Photosensitive TRPs

Retinal TRP channels: Cell-type-specific regulators of retinal homeostasis and multimodal integration

Transient receptor potential (TRP) channels are a widely expressed family of 28 evolutionarily conserved cationic ion channels that operate as primary detectors of chemical and physical stimuli and secondary effectors of metabotropic and ionotropic receptors. In vertebrates, the channels are grouped into six related families: TRPC, TRPV, TRPM, TRPA, TRPML, and TRPP. As sensory transducers, TRP channels are ubiquitously expressed across the body and the CNS, mediating critical functions in mechanosensation, nociception, chemosensing, thermosensing, and phototransduction. This article surveys current knowledge about the expression and function of the TRP family in vertebrate retinas, which, while dedicated to transduction and transmission of visual information, are highly susceptible to non-visual stimuli. Every retinal cell expresses multiple TRP subunits, with recent evidence establishing their critical roles in paradigmatic aspects of vertebrate vision that include TRPM1-dependent transduction of ON bipolar signaling, TRPC6/7-mediated ganglion cell phototransduction, TRP/TRPL phototransduction in Drosophila and TRPV4-dependent osmoregulation, mechanotransduction, and regulation of inner and outer blood-retina barriers. TRP channels tune light-dependent and independent functions of retinal circuits by modulating the intracellular concentration of the 2nd messenger calcium, with emerging evidence implicating specific subunits in the pathogenesis of debilitating diseases such as glaucoma, ocular trauma, diabetic retinopathy, and ischemia. Elucidation of TRP channel involvement in retinal biology will yield rewards in terms of fundamental understanding of vertebrate vision and therapeutic targeting to treat diseases caused by channel dysfunction or over-activation.

Extreme Membrane Tensile Loads Induce Half-Activation of the Thermosensitive TRPV1 Channel

Transient receptor potential (TRP) channels are sensors for a wide range of cellular and environmental signals, but elucidating how these channels convert a wide range of physical and chemical stimuli into channel opening is essential to understanding their normal function. Here, half-activation of thermosensitive TRPV1 channel under extreme membrane stretching from blast loads was provided by molecular dynamics simulations. The results show that such extreme membrane stretch loading will only lead to half-activation of the TRPV1 channel: that is, the upper gate is open for high-speed stretching (>15m/s), but the lower gate is still closed. The corresponding activation threshold also depends on both the tensile speed and the area strain. This means that the direct mechanical gating of TRP channels in one step is unlikely to occur.

Critical contributions of pre-S1 shoulder and distal TRP box in DAG-activated TRPC6 channel by PIP2 regulation

Phosphatidylinositol 4,5-bisphosphate (PI(4,5)P₂ or PIP₂) regulates the activities of numerous membrane proteins, including diacylglycerol(DAG)-activated TRPC3/6/7 channels. Although PIP₂ binding is known to support DAG-activated TRP channel activity, its binding site remains unknown. We screened for PIP₂ binding sites within TRPC6 channels through extensive mutagenesis. Using voltage-sensitive phosphatase (DrVSP), we found that Arg437 and Lys442, located in the channel’s pre-S1 domain/shoulder, are crucial for interaction with PIP₂. To gain structural insights, we conducted computer protein–ligand docking simulations with the pre-S1 domain/shoulder of TRPC6 channels. Further, the functional significance of PIP₂ binding to the pre-S1 shoulder was assessed for receptor-operated channel functions, cross-reactivity to DAG activation, and the kinetic model simulation. These results revealed that basic residues in the pre-S1 domain/shoulder play a central role in the regulation of PIP₂-dependent gating. In addition, neutralizing mutation of K771 in the distal TRP box reversed the effect of PIP₂ depletion from inhibiting to potentiating channel activity. A similar effect was seen in TRPV1 channels, which suggests that TRPC6 possesses a common but robust polarity switch mediating the PIP₂-dependent effect. Overall, these mutagenesis studies reveal functional and structural insights for how basic residues and channel segments in TRP channels are controlled through phosphoinositides recognition.

