Roles of cholecystokinin receptor phosphorylation in agonist-stimulated desensitization of pancreatic acinar cells and receptor-bearing Chinese hamster ovary cholecystokinin receptor cells

Contributing mechanisms underlying desensitization of cholecystokinin-induced activation of primary nodose ganglia neurons

Cholecystokinin (CCK) is a gut-derived peptide that potently promotes satiety and facilitates gastric function in part by activating G protein-coupled CCK1 receptors on primary vagal afferent neurons. CCK signaling is dynamic and rapidly desensitizes, due to decreases in either receptor function and the resulting signal cascade, ion channel effectors, or both. Here we report a decay-time analytical approach using fluorescent calcium imaging that relates peak and steady-state calcium responses in dissociated vagal afferent neurons, enabling discrimination between receptor and ion channel effector functions. We found desensitization of CCK-induced activation was predictable, consistent across cells, and strongly concentration dependent. The decay-time constant (tau) was inversely proportional to CCK concentration, apparently reflecting the extent of receptor activation. To test this possibility, we directly manipulated the ion channel effector(s) with either decreased bath calcium or the broad-spectrum pore blocker ruthenium red. Conductance inhibition diminished the magnitude of the CCK responses without altering decay kinetics, confirming changes in tau reflect changes in receptor function selectively. Next, we investigated the contributions of the PKC and PKA signaling cascades on the magnitude and decay-time constants of CCK calcium responses. While inhibition of either PKC or PKA increased CCK calcium response magnitude, only general PKC inhibition significantly decreased the decay-time constant. These findings suggest that PKC alters CCK receptor signaling dynamics, while PKA alters the ion channel effector of the CCK response. This analytical approach should prove useful in understanding receptor/effector changes underlying acute desensitization of G-protein coupled signaling and provide insight into CCK receptor dynamics.

CCK, PYY and PP: the control of energy balance

The control of food intake consists of neural and hormonal signals between the gut and central nervous system (CNS). Gut hormones such as CCK, PYY and PP signal to important areas in the CNS involved in appetite regulation to terminate a meal. These hormones can act directly via the circulation and activate their respective receptors in the hypothalamus and brainstem. In addition, gut vagal afferents also exist, providing an alternative pathway through which gut hormones can communicate with higher centres through the brainstem. Animal and human studies have demonstrated that peripheral administration of certain gut hormones reduces food intake and leads to weight loss. Gut hormones are therefore potential targets in the development of novel treatments for obesity and analogue therapies are currently under investigation.

Sensitivity of cholecystokinin receptors to membrane cholesterol content

Cholesterol represents a structurally and functionally important component of the eukaryotic cell membrane, where it increases lipid order, affects permeability, and influences the lateral mobility and conformation of membrane proteins. Several G protein-coupled receptors have been shown to be affected by the cholesterol content of the membrane, with functional impact on their ligand binding and signal transduction characteristics. The effects of cholesterol can be mediated directly by specific molecular interactions with the receptor and/or indirectly by altering the physical properties of the membrane. This review focuses on the importance and differential effects of membrane cholesterol on the activity of cholecystokinin (CCK) receptors. The type 1 CCK receptor is quite sensitive to its cholesterol environment, while the type 2 CCK receptor is not. The possible structural basis for this differential impact is explored and the implications of pathological states, such as metabolic syndrome, in which membrane cholesterol may be increased and CCK1R function may be abnormal are discussed. This is believed to have substantial potential importance for the development of drugs targeting the CCK receptor.

Ligand-induced internalization of the type 1 cholecystokinin receptor independent of recognized signaling activity

Receptor ligands, identified as antagonists, based on the absence of stimulation of signaling, can rarely stimulate receptor internalization. d-Tyr-Gly-[(Nle(28,31),d-Trp(30))CCK-26-32]-2-phenylethyl ester (d-Trp-OPE) is such a ligand that binds to the cholecystokinin (CCK) receptor and stimulates internalization. Here, the molecular basis of this trafficking event is explored, with the assumption that ligand binding initiates conformational change, exposing an epitope to direct endocytosis. Ligand-stimulated internalization was studied morphologically using fluorescent CCK and d-Trp-OPE. d-Trp-OPE occupation of Chinese hamster ovary cell receptors stimulated internalization into the same region as CCK. Arrestin-biased action was ruled out using morphological translocation of fluorescent arrestin 2 and arrestin 3, moving to the membrane in response to CCK, but not d-Trp-OPE. Possible roles of the carboxyl terminus were studied using truncated receptor constructs, eliminating the proline-rich distal tail, the serine/threonine-rich midregion, and the remainder to the vicinal cysteines. None of these constructs disrupted d-Trp-OPE-stimulated internalization. Possible contributions of transmembrane segments were studied using competitive inhibition with peptides that also had no effect. Intracellular regions were studied with a similar strategy using coexpressing cell lines. Peptides corresponding to ends of each loop region were studied, with only the peptide at the carboxyl end of the third loop inhibiting d-Trp-OPE-stimulated internalization but having no effect on CCK-stimulated internalization. The region contributing to this effect was refined to peptide 309-323, located below the recognized G protein-association motif. While a receptor in which this segment was deleted did internalize in response to d-Trp-OPE, it exhibited abnormal ligand binding and did not signal in response to CCK, suggesting an abnormal conformation and possible mechanism of internalization distinct from that being studied. This interpretation was further supported by the inability of peptide 309-323 to inhibit its d-Trp-OPE-stimulated internalization. Thus the 309-323 region of the type 1 CCK receptor affects antagonist-stimulated internalization of this receptor, although its mechanism and interacting partner are not yet clear.

