Melanosome

The melanosome is a specialized organelle nowadays in melanocytes that is associated with the endocytic pathway and is responsible for the production of pigment proteins.

From: The Allowed Response , 2006

Organizational Jail cell Biology

K. Fukuda , in Encyclopedia of Cell Biological science, 2016

Molecular Basis of Melanosome Biogenesis and Transport

Melanosomes are the melanin-containing organelles that are responsible for pigmentation of the pilus and skin of mammals. Because melanosomes are visible under a lite microscope, they are often regarded equally a representative model for analyzing LRO biogenesis and transport ( Nascimento et al., 2003; Wasmeier et al., 2008). Melanosomes are classified into stages I to Four based on their morphology and degree of maturation (Figure 2, top). Stage I and stage II melanosomes are immature melanosomes (besides called premelanosomes), and because of the absence of melanogenic enzymes they are non pigmented. Stage I melanosomes are not-pigmented endosomal compartments that incorporate few intraluminal vesicles and cytosolic coats and are besides called coated endosomes. Their intraluminal vesicles serve as a scaffold for polymerizing melanocyte-specific Pmel17 amyloid fibrils into sheets, which help to class rugby-ball-shaped stage Two melanosomes. Stage Iii melanosomes are less-pigmented melanosomes that contain 3 melanogenic enzymes, that is, tyrosinase (Tyr), tyrosinase-related protein 1 (Tyrp1), and dopachrome tautomerase (Dct), and other proteins required for melanosome maturation, including a putative transporter oculocutaneous albinism type 2 (OCA2, also called P protein), via mail service-Golgi trafficking or endosomal trafficking. These trafficking pathways are finely tuned by a set of molecules, including biogenesis of lysosome-related organelles complex-1~3 (BLOC-1~iii) (Dell'Angelica, 2004), adaptor protein (AP) complex (Bonifacino and Traub, 2003), and Rab small GTPases (Stenmark, 2009). Melanins synthesized by the melanogenic enzymes are deposited on Pmel17 sheets, and mature stage 4 melanosomes are formed.

Effigy ii. Biogenesis and send of melanosomes in mammalian epidermal melanocytes. (Peak) Schematic representation of the stepwise development of a mature melanosome. The phase I immature melanosome with intraluminal vesicles is derived from early endosomes. The phase II young melanosome contains Pmel17 fibrils. Melanogenic enzymes, for example, tyrosinase (Tyr) and tyrosinase-related protein 1 (Tyrp1), are transported to the stage III melanosome, where melanin synthesis takes identify. The stage IV melanosome is a mature melanosome. (Middle) Melanosomes form and mature around the nucleus. Mature melanosomes are transported to simply beneath the plasma membrane along 2 different cytoskeletons, microtubules and actin filaments. Melanosomes are somewhen transferred from the dendrites of melanocytes to neighboring keratinocytes. (Bottom) Mature melanosomes are transported to the cell periphery by motor protein complexes forth dissimilar cytoskeletons, that is, microtubules and actin filaments. An actin-based melanosome transport circuitous composed of Rab27A, Slac2-a/melanophilin, and myosin Va, deficiency of each of which causes the hypopigmentation of GS2/ashen, GS3/leaden, and GS1/dilute, respectively.

Considering melanosomes mature around the nucleus, they need to exist transported to the cell periphery along two unlike cytoskeletons and they are eventually transferred from the dendrites of melanocytes to neighboring keratinocytes and hair matrix cells for pigmentation of mammalian skin and pilus, respectively (Figure 2, middle). Mature melanosomes are first transported to the cell periphery along microtubules by a kinesin motor that powers anterograde motion on microtubules, then transferred to actin filaments, and then finally transported to only beneath the plasma membrane along actin filaments by an actin-based motor myosin-Va (Ohbayashi and Fukuda, 2012). Mature melanosomes anchored to the plasma membrane or actin filaments are somewhen transferred from melanocyte dendrites to neighboring keratinocytes and pilus matrix cells by mechanisms that are largely unknown (Wu and Hammer, 2014). In dissimilarity to mammalian melanosomes, the melanosomes in fish and amphibian melanophores, which are equivalent to the melanocytes in mammals, are speedily transported bidirectionally in response to extracellular stimuli and are not transferred to neighboring cells. Their bidirectional movements are regulated past coordination of two microtubule-based motors, kinesin and dynein. Rapid changes between two melanosome distribution states, melanosome aggregation around the nucleus and peripheral melanosome dispersion, in melanophores accounts for the changes in the torso color of fishes and amphibians that serve to camouflage them.

