Prevalence of Congenital Color Vision Deficiency in Southern Taiwan and Detection of Female Carriers by Visual Pigment Gene Analysis

This study aimed to investigate the prevalence of color vision deficiencies (CVDs) and determine whether carriers could be detected by analyzing the visual pigment genes. Materials and Methods: The data of students who underwent routine CVD screening using the Ishihara color test in Kaohsiung, Southern Taiwan were analyzed. Furthermore, the DNA samples of 80 randomly selected females and four obligate carriers were analyzed. The most upstream genes, downstream genes, and the most downstream genes in the red/green pigment gene arrays were amplified separately using polymerase chain reaction (PCR), and exon 5 of each gene was analyzed. The prevalence of congenital red-green CVD in this study was 3.46% in males and 0.14% in females. The PCR analysis of the first gene, downstream gene, and last gene revealed normal patterns in 73 normal cases. Seven unusual patterns were detected in two proton carriers and five deutan carriers. Among the randomly selected females, 8.8% (7/80) were CVD carriers. The prevalence of CVD among male Taiwanese students in this study was 3.46%. Female carriers of congenital CVD can be identified by molecular analysis of the visual pigment genes. The proportion of CVD carriers among the randomly selected females was 8.8%, which was slightly higher than expected and further studies are warranted.

Assessment of a Small Molecule Synthetic Lignan in Enhancing Oxidative Balance and Decreasing Lipid Accumulation in Human Retinal Pigment Epithelia

Visual function depends on the intimate structural, functional and metabolic interactions between the retinal pigment epithelium (RPE) and the neural retina. The daily phagocytosis of the photoreceptor outer segment tips by the overlaying RPE provides essential nutrients for the RPE itself and photoreceptors through intricate metabolic synergy. Age-related retinal changes are often characterized by metabolic dysregulation contributing to increased lipid accumulation and peroxidation as well as the release of proinflammatory cytokines. LGM2605 is a synthetic lignan secoisolariciresinol diglucoside (SDG) with free radical scavenging, antioxidant and anti-inflammatory properties demonstrated in diverse in vitro and in vivo inflammatory disease models. In these studies, we tested the hypothesis that LGM2605 may be an attractive small-scale therapeutic that protects RPE against inflammation and restores its metabolic capacity under lipid overload. Using an in vitro model in which loss of the autophagy protein, LC3B, results in defective phagosome degradation and metabolic dysregulation, we show that lipid overload results in increased gasdermin cleavage, IL-1 β release, lipid accumulation and decreased oxidative capacity. The addition of LGM2605 resulted in enhanced mitochondrial capacity, decreased lipid accumulation and amelioration of IL-1 β release in a model of defective lipid homeostasis. Collectively, these studies suggest that lipid overload decreases mitochondrial function and increases the inflammatory response, with LGM2605 acting as a protective agent.

Connective tissue growth factor promotes retinal pigment epithelium mesenchymal transition via the PI3K/AKT signaling pathway

Proliferative vitreoretinopathy (PVR) is a disease leading to the formation of contractile preretinal membranes (PRMs) and is one of the leading causes of blindness. Connective tissue growth factor (CTGF) has been identified as a possible key determinant of progressive tissue fibrosis and excessive scarring. Therefore, the present study investigated the role and mechanism of action of CTGF in PVR. Immunohistochemical staining was performed to detect the expression of CTGF, fibronectin and collagen type III in PRMs from patients with PVR. The effects and mechanisms of recombinant human CTGF and its upstream regulator, TGF‑β1, on epithelial‑mesenchymal transition (EMT) and the synthesis of extracellular matrix (ECM) by retinal pigment epithelium (RPE) cells were investigated using reverse transcription‑quantitative PCR, western blotting and a [3H]proline incorporation assay. The data indicated that CTGF, fibronectin and collagen type III were highly expressed in PRMs. In vitro, CTGF significantly decreased the expression of the epithelial markers ZO‑1 and E‑cadherin and increased that of the mesenchymal markers fibronectin, N‑cadherin and α‑smooth muscle actin in a concentration‑dependent manner. Furthermore, the expression of the ECM protein collagen type III was upregulated by CTGF. However, the trends in expression for the above‑mentioned markers were reversed after knocking down CTGF. The incorporation of [3H]proline into RPE cells was also increased by CTGF. In addition, 8‑Bromoadenosine cAMP inhibited CTGF‑stimulated collagen synthesis and transient transfection of RPE cells with a CTGF antisense oligonucleotide inhibited TGF‑β1‑induced collagen synthesis. The phosphorylation of PI3K and AKT in RPE cells was promoted by CTGF and TGF‑β1 and the latter promoted the expression of CTGF. The results of the present study indicated that CTGF may promote EMT and ECM synthesis in PVR via the PI3K/AKT signaling pathway and suggested that targeting CTGF signaling may have a therapeutic or preventative effect on PVR.

Colour perception in twins: individual variation beyond common genetic inheritance

BACKGROUND

The twin method was used to examine the genotype/phenotype relationship in colour vision, by determining concordance in colour perception within pairs of monozygotic (MZ) twins and dizygotic (DZ) twins. For MZ twins, whose photopigments are genetically identical, higher concordance in colour perception was expected; conversely, differences within each MZ pair would indicate a non-genetic contribution.

METHOD

Ratings of dissimilarity between successively presented colours were elicited from four MZ and three DZ twin pairs. A non-twin sibling pair and three unrelated normal trichromats were enrolled, for comparison. Concordance for each twin (sibling) pair was estimated by Spearman correlations (rs) between data matrices and by Procrustes distances (gl) between colour spaces, reconstructed from individual data using multidimensional scaling (MDS).

RESULTS

For MZ twins, rs) values (0.94-0.97) were comparable to intra-individual variability and significantly higher than those for DZ twins and siblings (0.72-0.82). Further, colour spaces for MZ co-twins were less discordant, with gl values (0.008-0.029) lower than for DZ co-twins (0.073-0.079) and siblings (0.052). Finally, concordances among all pairs of subjects were summarised by a geometrical 'subject space': the mean distance between MZ co-twins was 29 per cent of that between DZ and sibling pairs.

DISCUSSION

Lower concordance rates in DZ twins and siblings can be attributed to differences in the inherited arrays of photopigment genes. The high concordance for MZ twins is in line with their shared photopigment genotype, placing an upper limit on contributions to discordance from possible individual variations in non-genetic factors. Potential photoreceptor, ocular and cognitive sources of inter-twin variation are discussed.

The SNARE regulator Complexin3 is a target of the cone circadian clock

BMAL1 is a core component of the mammalian circadian clockwork. Removal of BMAL1 from the retina significantly affects visual information processing in both rod and cone pathways. To identify potential pathways and/or molecules through which BMAL1 alters signal transmission at the cone pedicle, we performed an RNA-seq differential expression analysis between cone-specific Bmal1 knockout cones (cone-Bmal1-/- ) and wild-type (WT) cones. We found 88 genes differentially expressed. Among these, Complexin3 (Cplx3), a SNARE regulator at ribbon synapses, was downregulated fivefold in the mutant cones. The purpose of this work was to determine whether BMAL1 and/or the cone clock controls CPLX3 protein expression at cone pedicles. We found that CPLX3 expression level was decreased twofold in cone-Bmal1-/- cones. Furthermore, CPLX3 expression was downregulated at night compared to the day in WT cones but remained constitutively low in mutant cones both day and night. The transcript and protein expression levels of Cplx4-the other complexin expressed in cones-were similar in WT and mutant cones; in WT cones, CPLX4 protein level did not change with the time of day. In silico analysis revealed four potential BMAL1:CLOCK binding sites upstream from exon one of Cplx3 and none upstream of exon one of Cplx4. Our results suggest that CPLX3 expression is regulated at the transcriptional level by the cone clock. The modulation of CPLX3 may be a mechanism by which the clock controls the cone synaptic transfer function to second-order cells and thereby impacts retinal signal processing during the day/night cycle.

