Role of calcium and prostaglandin (PGE 1 ) in the MSH-induced activation of adenylate cyclase in Xenopus laevis

Biochemical regulation of pigment motility in vertebrate chromatophores: a review of physiological color change mechanisms

The fundamental unit of rapid, physiological color change in vertebrates is the dermal chromatophore unit. This unit, comprised of cellular associations between different chromatophore types, is relatively conserved across the fish, amphibian, and reptilian species capable of physiological color change and numerous attempts have been made to understand the nature of the four major chromatophore types (melanophores, erythrophores, xanthophores, and iridophores) and their biochemical regulation. In this review, we attempt to describe the current state of knowledge regarding what classifies a pigment cell as a dynamic chromatophore, the unique characteristics of each chromatophore type, and how different hormones, neurotransmitters, or other signals direct pigment reorganization in a variety of vertebrate taxa.

Prostanoids-induced dispersion in the melanophores of a carp Labeo rohita (Ham.)

Effects of three prostaglandins (i.e., prostanoids) and one of its precursors, arachidonic acid, were examined on the melanophores of the fish Labeo rohita (Ham.). PGE₁, PGE₂, PGF(2α) and arachidonic acid elicit a concentration-related dispersion in the fish melanophores. In vitro analysis of melanophores was performed through incubation of the isolated fish scales in different agonists and antagonists solutions. Dispersal effect of prostanoids may be mediated directly through the typical receptors or indirectly through release of neurotransmitter substance(s) from the melanophore nerve endings. Denervation of fish melanophores rendered them insensitive to prostanoid (PGF(2α)). Propranolol and verapamil completely inhibited the dispersal effects of PGF(2α); theophylline and indomethacine blocked the effects of higher concentrations of PGF(2α). During dispersing influence of PGF(2α), a free flux of Ca²⁺ ions was required and the indirectly released substance(s) from melanophore nerve endings would be the catecholamines of adrenergic and purinergic in nature.

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.

Melanocyte stimulating hormone and melanin concentration hormone may be structurally and evolutionarily related

Two melanotropic peptides, melanin concentration hormone (MCH) and alpha-melanocyte stimulating hormone (alpha-MSH), exert opposing actions on melanosome (melanin granule) movements within teleost pigment cells, melanocytes (melanophores). MCH stimulates melanosome aggregation to the cell center whereas alpha-MSH stimulates pigment organelle dispersion out into the dendritic processes of the melanocytes. The actions of alpha-MSH are dependent upon extracellular calcium (Ca2+), whereas those of MCH are actually enhanced in the absence of the cation. At high concentrations (10(-5)-10(-8) M) MCH also exhibits MSH-like activity (autoantagonism), an effect which is abolished in the absence of Ca2+. Therefore, MCH exhibits MCH-like as well as MSH-like activity depending on the presence or absence of extracellular Ca2+. An analogue of MCH, [Ala5, Cys10]MCH, has been synthesized which is totally devoid of MCH activity but still exhibits MSH-like activity. These results suggest that the two melanotropic peptides share some component of structural similarity and may be evolutionarily related.

Inhibition by MIF-I of alpha-MSH induced increase of intraocular pressure and miosis in rabbits

The effects of intracameral (i.c.) administration of MIF-I on the ocular response to alpha-MSH were tested in rabbits. In confirmation of previous studies, i.c. alpha-MSH significantly increased intraocular pressure (IOP) and reduced pupillary diameter (PD). Concomitant administration of alpha-MSH and MIF-I antagonized both the IOP increase and miosis induced by alpha-MSH. Aqueous humor (AH) dynamics studies revealed that alpha-MSH increases IOP, possibly, by decreasing AH outflow. The decreased AH outflow induced by alpha-MSH was antagonized by concurrent administration of MIF-I and alpha-MSH. MIF-I did not affect IOP or PD when administered alone. These results add more support for a role of alpha-MSH in ocular function, and suggest that the ocular response to alpha-MSH may be subject to inhibitory control by MIF-I.

The role of calcium in MSH stimulated melanosome dispersion

We have examined the role of the calcium ion in the response of the melanophores of the lizard, Anolis carolinensis to alpha-melanocyte stimulating hormone (alpha-MSH) by a rate method which allows kinetic analysis of melanosome dispersion. Dose-response curves to alpha-MSH were compared in the presence of different calcium concentrations, and Schild regression plots were constructed. An avidly binding analogue of alpha-MSH, Nle4-D-Phe7-alpha-MSH, was used to demonstrate kinetically the involvement of calcium in the melanophore response both for MSH receptor binding and the subsequent transduction of the MSH signal across the melanophore membrane.