The best characterized melanosome transport system is the actin-dependent melanosome transport system, which, in vertebrates, is composed of three conserved molecules, Rab27A, Slac2-a (also called melanophilin), and myosin-Va (Fukuda, 2013). Rab27A is a member of the small GTPase Rab family unit and specifically localizes to mature melanosomes, when information technology binds GTP, that is, when Rab27A is activated. The agile Rab27A on the melanosome so recruits its specific effector molecule Slac2-a via the N-last Slp homology domain (SHD=Rab27A effector domain). Slac2-a also contains a myosin-Va-binding domain in the eye of the molecule and functions as a linker protein betwixt Rab27A on the melanosome and myosin-Va (Figure 2, bottom). The resulting tripartite protein complex composed of Rab27A, Slac2-a, and myosin-Va facilitates melanosome transfer from microtubules to actin filaments and regulates actin-dependent melanosome transport.

Although the above-described actin-dependent melanosome transport machinery is highly conserved both in mammalian melanocytes and in lower vertebrate melanophores, the microtubule-dependent melanosome ship systems of melanocytes and melanophores announced to differ, because heterotrimeric kinesin-2/Kif3 regulates microtubule-dependent anterograde melanosome transport in melanophores, whereas kinesin-1/Kif5 seems to be involved in microtubule-dependent anterograde melanosome transport in melanocytes. The precise machinery of microtubule-dependent anterograde melanosome transport in melanocytes remains elusive, but it has been suggested that a specific Rab isoform may be involved in the procedure, possibly as a cargo receptor for the kinesin motor (Ishida et al., 2012), in the same way that Rab27A functions equally a cargo receptor for actin-dependent melanosome transport.

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Ion Channels: Aqueduct Chemical Biology, Applied science, and Physiological Function

Jessica L. Scales , ... Elena Oancea , in Methods in Enzymology, 2021

1 Introduction

Melanosomes are unique, lysosome-related organelles found in cutaneous and ocular melanocytes, likewise as, retinal pigment epithelium cells, that synthesize and store melanin, the main mammalian paint ( d'Ischia et al., 2015; Hearing, 2000; Marks & Seabra, 2001). Defects in genes encoding melanosomal proteins upshot in oculocutaneous albinism (OCA), which is characterized by hypopigmentation of the skin, hair, and optics, in add-on to, dumb vision and increased susceptibility to skin and eye cancers (Rex, Hearing, Creel, & Oetting, 1995; Sturm, Teasdale, & Box, 2001). Interestingly, many of the genes defective in OCA patients encode transmembrane proteins predicted to regulate the ionic environment of melanosomes. Nevertheless, our understanding of the physiological ionic milieu in melanosomes and of ion transport beyond melanosomal membranes remains largely unknown. In fact, the composition of the intraluminal ionic milieu of not only melanosomes, merely of many cellular organelles, and its contribution to organelle function is a critically understudied, yet emerging surface area of subcellular biological science (Xu, Martinoia, & Szabo, 2015).

