Molecular Prerequisites for Diminished Cold Sensitivity in Ground Squirrels and Hamsters

Implications of TRPM3 and TRPM8 for sensory neuron sensitisation

Sensory neurons serve to receive and transmit a wide range of information about the conditions of the world around us as well as the external and internal state of our body. Sensitisation of these nerve cells, i.e. becoming more sensitive to stimuli or the emergence or intensification of spontaneous activity, for example in the context of inflammation or nerve injury, can lead to chronic diseases such as neuropathic pain. For many of these disorders there are only very limited treatment options and in order to find and establish new therapeutic approaches, research into the exact causes of sensitisation with the elucidation of the underlying mechanisms and the identification of the molecular components is therefore essential. These components include plasma membrane receptors and ion channels that are involved in signal reception and transmission. Members of the transient receptor potential (TRP) channel family are also expressed in sensory neurons and some of them play a crucial role in temperature perception. This review article focuses on the heat-sensitive TRPM3 and the cold-sensitive TRPM8 (and TRPA1) channels and their importance in sensitisation of dorsal root ganglion sensory neurons is discussed based on studies related to inflammation and injury- as well as chemotherapy-induced neuropathy.

Temperature-responsive hydrogel-grafted vessel-on-a-chip: Exploring cold-induced endothelial injury

Cold-induced vasoconstriction is a significant contributor that leads to chilblains and hypothermia in humans. However, current animal models have limitations in replicating cold-induced acral injury due to their low sensitivity to cold. Moreover, existing in vitro vascular chips composed of endothelial cells and perfusion systems lack temperature responsiveness, failing to simulate the vasoconstriction observed under cold stress. This study presents a novel approach where a microfluidic bioreactor of vessel-on-a-chip was developed by grafting the inner microchannel surface of polydimethylsiloxane with a thermosensitive hydrogel skin composed of N-isopropyl acrylamide and gelatin methacrylamide. With a lower critical solution temperature set at 30°C, the gel layer exhibited swelling at low temperatures, reducing the flow rate inside the channel by 10% when the temperature dropped from 37°C to 4°C. This well mimicked the blood stasis observed in capillary vessels in vivo. The vessel-on-a-chip was further constructed by culturing endothelial cells on the surface of the thermosensitive hydrogel layer, and a perfused medium was introduced to the cells to provide a physiological shear stress. Notably, cold stimulation of the vessel-on-a-chip led to cell necrosis, mitochondrial membrane potential (ΔΨm) collapse, cytoskeleton disaggregation, and increased levels of reactive oxygen species. In contrast, the static culture of endothelial cells showed limited response to cold exposure. By faithfully replicating cold-induced endothelial injury, this groundbreaking thermosensitive vessel-on-a-chip technology offers promising advancements in the study of cold-induced cardiovascular diseases, including pathogenesis and therapeutic drug screening.

Subcutaneous administration of a novel TRPM8 antagonist reverses cold hypersensitivity while attenuating the drop in core body temperature

BACKGROUND AND PURPOSE

We extend the characterization of the TRPM8 antagonist VBJ103 with tests of selectivity, specificity and distribution, therapeutic efficacy of systemic administration against oxaliplatin-induced cold hyperalgesia and the impact of systemic administration on core body temperature (CBT).

EXPERIMENTAL APPROACH

Selectivity at human TRPA1 and TRPV1 as well as in vitro safety profiling was determined. Effects of systemic administration of VBJ103 were evaluated in a model of oxaliplatin-induced cold hyperalgesia. Both peripheral and centrally mediated effects of VBJ103 on CBT were assessed with radiotelemetry.

KEY RESULTS

VBJ103 had no antagonist activity at TRPV1 and TRPA1, but low potency TRPA1 activation. The only safety liability detected was partial inhibition of the dopamine transporter (DAT). VBJ103 delivered subcutaneously dose-dependently attenuated cold hypersensitivity in oxaliplatin-treated mice at 3, 10 and 30 mg·kg-1 (n = 7, P < 0.05). VBJ103 (30 mg·kg-1) antinociception was influenced by neither the TRPA1 antagonist HC-030031 nor the DAT antagonist GBR12909. Subcutaneous administration of VBJ103 (3, 10 and 30 mg·kg-1, but not 100 or 300 mg·kg-1, n = 7) decreased CBT (2°C). Intraperitoneal (i.p.) administration of VBJ103 (3, 10 and 30 mg·kg-1) dose-dependently decreased CBT to an extent larger than that detected with subcutaneous administration. Intracerebroventricular (i.c.v.) administration (306 nmol/1 μL; n = 5) did not alter CBT.

CONCLUSIONS AND IMPLICATIONS

We achieve therapeutic efficacy with subcutaneous administration of a novel TRPM8 antagonist that attenuates deleterious influences on CBT, a side effect that has largely prevented the translation of TRPM8 as a target.

Evidence that the cold- and menthol-sensing functions of the human TRPM8 channel evolved separately

Transient receptor potential melastatin 8 (TRPM8) is a temperature- and menthol-sensitive ion channel that contributes to diverse physiological roles, including cold sensing and pain perception. Clinical trials targeting TRPM8 have faced repeated setbacks predominantly due to the knowledge gap in unraveling the molecular underpinnings governing polymodal activation. A better understanding of the molecular foundations between the TRPM8 activation modes may aid the development of mode-specific, thermal-neutral therapies. Ancestral sequence reconstruction was used to explore the origins of TRPM8 activation modes. By resurrecting key TRPM8 nodes along the human evolutionary trajectory, we gained valuable insights into the trafficking, stability, and function of these ancestral forms. Notably, this approach unveiled the differential emergence of cold and menthol sensitivity over evolutionary time, providing a fresh perspective on complex polymodal behavior. These studies provide a paradigm for understanding polymodal behavior in TRPM8 and other proteins with the potential to enhance our understanding of sensory receptor biology and pave the way for innovative therapeutic interventions.

Cold resistance of mammalian hibernators ∼ a matter of ferroptosis?

Most mammals adapt thermal physiology around 37°C and large deviations from their range, as observed in severe hypothermia and hyperthermia, resulting in organ dysfunction and individual death. A prominent exception is mammalian hibernation. Mammalian hibernators resist the long-term duration of severe low body temperature that is lethal to non-hibernators, including humans and mice. This cold resistance is supported, at least in part, by intrinsic cellular properties, since primary or immortalized cells from several hibernator species can survive longer than those from non-hibernators when cultured at cold temperatures. Recent studies have suggested that cold-induced cell death fulfills the hallmarks of ferroptosis, a type of necrotic cell death that accompanies extensive lipid peroxidation by iron-ion-mediated reactions. In this review, we summarize the current knowledge of cold resistance of mammalian hibernators at the cellular and molecular levels to organ and systemic levels and discuss key pathways that confer cold resistance in mammals.

