Red fluorescent protein-based cAMP indicator applicable to optogenetics and in vivo imaging

Functional Imaging of Learning-Induced Plasticity in the Central Nervous System with Genetically Encoded Reporters in Drosophila

Learning and memory allow animals to adjust their behavior based on the predictive value of their past experiences. Memories often exist in complex representations, spread across numerous cells and synapses in the brain. Studying relatively simple forms of memory provides insights into the fundamental processes that underlie multiple forms of memory. Associative learning occurs when an animal learns the relationship between two previously unrelated sensory stimuli, such as when a hungry animal learns that a particular odor is followed by a tasty reward. Drosophila is a particularly powerful model to study how this type of memory works. The fundamental principles are widely shared among animals, and there is a wide range of genetic tools available to study circuit function in flies. In addition, the olfactory structures that mediate associative learning in flies, such as the mushroom body and its associated neurons, are anatomically organized, relatively well-characterized, and readily accessible to imaging. Here, we review the olfactory anatomy and physiology of the olfactory system, describe how plasticity in the olfactory pathway mediates learning and memory, and explain the general principles underlying calcium imaging approaches.

A multicolor suite for deciphering population coding of calcium and cAMP in vivo

cAMP is a universal second messenger regulated by various upstream pathways including Ca2+ and G-protein-coupled receptors (GPCRs). To decipher in vivo cAMP dynamics, we rationally designed cAMPinG1, a sensitive genetically encoded green cAMP indicator that outperformed its predecessors in both dynamic range and cAMP affinity. Two-photon cAMPinG1 imaging detected cAMP transients in the somata and dendritic spines of neurons in the mouse visual cortex on the order of tens of seconds. In addition, multicolor imaging with a sensitive red Ca2+ indicator RCaMP3 allowed simultaneous measurement of population patterns in Ca2+ and cAMP in hundreds of neurons. We found Ca2+-related cAMP responses that represented specific information, such as direction selectivity in vision and locomotion, as well as GPCR-related cAMP responses. Overall, our multicolor suite will facilitate analysis of the interaction between the Ca2+, GPCR and cAMP signaling at single-cell resolution both in vitro and in vivo.

Network-level encoding of local neurotransmitters in cortical astrocytes

Astrocytes, the most abundant non-neuronal cell type in the mammalian brain, are crucial circuit components that respond to and modulate neuronal activity through calcium (Ca2+) signalling1-7. Astrocyte Ca2+ activity is highly heterogeneous and occurs across multiple spatiotemporal scales-from fast, subcellular activity3,4 to slow, synchronized activity across connected astrocyte networks8-10-to influence many processes5,7,11. However, the inputs that drive astrocyte network dynamics remain unclear. Here we used ex vivo and in vivo two-photon astrocyte imaging while mimicking neuronal neurotransmitter inputs at multiple spatiotemporal scales. We find that brief, subcellular inputs of GABA and glutamate lead to widespread, long-lasting astrocyte Ca2+ responses beyond an individual stimulated cell. Further, we find that a key subset of Ca2+ activity-propagative activity-differentiates astrocyte network responses to these two main neurotransmitters, and may influence responses to future inputs. Together, our results demonstrate that local, transient neurotransmitter inputs are encoded by broad cortical astrocyte networks over a minutes-long time course, contributing to accumulating evidence that substantial astrocyte-neuron communication occurs across slow, network-level spatiotemporal scales12-14. These findings will enable future studies to investigate the link between specific astrocyte Ca2+ activity and specific functional outputs, which could build a consistent framework for astrocytic modulation of neuronal activity.

Structure and dynamics determine G protein coupling specificity at a class A GPCR

G protein coupled receptors (GPCRs) exhibit varying degrees of selectivity for different G protein isoforms. Despite the abundant structures of GPCR-G protein complexes, little is known about the mechanism of G protein coupling specificity. The β2-adrenergic receptor is an example of GPCR with high selectivity for Gαs, the stimulatory G protein for adenylyl cyclase, and much weaker for the Gαi family of G proteins inhibiting adenylyl cyclase. By developing a new Gαi-biased agonist (LM189), we provide structural and biophysical evidence supporting that distinct conformations at ICL2 and TM6 are required for coupling of the different G protein subtypes Gαs and Gαi. These results deepen our understanding of G protein specificity and bias and can accelerate the design of ligands that select for preferred signaling pathways.

Regulation of long-range BMP gradients and embryonic polarity by propagation of local calcium-firing activity

Many amniote vertebrate species including humans can form identical twins from a single embryo, but this only occurs rarely. It has been suggested that the primitive-streak-forming embryonic region emits signals that inhibit streak formation elsewhere but the signals involved, how they are transmitted and how they act has not been elucidated. Here we show that short tracks of calcium firing activity propagate through extraembryonic tissue via gap junctions and prevent ectopic primitive streak formation in chick embryos. Cross-regulation of calcium activity and an inhibitor of primitive streak formation (Bone Morphogenetic Protein, BMP) via NF-κB and NFAT establishes a long-range BMP gradient spanning the embryo. This mechanism explains how embryos of widely different sizes can maintain positional information that determines embryo polarity. We provide evidence for similar mechanisms in two different human embryo models and in Drosophila, suggesting an ancient evolutionary origin.

Alternative splicing of latrophilin-3 controls synapse formation

The assembly and specification of synapses in the brain is incompletely understood1-3. Latrophilin-3 (encoded by Adgrl3, also known as Lphn3)-a postsynaptic adhesion G-protein-coupled receptor-mediates synapse formation in the hippocampus4 but the mechanisms involved remain unclear. Here we show in mice that LPHN3 organizes synapses through a convergent dual-pathway mechanism: activation of Gαs signalling and recruitment of phase-separated postsynaptic protein scaffolds. We found that cell-type-specific alternative splicing of Lphn3 controls the LPHN3 G-protein-coupling mode, resulting in LPHN3 variants that predominantly signal through Gαs or Gα12/13. CRISPR-mediated manipulation of Lphn3 alternative splicing that shifts LPHN3 from a Gαs- to a Gα12/13-coupled mode impaired synaptic connectivity as severely as the overall deletion of Lphn3, suggesting that Gαs signalling by LPHN3 splice variants mediates synapse formation. Notably, Gαs-coupled, but not Gα12/13-coupled, splice variants of LPHN3 also recruit phase-transitioned postsynaptic protein scaffold condensates, such that these condensates are clustered by binding of presynaptic teneurin and FLRT ligands to LPHN3. Moreover, neuronal activity promotes alternative splicing of the synaptogenic Gαs-coupled variant of LPHN3. Together, these data suggest that activity-dependent alternative splicing of a key synaptic adhesion molecule controls synapse formation by parallel activation of two convergent pathways: Gαs signalling and clustered phase separation of postsynaptic protein scaffolds.

Targeting cAMP in D1-MSNs in the nucleus accumbens, a new rapid antidepressant strategy

Studies have suggested that the nucleus accumbens (NAc) is implicated in the pathophysiology of major depression; however, the regulatory strategy that targets the NAc to achieve an exclusive and outstanding anti-depression benefit has not been elucidated. Here, we identified a specific reduction of cyclic adenosine monophosphate (cAMP) in the subset of dopamine D1 receptor medium spiny neurons (D1-MSNs) in the NAc that promoted stress susceptibility, while the stimulation of cAMP production in NAc D1-MSNs efficiently rescued depression-like behaviors. Ketamine treatment enhanced cAMP both in D1-MSNs and dopamine D2 receptor medium spiny neurons (D2-MSNs) of depressed mice, however, the rapid antidepressant effect of ketamine solely depended on elevating cAMP in NAc D1-MSNs. We discovered that a higher dose of crocin markedly increased cAMP in the NAc and consistently relieved depression 24 h after oral administration, but not a lower dose. The fast onset property of crocin was verified through multicenter studies. Moreover, crocin specifically targeted at D1-MSN cAMP signaling in the NAc to relieve depression and had no effect on D2-MSN. These findings characterize a new strategy to achieve an exclusive and outstanding anti-depression benefit by elevating cAMP in D1-MSNs in the NAc, and provide a potential rapid antidepressant drug candidate, crocin.

