Molecular basis of infrared detection by snakes

Organotopic organization of the primary Infrared Sensitive Nucleus (LTTD) in the western diamondback rattlesnake (Crotalus atrox)

Pit vipers (Crotalinae) have a specific sensory system that detects infrared radiation with bilateral pit organs in the upper jaw. Each pit organ consists of a thin membrane, innervated by three trigeminal nerve branches that project to a specific nucleus in the dorsal hindbrain. The known topographic organization of infrared signals in the optic tectum prompted us to test the implementation of spatiotopically aligned sensory maps through hierarchical neuronal levels from the peripheral epithelium to the first central site in the hindbrain, the nucleus of the lateral descending trigeminal tract (LTTD). The spatial organization of the anatomical connections was revealed in a novel in vitro whole-brain preparation of the western diamondback rattlesnake (Crotalus atrox) that allowed specific application of multiple neuronal tracers to identified pit-organ-supplying trigeminal nerve branches. After adequate survival times, the entire peripheral and central projections of fibers within the pit membrane and the LTTD became visible. This approach revealed a morphological partition of the pit membrane into three well-defined sensory areas with largely separated innervations by the three main branches. The peripheral segregation of infrared afferents in the sensory epithelium was matched by a differential termination of the afferents within different areas of the LTTD, with little overlap. This result demonstrates a topographic organizational principle of the snake infrared system that is implemented by maintaining spatially aligned representations of environmental infrared cues on the sensory epithelium through specific neuronal projections at the level of the first central processing stage, comparable to the visual system.

Functional diversity and evolutionary dynamics of thermoTRP channels

Animals have evolved sophisticated physiological systems for sensing ambient temperature since changes in environmental temperatures affect various biological processes. Thermosensitive transient receptor potential (thermoTRP) channels serve as thermal sensors in diverse animal species. They are multimodal receptors that are activated by temperature as well as other physical and chemical stimuli. Since thermoTRP channels are calcium permeable non-selective cation channels, their activation leads to an influx of calcium and sodium ions into the cell and triggers downstream signal transduction. ThermoTRP channels have been characterized in diverse animal species over the past several years, illuminating the diversification of thermoTRP channels in the course of evolution. The gene repertoires of thermoTRP channels differ among animal species. Additionally, in some cases, the temperature and chemical sensitivities among orthologous thermoTRP channels vary among species. The evolutionary flexibility of thermoTRP channels enabled them to contribute to unique physiological systems such as infrared sensation in snakes and bats and seasonal adaptation in silk moth. On the other hand, the functional differences of thermoTRP channels among species have been utilized for understanding the molecular basis for their activation (or inhibition) mechanisms, and amino acid residues (or domains) responsible for the respective channel properties have been identified in various thermoTRP channels. Here we summarize the current understanding of the functional diversity and evolutionary dynamics of thermoTRP channels.

Auto-oxidation products of epigallocatechin gallate activate TRPA1 and TRPV1 in sensory neurons

The sensation of astringency is elicited by catechins and their polymers in wine and tea. It has been considered that catechins in green tea are unstable and auto-oxidized to induce more astringent taste. Here, we examined how mammalian transient receptor potential V1 (TRPV1) and TRPA1, which are nociceptive sensors, are activated by green tea catechins during the auto-oxidation process. Neither TRPV1 nor TRPA1 could be activated by any of the freshly prepared catechin. When one of the major catechin, epigallocatechin gallate (EGCG), was preincubated for 3h in Hank's balanced salt solution, it significantly activated both TRP channels expressed in HEK293 cells. Even after incubation, other catechins showed much less effects. Results suggest that only oxidative products of EGCG activate both TRPV1 and TRPA1. Dorsal root ganglion (DRG) sensory neurons were also activated by the incubated EGCG through TRPV1 and TRPA1 channels. Liquid chromatography-mass spectrometry revealed that theasinensins A and D are formed during incubation of EGCG. We found that purified theasinensin A activates both TRPV1 and TRPA1, and that it stimulates DRG neurons through TRPV1 and TRPA1 channels. Results suggested a possibility that TRPV1 and TRPA1 channels are involved in the sense of astringent taste of green tea.

Human TRPA1 is intrinsically cold- and chemosensitive with and without its N-terminal ankyrin repeat domain

We have purified and reconstituted human transient receptor potential (TRP) subtype A1 (hTRPA1) into lipid bilayers and recorded single-channel currents to understand its inherent thermo- and chemosensory properties as well as the role of the ankyrin repeat domain (ARD) of the N terminus in channel behavior. We report that hTRPA1 with and without its N-terminal ARD (Δ1-688 hTRPA1) is intrinsically cold-sensitive, and thus, cold-sensing properties of hTRPA1 reside outside the N-terminal ARD. We show activation of hTRPA1 by the thiol oxidant 2-((biotinoyl)amino)ethyl methanethiosulfonate (MTSEA-biotin) and that electrophilic compounds activate hTRPA1 in the presence and absence of the N-terminal ARD. The nonelectrophilic compounds menthol and the cannabinoid Δ(9)-tetrahydrocannabiorcol (C16) directly activate hTRPA1 at different sites independent of the N-terminal ARD. The TRPA1 antagonist HC030031 inhibited cold and chemical activation of hTRPA1 and Δ1-688 hTRPA1, supporting a direct interaction with hTRPA1 outside the N-terminal ARD. These findings show that hTRPA1 is an intrinsically cold- and chemosensitive ion channel. Thus, second messengers, including Ca(2+), or accessory proteins are not needed for hTRPA1 responses to cold or chemical activators. We suggest that conformational changes outside the N-terminal ARD by cold, electrophiles, and nonelectrophiles are important in hTRPA1 channel gating and that targeting chemical interaction sites outside the N-terminal ARD provides possibilities to fine tune TRPA1-based drug therapies (e.g., for treatment of pain associated with cold hypersensitivity and cardiovascular disease).

Product and technology innovation: what can biomimicry inspire?

