Ca2+ Influx through TRPC Channels Is Regulated by Homocysteine-Copper Complexes

An elevated level of circulating homocysteine (Hcy) has been regarded as an independent risk factor for cardiovascular disease; however, the clinical benefit of Hcy lowering-therapy is not satisfying. To explore potential unrevealed mechanisms, we investigated the roles of Ca²⁺ influx through TRPC channels and regulation by Hcy-copper complexes. Using primary cultured human aortic endothelial cells and HEK-293 T-REx cells with inducible TRPC gene expression, we found that Hcy increased the Ca²⁺ influx in vascular endothelial cells through the activation of TRPC4 and TRPC5. The activity of TRPC4 and TRPC5 was regulated by extracellular divalent copper (Cu²⁺) and Hcy. Hcy prevented channel activation by divalent copper, but monovalent copper (Cu⁺) had no effect on the TRPC channels. The glutamic acids (E542/E543) and the cysteine residue (C554) in the extracellular pore region of the TRPC4 channel mediated the effect of Hcy-copper complexes. The interaction of Hcy-copper significantly regulated endothelial proliferation, migration, and angiogenesis. Our results suggest that Hcy-copper complexes function as a new pair of endogenous regulators for TRPC channel activity. This finding gives a new understanding of the pathogenesis of hyperhomocysteinemia and may explain the unsatisfying clinical outcome of Hcy-lowering therapy and the potential benefit of copper-chelating therapy.

Structural snapshots of the mechanism of TRPV2 channel activation by small-molecule agonists

Transient receptor potential (TRP) channels are polymodal sensors that play critical roles in various physiological processes in living organisms. These cation-permeable channels respond to a variety of physical and chemical stimuli, including cold and hot temperatures, acidic pH, and mechanical stress, often determining a sensory frontier of defense against hostile environments. Vanilloid (V) subfamily is the most studied category of TRP channels that includes six closely related members: highly calcium-selective TRPV5-6 and non-selective TRPV1-4. A remarkable feature of TRPV1-4 is their ability to sense heat, which makes them temperature-sensitive TRP channels or thermo-TRPs. TRPV channels are associated with a multitude of human diseases, including cancers, chronic pain, cardiovascular, neurological and nociceptive disorders. Despite the great clinical interest, pharmacology of TRPV channels remains largely undeveloped because of insufficient knowledge about the mechanisms of their regulation. For instance, activation of TRPV channels by small molecules or heat remains poorly understood. Numerous identified TRPV channel agonists, while effective in physiological experiments, appear limited in their ability to act in the conditions of structural biology experiments. In this regard, the recent study by Pumroy et al. [1] makes a significant contribution towards our understanding of TRPV2 structural dynamics that leads to opening of this channel in physiological conditions.

Electron Microscopic and Immunohistochemical Examination of the Effect of 2-Aminoethoxydiphenyl Borate on Optic Nerve Injury in A Rat Model

Objective

We conducted this study to explore the possible protective effect of 2-aminoethoxydiphenyl borate (2-APB) on experimentally induced optic nerve injury in an acute ischemia-reperfusion (AIR) model.

Materials and Methods

A total of 30 Wistar albino rats were randomly divided into sham, AIR, and AIR+treatment (AIR10) groups. In the sham group, AIR model was not created. In the AIR group, AIR model was created without the administration of drug. In the AIR10 group, 2-APB was administered 10 min before reperfusion.

Results

Tissue samples were subjected to histological, immunohistochemical, and electron microscopic procedures. Histopathological examination revealed intense hypertrophic cells, more glial cells, capillary dilatation, and intense demyelination areas in the AIR group compared to those in the sham and AIR10 groups. Immunohistochemical staining demonstrated an increase in Orai1 and STIM1 immunoreactivity in the AIR group but less intense staining in the AIR10 group. Electron microscopy revealed injury in optic nerve axons in the AIR group, whereas this type of injury occurred to a lesser extent in the AIR10 group.

Conclusion

In rats, store-operated Ca²⁺ entry in the cell had an essential role in optic nerve ischemia-reperfusion injury, and 2-ABP may have a protective effect on optic nerve injury caused due to AIR.

