Periodontal ligaments enhance neurite outgrowth in trigeminal ganglion neurons through Wnt5a production induced by mechanical stimulation

The peripheral sensory nerve must be maintained to perceive environmental changes. Daily physiological mechanical stimulations, like gravity, floor reaction force, and occlusal force, influence the nerve homeostasis directly or indirectly. Although the direct axonal membrane stretch enhances axon outgrowth via mechanosensitive channel activation, the indirect mechanisms remain to be elucidated. In this study, we identified the indirect pathways where Wnt5a was a molecular cue released by mechanically stimulated rat periodontal ligament (rPDL) cells. qRT-PCR and ELISA showed that mechanically stimulated rPDL cells enhanced Wnt5a expression level and Wnt5a protein in a Ca2+-dependent manner. The inhibitors of PI3K (LY294002) and MEK1/2 (U0126) suppressed the Akt/PKB and ERK1/2 phosphorylation, respectively, in Western blotting analysis and consequently abolished the increase in Wnt5a expression. Similarly, PF573228, a focal adhesion kinase inhibitor, attenuated Akt- and ERK1/2-phosphorylation and Wnt5a expression. Importantly, the culture medium of stretched PDL cells enhanced neurite elongation, sprouting, and branching in trigeminal ganglion neurons that project to PDL. Moreover, treatment with an anti-Wnt5a antibody (to neutralize Wnt5a activity), AP7677a (anti-Ryk antibody, to block Ryk receptor activity), or strictinin (Ror1 inhibitor) suppressed the morphological changes. These findings reveal the indirect mechanisms that Wnt5a, released from the connective tissues in response to mechanical stimulation, enhances the outgrowth of the peripheral nerves. Our study suggests that the peripheral connective tissues regulate peripheral nerve homeostasis and that Wnt5a signaling could be targeted for the treatment of peripheral nerve disorders.