Reactive Halogen Species: Role in Living Systems and Current Research Approaches

Reactive halogen species (RHS) are highly reactive compounds that are normally required for regulation of immune response, inflammatory reactions, enzyme function, etc. At the same time, hyperproduction of highly reactive compounds leads to the development of various socially significant diseases - asthma, pulmonary hypertension, oncological and neurodegenerative diseases, retinopathy, and many others. The main sources of (pseudo)hypohalous acids are enzymes from the family of heme peroxidases - myeloperoxidase, lactoperoxidase, eosinophil peroxidase, and thyroid peroxidase. Main targets of these compounds are proteins and peptides, primarily methionine and cysteine residues. Due to the short lifetime, detection of RHS can be difficult. The most common approach is detection of myeloperoxidase, which is thought to reflect the amount of RHS produced, but these methods are indirect, and the results are often contradictory. The most promising approaches seem to be those that provide direct registration of highly reactive compounds themselves or products of their interaction with components of living cells, such as fluorescent dyes. However, even such methods have a number of limitations and can often be applied mainly for in vitro studies with cell culture. Detection of reactive halogen species in living organisms in real time is a particularly acute issue. The present review is devoted to RHS, their characteristics, chemical properties, peculiarities of interaction with components of living cells, and methods of their detection in living systems. Special attention is paid to the genetically encoded tools, which have been introduced recently and allow avoiding a number of difficulties when working with living systems.

Temporally multiplexed imaging of dynamic signaling networks in living cells

Molecular signals interact in networks to mediate biological processes. To analyze these networks, it would be useful to image many signals at once, in the same living cell, using standard microscopes and genetically encoded fluorescent reporters. Here, we report temporally multiplexed imaging (TMI), which uses genetically encoded fluorescent proteins with different clocklike properties-such as reversibly photoswitchable fluorescent proteins with different switching kinetics-to represent different cellular signals. We linearly decompose a brief (few-second-long) trace of the fluorescence fluctuations, at each point in a cell, into a weighted sum of the traces exhibited by each fluorophore expressed in the cell. The weights then represent the signal amplitudes. We use TMI to analyze relationships between different kinase activities in individual cells, as well as between different cell-cycle signals, pointing toward broad utility throughout biology in the analysis of signal transduction cascades in living systems.

Network-level encoding of local neurotransmitters in cortical astrocytes

Astrocytes-the most abundant non-neuronal cell type in the mammalian brain-are crucial circuit components that respond to and modulate neuronal activity via calcium (Ca 2+ ) signaling 1-8 . Astrocyte Ca 2+ activity is highly heterogeneous and occurs across multiple spatiotemporal scales: from fast, subcellular activity 3,4 to slow, synchronized activity that travels across connected astrocyte networks 9-11 . Furthermore, astrocyte network activity has been shown to influence a wide range of processes 5,8,12 . While astrocyte network activity has important implications for neuronal circuit function, the inputs that drive astrocyte network dynamics remain unclear. Here we used ex vivo and in vivo two-photon Ca 2+ imaging of astrocytes while mimicking neuronal neurotransmitter inputs at multiple spatiotemporal scales. We find that brief, subcellular inputs of GABA and glutamate lead to widespread, long-lasting astrocyte Ca 2+ responses beyond an individual stimulated cell. Further, we find that a key subset of Ca 2+ activity-propagative events-differentiates astrocyte network responses to these two major neurotransmitters, and gates responses to future inputs. Together, our results demonstrate that local, transient neurotransmitter inputs are encoded by broad cortical astrocyte networks over the course of minutes, contributing to accumulating evidence across multiple model organisms that significant astrocyte-neuron communication occurs across slow, network-level spatiotemporal scales 13-15 . We anticipate that this study will be a starting point for future studies investigating the link between specific astrocyte Ca 2+ activity and specific astrocyte functional outputs, which could build a consistent framework for astrocytic modulation of neuronal activity.

Genetically encoded protein sensors for metal ion detection in biological systems: a review and bibliometric analysis

Metal ions are indispensable elements in living organisms and are associated with regulating various biological processes. An imbalance in metal ion content can lead to disorders in normal physiological functions of the human body and cause various diseases. Genetically encoded fluorescent protein sensors have the advantages of low biotoxicity, high specificity, and a long imaging time in vivo and have become a powerful tool to visualize or quantify the concentration level of biomolecules in vivo and in vitro, temporal and spatial distribution, and life activity process. This review analyzes the development status and current research hotspots in the field of genetically encoded fluorescent protein sensors by bibliometric analysis. Based on the results of bibliometric analysis, the research progress of genetically encoded fluorescent protein sensors for metal ion detection is reviewed, and the construction strategies, physicochemical properties, and applications of such sensors in biological imaging are summarized.

Structure-Activity Relationship Study of Cannabidiol-Based Analogs as Negative Allosteric Modulators of the μ-Opioid Receptor

The US faces an unprecedented surge in fatal drug overdoses. Naloxone, the only antidote for opiate overdose, competes at the mu opioid receptor (μOR) orthosteric site. Naloxone struggles against fentanyl-class synthetic opioids that now cause ∼80% of deaths. Negative allosteric modulators (NAMs) targeting secondary sites may noncompetitively downregulate μOR activation. (-)-Cannabidiol ((-)-CBD) is a candidate μOR NAM. To explore its therapeutic potential, we evaluated the structure-activity relationships among CBD analogs to identify NAMs with increased potency. Using a cyclic AMP assay, we characterize reversal of μOR activation by 15 CBD analogs, several of which proved more potent than (-)-CBD. Comparative docking investigations suggest that potent compounds interact with a putative allosteric pocket to stabilize the inactive μOR conformation. Finally, these compounds enhance naloxone displacement of fentanyl from the orthosteric site. Our results suggest that CBD analogs offer considerable potential for the development of next-generation antidotes for opioid overdose.

A collection of cannabinoid-related negative findings from autaptic hippocampal neurons

Autaptic hippocampal neurons are an architecturally simple model of neurotransmission that express several forms of cannabinoid signaling. Over the past twenty years this model has proven valuable for studies ranging from enzymatic control of endocannabinoid production and breakdown, to CB1 receptor structure/function, to CB2 signaling, understanding 'spice' (synthetic cannabinoid) pharmacology, and more. However, while studying cannabinoid signaling in these neurons, we have occasionally encountered what one might call 'interesting negatives', valid and informative findings in the context of our experimental design that, given the nature of scientific publishing, may not otherwise find their way into the scientific literature. In autaptic hippocampal neurons we have found that: (1) The fatty acid binding protein (FABP) blocker SBFI-26 does not alter CB1-mediated neuroplasticity. (2) 1-AG signals poorly relative to 2-AG in autaptic neurons. (3) Indomethacin is not a CB1 PAM in autaptic neurons. (4) The CB1-associated protein SGIP1a is not necessary for CB1 desensitization. We are presenting these negative or perplexing findings in the hope that they will prove beneficial to other laboratories and elicit fruitful discussions regarding their relevance and significance.

