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

Advances in the cellular biology, biochemistry, and molecular biology of acidocalcisomes

SUMMARYAcidocalcisomes are organelles conserved during evolution and closely related to the so-called volutin granules of bacteria and archaea, to the acidocalcisome-like vacuoles of yeasts, and to the lysosome-related organelles of animal species. All these organelles have in common their acidity and high content of polyphosphate and calcium. They are characterized by a variety of functions from storage of phosphorus and calcium to roles in Ca2+ signaling, osmoregulation, blood coagulation, and inflammation. They interact with other organelles through membrane contact sites or by fusion, and have several enzymes, pumps, transporters, and channels.

Arachidonic acid is a chemoattractant for Dictyostelium discoideum cells

Cyclic AMP (cAMP)is a natural chemoattractant of the social amoeba Dictyostelium discoideum. It is detected by cell surface cAMP receptors. Besides a signalling cascade involving phosphatidylinositol 3,4,5-trisphosphate (PIP3), Ca2+ signalling has been shown to have a major role in chemotaxis. Previously, we have shown that arachidonic acid (AA) induces an increase in the cytosolic Ca2+ concentration by causing the release of Ca2+ from intracellular stores and activating influx of extracellular Ca2+. Here we report that AA is a chemoattractant for D. discoideum cells differentiated for 8-9 h. Motility towards a glass capillary filled with an AA solution was dose-dependent and qualitatively comparable to cAMP-induced chemotaxis. Ca2+ played an important role in AA chemotaxis of wild-type Ax2 as ethyleneglycol-bis(b-aminoethyl)-N,N,N',N'-tetraacetic acid (EGTA) added to the extracellular buffer strongly inhibited motility. In the HM1049 mutant whose iplA gene encoding a putative Ins(1,4,5)P3 -receptor had been knocked out, chemotaxis was only slightly affected by EGTA. Chemotaxis in the presence of extracellular Ca2+ was similar in both strains. Unlike cAMP, addition of AA to a cell suspension did not change cAMP or cGMP levels. A model for AA chemotaxis based on the findings in this and previous work is presented.

Farnesyl diphosphate synthase is a cytosolic enzyme in Leishmania major promastigotes and its overexpression confers resistance to risedronate

Farnesyl diphosphate synthase is the most likely molecular target of aminobisphosphonates (e.g., risedronate), a set of compounds that have been shown to have antiprotozoal activity both in vitro and in vivo. This protein, together with other enzymes involved in isoprenoid biosynthesis, is an attractive drug target, yet little is known about the compartmentalization of the biosynthetic pathway. Here we show the intracellular localization of the enzyme in wild-type Leishmania major promastigote cells and in transfectants overexpressing farnesyl diphosphate synthase by using purified antibodies generated towards a homogenous recombinant Leishmania major farnesyl diphosphate synthase protein. Indirect immunofluorescence, together with immunoelectron microscopy, indicated that the enzyme is mainly located in the cytoplasm of both wild-type cells and transfectants. Digitonin titration experiments also confirmed this observation. Hence, while the initial step of isoprenoid biosynthesis catalyzed by 3-hydroxy-3-methylglutaryl-coenzyme A reductase is located in the mitochondrion, synthesis of farnesyl diphosphate by farnesyl diphosphate synthase is a cytosolic process. Leishmania major promastigote transfectants overexpressing farnesyl diphosphate synthase were highly resistant to risedronate, and the degree of resistance correlated with the increase in enzyme activity. Likewise, when resistance was induced by stepwise selection with the drug, the resulting resistant promastigotes exhibited increased levels of farnesyl diphosphate synthase. The overproduction of protein under different conditions of exposure to risedronate further supports the hypothesis that this enzyme is the main target of aminobisphosphonates in Leishmania cells.

