Low Nociceptor GRK2 Prolongs Prostaglandin E ₂ Hyperalgesia via Biased cAMP Signaling to Epac/Rap1, Protein Kinase Cε, and MEK/ERK

Abstract

Hyperexcitability of peripheral nociceptive pathways is often associated with inflammation and is an important mechanism underlying inflammatory pain. Here we describe a completely novel mechanism via which nociceptor G-protein-coupled receptor kinase 2 (GRK2) contributes to regulation of inflammatory hyperalgesia. We show that nociceptor GRK2 is downregulated during inflammation. In addition, we show for the first time that prostaglandin E ₂ (PGE ₂)-induced hyperalgesia is prolonged from <6 h in wild-type (WT) mice to 3 d in mice with low GRK2 in Na _v1.8 ⁺ nociceptors ( SNS–GRK2 ^+/− mice). This prolongation of PGE ₂ hyperalgesia in SNS–GRK2 ^+/− mice does not depend on changes in the sensitivity of the prostaglandin receptors because prolonged hyperalgesia also developed in response to 8-Br-cAMP. PGE ₂ or cAMP-induced hyperalgesia in WT mice is PKA dependent. However, PKA activity is not required for hyperalgesia in SNS–GRK2 ^+/− mice. SNS–GRK2 ^+/− mice developed prolonged hyperalgesia in response to the Exchange proteins directly activated by cAMP (Epac) activator 8-pCPT-2′- O-Me-cAMP (8-pCPT). Coimmunoprecipitation experiments showed that GRK2 binds to Epac1. In vitro, GRK2 deficiency increased 8-pCPT-induced activation of the downstream effector of Epac, Rap1, and extracellular signal-regulated kinase (ERK). In vivo, inhibition of MEK1 or PKCε prevented prolonged PGE ₂, 8-Br-cAMP, and 8-pCPT hyperalgesia in SNS–GRK2 ^+/− mice. In conclusion, we discovered GRK2 as a novel Epac1-interacting protein. A reduction in the cellular level of GRK2 enhances activation of the Epac–Rap1 pathway. In vivo, low nociceptor GRK2 leads to prolonged inflammatory hyperalgesia via biased cAMP signaling from PKA to Epac–Rap1, ERK/PKCε pathways.