Acidosis causes endoplasmic reticulum stress and caspase-12-mediated astrocyte death

Extraction, separation and efficacy of yam polysaccharide

Yam is used as common herbal remedy in traditional Chinese medicine and is grown all over Asia. According to previous research, one of the primary bioactive components of yam is yam polysaccharide. To shed light on the mechanism of yam polysaccharide in ulcerative colitis (UC), a yam heteropolysaccharide named CYP-3a was isolated and purified using ultrasonic extraction, the trichloroacetic acid technique, DEAE cellulose-52 and a Sephadex G75 column. CYP-3a comprises rhamnus: arabinose:galactose:mannose:galacturonic acid glucuronic acid, with a molar ratio of 2.25:4.17:3.30:0.09:0.13:0.26. CCK-8 and ELISA analysis results showed that CYP-3a increased the number of dextran sodium sulphate (DSS)-induced Caco-2 cells and could reduce and inhibit their inflammatory response by lowering the amounts of secreted TNF-α and IL-6. Western blot data demonstrated that CYP-3a at various doses could suppress the endoplasmic reticulum stress-mediated apoptotic pathway generated by DSS-induced UC and down-regulate the protein levels of GRP78, CHOP and NF-κB.

A proton-inhibited DEG/ENaC ion channel maintains neuronal ionstasis and promotes neuronal survival under stress

The nervous system participates in the initiation and modulation of systemic stress. Ionstasis is of utmost importance for neuronal function. Imbalance in neuronal sodium homeostasis is associated with pathologies of the nervous system. However, the effects of stress on neuronal Na⁺ homeostasis, excitability, and survival remain unclear. We report that the DEG/ENaC family member DEL-4 assembles into a proton-inactivated sodium channel. DEL-4 operates at the neuronal membrane and synapse to modulate Caenorhabditis elegans locomotion. Heat stress and starvation alter DEL-4 expression, which in turn alters the expression and activity of key stress-response transcription factors and triggers appropriate motor adaptations. Similar to heat stress and starvation, DEL-4 deficiency causes hyperpolarization of dopaminergic neurons and affects neurotransmission. Using humanized models of neurodegenerative diseases in C. elegans, we showed that DEL-4 promotes neuronal survival. Our findings provide insights into the molecular mechanisms by which sodium channels promote neuronal function and adaptation under stress.

Cerebral Glucose Metabolism and Potential Effects on Endoplasmic Reticulum Stress in Stroke

Ischemic stroke is an extremely common pathology with strikingly high morbidity and mortality rates. The endoplasmic reticulum (ER) is the primary organelle responsible for conducting protein synthesis and trafficking as well as preserving intracellular Ca2⁺ homeostasis. Mounting evidence shows that ER stress contributes to stroke pathophysiology. Moreover, insufficient circulation to the brain after stroke causes suppression of ATP production. Glucose metabolism disorder is an important pathological process after stroke. Here, we discuss the relationship between ER stress and stroke and treatment and intervention of ER stress after stroke. We also discuss the role of glucose metabolism, particularly glycolysis and gluconeogenesis, post-stroke. Based on recent studies, we speculate about the potential relationship and crosstalk between glucose metabolism and ER stress. In conclusion, we describe ER stress, glycolysis, and gluconeogenesis in the context of stroke and explore how the interplay between ER stress and glucose metabolism contributes to the pathophysiology of stroke.

Therapeutic Role of Sirtuins Targeting Unfolded Protein Response, Coagulation, and Inflammation in Hypoxia-Induced Thrombosis

Thrombosis remains one of the leading causes of morbidity and mortality across the world. Many pathological milieus in the body resulting from multiple risk factors escort thrombosis. Hypoxic condition is one such risk factor that disturbs the integrity of endothelial cells to cause an imbalance between anticoagulant and procoagulant proteins. Hypoxia generates reactive oxygen species (ROS) and triggers inflammatory pathways to augment the coagulation cascade. Hypoxia in cells also activates unfolded protein response (UPR) signaling pathways in the endoplasmic reticulum (ER), which tries to restore ER homeostasis and function. But the sustained UPR linked with inflammation, generation of ROS and apoptosis stimulates the severity of thrombosis in the body. Sirtuins, a group of seven proteins, play a vast role in bringing down inflammation, oxidative and ER stress and apoptosis. As a result, sirtuins might provide a therapeutic approach towards the treatment or prevention of hypoxia-induced thrombosis. Sirtuins modulate hypoxia-inducible factors (HIFs) and counteract ER stress-induced apoptosis by attenuating protein kinase RNA-like endoplasmic reticulum kinase (PERK)/Eukaryotic translation initiation factor 2α (eIF2α) pathway activation. It prevents ER-stress mediated inflammation by targeting X-Box Binding Protein 1 (XBP1) and inhibiting nuclear factor kappa-light-chain-enhancer of activated B cells (NF-κβ) signaling through deacetylation. Sirtuins also obstruct nucleotide-binding domain, leucine-rich-containing family, pyrin domain containing 3 (NLRP3) inflammasome activation to reduce the expression of several pro-inflammatory molecules. It protects cells against oxidative stress by targeting nuclear factor erythroid 2-related factor 2 (Nrf2), glutathione (GSH), forkhead box O3 (FOXO3), superoxide dismutase (SOD), catalase (CAT), peroxisome proliferator-activated receptor gamma coactivator 1-α (PGC-1α), glucose-6-phosphate dehydrogenase (G6PD), phosphoglucomutase-2 (PGAM2), and NF-κB, to name few. This review, thus, discusses the potential role of sirtuins as a new treatment for hypoxia-induced thrombosis that involves an intersection of UPR and inflammatory pathways in its pathological manifestation.

NMDAR in cultured astrocytes: Flux-independent pH sensor and flux-dependent regulator of mitochondria and plasma membrane-mitochondria bridging

Glutamate N-methyl-D-aspartate (NMDA) receptor (NMDAR) is critical for neurotransmission as a Ca²⁺ channel. Nonetheless, flux-independent signaling has also been demonstrated. Astrocytes express NMDAR distinct from its neuronal counterpart, but cultured astrocytes have no electrophysiological response to NMDA. We recently demonstrated that in cultured astrocytes, NMDA at pH6 (NMDA/pH6) acting through the NMDAR elicits flux-independent Ca²⁺ release from the Endoplasmic Reticulum (ER) and depletes mitochondrial membrane potential (mΔΨ). Here we show that Ca²⁺ release is due to pH6 sensing by NMDAR, whereas mΔΨ depletion requires both: pH6 and flux-dependent NMDAR signaling. Plasma membrane (PM) NMDAR guard a non-random distribution relative to the ER and mitochondria. Also, NMDA/pH6 induces ER stress, endocytosis, PM electrical capacitance reduction, mitochondria-ER, and -nuclear contacts. Strikingly, it also produces the formation of PM invaginations near mitochondria along with structures referred to here as PM-mitochondrial bridges (PM-m-br). These and earlier data strongly suggest PM-mitochondria communication. As proof of the concept of mass transfer, we found that NMDA/pH6 provoked mitochondria labeling by the PM dye FM-4-64FX. NMDA/pH6 caused PM depolarization, cell acidification, and Ca²⁺ release from most mitochondria. Finally, the MCU and microtubules were not involved in mΔΨ depletion, while actin cytoskeleton was partially involved. These findings demonstrate that NMDAR has concomitant flux-independent and flux-dependent actions in cultured astrocytes.

Protection of melatonin against acidosis-induced neuronal injuries

Acidosis, a common feature of cerebral ischaemia and hypoxia, plays a key role in these pathological processes by aggravating the ischaemic and hypoxic injuries. To explore the mechanisms, in this research, we cultured primary neurons in an acidic environment (potential of hydrogen [pH]6.2, 24 hours) to mimic the acidosis. By proteomic analysis, 69 differentially expressed proteins in the acidic neurons were found, mainly related to stress and cell death, synaptic plasticity and gene transcription. And, the acidotic neurons developed obvious alterations including increased neuronal death, reduced dendritic length and complexity, reduced synaptic proteins, tau hyperphosphorylation, endoplasmic reticulum (ER) stress activation, abnormal lysosome‐related signals, imbalanced oxidative stress/anti‐oxidative stress and decreased Golgi matrix proteins. Then, melatonin (1 × 10⁻⁴ mol/L) was used to pre‐treat the cultured primary neurons before acidic treatment (pH6.2). The results showed that melatonin partially reversed the acidosis‐induced neuronal death, abnormal dendritic complexity, reductions of synaptic proteins, tau hyperphosphorylation and imbalance of kinase/phosphatase. In addition, acidosis related the activations of glycogen synthase kinase‐3β and nuclear factor‐κB signals, ER stress and Golgi stress, and the abnormal autophagy‐lysosome signals were completely reversed by melatonin. These data indicate that melatonin is beneficial for neurons against acidosis‐induced injuries.

