Molecular basis of infrared detection by snakes

Drosophila menthol sensitivity and the Precambrian origins of transient receptor potential-dependent chemosensation

Transient receptor potential (TRP) cation channels are highly conserved, polymodal sensors which respond to a wide variety of stimuli. Perhaps most notably, TRP channels serve critical functions in nociception and pain. A growing body of evidence suggests that transient receptor potential melastatin (TRPM) and transient receptor potential ankyrin (TRPA) thermal and electrophile sensitivities predate the protostome-deuterostome split (greater than 550 Ma). However, TRPM and TRPA channels are also thought to detect modified terpenes (e.g. menthol). Although terpenoids like menthol are thought to be aversive and/or harmful to insects, mechanistic sensitivity studies have been largely restricted to chordates. Furthermore, it is unknown if TRP-menthol sensing is as ancient as thermal and/or electrophile sensitivity. Combining genetic, optical, electrophysiological, behavioural and phylogenetic approaches, we tested the hypothesis that insect TRP channels play a conserved role in menthol sensing. We found that topical application of menthol to Drosophila melanogaster larvae elicits a Trpm- and TrpA1-dependent nocifensive rolling behaviour, which requires activation of Class IV nociceptor neurons. Further, in characterizing the evolution of TRP channels, we put forth the hypotheses that three previously undescribed TRPM channel clades (basal, αTRPM and βTRPM), as well as TRPs with residues critical for menthol sensing, were present in ancestral bilaterians. This article is part of the Theo Murphy meeting issue 'Evolution of mechanisms and behaviour important for pain'.

Non-Analgesic Symptomatic or Disease-Modifying Potential of TRPA1

TRPA1, a versatile ion channel of the Transient Receptor Potential (TRP) channel family, detects a large variety of chemicals and can contribute to signal processing of other stimuli, e.g., due to its sensitivity to cytosolic calcium elevation or phosphoinositolphosphate modulation. At first, TRPA1 was found on sensory neurons, where it can act as a sensor for potential or actual tissue damage that ultimately may elicit pain or itch as warning symptoms. This review provides an update regarding the analgesic and antipruritic potential of TRPA1 modulation and the respective clinical trials. Furthermore, TRPA1 has been found in an increasing amount of other cell types. Therefore, the main focus of the review is to discuss the non-analgesic and particularly the disease-modifying potential of TRPA1. This includes diseases of the respiratory system, cancer, ischemia, allergy, diabetes, and the gastrointestinal system. The involvement of TRPA1 in the respective pathophysiological cascades is so far mainly based on pre-clinical data.

A Cell-Penetrating Scorpion Toxin Enables Mode-Specific Modulation of TRPA1 and Pain

TRPA1 is a chemosensory ion channel that functions as a sentinel for structurally diverse electrophilic irritants. Channel activation occurs through an unusual mechanism involving covalent modification of cysteine residues clustered within an amino-terminal cytoplasmic domain. Here, we describe a peptidergic scorpion toxin (WaTx) that activates TRPA1 by penetrating the plasma membrane to access the same intracellular site modified by reactive electrophiles. WaTx stabilizes TRPA1 in a biophysically distinct active state characterized by prolonged channel openings and low Ca²⁺ permeability. Consequently, WaTx elicits acute pain and pain hypersensitivity but fails to trigger efferent release of neuropeptides and neurogenic inflammation typically produced by noxious electrophiles. These findings provide a striking example of convergent evolution whereby chemically disparate animal- and plant-derived irritants target the same key allosteric regulatory site to differentially modulate channel activity. WaTx is a unique pharmacological probe for dissecting TRPA1 function and its contribution to acute and persistent pain.

What is new about mild temperature sensing? A review of recent findings

The superfamily of Transient Receptor Potential (TRP) channels is composed by a group of calcium-permeable ionic channels with a generally shared topology. The thermoTRP channels are a subgroup of 11 members, found in the TRPA, TRPV, TRPC, and TRPM subfamilies. Historically, members of this subgroup have been classified as cold, warm or hot-specific temperature sensors. Recently, new experimental results have shown that the role that has been given to the thermoTRPs in thermosensation is not necessarily strict. In addition, it has been shown that these channels activate over temperature ranges, which can have variations depending on the species and the interaction with a specific biological context. Investigation of these interactions could help to elucidate the mechanisms of activation by temperature, which remains uncertain. Abbreviations: Cryo-EM: Cryogenic electron microscopy; DRG: Dorsal root ganglia; H: Human; ROS: Reactive Oxygen Species; TG: Trigeminal ganglia; TRP: Transient Receptor Potential; TRPA: TRP ankyrin; TRPV: TRP vanilloid; TRPC: TRP canonical; TRPM: TRP melastatin.

Evolution of the human cold/menthol receptor, TRPM8

Genes showing versatile functions or subjected to fast expansion and contraction during the adaptation of species to specific ecological conditions, like sensory receptors for odors, pheromones and tastes, are characterized by a great plasticity through evolution. One of the most fascinating sensory receptors in the family of TRP channels, the cold and menthol receptor TRPM8, has received significant attention in the literature. Recent studies have reported the existence of TRPM8 channel isoforms encoded by alternative mRNAs transcribed from alternative promoters and processed by alternative splicing. Since the first draft of the human genome was accomplished in 2000, alternative transcription, alternative splicing and alternative translation have appeared as major sources of gene product diversity and are thought to participate in the generation of complexity in higher organisms. In this study, we investigate whether alternative transcription has been a driving force in the evolution of the human forms of the cold receptor TRPM8. We identified 33 TRPM8 alternative mRNAs (24 new sequences) and their associated protein isoforms in human tissues. Using comparative genomics, we described the evolution of the human TRPM8 sequences in eight ancestors since the origin of Amniota, and estimated in which ancestors the new TRPM8 variants originated. In order to validate the estimated origins of this receptor, we performed experimental validations of predicted exons in mouse tissues. Our results suggest a first diversification event of the cold receptor in the Boreoeutheria ancestor, and a subsequent divergence at the origin of Simiiformes.