Development of green fluorescent protein-based cAMP indicators for covering a wide range of cAMP concentrations

There is a wide range in the concentration of intracellular cyclic adenosine 3',5'-monophosphate (cAMP), which mediates specific effects as a second messenger in pathways affecting many physiological processes. Here, we developed green fluorescent cAMP indicators, named Green Falcan (Green fluorescent protein-based indicator visualizing cAMP dynamics) with various EC₅₀ values (0.3, 1, 3, 10 μM) for covering the wide range of intracellular cAMP concentrations. The fluorescence intensity of Green Falcans increased in a cAMP dose-dependent manner, with a dynamic range of over 3-fold. Green Falcans showed a high specificity for cAMP over its structural analogues. When we expressed Green Falcans in HeLa cells, these indicators were applicable for visualization of cAMP dynamics in the low concentration range compared to the previously developed cAMP indicators, and visualized distinct kinetics of cAMP in various pathways with high spatiotemporal resolution in living cells. Furthermore, we demonstrated that Green Falcans are applicable to dual-color imaging with R-GECO, a red fluorescent Ca²⁺ indicator, in the cytoplasm and the nucleus. This study shows that Green Falcans open up a new avenue for understanding hierarchal and cooperative interactions with other molecules in various cAMP signaling pathways by multi-color imaging.

Compartmentalised cAMP signalling in the primary cilium

cAMP is a universal second messenger that relies on precise spatio-temporal regulation to control varied, and often opposing, cellular functions. This is achieved via selective activation of effectors embedded in multiprotein complexes, or signalosomes, that reside at distinct subcellular locations. cAMP is also one of many pathways known to operate within the primary cilium. Dysfunction of ciliary signaling leads to a class of diseases known as ciliopathies. In Autosomal Dominant Polycystic Kidney Disease (ADPKD), a ciliopathy characterized by the formation of fluid-filled kidney cysts, upregulation of cAMP signaling is known to drive cystogenesis. For decades it has been debated whether the primary cilium is an independent cAMP sub-compartment, or whether it shares a diffusible pool of cAMP with the cell body. Recent studies now suggest it is a specific pool of cAMP generated in the cilium that propels cyst formation in ADPKD, supporting the notion that this antenna-like organelle is a compartment within which cAMP signaling occurs independently from cAMP signaling in the bulk cytosol. Here we present examples of cAMP function in the cilium which suggest this mysterious organelle is home to more than one cAMP signalosome. We review evidence that ciliary membrane localization of G-Protein Coupled Receptors (GPCRs) determines their downstream function and discuss how optogenetic tools have contributed to establish that cAMP generated in the primary cilium can drive cystogenesis.

Constrained catecholamines gain β2AR selectivity through allosteric effects on pocket dynamics

G protein-coupled receptors (GPCRs) within the same subfamily often share high homology in their orthosteric pocket and therefore pose challenges to drug development. The amino acids that form the orthosteric binding pocket for epinephrine and norepinephrine in the β1 and β2 adrenergic receptors (β1AR and β2AR) are identical. Here, to examine the effect of conformational restriction on ligand binding kinetics, we synthesized a constrained form of epinephrine. Surprisingly, the constrained epinephrine exhibits over 100-fold selectivity for the β2AR over the β1AR. We provide evidence that the selectivity may be due to reduced ligand flexibility that enhances the association rate for the β2AR, as well as a less stable binding pocket for constrained epinephrine in the β1AR. The differences in the amino acid sequence of the extracellular vestibule of the β1AR allosterically alter the shape and stability of the binding pocket, resulting in a marked difference in affinity compared to the β2AR. These studies suggest that for receptors containing identical binding pocket residues, the binding selectivity may be influenced in an allosteric manner by surrounding residues, like those of the extracellular loops (ECLs) that form the vestibule. Exploiting these allosteric influences may facilitate the development of more subtype-selective ligands for GPCRs.

A flexible and versatile system for multi-color fiber photometry and optogenetic manipulation

Here, we present simultaneous fiber photometry recordings and optogenetic stimulation based on a multimode fused fiber coupler for both light delivery and collection without the need for dichroic beam splitters. In combination with a multi-color light source and appropriate optical filters, our approach offers remarkable flexibility in experimental design and facilitates the exploration of new molecular tools in vivo at minimal cost. We demonstrate straightforward re-configuration of the setup to operate with green, red, and near-infrared calcium indicators with or without simultaneous optogenetic stimulation and further explore the multi-color photometry capabilities of the system. The ease of assembly, operation, characterization, and customization of this platform holds the potential to foster the development of experimental strategies for multi-color fused fiber photometry combined with optogenetics far beyond its current state.

Astrocyte heterogeneity and interactions with local neural circuits

Astrocytes are ubiquitous within the central nervous system (CNS). These cells possess many individual processes which extend out into the neuropil, where they interact with a variety of other cell types, including neurons at synapses. Astrocytes are now known to be active players in all aspects of the synaptic life cycle, including synapse formation and elimination, synapse maturation, maintenance of synaptic homeostasis and modulation of synaptic transmission. Traditionally, astrocytes have been studied as a homogeneous group of cells. However, recent studies have uncovered a surprising degree of heterogeneity in their development and function, suggesting that astrocytes may be matched to neurons to support local circuits. Hence, a better understanding of astrocyte heterogeneity and its implications are needed to understand brain function.

F-actin determines the time-dependent shift in docking dynamics of glucagon-like peptide-1 granules upon stimulation of secretion

Although exocytosis can be categorized into several forms based on docking dynamics, temporal regulatory mechanisms of the exocytotic forms are unclear. We explored the dynamics of glucagon-like peptide-1 (GLP-1) exocytosis in murine GLUTag cells (GLP-1-secreting enteroendocrine L-cells) upon stimulation with deoxycholic acid (DCA) or high K⁺ to elucidate the mechanisms regulating the balance between the different types of exocytotic forms (pre-docked with the plasma membrane before stimulation; docked after stimulation and subsequently fused; or rapidly recruited and fused after stimulation, without stable docking). GLP-1 exocytosis showed a biphasic pattern, and we found that most exocytosis was from the pre-docked granules with the plasma membrane before stimulation, or granules rapidly fused to the plasma membrane without docking after stimulation. In contrast, granules docked with the plasma membrane after stimuli and eventually fused were predominant thereafter. Inhibition of actin polymerization suppressed exocytosis of the pre-docked granules. These results suggest that the docking dynamics of GLP-1 granules shows a time-dependent biphasic shift, which is determined by interaction with F-actin.

In Vivo Calcium Imaging of Neuronal Ensembles in Networks of Primary Sensory Neurons in Intact Dorsal Root Ganglia

Ca2+ imaging can be used as a proxy for cellular activity, including action potentials and various signaling mechanisms involving Ca2+ entry into the cytoplasm or the release of intracellular Ca2+ stores. Pirt-GCaMP3-based Ca2+ imaging of primary sensory neurons of the dorsal root ganglion (DRG) in mice offers the advantage of simultaneous measurement of a large number of cells. Up to 1,800 neurons can be monitored, allowing neuronal networks and somatosensory processes to be studied as an ensemble in their normal physiological context at a populational level in vivo. The large number of neurons monitored allows the detection of activity patterns that would be challenging to detect using other methods. Stimuli can be applied to the mouse hindpaw, allowing the direct effects of stimuli on the DRG neuron ensemble to be studied. The number of neurons producing Ca2+ transients as well as the amplitude of Ca2+ transients indicates sensitivity to specific sensory modalities. The diameter of neurons provides evidence of activated fiber types (non-noxious mechano vs. noxious pain fibers, Aβ, Aδ, and C fibers). Neurons expressing specific receptors can be genetically labeled with td-Tomato and specific Cre recombinases together with Pirt-GCaMP. Therefore, Pirt-GCaMP3 Ca2+ imaging of DRG provides a powerful tool and model for the analysis of specific sensory modalities and neuron subtypes acting as an ensemble at the populational level to study pain, itch, touch, and other somatosensory signals.

