cAMP controls cytosolic Ca2+ levels in Dictyostelium discoideum

Neuropeptidergic integration of behavior in Trichoplax adhaerens, an animal without synapses

Trichoplax adhaerens is a flat, millimeter-sized marine animal that adheres to surfaces and grazes on algae. Trichoplax displays a repertoire of different feeding behaviors despite the apparent absence of a true nervous system with electrical or chemical synapses. It glides along surfaces to find food, propelled by beating cilia on cells at its ventral surface, and pauses during feeding by arresting ciliary beating. We found that when endomorphin-like peptides are applied to an animal, ciliary beating is arrested, mimicking natural feeding pauses. Antibodies against these neuropeptides label cells that express the neurosecretory proteins and voltage-gated calcium channels implicated in regulated secretion. These cells are embedded in the ventral epithelium, where they comprise only 4% of the total, and are concentrated around the edge of the animal. Each bears a cilium likely to be chemosensory and used to detect algae. Trichoplax pausing during feeding or spontaneously in the absence of food often induce their neighbors to pause as well, even neighbors not in direct contact. Pausing behavior propagates from animal to animal across distances much greater than the signal that diffuses from just one animal, so we presume that the peptides secreted from one animal elicit secretion from nearby animals. Signal amplification by peptide-induced peptide secretion explains how a small number of sensory secretory cells lacking processes and synapses can evoke a wave of peptide secretion across the entire animal to globally arrest ciliary beating and allow pausing during feeding.

Moving towards a paradigm: common mechanisms of chemotactic signaling in Dictyostelium and mammalian leukocytes

Chemotaxis, or directed migration of cells along a chemical gradient, is a highly coordinated process that involves gradient sensing, motility, and polarity. Most of our understanding of chemotaxis comes from studies of cells undergoing amoeboid-type migration, in particular the social amoeba Dictyostelium discoideum and leukocytes. In these amoeboid cells the molecular events leading to directed migration can be conceptually divided into four interacting networks: receptor/G protein, signal transduction, cytoskeleton, and polarity. The signal transduction network occupies a central position in this scheme as it receives direct input from the receptor/G protein network, as well as feedback from the cytoskeletal and polarity networks. Multiple overlapping modules within the signal transduction network transmit the signals to the actin cytoskeleton network leading to biased pseudopod protrusion in the direction of the gradient. The overall architecture of the networks, as well as the individual signaling modules, is remarkably conserved between Dictyostelium and mammalian leukocytes, and the similarities and differences between the two systems are the subject of this review.

Computational modelling suggests dynamic interactions between Ca2+, IP3 and G protein-coupled modules are key to robust Dictyostelium aggregation

Under conditions of starvation, Dictyostelium cells begin a programme of development during which they aggregate to form a multicellular structure by chemotaxis, guided by propagating waves of cyclic AMP that are relayed robustly from cell to cell. In this paper, we develop and analyse a new model for the intracellular and extracellular cAMP dependent processes that regulate Dictyostelium migration. The model allows, for the first time, a quantitative analysis of the dynamic interactions between calcium, IP(3) and G protein-dependent modules that are shown to be key to the generation of robust cAMP oscillations in Dictyostelium cells. The model provides a mechanistic explanation for the transient increase in cytosolic free Ca(2+) concentration seen in recent experiments with the application of the calmodulin inhibitor calmidazolium (R24571) to Dictyostelium cells, and also allows elucidation of the effects of varying both the conductivity of stretch-activated channels and the concentration of external phosphodiesterase on the oscillatory regime of an individual cell. A rigorous analysis of the robustness of the new model shows that interactions between the different modules significantly reduce the sensitivity of the resulting cAMP oscillations to variations in the kinetics of different Dictyostelium cells, an essential requirement for the generation of the spatially and temporally synchronised chemoattractant cAMP waves that guide Dictyostelium aggregation.

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.

