Calcium signaling and regulation of neutrophil functions: Still a long way to go

Neutrophils are the most abundant leukocytes in blood and disruption in their functions often results in an increased risk of serious infections and inflammatory autoimmune diseases. Following recent discoveries in their influence over disease progression, a resurgence of interest for neutrophil biology has taken place. The multitude of signaling pathways activated by the engagement of numerous types of receptors, with which neutrophils are endowed, reflects the functional complexity of these cells. It is therefore not surprising that there remains a huge lack in the understanding of molecular mechanisms underlining neutrophil functions. Moreover, studies on neutrophils are undoubtedly limited by the difficulty to efficiently edit the cell's genome. Over the past 30 years, compelling evidence has clearly highlighted that Ca²⁺ -signaling is governing the key processes associated with neutrophil functions. The confirmation of the role of an elevation of intracellular Ca²⁺ concentration has come from studies on NADPH oxidase activation and phagocytosis. In this review, we give an overview and update of our current knowledge on the role of Ca²⁺ mobilization in the regulation of pro-inflammatory functions of neutrophils. In particular, we stress the importance of Ca²⁺ in the formation of NETs and cytokine secretion in the light of newest findings. This will allow us to embrace how much further we have to go to understand the complex dynamics of Ca²⁺ -dependent mechanisms in order to gain more insights into the role of neutrophils in the pathogenesis of inflammatory diseases. The potential for therapeutics to regulate the neutrophil functions, such as Ca²⁺ influx inhibitors to prevent autoimmune and chronic inflammatory diseases, has been discussed in the last part of the review.

Regulation of neutrophil pro-inflammatory functions sheds new light on the pathogenesis of rheumatoid arthritis

For more than two centuries now, rheumatoid arthritis (RA) is under investigation intending to discover successful treatment. Despite decades of scientific advances, RA is still representing a challenge for contemporary medicine. Current drug therapies allow to improve significantly the quality of life of RA patients; however, they are still insufficient to reverse tissue injury and are often generating side-effects. The difficulty arises from the considerable fluctuation of the clinical course of RA among patients, making the predictive prognosis difficult. More and more studies underline the profound influence of the neutrophil multifaceted functions in the pathogenesis of RA. This renewed interest in the complexity of neutrophil functions in RA offers new exciting opportunities for valuable therapeutic targets as well as for safe and well-tolerated RA treatments. In this review, we aim to update the recent findings on the multiple facets of neutrophils in RA, in particular their impact in promoting the RA-based inflammation through the release of the cytokine-like S100A8/A9 protein complex, as well as the importance of NETosis in the disease progression and development. Furthermore, we delve into the complex question of neutrophil heterogeneity and plasticity and discuss the emerging role of miRNAs and epigenetic markers influencing the inflammatory response of neutrophils in RA and how they could constitute the starting point for novel attractive targets in RA therapy.

Store-operated Ca2+ channels formed by TRPC1, TRPC6 and Orai1 and non-store-operated channels formed by TRPC3 are involved in the regulation of NADPH oxidase in HL-60 granulocytes

Ca(2+) influx has been shown to be essential for NADPH oxidase activity which is involved in the inflammatory process. Ca(2+) conditions underlying the oxidative response are clearly delineated. Here, we show that store-operated Ca(2+) entry (SOCE) is required at the beginning of NADPH oxidase activation in response to fMLF (N-formyl-L-methionyl-L-leucyl-L-phenylalanine) in neutrophil-like HL-60 cells. When extracellular Ca(2+) is initially removed, early addition of Ca(2+) after stimulation causes a complete restoration of Ca(2+) entry and H(2)O(2) production. Both Ca(2+) entry and H(2)O(2) production are decreased by purported SOCE blockers, 2-aminoethoxydiphenyl borane (2-APB) and SK&F 96365. Endogenously expressed TRPC (transient receptor potential canonical) homologues and Orai1 were investigated for their role in supporting store-operated Ca(2+) channels activity. TRPC1, TRPC6 and Orai1 knock-out by siRNA resulted in the inhibition of Ca(2+) influx and H(2)O(2) production in response to fMLF and thapsigargin while suppression of TRPC3 had no effect on thapsigargin induced-SOCE. 2-APB and SK&F 96365 were able to amplify the reduction of fMLF-stimulated Ca(2+) entry and H(2)O(2) production observed in cells transfected by TRPC3 siRNA. In summary, Ca(2+) influx in HL-60 cells relies on different membrane TRPC channels and Orai1 for allowing NADPH oxidase activation. TRPC3 primarily mediates SOCE-independent pathways and TRPC1, TRPC6 and Orai1 exclusively contribute to SOCE.