Deficits in gastrointestinal responses controlling food intake and body weight

The gastrointestinal tract serves as a portal sensing incoming nutrients and relays mechanical and chemosensory signals of a meal to higher brain centers. Prolonged consumption of dietary fat causes adaptive changes within the alimentary, metabolic, and humoral systems that promote a more efficient process for energy metabolism from this rich source, leading to storage of energy in the form of adipose tissue. Furthermore, prolonged ingestion of dietary fats exerts profound effects on responses to signals involved in termination of a meal. This article reviews the effects of ingested fat on gastrointestinal motility, hormone release, and neuronal substrates. It focuses on changes in sensitivity to satiation signals resulting from chronic ingestion of high-fat diet, which may lead to disordered appetite and dysregulation of body weight.

Reduced sensitivity to cholecystokinin in male rats fed a high-fat diet is reversible

Adult rats chronically fed a high-fat (HF) diet maintain reduced sensitivity to cholecystokinin (CCK). We hypothesized that, similar to adult rats, pups fed a HF diet would also exhibit reduced sensitivity to CCK. To test this, male pups fed low-fat (LF) and HF isoenergetic (16.2 kJ/g) diets were administered CCK intraperitoneally (0.125-1 microg/kg) 1 wk following dietary adaptation. After receiving 0.5 microg/kg CCK, pups fed the HF diet suppressed food intake less (8.9 +/- 5.0%) than pups fed the LF diet (28.9 +/- 4.7%; P < 0.05) relative to intakes after saline administration. We then assessed the development and extinction of changes in CCK sensitivity by switching the diets between the groups. The HF-fed group, when switched to the LF diet, regained sensitivity by wk 4 and suppressed food intake following administration of 0.25 microg/kg CCK (33.1 +/- 5.7%; P < 0.05). The LF-fed group, when switched to the HF diet, lost sensitivity by wk 2 and did not suppress food intake after administrations of CCK compared with saline. Finally, we examined if HF-fed rats have an increased sensitivity to corn oil during brief access tests using a multibottle gustometer. At oil concentrations of 25, 75, and 100%, rats fed the HF diet sampled more oil than LF-fed rats (P < 0.05). These findings demonstrate that male rat pups fed a HF diet exhibit reduced sensitivity to CCK, the development of this reduced sensitivity is quicker than its extinction, and rats consuming a HF diet have increased oral sensitivity to oils.

Structural basis of cholecystokinin receptor binding and regulation

Two structurally-related guanine nucleotide-binding protein-coupled receptors for two related peptides, cholecystokinin (CCK) and gastrin, have evolved to exhibit substantial diversity in specificity of ligand recognition, in their molecular basis of binding these ligands, and in their mechanisms of biochemical and cellular regulation. Consistent with this, the CCK1 and CCK2 receptors also play unique and distinct roles in physiology and pathophysiology. The paradigms for ligand recognition and receptor regulation and function are reviewed in this article, and should be broadly applicable to many members of this remarkable receptor superfamily. This degree of specialization is instructive and provides an encouraging basis for the diversity of potential drugs targeting these receptors and their actions that can be developed.

Phosphorylation and regulation of a G protein-coupled receptor by protein kinase CK2

We demonstrate a role for protein kinase casein kinase 2 (CK2) in the phosphorylation and regulation of the M₃-muscarinic receptor in transfected cells and cerebellar granule neurons. On agonist occupation, specific subsets of receptor phosphoacceptor sites (which include the SASSDEED motif in the third intracellular loop) are phosphorylated by CK2. Receptor phosphorylation mediated by CK2 specifically regulates receptor coupling to the Jun-kinase pathway. Importantly, other phosphorylation-dependent receptor processes are regulated by kinases distinct from CK2. We conclude that G protein–coupled receptors (GPCRs) can be phosphorylated in an agonist-dependent fashion by protein kinases from a diverse range of kinase families, not just the GPCR kinases, and that receptor phosphorylation by a defined kinase determines a specific signalling outcome. Furthermore, we demonstrate that the M₃-muscarinic receptor can be differentially phosphorylated in different cell types, indicating that phosphorylation is a flexible regulatory process where the sites that are phosphorylated, and hence the signalling outcome, are dependent on the cell type in which the receptor is expressed.