Effect of photobiomodulation therapy on painful temporomandibular disorders

To evaluate the effect of photobiomodulation therapy (PBMT) on painful temporomandibular disorders (TMD) patients in a randomized, double-blinded, placebo-controlled manner. Participants were divided into a masseter myalgia group (n = 88) and a temporomandibular joint (TMJ) arthralgia group (n = 87) according to the Diagnostic Criteria for Temporomandibular Disorders (DC/TMD). Both groups randomly received PBMT or placebo treatment once a day for 7 consecutive days, one session. The PBMT was applied with a gallium-aluminum-arsenide (GaAlAs) laser (wavelength = 810 nm) at pre-determined points in the masseter muscle (6 J/cm², 3 regions, 60 s) or TMJ region (6 J/cm², 5 points, 30 s) according to their most painful site. Pain intensity was rated on a 0–10 numerical rating scale (NRS) and pressure pain thresholds (PPT) and mechanical sensitivity mapping were recorded before and after the treatment on day 1 and day 7. Jaw function was assessed by pain free jaw opening, maximum unassisted jaw opening, maximum assisted jaw opening, maximum protrusion and right and left excursion. Data were analyzed with a mixed model analysis of variance (ANOVA). Pain intensity in arthralgia patients decreased over time (P < 0.001) for both types of interventions, however, PBMT caused greater reduction in pain scores than placebo (P = 0.014). For myalgia patients, pain intensity decreased over time (P < 0.001) but without difference between interventions (P = 0.074). PPTs increased in both myalgia (P < 0.001) and TMJ arthralgia patients over time (P < 0.001) but without difference between interventions (P ≥ 0.614). Overall, PBMT was associated with marginally better improvements in range of motion compared to placebo in both myalgia and arthralgia patients. Pain intensity, sensory function and jaw movements improve after both PBMT and placebo treatments in myalgia and arthralgia patients indicating a substantial non-specific effect of PBMT.

Functional Expression of TRPV1 Ion Channel in the Canine Peripheral Blood Mononuclear Cells

TRPV1, known as a capsaicin receptor, is the best-described transient receptor potential (TRP) ion channel. Recently, it was shown to be expressed by non-excitable cells such as lymphocytes. However, the data regarding the functional expression of the TRPV1 channel in the immune cells are often contradictory. In the present study, we performed a phylogenetical analysis of the canine TRP ion channels, we assessed the expression of TRPV1 in the canine peripheral blood mononuclear cells (PBMC) by qPCR and Western blot, and we determined the functionality of TRPV1 by whole-cell patch-clamp recordings and calcium assay. We found high expression of TRPV2, -M2, and -M7 in the canine PBMCs, while expression of TRPV1, -V4 and, -M5 was relatively low. We confirmed that TRPV1 is expressed on the protein level in the PBMC and it localizes in the plasma membrane. The whole-cell patch-clamp recording revealed that capsaicin application caused a significant increase in the current density. Similarly, the results from the calcium assay show a dose-dependent increase in intracellular calcium level in the presence of capsaicin that was partially abolished by capsazepine. Our study confirms the expression of TRPV1 ion channel on both mRNA and protein levels in the canine PBMC and indicates that the ion channel is functional.

The Signalling Effects of Photobiomodulation on Osteoblast Proliferation, Maturation and Differentiation: A Review

Proliferation of osteoblasts is essential for maturation and mineralization of bone matrix. Ossification, the natural phase of bone-forming and hardening is a carefully regulated phase where deregulation of this process may result in insufficient or excessive bone mineralization or ectopic calcification. Osteoblasts can also be differentiated into osteocytes, populating short interconnecting passages within the bone matrix. Over the past few decades, we have seen a significant improvement in awareness and techniques using photobiomodulation (PBM) to stimulate cell function. One of the applications of PBM is the promotion of osteoblast proliferation and maturation. PBM research results on osteoblasts showed increased mitochondrial ATP production, increased osteoblast activity and proliferation, increased and pro-osteoblast expression in the presence of red and NIR radiation. Osteocyte differentiation was also accomplished using blue and green light, showing that different light parameters have various signalling effects. The current review addresses osteoblast function and control, a new understanding of PBM on osteoblasts and its therapeutic impact using various parameters to optimize osteoblast function that may be clinically important. Graphical Abstract.