Activation of pH-Sensing Receptor OGR1 (GPR68) Induces ER Stress Via the IRE1α/JNK Pathway in an Intestinal Epithelial Cell Model

Proton-sensing ovarian cancer G-protein coupled receptor (OGR1) plays an important role in pH homeostasis. Acidosis occurs at sites of intestinal inflammation and can induce endoplasmic reticulum (ER) stress and the unfolded protein response (UPR), an evolutionary mechanism that enables cells to cope with stressful conditions. ER stress activates autophagy, and both play important roles in gut homeostasis and contribute to the pathogenesis of inflammatory bowel disease (IBD). Using a human intestinal epithelial cell model, we investigated whether our previously observed protective effects of OGR1 deficiency in experimental colitis are associated with a differential regulation of ER stress, the UPR and autophagy. Caco-2 cells stably overexpressing OGR1 were subjected to an acidic pH shift. pH-dependent OGR1-mediated signalling led to a significant upregulation in the ER stress markers, binding immunoglobulin protein (BiP) and phospho-inositol required 1α (IRE1α), which was reversed by a novel OGR1 inhibitor and a c-Jun N-terminal kinase (JNK) inhibitor. Proton-activated OGR1-mediated signalling failed to induce apoptosis, but triggered accumulation of total microtubule-associated protein 1 A/1B-light chain 3, suggesting blockage of late stage autophagy. Our results show novel functions for OGR1 in the regulation of ER stress through the IRE1α-JNK signalling pathway, as well as blockage of autophagosomal degradation. OGR1 inhibition might represent a novel therapeutic approach in IBD.

Inhibiting Caspase-12 Mediated Inflammasome Activation protects against Oxygen-Glucose Deprivation Injury in Primary Astrocytes

Stroke is one of the leading causes of death worldwide. Accumulating evidence suggests that NLRP3 inflammasome activation plays an important role in ischemic stroke injury. However, the existence of the NLRP3 inflammasome in astrocytes remains controversial. In this study, we demonstrated the presence of the NLRP3 inflammasome in primary mouse astrocytes and investigated the role of caspase-12 in NLRP3 inflammasome activation and cell injury in an in vitro astrocyte oxygen-glucose deprivation (OGD) model. Astrocytes exposed to 2, 3, and 4 h of OGD exhibited increased cell injury and apoptosis, and the protein levels of caspase-12, cleaved caspase-3, NLRP3 inflammasome components, and IL-1β were also significantly elevated. Interestingly, pretreatment with the caspase-12-specific inhibitor Z-ATAD-FMK attenuated cell injury and apoptosis and decreased the levels of NLRP3, caspase-1, IL-1β, and cleaved caspase-3 in the OGD group. In conclusion, Z-ATAD-FMK protected astrocytes against OGD-induced cell death and inhibited NLPR3-inflammasome activation. Our results indicate that caspase-12 and its potential regulation of NLRP3 inflammasome activation might be a promising target for treatment of ischemic stroke.

Sirtuin 1 alleviates endoplasmic reticulum stress-mediated apoptosis of intestinal epithelial cells in ulcerative colitis

BACKGROUND

Sirtuin 1 (SIRT1) is a nicotinamide adenine dinucleotide (NAD⁺)-dependent protein deacetylase that is involved in various diseases, including cancers, metabolic diseases, and inflammation-associated diseases. However, the role of SIRT1 in ulcerative colitis (UC) is still confusing.

AIM

To investigate the role of SIRT1 in intestinal epithelial cells (IECs) in UC and further explore the underlying mechanisms.

METHODS

We developed a coculture model using macrophages and Caco-2 cells. After treatment with the SIRT1 activator SRT1720 or inhibitor nicotinamide (NAM), the expression of occludin and zona occludens 1 (ZO-1) was assessed by Western blot analysis. Annexin V-APC/7-AAD assays were performed to evaluate Caco-2 apoptosis. Dextran sodium sulfate (DSS)-induced colitis mice were exposed to SRT1720 or NAM for 7 d. Transferase-mediated dUTP nick-end labeling (TUNEL) assays were conducted to assess apoptosis in colon tissues. The expression levels of glucose-regulated protein 78 (GRP78), CCAAT/enhancer-binding protein homologous protein (CHOP), caspase-12, caspase-9, and caspase-3 in Caco-2 cells and the colon tissues of treated mice were examined by quantitative real-time PCR and Western blot.

RESULTS

SRT1720 treatment increased the protein levels of occludin and ZO-1 and inhibited Caco-2 apoptosis, whereas NAM administration caused the opposite effects. DSS-induced colitis mice treated with SRT1720 had a lower disease activity index (P < 0.01), histological score (P < 0.001), inflammatory cytokine levels (P < 0.01), and apoptotic cell rate (P < 0.01), while exposure to NAM caused the opposite effects. Moreover, SIRT1 activation reduced the expression levels of GRP78, CHOP, cleaved caspase-12, cleaved caspase-9, and cleaved caspase-3 in Caco-2 cells and the colon tissues of treated mice.

CONCLUSION

SIRT1 activation reduces apoptosis of IECs via the suppression of endoplasmic reticulum stress-mediated apoptosis-associated molecules CHOP and caspase-12. SIRT1 activation may be a potential therapeutic strategy for UC.

Mild Acidosis Protects Neurons during Oxygen-Glucose Deprivation by Reducing Loss of Mitochondrial Respiration

Brain ischemia is often accompanied by brain acidosis and this acidosis can affect ischemic neuronal injury. Ischemic neuronal injury is initiated by a decrease in ATP production which mainly relies on mitochondrial oxidative phosphorylation. Ischemia often causes mitochondrial dysfunction, and acidosis has been found to affect mitochondrial function, suggesting that acidosis accompanying ischemia may influence neurons by targeting mitochondrial metabolism. However, the effects of acidosis on mitochondrial energy metabolism during ischemia lacks thorough investigation. Here, we found that mild acidosis significantly reduced neuronal death possibly by slowing the process of ATP deprivation during oxygen-glucose deprivation (OGD), an in vitro ischemic model. The maintaining of neuronal ATP depended on protecting mitochondrial ATP production. Further investigation of mitochondrial function revealed that mild acidosis alleviated OGD-induced collapse of mitochondrial membrane potentials as well as damage to respiratory function, at least in part by reducing impacts on complex I and II activities. Inhibition of complex I activity aggravated neuronal death, which suggests that the contribution of mild acidosis to maintaining complex I activity promoted neuronal survival during OGD. Our findings reveal maintaining mitochondrial respiration as a new possible protective mechanism of mild acidosis during ischemia, on neurons.

Fate of Astrocytes in The Gerbil Hippocampus After Transient Global Cerebral Ischemia

Neuronal death and reactive gliosis are major features of brain tissue damage following transient global cerebral ischemia (tgCI). This study investigated long-term changes in neuronal death and astrogliosis in the gerbil hippocampus for 180 days after 5 min of tgCI. A massive loss of pyramidal neurons was found in the hippocampal CA1 area (CA1) area between 5 and 30 days after tgCI by Fluoro-Jade B (FJB, a marker for neuronal degeneration) histofluorescence staining, but pyramidal neurons in the CA2/3 area did not die. The reaction of astrocytes (astrogliosis) was examined by glial fibrillary acidic protein (GFAP) immunohistochemistry. Morphological change or degeneration (death) of the astrocytes was found in the CA1 area after tgCI, but, in the CA2/3 area, astrogliosis was hardly shown. GFAP immunoreactive astrocytes in the CA1 area was significantly increased in number with time and peaked at 30 days after tgCI, and they began to be degenerated or dead from 40 days after tgCI. The effect was examined by double immunofluorescence staining for FJB and GFAP. The number of FJB/GFAP⁺ cells (degenerating astrocytes) was gradually increased with time after tgCI. At 180 days after tgCI, FJB/GFAP⁺ cells were significantly decreased, but FJB⁺ cells (dead astrocytes) were significantly increased. In brief, 5 min of tgCI induced a progressive degeneration of CA1 pyramidal neurons from 5 until 30 days with an increase of reactive astrocytes, and, thereafter, astrocytes were degenerated with time and dead at later times. This phenomenon might be shown due to the death of neurons.

