Human ocular melanocytes and retinal pigment epithelial cells differ in their melanogenic properties in vivo and in vitro

Transcriptomic Analysis of the Ocular Posterior Segment Completes a Cell Atlas of the Human Eye

Although the visual system extends through the brain, most vision loss originates from defects in the eye. Its central element is the neural retina, which senses light, processes visual signals, and transmits them to the rest of the brain through the optic nerve (ON). Surrounding the retina are numerous other structures, conventionally divided into anterior and posterior segments. Here we used high-throughput single nucleus RNA sequencing (snRNA-seq) to classify and characterize cells in the extraretinal components of the posterior segment: ON, optic nerve head (ONH), peripheral sclera, peripapillary sclera (PPS), choroid, and retinal pigment epithelium (RPE). Defects in each of these tissues are associated with blinding diseases - for example, glaucoma (ONH and PPS), optic neuritis (ON), retinitis pigmentosa (RPE), and age-related macular degeneration (RPE and choroid). From ∼151,000 single nuclei, we identified 37 transcriptomically distinct cell types, including multiple types of astrocytes, oligodendrocytes, fibroblasts, and vascular endothelial cells. Our analyses revealed a differential distribution of many cell types among distinct structures. Together with our previous analyses of the anterior segment and retina, the new data complete a "Version 1" cell atlas of the human eye. We used this atlas to map the expression of >180 genes associated with the risk of developing glaucoma, which is known to involve ocular tissues in both anterior and posterior segments as well as neural retina. Similar methods can be used to investigate numerous additional ocular diseases, many of which are currently untreatable.

PAX6 Expression Patterns in the Adult Human Limbal Stem Cell Niche

Paired box 6 (PAX6), a nuclear transcription factor, determines the fate of limbal epithelial progenitor cells (LEPC) and maintains epithelial cell identity. However, the expression of PAX6 in limbal niche cells, primarily mesenchymal stromal cells (LMSC), and melanocytes is scarce and not entirely clear. To distinctly assess the PAX6 expression in limbal niche cells, fresh and organ-cultured human corneoscleral tissues were stained immunohistochemically. Furthermore, the expression of PAX6 in cultured limbal cells was investigated. Immunostaining revealed the presence of PAX6-negative cells which were positive for vimentin and the melanocyte markers Melan-A and human melanoma black-45 in the basal layer of the limbal epithelium. PAX6 staining was not observed in the limbal stroma. Moreover, the expression of PAX6 was observed by Western blot in cultured LEPC but not in cultured LMSC or LM. These data indicate a restriction of PAX6 expression to limbal epithelial cells at the limbal stem cell niche. These observations warrant further studies for the presence of other PAX isoforms in the limbal stem cell niche.

Evaluation of choroidal melanin-containing tissue in healthy Japanese subjects by polarization-sensitive optical coherence tomography

In this study, the choroidal melanin content in healthy eyes was evaluated with polarization-sensitive optical coherence tomography (PS-OCT). We evaluated 105 healthy eyes of 105 Japanese subjects. The mean thickness of melanin-containing tissue in the choroid (thickness of MeCh) and the choroidal melanin occupancy rate within a 5-mm circular region from the foveal center were calculated using the degree of polarization uniformity obtained by PS-OCT and compared with the choroidal thickness, patient age, and axial length. To evaluate regional variations, the 5-mm circular region was divided into a center area and an outer ring area, and the outer ring area was further divided into four areas (nasal, temporal, superior, and inferior). The mean thickness of MeCh showed a significant positive correlation with the choroidal thickness. The mean choroidal melanin occupancy rate showed a significant positive correlation with age. The mean choroidal melanin occupancy rate of the center area was significantly larger than that of the outer ring area. The mean thickness of MeCh and choroidal melanin occupancy rate of the nasal area were significantly lower than those of other areas. The distribution of melanin-containing tissue in the choroid varies significantly with age and location.

Three-Dimensional Distribution Of Fundus Depolarization and Associating Factors Measured Using Polarization-Sensitive Optical Coherence Tomography

Purpose

To investigate the three-dimensional distribution and associating demographic factors of depolarization, using polarization-sensitive optical coherence tomography (PS-OCT), to evaluate melanin pigmentation in the retinal pigment epithelium (RPE) and choroid in healthy eyes.

Methods

In total, 39 unaffected healthy eyes of 39 subjects were examined using a PS-OCT clinical prototype. The degree of depolarization, expressed as the polarimetric entropy, was assessed in the RPE, the superficial and the total choroid layer, especially in the center, the inner, or the outer areas centered at the fovea. The values and their association with the demographic data were analyzed. Near-infrared fundus autofluorescence (NIRAF) was also used, in the same manner, for the comparison. Twenty-eight of 39 eyes were measured twice to evaluate intrasession repeatability.

Results

Both the polarimetric entropy in the RPE and the gray level in NIRAF, decreased from the center to the periphery (P < 0.001). The polarimetric entropy in the RPE was significantly associated with age in each area (P ≤ 0.001). In the RPE and the superficial choroid, the polarimetric entropy was negatively associated with axial length in each area (P ≤ 0.002). The intraclass correlation coefficient of the polarimetric entropy in the same session was excellent in each area of the RPE, superficial choroid, or total choroid layer (0.94–0.98).

Conclusions

The distribution of fundus melanin pigment-related depolarization was evaluated using PS-OCT. The depolarization was associated with the subjects’ demographic data, such as age or axial length.

Translational Relevance

The presented information in healthy eyes provides an essential basis for the investigation into a variety of chorioretinal pathologies.

Evaluation of Retinal Pigment Epithelium and Choroidal Neovascularization in Rats Using Laser-Scanning Optical-Resolution Photoacoustic Microscopy

PURPOSE

To demonstrate the value of the laser-scanning optical-resolution (LSOR)-photoacoustic (PA) microscopy (PAM) system and the conventional multimodal imaging techniques in the evaluation of laser-induced retinal injury and choroidal neovascularization (CNV) in rats.

