PACmn for improved optogenetic control of intracellular cAMP

An Integrated Optogenetic and Bioelectronic Platform for Regulating Cardiomyocyte Function

Bioelectronic medicine is emerging as a powerful approach for restoring lost endogenous functions and addressing life-altering maladies such as cardiac disorders. Systems that incorporate both modulation of cellular function and recording capabilities can enhance the utility of these approaches and their customization to the needs of each patient. Here we report an integrated optogenetic and bioelectronic platform for stable and long-term stimulation and monitoring of cardiomyocyte function in vitro. Optical inputs are achieved through the expression of a photoactivatable adenylyl cyclase, that when irradiated with blue light causes a dose-dependent and time-limited increase in the secondary messenger cyclic adenosine monophosphate with subsequent rise in autonomous cardiomyocyte beating rate. Bioelectronic readouts are obtained through a multi-electrode array that measures real-time electrophysiological responses at 32 spatially-distinct locations. Irradiation at 27 µW mm-2 results in a 14% elevation of the beating rate within 20-25 min, which remains stable for at least 2 h. The beating rate can be cycled through "on" and "off" light states, and its magnitude is a monotonic function of irradiation intensity. The integrated platform can be extended to stretchable and flexible substrates, and can open new avenues in bioelectronic medicine, including closed-loop systems for cardiac regulation and intervention, for example, in the context of arrythmias.

All-optical presynaptic plasticity induction by photoactivated adenylyl cyclase targeted to axon terminals

Intracellular signaling plays essential roles in various cell types. In the central nervous system, signaling cascades are strictly regulated in a spatiotemporally specific manner to govern brain function; for example, presynaptic cyclic adenosine monophosphate (cAMP) can enhance the probability of neurotransmitter release. In the last decade, channelrhodopsin-2 has been engineered for subcellular targeting using localization tags, but optogenetic tools for intracellular signaling are not well developed. Therefore, we engineered a selective presynaptic fusion tag for photoactivated adenylyl cyclase (bPAC-Syn1a) and found its high localization at presynaptic terminals. Furthermore, an all-optical electrophysiological method revealed rapid and robust short-term potentiation by bPAC-Syn1a at brain stem-amygdala synapses in acute brain slices. Additionally, bPAC-Syn1a modulated mouse immobility behavior. These results indicate that bPAC-Syn1a can manipulate presynaptic cAMP signaling in vitro and in vivo. The all-optical manipulation technique developed in this study can help further elucidate the dynamic regulation of various cellular functions.

Engineering Bacteriophytochrome-coupled Photoactivated Adenylyl Cyclases for Enhanced Optogenetic cAMP Modulation

Sensory photoreceptors abound in nature and enable organisms to adapt behavior, development, and physiology to environmental light. In optogenetics, photoreceptors allow spatiotemporally precise, reversible, and non-invasive control by light of cellular processes. Notwithstanding the development of numerous optogenetic circuits, an unmet demand exists for efficient systems sensitive to red light, given its superior penetration of biological tissue. Bacteriophytochrome photoreceptors sense the ratio of red and far-red light to regulate the activity of enzymatic effector modules. The recombination of bacteriophytochrome photosensor modules with cyclase effectors underlies photoactivated adenylyl cyclases (PAC) that catalyze the synthesis of the ubiquitous second messenger 3', 5'-cyclic adenosine monophosphate (cAMP). Via homologous exchanges of the photosensor unit, we devised novel PACs, with the variant DmPAC exhibiting 40-fold activation of cyclase activity under red light, thus surpassing previous red-light-responsive PACs. Modifications of the PHY tongue modulated the responses to red and far-red light. Exchanges of the cyclase effector offer an avenue to further enhancing PACs but require optimization of the linker to the photosensor. DmPAC and a derivative for 3', 5'-cyclic guanosine monophosphate allow the manipulation of cyclic-nucleotide-dependent processes in mammalian cells by red light. Taken together, we advance the optogenetic control of second-messenger signaling and provide insight into the signaling and design of bacteriophytochrome receptors.

Advancements in the use of xenopus oocytes for modelling neurological disease for novel drug discovery

INTRODUCTION

Introduced about 50 years ago, the model of Xenopus oocytes for the expression of recombinant proteins has gained a broad spectrum of applications. The authors herein review the benefits brought from using this model system, with a focus on modeling neurological disease mechanisms and application to drug discovery.