Melanosomes are cell-type-specific organelles that originate from the endosomal system and serve specialized physiological functions inside their host cells (Mantegazza & Marks, 2016; Marks, Heijnen, & Raposo, 2013). Melanosomes, like all organelles of the endosomal system, require a unique intraluminal environment in order to deport out their functions. In particular, the activity of tyrosinase (TYR), the rate-limiting enzyme in melanin synthesis, requires a near-neutral pH and copper as a cofactor (Ancans, Hoogduijn, & Thody, 2001; Ancans, Tobin, et al., 2001; Halaban et al., 2002). Accordingly, during melanosome maturation, the organelle'south lumen changes from acidic to nigh-neutral pH values (Ancans, Tobin, et al., 2001; Bellono, Escobar, Lefkovith, Marks, & Oancea, 2014; Fuller, Spaulding, & Smith, 2001; Ito, Suzuki, Takebayashi, Commo, & Wakamatsu, 2013; Raposo, Tenza, Potato, Berson, & Marks, 2001) and acquires copper in add-on to other divalent cations (Bush & Simon, 2007; Hong & Simon, 2007; Liu et al., 2005). These changes in ionic composition are regulated by the selective incorporation of transmembrane ion channels and transporters into the melanosome membrane during maturation.

We recently advanced our agreement of melanosome physiology past identifying four regulators of melanosomal pH. Nosotros showed that OCA2, the product of the gene defective in OCA blazon 2 (Lee, Nicholls, Jong, Fukai, & Spritz, 1995; Rosemblat et al., 1994), functions every bit a chloride (Cl−) channel and reduces proton (H+) import into melanosomes, which neutralizes the luminal pH and activates the pH-sensitive TYR enzyme crucial for melanin synthesis (Bellono et al., 2014). Indeed, melanogenesis in OCA2-scarce melanocytes is restored by deacidification (Manga & Orlow, 2001).

In add-on to OCA2, the two-pore channel 2 (TPC2), a nonselective sodium- and calcium-permeable channel (Wang et al., 2012) associated with variation in pare, hair, and eye pigmentation (Sulem et al., 2008), conducts sodium ions from the melanosome into the cytosol. This action balances the positive luminal charges, allowing the vacuolar-type ATPase H+ pump to reestablish optimal pH for TYR activity (Ambrosio, Boyle, Aradi, Christian, & Di Pietro, 2016; Bellono et al., 2014; Bellono, Escobar, & Oancea, 2016; Sitaram et al., 2009).

Another regulator of pigmentation involved in melanosomal milieu regulation is SLC45A2, the product of the gene mutated in OCA type 4 (Newton et al., 2001). Our contempo data indicate that SLC45A2 localizes to the melanosome membrane where information technology directly functions to increase luminal pH (Le et al., 2020). While this function seems redundant with OCA2, we showed that SLC45A2 modulates H+ import into melanosomes at a later phase of melanosome maturation when OCA2 has been cleared from the melanosome delimiting membrane (Le et al., 2020). However, the mechanisms by which SLC45A2 regulates melanosomal pH remains unknown, due in function to technical limitations in the ability to reliably measure the luminal pH of melanosomes.

More recently, we showed that in melanocytes the lysosomal H+/Cl− antiporter CLC7 is as well localized to melanosomes, where information technology functions to acidify the organelle lumen via the inward ship of Cl−, thus opposing the activity of OCA2 (Koroma et al., 2021). For these studies, faced with the need to reliably measure out melanosomal pH in live cells, we designed and validated the ratiometric pH indicator, RpHiMEL (Ratiometric pH indicator for MELanosomes), and used information technology to develop an assay method for quantifying the pH of private melanosomes.

Together, these observations propose that melanosome pH is regulated via a complex network of putative channels and transporters. Future studies are necessary to identify other regulators of melanosome pH and to sympathize how these proteins cooperate to fine-tune the luminal environment of melanosomes, and ultimately, melanin synthesis. Consequently, investigation of such subcellular mechanisms will benefit from a robust ratiometric pH indicator and quantification method for unmarried-organelle measurements of melanosome pH in live cells.

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Bones Scientific discipline, Inherited Retinal Affliction, and Tumors

Gabriele Thumann , ... David R. Hinton , in Retina (4th Edition), 2006

Melanin pigment

Melanosomes, the organelles responsible for the biosynthesis of melanin, announced in the development of human RPE beginning at half-dozen weeks of gestational age. 184 It was previously idea that melanogenesis in vertebrates was restricted to the prenatal period because of the absence of the key enzyme tyrosinase in adults. 185–189 Show is now available that melanogenesis occurs in developed RPE cells of cattle, and gilt hamsters 190–192 and developed cultured human RPE. 7 , 43 In prison cell culture, 2 different pathways have been identified in melanogenesis; melanin can be synthesized with and without formation of pre-melanosomes. In the absence of pre-melanosomes, the formation of melanosomes takes place inside phagolysosomes and includes rest of degraded ROS. 190, 193, 194 Within the developed RPE cell the melanin granules are located in the apical portion of the cell, next to ROS.

