Light-activated channels targeted to ON bipolar cells restore visual function in retinal degeneration

Regenerative nanomedicine for vision restoration

Herein, we discuss recent applications of nanotechnology to ophthalmology, including nanoparticles for drug, gene, and trophic factor delivery; regenerative medicine (in the areas of optogenetics and optic nerve regeneration); and diagnostics (eg, minimally invasive biometric monitoring). Specific applications for the management of choroidal neovascularization, retinal neovascularization, oxidative damage, optic nerve damage, and retinal degenerative disease are considered. Nanotechnology will play an important role in early- and late-stage interventions in the management of blinding diseases.

Strategies for expanding the operational range of channelrhodopsin in optogenetic vision

Some hereditary diseases, such as retinitis pigmentosa, lead to blindness due to the death of photoreceptors, though the rest of the visual system might be only slightly affected. Optogenetics is a promising tool for restoring vision after retinal degeneration. In optogenetics, light-sensitive ion channels ("channelrhodopsins") are expressed in neurons so that the neurons can be activated by light. Currently existing variants of channelrhodopsin – engineered for use in neurophysiological research – do not necessarily support the goal of vision restoration optimally, due to two factors: First, the nature of the light stimulus is fundamentally different in "optogenetic vision" compared to "optogenetic neuroscience". Second, the retinal target neurons have specific properties that need to be accounted for, e.g. most retinal neurons are non-spiking. In this study, by using a computational model, we investigate properties of channelrhodopsin that might improve successful vision restoration. We pay particular attention to the operational brightness range and suggest strategies that would allow optogenetic vision over a wider intensity range than currently possible, spanning the brightest 5 orders of naturally occurring luminance. We also discuss the biophysical limitations of channelrhodopsin, and of the expressing cells, that prevent further expansion of this operational range, and we suggest design strategies for optogenetic tools which might help overcoming these limitations. Furthermore, the computational model used for this study is provided as an interactive tool for the research community.

Proceedings of the First International Optogenetic Therapies for Vision Symposium

Optogenetics is a research field that uses gene therapy to deliver a gene encoding a light-activated protein to cells providing light-regulated control of targeted cell pathways. The technology is a popular tool in many fields of neuroscience, used to transiently switch cells on and off, for example, to map neural circuits. In inherited retinal degenerative diseases, where loss of vision results from the loss of photoreceptors, optogenetics can be applied to either augment the function of surviving photoreceptors or confer light sensitivity to naturally nonlight sensitive retinal cells, such as a bipolar cells. This can be achieved either by the light sensitive protein integrating with native internal signaling pathways, or by using a dual function membrane protein that integrates light signaling with an ion channel or pump activity. Exposing treated cells to light of the correct wavelength activates the protein, resulting in cellular depolarization or hyperpolarization that triggers neurological signaling to the visual cortex. While there is a lot of interest in optogenetics as a pan-disease clinical treatment for end-stage application in the inherited degenerative diseases of the retina, research to date has been limited to nonhuman clinical studies. To address the clinical translational needs of this technology, the Foundation Fighting Blindness and Massachusetts Eye and Ear Infirmary cohosted an International Optogenetic Therapies for Vision Workshop, which was held at Massachusetts Eye and Ear Infirmary, Boston, Massachusetts on June 1, 2012.

A system for optically controlling neural circuits with very high spatial and temporal resolution

Optogenetics offers a powerful new approach for controlling neural circuits. It has a vast array of applications in both basic and clinical science. For basic science, it opens the door to unraveling circuit operations, since one can perturb specific circuit components with high spatial (single cell) and high temporal (millisecond) resolution. For clinical applications, it allows new kinds of selective treatments, because it provides a method to inactivate or activate specific components in a malfunctioning circuit and bring it back into a normal operating range [1-3]. To harness the power of optogenetics, though, one needs stimulating tools that work with the same high spatial and temporal resolution as the molecules themselves, the channelrhodopsins. To date, most stimulating tools require a tradeoff between spatial and temporal precision and are prohibitively expensive to integrate into a stimulating/recording setup in a laboratory or a device in a clinical setting [4, 5]. Here we describe a Digital Light Processing (DLP)-based system capable of extremely high temporal resolution (sub-millisecond), without sacrificing spatial resolution. Furthermore, it is constructed using off-the-shelf components, making it feasible for a broad range of biology and bioengineering labs. Using transgenic mice that express channelrhodopsin-2 (ChR2), we demonstrate the system's capability for stimulating channelrhodopsin-expressing neurons in tissue with single cell and sub-millisecond precision.

[Retinitis pigmentosa: eye sight restoration by optogenetic therapy]

Retinitis pigmentosa (RP) is a hereditary retinal disease leading to blindness, which affects two million people worldwide. Restoring vision in these blind patients was proposed by gene delivery of microbial light-activated ionic channels or pumps "optogenetic proteins" to transform surviving cells into artificial photoreceptors. This therapeutic strategy was validated in blind animal models of RP by recording at the level of the retina and cortex and by behavioural tests. The translational potentials of these optogenetic approaches have been evaluated using in vitro studies on post-mortem human retinal tissues. Here, we review these recent results and discuss the potential clinical applications of the optogenetic therapy for RP patients.

Holographic display system for restoration of sight to the blind

OBJECTIVE

We present a holographic near-the-eye display system enabling optical approaches for sight restoration to the blind, such as photovoltaic retinal prosthesis, optogenetic and other photoactivation techniques. We compare it with conventional liquid crystal displays (LCD) or digital light processing (DLP)-based displays in terms of image quality, field of view, optical efficiency and safety.

APPROACH

We detail the optical configuration of the holographic display system and its characterization using a phase-only spatial light modulator.

