Characterization of phospholipase activity in Dictyostelium discoideum. Identification of a Ca(2+)-dependent polyphosphoinositide-specific phospholipase C

A link of Ca2+ to cAMP oscillations in Dictyostelium: the calmodulin antagonist W-7 potentiates cAMP relay and transiently inhibits the acidic Ca2+-store

Background

During early differentiation of Dictyostelium the attractant cAMP is released periodically to induce aggregation of the cells. Here we pursue the question whether pulsatile cAMP signaling is coupled to a basic Ca²⁺-oscillation.

Results

We found that the calmodulin antagonist W-7 transiently enhanced cAMP spikes. We show that W-7 acts on an acidic Ca²⁺-store: it abolished ATP-dependent vesicular acidification, inhibited V-type H⁺ATPase activity more potently than the weaker antagonist W-5 and caused vesicular Ca²⁺-leakage. Concanamycin A, an inhibitor of the V-type H⁺-pump, blocked the Ca²⁺-leakage elicited by W-7 as well as cAMP-oscillations in the presence of W-7. Concanamycin A caused an increase of the cytosolic Ca²⁺-concentration whereas W-7 did not. In case of the latter, Ca²⁺ was secreted by the cells. In accord with our hypothesis that the link from Ca²⁺ to cAMP synthesis is mediated by a Ca²⁺-dependent phospholipase C we found that W-7 was not active in the phospholipase C knockout mutant.

Conclusion

We conclude that the potentiation of cAMP relay by W-7 is due to a transient inhibition of the acidic Ca²⁺-store. The inhibition of the proton pump by W-7 causes a leakage of Ca²⁺ that indirectly stimulates adenylyl cyclase activity via phospholipase C.

Fatty acids induce release of Ca2+ from acidosomal stores and activate capacitative Ca2+ entry in Dictyostelium discoideum

cAMP-induced Ca2+ fluxes in Dictyostelium discoideum largely depend on phospholipase A2 activity generating non-esterified fatty acids [Schaloske and Malchow (1997) Biochem. J. 327, 233-238]. In the present study the effect of fatty acids on Ca2+ homoeostasis in D. discoideum was investigated. Cytosolic free Ca2+ concentration ([Ca2+]i) was analysed by digital imaging of single fura2-dextran-loaded cells. Arachidonic acid and linoleic acid induced a transient increase in [Ca2+]i. The concentration of arachidonic acid determined the percentage of responding cells, with the mean height of the increase being dose-independent. In nominally Ca2+-free medium or in the presence of bis-(o-aminophenoxy)ethane-N, N,N',N'-tetra-acetic acid (BAPTA), no [Ca2+]i transient was detectable. In spite of this, we found that (1) arachidonic acid induced Ca2+ release from permeabilized cells and from vesicular fractions at concentrations that elicited Ca2+ influx in intact cells and (2) Ca2+ entry was inhibited by inhibitors of Ca2+-transport ATPases and V-type H+-ATPase, indicating that intracellular Ca2+ release precedes Ca2+ entry. Inhibition studies and mutant analysis point to the acidosomal Ca2+ stores as a target of fatty acids. Although fatty acids can substitute fully for cAMP with respect to Ca2+ influx in wild-type cells, experiments with a mutant strain revealed that cAMP also sensitizes the Ca2+-entry mechanism: cAMP-induced Ca2+ influx was normal in a phospholipase C knockout mutant but influx was fairly insensitive to arachidonic acid in this strain. This defect could be overcome by higher doses of arachidonic acid which cause sufficient Ca2+ to be released from the stores to trigger extracellular Ca2+ entry.

The role of calcium in aggregation and development of Dictyostelium

Changes in cytosolic Ca2+ play an important role in a wide array of cell types and the control of its concentration depends upon the interplay of many cellular constituents. Resting cells maintain cytosolic calcium ([Ca2+]i) at a low level in the face of steep gradients of extracellular and sequestered Ca2+. Many different signals can provoke the opening of calcium channels in the plasma membrane or in intracellular compartments and cause rapid influx of Ca2+ into the cytosol and elevation of [Ca2+]i. After such stimulation Ca2+ ATPases located in the plasma membrane and in the membranes of intracellular stores rapidly return [Ca2+]i to its basal level. Such responses to elevation of [Ca2+]i are a part of an important signal transduction mechanism that uses calcium (often via the binding protein calmodulin) to mediate a variety of cellular actions responsive to outside influences.

