Cardioprotective effects of Moku-boi-to and its impact on AngII-induced cardiomyocyte hypertrophy

Cardiomyocyte hypertrophy, induced by elevated levels of angiotensin II (AngII), plays a crucial role in cardiovascular diseases. Current therapeutic approaches aim to regress cardiac hypertrophy but have limited efficacy. Widely used Japanese Kampo medicines are highly safe and potential therapeutic agents. This study aims to explore the impact and mechanisms by which Moku-boi-to (MBT), a Japanese Kampo medicine, exerts its potential cardioprotective benefits against AngII-induced cardiomyocyte hypertrophy, bridging the knowledge gap and contributing to the development of novel therapeutic strategies. By evaluating the effects of six Japanese Kampo medicines with known cardiovascular efficiency on AngII-induced cardiomyocyte hypertrophy and cell death, we identified MBT as a promising candidate. MBT exhibited preventive effects against AngII-induced cardiomyocyte hypertrophy, cell death and demonstrated improvements in intracellular Ca2+ signaling regulation, ROS production, and mitochondrial function. Unexpectedly, experiments combining MBT with the AT1 receptor antagonist losartan suggested that MBT may target the AT1 receptor. In an isoproterenol-induced heart failure mouse model, MBT treatment demonstrated significant effects on cardiac function and hypertrophy. These findings highlight the cardioprotective potential of MBT through AT1 receptor-mediated mechanisms, offering valuable insights into its efficacy in alleviating AngII-induced dysfunction in cardiomyocytes. The study suggests that MBT holds promise as a safe and effective prophylactic agent for cardiac hypertrophy, providing a deeper understanding of its mechanisms for cardioprotection against AngII-induced dysfunction.

Cell death induction and protection by activation of ubiquitously expressed anion/cation channels. Part 3: the roles and properties of TRPM2 and TRPM7

Cell volume regulation (CVR) is a prerequisite for animal cells to survive and fulfill their functions. CVR dysfunction is essentially involved in the induction of cell death. In fact, sustained normotonic cell swelling and shrinkage are associated with necrosis and apoptosis, and thus called the necrotic volume increase (NVI) and the apoptotic volume decrease (AVD), respectively. Since a number of ubiquitously expressed ion channels are involved in the CVR processes, these volume-regulatory ion channels are also implicated in the NVI and AVD events. In Part 1 and Part 2 of this series of review articles, we described the roles of swelling-activated anion channels called VSOR or VRAC and acid-activated anion channels called ASOR or PAC in CVR and cell death processes. Here, Part 3 focuses on therein roles of Ca2+-permeable non-selective TRPM2 and TRPM7 cation channels activated by stress. First, we summarize their phenotypic properties and molecular structure. Second, we describe their roles in CVR. Since cell death induction is tightly coupled to dysfunction of CVR, third, we focus on their participation in the induction of or protection against cell death under oxidative, acidotoxic, excitotoxic, and ischemic conditions. In this regard, we pay attention to the sensitivity of TRPM2 and TRPM7 to a variety of stress as well as to their capability to physicall and functionally interact with other volume-related channels and membrane enzymes. Also, we summarize a large number of reports hitherto published in which TRPM2 and TRPM7 channels are shown to be involved in cell death associated with a variety of diseases or disorders, in some cases as double-edged swords. Lastly, we attempt to describe how TRPM2 and TRPM7 are organized in the ionic mechanisms leading to cell death induction and protection.

Sigma-1 receptor is involved in modification of ER-mitochondria proximity and Ca2+ homeostasis in cardiomyocytes

The Sigma-1 receptor (Sigmar1) is downregulated in heart failure model mice with mitochondrial dysfunction. However, the mechanism in detail has not been investigated. In this study, we investigated the role of Sigmar1 in ER-mitochondria proximity using Sigmar1-knockdown or -overexpressed neonatal rat ventricular myocytes (NRVMs). The endothelin-1 (ET-1)-induced cardiomyocyte hypertrophy was aggravated with the dysregulation of mitochondrial function and ER-mitochondrial junctional formation in Sigmar1-knockdown NRVMs, whereas improved in Sigmar1 overexpressed NRVMs. Our data suggests that the reduction of the cardiac Sigmar1 results in decrease mitochondrial Ca²⁺ influx and promotes mitochondrial fission, followed by reduced ER-mitochondria proximity, exacerbating ET-1-induced cardiomyocyte injury.

Intermediate conductance Ca2+-activated potassium channels are activated by functional coupling with stretch-activated nonselective cation channels in cricket myocytes

Cooperative gating of localized ion channels ranges from fine-tuning excitation-contraction coupling in muscle cells to controlling pace-making activity in the heart. Membrane deformation resulting from muscle contraction activates stretch-activated (SA) cation channels. The subsequent Ca2+ influx activates spatially localized Ca2+-sensitive K+ channels to fine-tune spontaneous muscle contraction. To characterize endogenously expressed intermediate conductance Ca2+-activated potassium (IK) channels and assess the functional relevance of the extracellular Ca2+ source leading to IK channel activity, we performed patch-clamp techniques on cricket oviduct myocytes and recorded single-channel data. In this study, we first investigated the identification of IK channels that could be distinguished from endogenously expressed large-conductance Ca2+-activated potassium (BK) channels by adding extracellular Ba2+. The single-channel conductance of the IK channel was 62 pS, and its activity increased with increasing intracellular Ca2+ concentration but was not voltage-dependent. These results indicated that IK channels are endogenously expressed in cricket oviduct myocytes. Second, the Ca2+ influx pathway that activates the IK channel was investigated. The absence of extracellular Ca2+ or the presence of Gd3+ abolished the activity of IK channels. Finally, we investigated the proximity between SA and IK channels. The removal of extracellular Ca2+, administration of Ca2+ to the microscopic region in a pipette, and application of membrane stretching stimulation increased SA channel activity, followed by IK channel activity. Membrane stretch-induced SA and IK channel activity were positively correlated. However, the emergence of IK channel activity and its increase in response to membrane mechanical stretch was not observed without Ca2+ in the pipette. These results strongly suggest that IK channels are endogenously expressed in cricket oviduct myocytes and that IK channel activity is regulated by neighboring SA channel activity. In conclusion, functional coupling between SA and IK channels may underlie the molecular basis of spontaneous rhythmic contractions.

