Identification of a photoreceptor-specific mRNA encoded by the gene responsible for retinal degeneration slow (rds)

Mutant mice homozygous for 'retinal degeneration slow' (rds/rds) are characterized phenotypically by abnormal development of photoreceptor outer segments in the retina, followed by gradual degeneration of photoreceptors. This process of degeneration is complete by one year, with preservation of all other retinal cells. The biochemical defect that leads to the mutant phenotype is not known. Our strategy for cloning the rds gene was based upon three previously reported observations. First, the rds locus maps to chromosome 17. Second, experimental rds/rds----+/+ and rds/+----+/+ tetra-parental mice manifest patchy photoreceptor changes in the retina, suggesting that the wild-type rds locus is expressed within cells of the photoreceptor lineage. Finally, the process of degeneration is specific to photoreceptors. On the basis of these observations, we predicted that the rds mRNA is encoded by a gene on chromosome 17 and is normally expressed exclusively within photoreceptors in the retina. We here present evidence that this is the case.     

Melanopsin-expressing ganglion cells in primate retina signal colour and irradiance and project to the LGN

Human vision starts with the activation of rod photoreceptors in dim light and short (S)-, medium (M)-, and long (L)- wavelength-sensitive cone photoreceptors in daylight. Recently a parallel, non-rod, non-cone photoreceptive pathway, arising from a population of retinal ganglion cells, was discovered in nocturnal rodents. These ganglion cells express the putative photopigment melanopsin and by signalling gross changes in light intensity serve the subconscious, 'non-image-forming' functions of circadian photoentrainment and pupil constriction. Here we show an anatomically distinct population of 'giant', melanopsin-expressing ganglion cells in the primate retina that, in addition to being intrinsically photosensitive, are strongly activated by rods and cones, and display a rare, S-Off, (L + M)-On type of colour-opponent receptive field. The intrinsic, rod and (L + M) cone-derived light responses combine in these giant cells to signal irradiance over the full dynamic range of human vision. In accordance with cone-based colour opponency, the giant cells project to the lateral geniculate nucleus, the thalamic relay to primary visual cortex. Thus, in the diurnal trichromatic primate, 'non-image-forming' and conventional 'image-forming' retinal pathways are merged, and the melanopsin-based signal might contribute to conscious visual perception.     

Retinal degeneration in the rd mouse is caused by a defect in the beta subunit of rod cGMP-phosphodiesterase

Mice homozygous for the rd mutation display hereditary retinal degeneration and the classic rd lines serve as a model for human retinitis pigmentosa. In affected animals the retinal rod photoreceptor cells begin degenerating at about postnatal day 8, and by four weeks no photoreceptors are left. Degeneration is preceded by accumulation of cyclic GMP in the retina and is correlated with deficient activity of the rod photoreceptor cGMP-phosphodiesterase. We have recently isolated a candidate complementary DNA for the rd gene from a mouse retinal library and completed the characterization of cDNAs encoding all subunits of bovine photoreceptor phosphodiesterase. The candidate cDNA shows strong homology with a cDNA encoding the bovine phosphodiesterase beta subunit. Here we present evidence that the candidate cDNA is the murine homologue of bovine phosphodiesterase beta cDNA. We conclude that the mouse rd locus encodes the rod photoreceptor cGMP-phosphodiesterase beta subunit.     

GABA and GAD immunoreactivity of photoreceptor terminals in primate retina

Within the vertebrate retina, two types of photoreceptor cells--the rods and cones--transduce visual signals and convey this information through synapses with bipolar and horizontal cells. Although the neurotransmitter at these first-order synapses has not been identified, electrophysiological studies suggest that it might be excitatory. In the present study, however, we have found photoreceptor terminals in the rhesus monkey retina which are immunoreactive with antibodies to either gamma-aminobutyric acid (GABA) or L-glutamic acid decarboxylase (GAD, an enzyme involved in the synthesis of GABA). In the perifoveal region of the retina, approximately 25% of presynaptic profiles having ultrastructural characteristics of either rod or cone terminals are immunoreactive with one or the other antibody. This evidence for a putatively inhibitory neurotransmitter in photoreceptor terminals challenges present understanding of retinal synaptic function.     

