SH oxidation coordinates subunits of rat brain ryanodine receptor channels activated by calcium and ATP

Amyloid β-Oligomers Inhibit the Nuclear Ca2+ Signals and the Neuroprotective Gene Expression Induced by Gabazine in Hippocampal Neurons

Hippocampal neuronal activity generates dendritic and somatic Ca2+ signals, which, depending on stimulus intensity, rapidly propagate to the nucleus and induce the expression of transcription factors and genes with crucial roles in cognitive functions. Soluble amyloid-beta oligomers (AβOs), the main synaptotoxins engaged in the pathogenesis of Alzheimer's disease, generate aberrant Ca2+ signals in primary hippocampal neurons, increase their oxidative tone and disrupt structural plasticity. Here, we explored the effects of sub-lethal AβOs concentrations on activity-generated nuclear Ca2+ signals and on the Ca2+-dependent expression of neuroprotective genes. To induce neuronal activity, neuron-enriched primary hippocampal cultures were treated with the GABAA receptor blocker gabazine (GBZ), and nuclear Ca2+ signals were measured in AβOs-treated or control neurons transfected with a genetically encoded nuclear Ca2+ sensor. Incubation (6 h) with AβOs significantly reduced the nuclear Ca2+ signals and the enhanced phosphorylation of cyclic AMP response element-binding protein (CREB) induced by GBZ. Likewise, incubation (6 h) with AβOs significantly reduced the GBZ-induced increases in the mRNA levels of neuronal Per-Arnt-Sim domain protein 4 (Npas4), brain-derived neurotrophic factor (BDNF), ryanodine receptor type-2 (RyR2), and the antioxidant enzyme NADPH-quinone oxidoreductase (Nqo1). Based on these findings we propose that AβOs, by inhibiting the generation of activity-induced nuclear Ca2+ signals, disrupt key neuroprotective gene expression pathways required for hippocampal-dependent learning and memory processes.

Ischemic brain injury in diabetes and endoplasmic reticulum stress

Diabetes is a widespread disease characterized by high blood glucose levels due to abnormal insulin activity, production, or both. Chronic diabetes causes many secondary complications including cardiovascular disease: a life-threatening complication. Cerebral ischemia-related mortality, morbidity, and the extent of brain injury are high in diabetes. However, the mechanism of increase in ischemic brain injury during diabetes is not well understood. Multiple mechanisms mediate diabetic hyperglycemia and hypoglycemia-induced increase in ischemic brain injury. Endoplasmic reticulum (ER) stress mediates both brain injury as well as brain protection after ischemia-reperfusion injury. The pathways of ER stress are modulated during diabetes. Free radical generation and mitochondrial dysfunction, two of the prominent mechanisms that mediate diabetic increase in ischemic brain injury, are known to stimulate the pathways of ER stress. Increased ischemic brain injury in diabetes is accompanied by a further increase in the activation of ER stress. As there are many metabolic changes associated with diabetes, differential activation of the pathways of ER stress may mediate pronounced ischemic brain injury in subjects suffering from diabetes. We presently discuss the literature on the significance of ER stress in mediating increased ischemia-reperfusion injury in diabetes.

Redox Post-translational Modifications of Protein Thiols in Brain Aging and Neurodegenerative Conditions-Focus on S-Nitrosation

Reactive oxygen species and reactive nitrogen species (RONS) are by-products of aerobic metabolism. RONS trigger a signaling cascade that can be transduced through oxidation-reduction (redox)-based post-translational modifications (redox PTMs) of protein thiols. This redox signaling is essential for normal cellular physiology and coordinately regulates the function of redox-sensitive proteins. It plays a particularly important role in the brain, which is a major producer of RONS. Aberrant redox PTMs of protein thiols can impair protein function and are associated with several diseases. This mini review article aims to evaluate the role of redox PTMs of protein thiols, in particular S-nitrosation, in brain aging, and in neurodegenerative diseases. It also discusses the potential of using redox-based therapeutic approaches for neurodegenerative conditions.

Redox modifications in synaptic components as biomarkers of cognitive status, in brain aging and disease

Aging is a natural process that includes several changes that gradually make organisms degenerate and die. Harman's theory proposes that aging is a consequence of the progressive accumulation of oxidative modifications mediated by reactive oxygen/nitrogen species, which plays an essential role in the development and progression of many neurodegenerative diseases. This review will focus on how abnormal redox modifications induced by age impair the functionality of neuronal redox-sensitive proteins involved in axonal elongation and guidance, synaptic plasticity, and intercellular communication. We will discuss post-transcriptional regulation of gene expression by microRNAs as a mechanism that controls the neuronal redox state. Finally, we will discuss how some brain-permeant antioxidants from the diet have a beneficial effect on cognition. Taken together, the evidence revised here indicates that oxidative-driven modifications of specific proteins and changes in microRNA expression may be useful biomarkers for aging and neurodegenerative diseases. Also, some specific antioxidant therapies have undoubtedly beneficial neuroprotective effects when administered in the correct doses, in the ideal formulation combination, and during the appropriate therapeutic window. The use of some antioxidants is, therefore, still poorly explored for the treatment of neurodegenerative diseases such as Alzheimer's disease.

