The superfamily of G protein-coupled receptors (GPCRs) is one of the largest families of membrane bound proteins in the human genome  comprising about 800 members. Being involved in a high number of physiological functions, including development, neurotransmission, metabolism, reproduction, immune responses, and behavior, GPCRs act as receptors for a great number of different signals, both endogenous - amines, peptides, proteins, lipids, nucleotides, neurotransmitters - and sensory, as organic odorants, pheromones, tastes and photons. According to the phylogenetic analysis of the entire human GPCR repertoire, five subfamilies make up the GRAFS classification system: Glutamate, Rhodopsin, Adhesion, Frizzled/Taste2 and Secretin. The phylogenetic grouping of Adhesion GPCRs, based on the 7TM regions, revealed that there are seven groups .
GPCRs are characterized by having seven α-helices that span the plasma membrane and form a receptor with a binding cavity for a ligand; the extracellular segment may also be able to bind a ligand. The main feature of the Adhesion family is the long N terminus with complex domain architecture which is thought to be highly glycosylated and form a rigid structure in the outer part of the protein. This extracellular portion contains the GPCR proteolytic site (GPS) and several various domains that can also be found in other proteins such as lectin, epidermal growth factor, olfactomedin, immunoglobulin, thrombospondin and cadherin domains . The GPS domain is referred to as an intracellular cleavage motif, pivotal for the protein transport from the endoplasmic reticulum to the membrane , while several other N terminal domains play important roles in the receptor-ligand binding as well as cell-to-cell and cell-to-matrix adhesion . Distinguishing themselves even more from other GPCRs, Adhesion GPCRs are genomically complex, each receptor having many isoforms , with multiple alternatively-spliceable introns and large genomic sizes, which makes them difficult to study . Only a few members of Adhesion GPCRs have been demonstrated to interact with G proteins [8, 9].
Beyond its evident role in digestion and adsorption, the gastrointestinal (GI) tract is involved in a variety of other physiological functions, such as endo- and exocrine secretion and immune responses [10–12]. Its autonomous neuronal network, referred to as the enteric nervous system or ENS, is also intimately linked with the brain in the brain-gut axis important for, among others, food intake regulation [13, 14]. GPCRs in the GI tract are known to be involved in nutrient balancing [15–17] and regulation of the immune system [18, 19]. In some cases their gross expression patterns in the GI tract have been established, but more subtle proximodistal variations in expression and their biological functions have yet to be determined . Furthermore, dysfunction of GPCRs is already known to contribute to certain diseases affecting the GI tract. For example, GPR49 overexpression has been related to increased incidence of human colon primary tumors , whereas overexpression of the Adhesion GPCR CD97 has been correlated to colorectal cancer , rectal adenocarcinoma recurrence and metastasis , and gastric carcinoma .
Though about 85% of Adhesion GPCRs are still orphans or lack biological characterization (details about known functions are provided in ), EMR receptors have been shown to be involved in immune responses : Crohn’s disease and ulcerative colitis are two of the main inflammatory bowel diseases affecting the GI tract, and there is a growing body of evidence indicating that the onset of these pathologies may be related to an immune system deregulation by the ENS or gut microbiota [11, 12]. The characterization of immunologically important Adhesion GPCRs in the GI tract may therefore lead to a greater understanding of these pathological conditions. Profiling of the entire GI tract, accounting for intra-regional differences is important since proximal-distal part of GI tract’s anatomical regions can serve different roles or be selectively affected by diseases; e.g. bile acids and vitamin B12 adsorption occur in the distal ileum, Barrett’s disease affects the distal esophagus , and gastric cancer occurs either proximally or distally . Likewise, Crohn’s disease more frequently affects the terminal ileum . Knowledge of the proximodistal expression pattern may therefore indirectly facilitate pinpointing the function of Adhesion GPCR orphans.
We present here the first complete analysis of mRNA expression of all members of the Adhesion GPCRs subfamily throughout the entire rat GI tract, which was divided into twelve subsegments (as described previously ). Using RT-qPCR with a validated range of housekeeping genes, we studied the expression in the esophagus, the corpus and the antrum of the stomach, the proximal and distal parts of the duodenum, jejunum, ileum and colon, and in the cecum.