Commentary and Perspective
Recent advances in optogenetics: Report for the session 12 at the 19th International Conference on Retinal Proteins
2023 Volume 20 Issue Supplemental Article ID: e201002
Details
2023 Volume 20 Issue Supplemental Article ID: e201002
When Deisseroth succeeded to express channelrhodopsin (ChR) in neurons [1], the door of the new biological field, optogenetics, opened. Finally, he succeeded to control the behavior of mouse by illumination of light on the ChR-expressed transgenic mouse [2]. Optical control of biological function becomes one of the powerful tools to understand the biological systems. In order to deepen and widen this emerging field, Prof. Yoshinori Shichida (Ritsumeikan) organized the PRESTO project of optical control. The session 12 “Optogenetics I” at the 19th International Conference on Retinal Proteins was planned as one of the major activities of the Shichida’s PRESTO, and Japan Science and Technology Agency (JST) sponsored the session. Three important elements for the establishment of optogenetics would be 1) manipulation and control by light; 2) observation by light and imaging; 3) application to real biological systems not only nerve or brain science but also the other wide fields of biology. In this session, the elements 1) and 3) are mainly focused, because retinal proteins are important tools for optical control after the discovery of ChR. However, they are not necessarily useful for the imaging or observation because their fluorescence is rather weak (element 2). Two young foreign researchers as well as three PRESTO members are invited as the speakers to cover the above-mentioned two elements.
Although ChR is a useful tool for optogenetics, some improvements are expected such as absorption band at higher wavelength, ion selectivity and slow decay kinetics. ChRmine discovered recently is a unique and interesting retinal protein. The primary structure is close to the pump-type rhodopsin rather than the channel-type one and whose absorption maximum (520 nm) is longer than ChR (470 nm). Dr. Yoon Seok Kim (Stanford) presented the structure-guided design of pump-like ChRs with more suitable properties than ChR for the optogenetic control in the brain. He introduced their solved structure of ChRmine by cryo-electron microscopy [3] and discussed the structure-based design of pump-like ChRs (PLCRs). The detailed examination of molecular structure around the chromophore, retinal, they selected the candidates of amino acid residues which control the absorption band. They succeeded to realize the engineered ChRmine with longer absorption maximum. They also explained in detail why ChRmine functions as light-triggered ion channel rather than light-driven ion pump despite that the structure is close to the pump-like rhodopsins. The structure-based design of PLCRs is expected to open the way to diverse applications in optogenetics. His work is highly collaborated with Dr. Hideaki Kato (U. Tokyo), one of the PRESTO members and an attendee of the conference.
The delivery of the gene of the protein for optogenetic tool such as ChR to the targeted cells is a key technique in optical control. Optogenetics was firstly applied to nerve and brain science, because it is possible to use neurotropic viral vectors and Cre recombinase-mediated gene expression systems in neurons. Dr. Kunio Kondoh (National Institute of Physiological Sciences, PRESTO) presented the trans-synaptic protein expression in the nervous system using trans-synaptic viral vectors. Since neurons are connected through synapse to form functional neural circuits, the manipulation of neurons connecting the targeted group of neurons through synapse is critical and desired. He focused on the trans-synaptic viruses, such as rabies virus (RV) and pseudorabies virus (PRV), that have an ability to travel to synaptically connected neurons. Although these viruses are suitable for the purpose, they have the serious disadvantage of the cytotoxicity. He overcame the drawback by the deletion of an immediate early gene IE180 of PRV. The modified IE180-deleted PRVs successfully expressed exogenous genes of interest such as ChR for a long time without obvious cytotoxicity. He also described the novel transsynaptic tracer system based on IE180-deleted PRVs. The new trans-synaptic viral vector system will be a powerful tool to expand the application of optogenetics from single populations of neurons to groups of connected neurons.