Melanin granules function to reduce light handful and block lite assimilation via the sclera, resulting in a meliorate prototype received by the retina. Melanin likewise absorbs radiant free energy (visible and ultraviolet spectrum) and dissipates the free energy in rut. 10 The assimilation spectrum and quantity of radiant energy absorbed is modified by the degree of melanin aggregation and redox state. 195, 196 In addition melanin can bind redox-active metal ions and sequester them in an inactive state, thus preventing oxidative damage to the retina by these potential photosensitizing agents. 189 The absorption of light by melanin acts as an effective antioxidant to protects the RPE cells and regulate their metabolic activity; in fact, RPE cells high in melanin content showroom significantly less formation of lipofuscin than cells low or devoid of melanin. 197 In case of an intense oxidative insult, structural or functional changes in RPE melanin may lead to loss of anti-oxidant chapters. 189

Tyrosinase, a copper-containing enzyme, catalyzes the oxidation of tyrosine to melanin. The RPE is capable of synthesizing tyrosinase and producing melanosomes and regular turnover of melanosomes takes place. In contrast to the epidermal melanosomes, the melanosomes of the RPE remain within the prison cell and are not transferred to other cells. Within the centre the melanin concentration of RPE cells decreases between the periphery and posterior pole and increases in the macular region. 6 With historic period, melanin content decreases in the RPE cells 6 and melanin granules become more uniformly distributed inside the cytoplasm. 189

The melanin in the RPE represents the unmarried most important source for heat in thermal photocoagulation. The affinity of certain drugs, such as quinine, for melanin results in ocular toxicity of these drugs. 198–201 The long-term application of chloroquin and certain phenothiazine derivatives bind to melanin and induce toxic reactions in melanin-containing cells via unknown molecular and cellular mechanisms. 200 , 202

In homo a congenital disorder in melanin production leads to albinism, which is associated with decreased vision, photophobia and nystagmus. Since albino individuals show foveal hypoplasia, it is suspected that melanin plays an important role in retinal development. 203, 204 In fact, in albinism, the misrouting of retinal ganglion cell fibers at the optic chiasm is believed to be caused by a lack of melanin during development 205, 206 Temporal retinal fibers cantankerous the chiasm when they should remain uncrossed resulting in inappropriate connections in the visual cortex and a lack of binocularly driven cells. 205

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Coat Color Mutations, Animals

V. Setaluri , A. Jayanthy , in Brenner'south Encyclopedia of Genetics (Second Edition), 2013

Mutations in Melanosome Biogenesis and Melanin Production

Melanosomes are lysosome-related organelles within which melanin-producing enzymes are present and where melanin is produced. They are assembled within melanocytes. Biogenesis of melanosomes involves a serial of poly peptide sorting and vesicular trafficking events mediated past a number of specialized proteins, some unique to melanocyte and others shared past other cell types. Accordingly, defects in the associates of melanosomes can manifest not only as coat color phenotypes but also other defects seemingly unrelated to glaze color.

The enzyme tyrosinase (TYR) catalyzes the get-go steps of melanin germination – the formation of l-iii,4-dihydroxyphenylalanine (l-DOPA) from the amino acrid l-tyrosine as well every bit dopaquinone from fifty-DOPA. Defects in the production or part of tyrosinase lead to a decrease or no production of both types of melanins. Studies of two sheep breeds showed that when tyrosinase activity was higher, the coat colors produced were darker and with a greater diversity, while a lower level of tyrosinase activity showed the opposite. Albinism is a disorder associated with the mutation of the tyrosinase gene. Equally described earlier, although both albinism and other glaze color defects have white coats in homozygous mutants and diluted or white-spotting phenotypes in heterozygous mutants, the effects of complete absence of tyrosinase activity also manifest in the retinal pigment epithelium, leading to pigmentation defects in of the optics, observed every bit the characteristic red eyes of albinos.

