Circularly Permuted Fluorescent Protein-Based Indicators: History, Principles, and Classification

Biosensor-based profiling to track cellular signalling in patient-derived models of dilated cardiomyopathy

Dilated cardiomyopathies (DCM) represent a diverse group of cardiovascular diseases impacting the structure and function of the myocardium. To better treat these diseases, we need to understand the impact of such cardiomyopathies on critical signalling pathways that drive disease progression downstream of receptors we often target therapeutically. Our understanding of cellular signalling events has progressed substantially in the last few years, in large part due to the design, validation and use of biosensor-based approaches to studying such events in cells, tissues and in some cases, living animals. Another transformative development has been the use of human induced pluripotent stem cells (hiPSCs) to generate disease-relevant models from individual patients. We highlight the importance of going beyond monocellular cultures to incorporate the influence of paracrine signalling mediators. Finally, we discuss the recent coalition of these approaches in the context of DCM. We discuss recent work in generating patient-derived models of cardiomyopathies and the utility of using signalling biosensors to track disease progression and test potential therapeutic strategies that can be later used to inform treatment options in patients.

Genetically Encoded Sensors to Study Metabolism in Drosophila

The rather recent development of genetically encoded metabolite sensors has changed the way we can study metabolism in living cells, ex vivo tissues, and in vivo immensely. In recent years, these sensors have also been adapted for use in Drosophila tissues. Here, we describe a standard protocol to image such sensors in ex vivo Drosophila larval brains using the glucose sensor FLII12Pglu-700μδ6. The protocol, however, can be adapted for the use of other sensors, tissues, and can even be used in vivo.

Design of a methotrexate-controlled chemical dimerization system and its use in bio-electronic devices

Natural evolution produced polypeptides that selectively recognize chemical entities and their polymers, ranging from ions to proteins and nucleic acids. Such selective interactions serve as entry points to biological signaling and metabolic pathways. The ability to engineer artificial versions of such entry points is a key goal of synthetic biology, bioengineering and bioelectronics. We set out to map the optimal strategy for developing artificial small molecule:protein complexes that function as chemically induced dimerization (CID) systems. Using several starting points, we evolved CID systems controlled by a therapeutic drug methotrexate. Biophysical and structural analysis of methotrexate-controlled CID system reveals the critical role played by drug-induced conformational change in ligand-controlled protein complex assembly. We demonstrate utility of the developed CID by constructing electrochemical biosensors of methotrexate that enable quantification of methotrexate in human serum. Furthermore, using the methotrexate and functionally related biosensor of rapamycin we developed a multiplexed bioelectronic system that can perform repeated measurements of multiple analytes. The presented results open the door for construction of genetically encoded signaling systems for use in bioelectronics and diagnostics, as well as metabolic and signaling network engineering.

Fluorescence-based sensing of the bioenergetic and physicochemical status of the cell

Fluorescence-based sensors play a fundamental role in biological research. These sensors can be based on fluorescent proteins, fluorescent probes or they can be hybrid systems. The availability of a very large dataset of fluorescent molecules, both genetically encoded and synthetically produced, together with the structural insights on many sensing domains, allowed to rationally design a high variety of sensors, capable of monitoring both molecular and global changes in living cells or in in vitro systems. The advancements in the fluorescence-imaging field helped researchers to obtain a deeper understanding of how and where specific changes occur in a cell or in vitro by combining the readout of the fluorescent sensors with the spatial information provided by fluorescent microscopy techniques. In this review we give an overview of the state of the art in the field of fluorescent biosensors and fluorescence imaging techniques, and eventually guide the reader through the choice of the best combination of fluorescent tools and techniques to answer specific biological questions. We particularly focus on sensors for probing the bioenergetics and physicochemical status of the cell.

