Profiling of G protein-coupled receptors in vagal afferents reveals novel gut-to-brain sensing mechanisms

Abstract

ObjectivesG protein-coupled receptors (GPCRs) act as transmembrane molecular sensors of neurotransmitters, hormones, nutrients, and metabolites. Because unmyelinated vagal afferents richly innervate the gastrointestinal mucosa, gut-derived molecules may directly modulate the activity of vagal afferents through GPCRs. However, the types of GPCRs expressed in vagal afferents are largely unknown. Here, we determined the expression profile of all GPCRs expressed in vagal afferents of the mouse, with a special emphasis on those innervating the gastrointestinal tract.MethodsUsing a combination of high-throughput quantitative PCR, RNA sequencing, and in situ hybridization, we systematically quantified GPCRs expressed in vagal unmyelinated Nav1.8-expressing afferents.ResultsGPCRs for gut hormones that were the most enriched in Nav1.8-expressing vagal unmyelinated afferents included NTSR1, NPY2R, CCK1R, and to a lesser extent, GLP1R, but not GHSR and GIPR. Interestingly, both GLP1R and NPY2R were coexpressed with CCK1R. In contrast, NTSR1 was coexpressed with GPR65, a marker preferentially enriched in intestinal mucosal afferents. Only few microbiome-derived metabolite sensors such as GPR35 and, to a lesser extent, GPR119 and CaSR were identified in the Nav1.8-expressing vagal afferents. GPCRs involved in lipid sensing and inflammation (e.g. CB1R, CYSLTR2, PTGER4), and neurotransmitters signaling (CHRM4, DRD2, CRHR2) were also highly enriched in Nav1.8-expressing neurons. Finally, we identified 21 orphan GPCRs with unknown functions in vagal afferents.ConclusionOverall, this study provides a comprehensive description of GPCR-dependent sensing mechanisms in vagal afferents, including novel coexpression patterns, and conceivably coaction of key receptors for gut-derived molecules involved in gut-brain communication.

Brain accessibility delineates the central effects of circulating ghrelin

“…Thus, constitutive GHSR activity and/or GHSR interactions with other receptors could signal in the brain and trigger a variety of physiological responses independently of plasma ghrelin. Alternatively, ghrelin could signal to the brain via its action on afferent vagal fibres derived from neurones located in the nodose ganglion; however, the role of vagal afferents mediating central ghrelin actions in mice is currently controversial . It has been also speculated that ghrelin could be generated within the brain after a GOAT‐mediated octanoylation of plasma‐derived des‐acyl‐ghrelin; nevertheless, such a possibility appears to be unlikely because GOAT mRNA levels in the brain are very low, if present at all …”

Regulatory peptides and systems biology: A new era of translational and reverse‐translational neuroendocrinology

“…The CVOs are brain structures that lack a BBB and express a number of receptors for circulating signals, being able to sense the blood‐borne levels of several regulatory peptides 63,64 . Similarly, vagal afferents also express a variety of receptors for circulating regulatory peptides, actively transporting receptor proteins to nerve terminals, and collecting information about local and circulating peptide levels 65,66 . Information about peripheral regulatory peptides is integrated with other CNS inputs at the CVOs and/or the nucleus tractus solitarius of the brain stem (from vagal afferents).…”

Seeing through sensory ganglia

“…Notably, aside from the genetic tools mentioned below, recent technological advances in microscopy now make it possible to visualize neurons and their projections throughout the entire mouse body (Cai et al, 2018). In addition, several laboratories have recently started to make use of the power of the Cre-LoxP technology to study the anatomy and functions of molecularly defined vagal neurons in the laboratory rodent (Chang, Strochlic, Williams, Umans, & Liberles, 2015;Egerod et al, 2008;Mastitskaya et al, 2012;Williams et al, 2016). The cover image is yet another example, taken from our laboratory, of how new tools in molecular neuroanatomy can help advance our understanding of the complexity of vagal afferents.…”