Regulation of adenylate cyclase by beta-melanotropin in the M2R melanoma cell line

We have examined adenylate cyclase (AC) in the M2R melanoma cell line, a novel clone of transplantable B16 melanoma cells. It has been found that activity of this enzyme is highly responsive to beta-melanotropin (beta-MSH) and other hormones possessing melanotropic activity (e.g., alpha-melanotropin (alpha-MSH) and adrenocorticotrophic hormone (ACTH1-24)). beta-MSH stimulation of adenylate cyclase, both in the intact cell and in a plasma membrane-enriched fraction derived thereof, was shown to be saturable and dose-dependent. In addition, prostaglandin E1 (PGE1) was found to be a potent stimulator of AC activity in these cells. Hormone stimulation of enzyme activity in the intact cell was strongly potentiated by forskolin which not only enhanced maximal AC activity 3-fold, but lowered by 40-fold the concentration of beta-MSH required for half-maximal stimulation. Using biologically active [125I]iodo-beta-MSH prepared in our laboratory we have examined the specificity of beta-MSH binding to its receptor in both intact M2R cells and plasma membranes derived thereof. Among a series of hormones tested only alpha-MSH and ACTH1-24 competed with [125I]iodo-beta-MSH for binding to the melanotropin receptor in accordance with the results obtained with AC. In contrast to the strong effect on cyclic 3',5'-adenosine monophosphate (cAMP) accumulation in M2R cells forskolin has no effect on [125I]iodo-beta-MSH binding. It appears that the kinetic properties of beta-MSH binding and beta-MSH stimulation of adenylate cyclase activity are essentially identical, the half-maximal effects of which are demonstrated at approximately 20 nM beta-MSH.

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.

The stimulatory effects of prostaglandins on the melanophores of the lizard, Anolis carolinensis

The melanosome dispersing activity of prostaglandins PGE1, PGE2, PGF1 alpha, PGF2 alpha, PGI2 and 6 beta PGI, was tested on the melanophores of Anolis carolinensis. Only PGE2 and PGE1 were active and while PGE2 was the most potent and acted synergistically with alpha-MSH, PGE1 was additive with alpha-MSH. Arachidonic acid also stimulated melanosome dispersion but its effect was blocked by indomethacin suggesting an action through its conversion to PGE1 or PGE2. The effect of alpha-MSH, on the other hand, was unaltered by indomethacin which suggests that alpha-MSH stimulated melanosome dispersion does not depend upon prostaglandin synthesis. Thus, while some prostaglandins may interact with alpha-MSH to stimulate melanosome dispersion they are unlikely to mediate its action.

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.

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.

Action of melanophore-stimulating hormone on melanophores of the cyprinid fish Zacco temmincki

1. Alpha MSH was extremely effective in inducing melanosome dispersion in both innervated and denervated melanophores in isolated scales of Zacco termmincki. 2. The sensitivity of the melanophores to MSH did not change after denervation. 3. The MSH action was blocked by mersalyl, a SH inhibitor, but not by any of alpha and beta adrenergic blockers. 4. Ca2+ was required for the MSH action, but not for melanosome dispersion itself, since the beta adrenergic response was normally produced in the absence of this ion. 5. Mg2+, Sr2+ and Ba2+ could not replace the Ca2+. 6. Mn2+ reversibly inhibited the MSH action.

Stimulatory effect of prostaglandin E1 on thyroliberin-induced alpha-melanotropin release from perifused neuro-intermediate lobes of frog pituitary gland

A possible direct effect of prostaglandins on alpha-melanotropin (alpha-MSH) release at the level of the intermediate lobe of the frog pituitary was investigated in vitro using a perifusion system technique. The effect of prostaglandins was studied on both spontaneous and TRH-stimulated alpha-MSH secretion. No significant effect of PGE1, PGE2, PGF1 alpha or PGF2 alpha on basal release of alpha-MSH could be detected. Indomethacin did not alter the alpha-MSH release induced by TRH. Conversely a significant increase in TRH-induced alpha-MSH secretion was observed in the presence of 1 x 10(-6) M PGE1. This magnifying effect was directly related to the concentration of TRH for doses ranging from 1 x 10(-8) M to 1 x 10(-6) M.

