A cell number-counting factor regulates the cytoskeleton and cell motility in Dictyostelium

An altered transcriptome underlies cln5-deficiency phenotypes in Dictyostelium discoideum

Mutations in CLN5 cause a subtype of neuronal ceroid lipofuscinosis (NCL) called CLN5 disease. The NCLs, commonly referred to as Batten disease, are a family of neurodegenerative lysosomal storage diseases that affect all ages and ethnicities globally. Previous research showed that CLN5 participates in a variety of cellular processes. However, the precise function of CLN5 in the cell and the pathway(s) regulating its function are not well understood. In the model organism Dictyostelium discoideum, loss of the CLN5 homolog, cln5, impacts various cellular and developmental processes including cell proliferation, cytokinesis, aggregation, cell adhesion, and terminal differentiation. In this study, we used comparative transcriptomics to identify differentially expressed genes underlying cln5-deficiency phenotypes during growth and the early stages of multicellular development. During growth, genes associated with protein ubiquitination/deubiquitination, cell cycle progression, and proteasomal degradation were affected, while genes linked to protein and carbohydrate catabolism were affected during early development. We followed up this analysis by showing that loss of cln5 alters the intracellular and extracellular amounts of proliferation repressors during growth and increases the extracellular amount of conditioned medium factor, which regulates cAMP signalling during the early stages of development. Additionally, cln5- cells displayed increased intracellular and extracellular amounts of discoidin, which is involved in cell-substrate adhesion and migration. Previous work in mammalian models reported altered lysosomal enzyme activity due to mutation or loss of CLN5. Here, we detected altered intracellular activities of various carbohydrate enzymes and cathepsins during cln5- growth and starvation. Notably, cln5- cells displayed reduced β-hexosaminidase activity, which aligns with previous work showing that D. discoideum Cln5 and human CLN5 can cleave the substrate acted upon by β-hexosaminidase. Finally, consistent with the differential expression of genes associated with proteasomal degradation in cln5- cells, we also observed elevated amounts of a proteasome subunit and reduced proteasome 20S activity during cln5- growth and starvation. Overall, this study reveals the impact of cln5-deficiency on gene expression in D. discoideum, provides insight on the genes and proteins that play a role in regulating Cln5-dependent processes, and sheds light on the molecular mechanisms underlying CLN5 disease.

The Use of Diffusion Calculations and Monte Carlo Simulations to Understand the Behavior of Cells in Dictyostelium Communities

Microbial communities are the simplest possible model of multicellular tissues, allowing studies of cell-cell interactions to be done with as few extraneous factors as possible. For instance, the eukaryotic microbe Dictyostelium discoideum proliferates as single cells, and when starved, the cells aggregate together and form structures of ~20,000 cells. The cells use a variety of signals to direct their movement, inform each other of their local cell density and whether they are starving, and organize themselves into groups of ~20,000 cells. Mathematical models and computational approaches have been a key check on, and guide of, the experimental work. In this minireview, I will discuss diffusion calculations and Monte Carlo simulations that were used for Dictyostelium studies that offer general paradigms for several aspects of cell-cell communication. For instance, computational work showed that diffusible secreted cell-density sensing (quorum) factors can diffuse away so quickly from a single cell that the local concentration will not build up to incorrectly cause the cell to sense that it is in the presence of a high density of other cells secreting that signal. In another example, computation correctly predicted a mechanism that allows a group of cells to break up into subgroups. These are thus some examples of the power and necessity of computational work in biology.

Collective regulation of cell motility using an accurate density-sensing system

The capacity of living cells to sense their population density and to migrate accordingly is essential for the regulation of many physiological processes. However, the mechanisms used to achieve such functions are poorly known. Here, based on the analysis of multiple trajectories of vegetative Dictyostelium discoideum cells, we investigate such a system extensively. We show that the cells secrete a high-molecular-weight quorum-sensing factor (QSF) in their medium. This extracellular signal induces, in turn, a reduction of the cell movements, in particular, through the downregulation of a mode of motility with high persistence time. This response appears independent of cAMP and involves a G-protein-dependent pathway. Using a mathematical analysis of the cells' response function, we evidence a negative feedback on the QSF secretion, which unveils a powerful generic mechanism for the cells to detect when they exceed a density threshold. Altogether, our results provide a comprehensive and dynamical view of this system enabling cells in a scattered population to adapt their motion to their neighbours without physical contact.

A telomerase with novel non-canonical roles: TERT controls cellular aggregation and tissue size in Dictyostelium

Telomerase, particularly its main subunit, the reverse transcriptase, TERT, prevents DNA erosion during eukaryotic chromosomal replication, but also has poorly understood non-canonical functions. Here, in the model social amoeba Dictyostelium discoideum, we show that the protein encoded by tert has telomerase-like motifs, and regulates, non-canonically, important developmental processes. Expression levels of wild-type (WT) tert were biphasic, peaking at 8 and 12 h post-starvation, aligning with developmental events, such as the initiation of streaming (~7 h) and mound formation (~10 h). In tert KO mutants, however, aggregation was delayed until 16 h. Large, irregular streams formed, then broke up, forming small mounds. The mound-size defect was not induced when a KO mutant of countin (a master size-regulating gene) was treated with TERT inhibitors, but anti-countin antibodies did rescue size in the tert KO. Although, conditioned medium (CM) from countin mutants failed to rescue size in the tert KO, tert KO CM rescued the countin KO phenotype. These and additional observations indicate that TERT acts upstream of smlA/countin: (i) the observed expression levels of smlA and countin, being respectively lower and higher (than WT) in the tert KO; (ii) the levels of known size-regulation intermediates, glucose (low) and adenosine (high), in the tert mutant, and the size defect's rescue by supplemented glucose or the adenosine-antagonist, caffeine; (iii) the induction of the size defect in the WT by tert KO CM and TERT inhibitors. The tert KO's other defects (delayed aggregation, irregular streaming) were associated with changes to cAMP-regulated processes (e.g. chemotaxis, cAMP pulsing) and their regulatory factors (e.g. cAMP; acaA, carA expression). Overexpression of WT tert in the tert KO rescued these defects (and size), and restored a single cAMP signaling centre. Our results indicate that TERT acts in novel, non-canonical and upstream ways, regulating key developmental events in Dictyostelium.

