Induction of granule release by intracellular application of calcium and guanosine-5'-O-(3-thiotriphosphate) in human eosinophils

Compound exocytosis and cumulative degranulation by eosinophils and their role in parasite killing

The killing of metazoan parasitic larvae by eosinophils occurs following cell adhesion and the secretion o f their cytotoxic proteins onto the surface o f these targets. In eosinophils, as in mast cells and neutrophils, stimulus-secretion coupling is mediated by GTP-binding proteins. In this article, Susanne Scepek Redwon Moqbel and Manfred Lindou summarize recent results indicating that the granule-fusion events activated by GTP-binding proteins lead to compound exocytosis and cumulative fusion. They propose that these exocytotic processes, following contact with opsonized larvae, may direct secretion to a restricted space defined by the site o f contact Such a focused release may be essential for effective targeting in parasite killing, thus preventing uncontrolled random diffusion of the secreted cytotoxic proteins with the possible undesirable consequences of damage to intact host tissue.

Characterization of platelet-activating factor-induced cytosolic calcium mobilization in human eosinophils

BACKGROUND

Activated eosinophils play an important role in the pathogenesis of bronchial asthma and other allergic diseases, and platelet-activating factor (PAF) is a potent activator of eosinophils.

OBJECTIVE

To characterize the cytosolic Ca2+ ([Ca2+]i) mobilization in human eosinophils in response to PAF.

METHODS

[Ca2+]i responses to PAF were examined in human eosinophils using a microscopic fura-2 fluorescence-ratio imaging system.

RESULTS

PAF caused a significant and dose-dependent increase in (Ca2+)i, which consisted of an initial rapid rise followed by a sustained elevation. This PAF-induced (Ca2+)i rise was inhibited by WEB 2086, a specific PAF receptor antagonist. The addition of 5 mM EGTA or 1 mM Ni2+ to a nominally Ca2+-free solution did not appreciably reduce the initial rise but significantly inhibited the sustained rise. The application of a protein kinase C inhibitor, Ro31-8220, augmented the sustained increase by PAF. Thapsigargin, a microsomal Ca2+ ATPase inhibitor, induced no appreciable change in a nominally Ca2+-free solution but induced a marked increase in (Ca2+)i when changed to a Ca2+-containing solution.

CONCLUSIONS

The initial rapid rise and the following sustained rise in (Ca2+)i by PAF depends on Ca2+ release from the intracellular Ca2+ stores and Ca2+ influx, respectively, which are regulated by protein kinase C in human eosinophils. Furthermore, the so called Ca2+-capacitative entry is possibly involved in the Ca2+ influx from the extracellular solution in human eosinophils.

Exocytotic Competence and Intergranular Fusion in Cord Blood-Derived Eosinophils During Differentiation

We studied degranulation of single cord blood-derived mononuclear cells differentiating to eosinophils in cultures containing recombinant human interleukin-5 (rhIL-5) and rhIL-3 by whole-cell patch-clamp capacitance measurements. As in mature cells, degranulation can be stimulated by intracellular application of guanosine-5′-O-(3-thiotriphosphate) (GTP) γS after 10 days in culture, simultaneously with the first morphological appearance of granules. These results demonstrate that the fusion machineries for exocytotic fusion are present and functional as soon as the granules are formed, presumably at the myeloblast stage. In the third week, the total amount of granules exocytosed upon stimulation is similar to that in mature eosinophils from peripheral blood. The capacitance step size distributions in promyelocytes and myelocytes confirm that mature large specific granules are formed by homotypic fusion of unit granules with similar size. Homotypic fusion is facilitated during early stages of differentiation associated with granulogenesis. Between day 10 and day 35 in culture the plasma membrane area of resting cells decreases from ≈700 μm2 to ≈400 μm2, approaching the value of mature cells from peripheral blood. The most prominent decrease occurs between day 25 and day 35 and is accompanied by the appearance of an exocytotic component due to small vesicles. This suggests that a class of small secretory vesicles is formed by endocytosis during a late phase in maturation.

