Adhesion GPCRs are widely expressed throughout the subsections of the gastrointestinal tract

Abstract

Background: G protein-coupled receptors (GPCRs) represent one of the largest families of transmembrane receptors and the most common drug target. The Adhesion subfamily is the second largest one of GPCRs and its several members are known to mediate neural development and immune system functioning through cell-cell and cell-matrix interactions. The distribution of these receptors has not been characterized in detail in the gastrointestinal (GI) tract. Here we present the first comprehensive anatomical profiling of mRNA expression of all 30 Adhesion GPCRs in the rat GI tract divided into twelve subsegments.

Methods: Using RT-qPCR, we studied the expression of Adhesion GPCRs in the esophagus, the corpus and antrum of the stomach, the proximal and distal parts of the duodenum, ileum, jejunum and colon, and the cecum.

Results: We found that twenty-one Adhesion GPCRs (70%) had a widespread (expressed in five or more segments) or ubiquitous (expressed in eleven or more segments) distribution, seven (23%) were restricted to a few segments of the GI tract and two were not expressed in any segment. Most notably, almost all Group III members were ubiquitously expressed, while the restricted expression was characteristic for the majority of group VII members, hinting at more specific/localized roles for some of these receptors.

Conclusions: Overall, the distribution of Adhesion GPCRs points to their important role in GI tract functioning and defines them as a potentially crucial target for pharmacological interventions.

Figure 1

GI sectioning approximately indicating the different sections of the rat GI tract used for analysis. The GI tract was divided into twelve segments: the esophagus, the corpus and the antrum of the stomach, the proximal and distal parts of the duodenum, ileum, jejunum and colon, and the cecum.

Figure 2

GPCRs expressed ubiquitously along the GI tract. Each panel refers to one GPCR showing three pieces of information: group of belonging, N terminal moieties and relative expression in twelve segments of the GI tract. Expression levels are relative for each gene (maximal level of expression set to 1). Values are plotted as the mean ± SD; n = 3. The phylogenetic grouping is based on the 7TM regions; the schematic representation of the domains in the N-termini as determined by RT-BLAST at NCBI is adapted from our previous work [3]. Abbreviations for N-termini moieties: GPS, GPCR proteolytic site; HBD, hormone-binding domain; EGF, epidermal growth factor; OLF, olfactomedin; GBL, galactose-binding lectin domain; LRR, leucine rich repeats; Ig, immunoglobulin; SEA, sperm protein, enterokinase, and agrin; glycosylation sites (NXS or NXT tripeptide sequences that conform to the consensus sequence for N-linked glycosylation) – shown as small circles attached to the N-termini stretches; E, esophagus; F, corpus of the stomach; A, antrum of the stomach; D1 and D2, proximal and distal parts of the duodenum; J1 and J2, proximal and distal parts of the jejunum; I1 and I2, proximal and distal parts of the ileum; C, cecum; K1 and K2, proximal and distal parts of the colon.

Figure 3

GPCRs with widespread expression along the GI tract. Each panel refers to one GPCR showing three pieces of information: group of belonging, N terminal moieties and relative expression in twelve segments of the GI tract. Expression levels are relative for each gene (maximal level of expression set to 1). Values are plotted as the mean ± SD; n = 3. Abbreviations for N-termini moieties: GPS, GPCR proteolytic site; HBD, hormone-binding domain; TSP1, thrombospondin; PTX, pentraxin domain; EGF, epidermal growth factor; CA, cadherin domains; LamG, laminin; glycosylation sites (NXS or NXT tripeptide sequences that conform to the consensus sequence for N-linked glycosylation) – shown as small circles attached to the N-termini stretches; E, esophagus; F, corpus of the stomach; A, antrum of the stomach; D1 and D2, proximal and distal parts of the duodenum; J1 and J2, proximal and distal parts of the jejunum; I1 and I2, proximal and distal parts of the ileum; C, cecum; K1 and K2, proximal and distal parts of the colon.

Figure 4

GPCRs with limited expression along the GI tract. Each panel refers to one GPCR showing three pieces of information: group of belonging, N terminal region and relative expression in twelve segments of the GI tract. Expression levels are relative for each gene (maximal level of expression set to 1). Values are plotted as the mean ± SD; n = 3. GPR111, GPR113, GPR115 were only expressed in one sub-segment: the expression value was arbitrarily set to 1 to indicate the presence of expression. Abbreviations for N-termini moieties: GPS, GPCR proteolytic site; HBD, hormone-binding domain; TSP1, thrombospondin; PTX, pentraxin domain. Glycosylation sites (NXS or NXT tripeptide sequences that conform to the consensus sequence for N-linked glycosylation) are shown as small circles attached to the N-termini stretches; E, esophagus; F, corpus of the stomach; A, antrum of the stomach; D1 and D2, proximal and distal parts of the duodenum; J1 and J2, proximal and distal parts of the jejunum; I1 and I2, proximal and distal parts of the ileum; C, cecum; K1 and K2, proximal and distal parts of the colon.

Figure 5

GPCRs whose high expression is associated with specific subsegments of the GI tract. The figure shows GPCRs whose levels were found to be markedly higher in a given segment relative to the expression level in other adjacent segments. For each region, two or three genes with the highest regionally specific expression levels are shown. To better illustrate the extent to which a given gene is highly expressed in a subsegment-specific manner compared with adjacent segments, a three-level system was adopted using the * symbol, assigning none, one or two symbols to each gene. The greater the deviation in expression level is from the surrounding expression pattern, the more asterisks have been assigned for a given gene. It should be noted that the distal duodenum and the cecum are not shown in the figure since no region-specific high gene expression levels were found for these two segments.