Melatonin-induced organelle movement in melanophores is coupled to tyrosine phosphorylation of a high molecular weight protein

Regulation of Aqueous Humor Secretion by Melatonin in Porcine Ciliary Epithelium

Secretion of melatonin, a natural hormone whose receptors are present in the ciliary epithelium, displays diurnal variation in the aqueous humor (AH), potentially contributing to the regulation of intraocular pressure. This study aimed to determine the effects of melatonin on AH secretion in porcine ciliary epithelium. The addition of 100 µM melatonin to both sides of the epithelium significantly increased the short-circuit current (Isc) by ~40%. Stromal administration alone had no effect on the Isc, but aqueous application triggered a 40% increase in Isc, similar to that of bilateral application without additive effect. Pre-treatment with niflumic acid abolished melatonin-induced Isc stimulation. More importantly, melatonin stimulated the fluid secretion across the intact ciliary epithelium by ~80% and elicited a sustained increase (~50-60%) in gap junctional permeability between pigmented ciliary epithelial (PE) cells and non-pigmented ciliary epithelial (NPE) cells. The expression of MT3 receptor was found to be >10-fold higher than that of MT1 and MT2 in porcine ciliary epithelium. Aqueous pre-treatment with MT1/MT2 antagonist luzindole failed to inhibit the melatonin-induced Isc response, while MT3 antagonist prazosin pre-treatment abolished the Isc stimulation. We conclude that melatonin facilitates Cl- and fluid movement from PE to NPE cells, thereby stimulating AH secretion via NPE-cell MT3 receptors.

The Flavor Enhancer Maltol Increases Pigment Aggregation in Dermal and Neural Melanophores in Xenopus laevis Tadpoles

Melanophores are pigmented cells that change the distribution of melanosomes, enabling animals to appear lighter or darker for camouflage, thermoregulation, and protection from ultraviolet radiation. A complex series of hormonal and neural mechanisms regulates melanophore pigment distribution, making these dynamic cells a valuable tool to screen toxicants as they rapidly respond to changes in the environment. We found that maltol, a naturally occurring flavor enhancer and fragrance agent, induces melanophore pigment aggregation in a dose-dependent manner in Xenopus laevis tadpoles. To determine if maltol affects camouflage adaptation, we placed tadpoles into maltol baths situated over either a white or a black background. Maltol induced pigment aggregation in a similar dose-dependent pattern regardless of background color. We also tested how maltol treatment compares to melatonin treatment and found that the degree of pigment aggregation induced by maltol is similar to treatment with melatonin but that maltol induces over a much longer time course. Last, maltol had no effect on mRNA expression in the brain of genes that regulate camouflage-related pigment aggregation. The present results suggest that maltol does not exert its effects via the camouflage adaptation mechanism or via melatonin-related mechanisms. These results are the first to identify a putative toxicological effect of maltol exposure in vivo and rule out several mechanisms by which maltol may exert its effects on pigment aggregation. Environ Toxicol Chem 2020;39:381-395. © 2019 SETAC.

Melatonin/polydopamine nanostructures for collective neuroprotection-based Parkinson's disease therapy