ZnT1 is a neuronal Zn2+/Ca2+ exchanger

Zinc transporter 1 (ZnT1; SLC30A1) is present in the neuronal plasma membrane, critically modulating NMDA receptor function and Zn²⁺ neurotoxicity. The mechanism mediating Zn²⁺ transport by ZnT1, however, has remained elusive. Here, we investigated ZnT1-dependent Zn²⁺ transport by measuring intracellular changes of this ion using the fluorescent indicator FluoZin-3. In primary mouse cortical neurons, which express ZnT1, transient addition of extracellular Zn²⁺ triggered a rise in cytosolic Zn²⁺, followed by its removal. Knockdown of ZnT1 by adeno associated viral (AAV)-short hairpin RNA (shZnT1) markedly increased rates of Zn²⁺ rise, and decreased rates of its removal, suggesting that ZnT1 is a primary route for Zn²⁺ efflux in neurons. Although Zn²⁺ transport by other members of the SLC30A family is dependent on pH gradients across cellular membranes, altered H⁺ gradients were not coupled to ZnT1-dependent transport. Removal of cytoplasmic Zn²⁺, against a large inward gradient during the initial loading phase, suggests that Zn²⁺ efflux requires a large driving force. We therefore asked if Ca²⁺ gradients across the membrane can facilitate Zn²⁺ efflux. Elimination of extracellular Ca²⁺ abolished Zn²⁺ efflux, while increased extracellular Ca²⁺ levels enhanced Zn²⁺ efflux. Intracellular Ca²⁺ rises, measured in GCaMP6 expressing neurons, closely paralleled cytoplasmic Zn²⁺ removal. Taken together, these results strongly suggest that ZnT1 functions as a Zn²⁺/Ca²⁺ exchanger, thereby regulating the transport of two ions of fundamental importance in neuronal signaling.

Connecting Artificial Proteolytic and Electrochemical Signaling Systems with Caged Messenger Peptides

Enzymatic polypeptide proteolysis is a widespread and powerful biological control mechanism. Over the last few years, substantial progress has been made in creating artificial proteolytic systems where an input of choice modulates the protease activity and thereby the activity of its substrates. However, all proteolytic systems developed so far have relied on the direct proteolytic cleavage of their effectors. Here, we propose a new concept where protease biosensors with a tunable input uncage a signaling peptide, which can then transmit a signal to an allosteric protein reporter. We demonstrate that both the cage and the regulatory domain of the reporter can be constructed from the same peptide-binding domain, such as calmodulin. To demonstrate this concept, we constructed a proteolytic rapamycin biosensor and demonstrated its quantitative actuation on fluorescent, luminescent, and electrochemical reporters. Using the latter, we constructed sensitive bioelectrodes that detect the messenger peptide release and quantitatively convert the recognition event into electric current. We discuss the application of such systems for the construction of in vitro sensory arrays and in vivo signaling circuits.

Circular permutated PQQ-glucose dehydrogenase as an ultrasensitive electrochemical biosensor

Protein biosensors play an increasingly important role as reporters for research and clinical applications. Here we present an approach for the construction of fully integrated but modular electrochemical biosensors based on the principal component of glucose monitors PQQ-glucose dehydrogenase (PQQ-GDH). We designed allosterically regulated circular permutated variants of PQQ-GDH that show large (>10 fold) changes in enzymatic activity following intramolecular scaffolding of the newly generated N- and C termini by ligand binding domain:ligand complexes. The developed biosensors demonstrated sub-nanomolar affinities for small molecules and proteins in colorimetric and electrochemical assays.  For instance, the concentration of Cyclosporine A could be measured in 1 ml of undiluted blood with the same accuracy as the leading diagnostic technique that uses 50 times more sample. We further used this biosensor to construct highly porous gold bioelectrodes capable of robustly detecting concentrations of Cyclosporine A as low as 20 pM and retained functionality in samples containing at least 60% human serum. These experiments suggest that the developed biosensor platform is generalizable and may be suitable for Point-of-Care diagnostics.

Fluorescent biosensors illuminating plant hormone research

Phytohormones act as key regulators of plant growth that coordinate developmental and physiological processes across cells, tissues and organs. As such, their levels and distribution are highly dynamic owing to changes in their biosynthesis, transport, modification and degradation that occur over space and time. Fluorescent biosensors represent ideal tools to track these dynamics with high spatiotemporal resolution in a minimally invasive manner. Substantial progress has been made in generating a diverse set of hormone sensors with recent FRET biosensors for visualising hormone concentrations complementing information provided by transcriptional, translational and degron-based reporters. In this review, we provide an update on fluorescent biosensor designs, examine the key properties that constitute an ideal hormone biosensor, discuss the use of these sensors in conjunction with in vivo hormone perturbations and highlight the latest discoveries made using these tools.

