Inhibition of Na+/H+ exchanger enhances low pH-induced L-selectin shedding and beta2-integrin surface expression in human neutrophils

Impact of real-time shedding on binding kinetics of membrane-remaining L-selectin to PSGL-1

L-selectin shedding induced by various cytokines is crucial in activating neutrophils (PMNs) in inflammatory cascade. While the real-time shedding in vivo lasts ~10 min after PMN activation, the impact of time-dependent shedding on binding kinetics of membrane-remaining L-selectins to its ligands is poorly understood at transient or steady state. Here, we developed an in vitro L-selectin shedding dynamics approach, together with competitive assays of cell adhesion, and proposed a theoretical model for quantifying the impact of real-time shedding on the binding kinetics of membrane-remaining L-selectins to P-selectin glycoprotein ligand-1 (PSGL-1). Our data indicated that the extent of L-selectin shedding on PMA activation is higher, but the terminating time is longer for Jurkat cells than those for human PMNs. Meanwhile, fMLF or IL-8 stimulation yields the longer terminating time than that on PMA stimulation but results in a similar shedding extent for PMNs. L-selectin shedding reduces L-selectin-PSGL-1-mediated cell adhesion in three ways: decreasing membrane-anchored L-selectins, increasing soluble L-selectins competitively binding to ligands, and presenting conformational alteration of membrane-remaining L-selectins themselves. Compared with those on intact cells, the binding affinities of membrane-remaining L-selectin-PSGL-1 pairs were all enhanced at initial and lowered at the late shedding phase for both PMN and Jurkat cells even with varied transition time points. The rolling velocities of both PMNs and Jurkat cells were increased following mechanically or biochemically induced shedding of L-selectin under shear flow. These findings help to further our understanding of the function of time-dependent L-selectin shedding during the inflammation cascade.

d(-) Lactic Acid-Induced Adhesion of Bovine Neutrophils onto Endothelial Cells Is Dependent on Neutrophils Extracellular Traps Formation and CD11b Expression

Bovine ruminal acidosis is of economic importance as it contributes to reduced milk and meat production. This phenomenon is mainly attributed to an overload of highly fermentable carbohydrate, resulting in increased d(−) lactic acid levels in serum and plasma. Ruminal acidosis correlates with elevated acute phase proteins in blood, along with neutrophil activation and infiltration into various tissues leading to laminitis and aseptic polysynovitis. Previous studies in bovine neutrophils indicated that d(−) lactic acid decreased expression of L-selectin and increased expression of CD11b to concentrations higher than 6 mM, suggesting a potential role in neutrophil adhesion onto endothelia. The two aims of this study were to evaluate whether d(−) lactic acid influenced neutrophil and endothelial adhesion and to trigger neutrophil extracellular trap (NET) production (NETosis) in exposed neutrophils. Exposure of bovine neutrophils to 5 mM d(−) lactic acid elevated NET release compared to unstimulated neutrophil negative controls. Moreover, this NET contains CD11b and histone H₄ citrullinated, the latter was dependent on PAD4 activation, a critical enzyme in DNA decondensation and NETosis. Furthermore, NET formation was dependent on d(−) lactic acid plasma membrane transport through monocarboxylate transporter 1 (MCT1). d(−) lactic acid enhanced neutrophil adhesion onto endothelial sheets as demonstrated by in vitro neutrophil adhesion assays under continuous physiological flow conditions, indicating that cell adhesion was a NET- and a CD11b/ICAM-1-dependent process. Finally, d(−) lactic acid was demonstrated for the first time to trigger NETosis in a PAD4- and MCT1-dependent manner. Thus, d(−) lactic acid-mediated neutrophil activation may contribute to neutrophil-derived pro-inflammatory processes, such as aseptic laminitis and/or polysynovitis in animals suffering acute ruminal acidosis.

Fever-range hyperthermia improves the anti-apoptotic effect induced by low pH on human neutrophils promoting a proangiogenic profile

