All-trans-Retinaldehyde Contributes to Retinal Vascular Permeability in Ischemia Reperfusion.

Purpose: Extracellular accumulation of all-trans-retinaldehyde (atRAL), a highly reactive visual cycle intermediate, is toxic to cells of the outer retina and contributes to retinal and macular degenerations. However, the contribution of atRAL to retinal capillary function has not been studied. We hypothesized that atRAL released from the outer retina can contribute to retinal vascular permeability. We, therefore, tested the contribution of atRAL to retinal ischemia-reperfusion (IR)-induced vascular permeability. Methods: IR was induced in mice by transient increase in intraocular pressure followed by natural reperfusion. The visual cycle was ablated in the Lrat-/- mice, reduced by dark adaptation or the use of the RPE65 inhibitor and atRAL scavenger emixustat. Accumulation of FITC-BSA was used to assess vascular permeability and DNA fragmentation quantified cell death after IR. Primary bovine retinal endothelial cell (BREC) culture was used to measure the direct effects of atRAL on endothelial permeability and cell death. Results: Inhibition of the visual cycle by Lrat-/-, dark adaptation, or with emixustat, all reduced approximately half of IR induced vascular permeability at 48 hours. An increase in BREC permeability with atRAL coincided with lactate dehydrogenase (LDH) release, a measure of cell death. Both permeability and toxicity were blocked by emixustat. Conclusions: Outer retinal pathology may contribute to vascular permeability by release of atRAL, which can act directly on vascular endothelial cells to alter barrier properties and induce cell death. These studies may have implications for a variety of blinding eye diseases that include outer retinal damage and retinal vascular permeability.

Signal Integration in Thalamus: Labeled Lines Go Cross-Eyed and Blurry.

The brain uses sensory information from the periphery to create percepts. In this issue of Neuron, Rompani et al. (2017) show that visual signals are combined in unexpected ways that vastly expand the possible representations of the outside world.

Horizontal cells expressing melanopsin x are novel photoreceptors in the avian inner retina.

In the vertebrate retina, three types of photoreceptors-visual photoreceptor cones and rods and the intrinsically photosensitive retinal ganglion cells (ipRGCs)-converged through evolution to detect light and regulate image- and nonimage-forming activities such as photic entrainment of circadian rhythms, pupillary light reflexes, etc. ipRGCs express the nonvisual photopigment melanopsin (OPN4), encoded by two genes: the Xenopus (Opn4x) and mammalian (Opn4m) orthologs. In the chicken retina, both OPN4 proteins are found in ipRGCs, and Opn4x is also present in retinal horizontal cells (HCs), which connect with visual photoreceptors. Here we investigate the intrinsic photosensitivity and functioning of HCs from primary cultures of embryonic retinas at day 15 by using calcium fluorescent fluo4 imaging, pharmacological inhibitory treatments, and Opn4x knockdown. Results show that HCs are avian photoreceptors with a retinal-based OPN4X photopigment conferring intrinsic photosensitivity. Light responses in HCs appear to be driven through an ancient type of phototransduction cascade similar to that in rhabdomeric photoreceptors involving a G-protein q, the activation of phospholipase C, calcium mobilization, and the release of the inhibitory neurotransmitter GABA. Based on their intrinsic photosensitivity, HCs may have a key dual function in the retina of vertebrates, potentially regulating nonvisual tasks together with their sister cells, ipRGCs, and with visual photoreceptors, modulating lateral interactions and retinal processing.

Tectal microcircuit generating visual selection commands on gaze-controlling neurons.

The optic tectum (called superior colliculus in mammals) is critical for eye-head gaze shifts as we navigate in the terrain and need to adapt our movements to the visual scene. The neuronal mechanisms underlying the tectal contribution to stimulus selection and gaze reorientation remains, however, unclear at the microcircuit level. To analyze this complex--yet phylogenetically conserved--sensorimotor system, we developed a novel in vitro preparation in the lamprey that maintains the eye and midbrain intact and allows for whole-cell recordings from prelabeled tectal gaze-controlling cells in the deep layer, while visual stimuli are delivered. We found that receptive field activation of these cells provide monosynaptic retinal excitation followed by local GABAergic inhibition (feedforward). The entire remaining retina, on the other hand, elicits only inhibition (surround inhibition). If two stimuli are delivered simultaneously, one inside and one outside the receptive field, the former excitatory response is suppressed. When local inhibition is pharmacologically blocked, the suppression induced by competing stimuli is canceled. We suggest that this rivalry between visual areas across the tectal map is triggered through long-range inhibitory tectal connections. Selection commands conveyed via gaze-controlling neurons in the optic tectum are, thus, formed through synaptic integration of local retinotopic excitation and global tectal inhibition. We anticipate that this mechanism not only exists in lamprey but is also conserved throughout vertebrate evolution.