Genetically encoded fluorescent sensors for imaging neuronal dynamics in vivo

The brain relies on many forms of dynamic activities in individual neurons, from synaptic transmission to electrical activity and intracellular signaling events. Monitoring these neuronal activities with high spatiotemporal resolution in the context of animal behavior is a necessary step to achieve a mechanistic understanding of brain function. With the rapid development and dissemination of highly optimized genetically encoded fluorescent sensors, a growing number of brain activities can now be visualized in vivo. To date, cellular calcium imaging, which has been largely used as a proxy for electrical activity, has become a mainstay in systems neuroscience. While challenges remain, voltage imaging of neural populations is now possible. In addition, it is becoming increasingly practical to image over half a dozen neurotransmitters, as well as certain intracellular signaling and metabolic activities. These new capabilities enable neuroscientists to test previously unattainable hypotheses and questions. This review summarizes recent progress in the development and delivery of genetically encoded fluorescent sensors, and highlights example applications in the context of in vivo imaging.

Serotonergic neurons control cortical neuronal intracellular energy dynamics by modulating astrocyte-neuron lactate shuttle

The central serotonergic system has multiple roles in animal physiology and behavior, including sleep-wake control. However, its function in controlling brain energy metabolism according to the state of animals remains undetermined. Through in vivo monitoring of energy metabolites and signaling, we demonstrated that optogenetic activation of raphe serotonergic neurons increased cortical neuronal intracellular concentration of ATP, an indispensable cellular energy molecule, which was suppressed by inhibiting neuronal uptake of lactate derived from astrocytes. Raphe serotonergic neuronal activation induced cortical astrocytic Ca²⁺ and cAMP surges and increased extracellular lactate concentrations, suggesting the facilitation of lactate release from astrocytes. Furthermore, chemogenetic inhibition of raphe serotonergic neurons partly attenuated the increase in cortical neuronal intracellular ATP levels as arousal increased in mice. Serotonergic neuronal activation promoted an increase in cortical neuronal intracellular ATP levels, partly mediated by the facilitation of the astrocyte-neuron lactate shuttle, contributing to state-dependent optimization of neuronal intracellular energy levels.

Essential role of the mitochondrial Na+/Ca2+ exchanger NCLX in mediating PDE2-dependent neuronal survival and learning

Impaired phosphodiesterase (PDE) function and mitochondrial Ca2+ (i.e., [Ca2+]m) lead to multiple health syndromes by an unknown pathway. Here, we fluorescently monitor robust [Ca2+]m efflux mediated by the mitochondrial Na+/Ca2+ exchanger NCLX in hippocampal neurons sequentially evoked by caffeine and depolarization. Surprisingly, neuronal depolarization-induced Ca2+ transients alone fail to evoke strong [Ca2+]m efflux in wild-type (WT) neurons. However, pre-treatment with the selective PDE2 inhibitor Bay 60-7550 effectively rescues [Ca2+]m efflux similarly to caffeine. Moreover, PDE2 acts by diminishing mitochondrial cAMP, thus promoting NCLX phosphorylation at its PKA site. We find that the protection of neurons against excitotoxic insults, conferred by PDE2 inhibition in WT neurons, is NCLX dependent. Finally, the administration of Bay 60-7550 enhances new object recognition in WT, but not in NCLX knockout (KO), mice. Our results identify a link between PDE and [Ca2+]m signaling that may provide effective therapy for cognitive and ischemic syndromes.

Differential Enantiomer-Specific Signaling of Cannabidiol at CB1 Receptors

The two main constituents of cannabis are Δ9-tetrahydrocannabinol (THC) and cannabidiol (CBD). While Δ9-THC pharmacology has been studied extensively, CBD-long considered inactive-is now the subject of vigorous research related to epilepsy, pain, and inflammation and is popularly embraced as a virtual cure-all. However, our understanding of CBD pharmacology remains limited, although CBD inhibits cannabinoid CB1 receptor signaling, likely as a negative allosteric modulator. Cannabis synthesizes (-)-CBD, but CBD can also exist as an enantiomer, (+)-CBD. We enantioselectively synthesized both CBD enantiomers using established conditions and describe here a new, practical, and reliable, NMR-based method for confirming the enantiomeric purity of two CBD enantiomers. We also investigated the pharmacology of (+)-CBD in autaptic hippocampal neurons, a well-characterized neuronal model of endogenous cannabinoid signaling, and in CHO-K1 cells. We report the inhibition constant for displacing CP55,940 at CB1 by (+)-CBD, is 5-fold lower than (-)-CBD. We find that (+)-CBD is ∼10 times more potent at inhibiting depolarization-induced suppression of excitation (DSE), a form of endogenous cannabinoid-mediated retrograde synaptic plasticity. (+)-CBD also inhibits CB1 suppression of cAMP accumulation but with less potency, indicating that the signaling profiles of the enantiomers differ in a pathway-specific manner. In addition, we report that (+)-CBD stereoselectively and potently activates the sphingosine-1 phosphate (S1P) receptors, S1P1 and S1P3 These results provide an attractive method for synthesizing and distinguishing enantiomers of CBD and related phytocannabinoids and provide further evidence that these enantiomers have their own unique and interesting signaling properties. SIGNIFICANCE STATEMENT: Cannabidiol (CBD) is the subject of considerable scientific and popular interest, but we know little of the enantiomers of CBD. We find that the enantiomer (+)-CBD is substantially more potent inhibitor of cannabinoid CB1 receptors and that it activates sphingosine-1-phosphate receptors in an enantiomer-specific manner; we have additionally developed an improved method for the synthesis of enantiomers of CBD and related compounds.

Sensitive genetically encoded sensors for population and subcellular imaging of cAMP in vivo

Cyclic adenosine monophosphate (cAMP) signaling integrates information from diverse G-protein-coupled receptors, such as neuromodulator receptors, to regulate pivotal biological processes in a cellular-specific and subcellular-specific manner. However, in vivo cellular-resolution imaging of cAMP dynamics remains challenging. Here, we screen existing genetically encoded cAMP sensors and further develop the best performer to derive three improved variants, called cAMPFIREs. Compared with their parental sensor, these sensors exhibit up to 10-fold increased sensitivity to cAMP and a cytosolic distribution. cAMPFIREs are compatible with both ratiometric and fluorescence lifetime imaging and can detect cAMP dynamics elicited by norepinephrine at physiologically relevant, nanomolar concentrations. Imaging of cAMPFIREs in awake mice reveals tonic levels of cAMP in cortical neurons that are associated with wakefulness, modulated by opioids, and differentially regulated across subcellular compartments. Furthermore, enforced locomotion elicits neuron-specific, bidirectional cAMP dynamics. cAMPFIREs also function in Drosophila. Overall, cAMPFIREs may have broad applicability for studying intracellular signaling in vivo.