Neutrophils have the shortest lifespan among leukocytes and usually die via apoptosis, limiting their deleterious potential. However, this tightly regulated cell death program can be modulated by pathogen-associated molecular patterns (PAMPs), danger-associated molecular pattern (DAMPs), and inflammatory cytokines. We have previously reported that low pH, a hallmark of inflammatory processes and solid tumors, moderately delays neutrophil apoptosis. Here we show that fever-range hyperthermia accelerates the rate of neutrophil apoptosis at neutral pH but markedly increases neutrophil survival induced by low pH. Interestingly, an opposite effect was observed in lymphocytes; hyperthermia plus low pH prevents lymphocyte activation and promotes the death of lymphocytes and lymphoid cell lines. Analysis of the mechanisms through which hyperthermia plus low pH increased neutrophil survival revealed that hyperthermia further decreases cytosolic pH induced by extracellular acidosis. The fact that two Na⁺/H⁺ exchanger inhibitors, 5-(N-ethyl-N-isopropyl) amiloride (EIPA) and amiloride, reproduced the effects induced by hyperthermia suggested that it prolongs neutrophil survival by inhibiting the Na⁺/H⁺ antiporter. The neutrophil anti-apoptotic effect induced by PAMPs, DAMPs, and inflammatory cytokines usually leads to the preservation of the major neutrophil effector functions such as phagocytosis and reactive oxygen species (ROS) production. In contrast, our data revealed that the anti-apoptotic effect induced by low pH and hyperthermia induced a functional profile characterized by a low phagocytic activity, an impairment in ROS production and a high ability to suppress T-cell activation and to produce the angiogenic factors VEGF, IL-8, and the matrix metallopeptidase 9 (MMP-9). These results suggest that acting together fever and local acidosis might drive the differentiation of neutrophils into a profile able to promote both cancer progression and tissue repair during the late phase of inflammation, two processes that are strongly dependent on the local production of angiogenic factors by infiltrating immune cells.

Time-dependent activity of Na+/H+ exchanger isoform 1 and homeostasis of intracellular pH in astrocytes exposed to CoCl2 treatment

Hypoxia causes injury to the central nervous system during stroke and has significant effects on pH homeostasis. Na+/H+ exchanger isoform 1 (NHE1) is important in the mechanisms of hypoxia and intracellular pH (pHi) homeostasis. As a well-established hypoxia-mimetic agent, CoCl2 stabilizes and increases the expression of hypoxia inducible factor‑1α (HIF-1α), which regulates several genes involved in pH balance, including NHE1. However, it is not fully understood whether NHE1 is activated in astrocytes under CoCl2 treatment. In the current study, pHi and NHE activity were analyzed using the pHi‑sensitive dye BCECF‑AM. Using cariporide (an NHE1‑specific inhibitor) and EIPA (an NHE nonspecific inhibitor), the current study demonstrated that it was NHE1, not the other NHE isoforms, that was important in regulating pHi homeostasis in astrocytes during CoCl2 treatment. Additionally, the present study observed that, during the early period of CoCl2 treatment (the first 2 h), NHE1 activity and pHi dropped immediately, and NHE1 mRNA expression was reduced compared with control levels, whereas expression levels of the NHE1 protein had not yet changed. In the later period of CoCl2 treatment, NHE1 activity and pHi significantly increased compared with the control levels, as did the mRNA and protein expression levels of NHE1. Furthermore, the cell viability and injury of astrocytes was not changed during the initial 8 h of CoCl2 treatment; their deterioration was associated with the higher levels of pHi and NHE1 activity. The current study concluded that NHE1 activity and pHi homeostasis are regulated by CoCl2 treatment in a time-dependent manner in astrocytes, and may be responsible for the changes in cell viability and injury observed under hypoxia-mimetic conditions induced by CoCl2 treatment.

Stable DNA Nanomachine Based on Duplex-Triplex Transition for Ratiometric Imaging Instantaneous pH Changes in Living Cells

DNA nanomachines are becoming useful tools for molecular recognition, imaging, and diagnostics and have drawn gradual attention. Unfortunately, the present application of most DNA nanomachines is limited in vitro, so expanding their application in organism has become a primary focus. Hence, a novel DNA nanomachine named t-switch, based on the DNA duplex-triplex transition, is developed for monitoring the intracellular pH gradient. Our strategy is based on the DNA triplex structure containing C(+)-G-C triplets and pH-dependent Förster resonance energy transfer (FRET). Our results indicate that the t-switch is an efficient reporter of pH from pH 5.3 to 6.0 with a fast response of a few seconds. Also the uptake of the t-switch is speedy. In order to protect the t-switch from enzymatic degradation, PEI is used for modification of our DNA nanomachine. At the same time, the dynamic range could be extended to pH 4.6-7.8. The successful application of this pH-depended DNA nanomachine and motoring spatiotemporal pH changes associated with endocytosis is strong evidence of the possibility of self-assembly DNA nanomachine for imaging, targeted therapies, and controllable drug delivery.

