Sarcoplasmic reticulum K(+) channels from human and sheep atrial cells display a specific electro-pharmacological profile

Fluorescence Imaging of Intracellular Glutathione Levels in the Endoplasmic Reticulum to Reveal the Inhibition Effect of Rutin on Ferroptosis

Ferroptosis is an emerging form of nonapoptotic cell death, and the search for novel ferroptosis inhibitors is of great importance to explore unique cytoprotective strategies against ferroptosis-relevant diseases. In this work, we present an endoplasmic reticulum-targeting fluorescent probe (ER-G) for the imaging of intracellular glutathione (GSH) levels and revealed the inhibition effect of rutin on ferroptosis. Structurally, ER-G utilized a cyclohexyl sulfonylurea as the endoplasmic reticulum-targeting unit, and single-crystal X-ray diffraction analysis confirmed that ER-G possessed a N-oxide pyridine sulfinyl group instead of sulfone. After the response of ER-G to GSH, the fluorescence intensity at 523 nm displayed a significant increase by 3900-fold. ER-G showed extreme sensitivity and selectivity to GSH. The fluorescence imaging results demonstrated that ER-G exhibited excellent endoplasmic reticulum-targeting properties and could be applied to monitor GSH levels in the endoplasmic reticulum during the erastin-induced ferroptosis process. By the fluorescence imaging of GSH levels in the endoplasmic reticulum, it was demonstrated that rutin could efficiently block the depletion of GSH during erastin-induced ferroptosis and potentially act as a novel ferroptosis inhibitor. Moreover, unlike traditional ferroptosis inhibitors, it was speculated that the inhibition mechanism of rutin to ferroptosis was the integration of the chelate effect on Fe(II) ions and antioxidant effect. We expect that fluorescence imaging of GSH levels in the endoplasmic reticulum could provide a convenient and feasible method to evaluate the inhibition effect of small molecules on ferroptosis.

Endoplasmic reticulum targeting fluorescent probes to image mobile Zn2

Zn2+ plays an important role in the normal function of the endoplasmic reticulum (ER) and its deficiency can cause ER stress, which is related to a wide range of diseases. In order to provide tools to better understand the role of mobile Zn2+ in ER processes, the first custom designed ER-localised fluorescent Zn2+ probes have been developed through the introduction of a cyclohexyl sulfonylurea as an ER-targeting unit with different Zn2+ receptors. Experiments in vitro and in cellulo show that both probes have a good fluorescence switch on response to Zn2+, high selectivity over other cations, low toxicity, ER-specific targeting ability and are efficacious imaging agents for mobile Zn2+ in four different cell lines. Probe 9 has been used to detect mobile Zn2+ changes under ER stress induced by both tunicamycin or thapsigargin, which indicates that the new probes should allow a better understanding of the mechanisms cells use to respond to dysfunction of zinc homeostasis in the ER and its role in the initiation and progression of diseases to be developed.

Sarcoplasmic Reticulum Ca2+ Release Uses a Cascading Network of Intra-SR and Channel Countercurrents

In muscle, Ca²⁺ release from the sarcoplasmic reticulum (SR) into the cytosol is mediated through the ryanodine receptors (RyRs) and sustained by countercurrents that keep the SR membrane potential near 0 mV. Likewise, Ca²⁺ reuptake by the sarco/endoplasmic reticulum Ca²⁺ ATPase pump requires countercurrent. Although evidence has suggested that TRIC K⁺ channels and/or RyR K⁺ influx provide these countercurrents, the exact sources have not yet been determined. We used an equivalent circuit compartment model of a cardiac SR, the surrounding cytosol, and the dyadic cleft to probe the sources of countercurrent during a complete cardiac cycle. By removing and relocating TRIC K⁺ channels, as well as limiting when they are active, we explored the various possible sources of SR countercurrent under many conditions. Our simulations indicate that no single channel type is essential for countercurrent. Rather, a cascading network of countercurrents is present with anion fluxes within the SR redistributing charges throughout the full SR volume. This allows ion channels in the entire SR membrane, far from the Ca²⁺ fluxes through the RyRs in the junctional SR and sarco/endoplasmic reticulum Ca²⁺ ATPase pump in the nonjunctional SR, to mediate countercurrents that support Ca²⁺ release and reuptake. This multifactorial network of countercurrents allows Ca²⁺ release to be remarkably robust.