METHODS

Different degrees of retinal injury were induced using laser photocoagulation. We compared the LSOR-PAM system with conventional imaging techniques in evaluating retinal injury with or without CNV. Six additional rats, treated with an anti-VEGF antibody or immunoglobulin G immediately after photocoagulation, were imaged 7 and 14 days after injection, and CNV lesion areas were compared.

RESULTS

In the retinal injury model, fundus autofluorescence showed well-defined hyperreflection, while the lesion displayed abundant PA signals demonstrating nonuniform melanin distribution in retinal pigment epithelium (RPE). RPE was detected with higher contrast in the PAM B-scan image than optical coherence tomography (OCT). Additionally, the CNV lesion was present with multiple PA signal intensities which distinctly characterized the location and area of CNV as found in fundus fluorescein angiography. Furthermore, the decreased PA signals extending from the CNV lesion were similar to those of the vascular bud in ex vivo imaging, which was invisible in other in vivo images. When treated with anti-VEGF agents, statistically significant differences can be demonstrated by PAM similar to other modalities.

CONCLUSIONS

LSOR-PAM can detect the melanin distribution of RPE in laser-induced retinal injury and CNV in rats. PAM imaging provides a potential new tool to evaluate the vitality and functionality of RPE in vivo as well as to monitor the development and treatment of CNV.

Characterization of a tissue-engineered choroid

The choroid of the eye is a vascularized and pigmented connective tissue lying between the retina and the sclera. Increasing evidence demonstrates that, beyond supplying nutrients to the outer retina, the different choroidal cells contribute to the retina's homeostasis, especially by paracrine signaling. However, the precise role of each cell type is currently unclear. Here, we developed a choroidal substitute using the self-assembly approach of tissue engineering. Retinal pigment epithelial (RPE) cells, as well as choroidal stromal fibroblasts, vascular endothelial cells and melanocytes, were isolated from human eye bank donor eyes. Fibroblasts were cultured in a medium containing serum and ascorbic acid. After six weeks, cells formed sheets of extracellular matrix (ECM), which were stacked to produce a tissue-engineered choroidal stroma (TECS). These stromal substitutes were then characterized and compared to the native choroid. Their ECM composition (collagens and proteoglycans) and biomechanical properties (ultimate tensile strength, strain and elasticity) were similar. Furthermore, RPE cells, human umbilical vein endothelial cells and choroidal melanocytes successfully repopulated the stromas. Physiological structures were established, such as a confluent monolayer of RPE cells, vascular-like structures and a pigmentation of the stroma. Our TECS thus recaptured the biophysical environment of the native choroid, and can serve as study models to understand the normal interactions between the RPE and choroidal cells, as well as their reciprocal exchanges with the ECM. This will consequently pave the way to derive accurate insight in the pathophysiological mechanisms of diseases affecting the choroid. STATEMENT OF SIGNIFICANCE: The choroid is traditionally known for supplying blood to the avascular outer retina. There has been a renewed attention directed towards the choroid partly due to its implication in the development of age-related macular degeneration (AMD), the leading cause of blindness in industrialized countries. Since AMD involves the dysfunction of the choroid/retinal pigment epithelium (RPE) complex, a three-dimensional (3D) model of RPE comprising the choroid layer is warranted. We used human choroidal cells to engineer a choroidal substitute. Our approach takes advantage of the ability of cells to recreate their own environment, without exogenous materials. Our model could help to better understand the role of each choroidal cell type as well as to advance the development of new therapeutics for AMD.

Calf melanin immunomodulates RPE cell attachment to extracellular matrix protein

PURPOSE

It is widely accepted that RPE melanin has a protective effect against oxidative damage in RPE cells. It is possible that an additional protective characteristic of melanin is the ability to modulate RPE cell immune response. In this study, in vitro modeling was used to probe the relationship between RPE pigmentation and immune response by monitoring IL-6 expression and secretion in calf melanin pigmented ARPE-19 cells seeded onto glycated extracellular matrix as a stressor.

METHODS

ARPE-19 cells were left unpigmented or were pigmented with either calf melanin or latex beads, and were then seeded onto RPE-derived extracellular matrix (ECM) or tissue culture-treated plates (no ECM). ECMs were modified by glycation. IL-6 expression was measured using qPCR and IL-6 secretion was determined using an ELISA, both at 30 min and 24 h after seeding. MTT assay was used to quantify cell attachment to glycated matrices 30 min after seeding. In unpigmented ARPE-19 cells, rate of cell attachment to substrate was monitored for 60 min after seeding using a hemacytometer to count unattached cells. Additionally, cell viability was evaluated using the Neutral Red assay 24 h after seeding.

RESULTS

A significant increase in IL-6 expression was observed in calf melanin pigmented cells versus latex bead and unpigmented controls (p < 0.0001) 30 min after seeding onto ECM. Twenty-four hours after seeding, a significant decrease in IL-6 expression was observed in calf melanin pigmented cells (p < 0.0001) versus controls, implicating down-regulation of the cytokine. Additionally, calf melanin pigmented cell populations showed significant increase in attachment compared to unpigmented controls on either no ECM or unmodified ECM.

CONCLUSIONS

Pigmentation of RPE cells with calf melanin resulted in significant changes in IL-6 expression regardless of ECM modification, in vitro. These findings suggest that melanin in the RPE may participate in immune response modulation in the retina with particular regard to cell attachment to protein substrates. The results of this study further implicate the role of chemical changes to melanin in regulating inflammation in retinal disease.

