Irreversible stimulation of Xenopus melanophores by photoaffinity labelling with p-azidophenylalanine13-alpha-melanotropin

Structure and chemistry of the peptide hormones of the intermediate lobe

The peptide hormones of the intermediate lobe are derived from a common precursor protein and are therefore biogenetically and structurally related. They represent a group of linear, flexible peptides which elicit a variety of physiological response. Structure-activity studies have shown that different segments of adjacent amino acid residues have a specific function (e.g. address, message, potentiation) in the interaction of each of these hormones with its receptor(s). This kind of organization of hormonal information is called sychnologic; it is the basic for the pleiotropic action of the opiomelanocortin peptides, i.e. the ability of related peptides to interact with different types of receptors in different target cells. Labelled peptide hormones with radioactive, fluorescent, or photolabile groups at defined sites are a prerequisite for studying hormone-receptor interaction. Multi-labelled derivatives of alpha-MSH are suitable for degradation and intracellular incorporation studies. Photoaffinity labelling of melanophore receptors with azidophenyl-containing analogues of alpha-MSH produces an irreversible stimulation of pigment cells. Covalent conjugates between peptide hormone receptors. These conjugates exhibit remarkable properties such as superpotency, strongly enhanced receptor affinity and prolonged action.

Melanophores: A model system for neuronal transport and exocytosis?

Black pigment cells, melanophores, from lower vertebrates are specialized in bidirectional and coordinated translocation of pigment granules, melanosomes, in the cytoplasm. Melanophores develop from the neuronal crest and are most abundant in the dermal and epidermal layers of the skin, where the intracellular distribution of the pigment significantly influences the color of the animal. The transport of pigment is dependent on an intact cytoskeleton and motor proteins associated with cytoskeletal components. The easily cultured melanophores have proved to be excellent models for organelle transport because the intracellular movements of pigment can be visualized via light microscopy, and the granules move in response to defined chemical signals. The ease of achieving a combination of morphological and functional transport studies is the advantage of the melanophore system, and studies on pigment cells have revealed new components of the transport machinery, including molecular motors, their adapters, and transfer of vesicles to other cells. Many cellular components are transported with a combination of the actin- and microtubule-based transport systems, and, since all eukaryotic organisms rely on functional intracellular transport and an intact cytoskeleton, studies on melanophores are important for many aspects of cell biology, including axonal transport. In this review, we present an overview of the research on the pigment transport system and the potential use of pigment cells as a model system.

Transformation of an irreversible MSH antagonist into an irreversible MSH agonist by differential receptor crosslinking using the photo-affinity technique

Photocrosslinking of receptors for alpha-melanocyte-stimulating hormone (alpha-MSH) on melanophores of frogs and lizards has been shown to induce long-lasting receptor stimulation whereby the photoreactive alpha-MSH may contain one or two photolabels in positions 1, 7, 9, or 13. The chemical synthesis and biological testing of an alpha-MSH analogue is now described which contains three photoreactive groups in positions 1, 9 and 13, one of which with a cleavable S-S disulphide bridge: [ApSSpr-Ser1, Trp(Naps)9 Pap13]-alpha-MSH. Photocrosslinking of MSH receptors on melanophores of Anolis carolinensis with this analogue led to almost complete receptor blockade which could be transformed into long-lasting receptor stimulation by exposure to a thiol reagent. By contrast, the analogue containing only two photoreactive groups in positions 9 and 13, [Trp(Naps)9, Pap13]-alpha-MSH, produced long-lasting receptor stimulation which was not altered by the thiol reagent. These results demonstrate that one and the same peptide ligand may contain structural information for both receptor activation and inhibition and that the receptor may become arrested in an activated or inhibited state by multiple photocrosslinking, depending on the relative positions of these crosslinks.