Other genes responsible for melanosome and melanin germination are ATP7A, Dct, GPNMB, MLANA, TYRP1, SLC45A2, and RAB38. Mutations in TYR and TYRP1 accept been studied extensively due to their role in melanin biosynthesis. TYRP1 or tyrosinase-related protein i is besides known as the brown locus protein. The production of TYRP1 is involved in the product of eumelanin, just it has besides been implicated in the maintenance of the ultrastructure of melanosomes, melanocyte proliferation, and apoptosis. The gene Tyrp2/Dct codes for an enzyme in the melanin biosynthetic pathway. Single-nucleotide polymorphisms in this cistron are associated with coat colour in blackness-boned sheep. SLC45A2 causes lighter skin pigmentation.

Another important gene that has recently been identified to play a role in pigmentation is the transient receptor potential M1 (Trpm1). It maintains calcium homeostasis and is involved in the suppression of melanoma metastasis. Reducing its expression decreases levels of tyrosinase in melanosomes. TRPM1 mutations are associated with congenital stationary dark blindness in horses and humans. Interestingly, although mutations in TRPM1 in Appaloosa horses causes both glaze colour phenotype and night incomprehension, no skin pigmentation defects are observed in human patients with built nighttime incomprehension, and similarly mice lacking TRPM1 show no coat colour defects.

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Laser–Tissue Interactions

Due east. Victor Ross , R. Rox Anderson , in Cutaneous and Cosmetic Laser Surgery, 2006

Pigmented Lesions

Melanosome heating conforms well with the basic theory of selective photothermolysis. 20 With very short pulses (ns) 1 observes immediate frostlike whitening at the surface (Fig. one.28). Although the verbal cause is unknown, information technology is well-nigh certainly related to the formation of gas bubbles that intensely scatter calorie-free. twenty Over several minutes, these bubbles deliquesce, causing the skin color to return to near normal. As the pulse duration increases, melanosome heating becomes more gentle, and at that place tends to be focal DE junction heating but with considerable improvidence outside the melanosome. Pigmented lesions can be divided into superficial and deep. Static epidermal pigmented lesions such every bit lentigos tend to be straightforward to treat. In most cases, the most complete removal (with only one treatment session) is achieved via Q-switched technologies. On the other hand, the least invasive and gentlest removal is via long pulse (ms) technologies in the visible light spectrum such as intense pulsed low-cal devices likewise is KTP lasers. Recently, long pulse alexandrite lasers have besides been employed likewise as long pulsed diode lasers to gently heat epidermal static pigmented lesions. 'Dermal' static pigmented lesions such as nevus of Ota respond best to Q-switched lasers. 95 This is consistent with the theory of selective photothermolysis. With longer pulses (ms domain), the dermal melanocytes, which are of relatively low concentration (compared to melanocytic nests in compound nevi or highly pigmented basal jail cell layers in lentigos), just practice not become hot enough to achieve pigment reduction. ix

For light lentigos, the contrast between the skin's background colour and the lesions may become too small for constructive reduction by long pulsed green–yellow (GY) sources. In these cases, the shorter pulses of Q-switched lasers are required for selective heating of pigmented lesions. Melasma is a challenging condition to treat via lasers, most likely due to its dynamic and inflammatory nature (compared to the static nature of lentigos). 96 Ablative lasers tin can sometimes result in improvement, withal the ablation normally has to be carried out deeply, or postinflammatory hyperpigmentation may outweigh any accomplishment gains. Q-switched lasers typically event in only temporary improvement (followed by postal service inflammatory hyperpigmentation that worsens the appearance!). 96 On the other hand, longer pulsed visible light laser technologies can sometimes achieve gradual comeback in melasma so long as the settings are not too high. 97, 98

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Coat Color Mutations, Animals☆