Ultrastructural evidence of a mechanosensory function of scale organs (sensilla) in sea snakes (Hydrophiinae)

The evolution of epidermal scales was a major innovation in lepidosaurs, providing a barrier to dehydration and physical stress, while functioning as a sensitive interface for detecting mechanical stimuli in the environment. In snakes, mechanoreception involves tiny scale organs (sensilla) that are concentrated on the surface of the head. The fully marine sea snakes (Hydrophiinae) are closely related to terrestrial hydrophiine snakes but have substantially more protruding (dome-shaped) scale organs that often cover a larger portion of the scale surface. Various divergent selection pressures in the marine environment could account for this morphological variation relating to detection of mechanical stimuli from direct contact with stimuli and/or indirect contact via water motion (i.e. 'hydrodynamic reception'), or co-option for alternate sensory or non-sensory functions. We addressed these hypotheses using immunohistochemistry, and light and electron microscopy, to describe the cells and nerve connections underlying scale organs in two sea snakes, Aipysurus laevis and Hydrophis stokesii. Our results show ultrastructural features in the cephalic scale organs of both marine species that closely resemble the mechanosensitive Meissner-like corpuscles that underlie terrestrial snake scale organs. We conclude that the scale organs of marine hydrophiines have retained a mechanosensory function, but future studies are needed to examine whether they are sensitive to hydrodynamic stimuli.

Functional properties of axolotl transient receptor potential ankyrin 1 revealed by the heterologous expression system

Transient receptor potential ankyrin 1 (TRPA1) from tetrapod vertebrates except rodents are activated by high temperature with a relatively clear threshold. Our recent investigation suggested that a gradual heat activation without clear threshold might be a common feature for TRPA1 of fish. To approach which animal first acquires TRPA1 as a threshold detector instead of a gradual heat sensor, here, we focused on TRPA1 from axolotls (Ambystoma mexicanum). We isolated a full-length cDNA of axolotl transient receptor potential ankyrin 1 (axTRPA1) and studied the functional properties by two-electrode voltage clamp method using Xenopus oocytes. Allyl isothiocyanate, caffeine, methyl anthranilate and carvacrol activated axTRPA1 channels. The results indicated that axTRPA1 is heat activated with the average threshold of 39.7°C, suggesting that axTRPA1 already has acquired the functional property of land animals.

Climate is a strong predictor of near-infrared reflectance but a poor predictor of colour in butterflies

Colour variation across climatic gradients is a common ecogeographical pattern; yet there is long-standing contention over underlying causes, particularly selection for thermal benefits. We tested the evolutionary association between climate gradients and reflectance of near-infrared (NIR) wavelengths, which influence heat gain but are not visible to animals. We measured ultraviolet (UVA), visible (Vis) and NIR reflectance from calibrated images of 372 butterfly specimens from 60 populations (49 species, five families) spanning the Australian continent. Consistent with selection for thermal benefits, the association between climate and reflectance was stronger for NIR than UVA-Vis wavelengths. Furthermore, climate predicted reflectance of the thorax and basal wing, which are critical to thermoregulation; but it did not predict reflectance of the entire wing, which has a variable role in thermoregulation depending on basking behaviour. These results provide evidence that selection for thermal benefits has shaped the reflectance properties of butterflies.

Mammalian Near-Infrared Image Vision through Injectable and Self-Powered Retinal Nanoantennae

Mammals cannot see light over 700 nm in wavelength. This limitation is due to the physical thermodynamic properties of the photon-detecting opsins. However, the detection of naturally invisible near-infrared (NIR) light is a desirable ability. To break this limitation, we developed ocular injectable photoreceptor-binding upconversion nanoparticles (pbUCNPs). These nanoparticles anchored on retinal photoreceptors as miniature NIR light transducers to create NIR light image vision with negligible side effects. Based on single-photoreceptor recordings, electroretinograms, cortical recordings, and visual behavioral tests, we demonstrated that mice with these nanoantennae could not only perceive NIR light, but also see NIR light patterns. Excitingly, the injected mice were also able to differentiate sophisticated NIR shape patterns. Moreover, the NIR light pattern vision was ambient-daylight compatible and existed in parallel with native daylight vision. This new method will provide unmatched opportunities for a wide variety of emerging bio-integrated nanodevice designs and applications. VIDEO ABSTRACT.

Potential seed dispersers: a new facet of the ecological role of Boa constrictor constrictor Linnaeus 1758

Abstract: The boa (Boa constrictor) is considered a top predator and its diet includes a wide variety of birds, mammals, and other reptiles, all related directly to their availability in the environment inhabited by the snake. Seven boas were found roadkilled on highways adjacent to conservation units in the semi-arid region of Rio Grande do Norte state, in northeastern Brazil. Their digestive tract was analyzed to identify food items and classify them according to their orientation in the tract. Among the food items found, the white-eared opossum (Didelphis albiventris) and the black-and-white tegu (Salvator merianae) were ingested head-first, while teeth of a punaré (Thrichomys laurentius) and a Spix's yellow-toothed cavy (Galea spixii) and hairs of an unidentified rodent were found in the intestinal tract. In addition, two novel items were identified: the plain-breasted ground-dove (Columbina minuta), which were ingested tail-first, and carnauba palm seeds (Copernicia prunifera). The orientation of the prey (head-first or tail-first) followed what was expected for each type of prey. In addition, the presence of carnauba palm seeds indicates that, while being a top predator, the boa may also be a potential disperser of seeds, which would constitute a previously unrecorded ecological role for this species.