A genetically encoded far-red fluorescent calcium ion biosensor derived from a biliverdin-binding protein

Far-red and near-infrared (NIR) genetically encoded calcium ion (Ca2+ ) indicators (GECIs) are powerful tools for in vivo and multiplexed imaging of neural activity and cell signaling. Inspired by a previous report to engineer a far-red fluorescent protein (FP) from a biliverdin (BV)-binding NIR FP, we have developed a far-red fluorescent GECI, designated iBB-GECO1, from a previously reported NIR GECI. iBB-GECO1 exhibits a relatively high molecular brightness, an inverse response to Ca2+ with ΔF/Fmin  = -13, and a near-optimal dissociation constant (Kd ) for Ca2+ of 105 nM. We demonstrate the utility of iBB-GECO1 for four-color multiplexed imaging in MIN6 cells and five-color imaging in HEK293T cells. Like other BV-binding GECIs, iBB-GECO1 did not give robust signals during in vivo imaging of neural activity in mice, but did provide promising results that will guide future engineering efforts. SIGNIFICANCE: Genetically encoded calcium ion (Ca2+ ) indicators (GECIs) compatible with common far-red laser lines (~630-640 nm) on commercial microscopes are of critical importance for their widespread application to deep-tissue multiplexed imaging of neural activity. In this study, we engineered a far-red excitable fluorescent GECI, designated iBB-GECO1, that exhibits a range of preferable specifications such as high brightness, large fluorescence response to Ca2+ , and compatibility with multiplexed imaging in mammalian cells.

Dopamine activates astrocytes in prefrontal cortex via α1-adrenergic receptors

The prefrontal cortex (PFC) is a hub for cognitive control, and dopamine profoundly influences its functions. In other brain regions, astrocytes sense diverse neurotransmitters and neuromodulators and, in turn, orchestrate regulation of neuroactive substances. However, basic physiology of PFC astrocytes, including which neuromodulatory signals they respond to and how they contribute to PFC function, is unclear. Here, we characterize divergent signaling signatures in mouse astrocytes of the PFC and primary sensory cortex, which show differential responsiveness to locomotion. We find that PFC astrocytes express receptors for dopamine but are unresponsive through the Gs/Gi-cAMP pathway. Instead, fast calcium signals in PFC astrocytes are time locked to dopamine release and are mediated by α1-adrenergic receptors both ex vivo and in vivo. Further, we describe dopamine-triggered regulation of extracellular ATP at PFC astrocyte territories. Thus, we identify astrocytes as active players in dopaminergic signaling in the PFC, contributing to PFC function though neuromodulator receptor crosstalk.

Pittolo et al. demonstrate that the neuromodulator dopamine targets astrocytes, a type of brain cell, via receptors specific to another neuromodulator—norepinephrine. This study provides groundwork on how dopamine affects non-neuronal brain cells and suggests that crosstalk between neuromodulatory pathways occurs in vivo, with possible clinical implications.

A high-performance genetically encoded fluorescent indicator for in vivo cAMP imaging

cAMP is a key second messenger that regulates diverse cellular functions including neural plasticity. However, the spatiotemporal dynamics of intracellular cAMP in intact organisms are largely unknown due to low sensitivity and/or brightness of current genetically encoded fluorescent cAMP indicators. Here, we report the development of the new circularly permuted GFP (cpGFP)-based cAMP indicator G-Flamp1, which exhibits a large fluorescence increase (a maximum ΔF/F₀ of 1100% in HEK293T cells), decent brightness, appropriate affinity (a K_d of 2.17 μM) and fast response kinetics (an association and dissociation half-time of 0.20 and 0.087 s, respectively). Furthermore, the crystal structure of the cAMP-bound G-Flamp1 reveals one linker connecting the cAMP-binding domain to cpGFP adopts a distorted β-strand conformation that may serve as a fluorescence modulation switch. We demonstrate that G-Flamp1 enables sensitive monitoring of endogenous cAMP signals in brain regions that are implicated in learning and motor control in living organisms such as fruit flies and mice.

Development of a red fluorescent protein-based cGMP indicator applicable for live-cell imaging

Cyclic guanosine 3', 5'-monophosphate (cGMP) is a second messenger that regulates a variety of physiological processes. Here, we develop a red fluorescent protein-based cGMP indicator, "Red cGull". The fluorescence intensity of Red cGull increase more than sixfold in response to cGMP. The features of this indicator include an EC₅₀ of 0.33 μM for cGMP, an excitation and emission peak at 567 nm and 591 nm, respectively. Live-cell imaging analysis reveal the utility of Red cGull for dual-colour imaging and its ability to be used in conjunction with optogenetics tools. Using enteroendocrine cell lines, Red cGull detects an increase in cGMP following the application of L-arginine. An increase in intracellular cGMP is found to be inhibited by Ca²⁺, and L-arginine-mediated hormone secretion is not potentiated. We propose that Red cGull will facilitate future research in cell signalling in relation to cGMP and its interplay with other signalling molecules.

Dopamine Negatively Regulates Insulin Secretion Through Activation of D1-D2 Receptor Heteromer

There is increasing evidence that dopamine (DA) functions as a negative regulator of glucose-stimulated insulin secretion; however, the underlying molecular mechanism remains unknown. Using total internal reflection fluorescence microscopy, we monitored insulin granule exocytosis in primary islet cells to dissect the effect of DA. We found that D1 receptor antagonists rescued the DA-mediated inhibition of glucose-stimulated calcium (Ca2+) flux, thereby suggesting a role of D1 in the DA-mediated inhibition of insulin secretion. Overexpression of D2, but not D1, alone exerted an inhibitory and toxic effect that abolished the glucose-stimulated Ca2+ influx and insulin secretion in β-cells. Proximity ligation and Western blot assays revealed that D1 and D2 form heteromers in β-cells. Treatment with a D1-D2 heteromer agonist, SKF83959, transiently inhibited glucose-induced Ca2+ influx and insulin granule exocytosis. Coexpression of D1 and D2 enabled β-cells to bypass the toxic effect of D2 overexpression. DA transiently inhibited glucose-stimulated Ca2+ flux and insulin exocytosis by activating the D1-D2 heteromer. We conclude that D1 protects β-cells from the harmful effects of DA by modulating D2 signaling. The finding will contribute to our understanding of the DA signaling in regulating insulin secretion and improve methods for preventing and treating diabetes.

A cAMP Sensor Based on Ligand-Dependent Protein Stabilization

cAMP is a ubiquitous second messenger with many functions in diverse organisms. Current cAMP sensors, including Föster resonance energy transfer (FRET)-based and single-wavelength-based sensors, allow for real time visualization of this small molecule in cultured cells and in some cases in vivo. Nonetheless the observation of cAMP in living animals is still difficult, typically requiring specialized microscopes and ex vivo tissue processing. Here we used ligand-dependent protein stabilization to create a new cAMP sensor. This sensor allows specific and sensitive detection of cAMP in living zebrafish embryos, which may enable new understanding of the functions of cAMP in living vertebrates.

A cAMP signalosome in primary cilia drives gene expression and kidney cyst formation

The primary cilium constitutes an organelle that orchestrates signal transduction independently from the cell body. Dysregulation of this intricate molecular architecture leads to severe human diseases, commonly referred to as ciliopathies. However, the molecular underpinnings how ciliary signaling orchestrates a specific cellular output remain elusive. By combining spatially resolved optogenetics with RNA sequencing and imaging, we reveal a novel cAMP signalosome that is functionally distinct from the cytoplasm. We identify the genes and pathways targeted by the ciliary cAMP signalosome and shed light on the underlying mechanisms and downstream signaling. We reveal that chronic stimulation of the ciliary cAMP signalosome transforms kidney epithelia from tubules into cysts. Counteracting this chronic cAMP elevation in the cilium by small molecules targeting activation of phosphodiesterase‐4 long isoforms inhibits cyst growth. Thereby, we identify a novel concept of how the primary cilium controls cellular functions and maintains tissue integrity in a specific and spatially distinct manner and reveal novel molecular components that might be involved in the development of one of the most common genetic diseases, polycystic kidney disease.