MAIN RESULTS

We describe approaches to controlling the zero diffraction order and speckle related issues in holographic display systems and assess the image quality of such systems. We show that holographic techniques offer significant advantages in terms of peak irradiance and power efficiency, and enable designs that are inherently safer than LCD or DLP-based systems. We demonstrate the performance of our holographic display system in the assessment of cortical response to alternating gratings projected onto the retinas of rats.

SIGNIFICANCE

We address the issues associated with the design of high brightness, near-the-eye display systems and propose solutions to the efficiency and safety challenges with an optical design which could be miniaturized and mounted onto goggles.

Alterations of sodium and potassium channels of RGCs in RCS rat with the development of retinal degeneration

All know that retinitis pigmentosa (RP) is a group of hereditary retinal degenerative diseases characterized by progressive dysfunction of photoreceptors and associated with progressive cells loss; nevertheless, little is known about how rods and cones loss affects the surviving inner retinal neurons and networks. Retinal ganglion cells (RGCs) process and convey visual information from retina to visual centers in the brain. The healthy various ion channels determine the normal reception and projection of visual signals from RGCs. Previous work on the Royal College of Surgeons (RCS) rat, as a kind of classical RP animal model, indicated that, at late stages of retinal degeneration in RCS rat, RGCs were also morphologically and functionally affected. Here, retrograde labeling for RGCs with Fluorogold was performed to investigate the distribution, density, and morphological changes of RGCs during retinal degeneration. Then, patch clamp recording, western blot, and immunofluorescence staining were performed to study the channels of sodium and potassium properties of RGCs, so as to explore the molecular and proteinic basis for understanding the alterations of RGCs membrane properties and firing functions. We found that the resting membrane potential, input resistance, and capacitance of RGCs changed significantly at the late stage of retinal degeneration. Action potential could not be evoked in a part of RGCs. Inward sodium current and outward potassium current recording showed that sodium current was impaired severely but only slightly in potassium current. Expressions of sodium channel protein were impaired dramatically at the late stage of retinal degeneration. The results suggested that the density of RGCs decreased, process ramification impaired, and sodium ion channel proteins destructed, which led to the impairment of electrophysiological functions of RGCs and eventually resulted in the loss of visual function.

Optogenetic cell control in experimental models of neurological disorders

The complexity of the brain, in which different neuronal cell types are interspersed and complexly interconnected, has posed a major obstacle in identifying pathophysiological mechanisms underlying prevalent neurological disorders. This is largely based in the inability of classical experimental approaches to target defined neural populations at sufficient temporal and spatial resolution. As a consequence, effective clinical therapies for prevalent neurological disorders are largely lacking. Recently developed optogenetic probes are genetically expressed photosensitive ion channels and pumps that in principal overcome these limitations. Optogenetic probes allow millisecond resolution functional control over selected optogenetically transduced neuronal populations targeted based on promoter activity. This optical cell control scheme has already been applied to answer fundamental questions pertaining to neurological disorders by allowing researchers to experimentally intercept, or induce, pathophysiological neuronal signaling activity in a highly controlled manner. Offering high temporal resolution control over neural activity at high cellular specificity, optogenetic tools constitute a game changer in research aiming at understanding pathophysiological signaling mechanisms in neurological disorders and in developing therapeutic strategies to correct these. In this regard, recent experimental work has provided new insights in underlying mechanisms, as well as preliminary proof-of-principle for optogenetic therapies, of several neurological disorders, including Parkinson's disease, epilepsy and progressive blindness. This review synthesizes experimental work where optogenetic tools have been applied to explore pathologic neural network activity in models of neurological disorders.

The effect of various levels of stroboscopic illumination on the growth of guinea pig eyes

BACKGROUND

The aim was to investigate various levels of stroboscopic illumination effect on the growth of guinea pig eyes.

METHODS

Thirty-six two-week-old guinea pigs were randomised to one of three treatment groups (n = 12 for each). Two stroboscopic-reared groups were raised with a duty diurnal cycle of 50 per cent at a flash rate of 0.5 Hz. Illumination intensity varied between zero-to-250 lux or zero-to-500 lux during each cycle in each group, respectively. The third control group was exposed to 250 lux illumination. Refraction and biometric measurements were taken for each animal prior to and after two, four, six and eight weeks of treatment. Finally, retinal microstructure was examined.

RESULTS

There was significant correlation between refractive errors and axial elongation. After eight weeks of treatment, illumination with flickering light 0-250 lux caused a larger myopic shift with increased axial length than illumination of continuous 250 lux. Stroboscopic illumination with zero-to-500 lux caused a further myopic shift and longer axial length than stroboscopic illumination with zero-to-250 lux. In animals raised in flickering light of zero-to-250 lux or zero-to-500 lux for eight weeks, the outer segment disc membranes in photoreceptor layers were found deformed and detached.

CONCLUSION

Chronic exposure to low-frequency temporally modulated illumination-induced histological damage in the retina and induced exaggerated axial length elongation.

Light scattering properties vary across different regions of the adult mouse brain

Recently developed optogenetic tools provide powerful approaches to optically excite or inhibit neural activity. In a typical in-vivo experiment, light is delivered to deep nuclei via an implanted optical fiber. Light intensity attenuates with increasing distance from the fiber tip, determining the volume of tissue in which optogenetic proteins can successfully be activated. However, whether and how this volume of effective light intensity varies as a function of brain region or wavelength has not been systematically studied. The goal of this study was to measure and compare how light scatters in different areas of the mouse brain. We delivered different wavelengths of light via optical fibers to acute slices of mouse brainstem, midbrain and forebrain tissue. We measured light intensity as a function of distance from the fiber tip, and used the data to model the spread of light in specific regions of the mouse brain. We found substantial differences in effective attenuation coefficients among different brain areas, which lead to substantial differences in light intensity demands for optogenetic experiments. The use of light of different wavelengths additionally changes how light illuminates a given brain area. We created a brain atlas of effective attenuation coefficients of the adult mouse brain, and integrated our data into an application that can be used to estimate light scattering as well as required light intensity for optogenetic manipulation within a given volume of tissue.