Tissue-specific contributions of Tmem79 to atopic dermatitis and mast cell-mediated histaminergic itch

Atopic dermatitis (AD) is the most common skin disease in children. It is characterized by relapsing inflammation, skin-barrier defects, and intractable itch. However, the pathophysiology of itch in AD remains enigmatic. Here, we examine the contribution of Tmem79, an orphan transmembrane protein linked to AD in both mice and humans. We show that Tmem79 is expressed by both keratinocytes and sensory neurons, but that loss of keratinocytic Tmem79 is sufficient to elicit robust scratching. Tmem79-/- mice demonstrate an accumulation of dermal mast cells, which are diminished following chronic treatment with cyclooxygenase inhibitors and an EP3 receptor antagonist. In Tmem79-/- mice, mast cell degranulation produces histaminergic itch in a histamine receptor 1/histamine receptor 4 (H4R/H1R)-dependent manner that may involve activation of TRPV1- afferents. TMEM79 has limited sequence homology to a family of microsomal glutathione transferases and confers protection from cellular accumulation of damaging reactive species, and may thus play a role in regulating oxidative stress. In any case, mechanistic insights from this model suggest that therapeutics targeting PGE2 and/or H1R/H4R histaminergic signaling pathways may represent useful avenues to treat Tmem79-associated AD itch. Our findings suggest that individuals with mutations in Tmem79 develop AD due to the loss of protection from oxidative stress.

Temperature-dependent behaviors of parasitic helminths

Parasitic helminth infections are the most common source of neglected tropical disease among impoverished global communities. Many helminths infect their hosts via an active, sensory-driven process in which environmentally motile infective larvae position themselves near potential hosts. For these helminths, host seeking and host invasion can be divided into several discrete behaviors that are regulated by both host-emitted and environmental sensory cues, including heat. Thermosensation is a critical sensory modality for helminths that infect warm-blooded hosts, driving multiple behaviors necessary for host seeking and host invasion. Furthermore, thermosensory cues influence the host-seeking behaviors of both helminths that parasitize endothermic hosts and helminths that parasitize insect hosts. Here, we discuss the role of thermosensation in guiding the host-seeking and host-infection behaviors of a diverse group of helminths, including mammalian-parasitic nematodes, entomopathogenic nematodes, and schistosomes. We also discuss the neural circuitry and molecular pathways that underlie thermosensory responses in these species.

TRPA1 ankyrin repeat six interacts with a small molecule inhibitor chemotype

TRPA1, a member of the transient receptor potential channel (TRP) family, is genetically linked to pain in humans, and small molecule inhibitors are efficacious in preclinical animal models of inflammatory pain. These findings have driven significant interest in development of selective TRPA1 inhibitors as potential analgesics. The majority of TRPA1 inhibitors characterized to date have been reported to interact with the S5 transmembrane helices forming part of the pore region of the channel. However, the development of many of these inhibitors as clinical drug candidates has been prevented by high lipophilicity, low solubility, and poor pharmacokinetic profiles. Identification of alternate compound interacting sites on TRPA1 provides the opportunity to develop structurally distinct modulators with novel structure-activity relationships and more desirable physiochemical properties. In this paper, we have identified a previously undescribed potent and selective small molecule thiadiazole structural class of TRPA1 inhibitor. Using species ortholog chimeric and mutagenesis strategies, we narrowed down the site of interaction to ankyrinR #6 within the distal N-terminal region of TRPA1. To identify the individual amino acid residues involved, we generated a computational model of the ankyrinR domain. This model was used predictively to identify three critical amino acids in human TRPA1, G238, N249, and K270, which were confirmed by mutagenesis to account for compound activity. These findings establish a small molecule interaction region on TRPA1, expanding potential avenues for developing TRPA1 inhibitor analgesics and for probing the mechanism of channel gating.

Lipid- and Cholesterol-Mediated Time-Scale-Specific Modulation of the Outer Membrane Protein X Dynamics in Lipid Bilayers

Membrane protein function fundamentally depends on lipid-bilayer fluidity and the composition of the biological membrane. Although dynamic interdependencies between membrane proteins and the surrounding lipids are suspected, a detailed description is still missing. To uncover lipid-modulated membrane protein backbone dynamics, time-scale-specific NMR relaxation experiments with residue-resolution were recorded. The data revealed that lipid order, modified either biochemically or biophysically, changes the dynamics of the immersed membrane protein in a specific and time-scale-dependent manner. A temperature-dependent dynamics analysis furthermore suggests a direct coupling between lipid and protein dynamics in the picosecond-nanosecond, microsecond, and millisecond time scales, caused by the lipid's trans-gauche isomerization, the segmental and rotational motion of lipids, and the fluidity of the lipid phase, respectively. These observations provide evidence of a direct modulatory capability of the membrane to regulate protein function through lipid dynamics ranging from picoseconds to milliseconds.

Synaptic convergence of afferent inputs in primary infrared-sensitive nucleus (LTTD) neurons of rattlesnakes (Crotalinae) as the origin for sensory contrast enhancement

Pitvipers have a specialized sensory system in the upper jaw to detect infrared (IR) radiation. The bilateral pit organs resemble simple pinhole cameras that map IR objects onto the sensory epithelium as blurred representations of the environment. Trigeminal afferents transmit information about changing temperature patterns as neuronal spike discharge in a topographic manner to the hindbrain nucleus of the lateral descending trigeminal tract (LTTD). A presumed, yet so far unknown neuronal connectivity within this central nucleus exerts a synaptic computation that constrains the relatively large receptive field of primary afferent fibers. Here, we used intracellular recordings of LTTD neurons in isolated rattlesnake brains to decipher the spatio-temporal pattern of excitatory and inhibitory responses following electrical stimulation of single and multiple peripheral pit organ-innervating nerve branches. The responses of individual neurons consisted of complex spike sequences that derived from spatially and temporally specific interactions between excitatory and inhibitory synaptic inputs from the same as well as from adjacent peripheral nerve terminal areas. This pattern complies with a central excitation that is flanked by a delayed lateral inhibition, thereby enhancing the contrast of IR sensory input, functionally reminiscent of the computations for contrast enhancement in the peripheral visual system.

