Effects of anoxia and aglycemia on cytosolic calcium regulation in rat sensory neurons

A fixed 20:1 combination of cafedrine/theodrenaline increases cytosolic Ca2+ concentration in human tracheal epithelial cells via ryanodine receptor-mediated Ca2+ release

Mucociliary clearance is a pivotal physiological mechanism that protects the lung by cleaning the airways from pollution and colonization, thereby preventing infection. Ciliary function is influenced by various signal transduction cascades, and Ca2+ represents a key second messenger. A fixed 20:1 combination of cafedrine and theodrenaline has been widely used to treat perioperative hypotension and emergency hypotensive states since the 1960s; however, its effect on the intracellular Ca2+ concentration ([Ca2+]i) of respiratory epithelium remains unknown. Therefore, human tracheal epithelial cells were exposed to the clinically applied 20:1 mixture of cafedrine/theodrenaline and the individual substances separately. [Ca2+]i was assessed by FURA-2 340/380 fluorescence ratio. Pharmacological inhibitors were applied to elucidate relevant signal transduction cascades, and reverse transcription polymerase chain reaction (RT-PCR) was performed on murine tracheal epithelium to analyze ryanodine receptor (RyR) subtype expression. All three pharmacological preparations instantaneously induced a steep increase in [Ca2+]i that quickly returned to its baseline value despite the persistence of each substance. Peak [Ca2+]i following the administration of 20:1 cafedrine/theodrenaline, cafedrine alone, and theodrenaline alone increased in a dose-dependent manner, with median effective concentrations of 0.35 mM (7.32 mM cafedrine and 0.35 mM theodrenaline), 3.14 mM, and 3.45 mM, respectively. When extracellular Ca2+ influx was inhibited using a Ca2+-free buffer solution, the peak [Ca2+]i following the administration of cafedrine alone and theodrenaline alone were reduced but not abolished. No alteration in [Ca2+]i compared with baseline [Ca2+]i was observed during β-adrenergic receptor inhibition. Depletion of caffeine-sensitive stores and inhibition of RyR, but not IP3 receptors, completely abolished any increase in [Ca2+]i. However, [Ca2+]i still increased following the depletion of mitochondrial Ca2+ stores using 2,4-dinitrophenol. RT-PCR revealed RyR-2 and RyR-3 expression on murine tracheal epithelium. Although our experiments showed that cafedrine/theodrenaline, cafedrine alone, or theodrenaline alone release Ca2+ from intracellular stores through mechanisms that are exclusively triggered by β-adrenergic receptor stimulation, which most probably lead to RyR activation, clinical plasma concentrations are considerably lower than those used in our experiments to elicit an increase in [Ca2+]i; therefore, further studies are needed to evaluate the ability of cafedrine/theodrenaline to alter mucociliary clearance in clinical practice.

Blockade of STAT3 signaling alleviates progression of acute kidney injury-to-chronic kidney disease through anti-apoptosis

Signal transducer and activator of transcription 3 (STAT3) is a pivotal mediator of IL-6-type cytokine signaling. However, the roles of its full-length and truncated isoforms in acute kidney injury (AKI) and its transition to chronic kidney disease (CKD) remain elusive. Herein, the role of STAT3 isoforms in AKI-to-CKD transition was characterized using an ischemia-reperfusion injury (IRI) mouse model. IRI was induced in C57BL/6 mice. Stattic^®, a STAT3 inhibitor, was administered to the mice 3 h prior to IRI. Intrarenal cytokine expression was quantified using real-time PCR, and FACS analysis was performed. The effect of Stattic^® on human tubular epithelial cells (TECs) cultured under hypoxic conditions was also evaluated. Phosphorylated STAT3 isoforms were detected by western blotting. Stattic^® treatment attenuated IRI-induced tubular damage and inflammatory cytokine/chemokine expression, while decreasing macrophage infiltration and fibrosis in mouse unilateral IRI and UUO models. Similarly, in vitro STAT3 inhibition downregulated fibrosis and apoptosis in 72-h hypoxia-induced human TECs and reduced pSTAT3α-mediated inflammation. Moreover, pSTAT3 expression was increased in human acute tubular necrosis and CKD tissues. STAT3 activation is associated with IRI progression, and STAT3-α may be a significant contributor. Hence, STAT3 may affect AKI-to-CKD transition, suggesting a novel strategy for AKI management with STAT3 inhibitors.