The kinetics of regeneration of rhodopsin under enzyme-limited availability of 11-cis retinoid.

In order to describe the regeneration of rhodopsin and the recovery of visual sensitivity following exposure of the eye to intense bleaching illumination, two models have been proposed, in which there is either a "resistive" or an "enzymatic" limit to the supply of retinoid. A solution has previously been derived for the resistive model, and here we derive an analytical solution for the enzymatic model and we investigate the form of this solution as a function of parameter values. We demonstrate that this enzymatic model provides a good fit to human post-bleach recovery, for four cases: for rhodopsin regeneration in normal subjects; for psychophysical scotopic dark adaptation in normal subjects; for rhodopsin regeneration and scotopic dark adaptation in fundus albipunctatus patients; and for cone pigment regeneration in normal subjects. Finally, we present arguments favouring the enzymatic model as the cellular basis for normal human rod and cone pigment regeneration.

Retinal phototransduction.

Vision is perhaps the most important of all our senses, and gives us an immense amount of information regarding the outside world. The initial format in which this information reaches the retina are photons; particles of energy radiation of a given wavelength emitted or reflected from our surroundings. The brain itself however, perceives information in electrical signals via action potentials and changes in electrochemical gradients. The processes involved in the transduction of photons into electrical potentials will be the focus of this article. This review article summarizes the recent advances in understanding these complex pathways and provides an overview of the main molecules involved in the neurobiology of vision.

Age-related macular degeneration.

Age-related macular degeneration (AMD) is the leading cause of blindness in the elderly, and the prevalence of the disease increases exponentially with every decade after age 50 years. It is a multifactorial disease involving a complex interplay of genetic, environmental, metabolic, and functional factors. Besides smoking, hypertension, obesity, and certain dietary habits, a growing body of evidence indicates that inflammation and the immune system may play a key role in the development of the disease. AMD may progress from the early form to the intermediate form and then to the advanced form, where two subtypes exist: the nonneovascular (dry) type and the neovascular (wet) type. The results from the Age-Related Eye Disease Study have shown that for the nonneovascular type of AMD, supplementation with high-dose antioxidants (vitamin C, vitamin E, and ß-carotene) and zinc is recommended for those with the intermediate form of AMD in one or both eyes or with advanced AMD or vision loss due to AMD in one eye. As for the neovascular type of the advanced AMD, the current standard of therapy is intravitreal injections of vascular endothelial growth factor inhibitors. In addition, lifestyle and dietary modifications including improved physical activity, reduced daily sodium intake, and reduced intake of solid fats, added sugars, cholesterol, and refined grain foods are recommended. To date, no study has demonstrated that AMD can be cured or effectively prevented. Clearly, more research is needed to fully understand the pathophysiology as well as to develop prevention and treatment strategies for this devastating disease.

Effects of different forms of monocular deprivation on primary visual cortex maps.

Monocular deprivation (MD) by lid suture is one of the classic paradigms for the study of developmental plasticity in the cerebral cortex, and we have detailed knowledge of its anatomical and physiological consequences as well as underlying molecular and cellular mechanisms. However, the effects of other forms of manipulating visual input through one eye on the functional architecture of the primary visual cortex (V1) have not yet been examined directly. We compared MD by lid suture with the effects of daily monocular lens wear using either a frosted lens or a neutral density (ND) filter. We used optical imaging of intrinsic signals and visually evoked potentials (VEPs) to assess responses in V1 to monocular stimulation. We found that loss of stimulus contrast through monocular frosted lens wear resulted in marked takeover of cortical territory by the nondeprived eye (NDE) similar to that caused by classic MD, and in virtual absence of orientation-selective responses following stimulation of the deprived eye (DE). Furthermore, amplitudes of VEPs in response to gratings of a range of spatial frequencies were significantly reduced in the DE compared to the NDE. In contrast, differences in luminance between two eyes caused by an ND filter in front of one eye did not affect ocular dominance and orientation maps, and there was no significant difference in the amplitude of VEPs elicited through the two eyes. Our results are consistent with previous electrophysiological studies in demonstrating that binocular pattern information is necessary to maintain normal functional maps in both eyes, while reduced luminance in one eye has little effect on the overall functional architecture and visual responses in V1.