Mutation of the phosphorylatable residue Thr429 in Glu of the human VPAC1 led to a constitutively desensitized receptor

The hVPAC1 receptor is rapidly phosphorylated and internalized by agonists but not re-expressed at the membrane after washing. Mutation of Ser/Thr residues in the C-terminus reduced phosphorylation but not internalization that was abolished only when all the phosphorylatable residues were mutated. Substitution of Thr429 by Glu mimicking a phosphothreonin led to a mutant with unchanged binding properties, decreased coupling to adenylate cyclase consisting in a reduced VIP potency, increased basal and VIP stimulated phosphorylation, preserved internalization followed by a rapid receptor re-expression. These are the expected characteristics of a constitutively desensitized receptor, putting forward the role of Thr429 phosphorylation in that process.

Contribution of the carboxyl terminus of the VPAC1 receptor to agonist-induced receptor phosphorylation, internalization, and recycling

When exposed to vasoactive intestinal peptide (VIP), the human wild type VPAC1 receptor expressed in Chinese hamster ovary (CHO) cells is rapidly phosphorylated, desensitized, and internalized in the endosomal compartment and is not re-expressed at the cell membrane within 2 h after agonist removal. The aims of the present work were first to correlate receptor phosphorylation level to internalization and recycling, measured by flow cytometry and in some cases by confocal microscopy using a monoclonal antibody that did not interfere with ligand binding, and second to identify the phosphorylated Ser/Thr residues. Combining receptor mutations and truncations allowed identification of Ser250 (in the second intracellular loop), Thr429, Ser435, Ser448 or Ser449, and Ser455 (all in the distal part of the C terminus) as candidates for VIP-stimulated phosphorylation. The effects of single mutations were not additive, suggesting alternative phosphorylation sites in mutated receptors. Replacement of all of the Ser/Thr residues in the carboxyl-terminal tail and truncation of the domain containing these residues completely inhibited VIP-stimulated phosphorylation and receptor internalization. There was, however, no direct correlation between receptor phosphorylation and internalization; in some truncated and mutated receptors, a 70% reduction in phosphorylation had little effect on internalization. In contrast to results obtained on the wild type and all of the mutated or truncated receptors that still underwent phosphorylation, internalization of the severely truncated receptor was reversed within 2 h of incubation in the absence of the agonist. Receptor recovery was blocked by monensin, an endosome inhibitor.

Differential Effects of Modification of Membrane Cholesterol and Sphingolipids on the Conformation, Function, and Trafficking of the G Protein-coupled Cholecystokinin Receptor

The lipid microenvironment of receptors can influence their conformation, function, and regulation. Cholecystokinin (CCK)-stimulated signaling is abnormal in some forms of hyperlipidemia, suggesting the possibility of unique sensitivity to its lipid environment. Here we examined the influence of cholesterol and sphingolipids on CCK receptors in model Chinese hamster ovary cell systems having lipid levels modified. Cholesterol was modulated chemically or metabolically, and sphingolipids were modulated using a temperature-sensitive cell line (SPB-1). Receptor conformation was probed with a fluorescent full agonist ligand, Alexa 488-conjugated Gly-[Nle(28,31)]CCK-(26-33), shown previously to decrease in anisotropy and lifetime when occupying a receptor in the active conformation (Harikumar, K. G., Pinon, D. L., Wessels, W. S., Prendergast, F. G., and Miller, L. J. (2002) J. Biol. Chem. 277, 18552-18560). Anisotropy and lifetime of this probe were increased and prolonged with cholesterol enrichment, and decreased and shortened with depletion of cholesterol or sphingolipids. The increase in these parameters with cholesterol enrichment may reflect change in CCK receptor conformation toward its inactive, uncoupled state. Indeed, cholesterol enrichment resulted in nonproductive agonist ligand binding, with affinity of binding higher than normal and calcium signaling in response to this reduced. In cholesterol- and sphingolipid-depleted states, the receptor moved into conformations that were less than optimal. With cholesterol depletion, both ligand binding and signaling were decreased, yet internalization and trafficking were unperturbed. With sphingolipid depletion, ligand binding and signaling were normal, but internalization and trafficking were markedly inhibited. Of note, normal transferrin receptor trafficking through the same clathrin-dependent pathway was maintained under these conditions. Thus, lipid microenvironment of the CCK receptor is particularly important, with different lipids having distinct effects.

Structure-activity function for binding and signaling in CHO-K1 and COS-7 cells expressing the cholecystokinin A receptor

Key amino acids of the cholecystokinin (CCK) peptide for receptor binding are sulfated Y27, W30, D32, and F33-NH(2). Three-dimensional modeling showed that the CCK-A receptor (CCK-AR) antagonist devazepide penetrated into the transmembrane (TM) domains, whereas CCK was placed on the surface of the CCK-AR. Four types of rat CCK-AR cDNAs were transfected into CHO-K1 and COS-7 cells: normal CCK-AR cDNA transfected cells (wild type, WT); K120 substituted with V; K130V; and R352V. Binding of [3H]CCK-8 was observed in WT and K130V, but not in K120V and R352V. CCK caused Ca(2+) spiking in WT and K130V, whereas K120V and R352V had no effect. Three chimeras including the CCK-AR/3ibeta2 adrenergic receptor (beta2AR), 3Nibeta2AR, and 3Cibeta2AR were constructed. Two groups of point mutations in the CCK-AR3i were also made: Y252V, S274V, S281V, and S289V (non-phospho-acceptor Y or S); S260V, S264V, S271V, and S275V (phospho-acceptor S). WT and CCK-AR/3Cibeta2AR increased [Ca(2+)](i) in response to CCK; 3Nibeta2AR was vice versa. CCK failed to increase [IP(3)] in phospho-acceptor S to V without affecting binding. Non-phospho-acceptor S or Y to V showed normal response. Thus, Lys120 outside the TM2 and Arg352 outside the TM6 of the CCK-AR are amino acids interacting with Tyr[SO(3)H]27 and Asp32 of the CCK peptide for binding. Phospho-acceptor Ser groups in the CCK-AR 3Ni are amino acids for initiating cell signaling.

