Pancreatic polypeptide receptors: affinity, sodium sensitivity and stability of agonist binding

Supraphysiological effects of pancreatic polypeptide on gastric motor function and nutrient tolerance in humans

Pancreatic polypeptide (PP) is known to affect food intake. In this exploratory study, we set out to investigate its supraphysiological effect on food tolerance, gastric accommodation, and emptying. In 12 healthy volunteers, 0, 3, or 10 pmol*kg-1 *min-1 PP was administered intravenously (PP0, PP3 or PP10). Thirty minutes thereafter, nutrient drink infusion (60 ml*min-1 ) through a nasogastric feeding tube was started until maximum satiation. Gastric accommodation was assessed by measuring the intragastric pressure (IGP; nasogastric manometry). In a separate test, the effect of PP0 or PP10 on gastric emptying was tested in 10 healthy volunteers and assessed using the 13 C breath test. Results are presented as mean ± SEM, and p < 0.05 was considered significant. For the IGP test, PP increased ingested nutrient volume: 886 ± 93, 1059 ± 124, and 1025 ± 125 ml for PP0, PP3, and PP10, respectively (p = 0.048). In all groups, Nadir IGP values were reached upon food intake (transformed values: 1.5 ± 0.2, 1.7 ± 0.3, and 1.6 ± 0.3 mmHg for PP0, PP3, and PP10, respectively; NS) to return to baseline thereafter. For the gastric emptying study, volunteers ingested a similar nutrient volume: 802 ± 119 and 1089 ± 128 ml (p = 0.016), and gastric half-emptying time was 281 ± 52 and 249 ± 37 min for PP0 and PP10, respectively (NS). No significant correlation between tolerated nutrient volume and IGP drop (R² < 0.01; p = 0.88 for PP0 vs. PP3 and R² =0.07; p = 0.40 for PP0 vs. PP10, respectively) or gastric half-emptying time (R² = 0.12; p = 0.32) was found. A supraphysiological PP dose enhances food tolerance; however, this effect is not mediated through gastric motility. CLINICAL TRIAL REGISTRY NUMBER: NCT03854708 is obtained from clinicaltrials.gov.

On the segregation of protein ionic residues by charge type

Based on ubiquitous presence of large ionic motifs and clusters in proteins involved in gene transcription and protein synthesis, we analyzed the distribution of ionizable sidechains in a broad selection of proteins with regulatory, metabolic, structural and adhesive functions, in agonist, antagonist, toxin and antimicrobial peptides, and in self-excising inteins and intron-derived proteins and sequence constructs. All tested groups, regardless of taxa or sequence size, show considerable segregation of ionizable sidechains into same type charge (homoionic) tracts. These segments in most cases exceed half of the sequence length and comprise more than two-thirds of all ionizable sidechains. This distribution of ionic residues apparently reflects a fundamental advantage of sorted electrostatic contacts in association of sequence elements within and between polypeptides, as well as in interaction with polynucleotides. While large ionic densities are encountered in highly interactive proteins, the average ionic density in most sets does not change appreciably with size of the homoionic segments, which supports the segregation as a modular feature favoring association.

Non-specific binding and general cross-reactivity of Y receptor agonists are correlated and should importantly depend on their acidic sectors

Non-specific binding of Y receptor agonists to intact CHO cells, and to CHO cell or rat brain particulates, is much greater for human neuropeptide Y (hNPY) compared to porcine peptide Y (pPYY), and especially relative to human pancreatic polypeptide (hPP). This binding of hNPY is reduced by alkali cations in preference to non-ionic chaotrope urea, while the much lower non-specific binding of pPYY is more sensitive to urea. The difference could mainly be due to the 10-16 stretch in 36-residue Y agonists (residues 8-14 in N-terminally clipped 34-peptides), located in the sector that contains all acidic residues of physiological Y agonists. Anionic pairs containing aspartate in the 10-16 zone could be principally responsible for non-specific attachments, but may also aid the receptor site binding. Two such pairs are found in hNPY, one in pPYY, and none in hPP. The hydroxyl amino acid residue at position 13 in mammalian PYY and PP molecules could lower conformational plasticity and the non-selective binding via intrachain hydrogen bonding. The acidity of this tract could also be important in agonist selectivity of the Y receptor subtypes. The differences point to an evolutionary reduction of promiscuous protein binding from NPY to PP, and should also be important for Y agonist selectivity within NPY receptor group, and correlate with partial agonism and out-of group cross-reactivity with other receptors.