Leptin protects placental cells from apoptosis induced by acidic stress

Development of the human placenta is critical for a successful pregnancy. The placenta allows the exchange of oxygen and carbon dioxide and is crucial to manage acid-base balance within a narrow pH. It is known that low pH levels are a risk of apoptosis in several tissues. However, there has been little discussion about the effect of acidic stress in the placenta. Leptin is produced by the placenta with a trophic autocrine effect. Previous results of our group have demonstrated that leptin prevents apoptosis of trophoblast cells under different stress conditions such as serum deprivation and hyperthermia. The purpose of the present work is to evaluate acidic stress consequences in trophoblast explant survival and to determine leptin action in these conditions. For this objective, term human trophoblast explants were cultured at physiological pH (pH 7.4) and at acidic pH (pH 6.8) in the presence or absence of leptin. Western blot assays were performed to study the abundance of active caspase-3 and the p89 fragment of PARP-1. Pro-apoptotic and pro-survival members of Bcl-2 family, as Bax, t-Bid, and Bcl-2, were studied. Moreover, p53 pathway was also evaluated including Mdm-2, the main p53 regulator. Active caspase-3 and cleaved PARP-1 abundances were increased at low extracellular pH. Moreover, t-Bid levels were also augmented as well as p53 expression and phosphorylation on S46. Leptin treatment prevents the consequences of acidosis, decreasing p53 expression and increasing Mdm-2 expression. In summary, this work demonstrated for first time that low pH induces apoptosis of human trophoblast explants involving apoptotic intrinsic pathway, and leptin impairs this effect.

Augmentation of poly(ADP-ribose) polymerase-dependent neuronal cell death by acidosis

Tissue acidosis is a key component of cerebral ischemic injury, but its influence on cell death signaling pathways is not well defined. One such pathway is parthanatos, in which oxidative damage to DNA results in activation of poly(ADP-ribose) polymerase and generation of poly(ADP-ribose) polymers that trigger release of mitochondrial apoptosis-inducing factor. In primary neuronal cultures, we first investigated whether acidosis per sé is capable of augmenting parthanatos signaling initiated pharmacologically with the DNA alkylating agent, N-methyl- N'-nitro- N-nitrosoguanidine. Exposure of neurons to medium at pH 6.2 for 4 h after N-methyl- N'-nitro- N-nitrosoguanidine washout increased intracellular calcium and augmented the N-methyl- N'-nitro- N-nitrosoguanidine-evoked increase in poly(ADP-ribose) polymers, nuclear apoptosis-inducing factor , and cell death. The augmented nuclear apoptosis-inducing factor and cell death were blocked by the acid-sensitive ion channel-1a inhibitor, psalmotoxin. In vivo, acute hyperglycemia during transient focal cerebral ischemia augmented tissue acidosis, poly(ADP-ribose) polymers formation, and nuclear apoptosis-inducing factor , which was attenuated by a poly(ADP-ribose) polymerase inhibitor. Infarct volume from hyperglycemic ischemia was decreased in poly(ADP-ribose) polymerase 1-null mice. Collectively, these results demonstrate that acidosis can directly amplify neuronal parthanatos in the absence of ischemia through acid-sensitive ion channel-1a . The results further support parthanatos as one of the mechanisms by which ischemia-associated tissue acidosis augments cell death.

Inhibition of autophagy blocks cathepsins-tBid-mitochondrial apoptotic signaling pathway via stabilization of lysosomal membrane in ischemic astrocytes

Our previous study and others have demonstrated that autophagy is activated in ischemic astrocytes and contributes to astrocytic cell death. However, the mechanisms of ischemia-induced autophagy remain largely unknown. In this study, we established a rat's model of permanent middle cerebral artery occlusion (pMCAO) and an in vitro oxygen and glucose deprivation (OGD) model. Autophagy was inhibited by either pharmacological treatment with 3-methyladenine (3-MA) and wortmannin (Wort) or genetic treatment with knockdown of Atg5 in primary cultured astrocytes and knockout of Atg5 in mouse embryonic fibroblast (MEF) cells, respectively. We found that pharmacological or genetic inhibition of autophagy reversed pMCAO or OGD-induced increase in LC3-II, active cathepsin B and L, tBid, active caspase-3 and cytoplastic cytochrome c (Cyt-c), and suppressed the injury-induced reduction in mitochondrial Cyt-c in ischemic cortex, in injured astrocytes and MEF cells. Immunofluorescence analysis showed that 3-MA or Wort treatment reversed OGD-induced release of cathepsin B and L from the lysosome to the cytoplasm and activation of caspase-3 in the astrocytes. Furthermore, treatment of 3-MA or Wort decreased OGD-induced increase in lysosomal membrane permeability and enhanced OGD-induced upregulation of lysosomal heat shock protein 70.1B (Hsp70.1B) in astrocytes. Inhibition of autophagy by 3-MA or Wort reduced infarction volume in rats and protected OGD-induced astrocytic cell injury. A non-selective caspase inhibitor z-VAD-fmk or a specific caspase-3 inhibitor Q-DEVD-OPh also rescued OGD-induced astrocytic cell injury. In conclusion, our presenting data suggest that inhibition of autophagy blocks cathepsins–tBid–mitochondrial apoptotic signaling pathway via stabilization of lysosomal membranes, possibly due to upregulation of the lysosomal Hsp70.1B in ischemic astrocytes.

Bicarbonate sensing in mouse cortical astrocytes during extracellular acid/base disturbances

KEY POINTS

The present study suggests that the electrogenic sodium-bicarbonate cotransporter, NBCe1, supported by carbonic anhydrase II, CAII, provides an efficient mechanism of bicarbonate sensing in cortical astrocytes. This mechanism is proposed to play a major role in setting the pH_i responses to extracellular acid/base challenges in astrocytes. A decrease in extracellular [HCO₃^- ] during isocapnic acidosis and isohydric hypocapnia, or an increase in intracellular [HCO₃^- ] during hypercapnic acidosis, was effectively sensed by NBCe1, which carried bicarbonate out of the cells under these conditions, and caused an acidification and sodium fall in WT astrocytes, but not in NBCe1-knockout astrocytes. Isocapnic acidosis, hypercapnic acidosis and isohydric hypocapnia evoked inward currents in NBCe1- and CAII-expressing Xenopus laevis oocytes, but not in native oocytes, suggesting that NBCe1 operates in the outwardly directed mode under these conditions consistent with our findings in astrocytes. We propose that bicarbonate sensing of astrocytes may have functional significance during extracellular acid/base disturbances in the brain, as it not only alters intracellular pH/[HCO₃^- ]-dependent functions of astrocytes, but also modulates the extracellular pH/[HCO₃^- ] in brain tissue.

