A novel capacitative calcium entry channel expressed in excitable cells

The olfactory bulb: A neuroendocrine spotlight on feeding and metabolism

Olfaction is the most ancient sense and is needed for food-seeking, danger protection, mating and survival. It is often the first sensory modality to perceive changes in the external environment, before sight, taste or sound. Odour molecules activate olfactory sensory neurons that reside on the olfactory epithelium in the nasal cavity, which transmits this odour-specific information to the olfactory bulb (OB), where it is relayed to higher brain regions involved in olfactory perception and behaviour. Besides odour processing, recent studies suggest that the OB extends its function into the regulation of food intake and energy balance. Furthermore, numerous hormone receptors associated with appetite and metabolism are expressed within the OB, suggesting a neuroendocrine role outside the hypothalamus. Olfactory cues are important to promote food preparatory behaviours and consumption, such as enhancing appetite and salivation. In addition, altered metabolism or energy state (fasting, satiety and overnutrition) can change olfactory processing and perception. Similarly, various animal models and human pathologies indicate a strong link between olfactory impairment and metabolic dysfunction. Therefore, understanding the nature of this reciprocal relationship is critical to understand how olfactory or metabolic disorders arise. This present review elaborates on the connection between olfaction, feeding behaviour and metabolism and will shed light on the neuroendocrine role of the OB as an interface between the external and internal environments. Elucidating the specific mechanisms by which olfactory signals are integrated and translated into metabolic responses holds promise for the development of targeted therapeutic strategies and interventions aimed at modulating appetite and promoting metabolic health.

Structural and accessibility studies highlight the differential binding of clemizole to TRPC5 and TRPC6

Transient Receptor Potential Canonical 5 (T RP C5) and T RP C6 channels play critical physiological roles in various cell types. Their involvement in numerous disease progression mechanisms has led to extensive searches for their inhibitors. Although several potent T RP C inhibitors have been developed and the structure of their binding sites were mapped using cryo electron microscopy, a comprehensive understanding of the molecular interactions within the inhibitor binding site of T RP Cs remains elusive. This study aimed to decipher the structural determinants and molecular mechanisms contributing to the differential binding of clemizole to T RP C5 and T RP C6, with a particular focus on the accessibility of binding site residues. This information can help better understand what molecular features allow for selective binding, which is a key characteristic of clinically effective pharmacological agents. Using computational methodologies, we conducted an in-depth molecular docking analysis of clemizole with T RP C5 and T RP C6 channels. The protein structures were retrieved from publicly accessible protein databases. Discovery Studio 2020 Client Visualizer and Chimera software facilitated our in-silico mutation experiments and enabled us to identify the critical structural elements influencing clemizole binding. Our study reveals key molecular determinants at the clemizole binding site, specifically outlining the role of residues' Accessible Surface Area (ASA) and Relative Accessible Surface Area (RASA) in differential binding. We found that lower accessibility of T RP C6 binding site residues, compared to those in T RP C5, could account for the lower affinity binding of clemizole to T RP C6. This work illuminates the pivotal role of binding site residue accessibility in determining the affinity of clemizole to T RP C5 and T RP C6. A nuanced understanding of the distinct binding properties between these homologous proteins may pave the way for the development of more selective inhibitors, promising improved therapeutic efficacy and fewer off-target effects. By demystifying the structural and molecular subtleties of T RP C inhibitors, this research could significantly accelerate the drug discovery process, offering hope to patients afflicted with T RP C-related diseases.

From zinc homeostasis to disease progression: Unveiling the neurodegenerative puzzle

Zinc is a crucial trace element in the human body, playing a role in various physiological processes such as oxidative stress, neurotransmission, protein synthesis, and DNA repair. The zinc transporters (ZnTs) family members are responsible for exporting intracellular zinc, while Zrt- and Irt-like proteins (ZIPs) are involved in importing extracellular zinc. These processes are essential for maintaining cellular zinc homeostasis. Imbalances in zinc metabolism have been linked to the development of neurodegenerative diseases. Disruptions in zinc levels can impact the survival and activity of neurons, thereby contributing to the progression of neurodegenerative diseases through mechanisms like cell apoptosis regulation, protein phase separation, ferroptosis, oxidative stress, and neuroinflammation. Therefore, conducting a systematic review of the regulatory network of zinc and investigating the relationship between zinc dysmetabolism and neurodegenerative diseases can enhance our understanding of the pathogenesis of these diseases. Additionally, it may offer new insights and approaches for the treatment of neurodegenerative diseases.

Pharmacological Differences between Native Homomeric Transient Receptor Potential Canonical Type 4 Channels and Heteromeric Transient Receptor Potential Canonical Type 1/4 Channels in Lateral Septal Neurons

Given the unique expression patterns and revelations of its critical involvement in a host of neurological disorders, the TRPC1/4/5 subgroup has become an intense target of drug development, and some compounds are now in clinical trials. However, little is known about the exact subunit composition of this subfamily of TRPC channels in various native tissues, and whether it has functional and pharmacological implications. In this study, we investigated the effects of two TRPC4 modulators located in the lateral septum, in which a metabotropic glutamate receptor (mGluR) agonist-induced plateau potential is mediated by TRPC channels composed of TRPC1 and TRPC4. Lateral septal neurons were recorded intracellularly in brain slices using sharp electrodes. Drugs were applied via bath superfusion. We showed that the plateau potential in mice lacking TRPC1 is modulated by ML204 and La3+ in a manner that is like homomeric TRPC4 channels in artificial expression systems. However, the plateau potential that is primarily mediated by heteromeric TRPC1/4 channels in lateral septal neurons in wildtype mice was modulated differently by ML204 and La3+. Our data suggest that native homomeric TRPC4 channels and heteromeric TRPC1/4 channels are pharmacologically distinct, and the current drug development strategy regarding TRPC1/4/5 may need to be reevaluated.

"ThermoTRP" Channel Expression in Cancers: Implications for Diagnosis and Prognosis (Practical Approach by a Pathologist)

Temperature-sensitive transient receptor potential (TRP) channels (so-called "thermoTRPs") are multifunctional signaling molecules with important roles in cell growth and differentiation. Several "thermoTRP" channels show altered expression in cancers, though it is unclear if this is a cause or consequence of the disease. Regardless of the underlying pathology, this altered expression may potentially be used for cancer diagnosis and prognostication. "ThermoTRP" expression may distinguish between benign and malignant lesions. For example, TRPV1 is expressed in benign gastric mucosa, but is absent in gastric adenocarcinoma. TRPV1 is also expressed both in normal urothelia and non-invasive papillary urothelial carcinoma, but no TRPV1 expression has been seen in invasive urothelial carcinoma. "ThermoTRP" expression can also be used to predict clinical outcomes. For instance, in prostate cancer, TRPM8 expression predicts aggressive behavior with early metastatic disease. Furthermore, TRPV1 expression can dissect a subset of pulmonary adenocarcinoma patients with bad prognosis and resistance to a number of commonly used chemotherapeutic agents. This review will explore the current state of this rapidly evolving field with special emphasis on immunostains that can already be added to the armoire of diagnostic pathologists.