N-Acetylcysteine Prevents the Spatial Memory Deficits and the Redox-Dependent RyR2 Decrease Displayed by an Alzheimer's Disease Rat Model

We have previously reported that primary hippocampal neurons exposed to synaptotoxic amyloid beta oligomers (AβOs), which are likely causative agents of Alzheimer’s disease (AD), exhibit abnormal Ca²⁺ signals, mitochondrial dysfunction and defective structural plasticity. Additionally, AβOs-exposed neurons exhibit a decrease in the protein content of type-2 ryanodine receptor (RyR2) Ca²⁺ channels, which exert critical roles in hippocampal synaptic plasticity and spatial memory processes. The antioxidant N-acetylcysteine (NAC) prevents these deleterious effects of AβOs in vitro. The main contribution of the present work is to show that AβOs injections directly into the hippocampus, by engaging oxidation-mediated reversible pathways significantly decreased RyR2 protein content but increased single RyR2 channel activation by Ca²⁺ and caused considerable spatial memory deficits. AβOs injections into the CA3 hippocampal region impaired rat performance in the Oasis maze spatial memory task, decreased hippocampal glutathione levels and overall content of plasticity-related proteins (c-Fos, Arc, and RyR2) and increased ERK1/2 phosphorylation. In contrast, in hippocampus-derived mitochondria-associated membranes (MAM) AβOs injections increased RyR2 levels. Rats fed with NAC for 3-weeks prior to AβOs injections displayed comparable redox potential, RyR2 and Arc protein contents, similar ERK1/2 phosphorylation and RyR2 single channel activation by Ca²⁺ as saline-injected (control) rats. NAC-fed rats subsequently injected with AβOs displayed the same behavior in the spatial memory task as control rats. Based on the present in vivo results, we propose that redox-sensitive neuronal RyR2 channels partake in the mechanism underlying AβOs-induced memory disruption in rodents.

High-Fat-Diet-Induced Obesity Produces Spontaneous Ventricular Arrhythmias and Increases the Activity of Ryanodine Receptors in Mice

Ventricular arrhythmias are a common cause of sudden cardiac death, and their occurrence is higher in obese subjects. Abnormal gating of ryanodine receptors (RyR2), the calcium release channels of the sarcoplasmic reticulum, can produce ventricular arrhythmias. Since obesity promotes oxidative stress and RyR2 are redox-sensitive channels, we investigated whether the RyR2 activity was altered in obese mice. Mice fed a high fat diet (HFD) became obese after eight weeks and exhibited a significant increase in the occurrence of ventricular arrhythmias. Single RyR2 channels isolated from the hearts of obese mice were more active in planar bilayers than those isolated from the hearts of the control mice. At the molecular level, RyR2 channels from HFD-fed mice had substantially fewer free thiol residues, suggesting that redox modifications were responsible for the higher activity. Apocynin, provided in the drinking water, completely prevented the appearance of ventricular arrhythmias in HFD-fed mice, and normalized the activity and content of the free thiol residues of the protein. HFD increased the expression of NOX4, an isoform of NADPH oxidase, in the heart. Our results suggest that HFD increases the activity of RyR2 channels via a redox-dependent mechanism, favoring the appearance of ventricular arrhythmias.

Stimulation of NOX2 in isolated hearts reversibly sensitizes RyR2 channels to activation by cytoplasmic calcium

The response of ryanodine receptor (RyR) channels to cytoplasmic free calcium concentration ([Ca(2+)]) is redox sensitive. Here, we report the effects of a mild oxidative stress on cardiac RyR (RyR2) channels in Langendorff perfused rat hearts. Single RyR2 channels from control ventricles displayed the same three responses to Ca(2+) reported in other mammalian tissues, characterized by low, moderate, or high maximal activation. A single episode of 5 min of global ischemia, followed by 1 min of reperfusion, enhanced 2.3-fold the activity of NOX2 compared to controls and changed the frequency distribution of the different responses of RyR2 channels to calcium, favoring the more active ones: high activity response increased and low activity response decreased with respect to controls. This change was fully prevented by perfusion with apocynin or VAS 2870 before ischemia and totally reversed by the extension of the reperfusion period to 15 min. In vitro activation of NOX2 in control SR vesicles mimicked the effect of the ischemia/reperfusion episode on the frequencies of emergence of single RyR2 channel responses to [Ca(2+)] and increased 2.2-fold the rate of calcium release in Ca(2+)-loaded SR vesicles. In vitro changes were reversed at the single channel level by DTT and in isolated SR vesicles by glutaredoxin. Our results indicate that in whole hearts a mild oxidative stress enhances the response of cardiac RyR2 channels to calcium via NOX2 activation, probably by S-glutathionylation of RyR2 protein. This change is transitory and fully reversible, suggesting a possible role of redox modification in the physiological response of cardiac RyR2 to cellular calcium influx.

Loss of endoplasmic reticulum Ca2+ homeostasis: contribution to neuronal cell death during cerebral ischemia

The loss of Ca(2+) homeostasis during cerebral ischemia is a hallmark of impending neuronal demise. Accordingly, considerable cellular resources are expended in maintaining low resting cytosolic levels of Ca(2+). These include contributions by a host of proteins involved in the sequestration and transport of Ca(2+), many of which are expressed within intracellular organelles, including lysosomes, mitochondria as well as the endoplasmic reticulum (ER). Ca(2+) sequestration by the ER contributes to cytosolic Ca(2+) dynamics and homeostasis. Furthermore, within the ER Ca(2+) plays a central role in regulating a host of physiological processes. Conversely, impaired ER Ca(2+) homeostasis is an important trigger of pathological processes. Here we review a growing body of evidence suggesting that ER dysfunction is an important factor contributing to neuronal injury and loss post-ischemia. Specifically, the contribution of the ER to cytosolic Ca(2+) elevations during ischemia will be considered, as will the signalling cascades recruited as a consequence of disrupting ER homeostasis and function.

