Interacting signaling pathways controlling multicellular development in Dictyostelium

Life, death, and self: Fundamental questions of primitive cognition viewed through the lens of body plasticity and synthetic organisms

Central to the study of cognition is being able to specify the Subject that is making decisions and owning memories and preferences. However, all real cognitive agents are made of parts (such as brains made of cells). The integration of many active subunits into a coherent Self appearing at a larger scale of organization is one of the fundamental questions of evolutionary cognitive science. Typical biological model systems, whether basal or advanced, have a static anatomical structure which obscures important aspects of the mind-body relationship. Recent advances in bioengineering now make it possible to assemble, disassemble, and recombine biological structures at the cell, organ, and whole organism levels. Regenerative biology and controlled chimerism reveal that studies of cognition in intact, "standard", evolved animal bodies are just a narrow slice of a much bigger and as-yet largely unexplored reality: the incredible plasticity of dynamic morphogenesis of biological forms that house and support diverse types of cognition. The ability to produce living organisms in novel configurations makes clear that traditional concepts, such as body, organism, genetic lineage, death, and memory are not as well-defined as commonly thought, and need considerable revision to account for the possible spectrum of living entities. Here, I review fascinating examples of experimental biology illustrating that the boundaries demarcating somatic and cognitive Selves are fluid, providing an opportunity to sharpen inquiries about how evolution exploits physical forces for multi-scale cognition. Developmental (pre-neural) bioelectricity contributes a novel perspective on how the dynamic control of growth and form of the body evolved into sophisticated cognitive capabilities. Most importantly, the development of functional biobots - synthetic living machines with behavioral capacity - provides a roadmap for greatly expanding our understanding of the origin and capacities of cognition in all of its possible material implementations, especially those that emerge de novo, with no lengthy evolutionary history of matching behavioral programs to bodyplan. Viewing fundamental questions through the lens of new, constructed living forms will have diverse impacts, not only in basic evolutionary biology and cognitive science, but also in regenerative medicine of the brain and in artificial intelligence.

Methylglyoxal upregulates Dictyostelium discoideum slug migration by triggering glutathione reductase and methylglyoxal reductase activity

Glutathione (GSH)-deprived Dictyostelium discoideum accumulates methylglyoxal (MG) and reactive oxygen species (ROS) during vegetative growth. However, the reciprocal effects of the production and regulation of these metabolites on differentiation and cell motility are unclear. Based on the inhibitory effects of γ-glutamylcysteine synthetase (gcsA) disruption and GSH reductase (gsr) overexpression on aggregation and culmination, respectively, we overexpressed GSH-related genes encoding superoxide dismutase (Sod2), catalase (CatA), and Gcs, in D. discoideum. Wild-type KAx3 and gcsA-overexpressing (gcsA^OE) slugs maintained GSH levels at levels of approximately 2.1-fold less than the reference GSH synthetase-overexpressing mutant; their GSH levels did not correlate with slug migration ability. Through prolonged KAx3 migration by treatment with MG and H₂O₂, we found that MG increased after the mound stage in this strain, with a 2.6-fold increase compared to early developmental stages; in contrast, ROS were maintained at high levels throughout development. While the migration-defective sod2- and catA-overexpressing mutant slugs (sod2^OE and catA^OE) decreased ROS levels by 50% and 53%, respectively, these slugs showed moderately decreased MG levels (36.2±5.8 and 40.7±1.6nmolg^-1 cells wet weight, P<0.05) compared to the parental strain (54.2±3.5nmolg^-1). Importantly, defects in the migration of gcsA^OE slugs decreased MG considerably (13.8±4.2nmolg^-1, P<0.01) along with a slight decrease in ROS. In contrast to the increase observed in migrating sod2^OE and catA^OE slugs by treatment with MG and H₂O₂, the migration of gcsA^OE slugs appeared unaffected. This behavior was caused by MG-triggered Gsr and NADPH-linked aldolase reductase activity, suggesting that GSH biosynthesis in gcsA^OE slugs is specifically used for MG-scavenging activity. This is the first report showing that MG upregulates slug migration via MG-scavenging-mediated differentiation.

Cell signaling during development of Dictyostelium

Continuous communication between cells is necessary for development of any multicellular organism and depends on the recognition of secreted signals. A wide range of molecules including proteins, peptides, amino acids, nucleic acids, steroids and polylketides are used as intercellular signals in plants and animals. They are also used for communication in the social ameba Dictyostelium discoideum when the solitary cells aggregate to form multicellular structures. Many of the signals are recognized by surface receptors that are seven-transmembrane proteins coupled to trimeric G proteins, which pass the signal on to components within the cytoplasm. Dictyostelium cells have to judge when sufficient cell density has been reached to warrant transition from growth to differentiation. They have to recognize when exogenous nutrients become limiting, and then synchronously initiate development. A few hours later they signal each other with pulses of cAMP that regulate gene expression as well as direct chemotactic aggregation. They then have to recognize kinship and only continue developing when they are surrounded by close kin. Thereafter, the cells diverge into two specialized cell types, prespore and prestalk cells, that continue to signal each other in complex ways to form well proportioned fruiting bodies. In this way they can proceed through the stages of a dependent sequence in an orderly manner without cells being left out or directed down the wrong path.

The Dictyostelium discoideum RACK1 orthologue has roles in growth and development

Background

The receptor for activated C-kinase 1 (RACK1) is a conserved protein belonging to the WD40 repeat family of proteins. It folds into a beta propeller with seven blades which allow interactions with many proteins. Thus it can serve as a scaffolding protein and have roles in several cellular processes.

Results

We identified the product of the Dictyostelium discoideum gpbB gene as the Dictyostelium RACK1 homolog. The protein is mainly cytosolic but can also associate with cellular membranes. DdRACK1 binds to phosphoinositides (PIPs) in protein-lipid overlay and liposome-binding assays. The basis of this activity resides in a basic region located in the extended loop between blades 6 and 7 as revealed by mutational analysis. Similar to RACK1 proteins from other organisms DdRACK1 interacts with G protein subunits alpha, beta and gamma as shown by yeast two-hybrid, pulldown, and immunoprecipitation assays. Unlike the Saccharomyces cerevisiae and Cryptococcus neoformans RACK1 proteins it does not appear to take over Gβ function in D. discoideum as developmental and other defects were not rescued in Gβ null mutants overexpressing GFP-DdRACK1. Overexpression of GFP-tagged DdRACK1 and a mutant version (DdRACK1mut) which carried a charge-reversal mutation in the basic region in wild type cells led to changes during growth and development.

Conclusion

DdRACK1 interacts with heterotrimeric G proteins and can through these interactions impact on processes specifically regulated by these proteins.

