Irreversible photoactivation of a pancreatic secretagogue receptor with cholecystokinin COOH-terminal octapeptides

Photodynamic Activation of Cholecystokinin 1 Receptor Is Conserved in Mammalian and Avian Pancreatic Acini

Cholecystokinin 1 receptor (CCK1R) is the only G protein coupled receptor that is activated in type II photodynamic action, but whether this is a property common to both mammalian and avian species is not known. In this work, pancreatic acini were isolated from the rat, mouse, and Peking duck, and photodynamic CCK1R activation was examined. Isolated pancreatic acini were exposed to photosensitizer sulphonated aluminum phthalocyanine (SALPC) and photodynamic action elicited by a brief light-emitting diode (LED 675 nm) pulse (1.5 min); photodynamic CCK1R activation was assessed by Fura-2 fluorescent calcium imaging. Photodynamic action was found to induce persistent calcium oscillations in rat, mouse, and Peking duck pancreatic acini, with the sensitivity order of mouse > rat > Peking duck. Photodynamically-activated CCK1R could be inhibited reversibly by CCK1R antagonist devazepide (1 μM); photodynamic CCK1R activation was blocked by pre-incubation with 1O2 quencher Trolox C (300 µM). The sensitivity of photodynamic CCK1R activation was correlated with the increasing size of the disordered region in intracellular loop 3. These data suggest that photodynamic CCK1R activation is conserved in both mammalian and avian species, as evidenced by the presence of the photodynamic activation motif “YFM” in transmembrane domain 3.

Photodynamic Activation of Cholecystokinin 1 Receptor Is Conserved in Mammalian and Avian Pancreatic Acini

Cholecystokinin 1 receptor (CCK1R) is the only G protein coupled receptor that is activated in type II photodynamic action, but whether this is a property common to both mammalian and avian species is not known. In this work, pancreatic acini were isolated from the rat, mouse, and Peking duck, and photodynamic CCK1R activation was examined. Isolated pancreatic acini were exposed to photosensitizer sulphonated aluminum phthalocyanine (SALPC) and photodynamic action elicited by a brief light-emitting diode (LED 675 nm) pulse (1.5 min); photodynamic CCK1R activation was assessed by Fura-2 fluorescent calcium imaging. Photodynamic action was found to induce persistent calcium oscillations in rat, mouse, and Peking duck pancreatic acini, with the sensitivity order of mouse > rat > Peking duck. Photodynamically-activated CCK1R could be inhibited reversibly by CCK1R antagonist devazepide (1 μM); photodynamic CCK1R activation was blocked by pre-incubation with 1O2 quencher Trolox C (300 µM). The sensitivity of photodynamic CCK1R activation was correlated with the increasing size of the disordered region in intracellular loop 3. These data suggest that photodynamic CCK1R activation is conserved in both mammalian and avian species, as evidenced by the presence of the photodynamic activation motif “YFM” in transmembrane domain 3.

Photodynamic Physiology-Photonanomanipulations in Cellular Physiology with Protein Photosensitizers

Singlet oxygen generated in a type II photodynamic action, due to its limited lifetime (1 μs) and reactive distance (<10 nm), could regulate live cell function nanoscopically. The genetically-encoded protein photosensitizers (engineered fluorescent proteins such as KillerRed, TagRFP, and flavin-binding proteins such as miniSOG, Pp2FbFP^L30M) could be expressed in a cell type- and/or subcellular organelle-specific manner for targeted protein photo-oxidative activation/desensitization. The newly emerged active illumination technique provides an additional level of specificity. Typical examples of photodynamic activation include permanent activation of G protein-coupled receptor CCK1 and photodynamic activation of ionic channel TRPA1. Protein photosensitizers have been used to photodynamically modulate major cellular functions (such as neurotransmitter release and gene transcription) and animal behavior. Protein photosensitizers are increasingly used in photon-driven nanomanipulation in cell physiology research.