Voltage-activated proton current in eosinophils from human blood

1. The resting membrane potential of freshly purified normodense human eosinophils bathed in and dialysed with quasi-physiological solutions was -63 +/- 2 mV (n = 100). 2. In voltage-clamp mode with quasi-physiological internal and external solutions, voltage steps from the holding potential of -60 mV to levels positive to +20 mV resulted in development of a quasi-instantaneous outward current and a slowly developing outward current. The instantaneous current was absent when the cells were bathed in and dialysed with K(+)-free solution. 3. The slow outward current persisted following simultaneous replacement of K+, Na+ and most of the Cl- with largely impermeant ions (tetraethylammonium, N-methyl-D-glucamine and methanesulphonate) and was augmented when the cell was dialysed with a solution of increased buffering capacity for protons. The observed reversal potential of the current closely followed the hydrogen equilibrium potential over a wide range of internal-external pH combinations, indicating that the conductance underlying the slow outward current was highly selective for H+ ions. 4. Acidification of the pipette solution (increasing [H+]i) augmented the outward H+ current and shifted its activation range negatively, whilst acidification of the external solution had the opposite effect. The voltage dependence of the current is modulated by the transmembrane pH gradient so the only outward current could be activated. However, when the outward current was activated by a voltage step, rapid acidification of external solution produced an inward H+ current which rapidly deactivated. 5. The proton current was reversibly inhibited in a voltage-dependent manner by extracellular application of Zn2+. The apparent dissociation constants were 8 nM (at +40 mV), 36 nM (at +70 mV) and 200 nM (at +100 mV). 6. The proton current was augmented by exposure to 10 microM arachidonic acid. This augmentation consisted of a shift of the voltage dependence of activation to more negative potentials and enhancement of maximum conductance (gH,max). The proton current recorded in eosinophils was significantly augmented under conditions of elevated cytosolic free calcium concentration ([Ca2+]i). The threshold level of [Ca2+]i associated with this effect lay between 0.1 and 1 microM and was not measurably affected by cytosolic acidification. 7. Eosinophils from human blood possess a voltage-dependent H+ conductance (gH) which normally allows protons to move outwards only; raising [Ca2+]i was associated with augmentation of gH and intracellular acidification or arachidonate shifted its activation range negatively towards physiological potentials.

A survey of GTP-binding proteins and other potential key regulators of exocytotic secretion in eosinophils. Apparent absence of rab3 and vesicle fusion protein homologues

We set out to identify potential key regulators of exocytotic fusion in the eosinophil, in the knowledge that granule exocytosis can be stimulated in these cells by intracellular application of nonhydrolyzable analogues of guanosine triphosphate, with Ca2+ acting as a modulator of guanine nucleotide-dependent secretion. To screen for GTP-binding proteins, guinea pig eosinophils were purified from peritoneal washings and subjected to western blotting analysis using specific immune sera raised against recombinant proteins or consensus peptide sequences within proteins of interest. We found a number of heterotrimeric G proteins (G alpha i3, G alpha o, G alpha q11, G alpha s and G beta subunits) and members of the small GTP-binding proteins expressed in eosinophils. Two subtypes of G-protein alpha subunits (G alpha i1 and G alpha z) could not be detected. Separation of subcellular organelles from homogenized eosinophils by density gradient centrifugation revealed that all of the detected GTP-binding proteins were mainly expressed in fractions containing peak plasma membrane and Golgi marker enzyme activities, while G beta subunits were also detected in secretory granule fractions. However, isoforms of Rab3, a putative GTP-binding regulator of exocytotic fusion, were undetectable in eosinophils. Neither, with the exception of syntaxin-3, could we detect any of the proteins belonging to the proposed synaptic vesicle fusion complex (SNAP-25; synaptobrevin (VAMP) and its non-neuronal homologue, cellubrevin; synaptophysin; synaptotagmin). The results from this study, based on western blotting, suggest that eosinophils express a different class of exocytotic fusion complex proteins from those found in neuronal tissues, although a number of potential candidates fulfilling the role of GE were identified in this important inflammatory cell.