Measurement of Intermolecular Distances for the Natural Agonist Peptide Docked at the Cholecystokinin Receptor Expressed in Situ Using Fluorescence Resonance Energy Transfer

Fluorescence resonance energy transfer is a powerful biophysical technique used to analyze the structure of membrane proteins. Here, we used this tool to determine the distances between a distinct position within a docked agonist and a series of distinct sites within the intramembranous confluence of helices and extracellular loops of the cholecystokinin (CCK) receptor. Pseudo-wild-type CCK receptor constructs having single reactive cysteine residues inserted into each of these sites were developed. The experimental strategy included the use of the full agonist, Alexa488-CCK, bound to these receptors as donor, with Alexa568 covalently bound to the specific sites within the CCK receptor as acceptor. Site-labeling was achieved by derivatization of intact cells with a novel fluorescent methanethiosulfonate reagent. A high degree of spectral overlap was observed between receptor-bound donor and receptor-derivatized acceptors, with no transfer observed for a series of controls representing saturation of the receptor binding site with nonfluorescent ligand and use of a null-reactive CCK receptor construct. The measured distances between the fluorophore within the docked agonist and the sites within the first (residue 102) and third (residue 341) extracellular loops of the receptor were shorter than those directed to the second loop (residue 204) or to intramembranous helix two (residue 94). These distances were accommodated well within a refined molecular model of the CCK-occupied receptor that is fully consistent with all existing structure-activity and photoaffinity-labeling studies. This approach provides the initial insights into the conformation of extracellular loop regions of this receptor and establishes clear differences from analogous loops in the rhodopsin crystal structure.

Neurochemical characterization of receptor-expressing cell populations by in vivo agonist-induced internalization: insights from the somatostatin sst2A receptor

Characterization of both neurochemical phenotype of G protein-coupled receptor (GPCR)-expressing cells and receptor compartmentalization is a prerequisite for the elucidation of receptor functions in the central nervous system. However, it is often prevented by the diffuse and homogeneous distribution of receptor immunoreactivity. This is particularly true for the somatostatin (SRIF) sst2A receptor, which is largely distributed in the mammalian brain. By using this receptor as a model, we investigated whether receptor internalization, a biochemical property shared by numerous GPCRs, would reveal sst2A-expressing cell populations in the rat dorsolateral septum (LSD), a region in which SRIF might play an important modulatory role. Thirty minutes to 1 hour after intracerebroventricular injection of the sst2A receptor agonist octreotide, numerous sst2A-immunoreactive neurons and processes became apparent due to intracytoplasmic accumulation of intensely stained granules. Double-immunolabeling experiments with synaptophysin and MAP2 provided evidence that internalized sst2A receptors are predominantly localized in the somatodendritic compartment. Revealing sst2A receptor-expressing cell bodies permitted to analyze their neurotransmitter content. Quantitative analysis demonstrated an extensive overlap (approximately 85%) between SRIF- and sst2A-expressing neuronal populations. Additionally, numerous SRIF-immunoreactive axon-like terminals were found in close apposition with sst2A-positive cell bodies and dendrites. Taken together, these data suggest that the sst2A receptor is predominantly expressed in LSD neurons as a postsynaptic autoreceptor, thus providing novel neuroanatomic clues to elucidate SRIF neurotransmission in this region. More generally, in vivo agonist-induced internalization appears as a rapid and powerful tool for the neurochemical characterization of GPCR-expressing cell populations in the mammalian brain.

Phosphorylation of the receptor for PTH and PTHrP is required for internalization and regulates receptor signaling

We have previously shown that agonist-dependent phosphorylation of the PTH/PTHrP receptor occurs on its carboxyl-terminal tail. Using site-directed mutagenesis, phosphopeptide mapping, and direct sequencing of cyanogen bromide-cleaved fragments of phosphoreceptors, we report here that PTH-dependent phosphorylation occurs on the serine residues at positions 491, 492, 493, 495, 501, and 504, and that the serine residue at position 489 is required for phosphorylation. When these seven sites were mutated to alanine residues, the mutant receptor was no longer phosphorylated after PTH stimulation. The phosphorylation-deficient receptor, stably expressed in LLCPK-1 cells, was impaired in PTH-dependent internalization and showed an increased sensitivity to PTH stimulation; the EC(50) for PTH-stimulated cAMP accumulation was decreased by 7-fold. Furthermore, PTH stimulation of the phosphorylation-deficient PTH/PTHrP receptor caused a sustained elevation in intracellular cAMP levels. These data indicate that agonist-dependent phosphorylation of the PTH/PTHrP receptor plays an important role in receptor function.

