Ionic channels formed by TRPC4

Silencing of transient receptor potential canonical channel 4 inhibits endothelial progenitor cell angiogenesis by suppressing VEGF and SDF-1

OBJECTIVES

Endothelial progenitor cells (EPCs) play a crucial role in acquired angiogenesis and endothelial injury repair. Transient receptor potential canonical channel 4 (TRPC4), a key component of store-operated calcium channels, is essential for EPC function. While the role of TRPCs has been clarified in vascular diseases, the relationship between TRPC4 and EPC function, along with the underlying molecular mechanisms, remains unclear and requires further elucidation.

METHODS

EPCs were isolated from canine bone marrow and identified by morphology and flow cytometry. TRPC4 was transfected into EPCs using lentivirus or negative control, and its expression was assessed using real-time polymerase chain reaction (RT-PCR). Proliferation, migration, and tube formation were evaluated using Cell Counting Kit-8 (CCK-8), Transwell, and Matrigel assays, respectively. Levels of vascular endothelial growth factor (VEGF) and stromal cell-derived factor-1 (SDF-1) were measured using enzyme-linked immunosorbent assay (ELISA).

RESULTS

TRPC4 mRNA expression was significantly reduced in TRPC4-short hairpin RNA (shRNA) transfected EPCs compared to the normal control (NC)-shRNA groups. Migration and tube formation were significantly decreased after TRPC4 silencing, while proliferation showed no difference. Additionally, levels of SDF-1 and VEGF in EPCs were markedly reduced following TRPC4 silencing.

CONCLUSION

TRPC4 plays a crucial role in regulating angiogenesis in EPCs. Silencing of TRPC4 can lead to decreased angiogenesis by inhibiting VEGF and SDF-1 expression, suggesting that TRPC4 knockdown might be a novel therapeutic strategy for vascular diseases.

Interactions between the Polysialylated Neural Cell Adhesion Molecule and the Transient Receptor Potential Canonical Channels 1, 4, and 5 Induce Entry of Ca2+ into Neurons

The neural cell adhesion molecule (NCAM) plays important functional roles in the developing and mature nervous systems. Here, we show that the transient receptor potential canonical (TRPC) ion channels TRPC1, -4, and -5 not only interact with the intracellular domains of the transmembrane isoforms NCAM140 and NCAM180, but also with the glycan polysialic acid (PSA) covalently attached to the NCAM protein backbone. NCAM antibody treatment leads to the opening of TRPC1, -4, and -5 hetero- or homomers at the plasma membrane and to the influx of Ca²⁺ into cultured cortical neurons and CHO cells expressing NCAM, PSA, and TRPC1 and -4 or TRPC1 and -5. NCAM-stimulated Ca²⁺ entry was blocked by the TRPC inhibitor Pico145 or the bacterial PSA homolog colominic acid. NCAM-stimulated Ca²⁺ influx was detectable neither in NCAM-deficient cortical neurons nor in TRPC1/4- or TRPC1/5-expressing CHO cells that express NCAM, but not PSA. NCAM-induced neurite outgrowth was reduced by TRPC inhibitors and a function-blocking TRPC1 antibody. A characteristic signaling feature was that extracellular signal-regulated kinase 1/2 phosphorylation was also reduced by TRPC inhibitors. Our findings indicate that the interaction of NCAM with TRPC1, -4, and -5 contributes to the NCAM-stimulated and PSA-dependent Ca²⁺ entry into neurons thereby influencing essential neural functions.

