THEMES
G Protein-Coupled Receptors in Gastrointestinal Physiology
I. CCK
receptors: an exemplary family*
Stephen A.
Wank
Digestive Diseases Branch, National Institute of Diabetes and
Digestive and Kidney Diseases, National Institutes of Health,
Bethesda, Maryland 20892-1804
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ABSTRACT |
The CCK and gastrin families of
peptides act as hormones and neuropeptides on central and peripheral
receptors to mediate secretion and motility in the gastrointestinal
tract in the physiological response to a normal meal. Thus far, two CCK
receptors have been molecularly identified to mediate the actions of
CCK and gastrin, CCK-A and CCK-B receptors (CCK-AR and CCK-BR,
respectively). The regulation of CCK-AR and CCK-BR affinity by guanine
nucleotides and the receptor activation of G protein-dependent
stimulation of phospholipase C and adenylyl cyclase suggested that they
were guanine nucleotide-binding protein-coupled receptors [G
protein-coupled receptors (GPCRs)]; however, the eventual cloning
of their cDNAs revealed their heptahelical structure and confirmed
their membership in the GPCR superfamily. The gastrointestinal system
is a rich source of neuroendocrine hormones that interact with a large
number of GPCRs to regulate the complex tasks of digestion, absorption, and excretion of a meal. This article focuses on the CCK family of
GPCRs, and its activities in the gastrointestinal system.
cholecystokinin; gastrin; expression; cDNA; signal transduction
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INTRODUCTION |
THE PHYSIOLOGICAL REGULATION of complex organisms
requires the interaction of extracellular regulators with cell surface
and intracellular receptors of the target cell. This review addresses the largest class of receptors, the structurally related superfamily of
guanine nucleotide-binding protein-coupled receptors [G
protein-coupled receptors (GPCRs)]. It is estimated that
1-2% of mammalian genes encode 1,000 or more different proteins
belonging to this superfamily of receptors. GPCRs share the signature
seven-transmembrane-spanning,
-helical domain motif and
the ability to transduce responses to a number of diverse extracellular
stimuli, including photons, odorants, hormones, neurotransmitters,
ions, and proteases, by activating heterotrimeric G proteins that
subsequently regulate intracellular effectors. The gastrointestinal
system is a rich source of neuroendocrine hormones that interact with
at least 10 families of GPCRs containing more than 30 known receptor
subtypes to regulate the complex tasks of digestion, absorption, and
excretion of a meal. This article focuses on a single family, the CCK
family of receptors. Although CCK receptors are widely distributed
outside the gastrointestinal system, especially in the nervous system, this review is limited predominantly to the gastrointestinal system.
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THE NATIVE LIGANDS: CCK AND GASTRIN |
CCK-like peptides are ancient brain-gut peptides conserved over a long
period of evolution, having been shown to be present in neurons and gut
endocrine cells as far back as protochordates. In humans, CCK occurs as
COOH-terminal amidated 58- and 8-amino acid major forms processed from
a 115-amino acid preprohormone and is expressed in neurons throughout
the central and peripheral autonomic nervous systems and in intestinal
endocrine cells and neurons, where it is released in response to a
meal. Full biological activity resides in the COOH-terminal seven amino
acids, although full potency requires the octapeptide sulfated at the
tyrosine in the seventh position from the COOH terminus.
-Amidation
of the COOH terminus is essential for biological activity. Gastrin also
occurs in multiple molecular forms processed from a 101-amino acid
preprohormone, with the COOH-terminal amidated 34- and 17-amino acid
processed forms predominating in the duodenum and gastric antrum,
respectively, where they are released in response to a meal. Full
biological activity resides in the COOH-terminal tetrapeptide, although
with much lower potency, and as with CCK,
-amidation is essential
for biological activity (Fig. 1) (27).

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Fig. 1.
Structure of CCK, gastrin, and subtype-selective nonpeptide agonists
and antagonists. A: primary sequence
of the most predominant mammalian forms of CCK: CCK-58, CCK-33, and
CCK-8. B: major sulfated form of
gastrin (~50% sulfated), gastrin-17-II (<E, denotes a pyroglutamic
acid). C-F: planar representations of
structures of substituted benzodiazepine CCK receptor antagonists
(C-E) L-364,718 [CCK-A
receptor (CCK-AR) preferring], L-365,260 [CCK-B receptor
(CCK-BR) preferring], and L-740,093 (second generation, CCK-BR
preferring) and CCK-AR agonist GW-5823
(F).
