Investigating the Role of Rhodopsin F45L Mutation in Mouse Rod Photoreceptor Signaling and Survival

Rhodopsin is the critical receptor molecule which enables vertebrate rod photoreceptor cells to detect a single photon of light and initiate a cascade of molecular events leading to visual perception. Recently, it has been suggested that the F45L mutation in the transmembrane helix of rhodopsin disrupts its dimerization in vitro To determine whether this mutation of rhodopsin affects its signaling properties in vivo, we generated knock-in mice expressing the rhodopsin F45L mutant. We then examined the function of rods in the mutant mice versus wild-type controls, using in vivo electroretinography and transretinal and single cell suction recordings, combined with morphologic analysis and spectrophotometry. Although we did not evaluate the effect of the F45L mutation on the state of dimerization of the rhodopsin in vivo, our results revealed that F45L-mutant mice exhibit normal retinal morphology, normal rod responses as measured both in vivo and ex vivo, and normal rod dark adaptation. We conclude that the F45L mutation does not affect the signaling properties of rhodopsin in its natural setting.

Maintaining the Chloroplast Redox Balance through the PGR5-Dependent Pathway and the Trx System Is Required for Light-Dependent Activation of Photosynthetic Reactions

Light-dependent activation of chloroplast enzymes is required for the rapid induction of photosynthesis after a shift from dark to light. The thioredoxin (Trx) system plays a central role in this process. In chloroplasts, the Trx system consists of two pathways: the ferredoxin (Fd)/Trx pathway and the nicotinamide adenine dinucleotide phosphate (NADPH)-Trx reductase C (NTRC) pathway. In Arabidopsis (Arabidopsis thaliana) mutants defective in either pathway, the photoreduction of thiol enzymes was impaired, resulting in decreased carbon fixation. The close relationship between the Fd/Trx pathway and proton gradient regulation 5 (PGR5)-dependent photosystem I cyclic electron transport (PSI CET) in the induction of photosynthesis was recently elucidated. However, how the PGR5-dependent pathway is involved in the NTRC pathway is unclear, although NTRC has been suggested to physically interact with PGR5. In this study, we analyzed Arabidopsis mutants lacking either the PGR5 or the chloroplast NADH dehydrogenase-like complex (NDH)-dependent PSI CET pathway in the ntrc mutant background. The ntrc pgr5 double mutant suppressed both the growth defects and the high non-photochemical quenching phenotype of the ntrc mutant when grown under long-day conditions. By contrast, the inactivation of NDH activity with the chlororespiratory reduction 2-2 mutant failed to suppress either phenotype. We discovered that the phenotypic rescue of ntrc by pgr5 is caused by the partial restoration of Trx-dependent reduction of thiol enzymes. These results suggest that electron partitioning to the PGR5-dependent pathway and the Trx system needs to be properly regulated for the activation of the Calvin-Benson-Bassham cycle enzymes during the induction of photosynthesis.

Insights into Gene Regulation of Jasmonate-Induced Whole-Plant Senescence of Tobacco under Non-Starvation Conditions

Jasmonate (JA)-induced plant senescence has been mainly studied with a dark/starvation-promoted system using detached leaves; yet, the induction of whole-plant senescence by JA remains largely unclear. This work reports the finding of a JA-induced whole-plant senescence of tobacco under light/non-starvation conditions and the investigation of underlying regulations. Methyl jasmonate (MeJA) treatment induces the whole-plant senescence of tobacco in a light-intensity-dependent manner, which is suppressed by silencing of NtCOI1 that encodes the receptor protein of JA-Ile (the bioactive derivative of JA). MeJA treatment could induce the senescence-specific cysteine protease gene SAG12 and another cysteine protease gene SAG-L1 to high expression levels in the detached leaf patches under dark conditions but failed to induce their expression in tobacco whole plants under light conditions. Furthermore, MeJA attenuates the RuBisCo activase (RCA) level in the detached leaves but has no effect on this protein in the whole plant under light conditions. A genome-wide transcriptional assay also supports the presence of a differential regulatory pattern of senescence-related genes during MeJA-induced whole-plant senescence under non-starvation conditions and results in the finding of a chlorophylase activity increase in this process. We also observed that the MeJA-induced senescence of tobacco whole plants is reversible, which is accompanied by a structural change of chloroplasts. This work provides novel insights into JA-induced plant senescence under non-starvation conditions and is helpful to dissect the JA-synchronized process of whole-plant senescence.

Deletion of the cytochrome bc complex from Heliobacterium modesticaldum results in viable but non-phototrophic cells

The heliobacteria, a family of anoxygenic phototrophs, possess the simplest known photosynthetic apparatus. Although they are photoheterotrophs in the light, the heliobacteria can also grow chemotrophically via pyruvate metabolism in the dark. In the heliobacteria, the cytochrome bc complex is responsible for oxidizing menaquinol and reducing cytochrome c₅₅₃ in the electron flow cycle used for phototrophy. However, there is no known electron acceptor for the mobile cytochrome c₅₅₃ other than the photochemical reaction center. We have, therefore, hypothesized that the cytochrome bc complex is necessary for phototrophy, but unnecessary for chemotrophic growth in the dark. We used a two-step method for CRISPR-based genome editing in Heliobacterium modesticaldum to delete the genes encoding the four major subunits of the cytochrome bc complex. Genotypic analysis verified the deletion of the petCBDA gene cluster encoding the catalytic components of the complex. Spectroscopic studies revealed that re-reduction of cytochrome c₅₅₃ after flash-induced photo-oxidation was over 100 times slower in the ∆petCBDA mutant compared to the wild-type. Steady-state levels of oxidized P₈₀₀ (the primary donor of the photochemical reaction center) were much higher in the ∆petCBDA mutant at every light level, consistent with a limitation in electron flow to the reaction center. The ∆petCBDA mutant was unable to grow phototrophically on acetate plus CO₂ but could grow chemotrophically on pyruvate as a carbon source similar to the wild-type strain in the dark. The mutants could be complemented by reintroduction of the petCBDA gene cluster on a plasmid expressed from the clostridial eno promoter.

Modulation of CHT7 Complexes during Light/Dark- and Nitrogen-Mediated Life Cycle Transitions of Chlamydomonas

The Chlamydomonas reinhardtii Compromised Hydrolysis of Triacylglycerols7 (CHT7) protein has been previously implicated in the regulation of DNA metabolism and cell-cycle-related gene expression during nitrogen (N) deprivation, and its predicted protein interaction domains are necessary for function. Here, we examined impacts of the cht7 mutation during the cell division cycle under nutrient deficiency in light-dark synchronized cultures. We explored the potential mechanisms affecting CHT7 complex activities during the cell cycle and N starvation, with a focus on the possible interaction between CHT7 and the C. reinhardtii retinoblastoma tumor suppressor (RB) protein homolog MAT3. Notably, the absence of CHT7 did not negatively impact the synchrony of cell division and cell cycle progression during diel growth. Although the majority of CHT7 and MAT3/RB proteins were observed in separate complexes by blue native-PAGE, the two proteins coimmunoprecipitated both during synchronized growth and following N deprivation, suggesting the presence of low abundance subcomplexes containing CHT7 and MAT3/RB. Furthermore, we observed several phosphorylated isoforms of CHT7 under these conditions. To test the potential role of phosphorylation on the structure and function of CHT7, we performed site-directed mutagenesis of previously identified phosphorylated amino acids within CHT7. These phosphorylated residues were dispensable for CHT7 function, but phosphorylated variants of CHT7 persisted, indicating that yet-unidentified residues within CHT7 are also likely phosphorylated. Based on the interaction of CHT7 and MAT3/RB, we postulate the presence of a low-abundance or transient regulatory complex in C. reinhardtii that may be similar to DREAM-like complexes in other organisms.