Differential PI(4,5)P2 sensitivities of TRPC4, C5 homomeric and TRPC1/4, C1/5 heteromeric channels

Transient receptor potential canonical (TRPC) 4 and TRPC5 channels are modulated by the Gα_q-PLC pathway. Since phosphatidylinositol 4,5-bisphosphate (PI(4,5)P₂) maintains TRPC4 and TRPC5 channel function, the Gα_q-PLC pathway inhibits channel activity by depleting PI(4,5)P₂. Here we investigated the difference in PI(4,5)P₂ sensitivity between homomeric and heteromeric TRPC channels. First, by using a Danio rerio voltage-sensing phosphatase (DrVSP), we show that PI(4,5)P₂ dephosphorylation robustly inhibits TRPC4α, TRPC4β, and TRPC5 homotetramer currents and also TRPC1/4α, TRPC1/4β, and TRPC1/5 heterotetramer currents. Secondly, sensitivity of channels to PI(4,5)P₂ dephosphorylation was suggested through the usage of FRET in combination with patch clamping. The sensitivity increased in the sequence TRPC4β < TRPC4α < TRPC5 in homotetramers, whereas when forming heterotetramers with TRPC1, the sensitivity was approximately equal between the channels. Thirdly, we determined putative PI(4,5)P₂ binding sites based on a TRPC4 prediction model. By neutralization of basic residues, we identified putative PI(4,5)P₂ binding sites because the mutations reduced FRET to a PI(4,5)P₂ sensor and reduced the current amplitude. Therefore, one functional TRPC4 has 8 pockets with the two main binding regions; K419, K664/R511, K518, H630. We conclude that TRPC1 channel function as a regulator in setting PI(4,5)P₂ affinity for TRPC4 and TRPC5 that changes PI(4,5)P₂ sensitivity.

Elevated Expression of TRPC4 in Cortical Lesions of Focal Cortical Dysplasia II and Tuberous Sclerosis Complex

Focal cortical dysplasia type II (FCD II) and tuberous sclerosis complex (TSC) are well-known causes of chronic refractory epilepsy in children. Canonical transient receptor potential channels (TRPCs) are non-selective cation channels that are commonly activated by phospholipase C (PLC) stimulation. Previous studies found that TRPC4 may participate in the process of epileptogenesis. This study aimed to examine the expression and distribution of TRPC4 in FCD II (n = 24) and TSC (n = 11) surgical specimens compared with that in age-matched autopsy control samples (n = 12). We found that the protein levels of TRPC4 and its upstream factor, PLC delta 1 (PLCD1), were elevated in FCD II and TSC samples compared to those of control samples. Immunohistochemistry assays revealed that TRPC4 staining was stronger in malformed cells, such as dysmorphic neurons, balloon cells and giant cells. Moderate-to-strong staining of the upstream factor PLCD1 was also identified in abnormal neurons. Moreover, double immunofluorescence staining revealed that TRPC4 was colocalised with glutamatergic and GABAergic neuron markers. Taken together, our results indicate that overexpression of TRPC4 protein may be involved in the epileptogenesis of FCD II and TSC.

Englerin A Agonizes the TRPC4/C5 Cation Channels to Inhibit Tumor Cell Line Proliferation

Englerin A is a structurally unique natural product reported to selectively inhibit growth of renal cell carcinoma cell lines. A large scale phenotypic cell profiling experiment (CLiP) of englerin A on ¬over 500 well characterized cancer cell lines showed that englerin A inhibits growth of a subset of tumor cell lines from many lineages, not just renal cell carcinomas. Expression of the TRPC4 cation channel was the cell line feature that best correlated with sensitivity to englerin A, suggesting the hypothesis that TRPC4 is the efficacy target for englerin A. Genetic experiments demonstrate that TRPC4 expression is both necessary and sufficient for englerin A induced growth inhibition. Englerin A induces calcium influx and membrane depolarization in cells expressing high levels of TRPC4 or its close ortholog TRPC5. Electrophysiology experiments confirmed that englerin A is a TRPC4 agonist. Both the englerin A induced current and the englerin A induced growth inhibition can be blocked by the TRPC4/C5 inhibitor ML204. These experiments confirm that activation of TRPC4/C5 channels inhibits tumor cell line proliferation and confirms the TRPC4 target hypothesis generated by the cell line profiling. In selectivity assays englerin A weakly inhibits TRPA1, TRPV3/V4, and TRPM8 which suggests that englerin A may bind a common feature of TRP ion channels. In vivo experiments show that englerin A is lethal in rodents near doses needed to activate the TRPC4 channel. This toxicity suggests that englerin A itself is probably unsuitable for further drug development. However, since englerin A can be synthesized in the laboratory, it may be a useful chemical starting point to identify novel modulators of other TRP family channels.