A fast Ca2+-induced Ca2+-release mechanism in Dictyostelium discoideum

In vertebrate cells calcium-induced calcium release (CICR) is thought to be responsible for rapid cytosolic Ca(2+) elevations despite the occurrence of strong Ca(2+) buffering within the cytosol. In Dictyostelium, a CICR mechanism has not been reported. While analyzing Ca(2+) regulation in a vesicular fraction of Dictyostelium rich in Ca(2+)-flux activity, containing contractile vacuoles (CV) as the main component of acidic Ca(2+) stores and ER, we detected a rapid Ca(2+) change upon addition of Ca(2+) (CIC). CIC was three times larger in active stores accumulating Ca(2+) than before Ca(2+) uptake and in inactivated stores. Ca(2+) release was demonstrated with the calmodulin antagonist W7 that inhibits the V-type H(+)ATPase activity and Ca(2+) uptake of acidic Ca(2+) stores. W7 caused a rapid and large increase of extravesicular Ca(2+) ([Ca(2+)](e)), much faster and larger than thapsigargin (Tg), a Ca(2+)-uptake inhibitor of the ER. W7 treatment blocked CIC indicating that a large part of CIC is due to Ca(2+) release. The height of CIC depended on the filling state of the Ca(2+) stores. CIC was virtually unchanged in the iplA(-) strain that lacks a putative IP(3) or ryanodine receptor thought to be located at the endoplasmic reticulum. By contrast, CIC was reduced in two mutants, HGR8 and lvsA(-), that are impaired in acidic Ca(2+)-store function. Purified Ca(2+) stores enriched in CV still displayed CIC, indicating that CV are a source of Ca(2+)-induced Ca(2+) release. CIC-defective mutants were altered in their oscillatory properties. The irregularity of the HGR8 oscillation suggests that the principal oscillator is affected in this mutant.

Influx of extracellular Ca2+ is necessary for electrotaxis in Dictyostelium

Intracellular free Ca2+ ([Ca2+](i)) is a pivotal signalling element in cell migration and is thought to be required for chemotaxis of Dictyostelium. Ca2+ signalling may also be important for electrotaxis. However this suggestion has been controversial. We show that electric fields direct Dictyostelium cells to migrate cathodally and increase [Ca2+](i) in Dictyostelium cells, as determined by Fluo-3 AM imaging and (45)Ca2+ uptake. Omission of extracellular Ca2+([Ca2+](e)) and incubation with EGTA abolished the electric-field-stimulated [Ca2+](i) rise and directional cell migration. This suggests a requirement for [Ca2+](e) in the electrotactic response. Deletion of iplA, a gene responsible for chemoattractant-induced [Ca2+](i) increase, had only a minor effect on the electric-field-induced [Ca2+](i) rise. Moreover, iplA-null Dictyostelium cells showed the same electrotactic response as wild-type cells. Therefore, iplA-independent Ca2+ influx is necessary for electrotactic cell migration. These results suggest that the [Ca2+](i) regulatory mechanisms induced by electric fields are different from those induced by cAMP and folic acid in Dictyostelium cells. Different roles of the iplA gene in chemoattractant-induced and electrically induced Ca2+ signalling, and different effects of [Ca2+](i) elevation on chemotaxis and electrotaxis indicate that the chemoattractant and electric cues activate distinctive initial signalling elements.

Like a `rolling stone': quantitative analysis of the body movement and skeletal dynamics of the sponge Tethya wilhelma

Although sponges (Porifera) are basal Metazoa without muscles and a central nervous system, they are able to locomote, which is generally correlated to drastic morphological changes. This behaviour has been known for more almost 150 years, but it is only partly understood. The sponge T. wilhelmadisplays extraordinary movement and rhythmic body contractions, and is thus a valuable model for the investigation of sponge movement. The aims of the present study were to track T. wilhelma quantitatively on natural and artificial substrates, to test for a peristaltic movement mechanism and to check for the influence of morphological changes. T. wilhelmadisplays a unique mode of locomotion among sponges, without reorganizing the whole sponge body. The overall morphology was stable, and skeletal rotation during movement was shown; this is the first time that such movement has been demonstrated in a sponge. The stability of the skeletal superstructure arrangement during movement suggests that only the cortical tissue is involved in movement, with only local tissue rearrangements. The movement track followed a straight direction for long periods, but directions could be altered instantly. It is most likely that environmental conditions play an important roll in induction of movement. In summary, T. wilhelmaresembles the proverbial `rolling stone' that stays at a given location if the conditions are favourable and starts moving when conditions change for the worse.