Calcium and endothelium-mediated vasodilator signaling

Vascular tone refers to the balance between arterial constrictor and dilator activity. The mechanisms that underlie tone are critical for the control of haemodynamics and matching circulatory needs with metabolism, and thus alterations in tone are a primary factor for vascular disease etiology. The dynamic spatiotemporal control of intracellular Ca(2+) levels in arterial endothelial and smooth muscle cells facilitates the modulation of multiple vascular signaling pathways. Thus, control of Ca(2+) levels in these cells is integral for the maintenance of tone and blood flow, and intimately associated with both physiological and pathophysiological states. Hence, understanding the mechanisms that underlie the modulation of vascular Ca(2+) activity is critical for both fundamental knowledge of artery function, and for the development of targeted therapies. This brief review highlights the role of Ca(2+) signaling in vascular endothelial function, with a focus on contact-mediated vasodilator mechanisms associated with endothelium-derived hyperpolarization and the longitudinal conduction of responses over distance.

High-frequency field stimulation of primary neurons enhances ryanodine receptor-mediated Ca2+ release and generates hydrogen peroxide, which jointly stimulate NF-κB activity

Neuronal electrical activity increases intracellular Ca(2+) concentration and generates reactive oxygen species. Here, we show that high frequency field stimulation of primary hippocampal neurons generated Ca(2+) signals with an early and a late component, and promoted hydrogen peroxide generation via a neuronal NADPH oxidase. Hydrogen peroxide generation required both Ca(2+) entry through N-methyl-D-aspartate receptors and Ca(2+) release mediated by ryanodine receptors (RyR). Field stimulation also enhanced nuclear translocation of the NF-κB p65 protein and NF-κB -dependent transcription, and increased c-fos mRNA and type-2 RyR protein content. Preincubation with inhibitory ryanodine or with the antioxidant N-acetyl L-cysteine abolished the increase in hydrogen peroxide generation and the late Ca(2+) signal component induced by electrical stimulation. Primary cortical cells behaved similarly as primary hippocampal cells. Exogenous hydrogen peroxide also activated NF-κB-dependent transcription in hippocampal neurons; inhibitory ryanodine prevented this effect. Selective inhibition of the NADPH oxidase or N-acetyl L-cysteine also prevented the enhanced translocation of p65 in hippocampal cells, while N-acetyl L-cysteine abolished the increase in RyR2 protein content induced by high frequency stimulation. In conclusion, the present results show that electrical stimulation induced reciprocal activation of ryanodine receptor-mediated Ca(2+) signals and hydrogen peroxide generation, which stimulated jointly NF-κB activity.

SNO-ing at the nociceptive synapse?

Nitric oxide is generally considered a pronociceptive retrograde transmitter that, by activation of soluble guanylyl cyclase-mediated cGMP production and activation of cGMP-dependent protein kinase, drives nociceptive hypersensitivity. The duality of its functions, however, is increasingly recognized. This review summarizes nitric-oxide-mediated direct S-nitrosylation of target proteins that may modify nociceptive signaling, including glutamate receptors and G-protein-coupled receptors, transient receptor potential channels, voltage-gated channels, proinflammatory enzymes, transcription factors, and redoxins. S-Nitrosylation events require close proximity of nitric oxide production and target proteins and a permissive redox state in the vicinity. Despite the diversity of potential targets and effects, three major schemes arise that may affect nociceptive signaling: 1) S-Nitrosylation-mediated changes of ion channel gating properties, 2) modulation of membrane fusion and fission, and thereby receptor and channel membrane insertion, and 3) modulation of ubiquitination, and thereby protein degradation or transcriptional activity. In addition, S-Nitrosylation may alter the production of nitric oxide itself.

A mathematical model of vasoreactivity in rat mesenteric arterioles. II. Conducted vasoreactivity

This study presents a multicellular computational model of a rat mesenteric arteriole to investigate the signal transduction mechanisms involved in the generation of conducted vasoreactivity. The model comprises detailed descriptions of endothelial (ECs) and smooth muscle (SM) cells (SMCs), coupled by nonselective gap junctions. With strong myoendothelial coupling, local agonist stimulation of the EC or SM layer causes local changes in membrane potential (V(m)) that are conducted electrotonically, primarily through the endothelium. When myoendothelial coupling is weak, signals initiated in the SM conduct poorly, but the sensitivity of the SMCs to current injection and agonist stimulation increases. Thus physiological transmembrane currents can induce different levels of local V(m) change, depending on cell's gap junction connectivity. The physiological relevance of current and voltage clamp stimulations in intact vessels is discussed. Focal agonist stimulation of the endothelium reduces cytosolic calcium (intracellular Ca(2+) concentration) in the prestimulated SM layer. This SMC Ca(2+) reduction is attributed to a spread of EC hyperpolarization via gap junctions. Inositol (1,4,5)-trisphosphate, but not Ca(2+), diffusion through homocellular gap junctions can increase intracellular Ca(2+) concentration in neighboring ECs. The small endothelial Ca(2+) spread can amplify the total current generated at the local site by the ECs and through the nitric oxide pathway, by the SMCs, and thus reduces the number of stimulated cells required to induce distant responses. The distance of the electrotonic and Ca(2+) spread depends on the magnitude of SM prestimulation and the number of SM layers. Model results are consistent with experimental data for vasoreactivity in rat mesenteric resistance arteries.

Ischemia enhances activation by Ca2+ and redox modification of ryanodine receptor channels from rat brain cortex