Different Isoforms of the Neuronal Guidance Molecule Slit2 Directly Cause Chemoattraction or Chemorepulsion of Human Neutrophils

The movement of neutrophils between blood and tissues appears to be regulated by chemoattractants and chemorepellents. Compared with neutrophil chemoattractants, relatively little is known about neutrophil chemorepellents. Slit proteins are endogenously cleaved into a variety of N- and C-terminal fragments, and these fragments are neuronal chemorepellents and inhibit chemoattraction of many cell types, including neutrophils. In this report, we show that the ∼140-kDa N-terminal Slit2 fragment (Slit2-N) is a chemoattractant and the ∼110-kDa N-terminal Slit2 fragment (Slit2-S) is a chemorepellent for human neutrophils. The effects of both Slit2 fragments were blocked by Abs to the Slit2 receptor Roundabout homolog 1 or the Slit2 coreceptor Syndecan-4. Slit2-N did not appear to activate Ras but increased phosphatidylinositol 3,4,5-triphosphate levels. Slit2-N-induced chemoattraction was unaffected by Ras inhibitors, reversed by PI3K inhibitors, and blocked by Cdc42 and Rac inhibitors. In contrast, Slit2-S activated Ras but did not increase phosphatidylinositol 3,4,5-triphosphate levels. Slit2-S-induced chemorepulsion was blocked by Ras and Rac inhibitors, not affected by PI3K inhibitors, and reversed by Cdc42 inhibitors. Slit2-N, but not Slit2-S, increased neutrophil adhesion, myosin L chain 2 phosphorylation, and polarized actin formation and single pseudopods at the leading edge of cells. Slit2-S induced multiple pseudopods. These data suggest that Slit2 isoforms use similar receptors but different intracellular signaling pathways and have different effects on the cytoskeleton and pseudopods to induce neutrophil chemoattraction or chemorepulsion.

An endogenous chemorepellent directs cell movement by inhibiting pseudopods at one side of cells

Eukaryotic chemoattraction signal transduction pathways, such as those used by Dictyostelium discoideum to move toward cAMP, use a G protein-coupled receptor to activate multiple conserved pathways such as PI3 kinase/Akt/PKB to induce actin polymerization and pseudopod formation at the front of a cell, and PTEN to localize myosin II to the rear of a cell. Relatively little is known about chemorepulsion. We previously found that AprA is a chemorepellent protein secreted by Dictyostelium cells. Here we used 29 cell lines with disruptions of cAMP and/or AprA signal transduction pathway components, and delineated the AprA chemorepulsion pathway. We find that AprA uses a subset of chemoattraction signal transduction pathways including Ras, protein kinase A, target of rapamycin (TOR), phospholipase A, and ERK1, but does not require the PI3 kinase/Akt/PKB and guanylyl cyclase pathways to induce chemorepulsion. Possibly as a result of not using the PI3 kinase/Akt/PKB pathway and guanylyl cyclases, AprA does not induce actin polymerization or increase the pseudopod formation rate, but rather appears to inhibit pseudopod formation at the side of cells closest to the source of AprA.

The Effect of Overexpressed DdRabS on Development, Cell Death, Vesicular Trafficking, and the Secretion of Lysosomal Glycosidase Enzymes

Rab GTPases are essential regulators of many cellular processes and play an important role in downstream signaling vital to proper cell function. We sought to elucidate the role of novel D. discoideum GTPase RabS. Cell lines over-expressing DdRabS and expressing DdRabS N137I (dominant negative (DN)) proteins were generated, and it was determined that DdRabS localized to endosomes, ER-Golgi membranes, and the contractile vacuole system. It appeared to function in vesicular trafficking, and the secretion of lysosomal enzymes. Interestingly, microscopic analysis of GFP-tagged DdRabS (DN) cells showed differential localization to lysosomes and endosomes compared to GFP-tagged DdRabS overexpressing cells. Both cell lines over-secreted lysosomal glycosidase enzymes, especially β-glucosidase. Furthermore, DdRabS overexpressing cells were defective in aggregation due to decreased cell–cell cohesion and sensitivity to cAMP, leading to abnormal chemotactic migration, the inability to complete development, and increased induced cell death. These data support a role for DdRabS in trafficking along the vesicular and biosynthetic pathways. We hypothesize that overexpression of DdRabS may interfere with GTP activation of related proteins essential for normal development resulting in a cascade of defects throughout these processes.

Protease activated-receptor 2 is necessary for neutrophil chemorepulsion induced by trypsin, tryptase, or dipeptidyl peptidase IV

Compared to neutrophil chemoattractants, relatively little is known about the mechanism neutrophils use to respond to chemorepellents. We previously found that the soluble extracellular protein dipeptidyl peptidase IV (DPPIV) is a neutrophil chemorepellent. In this report, we show that an inhibitor of the protease activated receptor 2 (PAR2) blocks DPPIV-induced human neutrophil chemorepulsion, and that PAR2 agonists such as trypsin, tryptase, 2f-LIGRL, SLIGKV, and AC55541 induce human neutrophil chemorepulsion. Several PAR2 agonists in turn block the ability of the chemoattractant fMLP to attract neutrophils. Compared to neutrophils from male and female C57BL/6 mice, neutrophils from male and female mice lacking PAR2 are insensitive to the chemorepulsive effects of DPPIV or PAR2 agonists. Acute respiratory distress syndrome (ARDS) involves an insult-mediated influx of neutrophils into the lungs. In a mouse model of ARDS, aspiration of PAR2 agonists starting 24 h after an insult reduce neutrophil numbers in the bronchoalveolar lavage (BAL) fluid, as well as the post-BAL lung tissue. Together, these results indicate that the PAR2 receptor mediates DPPIV-induced chemorepulsion, and that PAR2 agonists might be useful to induce neutrophil chemorepulsion.

Response of Dictyostelium discoideum to UV-C and involvement of poly (ADP-ribose) polymerase

OBJECTIVES

Radiation and chemical mutagens are direct DNA-damaging agents and ultraviolet (UV) radiation is frequently used in biological studies. Consequent to ozone depletion, UV-C could become a great challenge to living organisms on earth, in the near future. The present study has focused on the role of poly (ADP-ribose) polymerase (PARP) during UV-C-induced growth and developmental changes in Dictyostelium discoideum, a phylogenetically important unicellular eukaryote.

MATERIALS AND METHODS

Dictyostelium discoideum cells were exposed to different doses of UV-C and PARP activity, and effects of its inhibition were studied. Expression of developmentally regulated genes yakA, car1, aca, csA, regA, ctnA, ctnB, gp24, hspD and dsn were analysed using semiquantitative RT-PCR.

RESULTS

We report that the D. discoideum cells displayed PARP activation within 2 min of UV-C irradiation and there was increase in NO levels in a dose-dependent manner. UV-C-irradiated cells had impaired growth, delayed or blocked development and delayed germination compared to control cells. In our previous studies we have shown that inhibition of PARP recovered oxidative stress-induced changes in D. discoideum; however, intriguingly PARP inhibition did not correct all defects as effectively in UV-C-irradiated cells. This possibly was due to interplay with increased NO signalling.