Fluorescent toys 'n' tools lighting the way in fungal research

Although largely overlooked compared to bacterial infections, fungal infections pose a significant threat to the health of humans and other organisms. Many pathogenic fungi, especially Candida species, are extremely versatile and flexible in adapting to various host niches and stressful situations. This leads to high pathogenicity and increasing resistance to existing drugs. Due to the high level of conservation between fungi and mammalian cells, it is hard to find fungus-specific drug targets for novel therapy development. In this respect, it is vital to understand how these fungi function on a molecular, cellular as well as organismal level. Fluorescence imaging allows for detailed analysis of molecular mechanisms, cellular structures and interactions on different levels. In this manuscript, we provide researchers with an elaborate and contemporary overview of fluorescence techniques that can be used to study fungal pathogens. We focus on the available fluorescent labelling techniques and guide our readers through the different relevant applications of fluorescent imaging, from subcellular events to multispecies interactions and diagnostics. As well as cautioning researchers for potential challenges and obstacles, we offer hands-on tips and tricks for efficient experimentation and share our expert-view on future developments and possible improvements.

A single-wavelength flow cytometric approach using redox-sensitive green fluorescent protein probes for measuring redox stress in live cells

Cellular redox changes are common in apoptosis, immune function, signaling pathways and cancer. The authors aimed to develop a single-wavelength method using the superior fluorescence sensitivity of a flow cytometer for measuring redox-sensitive green fluorescent protein signal during oxidative stress in cell lines. The single-wavelength method was able to discern small differences in oxidative stress between cell lines and between the cytoplasmic and mitochondrial compartments within the same cell line. In Chinese hamster ovary cells, the mitochondrial matrix compartment was more sensitive to oxidative stress compared with MDA-MB-231 cells, and the rapid changes in redox state were followed by a slow recovery phase. The authors conclude that this simplified method is useful and preferred for studies where alterations in overall redox-sensitive green fluorescent protein expression are controlled.

Photo-transformable genetically-encoded optical probes for functional highlighting in vivo

Studying the brain requires knowledge about both structure (i.e., connectome) and function of its constituents (neurons and glia alike). This need has prompted the development of novel tools and techniques, in particular optical techniques to examine cells remotely. Early works (1900's) led to the development of novel cell-staining techniques that, when combined with the use of a very simple light microscope, visualized individual neurons and their subcellular compartments in fixed tissues. Today, highlighting of structure and function can be performed on live cells, notably in vivo, owing to discovery of GFP and subsequent development of genetically encoded fluorescent optical tools. In this review, we focus our attention on a subset of optical biosensors, namely probes whose emission can be modified by light. We designate them photo-transformable genetically encoded probes. The family of photo-transformable probes embraces current probes that undergo photoactivation (PA), photoconversion (PC) or photoswitching (PS). We argue that these are particularly suited for studying multiple features of neurons, such as structure, connectivity and function concomitantly, for functional highlighting of neurons in vivo.

Evaluation of multi-color genetically encoded Ca2+ indicators in filamentous fungi

Genetically encoded Ca²⁺ indicators (GECIs) enable long-term monitoring of cellular and subcellular dynamics of this second messenger in response to environmental and developmental cues without relying on exogenous dyes. Continued development and optimization in GECIs, combined with advances in gene manipulation, offer new opportunities for investigating the mechanism of Ca²⁺ signaling in fungi, ranging from documenting Ca²⁺ signatures under diverse conditions and genetic backgrounds to evaluating how changes in Ca²⁺ signature impact calcium-binding proteins and subsequent cellular changes. Here, we attempted to express multi-color (green, yellow, blue, cyan, and red) circularly permuted fluorescent protein (FP)-based Ca²⁺ indicators driven by multiple fungal promoters in Fusarium oxysporum, F. graminearum, and Neurospora crassa. Several variants were successfully expressed, with GCaMP5G driven by the Magnaporthe oryzae ribosomal protein 27 and F. verticillioides elongation factor-1α gene promoters being optimal for F. graminearum and F. oxysporum, respectively. Transformants expressing GCaMP5G were compared with those expressing YC3.60, a ratiometric Cameleon Ca²⁺ indicator. Wild-type and three Ca²⁺ signaling mutants of F. graminearum expressing GCaMP5G exhibited improved signal-to-noise and increased temporal and spatial resolution and are also more amenable to studies involving multiple FPs compared to strains expressing YC3.60.