Biomimicry (bio- meaning life in Greek, and -mimesis, meaning to copy) is a growing field that seeks to interpolate natural biological mechanisms and structures into a wide range of applications. The rise of interest in biomimicry in recent years has provided a fertile ground for innovation. This review provides an eco-system based analysis of biomimicry inspired technology and product innovation. A multi-disciplinary framework has been developed to accomplish this analysis and the findings focus on the areas that have been most strikingly affected by the application of biomimicry and also highlight the emerging trends and opportunity areas.

Sensory biology. Evolution of sweet taste perception in hummingbirds by transformation of the ancestral umami receptor

Sensory systems define an animal's capacity for perception and can evolve to promote survival in new environmental niches. We have uncovered a noncanonical mechanism for sweet taste perception that evolved in hummingbirds since their divergence from insectivorous swifts, their closest relatives. We observed the widespread absence in birds of an essential subunit (T1R2) of the only known vertebrate sweet receptor, raising questions about how specialized nectar feeders such as hummingbirds sense sugars. Receptor expression studies revealed that the ancestral umami receptor (the T1R1-T1R3 heterodimer) was repurposed in hummingbirds to function as a carbohydrate receptor. Furthermore, the molecular recognition properties of T1R1-T1R3 guided taste behavior in captive and wild hummingbirds. We propose that changing taste receptor function enabled hummingbirds to perceive and use nectar, facilitating the massive radiation of hummingbird species.

Neuronal mechanism for acute mechanosensitivity in tactile-foraging waterfowl

Relying almost exclusively on their acute sense of touch, tactile-foraging birds can feed in murky water, but the cellular mechanism is unknown. Mechanical stimuli activate specialized cutaneous end organs in the bill, innervated by trigeminal afferents. We report that trigeminal ganglia (TG) of domestic and wild tactile-foraging ducks exhibit numerical expansion of large-diameter mechanoreceptive neurons expressing the mechano-gated ion channel Piezo2. These features are not found in visually foraging birds. Moreover, in the duck, the expansion of mechanoreceptors occurs at the expense of thermosensors. Direct mechanical stimulation of duck TG neurons evokes high-amplitude depolarizing current with a low threshold of activation, high signal amplification gain, and slow kinetics of inactivation. Together, these factors contribute to efficient conversion of light mechanical stimuli into neuronal excitation. Our results reveal an evolutionary strategy to hone tactile perception in vertebrates at the level of primary afferents.

A molecular framework for temperature-dependent gating of ion channels

Perception of heat or cold in higher organisms is mediated by specialized ion channels whose gating is exquisitely sensitive to temperature. The physicochemical underpinnings of this temperature-sensitive gating have proven difficult to parse. Here, we took a bottom-up protein design approach and rationally engineered ion channels to activate in response to thermal stimuli. By varying amino acid polarities at sites undergoing state-dependent changes in solvation, we were able to systematically confer temperature sensitivity to a canonical voltage-gated ion channel. Our results imply that the specific heat capacity change during channel gating is a major determinant of thermosensitive gating. We also show that reduction of gating charges amplifies temperature sensitivity of designer channels, which accounts for low-voltage sensitivity in all known temperature-gated ion channels. These emerging principles suggest a plausible molecular mechanism for temperature-dependent gating that reconcile how ion channels with an overall conserved transmembrane architecture may exhibit a wide range of temperature-sensing phenotypes. :

GCY-8, PDE-2, and NCS-1 are critical elements of the cGMP-dependent thermotransduction cascade in the AFD neurons responsible for C. elegans thermotaxis

Certain thermoreceptor neurons are sensitive to tiny thermal fluctuations (0.01°C or less) and maintain their sensitivity across a wide range of ambient temperatures through a process of adaptation, but understanding of the biochemical basis for this performance is rudimentary. Prior studies of the AFD thermoreceptor in Caenorhabditis elegans revealed a signaling cascade that depends on a trio of receptor guanylate cyclases (rGCs), GCY-8, GCY-18, and GCY-23, and gives rise to warming-activated thermoreceptor currents (ThRCs) carried by cyclic GMP–gated ion channels. The threshold for ThRC activation adapts to the ambient temperature through an unknown calcium-dependent process. Here, we use in vivo whole-cell patch-clamp recording from AFD to show that loss of GCY-8, but not of GCY-18 or GCY-23, reduces or eliminates ThRCs, identifying this rGC as a crucial signaling element. To learn more about thermotransduction and adaptation, we used behavioral screens and analysis of gene expression patterns to identify phosphodiesterases (PDEs) likely to contribute to thermotransduction. Deleting PDE-2 decouples the threshold for ThRC activation from ambient temperature, altering adaptation. We provide evidence that the conserved neuronal calcium sensor 1 protein also regulates the threshold for ThRC activation and propose a signaling network to account for ThRC activation and adaptation. Because PDEs play essential roles in diverse biological processes, including vertebrate phototransduction and olfaction, and regulation of smooth muscle contractility and cardiovascular function, this study has broad implications for understanding how extraordinary sensitivity and dynamic range is achieved in cyclic nucleotide–based signaling networks.