Optogenetics is a powerful technique to understand biological systems. Moreover, there are some trials to apply optogenetics to medical treatment. A typical example is the application to diseases of retina such as retinitis pigmentosa. In retinal diseases, photoreceptor cells are degraded to lose light sensing. However, the retinal interneurons such as bipolar cells and ganglion cells remain almost intact. It is expected to restore vision with relatively high sensitivity to light by the expression of particular animal rhodopsins in bipolar cells, although the treatment leads the inversion of dark-light senses. Dr. Takashi Nagata (U. Tokyo, PRESTO) presented development of a dark-active, light-inactivated optogenetic tool for controlling G protein signaling toward the vision restoration. Jumping spider peropsin discovered by him and his colleagues [4] binds all-trans retinal, and the bound retinal isomerizes into 11-cis form upon light absorption, in contrast to the other visual opsins. By exchanging the third intracellular loop of peropsin into that of either Gs- or Gi-coupled opsin, he succeeded to drive the either Gs- or Gi-mediated signaling pathway in the dark, and to inactivate each pathway upon light illumination. Based on the success, he applied to control the Go-mediated signaling pathway of retinal bipolar cells, which is required for the medical application. He also touched on preliminary promising results of animal experiments with the chimeric Go-coupled peropsin. Optogenetic medical treatment using chimeric peropsin is expected to regenerate visual sensing with the normal dark-light sensation to patients of retinal diseases.
Memory is one of the most mysterious biological problems. It is expected to control memory by expression of light-triggered channel in a certain memory neural ensemble. For social animals, the discrimination of familiar individuals to novel or hostile individuals (social memory) is critical not only to exhibit appropriate social behaviors but also to maintain social groups such as family, village and country. Dr. Teruhiro Okuyama (U. Tokyo, PRESTO) presented social memory engram in the hippocampus. If the mechanism of social memory is understood, it contributes to the diagnosis and treatment of patients with social impairments such as autism spectrum disorder (ASD) [5]. Using mice, he succeeded to discriminate the social memory engram. The degree of memory of a test mouse is quantified by the comparison of the total duration of time spent by familiar mouse with that by novel mouse (social discrimination behavioral assay). He demonstrated that social memory is stored in vCA1 pyramidal neurons in the hippocampus [6]. It is surprising that the social memory engram is composed of a small number of neurons, and that the set of neurons is different among the engrams for different familiar mice. He also introduced that the social memory, which could not be naturally retrieved due to a long separation, can be restored by optogenetic treatment with ChR.
One of the powerful methods to understand the neuronal circuits is the silencing of synaptic transmission at the designed point. Optogenetic suppression of presynaptic neurotransmitter release is promising for the purpose. Since various G protein coupled pathways are working in nervous system, G protein coupled rhodopsins (optoGPCRs) would be an ideal tool. Dr. Jonas Wietek (Weizman Institute) presented bistable light-activated silencing of synaptic transmission. He listed up 11 candidates of promising optoGPCRs and examined their properties such as expression and membrane targeting, GIRK channel coupling efficiency, ability to suppress synaptic transmission and G protein coupling specificity. Furthermore, he carried out the behavioral experiments of mice and C. elegans for the screening. Finally, he obtained one new optoGPCR which can be reliably switched between active and inactive states with blue and green light, respectively. The optoGPCR allows highly effective silencing of synaptic transmission, but does not couple to Gi signaling, that is, it leaves cAMP signaling pathway unchanged.
We could survey the present status of optogenetics field through these presentations. These presentations strongly indicated the directions we should aim for. Developments of effective light absorption proteins superior to ChR have been sophisticating the optogenetic research. The use of animal opsin rather than bacterial opsin can enlarge the fields suitable for optogenetics. The effective gene delivery system is indispensable for optogenetics. Application of optogenetics to medical treatment [7] would be expected from general public. As the next stage of optogenetics, combination of imaging and observation system with the optical control is one of the expected directions for the more profound understanding of the functions of biological systems.
Due to COVID19, the conference is forced to postpone two years. Consequently, this session has a character of a part of the final report of the PRESTO, because the project has successfully completed in this October prior to the conference. We appreciate JST for the continuous support.