Recent Innovations in Medical and Surgical Retina

PURPOSE

To highlight recent innovations in the medical and surgical treatment of retinal disease.

DESIGN

Retrospective literature review.

METHODS

The peer-reviewed, published (during 2012) English literature was searched using the Medline database. Articles deemed relevant were selected and highlighted.

RESULTS

A number of important innovations in the treatment of retinal disease have occurred during the past year. We review developments in regenerative medicine (e.g., retinal prosthesis, optogenetics, cell-based therapy), diagnostics (e.g., swept source optical coherence tomography [OCT], intraoperative OCT, multimodal imaging), surgical innovations (e.g., chemical vitreolysis with ocriplasmin, 27-gauge vitrectomy), and pharmacological therapy of retinal vascular disease (e.g., use of ranibizumab and aflibercept in the treatment of macular edema associated with diabetic retinopathy and retinal vein occlusion).

CONCLUSIONS

Many important innovations in regenerative medicine, diagnostic and surgical instrumentation, and pharmacological therapy for retinal vascular disease have occurred during the past year. In view of the number of agents in early phase clinical trials (e.g., treatments for dry age-related macular degeneration) as well as technologies under development (e.g., microrobots for vitreous surgery), this trend will continue.

Retinal ganglion cells are resistant to photoreceptor loss in retinal degeneration

The rapid and massive degeneration of photoreceptors in retinal degeneration might have a dramatic negative effect on retinal circuits downstream of photoreceptors. However, the impact of photoreceptor loss on the morphology and function of retinal ganglion cells (RGCs) is not fully understood, precluding the rational design of therapeutic interventions that can reverse the progressive loss of retinal function. The present study investigated the morphological changes in several identified RGCs in the retinal degeneration rd1 mouse model of retinitis pigmentosa (RP), using a combination of viral transfection, microinjection of neurobiotin and confocal microscopy. Individual RGCs were visualized with a high degree of detail using an adeno-associated virus (AAV) vector carrying the gene for enhanced green fluorescent protein (EGFP), allowed for large-scale surveys of the morphology of RGCs over a wide age range. Interestingly, we found that the RGCs of nine different types we encountered were especially resistant to photoreceptor degeneration, and retained their fine dendritic geometry well beyond the complete death of photoreceptors. In addition, the RGC-specific markers revealed a remarkable degree of stability in both morphology and numbers of two identified types of RGCs for up to 18 months of age. Collectively, our data suggest that ganglion cells, the only output cells of the retina, are well preserved morphologically, indicating the ganglion cell population might be an attractive target for treating vision loss.

Holographic optogenetic stimulation of patterned neuronal activity for vision restoration

When natural photoreception is disrupted, as in outer-retinal degenerative diseases, artificial stimulation of surviving nerve cells offers a potential strategy for bypassing compromised neural circuits. Recently, light-sensitive proteins that photosensitize quiescent neurons have generated unprecedented opportunities for optogenetic neuronal control, inspiring early development of optical retinal prostheses. Selectively exciting large neural populations are essential for eliciting meaningful perceptions in the brain. Here we provide the first demonstration of holographic photo-stimulation strategies for bionic vision restoration. In blind retinas, we demonstrate reliable holographically patterned optogenetic stimulation of retinal ganglion cells with millisecond temporal precision and cellular resolution. Holographic excitation strategies could enable flexible control over distributed neuronal circuits, potentially paving the way towards high-acuity vision restoration devices and additional medical and scientific neuro-photonics applications.

rAAV-mediated subcellular targeting of optogenetic tools in retinal ganglion cells in vivo

Expression of optogenetic tools in surviving inner retinal neurons to impart retinal light sensitivity has been a new strategy for restoring vision after photoreceptor degeneration. One potential approach for restoring retinal light sensitivity after photoreceptor degeneration is to express optogenetic tools in retinal ganglion cells (RGCs). For this approach, restoration of ON and OFF center-surround receptive fields in RGCs, a key feature of visual information processing, may be important. A possible solution is to differentially express depolarizing and hyperpolarizing optogenetic tools, such as channelrhodopsin-2 and halorhodopsin, to the center and peripheral regions of the RGC dendritic field by using protein targeting motifs. Recombinant adeno-associated virus (rAAV) vectors have proven to be a powerful vehicle for in vitro and in vivo gene delivery, including in the retina. Therefore, the search for protein targeting motifs that can achieve rAAV-mediated subcellular targeted expression would be particularly valuable for developing therapeutic applications. In this study, we identified two protein motifs that are suitable for rAAV-mediated subcellular targeting for generating center-surround receptive fields while reducing the axonal expression in RGCs. Resulting morphological dendritic field and physiological response field by center-targeting were significantly smaller than those produced by surround-targeting. rAAV motif-mediated protein targeting could also be a valuable tool for studying physiological function and clinical applications in other areas of the central nervous system.

Gene therapy for blindness

Sight-restoring therapy for the visually impaired and blind is a major unmet medical need. Ocular gene therapy is a rational choice for restoring vision or preventing the loss of vision because most blinding diseases originate in cellular components of the eye, a compartment that is optimally suited for the delivery of genes, and many of these diseases have a genetic origin or genetic component. In recent years we have witnessed major advances in the field of ocular gene therapy, and proof-of-concept studies are under way to evaluate the safety and efficacy of human gene therapies. Here we discuss the concepts and recent advances in gene therapy in the retina. Our review discusses traditional approaches such as gene replacement and neuroprotection and also new avenues such as optogenetic therapies. We conjecture that advances in gene therapy in the retina will pave the way for gene therapies in other parts of the brain.

