A suicidal mechanism for the exquisite temperature sensitivity of TRPV1

Pathway-dependent cold activation of heat-responsive TRPV channels

The homotetrameric thermosensitive transient receptor potential vanilloid 1-4 (TRPV1-4) channels in sensory neurons are highly responsive to heat stimuli. However, their primary heat sensors or triggers for heat activation have not been examined for cold activation. In this study, cold activation of minimal TRPV1 without the pore turret was compared with that of full-length human TRPV3. The former followed a pathway from the putative heat activation starter, while the latter tracked a different pathway starting far from the assumed heat activation point. The results showed that the former shared temperature sensitivity with heat activation while the latter did not. Therefore, this mirrored thermosensitivity can be used to confirm the location of the primary thermal sensor for TRPV1 or TRPV3, and potentially define the primary thermal sensor of other thermosensitive proteins like TRPV2 or TRPV4 once the same heat capacity mechanism is applied.

TRPV1 and thermosensitivity

The capsaicin receptor TRPV1 was identified as the first heat-activated ion channel in 1997. Since then, numerous studies have been performed on its physiological functions and structure-function relationship, and chemicals targeting TRPV1 have been developed. It has been more than 27 years since the initial cloning of the TRPV1 gene and more than 11 years since the clarification of its structure at the atomic level using cryo-EM. However, we still lack good chemical antagonists of TRPV1 as medicines. TRPV1 is involved in body temperature regulation, but how TRPV1 antagonists cause hyperthermia and how TRPV1 is involved in body temperature regulation are not yet clearly understood. More research is needed in the thermal biology field.

Thermoring basis for thermo-gated TRPV2

The heat-responses of the homotetrameric thermosensitive transient receptor potential vanilloid (TRPV)1-4 channels are use-dependent. The initial short heat stimulus typically alters the temperature threshold and sensitivity for the subsequent one. The precise underlying structural motifs have not been identified except for TRPV1 and TRPV3. Since the release of lipid from the active vanilloid site is necessary for the initial heat activation of TRPV1 or TRPV3, the 3D cryo-EM structures of apo rat TRPV2 with or without any lipid in different gating states were analyzed using a highly sensitive thermoring model. The results indicated that two lipids in the voltage sensor-like domain and at the vanilloid site needed to be released to achieve theoretically and experimentally matched start and end thresholds and thermosensitivities during the first and second heat sensations. Therefore, this study further elucidated the role of lipids at various sites in the use-dependent heat responses of thermosensitive TRPV1-4 channels.

Chikungunya Particle and RNA Induce Mechanical and Heat Hypersensitivities in a TRPV1-Dependent Manner

Chikungunya virus (CHIKV), the causative agent of the chikungunya fever, is an alphavirus widely transmitted by the bite of the female mosquito of the genus Aedes sp., especially in tropical and subtropical regions. Brazil is the country most affected by the microorganism. CHIKV classically induces articular pain, which can become long lasting for even years in a great number of the infected individuals, reducing their quality of life. The mechanisms of CHIKV-induced pain are poorly understood, but recent evidence indicated a role for the transient receptor potential vanilloid 1 (TRPV1) in this pathology. Herein, we assessed the ability of intra-articularly injected inactivated CHIKV or its RNA to trigger nociception in mice. Both stimuli induced bilateral secondary hyperalgesia to mechanical and heat stimuli. These responses were attenuated by TRPV1 ablation or antagonism. Joint structural alterations and increased cartilage TRPV1 protein expression were detected in the ipsilateral knee joints injected with either CHIKV or viral RNA. However, the lack of this receptor did not influence the histological changes triggered by CHIKV or RNA. The results further support the role of TRPV1 in CHIKV-induced pain and highlight its importance in the chronic phase of the disease.