Periprosthetic hypoxia as consequence of TRPM7 mediated cobalt influx in osteoblasts

The reasons for the high number of loosened metal-on-metal (MoM) hip implants are still not fully understood. Hypoxia-inducible factor 1 (HIF-1) mediated signaling pathways, which normally modulate tissue metabolism under hypoxic circumstances, could be triggered by metallic wear debris and influence bone metabolism favoring osteolysis. This may lead to early loosening of the orthopedic implants. Immunhistochemical staining of periprosthetic tissues of failed artificial hip implants showed that the concentration of HIF-1α in the surrounding tissues of failed MoM hip implants was significantly higher in comparison to failed metal-on-polyethylene (MoP) hip implants and osteoarthritic tissues. Therefore, we examined the Co²⁺ -uptake mechanisms and the influence of Co²⁺ uptake on HIF-1α stabilization. Based on cobalt mediated quenching effects, calcium imaging experiments using fura-2 showed a concentration-dependent cobalt influx in MG-63 cells, which could be inhibited by the unspecific TRPM7 channel inhibitor 2-APB (20 μM) and TRPM7 specific siRNA. Western blots confirmed a dose dependent increase of HIF-1α upon stimulation with Co²⁺ . This effect could be abrogated by inhibition of cobalt influx using 2-APB. This study shows that chemical hypoxia originating from HIF-1α upregulation within the periprosthetic tissue is related to cobalt wear debris and highlights TRPM7 as an important key mediator in this context. © 2018 Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater 107B: 1806-1813, 2019.

The 2-aminoethoxydiphenyl borate analog, DPB161 blocks store-operated Ca2+ entry in acutely dissociated rat submandibular cells

Cellular Ca²⁺ signals play a critical role in cell physiology and pathology. In most non-excitable cells, store-operated Ca²⁺ entry (SOCE) is an important mechanism by which intracellular Ca²⁺ signaling is regulated. However, few drugs can selectively modulate SOCE. 2-Aminoethoxydiphenyl borate (2APB) and its analogs (DPB162 and DPB163) have been reported to inhibit SOCE. Here, we examined the effects of another 2-APB analog, DPB161 on SOCE in acutely-isolated rat submandibular cells. Both patch-clamp recordings and Ca²⁺ imaging showed that upon removal of extracellular Ca²⁺ ([Ca²⁺]_o=0), rat submandibular cells were unable to maintain ACh-induced Ca²⁺ oscillations, but restoration of [Ca²⁺]_o to refill Ca²⁺ stores enable recovery of these Ca²⁺ oscillations. However, addition of 50 μM DPB161 with [Ca²⁺]_o to extracellular solution prevented the refilling of Ca²⁺ store. Fura-2 Ca²⁺ imaging showed that DPB161 inhibited SOCE in a concentration-dependent manner. After depleting Ca²⁺ stores by thapsigargin treatment, bath perfusion of 1 mM Ca²⁺ induced [Ca²⁺]_i elevation in a manner that was prevented by DPB161. Collectively, these results show that the 2-APB analog DPB161 blocks SOCE in rat submandibular cells, suggesting that this compound can be developed as a pharmacological tool for the study of SOCE function and as a new therapeutic agent for treating SOCE-associated disorders.

Ca2+-Dependent Endoplasmic Reticulum Stress Regulates Mechanical Stress-Mediated Cartilage Thinning

Our previous study identified that endoplasmic reticulum stress (ERS) plays a critical role in chondrocyte apoptosis and mandibular cartilage thinning in response to compressive mechanical force, although the underlying mechanisms remain elusive. Because the endoplasmic reticulum (ER) is a primary site of intracellular Ca(2+) storage, we hypothesized that Ca(2+)-dependent ERS might be involved in mechanical stress-mediated mandibular cartilage thinning. In this study, we used in vitro and in vivo models to determine Ca(2+) concentrations, histological changes, subcellular changes, apoptosis, and the expression of ERS markers in mandibular cartilage and chondrocytes. The results showed that in chondrocytes, cytosolic Ca(2+) ([Ca(2+)]i) was dramatically increased by compressive mechanical force. Interestingly, the inhibition of Ca(2+) channels by ryanodine and 2-aminoethoxydiphenyl borate, inhibitors of ryanodine receptors and inositol trisphosphate receptors, respectively, partially rescued mechanical force-mediated mandibular cartilage thinning. Furthermore, chondrocyte apoptosis was also compromised by inhibiting the increase in [Ca(2+)]i that occurred in response to compressive mechanical force. Mechanistically, the ERS induced by compressive mechanical force was also repressed by [Ca(2+)]i inhibition, as demonstrated by a decrease in the expression of the ER stress markers 78 kDa glucose-regulated protein (GRP78) and 94 kDa glucose-regulated protein (GRP94) at both the mRNA and protein levels. Collectively, these data identified [Ca(2+)]i as a critical mediator of the pathological changes that occur in mandibular cartilage under compressive mechanical force and shed light on the treatment of mechanical stress-mediated cartilage degradation.