Channelrhodopsins: visual regeneration and neural activation by a light switch

The advent of optogenetics provides a new direction for the field of neuroscience and biotechnology, serving both as a refined investigative tool and as potential cure for many medical conditions via genetic manipulation. Although still in its infancy, recent advances in optogenetics has made it possible to remotely manipulate in vivo cellular functions using light. Coined Nature Methods' 'Method of the Year' in 2010, the optogenetic toolbox has the potential to control cell, tissue and even animal behaviour. This optogenetic toolbox consists of light-sensitive proteins that are able to modulate membrane potential in response to light. Channelrhodopsins (ChR) are light-gated microbial ion channels, which were first described in green algae. ChR2 (a subset of ChR) is a seven transmembrane α helix protein, which evokes membrane depolarization and mediates an action potential upon photostimulation with blue (470 nm) light. By contrast to other seven-transmembrane proteins that require second messengers to open ion channels, ChR2 form ion channels themselves, allowing ultrafast depolarization (within 50 milliseconds of illumination). It has been shown that integration of ChR2 into various tissues of mice can activate neural circuits, control heart muscle contractions, and even restore breathing after spinal cord injury. More compellingly, a plethora of evidence has indicated that artificial expression of ChR2 in retinal ganglion cells can reinstate visual perception in mice with retinal degeneration.

Light-triggered modulation of cellular electrical activity by ruthenium diimine nanoswitches

Ruthenium diimine complexes have previously been used to facilitate light-activated electron transfer in the study of redox metalloproteins. Excitation at 488 nm leads to a photoexcited state, in which the complex can either accept or donate an electron, respectively, in the presence of a soluble sacrificial reductant or oxidant. Here, we describe a novel application of these complexes in mediating light-induced changes in cellular electrical activity. We demonstrate that RubpyC17 ([Ru(bpy)(2)(bpy-C17)](2+), where bpy is 2,2'-bipyridine and bpy-C17 is 2,2'-4-heptadecyl-4'-methyl-bipyridine), readily incorporates into the plasma membrane of cells, as evidenced by membrane-confined luminescence. Excitable cells incubated in RubpyC17 and then illuminated at 488 nm in the presence of the reductant ascorbate undergo membrane depolarization leading to firing of action potentials. In contrast, the same experiment performed with the oxidant ferricyanide, instead of ascorbate, leads to hyperpolarization. These experiments suggest that illumination of membrane-associated RubpyC17 in the presence of ascorbate alters the cell membrane potential by increasing the negative charge on the outer face of the cell membrane capacitor, effectively depolarizing the cell membrane. We rule out two alternative explanations for light-induced membrane potential changes, using patch clamp experiments: (1) light-induced direct interaction of RubpyC17 with ion channels and (2) light-induced membrane perforation. We show that incorporation of RubpyC17 into the plasma membrane of neuroendocrine cells enables light-induced secretion as monitored by amperometry. While the present work is focused on ruthenium diimine complexes, the findings point more generally to broader application of other transition metal complexes to mediate light-induced biological changes.

Optogenetic manipulation of neural and non-neural functions

Optogenetic manipulation of the neuronal activity enables one to analyze the neuronal network both in vivo and in vitro with precise spatio-temporal resolution. Channelrhodopsins (ChRs) are light-sensitive cation channels that depolarize the cell membrane, whereas halorhodopsins and archaerhodopsins are light-sensitive Cl(-) and H(+) transporters, respectively, that hyperpolarize it when exogenously expressed. The cause-effect relationship between a neuron and its function in the brain is thus bi-directionally investigated with evidence of necessity and sufficiency. In this review we discuss the potential of optogenetics with a focus on three major requirements for its application: (i) selection of the light-sensitive proteins optimal for optogenetic investigation, (ii) targeted expression of these selected proteins in a specific group of neurons, and (iii) targeted irradiation with high spatiotemporal resolution. We also discuss recent progress in the application of optogenetics to studies of non-neural cells such as glial cells, cardiac and skeletal myocytes. In combination with stem cell technology, optogenetics may be key to successful research using embryonic stem cells (ESCs) and induced pluripotent stem cells (iPSCs) derived from human patients through optical regulation of differentiation-maturation, through optical manipulation of tissue transplants and, furthermore, through facilitating survival and integration of transplants.

Age-related macular degeneration revisited--piecing the puzzle: the LXIX Edward Jackson memorial lecture

PURPOSE

To present the current understanding of age-related macular degeneration (AMD) pathogenesis, based on clinical evidence, epidemiologic data, histopathologic examination, and genetic data; to provide an update on current and emerging therapies; and to propose an integrated model of the pathogenesis of AMD.

DESIGN

Review of published clinical and experimental studies.

METHODS

Analysis and synthesis of clinical and experimental data.

RESULTS

We are closer to a complete understanding of the pathogenesis of AMD, having progressed from clinical observations to epidemiologic observations and clinical pathologic correlation. More recently, modern genetic and genomic studies have facilitated the exploration of molecular pathways. It seems that AMD is a complex disease that results from the interaction of genetic susceptibility with aging and environmental factors. Disease progression also seems to be driven by a combination of genetic and environmental factors.

CONCLUSIONS

Therapies based on pathophysiologic features have changed the paradigm for treating neovascular AMD. With improved understanding of the underlying genetic susceptibility, we can identify targets to halt early disease and to prevent progression and vision loss.