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CCK RECEPTOR CLASSIFICATION |
Receptors for CCK were first characterized on pancreatic acinar cells
and identified as CCK type A receptors (CCK-AR), with the subsequent
discovery in the same year of a second receptor with a different
pharmacology in the brain, CCK type B receptors (CCK-BR). These two
types of CCK receptor could be pharmacologically distinguished on the
basis of their affinity for the agonists CCK and gastrin, which share
the same COOH-terminal pentapeptide amide sequence but differ in
sulfation at the sixth (gastrin) and seventh (CCK) tyrosyl residues,
and by recently developed subtype-specific antagonists. CCK-AR are
highly selective (500- to 1,000-fold) for sulfated analogs of CCK,
whereas CCK-BR have similarly high affinity for both sulfated and
nonsulfated peptide analogs of CCK and gastrin peptides. Formerly, the
gastrin receptor mediating acid secretion in the stomach was thought to
constitute a third type of high-affinity receptor on the basis of its
location, small differences in affinity for CCK and gastrin-like
peptides, and the reversal in relative affinityfor receptor
subtype-selective antagonists in canine gastric glands. Subsequent
cloning of gastrin receptors from canine stomach and CCK-BR from canine
brain revealed their molecular identity, leading to the classification
of gastrin receptors as CCK-BR (27).
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LOCATION AND FUNCTION OF CCK RECEPTORS |
Gastrin and CCK are released from gastrointestinal endocrine cells
lining the mucosa of the stomach (antral G cells) and upper small bowel
(I cells), respectively, in response to protein and lipid in a meal and
bind CCK-AR and CCK-BR present on a variety of gastrointestinal target
tissues to regulate digestion and absorption (Table
1). Gastrin regulates the
release of acid by directly activating CCK-BR on parietal cells and,
more importantly, CCK-BR on nearby enterochromaffin-like (ECL) cells
that release histamine to stimulate acid via
H2 histamine receptors on parietal
cells. Both CCK-AR and CCK-BR are present on gastric
somatostatin-releasing D cells, where CCK affects the CCK-AR to
stimulate the inhibition of acid secretion through the release of
somatostatin and subsequent inhibition of both gastrin and histamine
via somatostatin type 2 receptors on G and ECL cells. CCK-AR regulate
the release of pepsinogen from gastric chief cells. CCK-AR on smooth
muscle cells and neural intermediates cause relaxation of the lower
esophageal sphincter, increased pyloric sphincter tone, and relaxation
of the gastric corpus, resulting in delayed gastric emptying,
gallbladder contraction, relaxation of the sphincter of Oddi, decreased
small bowel transit time, and increased colonic transit time. CCK-AR on
duodenal vagal afferent neurons and to a lesser degree on pancreatic acinar cells regulate enzyme secretion in animals, whereas CCK-BR, presumably on vagal afferent neurons and intrapancreatic cholinergic neurons, regulate secretion in humans. Gastrin and CCK have trophic effects on parietal and ECL cells in the gastric mucosa and on pancreatic acini, respectively, as well as on a number of
gastrointestinal and nongastrointestinal tumoral cell lines (27).
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GASTROINTESTINAL REGULATION OF RENAL FUNCTION |
After the cloning of the human CCK-BR, a search for previously
unappreciated tissue expression of the CCK-BR by Northern blot hybridization revealed relatively high expression in the kidney. Although it had been hypothesized for many years that gastrointestinal hormones such as gastrin that become elevated in response to a meal
play a significant role in the acute regulation of the renal handling
of absorbed nutrients, the actual hormones involved as well as their
site and mechanism of action have remained elusive. Pisegna et al. (17)
describe the distribution of CCK-BR to proximal tubules in the kidney
by immunohistochemistry and RT-PCR in a pattern consistent with this
integrative function. Elevation of serum gastrin in rats either by
stimulation of a gavaged meal or infusion of physiological doses
resulted in a fourfold increase in urinary
Na+ excretion and fractional
Na+ excretion and an eightfold
increase in urine output that could be inhibited by the CCK-BR-specific
antagonist L-365,260. The studies of Pisegna et al. (17) provide, for
the first time, direct evidence for gastrointestinal hormones as
mediators of the gastrointestinal-renal axis and have profound
implications in understanding the influence of feeding on renal
function.