Becoming Multicellular by Aggregation; The Morphogenesis of the Social Amoebae Dicyostelium discoideum

The organisation and form of most organisms is generated during theirembryonic development and involves precise spatial and temporal controlof cell division, cell death, cell differentiation and cell movement.Differential cell movement is a particularly important mechanism in thegeneration of form. Arguably the best understood mechanism of directedmovement is chemotaxis. Chemotaxis plays a major role in the starvationinduced multicellular development of the social amoebae Dictyostelium.Upon starvation up to 10(5) individual amoebae aggregate to form afruiting body. In this paper we review the evidence that the movement ofthe cells during all stages of Dictyostelium development is controlled bypropagating waves of cAMP which control the chemotactic movement ofthe cells. We analyse the complex interactions between cell-cell signallingresulting in cAMP waves of various geometries and cell movement whichresults in a redistribution of the signalling sources and therefore changes thegeometry of the waves. We proceed to show how the morphogenesis,including aggregation stream and mound formation, slug formation andmigration, of this relatively simple organism is beginning to be understoodat the level of rules for cell behaviour, which can be tested experimentallyand theoretically by model calculations.

Coupling mechanism of a GPCR and a heterotrimeric G protein during chemoattractant gradient sensing in Dictyostelium

The coupling of heterotrimeric guanine nucleotide-binding protein (G protein)-coupled receptors (GPCRs) with G proteins is fundamental for GPCR signaling; however, the mechanism of coupling is still debated. Moreover, how the proposed mechanisms affect the dynamics of downstream signaling remains unclear. Here, through experiments involving fluorescence recovery after photobleaching and single-molecule imaging, we directly measured the mobilities of cyclic adenosine monophosphate (cAMP) receptor 1 (cAR1), a chemoattractant receptor, and a G protein βγ subunit in live cells. We found that cAR1 diffused more slowly in the plasma membrane than did Gβγ. Upon binding of ligand to the receptor, the mobility of cAR1 was unchanged, whereas the speed of a fraction of the faster-moving Gβγ subunits decreased. Our measurements showed that cAR1 was relatively immobile and Gβγ diffused freely, suggesting that chemoattractant-bound cAR1 transiently interacted with G proteins. Using models of possible coupling mechanisms, we computed the temporal kinetics of G protein activation. Our fluorescence resonance energy transfer imaging data showed that fully activated cAR1 induced the sustained dissociation of G protein α and βγ subunits, which indicated that ligand-bound cAR1 activated G proteins continuously. Finally, simulations indicated that a high-affinity coupling of ligand-bound receptors and G proteins was essential for cAR1 to translate extracellular gradient signals into directional cellular responses. We suggest that chemoattractant receptors use a ligand-induced coupling rather than a precoupled mechanism to control the activation of G proteins during chemotaxis.

The ROCO kinase QkgA is necessary for proliferation inhibition by autocrine signals in Dictyostelium discoideum

AprA and CfaD are secreted proteins that function as autocrine signals to inhibit cell proliferation in Dictyostelium discoideum. Cells lacking AprA or CfaD proliferate rapidly, and adding AprA or CfaD to cells slows proliferation. Cells lacking the ROCO kinase QkgA proliferate rapidly, with a doubling time 83% of that of the wild type, and overexpression of a QkgA-green fluorescent protein (GFP) fusion protein slows cell proliferation. We found that qkgA(-) cells accumulate normal levels of extracellular AprA and CfaD. Exogenous AprA or CfaD does not slow the proliferation of cells lacking qkgA, and expression of QkgA-GFP in qkgA(-) cells rescues this insensitivity. Like cells lacking AprA or CfaD, cells lacking QkgA tend to be multinucleate, accumulate nuclei rapidly, and show a mass and protein accumulation per nucleus like those of the wild type, suggesting that QkgA negatively regulates proliferation but not growth. Despite their rapid proliferation, cells lacking AprA, CfaD, or QkgA expand as a colony on bacteria less rapidly than the wild type. Unlike AprA and CfaD, QkgA does not affect spore viability following multicellular development. Together, these results indicate that QkgA is necessary for proliferation inhibition by AprA and CfaD, that QkgA mediates some but not all of the effects of AprA and CfaD, and that QkgA may function downstream of these proteins in a signal transduction pathway regulating proliferation.

KeaA, a Dictyostelium Kelch-domain protein that regulates the response to stress and development

Background

The protein kinase YakA is responsible for the growth arrest and induction of developmental processes that occur upon starvation of Dictyostelium cells. yakA^- cells are aggregation deficient, have a faster cell cycle and are hypersensitive to oxidative and nitrosoative stress. With the aim of isolating members of the YakA pathway, suppressors of the death induced by nitrosoative stress in the yakA^- cells were identified. One of the suppressor mutations occurred in keaA, a gene identical to DG1106 and similar to Keap1 from mice and the Kelch protein from Drosophila, among others that contain Kelch domains.

Results

A mutation in keaA suppresses the hypersensitivity to oxidative and nitrosoative stresses but not the faster growth phenotype of yakA^- cells. The growth profile of keaA deficient cells indicates that this gene is necessary for growth. keaA deficient cells are more resistant to nitrosoative and oxidative stress and keaA is necessary for the production and detection of cAMP. A morphological analysis of keaA deficient cells during multicellular development indicated that, although the mutant is not absolutely deficient in aggregation, cells do not efficiently participate in the process. Gene expression analysis using cDNA microarrays of wild-type and keaA deficient cells indicated a role for KeaA in the regulation of the cell cycle and pre-starvation responses.

Conclusions

KeaA is required for cAMP signaling following stress. Our studies indicate a role for kelch proteins in the signaling that regulates the cell cycle and development in response to changes in the environmental conditions.

Identifying the molecular basis of functions in the transcriptome of the social amoeba Dictyostelium discoideum

The unusual life cycle of Dictyostelium discoideum, in which an extra-cellular stressor such as starvation induces the development of a multicellular fruiting body consisting of stalk cells and spores from a culture of identical amoebae, provides an excellent model for investigating the molecular control of differentiation and the transition from single- to multi-cellular life, a key transition in development. We utilized serial analysis of gene expression (SAGE), a molecular method that is unbiased by dependence on previously identified genes, to obtain a transcriptome from a high-density culture of amoebae, in order to examine the transition to multi-cellular development. The SAGE method provides relative expression levels, which allows us to rank order the expressed genes. We found that a large number of ribosomal proteins were expressed at high levels, while various components of the proteosome were expressed at low levels. The only identifiable transmembrane signaling system components expressed in amoebae are related to quorum sensing, and their expression levels were relatively low. The most highly expressed gene in the amoeba transcriptome, dutA untranslated RNA, is a molecule with unknown function that may serve as an inhibitor of translation. These results suggest that high-density amoebae have not initiated development, and they also suggest a mechanism by which the transition into the development program is controlled.