Optogenetics in neuroscience: what we gain from studies in mammals

Optogenetics is a newly-introduced technology in the life sciences and is gaining increasing attention. It refers to the combination of optical technologies and genetic methods to control the activity of specific cell groups in living tissue, during which high-resolution spatial and temporal manipulation of cells is achieved. Optogenetics has been applied to numerous regions, including cerebral cortex, hippocampus, ventral tegmental area, nucleus accumbens, striatum, spinal cord, and retina, and has revealed new directions of research in neuroscience and the treatment of related diseases. Since optogenetic tools are controllable at high spatial and temporal resolution, we discuss its applications in these regions in detail and the recent understanding of higher brain functions, such as reward-seeking, learning and memory, and sleep. Further, the possibilities of improved utility of this newly-emerging technology are discussed. We intend to provide a paradigm of the latest advances in neuroscience using optogenetics.

Photochemical restoration of visual responses in blind mice

Retinitis pigmentosa (RP) and age-related macular degeneration (AMD) are degenerative blinding diseases caused by the death of rods and cones, leaving the remainder of the visual system intact but largely unable to respond to light. Here, we show that AAQ, a synthetic small molecule photoswitch, can restore light sensitivity to the retina and behavioral responses in vivo in mouse models of RP, without exogenous gene delivery. Brief application of AAQ bestows prolonged light sensitivity on multiple types of retinal neurons, resulting in synaptically amplified responses and center-surround antagonism in arrays of retinal ganglion cells (RGCs). Intraocular injection of AAQ restores the pupillary light reflex and locomotory light avoidance behavior in mice lacking retinal photoreceptors, indicating reconstitution of light signaling to brain circuits. AAQ and related photoswitch molecules present a potential drug strategy for restoring retinal function in degenerative blinding diseases.

The neuronal organization of the retina

The mammalian retina consists of neurons of >60 distinct types, each playing a specific role in processing visual images. They are arranged in three main stages. The first decomposes the outputs of the rod and cone photoreceptors into ∼12 parallel information streams. The second connects these streams to specific types of retinal ganglion cells. The third combines bipolar and amacrine cell activity to create the diverse encodings of the visual world--roughly 20 of them--that the retina transmits to the brain. New transformations of the visual input continue to be found: at least half of the encodings sent to the brain (ganglion cell response selectivities) remain to be discovered. This diversity of the retina's outputs has yet to be incorporated into our understanding of higher visual function.

Synthetic biology with surgical precision: targeted reengineering of signaling proteins

The complexity of living systems exceeds everything else studied by natural sciences. Sophisticated networks of intimately intertwined signaling pathways coordinate cellular functions. Clear understanding how the integration of multiple inputs produces coherent behavior is one of the major challenges of cell biology. Integration via perfectly timed highly regulated protein-protein interactions and precise targeting of the "output" proteins to particular substrates is emerging as a common theme of signaling regulation. This often involves specialized scaffolding proteins, whose key function is to ensure that correct partners come together in an appropriate place at the right time. Defective or faulty signaling underlies many congenital and acquired human disorders. Several pioneering studies showed that ectopic expression of existing proteins or their elements can restore functions destroyed by mutations or normalize the signaling pushed out of balance by disease and/or current small molecule-based therapy. Several recent studies show that proteins with new functional modalities can be generated by mixing and matching existing domains, or via functional recalibration and fine-tuning of existing proteins by precisely targeted mutations. Using arrestins as an example, we describe how manipulation of individual functions yields signaling-biased proteins. Creative protein redesign generates novel tools valuable for unraveling the intricacies of cell biology. Engineered proteins with specific functional changes also have huge therapeutic potential in disorders associated with inherited or acquired signaling errors.

Channelrhodopsins-Their potential in gene therapy for neurological disorders

Recently, channelrhodopsins (ChRs) have begun to be used to manipulate the neuronal activity, since they can be targeted to specific neurons or neural circuits using genetic methods. To advance the potential applications in the investigation and treatment of neurological disorders, the following types of basic research should receive extensive financial support. The spectral and kinetic properties of ChRs should be optimized according to the application by generating variants of ChRs or exploring new rhodopsins from other species. These ChRs should be targeted to the specific types of neurons involved in the neurological disorders through a gene expression system using cell- or tissue-specific promoters/enhancers as well as gene delivery systems with modified virus vectors. The methods have to be developed to apply the genes of interest with safety and long-term effectiveness. Sophisticated opto-electrical devices should be developed. Appropriate primate animal model systems should be established to minimize the structural differences between small animals such as rodents and human beings. In this paper, we will review the current progress in the basic research concerned with the potential clinical application of ChRs and discuss the future directions of research on ChRs so that they could be applied for human welfare.

Cellular Resolution Panretinal Imaging of Optogenetic Probes Using a Simple Funduscope

PURPOSE

To acquire and characterize cellular-resolved in vivo fluorescence images of optogenetic probes expressed in rodent retinal ganglion cells, by adapting a low-cost and simple fundus system based on a topical endoscope.

METHODS

A custom endoscope-based fundus system was constructed (adapted from the design of Paques and colleagues). Bright field and fluorescence images were acquired from head-fixed transgenic mice expressing Channelrhodopsin2-eYFP, and Sprague Dawley rats virally transfected with the optogenetic probe GCaMP3. Images were compared to in vitro images of the same structures and were analyzed.

RESULTS

The fundus system provides high-quality, high-resolution fluorescence images of the eye fundus that span the whole retina. The images allow resolving individual cells and axon bundles in the Channelrhodopsin2-eYFP mice and cellular-scale structures in the GCaMP3 expressing rats. The resolution in mouse eyes was estimated to be better than 20 μm (full width at half maximum) and is only marginally dependent on movement-related blurring.

