Human Brain Region-Specific Alternative Splicing of TRPC3, the Type 3 Canonical Transient Receptor Potential Non-Selective Cation Channel

TRPC Channels Activated by G Protein-Coupled Receptors Drive Ca2+ Dysregulation Leading to Secondary Brain Injury in the Mouse Model

Canonical transient receptor potential (TRPC) non-selective cation channels, particularly those assembled with TRPC3, TRPC6, and TRPC7 subunits, are coupled to Gαq-type G protein-coupled receptors for the major classes of excitatory neurotransmitters. Sustained activation of this TRPC channel-based pathophysiological signaling hub in neurons and glia likely contributes to prodigious excitotoxicity-driven secondary brain injury expansion. This was investigated in mouse models with selective Trpc gene knockout (KO). In adult cerebellar brain slices, application of glutamate and the class I metabotropic glutamate receptor agonist (S)-3,5-dihydroxyphenylglycine to Purkinje neurons expressing the GCaMP5g Ca2+ reporter demonstrated that the majority of the Ca2+ loading in the molecular layer dendritic arbors was attributable to the TRPC3 effector channels (Trpc3KO compared with wildtype (WT)). This Ca2+ dysregulation was associated with glutamate excitotoxicity causing progressive disruption of the Purkinje cell dendrites (significantly abated in a GAD67-GFP-Trpc3KO reporter brain slice model). Contribution of the Gαq-coupled TRPC channels to secondary brain injury was evaluated in a dual photothrombotic focal ischemic injury model targeting cerebellar and cerebral cortex regions, comparing day 4 post-injury in WT mice, Trpc3KO, and Trpc1/3/6/7 quadruple knockout (TrpcQKO), with immediate 2-h (primary) brain injury. Neuroprotection to secondary brain injury was afforded in both brain regions by Trpc3KO and TrpcQKO models, with the TrpcQKO showing greatest neuroprotection. These findings demonstrate the contribution of the Gαq-coupled TRPC effector mechanism to excitotoxicity-based secondary brain injury expansion, which is a primary driver for mortality and morbidity in stroke, traumatic brain injury, and epilepsy.

Phenotypical, genotypical and pathological characterization of the moonwalker mouse, a model of ataxia

We performed a comprehensive study of the morphological, functional, and genetic features of moonwalker (MWK) mice, a mouse model of spinocerebellar ataxia caused by a gain of function of the TRPC3 channel. These mice show numerous behavioral symptoms including tremor, altered gait, circling behavior, impaired motor coordination, impaired motor learning and decreased limb strength. Cerebellar pathology is characterized by early and almost complete loss of unipolar brush cells as well as slowly progressive, moderate loss of Purkinje cell (PCs). Structural damage also includes loss of synaptic contacts from parallel fibers, swollen ER structures, and degenerating axons. Interestingly, no obvious correlation was observed between PC loss and severity of the symptoms, as the phenotype stabilizes around 2 months of age, while the cerebellar pathology is progressive. This is probably due to the fact that PC function is severely impaired much earlier than the appearance of PC loss. Indeed, PC firing is already impaired in 3 weeks old mice. An interesting feature of the MWK pathology that still remains to be explained consists in a strong lobule selectivity of the PC loss, which is puzzling considering that TRPC is expressed in every PC. Intriguingly, genetic analysis of MWK cerebella shows, among other alterations, changes in the expression of both apoptosis inducing and resistance factors possibly suggesting that damaged PCs initiate specific cellular pathways that protect them from overt cell loss.

Memantine suppresses the excitotoxicity but fails to rescue the ataxic phenotype in SCA1 model mice

Spinocerebellar ataxia type 1 (SCA1) is a debilitating neurodegenerative disorder of the cerebellum and brainstem. Memantine has been proposed as a potential treatment for SCA1. It blocks N-methyl-D-aspartate (NMDA) receptors on neurons, reduces excitotoxicity and decreases neurodegeneration in Alzheimer models. However, in cerebellar neurodegenerative diseases, the potential value of memantine is still unclear. We investigated the effects of memantine on motor performance and synaptic transmission in the cerebellum in a mouse model where mutant ataxin 1 is specifically targeted to glia. Lentiviral vectors (LVV) were used to express mutant ataxin 1 selectively in Bergmann glia (BG). In mice transduced with the mutant ataxin 1, chronic treatment with memantine improved motor activity during initial tests, presumably due to preserved BG and Purkinje cell (PC) morphology and numbers. However, mice were unable to improve their rota rod scores during next days of training. Memantine also compromised improvement in the rota rod scores in control mice upon repetitive training. These effects may be due to the effects of memantine on plasticity (LTD suppression) and NMDA receptor modulation. Some effects of chronically administered memantine persisted even after its wash-out from brain slices. Chronic memantine reduced morphological signs of neurodegeneration in the cerebellum of SCA1 model mice. This resulted in an apparent initial reduction of ataxic phenotype, but memantine also affected cerebellar plasticity and ultimately compromised motor learning. We speculate that that clinical application of memantine in SCA1 might be hampered by its ability to suppress NMDA-dependent plasticity in cerebellar cortex.