Improvement of Storage Medium for Cultured Human Retinal Pigment Epithelial Cells Using Factorial Design

Storage of human retinal pigment epithelium (hRPE) can contribute to the advancement of cell-based RPE replacement therapies. The present study aimed to improve the quality of stored hRPE cultures by identifying storage medium additives that, alone or in combination, contribute to enhancing cell viability while preserving morphology and phenotype. hRPE cells were cultured in the presence of the silk protein sericin until pigmentation. Cells were then stored for 10 days in storage medium plus sericin and either one of 46 different additives. Individual effects of each additive on cell viability were assessed using epifluorescence microscopy. Factorial design identified promising additive combinations by extrapolating their individual effects. Supplementing the storage medium with sericin combined with adenosine, L-ascorbic acid and allopurinol resulted in the highest cell viability (98.6 ± 0.5%) after storage for three days, as measured by epifluorescence microscopy. Flow cytometry validated the findings. Proteomics identified 61 upregulated and 65 downregulated proteins in this storage group compared to the unstored control. Transmission electron microscopy demonstrated the presence of melanosomes after storage in the optimized medium. We conclude that the combination of adenosine, L-ascorbic acid, allopurinol and sericin in minimal essential medium preserves RPE pigmentation while maintaining cell viability during storage.

Differential expression of TYRP1 in adult human retinal pigment epithelium and uveal melanoma cells

Uveal melanoma (UM) is the most frequently occurring primary intraocular malignancy in adults. Tyrosinase (TYR) is a copper-containing enzyme and a type I membrane protein that is involved in the generation of melanin, the main pigment in vertebrates. TYR-related protein 1 (TYRP1) is regarded to have a crucial role in the immunotherapy of melanoma. As biomarkers, the TYR-related proteins, TYRP1 and TYRP2, exhibit specific expression in melanocytes, while also contributing to melanin synthesis within melanosomes. In the present study, the differential expression of TYRP1 was investigated at the mRNA, protein and morphological levels in four human UM cell lines (SP6.5, OM431, OCM1 and OCM290) and the human retinal pigment epithelium (RPE) cell line, using polymerase chain reaction, western blotting, immunocytochemistry and immunofluorescence staining. It was found that SP6.5 cells expressed the highest level of TYRP1, in comparison to SP6.5 OCM1 and OM431 cells, which produced less TYRP1, and OCM290 cells, which produced almost no TYRP1. No TYRP1 protein expression was identified in the RPE cell line. These findings indicate the potential use of TYRP1 in the development of therapy for UM.

Effect of Storage Temperature on Key Functions of Cultured Retinal Pigment Epithelial Cells

Purpose. Replacement of the diseased retinal pigment epithelium (RPE) with cells capable of performing the specialized functions of the RPE is the aim of cell replacement therapy for treatment of macular degenerative diseases. A storage method for RPE is likely to become a prerequisite for the establishment of such treatment. Herein, we analyze the effect of storage temperature on key functions of cultured RPE cells. Methods. Cultured ARPE-19 cells were stored in Minimum Essential Medium at 4°C, 16°C, and 37°C for seven days. Total RNA was isolated and the gene expression profile was determined using DNA microarrays. Comparison of the microarray expression values with qRT-PCR analysis of selected genes validated the results. Results. Expression levels of several key genes involved in phagocytosis, pigment synthesis, the visual cycle, adherens, and tight junctions, and glucose and ion transport were maintained close to control levels in cultures stored at 4°C and 16°C. Cultures stored at 37°C displayed regulational changes in a larger subset of genes related to phagocytosis, adherens, and tight junctions. Conclusion. RPE cultures stored at 4°C and 16°C for one week are capable of maintaining the expression levels of genes important for key RPE functions close to control levels.

Studying melanin and lipofuscin in RPE cell culture models

The retinal pigment epithelium contains three major types of pigment granules; melanosomes, lipofuscin and melanolipofuscin. Melanosomes in the retinal pigment epithelium (RPE) are formed during embryogenesis and mature during early postnatal life while lipofuscin and melanolipofuscin granules accumulate as a function of age. The difficulty in studying the formation and consequences of melanosomes and lipofuscin granules in RPE cell culture is compounded by the fact that these pigment granules do not normally occur in established RPE cell lines and pigment granules are rapidly lost in adult human primary culture. This review will consider options available for overcoming these limitations and permitting the study of melanosomes and lipofuscin in cell culture and will briefly evaluate the advantages and disadvantages of the different protocols.

Foveal sparing in patients with Japanese Stargardt's disease and good visual acuity

PURPOSE

Patients with Stargardt's disease usually have a poor visual prognosis. However, in our clinical practice we have observed some patients with dark-red foveal pigmentation (sparing) who had good best corrected visual acuity (BCVA) despite the presence of a late-stage disease. The purpose of this study was to investigate the BCVA outcomes in Japanese patients with Stargardt's disease with foveal sparing.

METHODS

Eighteen consecutive patients (36 eyes) with Stargardt's disease underwent ophthalmoscopy, fluorescein angiography, fundus autofluorescence imaging, and near-infrared fundus autofluorescence (NIA) imaging. The patients were divided into two groups based on the presence or absence of foveal sparing. The presence of foveal sparing was determined based on ophthalmoscopy and NIA imaging results. The association between foveal sparing and BCVA was assessed statistically by Students' t test.

RESULTS

Of the 36 eyes, ten (27.8 %) had dark-red foveal sparing. The mean BCVA of the group with sparing was 0.16 ± 0.31, expressed in logarithm of the minimum angle of resolution (logMAR) units, and that of the group without sparing was 1.03 ± 0.39 logMAR units, which is a significant difference (p = 0.0000002507).

CONCLUSION

A subgroup of Japanese patients with late-stage Stargardt's disease and dark-red foveal sparing maintained a relatively good BCVA. The pigmentation was clearly observed using NIA and proved useful for assessing the BCVA prognosis.

Multimodal imaging of dry age-related macular degeneration

PURPOSE

The purpose of this study was to understand clinical significance of near-infrared reflectance (NIR), blue fundus autofluorescence (FAF) and near-infrared autofluorescence (NIA) in dry age-related macular degeneration (AMD), by correlation with fluorescein angiography (FA) and cross-sectional spectral domain optical coherence tomography (SD OCT).