CONCLUSIONS

The fluorescence-endoscopy fundus system provides a powerful yet simple and widely accessible tool for obtaining cellular resolved fluorescent images of optogenetic and other fluorescent probes.

TRANSLATIONAL RELEVANCE

The new system could prove to be a basic tool for non-invasive in vivo small animal retinal imaging in a wide array of translational vision applications, including the tracking of fluorescently tagged cells and the expression of gene-therapy and optogenetic vectors.

Optogenetics: a novel optical manipulation tool for medical investigation

Optogenetics is a new and rapidly evolving gene and neuroengineering technology that allows optical control of specific populations of neurons without affecting other neurons in the brain at high temporal and spatial resolution. By heterologous expression of the light-sensitive membrane proteins, cell type-specific depolarization or hyperpolarization can be optically induced on a millisecond time scale. Optogenetics has the higher selectivity and specificity compared to traditional electrophysiological techniques and pharmaceutical methods. It has been a novel promising tool for medical research. Because of easy handling, high temporal and spatial precision, optogenetics has been applied to many aspects of nervous system research, such as tactual neural circuit, visual neural circuit, auditory neural circuit and olfactory neural circuit, as well as research of some neurological diseases. The review highlights the recent advances of optogenetics in medical study.

Development and degeneration of cone bipolar cells are independent of cone photoreceptors in a mouse model of retinitis pigmentosa

Retinal photoreceptors die during retinal synaptogenesis in a portion of retinal degeneration. Whether cone bipolar cells establish regular retinal mosaics and mature morphologies, and resist degeneration are not completely understood. To explore these issues, we backcrossed a transgenic mouse expressing enhanced green fluorescent protein (EGFP) in one subset of cone bipolar cells (type 7) into rd1 mice, a classic mouse model of retinal degeneration, to examine the development and survival of cone bipolar cells in a background of retinal degeneration. Our data revealed that both the development and degeneration of cone bipolar cells are independent of the normal activity of cone photoreceptors. We found that type 7 cone bipolar cells achieved a uniform tiling of the retinal surface and developed normal dendritic and axonal arbors without the influence of cone photoreceptor innervation. On the other hand, degeneration of type 7 cone bipolar cells, contrary to our belief of central-to-peripheral progression, was spatially uniform across the retina independent of the spatiotemporal pattern of cone degeneration. The results have important implications for the design of more effective therapies to restore vision in retinal degeneration.

Recording from defined populations of retinal ganglion cells using a high-density CMOS-integrated microelectrode array with real-time switchable electrode selection

In order to understand how retinal circuits encode visual scenes, the neural activity of defined populations of retinal ganglion cells (RGCs) has to be investigated. Here we report on a method for stimulating, detecting, and subsequently targeting defined populations of RGCs. The possibility to select a distinct population of RGCs for extracellular recording enables the design of experiments that can increase our understanding of how these neurons extract precise spatio-temporal features from the visual scene, and how the brain interprets retinal signals. We used light stimulation to elicit a response from physiologically distinct types of RGCs and then utilized the dynamic-configurability capabilities of a microelectronics-based high-density microelectrode array (MEA) to record their synchronous action potentials. The layout characteristics of the MEA made it possible to stimulate and record from multiple, highly overlapping RGCs simultaneously without light-induced artifacts. The high-density of electrodes and the high signal-to-noise ratio of the MEA circuitry allowed for recording of the activity of each RGC on 14±7 electrodes. The spatial features of the electrical activity of each RGC greatly facilitated spike sorting. We were thus able to localize, identify and record from defined RGCs within a region of mouse retina. In addition, we stimulated and recorded from genetically modified RGCs to demonstrate the applicability of optogenetic methods, which introduces an additional feature to target a defined cell type. The developed methodologies can likewise be applied to other neuronal preparations including brain slices or cultured neurons.

Undersized dendritic arborizations in retinal ganglion cells of the rd1 mutant mouse: a paradigm of early onset photoreceptor degeneration

Retinitis pigmentosa (RP) is a family of inherited diseases causing progressive photoreceptor death. Retinal ganglion cells (RGCs) form the biological substrate for various therapeutic approaches designed to restore vision in RP individuals. Assessment of survival and preservation of RGCs in animal paradigms mimicking the human disease is of key importance for appropriate implementation of vision repair strategies. Here we studied the survival of RGCs in the rd1 mutant mouse, a known model of early onset, autosomic recessive RP, at various stages of photoreceptor degeneration. Furthermore, we analyzed the morphology of various types of RGCs using the newly generated transgenic mouse rd1/Thy1-GFP, in which the rd1 mutation is associated with green fluorescent protein (GFP) expression in a small population of different RGCs. We found excellent survival of cells at up to 1 year of age, a time at which the inner retina is known to have severely reorganized and partially degenerated. However, 50% of the cells analyzed within all RGC types exhibit an undersized dendritic tree, spanning about half of the normal area. Undersized cells are found both in adult and in very young (1-month-old) mice. This suggests that their aberrant phenotype is due to incomplete dendritic development, possibly as a consequence of altered visual input at the time of dendritic arbor refinement. These data show the importance of the timing of photoreceptor death in RGC dendritic development.