Cerebral ischemia stimulates Ca2+ influx and thus increases neuronal intracellular free [Ca2+]. Using a rat model of cerebral ischemia without recirculation, we tested whether ischemia enhances the activation by Ca2+ of ryanodine receptor (RyR) channels, a requisite feature of RyR-mediated Ca2+-induced Ca2+ release (CICR). To this aim, we evaluated how single RyR channels from endoplasmic reticulum vesicles, fused into planar lipid bilayers, responded to cytoplasmic [Ca2+] changes. Endoplasmic reticulum vesicles were isolated from the cortex of rat brains incubated without blood flow for 5 min at 37 degrees C (ischemic) or at 4 degrees C (control). Ischemic brains displayed increased oxidative intracellular conditions, as evidenced by a lower ratio (approximately 130:1) of reduced/oxidized glutathione than controls (approximately 200:1). Single RyR channels from ischemic or control brains displayed the same three responses to Ca2+ reported previously, characterized by low, moderate, or high maximal activity. Relative to controls, RyR channels from ischemic brains displayed with increased frequency the high activity response and with lower frequency the low activity response. Both control and ischemic cortical vesicles contained the RyR2 and RyR3 isoforms in a 3:1 proportion, with undetectable amounts of RyR1. Ischemia reduced [3H]ryanodine binding and total RyR protein content by 35%, and increased at least twofold endogenous RyR2 S-nitrosylation and S-glutathionylation without affecting the corresponding RyR3 endogenous levels. In vitro RyR S-glutathionylation but not S-nitrosylation favored the emergence of high activity channels. We propose that ischemia, by enhancing RyR2 S-glutathionylation, allows RyR2 to sustain CICR; the resulting amplification of Ca2+ entry signals may contribute to cortical neuronal death.

Crosstalk between calcium and redox signaling: from molecular mechanisms to health implications

Studies done many years ago established unequivocally the key role of calcium as a universal second messenger. In contrast, the second messenger roles of reactive oxygen and nitrogen species have emerged only recently. Therefore, their contributions to physiological cell signaling pathways have not yet become universally accepted, and many biological researchers still regard them only as cellular noxious agents. Furthermore, it is becoming increasingly apparent that there are significant interactions between calcium and redox species, and that these interactions modify a variety of proteins that participate in signaling transduction pathways and in other fundamental cellular functions that determine cell life or death. This review article addresses first the central aspects of calcium and redox signaling pathways in animal cells, and continues with the molecular mechanisms that underlie crosstalk between calcium and redox signals under a number of physiological or pathological conditions. To conclude, the review focuses on conditions that, by promoting cellular oxidative stress, lead to the generation of abnormal calcium signals, and how this calcium imbalance may cause a variety of human diseases including, in particular, degenerative diseases of the central nervous system and cardiac pathologies.

The N-terminus of presenilin-2 increases single channel activity of brain ryanodine receptors through direct protein-protein interaction

Presenilin-1 (PS1) and presenilin-2 (PS2) form the catalytic core in gamma-secretase complexes and mutations in these proteins result in aberrant cleavage of amyloid precursor protein leading to accumulation of the beta-amyloid in the brain of familial Alzheimer Disease patients. PS2 possesses a hydrophilic cytoplasmic N-terminal domain (PS2 NTF1-87) dispensable for gamma-secretase activity with physiological functions yet to be determined. The effects of this soluble 87 amino acid fragment of mouse PS2 on single channel activity of mouse brain ryanodine receptors (RyR) were determined. PS2 NTF1-87 application to the cytoplasmic side of the RyR significantly increased single channel activity by favoring higher sublevel openings. The Ca(2+) activation and desensitization ranges for RyRs were unchanged. We demonstrate facilitation of RyR gating by PS2 NTF1-87, which might represent a general mechanism of RyR regulation by presenilins potentially prone to be affected by mutations or external stimuli contributing to the development of neurodegenerative diseases.

A glutathione deficit alters dopamine modulation of L-type calcium channels via D2 and ryanodine receptors in neurons

Synthesis of glutathione, a major redox regulator, is compromised in schizophrenia. We postulated that the resulting glutathione deficit via its effect on redox-sensitive proteins could contribute to dysfunction of some neurotransmitter systems in schizophrenia. We investigated whether a glutathione deficit, induced by a blocker of glutathione synthesis, L-buthionine-(S,R)-sulfoximine, affects intracellular pathways implicated in dopamine signaling in neurons, namely dopamine modulation of calcium responses to NMDA. Such a glutathione deficit changed the modulation of responses by dopamine, from enhanced responses in control neurons (likely via D1-type receptors) to decreased responses in low-glutathione neurons (via D2-type receptors). This difference in dopamine modulation was due to a different modulation of L-type calcium channels activated during NMDA stimulation: dopamine enhanced function of these channels in control neurons but decreased it in low-glutathione neurons. The effect of a glutathione deficit on dopamine signaling was dependent on the redox-sensitive ryanodine receptors (RyRs), whose function was enhanced in low-glutathione neurons. This suggests that enhanced RyRs in low-glutathione neurons strengthens intracellular calcium-dependent pathways following activation of D2-type receptors and causes a decrease in function of L-type channels. This represents a mechanism by which dopaminergic systems could be dysfunctional under conditions of impaired glutathione synthesis as in schizophrenia.

Allosterically coupled calcium and magnesium binding sites are unmasked by ryanodine receptor chimeras

We studied cation regulation of wild-type ryanodine receptor type 1 ((WT)RyR1), type 3 ((WT)RyR3), and RyR3/RyR1 chimeras (Ch) expressed in 1B5 dyspedic myotubes. Using [(3)H]ryanodine binding to sarcoplasmic reticulum (SR) membranes, Ca(2+) titrations with (WT)RyR3 and three chimeras show biphasic activation that is allosterically coupled to an attenuated inhibition relative to (WT)RyR1. Chimeras show biphasic Mg(2+) inhibition profiles at 3 and 10 microM Ca(2+), no observable inhibition at 20 microM Ca(2+) and monophasic inhibition at 100 microM Ca(2+). Ca(2+) imaging of intact myotubes expressing Ch-4 exhibit caffeine-induced Ca(2+) transients with inhibition kinetics that are significantly slower than those expressing (WT)RyR1 or (WT)RyR3. Four new aspects of RyR regulation are evident: (1) high affinity (H) activation and low affinity (L) inhibition sites are allosterically coupled, (2) Ca(2+) facilitates removal of the inherent Mg(2+) block, (3) (WT)RyR3 exhibits reduced cooperativity between H activation sites when compared to (WT)RyR1, and (4) uncoupling of these sites in Ch-4 results in decreased rates of inactivation of caffeine-induced Ca(2+) transients.

