Pyrazolopyrimidines as Potent Stimulators for Transient Receptor Potential Canonical 3/6/7 Channels

Pathophysiological significance and modulation of the transient receptor potential canonical 3 ion channel

Transient receptor potential canonical 3 (TRPC3) protein belongs to the TRP family of nonselective cation channels. Its activation occurs by signaling through a G protein-coupled receptor (GPCR) and a phospholipase C-dependent (PLC) pathway. Perturbations in the expression of TRPC3 are associated with a plethora of pathophysiological conditions responsible for disorders of the cardiovascular, immune, and central nervous systems. The recently solved cryo-EM structure of TRPC3 provides detailed inputs about the underlying mechanistic aspects of the channel, which in turn enables more efficient ways of designing small-molecule modulators. Pharmacologically targeting TRPC3 in animal models has demonstrated great efficacy in treating diseases including cancers, neurological disorders, and cardiovascular diseases. Despite extensive scientific evidence supporting some strong correlations between the expression and activity of TRPC3 and various pathophysiological conditions, therapeutic strategies based on its pharmacological modulations have not led to clinical trials. The development of small-molecule TRPC3 modulators with high safety, sufficient brain penetration, and acceptable drug-like profiles remains in progress. Determining the pathological mechanisms for TRPC3 involvement in human diseases and understanding the requirements for a drug-like TRPC3 modulator will be valuable in advancing small-molecule therapeutics to future clinical trials. In this review, we provide an overview of the origin and activation mechanism of TRPC3 channels, diseases associated with irregularities in their expression, and new development in small-molecule modulators as potential therapeutic interventions for treating TRPC3 channelopathies.

Small molecules targeting canonical transient receptor potential channels: an update

Transient receptor potential canonical (TRPC) channels belong to an important class of non-selective cation channels. This channel family consists of multiple members that widely participate in various physiological and pathological processes. Previous studies have uncovered the intricate regulation of these channels, as well as the spatial arrangement of TRPCs and the binding sites for various small molecule compounds. Multiple small molecules have been identified as selective agonists or inhibitors targeting different subtypes of TRPC, including potential preclinical drug candidates. This review covers recent advancements in the understanding of TRPC regulation and structure and the discovery of TRPC small molecules over the past few years, with the aim of facilitating research on TRPCs and small-molecule drug discovery.

Discovery of N-alkyl-N-benzyl thiazoles as novel TRPC antagonists for the treatment of glioblastoma multiforme

Glioblastoma multiforme represents a substantial clinical challenge. Transient receptor potential channel (TRPC) antagonists might provide new therapeutic options for this aggressive cancer. In this study, a series of N-alkyl-N-benzoyl and N-alkyl-N-benzyl thiazoles were designed and prepared using a scaffold-hopping strategy and evaluated as TRPC6 antagonists. This resulted in the discovery of 15g, a potent TRPC antagonist that exhibited suitable inhibitory micromolar activities against TRPC3, TRPC4, TRPC5, TPRC6, and TRPC7 and displayed noteworthy anti-glioblastoma efficacy in vitro against U87 cell lines. In addition, 15g featured an acceptable pharmacokinetic profile and exhibited better in vivo potency (25 mg/kg/d) than the frontline therapeutic agent temozolomide (50 mg/kg/d) in xenograft models. Taken together, the TRPC antagonist 15g represents a promising lead compound for developing new anti-glioblastoma agents.

The role of TRPV ion channels in adipocyte differentiation: What is the evidence?

Obesity is a complex disorder, and the incidence of obesity continues to rise at an alarming rate worldwide. In particular, the growing incidence of overweight and obesity in children is a major health concern. However, the underlying mechanisms of obesity remain unclear and the efficacy of several approaches for weight loss is limited. As an important calcium-permeable temperature-sensitive cation channel, transient receptor potential vanilloid (TRPV) ion channels directly participate in thermo-, mechano-, and chemosensory responses. Modulation of TRPV ion channel activity can alter the physiological function of the ion channel, leading to neurodegenerative diseases, chronic pain, cancer, and skin disorders. In recent years, increasing studies have demonstrated that TRPV ion channels are abundantly expressed in metabolic organs, including the liver, adipose tissue, skeletal muscle, pancreas, and central nervous system, which has been implicated in various metabolic diseases, including obesity and diabetes mellitus. In addition, as an important process for the pathophysiology of adipocyte metabolism, adipocyte differentiation plays a critical role in obesity. In this review, we focus on the role of TRPV ion channels in adipocyte differentiation to broaden the ideas for prevention and control strategies for obesity.

Discovery of potential novel TRPC5 inhibitors by virtual screening and bioassay

The transient receptor potential canonical channel 5 (TRPC5), a member of the TRPC family, plays a crucial role in the regulation of various physiological activities and diseases, including those related to the central nervous system, cardiovascular system, kidney, and cancer. As a nonselective cation channel, TRPC5 mainly controls the influx of extracellular Ca2+ into cells, thereby modulating cellular depolarization and intracellular ion concentration. Inhibition of TRPC5 by small molecules presents a promising approach for the treatment of TRPC5-associated diseases. In this study, we conducted a comprehensive virtual screening of more than 1.5 million molecules from the Chemdiv database (https://www.chemdiv.com) to identify potential inhibitors of hTRPC5, utilizing the published structures and binding sites of hTRPC5 as a basis. Lipinski's rule, Veber's rule, PAINS filters, pharmacophore analysis, molecular docking, ADMET evaluation and cluster analysis methods were applied for the screening. From this rigorous screening process, 18 candidates exhibiting higher affinities to hTRPC5 were subsequently evaluated for their inhibitory effects on Ca2+ influx using a fluorescence-based assay. Notably, two molecules, namely SML-1 and SML-13, demonstrated significant inhibition of intracellular Ca2+ levels in hTRPC5-overexpressing HEK 293T cells, with IC50 values of 10.2 μM and 10.3 μM, respectively. These findings highlight SML-1 and SML-13 as potential lead molecules for the development of therapeutics targeting hTRPC5 and its associated physiological activities and diseases.