A helping hand: roles for accessory cells in the sense of touch across species

During touch, mechanical forces are converted into electrochemical signals by tactile organs made of neurons, accessory cells, and their shared extracellular spaces. Accessory cells, including Merkel cells, keratinocytes, lamellar cells, and glia, play an important role in the sensation of touch. In some cases, these cells are intrinsically mechanosensitive; however, other roles include the release of chemical messengers, the chemical modification of spaces that are shared with neurons, and the tuning of neural sensitivity by direct physical contact. Despite great progress in the last decade, the precise roles of these cells in the sense of touch remains unclear. Here we review the known and hypothesized contributions of several accessory cells to touch by incorporating research from multiple organisms including C. elegans, D. melanogaster, mammals, avian models, and plants. Several broad parallels are identified including the regulation of extracellular ions and the release of neuromodulators by accessory cells, as well as the emerging potential physical contact between accessory cells and sensory neurons via tethers. Our broader perspective incorporates the importance of accessory cells to the understanding of human touch and pain, as well as to animal touch and its molecular underpinnings, which are underrepresented among the animal welfare literature. A greater understanding of touch, which must include a role for accessory cells, is also relevant to emergent technical applications including prosthetics, virtual reality, and robotics.

Across two continents: the genomic basis of environmental adaptation in house mice (Mus musculus domesticus) from the Americas

Parallel clines across environmental gradients can be strong evidence of adaptation. House mice (Mus musculus domesticus) were introduced to the Americas by European colonizers and are now widely distributed from Tierra del Fuego to Alaska. Multiple aspects of climate, such as temperature, vary predictably across latitude in the Americas. Past studies of North American populations across latitudinal gradients provided evidence of environmental adaptation in traits related to body size, metabolism, and behavior and identified candidate genes using selection scans. Here, we investigate genomic signals of environmental adaptation on a second continent, South America, and ask whether there is evidence of parallel adaptation across multiple latitudinal transects in the Americas. We first identified loci across the genome showing signatures of selection related to climatic variation in mice sampled across a latitudinal transect in South America, accounting for neutral population structure. Consistent with previous results, most candidate SNPs were in regulatory regions. Genes containing the most extreme outliers relate to traits such as body weight or size, metabolism, immunity, fat, and development or function of the eye as well as traits associated with the cardiovascular and renal systems. We then combined these results with published results from two transects in North America. While most candidate genes were unique to individual transects, we found significant overlap among candidate genes identified independently in the three transects, providing strong evidence of parallel adaptation and identifying genes that likely underlie recent environmental adaptation in house mice across North and South America.

Differential AMPK-mediated metabolic regulation observed in hibernation-style polymorphisms in Siberian chipmunks

Hibernation is a unique physiological phenomenon allowing extreme hypothermia in endothermic mammals. Hypometabolism and hypothermia tolerance in hibernating animals have been investigated with particular interest; recently, studies of cultured cells and manipulation of the nervous system have made it possible to reproduce physiological states related to hypothermia induction. However, much remains unknown about the periodic regulation of hibernation. In particular, the physiological mechanisms facilitating the switch from an active state to a hibernation period, including behavioral changes and the acquisition of hypothermia tolerance remain to be elucidated. AMPK is a protein known to play a central role not only in feeding behavior but also in metabolic regulation in response to starvation. Our previous research has revealed that chipmunks activate AMPK in the brain during hibernation. However, whether AMPK is activated during winter in non-hibernating animals is unknown. Previous comparative studies between hibernating and non-hibernating animals have often been conducted between different species, consequently it has been impossible to account for the effects of phylogenetic differences. Our long-term monitoring of siberian chipmunks, has revealed intraspecific variation between those individuals that hibernate annually and those that never become hypothermic. Apparent differences were found between hibernating and non-hibernating types with seasonal changes in lifespan and blood HP levels. By comparing seasonal changes in AMPK activity between these polymorphisms, we clarified the relationship between hibernation and AMPK regulation. In hibernating types, phosphorylation of p-AMPK and p-ACC was enhanced throughout the brain during hibernation, indicating that AMPK-mediated metabolic regulation is activated. In non-hibernating types, AMPK and ACC were not seasonally activated. In addition, AMPK activation in the hypothalamus had already begun during high Tb before hibernation. Changes in AMPK activity in the brain during hibernation may be driven by circannual rhythms, suggesting a hibernation-regulatory mechanism involving AMPK activation independent of Tb. The differences in brain AMPK regulation between hibernators and non-hibernators revealed in this study were based on a single species thus did not involve phylogenetic differences, thereby supporting the importance of brain temperature-independent AMPK activation in regulating seasonal metabolism in hibernating animals.

Ground squirrels - experts in thermoregulatory adaptation

Ground squirrels exemplify one of the most extreme forms of mammalian hibernation and a convenient model for studying its mechanisms. Their thermoregulatory system demonstrates remarkable adaptive capabilities by maintaining optimal levels of body temperature both in active and hibernation states. Here, we review recent findings and unresolved issues regarding the neural mechanisms of body temperature control in ground squirrels.

Molecular determinants of TRPM8 function: key clues for a cool modulation

Cold thermoreceptor neurons detect temperature drops with highly sensitive molecular machinery concentrated in their peripheral free nerve endings. The main molecular entity responsible for cold transduction in these neurons is the thermo-TRP channel TRPM8. Cold, cooling compounds such as menthol, voltage, and osmolality rises activate this polymodal ion channel. Dysregulation of TRPM8 activity underlies several physiopathological conditions, including painful cold hypersensitivity in response to axonal damage, migraine, dry-eye disease, overactive bladder, and several forms of cancer. Although TRPM8 could be an attractive target for treating these highly prevalent diseases, there is still a need for potent and specific modulators potentially suitable for future clinical trials. This goal requires a complete understanding of the molecular determinants underlying TRPM8 activation by chemical and physical agonists, inhibition by antagonists, and the modulatory mechanisms behind its function to guide future and more successful treatment strategies. This review recapitulates information obtained from different mutagenesis approaches that have allowed the identification of specific amino acids in the cavity comprised of the S1-S4 and TRP domains that determine modulation by chemical ligands. In addition, we summarize different studies revealing specific regions within the N- and C-terminus and the transmembrane domain that contribute to cold-dependent TRPM8 gating. We also highlight the latest milestone in the field: cryo-electron microscopy structures of TRPM8, which have provided a better comprehension of the 21 years of extensive research in this ion channel, shedding light on the molecular bases underlying its modulation, and promoting the future rational design of novel drugs to selectively regulate abnormal TRPM8 activity under pathophysiological conditions.