The behaviour of myo-inositol hexakisphosphate in the presence of magnesium(II) and calcium(II): protein-free soluble InsP6 is limited to 49 microM under cytosolic/nuclear conditions

Progress in the biology of myo-inositol hexakisphosphate (InsP(6)) has been delayed by the lack of a quantitative description of its multiple interactions with divalent cations. Our recent initial description of these [J. Torres, S. Dominguez, M.F. Cerda, G. Obal, A. Mederos, R.F. Irvine, A. Diaz, C. Kremer, J. Inorg. Biochem. 99 (2005) 828-840] predicted that under cytosolic/nuclear conditions, protein-free soluble InsP(6) occurs as Mg(5)(H(2)L), a neutral complex that exists thanks to a significant, but undefined, window of solubility displayed by solid Mg(5)(H(2)L).22H(2)O (L is fully deprotonated InsP(6)). Here we complete the description of the InsP(6)-Mg(2+)-Ca(2+) system, defining the solubilities of the Mg(2+) and Ca(2+) (Ca(5)(H(2)L).16H(2)O) solids in terms of K(s0)=[M(2+)](5)[H(2)L(10-)], with pK(s0)=32.93 for M=Mg and pK(s0)=39.3 for M=Ca. The concentration of soluble Mg(5)(H(2)L) at 37 degrees C and I=0.15M NaClO(4) is limited to 49muM, yet InsP(6) in mammalian cells may reach 100muM. Any cytosolic/nuclear InsP(6) in excess of 49muM must be protein- or membrane-bound, or as solid Mg(5)(H(2)L).22H(2)O, and any extracellular InsP(6) (e.g. in plasma) is surely protein-bound.

Formation of a parasitophorous vacuole in a nonadequate experimental host: electron microscopical and X-ray microanalytical study

An unusual mechanism of formation of a parasitophorous vacuole as a result of interaction between an invasive stage of a parasite (merozoites of a protozoon, Mattesia dispora) and defense response of an insect host, Galleria mellonella is reported. The entire ontogenesis of parasitophorous vacuole can be divided into five morphologically clearly discernible stages. They differed, e.g., in the contents and distribution of elements at subcellular level, as determined by direct in situ elemental analysis of single organelles (electron microprobe X-ray analysis). The method was used in conjunction with electron microscopy to investigate the relationship between the host and the parasite.

Ca2+ regulation in the absence of the iplA gene product in Dictyostelium discoideum

Background

Stimulation of Dictyostelium discoideum with cAMP evokes an elevation of the cytosolic free Ca²⁺ concentration ([Ca²⁺]_i). The [Ca²⁺]_i-change is composed of liberation of stored Ca²⁺ and extracellular Ca²⁺-entry. The significance of the [Ca²⁺]_i-transient for chemotaxis is under debate. Abolition of chemotactic orientation and migration by Ca²⁺-buffers in the cytosol indicates that a [Ca²⁺]_i-increase is required for chemotaxis. Yet, the iplA^- mutant disrupted in a gene bearing similarity to IP₃-receptors of higher eukaryotes aggregates despite the absence of a cAMP-induced [Ca²⁺]_i-transient which favours the view that [Ca²⁺]_i-changes are insignificant for chemotaxis.

Results

We investigated Ca²⁺-fluxes and the effect of their disturbance on chemotaxis and development of iplA^- cells. Differentiation was altered as compared to wild type amoebae and sensitive towards manipulation of the level of stored Ca²⁺. Chemotaxis was impaired when [Ca²⁺]_i-transients were suppressed by the presence of a Ca²⁺-chelator in the cytosol of the cells. Analysis of ion fluxes revealed that capacitative Ca²⁺-entry was fully operative in the mutant. In suspensions of intact and permeabilized cells cAMP elicited extracellular Ca²⁺-influx and liberation of stored Ca²⁺, respectively, yet to a lesser extent than in wild type. In suspensions of partially purified storage vesicles ATP-induced Ca²⁺-uptake and Ca²⁺-release activated by fatty acids or Ca²⁺-ATPase inhibitors were similar to wild type. Mn²⁺-quenching of fura2 fluorescence allows to study Ca²⁺-influx indirectly and revealed that the responsiveness of mutant cells was shifted to higher concentrations: roughly 100 times more Mn²⁺ was necessary to observe agonist-induced Mn²⁺-influx. cAMP evoked a [Ca²⁺]_i-elevation when stores were strongly loaded with Ca²⁺, again with a similar shift in sensitivity in the mutant. In addition, basal [Ca²⁺]_i was significantly lower in iplA^- than in wild type amoebae.