Diminished satiation in rats exposed to elevated levels of endogenous or exogenous cholecystokinin

Rats maintained on a high-fat (HF) diet exhibit reduced sensitivity to the satiation-producing effect of exogenous CCK. Because more CCK is released in response to HF meals than low-fat (LF) meals, we hypothesized that increased circulating CCK associated with ingestion of HF diets contributes to the development of decreased CCK sensitivity. To test this hypothesis, we implanted osmotic minipumps filled with either NaCl or CCK octapeptide into the peritoneal cavity. Subsequently, we examined the effect of intraperitoneal NaCl or CCK (0.5 microg/kg) injection on 30-min food intake. CCK significantly reduced 30-min food intake less in rats implanted with CCK-releasing minipumps compared with those with NaCl-releasing minipumps. Because dietary protein is a potent releaser of endogenous CCK, we hypothesized that rats adapted to a high-protein (HP) diet might also exhibit reduced sensitivity to exogenous CCK. Therefore, in a second experiment, we examined CCK-induced reduction of food intake in rats maintained on LF and rats maintained on HF or HP. Ingestion of LF stimulates very little endogenous CCK secretion, whereas both HF and HP markedly increase plasma CCK concentrations. Both doses of CCK reduced food intake significantly less in HF and HP rats compared with LF rats. There were no differences in 24-h food intake, body weight, or body fat composition among LF-, HF-, and HP-fed rats. These results are consistent with the hypothesis that sustained elevation of CCK either by infusion of exogenous CCK or by dietary-induced elevation of plasma CCK contributes to the development of reduced sensitivity to exogenous CCK.

Impaired resensitization and recycling of the cholecystokinin receptor by co-expression of its second intracellular loop

Intermolecular interaction represents an important theme in regulation of intracellular trafficking of organelles that can be interrupted by competitive overexpression of a relevant molecular domain. We attempted to identify the functional importance of intracellular domains of the cholecystokinin (CCK) receptor by their over-expression in receptor-bearing Chinese hamster ovary (CHO-CCKR) cell lines. Although clathrin-dependent endocytosis and recycling of this receptor are well-established (J Cell Biol 128:1029-1042, 1995), any influence of distinct receptor domains is not understood. In this work, constructs representing each of the intracellular domains of the CCK receptor were coexpressed with wild-type receptor, and stable clonal cell lines were selected. Each was characterized for ligand binding and agonist-stimulated biological activity (inositol 1,4,5-trisphosphate generation), desensitization, resensitization, receptor internalization, and recycling. Each cell line expressed normal CCK radioligand binding, signaling, internalization, and desensitization. Three independent cell lines that coexpressed the 25-residue second intracellular loop domain exhibited deficient resensitization. In morphological assessment of receptor trafficking, this construct was also shown to interfere with receptor recycling to the plasma membrane. As a control, recycling of an unrelated G protein-coupled receptor was demonstrated to occur normally in this cell line. These observations suggest that rather than representing passive cargo within an endosome, a receptor can influence its own trafficking within the cell.

CCK receptor phosphorylation exposes regulatory domains affecting phosphorylation and receptor trafficking

Agonist-stimulated phosphorylation of guanine nucleotide-binding protein (G protein)-coupled receptors has been recognized as an important mechanism for desensitization by interfering with coupling of the activated receptor with its G protein. We recently described a mutant of the CCK receptor that modified two of five key sites of phosphorylation (S260,264A) and eliminated agonist-stimulated receptor phosphorylation, despite normal ligand binding and signaling (20). As expected, this nonphosphorylated mutant had impaired rapid desensitization but was ultimately able to be desensitized by normal receptor internalization. Here we demonstrate that this mutant receptor is also defective in resensitization, with abnormal recycling to the cell surface. To explore this, another receptor mutant was prepared, replacing the same serines with aspartates to mimic the charge of serine-phosphate (S260,264D). This mutant was expressed in a Chinese hamster ovary cell line and shown to bind CCK normally. It had accelerated kinetics of signaling and desensitization and was phosphorylated in response to agonist occupation, with all other normal sites of phosphorylation modified. It was internalized like wild-type receptors and was resensitized and trafficked normally. This provides evidence for an additional important function for phosphorylation of G protein-coupled receptors. Phosphorylation may induce a conformational change in the receptor to expose other potential sites of phosphorylation and to expose domains involved in the targeting and trafficking of endosomes. The hierarchical phosphorylation of these sites may play a key role in receptor regulation.