ABSTRACT

Extracellular acid/base status of the mammalian brain undergoes dynamic changes during many physiological and pathological events. Although intracellular pH (pH_i ) of astrocytes responds to extracellular acid/base changes, the mechanisms mediating these changes have remained unresolved. We have previously shown that the electrogenic sodium-bicarbonate cotransporter, NBCe1, is a high-affinity bicarbonate carrier in cortical astrocytes. In the present study, we investigated whether NBCe1 plays a role in bicarbonate sensing in astrocytes, and in determining the pH_i responses to extracellular acid/base challenges. We measured changes in intracellular H⁺ and Na⁺ in astrocytes from wild-type (WT) and from NBCe1-knockout (KO) mice, using ion-selective dyes, during isocapnic acidosis, hypercapnic acidosis and hypocapnia. We also analysed NBCe1-mediated membrane currents in Xenopus laevis oocytes under similar conditions. Comparing WT and NBCe1-KO astrocytes, we could dissect the contribution of NBCe1, of diffusion of CO₂ across the cell membrane and, after blocking carbonic anhydrase (CA) activity with ethoxyzolamide, of the role of CA, for the amplitude and rate of acid/base fluxes. Our results suggest that NBCe1 transport activity in astrocytes, supported by CA activity, renders astrocytes bicarbonate sensors in the mouse cortex. NBCe1 carried bicarbonate into and out of the cell by sensing the variations of transmembrane [HCO₃^- ], irrespective of the changes in intra- and extracellular pH, and played a major role in setting pH_i responses to the extracellular acid/base challenges. We propose that bicarbonate sensing of astrocytes may have potential functional significance during extracellular acid/base alterations in the brain.

Acidosis Activates Endoplasmic Reticulum Stress Pathways through GPR4 in Human Vascular Endothelial Cells

Acidosis commonly exists in the tissue microenvironment of various pathophysiological conditions such as tumors, inflammation, ischemia, metabolic disease, and respiratory disease. For instance, the tumor microenvironment is characterized by acidosis and hypoxia due to tumor heterogeneity, aerobic glycolysis (the "Warburg effect"), and the defective vasculature that cannot efficiently deliver oxygen and nutrients or remove metabolic acid byproduct. How the acidic microenvironment affects the function of blood vessels, however, is not well defined. GPR4 (G protein-coupled receptor 4) is a member of the proton-sensing G protein-coupled receptors and it has high expression in endothelial cells (ECs). We have previously reported that acidosis induces a broad inflammatory response in ECs. Acidosis also increases the expression of several endoplasmic reticulum (ER) stress response genes such as CHOP (C/EBP homologous protein) and ATF3 (activating transcription factor 3). In the current study, we have examined acidosis/GPR4- induced ER stress pathways in human umbilical vein endothelial cells (HUVEC) and other types of ECs. All three arms of the ER stress/unfolded protein response (UPR) pathways were activated by acidosis in ECs as an increased expression of phosphorylated eIF2α (eukaryotic initiation factor 2α), phosphorylated IRE1α (inositol-requiring enzyme 1α), and cleaved ATF6 upon acidic pH treatment was observed. The expression of other downstream mediators of the UPR, such as ATF4, ATF3, and spliced XBP-1 (X box-binding protein 1), was also induced by acidosis. Through genetic and pharmacological approaches to modulate the expression level or activity of GPR4 in HUVEC, we found that GPR4 plays an important role in mediating the ER stress response induced by acidosis. As ER stress/UPR can cause inflammation and cell apoptosis, acidosis/GPR4-induced ER stress pathways in ECs may regulate vascular growth and inflammatory response in the acidic microenvironment.

Perturbation of Akt Signaling, Mitochondrial Potential, and ADP/ATP Ratio in Acidosis-Challenged Rat Cortical Astrocytes

Cells switch to anaerobic glycolysis when there is a lack of oxygen during brain ischemia. Extracellular pH thus drops and such acidosis causes neuronal cell death. The fate of astrocytes, mechanical, and functional partners of neurons, in acidosis is less studied. In this report, we investigated the signaling in acidosis-challenged rat cortical astrocytes and whether these signals were related to mitochondrial dysfunction and cell death. Exposure to acidic pH (6.8, 6.0) caused Ca²⁺ release and influx, p38 MAPK activation, and Akt inhibition. Mitochondrial membrane potential was hyperpolarized after astrocytes were exposed to acidic pH as soon as 1 h and lasted for 24 h. Such mitochondrial hyperpolarization was prevented by SC79 (an Akt activator) but not by SB203580 (a p38 inhibitor) nor by cytosolic Ca²⁺ chelation by BAPTA, suggesting that only the perturbation in Akt signaling was causally related to mitochondrial hyperpolarization. SC79, SB203580, and BAPTA did not prevent acidic pH-induced cell death. Acidic pH suppressed ROS production, thus ruling out the role of ROS in cytotoxicity. Interestingly, pH 6.8 caused an increase in ADP/ATP ratio and apoptosis; pH 6.0 caused a further increase in ADP/ATP ratio and necrosis. Therefore, astrocyte cell death in acidosis did not result from mitochondrial potential collapse; in case of acidosis at pH 6.0, necrosis might partly result from mitochondrial hyperpolarization and subsequent suppressed ATP production. J. Cell. Biochem. 118: 1108-1117, 2017. © 2016 Wiley Periodicals, Inc.

Early Administration of Glutamine Protects Cardiomyocytes from Post-Cardiac Arrest Acidosis

Postcardiac arrest acidosis can decrease survival. Effective medications without adverse side effects are still not well characterized. We aimed to analyze whether early administration of glutamine could improve survival and protect cardiomyocytes from postcardiac arrest acidosis using animal and cell models. Forty Wistar rats with postcardiac arrest acidosis (blood pH < 7.2) were included. They were divided into study (500 mg/kg L-alanyl-L-glutamine, n = 20) and control (normal saline, n = 20) groups. Each of the rats received resuscitation. The outcomes were compared between the two groups. In addition, cardiomyocytes derived from human induced pluripotent stem cells were exposed to HBSS with different pH levels (7.3 or 6.5) or to culture medium (control). Apoptosis-related markers and beating function were analyzed. We found that the duration of survival was significantly longer in the study group (p < 0.05). In addition, in pH 6.5 or pH 7.3 HBSS buffer, the expression levels of cell stress (p53) and apoptosis (caspase-3, Bcl-xL) markers were significantly lower in cardiomyocytes treated with 50 mM L-glutamine than those without L-glutamine (RT-PCR). L-glutamine also increased the beating function of cardiomyocytes, especially at the lower pH level (6.5). More importantly, glutamine decreased cardiomyocyte apoptosis and increased these cells' beating function at a low pH level.

Placental endoplasmic reticulum stress in gestational diabetes: the potential for therapeutic intervention with chemical chaperones and antioxidants

AIMS/HYPOTHESIS

The aim of this work was to determine whether placental endoplasmic reticulum (ER) stress may contribute to the pathophysiology of gestational diabetes mellitus (GDM) and to test the efficacy of chemical chaperones and antioxidant vitamins in ameliorating that stress in a trophoblast-like cell line in vitro.

METHODS

Placental samples were obtained from women suffering from GDM and from normoglycaemic controls and were frozen immediately. Women with GDM had 2 h serum glucose levels > 9.0 mmol/l following a 75 g oral glucose tolerance test and were treated with diet and insulin when necessary. Western blotting was used to assess markers of ER stress. To test the effects of hyperglycaemia on the generation of ER stress, a new trophoblast-like cell line, BeWo-NG, was generated by culturing in a physiological glucose concentration of 5.5 mmol/l (over 20 passages) before challenging with 10 or 20 mmol/l glucose.

RESULTS

All GDM patients were well-controlled (HbA1c 5.86 ± 0.55% or 40.64 ± 5.85 mmol/mol, n = 11). Low-grade ER stress was observed in the placental samples, with dilation of ER cisternae and increased phosphorylation of eukaryotic initiation factor 2 subunit α. Challenge of BeWo-NG with high glucose activated the same pathways, but this was as a result of acidosis of the culture medium rather than the glucose concentration per se. Addition of chemical chaperones 4-phenylbutyrate and tauroursodeoxycholic acid and vitamins C and E ameliorated the ER stress.

CONCLUSIONS/INTERPRETATION

This is the first report of placental ER stress in GDM patients. Chemical chaperones and antioxidant vitamins represent potential therapeutic interventions for GDM.

Mesenchymal stem cell proliferation and mineralization but not osteogenic differentiation are strongly affected by extracellular pH

Osteomyelitis is a serious complication in oral and maxillofacial surgery affecting bone healing. Bone remodeling is not only controlled by cellular components but also by ionic and molecular composition of the extracellular fluids in which calcium phosphate salts are precipitated in a pH dependent manner.

OBJECTIVE

To determine the effect of pH on self-renewal, osteogenic differentiation and matrix mineralization of mesenchymal stem cells (MSCs).