AREAS COVERED

Using multiple examples spanning from ligand gated ion channels to transporters, this review presents, in the light of the latest publications, the benefits offered from using Xenopus oocytes. Studies range from the characterization of gene mutations to the discovery of novel treatments for disorders of the central nervous system (CNS).

EXPERT OPINION

Development of new drugs targeting CNS disorders has been marked by failures in the translation from preclinical to clinical studies. As progress in genetics and molecular biology highlights large functional differences arising from a single to a few amino acid exchanges, the need for drug screening and functional testing against human proteins is increasing. The use of Xenopus oocytes to enable precise modeling and characterization of clinically relevant genetic variants constitutes a powerful model system that can be used to inform various aspects of CNS drug discovery and development.

Optimized design and in vivo application of optogenetically functionalized Drosophila dopamine receptors

Neuromodulatory signaling via G protein-coupled receptors (GPCRs) plays a pivotal role in regulating neural network function and animal behavior. The recent development of optogenetic tools to induce G protein-mediated signaling provides the promise of acute and cell type-specific manipulation of neuromodulatory signals. However, designing and deploying optogenetically functionalized GPCRs (optoXRs) with accurate specificity and activity to mimic endogenous signaling in vivo remains challenging. Here we optimize the design of optoXRs by considering evolutionary conserved GPCR-G protein interactions and demonstrate the feasibility of this approach using two Drosophila Dopamine receptors (optoDopRs). These optoDopRs exhibit high signaling specificity and light sensitivity in vitro. In vivo, we show receptor and cell type-specific effects of dopaminergic signaling in various behaviors, including the ability of optoDopRs to rescue the loss of the endogenous receptors. This work demonstrates that optoXRs can enable optical control of neuromodulatory receptor-specific signaling in functional and behavioral studies.

Exaptation and Evolutionary Adaptation in Nociceptor Mechanisms Driving Persistent Pain

BACKGROUND

Several evolutionary explanations have been proposed for why chronic pain is a major clinical problem. One is that some mechanisms important for driving chronic pain, while maladaptive for modern humans, were adaptive because they enhanced survival. Evidence is reviewed for persistent nociceptor hyperactivity (PNH), known to promote chronic pain in rodents and humans, being an evolutionarily adaptive response to significant bodily injury, and primitive molecular mechanisms related to cellular injury and stress being exapted (co-opted or repurposed) to drive PNH and consequent pain.

SUMMARY

PNH in a snail (Aplysia californica), squid (Doryteuthis pealeii), fruit fly (Drosophila melanogaster), mice, rats, and humans has been documented as long-lasting enhancement of action potential discharge evoked by peripheral stimuli, and in some of these species as persistent extrinsically driven ongoing activity and/or intrinsic spontaneous activity (OA and SA, respectively). In mammals, OA and SA are often initiated within the protected nociceptor soma long after an inducing injury. Generation of OA or SA in nociceptor somata may be very rare in invertebrates, but prolonged afterdischarge in nociceptor somata readily occurs in sensitized Aplysia. Evidence for the adaptiveness of injury-induced PNH has come from observations of decreased survival of injured squid exposed to predators when PNH is blocked, from plausible survival benefits of chronic sensitization after severe injuries such as amputation, and from the functional coherence and intricacy of mammalian PNH mechanisms. Major contributions of cAMP-PKA signaling (with associated calcium signaling) to the maintenance of PNH both in mammals and molluscs suggest that this ancient stress signaling system was exapted early during the evolution of nociceptors to drive hyperactivity following bodily injury. Vertebrates have retained core cAMP-PKA signaling modules for PNH while adding new extracellular modulators (e.g., opioids) and cAMP-regulated ion channels (e.g., TRPV1 and Nav1.8 channels).

KEY MESSAGES

Evidence from multiple phyla indicates that PNH is a physiological adaptation that decreases the risk of attacks on injured animals. Core cAMP-PKA signaling modules make major contributions to the maintenance of PNH in molluscs and mammals. This conserved signaling has been linked to ancient cellular responses to stress, which may have been exapted in early nociceptors to drive protective hyperactivity that can persist while bodily functions recover after significant injury.