CONCLUSIONS

Our results signify that UV-C and oxidative stress affected growth and development in D. discoideum by different mechanisms; these studies could provide major clues to complex mechanisms of growth and development in higher organisms.

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.

A cell number counting factor alters cell metabolism

It is still not clear how organisms regulate the size of appendages or organs during development. During development, Dictyostelium discoideum cells form groups of approximately 2 x 10(4) cells. The cells secrete a protein complex called counting factor (CF) that allows them to sense the local cell density. If there are too many cells in a group, as indicated by high extracellular concentrations of CF, the cells break up the group by decreasing cell-cell adhesion and increasing random cell motility. As a part of the signal transduction pathway, CF decreases the activity of glucose-6-phosphatase to decrease internal glucose levels. CF also decreases the levels of fructose-1,6-bisphosphate and increases the levels of glucose-6-phosphate and fructose-6-phosphate. In this report, we focus on how a secreted signal used to regulate the size of a group of cells regulates many basic aspects of cell metabolism, including the levels of pyruvate, lactate, and ATP, and oxygen consumption.

CnrN regulates Dictyostelium group size using a counting factor-independent mechanism

One of the simplest examples of a complex behavior is the aggregation of solitary Dictyostelium discoideum amoebae to form a 20,000-cell fruiting body. A field of starving amoebae first breaks up into territories. In each territory, the cells form a spider-like pattern of streams of cells. As part of a negative feedback loop, counting factor (CF), a secreted protein complex whose concentration increases with the size of the stream, prevents over-sized fruiting bodies from being formed by increasing cell motility and decreasing cell-cell adhesion, which causes the breakup of excessively large streams. Cells lacking the phosphatase CnrN (cnrN(-) cells) form small aggregation territories and few streams.1 In this report, we present computer simulations that suggest that in the absence of stream formation, CF should be unable to affect group size. As predicted, cnrN(-) group size is insensitive to the addition or depletion of CF. Together, the data indicate that CnrN regulates group size by regulating both the break-up of a field of cells into aggregation territories and stream formation during development, and that CnrN-mediated and CF-mediated group size regulation use different mechanisms.

Group formation and the evolution of sociality

In spite of its intrinsic evolutionary instability, altruistic behavior in social groups is widespread in nature, spanning from organisms endowed with complex cognitive abilities to microbial populations. In this study, we show that if social individuals have an enhanced tendency to form groups and fitness increases with group cohesion, sociality can evolve and be maintained in the absence of actively assortative mechanisms such as kin recognition or nepotism toward other carriers of the social gene. When explicitly taken into account in a game-theoretical framework, the process of group formation qualitatively changes the evolutionary dynamics with respect to games played in groups of constant size and equal grouping tendencies. The evolutionary consequences of the rules underpinning the group size distribution are discussed for a simple model of microbial aggregation by differential attachment, indicating a way to the evolution of sociality bereft of peer recognition.

A quorum-sensing factor in vegetative Dictyostelium discoideum cells revealed by quantitative migration analysis

Background

Many cells communicate through the production of diffusible signaling molecules that accumulate and once a critical concentration has been reached, can activate or repress a number of target genes in a process termed quorum sensing (QS). In the social amoeba Dictyostelium discoideum, QS plays an important role during development. However little is known about its effect on cell migration especially in the growth phase.

Methods and Findings

To investigate the role of cell density on cell migration in the growth phase, we use multisite timelapse microscopy and automated cell tracking. This analysis reveals a high heterogeneity within a given cell population, and the necessity to use large data sets to draw reliable conclusions on cell motion. In average, motion is persistent for short periods of time (), but normal diffusive behavior is recovered over longer time periods. The persistence times are positively correlated with the migrated distances. Interestingly, the migrated distance decreases as well with cell density. The adaptation of cell migration to cell density highlights the role of a secreted quorum sensing factor (QSF) on cell migration. Using a simple model describing the balance between the rate of QSF generation and the rate of QSF dilution, we were able to gather all experimental results into a single master curve, showing a sharp cell transition between high and low motile behaviors with increasing QSF.

Conclusion

This study unambiguously demonstrates the central role played by QSF on amoeboid motion in the growth phase.

Copine A is expressed in prestalk cells and regulates slug phototaxis and thermotaxis in developing Dictyostelium

Copines are calcium-dependent membrane-binding proteins found in many eukaryotic organisms. We are studying the function of copines using the model organism, Dictyostelium discoideum. When under starvation conditions, Dictyostelium cells aggregate into mounds that become migrating slugs, which can move toward light and heat before culminating into a fruiting body. Previously, we showed that Dictyostelium cells lacking the copine A (cpnA) gene are not able to form fruiting bodies and instead arrest at the slug stage. In this study, we compared the slug behavior of cells lacking the cpnA gene to the slug behavior of wild-type cells. The slugs formed by cpnA- cells were much larger than wild-type slugs and exhibited no phototaxis and negative thermotaxis in the same conditions that wild-type slugs exhibited positive phototaxis and thermotaxis. Mixing as little as 5% wild-type cells with cpnA- cells rescued the phototaxis and thermotaxis defects, suggesting that CpnA plays a specific role in the regulation of the production and/or release of a signaling molecule. Reducing extracellular levels of ammonia also partially rescued the phototaxis and thermotaxis defects of cpnA- slugs, suggesting that CpnA may have a specific role in regulating ammonia signaling. Expressing the lacZ gene under the cpnA promoter in wild-type cells indicated cpnA is preferentially expressed in the prestalk cells found in the anterior part of the slug, which include the cells at the tip of the slug that regulate phototaxis, thermotaxis, and the initiation of culmination into fruiting bodies. Our results suggest that CpnA plays a role in the regulation of the signaling pathways, including ammonia signaling, necessary for sensing and/or orienting toward light and heat in the prestalk cells of the Dictyostelium slug.

Cell density sensing and size determination

The social amoeba Dictyostelium discoideum is one of the leading model systems used to study how cells count themselves to determine the number and/or density of cells. In this review, we describe work on three different cell-density sensing systems used by Dictyostelium. The first involves a negative feedback loop in which two secreted signals inhibit cell proliferation during the growth phase. As the cell density increases, the concentrations of the secreted factors concomitantly increase, allowing the cells to sense their density. The two signals act as message authenticators for each other, and the existence of two different signals that require each other for activity may explain why previous efforts to identify autocrine proliferation-inhibiting signals in higher eukaryotes have generally failed. The second system involves a signal made by growing cells that is secreted only when they starve. This then allows cells to sense the density of just the starving cells, and is an example of a mechanism that allows cells in a tissue to sense the density of one specific cell type. The third cell density counting system involves cells in aggregation streams secreting a signal that limits the size of fruiting bodies. Computer simulations predicted, and experiments then showed, that the factor increases random cell motility and decreases cell-cell adhesion to cause streams to break up if there are too many cells in the stream. Together, studies on Dictyostelium cell density counting systems will help elucidate how higher eukaryotes regulate the size and composition of tissues.