Cytoskeletal Arrest: An Anoxia Tolerance Mechanism

Polymerization of actin filaments and microtubules constitutes a ubiquitous demand for cellular adenosine-5′-triphosphate (ATP) and guanosine-5′-triphosphate (GTP). In anoxia-tolerant animals, ATP consumption is minimized during overwintering conditions, but little is known about the role of cell structure in anoxia tolerance. Studies of overwintering mammals have revealed that microtubule stability in neurites is reduced at low temperature, resulting in withdrawal of neurites and reduced abundance of excitatory synapses. Literature for turtles is consistent with a similar downregulation of peripheral cytoskeletal activity in brain and liver during anoxic overwintering. Downregulation of actin dynamics, as well as modification to microtubule organization, may play vital roles in facilitating anoxia tolerance. Mitochondrial calcium release occurs during anoxia in turtle neurons, and subsequent activation of calcium-binding proteins likely regulates cytoskeletal stability. Production of reactive oxygen species (ROS) formation can lead to catastrophic cytoskeletal damage during overwintering and ROS production can be regulated by the dynamics of mitochondrial interconnectivity. Therefore, suppression of ROS formation is likely an important aspect of cytoskeletal arrest. Furthermore, gasotransmitters can regulate ROS levels, as well as cytoskeletal contractility and rearrangement. In this review we will explore the energetic costs of cytoskeletal activity, the cellular mechanisms regulating it, and the potential for cytoskeletal arrest being an important mechanism permitting long-term anoxia survival in anoxia-tolerant species, such as the western painted turtle and goldfish.

Caspofungin induces the release of Ca2+ ions from internal stores by activating ryanodine receptor-dependent pathways in human tracheal epithelial cells

The antimycotic drug caspofungin is known to alter the cell function of cardiomyocytes and the cilia-bearing cells of the tracheal epithelium. The objective of this study was to investigate the homeostasis of intracellular Ca²⁺ concentration ([Ca²⁺]_i) after exposure to caspofungin in isolated human tracheal epithelial cells. The [Ca²⁺]_i was measured using the ratiometric fluoroprobe FURA-2 AM. We recorded two groups of epithelial cells with distinct responses to caspofungin exposure, which demonstrated either a rapid transient rise in [Ca²⁺]_i or a sustained elevation of [Ca²⁺]_i. Both patterns of Ca²⁺ kinetics were still observed when an influx of transmembraneous Ca²⁺ ions was pharmacologically inhibited. Furthermore, in extracellular buffer solutions without Ca²⁺ ions, caspofungin exposure still evoked this characteristic rise in [Ca²⁺]_i. To shed light on the origin of the Ca²⁺ ions responsible for the elevation in [Ca²⁺]_i we investigated the possible intracellular storage of Ca²⁺ ions. The depletion of mitochondrial Ca²⁺ stores using 25 µM 2,4-dinitrophenol (DNP) did not prevent the caspofungin-induced rise in [Ca²⁺]_i, which was rapid and transient. However, the application of caffeine (30 mM) to discharge Ca²⁺ ions that were presumably stored in the endoplasmic reticulum (ER) prior to caspofungin exposure completely inhibited the caspofungin-induced changes in [Ca²⁺]_i levels. When the ER-bound IP₃ receptors were blocked by 2-APB (40 µM), we observed a delayed transient rise in [Ca²⁺]_i as a response to the caspofungin. Inhibition of the ryanodine receptors (RyR) using 40 µM ryanodine completely prevented the caspofungin-induced elevation of [Ca²⁺]_i. In summary, caspofungin has been shown to trigger an increase in [Ca²⁺]_i independent from extracellular Ca²⁺ ions by liberating the Ca²⁺ ions stored in the ER, mainly via a RyR pathway.