Latent generative landscapes as maps of functional diversity in protein sequence space

Variational autoencoders are unsupervised learning models with generative capabilities, when applied to protein data, they classify sequences by phylogeny and generate de novo sequences which preserve statistical properties of protein composition. While previous studies focus on clustering and generative features, here, we evaluate the underlying latent manifold in which sequence information is embedded. To investigate properties of the latent manifold, we utilize direct coupling analysis and a Potts Hamiltonian model to construct a latent generative landscape. We showcase how this landscape captures phylogenetic groupings, functional and fitness properties of several systems including Globins, β-lactamases, ion channels, and transcription factors. We provide support on how the landscape helps us understand the effects of sequence variability observed in experimental data and provides insights on directed and natural protein evolution. We propose that combining generative properties and functional predictive power of variational autoencoders and coevolutionary analysis could be beneficial in applications for protein engineering and design.

Molecular mechanisms of adaptive evolution in wild animals and plants

Wild animals and plants have developed a variety of adaptive traits driven by adaptive evolution, an important strategy for species survival and persistence. Uncovering the molecular mechanisms of adaptive evolution is the key to understanding species diversification, phenotypic convergence, and inter-species interaction. As the genome sequences of more and more non-model organisms are becoming available, the focus of studies on molecular mechanisms of adaptive evolution has shifted from the candidate gene method to genetic mapping based on genome-wide scanning. In this study, we reviewed the latest research advances in wild animals and plants, focusing on adaptive traits, convergent evolution, and coevolution. Firstly, we focused on the adaptive evolution of morphological, behavioral, and physiological traits. Secondly, we reviewed the phenotypic convergences of life history traits and responding to environmental pressures, and the underlying molecular convergence mechanisms. Thirdly, we summarized the advances of coevolution, including the four main types: mutualism, parasitism, predation and competition. Overall, these latest advances greatly increase our understanding of the underlying molecular mechanisms for diverse adaptive traits and species interaction, demonstrating that the development of evolutionary biology has been greatly accelerated by multi-omics technologies. Finally, we highlighted the emerging trends and future prospects around the above three aspects of adaptive evolution.

The role of TRPA1 channels in thermosensation

Transient receptor potential ankyrin 1 (TRPA1) is a polymodal nonselective cation channel sensitive to different physical and chemical stimuli. TRPA1 is associated with many important physiological functions in different species and thus is involved in different degrees of evolution. TRPA1 acts as a polymodal receptor for the perceiving of irritating chemicals, cold, heat, and mechanical sensations in various animal species. Numerous studies have supported many functions of TRPA1, but its temperature-sensing function remains controversial. Although TRPA1 is widely distributed in both invertebrates and vertebrates, and plays a crucial role in tempreture sensing, the role of TRPA1 thermosensation and molecular temperature sensitivity are species-specific. In this review, we summarize the temperature-sensing role of TRPA1 orthologues in terms of molecular, cellular, and behavioural levels.

Selection-driven adaptation to the extreme Antarctic environment in the Emperor penguin

The eco-evolutionary history of penguins is characterised by shifting from temperate to cold environments. Breeding in Antarctica, the Emperor penguin appears as an extreme outcome of this process, with unique features related to insulation, heat production and energy management. However, whether this species actually diverged from a less cold-adapted ancestor, more ecologically similar to its sister species, the King penguin, is still an open question. As the Antarctic colonisation likely resulted in vast changes in selective pressure experienced by the Emperor penguin, the relative quantification of the genomic signatures of selection, unique to each sister species, could answer this question. Applying phylogeny-based selection tests on 7651 orthologous genes, we identified a more pervasive selection shift in the Emperor penguin than in the King penguin, supporting the hypothesis that its extreme cold adaptation is a derived state. Furthermore, among candidate genes under selection, four (TRPM8, LEPR, CRB1, and SFI1) were identified before in other cold-adapted homeotherms, like the woolly Mammoth, while other 161 genes can be assigned to biological functions relevant to cold adaptation identified in previous studies. Location and structural effects of TRPM8 substitutions in Emperor and King penguin lineages support their functional role with putative divergent effects on thermal adaptation. We conclude that extreme cold adaptation in the Emperor penguin largely involved unique genetic options which, however, affect metabolic and physiological traits common to other cold-adapted homeotherms.

Mutations of TRPM8 channels: Unraveling the molecular basis of activation by cold and ligands

The cation nonselective channel TRPM8 is activated by multiple stimuli, including moderate cold and various chemical compounds (i.e., menthol and icilin [Fig. 1], among others). While research continues growing on the understanding of the physiological involvement of TRPM8 channels and their role in various pathological states, the information available on its activation mechanisms has also increased, supported by mutagenesis and structural studies. This review compiles known information on specific mutations of channel residues and their consequences on channel viability and function. Besides, the comparison of sequence of animals living in different environments, together with chimera and mutagenesis studies are helping to unravel the mechanism of adaptation to different temperatures. The results of mutagenesis studies, grouped by different channel regions, are compared with the current knowledge of TRPM8 structures obtained by cryo-electron microscopy. Trying to make this review self-explicative and highly informative, important residues for TRPM8 function are summarized in a figure, and mutants, deletions and chimeras are compiled in a table, including also the observed effects by different methods of activation and the corresponding references. The information provided by this review may also help in the design of new ligands for TRPM8, an interesting biological target for therapeutic intervention.

A cross-taxonomic perspective on the integration of temperature cues in vertebrate seasonal neuroendocrine pathways

The regulation of seasonality has been an area of interest for decades, yet global climate change has created extra urgency in the quest to understand how sensory circuits and neuroendocrine control systems interact to generate flexibility in biological timekeeping. The capacity of temperature to alter endogenous or photoperiod-regulated neuroendocrine mechanisms driving seasonality, either as a direct cue or through temperature-dependent effects on energy and metabolism, is at the heart of this phenological flexibility. However, until relatively recently, little research had been done on the integration of temperature information in canonical seasonal neuroendocrine pathways, particularly in vertebrates. We review recent advances from research in vertebrates that deepens our understanding of how temperature cues are perceived and integrated into seasonal hypothalamic thyroid hormone (TH) signaling, which is a critical regulator of downstream seasonal phenotypic changes such as those regulated by the BPG (brain-pituitary-gonadal) axis. Temperature perception occurs through cutaneous transient receptor potential (TRP) neurons, though sensitivity of these neurons varies markedly across taxa. Although photoperiod is the dominant cue used to trigger seasonal physiology or entrain circannual clocks, across birds, mammals, fish, reptiles and amphibians, seasonality appears to be temperature sensitive and in at least some cases this appears to be related to phylogenetically conserved TH signaling in the hypothalamus. Nevertheless, the exact mechanisms through which temperature modulates seasonal neuroendocrine pathways remains poorly understood.