METHODS

We evaluated 110 eyes (62 patients, mean age: 64 ± 8 years) diagnosed with dry AMD between January 2010 and December 2010, which underwent NIR (λ = 830 nm), FAF and FA (excitation λ = 488 nm; emission λ > 500 nm), NIA (excitation λ = 787 nm; emission λ > 800 nm), and simultaneous SD OCT scanning using a combined confocal scanning laser ophthalmoscope/SD OCT device (Spectralis HRA + OCT; Heidelberg Engineering, Heidelberg, Germany).

RESULTS

Drusen showed variable increased/decreased NIR, FAF, NIA and FA, which corresponded to variable increased/decreased thickness of the retinal pigment epithelium (RPE) and possible presence of subretinal deposits on SD OCT. Geographic atrophy (GA) was present in 43/110 eyes (39.0%) and showed increased NIR and fluorescence (FA), absent FAF and NIA, and loss of RPE on SD OCT. The hyperautofluorescence of the GA margin was never larger in FAF than that in NIA, while in 16.2% of cases, it was larger in NIA than that in FAF and corresponded to mild choroidal hyperreflectivity on SD OCT.

CONCLUSIONS

Simultaneous recording of SD OCT scans provided ultrastructural data for the evaluation of NIR, FAF, NIA and FA in dry AMD. Near-infrared autofluorescence might detect earlier than FAF areas of RPE cell loss at the GA margin.

In vitro differentiation of retinal pigment epithelium from adult retinal stem cells

One of the limitations in molecular and functional studies of the retinal pigment epithelium (RPE) has been the lack of an in vitro system retaining all the features of in vivo RPE cells. Retinal pigment epithelium cell lines do not show characteristics typical of a functional RPE, such as pigmentation and expression of specific markers. The present study was aimed at the development of culture conditions to differentiate, in vitro, retinal stem cells (RSC), derived from the adult ciliary body, into a functional RPE. Retinal stem cells were purified from murine eyes, grown as pigmented neurospheres and induced to differentiate into RPE on an extracellular matrix substrate using specific culture conditions. After 7-15 days of culture, pigmented cells with an epithelial morphology showed a polarized organization and a capacity for phagocytosis. We detected different stages of melanogenesis in cells at 7 days of differentiation, whereas RPE at 15 days contained only mature melanosomes. These data suggest that our protocol to differentiate RPE in vitro can provide a useful model for molecular and functional studies.

Lipofuscin- and melanin-related fundus autofluorescence in patients with ABCA4-associated retinal dystrophies

PURPOSE

To compare melanin-related near-infrared fundus autofluorescence (NIA; excitation 787 nm, emission > 800 nm) to lipofuscin-related fundus autofluorescence (FAF; excitation 488 nm, emission > 500 nm) in patients with retinal dystrophies associated with ABCA4 gene mutations (ABCA4-RD).

DESIGN

Observational case series.

METHODS

Sixteen consecutive patients with ABCA4-RD diagnosed in one institution were included. FAF and NIA imaging were performed with a confocal scanning laser ophthalmoscope (Heidelberg Retina Angiograph 2; Heidelberg Engineering, Heidelberg, Germany). The pattern and size of retinal pigment epithelial (RPE) alterations detected with FAF and NIA were evaluated.

RESULTS

FAF and NIA alterations were detected in all patients. In 7 of 16 patients, the alterations progressed beyond the vascular arcades, and in 9 of 16, they were confined to the macula. Spots of increased NIA (4/16) were less frequent compared with spots of increased FAF (15/16). Confluent patches of reduced NIA were frequent (12/16), and severely reduced NIA was observed in 3 cases. Areas with reduced NIA corresponded to either increased or reduced FAF. Preservation of subfoveal FAF or NIA corresponded to visual acuity > or = 0.4. Abnormalities detected with NIA were more extensive or more severe compared to FAF in 15 of 16 patients.

CONCLUSION

Patterns of FAF and NIA indicate different involvement of lipofuscin and melanin and their derivates in the pathophysiologic process of ABCA4-RD. NIA imaging provides a noninvasive in vivo visualization of RPE abnormalities that may precede FAF alterations during the degenerative process. Combined FAF and NIA imaging will provide further insight in the development of ABCA4-RD and could help to monitor future therapeutic interventions.

Lipofuscin- and melanin-related fundus autofluorescence visualize different retinal pigment epithelial alterations in patients with retinitis pigmentosa

AIMS

To compare melanin-related near-infrared fundus autofluorescence (FAF; NIA, excitation 787 nm, emission >800 nm) with lipofuscin-related FAF (excitation 488 nm, emission >500 nm) in retinitis pigmentosa (RP).

METHODS

Thirty-three consecutive RP patients with different modes of inheritance were diagnosed clinically, with full-field ERG, and if possible with molecular genetic methods. FAF and NIA imaging were performed with a confocal scanning laser ophthalmoscope (Heidelberg Retina Angiograph 2).

RESULTS

Rings of increased FAF were present within an area of preserved retinal pigment epithelium (RPE) at the posterior pole (31/33). Rings of increased NIA were located in the same region as rings of increased FAF. In contrast to FAF, NIA showed a precipitous decline of NIA peripheral to the ring. In larger areas of preserved NIA (11/31), pericentral and foveal NIA were of similar intensity with an area of lower NIA in between. In smaller areas of preserved NIA (20/31), NIA was homogeneous from the perifovea to the fovea. In one patient without a ring of increased FAF, NIA distribution was normal. In the remaining patient with severely advanced RP, no residual RPE as well as no FAF and NIA were detectable.

CONCLUSION

Characteristic features for FAF and NIA alterations in a heterogeneous group of RP patients indicate a common pathway of RPE degeneration. Patterns of NIA and FAF indicate different pathophysiologic processes involving melanin and lipofuscin. Combined NIA and FAF imaging will provide further insight into the pathogenesis of RP and non-invasive monitoring of future therapeutic interventions.