Ligand-induced cold activation of TRPV3

Both hot and cold sensation of the homotetrameric thermosensitive transient receptor potential vanilloid 1-4 (TRPV1-4) channels have been predicted by a single Gibbs-Helmholtz equation for a change in molar heat capacity. However, cold activation has not been confirmed for those heat-responsive TRPV1-4 channels. Given the cooperative heat unfolding and non-cooperative cold unfolding behaviors in proteins, two different open states at low and high temperatures should be detected in TRPV1-4 channels. To test this hypothesis, the temperature-dependent quaternary and tertiary structures of oxidized TRPV3 in the presence and absence of the natural cannabinoid tetrahydrocannabivarin (THCV) were characterized along a lipid-dependent minimal gating pathway. Further thermoring analyses showed that gating state-dependent thermostability allowed oxidized TRPV3 to be activated and then inactivated only below 30°C. However, no inactivation would be observed above 30°C once the lipid at the active vanilloid site was released by THCV binding. Therefore, such two temperature-dependent gating pathways of oxidized TRPV3 actually resulted from cold and heat activation. (161 words).

Protein dynamics underlies strong temperature dependence of heat receptors

Ion channels are generally allosteric proteins, involving specialized stimulus sensor domains conformationally linked to the gate to drive channel opening. Temperature receptors are a group of ion channels from the transient receptor potential family. They exhibit an unprecedentedly strong temperature dependence and are responsible for temperature sensing in mammals. Despite intensive studies, however, the nature of the temperature sensor domain in these channels remains elusive. By direct calorimetry of TRPV1 proteins, we have recently provided a proof of principle that temperature sensing by ion channels may diverge from the conventional allosterity theory; rather it is intimately linked to inherent thermal instability of channel proteins. Here, we tackle the generality of the hypothesis and provide key molecular pieces of evidence on the coupling of thermal transitions in the channels. We show that while wild-type channels possess a single concerted thermal transition peak, the chimera, in which strong temperature dependence becomes disrupted, results in multitransition peaks, and the activation enthalpies are accordingly reduced. The data show that the coupling with protein unfolding drives up the energy barrier of activation, leading to a strong temperature dependence of opening. Furthermore, we pinpoint the proximal N-terminus of the channels as a linchpin in coalescing different parts of the channels into concerted activation. Thus, we suggest that coupled interaction networks in proteins underlie the strong temperature dependence of temperature receptors.

ATP-dependent thermoring basis for the heat unfolding of the first nucleotide-binding domain isolated from human CFTR

Traditionally, the thermostability of a protein is defined by a melting temperature, at which half of the protein is unfolded. However, this definition cannot indicate the structural origin of a heat-induced unfolding pathway. Here, the thermoring structures were studied on the ATP-dependent heat-induced unfolding of the first nucleotide-binding domain from the human cystic fibrosis transmembrane conductance regulator. The results showed that initial theoretical and experimental melting thresholds aligned well after three structural perturbations including the F508del mutation, the most common cause of cystic fibrosis. This alignment further demonstrated that the heat-induced unfolding process began with the disruption of the least-stable noncovalent interaction within the biggest thermoring along the single peptide chain. The C-terminal region, which was related to the least-stable noncovalent interaction and the ATP-dependent dimerization of two nucleotide-binding domains, emerged as a crucial determinant of the thermal stability of the isolated protein and a potential interfacial drug target to alleviate the thermal defect caused by the F508del mutation. This groundbreaking discovery significantly advances our understanding of protein activity, thermal stability, and molecular pathology.

