Photodimerization systems for regulating protein-protein interactions with light

Multiplexing light-inducible recombinases to control cell fate, Boolean logic, and cell patterning in mammalian cells

Light-inducible regulatory proteins are powerful tools to interrogate fundamental mechanisms driving cellular behavior. In particular, genetically encoded photosensory domains fused to split proteins can tightly modulate protein activity and gene expression. While light-inducible split protein systems have performed well individually, few multichromatic and orthogonal gene regulation systems exist in mammalian cells. The design space for multichromatic circuits is limited by the small number of orthogonally addressable optogenetic switches and the types of effectors that can be actuated by them. We developed a library of red light-inducible recombinases and directed patterned myogenesis in a mesenchymal fibroblast-like cell line. To address the limited number of light-inducible domains (LIDs) responding to unique excitation spectra, we multiplexed light-inducible recombinases with our "Boolean logic and arithmetic through DNA excision" (BLADE) platform. Multiplexed optogenetic tools will be transformative for understanding the role of multiple interacting genes and their spatial context in endogenous signaling networks.

Biomolecular Actuators for Soft Robots

Biomolecules present promising stimuli-responsive mechanisms to revolutionize soft actuators. Proteins, peptides, and nucleic acids foster specific intermolecular interactions, and their boundless sequence design spaces encode precise actuation capabilities. Drawing inspiration from nature, biomolecular actuators harness existing stimuli-responsive properties to meet the needs of diverse applications. This review features biomolecular actuators that respond to a wide variety of stimuli to drive both user-directed and autonomous actuation. We discuss how advances in biomaterial fabrication accelerate prototyping of precise, custom actuators, and we identify biomolecules with untapped actuation potential. Finally, we highlight opportunities for multifunctional and reconfigurable biomolecules to improve the versatility and sustainability of next-generation soft actuators.

Chemistries on the inner leaflet of the cell membrane

The cell membrane, characterized by its inherent asymmetry, functions as a dynamic barrier that regulates numerous cellular activities. This Highlight aims to provide the chemistry community with a comprehensive overview of the intriguing and underexplored inner leaflet, encompassing both fundamental biology and emerging synthetic modification strategies. We begin by describing the asymmetric nature of the plasma membrane, with a focus on the distinct roles of lipids, proteins, and glycan chains, highlighting the composition and biofunctions of the inner leaflet and the biological mechanisms that sustain membrane asymmetry. Next, we explore chemical biological strategies for engineering the inner leaflet, including genetic engineering, transmembrane peptides, and liposome fusion-based transport. In the perspective section, we discuss the challenges in developing chemistries for the inner leaflet of the cell membrane, aiming to inspire researchers and collaborators to explore this field and address its unanswered biological questions.

Plant physiology: Rethinking CRY photoreceptors

The CRY2 photoreceptor is known to form homotetramers that bind to transcription regulators to affect gene expression in response to light. A new study provides evidence that the CRY2 monomer binds different transcription regulators to affect gene expression in darkness, suggesting that photoreceptors change activity in response to light.

Engineering of LOV-domains for their use as protein tags

Light-Oxygen-Voltage (LOV) domains are the protein-based light switches used in nature to trigger and regulate various processes. They allow light signals to be converted into metabolic signaling cascades. Various LOV-domain proteins have been characterized in the last few decades and have been used to develop light-sensitive tools in cell biology research. LOV-based applications exploit the light-driven regulation of effector elements to activate signaling pathways, activate genes, or locate proteins within cells. A relatively new application of an engineered small LOV-domain protein called miniSOG (mini singlet oxygen generator) is based on the light-induced formation of reactive oxygen species (ROS). The first miniSOG was engineered from a LOV domain from Arabidopsis thaliana. This engineered 14 kDa light-responsive flavin-containing protein can be exploited as protein tag for the light-triggered localized production of ROS. Such tunable ROS production by miniSOG or similarly redesigned LOV-domains can be of use in studies focused on subcellular phenomena but may also allow new light-fueled catalytic processes. This review provides an overview of the discovery of LOV domains and their development into tools for cell biology. It also highlights recent advancements in engineering LOV domains for various biotechnological applications and cell biology studies.