Transient receptor potential channel ankyrin-1 is not a cold sensor for autonomic thermoregulation in rodents

The rodent transient receptor potential ankyrin-1 (TRPA1) channel has been hypothesized to serve as a temperature sensor for thermoregulation in the cold. We tested this hypothesis by using deletion of the Trpa1 gene in mice and pharmacological blockade of the TRPA1 channel in rats. In both Trpa1(-/-) and Trpa1(+/+) mice, severe cold exposure (8°C) resulted in decreases of skin and deep body temperatures to ∼8°C and 13°C, respectively, both temperatures being below the reported 17°C threshold temperature for TRPA1 activation. Under these conditions, Trpa1(-/-) mice had the same dynamics of body temperature as Trpa1(+/+) mice and showed no weakness in the tail skin vasoconstriction response or thermogenic response to cold. In rats, the effects of pharmacological blockade were studied by using two chemically unrelated TRPA1 antagonists: the highly potent and selective compound A967079, which had been characterized earlier, and the relatively new compound 43 ((4R)-1,2,3,4-tetrahydro-4-[3-(3-methoxypropoxy)phenyl]-2-thioxo-5H-indeno[1,2-d]pyrimidin-5-one), which we further characterized in the present study and found to be highly potent (IC50 against cold of ∼8 nm) and selective. Intragastric administration of either antagonist at 30 mg/kg before severe (3°C) cold exposure did not affect the thermoregulatory responses (deep body and tail skin temperatures) of rats, even though plasma concentrations of both antagonists well exceeded their IC50 value at the end of the experiment. In the same experimental setup, blocking the melastatin-8 (TRPM8) channel with AMG2850 (30 mg/kg) attenuated cold-defense mechanisms and led to hypothermia. We conclude that TRPA1 channels do not drive autonomic thermoregulatory responses to cold in rodents.

Directionality of temperature activation in mouse TRPA1 ion channel can be inverted by single-point mutations in ankyrin repeat six

Several transient receptor potential (TRP) ion channels are activated with high sensitivity by either cold or hot temperatures. However, structures and mechanism that determine temperature directionality (cold versus heat) are not established. Here we screened 12,000 random mutant clones of the cold-activated mouse TRPA1 ion channel with a heat stimulus. We identified three single-point mutations that are individually sufficient to make mouse TRPA1 warm activated, while leaving sensitivity to chemicals unaffected. Mutant channels have high temperature sensitivity of voltage activation, specifically of channel opening, but not channel closing, which is reminiscent of other heat-activated TRP channels. All mutations are located in ankyrin repeat six, which identifies this domain as a sensitive modulator of thermal activation. We propose that a change in the coupling of temperature sensing to channel gating generates this sensitivity to warm temperatures. Our results demonstrate that minimal changes in protein sequence are sufficient to generate a wide diversity of thermal sensitivities in TRPA1.

Species differences and molecular determinant of TRPA1 cold sensitivity

TRPA1 is an ion channel and has been proposed as a thermosensor across species. In invertebrate and ancestral vertebrates such as fly, mosquito, frog, lizard and snakes, TRPA1 serves as a heat receptor, a sensory input utilized for heat avoidance or infrared detection. However, in mammals, whether TRPA1 is a receptor for noxious cold is highly controversial, as channel activation by cold was observed by some groups but disputed by others. Here we attribute the discrepancy to species differences. We show that cold activates rat and mouse TRPA1 but not human or rhesus monkey TRPA1. At the molecular level, a single residue within the S5 transmembrane domain (G878 in rodent but V875 in primate) accounts for the observed difference in cold sensitivity. This residue difference also underlies the species-specific effects of menthol. Together, our findings identify the species-specific cold activation of TRPA1 and reveal a molecular determinant of cold-sensitive gating.

TRPA1 ion channels act as thermosensors across different species; however, studies on their role in noxious cold sensation have provided conflicting results in mammals. Chen et al. show that these discrepancies arise because cold activates rat and mouse TRPA1 but not human or rhesus monkey TRPA1.

Discovery of a novel accessory structure of the pitviper infrared receptor organ (serpentes: viperidae)

The facial pits of rattlesnakes, copperheads, lanceheads, bushmasters and other American and Asian pitvipers (Crotalinae) are highly innervated and densely vascularized infrared (IR) receptor organs. For over a century, studies have focused on a small sample of model species from North America and Asia. Based on an expanded survey of Central and South American crotalines, we report a conspicuous accessory structure composed of well-defined papillae that project from the anterior orbital adnexa. The papillae are continuous with the inner chamber of the IR receptor organ and our histological and ultrastructural data suggest that they possess a well-developed nervous network and extensive vascularization; however, they lack the characteristic IR-sensitive terminal nerve masses found in the IR-receptive pit membrane. The function of the IR receptor organ papillae is unknown.

Evolutionarily conserved, multitasking TRP channels: lessons from worms and flies

The Transient Receptor Potential (TRP) channel family is comprised of a large group of cation-permeable channels, which display an extraordinary diversity of roles in sensory signaling. TRPs allow animals to detect chemicals, mechanical force, light, and changes in temperature. Consequently, these channels control a plethora of animal behaviors. Moreover, their functions are not limited to the classical senses, as they are cellular sensors, which are critical for ionic homeostasis and metabolism. Two genetically tractable invertebrate model organisms, Caenorhabditis elegans and Drosophila melanogaster, have led the way in revealing a wide array of sensory roles and behaviors that depend on TRP channels. Two overriding themes have emerged from these studies. First, TRPs are multitasking proteins, and second, many functions and modes of activation of these channels are evolutionarily conserved, including some that were formerly thought to be unique to invertebrates, such as phototransduction. Thus, worms and flies offer the potential to decipher roles for mammalian TRPs, which would otherwise not be suspected.

Thermal effects and sensitivity of biological membranes

Temperature is one of the key parameters that controlled the origin and evolution of life on earth and it continues to be a principal regulator of the functions of organisms. Some aspects of the response of simple and complex organisms to temperature variations are encoded in the physical properties of the cell components, with the all-important plasma membrane playing a principal role. Other responses to temperature are more specific and through evolution, specialized receptors with particular temperature sensitivities have appeared to mediate this signaling. While some of these receptors are ancient and can be found in very primitive organisms, it seems that the mechanisms used by prokaryotes and eukaryotes are very different, indicating that temperature sensitivity has evolved in more than one occasion during evolution.

