Intraarticular Adeno-Associated Virus Serotype AAV-PHP.S–Mediated Chemogenetic Targeting of Knee-Innervating Dorsal Root Ganglion Neurons Alleviates Inflammatory Pain in Mice

Objective: Joint pain is the major clinical symptom of arthritis that affects millions of people. Controlling the excitability of knee-innervating dorsal root ganglion (DRG) neurons (knee neurons) could potentially provide pain relief. We undertook this study to evaluate whether the newly engineered adeno-associated virus (AAV) serotype, AAV-PHP.S, can deliver functional artificial receptors to control knee neuron excitability following intraarticular knee injection. Methods: The AAV-PHP.S virus, packaged with dTomato fluorescent protein and either excitatory (Gq) or inhibitory (Gi) designer receptors exclusively activated by designer drugs (DREADDs), was injected into the knee joints of adult mice. Labeling of DRG neurons with AAV-PHP.S from the knee was evaluated using immunohistochemistry. The functionality of Gq- and Gi-DREADDs was evaluated using whole-cell patch clamp electrophysiology on acutely cultured DRG neurons. Pain behavior in mice was assessed using a digging assay, dynamic weight bearing, and rotarod performance, before and after intraperitoneal administration of the DREADD activator, Compound 21. Results: We showed that AAV-PHP.S can deliver functional genes into ~7% of lumbar DRG neurons when injected into the knee joint in a similar manner to the well-established retrograde tracer, fast blue. Short-term activation of AAV-PHP.S–delivered Gq-DREADD increased excitability of knee neurons in vitro (P = 0.02 by unpaired t-test), without inducing overt pain in mice when activated in vivo. By contrast, in vivo Gi-DREADD activation alleviated digging deficits induced by Freund's complete adjuvant–mediated knee inflammation (P = 0.0002 by repeated-measures analysis of variance [ANOVA] followed by Holm-Sidak multiple comparisons test). A concomitant decrease in knee neuron excitability was observed in vitro (P = 0.005 by ANOVA followed by Holm-Sidak multiple comparisons test). Conclusion: We describe an AAV-mediated chemogenetic approach to specifically control joint pain, which may be utilized in translational arthritic pain research.