From optogenetic to thermogenetic gene therapy for vision restoration, perspectives article

Retinal degeneration diseases, including age-related macular degeneration and retinitis pigmentosa, lead to irreversible vision loss due to the degeneration of photoreceptor cells. While current treatments are limited, innovative approaches in gene therapy are emerging, with optogenetics offering promising strategies for vision restoration. The imitation of blue opsin led to the development of red-shifted variants with the advantage of penetrating deeper into tissues with less absorption. Meanwhile, thermogenetics leverages temperature-sensitive ion channels that can be activated by thermal stimuli, offering a potentially non-invasive approach to modulate retinal cell activity. This article discusses optogenetic strategies in retinal degeneration, focusing on the emerging role of red-shifted variants in optogenetic therapies. We also investigate how optogenetic gene therapy paves the way for thermogenetic gene therapy, offering promising and minimally invasive treatment for retinal diseases. Finally, we introduce new potential thermogenetic tools for future thermogenetic therapy. Degenerative eye diseases such as retinitis pigmentosa (RP), age-related macular degeneration (AMD), and diabetic retinopathy cause partial or complete blindness through photoreceptor degeneration. Traditional treatments, including pharmacotherapy[1–3] and gene replacement therapy,[4–6] have been explored but are often ineffective once the disease progresses to advanced stages, where photoreceptor cells are already lost. To address these limitations, researchers have developed innovative strategies such as stem cell therapy,[7–9] electronic prostheses,[10] synthetic photoswitchable ligands,[11] and optogenetics,[12–15] all of which aim to restore vision even after significant photoreceptor damage. Optogenetic gene therapy, in particular, combines genetic and optical methods to control neuron function. This technique has the potential to deliver high-resolution vision across the visual field by transferring genes encoding light-sensitive proteins (opsins) into retinal cells, effectively replacing the lost photoreceptor function. Similarly, thermogenetic gene therapy, which uses temperature-sensitive proteins to stimulate cells, holds promise as a complementary approach. Together, these therapies represent a revolutionary step toward long-lasting vision restoration for patients with degenerative eye diseases. This article explores innovative therapeutic strategies for retinal degeneration, focusing on advancements in optogenetic approaches. It highlights the emerging role of red-shifted optogenetic variants in improving therapy efficacy and safety by leveraging their ability to respond to longer wavelengths of light. Furthermore, the article examines the transition from optogenetic to thermogenetic gene therapy, emphasizing its potential as a minimally invasive treatment option. Finally, it introduces and discusses the development of novel thermogenetic tools, laying the groundwork for future applications in retinal disease therapies. Optogenetic Science Development In 2003, Nagel et al. introduced Channelrhodopsin-2 (ChR2) as a light-activated ion channel, showcasing its functional expression in Xenopus laevis oocytes and mammalian cells.[16] Two years later, in 2005, Boyden, Deisseroth, and Nagel developed optogenetics, a groundbreaking technology that enables precise, noninvasive, and genetically targeted control of neuronal activity.[17] The transformative potential of optogenetics has earned it significant recognition, including prestigious awards like the Brain Prize in 2013 and the Warren Alpert Foundation Prize in 2019. First Optogenetic Vision Restoration In 2005, the Zhuo-Hua Pan research group first explored optogenetics science as a strategy for vision restoration. This pioneering study involved expressing ChR2, a light-sensitive ion channel, in the retinas of rodent models that lacked functional photoreceptors. The researchers demonstrated that ChR2 expression in photoreceptor-deficient rd1/rd1 mice met several essential criteria for effective vision restoration. First, ChR2 showed prolonged expression in retinal cells, which is crucial for long-term therapeutic potential. Second, the optogenetically modified retinal cells successfully generated action potentials in response to light within the retina and the visual cortex, suggesting that the brain could process these new light-activated signals. This provided a foundation for using ChR2 as a light sensor in retinal neurons, offering a novel approach to restoring visual function in cases where natural photoreceptors are degenerated or absent.[18] Challenge of Blue-light Stimulation and the Development of Red-shifted Opsins ChR2, the first introduced opsin in optogenetics, is optimized for blue light with a peak absorption at wavelengths of 460–470 nm.