Modulation and Regulation of Canonical Transient Receptor Potential 3 (TRPC3) Channels

Canonical transient receptor potential 3 (TRPC3) channel is a non-selective cation permeable channel that plays an essential role in calcium signalling. TRPC3 is highly expressed in the brain and also found in endocrine tissues and smooth muscle cells. The channel is activated directly by binding of diacylglycerol downstream of G-protein coupled receptor activation. In addition, TRPC3 is regulated by endogenous factors including Ca2+ ions, other endogenous lipids, and interacting proteins. The molecular and structural mechanisms underlying activation and regulation of TRPC3 are incompletely understood. Recently, several high-resolution cryogenic electron microscopy structures of TRPC3 and the closely related channel TRPC6 have been resolved in different functional states and in the presence of modulators, coupled with mutagenesis studies and electrophysiological characterisation. Here, we review the recent literature which has advanced our understanding of the complex mechanisms underlying modulation of TRPC3 by both endogenous and exogenous factors. TRPC3 plays an important role in Ca2+ homeostasis and entry into cells throughout the body, and both pathological variants and downstream dysregulation of TRPC3 channels have been associated with a number of diseases. As such, TRPC3 may be a valuable therapeutic target, and understanding its regulatory mechanisms will aid future development of pharmacological modulators of the channel.

Transient Receptor Potential Canonical 6 (TRPC6) Channel in the Pathogenesis of Diseases: A Jack of Many Trades

The mammalian Transient Receptor Potential Canonical (TRPC) subfamily comprises seven transmembrane proteins (TRPC1-7) forming cation channels in the plasma membrane of mammalian cells. TRPC channels mediate Ca²⁺ and Na⁺ influx into the cells. Amongst TRPCs, TRPC6 deficiency or increased activity due to gain-of-function mutations has been associated with a multitude of diseases, such as kidney disease, pulmonary disease, and neurological disease. Indeed, the TRPC6 protein is expressed in various organs and is involved in diverse signalling pathways. The last decade saw a surge in the investigative studies concerning the physiological roles of TRPC6 and describing the development of new pharmacological tools modulating TRPC6 activity. The current review summarizes the progress achieved in those investigations.

Interaction Patterns of Pyrazolopyrimidines with Receptor Proteins

Heterocyclic compounds have a prominent role in medicinal chemistry and drug design. They are not only useful as medicinally active compounds but also as a modular structural scaffold for drug design. Therefore, heterocycles are present in many ligands that exhibit a broad spectrum of biological activities. Pyazolopyrimidines are nitrogen heterocycles and are part of many biologically active compounds and marketed drugs. This study examines the non-covalent interactions between the pyrazolopyrimidine rings and receptor proteins through data mining and analysis of high-resolution crystal structures deposited in the Protein Data Bank. The Protein Data Bank contains 471 crystal structures with pyrazolopyrimidine derivatives as ligands, among which 50% contains 1H-pyrazolo[3,4-d]pyrimidines (Pyp1), while 38% contains pyrazolo[1,5-a] pyrimidines (Pyp2). 1H-Pyrazolo[4,3-d]pyrimidines (Pyp3) are found in 11% of the structures, and no structural data is available for pyrazolo[1,5-c]pyrimidine isomers (Pyp4). Among receptor proteins, transferases are found in most examples (67.5%), followed by hydrolases (13.4%) and oxidoreductases (8.9%). Detailed analysis of structures to identify the most prevalent interactions of pyrazolopyrimidines with proteins shows that aromatic π···π interactions are present in ∼91% of the structures and hydrogen bonds/other polar contacts are present in ∼73% of the structures. The centroid-centroid distances (d_cent) between the pyrazolopyrimidine rings and aromatic side chains of the proteins have been retrieved from crystal structures recorded at a high resolution (data resolution <2.0 Å). The average value of d_cent in pyrazolopyrimidine-protein complexes is 5.32 Å. The information on the geometric parameters of aromatic interactions between the core pyrazolopyrimidine ring and the protein would be helpful in future in silico modeling studies on pyrazolopyrimidine-receptor complexes.

TRPV3: Structure, Diseases and Modulators

Transient receptor potential vanillin 3 (TRPV3) is a member of the transient receptor potential (TRP) superfamily. As a Ca²⁺-permeable nonselective cation channel, TRPV3 can recognize thermal stimulation (31-39 °C), and it plays an important regulatory role in temperature perception, pain transduction, skin physiology, inflammation, cancer and other diseases. TRPV3 is not only activated by the changes in the temperature, but it also can be activated by a variety of chemical and physical stimuli. Selective TRPV3 agonists and antagonists with regulatory effects and the physiological functions for clinical application are highly demanded. In recent years, significant progress has been made in the study of TRPV3, but there is still a lack of modulators with a strong affinity and excellent selectivity. This paper reviews the functional characteristics of TRPV3 in terms of the structure, diseases and the research on TRPV3 modulators.