Retinal prosthetic strategy with the capacity to restore normal vision

Retinal prosthetics offer hope for patients with retinal degenerative diseases. There are 20-25 million people worldwide who are blind or facing blindness due to these diseases, and they have few treatment options. Drug therapies are able to help a small fraction of the population, but for the vast majority, their best hope is through prosthetic devices [reviewed in Chader et al. (2009) Prog Brain Res 175:317-332]. Current prosthetics, however, are still very limited in the vision that they provide: for example, they allow for perception of spots of light and high-contrast edges, but not natural images. Efforts to improve prosthetic capabilities have focused largely on increasing the resolution of the device's stimulators (either electrodes or optogenetic transducers). Here, we show that a second factor is also critical: driving the stimulators with the retina's neural code. Using the mouse as a model system, we generated a prosthetic system that incorporates the code. This dramatically increased the system's capabilities--well beyond what can be achieved just by increasing resolution. Furthermore, the results show, using 9,800 optogenetically stimulated ganglion cell responses, that the combined effect of using the code and high-resolution stimulation is able to bring prosthetic capabilities into the realm of normal image representation.

Light-controlled tools

Spatial and temporal control over chemical and biological processes plays a key role in life, where the whole is often much more than the sum of its parts. Quite trivially, the molecules of a cell do not form a living system if they are only arranged in a random fashion. If we want to understand these relationships and especially the problems arising from malfunction, tools are necessary that allow us to design sophisticated experiments that address these questions. Highly valuable in this respect are external triggers that enable us to precisely determine where, when, and to what extent a process is started or stopped. Light is an ideal external trigger: It is highly selective and if applied correctly also harmless. It can be generated and manipulated with well-established techniques, and many ways exist to apply light to living systems--from cells to higher organisms. This Review will focus on developments over the last six years and includes discussions on the underlying technologies as well as their applications.

AAV-mediated gene therapy in mouse models of recessive retinal degeneration

In recent years, more and more mutant genes that cause retinal diseases have been detected. At the same time, many naturally occurring mouse models of retinal degeneration have also been found, which show similar changes to human retinal diseases. These, together with improved viral vector quality allow more and more traditionally incurable inherited retinal disorders to become potential candidates for gene therapy. Currently, the most common vehicle to deliver the therapeutic gene into target retinal cells is the adenoassociated viral vector (AAV). Following delivery to the immuno-privileged subretinal space, AAV-vectors can efficiently target both retinal pigment epithelium and photoreceptor cells, the origin of most retinal degenerations. This review focuses on the AAV-based gene therapy in mouse models of recessive retinal degenerations, especially those in which delivery of the correct copy of the wild-type gene has led to significant beneficial effects on visual function, as determined by morphological, biochemical, electroretinographic and behavioral analysis. The past studies in animal models and ongoing successful LCA2 clinical trials, predict a bright future for AAV gene replacement treatment for inherited recessive retinal diseases.

High-frequency limit of neural stimulation with ChR2

Optogenetic technology based on light activation of genetically targeted single component opsins such as Channelrhodopsin-2 (ChR2) has been changing the way neuroscience research is conducted. This technology is becoming increasingly important for neural engineering as well. The efficiency of neural stimulation with ChR2 drops at high frequencies, often before the natural limit of the neuron is reached. This study aims to investigate the underlying mechanisms that limit the efficiency of the stimulation at high frequencies. The study analyzes the dynamics of the spikes induced by ChR2 in comparison to control stimulations using patch clamp current injection. It shows that the stimulation dynamics is limited by two mechanisms: 1) a frequency independent reduction in the conductance-to-irradiance yield due to the ChR2 light adaptation process and 2) a frequency dependent reduction in the conductance-to-current yield due to a decrease in membrane re-polarization level between spikes that weakens the ionic driving force. The effect of the first mechanism can be minimized by using ChR2 mutants with lower irradiance threshold. In contrast the effect of the second mechanism is fundamentally limited by the rate the native ion channels re-polarize the membrane potential.

Challenges in neuroprotective nanomedicine development: progress towards noninvasive gene therapy of glaucoma

Over the past decade the application of gene therapy of retinal diseases such as glaucoma has produced promising results. However, optic nerve regeneration and restoration of vision in patients with glaucoma is still far from reality. Neuroprotective approaches in the form of gene therapy may provide significant advantages, but are still limited by many factors both at the organ and cellular levels. In general, gene delivery systems for eye diseases range from simple eye drops and ointments to more advanced bio- and nanotechnology-based systems such as muco-adhesive systems, polymers, liposomes and ocular inserts. Most of these technologies were developed for front-of-the-eye ophthalmic therapies and are not applicable as back-of-the-eye delivery systems. Currently, only the invasive intravitreal injections are capable of successfully delivering genes to the retina. Here we review the challenges and possible strategies for the noninvasive gene therapy of glaucoma including the barriers in the eye and in neural cells, and present a cross-sectional view of gene delivery as it pertains to the prevention and treatment of glaucoma.

Spontaneous activity promotes synapse formation in a cell-type-dependent manner in the developing retina

Spontaneous activity is thought to regulate synaptogenesis in many parts of the developing nervous system. In vivo evidence for this regulation, however, is scarce and comes almost exclusively from experiments in which normal activity was reduced or blocked completely. Thus, whether spontaneous activity itself promotes synaptogenesis or plays a purely permissive role remains uncertain. In addition, how activity influences synapse dynamics to shape connectivity and whether its effects among neurons are uniform or cell-type-dependent is unclear. In mice lacking the cone-rod homeobox gene (Crx), photoreceptors fail to establish normal connections with bipolar cells (BCs). Here, we find that retinal ganglion cells (RGCs) in Crx⁻/⁻ mice become rhythmically hyperactive around the time of eye opening as a result of increased spontaneous glutamate release from BCs. This elevated neurotransmission enhances synaptogenesis between BCs and RGCs, without altering the overall circuit architecture. Using live imaging, we discover that spontaneous activity selectively regulates the rate of synapse formation, not elimination, in this circuit. Reconstructions of the connectivity patterns of three BC types with a shared RGC target further revealed that neurotransmission specifically promotes the formation of multisynaptic appositions from one BC type without affecting the maintenance or elimination of connections from the other two. Although hyperactivity in Crx⁻/⁻ mice persists, synapse numbers do not increase beyond 4 weeks of age, suggesting closure of a critical period for synaptic refinement in the inner retina. Interestingly, despite their hyperactivity, RGC axons maintain normal eye-specific territories and cell-type-specific layers in the dorsal lateral geniculate nucleus.