Collaboration between NDH and KEA3 Allows Maximally Efficient Photosynthesis after a Long Dark Adaptation

In angiosperms, the NADH dehydrogenase-like (NDH) complex mediates cyclic electron transport around PSI (CET). K⁺ Efflux Antiporter3 (KEA3) is a putative thylakoid H⁺/K⁺ antiporter and allows an increase in membrane potential at the expense of the ∆pH component of the proton motive force. In this study, we discovered that the chlororespiratory reduction2-1 (crr2-1) mutation, which abolished NDH-dependent CET, enhanced the kea3-1 mutant phenotypes in Arabidopsis (Arabidopsis thaliana). The NDH complex pumps protons during CET, further enhancing ∆pH, but its physiological function has not been fully clarified. The observed effect only took place upon exposure to light of 110 µmol photons m^-2 s^-1 after overnight dark adaptation. We propose two distinct modes of NDH action. In the initial phase, within 1 min after the onset of actinic light, the NDH-dependent CET engages with KEA3 to enhance electron transport efficiency. In the subsequent phase, in which the ∆pH-dependent down-regulation of the electron transport is relaxed, the NDH complex engages with KEA3 to relax the large ∆pH formed during the initial phase. We observed a similar impact of the crr2-1 mutation in the genetic background of the PROTON GRADIENT REGULATION5 overexpression line, in which the size of ∆pH was enhanced. When photosynthesis was induced at 300 µmol photons m^-2 s^-1, the contribution of KEA3 was negligible in the initial phase and the ∆pH-dependent down-regulation was not relaxed in the second phase. In the crr2-1 kea3-1 double mutant, the induction of CO₂ fixation was delayed after overnight dark adaptation.

Molecular interactions and mutational impact upon rhodopsin (G90→D90) for hindering dark adaptation of eye: A comparative structural level outlook for signaling mechanism in night blindness

For night blindness, a detailed structural exploration of the interactions among G-protein receptor rhodopsin, transducin and arrestin was performed. Rhodopsin is responsible for dim light vision while a point mutation (G90→D90) results in an adverse change in its photo-transduction. The validated 3D models of the three proteins were utilized, and upon mutation and interactions, rhodopsin attained higher stability (evaluated through thermodynamic energy calculations, electrostatic surface potential and solvent accessible area), thereby participating strongly with transducin. Conformational switches in mutated rhodopsin also depicted a firm conformation with few 3₁₀ helices accompanied by increased percentage of pure α-helices and sheets. All evaluations were corroborated through paired T-tests. Glu33 (glycosylated unit in the N-terminal zone) of rhodopsin plays a chief role in the overall interaction pattern. Arg69 and Glu33 from wild-type rhodopsin participated in ionic interactions, while the latter set of ionic interaction remained preserved even after mutation. Cys323 (C-terminal residue) and Arg69 formed H-bonds from the wild-type rhodopsin. Cys323 exceptionally supports cellular signaling pattern in the non-mutated situation and for the non-sufferers of night-blindness. Ser297 and Tyr43 from mutated rhodopsin reside in helices and interact with Thr32 of transducin, preserving the steady conformation in activated interacted state, even in the dark. Ser297 lies adjoined to Lys296 (retinal attachment site), which resides in NPXXY motif (an "activation switch" for signal transduction). Thus, the molecular facet for involvement of photo-transduction, which holds a paramount zone in ophthalmology, was dealt with. This might instigate the future prospect for drug discovery to prevent such mutations.

Accelerated Evolution and Functional Divergence of the Dim Light Visual Pigment Accompanies Cichlid Colonization of Central America

Cichlids encompass one of the most diverse groups of fishes in South and Central America, and show extensive variation in life history, morphology, and colouration. While studies of visual system evolution in cichlids have focussed largely on the African rift lake species flocks, Neotropical cichlids offer a unique opportunity to investigate visual system evolution at broader temporal and geographic scales. South American cichlid colonization of Central America has likely promoted accelerated rates of morphological evolution in Central American lineages as they encountered reduced competition, renewed ecological opportunity, and novel aquatic habitats. To investigate whether such transitions have influenced molecular evolution of vision in Central American cichlids, we sequenced the dim-light rhodopsin gene in 101 Neotropical cichlid species, spanning the diversity of the clade. We find strong evidence for increased rates of evolution in Central American cichlid rhodopsin relative to South American lineages, and identify several sites under positive selection in rhodopsin that likely contribute to adaptation to different photic environments. We expressed a Neotropical cichlid rhodopsin protein invitro for the first time, and found that while its spectral tuning properties were characteristic of typical vertebrate rhodopsin pigments, the rate of decay of its active signalling form was much slower, consistent with dim light adaptation in other vertebrate rhodopsins. Using site-directed mutagenesis combined with spectroscopic assays, we found that a key amino acid substitution present in some Central American cichlids accelerates the rate of decay of active rhodopsin, which may mediate adaptation to clear water habitats.

Normal radial migration and lamination are maintained in dyslexia-susceptibility candidate gene homolog Kiaa0319 knockout mice

Developmental dyslexia is a common disorder with a strong genetic component, but the underlying molecular mechanisms are still unknown. Several candidate dyslexia-susceptibility genes, including KIAA0319, DYX1C1, and DCDC2, have been identified in humans. RNA interference experiments targeting these genes in rat embryos have shown impairments in neuronal migration, suggesting that defects in radial cortical migration could be involved in the disease mechanism of dyslexia. Here we present the first characterisation of a Kiaa0319 knockout mouse line. Animals lacking KIAA0319 protein do not show anatomical abnormalities in any of the layered structures of the brain. Neurogenesis and radial migration of cortical projection neurons are not altered, and the intrinsic electrophysiological properties of Kiaa0319-deficient neurons do not differ from those of wild-type neurons. Kiaa0319 overexpression in cortex delays radial migration, but does not affect final neuronal position. However, knockout animals show subtle differences suggesting possible alterations in anxiety-related behaviour and in sensorimotor gating. Our results do not reveal a migration disorder in the mouse model, adding to the body of evidence available for Dcdc2 and Dyx1c1 that, unlike in the rat in utero knockdown models, the dyslexia-susceptibility candidate mouse homolog genes do not play an evident role in neuronal migration. However, KIAA0319 protein expression seems to be restricted to the brain, not only in early developmental stages but also in adult mice, indicative of a role of this protein in brain function. The constitutive and conditional knockout lines reported here will be useful tools for further functional analyses of Kiaa0319.

Circadian clock control of connexin36 phosphorylation in retinal photoreceptors of the CBA/CaJ mouse strain

The gap-junction-forming protein connexin36 (Cx36) represents the anatomical substrate of photoreceptor electrical coupling in mammals. The strength of coupling is directly correlated to the phosphorylation of Cx36 at two regulatory sites: Ser110 and Ser293. Our previous work demonstrated that the extent of biotinylated tracer coupling between photoreceptor cells, which provides an index of the extent of electrical coupling, depends on the mouse strain. In the C57Bl/6J strain, light or dopamine reduces tracer coupling and Cx36 phosphorylation in photoreceptors. Conversely, darkness or a dopaminergic antagonist increases tracer coupling and Cx36 phosphorylation, regardless of the daytime. In the CBA/CaJ strain, photoreceptor tracer coupling is not only regulated by light and dopamine, but also by a circadian clock, a type of oscillator with a period close to 24 h and intrinsic to the retina, so that under prolonged dark-adapted conditions tracer coupling is broader at night compared to daytime. In the current study, we examined whether the modulation of photoreceptor coupling by a circadian clock in the CBA/CaJ mouse photoreceptors reflected a change in Cx36 protein expression and/or phosphorylation. We found no significant change in Cx36 expression or in the number of Cx36 gap junction among the conditions examined. However, we found that Cx36 phosphorylation is higher under dark-adapted conditions at night than in the daytime, and is the lowest under prolonged illumination at any time of the day/night cycle. Our observations are consistent with the view that the circadian clock regulation of photoreceptor electrical coupling is mouse strain-dependent and highlights the critical position of Cx36 phosphorylation in the control of photoreceptor coupling.