Dual depolarization responses generated within the same lateral septal neurons by TRPC4-containing channels

In the central nervous system, canonical transient receptor potential (TRPC) channels have been implicated in mediating neuronal excitation induced by stimulating metabotropic receptors, including group 1 metabotropic glutamate receptors (mGluRs). Lateral septal (LS) neurons express high levels of TRPC4 and group I mGluRs. However, to what extent native TRPC4-containing channels (TRPC4-cc) are activated as well as the impact of different levels of TRPC4-cc activation on neuronal excitability remain elusive. Here, we report that stimulating LS neurons with group I mGluR agonist, (S)-3,5-DHPG, causes either an immediate increase in firing rate or an initial burst followed by a pause of firing, which can be correlated with below-threshold-depolarization (BTD) or above-threshold-plateau-depolarization (ATPD), respectively, in whole-cell recordings. The early phase of BTD and the entire ATPD are completely absent in neurons from TRPC4−/− mice. Moreover, in the same LS neurons, BTD can be converted to ATPD at more depolarized potentials or with a brief current injection, suggesting that BTD and ATPD may represent partial and full activations of TRPC4-cc, respectively. We show that coincident mGluR stimulation and depolarization is required to evoke strong TRPC4-cc current, and Na+ and Ca2+ influx, together with dynamic changes of intracellular Ca(2+), are essential for ATPD induction. Our results suggest that TRPC4-cc integrates metabotropic receptor stimulation with intracellular Ca(2+) signals to generate two interconvertible depolarization responses to affect excitability of LS neurons in distinct fashions.

Brain transient receptor potential channels and stroke

Transient receptor potential (TRP) channels have been increasingly implicated in the pathological mechanisms of CNS disorders. TRP expression has been detected in neurons, astrocytes, oligodendrocytes, microglia, and ependymal cells as well as in the cerebral vascular endothelium and smooth muscle. In stroke, TRPC3/4/6, TRPM2/4/7, and TRPV1/3/4 channels have been found to participate in ischemia-induced cell death, whereas other TRP channels, in particular those expressed in nonneuronal cells, have been less well studied. This review summarizes the current knowledge on the expression and functions of the TRP channels in various cell types in the brain and our current understanding of TRP channels in stroke pathophysiology. In an aging society, the occurrence of stroke is expected to increase steadily, and there is an urgent requirement to improve the current stroke management strategy. Therefore, elucidating the roles of TRP channels in stroke could shed light on the development of novel therapeutic strategies and ultimately improve stroke outcome.