Photoactivable CRISPR for Biosensing and Cancer Therapy

Photoactivable CRISPR technology represents a transformative approach in the field of genome editing, offering unprecedented control over gene editing with high spatial and temporal precision. By harnessing the power of light to modulate the activity of CRISPR components, this innovative strategy enables precise regulation of Cas proteins, guide RNAs, and ribonucleoprotein complexes. Recent advancements in optical control methodologies, including the development of photoactivable nanocarriers, have significantly expanded the potential applications of CRISPR in biomedical fields. This Concept highlights the latest developments in designing photoactivable CRISPR systems and their promising applications in biosensing and cancer therapy. Additionally, the remaining challenges and future trends are also discussed. It is expected that the photoactivable CRISPR would facilitate translating more precise gene therapies into clinical use.

Studying CaMKII: Tools and standards

The Ca2+/calmodulin (CaM)-dependent protein kinase II (CaMKII) is a ubiquitous mediator of cellular Ca2+ signals with both enzymatic and structural functions. Here, we briefly introduce the complex regulation of CaMKII and then provide a comprehensive overview of the expanding toolbox to study CaMKII. Beyond a variety of distinct mutants, these tools now include optical methods for measurement and manipulation, with the latter including light-induced inhibition, stimulation, and sequestration. Perhaps most importantly, there are now three mechanistically distinct classes of specific CaMKII inhibitors, and their combined use enables the interrogation of CaMKII functions in a manner that is powerful and sophisticated yet also accessible. This review aims to provide guidelines for the interpretation of the results obtained with these tools, with careful consideration of their direct and indirect effects.

Spatiotemporal functional assembly of split protein pairs through a light-activated SpyLigation

Proteins provide essential functional regulation of many bioprocesses across all scales of life; however, new techniques to specifically modulate protein activity within living systems and in engineered biomaterials are needed to better interrogate fundamental cell signalling and guide advanced decisions of biological fate. Here we establish a generalizable strategy to rapidly and irreversibly activate protein function with full spatiotemporal control. Through the development of a genetically encoded and light-activated SpyLigation (LASL), bioactive proteins can be stably reassembled from non-functional split fragment pairs following brief exposure (typically minutes) to cytocompatible light. Employing readily accessible photolithographic processing techniques to specify when, where and how much photoligation occurs, we demonstrate precise protein activation of UnaG, NanoLuc and Cre recombinase using LASL in solution, biomaterials and living mammalian cells, as well as optical control over protein subcellular localization. Looking forward, we expect that these photoclick-based optogenetic approaches will find tremendous utility in probing and directing complex cellular fates in both time and three-dimensional space.

Static and photoresponsive dynamic materials to dissect physical regulation of cellular functions

Recent progress in mechanobiology has highlighted the importance of physical cues, such as mechanics, geometry (size), topography, and porosity, in the determination of cellular activities and fates, in addition to biochemical factors derived from their surroundings. In this review, we will first provide an overview of how such fundamental insights are identified by synchronizing the hierarchical nature of biological systems and static materials with tunable physical cues. Thereafter, we will explain the photoresponsive dynamic biomaterials to dissect the spatiotemporal aspects of the dependence of biological functions on physical cues.

Imaging translational control by Argonaute with single-molecule resolution in live cells

A major challenge to our understanding of translational control has been deconvolving the individual impact specific regulatory factors have on the complex dynamics of mRNA translation. MicroRNAs (miRNAs), for example, guide Argonaute and associated proteins to target mRNAs, where they direct gene silencing in multiple ways that are not well understood. To better deconvolve these dynamics, we have developed technology to directly visualize and quantify the impact of human Argonaute2 (Ago2) on the translation and subcellular localization of individual reporter mRNAs in living cells. We show that our combined translation and Ago2 tethering sensor reflects endogenous miRNA-mediated gene silencing. Using the sensor, we find that Ago2 association leads to progressive silencing of translation at individual mRNA. Silencing was occasionally interrupted by brief bursts of translational activity and took 3–4 times longer than a single round of translation, consistent with a gradual increase in the inhibition of translation initiation. At later time points, Ago2-tethered mRNAs cluster and coalesce with P-bodies, where a translationally silent state is maintained. These results provide a framework for exploring miRNA-mediated gene regulation in live cells at the single-molecule level. Furthermore, our tethering-based, single-molecule reporter system will likely have wide-ranging application in studying RNA-protein interactions.

Guided by miRNA, Argonaute proteins silence mRNA in multiple ways that are not well understood. Here, the authors develop live-cell biosensors to image the impact tethered regulatory factors, such as Argonaute, have on single-mRNA translation dynamics.

