TRPC3 mediates hyperexcitability and epileptiform activity in immature cortex and experimental cortical dysplasia

TRPC3 suppression ameliorates synaptic dysfunctions and memory deficits in Alzheimer's disease

Transient receptor potential canonical (TRPC) channels are widely expressed in the brain; however, their precise roles in neurodegeneration, such as Alzheimer's disease (AD) remain elusive. Bioinformatic analysis of the published single-cell RNA-seq data collected from AD patient cohorts indicates that the Trpc3 gene is uniquely upregulated in excitatory neurons. TRPC3 expression is also upregulated in post-mortem AD brains, and in both acute and chronic mouse models of AD. Functional screening of TRPC3 antagonists resulted in a lead inhibitor JW-65, which completely rescued Aβ-induced neurotoxicity, impaired synaptic plasticity (e.g., LTP), and learning memory in acute and chronic experimental AD models. In cultured rat hippocampal neurons, we found that treatment with soluble β-amyloid oligomers (AβOs) induces rapid and sustained upregulation of the TRPC3 expression selectively in excitatory neurons. This aberrantly upregulated TRPC3 contributes to AβOs-induced Ca 2+ overload through the calcium entry and store-release mechanisms. The neuroprotective action of JW-65 is primarily mediated via restoring AβOs-impaired Ca 2+ /calmodulin-mediated signaling pathways, including calmodulin kinases CaMKII/IV and calcineurin (CaN). The synaptic protective mechanism via TRPC3 inhibition was further supported by hippocampal RNA-seq data from the symptomatic 5xFAD mice after chronic treatment with JW-65. Overall, these findings not only validate TRPC3 as a novel therapeutic target for treating synaptic dysfunction of AD but most importantly, disclose a distinct role of upregulated TRPC3 in AD pathogenesis in mediating Ca 2+ dyshomeostasis.

Inhibition of TRPC3 channels suppresses seizure susceptibility in the genetically-epilepsy prone rats

Transient receptor potential canonical 3 (TRPC3) channels are important in regulating Ca2+ homeostasis and have been implicated in the pathophysiology of chemically induced seizures. Inherited seizure susceptibility in genetically epilepsy-prone rats (GEPR-3s) has been linked to increased voltage-gated Ca2+ channel currents in the inferior colliculus neurons, which can affect intraneuronal Ca2+ homeostasis. However, whether TRPC3 channels also contribute to inherited seizure susceptibility in GEPR-3s is unclear. This study investigated the effects of JW-65, a potent and selective inhibitor of TRPC3 channels, on acoustically evoked seizure susceptibility in adult male and female GEPR-3s. These seizures consisted of wild running seizures (WRSs) that evolved into generalized tonic-clonic seizures (GTCSs). The results showed that acute administration of low doses of JW-65 significantly decreased by 55-89% the occurrence of WRSs and GTCSs and the seizure severity in both male and female GEPR-3s. This antiseizure effect was accompanied by increased seizure latency and decreased seizure duration. Additionally, female GEPR-3s were more responsive to JW-65's antiseizure effects than males. Moreover, JW-65 treatment for five consecutive days completely suppressed acoustically evoked seizures in male and female GEPR-3s. These findings suggest that inhibiting TRPC3 channels could be a promising antiseizure strategy targeting Ca2+ signaling mechanisms in inherited generalized tonic-clonic epilepsy.

Investigating Contributions of Canonical Transient Receptor Potential Channel 3 to Hippocampal Hyperexcitability and Seizure-Induced Neuronal Cell Death

Canonical transient receptor potential channel 3 (TRPC3) is the most abundant TRPC channel in the brain and is highly expressed in all subfields of the hippocampus. Previous studies have suggested that TRPC3 channels may be involved in the hyperexcitability of hippocampal pyramidal neurons and seizures. Genetic ablation of TRPC3 channel expression reduced the intensity of pilocarpine-induced status epilepticus (SE). However, the underlying cellular mechanisms remain unexplored and the contribution of TRPC3 channels to SE-induced neurodegeneration is not determined. In this study, we investigated the contribution of TRPC3 channels to the electrophysiological properties of hippocampal pyramidal neurons and hippocampal synaptic plasticity, and the contribution of TRPC3 channels to seizure-induced neuronal cell death. We found that genetic ablation of TRPC3 expression did not alter basic electrophysiological properties of hippocampal pyramidal neurons and had a complex impact on epileptiform bursting in CA3. However, TRPC3 channels contribute significantly to long-term potentiation in CA1 and SE-induced neurodegeneration. Our results provided further support for therapeutic potential of TRPC3 inhibitors and raised new questions that need to be answered by future studies.