G{alpha}5 subunit-mediated signalling requires a D-motif and the MAPK ERK1 in Dictyostelium

The Dictyostelium Galpha5 subunit has been shown to reduce cell viability, inhibit folate chemotaxis and accelerate tip morphogenesis and gene expression during multicellular development. Alteration of the D-motif (mitogen-activated protein kinase docking site) at the amino terminus of the Galpha 5 subunit or the loss of extracellular signal-regulated kinase (ERK)1 diminished the lethality associated with the overexpression or constitutive activation of the Galpha5 subunit. The amino-terminal D-motif of the Galpha5 subunit was also found to be necessary for the reduced cell size, small aggregate formation and precocious developmental gene expression associated with Galpha5 subunit overexpression. This D-motif also contributed to the aggregation delay in cells expressing a constitutively active Galpha5 subunit, but the D-motif was not necessary for the inhibition of folate chemotaxis. These results suggest that the amino-terminal D-motif is required for some but not all phenotypes associated with elevated Galpha5 subunit functions during growth and development and that ERK1 can function in Galpha5 subunit-mediated signal transduction.

Developmental morphology and chemotactic responses are dependent on G alpha subunit specificity in Dictyostelium

Dictyostelium discoideum expresses multiple G alpha subunits but only a single G beta and G gamma subunit suggesting that the specific response to an external signal depends largely on G alpha subunit function or G protein-independent signaling from the receptor. To test the contribution of G alpha subunit functional specificity, the chimeric G alpha subunits, G alpha2/4 and G alpha5/4, were created and analyzed along with wild-type subunits for the ability to substitute for the G alpha4 subunit in mediating responses from folate receptors. The G alpha2/4 subunit, but not the G alpha2 or G alpha5/4 subunits, partly rescued chemotaxis and cGMP accumulation in folate-stimulated g alpha4(-) cells. Expression of the G alpha5/4 or G alpha5 subunits resulted in an inhibition of g alpha4- and wild-type cell movement and a reduced aggregate size in developing wild-type and g alpha5- cells suggesting these subunits mediate similar responses. Only the G alpha4 subunit was capable of correcting developmental morphology in g alpha4- multicellular aggregates suggesting that the chimeric G alpha2/4 or G alpha5/4 subunits were insufficient to provide the G alpha4 function necessary for proper development. These results indicate that Dictyostelium G alpha subunit specificity is not limited to receptor coupling and that G alpha subunit sequences outside of the carboxyl terminus are important for cell movement and developmental processes.

Frequency of intrinsic pulsatile gonadotropin-releasing hormone secretion is regulated by the expression of cyclic nucleotide-gated channels in GT1 cells

Cultures of endogenous GnRH neurons and the GT1 GnRH neuronal cell line release GnRH in pulses (intrinsic pulsatile release) with an interpulse frequency similar to that seen in castrated animals. In both GT1 cells and transgenic rats, lowering cAMP levels by expression of a phosphodiesterase decreased the frequency of intrinsic GnRH pulsatility. We asked whether the cyclic nucleotide-gated cation (CNG) channels expressed in GT1 cells participated in cAMP modulation of intrinsic GnRH pulsatility. Because expression of the CNGA2 subunit is essential for formation of functional CNG channels, we developed an adenovirus (Ad) vector expressing a short interference RNA (siRNA) to the CNGA2 subunit (Ad-CNG-siRNA) or as an infection control, to the coding region of luciferase (Ad-Luc-siRNA). Infection with the Ad-CNG-siRNA of COS cells transfected with a CNGA2 expression vector significantly inhibited CNGA2 protein levels by 74% by Western blot. Infection of GT1-1 cells with Ad-CNG-siRNA resulted in a 68% decrease in the levels of CNGA2 mRNA, a 44% decrease in protein levels, and a clear decrease in immunostaining with an antibody to CNGA2. Infection of GT1-1 cells with Ad-CNG-siRNA decreased spontaneous Ca2+ oscillations compared with Ad-Luc-siRNA-infected or uninfected cells by 71%. Furthermore infection with Ad-CNG-siRNA resulted in a 2-fold increase in the interpulse interval in GnRH secretion (49.4+/-9.1 min) compared with uninfected cells (25.9+/-2.5 min) or Ad-Luc-siRNA (29.3+/-2.8 min)-infected cells. These data provide the first direct evidence that the CNG channel is a downstream signaling molecule in the regulation of the frequency of intrinsic GnRH pulsatility by cAMP.

Reduced glutathione levels affect the culmination and cell fate decision in Dictyostelium discoideum

Glutaredoxins have been known to be glutathione-dependent oxidoreductases that participate in the redox regulation of various cellular processes. To understand the role of glutaredoxins in the development, we examined glutaredoxin 1 (Grx1) of Dictyostelium discoideum. Its mRNA was highly accumulated at the mound and the culmination stages. When Grx1-overexpressing cells were developed, their culmination was delayed, and the expression of marker genes for prespore and spore decreased. Interestingly, they had about 1.5-fold higher amount of reduced glutathione (GSH) compared with parental cells and their prolonged migration was repressed by the oxidant such as hydrogen peroxide. To confirm the effect of GSH on the culmination, glutathione reductase (Gsr) was overexpressed or underexpressed. Similar to Grx1-overexpressing cells, Gsr-overexpressing cells contained about 1.5-fold higher amount of GSH and exhibited the delayed culmination. In contrast, the knockdown mutant of Gsr had nearly 50% lower amount of GSH and showed accelerated culmination. Taken together, these data suggest that the culmination of Dictyostelium is controlled by GSH. In addition, the cells having higher GSH levels showed a prestalk tendency in the chimeric slugs with parental cells, indicating that the difference in the amount of GSH may affect the determination of cell fate.

Transcriptional regulation of post-aggregation genes in Dictyostelium by a feed-forward loop involving GBF and LagC

Expression profiles of developmental genes in Dictyostelium were determined on microarrays during development of wild type cells and mutant cells lacking either the DNA binding protein GBF or the signaling protein LagC. We found that the mutant strains developed in suspension with added cAMP expressed the pulse-induced and early adenylyl cyclase (ACA)-dependent genes, but not the later ACA-dependent, post-aggregation genes. Since expression of lagC itself is dependent on GBF, expression of the post-aggregation genes might be controlled only by signaling from LagC. However, expression of lagC in a GBF-independent manner in a gbfA- null strain did not result in expression of the post-aggregation genes. Since GBF is necessary for accumulation of LagC and both the DNA binding protein and the LagC signal transduction pathway are necessary for expression of post-aggregation genes, GBF and LagC form a feed-forward loop. Such network architecture is a common motif in diverse organisms and can act as a filter for noisy inputs. Breaking the feed-forward loop by expressing lagC in a GBF-independent manner in a gbfA+ strain does not significantly affect the patterns of gene expression for cells developed in suspension with added cAMP, but results in a significant delay at the mound stage and asynchronous development on solid supports. This feed-forward loop can integrate temporal information with morphological signals to ensure that post-aggregation genes are only expressed after cell contacts have been made.