Green fluorescent cAMP indicator of high speed and specificity suitable for neuronal live-cell imaging

Cyclic adenosine monophosphate (cAMP) is a canonical intracellular messenger playing diverse roles in cell functions. In neurons, cAMP promotes axonal growth during early development, and mediates sensory transduction and synaptic plasticity after maturation. The molecular cascades of cAMP are well documented, but its spatiotemporal profiles associated with neuronal functions remain hidden. Hence, we developed a genetically encoded cAMP indicator based on a bacterial cAMP-binding protein. This indicator "gCarvi" monitors [cAMP]_i at 0.2 to 20 µM with a subsecond time resolution and a high specificity over cyclic guanosine monophosphate (cGMP). gCarvi can be converted to a ratiometric probe for [cAMP]_i quantification and its expression can be specifically targeted to various subcellular compartments. Monomeric gCarvi also enables simultaneous multisignal monitoring in combination with other indicators. As a proof of concept, simultaneous cAMP/Ca²⁺ imaging in hippocampal neurons revealed a tight linkage of cAMP to Ca²⁺ signals. In cerebellar presynaptic boutons, forskolin induced nonuniform cAMP elevations among boutons, which positively correlated with subsequent increases in the size of the recycling pool of synaptic vesicles assayed using FM dye. Thus, the cAMP domain in presynaptic boutons is an important determinant of the synaptic strength.

Advantages, Pitfalls, and Developments of All Optical Interrogation Strategies of Microcircuits in vivo

The holy grail for every neurophysiologist is to conclude a causal relationship between an elementary behaviour and the function of a specific brain area or circuit. Our effort to map elementary behaviours to specific brain loci and to further manipulate neural activity while observing the alterations in behaviour is in essence the goal for neuroscientists. Recent advancements in the area of experimental brain imaging in the form of longer wavelength near infrared (NIR) pulsed lasers with the development of highly efficient optogenetic actuators and reporters of neural activity, has endowed us with unprecedented resolution in spatiotemporal precision both in imaging neural activity as well as manipulating it with multiphoton microscopy. This readily available toolbox has introduced a so called all-optical physiology and interrogation of circuits and has opened new horizons when it comes to precisely, fast and non-invasively map and manipulate anatomically, molecularly or functionally identified mesoscopic brain circuits. The purpose of this review is to describe the advantages and possible pitfalls of all-optical approaches in system neuroscience, where by all-optical we mean use of multiphoton microscopy to image the functional response of neuron(s) in the network so to attain flexible choice of the cells to be also optogenetically photostimulated by holography, in absence of electrophysiology. Spatio-temporal constraints will be compared toward the classical reference of electrophysiology methods. When appropriate, in relation to current limitations of current optical approaches, we will make reference to latest works aimed to overcome these limitations, in order to highlight the most recent developments. We will also provide examples of types of experiments uniquely approachable all-optically. Finally, although mechanically non-invasive, all-optical electrophysiology exhibits potential off-target effects which can ambiguate and complicate the interpretation of the results. In summary, this review is an effort to exemplify how an all-optical experiment can be designed, conducted and interpreted from the point of view of the integrative neurophysiologist.

The N Terminus of Adhesion G Protein–Coupled Receptor GPR126/ADGRG6 as Allosteric Force Integrator

The adhesion G protein–coupled receptor (aGPCR) GPR126/ADGRG6 plays an important role in several physiological functions, such as myelination or peripheral nerve repair. This renders the receptor an attractive pharmacological target. GPR126 is a mechano-sensor that translates the binding of extracellular matrix (ECM) molecules to its N terminus into a metabotropic intracellular signal. To date, the structural requirements and the character of the forces needed for this ECM-mediated receptor activation are largely unknown. In this study, we provide this information by combining classic second-messenger detection with single-cell atomic force microscopy. We established a monoclonal antibody targeting the N terminus to stimulate GPR126 and compared it to the activation through its known ECM ligands, collagen IV and laminin 211. As each ligand uses a distinct mode of action, the N terminus can be regarded as an allosteric module that can fine-tune receptor activation in a context-specific manner.

An Improved Genetically Encoded Fluorescent cAMP Indicator for Sensitive cAMP Imaging and Fast Drug Screening

Cyclic adenosine 3′,5′-monophosphate (cAMP) is an important intracellular second messenger molecule downstream of many G protein-coupled receptors (GPCRs). Fluorescence imaging with bright and sensitive cAMP indicators allows not only dissecting the spatiotemporal dynamics of intracellular cAMP, but also high-content screening of compounds against GPCRs. We previously reported the high-performance circularly permuted GFP (cpGFP)-based cAMP indicator G-Flamp1. Here, we developed improved G-Flamp1 variants G-Flamp2 and G-Flamp2b. Compared to G-Flamp1, G-Flamp2 exhibited increased baseline fluorescence (1.6-fold) and larger fluorescence change (ΔF/F₀) (1,300% vs. 1,100%) in HEK293T cells, while G-Flamp2b showed increased baseline fluorescence (3.1-fold) and smaller ΔF/F₀ (400% vs. 1,100%). Furthermore, live cell imaging of mitochondrial matrix–targeted G-Flamp2 confirmed cytosolic cAMP was able to enter the mitochondrial matrix. G-Flamp2 imaging also showed that adipose tissue extract activated the Gi protein-coupled orphan GPCR GPR50 in HEK293T cells. Taken together, our results showed that the high-performance of G-Flamp2 would facilitate sensitive intracellular cAMP imaging and activity measurement of compounds targeting GPCR-cAMP signaling pathway during early drug development.

Taste Cells of the Type III Employ CASR to Maintain Steady Serotonin Exocytosis at Variable Ca2+ in the Extracellular Medium

Type III taste cells are the only taste bud cells which express voltage-gated (VG) Ca²⁺ channels and employ Ca²⁺-dependent exocytosis to release neurotransmitters, particularly serotonin. The taste bud is a tightly packed cell population, wherein extracellular Ca²⁺ is expected to fluctuate markedly due to the electrical activity of taste cells. It is currently unclear whether the Ca²⁺ entry-driven synapse in type III cells could be reliable enough at unsteady extracellular Ca². Here we assayed depolarization-induced Ca²⁺ signals and associated serotonin release in isolated type III cells at varied extracellular Ca²⁺. It turned out that the same depolarizing stimulus elicited invariant Ca²⁺ signals in type III cells irrespective of bath Ca²⁺ varied within 0.5–5 mM. The serotonin release from type III cells was assayed with the biosensor approach by using HEK-293 cells co-expressing the recombinant 5-HT4 receptor and genetically encoded cAMP sensor Pink Flamindo. Consistently with the weak Ca²⁺ dependence of intracellular Ca²⁺ transients produced by VG Ca²⁺ entry, depolarization-triggered serotonin secretion varied negligibly with bath Ca²⁺. The evidence implicated the extracellular Ca²⁺-sensing receptor in mediating the negative feedback mechanism that regulates VG Ca²⁺ entry and levels off serotonin release in type III cells at deviating Ca²⁺ in the extracellular medium.

cAMP-Dependent Synaptic Plasticity at the Hippocampal Mossy Fiber Terminal

Cyclic adenosine monophosphate (cAMP) is a crucial second messenger involved in both pre- and postsynaptic plasticity in many neuronal types across species. In the hippocampal mossy fiber (MF) synapse, cAMP mediates presynaptic long-term potentiation and depression. The main cAMP-dependent signaling pathway linked to MF synaptic plasticity acts via the activation of the protein kinase A (PKA) molecular cascade. Accordingly, various downstream putative synaptic PKA target proteins have been linked to cAMP-dependent MF synaptic plasticity, such as synapsin, rabphilin, synaptotagmin-12, RIM1a, tomosyn, and P/Q-type calcium channels. Regulating the expression of some of these proteins alters synaptic release probability and calcium channel clustering, resulting in short- and long-term changes to synaptic efficacy. However, despite decades of research, the exact molecular mechanisms by which cAMP and PKA exert their influences in MF terminals remain largely unknown. Here, we review current knowledge of different cAMP catalysts and potential downstream PKA-dependent molecular cascades, in addition to non-canonical cAMP-dependent but PKA-independent cascades, which might serve as alternative, compensatory or competing pathways to the canonical PKA cascade. Since several other central synapses share a similar form of presynaptic plasticity with the MF, a better description of the molecular mechanisms governing MF plasticity could be key to understanding the relationship between the transcriptional and computational levels across brain regions.