Sensory TRP Channels in Three Dimensions

Transient receptor potential (TRP) ion channels are sophisticated signaling machines that detect a wide variety of environmental and physiological signals. Every cell in the body expresses one or more members of the extended TRP channel family, which consists of over 30 subtypes, each likely possessing distinct pharmacological, biophysical, and/or structural attributes. While the function of some TRP subtypes remains enigmatic, those involved in sensory signaling are perhaps best characterized and have served as models for understanding how these excitatory ion channels serve as polymodal signal integrators. With the recent resolution revolution in cryo-electron microscopy, these and other TRP channel subtypes are now yielding their secrets to detailed atomic analysis, which is beginning to reveal structural underpinnings of stimulus detection and gating, ion permeation, and allosteric mechanisms governing signal integration. These insights are providing a framework for designing and evaluating modality-specific pharmacological agents for treating sensory and other TRP channel-associated disorders.

Ground squirrels

Pra et al. provide an overview of ground squirrels and the physiological adaptations these animals have evolved to contend with harsh climates.

The acquisition of cold sensitivity during TRPM8 ion channel evolution

SignificanceAdaptation to more severe ambient temperature fluctuations can be considered one of the key innovations of terrestrial tetrapods. Our study shows the formation of the functional MHR1-3 domain in transient receptor potential melastatin 8 (TRPM8) bestowed the channel with cold sensitivity during the water-to-land transition. The evolved MHR1-3 domain found in terrestrial tetrapods serves as an independent apparatus with cold sensitivity. Furthermore, this domain with independent cold sensitivity is necessary for the regulatory mechanism of the pore domain, where the efficacy of cold activation is largely altered by evolutionary tuning of the hydrophobicity of several residues during the diversification of terrestrial tetrapods. Our findings advance the understanding of cold-sensing emergence during evolution and the thermodynamic basis of TRPM8 cold activation.

Glutamate-aspartate transporter 1 attenuates oxygen-glucose deprivation-induced injury by promoting glutamate metabolism in primary cortical neurons

Ischemic stroke is a common cerebral disease. However, the treatment for the disease is limited. Daurian ground squirrel (GS; Spermophilus dauricus), a hibernating mammalian species, is highly tolerant to ischemia. In the present study, GS neurons in a non-hibernating state were found to be more resistant to oxygen-glucose deprivation (OGD), an ischemic model in vitro. We leveraged the differences in the endurance capacity of GS and rats to investigate the mechanisms of resistance to ischemia in GS neurons. We first identified glutamate-aspartate transporter 1 (GLAST) as a cytoprotective factor that contributed to tolerance against OGD injury of GS neurons. The expression of GLAST in GS neurons was much higher than that in rat neurons. Overexpression of GLAST rescued viability in rat neurons, and GS neurons exhibited decreased viability following GLAST knockdown under OGD conditions. Mechanistically, more glutamate was transported into neurons after GLAST overexpression and served as substrates for ATP production. Furthermore, eukaryotic transcription initiation factor 4E binding protein 1 was downregulated by GLAST to rescue neuronal viability. Our findings not only revealed an important molecular mechanism underlying the survival of hibernating mammals but also suggested that neuronal GLAST may be a potential target for ischemic stroke therapy.

Towards understanding the neural origins of hibernation

Hibernators thrive under harsh environmental conditions instead of initiating canonical behavioral and physiological responses to promote survival. Although the physiological changes that occur during hibernation have been comprehensively researched, the role of the nervous system in this process remains relatively underexplored. In this Review, we adopt the perspective that the nervous system plays an active, essential role in facilitating and supporting hibernation. Accumulating evidence strongly suggests that the hypothalamus enters a quiescent state in which powerful drives to thermoregulate, eat and drink are suppressed. Similarly, cardiovascular and pulmonary reflexes originating in the brainstem are altered to permit the profoundly slow heart and breathing rates observed during torpor. The mechanisms underlying these changes to the hypothalamus and brainstem are not currently known, but several neuromodulatory systems have been implicated in the induction and maintenance of hibernation. The intersection of these findings with modern neuroscience approaches, such as optogenetics and in vivo calcium imaging, has opened several exciting avenues for hibernation research.

OUP accepted manuscript

Animals rely on their sensory systems to inform them of ecologically relevant environmental variation. In the Southern Ocean, the thermal environment has remained between −1.9 and 5 °C for 15 Myr, yet we have no knowledge of how an Antarctic marine organism might sense their thermal habitat as we have yet to discover a thermosensitive ion channel that gates (opens/closes) below 10 °C. Here, we investigate the evolutionary dynamics of transient receptor potential (TRP) channels, which are the primary thermosensors in animals, within cryonotothenioid fishes—the dominant fish fauna of the Southern Ocean. We found cryonotothenioids have a similar complement of TRP channels as other teleosts (∼28 genes). Previous work has shown that thermosensitive gating in a given channel is species specific, and multiple channels act together to sense the thermal environment. Therefore, we combined evidence of changes in selective pressure, gene gain/loss dynamics, and the first sensory ganglion transcriptome in this clade to identify the best candidate TRP channels that might have a functional dynamic range relevant for frigid Antarctic temperatures. We concluded that TRPV1a, TRPA1b, and TRPM4 are the likeliest putative thermosensors, and found evidence of diversifying selection at sites across these proteins. We also put forward hypotheses for molecular mechanisms of other cryonotothenioid adaptations, such as reduced skeletal calcium deposition, sensing oxidative stress, and unusual magnesium homeostasis. By completing a comprehensive and unbiased survey of these genes, we lay the groundwork for functional characterization and answering long-standing thermodynamic questions of thermosensitive gating and protein adaptation to low temperatures.

Thermodynamic and structural basis of temperature-dependent gating in TRP channels

Living organisms require detecting the environmental thermal clues for survival, allowing them to avoid noxious stimuli or find prey moving in the dark. In mammals, the Transient Receptor Potential ion channels superfamily is constituted by 27 polymodal receptors whose activity is controlled by small ligands, peptide toxins, protons and voltage. The thermoTRP channels subgroup exhibits unparalleled temperature dependence -behaving as heat and cold sensors. Functional studies have dissected their biophysical features in detail, and the advances of single-particle Cryogenic Electron microscopy provided the structural framework required to propose detailed channel gating mechanisms. However, merging structural and functional evidence for temperature-driven gating of thermoTRP channels has been a hard nut to crack, remaining an open question nowadays. Here we revisit the highlights on the study of heat and cold sensing in thermoTRP channels in the light of the structural data that has emerged during recent years.

Early origin of sweet perception in the songbird radiation

Early events in the evolutionary history of a clade can shape the sensory systems of descendant lineages. Although the avian ancestor may not have had a sweet receptor, the widespread incidence of nectar-feeding birds suggests multiple acquisitions of sugar detection. In this study, we identify a single early sensory shift of the umami receptor (the T1R1-T1R3 heterodimer) that conferred sweet-sensing abilities in songbirds, a large evolutionary radiation containing nearly half of all living birds. We demonstrate sugar responses across species with diverse diets, uncover critical sites underlying carbohydrate detection, and identify the molecular basis of sensory convergence between songbirds and nectar-specialist hummingbirds. This early shift shaped the sensory biology of an entire radiation, emphasizing the role of contingency and providing an example of the genetic basis of convergence in avian evolution.

