The word ‘optogenetics’ was originally utilized with reference to the field of neuroscience, to depict the method of using light to image and control neuronal function in the entire living mind. The notion of controlling biological activity with the use of light has been around for quite a while. In fact, the utilization of the method as a truly empowering tool in the area of neuroscience had been predicted beforehand. Over the previous decade, the remarkable potential of optogenetics has received widespread acknowledgment. The achievements of optogenetics in neuroscience have caught the interest of numerous researchers and specialists in different fields, and now the meaning of optogenetics has expanded to incorporate the general field of biotechnology that merges genetic engineering and optics to allow the gain or loss of well-defined activities in the intact animal.
In recent years, the optogenetics technique has had a significant effect on neuroscience research by allowing the control of specific types of cells in the intact brain. A noteworthy development that allowed using these tools was the capability to control specific cell types in intact frameworks with high spatial and temporal resolution. Another possibly transformational prospect would be the combination of optical control strategies with different detection techniques to allow closed-loop control of biological frameworks. A few studies have already demonstrated closed-loop control of neural circuits and feedback-regulated control of cell signaling and degrees of gene expression. The research proposed that optogenetic techniques will allow researchers to uncover the underlying function behind complex intracellular signaling frameworks or multicellular dynamics and precisely alter it to accomplish an ideal outcome. They will likewise enable us to create and analyze models of intact, complex biological processes, beginning a period of constant ‘process engineering’ in biological frameworks. Optogenetics may allow the utilization of highly developed process engineering methodologies generally utilized in chemical and electrical processes to biological processes.
Successful advancement of new optogenetic devices and their implementation is an interdisciplinary endeavor that calls for devices custom-made to the particular biological method being examined. Targeting and acquiring optimal expression in the preferred cell type should be tested, and the biophysical attributes of photoreceptors used should be refined according to the spatial and temporal characteristics to be controlled. Hence, users of optogenetic instruments need to consider biophysical qualities carefully, and developers of the optogenetic tools need to find ways to adjust them.
Cell-type specificity is an important aspect of optogenetic research. In various ways, gain or loss of function tests on distinctive cell types are similar to genetic knock-in or knockout analyses in molecular biology. Nevertheless, while the meaning of gene is clear, the meaning of cell type can be vague and unpredictable. This is especially valid in exceedingly complex frameworks, for example, the mammalian neocortex. To justify such high complexity, numerous parameters have been utilized to depict every cell type, for example, developmental lineage, anatomical position, gene expression, electrophysiology, and dendritic and axonal morphology. In any case, the latest developments in single-cell analysis procedures have given proof that gene expression patterns can represent a considerable number of these diverse cell phenotypes. Consequently, the utilization of genetically encoded instruments is all-around acceptable for examining the function of each cell type and is probably going to grow as we collect more information on gene expression.
In a set of extraordinary experiments, Liu and others demonstrated that neurons that increase movement during particular behavioral tasks in rodents could be explicitly reactivated utilizing the light-gated ion channel channelrhodopsin-2 (ChR2) propelled by the activity-dependent promoter of c-Fos. This reactivation reproduced the original behavior with just light activation, showing optically controlled memory recall. Ramirez and others later utilized a similar way to deal with trigger fear response by optically stimulating a lot of neurons activated during fear conditioning, indicating the formation of false memory. Theoretically, these demonstrations reveal that optogenetics can be utilized to reactivate cell populations particularly stimulated during a behavioral task, empowering activity-based targeting. In view of the diverse performance of transcription factors that can detect a wide variety of input signals, such responsive transcription might be a generalizable way to express optogenetic devices to control cell populations that direct a particular functional output.
The utilization of microbial rhodopsins in mammalian neural cells is an inspirational depiction of altering photoreceptors that exist in nature for controlling physiological characteristics in an entirely new context. To account for the diverse modes of activity originating in photoreceptors, the idea of repurposing parts that exist in nature to manage new biological functions has been employed in numerous applications and is estimated to grow further as new photoreceptors keep on being found. Microbial rhodopsins (type 1 rhodopsins) arbitrate either transmembrane ion transport or light-sensing through signal transduction. Microbial rhodopsins that arbitrate light-driven ion transport have been extensively utilized to control membrane potential in mammalian neurons. They can be classified into two mechanistically distinctive structures: ion pumps that can carry ions against their gradient and channels that passively transport ions along the gradient formed by other dynamic transporters.