D. Mikheil , ... Vijayasaradhi Setaluri , in Reference Module in Life Sciences, 2017

Mutations in Melanosome Biogenesis and Melanin Product

Melanosomes are lysosome-related organelles inside which melanin-producing enzymes are present and where melanin is produced. They are assembled inside melanocytes. Biogenesis of melanosomes involves a series of protein sorting and vesicular trafficking events mediated past a number of specialized proteins, some unique to melanocyte and others shared by other jail cell types. Accordingly, defects in the assembly of melanosomes can manifest non only as coat colour phenotypes but also other defects seemingly unrelated to coat color. The enzyme tyrosinase (TYR) catalyzes the first steps of melanin formation – the formation of fifty-3,4-dihydroxyphenylalanine (50-DOPA) from the amino acid l-tyrosine also equally dopaquinone from l-DOPA. Defects in the product or function of tyrosinase lead to a decrease or no production of both types of melanins. Studies of two sheep breeds showed that when tyrosinase activity was higher, the glaze colors produced were darker and with a greater variety, while a lower level of tyrosinase activity showed the contrary. Albinism is a disorder associated with the mutation of the tyrosinase factor. As described before, although both albinism and other glaze color defects have white coats in homozygous mutants and diluted or white-spotting phenotypes in heterozygous mutants, the furnishings of consummate absence of tyrosinase activity also manifest in the retinal paint epithelium, leading to pigmentation defects in of the eyes, observed as the characteristic carmine eyes of albinos.

Mutations in TYR and TYRP1 take been studied extensively due to their function in melanin biosynthesis. TYRP1 or tyrosinase-related protein i is also known as the brown locus protein. The product of TYRP1 is involved in the production of eumelanin, only information technology has besides been implicated in the maintenance of the ultrastructure of melanosomes, melanocyte proliferation, and apoptosis.

White lions are not albinos, but have visible hazel to green-gray paint in the eyes, paw pads and lips. This white trait is due to a recessive missense mutation, TYR260G>A, in the tyrosinase gene, that causes the R87Q amino acrid change in the central domain of the tyrosinase enzyme (Cho et al., 2013). A genome-wide association mapping in Copper-necked goats has identified TYRP1 to be associated with the chocolate-brown glaze color phenotype (Becker et al., 2015). Furthermore, the dominant mode of inheritance of a TYRP1 allele, c.1487G>A (p.Gly496Asp), was confirmed in a targeted mating experiment to cause the brown coat color in these goats (Dietrich et al., 2015). TYRP1 gene sequencing and genotyping studies on European rabbit have identified a premature stop codon mutation one thousand.41360196G>A (p.Trp190ter) to be associated with the brown coat colour (Utzeri et al., 2014).

The gene Tyrp2/Dct codes for an enzyme in the melanin biosynthetic pathway. Single-nucleotide polymorphisms inTyrp2/Dct gene are associated with glaze colour in blackness-boned sheep.

Another important gene that has recently been identified to play a role in pigmentation is the transient receptor potential M1 (Trpm1). It maintains calcium homeostasis and is involved in the suppression of melanoma metastasis. Reducing its expression decreases levels of tyrosinase in melanosomes. TRPM1 mutations are associated with congenital stationary night blindness in horses and humans. Interestingly, although mutations in TRPM1 in Appaloosa horses causes both glaze color phenotype and night incomprehension, no skin pigmentation defects are observed in man patients with congenital night blindness, and similarly mice lacking TRPM1 show no coat colour defects.

Other genes responsible for melanosome and melanin formation are SLC45A2, PMEL, ATP7A, Dct, GPNMB, MLANA, and RAB38.

SLC45A2 causes lighter skin pigmentation. White coat color in White Doberman pinscher (WDP) was linked to a 4081 base pair deletion (chr4:77,062,968–77,067,051) in the SLC45A2 gene that results in the partial loss of exon 7. This deletion likewise causes oculocutaneous albinism in these dogs (Winkler et al., 2014). In domestic yaks, a mutation in PMEL (p.Leu18del) has accounted for the brown glaze colour in 60% of the yaks (Zhang et al., 2014). Unmarried marker regression analysis by Meszaros et al. (2015) has identified PMEL amidst regions significantly associated with coat color saturation in Fleckvieh cattle.