Parkinson's disease (PD) is characterized by the loss of dopaminergic neurons in the substantia nigra and localized deposition of cytoplasmic fibrillary inclusions as Lewy bodies in the brain. The aberrant phosphorylation of α-synuclein at serine 129 is the key process on its early onset, which alters the cellular conformation to oligomers and insoluble aggregates, underpinning cellular oxidative stress and mitochondrial dysfunction, leading to devastating PD synucleinopathy. The multiple neuroprotective roles of dopamine and melatonin are often demonstrated separately; however, this approach suffers from low and short bioavailability and is associated with side-effects upon overdosing. Herein, highly pleiotropic melatonin-enriched polydopamine nanostructures were fabricated, which showed efficient brain tissue retention, sustainable and prolonged melatonin release, and prevented neuroblastoma cell death elicited by Parkinson's disease-associated and mitochondrial damaging stimuli. The synergistic neuroprotection re-established the mitochondrial membrane potential, reduced the generation of cellular reactive oxygen species (ROS), inhibited the activation of both the caspase-dependent and independent apoptotic pathways, and exhibited an anti-inflammatory effect. At the molecular level, it suppressed α-synuclein phosphorylation at Ser 129 and reduced the cellular deposition of high molecular weight oligomers. The therapeutic assessment on ex vivo organotypic brain slice culture, and in vivo experimental PD model confirmed the superior brain targeting, collective neuroprotection on dopaminergic neurons with reduced alpha-synuclein phosphorylation and deposition in the hippocampal and substantia nigra region of the brain. Thus, nature-inspired melatonin-enriched polydopamine nanostructures conferring collective neuroprotective effects attributes activation of anti-oxidative, anti-inflammatory, and anti-apoptotic pathways may be superior for application in a nanomedicine-based PD therapy.

Modulation of serine/threonine phosphatases by melatonin: therapeutic approaches in neurodegenerative diseases

Melatonin is an endogenous hormone produced by the pineal gland as well as many other tissues and organs. The natural decline in melatonin levels with ageing contributes significantly to the development of neurodegenerative disorders. Neurodegenerative diseases share common mechanisms of toxicity such as proteinopathy, mitochondrial dysfunction, metal dyshomeostasis, oxidative stress, neuroinflammation and an imbalance in the phosphorylation/dephosphorylation ratio. Several reports have proved the usefulness of melatonin in counteracting the events that lead to a neurodegenerative scenario. In this review, we have focused on the fact that melatonin could rectify the altered phosphorylation/dephosphorylation rate found in some neurodegenerative diseases by influencing the activity of phosphoprotein phosphatases. We analyse whether melatonin offers any protective activity towards these enzymes through a direct interaction. LINKED ARTICLES: This article is part of a themed section on Recent Developments in Research of Melatonin and its Potential Therapeutic Applications. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v175.16/issuetoc.

Myosin Va's adaptor protein melanophilin enforces track selection on the microtubule and actin networks in vitro

Pigment organelles, or melanosomes, are transported by kinesin, dynein, and myosin motors. As such, melanosome transport is an excellent model system to study the functional relationship between the microtubule- and actin-based transport systems. In mammalian melanocytes, it is well known that the Rab27a/melanophilin/myosin Va complex mediates actin-based transport in vivo. However, pathways that regulate the overall directionality of melanosomes on the actin/microtubule networks have not yet been delineated. Here, we investigated the role of PKA-dependent phosphorylation on the activity of the actin-based Rab27a/melanophilin/myosin Va transport complex in vitro. We found that melanophilin, specifically its C-terminal actin-binding domain (ABD), is a target of PKA. Notably, in vitro phosphorylation of the ABD closely recapitulated the previously described in vivo phosphorylation pattern. Unexpectedly, we found that phosphorylation of the ABD affected neither the interaction of the complex with actin nor its movement along actin tracks. Surprisingly, the phosphorylation state of melanophilin was instead important for reversible association with microtubules in vitro. Dephosphorylated melanophilin preferred binding to microtubules even in the presence of actin, whereas phosphorylated melanophilin associated with actin. Indeed, when actin and microtubules were present simultaneously, melanophilin's phosphorylation state enforced track selection of the Rab27a/melanophilin/myosin Va transport complex. Collectively, our results unmasked the regulatory dominance of the melanophilin adaptor protein over its associated motor and offer an unexpected mechanism by which filaments of the cytoskeletal network compete for the moving organelles to accomplish directional transport on the cytoskeleton in vivo.