Conclusion

These results support the view that [Ca²⁺]_i-transients are essential for chemotaxis and differentiation. Moreover, capacitative and agonist-activated ion fluxes are regulated by separate pathways that are mediated either by two types of channels in the plasma membrane or by distinct mechanisms coupling Ca²⁺-release from stores to Ca²⁺-entry in Dictyostelium. The iplA^- strain retains the capacitative Ca²⁺-entry pathway and an impaired agonist-activated pathway that operates with reduced efficiency or at higher ionic pressure.

cAMP controls cytosolic Ca2+ levels in Dictyostelium discoideum

BACKGROUND

Differentiating Dictyostelium discoideum amoebae respond upon cAMP-stimulation with an increase in the cytosolic free Ca2+ concentration ([Ca2+]i) that is composed of liberation of stored Ca2+ and extracellular Ca2+-influx. In this study we investigated whether intracellular cAMP is involved in the control of [Ca2+]i.

RESULTS

We analyzed Ca2+-fluxes in a mutant that is devoid of the main cAMP-phosphodiesterase (PDE) RegA and displays an altered cAMP metabolism. In suspensions of developing cells cAMP-activated influx of extracellular Ca2+ was reduced as compared to wild type. Yet, single cell [Ca2+]i-imaging of regA- amoebae revealed a cAMP-induced [Ca2+]i increase even in the absence of extracellular Ca2+. The cytosolic presence of the cAMP PDE inhibitor 3-isobutyl-1-methylxanthine (IBMX) induced elevated basal [Ca2+]i in both, mutant and wild type cells. Under this condition wild type cells displayed cAMP-activated [Ca2+]i-transients also in nominally Ca2+-free medium. In the mutant strain the amplitude of light scattering oscillations and of accompanying cAMP oscillations were strongly reduced to almost basal levels. In addition, chemotactic performance during challenge with a cAMP-filled glass capillary was altered by EGTA-incubation. Cells were more sensitive to EGTA treatment than wild type: already at 2 mM EGTA only small pseudopods were extended and chemotactic speed was reduced.

CONCLUSION

We conclude that there is a link between the second messengers cAMP and Ca2+. cAMP-dependent protein kinase (PKA) could provide for this link as a membrane-permeable PKA-activator also increased basal [Ca2+]i of regA- cells. Intracellular cAMP levels control [Ca2+]i by regulating Ca2+-fluxes of stores which in turn affect Ca2+-influx, light scattering oscillations and chemotactic performance.

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.

Cytosolic [Ca2+] transients in dictyostelium discoideum depend on the filling state of internal stores and on an active sarco/endoplasmic reticulum calcium ATPase (SERCA) Ca2+ pump

Stimulation of Dictyostelium discoideum with cAMP evokes a change of the cytosolic free Ca(2+) concentration ([Ca(2+)](i)). We analyzed the role of the filling state of Ca(2+) stores for the [Ca(2+)] transient. Parameters tested were the height of the [Ca(2+)](i) elevation and the percentage of responding amoebae. After loading stores with Ca(2+), cAMP induced a [Ca(2+)](i) transient in many cells. Without prior loading, cAMP evoked a [Ca(2+)](i) change in a few cells only. This indicates that the [Ca(2+)](i) elevation is not mediated exclusively by Ca(2+) influx but also by Ca(2+) release from stores. Reducing the Ca(2+) content of the stores by EGTA preincubation led to a cAMP-activated [Ca(2+)](i) increase even at low extracellular [Ca(2+)]. Moreover, the addition of Ca(2+) itself elicited a capacitative [Ca(2+)](i) elevation. This effect was not observed when stores were emptied by the standard technique of inhibiting internal Ca(2+) pumps with 2,5-di-(t-butyl)-1,4-hydroquinone. Therefore, in Dictyostelium, an active internal Ca(2+)-ATPase is absolutely required to allow for Ca(2+) entry. No influence of the filling state of stores on Ca(2+) influx characteristics was found by the Mn(2+)-quenching technique, which monitors the rate of Ca(2+) entry. Both basal and cAMP-activated Mn(2+) influx rates were similar in control cells and cells with empty stores. By contrast, determination of extracellular free Ca(2+) concentration ([Ca(2+)](e)) changes, which represent the sum of Ca(2+) influx and efflux, revealed a higher rate of [Ca(2+)](e) decrease in EGTA-treated than in control amoebae. We conclude that emptying of Ca(2+) stores does not change the rate of Ca(2+) entry but results in inhibition of the plasma membrane Ca(2+)-ATPase. Furthermore, the activities of the Ca(2+) transport ATPases of the stores are of crucial importance for the regulation of [Ca(2+)](i) changes.