The effect of the divalent cation ionophore A23187 on amphibian melanophores and iridophores

Calcium ions are required in the medium for the full darkening action of melanocyte-stimulating hormone, but not for its second messenger, cyclic adenosine monophosphate, on isolated frog skin. Thus, the possible effect of the divalent cation ionophore A23187 was studied, using the skin of Rana pipiens. It is a potent darkening agent over the range of 1-2 micrometer, as measured by reflectance change and microscopic observation. During the darkening response of A23187, melanosome dispersion takes place in both dermal and epidermal melanophores, as well as aggregation of iridophore reflecting platelets. The darkening is slowly reversible in Ringer's solution, thus it is not due to toxicity. The darkening is partially dependent on the presence of calcium in the medium under certain conditions, suggesting that the response involves both the uptake and release of calcium ions. Cytochalasin B reversibly inhibits the darkening produced by A23187. Since this drug is known to bring about the breakdown of microfilaments, its inhibitory action is in accord with the concept that the ionophore is stimulating microfilament function. A23187 administered to tissue cultured embryonic salamander (Ambystoma maculatum) melanophores produced an irreversible rounding up of the cells, suggesting a toxic effect. The results with frog skin are interpreted as supporting the concept that the action of melanocyte-stimulating hormone involves the production of pigment granule movements as a result of the interaction of calcium ions with intracellular microfilaments and possible also the breakdown of microtubules. The mobilization of calcium ions could be brought about by cyclic adenosine monophosphate.

Presence of adenylate cyclase activity in Golgi and other fractions from rat liver. I. Biochemical determination

The distribution of adenylate cyclase (AC) in Golgi and other cell fractions from rat liver was studied using the Golgi isolation procedure of Ehrenreich et al. In liver homogenate the AC activity was found to decay with time, but addition of 1 mM EGTA reduced the rate of enzyme loss. The incorporation of 1 mM EGTA into the sucrose medium used in the initial two centrifugal steps of the Golgi isolation method stabilized the enzyme activity throughout the entire procedure and resulted in good enzyme recovery. In such preparations, AC activity was demonstrated to be associated not only with plasma membranes but also with Golgi membranes and smooth microsomal membranes as well. Furthermore, under the conditions used, enzyme activity was also associated with the 105,000 g x 90 min supernatant fraction. The specific activity of the liver homogenate was found to be 2.9 pmol-mg protein-1-min-1, the nonsedimentabel and microsomal activity was of the same order of magnitude, but the Golgi and plasma membrane activities were much higher. The specific activity of plasma membrane AC was 29 pmol-mg proten-1-min-1. The Golgi activity varied in the three fractions, with the highest activity (14 pmol) in GF1 lowest activity (1.8) in GF2, and intermediate activity (5.5) in GF3, when the Golgi activity was corrected for the presence of content protein, the activity in GF1 became much higher (9 x) than that of the plasma membrane while the activities in GF2 and GF3 were comparable to that of plasma membrane. In all locations studied, the AC was sensitive to NaF stimulation, especially the enzyme associated with Golgi membranes. The activities in plasma and microsomal membranes were stimulated by glucagon, whereas the Golgi and nonsedimentable AC were not.

Facilitation by imidazole of the aqueous flare response to alpha-melanocyte stimulating hormone

MSH, like traumata to the eye, cause a permeability disturbance in rabbits, with protein leakage into the aqueous. The MSH effect was enormously increased by instillation of imidazole or pilocarpine. The MSH effect seems to engage a different mechanism than the prostaglandin-dependent action of other agents.

Chemical messengers in development: a hypothesis

The hypothesis that physiological and developmental regulatory mechanisms are similar has been presented. Well-known developmental systems chosen illustrate the capability of the model to suggest a simple mechanism underlying the effects on development of a diverse group of chemicals. This hypothesis might be applied to other systems including the induction of the lens, limb regeneration, and the induction of the head of hydra (124). I have proposed this hypothesis not only because it permits consideration of a complex and varied array of experimental observations as reflections of a simple basic biochemical mechanism, but because recent technical advances in instrumentation and methods allow it to be directly tested. The fluorescent antibody method for the cytochemical measurement of cyclic nucleotides provides a means for investigating changes in the concentrations of cyclic nucleotides in developing cells and could also be used to detect neurotransmitters in developing cells. Similarly, the scanning electron microscope in the emitted x-ray mode provides a method for measuring changes in the content and distribution of cations within developing cells. The hypothesis presented here suggests pleasing asceticism on the part of eukaryotes. It suggests that simple derivatives of metabolites, including neurotransmitters and cyclic nucleotides, are linked together as regulatory molecules throughout the eukaryotes. The neurotransmitters are suggested to have a more general role in information transmission in eukaryotes than is generally accepted. They are hypothesized to have progressed during evolution from being intracellular messengers to a role as intercellular messengers for the relatively slow communication of developmental informatbn; and, finally, this process has culminated with their participation in the rapid intercellular communication mediated by nerves. The thought that the complex pictures of physiological regulation and of the construction of a complex multicellular organism like man might be painted with so few colors is quite satisfying.