[Vision restoration: science fiction or reality?] / Restauration de la vision: Science-fiction ou réalité ?

Visual prostheses aim at restoring useful vision to patients who have become blind. This useful vision should enable them to regain autonomy in society for navigation, face recognition or reading. Two retinal prostheses have already obtained market authorization for patients affected by retinal dystrophies while a new device is in clinical trials for patients affected by age-related macular degeneration. Various prostheses, in particular cortical prostheses, are currently in clinical trials for optic neuropathies (glaucoma). Optogenetic therapy, an alternative strategy, has now reached the stage of clinical trials at the retinal level while moving forward at the cortical level. Other innovating strategies have obtained proofs of concepts in rodents but require a further validation in large animals prior to their evaluation on patients. Restoring vision should therefore become a reality for many patients even if this vision will not be as extensive and perfect as natural vision.

TITLE: Restauration de la vision: Science-fiction ou réalité ? ABSTRACT: Les prothèses visuelles ont pour objet de redonner une vision utile aux patients devenus aveugles. Cette vision utile doit leur permettre de retrouver une autonomie dans la société pour leurs déplacements, la reconnaissance des visages ou la lecture. Plusieurs prothèses rétiniennes ont déjà obtenu l'autorisation de mise sur le marché pour les dystrophies rétiniennes alors qu'un nouveau dispositif est en essai clinique pour la dégénérescence maculaire liée à l'âge. D'autres prothèses, notamment corticales, sont en essai clinique pour les neuropathies optiques (glaucome). Des stratégies alternatives, comme la thérapie optogénétique, ont également atteint le stade des essais cliniques. D'autres ont été évaluées sur les rongeurs, attendant leur validation sur le gros animal. Revoir devrait donc prochainement devenir une réalité pour de nombreux patients, même si cette vision ne sera ni aussi étendue, ni aussi parfaite que la vision naturelle.

Contemporary approaches to visual prostheses.

Visual prostheses serve to restore visual function following acquired blindness. Acquired blindness (as opposed to congenital blindness) has many causes, including diseases such as retinitis pigmentosa, glaucoma, and macular degeneration, or trauma such as caused by automobile accident or blast damage from explosions. Many of the blindness-causing diseases target the retina or other ocular structure. Often, despite the loss of sensitivity to light, the remainder of the visual pathway is still functional, enabling electrical devices to deliver effective and meaningful visual information to the brain via arrays of electrodes. These arrays can be placed in any part of the early visual pathway, such as the retina, optic nerve, lateral geniculate nucleus, or visual cortex. A camera or other imaging source is used to drive electrical stimulation of remaining healthy cells or structures to create artificial vision and provide restoration of function. In this review, each approach to visual prostheses is described, including advantages and disadvantages as well as assessments of the current state of the art. Most of the work to-date has been targeting stimulation of (a) the retina, with three devices approved for general use and two more in clinical testing; (b) the lateral geniculate nucleus, with efforts still in the pre-clinical stage; and (c) the cortex, with three devices in clinical testing and none currently approved for general use despite the longest history of investigation of the three major approaches. Each class of device has different medical indications, and different levels of invasiveness required for implantation. All contemporary devices deliver relatively poor vision. There has been remarkable progress since the first proof-of-concept demonstration that used stimulation of the primary visual cortex, with the field exploring all viable options for restoration of function. Much of the progress has been recent, driven by advances in microelectronics and biocompatibility. With three devices currently approved for general use in various parts of the world, and a handful of additional devices well along in the pipeline toward approval, prospects for wide deployment of a device-based therapy to treat acquired blindness are good.

Optic nerve and retinal electrostimulation in rats: direct activation of the retinal ganglion cells.

Visual prosthesis is competing with biological approaches to restore vision to the blind. Understanding and developing the ability to replicate the neural code of the retina are key factors that can bring bionic vision significant advantage. Here, electrically evoked potentials were recorded in anesthetized rats from the dorsal surface of the superior colliculus. Electrical stimuli of different amplitudes were delivered at the retina and the optic nerve. An evoked potential appeared in both cases within the first 5 ms post-stimulus suggesting that this component of the response was initiated by direct activation of the retinal ganglion cells. However, in the case of retinal neurostimulation, a second evoked potential occurred $9.0 \pm 3.4$ ms after the stimulus delivery. Because this component was not present in the case of optic nerve electrostimulation, it is expected to be originated by the activation of other cells in the retinal network.