A cysteine-rich extracellular protein containing a PA14 domain mediates quorum sensing in Dictyostelium discoideum

Much remains to be understood about quorum-sensing factors that allow cells to sense their local density. Dictyostelium discoideum is a simple eukaryote that grows as single-celled amoebae and switches to multicellular development when food becomes limited. As the growing cells reach a high density, they begin expressing discoidin genes. The cells secrete an unknown factor, and at high cell densities the concomitant high levels of the factor induce discoidin expression. We report here the enrichment of discoidin-inducing complex (DIC), an approximately 400-kDa protein complex that induces discoidin expression during growth and development. Two proteins in the DIC preparation, DicA1 and DicB, were identified by sequencing proteolytic digests. DicA1 and DicB were expressed in Escherichia coli and tested for their ability to induce discoidin during growth and development. Recombinant DicB was unable to induce discoidin expression, while recombinant DicA1 was able to induce discoidin expression. This suggests that DicA1 is an active component of DIC and indicates that posttranslational modification is dispensable for activity. DicA1 mRNA is expressed in vegetative and developing cells. The mature secreted form of DicA1 has a molecular mass of 80 kDa and has a 24-amino-acid cysteine-rich repeat that is similar to repeats in Dictyostelium proteins, such as the extracellular matrix protein ecmB/PstA, the prespore cell-inducing factor PSI, and the cyclic AMP phosphodiesterase inhibitor PDI. Together, the data suggest that DicA1 is a component of a secreted quorum-sensing signal regulating discoidin gene expression during Dictyostelium growth and development.

LrrA, a novel leucine-rich repeat protein involved in cytoskeleton remodeling, is required for multicellular morphogenesis in Dictyostelium discoideum

Cell sorting by differential cell adhesion and movement is a fundamental process in multicellular morphogenesis. We have identified a Dictyostelium discoideum gene encoding a novel protein, LrrA, which composes almost entirely leucine-rich repeats (LRRs) including a putative leucine zipper motif. Transcription of lrrA appeared to be developmentally regulated with robust expression during vegetative growth and early development. lrrA null cells generated by homologous recombination aggregated to form loose mounds, but subsequent morphogenesis was blocked without formation of the apical tip. The cells adhered poorly to a substratum and did not form tight cell-cell agglomerates in suspension; in addition, they were unable to polarize and exhibit chemotactic movement in the submerged aggregation and Dunn chamber chemotaxis assays. Fluorescence-conjugated phalloidin staining revealed that both vegetative and aggregation competent lrrA(-) cells contained numerous F-actin-enriched microspikes around the periphery of cells. Quantitative analysis of the fluorescence-stained F-actin showed that lrrA(-) cells exhibited a dramatically increase in F-actin as compared to the wild-type cells. When developed together with wild-type cells, lrrA(-) cells were unable to move to the apical tip and sorted preferentially to the rear and lower cup regions. These results indicate that LrrA involves in cytoskeleton remodeling, which is needed for normal chemotactic aggregation and efficient cell sorting during multicellular morphogenesis, particularly in the formation of apical tip.

A cAMP receptor-like G protein-coupled receptor with roles in growth regulation and development

Dictyostelium discoideum uses G protein-mediated signal transduction for many vegetative and developmental functions, suggesting the existence of G protein-coupled receptors (GPCRs) other than the four known cyclic adenosine monophosphate (cAMP) receptors (cAR1-4). Sequences of the cAMP receptors were used to identify Dictyostelium genes encoding cAMP receptor-like proteins, CrlA-C. Limited sequence identity between these putative GPCRs and the cAMP receptors suggests the Crl receptors are unlikely to be receptors for cAMP. The crl genes are expressed at various times during growth and the developmental life cycle. Disruption of individual crl genes did not impair chemotactic responses to folic acid or cAMP or alter cAMP-dependent aggregation. However, crlA(-) mutants grew to a higher cell density than did wild-type cells and high-copy-number crlA expression vectors were detrimental to cell viability, suggesting that CrlA is a negative regulator of cell growth. In addition, crlA(-) mutants produce large aggregates with delayed anterior tip formation indicating a role for the CrlA receptor in the development of the anterior prestalk cell region. The scarcity of GFP-expressing crlA(-) mutants in the anterior prestalk cell region of chimeric organisms supports a cell-autonomous role for the CrlA receptor in prestalk cell differentiation.

Eme1 is involved in DNA damage processing and maintenance of genomic stability in mammalian cells

Yeast and human Eme1 protein, in complex with Mus81, constitute an endonuclease that cleaves branched DNA structures, especially those arising during stalled DNA replication. We identified mouse Eme1, and show that it interacts with Mus81 to form a complex that preferentially cleaves 3'-flap structures and replication forks rather than Holliday junctions in vitro. We demonstrate that Eme1-/- embryonic stem (ES) cells are hypersensitive to the DNA cross-linking agents mitomycin C and cisplatin, but only mildly sensitive to ionizing radiation, UV radiation and hydroxyurea treatment. Mammalian Eme1 is not required for the resolution of DNA intermediates that arise during homologous recombination processes such as gene targeting, gene conversion and sister chromatid exchange (SCE). Unlike Blm-deficient ES cells, increased SCE was seen only following induced DNA damage in Eme1-deficient cells. Most importantly, Eme1 deficiency led to spontaneous genomic instability. These results reveal that mammalian Eme1 plays a key role in DNA repair and the maintenance of genome integrity.

Role for YakA, cAMP, and protein kinase A in regulation of stress responses of Dictyostelium discoideum cells

The Dictyostelium protein kinase YakA is required for the growth-to-development transition. During growth YakA controls the cell cycle, regulating the intervals between cell divisions. When starved for nutrients Dictyostelium cells arrest growth and undergo changes in gene expression, decreasing vegetative mRNAs and inducing the expression of pkaC. YakA is an effector of these changes, being necessary for the decrease of vegetative mRNA expression and the increase of protein kinase A (PKA) activity that will ultimately regulate expression of adenylyl cyclase, cAMP synthesis, and the induction of development. We report a role for this kinase in the response to nitrosoative or oxidative stress of Dictyostelium cells. Hydrogen peroxide and sodium nitroprusside arrest the growth of cells and trigger cAMP synthesis and activation of PKA in a manner similar to the well-established response to nutrient starvation. We have found that yakA null cells are hypersensitive to nitrosoative/oxidative stress and that a second-site mutation in pkaC suppresses this sensitivity. The response to different stresses has been investigated and YakA, cAMP, and PKA have been identified as components of the pathway that regulate the growth arrest that follows treatment with compounds that generate reactive oxygen species. The effect of different types of stress was evaluated in Dictyostelium and the YakA/PKA pathway was also implicated in the response to heat stress.

cAMP receptor affinity controls wave dynamics, geometry and morphogenesis inDictyostelium

Serpentine G-protein-coupled cAMP receptors are key components in the detection and relay of the extracellular cAMP waves that control chemotactic cell movement during Dictyostelium development. During development the cells sequentially express four closely related cAMP receptors of decreasing affinity. In this study, we investigated the effect of cAMP receptor type and affinity on the dynamics of cell-cell signalling in vivo, by measuring the dynamics of wave initiation and propagation in a variety of cAMP receptor mutants. We found that receptor affinity controls the frequency of wave initiation, but it does not determine wave propagation velocity, thus resulting in dramatic changes in wave geometry. In the limiting case, the affinity of the receptor is so low that waves can still be initiated but no stable centres form - thus, the cells cannot aggregate. In mounds, expression of low affinity receptors results in slow concentric waves instead of the normally observed multi-armed spiral waves. Under these conditions there is no rotational cell movement and the hemispherical mounds cannot transform into slugs. These results highlight the importance of receptor number and affinity in the proper control of cell-cell signalling dynamics required for the successful completion of development.

