Astrocytic STAT3 activation and chronic itch require IP₃R1/TRPC-dependent Ca²⁺ signals in mice

Background: Chronic itch is a debilitating symptom of inflammatory skin diseases, but the underlying mechanism is poorly understood. We have recently demonstrated that astrocytes in the spinal dorsal horn become reactive in models of atopic and contact dermatitis via activation of the transcription factor signal transducer and activator of transcription 3 (STAT3) and critically contribute to chronic itch. In general, STAT3 is transiently activated; however, STAT3 activation in reactive astrocytes of chronic itch model mice persistently occurs via an unknown mechanism. Objective: We aimed to determine the mechanisms of persistent activation of astrocytic STAT3 in chronic itch conditions. Methods: To determine the factors that are required for persistent activation of astrocytic STAT3, Western blotting and calcium imaging with cultured astrocytes or spinal cord slices were performed. Thereafter, chronic itch model mice were used for genetic and behavioral experiments to confirm the role of the factors determined to mediate persistent STAT3 activation from in vitro and ex vivo experiments in chronic itch. Results: IP₃ receptor type 1 (IP₃R1) knockdown in astrocytes suppressed IL-6–induced persistent STAT3 activation and expression of lipocalin-2 (LCN2), an astrocytic STAT3-dependent inflammatory factor that is required for chronic itch. IP₃R1-dependent astrocytic Ca²⁺ responses involved Ca²⁺ influx through the cation channel transient receptor potential canonical (TRPC), which was required for persistent STAT3 activation evoked by IL-6. IL-6 expression was upregulated in dorsal root ganglion neurons in a mouse model of chronic itch. Dorsal root ganglion neuron–specific IL-6 knockdown, spinal astrocyte–specific IP₃R1 knockdown, and pharmacologic spinal TRPC inhibition attenuated LCN2 expression and chronic itch. Conclusion: Our findings suggest that IP₃R1/TRPC channel–mediated Ca²⁺ signals elicited by IL-6 in astrocytes are necessary for persistent STAT3 activation, LCN2 expression, and chronic itch, and they may also provide new targets for therapeutic intervention.