The differing embryonic origins of retinal and uveal (iris/ciliary body and choroid) melanosomes are mirrored by their phospholipid composition

Hyperpigmented spots at fundus examination: a new ocular sign in Neurofibromatosis Type I

BACKGROUND

Neurofibromatosis Type I (NF1), also termed von Recklinghausen disease, is a rare genetic disorder that is transmitted by autosomal dominant inheritance, with complete penetrance and variable expressivity. It is caused by mutation in the NF1 gene on chromosome 17 encoding for neurofibromin, a protein with oncosuppressive activity, and it is 50% sporadic or inherited. The disease is characterized by a broad spectrum of clinical manifestations, mainly involving the nervous system, the eye and skin, and a predisposition to develop multiple benign and malignant neoplasms. Ocular diagnostic hallmarks of NF1 include optic gliomas, iris Lisch nodules, orbital and eyelid neurofibromas, eyelid café-au-lait spots. Choroidal nodules and microvascular abnormalities have recently been identified as additional NF1-related ocular manifestations. The present study was designed to describe the features and clinical significance of a new sign related to the visual apparatus in NF-1, represented by hyperpigmented spots (HSs) of the fundus oculi.

RESULTS

HSs were detected in 60 (24.1%) out of 249 patients with NF1, with a positive predictive value of 100% and a negative predictive value of 44.2%. None of the healthy subjects (150 subjects) showed the presence of HSs. HSs were visible under indirect ophthalmoscopy, ultra-wide field (UWF) pseudocolor imaging and red-only laser image, near-infrared reflectance (NIR)-OCT, but they were not appreciable on UWF green reflectance. The location and features of pigmentary lesions matched with the already studied NF1-related choroidal nodules. No significant difference was found between the group of patients (n = 60) with ocular HSs and the group of patients (n = 189) without ocular pigmented spots in terms of age, gender or severity grading of the disease. A statistically significant association was demonstrated between the presence of HSs and neurofibromas (p = 0.047), and between the presence of HSs and NF1-related retinal microvascular abnormalities (p = 0.017).

CONCLUSIONS

We described a new ocular sign represented by HSs of the fundus in NF1. The presence of HSs was not a negative prognostic factor of the disease. Following multimodal imaging, we demonstrated that HSs and choroidal nodules were consistent with the same type of lesion, and simple indirect ophthalmoscopy allowed for screening of HSs in NF1.

Implications of melanin binding in ocular drug delivery

Pigmented ocular tissues contain melanin within the intracellular melanosomes. Drugs bind to melanin at varying extent that ranges from no binding to extensive binding. Binding may lead to drug accumulation to the pigmented tissues and prolonged drug retention in the melanin containing cells. Therefore, melanin binding is an important feature that affects ocular drug delivery and biodistribution, but this topic has not been reviewed since 1998. In this review, we present current knowledge on ocular melanin, melanosomes and binding of drugs to pigmented cells and tissues. In vitro, in vivo and in silico methods in the field were critically evaluated, because the literature in this field can be confusing if the reader does not properly understand the methodological aspects. Literature analysis includes a comprehensive table of literature data on melanin binding of drugs. Furthermore, we aimed to give some insights beyond the current literature by making a chemical structure based classification model for melanin binding of drugs and kinetic simulations that revealed significant interplay between melanin binding and drug permeability across the melanosomal and plasma membranes. Overall, more mechanistic and systematic research is needed before the impact of melanin binding on ocular drug delivery can be properly understood and predicted.

The Functional Amyloid Orb2A Binds to Lipid Membranes

Lipid membranes interact with and influence the aggregation of many amyloid-forming proteins. Orb2 is a cytoplasmic polyadenylation element-binding protein homolog in Drosophila melanogaster that forms functional amyloids necessary for long-term memory. One isoform, Orb2A, has a unique N-terminus that has been shown to be important for the formation of amyloid-like aggregates and long-term memory in vivo. Orb2A is also found enriched in the synaptic membrane fraction. Our sequence and hydropathy analysis suggests that it can form an amphipathic helix, which is ideal for lipid membrane interaction. We used circular dichroism and site-directed spin labeling coupled with electron paramagnetic resonance to test the first 88 amino acids of Orb2A for lipid interaction. We show that Orb2A1-88 interacts with anionic lipid membranes using an amphipathic helix at its unique N-terminus. This interaction depends on the charge of the lipid membrane and the degree of membrane curvature. We used transmission electron microscopy and electron paramagnetic resonance to show that the presence of anionic small unilamellar vesicles inhibits amyloid fibril formation by Orb2A. This inhibition by anionic membranes could be a potential mechanism regulating Orb2A amyloid formation in vivo.

