Delivery of 9-Cis Retinal to Photoreceptors from Bovine Serum Albumin

An Ex Vivo Electroretinographic Apparatus for the mL-Scale Testing of Drugs to One Day and Beyond

When screening new drugs to treat retinal diseases, ex vivo electroretinography (ERG) potentially combines the experimental throughput of its traditional in vivo counterpart, with greater mechanistic insight and reproducible delivery. To date, this technique was used in experiments with open loop superfusion and lasting up to a few hours. Here, we present a compact apparatus that provides continuous and simultaneous recordings of the scotopic a-waves from four mouse retinas for much longer durations. Crucially, each retina can be incubated at 37 °C in only 2 mL of static medium, enabling the testing of very expensive drugs or nano devices. Light sensitivity and response kinetics of these preparations remain in the physiological range throughout incubation, displaying only very slow drifts. As an example application, we showed that barium, a potassium channel blocker used to abolish the glial component of the ERG, displayed no overt side effects on photoreceptors over several hours. In another example, we fully regenerated a partially bleached retina using a minimal quantity of 9-cis-retinal. Finally, we demonstrated that including antibiotic in the incubation medium extends physiological light responses to over one day. This system represents a necessary stepping stone towards the goal of combining ERG recordings with organotypically cultured retinas.

A Cambrian origin for vertebrate rods

Vertebrates acquired dim-light vision when an ancestral cone evolved into the rod photoreceptor at an unknown stage preceding the last common ancestor of extant jawed vertebrates (∼420 million years ago Ma). The jawless lampreys provide a unique opportunity to constrain the timing of this advance, as their line diverged ∼505 Ma and later displayed high-morphological stability. We recorded with patch electrodes the inner segment photovoltages and with suction electrodes the outer segment photocurrents of Lampetra fluviatilis retinal photoreceptors. Several key functional features of jawed vertebrate rods are present in their phylogenetically homologous photoreceptors in lamprey: crucially, the efficient amplification of the effect of single photons, measured by multiple parameters, and the flow of rod signals into cones. These results make convergent evolution in the jawless and jawed vertebrate lines unlikely and indicate an early origin of rods, implying strong selective pressure toward dim-light vision in Cambrian ecosystems.

DOI:http://dx.doi.org/10.7554/eLife.07166.001

The eyes of humans and many other animals with backbones contain two different types of cells that can detect light, which are known as rod and cone cells. Rod cells are much more sensitive to light than cone cells. The rods allow us to see in dim light by amplifying weak light signals and transmitting information to other cells, including the cones themselves. It is thought that the rod cell evolved from the cone cell in the common ancestors of mammals, fish, and other animals with backbones and jaws at least 420 million years ago.

Lampreys are jawless fish that diverged from the ancestors of jawed animals around 505 million years ago, in the middle of a period of great evolutionary innovation called the Cambrian. They have changed relatively little since that time so they provide a snapshot of what our ancestors' eyes might have been like back then. Like the rod and cone cells of jawed animals, the eyes of adult lampreys also have two types of photoreceptors. However, it was not clear whether the lamprey photoreceptor cells work in a similar way to rod and cone cells. Asteriti et al. collected lampreys in Sweden and France during their breeding season and used patch and suction electrodes to measure the activity of their photoreceptor cells. The experiments show that the short photoreceptor cells are more sensitive to light than the long photoreceptors and are able to amplify weak light signals. Also, the short photoreceptors send signals to the long photoreceptors in a similar way to how rod cells send information to cone cells.

The similarities between lamprey photoreceptor cells and those of jawed animals support the idea that they have a common origin in evolutionary history. Therefore, Asteriti et al. conclude that the ability to see in low light evolved before these groups of animals diverged about 505 million years ago. The picture that emerges is one in which our remote ancestors inhabiting the Cambrian seas already possessed dim-light vision. This would have allowed them to colonize deep waters or to move at twilight, an adaptation suggestive of intense competition or predation from other life forms.

DOI:http://dx.doi.org/10.7554/eLife.07166.002

Nanoparticle-based technologies for retinal gene therapy

For patients with hereditary retinal diseases, retinal gene therapy offers significant promise for the prevention of retinal degeneration. While adeno-associated virus (AAV)-based systems remain the most popular gene delivery method due to their high efficiency and successful clinical results, other delivery systems, such as non-viral nanoparticles (NPs) are being developed as additional therapeutic options. NP technologies come in several categories (e.g., polymer, liposomes, peptide compacted DNA), several of which have been tested in mouse models of retinal disease. Here, we discuss the key biochemical features of the different NPs that influence how they are internalized into cells, escape from endosomes, and are delivered into the nucleus. We review the primary mechanism of NP uptake by retinal cells and highlight various NPs that have been successfully used for in vivo gene delivery to the retina and RPE. Finally, we consider the various strategies that can be implemented in the plasmid DNA to generate persistent, high levels of gene expression.

