Long-term cultivation of amphibian melanophores. In vitro ageing and spontaneous transformation to a continuous cell line

Control of melanosome movement in intact and cultured melanophores in the bitterling, Acheilognathus lanceolatus

The melanophores in the dermis on scales in the bitterling, Acheilognathus lanceolatus were studies to obtain information about the control mechanism of aggregation and dispersion using intact, membrane-permeabilized and cultured cells. The cultured melanophores showed supersensitivity, namely, they responded to norepinephrine with much higher sensitivity than intact cells. The cultured melanophores failed to respond to high KCl. Melatonin aggregated and adenosine dispersed melanosomes within a cell. Digitonin permeabilized cells showed aggregation with Ca ions and dispersion by cyclic adenosine 3',5'-monophosphate (cAMP) in the presence of ATP. Movement of melanosomes was observed under the high magnification of light microscope and the tracks of each pigment granule were followed. The granules moved fast and linearly during aggregation, whereas they showed to-and-fro movement during dispersion.

Anterior/posterior influences on neural crest-derived pigment cell differentiation

The neural crest of vertebrate embryos has been used to elucidate steps involved in early embryonic cellular processes such as differentiation and migration. Neural crest cells form a ridge along the dorsal midline and subsequently they migrate throughout the embryo and differentiate into a wide variety of cell types. Intrinsic factors and environmental cues distributed along the neural tube, along the migratory pathways, and/or at the location of arrest influence the fate of neural crest cells. Although premigratory cells of the cranial and trunk neural crest exhibit differences in their differentiation potentials, premigratory trunk neural crest cells are generally assumed to have equivalent developmental potentials. Axolotl neural crest cells from different regions of origin, different stages of development, and challenged with different culture media have been analyzed for differentiation preferences pertaining to the pigment cell lineages. We report region-dependent differentiation of chromatophores from trunk neural crest at two developmental stages. Also, dosage with guanosine produces region-specific influences on the production of xanthophores from wild-type embryos. Our results support the hypothesis that spatial and temporal differences among premigratory trunk neural crest cells found along the anteroposterior axis influence developmental potentials and diminish the equivalency of axolotl neural crest cells.

Cellular plasticity among axolotl neural crest-derived pigment cell lineages

Many of the factors and mechanisms guiding the migration/differentiation of neural crest cells that give rise to a number of distinguishable cell types, including all dermal and epidermal pigment cells, remain unknown. The axolotl possesses three pigment cell types that differentiate according to specific developmentally programmed sequences and contribute to pigment pattern in the adult. A single lineage of the crest that becomes restricted to one of three pigment cell types gives us the opportunity to examine the existence of a neural crest stem cell population and the potential for trans-differentiation events. Interpretations of experiments involving drug-treated and mutant axolotls implicate cellular plasticity leading to observed phenotypes. We present results from recent in vitro studies designed to identify parameters influencing differentiation events of individual neural crest-derived pigment cell lineages. We demonstrate that the differentiation of xanthophores is enhanced, while that of the melanophores are inhibited in guanosine-supplemented neural crest cell cultures. Data suggest that the increase in one pigment cell population is at the expense of another, indicative of cellular plasticity. Videomicroscopy used in this study agrees with an abundance of correlative evidence supporting the hypothesis of transdifferentiation events among neural crest-derived pigment cell populations. The embryonic neural crest-derived pigment cell system is an ideal model to study differentiation of multipotential stem cells that play critical roles in patterning.

Structure-based search for peptide ligands that cross-react with melanocortin receptors

PURPOSE

To define sequence motifs that can be used to identify peptide ligands of the melanocortin receptor (MCR).

METHODS

Screening of combinatorial libraries has led to identification of D-Trp-Nle-NH2 (Nle, norleucine) and D-Trp-Arg-NH2 as the smallest structures known to antagonize the amphibian MCR (1). As the basis of a search paradigm, peptide-ligands containing these or similar motifs within their larger primary structure were examined for ability to antagonize amphibian and recombinant human MCRs. Compounds examined include analogs of substance P, leutinizing-hormone releasing-hormone, endothelin, neurotensin, and opioid-somatostatin.