Engineering Light-Control in Biology

Unraveling the transformative power of optogenetics in biology requires sophisticated engineering for the creation and optimization of light-regulatable proteins. In addition, diverse strategies have been used for the tuning of these light-sensitive regulators. This review highlights different protein engineering and synthetic biology approaches, which might aid in the development and optimization of novel optogenetic proteins (Opto-proteins). Focusing on non-neuronal optogenetics, chromophore availability, general strategies for creating light-controllable functions, modification of the photosensitive domains and their fusion to effector domains, as well as tuning concepts for Opto-proteins are discussed. Thus, this review shall not serve as an encyclopedic summary of light-sensitive regulators but aims at discussing important aspects for the engineering of light-controllable proteins through selected examples.

Light-activated receptor tyrosine kinases: Designs and applications

Receptor tyrosine kinases (RTKs) are a large and essential membrane receptor family. The molecular mechanisms and physiological consequences of RTK activation depend on, for example, ligand identity, subcellular localization, and developmental or disease stage. In the past few years, genetically-encoded light-activated RTKs (Opto-RTKs) have been developed to dissect these complexities by providing reversible and spatio-temporal control over cell signaling. These methods have very recently matured to include highly-sensitive multi-color actuators. The new ability to regulate RTK activity with high precision has been recently harnessed to gain mechanistic insights in subcellular, tissue, and animal models. Because of their sophisticated engineering, Opto-RTKs may only mirror some aspects of natural activation mechanisms but nevertheless offer unique opportunities to study RTK signaling and physiology.

Optophysiology: Illuminating cell physiology with optogenetics

Optogenetics combines light and genetics to enable precise control of living cells, tissues, and organisms with tailored functions. Optogenetics has the advantages of noninvasiveness, rapid responsiveness, tunable reversibility, and superior spatiotemporal resolution. Following the initial discovery of microbial opsins as light-actuated ion channels, a plethora of naturally occurring or engineered photoreceptors or photosensitive domains that respond to light at varying wavelengths has ushered in the next chapter of optogenetics. Through protein engineering and synthetic biology approaches, genetically-encoded photoswitches can be modularly engineered into protein scaffolds or host cells to control a myriad of biological processes, as well as to enable behavioral control and disease intervention in vivo. Here, we summarize these optogenetic tools on the basis of their fundamental photochemical properties to better inform the chemical basis and design principles. We also highlight exemplary applications of opsin-free optogenetics in dissecting cellular physiology (designated "optophysiology"), and describe the current progress, as well as future trends, in wireless optogenetics, which enables remote interrogation of physiological processes with minimal invasiveness. This review is anticipated to spark novel thoughts on engineering next-generation optogenetic tools and devices that promise to accelerate both basic and translational studies.

Directed evolution approaches for optogenetic tool development

Photoswitchable proteins enable specific molecular events occurring in complex biological settings to be probed in a rapid and reversible fashion. Recent progress in the development of photoswitchable proteins as components of optogenetic tools has been greatly facilitated by directed evolution approaches in vitro, in bacteria, or in yeast. We review these developments and suggest future directions for this rapidly advancing field.

Modular and Molecular Optimization of a LOV (Light-Oxygen-Voltage)-Based Optogenetic Switch in Yeast

Optogenetic switches allow light-controlled gene expression with reversible and spatiotemporal resolution. In Saccharomyces cerevisiae, optogenetic tools hold great potential for a variety of metabolic engineering and biotechnology applications. In this work, we report on the modular optimization of the fungal light-oxygen-voltage (FUN-LOV) system, an optogenetic switch based on photoreceptors from the fungus Neurospora crassa. We also describe new switch variants obtained by replacing the Gal4 DNA-binding domain (DBD) of FUN-LOV with nine different DBDs from yeast transcription factors of the zinc cluster family. Among the tested modules, the variant carrying the Hap1p DBD, which we call "HAP-LOV", displayed higher levels of luciferase expression upon induction compared to FUN-LOV. Further, the combination of the Hap1p DBD with either p65 or VP16 activation domains also resulted in higher levels of reporter expression compared to the original switch. Finally, we assessed the effects of the plasmid copy number and promoter strength controlling the expression of the FUN-LOV and HAP-LOV components, and observed that when low-copy plasmids and strong promoters were used, a stronger response was achieved in both systems. Altogether, we describe a new set of blue-light optogenetic switches carrying different protein modules, which expands the available suite of optogenetic tools in yeast and can additionally be applied to other systems.