An S-opsin knock-in mouse (F81Y) reveals a role for the native ligand 11-cis-retinal in cone opsin biosynthesis.

In absence of their natural ligand, 11-cis-retinal, cone opsin G-protein-coupled receptors fail to traffic normally, a condition associated with photoreceptor degeneration and blindness. We created a mouse with a point mutation (F81Y) in cone S-opsin. As expected, cones with this knock-in mutation respond to light with maximal sensitivity red-shifted from 360 to 420 nm, consistent with an altered interaction between the apoprotein and ligand, 11-cis-retinal. However, cones expressing F81Y S-opsin showed an ∼3-fold reduced absolute sensitivity that was associated with a corresponding reduction in S-opsin protein expression. The reduced S-opsin expression did not arise from decreased S-opsin mRNA or cone degeneration, but rather from enhanced endoplasmic reticulum (ER)-associated degradation of the nascent protein. Exogenously increased 11-cis-retinal restored F81Y S-opsin protein expression to normal levels, suggesting that ligand binding in the ER facilitates proper folding. Immunohistochemistry and electron microscopy of normal retinas showed that Mueller cells, which synthesize a precursor of 11-cis-retinal, are closely adjoined to the cone ER, so they could deliver the ligand to the site of opsin synthesis. Together, these results suggest that the binding of 11-cis-retinal in the ER is important for normal folding during cone opsin biosynthesis.

Differential expression of the demosponge (Suberites domuncula) carotenoid oxygenases in response to light: protection mechanism against the self-produced toxic protein (Suberitine).

The demosponge Suberites domuncula has been described to contain high levels of a proteinaceous toxin, Suberitine, that displays haemolytic activityIn the present study this 7-8 kDa polypeptide has been isolated and was shown to exhibit also cytotoxic effects on cells of the same species. Addition of retinal, a recently identified metabolite of ß-carotene that is abundantly present in S. domuncula was found to reduce both the haemolytic and the cell toxic activity of Suberitine at a molar ratio of 1:1. Spectroscopic analyses revealed that the interaction between ß-carotene and Suberitine can be ascribed to a reversible energy transfer reaction. The enzyme that synthesises retinal in the sponge system is the ß,ß-carotene-15,15'-dioxygenase [carotene dioxygenase]. In order to clarify if this enzyme is the only ß-carotene-metabolizing enzyme a further oxygenase had been identified and cloned, the (related) carotenoid oxygenase. In contrast to the dioxygenase, the carotenoid oxygenase could not degrade ß-carotene or lycopene in Escherichia coli strains that produced these two carotenoids; therefore it had been termed related-carotenoid oxygenase. Exposure of primmorphs to light of different wavelengths from the visible spectrum resulted after 3 days in a strong upregulation of the dioxygenase in those 3D-cell aggregates that had been incubated with ß-carotene. The strongest effect is seen with blue light at a maximum around 490 nm. It is concluded that the toxin Suberitine is non-covalently modified by retinal, the cleavage product from ß-carotene via the enzyme carotene dioxygenase, a light inducible oxygenase. Hence, this study highlights that in S. domuncula the bioactive metabolite, retinal, has the property to detoxify its homologous toxin.

Rapid release of retinal from a cone visual pigment following photoactivation.

As part of the visual cycle, the retinal chromophore in both rod and cone visual pigments undergoes reversible Schiff base hydrolysis and dissociation following photobleaching. We characterized light-activated release of retinal from a short-wavelength-sensitive cone pigment (VCOP) in 0.1% dodecyl maltoside using fluorescence spectroscopy. The half-time (t(1/2)) of release of retinal from VCOP was 7.1 s, 250-fold faster than that of rhodopsin. VCOP exhibited pH-dependent release kinetics, with the t(1/2) decreasing from 23 to 4 s with the pH decreasing from 4.1 to 8, respectively. However, the Arrhenius activation energy (E(a)) for VCOP derived from kinetic measurements between 4 and 20 °C was 17.4 kcal/mol, similar to the value of 18.5 kcal/mol for rhodopsin. There was a small kinetic isotope (D(2)O) effect in VCOP, but this effect was smaller than that observed in rhodopsin. Mutation of the primary Schiff base counterion (VCOP(D108A)) produced a pigment with an unprotonated chromophore (λ(max) = 360 nm) and dramatically slowed (t(1/2) ~ 6.8 min) light-dependent retinal release. Using homology modeling, a VCOP mutant with two substitutions (S85D and D108A) was designed to move the counterion one α-helical turn into the transmembrane region from the native position. This double mutant had a UV-visible absorption spectrum consistent with a protonated Schiff base (λ(max) = 420 nm). Moreover, the VCOP(S85D/D108A) mutant had retinal release kinetics (t(1/2) = 7 s) and an E(a) (18 kcal/mol) similar to those of the native pigment exhibiting no pH dependence. By contrast, the single mutant VCOP(S85D) had an ~3-fold decreased retinal release rate compared to that of the native pigment. Photoactivated VCOP(D108A) had kinetics comparable to those of a rhodopsin counterion mutant, Rho(E113Q), both requiring hydroxylamine to fully release retinal. These results demonstrate that the primary counterion of cone visual pigments is necessary for efficient Schiff base hydrolysis. We discuss how the large differences in retinal release rates between rod and cone visual pigments arise, not from inherent differences in the rate of Schiff base hydrolysis but rather from differences in the properties of noncovalent binding of the retinal chromophore to the protein.