A perspective on astrocyte regulation of neural circuit function and animal behavior

Studies over the past two decades have demonstrated that astrocytes are tightly associated with neurons and play pivotal roles in neural circuit development, operation, and adaptation in health and disease. Nevertheless, precisely how astrocytes integrate diverse neuronal signals, modulate neural circuit structure and function at multiple temporal and spatial scales, and influence animal behavior or disease through aberrant excitation and molecular output remains unclear. This Perspective discusses how new and state-of-the-art approaches, including fluorescence indicators, opto- and chemogenetic actuators, genetic targeting tools, quantitative behavioral assays, and computational methods, might help resolve these longstanding questions. It also addresses complicating factors in interpreting astrocytes’ role in neural circuit regulation and animal behavior, such as their heterogeneity, metabolism, and inter-glial communication. Research on these questions should provide a deeper mechanistic understanding of astrocyte-neuron assemblies’ role in neural circuit function, complex behaviors, and disease.

Gαi2-induced conductin/axin2 condensates inhibit Wnt/β-catenin signaling and suppress cancer growth

Conductin/axin2 is a scaffold protein negatively regulating the pro-proliferative Wnt/β-catenin signaling pathway. Accumulation of scaffold proteins in condensates frequently increases their activity, but whether condensation contributes to Wnt pathway inhibition by conductin remains unclear. Here, we show that the Gαi2 subunit of trimeric G-proteins induces conductin condensation by targeting a polymerization-inhibiting aggregon in its RGS domain, thereby promoting conductin-mediated β-catenin degradation. Consistently, transient Gαi2 expression inhibited, whereas knockdown activated Wnt signaling via conductin. Colorectal cancers appear to evade Gαi2-induced Wnt pathway suppression by decreased Gαi2 expression and inactivating mutations, associated with shorter patient survival. Notably, the Gαi2-activating drug guanabenz inhibited Wnt signaling via conductin, consequently reducing colorectal cancer growth in vitro and in mouse models. In summary, we demonstrate Wnt pathway inhibition via Gαi2-triggered conductin condensation, suggesting a tumor suppressor function for Gαi2 in colorectal cancer, and pointing to the FDA-approved drug guanabenz for targeted cancer therapy.

Acute pyelonephritis: Increased plasma membrane targeting of renal aquaporin-2

AIM

Aquaporin-2 (AQP2) shuttling between intracellular vesicles and the apical plasma membrane is pivotal in arginine vasopressin-mediated urine concentration and is dysregulated in multiple diseases associated with water balance disorders. Children and adults with acute pyelonephritis have a urinary concentration defect and studies in children revealed increased AQP2 excretion in the urine. This study aimed to analyse AQP2 trafficking in response to acute pyelonephritis.

METHODS

Immunofluorescence analysis was used to evaluate subcellular localization of AQP2 and AQP2-S256A (mimicking non-phosphorylated AQP2 on serine 256) in cells stimulated with bacterial lysates and of AQP2 and pS256-AQP2 in a mouse model at day 5 of acute pyelonephritis. Western blotting was used to evaluate AQP2 levels and AQP2 phosphorylation on S256 upon incubation with bacterial lysates. Time-lapse imaging was used to measure intracellular cAMP levels in response to incubation with bacterial lysates.

RESULTS

In cell cultures, lysates from both uropathogenic and nonpathogenic bacteria-mediated AQP2 plasma membrane targeting and increased AQP2 phosphorylation on serine 256 (pS256) without increasing cAMP levels. Both bacterial lysates induced plasma membrane targeting of AQP2-S256A. Immunofluorescence analysis of renal sections from mice after 5 days of acute pyelonephritis revealed apical plasma membrane targeting of AQP2 and pS256-AQP2 in inner medullary collecting ducts.

CONCLUSION

Uropathogenic bacteria induce AQP2 plasma membrane targeting in vitro and in vivo. cAMP levels were not elevated by the bacterial lysates and AQP2 plasma membrane targeting could occur without S256 phosphorylation. This may explain increased AQP2 excretion in the urine during acute pyelonephritis.

Monitoring Cellular Responses to Infection with Fluorescent Biosensors

Fluorescent biosensors are chemically or genetically encoded reporters of cellular processes, signaling pathways, or biomolecule concentration, whose output is quantified using fluorescence microscopy or fluorescence spectrometry. These biosensors can detect the target activity or metabolites via mechanisms including conversion between nonfluorescent and fluorescent forms, changes in reporter intensity, changes in the intensity ratio across fluorescence channels, alterations to the subcellular localization of the bioreporter, and by fluorescence resonance energy transfer. Here, we describe the use of a chemical photoconverting biosensor, and genetically encoded localization and ratiometric biosensors, for monitoring the cellular and signaling processes involved in pathogen-induced apoptosis and the resulting destruction of the pathogen. While this study uses biosensors to monitor responses to infection, these approaches can be readily translated to other cellular systems and other fluorescent biosensors.

Glucosylsphingosine evokes pruritus via activation of 5-HT2A receptor and TRPV4 in sensory neurons

BACKGROUND AND PURPOSE

Glucosylsphingosine (GS), an endogenous sphingolipid, is highly accumulated in the epidermis of patients with atopic dermatitis (AD) due to abnormal ceramide metabolism. More importantly, GS can evoke scratching behaviors. However, the precise molecular mechanism by which GS induces pruritus has been elusive. Thus, the present study aimed to elucidate the molecular signaling pathway of GS, especially at the peripheral sensory neuronal levels.

EXPERIMENTAL APPROACH

Calcium imaging was used to investigate the responses of HEK293T cells or mouse dorsal root ganglion (DRG) neurons to application of GS. Scratching behavior tests were also performed with wild-type and Trpv4 knockout mice.

KEY RESULTS

GS activated DRG neurons in a manner involving both the 5-HT_2A receptor and TRPV4. Furthermore, GS-induced responses were significantly suppressed by various inhibitors, including ketanserin (5-HT_2A receptor antagonist), YM254890 (Gα_q/11 inhibitor), gallein (Gβγ complex inhibitor), U73122 (phospholipase C inhibitor), bisindolylmaleimide I (PKC inhibitor), and HC067047 (TRPV4 antagonist). Moreover, DRG neurons from Trpv4 knockout mice exhibited significantly reduced responses to GS. Additionally, GS-evoked scratching behaviors were greatly decreased by pretreatment with inhibitors of either 5-HT_2A receptor or TRPV4. As expected, GS-evoked scratching behavior was also significantly decreased in Trpv4 knockout mice.

CONCLUSION AND IMPLICATIONS

Overall, the present study provides evidence for a novel molecular signaling pathway for GS-evoked pruritus, which utilizes both 5-HT_2A receptor and TRPV4 in mouse sensory neurons. Considering the high accumulation of GS in the epidermis of patients with AD, GS could be another pruritogen in patients with AD.