Different regulation of atrial ANP release through neuropeptide Y2 and Y4 receptors

Neuropeptide Y (NPY) receptors are present in cardiac membranes. However, its physiological roles in the heart are not clear. The aim of this study was to define the direct effects of pancreatic polypeptide (PP) on atrial dynamics and atrial natriuretic peptide (ANP) release in perfused beating atria. Pancreatic polypeptides, a NPY Y(4) receptor agonist, decreased atrial contractility but was not dose-dependent. The ANP release was stimulated by PP in a dose-dependent manner. GR 23118, a NPY Y(4) receptor agonist, also increased the ANP release and the potency was greater than PP. In contrast, peptide YY (3-36) (PYY), an NPY Y(2) receptor agonist, suppressed the release of ANP with positive inotropy. NPY, an agonist for Y(1, 2, 5) receptor, did not cause any significant changes. The pretreatment of NPY (18-36), an antagonist for NPY Y(3) receptor, markedly attenuated the stimulation of ANP release by PP but did not affect the suppression of ANP release by PYY. BIIE0246, an antagonist for NPY Y(2) receptor, attenuated the suppression of ANP release by PYY. The responsiveness of atrial contractility to PP or PYY was not affected by either of the antagonists. These results suggest that NPY Y(4) and Y(2) receptor differently regulate the release of atrial ANP.

Determination of Affinity and Activity of Ligands at the Human Neuropeptide Y Y4Receptor by Flow Cytometry and Aequorin Luminescence

Fluorescence-labeled neuropeptide Y (NPY) has been used in flow cytometric binding assays for the determination of affinity constants of NPY Y1, Y2, and Y5 receptor ligands. Because the binding of fluorescent NPY is insufficient for competition studies at the human Y4 receptor (hY4R), we replaced Glu-4 in hPP with Lys for the derivatization with cyanine-5. Because cy5-[K(4)]hPP has high affinity (Kd 5.6 nM) to the hY4R, it was used as a probe in a flow cytometric binding assay. Specific binding of cy5-[K(4)]hPP to hY4R was visualized by confocal microscopy. The hY(4)R, the chimeric G protein G(qi5) and mitochondrially targeted apoaequorin were stably coexpressed in CHO cells. Aequorin luminescence was quantified in a microplate reader and by a CCD camera. By application of these methods 3-cyclohexyl-N-[(3-1H-imidazol-4-ylpropylamino)(imino)methyl]propanamide (UR-AK49) was discovered as the first nonpeptidic Y4R antagonist (pKi 4.17), a lead to be optimized in terms of potency and selectivity.

An ion-responsive motif in the second transmembrane segment of rhodopsin-like receptors

A L(M)xxxD(N, E) motif (x=a non-ionic amino acid residue, most frequently A, S, L or F; small capitals indicating a minor representation) is found in the second transmembrane (tm2) segment of most G-protein coupling metazoan receptors of the rhodopsin family (Rh-GPCRs). Changes in signal transduction, agonist binding and receptor cycling are known for numerous receptors bearing evolved or experimentally introduced mutations in this tm2 motif, especially of its aspartate residue. The [Na(+)] sensitivity of the receptor-agonist interaction relates to this aspartate in a number of Rh-GPCRs. Native non-conservative mutations in the tm2 motif only rarely coincide with significant changes in two other ubiquitous features of the rhodopsin family, the seventh transmembrane N(D)PxxY(F) motif and the D(E)RY(W,F) or analogous sequence at the border of the third transmembrane helix and the second intracellular loop. Native tm2 mutations with Rh-GPCRs frequently result in constitutive signaling, and with visual opsins also in shifts to short-wavelength sensitivity. Substitution of a strongly basic residue for the tm2 aspartate in Taste-2 receptors could be connected to a lack of sodium sensing by these receptors. These properties could be consistent with ionic interactions, and even of ion transfer, that involve the tm2 motif. A decrease in cation sensing by this motif is usually connected to an enhanced constitutive interaction of the mutated receptors with cognate G- proteins, and also relates to both the constitutive and the overall activity of the short-wavelength opsins.

Self-regulation of agonist activity at the Y receptors

Neuropeptide Y (NPY) is one of the most abundant neuropeptides, and is likely to be present at nanomolar levels over extended periods in the synaptic space of many forebrain areas. This might be linked to an evolved generalized toning activity through a number of other peptide receptors that use C-terminally amidated agonists (with LHRH and orexin receptors and GIR as examples). However, the Y1 and Y2 receptors (which constitute the bulk of Y receptors active in the neural matrix) possess subnanomolar affinities that, at saturating NPY levels, could produce excessive signaling, as well as receptor losses via repeated endocytosis. The related Y4 receptor shows an even higher agonist affinity, and faces the same problem in visceral and neural locations accessible to pancreatic polypeptide (PP). An examination of agonist peptide interaction with Y receptors shows that Y1 and Y4 receptors in particular (as located on either the intact cells, or on particulates derived from various cell types) develop a blockade dependent on ligand concentration, with the blocking ranks of [NPY]>>[peptide YY] (PYY) for the Y1, and [human PP]>>>[PYY-related Y4 agonist] for the Y4 receptor. This blockade is also echoed in a concentration-related reduction in biological activity of primary agonists (NPY and PP), resembling a partial agonism, and is influenced especially by the allosteric interactivity of agonists. With the Y2 receptor, the blocking by agonists is less pronounced, but the signaling by NPY-related peptides is apparently less than with PYY-related agonists. The extended occupancy and self-attenuation of primary agonist activity at Y receptors could represent an evolutionary solution contributing to a balancing of metabolic signaling, agonist clearance and receptor conservation.