Reactive oxygen species produced up- or downstream of calcium influx regulate proinflammatory mediator release from mast cells: role of NADPH oxidase and mitochondria

Earlier studies have demonstrated that mast cells produce reactive oxygen species (ROS), which play a role in regulating Ca(2+) influx, while in other cell types ROS are produced in a Ca(2+)-dependent manner. We sought to determine whether ROS are produced downstream of the extracellular Ca(2+) entry in mast cells. Thapsigargin (TG), a receptor-independent agonist, could evoke a robust burst of intracellular ROS. However, this response was distinct from the antigen-induced burst of ROS with respect to time course and dependence on Ca(2+) and phosphatidylinositol-3-kinase (PI3K). The antigen-induced ROS generation occurred immediately, while the TG-induced ROS generation occurred with a significant lag time (~2 min). Antigen but not TG caused extracellular release of superoxide (O(2)(*-))/hydrogen peroxide (H(2)O(2)), which was blocked by diphenyleneiodonium, apocynin, and wortmannin. A capacitative Ca(2+) entry resulted in the generation of O(2)(*-) in the mitochondria in a PI3K-independent manner. Blockade of ROS generation inhibited TG-induced mediator release. Finally, when used together, antigen and TG evoked the release of leukotriene C(4), tumor necrosis factor-alpha, and interleukin-13 as well as ROS generation synergistically. These results suggest that ROS produced upstream of Ca(2+) influx by NADPH oxidase and downstream of Ca(2+) influx by the mitochondria regulate the proinflammatory response of mast cells.

A mathematical model of plasma membrane electrophysiology and calcium dynamics in vascular endothelial cells

Vascular endothelial cells (ECs) modulate smooth muscle cell (SMC) contractility, assisting in vascular tone regulation. Cytosolic Ca2+ concentration ([Ca2+]i) and membrane potential ( Vm) play important roles in this process by controlling EC-dependent vasoactive signals and intercellular communication. The present mathematical model integrates plasmalemma electrophysiology and Ca2+ dynamics to investigate EC responses to different stimuli and the controversial relationship between [Ca2+]i and Vm. The model contains descriptions for the intracellular balance of major ionic species and the release of Ca2+ from intracellular stores. It also expands previous formulations by including more detailed transmembrane current descriptions. The model reproduces Vm responses to volume-regulated anion channel (VRAC) blockers and extracellular K+ concentration ([K+]o) challenges, predicting 1) that Vm changes upon VRAC blockade are [K+]o dependent and 2) a biphasic response of Vm to increasing [K+]o. Simulations of agonist-induced Ca2+ mobilization replicate experiments under control and Vm hyperpolarization blockade conditions. They show that peak [Ca2+]i is governed by store Ca2+ release while Ca2+ influx (and consequently Vm) impacts more the resting and plateau [Ca2+]i. The Vm sensitivity of rest and plateau [Ca2+]i is dictated by a [Ca2+]i “buffering” system capable of masking the Vm-dependent transmembrane Ca2+ influx. The model predicts plasma membrane Ca2+-ATPase and Ca2+ permeability as main players in this process. The heterogeneous Vm impact on [Ca2+]i may elucidate conflicting reports on how Vm influences EC Ca2+. The present study forms the basis for the development of multicellular EC-SMC models that can assist in understanding vascular autoregulation in health and disease.

Mechanisms initiating integrin-stimulated flow recruitment in arteriolar networks

Our purpose was to investigate the local mechanisms involved in network-wide flow and diameter changes observed with localized downstream vitronectin receptor ligation; we tested specific K or Cl channels known to be involved in either dilation or elevated permeability following vitronectin receptor activation and tested integrin-linked pathway elements of tyrosine phosphorylation and protein kinase C (PKC). Arteriolar networks were observed in the cheek pouch tissue of anesthetized (pentobarbital sodium, 70 mg/kg) hamsters ( n = 86) using intravital microscopy. Terminal arteriolar branches of the networks were stimulated with micropipette LM609 (0.5–10 μg/ml, 60 s) alone or with inhibitors (separate micropipette). Hemodynamic changes (diameter, red blood cell flux, velocity) were observed at the upstream entrance to the network. LM609 alone stimulated first an increase in wall shear stress (WSS), followed by a dilation that recovered WSS to baseline or below. K channel inhibition (glybenclamide, 4-AP) had no effect on the initial peak in WSS, but decreased remote vasodilation. Cl channel inhibition (DIDS, IAA-94, niflumic acid) or inhibition of PKC (chelerythrine) prevented the initial peak in WSS and decreased remote vasodilation. Inhibition of tyrosine phosphorylation (genistein) prevented both. With the use of nitro-arginine at the observation site, the initial peak in WSS was not affected, but remote vasodilation was decreased. We conclude the remote response consists of an initial peak in WSS that relies on both PKC activity and depolarization downstream, leading to an upstream flow mediated dilation and a secondary remote dilation that relies on hyperpolarization downstream at the stimulus site; both components require tyrosine phosphorylation downstream.