The first case of Oguchi disease, type 2 in a Polish patient with confirmed GRK1 gene mutation

Oguchi disease type 2 is a rare autosomal recessive form of congenital stationary night blindness. A typical feature of this disorder is a golden-brown discoloration of the fundus called Mizuo-Nakamura phenomenon, which disappears after prolonged dark adaptation and reappears shortly after the onset of light.

MATERIAL AND METHODS

A 13-year-old boy exhibiting the clinical features of congenital stationary night blindness, was examined. Ophthalmic examination including slit-lamp biomicroscopy, perimetry and funduscopy was performed. Additionally, the full-field electroretinography and molecular testing for congenital stationary night blindness using the Single Nucleotide Polymorphism microarray technique were performed.

RESULTS

The ophthalmic examination showed normal visual acuity, normal anterior segment of both eyes and full visual fields. The eye fundus examination showed a typical golden-brownish discoloration of the peripheral retina (disappearing after long dark adaptation) with no pigment deposits. Full-field electroretinography showed reduced amplitudes of both waves under scotopic conditions, while under photopic conditions both shape and parameters of the record were within the normal limits. The Single Nucleotide Polymorphism microarray revealed a homozygous deletion: c.1607161 OdelCGGA in GRK1 gene. This frameshift mutation introduces a stop codon (p.Asp537Valfs*542) and results in deletion of terminal 22 amino acid residues of retinal kinase protein.

CONCLUSIONS

This is the first molecular evidence for GRK1 gene mutation in a Polish patient with Oguchi disease type 2. The identification of the c.1607_1610delCGGA mutation in a patient with Oguchi disease confirms the pathogenicity of this variant.

Neuropeptidergic signaling partitions arousal behaviors in zebrafish

Animals modulate their arousal state to ensure that their sensory responsiveness and locomotor activity match environmental demands. Neuropeptides can regulate arousal, but studies of their roles in vertebrates have been constrained by the vast array of neuropeptides and their pleiotropic effects. To overcome these limitations, we systematically dissected the neuropeptidergic modulation of arousal in larval zebrafish. We quantified spontaneous locomotor activity and responsiveness to sensory stimuli after genetically induced expression of seven evolutionarily conserved neuropeptides, including adenylate cyclase activating polypeptide 1b (adcyap1b), cocaine-related and amphetamine-related transcript (cart), cholecystokinin (cck), calcitonin gene-related peptide (cgrp), galanin, hypocretin, and nociceptin. Our study reveals that arousal behaviors are dissociable: neuropeptide expression uncoupled spontaneous activity from sensory responsiveness, and uncovered modality-specific effects upon sensory responsiveness. Principal components analysis and phenotypic clustering revealed both shared and divergent features of neuropeptidergic functions: hypocretin and cgrp stimulated spontaneous locomotor activity, whereas galanin and nociceptin attenuated these behaviors. In contrast, cart and adcyap1b enhanced sensory responsiveness yet had minimal impacts on spontaneous activity, and cck expression induced the opposite effects. Furthermore, hypocretin and nociceptin induced modality-specific differences in responsiveness to changes in illumination. Our study provides the first systematic and high-throughput analysis of neuropeptidergic modulation of arousal, demonstrates that arousal can be partitioned into independent behavioral components, and reveals novel and conserved functions of neuropeptides in regulating arousal.

Neurobehavioral characterization of Endonuclease G knockout mice reveals a new putative molecular player in the regulation of anxiety

Endonuclease G (EndoG) has been largely related with a role in the modulation of a caspase-independent cell death pathway in many cellular systems. However, whether this protein plays a specific role in the brain remains to be elucidated. Here we have characterized the behavioral phenotype of EndoG(-/-) null mice and the expression of the nuclease among brain regions. EndoG(-/-) mice showed normal neurological function, learning, motor coordination and spontaneous behaviors. However, these animals displayed lower activity in a running wheel and, strikingly, they were consistently less anxious compared to EndoG(+/+) mice in different tests for anxiety such as plus maze and dark-light test. We next evaluated the expression of EndoG in different brain regions of wild type mice and found that it was expressed in all over but specially enriched in the striatum. Further, subcellular biochemical experiments in neocortical samples from wild type mice revealed that EndoG is localized in pre-synaptic compartments but not in post-synaptic compartments. Altogether these findings suggest that EndoG could play a highly specific role in the regulation of anxiety by modulating synaptic components.

Fatp1 deficiency affects retinal light response and dark adaptation, and induces age-related alterations

FATP1 is involved in lipid transport into cells and in intracellular lipid metabolism. We showed previously that this protein interacts with and inhibits the limiting-step isomerase of the visual cycle RPE65. Here, we aimed to analyze the effect of Fatp1-deficiency in vivo on the visual cycle, structure and function, and on retinal aging. Among the Fatp family members, we observed that only Fatp1 and 4 are expressed in the control retina, in both the neuroretina and the retinal pigment epithelium. In the neuroretina, Fatp1 is mostly expressed in photoreceptors. In young adult Fatp1^−/− mice, Fatp4 expression was unchanged in retinal pigment epithelium and reduced two-fold in the neuroretina as compared to Fatp1^+/+ mice. The Fatp1^−/− mice had a preserved retinal structure but a decreased electroretinogram response to light. These mice also displayed a delayed recovery of the b-wave amplitude after bleaching, however, visual cycle speed was unchanged, and both retinal pigment epithelium and photoreceptors presented the same fatty acid pattern compared to controls. In 2 year-old Fatp1^−/− mice, transmission electron microscopy studies showed specific abnormalities in the retinas comprising choroid vascularization anomalies and thickening of the Bruch membrane with material deposits, and sometimes local disorganization of the photoreceptor outer segments. These anomalies lead us to speculate that the absence of FATP1 accelerates the aging process.

Transsynaptic signaling by activity-dependent cleavage of neuroligin-1

Adhesive contact between pre- and postsynaptic neurons initiates synapse formation during brain development and provides a natural means of transsynaptic signaling. Numerous adhesion molecules and their role during synapse development have been described in detail. However, once established, the mechanisms of adhesive disassembly and its function in regulating synaptic transmission have been unclear. Here, we report that synaptic activity induces acute proteolytic cleavage of neuroligin-1 (NLG1), a postsynaptic adhesion molecule at glutamatergic synapses. NLG1 cleavage is triggered by NMDA receptor activation, requires Ca2+ /calmodulin-dependent protein kinase, and is mediated by proteolytic activity of matrix metalloprotease 9 (MMP9). Cleavage of NLG1 occurs at single activated spines, is regulated by neural activity in vivo, and causes rapid destabilization of its presynaptic partner neurexin-1β (NRX1β). In turn, NLG1 cleavage depresses synaptic transmission by abruptly reducing presynaptic release probability. Thus, local proteolytic control of synaptic adhesion tunes synaptic transmission during brain development and plasticity.

Arrestin translocation is stoichiometric to rhodopsin isomerization and accelerated by phototransduction in Drosophila photoreceptors

Upon illumination, visual arrestin translocates from photoreceptor cell bodies to rhodopsin and membrane-rich photosensory compartments, vertebrate outer segments or invertebrate rhabdomeres, where it quenches activated rhodopsin. Both the mechanism and function of arrestin translocation are unresolved and controversial. In dark-adapted photoreceptors of the fruitfly Drosophila, confocal immunocytochemistry shows arrestin (Arr2) associated with distributed photoreceptor endomembranes. Immunocytochemistry and live imaging of GFP-tagged Arr2 demonstrate rapid reversible translocation to stimulated rhabdomeres in stoichiometric proportion to rhodopsin photoisomerization. Translocation is very rapid in normal photoreceptors (time constant <10 s) and can also be resolved in the time course of electroretinogram recordings. Genetic elimination of key phototransduction proteins, including phospholipase C (PLC), Gq, and the light-sensitive Ca2+-permeable TRP channels, slows translocation by 10- to 100-fold. Our results indicate that Arr2 translocation in Drosophila photoreceptors is driven by diffusion, but profoundly accelerated by phototransduction and Ca2+ influx.