Reflection of near-infrared light confers thermal protection in birds

Biologists have focused their attention on the optical functions of light reflected at ultraviolet and human-visible wavelengths. However, most radiant energy in sunlight occurs in 'unseen' near-infrared (NIR) wavelengths. The capacity to reflect solar radiation at NIR wavelengths may enable animals to control heat gain and remain within their critical thermal limits. Here, using a continent-wide phylogenetic analysis of Australian birds, we show that species occupying hot, arid environments reflect more radiant energy in NIR wavelengths than species in thermally benign environments, even when controlling for variation in visible colour. Biophysical models confirm that smaller species gain a greater advantage from high NIR reflectivity in hot, arid environments, reducing water loss from compensatory evaporative cooling by up to 2% body mass per hour. These results highlight the importance of NIR reflectivity for thermal protection, which may become increasingly critical as the frequency of extreme climatic events increases.

Ethical Issues of Using CRISPR Technologies for Research on Military Enhancement

This paper presents an overview of the key ethical questions of performing gene editing research on military service members. The recent technological advance in gene editing capabilities provided by CRISPR/Cas9 and their path towards first-in-human trials has reinvigorated the debate on human enhancement for non-medical purposes. Human performance optimization has long been a priority of military research in order to close the gap between the advancement of warfare and the limitations of human actors. In spite of this focus on temporary performance improvement, biomedical enhancement is an extension of these endeavours and the ethical issues of such research should be considered. In this paper, we explore possible applications of CRISPR to military human gene editing research and how it could be specifically applied towards protection of service members against biological or chemical weapons. We analyse three normative areas including risk-benefit analysis, informed consent, and inequality of access as it relates to CRISPR applications for military research to help inform and provide considerations for military institutional review boards and policymakers.

Electrostatic Tuning of a Potassium Channel in Electric Fish

Molecular variation contributes to the evolution of adaptive phenotypes, though it is often difficult to understand precisely how. The adaptively significant electric organ discharge behavior of weakly electric fish is the direct result of biophysical membrane properties set by ion channels. Here, we describe a voltage-gated potassium-channel gene in African electric fishes that is under positive selection and highly expressed in the electric organ. The channel produced by this gene shortens electric organ action potentials by activating quickly and at hyperpolarized membrane potentials. The source of these properties is a derived patch of negatively charged amino acids in an extracellular loop near the voltage sensor. We demonstrate that this negative patch acts by contributing to the global surface charge rather than by local interactions with specific amino acids in the channel's extracellular face. We suggest a more widespread role for this loop in the evolutionary tuning of voltage-dependent channels.

The microstructure of white feathers predicts their visible and near-infrared reflectance properties

Research on the optical properties of animal integuments, including fur, feather, skin and cuticle, has focussed almost exclusively on animal-visible wavelengths within the narrow range of 300-700 nm. By contrast, the near-infrared (NIR) portion of direct sunlight, spanning 700-2600 nm, has been largely ignored despite its potentially important thermal consequences. We quantified variation in visible and NIR reflectance and transmission for white body contour feathers of 50 bird species, and examined how well they are predicted by feather macro- and micro-structural morphology. Both visible and NIR reflectance of the feathers varied substantially across species. Larger, thicker, and sparser feathers that are characteristic of larger species, and feathers with rounder barbs and more closely spaced barbules, had high average reflectance, particularly within avian-visible wavelengths (300-700 nm). Feathers with rounder barbs and more closely situated barbules also had high average reflectance, particularly for NIR wavelengths. Barb roundness and barbule density were the only predictors of NIR reflectance after accounting for variation in visible reflectance and body size. Our results highlight the potential for adaptive variation in NIR reflectance mediated by feather structure, which may inform the design of functional materials to control light and heat.

Bioinspired Infrared Sensing Materials and Systems

Bioinspired engineering offers a promising alternative approach in accelerating the development of many man-made systems. Next-generation infrared (IR) sensing systems can also benefit from such nature-inspired approach. The inherent compact and uncooled operation of biological IR sensing systems provides ample inspiration for the engineering of portable and high-performance artificial IR sensing systems. This review overviews the current understanding of the biological IR sensing systems, most of which are thermal-based IR sensors that rely on either bolometer-like or photomechanic sensing mechanism. The existing efforts inspired by the biological IR sensing systems and possible future bioinspired approaches in the development of new IR sensing systems are also discussed in the review. Besides these biological IR sensing systems, other biological systems that do not have IR sensing capabilities but can help advance the development of engineered IR sensing systems are also discussed, and the related engineering efforts are overviewed as well. Further efforts in understanding the biological IR sensing systems, the learning from the integration of multifunction in biological systems, and the reduction of barriers to maximize the multidiscipline collaborations are needed to move this research field forward.

Neural basis of trigeminal chemo- and thermonociception in brown treesnakes, Boiga irregularis (Squamata: Colubridae)

To elucidate the nociceptive system of the brown treesnake, Boiga irregularis, we exposed isolated brown treesnake trigeminal neurons to thermal and chemical stimulation. We measured responses as changes in intracellular calcium using ratiometric fluorescent calcium imaging. Responses to aversive thermal and chemical identified several classes of putative nociceptors. Compounds that were aversive excited many trigeminal neurons, putative chemonociceptors. Identification as nociceptors was further supported by lack of activation by compounds that were not aversive. Brown treesnake neurons had thermal thresholds ranging from 32 to 49 °C. The distribution was discontinuous, with a population of thresholds from 32 to 45 °C and a population with thresholds > 48 °C. Thermal stimulation of 48 °C has been shown to be strongly aversive to brown treesnakes, is lethal, and suggests the presence of thermonociceptors. Thermal sensitivity of brown treesnake trigeminal neurons greatly overlaps with chemical sensitivity; only 1.1% of neurons were sensitive to only thermal stimulation. 50% of brown treesnake trigeminal neurons tested with both > 48 °C and cinnamaldehyde responded to both stimuli, identifying putative polymodal nociceptors. Although a previous study found brown treesnakes insensitive to capsicum extract containing capsaicin, brown treesnake trigeminal neurons responded to capsaicin. These findings are of evolutionary interest as well as providing potential insights into managing this significant pest species.