Melatonin signaling and cell protection function

Besides its well-known regulatory role on circadian rhythm, the pineal gland hormone melatonin has other biological functions and a distinct metabolism in various cell types and peripheral tissues. In different tissues and organs, melatonin has been described to act as a paracrine and also as an intracrine and autocrine agent with overall homeostatic functions and pleiotropic effects that include cell protection and prosurvival factor. These latter effects, documented in a number of in vitro and in vivo studies, are sustained through both receptor-dependent and -independent mechanisms that control detoxification and stress response genes, thus conferring protection against a number of xenobiotics and endobiotics produced by acute and chronic noxious stimuli. Redox-sensitive components are included in the cell protection signaling of melatonin and in the resulting transcriptional response that involves the control of NF-κB, AP-1, and Nrf2. By these pathways, melatonin stimulates the expression of antioxidant and detoxification genes, acting in turn as a glutathione system enhancer. A further and converging mechanism of cell protection by this indoleamine described in different models seems to lie in the control of damage and signaling function of mitochondria that involves decreased production of reactive oxygen species and activation of the antiapoptotic and redox-sensitive element Bcl2. Recent evidence suggests that upstream components in this mitochondrial route include the calmodulin pathway with its central role in melatonin signaling and the survival-promoting component of MAPKs, ERK1/2. In this review article, we will discuss these and other molecular aspects of melatonin signaling relevant to cell protection and survival mechanisms.

Frog melanophores cultured on fluorescent microbeads: biomimic-based biosensing

Melanophores are pigmented cells in lower vertebrates capable of quick color changes and thereby suitable as whole cell biosensors. In the frog dermis skin layer, the large and dark pigmented melanophore surrounds a core of other pigmented cells. Upon hormonal stimulation the black-brown pigment organelles will redistribute within the melanophore, and thereby cover or uncover the core, making complex color changes possible in the dermis. Previously, melanophores have only been cultured on flat surfaces. Here we mimic the three dimensional biological geometry in the frog dermis by culturing melanophores on fluorescent plastic microbeads. To demonstrate biosensing we use the hormones melatonin and alpha-melanocyte stimulating hormone (alpha-MSH) as lightening or darkening stimuli, respectively. Cellular responses were successfully demonstrated on single cell level by fluorescence microscopy, and in cell suspension by a fluorescence microplate reader and a previously demonstrated computer screen photo-assisted technique. The demonstrated principle is the first step towards "single well/multiple read-out" biosensor arrays based on suspensions of different selective-responding melanophores, each cultured on microbeads with distinctive spectral characteristics. By applying small amount of a clinical sample, or a candidate substance in early drug screening, to a single well containing combinations of melanophores on beads, multiple parameter read-outs will be possible.

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.

Constitutive secretion of serum albumin requires reversible protein tyrosine phosphorylation events intrans-Golgi

Serum albumin secretion from rat hepatocytes proceeds via the constitutive pathway. Although much is known about the role of protein tyrosine phosphorylation in regulated secretion, nothing is known about its function in the constitutive process. Here we show that albumin secretion is inhibited by the tyrosine kinase inhibitor genistein but relatively insensitive to subtype-selective inhibitors or treatments. Secretion is also blocked in a physiologically identical manner by the tyrosine phosphatase inhibitors pervanadate and bisperoxo(1,10-phenanthroline)-oxovanadate. Inhibition of either the kinase(s) or phosphatase(s) leads to the accumulation of albumin between the trans-Golgi and the plasma membrane, whereas the immediate precursor proalbumin builds up in a proximal compartment. The trans-Golgi marker TGN38 is rapidly dispersed under conditions that inhibit tyrosine phosphatase action, whereas the distribution of the cis-Golgi marker GM130 is insensitive to genistein or pervanadate. By using a specifically reactive biotinylation probe, we detected protein tyrosine phosphatases in highly purified rat liver Golgi membranes. These membranes also contain both endogenous tyrosine kinases and their substrates, indicating that enzymes and substrates for reversible tyrosine phosphorylation are normal membrane-resident components of this trafficking compartment. In the absence of perturbation of actin filaments and microtubules, we conclude that reversible protein tyrosine phosphorylation in the trans-Golgi network is essential for albumin secretion and propose that the constitutive secretion of albumin is in fact a regulated process.

