Role of stromal interaction molecule 2 in chronic itch

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Abstract Chronic itch is a debilitating condition and represents a clinical challenge. Increase evidence has demonstrated that neuronal and astrocytic signaling pathways in the spinal cord play a crucial role in chronic itch. Although several molecules have been identified as key itch mediators/modulators, molecular mechanisms underlying chronic itch remain incompletely understood. Our research has been focused on role of store-operated calcium channels (SOCs) in pain and itch. We have shown that the primary SOC family members STIM1 and STIM2 (endoplasmic reticulum Ca2+ sensors) are expressed in dorsal horn neurons and astrocytes, but their functional significance in the spinal cord is still elusive. We recently generated vesicular glutamate transporters 2 (Vglut2, an excitatory neuron marker) conditional STIM1 and STIM2 knockout (KO) mice. Our pilot data showed that deletion of STIM1 in Vglut2+ neurons attenuated nociception, but did not affect pruritogen-induced scratches. In contrast, ablation of STIM2 in Vglut2+ neurons (v-STIM2 KO) markedly attenuated pruritogen-induced scratches, but had no effect on nociception. These exciting pilot work suggests that STIM1 and STIM2 play distinct roles in pain and itch. Our pilot data also show that inhibition of STIMs by intrathecal, not intradermal injection, decreased histamine (His)- and chloroquine (CQ)-induced itch behavior. Moreover, inhibition or deletion of excitatory neuronal STIM2 attenuated FITC- and DNFB-induced chronic itch, suggesting that neuronal STIM2 play a role in both acute and chronic itch. Importantly, STIM2 expression was increased in the dorsal horn from diphenylcyclopropenone (DCP)- and 1-fluoro-2, 4-dinitrobenzene (DNFB)-induced mouse models of chronic itch. GRPR plays a critical role in itch transmission. Our pilot data showed that deletion of STIM2 in Vglut2 or Lbx1 (specifically expressed in spinal and hindbrain neurons)-expressing neurons drastically attenuated GRP-induced Ca2+ responses and itch behavior. It is known that signal transducer and activator of transcription 3 (STAT3) and its downstream lipocalin-2 (LCN2) are key modulators of chronic itch. We found that activation of TLR4 upregulated STIM2 expression via STAT3 signaling. Based on these findings, we hypothesize that spinal STIM2 plays a crucial role in chronic itch through a combination of neuronal and astrocytic signaling pathways. In Aim 1, we will determine whether ablation or knockdown of spinal STIM2 attenuates chronic itch. In Aim 2, we will examine the expression and function of STIM2 under chronic itch conditions. In Aim 3, we will investigate the mechanisms underlying spinal STIM2 involvement in chronic itch. The proposed work will establish a novel role of STIM2 in itch sensation and will provide new insights into the mechanisms of GRPR in itch transmission. Successful completion of this research will not only identify STIM2 as a key modulator in chronic itch, but also decipher the cellular and molecular mechanisms underlying chronic itch.