Monovalent cationic channel activity in the inner membrane of nuclei from skeletal muscle fibers

Nuclear ion channels remain among the least studied and biophysically characterized channels. Although considerable progress has been made in characterizing calcium release channels in the nuclear membrane, very little is known regarding the properties of nuclear monovalent cationic channels. Here, we describe a method to isolate nuclei from adult skeletal muscle fibers that are suitable for electrophysiological experiments. Using this approach, we show for the first time, to our knowledge, that a nuclear monovalent cationic channel (NMCC) is prominently expressed in the inner membrane of nuclei isolated from flexor digitorum brevis skeletal muscle fibers of adult mice. In isotonic 140 mM KCl, the skeletal muscle NMCC exhibits a unitary conductance of ?160 pS and high, voltage-independent open probability. Based on single-channel reversal potential measurements, NMCCs are slightly more permeable to potassium ions over sodium (PK/PNa = 2.68 ± 0.21) and cesium (PK/PCs = 1.39 ± 0.03) ions. In addition, NMCCs do not permeate divalent cations, are inhibited by calcium ions, and demonstrate weak rectification in asymmetric Ca(2+)-containing solutions. Together, these studies characterize a voltage-independent NMCC in skeletal muscle, the properties of which are ideally suited to serve as a countercurrent mechanism during calcium release from the nuclear envelope.

Evidence for a KATP Channel in Rough Endoplasmic Reticulum (rerKATP Channel) of Rat Hepatocytes

We report in a previous study the presence of a large conductance K⁺ channel in the membrane of rough endoplasmic reticulum (RER) from rat hepatocytes incorporated into lipid bilayers. Channel activity in this case was found to decrease in presence of ATP 100 µM on the cytoplasmic side and was totally inhibited at ATP concentrations greater than 0.25 mM. Although such features would be compatible with the presence of a K_ATP channel in the RER, recent data obtained from a brain mitochondrial inner membrane preparation have provided evidence for a Maxi-K channel which could also be blocked by ATP within the mM concentration range. A series of channel incorporation experiments was thus undertaken to determine if the ATP-sensitive channel originally observed in the RER corresponds to K_ATP channel. Our results indicate that the gating and permeation properties of this channel are unaffected by the addition of 800 nM charybdotoxin and 1 µM iberiotoxin, but appeared sensitive to 10 mM TEA and 2.5 mM ATP. Furthermore, adding 100 µM glibenclamide at positive potentials and 400 µM tolbutamide at negative or positive voltages caused a strong inhibition of channel activity. Finally Western blot analyses provided evidence for Kir6.2, SUR1 and/or SUR2B, and SUR2A expression in our RER fractions. It was concluded on the basis of these observations that the channel previously characterized in RER membranes corresponds to K_ATP, suggesting that opening of this channel may enhance Ca²⁺ releases, alter the dynamics of the Ca²⁺ transient and prevent accumulation of Ca²⁺ in the ER during Ca²⁺ overload.

Sarcoplasmic reticulum K(+) (TRIC) channel does not carry essential countercurrent during Ca(2+) release

The charge translocation associated with sarcoplasmic reticulum (SR) Ca(2+) efflux is compensated for by a simultaneous SR K(+) influx. This influx is essential because, with no countercurrent, the SR membrane potential (Vm) would quickly (<1 ms) reach the Ca(2+) equilibrium potential and SR Ca(2+) release would cease. The SR K(+) trimeric intracellular cation (TRIC) channel has been proposed to carry the essential countercurrent. However, the ryanodine receptor (RyR) itself also carries a substantial K(+) countercurrent during release. To better define the physiological role of the SR K(+) channel, we compared SR Ca(2+) transport in saponin-permeabilized cardiomyocytes before and after limiting SR K(+) channel function. Specifically, we reduced SR K(+) channel conduction 35 and 88% by replacing cytosolic K(+) for Na(+) or Cs(+) (respectively), changes that have little effect on RyR function. Calcium sparks, SR Ca(2+) reloading, and caffeine-evoked Ca(2+) release amplitude (and rate) were unaffected by these ionic changes. Our results show that countercurrent carried by SR K(+) (TRIC) channels is not required to support SR Ca(2+) release (or uptake). Because K(+) enters the SR through RyRs during release, the SR K(+) (TRIC) channel most likely is needed to restore trans-SR K(+) balance after RyRs close, assuring SR Vm stays near 0 mV.