Purinergic signaling negatively regulates activity of an olfactory receptor in an odorant-dependent manner

Extracellular purines and pyrimidines are important signaling molecules that mediate diverse biological functions via cell surface purinergic receptors. Although purinergic modulation to olfactory activity has been reported, cell-specific expression and action of purinergic receptors deserve further exploration. We physiologically characterized expression of purinergic receptors in a set of olfactory sensory neurons that are responsive to both acetophenone and benzaldehyde (AB-OSNs). Sparsely distributed in the most ventral olfactory receptor zone, AB-OSNs were activated by P2 purinergic receptor agonists but not by P1 purinergic receptor agonist adenosine. Both P2X-selective agonist α,β-methylene ATP and P2Y-selective agonist uridine 5'-triphosphate (UTP) were stimulatory to AB-OSNs, indicating expression of both P2X and P2Y purinergic receptors in AB-OSNs. Pharmacological characterization of receptor specificity using various P2X and P2Y agonists and antagonists illustrated that P2X1 and P2Y2 receptors played major roles in purinergic signaling in AB-OSNs. Interestingly, the results of purinergic modulation to acetophenone-evoked responses were different from those to benzaldehyde-evoked responses within the same neurons. Activation of P2X1 receptors had more profound inhibitory effects on benzaldehyde-evoked intracellular calcium elevation than on acetophenone-evoked responses within the same neurons, and the reverse was true when P2Y2 receptors were activated. Cross-adaptation data showed that acetophenone and benzaldehyde bound to the same olfactory receptor. Thus, our study has demonstrated that purinergic signaling of P2X and P2Y receptors has different effects on olfactory transduction mediated by a defined olfactory receptor and the consequences of purinergic modulation of olfactory activity might depend on stereotypic structures of the odorant-receptor complex.

BAFF activates Erk1/2 promoting cell proliferation and survival by Ca2+-CaMKII-dependent inhibition of PP2A in normal and neoplastic B-lymphoid cells

B-cell activating factor (BAFF) is involved in not only the physiology of normal B cells, but also the pathophysiology of aggressive B cells related to malignant and autoimmune diseases. However, how excessive BAFF promotes aggressive B-cell proliferation and survival is not well understood. Here we show that excessive human soluble BAFF (hsBAFF) enhanced cell proliferation and survival in normal and B-lymphoid (Raji) cells, which was associated with suppression of PP2A, resulting in activation of Erk1/2. This is supported by the findings that pretreatment with U0126 or PD98059, expression of dominant negative MKK1, or overexpression of PP2A prevented hsBAFF-induced activation of Erk1/2 and cell proliferation/viability in the cells. It appears that hsBAFF-mediated PP2A-Erk1/2 pathway and B-cell proliferation/viability was Ca(2+)-dependent, as pretreatment with BAPTA/AM, EGTA or 2-APB significantly attenuated these events. Furthermore, we found that inhibiting CaMKII with KN93 or silencing CaMKII also attenuated hsBAFF-mediated PP2A-Erk1/2 signaling and B-cell proliferation/viability. The results indicate that BAFF activates Erk1/2, in part through Ca(2+)-CaMKII-dependent inhibition of PP2A, increasing cell proliferation/viability in normal and neoplastic B-lymphoid cells. Our data suggest that inhibitors of CaMKII and Erk1/2, activator of PP2A or manipulation of intracellular Ca(2+) may be exploited for prevention of excessive BAFF-induced aggressive B-cell malignancies and autoimmune diseases.