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CCK RECEPTOR COUPLING TO G PROTEINS |
The regulation of CCK-AR and CCK-BR affinity by guanine nucleotides and
the receptor activation of G protein-dependent stimulation of
phospholipase C (PLC) and adenylyl cyclase suggested that they were
GPCRs. Cloning of the CCK-AR from pancreas (28) and the CCK-BR from
stomach and brain (8, 29) revealed their heptahelical structure (Fig.
2) and confirmed their
membership in the GPCR superfamily. The insensitivity of CCK-AR and
CCK-BR inositol phosphate signaling to pertussis toxin suggested
coupling through the Gq family of
G proteins. More recently, a study using both PLC and G protein
-subunit-specific antibodies in pancreatic cell membranes has
indicated that both Gq and
G11
are present in pancreas and that the CCK-AR couples to Gq/11
to activate PLC-
1, as measured by CCK-8-stimulated increase in phosphatidylinositol
4,5-bisphosphate-specific PLC activity (16). Another study of CCK-AR in
acutely dispersed substantia nigra dopaminergic neurons confirmed the
presence of Gq/11 by RT-PCR and
used Gq/11-specific antibodies to
inhibit CCK-8-evoked cationic currents of whole cell patch-clamp
recordings (31). The region of the CCK-BR interacting with
Gq was determined in mutated
CCK-BR transiently expressed into COS-7 cells and
Xenopus oocytes. K333M, K334T, and
R335L (Fig. 2) mutations resulted in the loss of
Gq activation without affecting
receptor affinity (25). The COOH-terminal portion of the third
cytoplasmic loop (Ci3) of most GPCRs contains a stretch of charged
residues (including CCK-AR and CCK-BR) that are thought to form an
amphipathic
-helical extension of the sixth transmembrane domain (TM
VI) in a critical orientation for G protein activation (30); thus these
results suggest that the CCK-BR Ci3 is also likely to
form an
-helical extension of TM VI necessary for the specific
activation of Gq. Similar to
several other GPCRs, CCK-AR is capable of coupling to both PLC and
adenylyl cyclase at physiological concentrations in native cells (10).
It is not clear whether this is a result of dual coupling to
Gs and
Gq or simply the result of G
protein 
-subunit activation of adenylyl cyclase. However, a
recent CCK receptor chimeric study bearing on this issue exchanged the
first intracellular loop between the CCK-AR and CCK-BR and found that Arg68 and
Asn69 in the first intracellular
loop of the CCK-AR were important for activation of adenylyl cyclase
without affecting Gq coupling. These results support the direct coupling of the CCK-AR to both Gs and
Gq (32).

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Fig. 2.
Schematic models of rat CCK-AR (A) and CCK-BR
(B). Deduced primary amino acid sequences of rat CCK-AR and
CCK-BR showing putative transmembrane helices,
NH2-linked glycosylation sites
(tridents), protein kinase C and protein kinase A
(-PO3) phosphorylation sites,
potential disulfide bridge (-S-S-) between conserved cysteines in the
first and second extracellular loops, and a potential palmitoylated
(jagged line) conserved cysteine in the cytoplasmic tail
(-NH2, amino terminus; -COOH,
carboxy terminus). , Conserved (identical) residues between CCK-AR
and CCK-BR (27).
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A NEW POTENT AND SELECTIVE CCK-BR ANTAGONIST |
The development of radioimmunoassays, radioligands with full efficacy
and potency, immunocytochemistry, and more recently receptor
subtype-specific agonists and antagonists has greatly accelerated
discoveries in gastrointestinal endocrinology. It is clear from the
distribution of CCK receptors described above that many tissues and
even single cells express both CCK-AR and CCK-BR and that gastrin is
the only native selective agonist. Therefore, the recently available
selective CCK-AR agonist, A-71378, and CCK-AR and CCK-BR
subtype-selective substituted 1,4-benzodiazepine antagonists such as
L-364,718 and L-365,260, respectively (Fig. 1), have been extremely
useful in sorting out the location and physiological functions
attributed to each receptor subtype (4). The usefulness of L-365,260
has been limited by its relatively low selectivity for CCK-BR vs.
CCK-AR (80- to 280-fold depending on the species, and even a reversal
of affinity in canines) and its partial agonist activity. Recently, a
second generation benzodiazepine antagonist, L-740,093 (Fig. 1), has
been developed with higher affinity
(IC50 = 0.1 nM for acid
secretion), higher selectivity (16,000-fold) for CCK-BR, and improved
aqueous solubility (15).