[16] However, blue-light stimulation presents a significant challenge in therapeutic applications for vision restoration. Exposure to blue wavelengths can increase the risk of photochemical damage in the retina and retinal pigment epithelium due to the generation of reactive oxygen species, which leads to oxidative stress and potential cellular damage.[19] In addition, blue light scatters more readily and is strongly absorbed by natural chromophores in retinal tissue, which limits its penetration depth and effectiveness for reaching deeper retinal layers.[20] Moreover, red-shifted opsins allow for combinatorial control of different cell populations within the same retinal area by using both blue and red light, enabling more sophisticated experimental and therapeutic approaches.[21] This has driven the development of various red-shifted opsin variants, including VChR1, C1V1-ET, ReaChR, Chrimson, and ChrimsonR, SOUL, ROM19, ROM18 and ROM17 [Table 1]. These red-shifted opsins allow for deeper tissue penetration and reduce the risks associated with blue light exposure. In vivo, preclinical testing of red-shifted variants for vision restoration is typically performed by viral vector delivery in retinal disease model mice [Table 2].Table 1: Red-shifted opsins were introduced until 2025Table 2: In vivo preclinical study of red-shifted variants for vision restorationClinical Trial of Red-shifted Opsin, ChrimsonR Preclinical studies of the red-shifted opsin ChrimsonR were conducted in non-human primates to assess its effectiveness and compare it to the more widely used ChR2. In these studies, ChrimsonR successfully elicited visual responses in the retinal cells of non-human primates, demonstrating its potential for deeper tissue penetration and its suitability for vision restoration applications. These promising results provided the necessary foundation to initiate a clinical trial using the AAV2.7 m8-ChrimsonR-tdTomato (tdT) vector in humans with RP, a degenerative eye disease that leads to severe vision loss.[31] In 2017, Phase 1 and Phase 2 clinical trials (ClinicalTrials.gov identifier: NCT03326336) were launched to evaluate the safety and tolerability of intravitreal injection of the rAAV2.7 m8-CAG-ChrimsonR-tdTomato vector (GS030-DP) using light-stimulating goggles (GS030-MD) in patients with advanced RP. This gene therapy approach involves delivering ChrimsonR to retinal cells via an adeno-associated viral vector engineered for high efficiency, allowing surviving retinal cells to respond to red light and bypass degenerated photoreceptors. It has been demonstrated that two patients had preliminary partial functional recovery as measured by locating and touching small objects on a table and a good safety profile after 2.5 years of follow-up.[12,13] Overall, the current evidence supports the potential of this red-shifted opsin, ChrimsonR, for vision restoration in humans. Evolution of Optogenetic Tools Leading to Thermogenetic Applications Thermogenetic gene therapy utilizes temperature-sensitive proteins, or thermogenetic opsins, to modulate cellular activity in response to controlled temperature variations. Unlike optogenetics, which relies on light to activate specific ion channels, thermogenetics enables temperature shifts to trigger cellular responses, providing a more natural and less invasive method for stimulation. These temperature-sensitive proteins can be introduced into target tissues through gene therapy, allowing for targeted activation or inhibition of neurons, including retinal cells in vision restoration. In vision restoration, thermogenetics presents several advantages over traditional optogenetics. Thermogenetic stimulation can be achieved by controlling the temperature of tissues, removing the dependency on external light sources (like lasers or Light-Emitting Diode (LEDs)) often used in optogenetic treatments. In addition, thermogenetics relies on thermal gradients rather than implanted light delivery systems; it may offer a less invasive solution for the long term. Furthermore, temperature-based stimulation mimics the body’s natural thermosensation processes, offering a more physiologically relevant way to control cellular activity.[32] By leveraging thermal gradients, thermogenetic gene therapy may offer a more adaptable and less technically demanding alternative for vision restoration therapies. First Thermogenetic Vision Restoration TRPA1 are the most heat-sensitive vertebrate ion channel in the pit organs of snakes. The pit organ receives infrared signals and is transduced into the nerve. Pit organs are located between the eyes and the nostril on either side of the viper’s face. The infrared sensation is exquisitely sensitive and vital for predator avoidance and thermoregulatory behavior.