Molecular properties of solubilized CCK receptor from guinea-pig pancreas

In order to characterize the CCK receptor in guinea-pig pancreas, iodinated CCK-39 was bound to pancreatic membranes and the reversible complex was solubilized using various non-denaturing detergents. In term of recovery of ligand stabilized receptors, the relative potencies were Zwittergent 3-14 greater than CHAPS = CHAPSO greater than digitonin greater than MEGA 10 greater than octyl beta-D-glucopyranoside. The stability of receptor complexes was increased by glycerol. Chromatographic analysis revealed that digitonin was the most efficient detergent for disaggregation of CCK receptor complex since it yielded a 76 kDa component in addition to the large components obtained after solubilization with CHAPS and Zwittergent. Furthermore, CCK receptors were covalently labelled using dissuccinimidyl suberate or UV irradiation of labelled membranes by photoactivable radioiodinated CCK-39 and subsequently solubilized by CHAPS + SDS or by SDS alone. A predominant molecule was characterized by chromatography (76 kDa) and SDS-PAGE (89 kDa). In addition to this component, other components having molecular masses of 130-150 kDa, 57 kDa and 40 kDa were detected by SDS-PAGE. They correspond to minor bands. These bands, except the 40 kDa band, were protected from covalent labelling by the presence of CCK-39 (10(-6) M) during initial incubation. Reduction under beta-mercaptoethanol mainly resulted in the decrease of high molecular weight aggregates (Mr greater than 200 kDa). We concluded that for a given detergent a specific molecular weight pattern of solubilized CCK receptor complex is achieved. The minimal component had a molecular mass of 71-84 kDa according to the method of biochemical analysis used.

Photoaffinity labelling of MSH receptors on Anolis melanophores: effects of catecholamines, calcium and forskolin

Photoaffinity labelling of MSH receptors on Anolis melanophores was used as a tool for studying the effects of catecholamines, calcium and forskolin on hormone-receptor interaction and receptor-adenylate cyclase coupling. Covalent attachment of photoreactive alpha-MSH to its receptor was suppressed in calcium-free buffer but was hardly influenced by catecholamines or forskolin. The longlasting signal generated by the covalent MSH-receptor complex was readily and reversibly abolished by adrenaline, noradrenaline, dopamine or clonidine or by the absence of calcium. The suppression of pigment dispersion by catecholamines was blocked by the simultaneous presence of yohimbine but not prazosin, indicating that the catecholamines antagonize the alpha-MSH signal by inhibitory action on the adenylate cyclase system through an alpha-2 receptor. Forskolin, which stimulates melanophores by direct action on the catalytic unit of the adenylate cyclase and at about the same speed as alpha-MSH, produced a slower and weaker response in the presence of noradrenaline. If MSH receptors were covalently labelled and then exposed to noradrenaline, the characteristics of the forskolin-induced response were identical to those of unlabelled cells that had not been exposed to noradrenaline. This may point to a partial restoration of receptor-adenylate cyclase coupling by forskolin. The results show that the longlasting stimulation of Anolis melanophores by photoaffinity labelling proceeds via a permanently stimulated adenylate-cyclase system whose coupling to the receptor depends on calcium and is abolished by alpha-2 receptor agonists. Calcium is also essential for hormone-receptor binding.

Preparation and characterization of photoreactive derivatives of GnRH. Persistent activation of function by photoaffinity labeling

A photoreactive derivative of the highly potent gonadotropin releasing hormone (GnRH) agonist, D-Lys6-GnRH(1-9)-ethylamide, was prepared by selective modification of the epsilon-amino group with 2-nitro-4-azidophenyl sulfenyl chloride (2,4-NAPS C1). The modified peptide [D-Lys(NAPS)]6-GnRH-(1-9)-ethylamide was found to be a full agonist of LH release from rat pituitary cells with a relative potency 23 compared to GnRH. Covalent attachment of the photoreactive analog to rat pituitary cells resulted in prolonged activation of LH secretion which could not be inhibited by a potent GnRH antagonist. Persistent stimulation of pituitary gonadotrophs caused by covalently bound hormone led to desensitization of the LH releasing mechanism.