Pathophysiological Roles of Transient Receptor Potential (Trp) Channels and Zinc Toxicity in Brain Disease

Maintaining the correct ionic gradient from extracellular to intracellular space via several membrane-bound transporters is critical for maintaining overall cellular homeostasis. One of these transporters is the transient receptor potential (TRP) channel family that consists of six putative transmembrane segments systemically expressed in mammalian tissues. Upon the activation of TRP channels by brain disease, several cations are translocated through TRP channels. Brain disease, especially ischemic stroke, epilepsy, and traumatic brain injury, triggers the dysregulation of ionic gradients and promotes the excessive release of neuro-transmitters and zinc. The divalent metal cation zinc is highly distributed in the brain and is specifically located in the pre-synaptic vesicles as free ions, usually existing in cytoplasm bound with metallothionein. Although adequate zinc is essential for regulating diverse physiological functions, the brain-disease-induced excessive release and translocation of zinc causes cell damage, including oxidative stress, apoptotic cascades, and disturbances in energy metabolism. Therefore, the regulation of zinc homeostasis following brain disease is critical for the prevention of brain damage. In this review, we summarize recent experimental research findings regarding how TRP channels (mainly TRPC and TRPM) and zinc are regulated in animal brain-disease models of global cerebral ischemia, epilepsy, and traumatic brain injury. The blockade of zinc translocation via the inhibition of TRPC and TRPM channels using known channel antagonists, was shown to be neuroprotective in brain disease. The regulation of both zinc and TRP channels may serve as targets for treating and preventing neuronal death.

Transient Receptor Potential (TRP) Channels in Pain, Neuropsychiatric Disorders, and Epilepsy

Pharmacomodulation of membrane channels is an essential topic in the study of physiological conditions and disease status. Transient receptor potential (TRP) channels are one such family of nonselective cation channels that have an important influence. In mammals, TRP channels consist of seven subfamilies with a total of twenty-eight members. Evidence shows that TRP channels mediate cation transduction in neuronal signaling, but the full implication and potential therapeutic applications of this are not entirely clear. In this review, we aim to highlight several TRP channels which have been shown to mediate pain sensation, neuropsychiatric disorders, and epilepsy. Recent findings suggest that TRPM (melastatin), TRPV (vanilloid), and TRPC (canonical) are of particular relevance to these phenomena. The research reviewed in this paper validates these TRP channels as potential targets of future clinical treatment and offers patients hope for more effective care.

Inhibition of TRPC3 channels by a novel pyrazole compound confers antiseizure effects

OBJECTIVE

As a key member of the transient receptor potential (TRP) superfamily, TRP canonical 3 (TRPC3) regulates calcium homeostasis and contributes to neuronal excitability. Ablation of TRPC3 lessens pilocarpine-induced seizures in mice, suggesting that TRPC3 inhibition might represent a novel antiseizure strategy. Among current TRPC3 inhibitors, pyrazole 3 (Pyr3) is most selective and potent. However, Pyr3 only provides limited benefits in pilocarpine-treated mice, likely due to its low metabolic stability and potential toxicity. We recently reported a modified pyrazole compound 20 (or JW-65) that has improved stability and safety. The objective of this study was to explore the effects of TRPC3 inhibition by our current lead compound JW-65 on seizure susceptibility.

METHODS

We first examined the pharmacokinetic properties including plasma half-life and brain to plasma ratio of JW-65 after systemic administration in mice. We then investigated the effects of TRPC3 inhibition by JW-65 on behavioral and electrographic seizures in mice treated with pilocarpine. To ensure our findings are not model specific, we assessed the susceptibility of JW-65-treated mice to pentylenetetrazole (PTZ)-induced seizures with phenytoin as a comparator.