TRIC-B channels display labile gating: evidence from the TRIC-A knockout mouse model

Sarcoplasmic/endoplasmic reticulum (SR) and nuclear membranes contain two related cation channels named TRIC-A and TRIC-B. In many tissues, both subtypes are co-expressed, making it impossible to distinguish the distinct single-channel properties of each subtype. We therefore incorporated skeletal muscle SR vesicles derived from Tric-a-knockout mice into bilayers in order to characterise the biophysical properties of native TRIC-B without possible misclassification of the channels as TRIC-A, and without potential distortion of functional properties by detergent purification protocols. The native TRIC-B channels were ideally selective for cations. In symmetrical 210 mM K⁺, the maximum (full) open channel level (199 pS) was equivalent to that observed when wild-type SR vesicles were incorporated into bilayers. Analysis of TRIC-B gating revealed complex and variable behaviour. Four main sub-conductance levels were observed at approximately 80 % (161 pS), 60 % (123 pS), 46 % (93 pS), and 30 % (60 pS) of the full open state. Seventy-five percent of the channels were voltage sensitive with Po being markedly reduced at negative holding potentials. The frequent, rapid transitions between TRIC-B sub-conductance states prevented development of reliable gating models using conventional single-channel analysis. Instead, we used mean-variance plots to highlight key features of TRIC-B gating in a more accurate and visually useful manner. Our study provides the first biophysical characterisation of native TRIC-B channels and indicates that this channel would be suited to provide counter current in response to Ca²⁺ release from the SR. Further experiments are required to distinguish the distinct functional properties of TRIC-A and TRIC-B and understand their individual but complementary physiological roles.

Emerging roles of inositol 1,4,5-trisphosphate signaling in cardiac myocytes

Inositol 1,4,5-trisphosphate (IP(3)) is a ubiquitous intracellular messenger regulating diverse functions in almost all mammalian cell types. It is generated by membrane receptors that couple to phospholipase C (PLC), an enzyme which liberates IP(3) from phosphatidylinositol 4,5-bisphosphate (PIP(2)). The major action of IP(3), which is hydrophilic and thus translocates from the membrane into the cytoplasm, is to induce Ca(2+) release from endogenous stores through IP(3) receptors (IP(3)Rs). Cardiac excitation-contraction coupling relies largely on ryanodine receptor (RyR)-induced Ca(2+) release from the sarcoplasmic reticulum. Myocytes express a significantly larger number of RyRs compared to IP(3)Rs (~100:1), and furthermore they experience substantial fluxes of Ca(2+) with each heartbeat. Therefore, the role of IP(3) and IP(3)-mediated Ca(2+) signaling in cardiac myocytes has long been enigmatic. Recent evidence, however, indicates that despite their paucity cardiac IP(3)Rs may play crucial roles in regulating diverse cardiac functions. Strategic localization of IP(3)Rs in cytoplasmic compartments and the nucleus enables them to participate in subsarcolemmal, bulk cytoplasmic and nuclear Ca(2+) signaling in embryonic stem cell-derived and neonatal cardiomyocytes, and in adult cardiac myocytes from the atria and ventricles. Intriguingly, expression of both IP(3)Rs and membrane receptors that couple to PLC/IP(3) signaling is altered in cardiac disease such as atrial fibrillation or heart failure, suggesting the involvement of IP(3) signaling in the pathology of these diseases. Thus, IP(3) exerts important physiological and pathological functions in the heart, ranging from the regulation of pacemaking, excitation-contraction and excitation-transcription coupling to the initiation and/or progression of arrhythmias, hypertrophy and heart failure.