Peptides containing multiple photolabels: a new tool for the analysis of ligand-receptor interactions. Reversible long-lasting stimulation and inhibition of MSH receptors by multiple photocrosslinks with alpha-MSH

Crosslinking of MSH receptors on melanophores of the lizard Anolis carolinensis with analogues of alpha-MSH containing a photoreactive group in position 1, 7, 9 or 13 leads to long-lasting receptor stimulation. Reversibility of this long-lasting stimulation is obtained by employing a disulfide-containing photoreactive group which can be cleaved from the receptor by thiol reagents [Ref. 3]. When two photoreactive groups are simultaneously present on the alpha-MSH molecule (e.g in positions 1 + 9; 1 + 13; 7 + 13, or 9 + 13), identical results were obtained and long-lasting receptor stimulation was not altered after cleavage of one single crosslink. alpha-MSH analogues with three photoreactive groups in positions 1 + 7 + 13 led to irreversible receptor stimulation whereas one compound with the photoreactive groups in positions 1 + 9 + 13 induced reversible receptor inactivation which could be changed into long-lasting stimulation by cleaving the crosslink at position 1 of alpha-MSH. These results demonstrate that one and the same peptide ligand may contain structural information for both receptor activation and inhibition and that the receptor may become arrested in an activated or inhibited state by multiple photocrosslinking, depending on the relative positions of these crosslinks.

The protein-phosphatase inhibitor okadaic acid mimics MSH-induced and melatonin-reversible melanosome dispersion in Xenopus laevis melanophores

The present study describes the ability of 315 nM okadaic acid to induce melanosome dispersion in cultured Xenopus laevis melanophores. This effect of okadaic acid is similar to that of a-melanocyte stimulating hormone (MSH) and can be reversed by melatonin treatment; it indicates that a member of the protein-phosphatase 1 or 2A families must be active for maintenance of the aggregated state. Higher concentrations of okadaic acid (1 microM) attenuate the response of Xenopus melanophores to melatonin leading to the hypothesis that melatonin action is mediated by the calcium/calmodulin activated phosphatase 2B. This hypothesis seems unlikely, however, since the calcium/calmodulin inhibitors TFP and W7 do not prevent melatonin-induced pigment aggregation, but instead induce aggregation on their own.

Photoaffinity labelling of melanoma cell MSH receptors

UV-irradiation at 365 nm of cultured Cloudman S91 mouse melanoma cells in the presence of photoreactive alpha-MSH analogues induced longlasting receptor stimulation as revealed by the ensuring activation of tyrosinase. Receptor labelling was more efficient with 4-diazirinophenyl and 2-nitro-4-azidophenyl photolabels than with 4-azidophenyl, and was further increased when superpotent [Nle4,D-Phe7]-alpha-MSH was used as ligand. Incubation of B16 melanoma cell membranes with mono-iodinated [Nle4,D-Phe7,Trp-(Naps)9]-alpha-MSH followed by UV-irradiation at 310-550 nm labelled a single band on SDS-PAGE with a molecular mass approximately or equal to 45 kDa. The displacement curve obtained in a competitive photolabelling experiment paralleled that of the binding assay, demonstrating that the labelling was specific.

Stabilized complexes of epidermal growth factor and its receptor on the cell surface stimulate RNA synthesis but not mitogenesis

Treatment of mouse fibroblasts, prelabeled at 4 degrees C with 125I-labeled epidermal growth factor (EGF), with the lectin concanavalin A (Con A) stabilized the 125I-labeled EGF-receptor complex to dissociation and prevented receptor-mediated endocytosis; after 5 hr at 37 degrees C, approximately 50% of the 125I-labeled EGF initially bound at 4 degrees C remained cell associated, compared to less than 15% in control cells. The radioactivity lost from the Con A-treated cells was found as intact hormone in the medium, with almost no hormone degradation evident, whereas in control cells most of the medium radioactivity was in the form of low molecular weight degradation products. The trimolecular complex Con A-EGF-receptor was capable of stimulating RNA synthesis to levels greater than control (untreated) or EGF alone and maintained this stimulation for prolonged periods of time. However, there was no effect of Con A treatment on the stimulation of DNA synthesis induced by EGF prebound at 4 degrees C. Thus, maintaining the EGF-receptor complex at the cell surface allows enhanced stimulation of an acute biological response to EGF (RNA synthesis) but not stimulation of DNA synthesis. These data support the idea that processing subsequent to receptor binding is necessary to produce the mitogenic signal.