The Underlying Mechanism of Modulation of Transient Receptor Potential Melastatin 3 by protons

Transient receptor potential melastatin 3 channel (TRPM3) is a calcium-permeable nonselective cation channel that plays an important role in modulating glucose homeostasis in the pancreatic beta cells. However, how TRPM3 is regulated under physiological and pathological conditions is poorly understood. In this study, we found that both intracellular and extracellular protons block TRPM3 through its binding sites in the pore region. We demonstrated that external protons block TRPM3 with an inhibitory pH₅₀ of 5.5. whereas internal protons inhibit TRPM3 with an inhibitory pH₅₀ of 6.9. We identified three titratable residues, D1059, D1062, and D1073, at the vestibule of the channel pore that contributes to pH sensitivity. The mutation of D1073Q reduced TRPM3 current by low external pH 5.5 from 62 ± 3% in wildtype to 25 ± 6.0% in D1073Q mutant. These results indicate that D1073 is essential for pH sensitivity. In addition, we found that a single mutation of D1059 or D1062 enhanced pH sensitivity. In summary, our findings identify molecular determinants respionsible for the pH regulation of TRPM3. The inhibition of TRPM3 by protons may indicate an endogenous mechanism governing TRPM3 gating and its physiological/pathological functions.

Effect of different treatments on recurrent aphthous stomatitis: laser versus medication

Recurrent aphthous stomatitis (RAS) is a common disease with ulcers in oral cavity which may trigger chewing, speaking, and swallowing difficulties to patients. Treatment of RAS is primarily aimed at pain relief and the promotion of wound healing. However, few agents have been found to have definite effect in the management of RAS and most of the medicinal products may cause adverse reactions or other disadvantages, which makes their clinical usage questionable. The purpose of this randomized controlled clinical trial (RCT) was to assess the clinical effect of diode laser and traditional medication treatment on RAS. In this study, 56 patients were randomly assigned to two groups (n = 28). Laser group was treated using diode laser (810 nm, 1.0 W, CW, irradiation time 20 s for 3 applications) once daily for continuous 3 days. Medication group was treated with triamcinolone acetonide 0.1% three times a day until the lesion was healed. Spontaneous and functional pain level on the third day of treatment was significantly less in the laser group. Significant difference was observed with respect to healing time; however, the order of difference is small albeit of statistical significance. Diode laser with the chosen parameters had better effects on pain relief and no distinct advantage on wound healing comparing with medication. Trial registration number: ChiCTR2000030298; date of registration: 26 February 2020 (retrospectively registered).

Phosphoinositides in Retinal Function and Disease

Phosphatidylinositol and its phosphorylated derivatives, the phosphoinositides, play many important roles in all eukaryotic cells. These include modulation of physical properties of membranes, activation or inhibition of membrane-associated proteins, recruitment of peripheral membrane proteins that act as effectors, and control of membrane trafficking. They also serve as precursors for important second messengers, inositol (1,4,5) trisphosphate and diacylglycerol. Animal models and human diseases involving defects in phosphoinositide regulatory pathways have revealed their importance for function in the mammalian retina and retinal pigmented epithelium. New technologies for localizing, measuring and genetically manipulating them are revealing new information about their importance for the function and health of the vertebrate retina.

Applications of photobiomodulation in hearing research: from bench to clinic

Hearing loss is very common and economically burdensome. No accepted therapeutic modality for sensorineural hearing loss is yet available; most clinicians emphasize rehabilitation, placing hearing aids and cochlear implants. Photobiomodulation (PBM) employs light energy to enhance or modulate the activities of specific organs, and is a popular non-invasive therapy used to treat skin lesions and neurodegenerative disorders. Efforts to use PBM to improve hearing have been ongoing for several decades. Initial in vitro studies using cell lines and ex vivo culture techniques have now been supplanted by in vivo studies in animals; PBM protects the sensory epithelium and triggers neural regeneration. Many reports have used PBM to treat tinnitus. In this brief review, we introduce PBM applications in hearing research, helpful protocols, and relevant background literature.

