Actin-dependent motility of melanosomes from fish retinal pigment epithelial (RPE) cells investigated using in vitro motility assays

Melanocytes in black-boned chicken have immune contribution under infectious bursal disease virus infection

In black-boned chicken, melanocytes are widely distributed in their inner organs. However, the roles of these cells are not fully elucidated. In this study, we used 3-wk-old female Silky Fowl to investigate the functions of melanocytes under infection with infectious bursal disease virus (IBDV). We found the melanocytes in the bursa of Fabricius involved in IBDV infection shown as abundant melanin were transported into the nodule and lamina propria where obvious apoptotic cells and higher expression of BAX were detected. Genes related to the toll-like receptor (TLR) signaling pathway were highly detected by quantitative PCR, including TLR1, TLR3, TLR4, TLR15, myeloid differential protein-88, interferon-α, and interferon-β. We then isolated and infected primary melanocytes with IBDV in vitro and found that higher expressions of immune genes were detected at 24 and 48 h after infection; the upregulated innate and adaptive immune genes were involved in the pathogenesis of IBDV infection, including TLR3, TLR7, interleukin 15 (IL15), IL18, IL1rap, CD7, BG2, ERAP1, and SLA2. These changes in gene expression were highly associated with microtubule-based movement, antigen processing and presentation, defense against viruses, and innate immune responses. Our results indicated that the widely distributed melanocytes in Silky Fowl could migrate to play important innate immune roles during virus infection.

The cell biology of the retinal pigment epithelium

The retinal pigment epithelium (RPE), a monolayer of post-mitotic polarized epithelial cells, strategically situated between the photoreceptors and the choroid, is the primary caretaker of photoreceptor health and function. Dysfunction of the RPE underlies many inherited and acquired diseases that cause permanent blindness. Decades of research have yielded valuable insight into the cell biology of the RPE. In recent years, new technologies such as live-cell imaging have resulted in major advancement in our understanding of areas such as the daily phagocytosis and clearance of photoreceptor outer segment tips, autophagy, endolysosome function, and the metabolic interplay between the RPE and photoreceptors. In this review, we aim to integrate these studies with an emphasis on appropriate models and techniques to investigate RPE cell biology and metabolism, and discuss how RPE cell biology informs our understanding of retinal disease.

Regulation of melanosome number, shape and movement in the zebrafish retinal pigment epithelium by OA1 and PMEL

Analysis of melanosome biogenesis in the retinal pigment epithelium (RPE) is challenging because it occurs predominantly in a short embryonic time window. Here, we show that the zebrafish provides an ideal model system for studying this process because in the RPE the timing of melanosome biogenesis facilitates molecular manipulation using morpholinos. Morpholino-mediated knockdown of OA1 (also known as GPR143), mutations in the human homologue of which cause the most common form of human ocular albinism, induces a major reduction in melanosome number, recapitulating a key feature of the mammalian disease where reduced melanosome numbers precede macromelanosome formation. We further show that PMEL, a key component of mammalian melanosome biogenesis, is required for the generation of cylindrical melanosomes in zebrafish, which in turn is required for melanosome movement into the apical processes and maintenance of photoreceptor integrity. Spherical and cylindrical melanosomes containing similar melanin volumes co-exist in the cell body but only cylindrical melanosomes enter the apical processes. Taken together, our findings indicate that melanosome number and shape are independently regulated and that melanosome shape controls a function in the RPE that depends on localisation in the apical processes.

Orientation of actin filaments in teleost retinal pigment epithelial cells, and the effect of the lectin, Concanavalin A, on melanosome motility

Retinal pigment epithelial cells of teleosts contain numerous melanosomes (pigment granules) that exhibit light-dependent motility. In light, melanosomes disperse out of the retinal pigment epithelium (RPE) cell body (CB) into long apical projections that interdigitate with rod photoreceptors, thus shielding the photoreceptors from bleaching. In darkness, melanosomes aggregate through the apical projections back into the CB. Previous research has demonstrated that melanosome motility in the RPE CB requires microtubules, but in the RPE apical projections, actin filaments are necessary and sufficient for motility. We used myosin S1 labeling and platinum replica shadowing of dissociated RPE cells to determine actin filament polarity in apical projections. Actin filament bundles within RPE apical projections are uniformly oriented with barbed ends toward the distal tips. Treatment of RPE cells with the tetravalent lectin, Concanavalin A, which has been shown to suppress cortical actin flow by crosslinking of cell-surface proteins, inhibited melanosome aggregation and stimulated ectopic filopodia formation but did not block melanosome dispersion. The polarity orientation of F-actin in apical projections suggests that a barbed-end directed myosin motor could effect dispersion of melanosomes from the CB into apical projections. Inhibition of aggregation, but not dispersion, by ConA confirms that different actin-dependent mechanisms control these two processes and suggests that melanosome aggregation is sensitive to treatments previously shown to disrupt actin cortical flow.