TRPs et al.: a molecular toolkit for thermosensory adaptations

The ability to sense temperature is crucial for the survival of an organism. Temperature influences all biological operations, from rates of metabolic reactions to protein folding, and broad behavioral functions, from feeding to breeding, and other seasonal activities. The evolution of specialized thermosensory adaptations has enabled animals to inhabit extreme temperature niches and to perform specific temperature-dependent behaviors. The function of sensory neurons depends on the participation of various types of ion channels. Each of the channels involved in neuronal excitability, whether through the generation of receptor potential, action potential, or the maintenance of the resting potential have temperature-dependent properties that can tune the neuron's response to temperature stimuli. Since the function of all proteins is affected by temperature, animals need adaptations not only for detecting different temperatures, but also for maintaining sensory ability at different temperatures. A full understanding of the molecular mechanism of thermosensation requires an investigation of all channel types at each step of thermosensory transduction. A fruitful avenue of investigation into how different molecules can contribute to the fine-tuning of temperature sensitivity is to study the specialized adaptations of various species. Given the diversity of molecular participants at each stage of sensory transduction, animals have a toolkit of channels at their disposal to adapt their thermosensitivity to their particular habitats or behavioral circumstances.

Hologenomic adaptations underlying the evolution of sanguivory in the common vampire bat

Adaptation to specialized diets often requires modifications at both genomic and microbiome levels. We applied a hologenomic approach to the common vampire bat (Desmodus rotundus), one of the only three obligate blood-feeding (sanguivorous) mammals, to study the evolution of its complex dietary adaptation. Specifically, we assembled its high-quality reference genome (scaffold N50 = 26.9 Mb, contig N50 = 36.6 kb) and gut metagenome, and compared them against those of insectivorous, frugivorous and carnivorous bats. Our analyses showed a particular common vampire bat genomic landscape regarding integrated viral elements, a dietary and phylogenetic influence on gut microbiome taxonomic and functional profiles, and that both genetic elements harbour key traits related to the nutritional (for example, vitamin and lipid shortage) and non-nutritional (for example, nitrogen waste and osmotic homeostasis) challenges of sanguivory. These findings highlight the value of a holistic study of both the host and its microbiota when attempting to decipher adaptations underlying radical dietary lifestyles.

Thermally activated TRP channels: molecular sensors for temperature detection

Temperature sensing is one of the oldest capabilities of living organisms, and is essential for sustaining life, because failure to avoid extreme noxious temperatures can result in tissue damage or death. A subset of members of the transient receptor potential (TRP) ion channel family is finely tuned to detect temperatures ranging from extreme cold to noxious heat, giving rise to thermoTRP channels. Structural and functional experiments have shown that thermoTRP channels are allosteric proteins, containing different domains that sense changes in temperature, among other stimuli, triggering pore opening. Although temperature-dependence is well characterized in thermoTRP channels, the molecular nature of temperature-sensing elements remains unknown. Importantly, thermoTRP channels are involved in pain sensation, related to pathological conditions. Here, we provide an overview of thermoTRP channel activation. We also discuss the structural and functional evidence supporting the existence of an intrinsic temperature sensor in this class of channels, and we explore the basic thermodynamic principles for channel activation. Finally, we give a view of their role in painful pathophysiological conditions.

Heat activation is intrinsic to the pore domain of TRPV1

The TRPV1 channel is a sensitive detector of pain-producing stimuli, including noxious heat, acid, inflammatory mediators, and vanilloid compounds. Although binding sites for some activators have been identified, the location of the temperature sensor remains elusive. Using available structures of TRPV1 and voltage-activated potassium channels, we engineered chimeras wherein transmembrane regions of TRPV1 were transplanted into the Shaker Kv channel. Here we show that transplanting the pore domain of TRPV1 into Shaker gives rise to functional channels that can be activated by a TRPV1-selective tarantula toxin that binds to the outer pore of the channel. This pore-domain chimera is permeable to Na+, K+, and Ca2+ ions, and remarkably, is also robustly activated by noxious heat. Our results demonstrate that the pore of TRPV1 is a transportable domain that contains the structural elements sufficient for activation by noxious heat.

Molecular basis of peripheral innocuous warmth sensitivity

The perception of innocuous warmth is a sensory capability that facilitates thermoregulatory, social, hedonic, and even predatory functions. It has long been recognized that innocuous warmth perception is triggered by activation of a subpopulation of specially tuned peripheral thermosensory neurons. In addition, there is growing evidence that thermotransduction by nonneuronal cells, such as skin keratinocytes, might contribute to or modulate our thermosensory experience. Yet, the precise molecular mechanisms underlying warmth transduction are only now being uncovered. Recent molecular genetics approaches have led to the identification of multiple candidate warmth-transducing molecules that appear to confer thermosensitivity upon innocuous warmth afferents and/or neighboring cell types. Most, but not all, of these candidate transducers are members of the transient receptor potential (TRP) ion channel family. Among the latter, evidence supporting a function in innocuous warmth sensation is strongest for TRPV1 and TRPM2 in mammals and for TRPA1 in nonmammalian species.

Differences in ion channel phenotype and function between humans and animal models

Ion channels play crucial roles in regulating a broad range of physiological processes. They form a very large family of transmembrane proteins. Their diversity results from not only a large number of different genes encoding for ion channel subunits but also the ability of subunits to assemble into homo- or heteromultimers, the existence of splice variants, and the expression of different regulatory subunits. These characteristics and the existence of very selective modulators make ion channels very attractive targets for therapy in a wide variety of pathologies. Some ion channels are already being targeted in the clinic while many more are being evaluated as novel drug targets in both clinical and preclinical studies. Advancing ion channel modulators from the bench to the clinic requires their assessment for safety and efficacy in animal models. While extrapolating results from one species to another is tempting, doing such without careful evaluation of the ion channels in different species presents a risk as the translation is not always straightforward. Here, we discuss differences between species in terms of ion channels expressed in selected tissues, differing roles of ion channels in some cell types, variable response to pharmacological agents, and human channelopathies that cannot fully be replicated in animal models.