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CCK-AR AND SATIETY |
CCK acting at the CCK-AR on capsaicin-sensitive vagal afferent neurons
has been shown to reduce food intake in animals and humans (4). An
exciting recent study demonstrated that a subthreshold intraperitoneal
dose of CCK-8 acting at vagal afferent CCK-AR synergized with low doses
of leptin to cause an accelerated dose-dependent decrease in food
intake in lean mice (2). Although the peptide A-71378
[desamino-Tyr(SO4)-Nle-Gly-Trp-Nle-(N-methyl)Asp-Phe-NH2] is a high-affinity, selective agonist at the CCK-AR, an orally active
(nonpeptide) CCK-AR-selective agonist would be more useful in the
treatment of human obesity. Evaluation of a series of
3-(1H-indazol-3-yl-methyl)-1,5-benzodiazepines,
using an in vivo mouse gallbladder emptying assay, identified a CCK-AR
agonist, GW-5823 (Fig. 1), with relatively high potency
(EC50 = 70 nM), full efficacy, and
a modest 50-fold selectivity for CCK-AR vs. CCK-BR. Although these
compounds have poor oral bioavailability and high clearance, in part
due to delayed gastric emptying and rapid liver metabolism, they were
still potent and efficacious as oral anorectic agents in a rat
conditioned feeding model (5). With further improvement these
1,5-substituted benzodiazepine analogs may become the first orally
effective CCK-AR-mediated satiety agents for the treatment of human
obesity.
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DISEASE-CAUSING MUTATIONS IN GPCR |
Advances in molecular biology have led to an increased understanding of
inherited and spontaneously mutated gene-based diseases. Cloning of
mutant GPCRs, for example, has revealed the molecular basis
of human diseases due to decreased receptor function, such as color
blindness and retinitis pigmentosa (cone opsins and rhodopsin mutants),
hormone-resistant X-linked nephrogenic diabetes insipidus (V2 vasopressin receptor),
familial hypocalciuric hypercalcemia (Ca2+-sensing receptor),
hereditary isolated glucocorticoid deficiency (ACTH receptor), partial
thyrotropin resistance syndrome [thyrotropin-stimulating hormone
receptor (TSHR)], and familial Leydig cell hyperplasia (luteinizing hormone receptor). Activating mutations of
GPCRs can also cause diseases such as retinitis pigmentosa and
congenital night blindness (rhodopsin), familial male precocious
puberty (luteinizing hormone receptor), hyperfunctional
thyroid adenomas, hyperfunctional thyroid hyperplasia (TSHR), and
autosomal dominant hypocalcemia
(Ca2+-sensing receptor) (22). It
was hoped that cloning of gastrointestinal hormone receptors,
especially the widely distributed CCK receptors, would lead to
establishing the molecular basis for diseases affecting many of the
gastrointestinal organs. However, GPCR mutation-based diseases tend to
be quite rare. An interesting case has been reported of a woman with
gallstones and obesity ascribed to abnormal processing of transcripts
from a normal CCK-AR gene, resulting in the predominance of mRNA with a
262-bp deletion corresponding to the third exon. Unfortunately, other
affected family members were not examined, and expected splicing
abnormalities in messages for other genes were not studied, so that an
association could only be established between the common phenotype of
gallstones and obesity and the putative RNA processing abnormality in
the affected patient (11).
The rodent Mastomys natalensis, with a
genetic predisposition toward development of gastric carcinoids of ECL
origin, has served as a model for studying the neoplastic potential of
hypergastrinemia acting at the ECL CCK-BR. Hypergastrinemia acclerates
the development of Mastomys gastric
carcinoids in which the ECL cells have increased CCK-BR expression
compared with native ECL cells. This association has prompted further
investigation into the possible growth-promoting role of CCK-BR in the
transformation of ECL cells (12). Schaffer et al. (21) recently
demonstrated a higher basal level of agonist-independent signaling of
Mastomys CCK-BR in transfected COS
cells compared with human CCK-BR, due to a four-amino acid species
polymorphism in the sixth transmembrane domain, suggesting a possible
contributing mechanism underlying the propensity of
Mastomys to develop gastric carcinoids. Whether these results are applicable to human disease is
uncertain; however, hypergastrinemia, due either to achlorhydria or
sporadic Zollinger-Ellison syndrome, rarely results in ECL carcinoids.