Chromatic sensitivity of ganglion cells in the peripheral primate retina

Visual abilities change over the visual field. For example, our ability to detect movement is better in peripheral vision than in foveal vision, but colour discrimination is markedly worse. The deterioration of colour vision has been attributed to reduced colour specificity in cells of the midget, parvocellular (PC) visual pathway in the peripheral retina. We have measured the colour specificity (red-green chromatic modulation sensitivity) of PC cells at eccentricities between 20 and 50 degrees in the macaque retina. Here we show that most peripheral PC cells have red-green modulation sensitivity close to that of foveal PC cells. This result is incompatible with the view that PC pathway cells in peripheral retina make indiscriminate connections ('random wiring') with retinal circuits devoted to different spectral types of cone photoreceptors. We show that selective cone connections can be maintained by dendritic field anisotropy, consistent with the morphology of PC cell dendritic fields in peripheral retina. Our results also imply that postretinal mechanisms contribute to the psychophysically demonstrated deterioration of colour discrimination in the peripheral visual field.     

Photoreceptor degeneration in inherited retinal dystrophy delayed by basic fibroblast growth factor

Numerous inherited retinal degenerations exist in animals and humans, in which photoreceptors inexplicably degenerate and disappear. In RCS rats with inherited retinal dystrophy, the mutant gene is expressed in the retinal pigment epithelial (RPE) cell, and leads to the loss of photoreceptor cells. Photoreceptors can be rescued from degeneration if they are juxtaposed to wild-type RPE cells in experimental chimaeras or by the transplantation of RPE cells from normal rats. In both cases, the rescue effect extends beyond the immediate boundaries of the normal RPE cells, suggesting trophic action of a diffusible factor(s) from the normal RPE cells. We considered that the fibroblast growth factors, aFGF and bFGF, might have such a trophic role as they are found in the retina and RPE cells; bFGF acts as a neurotrophic agent after axonal injury in several regions of the central nervous system, and bFGF induces retinal regeneration from developing RPE cells. Here we report that subretinal injection of bFGF results in extensive rescue of photoreceptors in RCS rats for at least two months after the injection, and that intravitreal injection of bFGF results in even more widespread rescue, across almost the entire retina. The findings demonstrate for the first time that bFGF can act as a survival-promoting neurotrophic factor in a hereditary neuronal degeneration of the central nervous system.     

Simulating Human Visual Perception in Nighttime Illumination

This paper presents an image-based algorithm for simulating the visual adaptation of the human visual system to various illuminations,especially in dark nighttime conditions.The human visual system exhibits different characteristics depending on the illumination intensity,with photopic vision in bright conditions,scotopic vision in dark conditions,and mesopic vision between these two.A computational model is designed to simulate multiple features of mesopic vision and scotopic vision,including the chromaticity change,luminance change,and visual acuity loss.The system uses a source image under bright illumination as input.Then assuming that the viewer has already adapted to the new conditions,the color spectrum of the input image is reconstructed to replace the source with modifications of the chromaticity and the luminance of the relighted scene.A bilateral filter is used to simulate the visual acuity loss.The model parameters have clear physical meanings and can be obtained from experimental data to achieve realistic results.The algorithm can be used not only for visual perception simulation,but also as a day-for-night tool to produce realistic nighttime images from daytime images.     

褐菖鲉视网膜感受系统及其适应特性研究

褐菖鲉Sebastiscus marmoratus的暗适应时程特别长,用LogIs=0的光强漂白后,阈值需要经4.5h暗适应才能回到原来暗视水平.而明适应进程很短,只需要3min就达稳定,这与网膜变化一致。它的网膜具有视杆和视维两种不同光感受系统,但是,暗适应曲线无明显平台和转折,强光明适应曲线也未出现R型变化.因此,其视锥感光功能可能在某种程度上发生退化,适于弱光视觉,这些视觉特性可能与其长期生活于近海底层礁石间弱光环境的习性相适应。     

Signal clipping by the rod output synapse

The properties of synapses between retinal neurons make an essential contribution to early visual processing. Light produces a graded hyperpolarization in photoreceptors, up to 25 mV in amplitude, and it is conventionally assumed that all of this response range is available for coding visual information. We report here, however, that the rod output synapse rectifies strongly, so that only potential changes within 5 mV of the rod dark potential are transmitted effectively to postsynaptic horizontal cells. This finding is consistent with the voltage-dependence of the calcium current presumed to control neurotransmitter release from rods. It suggests functional roles for the strong electrical coupling of adjacent rods and the weak electrical coupling of adjacent rods and cones. The existence of photoreceptor coupling resolves the apparent paradox that rods have a 25 mV response range, while signals greater than 5 mV in amplitude are clipped during synaptic transmission. We predict that the strengths of rod-rod and rod-cone coupling are quantitatively linked to the relationship between the rod response range and the synapse operating range.     

