Adenylyl cyclase localization regulates streaming during chemotaxis

Dictyostelium morphogenesis

During starvation-induced Dictyostelium development, up to several hundred thousand amoeboid cells aggregate, differentiate and form a fruiting body. The chemotactic movement of the cells is guided by the rising phase of the outward propagating cAMP waves and results in directed periodic movement towards the aggregation centre. In the mound and slug stages of development, cAMP waves continue to play a major role in the coordination of cell movement, cell-type-specific gene expression and morphogenesis; however, in these stages where cells are tightly packed, cell-cell adhesion/contact-dependent signalling mechanisms also play important roles in these processes.

Breaking symmetries: regulation of Dictyostelium development through chemoattractant and morphogen signal-response

Dictyostelium discoideum grow unicellularly, but develop as multicellular organisms. At two stages of development, their underlying symmetrical pattern of cellular organization becomes disrupted. During the formation of the multicellular aggregate, individual non-polarized cells re-organize their cytoskeletal structures to sequester specific intracellular signaling elements for activation by and directed movement within chemoattractant gradients. Subsequently, response to secreted morphogens directs undifferentiated populations to adopt different cell fates. Using a combination of cellular, biochemical and molecular approaches, workers have now begun to understand the mechanisms that permit Dictyostelium (and other chemotactic cells) to move directionally in shallow chemoattractant gradients and the transcriptional regulatory pathways that polarize cell-fate choice and initiate pattern formation.

Chemoattractant signaling in dictyostelium discoideum

Dictyostelium is an accessible organism for studies of signaling via chemoattractant receptors. Chemoattractant-mediated signaling events and components are reviewed and presented as a series of connected modules, including excitation, inhibition, G protein-independent responses, early gene expression, inositol lipids, PH domain-containing proteins, cyclic AMP signaling, polarization acquisition, actin polymerization, and cortical myosin. The network incorporates information from biochemical, genetic, and cell biological experiments carried out on living cells. The modules and connections represent current understanding, and future information is expected to modify and build upon this structure.

The PI3K-Mediated Activation of CRAC Independently Regulates Adenylyl Cyclase Activation and Chemotaxis

The ability of a cell to detect an external chemical signal and initiate a program of directed migration along a gradient comprises the fundamental process called chemotaxis. Investigations in Dictyostelium discoideum and neutrophils have established that pleckstrin homology (PH) domain-containing proteins that bind to the PI3K products PI(3,4)P2 and PI(3,4,5)P3, such as CRAC (cytosolic regulator of adenylyl cyclase) and Akt/PKB, translocate specifically to the leading edge of chemotaxing cells. CRAC is essential for the chemoattractant-mediated activation of the adenylyl cyclase ACA, which converts ATP into cAMP, the primary chemoattractant for D. discoideum. The mechanisms by which CRAC activates ACA remain to be determined. We now show that in addition to its essential role in the activation of ACA, CRAC is involved in regulating chemotaxis. Through mutagenesis, we show that these two functions are independently regulated downstream of PI3K. A CRAC mutant that has lost the capacity to bind PI3K products does not support chemotaxis and shows minimal ACA activation. Finally, overexpression of CRAC and various CRAC mutants show strong effects on ACA activation with little effect on chemotaxis. These findings establish that chemoattractant-mediated activation of PI3K is important for the CRAC-dependent regulation of both chemotaxis and adenylyl cyclase activation.