Deletion of the serotonin 5-HT2C receptor PDZ recognition motif prevents receptor phosphorylation and delays resensitization of receptor responses

Phosphorylation-deficient serotonin 5-HT(2C) receptors were generated to determine whether phosphorylation promotes desensitization of receptor responses. Phosphorylation of mutant 5-HT(2C) receptors that lack the carboxyl-terminal PDZ recognition motif (Ser(458)-Ser-Val-COOH; DeltaPDZ) was not detectable based on a band-shift phosphorylation assay and incorporation of (32)P. Treatment of cells stably expressing DeltaPDZ or wild-type 5-HT(2C) receptors with serotonin produced identical maximal responses and EC(50) values for eliciting [(3)H]inositol phosphate formation. In calcium imaging studies, treatment of cells expressing DeltaPDZ or wild-type 5-HT(2C) receptors with 100 nm serotonin elicited initial maximal responses and decay rates that were indistinguishable. However, a second application of serotonin 2.5 min after washout caused maximal responses that were approximately 5-fold lower with DeltaPDZ receptors relative to wild-type 5-HT(2C) receptors. After 10 min, responses of DeltaPDZ receptors recovered to wild-type 5-HT(2C) receptor levels. Receptors with single mutations at Ser(458) (S458A) or Ser(459) (S459A) decreased serotonin-mediated phosphorylation to 50% of wild-type receptor levels. Furthermore, subsequent calcium responses of S459A receptors were diminished relative to S458A and wild-type receptors. These results establish that desensitization occurs in the absence of 5-HT(2C) receptor phosphorylation and suggest that receptor phosphorylation at Ser(459) enhances resensitization of 5-HT(2C) receptor responses.

Secretagogues cause ubiquitination and down-regulation of inositol 1, 4,5-trisphosphate receptors in rat pancreatic acinar cells

BACKGROUND & AIMS

The action of several exocrine pancreas secretagogues depends on the second messenger inositol 1,4, 5-trisphosphate (IP3), which, via endoplasmic reticulum-located IP3 receptors, mobilizes intracellular Ca2+ stores. Signaling pathways like this one are regulated at multiple loci. To determine whether IP3 receptors are one of these loci, we measured IP3 receptor concentration, distribution, and modification in secretagogue-stimulated rat pancreatic acinar cells.

METHODS

Isolated rat pancreatic acinar cells were exposed to cholecystokinin and other secretagogues, or rats were injected intraperitoneally with cerulein. Then samples of cells or pancreata were probed for IP3 receptor content and distribution as well as for ubiquitin association with IP3 receptors.

RESULTS

Secretagogues rapidly down-regulated acinar cell IP3 receptors both in vitro and in vivo. They also elicited receptor redistribution and caused receptors to become ubiquitinated, indicating that the ubiquitin/proteasome proteolytic pathway contributes to the down-regulation. Surprisingly, however, proteasome inhibitors did not block IP3 receptor down-regulation, and phospholipase Cbeta1 and protein kinase C also were down-regulated. Thus, secretagogues simultaneously activate an additional proteolytic pathway.

CONCLUSIONS

Secretagogues rapidly down-regulate IP3 receptors and other proteins involved in intracellular signaling by a mechanism that involves, but is not limited to, the ubiquitin/proteasome pathway. Loss of these proteins may account for the disruption of Ca2+ mobilization that occurs in models of acute pancreatitis, and may contribute to cell adaptation under physiological conditions.

Bradykinin-induced internalization of the human B2 receptor requires phosphorylation of three serine and two threonine residues at its carboxyl tail

The binding of bradykinin (BK) to B2 receptor triggers the internalization of the agonist-receptor complex. To investigate the mechanisms and the receptor structures involved in this fundamental process of receptor regulation, the human B2 receptor was mutated within its cytoplasmic tail by complementary strategies of truncation, deletion, and amino acid substitution. Ligand binding, signal transduction, internalization as well as phosphorylation were studied for the mutated receptors expressed in COS, CHO, and HEK 293 cells. Truncation of 44 out of 55 amino acid residues of the receptor's cytoplasmic tail corresponding to positions 321-364 did not alter the kinetics of BK binding and the receptor coupling to phospholipase C and phospholipase A2. By contrast, truncations after positions 320 and 334, deletions within the segment covering positions 335-351, as well as alanine substitution of serine and threonine residues within segment 335-351 diminished the internalization capacity of the mutant receptors. Mutants with a markedly reduced internalization potential failed to produce BK-induced receptor phosphorylation suggesting that phosphorylation may be involved in receptor internalization. The mutagenesis approaches converged at the conclusion that three serines in positions 339, 346, and 348 and two threonines in positions 342 and 345, contained in a sequence segment that is highly conserved between species, have a critical role in the ligand-dependent internalization and phosphorylation of kinin receptors and can intervene in these processes in an alternative manner. However, mutants lacking these residues were still sensitive to dominant-negative forms of beta-arrestin and dynamin, suggesting the existence of additional receptor structure(s) involved in the receptor sequestration through clathrin-coated vesicles.