花背蟾蜍角膜的发育和诱导

以花背蟾蜍的胚胎和变态期蝌蚪为材料,采用光镜、电镜、免疫荧光组织化学、放射自显曩、电镜细胞化学、皮肤片移植及类坏死处理等技术,研究：（１）在胚胎发育及变态期间角膜的发育过程;（２）细胞外基质的合成变化及其对角膜发育和诱导的作用;（３）角膜诱导期间的超微结构变化及类坏死对角膜诱导的影响;（４）在角膜下沉发育和诱导过程中的组织化学和视网膜感光细胞特异视蛋白的出现及二者间的可能关系。还提出角膜诱导的可能     

Spatial structure of cone inputs to receptive fields in primate lateral geniculate nucleus.

Human colour vision depends on three classes of cone photoreceptors, those sensitive to short (S), medium (M) or long (L) wavelengths, and on how signals from these cones are combined by neurons in the retina and brain. Macaque monkey colour vision is similar to human, and the receptive fields of macaque visual neurons have been used as an animal model of human colour processing. P retinal ganglion cells and parvocellular neurons are colour-selective neurons in macaque retina and lateral geniculate nucleus. Interactions between cone signals feeding into these neurons are still unclear. On the basis of experimental results with chromatic adaptation, excitatory and inhibitory inputs from L and M cones onto P cells (and parvocellular neurons) were thought to be quite specific (Fig. 1a). But these experiments with spatially diffuse adaptation did not rule out the 'mixed-surround' hypothesis: that there might be one cone-specific mechanism, the receptive field centre, and a surround mechanism connected to all cone types indiscriminately (Fig. 1e). Recent work has tended to support the mixed-surround hypothesis. We report here the development of new stimuli to measure spatial maps of the linear L-, M- and S-cone inputs to test the hypothesis definitively. Our measurements contradict the mixed-surround hypothesis and imply cone specificity in both centre and surround.     

Targeted misexpression of a Drosophila opsin gene leads to altered visual function

Drosophila mutants transformed with a chimaeric gene that expresses the ocellar visual pigment in the major class of photoreceptor cells of the retina were used to investigate the properties of this minor pigment. The photoreceptor cells in which this opsin was misexpressed showed new spectral characteristics and physiology.     

Drosophila Crumbs is a positional cue in photoreceptor adherens junctions and rhabdomeres

Drosophila Crumbs (Crb) is required for apical-basal polarity and is an apical determinant in embryonic epithelia. Here, we describe properties of Crb that control the position and integrity of the photoreceptor adherens junction and photosensitive organ, or rhabdomere. In contrast to normal photoreceptor adherens junctions and rhabdomeres, which span the depth of the retina, adherens junctions and rhabdomeres of Crb-deficient photoreceptors initially accumulate at the top of the retina and fail to maintain their integrity as they stretch to the retinal floor. We show that Crb controls localization of the adherens junction through its intracellular domain containing a putative binding site for a protein 4.1 superfamily protein (FERM). Although loss of Crb or overexpression of the FERM binding domain causes mislocalization of adherens junctions, they do not result in a significant loss of photoreceptor polarity. Mutations in CRB1, a human homologue of crb, are associated with photoreceptor degeneration in retinitis pigmentosa 12 (RP12) and Leber congenital amaurosis (LCA). The intracellular domain of CRB1 behaves similarly to its Drosophila counterpart when overexpressed in the fly eye. Our studies may provide clues for mechanisms of photoreceptor degeneration in RP12 and LCA.     

A cyclic GMP-activated channel in dissociated cells of the chick pineal gland.