Retina, retinol, retinal and the natural history of vitamin A as a light sensor.

Light is both the ultimate energy source for most organisms and a rich information source. Vitamin A-based chromophore was initially used in harvesting light energy, but has become the most widely used light sensor throughout evolution from unicellular to multicellular organisms. Vitamin A-based photoreceptor proteins are called opsins and have been used for billions of years for sensing light for vision or the equivalent of vision. All vitamin A-based light sensors for vision in the animal kingdom are G-protein coupled receptors, while those in unicellular organisms are light-gated channels. This first major switch in evolution was followed by two other major changes: the switch from bistable to monostable pigments for vision and the expansion of vitamin A's biological functions. Vitamin A's new functions such as regulating cell growth and differentiation from embryogenesis to adult are associated with increased toxicity with its random diffusion. In contrast to bistable pigments which can be regenerated by light, monostable pigments depend on complex enzymatic cycles for regeneration after every photoisomerization event. Here we discuss vitamin A functions and transport in the context of the natural history of vitamin A-based light sensors and propose that the expanding functions of vitamin A and the choice of monostable pigments are the likely evolutionary driving forces for precise, efficient, and sustained vitamin A transport.

Retinaldehyde is a substrate for human aldo-keto reductases of the 1C subfamily.

Human AKR (aldo-keto reductase) 1C proteins (AKR1C1-AKR1C4) exhibit relevant activity with steroids, regulating hormone signalling at the pre-receptor level. In the present study, investigate the activity of the four human AKR1C enzymes with retinol and retinaldehyde. All of the enzymes except AKR1C2 showed retinaldehyde reductase activity with low Km values (~1 µM). The kcat values were also low (0.18-0.6 min-1), except for AKR1C3 reduction of 9-cis-retinaldehyde whose kcat was remarkably higher (13 min-1). Structural modelling of the AKR1C complexes with 9-cis-retinaldehyde indicated a distinct conformation of Trp227, caused by changes in residue 226 that may contribute to the activity differences observed. This was partially supported by the kinetics of the AKR1C3 R226P mutant. Retinol/retinaldehyde conversion, combined with the use of the inhibitor flufenamic acid, indicated a relevant role for endogenous AKR1Cs in retinaldehyde reduction in MCF-7 breast cancer cells. Overexpression of AKR1C proteins depleted RA (retinoic acid) transactivation in HeLa cells treated with retinol. Thus AKR1Cs may decrease RA levels in vivo. Finally, by using lithocholic acid as an AKR1C3 inhibitor and UVI2024 as an RA receptor antagonist, we provide evidence that the pro-proliferative action of AKR1C3 in HL-60 cells involves the RA signalling pathway and that this is in part due to the retinaldehyde reductase activity of AKR1C3.

Toll-like receptor 3 is required for development of retinopathy caused by impaired all-trans-retinal clearance in mice.