The design and evolution of fluorescent protein-based sensors for monoatomic ions in biology

Living cells rely on a finely tuned symphony of inorganic ion gradients composed of both cations and anions. This delicate balance is maintained by biological receptors all acting in concert to selectively recognize and position ions for homeostasis. These dynamic processes can be intercepted and visualized with optical microscopy at the organismal, tissue, cellular and subcellular levels using fluorescent protein-based biosensors. Since the first report of such tool for calcium (Ca2+) in 1997, outstanding biological questions and innovations in protein engineering along with associated fields have driven the development of new biosensors for Ca2+ and beyond. In this Review, we summarize a workflow that can be used to generate fluorescent protein-based biosensors to study monoatomic ions in biology. To showcase the scope of this approach, we highlight recent advances reported for Ca2+ biosensors and in detail discuss representative case studies of biosensors reported in the last four years for potassium (K+), magnesium (Mg2+), copper (Cu2+/+), lanthanide (Ln3+) and chloride (Cl-) ions.

Structure- and mechanism-guided design of single fluorescent protein-based biosensors

Intensiometric genetically encoded biosensors, based on allosteric modulation of the fluorescence of a single fluorescent protein, are powerful tools for enabling imaging of neural activities and other cellular biochemical events. The archetypical example of such biosensors is the GCaMP series of Ca²⁺ biosensors, which have been steadily improved over the past two decades and are now indispensable tools for neuroscience. However, no other biosensors have reached levels of performance, or had revolutionary impacts within specific disciplines, comparable to that of the Ca²⁺ biosensors. Of the many reasons why this has been the case, a critical one has been a general black-box view of biosensor structure and mechanism. With this Perspective, we aim to summarize what is known about biosensor structure and mechanisms and, based on this foundation, provide guidelines to accelerate the development of a broader range of biosensors with performance comparable to that of the GCaMP series.

Emergent Biosensing Technologies Based on Fluorescence Spectroscopy and Surface Plasmon Resonance

The purpose of this work is to provide an exhaustive overview of the emerging biosensor technologies for the detection of analytes of interest for food, environment, security, and health. Over the years, biosensors have acquired increasing importance in a wide range of applications due to synergistic studies of various scientific disciplines, determining their great commercial potential and revealing how nanotechnology and biotechnology can be strictly connected. In the present scenario, biosensors have increased their detection limit and sensitivity unthinkable until a few years ago. The most widely used biosensors are optical-based devices such as surface plasmon resonance (SPR)-based biosensors and fluorescence-based biosensors. Here, we will review them by highlighting how the progress in their design and development could impact our daily life.

Genetically Encoded Biosensors Based on Fluorescent Proteins

Genetically encoded biosensors based on fluorescent proteins (FPs) allow for the real-time monitoring of molecular dynamics in space and time, which are crucial for the proper functioning and regulation of complex cellular processes. Depending on the types of molecular events to be monitored, different sensing strategies need to be applied for the best design of FP-based biosensors. Here, we review genetically encoded biosensors based on FPs with various sensing strategies, for example, translocation, fluorescence resonance energy transfer (FRET), reconstitution of split FP, pH sensitivity, maturation speed, and so on. We introduce general principles of each sensing strategy and discuss critical factors to be considered if available, then provide representative examples of these FP-based biosensors. These will help in designing the best sensing strategy for the successful development of new genetically encoded biosensors based on FPs.

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.

Imaging Neurotransmitter and Neuromodulator Dynamics In Vivo with Genetically Encoded Indicators

The actions of neuromodulation are thought to mediate the ability of the mammalian brain to dynamically adjust its functional state in response to changes in the environment. Altered neurotransmitter (NT) and neuromodulator (NM) signaling is central to the pathogenesis or treatment of many human neurological and psychiatric disorders, including Parkinson's disease, schizophrenia, depression, and addiction. To reveal the precise mechanisms by which these neurochemicals regulate healthy and diseased neural circuitry, one needs to measure their spatiotemporal dynamics in the living brain with great precision. Here, we discuss recent development, optimization, and applications of optical approaches to measure the spatial and temporal profiles of NT and NM release in the brain using genetically encoded sensors for in vivo studies.

