Crystal structure of the globular domain of C1QTNF5: Implications for late-onset retinal macular degeneration.

Autosomal dominant late-onset retinal macular degeneration (L-ORMD) is caused by a single S163R mutation in the C1q and tumor necrosis factor-related protein 5 (C1QTNF5) gene. The C1QTNF5 gene encodes a secreted and membrane-associated protein involved in adhesion of retinal pigmented epithelial cells (RPE) to Bruch's membrane. The crystal structure of the trimeric globular domain of human C1QTNF5 at 1.34Å resolution reveals unique features of this novel C1q family member. It lacks a Ca²âº-binding site, displays a remarkable non-uniform distribution of surface electrostatic potentials and possesses a unique sequence (F181F182G183G184W185P186) that forms a hydrophobic plateau surrounded by Lys and Arg residues with a solvent cavity underneath. S163 forms a hydrogen bond with F182 in a hydrophobic area extending to the hydrophobic plateau. The pathogenic mutation S163R disrupts this hydrogen bonding and positively charges these hydrophobic areas. Thus, our analysis provides insights into the structural basis of the L-ORMD disease mechanism.

Retinal pigment epithelium lipofuscin proteomics.

Lipofuscin accumulates with age in the retinal pigment epithelium (RPE) in discrete granular organelles and may contribute to age-related macular degeneration. Because previous studies suggest that lipofuscin contains protein that may impact pathogenic mechanisms, we pursued proteomics analysis of lipofuscin. The composition of RPE lipofuscin and its mechanisms of pathogenesis are poorly understood in part because of the heterogeneity of isolated preparations. We purified RPE lipofuscin granules by treatment with proteinase K or SDS and showed by light, confocal, and transmission electron microscopy that the purified granules are free of extragranular material and associated membranes. Crude and purified lipofuscin preparations were quantitatively compared by (i) LC MS/MS proteomics analyses, (ii) immunoanalyses of oxidative protein modifications, (iii) amino acid analysis, (iv) HPLC of bisretinoids, and (v) assaying phototoxicity to RPE cells. From crude lipofuscin preparations 186 proteins were identified, many of which appeared to be modified. In contrast, very little protein ( approximately 2% (w/w) by amino acid analysis) and no identifiable protein were found in the purified granules, which retained full phototoxicity to cultured RPE cells. Our analyses showed that granules in purified and crude lipofuscin preparations exhibit no statistically significant differences in diameter or circularity or in the content of the bisretinoids A2E, isoA2E, and all-trans-retinal dimer-phosphatidylethanolamine. The finding that the purified granules contain minimal protein yet retain phototoxic activity suggests that RPE lipofuscin pathogenesis is largely independent of associated protein. The purified granules also exhibited oxidative protein modifications, including nitrotyrosine generated from reactive nitrogen oxide species and carboxyethylpyrrole and iso[4]levuglandin E(2) adducts generated from reactive lipid fragments. This finding is consistent with previous studies demonstrating RPE lipofuscin to be a potent generator of reactive oxygen species and supports the hypothesis that such species, including reactive fragments from lipids and retinoids, contribute to the mechanisms of RPE lipofuscin pathogenesis.

The all-trans-retinal dimer series of lipofuscin pigments in retinal pigment epithelial cells in a recessive Stargardt disease model.

The bis-retinoid pigments that accumulate in retinal pigment epithelial cells as lipofuscin are associated with inherited and age-related retinal disease. In addition to A2E and related cis isomers, we previously showed that condensation of two molecules of all-trans-retinal leads to the formation of a protonated Schiff base conjugate, all-trans-retinal dimer-phosphatidylethanolamine. Here we report the characterization of the related pigments, all-trans-retinal dimer-ethanolamine and unconjugated all-trans-retinal dimer, in human and mouse retinal pigment epithelium. In eyecups of Abcr(-/-) mice, a model of recessive Stargardt macular degeneration, all-trans-retinal dimer-phosphatidylethanolamine was increased relative to wild type and was more abundant than A2E. Total pigment of the all-trans-retinal dimer series (sum of all-trans-retinal dimer-phosphatidylethanolamine, all-trans-retinal dimer-ethanolamine, and all-trans-retinal dimer) increased with age in Abcr(-/-) mice and was modulated by amino acid variants in Rpe65. In in vitro assays, enzyme-mediated hydrolysis of all-trans-retinal dimer-phosphatidylethanolamine generated all-trans-retinal dimer-ethanolamine, and protonation/deprotonation of the Schiff base nitrogen of all-trans-retinal dimer-ethanolamine was pH-dependent. Unconjugated all-trans-retinal dimer was a more efficient generator of singlet oxygen than A2E, and the all-trans-retinal dimer series was more reactive with singlet oxygen than was A2E. By analyzing chromatographic properties and UV-visible spectra together with mass spectrometry, mono- and bis-oxygenated all-trans-retinal dimer photoproducts were detected in Abcr(-/-) mice. The latter findings are significant to an understanding of the adverse effects of retinal pigment epithelial cell lipofuscin.