Internalization of cloned pancreatic polypeptide receptors is accelerated by all types of Y4 agonists

Internalization of cloned rat or human Y4 receptors expressed in Chinese hamster ovary (CHO) cells increased with concentration of all types of Y4 agonists, including human and rat pancreatic polypeptides, the Y1 receptor group co-agonists possessing C-terminal TRPRY.NH2 pentapeptide, and a C-terminally amidated dimeric nonapeptide related to neuropeptide Y, GR231118. These peptides also inhibited forskolin-stimulated adenylyl cyclase activity in Y4 receptor-expressing cells, and stimulated the binding of 35S-labeled GTP-gamma-S to pertussis toxin-sensitive G-proteins in particulates from these cells. Peptide VD-11 (differing from GR231118 only by C-terminal oxymethylation) acted as a competitive antagonist in all of the above processes. Agonist-induced stimulation of the Y4 receptor internalization persisted in the presence of allosteric inhibitors of hPP binding, N5-substituted amilorides, which also were relatively little active in G-protein stimulation and cyclase inhibition by Y4 agonists. Acceleration of Y4 receptor internalization by agonists apparently is related to relaxation of allosteric constraints to ligand attachment and sequestration of the receptor-ligand complex.

Neuropeptide Y as a partial agonist of the Y1 receptor

In absence of receptor cycling, human/rat neuropeptide Y was found to persistently occupy the guinea pig neuropeptide Y Y1 receptors expressed on the surface of Chinese hamster ovary (CHO) cells (IC50 approximately 8 nM); a lasting occupancy was also evident with active receptor cycling. A similar blockade was obtained with the human neuropeptide Y Y1 receptor (in CHO or SK-N-MC cells). Peptidic antagonists GR238118 (1229U91) and VD-11 blocked the Y1 receptor in the same molarity range. A neuropeptide Y-related Y1 agonist, (Leu31Pro34) human neuropeptide Y, also strongly adhered to the Y1 site. Similar blockade-like occupancy by neuropeptide Y was found with particulates from Y1-expressing CHO cells, and with native neuropeptide Y Y1 receptors of rat synaptosomes. Peptide YY and a related Y1-selective agonist, (Leu31Pro34) human peptide YY, showed a much less stable binding to the neuropeptide Y Y1 receptor with either the intact cells or particulates. The Y1 binding of neuropeptide Y was also less sensitive to chaotropic agents and guanine nucleotides than the binding of peptide YY, indicating a larger stability for association of neuropeptide Y with the receptor. Inhibition of forskolin-stimulated adenylyl cyclase showed a distinctly attenuating agonism for neuropeptide Y, with an activity similar to peptide YY below 1 nM, but considerably lower above 3 nM of the peptides. This activity was largely exerted via pertussis toxin-sensitive G-proteins of Y1-CHO cells. Our findings indicate that signaling by neuropeptide Y via its Y1 receptor could be self-restricting at higher levels of the peptide, in relation to a strong association of the agonist with the Y1 binding site.

Internalization of neuropeptide Y Y1 and Y5 and of pancreatic polypeptide Y4 receptors is inhibited by lithium in preference to sodium and potassium ions

The receptor-linked internalization of [125I] human neuropeptide Y (NPY) in Chinese hamster ovary (CHO) cells expressing the guinea-pig Y1 receptors or in human endometrial carcinoma-1B (Hec-1B) cells expressing the human Y5 receptor, as well as the receptor-linked internalization of human pancreatic polypeptide (hPP) receptor expressed in CHO cells, is selectively inhibited by low molarities of the Li+ cation. The Na+ and K+ cations decreased the receptor-linked internalization of agonist peptides only at high molar inputs, and largely in proportion to the reduction of cell surface binding of Y ligand peptides, dependent on ion concentration and the type of Y receptor examined. With particulates isolated from disrupted cells, there was no preferential inhibition by Li+ relative to Na+ in the binding of type-specific ligand peptides to Y receptors of any type. The observed difference could be connected to the known ability of Li+ to modify active conformations of signal transducers, which may also directly or indirectly affect the internalization motors. The decrease in the rate of Y receptor internalization by Li+ also points to a possible alteration of Y receptor signaling in vivo by lithium at acute therapeutically employed dose levels.

