nerve fibers in the porous endplate of LSI mice. Behavior test experiments showed an increased spinal hypersensitivity in LSI mice. Moreover, we found an increased cyclooxygenase 2 (COX2) expression and an elevated prostaglandin E2 (PGE2) concentration in the porous endplate of LSI mice. Immunofluorescent staining showed the colocalization of E-prostanoid 4 (EP4)/transient receptor potential vanilloid 1 (TRPV1) and CGRP in the nerve endings in the endplate and in the dorsal root ganglion (DRG) neurons, and western blotting analysis demonstrated that EP4 and TRPV1 expression significantly increased in the LSI group. Our patch clamp study further showed that LSI surgery significantly enhanced the current density of the TRPV1 channel in small-size DRG neurons. A selective EP4 receptor antagonist, L161982, reduced the spinal hypersensitivity of LSI mice by blocking the PGE2/EP4 pathway. In addition, TRPV1 current and neuronal excitability in DRG neurons were also significantly decreased by L161982 treatment. In summary, the PGE2/EP4 pathway in the porous endplate could activate the TRPV1 channel in DRG neurons to cause spinal hypersensitivity in LSI mice. L161982, a selective EP4 receptor antagonist, could turn down the TRPV1 current and decrease the neuronal excitability of DRG neurons to reduce spinal pain.

Intervertebral disc degeneration and how it leads to low back pain

“… 219 Activation of the TRPV1 channel by the PGE2/EP4 cell signaling pathway induces spinal hypersensitivity in a mouse model of CEP degeneration. 220 …”

“…The reduced osmolarity in the degenerate IVD due to depletion of aggrecan increases TRPV4 expression and pro‐inflammatory cytokine production by IVD cells 219 . Activation of the TRPV1 channel by the PGE2/EP4 cell signaling pathway induces spinal hypersensitivity in a mouse model of CEP degeneration 220 …”

Inhibition of PGE2 in Subchondral Bone Attenuates Osteoarthritis

“…PGE2/EP4 pathway-induced activation of the transient receptor potential cation channel subfamily V member 1 (TRPV1) may be another subchondral bone homeostasis and osteoarthritis pain mechanism. A previous study has shown that the PGE2/EP4 pathway leads to the activation of the TRPV1 channel, further inducing spinal hypersensitivity and spinal pain in the DRG neurons of lumbar spine instability mice [ 48 ]. In addition, the activation of the TRPV1 channel, induced by zoledronic acid, promotes osteoblast mineralization [ 49 ].…”

Pathophysiological mechanism of acute bone loss after fracture

“…PGE2 is also involved in pain processes in diseases such as osteoarthritis and rheumatoid arthritis [166] . For patients with low back pain, the increase in PGE2 in the porous endplate stimulates sensory nerves, leading to spinal cord pain [167] , [169] . The use of low-dose celecoxib can inhibit endplate porosity, sensory innervation, as well as spinal pain and allergies [170] .…”