TRPC4- and TRPC4-containing channels

TRPC4 proteins comprise six transmembrane domains, a putative pore-forming region, and an intracellularly located amino- and carboxy-terminus. Among eleven splice variants identified so far, TRPC4α and TRPC4β are the most abundantly expressed and functionally characterized. TRPC4 is expressed in various organs and cell types including the soma and dendrites of numerous types of neurons; the cardiovascular system including endothelial, smooth muscle, and cardiac cells; myometrial and skeletal muscle cells; kidney; and immune cells such as mast cells. Both recombinant and native TRPC4-containing channels differ tremendously in their permeability and other biophysical properties, pharmacological modulation, and mode of activation depending on the cellular environment. They vary from inwardly rectifying store-operated channels with a high Ca(2+) selectivity to non-store-operated channels predominantly carrying Na(+) and activated by Gαq- and/or Gαi-coupled receptors with a complex U-shaped current-voltage relationship. Thus, individual TRPC4-containing channels contribute to agonist-induced Ca(2+) entry directly or indirectly via depolarization and activation of voltage-gated Ca(2+) channels. The differences in channel properties may arise from variations in the composition of the channel complexes, in the specific regulatory pathways in the corresponding cell system, and/or in the expression pattern of interaction partners which comprise other TRPC proteins to form heteromultimeric channels. Additional interaction partners of TRPC4 that can mediate the activity of TRPC4-containing channels include (1) scaffolding proteins (e.g., NHERF) that may mediate interactions with signaling molecules in or in close vicinity to the plasma membrane such as Gα proteins or phospholipase C and with the cytoskeleton, (2) proteins in specific membrane microdomains (e.g., caveolin-1), or (3) proteins on cellular organelles (e.g., Stim1). The diversity of TRPC4-containing channels hampers the development of specific agonists or antagonists, but recently, ML204 was identified as a blocker of both recombinant and endogenous TRPC4-containing channels with an IC50 in the lower micromolar range that lacks activity on most voltage-gated channels and other TRPs except TRPC5 and TRPC3. Lanthanides are specific activators of heterologously expressed TRPC4- and TRPC5-containing channels but can block individual native TRPC4-containing channels. The biological relevance of TRPC4-containing channels was demonstrated by knockdown of TRPC4 expression in numerous native systems including gene expression, cell differentiation and proliferation, formation of myotubes, and axonal regeneration. Studies of TRPC4 single and TRPC compound knockout mice uncovered their role for the regulation of vascular tone, endothelial permeability, gastrointestinal contractility and motility, neurotransmitter release, and social exploratory behavior as well as for excitotoxicity and epileptogenesis. Recently, a single-nucleotide polymorphism (SNP) in the Trpc4 gene was associated with a reduced risk for experience of myocardial infarction.

When a TRP goes bad: transient receptor potential channels in addiction

Drug addiction is a psychiatric disease state, wherein a drug is impulsively and compulsively self-administered despite negative consequences. This repeated administration results in permanent changes to nervous system physiology and architecture. The molecular pathways affected by addictive drugs are complex and inter-dependent on each other. Recently, various new proteins and protein families have been discovered to play a role in drug abuse. Emerging players in this phenomenon include TRP (Transient Receptor Potential) family channels, which are primarily known to function in sensory systems. Several TRP family channels identified in both vertebrates and invertebrates are involved in psychostimulant-induced plasticity, suggesting their involvement in drug dependence. This review summarizes various observations, both from studies in humans and other organisms, which support a role for these channels in the development of drug-related behaviors.

The Role of TRP Proteins in Mast Cells

Transient receptor potential (TRP) proteins form cation channels that are regulated through strikingly diverse mechanisms including multiple cell surface receptors, changes in temperature, in pH and osmolarity, in cytosolic free Ca(2+) concentration ([Ca(2+)](i)), and by phosphoinositides which makes them polymodal sensors for fine tuning of many cellular and systemic processes in the body. The 28 TRP proteins identified in mammals are classified into six subfamilies: TRPC, TRPV, TRPM, TRPA, TRPML, and TRPP. When activated, they contribute to cell depolarization and Ca(2+) entry. In mast cells, the increase of [Ca(2+)](i) is fundamental for their biological activity, and several entry pathways for Ca(2+) and other cations were described including Ca(2+) release activated Ca(2+) (CRAC) channels. Like in other non-excitable cells, TRP channels could directly contribute to Ca(2+) influx via the plasma membrane as constituents of Ca(2+) conducting channel complexes or indirectly by shifting the membrane potential and regulation of the driving force for Ca(2+) entry through independent Ca(2+) entry channels. Here, we summarize the current knowledge about the expression of individual Trp genes with the majority of the 28 members being yet identified in different mast cell models, and we highlight mechanisms how they can regulate mast cell functions. Since specific agonists or blockers are still lacking for most members of the TRP family, studies to unravel their function and activation mode still rely on experiments using genetic approaches and transgenic animals. RNAi approaches suggest a functional role for TRPC1, TRPC5, and TRPM7 in mast cell derived cell lines or primary mast cells, and studies using Trp gene knock-out mice reveal a critical role for TRPM4 in mast cell activation and for mast cell mediated cutaneous anaphylaxis, whereas a direct role of cold- and menthol-activated TRPM8 channels seems to be unlikely for the development of cold urticaria at least in mice.