Molecular basis of peripheral innocuous cold sensitivity

Of somatosensory modalities cold is one of the more ambiguous percepts, evoking the pleasant sensation of cooling, the stinging bite of cold pain, and welcome relief from chronic pain. Moreover, unlike the precipitous thermal thresholds for heat activation of thermosensitive afferent neurons, thresholds for cold fibers are across a range of cool to cold temperatures that spans over 30°C. Until recently, how cold produces this myriad of biologic effects was unknown. However, recent advances in our understanding of cold mechanisms at the behavioral, physiologic, and cellular level have begun to provide insights into this sensory modality. The identification of a number of ion channels that either serve as the principal detectors of a cold stimulus in the peripheral nervous system, or are part of a differential expression pattern of channels that maintain cell excitability in the cold, endows select neurons with properties that are amenable to electric signaling in the cold. This chapter highlights the current understanding of the molecules involved in cold transduction in the mammalian peripheral nervous system, as well as presenting a hypothetic model to account for the broad range of cold thermal thresholds and distinct functions of cold fibers in perception, pain, and analgesia.

Cooler snakes respond more strongly to infrared stimuli, but we have no idea why

The pit organ defining pit vipers (Crotalinae) contains a membrane covered with temperature receptors that detect thermal radiation from environmental surfaces. Temperature is both the environmental parameter being sensed and the mechanism by which the pit membrane detects the signal. As snakes are ectotherms, temperature also has a strong influence on neurological and locomotor responses to the signal. This study of Pacific Rattlesnakes (Crotalus oreganus) systematically examined the effect of body, target, and background temperatures on response to a moving target. We presented each snake with a moving pendulum bob regulated at a series of 6 temperatures against a uniform background regulated at one of 3 temperatures. Snake body temperatures varied from 18° to 36°C. As expected, we found stronger responses to positive contrasts (target warmer than background) than to negative contrasts, and stronger responses to greater contrasts. However, the effect of body temperature was contrary to expectations based on studies of the TRPA1 ion channel (believed to be the molecular basis for pit membrane temperature receptors) and typical thermal reaction norms for neural and motor performance. These predict (1) no response below the threshold where the TRPA1 channel opens, (2) response increasing as temperature increases, peaking near preferred body temperature, and (3) declining thereafter. Remarkably, this behavioral response decreased as body temperature increased from 18°C to 36°C, with no threshold or peak in this range. We review various possible physiological mechanisms related to body temperature proposed in the literature, but find none that can satisfactorily explain this result.

Do free-ranging rattlesnakes use thermal cues to evaluate prey?

Rattlesnakes use infrared radiation to detect prey animals such as small mammals and lizards. Because ectotherm locomotor performance depends on temperature, rattlesnakes could use prey temperature to evaluate the potential of lizards to evade attacks. Here, we tested whether hunting rattlesnakes use infrared information to (1) detect and (2) evaluate prey before attack. We expected thermal contrast between prey and background to be the best predictor of predatory behaviour under the prey detection hypothesis, and absolute prey temperature under the prey evaluation hypothesis. We presented lizard carcasses of varying temperatures to free-ranging sidewinder rattlesnakes (Crotalus cerastes) and scored behavioural responses as a function of thermal contrast, absolute lizard temperature, and light level. Thermal contrast and light level were the most salient predictors of snake behaviour. Snakes were more likely to respond to lizards and/or respond at greater distances at night and when thermal contrast was high, supporting the known prey detection function of infrared sensing. Absolute lizard temperature was not an important predictor of snake behaviour; thus, we found no evidence for temperature-based prey evaluation. Infrared sensing is still poorly understood in ecologically relevant contexts; future research will test whether rattlesnakes learn to evaluate prey based on temperature with experience.

Activation of planarian TRPA1 by reactive oxygen species reveals a conserved mechanism for animal nociception

All animals must detect noxious stimuli to initiate protective behavior, but the evolutionary origin of nociceptive systems is not well understood. Here we show that noxious heat and irritant chemicals elicit robust escape behaviors in the planarian Schmidtea mediterranea and that the conserved ion channel TRPA1 is required for these responses. TRPA1-mutant Drosophila flies are also defective in noxious-heat responses. We find that either planarian or human TRPA1 can restore noxious-heat avoidance to TRPA1-mutant Drosophila, although neither is directly activated by heat. Instead, our data suggest that TRPA1 activation is mediated by H2O2 and reactive oxygen species, early markers of tissue damage rapidly produced as a result of heat exposure. Together, our data reveal a core function for TRPA1 in noxious heat transduction, demonstrate its conservation from planarians to humans, and imply that animal nociceptive systems may share a common ancestry, tracing back to a progenitor that lived more than 500 million years ago.

Membrane ion Channels and Receptors in Animal lifespan Modulation

Acting in the interfaces between environment and membrane compartments, membrane ion channels, and receptors transduce various physical and chemical cues into downstream signaling events. Not surprisingly, these membrane proteins play essential roles in a wide range of cellular processes such as sensory perception, synaptic transmission, cellular growth and development, fate determination, and apoptosis. However, except insulin and insulin-like growth factor receptors, the functions of membrane receptors in animal lifespan modulation have not been well appreciated. On the other hand, although ion channels are popular therapeutic targets for many age-related diseases, their potential roles in aging itself are largely neglected. In this review, we will discuss our current understanding of the conserved functions and mechanisms of membrane ion channels and receptors in the modulation of lifespan across multiple species including Caenorhabditis elegans, Drosophila, mouse, and human.