Widespread alternative splicing dysregulation occurs presymptomatically in CAG expansion spinocerebellar ataxias

The spinocerebellar ataxias (SCAs) are a group of dominantly inherited neurodegenerative diseases, several of which are caused by CAG expansion mutations (SCAs 1, 2, 3, 6, 7 and 12) and more broadly belong to the large family of over 40 microsatellite expansion diseases. While dysregulation of alternative splicing is a well defined driver of disease pathogenesis across several microsatellite diseases, the contribution of alternative splicing in CAG expansion SCAs is poorly understood. Furthermore, despite extensive studies on differential gene expression, there remains a gap in our understanding of presymptomatic transcriptomic drivers of disease. We sought to address these knowledge gaps through a comprehensive study of 29 publicly available RNA-sequencing datasets. We identified that dysregulation of alternative splicing is widespread across CAG expansion mouse models of SCAs 1, 3 and 7. These changes were detected presymptomatically, persisted throughout disease progression, were repeat length-dependent, and were present in brain regions implicated in SCA pathogenesis including the cerebellum, pons and medulla. Across disease progression, changes in alternative splicing occurred in genes that function in pathways and processes known to be impaired in SCAs, such as ion channels, synaptic signalling, transcriptional regulation and the cytoskeleton. We validated several key alternative splicing events with known functional consequences, including Trpc3 exon 9 and Kcnma1 exon 23b, in the Atxn1154Q/2Q mouse model. Finally, we demonstrated that alternative splicing dysregulation is responsive to therapeutic intervention in CAG expansion SCAs with Atxn1 targeting antisense oligonucleotide rescuing key splicing events. Taken together, these data demonstrate that widespread presymptomatic dysregulation of alternative splicing in CAG expansion SCAs may contribute to disease onset, early neuronal dysfunction and may represent novel biomarkers across this devastating group of neurodegenerative disorders.

Calcium transport and sensing in TRPC channels - New insights into a complex feedback regulation

Canonical TRP (TRPC) channels are a still enigmatic family of signaling molecules with multimodal sensing features. These channels enable Ca2+ influx through the plasma membrane to control a diverse range of cellular functions. Based on both regulatory- and recently uncovered structural features, TRPC channels are considered to coordinate Ca2+ and other divalent cations not only within the permeation path but also at additional sensory sites. Analysis of TRPC structures by cryo-EM identified multiple regulatory ion binding pockets. With this review, we aim at an overview and a critical discussion of the current concepts of divalent sensing by TRPC channels.

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.

TRPM4 Drives Cerebral Edema by Switching to Alternative Splicing Isoform After Experimental Traumatic Brain Injury

Traumatic brain injury (TBI) affects persons of all ages and is recognized as a major cause of death and disability worldwide; it also brings heavy life burden to patients and their families. The treatment of those with secondary injury after TBI is still scarce, however. Alternative splicing (AS) is a crucial post-transcriptional regulatory mechanism associated with various physiological processes, while the contribution of AS in treatment after TBI is poorly illuminated. In this study, we performed and analyzed the transcriptome and proteome datasets of brain tissue at multiple time points in a controlled cortical impact (CCI) mouse model. We found that AS, as an independent change against the transcriptional level, is a novel mechanism linked to cerebral edema after TBI. Bioinformatics analysis further indicated that the transformation of splicing isoforms after TBI was related to cerebral edema. Accordingly, we found that the fourth exon of transient receptor potential channel melastatin 4 (Trpm4) abrogated skipping at 72 h after TBI, resulting in a frameshift of the encoded amino acid and an increase in the proportion of spliced isoforms. Using magnetic resonance imaging (MRI), we have shown the numbers of 3nEx isoforms of Trpm4 may be positively correlated with volume of cerebral edema. Thus alternative splicing of Trpm4 becomes a noteworthy mechanism of potential influence on edema. In summary, alternative splicing of Trpm4 may drive cerebral edema after TBI. Trpm4 is a potential therapeutic targeting cerebral edema in patients with TBI.

Increase in membrane surface expression and phosphorylation of TRPC3 related to olfactory dysfunction in α-synuclein transgenic mice

Olfactory impairment is an initial non-motor symptom of Parkinson's disease that causes the deposition of aggregated α-synuclein (α-syn) in olfactory neurons. Transient receptor potential canonical (TRPC) channels are a diverse group of non-selective Ca²⁺ entry channels involved in the progression or pathogenesis of PD via Ca²⁺ homeostatic regulation. However, the relationship between TRPC and α-syn pathology in an olfactory system remains unclear. To address this issue, we assessed the olfactory function in α-syn transgenic mice. In contrast with control mice, the transgenic mice exhibited impaired olfaction, TRPC3 activation and apoptotic neuronal cell death in the olfactory system. Similar results were observed in primary cultures of olfactory neurons, that is TRPC3 activation, increasing intracellular Ca²⁺ concentration and apoptotic cell death in the α-syn-overexpressed neurons. These changes were significantly attenuated by TRPC3 knockdown. Therefore, our findings suggest that TRPC3 activation and calcium dyshomeostasis play a key role in α-syn-induced olfactory dysfunction in mice.