Optogenetic Methods in Plant Biology

Optogenetics is a technique employing natural or genetically engineered photoreceptors in transgene organisms to manipulate biological activities with light. Light can be turned on or off, and adjusting its intensity and duration allows optogenetic fine-tuning of cellular processes in a noninvasive and spatiotemporally resolved manner. Since the introduction of Channelrhodopsin-2 and phytochrome-based switches nearly 20 years ago, optogenetic tools have been applied in a variety of model organisms with enormous success, but rarely in plants. For a long time, the dependence of plant growth on light and the absence of retinal, the rhodopsin chromophore, prevented the establishment of plant optogenetics until recent progress overcame these difficulties. We summarize the recent results of work in the field to control plant growth and cellular motion via green light-gated ion channels and present successful applications to light-control gene expression with single or combined photoswitches in plants. Furthermore, we highlight the technical requirements and options for future plant optogenetic research.

Optogenetic activation of UCP1-dependent thermogenesis in brown adipocytes

Brown adipocytes are unique in that they expend energy and produce heat to maintain euthermia through expression of uncoupling protein-1 (UCP1). Given their propensity to stimulate weight loss and promote resistance to obesity, they are a compelling interventional target for obesity-related disorders. Here, we tested whether an optogenetic approach could be used to activate UCP1-dependent thermogenesis in brown adipocytes. We generated brown adipocytes expressing a bacterial-derived photoactivatable adenylyl cyclase (bPAC) that, upon blue light stimulation, increases UCP1 expression, fuel uptake and thermogenesis. This unique system allows for precise, chemical free, temporal control of UCP1-dependent thermogenesis, which can aid in our understanding of brown adipocyte biology and development of therapies that target obesity-related disorders.

Photoactivated adenylyl cyclases attenuate sepsis-induced cardiomyopathy by suppressing macrophage-mediated inflammation

Sepsis-induced myocardiopathy, characterized by innate immune cells infiltration and proinflammatory cytokines release, may lead to perfusion failure or even life-threatening cardiogenic shock. Macrophages-mediated inflammation has been shown to contribute to sepsis-induced myocardiopathy. In the current study, we introduced two photoactivated adenylyl cyclases (PACs), Beggiatoa sp. PAC (bPAC) and Beggiatoa sp. IS2 PAC (biPAC) into macrophages by transfection to detect the effects of light-induced regulation of macrophage pro-inflammatory response and LPS-induced sepsis-induced myocardiopathy. By this method, we uncovered that blue light-induced bPAC or biPAC activation considerably inhibited the production of pro-inflammatory cytokines IL-1 and TNF-α, both at mRNA and protein levels. Further, we assembled a GelMA-Macrophages-LED system, which consists of GelMA—a type of light crosslink hydrogel, gene modulated macrophages and wireless LED device, to allow light to regulate cardiac inflammation in situ with murine models of LPS-induced sepsis. Our results showed significant inhibition of leukocytes infiltration, especially macrophages and neutrophils, suppression of pro-inflammatory cytokines release, and alleviation of sepsis-induced cardiac dysfunction. Thus, our study may represent an emerging means to treat sepsis-induced myocardiopathy and other cardiovascular diseases by photo-activated regulating macrophage function.

Emerging molecular technologies for light-mediated modulation of pancreatic beta-cell function

Background

Optogenetic modalities as well as optochemical and photopharmacological strategies, collectively termed optical methods, have revolutionized the control of cellular functions via light with great spatiotemporal precision. In comparison to the major advances in the photomodulation of signaling activities noted in neuroscience, similar applications to endocrine cells of the pancreas, particularly insulin-producing β-cells, have been limited. The availability of tools allowing light-mediated changes in the trafficking of ions such as K⁺ and Ca²⁺ and signaling intermediates such as cyclic adenosine monophosphate (cAMP), renders β-cells and their glucose-stimulated insulin secretion (GSIS) amenable to optoengineering for drug-free control of blood sugar.

Scope of review

The molecular circuit of the GSIS in β-cells is described with emphasis on intermediates which are targetable for optical intervention. Various pharmacological agents modifying the release of insulin are reviewed along with their documented side effects. These are contrasted with optical approaches, which have already been employed for engineering β-cell function or are considered for future such applications. Principal obstacles are also discussed as the implementation of optogenetics is pondered for tissue engineering and biology applications of the pancreas.

Major Conclusions

Notable advances in optogenetic, optochemical and photopharmacological tools are rendering feasible the smart engineering of pancreatic cells and tissues with light-regulated function paving the way for novel solutions for addressing pancreatic pathologies including diabetes.