alpha-Melanotropin-induced changes in protein phosphorylation in melanophores

To investigate a possible role of protein phosphorylation in the mechanism of action of alpha-MSH, excised tail-fins of Xenopus tadpoles were incubated with or without alpha-MSH. After homogenization, in vitro endogenous protein phosphorylation was assayed using [gamma-32P]ATP. alpha-MSH treatment of intact tail-fins, producing full pigment dispersion, resulted in a 5-fold increase in 32P-incorporation into a 53 kDa protein band. This increase in 53 kDa phosphorylation was completely reversible. The increase was not found in homogenates from the melanophore-free part of the alpha-MSH-treated tail-fins. Phosphorylation of the 53 kDa protein could be detected in homogenates of alpha-MSH-treated primary cultured melanophores. Incubation of tail-fins with ACTH1-24, an alpha-MSH-like peptide producing full pigment dispersion, also induced an increase in 53 kDa phosphorylation. A structurally related peptide (ACTH15-24) and an unrelated peptide (LH-RH), neither of which induced pigment dispersion, were ineffective in stimulating 53 kDa phosphorylation. Injection of white adapted tadpoles with 1 micrograms of alpha-MSH or adaptation of tadpoles to a black background also resulted in a significant increase in 53 kDa phosphorylation. alpha-MSH added to the homogenates did not affect 53 kDa phosphorylation, indicating that alpha-MSH acts through a receptor-mediated mechanism. The increase in 53 kDa phosphorylation measured in vitro (post hoc), most likely reflects an alpha-MSH-induced decrease in 53 kDa phosphorylation in vivo. Our results strongly suggest that a decrease in 53 kDa phosphorylation is involved in the mechanism of action of alpha-MSH on melanophores.

Characterization of alpha-MSH-induced changes in the phosphorylation of a 53 kDa protein in Xenopus melanophores

alpha-Melanotropin has been shown to induce specific changes in the degree of phosphorylation of a 53 kDa melanophore protein, concomitant with pigment dispersion. To further characterize the alpha-MSH-induced changes in 53 kDa phosphorylation in melanophores from the ventral tail-fin of Xenopus tadpoles, we investigated the concentration and time dependency of the effect. A significant increase in 53 kDa phosphorylation was detectable at 5 X 10(-8) M alpha-MSH. The maximal increase in 53 kDa phosphorylation was found after an incubation time of 10-15 min, whereas pigment dispersion was optimal after 60 min. The phosphorylated 53 kDa band showed clear cross-reactivity with monoclonal anti-beta-tubulin, and migrates as a single protein after two-dimensional (2D) separation. On a 2D-separation system the 53 kDa protein (IEP 5.1) migrated in the acidic tail of purified beta-tubulin. Our data strongly indicate that the 53 kDa protein is a beta-tubulin-like protein. We suggest that the degree of 53 kDa phosphorylation may be an important factor in the regulation of microtubule function in melanophores.

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.

Photoaffinity labelling of MSH receptors on Anolis melanophores: irradiation technique and MSH photolabels for irreversible stimulation

Excised dorsal skin of Anolis carolinensis was exposed to high intensity UV-irradiation in the presence of different photoreactive alpha-MSH derivatives. The resulting covalent binding of the hormone to its receptor induced irreversible pigment dispersion. The duration of the longlasting response depended on the type and length of irradiation; it was maximal after two 5 min irradiation phases with a light intensity of approximately 180 mW/cm2 and a spectrum from 310 to 550 nm, fresh hormone being added after the first phase. [N alpha-(4-Azidophenylacetyl-serine1]-alpha-MSH (I), [2'-(2-nitro-4-azidophenylsulphenyl)-tryptophan9]-alpha-MSH (II) and [p-azidophenylalanine13]-alpha-MSH (III) all inserted into the receptor to about the same extent, as judged from the persistence of the longlasting signal. In contrast, [D-alanine1, p-azidophenylalanine2, norvaline4]-alpha-MSH (IV) and [N alpha-(4-azidophenylacetyl)-serine1, leucine9]-alpha-MSH (V) gave much less insertion and [leucine9, p-azidophenylalanine13]-alpha-MSH (VI) hardly any insertion when applied in the same relative excess (5-fold the concentration inducing a maximal response). Covalent attachment of the cleavable photolabel [N alpha-(4-azidophenyl)-1, 3'-dithio-propionyl-serine1]-alpha-MSH (VII) and subsequent washing of the skin in buffer containing 1% beta-mercaptoethanol released the peptide from the receptor. Insertion of the C-terminal photolabel [p-azidophenylalanine13]-alpha-MSH was reduced by the weak antagonist H-Phe-Ala-Trp-Gly-Gly-Pro-Val-NH2. These experiments prove that hormone receptors can be covalently labelled in tissue with very limited light transparency.