Calcium release by ryanodine receptors mediates hydrogen peroxide-induced activation of ERK and CREB phosphorylation in N2a cells and hippocampal neurons

Hydrogen peroxide, which stimulates ERK phosphorylation and synaptic plasticity in hippocampal neurons, has also been shown to stimulate calcium release in muscle cells by promoting ryanodine receptor redox modification (S-glutathionylation). We report here that exposure of N2a cells or rat hippocampal neurons in culture to 200 microM H2O2 elicited calcium signals, increased ryanodine receptor S-glutathionylation, and enhanced both ERK and CREB phosphorylation. In mouse hippocampal slices, H2O2 (1 microM) also stimulated ERK and CREB phosphorylation. Preincubation with ryanodine (50 microM) largely prevented the effects of H2O2 on calcium signals and ERK/CREB phosphorylation. In N2a cells, the ERK kinase inhibitor U0126 suppressed ERK phosphorylation and abolished the stimulation of CREB phosphorylation produced by H2O2, suggesting that H2O2 enhanced CREB phosphorylation via ERK activation. In N2a cells in calcium-free media, 200 microM H2O2 stimulated ERK and CREB phosphorylation, while preincubation with thapsigargin prevented these enhancements. These combined results strongly suggest that H2O2 promotes ryanodine receptors redox modification; the resulting calcium release signals, by enhancing ERK activity, would increase CREB phosphorylation. We propose that ryanodine receptor stimulation by activity-generated redox species produces calcium release signals that may contribute significantly to hippocampal synaptic plasticity, including plasticity that requires long-lasting ERK-dependent CREB phosphorylation.

Effects of ATP, Mg2+, and redox agents on the Ca2+ dependence of RyR channels from rat brain cortex

Despite their relevance for neuronal Ca(2+)-induced Ca(2+) release (CICR), activation by Ca(2+) of ryanodine receptor (RyR) channels of brain endoplasmic reticulum at the [ATP], [Mg(2+)], and redox conditions present in neurons has not been reported. Here, we studied the effects of varying cis-(cytoplasmic) free ATP concentration ([ATP]), [Mg(2+)], and RyR redox state on the Ca(2+) dependence of endoplasmic reticulum RyR channels from rat brain cortex. At pCa 4.9 and 0.5 mM adenylylimidodiphosphate (AMP-PNP), increasing free [Mg(2+)] up to 1 mM inhibited vesicular [(3)H]ryanodine binding; incubation with thimerosal or dithiothreitol decreased or enhanced Mg(2+) inhibition, respectively. Single RyR channels incorporated into lipid bilayers displayed three different Ca(2+) dependencies, defined by low, moderate, or high maximal fractional open time (P(o)), that depend on RyR redox state, as we have previously reported. In all cases, cis-ATP addition (3 mM) decreased threshold [Ca(2+)] for activation, increased maximal P(o), and shifted channel inhibition to higher [Ca(2+)]. Conversely, at pCa 4.5 and 3 mM ATP, increasing cis-[Mg(2+)] up to 1 mM inhibited low activity channels more than moderate activity channels but barely modified high activity channels. Addition of 0.5 mM free [ATP] plus 0.8 mM free [Mg(2+)] induced a right shift in Ca(2+) dependence for all channels so that [Ca(2+)] <30 microM activated only high activity channels. These results strongly suggest that channel redox state determines RyR activation by Ca(2+) at physiological [ATP] and [Mg(2+)]. If RyR behave similarly in living neurons, cellular redox state should affect RyR-mediated CICR.

A role for reactive oxygen/nitrogen species and iron on neuronal synaptic plasticity

A great body of experimental evidence collected over many years indicates that calcium has a central role in a variety of neuronal functions. In particular, calcium participates in synaptic plasticity, a neuronal process presumably correlated with cognitive brain functions such as learning and memory. In contrast, only recently, evidence has begun to emerge supporting a physiological role of reactive oxygen (ROS) and nitrogen (RNS) species in synaptic plasticity. This subject will be the central topic of this review. The authors also present recent results showing that, in hippocampal neurons, ROS/RNS, including ROS generated by iron through the Fenton reaction, stimulate ryanodine receptor-mediated calcium release, and how the resulting calcium signals activate the signaling cascades that lead to the transcription of genes known to participate in synaptic plasticity. They discuss the possible participation of ryanodine receptors jointly stimulated by calcium and ROS/RNS in the normal signaling cascades needed for synaptic plasticity, and how too much ROS production may contribute to neurodegeneration via excessive calcium release. In addition, the dual role of iron as a necessary, but potentially toxic, element for normal neuronal function is discussed.

Redox regulation of the ryanodine receptor/calcium release channel

The RyR (ryanodine receptor)/calcium release channel contains a number of highly reactive thiol groups that endow it with redox sensitivity. In general, oxidizing conditions favour channel opening, while reducing conditions have the opposite effect. Thiol modification affects the channel sensitivity to its principal effectors, Ca2+, Mg2+ and ATP, and alters RyR protein interactions. Here, we give a brief account of the major findings and prevailing views in the field.

Assessment of renal autoregulation

The kidney displays highly efficient autoregulation so that under steady-state conditions renal blood flow (RBF) is independent of blood pressure over a wide range of pressure. Autoregulation occurs in the preglomerular microcirculation and is mediated by two, perhaps three, mechanisms. The faster myogenic mechanism and the slower tubuloglomerular feedback contribute both directly and interactively to autoregulation of RBF and of glomerular capillary pressure. Multiple experiments have been used to study autoregulation and can be considered as variants of two basic designs. The first measures RBF after multiple stepwise changes in renal perfusion pressure to assess how a biological condition or experimental maneuver affects the overall pressure-flow relationship. The second uses time-series analysis to better understand the operation of multiple controllers operating in parallel on the same vascular smooth muscle. There are conceptual and experimental limitations to all current experimental designs so that no one design adequately describes autoregulation. In particular, it is clear that the efficiency of autoregulation varies with time and that most current techniques do not adequately address this issue. Also, the time-varying and nonadditive interaction between the myogenic mechanism and tubuloglomerular feedback underscores the difficulty of dissecting their contributions to autoregulation. We consider the modulation of autoregulation by nitric oxide and use it to illustrate the necessity for multiple experimental designs, often applied iteratively.