In Dictyostelium discoideum inositol 1,3,4,5-tetrakisphosphate is dephosphorylated by a 3-phosphatase and a 1-phosphatase

The degradation of Ins(1,3,4,5)P4 in Dictyostelium was investigated using a mixture of [3H]Ins(1,3,4,5)P4 and [3-32P]Ins-(1,3,4,5)P4. After incubation of this mixture with a Dictyostelium homogenate the 32P/3H ratio found in the InsP3 product was reduced to 24% of the ratio in the substrate. 32P-labelled inorganic phosphate was found as well, whereas hardly any InsP2 was detected. This indicates that Ins(1,3,4,5)P4 is mainly degraded by a 3-phosphatase. The other enzyme was characterized by identification of the 32P-labelled InsP3 isomer. This isomer did not co-elute with Ins(1,3,4)P3, indicating that no 5-phosphatase was present in Dictyostelium. The 32P-labelled InsP3 could be oxidized using NaIO4. The only InsP3 isomer that has these characteristics is Ins(3,4,5)P3, indicating 1-phosphatase activity. The 1-phosphatase appeared to be dependent on MgCl2, whereas the 3-phosphatase was still active in the absence of MgCl2. An analogue of Ins(1,3,4,5)P4 with a thiophosphate substitution at the 1-position was found to be almost completely resistant to hydrolysis by the 1-phosphatase, but was degraded by the 3-phosphatase.

The role of Ca2+ during spore germination in Dictyostelium: autoactivation is mediated by the mobilization of Ca2+ while amoebal emergence requires entry of external Ca2+

One of the developmental pathways used by the social amoeba Dictyostelium discoideum produces dormant spores. As with any temporary resistant stage, these spores must be able to germinate rapidly in response to positive environmental stimuli. One such stimulus is the autoactivator, an endogenous, diffusible molecule that is secreted by spores. Previous work has shown that three phases of germination, autoactivation, spore swelling and amoebal emergence, require the activity of the Ca(2+)-dependent, regulatory protein calmodulin, implicating Ca2+ as an essential cation during germination. In this study we used a pharmacological approach coupled with the direct measurement of Ca2+ levels in germinating spore populations by atomic adsorption to examine Ca(2+)-dependent signal transduction during spore activation and germination in D. discoideum. Inhibitors of both phospholipase C and internal Ca2+ release inhibited autoactivation while exogenously added Ins(1,4,5)P3, acted synergistically with the autoactivator. The antagonists specifically affected spore activation as mediated by the autoactivator, since neither had any effect on heat-activated spores. In contrast, La3+, an inhibitor of Ca2+ uptake, had little or no effect on either autoactivation or the swelling of autoactivated spores. However, an inhibition of Ca2+ influx by La3+ inhibited both the swelling of heat-activated spores and amoebal emergence following each period of autoactivation or heat activation. Ca2+ levels change in the spore population during germination. During activation and swelling, Ca2+ efflux occurs from the spores. Both of the activating stimuli used here, the autoactivator and heat, caused this Ca2+ efflux. The efflux is reversed during emergence when there is a net Ca2+ uptake by the spores and cells from the medium. Together these data provide the first evidence that autoactivation is mediated by Ca(2+)-dependent signal transduction, leading to Ca2+ efflux, and that the late event of germination, amoebal emergence, requires Ca2+ uptake to proceed. The data also suggest that the responses of the spore to the each of autoactivator and heat, i.e. Ca2+ movements and germination, are mediated by different mechanisms.

Calcium-sequestering organelles of Dictyostelium discoideum: changes in element content during early development as measured by electron probe X-ray microanalysis

Starving Dictyostelium discoideum amoebae aggregate within a few hours by chemotaxis towards the attractant cAMP to form a multicellular organism. The differentiating cells possess rapid and efficient calcium buffering and sequestration systems which enable them to restrict changes in the cytosolic free calcium concentration temporally and spatially during their chemotactic reaction and allow the continuous accumulation of Ca2+ during development. In order to identify and to characterize calcium storage compartments, we analyzed the element content of amoebae at three consecutive stages of differentiation. Determination of the element distribution was done using energy-dispersive X-ray microanalysis of freeze-dried cryosections of rapid-frozen cells. Amoebae were frozen in the vegetative and aggregation-competent state and after formation of aggregates. Aggregation-competent as well as aggregated cells contained mass dense granules with large amounts of calcium together with phosphorous and either potassium or magnesium: in aggregation-competent cells calcium was colocalized with potassium, whereas in aggregated cells the mass dense granules contained calcium and magnesium. Although mass dense granules were also present in undifferentiated, vegetative cells, they contained only low amounts of phosphorous and potassium together with little Ca and Mg. We conclude that during their differentiation D. discoideum cells use an intracellular storage compartment to sequester Ca and other cations constantly throughout development.

