Energy requirements for pigment aggregation in fundulus melanophores

Pigment cells: a model for the study of organelle transport

Eukaryotic organisms rely on intracellular transport to position organelles and other components within their cells. Pigment cells provide an excellent model to study organelle transport as they specialize in the translocation of pigment granules in response to defined chemical signals. Pigment cells of lower vertebrates have traditionally been used as a model for these studies because these cells transport pigment organelles in a highly coordinated fashion, are easily cultured and transfected, are ideal for microsurgery, and are good for biochemical experiments, including in vitro analysis of organelle motility. Many important properties of organelle transport, for example, the requirement of two cytoskeletal filaments (actin and microtubules), the motor proteins involved, and the mechanisms of their regulation and interactions, have been studied using pigment cells of lower vertebrates. Genetic studies of mouse melanocytes allowed the discovery of essential elements involved in organelle transport including the myosin-Va motor and its receptor and adaptor molecules on the organelle surface. Future studies of pigment cells will contribute to our understanding of issues such as the cooperation among multiple motor proteins and the mechanisms of regulation of microtubule motors.

Morphological studies on the mechanisms of pigmentary organelle transport in fish xanthophores and melanophores

Pigmentary organelle translocations within fish chromatophores undergo physiological color changes when exposed to external signals. Chromatophores can be isolated in high yields, and their pigmentary organelles can be tracked readily by microscopy. The combined efforts of morphology and biomolecular chemistry have led to the identification of and determination of the interrelationships between cytoskeletal elements and accessory proteins, motor molecules, cytomatrix, and pigmentary organelles of various sizes. Fish chromatophores have been classified as fast, intermediate, and slow translocators, based on the relative numbers of microtubules. Studies on cultured goldfish (Carassius auratus L.) xanthophores for over 20 years have demonstrated that in this slow translocator, tubulovesicular structures of the smooth endoplasmic reticular (SER) cisternae are involved in the disperson and aggregation of associated carotenoid droplets (CD) with some involvement of cytoskeletal elements. Killifish (Fundulus heteroclitus L.) melanophore, a fast translocator, was also examined. Recent work demonstrates a bright fluorescent "starburst"-like spot that we call an actin filament-organizing center (AFOC) with radiating microfilaments, akin to the microtubule-organizing center (MTOC) with radiating microtubules. Melanosomes translocate single-file on microtubules and are not associated with SER cisternae. Slower CD dispersion or aggregation in goldfish xanthophores seems to be predominantly microfilament-based transport, or microfilament- and microtubule-based transport, respectively. Faster melanosome translocations in killifish melanophores are based on microtubules, with our evidence indicating microfilament involvement. Neural crest-derived chromatophores are models for vesicular transport in axons, and immunocytochemical and imaging technologies may help to elucidate the cellular transport mechanisms.

Morphological studies on microfilaments and their organizing center in killifish (Fundulus heteroclitus L.) melanophores

Fish chromatophores serve as excellent study models for cytoskeleton-dependent organelle translocations because the distribution of pigmentary organelles can be observed against a time frame by microscopy. In this study the distribution of microfilaments along with microtubules in cultured melanophores of the killifish (Fundulus heteroclitus Linneaus) are examined using whole-cell transmission electron microscopy (WCTEM), fluorescence, and laser scanning confocal microscopy. Dispersing, dispersed, aggregating and aggregated states of pigment are induced by adding either caffeine (for dispersion) or epinephrine (for aggregation) to the cells in a standard culture medium. The cells that exhibited a random melanosome distribution in the standard culture media without these two reagents, served as the control. The results indicate that: (i) a structure considered to be the actin-filament organizing center (AFOC) is in close proximity to the microtubule-organizing center (MTOC); (ii) the radial layout of microfilaments remains similar over four physiological states of pigmentary response with the exception of epinephrine-aggregated pigment, in which the aggregate blocks the viewing of the AFOC and central microfilament rays, yet radial microfilaments, whether central and/or peripheral, are apparent in all physiological states of distribution; and (iii) microfilaments serve, together with microtubules, as scaffolding for melanosomes which migrate in bi-directional rows on cross-bridges, thus shedding light on the mechanisms for orderly melanosome translocations in a structural continuum.