Modeling the model organism Dictyostelium discoideum

The cellular slime mold Dictyostelium discoideum is a fascinating organism, not only for biologists, but also for physicists. Since the Belousov-Zhabotinskii reaction pattern, a well-known non-linear phenomenon in chemistry, was observed during aggregation of Dictyostelium amoebae, Dictyostelium has been one of the major subjects of non-linear dynamics studies. Macroscopic theory, such as continuous cell density approximation, has been a common approach to studying pattern formation since the pioneering work of Turing. Recently, promising microscopic approaches, such as the cellular dynamics method, have emerged. They have shown that Dictyostelium is useful as a model system in biology, The synchronization mechanism of oscillatory production of cyclic adenosine 3',5'-monophosphate in Dictyostelium is discussed in detail to show how it is a universal feature that can explain synchronization in other organisms.

Related Galpha subunits play opposing roles during Dictyostelium development

Of the several known Dictyostelium G protein subunits, the Galpha4 and Galpha5 subunits are the most closely related pair based on phylogenetic analysis and expression patterns, but these subunits perform different roles during development. To investigate potential relationships between these subunits with respect to cell differentiation, chimeric organisms composed of strains lacking or overexpressing either subunit were created and examined for developmental morphogenesis and spore production. Chimeras of galpha4 null and galpha5 null strains or Galpha4 and Galpha5 overexpression strains displayed compensatory morphogenesis, implying that the subunits promote complementary developmental processes. However, chimeras composed of galpha4 null and Galpha5 overexpression strains or galpha5 null and Galpha4 overexpression strains displayed distorted tip morphogenesis, suggesting the strains of these chimeras share common developmental deficiencies. Cells lacking the Galpha5 subunit localized to the prespore region of chimeras similar to the pattern observed for cells overexpressing the Galpha4 subunit, and cells overexpressing the Galpha5 subunit displayed localization patterns similar to galpha4 null mutants. A strain overexpressing both subunits displayed a partial suppression of morphology, gene expression, and cell localization phenotypes associated with the overexpression of the individual Galpha subunit genes, suggesting that each Galpha subunits can inhibit signaling mediated by the other subunit. Overexpression of the Galpha5 subunit inhibited chemotaxis and cGMP accumulation in response to folic acid, indicating that the Galpha5 subunit can inhibit early steps in the Galpha4-mediated signal transduction pathway. The contrasting phenotypes of the Galpha mutants suggest the Galpha4 and Galpha5 subunits provide opposing functions in cell differentiation, localization, and chemotactic responses to folic acid.

Mutant Rac1B expression in Dictyostelium: effects on morphology, growth, endocytosis, development, and the actin cytoskeleton

Rac1 is a small G-protein in the Ras superfamily that has been implicated in the control of cell growth, adhesion, and the actin-based cytoskeleton. To investigate the role of Rac1 during motile processes, we have established Dictyostelium cell lines that conditionally overexpress epitope-tagged Dictyostelium discoideum wild-type Rac1B (DdRac1B) or a mutant DdRac1B protein. Expression of endogenous levels of myc- or GFP-tagged wild-type DdRac1B had minimal effect on cellular morphologies and behaviors. By contrast, expression of a constitutively active mutant (G12-->V or Q61-->L) or a dominant negative mutant (T17-->N) generated amoebae with characteristic cellular defects. The morphological appearance of actin-containing structures, intracellular levels of F-actin, and cellular responses to chemoattractant closely paralleled the amount of active DdRac1B, indicating a role in upregulating actin cytoskeletal activities. Expression of any of the three mutants inhibited cell growth and cytokinesis, and delayed multicellular development, suggesting that DdRac1B plays important regulatory role(s) during these processes. No significant effects were observed on binding or internalization of latex beads in suspension or on intracellular membrane trafficking. Cells expressing DdRac1B-G12V exhibited defects in fluid-phase endocytosis and the longest developmental delays; DdRac1B-Q61L produced the strongest cytokinesis defect; and DdRac1B-T17N generated intermediate phenotypes. These conditionally expressed DdRac1B proteins should facilitate the identification and characterization of the Rac1 signaling pathway in an organism that is amenable to both biochemical and molecular genetic manipulations.

The control of chemotactic cell movement during Dictyostelium morphogenesis

Differential cell movement is an important mechanism in the development and morphogenesis of many organisms. In many cases there are indications that chemotaxis is a key mechanism controlling differential cell movement. This can be particularly well studied in the starvation-induced multicellular development of the social amoeba Dictyostelium discoideum. Upon starvation, up to 10(5) individual amoebae aggregate to form a fruiting body The cells aggregate by chemotaxis in response to propagating waves of cAMP, initiated by an aggregation centre. During their chemotactic aggregation the cells start to differentiate into prestalk and prespore cells, precursors to the stalk and spores that form the fruiting body. These cells enter the aggregate in a random order but then sort out to form a simple axial pattern in the slug. Our experiments strongly suggest that the multicellular aggregates (mounds) and slugs are also organized by propagating cAMP waves and, furthermore, that cell-type-specific differences in signalling and chemotaxis result in cell sorting, slug formation and movement.

tip genes act in parallel pathways of early Dictyostelium development

Analysis of Dictyostelium strains carrying null mutations in tipA showed a primary defect in cell sorting and the formation of tips on the developing mound. To study the process affected in tipA- mutants further, other mutants with a similar phenotype were isolated and characterized. These studies showed three new Dictyostelium genes: tipB, tipC, and tipD. All the tip mutants aggregate into larger than average mounds, which split up and form many lips on their surfaces. Furthermore, each mutant exhibits reduced or aberrant cell-sorting behavior, never makes migrating slugs, and has severely reduced fruiting body and spore production. The mRNA of each tip gene is present in vegetative cells and does not vary significantly with development. Prespore and prestalk gene expression is reduced or delayed in the tip mutants indicating cell type differentiation is dependent on the function of these genes. Developing mutant cells in chimeric mixtures with wildtype cells demonstrated that the defects in each tip mutant behave cell autonomously. The overexpression of TipA in a tipB- background and the overexpression of TipB in a tipA- background significantly improved the morphogenesis of these mutants. These were the only situations in which the expression of one tip gene could compensate for the lack of a different tip gene. Except for the tipA-/tipB- strain, double mutations in the tip genes have additive effects, causing a more severe mutant phenotype with defects earlier in development than single mutants. The tipA-/tipB- double mutant does not show additive effects and is very similar to the tipA-single mutant. Analysis of the effects of double mutations and overexpression indicates that members of this class of genes appear to act through parallel pathways of differentiation and tip formation in early Dictyostelium development. Furthermore, TipA and TipB appear to have some overlapping functions or are involved in the same pathway. The multitipped phenotype observed in all the mutants may be a general result of perturbing early developmental events such as cell type differentiation and cell type proportioning.