Properties, Structures, and Physiological Roles of Three Types of Anion Channels Molecularly Identified in the 2010's

Molecular identification was, at last, successfully accomplished for three types of anion channels that are all implicated in cell volume regulation/dysregulation. LRRC8A plus LRRC8C/D/E, SLCO2A1, and TMEM206 were shown to be the core or pore-forming molecules of the volume-sensitive outwardly rectifying anion channel (VSOR) also called the volume-regulated anion channel (VRAC), the large-conductance maxi-anion channel (Maxi-Cl), and the acid-sensitive outwardly rectifying anion channel (ASOR) also called the proton-activated anion channel (PAC) in 2014, 2017, and 2019, respectively. More recently in 2020 and 2021, we have identified the S100A10-annexin A2 complex and TRPM7 as the regulatory proteins for Maxi-Cl and VSOR/VRAC, respectively. In this review article, we summarize their biophysical and structural properties as well as their physiological roles by comparing with each other on the basis of their molecular insights. We also point out unsolved important issues to be elucidated soon in the future.

Vasopressin Neurons Respond to Hyperosmotic Stimulation with Regulatory Volume Increase and Secretory Volume Decrease by Activating Ion Transporters and Ca2+ Channels

BACKGROUND/AIMS

Arginine vasopressin (AVP) neurons play an important role for sensing a change in the plasma osmolarity and thereby responding with regulated AVP secretion in order to maintain the body fluid homeostasis. The osmo-sensing processes in magnocellular neurosecretory cells (MNCs) including AVP and oxytocin (OXT) neurons of the hypothalamus were reported to be coupled to sustained osmotic shrinkage or swelling without exhibiting discernible cell volume regulation. Since increasing evidence has shown some important differences in properties between AVP and OXT neurons, osmotic volume responses are to be reexamined with distinguishing these cell types from each other. We previously reported that AVP neurons identified by transgenic expression of enhanced green fluorescence protein (eGFP) possess the ability of regulatory volume decrease (RVD) after hypoosmotic cell swelling. Thus, in the present study, we examined the ability of regulatory volume increase (RVI) after hyperosmotic cell shrinkage in AVP neurons.

METHODS

Here, we used eGFP-identified AVP neurons acutely dissociated from AVP-eGFP transgenic rats. We performed single-cell size measurements, cytosolic RT-PCR analysis, AVP secretion measurements, and patch-clamp studies.

RESULTS

The AVP neurons were found to respond to a hyperosmotic challenge with physiological cell shrinkage caused by massive secretion of AVP, called a secretory volume decrease (SVD), superimposed onto physical osmotic cell shrinkage, and also to exhibit the ability of RVI coping with osmotic and secretory cell shrinkage. Furthermore, our pharmacological and molecular examinations indicated that AVP secretion and its associated SVD event are triggered by activation of T-type Ca²⁺ channels, and the RVI event is attained by parallel operation of Na⁺/H⁺ exchanger and Cl^-/HCO₃^- anion exchanger.

CONCLUSION

Thus, it is concluded that AVP neurons respond to hyperosmotic stimulation with the regulatory volume increase and the secretory volume increase by activating ion transporters and Ca²⁺ channels, respectively.

Cell Death Induction and Protection by Activation of Ubiquitously Expressed Anion/Cation Channels. Part 2: Functional and Molecular Properties of ASOR/PAC Channels and Their Roles in Cell Volume Dysregulation and Acidotoxic Cell Death

For survival and functions of animal cells, cell volume regulation (CVR) is essential. Major hallmarks of necrotic and apoptotic cell death are persistent cell swelling and shrinkage, and thus they are termed the necrotic volume increase (NVI) and the apoptotic volume decrease (AVD), respectively. A number of ubiquitously expressed anion and cation channels play essential roles not only in CVR but also in cell death induction. This series of review articles address the question how cell death is induced or protected with using ubiquitously expressed ion channels such as swelling-activated anion channels, acid-activated anion channels, and several types of TRP cation channels including TRPM2 and TRPM7. In the Part 1, we described the roles of swelling-activated VSOR/VRAC anion channels. Here, the Part 2 focuses on the roles of the acid-sensitive outwardly rectifying (ASOR) anion channel, also called the proton-activated chloride (PAC) anion channel, which is activated by extracellular protons in a manner sharply dependent on ambient temperature. First, we summarize phenotypical properties, the molecular identity, and the three-dimensional structure of ASOR/PAC. Second, we highlight the unique roles of ASOR/PAC in CVR dysfunction and in the induction of or protection from acidotoxic cell death under acidosis and ischemic conditions.

BK Channels Are Activated by Functional Coupling With L-Type Ca2+ Channels in Cricket Myocytes

Large-conductance calcium (Ca2+)-activated potassium (K+) (BK) channel activation is important for feedback control of Ca2+ influx and cell excitability during spontaneous muscle contraction. To characterize endogenously expressed BK channels and evaluate the functional relevance of Ca2+ sources leading to BK activity, patch-clamp electrophysiology was performed on cricket oviduct myocytes to obtain single-channel recordings. The single-channel conductance of BK channels was 120 pS, with increased activity resulting from membrane depolarization or increased intracellular Ca2+ concentration. Extracellular application of tetraethylammonium (TEA) and iberiotoxin (IbTX) suppressed single-channel current amplitude. These results indicate that BK channels are endogenously expressed in cricket oviduct myocytes. Ca2+ release from internal Ca2+ stores and Ca2+ influx via the plasma membrane, which affect BK activity, were investigated. Extracellular Ca2+ removal nullified BK activity. Administration of ryanodine and caffeine reduced BK activity. Administration of L-type Ca2+ channel activity regulators (Bay K 8644 and nifedipine) increased and decreased BK activity, respectively. Finally, the proximity between the L-type Ca2+ channel and BK was investigated. Administration of Bay K 8644 to the microscopic area within the pipette increased BK activity. However, this increase was not observed at a sustained depolarizing potential. These results show that BK channels are endogenously expressed in cricket oviduct myocytes and that BK activity is regulated by L-type Ca2+ channel activity and Ca2+ release from Ca2+ stores. Together, these results show that functional coupling between L-type Ca2+ and BK channels may underlie the molecular basis of spontaneous rhythmic contraction.

Cell Death Induction and Protection by Activation of Ubiquitously Expressed Anion/Cation Channels. Part 1: Roles of VSOR/VRAC in Cell Volume Regulation, Release of Double-Edged Signals and Apoptotic/Necrotic Cell Death

Cell volume regulation (CVR) is essential for survival and functions of animal cells. Actually, normotonic cell shrinkage and swelling are coupled to apoptotic and necrotic cell death and thus called the apoptotic volume decrease (AVD) and the necrotic volume increase (NVI), respectively. A number of ubiquitously expressed anion and cation channels are involved not only in CVD but also in cell death induction. This series of review articles address the question how cell death is induced or protected with using ubiquitously expressed ion channels such as swelling-activated anion channels, acid-activated anion channels and several types of TRP cation channels including TRPM2 and TRPM7. The Part 1 focuses on the roles of the volume-sensitive outwardly rectifying anion channels (VSOR), also called the volume-regulated anion channel (VRAC), which is activated by cell swelling or reactive oxygen species (ROS) in a manner dependent on intracellular ATP. First we describe phenotypical properties, the molecular identity, and physical pore dimensions of VSOR/VRAC. Second, we highlight the roles of VSOR/VRAC in the release of organic signaling molecules, such as glutamate, glutathione, ATP and cGAMP, that play roles as double-edged swords in cell survival. Third, we discuss how VSOR/VRAC is involved in CVR and cell volume dysregulation as well as in the induction of or protection from apoptosis, necrosis and regulated necrosis under pathophysiological conditions.