Diminished Cold Avoidance Behaviours after Chronic Cold Exposure - Potential Involvement of TRPM8

Ambient temperature changes trigger plastic biological responses. Cold temperature is detected by the somatosensory system and evokes perception of cold together with adaptive physiological responses. We addressed whether chronic cold exposure induces adaptive adjustments of (1) thermosensory behaviours, and (2) the principle molecular cold sensor in the transduction machinery, transient receptor potential melastatin subtype 8 (TRPM8). Mice in two groups were exposed to either cold (6 °C) or thermoneutral (27 °C) ambient temperatures for 4 weeks and subjected to thermosensory behavioural testing. Cold group mice behaved different from Thermoneutral group in the Thermal Gradient Test: the former occupied a wider temperature range and was less cold avoidant. Furthermore, subcutaneous injection of the TRPM8 agonist icilin, enhanced cold avoidance in both groups in the Thermal Gradient Test, but Cold group mice were significantly less affected by icilin. Primary sensory neuron soma are located in dorsal root ganglia (DRGs), and western blotting showed diminished TRPM8 levels in DRGs of Cold group mice, as compared to the Thermoneutral group. We conclude that acclimation to chronic cold altered thermosensory behaviours, so that mice appeared less cold sensitive, and potentially, TRPM8 is involved.

Molecular sensors for temperature detection during behavioral thermoregulation in turtle embryos

Temperature sensing is essential for the survival of living organisms. Some reptile embryos can reposition themselves within the egg to seek optimal temperatures, but the molecular sensors involved in this temperature detection remain unknown. Here, we show that such thermotaxic behavior is directly determined by the activation of two heat-sensitive ion channels of the turtle: the transient receptor potential ankyrin 1 (MrTRPA1) and transient receptor potential vanilloid-1 (MrTRPV1). These two TRP channels were found to exhibit distinctive distributions among turtle dorsal root ganglion (DRG) neurons. Additionally, our laser irradiation assays illustrated that the heat activation thresholds of MrTRPA1 and MrTRPV1 are consistent with the mild (28-33°C) and noxious (>33°C) heat determined by behavioral tests, respectively. Further pharmacological studies have demonstrated that ligand-induced intervention of MrTRPA1 or MrTRPV1 is sufficient to mimic heat stimuli or block temperature signaling, causing changes in embryo movement. These findings indicate that the initiation of thermotaxic response in turtle embryos relies on a delicate functional balance between the heat activation of MrTRPA1 and MrTRPV1. Our study reveals, for the first time, a unique molecular mechanism underlying thermal detection: the two TRP channels act as a physiological tandem to control the thermotaxic behavior of turtle embryos.

TMEM163 Regulates ATP-Gated P2X Receptor and Behavior

Fast purinergic signaling is mediated by ATP and ATP-gated ionotropic P2X receptors (P2XRs), and it is implicated in pain-related behaviors. The properties exhibited by P2XRs vary between those expressed in heterologous cells and in vivo. Several modulators of ligand-gated ion channels have recently been identified, suggesting that there are P2XR functional modulators in vivo. Here, we establish a genome-wide open reading frame (ORF) collection and perform functional screening to identify modulators of P2XR activity. We identify TMEM163, which specifically modulates the channel properties and pharmacology of P2XRs. We also find that TMEM163 is required for full function of the neuronal P2XR and a pain-related ATP-evoked behavior. These results establish TMEM163 as a critical modulator of P2XRs in vivo and a potential target for the discovery of drugs for treating pain.

The genomics of rapid climatic adaptation and parallel evolution in North American house mice

Parallel changes in genotype and phenotype in response to similar selection pressures in different populations provide compelling evidence of adaptation. House mice (Mus musculus domesticus) have recently colonized North America and are found in a wide range of environments. Here we measure phenotypic and genotypic differentiation among house mice from five populations sampled across 21° of latitude in western North America, and we compare our results to a parallel latitudinal cline in eastern North America. First, we show that mice are genetically differentiated between transects, indicating that they have independently colonized similar environments in eastern and western North America. Next, we find genetically-based differences in body weight and nest building behavior between mice from the ends of the western transect which mirror differences seen in the eastern transect, demonstrating parallel phenotypic change. We then conduct genome-wide scans for selection and a genome-wide association study to identify targets of selection and candidate genes for body weight. We find some genomic signatures that are unique to each transect, indicating population-specific responses to selection. However, there is significant overlap between genes under selection in eastern and western house mouse transects, providing evidence of parallel genetic evolution in response to similar selection pressures across North America.

Human torpor: translating insights from nature into manned deep space expedition

During a long-duration manned spaceflight mission, such as flying to Mars and beyond, all crew members will spend a long period in an independent spacecraft with closed-loop bioregenerative life-support systems. Saving resources and reducing medical risks, particularly in mental heath, are key technology gaps hampering human expedition into deep space. In the 1960s, several scientists proposed that an induced state of suppressed metabolism in humans, which mimics 'hibernation', could be an ideal solution to cope with many issues during spaceflight. In recent years, with the introduction of specific methods, it is becoming more feasible to induce an artificial hibernation-like state (synthetic torpor) in non-hibernating species. Natural torpor is a fascinating, yet enigmatic, physiological process in which metabolic rate (MR), body core temperature (T_b ) and behavioural activity are reduced to save energy during harsh seasonal conditions. It employs a complex central neural network to orchestrate a homeostatic state of hypometabolism, hypothermia and hypoactivity in response to environmental challenges. The anatomical and functional connections within the central nervous system (CNS) lie at the heart of controlling synthetic torpor. Although progress has been made, the precise mechanisms underlying the active regulation of the torpor-arousal transition, and their profound influence on neural function and behaviour, which are critical concerns for safe and reversible human torpor, remain poorly understood. In this review, we place particular emphasis on elaborating the central nervous mechanism orchestrating the torpor-arousal transition in both non-flying hibernating mammals and non-hibernating species, and aim to provide translational insights into long-duration manned spaceflight. In addition, identifying difficulties and challenges ahead will underscore important concerns in engineering synthetic torpor in humans. We believe that synthetic torpor may not be the only option for manned long-duration spaceflight, but it is the most achievable solution in the foreseeable future. Translating the available knowledge from natural torpor research will not only benefit manned spaceflight, but also many clinical settings attempting to manipulate energy metabolism and neurobehavioural functions.