Recent advances in neurotechnologies with broad potential for neuroscience research

Interest in deciphering the fundamental mechanisms and processes of the human mind represents a central driving force in modern neuroscience research. Activities in support of this goal rely on advanced methodologies and engineering systems that are capable of interrogating and stimulating neural pathways, from single cells in small networks to interconnections that span the entire brain. Recent research establishes the foundations for a broad range of creative neurotechnologies that enable unique modes of operation in this context. This review focuses on those systems with proven utility in animal model studies and with levels of technical maturity that suggest a potential for broad deployment to the neuroscience community in the relatively near future. We include a brief summary of existing and emerging neuroscience techniques, as background for a primary focus on device technologies that address associated opportunities in electrical, optical and microfluidic neural interfaces, some with multimodal capabilities. Examples of the use of these technologies in recent neuroscience studies illustrate their practical value. The vibrancy of the engineering science associated with these platforms, the interdisciplinary nature of this field of research and its relevance to grand challenges in the treatment of neurological disorders motivate continued growth of this area of study.

In Vivo Imaging with Genetically Encoded Redox Biosensors

Redox reactions are of high fundamental and practical interest since they are involved in both normal physiology and the pathogenesis of various diseases. However, this area of research has always been a relatively problematic field in the context of analytical approaches, mostly because of the unstable nature of the compounds that are measured. Genetically encoded sensors allow for the registration of highly reactive molecules in real-time mode and, therefore, they began a new era in redox biology. Their strongest points manifest most brightly in in vivo experiments and pave the way for the non-invasive investigation of biochemical pathways that proceed in organisms from different systematic groups. In the first part of the review, we briefly describe the redox sensors that were used in vivo as well as summarize the model systems to which they were applied. Next, we thoroughly discuss the biological results obtained in these studies in regard to animals, plants, as well as unicellular eukaryotes and prokaryotes. We hope that this work reflects the amazing power of this technology and can serve as a useful guide for biologists and chemists who work in the field of redox processes.

Genetically encoded thiol redox-sensors in the zebrafish model: lessons for embryonic development and regeneration

Important roles for reactive oxygen species (ROS) and redox signaling in embryonic development and regenerative processes are increasingly recognized. However, it is difficult to obtain information on spatiotemporal dynamics of ROS production and signaling in vivo. The zebrafish is an excellent model for in vivo bioimaging and possesses a remarkable regenerative capacity upon tissue injury. Here, we review data obtained in this model system with genetically encoded redox-sensors targeting H₂O₂ and glutathione redox potential. We describe how such observations have prompted insight into regulation and downstream effects of redox alterations during tissue differentiation, morphogenesis and regeneration. We also discuss the properties of the different sensors and their consequences for the interpretation of in vivo imaging results. Finally, we highlight open questions and additional research fields that may benefit from further application of such sensor systems in zebrafish models of development, regeneration and disease.

Triclosan disrupts immune cell function by depressing Ca2+ influx following acidification of the cytoplasm

Triclosan (TCS) is an antimicrobial agent that was effectively banned by the FDA from hand soaps in 2016, hospital soaps in 2017, and hand sanitizers in 2019; however, TCS can still be found in a few products. At consumer-relevant, non-cytotoxic doses, TCS inhibits the functions of both mitochondria and mast cells, a ubiquitous cell type. Via the store-operated Ca2+ entry mechanism utilized by many immune cells, mast cells undergo antigen-stimulated Ca2+ influx into the cytosol, for proper function. Previous work showed that TCS inhibits Ca2+ dynamics in mast cells, and here we show that TCS also inhibits Ca2+ mobilization in human Jurkat T cells. However, the biochemical mechanism behind the Ca2+ dampening has yet to be elucidated. Three-dimensional super-resolution microscopy reveals that TCS induces mitochondrial swelling, in line with and extending the previous finding of TCS inhibition of mitochondrial membrane potential via its proton ionophoric activity. Inhibition of plasma membrane potential (PMP) by the canonical depolarizer gramicidin can inhibit mast cell function. However, use of the genetically encoded voltage indicators (GEVIs) ArcLight (pH-sensitive) and ASAP2 (pH-insensitive), indicates that TCS does not disrupt PMP. In conjunction with data from a plasma membrane-localized, pH-sensitive reporter, these results indicate that TCS, instead, induces cytosolic acidification in mast cells and T cells. Acidification of the cytosol likely inhibits Ca2+ influx by uncoupling the STIM1/ORAI1 interaction that is required for opening of plasma membrane Ca2+ channels. These results provide a mechanistic explanation of TCS disruption of Ca2+ influx and, thus, of immune cell function.