Kinesin is responsible for centrifugal movement of pigment granules in melanophores

Kinesin is a mechanochemical ATPase that induces translocation of latex beads along microtubules and microtubule gliding on a glass surface. This protein is thought to be a motor for the movement of membranous organelles in cells. Recently Hollenbeck and Swanson [Hollenbeck, P. J. & Swanson, J. A. (1990) Nature (London) 346, 864-866] showed that kinesin is involved in the positioning of tubular lysosomes in macrophages. However, the role of this protein in the movement of organelles was not yet clear. We used a polyclonal antibody against the kinesin heavy chain that inhibited kinesin-dependent microtubule gliding in vitro to study the role of kinesin in the movement of pigment granules in melanophores of the teleost black tetra (Gymnocorymbus ternetzi). Microinjection of the antibody into cultured melanophores did not produce any specific effect on the aggregation of pigment granules in melanophores, but it did result in a strong dose-dependent inhibition of the dispersion. Immunoblotting of melanophore extracts showed that the kinesin antibody reacted in these cells with a single protein component with a molecular mass of 135 kDa. Thus, kinesin is responsible for the movement of pigment granules from the center to the periphery of the melanophore.

Epinephrine-induced pigment aggregation in goldfish melanophoroma cells: apparent involvement of an unknown second messenger

Using a goldfish-derived melanized cell line, we attempted to determine the identity of the signal transduction system/second messenger for epinephrine-induced aggregation of melanosomes in a goldfish cell line. The results show that the second messenger is unknown. It is not 1) influx of extracellular calcium, 2) release of intracellular stored calcium via the phosphoinositide pathway, 3) cGMP, or 4) decrease of cAMP. These results suggest that there is an unknown second messenger for this activity of epinephrine.

Role of microtubules in pigment granule migration in teleost retinal pigment epithelial cells

In cells of the teleost retinal pigment epithelium (RPE), melanin pigment granules migrate in response to changes in environmental light conditions. Melanin granules disperse into the RPE cell's long apical projections in response to the onset of light, and aggregate towards the base of the RPE cell in response to the onset of darkness. The RPE cells possess numerous microtubules and actin filaments, which in the apical projections are aligned longitudinally. Previous cytochalasin studies have shown that intact actin filaments are required for pigment granule dispersion and maintenance of the dispersed state (Burnside, Adler and O'Connor (1983). Invest. Ophthalmol. Vis. Sci. 24, 1). We report here that pigment granule aggregation is strongly inhibited when the highly stable microtubules of RPE apical projections are disrupted by a combination of cold and nocodazole treatments. Pigment dispersion and maintenance of the dispersed and aggregated states are unaffected by microtubule disruption. These results indicate that microtubules are required for RPE pigment aggregation but not for dispersion.

Lysed chromatophores: a model system for the study of bidirectional organelle transport

The development of procedures to lyse and reactivate pigment granule movements in chromatophores has provided the only information to date concerning the mechanisms by which cells regulate the direction of organelle transport. Continued analysis of motility in these models as well as in a reconstituted system containing only the pigment granules, the appropriate cytoskeletal structures, and defined soluble cell components should contribute to our understanding of the mechanisms by which protein phosphorylation and dephosphorylation or Ca2+ regulate direction of transport and to the identification and characterization of the force-generating proteins responsible for producing bidirectional organelle movements.

Calcium regulation of pigment transport in vitro

Calcium has been implicated in the regulation of many cellular motility events. In this study we have examined the role of different Ca2+ concentrations on the in vitro transport of pigment within cultured chromatophores. Cells treated with Brij detergent for 1-2 min were stripped of their plasma membranes, leaving their cytoskeleton and associated pigment granules exposed to the external milieu. We found that retrograde pigment transport (aggregation) is induced upon addition of 1 mM MgATP2- with 10(-7) M free Ca2+, while an orthograde transport (redispersal) of pigment results from lowering the concentration of free Ca2+ to 10(-8) M while maintaining 1 mM MgATP2-. These Ca2+-regulated movements are ATP dependent but are apparently independent of cAMP and insensitive to calmodulin inhibitors. The observations reported here provide novel evidence that the concentration of free Ca2+ acts to regulate the direction of intracellular organelle transport.

On the mechanism of anaphase A: evidence that ATP is needed for microtubule disassembly and not generation of polewards force

As anaphase began, mitotic PtK1 and newt lung epithelial cells were permeabilized with digitonin in permeabilization medium (PM). Permeabilization stopped cytoplasmic activity, chromosome movement, and cytokinesis within about 3 min, presumably due to the loss of endogenous ATP. ATP, GTP, or ATP-gamma-S added in the PM 4-7 min later restarted anaphase A while kinetochore fibers shortened. AMPPNP could not restart anaphase A; ATP was ineffective if the spindle was stabilized in PM + DMSO. Cells permeabilized in PM + taxol varied in their response to ATP depending on the stage of anaphase reached: one mid-anaphase cell showed initial movement of chromosomes back to the metaphase plate upon permeabilization but later, anaphase A resumed when ATP was added. Anaphase A was also reactivated by cold PM (approximately 16 degrees C) or PM containing calcium (1-10 mM). Staining of fixed cells with antitubulin showed that microtubules (MTs) were relatively stable after permeabilization and MT assembly was usually promoted in asters. Astral and kinetochore MTs were sensitive to MT disassembly conditions, and shortening of kinetochore MTs always accompanied reactivation of anaphase A. Interphase and interzonal spindle MTs were relatively stable to cold and calcium until extraction of cells was promoted by longer periods in the PM, or by higher concentrations of detergent. Since we cannot envisage how both cold treatment or relatively high calcium levels can reactivate spindle motility in quiescent, permeabilized, and presumably energy-depleted cells, we conclude that anaphase A is powered by energy stored in the spindle. The nucleotide triphosphates effective in reactivating anaphase A could be necessary for the kinetochore MT disassembly without which anaphase movement cannot proceed.