RESULTS

Of seven compounds tested containing the predetermined search motif D-Trp-AAx (where AAx is Arg, Leu, Nle, or Ile), six were found to have previously unrecognized antagonist activity at the amphibian MCR (Kd 30 to 5000 nM). In contrast, of 14 similar control peptides lacking the D-Trp-AAx search motif, only somatostatin displayed measurable antagonist potency. The anticancer peptide, [Arg8, D-Trp7.9, N-methyl-Phe8]-substance P, was the most potent of the motif-containing peptides with a Kd of 31 nM. The mu-opioid antagonist D-Phe-cyclic[Cys-Tyr-D-Trp-Arg-Thr-Pen]-Thr-NH2 (CTAP) also blocked the amphibian MCR (Kd 1 microM), but the related mu-antagonist CTOP, different only by only by substitution of Arg with ornithine within the search motif, was found to agonize the amphibian MCR (EC50 67 nM). CTAP and the anticancer peptide were also tested on human MCRs (hMCRs); while CTAP competed with alpha-MSH at the hMC1 receptor, the anticancer peptide had no effect or was slightly stimulatory.

CONCLUSIONS

We have identified dipeptide motifs that help distinguish antagonist ligands of the amphibian MCR from ligands known to interact with other G-protein coupled receptors. This approach might be generally applicable if motifs can identified for other receptors and their subtypes. In studies employing CTAP and CTOP, analogs previously considered highly selective for the mu-opioid receptor, cross-reaction with MCRs must be considered.

Action of light on frog pigment cells in culture

Solar radiation induces numerous biologic effects in skin but the mechanism underlying these responses is poorly understood. To study the etiology of these phenomena, we investigated the effect of light on cultured Xenopus laevis melanophores. Visible light stimulated a marked increase in intracellular cAMP levels within the first minute of irradiation. This light-induced elevation in cAMP was blocked by melatonin and was not seen in fibroblasts irradiated in a similar manner. These data show that the photoresponse of pigment cells from amphibian skin can be mediated by a cAMP-dependent mechanisms and suggest that a unique member of the rhodopsin family is involved in this process.

Olfaction by melanophores: what does it mean?

Hypotheses on general olfaction can be divided into two broad groups: those that predict the existence of olfactory-specific olfactory receptor proteins and those that do not. Recently, much attention has been paid to the discovery of an odorant-stimulated adenylate cyclase in purified olfactory cilia. This finding has, for the most part, been accepted as evidence that the former hypotheses are correct. Here we report that frog melanophores, which are nonolfactory in nature, disperse their melanosomes in response to the same types and concentrations of odorants used in the investigations of olfactory cilia and that pigment dispersion is accompanied by rises in intracellular cAMP levels. The effects show that the existence of a cAMP-based second messenger system in olfactory cilia is not in itself proof of the existence of olfactory-specific olfactory receptor proteins. Also they explain the basis of Ottoson's pioneering work of 30 years ago on the electrical responses of frog olfactory epithelium to stimulation with alcohols. The results suggest that there could be two mechanisms that are important for the detection of odorants: one based on specific receptors, the other nonspecific, but both working through activation of cAMP.

Effects of exogenous guanosine on chromatophore differentiation in the axolotl

Guanosine is shown to dramatically alter the pigment phenotype of axolotls by suppressing melanization and enhancing the biosynthesis and deposition of purine-derived pigments. Phenotypic changes caused by guanosine are manifested by altered chromatophore differentiation patterns such that few black pigment cells (melanophores) differentiate (and those that do are punctate and necrotic in appearance), whereas the development of yellow (xanthophore) and reflecting (iridophore) pigment cells is enhanced. Mechanisms for changes in chromatophore differentiation, and thus pattern formation, are discussed, including the possibility that pigment cells may undergo transdifferentiation in vivo.

The effects of various nutritional supplements on the growth, migration and differentiation of Xenopus laevis neural crest cells in vitro

This study investigates the nutritional requirements of Xenopus laevis neural crest cells and melanophores developing in vitro. A comparison is made between the growth and differentiation of cells in serum-containing medium and a chemically defined, serum-free medium that we have designed. Our chemically defined medium is more efficient than serum-supplemented medium in promoting proliferation of these cells. Several supplements are required to enhance culture development. These include insulin, alpha-melanocyte stimulating hormone, somatotropin, luteotrophic hormone, linoleic acid, uridine, and putrescine. In addition, collagen and fibronectin provide the most conducive environment tested for cell migration and adhesion.

Fish melanin-concentrating hormone disperses melanin in amphibian melanophores

Melanin-concentrating hormone (MCH), which was isolated from salmon pituitary and caused melanin concentration in fish scale melanophores, has been tested on cultured chromatophores of an amphibian, the bullfrog tadpole. MCH induced dispersion of melanin in cultured melanophores of the tadpole. The duration of the dispersing effect of MCH was relatively short compared with that of alpha-melanocyte-stimulating hormone (alpha-MSH). MCH also induced the concentration of cultured iridophores of bullfrog tadpole.