Genetically encoded sensors towards imaging cAMP and PKA activity in vivo

Cyclic adenosine monophosphate (cAMP) is a universal second messenger that plays a crucial role in diverse biological functions, ranging from transcription to neuronal plasticity, and from development to learning and memory. In the nervous system, cAMP integrates inputs from many neuromodulators across a wide range of timescales - from seconds to hours - to modulate neuronal excitability and plasticity in brain circuits during different animal behavioral states. cAMP signaling events are both cell-specific and subcellularly compartmentalized. The same stimulus may result in different, sometimes opposite, cAMP dynamics in different cells or subcellular compartments. Additionally, the activity of protein kinase A (PKA), a major cAMP effector, is also spatiotemporally regulated. For these reasons, many laboratories have made great strides toward visualizing the intracellular dynamics of cAMP and PKA. To date, more than 80 genetically encoded sensors, including original and improved variants, have been published. It is starting to become possible to visualize cAMP and PKA signaling events in vivo, which is required to study behaviorally relevant cAMP/PKA signaling mechanisms. Despite significant progress, further developments are needed to enhance the signal-to-noise ratio and practical utility of these sensors. This review summarizes the recent advances and challenges in genetically encoded cAMP and PKA sensors with an emphasis on in vivo imaging in the brain during behavior.

An Evaluation of Understudied Phytocannabinoids and Their Effects in Two Neuronal Models

Cannabis contains more than 100 phytocannabinoids. Most of these remain poorly characterized, particularly in neurons. We tested a panel of five phytocannabinoids-cannabichromene (CBC), cannabidiolic acid (CBDA), cannabidivarin (CBDV), cannabidivarinic acid (CBDVA), and Δ9-tetrahydrocannabivarin (THCV) in two neuronal models, autaptic hippocampal neurons and dorsal root ganglion (DRG) neurons. Autaptic neurons expressed a form of CB1-dependent retrograde plasticity while DRGs expressed a variety of transient receptor potential (TRP) channels. CBC, CBDA, and CBDVA had little or no effect on neuronal cannabinoid signaling. CBDV and THCV differentially inhibited cannabinoid signaling. THCV inhibited CB1 receptors presynaptically while CBDV acted post-synaptically, perhaps by inhibiting 2-AG production. None of the compounds elicited a consistent DRG response. In summary, we find that two of five 'minor' phytocannabinoids tested antagonized CB1-based signaling in a neuronal model, but with very different mechanisms. Our findings highlight the diversity of potential actions of phytocannabinoids and the importance of fully evaluating these compounds in neuronal models.

Versatile Surface Electrodes for Combined Electrophysiology and Two-Photon Imaging of the Mouse Central Nervous System

Understanding and modulating CNS function in physiological as well as pathophysiological contexts remains a significant ambition in research and clinical applications. The investigation of the multifaceted CNS cell types including their interactions and contributions to neural function requires a combination of the state-of-the-art in vivo electrophysiology and imaging techniques. We developed a novel type of liquid crystal polymer (LCP) surface micro-electrode manufactured in three customized designs with up to 16 channels for recording and stimulation of brain activity. All designs include spare central spaces for simultaneous 2P-imaging. Nanoporous platinum-plated contact sites ensure a low impedance and high current transfer. The epidural implantation of the LCP micro-electrodes could be combined with standard cranial window surgery. The epidurally positioned electrodes did not only display long-term biocompatibility, but we also observed an additional stabilization of the underlying CNS tissue. We demonstrate the electrode’s versatility in combination with in vivo 2P-imaging by monitoring anesthesia-awake cycles of transgenic mice with GCaMP3 expression in neurons or astrocytes. Cortical stimulation and simultaneous 2P Ca²⁺ imaging in neurons or astrocytes highlighted the astrocytes’ integrative character in neuronal activity processing. Furthermore, we confirmed that spontaneous astroglial Ca²⁺ signals are dampened under anesthesia, while evoked signals in neurons and astrocytes showed stronger dependency on stimulation intensity rather than on various levels of anesthesia. Finally, we show that the electrodes provide recordings of the electrocorticogram (ECoG) with a high signal-to noise ratio and spatial signal differences which help to decipher brain activity states during experimental procedures. Summarizing, the novel LCP surface micro-electrode is a versatile, convenient, and reliable tool to investigate brain function in vivo.

The Cutting Edge of Disease Modeling: Synergy of Induced Pluripotent Stem Cell Technology and Genetically Encoded Biosensors

The development of cell models of human diseases based on induced pluripotent stem cells (iPSCs) and a cell therapy approach based on differentiated iPSC derivatives has provided a powerful stimulus in modern biomedical research development. Moreover, it led to the creation of personalized regenerative medicine. Due to this, in the last decade, the pathological mechanisms of many monogenic diseases at the cell level have been revealed, and clinical trials of various cell products derived from iPSCs have begun. However, it is necessary to reach a qualitatively new level of research with cell models of diseases based on iPSCs for more efficient searching and testing of drugs. Biosensor technology has a great application prospect together with iPSCs. Biosensors enable researchers to monitor ions, molecules, enzyme activities, and channel conformation in live cells and use them in live imaging and drug screening. These probes facilitate the measurement of steady-state concentrations or activity levels and the observation and quantification of in vivo flux and kinetics. Real-time monitoring of drug action in a specific cellular compartment, organ, or tissue type; the ability to screen at the single-cell resolution; and the elimination of the false-positive results caused by low drug bioavailability that is not detected by in vitro testing methods are a few of the benefits of using biosensors in drug screening. Here, we discuss the possibilities of using biosensor technology in combination with cell models based on human iPSCs and gene editing systems. Furthermore, we focus on the current achievements and problems of using these methods.

Reverse optogenetics of G protein signaling by zebrafish non-visual opsin Opn7b for synchronization of neuronal networks

Opn7b is a non-visual G protein-coupled receptor expressed in zebrafish. Here we find that Opn7b expressed in HEK cells constitutively activates the Gi/o pathway and illumination with blue/green light inactivates G protein-coupled inwardly rectifying potassium channels. This suggests that light acts as an inverse agonist for Opn7b and can be used as an optogenetic tool to inhibit neuronal networks in the dark and interrupt constitutive inhibition in the light. Consistent with this prediction, illumination of recombinant expressed Opn7b in cortical pyramidal cells results in increased neuronal activity. In awake mice, light stimulation of Opn7b expressed in pyramidal cells of somatosensory cortex reliably induces generalized epileptiform activity within a short (<10 s) delay after onset of stimulation. Our study demonstrates a reversed mechanism for G protein-coupled receptor control and Opn7b as a tool for controlling neural circuit properties.

Development of red genetically encoded biosensor for visualization of intracellular glucose dynamics

Glucose is the main source of energy for organisms, and it is important to understand the spatiotemporal dynamics of intracellular glucose. Single fluorescent protein-based glucose indicators, named "Red Glifons" have been developed that apply to live-cell and dual-color imaging. These indicators exhibited more than 3-fold increase in fluorescence intensity in the presence of 10 mM glucose. The two Red Glifons developed have different half-maximal effective concentration (EC₅₀) values for glucose (300 μM and 3,000 μM) and are able to monitor a wide range of glucose dynamics. Red Glifon combined with green indicators allowing visualization of the interplay between glucose and ATP, lactate, or pyruvate. Glucose influx in the pharyngeal muscle of Caenorhabditis elegans, enteroendocrine cells, and human iPS cell-derived cardiac myocytes was observed using the Red Glifons. Thus these red glucose indicators serve as a multi-color imaging toolkit for investigating complex interactions in energy metabolism.