TRPM7 channel regulates PDGF-BB-induced proliferation of hepatic stellate cells via PI3K and ERK pathways

TRPM7, a non-selective cation channel of the TRP channel superfamily, is implicated in diverse physiological and pathological processes including cell proliferation. Recently, TRPM7 has been reported in hepatic stellate cells (HSCs). Here, we investigated the contribution role of TRPM7 in activated HSC-T6 cell (a rat hepatic stellate cell line) proliferation. TRPM7 mRNA and protein were measured by RT-PCR and Western blot in rat model of liver fibrosis in vivo and PDGF-BB-activated HSC-T6 cells in vitro. Both mRNA and protein of TRPM7 were dramatically increased in CCl4-treated rat livers. Stimulation of HSC-T6 cells with PDGF-BB resulted in a time-dependent increase of TRPM7 mRNA and protein. However, PDGF-BB-induced HSC-T6 cell proliferation was inhibited by non-specific TRPM7 blocker 2-aminoethoxydiphenyl borate (2-APB) or synthetic siRNA targeting TRPM7, and this was accompanied by downregulation of cell cycle proteins, cyclin D1, PCNA and CDK4. Blockade of TRPM7 channels also attenuated PDGF-BB induced expression of myofibroblast markers as measured by the induction of α-SMA and Col1α1. Furthermore, the phosphorylation of ERK and AKT, associated with cell proliferation, decreased in TRPM7 deficient HSC-T6 cells. These observations suggested that TRPM7 channels contribute to perpetuated fibroblast activation and proliferation of PDGF-BB induced HSC-T6 cells via the activation of ERK and PI3K pathways. Therefore, TRPM7 may constitute a useful target for the treatment of liver fibrosis.

Activation of rat transient receptor potential cation channel subfamily V member 1 channels by 2-aminoethoxydiphenyl borate

BACKGROUND

The transient receptor potential cation channel subfamily V member 1 (TRPV1) channel has been proved to be a molecular integrator of inflammatory pain sensation. 2-Aminoethoxydiphenyl borate (2-APB) and its analogs have been noticed as attractive candidates for the development of a selective TRPV1 agonist and/or antagonist. However, selectivity and effectiveness, species dependence, and the binding site(s) of 2-APB on TRPV1 channel protein remain controversial.

METHODS

The present study aimed to characterize acting sites of 2-APB on heterologously expressed rat TRPV1 (rTRPV1) channels in HEK 293 cells. Rat TRPV1 currents were recorded by cell-free, excised patch clamp techniques.

RESULTS

In inside-out and outside-out patch modes, 2-APB applied either side of the membrane dose-dependently activated rTRPV1 channels. 2-APB dose-dependently potentiated rTRPV1 currents, that activated by capsaicin, protons, or noxious heat. 2-APB potentiated the capsaicin-activated rTRPV1 current from both side of the patch membrane. A structural analogue of 2-APB, diphenylboronic anhydride, showed the same potentiation effect on the capsaicin-activated rTRPV1 current.

CONCLUSION

It is suggested that 2-APB directly opens rTRPV1 channels from both sides of the membrane and potentiates the opening of channels by inflammatory stimuli.

Evodiamine, a plant alkaloid, induces calcium/JNK-mediated autophagy and calcium/mitochondria-mediated apoptosis in human glioblastoma cells. Chem Biol Interact. 2013;205:20-28. [PMID: 23774672 DOI: 10.1016/j.cbi.2013.06.004]

Glioblastomas, the most common primary gliomas, are characterized by increased invasion and difficult therapy. Major clinical medicines for treating gliomas merely extend the survival time for a number of months. Therefore, development of new agents against gliomas is important. Autophagy, a process for degrading damaged organelles and proteins, is an adaptive response to environmental stress. However, the role of autophagy in glioblastoma development still needs to be further investigated. Evodiamine, a major alkaloid isolated from Evodia rutaecarpa Bentham, has various pharmacological activities, such as inhibiting tumor growth and metastatic properties. However, the effects of evodiamine on glioblastomas and their detailed molecular mechanisms and autophagy formation are not well understood. In this study, we observed that evodiamine induced dose- and time-dependent apoptosis in glioma cells. Blockade of calcium channels in endoplasmic reticulum (ER) significantly reduced evodiamine-induced cytosolic calcium elevation, apoptosis, and mitochondrial depolarization, which suggests that evodiamine induces a calcium-mediated intrinsic apoptosis pathway. Interestingly, autophagy was also enhanced by evodiamine, and had reached a plateau by 24h. Pharmacological inhibition of autophagy resulted in increased apoptosis and reduced cell viability. Inhibition of ER calcium channel activation also significantly reduced evodiamine-induced autophagy. Inactivation of c-Jun N-terminal kinases (JNK) suppressed evodiamine-mediated autophagy accompanied by increased apoptosis. Furthermore, evodiamine-mediated JNK activation was abolished by BAPTA-AM, an intracellular calcium scavenger, suggesting that evodiamine mediates autophagy via a calcium-JNK signaling pathway. Collectively, these results suggest that evodiamine induces intracellular calcium/JNK signaling-mediated autophagy and calcium/mitochondria-mediated apoptosis in glioma cells.