Myosin II and Rho kinase activity are required for melanosome aggregation in fish retinal pigment epithelial cells

In the retinal pigment epithelium (RPE) of fish, melanosomes (pigment granules) migrate long distances through the cell body into apical projections in the light, and aggregate back into the cell body in the dark. RPE cells can be isolated from the eye, dissociated, and cultured as single cells in vitro. Treatment of isolated RPE cells with cAMP or the phosphatase inhibitor, okadaic acid (OA), stimulates melanosome aggregation, while cAMP or OA washout in the presence of dopamine triggers dispersion. Previous studies have shown that actin filaments are both necessary and sufficient for aggregation and dispersion of melanosomes within apical projections of isolated RPE. The role of myosin II in melanosome motility was investigated using the myosin II inhibitor, blebbistatin, and a specific rho kinase (ROCK) inhibitor, H-1152. Blebbistatin and H-1152 partially blocked melanosome aggregation triggered by cAMP in dissociated, isolated RPE cells and isolated sheets of RPE. In contrast, neither drug affected melanosome dispersion. In cells exposed to either blebbistatin or H-1152, then triggered to aggregate using OA, melanosome aggregation was completely inhibited. These results demonstrate that (1) melanosome aggregation and dispersion occur through different, actin-dependent mechanisms; (2) myosin II and ROCK activity are required for full melanosome aggregation, but not dispersion; (3) partial aggregation that occurred despite myosin II or ROCK inhibition suggests a second component of aggregation that is dependent on cAMP signaling, but independent of ROCK and myosin II.

Myosins in melanocytes: to move or not to move?

The actin network has been implicated in the intracellular transport and positioning of the melanosomes, organelles that are specialized in the biosynthesis and the storage of melanin. It contributes also to molecular mechanisms that underlie the intracellular membrane dynamics and thereby can control the biogenesis of melanosomes. Two mechanisms for actin-based movements have been identified: one is dependent on the motors associated to actin namely the myosins; the other is dependent on actin polymerization. This review will focus on to the role of the actin cytoskeleton and myosins in the transport and in the biogenesis of melanosomes. Myosins involved in membrane traffic are largely seen as transporters of organelles or membrane vesicles containing cargos along the actin networks. Yet increasing evidence suggests that some of the myosins contribute to the dynamics of internal membrane by using other mechanisms. The role of the myosins and the different molecular mechanisms by which they contribute or may contribute to the distribution, the movement and the biogenesis of the melanosomes in epidermal melanocytes and retinal pigmented epithelial (RPE) cells will be discussed.

Proteomic analysis of mature melanosomes from the retinal pigmented epithelium

The protein content of melanosomes in the retinal pigment epithelium (RPE) was analyzed by mass spectrometry. More than 100 proteins were found to be common to two out of three variations of sample preparation. Some proteins normally associated with other organelles were detected. Several lysosomal enzymes were detected, with the presence of cathepsin D confirmed by immunoelectron microscopy, thus supporting the previously suggested notion that melanosomes may contribute to the degradation of ingested photoreceptor outer segment disks.

Caveolin-1alpha and -1beta perform nonredundant roles in early vertebrate development

Caveolins are integral membrane proteins that localize predominantly to lipid rafts, where they oligomerize to form flask-shaped organelles termed caveolae and play important roles in membrane trafficking, signal transduction, and other cellular processes. To investigate potential roles for caveolin-1 (cav-1) in development, cav-1alpha and -1beta cDNAs were functionally characterized in the zebrafish. Cav-1alpha and -1beta mRNAs exhibited overlapping but distinct expression patterns throughout embryogenesis. Targeted depletion of either Cav-1 isoform, using antisense morpholino oligomers, resulted in a substantial loss of caveolae and dramatic neural, eye, and somite defects by 12 hours after fertilization, the time at which mRNA levels of both isoforms substantially increased in wild-type animals. Morphant phenotypes were rescued by injection of homotypic (cav-1alpha/cav-1alpha) but not heterotypic (cav-1alpha/cav-1beta) zebrafish and human cav-1 cRNAs, revealing nonredundant and evolutionarily conserved functions for the individual Cav-1 isoforms. Mutation of a known Cav-1 phosphorylation site unique to Cav-1alpha (Y14-->F) resulted in a failure to rescue the cav-1alpha morphant phenotype, verifying an essential role for Cav-1alpha specifically and implicating this residue in early developmental functions. Cav-1alpha and -1beta morphants also exhibited disruption in the actin cytoskeleton. These results support important and previously unanticipated roles for the Caveolin-1 isoforms in vertebrate organogenesis.

Shroom2 (APXL) regulates melanosome biogenesis and localization in the retinal pigment epithelium

Shroom family proteins have been implicated in the control of the actin cytoskeleton, but so far only a single family member has been studied in the context of developing embryos. Here, we show that the Shroom-family protein,Shroom2 (previously known as APXL) is both necessary and sufficient to govern the localization of pigment granules at the apical surface of epithelial cells. In Xenopus embryos that lack Shroom2 function, we observed defects in pigmentation of the eye that stem from failure of melanosomes to mature and to associate with the apical cell surface. Ectopic expression of Shroom2 in naïve epithelial cells facilitates apical pigment accumulation, and this activity specifically requires the Rab27a GTPase. Most interestingly, we find that Shroom2, like Shroom3 (previously called Shroom),is sufficient to induce a dramatic apical accumulation of the microtubule-nucleating protein γ-tubulin at the apical surfaces of naïve epithelial cells. Together, our data identify Shroom2 as a central regulator of RPE pigmentation, and suggest that, despite their diverse biological roles, Shroom family proteins share a common activity. Finally,because the locus encoding human SHROOM2 lies within the critical region for two distinct forms of ocular albinism, it is possible that SHROOM2mutations may be a contributing factor in these human visual system disorders.