Propagation of calcium waves along endothelium of hamster feed arteries

An increase in tissue blood flow requires relaxation of smooth muscle cells along entire branches of the resistance vasculature. Whereas the spread of hyperpolarization along the endothelium can coordinate smooth muscle cell relaxation, complementary signaling events have been implicated in the conduction of vasodilation. We tested the hypothesis that Ca(2+) waves propagate from cell to cell along the endothelium of feed arteries exhibiting spontaneous vasomotor tone. Feed arteries of the hamster retractor muscle were isolated, pressurized to 75 mmHg at 37 degrees C, and developed myogenic tone spontaneously. Smooth muscle cells and endothelial cells were loaded with the Ca(2+) indicator Fluo-4. An acetylcholine stimulus was delivered locally using microiontophoresis (1-microm pipette tip, 1 microA, 1 s), and Ca(2+) signaling within and along respective cell layers was determined using laser-scanning confocal microscopy. Acetylcholine triggered an increase in intracellular Ca(2+) concentration ([Ca(2+)](i)) of endothelial cells at the site of stimulation that preceded two distinct events: 1) a rapid synchronous decrease in smooth muscle [Ca(2+)](i) along the entire vessel and 2) an ensuing Ca(2+) wave that propagated bidirectionally along the endothelium at approximately 111 microm/s for distances exceeding 1 mm. Maximal dilation of vessels with either nifedipine (1 microM) or sodium nitroprusside (SNP, 100 microM) reduced the distance that Ca(2+) waves traveled to approximately 300 microm (P < 0.05). Thus Ca(2+) waves propagate along the endothelium of resistance vessels with vasomotor tone, and this signaling pathway is compromised during maximal dilation with nifedipine or SNP.

Cross talk between Ca2+ and redox signalling cascades in muscle and neurons through the combined activation of ryanodine receptors/Ca2+ release channels

Calcium release mediated by the ryanodine receptors (RyR) Ca2+ release channels is required for muscle contraction and contributes to neuronal plasticity. In particular, Ca2+ activation of RyR-mediated Ca2+ release can amplify and propagate Ca2+ signals initially generated by Ca2+ entry into cells. Redox modulation of RyR function by a variety of non-physiological or endogenous redox molecules has been reported. The effects of RyR redox modification on Ca2+ release in skeletal muscle as well as the activation of signalling cascades and transcription factors in neurons will be reviewed here. Specifically, the different effects of S-nitrosylation or S-glutathionylation of RyR cysteines by endogenous redox-active agents on the properties of skeletal muscle RyRs will be discussed. Results will be presented indicating that these cysteine modifications change the activity of skeletal muscle RyRs, modify their behaviour towards both activators and inhibitors and affect their interactions with FKBP12 and calmodulin. In the hippocampus, sequential activation of ERK1/2 and CREB is a requisite for Ca2+-dependent gene expression associated with long-lasting synaptic plasticity. The effects of reactive oxygen/nitrogen species on RyR channels from neurons and RyR-mediated sequential activation of neuronal ERK1/2 and CREB produced by hydrogen peroxide and other stimuli will be discussed as well.

Intracellular Ca(2+) and Zn(2+) signals during monochloramine-induced oxidative stress in isolated rat colon crypts

During acute exacerbations of inflammatory bowel diseases, oxidants are generated through the interactions of bacteria in the lumen, activated granulocytes, and cells of the colon mucosa. In this study we explored the ability of one such class of oxidants, represented by monochloramine (NH(2)Cl), to serve as agonists of Ca(2+) and Zn(2+) accumulation within the colonocyte. Individual colon crypts prepared from Sprague-Dawley rats were mounted in perfusion chambers after loading with fluorescent reporters fura 2-AM and fluozin 3-AM. These reporters were characterized, in situ, for responsiveness to Ca(2+) and Zn(2+) in the cytoplasm. Responses to different concentrations of NH(2)Cl (50, 100, and 200 microM) were monitored. Subsequent studies were designed to identify the sources and mechanisms of NH(2)Cl-induced increases in Ca(2+) and Zn(2+) in the cytoplasm. Exposure to NH(2)Cl led to dose-dependent increases in intracellular Ca(2+) concentration ([Ca(2+)](i)) in the range of 200-400 nM above baseline levels. Further studies indicated that NH(2)Cl-induced accumulation of Ca(2+) in the cytoplasm is the result of release from intracellular stores and basolateral entry of extracellular Ca(2+) through store-operated channels. In addition, exposure to NH(2)Cl resulted in dose-dependent and sustained increases in intracellular Zn(2+) concentration ([Zn(2+)](i)) in the nanomolar range. These alterations were neutralized by dithiothreitol, which shields intracellular thiol groups from oxidation. We conclude that Ca(2+)- and Zn(2+)-handling proteins are susceptible to oxidation by chloramines, leading to sustained, but not necessarily toxic, increases in [Ca(2+)](i) and [Zn(2+)](i). Under certain conditions, NH(2)Cl may act not as a toxin but as an agent that activates intracellular signaling pathways.

The ryanodine receptors Ca2+ release channels: cellular redox sensors?

The release of Ca2+ from intracellular stores mediated by ryanodine receptors (RyR) Ca2+ release channels is essential for striated muscle contraction and contributes to diverse neuronal functions including synaptic plasticity. Through Ca2+-induced Ca2+-release, RyR can amplify and propagate Ca2+ signals initially generated by Ca2+ entry into cardiac muscle cells or neurons. In contrast, RyR activation in skeletal muscle is under membrane potential control and does not require Ca2+ entry. Non-physiological or endogenous redox molecules can change RyR function via modification of a few RyR cysteine residues. This critical review will address the functional effects of RyR redox modification on Ca2+ release in skeletal muscle and cardiac muscle as well as in the activation of signaling cascades and transcriptional regulators required for synaptic plasticity in neurons. Specifically, the effects of endogenous redox-active agents, which induce S-nitrosylation or S-glutathionylation of particular channel cysteine residues, on the properties of muscle RyRs will be discussed. The effects of endogenous redox RyR modifications on cardiac preconditioning will be analyzed as well. In the hippocampus, sequential activation of ERKs and CREB is a requisite for Ca2+-dependent gene expression associated with long lasting synaptic plasticity. Results showing that reactive oxygen/nitrogen species modify RyR channels from neurons and the RyR-mediated sequential activation of neuronal ERKs and CREB produced by hydrogen peroxide and other stimuli will be also discussed.