Evolution of Insect Color Vision: From Spectral Sensitivity to Visual Ecology

Color vision is widespread among insects but varies among species, depending on the spectral sensitivities and interplay of the participating photoreceptors. The spectral sensitivity of a photoreceptor is principally determined by the absorption spectrum of the expressed visual pigment, but it can be modified by various optical and electrophysiological factors. For example, screening and filtering pigments, rhabdom waveguide properties, retinal structure, and neural processing all influence the perceived color signal. We review the diversity in compound eye structure, visual pigments, photoreceptor physiology, and visual ecology of insects. Based on an overview of the current information about the spectral sensitivities of insect photoreceptors, covering 221 species in 13 insect orders, we discuss the evolution of color vision and highlight present knowledge gaps and promising future research directions in the field.

Origin and adaptation of green-sensitive (RH2) pigments in vertebrates

One of the critical times for the survival of animals is twilight where the most abundant visible lights are between 400 and 550 nanometres (nm). Green-sensitive RH2 pigments help nonmammalian vertebrate species to better discriminate wavelengths in this blue-green region. Here, evaluation of the wavelengths of maximal absorption (λmax s) of genetically engineered RH2 pigments representing 13 critical stages of vertebrate evolution revealed that the RH2 pigment of the most recent common ancestor of vertebrates had a λmax of 503 nm, while the 12 ancestral pigments exhibited an expanded range in λmax s between 474 and 524 nm, and present-day RH2 pigments have further expanded the range to ~ 450-530 nm. During vertebrate evolution, eight out of the 16 significant λmax shifts (or |Δλmax | ≥ 10 nm) of RH2 pigments identified were fully explained by the repeated mutations E122Q (twice), Q122E (thrice) and M207L (twice), and A292S (once). Our data indicated that the highly variable λmax s of teleost RH2 pigments arose from gene duplications followed by accelerated amino acid substitution.

N-Terminomics identifies HtrA1 cleavage of thrombospondin-1 with generation of a proangiogenic fragment in the polarized retinal pigment epithelial cell model of age-related macular degeneration

Age-related macular degeneration (AMD) is the leading cause of irreversible blindness in the elderly population. Variants in the HTRA1-ARMS2 locus have been linked to increased AMD risk. In the present study we investigated the impact of elevated HtrA1 levels on the retina pigment epithelial (RPE) secretome using a polarized culture system. Upregulation of HtrA1 alters the abundance of key proteins involved in angiogenesis and extracellular matrix remodeling. Thrombospondin-1, an angiogenesis modulator, was identified as a substrate for HtrA1 using terminal amine isotope labeling of substrates in conjunction with HtrA1 specificity profiling. HtrA1 cleavage of thrombospondin-1 was further corroborated by in vitro cleavage assays and targeted proteomics together with small molecule inhibition of HtrA1. While thrombospondin-1 is anti-angiogenic, the proteolytically released N-terminal fragment promotes the formation of tube-like structure by endothelial cells. Taken together, our findings suggest a mechanism by which increased levels of HtrA1 may contribute to AMD pathogenesis. The proteomic data has been deposited to the ProteomeXchange Consortium via the PRIDE partner repository with the data set identifier. For quantitative secretome analysis, project accession: PXD007691, username: reviewer45093@ebi.ac.uk, password: 1FUpS6Yq. For TAILS analysis, project accession: PXD007139, username: reviewer76731@ebi.ac.uk, password: sNbMp7xK.

Spectral Tuning Mechanism of Primate Blue-sensitive Visual Pigment Elucidated by FTIR Spectroscopy

Protein-bound water molecules are essential for the structure and function of many membrane proteins, including G-protein-coupled receptors (GPCRs). Our prior work focused on studying the primate green- (MG) and red- (MR) sensitive visual pigments using low-temperature Fourier transform infrared (FTIR) spectroscopy, which revealed protein-bound waters in both visual pigments. Although the internal waters are located in the vicinity of both the retinal Schiff base and retinal β-ionone ring, only the latter showed differences between MG and MR, which suggests their role in color tuning. Here, we report FTIR spectra of primate blue-sensitive pigment (MB) in the entire mid-IR region, which reveal the presence of internal waters that possess unique water vibrational signals that are reminiscent of a water cluster. These vibrational signals of the waters are influenced by mutations at position Glu113 and Trp265 in Rh, which suggest that these waters are situated between these two residues. Because Tyr265 is the key residue for achieving the spectral blue-shift in λmax of MB, we propose that these waters are responsible for the increase in polarity toward the retinal Schiff base, which leads to the localization of the positive charge in the Schiff base and consequently causes the blue-shift of λmax.

Alouatta trichromatic color vision: cone spectra and physiological responses studied with microspectrophotometry and single unit retinal electrophysiology

The howler monkeys (Alouatta sp.) are the only New World primates to exhibit routine trichromacy. Both males and females have three cone photopigments. However, in contrast to Old World monkeys, Alouatta has a locus control region upstream of each opsin gene on the X-chromosome and this might influence the retinal organization underlying its color vision. Post-mortem microspectrophotometry (MSP) was performed on the retinae of two male Alouatta to obtain rod and cone spectral sensitivities. The MSP data were consistent with only a single opsin being expressed in each cone and electrophysiological data were consistent with this primate expressing full trichromacy. To study the physiological organization of the retina underlying Alouatta trichromacy, we recorded from retinal ganglion cells of the same animals used for MSP measurements with a variety of achromatic and chromatic stimulus protocols. We found MC cells and PC cells in the Alouatta retina with similar properties to those previously found in the retina of other trichromatic primates. MC cells showed strong phasic responses to luminance changes and little response to chromatic pulses. PC cells showed strong tonic response to chromatic changes and small tonic response to luminance changes. Responses to other stimulus protocols (flicker photometry; changing the relative phase of red and green modulated lights; temporal modulation transfer functions) were also similar to those recorded in other trichromatic primates. MC cells also showed a pronounced frequency double response to chromatic modulation, and with luminance modulation response saturation accompanied by a phase advance between 10-20 Hz, characteristic of a contrast gain mechanism. This indicates a very similar retinal organization to Old-World monkeys. Cone-specific opsin expression in the presence of a locus control region for each opsin may call into question the hypothesis that this region exclusively controls opsin expression.

Extraordinarily low evolutionary rates of short wavelength-sensitive opsin pseudogenes

Aquatic organisms such as cichlids, coelacanths, seals, and cetaceans are active in UV-blue color environments, but many of them mysteriously lost their abilities to detect these colors. The loss of these functions is a consequence of the pseudogenization of their short wavelength-sensitive (SWS1) opsin genes without gene duplication. We show that the SWS1 gene (BdenS1ψ) of the deep-sea fish, pearleye (Benthalbella dentata), became a pseudogene in a similar fashion about 130 million years ago (Mya) yet it is still transcribed. The rates of nucleotide substitution (~1.4 × 10(-9)/site/year) of the pseudogenes of these aquatic species as well as some prosimian and bat species are much smaller than the previous estimates for the globin and immunoglobulin pseudogenes.