METHODS

We selected three different pH values; acidic (6.3, 6.7), physiological (7.0-8.0) and severe alkaline (8.5). MSCs were cultured at different pH ranges, cell viability measured by WST-1, apoptosis detected by JC-1, senescence was analyzed by β-galactosidase whereas mineralization was detected by Alizarin Red and osteogenic differentiation analyzed by Real-time PCR.

RESULTS

Self-renewal was affected by pH as well as matrix mineralization in which pH other than physiologic inhibited the deposition of extracellular matrix but did not affect MSCs differentiation as osteoblast markers were upregulated. The expression of osteocalcin and alkaline phosphatase activity was upregulated whereas osteopontin was downregulated under acidic pH.

CONCLUSION

pH affected MSCs self-renewal and mineralization without influencing osteogenic differentiation. Thus, future therapies, based on shifting acid-base balance toward the alkaline direction might be beneficial for prevention or treatment of osteomyelitis.

Crosstalk Between Endoplasmic Reticulum Stress, Oxidative Stress, and Autophagy: Potential Therapeutic Targets for Acute CNS Injuries

Endoplasmic reticulum (ER) stress induces a variety of neuronal cell death pathways that play a critical role in the pathophysiology of stroke. ER stress occurs when unfolded/misfolded proteins accumulate and the folding capacity of ER chaperones exceeds the capacity of ER lumen to facilitate their disposal. As a consequence, a complex set of signaling pathways will be induced that transmit from ER to cytosol and nucleus to compensate damage and to restore the normal cellular homeostasis, collectively known as unfolded protein response (UPR). However, failure of UPR due to severe or prolonged stress leads to cell death. Following acute CNS injuries, chronic disturbances in protein folding and oxidative stress prolong ER stress leading to sustained ER dysfunction and neuronal cell death. While ER stress responses have been well studied after stroke, there is an emerging need to study the association of ER stress with other cell pathways that exacerbate neuronal death after an injury. In this review, we summarize the current understanding of the role for ER stress in acute brain injuries, highlighting the diverse molecular mechanisms associated with ER stress and its relation to oxidative stress and autophagy. We also discussed the existing and developing therapeutic options aimed to reduce ER stress to protect the CNS after acute injuries.

Role of soluble adenylyl cyclase in cell death and growth

cAMP signaling is an evolutionarily conserved intracellular communication system controlling numerous cellular functions. Until recently, transmembrane adenylyl cyclase (tmAC) was considered the major source for cAMP in the cell, and the role of cAMP signaling was therefore attributed exclusively to the activity of this family of enzymes. However, increasing evidence demonstrates the role of an alternative, intracellular source of cAMP produced by type 10 soluble adenylyl cyclase (sAC). In contrast to tmAC, sAC produces cAMP in various intracellular microdomains close to specific cAMP targets, e.g., in nucleus and mitochondria. Ongoing research demonstrates involvement of sAC in diverse physiological and pathological processes. The present review is focused on the role of cAMP signaling, particularly that of sAC, in cell death and growth. Although the contributions of sAC to the regulation of these cellular functions have only recently been discovered, current data suggest that sAC plays key roles in mitochondrial bioenergetics and the mitochondrial apoptosis pathway, as well as cell proliferation and development. Furthermore, recent reports suggest the importance of sAC in several pathologies associated with apoptosis as well as in oncogenesis. This article is part of a Special Issue entitled: The role of soluble adenylyl cyclase in health and disease.

Glucose-regulated protein 78 (Grp78) confers chemoresistance to tumor endothelial cells under acidic stress

OBJECTIVES

This study was designed to investigate the activation of the unfolded protein response (UPR) in tumor associated endothelial cells (TECs) and its association with chemoresistance during acidic pH stress.

MATERIALS AND METHODS

Endothelial cells from human oral squamous cell carcinomas (OSCC) were excised by laser capture microdissection (LCM) followed by analysis of UPR markers (Grp78, ATF4 and CHOP) using quantitative PCR. Grp78 expression was also determined by immunostaining. Acidic stress was induced in primary human dermal microvascular endothelial cells (HDMECs) by treatment with conditioned medium (CM) from tumor cells grown under hypoxic conditions or by adjusting medium pH to 6.4 or 7.0 using lactic acid or hydrochloric acid (HCl). HDMEC resistance to the anti-angiogenic drug Sunitinib was assessed with SRB assay.

RESULTS

UPR markers, Grp78, ATF4 and CHOP were significantly upregulated in TECs from OSCC compared to HDMECs. HDMECs cultured in acidic CM (pH 6.0-6.4) showed increased expression of the UPR markers. However, severe acidosis led to marked cell death in HDMECs. Alternatively, HDMECs were able to adapt when exposed to chronic acidosis at pH 7.0 for 7 days, with concomittant increase in Grp78 expression. Chronic acidosis also confers drug resistance to HDMECs against Sunitinib. Knockdown of Grp78 using shRNA resensitizes HDMECs to drug treatment.

CONCLUSIONS

UPR induction in ECs under acidic pH conditions is related to chemoresistance and may contribute to therapeutic failures in response to chemotherapy. Targeting Grp78, the key component of the UPR pathway, may provide a promising approach to overcome ECs resistance in cancer therapy.

Inhibition of cysteine cathepsin B and L activation in astrocytes contributes to neuroprotection against cerebral ischemia via blocking the tBid-mitochondrial apoptotic signaling pathway

The roles of cathepsins in the ischemic astrocytic injury remain unclear. Here, we test the hypothesis that activation of cathepsin B and L contributes to the ischemic astrocyte injury via the tBid-mitochondrial apoptotic signaling pathways. In the rat models of pMCAO, CA-074Me or Clik148, a selective inhibitor of cathepsin B or cathepsin L, reduced the infarct volume, improved the neurological deficits and increased the MAP2 and GFAP levels. In OGD-induced astrocyte injury, CA-074Me or Clik148 decreased the LDH leakage and increased the GFAP levels. In the ischemic cortex or OGD-induced astrocytes injury, Clik148 or CA-074Me reversed pMCAO or OGD-induced increase in active cathepsin L or cathepsin B at 3 h or 6 h, increase in tBid, reduction in mitochondrial cytochrome-c (Cyt-c) and increase in cytoplastic Cyt-c and active caspase-3 at 12-24 h of the late stage of pMCAO or OGD. CA-074Me or Clik148 also reduced cytosolic and mitochondrial tBid, increased mitochondrial Cyt-c and decreased cytoplastic Cyt-c and active caspase-3 at 6 h of the early stage of Bid activation. CA-074Me or Clik148 blocked the pMCAO-induced release of cathepsin B or L from the lysosomes into the cytoplasm and activation of caspase-3 in ischemic astrocytes at 12 h after ischemia. Concurrent inhibition of cathepsin B and cathepsin L provided better protection on the OGD-induced astrocytic apoptosis than obtained with separate use of each inhibitor. These results suggest that inhibition of the cysteine cathepsin B and cathepsin L activation in ischemic astrocytes contributes to neuroprotection via blocking the tBid-mitochondrial apoptotic signaling pathway.

Intracellular pH reduction prevents excitotoxic and ischemic neuronal death by inhibiting NADPH oxidase

Sustained activation of N-methyl-d-aspartate (NMDA) -type glutamate receptors leads to excitotoxic neuronal death in stroke, brain trauma, and neurodegenerative disorders. Superoxide production by NADPH oxidase is a requisite event in the process leading from NMDA receptor activation to excitotoxic death. NADPH oxidase generates intracellular H(+) along with extracellular superoxide, and the intracellular H(+) must be released or neutralized to permit continued NADPH oxidase function. In cultured neurons, NMDA-induced superoxide production and neuronal death were prevented by intracellular acidification by as little as 0.2 pH units, induced by either lowered medium pH or by inhibiting Na(+)/H(+) exchange. In mouse brain, superoxide production induced by NMDA injections or ischemia-reperfusion was likewise prevented by inhibiting Na(+)/H(+) exchange and by reduced expression of the Na(+)/H(+) exchanger-1 (NHE1). Neuronal intracellular pH and neuronal Na(+)/H(+) exchange are thus potent regulators of excitotoxic superoxide production. These findings identify a mechanism by which cell metabolism can influence coupling between NMDA receptor activation and superoxide production.

