Optogenetic Stimulation for Restoring Vision to Patients Suffering From Retinal Degenerative Diseases: Current Strategies and Future Directions

Optogenetic therapy for retinal degenerative diseases: A review

Optogenetics, a cutting-edge tool in novel gene manipulation and drug discovery, holds significant therapeutic potential for a variety of neurological disorders, including retinal diseases. Retinal diseases such as age-related macular degeneration (AMD) and retinitis pigmentosa (RP), significantly impair quality of life and cause severe visual impairment due to limited treatment options and a general lack of awareness. The increasing incidence of these degenerative conditions underscores the need for innovative solutions, such as optogenetics. Optogenetic therapy introduces genes coding for light-sensitive proteins, which are controlled by light signals to make neurons photosensitive. This precise targeting approach does not require specific gene intervention and can bypass dysfunctional photoreceptors, offering a treatment option for various degenerative and dystrophic eye diseases. Successful outcomes in patients with late-stage genetic retinal diseases and numerous clinical trials suggest that optogenetics could be an effective treatment for humans. This review provides an overview of the current landscape of optogenetic therapy, discusses its challenges, and summarizes the findings of ongoing clinical trials for neural and visual restoration.

Combined optogenetic and electrical stimulation of the sciatic nerve for selective control of sensory fibers

Introduction

Electrical stimulation offers a drug-free alternative for the treatment of many neurological conditions, such as chronic pain. However, it is not easy to selectively activate afferent or efferent fibers of mixed nerves, nor their functional subtypes. Optogenetics overcomes these issues by controlling activity selectively in genetically modified fibers, however the reliability of responses to light are poor compared to electrical stimulation and the high intensities of light required present considerable translational challenges. In this study we employed a combined protocol of optical and electrical stimulation to the sciatic nerve in an optogenetic mouse model to allow for better selectivity, efficiency, and safety to overcome fundamental limitations of electrical-only and optical-only stimulation.

Methods

The sciatic nerve was surgically exposed in anesthetized mice (n = 12) expressing the ChR2-H134R opsin via the parvalbumin promoter. A custom-made peripheral nerve cuff electrode and a 452 nm laser-coupled optical fiber were used to elicit neural activity utilizing optical-only, electrical-only, or combined stimulation. Activation thresholds for the individual and combined responses were measured.

Results

Optically evoked responses had a conduction velocity of 34.3 m/s, consistent with ChR2-H134R expression in proprioceptive and low-threshold mechanoreceptor (Aα/Aβ) fibers which was also confirmed via immunohistochemical methods. Combined stimulation, utilizing a 1 ms near-threshold light pulse followed by an electrical pulse 0.5 ms later, approximately halved the electrical threshold for activation (p = 0.006, n = 5) and resulted in a 5.5 dB increase in the Aα/Aβ hybrid response amplitude compared to the electrical-only response at equivalent electrical levels (p = 0.003, n = 6). As a result, there was a 3.25 dB increase in the therapeutic stimulation window between the Aα/Aβ fiber and myogenic thresholds (p = 0.008, n = 4).

Discussion

The results demonstrate that light can be used to prime the optogenetically modified neural population to reside near threshold, thereby selectively reducing the electrical threshold for neural activation in these fibers. This reduces the amount of light needed for activation for increased safety and reduces potential off-target effects by only stimulating the fibers of interest. Since Aα/Aβ fibers are potential targets for neuromodulation in chronic pain conditions, these findings could be used to develop effective strategies to selectively manipulate pain transmission pathways in the periphery.

Artificial intelligence techniques for retinal prostheses: a comprehensive review and future direction

Objective. Retinal prostheses are promising devices to restore vision for patients with severe age-related macular degeneration or retinitis pigmentosa disease. The visual processing mechanism embodied in retinal prostheses play an important role in the restoration effect. Its performance depends on our understanding of the retina's working mechanism and the evolvement of computer vision models. Recently, remarkable progress has been made in the field of processing algorithm for retinal prostheses where the new discovery of the retina's working principle and state-of-the-arts computer vision models are combined together.Approach. We investigated the related research on artificial intelligence techniques for retinal prostheses. The processing algorithm in these studies could be attributed to three types: computer vision-related methods, biophysical models, and deep learning models.Main results. In this review, we first illustrate the structure and function of the normal and degenerated retina, then demonstrate the vision rehabilitation mechanism of three representative retinal prostheses. It is necessary to summarize the computational frameworks abstracted from the normal retina. In addition, the development and feature of three types of different processing algorithms are summarized. Finally, we analyze the bottleneck in existing algorithms and propose our prospect about the future directions to improve the restoration effect.Significance. This review systematically summarizes existing processing models for predicting the response of the retina to external stimuli. What's more, the suggestions for future direction may inspire researchers in this field to design better algorithms for retinal prostheses.