Spatiotemporal localization of the calcium-stimulated adenylate cyclases, AC1 and AC8, during mouse brain development

Type 1 and type 8 adenylate cyclases, AC1 and AC8, are membrane bound enzymes that produce cAMP in response to calcium entry and could thus control a large number of developmental processes. We provide a detailed spatiotemporal localization of these genes in the mouse brain during embryonic and postnatal life using in situ hybridization. AC1 gene expression begins early in embryonic life (before E13), and its expression is much more widespread than in adults. Transient expression of AC1 is found in the striatum, the dorsal thalamus, the trigeminal nerve nuclei, the Purkinje cells of the cerebellum, the interneurons of the hippocampus, and the retinal ganglion cells. In all these structures, the peak of AC1 gene expression occurs during early postnatal life, decreasing by P10. After P15, AC1 expression is confined to the hippocampus, the cerebral cortex, and to the granule cells of the cerebellum. AC8 gene expression also begins early in embryonic life (E12)--but in a more limited number of regions than in adults. AC8 expression is initially restricted to the epithalamus, the hypothalamus, the superior colliculus, the cerebellar anlage the proliferative zone of the rhombic lip, and the spinal cord. The expression increases and broadens during postnatal life, particularly in the thalamus and the cerebral cortex. A transient peak of AC8 expression is found in layer IV of the somatosensory cortex. Thus, AC1 and AC8 have an early developmental onset with complementary spatiotemporal distribution patterns: AC1 is most broadly distributed in embryonic life, whereas AC8 is most broadly expressed in adulthood. Transient expression of these genes designate areas that may be particularly sensitive to neural activity/calcium-modulated cAMP responses during development.

Activation of soluble guanylyl cyclase at the leading edge during Dictyostelium chemotaxis

Dictyostelium contains two guanylyl cyclases, GCA, a 12-transmembrane enzyme, and sGC, a homologue of mammalian soluble adenylyl cyclase. sGC provides nearly all chemoattractant-stimulated cGMP formation and is essential for efficient chemotaxis toward cAMP. We show that in resting cells the major fraction of the sGC-GFP fusion protein localizes to the cytosol, and a small fraction is associated to the cell cortex. With the artificial substrate Mn2+/GTP, sGC activity and protein exhibit a similar distribution between soluble and particulate fraction of cell lysates. However, with the physiological substrate Mg2+/GTP, sGC in the cytosol is nearly inactive, whereas the particulate enzyme shows high enzyme activity. Reconstitution experiments reveal that inactive cytosolic sGC acquires catalytic activity with Mg2+/GTP upon association to the membrane. Stimulation of cells with cAMP results in a twofold increase of membrane-localized sGC-GFP, which is accompanied by an increase of the membrane-associated guanylyl cyclase activity. In a cAMP gradient, sGC-GFP localizes to the anterior cell cortex, suggesting that in chemotacting cells, sGC is activated at the leading edge of the cell.

Chemotaxis: signalling modules join hands at front and tail

Chemotaxis is the result of a refined interplay among various intracellular molecules that process spatial and temporal information. Here we present a modular scheme of the complex interactions between the front and the back of cells that allows them to navigate. First, at the front of the cell, activated Rho-type GTPases induce actin polymerization and pseudopod formation. Second, phosphatidylinositol-3,4,5-trisphosphate (PtdIns(3,4,5)P(3)) is produced in a patch at the leading edge, where it binds pleckstrin-homology-domain-containing proteins, which enhance actin polymerization and translocation of the pseudopod. Third, in Dictyostelium amoebae, a cyclic-GMP-signalling cascade has been identified that regulates myosin filament formation in the posterior of the cell, thereby inhibiting the formation of lateral pseudopodia that could misdirect the cell.

Making all the right moves: chemotaxis in neutrophils and Dictyostelium

Neutrophils and Dictyostelium discoideum share the ability to migrate directionally in response to external chemoattractant gradients. The binding of chemoattractants to specific receptors that are coupled to heterotrimeric G proteins leads to a wide range of biochemical responses that become highly localized as cells polarize and migrate by chemotaxis. The signaling mechanisms that lead to the predominant polymerization of F-actin at the front of cells for propulsion and to myosin II assembly at the sides to suppress lateral pseudopod formation and at the back for retraction are now beginning to emerge.

