Molecular mechanisms of pigment transport in melanophores

Actin-dependent, retrograde motility of surface-attached beads and aggregating pigment granules in dissociated teleost retinal pigment epithelial cells

Teleost retinal pigment epithelial (RPE) cells contain pigment granules within apical projections which undergo actin-dependent, bi-directional motility. Dissociated RPE cells in culture attach to the substrate and extend apical projections in a radial array from the central cell body. Pigment granules within projections can be triggered to aggregate or disperse by the presence or absence of 1 mM cAMP. Aminated, fluorescent latex beads attached to the dorsal surface of apical projections and moved in the retrograde direction, towards the cell body. Bead rates on RPE cells with aggregating or fully aggregated pigment granules were 2.2 +/- 0.5 and 2.6 +/- 0.2 microm/min (mean +/- SEM), respectively, similar to rates of aggregating (retrograde) pigment granule movement (2.0 +/- 0.4 microm/min). Bead rates were slightly slower on cells with fully dispersed or dispersing pigment granules (1.5 +/- 0.1 and 1.5 +/- 0.4 microm/min). Movements of surface-attached beads and aggregating pigment granules were closely correlated in the distal portions of apical projections, but were more independent of each other in proximal regions of the projections. The actin disrupting drug, cytochalasin D (CD), reversibly halted retrograde bead movements, suggesting that motility of surface-attached particles is actin-dependent. In contrast, the microtubule depolymerizing drug, nocodazole, had no effect on retrograde bead motility. The similar characteristics and actin-dependence of retrograde bead movements and aggregating pigment granules suggest a correlation between these two processes.

Expression of constructs of the neuronal isoform of myosin-Va interferes with the distribution of melanosomes and other vesicles in melanoma cells

Myosin-Va has been implicated in melanosome translocation, but the exact molecular mechanisms underlying this function are not known. In the dilute, S91 melanoma cells, melanosomes move to the cell periphery but do not accumulate in the tips of dendrites as occurs in wild-type B16 melanocytes; rather, they return and accumulate primarily at the pericentrosomal region in a microtubule-dependent manner. Expression of the full-length neuronal isoform of myosin-Va in S91 cells causes melanosomes to disperse, occupying a cellular area approximately twice that observed in non-transfected cells, suggesting a partial rescue of the dilute phenotype. Overexpression of the full tail domain in S91 cells is not sufficient to induce melanosome dispersion, rather it causes melanosomal clumping. Overexpression of the head and head-neck domains of myosin-Va in B16 cells does not alter the melanosome distribution. However, overexpression of the full tail domain in these cells induces melanosome aggregation and the appearance of tail-associated, aggregated particles or vesicular structures that exhibit variable degrees of staining for melanosomal and Golgi beta-COP markers, as well as colocalization with the endogenous myosin-Va. Altogether, the present data suggest that myosin-Va plays a role in regulating the direction of microtubule-dependent melanosome translocation, in addition to promoting the capture of melanosomes at the cell periphery as suggested by previous studies. These studies also reinforce the notion that myosin-V has a broader function in melanocytes by acting on vesicular targeting or intracellular protein trafficking.

Fish chromatophores as cytosensors in a microscale device: detection of environmental toxins and bacterial pathogens

Fish chromatophores from Betta splendens are used as the cytosensor element in the development of a portable microscale device capable of detecting certain environmental toxins and bacterial pathogens by monitoring changes in pigment granule distribution. The adaptation of chromatophores to a microscale environment has required the development of enabling technologies to produce miniaturized culture chambers, to integrate microfluidics for sample delivery, to miniaturize image capture, and to design new statistical methods for image analyses. Betta splendens chromatophores were selected as the cytosensor element because of their moderate size, their toleration of close contact, and most importantly, for their responses to a broad range of chemicals and pathogenic bacteria. A miniaturized culture chamber has been designed that supports chromatophore viability for as long as 3 months, and that can be easily transported without damage to the cells. New statistical methods for image analyses have been developed that increase sensitivity and also decrease the time required for detection of significant changes in pigment granule distribution. Betta chromatophores have been tested for their responses to selected pathogenic bacteria and chemical agents. We discuss in detail the aggregation of pigment granules seen when chromatophores are incubated with Bacillus cereus, a common cause of food poisoning. Also described are the more subtle responses of chromatophores to a class of environmental chemical toxins, polynuclear aromatic hydrocarbons. We show that the chromatophores are able to detect the presence of certain polynuclear aromatic hydrocarbons at concentrations lower than the Environment Protection Agency (EPA) 550.1 standards.

High affinity binding of brain myosin-Va to F-actin induced by calcium in the presence of ATP

Brain myosin-Va consists of two heavy chains, each containing a neck domain with six tandem IQ motifs that bind four to five calmodulins and one to two essential light chains. Previous studies demonstrated that myosin-Va exhibits an unusually high affinity for F-actin in the presence of ATP and that its MgATPase activity is stimulated by micromolar Ca(2+) in a highly cooperative manner. We demonstrate here that Ca(2+) also induces myosin-Va binding to and cosedimentation with F-actin in the presence of ATP in a similar cooperative manner and calcium concentration range as that observed for the ATPase activity. Neither hydrolysis of ATP nor buildup of ADP was required for Ca(2+)-induced cosedimentation. The Ca(2+)-induced binding was inhibited by low temperature or by 0.6 m NaCl, but not by 1% Triton X-100. Tight binding between myosin-Va and pyrene-labeled F-actin in the presence of ATP and Ca(2+) was also detected by quenching of the pyrene fluorescence. Negatively stained preparations of actomyosin-Va under Ca(2+)-induced binding conditions showed tightly packed F-actin bundles cross-linked by myosin-Va. Our data demonstrate that high affinity binding of myosin-Va and F-actin in the presence of ATP or 5'-O-(thiotriphosphate) is induced by micromolar concentrations of Ca(2+). Since Ca(2+) regulates both the actin binding properties and actin-activated ATPase of myosin-Va over the same concentration range, we suggest that the calcium signal may regulate the mechanism of processivity of myosin Va.