Specific role of lymphatic marker podoplanin in retinal pigment epithelial cells.

Podoplanin is a small transmembrane glycoprotein widely known to be a marker for lymphatic endothelial cells. In this study, we identify a novel localization of podoplanin in the retinal pigment epithelium (RPE), a cellular monolayer critically involved in the visual process. Using a small interfering RNA (siRNA)-mediated gene silencing approach, we have also demonstrated, for the first time, that podoplanin depletion in human RPE cells leads to a marked reduction of cell aggregates and tight junctions. Additionally, the podoplanin-depleted cells also exhibit a significantly lower rate of proliferation. These data together indicate that podoplanin plays a crucial role in RPE cell functions. Further investigation on this factor may reveal novel mechanisms and therapeutic strategies for RPE-related eye diseases, such as proliferative retinopathy and age-related macular degeneration.

Retinal degeneration in choroideremia: deficiency of rab geranylgeranyl transferase.

Rab geranylgeranyl transferase (GG transferase) is a two-component enzyme that attaches 20-carbon isoprenoid groups to cysteine residues in Rab proteins, a family of guanosine triphosphate-binding proteins that regulate vesicular traffic. The mutant gene in human choroideremia, an X-linked form of retinal degeneration, encodes a protein that resembles component A of rat Rab GG transferase. Lymphoblasts from choroideremia subjects showed a marked deficiency in the activity of component A, but not component B, of Rab GG transferase. The deficiency was more pronounced when the substrate was Rab3A, a synaptic vesicle protein, than it was when the substrate was Rab1A, a protein of the endoplasmic reticulum. The data imply the existence of multiple component A proteins, one of which is missing in choroideremia.

[Specification of reflectometric measurement complex to evaluate optic density of macular pigments and concentration of phototoxic chemicals in retina].

Suggestion is to specify reflectometric measurement complex based on digital multisensor imaginery fundus-camera, in order to evaluate optic density of macular pigments and concentration of phototoxic chemicals in human retina. The authors presented a review of role played by macular pigments (zeaxanthine and lutein) in human eye viability, analyzed yellow spot as a protective light filter against harmful effects of short-wave light, increasing optic image quality in human eye and responsible for colour vision. Role of evaluating the individual density of macular pigments was stressed as a forecasting efficient criterion of occupational selection in operators performing visual tasks of detection, distance and dimensions measurement for remote objects, monitoring the changeable circumstances.

Sub-retinal drusenoid deposits in human retina: organization and composition.

We demonstrate histologically sub-retinal drusenoid debris in three aged human eyes, two of them affected by age-related maculopathy. By postmortem fundus examination, the lesions were drusen-like, i.e., they were pale spots apparently at the level of the retinal pigment epithelium (RPE). Light and electron microscopy revealed aggregations of membranous debris, the principal constituent of soft drusen, in the sub-retinal space. Immunohistochemistry and confocal microscopy confirmed the presence of molecules typically associated with drusen (positive for unesterified cholesterol, apoE, complement factor H, and vitronectin) without evidence for molecules associated with photoreceptors (lectin-binding disaccharide bridges and opsins), Müller cells (glial fibrillary acid protein and cellular retinal binding protein, CRALPB), or RPE (CRALPB). The fact that a drusenoid material, sharing some markers with conventional drusen, can occur on opposite faces of the RPE, suggests deranged polarity of normally highly vectorial processes for basolateral secretion from RPE, and that overproduction of secreted materials and direction of secretion are independently specified processes. In the future, drusenoid sub-retinal debris might be more frequently revealed by emerging high-resolution imaging techniques.

Hydrodynamic properties of porcine bestrophin-1 in Triton X-100.

