BDNF-TrκB-PLCγ signaling drives BPA-induced mechanical allodynia through activating mechanosensitive Piezo2 channels

Brachial plexus avulsion (BPA) induced neuropathic pain (NP) affects up to 90% patients with BPA, which is often characterized by mechanical allodynia. The gradual chronicity of the pain and the lack of effective treatment methods make it a challenging issue for researchers and clinicians. Mechanosensitive channel, Piezo2, has been reported to participate in several physiological processes including NP. However, whether it plays a role in BPA-induced NP and related mechanisms at the cervical dorsal root ganglion (DRG) level remains unclear. In this study, we focused on the peripheral level Piezo2 and intended to explore its role in BPA-induced NP.

A novel BPA mouse model was established, and pain behavioral tests were performed. CatWalk gait analysis was used to evaluate the pain and motor function. Qualitative and quantitative analyses were used to evaluate the expression of Piezo2 in cervical DRG via Western blot analysis and immunohistochemistry (IHC) technology. Furthermore, Piezo2-related signaling was also assessed using Western blot and pharmacological methods. Whole-cell patch-clamp was employed to directly record the excitation state of DRG neurons, and function modulation was tested through antagonists, and genetic knockdown was used to confirm the role of Piezo2 at the peripheral level. In addition, avulsed DRG tissues derived from patients with BPA were also analyzed through Western blot and IHC technology.

The BPA mouse model revealed a substantial reduction in the mechanical withdrawal threshold of the affected forepaw, and CatWalk gait analysis demonstrated that a single C7 root avulsion did not affect the motor function of the mice. An increased expression level of Piezo2 was identified, and Piezo2 was primarily expressed in nociceptors in cervical DRGs under pain conditions. Piezo2 positive (Piezo2⁺) neurons in the DRG also had a high colocalization rate with TrκB, the high affinity receptor of brain-derived neurotrophic factor (BDNF), which indicates the possibility of molecular crosstalk. Neurons in the cervical DRG of BPA mice also exhibited a hyperexcitation state through patch-clamp recording, and this hyperexcitation could be inhibited by GsMTx4, a Piezo2 inhibitor, and BDNF genetic knockdown. Furthermore, the mechanical pain behavior of BPA mice was relieved by these hyperexcitability inhibition methods. BDNF-TrκB-PLCγ signaling was shown to promote BPA-induced NP via downstream Piezo2 channels using Western blots, Co-immunoprecipitation, and pharmacological analyses. As a branch experiment, tissues derived from patients with BPA also exhibited a comparable change in molecular profile to that of the BPA mice.

The current study proves the facilitative role of Piezo2 in BPA-induced NP, possibly via BDNF-TrκB-PLCγ signaling at the peripheral level. Piezo2 is expressed in different diameter neurons of the cervical DRGs and is particularly highly colocalized with low-threshold mechanoreceptors (i.e., TrκB⁺ neurons). Consistency of the results in both rodent and human experiments suggests the potential for clinical translation in the future. Limitations concerning more specific pharmacological inhibition of Piezo2 channels and a direct mechanistic link between PLCγ activation and Piezo2 function require future investigations.