The impact of severe acidemia on neurologic outcome of cardiac arrest survivors undergoing therapeutic hypothermia

INTRODUCTION

Therapeutic Hypothermia (TH) has become a standard of care in improving neurological outcomes in cardiac arrest (CA) survivors. Previous studies have defined severe acidemia as plasma pH<7.20. We investigated the influence of severe acidemia at the time of initiation of TH on neurological outcome in CA survivors.

METHODS

A retrospective analysis was performed on 196 consecutive CA survivors (out-of-hospital CA and in-hospital CA) who underwent TH with endovascular cooling between January 2007 and October 2012. Arterial blood gas drawn prior to initiation of TH was utilized to measure pH in all patients. Shockable and non-shockable CA patients were divided into two sub-groups based on pH (pH<7.2 and pH≥7.2). The primary end-point was measured using the Pittsburgh Cerebral Performance Category (CPC) scale prior to discharge from the hospital: good (CPC 1 and 2) and poor (CPC 3 to 5) neurologic outcome.

RESULTS

Sixty-two percent of shockable CA patients with pH≥7.20 had good neurological outcome as compared to 34% patients with pH<7.20. Shockable CA patients with pH≥7.20 were 3.3 times more likely to have better neurological outcome when compared to those with pH <7.20 [p=0.013, OR 3.3, 95% CI (1.28-8.45)]. In comparison, non-shockable CA patients with p≥7.20 did not have a significantly different neurological outcome as compared to those with pH<7.20 [p=0.97, OR 1.02, 95% CI (0.31-3.3)].

CONCLUSION

Presence of severe acidemia at initiation of TH in shockable CA survivors is significantly associated with poor neurological outcomes. This effect was not observed in the non-shockable CA survivors.

Bax Inhibitor-1-mediated Ca2+ leak is decreased by cytosolic acidosis

Bax Inhibitor-1 (BI-1) is an evolutionarily conserved six-transmembrane domain endoplasmic reticulum (ER)-localized protein that protects against ER stress-induced apoptotic cell death. This function is closely connected to its ability to lower steady-state ER Ca2+ levels. Recently, we elucidated BI-1's Ca(2+)-channel pore in the C-terminal part of the protein and identified the critical amino acids of its pore. Based on these insights, a Ca(2+)-channel pore-dead mutant BI-1 (BI-1(D213R)) was developed. We determined whether BI-1 behaves as a bona fide H+/Ca2+ antiporter or as an ER Ca(2+)-leak channel by investigating the effect of pH on unidirectional Ca(2+)-efflux rates. At pH 6.8, wild-type BI-1 expression in BI-1(-/-) cells increased the ER Ca(2+)-leak rate, correlating with its localization in the ER compartment. In contrast, BI-1(D231R) expression in BI-1(-/-), despite its ER localization, did not increase the ER Ca(2+)-leak rate. However, at pH < 6.8, the BI-1-mediated ER Ca2+ leak was blocked. Finally, a peptide representing the Ca(2+)-channel pore of BI-1 promoting Ca2+ flux from the ER was used. Lowering the pH from 6.8 to 6.0 completely abolished the ability of the BI-1 peptide to mediate Ca2+ flux from the ER. We propose that this pH dependence is due to two aspartic acid residues critical for the function of the Ca(2+)-channel pore and located in the ER membrane-dipping domain, which facilitates the protonation of these residues.

Metabolic signatures of amyotrophic lateral sclerosis reveal insights into disease pathogenesis

Metabolic dysfunction is an important modulator of disease course in amyotrophic lateral sclerosis (ALS). We report here that a familial mouse model (transgenic mice over-expressing the G93A mutation of the Cu/Zn superoxide dismutase 1 gene) of ALS enters a progressive state of acidosis that is associated with several metabolic (hormonal) alternations that favor lipolysis. Extensive investigation of the major determinants of H(+) concentration (i.e., the strong ion difference and the strong ion gap) suggests that acidosis is also due in part to the presence of an unknown anion. Consistent with a compensatory response to avert pathological acidosis, ALS mice harbor increased accumulation of glycogen in CNS and visceral tissues. The altered glycogen is associated with fluctuations in lysosomal and neutral α-glucosidase activities. Disease-related changes in glycogen, glucose, and α-glucosidase activity are also found in spinal cord tissue samples of autopsied patients with ALS. Collectively, these data provide insights into the pathogenesis of ALS as well as potential targets for drug development.

The unfolded protein response triggered by environmental factors

Endoplasmic reticulum (ER) stress and consequent unfolded protein response (UPR) are involved in a diverse range of pathologies including ischemic diseases, neurodegenerative disorders, and metabolic diseases, such as diabetes mellitus. The UPR is also triggered by various environmental factors; e.g., pollutants, infectious pathogens, therapeutic drugs, alcohol, physical stress, and malnutrition. This review summarizes current knowledge on environmental factors that induce ER stress and describes how the UPR is linked to particular pathological states after exposure to environmental triggers.

Pharmacological modulation of the retinal unfolded protein response in Bardet-Biedl syndrome reduces apoptosis and preserves light detection ability

Ciliopathies, a class of rare genetic disorders, present often with retinal degeneration caused by protein transport defects between the inner segment and the outer segment of the photoreceptors. Bardet-Biedl syndrome is one such ciliopathy, genetically heterogeneous with 17 BBS genes identified to date, presenting early onset retinitis pigmentosa. By investigating BBS12-deprived retinal explants and the Bbs12(-/-) murine model, we show that the impaired intraciliary transport results in protein retention in the endoplasmic reticulum. The protein overload activates a proapoptotic unfolded protein response leading to a specific Caspase12-mediated death of the photoreceptors. Having identified a therapeutic window in the early postnatal retinal development and through optimized pharmacological modulation of the unfolded protein response, combining three specific compounds, namely valproic acid, guanabenz, and a specific Caspase12 inhibitor, achieved efficient photoreceptor protection, thereby maintaining light detection ability in vivo.

Ectopic ATP synthase blockade suppresses lung adenocarcinoma growth by activating the unfolded protein response

Ectopic expression of the mitochondrial F(1)F(0)-ATP synthase on the plasma membrane has been reported to occur in cancer, but whether it exerts a functional role in this setting remains unclear. Here we show that ectopic ATP synthase and the electron transfer chain exist on the plasma membrane in a punctuated distribution of lung adenocarcinoma cells, where it is critical to support cancer cell proliferation. Applying ATP synthase inhibitor citreoviridin induced cell cycle arrest and inhibited proliferation and anchorage-independent growth of lung cancer cells. Analysis of protein expression profiles after citreoviridin treatment suggested this compound induced the unfolded protein response (UPR) associated with phosphorylation the translation initiation factor 2α (eIF2α), triggering cell growth inhibition. Citreoviridin-enhanced eIF2α phosphorylation could be reversed by siRNA-mediated attenuation of the UPR kinase PKR-like endoplasmic reticulum kinase (PERK) combined with treatment with the antioxidant N-acetylcysteine, establishing that reactive oxygen species (ROS) boost UPR after citreoviridin treatment. Thus, a coordinate elevation of UPR and ROS initiates a positive feedback loop that convergently blocks cell proliferation. Our findings define a molecular function for ectopic ATP synthase at the plasma membrane in lung cancer cells and they prompt further study of its inhibition as a potential therapeutic approach.