Dual Systems for Enhancing Control of Protein Activity through Induced Dimerization Approaches

To reveal the underpinnings of complex biological systems, a variety of approaches have been developed that allow switchable control of protein function. One powerful approach for switchable control is the use of inducible dimerization systems, which can be configured to control activity of a target protein upon induced dimerization triggered by chemicals or light. Individually, many inducible dimerization systems suffer from pre-defined dynamic ranges and overwhelming sensitivity to expression level and cellular context. Such systems often require extensive engineering efforts to overcome issues of background leakiness and restricted dynamic range. To address these limitations, recent tool development efforts have explored overlaying dimerizer systems with a second layer of regulation. Albeit more complex, the resulting layered systems have enhanced functionality, such as tighter control that can improve portability of these tools across platforms.

A light way for nuclear cell biologists

The nucleus is a very complex organelle present in eukaryotic cells. Having the crucial task to safeguard, organize and manage the genetic information, it must tightly control its molecular constituents, its shape and its internal architecture at any given time. Despite our vast knowledge of nuclear cell biology, much is yet to be unravelled. For instance, only recently we came to appreciate the existence of a dynamic nuclear cytoskeleton made of actin filaments that regulates processes such as gene expression, DNA repair and nuclear expansion. This suggests further exciting discoveries ahead of us. Modern cell biologists embrace a new methodology relying on precise perturbations of cellular processes that require a reversible, highly spatially confinable, rapid, inexpensive and tunEable external stimulus: light. In this review, we discuss how optogenetics, the state-of-the-art technology that uses genetically encoded light-sensitive proteins to steer biological processes, can be adopted to specifically investigate nuclear cell biology.

Visualization and Manipulation of Intracellular Signaling

Cells respond to a wide range of extracellular stimuli, and process the input information through an intracellular signaling system comprised of biochemical and biophysical reactions, including enzymatic and protein-protein interactions. It is essential to understand the molecular mechanisms underlying intracellular signal transduction in order to clarify not only physiological cellular functions but also pathological processes such as tumorigenesis. Fluorescent proteins have revolutionized the field of life science, and brought the study of intracellular signaling to the single-cell and subcellular levels. Much effort has been devoted to developing genetically encoded fluorescent biosensors based on fluorescent proteins, which enable us to visualize the spatiotemporal dynamics of cell signaling. In addition, optogenetic techniques for controlling intracellular signal transduction systems have been developed and applied in recent years by regulating intracellular signaling in a light-dependent manner. Here, we outline the principles of biosensors for probing intracellular signaling and the optogenetic tools for manipulating them.

The rise and shine of yeast optogenetics

Optogenetics refers to the control of biological processes with light. The activation of cellular phenomena by defined wavelengths has several advantages compared with traditional chemically inducible systems, such as spatiotemporal resolution, dose-response regulation, low cost, and moderate toxic effects. Optogenetics has been successfully implemented in yeast, a remarkable biological platform that is not only a model organism for cellular and molecular biology studies, but also a microorganism with diverse biotechnological applications. In this review, we summarize the main optogenetic systems implemented in the budding yeast Saccharomyces cerevisiae, which allow orthogonal control (by light) of gene expression, protein subcellular localization, reconstitution of protein activity, and protein sequestration by oligomerization. Furthermore, we review the application of optogenetic systems in the control of metabolic pathways, heterologous protein production and flocculation. We then revise an example of a previously described yeast optogenetic switch, named FUN-LOV, which allows precise and strong activation of the target gene. Finally, we describe optogenetic systems that have not yet been implemented in yeast, which could therefore be used to expand the panel of available tools in this biological chassis. In conclusion, a wide repertoire of optogenetic systems can be used to address fundamental biological questions and broaden the biotechnological toolkit in yeast.

Improvement of Phycocyanobilin Synthesis for Genetically Encoded Phytochrome-Based Optogenetics