Genetic dissection of rod and cone pathways in the dark-adapted mouse retina

A monumental task of the mammalian retina is to encode an enormous range (>10(9)-fold) of light intensities experienced by the animal in natural environments. Retinal neurons carry out this task by dividing labor into many parallel rod and cone synaptic pathways. Here we study the operational plan of various rod- and cone-mediated pathways by analyzing electroretinograms (ERGs), primarily b-wave responses, in dark-adapted wildtype, connexin36 knockout, depolarizing rod-bipolar cell (DBCR) knockout, and rod transducin alpha-subunit knockout mice [WT, Cx36(-/-), Bhlhb4(-/-), and Tralpha(-/-)]. To provide additional insight into the cellular origins of various components of the ERG, we compared dark-adapted ERG responses with response dynamic ranges of individual retinal cells recorded with patch electrodes from dark-adapted mouse retinas published from other studies. Our results suggest that the connexin36-mediated rod-cone coupling is weak when light stimulation is weak and becomes stronger as light stimulation increases in strength and that rod signals may be transmitted to some DBCCs via direct chemical synapses. Moreover, our analysis indicates that DBCR responses contribute about 80% of the overall DBC response to scotopic light and that rod and cone signals contribute almost equally to the overall DBC responses when stimuli are strong enough to saturate the rod bipolar cell response. Furthermore, our study demonstrates that analysis of ERG b-wave of dark-adapted, pathway-specific mutants can be used as an in vivo tool for dissecting rod and cone synaptic pathways and for studying the functions of pathway-specific gene products in the retina.

[Usher syndrome]

We present the case report of two brothers, PF-21 years old and PN-19 years old, to whom the fundus examination, perimetry and dark adaptation established the diagnosis of Retinitis Pigmentosa. The otorhinolaryngology exam and the audiogram revealed, in both cases, bilateral sensorineural deafness. The simultaneous presence of these two conditions completes the clinical findings of Usher syndrome. The common ectodermic origin of the retina and the inner ear could explain this pathological association.

Gbeta5 is required for normal light responses and morphology of retinal ON-bipolar cells

Gbeta5 exists as two splice variants, Gbeta5-S and Gbeta5-L, which interact with and stabilize the R7 members of the regulators of G-protein signaling (RGSs): RGS6, RGS7, RGS9, and RGS11. Although the role of Gbeta5-L and RGS9-1 is established in photoreceptors, the physiological functions of Gbeta5-S and other R7 RGS proteins remain unclear. We found that the electroretinogram of Gbeta5-/- mice lacks the b-wave component and that Gbeta5-S and RGS11 colocalize with Go alpha at the tips of the ON-bipolar cell dendrites. Unexpectedly, we found a significant reduction in the number of synaptic triads in the outer plexiform layer (OPL) of the Gbeta5-/- mice, which is evident at postnatal day 14. Transgenic expression of Gbeta5-L in rods failed to rescue the b-wave or the OPL defects. These results indicate that Gbeta5-S is indispensable for OPL integrity and normal light responses of the retina.

Constitutive excitation by Gly90Asp rhodopsin rescues rods from degeneration caused by elevated production of cGMP in the dark

Previous experiments indicate that congenital human retinal degeneration caused by genetic mutations that change the Ca(2+) sensitivity of retinal guanylyl cyclase (retGC) can result from an increase in concentration of free intracellular cGMP and Ca(2+) in the photoreceptors. To rescue degeneration in transgenic mouse models having either the Y99C or E155G mutations of the retGC modulator guanylyl cyclase-activating protein 1 (GCAP-1), which produce elevated cGMP synthesis in the dark, we used the G90D rhodopsin mutation, which produces constitutive stimulation of cGMP hydrolysis. The effects of the G90D transgene were evaluated by measuring retGC activity biochemically, by recording single rod and electroretinogram (ERG) responses, by intracellular free Ca(2+) measurement, and by retinal morphological analysis. Although the G90D rhodopsin did not alter the abnormal Ca(2+) sensitivity of retGC in the double-mutant animals, the intracellular free cGMP and Ca(2+) concentrations returned close to normal levels, consistent with constitutive activation of the phosphodiesterase PDE6 cascade in darkness. G90D decreased the light sensitivity of rods but spared them from severe retinal degeneration in Y99C and E155G GCAP-1 mice. More than half of the photoreceptors remained alive, appeared morphologically normal, and produced electrical responses, at the time when their siblings lacking the G90D rhodopsin transgene lost the entire retinal outer nuclear layer and no longer responded to illumination. These experiments indicate that mutations that lead to increases in cGMP and Ca(2+) can trigger photoreceptor degeneration but that constitutive activation of the transduction cascade in these animals can greatly enhance cell survival.

The developing and evolving retina: using time to organize form

Evolutionary and other functional accounts of the retina and its normal development highlight different aspects of control of its growth and form than genomic and mechanistic accounts. Discussing examples from opsin expression, developmental regulation of the eye's size and optical quality, regulation of eye size with respect to brain and body size, and the development of the fovea, these different aspects of control are contrasted. Contributions of mouse models, particularly with regard to relative timing of events in different species are reviewed, introducing a Web-based utility for exploration of timing issues (www.translatingtime.net). Variation at the individual level, in early experience, and also across species is an essential source of information to understand normal development and its pathologies.

Knockdown of cone-specific kinase GRK7 in larval zebrafish leads to impaired cone response recovery and delayed dark adaptation

Phosphorylation of rhodopsin by rhodopsin kinase GRK1 is an important desensitization mechanism in scotopic vision. For cone vision GRK1 is not essential. However, cone opsin is phosphorylated following light stimulation. In cone-dominant animals as well as in humans, but not in rodents, GRK7, a cone-specific homolog of GRK1, has been identified in cone outer segments. To investigate the function of GRK7 in vivo, we cloned two orthologs of grk7 in zebrafish and knocked down gene expression of grk7a in zebrafish larvae by morpholino antisense nucleotides. Photoresponse recovery in Grk7a-deficient larvae was delayed in electroretinographic measurements, and temporal contrast sensitivity was reduced, particularly under bright-light conditions. These results show that function of a cone-specific kinase is essential for cone vision in the zebrafish retina and argue that pigment bleaching and spontaneous decay alone are not sufficient for light adaptation and rapid cone response inactivation.

Extreme hyperopia is the result of null mutations in MFRP, which encodes a Frizzled-related protein

Nanophthalmos is a rare disorder of eye development characterized by extreme hyperopia (farsightedness), with refractive error in the range of +8.00 to +25.00 diopters. Because the cornea and lens are normal in size and shape, hyperopia occurs because insufficient growth along the visual axis places these lensing components too close to the retina. Nanophthalmic eyes show considerable thickening of both the choroidal vascular bed and scleral coat, which provide nutritive and structural support for the retina. Thickening of these tissues is a general feature of axial hyperopia, whereas the opposite occurs in myopia. We have mapped recessive nanophthalmos to a unique locus at 11q23.3 and identified four independent mutations in MFRP, a gene that is selectively expressed in the eye and encodes a protein with homology to Tolloid proteases and the Wnt-binding domain of the Frizzled transmembrane receptors. This gene is not critical for retinal function, as patients entirely lacking MFRP can still have good refraction-corrected vision, produce clinically normal electro-retinograms, and show only modest anomalies in the dark adaptation of photoreceptors. MFRP appears primarily devoted to regulating axial length of the eye. It remains to be determined whether natural variation in its activity plays a role in common refractive errors.