Autofluorescence imaging with near-infrared excitation:normalization by reflectance to reduce signal from choroidal fluorophores

PURPOSE

We previously developed reduced-illuminance autofluorescence imaging (RAFI) methods involving near-infrared (NIR) excitation to image melanin-based fluorophores and short-wavelength (SW) excitation to image lipofuscin-based flurophores. Here, we propose to normalize NIR-RAFI in order to increase the relative contribution of retinal pigment epithelium (RPE) fluorophores.

METHODS

Retinal imaging was performed with a standard protocol holding system parameters invariant in healthy subjects and in patients. Normalized NIR-RAFI was derived by dividing NIR-RAFI signal by NIR reflectance point-by-point after image registration.

RESULTS

Regions of RPE atrophy in Stargardt disease, AMD, retinitis pigmentosa, choroideremia, and Leber congenital amaurosis as defined by low signal on SW-RAFI could correspond to a wide range of signal on NIR-RAFI depending on the contribution from the choroidal component. Retinal pigment epithelium atrophy tended to always correspond to high signal on NIR reflectance. Normalizing NIR-RAFI reduced the choroidal component of the signal in regions of atrophy. Quantitative evaluation of RPE atrophy area showed no significant differences between SW-RAFI and normalized NIR-RAFI.

CONCLUSIONS

Imaging of RPE atrophy using lipofuscin-based AF imaging has become the gold standard. However, this technique involves bright SW lights that are uncomfortable and may accelerate the rate of disease progression in vulnerable retinas. The NIR-RAFI method developed here is a melanin-based alternative that is not absorbed by opsins and bisretinoid moieties, and is comfortable to view. Further development of this method may result in a nonmydriatic and comfortable imaging method to quantify RPE atrophy extent and its expansion rate.

Lysophospholipid-containing membranes modulate the fibril formation of the repeat domain of a human functional amyloid, pmel17

Pmel17 is an important protein for pigmentation in human skin and eyes. Proteolytic fragments from Pmel17 form fibrils upon which melanin is deposited in melanosomes. The repeat domain (RPT) derived from Pmel17 only forms fibrils under acidic melanosomal conditions. Here, we examined the effects of lipids on RPT aggregation to explore whether intramelanosomal vesicles can facilitate fibrillogenesis. Using transmission electron microscopy, circular dichroism, and fluorescence spectroscopy, we monitored fibril formation at the ultrastructural, secondary conformational, and local levels, respectively. Phospholipid vesicles and lysophospholipid (lysolipid) micelles were employed as membrane mimics. The surfactant-like lysolipids are particularly pertinent due to their high content in melanosomal membranes. Interestingly, RPT aggregation kinetics were influenced only by lysolipid-containing phospholipid vesicles. While both vesicles containing either anionic lysophosphatidylglycerol (LPG) or zwitterionic lysophosphatidylcholine (LPC) stimulate aggregation, LPG exerted a greater effect on reducing the apparent nucleation time. A detailed comparison showed distinct behaviors of LPG versus LPC monomers and micelles plausibly originating from their headgroup hydrogen bonding capabilities. Acceleration and retardation of aggregation were observed for LPG monomers and micelles, respectively. Because a specific interaction between LPG and RPT was identified by intrinsic W423 fluorescence and induced α-helical structure, it is inferred that binding of LPG near the C-terminal amyloid core initiates intermolecular association, whereas stabilization of α-helical conformation inhibits β-sheet formation. Contrastingly, LPC promotes RPT aggregation at both submicellar and micellar concentrations via non-specific binding with undetectable secondary structural change. Our findings suggest that protein-lysolipid interactions within melanosomes may regulate amyloid formation in vivo.

Genetic modulation of the iris transillumination defect: a systems genetics analysis using the expanded family of BXD glaucoma strains

We investigated the contributions of Tyrp1 and Gpnmb to the iris transillumination defect (TID) in five age cohorts of BXD mice. Using systems genetics, we also evaluated the role of other known pigmentation genes (PGs). Mapping studies indicate that Tyrp1 contributes to the phenotype at all ages, yet the TID maps to Gpnmb only in the oldest cohort. Composite interval mapping reveals secondary loci viz. Oca2, Myo5a, Prkcz, and Zbtb20 that modulate the phenotype in the age groups up to 10–13 months. The contributions of Tyrp1 and Gpnmb were highly significant in all age cohorts. Moreover, in young mice, all six gene candidates had substantial interactions in our model. Our model accounted for 71–88% of the explained variance of the TID phenotype across the age bins. These results demonstrate that along with Tyrp1 and Gpnmb, Oca2, Myo5a, Prkcz, and Zbtb20 modulate the TID in an age-dependent manner.