Cell Adhesion, Polarity, and Epithelia in the Dawn of Metazoans

Transporting epithelia posed formidable conundrums right from the moment that Du Bois Raymond discovered their asymmetric behavior, a century and a half ago. It took a century and a half to start unraveling the mechanisms of occluding junctions and polarity, but we now face another puzzle: lest its cells died in minutes, the first high metazoa (i.e., higher than a sponge) needed a transporting epithelium, but a transporting epithelium is an incredibly improbable combination of occluding junctions and cell polarity. How could these coincide in the same individual organism and within minutes? We review occluding junctions (tight and septate) as well as the polarized distribution of Na+-K+-ATPase both at the molecular and the cell level. Junctions and polarity depend on hosts of molecular species and cellular processes, which are briefly reviewed whenever they are suspected to have played a role in the dawn of epithelia and metazoan. We come to the conclusion that most of the molecules needed were already present in early protozoan and discuss a few plausible alternatives to solve the riddle described above.

Chemotaxis: signalling the way forward

During random locomotion, human neutrophils and Dictyostelium discoideum amoebae repeatedly extend and retract cytoplasmic processes. During directed cell migration--chemotaxis--these pseudopodia form predominantly at the leading edge in response to the local accumulation of certain signalling molecules. Concurrent changes in actin and myosin enable the cell to move towards the stimulus. Recent studies are beginning to identify an intricate network of signalling molecules that mediate these processes, and how these molecules become localized in the cell is now becoming clear.

A homologue of Cdk8 is required for spore cell differentiation in Dictyostelium

The Cdk8 proteins are kinases which phosphorylate the carboxy terminal domain (CTD) of RNA polymerase II (Pol II) as well as some transcription factors and, therefore, are involved in the regulation of transcription. Here, we report that a Cdk8 homologue from Dictyostelium discoideum is localized in the nucleus where it forms part of a high molecular weight complex that has CTD kinase activity. Insertional mutagenesis was used to abrogate gene function, and analysis of the null strain revealed that the DdCdk8 protein plays an important role in spore formation during late development. As previously reported [Dev. Growth Differ. 44 (2002) 213] Ddcdk8- cells also exhibit impaired aggregation, although we report that the severity of the defect depends upon experimental conditions. When aggregation occurs, Ddcdk8- cells form abnormal terminally differentiated structures within which the Ddcdk8- cells differentiate into stalk cells but fail to form spores, indicating a role for DdCdk8 in cell differentiation. When Ddcdk8 is expressed from its own promoter, the protein is able to rescue both the late developmental defect and the impaired aggregation. However, when expressed from an heterologous promoter, only the impaired aggregation is rescued. This result demonstrates that the defect during late development is not a consequence of impaired aggregation and indicates a direct role for DdCdk8 in spore formation.

Cell cycle-dependent dynamic localization of a bacterial response regulator with a novel di-guanylate cyclase output domain

Pole development is coordinated with the Caulobacter crescentus cell cycle by two-component signaling proteins. We show that an unusual response regulator, PleD, is required for polar differentiation and is sequestered to the cell pole only when it is activated by phosphorylation. Dynamic localization of PleD to the cell pole provides a mechanism to temporally and spatially control the signaling output of PleD during development. Targeting of PleD to the cell pole is coupled to the activation of a C-terminal guanylate cyclase domain, which catalyzes the synthesis of cyclic di-guanosine monophosphate. We propose that the local action of this novel-type guanylate cyclase might constitute a general regulatory principle in bacterial growth and development.