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TARGETED DISRUPTION OF THE CCK-BR GENE IN TRANSGENIC MICE |
The development of transgenic animals has already provided insight into
tissue and temporal expression of hormones such as gastrin (26). More
recently, transgenic animals with targeted disruption of individual
genes for gastrointestinal hormones and their GPCRs have been created
with the hope of revealing previously unknown sites of expression and
physiological function as well as confirming our present knowledge.
Targeted disruption of the CCK-BR caused selective decreased growth of
gastric parietal and ECL cells, confirming the growth-promoting effects
of gastrin at the CCK-BR seen in patients with hypergastrinemia due to
Zollinger-Ellison syndrome. Also, as expected, these mice were
hypochlorhydric and hypergastrinemic (13). Perhaps more interesting and
somewhat disappointing was the absence of other abnormalities, given
the wide expression and wide range of physiological functions
attributed to CCK-BR both within and outside the gastrointestinal
system. This absence may be due to compensation by other perhaps
redundant hormones and their receptors serving the same function. The
effect of disrupting the CCK-BR on gastric parietal and ECL cells on acid production was recently reinforced by Koh et al. (7) by disruption
of the gastrin gene, which also reduced the number of chief
cells. In addition, there was a decreased rate of
proliferation of the colonic mucosa, although without a change in
histological appearance (7). The effects on the colonic mucosa are
likely to be the result of progastrin and glycine-extended gastrin
acting on a non-CCK-BR.
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STRUCTURAL BASIS FOR CCK AND GASTRIN BINDING TO CCK RECEPTORS |
GPCRs are activated by a chemically diverse group of ligands, despite
their overall structural similarity. Unfortunately, because GPCRs are
integral membrane proteins, generation of high-quality crystals
necessary for detailed X-ray structural analysis to help explain this
observed diversity has so far been unsuccessful. However, using the
low-resolution electron cryomicroscopic structure of rhodopsin and
sequence comparison of more than 200 GPCRs, a general working model of
transmembrane helical orientation, packing, and alignment has been
proposed (1), and the usefulness of its basic features has been
confirmed by a number of biochemical and molecular genetic mutational
studies. From this model, it has been determined that almost all small
ligand-interacting amino acids are oriented toward the central,
hydrophilic cleft of the receptor. Although the structural features of
the transmembrane core of the receptor are becoming clearer, the
structure of the more variable and less ordered extracellular surface
of GPCRs remains unclear. Peptide ligands are thought to interact with amino acids at the top of the transmembrane domains and in the less
well-characterized extracellular loops of GPCRs (3).
A study examining 58 chimeric receptors, in which one to four divergent
amino acids in the transmembrane domain of the CCK-BR were replaced
with the corresponding amino acids from the CCK-AR, identified only a
single residue,
Ser131, at the top
of TM III, conferring approximately sixfold subtype selectivity for the
peptide agonist gastrin vs. CCK-8 (9).
Elegant studies examining the extracellular
NH2 terminus of the CCK-AR,
utilizing first a 42-amino acid
NH2-terminal truncation of the
human CCK-AR and subsequently site-directed mutants in the region near
the top of TM I, suggested the interaction of amino acid residues
Trp39 and
Gln40 with CCK (Fig. 2). Further
binding between wild-type,
Trp39-Phe and
Gln40-Asn mutant CCK-AR and a
series of NH2-terminally modified
CCK analogs that were applied to a model of the CCK-AR (based on data from bacteriorhodopsin, rhodopsin, and the
-adrenergic receptors) suggested that the NH2-terminal
moiety of CCK-8 interacts via hydrogen bonding to
Trp39 and
Gln40 (6).
The interaction of CCK with the CCK-AR was further modeled in a study
(24) describing the separate single amino acid mutations of
Lys105-Val and
Arg337-Val (Fig. 2) in the CCK-AR
expressed in CHO cells that resulted in a loss in CCK-8-stimulated
Ca2+ release. The loss of
Ca2+ release in the cells
expressing CCK-AR Lys105-Val and
Arg337-Val was attributed to the
loss of interaction with CCK at
Tyr(SO3H)52
and Asp57 (Fig. 1; CCK-58),
respectively (24).
A study of the rat CCK-BR using chimeric CCK-AR/CCK-BR to determine the
structural basis of CCK-BR subtype selectivity for gastrin vs. CCK-8
identified the importance of the second extracellular loop.
Site-directed mutagenesis of the second extracellular loop suggested
that a segment of five amino acids (near
Cys205, which putatively forms a
disulfide bridge with Cys127 at
the top of TM III) (Fig. 2) was important for gastrin selectivity (19).