Dictyostelium cells bind a secreted autocrine factor that represses cell proliferation

BACKGROUND

Dictyostelium cells secrete the proteins AprA and CfaD. Cells lacking either AprA or CfaD proliferate faster than wild type, while AprA or CfaD overexpressor cells proliferate slowly, indicating that AprA and CfaD are autocrine factors that repress proliferation. CfaD interacts with AprA and requires the presence of AprA to slow proliferation. To determine if CfaD is necessary for the ability of AprA to slow proliferation, whether AprA binds to cells, and if so whether the binding requires the presence of CfaD, we examined the binding and effect on proliferation of recombinant AprA.

RESULTS

We find that the extracellular accumulation of AprA increases with cell density and reaches a concentration of 0.3 microg/ml near a stationary cell density. When added to wild-type or aprA- cells, recombinant AprA (rAprA) significantly slows proliferation at 0.1 microg/ml and higher concentrations. From 4 to 64 microg/ml, the effect of rAprA is at a plateau, slowing but not stopping proliferation. The proliferation-inhibiting activity of rAprA is roughly the same as that of native AprA in conditioned growth medium. Proliferating aprA- cells show saturable binding of rAprA to 92,000 +/- 11,000 cell-surface receptors with a KD of 0.03 +/- 0.02 microg/ml. There appears to be one class of binding site, and no apparent cooperativity. Native AprA inhibits the binding of rAprA to aprA- cells with a Ki of 0.03 mug/ml, suggesting that the binding kinetics of rAprA are similar to those of native AprA. The proliferation of cells lacking CrlA, a cAMP receptor-like protein, or cells lacking CfaD are not affected by rAprA. Surprisingly, both cell types still bind rAprA.

CONCLUSION

Together, the data suggest that AprA functions as an autocrine proliferation-inhibiting factor by binding to cell surface receptors. Although AprA requires CfaD for activity, it does not require CfaD to bind to cells, suggesting the possibility that cells have an AprA receptor and a CfaD receptor, and activation of both receptors is required to slow proliferation. We previously found that crlA- cells are sensitive to CfaD. Combined with the results presented here, this suggests that CrlA is not the AprA or CfaD receptor, and may be the receptor for an unknown third factor that is required for AprA and CfaD activity.

Combining experiments and modelling to understand size regulation in Dictyostelium discoideum

Little is known about how the sizes of specific organs and tissues are regulated. To try to understand these mechanisms, we have been using a combination of modelling and experiments to study the simple system Dictyostelium discoideum, which forms approximately 20000 cell groups. We found that cells secrete a factor, and as the number of cells increases, the concentration of the factor increases. Diffusion calculations indicated that this lets cells sense the local cell density. Computer simulations predicted, and experiments then showed, that this factor decreases cell-cell adhesion and increases random cell motility. In a group, adhesion forces keep cells together, while random motility forces cause cells to pull apart and separate from each other. As the group size increases above a threshold, the factor concentration goes above a threshold and the cells switch from an adhered state to a separated state. This causes excessively large groups to break apart and/or dissipate, creating an upper limit to group size. In this review, we focus on how computer simulations made testable predictions that led the way to understanding the size regulation mechanism mediated by this factor.

A protein with similarity to PTEN regulates aggregation territory size by decreasing cyclic AMP pulse size during Dictyostelium discoideum development

An interesting but largely unanswered biological question is how eukaryotic organisms regulate the size of multicellular tissues. During development, a lawn of Dictyostelium cells breaks up into territories, and within the territories the cells aggregate in dendritic streams to form groups of approximately 20,000 cells. Using random insertional mutagenesis to search for genes involved in group size regulation, we found that an insertion in the cnrN gene affects group size. Cells lacking CnrN (cnrN(-)) form abnormally small groups, which can be rescued by the expression of exogenous CnrN. Relayed pulses of extracellular cyclic AMP (cAMP) direct cells to aggregate by chemotaxis to form aggregation territories and streams. cnrN(-) cells overaccumulate cAMP during development and form small territories. Decreasing the cAMP pulse size by treating cnrN(-) cells with cAMP phosphodiesterase or starving cnrN(-) cells at a low density rescues the small-territory phenotype. The predicted CnrN sequence has similarity to phosphatase and tensin homolog (PTEN), which in Dictyostelium inhibits cAMP-stimulated phosphatidylinositol 3-kinase signaling pathways. CnrN inhibits cAMP-stimulated phosphatidylinositol 3,4,5-trisphosphate accumulation, Akt activation, actin polymerization, and cAMP production. Our results suggest that CnrN is a protein with some similarities to PTEN and that it regulates cAMP signal transduction to regulate territory size.

The secreted Dictyostelium protein CfaD is a chalone

Dictyostelium discoideum cells secrete CfaD, a protein that is similar to cathepsin proteases. Cells that lack cfaD proliferate faster and reach a higher stationary-phase density than wild-type cells, whereas cells that overexpress CfaD proliferate slowly and reach the stationary phase when at a low density. On a per-nucleus basis, CfaD affects proliferation but not growth. The drawback of not having CfaD is a reduced spore viability. Recombinant CfaD has no detectable protease activity but, when added to cells, inhibits the proliferation of wild-type and cfaD(-) cells. The secreted protein AprA also inhibits proliferation. AprA is necessary for the effect of CfaD on proliferation. Molecular-sieve chromatography indicates that in conditioned growth medium, the 60 kDa CfaD is part of a approximately 150 kDa complex, and both chromatography and pull-down assays suggest that CfaD interacts with AprA. These results suggest that two interacting proteins may function together as a chalone signal in a negative feedback loop that slows Dictyostelium cell proliferation.