Modulation by cADPr of Ca2+ mobilization and oxidative response in dimethylsulfoxide- or retinoic acid-differentiated HL-60 cells

In human phagocytic cells, reactive oxygen species (ROS) generation in response to N-formyl-L-Methionyl-L-Leucyl-L-Phenylalanine (fMLF) is largely dependent on cytosolic free calcium concentration ([Ca2+]i). Cyclic ADP-ribose (cADPr) is able to regulate Ca2+ release from intracellular stores through the ryanodine receptor but its potential role in biological responses has so far not been determined. In this study, we examined whether extracellular and intracellular cADPr is required in fMLF-induced [Ca2+]i rise and consequently in the oxidative response in human neutrophil-like HL-60 cells differentiated with dimethylsulfoxide or all-trans-retinoic acid (ATRA). We establish that extracellular cADPr cannot elicit [Ca2+]i elevation. Furthermore, we demonstrate that 8-Br-cADPr, a functional antagonist of cADPr, inhibits Ca2+ entry into HL-60 cells differentiated with ATRA and stimulated with fMLF (95+/-4 and 148+/-5 nM respectively, n=3). Finally, we show that this partial inhibition of Ca2+ mobilization is unrelated to ROS production (10.0+/-0.3 vs. 9.6+/-0.5 A.U., n=3). In conclusion, we showed that cADPr can control fMLF-induced Ca2+ influx but is unable to regulate a Ca2+-dependent biological response, i.e. H2O2 production.

Interleukin-8 primes oxidative burst in neutrophil-like HL-60 through changes in cytosolic calcium

In response to a variety of stimuli, neutrophils release large amount of reactive oxygen species (ROS) generated by NADPH oxidase. This process known as the respiratory burst is dependent on cytosolic free calcium concentration ([Ca(2+)](i)). Proinflammatory cytokines such as interleukin-8 (IL-8) may modulate ROS generation through a priming phenomenon. The aim of this study was to determine the effect of human IL-8 on ROS production in neutrophil-like dimethylsulfoxide-differentiated HL-60 cells (not equalHL-60 cells) and further to examine the role of Ca(2+) mobilization during the priming. IL-8 at 10 nM induced no ROS production but a [Ca(2+)](i) rise (254 +/- 36 nM). IL-8 induced a strongly enhanced (2 fold) ROS release during stimulation with 1 microM of N-formyl-L-methionyl-L-leucyl-L-phenylalanine (fMLF). This potentiation of ROS production is dependent of extracellular Ca(2+) (17.0+/-4.5 arbitrary units (A.U.) in the absence of Ca(2+) versus 56.6 +/- 3.9 A.U. in the presence of 1.25 mM of Ca(2+)). Also, IL-8 enhanced fMLF-stimulated increase in [Ca(2+)](i) (375 +/- 35 versus 245 +/- 21 nM, 0.1 microM of fMLF). IL-8 had no effect on not equalHL-60 cells in response to 1 microM of thapsigargin (472 +/- 66 versus 470 +/- 60 nM). In conclusion, Ca(2+) influx is necessary for a full induction of neutrophil priming by IL-8.

Correlation between tyrosine phosphorylation intensity of a 107 kDa protein band and A23187-induced acrosome reaction in human spermatozoa

This study, performed using semen samples from 10 men, investigated the relationship between sperm protein tyrosine phosphorylation and acrosomal status in conditions supporting in vitro capacitation. Percoll-selected spermatozoa (cells from the 95% fraction) were incubated for 3 h at 37 degrees C under an atmosphere of 5% CO2 in air, in a polyvinyl alcohol (1 mg ml(-1)) containing Biggers-Whitten-Whittingham's medium, nonsupplemented or supplemented with either bovine serum albumin (BSA; fatty acid free, 3 mg ml(-1)) or 2-hydroxy-propyl-beta-cyclodextrin (2-OH-p-beta-CD; 0.5, 1, 2 mmol l(-1)). Sperm suspension in each medium was split into two aliquots. The first was used to evaluate the acrosomal status by staining with the fluorescein isothiocyanate Pisum sativum agglutinin after induction of the acrosome reaction (AR) for 45 min with 10 micromol l(-1) of A23187 calcium ionophore. The second aliquot was used for sodium dodecyl sulphate polyacrylamide gel electrophoresis and immunoblotting, followed by a densitometric analysis. Compared with the nonsupplemented medium, BSA- or 2-OH-p-beta-CD-supplementation induced an increase in both the percentage of live acrosome-reacted sperm and the tyrosine phosphorylation intensity of the main phosphorylated 107 kDa protein. A correlation between the percentage of live acrosome-reacted sperm and the 107-kDa protein phosphotyrosine intensity was observed. Therefore, the 107 kDa protein-phosphotyrosine level measurement would bring additional information to conventional semen parameters in the assessment of the human sperm functionality.