Anion sensitivity and spectral tuning of middle- and long-wavelength-sensitive (MWS/LWS) visual pigments

The long-wavelength-sensitive (LWS) opsins form one of four classes of vertebrate cone visual pigment and exhibit peak spectral sensitivities (λ(max)) that generally range from 525 to 560 nm for rhodopsin/vitamin-A(1) photopigments. Unique amongst the opsin classes, many LWS pigments show anion sensitivity through the interaction of chloride ions with a histidine residue at site 197 (H197) to give a long-wavelength spectral shift in peak sensitivity. Although it has been shown that amino acid substitutions at five sites (180, 197, 277, 285 and 308) are useful in predicting the λ(max) values of the LWS pigment class, some species, such as the elephant shark and most marine mammals, express LWS opsins that possess λ(max) values that are not consistent with this 'five-site' rule, indicating that other interactions may be involved. This study has taken advantage of the natural mutation at the chloride-binding site in the mouse LWS pigment. Through the use of a number of mutant pigments generated by site-directed mutagenesis, a new model has been formulated that takes into account the role of charge and steric properties of the side chains of residues at sites 197 and 308 in the function of the chloride-binding site in determining the peak sensitivity of LWS photopigments.

Optical silencing of C. elegans cells with arch proton pump

BACKGROUND

Optogenetic techniques using light-driven ion channels or ion pumps for controlling excitable cells have greatly facilitated the investigation of nervous systems in vivo. A model organism, C. elegans, with its small transparent body and well-characterized neural circuits, is especially suitable for optogenetic analyses.

METHODOLOGY/PRINCIPAL FINDINGS

We describe the application of archaerhodopsin-3 (Arch), a recently reported optical neuronal silencer, to C. elegans. Arch::GFP expressed either in all neurons or body wall muscles of the entire body by means of transgenes were localized, at least partially, to the cell membrane without adverse effects, and caused locomotory paralysis of worms when illuminated by green light (550 nm). Pan-neuronal expression of Arch endowed worms with quick and sustained responsiveness to such light. Worms reliably responded to repeated periods of illumination and non-illumination, and remained paralyzed under continuous illumination for 30 seconds. Worms expressing Arch in different subsets of motor neurons exhibited distinct defects in the locomotory behavior under green light: selective silencing of A-type motor neurons affected backward movement while silencing of B-type motor neurons affected forward movement more severely. Our experiments using a heat-shock-mediated induction system also indicate that Arch becomes fully functional only 12 hours after induction and remains functional for more than 24 hour. CONCLUSIONS/SGNIFICANCE: Arch can be used for silencing neurons and muscles, and may be a useful alternative to currently widely used halorhodopsin (NpHR) in optogenetic studies of C. elegans.

Spectral tuning and evolution of primate short-wavelength-sensitive visual pigments

The peak sensitivities (λ(max)) of the short-wavelength-sensitive-1 (SWS1) pigments in mammals range from the ultraviolet (UV) (360-400 nm) to the violet (400-450 nm) regions of the spectrum. In most cases, a UV or violet peak is determined by the residue present at site 86, with Phe conferring UV sensitivity (UVS) and either Ser, Tyr or Val causing a shift to violet wavelengths. In primates, however, the tuning mechanism of violet-sensitive (VS) pigments would appear to differ. In this study, we examine the tuning mechanisms of prosimian SWS1 pigments. One species, the aye-aye, possesses a pigment with Phe86 but in vitro spectral analysis reveals a VS rather than a UVS pigment. Other residues (Cys, Ser and Val) at site 86 in prosimians also gave VS pigments. Substitution at site 86 is not, therefore, the primary mechanism for the tuning of VS pigments in primates, and phylogenetic analysis indicates that substitutions at site 86 have occurred at least five times in primate evolution. The sole potential tuning site that is conserved in all primate VS pigments is Pro93, which when substituted by Thr (as found in mammalian UVS pigments) in the aye-aye pigment shifted the peak absorbance into the UV region with a λ(max) value at 371 nm. We, therefore, conclude that the tuning of VS pigments in primates depends on Pro93, not Tyr86 as in other mammals. However, it remains uncertain whether the initial event that gave rise to the VS pigment in the ancestral primate was achieved by a Thr93Pro or a Phe86Tyr substitution.

[Molecular-cellular mechanisms of retina pathology development in people of various age]

The review considers the molecular-cellular mechanisms of retina pathology in people of various age. Dysfunction of retinal cells (retinal pigment epithelium, photoreceptors, neurons) causes the development of age-related macular degeneration, retinal ischemia and a variety of hereditary diseases. This is the description of involvement of genes and signaling molecules in the dysfunction of retinal cell types. It is established that a breach of RPE65 gene expression leads to age-related macular degeneration, retinitis pigmentosa and Leber's congenital amaurosis. Mutations in the CRX gene are the cause of progressive states such as cone-rod dystrophy. In addition, more than 100 mutations in RHO have been identified, leading to different variants of retinitis pigmentosa. The involvement of TGF-(beta2 in the formation of retinal cells and the regulation of secretion of vascular endothelial growth factor VEGF, which synthesis is increased by ischemic lesions of the retina, is described.

Conserved residues in the extracellular loops of short-wavelength cone visual pigments

The role of the extracellular loop region of a short-wavelength sensitive pigment, Xenopus violet cone opsin, is investigated via computational modeling, mutagenesis, and spectroscopy. The computational models predict a complex H-bonding network that stabilizes and connects the EC2-EC3 loop and the N-terminus. Mutations that are predicted to disrupt the H-bonding network are shown to produce visual pigments that do not stably bind chromophore and exhibit properties of a misfolded protein. The potential role of a disulfide bond between two conserved Cys residues, Cys(105) in TM3 and Cys(182) in EC2, is necessary for proper folding and trafficking in VCOP. Lastly, certain residues in the EC2 loop are predicted to stabilize the formation of two antiparallel β-strands joined by a hairpin turn, which interact with the chromophore via H-bonding or van der Waals interactions. Mutations of conserved residues result in a decrease in the level of chromophore binding. These results demonstrate that the extracellular loops are crucial for the formation of this cone visual pigment. Moreover, there are significant differences in the structure and function of this region in VCOP compared to that in rhodopsin.

Evolution and mechanism of spectral tuning of blue-absorbing visual pigments in butterflies

The eyes of flower-visiting butterflies are often spectrally highly complex with multiple opsin genes generated by gene duplication, providing an interesting system for a comparative study of color vision. The Small White butterfly, Pieris rapae, has duplicated blue opsins, PrB and PrV, which are expressed in the blue (λ_max = 453 nm) and violet receptors (λ_max = 425 nm), respectively. To reveal accurate absorption profiles and the molecular basis of the spectral tuning of these visual pigments, we successfully modified our honeybee opsin expression system based on HEK293s cells, and expressed PrB and PrV, the first lepidopteran opsins ever expressed in cultured cells. We reconstituted the expressed visual pigments in vitro, and analysed them spectroscopically. Both reconstituted visual pigments had two photointerconvertible states, rhodopsin and metarhodopsin, with absorption peak wavelengths 450 nm and 485 nm for PrB and 420 nm and 482 nm for PrV. We furthermore introduced site-directed mutations to the opsins and found that two amino acid substitutions, at positions 116 and 177, were crucial for the spectral tuning. This tuning mechanism appears to be specific for invertebrates and is partially shared by other pierid and lycaenid butterfly species.