Transcorneal electrical stimulation promotes survival of retinal ganglion cells after optic nerve transection in rats accompanied by reduced microglial activation and TNF-α expression.

Microglial activation plays a crucial role in the pathological processes of various retinal and optic nerve diseases. TNF-α is a pro-inflammatory cytokine that is rapidly upregulated and promotes retinal ganglion cells (RGCs) death after optic nerve injury. However, the cellular source of TNF-α after optic nerve injury remains unclear. Thus, we aimed to determine the changes of retinal microglial activation in a rat model of optic nerve transection (ONT) after transcorneal electrical stimulation (TES). Furthermore, we assessed TNF-α expression after ONT and evaluated the effects of TES on TNF-α production. Rats were divided into 2 control groups receiving a sham surgery procedure, 2 ONT+Sham TES groups, and 2 ONT+TES groups. The rats were sacrificed on day 7 or 14 after ONT. RGCs were retrogradely labelled by Fluorogold (FG) 7 days before ONT, one TES group and corresponding controls were stimulated on day 0, 4, and the second were stimulated on day 0, 4, 7, 10. Whole-mount immunohistofluorescence, quantification of RGCs and microglia, and western blot analysis were performed on day 7 and 14 after ONT. TES significantly increased RGCs survival on day 7 and 14 after ONT, which was accompanied by reduced microglia on day 7, but not 14. TNF-α was co-localized with ameboid microglia and significantly increased on day 7 and 14 after ONT. TES significantly reduced TNF-α production on day 7 and 14 after ONT. Our study demonstrated that TES promotes RGCs survival after ONT accompanied by reduced microglial activation and microglia-derived TNF-α production.

Aligned Nanofibers from Polypyrrole/Graphene as Electrodes for Regeneration of Optic Nerve via Electrical Stimulation.

The damage of optic nerve will cause permanent visual field loss and irreversible ocular diseases, such as glaucoma. The damage of optic nerve is mainly derived from the atrophy, apoptosis or death of retinal ganglion cells (RGCs). Though some progress has been achieved on electronic retinal implants that can electrically stimulate undamaged parts of RGCs or retina to transfer signals, stimulated self-repair/regeneration of RGCs has not been realized yet. The key challenge for development of electrically stimulated regeneration of RGCs is the selection of stimulation electrodes with a sufficient safe charge injection limit (Q(inj), i.e., electrochemical capacitance). Most traditional electrodes tend to have low Q(inj) values. Herein, we synthesized polypyrrole functionalized graphene (PPy-G) via a facile but efficient polymerization-enhanced ball milling method for the first time. This technique could not only efficiently introduce electron-acceptor nitrogen to enhance capacitance, but also remain a conductive platform-the π-π conjugated carbon plane for charge transportation. PPy-G based aligned nanofibers were subsequently fabricated for guided growth and electrical stimulation (ES) of RGCs. Significantly enhanced viability, neurite outgrowth and antiaging ability of RGCs were observed after ES, suggesting possibilities for regeneration of optic nerve via ES on the suitable nanoelectrodes.

Injured adult retinal axons with Pten and Socs3 co-deletion reform active synapses with suprachiasmatic neurons.

Despite advances in promoting axonal regeneration after adult central nervous system injury, elicitation of a large number of lesion-passing axons reform active synaptic connections with natural target neurons remains limited. By deleting both Pten and Socs3 in retinal ganglion cells, we report that optic nerve axons after prechiasm lesion robustly reinnervate the hypothalamus, form new synapses with neurons in the suprachiasmatic nucleus (SCN), and re-integrate with the existing circuitry. Photic or electric stimulation of the retinal axons induces neuronal response in SCN. However both the innervation pattern and evoked responses are not completely restored by the regenerating axons, suggesting that combining with other strategies is necessary to overcome the defective rewiring. Our results support that boosting the intrinsic growth capacity in injured neurons promotes axonal reinnervation and rewiring.

Pharmacological inhibition of caspase-2 protects axotomised retinal ganglion cells from apoptosis in adult rats.