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Basic Science, Inherited Retinal Disease, and Tumors

Daniel Palanker , ... John J. Weiter , in Retina (Fourth Edition), 2006

Selective therapy of retinal pigment epithelium

Calorie-free is strongly absorbed in the melanosomes in the RPE (μ a ≈ 8000 cm−ane). 40 Awarding of brusk (sub-microsecond) laser pulses allows for confinement of the thermal and mechanical furnishings of this absorption within the RPE layer, thus sparing the neurosensory retina. It has been demonstrated that awarding of repetitive pulses of microsecond and sub-microsecond elapsing results in selective damage to RPE presumably due to formation of small cavitation bubbles around melanosomes. 40 Collateral damage to next photoreceptors, to underlying Bruch's membrane and choroid is minimized (Fig. 21-19) Selective RPE treatment (SRT) has been limited then far by the lack of visible change in the retinal advent making it difficult to assess adequate laser dosimetry as the applications are being placed. Acousto-optic systems are currently under development that may help to assess the threshold energy density required for cavitation in RPE. 21

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Retinal Laser Therapy

Daniel Palanker , Mark S. Blumenkranz , in Retina (Fifth Edition), 2013

Selective retina therapy (SRT)

Light is strongly absorbed in the melanosomes in the RPE (µ a ≈ 8000 cm-1). 59 The application of short (submicrosecond) light amplification by stimulated emission of radiation pulses allows for confinement of the thermal and mechanical effects of this assimilation within the RPE layer, thus sparing the photoreceptors and the inner retina 60,61 (Fig. 39.nineteen). Information technology has been demonstrated that the application of repetitive pulses of microsecond and submicrosecond duration results in selective impairment to RPE, presumably due to the formation of small cavitation bubbles around melanosomes. 59 Subsequent RPE proliferation and migration restore continuity of the RPE layer. Several pocket-sized clinical studies have shown the efficacy of SRT in diabetic maculopathy, cardinal serous choroidopathy, 62,63 and subfoveal fluid after rhegmatogenous retinal detachment. 64,65 Despite its clinical hope, this technique has not been commercialized. One of the difficulties with SRT is the lack of visible change in the retinal appearance, making it difficult to assess adequate laser dosimetry as the applications are existence placed. An acousto-optical system is currently under evolution that may help to assess the threshold free energy density required for cavitation in RPE. 66 An culling arroyo to SRT is the rapid scanning of a laser beam providing microsecond exposures, sufficiently short for selective treatment of the RPE. 67,68

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Retinomotor Movements☆

B. Burnside , C. King-Smith , in Reference Module in Neuroscience and Biobehavioral Psychology, 2017

The RPE Cytoskeleton and Forcefulness Production for Pigment Granule Dispersion and Aggregation

Light-induced dispersion of melanin pigment granules (melanosomes) out into the upmost projections of fish RPE cells and dark-induced aggregation of granules back into the RPE cell body results from translocation of individual pigment granules within apical projections of the RPE cells, non from extension and retraction of the upmost projections themselves. Apical projections extend from the RPE prison cell upmost surface out into the subretinal space where they interdigitate with photoreceptor inner and outer segments. Thus, when the membrane-bound pigment granules are dispersed, they shield and protect rod outer segments from full bleach in bright light. RPE apical projections may be up to 100  μm long in some fish species and they remain extended throughout the low-cal and dark phases of pigment granule migration.

Apical projections of fish RPE cells contain both actin filaments and microtubules. Microtubule disruption by intraocular injection of inhibitors in vivo blocks movement of pigment granules both in the cell body and in the apical projections; this finding would propose that melanosome movement is microtubule dependent. However, when inhibitor studies are carried out using dissociated RPE cells in culture, microtubule disruption has no effect on melanosome aggregation or dispersion. Presumably, this discrepancy results from the need for microtubules to provide structural support for upmost projections in vivo, or to maintain patent tracks or pathways through which pigment granules can move. In isolated cells, on the other manus, the attachment of the projections to the substrate likely keeps projections intact enough to permit pigment granule translocation. Isolated cell studies therefore suggest that microtubules are needed for the structural integrity of RPE apical projections, but the forces that actually power pigment granule translocation are microtubule independent.