SrfB, a member of the Serum Response Factor family of transcription factors, regulates starvation response and early development in Dictyostelium

The Serum Response Factor (SRF) is an important regulator of cell proliferation and differentiation. Dictyostelium discoideum srfB gene codes for an SRF homologue and is expressed in vegetative cells and during development under the control of three alternative promoters, which show different cell-type specific patterns of expression. The two more proximal promoters directed gene transcription in prestalk AB, stalk and lower-cup cells. The generation of a strain where the srfB gene has been interrupted (srfB⁻) has shown that this gene is required for regulation of actin–cytoskeleton-related functions, such as cytokinesis and macropinocytosis. The mutant failed to develop well in suspension, but could be rescued by cAMP pulsing, suggesting a defect in cAMP signaling. srfB⁻ cells showed impaired chemotaxis to cAMP and defective lateral pseudopodium inhibition. Nevertheless, srfB⁻ cells aggregated on agar plates and nitrocellulose filters 2 h earlier than wild type cells, and completed development, showing an increased tendency to form slug structures. Analysis of wild type and srfB⁻ strains detected significant differences in the regulation of gene expression upon starvation. Genes coding for lysosomal and ribosomal proteins, developmentally-regulated genes, and some genes coding for proteins involved in cytoskeleton regulation were deregulated during the first stages of development.

Dynamical quorum sensing: Population density encoded in cellular dynamics

Mutual synchronization by exchange of chemicals is a mechanism for the emergence of collective dynamics in cellular populations. General theories exist on the transition to coherence, but no quantitative, experimental demonstration has been given. Here, we present a modeling and experimental analysis of cell-density-dependent glycolytic oscillations in yeast. We study the disappearance of oscillations at low cell density and show that this phenomenon occurs synchronously in all cells and not by desynchronization, as previously expected. This study identifies a general scenario for the emergence of collective cellular oscillations and suggests a quorum-sensing mechanism by which the cell density information is encoded in the intracellular dynamical state.

A cell number-counting factor regulates levels of a novel protein, SslA, as part of a group size regulation mechanism in Dictyostelium

Developing Dictyostelium cells form aggregation streams that break into groups of approximately 2 x 10(4) cells. The breakup and subsequent group size are regulated by a secreted multisubunit counting factor (CF). To elucidate how CF regulates group size, we isolated second-site suppressors of smlA(-), a transformant that forms small groups due to oversecretion of CF. smlA(-) sslA1(CR11) cells form roughly wild-type-size groups due to an insertion in the beginning of the coding region of sslA1, one of two highly similar genes encoding a novel protein. The insertion increases levels of SslA. In wild-type cells, the sslA1(CR11) mutation forms abnormally large groups. Reducing SslA levels by antisense causes the formation of smaller groups. The sslA(CR11) mutation does not affect the extracellular accumulation of CF activity or the CF components countin and CF50, suggesting that SslA does not regulate CF secretion. However, CF represses levels of SslA. Wild-type cells starved in the presence of smlA(-) cells, recombinant countin, or recombinant CF50 form smaller groups, whereas sslA1(CR11) cells appear to be insensitive to the presence of smlA(-) cells, countin, or CF50, suggesting that the sslA1(CR11) insertion affects CF signal transduction. We previously found that CF reduces intracellular glucose levels. sslA(CR11) does not significantly affect glucose levels, while glucose increases SslA levels. Together, the data suggest that SslA is a novel protein involved in part of a signal transduction pathway regulating group size.

Polysialylation increases lateral diffusion of neural cell adhesion molecule in the cell membrane

Polysialic acid (PSA) is a polymer of N-acetylneuraminic acid residues added post-translationally to the membrane-bound neural cell adhesion molecule (NCAM). The large excluded volume created by PSA polymer is thought to facilitate cell migration by decreasing cell adhesion. Here we used live cell imaging (spot fluorescence recovery after photobleaching and fluorescence correlation spectroscopy) combined with biochemical approaches in an attempt to uncover a link between cell motility and the impact of polysialylation on NCAM dynamics. We show that PSA regulates specifically NCAM lateral diffusion and this is dependent on the integrity of the cytoskeleton. However, whereas the glial-derivative neurotrophic factor chemotactic effect is dependent on PSA, the molecular dynamics of PSA-NCAM is not directly affected by glial-derivative neurotrophic factor. These findings reveal a new intrinsic mechanism by which polysialylation regulates NCAM dynamics and thereby a biological function like cell migration.

Proteomic and microarray analyses of the Dictyostelium Zak1-GSK-3 signaling pathway reveal a role in early development

GskA, the Dictyostelium GSK-3 orthologue, is modified and activated by the dual-specificity tyrosine kinase Zak1, and the two kinases form part of a signaling pathway that responds to extracellular cyclic AMP. We identify potential cellular effectors for the two kinases by analyzing the corresponding null mutants. There are proteins and mRNAs that are altered in abundance in only one or the other of the two mutants, indicating that each kinase has some unique functions. However, proteomic and microarray analyses identified a number of proteins and genes, respectively, that are similarly misregulated in both mutant strains. The positive correlation between the array data and the proteomic data is consistent with the Zak1-GskA signaling pathway's functioning by directly or indirectly regulating gene expression. The discoidin 1 genes are positively regulated by the pathway, while the abundance of the H5 protein is negatively regulated. Two of the targets, H5 and discoidin 1, are well-characterized markers for early development, indicating that the Zak1-GskA pathway plays a role in development earlier than previously observed.

A protein in crude cytosol regulates glucose-6-phosphatase activity in crude microsomes to regulate group size in Dictyostelium

Dictyostelium discoideum form groups of approximately 2 x 10(4) cells. The group size is regulated in part by a negative feedback pathway mediated by a secreted multipolypeptide complex called counting factor (CF). The CF signal transduction pathway involves CF-repressing internal glucose levels by increasing the K(m) of glucose-6-phosphatase. Little is known about how this enzyme is regulated. Glucose-6-phosphatase is associated with microsomes in both Dictyostelium and mammals. We find that the activity of glucose-6-phosphatase in crude microsomes from cells with high, normal, or low CF activity had a negative correlation with the amount of CF present in these cell lines. In crude cytosols (supernatants from ultracentrifugation of cell lysates), the glucose-6-phosphatase activity had a positive correlation with CF accumulation. The crude cytosols were further fractionated into a fraction containing molecules greater than 10 kDa (S>10K) and molecules less than 10 KDa (S<10K). S>10K from wild-type cells strongly repressed the activity of glucose-6-phosphatase in wild-type microsomes, whereas S>10K from countin(-) cells (cells with low CF activity) significantly increased the activity of glucose-6-phosphatase in wild-type microsomes by decreasing K(m). The regulatory activities in the wild-type and countin(-) S>10Ks are heat-labile and protease-sensitive, suggesting that they are proteins. S<10K from both wild-type and countin(-) cells did not significantly change glucose-6-phosphatase activity. Together, the data suggest that, as a part of a pathway modulating multicellular group size, CF regulates one or more proteins greater than 10 KDa in crude cytosol that affect microsome-associated glucose-6-phosphatase activity.