Molecular basis for the sensitivity of TRP channels to polyunsaturated fatty acids

Transient receptor potential (TRP) channels represent a superfamily of unselective cation channels that are subdivided into seven subfamilies based on their sequence homology and differences in gating and functional properties. Little is known about the molecular mechanisms of TRP channel regulation, particularly of the “canonical” TRP (TRPC) subfamily and their activation by polyunsaturated fatty acids (PUFAs). Here, we analyzed the structure-function relationship of Drosophila fruit fly TRPC channels. The primary aim was to uncover the molecular basis of PUFA sensitivity of Drosophila TRP-like (TRPL) and TRPgamma channels. Amino acid (aa) sequence alignment of the three Drosophila TRPC channels revealed 50 aa residues highly conserved in PUFA-sensitive TRPL and TRPgamma channels but not in the PUFA-insensitive TRP channel. Substitution of respective aa in TRPL by corresponding aa of TRP identified 18 residues that are necessary for PUFA-mediated activation of TRPL. Most aa positions are located within a stretch comprising transmembrane domains S2–S4, whereas six aa positions have been assigned to the proximal cytosolic C-terminus. Interestingly, residues I465 and S471 are required for activation by 5,8,11,14-eicosatetraynoic acid (ETYA) but not 5,8,11-eicosatriynoic acid (ETI). As proof of concept, we generated a PUFA-sensitive TRP channel by exchanging the corresponding aa from TRPL to TRP. Our study demonstrates a specific aa pattern in the transmembrane domains S2–S4 and the proximal C-terminus essential for TRP channel activation by PUFAs.

Distinct roles of light-activated channels TRP and TRPL in photoreceptors of Periplaneta americana

Electrophysiological studies in Drosophila melanogaster and Periplaneta americana have found that the receptor current in their microvillar photoreceptors is generated by two light-activated cationic channels, TRP (transient receptor potential) and TRPL (TRP-like), each having distinct properties. However, the relative contribution of the two channel types to sensory information coding by photoreceptors remains unclear. We recently showed that, in contrast to the diurnal Drosophila in which TRP is the principal phototransduction channel, photoreceptors of the nocturnal P. americana strongly depend on TRPL. Here, we perform a functional analysis, using patch-clamp and intracellular recordings, of P. americana photoreceptors after RNA interference to knock down TRP (TRPkd) and TRPL (TRPLkd). Several functional properties were changed in both knockdown phenotypes: cell membrane capacitance was reduced 1.7-fold, light sensitivity was greatly reduced, and amplitudes of sustained light-induced currents and voltage responses decreased more than twofold over the entire range of light intensities. The information rate (IR) was tested using a Gaussian white-noise modulated light stimulus and was lower in TRPkd photoreceptors (28 ± 21 bits/s) than in controls (52 ± 13 bits/s) because of high levels of bump noise. In contrast, although signal amplitudes were smaller than in controls, the mean IR of TRPLkd photoreceptors was unchanged at 54 ± 29 bits/s¹ because of proportionally lower noise. We conclude that TRPL channels provide high-gain/high-noise transduction, suitable for vision in dim light, whereas transduction by TRP channels is relatively low-gain/low-noise and allows better information transfer in bright light.

Proposed Mechanisms of Photobiomodulation or Low-Level Light Therapy

Photobiomodulation (PBM) also known as low-level laser (or light) therapy (LLLT), has been known for almost 50 years but still has not gained widespread acceptance, largely due to uncertainty about the molecular, cellular, and tissular mechanisms of action. However, in recent years, much knowledge has been gained in this area, which will be summarized in this review. One of the most important chromophores is cytochrome c oxidase (unit IV in the mitochondrial respiratory chain), which contains both heme and copper centers and absorbs light into the near-infra-red region. The leading hypothesis is that the photons dissociate inhibitory nitric oxide from the enzyme, leading to an increase in electron transport, mitochondrial membrane potential and ATP production. Another hypothesis concerns light-sensitive ion channels that can be activated allowing calcium to enter the cell. After the initial photon absorption events, numerous signaling pathways are activated via reactive oxygen species, cyclic AMP, NO and Ca2+, leading to activation of transcription factors. These transcription factors can lead to increased expression of genes related to protein synthesis, cell migration and proliferation, anti-inflammatory signaling, anti-apoptotic proteins, antioxidant enzymes. Stem cells and progenitor cells appear to be particularly susceptible to LLLT.