New Positive TRPC6 Modulator Penetrates Blood–Brain Barrier, Eliminates Synaptic Deficiency and Restores Memory Deficit in 5xFAD Mice

Synapse loss in the brain of Alzheimer’s disease patients correlates with cognitive dysfunctions. Drugs that limit synaptic loss are promising pharmacological agents. The transient receptor potential cation channel, subfamily C, member 6 (TRPC6) regulates the formation of an excitatory synapse. Positive regulation of TRPC6 results in increased synapse formation and enhances learning and memory in animal models. The novel selective TRPC6 agonist, 3-(3-,4-Dihydro-6,7-dimethoxy-3,3-dimethyl-1-isoquinolinyl)-2H-1-benzopyran-2-one, has recently been identified. Here we present in silico, in vitro, ex vivo, pharmacokinetic and in vivo studies of this compound. We demonstrate that it binds to the extracellular agonist binding site of the human TRPC6, protects hippocampal mushroom spines from amyloid toxicity in vitro, efficiently recovers synaptic plasticity in 5xFAD brain slices, penetrates the blood–brain barrier and recovers cognitive deficits in 5xFAD mice. We suggest that C20 might be recognized as the novel TRPC6-selective drug suitable to treat synaptic deficiency in Alzheimer’s disease-affected hippocampal neurons.

Synthesis of AC1903 analogs as potent transient receptor potential canonical channel 4/5 inhibitors and biological evaluation

Transient receptor potential canonical (TRPC) channels are a class of non-selective cation channels expressed in a variety of tissues and organ systems where they functionally regulate physiological and pathological processes. TRPC5 has been shown to be a promising target for focal segmental glomerulosclerosis treatment. In this study, we report the synthesis and biological evaluation of a novel series of benzimidazole-based TRPC5 inhibitors. One compound, 8b, is 100-fold more potent than the parent compound, AC1903, in the suppression of TRPC5 channel activity. Interestingly, both AC1903 and 8b also suppressed TRPC4 channel activity with similar potency. Compound 8b also significantly blunts protamine sulfate-induced reorganization of podocyte cytoskeleton, interleukin (IL)-17-induced cell proliferation, and the expression of proinflammatory mediators in human keratinocyte HaCaT cells.

Structural mechanism of human TRPC3 and TRPC6 channel regulation by their intracellular calcium-binding sites

TRPC3 and TRPC6 channels are calcium-permeable non-selective cation channels that are involved in many physiological processes. The gain-of-function (GOF) mutations of TRPC6 lead to familial focal segmental glomerulosclerosis (FSGS) in humans, but their pathogenic mechanism remains elusive. Here, we report the cryo-EM structures of human TRPC3 in both high-calcium and low-calcium conditions. Based on these structures and accompanying electrophysiological studies, we identified both inhibitory and activating calcium-binding sites in TRPC3 that couple intracellular calcium concentrations to the basal channel activity. These calcium sensors are also structurally and functionally conserved in TRPC6. We uncovered that the GOF mutations of TRPC6 activate the channel by allosterically abolishing the inhibitory effects of intracellular calcium. Furthermore, structures of human TRPC6 in complex with two chemically distinct inhibitors bound at different ligand-binding pockets reveal different conformations of the transmembrane domain, providing templates for further structure-based drug design targeting TRPC6-related diseases such as FSGS.

Pharmacology of TRPC Channels and Its Potential in Cardiovascular and Metabolic Medicine

Transient receptor potential canonical (TRPC) proteins assemble to form homo- or heterotetrameric, nonselective cation channels permeable to K⁺, Na⁺, and Ca²⁺. TRPC channels are thought to act as complex integrators of physical and chemical environmental stimuli. Although the understanding of essential physiological roles of TRPC channels is incomplete, their implication in various pathological mechanisms and conditions of the nervous system, kidneys, and cardiovascular system in combination with the lack of major adverse effects of TRPC knockout or TRPC channel inhibition is driving the search of TRPC channel modulators as potential therapeutics. Here, we review the most promising small-molecule TRPC channel modulators, the understanding of their mode of action, and their potential in the study and treatment of cardiovascular and metabolic disease.

Ion channels alterations in the forebrain of high-fat diet fed rats

Evidence suggests that transient receptor potential (TRP) ion channels dysfunction significantly contributes to the physiopathology of metabolic and neurological disorders. Dysregulation in functions and expression in genes encoding the TRP channels cause several inherited diseases in humans (the so-called 'TRP channelopathies'), which affect the cardiovascular, renal, skeletal, and nervous systems. This study aimed to evaluate the expression of ion channels in the forebrain of rats with diet-induced obesity (DIO). DIO rats were studied after 17 weeks under a hypercaloric diet (high-fat diet, HFD) and were compared to the control rats with a standard diet (CHOW). To determine the systemic effects of HFD exposure, we examined food intake, fat mass content, fasting glycemia, insulin levels, cholesterol, and triglycerides. qRT-PCR, Western blot, and immunochemistry analysis were performed in the frontal cortex (FC) and hippocampus (HIP). After 17 weeks of HFD, DIO rats increased their body weight significantly compared to the CHOW rats. In DIO rats, TRPC1 and TRPC6 were upregulated in the HIP, while they were downregulated in the FC. In the case of TRPM2 expression, instead was increased both in the HIP and in the FC. These could be related to the increase of proteins and nucleic acid oxidation. TRPV1 and TRPV2 gene expression showed no differences both in the FC and HIP. In general, qRT-PCR analyses were confirmed by Western blot analysis. Immunohistochemical procedures highlighted the expression of the channels in the cell body of neurons and axons, particularly for the TRPC1 and TRPC6. The alterations of TRP channel expression could be related to the activation of glial cells or the neurodegenerative process presented in the brain of the DIO rat highlighted with post synaptic protein (PSD 95) alterations. The availability of suitable animal models may be useful for studying possible pharmacological treatments to counter obesity-induced brain injury. The identified changes in DIO rats may represent the first insight to characterize the neuronal alterations occurring in obesity. Further investigations are necessary to characterize the role of TRP channels in the regulation of synaptic plasticity and obesity-related cognitive decline.