Marriage of resistance and conduit arteries breeds critical limb ischemia

Atherosclerosis in a major leg artery leads to impaired blood supply, which normally progresses to critical limb ischemia. Atherosclerosis produces substantial alterations of structure and endothelial function in the large conduit arteries. Pressure unloading and ischemia in the distal vasculature bring about alterations in microvascular function. Resistance arteries undergo significant wall thinning and changes in their contractile regulation. Optimization of large artery dimensions by the small arteries through flow-mediated vasodilation is impaired. Angiogenesis is stimulated, which can result in the formation of major collateral feeder vessels in addition to small nutritive blood vessels. However, angiogenesis can also contribute to instability of atherosclerotic plaques, which ultimately leads to further deterioration in blood supply. Surgical bypass grafting to restore blood supply to the distal leg generates a sudden increase of pressure in the weakened resistance vasculature, leading to uncontrolled changes in capillary hydrostatic pressure, extravasation of fluid, and tissue edema. This review aims to highlight the importance of the resistance vasculature in critical limb ischemia and the interdependence of pathophysiological changes in the large conduit and small resistance arteries. The major unresolved question is why the physiological mechanisms that regulate vascular structure and function ultimately break down, leading to circulatory failure within the distal limb.

Connexins: gaps in our knowledge of vascular function

Gap junctions are common features in the vasculature, long thought to provide a pathway for cell-cell signaling. Emerging understanding of the gap-junctional proteins (connexins) and new tools for their investigation now offer the opportunity to explore the vital role that the gap junctions may play in cardiovascular homeostasis and pathophysiology.

Calcium mobilization is required for peroxynitrite-mediated enhancement of spontaneous transient outward currents in arteriolar smooth muscle cells

Transiently local release of Ca(2+) from the sarcoplasmic reticulum (SR) activates nearby Ca(2+)-activated K(+) channels to produce spontaneous transient outward currents (STOCs) in smooth muscle cells. The purpose of the present study was to investigate the possible effect of peroxynitrite (ONOO(-)) on STOCs in mesenteric arteriolar smooth muscle cells (ASMCs) and decide whether Ca(2+) mobilization was involved in STOCs alteration by ONOO(-). STOCs were recorded and characterized using the perforated whole-cell patch-clamp configuration. The results demonstrated that STOCs activity was greatly suppressed by removal of extracellular Ca(2+); by addition of nifedipine, a specific inhibitor of L-type voltage-gated Ca(2+) channels (VGCCs); or by addition of ryanodine, a SR ryanodine receptors (RyRs) blocker. In contrast, both caffeine, a RyR activator, and 2-aminoethoxydiphenylborate (2-APB), a membrane-permeable inositol 1,4,5-trisphosphate receptors, (IP3R) antagonist, increased STOCs activity. 3-morpholinosydnonimine (SIN-1), an ONOO(-) donor, at concentrations of 20-200 microM, induced a dose-dependent enhancement of STOCs in ASMCs and led to conspicuous increases in STOCs frequency and amplitude, which were prevented by prior exposure to low external Ca(2+) (200 nM), ryanodine (10 microM), or nifedipine (10 microM). In contrast, caffeine (0.5 mM) did not further stimulate STOCs in ASMCs preincubated with SIN-1, and pretreatment with 2-APB (50 microM) had little effect on ONOO(-) -induced STOCs activation. These findings suggest that complex Ca(2+)-mobilizing pathways, including external Ca2+ influx through VGCCs activation and subsequent internal Ca(2+) release through RyRs but not IP3Rs, are involved in ONOO(-)mediated STOCs enhancement in ASMCs.

Identification of hyperreactive cysteines within ryanodine receptor type 1 by mass spectrometry

The skeletal-type ryanodine receptor (RyR1) undergoes covalent adduction by nitric oxide (NO), redox-induced shifts in cation regulation, and non-covalent interactions driven by the transmembrane redox potential that enable redox sensing. Tight redox regulation of RyR1 is thought to be primarily mediated through highly reactive (hyperreactive) cysteines. Of the 100 cysteines per subunit of RyR1, approximately 25-50 are reduced, with 6-8 considered hyperreactive. Thus far, only Cys-3635, which undergoes selective adduction by NO, has been identified. In this report, RyR1-enriched junctional sarcoplasmic reticulum is labeled with 7-diethylamino-3-(4'-maleimidylphenyl)-4-methylcoumarin (CPM, 1 pmol/microg of protein) in the presence of 10 mm Mg(2+), conditions previously shown to selectively label hyperreactive sulfhydryls and eliminate redox sensing. The CPM-adducted RyR1 is separated by gel electrophoresis and subjected to in-gel tryptic digestion. Isolation of CPM-adducted peptides is achieved by analytical and microbore high-performance liquid chromatography utilizing fluorescence and UV detection. Subsequent analysis using two direct and one tandem mass spectrometry methods results in peptide masses and sequence data that, compared with the known primary sequence of RyR1, enable unequivocal identification of CPM-adducted cysteines. This work is the first to directly identify seven hyperreactive cysteines: 1040, 1303, 2436, 2565, 2606, 2611, and 3635 of RyR1. In addition to Cys-3635, the nitrosylation site, six additional cysteines may contribute toward redox regulation of the RyR1 complex.