ATP regulation of a large conductance voltage-gated cation channel in rough endoplasmic reticulum of rat hepatocytes

ATP-sensitive K+ channels play an important role in regulating membrane potential during metabolic stress. In this work we report the effect of ATP and ADP-Mg on a K+ channel present in the membrane of rough endoplasmic reticulum (RER) from rat hepatocytes incorporated into lipid bilayers. Channel activity was found to decrease in presence of ATP 100 microM on the cytoplasmic side and was totally inhibited at ATP concentrations greater than 0.25mM. The effect appeared voltage dependent, suggesting that the ATP binding site was becoming available upon channel opening. Channel activity was suppressed by the nonhydrolyzable ATP analog (ATPgammaS), ruling out a phosphorylation-based mechanism. Notably addition of 2.5mM ADP-Mg to the cytosolic side increased the channel open probability at negative potentials. We conclude that the large conductance voltage-gated cation channel in RER of rat hepatocytes is an ATP and ADP sensitive channel likely to be involved in cellular processes such as Ca(2+) signaling or control of membrane potential across the endoplasmic reticulum membrane.

Calcium and arrhythmogenesis

Triggered activity in cardiac muscle and intracellular Ca2+ have been linked in the past. However, today not only are there a number of cellular proteins that show clear Ca2+ dependence but also there are a number of arrhythmias whose mechanism appears to be linked to Ca2+-dependent processes. Thus we present a systematic review of the mechanisms of Ca2+ transport (forward excitation-contraction coupling) in the ventricular cell as well as what is known for other cardiac cell types. Second, we review the molecular nature of the proteins that are involved in this process as well as the functional consequences of both normal and abnormal Ca2+ cycling (e.g., Ca2+ waves). Finally, we review what we understand to be the role of Ca2+ cycling in various forms of arrhythmias, that is, those associated with inherited mutations and those that are acquired and resulting from reentrant excitation and/or abnormal impulse generation (e.g., triggered activity). Further solving the nature of these intricate and dynamic interactions promises to be an important area of research for a better recognition and understanding of the nature of Ca2+ and arrhythmias. Our solutions will provide a more complete understanding of the molecular basis for the targeted control of cellular calcium in the treatment and prevention of such.

Evidence for a large conductance voltage gated cationic channel in rough endoplasmic reticulum of rat hepatocytes

In this work, we report the single channel characterization of a voltage gated cationic channel from rough endoplasmic reticulum (RER) membranes of rat hepatocytes incorporated into a planar lipid bilayer. The channel was found to be cation selective with a main conductance of 598+/-20 pS in 200 mM KCl cis/50 mM KCl trans. The channel open probability appeared voltage dependent with a voltage for half activation (V(1/2)) of 38 mV and an effective gating charge z of -6.66. Adding either 4-AP (5 mM) or ATP (2.5 mM) to the side corresponding to the cell internal medium caused a strong inhibition of the channel activity. This channel is likely to be involved in maintaining proper cation homeostasis in the RER of hepatocytes.

Spontaneously active and InsP3-activated ion channels in cell nuclei from rat cerebellar Purkinje and granule neurones

Increases in Ca(2+) concentration in the nucleus of neurones modulate gene transcription and may be involved in activity-dependent long-term plasticity, apoptosis, and neurotoxicity. Little is currently known about the regulation of Ca(2+) in the nuclei of neurones. Investigation of neuronal nuclei is hampered by the cellular heterogeneity of the brain where neurones comprise no more than 10% of the cells. The situation is further complicated by large differences in properties of different neurones. Here we report a method for isolating nuclei from identified central neurones. We employed this technique to study nuclei from rat cerebellar Purkinje and granule neurones. Patch-clamp recording from the nuclear membrane of Purkinje neurones revealed numerous large-conductance channels selective for monovalent cations. The nuclear membrane of Purkinje neurones also contained multiple InsP(3)- activated ion channels localized exclusively in the inner nuclear membrane with their receptor loci facing the nucleoplasm. In contrast, the nuclear membrane of granule neurones contained only a small number of mainly anion channels. Nuclear InsP(3) receptors (InsP(3)Rs) were activated by InsP(3) with EC(50) = 0.67 microm and a Hill coefficient of 2.5. Ca(2+) exhibited a biphasic effect on the receptors elevating its activity at low concentrations and inhibiting it at micromolar concentrations. InsP(3) in saturating concentrations did not prevent the inhibitory effect of Ca(2+), but strongly increased InsP(3)R activity at resting Ca(2+) concentrations. These data are the first evidence for the presence of intranuclear sources of Ca(2+) in neurones. Ca(2+) release from the nuclear envelope may amplify Ca(2+) transients penetrating the nucleus from the cytoplasm or generate Ca(2+) transients in the nucleus independently of the cytoplasm.