The Y99C mutation in guanylyl cyclase-activating protein 1 increases intracellular Ca2+ and causes photoreceptor degeneration in transgenic mice

Guanylyl cyclase-activating proteins (GCAPs) are Ca2+-binding proteins that activate guanylyl cyclase when free Ca2+ concentrations in retinal rods and cones fall after illumination and inhibit the cyclase when free Ca2+ reaches its resting level in the dark. Several forms of retinal dystrophy are caused by mutations in GUCA1A, the gene coding for GCAP1. To investigate the cellular mechanisms affected by the diseased state, we created transgenic mice that express GCAP1 with a Tyr99Cys substitution (Y99C GCAP1) found in human patients with a late-onset retinal dystrophy (Payne et al., 1998). Y99C GCAP1 shifted the Ca2+ sensitivity of the guanylyl cyclase in photoreceptors, keeping it partially active at 250 nM free Ca2+, the normal resting Ca2+ concentration in darkness. The enhanced activity of the cyclase in the dark increased cyclic nucleotide-gated channel activity and elevated the rod outer segment Ca2+ concentration in darkness, measured by using fluo-5F and laser spot microscopy. In different lines of transgenic mice the magnitude of this effect rose with the Y99C GCAP1 expression. Surprisingly, there was little change in the rod photoresponse, indicating that dynamic Ca2+-dependent regulation of cGMP synthesis was preserved. However, the photoreceptors in these mice degenerated, and the rate of the cell loss increased with the level of the transgene expression, unlike in transgenic mice that overexpressed normal GCAP1. These results provide the first direct evidence that a mutation linked to congenital blindness increases Ca2+ in the outer segment, which may trigger the apoptotic process.

The R172W mutation in peripherin/rds causes a cone-rod dystrophy in transgenic mice

Peripherin/rds (P/rds) is a membrane glycoprotein essential for the photoreceptor outer segment disc morphogenesis and maintenance. More than half of the disease-causing mutations in P/rds have been linked to different forms of macular dystrophy; the most common one is substitution of tryptophan for arginine at position 172 (R172W). Here we confirm the patient phenotype associated with the expression of R172W mutation in transgenic mice. Functional, structural and biochemical analyses showed that, while R172W P/rds is appropriately localized, a direct correlation exists between transgene expression levels and the onset/severity of the phenotype. In the wild-type background, both cone and rod photoreceptors' structure and function were significantly diminished, which indicates a dominant-negative, cone-rod defect. Whereas rds(+/-) mice maintained the normal cone function at early ages, cone responses in R172W/rds(+/-) mice were diminished to 41% of the wild-type level signifying a preferential damaging effect of the mutation on cones. Conversely, R172W/rds(+/-) mice showed a significant rescue of rod function and improvement of rod outer segment structure. Although rds(-/-) mice have no detectable rod or cone responses, R172W/rds(-/-) animals retained 30% of wild-type structure and rod function, but no significant rescue of cone function was detected at 1 month of age. No biochemical abnormalities were observed in complex formation and association with Rom-1; however, R172W protein was more sensitive to tryptic digestion, indicative of a change in protein conformation, possibly contributing to the cone-dominated phenotype. As the first animal model for P/rds-associated cone-rod dystrophy, R172W mice provide a valuable tool for studying the pathophysiology of P/rds-associated human retinal dystrophies and the development of therapeutic strategies to intervene in these diseases.

Localization of dopamine D1-receptor to A-type horizontal cells in the rabbit retina by single cell RT-PCR

Dopamine is an important neurotransmitter regulating light-dark adaptation in the retina. The effect of dopamine is widespread and dependent on the localization of its receptors. We performed single cell RT-PCR on A-type horizontal cells visually selected from dissociated rabbit retinal neurons. Dopamine D1-receptor mRNA was positively identified. Subsequent sequencing of the fragment showed 82% homology with rat D1 receptor and 87% homology with human D1 receptor. This study supported previous observations that dopamine regulated A-type horizontal cell coupling via D1 receptors at the level of transcription.

Increase of hippocampal acetylcholine release at the onset of dark phase is suppressed in a mutant mice model of evening-type individuals

We have previously reported that clock mutant mice on Jcl:ICR background show about 2-h delayed circadian profiles in body temperature, spontaneous activity and sleep-wake rhythm, and that they appear to be useful as a model of evening-type of individual. Hippocampal acetylcholine (ACh) release which is positively correlated with attention, learning and memory shows a circadian variation. In this study, changes in hippocampal ACh release in transitional phase from light (rest) to dark (active) period in clock mutant mice were monitored using an in vivo microdialysis method. Compared with wild mice, the increase in hippocampal ACh in the first 2 h of the active period in the mutant mice was suppressed in parallel with peak frequency in electroencephalogram theta rhythm. The molecular basis of the circadian system appears to have a strong effect on hippocampal cholinergic function, and is probably associating with individual temporal differences in voluntary behavior, cognition, learning and/or memory performance.

Experience-dependent slow-wave sleep development

Sleep enhances plasticity in neocortex, and thereby improves sensory learning. Here we show that sleep itself undergoes changes as a consequence of waking experience during a late critical period in cats and mice. Dark-rearing produced a robust and reversible decrement of slow-wave electrical activity during sleep that was restricted to visual cortex and impaired by gene-targeted reduction of NMDA receptor function.

Light exposure during daytime modulates expression of Per1 and Per2 clock genes in the suprachiasmatic nuclei of mice

The suprachiasmatic nuclei (SCN) of the hypothalamus contain the master circadian clock in mammals. Nocturnal light pulses that reset the circadian clock also lead to rapid increases in levels of Per1 and Per2 mRNA in the SCN, suggesting that these genes are involved in the synchronization to light. During the day, when light has no phase-shifting effects in nocturnal rodents, the consequences of light exposure for Per expression have been less thoroughly studied. Therefore, the effects of light exposure during the day were assessed on Per1 and Per2 mRNA in the SCN of mice. Expression of Per1 and Per2 was generally increased by 30-min light pulses during the subjective day, with more pronounced effects in the morning. One exception was noted for a transient decrease in Per2 expression after a short light pulse applied at midday. Prolonged light exposure (up to 3 hr) starting at midday markedly increased Per2 expression but not that of Per1. Moreover, the amplitude of the daily variations of both Per and the duration of Per1 peak was increased in mice exposed to a light-dark cycle compared with those transferred to constant darkness. Finally, the amplitude of the daily variations of both Per and the basal level of Per1 were increased in mice under a light-dark cycle compared with animals synchronized to a skeleton photoperiod (i.e., with daily dawn and dusk 1-hr exposures to light). Taken together, the results indicate that prolonged light exposure during daytime positively modulates daily levels of Per1 and Per2 mRNA in the SCN of mice.

Dual regulation of cryptochrome 1 mRNA expression in chicken retina by light and circadian oscillators

The localization and regulation of chicken cryptochrome 1 (cCry1) mRNA expression in retina was investigated by laser capture microdissection and quantitative real-time RT-PCR. Laser capture microdissection (LCM) of retinal cell layers showed the highest level of cCry1 expression in the ganglion cell and photoreceptor layers. In both layers, expression was high during the daytime and low at night in subjects exposed to a 12:12 h light:dark cycle. Robust circadian oscillations of cCry1 mRNA levels were observed in constant (24 h day) light, but not in constant darkness, with the highest expression during daytime at zeitgeber time (ZT) 8. Unlike cCry1, circadian rhythms of the melatonin-synthesizing enzyme, arylalkylamine N-acetyltransferase, persisted in constant darkness, suggesting that rhythmic cCry1 expression is not essential for circadian clock function or output. On the second day of constant darkness, when cCry1 expression is arrhythmic, light exposure for 2 h significantly increased retinal cCry1 mRNA levels at ZT 4 and 8, times that cCry1 expression is induced in LD and LL. Similar light exposure ending at ZT 20 had no significant effect. Thus, expression of cCry1 mRNA is regulated dually by light and circadian clocks.