Dictyostelium amoebae lacking an F-box protein form spores rather than stalk in chimeras with wild type

Using a selection for Dictyostelium mutants that preferentially form spores, we have recovered a mutant called CheaterA. In chimeras with isogenic wild-type cells, the CheaterA mutant preferentially forms viable spores rather than inviable stalk cells. The mutant causes wild-type cells that have begun to express spore-specific genes to accumulate in the prestalk compartment of the developing organism. In the wild-type cells, the chtA transcript is absent in growing cells and appears early in development. No transcript was detected in the mutant by Northern blot. The chtA gene codes for a protein with an F-box and WD40 domains. This class of protein usually forms part of an Skp1, cullin, F-box (SCF) complex that targets specific protein substrates for ubiquitination and degradation.

Dictyostelium amoebae lacking an F-box protein form spores rather than stalk in chimeras with wild type

Using a selection for Dictyostelium mutants that preferentially form spores, we have recovered a mutant called CheaterA. In chimeras with isogenic wild-type cells, the CheaterA mutant preferentially forms viable spores rather than inviable stalk cells. The mutant causes wild-type cells that have begun to express spore-specific genes to accumulate in the prestalk compartment of the developing organism. In the wild-type cells, the chtA transcript is absent in growing cells and appears early in development. No transcript was detected in the mutant by Northern blot. The chtA gene codes for a protein with an F-box and WD40 domains. This class of protein usually forms part of an Skp1, cullin, F-box (SCF) complex that targets specific protein substrates for ubiquitination and degradation.

Functional classification of cNMP-binding proteins and nucleotide cyclases with implications for novel regulatory pathways in Mycobacterium tuberculosis

We have analyzed the cyclic nucleotide (cNMP)-binding protein and nucleotide cyclase superfamilies using Bayesian computational methods of protein family identification and classification. In addition to the known cNMP-binding proteins (cNMP-dependent kinases, cNMP-gated channels, cAMP-guanine nucleotide exchange factors, and bacterial cAMP-dependent transcription factors), new functional groups of cNMP-binding proteins were identified, including putative ABC-transporter subunits, translocases, and esterases. Classification of the nucleotide cyclases revealed subtle differences in sequence conservation of the active site that distinguish the five classes of cyclases: the multicellular eukaryotic adenylyl cyclases, the eukaryotic receptor-type guanylyl cyclases, the eukaryotic soluble guanylyl cyclases, the unicellular eukaryotic and prokaryotic adenylyl cyclases, and the putative prokaryotic guanylyl cyclases. Phylogenetic distribution of the cNMP-binding proteins and cyclases was analyzed, with particular attention to the 22 complete archaeal and eubacterial genome sequences. Mycobacterium tuberculosis H37Rv and Synechocystis PCC6803 were each found to encode several more putative cNMP-binding proteins than other prokaryotes; many of these proteins are of unknown function. M. tuberculosis also encodes several more putative nucleotide cyclases than other prokaryotic species.

Integration of signaling networks that regulate Dictyostelium differentiation

In Dictyostelium amoebae, cell-type differentiation, spatial patterning, and morphogenesis are controlled by a combination of cell-autonomous mechanisms and intercellular signaling. A chemotactic aggregation of approximately 10(5) cells leads to the formation of a multicellular organism. Cell-type differentiation and cell sorting result in a small number of defined cell types organized along an anteroposterior axis. Finally, a mature fruiting body is created by the terminal differentiation of stalk and spore cells. Analysis of the regulatory program demonstrates a role for several molecules, including GSK-3, signal transducers and activators of transcription (STAT) factors, and cAMP-dependent protein kinase (PKA), that control spatial patterning in metazoans. Unexpectedly, two component systems containing histidine kinases and response regulators also play essential roles in controlling Dictyostelium development. This review focuses on the role of cAMP, which functions intracellularly to mediate the activity of PKA, an essential component in aggregation, cell-type specification, and terminal differentiation. Cytoplasmic cAMP levels are controlled through both the regulated activation of adenylyl cyclases and the degradation by a phosphodiesterase containing a two-component system response regulator. Extracellular cAMP regulates G-protein-dependent and -independent pathways to control aggregation as well as the activity of GSK-3 and the transcription factors GBF and STATa during multicellular development. The integration of these pathways with others regulated by the morphogen DIF-1 to control cell fate decisions are discussed.

Regulation of cell-fate determination in Dictyostelium

A key step in the development of all multicellular organisms is the differentiation of specialized cell types. The eukaryotic microorganism Dictyostelium discoideum provides a unique experimental system for studying cell-type determination and spatial patterning in a developing multicellular organism. Unlike metazoans, which become multicellular by undergoing many rounds of cell division after fertilization of an egg, the social amoeba Dictyostelium achieves multicellularity by the aggregation of approximately 10(5) cells in response to nutrient depletion. Following aggregation, cell-type differentiation and morphogenesis result in a multicellular organism with only a few cell types that exhibit a defined patterning along the anterior-posterior axis of the organism. Analysis of the mechanisms that control these processes is facilitated by the relative simplicity of Dictyostelium development and the availability of molecular, genetic, and cell biological tools. Interestingly, analysis has shown that many molecules that play integral roles in the development of higher eukaryotes, such as PKA, STATs, and GSK-3, are also essential for cell-type differentiation and patterning in Dictyostelium. The role of these and other signaling pathways in the induction, maintenance, and patterning of cell types during Dictyostelium development is discussed.

A putative receptor mediating cell-density sensing in Dictyostelium

When Dictyostelium cells starve, they begin secreting a glycoprotein called conditioned medium factor (CMF). When there is a high density of starved cells, as indicated by a high concentration of CMF, the cells begin expressing some genes and aggregate using pulses of cAMP as a chemoattractant. CMF regulates gene expression via a G protein-independent pathway, whereas CMF regulates cAMP signal transduction via a G protein-dependent pathway. To elucidate receptors mediating cell density sensing, we used CMF-Sepharose to isolate membrane proteins that bind CMF. We identified a 50-kDa protein, CMFR1, that is sensitive to trypsin treatment of whole cells. We obtained partial amino acid sequence of CMFR1 and isolated the cDNA encoding it. The derived amino acid sequence has no significant similarity to known proteins and has two or three predicted transmembrane domains. Expression of CMFR1 in insect cells caused an increase in CMF binding. Repression of CMFR1 in Dictyostelium by gene disruption resulted in a approximately 50% decrease of the CMF binding and a loss of CMF-induced G protein-independent gene expression. The G protein-dependent CMF signal transduction pathways appear to be functional in cmfr1 cells, suggesting that cells sense the density-sensing factor CMF using two or more different receptors.