Bestrophin-1 (Best-1) is an integral membrane protein, defects in which cause Best vitelliform macular dystrophy. Best-1 is proposed to function as a Cl- channel and/or a regulator of Ca++ channels. A tetrameric (or pentameric) stoichiometry has been reported for recombinant best-1. Using a combination of gel exclusion chromatography and velocity sedimentation we examined the quaternary structure of native best-1 and found that it migrates as a single species with a Stokes radius of 7.3 nm, sedimentation coefficient (S20,w) of 4.9, and partial specific volume (nu) of 0.80 ml/g. The mass of the protein-detergent complex is calculated to be 206 kDa, with the protein component estimated to be approximately 138 kDa. Given a monomeric mass of 68 kDa, we conclude that native best-1 solubilized with Triton X-100 is a homodimer. The differences between this observation and a prior report were examined by comparing recombinant best-1 with tissue derived best-1 using gel exclusion chromatography. Much of the recombinant best-1 eluted in the column void (Vo) fraction, unlike that extracted from RPE cells. We conclude that the minimal functional unit of best-1 is dimeric. This stoichiometry differs from that previously measured for recombinant best-1, suggesting that further studies are necessary to determine the stoichiometry of functional best-1 in RPE membranes.

Expression of molecular equivalent of hypothalamic-pituitary-adrenal axis in adult retinal pigment epithelium.

We have investigated expression of molecular elements of the hypothalamic-pituitary-adrenal (HPA) axis in the human retinal pigment epithelium (RPE) cells. The presence of corticotropin-releasing factor (CRF); urocortins I, II and III; CRF receptor type 1 (CRFR1); POMC and prohormone convertases 1 and 2 (PC1 and PC2) mRNAs were shown by RT-PCR; the protein products were detected by ELISA, western blot or immunocytochemical methods in an ARPE-19 cell line derived from an adult human donor. CRFR2 was below the level of detectability. The CRFR1 was functional as evidenced by CRF stimulation of cAMP and inositol triphosphate production as well as by ligand induction of transcriptional activity of inducible cis-elements cAMP responsive element (CRE), activator protein 1 responsive element (AP-1) and POMC promoter) in ARPE-19 using luciferase reporter assay. Immunoreactivities representative of CRF, pre-urocortin, CRFR1 receptor and ACTH were also detected in mouse retina by in situ immunocytochemistry. Finally, using RT-PCR, we detected expression of genes encoding four key enzymes participating in steroids synthesis (CYP11A1, CYP11B1, CYP17 and CYP21A2) and showed transformation of progesterone into cortisol-immunoreactivity in cultured ARPE-19 cells. Therefore, we suggest that ocular tissue expresses CRF-driven signalling system that follows organisational structure of the HPA axis.

Overexpression of FasL in retinal pigment epithelial cells reduces choroidal neovascularization.

Choroidal neovascularization (CNV) is responsible for the severe visual loss in age-related macular degeneration. CNV formation is considered to be due to an imbalance between pro- and antiangiogenic factors that lead to neovascular growth from the choriocapillaris into the subretinal space. To define whether FasL overexpression in retinal pigment epithelial cells (RPE) can inhibit choroidal neovascularization through Fas-FasL-mediated apoptosis, we examined the role of this pathway in a mouse model of laser-induced choroidal neovascularization. FasL was expressed in the retinal pigment epithelium of transgenic mice. Polymerase chain reaction (PCR), immunoblot, and immunohistochemistry confirmed that the transgene FasL was specifically expressed in RPE. The established laser model was used to induce choroidal neovascularization (CNV) in wild-type (WT) and transgenic mice. CNV formation was compared with respect to fluorescein angiographic leakage (at days 0 and 14 after laser injury) and histological appearance. The lesions were assessed on RPE-choroidal flatmounts after CD31-labeling and with confocal microscopy after perfusion with rhodamine-labeled concanavalin A (Con A). Apoptosis was quantified by TUNEL positivity and caspase activation. FasL mRNA and protein were highly expressed in the RPE of the transgenic mice before and after laser photocoagulation. In contrast, FasL was only weakly expressed in the RPE layer of WT C57BL/6J mice. While ruptures of Bruch's membrane and CNV formation were observed histologically two weeks after laser photocoagulation in transgenic as well as control eyes, the shape and size of CNV lesions were reduced in the transgenic mice. The area of leakage was decreased by 70% in FasL transgenic mice compared with WT mice (P<0.005). The number of TUNEL-positive cells was greater in FasL-overexpressing mice and correlated with the expression of activated caspases. Th expression of other antiangiogenic factors such as PEDF remained unchanged. The specific overexpression of FasL in RPE layer reduced CNV formation in our laser model. Our results strongly point to the FasL-Fas pathway as a potential therapeutic target in controlling pathological choroidal neovascularization.