NeuroSEE: A Neuromorphic Energy Efficient Processing Framework for Visual Prostheses

Visual prostheses with both comprehensive visual signal processing capability and energy efficiency are becoming increasingly demanded in the age of intelligent personal healthcare, particularly with the rise of wearable and implantable devices. To address this trend, we propose NeuroSEE, a neuromorphic energy-efficient processing framework that combines a spike representation encoding technique and a bio-inspired processing method. This framework first utilizes sparse spike trains to represent visual information, and then a bio-inspired spiking neural network (SNN) is adopted to process the spike trains. The SNN model makes use of an IF neuron with multiple spikefiring rates to decrease the energy consumption without compensating for prediction performance. The experimental results indicate that when predicting the response of the primary visual cortex, the framework achieves a state-ofthe- art Pearson correlation coefficient performance. Spikebased recording and processing methods simplify the storage and transmission of redundant scene information and complex calculation processes. It could reduce power consumption by 15 times compared with the existing Convolutional neural network (CNN) processing framework. The proposed NeuroSEE framework predicts the response of the primary visual cortex in an energy efficient manner, making it a powerful tool for visual prostheses.

Gene Therapy Approaches to Slow or Reverse Blindness From Inherited Retinal Degeneration: Growth Factors and Optogenetics

To date, clinical gene therapy efforts for inherited retinal degeneration (IRD) have focused largely on gene replacement. The large number of genes and alleles causing IRD, however, makes this approach practical only for the most common causes. Additionally, gene replacement therapy cannot reverse existing retinal degeneration. Viral-mediated gene therapy can be used for two other approaches to slow or reverse IRD. First, by driving intraocular expression of growth factors or neuroprotective proteins, retinal degeneration can be slowed. Second, by expressing light-sensitive proteins (either microbial channelopsins or mammalian G-protein coupled opsins) in preserved inner retinal neurons, light sensitivity can be restored to the blind retina. Both approaches have advanced substantially in the past decade, and both are nearing clinical tests. This review surveys recent progress in these approaches.

Neuromodulation using electroosmosis

Objective.Our laboratory has proposed chemical stimulation of retinal neurons using exogenous glutamate as a biomimetic strategy for treating vision loss caused by photoreceptor (PR) degenerative diseases. Although our previousin-vitrostudies using pneumatic actuation indicate that chemical retinal stimulation is achievable, an actuation technology that is amenable to microfabrication, as needed for anin-vivoimplantable device, has yet to be realized. In this study, we sought to evaluate electroosmotic flow (EOF) as a mechanism for delivering small quantities of glutamate to the retina. EOF has great potential for miniaturization.Approach.An EOF device to dispense small quantities of glutamate was constructed and its ability to drive retinal output tested in anin-vitropreparation of PR degenerate rat retina.Main results.We built and tested an EOF microfluidic system, with 3D printed and off-the-shelf components, capable of injecting small volumes of glutamate in a pulsatile fashion when a low voltage control signal was applied. With this device, we produced excitatory and inhibitory spike rate responses in PR degenerate rat retinae. Glutamate evoked spike rate responses were also observed to be voltage-dependent and localized to the site of injection.Significance.The EOF device performed similarly to a previously tested conventional pneumatic microinjector as a means of chemically stimulating the retina while eliminating the moving plunger of the pneumatic microinjector that would be difficult to miniaturize and parallelize. Although not implantable, the prototype device presented here as a proof of concept indicates that a retinal prosthetic based on EOF-driven chemical stimulation is a viable and worthwhile goal. EOF should have similar advantages for controlled dispensing of charged neurochemicals at any neural interface.