Using optogenetics to assess neuroendocrine modulation of heart rate in Drosophila melanogaster larvae

The Effect of Magnesium Concentration on Myogenic Cardiac Function: Larval Drosophila

The heart of larval Drosophila serves as a model preparation in addressing cardiac function, as known genetic mutations can be mimicked to examine therapies. Pharmacological agents and function of proteins, like TRPA1, which affect ionic transport and ion concentrations can be investigated for their action on cardiac function in this model. To maintain in-situ function, the larval heart tube needs to remain viable; thus, a physiological saline is required. It was found that a reduced Mg 2+ level from the standard saline provides a more stable heartbeat, even in stressful conditions such as heat and increased expression levels of TRPA1 proteins.

The effects of Gram-positive and Gram-negative bacterial toxins (LTA & LPS) on cardiac function in Drosophila melanogaster larvae

The effects of Gram negative and positive bacterial sepsis depend on the type of toxins released, such as lipopolysaccharides (LPS) or lipoteichoic acid (LTA). Previous studies show LPS to rapidly hyperpolarize larval Drosophila skeletal muscle, followed by desensitization and return to baseline. In larvae, heart rate increased then decreased with exposure to LPS. However, responses to LTA, as well as the combination of LTA and LPS, on the larval Drosophila heart have not been previously examined. This study examined the effects of LTA and a cocktail of LTA and LPS on heart rate. The combined effects were examined by first treating with either LTA or LPS only, and then with the cocktail. The results showed a rapid increase in heart rate upon LTA application, followed by a gradual decline over time. When applying LTA followed by the cocktail, an increase in the rate occurred. However, if LPS was applied before the cocktail, the rate continued declining. These responses indicate the receptors or cellular cascades responsible for controlling heart rate within seconds and the rapid desensitization are affected by LTA or LPS and a combination of the two. The mechanisms for rapid changes which are not regulated by gene expression by exposure to LTA or LPS or associated bacterial peptidoglycans have yet to be identified in cardiac tissues of any organism.

An in-vivo microfluidic assay reveals cardiac toxicity of heavy metals and the protective effect of metal responsive transcription factor (MTF-1) in Drosophila model

Previous toxicity assessments of heavy metals on Drosophila are limited to investigating the survival, development rate, and climbing behaviour by oral administration while cardiac toxicity of these elements have not been investigated. We utilized a microfluidic device to inject known dosages of zinc (Zn) or cadmium (Cd) into the larvae's hemolymph to expose their heart directly and study their heart rate and arrhythmicity. The effect of heart-specific overexpression of metal responsive transcription factor (MTF-1) on different heartbeat parameters and survival of Drosophila larvae was investigated. The heart rate of wild-type larvae decreased by 24.8% or increased by 11.9%, 15 min after injection of 40 nL of 100 mM Zn or 10 mM Cd solution, respectively. The arrhythmicity index of wild-type larvae increased by 58.2% or 76.8%, after injection of Zn or Cd, respectively. MTF-1 heart overexpression ameliorated these effects completely. Moreover, it increased larvae's survival to pupal and adulthood stages and prolonged the longevity of flies injected with Zn and Cd. Our microfluidic-based cardiac toxicity assay illustrated that heart is an acute target of heavy metals toxicity, and MTF-1 overexpression in this tissue can ameliorate cardiac toxicity of Zn and Cd. The method can be used for cardiotoxicity assays with other pollutants in the future.

Supplementary Information

The online version contains supplementary material available at 10.1007/s13205-022-03336-7.

Examining the effect of manganese on physiological processes: Invertebrate models

Manganese (Mn²⁺ as MnSO₄ &/or MnCl₂) is a common and essential element for maintaining life in plants and animals and is found in soil, fresh waters and marine waters; however, over exposure is toxic to organisms. MnSO₄ is added to soil for agricultural purposes and people are exposed to Mn²⁺ in the mining industry. Hypermanganesemia in mammals is associated with neurological issues mimicking Parkinson's disease (PD) and appears to target dopaminergic neural circuits. However, it also seems that hypermanganesemia can affect many aspects of health besides dopaminergic synapses. We examined the effect on development, behavior, survival, cardiac function, and glutamatergic synaptic transmission in the Drosophila melanogaster. In addition, we examined the effect of Mn²⁺ on a sensory proprioceptive organ and nerve conduction in a marine crustacean and synaptic transmission at glutamatergic neuromuscular junctions of freshwater crayfish. A dose-response effect of higher Mn²⁺ retards development, survival and cardiac function in larval Drosophila and survival in larvae and adults. MnSO₄ as well as MnCl₂ blocks stretch activated responses in primary proprioceptive neurons in a dose-response manner. Mn²⁺ blocks glutamatergic synaptic transmission in Drosophila as well as crayfish via presynaptic action. This study is relevant in demonstrating the effects of Mn²⁺ on various physiological functions in order to learn more about acute and long-term consequences Mn²⁺ exposure.