Role of ocular melanin in ophthalmic physiology and pathology

The mammalian eye consists of several layers of pigmented tissues that contain melanin. The eye is a unique organ for pigment cell research because one can isolate and compare melanosomes from different tissues and embryonic origins. Retinal, iris and ciliary pigment epithelial cells are derived from the neural ectoderm, more specifically from the extremity of the embryonic optical cup, which is also the origin of the retina. In contrast, the pigment-generating cells in the choroid and in the stroma of the iris and ciliary body, uveal melanocytes, are developed from the neural crest, the same origin as the melanocytes in skin and hair. This review examines the potential functions of ocular melanin in the human eye. Following a discussion of the role of melanins in the pigment epithelium and uveal melanocytes, three specific topics are explored in detail-photo-screening protective effects, biophysical and biochemical protective effects, and the biologic and photobiologic effects of the two main classes of melanins (generally found as mixtures in ocular melanosomes)--eumelanin and pheomelanin.

Chloroquine retinopathy: lipofuscin- and melanin-related fundus autofluorescence, optical coherence tomography and multifocal electroretinography

PURPOSE

To evaluate melanin-related near-infrared fundus autofluorescence (NIA, excitation 787 nm, emission > 800 nm), lipofuscin-related fundus autofluorescence (FAF, excitation 488 nm, emission >500 nm), optical coherence tomography (OCT), and multifocal electroretinography (mfERG) in patients with chloroquine (CQ) retinopathy.

METHODS

Two patients with progressed CQ retinopathy underwent clinical examination, ISCEV mfERG evaluation, and FAF and NIA imaging using a confocal scanning laser ophthalmoscope (Heidelberg Retina Angiograph 2) with either a 30 degrees or wide-angle field-of-view. OCT3 imaging was performed in one of these patients.

RESULTS

In the foveola, FAF and NIA were relatively normal. Parafoveal loss of retinal pigment epithelium (RPE) was indicated by absent FAF and NIA. An area of reduced FAF and NIA surrounded the parafoveal region of RPE loss. In the adjacent area, FAF was increased and increased NIA marked the peripheral border of increased FAF. Wide-field imaging revealed increased FAF in association with retinal vessels. Retinal thickness was markedly reduced in the OCT predominantly in the parafoveal region. Visual field loss and mfERG amplitude reduction corresponded to areas with increased or reduced FAF and NIA.

CONCLUSION

Patterns of FAF and NIA indicate different stages of pathophysiologic processes involving lipofuscin and melanin in the RPE. Combined retinal imaging and functional testing provides further insights in the pathogenesis and development of retinal degenerative disease. An association of CQ retinopathy with retinal vessels architecture is hypothesized.

Melanosome maturation defect in Rab38-deficient retinal pigment epithelium results in instability of immature melanosomes during transient melanogenesis

Pathways of melanosome biogenesis in retinal pigment epithelial (RPE) cells have received less attention than those of skin melanocytes. Although the bulk of melanin synthesis in RPE cells occurs embryonically, it is not clear whether adult RPE cells continue to produce melanosomes. Here, we show that progression from pmel17-positive premelanosomes to tyrosinase-positive mature melanosomes in the RPE is largely complete before birth. Loss of functional Rab38 in the "chocolate" (cht) mouse causes dramatically reduced numbers of melanosomes in adult RPE, in contrast to the mild phenotype previously shown in skin melanocytes. Choroidal melanocytes in cht mice also have reduced melanosome numbers, but a continuing low level of melanosome biogenesis gradually overcomes the defect, unlike in the RPE. Partial compensation by Rab32 that occurs in skin melanocytes is less effective in the RPE, presumably because of the short time window for melanosome biogenesis. In cht RPE, premelanosomes form but delivery of tyrosinase is impaired. Premelanosomes that fail to deposit melanin are unstable in both cht and tyrosinase-deficient RPE. Together with the high levels of cathepsin D in immature melanosomes of the RPE, our results suggest that melanin deposition may protect the maturing melanosome from the activity of lumenal acid hydrolases.

Serum-free cultivation of adult normal human choroidal melanocytes

BACKGROUND

Cultures of normal choroidal melanocytes are useful in vitro models for the study of melanocyte biology. Current protocols involve the supplementation of culture media with serum, toxins, and phorbol esters, the latter being known as tumour-inducing agents. We therefore sought to establish a protocol to cultivate normal human choroidal melanocytes (NHCMs) without these supplements.

METHODS

NHCMs were isolated by dispase II treatment, after isolation of retinal pigment epithelial (RPE) cells, and seeded in serum-free Melanocyte Growth Medium M2 in uncoated T25 cell culture flasks. Purity of the established cultures was proven by immunocytochemistry. Morphology of the cultured cells was evaluated throughout the entire cultivation period. Eventually, cells were cryopreserved in liquid nitrogen.

RESULTS

The cultures underwent at least 11.5 +/- 4.5 doublings before they became senescent or culture was deliberately terminated. Mean generation time was 95.0 +/- 27.7 h. After cryopreservation, generation time was markedly increased, but proliferative capacity of the cells was not impaired. Cultured cells showed bipolar to dendritic morphology; sometimes flattened triangular cells were seen in the cultures. Cultured cells lost pigment after initial seeding but displayed continuous melanogenesis. All cultures stained positive for HMB45 antigen and S-100 and negative for RPE-specific cytokeratins 8, 18. Only few cultures contained single cells that were weakly positive for matrix metalloproteinase (MMP)-2 or MMP-9.

CONCLUSIONS

The cultivation protocol yields pure cultures of NHCMs that can successfully be maintained for several months without the use of serum, tumour-inducing substances, or toxins.

Tyrosinase biosynthesis and trafficking in adult human retinal pigment epithelial cells

BACKGROUND

Tyrosinase (EC 1.14.18.1) is the key enzyme of melanin pigment formation and it is unclear whether it is synthesized in human postnatal retinal pigment epithelium (RPE). In this study, we investigated if phagocytosis of rod outer segments (ROS) can increase tyrosinase expression in vitro.