Why wild giant pandas frequently roll in horse manure

Attraction to feces in wild mammalian species is extremely rare. Here we introduce the horse manure rolling (HMR) behavior of wild giant pandas (Ailuropoda melanoleuca). Pandas not only frequently sniffed and wallowed in fresh horse manure, but also actively rubbed the fecal matter all over their bodies. The frequency of HMR events was highly correlated with an ambient temperature lower than 15 °C. BCP/BCPO (beta-caryophyllene/caryophyllene oxide) in fresh horse manure was found to drive HMR behavior and attenuated the cold sensitivity of mice by directly targeting and inhibiting transient receptor potential melastatin 8 (TRPM8), an archetypical cold-activated ion channel of mammals. Therefore, horse manure containing BCP/BCPO likely bestows the wild giant pandas with cold tolerance at low ambient temperatures. Together, our study described an unusual behavior, identified BCP/BCPO as chemical inhibitors of TRPM8 ion channel, and provided a plausible chemistry-auxiliary mechanism, in which animals might actively seek and utilize potential chemical resources from their habitat for temperature acclimatization.

Cellular, Molecular, and Physiological Adaptations of Hibernation: The Solution to Environmental Challenges

Thriving in times of resource scarcity requires an incredible flexibility of behavioral, physiological, cellular, and molecular functions that must change within a relatively short time. Hibernation is a collection of physiological strategies that allows animals to inhabit inhospitable environments, where they experience extreme thermal challenges and scarcity of food and water. Many different kinds of animals employ hibernation, and there is a spectrum of hibernation phenotypes. Here, we focus on obligatory mammalian hibernators to identify the unique challenges they face and the adaptations that allow hibernators to overcome them. This includes the cellular and molecular strategies used to combat low environmental and body temperatures and lack of food and water. We discuss metabolic, neuronal, and hormonal cues that regulate hibernation and how they are thought to be coordinated by internal clocks. Last, we touch on questions that are left to be addressed in the field of hibernation research. Studies from the last century and more recent work reveal that hibernation is not simply a passive reduction in body temperature and vital parameters but rather an active process seasonally regulated at the molecular, cellular, and organismal levels.

Functional evolution of vertebrate sensory receptors

Sensory receptors enable animals to perceive their external world, and functional properties of receptors evolve to detect the specific cues relevant for an organism's survival. Changes in sensory receptor function or tuning can directly impact an organism's behavior. Functional tests of receptors from multiple species and the generation of chimeric receptors between orthologs with different properties allow for the dissection of the molecular basis of receptor function and identification of the key residues that impart functional changes in different species. Knowledge of these functionally important sites facilitates investigation into questions regarding the role of epistasis and the extent of convergence, as well as the timing of sensory shifts relative to other phenotypic changes. However, as receptors can also play roles in non-sensory tissues, and receptor responses can be modulated by numerous other factors including varying expression levels, alternative splicing, and morphological features of the sensory cell, behavioral validation can be instrumental in confirming that responses observed in heterologous systems play a sensory role. Expression profiling of sensory cells and comparative genomics approaches can shed light on cell-type specific modifications and identify other proteins that may affect receptor function and can provide insight into the correlated evolution of complex suites of traits. Here we review the evolutionary history and diversity of functional responses of the major classes of sensory receptors in vertebrates, including opsins, chemosensory receptors, and ion channels involved in temperature-sensing, mechanosensation and electroreception.

Keratinocytes contribute to normal cold and heat sensation

Keratinocytes are the most abundant cell type in the epidermis, the most superficial layer of skin. Historically, epidermal-innervating sensory neurons were thought to be the exclusive detectors and transmitters of environmental stimuli. However, recent work from our lab (Moehring et al., 2018) and others (Baumbauer et al., 2015) has demonstrated that keratinocytes are also critical for normal mechanotransduction and mechanically-evoked behavioral responses in mice. Here, we asked whether keratinocyte activity is also required for normal cold and heat sensation. Using calcium imaging, we determined that keratinocyte cold activity is conserved across mammalian species and requires the release of intracellular calcium through one or more unknown cold-sensitive proteins. Both epidermal cell optogenetic inhibition and interruption of ATP-P2X4 signaling reduced reflexive behavioral responses to cold and heat stimuli. Based on these data and our previous findings, keratinocyte purinergic signaling is a modality-conserved amplification system that is required for normal somatosensation in vivo.

Current View of Ligand and Lipid Recognition by the Menthol Receptor TRPM8

Transient receptor potential (TRP) melastatin member 8 (TRPM8), which is a calcium-permeable ion channel, functions as the primary molecular sensor of cold and menthol in humans. Recent cryoelectron microscopy (cryo-EM) studies of TRPM8 have shown distinct structural features in its architecture and domain assembly compared with the capsaicin receptor TRP vanilloid member 1 (TRPV1). Moreover, ligand-bound TRPM8 structures have uncovered unforeseen binding sites for both cooling agonists and membrane lipid phosphatidylinositol 4,5-bisphosphate [PI(4,5)P₂]. These complex structures unveil the molecular basis of cooling agonist sensing by TRPM8 and the allosteric role of PI(4,5)P₂ in agonist binding for TRPM8 activation. Here, we review the recent advances in TRPM8 structural biology and investigate the molecular principles governing the distinguishing role of TRPM8 as the evolutionarily conserved menthol receptor.

A specialized pore turret in the mammalian cation channel TRPV1 is responsible for distinct and species-specific heat activation thresholds

The transient receptor potential vanilloid 1 (TRPV1) channel is a heat-activated cation channel that plays a crucial role in ambient temperature detection and thermal homeostasis. Although several structural features of TRPV1 have been shown to be involved in heat-induced activation of the gating process, the physiological significance of only a few of these key elements has been evaluated in an evolutionary context. Here, using transient expression in HEK293 cells, electrophysiological recordings, and molecular modeling, we show that the pore turret contains both structural and functional determinants that set the heat activation thresholds of distinct TRPV1 orthologs in mammals whose body temperatures fluctuate widely. We found that TRPV1 from the bat Carollia brevicauda exhibits a lower threshold temperature of channel activation than does its human ortholog and three bat-specific amino acid substitutions located in the pore turret are sufficient to determine this threshold temperature. Furthermore, the structure of the TRPV1 pore turret appears to be of physiological and evolutionary significance for differentiating the heat-activated threshold among species-specific TRPV1 orthologs. These findings support a role for the TRPV1 pore turret in tuning the heat-activated threshold, and they suggest that its evolution was driven by adaption to specific physiological traits among mammals exposed to variable temperatures.

A paradigm of thermal adaptation in penguins and elephants by tuning cold activation in TRPM8

Sensing temperature is critical for the survival of all living beings. Here, we show that during cold-induced activation of the archetypical temperature-sensitive TRPM8 ion channel, there are hydrophobic residues in the pore domain stabilized in the exposed state. Tuning hydrophobicity of these residues specifically alters cold response in TRPM8. Furthermore, TRPM8 orthologs in vertebrates evolved to employ such a mechanism, which physiologically tunes cold tolerance for better thermal adaptation. Our findings not only advance the understanding of the cold-induced activation mechanism of TRPM8 but also bring insights to the molecular evolution strategy for ambient-temperature adaptation in vertebrates.