TRPC3 governs the spatiotemporal organization of cellular Ca2+ signatures by functional coupling to IP3 receptors

Communication between TRPC channels and IP₃ receptors (IP₃R) is considered pivotal in the generation of spatiotemporal Ca²⁺signaling patterns. Here we revisited the role of TRPC3-IP₃R coupling for local Ca²⁺ signaling within TRPC3-harbouring micro/nanodomains using R-GECO as a reporter, fused to the channel´s C-terminus. Cytoplasmic Ca²⁺ changes at TRPC3 originated from both the entry of Ca²⁺ through the TRPC channel and Ca²⁺ mobilization via IP₃R. Local Ca²⁺ changes at TRPC3 channels expressed in HEK293 cells were predominantly biphasic with IP₃R-dependent initial Ca²⁺ transients, while exclusively monophasic signals were recorded when all three IP₃R isoforms were lacking. Abrogation of Ca²⁺ entry through TRPC3 by point mutations, which impair Ca²⁺ permeability (E630Q), cation permeation (E630K), or DAG sensitivity (G652A), promoted microdomain Ca²⁺ oscillations. Ca²⁺ signals at E630Q, E630K, and G652A channels featured initial Ca²⁺ transients along with oscillatory activity. Similarly, when extracellular Ca²⁺ was omitted, IP₃R-mediated Ca²⁺ transients and Ca²⁺ oscillations were promoted at the cytoplasmic face of wild-type TRPC3 channels. By contrast, oscillations, as well as initial Ca²⁺ transients, were virtually lacking, when the TRPC3 channels were sensitized by preexposure to low-level PLC activity. TIRF imaging provided evidence for dynamic colocalization of TRPC3 and IP₃R. We suggest that TRPC3-mediated Ca²⁺ entry controls IP₃R activity at ER-PM junctions to determine Ca²⁺ signaling signatures and enable specificity of downstream signaling.

Spinal TRPA1 Contributes to the Mechanical Hypersensitivity Effect Induced by Netrin-1

Netrin-1, a chemoattractant expressed by floor plate cells, and one of its receptors (deleted in colorectal cancer) has been associated with pronociceptive actions in a number of pain conditions. Here, we addressed the question of whether spinal TRPC4/C5 or TRPA1 are among the downstream receptors contributing to pronociceptive actions induced by netrin-1. The experiments were performed on rats using a chronic intrathecal catheter for administration of netrin-1 and antagonists of TRPC4/C5 or TRPA1. Pain sensitivity was assessed behaviorally by using mechanical and heat stimuli. Effect on the discharge rate of rostral ventromedial medullary (RVM) pain control neurons was studied in lightly anesthetized animals. Netrin-1, in a dose-related fashion, induced mechanical hypersensitivity that lasted up to three weeks. Netrin-1 had no effect on heat nociception. Mechanical hypersensitivity induced by netrin-1 was attenuated by TRPA1 antagonist Chembridge-5861528 and by the control analgesic compound pregabalin both during the early (first two days) and late (third week) phase of hypersensitivity. TRPC4/C5 antagonist ML-204 had a weak antihypersensitivity effect that was only in the early phase, whereas TRPC4/C5 antagonist HC-070 had no effect on hypersensitivity induced by netrin-1. The discharge rate in pronociceptive ON-like RVM neurons was increased by netrin-1 during the late but not acute phase, whereas netrin-1 had no effect on the discharge rate of antinociceptive RVM OFF-like neurons. The results suggest that spinal TRPA1 receptors and pronociceptive RVM ON-like neurons are involved in the maintenance of submodality-selective pronociceptive actions induced by netrin-1 in the spinal cord.

Minireview: Insights into the role of TRP channels in the retinal circulation and function

Consistent with their wide distribution throughout the CNS, transcripts of all transient receptor potential (TRP) cation channel superfamily members have been detected in both neuronal and non-neuronal cells of the mammalian retina. Evidence shows that members of the TRPC (canonical, TRPC1/4/5/6), TRPV (vanilloid, TRPV1/2/4), TRPM (melastatin, TRPM1/2/3/5), TRPA (ankyrin, TRPA1), and TRPP (polycystin, TRPP2) subfamilies contribute to retinal function and circulation in health and disease, but the relevance of most TRPs has yet to be determined. Their principal role in light detection is far better understood than their participation in the control of intraocular pressure, retinal blood flow, oxidative stress, ion homeostasis, and transmitter signaling for retinal information processing. Moreover, if the therapeutic potential of targeting some TRPs to treat various retinal diseases remains speculative, recent studies highlight that vision restoration strategies are very likely to benefit from the thermo- and mechanosensitive properties of TRPs. This minireview focuses on the evidence of the past 5 years about the role of TRPs in the retina and retinal circulation, raises some possibilities about the function of TRPs in the retina, and discusses the potential sources of endogenous stimuli for TRPs in this tissue, as a reflection for future studies.

Novel localizations of TRPC5 channels suggest novel and unexplored roles: A study in the chick embryo brain

Mammalian TRPC5 channels are predominantly expressed in the brain, where they increase intracellular Ca²⁺ and induce depolarization. Because they augment presynaptic vesicle release, cause persistent neural activity, and show constitutive activity, TRPC5s could play a functional role in late developmental brain events. We used immunohistochemistry to examine TRPC5 in the chick embryo brain between 8 and 20 days of incubation, and provide the first detailed description of their distribution in birds and in the whole brain of any animal species. Stained areas substantially increased between E8 and E16, and staining intensity in many areas peaked at E16, a time when chick brains first show organized patterns of whole-brain metabolic activation like what is seen consistently after hatching. Areas showing cell soma staining match areas showing Trpc5 mRNA or protein in adult rodents (cerebral cortex, hippocampus, amygdala, cerebellar Purkinje cells). Chick embryos show protein staining in the optic tectum, cerebellar nuclei, and several brainstem nuclei; equivalent areas in the Allen Institute mouse maps express Trpc5 mRNA. The strongest cell soma staining was found in a dorsal hypothalamic area (matching a group of parvicellular arginine vasotocin neurons and a pallial amygdalohypothalamic cell corridor) and the vagal motor complex. Purkinje cells showed strong dendritic staining at E20. Unexpectedly, we also describe neurite staining in the septum, several hypothalamic nuclei, and a paramedian raphe area; the strongest neurite staining was in the median eminence. These novel localizations suggest new unexplored TRPC5 functions, and possible roles in late embryonic brain development.

Structural basis for human TRPC5 channel inhibition by two distinct inhibitors

TRPC5 channel is a nonselective cation channel that participates in diverse physiological processes. TRPC5 inhibitors show promise in the treatment of anxiety disorder, depression, and kidney disease. However, the binding sites and inhibitory mechanism of TRPC5 inhibitors remain elusive. Here, we present the cryo-EM structures of human TRPC5 in complex with two distinct inhibitors, namely clemizole and HC-070, to the resolution of 2.7 Å. The structures reveal that clemizole binds inside the voltage sensor-like domain of each subunit. In contrast, HC-070 is wedged between adjacent subunits and replaces the glycerol group of a putative diacylglycerol molecule near the extracellular side. Moreover, we found mutations in the inhibitor binding pockets altered the potency of inhibitors. These structures suggest that both clemizole and HC-070 exert the inhibitory functions by stabilizing the ion channel in a nonconductive closed state. These results pave the way for further design and optimization of inhibitors targeting human TRPC5.

Effects of Transient Receptor Potential Cation 5 (TRPC5) Inhibitor, NU6027, on Hippocampal Neuronal Death after Traumatic Brain Injury

Traumatic brain injury (TBI) can cause physical, cognitive, social, and behavioral changes that can lead to permanent disability or death. After primary brain injury, translocated free zinc can accumulate in neurons and lead to secondary events such as oxidative stress, inflammation, edema, swelling, and cognitive impairment. Under pathological conditions, such as ischemia and TBI, excessive zinc release, and accumulation occurs in neurons. Based on previous research, it hypothesized that calcium as well as zinc would be influx into the TRPC5 channel. Therefore, we hypothesized that the suppression of TRPC5 would prevent neuronal cell death by reducing the influx of zinc and calcium. To test our hypothesis, we used a TBI animal model. After the TBI, we immediately injected NU6027 (1 mg/kg, intraperitoneal), TRPC5 inhibitor, and then sacrificed animals 24 h later. We conducted Fluoro-Jade B (FJB) staining to confirm the presence of degenerating neurons in the hippocampal cornus ammonis 3 (CA3). After the TBI, the degenerating neuronal cell count was decreased in the NU6027-treated group compared with the vehicle-treated group. Our findings suggest that the suppression of TRPC5 can open a new therapeutic window for a reduction of the neuronal death that may occur after TBI.