Microscopic characterization of rat retinal progenitor cells.

A progenitor cell line was developed from a postnatal day 2 (P2) rat retina to study the effects of secreted proteins of the retinal pigment epithelium (RPE) on isolated retinal progenitor cells and markers for immature and differentiated retinal cells. Progenitor cells were cloned from a P2 explant grown in secreted proteins of cultured RPE cells. A cell line was cloned from a single progenitor cell. During the period of RPE-secreted protein stimulation the cells were transformed with the psi AE1A virus. Progenitor cells formed extensive processes when grown in serum and proliferated from the explant when grown in secreted proteins of RPE cells as demonstrated by bromodeoxyuridine (BrdU). All progenitor cells at early and late passages including a cloned cell line (D4) expressed Pax6, a transcription factor essential for eye development, which was verified by Western blotting. All cells expressed nestin, an early neuroepithelial cell marker. These two traits showed the immature character of these rat retinal progenitor cells. All cells expressed the intermediate filament protein vimentin, an intermediate filament protein. Interestingly, most progenitor cells grown in serum expressed the mature cell markers opsin, but few cells expressed glial fibrillary acidic protein (GFAP). The progenitor cells responded to proteins secreted by cultured RPE cells by forming large clusters, while cells grown in retinoic acid formed long thin processes that extended from a round cell body. These progenitor cells, following treatment with secreted proteins of the RPE, will be tested for their therapeutic effect in diseased rat retinas.

Hypoxia-inducible factor expression in human RPE cells.

BACKGROUND: Hypoxia-inducible factor (HIF) is a common transcription factor for many angiogenic proteins. Retinal pigment epithelial (RPE) cells are an important source of angiogenic factors in the retina. The expression of HIF, its regulation by proline hydroxylase (PHD) enzymes, and its downstream regulation of angiogenic factors like vascular endothelial growth factor (VEGF) and erythropoietin (EPO) was studied in RPE cells in order to determine some of the molecular mechanisms underlying ischaemic retinal disease. METHODS: ARPE-19 cells were cultured for various times under hypoxic conditions. Cellular HIF and PHD isoforms were analysed and quantified using western blot and densitometry. VEGF and EPO secreted into the media were assayed using enzyme-linked immunosorbent assay (ELISA). Messenger RNA (mRNA) was quantified using real-time quantitative reverse transcriptase polymerase chain reaction (qPCR). RNA interference was achieved using siRNA techniques. RESULTS: HIF-1 alpha was readily produced by ARPE-19 cells under hypoxia, but HIF-2 alpha and HIF-3 alpha could not be detected even after HIF-1 alpha silencing. HIF-1 alpha protein levels showed an increasing trend for the first 24 h while HIF-1 alpha mRNA levels fluctuated during this time. After 36 h HIF-1 alpha protein levels declined to baseline levels, a change that was coincident with a rise in both PHD2 and PHD3. Silencing HIF-1 alpha significantly decreased VEGF secretion. Significant production of EPO could not be detected at the protein or mRNA level. CONCLUSIONS: HIF-1 alpha appears to be the main isoform of HIF functioning in ARPE-19 cells. Under hypoxia, HIF-1 alpha levels are likely self-regulated by a feedback loop that involves both transcriptional and post-translational mechanisms. VEGF production by human RPE cells is regulated by HIF-1 alpha. EPO was not produced in significant amounts by RPE cells under hypoxic conditions, suggesting that other cells and/or transcription factors in the retina are responsible for its production.

Near-infrared autofluorescence imaging of the fundus: visualization of ocular melanin.