Neurohumoral Cardiac Regulation: Optogenetics Gets Into the Groove

The cardiac autonomic nervous system (ANS) is the main modulator of heart function, adapting contraction force, and rate to the continuous variations of intrinsic and extrinsic environmental conditions. While the parasympathetic branch dominates during rest-and-digest sympathetic neuron (SN) activation ensures the rapid, efficient, and repeatable increase of heart performance, e.g., during the "fight-or-flight response." Although the key role of the nervous system in cardiac homeostasis was evident to the eyes of physiologists and cardiologists, the degree of cardiac innervation, and the complexity of its circuits has remained underestimated for too long. In addition, the mechanisms allowing elevated efficiency and precision of neurogenic control of heart function have somehow lingered in the dark. This can be ascribed to the absence of methods adequate to study complex cardiac electric circuits in the unceasingly moving heart. An increasing number of studies adds to the scenario the evidence of an intracardiac neuron system, which, together with the autonomic components, define a little brain inside the heart, in fervent dialogue with the central nervous system (CNS). The advent of optogenetics, allowing control the activity of excitable cells with cell specificity, spatial selectivity, and temporal resolution, has allowed to shed light on basic neuro-cardiology. This review describes how optogenetics, which has extensively been used to interrogate the circuits of the CNS, has been applied to untangle the knots of heart innervation, unveiling the cellular mechanisms of neurogenic control of heart function, in physiology and pathology, as well as those participating to brain-heart communication, back and forth. We discuss existing literature, providing a comprehensive view of the advancement in the understanding of the mechanisms of neurogenic heart control. In addition, we weigh the limits and potential of optogenetics in basic and applied research in neuro-cardiology.

Optogenetic approaches for understanding homeostatic and degenerative processes in Drosophila

Many organs and tissues have an intrinsic ability to regenerate from a dedicated, tissue-specific stem cell pool. As organisms age, the process of self-regulation or homeostasis begins to slow down with fewer stem cells available for tissue repair. Tissues become more fragile and organs less efficient. This slowdown of homeostatic processes leads to the development of cellular and neurodegenerative diseases. In this review, we highlight the recent use and future potential of optogenetic approaches to study homeostasis. Optogenetics uses photosensitive molecules and genetic engineering to modulate cellular activity in vivo, allowing precise experiments with spatiotemporal control. We look at applications of this technology for understanding the mechanisms governing homeostasis and degeneration as applied to widely used model organisms, such as Drosophila melanogaster, where other common tools are less effective or unavailable.

Open access tool and microfluidic devices for phenotypic quantification of heart function of intact fruit fly and zebrafish larvae

In this paper, the heartbeat parameters of small model organisms, i.e. Drosophila melanogaster (fruit fly) and Danio rerio (zebrafish), were quantified in-vivo in intact larvae using microfluidics and a novel MATLAB-based software. Among different developmental stages of flies and zebrafish, the larval stage is privileged due to biological maturity, optical accessibility, and the myogenic nature of the heart. Conventional methods for parametric quantification of heart activities are complex and mostly done on dissected, irreversibly immobilized, or anesthetized larvae. Microfluidics has helped with reversible immobilization without the need for anesthesia, but heart monitoring is still done manually due to challenges associated with the movement of floating organs and cardiac interruptions. In our MATLAB software applied to videos recorded in microfluidic-based whole-organism assays, we have used image segmentation to automatically detect the heart and extract the heartbeat signal based on pixel intensity variations of the most contractile region of the heart tube. The smoothness priors approach (SPA) was applied to remove the undesired low-frequency noises caused by environmental light changes or heart movement. Heart rate and arrhythmicity were automatically measured from the detrended heartbeat signal while other parameters including end-diastolic and end-systolic diameters, shortening distance, shortening time, fractional shortening, and shortening velocity were quantified for the first time in intact larvae, using M-mode images under bright field microscopy. The software was able to detect more than 94% of the heartbeats and the cardiac arrests in intact Drosophila larvae. Our user-friendly software enables in-vivo quantification of D. melanogaster and D. rerio larval heart functions in microfluidic devices, with the potential to be applied to other biological models and used for automatic screening of drugs and alleles that affect their heart.