The melanosome: membrane dynamics in black and white

Melanosomes are morphologically and functionally unique organelles within which melanin pigments are synthesized and stored. Melanosomes share some characteristics with lysosomes, but can be distinguished from them in many ways. The biogenesis and intracellular movement of melanosomes and related organelles are disrupted in several genetic disorders in mice and humans. The recent characterization of genes defective in these diseases has reinvigorated interest in the melanosome as a model system for understanding the molecular mechanisms that underlie intracellular membrane dynamics.

Self-organization of a radial microtubule array by dynein-dependent nucleation of microtubules

Polarized radial arrays of cytoplasmic microtubules (MTs) with minus ends clustered at the cell center define the organization of the cytoplasm through interaction with microtubule motors bound to membrane organelles or chromosomes. It is generally assumed that the radial organization results from nucleation of MTs at the centrosome. However, radial MT array can also be attained through self-organization that requires the activity of a minus-end-directed MT motor, cytoplasmic dynein. In this study we examine the role of cytoplasmic dynein in the self-organization of a radial MT array in cytoplasmic fragments of fish melanophores lacking the centrosome. After activation of dynein motors bound to membrane-bound organelles, pigment granules, the fragments rapidly form polarized radial arrays of MTs and position pigment aggregates at their centers. We show that rearrangement of MTs in the cytoplasm is achieved through dynein-dependent MT nucleation. The radial pattern is generated by continuous disassembly and reassembly of MTs and concurrent minus-end-directed transport of pigment granules bearing the nucleation sites.

Myosin V-mediated vacuole distribution and fusion in fission yeast

The class V myosins are actin-based motors that move a variety of cellular cargoes [1]. In budding yeast, their activity includes the relocation of a portion of the vacuole from the mother cell to the bud [2, 3]. Fission yeast cells contain numerous (approximately 80) small vacuoles. When S. pombe cells are placed in water, vacuoles fuse in response to osmotic stress [4]. Fission yeast possess two type V myosin genes, myo51(+) and myo52(+) [5]. In a myo51Delta strain, vacuoles were distributed throughout the cell, and mean vacuole diameter was identical to that seen in wild-type cells. When myo51Delta and wild-type cells were placed in water, vacuoles enlarged by fusion. In myo52Delta cells, by contrast, vacuoles were smaller and mostly clustered around the nucleus, and fusion in water was largely inhibited. When cells containing GFP-Myo52 were placed in water, Myo52 was seen to redistribute from the cell poles to the surface of the fusing vacuoles. Vacuole fusion in fission yeast was inhibited by the microtubule drug thiabendazole (TBZ) but not by the actin inhibitor latrunculin B. This is the first demonstration of the involvement of a type V myosin, possibly via an interaction with microtubules, in homotypic membrane fusion.

In vitro reconstitution of fish melanophore pigment aggregation

Movement and positioning of melanophore pigment organelles depend on microtubule- and actin-dependent motors, but the regulation of these forces are poorly understood. Here, we describe a cell free and fixed time motility assay for the study of the regulation of microtubule-dependent pigment organelle positioning in vitro. The assay involves introduction of microtubule-asters assembled in clam oocyte lysates into lysates prepared from Fundulus heteroclitus melanophores with either aggregated or dispersed pigment. When asters were introduced in lysates prepared from melanophores with dispersed pigment, pigment organelles bound astral microtubules and were evenly distributed throughout the aster. In contrast, when asters were introduced into lysates prepared from melanophores with aggregated pigment, pigment organelles accumulated around the centrosome, mimicking a pigment aggregate. The addition of anti-dynein intermediate chain antibody (m74-1), previously shown to interfere with binding of dynactin to dynein and thereby causing detachment of dynein from organelles, blocked the ATP-dependent aggregation of pigment in vitro and induced a depletion of pigment from the centrosomal area. The results show that dynein is essential for pigment aggregation and involved in maintenance of evenly dispersed pigment in vitro, analogous to cellular evidence, and suggest a possible role for dynactin in these processes as well.

Dynein, dynactin, and kinesin II's interaction with microtubules is regulated during bidirectional organelle transport

The microtubule motors, cytoplasmic dynein and kinesin II, drive pigmented organelles in opposite directions in Xenopus melanophores, but the mechanism by which these or other motors are regulated to control the direction of organelle transport has not been previously elucidated. We find that cytoplasmic dynein, dynactin, and kinesin II remain on pigment granules during aggregation and dispersion in melanophores, indicating that control of direction is not mediated by a cyclic association of motors with these organelles. However, the ability of dynein, dynactin, and kinesin II to bind to microtubules varies as a function of the state of aggregation or dispersion of the pigment in the cells from which these molecules are isolated. Dynein and dynactin bind to microtubules when obtained from cells with aggregated pigment, whereas kinesin II binds to microtubules when obtained from cells with dispersed pigment. Moreover, the microtubule binding activity of these motors/dynactin can be reversed in vitro by the kinases and phosphatase that regulate the direction of pigment granule transport in vivo. These findings suggest that phosphorylation controls the direction of pigment granule transport by altering the ability of dynein, dynactin, and kinesin II to interact with microtubules.