Mathematically modelling the effects of counting factor in Dictyostelium discoideum

Size regulation is a crucial feature in many biological systems, with misregulation leading to dysplasia or hyperplasia. The recent discovery of counting factor (CF) in Dictyostelium discoideum will lead to a greater understanding of how the system regulates the size of a group of cells. In this paper we mathematically model the known effects of CF using two different models: a cellular automata model and a discrete continuum hybrid model. With the use of these models we are able to understand how modulation of adhesion and motile forces by CF can facilitate stream breakup. In addition, the modelling suggests a new possible mechanism for stream breakup involving the frequency of cell reorientation.

Signal relay during the life cycle of Dictyostelium

A fundamental property of multicellular organisms is signal relay, the process by which information is transmitted from one cell to another. The integration of external information, such as nutritional status or developmental cues, is critical to the function of organisms. In addition, the spatial organizations of multicellular organisms require intricate signal relay mechanisms. Signal relay is remarkably exhibited during the life cycle of the social amoebae Dictyostelium discoideum, a eukaryote that retains a simple way of life, yet it has greatly contributed to our knowledge of the mechanisms cells use to communicate and integrate information. This chapter focuses on the molecules and mechanisms that Dictyostelium employs during its life cycle to relay temporal and spatial cues that are required for survival.

A secreted factor represses cell proliferation in Dictyostelium

Many cells appear to secrete factors called chalones that limit their proliferation, but in most cases the factors have not been identified. We found that growing Dictyostelium cells secrete a 60 kDa protein called AprA for autocrine proliferation repressor. AprA has similarity to putative bacterial proteins of unknown function. Compared with wild-type cells, aprA-null cells proliferate faster, while AprA overexpressing cells proliferate slower. Growing wild-type cells secrete a factor that inhibits the proliferation of wild-type and aprA- cells; this activity is not secreted by aprA- cells. AprA purified by immunoprecipitation also slows the proliferation of wild-type and aprA- cells. Compared with wild type, there is a higher percentage of multinucleate cells in the aprA- population, and when starved, aprA- cells form abnormal structures that contain fewer spores. AprA may thus decrease the number of multinucleate cells and increase spore production. Together, the data suggest that AprA functions as part of a Dictyostelium chalone.

Gamete fusion and cytokinesis preceding zygote establishment in the sexual process of Dictyostelium discoideum

Cells of Dictyostelium discoideum become sexually mature under submerged and dark conditions, and fuse with opposite mating-type cells to form zygote giant cells, which gather surrounding cells and finally develop into dormant structures called macrocysts. In the present study, we found that the multinuclear fused cells formed during this process frequently underwent cytokinesis driven by random local movements. The split cells were capable of re-fusion, and repeated cytokinesis. These radical behaviors continued until the extensive cell aggregation started around the giant cells. Thus, gamete fusion and initiation of zygote development do not coincide in the mating of D. discoideum. Analyses by confocal microscopy and flow cytometry indicated that the cessation of the random movement followed pronuclear fusion, and that microtubule organizing centers (MTOC), abundant in the fused cells at the beginning, gradually decreased and only one of them remained within the developed macrocyst. Some of the genes known to control cell movement, such as rasGEFB and rasS, increased shortly before the cessation of repeated fusion-cytokinesis and initiation of phagocytosis. These results suggest that the sequential molecular events are necessary in D. discoideum after gamete fusion to establish a new individuality of zygotes.

Exposure of cells to a cell number-counting factor decreases the activity of glucose-6-phosphatase to decrease intracellular glucose levels in Dictyostelium discoideum

The development of Dictyostelium discoideum is a model for tissue size regulation, as these cells form groups of approximately 2 x 10(4) cells. The group size is regulated in part by a negative feedback pathway mediated by a secreted multipolypeptide complex called counting factor (CF). CF signal transduction involves decreasing intracellular CF glucose levels. A component of CF, countin, has the bioactivity of the entire CF complex, and an 8-min exposure of cells to recombinant countin decreases intracellular glucose levels. To understand how CF regulates intracellular glucose, we examined the effect of CF on enzymes involved in glucose metabolism. Exposure of cells to CF has little effect on amylase or glycogen phosphorylase, enzymes involved in glucose production from glycogen. Glucokinase activity (the first specific step of glycolysis) is inhibited by high levels of CF but is not affected by an 8-min exposure to countin. The second enzyme specific for glycolysis, phosphofructokinase, is not regulated by CF. There are two corresponding enzymes in the gluconeogenesis pathway, fructose-1,6-bisphosphatase and glucose-6-phosphatase. The first is not regulated by CF or countin, whereas glucose-6-phosphatase is regulated by both CF and an 8-min exposure to countin. The countin-induced changes in the Km and Vmax of glucose-6-phosphatase cause a decrease in glucose production that can account for the countin-induced decrease in intracellular glucose levels. It thus appears that part of the CF signal transduction pathway involves inhibiting the activity of glucose-6-phosphatase, decreasing intracellular glucose levels and affecting the levels of other metabolites, to regulate group size.

A cell number counting factor regulates Akt/protein kinase B to regulate Dictyostelium discoideum group size

Little is known about how individual cells can organize themselves to form structures of a given size. During development, Dictyostelium discoideum aggregates in dendritic streams and forms groups of approximately 20,000 cells. D. discoideum regulates group size by secreting and simultaneously sensing a multiprotein complex called counting factor (CF). If there are too many cells in a stream, the associated high concentration of CF will decrease cell-cell adhesion and increase cell motility, causing aggregation streams to break up. The pulses of cyclic AMP (cAMP) that mediate aggregation cause a transient translocation of Akt/protein kinase B (Akt/PKB) to the leading edge of the plasma membrane and a concomitant activation of the kinase activity, which in turn stimulates motility. We found that countin- cells (which lack bioactive CF) and wild-type cells starved in the presence of anticountin antibodies (which block CF activity) showed a decreased level of cAMP-stimulated Akt/PKB membrane translocation and kinase activity compared to parental wild-type cells. Recombinant countin has the bioactivity of CF, and a 1-min treatment of cells with recombinant countin potentiated Akt/PKB translocation to membranes and Akt/PKB activity. Western blotting of total cell lysates indicated that countin does not affect the total level of Akt/PKB. Fluorescence microscopy of cells expressing an Akt/PKB pleckstrin homology domain-green fluorescent protein (PH-GFP) fusion protein indicated that recombinant countin and anti-countin antibodies do not obviously alter the distribution of Akt/PKB PH-GFP when it translocates to the membrane. Our data indicate that CF increases motility by potentiating the cAMP-stimulated activation and translocation of Akt/PKB.

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.