A G alpha-dependent pathway that antagonizes multiple chemoattractant responses that regulate directional cell movement

Chemotactic cells, including neutrophils and Dictyostelium discoideum, orient and move directionally in very shallow chemical gradients. As cells polarize, distinct structural and signaling components become spatially constrained to the leading edge or rear of the cell. It has been suggested that complex feedback loops that function downstream of receptor signaling integrate activating and inhibiting pathways to establish cell polarity within such gradients. Much effort has focused on defining activating pathways, whereas inhibitory networks have remained largely unexplored. We have identified a novel signaling function in Dictyostelium involving a Galpha subunit (Galpha9) that antagonizes broad chemotactic response. Mechanistically, Galpha9 functions rapidly following receptor stimulation to negatively regulate PI3K/PTEN, adenylyl cyclase, and guanylyl cyclase pathways. The coordinated activation of these pathways is required to establish the asymmetric mobilization of actin and myosin that typifies polarity and ultimately directs chemotaxis. Most dramatically, cells lacking Galpha9 have extended PI(3,4,5)P(3), cAMP, and cGMP responses and are hyperpolarized. In contrast, cells expressing constitutively activated Galpha9 exhibit a reciprocal phenotype. Their second message pathways are attenuated, and they have lost the ability to suppress lateral pseudopod formation. Potentially, functionally similar Galpha-mediated inhibitory signaling may exist in other eukaryotic cells to regulate chemoattractant response.

Cell polarity and Dictyostelium development

Cell polarity is essential for unicellular and multicellular stages of Dictyostelium development. Chemotaxis during early development requires each cell to rapidly reorganize its cytoskeleton to point towards a source of cAMP. This involves a balance between local induction of F-actin polymerization and suppression of pseudopods that point in other directions. Both the lipid phosphatidylinositol (3,4,5) trisphosphate and the soluble signal cGMP have been implicated in these processes, in addition to conserved and novel proteins. During later development cells adopt newly discovered, alternative modes of movement and interact through adhesion molecules. Finally, cells polarize secretion to particular regions of their surface.

Leading the way: Directional sensing through phosphatidylinositol 3-kinase and other signaling pathways

Chemoattractant-responsive cells are able to translate a shallow extracellular chemical gradient into a steep intracellular gradient resulting in the localization of F-actin assembly at the front and an actomyosin network at the rear that moves the cell forward. Recent evidence suggests that one of the first asymmetric cellular responses is the localized accumulation of PtdIns(3,4,5)P3, the product of class I phosphoinositide 3-kinase (PI3K) at the site of the new leading edge. The strong accumulation of PtdIns(3,4,5)P3 results from the localized activation of PI3K and also from feedback loops that amplify PtdIns(3,4,5)P3 synthesis at the front and control its degradation at the side and back of cells. These different pathways are temporally and spatially regulated and integrate with other signaling pathways during directional sensing and chemotaxis.

The signal to move: D. discoideum go orienteering

Cells migrating directionally toward a chemoattractant source display a highly polarized cytoskeletal organization, with F-actin localized predominantly at the anterior and myosin II at the lateral and posterior regions. Dictyostelium discoideum has proven a useful system for elucidating signaling pathways that regulate this chemotactic response. During development, extracellular adenosine 3', 5' monophosphate (cAMP) functions as a primary signal to activate cell surface cAMP receptors (cARs). These receptors transduce different signals depending on whether or not they are coupled to heterotrimeric guanine nucleotide-binding proteins (G proteins) (see the STKE Connections Maps). Multiple G protein-stimulated pathways interact to establish polarity in chemotaxing D. discoideum cells by localizing F-actin at their leading edge and by regulating the phosphorylation state and assembly of myosin II. Many of the molecular interactions described are fundamental to the regulation of chemotaxis in other eukaryotic cells.

Dispatch. Dictyostelium chemotaxis: fascism through the back door?

Aggregating Dictyostelium cells secrete cyclic AMP to attract their neighbours by chemotaxis. It has now been shown that adenylyl cyclase is enriched in the rear of cells, and this localisation is required for normal aggregation.

Follow the leader

Upon starvation, individual Dictyostelium amoebae chemotax toward aggregation centers in tightly packed streams in which cells are organized in head-to-tail chains. A recent report in Cell shows that this behavior requires localization of adenylyl cyclase and the production and secretion of the chemoattractant cAMP at the posterior of individual cells. These findings suggest a relay and communication system to regulate the long-range coordinated movement of cells.