Expression and functions of N-type Cav2.2 and T-type Cav3.1 channels in rat vasopressin neurons under normotonic conditions

Arginine vasopressin (AVP) neurons play essential roles in sensing the change in systemic osmolarity and regulating AVP release from their neuronal terminals to maintain the plasma osmolarity. AVP exocytosis depends on the Ca2+ entry via voltage-gated Ca2+ channels (VGCCs) in AVP neurons. In this study, suppression by siRNA-mediated knockdown and pharmacological sensitivity of VGCC currents evidenced molecular and functional expression of N-type Cav2.2 and T-type Cav3.1 in AVP neurons under normotonic conditions. Also, both the Cav2.2 and Cav3.1 currents were found to be sensitive to flufenamic acid (FFA). TTX-insensitive spontaneous action potentials were suppressed by FFA and T-type VGCC blocker Ni2+. However, Cav2.2-selective ω-conotoxin GVIA failed to suppress the firing activity. Taken together, it is concluded that Cav2.2 and Cav3.1 are molecularly and functionally expressed and both are sensitive to FFA in unstimulated rat AVP neurons. Also, it is suggested that Cav3.1 is primarily involved in their action potential generation.

Elucidation of the Mechanisms for the Underlying Depolarization and Reversibility by Photoactive Molecule

BACKGROUND/AIMS

Light-induced control of the cell membrane potential has enabled important advances in the study of biological processes involving the nervous system and muscle activity. The use of these light-induced modifications is expected in various medical applications, including the control of physiological responses and the recovery of lost functions by regulating nerve activity. In particular, charge-separating linkage molecules (Charge-Separation (CS) molecules) can depolarize cells by photoexcitation without genetic processing. However, the molecular mechanisms underlying cell membrane depolarization are unknown and have hindered its application. Here, we show that CS molecules localized in the cell membrane of PC12 cells using a high-density lipoprotein (HDL)-based drug carrier can excite the cells through a novel membrane current regulation mechanism by light irradiation.

METHODS

Membrane potential, channel activity, and membrane capacitance were measured by patch clamp method in rat adrenal gland pheochromocytoma (PC12) cells and K_V-overexpressing PC12 cells. CS molecules localized in the cell membrane of PC12 cells using HDL-based drug carrier. The localization of CS molecule was measured by a confocal microscopy. The mRNA expression was tested by RT-PCR.

RESULTS

Current clamp measurements revealed that the photo-activated CS molecule causes a sharp depolarization of about 15 mV. Furthermore, it was shown by voltage clamp measurement that this mechanism inactivates the voltage-dependent potassium current and simultaneously generates photo-activated CS molecule induced (PACS) current owing to the loss of the cell membrane capacitance. This activity continues the depolarization of the target cell, but is reversible via a regenerative mechanism such as endocytosis and exocytosis because the cell membrane is intact.

CONCLUSION

Thus, the mechanism of photo-induced depolarization concludes that photo-activated TC1 causes depolarization by generating PACS current in parallel with the suppression of the K⁺ current. Moreover, the depolarization slowly restores by internalization of TC1 from the membrane and insertion of new lipids into the cell membrane, resulting in the restoration of K_V to normal activity and eliminating PACS currents, without cell damage. These results suggest the possibility of medical application that can safely control membrane excitation.

O2-Dependent Protein Internalization Underlies Astrocytic Sensing of Acute Hypoxia by Restricting Multimodal TRPA1 Channel Responses

Hypoxia sensors are essential for regulating local oxygen (O₂) homeostasis within the body. This is especially pertinent within the CNS, which is particularly vulnerable to O₂ deprivation due to high energetic demand. Here, we reveal hypoxia-monitoring function exerted by astrocytes through an O₂-regulated protein trafficking mechanism within the CNS. Strikingly, cultured mouse astrocytes isolated from the parafacial respiratory group (pFRG) and retrotrapezoid nucleus (RTN) region are capable of rapidly responding to moderate hypoxia via the sensor cation channel transient receptor potential (TRP) A1 but, unlike multimodal sensory neurons, are inert to hyperoxia and other TRPA1 activators (carbon dioxide, electrophiles, and oxidants) in normoxia. Mechanistically, O₂ suppresses TRPA1 channel activity by protein internalization via O₂-dependent proline hydroxylation and subsequent ubiquitination by an E3 ubiquitin ligase, NEDD4-1 (neural precursor cell-expressed developmentally down-regulated protein 4). Hypoxia inhibits this process and instantly accumulates TRPA1 proteins at the plasma membrane, inducing TRPA1-mediated Ca²⁺ influx that triggers ATP release from pFRG/RTN astrocytes, potentiating respiratory center activity. Furthermore, astrocyte-specific Trpa1 disruption in a mouse brainstem-spinal cord preparation impedes the amplitude augmentation of the central autonomic respiratory output during hypoxia. Thus, reversible coupling of the TRPA1 channels with O₂-dependent protein translocation allows astrocytes to act as acute hypoxia sensors in the medullary respiratory center.

TRPM7 is involved in acid-induced necrotic cell death in a manner sensitive to progesterone in human cervical cancer cells

Because intravaginal pH is strongly acidic, it is important to investigate the effects of acidosis on cervical cancer cells. Recently, in response to strong acidosis, human cervical cancer HeLa cells were shown to exhibit necrosis after showing persistent cell swelling induced by Cl⁻ influx. Since cation influx should be accompanied with Cl⁻ influx to drive water inflow causing cell swelling, we here studied on the nature of acidotoxic cation conductance. The mRNA/protein expression was assessed by RT‐PCR and Western blotting. Ionic currents were measured by patch‐clamping techniques. Cell counting/viability and colorimetric assays were applied to assess proliferation rate and caspase 3/7 activity, respectively. Cell volume and size were measured by electronic sizing and video‐microscopic measurements, respectively. Acid exposure enhanced TRPM7 activity endogenously expressed in HeLa cells and exogenously overexpressed in HEK293T cells. Gene silencing of TRPM7 abolished acid‐induced cell swelling and necrosis but rather induced activation of apoptotic caspase 3/7 in HeLa cells. Overexpression with the pore charge‐neutralizing D1054A mutant suppressed acid‐enhanced cation currents, acid‐induced cell swelling, and acidotoxic necrosis in HEK293T cells. Progesterone treatment was surprisingly found to suppress molecular and functional expression of TRPM7 and cell proliferation in HeLa cells. Furthermore, in the progesterone‐treated cells, acid exposure did not induce persistent cell swelling followed by necrosis but induced persistent cell shrinkage and apoptotic cell death. These results indicate that in the human cervical cancer cells, TRPM7 is essentially involved in acidotoxic necrotic cell death, and progesterone inhibits TRPM7 expression thereby inhibiting acidotoxic necrosis by switching to apoptosis.