Phospholipase C in Dictyostelium discoideum. Cyclic AMP surface receptor and G-protein-regulated activity in vitro

The cellular slime mould Dictyostelium discoideum shows several responses after stimulation with the chemoattractant cAMP, including a transient rise in cyclic AMP (cAMP), cGMP and Ins(1,4,5)P3. In this paper the regulation of phospholipase C in vitro is described. Under our experimental conditions commercial PtdIns(4,5)P2 cannot be used to analyse phospholipase C activity in Dictyostelium lysates, because it is hydrolysed mainly to glycerophosphoinositol instead of Ins(1,4,5)P3. Enzyme activity was determined with endogenous unlabelled PtdInsP2 as a substrate. The product was measured by isotope-dilution assay and identified as authentic Ins(1,4,5)P3. Since phospholipase C is strictly Ca(2+)-dependent, with an optimal concentration range of 1-100 microM, cell lysates were prepared in EGTA and the enzyme reaction was started by adding 10 microM free Ca2+. Phospholipase C activity increased 2-fold during Dictyostelium development up to 8 h of starvation, after which the activity declined to less than 10% of the vegetative level. Enzyme activity in vitro increased up to 2-fold after stimulation of cells with the agonist cAMP in vivo. Addition of 10 microM guanosine 5'-[gamma-thio]triphosphate during lysis activated the enzyme to the same extent, and this effect was antagonized by guanosine 5'-[beta-thio]diphosphate. These results strongly suggest that surface cAMP receptors and G-proteins regulate phospholipase C during Dictyostelium development.

Cyclic AMP- and Ins(1,4,5)P3-induced Ca2+ fluxes in permeabilised cells of Dictyostelium discoideum: cGMP regulates Ca2+ entry across the plasma membrane

Transduction of chemotactic signals in Dictyostelium discoideum apparently involves a precise regulation of the cytosolic Ca2+ concentration. Cyclic AMP stimulation causes Ca2+ influx followed by Ca2+ extrusion, the magnitude of extrusion depending on the state of differentiation. Here, we show that the cAMP receptor controls Ca2+ influx both at the level of entry across the plasma membrane and at the level of transport into Ca2+-sequestering organelles. The use of permeabilised cells allowed us to discriminate between both fluxes. Permeabilised cells still showed the cAMP-induced Ca2+ uptake. The flux across the plasma membrane was more sensitive to Ba2+ and Mn2+, respectively, than Ca2+ sequestration. We have shown previously, using stmF mutants, that cGMP regulates Ca2+ influx. We confirmed this result with the membrane-permeant cGMP-analogue, Sp-8-Br-cGMPS, which enhanced the cAMP-induced Ca2+ influx in intact cells but not the uptake in permeabilised cells, indicating that cGMP regulates Ca2+ influx across the plasma membrane. Occasionally, a fast transient Ca2+ efflux, preceding the influx, occurred in intact cells. A small cAMP-induced Ca2+ release was also found in permeabilised cells. A similarly sized Ca2+ release was elicited by Ins(1,4,5)P3 and could be substituted for by GTP or GTPgammaS. This result suggests that rapid Ca2+ release can be mediated by Ins(1,4,5)P3.

Crystallization of phosphatidylinositol-specific phospholipase C from Bacillus cereus

Phosphatidylinositol-specific phospholipase C (PI-PLC) cleaves phosphoinositides into two parts, lipid-soluble diacylglycerol and the water-soluble phosphorylated inositol. Two crystal forms of Bacillus cereus PI-PLC have been obtained by the vapor diffusion technique. Hexagonal crystals were grown from solutions containing polyethylene glycol (PEG; 4,000 to 8,000 D). The space group of these hexagonal crystals is P6(1)22 (or the enantiomorphic space group P6(5)22), with cell constants a = b = 133 A, and c = 231 A. The crystals diffract to 2.8 A. The second crystalline form was grown from a two-phase PEG (600 D)-sodium citrate solution. The phase diagram and PI-PLC distribution between phases has been determined. The enzyme crystallizes from the PEG-rich phase. The crystals are orthorhombic with space group P2(1)2(1)2(1) (a = 45 A, b = 46 A, c = 160 A), and contain one PI-PLC monomer per asymmetric unit. The orthorhombic crystals diffract to 2.5 A. Both the hexagonal and orthorhombic forms are suitable for crystallographic studies.