Direct protection of neurons and astrocytes by matrine via inhibition of the NF-κB signaling pathway contributes to neuroprotection against focal cerebral ischemia

Matrine (Mat) and oxymatrine are two major alkaloids of the Chinese herb Sophora flavescens Ait. (Leguminosae). Previous study has demonstrated that Mat reduces brain edema induced by focal cerebral ischemia. More recently, oxymatrine has been reported to produce neuroprotective effects against focal cerebral ischemia via inhibiting the activation of NF-κB in the ischemic brain tissue. In the current study, we investigated whether direct protection on neurons and astrocytes via inhibition of NF-κB signaling pathway is associated with Mat's neuroprotective effects against cerebral ischemia. In a model of permanent middle cerebral artery occlusion (pMCAO), Mat (12.5, 25 and 50 mg/kg) reduced the infarction volume and improved the neurological deficits in a dose-dependent manner, administered 10 min, 3h and even 6h following pMCAO. Mat 50 mg/kg also decreased the hemispheric water content. The number of GFAP-positive cells was markedly decreased in the ischemic cortex at 12h after ischemia. In contrast, Mat increased the number of GFAP-positive cells. Mat 50mg/kg has no effect on the cerebral blood flow (CBF). Primary neuron or astrocyte cultures were exposed to a paradigm of ischemic insult by using an oxygen-glucose deprivation (OGD), Mat (50-200 μM) reduced LDH leakage and the number of neuronal and astrocytic apoptosis, and increased the number of MAP2-positive and GFAP-positive cells. Further observations revealed that Mat increased the protein levels of IκBα, and blocked the translocation of NF-κB p65 from the cytosol to the nucleus in the ischemic cortex and injured neurons and astrocytes induced by in vitro OGD. Moreover, Mat could down-regulate NF-κB p65 downstream pro-apoptotic gene p53 and/or c-Myc in the injured neurons and astrocytes induced by OGD. The present findings suggest that Mat, even when administrated 6h after ischemia, has neuroprotective effects against focal cerebral ischemia and directly protects neurons and astrocytes via inhibition of NF-κB signaling pathway, contributing to matrine's neuroprotection against focal cerebral ischemia.

Sodium phenylbutyrate ameliorates focal cerebral ischemic/reperfusion injury associated with comorbid type 2 diabetes by reducing endoplasmic reticulum stress and DNA fragmentation

Endoplasmic reticulum (ER) stress has been postulated to play a crucial role in the pathophysiology of cerebral ischemic/reperfusion (I/R) injury and diabetes. Diabetes is a major risk factor and also common amongst the people who suffer from stroke. In this study, we have investigated the neuroprotective potential of sodium 4-phenylbutyrate (SPB; 30-300mg/kg), a chemical chaperone by targeting ER stress in a rat model of transient focal cerebral ischemia associated with comorbid type 2 diabetes. Intraperitoneal treatment with SPB (100 and 300mg/kg) significantly ameliorated brain I/R damage as evidenced by reduction in cerebral infarct and edema volume. It also significantly improved the functional recovery of various neurobehavioral impairments (neurological deficit score, grip strength and rota rod) evoked by I/R compared with vehicle-treatment. Further, SPB (100mg/kg) significantly reduced the DNA fragmentation as shown by prominent reduction in terminal deoxynucleotidyl transferase-mediated dUTP nick end labeling (TUNEL)-positive cells. This effect was observed concomitantly with significant attenuation in upregulation of 78kDa glucose regulated protein (GRP78), CCAAT/enhancer binding protein homologous protein or growth arrest DNA damage-inducible gene 153 (CHOP/GADD153) and activation of caspase-12, specific markers of ER stress/apoptosis. The neuroprotection observed with SPB was independent of its effect on cerebral blood flow and blood glucose. In conclusion, this study demonstrates the neuroprotective effect of SPB owing to amelioration of ER stress and DNA fragmentation. It also suggest that targeting ER stress might offer a promising therapeutic approach and benefits against ischemic stroke associated with comorbid type 2 diabetes.

The low viability of human CD34+ cells under acidic conditions is improved by exposure to thrombopoietin, stem cell factor, interleukin-3, or increased cyclic adenosine monophosphate levels

BACKGROUND

Transplanted hematopoietic progenitor cells (CD34+) have shown great promise in regenerative medicine. However, the therapeutic potential of transplanted cells is limited by their poor viability. It is well known that the microenvironment in which progenitors reside substantially affects their behavior. Because extracellular acidosis is a common feature of injured tissues or the tumor microenvironment and is a critical regulator of cell survival and activation, we evaluated the impact of acidosis on CD34+ cell biology.

STUDY DESIGN AND METHODS

Apoptosis was evaluated by fluorescence microscopy and binding of annexin V, hypodiploid cells, Bcl-xL expression, active caspase-3, and mitochondrial membrane potential was determined by flow cytometry. Colony-forming units were studied by clonogenic assays, and cell cycle was evaluated by flow cytometry.

RESULTS

Exposure of CD34+ cells to low pH (7.0-6.5) caused intracellular acidification, decreased cell proliferation, and triggered apoptosis via the mitochondrial pathway. Whereas exposure to thrombopoietin (TPO), stem cell factor (SCF), interleukin (IL)-3 or increases in cyclic adenosine monophosphate (cAMP) levels prevented CD34+ cell death induced by acidic conditions, granulocyte-macrophage-colony-stimulating factor, FMS-like tyrosine kinase 3-ligand, erythropoietin, and vascular endothelial growth factor had no effect. Despite their cytoprotective effect, CD34+ cell expansion triggered by TPO, SCF, or IL-3 was significantly impaired at low pH. However, a cocktail of these three cytokines synergistically supported proliferation, cell cycle progression, and colony formation.

DISCUSSION

Our findings indicate that an acidic milieu is deleterious for CD34+ cells and that a combination of certain cytokines and cAMP donors may improve cell viability and function. These data may be useful to develop new therapeutic strategies or to optimize protocols for regenerative medicine.

Lactic acid induces aberrant amyloid precursor protein processing by promoting its interaction with endoplasmic reticulum chaperone proteins

Background

Lactic acid, a natural by-product of glycolysis, is produced at excess levels in response to impaired mitochondrial function, high-energy demand, and low oxygen availability. The enzyme involved in the production of β-amyloid peptide (Aβ) of Alzheimer's disease, BACE1, functions optimally at lower pH, which led us to investigate a potential role of lactic acid in the processing of amyloid precursor protein (APP).

Methodology/Principal Findings

Lactic acid increased levels of Aβ40 and 42, as measured by ELISA, in culture medium of human neuroblastoma cells (SH-SY5Y), whereas it decreased APP metabolites, such as sAPPα. In cell lysates, APP levels were increased and APP was found to interact with ER-chaperones in a perinuclear region, as determined by co-immunoprecipitation and fluorescence microscopy studies. Lactic acid had only a very modest effect on cellular pH, did increase the levels of ER chaperones Grp78 and Grp94 and led to APP aggregate formation reminiscent of aggresomes.

Conclusions/Significance

These findings suggest that sustained elevations in lactic acid levels could be a risk factor in amyloidogenesis related to Alzheimer's disease through enhanced APP interaction with ER chaperone proteins and aberrant APP processing leading to increased generation of amyloid peptides and APP aggregates.

SLC4A7 sodium bicarbonate co-transporter controls mitochondrial apoptosis in ischaemic coronary endothelial cells

AIMS

Bicarbonate transport has been shown to participate in apoptosis under ischaemic stress. However, the precise transporting mechanisms involved in ischaemic apoptosis are unknown and were thus the aim of the present study.

METHODS AND RESULTS

Rat coronary endothelial cells (EC) were exposed to simulated in vitro ischaemia for 2 h, and apoptosis was subsequently determined by chromatin staining and caspase-3 activity analysis. By examining the expression of bicarbonate transporters (BT) in EC by reverse transcriptase polymerase chain reaction and western blotting, a marked expression of the electroneutral sodium bicarbonate co-transporter (SLC4A7) was defined. To analyse the potential role of this transporter during apoptosis, a selective inhibitor (S0859, Sanofi-Aventis) was applied. Treatment with S0859 significantly increased caspase-3 activity and elevated the number of apoptotic EC. These results were comparable with an unselective inhibition of all BT due to withdrawal of bicarbonate in the anoxic medium. Knockdown of SLC4A7 in EC by transfecting appropriate siRNA similarly increased apoptosis of EC under simulated ischaemia. The initial characterization of the participating mechanisms of SLC4A7-dependent apoptosis revealed an activation of the mitochondrial pathway of apoptosis, i.e. cleavage of caspase-9 and binding of Bax to mitochondria. In contrast, no activation of the endoplasmic reticulum-dependent pathway (caspase-12 cleavage) or the extrinsic apoptotic pathway (caspase-8 cleavage) was found. Finally, a mitochondrial localization of SLC4A7 was demonstrated.