Cryo-EM structure of the volume-regulated anion channel LRRC8D isoform identifies features important for substrate permeation

Members of the leucine-rich repeat-containing 8 (LRRC8) protein family, composed of the five LRRC8A-E isoforms, are pore-forming components of the volume-regulated anion channel (VRAC). LRRC8A and at least one of the other LRRC8 isoforms assemble into heteromers to generate VRAC transport activities. Despite the availability of the LRRC8A structures, the structural basis of how LRRC8 isoforms other than LRRC8A contribute to the functional diversity of VRAC has remained elusive. Here, we present the structure of the human LRRC8D isoform, which enables the permeation of organic substrates through VRAC. The LRRC8D homo-hexamer structure displays a two-fold symmetric arrangement, and together with a structure-based electrophysiological analysis, revealed two key features. The pore constriction on the extracellular side is wider than that in the LRRC8A structures, which may explain the increased permeability of organic substrates. Furthermore, an N-terminal helix protrudes into the pore from the intracellular side and may be critical for gating.

SGLT2 inhibitor ipragliflozin attenuates breast cancer cell proliferation

Cancer is currently one of the major causes of death in patients with type 2 diabetes mellitus. We previously reported the beneficial effects of the glucagon-like peptide-1 receptor agonist exendin-4 against prostate and breast cancer. In the present study, we examined the anti-cancer effect of the sodium-glucose cotransporter 2 (SGLT2) inhibitor ipragliflozin using a breast cancer model. In human breast cancer MCF-7 cells, SGLT2 expression was detected using both RT-PCR and immunohistochemistry. Ipragliflozin at 1-50 μM significantly and dose-dependently suppressed the growth of MCF-7 cells. BrdU assay also revealed that ipragliflozin attenuated the proliferation of MCF-7 cells in a dose-dependent manner. Because the effect of ipragliflozin against breast cancer cells was completely canceled by knocking down SGLT2, ipragliflozin could act via inhibiting SGLT2. We next measured membrane potential and whole-cell current using the patch clamp technique. When we treated MCF-7 cells with ipragliflozin or glucose-free medium, membrane hyperpolarization was observed. In addition, glucose-free medium and knockdown of SGLT2 by siRNA suppressed the glucose-induced whole-cell current of MCF-7 cells, suggesting that ipragliflozin inhibits sodium and glucose cotransport through SGLT2. Furthermore, JC-1 green fluorescence was significantly increased by ipragliflozin, suggesting the change of mitochondrial membrane potential. These findings suggest that the SGLT2 inhibitor ipragliflozin attenuates breast cancer cell proliferation via membrane hyperpolarization and mitochondrial membrane instability.

Herbal components of Japanese Kampo medicines exert laxative actions in colonic epithelium cells via activation of BK and CFTR channels

Japanese Kampo medicines Junchoto and Mashiningan are mixtures of numerous herbal plant extracts and empirically known to exert laxative actions by stimulating fluid secretion in the colonic epithelium. However, it is unknown which and how the herbal components of these crude Kampo drugs are effective to stimulate ion effluxes causing fluid secretion. Here, we selected four herbal components of Junchoto and Mashiningan, Mashinin (MSN), Kyonin (KYN), Tonin (TON), and Daio (DIO), which are putatively laxatives, and examined their effects on the ion channel activity of human colonic epithelial Caco-2 cells. Patch clamp analyses revealed that MSN activated whole-cell current characteristics of the cystic fibrosis transmembrane conductance regulator (CFTR) channel, whereas KYN, TON, and DIO activated the large-conductance and voltage-activated K⁺ (BK) channel. Furthermore, electronic cell sizing showed that MSN induced secretory volume decrease (SVD) sensitivity to a CFTR blocker, whereas TON, KYN, and DIO induced SVD sensitivity to a K⁺ channel blocker. In conclusion, MSN and TON, KYN, and DIO promote fluid secretion from colonic epithelial cells by activating CFTR and BK channels. Thus, Japanese Kampo medicines, Junchoto and Mashiningan, exert anti-constipation actions by inducing KCl efflux through the combined actions of CFTR- and BK-stimulating herbal components.

Short TRPM2 prevents the targeting of full-length TRPM2 to the surface transmembrane by hijacking to ER associated degradation

Membrane proteins are targeted to the surface transmembrane after folding and assembling in the endoplasmic reticulum (ER). Misfolded- and unassembled-proteins are degraded by proteasomes following ubiquitination, termed ER-associated degradation (ERAD). Transient receptor potential melastatin 2 (TRPM2) is an oxidative stress-sensitive channel. One of the TRPM2 splicing variants, short TRPM2 (TRPM2-S) having only the N-terminus and first two transmembrane domains, was reported to prevent full-length TRPM2 (TRPM2-L) activation. Although TRPM2-S interacts with TRPM2-L, the inhibitory mechanisms of TRPM2-S are unclear. We found that TRPM2-S prevents transmembrane expression of TRPM2-L by targeting ERAD. TRPM2-S expression was lower than that of TRPM2-L, and was increased by an ERAD inhibitor. TRPM2-S was not expressed at the transmembrane. This suggests that TRPM2-S is a substrate for ERAD. Upon the simultaneous expression of TRPM2-S, the transmembrane expression of TRPM2-L was attenuated and the poly-ubiquitination of TRPM2-L was facilitated. Our study may clarify why TRPM2-S inhibits oxidative stress-induced TRPM2-L activation.

Construction of a Fluorescent Screening System of Allosteric Modulators for the GABAA Receptor Using a Turn-On Probe

γ-Aminobutyric acid (GABA) is the major inhibitory neurotransmitter in the central nervous system. The fast inhibitory actions of GABA are mainly mediated by GABA_A receptors (GABA_ARs), which are widely recognized as clinically relevant drug targets. However, it remains difficult to create screening systems for drug candidates that act on GABA_ARs because of the existence of multiple ligand-binding sites and the delicate pentameric structures of GABA_ARs. We here developed the first turn-on fluorescent imaging probe for GABA_ARs, which can be used to quantitatively evaluate ligand-receptor interactions under live cell conditions. Using noncovalent labeling of GABA_ARs with this turn-on probe, a new imaging-based ligand assay system, which allows discovery of positive allosteric modulators (PAMs) for the GABA_AR, was successfully constructed. Our system is applicable to high-throughput ligand screening, and we discovered new small molecules that function as PAMs for GABA_ARs. These results highlight the power of the use of a turn-on fluorescent probe to screen drugs for complicated membrane proteins that cannot be addressed by conventional methods.