Severing the axons of retinal ganglion cells (RGC) by crushing the optic nerve (ONC) causes the majority of RGC to degenerate and die, primarily by apoptosis. We showed recently that after ONC in adult rats, caspase-2 activation occurred specifically in RGC while no localisation of caspase-3 was observed in ganglion cells but in cells of the inner nuclear layer. We further showed that inhibition of caspase-2 using a single injection of stably modified siRNA to caspase-2 protected almost all RGC from death at 7 days, offering significant protection for up to 1 month after ONC. In the present study, we confirmed that cleaved caspase-2 was localised and activated in RGC (and occasional neurons in the inner nuclear layer), while TUNEL⁺ RGC were also observed after ONC. We then investigated if suppression of caspase-2 using serial intravitreal injections of the pharmacological inhibitor z-VDVAD-fmk (z-VDVAD) protected RGC from death for 15 days after ONC. Treatment of eyes with z-VDVAD suppressed cleaved caspase-2 activation by >85% at 3-4 days after ONC. Increasing concentrations of z-VDVAD protected greater numbers of RGC from death at 15 days after ONC, up to a maximum of 60% using 4000 ng/ml of z-VDVAD, compared to PBS treated controls. The 15-day treatment with 4000 ng/ml of z-VDVAD after ONC suppressed levels of cleaved caspase-2 but no significant changes in levels of cleaved caspase-3, -6, -7 or -8 were detected. Although suppression of caspase-2 protected 60% of RGC from death, RGC axon regeneration was not promoted. These results suggest that caspase-2 specifically mediates death of RGC after ONC and that suppression of caspase-2 may be a useful therapeutic strategy to enhance RGC survival not only after axotomy but also in diseases where RGC death occurs such as glaucoma and optic neuritis.

Hyperbaric oxygen preconditioning promotes survival of retinal ganglion cells in a rat model of optic nerve crush.

In this study we tested the hypothesis that hyperbaric oxygen preconditioning (HBO-PC) reduces retinal neuronal death due to optic nerve crush (ONC). Adult male Sprague-Dawley rats were subjected to ONC accompanied by a contralateral sham operation. HBO-PC was conducted four times by giving 100% oxygen at 2.5 atmospheres absolute (ATA) for 1 h every 12 h for 2 days prior to ONC. The rats were euthanized at 1 or 2 weeks after ONC. Retinal ganglion cell (RGC) density was counted by hematoxylin and eosin (H&E) staining of the retina and retrograde labeling with FluoroGold application to the superior colliculus. Visual function was assessed by flash visual evoked potentials (FVEP). TUNEL straining and caspase-3 and caspase-9 activity in the retinas were assessed. The RGC density in the retinas of ONC HBO-PC-treated rats was significantly higher than that of the corresponding ONC-only rats (the survival rate was 67.2% versus 49.7% by H&E staining, and 60.3% versus 28.9% by retrograde labeling with FluoroGold, respectively; p < 0.01) at 2 weeks after ONC. FVEP measurements indicated a significantly better preserved latency and amplitude of the P1 wave in the ONC HBO-PC-treated rats than the ONC-only rats (92 +/- 7 msec, 21 +/- 3 microv in the sham-operated group, 117 +/- 12 msec, 14 +/- 2 microv in the HBO-PC-treated group, and 169 +/- 15 msec, 7 +/- 1 microv in the corresponding ONC group; p < 0.01). TUNEL assays showed fewer apoptotic cells in the HBO-PC-treated group, accompanied by the suppression of caspase-3 and caspase-9 activity. These results demonstrate that HBO-PC appears to be neuroprotective against ONC insult via inhibition of neuronal apoptosis pathways.

Effectiveness of iron repletion in the diet for the optic nerve development of anaemic rats.