RESULTS

JW-65 showed adequate half-life and brain penetration in mice, justifying its use for central nervous system conditions. Systemic treatment with JW-65 before pilocarpine injection in mice markedly impaired the initiation of behavioral seizures. This antiseizure action was recapitulated when JW-65 was administered after pilocarpine-induced behavioral seizures were well established and was confirmed by time-locked electroencephalographic monitoring and synchronized video. Moreover, JW-65-treated mice showed substantially decreased susceptibility to PTZ-induced seizures in a dose-dependent manner.

SIGNIFICANCE

These results suggest that pharmacological inhibition of the TRPC3 channels by our novel compound JW-65 might represent a new antiseizure strategy engaging a previously undrugged mechanism of action. Hence, this proof-of-concept study establishes TRPC3 as a novel feasible therapeutic target for the treatment of some forms of epilepsy.

Functional Importance of Transient Receptor Potential (TRP) Channels in Neurological Disorders

Transient receptor potential (TRP) channels are transmembrane protein complexes that play important roles in the physiology and pathophysiology of both the central nervous system (CNS) and the peripheral nerve system (PNS). TRP channels function as non-selective cation channels that are activated by several chemical, mechanical, and thermal stimuli as well as by pH, osmolarity, and several endogenous or exogenous ligands, second messengers, and signaling molecules. On the pathophysiological side, these channels have been shown to play essential roles in the reproductive system, kidney, pancreas, lung, bone, intestine, as well as in neuropathic pain in both the CNS and PNS. In this context, TRP channels have been implicated in several neurological disorders, including Alzheimer’s disease, Parkinson’s disease, Huntington’s disease, amyotrophic lateral sclerosis, and epilepsy. Herein, we focus on the latest involvement of TRP channels, with a special emphasis on the recently identified functional roles of TRP channels in neurological disorders related to the disruption in calcium ion homeostasis.

Roles of TRP Channels in Neurological Diseases

Transient receptor potential (TRP) proteins consist of a superfamily of cation channels that have been involved in diverse physiological processes in the brain as well as in the pathogenesis of neurological disease. TRP channels are widely expressed in the brain, including neurons and glial cells, as well as in the cerebral vascular endothelium and smooth muscle. Members of this channel superfamily show a wide variety of mechanisms ranging from ligand binding to voltage, physical, and chemical stimuli, implying the promising therapeutic potential of TRP in neurological diseases. In this review, we focus on the physiological functions of TRP channels in the brain and the pathological roles in neurological disorders to explore future potential neuroprotective strategies.

Therapeutic potential of pharmacological agents targeting TRP channels in CNS disorders

Central nervous system (CNS) disorders like Alzheimer's disease (AD), Parkinson disease (PD), stroke, epilepsy, depression, and bipolar disorder have a high impact on both medical and social problems due to the surge in their prevalence. All of these neuronal disorders share some common etiologies including disruption of Ca²⁺ homeostasis and accumulation of misfolded proteins. These misfolded proteins further disrupt the intracellular Ca²⁺ homeostasis by disrupting the activity of several ion channels including transient receptor potential (TRP) channels. TRP channel families include non-selective Ca²⁺ permeable channels, which act as cellular sensors activated by various physio-chemical stimuli, exogenous, and endogenous ligands responsible for maintaining the intracellular Ca²⁺ homeostasis. TRP channels are abundantly expressed in the neuronal cells and disturbance in their activity leads to various neuronal diseases. Under the pathological conditions when the activity of TRP channels is perturbed, there is a disruption of the neuronal homeostasis through increased inflammatory response, generation of reactive oxygen species, and mitochondrial dysfunction. Therefore, there is a potential of pharmacological interventions targeting TRP channels in CNS disorders. This review focuses on the role of TRP channels in neurological diseases; also, we have highlighted the current insights into the pharmacological modulators targeting TRP channels.