GSK1702934A and M085 directly activate TRPC6 via a mechanism of stimulating the extracellular cavity formed by the pore helix and transmembrane helix S6

Transient receptor potential canonical (TRPC) channels, as important membrane proteins regulating intracellular calcium (Ca2+i) signaling, are involved in a variety of physiological and pathological processes. Activation and regulation of TRPC are more dependent on membrane or intracellular signals. However, how extracellular signals regulate TRPC6 function remains to be further investigated. Here, we suggest that two distinct small molecules, M085 and GSK1702934A, directly activate TRPC6, both through a mechanism of stimulation of extracellular sites formed by the pore helix (PH) and transmembrane (TM) helix S6. In silico docking scanning of TRPC6 identified three extracellular sites that can bind small molecules, of which only mutations on residues of PH and S6 helix significantly reduced the apparent affinity of M085 and GSK1702934A and attenuated the maximal response of TRPC6 to these two chemicals by altering channel gating of TRPC6. Combing metadynamics, molecular dynamics simulations, and mutagenesis, we revealed that W679, E671, E672, and K675 in the PH and N701 and Y704 in the S6 helix constitute an orthosteric site for the recognition of these two agonists. The importance of this site was further confirmed by covalent modification of amino acid residing at the interface of the PH and S6 helix. Given that three structurally distinct agonists M085, GSK1702934A, and AM-0883, act at this site, as well as the occupancy of lipid molecules at this position found in other TRP subfamilies, it is suggested that the cavity formed by the PH and S6 has an important role in the regulation of TRP channel function by extracellular signals.

Benzothiazole Amides as TRPC3/6 Inhibitors for Gastric Cancer Treatment

Transient receptor potential canonical channel 6 (TRPC6) has been implicated in many kinds of malignant tumors, but very few potent TRPC6 antagonists are available. In this study, a benzothiazole amide derivative 1a was discovered as a TRPC6 activator in a cell-based high-throughput screening. A series of benzothiazole amide derivatives were designed and synthesized. The docking analyses indicated that the conformations of the compounds bound to TRPC6 determined the agonistic or antagonistic activity of the compounds against TRPC6, and compound 1s with the tetrahydronaphthalene group in R₁ position fit well into the binding pocket of the antagonist-bound conformation of TRPC6. Compound 1s showed an inhibitory potency order of TRPC3 (IC₅₀ 3.3 ± 0.13 μM) ≈ C6 (IC₅₀ 4.2 ± 0.1 μM) > C7 with good anti-gastric cancer activity in a micromolecular range against AGS and MKN-45, respectively. In addition, 1s inhibited the invasion and migration of MKN-45 cells in vitro.

Canonical transient receptor potential channels and their modulators: biology, pharmacology and therapeutic potentials

Canonical transient receptor potential channels (TRPCs) are nonselective, high calcium permeability cationic channels. The TRPCs family includes TRPC1, TRPC2, TRPC3, TRPC4, TRPC5, TRPC6, and TRPC7. These channels are widely expressed in the cardiovascular and nervous systems and exist in many other human tissues and cell types, playing several crucial roles in the human physiological and pathological processes. Hence, the emergence of TRPCs modulators can help investigate these channels’ applications in health and disease. It is worth noting that the TRPCs subfamilies have structural and functional similarities, which presents a significant difficulty in screening and discovering of TRPCs modulators. In the past few years, only a limited number of selective modulators of TRPCs were detected; thus, additional research on more potent and more selective TRPCs modulators is needed. The present review focuses on the striking desired therapeutic effects of TRPCs modulators, which provides intel on the structural modification of TRPCs modulators and further pharmacological research. Importantly, TRPCs modulators can significantly facilitate future studies of TRPCs and TRPCs related diseases.

Potential Drug Candidates to Treat TRPC6 Channel Deficiencies in the Pathophysiology of Alzheimer's Disease and Brain Ischemia

Alzheimer’s disease and cerebral ischemia are among the many causative neurodegenerative diseases that lead to disabilities in the middle-aged and elderly population. There are no effective disease-preventing therapies for these pathologies. Recent in vitro and in vivo studies have revealed the TRPC6 channel to be a promising molecular target for the development of neuroprotective agents. TRPC6 channel is a non-selective cation plasma membrane channel that is permeable to Ca²⁺. Its Ca²⁺-dependent pharmacological effect is associated with the stabilization and protection of excitatory synapses. Downregulation as well as upregulation of TRPC6 channel functions have been observed in Alzheimer’s disease and brain ischemia models. Thus, in order to protect neurons from Alzheimer’s disease and cerebral ischemia, proper TRPC6 channels modulators have to be used. TRPC6 channels modulators are an emerging research field. New chemical structures modulating the activity of TRPC6 channels are being currently discovered. The recent publication of the cryo-EM structure of TRPC6 channels should speed up the discovery process even more. This review summarizes the currently available information about potential drug candidates that may be used as basic structures to develop selective, highly potent TRPC6 channel modulators to treat neurodegenerative disorders, such as Alzheimer’s disease and cerebral ischemia.