PURPOSE: To evaluate the origin of the near-infrared autofluorescence (AF) of the fundus detected by scanning laser ophthalmoscopy and compare the distribution of this AF with that of lipofuscin. METHODS: AF [787] fundus images (excitation [Exc.] 787 nm; emission [Emi.] >800 nm) were recorded with a confocal scanning laser ophthalmoscope, in 85 normal subjects (ages: 11-77 years) and in 25 patients with AMD and other retinal diseases. Standard AF [488] images (Exc. 488 nm; Emi. >500 nm) were recorded in a subset of the population. RESULTS: The fovea exhibits higher AF[787] than the perifovea in an area approximately 8 degrees in diameter, roughly equivalent to the area of higher RPE melanin seen in AF[488] and color images. The ratio of foveal to perifoveal AF[787] decreases with age (P < 0.0001) and is higher in subjects with light irides (P = 0.04). Higher AF[787] emanates from hyperpigmentation, from the choroidal pigment (nevi, outer layers) and from the pigment epithelium and stroma of the iris. Low AF[787] is observed in geographic atrophy particularly in subjects with light irides. CONCLUSIONS: AF[787] originates from the RPE and to a varying degree from the choroid. Oxidized melanin, or compounds closely associated with melanin, contributes substantially to this AF, but other fluorophores cannot be excluded at this stage. Confocal AF[787] imaging may provide a new modality to visualize pathologic features of the RPE and the choroid, and, together with AF[488] imaging, offers a new tool to study biological changes associated with aging of the RPE and pathology.

Localisation of SDF-1 and its receptor CXCR4 in retina and choroid of aged human eyes and in eyes with age related macular degeneration.

AIM: To examine the immunolocalisation of stromal cell derived factor 1 (SDF-1) and its receptor CXCR4 in aged control human donor eyes and eyes with age related macular degeneration (AMD). METHODS: Postmortem eyes from eight aged control donors (mean age 79.8 years) and from 12 donors with AMD (mean age 83.9 years) were cryopreserved and sectioned through the macular region. SDF-1 and CXCR4 were localised using streptavidin alkaline phosphatase immunohistochemistry and then sections were bleached. Three independent masked observers scored the immunohistochemical reaction product. RESULTS: In aged control retinas, SDF-1 immunoreactivity was most intense in inner photoreceptor matrix (IPM). CXCR4 showed a similar pattern of immunostaining, but was more prominent in inner segments of photoreceptors. In aged control and AMD choroid, SDF-1 and CXCR4 localisations were most prominent in retinal pigment epithelial (RPE) cells and choroidal stroma. However, the intensity for SDF-1 was significantly reduced in RPE (p < 0.0001) and choroidal stroma (p < 0.05) in late AMD eyes. SDF-1 and CXCR4 immunoreactivities were weak or nearly absent in disciform scars with choroidal neovascularisation (CNV). Circulating cells, presumably leucocytes, were most intensely positive for CXCR4. CONCLUSIONS: These results show that changes in distribution and relative levels of SDF-1/CXCR4 were not evident in early AMD. This suggests that SDF-1/CXCR4 may not contribute to the formation of CNV in AMD, in that CXCR4+ cells were not incorporated into neovascularisation. However, the examples of CNV studied were within disciform scars, so the authors cannot comment on the role of SDF-1/CXCR4 in the early stages of CNV formation.

Characterization of the 3' UTR sequence encoded by the AQP-1 gene in human retinal pigment epithelium.

The complete 3' UTR sequence encoded by the human aquaporin-1 gene is reported. The sequence encompassed by two cDNA clones showed, 33 nucleotides of 5' UTR sequence, a coding sequence of 807 nucleotides and 1886 nucleotides corresponding to the complete 3' UTR sequence. High similarity with 3' UTR sequences from rat and mouse counterparts was found. Northern blot analysis of several human tissues revealed a 2.8 kbp transcript. These data confirm the existence of water channels in the human retinal pigment epithelium.

Sequence and expression analysis of bovine pigment epithelium-derived factor.

PEDF, a member of the serpin superfamily of proteins related through their highly conserved folded conformation, has neurotrophic properties, including promotion of neurite-outgrowth and neuronal survival. Previously, we have purified and characterized PEDF protein from extracellular matrixes of bovine eyes. Here, we show the cDNA sequence and expression analysis of bovine PEDF. Northern analysis of RNA from bovine retinal pigment epithelium (RPE) and neural retina using a human PEDF cDNA fragment reveals expression of the PEDF gene only for RPE. Sequence analysis of a cDNA clone isolated from bovine RPE predicts a polypeptide of 416 amino acid residues that shares 88.6% and 85% amino acid identity with human and mouse PEDF, respectively. It has an N-terminal signal peptide, a consensus glycosylation site and homology with serpins including the conserved residues required for maintaining the serpin tertiary structure. Cell-free expression of the bovine PEDF cDNA by in vitro transcription and translation yields a precursor polypeptide of 45,000-Mr that immunoprecipitates with an antibody to human PEDF. Expression analysis in stably transfected baby hamster kidney cells shows that the recombinant bovine protein is secreted to the culture media as a mature 50,000-Mr protein, which induces neurite-outgrowth on retinoblastoma cells, like the naturally-occurring PEDF protein. Thus, the bovine PEDF cDNA isolated here codes for a functional soluble secreted PEDF glycoprotein.