Acidocalcisomes are functionally linked to the contractile vacuole of Dictyostelium discoideum

The mass-dense granules of Dictyostelium discoideum were shown to contain large amounts of phosphorus, magnesium, and calcium, as determined by x-ray microanalysis, either in situ or when purified using iodixanol gradient centrifugation. The high phosphorus content was due to the presence of pyrophosphate and polyphosphate, which were also present in the contractile vacuoles. Both organelles also possessed a vacuolar H(+)-ATPase, an H(+)-pyrophosphatase, and a Ca(2+)-ATPase, as determined by biochemical methods or by immunofluorescence microscopy. The H(+)-pyrophosphatase activity of isolated mass-dense granules was stimulated by potassium ions and inhibited by the pyrophosphate analogs aminomethylenediphosphonate and imidodiphosphate and by KF and N-ethylmaleimide in a dose-dependent manner. The mass-dense granules and the contractile vacuole appeared to contact each other when the cells were submitted to hyposmotic stress. Acetazolamide inhibited the carbonic anhydrase activity of the contractile vacuoles and prolonged their contraction cycle in a dose-dependent manner. Similar effects were observed with the anion exchanger inhibitor 4,4' -diisothiocyanatodihydrostilbene-2, 2' -disulfonic acid and the vacuolar H(+)-ATPase inhibitor bafilomycin A(1). Together, these results suggest that the mass-dense granules of D. discoideum are homologous to the acidocalcisomes described in protozoan parasites and are linked to the function of the contractile vacuole.

Calcium stores in differentiated Dictyostelium discoideum: prespore cells sequester calcium more efficiently than prestalk cells

Dictyostelium discoideum pseudoplasmodia exhibit a gradient of the cytosolic free Ca2+-concentration ([Ca2+]i) along their anterior-posterior axis involved in cell-type specific differentiation. [Ca2+]i is high in prestalk and low in prespore cells. We determined the content and localization of calcium and other elements in cryosectioned cells of pseudoplasmodia and fruiting bodies by X-ray microanalysis. Granular stores rich in Ca, Mg and P were identified. Average Ca was higher in prespore than prestalk granules (225vs 111 mmol/kg dry weight). Total Ca stored in granules was also higher in prespore than prestalk cells. The amount of P and S in granules differed between the two cell types indicating different store composition. In spores mean granular Ca was 120 mmol/kg dry weight. Stalk cells had smaller granules with 360 mmol Ca/kg dry weight. Complementary to microanalysis, vesicular Ca2+-fluxes were studied in fractionated cell homogenates. The rate of Ca2+-uptake was higher in pellet fractions of prespore than prestalk amoebae (4.7 vs 3.4 nmol/min x mg). Ca2+-release was greater in supernatant fractions from prestalk than prespore cells (16.5vs 7.7 nmol/10(8)cells). In summary, prestalk and prespore cells possess qualitatively different, high-capacity stores containing distinct amounts of Ca and probably being involved in regulation of the anterior-posterior [Ca2+]i-gradient.

A plasma membrane-type Ca(2+)-ATPase co-localizes with a vacuolar H(+)-pyrophosphatase to acidocalcisomes of Toxoplasma gondii

Ca(2+)-ATPases are likely to play critical roles in the biochemistry of Toxoplasma gondii, since these protozoa are obligate intracellular parasites and the Ca(2+) concentration in their intracellular location is three orders of magnitude lower than in the extracellular medium. Here, we report the cloning and sequencing of a gene encoding a plasma membrane-type Ca(2+)-ATPase (PMCA) of T.gondii (TgA1). The predicted protein (TgA1) exhibits 32-36% identity to vacuolar Ca(2+)-ATPases of Trypanosoma cruzi, Saccharomyces cerevisiae, Entamoeba histolytica and Dictyostelium discoideum. Sequencing of both cDNA and genomic DNA from T.gondii indicated that TgA1 contains two introns near the C-terminus. A hydropathy profile of the protein suggests 10 transmembrane domains. TgA1 suppresses the Ca(2+) hypersensitivity of a mutant of S.cerevisiae that has a defect in vacuolar Ca(2+) accumulation. Indirect immunofluorescence and immunoelectron microscopy analysis indicate that TgA1 localizes to the plasma membrane and co-localizes with the vacuolar H(+)-pyrophosphatase to intracellular vacuoles identified morphologically and by X-ray microanalysis as the acidocalcisomes. This vacuolar-type Ca(2+)-ATPase could play an important role in Ca(2+) homeostasis in T.gondii.