Loss of Orai2-Mediated Capacitative Ca 2+ Entry Is Neuroprotective in Acute Ischemic Stroke

Background and Purpose—

Ischemic stroke is one of the leading causes of disability and death. The principal goal of acute stroke treatment is the recanalization of the occluded cerebral arteries, which is, however, only effective in a very narrow time window. Therefore, neuroprotective treatments that can be combined with recanalization strategies are needed. Calcium overload is one of the major triggers of neuronal cell death. We have previously shown that capacitative Ca 2+ entry, which is triggered by the depletion of intracellular calcium stores, contributes to ischemia-induced calcium influx in neurons, but the responsible Ca 2+ channel is not known.

Methods—

Here, we have generated mice lacking the calcium channel subunit Orai2 and analyzed them in experimental stroke.

Results—

Orai2-deficient mice were protected from ischemic neuronal death both during acute ischemia under vessel occlusion and during ischemia/reperfusion upon successful recanalization. Calcium signals induced by calcium store depletion or oxygen/glucose deprivation were significantly diminished in Orai2-deficient neurons demonstrating that Orai2 is a central mediator of neuronal capacitative Ca 2+ entry and is involved in calcium overload during ischemia.

Conclusions—

Our experimental data identify Orai2 as an attractive target for pharmaceutical intervention in acute stroke.

PKCγ and PKCε are Differentially Activated and Modulate Neurotoxic Signaling Pathways During Oxygen Glucose Deprivation in Rat Cortical Slices

Cerebral ischemia is known to trigger a series of intracellular events such as changes in metabolism, membrane function and intracellular transduction, which eventually leads to cell death. Many of these processes are mediated by intracellular signaling cascades that involve protein kinase activation. Among all the kinases activated, the serine/threonine kinase family, protein kinase C (PKC), particularly, has been implicated in mediating cellular response to cerebral ischemic and reperfusion injury. In this study, using oxygen-glucose deprivation (OGD) in acute cortical slices as an in vitro model of cerebral ischemia, I show that PKC family of isozymes, specifically PKCγ and PKCε are differentially activated during OGD. Detecting the expression and activation levels of these isozymes in response to different durations of OGD insult revealed an early activation of PKCε and delayed activation of PKCγ, signifying their roles in response to different durations and stages of ischemic stress. Specific inhibition of PKCγ and PKCε significantly attenuated OGD induced cytotoxicity, rise in intracellular calcium, membrane depolarization and reactive oxygen species formation, thereby enhancing neuronal viability. This study clearly suggests that PKC family of isozymes; specifically PKCγ and PKCε are involved in OGD induced intracellular responses which lead to neuronal death. Thus isozyme specific modulation of PKC activity may serve as a promising therapeutic route for the treatment of acute cerebral ischemic injury.

Effect of activation of the Ca(2+)-permeable acid-sensing ion channel 1a on focal cerebral ischemia in diabetic rats

We investigated the role of acid-sensing ion channel Ia (ASIC1a) expression and changes in intracellular Ca(2+) concentration ([Ca(2+)]) in focal cerebral ischemia after middle cerebral artery occlusion (MCAO) in a rat model of diabetes mellitus (DM). Male Wistar rats (n = 108) were divided into three groups: the MCAO, DM + MCAO, and DM + MCAO + fasudil groups (n = 36 each). Samples were obtained 1, 3, 6, and 24 h after ischemia induction (n = 9). Rats in the DM + MCAO + fasudil group were treated with 1 mg/kg fasudil, a Rho-kinase inhibitor, by caudal vein injection 30 min after MCAO was performed. ASIC1a expression gradually increased with time in the MCAO and DM + MCAO groups (0.71 ± 0.10 nM, 0.80 ± 0.11 nM, 0.86 ± 0.08 nM, 0.93 ± 0.09 nM; 0.86 ± 0.11 nM, 1.05 ± 0.51 nM, 2.42 ± 0.08 nM, 2.78 ± 0.04 nM; pairwise comparisons at each time point, P < 0.05), and was higher in the DM + MCAO than the MCAO group (P < 0.05). [Ca(2+)] gradually increased in the DM + MCAO group (106.32 ± 18.6 nM, 137.84 ± 14.32 nM, 151.94 ± 18.38 nM, 183.61 ± 7.96 nM, P < 0.05). ASIC1a expression and calcium currents were reduced in the DM + MCAO + fasudil group. The overload of intracellular [Ca(2+)] caused by ASIC1a activation could be one mechanism for the aggravation of focal cerebral ischemia in diabetes.