Transient Receptor Potential Canonical (TRPC) Channels: Then and Now

Twenty-five years ago, the first mammalian Transient Receptor Potential Canonical (TRPC) channel was cloned, opening the vast horizon of the TRPC field. Today, we know that there are seven TRPC channels (TRPC1-7). TRPCs exhibit the highest protein sequence similarity to the Drosophila melanogaster TRP channels. Similar to Drosophila TRPs, TRPCs are localized to the plasma membrane and are activated in a G-protein-coupled receptor-phospholipase C-dependent manner. TRPCs may also be stimulated in a store-operated manner, via receptor tyrosine kinases, or by lysophospholipids, hypoosmotic solutions, and mechanical stimuli. Activated TRPCs allow the influx of Ca2+ and monovalent alkali cations into the cytosol of cells, leading to cell depolarization and rising intracellular Ca2+ concentration. TRPCs are involved in the continually growing number of cell functions. Furthermore, mutations in the TRPC6 gene are associated with hereditary diseases, such as focal segmental glomerulosclerosis. The most important recent breakthrough in TRPC research was the solving of cryo-EM structures of TRPC3, TRPC4, TRPC5, and TRPC6. These structural data shed light on the molecular mechanisms underlying TRPCs' functional properties and propelled the development of new modulators of the channels. This review provides a historical overview of the major advances in the TRPC field focusing on the role of gene knockouts and pharmacological tools.

The Expression of Cold-Inducible RNA-Binding Protein mRNA in Sow Genital Tract Is Modulated by Natural Mating, But Not by Seminal Plasma

The RNA-binding proteins (RBPs), some of them induced by transient receptor potential (TRP) ion channels, are crucial regulators of RNA function that can contribute to reproductive pathogenesis, including inflammation and immune dysfunction. This study aimed to reveal the influence of spermatozoa, seminal plasma, or natural mating on mRNA expression of RBPs and TRP ion channels in different segments of the internal genital tract of oestrous, preovulatory sows. Particularly, we focused on mRNA expression changes of the cold-inducible proteins (CIPs) and related TRP channels. Pre-ovulatory sows were naturally mated (NM) or cervically infused with semen (Semen-AI) or sperm-free seminal plasma either from the entire ejaculate (SP-TOTAL) or the sperm-rich fraction (SP-AI). Samples (cervix to infundibulum) were collected by laparotomy under general anaesthesia for transcriptomic analysis (GeneChip^® Porcine Gene 1.0 ST Array) 24 h after treatments. The NM treatment induced most of the mRNA expression changes, compared to Semen-AI, SP-AI, and SP-TOTAL treatments including unique significative changes in CIRBP, RBM11, RBM15B, RBMS1, TRPC1, TRPC4, TRPC7, and TRPM8. The findings on the differential mRNA expression on RBPs and TRP ion channels, especially to CIPs and related TRP ion channels, suggest that spermatozoa and seminal plasma differentially modulated both protein families during the preovulatory phase, probably related to a still unknown early signalling mechanism in the sow reproductive tract.

How TRPC Channels Modulate Hippocampal Function

Transient receptor potential canonical (TRPC) proteins constitute a group of receptor-operated calcium-permeable nonselective cationic membrane channels of the TRP superfamily. They are largely expressed in the hippocampus and are able to modulate neuronal functions. Accordingly, they have been involved in different hippocampal functions such as learning processes and different types of memories, as well as hippocampal dysfunctions such as seizures. This review covers the mechanisms of activation of these channels, how these channels can modulate neuronal excitability, in particular the after-burst hyperpolarization, and in the persistent activity, how they control synaptic plasticity including pre- and postsynaptic processes and how they can interfere with cell survival and neurogenesis.

Transient Receptor Potential Canonical 5-Scramblase Signaling Complex Mediates Neuronal Phosphatidylserine Externalization and Apoptosis

Phospholipid scramblase 1 (PLSCR1), a lipid-binding and Ca²⁺-sensitive protein located on plasma membranes, is critically involved in phosphatidylserine (PS) externalization, an important process in cell apoptosis. Transient receptor potential canonical 5 (TRPC5), is a nonselective Ca²⁺ channel in neurons that interacts with many downstream molecules, participating in diverse physiological functions including temperature or mechanical sensation. The interaction between TRPC5 and PLSCR1 has never been reported. Here, we showed that PLSCR1 interacts with TRPC5 through their C-termini in HEK293 cells and mouse cortical neurons. Formation of TRPC5-PLSCR1 complex stimulates PS externalization and promotes cell apoptosis in HEK293 cells and mouse cerebral neurons. Furthermore, in vivo studies showed that PS externalization in cortical neurons induced by artificial cerebral ischemia-reperfusion was reduced in TRPC5 knockout mice compared to wild-type mice, and that the percentage of apoptotic neurons was also lower in TRPC5 knockout mice than in wild-type mice. Collectively, the present study suggested that TRPC5-PLSCR1 is a signaling complex mediating PS externalization and apoptosis in neurons and that TRPC5 plays a pathological role in cerebral-ischemia reperfusion injury.

TRPC1 Regulates the Activity of a Voltage-Dependent Nonselective Cation Current in Hippocampal CA1 Neurons

The cation channel subunit TRPC1 is strongly expressed in central neurons including neurons in the CA1 region of the hippocampus where it forms complexes with TRPC4 and TRPC5. To investigate the functional role of TRPC1 in these neurons and in channel function, we compared current responses to group I metabotropic glutamate receptor (mGluR I) activation and looked for major differences in dendritic morphology in neurons from TRPC1^+/+ and TRPC1^-/- mice. mGluR I stimulation resulted in the activation of a voltage-dependent nonselective cation current in both genotypes. Deletion of TRPC1 resulted in a modification of the shape of the current-voltage relationship, leading to an inward current increase. In current clamp recordings, the percentage of neurons that responded to depolarization in the presence of an mGluR I agonist with a plateau potential was increased in TRPC1^-/- mice. There was also a small increase in the minor population of CA1 neurons that have more than one apical dendrite in TRPC1^-/- mice. We conclude that TRPC1 has an inhibitory effect on receptor-operated nonselective cation channels in hippocampal CA1 neurons probably as a result of heterotetramer formation with other TRPC isoforms, and that TRPC1 deletion has only minor effects on dendritic morphology.

TRPC5 regulates axonal outgrowth in developing retinal ganglion cells

The TRPC5 ion channel is activated upon depletion of intracellular calcium stores, as well as by various stimuli such as nitric oxide (NO), membrane stretch, and cold temperatures. TRPC5 is abundantly expressed in the central nervous system where it has important neuronal functions. In the chick retina, TRPC5 expression was shown to be restricted to amacrine cells (ACs) and Müller glial cells, although its expression was also observed in the ganglion cell layer (GCL) in displaced ACs, as determined by their characteristic cell morphology. However, it is possible that this expression analysis alone might be insufficient to fully understand the expression of TRPC5 in retinal ganglion cells (RGCs). Hence, we analyzed TRPC5 expression by in situ hybridization and immunostaining in the developing mouse retina, and for the first time identified that developing and mature RGCs strongly express TRPC5. The expression begins at E14.5, and is restricted to ACs and RGCs. It was reported that TRPC5 negatively regulates axonal outgrowth in hippocampal neurons. We thus hypothesized that TRPC5 might have similar functions in RGCs since they extend very long axons toward the brain, and this characteristic significantly differs from other retinal cell types. To elucidate its possible involvement in axonal outgrowth, we inhibited TRPC5 activity in developing RGCs which significantly increased RGC axon length. In contrast, overexpression of TRPC5 inhibited axonal outgrowth in developing RGCs. These results indicate that TRPC5 is an important negative regulator of RGC axonal outgrowth. Since TRPC5 is a mechanosensor, it might function to sense abnormal intraocular pressure changes, and could contribute to the death of RGCs in diseases such as glaucoma. In this case, excessive Ca²⁺ entry through TRPC5 might induce dendritic and axonal remodeling, which could lead to cell death, as our findings clearly indicate that TRPC5 is an important regulator of neurite remodeling.