Heat treatment decreases melanin synthesis via protein phosphatase 2A inactivation

In the present study, we investigated the effects of heat treatment on melanogenesis in a mouse melanocyte cell line (Mel-Ab). It has been reported that activated extracellular signal-regulated kinase (ERK) is responsible for microphthalmia-associated transcription factor (MITF) degradation, which leads to a reduction in tyrosinase protein production and melanin synthesis. Here we demonstrate that heat treatment induces sustained ERK activation, which may inhibit melanogenesis. However, the specific ERK pathway inhibitors, PD98059 or U0126 did not restore heat-induced hypopigmentation. Furthermore, PD98059 or U0126 hardly blocked the heat-induced activation of ERK. These results suggest that heat treatment may inactivate protein phosphatase, and thus ERK activation is maintained. To support this hypothesis, we examined the effects of heat treatment on protein phosphatase 2A (PP2A) activity. The results obtained show that heat treatment inactivates PP2A, which may subsequently cause ERK activation and that heat treatment inhibits MITF promoter activity. Overall, our results demonstrate that heat treatment reduces melanin production in a temperature-dependent manner.

Phosphoinositide 3-kinase is involved in Xenopus and Labrus melanophore aggregation

Melanophores are pigmented cells capable of quick colour changes through coordinated transport of their intracellular pigment granules. We demonstrate the involvement of phosphoinositide 3-kinase (PI3-K) in Xenopus and Labrus aggregation by the use of the PI3-K inhibitor, LY-294002. In Xenopus, wortmannin-insensitive PI3-K was found to be essential for the aggregation, mitogen-activated protein kinase (MAPK) activation and tyrosine phosphorylation of a 280-kDa protein, and for the maintenance of low cyclic adenosine 3':5'-monophosphate (cAMP) during the aggregated state. Pre-aggregated cells disperse completely to LY-294002 at 50-100 muM, involving a transient elevation in cAMP due to adenylate cyclase (AC) stimulation or to inhibition of cyclic nucleotide phosphodiesterase (PDE). The inactive analogue LY-303511 did not induce dispersion at the same concentrations. PDE4 and/or PDE2 was found to be involved in melanosome aggregation. The similar kinetics of LY-294002 and various PDE inhibitors indicates that the elevation of cAMP might be due to inhibition of PDE. In Labrus melanophores, LY-294002 had a less dramatic effect, probably due to less dependence on PDE in regulation of cAMP levels. In Xenopus aggregation, we suggest that melatonin stimulation of the Mel1c receptor via G(beta gamma) activates PI3-K that, directly or indirectly via MAPK, activates PDE.

A Role for Spectrin in Dynactin-dependent Melanosome Transport in Xenopus laevis Melanophores

The bi-directional movement of pigment granules in frog melanophores involves the microtubule-based motors cytoplasmic dynein, which is responsible for aggregation, and kinesin II and myosin V, which are required for dispersion of pigment. It was recently shown that dynactin acts as a link between dynein and kinesin II and melanosomes, but it is not fully understood how this is regulated and if more proteins are involved. Here, we suggest that spectrin, which is known to be associated with Golgi vesicles as well as synaptic vesicles in a number of cells, is of importance for melanosome movements in Xenopus laevis melanophores. Large amounts of spectrin were found on melanosomes isolated from both aggregated and dispersed melanophores. Spectrin and two components of the oligomeric dynactin complex, p150(glued) and Arp1/centractin, co-localized with melanosomes during aggregation and dispersion, and the proteins were found to interact as determined by co-immunoprecipitation. Spectrin has been suggested as an important link between cargoes and motor proteins in other cell types, and our new data indicate that spectrin has a role in the specialized melanosome transport processes in frog melanophores, in addition to a more general vesicle transport.