Ligand internalization by cloned neuropeptide Y Y5 receptors excludes Y2 and Y4 receptor-selective peptides

In human embryonic kidney-293 (HEK-293) cells, the cloned human neuropeptide Y Y5 receptor saturably internalized agonists, with the rank order of neuropeptide Y-(19-23)-[Gly1,Ser3,Gln4,Thr6,Ala31,Aib32,Gln34]human pancreatic polypeptide (neuropeptide Y-Aib-pancreatic polypeptide)>human neuropeptide Y>porcine peptide YY>[Pro34]human peptide YY>[Leu31,Pro34]human peptide YY>>human peptide YY-(3-36). Human pancreatic polypeptide competed [125I]neuropeptide Y binding and internalization in neuropeptide Y Y5 receptor-expressing cells, but itself showed no internalization. The internalization was strongly dependent on temperature. The surface binding, and especially the internalization, of human neuropeptide Y were highly sensitive to the clathrin network inhibitor phenylarsine oxide, and to the cholesterol-complexing antibiotic filipin III. The internalized ligands were present in particles corresponding to secondary endosomes in Percoll gradients, but especially in particles banding with the acid hexosaminidase lysosomal marker. At any temperature tested, internalization of the neuropeptide Y Y5 receptor driven by human neuropeptide Y in HEK-293 cells was much slower than the internalization of the neuropeptide Y Y1 receptor reported in the same cells, or in Chinese hamster ovary (CHO) cells. The neuropeptide Y Y5 receptor subtype could be the metabotropic receptor responding to protracted challenges by neuropeptide Y-like peptides, and its density could be little sensitive to concentration of extracellular agonists.

Internalization of pancreatic polypeptide Y4 receptors: correlation of receptor intake and affinity

Unlike neuropeptide Y receptors, the pancreatic polypeptide Y4 receptors display considerable differences in sequence and ligand-binding affinity across mammalian species. This could produce different receptor turnover rates in the same cellular membrane environment. Comparing rat, human and guinea-pig Y4 receptors expressed in Chinese hamster ovary (CHO) cells (K(d) with human pancreatic polypeptide 14, 45 and 116 pM, respectively), we indeed found human pancreatic polypeptide internalization in the rank order of receptor affinities. A large fraction of the internalized human pancreatic polypeptide, similar across the Y4 species, was associated with secondary endosomes (density approximately 1.05 in Percoll gradients) and lysosomes (density approximately 1.11). For all Y4 receptors examined, this intake was potently and selectively inhibited by cholesterol-complexing polyene antibiotic filipin III and also by clathrin lattice formation inhibitor, phenylarsine oxide. Internalization differences found across Y4 receptor species to a degree compare with those observed for the cloned guinea-pig neuropeptide Y Y1 and human neuropeptide Y Y5 receptors and, generally, support ligand-binding affinities as important determinants of internalization for neuropeptide receptors.

Bovine pancreatic polypeptide (bPP) undergoes significant changes in conformation and dynamics upon binding to DPC micelles

The pancreatic polypeptide (PP), a 36-residue, C-terminally amidated polypeptide hormone is a member of the neuropeptide Y (NPY) family. Here, we have studied the structure and dynamics of bovine pancreatic polypeptide (bPP) when bound to DPC-micelles as a membrane-mimicking model as well as the dynamics of bPP in solution. The comparison of structure and dynamics of bPP in both states reveals remarkable differences. The overall correlation time of 5.08ns derived from the 15N relaxation data proves unambiguously that bPP in solution exists as a dimer. Therein, intermolecular as well as intramolecular hydrophobic interactions from residues of both the amphiphilic helix and of the back-folded N terminus contribute to the stability of the PP fold. The overall rigidity is well-reflected in positive values for the heteronuclear NOE for residues 4-34. The membrane-bound species displays a partitioning into a more flexible N-terminal region and a well-defined alpha-helical region comprising residues 17-31. The average RMSD value for residues 17-31 is 0.22(+/-0.09)A. The flexibility of the N terminus is compatible with negative values of the heteronuclear NOE observed for the N-terminal residues 4-12 and low values of the generalized order parameter S(2). The membrane-peptide interface was investigated by micelle-integrating spin-labels and H,2H exchange measurements. It is formed by those residues which make contacts between the C-terminal alpha-helix and the polyproline helix. In contrast to pNPY, also residues from the N terminus display spatial proximity to the membrane interface. Furthermore, the orientation of the C terminus, that presumably contains residues involved in receptor binding, is different in the two environments. We speculate that this pre-positioning of residues could be an important requirement for receptor activation. Moreover, we doubt that the PP fold is of functional relevance for binding at the Y(4) receptor.