Protective Role of Tangshen Formula on the Progression of Renal Damage in db/db Mice by TRPC6/Talin1 Pathway in Podocytes

Tangshen Formula (TSF) is a Chinese Medicine formula that has been reported to alleviate proteinuria and protect renal function in humans and animals with diabetic kidney disease (DKD). However, little is known about its mechanism in improving proteinuria. The dysregulation of podocyte cell-matrix adhesion has been demonstrated to play an important role in the pathogenesis and progression of proteinuric kidney diseases including DKD. In the present study, the underlying protective mechanism of TSF on podocytes was investigated using the murine model of type 2 DKD db/db mice in vivo and advanced glycation end products (AGEs)-stimulated primary mice podocytes in vitro. Results revealed that TSF treatment could significantly mitigate reduction of podocyte numbers and foot process effacement, reduce proteinuria, and protect renal function in db/db mice. There was a significant increase in expression of transient receptor potential canonical channel 6 (TRPC6) and a decrease in expression of talin1 in podocytes of db/db mice. The results of AGEs-stimulated primary mice podocytes showed increased cell migration and actin-cytoskeleton rearrangement. Moreover, primary mice podocytes stimulated by AGEs displayed an increase in TRPC6-dependent Ca2+ influx, a loss of talin1, and translocation of nuclear factor of activated T cell (NFATC) 2. These dysregulations in mice primary podocytes stimulated by AGEs could be significantly attenuated after TSF treatment. 1-Oleoyl-2-acetyl-sn-glycerol (OAG), a TRPC6 agonist, blocked the protective role of TSF on podocyte cell-matrix adherence. In conclusion, TSF could protect podocytes from injury and reduce proteinuria in DKD, which may be mediated by the regulation of the TRPC6/Talin1 pathway in podocytes.

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.

Huangkui Capsule Ameliorates Renal Fibrosis in a Unilateral Ureteral Obstruction Mouse Model Through TRPC6 Dependent Signaling Pathways

Renal fibrosis is the final common pathological manifestation of almost all progressive chronic kidney diseases (CKD). Transient receptor potential canonical (TRPC) channels, especially TRPC3/6, were proposed to be essential therapeutic targets for kidney injury. Huangkui capsule (HKC), an important adjuvant therapy for CKD, showed superior efficacy for CKD at stages 1–2 in clinical practice. However, its anti-fibrotic effect and the underlying mechanisms remain to be investigated. In the present study, we evaluated the efficacy of HKC on renal fibrosis in a mouse model of unilateral ureteral obstruction (UUO) and explored the potential underlying mechanism. Administration of HKC by intragastric gavage dose-dependently suppressed UUO-induced kidney injury and tubulointerstitial fibrosis. Similarly, HKC suppressed the expression level of α-smooth muscle actin (α-SMA), increased the expression of E-cadherin, and suppressed the mRNA expression of a plethora of proinflammatory mediators that are necessary for the progression of renal fibrosis. Mechanistically, HKC suppressed both canonical and non-canonical TGF-β signaling pathways in UUO mice as well as the TRPC6/calcineurin A (CnA)/nuclear factor of activated T cells (NFAT) signaling axis. In addition, TRPC6 knockout mice and HKC treated wild type mice displayed comparable protection on UUO-triggered kidney tubulointerstitial injury, interstitial fibrosis, and α-SMA expression. More importantly, HKC had no additional protective effect on UUO-triggered kidney tubulointerstitial injury and interstitial fibrosis in TRPC6 knockout mouse. Further investigation demonstrated that HKC could directly suppress TRPC3/6 channel activities. Considered together, these data demonstrated that the protective effect of HKC on renal injury and interstitial fibrosis is dependent on TRPC6, possibly through direct inhibition of TRPC6 channel activity and indirect suppression of TRPC6 expression.

TRPC4 as a coincident detector of Gi/o and Gq/11 signaling: mechanisms and pathophysiological implications

TRPC channels are Ca²⁺-permeable nonselective cation channels activated downstream from phospholipase C (PLC). Although most TRPC channels can be activated by stimulating G_q/11-coupled receptors, TRPC4 requires simultaneous stimulation of G_i/o-coupled receptors, making it a perfect detector of coincident G_i/o and G_q/11 signaling. Evidence shows that activated Gα_i/o proteins work together with PLCδ1 to induce robust TRPC4 activation and the process is accelerated by stimulation of other PLC isozymes, such as PLCβ through G_q/11 proteins. Mechanistically, G_q/11-PLCβ activation produces triggering proton and calcium signals to initiate self-propagating PLCδ1 activity, crucial for G_i/o-mediated TRPC4 function. Thus, TRPC4-containing channels are activated under conditions not only when coincident G_i/o and G_q/11 stimulation occurs, but also when G_i/o stimulation coincides with proton and Ca²⁺ signals. The resulting cytosolic Ca²⁺ rise and membrane depolarization switch the inhibitory G_i/o response to excitation. The conditions and implications of G_i/o-mediated TRPC4 activation in physiology and pathophysiology warrant further investigation.

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.