Calcium requirement for alpha-MSH action on melanophores: studies with forskolin

alpha-MSH-induced pigment dispersion in melanophores shows an absolute requirement for extracellular Ca2+. To localize Ca2+ sites involved in the mechanism of action of alpha-MSH we studied the effects of Ca2+ deprivation on alpha-MSH and forskolin-induced melanophore responses. In an in vitro melanophore system employing ventral tailfins of Xenopus tadpoles, melanophore responses were assayed in terms of pigment dispersion and the phosphorylation state of a 53 kDa melanophore-specific protein. In the same melanophore system alpha-MSH has been shown to specifically increase the phosphorylation of this 53 kDa protein. Forskolin induces a dose-dependent pigment dispersion (EC50 7 X 10(-7) M). In contrast to the dispersion induced by alpha-MSH forskolin-induced dispersion does not require extracellular Ca2+. Moreover, in a Ca2+-free medium melanophores with permanently activated MSH-receptors aggregate, but can be redispersed by the addition of forskolin. Forskolin increases 53 kDa phosphorylation in a dose-dependent manner. Maximal stimulation with forskolin (10(-5) M) is four-fold and equals maximal 53 kDa phosphorylation obtainable with alpha-MSH. The MSH-induced increase in 53 kDa phosphorylation is inhibited by Ca2+ deprivation, whereas the forakolin-induced increase is unaffected. Our results suggest that alpha-MSH and forskolin stimulate melanophores through a common pathway and confirm that cAMP is a second messenger in alpha-MSH action in this system. We conclude that the Ca2+ sites in the mechanism of alpha-MSH action on melanophores precede adenylate cyclase activation.

A new in vitro melanophore bioassay for MSH using tail-fins of Xenopus tadpoles

A new in vitro melanophore system is described, which employs pieces from the ventral tail-fin of Xenopus laevis tadpoles. Tail-fin melanophores in vitro retain the ability to disperse their pigment in darkness and to reaggregate it upon illumination. In the light, alpha-MSH, cAMP, dibutyryl-cAMP and theophylline induce a concentration-dependent pigment dispersion. The log dose-response curve obtained with alpha-MSH is sigmoidal with a linear portion between 0.5 and 2.0 ng alpha-MSH/ml. In this range, the log dose-response curve can be used as the standard curve in a bioassay for melanotropic activity, applying either the melanophore index (assay I) or a photometric transmittance measurement (assay II) for the quantification of the melanophore response. To prevent interference from the light/darkness response, light of 400-500 nm (to which the melanophores are most sensitive) was used during the assay. Both assays show high precision (lambda I = 0.13, lambda II = 0.11). Several peptides derived from alpha-MSH were tested for their melanotropic activity. The in vitro Xenopus melanophore system offers unique properties for the study of alpha-MSH action: (1) the melanophore system is uncontaminated with other chromatophores; (2) to date it is the only system suitable for photoaffinity labelling of alpha-MSH receptors; and (3) the melanophore receptor requirements differ from those of Rana.

Covalent linking of photoreactive gonadotropin-releasing hormone to gonadotropes produces a prolonged signal

A bioactive, photoreactive derivative of gonadotropin-releasing hormone (GnRH; gonadoliberin), [azidobenzoyl-D-Lys6]GnRH, was shown to bind covalently to dispersed pituitary cells after irradiation. Approximately 7% of the total cell-associated radioactivity was covalently bound to the receptors. Photolysis of cultured pituitary cells in the presence of the photoreactive derivative resulted in persistent activation of luteinizing hormone (LH; lutropin) release. This persistent response was time dependent and concentration dependent. No increase in the basal rate of LH release was observed with cells incubated in the presence of photoreactive GnRH analog and maintained in the dark or with hormone derivatives that lack the photoreactive azido group. These results suggest that only the covalently bound cell surface receptors account for the persistent activation of LH release after photolysis.