Rational Design of Novel Fluorescent Enzyme Biosensors for Direct Detection of Strigolactones

Strigolactones are plant hormones and rhizosphere signaling molecules with key roles in plant development, mycorrhizal fungal symbioses, and plant parasitism. Currently, sensitive, specific, and high-throughput methods of detecting strigolactones are limited. Here, we developed genetically encoded fluorescent strigolactone biosensors based on the strigolactone receptors DAD2 from Petunia hybrida, and HTL7 from Striga hermonthica. The biosensors were constructed via domain insertion of circularly permuted GFP. The biosensors exhibited loss of cpGFP fluorescence in vitro upon treatment with the strigolactones 5-deoxystrigol and orobanchol, or the strigolactone analogue rac-GR24, and the ShHTL7 biosensor also responded to a specific antagonist. To overcome biosensor sensitivity to changes in expression level and protein degradation, an additional strigolactone-insensitive fluorophore, LSSmOrange, was included as an internal normalization control. Other plant hormones and karrikins resulted in no fluorescence change, demonstrating that the biosensors report on compounds that specifically bind the SL receptors. The DAD2 biosensor likewise responded to strigolactones in an in vivo protoplast system, and retained strigolactone hydrolysis activity. These biosensors have applications in high-throughput screening for agrochemical compounds, and may also have utility in understanding strigolactone mediated signaling in plants.

Genetically Encoded Tools for Research of Cell Signaling and Metabolism under Brain Hypoxia

Hypoxia is characterized by low oxygen content in the tissues. The central nervous system (CNS) is highly vulnerable to a lack of oxygen. Prolonged hypoxia leads to the death of brain cells, which underlies the development of many pathological conditions. Despite the relevance of the topic, different approaches used to study the molecular mechanisms of hypoxia have many limitations. One promising lead is the use of various genetically encoded tools that allow for the observation of intracellular parameters in living systems. In the first part of this review, we provide the classification of oxygen/hypoxia reporters as well as describe other genetically encoded reporters for various metabolic and redox parameters that could be implemented in hypoxia studies. In the second part, we discuss the advantages and disadvantages of the primary hypoxia model systems and highlight inspiring examples of research in which these experimental settings were combined with genetically encoded reporters.

Expanding the Chemogenetic Toolbox by Circular Permutation

To expand the repertoire of chemogenetic tools tailored for molecular and cellular engineering, we describe herein the design of cpRAPID as a circularly permuted rapamycin-inducible dimerization system composed of the canonical FK506-binding protein (FKBP) and circular permutants of FKBP12-rapamycin binding domain (cpFRB). By permuting the topology of the four helices within FRB, we have created cpFRB-FKBP pairs that respond to ligand with varying activation kinetics and dynamics. The cpRAPID system enables chemical-controllable subcellular redistribution of proteins, as well as inducible transcriptional activation when coupled with the CRISPR activation (CRISPRa) technology to induce a GFP reporter and endogenous gene expression. We have further demonstrated the use of cpRAPID to generate chemically switchable split nanobody (designated Chessbody) for ligand-gated antigen recognition in living cells. Collectively, the circular permutation approach offers a powerful means for diversifying the chemogenetics toolbox to benefit the burgeoning synthetic biology field.

A Bright and Colorful Future for G-Protein Coupled Receptor Sensors

Neurochemicals have a large impact on brain states and animal behavior but are notoriously hard to detect accurately in the living brain. Recently developed genetically encoded sensors obtained from engineering a circularly permuted green fluorescent protein into G-protein coupled receptors (GPCR) provided a vital boost to neuroscience, by innovating the way we monitor neural communication. These new probes are becoming widely successful due to their flexible combination with state of the art optogenetic tools and in vivo imaging techniques, mainly fiber photometry and 2-photon microscopy, to dissect dynamic changes in brain chemicals with unprecedented spatial and temporal resolution. Here, we highlight current approaches and challenges as well as novel insights in the process of GPCR sensor development, and discuss possible future directions of the field.

Fluorescent sensors for neuronal signaling

Dissecting neuronal structure and function in relation to behavior is an immense undertaking. Researchers require imaging tools to study neuronal activity and biochemical signaling in situ in order to study the roles of neuronal and biochemical activity in information processing. A large number of genetically encoded fluorescent biosensors have been reported in the literature over the past few years as there is a push to develop new technology in neuroscience. Here, we review the classes and characteristics of fluorescent biosensors and highlight some considerations that investigators should keep in mind when choosing their tool. In addition, we discuss recent advances in biosensor development.