TRPC1 mediates slow excitatory synaptic transmission in hippocampal oriens/alveus interneurons

Hippocampal GABAergic interneurons play key roles in regulating principal cell activity and plasticity. Interneurons located in stratum oriens/alveus (O/A INs) receive excitatory inputs from CA1 pyramidal cells and express a Hebbian form of long-term potentiation (LTP) at their excitatory input synapses. This LTP requires the activation of metabotropic glutamate receptors 1a (mGluR1a) and Ca²⁺ entry via transient receptor potential (TRP) channels. However, the type of TRP channels involved in synaptic transmission at these synapses remains largely unknown. Using patch-clamp recordings, we show that slow excitatory postsynaptic currents (EPSCs) evoked in O/A INs are dependent on TRP channels but may be independent of phospholipase C. Using reverse transcription polymerase chain reaction (RT-PCR) we found that mRNA for TRPC 1, 3–7 was present in CA1 hippocampus. Using single-cell RT-PCR, we found expression of mRNA for TRPC 1, 4–7, but not TRPC3, in O/A INs. Using co-immunoprecipitation assays in HEK-293 cell expression system, we found that TRPC1 and TRPC4 interacted with mGluR1a. Co-immunoprecipitation in hippocampus showed that TRPC1 interacted with mGluR1a. Using immunofluorescence, we found that TRPC1 co-localized with mGluR1a in O/A IN dendrites, whereas TRPC4 localization appeared limited to O/A IN cell body. Down-regulation of TRPC1, but not TRPC4, expression in O/A INs using small interfering RNAs prevented slow EPSCs, suggesting that TRPC1 is an obligatory TRPC subunit for these EPSCs. Our findings uncover a functional role of TRPC1 in mGluR1a-mediated slow excitatory synaptic transmission onto O/A INs that could be involved in Hebbian LTP at these synapses.

TRPC channels: Structure, function, regulation and recent advances in small molecular probes

Transient receptor potential canonical (TRPC) channels constitute a group of receptor-operated calcium-permeable nonselective cation channels of the TRP superfamily. The seven mammalian TRPC members, which can be further divided into four subgroups (TRPC1, TRPC2, TRPC4/5, and TRPC3/6/7) based on their amino acid sequences and functional similarities, contribute to a broad spectrum of cellular functions and physiological roles. Studies have revealed complexity of their regulation involving several components of the phospholipase C pathway, G_i and G_o proteins, and internal Ca²⁺ stores. Recent advances in cryogenic electron microscopy have provided several high-resolution structures of TRPC channels. Growing evidence demonstrates the involvement of TRPC channels in diseases, particularly the link between genetic mutations of TRPC6 and familial focal segmental glomerulosclerosis. Because TRPCs were discovered by the molecular identity first, their pharmacology had lagged behind. This is rapidly changing in recent years owning to great efforts from both academia and industry. A number of potent tool compounds from both synthetic and natural products that selective target different subtypes of TRPC channels have been discovered, including some preclinical drug candidates. This review will cover recent advancements in the understanding of TRPC channel regulation, structure, and discovery of novel TRPC small molecular probes over the past few years, with the goal of facilitating drug discovery for the study of TRPCs and therapeutic development.

TRPC channels regulate Ca2+-signaling and short-term plasticity of fast glutamatergic synapses

Transient receptor potential (TRP) proteins form Ca²⁺-permeable, nonselective cation channels, but their role in neuronal Ca²⁺ homeostasis is elusive. In the present paper, we show that TRPC channels potently regulate synaptic plasticity by changing the presynaptic Ca²⁺-homeostasis of hippocampal neurons. Specifically, loss of TRPC1/C4/C5 channels decreases basal-evoked secretion, reduces the pool size of readily releasable vesicles, and accelerates synaptic depression during high-frequency stimulation (HFS). In contrast, primary TRPC5 channel-expressing neurons, identified by a novel TRPC5–τ-green fluorescent protein (τGFP) knockin mouse line, show strong short-term enhancement (STE) of synaptic signaling during HFS, indicating a key role of TRPC5 in short-term plasticity. Lentiviral expression of either TRPC1 or TRPC5 turns classic synaptic depression of wild-type neurons into STE, demonstrating that TRPCs are instrumental in regulating synaptic plasticity. Presynaptic Ca²⁺ imaging shows that TRPC activity strongly boosts synaptic Ca²⁺ dynamics, showing that TRPC channels provide an additional presynaptic Ca²⁺ entry pathway, which efficiently regulates synaptic strength and plasticity.

Transient receptor potential (TRP) proteins can form non-selective cation channels, but their role in synaptic transmission is poorly understood. This study shows that calcium-permeable TRPC channels provide an additional calcium entry pathway at presynaptic sites and are efficient regulators of synaptic strength and plasticity.

A simple and dual expression plasmid system in prokaryotic (E. coli) and mammalian cells

We introduce a simple and universal cloning plasmid system for gene expression in prokaryotic (Escherichia coli) and mammalian cells. This novel system has two expression modes: the (subcloning) prokaryotic and mammalian modes. This system streamlines the process of producing mammalian gene expression plasmids with desired genes. The plasmid (prokaryotic mode) has an efficient selection system for DNA insertion, multiple component genes with rare restriction sites at both ends (termed “units”), and a simple transformation to mammalian expression mode utilizing rare restriction enzymes and re-ligation (deletion step). The new plasmid contains the lac promoter and operator followed by a blunt-end EcoRV recognition site, and a DNA topoisomerase II toxin-originated gene for effective selection with isopropyl-β-D-thiogalactoside (IPTG) induction. This system is highly efficient for the subcloning of blunt-end fragments, including PCR products. After the insertion of the desired gene, protein encoded by the desired gene can be detected in E. coli with IPTG induction. Then, the lac promoter and operator are readily deleted with 8-nucleotide rare-cutter blunt-end enzymes (deletion step). Following re-ligation and transformation, the plasmid is ready for mammalian expression analysis (mammalian mode). This idea (conversion from prokaryotic to mammalian mode) can be widely adapted. The pgMAX system overwhelmingly simplifies prokaryotic and mammalian gene expression analyses.

TRPC5 ion channel permeation promotes weight gain in hypercholesterolaemic mice

Transient Receptor Potential Canonical 5 (TRPC5) is a subunit of a Ca²⁺-permeable non-selective cationic channel which negatively regulates adiponectin but not leptin in mice fed chow diet. Adiponectin is a major anti-inflammatory mediator and so we hypothesized an effect of TRPC5 on the inflammatory condition of atherosclerosis. Atherosclerosis was studied in aorta of ApoE^−/− mice fed western-style diet. Inhibition of TRPC5 ion permeation was achieved by conditional transgenic expression of a dominant negative ion pore mutant of TRPC5 (DNT5). Gene expression analysis in adipose tissue suggested that DNT5 increases transcript expression for adiponectin while decreasing transcript expression of the inflammatory mediator Tnfα and potentially decreasing Il6, Il1β and Ccl2. Despite these differences there was mild or no reduction in plaque coverage in the aorta. Unexpectedly DNT5 caused highly significant reduction in body weight gain and reduced adipocyte size after 6 and 12 weeks of western-style diet. Steatosis and circulating lipids were unaffected but mild effects on regulators of lipogenesis could not be excluded, as indicated by small reductions in the expression of Srebp1c, Acaca, Scd1. The data suggest that TRPC5 ion channel permeation has little or no effect on atherosclerosis or steatosis but an unexpected major effect on weight gain.

