Inflammation-induced abnormalities in the subcellular localization and trafficking of the neurokinin 1 receptor in the enteric nervous system

Activated G protein-coupled receptors traffic to endosomes and are sorted to recycling or degradative pathways. Endosomes are also a site of receptor signaling of sustained and pathophysiologically important processes, including inflammation. However, the mechanisms of endosomal sorting of receptors and the impact of disease on trafficking have not been fully defined. We examined the effects of inflammation on the subcellular distribution and trafficking of the substance P (SP) neurokinin 1 receptor (NK₁R) in enteric neurons. We studied NK₁R trafficking in enteric neurons of the mouse colon using immunofluorescence and confocal microscopy. The impact of inflammation was studied in IL10^-/--piroxicam and trinitrobenzenesulfonic acid colitis models. NK₁R was localized to the plasma membrane of myenteric and submucosal neurons of the uninflamed colon. SP evoked NK₁R endocytosis and recycling. Deletion of β-arrestin2, which associates with the activated NK₁R, accelerated recycling. Inhibition of endothelin- converting enzyme-1 (ECE-1), which degrades endosomal SP, prevented recycling. Inflammation was associated with NK1R endocytosis in myenteric but not submucosal neurons. Whereas the NK₁R in uninflamed neurons recycled within 60 min, NK₁R recycling in inflamed neurons was delayed for >120 min, suggesting defective recycling machinery. Inflammation was associated with β-arrestin2 upregulation and ECE-1 downregulation, which may contribute to the defective NK₁R recycling. We conclude that inflammation evokes redistribution of NK₁R from the plasma membrane to endosomes of myenteric neurons through enhanced SP release and defective NK₁R recycling. Defective recycling may be secondary to upregulation of β-arrestin2 and downregulation of ECE-1. Internalized NK₁R may generate sustained proinflammatory signals that disrupt normal neuronal functions.