Optogenetics is a powerful technique using photoresponsive proteins, and the light-inducible dimerization (LID) system, an optogenetic tool, allows to manipulate intracellular signaling pathways. One of the red/far-red responsive LID systems, phytochrome B (PhyB)-phytochrome interacting factor (PIF), has a unique property of controlling both association and dissociation by light on the second time scale, but PhyB requires a linear tetrapyrrole chromophore such as phycocyanobilin (PCB), and such chromophores are present only in higher plants and cyanobacteria. Here, we report that we further improved our previously developed PCB synthesis system (SynPCB) and successfully established a stable cell line containing a genetically encoded PhyB-PIF LID system. First, four genes responsible for PCB synthesis, namely, PcyA, HO1, Fd, and Fnr, were replaced with their counterparts derived from thermophilic cyanobacteria. Second, Fnr was truncated, followed by fusion with Fd to generate a chimeric protein, tFnr-Fd. Third, these genes were concatenated with P2A peptide cDNAs for polycistronic expression, resulting in an approximately 4-fold increase in PCB synthesis compared with the previous version. Finally, we incorporated the PhyB, PIF, and SynPCB system into drug inducible lentiviral and transposon vectors, which enabled us to induce PCB synthesis and the PhyB-PIF LID system by doxycycline treatment. These tools provide a new opportunity to advance our understanding of the causal relationship between intracellular signaling and cellular functions.

Lighting up single-mRNA translation dynamics in living cells

Over the past five years, technological advances have made it possible to image the translation of single mRNA in the natural context of living cells. With these advances, researchers are beginning to shed light on when, where, and to what degree mRNA are translated with single-molecule precision. These works provide insight into the heterogeneity of translation amongst single transcripts, behavior that is averaged out in complementary bulk assays. In this review, we discuss the rapidly maturing field of live-cell, single-mRNA imaging of translation, beginning with a brief overview of recent technological advances. The remainder of the review focuses on the new biological insights gained from these technologies. We conclude with a discussion of the future of this technology.

Optogenetics reveals Cdc42 local activation by scaffold-mediated positive feedback and Ras GTPase

Local activity of the small GTPase Cdc42 is critical for cell polarization. Whereas scaffold-mediated positive feedback was proposed to break symmetry of budding yeast cells and produce a single zone of Cdc42 activity, the existence of similar regulation has not been probed in other organisms. Here, we address this problem using rod-shaped cells of fission yeast Schizosaccharomyces pombe, which exhibit zones of active Cdc42-GTP at both cell poles. We implemented the CRY2-CIB1 optogenetic system for acute light-dependent protein recruitment to the plasma membrane, which allowed to directly demonstrate positive feedback. Indeed, optogenetic recruitment of constitutively active Cdc42 leads to co-recruitment of the guanine nucleotide exchange factor (GEF) Scd1 and endogenous Cdc42, in a manner dependent on the scaffold protein Scd2. We show that Scd2 function is dispensable when the positive feedback operates through an engineered interaction between the GEF and a Cdc42 effector, the p21-activated kinase 1 (Pak1). Remarkably, this rewired positive feedback confers viability and allows cells to form 2 zones of active Cdc42 even when otherwise essential Cdc42 activators are lacking. These cells further revealed that the small GTPase Ras1 plays a role in both localizing the GEF Scd1 and promoting its activity, which potentiates the positive feedback. We conclude that scaffold-mediated positive feedback, gated by Ras activity, confers robust polarization for rod-shape formation.

The small GTPase Cdc42 is a key regulator of cell polarization. This study uses optogenetic and genetic strategies to show that Cdc42 is under positive feedback regulation potentiated by Ras GTPase activity.

Deep Learning in the Study of Protein-Related Interactions

Protein-related interaction prediction is critical to understanding life processes, biological functions, and mechanisms of drug action. Experimental methods used to determine proteinrelated interactions have always been costly and inefficient. In recent years, advances in biological and medical technology have provided us with explosive biological and physiological data, and deep learning-based algorithms have shown great promise in extracting features and learning patterns from complex data. At present, deep learning in protein research has emerged. In this review, we provide an introductory overview of the deep neural network theory and its unique properties. Mainly focused on the application of this technology in protein-related interactions prediction over the past five years, including protein-protein interactions prediction, protein-RNA\DNA, Protein- drug interactions prediction, and others. Finally, we discuss some of the challenges that deep learning currently faces.

Synthesis and application of light-switchable arylazopyrazole rapamycin analogs

Rapamycin-induced dimerization of FKBP and FRB has been utilized as a tool for co-localizing two proteins of interest in numerous applications. Due to the tight binding interaction of rapamycin with FKBP and FRB, the ternary complex formation is essentially irreversible. Since biological processes occur in a highly dynamic fashion with cycles of protein association and dissociation to generate a cellular response, it is useful to have chemical tools that function in a similar manner. We have developed arylazopyrazole-modified rapamycin analogs which undergo a configurational change upon light exposure and we observed enhanced ternary complex formation for the cis-isomer over the trans-isomer for one of the analogs.