Molecular basis of amplification in Drosophila phototransduction: roles for G protein, phospholipase C, and diacylglycerol kinase

In Drosophila photoreceptors, the amplification responsible for generating quantum bumps in response to photoisomerization of single rhodopsin molecules has been thought to be mediated downstream of phospholipase C (PLC), since bump amplitudes were reportedly unaffected in mutants with greatly reduced levels of either G protein or PLC. We now find that quantum bumps in such mutants are reduced approximately 3- to 5-fold but are restored to near wild-type values by mutations in the rdgA gene encoding diacylglycerol kinase (DGK) and also by depleting intracellular ATP. The results demonstrate that amplification requires activation of multiple G protein and PLC molecules, identify DGK as a key enzyme regulating amplification, and implicate diacylglycerol as a messenger of excitation in Drosophila phototransduction.

Effect of melatonin administration on qPer2, qPer3, and qClock gene expression in the suprachiasmatic nucleus of Japanese quail

Temporal changes of mRNA expression of three clock genes, qPer2, qPer3 and qClock, were studied in the suprachiasmatic nucleus (SCN) of Japanese quail under different light conditions, as well as under the condition of continuous melatonin. In addition, the expression of melatonin receptor genes, Mel1a and Mel1c, in the SCN were also examined. The expression of qPer2 mRNA showed robust oscillation during both light and dark (LD) 12:12 cycles and under constant dark conditions (DD), but did not exhibit circadian rhythmicity in constant light conditions (LL), instead being expressed at a consistently high level. Expression of qPer3 also showed robust oscillation under both LD and DD conditions. Unlike qPer2 however, qPer3 mRNA expression remained rhythmic under LL conditions. Contrary to the findings on the other clock genes, no remarkable rhythmicity was detectable in either light condition. Both Mel1a and Mel1c mRNAs were detected in the SCN, however, Mel1a mRNA levels were higher than Mel1c and showed daily rhythmicity. Although implantation of melatonin tubes caused constant high levels of plasma melatonin and consequently masked the endogenous daily melatonin rhythm, no significant differences in the expression pattern of any of the three clock genes were observed between birds with and without constant melatonin. In addition, a single injection of melatonin did not affect mRNA expression of these clock genes. These results suggest that melatonin does not affect transcription of clock genes, but may act on the mechanism of synchronization among SCN oscillatory cells.

Differential induction and localization of mPer1 and mPer2 during advancing and delaying phase shifts

The mechanism whereby brief light exposure resets the mammalian circadian clock in a phase dependent manner is not known, but is thought to involve Per gene expression. At the behavioural level, a light pulse produces phase delays in early subjective night, phase advances in late subjective night, and no phase shifts in mid-subjective night or subjective day. To understand the relationship between Per gene activity and behavioural phase shifts, we examined light-induced mPer1 and mPer2 expression in the suprachiasmatic nucleus (SCN) of the mouse, in the subjective night, with a view to understanding SCN heterogeneity. In the VIP-containing region of the SCN (termed 'core'), light-induced mPer1 expression occurs at all times of the subjective night, while mPer2 induction is seen only in early subjective night. In the remaining regions of the SCN (termed 'shell'), a phase delaying light pulse produces no mPer1 but significant mPer2 expression, while a phase advancing light pulse produces no mPer2 but substantial mPer1 induction. Moreover, following a light pulse during mid-subjective night, neither mPer1 nor mPer2 are induced in the shell. The results reveal that behavioural phase shifts occur only when light-induced Per gene expression spreads from the core to the shell SCN, with mPer1 expression in shell corresponding to phase advances, and mPer2 corresponding to phase delays. The results indicate that the time course and the localization of light-induced Per gene expression in SCN reveals important aspects of intra-SCN communication.

Immediate early gene expression in the visual cortex of normal and dark reared cats: differences between fos and egr-1

Northern blotting indicated that the level of induced fos was higher in normal than dark reared cat visual cortex (VC) at 5 weeks of age, comparable at 10 weeks, and higher in dark reared than normal cat visual cortex at 20 weeks. Fos egr-1, induction was higher in dark reared visual cortex at all ages. Thus, dark rearing has similar effects on fos, but not egr-1, expression as it has on physiological neuronal plasticity during the VC critical period.

Diurnal changes in arginine vasopressin gene transcription in the rat suprachiasmatic nucleus

The diurnal changes in arginine vasopressin (AVP) mRNA and heteronuclear (hn) RNA, an indicator for gene transcription, were examined in the hypothalamus of Sprague-Dawley rats using in situ hybridization. AVP hnRNA levels in the suprachiasmatic nucleus (SCN) varied during a 24-h cycle and showed a peak at day-time [Zeitgeber time (ZT) 5], which preceded the peak in AVP mRNA levels by 4 h. AVP hnRNA was undetectable at ZT 13 and 17, indicating that the gene transcription was almost shut down at these time points. AVP mRNA levels in the SCN continued to decrease at night (ZT 13, 17 and 21) when there were minimal changes in transcription, suggesting rapid turnover of mRNA. Similar diurnal changes in AVP hnRNA levels were observed without photic cues. On the other hand, AVP hnRNA or mRNA levels in the supraoptic nucleus, where AVP is synthesized in response to plasma osmolarity and/or volume, did not show any circadian rhythm. These data suggest that both dynamic changes in AVP gene transcription and rapid turnover of mRNA contribute to the diurnal variation in AVP mRNA levels in the SCN.

Effects of nursing mothers on rPer1 and rPer2 circadian expressions in the neonatal rat suprachiasmatic nuclei vary with developmental stage

The ability of nursing mothers to entrain the circadian pacemaker of rat pups was examined by measuring the rat Per1 (rPer1) and rPer2 expression levels in the suprachiasmatic nuclei (SCN). Newborn rats from mothers under a light-dark cycle (LD) were blinded immediately after birth and reared by foster mothers under either LD (LD blind pups) or reversed light-dark cycle (DL; DL blind pups). At postnatal day (P)6, small but significant phase differences were observed in the circadian gene expression rhythms of the SCN not only between the blind and sighted pups, but also between the two groups of blind pups, indicating the involvement of both free-running and maternal influence in phase-resetting the circadian rhythms of blind pups. However, from P6 to P13 the circadian rhythms of both LD and DL blind pups showed phase delays of similar extent, which suggests that the influence of nursing mothers was lost. From P13 to P20 (the day of weaning), the rPer1 and rPer2 rhythms phase-shifted in a different manner, the rPer2 rhythm being related more closely to the behavioural rhythm than was the rPer1. This finding suggests a differential influence of mothers on the rPer1 and rPer2 rhythms in the third week of life. It is concluded that the ability of nursing mothers to entrain pup circadian oscillation depends on the developmental stage.

Electrical silencing of Drosophila pacemaker neurons stops the free-running circadian clock

Electrical silencing of Drosophila circadian pacemaker neurons through targeted expression of K+ channels causes severe deficits in free-running circadian locomotor rhythmicity in complete darkness. Pacemaker electrical silencing also stops the free-running oscillation of PERIOD (PER) and TIMELESS (TIM) proteins that constitutes the core of the cell-autonomous molecular clock. In contrast, electrical silencing fails to abolish PER and TIM oscillation in light-dark cycles, although it does impair rhythmic behavior. On the basis of these findings, we propose that electrical activity is an essential element of the free-running molecular clock of pacemaker neurons along with the transcription factors and regulatory enzymes that have been previously identified as required for clock function.