Evidence for the presence of an NF-kappaB signal transduction system in Dictyostelium discoideum

The Rel/NF-kappaB family of transcription factors and regulators has so far only been described in vertebrates and arthropods, where they mediate responses to many extracellular signals. No counterparts of genes coding for such proteins have been identified in the Caenorhabditis elegans genome and no NF-kappaB activity was found in Saccharomyces cerevisiae. We describe here the presence of an NF-kappaB transduction pathway in the lower eukaryote Dictyostelium discoideum. Using antibodies raised against components of the mammalian NF-kappaB pathway, we demonstrate in Dictyostelium cells extracts the presence of proteins homologous to Rel/NF-kappaB, IkappaB and IKK components. Using gel-shift experiments in nuclear extracts of developing Dictyostelium cells, we demonstrate the presence of proteins binding to kappaB consensus oligonucleotides and to a GC-rich kappaB-like sequence, lying in the promoter of cbpA, a developmentally regulated Dictyostelium gene encoding the Ca(2+)-binding protein CBP1. Using immunofluorescence, we show specific nuclear translocation of the p65 and p50 homologues of the NF-kappaB transcription factors as vegetatively growing cells develop to the slug stage. Taken together, our results strongly indicate the presence of a complete NF-kappaB signal transduction system in Dictyostelium discoideum that could be involved in the developmental process.

A negative regulatory element in a prespore-specific promoter of dictyostelium discoideum(1)

We previously isolated several 'promoter-trap' transformants in which insertion of a promoterless beta-galactosidase gene into the genome caused expression of beta-galactosidase in specific cell types. The upstream flanking region was rescued from one transformant specifically expressing beta-galactosidase in prespore cells. We sequenced the promoter of the gene that is fused in-frame with lacZ and characterised a negative element that inhibits expression in pstO cells (a subtype of prestalk cells). Gel-retardation assays show that a developmentally regulated factor(s) recognises and binds to this element.

A Search for the Origins of Animals and Fungi: Comparing and Combining Molecular Data

Green plants, animals, and fungi have long held our interest as complex, largely multicellular eukaryotes of indeterminate origin. Considerable progress has now been made toward understanding the evolutionary relationships among these taxa as well as identifying their closest protistan relatives. An exclusive animal-fungal clade (the Opisthokonta) is now widely accepted based on an insertion in the protein synthesis elongation factor 1alpha (EF-1alpha) and molecular phylogenies of ribosomal RNAs and the conservative proteins actin, alpha-tubulin, beta-tubulin, and EF-1alpha. Protein data also suggest that the cellular (dictyostelid) and acellular (myxogastrid) slime molds are a close outgroup to the animal-fungal clade. Subsequent sequencing and phylogenetic analysis of EF-1alpha sequences very strongly support a monophyletic slime mold clade (the Mycetozoa or Eumycetozoa), which also includes the lesser-known protostelid slime molds. Monophyly of the opisthokont and mycetozoan clades, exclusive of green plants, is suggested by individual analyses of EF-1alpha and actin and given strong support by concatenated protein data. Neither the monophyly of the slime molds nor their close relationship to animals and fungi are consistently supported by ribosomal RNA data. Thus, it appears unlikely that any single molecule will accurately reconstruct all higher-order taxonomy.

The RdeA-RegA system, a eukaryotic phospho-relay controlling cAMP breakdown

The regA and rdeA gene products of Dictyostelium are involved in the regulation of cAMP signaling. The response regulator, RegA, is composed of an N-terminal receiver domain linked to a C-terminal cAMP-phosphodiesterase domain. RdeA may be a phospho-transfer protein that supplies phosphates to RegA. We show genetically that phospho-RegA is the activated form of the enzyme in vivo, in that the predicted site of aspartate phosphorylation is required for full activity. We show biochemically that RdeA and RegA communicate, as evidenced by phospho-transfer between the two proteins in vitro. Phospho-transfer is dependent on the presumed phospho-accepting amino acids, histidine 65 of RdeA and aspartate 212 of RegA, and occurs in both directions. Phosphorylation of RegA by a heterologous phospho-donor protein activates RegA phosphodiesterase activity at least 20-fold. Our results suggest that the histidine phosphotransfer protein, RdeA, and the response regulator, RegA, constitute two essential elements in a eukaryotic His-Asp phospho-relay network that regulates Dictyostelium development and fruiting body maturation.

Chemoattractant-mediated transient activation and membrane localization of Akt/PKB is required for efficient chemotaxis to cAMP in Dictyostelium

Chemotaxis-competent cells respond to a variety of ligands by activating second messenger pathways leading to changes in the actin/myosin cytoskeleton and directed cell movement. We demonstrate that Dictyostelium Akt/PKB, a homologue of mammalian Akt/PKB, is very rapidly and transiently activated by the chemoattractant cAMP. This activation takes place through G protein-coupled chemoattractant receptors via a pathway that requires homologues of mammalian p110 phosphoinositide-3 kinase. pkbA null cells exhibit aggregation-stage defects that include aberrant chemotaxis, a failure to polarize properly in a chemoattractant gradient and aggregation at low densities. Mechanistically, we demonstrate that the PH domain of Akt/PKB fused to GFP transiently translocates to the plasma membrane in response to cAMP with kinetics similar to those of Akt/PKB kinase activation and is localized to the leading edge of chemotaxing cells in vivo. Our results indicate Akt/PKB is part of the regulatory network required for sensing and responding to the chemoattractant gradient that mediates chemotaxis and aggregation.

Induction of optical density waves and chemotactic cell movement in Dictyostelium discoideum by microinjection of cAMP pulses

The development of most multicellular organisms involves coordinated cell movement. The early aggregation of Dictyostelium cells has been shown to be mediated by chemotactic movement to propagating waves of cAMP. We have proposed that propagating waves of a chemoattractant, most likely cAMP, also control the movement of cells in mounds and slugs. We have now used periodic pressure injection of pulses of cAMP in the extracellular space of aggregation streams, mounds, and slugs to investigate whether these signals can be relayed and control cell movement, using quantitative digital time-lapse microscopy. Our major findings are (1) short (0.1 s) pulses of cAMP (10(7) molecules) were able to elicit optical density (OD) waves in fields of aggregating amoebae. They propagate from the micropipet outward and interact with endogenous OD waves. (2) Periodic injection of cAMP pulses into aggregation streams blocked the pulses coming from the center and led to the rapid accumulation of cells downstream of the pipet around the pipet. (3) Injection of pulses of cAMP into mounds elicited OD waves, which propagated from the pipet outward and interacted with the endogenous waves, indicating that the same propagator carries them. (4) Periodic microinjection of cAMP in the prespore zone of slugs led to accumulation of anterior-like cells around the micropipet followed by tip formation. Furthermore, the cAMP signal could control the spacing of the endogenous sorting pattern. These results strongly support the hypothesis that the optical density waves observed during early development up to the mound stage represent propagating cAMP waves. They suggest furthermore that cAMP is the morphogen that controls cell movements in slugs.