A sustained increase in the intracellular Ca²⁺ concentration induces proteolytic cleavage of EAG2 channel

Voltage-gated EAG2 channel is abundant in the brain and enhances cancer cell growth by controlling cell volume. The channel contains a cyclic nucleotide-binding homology (CNBH) domain and multiple calmodulin-binding motifs. Here we show that a raised intracellular Ca(2+) concentration causes proteolytic digestion of heterologously expressed and native EAG2 channels. A treatment of EAG2-expressing cells with the Ca(2+) ionophore A23187 for 1h reduces the full-length protein by ∼80% with a concomitant appearance of 30-35-kDa peptides. Similarly, a treatment with the Ca(2+)-ATPase inhibitor thapsigargin for 3h removes 30-35-kDa peptides from ∼1/3 of the channel protein. Moreover, an incubation of the isolated rat brain membrane with CaCl2 leads to the generation of fragments with similar sizes. This Ca(2+)-induced digestion is not seen with EAG1. Mutations in a C-terminal calmodulin-binding motif alter the degrees and positions of the cleavage. Truncated channels that mimic the digested proteins exhibit a reduced current density and altered channel gating. In particular, these shorter channels lack a rapid activation typical in EAG channels with more than 20-mV positive shifts in voltage dependence of activation. The truncation also eliminates the ability of EAG2 channel to reduce cell volume. These results suggest that a sustained increase in the intracellular Ca(2+) concentration leads to proteolytic cleavage at the C-terminal cytosolic region following the CNBH domain by altering its interaction with calmodulin. The observed Ca(2+)-induced proteolytic cleavage of EAG2 channel may act as an adaptive response under physiological and/or pathological conditions.

Cytosolic calcium regulation in rat afferent vagal neurons during anoxia

Sensory neurons are able to detect tissue ischaemia and both transmit information to the brainstem as well as release local vasoactive mediators. Their ability to sense tissue ischaemia is assumed to be primarily mediated through proton sensing ion channels, lack of oxygen however may also affect sensory neuron function. In this study we investigated the effects of anoxia on isolated capsaicin sensitive neurons from rat nodose ganglion. Acute anoxia triggered a reversible increase in [Ca2+]i that was mainly due to Ca2+-efflux from FCCP sensitive stores and from caffeine and CPA sensitive ER stores. Prolonged anoxia resulted in complete depletion of ER Ca2+-stores. Mitochondria were partially depolarised by acute anoxia but mitochondrial Ca2+-uptake/buffering during voltage-gated Ca2+-influx was unaffected. The process of Ca2+-release from mitochondria and cytosolic Ca2+-clearance following Ca2+ influx was however significantly slowed. Anoxia was also found to inhibit SERCA activity and, to a lesser extent, PMCA activity. Hence, anoxia has multiple influences on [Ca2+]i homeostasis in vagal afferent neurons, including depression of ATP-driven Ca2+-pumps, modulation of the kinetics of mitochondrial Ca2+ buffering/release and Ca2+-release from, and depletion of, internal Ca2+-stores. These effects are likely to influence sensory neuronal function during ischaemia.