TRPC3, but not TRPC1, as a good therapeutic target for standalone or complementary treatment of DMD

Background

Duchenne muscular dystrophy (DMD) is an X-linked inherited disease caused by mutations in the gene encoding dystrophin that leads to a severe and ultimately life limiting muscle-wasting condition. Recombinant adeno-associated vector (rAAV)-based gene therapy is promising, but the size of the full-length dystrophin cDNA exceeds the packaging capacity of a rAAV. Alternative or complementary strategies that could treat DMD patients are thus needed. Intracellular calcium overload due to a sarcolemma permeability to calcium (SPCa) increase is an early and critical step of the DMD pathogenesis. We assessed herein whether TRPC1 and TRPC3 calcium channels may be involved in skeletal muscle SPCa alterations and could represent therapeutic targets to treat DMD.

Methods

All experiments were conducted in the DMD^mdx rat, an animal model that closely reproduces the human DMD disease. We measured the cytosolic calcium concentration ([Ca²⁺]_c) and SPCa in EDL (Extensor Digitorum Longus) muscle fibers from age-matched WT and DMD^mdx rats of 1.5 to 7 months old. TRPC1 and TRPC3 expressions were measured in the EDL muscles at both the mRNA and protein levels, by RT-qPCR, western blot and immunocytofluorescence analysis.

Results

As expected from the malignant hyperthermia like episodes observed in several DMD^mdx rats, calcium homeostasis alterations were confirmed by measurements of early increases in [Ca²⁺]_c and SPCa in muscle fibers. TRPC3 and TRPC1 protein levels were increased in DMD^mdx rats. This was observed as soon as 1.5 months of age for TRPC3 but only at 7 months of age for TRPC1. A slight but reliable shift of the TRPC3 apparent molecular weight was observed in DMD^mdx rat muscles. Intracellular localization of both channels was not altered. We thus focused our attention on TRPC3. Application of Pyr10, a specific inhibitor of TRPC3, abolished the differences between SPCa values measured in WT and DMD^mdx. Finally, we showed that a rAAV-microdystrophin based treatment induced a high microdystrophin expression but only partial prevention of calcium homeostasis alterations, skeletal muscle force and TRPC3 protein increase.

Conclusions

All together our results show that correcting TRPC3 channel expression and/or activity appear to be a promising approach as a single or as a rAAV-based complementary therapy to treat DMD.

Contribution of TRPC Channels in Neuronal Excitotoxicity Associated With Neurodegenerative Disease and Ischemic Stroke

The seven canonical members of transient receptor potential (TRPC) proteins form cation channels that evoke membrane depolarization and intracellular calcium concentration ([Ca²⁺] _i ) rise, which are not only important for regulating cell function but their deregulation can also lead to cell damage. Recent studies have implicated complex roles of TRPC channels in neurodegenerative diseases including ischemic stroke. Brain ischemia reduces oxygen and glucose supply to neurons, i.e., Oxygen and Glucose Deprivation (OGD), resulting in [Ca²⁺] _i elevation, ion dyshomeostasis, and excitotoxicity, which are also common in many forms of neurodegenerative diseases. Although ionotropic glutamate receptors, e.g., N-methyl-D-aspartate receptors, are well established to play roles in excitotoxicity, the contribution of metabotropic glutamate receptors and their downstream effectors, i.e., TRPC channels, should not be neglected. Here, we summarize the current findings about contributions of TRPC channels in neurodegenerative diseases, with a focus on OGD-induced neuronal death and rodent models of cerebral ischemia/reperfusion. TRPC channels play both detrimental and protective roles to neurodegeneration depending on the TRPC subtype and specific pathological conditions involved. When illustrated the mechanisms by which TRPC channels are involved in neuronal survival or death seem differ greatly, implicating diverse and complex regulation. We provide our own data showing that TRPC1/C4/C5, especially TRPC4, may be generally detrimental in OGD and cerebral ischemia/reperfusion. We propose that although TRPC channels significantly contribute to ischemic neuronal death, detailed mechanisms and specific roles of TRPC subtypes in brain injury at different stages of ischemia/reperfusion and in different brain regions need to be carefully and systematically investigated.

Alternative Splicing Regulation of an Alzheimer's Risk Variant in CLU

Clusterin (CLU) is one of the risk genes most associated with late onset Alzheimer’s disease (AD), and several genetic variants in CLU are associated with AD risk. However, the functional role of known AD risk genetic variants in CLU has been little explored. We investigated the effect of an AD risk variant (rs7982) in the 5th exon of CLU on alternative splicing by using an integrative approach of brain-tissue-based RNA-Seq and whole genome sequencing data from Accelerating Medicines Partnership—Alzheimer’s Disease (AMP-AD). RNA-Seq data were generated from three regions in the temporal lobe of the brain—the temporal cortex, superior temporal gyrus, and parahippocampal gyrus. The rs7982 was significantly associated with intron retention (IR) of the 5th exon of CLU; as the number of alternative alleles (G) increased, the IR rates decreased more significantly in females than in males. Our results suggest a sex-dependent role of rs7982 in AD pathogenesis via splicing regulation.

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.