Cellular retinol binding protein 1 modulates photoreceptor outer segment folding in the isolated eye

In a previous study, we used differential proteomics to identify retinal proteins whose steady-state levels were altered in an experimental system in which photoreceptor outer segments were improperly folded. We determined that the steady-state level of cellular retinol binding protein 1 (CRBP1) was downregulated in eyes lacking organized outer segments. The purpose of this study was to determine if CRBP1 is a plausible candidate for regulating outer segment assembly. We used Morpholinos to directly test the hypothesis that a decreased level of CRBP1 protein was associated with the misfolding of outer segments. Results from these studies indicate that downregulation of CRBP1 protein resulted in aberrant assembly of outer segments. Because CRBP1 plays a dual role in the retina-retinal recycling and generation of retinoic acid-we evaluated both possibilities. Our data demonstrate that outer segment folding was not modified by 11-cis retinal supplementation, suggesting that CRBP1 influences outer segment assembly through a mechanism unrelated to rhodopsin regeneration. In contrast, retinoic acid is required for the proper organization of nascent outer segment membranes. The localization of CRBP1 within Muller cells and the RPE and its demonstrated role in modulating the proper folding of nascent outer segment membranes through retinoic acid further elucidates the role of these cells in directly influencing photoreceptor physiology.

Substitution of 5-HT1A receptor signaling by a light-activated G protein-coupled receptor

Understanding serotonergic (5-HT) signaling is critical for understanding human physiology, behavior, and neuropsychiatric disease. 5-HT mediates its actions via ionotropic and metabotropic 5-HT receptors. The 5-HT(1A) receptor is a metabotropic G protein-coupled receptor linked to the G(i/o) signaling pathway and has been specifically implicated in the pathogenesis of depression and anxiety. To understand and precisely control 5-HT(1A) signaling, we created a light-activated G protein-coupled receptor that targets into 5-HT(1A) receptor domains and substitutes for endogenous 5-HT(1A) receptors. To induce 5-HT(1A)-like targeting, vertebrate rhodopsin was tagged with the C-terminal domain (CT) of 5-HT(1A) (Rh-CT(5-HT1A)). Rh-CT(5-HT1A) activates G protein-coupled inward rectifying K(+) channels in response to light and causes membrane hyperpolarization in hippocampal neurons, similar to the agonist-induced responses of the 5-HT(1A) receptor. The intracellular distribution of Rh-CT(5-HT1A) resembles that of the 5-HT(1A) receptor; Rh-CT(5-HT1A) localizes to somatodendritic sites and is efficiently trafficked to distal dendritic processes. Additionally, neuronal expression of Rh-CT(5-HT1A), but not Rh, decreases 5-HT(1A) agonist sensitivity, suggesting that Rh-CT(5-HT1A) and 5-HT(1A) receptors compete to interact with the same trafficking machinery. Finally, Rh-CT(5-HT1A) is able to rescue 5-HT(1A) signaling of 5-HT(1A) KO mice in cultured neurons and in slices of the dorsal raphe showing that Rh-CT(5-HT1A) is able to functionally compensate for native 5-HT(1A). Thus, as an optogenetic tool, Rh-CT(5-HT1A) has the potential to directly correlate in vivo 5-HT(1A) signaling with 5-HT neuron activity and behavior in both normal animals and animal models of neuropsychiatric disease.

Isorhodopsin rather than rhodopsin mediates rod function in RPE65 knock-out mice

The chromophore of visual pigments is 11-cis-retinal and, thus, in its absence, opsin is not photosensitive and no visual function exists. However, in the RPE65 knockout (Rpe65-/-) mouse, where synthesis of 11-cis-retinal does not occur, a minimal visual response from rod photoreceptors is obtained. We have examined if an alternative pathway exists for cis-retinoid generation in the absence of RPE65. Cyclic-light-reared, 2-month-old Rpe65-/- mice were placed in complete darkness. No exogenous retinoids were administered. After 4 weeks, enhanced a- and b-wave amplitudes were obtained, increasing >10-fold for the a-wave and >3-fold for the b-wave as compared with cyclic-light-reared Rpe65-/- mice. Visual-pigment levels increased to approximately 10 pmol per retina, compared with no measurable pigment for cyclic-light-reared Rpe65-/- mice. The lambdamax of the isolated pigment was 487 nm, characteristic for isorhodopsin. Retinoid extractions confirmed the presence of 9-cis-retinal and the absence of 11-cis-retinal. Once the Rpe65-/- mice were returned to cyclic light, within 48 h the electroretinogram function returned to levels found in Rpe65-/- mice maintained in cyclic light. This dark-mediated pathway is also operational in older animals, because 13-month-old Rpe65-/- mice kept in prolonged darkness (12 weeks) had increased isorhodopsin levels and electroretinogram a- and b-wave amplitudes. These studies demonstrate that a pathway exists in the eye for the generation of 9-cis-retinal that is independent of RPE65 and light.

Confronting complexity: the interlink of phototransduction and retinoid metabolism in the vertebrate retina

Absorption of light by rhodopsin or cone pigments in photoreceptors triggers photoisomerization of their universal chromophore, 11-cis-retinal, to all-trans-retinal. This photoreaction is the initial step in phototransduction that ultimately leads to the sensation of vision. Currently, a great deal of effort is directed toward elucidating mechanisms that return photoreceptors to the dark-adapted state, and processes that restore rhodopsin and counterbalance the bleaching of rhodopsin. Most notably, enzymatic isomerization of all-trans-retinal to 11-cis-retinal, called the visual cycle (or more properly the retinoid cycle), is required for regeneration of these visual pigments. Regeneration begins in rods and cones when all-trans-retinal is reduced to all-trans-retinol. The process continues in adjacent retinal pigment epithelial cells (RPE), where a complex set of reactions converts all-trans-retinol to 11-cis-retinal. Although remarkable progress has been made over the past decade in understanding the phototransduction cascade, our understanding of the retinoid cycle remains rudimentary. The aim of this review is to summarize recent developments in our current understanding of the retinoid cycle at the molecular level, and to examine the relevance of these reactions to phototransduction.