Contribution of endoplasmic reticulum to Ca(2+) signals in Dictyostelium depends on extracellular Ca(2+)

We recently reported the first molecular genetic evidence that Dictyostelium Ca(2+) responses to chemoattractants include a contribution from the endoplasmic reticulum (ER) - responses are enhanced in mutants lacking calreticulin or calnexin, two major Ca(2+)-binding proteins in the ER, even though the influx of Ca(2+) into the mutants is reduced. Compared with wild-type cells, the ER in the mutants contributes at least 30-70 nM additional Ca(2+) to the responses. Here we report that this additional ER contribution to the cytosolic Ca(2+) signal depends upon extracellular Ca(2+)- it does not occur in the absence of extracellular Ca(2+), increases to a maximum as the extracellular Ca(2+) levels rise to 10 microM and then remains constant at extracellular Ca(2+) concentrations up to at least 250 microM. These results suggest that Ca(2+) influx causes the intracellular release, in the simplest scenario by a mechanism involving Ca(2+)-induced Ca(2+) release from the ER. By way of contrast, we show that Ca(2+) responses to mechanical stimulation are reduced, but still occur in the absence of extracellular Ca(2+). Unlike the responses to chemoattractants, mechanoresponses thus include contributions from the ER that are independent of extracellular Ca(2+).

The contractile vacuole in Ca2+-regulation in Dictyostelium: its essential function for cAMP-induced Ca2+-influx

BACKGROUND

cAMP-induced Ca2+-influx in Dictyostelium is controlled by at least two non-mitochondrial Ca2+-stores: acidic stores and the endoplasmic reticulum (ER). The acidic stores may comprise the contractile vacuole network (CV), the endosomal compartment and acidocalcisomes. Here the role of CV in respect to function as a potential Ca2+-store was investigated.

RESULTS

Dajumin-GFP labeled contractile vacuoles were purified 7-fold by anti-GFP-antibodies in a magnetic field. The purified CV were shown for the first time to accumulate and release Ca2+. Release of Ca2+ was elicited by arachidonic acid or the calmodulin antagonist W7, the latter due to inhibition of the pump. The characteristics of Ca2+-transport and Ca2+-release of CV were compared to similarly purified vesicles of the ER labeled by calnexin-GFP. Since the CV proved to be a highly efficient Ca2+-compartment we wanted to know whether or not it takes part in cAMP-induced Ca2+-influx. We made use of the LvsA--mutant expected to display reduced Ca2+-transport due to loss of calmodulin. We found a severe reduction of cAMP-induced Ca2+-influx into whole cells.

CONCLUSION

The contractile vacuoles in Dictyostelium represent a highly efficient acidic Ca2+-store that is required for cAMP-induced Ca2+-influx.

Neuroactive substances specifically modulate rhythmic body contractions in the nerveless metazoon Tethya wilhelma (Demospongiae, Porifera)

BACKGROUND

Sponges (Porifera) are nerve- and muscleless metazoa, but display coordinated motor reactions. Therefore, they represent a valuable phylum to investigate coordination systems, which evolved in a hypothetical Urmetazoon prior to the central nervous system (CNS) of later metazoa. We have chosen the contractile and locomotive species Tethya wilhelma (Demospongiae, Hadromerida) as a model system for our research, using quantitative analysis based on digital time lapse imaging. In order to evaluate candidate coordination pathways, we extracorporeally tested a number of chemical messengers, agonists and antagonists known from chemical signalling pathways in animals with CNS.

RESULTS

Sponge body contraction of T. wilhelma was induced by caffeine, glycine, serotonine, nitric oxide (NO) and extracellular cyclic adenosine monophosphate (cAMP). The induction by glycine and cAMP followed patterns varying from other substances. Induction by cAMP was delayed, while glycine lead to a bi-phasic contraction response. The frequency of the endogenous contraction rhythm of T. wilhelma was significantly decreased by adrenaline and NO, with the same tendency for cAMP and acetylcholine. In contrast, caffeine and glycine increased the contraction frequency. The endogenous rhythm appeared irregular during application of caffeine, adrenaline, NO and cAMP. Caffeine, glycine and NO attenuated the contraction amplitude. All effects on the endogenous rhythm were neutralised by the washout of the substances from the experimental reactor system.

CONCLUSION

Our study demonstrates that a number of chemical messengers, agonists and antagonists induce contraction and/or modulate the endogenous contraction rhythm and amplitude of our nerveless model metazoon T. wilhelma. We conclude that a relatively complex system of chemical messengers regulates the contraction behaviour through auto- and paracrine signalling, which is presented in a hypothetical model. We assume that adrenergic, adenosynergic and glycinergic pathways, as well as pathways based on NO and extracellular cAMP are candidates for the regulation and timing of the endogenous contraction rhythm within pacemaker cells, while GABA, glutamate and serotonine are candidates for the direct coordination of the contractile cells.