Direct Activation of TRPC3 Channels by the Antimalarial Agent Artemisinin

(1) Background: Members of the TRPC3/TRPC6/TRPC7 subfamily of canonical transient receptor potential (TRP) channels share an amino acid similarity of more than 80% and can form heteromeric channel complexes. They are directly gated by diacylglycerols in a protein kinase C-independent manner. To assess TRPC3 channel functions without concomitant protein kinase C activation, direct activators are highly desirable. (2) Methods: By screening 2000 bioactive compounds in a Ca²⁺ influx assay, we identified artemisinin as a TRPC3 activator. Validation and characterization of the hit was performed by applying fluorometric Ca²⁺ influx assays and electrophysiological patch-clamp experiments in heterologously or endogenously TRPC3-expressing cells. (3) Results: Artemisinin elicited Ca²⁺ entry through TRPC3 or heteromeric TRPC3:TRPC6 channels, but did not or only weakly activated TRPC6 and TRPC7. Electrophysiological recordings confirmed the reversible and repeatable TRPC3 activation by artemisinin that was inhibited by established TRPC3 channel blockers. Rectification properties and reversal potentials were similar to those observed after stimulation with a diacylglycerol mimic, indicating that artemisinin induces a similar active state as the physiological activator. In rat pheochromocytoma PC12 cells that endogenously express TRPC3, artemisinin induced a Ca²⁺ influx and TRPC3-like currents. (4) Conclusions: Our findings identify artemisinin as a new biologically active entity to activate recombinant or native TRPC3-bearing channel complexes in a membrane-confined fashion.

3'-O-Methylorobol Inhibits the Voltage-Gated Sodium Channel Nav1.7 with Anti-Itch Efficacy in A Histamine-Dependent Itch Mouse Model

An itch is a clinical complication that affects millions of patients. However, few treatment options are available. The voltage-gated sodium channel Nav1.7 is predominantly expressed in peripheral sensory neurons and is responsible for the rising phase of action potentials, thereby mediating nociceptive conduction. A gain-of-function mutation of Nav1.7 results in the hyperexcitability of sensory neurons and causes the inherited paroxysmal itch. Conversely, a monoclonal antibody that selectively inhibits Nav1.7 is able to effectively suppress the histamine-dependent itch in mice. Therefore, Nav1.7 inhibitors may possess the potential to relieve the itch. In the present study, using whole-cell voltage-clamp recordings, we demonstrated that 3’-O-methylorobol inhibited Na⁺ currents in Nav1.7-CHO cells and tetrodotoxin-sensitive Na⁺ currents in mouse dorsal root ganglion (DRG) neurons with IC₅₀ (half-maximal inhibitory concentration) values of 3.46 and 6.60 μM, respectively. 3’-O-methylorobol also suppressed the tetrodotoxin-resistant Na⁺ currents in DRG neurons, though with reduced potency (~43% inhibition at 30 µM). 3’-O-methylorobol (10 µM) affected the Nav1.7 by shifting the half-maximal voltage (V_1/2) of activation to a depolarizing direction by ~6.76 mV, and it shifted the V_1/2 of inactivation to a hyperpolarizing direction by ~16.79 mV. An analysis of 3’-O-methylorobol activity toward an array of itch targets revealed that 3’-O-methylorobol was without effect on histamine H₁ receptor, TRPV1, TRPV3, TRPV4, TRPC4 and TRPM8. The intrathecal administration of 3’-O-methylorobol significantly attenuated compound 48/80-induced histamine-dependent spontaneous scratching bouts and the expression level of c-fos in the nuclei of spinal dorsal horn neurons with a comparable efficacy to that of cyproheptadine. Our data illustrated the therapeutic potential for 3’-O-methylorobol for histamine-dependent itching, and the small molecule inhibition of Nav1.7 may represent a useful strategy to develop novel therapeutics for itching.

Derivatisation of parthenolide to address chemoresistant chronic lymphocytic leukaemia

Parthenolide is a natural product that exhibits anti-leukaemic activity, however, its clinical use is limited by its poor bioavailability. It may be extracted from feverfew and protocols for growing, extracting and derivatising it are reported. A novel parthenolide derivative with good bioavailability and pharmacological properties was identified through a screening cascade based on in vitro anti-leukaemic activity and calculated "drug-likeness" properties, in vitro and in vivo pharmacokinetics studies and hERG liability testing. In vitro studies showed the most promising derivative to have comparable anti-leukaemic activity to DMAPT, a previously described parthenolide derivative. The newly identified compound was shown to have pro-oxidant activity and in silico molecular docking studies indicate a prodrug mode of action. A synthesis scheme is presented for the production of amine 7 used in the generation of 5f.

Activation of TRPC6 channels contributes to (+)-conocarpan-induced apoptotic cell death in HK-2 cells

(+)-Conocarpan (CNCP), a neolignan frequently found in many medicinal and edible plants displays a broad spectrum of bioactivity. Here, we demonstrated that CNCP induced apoptotic cell death in human kidney-2 (HK-2) cells in a concentration-dependent manner (IC₅₀ = 19.3 μM) and led to the sustained elevation of intracellular Ca²⁺ ([Ca²⁺]_i). Lower extracellular Ca²⁺ concentrations from 2.3 mM to 0 mM significantly suppressed the CNCP-induced Ca²⁺ response by 69.1%. Moreover, the depletion of intracellular Ca²⁺ stores using thapsigargin normalized CNCP-induced Ca²⁺ release from intracellular Ca²⁺ stores, suggesting that the CNCP-induced Ca²⁺ response involved both extracellular Ca²⁺ influx and Ca²⁺ release from intracellular Ca²⁺ stores. SAR7334, a TRPC3/6/7 channel inhibitor, but neither Pyr3, a selective TRPC3 channel inhibitor, nor Pico145, a TRPC1/4/5 inhibitor, suppressed the CNCP-induced Ca²⁺ response by 57.2% and decreased CNCP-induced cell death by 53.4%, suggesting a critical role for TRPC6 channels in CNCP-induced Ca²⁺ influx and apoptotic cell death. Further electrophysiological recording demonstrated that CNCP directly activated TRPC6 channels by increasing channel open probability with an EC₅₀ value of 6.01 μM. Considered together, these data demonstrate that the direct activation of TRPC6 channels contributes to CNCP-induced apoptotic cell death in HK-2 cells. Our data point out the potential risk of renal toxicity from CNCP if used as a therapeutic agent.