Structure of thermally activated TRP channels

Temperature sensation is important for adaptation and survival of organisms. While temperature has the potential to affect all biological macromolecules, organisms have evolved specific thermosensitive molecular detectors that are able to transduce temperature changes into physiologically relevant signals. Among these thermosensors are ion channels from the transient receptor potential (TRP) family. Prime candidates include TRPV1-4, TRPA1, and TRPM8 (the so-called "thermoTRP" channels), which are expressed in sensory neurons and gated at specific temperatures. Electrophysiological and thermodynamic approaches have been employed to determine the nature by which thermoTRPs detect temperature and couple temperature changes to channel gating. To further understand how thermoTRPs sense temperature, high-resolution structures of full-length thermoTRPs channels will be required. Here, we will discuss current progress in unraveling the structures of thermoTRP channels.

TRPA1 channels: chemical and temperature sensitivity

Transient receptor potential ankyrin 1 (TRPA1) is a polymodal excitatory ion channel found in sensory neurons of different organisms, ranging from worms to humans. Since its discovery as an uncharacterized transmembrane protein in human fibroblasts, TRPA1 has become one of the most intensively studied ion channels. Its function has been linked to regulation of heat and cold perception, mechanosensitivity, hearing, inflammation, pain, circadian rhythms, chemoreception, and other processes. Some of these proposed functions remain controversial, while others have gathered considerable experimental support. A truly polymodal ion channel, TRPA1 is activated by various stimuli, including electrophilic chemicals, oxygen, temperature, and mechanical force, yet the molecular mechanism of TRPA1 gating remains obscure. In this review, we discuss recent advances in the understanding of TRPA1 physiology, pharmacology, and molecular function.

Temperature sensing by thermal TRP channels: thermodynamic basis and molecular insights

All organisms need to sense temperature in order to survive and adapt. But how they detect and perceive temperature remains poorly understood. Recent discoveries of thermal Transient Receptor Potential (TRP) ion channels have shed light on the problem and unravel molecular entities for temperature detection and transduction in mammals. Thermal TRP channels belong to the large family of transient receptor potential channels. They are directly activated by heat or cold in physiologically relevant temperature ranges, and the activation is exquisitely sensitive to temperature changes. Thermodynamically, this strong temperature dependence of thermal channels occurs due to large enthalpy and entropy changes associated with channel opening. Thus understanding how the channel proteins obtain their exceptionally large energetics is central toward determining functional mechanisms of thermal TRP channels. The purpose of this chapter is to provide a comprehensive review on critical issues and challenges facing the problem, with emphases on underlying biophysical and molecular mechanisms.

What do we really know and what do we need to know: some controversies, perspectives, and surprises

TRP channels comprise one of the most rapid growing research topics in ion channel research, in fields related to ion channels including channelopathies and translational medicine. We provide here a critical survey on our current knowledge of TRP channels and highlight some of the still open or controversial questions. This comprises questions related to evolution of TRP channels; biophysics, i.e., permeation; pore properties and gating; modulation; the still-elusive 3D structure; and channel subunits but also their role as general sensory channels and in human diseases. We will conclude that our knowledge on TRP channels is still at the very beginning of an exciting research journey.

Gating of thermally activated channels

A class of ion channels that belongs to the transient receptor potential (TRP) superfamily and is present in specialized neurons that project to the skin has evolved as temperature detectors. These channels are classified into subfamilies, namely canonical (TRPC), melastatin (TRPM), ankyrin (TRPA), and vanilloid (TRPV). Some of these channels are activated by heat (TRPM2/4/5, TRPV1-4), while others by cold (TRPA1, TRPC5, and TRPM8). The general structure of these channels is closely related to that of the voltage-dependent K(+) channels, with their subunits containing six transmembrane segments that form tetramers. Thermal TRP channels are polymodal receptors. That is, they can be activated by temperature, voltage, pH, lipids, and agonists. The high temperature sensitivity in these thermal TRP channels is due to a large enthalpy change (∼100 kcal/mol), which is about five times the enthalpy change in voltage-dependent gating. The characterization of the macroscopic currents and single-channel analysis demonstrated that gating by temperature is complex and best described by branched or allosteric models containing several closed and open states. The identification of molecular determinants of temperature sensitivity in TRPV1, TRPA1, and TRPV3 strongly suggest that thermal sensitivity arises from a specific protein domain.

The role of allosteric coupling on thermal activation of thermo-TRP channels

Thermo-transient receptor potential channels display outstanding temperature sensitivity and can be directly gated by low or high temperature, giving rise to cold- and heat-activated currents. These constitute the molecular basis for the detection of changes in ambient temperature by sensory neurons in animals. The mechanism that underlies the temperature sensitivity in thermo-transient receptor potential channels remains unknown, but has been associated with large changes in standard-state enthalpy (ΔH(o)) and entropy (ΔS(o)) upon channel gating. The magnitude, sign, and temperature dependence of ΔH(o) and ΔS(o), the last given by an associated change in heat capacity (ΔCp), can determine a channel's temperature sensitivity and whether it is activated by cooling, heating, or both, if ΔCp makes an important contribution. We show that in the presence of allosteric gating, other parameters, besides ΔH(o) and ΔS(o), including the gating equilibrium constant, the strength- and temperature dependence of the coupling between gating and the temperature-sensitive transitions, as well as the ΔH(o)/ΔS(o) ratio associated with them, can also determine a channel's temperature-dependent activity, and even give rise to channels that respond to both cooling and heating in a ΔCp-independent manner.

TRP channels and pain

Nociception is the process whereby primary afferent nerve fibers of the somatosensory system detect noxious stimuli. Pungent irritants from pepper, mint, and mustard plants have served as powerful pharmacological tools for identifying molecules and mechanisms underlying this initial step of pain sensation. These natural products have revealed three members of the transient receptor potential (TRP) ion channel family--TRPV1, TRPM8, and TRPA1--as molecular detectors of thermal and chemical stimuli that activate sensory neurons to produce acute or persistent pain. Analysis of TRP channel function and expression has validated the existence of nociceptors as a specialized group of somatosensory neurons devoted to the detection of noxious stimuli. These studies are also providing insight into the coding logic of nociception and how specification of nociceptor subtypes underlies behavioral discrimination of noxious thermal, chemical, and mechanical stimuli. Biophysical and pharmacological characterization of these channels has provided the intellectual and technical foundation for developing new classes of analgesic drugs.