Among some of the more popular microbial rhodopsins are channelrhodopsins originating in the green alga Chlamydomonas reinhardtii, which direct cations including proton, sodium, potassium, and calcium particles. Despite the fact that the conductance of these channels is moderately low, i.e. around 100-fold lesser than high-conductance ion channels), their current is able to depolarize mammalian neurons over their threshold to begin action potentials. Lately, more than 60 channelrhodopsin homologs were discovered by carrying out a methodical investigation of transcriptomes from 127 types of alga. This examination led to a high sensitivity channelrhodopsin with tremendously quick channel kinetics known as Chronos, and a red-sensitive channelrhodopsin named Chrimson. Concurrently, these channelrhodopsins facilitated independent multicolor stimulation of two distinct neural populations. Another up to date research discovered anion transporting light-gated channels in the cryptophyte Guillardia theta that allowed fast and reversible optical silencing of rodent neurons. Up until this point, microbial sensory rhodopsins have not been involved in optogenetic tests, maybe because of the transmembrane requisite or soluble transducers that are not easily compatible with other cell types.
In addition, animal rhodopsins form the biggest GPCR family—more than 700 rhodopsin-like GPCRs have been discovered in humans. In view of the widespread structure-function relationship studies of GPCRs, Khorana and colleagues established that the cytoplasmic areas of bovine rhodopsin might be supplanted by analogous sequences from a non-light sensitive GPCR, for example, β 2 – adrenergic receptor (β 2 – AR) to produce a chimeric light-sensitive β 2 – AR.
This technique has been expanded to generate light-driven rhodopsins coupled to Gq and Gs signaling pathways; Gi/o coupled pathways.
Apart from rhodopsin-based photoreceptors, light-sensitive proteins that exist in plants and microbes have been involved in directing cell signaling in various cell types including mammalian cells, yeast, and bacteria. This type of light-activated uncaging technique has been effectively applied in quite a lot of synthetic constructs such as a photoactivatable GTPase Rac1 and Cdc42, peptide-binding motifs, and tethered toxins. Despite the fact that such a photo-uncaging technique is by all accounts a generally relevant approach, in several incidents, it needs more than a few levels of optimization modified to every molecule for efficient control. While conformation-dependent exchanges are difficult to be anticipated, the design of a photoactivatable structure still poses a challenge. Such challenges might be solved by utilizing Dronpa, which is a photoactivated fluorescent protein that appears to permit the modular design of optical control. Upon activation by light, Dronpa alters fluorescence, as well as monomerizes through the unfolding of a β-sheet. This element has been utilized to design photoactivatable GTPases and proteases.
An added mode of activity that exists in photoreceptor domains is interaction upon inducing light. Several LOV domains, for example, the fungal LOV domain Vivid (VVD) and bacterial LOV domain EL222, homodimerize at the time of light activation. Plant photoreceptors, for example, Arabidopsis thaliana UVR8 exist as dimers in a dark environment however monomerize after being illuminated by UV light. Other photoreceptor domains, for example, Arabidopsis thaliana Cryptochrome-2 (CRY2) and Phytochrome B (PhyB) heterodimerize with a particular associate, cryptochrome-interacting basic helix– loop– helix 1 (CIB1) and phytochrome interacting factor (PIF), correspondingly. Such light-activated interactions have been utilized basically to direct intracellular signaling, by selecting signaling proteins to or away from a particular intracellular activity location, leading to signal activation or obstruction through sequestration. A few experiments additionally displayed optical control of DNA transcription utilizing light-induced binders that arbitrate the selection of transcription activation domains or activation of Cre recombinase.
Despite the fact that natural photoreceptor frameworks appear to depend on the modularity of light-induced protein domains for controlling various signaling pathways, the development of a new optogenetic device generally entails pronounced engineering efforts for efficient control. One major hurdle is accomplishing ample expression of optogenetic devices. As found in the case of microbial opsins, the expression level of an optogenetic tool might be one of the restricting components in accomplishing successful control. Heterologous expression of photoreceptor domains relies upon the host cell type and can result in decreased levels of the functionally active form of the optogenetic device. For instance, an examination that analytically evaluated channelrhodopsin homologs discovered that below 50% of them displayed discernible ion conduction when expressed in a mammalian cell line, and even lower amounts were functionally active in neurons. One approach to enhancing functional expression is to improve the protein trafficking to the preferred compartment utilizing targeting sequences. In any case, this methodology isn’t generalizable, and accomplishing sufficient expression of optogenetic instruments in the desired cell type remains a challenge. Another significant hurdle is the fact that optogenetic devices need multiple properties to be optimized. For instance, the basic properties of channelrhodopsins that affect their efficiency in controlling neural movement consist of ion conductance, light sensitivity, spectral sensitivity, channel kinetics, and ion selectivity.