Smoking flies: testing the effect of tobacco cigarettes on heart function of Drosophila melanogaster

Studies about the relationship between substances consumed by humans and their impact on health, in animal models, have been a challenge due to differences between species in the animal kingdom. However, the homology of certain genes has allowed extrapolation of certain knowledge obtained in animals. Drosophila melanogaster, studied for decades, has been widely used as model for human diseases as well as to study responses associated with the consumption of several substances. In the present work we explore the impact of tobacco consumption on a model of 'smoking flies'. Throughout these experiments, we aim to provide information about the effects of tobacco consumption on cardiac physiology. We assessed intracellular calcium handling, a phenomenon underlying cardiac contraction and relaxation. Flies chronically exposed to tobacco smoke exhibited an increased heart rate and alterations in the dynamics of the transient increase of intracellular calcium in myocardial cells. These effects were also evident under acute exposure to nicotine of the heart, in a semi-intact preparation. Moreover, the alpha 1 and 7 subunits of the nicotinic receptors are involved in the heart response to tobacco and nicotine under chronic (in the intact fly) as well as acute exposure (in the semi-intact preparation). The present data elucidate the implication of the intracellular cardiac pathways affected by nicotine on the heart tissue. Based on the probed genetic and physiological similarity between the fly and human heart, cardiac effects exerted by tobacco smoke in Drosophila advances our understanding of the impact of it in the human heart. Additionally, it may also provide information on how nicotine-like substances, e.g. neonicotinoids used as insecticides, affect cardiac function.This article has an associated First Person interview with the first author of the paper.

Effect of Temperature on Heart Rate for Phaenicia sericata and Drosophila melanogaster with Altered Expression of the TrpA1 Receptors

The transient receptor potential (TrpA-ankyrin) receptor has been linked to pathological conditions in cardiac function in mammals. To better understand the function of the TrpA1 in regulation of the heart, a Drosophila melanogaster model was used to express TrpA1 in heart and body wall muscles. Heartbeat of in intact larvae as well as hearts in situ, devoid of hormonal and neural input, indicate that strong over-expression of TrpA1 in larvae at 30 or 37 °C stopped the heart from beating, but in a diastolic state. Cardiac function recovered upon cooling after short exposure to high temperature. Parental control larvae (UAS-TrpA1) increased heart rate transiently at 30 and 37 °C but slowed at 37 °C within 3 min for in-situ preparations, while in-vivo larvae maintained a constant heart rate. The in-situ preparations maintained an elevated rate at 30 °C. The heartbeat in the TrpA1-expressing strains could not be revived at 37 °C with serotonin. Thus, TrpA1 activation may have allowed enough Ca2+ influx to activate K(Ca) channels into a form of diastolic stasis. TrpA1 activation in body wall muscle confirmed a depolarization of membrane. In contrast, blowfly Phaenicia sericata larvae increased heartbeat at 30 and 37 °C, demonstrating greater cardiac thermotolerance.

Cardiac optogenetics: a decade of enlightenment

The electromechanical function of the heart involves complex, coordinated activity over time and space. Life-threatening cardiac arrhythmias arise from asynchrony in these space-time events; therefore, therapies for prevention and treatment require fundamental understanding and the ability to visualize, perturb and control cardiac activity. Optogenetics combines optical and molecular biology (genetic) approaches for light-enabled sensing and actuation of electrical activity with unprecedented spatiotemporal resolution and parallelism. The year 2020 marks a decade of developments in cardiac optogenetics since this technology was adopted from neuroscience and applied to the heart. In this Review, we appraise a decade of advances that define near-term (immediate) translation based on all-optical electrophysiology, including high-throughput screening, cardiotoxicity testing and personalized medicine assays, and long-term (aspirational) prospects for clinical translation of cardiac optogenetics, including new optical therapies for rhythm control. The main translational opportunities and challenges for optogenetics to be fully embraced in cardiology are also discussed.