The exceptionally high reactivity of Cys 621 is critical for electrophilic activation of the sensory nerve ion channel TRPA1

Electrophiles produced during oxidative stress trigger pain responses by reacting with TRPA1 ion channels on sensory nerves. Bahia et al. show that residue C621 on TRPA1 has remarkable reactivity with electrophiles—more than cellular antioxidants—and is crucial for this sensory response.

Activation of the sensory nerve ion channel TRPA1 by electrophiles is the key mechanism that initiates nociceptive signaling, and leads to defensive reflexes and avoidance behaviors, during oxidative stress in mammals. TRPA1 is rapidly activated by subtoxic levels of electrophiles, but it is unclear how TRPA1 outcompetes cellular antioxidants that protect cytosolic proteins from electrophiles. Here, using physiologically relevant exposures, we demonstrate that electrophiles react with cysteine residues on mammalian TRPA1 at rates that exceed the reactivity of typical cysteines by 6,000-fold and that also exceed the reactivity of antioxidant enzymes. We show that TRPA1 possesses a complex reactive cysteine profile in which C621 is necessary for electrophile-induced binding and activation. Modeling of deprotonation energies suggests that K620 contributes to C621 reactivity and mutation of K620 alone greatly reduces the effect of electrophiles on TRPA1. Nevertheless, binding of electrophiles to C621 is not sufficient for activation, which also depends on the function of another reactive cysteine (C665). Together, our results demonstrate that TRPA1 acts as an effective electrophilic sensor because of the exceptionally high reactivity of C621.

Ascospore release in apple scab underlies infrared sensation

The agent of apple scab disease (Venturia inaequalis) is the most common pathogen in apple cultivation. Its ascospores are released in spring, mainly during daylight hours and triggered by rain events. To investigate the causes of diurnal rhythm of ascospore dissemination of the apple scab fungus ascospore releases were examined continuously with spore traps in the orchard and with laboratory assays. One of the spore traps was illuminated at night with different light sources in each year during 2011-2015. The laboratory assays were performed with different light sources with varying wavelengths and intensities. In field and laboratory conditions only light including infrared radiation stimulated ascospore release, but not with light in the visible spectrum only. Artificial illumination during night was correlated with an increase of up to 46 % of ascospores released overnight in the field. We proved that infrared radiation induces V. inaequalis to release its spores. This is the first report in which spore discharge could be stimulated during night under field conditions.

The thermal background determines how the infrared and visual systems interact in pit vipers

The thermal infrared (IR) sensing system of pit vipers is believed to complement vision and provide a substitute imaging system in dark environments. Theoretically, the IR system would best image a scene consisting of a homothermal target in cold surroundings as a bright spot on a dark background. To test this hypothesis, we evaluated how the pit viper (Gloydius brevicaudus) discriminates and strikes prey when the background temperature is either higher or lower than that of the prey (approximately 32–33°C) in different parts of the scene. Snakes were tested in a modified predation cage in which background temperatures were set to 26°C on one side and either 33 or 40°C on the opposite side when the eyes, the pit organs or neither sensory system was occluded. When the eyes were blocked, snakes preferred to strike prey on the 26°C side rather than on the 33°C side but showed no bias in the other conditions. Snakes showed no preference for 26 versus 40°C background temperature, although more missed strikes occurred when the eyes were occluded. The results thus revealed that the pit viper IR system can accomplish a ‘brightness constancy’ computation reflecting the difference between the target and background temperatures, much as the visual system compares the luminance of a figure and the background. Furthermore, the results show that the IR system performs less well for locating prey when the background is warmer than the target.

Multiple Routes of Light Signaling during Root Photomorphogenesis

Plants dynamically adjust their architecture to optimize growth and performance under fluctuating light environments, a process termed photomorphogenesis. A variety of photomorphogenic responses have been studied extensively in the shoots, where diverse photoreceptors and signaling molecules have been functionally characterized. Notably, accumulating evidence demonstrates that the underground roots also undergo photomorphogenesis, raising the question of how roots perceive and respond to aboveground light. Recent findings indicate that root photomorphogenesis is mediated by multiple signaling routes, including shoot-to-root transmission of mobile signaling molecules, direct sensing of light by the roots, and light channeling through the plant body. In this review we discuss recent advances in how light signals are transmitted to the roots to trigger photomorphogenic responses.

Ethyl Vanillin Activates TRPA1

The nonselective cation channel transient receptor potential ankryn subtype family 1 (TRPA1) is expressed in neurons of dorsal root ganglia and trigeminal ganglia and also in vagal afferent neurons that innervate the lungs and gastrointestinal tract. Many TRPA1 agonists are reactive electrophilic compounds that form covalent adducts with TRPA1. Allyl isothiocyanate (AITC), the common agonist used to identify TRPA1, contains an electrophilic group that covalently binds with cysteine residues of TRPA1 and confers a structural change on the channel. There is scientific motivation to identify additional compounds that can activate TRPA1 with different mechanisms of channel gating. We provide evidence that ethyl vanillin (EVA) is a TRPA1 agonist. Using fluorescent calcium imaging and whole-cell patch-clamp electrophysiology on dissociated rat vagal afferent neurons and TRPA1-transfected COS-7 cells, we discovered that EVA activates cells also activated by AITC. Both agonists display similar current profiles and conductances. Pretreatment with A967079, a selective TRPA1 antagonist, blocks the EVA response as well as the AITC response. Furthermore, EVA does not activate vagal afferent neurons from TRPA1 knockout mice, showing selectivity for TRPA1 in this tissue. Interestingly, EVA appears to be pharmacologically different from AITC as a TRPA1 agonist. When AITC is applied before EVA, the EVA response is occluded. However, they both require intracellular oxidation to activate TRPA1. These findings suggest that EVA activates TRPA1 but via a distinct mechanism that may provide greater ease for study in native systems compared with AITC and may shed light on differential modes of TRPA1 gating by ligand types.