Regulation of lateral mobility and cellular trafficking of the CCK receptor by a partial agonist

Partial agonists are effective tools for advancing development of highly selective drugs and providing insights into molecular regulation of cellular functions. Here, we explore the impact of a partial agonist on key aspects of cholecystokinin (CCK) receptor regulation, its lateral mobility and cellular trafficking, in native pancreatic acinar cells and Chinese hamster ovary cells expressing CCK receptor (CHO-CCKR). We developed and characterized a novel fluorescent partial agonist, rhodamine-Gly-[(Nle28, 31)CCK-26-32]-phenethyl ester, that binds specifically and with high affinity to CCK receptors. Such analogs are fully efficacious pancreatic acinar cell secretagogues without supramaximal inhibition that mobilize intracellular calcium with little or no increase in phospholipase C (PLC) activity. Despite minimal phosphorylation of CCK receptors in response to this partial agonist, receptor trafficking was the same as that observed with full agonist (CCK). This included normal internalization via clathrin-dependent endocytosis in CHO-CCKR cells and insulation on the surface of pancreatic acinar cells. Also, as with CCK-occupied receptor, fluorescence recovery after photobleaching of partial agonist-occupied receptor on the acinar cell surface demonstrated a marked temperature-dependent slowing of its rate of diffusion. This was similarly associated with resistance to acid-induced dissociation of ligand. Thus some key molecular regulatory mechanisms for CCK receptor internalization and insulation may be initiated by cellular signaling cascades that are not dependent on PLC activation or receptor phosphorylation.

Somatostatin receptor desensitization in NG108-15 cells. A consequence of receptor sequestration

In NG108-15 cells inhibition of both N-type calcium channel current and adenylyl cyclase by somatostatin (SRIF) was not sustained but rapidly desensitized in the continued presence of the drug. The degree and rate of desensitization were concentration-dependent, and the desensitization was homologous with respect to the delta-opioid receptor. We have been unable to obtain evidence for the involvement of G protein-coupled receptor kinases (GRKs) in this desensitization. SRIF-induced desensitization of N-type calcium channel currents was not reduced in cells stably overexpressing a dominant negative mutant of GRK2 or following intracellular dialysis with GRK2- and GRK3-blocking peptides or with heparin. Inhibitors of protein kinase A, protein kinase C, and protein kinase G were also without effect. In contrast, both the rate and degree of SRIF-induced desensitization were reduced by pretreatment with phenylarsine oxide or concanavalin A, both inhibitors of receptor endocytosis. Furthermore, SRIF-induced desensitization was enhanced by monensin, which prevents receptor recycling back to the plasma membrane. Similarly, SRIF-induced desensitization of adenylyl cyclase inhibition was not reduced in cells stably overexpressing dominant negative mutant GRK2 but was reduced in cells pretreated with the receptor endocytosis inhibitor hyperosmotic sucrose or concanavalin A. These data are consistent with the view that SRIF-induced desensitization in NG108-15 cells results from receptor internalization.

Adenosine deaminase and A1 adenosine receptors internalize together following agonist-induced receptor desensitization

A1 adenosine receptors (A1Rs) and adenosine deaminase (ADA; EC 3.5.4. 4) interact on the cell surface of DDT1MF-2 smooth muscle cells. The interaction facilitates ligand binding and signaling via A1R, but it is not known whether it has a role in homologous desensitization of A1Rs. Here we show that chronic exposure of DDT1MF-2 cells to the A1R agonist, N6-(R)-(phenylisopropyl)adenosine (R-PIA), caused a rapid aggregation or clustering of A1 receptor molecules on the cell membrane, which was enhanced by pretreatment with ADA. Colocalization between A1R and ADA occurred in the R-PIA-induced clusters. Interestingly, colocalization between A1R and ADA also occurred in intracellular vesicles after internalization of both protein molecules in response to R-PIA. Agonist-induced aggregation of A1Rs was mediated by phosphorylation of A1Rs, which was enhanced and accelerated in the presence of ADA. Ligand-induced second-messenger desensitization of A1Rs was also accelerated in the presence of exogenous ADA, and it correlated well with receptor phosphorylation. However, although phosphorylation of A1R returned to its basal state within minutes, desensitization continued for hours. The loss of cell-surface binding sites (sequestration) induced by the agonist was time-dependent (t1/2= 10 +/- 1 h) and was accelerated by ADA. All of these results strongly suggest that ADA plays a key role in the regulation of A1Rs by accelerating ligand-induced desensitization and internalization and provide evidence that the two cell surface proteins internalize via the same endocytic pathway.

Identification of a domain in the carboxy terminus of CCK receptor that affects its intracellular trafficking

The carboxy-terminal region of many guanine nucleotide-binding protein (G protein)-coupled receptors contains important regulatory sequences such as an NP(x)2-3Y motif, a site of fatty acid acylation, and serine- and threonine-rich domains. The type A CCK receptor contains all of these, yet their significance has not been examined. We have, therefore, constructed a series of receptor site mutants and truncations that interfere with each of these motifs and expressed each in Chinese hamster ovary cells where they were studied for radioligand binding, cell signaling, receptor internalization, and intracellular trafficking. Each construct was synthesized and transported appropriately to the cell surface, where CCK bound with high affinity, elicited an inositol 1,4, 5-trisphosphate response, and resulted in internalization and normal trafficking. Thus modification or elimination of each of these established sequence motifs had no substantial effect on any of these parameters of receptor and cellular function. However, an additional construct that truncated the carboxy terminus, eliminating an additional 15-amino-acid segment devoid of any currently recognized sequence motifs, resulted in a marked change in receptor trafficking, with all other parameters of receptor function normal. This mutant receptor construct was delayed at the stage of early endosomes, delaying its progress to the lysosome-enriched perinuclear compartment from the rapid time course followed by wild-type receptor and all of the other constructs. It is proposed that this region of the CCK receptor tail contains a new motif important for intracellular receptor trafficking.