To adapt to habitat temperature, vertebrates have developed sophisticated physiological and ecological mechanisms through evolution. Transient receptor potential melastatin 8 (TRPM8) serves as the primary sensor for cold. However, how cold activates TRPM8 and how this sensor is tuned for thermal adaptation remain largely unknown. Here we established a molecular framework of how cold is sensed in TRPM8 with a combination of patch-clamp recording, unnatural amino acid imaging, and structural modeling. We first observed that the maximum cold activation of TRPM8 in eight different vertebrates (i.e., African elephant and emperor penguin) with distinct side-chain hydrophobicity (SCH) in the pore domain (PD) is tuned to match their habitat temperature. We further showed that altering SCH for residues in the PD with solvent-accessibility changes leads to specific tuning of the cold response in TRPM8. We also observed that knockin mice expressing the penguin’s TRPM8 exhibited remarkable tolerance to cold. Together, our findings suggest a paradigm of thermal adaptation in vertebrates, where the evolutionary tuning of the cold activation in the TRPM8 ion channel through altering SCH and solvent accessibility in its PD largely contributes to the setting of the cold-sensitive/tolerant phenotype.

Structure-Based Design of Novel Biphenyl Amide Antagonists of Human Transient Receptor Potential Cation Channel Subfamily M Member 8 Channels with Potential Implications in the Treatment of Sensory Neuropathies

Structure-activity relationship studies of a reported menthol-based transient receptor potential cation channel subfamily M member 8 channel (TRPM8) antagonist, guided by computational simulations and structure-based design, uncovers a novel series of TRPM8 antagonists with >10-fold selectivity versus related TRP subtypes. Spiro[4.5]decan-8-yl analogue 14 inhibits icilin-evoked Ca²⁺ entry in HEK-293 cells stably expressing human TRPM8 (hTRPM8) with an IC₅₀ of 2.4 ± 1.0 nM, while in whole-cell patch-clamp recordings this analogue inhibits menthol-evoked currents with a hTRPM8 IC₅₀ of 64 ± 2 nM. Molecular dynamics (MD) simulations of compound 14 in our homology model of hTRPM8 suggest that this antagonist forms extensive hydrophobic contacts within the orthosteric site. In the wet dog shakes (WDS) assay, compound 14 dose-dependently blocks icilin-triggered shaking behaviors in mice. Upon local administration, compound 14 dose dependently inhibits cold allodynia evoked by the chemotherapy oxaliplatin in a murine model of peripheral neuropathy at microgram doses. Our findings suggest that 14 and other biphenyl amide analogues within our series can find utility as potent antagonist chemical probes derived from (-)-menthol as well as small molecule therapeutic scaffolds for chemotherapy-induced peripheral neuropathy (CIPN) and other sensory neuropathies.

Molecular mechanisms of cold pain

The sensation of cooling is essential for survival. Extreme cold is a noxious stimulus that drives protective behaviour and that we thus perceive as pain. However, chronic pain patients suffering from cold allodynia paradoxically experience innocuous cooling as excruciating pain. Peripheral sensory neurons that detect decreasing temperature express numerous cold-sensitive and voltage-gated ion channels that govern their response to cooling in health and disease. In this review, we discuss how these ion channels control the sense of cooling and cold pain under physiological conditions, before focusing on the molecular mechanisms by which ion channels can trigger pathological cold pain. With the ever-rising number of patients burdened by chronic pain, we end by highlighting the pressing need to define the cells and molecules involved in cold allodynia and so identify new, rational drug targets for the analgesic treatment of cold pain.

Structural insights into TRPM8 inhibition and desensitization

The transient receptor potential melastatin 8 (TRPM8) ion channel is the primary detector of environmental cold and an important target for treating pathological cold hypersensitivity. Here, we present cryo-electron microscopy structures of TRPM8 in ligand-free, antagonist-bound, or calcium-bound forms, revealing how robust conformational changes give rise to two nonconducting states, closed and desensitized. We describe a malleable ligand-binding pocket that accommodates drugs of diverse chemical structures, and we delineate the ion permeation pathway, including the contribution of lipids to pore architecture. Furthermore, we show that direct calcium binding mediates stimulus-evoked desensitization, clarifying this important mechanism of sensory adaptation. We observe large rearrangements within the S4-S5 linker that reposition the S1-S4 and pore domains relative to the TRP helix, leading us to propose a distinct model for modulation of TRPM8 and possibly other TRP channels.

Role of brown adipose tissue in body temperature control during the early postnatal period in Syrian hamsters and mice

Brown adipose tissue (BAT) contributes to non-shivering thermogenesis and plays an important role in body temperature control. The contribution of BAT thermogenesis to body temperature control in a non-cold environment was evaluated using developing hamsters. Immunostaining for uncoupling protein 1 (UCP1), a mitochondrial protein responsible for BAT thermogenesis, indicated that interscapular fat tissue had matured as BAT at day 14. When pups were placed on a thermal plate kept at 23°C, the body surface temperature decreased in day 7- and 10-day-old pups but was maintained at least for 15 min in 14-day-old pups, indicating that hamsters are unable to maintain their body temperature until around day 14 even in a non-cold environment. Body temperature maintenance was also evaluated in UCP1-deficient mice. BAT analysis showed that the UCP1 protein level in Ucp1^+/− Hetero mice was 61.3 ± 1.4% of that in wild-type (WT) mice and was undetected in Ucp1^−/− knockout (KO) mice. When 12-day-old pups were place on a thermal plate at 23°C, body surface temperature was maintained for at least 15 min in WT and Hetero mice but gradually dropped by 2.4 ± 0.2°C in 15 min in KO mice. It is concluded that BAT thermogenesis is indispensable for body temperature maintenance in pups of hamsters and mice, even in the non-cold circumstances. The early life poikilothermy and the later acquirement of homeothermy in hamsters may be because of the postnatal development of BAT.

Searching for Signatures of Cold Climate Adaptation in TRPM8 Gene in Populations of East Asian Ancestry

Dispersal of Homo sapiens across the globe during the last 200,000 years was accompanied by adaptation to local climatic conditions, with severe winter temperatures being probably one of the most significant selective forces. The TRPM8 gene codes for a cold-sensing ion channel, and adaptation to low temperatures is the major determinant of its molecular evolution. Here, our aim was to search for signatures of cold climate adaptation in TRPM8 gene using a combined data set of 19 populations of East Asian ancestry from the 1000 Genomes Project and Human Genome Diversity Project. As a result, out of a total of 60 markers under study, none showed significant association with the average winter temperatures at the locations of the studied populations considering the multiple testing thresholds. This might suggest that the principal mode of TRPM8 evolution may be different from widespread models, where adaptive alleles are additive, dominant or recessive, at least in populations with the predominant East Asian component. For example, evolution by means of selectively preferable epistatic interactions among amino acids may have taken place. Despite the lack of strong signals of association, however, a very promising single nucleotide polymorphism (SNP) was found. The SNP rs7577262 is considered the best candidate based on its allelic correlations with winter temperatures, signatures of selective sweep and physiological evidences. The second top SNP, rs17862920, may participate in adaptation as well. Additionally, to assist in interpreting the nominal associations, the other markers reached, we performed SNP prioritization based on functional evidences found in literature and on evolutionary conservativeness.