Acidocalcisome: A novel Ca2+ storage compartment in trypanosomatids and apicomplexan parasites

Acidocalcisomes are novel acidic Ca2+ storage organelles found in trypanosomatids and apicomplexan parasites, abundant in the intracellular stages of these parasites, and characterized by their high electron density, and high content of phosphorus, Ca2+, Mg2+, Na+ and Zn2+. A number of energy-utilizing pumps and exchangers have been found in these organelles, which underlines their importance in the homeostasis of different elements, as discussed here by Roberto Docampo and Silvia Moreno.

Presence of a plant-like proton-pumping pyrophosphatase in acidocalcisomes of Trypanosoma cruzi

The vacuolar-type proton-translocating pyrophosphatase (V-H+-PPase) is an enzyme previously described in detail only in plants. This paper demonstrates its presence in the trypanosomatid Trypanosoma cruzi. Pyrophosphate promoted organellar acidification in permeabilized amastigotes, epimastigotes, and trypomastigotes of T. cruzi. This activity was stimulated by K+ ions and was inhibited by Na+ ions and pyrophosphate analogs, as is the plant activity. Separation of epimastigote extracts on Percoll gradients yielded a dense fraction that contained H+-PPase activity measured both by proton uptake and phosphate release but lacked markers for mitochondria, lysosomes, glycosomes, cytosol, and plasma membrane. Antiserum raised against specific sequences of the plant V-H+-PPase cross-reacted with a T. cruzi protein, which was also detectable in the dense Percoll fraction. The organelles in this fraction appeared by electron microscopy to consist mainly of acidocalcisomes (acidic calcium storage organelles). This identification was confirmed by x-ray microanalysis. Immunofluorescence and immunoelectron microscopy indicated that the V-H+-PPase was located in the plasma membrane and acidocalcisomes of the three different forms of the parasite. Pyrophosphate was able to drive calcium uptake in permeabilized T. cruzi. This uptake depended upon a proton gradient and was reversed by a specific V-H+-PPase inhibitor. Our results imply that the phylogenetic distribution of V-H+-PPases is much wider than previously perceived but that the enzyme has a unique subcellular location in trypanosomes.

In situ compositional analysis of acidocalcisomes in Trypanosoma cruzi

We measured the elemental content of different compartments in Trypanosoma cruzi epimastigotes using quick freezing, ultracryomicrotomy, and electron probe microanalysis. Vacuoles identified by high electron density contained (in units of mmol/kg dry weight +/- S.E.) large amounts of phosphorus (1390 +/- 13), magnesium (646 +/- 19), calcium (171 +/- 5), sodium (161 +/- 18), and zinc (148 +/- 6). No other compartment had appreciable calcium or zinc content. Iron (128 +/- 16 mmol/kg) was detected only in vacuoles distinct from the electron-dense vacuoles and other organelles. Incubation of cells for 70 min in culture medium in the presence of ionomycin plus nigericin led to a very significant 3- or 2-fold increase in potassium in the electron-dense vacuoles and the iron-rich vacuoles, respectively, with no significant change in the other elements investigated. This indicated the acidic nature of the vacuoles and demonstrated that the electron-dense vacuoles correspond to what were described previously as acidocalcisomes, i.e. acidic compartments rich in Ca2+. The acidocalcisomes were investigated by separation of epimastigote fractions on Percoll gradients in combination with Triton WR-1339 treatment. This detergent caused a rapid vacuolation; these vacuoles were shown by electron microscopy to be largely transparent, with a diffuse matrix. Percoll gradient fractionation demonstrated decreases in the density of various organelle markers in detergent-treated cells compared with controls. Large decreases in the density of the acidocalcisome and the mitochondrion were seen, as well as smaller decreases in the density of the other markers. Conventional electron microscopy of epimastigotes loaded with gold-labeled transferrin indicated that the endosomal system was separate from vacuoles that probably corresponded to the calcium-containing organelles detected by electron probe microanalysis. The combined results provide evidence that acidocalcisomes are organelles different from lysosomes or other organelles previously described in these parasites.