CONCLUSION

The electroneutral sodium bicarbonate co-transporter SLC4A7 localizes in mitochondria and suppresses the ischaemia-induced activation of the mitochondrial pathway of apoptosis in coronary EC.

Time course Analysis of Gene expression patterns in ZebrafIsh Eye during Optic Nerve Regeneration

It is well-established that neurons in the adult mammalian central nervous system (CNS) are terminally differentiated and, if injured, will be unable to regenerate their connections. In contrast to mammals, zebrafish and other teleosts display a robust neuroregenerative response. Following optic nerve crush (ONX), retinal ganglion cells (RGC) regrow their axons to synapse with topographically correct targets in the optic tectum, such that vision is restored in ~21 days. What accounts for these differences between teleostean and mammalian responses to neural injury is not fully understood. A time course analysis of global gene expression patterns in the zebrafish eye after ONX can help to elucidate cellular and molecular mechanisms that contribute to a successful neuroregeneration. To define different phases of regeneration after ONX, alpha tubulin 1 ( tuba1) and growth-associated protein 43 ( gap43), markers previously shown to correspond to morphophological events, were measured by real time quantitative PCR (qPCR). Microarray analysis was then performed at defined intervals (6 hours, 1, 4, 12, and 21 days) post-ONX and compared to SHAM. Results show that optic nerve damage induces multiple, phase-related transcriptional programs, with the maximum number of genes changed and highest fold-change occurring at 4 days. Several functional groups affected by optic nerve regeneration, including cell adhesion, apoptosis, cell cycle, energy metabolism, ion channel activity, and calcium signaling, were identified. Utilizing the whole eye allowed us to identify signaling contributions from the vitreous, immune and glial cells as well as the neural cells of the retina. Comparisons between our dataset and transcriptional profiles from other models of regeneration in zebrafish retina, heart and fin revealed a subset of commonly regulated transcripts, indicating shared mechanisms in different regenerating tissues. Knowledge of gene expression patterns in all components of the eye in a model of successful regeneration provides an entry point for functional analyses, and will help in devising hypotheses for testing normal and toxic regulatory factors.

Astrocyte cultures exhibit P2X7 receptor channel opening in the absence of exogenous ligands

P2X7 receptors (P2X7Rs) gate the opening of large channels when activated by ATP or other ligands. P2X7Rs are expressed by astrocytes in culture and by reactive astrocytes in vivo, and astrocytes in culture have been shown to release glutamate and ATP in response to P2X7R activation. However, P2X7Rs are activated by ATP only at concentrations greater than 1 mM. The conditions under which astrocyte P2X7Rs would be activated in vivo are, thus, unclear. Here we show that astrocytes in culture exhibit basal P2X7R activity. Primary mouse astrocytes were found to take up the P2X7R permeant dyes YO-PRO-1 (YP) and propidium iodide in absence of any added ligands. By contrast, cultured rat astrocytes took up very little YP, consistent with their much lower level of P2X7R expression. The uptake by mouse astrocytes was inhibited by oxATP, suramin, KN-62 and brilliant blue G, and by siRNA knock-down of P2X7R. Astrocyte uptake of YP was also inhibited by phenol red at concentrations above 50 muM, suggesting that phenol red present in standard cell culture media may influence P2X7R channel activity. Treatment with apyrase, an enzyme that degrades extracellular ATP, partially decreased YP uptake in astrocytes. Conversely, exposure to the ectonucleotidase inhibitor ARL67156 enhanced YP uptake and astrocytes plated without contiguous neighboring astrocytes showed reduced basal YP uptake. These results suggest that the basal uptake of YP may be due to activation of P2X7R by release of ATP by astrocytes themselves into intercellular spaces.

Soluble adenylyl cyclase controls mitochondria-dependent apoptosis in coronary endothelial cells

The cAMP signaling pathway plays an essential role in modulating the apoptotic response to various stress stimuli. Until now, it was attributed exclusively to the activity of the G-protein-responsive transmembrane adenylyl cyclase. In addition to transmembrane AC, mammalian cells possess a second source of cAMP, the ubiquitously expressed soluble adenylyl cyclase (sAC). However, the role of this cyclase in apoptosis was unknown. A mitochondrial localization of this cyclase has recently been demonstrated, which led us to the hypothesis that sAC may play a role in apoptosis through modulation of mitochondria-dependent apoptosis. To prove this hypothesis, apoptosis was induced by simulated in vitro ischemia or by acidosis, which is an important component of ischemia. Suppression of sAC activity with the selective inhibitor KH7 or sAC knockdown by small interfering RNA transfection abolished endothelial apoptosis. Furthermore, pharmacological inhibition or knockdown of protein kinase A, an important cAMP target, demonstrated a significant anti-apoptotic effect. Analysis of the underlying mechanisms revealed (i) the translocation of sAC to mitochondria under acidic stress and (ii) activation of the mitochondrial pathway of apoptosis, i.e. cytochrome c release and caspase-9 cleavage. sAC inhibition or knockdown abolished the activation of the mitochondrial pathway of apoptosis. Analysis of mitochondrial co-localization of Bcl-2 family proteins demonstrated sAC- and protein kinase A-dependent translocation of Bax to mitochondria. Taken together, these results suggest the important role of sAC in modulating the mitochondria-dependent pathway of apoptosis in endothelial cells.

Acidic preconditioning protects endothelial cells against apoptosis through p38- and Akt-dependent Bcl-xL overexpression

To analyze the underlying cellular mechanisms of adaptation to ischemia-induced apoptosis through short acidic pretreatment, i.e. acidic preconditioning (APC), Wistar rat coronary endothelial cells (EC) were exposed for 40 min to acidosis (pH 6.4) followed by a 14 h recovery period (pH 7.4) and finally treated for 2 h with simulated in vitro ischemia (glucose-free anoxia at pH 6.4). APC led to a transient activation of p38 and Akt kinases, but not of JNK and ERK1/2 kinases, which was accompanied by significant reduction of the apoptotic cell number, caspase-12/-3 cleavage and Bcl-xL overexpression. These effects of APC were completely abolished by prevention of Akt- or p38-phosphorylation during APC. Furthermore, knock-down of Bcl-xL by siRNA-transfection also abolished the anti-apoptotic effect of APC. Therefore, APC leads to protection of EC against ischemic apoptosis by activation of Akt and p38 followed by overexpression of Bcl-xL, which is a key anti-apoptotic mechanism of APC.

In vivo administration of calpeptin attenuates calpain activation and cardiomyocyte loss in pressure-overloaded feline myocardium

Calpain activation is linked to the cleavage of several cytoskeletal proteins and could be an important contributor to the loss of cardiomyocytes and contractile dysfunction during cardiac pressure overload (PO). Using a feline right ventricular (RV) PO model, we analyzed calpain activation during the early compensatory period of cardiac hypertrophy. Calpain enrichment and its increased activity with a reduced calpastatin level were observed in 24- to 48-h-PO myocardium, and these changes returned to basal level by 1 wk of PO. Histochemical studies in 24-h-PO myocardium revealed the presence of TdT-mediated dUTP nick-end label (TUNEL)-positive cardiomyocytes, which exhibited enrichment of calpain and gelsolin. Biochemical studies showed an increase in histone H2B phosphorylation and cytoskeletal binding and cleavage of gelsolin, which indicate programmed cardiomyocyte cell death. To test whether calpain inhibition could prevent these changes, we administered calpeptin (0.6 mg/kg iv) by bolus injections twice, 15 min before and 6 h after induction of 24-h PO. Calpeptin blocked the following PO-induced changes: calpain enrichment and activation, decreased calpastatin level, caspase-3 activation, enrichment and cleavage of gelsolin, TUNEL staining, and histone H2B phosphorylation. Although similar administration of a caspase inhibitor, N-benzoylcarbonyl-Val-Ala-Asp-fluoromethylketone (Z-VD-fmk), blocked caspase-3 activation, it did not alleviate other aforementioned changes. These results indicate that biochemical markers of cardiomyocyte cell death, such as sarcomeric disarray, gelsolin cleavage, and TUNEL-positive nuclei, are mediated, at least in part, by calpain and that calpeptin may serve as a potential therapeutic agent to prevent cardiomyocyte loss and preserve myocardial structure and function during cardiac hypertrophy.