Research ArticlePain

Neurokinin 1 receptor signaling in endosomes mediates sustained nociception and is a viable therapeutic target for prolonged pain relief

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Science Translational Medicine  31 May 2017:
Vol. 9, Issue 392, eaal3447
DOI: 10.1126/scitranslmed.aal3447

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Targeting the enemy within endosomes

With opioid addiction on the rise, there is a great need for effective nonopioid approaches to treat pain. Jensen et al. examined the function of substance P neurokinin 1 receptor, which plays a role in the transmission of pain signals in the central nervous system. The authors demonstrated that endocytosis of this receptor is required for the transmission of pain signals. Although systemic inhibition of endocytosis would not be feasible in a living organism, the authors discovered another way to take advantage of this information. They conjugated neurokinin 1 receptor antagonists to cholestanol, promoting their incorporation into endosomes, where they successfully inhibited their target to block pain transmission.

Abstract

Typically considered to be cell surface sensors of extracellular signals, heterotrimeric GTP-binding protein (G protein)–coupled receptors (GPCRs) control many pathophysiological processes and are the target of 30% of therapeutic drugs. Activated receptors redistribute to endosomes, but researchers have yet to explore whether endosomal receptors generate signals that control complex processes in vivo and are viable therapeutic targets. We report that the substance P (SP) neurokinin 1 receptor (NK1R) signals from endosomes to induce sustained excitation of spinal neurons and pain transmission and that specific antagonism of the NK1R in endosomes with membrane-anchored drug conjugates provides more effective and sustained pain relief than conventional plasma membrane–targeted antagonists. Pharmacological and genetic disruption of clathrin, dynamin, and β-arrestin blocked SP-induced NK1R endocytosis and prevented SP-stimulated activation of cytosolic protein kinase C and nuclear extracellular signal–regulated kinase, as well as transcription. Endocytosis inhibitors prevented sustained SP-induced excitation of neurons in spinal cord slices in vitro and attenuated nociception in vivo. When conjugated to cholestanol to promote endosomal targeting, NK1R antagonists selectively inhibited endosomal signaling and sustained neuronal excitation. Cholestanol conjugation amplified and prolonged the antinociceptive actions of NK1R antagonists. These results reveal a critical role for endosomal signaling of the NK1R in the complex pathophysiology of pain and demonstrate the use of endosomally targeted GPCR antagonists.

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