Mapping of bovine prolactin and rhodopsin genes in hybrid somatic cells

The genes encoding bovine prolactin and rhodopsin were assigned to syntenic groups on the basis of hybridization of DNA from a panel of bovine-hamster hybrid somatic cell lines with cloned prolactin and rhodopsin gene probes. Prolactin was found to be syntenic with previously mapped glyoxalase, BoLA and 21-hydroxylase genes, establishing a syntenic conservation with human chromosome 6. The presence of bovine rhodopsin sequences among the various hybrid cell lines was not concordant with any gene previously assigned to one of the 23 defined autosomal syntenic groups. Thus, rhodopsin marks a new bovine syntenic group, U24, leaving only five cattle autosomes unmarked by at least one biochemical or molecular marker.

Photoreceptive systems in ascidians

The brain vesicle of the tadpole larva of ascidians, simple basal chordates, contains an eye-spot (ocellus), which is responsible for the photic swimming behavior. Ascidian adults also exhibit several types of light-responsive behaviors. Molecular phylogenetic studies have suggested that ascidians are the closest living relatives of vertebrates, and therefore, understanding the photoreceptive systems in ascidians is a key to uncover the origin and evolution of the vertebrate eyes. The ocellus of the ascidian larva has ciliary photoreceptors resembling those of the retina and pineal eye of vertebrates. Recent studies have indicated that the ascidian larva has phototransduction and visual cycle systems similar to those of vertebrate eyes. Comparative studies on photoreceptor systems between ascidians and vertebrates provide us clues to reconstructing the evolutionary pathway leading to the lateral and median eyes of vertebrates.

Signaling properties of a short-wave cone visual pigment and its role in phototransduction

Although visual pigments play key structural and functional roles in photoreceptors, the relationship between the properties of mammalian cone pigments and those of mammalian cones is not well understood. We generated transgenic mice with rods expressing mouse short-wave cone opsin (S-opsin) to test whether cone pigment can substitute for the structural and functional roles of rhodopsin and to investigate how the biophysical and signaling properties of the short-wave cone pigment (S-pigment) contribute to the specialized function of cones. The transgenic S-opsin was targeted to rod outer segments, and formed a pigment with peak absorption at 360 nm. Expression of S-opsin in rods lacking rhodopsin (rho-/-) promoted outer segment growth and cell survival and restored their ability to respond to light while shifting their action spectrum to 355 nm. Using the spectral separation between S-pigment and rhodopsin, we found that the two pigments produced similar photoresponses. Dark noise did not increase in transgenic rods, indicating that thermal activation of S-pigment might not contribute to the low sensitivity of mouse S-cones. Using rod arrestin knock-out animals (arr1-/-), we found that the physiologically active (meta II) state of S-pigment decays 40 times faster than that of rhodopsin. Interestingly, rod arrestin was efficient in deactivating S-pigment in rods, but its deletion did not have any obvious effect on dim-flash response shutoff in cones. Furthermore, transgenic cone arrestin was not able to rescue the slow shutoff of S-pigment dim-flash response in arr1-/- rods. Thus, the connection between rod/cone arrestins and S-pigment shutoff remains unclear.

Color vision: mice see hue too

A transgenic mouse has been generated with three cone types, instead of the normal murine two. Remarkably, some of these mice use the extra cone to make trichromatic color discriminations similar to those that are the basis of human color vision.

Physiological properties of rod photoreceptor cells in green-sensitive cone pigment knock-in mice

Rod and cone photoreceptor cells that are responsible for scotopic and photopic vision, respectively, exhibit photoresponses different from each other and contain similar phototransduction proteins with distinctive molecular properties. To investigate the contribution of the different molecular properties of visual pigments to the responses of the photoreceptor cells, we have generated knock-in mice in which rod visual pigment (rhodopsin) was replaced with mouse green-sensitive cone visual pigment (mouse green). The mouse green was successfully transported to the rod outer segments, though the expression of mouse green in homozygous retina was approximately 11% of rhodopsin in wild-type retina. Single-cell recordings of wild-type and homozygous rods suggested that the flash sensitivity and the single-photon responses from mouse green were three to fourfold lower than those from rhodopsin after correction for the differences in cell volume and levels of several signal transduction proteins. Subsequent measurements using heterozygous rods expressing both mouse green and rhodopsin E122Q mutant, where these pigments in the same rod cells can be selectively irradiated due to their distinctive absorption maxima, clearly showed that the photoresponse of mouse green was threefold lower than that of rhodopsin. Noise analysis indicated that the rate of thermal activations of mouse green was 1.7 x 10(-7) s(-1), about 860-fold higher than that of rhodopsin. The increase in thermal activation of mouse green relative to that of rhodopsin results in only 4% reduction of rod photosensitivity for bright lights, but would instead be expected to severely affect the visual threshold under dim-light conditions. Therefore, the abilities of rhodopsin to generate a large single photon response and to retain high thermal stability in darkness are factors that have been necessary for the evolution of scotopic vision.

Introduction of a rod pigment aromatic cluster does not improve the structural stability of the human green cone pigment

In the course of our studies on the structure/function relationship of visual pigments, we have expressed the human green cone pigment in the baculovirus/insect cell expression system. Purification of the human green cone pigment, however, has so far proven to be severely hampered by the low thermal stability of this receptor in a solubilized state. In order to overcome this problem, we tested a variety of chemical compounds that have been described to improve protein stability in various applications. The presence of glycerol, sucrose, trehalose and lipids during extraction improved the thermal stability of the recombinant green cone pigment up to twofold. We also analyzed the effect of mutation of residues Met208, Cys212 and Cys273 into Phe in all combinations. These mutants were designed in an attempt to increase the thermal stability by replacing weakly interacting side chains in the green pigment with their counterparts in rhodopsin with strong aromatic stacking interaction. All mutants produced wild-type levels of functional pigment, but none showed an increase in thermal stability.

The Molecular Evolution of Avian Ultraviolet- and Violet-Sensitive Visual Pigments

The shortwave-sensitive SWS1 class of vertebrate visual pigments range in lambda(max) from the violet (385-445 nm) to the ultraviolet (UV) (365-355 nm), with UV-sensitivity almost certainly ancestral. In birds, however, the UV-sensitive pigments present in a number of species have evolved secondarily from an avian violet-sensitive (VS) pigment. All avian VS pigments expressed in vitro to date encode Ser86 whereas Phe86 is present in all non-avian ultraviolet sensitive (UVS) pigments. In this paper, we show by site directed mutagenesis of avian VS pigments that Ser86 is required in an avian VS pigment to maintain violet-sensitivity and therefore underlies the evolution of avian VS pigments. The major mechanism for the evolution of avian UVS pigments from an ancestral avian VS pigment is undoubtedly a Ser90Cys substitution. However, Phe86, as found in the Blue-crowned trogon, will also short-wave shift the pigeon VS pigment into the UV whereas Ala86 and Cys86 which are also found in natural avian pigments do not generate short-wave shifts when substituted into the pigeon pigment. From available data on avian SWS1 pigments, it would appear that UVS pigments have evolved on at least 5 separate occasions and utilize 2 different mechanisms for the short-wave shift.