Molecular mechanism of TRP channels

Transient receptor potential (TRP) channels are cellular sensors for a wide spectrum of physical and chemical stimuli. They are involved in the formation of sight, hearing, touch, smell, taste, temperature, and pain sensation. TRP channels also play fundamental roles in cell signaling and allow the host cell to respond to benign or harmful environmental changes. As TRP channel activation is controlled by very diverse processes and, in many cases, exhibits complex polymodal properties, understanding how each TRP channel responds to its unique forms of activation energy is both crucial and challenging. The past two decades witnessed significant advances in understanding the molecular mechanisms that underlie TRP channels activation. This review focuses on our current understanding of the molecular determinants for TRP channel activation.

Tools, methods, and applications for optophysiology in neuroscience

The advent of optogenetics and genetically encoded photosensors has provided neuroscience researchers with a wealth of new tools and methods for examining and manipulating neuronal function in vivo. There exists now a wide range of experimentally validated protein tools capable of modifying cellular function, including light-gated ion channels, recombinant light-gated G protein-coupled receptors, and even neurotransmitter receptors modified with tethered photo-switchable ligands. A large number of genetically encoded protein sensors have also been developed to optically track cellular activity in real time, including membrane-voltage-sensitive fluorophores and fluorescent calcium and pH indicators. The development of techniques for controlled expression of these proteins has also increased their utility by allowing the study of specific populations of cells. Additionally, recent advances in optics technology have enabled both activation and observation of target proteins with high spatiotemporal fidelity. In combination, these methods have great potential in the study of neural circuits and networks, behavior, animal models of disease, as well as in high-throughput ex vivo studies. This review collects some of these new tools and methods and surveys several current and future applications of the evolving field of optophysiology.

Crocodylians evolved scattered multi-sensory micro-organs

BACKGROUND

During their evolution towards a complete life cycle on land, stem reptiles developed both an impermeable multi-layered keratinized epidermis and skin appendages (scales) providing mechanical, thermal, and chemical protection. Previous studies have demonstrated that, despite the presence of a particularly armored skin, crocodylians have exquisite mechanosensory abilities thanks to the presence of small integumentary sensory organs (ISOs) distributed on postcranial and/or cranial scales.

RESULTS

Here, we analyze and compare the structure, innervation, embryonic morphogenesis and sensory functions of postcranial, cranial, and lingual sensory organs of the Nile crocodile (Crocodylus niloticus) and the spectacled caiman (Caiman crocodilus). Our molecular analyses indicate that sensory neurons of crocodylian ISOs express a large repertoire of transduction channels involved in mechano-, thermo-, and chemosensory functions, and our electrophysiological analyses confirm that each ISO exhibits a combined sensitivity to mechanical, thermal and pH stimuli (but not hyper-osmotic salinity), making them remarkable multi-sensorial micro-organs with no equivalent in the sensory systems of other vertebrate lineages. We also show that ISOs all exhibit similar morphologies and modes of development, despite forming at different stages of scale morphogenesis across the body.

CONCLUSIONS

The ancestral vertebrate diffused sensory system of the skin was transformed in the crocodylian lineages into an array of discrete multi-sensory micro-organs innervated by multiple pools of sensory neurons. This discretization of skin sensory expression sites is unique among vertebrates and allowed crocodylians to develop a highly-armored, but very sensitive, skin.

Genetic and neural mechanisms that inhibit Drosophila from mating with other species

Genetically hard-wired neural mechanisms must enforce behavioral reproductive isolation because interspecies courtship is rare even in sexually naïve animals of most species. We find that the chemoreceptor Gr32a inhibits male D. melanogaster from courting diverse fruit fly species. Gr32a recognizes nonvolatile aversive cues present on these reproductively dead-end targets, and activity of Gr32a neurons is necessary and sufficient to inhibit interspecies courtship. Male-specific Fruitless (Fru(M)), a master regulator of courtship, also inhibits interspecies courtship. Gr32a and Fru(M) are not coexpressed, but Fru(M) neurons contact Gr32a neurons, suggesting that these genes influence a shared neural circuit that inhibits interspecies courtship. Gr32a and Fru(M) also suppress within-species intermale courtship, but we show that distinct mechanisms preclude sexual displays toward conspecific males and other species. Although this chemosensory pathway does not inhibit interspecies mating in D. melanogaster females, similar mechanisms appear to inhibit this behavior in many other male drosophilids.

From the ultrasonic to the infrared: molecular evolution and the sensory biology of bats

Great advances have been made recently in understanding the genetic basis of the sensory biology of bats. Research has focused on the molecular evolution of candidate sensory genes, genes with known functions [e.g., olfactory receptor (OR) genes] and genes identified from mutations associated with sensory deficits (e.g., blindness and deafness). For example, the FoxP2 gene, underpinning vocal behavior and sensorimotor coordination, has undergone diversification in bats, while several genes associated with audition show parallel amino acid substitutions in unrelated lineages of echolocating bats and, in some cases, in echolocating dolphins, representing a classic case of convergent molecular evolution. Vision genes encoding the photopigments rhodopsin and the long-wave sensitive opsin are functional in bats, while that encoding the short-wave sensitive opsin has lost functionality in rhinolophoid bats using high-duty cycle laryngeal echolocation, suggesting a sensory trade-off between investment in vision and echolocation. In terms of olfaction, bats appear to have a distinctive OR repertoire compared with other mammals, and a gene involved in signal transduction in the vomeronasal system has become non-functional in most bat species. Bitter taste receptors appear to have undergone a "birth-and death" evolution involving extensive gene duplication and loss, unlike genes coding for sweet and umami tastes that show conservation across most lineages but loss in vampire bats. Common vampire bats have also undergone adaptations for thermoperception, via alternative splicing resulting in the evolution of a novel heat-sensitive channel. The future for understanding the molecular basis of sensory biology is promising, with great potential for comparative genomic analyses, studies on gene regulation and expression, exploration of the role of alternative splicing in the generation of proteomic diversity, and linking genetic mechanisms to behavioral consequences.