Oxygen and glucose deprivation (OGD)-induced spreading depression in the Substantia Nigra

Spreading depression (SD) is a profound depolarization of neurons and glia that propagates in a wave-like manner across susceptible brain regions, and can develop during periods of compromised cellular energy such as ischemia, when it influences the severity of acute neuronal damage. Although SD has been well characterized in the cerebral cortex and hippocampus, little is known of this event in the Substantia Nigra (SN), a brainstem nucleus engaged in motor control and reward-related behavior. Transverse brain slices (250 μm; P21-23 rats) containing the SN were subject to oxygen and glucose deprivation (OGD) tests, modeling brain ischemia. SD developed in lateral aspects of the SN within 3.3±0.2 min of OGD onset, and spread through the Substantia Nigra pars reticulata (SNr), as indicated by fast-occurring and propagating increased tissue light transmittance and negative shift of extracellular DC potential. These events were associated with profound mitochondrial membrane depolarization (ΔΨm) throughout the SN, as demonstrated by increased Rhodamine 123 fluorescence. Extracellular recordings from individual SNr neurons indicated rapid depolarization followed by depolarizing block, while dopaminergic neurons in the Substantia Nigra pars compacta (SNc) showed inhibition of firing associated with hyperpolarization. SD evoked in the SNr was similar to OGD-induced SD in the CA1 region in hippocampal slices. In the hippocampus, SD also developed during anoxia or aglycemia alone (associated with less profound ΔΨm than OGD), while these conditions rarely led to SD in the SNr. Our results demonstrate that OGD consistently evokes SD in the SN, and that this phenomenon only involves the SNr. It remains to be established whether nigral SD contributes to neuronal damage associated with a sudden-onset form of Parkinson's disease known as 'vascular parkinsonism'.

Painful nerve injury decreases sarco-endoplasmic reticulum Ca²⁺-ATPase activity in axotomized sensory neurons

The sarco-endoplasmic reticulum Ca(2+)-ATPase (SERCA) is a critical pathway by which sensory neurons sequester cytosolic Ca(2+) and thereby maintain intracellular Ca(2+) homeostasis. We have previously demonstrated decreased intraluminal endoplasmic reticulum Ca(2+) concentration in traumatized sensory neurons. Here we examine SERCA function in dissociated sensory neurons using Fura-2 fluorometry. Blocking SERCA with thapsigargin (1 μM) increased resting [Ca(2+)](c) and prolonged recovery (τ) from transients induced by neuronal activation (elevated bath K(+)), demonstrating SERCA contributes to control of resting [Ca(2+)](c) and recovery from transient [Ca(2+)](c) elevation. To evaluate SERCA in isolation, plasma membrane Ca(2+) ATPase was blocked with pH 8.8 bath solution and mitochondrial buffering was avoided by keeping transients small (≤ 400 nM). Neurons axotomized by spinal nerve ligation (SNL) showed a slowed rate of transient recovery compared to control neurons, representing diminished SERCA function, whereas neighboring non-axotomized neurons from SNL animals were unaffected. Injury did not affect SERCA function in large neurons. Repeated depolarization prolonged transient recovery, showing that neuronal activation inhibits SERCA function. These findings suggest that injury-induced loss of SERCA function in small sensory neurons may contribute to the generation of pain following peripheral nerve injury.

Orais and STIMs: physiological mechanisms and disease

The stromal interaction molecules STIM1 and STIM2 are Ca²⁺ sensors, mostly located in the endoplasmic reticulum, that detect changes in the intraluminal Ca²⁺ concentration and communicate this information to plasma membrane store-operated channels, including members of the Orai family, thus mediating store-operated Ca²⁺ entry (SOCE). Orai and STIM proteins are almost ubiquitously expressed in human cells, where SOCE has been reported to play a relevant functional role. The phenotype of patients bearing mutations in STIM and Orai proteins, together with models of STIM or Orai deficiency in mice, as well as other organisms such as Drosophila melanogaster, have provided compelling evidence on the relevant role of these proteins in cellular physiology and pathology. Orai1-deficient patients suffer from severe immunodeficiency, congenital myopathy, chronic pulmonary disease, anhydrotic ectodermal dysplasia and defective dental enamel calcification. STIM1-deficient patients showed similar abnormalities, as well as autoimmune disorders. This review summarizes the current evidence that identifies and explains diseases induced by disturbances in SOCE due to deficiencies or mutations in Orai and STIM proteins.