Independent human chimeric CCK-AR/CCK-BR studies based on exon
shuffling of the respective receptor genes also demonstrated the
importance of this area near the top of TM III for conferring high
gastrin affinity (32).
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CCK RECEPTOR REGULATION |
Physiological systems regulate signaling between extracellular stimuli
and cellular GPCRs at several levels, including secretion, clearance,
and degradation of the signaling molecule; desensitization, endocytosis, and downregulation of the receptor; and modulation of
postreceptor effectors. Desensitization, the earliest process in the
attenuation of GPCR signaling, has been best characterized for
rhodopsin and
-adrenergic receptors, where phosphorylation by a
family of G protein-coupled receptor kinases (GRK) and second messenger
kinases such as protein kinase A (PKA) [and for some other
receptors, protein kinase C (PKC)] leads to the uncoupling of the
receptors from their respective G proteins. For the CCK-AR, CCK at
supraphysiological concentrations causes rapid phosphorylation by both
PKC and GRK of native receptors in pancreatic acini and by PKC alone in
recombinant transfected CHO cells (14). CCK-AR phosphorylation accounts
for ~50% of the first 1- to 2-min component of desensitization of
the CCK-AR expressed in CHO cells, with longer-term and near-complete
attenuation attributed to internalization to either a site of
"insulation" indistinguishable from the plasma membrane in
pancreatic acinar cells or to a deeper endocytic compartment in CHO
cells (20). Internalization is independent of the state of
phosphorylation and the COOH-terminal tail of the CCK-AR (18, 20),
unlike the CCK-BR, which is ~80% dependent on potential Ser/Thr
residues in the COOH terminus for internalization (18). Fusion of the
green fluorescent protein (GFP) from jellyfish to the COOH terminus of
the CCK-AR did not alter ligand affinity, signal transduction,
expression, or internalization and allowed for real-time simultaneous
studies of both fluorescent ligand and receptor trafficking. Studies of
CCK-AR/GFP stably expressed in CHO, NIH/3T3, and HeLa cells and
transiently expressed in COS-1 cells demonstrated that receptor
internalization was predominantly ligand dependent, except in NIH/3T3
cells, where it was constitutive but inhibitable by the peptide and
nonpeptide antagonists CCK-(27
32) amide and L-364,718, respectively.
The fluorescent ligand, Cy3.29-CCK-8, dissociated from the receptor
with a half time of ~25 min, with subsequent recycling of the
receptor back to the cell membrane in ~60 min, while only
Cy3.29-CCK-8 sorted to the lysosomes (23). GFP tagging should be
applicable for the study in real time of the regulation and trafficking
of other GPCR receptors and their ligands in living cells.
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CONCLUSIONS |
The gastrointestinal system consists of a variety of tissues, each with
a specific function necessary for the effective handling of a meal. The
coordination of these separate functions is achieved in large part by
molecules of neuroendocrine origin that regulate their target cells
through GPCR-mediated signaling. The large number of GPCR families and
their subtypes allows for a specificity necessary for the complexity of
the task. Recent advances in the identification and cloning of many of
the GPCRs have accelerated our understanding of how the physiological
processes within the gastrointestinal system are regulated. GPCRs share
structural features and signal-transduction mechanisms that allow for
some generalization across families; however, there are significant differences even between subtypes and the cell system in which they are
expressed that necessitate the specific analysis of individual receptors. The understanding of CCK receptors as a single family of
GPCRs that is representative of numerous other GPCR families widely
distributed throughout the gastrointestinal system has benefitted from
the early development of radioimmunoassays for their peptide ligands
and more recently from radioligands and specific antagonists. Most
recently, the molecular cloning of their cDNAs and genes has allowed
new approaches for determining receptor localization, the structural
basis for receptor ligand and effector interactions in transfected cell
systems, and the assignment of physiological roles through their
molecular manipulation in transgenic animals. Ultimately these new
approaches to the understanding of GPCRs hold the promise of improved
diagnosis and therapy of diseases resulting from receptor hypo- and
hyperfunctioning states, some of which may be gene based.
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FOOTNOTES |
*
First in a series of invited articles on G Protein-Coupled
Receptors in Gastrointestinal Physiology.
Address reprint requests to Bldg. 10, Rm. 9C103, National
Institutes of Health, Bethesda, MD 20892-1804.
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AJP Gastroint Liver Physiol 274(4):G607-G613