TRPA1 and TRPV1 are differentially involved in heat nociception of mice

BACKGROUND

Two transient receptor potential (TRP) channels, TRPV1 and TRPA1, have been physiologically studied with regard to noxious heat transduction. Evidence argues against these channels as sole transducers of noxious heat or cold, respectively. Moreover, in submammalian species the TRPA1 orthologue shows heat sensitivity.

METHODS

In vitro, single-fibre and compound action potential recordings from C-fibres as well as measurements of stimulated cutaneous CGRP release are combined with behavioural experiments to assess heat responsiveness in wild type mice, TRPA1 and TRPV1 as well as double-null mutants.

RESULTS

Heat thresholds of cutaneous C-mechano-heat sensitive fibres were significantly higher in TRPA1-/- (43 °C) than +/+ (40 °C) mice, and averaged heat responses were clearly weaker, whereas TRPV1-/- showed normal heat thresholds and responses (up to 46 °C). Compound action potential recordings revealed much less activity-dependent slowing of conduction velocity upon noxious heat stimulation in TRPA1-/- and a delayed deficit in TRPV1-/- in comparison to controls. Heat-induced calcitonin gene-related peptide release was reduced in TRPV1-/- but not TRPA1-/- animals. Paw withdrawal latencies to radiant heat were significantly elevated in TRPA1-/-, more so in TRPV1-/- animals. In general, double-null mutants were similar to TRPV1-/- except for the single-fibre heat responses which appeared as weak as in TRPA1-/-.

CONCLUSIONS

Our results indicate that in addition to TRPV1, TRPA1 plays a role in heat nociception, in particular in definition of the heat threshold, and might therefore serve as a therapeutic target in acute inflammatory pain.

The zebrafish ortholog of TRPV1 is required for heat-induced locomotion

The ability to detect hot temperatures is critical to maintaining body temperature and avoiding injury in diverse animals from insects to mammals. Zebrafish embryos, when given a choice, actively avoid hot temperatures and display an increase in locomotion similar to that seen when they are exposed to noxious compounds such as mustard oil. Phylogenetic analysis suggests that the single zebrafish ortholog of TRPV1/2 may have arisen from an evolutionary precursor of the mammalian TRPV1 and TRPV2. As opposed to TRPV2, mammalian TRPV1 is essential for environmentally relevant heat sensation. In the present study, we provide evidence that the zebrafish TRPV1 ion channel is also required for the sensation of heat. Contrary to development in mammals, zebrafish TRPV1(+) neurons arise during the first wave of somatosensory neuron development, suggesting a vital importance of thermal sensation in early larval survival. In vitro analysis showed that zebrafish TRPV1 acts as a molecular sensor of environmental heat (≥25°C) that is distinctly lower than the sensitivity of the mammalian form (≥42°C) but consistent with thresholds measured in behavioral assays. Using in vivo calcium imaging with the genetically encoded calcium sensor GCaMP3, we show that TRPV1-expressing trigeminal neurons are activated by heat at behaviorally relevant temperatures. Using knock-down studies, we also show that TRPV1 is required for normal heat-induced locomotion. Our results demonstrate for the first time an ancient role for TRPV1 in the direct sensation of environmental heat and show that heat sensation is adapted to reflect species-dependent requirements in response to environmental stimuli.

How keeping active pays off in the olfactory system

A protein that is found in the main olfactory epithelium of mice ensures that odour-sensing neurons that are active to have longer lifespans than those that are inactive.

Residues in the pore region of Drosophila transient receptor potential A1 dictate sensitivity to thermal stimuli

The capacity to sense temperature is essential for the survival of all animals. At the molecular level, ion channels belonging to the transient receptor potential (TRP) family of channels function as temperature sensors in animals across several phyla. TRP channels are opened directly by changes in temperature and show pronounced sensitivity at their activation range. To determine how temperature activates these channels, we analysed channels belonging to the TRPA family, which detect heat in insects and cold in mammals. By constructing chimeric proteins consisting of human and Drosophila TRPA1 channels, we mapped regions that regulate thermal activation and identified residues in the pore helix that invert temperature sensitivity of TRPA1. From analysis of individual channels we defined the gating reaction of Drosophila TRPA1 and determined how mutagenesis alters the energy landscape for channel opening. Our results reveal specific molecular requirements for thermal activation of TRPA1 and provide mechanistic insight into this process.

Current perspectives on the modulation of thermo-TRP channels: new advances and therapeutic implications

The thermo transient receptor potential (TRP) ion channels, a recently discovered family of ion channels activated by temperature, are expressed in primary sensory nerve terminals, where they provide information regarding thermal changes in the environment. Six thermo-TRPs have been characterized to date: TRPV1-4, which respond to different levels of warmth and heat, and TRPM8 and TRPA1, which respond to cool temperatures. We review the current state of knowledge of thermo-TRPs, and of the modulation of their thermal thresholds by a range of inflammatory mediators. Blockers of these channels are likely to have therapeutic uses as novel analgesics but may also cause unacceptable side effects. Controlling the modulation of thermo-TRPs by inflammatory mediators may be a useful alternative strategy in developing novel analgesics.

Snakes antibodies

Immunoglobulins are basic molecules of the immune system of vertebrates. In previous studies we described the immunoglobulins found in two squamata reptiles, Anolis carolinensis and Eublepharis macularius. Snakes are squamata reptiles too but they have undergone an extreme evolutionary process. We therefore wanted to know how these changes affected their immunoglobulin coding genes. To perform this analysis we studied five snake transcriptomes and two genome draft sequences. Sequences coding for immunoglobulin M (IgM), immunoglobulin D (IgD) and two classes of immunoglobulin Y (IgY - named IgYa and IgYb-) were found in all of them. Moreover, the Thamnophis elegans transcriptome and Python molurus genome draft sequences showed a third class of IgY, the IgYc, whose constant region only presents three domains and lacks the CH2. All data suggest that the IgYb is the evolutionary origin of this IgYc. An exhaustive search of the light chains were carried out, being lambda the only light chain found in snakes. The results provide a clear picture of the immunoglobulins present in the suborder Serpentes.