Autofluorescence characteristics of normal foveas and reconstruction of foveal autofluorescence from limited data subsets.

PURPOSE: To develop mathematical and geometric models of the nonuniform autofluorescence (AF) patterns of foveas of normal subjects and to reconstruct these models from limited subsets of data. METHODS: Confocal scanning laser ophthalmoscope (cSLO) AF fundus images of normal maculae were obtained from both eyes of 10 middle-aged subjects. They were filtered and contrast enhanced, to obtain elliptical isobars of equal gray levels (GLs) and determine the isobars' resolutions, eccentricities, and angles of orientation. The original image data were fit with a mathematical model of elliptic quadratic polynomials in two equal zones: the center and the remaining annulus. RESULTS: The AF images segmented into nested concentric GL isobars with GLs that increased radially from the least-fluorescent center. The mean isobar resolution was 31 +/- 7 mum. The geometric eccentricity of the ellipses increased from 0.42 +/- 0.12 centrally to 0.52 +/- 0.14 peripherally (P = 0.0005), with mean axes of orientation peripherally 97.12 +/- 15.46 degrees . The model fits to the complete image data had mean absolute normalized errors ranging from 3.6% +/- 3.7% to 7.3% +/- 7.1%. The model fits to small subsets (1% to 2% of total image data) had mean absolute errors ranging from 3.7% +/- 3.8% to 7.3% +/- 7.2%. CONCLUSIONS: Normal AF fundus images show finely resolved, concentric, elliptical foveal patterns consistent with the anatomic distribution of fluorescent lipofuscin, light-attenuating macular pigment (MP), cone photopigment, and retinal pigment epithelial (RPE) pigment in the fovea. A two-zone, elliptic, quadratic polynomial model can accurately model foveal data. This model may be useful for image analysis and for automated segmentation of pathology.

Examining the proteins of functional retinal lipofuscin using proteomic analysis as a guide for understanding its origin.

PURPOSE: To elucidate the origins of biologically active retinal lipofuscin (RLF) by examining its protein composition. METHODS: Total protein and total lipid were extracted and quantified. Proteins in this lipoprotein granule were identified by limited-scale proteomic analysis using both two-dimensional (2D) gel electrophoresis and SDS-PAGE coupled with MALDI-QqToF MSMS and automated LCMSMS, respectively. RESULTS: RLF granules were 44% protein and 50% lipid. Proteomic analyses identified 41 constituent proteins. Hydrophobic proteins and several proteins specific to photoreceptors, including rhodopsin, that have not previously been reported, were identified. Extensive protein modification, especially oxidative damage, was observed. CONCLUSIONS: Proteins identified support the model that RLF accumulates in RPE cells as a result of the buildup of undigested material from the phagocytosis of photoreceptor outer segments. Perhaps oxidative damage renders some of these proteins indigestible and thus leads to the accumulation of RLF granules.

Spheramine Titan/Schering.

Spheramine, which is composed of microcarriers coated with dopamine-producing human retinal pigment epithelial cells, is being developed by Titan and Schering for the potential treatment of advanced Parkinson's disease. Phase II trials were ongoing in March 2005 and, at this time, were expected to be completed in the second half of 2006.

Dietary lutein and zeaxanthin: possible effects on visual function.

Of the many carotenoids circulating in human sera, only lutein and zeaxanthin are accumulated throughout the tissues of the eye. Within the eye, they reach their highest concentration in the central retina, where they are clinically referred to as the macula lutea. Lutein and zeaxanthin, more commonly referred to as macular pigments, may serve a variety of roles in the specialized vision of higher primates. This paper reviews recent studies investigating the influence of macular pigments on human visual performance. Such studies have offered insight into why lutein and zeaxanthin are uniquely concentrated in ocular tissues.