Recent studies indicate that membrane cholesterol can associate with G protein-coupled receptors (GPCRs) and affect their function. Previously, we reported that manipulation of membrane cholesterol affects ligand binding and signal transduction of the type 1 cholecystokinin receptor (CCK1R), a Class A GPCR. We now demonstrate that the closely related type 2 cholecystokinin receptor (CCK2R) does not share this cholesterol sensitivity. The sequences of both receptors reveal almost identical cholesterol interaction motifs in analogous locations in transmembrane segments two, three, four, and fi ve. The disparity in cholesterol sensitivity between these receptors, despite their close structural relationship, provides a unique opportunity to defi ne the possible structural basis of cholesterol sensitivity of CCK1R. To evaluate the relative contributions of different regions of CCK1R to cholesterol sensitivity, we performed ligand binding studies and biological activity assays of wildtype and CCK2R/CCK1R chimeric receptor-bearing Chinese hamster ovary cells after manipulation of membrane cholesterol. We also extended these studies to site-directed mutations within the cholesterol interaction motifs. The results contribute to a better understanding of the structural requirements for cholesterol sensitivity in CCK1R and provides insight into the function of other cholesterol-sensitive Class A GPCRs. Cholesterol is an important lipid component of the eukaryotic plasma membrane that has substantial effects on the physicochemical characteristics of the membrane. These include effects on the membrane rigidity and fl uidity, as well as its dimensions ( 1-3 ).

Background & Aims Corticotropin-releasing factor receptor-1 (CRF1) mediates the stress-induced colonic motor activity. Less is known about the role of CRF2 in the colonic response to stress. Methods We studied colonic contractile activity (CCA) in rats and CRF2-/-, CRF-overexpressing, and wild-type mice using still manometry; we analyzed defecation induced by acute, partial-restraint stress (PRS), and/or intraperitoneal (IP) injection of CRF ligands. In rats, we monitored activation of the colonic longitudinal muscle myenteric plexus (LMMP) neurons and localization of CRF1 and CRF2 using immunohistochemical and immunoblot analyses. We measured phosphorylation of ERK1/2 by CRF ligands in primary cultures of LMMP-neurons (PC-LMMPn) and cAMP production in HEK-293 cells transfected with CRF1 and/or CRF2. Results In rats, a selective agonist of CRF2 (urocortin 2) reduced CRF-induced defecation (>50%), CCA, and Fos expression in the colonic LMMP. A selective antagonist of CRF2 (astressin2-B) increased these responses. Urocortin 2 reduced PRS-induced CCA in wild-type and CRF-overexpressing mice, whereas disruption of CRF2 increased PRS-induced CCA and CRF-induced defecation. CRF2 co-localized with CRF1 and neuronal nitric oxide synthase in the rat colon, LMMP, and PC-LMMPn. CRF-induced phosphorylation of ERK in PC-LMMPn; this was inhibited or increased by a selective antagonist of CRF1 (NBI35965) or astressin2-B, respectively. The EC50 for the CRF-induced cAMP response was 8.6 nM in HEK-293 cells that express only CRF1; this response was suppressed 10-fold in cells that express CRF1 and CRF2. Conclusions In colon tissues of rodents, CRF2 activation inhibits CRF1 signaling in myenteric neurons and the stress-induced colonic motor responses. Disruption of CRF2 function impairs colonic coping responses to stress.

The active form of vitamin D, 1,25-dihydroxyvitamin D3, [1,25(OH)2D3] has potent actions on innate and adaptive immunity. Although endocrine synthesis of 1,25(OH)2D3 takes place in the kidney, the enzyme that catalyzes this, 25-hydroxyvitamin D-1alpha-hydroxylase (CYP27b1 in humans, Cyp27b1 in mice), is expressed at many extra-renal sites including the colon. We have shown previously that colonic expression of CYP27b1 may act to protect against the onset of colitis. To investigate this further, we firstly characterized changes in Cyp27b1 expression in a mouse model of colitis. Mice treated with dextran sodium sulfate (DSS) showed weight loss, histological evidence of colitis, and increased expression of inflammatory cytokines. This was associated with decreased renal expression of Cyp27b1 (5-fold, P=0.013) and lower serum 1,25(OH)2D3 (51.8+/-5.9 pg/nl vs. 65.1+/-1.6 in controls, P<0.001). However, expression of CYP27b1 was increased in the proximal colon of DSS mice (4-fold compared with controls, P<0.001). Further studies were carried out using Cyp27b1 null (-/-) mice. Compared with+/-controls the Cyp27b1-/-mice showed increased weight loss (4.9% vs. 22.8%, P<0.001) and colitis. This was associated with raised IL-1 in the distal colon and IL-17 in the proximal and distal colon. Conversely, DSS-treated Cyp27b1-/-mice exhibited lower IL-10 in the proximal colon and toll-like receptors 2 and 4 in the distal colon. These data indicate that both local and endocrine synthesis of 1,25(OH)2D3 affect colitis in DSS-treated mice. Lack of Cyp27b1 exacerbates disease in this model, suggesting that similar effects may occur with vitamin D deficiency.

The rat esophagus shares some cellular features with skin squamous epithelium and striated muscle that express high levels of corticotropin-releasing factor type 2 (CRF2) receptors or their cognate ligand urocortin (Ucn) 1, 2, and 3. We investigated the expression and cell signaling of CRF2 receptors and ligands in the rat esophagus and lower esophageal sphincter (LES) by RT-PCR and quantitative PCR in normal and corticosterone-treated whole esophageal tissue, laser capture microdissected layers, and isolated esophageal cells. The expression of CRF2 receptor protein and intracellular cAMP and ERK1/2 responses to CRF agonists and CRF2 antagonist were determined in cultured esophageal cells and HEK-293 cells transfected with CRF2b receptors. CRF2 was abundantly expressed in the mucosa and longitudinal muscle layers of the esophagus and LES, whereas CRF1 expression was scarce. CRF2b wild-type transcript was predominantly expressed in the esophagus, and in addition, several new CRF2 splice variants including six CRF2a isoforms were identified. Expression of Ucn 1, Ucn 2, and to a smaller extent Ucn 3, but not CRF mRNA, was detected in the esophagus and LES. Ucn 1 and Ucn 2 stimulated dose-dependent cAMP production and ERK1/2 phosphorylation in the esophageal cells, whereas CRF and CRF1 agonist, cortagine, had less potent effects. In addition, Ucn 2-stimulated cAMP and ERK responses were blocked by the CRF2 antagonist, astressin2-B. These data established the presence of a prominent CRF2 signaling system in the esophagus and LES-encompassing multiple CRF2 receptor variants and Ucn, suggesting a functional role in secretomotor activity and epithelial and muscle cell proliferation.