Insights into melanosomes and melanin from some interesting spatial and temporal properties

Melanosomes are organelles found in a wide variety of tissues throughout the animal kingdom and exhibit a range of different shapes: spheres of up to approximately 1 mum diameters and ellipsoids with lengths of up to approximately 2 mum and varying aspect ratios. The functions of melanosomes include photoprotection, mitigation of the effects of reactive oxygen species, and metal chelation. The melanosome contains a variety of biological molecules, e.g., proteins and lipids, but the dominant constituent is the pigment melanin, and the functions ascribed to melanosomes are uniquely enabled by the chemical properties of the melanins they contain. In the past decade, there has been significant progress in understanding melanins and their impact on human health. While the molecular details of melanin production and how the pigment is organized within the melanosome determine its properties and biological functions, the physical and chemical properties of the surface of the melanosome are central to their range of ascribed functions. Surprisingly, few studies designed to probe this biological surface have been reported. In this article, we discuss recent work using surface-sensitive analytic, spectroscopic, and imaging techniques to examine the structural and chemical properties of many types of natural pigments: sepia melanin granules, human and bovine ocular melanosomes, human hair melanosomes, and neuromelanin. N 2 adsorption/desorption measurements and atomic force microscopy provide novel insights into surface morphology. The chemical properties of the melanins present on the surface are revealed by X-ray photoelectron spectroscopy and photoemission electron microscopy. These technologies are also applied to elucidate changes in surface properties that occur with aging. Specifically, studies of the surface properties of human retinal pigment epithelium melanosomes as a function of age are stimulating the development of models for their age-dependent behaviors. The article concludes with a brief discussion of important unanswered research questions in this field.

Quantification of naturally occurring pyrrole acids in melanosomes

Three naturally occurring pyrrole acids were found in Sepia, human black hair, and bovine choroid and iris melanosomes using high-performance liquid chromatography and mass spectrometry--pyrrole-2,3-dicarboxylic acid (PDCA), pyrrole-2,3,5-tricarboxylic acid (PTCA) and pyrrole-2,3,4,5-tetracarboxylic acid (PTeCA). PDCA and PTCA are common markers quantified from oxidative degradation of eumelanins. Using standards, the amounts of naturally occurring PDCA and PTCA were determined and compared to those obtained following peroxide oxidation of the same samples. Because the naturally occurring acids are water soluble, these results indicate that care must be exercised when comparing PDCA and PTCA yields from the degradation analyses of melanins isolated and prepared by different methods. This work also establishes that PTeCA is a naturally occurring pyrrole acid in melanosomes.

Current understanding of the binding sites, capacity, affinity, and biological significance of metals in melanin

Metal chelation is often invoked as one of the main biological functions of melanin. In order to understand the interaction between metals and melanin, extensive studies have been carried out to determine the nature of the metal binding sites, binding capacity, and affinity. These data are central to efforts aimed at elucidating the role metal binding plays in determining the physical, structural, biological, and photochemical properties of melanin. This article examines the current state of understanding of this field.

Iris phenotypes and pigment dispersion caused by genes influencing pigmentation

Spontaneous mutations altering mouse coat colors have been a classic resource for discovery of numerous molecular pathways. Although often overlooked, the mouse iris is also densely pigmented and easily observed, thus representing a similarly powerful opportunity for studying pigment cell biology. Here, we present an analysis of iris phenotypes among 16 mouse strains with mutations influencing melanosomes. Many of these strains exhibit biologically and medically relevant phenotypes, including pigment dispersion, a common feature of several human ocular diseases. Pigment dispersion was identified in several strains with mutant alleles known to influence melanosomes, including beige, light, and vitiligo. Pigment dispersion was also detected in the recently arising spontaneous coat color variant, nm2798. We have identified the nm2798 mutation as a missense mutation in the Dct gene, an identical re-occurrence of the slaty light mutation. These results suggest that dysregulated events of melanosomes can be potent contributors to the pigment dispersion phenotype. Combined, these findings illustrate the utility of studying iris phenotypes as a means of discovering new pathways, and re-linking old ones, to processes of pigmented cells in health and disease.