Emerging Roles of Diacylglycerol-Sensitive TRPC4/5 Channels

Transient receptor potential classical or canonical 4 (TRPC4) and TRPC5 channels are members of the classical or canonical transient receptor potential (TRPC) channel family of non-selective cation channels. TRPC4 and TRPC5 channels are widely accepted as receptor-operated cation channels that are activated in a phospholipase C-dependent manner, following the G_q/11 protein-coupled receptor activation. However, their precise activation mechanism has remained largely elusive for a long time, as the TRPC4 and TRPC5 channels were considered as being insensitive to the second messenger diacylglycerol (DAG) in contrast to the other TRPC channels. Recent findings indicate that the C-terminal interactions with the scaffolding proteins Na⁺/H⁺ exchanger regulatory factor 1 and 2 (NHERF1 and NHERF2) dynamically regulate the DAG sensitivity of the TRPC4 and TRPC5 channels. Interestingly, the C-terminal NHERF binding suppresses, while the dissociation of NHERF enables, the DAG sensitivity of the TRPC4 and TRPC5 channels. This leads to the assumption that all of the TRPC channels are DAG sensitive. The identification of the regulatory function of the NHERF proteins in the TRPC4/5-NHERF protein complex offers a new starting point to get deeper insights into the molecular basis of TRPC channel activation. Future studies will have to unravel the physiological and pathophysiological functions of this multi-protein channel complex.

Electrophysiological Mechanism of Peripheral Hormones and Nutrients Regulating Energy Homeostasis

In organism, energy homeostasis is a biological process that involves the coordinated homeostatic regulation of energy intake (food intake) and energy expenditure. The human brain, particularly the hypothalamic proopiomelanocortin (POMC)- and agouti-related protein/neuropeptide Y (AgRP/NPY)-expressing neurons in the arcuate nucleus, plays an essential role in regulating energy homeostasis. The regulation process is mainly dependent upon peripheral hormones such as leptin and insulin, as well as nutrients such as glucose, amino acids, and fatty acids. Although many studies have attempted to illustrate the exact mechanisms of glucose and hormones action on these neurons, we still cannot clearly see the full picture of this regulation action. Therefore, in this review we will mainly discuss those established theories and recent progresses in this area, demonstrating the possible physiological mechanism by which glucose, leptin, and insulin affect neuronal excitability of POMC and AgRP neurons. In addition, we will also focus on some important ion channels which are expressed by POMC and AgRP neurons, such as K_ATP channels and TRPC channels, and explain how these channels are regulated by peripheral hormones and nutrients and thus regulate energy homeostasis.

Calcium signaling in Alzheimer's disease & therapies

Alzheimer's disease (AD) is the most common type of dementia and is characterized by the accumulation of amyloid (Aβ) plaques and neurofibrillary tangles in the brain. Much attention has been given to develop AD treatments based on the amyloid cascade hypothesis; however, none of these drugs had good efficacy at improving cognitive functions in AD patients suggesting that Aβ might not be the disease origin. Thus, there are urgent needs for the development of new therapies that target on the proximal cause of AD. Cellular calcium (Ca²⁺) signals regulate important facets of neuronal physiology. An increasing body of evidence suggests that age-related dysregulation of neuronal Ca²⁺ homeostasis may play a proximal role in the pathogenesis of AD as disrupted Ca²⁺ could induce synaptic deficits and promote the accumulation of Aβ plaques and neurofibrillary tangles. Given that Ca²⁺ disruption is ubiquitously involved in all AD pathologies, it is likely that using chemical agents or small molecules specific to Ca²⁺ channels or handling proteins on the plasma membrane and membranes of intracellular organelles to correct neuronal Ca²⁺ dysregulation could open up a new approach to AD prevention and treatment. This review summarizes current knowledge on the molecular mechanisms linking Ca²⁺ dysregulation with AD pathologies and discusses the possibility of correcting neuronal Ca²⁺ disruption as a therapeutic approach for AD.

Redox regulation of transient receptor potential channels in the endothelium

ROS and RNS are important mediators of signaling pathways in the endothelium. Specific members of the TRP superfamily of cation channels act as important Ca²⁺ influx pathways in endothelial cells and are involved in endothelium-dependent vasodilation, regulation of barrier permeability, and angiogenesis. ROS and RNS can modulate the activity of certain TRP channels mainly by modifying specific cysteine residues or by stimulating the production of second messengers. In this review, we highlight the recent literature describing redox regulation of TRP channel activity in endothelial cells as well as the physiological importance of these pathways and implication for cardiovascular diseases.

Nutrient Sensing: Another Chemosensitivity of the Olfactory System

Olfaction is a major sensory modality involved in real time perception of the chemical composition of the external environment. Olfaction favors anticipation and rapid adaptation of behavioral responses necessary for animal survival. Furthermore, recent studies have demonstrated that there is a direct action of metabolic peptides on the olfactory network. Orexigenic peptides such as ghrelin and orexin increase olfactory sensitivity, which in turn, is decreased by anorexigenic hormones such as insulin and leptin. In addition to peptides, nutrients can play a key role on neuronal activity. Very little is known about nutrient sensing in olfactory areas. Nutrients, such as carbohydrates, amino acids, and lipids, could play a key role in modulating olfactory sensitivity to adjust feeding behavior according to metabolic need. Here we summarize recent findings on nutrient-sensing neurons in olfactory areas and delineate the limits of our knowledge on this topic. The present review opens new lines of investigations on the relationship between olfaction and food intake, which could contribute to determining the etiology of metabolic disorders.

Muscling in on TRP channels in vascular smooth muscle cells and cardiomyocytes

The human TRP protein family comprises a family of 27 cation channels with diverse permeation and gating properties. The common theme is that they are very important regulators of intracellular Ca²⁺ signaling in diverse cell types, either by providing a Ca²⁺ influx pathway, or by depolarising the membrane potential, which on one hand triggers the activation of voltage-gated Ca²⁺ channels, and on the other limits the driving force for Ca²⁺ entry. Here we focus on the role of these TRP channels in vascular smooth muscle and cardiac striated muscle. We give an overview of highlights from the recent literature, and highlight the important and diverse roles of TRP channels in the pathophysiology of the cardiovascular system. The discovery of the superfamily of Transient Receptor Potential (TRP) channels has significantly enhanced our knowledge of multiple signal transduction mechanisms in cardiac muscle and vascular smooth muscle cells (VSMC). In recent years, multiple studies have provided evidence for the involvement of these channels, not only in the regulation of contraction, but also in cell proliferation and remodeling in pathological conditions. The mammalian family of TRP cation channels is composed by 28 genes which can be divided into 6 subfamilies groups based on sequence similarity: TRPC (Canonical), TRPM (Melastatin), TRPML (Mucolipins), TRPV (Vanilloid), TRPP (Policystin) and TRPA (Ankyrin-rich protein). Functional TRP channels are believed to form four-unit complexes in the plasma, each of them expressed with six transmembrane domain and intracellular N and C termini. Here we review the current knowledge on the expression of TRP channels in both muscle types, and discuss their functional properties and role in physiological and pathophysiological processes.

The stoichiometry of the TMEM16A ion channel determined in intact plasma membranes of COS-7 cells using liquid-phase electron microscopy

TMEM16A is a membrane protein forming a calcium-activated chloride channel. A homodimeric stoichiometry of the TMEM16 family of proteins has been reported but an important question is whether the protein resides always in a dimeric configuration in the plasma membrane or whether monomers of the protein are also present in its native state within in the intact plasma membrane. We have determined the stoichiometry of the human (h)TMEM16A within whole COS-7 cells in liquid. For the purpose of detecting TMEM16A subunits, single proteins were tagged by the streptavidin-binding peptide within extracellular loops accessible by streptavidin coated quantum dot (QD) nanoparticles. The labeled proteins were then imaged using correlative light microscopy and environmental scanning electron microscopy (ESEM) using scanning transmission electron microscopy (STEM) detection. The locations of 19,583 individual proteins were determined of which a statistical analysis using the pair correlation function revealed the presence of a dimeric conformation of the protein. The amounts of detected label pairs and single labels were compared between experiments in which the TMEM16A SBP-tag position was varied, and experiments in which tagged and non-tagged TMEM16A proteins were present. It followed that hTMEM16A resides in the plasma membrane as dimer only and is not present as monomer. This strategy may help to elucidate the stoichiometry of other membrane protein species within the context of the intact plasma membrane in future.