Phototransduction in the vertebrate retina is dependent in part on a cyclic GMP-activated ionic channel in the plasma membrane of rods and cones. But other vertebrate cells are also photosensitive. Cells of the chick pineal gland have a photosensitive circadian rhythm in melatonin secretion that persists in dissociated cell culture. Exposure to light causes inhibition of melatonin secretion, and entrainment of the intrinsic circadian oscillator. Chick pinealocytes express several 'retinal' proteins, including arrestin, transducin and a protein similar to the visual pigment rhodopsin. Pinealocytes of lower vertebrates display hyperpolarizing responses to brief pulses of light. Thus it is possible that some of the mechanisms of phototransduction are similar in retinal and pineal photoreceptors. We report here the first recordings of cyclic GMP-activated channels in an extraretinal photoreceptor. Application of GMP, but not cyclic AMP, to excised inside-out patches caused activation of a 15-25 pS cationic channel. These channels may be essential for phototransduction in the chick pineal gland.     

Development of the light response in neonatal mammalian rods

The sensitivity to light is low in many neonatal mammals when compared with that in the adult. In human infants at one month of age, for example, the dark-adapted sensitivity for detection of large stimuli is 50 times lower than in the adult, and in rats the overall sensitivity of the neonatal retina is also low compared with the adult. This low sensitivity in the neonate has been attributed to a number of factors, but the possibility that the photoreceptors themselves might be an important limitation on the overall visual sensitivity has not so far been clearly established. Here we record the light response of single neonatal rat rods and find that the sensitivity is considerably lower than in the adult. The response to a single photoisomerization is normal in the neonate, and the sensitivity deficit can therefore be attributed to a low level of functional rhodopsin. Opsin, the protein component of rhodopsin, must be present in normal amounts, as the sensitivity can be restored to adult levels by treating the retina with 9-cis retinal, an active homologue of the native chromophore 11-cis retinal. The low sensitivity of photoreceptors in the neonate can therefore be attributed mainly to a low concentration of 11-cis retinal in the developing retina.     

ON and OFF pathways in Drosophila motion vision

Motion vision is a major function of all visual systems, yet the underlying neural mechanisms and circuits are still elusive. In the lamina, the first optic neuropile of Drosophila melanogaster, photoreceptor signals split into five parallel pathways, L1-L5. Here we examine how these pathways contribute to visual motion detection by combining genetic block and reconstitution of neural activity in different lamina cell types with whole-cell recordings from downstream motion-sensitive neurons. We find reduced responses to moving gratings if L1 or L2 is blocked; however, reconstitution of photoreceptor input to only L1 or L2 results in wild-type responses. Thus, the first experiment indicates the necessity of both pathways, whereas the second indicates sufficiency of each single pathway. This contradiction can be explained by electrical coupling between L1 and L2, allowing for activation of both pathways even when only one of them receives photoreceptor input. A fundamental difference between the L1 pathway and the L2 pathway is uncovered when blocking L1 or L2 output while presenting moving edges of positive (ON) or negative (OFF) contrast polarity: blocking L1 eliminates the response to moving ON edges, whereas blocking L2 eliminates the response to moving OFF edges. Thus, similar to the segregation of photoreceptor signals in ON and OFF bipolar cell pathways in the vertebrate retina, photoreceptor signals segregate into ON-L1 and OFF-L2 channels in the lamina of Drosophila.     

Photoreceptor-specific degeneration caused by tunicamycin

The antibiotic tunicamycin inhibits the biosynthesis of N-acetylglucosaminylpyrophosphoryl polyisoprenol, a key intermediate in the formation of the asparagine-linked oligosaccharides of glycoproteins. The effects of tunicamycin have been studied in various biological systems, primarily with the aim of elucidating the role of the carbohydrate moieties in the cellular function of glycoproteins. Rhodopsin, the visual pigment of retinal rod photoreceptor cells, is a membrane glycoprotein which consists of a single polypeptide chain (opsin) to which a chromophoric prosthetic group (II-cis-retinaldehyde) and two asparagine-linked oligosaccharide chains are covalently attached. The glycosylation of opsin can be blocked with tunicamycin in vitro in conditions where polypeptide synthesis is only slightly decreased. We have reported that tunicamycin can disrupt the normal assembly of rod outer segment membranes in vitro without significantly inhibiting the biosynthesis or intracellular transport of opsin. Here we report that intraocular injection of tunicamycin produces a photoreceptor-specific degeneration characterized by progressive shortening of rod outer segment, decreased membrane assembly, and eventual photoreceptor cell death.