Maximizing neuroprotection: where do we stand?

Brain and spinal cord traumas include blunt and penetrating trauma, disease, and required surgery. Such traumas trigger events such as inflammation, infiltration of inflammatory and other cells, oxidative stress, acidification, excitotoxicity, ischemia, and the loss of calcium homeostasis, all of which cause neurotoxicity and neuron death. To prevent trauma-induced neurological deficits and death, each of the many neurotoxic events that occur in parallel or sequentially must be minimized or prevented. Although neuroprotective techniques have been developed that block single neurotoxic events, most provide only limited neuroprotection and are only applied singly. However, because many neurotoxicity triggers arise from common events, an approach for invoking more effective neuroprotection is to apply multiple neuroprotective methods simultaneously before the many neurotoxic triggers and cascades are initiated and become irreversible. This paper first discusses some triggers of neurotoxicity and neuroprotective mechanisms that block them, including hypothermia, alkalinization, and the administration of adenosine. It then examines how the simultaneous application of these techniques provides significantly greater neuroprotection than is provided by any technique alone. The paper also stresses the importance of determining whether the neuroprotection provided by these techniques can be further enhanced by combining them with additional techniques, such as the systemic administration of glucocorticoids. Finally, the paper stresses the absolute critical importance of applying these techniques within the "golden hour" following trauma, before the many neurotoxic events and cascades are manifest and before the neurotoxic cascades become irreversible.

Hypoxic Pulmonary Vasoconstriction

It has been known for more than 60 years, and suspected for over 100, that alveolar hypoxia causes pulmonary vasoconstriction by means of mechanisms local to the lung. For the last 20 years, it has been clear that the essential sensor, transduction, and effector mechanisms responsible for hypoxic pulmonary vasoconstriction (HPV) reside in the pulmonary arterial smooth muscle cell. The main focus of this review is the cellular and molecular work performed to clarify these intrinsic mechanisms and to determine how they are facilitated and inhibited by the extrinsic influences of other cells. Because the interaction of intrinsic and extrinsic mechanisms is likely to shape expression of HPV in vivo, we relate results obtained in cells to HPV in more intact preparations, such as intact and isolated lungs and isolated pulmonary vessels. Finally, we evaluate evidence regarding the contribution of HPV to the physiological and pathophysiological processes involved in the transition from fetal to neonatal life, pulmonary gas exchange, high-altitude pulmonary edema, and pulmonary hypertension. Although understanding of HPV has advanced significantly, major areas of ignorance and uncertainty await resolution.

Impaired hippocampal Ca2+ homeostasis and concomitant K+ channel dysfunction in a mouse model of Rett syndrome during anoxia

Methyl-CpG-binding protein 2 (MeCP2) deficiency causes Rett syndrome (RTT), a neurodevelopmental disorder characterized by severe cognitive impairment, synaptic dysfunction, and hyperexcitability. Previously we reported that the hippocampus of MeCP2-deficient mice (Mecp2(-/y)), a mouse model for RTT, is more susceptible to hypoxia. To identify the underlying mechanisms we now focused on the anoxic responses of wildtype (WT) and Mecp2(-/y) CA1 neurons in acute hippocampal slices. Intracellular recordings revealed that Mecp2(-/y) neurons show only reduced or no hyperpolarizations early during cyanide-induced anoxia, suggesting potassium channel (K(+) channel) dysfunction. Blocking adenosine-5'-triphosphate-sensitive K(+) channels (K(ATP-)) and big-conductance Ca(2+)-activated K(+) channels (BK-channels) did not affect the early anoxic hyperpolarization in either genotype. However, blocking Ca(2+) release from the endoplasmic reticulum almost abolished the anoxic hyperpolarizations in Mecp2(-/y) neurons. Single-channel recordings confirmed that neither K(ATP)- nor BK-channels are the sole mediators of the early anoxic hyperpolarization. Instead, anoxia Ca(2+)-dependently activated various small/intermediate-conductance K(+) channels in WT neurons, which was less evident in Mecp2(-/y) neurons. Yet, pharmacologically increasing the Ca(2+) sensitivity of small/intermediate-conductance K(Ca) channels fully restored the anoxic hyperpolarization in Mecp2(-/y) neurons. Furthermore, Ca(2+) imaging unveiled lower intracellular Ca(2+) levels in resting Mecp2(-/y) neurons and reduced anoxic Ca(2+) transients with diminished Ca(2+) release from intracellular stores. In conclusion, the enhanced hypoxia susceptibility of Mecp2(-/y) hippocampus is primarily associated with disturbed Ca(2+) homeostasis and diminished Ca(2+) rises during anoxia. This secondarily attenuates the activation of K(Ca) channels and thereby increases the hypoxia susceptibility of Mecp2(-/y) neuronal networks. Since cytosolic Ca(2+) levels also determine neuronal excitability and synaptic plasticity, Ca(2+) homeostasis may constitute a promising target for pharmacotherapy in RTT.