Varieties of behavioral natural variation

Behavior is flexible at different timescales, modifiable by experience in the short term and by evolution in the long term. In order to understand how behavior evolves, we must both understand how trait differences between individuals are inherited and how a subset of these differences get fixed within a species' lineage. Work over the past few decades has shown that this will not be easy; the genetic basis of heritable behavioral differences between two individuals is typically complex, caused by multiple genetic variants of small effect. Here I describe how the underlying genetic networks impact the types of genetic variants that can be selected for by evolution.

Analytical methods for the geometric optics of thermal vision illustrated with four species of pitvipers

The pitviper facial pit is a pinhole camera-like sensory organ consisting of a flask-shaped cavity divided into two chambers by a suspended membrane. Neurophysiological studies and simplified optical models suggest that facial pits detect thermal radiation and form an image that is combined with visual input in the optic tectum to form a single multispectral image. External pit anatomy varies markedly among taxonomic groups. However, optical function depends on unknown internal anatomy. Therefore, we developed methods for relating anatomy to optical performance. To illustrate, we constructed detailed anatomical models of the internal anatomy of the facial pits of four individuals of four pitviper species using X-ray tomography sections of fresh material. We used these models to define the point spread function, i.e. the distribution of radiation from a point source over the pit membrane, for each species. We then used optical physics, heat transfer physics and computational image processing to define the thermal image formed on the pit membrane for each species. Our computed pit membrane images are consistent with behavioral observations if the sensitivity of membrane receptors equals the most sensitive (ca. 0.001°C) laboratory estimates. Vignetting (variation in optical aperture size with view angle) and differences between body and environmental temperatures can create temperature variation across the membrane that greatly exceeds image temperature contrasts, potentially impairing imaging. Spread functions plotted versus source point azimuth and elevation show distinct patterns that suggest new research directions into the relationships among the optical anatomy, ecology, behavior and sensory neurophysiology of pitvipers.

Directional sensitivity in the thermal response of the facial pit in western diamondback rattlesnakes (Crotalus atrox)

Recent work published in the accompanying paper used a combination of 3D morphological reconstruction to define optical spread functions and heat transfer physics to study how external heat energy would reach the sensory membrane within the facial pit of pitvipers. The results from all of the species examined indicated asymmetric directional sensitivity, e.g. the pit would preferentially respond to stimuli located below and behind the snake. The present study was intended as a test of these findings through a quantitative neurophysiological analysis of directional sensitivity in the facial pit of the western diamondback rattlesnake, Crotalus atrox. An infrared emitter was positioned through a coordinate system (with varying angular orientations and distances) and the response it evoked measured through neurophysiological recordings of a trigeminal nerve branch composed of the afferents from the sensory membrane of the facial pit. Significant differences were found in the strength of the membrane's neural response to a constant stimulus presented at different orientations (relative to the facial pit opening) and over different distances. The peak sensitivity (at 12 deg above and 20 deg in front of the facial pit opening) was in good agreement with the predicted directional sensitivities based on optical spread functions and 3D topography. These findings support the hypothesis that the topography, and functional performance, of the facial pit has undergone an adaptive radiation within the pit vipers, and that differences in the behavioral ecology of the pit vipers (i.e. terrestrial versus arboreal) are reflected within the facial pits.

Analysis of transient receptor potential ankyrin 1 (TRPA1) in frogs and lizards illuminates both nociceptive heat and chemical sensitivities and coexpression with TRP vanilloid 1 (TRPV1) in ancestral vertebrates

Transient receptor potential ankyrin 1 (TRPA1) and TRP vanilloid 1 (V1) perceive noxious temperatures and chemical stimuli and are involved in pain sensation in mammals. Thus, these two channels provide a model for understanding how different genes with similar biological roles may influence the function of one another during the course of evolution. However, the temperature sensitivity of TRPA1 in ancestral vertebrates and its evolutionary path are unknown as its temperature sensitivities vary among different vertebrate species. To elucidate the functional evolution of TRPA1, TRPA1s of the western clawed (WC) frogs and green anole lizards were characterized. WC frog TRPA1 was activated by heat and noxious chemicals that activate mammalian TRPA1. These stimuli also activated native sensory neurons and elicited nocifensive behaviors in WC frogs. Similar to mammals, TRPA1 was functionally co-expressed with TRPV1, another heat- and chemical-sensitive nociceptive receptor, in native sensory neurons of the WC frog. Green anole TRPA1 was also activated by heat and noxious chemical stimulation. These results suggest that TRPA1 was likely a noxious heat and chemical receptor and co-expressed with TRPV1 in the nociceptive sensory neurons of ancestral vertebrates. Conservation of TRPV1 heat sensitivity throughout vertebrate evolution could have changed functional constraints on TRPA1 and influenced the functional evolution of TRPA1 regarding temperature sensitivity, whereas conserving its noxious chemical sensitivity. In addition, our results also demonstrated that two mammalian TRPA1 inhibitors elicited different effect on the TRPA1s of WC frogs and green anoles, which can be utilized to clarify the structural bases for inhibition of TRPA1.

Metabolic and thermal stimuli control K(2P)2.1 (TREK-1) through modular sensory and gating domains

The two-pore domain potassium channel K_2P2.1 (TREK-1) responds to extracellular and intracellular stimuli, including pH and temperature. This study elucidates how the intracellular sensor element relays metabolic and thermal stimuli to the extracellular C-type gating element.