Chronic inflammation is an important component that contributes to many age-related neurodegenerative diseases, including macular degeneration. Here, we report a role for toll-like receptor 3 (TLR3) in cone-rod dystrophy (CORD) of mice lacking ATP-binding cassette transporter 4 (ABCA4) and retinol dehydrogenase 8 (RDH8), proteins critical for all-trans-retinal clearance in the retina. Increased expression of toll-like receptor-signaling elements and inflammatory changes were observed in Rdh8(-/-)Abca4(-/-) eyes by RNA expression analysis. Unlike 3-month-old Rdh8(-/-)Abca4(-/-) mice that developed CORD, 6-month-old Tlr3(-/-)Rdh8(-/-)Abca4(-/-) mice did not evidence an abnormal retinal phenotype. Light-induced retinal degeneration in Tlr3(-/-)Rdh8(-/-)Abca4(-/-) mice was milder than that in Rdh8(-/-)Abca4(-/-) mice, and a 2-fold increased TLR3 expression was detected in light-illuminated retinas of Rdh8(-/-)Abca4(-/-) mice compared with nonilluminated retinas. Poly(I-C), a TLR3 ligand, caused caspase-8-independent cellular apoptosis. Whereas poly(I-C) induced retinal cell death in Rdh8(-/-)Abca4(-/-) and WT mice both in vivo and ex vivo, this was not seen in mice lacking Tlr3. Far fewer invasive macrophage/microglial cells in the subretinal space and weaker activation of Muller glial cells were exhibited by Tlr3(-/-)Rdh8(-/-) Abca4(-/-) mice compared with Rdh8(-/-)Abca4(-/-) mice at 3 and 6 months of age, indicating that loss of TLR3 inhibits local inflammation in the retina. Both poly(I-C) and endogenous products emanating from dying/dead retinal cells induced NF-κB and IRF3 activation. These findings demonstrate that endogenous products from degenerating retina stimulate TLR3 that causes cellular apoptosis and retinal inflammation and that loss of TLR3 protects mice from CORD.

Regulation of gene expression by retinoids.

Vitamin A serves as substrate for the biosynthesis of several derivates (retinoids) which are important for cell growth and cell differentiation as well as for vision. Retinoic acid is the major physiologically active form of vitamin A regulating the expression of different genes. At present, hundreds of genes are known to be regulated by retinoic acid. This regulation is very complex and is, in turn, regulated on many levels. To date, two families of retinoid nuclear receptors have been identified: retinoic acid receptors and retinoid X receptors, which are members of the steroid hormone receptor superfamily of ligand-activated transcription factors. In order to regulate gene expression, all-trans retinal needs to be oxidized to retinoic acid. All-trans retinal, in turn, can be produced during oxidation of all-trans retinol or in a retinol-independent metabolic pathway through cleavage of ß-carotene with all-trans retinal as an intermediate metabolite. Recently it has been shown that not only retinoic acid is an active form of vitamin A, but also that all-trans retinal can play an important role in gene regulation. In this review we comprehensively summarize recent literature on regulation of gene expression by retinoids, biochemistry of retinoid receptors, and molecular mechanisms of retinoid-mediated effects on gene regulation.

Probing potassium channel function in vivo by intracellular delivery of antibodies in a rat model of retinal neurodegeneration.

Inward rectifying potassium (Kir) channels participate in regulating potassium concentration (K(+)) in the central nervous system (CNS), including in the retina. We explored the contribution of Kir channels to retinal function by delivering Kir antibodies (Kir-Abs) into the rat eye in vivo to interrupt channel activity. Kir-Abs were coupled to a peptide carrier to reach intracellular epitopes. Functional effects were evaluated by recording the scotopic threshold response (STR) and photopic negative response (PhNR) of the electroretinogram (ERG) noninvasively with an electrode on the cornea to determine activity of the rod and cone pathways, respectively. Intravitreal delivery of Kir2.1-Ab coupled to the peptide carrier diminished these ERG responses equivalent to dimming the stimulus 10- to 100-fold. Immunohistochemistry (IHC) showed Kir2.1 immunostaining of retinal bipolar cells (BCs) matching the labeling pattern obtained with conventional IHC of applying Kir2.1-Ab to fixed retinal sections postmortem. Whole-cell voltage-clamp BC recordings in rat acute retinal slices showed suppression of barium-sensitive Kir2.1 currents upon inclusion of Kir2.1-Ab in the patch pipette. The in vivo functional and structural results implicate a contribution of Kir2.1 channel activity in these electronegative ERG potentials. Studies with Kir4.1-Ab administered in vivo also suppressed the ERG components and showed immunostaining of Müller cells. The strategy of administering Kir antibodies in vivo, coupled to a peptide carrier to facilitate intracellular delivery, identifies roles for Kir2.1 and Kir4.1 in ERG components arising in the proximal retina and suggests this approach could be of further value in research.

Cortical dynamics of navigation and steering in natural scenes: Motion-based object segmentation, heading, and obstacle avoidance.