Calmidazolium leads to an increase in the cytosolic Ca2+ concentration in Dictyostelium discoideum by induction of Ca2+ release from intracellular stores and influx of extracellular Ca2+

The Ca2+ stores of Dictyostelium discoideum amoebae take part in control of homoeostasis of the cytosolic free Ca2+ concentration ([Ca2+]i) and the cyclic-AMP-induced [Ca2+]i-signalling cascade. In order to characterize regulatory mechanisms of these stores, we incubated cells with the calmodulin antagonist calmidazolium. Measurement of permeabilized and intact cells in suspension with a Ca(2+)-sensitive electrode revealed that calmidazolium induced Ca2+ release from intracellular stores, influx of Ca2+ across the plasma membrane and subsequent efflux. In single fura-2-loaded cells calmidazolium evoked rapid and global transient elevations of [Ca2+]i. Other calmodulin antagonists (trifluoperazine, chlorpromazine, fendiline and W7) also induced transient elevations of [Ca2+]i, which were, however, slower and observed in fewer cells. The calmidazolium-induced influx of extracellular Ca2+ was inhibited by preincubation with 2,5-di-(t-butyl)-1, 4-hydroquinone (BHQ) and 7-chloro-4-nitrobenz-2-oxa-1,3-diazole (NBD-Cl), both known to interact with pumps of the inositol 1,4,5-trisphosphate (IP3)-sensitive store, and by the V-type H(+)-ATPase inhibitor bafilomycin A1, which affects the acidosomal Ca2+ store. Incubation with pump inhibitors did not itself induce changes in [Ca2+]i. We conclude that the effects of calmidazolium are, at least in part, mediated by its calmodulin-antagonizing properties, that it acts by inducing Ca2+ release from filled storage compartments, and that its target of action is both the IP3-sensitive store and the acidosome; emptying of these stores leads to influx of extracellular Ca2+.

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.

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.

Cloning and expression in Escherichia coli of a cDNA encoding a developmentally regulated Ca(2+)-binding protein from Dictyostelium discoideum

We have cloned a full-length cDNA from Dictyostelium discoideum which encodes a new Ca(2+)-binding protein. The deduced protein (termed CBP1) is composed of 156 amino acids and contains four consensus metal-ligating loop sequences found in helix-loop-helix motifs of many Ca(2+)-binding proteins. When expressed in bacteria as a GST fusion protein, CBP1 binds Ca2+ in a 45Ca2+ overlay assay. CBP1 exhibits little amino acid sequence homology with Dictyostelium calmodulin or calfumirin-1 (CAF-1) except in the putative Ca(2+)-binding regions. Moreover, unlike calmodulin and CAF-1 expression, CBP1 mRNA is expressed preferentially during the multicellular stages of development.

Introduction of calcium buffers into the cytosol of Dictyostelium discoideum amoebae alters cell morphology and inhibits chemotaxis

Differentiating Dictyostelium discoideum amoebae respond chemotactically towards the attractant cAMP. To test whether chemotaxis requires the establishment of a spatial gradient of the cytosolic calcium concentration ([Ca2+]i) we scrape-loaded calcium chelating agents with different affinities for Ca2+ into the cytosol of the cells. The buffers were 1,2-bis(o-aminophenoxy)-ethane-N,N,N',N'-tetraacetic acid (BAPTA) and its derivatives. Parameters analyzed were general cell morphology and the capability to protrude pseudopods and to migrate towards a cAMP-filled capillary. The chelators dose- and time-dependently inhibited spreading of the amoebae on the substrate. Both oriented pseudopod formation and locomotion of the cells were reduced. This effect was overcome by extracellular Ca2+, but not Mg2+. The effects of BAPTA derivatives were compared to the inhibition by BAPTA. A dose-response curve was obtained; 5,5'-difluoro-BAPTA was the most potent analogue. We conclude that a [Ca2+]i-gradient is necessary for orientation and locomotion. Chemotaxis experiments performed in the presence of extracellular EGTA revealed that liberation of Ca2+ from intracellular stores is sufficient for pseudopod formation; yet under physiological conditions influx of extracellular Ca2+ is also used to establish the gradient.