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.

Calcium requirement for alpha-MSH action on tail-fin melanophores of xenopus tadpoles

The role of Ca2+ in alpha-MSH action on melanophores was studied, in vitro, with a bioassay on ventral tail-fin pieces from tadpoles of Xenopus laevis. Melanosome dispersion induced by alpha-MSH required 1-2 mM extracellular Ca2+. Gradual lowering of the extracellular Ca2+ levels produced a concentration-dependent inhibition of the alpha-MSH response; complete inhibition was obtained in a Ca2+-free medium containing 10-4 M EGTA. In Mg2+-free medium, normal dispersion was observed. The Ca2+ antagonists verapamil (10-4 M), methoxy-verapamil (10-4 M) and La3+ (10-3 M) inhibited the dispersion induced by 3 X 10-9 M alpha-MSH, whereas ruthenium red (10-3 M) was without effect. The ionophore A23187 mimicked the effect of the hormone. Melanosome movement per se was evidently independent of Ca2+, because cAMP and dibutyryl-cAMP induced a full dispersion in the absence of Ca2+. These results show that extracellular Ca2+ is specifically required for alpha-MSH action on tail-fin melanophores in vitro and suggests a Ca2+ influx concomitant with the action of the hormone. Possible intra- and extra-cellular Ca2+ sites are discussed.

Calcium sites in MSH stimulation of xenopus melanophores: studies with photoreactive alpha-MSH

Photo-affinity labelling of MSH receptors on tail-fin melanophores of Xenopus tadpoles with p-azidophenylalanine 13-alpha-MSH (Pap13)-alpha-MSH) or p-azidophenylacetyl-serine1-alpha-MSH ([Apac-Ser1]-alpha-MSH) resulted in a long-lasting stimulation of the melanophores which cannot be reversed despite continuous washing. The generation of this irreversible response is inhibited when photo-affinity labelling is performed in a Ca2+-free medium or in the presence of Ca2+ antagonists. The irreversible stimulation produced in normal medium is completely reversed upon removal of Ca2+ , but is not affected by Ca2+ antagonists or melatonin. Re-addition of Ca2+ after temporary removal restores to irreversible stimulation, even in the presence of Ca2+ antagonists or melatonin. This proves that covalent alpha-MSH-receptor complexes remain fully functional despite temporary deprivation of ca2+. Racemized alpha-MSH, which binds 'tightly' to the receptor and produces a long-lasting effect, is removed from the receptor in Ca2+-free medium, but not by Ca2+ antagonists or melatonin. These results confirm earlier results showing that at least 2 Ca2+ sites are involved in alpha-MSH action, one associated with MSH-receptor binding and the other with the subsequent generation of the effect. The dual role of Ca2+ is not the result of EGTA present; it is specific (Mg2+ has no effect) and is identical for the two different photoreactive alpha-MSH derivatives.

Calcium-dependent prolonged effects on melanophores of [4-norleucine, 7-D-phenylalanine]-alpha-melanotropin

A single injection of the melanotropin analog [4-norleucine, 7-D-phenylalanine]-alpha-melanotropin into frogs (Rana pipiens) caused near maximum darkening of the skins of the frogs for at least 6 weeks. Injections of the natural hormone alpha-melanotropin or of the analog [Nle4]-alpha-melanotropin also caused darkening, but this effect lasted only a few days. Morphological examination of the skins of frogs injected with [Nle4, D-Phe7]-alpha-melanotropin revealed that both dermal and epidermal melanophores were dispersed during the entire 6-week period. In vitro [Nle4, D-Phe7]-alpha-melanotropin also causes prolonged darkening of the skin of the lizard Anolis carolinensis. In the absence of the melanotropin, skins previously darkened with the analog could be lightened by removal of calcium from the incubation medium but could then be redarkened by adding calcium. The cycle could be repeated indefinitely without addition of melanotropin. These results demonstrate the role of calcium in receptor signal transduction and the prolonged biological effects of [Nle4, D-Phe7]-alpha-melanotropin long after its removal from the assay medium.