Redox control of endoplasmic reticulum function

The lumen of the endoplasmic reticulum constitutes a separate intracellular compartment with a special proteome and metabolome. The redox conditions of the organelle are also characteristically different from those of the other subcellular compartments. The luminal environment has been considered more oxidizing than the cytosol due to the presence of oxidative protein folding. However, recent observations suggest that redox systems in reduced and oxidized states are present simultaneously. The concerted action of membrane transporters and oxidoreductase enzymes maintains the oxidized state of the thiol-disulfide and the reduced state of the pyridine nucleotide redox systems, which are prerequisites for the normal redox reactions localized in the organelle. The powerful thiol-oxidizing machinery of oxidative protein folding continuously challenges the local antioxidant defense. Alterations of the luminal redox conditions, either in oxidizing or reducing direction, affect protein processing, are sensed by the accumulation of misfolded/unfolded proteins, and may induce endoplasmic reticulum stress and unfolded protein response. The activated signaling pathways attempt to restore the balance between protein loading and processing and induce programmed cell death if these attempts fail. Recent findings strongly support the involvement of redox-based endoplasmic reticulum stress in a plethora of human diseases, either as causative agents or as complications.

Acid-evoked Ca2+ signalling in rat sensory neurones: effects of anoxia and aglycaemia

Ischaemia excites sensory neurones (generating pain) and promotes calcitonin gene-related peptide release from nerve endings. Acidosis is thought to play a key role in mediating excitation via the activation of proton-sensitive cation channels. In this study, we investigated the effects of acidosis upon Ca²⁺ signalling in sensory neurones from rat dorsal root ganglia. Both hypercapnic (pH_o 6.8) and metabolic–hypercapnic (pH_o 6.2) acidosis caused a biphasic increase in cytosolic calcium concentration ([Ca²⁺]_i). This comprised a brief Ca²⁺ transient (half-time approximately 30 s) caused by Ca²⁺ influx followed by a sustained rise in [Ca²⁺]_i due to Ca²⁺ release from caffeine and cyclopiazonic acid-sensitive internal stores. Acid-evoked Ca²⁺ influx was unaffected by voltage-gated Ca²⁺-channel inhibition with nickel and acid sensing ion channel (ASIC) inhibition with amiloride but was blocked by inhibition of transient receptor potential vanilloid receptors (TRPV1) with (E)-3-(4-t-butylphenyl)-N-(2,3-dihydrobenzo[b][1,4] dioxin-6-yl)acrylamide (AMG 9810; 1 μM) and N-(4-tertiarybutylphenyl)-4-(3-cholorphyridin-2-yl) tetrahydropryazine-1(2H)-carbox-amide (BCTC; 1 μM). Combining acidosis with anoxia and aglycaemia increased the amplitude of both phases of Ca²⁺ elevation and prolonged the Ca²⁺ transient. The Ca²⁺ transient evoked by combined acidosis, aglycaemia and anoxia was also substantially blocked by AMG 9810 and BCTC and, to a lesser extent, by amiloride. In summary, the principle mechanisms mediating increase in [Ca²⁺]_i in response to acidosis are a brief Ca²⁺ influx through TRPV1 followed by sustained Ca²⁺ release from internal stores. These effects are potentiated by anoxia and aglycaemia, conditions also prevalent in ischaemia. The effects of anoxia and aglycaemia are suggested to be largely due to the inhibition of Ca²⁺-clearance mechanisms and possible increase in the role of ASICs.