Thyroid-beta2 and the retinoid RAR-alpha, RXR-gamma and ROR-beta2 receptor mRNAs; expression profiles in mouse retina, retinal explants and neocortex

In neonatal retinal explants cultured long-term green cones are missing. Recently it was reported that thyroid hormone beta2 receptors (TR-beta2) are essential for these green cones to differentiate. Therefore transcript level of these receptors was investigated in our mouse retinal explants. However, thyroid receptors function as heterodimers with retinoid receptors (RR); so the fate of selected RRs was similarly analyzed using semi-quantitative RT-PCR. Loss of TR-beta2 and RR (RXR-gamma and ROR-beta2) mRNAs was observed after culturing the neonatal retina for 12 days. This indicates that these proteins are involved in determination of green cone identity. In addition, levels of the selected RR transcripts are differentially affected by short- or long-term culture. In the latter case an attached retinal pigment epithelium seems to play a protective role. Furthermore, divergent diurnal peaks of RR mRNAs are present in young as well as aged mouse retina and neocortex. This data might be relevant in the context of human ageing disorders.

Newfoundland rod-cone dystrophy, an early-onset retinal dystrophy, is caused by splice-junction mutations in RLBP1

Some isolated populations exhibit an increased prevalence of rare recessive diseases. The island of Newfoundland is a characteristic geographic isolate, settled by a small number of families primarily during the late 1700s and early 1800s. During our studies of this population, we identified a group of families exhibiting a retinal dystrophy reminiscent of retinitis punctata albescens but with a substantially lower age at onset and more-rapid and distinctive progression, a disorder that we termed "Newfoundland rod-cone dystrophy" (NFRCD). The size of one of these families was sufficient to allow us to perform a genomewide screen to map the NFRCD locus. We detected significant linkage to markers on the long arm of chromosome 15, in a region encompassing RLBP1, the gene encoding the cellular retinaldehyde-binding protein. Previously, mutations in RLBP1 have been associated with other retinal dystrophies, leading us to hypothesize that RLBP1 mutations might also cause NFRCD. To test this hypothesis, we sequenced all coding exons and splice junctions of RLBP1. We detected two sequence alterations, each of which is likely to be pathogenic, since each segregates with the disease and is predicted to interfere with mRNA splicing. In contrast to some previously reported RLBP1 mutations, which yield a protein that may retain some residual activity, each NFRCD mutation is likely to give rise to a null allele. This difference may account for the severe phenotype in these families and exemplifies the molecular continuum that underlies clinically distinct but genetically related entities.

Targeted disruption of the mouse cis-retinol dehydrogenase gene: visual and nonvisual functions

It has been proposed that cis-retinol dehydrogenase (cRDH) acts within the body to catalyze the oxidation of 9-cis-retinol, an oxidative step needed for 9-cis-retinoic acid synthesis, the oxidation of 11-cis-retinol [an oxidative step needed for 11-cis-retinal (visual chromophore) synthesis], and 3 alpha-hydroxysteroid transformations. To assess in vivo the physiological importance of each of these proposed actions of cRDH, we generated cRDH-deficient (cRDH-/-) mice. The cRDH-/- mice reproduce normally and appear to be normal. However, the mutant mice do have a mild visual phenotype of impaired dark adaptation. This phenotype is evidenced by electroretinagram analysis of the mice and by biochemical measures of eye levels of retinoid intermediates during recovery from an intense photobleach. Although it is thought that cRDH is expressed in the eye almost solely in retinal pigment epithelial cells, we detected cRDH expression in other retinal cells, including ganglion cells, amacrine cells, horizontal cells, and the inner segments of the rod photoreceptor cells. Aside from the eye, there are no marked differences in retinoid levels in other tissues throughout the body for cRDH-/- compared with cRDH+/+ mice. Moreover, we did not detect any non-visual phenotypic changes for cRDH-/- mice, suggesting that these mice do not have problems in metabolizing 3 alpha-hydroxysteroids.Thus, cRDH may act essentially in the visual cycle but is redundant for catalyzing 9-cis-retinoic acid formation and 3 alpha-hydroxysteroid metabolism.

Synaptic vesicle alterations in rod photoreceptors of synaptophysin-deficient mice

The abundance of the integral membrane protein synaptophysin in synaptic vesicles and its multiple possible functional contributions to transmitter exocytosis and synaptic vesicle formation stand in sharp contrast to the observed lack of defects in synaptophysin knockout mice. Assuming that deficiencies are compensated by the often coexpressed synaptophysin isoform synaptoporin, we now show that retinal rod photoreceptors, which do not synthesize synaptoporin either in wild-type or in knockout mice, are affected by the loss of synaptophysin. Multiple pale-appearing photoreceptors, as seen by electron microscopy, possess reduced cytoplasmic electron density, swollen mitochondria, an enlarged cell surface area, and, most importantly, a significantly reduced number of synaptic vesicles with an unusually bright interior. Quantification of the number of synaptic vesicles per unit area, not only in these, but also in all other rod terminals of knockout animals, reveals a considerable reduction in vesicles that is even more pronounced during the dark period, i.e., at times of highest synaptic activity. Moreover, activity-dependent reduction in synaptic vesicle diameter, typically occurring in wild-type mice, is not detected in knockout animals. The large number of clathrin-coated pits and vesicles in dark-adapted synaptophysin knockout mice is taken as an indication of compensatory usage of synaptophysin-independent pathway(s), and, conversely, in view of the overall reduction in the number of synaptic vesicles, as an indication for the presence of another synaptophysin-dependent synaptic vesicle recycling pathway. Our results provide in vivo evidence for the importance of the integral membrane protein synaptophysin for synaptic vesicle recycling and formation.

Behavioral characterization and genetic analysis of the Drosophila melanogaster larval response to light as revealed by a novel individual assay

A new assay was designed, named checker, that measures the individual response to light in the fruitfly Drosophila melanogaster larva. In this assay the Drosophila larva apparently modulates its pattern of locomotion when faced with a choice between a dark and lit environment by orienting its movement towards the dark environment. We show that, in this assay, a response to light can be measured as an increase in residence time in the dark versus the lit quadrant. Mutations that disrupt phototransduction in the adult Drosophila abolish the larval response to light, demonstrating that this larval visual function is similar to that of the adult fly. Similarly, no response to light was detected in strains where the larval visual system (photoreceptors and target area) was disrupted by a mutation in the homeobox containing gene sine oculis (so) gene. Ablation of photoreceptors by the targeted expression of the cell death gene hid under the control of the photoreceptor-specific transcription factor glass (gl) abolishes this response entirely. Finally, we demonstrate that this response to light can be mediated by rhodopsins other than the blue absorbing Rh1.

Kinetics of visual pigment regeneration in excised mouse eyes and in mice with a targeted disruption of the gene encoding interphotoreceptor retinoid-binding protein or arrestin

Photoisomerization of 11-cis-retinal to all-trans-retinal and reduction to all-trans-retinol occur in photoreceptor outer segments whereas enzymatic esterification of all-trans-retinol, isomerization to 11-cis-retinol, and oxidation to 11-cis-retinal occur in adjacent cells. The processes are linked into a visual cycle by intercellular diffusion of retinoids. Knowledge of the mechanistic aspects of the visual cycle is very limited. In this study, we utilize chemical analysis of visual cycle retinoids to assess physiological roles for components inferred from in vitro experiments and to understand why excised mouse eyes fail to regenerate their bleached visual pigment. Flash illumination of excised mouse eyes or eyecups, in which regeneration of rhodopsin does not occur, produced a block in the visual cycle after all-trans-retinal formation; constant illumination of eyecups produced a block in the cycle after all-trans-retinol formation; and constant illumination of whole excised eyes resulted in a block of the cycle after formation of all-trans-retinyl ester. These blocks emphasize the role of cellular metabolism in the visual cycle. Interphotoreceptor retinoid-binding protein (IRBP) has been postulated to play a role in intercellular retinoid transfer in the retina; however, the rates of recovery of 11-cis-retinal and of regeneration of rhodopsin in the dark in IRBP-/- mice were very similar to those found with wild-type (wt) mice. Thus, IRBP is necessary for photoreceptor survival but is not essential for a normal rate of visual pigment turnover. Arrestin forms a complex with activated rhodopsin, quenches its activity, and affects the release of all-trans-retinal in vitro. The rate of recovery of 11-cis-retinal in arrestin-/- mice was modestly delayed relative to wt, and the rate of rhodopsin recovery was approximately 80% of that observed with wt mice. Thus, the absence of arrestin appeared to have a minor effect on the kinetics of the visual cycle.