TRPM7 channels mediate spontaneous Ca2+ fluctuations in growth plate chondrocytes that promote bone development

During endochondral ossification of long bones, the proliferation and differentiation of chondrocytes cause them to be arranged into layered structures constituting the epiphyseal growth plate, where they secrete the cartilage matrix that is subsequently converted into trabecular bone. Ca²⁺ signaling has been implicated in chondrogenesis in vitro. Through fluorometric imaging of bone slices from embryonic mice, we demonstrated that live growth plate chondrocytes generated small, cell-autonomous Ca²⁺ fluctuations that were associated with weak and intermittent Ca²⁺ influx. Several genes encoding Ca²⁺-permeable channels were expressed in growth plate chondrocytes, but only pharmacological inhibitors of transient receptor potential cation channel subfamily M member 7 (TRPM7) reduced the spontaneous Ca²⁺ fluctuations. The TRPM7-mediated Ca²⁺ influx was likely activated downstream of basal phospholipase C activity and was potentiated upon cell hyperpolarization induced by big-conductance Ca²⁺-dependent K⁺ channels. Bones from embryos in which Trpm7 was conditionally knocked out during ex vivo culture exhibited reduced outgrowth and displayed histological abnormalities accompanied by insufficient autophosphorylation of Ca²⁺/calmodulin-dependent protein kinase II (CaMKII) in the growth plate. The link between TRPM7-mediated Ca²⁺ fluctuations and CaMKII-dependent chondrogenesis was further supported by experiments with chondrocyte-specific Trpm7 knockout mice. Thus, growth plate chondrocytes generate spontaneous, TRPM7-mediated Ca²⁺ fluctuations that promote self-maturation and bone development.

Cell Volume-Activated and Volume-Correlated Anion Channels in Mammalian Cells: Their Biophysical, Molecular, and Pharmacological Properties

There are a number of mammalian anion channel types associated with cell volume changes. These channel types are classified into two groups: volume-activated anion channels (VAACs) and volume-correlated anion channels (VCACs). VAACs can be directly activated by cell swelling and include the volume-sensitive outwardly rectifying anion channel (VSOR), which is also called the volume-regulated anion channel; the maxi-anion channel (MAC or Maxi-Cl); and the voltage-gated anion channel, chloride channel (ClC)-2. VCACs can be facultatively implicated in, although not directly activated by, cell volume changes and include the cAMP-activated cystic fibrosis transmembrane conductance regulator (CFTR) anion channel, the Ca²⁺-activated Cl^- channel (CaCC), and the acid-sensitive (or acid-stimulated) outwardly rectifying anion channel. This article describes the phenotypical properties and activation mechanisms of both groups of anion channels, including accumulating pieces of information on the basis of recent molecular understanding. To that end, this review also highlights the molecular identities of both anion channel groups; in addition to the molecular identities of ClC-2 and CFTR, those of CaCC, VSOR, and Maxi-Cl were recently identified by applying genome-wide approaches. In the last section of this review, the most up-to-date information on the pharmacological properties of both anion channel groups, especially their half-maximal inhibitory concentrations (IC₅₀ values) and voltage-dependent blocking, is summarized particularly from the standpoint of pharmacological distinctions among them. Future physiologic and pharmacological studies are definitely warranted for therapeutic targeting of dysfunction of VAACs and VCACs.

TRPM7-mediated spontaneous Ca2+ entry regulates the proliferation and differentiation of human leukemia cell line K562

Continuous Ca2+ influx is essential to maintain intracellular Ca2+ homeostasis and its dysregulation leads to a variety of cellular dysfunctions. In this study, we explored the functional roles of spontaneous Ca2+ influx for the proliferation and differentiation of a human erythromyeloid leukemia cell line K562. mRNA/protein expressions were assessed by the real-time RT-PCR, western blotting, and immunocytochemical staining. Intracellular Ca2+ concentration ([Ca2+ ]i ) and ionic currents were measured by fluorescent imaging and patch clamping techniques, respectively. Cell counting/viability and colorimetric assays were applied to assess proliferation rate and hemoglobin synthesis, respectively. Elimination of extracellular Ca2+ decreased basal [Ca2+ ]i in proliferating K562 cells. Cation channel blockers such as SK&F96365, 2-APB, Gd3+ , and FTY720 dose dependently decreased basal [Ca2+ ]i . A spontaneously active inward current (Ispont ) contributive to basal [Ca2+ ]i was identified by the nystatin-perforated whole-cell recording. Ispont permeated Ca2+ comparably to Na+ , and was greatly eliminated by siRNA targeting TRPM7, a melastatin member of the transient receptor potential (TRP) superfamily. Consistent with these findings, TRPM7 immune reactivity was detected by western blotting, and immunofluorescence representing TRPM7 was found localized to the K562 cell membrane. Strikingly, all these procedures, that is, Ca2+ removal, TRPM7 blockers and siRNA-mediated TRPM7 knockdown significantly retarded the growth and suppressed hemin-induced γ-globin and hemoglobin syntheses in K562 cells, respectively, both of which appeared associated with the inhibition of ERK activation. These results collectively suggest that spontaneous Ca2+ influx through constitutively active TRPM7 channels may critically regulate both proliferative and erythroid differentiation potentials of K562 cells.

Uncovering the arrhythmogenic potential of TRPM4 activation in atrial-derived HL-1 cells using novel recording and numerical approaches

Aims

Transient receptor potential cation channel subfamily melastatin member 4 (TRPM4), a Ca2+-activated nonselective cation channel abundantly expressed in the heart, has been implicated in conduction block and other arrhythmic propensities associated with cardiac remodelling and injury. The present study aimed to quantitatively evaluate the arrhythmogenic potential of TRPM4.