[33] When the pit organ receives infrared signals, they are converted into nerve impulses, allowing the snake to sense the thermal environment even in complete darkness. Inspired by this natural infrared detection system, researchers have explored the potential of using TRPA1 channels for vision restoration. By expressing TRPA1 channels in retinal cone cells of rd1 mice – a model of retinal degeneration – scientists have enabled these cells to respond to near-infrared (NIR) light. NIR stimulation in TRPA1-expressing rd1 cone cells successfully increased activity in multiple layers of the visual system, including cone cells, ganglion cell layer neurons, and even cortical neurons, indicating that the brain could process these artificially induced signals. Remarkably, this approach allowed the mice to perform learned behaviors driven by light, suggesting that TRPA1 channels could restore vision by making retinal cells sensitive to NIR light.[34] This strategy, which repurposes the natural infrared-detecting function of TRPA1, offers an innovative approach to bypass damaged photoreceptors and restore light sensitivity to the retina using NIR light. Such an approach could lead to new therapeutic applications for patients with advanced retinal degeneration, where traditional photoreceptor-based therapies are no longer effective. A recent study approved the TRPA1 as a thermogenetic neuromodulation in brain research and treatment of neuropathologies.[35–37] Propose Thermosensitive Opsins for Future Vision Restoration Research Introducing the first thermosensitive opsin for vision restoration marks a breakthrough in retinal therapy, demonstrating the feasibility of temperature-based stimulation to restore vision. This foundational development inspires us to introduce new potential thermosensitive opsins with optimized properties, paving the way for advanced studies in retinal restoration and broader applications in vision restoration therapies. Fish-specific Thermoreceptors and Their Potential for Vision Restoration Environmental temperature sensing is a critical survival function across all organisms, often involving specialized thermoreceptors adapted to specific habitats. In fish, thermosensitivity is closely linked to environmental illumination. For example, in clear oceanic waters, blue light can penetrate to depths of 300–500 m, and many fish species in these ecosystems have evolved eyes with peak sensitivity around 470 nm, matching the blue light available at those depths. However, in turbid waters with limited visibility, some fish have developed unique thermosensory adaptations, allowing them to sense red and NIR ranges of light (thermal cues) as part of their visual processing. Unlike most terrestrial animals, fish use their eyes, rather than the pineal organ, for thermosensation, employing this capability in critical behaviors such as prey detection, intraspecific communication, and sexual selection. Thermosensitive opsins have been introduced in various fish species, including guppies, green swordtails, zebrafish, Nile tilapia, and Mozambique tilapia. Interestingly, studies show that Nile tilapia and Mozambique tilapia (930 nm) exhibit higher thermosensitivity than other species like zebrafish, guppies (845–910 nm), and green swordtails (825–845 nm). These differences may reflect evolutionary adaptations to specific environmental conditions, suggesting that thermosensitive opsins are functionally versatile and could be diverse.[38] The unique thermosensory capabilities of these fish open intriguing possibilities for future vision restoration research. By exploring how thermosensitive opsins function in aquatic species, scientists could potentially adapt these proteins for use in mammalian systems, where restoring temperature-sensitive or NIR sensitivity to retinal cells might provide an alternative means of vision restoration. Such an approach could benefit individuals with advanced retinal degeneration, where traditional photoreceptor-based therapies are less effective. Thermo-sensitive opsins, if successfully integrated into retinal cells, might allow these cells to detect thermal cues or infrared light, thus bypassing damaged photoreceptors and restoring functional visual responses. Thermosensitivity in Mammalian: Evolution and Potential for Vision Restoration In mammals, sensory neurons play a crucial role in conveying thermal information to the central nervous system, enabling organisms to respond to changes in environmental temperature [Table 3]. This thermal sensation is mediated mainly by a group of membrane-bound cation channels known as transient receptor potential (TRP) channels. TRP channels consist of 28 channels grouped into six subfamilies: TRPC (Canonical), TRPV (Vanilloid), TRPM (Melastatin), TRPA (Ankyrin), TRPML (Mucolipin), and TRPP (Polycystin). These channels are primarily conserved across species, from simple organisms like nematodes to complex vertebrates, including humans, highlighting