Infrared light detection by the haller's organ of adult american dog ticks, Dermacentor variabilis (Ixodida: Ixodidae)

The Haller's organ (HO), unique to ticks and mites, is found only on the first tarsus of the front pair of legs. The organ has an unusual morphology consisting of an anterior pit (AP) with protruding sensilla and a posterior capsule (Cp). The current thinking is that the HO's main function is chemosensation analogous to the insect antennae, but the functionality of its atypical structure (exclusive to the Acari) is unexplained. We provide the first evidence that the HO allows the American dog tick, Dermacentor variabilis, to respond to infrared (IR) light. Unfed D. variabilis adults with their HOs present were positively phototactic to IR. However, when the HOs were removed, no IR response was detected. Ticks in these experiments were also attracted to white light with and without the HOs, but were only positively phototactic to white light when the ocelli (primitive eyes) were unobstructed. Covering the eyes did not prevent IR attraction. A putative TRPA1 receptor was characterized from a D. variabilis-specific HO transcriptome we constructed. This receptor was homologous to transient receptor potential cation channel, subfamily A, member 1 (TRPA1) from the pit organ of the pit viper, python, and boa families of snakes, the only receptor identified so far for IR detection. HO scanning electron microscopy (SEM) studies in the American dog tick showed the AP and Cp but also novel structures not previously described; the potential role of these structures in IR detection is discussed. The ability of ticks to use IR for host finding is consistent with their obligatory hematophagy and has practical applications in tick trapping and the development of new repellents.

Thermogenetic neurostimulation with single-cell resolution

Thermogenetics is a promising innovative neurostimulation technique, which enables robust activation of neurons using thermosensitive transient receptor potential (TRP) cation channels. Broader application of this approach in neuroscience is, however, hindered by a limited variety of suitable ion channels, and by low spatial and temporal resolution of neuronal activation when TRP channels are activated by ambient temperature variations or chemical agonists. Here, we demonstrate rapid, robust and reproducible repeated activation of snake TRPA1 channels heterologously expressed in non-neuronal cells, mouse neurons and zebrafish neurons in vivo by infrared (IR) laser radiation. A fibre-optic probe that integrates a nitrogen-vacancy (NV) diamond quantum sensor with optical and microwave waveguide delivery enables thermometry with single-cell resolution, allowing neurons to be activated by exceptionally mild heating, thus preventing the damaging effects of excessive heat. The neuronal responses to the activation by IR laser radiation are fully characterized using Ca2+ imaging and electrophysiology, providing, for the first time, a complete framework for a thermogenetic manipulation of individual neurons using IR light.

Characterization of TRPA channels in the starfish Patiria pectinifera: involvement of thermally activated TRPA1 in thermotaxis in marine planktonic larvae

The vast majority of marine invertebrates spend their larval period as pelagic plankton and are exposed to various environmental cues. Here we investigated the thermotaxis behaviors of the bipinnaria larvae of the starfish, Patiria pectinifera, in association with TRPA ion channels that serve as thermal receptors in various animal species. Using a newly developed thermotaxis assay system, we observed that P. pectinifera larvae displayed positive thermotaxis toward high temperatures, including toward temperatures high enough to cause death. In parallel, we identified two TRPA genes, termed PpTRPA1 and PpTRPA basal, from this species. We examined the phylogenetic position, spatial expression, and channel properties of each PpTRPA. Our results revealed the following: (1) The two genes diverged early in animal evolution; (2) PpTRPA1 and PpTRPA basal are expressed in the ciliary band and posterior digestive tract of the larval body, respectively; and (3) PpTRPA1 is activated by heat stimulation as well as by known TRPA1 agonists. Moreover, knockdown and rescue experiments demonstrated that PpTRPA1 is involved in positive thermotaxis in P. pectinifera larvae. This is the first report to reveal that TRPA1 channels regulate the behavioral response of a marine invertebrate to temperature changes during its planktonic larval period.

Coagulating Colubrids: Evolutionary, Pathophysiological and Biodiscovery Implications of Venom Variations between Boomslang (Dispholidus typus) and Twig Snake (Thelotornis mossambicanus)

Venoms can deleteriously affect any physiological system reachable by the bloodstream, including directly interfering with the coagulation cascade. Such coagulopathic toxins may be anticoagulants or procoagulants. Snake venoms are unique in their use of procoagulant toxins for predatory purposes. The boomslang (Dispholidus typus) and the twig snakes (Thelotornis species) are iconic African snakes belonging to the family Colubridae. Both species produce strikingly similar lethal procoagulant pathologies. Despite these similarities, antivenom is only produced for treating bites by D. typus, and the mechanisms of action of both venoms have been understudied. In this study, we investigated the venom of D. typus and T. mossambicanus utilising a range of proteomic and bioactivity approaches, including determining the procoagulant properties of both venoms in relation to the human coagulation pathways. In doing so, we developed a novel procoagulant assay, utilising a Stago STA-R Max analyser, to accurately detect real time clotting in plasma at varying concentrations of venom. This approach was used to assess the clotting capabilities of the two venoms both with and without calcium and phospholipid co-factors. We found that T. mossambicanus produced a significantly stronger coagulation response compared to D. typus. Functional enzyme assays showed that T. mossambicanus also exhibited a higher metalloprotease and phospholipase activity but had a much lower serine protease activity relative to D. typus venom. The neutralising capability of the available boomslang antivenom was also investigated on both species, with it being 11.3 times more effective upon D. typus venom than T. mossambicanus. In addition to being a faster clotting venom, T. mossambicanus was revealed to be a much more complex venom composition than D. typus. This is consistent with patterns seen for other snakes with venom complexity linked to dietary complexity. Consistent with the external morphological differences in head shape between the two species, CT and MRI analyses revealed significant internal structural differences in skull architecture and venom gland anatomy. This study increases our understanding of not only the biodiscovery potential of these medically important species but also increases our knowledge of the pathological relationship between venom and the human coagulation cascade.