A molecular network that produces spontaneous oscillations in excitable cells of Dictyostelium

A network of interacting proteins has been found that can account for the spontaneous oscillations in adenylyl cyclase activity that are observed in homogenous populations of Dictyostelium cells 4 h after the initiation of development. Previous biochemical assays have shown that when extracellular adenosine 3',5'-cyclic monophosphate (cAMP) binds to the surface receptor CAR1, adenylyl cyclase and the MAP kinase ERK2 are transiently activated. A rise in the internal concentration of cAMP activates protein kinase A such that it inhibits ERK2 and leads to a loss-of-ligand binding by CAR1. ERK2 phosphorylates the cAMP phosphodiesterase REG A that reduces the internal concentration of cAMP. A secreted phosphodiesterase reduces external cAMP concentrations between pulses. Numerical solutions to a series of nonlinear differential equations describing these activities faithfully account for the observed periodic changes in cAMP. The activity of each of the components is necessary for the network to generate oscillatory behavior; however, the model is robust in that 25-fold changes in the kinetic constants linking the activities have only minor effects on the predicted frequency. Moreover, constant high levels of external cAMP lead to attenuation, whereas a brief pulse of cAMP can advance or delay the phase such that interacting cells become entrained.

A novel, putative MEK kinase controls developmental timing and spatial patterning in Dictyostelium and is regulated by ubiquitin-mediated protein degradation

We have identified a developmentally regulated, putative MEK kinase (MEKKalpha) that contains an F-box and WD40 repeats and plays a complex role in regulating cell-type differentiation and spatial patterning. Cells deficient in MEKKalpha develop precociously and exhibit abnormal cell-type patterning with an increase in one of the prestalk compartments (pstO), a concomitant reduction in the prespore domain, and a loss of the sharp compartment boundaries, resulting in overlapping prestalk and prespore domains. Overexpression of MEKKalpha or MEKKalpha lacking the WD40 repeats results in very delayed development and a severe loss of compartment boundaries. Prespore and prestalk cells are interspersed throughout the slug. Analysis of chimeric organisms suggests that MEKKalpha function is required for the proper induction and maintenance of prespore cell differentiation. We show that the WD40 repeats target MEKKalpha to the cortical region of the cell, whereas the F-box/WD40 repeats direct ubiquitin-mediated MEKKalpha degradation. We identify a UBC and a UBP (ubiquitin hydrolase) that interact with the F-box/WD40 repeats. Our findings indicate that cells lacking the ubiquitin hydrolase have phenotypes similar to those of MEKKalpha null (mekkalpha-) cells, further supporting a direct genetic and biochemical interaction between MEKKalpha, the UBC, and the UBP. We demonstrate that UBC and UBP differentially control MEKKalpha ubiquitination/deubiquitination and degradation through the F-box/WD40 repeats in a cell-type-specific and temporally regulated manner. Our results represent a novel mechanism that includes targeted protein degradation by which MAP kinase cascade components can be controlled. More importantly, our findings suggest a new paradigm of spatial and temporal control of the kinase activity controlling spatial patterning during multicellular development, which parallels the temporally regulated degradation of proteins required for cell-cycle progression.

Cell-type-specific rescue of myosin function during Dictyostelium development defines two distinct cell movements required for culmination

Mutant Dictyostelium cells lacking any of the component polypeptides of myosin II exhibit developmental defects. To define myosin's role in establishing Dictyostelium's developmental pattern, we have rescued myosin function in a myosin regulatory light chain null mutant (mlcR-) using cell-type-specific promoters. While mlcR- cells fail to progress beyond the mound stage, expression of RLC from the prestalk promoter, ecmA, produces culminants with normal stalks but with defects in spore cell localization. When GFP-marked prestalk and prespore cells expressing ecmA-RLC are mixed with wild-type cells, the mislocalization of prestalk cells, but not prespore cells, is rescued. Time-lapse video recording of ecmA-RLC cells showed that the posterior prespore zone failed to undergo a contraction important for the upward movement of prespore cells. Prespore cells marked with green fluorescent protein (GFP) failed to move toward the tip with the spiral motion typical of wild type. In contrast, expression of RLC in prespore cells using the psA promoter produced balloon-like structures reminiscent of sorocarps but lacking stalks. GFP-labeled prespore cells showed a spiral movement toward the top of the structures. Expression of RLC from the psA promoter restores the normal localization of psA-GFP cells, but not ecmA-GFP cells. These results define two distinct, myosin-dependent movements that are required for establishing a Dictyostelium fruiting body: stalk extension and active movement of the prespore zone that ensures proper placement of the spores atop the stalk. The approach used in these studies provides a direct means of testing the role of cell motility in distinct cell types during a morphogenetic program.

Cell-cell signaling during Dictyostelium development

Specific proteins and peptides, as well as cAMP, are used as intercellular signals in Dictyostelium. Our understanding of the signal transduction pathways activated by these signals has been expanded by inclusion of newly characterized proteins. cAMP-dependent protein kinase (PKA) and its associated phosphodiesterase, RegA, play multiple roles in these pathways.

A developmentally regulated kinesin-related motor protein from Dictyostelium discoideum

The cellular slime mold Dictyostelium discoideum is an attractive system for studying the roles of microtubule-based motility in cell development and differentiation. In this work, we report the first molecular characterization of kinesin-related proteins (KRPs) in Dictyostelium. A PCR-based strategy was used to isolate DNA fragments encoding six KRPs, several of which are induced during the developmental program that is initiated by starvation. The complete sequence of one such developmentally regulated KRP (designated K7) was determined and found to be a novel member of the kinesin superfamily. The motor domain of K7 is most similar to that of conventional kinesin, but unlike conventional kinesin, K7 is not predicted to have an extensive alpha-helical coiled-coil domain. The nonmotor domain is unusual and is rich in Asn, Gln, and Thr residues; similar sequences are found in other developmentally regulated genes in Dictyostelium. K7, expressed in Escherichia coli, supports plus end-directed microtubule motility in vitro at a speed of 0.14 micron/s, indicating that it is a bona fide motor protein. The K7 motor is found only in developing cells and reaches a peak level of expression between 12 and 16 h after starvation. By immunofluorescence microscopy, K7 localizes to a membranous perinuclear structure. To examine K7 function, we prepared a null cell line but found that these cells show no gross developmental abnormalities. However, when cultivated in the presence of wild-type cells, the K7-null cells are mostly absent from the prestalk zone of the slug. This result suggests that in a population composed largely of wild-type cells, the absence of the K7 motor protein interferes either with the ability of the cells to localize to the prestalk zone or to differentiate into prestalk cells.