Spatial and temporal expression of AP-1 responsive rod photoreceptor genes and bZIP transcription factors during development of the rat retina

PURPOSE

The promoter region of the rod-specific beta subunit of cGMP PDE (beta-PDE) and opsin genes contains highly conserved cis-acting elements, which include an AP-1 and/or Nrl response element (NRE: An extended AP-1 like sequence). Transactivation of AP-1 or NRE appears necessary to drive expression of these rod-specific genes during adulthood, however, their role during development is relatively unknown. Therefore, we determined the spatial and temporal relationships between rod morphological and functional development, rod-specific gene expression, and expression of the bZIP transcription factors c-fos, junD and Nrl.

METHODS

Retinas from 0-45 day old (PN0-45) dark- and light-adapted Long-Evans rats were used. Morphological development was monitored by light and electron microscopy. Whole retinal trypsin-activated cGMP-PDE activity and rhodopsin content were measured biochemically. The expression of opsin, beta-PDE, c-fos, junD and Nrl mRNAs were determined by Northern blot analysis. The cellular localization of Nrl was examined with in situ hybridization.

RESULTS

The mRNAs for opsin, beta-PDE and c-fos were observed at PN0-2, while cGMP-PDE activity and rhodopsin were detected first at PN5: coincident with rod outer segment development. The developmental pattern of cGMP-PDE activity and rhodopsin accumulation paralleled the expression of beta-PDE and opsin mRNA and all reached their maximal levels by PN45. Nrl expression, for all three transcripts found in the rat retina, was low on PN2 and reached its maximal level at PN14. The c-fos and Nrl expression preceded beta-PDE and opsin mRNA expression by 1-2 days. Nrl expression was detected first in the distal post-mitotic retina at PN5 and then in all nuclear layers during retinal development. Maximal expression shifted from the ganglion cells to the outer nuclear layer as the neural retina matured. In contrast, junD expression was highest at PN0 and declined to a stable level by PN10.

CONCLUSIONS

Colocalization of Nrl and c-Fos suggests that expression of rod-specific genes, which utilize AP-1 or NRE sites in their promoter, could be regulated through the formation of Nrl-Fos dimers. We hypothesize that Nrl and c-Fos play a fundamental role in the initiation and regulation of the rod-specific gene expression in developing and adult rod photoreceptors.

[Incomplete congenital stationary night blindness (CSNB). An important differential diagnosis of congenital nystagmus]

BACKGROUND

Classifying congenital nystagmus in the absence of biomicroscopically detectable abnormalities of the eye, and in an otherwise healthy child is difficult, especially early in life. At that age, nystagmus and visual loss may be the predominant symptoms of congenital stationary night-blindness. Unless night-blindness is specifically asked for or an ERG performed the correct diagnosis may be missed.

PATIENTS AND METHODS

We present the clinical data of two families with X-linked incomplete CSNB previously undiagnosed. ERG recordings in both families were suggestive of CSNB. The ERG of the obligate carrier was normal. In an attempt to distinguish between the complete and the incomplete type, and to identify further carrier signs, scotopic perimetry and dark adaptation were performed in both affected males and carriers. Scotopic perimetry allows to test the rod-mediated visual pathway in its spatial distribution.

RESULTS

In affected males with non-recordable ERGs scotopic perimetry and dark adaptation disclosed residual rod function indicating an incomplete type. In carriers, there was a sensitivity loss at 600 nm, which may be a new carrier sign.

CONCLUSIONS

The correct diagnosis of the different forms of CSNB together with the identification of carriers is important for (1) genetic counselling and (2) linkage studies to identify the gene(s) for CSNB.

X linked progressive cone dystrophy. Localisation of the gene locus to Xp21-p11.1 by linkage analysis

Six affected males, three female carriers, and two possible carriers were evaluated from a three generation pedigree with X linked progressive cone dystrophy. The affected males presented with progressive decrease of visual acuity, impairment of colour vision, and deterioration of electroretinogram, which ranged from absent response to red light in all young patients to abnormal cone-rod responses in the elderly ones. In most affected males dark adaptation curves were monophasic and the electro-oculogram values were reduced. While some obligate carriers showed functional anomalies, they all had reduced electroretinogram response to red light. The a1/aT ratio for 1 joule white light was an appropriate indicator for carrier state. The family was studied with seven DNA markers from the proximal part of the short arm of the human X chromosome. So far, significant linkage has been found between three DNA markers and COD1, which assigns the progressive cone dystrophy gene (COD1) in this family to Xp21-p11.1. Differential diagnosis with congenital cone dystrophies is discussed.

Failure to induce rapid eye movement sleep by dark pulses in pigmented inbred rat strains

Studies of albino Lewis rats, pigmented Brown Norway rats, and their F2 backcross progeny have demonstrated that the ability to trigger rapid eye movement (REM) sleep by turning off cage lights (dark pulses) is associated with albinism in these rat strains. Other studies have shown that pigmented inbred rats show REM sleep induction in the dark portion of short light:dark cycles or skin temperature changes. In the present study, these same pigmented breeds, Dark Agouti and hooded Long-Evans rats, were subjected to 5-min dark pulses and failed to show any evidence of REM sleep triggering. In fact, they showed trends towards REM sleep suppression during dark pulses. These results extend the finding that dark pulse triggering of REM sleep, readily evoked in albino rats, does not appear in pigmented rat strains.

Elevated dark-adapted thresholds in hypopigmented mice measured with a water maze screening apparatus

In previous electrophysiological experiments from hypopigmented animals (mice, rats, rabbits), single-unit recordings from both retinal ganglion axons and cells in the superior colliculus have demonstrated an increase in threshold in the dark-adapted state which is roughly proportional to the ocular melanin concentration. In the present study we compared an albino mouse strain which is relatively resistant to light damage and the beige mouse mutant to their wild-type controls in a situation that involved unanesthetized, unrestrained mice as a control to the electrophysiological single unit experiments. We used a six-chambered water maze. Animals were trained to swim to an illuminated ramp until their performances leveled off (about 10 days). The animals were then dark-adapted for 24 h and tested after reducing the luminance level of the water maze. We found that the albino mice failed to find the ramp when the luminance fell to 1.58 x 10(-3) cd/m2 (p < or = .0001), the beige mice failed at 2.00 x 10(-4) cd/m2 (p < or = .0001), and the normally pigmented controls performed to 5.00 x 10(-5) cd/m2 (p < or = .0001). These results support our previous findings that the sensitivity defect in hypopigmented animals is proportional to the degree of ocular hypopigmentation.

Rat strain differences in response to dark pulse triggering of paradoxical sleep

Previous studies of inbred rats have shown that Brown Norway (BN) rats had more daily paradoxical sleep (PS) than Lewis (L) rats, while F1 progeny had intermediate amounts, suggesting codominant or polygenic transmission. Amount of PS and the induction of PS episodes may be under separate genetic control. Earlier work had shown that five-minute exposures to cage lights-off every half-hour can trigger PS in outbred albino strains. To explore the genetic controls for PS induction, PS triggering by dark pulse stimulation was examined in L and BN rats. L rats showed a five-fold increase in PS during dark pulse stimulation. Although, as in the earlier study, BN rats had more total daily PS than L rats, they exhibited no dark pulse triggering of PS. Thus L and BN rats show significant strain differences in two independent parameters of PS, and may be a useful model for studying genetic and neurologic factors which regulate PS.