METHODS

Primary cultures of human RPE cells were fed with isolated ROS from cattle and with latex particles. After phagocytosis, RPE cells were tested for tyrosinase presence and activity with several independent methods: (1) immunocytochemistry with anti-tyrosinase antibodies and (2) ultrastructural as well as light microscopic DOPA histochemistry; (3) mRNA was isolated from human RPE before incubation with ROS and 5, 20 and 40 h after feeding with ROS. The amount of tyrosinase mRNA was determined quantitatively by real-time reverse transcription polymerase chain reaction (RT-PCR), and the tyrosinase activity was investigated by measuring tyrosine hydroxylase activity using [(3)H]tyrosine.

RESULTS

Tyrosinase was found in fed RPE cells using these methods, but was absent without feeding. Furthermore, we showed co-localization of rhodopsin and tyrosinase in the fed RPE cells. Contrary to tyrosinase activity, the mRNA for tyrosinase was clearly present in the cultured RPE cells which had not been exposed to ROS, decreased significantly from 5 h after exposure to ROS and returned to its original non-fed level 40 h after ROS feeding.

CONCLUSION

Our study does not present new evidence that de novo melanogenesis takes place in the adult differentiated RPE. However, in contrast to the classic hypothesis, which states that tyrosinase is only detected in embryos, we provide evidence with several independent methods that the expression of tyrosinase and its enzymatic activity are induced in cultured human adult RPE by phagocytosis of ROS.

Tyrosinase biosynthesis in adult mammalian retinal pigment epithelial cells

Tyrosinase (EC 1.14.18.1) is the rate limiting enzyme of melanogenesis and it is unclear whether it is synthesized in postnatal retinal pigment epithelium (RPE). Cultured RPE cells from cattle were fed with isolated rod outer segments (ROS). After phagocytosis, RPE cells were tested for tyrosinase presence and activity with three independent methods: (1) ultrastructural DOPA (l-3,4-dihydroxyphenylalanine) histochemistry (2) immunocytochemistry with anti-tyrosinase antibodies (3) measuring tyrosine hydroxylase activity using [(3)H]tyrosine. With all three methods tyrosinase was found in RPE cells after ROS-feeding but was absent without feeding. In contrast to the classical hypothesis, we demonstrated with three independent methods that the expression of tyrosinase and its enzymatic activity are induced in cultured adult RPE by phagocytosis of rod outer segments (ROS) in vitro.

Dopa oxidase activity in the hair, skin and ocular melanocytes is increased in the presence of stressed fibroblasts

We previously reported that mesenchymal cells (dermal fibroblasts and dermal papilla cells) can stimulate dopa oxidase activity in the skin melanocytes. This study extends the investigation of the influence of the fibroblast in a comparative study of melanogenesis in melanocytes from the hair, the skin and the eye. Culture of melanocytes with normal proliferative dermal fibroblasts slightly increased dopa oxidase activity of the hair, skin and ocular melanocytes (by 17, 11 and 28%, respectively), but co-culture with fibroblasts recovering from storage in liquid nitrogen or growth-arrested by means of gamma radiation showed much greater effects. Most dramatic results were obtained with fibroblasts, which had been both gamma-irradiated and then frozen in liquid nitrogen, where increases in dopa oxidase activity of 125, 227 and 185% for melanocytes of the hair, the skin and the eye, respectively, were seen. Experiments by using transwell cultures of melanocytes and fibroblasts and by using fibroblast-conditioned medium showed that a large proportion of this fibroblast influence could be mediated by diffusible factors, of which a good proportion was attributable to basic Fibroblast Growth Factor (bFGF). The addition of bFGF significantly increased dopa oxidase activity of the skin melanocytes, when fibroblasts were present, but not in their absence. These data show that fibroblasts in vitro, particularly when deliberately stressed, have the ability to increase dopa oxidase activity in melanocytes of the hair, the skin and the eye and further suggest that this effect is mediated by bFGF acting in combination with some other fibroblast-derived factors.

Ocular Melanogenesis: The Role of Antioxidants

Given the propensity of a large number of melanogenic pathways that can be modulated by cellular redox status, a causal role of the deficiency of ocular pigments such as melanin in the pathogenesis of age-related macular degeneration and evidence that melanin production does occur in the adult eye, it seems not improbable that antioxidants (or agents that modify cellular redox status) may have melanin stimulatory (or inhibitory) effects that are superimposible on their effects as mere free radical scavengers. More empirical studies are needed to investigate this phenomenon so that antioxidant therapy may prove more beneficial to patients with ocular degenerative diseases.

alpha-Melanocyte-stimulating hormone, MSH 11-13 KPV and adrenocorticotropic hormone signalling in human keratinocyte cells

alpha-MSH signals by binding to the melanocortin-1 receptor (MC-1R) and elevating cyclic AMP in several different cells. The anti-inflammatory properties of this peptide are also believed to be cyclic AMP dependent. The carboxyl terminal tripeptides of alpha-MSH (KPV / KP-D-V) are the smallest minimal sequences reported to prevent inflammation but it is not known if they operate via MC-1R or cyclic AMP. The aim of this study was to examine the intracellular signalling of key MSH and ACTH peptides in human keratinotocytes. No elevation in cyclic AMP was detected in either HaCaT or normal human keratinocytes in response to alpha-MSH, KPV or ACTH peptides. Rapid and acute intracellular calcium, however, were observed in HaCaT keratinocytes in response to alpha-MSH (10(-15)-10(-7) M), KPV (10(-15)-10(-7) M), KP-D-V (10(-15)-10(-7) M) and ACTH (10(-15)-10(-7) M), but only in the presence of PIA, an adenosine agonist that inhibits the cyclic AMP pathway. Normal keratinocytes responded to all the above peptides but in addition responded to ACTH 1-17 (10(-13)-10(-7) M) in contrast to the HaCaT keratinocytes. Stable transfection of Chinese hamster ovary cells with the MC-1 receptor showed that alpha-MSH and the KPV peptides elevated intracellular calcium.