Major contribution of the 3/6/7 class of TRPC channels to myocardial ischemia/reperfusion and cellular hypoxia/reoxygenation injuries

The injury phase after myocardial infarcts occurs during reperfusion and is a consequence of calcium release from internal stores combined with calcium entry, leading to cell death by apoptopic and necrotic processes. The mechanism(s) by which calcium enters cells has(ve) not been identified. Here, we identify canonical transient receptor potential channels (TRPC) 3 and 6 as the cation channels through which most of the damaging calcium enters cells to trigger their death, and we describe mechanisms activated during the injury phase. Working in vitro with H9c2 cardiomyoblasts subjected to 9-h hypoxia followed by 6-h reoxygenation (H/R), and analyzing changes occurring in areas-at-risk (AARs) of murine hearts subjected to a 30-min ischemia followed by 24-h reperfusion (I/R) protocol, we found: (i) that blocking TRPC with SKF96365 significantly ameliorated damage induced by H/R, including development of the mitochondrial permeability transition and proapoptotic changes in Bcl2/BAX ratios; and (ii) that AAR tissues had increased TUNEL⁺ cells, augmented Bcl2/BAX ratios, and increased p(S240)NFATc3, p(S473)AKT, p(S9)GSK3β, and TRPC3 and -6 proteins, consistent with activation of a positive-feedback loop in which calcium entering through TRPCs activates calcineurin-mediated NFATc3-directed transcription of TRPC genes, leading to more Ca²⁺ entry. All these changes were markedly reduced in mice lacking TRPC3, -6, and -7. The changes caused by I/R in AAR tissues were matched by those seen after H/R in cardiomyoblasts in all aspects except for p-AKT and p-GSK3β, which were decreased after H/R in cardiomyoblasts instead of increased. TRPC should be promising targets for pharmacologic intervention after cardiac infarcts.

Functional Characterization of Transient Receptor Potential (TRP) Channel C5 in Female Murine Gonadotropes

Gonadotrope cells in the anterior pituitary gland secrete gonadotropins regulating gonadal function in mammals. Recent results have implicated transient receptor potential (TRP) cation channels in pituitary physiology; however, if and how TRP channels contribute to gonadotrope function is not known. Here, we report that 14 out of 28 TRP channels encoded in the mouse genome are expressed in murine gonadotropes with highest expression levels found for canonical TRP (TRPC) channel 5 in juvenile females. We show that TRP channel expression in these cells exhibits considerable plasticity and that it depends on the sex and the developmental and hormonal status of the animal. We then combine different genetic strategies including genetic confocal Ca2+ imaging in whole-mount pituitary gland preparations to characterize TRPC5 channel function in gonadotropes from juvenile females. We show that the TRPC5 agonist Englerin A activates a cytosolic Ca2+ signal and a whole-cell current in these cells, which is absent in TRPC5-deficient mice, and demonstrate that TRPC5 forms functional heteromultimers with TRPC1 in gonadotropes. We further show that the Englerin A-activated TRPC5-dependent Ca2+ signal is mediated by Ca2+ influx both via TRPC5 and via l-type voltage-gated Ca2+ channels, activated by the depolarization through TRPC5-mediated cation influx. Finally, we demonstrate that the gonadotropin-releasing hormone (GnRH)-mediated net depolarization is significantly reduced in gonadotropes isolated from TRPC5-deficient mice. In conclusion, our data suggest that TRPC5 contributes to depolarization of the plasma membrane in gonadotropes upon GnRH stimulation and increases the intracellular Ca2+ concentration via its own Ca2+ permeability and via the activation of voltage-gated Ca2+ channels.

Picomolar, selective, and subtype-specific small-molecule inhibition of TRPC1/4/5 channels

The concentration of free cytosolic Ca²⁺ and the voltage across the plasma membrane are major determinants of cell function. Ca²⁺-permeable non-selective cationic channels are known to regulate these parameters, but understanding of these channels remains inadequate. Here we focus on transient receptor potential canonical 4 and 5 proteins (TRPC4 and TRPC5), which assemble as homomers or heteromerize with TRPC1 to form Ca²⁺-permeable non-selective cationic channels in many mammalian cell types. Multiple roles have been suggested, including in epilepsy, innate fear, pain, and cardiac remodeling, but limitations in tools to probe these channels have restricted progress. A key question is whether we can overcome these limitations and develop tools that are high-quality, reliable, easy to use, and readily accessible for all investigators. Here, through chemical synthesis and studies of native and overexpressed channels by Ca²⁺ and patch-clamp assays, we describe compound 31, a remarkable small-molecule inhibitor of TRPC1/4/5 channels. Its potency ranged from 9 to 1300 pm, depending on the TRPC1/4/5 subtype and activation mechanism. Other channel types investigated were unaffected, including TRPC3, TRPC6, TRPV1, TRPV4, TRPA1, TRPM2, TRPM8, and store-operated Ca²⁺ entry mediated by Orai1. These findings suggest identification of an important experimental tool compound, which has much higher potency for inhibiting TRPC1/4/5 channels than previously reported agents, impressive specificity, and graded subtype selectivity within the TRPC1/4/5 channel family. The compound should greatly facilitate future studies of these ion channels. We suggest naming this TRPC1/4/5-inhibitory compound Pico145.

Neurological and Motor Disorders: Neuronal Store-Operated Ca2+ Signaling: An Overview and Its Function

Calcium (Ca2+) is a ubiquitous second messenger that performs significant physiological task such as neurosecretion, exocytosis, neuronal growth/differentiation, and the development and/or maintenance of neural circuits. An important regulatory aspect of neuronal Ca2+ homeostasis is store-operated Ca2+ entry (SOCE) which, in recent years, has gained much attention for influencing a variety of nerve cell responses. Essentially, activation of SOCE ensues following the activation of the plasma membrane (PM) store-operated Ca2+ channels (SOCC) triggered by the depletion of endoplasmic reticulum (ER) Ca2+ stores. In addition to the TRPC (transient receptor potential canonical) and the Orai family of ion channels, STIM (stromal interacting molecule) proteins have been baptized as key molecular regulators of SOCE. Functional significance of the TRPC channels in neurons has been elaborately studied; however, information on Orai and STIM components of SOCE, although seems imminent, is currently limited. Importantly, perturbations in SOCE have been implicated in a spectrum of neuropathological conditions. Hence, understanding the precise involvement of SOCC in neurodegeneration would presumably unveil avenues for plausible therapeutic interventions. We thus review the role of SOCE-regulated neuronal Ca2+ signaling in selecting neurodegenerative conditions.

TRP channels in oxygen physiology: distinctive functional properties and roles of TRPA1 in O2 sensing

Transient Receptor Potential (TRP) proteins form cation channels characterized by a wide variety of activation triggers. Here, we overview a group of TRP channels that respond to reactive redox species to transduce physiological signals, with a focus on TRPA1 and its role in oxygen physiology. Our systematic evaluation of oxidation sensitivity using cysteine-selective reactive disulphides with different redox potentials reveals that TRPA1 has the highest sensitivity to oxidants/electrophiles among the TRP channels, which enables it to sense O₂. Proline hydroxylation by O₂-dependent hydroxylases also regulates the O₂-sensing function by inhibiting TRPA1 in normoxia; TRPA1 is activated by hypoxia through relief from the inhibition and by hyperoxia through cysteine oxidation that overrides the inhibition. TRPA1 enhances neuronal discharges induced by hyperoxia and hypoxia in the vagus to underlie respiratory adaptation to changes in O₂ availability. This importance of TRPA1 in non-carotid body O₂ sensors can be extended to the universal significance of redox-sensitive TRP channels in O₂ adaptation.