An association of actin isoforms with the expression of motile response in pigmentation variants induced from goldfish erythrophoroma cells

Comparison of actin isoforms in unpigmented goldfish cells (a normal dermal fibroblast-like cell line, and an unpigmented erythrophoroma cell line capable of being induced to undergo melanization) and in normal and neoplastic melanized goldfish cells shows that the melanized phenotype is accompanied by the presence of multiple actin isoforms. In contrast, the unpigmented cells have only beta-actin. The possible significance of this to pigment organelle translocation is discussed.

Reactivated melanophore motility: differential regulation and nucleotide requirements of bidirectional pigment granule transport

To study the molecular basis for organized pigment granule transport, procedures were developed to lyse melanophores of Tilapia mossambica under conditions in which pigment granule movements could be reactivated. Gentle lysis of the melanophores resulted in a permeabilized cell model, which, in the absence of exogenous ATP, could undergo multiple rounds of pigment granule aggregation and dispersion when sequentially challenged with epinephrine and cAMP. Both directions of transport required ATP, since aggregation or dispersion in melanophores depleted of nucleotides could be reactivated only upon addition of MgATP or MgATP plus cAMP, respectively. Differences between the nucleotide sensitivities for aggregation and dispersion were demonstrated by observations that aggregation had a lower apparent Km for ATP than did dispersion and could be initiated at a lower ATP concentration. Moreover, aggregation could be initiated by ADP, but only dispersion could be reactivated by the thiophosphate ATP analog, ATP gamma S. The direction of pigment transport was determined solely by cAMP, since pigment granules undergoing dispersion reaggregated when cAMP was removed, and those undergoing aggregation dispersed when cAMP was added. These results provide evidence that pigment granule motility may be based on two distinct mechanisms that are differentially activated and regulated to produce bidirectional movements.

Metabolic inhibitors block anaphase A in vivo

Anaphase in dividing guard mother cells of Allium cepa and stamen hair cells of Tradescantia virginiana consists almost entirely of chromosome-to-pole motion, or anaphase A. Little or no separation of the poles (anaphase B) occurs. Anaphase is reversibly blocked at any point by azide or dinitrophenol, with chromosome motion ceasing 1-10 min after application of the drugs. Motion can be stopped and restarted several times in the same cell. Prometaphase, metaphase, and cytoplasmic streaming are also arrested. Carbonyl cyanide m-chlorophenyl hydrazone also stops anaphase, but its effects are not reversible. Whereas the spindle collapses in the presence of colchicine, the chromosomes seem to "freeze" in place when cells are exposed to respiratory inhibitors. Electron microscope examination of dividing guard mother cells fixed during azide and dinitrophenol treatment reveals that spindle microtubules are still present. Our results show that chromosome-to-pole motion in these cells is sensitive to proton ionophores and electron transport inhibitors. They therefore disagree with recent reports that anaphase A does not require a continuous supply of energy. It is possible, however, that anaphase does not directly use ATP but instead depends on the energy of chemical and/or electrical gradients generated by cellular membranes.

The cytomatrix regulates "resolute" transport in erythrophores

Light microscopic studies have indicated that most microtubule-directed transport is either saltatory or resolute in nature. The latter form of transport is an intriguing phenomenon, because it commonly involves the unidirectional bulk motion of an organelle(s) such as chromosomes in dividing cells or pigment granules in chromatophores. We have investigated the ultrastructural and biochemical basis for the resolute transport of pigment in chromatophores. Light and EM studies of erythrophores in situ have clearly shown that when the microtubules were completely removed with nocodazole, resolute transport continued and was stimulated by aggregating and dispersing agents. Light and electron microscopic studies of cultured erythrophores permeabilized with digitonin indicated that resolute motion was produced by a cytomatrix of 3 to 7 nm filaments. Immunofluorescent analysis with several monoclonal antibodies raised against MAP-2 further demonstrated that MAP-2 was an important component of the contractile cytomatrix that powers pigment aggregation and dispersion. We conclude that a microtubule-associated cytomatrix normally produces resolute pigment transport in chromatophores.