In contrast, if the actin cytoskeleton is disrupted past cytochalasin D in dissociated RPE cells, both aggregation and dispersion of pigment granules are reversibly blocked, and maintenance of full aggregation or full dispersion is compromised. Thus, actin filaments are necessary and sufficient for both assemblage and dispersion. The actin dependence of pigment granule translocation in RPE cells suggests that myosin motor proteins are likely to play a role in force production. Actin decoration using myosin S1 subfragments and platinum replica shadowing demonstrated that actin filaments are uniformly oriented in fish RPE upmost projections with spinous (plus) ends distal, suggesting that a plus-finish directed motor could effect pigment granule dispersion. Polymerase chain reaction (PCR) screens of fish RPE have identified xi distinct myosins expressed in RPE. Immunocytochemistry with antibodies to four of these myosins indicated that the fish RPE has abundant levels of myosins IIIA, Half-dozen, and IXb, and lower levels of myosin VIIa. Myosin VIIa has been immunolocalized on pigment granules in RPE of humans, rodents, and fish, and defective localization of pigment granules to the RPE apical projections in mice with a mutant myosin VIIA gene has been reported. In vitro motility assays have shown that isolated teleost RPE pigment granules do support plus-end-directed, actin-dependent motility. Interestingly, RPE of zebrafish mariner mutants having a defective myosin VIIa demonstrate normal light- and dark-adaptive paint granule movements, although constant light produces increased photoreceptor cell death. Thus the part of myosin VIIa in fish RPE pigment granule motility is not clear.

A function for nonmuscle myosin 2 in RPE pigment granule motion was implicated in studies using the myosin II inhibitor, blebbistatin, which partially blocked pigment granule aggregation in isolated sunfish RPE cells, but did not affect dispersion. Similarly, inhibitors of Rho guanosine triphosphate (GTP)-binding protein-activated kinase (Stone), which activates myosin II, also blocked assemblage. These results suggest myosin 2 may contribute to paint granule aggregation, perhaps by contracting an actin network with which paint granules are associated. Similarly, treatment of fish RPE with the tetravalent lectin, Conconavalin A, which blocks cortical actin flow by crosslinking of cell-surface proteins, likewise blocked pigment granule aggregation, but not dispersion, further supporting a role for myosin IIa-mediated actin network wrinkle equally a mechanism for pigment granule aggregation. Myosin IIa was identified in striped bass RPE by PCR distension of a complementary DNA (cDNA) library. Fig. iv.

Figure four. Schematic illustration of teleost RPE retinomotor pigment migration and cytoskeleton. Pigment granules aggregate into the cell torso in the dark and disperse into apical RPE projections in the low-cal. Both microtubules and actin filaments are found in the prison cell body and the apical projections.

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Albinism

Gerald F Cox , Anne B Fulton , in Ocular Affliction, 2010

OA1

Abnormalities in the OA1 protein, GRP143, lead to a defect in melanosome biogenesis and the regulation of melanosome size; consequently, stage 3 and IV melanosomes are decreased in number and unusually large (megamelanosomes). They are institute in most carrier females in a mosaic pattern in tissues because of random 10-linked inactivation, which allows for only one 10 chromosome to exist expressed in a given cell.

Most of the circuitous forms of albinism involve non only melanosomes, but other organelles too. Hermansky–Pudlak syndrome is a genetically heterogeneous group of disorders, all of which involve abnormalities of vesicles of the lysosomal lineage that grade melanosomes, lysosomes, and platelet dumbo bodies. Decreased or absent-minded platelet dense bodies can be demonstrated by electron microscopy of fresh blood. Chédiak–Higashi syndrome is caused by a defect in the CHS1 (LYST) factor, which is idea to play a role in vesicular formation and transport. Equally in OA1, megamelanosomes are nowadays in Chédiak–Higashi syndrome.

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