Regulation of sarcoendoplasmic reticulum Ca2+-ATPase (SERCA) in turtle muscle and liver during acute exposure to anoxia

The freshwater turtle Trachemys scripta elegans naturally tolerates extended periods of anoxia during winter hibernation at the bottom of ice-locked ponds. Survival in this anoxic state is facilitated by a profound depression of metabolic rate. As calcium levels are known to be elevated in anoxic turtles, and ion pumping is an ATP-expensive process, we proposed that activity of the sarcoendoplasmic reticulum Ca2+-ATPase (SERCA) would be reduced in muscle and liver of T. s. elegans during acute (up to 20 h) exposure to anoxia. SERCA activity decreased ∼30% in liver and ∼40% in muscle after 1 h anoxia exposure and was ∼50% lower after 20 h of anoxia exposure in both tissues, even though SERCA protein levels did not change. SERCA kinetic parameters (increased substrate Km values, increased Arrhenius activation energy) were indicative of a less active enzyme form under anoxic conditions. Interestingly, the less active SERCA in anoxic turtles featured greater stability than the enzyme from normoxic animals as determined by both kinetic analysis (effect of low pH and low temperatures on Km MgATP) and conformational resistance to urea denaturation. The quick time course of deactivation and the stable changes in kinetic parameters that resulted suggested that SERCA was regulated by a post-translational mechanism. In vitro experiments indicated that SERCA activity could be blunted by protein phosphorylation and enhanced by dephosphorylation in a tissue-specific manner.

STIM2 regulates capacitive Ca2+ entry in neurons and plays a key role in hypoxic neuronal cell death

Excessive cytosolic calcium ion (Ca(2+)) accumulation during cerebral ischemia triggers neuronal cell death, but the underlying mechanisms are poorly understood. Capacitive Ca(2+) entry (CCE) is a process whereby depletion of intracellular Ca(2+) stores causes the activation of plasma membrane Ca(2+) channels. In nonexcitable cells, CCE is controlled by the endoplasmic reticulum (ER)-resident Ca(2+) sensor STIM1, whereas the closely related protein STIM2 has been proposed to regulate basal cytosolic and ER Ca(2+) concentrations and make only a minor contribution to CCE. Here, we show that STIM2, but not STIM1, is essential for CCE and ischemia-induced cytosolic Ca(2+) accumulation in neurons. Neurons from Stim2(-/-) mice showed significantly increased survival under hypoxic conditions compared to neurons from wild-type controls both in culture and in acute hippocampal slice preparations. In vivo, Stim2(-/-) mice were markedly protected from neurological damage in a model of focal cerebral ischemia. These results implicate CCE in ischemic neuronal cell death and establish STIM2 as a critical mediator of this process.

Redox-based endoplasmic reticulum dysfunction in neurological diseases

The redox homeostasis of the endoplasmic reticulum lumen is characteristically different from that of the other subcellular compartments. The concerted action of membrane transport processes and oxidoreductase enzymes maintain the oxidized state of the thiol-disulfide and the reducing state of the pyridine nucleotide redox systems, which are prerequisites for the normal functions of the organelle. The powerful thiol-oxidizing machinery allows oxidative protein folding but continuously challenges the local antioxidant defense. Alterations of the cellular redox environment either in oxidizing or reducing direction affect protein processing and may induce endoplasmic reticulum stress and unfolded protein response. The activated signaling pathways attempt to restore the balance between protein loading and processing and induce apoptosis if the attempt fails. Recent findings strongly support the involvement of this mechanism in brain ischemia, neuronal degenerative diseases and traumatic injury. The redox changes in the endoplasmic reticulum are integral parts of the pathomechanism of neurological diseases, either as causative agents, or as complications.