Temperature activated transient receptor potential ion channels from Antarctic fishes

Functional characterization of thermosensitive TRPV channels from holocephalan elephant shark (Callorhinchus milii) illuminate the ancestral thermosensory system in vertebrates

Homeostasis and survival of various animal species have been affected by changes in environmental temperature, causing animals to evolve physiological systems for sensing ambient and body temperature. Temperature-sensitive transient receptor potential (TRP) channels have multimodal properties that are activated by physical stimuli such as temperature, as well as by various chemical substances. Our goal is to understand the diversity of the vertebrate thermosensory system by characterizing the temperature-sensitive TRPV channels of the elephant shark, which belongs to the Holocephali of the cartilaginous fishes. Since elephant sharks are basal jawed vertebrates, analysis of elephant shark TRPs is critical to understanding the evolution of thermosensory systems in vertebrate lineages. We found that temperature stimulation activated elephant shark TRPVs in an electrophysiological analysis similarly to the mammalian ortholog. The thermal activation threshold of elephant shark TRPV1 (31°C) was similar to the thresholds reported for several other fish species, but was much lower than that of mammalian orthologs. Strikingly, the elephant shark TRPV4 was a cooling-activated channel with a threshold of 20°C, whereas, in several tetrapods, it is activated by warmth. These results suggest that the temperature sensitivity of TRPV4 has changed in vertebrate evolutionary lineages. Furthermore, we also found the elephant shark possesses heat-evoked TRPV3 with a threshold of 42°C, which is absent in more derived teleost fishes. Taken together, our findings elucidate that the vertebrate-type thermosensory system has already emerged in the common ancestor of jawed vertebrates, although their temperature-sensing ranges were different from those of mammals.

Evolution of Temperature Receptors and Their Roles in Sensory Diversification and Adaptation

Among various environmental factors, temperature is one of the critical factors for organisms since it can affect most, if not all, biological processes. Therefore, animals precisely sense ambient and body temperatures and physiologically and behaviorally respond to temperature changes. Taking such nature into consideration, alteration of thermal perception should have played a pivotal role in adaptation to diverse thermal niches. Temperature as well as other physical and chemical stimuli are perceived by the primary afferent neurons where transient receptor potential (TRP) channels are expressed, and these channels serve as multimodal receptors in the somatosensory system. To understand the roles of TRP channels in the evolution of sensory perception, comparative analyses have been performed using various animal species, and their functional diversity has been well documented over the past 2 decades. Furthermore, in recent years, species differences in the thermal responses of TRP channels have been found among closely related species inhabiting different thermal niches, which have uncovered the contributions of TRP channels to environmental adaptation in various vertebrate species. The purpose of this review is to summarize the studies that addressed the functional evolution of TRP channels associated with sensory diversification and environmental adaptation.

Vertebrate behavioral thermoregulation: knowledge and future directions

Thermoregulation is critical for survival across species. In animals, the nervous system detects external and internal temperatures, integrates this information with internal states, and ultimately forms a decision on appropriate thermoregulatory actions. Recent work has identified critical molecules and sensory and motor pathways controlling thermoregulation. However, especially with regard to behavioral thermoregulation, many open questions remain. Here, we aim to both summarize the current state of research, the "knowledge," as well as what in our mind is still largely missing, the "future directions." Given the host of circuit entry points that have been discovered, we specifically see that the time is ripe for a neuro-computational perspective on thermoregulation. Such a perspective is largely lacking but is increasingly fueled and made possible by the development of advanced tools and modeling strategies.

Transient receptor potential (TRP) channels in Sebastes schlegelii: Genome-wide identification and ThermoTRP expression analysis under high-temperature

Transient Receptor Potential (TRP) channels, essential for sensing environmental stimuli, are widely distributed. Among them, thermosensory TRP channels play a crucial role in temperature sensing and regulation. Sebastes schlegelii, a significant aquatic economic species, exhibits sensitivity to temperature across multiple aspects. In this study, we identified 18 SsTRP proteins using whole-genome scanning. Motif analysis revealed motif 2 in all TRP proteins, with conserved motifs in subfamilies. TRP-related domains, anchored repeats, and ion-transmembrane domains were found. Chromosome analysis showed 18 TRP genes on 11 chromosomes and a scaffold. Phylogenetics classified SsTRPs into four subfamilies: TRPM, TRPA, TRPV, and TRPC. In diverse organisms, four monophyletic subfamilies were identified. Additionally, we identified key TRP genes with significantly upregulated transcription levels under short-term (30 min) and long-term (3 days) exposure at 24 °C (optimal elevated temperature) and 27 °C (critical high temperature). We propose that genes upregulated at 30 min may be involved in the primary response process of temperature sensing, while genes upregulated at 3 days may participate in the secondary response process of temperature perception. This study lays the foundation for understanding the regulatory mechanisms of TRPs responses to environmental stimuli in S. schlegelii and other fishes.

Temperature activated transient receptor potential ion channels from Antarctic fishes

Antarctic notothenioid fishes (cryonotothenioids) live in waters that range between -1.86°C and an extreme maximum +4°C. Evidence suggests these fish sense temperature peripherally, but the molecular mechanism of temperature sensation in unknown. Previous work identified transient receptor potential (TRP) channels TRPA1b, TRPM4 and TRPV1a as the top candidates for temperature sensors. Here, cryonotothenioid TRPA1b and TRPV1a are characterized using Xenopus oocyte electrophysiology. TRPA1b and TRPV1a showed heat-evoked currents with Q10s of 11.1 ± 2.2 and 20.5 ± 2.4, respectively. Unexpectedly, heat activation occurred at a threshold of 22.9 ± 1.3°C for TRPA1b and 32.1 ± 0.6°C for TRPV1a. These fish have not experienced such temperatures for at least 15 Myr. Either (1) another molecular mechanism underlies temperature sensation, (2) these fishes do not sense temperatures below these thresholds despite having lethal limits as low as 5°C, or (3) native cellular conditions modify the TRP channels to function at relevant temperatures. The effects of osmolytes, pH, oxidation, phosphorylation, lipids and accessory proteins were tested. No conditions shifted the activity range of TRPV1a. Oxidation in combination with reduced cholesterol significantly dropped activation threshold of TRPA1b to 11.3 ± 2.3°C, it is hypothesized the effect may be due to lipid raft disruption.