TRP channels: a missing bond in the entrainment mechanism of peripheral clocks throughout evolution

Circadian rhythm may be understood as a temporal organization that works to orchestrate physiological processes and behavior in a period of approximately 24 h. Because such temporal organization has evolved in the presence of predictable environmental clues, such as day length, tides, seasons, and temperature, the organism has confronted the natural selection in highly precise intervals of opportunities and risks, generating temporal programs and resetting mechanisms, which are well conserved among different taxa of animals. The present review brings some evidence of how these programs may have co-evolved in systems able to deal with 2 or more environmental clues, and how they similarly function in different group of animals, stressing how important temperature and light were to establish the temporal organizations. For example, melanopsin and rhodopsin, photopigments present respectively in circadian and visual photoreceptors, are required for temperature discrimination in Drosophila melanogaster. These pigments may signal light and temperature via activation of cationic membrane channel, named transient-receptor potential channel (TRP). In fact, TRPs have been suggested to function as thermal sensor for various groups of animals. Another example is the clock machinery at the molecular level. A set of very-well conserved proteins, known as clock proteins, function as transcription factors in positive and negative auto-regulatory loops generating circadian changes of their expression, and of clock-controlled genes. Similar molecular machinery is present in organisms as diverse as cyanobacteria (Synechococcus), fungi (Neurospora), insects (Drosophila), and vertebrates including humans.

Melanopsins: Localization and Phototransduction in Xenopus laevis Melanophores

Xenopus laevis melanophores express two melanopsins, Opn4x and Opn4m. We identified Opn4x immunoreactivity throughout the melanophore cytoplasm and in the cell membrane. The strongest immunopositivity for Opn4m was observed in the nuclear region, and no labeling was seen in the cell membrane. This immunodistribution suggests Opn4x as the functional photopigment. In X. laevis melanophores, light triggers pigment dispersion and clock gene induction at blue wavelength, which maximally activates melanopsins. Although light stimulation activates phospholipase C and increases intracellular calcium and cGMP, this nucleotide does not participate in photo-induced melanin dispersion. Nevertheless, the guanylyl cyclase activator YC-1 stimulates Per1 expression, similar to blue light pulse, and the use of pharmacological inhibitors indicates the participation of the phosphoinositide cascade. Since cAMP levels does not change after blue light stimulation, the cAMP/PKA pathway most probably is not involved in blue light induction of Per in X. laevis melanophores. Given the localization of melanopsins and our pharmacological data, the light-induced clock gene expression seems to be mediated by Opn4x through phosphoinositide cascade and rise in cGMP, thus leading to the reset of the biological clock in our model.

Forcing open TRP channels: Mechanical gating as a unifying activation mechanism

Transient receptor potential (TRP) proteins are cation channels that comprise a superfamily of molecular sensors that enable animals to detect a wide variety of environmental stimuli. This versatility enables vertebrate and invertebrate TRP channels to function in a diversity of senses, ranging from vision to taste, smell, touch, hearing, proprioception and thermosensation. Moreover, many individual TRP channels are activated through a surprising range of sensory stimuli. The multitasking nature of TRP channels raises the question as to whether seemingly disparate activators gate TRPs through common strategies. In this regard, a recent major advance is the discovery that a phospholipase C (PLC)-dependent signaling cascade activates the TRP channels in Drosophila photoreceptor cells through generation of force in the lipid-bilayer. The premise of this review is that mechanical force is a unifying, common strategy for gating TRP channels. In addition to several TRP channels that function in mechanosensation and are gated by force applied to the cells, changes in temperature or alterations in the concentration of lipophilic second messengers through stimulation of signaling cascades, cause architectural modifications of the cell membrane, which in turn activate TRP channels through mechanical force. Consequently, TRPs are capable of functioning as stretch-activated channels, even in cases in which the stimuli that initiate the signaling cascades are not mechanical. We propose that most TRPs are actually mechanosensitive channels (MSCs), which undergo conformational changes in response to tension imposed on the lipid bilayer, resulting in channel gating.