Evidence of a Role for the TRPC Subfamily in Mediating Oxidative Stress in Parkinson's Disease

Parkinson's disease (PD) represents one of the most common multifactorial neurodegenerative disorders affecting the elderly population. It is associated with the aggregation of α-synuclein protein and the loss of dopaminergic neurons in the substantia nigra pars compacta of the brain. The disease is mainly represented by motor symptoms, such as resting tremors, postural instability, rigidity, and bradykinesia, that develop slowly over time. Parkinson's disease can also manifest as disturbances in non-motor functions. Although the pathology of PD has not yet been fully understood, it has been suggested that the disruption of the cellular redox status may contribute to cellular oxidative stress and, thus, to cell death. The generation of reactive oxygen species and reactive nitrogen intermediates, as well as the dysfunction of dopamine metabolism, play important roles in the degeneration of dopaminergic neurons. In this context, the transient receptor potential channel canonical (TRPC) sub-family plays an important role in neuronal degeneration. Additionally, PD gene products, including DJ-1, SNCA, UCH-L1, PINK-1, and Parkin, also interfere with mitochondrial function leading to reactive oxygen species production and dopaminergic neuronal vulnerability to oxidative stress. Herein, we discuss the interplay between these various biochemical and molecular events that ultimately lead to dopaminergic signaling disruption, highlighting the recently identified roles of TRPC in PD.

Expression of TRPC3 in cortical lesions from patients with focal cortical dysplasia

Focal cortical dysplasia (FCD) is one of the main causes of medically intractable epilepsy. Some studies have reported that transient receptor potential canonical channel 3 (TRPC3) may play an important role in the occurrence of seizures. In this study, we investigated the expression patterns of TRPC3 in different types of FCD. Forty-five FCD specimens and 12 control samples from autopsies were used in our study. Western blotting, immunohistochemistry, and immunofluorescence staining were employed to detect protein expression and distribution. The amount of TRPC3 protein was markedly elevated in the FCD group. The immunohistochemistry results revealed that TRPC3 staining was strong in the malformed cells and microcolumns. Most of the TRPC3-positive cells were colabeled with glutamatergic and GABAergic markers. The overexpression and altered cellular distribution of TRPC3 in the FCD samples suggest that TRPC3 may be related to epileptogenesis in FCD.

Trpm4 ion channels in pre-Bötzinger complex interneurons are essential for breathing motor pattern but not rhythm

Inspiratory breathing movements depend on pre-Bötzinger complex (preBötC) interneurons that express calcium (Ca2+)-activated nonselective cationic current (ICAN) to generate robust neural bursts. Hypothesized to be rhythmogenic, reducing ICAN is predicted to slow down or stop breathing; its contributions to motor pattern would be reflected in the magnitude of movements (output). We tested the role(s) of ICAN using reverse genetic techniques to diminish its putative ion channels Trpm4 or Trpc3 in preBötC neurons in vivo. Adult mice transduced with Trpm4-targeted short hairpin RNA (shRNA) progressively decreased the tidal volume of breaths yet surprisingly increased breathing frequency, often followed by gasping and fatal respiratory failure. Mice transduced with Trpc3-targeted shRNA survived with no changes in breathing. Patch-clamp and field recordings from the preBötC in mouse slices also showed an increase in the frequency and a decrease in the magnitude of preBötC neural bursts in the presence of Trpm4 antagonist 9-phenanthrol, whereas the Trpc3 antagonist pyrazole-3 (pyr-3) showed inconsistent effects on magnitude and no effect on frequency. These data suggest that Trpm4 mediates ICAN, whose influence on frequency contradicts a direct role in rhythm generation. We conclude that Trpm4-mediated ICAN is indispensable for motor output but not the rhythmogenic core mechanism of the breathing central pattern generator.

TRPC Channels and Epilepsy

Accumulating evidence suggest that TRPC channels play critical roles in various aspects of epileptogenesis. TRPC1/4 channels are major contributors to nonsynaptically derived epileptiform burst firing in the CA1 and the lateral septum. TRPC7 channels play a critical role in synaptically derived epileptiform burst firing. The reduction of spontaneous epileptiform bursting in the CA3 is correlated to a reduction in pilocarpine-induced SE in vivo in TRPC7 knockout mice. TRPC channels are also significant contributors to SE-induced neuronal cell death. Although the pilocarpine-induced SE itself is not significantly reduced, the SE-induced neuronal cell death is significantly reduced in the CA1 and the lateral septum, indicating that TRPC1/4 channels directly contribute to SE-induced neuronal cell death. Genetic ablation of TRPC5 also reduces SE-induced neuronal cell death in the CA1 and CA3 areas of the hippocampus.