Methods and results

Patch clamp and biochemical analyses were performed using expression system and an immortalized atrial cardiomyocyte cell line (HL-1), and numerical model simulation was employed. After rapid desensitization, robust reactivation of TRPM4 channels required high micromolar concentrations of Ca2+. However, upon evaluation with a newly devised, ionomycin-permeabilized cell-attached (Iono-C/A) recording technique, submicromolar concentrations of Ca2+ (apparent Kd = ∼500 nM) were enough to activate this channel. Similar submicromolar Ca2+ dependency was also observed with sharp electrode whole-cell recording and in experiments coexpressing TRPM4 and L-type voltage-dependent Ca2+ channels. Numerical simulations using a number of action potential (AP) models (HL-1, Nygren, Luo-Rudy) incorporating the Ca2+- and voltage-dependent gating parameters of TRPM4, as assessed by Iono-C/A recording, indicated that a few-fold increase in TRPM4 activity is sufficient to delay late AP repolarization and further increases (≥ six-fold) evoke early afterdepolarization. These model predictions are consistent with electrophysiological data from angiotensin II-treated HL-1 cells in which TRPM4 expression and activity were enhanced.

Conclusions

These results collectively indicate that the TRPM4 channel is activated by a physiological range of Ca2+ concentrations and its excessive activity can cause arrhythmic changes. Moreover, these results demonstrate potential utility of the first AP models incorporating TRPM4 gating for in silico assessment of arrhythmogenicity in remodelling cardiac tissue.

Cellular mechanism for herbal medicine Junchoto to facilitate intestinal Cl-/water secretion that involves cAMP-dependent activation of CFTR

Constipation is a common symptom frequently compromising the quality of daily life. Several mechanistically different drugs have been used to mitigate constipation, including Japanese herbal (Kampo) medicines. However, the mechanisms of their actions are often not well understood. Here we aimed to investigate the molecular mechanisms underlying the effects of Junchoto (JCT), a Kampo medicine empirically prescribed for chronic constipation. Cl⁻ channel activity was measured by the patch-clamp method in human cystic fibrosis transmembrane conductance regulator (CFTR)-expressing HEK293T cells and human intestinal Caco-2 cells. cAMP was measured by a luciferase-based assay. Cell volume change was measured by a particle-sizing and particle-counting analyzer and video-microscopic measurement. In both CFTR-expressing HEK293T and Caco-2 cells, JCT dose-dependently induced whole-cell currents showing typical biophysical and pharmacological features of CFTR. Robust expression of CFTR was confirmed by RT-PCR and Western blotting in Caco-2 cells. Luciferase-based measurement revealed that JCT increases intracellular cAMP levels. Administration of the adenylate cyclase inhibitor SQ22536 or CFTR inhibitor-172, or treatment with small interfering RNAs (siRNA) targeting CFTR, abolished JCT-induced whole-cell currents, suggesting that elevated intracellular cAMP by JCT causes activation of CFTR in Caco-2 cells. Finally, blockade of CFTR activity by CFTR inhibitor-172 or siRNA-knockdown of CFTR or application of SQ22536 markedly reduced the degree of cell volume decrease induced by JCT. JCT can induce a Cl⁻ efflux through the CFTR channel to promote water secretion, and this effect is likely mediated by increased cAMP production.

Sézary syndrome managed with histone deacetylase inhibitor followed by anti-CCR4 monoclonal antibody

A 70-year-old man presented to our clinic with a 10-year history of recurrent pruritic erythema and plaques on his trunk and limbs. Based on the pathological findings and monoclonal rearrangement of the T-cell receptor (TCR)-Cβ1 gene, mycosis fungoides (T2N0M0B0 stage IB) was diagnosed. Despite combination therapy including histone deacetylase inhibitor (vorinostat), the symptoms slowly evolved into Sézary syndrome (SS; T4N1M0B2) over 4 years, with dense infiltrates due to atypical lymphocytes expressing CCR4 developing in the entire dermis. Anti-CCR4 monoclonal antibody (mogamulizumab) treatment was started. After seven courses, the CCR4-positive atypical lymphocytes decreased in the dermis to levels below those seen at the outset of treatment. To our knowledge, there is no previous report of a case of SS managed with vorinostat followed by mogamulizumab demonstrating such a remarkable change in the pathological state following treatment.

Distinct Mechanism of Cysteine Oxidation-Dependent Activation and Cold Sensitization of Human Transient Receptor Potential Ankyrin 1 Channel by High and Low Oxaliplatin

Oxaliplatin, a third-generation platinum-based chemotherapeutic agent, displays unique acute peripheral neuropathy triggered or enhanced by cold, and accumulating evidence suggests that transient receptor potential ankyrin 1 (TRPA1) is responsible. TRPA1 is activated by oxaliplatin via a glutathione-sensitive mechanism. However, oxaliplatin interrupts hydroxylation of a proline residue located in the N-terminal region of TRPA1 via inhibition of prolyl hydroxylase (PHD), which causes sensitization of TRPA1 to reactive oxygen species (ROS). Furthermore, PHD inhibition endows cold-insensitive human TRPA1 (hTRPA1) with ROS-dependent cold sensitivity. Since cysteine oxidation and proline hydroxylation regulate its activity, their association with oxaliplatin-induced TRPA1 activation and acquirement of cold sensitivity were investigated in the present study. A high concentration of oxaliplatin (1 mM) induced outward-rectifier whole-cell currents and increased the intracellular Ca²⁺ concentration in hTRPA1-expressing HEK293 cells, but did not increase the probability of hTRPA1 channel opening in the inside-out configuration. Oxaliplatin also induced the rapid generation of hydrogen peroxide, and the resultant Ca²⁺ influx was prevented in the presence of glutathione and in cysteine-mutated hTRPA1 (Cys641Ser)-expressing cells, whereas proline-mutated hTRPA1 (Pro394Ala)-expressing cells showed similar whole-cell currents and Ca²⁺ influx. By contrast, a lower concentration of oxaliplatin (100 μM) did not increase the intracellular Ca²⁺ concentration but did confer cold sensitivity on hTRPA1-expressing cells, and this was inhibited by PHD2 co-overexpression. Cold sensitivity was abolished by the mitochondria-targeting ROS scavenger mitoTEMPO and was minimal in cysteine-mutated hTRPA1 (Cys641Ser or Cys665Ser)-expressing cells. Thus, high oxaliplatin evokes ROS-mediated cysteine oxidation-dependent hTRPA1 activation independent of PHD activity, while a lower concentration induces cold-induced cysteine oxidation-dependent opening of hTRPA1 via PHD inhibition.

Distinct contributions of LRRC8A and its paralogs to the VSOR anion channel from those of the ASOR anion channel

Volume- and acid-sensitive outwardly rectifying anion channels (VSOR and ASOR) activated by swelling and acidification exhibit voltage-dependent inactivation and activation time courses, respectively. Recently, LRRC8A and some paralogs were shown to be essentially involved in the activity and inactivation kinetics of VSOR currents in human colonic HCT116 cells. In human cervix HeLa cells, here, inactivation of VSOR currents was found to become accelerated by RNA silencing only of LRRC8A but never decelerated by that of any LRRC8 isoform. These data suggest that LRRC8A is associated with the deceleration mechanism of VSOR inactivation, while none of LRRC8 members is related to the acceleration mechanism. Activation kinetics of ASOR currents was unaffected by knockdown of any LRRC8 family member. Double, triple and quadruple gene-silencing studies indicated that combinatory expression of LRRC8A with LRRC8D and LRRC8C is essential for VSOR activity, whereas none of LRRC8 family members is involved in ASOR activity.