Transcriptome analysis and RNA interference of cockroach phototransduction indicate three opsins and suggest a major role for TRPL channels

Our current understanding of insect phototransduction is based on a small number of species, but insects occupy many different visual environments. We created the retinal transcriptome of a nocturnal insect, the cockroach, Periplaneta americana to identify proteins involved in the earliest stages of compound eye phototransduction, and test the hypothesis that different visual environments are reflected in different molecular contributions to function. We assembled five novel mRNAs: two green opsins, one UV opsin, and one each TRP and TRPL ion channel homologs. One green opsin mRNA (pGO1) was 100–1000 times more abundant than the other opsins (pGO2 and pUVO), while pTRPL mRNA was 10 times more abundant than pTRP, estimated by transcriptome analysis or quantitative PCR (qPCR). Electroretinograms were used to record photoreceptor responses. Gene-specific in vivo RNA interference (RNAi) was achieved by injecting long (596–708 bp) double-stranded RNA into head hemolymph, and verified by qPCR. RNAi of the most abundant green opsin reduced both green opsins by more than 97% without affecting UV opsin, and gave a maximal reduction of 75% in ERG amplitude 7 days after injection that persisted for at least 19 days. RNAi of pTRP and pTRPL genes each specifically reduced the corresponding mRNA by 90%. Electroretinogram (ERG) reduction by pTRPL RNAi was slower than for opsin, reaching 75% attenuation by 21 days, without recovery at 29 days. pTRP RNAi attenuated ERG much less; only 30% after 21 days. Combined pTRP plus pTRPL RNAi gave only weak evidence of any cooperative interactions. We conclude that silencing retinal genes by in vivo RNAi using long dsRNA is effective, that visible light transduction in Periplaneta is dominated by pGO1, and that pTRPL plays a major role in cockroach phototransduction.

Polymodal roles of transient receptor potential channels in the control of ocular function

Maintenance of intracellular Ca²⁺ levels at orders of magnitude below those in the extracellular environment is a requisite for preserving cell viability. Membrane channels contribute to such control through modulating their time-dependent opening and closing behaviour. Such regulation requires Ca²⁺ to serve as a second messenger mediating receptor control of numerous life-sustaining responses. Transient receptor potential (TRP) channels signal transduce a wide variety of different sensory stimuli to induce responses modulating cellular function. These channels are non-selective cation channels with variable Ca²⁺ selectivity having extensive sequence homology. They constitute a superfamily made up of 28 different members that are subdivided into 7 different subfamilies based on differences in sequence homology. Some of these TRP channel isotypes are expressed in the eye and localized to both neuronal and non-neuronal cell membranes. Their activation generates intracellular Ca²⁺ transients and other downstream-linked signalling events that affect numerous responses required for visual function. As there is an association between changes in functional TRP expression in various ocular diseases, there are efforts underway to determine if these channels can be used as drug targets to reverse declines in ocular function. We review here our current knowledge about the expression, function and regulation of TRPs in different eye tissues in health and disease. Furthermore, some of the remaining hurdles are described to developing safe and efficacious TRP channel modulators for use in a clinical setting.

Role of TRP channels in the induction of heat shock proteins (Hsps) by heating skin

Transient receptor potential (TRP) channels in skin are crucial for achieving temperature sensitivity to maintain internal temperature balance and thermal homeostasis, as well as to protect skin cells from environmental stresses such as infrared (IR) or near-infrared (NIR) radiation via heat shock protein (Hsp) production. However, the mechanisms by which IR and NIR activate TRP channels and produce Hsps intracellularly have been independently reported. In this review, we discuss the relationship between TRP channel activation and Hsp production, and introduce the roles of several skin TRP channels in the regulation of HSP production by IR and NIR exposure.