Review of Transient Receptor Potential Canonical (TRPC5) Channel Modulators and Diseases

Transient receptor potential canonical (TRPC) channels are highly homologous, nonselective cation channels that form many homo- and heterotetrameric channels. These channels are highly abundant in the brain and kidney and have been implicated in numerous diseases, such as depression, addiction, and chronic kidney disease, among others. Historically, there have been very few selective modulators of the TRPC family in order to fully understand their role in disease despite their physiological significance. However, that has changed recently and there has been a significant increase in interest in this family of channels which has led to the emergence of selective tool compounds, and even preclinical drug candidates, over the past few years. This review will cover these new advancements in the discovery of TRPC modulators and the emergence of newly reported structural information which will undoubtedly lead to even greater advancements.

TRPC-mediated Ca2+ signaling and control of cellular functions

Canonical members of the TRP superfamily of ion channels have long been recognized as key elements of Ca²⁺ handling in a plethora of cell types. The emerging role of TRPC channels in human physiopathology has generated considerable interest in their pharmacological targeting, which requires detailed understanding of their molecular function. Although consent has been reached that receptor-phospholipase C (PLC) pathways and generation of lipid mediators constitute the prominent upstream signaling process that governs channel activity, multimodal sensing features of TRPC complexes have been demonstrated repeatedly. Downstream signaling by TRPC channels is similarly complex and involves the generation of local and global cellular Ca²⁺ rises, which are well-defined in space and time to govern specific cellular functions. These TRPC-mediated Ca²⁺ signals rely in part on Ca²⁺ permeation through the channels, but are essentially complemented by secondary mechanisms such as Ca²⁺ mobilization from storage sites and Na⁺/Ca²⁺ exchange, which involve coordinated interaction with signaling partners. Consequently, the control of cell functions by TRPC molecules is critically determined by dynamic assembly and subcellular targeting of the TRPC complexes. The very recent availability of high-resolution structure information on TRPC channel complexes has paved the way towards a comprehensive understanding of signal transduction by TRPC channels. Here, we summarize current concepts of cation permeation in TRPC complexes, TRPC-mediated shaping of cellular Ca²⁺ signals and the associated control of specific cell functions.

Novel near-infrared II aggregation-induced emission dots for in vivo bioimaging

Near-infrared II fluorescence imaging holds great promise for in vivo imaging and imaging-guided surgery with deep penetration and high spatiotemporal resolution. However, most NIR-II aromatic luminophores suffer from the notorious aggregation-caused quenching (ACQ) effect in the aqueous solution, which largely hinders their biomedical application in vivo. In this study, the first NIR-II organic aggregation-induced emission (AIE) fluorophore (HLZ-BTED), encapsulated as nanoparticles (HLZ-BTED dots) for in vivo biomedical imaging, was designed and synthesized. The NIR-II AIE HLZ-BTED dots showed high temporal resolution, high photostability, outstanding water-solubility and biocompatibility in vitro and in vivo. The HLZ-BTED dots were further used for long-term breast tumor imaging and visualizing tumor-feeding blood vessels, long-term hind limb vasculature and incomplete hind limb ischemia. More importantly, as a proof-of-concept, this is the first time that non-invasive and real-time NIR-II imaging of the gastrointestinal tract in health and disease has been performed, making the AIE dots a promising tool for gastrointestinal (GI) tract research, such as understanding the healthy status of GI peristalsis, diagnosing and evaluating intestinal motility dysfunction, and assessing drug effects on intestinal obstruction.

Discovery and characterization of a positive allosteric modulator of transient receptor potential canonical 6 (TRPC6) channels

The non-selective second messenger-gated cation channel TRPC6 (transient receptor potential canonical 6) is activated by diacylglycerols (DAG) in a PKC-independent manner and plays important roles in a variety of physiological processes and diseases. In order to facilitate novel therapies, the development of potent inhibitors as well as channel-activating agents is of great interest. The screening of a chemical library, comprising about 17,000 small molecule compounds, revealed an agent, which induced increases in intracellular Ca²⁺ concentrations ([Ca²⁺]_i) in a concentration-dependent manner (EC₅₀ = 2.37 ± 0.25 μM) in stably TRPC6-expressing HEK293 cells. This new compound (C20) selectively acts on TRPC6, unlike OAG (1-oleoyl-1-acetyl-sn-glycerol), which also activates PKC and does not discriminate between TRPC6 and the closely related channels TRPC3 and TRPC7. Further evaluation by Ca²⁺ assays and electrophysiological studies revealed that C20 rather operated as an enhancer of channel activation than as an activator by itself and led to the assumption that the compound C20 is an allosteric modulator of TRPC6, enabling low basal concentrations of DAG to induce activation of the ion channel. Furthermore, C20 was tested in human platelets that express TRPC6. A combined activation of TRPC6 with C20 and OAG elicited a robust increase in [Ca²⁺]_i in human platelets. This potentiated channel activation was sensitive to TRPC6 channel blockers. To achieve sufficient amounts of C20 for biological studies, we applied a one-pot synthesis strategy. With regard to studies in native systems, the sensitizing ability of C20 can be a valuable pharmacological tool to selectively exaggerate TRPC6-dependent signals.