Melatonin protects SH-SY5Y neuroblastoma cells from calyculin A-induced neurofilament impairment and neurotoxicity

Hyperphosphorylation of cytoskeletal proteins seen in Alzheimer's disease is most probably the result of an imbalanced regulation in protein kinases and protein phosphatases (PP) in the affected neurons. Previous studies have revealed that PP-2A and PP-1 play important roles in the pathogenesis. Employing human neuroblastoma cells, we found that 10 nM calyculin A (CA), a selective inhibitor of PP-2A and PP-1, significantly increased phosphorylation and accumulation of neurofilament (NF) in the cells. Levels of NF-M (middle chain) and NF-L (light chain) mRNA decreased after CA treatment. Additionally, CA led to a decreased cell viability determined by MTT and crystal violet assay. Melatonin efficiently protects the cell from CA-induced alterations in NF hyperphosphorylation and accumulation, suppressed NF gene expression as well as decreased cell viability. It is concluded that inhibition of PP-2A/PP-1 by CA induces abnormalities in NF metabolism and cell survival, and melatonin efficiently arrests the lesions.

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.

Regulation of melanosome movement by MAP kinase

Our objectives were to further characterize the signaling pathways in melatonin-induced aggregation in Xenopus melanophores, specifically to investigate a possible role of mitogen-activated protein kinase (MAPK). By Western blotting we found that melatonin activates MAPK, which precedes melanosome aggregation measured in a microplate reader. Activation of MAPK, tyrosine phosphorylation of a previously described 280-kDa protein, and melanosome aggregation are sensitive to PD98059, a selective inhibitor of MAPK kinase. The MAPK activation is also decreased by the adenylate cyclase stimulant forskolin. In summary, we found that MAPK is activated during melatonin-induced melanosome aggregation. Activation was decreased by an inhibitor of MAPK kinase, and by forskolin. In addition to inhibition of cyclic adenosine 3',5'-monophosphate (cAMP), reduction in protein kinase A activity (PKA), and activation of protein phosphatase 2A, we suggest that melatonin receptors activate the MAPK cascade and tyrosine phosphorylation of the 280-kDa protein. Although the cAMP/PKA signaling pathway is the most prominent, our data suggest that simultaneous activation of the MAPK cascade is of importance to obtain a completely aggregated state. This new regulatory mechanism of organelle transport by the MAPK cascade might be important in other eukaryotic cells.

Noradrenaline- and melatonin-mediated regulation of pigment aggregation in fish melanophores

The effects of melatonin and noradrenaline (NA) on bi-directional melanosome transport were analysed in primary cultures of melanophores from the Atlantic cod. Both agents mediated rapid melanosome aggregation, and by using receptor antagonists, melatonin was found to bind to a melatonin receptor whereas NA binds to an alpha2-adrenoceptor. It has previously been stated that melatonin-mediated melanosome aggregation in Xenopus is coupled with tyrosine phosphorylation of a so far unidentified high molecular weight protein and we show that although acting through different receptors and through somewhat different downstream signalling events, tyrosine phosphorylation is of the utmost importance for melanosome aggregation mediated by both NA and melatonin in cod melanophores. Together with cyclic adenosine 3-phosphate-fluctuations, tyrosine phosphorylation functions as a switch signal for melanosome aggregation and dispersion in these cells.

Melanophore aggregation in strong static magnetic fields

Contradicting results can be found in the literature on effects from magnetic exposure to pigment cells. We have studied the influence of strong, static, homogenous magnetic fields of 8 and 14 T on melanophore aggregation during exposure to the field. Melanophores, black pigment cells, in fish are large flat cells having intracellular black pigment granules. Due to large size, high optical contrast, and quick response to drugs, melanophores are attractive as biosensors as well as for model studies of intracellular processes. This is especially true for modeling nerve cells, since melanophores share stem cells with axons. Twenty experiments on black tetra fish fins were carried out in the two magnetic flux densities. The same number of control experiments were carried out in the magnet with the magnetic field turned off. Several factors, such as the degree of maximal aggregation, speed of aggregation, and irregularity of the speed, were examined. The statistical analysis showed no significant field effects on the aggregation, with one exception: the irregularity in aggregation speed in the 8 T field, compared to control. The believed reorientation of the cytoskeleton (microtubules) in the field or the induced Lorentz force on moving pigment granules, did not affect the aggregation.