Optogenetics at the presynapse

Optogenetic actuators enable highly precise spatiotemporal interrogation of biological processes at levels ranging from the subcellular to cells, circuits and behaving organisms. Although their application in neuroscience has traditionally focused on the control of spiking activity at the somatodendritic level, the scope of optogenetic modulators for direct manipulation of presynaptic functions is growing. Presynaptically localized opsins combined with light stimulation at the terminals allow light-mediated neurotransmitter release, presynaptic inhibition, induction of synaptic plasticity and specific manipulation of individual components of the presynaptic machinery. Here, we describe presynaptic applications of optogenetic tools in the context of the unique cell biology of axonal terminals, discuss their potential shortcomings and outline future directions for this rapidly developing research area.

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.

Long-term in vivo application of a potassium channel-based optogenetic silencer in the healthy and epileptic mouse hippocampus

BACKGROUND

Optogenetic tools allow precise manipulation of neuronal activity via genetically encoded light-sensitive proteins. Currently available optogenetic inhibitors are not suitable for prolonged use due to short-lasting photocurrents, tissue heating, and unintended changes in ion distributions, which may interfere with normal neuron physiology. To overcome these limitations, a novel potassium channel-based optogenetic silencer, named PACK, was recently developed. The PACK tool has two components: a photoactivated adenylyl cyclase from Beggiatoa (bPAC) and a cAMP-dependent potassium channel, SthK, which carries a large, long-lasting potassium current in mammalian cells. Previously, it has been shown that activating the PACK silencer with short light pulses led to a significant reduction of neuronal firing in various in vitro and acute in vivo settings. Here, we examined the viability of performing long-term studies in vivo by looking at the inhibitory action and side effects of PACK and its components in healthy and epileptic adult male mice.

RESULTS

We targeted hippocampal cornu ammonis (CA1) pyramidal cells using a viral vector and enabled illumination of these neurons via an implanted optic fiber. Local field potential (LFP) recordings from CA1 of freely moving mice revealed significantly reduced neuronal activity during 50-min intermittent (0.1 Hz) illumination, especially in the gamma frequency range. Adversely, PACK expression in healthy mice induced chronic astrogliosis, dispersion of pyramidal cells, and generalized seizures. These side effects were independent of the light application and were also present in mice expressing bPAC without the potassium channel. Light activation of bPAC alone increased neuronal activity, presumably via enhanced cAMP signaling. Furthermore, we applied bPAC and PACK in the contralateral hippocampus of chronically epileptic mice following a unilateral injection of intrahippocampal kainate. Unexpectedly, the expression of bPAC in the contralateral CA1 area was sufficient to prevent the spread of spontaneous epileptiform activity from the seizure focus to the contralateral hippocampus.

CONCLUSION

Our study highlights the PACK tool as a potent optogenetic inhibitor in vivo. However, further refinement of its light-sensitive domain is required to avoid unexpected physiological changes.

Characterization and Modification of Light-Sensitive Phosphodiesterases from Choanoflagellates

Enzyme rhodopsins, including cyclase opsins (Cyclops) and rhodopsin phosphodiesterases (RhoPDEs), were recently discovered in fungi, algae and protists. In contrast to the well-developed light-gated guanylyl/adenylyl cyclases as optogenetic tools, ideal light-regulated phosphodiesterases are still in demand. Here, we investigated and engineered the RhoPDEs from Salpingoeca rosetta, Choanoeca flexa and three other protists. All the RhoPDEs (fused with a cytosolic N-terminal YFP tag) can be expressed in Xenopus oocytes, except the AsRhoPDE that lacks the retinal-binding lysine residue in the last (8th) transmembrane helix. An N296K mutation of YFP::AsRhoPDE enabled its expression in oocytes, but this mutant still has no cGMP hydrolysis activity. Among the RhoPDEs tested, SrRhoPDE, CfRhoPDE1, 4 and MrRhoPDE exhibited light-enhanced cGMP hydrolysis activity. Engineering SrRhoPDE, we obtained two single point mutants, L623F and E657Q, in the C-terminal catalytic domain, which showed ~40 times decreased cGMP hydrolysis activity without affecting the light activation ratio. The molecular characterization and modification will aid in developing ideal light-regulated phosphodiesterase tools in the future.