Latanoprost-induced pigmentation in human iridial melanocytes is fibroblast dependent

The prostaglandin F2alpha derivative, latanoprost (LT), used in glaucoma treatment, can induce pigmentation in irises of patients with hazel or heterochromatic eye colour. The mechanism by which LT induces pigmentation in the iris is not yet established, although it does not appear to induce proliferation of iridial melanocytes. The purpose of this study was to develop an in vitro model in which to investigate this mechanism. The pigmentary responses to LT and prostaglandin F(2alpha) (PGF(2alpha)) were examined in human iridial melanocytes alone or in co-culture with epithelial cells (non-ocular human epidermal keratinocytes and iris pigment epithelial cells) or mesenchymal cells (non-ocular dermal fibroblasts or iridial fibroblasts). Melanogenesis was assessed after 4 days culture with prostanoids, using dopa oxidase activity. Prostaglandin FP expression on human iridial fibroblasts and melanocytes was investigated using an immunofluorescent technique employing antibody to PGF(2alpha) receptor and RT-PCR. Iridial melanocytes did not show a convincing increase in dopa oxidase when cultured alone but in the presence of fibroblasts (ocular or non-ocular) there was a significant increase (25-30%) in dopa oxidase activity in response to 10(-7)-10(-5)m LT and PGF(2alpha). Co-culture of melanocytes with epithelial cells, while leading to increased dopa oxidase activity, did not lead to any melanogenic response to LT or PGF(2alpha). FP receptor expression was detected on fibroblasts but not iridial melanocytes by immunocytochemistry and RT-PCR. The melanocyte/fibroblast co-culture model developed in this study also showed that LT and PGF(2alpha) increased dopa oxidase activity in melanocytes from donors with brown but not blue eyes. These results suggest that LT may be inducing pigmentation in the human iris indirectly through the FP receptor on adjacent fibroblasts.

Latanoprost-induced iris darkening: a morphometric study of human peripheral iridectomies

PURPOSE

This microscopic study was undertaken to compare the melanocytes of peripheral iridectomy specimens from two eyes that had latanoprost-induced iris darkening (LIID) with iridectomies taken from the fellow untreated eyes.

METHODS

The two patients in this study were the ones who underwent LIID in the latanoprost treated eye from a series of 17 patients requiring bilateral trabeculectomy. The first trabeculectomy procedure provided a control peripheral iridectomy for each patient, whereas the second eye was treated with once daily 50 microg ml(-1) latanoprost drops for 6 months. The four peripheral iridectomy specimens from the two LIID patients were subjected to quantitative morphometric analysis by light microscopy of iris cellularity, and electron microscopy of iris melanocyte immature melanosomes and mature melanin granules.

RESULTS

There was no significant difference in stromal cellularity between the LIIDs and their respective controls nor were there significant differences in the numbers of immature melanosomes or melanin granules in the melanocytes. However, there was a significant increase in the diameter of melanin granules that was more pronounced in the anterior border layer than the deeper stroma. With the anterior border melanocytes, the increase in melanin granule size was associated with significant increases in granule area and the percentage of cell cytoplasm occupied by melanin (granularity).

CONCLUSIONS

The only morphological change identified in two peripheral iridectomies that had LIID when compared to untreated fellow eye specimens was a modest increase in the size of stromal melanocyte melanin granules that was more pronounced in the cells of the anterior border region.

Retinal pigment epithelium melanin and ocular toxicity

Ocular morphology is specifically directed at facilitating the transmittance of visible light to the retina for the purposes of photoreceptor absorption and phototransduction, thereby initiating the process of vision. By absorbing excess radiation, melanin significantly enhances this process. It can also act as a photoprotector by quenching reactive oxygen species and other radicals produced as a result of the high oxygen dependency of the retina for its metabolism. However, melanin also binds numerous pharmaceuticals, a process that can result in ocular toxicity. Although our understanding of this binding remains somewhat limited, melanin chemistry, its distribution, and other factors influencing binding appear to play a significant role in predisposing ocular tissues, such as the choroid and retinal pigment epithelium, to toxicological insult. Many of the drugs that have been identified as causing these effects are known photosensitizers in which radiation plays a significant role in eliciting the pathologies. The phototoxic effects range from minor histological/chemical changes, which do not impact the quality of vision, to pigmentary retinopathies, which could potentially involve the loss of sight. Such effects, resulting from photosensitizer-drug binding to melanin, are to be separated from toxic effects, such as some ganglion cell abnormalities, that result from pharmaceuticals affecting ocular tissues directly.

Uveal melanocytes, ocular pigment epithelium, and Müller cells in culture: in vitro toxicology

Uveal melanocytes and the ocular pigment epithelium are located in the middle and inner layers of the eye. Müller cells (a type of glial cell) are located in the neural retina. Melanocytes, retinal pigment epithelium (RPE), and Müller cells do not participate directly in the detection or transfer of visual information, but they have various functions that support the neural retina and are essential for the maintenance of vision. Methods for the isolation and cultivation of melanocytes, RPE, and Müller cells have been established by us and other investigators. These cultured cells can be used as in vitro model systems for studying the toxicology of visible light, ultraviolet (UV) radiation, drugs, and other potentially toxic agents. Toxic effects on these cells may give rise to altered retinal function and result in impaired vision. Both melanocytes and pigment epithelium contain melanin, which has the ability to bind organic amines and metal ions. This results in the accumulation of these substances in the eye. Melanin may protect cells from chemical stress by binding toxic chemicals; but in chronic exposure, increased and lengthy binding may cause damage to these cells. Two different types of melanin are found in the eye: eumelanin and pheomelanin, which may have photoprotective and phototoxic effects, respectively. Pigment epithelium contains mainly eumelanin, whereas melanocytes contain both eumelanin and pheomelanin. Melanin is an antioxidant and with age, the antioxidant properties may diminish to the point that it may even become a prooxidant. There are also other functions of pigment epithelium and uveal melanocytes not related to melanin and there are also several functions of Muller cells that play a role in the toxicological aspects of the eye. Cultured uveal melanocytes, pigment epithelial cells, and Müller cells can be used to study the toxicology of these cells in vitro.