Intracrine endothelin signaling evokes IP3-dependent increases in nucleoplasmic Ca²⁺ in adult cardiac myocytes

Endothelin receptors are present on the nuclear membranes in adult cardiac ventricular myocytes. The objectives of the present study were to determine 1) which endothelin receptor subtype is in cardiac nuclear membranes, 2) if the receptor and ligand traffic from the cell surface to the nucleus, and 3) the effect of increased intracellular ET-1 on nuclear Ca(2+) signaling. Confocal microscopy using fluorescently-labeled endothelin analogs confirmed the presence of ETB at the nuclear membrane of rat cardiomyocytes in skinned-cells and isolated nuclei. Furthermore, in both cardiac myocytes and aortic endothelial cells, endocytosed ET:ETB complexes translocated to lysosomes and not the nuclear envelope. Although ETA and ETB can form heterodimers, the presence or absence of ETA did not alter ETB trafficking. Treatment of isolated nuclei with peptide: N-glycosidase F did not alter the electrophoretic mobility of ETB. The absence of N-glycosylation further indicates that these receptors did not originate at the cell surface. Intracellular photolysis of a caged ET-1 analog ([Trp-ODMNB(21)]ET-1) evoked an increase in nucleoplasmic Ca(2+) ([Ca(2+)]n) that was attenuated by inositol 1,4,5-trisphosphate receptor inhibitor 2-aminoethoxydiphenyl borate and prevented by pre-treatment with ryanodine. A caged cell-permeable analog of the ETB-selective antagonist IRL-2500 blocked the ability of intracellular cET-1 to increase [Ca(2+)]n whereas extracellular application of ETA and ETB receptor antagonists did not. These data suggest that 1) the endothelin receptor in the cardiac nuclear membranes is ETB, 2) ETB traffics directly to the nuclear membrane after biosynthesis, 3) exogenous endothelins are not ligands for ETB on nuclear membranes, and 4) ETB associated with the nuclear membranes regulates nuclear Ca(2+) signaling.

Inositol 1,4,5-trisphosphate-induced calcium release is necessary for generating the entire light response of limulus ventral photoreceptors

The experiments reported here were designed to answer the question of whether inositol 1,4,5-trisphosphate (IP3)-induced calcium release is necessary for generating the entire light response of Limulus ventral photoreceptors. For this purpose the membrane-permeable IP3 receptor antagonist 2-aminoethoxydiphenyl borate (2APB) (Maruyama, T., T. Kanaji, S. Nakade, T. Kanno, and K. Mikoshiba. 1997. J. Biochem. (Tokyo). 122:498-505) was used. Previously, 2APB was found to inhibit the light activated current of Limulus ventral photoreceptors and reversibly inhibit both light and IP3 induced calcium release as well as the current activated by pressure injection of calcium into the light sensitive lobe of the photoreceptor (Wang, Y., M. Deshpande, and R. Payne. 2002. Cell Calcium. 32:209). In this study 2APB was found to inhibit the response to a flash of light at all light intensities and to inhibit the entire light response to a step of light, that is, both the initial transient and the steady-state components of the response to a step of light were inhibited. The light response in cells injected with the calcium buffer 1,2-bis(o-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid (BAPTA) was reversibly inhibited by 2APB, indicating that these light responses result from IP3-mediated calcium release giving rise to an increase in Cai. The light response obtained from cells after treatment with 100 microM cyclopiazonic acid (CPA), which acts to empty intracellular calcium stores, was reversibly inhibited by 2APB, indicating that the light response after CPA treatment results from IP3-mediated calcium release and a consequent rise in Cai. Together these findings imply that IP3-induced calcium release is necessary for generating the entire light response of Limulus ventral photoreceptors.