Multiple target of hAmylin on rat primary hippocampal neurons

Alzheimer's disease (AD) and type II diabetes mellitus (DM2) are the most common aging-related diseases and are characterized by β-amyloid and amylin accumulation, respectively. Multiple studies have indicated a strong correlation between these two diseases. Amylin oligomerization in the brain appears to be a novel risk factor for developing AD. Although amylin aggregation has been demonstrated to induce cytotoxicity in neurons through altering Ca²⁺ homeostasis, the underlying mechanisms have not been fully explored. In this study, we investigated the effects of amylin on rat hippocampal neurons using calcium imaging and whole-cell patch clamp recordings. We demonstrated that the amylin receptor antagonist AC187 abolished the Ca²⁺ response induced by low concentrations of human amylin (hAmylin). However, the Ca²⁺ response induced by higher concentrations of hAmylin was independent of the amylin receptor. This effect was dependent on extracellular Ca²⁺. Additionally, blockade of L-type Ca²⁺ channels partially reduced hAmylin-induced Ca²⁺ response. In whole-cell recordings, hAmylin depolarized the membrane potential. Moreover, application of the transient receptor potential (TRP) channel antagonist ruthenium red (RR) attenuated the hAmylin-induced increase in Ca²⁺. Single-cell RT-PCR demonstrated that transient receptor potential vanilloid 4 (TRPV4) mRNA was expressed in most of the hAmylin-responsive neurons. In addition, selective knockdown of TRPV4 channels inhibited the hAmylin-evoked Ca²⁺ response. These results indicated that different concentrations of hAmylin act through different pathways. The amylin receptor mediates the excitatory effects of low concentrations of hAmylin. In contrast, for high concentrations of hAmylin, hAmylin aggregates precipitated on the neuronal membrane, activated TRPV4 channels and subsequently triggered membrane voltage-gated calcium channel opening followed by membrane depolarization. Therefore, our data suggest that TRPV4 is a key molecular mediator for the cytotoxic effects of hAmylin on hippocampal neurons.

PERK regulates Gq protein-coupled intracellular Ca2+ dynamics in primary cortical neurons

PERK (EIF2AK3) is an ER-resident eIF2α kinase required for behavioral flexibility and metabotropic glutamate receptor-dependent long-term depression via its translational control. Motivated by the recent discoveries that PERK regulates Ca²⁺ dynamics in insulin-secreting β-cells underlying glucose-stimulated insulin secretion, and modulates Ca²⁺ signals-dependent working memory, we explored the role of PERK in regulating G_q protein-coupled Ca²⁺ dynamics in pyramidal neurons. We found that acute PERK inhibition by the use of a highly specific PERK inhibitor reduced the intracellular Ca²⁺ rise stimulated by the activation of acetylcholine, metabotropic glutamate and bradykinin-2 receptors in primary cortical neurons. More specifically, acute PERK inhibition increased IP₃ receptor mediated ER Ca²⁺ release, but decreased receptor-operated extracellular Ca²⁺ influx. Impaired G_q protein-coupled intracellular Ca²⁺ rise was also observed in genetic Perk knockout neurons. Taken together, our findings reveal a novel role of PERK in neurons, which is eIF2α-independent, and suggest that the impaired working memory in forebrain-specific Perk knockout mice may stem from altered G_q protein-coupled intracellular Ca²⁺ dynamics in cortical pyramidal neurons.

Calcium-permeable ion channels in the kidney

Calcium ions (Ca(2+)) are crucial for a variety of cellular functions. The extracellular and intracellular Ca(2+) concentrations are thus tightly regulated to maintain Ca(2+) homeostasis. The kidney, one of the major organs of the excretory system, regulates Ca(2+) homeostasis by filtration and reabsorption. Approximately 60% of the Ca(2+) in plasma is filtered, and 99% of that is reabsorbed by the kidney tubules. Ca(2+) is also a critical signaling molecule in kidney development, in all kidney cellular functions, and in the emergence of kidney diseases. Recently, studies using genetic and molecular biological approaches have identified several Ca(2+)-permeable ion channel families as important regulators of Ca(2+) homeostasis in kidney. These ion channel families include transient receptor potential channels (TRP), voltage-gated calcium channels, and others. In this review, we provide a brief and systematic summary of the expression, function, and pathological contribution for each of these Ca(2+)-permeable ion channels. Moreover, we discuss their potential as future therapeutic targets.

Sensing of redox status by TRP channels

Cellular redox status is maintained by the balance between series of antioxidant systems and production of reactive oxygen/nitrogenous species. Cells utilize this redox balance to mediate diverse physiological functions. Transient receptor potential (TRP) channels are non-selective cation channels that act as biosensors for environmental and noxious stimuli, such as capsaicin and allicin, as well as changes in temperature and conditions inside the cell. TRP channels also have an emerging role as essential players in detecting cellular redox status to regulate cellular signals mediating physiological phenomena. Reactive species activate TRP channels either directly through oxidative amino acid modifications or indirectly through second messengers. For instance, TRPA1, TRPV1 and TRPC5 channels are directly activated by oxidizing agents through cysteine modification; whereas, TRPM2 channel is indirectly activated by production of ADP-ribose. One intriguing property of several TRP channels is susceptibility to both oxidizing and reducing stimuli, suggesting TRP channels could potentially act as a bidirectional sensor for detecting deviations in redox status. In this review, we discuss the unique chemical physiologies of redox sensitive TRP channels and their physiological significance in Ca(2+) signaling.

Transient receptor potential channel C5 in cancer chemoresistance

The transient receptor potential (TRP) superfamily contains at least 28 homologs in mammalian. These proteins form TRP channels are permeable to monovalent and divalent cations and participate in a variety of physiological functions. Dysregulation of TRP channels is responsible for numerous diseases. This review provides a brief short overview of mammalian TRP channels with a focus on TRPC5 and its role in cancers. Dysregulation of TRPC5 interrupts Ca(2+) homeostasis in cancer cells, which activates signaling pathways that are highly associated with cancer progression, especially cancer chemoresistance. Based on the important role of TRPC5, we also discuss the potential of TRPC5 as a target for therapeutic intervention. Either direct targeting of TRPC5 or indirect interruption of TRPC5-related signaling pathways may effectively overcome cancer chemoresistance.

From GTP and G proteins to TRPC channels: a personal account

By serendipity and good fortune, as a postdoctoral fellow in 1967, I landed at the right place at the right time, as I was allowed to investigate the mechanism by which hormones activate the enzyme adenylyl cyclase (then adenyl cyclase) in Martin Rodbell's Laboratory at the NIH in Bethesda, Maryland. The work uncovered first, the existence of receptors separate from the enzyme and then, the existence of transduction mechanisms requiring guanosine-5'-triphosphate (GTP) and Mg(2+). With my laboratory colleagues first and postdoctoral fellows after leaving NIH, I participated in the development of the field "signal transduction by G proteins," uncovered by molecular cloning several G-protein-coupled receptors (GPCRs) and became interested in both the molecular makeup of voltage-gated Ca channels and Ca2+ homeostasis downstream of activation of phospholipase C (PLC) by the Gq/11 signaling pathway. We were able to confirm the hypothesis that there would be mammalian homologues of the Drosophila "transient receptor potential" channel and discovered the existence of six of the seven mammalian genes, now called transient receptor potential canonical (TRPC) channels. In the present article, I summarize from a bird's eye view of what I feel were key findings along this path, not only from my laboratory but also from many others, that allowed for the present knowledge of cell signaling involving G proteins to evolve. Towards the end, I summarize roles of TRPC channels in health and disease.

Transcriptomic differences between day and night in Acropora millepora provide new insights into metabolite exchange and light-enhanced calcification in corals

The evolutionary success of reef-building corals is often attributed to their symbiotic relationship with photosynthetic dinoflagellates of the genus Symbiodinium, but metabolic interactions between the partners and the molecular bases of light-enhanced calcification (LEC) are not well understood. Here, the metabolic bases of the interaction between the coral Acropora millepora and its dinoflagellate symbiont were investigated by comparing gene expression levels under light and dark conditions at the whole transcriptome level. Among the 497 differentially expressed genes identified, a suite of genes involved in cholesterol transport was found to be upregulated under light conditions, confirming the significance of this compound in the coral symbiosis. Although ion transporters likely to have roles in calcification were not differentially expressed in this study, expression levels of many genes associated with skeletal organic matrix composition and organization were higher in light conditions. This implies that the rate of organic matrix synthesis is one factor limiting calcification at night. Thus, LEC during the day is likely to be a consequence of increases in both matrix synthesis and the supply of precursor molecules as a result of photosynthetic activity.