Is photosensitive epilepsy less common in males due to variation in X chromosome photopigment genes?

Photosensitive epilepsy is less frequent among males than females. Red is the most epileptogenic color. The X-linked red pigment gene contains the polymorphism Ser180Ala; the Ser180 allele increases red sensitivity. We hypothesized that the paucity of males with photosensitive epilepsy is explained by the distribution of this sex-linked allele, and predicted photosensitive males would have a low frequency of this allele. We genotyped 35 males with photosensitive epilepsy and 84 male controls. Allele frequencies did not differ between these groups. The hypothesis was not supported, so alternate reasons for the sex bias in photosensitive epilepsy must be sought.

Regulation of photoactivation in vertebrate short wavelength visual pigments: protonation of the retinylidene Schiff base and a counterion switch

Xenopus violet cone opsin (VCOP) and its counterion variant (VCOP-D108A) are expressed in mammalian COS1 cells and regenerated with 11-cis-retinal. The phototransduction process in VCOP-D108A is investigated via cryogenic electronic spectroscopy, homology modeling, molecular dynamics, and molecular orbital theory. The VCOP-D108A variant is a UV-like pigment that displays less efficient photoactivation than the mouse short wavelength sensitive visual pigment (MUV) and photobleaching properties that are significantly different. Theoretical calculations trace the difference to the protonation state of the nearby glutamic acid residue E176, which is the homology equivalent of E181 in rhodopsin. We find that E176 is negatively charged in MUV but neutral (protonated) in VCOP-D108A. In the dark state, VCOP-D108A has an unprotonated Schiff base (SB) chromophore (lambdamax = 357 nm). Photolysis of VCOP-D108A at 70 K generates a bathochromic photostationary state (lambdamax = 380 nm). We identify two lumi intermediates, wherein the transitions from batho to the lumi intermediates are temperature- and pH-dependent. The batho intermediate decays to a more red-shifted intermediate called lumi I. The SB becomes protonated during the lumi I to lumi II transition. Decay of lumi II forms meta I, followed by the formation of meta II. We conclude that even in the absence of a primary counterion in VCOP-D108A, the SB becomes protonated during the photoactivation cascade. We examine the relevance of this observation to the counterion switch mechanism of visual pigment activation.

Emergence of novel color vision in mice engineered to express a human cone photopigment

Changes in the genes encoding sensory receptor proteins are an essential step in the evolution of new sensory capacities. In primates, trichromatic color vision evolved after changes in X chromosome-linked photopigment genes. To model this process, we studied knock-in mice that expressed a human long-wavelength-sensitive (L) cone photopigment in the form of an X-linked polymorphism. Behavioral tests demonstrated that heterozygous females, whose retinas contained both native mouse pigments and human L pigment, showed enhanced long-wavelength sensitivity and acquired a new capacity for chromatic discrimination. An inherent plasticity in the mammalian visual system thus permits the emergence of a new dimension of sensory experience based solely on gene-driven changes in receptor organization.

Spectral Tuning of Shortwave-sensitive Visual Pigments in Vertebrates†

Of the four classes of vertebrate cone visual pigments, the shortwave-sensitive SWS1 class shows some of the largest shifts in lambda(max), with values ranging in different species from 390-435 nm in the violet region of the spectrum to < 360 nm in the ultraviolet. Phylogenetic evidence indicates that the ancestral pigment most probably had a lambda(max) in the UV and that shifts between violet and UV have occurred many times during evolution. In violet-sensitive (VS) pigments, the Schiff base is protonated whereas in UV-sensitive (UVS) pigments, it is almost certainly unprotonated. The generation of VS pigments in amphibia, birds and mammals from ancestral UVS pigments must involve therefore the stabilization of protonation. Similarly, stabilization must be lost in the evolution of avian UVS pigments from a VS ancestral pigment. The key residues in the opsin protein for these shifts are at sites 86 and 90, both adjacent to the Schiff base and the counterion at Glu113. In this review, the various molecular mechanisms for the UV and violet shifts in the different vertebrate groups are presented and the changes in the opsin protein that are responsible for the spectral shifts are discussed in the context of the structural model of bovine rhodopsin.

An adaptation of the Cambridge Colour Test for use with animals

Recently, molecular biological techniques have presented new opportunities for addressing questions concerning the neural mechanisms involved in color coding, thereby rousing renewed interest in animal color vision testing. We have modified a computer-based assessment tool, the Cambridge Colour Test, to make it suitable for use with animals. Here, the validity and reliability of the testing method were evaluated using squirrel monkeys. Because the chromatic stimuli and the achromatic backgrounds of the test consist of dots that vary in lightness, the stimulus parameters can be adjusted so that animals are not able to use luminance differences to make correct discriminations. Thus, in contrast to methods used previously, this test does not require that time be spent equating the luminance of each chromatic stimulus examined. Furthermore, the computer video-display based design of the testing apparatus can be easily replicated and adapted for use with many species in a variety of settings. In the present experiments, the squirrel monkeys' behavioral results agreed with the predictions for their color vision based on genetic analysis and electroretinography (ERG) spectral sensitivity data. Repeated measurements were highly consistent. Thus, an adaptation of the Cambridge Colour Test provides a valid and reliable method for testing color vision in animals.

The visual pigments of the West Indian manatee (Trichechus manatus)

Manatees are unique among the fully aquatic marine mammals in that they are herbivorous creatures, with hunting strategies restricted to grazing on sea-grasses. Since the other groups of (carnivorous) marine mammals have been found to possess various visual system adaptations to their unique visual environments, it was of interest to investigate the visual capability of the manatee. Previous work, both behavioral (Griebel & Schmid, 1996), and ultrastructural (Cohen, Tucker, & Odell, 1982; unpublished work cited by Griebel & Peichl, 2003), has suggested that manatees have the dichromatic color vision typical of diurnal mammals. This study uses molecular techniques to investigate the cone visual pigments of the manatee. The aim was to clone and sequence cone opsins from the retina, and, if possible, express and reconstitute functional visual pigments to perform spectral analysis. Both LWS and SWS cone opsins were cloned and sequenced from manatee retinae, which, upon expression and spectral analysis, had lambda(max) values of 555 and 410 nm, respectively. The expression of both the LWS and SWS cone opsin in the manatee retina is unique as both pinnipeds and cetaceans only express a cone LWS opsin.

Novel form of a single X-linked visual pigment gene in a unique dichromatic color-vision defect

In normal trichromats, the long- (L) and middle-wavelength-sensitive (M) pigment genes are arranged in a head-to-tandem array on the X chromosome. Two amino acids at positions 277 and 285, encoded by exon 5 of the L and M genes, respectively, are essential for the spectral difference between L and M pigments whose spectral peaks are at approximately 560 and 530 nm. Intragenic or intergenic unequal crossing-over commonly occurs between the highly homologous L and M genes, resulting in red-green color vision deficiencies. The dichromacy is usually associated with a single L gene for deuteranopia or a single 5' L-M 3' hybrid gene with M-gene exon 5 for protanopia. We clinically diagnosed a total of 88 male dichromats using a Nagel model I anomaloscope, which included one unclassified subject in addition to 31 protanopes and 56 deuteranopes. The objective of this study was to characterize the phenotype of the subject and to determine the genotype of his X-linked pigment genes. The subject accepted not only any red-green mixture but also an extended yellow-scale range at each matching point (i.e. 20 to 32 scale units at the green primary and 3.5 to 6 scale units at the red primary). The slopes of regression lines were in the range of -0.34 to -0.23, while the mean slopes for the protanopes and deuteranopes were -0.38 and -0.01, respectively. Spectral sensitivity tests showed that the subject's curve was shifted between the protanope and deuteranope curves. Molecular analysis revealed a novel form of a single pigment gene with a unique arrangement of exon 5 (Y277 from the L gene and A285 from the M gene). The predicted lambdamax (541 to 546 nm) of the unique pigment was closer to the M than to the L pigment. Our outcome suggests that intragenic unequal crossing-over may have occurred between amino acid positions 279 and 283.