Optical control of neuronal firing via photoinduced electron transfer in donor-acceptor conjugates

A rationally designed donor–acceptor conjugate efficiently generates a photoinduced charge-separated state in a cellular environment, achieving photoinduction of neuronal firing.

A series of porphyrin–fullerene linked molecules has been synthesized to evaluate the effects of substituents and molecular structures on their charge-separation yield and the lifetime of a final charge-separated state in various hydrophilic environments. The selected high-performance molecule effectively achieved depolarization in a plasma cell membrane by visible light as well as two-photon excitation using a near-infrared light laser. Moreover, it was revealed that the depolarization can trigger neuronal firing in rat hippocampal neurons, demonstrating the potential and versatility for controlling cell functions using light.

Different Contribution of Redox-Sensitive Transient Receptor Potential Channels to Acetaminophen-Induced Death of Human Hepatoma Cell Line

Acetaminophen (APAP) is a safe analgesic antipyretic drug at prescribed doses. Its overdose, however, can cause life-threatening liver damage. Though, involvement of oxidative stress is widely acknowledged in APAP-induced hepatocellular death, the mechanism of this increased oxidative stress and the associated alterations in Ca²⁺ homeostasis are still unclear. Among members of transient receptor potential (TRP) channels activated in response to oxidative stress, we here identify that redox-sensitive TRPV1, TRPC1, TRPM2, and TRPM7 channels underlie Ca²⁺ entry and downstream cellular damages induced by APAP in human hepatoma (HepG2) cells. Our data indicate that APAP treatment of HepG2 cells resulted in increased reactive oxygen species (ROS) production, glutathione (GSH) depletion, and Ca²⁺ entry leading to increased apoptotic cell death. These responses were significantly suppressed by pretreatment with the ROS scavengers N-acetyl-L-cysteine (NAC) and 4,5-dihydroxy-1,3-benzene disulfonic acid disodium salt monohydrate (Tiron), and also by preincubation of cells with the glutathione inducer Dimethylfumarate (DMF). TRP subtype-targeted pharmacological blockers and siRNAs strategy revealed that suppression of either TRPV1, TRPC1, TRPM2, or TRPM7 reduced APAP-induced ROS formation, Ca²⁺ influx, and cell death; the effects of suppression of TRPV1 or TRPC1, known to be activated by oxidative cysteine modifications, were stronger than those of TRPM2 or TRPM7. Interestingly, TRPV1 and TRPC1 were labeled by the cysteine-selective modification reagent, 5,5′-dithiobis (2-nitrobenzoic acid)-2biotin (DTNB-2Bio), and this was attenuated by pretreatment with APAP, suggesting that APAP and/or its oxidized metabolites act directly on the modification target cysteine residues of TRPV1 and TRPC1 proteins. In human liver tissue, TRPV1, TRPC1, TRPM2, and TRPM7 channels transcripts were localized mainly to hepatocytes and Kupffer cells. Our findings strongly suggest that APAP-induced Ca²⁺ entry and subsequent hepatocellular death are regulated by multiple redox-activated cation channels, among which TRPV1 and TRPC1 play a prominent role.

TRPM2 channels in alveolar epithelial cells mediate bleomycin-induced lung inflammation

Lung inflammation is a major adverse effect of therapy with the antitumor drug bleomycin (BLM). Transient receptor potential melastatin 2 (TRPM2) is a Ca(2+)-permeable channel that is activated by oxidative stress through the production of ADP-ribose. We herein investigated whether TRPM2 channels contributed to BLM-induced lung inflammation. The intratracheal instillation of BLM into wild-type (WT) mice increased the number of polymorphonuclear leukocytes (PMNs) and inflammatory cytokine levels in the lung. Increases in inflammatory markers in WT mice were markedly reduced in trpm2 knockout (KO) mice, which demonstrated that the activation of TRPM2 channels was involved in BLM-induced lung inflammation. The expression of TRPM2 mRNA was observed in alveolar macrophages, alveolar epithelial cells, and lung fibroblasts. Actually, TRPM2 protein was expressed in lung tissues. Of these, TRPM2 channels in epithelial cells were activated by the addition of H2O2 following a BLM pretreatment, resulting in the secretion of macrophage inflammatory protein-2 (MIP-2). The H2O2-induced activation of TRPM2 by the BLM pretreatment was blocked by the poly(ADP-ribose) polymerase (PARP) inhibitors PJ34 and 3-aminobenzamide. The accumulation of poly(ADP-ribose) in the nucleus, a marker for ADP-ribose production, was strongly induced by H2O2 following the BLM pretreatment. Furthermore, administration of PRAP inhibitors into WT mice markedly reduced recruitment of inflammatory cells and MIP-2 secretion induced by BLM instillation. These results suggest that the induction of MIP-2 secretion through the activation of TRPM2 channels in alveolar epithelial cells is an important mechanism in BLM-induced lung inflammation, and the TRPM2 activation is likely to be mediated by ADP-ribose production via PARP pathway. TRPM2 channels may be new therapeutic target for BLM-induced lung inflammation.

Sensitization of H2O2-induced TRPM2 activation and subsequent interleukin-8 (CXCL8) production by intracellular Fe(2+) in human monocytic U937 cells

Transient receptor potential melastatin 2 (TRPM2) is an oxidative stress-sensitive Ca(2+)-permeable channel. In monocytes/macrophages, H2O2-induced TRPM2 activation causes cell death and/or production of chemokines that aggravate inflammatory diseases. However, relatively high concentrations of H2O2 are required for activation of TRPM2 channels in vitro. Thus, in the present study, factors that sensitize TRPM2 channels to H2O2 were identified and subsequent physiological responses were examined in U937 human monocytes. Temperature increase from 30°C to 37°C enhanced H2O2-induced TRPM2-mediated increase in intracellular free Ca(2+) ([Ca(2+)]i) in TRPM2-expressing HEK 293 cells (TRPM2/HEK cells). The H2O2-induced TRPM2 activation enhanced by the higher temperature was dramatically sensitized by intracellular Fe(2+)-accumulation following pretreatment with FeSO4. Thus intracellular Fe(2+)-accumulation sensitizes H2O2-induced TRPM2 activation at around body temperature. Moreover, intracellular Fe(2+)-accumulation increased poly(ADP-ribose) levels in nuclei by H2O2 treatment, and the sensitization of H2O2-induced TRPM2 activation were almost completely blocked by poly(ADP-ribose) polymerase inhibitors, suggesting that intracellular Fe(2+)-accumulation enhances H2O2-induced TRPM2 activation by increase of ADP-ribose production through poly(ADP-ribose) polymerase pathway. Similarly, pretreatment with FeSO4 stimulated H2O2-induced TRPM2 activation at 37°C in U937 cells and enhanced H2O2-induced ERK phosphorylation and interleukin-8 (CXCL8) production. Although the addition of H2O2 to cells under conditions of intracellular Fe(2+)-accumulation caused cell death, concentration of H2O2 required for CXCL8 production was lower than that resulting in cell death. These results indicate that intracellular Fe(2+)-accumulation sensitizes TRPM2 channels to H2O2 and subsequently produces CXCL8 at around body temperature. It is possible that sensitization of H2O2-induced TRPM2 channels by Fe(2+) may implicated in hemorrhagic brain injury via aggravation of inflammation, since Fe(2+) is released by heme degradation under intracerebral hemorrhage.