Association study of TRPC4 as a candidate gene for generalized epilepsy with photosensitivity

Photoparoxysmal response (PPR) is characterized by abnormal visual sensitivity of the brain to photic stimulation. Frequently associated with idiopathic generalized epilepsies (IGEs), it might be an endophenotype for cortical excitability. Transient receptor potential cation (TRPC) channels are involved in the generation of epileptiform discharges, and TRPC4 constitutes the main TRPC channel in the central nervous system. The present study investigated an association of PPR with sequence variations of the TRPC4 gene. Thirty-five single nucleotide polymorphisms (SNP) within TRPC4 were genotyped in 273 PPR probands and 599 population controls. Association analyses were performed for the broad PPR endophenotype (PPR types I-IV; n = 273), a narrow model of affectedness (PPR types III and IV; n = 214) and PPR associated with IGE (PPR/IGE; n = 106) for each SNP and for corresponding haplotypes. Association was found between the intron 5 SNP rs10507456 and PPR/IGE both for single markers (P = 0.005) and haplotype level (P = 0.01). Three additional SNPs (rs1535775, rs10161932 and rs7338118) within the same haplotype block were associated with PPR/IGE at P < 0.05 (uncorrected) as well as two more markers (rs10507457, rs7329459) located in intron 3. Again, the corresponding haplotype also showed association with PPR/IGE. Results were not significant following correction for multiple comparisons by permutation analysis for single markers and Bonferroni-Holm for haplotypes. No association was found between variants in TRPC4 and other phenotypes. Our results showed a trend toward association of TRPC4 variants and PPR/IGE. Further studies including larger samples of photosensitive probands are required to clarify the relevance of TRPC4 for PPR and IGE.

Histamine-induced Ca2+ entry in human astrocytoma U373 MG cells: evidence for involvement of store-operated channels

Glial and glia-derived cells express a variety of receptors for neurotransmitters and hormones, the majority of which evoke both Ca(2+) release from intracellular stores and Ca(2+) entry across the plasma membrane. We investigated the links between histamine H(1) receptor activation, Ca(2+) release from intracellular stores and Ca(2+) influx in human astrocytoma U373 MG cells. Histamine, through a H(1) receptor-mediated effect, evoked an increase in cytoplasmic free calcium concentration ([Ca(2+)](i)) that occurred in two phases: an initial, transient, increase owing to Ca(2+) mobilization from intracellular pools, and a second, sustained increase dependent on both Ca(2+) influx and continuous receptor occupancy. The characteristics of histamine-induced increases in [Ca(2+)](i) were similar to the capacitative entry evoked by emptying of the Ca(2+) stores with thapsigargine, and different from that observed when Ca(2+) influx was activated with OAG (1-oleoyl-2-acetyl-sn-glycerol), a diacylglycerol (DAG) analog. OAG application or increased endogenous DAG, resulting from DAG kinase inhibition, reduced the histamine-induced response. Furthermore, activation of the DAG target, protein kinase C (PKC), by TPA (12-O-tetradecanoyl 4beta-phorbol 13alpha-acetate) resulted in inhibition of the histamine-induced Ca(2+) response, an action prevented by PKC inhibitors. By using reverse transcriptase-polymerase chain reaction analysis, mRNAs for transient receptor potential channels (TRPCs) 1, 4, and 6 as well as for STIM1 (stromal-interacting molecule) and Orai1 were found to be expressed in the U373 MG cells, and confocal microscopy using specific antibodies revealed the presence of the corresponding proteins. Therefore, TRPCs may be candidate proteins forming store-operated channels in the U373 MG cell line. Further, our results confirm the involvement of PKC in the regulation of H(1) receptor-induced responses and point out to the existence of a feedback mechanism acting via PKC to limit the increase in [Ca(2+)](i).