their evolutionary importance in sensory functions.Table 3: Mammalian thermosensitive proteinEach TRP subfamily is specialized for detecting different types of stimuli, including temperature, mechanical forces, and chemical signals. For instance, TRPV channels are known for their sensitivity to heat, with specific members such as TRPV1 and TRPV2 being activated by warm or hot temperatures above 42°C. In contrast, TRPV3 and TRPV4 are activated by milder warmth, typically between 31°C and 39°C. These channels allow ions such as calcium and sodium to flow into sensory neurons upon activation, initiating electrical signals processed in the brain as thermal sensations. The TRPM subfamily of TRP channels includes key members like TRPM8 and TRPM3, which are involved in sensing temperature changes. TRPM8 is primarily activated by cold temperatures below 28°C, making it essential for detecting cool and cold sensations. This channel also responds to cooling agents like menthol. In contrast, TRPM3 is sensitive to heat, activating at temperatures above 42°C. It plays a crucial role in detecting heat-induced pain and is involved in thermoregulation, helping the body manage temperature changes. Together, these channels enable the detection of both cold and heat, contributing to the organism’s ability to sense and respond to environmental temperature shifts. TRPA1, another member of the TRP family, detects painful cold stimuli, particularly at temperatures below 10°C. It is also known for its high sensitivity to temperature in certain species that have evolved specialized thermoreception, such as pit vipers. TRPC channels, particularly TRPC5, are involved in detecting cold temperatures. These channels are activated within a temperature range of 25°C–37°C, pivotal in moderating the body’s response to cooler conditions. By contributing to the sensation of a mild cold, TRPC5 channels help organisms adjust to changes in environmental temperature, ensuring appropriate physiological reactions to moderate cooling. Understanding the mechanisms of TRP-mediated thermosensation in mammals sheds light on fundamental sensory processes and opens up possibilities for therapeutic applications. For instance, some TRP channels have been investigated for their potential to restore sensory function in retinal cells, where artificially activating these channels with light or temperature could provide new ways to bypass damaged photoreceptors and convey visual information.[39] In addition to the well-known temperature-sensitive TRP channels, specific non-TRP ion channels are also intrinsically sensitive to temperature changes and play essential roles in modulating the activation of sensory neurons. Among the non-TRP ion channels, specific sodium (Na) and potassium (K) channels in the K2P (two-pore domain potassium) channel family stand out for their thermosensitive properties. The K2P family includes three subfamilies – TREK1, TREK2, and TRAAK – all contributing to membrane hyperpolarization in response to temperature fluctuations. These channels exhibit distinctive responses to thermal stimuli. For example, under noxious heat, TREK and TRAAK channels can hyperpolarize cell membranes, thereby reducing neuronal excitability and modulating the sensation of heat. In contrast, exposure to noxious cold depolarizes thermoreceptors, increasing their excitability and thus enhancing the sensory perception of cold. This temperature-dependent activity of K2P channels adds an additional layer of regulation in thermosensory neurons, contributing to a fine-tuned balance between excitation and inhibition in response to extreme temperatures.[40] These channels, sensitive to cold and warm temperatures, play a crucial role in regulating neuronal excitability. This temperature-sensitive behavior of K2P (two-pore domain potassium) channels makes them a promising therapeutic approach for controlling cell activation and inhibition in retinal restoration. Another specific non-TRP ion channel, Anoctamin 1 (ANO1), is a calcium-activated chloride (Cl⁻) channel critical to temperature sensation. ANO1 is activated at temperatures above 42°C, facilitating the depolarization of sensory neurons in response to noxious heat. When intracellular calcium levels rise, ANO1 channels open, allowing chloride ions to flow across the cell membrane. This depolarizing effect increases neuronal excitability, thus contributing to the sensation of heat. Given its sensitivity to high temperatures, ANO1 has been studied for potential applications in sensory modulation and pain research, in involving thermal sensitivity to painful ability to activate sensory neurons under specific temperature makes it a promising for future sensory restoration and modulation Two sodium channels, and are in sensory neurons and play an essential role in detecting and to cool temperatures. 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