Temperature sensitivity of acid-sensitive outwardly rectifying (ASOR) anion channels in cortical neurons is involved in hypothermic neuroprotection against acidotoxic necrosis

The acid-sensitive outwardly rectifying (ASOR) anion channel has been found in non-neuronal cell types and was shown to be involved in acidotoxic death of epithelial cells. We have recently shown that the ASOR channel is sensitive to temperature. Here, we extend those results to show that temperature-sensitive ASOR anion channels are expressed in cortical neurons and involved in acidotoxic neuronal cell death. In cultured mouse cortical neurons, reduction of extracellular pH activated anionic currents exhibiting phenotypic properties of the ASOR anion channel. The neuronal ASOR currents recorded at pH 5.25 were augmented by warm temperature, with a threshold temperature of 26 °C and the Q(10) value of 5.6. After 1 h exposure to acidic solution at 37 °C, a large population of neurons suffered from necrotic cell death which was largely protected not only by ASOR channel blockers but also by reduction of temperature to 25 °C. Thus, it is suggested that high temperature sensitivity of the neuronal ASOR anion channel provides, at least in part, a basis for hypothermic neuroprotection under acidotoxic situations associated with a number of pathological brain states.

Extraction of response waveforms of heartbeat and blood pressure to swallowing. Using mixed signal processing of time domain and respiratory phase domain

BACKGROUND

Evaluating the accurate responses of the cardiovascular system to external stimuli is important for a deeper understanding of cardiovascular homeostasis. However, the responses should be distorted by the conventional time domain analysis when a frequency of the effect of external stimuli matches that of intrinsic fluctuations.

OBJECTIVES

The purpose of this study is to propose a mixed signal processing of time domain and respiratory phase domain to extract the response waveforms of heartbeat and blood pressure (BP) to external stimuli and to clarify the physiological mechanisms of swallowing effects on the cardiovascular system.

METHODS

Measurements were conducted on 12 healthy humans in the sitting and standing positions, with each subject requested to swallow every 30 s between expiration and inspiration. Waveforms of respiratory sinus arrhythmia (RSA) and respiratory-related BP variations were extracted as functions of the respiratory phase. Then, respiratory effects were subtracted from response waveforms with reference to the respiratory phase in the time domain.

RESULTS

As a result, swallowing induced tachycardia, which peaked within 3 s and recovered within 8 s. Tachycardia was greater in the sitting position than during standing. Furthermore, systolic BP and pulse pressure immediately decreased and diastolic BP increased coincident with the occurrence of tachycardia. Subsequently, systolic BP and pulse pressure recovered faster than the R-R interval.

CONCLUSIONS

We conclude that swallowing-induced tachycardia arises largely from the decrease of vagal activity and the baroreflex would yield fast oscillatory responses in recovery.

Novel HCN2 mutation contributes to febrile seizures by shifting the channel's kinetics in a temperature-dependent manner

Hyperpolarization-activated cyclic nucleotide-gated (HCN) channel-mediated currents, known as I h, are involved in the control of rhythmic activity in neuronal circuits and in determining neuronal properties including the resting membrane potential. Recent studies have shown that HCN channels play a role in seizure susceptibility and in absence and limbic epilepsy including temporal lobe epilepsy following long febrile seizures (FS). This study focused on the potential contributions of abnormalities in the HCN2 isoform and their role in FS. A novel heterozygous missense mutation in HCN2 exon 1 leading to p.S126L was identified in two unrelated patients with FS. The mutation was inherited from the mother who had suffered from FS in a pedigree. To determine the effect of this substitution we conducted whole-cell patch clamp electrophysiology. We found that mutant channels had elevated sensitivity to temperature. More specifically, they displayed faster kinetics at higher temperature. Kinetic shift by change of temperature sensitivity rather than the shift of voltage dependence led to increased availability of I h in conditions promoting FS. Responses to cyclic AMP did not differ between wildtype and mutant channels. Thus, mutant HCN2 channels cause significant cAMP-independent enhanced availability of I h during high temperatures, which may contribute to hyperthermia-induced neuronal hyperexcitability in some individuals with FS.

Active zone protein Bassoon co-localizes with presynaptic calcium channel, modifies channel function, and recovers from aging related loss by exercise

The P/Q-type voltage-dependent calcium channels (VDCCs) are essential for synaptic transmission at adult mammalian neuromuscular junctions (NMJs); however, the subsynaptic location of VDCCs relative to active zones in rodent NMJs, and the functional modification of VDCCs by the interaction with active zone protein Bassoon remain unknown. Here, we show that P/Q-type VDCCs distribute in a punctate pattern within the NMJ presynaptic terminals and align in three dimensions with Bassoon. This distribution pattern of P/Q-type VDCCs and Bassoon in NMJs is consistent with our previous study demonstrating the binding of VDCCs and Bassoon. In addition, we now show that the interaction between P/Q-type VDCCs and Bassoon significantly suppressed the inactivation property of P/Q-type VDCCs, suggesting that the Ca²⁺ influx may be augmented by Bassoon for efficient synaptic transmission at NMJs. However, presynaptic Bassoon level was significantly attenuated in aged rat NMJs, which suggests an attenuation of VDCC function due to a lack of this interaction between VDCC and Bassoon. Importantly, the decreased Bassoon level in aged NMJs was ameliorated by isometric strength training of muscles for two months. The training increased Bassoon immunoreactivity in NMJs without affecting synapse size. These results demonstrated that the P/Q-type VDCCs preferentially accumulate at NMJ active zones and play essential role in synaptic transmission in conjunction with the active zone protein Bassoon. This molecular mechanism becomes impaired by aging, which suggests altered synaptic function in aged NMJs. However, Bassoon level in aged NMJs can be improved by muscle exercise.