Differential actin-dependent localization modulates the evolutionarily conserved activity of Shroom family proteins

Shroom is an actin-associated determinant of cell morphology that is required for neural tube closure in both mice and frogs. Shroom regulates this process by causing apical constriction of epithelial cells via a pathway involving myosin II. Here we report on characterization of the Shroom-related proteins Apxl and KIAA1202 and their role in cell architecture. Shroom, Apxl, and KIAA1202 exhibit differing abilities to interact with the actin cytoskeleton. In fibroblasts, Shroom readily associates with actin stress fibers and induces bundling, Apxl is found on cortical actin, and KIAA1202 is localized to a cytoplasmic population of F-actin. In epithelial cells, Apxl and KIAA1202 do not induce apical constriction as Shroom does, but have the capacity to do so if targeted to the apical junctional complex. To determine whether the activity of Shroom-like proteins is conserved in invertebrates, we have tested the ability of the lone Shroomrelated protein in Drosophila, CG8603, to activate the constriction pathway. A chimeric protein consisting of the Shroom targeting domain and the Drosophila protein elicits constriction. Finally, we show that Apxl is involved in regulating the cytoskeletal organization and architecture of endothelial cells. We predict that the ability of Shroom-like proteins to regulate cellular morphology is conserved in evolution and is regulated in part by subcellular localization.

Directed motility of phagosomes inTetrahymena thermophila requires actin and Myo1p, a novel unconventional myosin

The phagosome cycle was investigated in Tetrahymena thermophila, which had internalized fluorescent latex beads. Confocal microscopy of cells from a GFP-actin strain revealed actin filaments that extended 3-5 mum from the periphery of fluorescent phagosomes. In GFP-actin cells and in wild-type cells, motility of fluorescent phagosomes was directed from the oral cavity to the posterior end of the cell. Although 60% of fluorescent phagosomes in the MYO1-knockout strain were motile, movement of phagosomes was not directed toward the posterior end of the cell and was random. Forty percent of fluorescent phagosomes in knockout cells were non-motile in contrast to only 20% non-motile phagosomes in wild-type cells. The increased incidence of non-motile phagosomes in the knockout strain could reflect absence of Myo1p as a motor. Another myosin or other molecular motors could power random movement of phagosomes in the MYO1-knockout strain. In latrunculin-treated GFP-actin cells, movement of fluorescent phagosomes was random. Average velocity of random movement of fluorescent phagosomes in the knockout strain and in latrunculin-treated cells was statistically the same as the average velocity (2.0 +/- 1.9 microm/min) of phagosomes in GFP-actin cells. These findings are an indication that dynamic actin and Myo1p are required for directed motility of phagosomes.

Role of myosin VIIa and Rab27a in the motility and localization of RPE melanosomes

Myosin VIIa functions in the outer retina, and loss of this function causes human blindness in Usher syndrome type 1B (USH1B). In mice with mutant Myo7a, melanosomes in the retinal pigmented epithelium (RPE) are distributed abnormally. In this investigation we detected many proteins in RPE cells that could potentially participate in melanosome transport, but of those tested, only myosin VIIa and Rab27a were found to be required for normal distribution. Two other expressed proteins, melanophilin and myosin Va, both of which are required for normal melanosome distribution in melanocytes, were not required in RPE, despite the association of myosin Va with the RPE melanosome fraction. Both myosin VIIa and myosin Va were immunodetected broadly in sections of the RPE, overlapping with a region of apical filamentous actin. Some 70-80% of the myosin VIIa in RPE cells was detected on melanosome membranes by both subcellular fractionation of RPE cells and quantitative immunoelectron microscopy, consistent with a role for myosin VIIa in melanosome motility. Time-lapse microscopy of melanosomes in primary cultures of mouse RPE cells demonstrated that the melanosomes move in a saltatory manner, interrupting slow movements with short bursts of rapid movement (>1 RR01183m/second). In RPE cells from Myo7a-null mice, both the slow and rapid movements still occurred, except that more melanosomes underwent rapid movements, and each movement extended approximately five times longer (and further). Hence, our studies demonstrate the presence of many potential effectors of melanosome motility and localization in the RPE, with a specific requirement for Rab27a and myosin VIIa, which function by transporting and constraining melanosomes within a region of filamentous actin. The presence of two distinct melanosome velocities in both control and Myo7a-null RPE cells suggests the involvement of at least two motors other than myosin VIIa in melanosome motility, most probably, a microtubule motor and myosin Va.