Non-invasive red-light optogenetic control of Drosophila cardiac function

Drosophila is a powerful genetic model system for cardiovascular studies. Recently, optogenetic pacing tools have been developed to control Drosophila heart rhythm noninvasively with blue light, which has a limited penetration depth. Here we developed both a red-light sensitive opsin expressing Drosophila system and an integrated red-light stimulation and optical coherence microscopy (OCM) imaging system. We demonstrated noninvasive control of Drosophila cardiac rhythms using a single light source, including simulated tachycardia in ReaChR-expressing flies and bradycardia and cardiac arrest in halorhodopsin (NpHR)-expressing flies at multiple developmental stages. By using red excitation light, we were able to pace flies at higher efficiency and with lower power than with equivalent blue light excitation systems. The recovery dynamics after red-light stimulation of NpHR flies were observed and quantified. The combination of red-light stimulation, OCM imaging, and transgenic Drosophila systems provides a promising and easily manipulated research platform for noninvasive cardiac optogenetic studies.

Men et al. develop an optogenetic pacing tool to control Drosophila heart rhythm noninvasively with red light. Using optical coherence microscopy imaging, they demonstrate effective light-induced tachypacing, bradypacing, and restorable cardiac arrest in transgenic fly models. This study provides a user-friendly research platform for noninvasive cardiac optogenetic studies.

Feasibility of Using Adjunctive Optogenetic Technologies in Cardiomyocyte Phenotyping - from the Single Cell to the Whole Heart

In 1791, Galvani established that electricity activated excitable cells. In the two centuries that followed, electrode stimulation of neuronal, skeletal and cardiac muscle became the adjunctive method of choice in experimental, electrophysiological, and clinical arenas. This approach underpins breakthrough technologies like implantable cardiac pacemakers that we currently take for granted. However, the contact dependence, and field stimulation that electrical depolarization delivers brings inherent limitations to the scope and experimental scale that can be achieved. Many of these were not exposed until reliable in vitro stem-cell derived experimental materials, with genotypes of interest, were produced in the numbers needed for multi-well screening platforms (for toxicity or efficacy studies) or the 2D or 3D tissue surrogates required to study propagation of depolarization within multicellular constructs that mimic clinically relevant arrhythmia in the heart or brain. Here the limitations of classical electrode stimulation are discussed. We describe how these are overcome by optogenetic tools which put electrically excitable cells under the control of light. We discuss how this enables studies in cardiac material from the single cell to the whole heart scale. We review the current commercial platforms that incorporate optogenetic stimulation strategies, and summarize the global literature to date on cardiac applications of optogenetics. We show that the advantages of optogenetic stimulation relevant to iPS-CM based screening include independence from contact, elimination of electrical stimulation artefacts in field potential measuring approaches such as the multi-electrode array, and the ability to print re-entrant patterns of depolarization at will on 2D cardiomyocyte monolayers.

The Effects of Chloride Flux on Drosophila Heart Rate

Approaches are sought after to regulate ionotropic and chronotropic properties of the mammalian heart. Electrodes are commonly used for rapidly exciting cardiac tissue and resetting abnormal pacing. With the advent of optogenetics and the use of tissue-specific expression of light-activated channels, cardiac cells cannot only be excited but also inhibited with ion-selective conductance. As a proof of concept for the ability to slow down cardiac pacing, anion-conducting channelrhodopsins (GtACR1/2) and the anion pump halorhodopsin (eNpHR) were expressed in hearts of larval Drosophila and activated by light. Unlike body wall muscles in most animals, the equilibrium potential for Cl⁻ is more positive as compared to the resting membrane potential in larval Drosophila. As a consequence, upon activating the two forms of GtACR1 and 2 with low light intensity the heart rate increased, likely due to depolarization and opening of voltage-gated Ca²⁺ channels. However, with very intense light activation the heart rate ceases, which may be due to Cl^– shunting to the reversal potential for chloride. Activating eNpHR hyperpolarizes body wall and cardiac muscle in larval Drosophila and rapidly decreases heart rate. The decrease in heart rate is related to light intensity. Intense light activation of eNpHR stops the heart from beating, whereas lower intensities slowed the rate. Even with upregulation of the heart rate with serotonin, the pacing of the heart was slowed with light. Thus, regulation of the heart rate in Drosophila can be accomplished by activating anion-conducting channelrhodopsins using light. These approaches are demonstrated in a genetically amenable insect model.