Involvement of a Rab8-like protein of Dictyostelium discoideum, Sas1, in the formation of membrane extensions, secretion and adhesion during development

Establishment of cell-cell adhesions, regulation of actin, and secretion are critical during development. Rab8-like GTPases have been shown to modulate these cellular events, suggesting an involvement in developmental processes. To further elucidate the function of Rab8-like GTPases in a developmental context, a Rab8-related protein (Sas1) of Dictyostelium discoideum was examined, the expression of which increases at the onset of development. Dictyostelium cell lines expressing inactive (N128I mutant) and constitutively active (Q74L mutant) Sas1 as green fluorescent protein (GFP)-Sas1 chimeras were generated. Cells expressing Sas1Q74L displayed numerous actin-rich membrane protrusions, increased secretion, and were unable to complete development. In particular, these cells demonstrated a reduction in adhesion as well as in the levels of a cell adhesion molecule, gp24 (DdCAD-1). In contrast, cells expressing Sas1N128I exhibited increased cell-cell adhesion and increased levels of gp24. Counting factor is a multisubunit signalling complex that is secreted in early development and controls aggregate size by negatively regulating the levels of cell adhesion molecules, including gp24. Interestingly, the Sas1Q74L mutant demonstrated increased levels of extracellular countin, a subunit of counting factor, suggesting that Sas1 may regulate trafficking of counting factor components. Together, the data suggest that Sas1 may be a key regulator of actin, adhesion and secretion during development.

Cell motility mediates tissue size regulation in Dictyostelium

Little is known about how organisms regulate the size of multicellular structures. This review condenses some of the observations about how Dictyostelium regulates the size of fruiting bodies. Very large fruiting bodies tend to fall over, and one of the ways Dictyostelium cells prevent this is by breaking up the aggregation streams when there is an excessive number of cells in the stream. Developing cells simultaneously secrete and sense counting factor (CF), a 450 kDa complex of proteins. Diffusion calculations showed that as the number of cells in a stream or group increases, the local concentration of CF will increase, allowing the cells to sense the number of cells in the stream or group. Computer simulations predicted that a high level of CF could trigger stream breakup by decreasing cell-cell adhesion and/or increasing cell motility, effectively causing the stream to dissipate and begin to fall apart. The prediction that adhesion and motility affect group size is supported by observations that decreasing adhesion by adding antibodies that bind to adhesion protein causes the formation of smaller groups, while increasing adhesion by overexpressing adhesion proteins, or decreasing motility with drugs that disrupt actin function both cause the formation of larger groups. CF both decreases adhesion and increases motility. CF increases motility in part by increasing actin polymerization and myosin phosphorylation, and decreasing myosin polymerization. New observations using a fusion of a green fluorescent protein to a protein fragment that binds polymerized actin show that in live cells CF does not affect the distribution of polymerized actin. CF increases the levels of ABP-120, an actin-bundling protein, and new observations indicate that very low levels of CF cause an increase in levels of myoB, an unconventional myosin. Our current understanding of group size regulation in Dictyostelium is thus that motility plays a key role, and that to regulate group size cells regulate the expression of at least two proteins, as well as regulating the polymerization of both actin and myosin.

Two components of a secreted cell number-counting factor bind to cells and have opposing effects on cAMP signal transduction in Dictyostelium

A secreted 450-kDa complex of proteins called counting factor (CF) is part of a negative feedback loop that regulates the size of the groups formed by developing Dictyostelium cells. Two components of CF are countin and CF50. Both recombinant countin and recombinant CF50 decrease group size in Dictyostelium. countin- cells have a decreased cAMP-stimulated cAMP pulse, whereas recombinant countin potentiates the cAMP pulse. We find that CF50 cells have an increased cAMP pulse, whereas recombinant CF50 decreases the cAMP pulse, suggesting that countin and CF50 have opposite effects on cAMP signal transduction. In addition, countin and CF50 have opposite effects on cAMP-stimulated Erk2 activation. However, like recombinant countin, recombinant CF50 increases cell motility. We previously found that cells bind recombinant countin with a Hill coefficient of approximately 2, a KH of 60 pm, and approximately 53 sites/cell. We find here that cells also bind 125I-recombinant CF50, with a Hill coefficient of approximately 2, a KH of approximately 15 ng/ml (490 pm), and approximately 56 sites/cell. Countin and CF50 require each other's presence to affect group size, but the presence of countin is not necessary for CF50 to bind to cells, and CF50 is not necessary for countin to bind to cells. Our working hypothesis is that a signal transduction pathway activated by countin binding to cells modulates a signal transduction pathway activated by CF50 binding to cells and vice versa and that these two pathways can be distinguished by their effects on cAMP signal transduction.

Disruption of aldehyde reductase increases group size in dictyostelium

Developing Dictyostelium cells form structures containing approximately 20,000 cells. The size regulation mechanism involves a secreted counting factor (CF) repressing cytosolic glucose levels. Glucose or a glucose metabolite affects cell-cell adhesion and motility; these in turn affect whether a group stays together, loses cells, or even breaks up. NADPH-coupled aldehyde reductase reduces a wide variety of aldehydes to the corresponding alcohols, including converting glucose to sorbitol. The levels of this enzyme previously appeared to be regulated by CF. We find that disrupting alrA, the gene encoding aldehyde reductase, results in the loss of alrA mRNA and AlrA protein and a decrease in the ability of cell lysates to reduce both glyceraldehyde and glucose in an NADPH-coupled reaction. Counterintuitively, alrA- cells grow normally and have decreased glucose levels compared with parental cells. The alrA- cells form long unbroken streams and huge groups. Expression of AlrA in alrA- cells causes cells to form normal fruiting bodies, indicating that AlrA affects group size. alrA- cells have normal adhesion but a reduced motility, and computer simulations suggest that this could indeed result in the formation of large groups. alrA- cells secrete low levels of countin and CF50, two components of CF, and this could partially account for why alrA- cells form large groups. alrA- cells are responsive to CF and are partially responsive to recombinant countin and CF50, suggesting that disrupting alrA inhibits but does not completely block the CF signal transduction pathway. Gas chromatography/mass spectroscopy indicates that the concentrations of several metabolites are altered in alrA- cells, suggesting that the Dictyostelium aldehyde reductase affects several metabolic pathways in addition to converting glucose to sorbitol. Together, our data suggest that disrupting alrA affects CF secretion, causes many effects on cellular metabolism, and has a major effect on group size.