Protein Dynamics Underlies Strong Temperature Dependence of Heat Receptors

Ion channels are generally allosteric proteins, involving specialized stimulus sensor domains conformationally linked to the gate to drive channel opening. Temperature receptors are a group of ion channels from the transient receptor potential (TRP) family. They exhibit an unprecedentedly strong temperature dependence and are responsible for temperature sensing in mammals. Despite intensive studies, however, the nature of the temperature sensor domain in these channels remains elusive. By direct calorimetry of TRPV1 proteins, we have recently provided a proof of principle that temperature sensing by ion channels may diverge from the conventional allosterity theory; rather it is intimately linked to inherent thermal instability of channel proteins. Here we tackle the generality of the hypothesis and provide key molecular evidences on the coupling of thermal transitions in the channels. We show that while wild-type channels possess a single concerted thermal transition peak, the chimera, in which strong temperature dependence becomes disrupted, results in multi-transition peaks, and the activation enthalpies are accordingly reduced. The data show that the coupling with protein unfolding drives up the energy barrier of activation, leading to a strong temperature dependence of opening. Furthermore, we pinpoint the proximal N-terminus of the channels as a linchpin in coalescing different parts of the channels into concerted activation. Thus, we suggest that coupled interaction networks in proteins underlie the strong temperature dependence of temperature receptors.

Oleic acid released by sensory neurons inhibits TRPV1-mediated thermal hypersensitivity via GPR40

Noxious stimuli activate nociceptive sensory neurons, causing action potential firing and the release of diverse signaling molecules. Several peptides have already been identified to be released by sensory neurons and shown to modulate inflammatory responses and inflammatory pain. However, it is still unclear whether lipid mediators can be released upon sensory neuron activation to modulate intercellular communication. Here, we analyzed the lipid secretome of capsaicin-stimulated nociceptive neurons with LC-HRMS, revealing that oleic acid is strongly released from sensory neurons by capsaicin. We further demonstrated that oleic acid inhibits capsaicin-induced calcium transients in sensory neurons and reverses bradykinin-induced TRPV1 sensitization by a calcineurin (CaN) and GPR40 (FFAR1) dependent pathway. Additionally, oleic acid alleviated zymosan-mediated thermal hypersensitivity via the GPR40, suggesting that the capsaicin-mediated oleic acid release from sensory neurons acts as a protective and feedback mechanism, preventing sensory neurons from nociceptive overstimulation via the GPR40/CaN/TRPV1-axis.

The Variety of Mechanosensitive Ion Channels in Retinal Neurons

Alterations in intraocular and external pressure critically involve the pathogenesis of glaucoma, traumatic retinal injury (TRI), and other retinal disorders, and retinal neurons have been reported to express multiple mechanical-sensitive channels (MSCs) in recent decades. However, the role of MSCs in visual functions and pressure-related retinal conditions has been unclear. This review will focus on the variety and functional significance of the MSCs permeable to K+, Na+, and Ca2+, primarily including the big potassium channel (BK); the two-pore domain potassium channels TRAAK and TREK; Piezo; the epithelial sodium channel (ENaC); and the transient receptor potential channels vanilloid TRPV1, TRPV2, and TRPV4 in retinal photoreceptors, bipolar cells, horizontal cells, amacrine cells, and ganglion cells. Most MSCs do not directly mediate visual signals in vertebrate retinas. On the other hand, some studies have shown that MSCs can open in physiological conditions and regulate the activities of retinal neurons. While these data reasonably predict the crossing of visual and mechanical signals, how retinal light pathways deal with endogenous and exogenous mechanical stimulation is uncertain.

Degrees of freedom: temperature's influence on developmental rate

Temperature exerts a fundamental influence across scales of biology, from the biophysical nature of molecules, to the sensitivity of cells, and the coordinated progression of development in embryos. Species-specific developmental rates and temperature-induced acceleration of development indicate that these sensing mechanisms are harnessed to influence developmental dynamics. Tracing how temperature sensitivity propagates through biological scales to influence the pace of development can therefore reveal how embryogenesis remains robust to environmental influences. Cellular protein homeostasis (proteostasis), and cellular metabolic rate are linked to both temperature-induced and species-specific developmental tempos in specific cell types, hinting toward generalized mechanisms of timing control. New methods to extract timing information from single-cell profiling experiments are driving further progress in understanding how mechanisms of temperature sensitivity can direct cell-autonomous responses, coordination across cell types, and evolutionary modifications of developmental timing.