A Light Wand to Untangle the Myocardial Cell Network

The discovery of optogenetics has revolutionized research in neuroscience by providing the tools for noninvasive, cell-type selective modulation of membrane potential and cellular function in vitro and in vivo. Rhodopsin-based optogenetics has later been introduced in experimental cardiology studies and used as a tool to photoactivate cardiac contractions or to identify the sites, timing, and location most effective for defibrillating impulses to interrupt cardiac arrhythmias. The exploitation of cell-selectivity of optogenetics, and the generation of model organisms with myocardial cell type targeted expression of opsins has started to yield novel and sometimes unexpected notions on myocardial biology. This review summarizes the main results, the different uses, and the prospective developments of cardiac optogenetics.

The effects of bacterial endotoxin (LPS) on cardiac function in a medicinal blow fly (Phaenicia sericata) and a fruit fly (Drosophila melanogaster)

The bacterial endotoxins, lipopolysaccharides (LPS), are known to have direct effects on mammalian heart cells; thus, LPS is likely to have some effects in other cardiac models. Drosophila melanogaster was used since it serves as a model for cardiac physiology. Larvae of blow flies (Phaenicia sericata) commonly used as therapy for debriding dead tissue, are exposed to high levels of bacterial endotoxins, but their mechanisms of LPS resistance are not entirely understood. Comparative effects of LPS on heart rate (HR) were examined for both Drosophila and blowfly larvae. Acute 10-min direct exposure of in situ heart tubes with saline containing 1, 100, and 500 μg/ml LPS from two common bacterial stains (Pseudomonas aeruginosa and Serratia marcescens) revealed a dose-dependent effect. The effects differed between the two fly models. Larval hearts of Drosophila stopped rapidly in low Ca²⁺ containing saline, but the hearts of blow flies appear unaffected for >30 min. S. marcescens increased HR initially in Drosophila followed by a reduction for low and high doses, but no change was observed in larvae of blow flies. Whereas P. aeruginosa at a high dose decreased HR in larvae of Drosophila but increased HR in larvae of blow flies. The goal of this study is to better the understanding in the direct action of LPS on HR. Knowing the acute and direct actions of LPS exposure on HR in different species of larvae may aid in understanding the underlying mechanisms in other animals during septicemia.

Hyperpolarization by activation of halorhodopsin results in enhanced synaptic transmission: Neuromuscular junction and CNS circuit

Optogenetics offers a unique method to regulate the activity of select neural circuits. However, the electrophysiological consequences of targeted optogenetic manipulation upon the entire circuit remain poorly understood. Analysis of the sensory-CNS-motor circuit in Drosophila larvae expressing eHpHR and ChR2-XXL revealed unexpected patterns of excitability. Optical stimulation of motor neurons targeted to express eNpHR resulted in inhibition followed by excitation of body wall contraction with repetitive stimulation in intact larvae. In situ preparations with direct electrophysiological measures showed an increased responsiveness to excitatory synaptic activity induced by sensory stimulation within a functional neural circuit. To ensure proper function of eNpHR and ChR2-XXL they were expressed in body wall muscle and direct electrophysiological measurements were obtained. Under eNpHR induced hyperpolarization the muscle remained excitable with increased amplitude of excitatory postsynaptic synaptic potentials. Theoretical models to explain the observations are presented. This study aids in increasing the understanding of the varied possible influences with light activated proteins within intact neural circuits.

Considerations in repetitive activation of light sensitive ion channels for long-term studies: Channel rhodopsin in the Drosophila model

Optogenetics is a technique used in various animal models and holds a potential for therapeutic possibilities in mammals. There are technical issues with the use of light sensitive ion channels: reproducible effects over time, controlling where the non-native proteins are targeted within the cell and changes in the biophysical properties of the cells they are expressed in. We used a variant of channel rhodopsin (ChR2-XXL) and targeted expression in neurons of larval Drosophila to investigate the acute and chronic activation, with light pulses, of the channels on synaptic function. The rhodopsin channel modifier all trans retinal (ATR) also plays a role in the sensitivity of the channel to light. Periods of acute, repetitive, and pulsatile blue light exposure over larval development produced attenuated responses. These blue light sensitive ion channels, with ATR, show accommodation and produce an electrical refractory period in inducing synaptic responses. The biological significance and aim of this study is to demonstrate that in controlling particular neurons or neuronal circuits with optogenetics, over time and throughout development, one will have to understand the dynamic nature of activating and silencing the light sensitive channels as well as the biophysical effects on neuronal activity.