Mutually exclusive expression of the L and M pigment genes in the human retinoblastoma cell line WERI: Resetting by cell division

The key steps in the evolution of full trichromatic color vision in primates include duplication of the ancestral pigment gene to form the L and M pigment gene array on the X chromosome, mutually exclusive expression of the L and M pigment genes in cone photoreceptors, and formation of a retinal mosaic with randomly distributed L and M cones. Previous work using transgenic mice has indicated that a locus control region adjacent to this array of genes plays an important role in their mutually exclusive expression in respective cone cells (Smallwood et al., 2002). However, the mechanism by which this is accomplished is unknown. We searched for a cellular model system to investigate the mechanism of this mutually exclusive expression. We previously showed that the undifferentiated human retinoblastoma cell line WERI expresses L and M cone opsin but not rod opsin genes. We now show that WERI cells express the L and M pigment genes in a mutually exclusive manner, in that either L or M pigment mRNA is expressed in a single cell. Importantly, clonal analysis showed that single WERI cells that express either L or M generate, upon cell division produce, a mixed population of L- or M-expressing cells. These results indicate, first, that cell division resets L or M pigment gene expression, most likely due to disassembly and reassembly of LCR-promoter DNA-protein complexes during cell division. Second, a retinal mosaic with near-random distribution of L and M cones may have been generated automatically after duplication of the ancestral gene to form the L and M pigment genes. Third, determination of L and M cone identity may not require external molecular cues during differentiation, and is consistent with the idea that L and M cones are not intrinsically different.

L and M cone proportions in polymorphic New World monkeys

Platyrrhine monkeys typically have only a single X-chromosome opsin gene. Alleles of this gene code for multiple versions of middle- to long-wavelength cone photopigments. X-chromosome inactivation provides heterozygous females with a retinal mosaic of cones containing either of two types of M and L pigment, thus establishing the photopigment basis for trichromatic color vision. This study examined the proportions of L and M cones created by this process. For that purpose, electroretinogram flicker photometry was used to obtain complete spectral sensitivity functions from 60 heterozygous female monkeys drawn from seven genera of platyrrhine monkeys. To obtain estimates of cone proportions, these functions were subsequently fit with linear combinations of L and M cone fundamentals that were derived from similar recordings made on conspecific animals having only one type of M/L pigment. Consistent with a random X-chromosome inactivation process, the average L:M cone weighting across the sample was close to unity. At the same time, there were significant individual variations in L:M cone proportions. The genesis of this variation and its implications for seeing are discussed.

Bone morphogenetic protein 7 increases chick photoreceptor outer segment initiation

PURPOSE

The purpose of this study was to investigate the regulation of photoreceptor differentiation and outer segment elongation by the growth factor BMP7.

METHODS

Dissociated low-density embryonic day 6 (E6) chick retinal cultures were grown for 6 days in the presence of BMP7, other members of the TGF-beta family of growth factors, or control vehicle. Cultured cells were characterized using microscopy, immunocytochemistry, and RT-PCR. Antibodies against visinin and GABA were used to distinguish photoreceptors from nonphotoreceptor cells, and monoclonal antibodies rhodopsin (rho) 4D2, OS-2, and COS-1 were used to distinguish subpopulations of cones and rods. RT-PCR was used to investigate mRNAs encoding visual pigments.

RESULTS

Photoreceptors treated with BMP7 initiated outer segment elongation more frequently than photoreceptors in control cultures. The effect on outer segment initiation was confined to rods and to green opsin-expressing cones and appeared not to involve an increase in outer segment length. BMP7 did not appear to affect the survival, proliferation, or differentiation of progenitors or the fate of photoreceptors or amacrine cells in vitro. BMP5 and GDF5 showed weaker stimulatory effects than BMP7 on outer segment formation, whereas activin, BMP2, and BMP4 inhibited visual pigment expression and outer segment formation, and BMP6 had no detectable effects.

CONCLUSIONS

BMP7 must be added to the list of candidate molecules capable of stimulating outer segment formation.

Protan color vision deficiency with a unique order of green-red as the first two genes of a visual pigment array

Normal visual pigment gene arrays on the human X chromosome have a red gene at the first and a green gene at the second positions. More than half of the arrays have additional green genes downstream, but only the first two genes of the array are likely to be expressed in the retina. An array consisting of four genes in two Japanese participants, A121 and A447, was detected either by pulsed field gel electrophoresis and subsequent Southern hybridization or by single nucleotide primer extension reaction. In both participants, the first gene of the array was green, downstream genes were red and green, and the fourth gene was green. The red gene was determined to be at the second position by comparison of polymorphic sites among the intergenic regions that had been amplified by long-range PCR. Such an array with a reverse normal order of pigment genes, green-red as the first two, has never been reported before. They were expected to have normal color vision but showed protan deficiency (protanomaly), a phenotype lacking the red pigment. The red gene had no mutations in the exons and exon/intron boundaries, but had an A-71C substitution in the promoter in both participants.

Generation of knock-in mice carrying third cones with spectral sensitivity different from S and L cones

Red-green color vision in primates is unique in the sense that it is mediated by two photoreceptor cells that are indistinguishable in all aspects except for their visual pigments. In order to generate an animal model for investigation of the interaction between red-green inputs at the molecular level, we applied knock-in technology and X-chromosome inactivation machinery to make a mouse model with cone cells possessing visual pigments with different spectral sensitivities. We introduced a S308A point mutation into the Green opsin gene allele on the X-chromosome. This manipulation generated a 24 nm red-shift of absorption maximum in the cone pigment with negligible functional differences in other molecular properties. Amplitudes of responses in ERG and ganglion cell recordings of homozygotes were similar to those of wild-types, although the spectral sensitivities differed. Heterozygotes showed variable spectral sensitivities of ganglion cell responses due to the different integration of the native and the S308A cone inputs on the dendritic fields. In situ hybridization experiments showed that cone cells with respective pigments formed patch-like clusters of specific L cone-types, approximately 30 mum in diameter, which were randomly distributed in the dorsal region of the retinas. Since the patch-like clustering was arranged by X-inactivation, such clustering could be present in the peripheral retinas of New World monkeys with polymorphic L pigments, indicating that our mice would be a suitable model to study evolution of the mammalian color vision system.