Redox-sensitive transient receptor potential channels in oxygen sensing and adaptation

Regulation of ion channels is central to the mechanisms that underlie immediate acute physiological responses to changes in the availability of molecular oxygen (O₂). A group of cation-permeable channels that are formed by transient receptor potential (TRP) proteins have been characterized as exquisite sensors of redox reactive species and as efficient actuators of electric/ionic signals in vivo. In this review, we first discuss how redox-sensitive TRP channels such as TRPA1 have recently emerged as sensors of the relatively inert oxidant O₂. With regard to the physiological significance of O₂ sensor TRP channels, vagal TRPA1 channels are mainly discussed with respect to their role in respiratory regulation in comparison with canonical pathways in glomus cells of the carotid body, which is a well-established O₂-sensing organ. TRPM7 channels are discussed regarding hypoxia-sensing function in ischemic cell death. Also, ubiquitous expression of TRPA1 and TRPM7 together with their physiological relevance in the body is examined. Finally, based upon these studies on TRP channels, we propose a hypothesis of “O₂ remodeling.” The hypothesis is that cells detect deviation of O₂ availability from appropriate levels via sensors and adjust local O₂ environments in vivo by controlling supply and consumption of O₂ via pathways comprising cellular signals and transcription factors downstream of sensors, which consequently optimize physiological functions. This new insight into O₂ adaptation through ion channels, particularly TRPs, may foster a paradigm shift in our understanding in the biological significance of O₂.

STATE TRANSITION7-Dependent Phosphorylation Is Modulated by Changing Environmental Conditions, and Its Absence Triggers Remodeling of Photosynthetic Protein Complexes

In plants and algae, the serine/threonine kinase STN7/STT7, orthologous protein kinases in Chlamydomonas reinhardtii and Arabidopsis (Arabidopsis thaliana), respectively, is an important regulator in acclimation to changing light environments. In this work, we assessed STT7-dependent protein phosphorylation under high light in C. reinhardtii, known to fully induce the expression of light-harvesting complex stress-related protein3 (LHCSR3) and a nonphotochemical quenching mechanism, in relationship to anoxia where the activity of cyclic electron flow is stimulated. Our quantitative proteomics data revealed numerous unique STT7 protein substrates and STT7-dependent protein phosphorylation variations that were reliant on the environmental condition. These results indicate that STT7-dependent phosphorylation is modulated by the environment and point to an intricate chloroplast phosphorylation network responding in a highly sensitive and dynamic manner to environmental cues and alterations in kinase function. Functionally, the absence of the STT7 kinase triggered changes in protein expression and photoinhibition of photosystem I (PSI) and resulted in the remodeling of photosynthetic complexes. This remodeling initiated a pronounced association of LHCSR3 with PSI-light harvesting complex I (LHCI)-ferredoxin-NADPH oxidoreductase supercomplexes. Lack of STT7 kinase strongly diminished PSII-LHCII supercomplexes, while PSII core complex phosphorylation and accumulation were significantly enhanced. In conclusion, our study provides strong evidence that the regulation of protein phosphorylation is critical for driving successful acclimation to high light and anoxic growth environments and gives new insights into acclimation strategies to these environmental conditions.

Thermosensitive transient receptor potential (TRP) channel agonists and their role in mechanical, thermal and nociceptive sensations as assessed using animal models

INTRODUCTION

The present paper summarizes research using animal models to investigate the roles of thermosensitive transient receptor potential (TRP) channels in somatosensory functions including touch, temperature and pain. We present new data assessing the effects of eugenol and carvacrol, agonists of the warmth-sensitive TRPV3, on thermal, mechanical and pain sensitivity in rats.

METHODS

Thermal sensitivity was assessed using a thermal preference test, which measured the amount of time the animal occupied one of two adjacent thermoelectric plates set at different temperatures. Pain sensitivity was assessed as an increase in latency of hindpaw withdrawal away from a noxious thermal stimulus directed to the plantar hindpaw (Hargreaves test). Mechanical sensitivity was assessed by measuring the force exerted by an electronic von Frey filament pressed against the plantar surface that elicited withdrawal.

RESULTS

Topical application of eugenol and carvacrol did not significantly affect thermal preference, although there was a trend toward avoidance of the hotter surface in a 30 vs. 45°C preference test for rats treated with 1 or 10% eugenol and carvacrol. Both eugenol and carvacrol induced a concentration-dependent increase in thermal withdrawal latency (analgesia), with no significant effect on mechanosensitivity.

CONCLUSIONS

The analgesic effect of eugenol and carvacrol is consistent with previous studies. The tendency for these chemicals to increase the avoidance of warmer temperatures suggests a possible role for TRPV3 in warmth detection, also consistent with previous studies. Additional roles of other thermosensitive TRP channels (TRPM8 TRPV1, TRPV2, TRPV4, TRPM3, TRPM8, TRPA1, TRPC5) in touch, temperature and pain are reviewed.

Redox regulation of transient receptor potential channels

SIGNIFICANCE

Environmental and endogenous reactive species such as reactive oxygen species (ROS), reactive nitrogen species (RNS), and other electrophiles are not only known to exert toxic effects on organisms, but are also emerging as molecules that mediate cell signaling responses. However, the mechanisms underlying this cellular redox signaling by reactive species remains largely uncharacterized.

RECENT ADVANCES

Ca2+-permeable cation channels encoded by the transient receptor potential (trp) gene superfamily are characterized by a wide variety of activation triggers that act from outside and inside the cell. Recent studies have revealed that multiple TRP channels sense reactive species and induce diverse physiological and pathological responses, such as cell death, chemokine production, and pain transduction. TRP channels sense reactive species either indirectly through second messengers or directly via oxidative modification of cysteine residues. In this review, we describe the activation mechanisms and biological roles of redox-sensitive TRP channels, including TRPM2, TRPM7, TRPC5, TRPV1, and TRPA1.

CRITICAL ISSUES

The sensitivity of TRP channels to reactive species in vitro has been well characterized using molecular and pharmacological approaches. However, the precise activation mechanism(s) and in vivo function(s) of ROS/RNS-sensitive TRP channels remain elusive.

FUTURE DIRECTIONS

Redox sensitivity of TRP channels has been shown to mediate previously unexplained biological phenomena and is involved in various pathologies. Understanding the physiological significance and activation mechanisms of TRP channel regulation by reactive species may lead to TRP channels becoming viable pharmacological targets, and modulators of these channels may offer therapeutic options for previously untreatable diseases.

Physiological Function and Characterization of TRPCs in Neurons

Ca²⁺ entry is essential for regulating vital physiological functions in all neuronal cells. Although neurons are engaged in multiple modes of Ca²⁺ entry that regulates variety of neuronal functions, we will only discuss a subset of specialized Ca²⁺-permeable non-selective Transient Receptor Potential Canonical (TRPC) channels and summarize their physiological and pathological role in these excitable cells. Depletion of endoplasmic reticulum (ER) Ca²⁺ stores, due to G-protein coupled receptor activation, has been shown to activate TRPC channels in both excitable and non-excitable cells. While all seven members of TRPC channels are predominately expressed in neuronal cells, the ion channel properties, mode of activation, and their physiological responses are quite distinct. Moreover, many of these TRPC channels have also been suggested to be associated with neuronal development, proliferation and differentiation. In addition, TRPCs also regulate neurosecretion, long-term potentiation and synaptic plasticity. Similarly, perturbations in Ca²⁺ entry via the TRPC channels have been also suggested in a spectrum of neuropathological conditions. Hence, understanding the precise involvement of TRPCs in neuronal function and in neurodegenerative conditions would presumably unveil avenues for plausible therapeutic interventions for these devastating neuronal diseases.