K_2P2.1 (TREK-1) is a polymodal two-pore domain leak potassium channel that responds to external pH, GPCR-mediated phosphorylation signals, and temperature through the action of distinct sensors within the channel. How the various intracellular and extracellular sensory elements control channel function remains unresolved. Here, we show that the K_2P2.1 (TREK-1) intracellular C-terminal tail (Ct), a major sensory element of the channel, perceives metabolic and thermal commands and relays them to the extracellular C-type gate through transmembrane helix M4 and pore helix 1. By decoupling Ct from the pore-forming core, we further demonstrate that Ct is the primary heat-sensing element of the channel, whereas, in contrast, the pore domain lacks robust temperature sensitivity. Together, our findings outline a mechanism for signal transduction within K_2P2.1 (TREK-1) in which there is a clear crosstalk between the C-type gate and intracellular Ct domain. In addition, our findings support the general notion of the existence of modular temperature-sensing domains in temperature-sensitive ion channels. This marked distinction between gating and sensory elements suggests a general design principle that may underlie the function of a variety of temperature-sensitive channels.

The dynamic TRPA1 channel: a suitable pharmacological pain target?

Acute pain detection is vital to navigate and survive in one's environment. Protection and preservation occur because primary afferent nociceptors transduce adverse environmental stimuli into electrical impulses that are transmitted to and interpreted within high levels of the central nervous system. Therefore, it is critical that the molecular mechanisms that convert noxious information into neural signals be identified, and their specific functional roles delineated in both acute and chronic pain settings. The Transient Receptor Potential (TRP) channel family member TRP ankyrin 1 (TRPA1) is an excellent candidate molecule to explore and intricately understand how single channel properties can tailor behavioral nociceptive responses. TRPA1 appears to dynamically respond to an amazingly wide range of diverse stimuli that include apparently unrelated modalities such as mechanical, chemical and thermal stimuli that activate somatosensory neurons. How such dissimilar stimuli activate TRPA1, yet result in modality-specific signals to the CNS is unclear. Furthermore, TRPA1 is also involved in persistent to chronic painful states such as inflammation, neuropathic pain, diabetes, fibromyalgia, bronchitis and emphysema. Yet how TRPA1's role changes from an acute sensor of physical stimuli to its contribution to these diseases that are concomitant with implacable, chronic pain is unknown. TRPA1's involvement in the nociceptive machinery that relays the adverse stimuli during painful disease states is of considerable interest for drug delivery and design by many pharmaceutical entities. In this review, we will assess the current knowledge base of TRPA1 in acute nociception and persistent inflammatory pain states, and explore its potential as a therapeutic pharmacological target in chronic pervasive conditions such neuropathic pain, persistent inflammation and diabetes.

Conservation of tubulin-binding sequences in TRPV1 throughout evolution

Background

Transient Receptor Potential Vanilloid sub type 1 (TRPV1), commonly known as capsaicin receptor can detect multiple stimuli ranging from noxious compounds, low pH, temperature as well as electromagnetic wave at different ranges. In addition, this receptor is involved in multiple physiological and sensory processes. Therefore, functions of TRPV1 have direct influences on adaptation and further evolution also. Availability of various eukaryotic genomic sequences in public domain facilitates us in studying the molecular evolution of TRPV1 protein and the respective conservation of certain domains, motifs and interacting regions that are functionally important.

Methodology and Principal Findings

Using statistical and bioinformatics tools, our analysis reveals that TRPV1 has evolved about ∼420 million years ago (MYA). Our analysis reveals that specific regions, domains and motifs of TRPV1 has gone through different selection pressure and thus have different levels of conservation. We found that among all, TRP box is the most conserved and thus have functional significance. Our results also indicate that the tubulin binding sequences (TBS) have evolutionary significance as these stretch sequences are more conserved than many other essential regions of TRPV1. The overall distribution of positively charged residues within the TBS motifs is conserved throughout evolution. In silico analysis reveals that the TBS-1 and TBS-2 of TRPV1 can form helical structures and may play important role in TRPV1 function.

Conclusions and Significance

Our analysis identifies the regions of TRPV1, which are important for structure – function relationship. This analysis indicates that tubulin binding sequence-1 (TBS-1) near the TRP-box forms a potential helix and the tubulin interactions with TRPV1 via TBS-1 have evolutionary significance. This interaction may be required for the proper channel function and regulation and may also have significance in the context of Taxol®-induced neuropathy.

Warmth suppresses and desensitizes damage-sensing ion channel TRPA1

Background

Acute or chronic tissue damage induces an inflammatory response accompanied by pain and alterations in local tissue temperature. Recent studies revealed that the transient receptor potential A1 (TRPA1) channel is activated by a wide variety of substances that are released following tissue damage to evoke nociception and neurogenic inflammation. Although the effects of a noxious range of cold temperatures on TRPA1 have been rigorously studied, it is not known how agonist-induced activation of TRPA1 is regulated by temperature over an innocuous range centred on the normal skin surface temperature. This study investigated the effect of temperature on agonist-induced currents in human embryonic kidney (HEK) 293 cells transfected with rat or human TRPA1 and in rat sensory neurons.

Results

Agonist-induced TRPA1 currents in HEK293 cells were strongly suppressed by warm temperatures, and almost abolished at 39°C. Such inhibition occurred when TRPA1 was activated by either electrophilic or non-electrophilic agonists. Warming not only decreased the apparent affinity of TRPA1 for mustard oil (MO), but also greatly enhanced the desensitization and tachyphylaxis of TRPA1. Warming also attenuated MO-induced ionic currents in sensory neurons. These results suggest that the extent of agonist-induced activity of TRPA1 may depend on surrounding tissue temperature, and local hyperthermia during acute inflammation could be an endogenous negative regulatory mechanism to attenuate persistent pain at the site of injury.

Conclusion

These results indicate that warmth suppresses and desensitizes damage-sensing ion channel TRPA1. Such warmth-induced suppression of TRPA1 may also explain, at least in part, the mechanistic basis of heat therapy that has been widely used as a supplemental anti-nociceptive approach.