Cone visual pigments of aquatic mammals

It has long been hypothesized that the visual systems of animals are evolutionarily adapted to their visual environment. The entrance many millions of years ago of mammals into the sea gave these new aquatic mammals completely novel visual surroundings with respect to light availability and predominant wavelengths. This study examines the cone opsins of marine mammals, hypothesizing, based on previous studies [Fasick et al. (1998) and Levenson & Dizon (2003)], that the deep-dwelling marine mammals would not have color vision because the pressure to maintain color vision in the dark monochromatic ocean environment has been relaxed. Short-wavelength-sensitive (SWS) and long-wavelength-sensitive (LWS) cone opsin genes from two orders (Cetacea and Sirenia) and an additional suborder (Pinnipedia) of aquatic mammals were amplified from genomic DNA (for SWS) and cDNA (for LWS) by PCR, cloned, and sequenced. All animals studied from the order Cetacea have SWS pseudogenes, whereas a representative from the order Sirenia has an intact SWS gene, for which the corresponding mRNA was found in the retina. One of the pinnipeds studied (harp seal) has an SWS pseudogene, while another species (harbor seal) appeared to have an intact SWS gene. However, no SWS cone opsin mRNA was found in the harbor seal retina, suggesting a promoter or splice site mutation preventing transcription of the gene. The LWS opsins from the different species were expressed in mammalian cells and reconstituted with the 11-cis-retinal chromophore in order to determine maximal absorption wavelengths (lambda(max)) for each. The deeper dwelling Cetacean species had blue shifted lambda(max) values compared to shallower-dwelling aquatic species. Taken together, these findings support the hypothesis that in the monochromatic oceanic habitat, the pressure to maintain color vision has been relaxed and mutations are retained in the SWS genes, resulting in pseudogenes. Additionally, LWS opsins are retained in the retina and, in deeper-dwelling animals, are blue shifted in lambda(max).

Visual pigments of marine carnivores: pinnipeds, polar bear, and sea otter

Rod and cone visual pigments of 11 marine carnivores were evaluated. Rod, middle/long-wavelength sensitive (M/L) cone, and short-wavelength sensitive (S) cone opsin (if present) sequences were obtained from retinal mRNA. Spectral sensitivity was inferred through evaluation of known spectral tuning residues. The rod pigments of all but one of the pinnipeds were similar to those of the sea otter, polar bear, and most other terrestrial carnivores with spectral peak sensitivities (lambda(max)) of 499 or 501 nm. Similarly, the M/L cone pigments of the pinnipeds, polar bear, and otter had inferred lambda(max) of 545 to 560 nm. Only the rod opsin sequence of the elephant seal had sensitivity characteristic of adaptation for vision in the marine environment, with an inferred lambda(max) of 487 nm. No evidence of S cones was found for any of the pinnipeds. The polar bear and otter had S cones with inferred lambda(max) of approximately 440 nm. Flicker-photometric ERG was additionally used to examine the in situ sensitivities of three species of pinniped. Despite the use of conditions previously shown to evoke cone responses in other mammals, no cone responses could be elicited from any of these pinnipeds. Rod photoreceptor responses for all three species were as predicted by the genetic data.

Photoreceptor layer of salmonid fishes: Transformation and loss of single cones in juvenile fish

The retinas of many vertebrates have cone photoreceptors that express multiple visual pigments. In many of these animals, including humans, the original cones to appear in the retina (which express UV or blue opsin) may change opsin types, giving rise to new spectral phenotypes. Here we used microspectrophotometry and in situ hybridization with cDNA probes to study the distribution of UV and blue cones in the retinas of four species of Pacific salmon (coho, chum, chinook, and pink salmon), in the Atlantic salmon, and in the rainbow/steelhead trout. In Pacific salmon and in the trout, all single cones express a UV opsin at hatching (lambda(max) of the visual pigment approximately 365 nm), and these cones later transform into blue cones by opsin changeover (lambda(max) of the blue visual pigment approximately 434 nm). Cones undergoing UV opsin downregulation exhibit either of two spectral absorbance profiles. The first is characterized by UV and blue absorbance peaks, with blue absorbance dominating the base of the outer segment. The second shows UV absorbance diminishing from the outer segment tip to the base, with no sign of blue absorbance. The first absorbance profile indicates a transformation from UV to blue phenotype by opsin changeover, while the second type suggests that the cone is undergoing apoptosis. These two events (transformation and loss of corner cones) are closely associated in time and progress from ventral to dorsal retina. Each double cone member contains green (lambda(max) approximately 510 nm) or red (lambda(max) approximately 565 nm) visual pigment (double cones are green/red pairs), and, like the rods (lambda(max) approximately 508 nm), do not exhibit opsin changeover. Unlike Pacific salmonids, the Atlantic salmon shows a mixture of UV and blue cones and a partial loss of corner cones at hatching. This study establishes the UV-to-blue cone transformation as a general feature of retinal growth in Pacific salmonids (genus Oncorhynchus).

Color vision pigment frequencies in wild tamarins (Saguinus spp.)

The adaptive importance of polymorphic color vision found in many New World and some prosimian primates has been discussed for many years. Polymorphism is probably maintained in part through a heterozygote advantage for trichromatic females, as such individuals are observed to have greater foraging success when selecting ripe fruits against a background of forest leaves. However, recent work also suggests there are some situations in which dichromatic individuals may have an advantage, and that variation in color vision among individuals possessing different alleles may also be significant. Alleles that confer a selective advantage to individuals are expected to occur at a higher frequency in populations than those that do not. Therefore, analyzing the frequencies of color vision alleles in wild populations can add to our understanding of the selective advantages of some color vision phenotypes over others. With this aim, we used molecular techniques to determine the frequencies of color vision alleles in 12 wild tamarin groups representing three species of the genus Saguinus. Our results show that allele frequencies are not equal, possibly reflecting different selective regimes operating on different color vision phenotypes.

Color-vision polymorphism in wild capuchins (Cebus capucinus) and spider monkeys (Ateles geoffroyi) in Costa Rica

New World monkeys are unique in exhibiting a color-vision polymorphism due to an allelic variation of the red-green visual pigment gene. This makes these monkeys excellent subjects for studying the adaptive evolution of the visual system from both molecular and ecological viewpoints. However, the allele frequencies of the pigments within a natural population have not been well investigated. As a first step toward understanding the relationship between vision and behavior, we conducted color-vision typing by analyzing fecal DNA from two wild groups of white-faced capuchin monkeys (Cebus capucinus) and one group of black-handed spider monkeys (Ateles geoffroyi) inhabiting Santa Rosa National Park of Costa Rica. All color-typed monkeys were individually identified. In C. capucinus and A. geoffroyi we found three and two pigment types, respectively, and the spectral mechanism that created one of the two Ateles pigments was found to be novel. In one Cebus group and the Ateles group, all alleles were present, whereas in the other Cebus group only two alleles were found, with one allele predominating. This was likely due to the effect of close inbreeding, indicating that wild populations can exhibit a variety of allele compositions. This result also suggests that the color-vision polymorphism can be easily distorted by natural factors, such as inbreeding, skewing the population structure.

Demonstration of a genotype-phenotype correlation in the polymorphic color vision of a non-callitrichine New World monkey, capuchin (Cebus apella)

Color-vision polymorphism in New World monkeys occurs because of an allelic polymorphism of the single-copy red-green middle-to-long-wavelength-sensitive (M/LWS) opsin gene on the X chromosome. Because color-vision types can readily be estimated from allelic types of the M/LWS opsin gene, this polymorphic system offers researchers an excellent opportunity to study the association between vision and behavior. As a prerequisite for such studies, genetically determined color-vision types must be concordant with phenotypes determined directly by behavioral criteria (e.g., by a color discrimination test). However, such correlations between genotypes and phenotypes have been studied only for callitrichine species. Using genetic, electrophysiological, and behavioral approaches, we evaluated the color vision of brown capuchin monkeys (Cebus apella), a representative non-callitrichine model animal for physiology and behavior. Two allelic M/LWS opsins-P545 and P530-were identified in the studied captive population. Females had one or both of the alleles, and males had either one. The retinal sensitivity in P530 dichromats was short-wave shifted relative to that in P545 dichromats, whereas that in P530/P545 trichromats was between the two groups. In a discrimination task using Ishihara pseudo-isochromatic plates, P530/P545 trichromats were successful in discriminating stimuli that P530 and P545 dichromats were unable to discriminate. In a food-search task, P530/P545 trichromats were able to locate red targets among green distracters as quickly as among white distracters, whereas both types of dichromats took longer. These results demonstrate the mutual consistency between genotypes and phenotypes of color vision, and provide a solid genetic basis on which the ecology and evolution of color vision can be investigated.