Mutational analysis of the potential phosphorylation sites for protein kinase C on the CCK(A) receptor

1. Many G protein-coupled receptors contain potential phosphorylation sites for protein kinase C (PKC), the exact role of which is poorly understood. In the present study, a mutant cholecystokininA (CCK(A)) receptor was generated in which the four consensus sites for PKC action were changed in an alanine. Both the wild-type (CCK(A)WT) and mutant (CCK(A)MT) receptor were stably expressed in Chinese hamster ovary (CHO) cells. 2. Binding of [3H]-cholecystokinin-(26-33)-peptide amide (CCK-8) to membranes prepared from CHO-CCK(A)WT cells and CHO-CCK(A)MT cells revealed no difference in binding affinity (Kd values of 0.72 nM and 0.86 nM CCK-8, respectively). 3. The dose-response curves for CCK-8-induced cyclic AMP accumulation and inositol 1,4,5-trisphosphate (Ins(1,4,5)P3) formation were shifted to the left in CHO-CCK(A)MT cells. This leftward shift was mimicked by the potent inhibitor of protein kinase activity, staurosporine. However, the effect of staurosporine was restricted to CHO-CCK(A)WT cells. This demonstrates that attenuation of CCK-8-induced activation of adenylyl cyclase and phospholipase C-beta involves a staurosporine-sensitive kinase, which acts directly at the potential sites of PKC action on the CCK(A) receptor in CCK-8-stimulated CHO-CCK(A)WT cells. 4. The potent PKC activator, 12-O-tetradecanoylphorbol 13-acetate (TPA), evoked a rightward shift of the dose-response curve for CCK-8-induced cyclic AMP accumulation in CHO-CCK(A)WT cells but not CHO-CCK(A)MT cells. This is in agreement with the idea that PKC acts directly at the CCK(A) receptor to attenuate adenylyl cyclase activation. 5. In contrast, TPA evoked a rightward shift of the dose-response curve for CCK-8-induced Ins(1,4,5)P3 formation in both cell lines. This demonstrates that high-level PKC activation inhibits CCK-8-induced Ins(1,4,5)P3 formation also at a post-receptor site. 6. TPA inhibition of agonist-induced Ca2+ mobilization was only partly reversed in CHO-CCK(A)MT cells. TPA also inhibited Ca2+ mobilization in response to the G protein activator, Mas-7. These findings are in agreement with the idea that partial reversal of agonist-induced Ca2+ mobilization is due to the presence of an additional site of PKC inhibition downstream of the receptor and that the mutant receptor itself is not inhibited by the action of PKC. 7. The data presented demonstrate that the predicted sites for PKC action on the CCK(A) receptor are the only sites involved in TPA-induced uncoupling of the receptor from its G proteins. In addition, the present study unveils a post-receptor site of PKC action, the physiological relevance of which may be that it provides a means for the cell to inhibit phospholipase C-beta activation by receptors that are not phosphorylated by PKC.

Signal transduction, desensitization, and recovery of responses to thyrotropin-releasing hormone after inhibition of receptor internalization

Three independent methods were used to block internalization of the TRH receptor: cells were infected with vaccinia virus encoding a dominant negative dynamin, incubated in hypertonic sucrose, or stably transfected with a receptor lacking the C-terminal tail. Internalization was blocked in all three paradigms as judged by microscopy using a fluorescently labeled TRH agonist and biochemically. The initial inositol trisphosphate (IP3) and Ca2+ responses to TRH were normal when internalization was inhibited. The IP3 increase was sustained rather than transient, however, in cells expressing the truncated TRH receptor, implying that the C-terminal tail of the receptor may be important for uncoupling from phospholipase C. After withdrawal of TRH, cells were refractory to TRH until both ligand dissociation and resensitization of the receptor had occurred. When surface-bound TRH was removed by a mild acid wash, which did not impair receptor function, neither wild-type nor truncated receptors were able to generate full IP3 responses for about 10 min. The rate of recovery was not altered by blocking internalization. Recovery of intracellular Ca2+ responses also depended on the rate of Ca2+ pool refilling. In summary, in the continued presence of TRH, phospholipase C activity declines quickly due to receptor uncoupling; this desensitization does not take place for the truncated receptor. After TRH is withdrawn, cells are refractory to TRH. Before cells can respond, TRH must dissociate and a resensitization step, which takes place on the plasma membrane and does not require the C-terminal tail of the receptor, must occur.

G protein-coupled receptors in gastrointestinal physiology. I. CCK receptors: an exemplary family

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.