Structure-guided mutagenesis experiments have led to enhanced channelrhodopsins with rapid kinetics, improved photocurrent, red-shifted spectral sensitivity, and modified ion selectivity. These investigations showed that vital properties could be adjusted utilizing mutagenesis, but in addition, discovered that a single mutation frequently influences multiple properties. For instance, mutations at E123 in ChR2 that are analogous to D85, which is the Schiff base counter-ion in bacteriorhodopsin, influence channel kinetics and red-shifts the motion spectra. Mutation H134R in ChR2 improves photocurrent maybe because of enhanced membrane expression; however, decreases channel kinetics. Mutations C128 and D156 decrease the channel kinetics significantly which facilitates keeping open channels for longer timeframes and successfully improves the light sensitivity. The pattern of multiple property modulation by a sole mutation is likewise found in LOV domains. For instance, in the Avena sativa phototropin 1 LOV2, mutations in the extremely persevered residue Q513 lessen the structural modifications between the dark and the illumination state and decrease the dark state return rate. Likewise, in the LOV domain VVD, mutations in M135 and M165 reduce recovery kinetics and improve the affinity of the illumination state VVD dimer.
In this way, optimization of an optogenetic instrument requires multidimensional characterizations to quantify every basic parameter, and in specific cases, individual properties might not be independently optimized. So, the fitness landscape of photoreceptors is a multidimensional space made out of possibly dependent factors. Since characteristic screens utilized for protein engineering depend on the quick estimation of a couple of parameters, techniques including directed evolution to optimize one parameter may lead to de-optimization of others that are necessary for device performance. As of now, the general approach is to unite arbitrary mutagenesis-based techniques with structure-guided methodologies and depend on the additive valuable effects of multiple mutations to produce an ideal construct. Furthermore, screens that can characterize multiple parameters might, in the future, be produced to execute multidimensional optimizations.
Another significant pathway in optogenetics has been to utilize various colors of light to control two autonomous cell types or procedures utilizing photoreceptor domains that have unique spectral sensitivities. This methodology has been investigated at length by utilizing microbial opsins. For example, ChR2, which has maximal sensitivity to blue light, has been merged with a halorhodopsin which can be stimulated with yellow light to allow bidirectional control of neural movement. Proton pumps with a division in their spectral sensitivity have been utilized to accomplish two-color neural silencing. Despite the fact that these experiments exhibited that activating or reducing action potentials in neurons can be controlled by using the diversity in spectral sensitivity of microbial rhodopsins, a new study demonstrated that accomplishing multicolor control depending on spectral separation alone may lead to cross talk, because of the innate blue-light sensitivity found in rhodopsins.
In fact, the red-sensitive channelrhodopsin Chrimson appears to activate action potentials under powerful blue light in neurons with high expressions levels. Nevertheless, an in-depth biophysical characterization of Chrimson showed that its channel opening rate under blue light is considerably slower than that under red light, and is likewise reliant on light intensity. As a result, cross-activation of neurons under blue light was reduced by combining Chrimson with the quick and light-sensitive channelrhodopsin Chronos and restricting the power and duration of blue light. The blue-light sensitivity of microbial rhodopsins may not be totally eliminated except if the chromophore itself is changed. Numerous photoreceptors, including LOV domains and cryptochromes, are likewise maximally sensitive to blue light, which might be difficult to eradicate as in the case of microbial rhodopsins. Thus, techniques other than depending on spectral separation alone, for instance, using differences in light sensitivity and kinetics under blue light might be important to lessen cross talk for multicolor control utilizing different photoreceptors.
Despite the fact that the concept of multicolor control in optogenetics may appear to be similar to multicolor fluorescence imaging, there are practical differences that entail caution in designing optogenetic studies with various light sources. In multicolor fluorescence imaging, cross-activation of fluorophores happens frequently; however, the outflow from multiple fluorophores can be filtered to acquire cross-talk free images. In studies utilizing light-inducing proteins, any cross-activation may lead to changes that cannot be filtered or removed.
Contingent upon the biological investigation being performed, small changes triggered by cross-activation may become significant, for example, in studies that emphasize subthreshold changes in membrane potential. Hence, when designing studies that entail various sources of light, for example, utilizing light-gated ion channels with calcium reporters, the wavelength and force of light utilized for imaging ought to be tested for cross-activation of optical actuators. It is noteworthy that blue-sensitive channelrhodopsins that appear to have insignificant red sensitivity can depolarize membrane potential under strong red light. Therefore, techniques that capitalize on all the characteristics of biophysical properties of photoreceptors might be utilized to stop cross-activation but may be hard to entirely remove small cross talks induce entirely.