Polymorphism of visual pigment genes in the muriqui (Primates, Atelidae)

Colour vision varies within the family Atelidae (Primates, Platyrrhini), which consists of four genera with the following cladistic relationship: {Alouatta[Ateles (Lagothrix and Brachyteles)]}. Spider monkeys (Ateles) and woolly monkeys (Lagothrix) are characteristic of platyrrhine monkeys in possessing a colour vision polymorphism. The polymorphism results from allelic variation of the single-locus middle-to-long wavelength (M/L) cone opsin gene on the X-chromosome. The presence in the population of alleles coding for different M/L photopigments results in a variety of colour vision phenotypes. Such a polymorphism is absent in howling monkeys (Alouatta), which, alone among platyrrhines, acquired uniform trichromatic vision similar to that of Old World monkeys, apes, and humans through opsin gene duplication. Dietary and morphological similarities between howling monkeys and muriquis (Brachyteles) raise the possibility that the two genera share a similar form of colour vision, uniform trichromacy. Yet parsimony predicts that the colour vision of Brachyteles will resemble the polymorphism present in Lagothrix and Ateles. Here we test this assumption. We obtained DNA from the blood or faeces of 18 muriquis and sequenced exons 3 and 5 of the M/L opsin gene. Our results affirm the existence of a single M/L cone opsin gene in the genus Brachyteles. We detected three alleles with predicted lambdamax values of 530, 550, and 562 nm. Two females were heterozygous and are thus predicted to have different types of M/L cone pigment. We discuss the implication of this result towards understanding the evolutionary ecology of trichromatic vision.

Heritability of macular pigment: a twin study

PURPOSE

Several studies have reported higher levels of macular pigment (MP) in association with reduced risk for age-related macular degeneration (ARMD), a disease to which there is a genetic predisposition. A classic twin study was performed to determine the heritability of MP in the healthy eye.

METHODS

One hundred fifty twin pairs (76 monozygotic [MZ] and 74 dizygotic [DZ]), aged 18 to 50 years, participated. MP optical density was measured psychophysically with heterochromatic flicker photometry (HFP) and also with an imaging method involving fundus autofluorescence (AF). The covariance of MP within MZ and DZ twin pairs was compared, and genetic modeling techniques were used to determine the relative contributions of genes and environment to the variation in MP.

RESULTS

The mean MP optical density, measured using HFP, was 0.43 +/- 0.21. Using AF, the mean MP optical density, measured at 1 degrees eccentricity, was 0.28 +/- 0.11. MP optical densities correlated more highly in MZ twins than in DZ twins, according to both HFP (MZ: 0.65; DZ: 0.24) and AF (MZ: 0.83; DZ: 0.50). A model combining additive genetic and unique environmental effects provided the best fit and resulted in MP heritability estimates of 0.67 (95% CI, 0.52-0.77) and 0.85 (95% CI, 0.78-0.90) for HFP and AF readings, respectively.

CONCLUSIONS

This classic twin study demonstrates that genetic background is an important determinant of MP optical density, reflected in heritability estimates of 0.67 and 0.85 for HFP and AF measures, respectively.

Tertiary structure and spectral tuning of UV and violet pigments in vertebrates

Many vertebrate species use ultraviolet (UV) vision for such behaviors as mating, foraging, and communication. UV vision is mediated by UV-sensitive visual pigments, which have the wavelengths of maximal absorption (lambda max) at approximately 360 nm, whereas violet (or blue) vision is mediated by orthologous pigments with lambda max values of 390-440 nm. It is widely believed that amino acids in transmembrane (TM) I-III are solely responsible for the spectral tuning of these SWS1 pigments. Recent molecular analyses of SWS1 pigments, however, show that amino acids in TM IV-VII are also involved in the spectral tuning of these pigments through synergistic interactions with those in TM I-III. Comparisons of the tertiary structures of UV and violet pigments reveal that the distance between the counterion E113 in TM III and amino acid sites 87-93 in TM II is narrower for UV pigments than for violet pigments, which may restrict the access of water molecules to the Schiff base pocket and deprotonate the Schiff base nitrogen. Both mutagenesis analyses of E113Q and quantum chemical calculations strongly suggest that unprotonated Schiff base-linked chromophore is responsible for detecting UV light.

Spectral heterogeneity of honeybee ommatidia

The honeybee compound eye is equipped with ultraviolet, blue, and green receptors, which form the physiological basis of a trichromatic color vision system. We studied the distribution of the spectral receptors by localizing the three mRNAs encoding the opsins of the ultraviolet-, blue- and green-absorbing visual pigments. The expression patterns of the three opsin mRNAs demonstrated that three distinct types ommatidia exist, refuting the common assumption that the ommatidia composing the bee compound eye contain identical sets of spectral receptors. We found that type I ommatidia contain one ultraviolet and one blue receptor, type II ommatidia contain two ultraviolet receptors, and type III ommatidia have two blue receptors. All the three ommatidial types contain six green receptors. The ommatidia appear to be distributed rather randomly over the retina. The ratio of type I, II, and III ommatidia was about 44:46:10. Type III ommatidia appeared to be slightly more frequent (18%) in the anterior part of the ventral region of the eye. Retinal heterogeneity and ommatidial randomness, first clearly demonstrated in butterflies, seems to be a common design principle of the eyes of insects.

The signaling pathway in photoresponses that may be mediated by visual pigments in erythrophores of Nile tilapia

The ability to increase the synthesis or vary the distribution of pigment in response to light is an important feature of many pigment cells. Unlike other light-sensitive pigment cells, erythrophores of Nile tilapia change the direction of pigment migration depending on the peak wavelength of incident light: light near 365, 400 or 600 nm induces pigment aggregation, while dispersion occurs in response to light at 500 nm. How these phenomena are achieved is currently unknown. In the present study, the phototransduction involved in the pigment dispersion caused by light at 500 nm or the aggregation by light at 600 nm was examined, using pertussis toxin, cholera toxin, blockers of ion channels, various chemicals affecting serial steps of signaling pathways and membrane-permeable cAMP analog. The results show that light-induced bidirectional movements in tilapia erythrophores may be controlled by cytosolic cAMP levels via Gi- or Gs-type G proteins. In addition, RT-PCR demonstrated for the first time the expression of mRNAs encoding red and green opsins in tilapia fins, only where erythrophores exist. Here, we suggest that multiple cone-type visual pigments may be present in the erythrophores, and that unique cascades in which such opsins couple to Gi or Gs-type G proteins are involved in the photoresponses in these pigment cells. Thus, tilapia erythrophore system seems to be a nice model for understanding the photoresponses of cells other than visual cells.