Retinal remodeling

Retinal photoreceptor degeneration takes many forms. Mutations in rhodopsin genes or disorders of the retinal pigment epithelium, defects in the adenosine triphosphate binding cassette transporter, ABCR gene defects, receptor tyrosine kinase defects, ciliopathies and transport defects, defects in both transducin and arrestin, defects in rod cyclic guanosine 3',5'-monophosphate phosphodiesterase, peripherin defects, defects in metabotropic glutamate receptors, synthetic enzymatic defects, defects in genes associated with signaling, and many more can all result in retinal degenerative disease like retinitis pigmentosa (RP) or RP-like disorders. Age-related macular degeneration (AMD) and AMD-like disorders are possibly due to a constellation of potential gene targets and gene/gene interactions, while other defects result in diabetic retinopathy or glaucoma. However, all of these insults as well as traumatic insults to the retina result in retinal remodeling. Retinal remodeling is a universal finding subsequent to retinal degenerative disease that results in deafferentation of the neural retina from photoreceptor input as downstream neuronal elements respond to loss of input with negative plasticity. This negative plasticity is not passive in the face of photoreceptor degeneration, with a phased revision of retinal structure and function found at the molecular, synaptic, cell, and tissue levels involving all cell classes in the retina, including neurons and glia. Retinal remodeling has direct implications for the rescue of vision loss through bionic or biological approaches, as circuit revision in the retina corrupts any potential surrogate photoreceptor input to a remnant neural retina. However, there are a number of potential opportunities for intervention that are revealed through the study of retinal remodeling, including therapies that are designed to slow down photoreceptor loss, interventions that are designed to limit or arrest remodeling events, and optogenetic approaches that target appropriate classes of neurons in the remnant neural retina.

Electrical stimulation of retinal neurons in epiretinal and subretinal configuration using a multicapacitor array

Electrical stimulation of retinal neurons offers the possibility of partial restoration of visual function. Challenges in neuroprosthetic applications are the long-term stability of the metal-based devices and the physiological activation of retinal circuitry. In this study, we demonstrate electrical stimulation of different classes of retinal neurons with a multicapacitor array. The array—insulated by an inert oxide—allows for safe stimulation with monophasic anodal or cathodal current pulses of low amplitude. Ex vivo rabbit retinas were interfaced in either epiretinal or subretinal configuration to the multicapacitor array. The evoked activity was recorded from ganglion cells that respond to light increments by an extracellular tungsten electrode. First, a monophasic epiretinal cathodal or a subretinal anodal current pulse evokes a complex burst of action potentials in ganglion cells. The first action potential occurs within 1 ms and is attributed to direct stimulation. Within the next milliseconds additional spikes are evoked through bipolar cell or photoreceptor depolarization, as confirmed by pharmacological blockers. Second, monophasic epiretinal anodal or subretinal cathodal currents elicit spikes in ganglion cells by hyperpolarization of photoreceptor terminals. These stimuli mimic the photoreceptor response to light increments. Third, the stimulation symmetry between current polarities (anodal/cathodal) and retina-array configuration (epi/sub) is confirmed in an experiment in which stimuli presented at different positions reveal the center-surround organization of the ganglion cell. A simple biophysical model that relies on voltage changes of cell terminals in the transretinal electric field above the stimulation capacitor explains our results. This study provides a comprehensive guide for efficient stimulation of different retinal neuronal classes with low-amplitude capacitive currents.

Restoration of vision after transplantation of photoreceptors

Cell transplantation is a potential strategy for treating blindness caused by the loss of photoreceptors. Although transplanted rod-precursor cells are able to migrate into the adult retina and differentiate to acquire the specialized morphological features of mature photoreceptor cells, the fundamental question remains whether transplantation of photoreceptor cells can actually improve vision. Here we provide evidence of functional rod-mediated vision after photoreceptor transplantation in adult Gnat1−/− mice, which lack rod function and are a model of congenital stationary night blindness. We show that transplanted rod precursors form classic triad synaptic connections with second-order bipolar and horizontal cells in the recipient retina. The newly integrated photoreceptor cells are light-responsive with dim-flash kinetics similar to adult wild-type photoreceptors. By using intrinsic imaging under scotopic conditions we demonstrate that visual signals generated by transplanted rods are projected to higher visual areas, including V1. Moreover, these cells are capable of driving optokinetic head tracking and visually guided behaviour in the Gnat1−/− mouse under scotopic conditions. Together, these results demonstrate the feasibility of photoreceptor transplantation as a therapeutic strategy for restoring vision after retinal degeneration.

Engineering therapies in the CNS: what works and what can be translated

Engineering is the art of taking what we know and using it to solve problems. As engineers, we build tool chests of approaches; we attempt to learn as much as possible about the problem at hand, and then we design, build, and test our approaches to see how they impact the system. The challenge of applying this approach to the central nervous system (CNS) is that we often do not know the details of what is needed from the biological side. New therapeutic options for treating the CNS range from new biomaterials to make scaffolds, to novel drug-delivery techniques, to functional electrical stimulation. However, the reality is that translating these new therapies and making them widely available to patients requires collaborations between scientists, engineers, clinicians, and patients to have the greatest chance of success. Here we discuss a variety of new treatment strategies and explore the pragmatic challenges involved with engineering therapies in the CNS.