Oxidative response of neutrophils to platelet-activating factor is altered during acute ruminal acidosis induced by oligofructose in heifers

Reactive oxygen species (ROS) production is one of the main mechanisms used to kill microbes during innate immune response. D-lactic acid, which is augmented during acute ruminal acidosis, reduces platelet activating factor (PAF)-induced ROS production and L-selectin shedding in bovine neutrophils in vitro. This study was conducted to investigate whether acute ruminal acidosis induced by acute oligofructose overload in heifers interferes with ROS production and L-selectin shedding in blood neutrophils. Blood neutrophils and plasma were obtained by jugular venipuncture, while ruminal samples were collected using rumenocentesis. Lactic acid from plasma and ruminal samples was measured by HPLC. PAF-induced ROS production and L-selectin shedding were measured in vitro in bovine neutrophils by a luminol chemiluminescence assay and flow cytometry, respectively. A significant increase in ruminal and plasma lactic acid was recorded in these animals. Specifically, a decrease in PAF-induced ROS production was observed 8 h after oligofructose overload, and this was sustained until 48 h post oligofructose overload. A reduction in PAF-induced L-selectin shedding was observed at 16 h and 32 h post oligofructose overload. Overall, the results indicated that neutrophil PAF responses were altered in heifers with ruminal acidosis, suggesting a potential dysfunction of the innate immune response.

Role of inflammation and its mediators in acute ischemic stroke

Inflammation plays an important role in the pathogenesis of ischemic stroke and other forms of ischemic brain injury. Increasing evidence suggests that inflammatory response is a double-edged sword, as it not only exacerbates secondary brain injury in the acute stage of stroke but also beneficially contributes to brain recovery after stroke. In this article, we provide an overview on the role of inflammation and its mediators in acute ischemic stroke. We discuss various pro-inflammatory and anti-inflammatory responses in different phases after ischemic stroke and the possible reasons for their failures in clinical trials. Undoubtedly, there is still much to be done in order to translate promising pre-clinical findings into clinical practice. A better understanding of the dynamic balance between pro- and anti-inflammatory responses and identifying the discrepancies between pre-clinical studies and clinical trials may serve as a basis for designing effective therapies.

K+ efflux through two-pore domain K+ channels is required for mouse embryonic development

Numerous studies have suggested that K+ channels regulate a wide range of physiological processes in mammalian cells. However, little is known about the specific function of K+ channels in germ cells. In this study, mouse zygotes were cultured in a medium containing K+ channel blockers to identify the functional role of K+ channels in mouse embryonic development. Voltage-dependent K+ channel blockers, such as tetraethylammonium and BaCl2, had no effect on embryonic development to the blastocyst stage, whereas K2P channel blockers, such as quinine, selective serotonin reuptake inhibitors (fluoxetine, paroxetine, and citalopram), gadolinium trichloride, anandamide, ruthenium red, and zinc chloride, significantly decreased blastocyst formation (P<0.05). RT-PCR data showed that members of the K2P channel family, specifically KCNK2, KCNK10, KCNK4, KCNK3, and KCNK9, were expressed in mouse oocytes and embryos. In addition, their mRNA expression levels, except Kcnk3, were up-regulated by above ninefold in morula-stage embryos compared with 2-cell stage embryos (2-cells). Immunocytochemical data showed that KCNK2, KCNK10, KCNK4, KCNK3, and KCNK9 channel proteins were expressed in the membrane of oocytes, 2-cells, and blastocysts. Each siRNA injection targeted at Kcnk2, Kcnk10, Kcnk4, Kcnk3, and Kcnk9 significantly decreased blastocyst formation by ∼38% compared with scrambled siRNA injection (P<0.05). The blockade of K2P channels acidified the intracellular pH and depolarized the membrane potential. These results suggest that K2P channels could improve mouse embryonic development through the modulation of gating by activators.

Binding of L-selectin to its vascular and extravascular ligands is differentially regulated by pH

Ligands for L-selectin, a leukocyte adhesion molecule, are expressed in high endothelial venules (HEVs) in lymph nodes and extravascular tissues, such as renal tubules. Here, we report that the binding of L-selectin to its vascular and extravascular ligands is differentially regulated by pH. The optimal L-selectin-dependent binding of leukocytes to HEVs was observed at pH 7.4, a physiological pH in the blood. In contrast, the optimal binding of leukocytes to the renal tubules was observed at pH 5.6. Consistently, optimal binding of soluble recombinant L-selectin to a major vascular ligand, 6-sulfo sialyl Lewis X, was observed at pH 7.4. Binding to extravascular ligands, such as chondroitin sulfate (CS) B, CS E and heparan sulfate, occurred at pH 5.6. Under physiological shear stress ranging from 1 to 2 dynes/cm(2), maximal leukocyte rolling on vascular ligands was observed at pH 6.8 to 7.4, and no rolling was detected at pH conditions below 5.6. These findings suggest that the pH environment is one important factor that determines leukocyte trafficking under physiological and pathological conditions.