Visually guided navigation through a cluttered natural scene is a challenging problem that animals and humans accomplish with ease. The ViSTARS neural model proposes how primates use motion information to segment objects and determine heading for purposes of goal approach and obstacle avoidance in response to video inputs from real and virtual environments. The model produces trajectories similar to those of human navigators. It does so by predicting how computationally complementary processes in cortical areas MT(-)/MSTv and MT(+)/MSTd compute object motion for tracking and self-motion for navigation, respectively. The model's retina responds to transients in the input stream. Model V1 generates a local speed and direction estimate. This local motion estimate is ambiguous due to the neural aperture problem. Model MT(+) interacts with MSTd via an attentive feedback loop to compute accurate heading estimates in MSTd that quantitatively simulate properties of human heading estimation data. Model MT(-) interacts with MSTv via an attentive feedback loop to compute accurate estimates of speed, direction and position of moving objects. This object information is combined with heading information to produce steering decisions wherein goals behave like attractors and obstacles behave like repellers. These steering decisions lead to navigational trajectories that closely match human performance.

Cone outer segment morphology and cone function in the Rpe65-/- Nrl-/- mouse retina are amenable to retinoid replacement.

PURPOSE: RPE65, a major retinal pigment epithelium protein, is essential in generating 11-cis retinal, the chromophore for all opsins. Without chromophore, cone opsins are mislocalized and cones degenerate rapidly (e.g., Rpe65(-/-) mouse). Function, survival, and correct targeting of opsins is increased in Rpe65(-/-) cones on supplying 11-cis retinal. Here, we determine the consequences of 11-cis retinal withdrawal and supplementation on cone development in the all-cone Nrl(-/-) retina. METHODS: Rpe65(-/-) Nrl(-/-), Nrl(-/-), and wild-type mice were examined. Cone structure was analyzed by using TUNEL assay, electron microscopy, and cone-specific antibodies. Cone function was assessed with light-adapted single-flash ERGs. RESULTS: Rpe65(-/-)Nrl(-/-) mice had an increased number of TUNEL-positive photoreceptors during programmed cell death compared with Nrl(-/-) mice, in addition to accelerated age-related degeneration. Cone function in Rpe65(-/-)Nrl(-/-) mice was minimal, and opsins were mislocalized. Treatment with 11-cis retinal restored cone function, promoted outer segment formation, and enabled opsin trafficking to outer segments. Eliminating Rpe65 prevented rosette formation in Nrl(-/-) retinas; supplementation of Rpe65(-/-)Nrl(-/-) mice with 11-cis retinal resulted in their reoccurrence. CONCLUSIONS: Taken together, function and opsin trafficking in Nrl(-/-) and wild-type cones are comparable, confirming and extending our findings that cone maturation and outer segment development are dependent on the presence of chromophore. The data on age-related cone death in Rpe65(-/-)Nrl(-/-) mice and the reintroduction of rosettes after 11-cis retinal injections confirm that outer segments, which for steric reasons appear to introduce rosettes in an all-cone retina, are essential for cell survival. These results are important for understanding and treating chromophore-related cone dystrophies.

Rlbp1 promoter drives robust Müller glial GFP expression in transgenic mice.

PURPOSE: Müller glia are essential for maintaining retinal homeostasis and exhibit neuroprotective and deleterious responses during retinal degeneration. Having the ability to visualize and genetically manipulate Müller glia in vivo will facilitate a better understanding of how these cells contribute to these processes. The goal of this study was to determine whether regulatory elements of the retinaldehyde binding protein 1 (Rlbp1; formerly Cralbp) gene can drive robust Müller glial gene expression in vivo. METHODS: Transgenic mice were generated by pronuclear injection of a construct carrying a 3-kilobase (kb) region of the Rlbp1 gene and 5'-flanking sequences linked to the enhanced green fluorescent protein (GFP) cDNA. GFP expression was analyzed by immunohistology in regions of the central nervous system in which RLBP1 protein is expressed, in retinas from wild-type and retinal degeneration 1 (rd1) mice, and during retinal development. RESULTS: Three transgenic lines were generated, and the one with the strongest and most consistent GFP expression was characterized further. Müller glia displayed robust GFP expression at all postnatal developmental stages and in the rd1 retina. Onset of expression occurred by birth in retinal progenitor cells. CONCLUSIONS: Regulatory elements in a restricted region of the Rlbp1 gene are sufficient to drive GFP expression in vivo. This transgenic line provides robust GFP expression that can be used to visualize retinal progenitor cells during postnatal development and Müller glia during their differentiation and in the healthy or degenerating adult retina.