PURPOSE: To compare the process of myelination in the developing optic nerve (ON) of anaemic rats with the subsequent recovery after being fed an iron-recovery diet. METHODS: In this study, the morphometrical parameters in the ON were assessed by electron microscopy in Wistar rats that were on an iron-deficient diet for 32 days or for 21 days followed by 10 days on an iron-recovery diet. Qualitative and quantitative analyses were performed using representative electron ultramicrographs. Data were analysed by one-way analysis of variance (ANOVA). When differences were detected, comparisons were made using Tukey's post hoc test (P<0.05 was considered to be significant). RESULTS: Qualitative analysis of the ONs in anaemic and recovered animals showed a higher rate of deformed axons and increased lamellar separation in the myelin sheath when compared with the respective control group. The ON of the anaemic group showed a reduced mean density of myelinated fibres when compared with the control group. The fibre area ratio, axon area ratio, and myelin area ratio of large axons/small axons in the ONs of the control group showed the highest values for the myelin areas, axon areas, and total fibre areas. The control group showed a significantly higher myelin sheath thickness when compared with the anaemic and recovered groups. CONCLUSIONS: Our data indicate that iron is necessary for maintenance of the ON cell structure, and that morphological damage from iron deficiency is not easily reverted by iron repletion.

Premise and prediction-how optic nerve head biomechanics underlies the susceptibility and clinical behavior of the aged optic nerve head.

We propose that age-related alterations in optic nerve head (ONH) biomechanics underlie the clinical behavior and increased susceptibility of the aged ONH to glaucomatous damage. The literature which suggests that the aged ONH is more susceptible to glaucomatous damage at all levels of intraocular pressure is reviewed. The relevant biomechanics of the aged ONH are discussed and a biomechanical explanation for why, on average, the stiffened peripapillary scleral and lamina cribrosa connective tissues of the aged eye should lead to a shallow (senile sclerotic) form of cupping is proposed. A logic for why age-related axon loss and the optic neuropathy of glaucoma in the aged eye may overlap is discussed. Finally, we argue for a need to characterize all forms of clinical cupping into prelaminar and laminar components so as to add precision to the discussion of clinical cupping which does not currently exist. Such characterization may lead to the early detection of ONH axonal and connective tissue pathology in ocular hypertension and eventually aid in the assessment of etiology in all forms of optic neuropathy including those that may be purely age-related.

Parameters of optic nerve electrical stimulation affecting neuroprotection of axotomized retinal ganglion cells in adult rats.

We previously showed the enhancement of survival of retinal ganglion cells (RGCs) by electrical stimulation (ES) of the optic nerve (ON) stump in adult rats. To elucidate the mechanisms underlying the survival enhancement, we determined whether the neuroprotective effect of ES is affected by the following parameters: stimulation time, frequency of current pulses and starting of ES. ES for 10min immediately after ON transection was not effective in increasing the number of surviving RGCs 7 days after the transection, but that for 30min was effective. ES at 20Hz was the most effective, when applied just after axotomy. When the starting of ES to the ON was shifted either 3h after or 4h before the axotomy, the neuroprotective effect of ES was not observed. These results suggest that the electrical activation of RGCs and/or the transected ON interfere with early events after axotomy that leads to RGC death.

A silicon retina that reproduces signals in the optic nerve.

Prosthetic devices may someday be used to treat lesions of the central nervous system. Similar to neural circuits, these prosthetic devices should adapt their properties over time, independent of external control. Here we describe an artificial retina, constructed in silicon using single-transistor synaptic primitives, with two forms of locally controlled adaptation: luminance adaptation and contrast gain control. Both forms of adaptation rely on local modulation of synaptic strength, thus meeting the criteria of internal control. Our device is the first to reproduce the responses of the four major ganglion cell types that drive visual cortex, producing 3600 spiking outputs in total. We demonstrate how the responses of our device's ganglion cells compare to those measured from the mammalian retina. Replicating the retina's synaptic organization in our chip made it possible to perform these computations using a hundred times less energy than a microprocessor-and to match the mammalian retina in size and weight. With this level of efficiency and autonomy, it is now possible to develop fully implantable intraocular prostheses.

Reduction of GABAergic neurons within the tectofugal visual system of white zebra finches.

The central visual system of white zebra finches is physiologically and anatomically different from normally coloured (wild type) animals. The main difference to normal birds is an enhanced response to ipsilateral stimulation in all areas of the tectofugal visual pathway. Previous experiments indicated that besides an enhancement of recrossing fibers, it might be a lack of inhibition which causes this effect. We show here that such an explanation can only be true for a part of the entopallium, the telencephalic station of this projection. Only within the so-called perientopallium, the number of GABAergic neurons is strongly reduced, while there is no significant difference between white and wild type birds in the other visual areas. It is speculated that these neurons in normal birds inhibit ipsilateral input conveyed by the second visual projection in birds, the thalamofugal pathway.