Polymethine Thiopyrylium Fluorophores with Absorption beyond 1000 nm for Biological Imaging in the Second Near-Infrared Subwindow

Small-molecule fluorescence imaging in the second near-infrared (NIR-II, 1000-1700 nm) window has gained increasing interest in clinical application. Till now, very few studies have been exploited in the small-molecule fluorophores with both excitation and emission in the NIR-II window. Inspired by the indocyanine green structure, a series of polymethine dyes with both absorption and emission in the NIR-II window have been developed for NIR-II imaging, providing the feasibility to directly compare optical imaging in the NIR-IIa (1300-1400 nm) subwindow under 1064 nm excitation with that in the NIR-II window under 808 nm excitation. The signal-background ratio and the tumor-normal tissue ratio achieved great improvement under 1064 nm excitation in the imaging of mouse blood pool and U87 glioma tumors. Our study not only introduces a broadband emission fluorophore for both NIR-II and NIR-IIa imaging, but also reveals the advantages of NIR-II excitation over NIR-I in in vivo imaging.

Pyrazolo[1,5-a]pyrimidine TRPC6 antagonists for the treatment of gastric cancer

Transient receptor potential canonical 6 (TRPC6) proteins form receptor-operated Ca2+-permeable channels, which have been thought to bring benefit to the treatment of diseases, including cancer. However, selective antagonists for TRPC channels are rare and none of them has been tested against gastric cancer. Compound 14a and analogs were synthesized by chemical elaboration of previously reported TRPC3/6/7 agonist 4o. 14a had very weak agonist activity at TRPC6 expressed in HEK293 cells but exhibited strong inhibition on both 4o-mediated and receptor-operated activation of TRPC6 with an IC50 of about 1 μM. When applied to the culture media, 14a suppressed proliferation of AGS and MKN45 cells with IC50 values of 17.1 ± 0.3 and 18.5 ± 1.0 μM, respectively, and inhibited tube formation and migration of cultured human endothelial cells. This anti-tumor effect on gastric cancer was further verified in xenograft models using nude mice. This study has found a new tool compound which shows excellent therapeutic potential against human gastric cancer most likely through targeting TRPC6 channels.

Near-Infrared II Dye-Protein Complex for Biomedical Imaging and Imaging-Guided Photothermal Therapy

The development of novel biodegradable and nontoxic fluorophores that integrate diagnosis and therapy for effective cancer treatment has obtained tremendous attention in the past decades. In this report, water-soluble and biocompatible small-molecule near-infrared II (NIR-II) fluorescent dye H2a-4T complexed with fetal bovine serum (FBS) and Cetuximab proteins with excellent optical properties and targeting ability is prepared. High spatial and temporal resolution imaging of hind limb vasculature and the lymphatic system of living mice using H2a-4T@FBS complex is demonstrated in precise NIR-II imaging-guided sentinel lymph node surgery. More importantly, H2a-4T@Cetuximab complex not only exhibits a remarkable cell-killing ability but also achieves highly active tumor targeting efficiency for epidermal growth factor receptor, overexpressing colorectal cancer which is beneficial to in vivo NIR-II fluorescent imaging-guided photothermal therapy of colon tumors. To the best of our knowledge, it is the first time that the concept of light-harvesting complex is exploited for enhancing the NIR-II signals and photothermal energy conversion in molecule-protein complex theranostic agent, making them a promising candidate for future clinical applications in cancer theranostics.

TRPC3 as a Target of Novel Therapeutic Interventions

TRPC3 is one of the classical members of the mammalian transient receptor potential (TRP) superfamily of ion channels. TRPC3 is a molecule with intriguing sensory features including the direct recognition of and activation by diacylglycerols (DAG). Although TRPC3 channels are ubiquitously expressed, they appear to control functions of the cardiovascular system and the brain in a highly specific manner. Moreover, a role of TRPC3 in immunity, cancer, and tissue remodeling has been proposed, generating much interest in TRPC3 as a target for pharmacological intervention. Advances in the understanding of molecular architecture and structure-function relations of TRPC3 have been the foundations for novel therapeutic approaches, such as photopharmacology and optochemical genetics of TRPC3. This review provides an account of advances in therapeutic targeting of TRPC3 channels.

Ribemansides A and B, TRPC6 Inhibitors from Ribes manshuricum That Suppress TGF-β1-Induced Fibrogenesis in HK-2 Cells

Two new acylated β-hydroxynitrile glycosides, ribemansides A (1) and B (2), were isolated from the aerial parts of Ribes manshuricum. Their structures were elucidated by comprehensive spectroscopic analysis. Ribemansides A and B inhibited transforming growth factor β1 (TGF-β1)-induced expression of α-smooth muscle actin, fibronectin release, and changes in cell morphology in the human proximal tubular epithelial cell line (human kidney-2, HK-2). Further biological evaluation demonstrated that both 1 and 2 inhibit the activity of canonical transient receptor potential cation channel 6 (TRPC6), with IC₅₀ values of 24.5 and 25.6 μM, respectively. The antifibrogenic effect of these compounds appears to be mediated through TRPC6 inhibition, since the TRPC6 inhibitor, SAR7334, also suppressed TGF-β1-induced fibrogenesis in HK-2 cells.