The cytoplasmic domains of TNFalpha-converting enzyme (TACE/ADAM17) and L-selectin are regulated differently by p38 MAPK and PKC to promote ectodomain shedding

L-selectin mediates the initial tethering and subsequent rolling of leucocytes along luminal walls of inflamed venules. TACE [TNFalpha (tumour necrosis factor alpha)-converting enzyme] is responsible for cleaving the membrane-proximal extracellular domain of L-selectin (also known as shedding), which reduces the efficiency of leucocyte recruitment to sites of inflammation. Many reports have highlighted roles for PKC (protein kinase C) and p38 MAPK (mitogen-activated protein kinase) in promoting L-selectin shedding with little insight into the mechanism involved. By using PMA and the phosphatase inhibitors cantharidin and calyculin A, we could selectively activate PKC or p38 MAPK respectively to promote TACE-dependent shedding of L-selectin. Interestingly, the intracellular mechanisms leading to the shedding event differed dramatically. For example, regulatory elements within the L-selectin cytoplasmic tail, such as ERM (ezrin/radixin/moesin)-binding and serine residues, were important for PKC- but not p38 MAPK-dependent shedding. Also, increased and sustained cell surface levels of TACE, and phosphorylation of its cytoplasmic tail (a hallmark of TACE activation), occurred in lymphocytes and monocytes following p38 MAPK activation. Finally, we showed that TNFalpha-induced shedding of L-selectin in monocytes was strikingly similar to cantharidin-induced shedding and suggest that this newly characterized mechanism could be physiologically relevant in inflammatory cells.

Human neutrophils do not express purinergic P2X7 receptors

It has been reported that in human neutrophils, external ATP activates plasma membrane purinergic P2X(7) receptors (P2X(7)R) to elicit Ca(2+) entry, production of reactive oxygen species (ROS), processing and release of pro-inflammatory cytokines, shedding of adhesion molecules and uptake of large molecules. However, the expression of P2X(7)R at the plasma membrane of neutrophils has also been questioned since these putative responses are not always reproduced. In this work, we used electrophysiological recordings to measure functional responses associated with the activation of membrane receptors, spectrofluorometric measurements of ROS production and ethidium bromide uptake to asses coupling of P2X(7)R activation to downstream effectors, immune-labelling of P2X(7)R using a fluorescein isothiocyanate-conjugated antibody to detect the receptors at the plasma membrane, RT-PCR to determine mRNA expression of P2X(7)R and Western blot to determine protein expression in neutrophils and HL-60 cells. None of these assays reported the presence of P2X(7)R in the plasma membrane of neutrophils and non-differentiated or differentiated HL-60 cells-a model cell for human neutrophils. We concluded that P2X(7)R are not present at plasma membrane of human neutrophils and that the putative physiological responses triggered by external ATP should be reconsidered.

Inhibition of Na(+)-H(+) exchange before resuscitation following hemorrhagic shock is cardioprotective in rats

BACKGROUND

Stimulation of the Na(+)-H(+) exchanger during resuscitation following hemorrhagic shock results in myocardial injury and dysfunction. Inhibition of the Na(+)-H(+) exchanger appears to be a new pharmacological tool for myocardial protection following ischemia-reperfusion. Our lab showed that inhibition of the Na(+)-H(+) exchanger, using amiloride and dimethyl amiloride, before ex vivo resuscitation of isolated perfused hearts protected the myocardium and improved the post-resuscitation myocardial function. The purpose of the present study was to examine the myocardial protective effects of treating the hemorrhagic shocked rats by intra-arterial injection of 20 μM dimethyl amiloride (DMA), a specific Na(+)-H(+) exchanger blocker, before in vivo resuscitation.

METHODS

Sprague-Dawley rats were assigned to hemorrhagic treated or untreated groups (n = 4 per group). After 60 min of hemorrhagic shock, rats were treated or not by injection of 20 μM 5-(N,N-dimethyl)-amiloride (DMA) intra-arterially. Rats were then resuscitated in vivo and monitored for 30 min. Then hearts were harvested and perfused in the Langendorff system for 60 min for measurements of hemodynamic function.

RESULTS

Administration of DMA before in vivo resuscitation following 60 min of hemorrhagic shock and 30 min of in vivo resuscitation, 20 μM DMA intra-arterially significantly improved post-resuscitation myocardial function.

CONCLUSION

Our results suggest that DMA protects the heart against post-resuscitation myocardial injury.