Evaluation of extraocular electrodes for a retinal prosthesis using evoked potentials in cat visual cortex.

OBJECTIVE: To assess the efficacy of a device using extraocular electrodes as a retinal prosthesis by evaluating the responses evoked in the visual cortex to electrical stimulation. METHODS: In anaesthetised cats, a lateral orbital dissection and ipsilateral parietal craniotomy was performed. Two extraocular retinal prosthesis (ERP) disc electrodes were sutured to the sclera on the lateral and superior aspects of the globe. Retinal stimulation was performed with charge-balanced constant-current pulses. Potentials evoked in the visual cortex were measured with a ball electrode placed on the lateral gyrus after removal of the dura. RESULTS: Stable attachment of the ERP electrodes to the globe was achieved with scleral sutures. Visual cortex responses were recorded with the electrodes in bipolar and monopolar configurations. The evoked response consisted of an early component with a peak around 8 ms, and a late component with a peak after 50 ms. Thresholds for evoking a response occurred at current intensities as low as 500 microA. Through extrapolation from evoked response amplitude data, thresholds as low as 300 microA were calculated. Cathodal monopolar stimulation demonstrated lower thresholds than anodal stimulation for evoking cortical responses. CONCLUSIONS: The ERP electrodes can be easily attached to the globe and are effective in electrically stimulating the retina, evoking responses in the primary visual cortex. Threshold charge-density was within safe limits for neural stimulation.

Simulation of intra-orbital optic nerve electrical stimulation.

In blind subjects who still have functional retinal ganglion cells, electrical stimuli applied to the optic nerve can produce localised visual sensations. This has been demonstrated with an intracranially implanted self-sizing spiral cuff electrode, but, to avoid skull opening, intra-orbital cuff implantation is now considered. In its orbital segment, the optic nerve is surrounded by subarachnoidal cerebrospinal fluid (CSF) and dura mater. Dura mater is a tough fibrous tissue that can impede electrical stimulation. In the study, the issue of whether or not to remove the dura mater at the implantation site was addressed using simulation on numerical models. Several volume conductor models were built representing, respectively: the cuff implanted directly around the nerve; the cuff over the nerve after connective tissue encapsulated the implant; and the cuff electrode placed around the dura mater. Stimulation-induced electric potential fields were computed for these configurations using a full 3D finite elements software. Responses of fibres within the nerve were computed. A large range of dural conductivities and several CSF thicknesses were considered. In all simulated conditions, the presence of dura mater around a layer of CSF increased excitation thresholds. Selectivity performance also decreased, but was found to be independent of the CSF thickness. However, simulations showed that, if the diameter of the cuff electrode is adapted to the target nerve, the injected charge associated with activation is limited within a reasonable range. Electrical stimulation of the optic nerve with a cuff electrode implanted around the dura mater should therefore be feasible.

A history of the optic nerve and its diseases.

We will trace the history of ideas about optic nerve anatomy and function in the Western world from the ancient Greeks to the early 20th century and show how these influenced causal theories of optic nerve diseases. Greek and Roman humoral physiology needed a hollow optic nerve, the obstruction of which prevented the flow of visual spirit to and from the brain and resulted in blindness. Medieval physicians understood that the presence of a fixed dilated pupil indicated optic nerve obstruction, preventing the passage of visual spirit, and that cataract surgery in such cases would not restore sight. During the Renaissance, the organ of vision was transferred from the lens to the optic nerve, which was generally believed to be on the axis of the eye. The acuity of central vision (at the optic disc) was explained by the concentration of visual spirit where the optic nerve met the retina. The growth of anatomy and influence of mechanical philosophy from the 17th century led to visual spirit being replaced with the concept of nerve force, which later became associated with electricity travelling along nerve fibres. This coincided with discourse about the nature of the nervous system and a shift in orientation from understanding illness holistically in terms of an individual's humoral imbalance to the concept of organ-based diseases. Both the microscope and the ophthalmoscope allowed visualisation of the optic nerve, but problems of interpretation persisted until conceptual transformations in medical science were made.

Effect of high dosage of methylprednisolone on rat retinal ganglion cell apoptosis after optic nerve crush.