One-step isocratic high-performance liquid chromatographic purification of radioiodinated and radioiodinated-photoactivable derivatives of cholecystokinin

N-Hydroxysuccinimidyl-3-(4-hydroxy-3-[125I]iodophenyl)propionate (the Bolton-Hunter reagent) was conjugated with (Thr 34, NLeu 37) cholecystokinin (CCK) 31-39 in anhydrous dimethylformamide-pyridine in 20% yield. The radiolabelled peptide was purified from the reaction mixture in one step, by isocratic elution from a C18 high-performance liquid chromatographic (HPLC) column with 35% aqueous acetonitrile-0.13% heptafluorobutyric acid as eluent. The concentration of the radiolabelled peptide was estimated by UV monitoring. The acylating photoactivable radioiodinated reagent N-hydroxysuccinimidyl-N-(4-azido-2-nitrophenyl)-3-[125I]iodotyrosi ne was synthesized, purified on a C18 HPLC column by isocratic elution with 65% acetonitrile-1 mM hydrochloric acid, then conjugated with (Thr 34, NLeu 37) CCK31-39. The resulting photoactivable radioiodinated CCK analogue was purified by isocratic elution on a C18 HPLC column with 39% aqueous acetonitrile-0.1 M triethylamine phosphate (pH 3.5). The binding ability of both tracers and their non-radioactive analogues to CCK receptors was tested on rat pancreatic plasma membranes. As compared to a KD of 4.5 nM for unmodified (Thr 34, NLeu 37) CCK31-39, the KD of the radioiodinated Bolton-Hunter derivative was 3 nM, and that of the photoactivable radioiodinated derivative was 19 nM.

Photoaffinity labelling of peptide hormone receptors

Photoreactive peptide derivatives for the labelling of hormone receptors are usually prepared by inserting a chemically stable aryl azide or nitroaryl azide into a specific site of the molecule, such as an alpha or omega amino or carboxyl group, or into the side-chain of an Arg, Cys, His, Trp or Tyr. With p-azidophenylalanine (Pap), a more or less isosteric replacement of Tyr or Phe can be achieved when other alterations would impair the biological activity of the hormone. Reversible attachment of photoreactive groups via S-S linkage to, e.g., SH-Trp makes it possible to release the covalently bound hormone from the receptor. This is an advantage when photolabelling is used for the isolation of receptors. Photoaffinity labelling of intact cell systems may reveal insights into dynamic aspects, such as receptor inactivation and turnover or the temporal involvement of second messengers, and may be particularly useful for the study of peptide action on target cells occurring only in small numbers. The Xenopus MSH-melanophore system which is stimulated irreversibly upon UV-irradiation of photoreactive alpha-MSH represents such an example.

Specific photoaffinity crosslinking of [125I]cholecystokinin to pancreatic plasma membranes. Evidence for a disulfide-linked Mr 76 000 peptide in cholecystokinin receptors

In rat pancreatic plasma membranes, preincubated with [125I]cholecystokinin-33 (CCK-33) and washed free of unbound tracer, the irradiation by UV light induced the irreversible binding of radioactivity to high molecular weight peptides as shown by polyacrylamide gel electrophoresis in the presence of sodium dodecyl sulfate (SDS) and autoradiography. This was not observed when the membranes were preincubated in the simultaneous presence of [125I]CCK-33 and of either an excess of unlabelled CCK-8 or of guanosine 5'-(beta, gamma-imido)-triphosphate. The radioactivity was mostly crosslinked with a Mr 96,000 peptide and peptide species of Mr greater than 200,000, after SDS solubilization in the absence of beta-mercaptoethanol. Peptide reduction with beta-mercaptoethanol converted the high molecular weight radioactive species into a Mr 76,000 peptide that contained as much as 65% of the radioactivity crosslinked. The Mr 76,000 peptide appears, therefore, to be a disulfide-linked constituent of rat pancreatic cholecystokinin receptors.