Piezo2 integrates mechanical and thermal cues in vertebrate mechanoreceptors

Tactile information is detected by thermoreceptors and mechanoreceptors in the skin and integrated by the central nervous system to produce the perception of somatosensation. Here we investigate the mechanism by which thermal and mechanical stimuli begin to interact and report that it is achieved by the mechanotransduction apparatus in cutaneous mechanoreceptors. We show that moderate cold potentiates the conversion of mechanical force into excitatory current in all types of mechanoreceptors from mice and tactile-specialist birds. This effect is observed at the level of mechanosensitive Piezo2 channels and can be replicated in heterologous systems using Piezo2 orthologs from different species. The cold sensitivity of Piezo2 is dependent on its blade domains, which render the channel resistant to cold-induced perturbations of the physical properties of the plasma membrane and give rise to a different mechanism of mechanical activation than that of Piezo1. Our data reveal that Piezo2 is an evolutionarily conserved mediator of thermal-tactile integration in cutaneous mechanoreceptors.

The role of π-helices in TRP channel gating

Transient Receptor Potential (TRP) channels are a large superfamily of polymodal ion channels, which perform important roles in numerous physiological processes. The architecture of their transmembrane (TM) domains closely resembles that of voltage-gated potassium channels (K_V). However, recent cryoEM and crystallographic studies of TRP channels have identified π-helices in functionally important regions, and it is increasingly recognized that they utilize a distinct mechanism of gating that relies on α-to-π secondary structure transitions. Here we review our current understanding of the role of π-helices in TRP channel function and their broader impact on different classes of ion channels.

An evolutionary perspective on immunometabolism

Metabolism is at the core of all biological functions. Anabolic metabolism uses building blocks that are either derived from nutrients or synthesized de novo to produce the biological infrastructure, whereas catabolic metabolism generates energy to fuel all biological processes. Distinct metabolic programs are required to support different biological functions. Thus, recent studies have revealed how signals regulating cell quiescence, proliferation, and differentiation also induce the appropriate metabolic programs. In particular, a wealth of new studies in the field of immunometabolism has unveiled many examples of the connection among metabolism, cell fate decisions, and organismal physiology. We discuss these findings under a unifying framework derived from the evolutionary and ecological principles of life history theory.

Structural and Evolutionary Insights Point to Allosteric Regulation of TRP Ion Channels

The familiar pungent taste of spicy food, the refreshing taste of mint, and many other physiological phenomena are mediated by transient receptor potential (TRP) ion channels. TRP channels are a superfamily of ion channels that are sensitive to diverse chemical and physical stimuli and play diverse roles in biology. In addition to chemical regulation, some family members also sense common physical stimuli, such as temperature or pressure. Since their discovery and cloning in the 1990s and 2000s, understanding the molecular mechanisms governing TRP channel function and polymodal regulation has been a consistent but challenging goal. Until recently, a general lack of high-resolution TRP channel structures had significantly limited a molecular understanding of their function. In the past few years, a flood of TRP channel structures have been released, made possible primarily by advances in cryo-electron microscopy (cryo-EM). The boon of many structures has unleashed unparalleled insight into TRP channel architecture. Substantive comparative studies between TRP structures provide snapshots of distinct states such as ligand-free, stabilized by chemical agonists, or antagonists, partially illuminating how a given channel opens and closes. However, the now ∼75 TRP channel structures have ushered in surprising outcomes, including a lack of an apparent general mechanism underlying channel opening and closing among family members. Similarly, the structures reveal a surprising diversity in which chemical ligands bind TRP channels. Several TRP channels are activated by temperature changes in addition to ligand binding. Unraveling mechanisms of thermosensation has proven an elusive challenge to the field. Although some studies point to thermosensitive domains in the transmembrane region of the channels, results have sometimes been contradictory and difficult to interpret; in some cases, a domain that proves essential for thermal sensitivity in one context can be entirely removed from the channel without affecting thermosensation in another context. These results are not amenable to simple interpretations and point to allosteric networks of regulation within the channel structure. TRP channels have evolved to be fine-tuned for the needs of a species in its environmental niche, a fact that has been both a benefit and burden in unlocking their molecular features. Functional evolutionary divergence has presented challenges for studying TRP channels, as orthologs from different species can give conflicting experimental results. However, this diversity can also be examined comparatively to decipher the basis for functional differences. As with structural biology, untangling the similarities and differences resulting from evolutionary pressure between species has been a rich source of data guiding the field. This Account will contextualize the existing biochemical and functional data with an eye to evolutionary data and couple these insights with emerging structural biology to better understand the molecular mechanisms behind chemical and physical regulation of TRP channels.

Remodeling Ancestral Phenotypic Plasticity in Local Adaptation: A New Framework to Explore the Role of Genetic Compensation in the Evolution of Homeostasis

Phenotypic plasticity is not universally adaptive. In certain cases, plasticity can result in phenotypic shifts that reduce fitness relative to the un-induced state. A common cause of such maladaptive plasticity is the co-option of ancestral developmental and physiological response systems to meet novel challenges. Because these systems evolved to meet specific challenges in an ancestral environment (e.g., localized and transient hypoxia), their co-option to meet a similar, but novel, stressor (e.g., reductions in ambient pO2 at high elevation) can lead to misdirected responses that reduce fitness. In such cases, natural selection should act to remodel phenotypic plasticity to suppress the expression of these maladaptive responses. Because these maladaptive responses reduce the fitness of colonizers in new environments, this remodeling of ancestral plasticity may be among the earliest steps in adaptive walks toward new local optima. Genetic compensation has been proposed as a general form of adaptive evolution that leads to the suppression of maladaptive plasticity to restore the ancestral trait value in the face of novel stimuli. Given their central role in the regulation of basic physiological functions, we argue that genetic compensation may often be achieved by modifications of homeostatic regulatory systems. We further suggest that genetic compensation to modify homeostatic systems can be achieved by two alternative strategies that differ in their mechanistic underpinnings; to our knowledge, these strategies have not been formally recognized by previous workers. We then consider how the mechanistic details of these alternative strategies may constrain their evolution. These considerations lead us to argue that genetic compensation is most likely to evolve by compensatory physiological changes that safeguard internal homeostatic conditions to prevent the expression of maladaptive portions of conserved reaction norms, rather than direct evolution of plasticity itself. Finally, we outline a simple experimental framework to test this hypothesis. Our goal is to stimulate research aimed at providing a deeper mechanistic understanding of whether and how phenotypic plasticity can be remodeled following environmental shifts that render ancestral responses maladaptive, an issue with increasing importance in our current era of rapid environmental change.