PURPOSE: To investigate the effect of high dose methylprednisolone (MP) on retinal ganglion cells (RGC) apoptosis. METHODS: One hundred and twenty six Wistar rats were divided into normal control, crush control and MP treatment groups. High dose MP and 0.9% sodium chloride were injected intravenously at 1 h after injury in the treatment group and crush control group, respectively. Apoptotic cells, Bcl-2 and Bax positive cells were located and calculated by terminal-deoxynucleotidyl transferase mediated dUTP-biotin nick end labeling (TUNEL), immunohistochemical method and retinal whole-mount combined with section technique at day 4, 7, 14 after crush, respectively. RESULTS: The TUNEL-labeling cells, Bcl-2 and Bax positive cells were detected in the ganglion cell layer of retina. Few apoptotic cells, a few of Bcl-2 and Bax positive cells were observed in normal control groups. The numbers of positive cells increased gradually after crush. The number of apoptotic cells and Bax positive tells in treatment groups was less than that in crush group significantly (P < 0.05), while the number of Bcl-2 positive cells in treatment group was more than that in crush group at day 4, 7 and 14 after crush significantly (P < 0.05). CONCLUSIONS: These results suggest that high dose MP can inhibit the apoptosis of RGC after optic nerve crush probably through up-regulation of Bcl-2 expression and down-regulation of Bax expression.

Role of purpurin as a retinol-binding protein in goldfish retina during the early stage of optic nerve regeneration: its priming action on neurite outgrowth.

Unlike mammals, the fish optic nerve can regenerate after injury. So far, many growth or trophic factors have been shown as an axon-regenerating molecule. However, it is totally unknown what substance regulates or triggers the activity of these factors on axonal elongation. Therefore, we constructed a goldfish retina cDNA library prepared from the retina treated with optic nerve transection 5 d previously, when it was just before regrowing optic axons after injury. A cDNA clone for goldfish purpurin for which expression was upregulated during the early stage of optic nerve regeneration was isolated from the retina cDNA library. Purpurin was discovered as a secretory retinol-binding protein in developing chicken retinas. Levels of purpurin mRNA and protein transiently increased and rapidly decreased 2-5 d and 10 d after axotomy, respectively. Purpurin mRNA was localized to the photoreceptor cells, whereas the protein was diffusely found in all of the retinal layers. A recombinant purpurin alone did not affect any change of neurite outgrowth in explant culture of the control retina, whereas a concomitant addition of the recombinant purpurin and retinol first induced a drastic enhancement of neurite outgrowth. Furthermore, the action of retinol-bound purpurin was effective only in the control (untreated) retinas but not in those primed (treated) with a previous optic nerve transection. Thus, purpurin with retinol is the first candidate molecule of priming neurite outgrowth in the early stage of optic nerve regeneration in fish.

Regulation of cyclic adenosine 3',5'- monophosphate signaling and pulsatile neurosecretion by Gi-coupled plasma membrane estrogen receptors in immortalized gonadotrophin-releasing hormone neurons.

Immortalized GnRH neurons (GT1-7) express receptors for estrogen [estrogen receptor-alpha and-13(ERa and ERI3)] and progesterone (progesterone receptor A) and exhibit positive immunostaining for both intracellular and plasma membrane ERs. Exposure of GT1-7 cells to picomolar estradiol concentrations for 5-60 min caused rapid, sustained,and dose-dependent inhibition of cAMP production. In contrast, treatment with nanomolar estradiol concentrations for 60 min increased cAMP production. The inhibitory and stimulatory actions of estradiol on cAMP formation were abolished by the ER antagonist, ICI 182,780. The estradiol-induced inhibition of cAMP production was prevented by treatment with pertussis toxin, consistent with coupling of the plasma membrane ER to an inhibitory G protein. Coimmunoprecipitation studies demonstrated an estradiol-regulated stimulatory interaction between ERa and G,3 that was prevented by the ER antagonist, ICI 182,780. Exposure of perifused GT1-7 cells and hypothalamic neurons to picomolar estradiol levels increased the GnRH peak interval, shortened peak duration, and increased peak amplitude. These findings indicate that occupancy of the plasma membrane-associated ERs expressed in GT1-7 neurons by physio-logical estradiol levels causes activation of a G, protein and modulates cAMP signaling and neuropeptide secretion.