Metabolism of protein-bound DOPA in mammals

Protein-bound 3,4-dihydroxyphenylalanine (DOPA) can be generated in mammalian cells by both controlled enzymatic pathways, and by uncontrolled radical reactions. Protein-bound DOPA (PB-DOPA) has reducing activity and the capacity to inflict secondary damage on other important biomolecules such as DNA. This may be mediated through replenishment of transition metals or from catechol-quinone-catechol redox cycles in the presence of cellular components such as ascorbate or cysteine, resulting in amplification of radical damaging events. The generation of PB-DOPA confers on protein the ability to chelate transition metals generating protein 'oxychelates'; this may be amongst the factors, which localise such damage. Tissue levels of PB-DOPA are increased in a number of age-related pathologies such as atherosclerosis and cataract formation. We discuss the detoxification, and the subsequent proteolysis and excretion of components of PB-DOPA. We contrast the fact that in marine organisms, and particularly in extracellular proteins, PB-DOPA and other DOPA-polymers can play important functional roles in adhesion and the provision of tensile properties.

A comparative study of the effect of pigment on drug toxicity in human choroidal melanocytes and retinal pigment epithelial cells

The aim of this study was to investigate whether the presence of pigment affects the sensitivity of pigmented cells of the eye, retinal pigment epithelium (RPE) and choroidal melanocytes (CMs) to the cytotoxic effects of xenobiotic drugs. Two approaches were used to compare pigmented versus unpigmented cells: RPE cells were repigmented by phagocytosis of synthetic melanin; UVB irradiation was used to induce an increase in pigment in both RPE and CMs. Three drugs known to induce toxicity in the eye, tamoxifen, chloroquine and thioridazine, were used to assess the sensitivity of cells to xenobiotic drugs. RPE cells were more resistant than CMs to the cytotoxic effects of all three drugs by a factor of 5-fold for tamoxifen, 7-fold for thioridazine and 30-fold for chloroquine. When RPE cells were repigmented using synthetic melanin, their sensitivity to tamoxifen was unchanged, they showed a slightly improved response to thioridazine (after 3 days of incubation with this drug), but they showed greatly increased toxicity to chloroquine (after 1 and 3 days of exposure to the drug), suggesting accumulation of this latter drug on the synthetic melanin. UVB irradiation was used to achieve an increase in the pigment content of both RPE and CMs. CMs were much more sensitive to UVB than RPE cells. CMs appeared to synthesise pigment via DOPA oxidase activity; RPE cells showed an increase in fluorescent material independent of any detectable DOPA oxidase activity. Irrespective of the nature of the pigment that UVB induced in melanocytes and RPE cells, their subsequent response to thioridazine and chloroquine was unchanged by the presence of this pigment.

High expression of immunotherapy candidate proteins gp100, MART-1, tyrosinase and TRP-1 in uveal melanoma

In the treatment of cutaneous melanoma, provisional therapeutic strategies have been designed to combat tumour load using T cells that are sensitized with peptides derived from melanoma autoantigens, such as glycoprotein 100 (gp100), melanoma antigen recognized by T cells 1 (MART-1 or MelanA), tyrosinase and tyrosinase-related protein 1 (TRP-1). We recently found that gp100, MART-1 and tyrosinase are heterogeneously expressed in human cutaneous melanoma (De Vries et al (1997) Cancer Res 57: 3223-3229). Here, we extended our investigations on expression of these immunotherapy candidate proteins to uveal melanoma lesions. Cryostat sections from 11 spindle-type, 21 mixed and epithelioid tumours and four metastasis lesions were stained with antibodies specifically recognizing gp100, MART-1, tyrosinase and TRP-1. In addition, we used the DOPA reaction to detect tyrosinase enzyme activity as a confirmation of the tyrosinase immunohistochemical results. High expression of gp100, MART-1 and tyrosinase was found in the uveal melanoma lesions: 80% of the lesions displayed 75-100% positive tumour cells. TRP-1 positivity was slightly less: approximately 65% of the lesions stained in the 75-100% positive tumour cell category. All uveal melanoma lesions were positive for the four markers studied, this being in contrast to cutaneous melanoma where 17% of the advanced primary lesions and metastases were negative. The presence of these antigens was a little lower in metastases. We conclude that uveal melanomas and their metastases express melanocyte-lineage immunotherapy candidate proteins very abundantly. Uveal melanomas differ in this respect from cutaneous melanoma, in which the expression of these immunotherapy antigens was much more heterogeneous. This makes uveal melanoma a suitable candidate tumour for immunotherapeutic approaches.

The influence of extracellular matrix proteins on cutaneous and uveal melanocytes

Cutaneous and ocular melanocytes are routinely cultured in complex mitogen-rich media. The physiological regulation of melanocyte proliferation and differentiation is not yet fully defined and this study summarises several separate lines of evidence which suggest that, in vivo, some of the signals required for melanocyte proliferation and differentiation may derive from extracellular matrix (ECM) proteins adjacent to these cells. Culture of cutaneous and uveal melanocytes on cell-derived and individual ECM proteins was found to influence cell morphology with such effects being most noticeable in mitogen-deficient media. Similarly, cell-derived and individual ECM proteins increased tyrosinase activity in normal cutaneous melanocytes and effects of these ECM proteins were seen most consistently in mitogen-deficient media. Uveal melanocytes (as has been reported for cutaneous melanocytes) showed preferential attachment to fibronectin over other ECM substrates. This attachment was particularly sensitive to drugs which affected intracellular calcium or calmodulin activity. Acute addition of fibronectin to coverslips of uveal melanocytes loaded with Fura-2 produced an acute and transient increase in intracellular calcium which was more prevalent in low density than higher density cells. We conclude that ECM proteins in vitro are capable of influencing melanocyte morphology, tyrosinase activity, and proliferation and that an ECM-induced elevation in intracellular calcium may be part of the signalling system that transmits ECM information into the cell.