CF45-1, a secreted protein which participates in Dictyostelium group size regulation

Developing Dictyostelium cells aggregate to form fruiting bodies containing typically 2 x 10(4) cells. To prevent the formation of an excessively large fruiting body, streams of aggregating cells break up into groups if there are too many cells. The breakup is regulated by a secreted complex of polypeptides called counting factor (CF). Countin and CF50 are two of the components of CF. Disrupting the expression of either of these proteins results in cells secreting very little detectable CF activity, and as a result, aggregation streams remain intact and form large fruiting bodies, which invariably collapse. We find that disrupting the gene encoding a third protein present in crude CF, CF45-1, also results in the formation of large groups when cells are grown with bacteria on agar plates and then starve. However, unlike countin(-) and cf50(-) cells, cf45-1(-) cells sometimes form smaller groups than wild-type cells when the cells are starved on filter pads. The predicted amino acid sequence of CF45-1 has some similarity to that of lysozyme, but recombinant CF45-1 has no detectable lysozyme activity. In the exudates from starved cells, CF45-1 is present in a approximately 450-kDa fraction that also contains countin and CF50, suggesting that it is part of a complex. Recombinant CF45-1 decreases group size in colonies of cf45-1(-) cells with a 50% effective concentration (EC(50)) of approximately 8 ng/ml and in colonies of wild-type and cf50(-) cells with an EC(50) of approximately 40 ng/ml. Like countin(-) and cf50(-) cells, cf45-1(-) cells have high levels of cytosolic glucose, high cell-cell adhesion, and low cell motility. Together, the data suggest that CF45-1 participates in group size regulation in Dictyostelium.

IfkA, a presumptive eIF2 alpha kinase of Dictyostelium, is required for proper timing of aggregation and regulation of mound size

BACKGROUND

The transition from growth to development in Dictyostelium is initiated by amino acid starvation of growing amobae. In other eukaryotes, a key sensor of amino acid starvation and mediator of the resulting physiological responses is the GCN2 protein, an eIF2alpha kinase. GCN2 downregulates the initiation of translation of bulk mRNA and enhances translation of specific mRNAs by phosphorylating the translation initiation factor eIF2alpha. Two eIF2alpha kinases were identified in Dictyostelium and studied herein.

RESULTS

Neither of the eIF2alpha kinases appeared to be involved in sensing amino acid starvation to initiate development. However, one of the kinases, IfkA, was shown to phosphorylate eIF2alpha from 1 to 7 hours after the onset of development, resulting in a shift from polysomes to free ribosomes for bulk mRNA. In the absence of the eIF2alpha phosphorylation, ifkA null cells aggregated earlier than normal and formed mounds and ultimately fruiting bodies that were larger than normal. The early aggregation phenotype in ifkA null cells reflected an apparent, earlier than normal establishment of the cAMP pulsing system. The large mound phenotype resulted from a reduced extracellular level of Countin, a component of the counting factor that regulates mound size. In wild type cells, phosphorylation of eIF2alpha by IfkA resulted in a specific stabilization and enhanced translational efficiency of countin mRNA even though reduced translation resulted for bulk mRNA.

CONCLUSIONS

IfkA is an eIF2alpha kinase of Dictyostelium that normally phosphorylates eIF2alpha from 1 to 7 hours after the onset of development, or during the preaggregation phase. This results in an overall reduction in the initiation of protein synthesis during this time frame and a concomitant reduction in the number of ribosomes associated with most mRNAs. For some mRNAs, however, initiation of protein synthesis is enhanced or stabilized under the conditions of increased eIF2alpha phosphorylation. This includes countin mRNA.

A secreted cell number counting factor represses intracellular glucose levels to regulate group size in dictyostelium

Developing Dictyostelium cells form evenly sized groups of approximately 2 x 10(4) cells. A secreted 450-kDa protein complex called counting factor (CF) regulates group size by repressing cell-cell adhesion and myosin polymerization and by increasing cAMP-stimulated cAMP production, actin polymerization, and cell motility. We find that CF regulates group size in part by repressing internal glucose levels. Transformants lacking bioactive CF and wild-type cells with extracellular CF depleted by antibodies have high glucose levels, whereas transformants oversecreting CF have low glucose levels. A component of CF, countin, affects group size in a manner similar to CF, and a 1-min exposure of cells to countin decreases glucose levels. Adding 1 mm exogenous glucose negates the effect of high levels of extracellular CF on group size and mimics the effect of depleting CF on glucose levels, cell-cell adhesion, cAMP pulse size, actin polymerization, myosin assembly, and motility. These results suggest that glucose is a downstream component in part of the CF signaling pathway and may be relevant to the observed role of the insulin pathway in tissue size regulation in higher eukaryotes.

Cells respond to and bind countin, a component of a multisubunit cell number counting factor

In Dictyostelium discoideum counting factor (CF), a secreted approximately 450-kDa complex of polypeptides, inhibits group and fruiting body size. When the gene encoding countin (a component of CF) was disrupted, cells formed large groups. We find that recombinant countin causes developing cells to form small groups, with an EC(50) of approximately 3 ng/ml, and affects cAMP signal transduction in the same manner as semipurified CF. Recombinant countin increases cell motility, decreases cell-cell adhesion, and regulates gene expression in a manner similar to the effect of CF. However, countin does not decrease adhesion or group size to the extent that semipurified CF does. A 1-min exposure of developing cells to countin causes an increase in F-actin polymerization and myosin phosphorylation and a decrease in myosin polymerization, suggesting that countin activates a rapid signal transduction pathway. (125)I-Labeled countin has countin bioactivity, and binding experiments suggest that vegetative and developing cells have approximately 53 cell-surface sites that bind countin with a K(D) of approximately 1.5 ng/ml or 60 pm. We hypothesize that countin regulates cell development through the same pathway as CF and that other proteins within the complex may modify the activity of countin and/or have independent size-regulating activities.

The different components of a multisubunit cell number-counting factor have both unique and overlapping functions

Dictyostelium aggregation streams break up into groups of 103 to 2×104 cells. The cells sense the number of cells in a stream or group by the level of a secreted counting factor (CF). CF is a complex of at least 5 polypeptides. When the gene encoding countin (one of the CF polypeptides) was disrupted, the cells could not sense each other’s presence, resulting in non-breaking streams that coalesced into abnormally large groups. To understand the function of the components of CF, we have isolated cDNA sequences encoding a second component of CF, CF50. CF50 is 30% identical to lysozyme (but has very little lysozyme activity) and contains distinctive serine-glycine motifs. Transformants with a disrupted cf50 gene, like countin– cells, form abnormally large groups. Addition of recombinant CF50 protein to developing cf50– cells rescues their phenotype by decreasing group size. Abnormalities seen in aggregating countin– cells (such as high cell-cell adhesion and low motility) are also observed in the cf50– cells. Western blot analysis of conditioned medium sieve column fractions showed that the CF50 protein is present in the same fraction as the 450 kDa CF complex. In the absence of CF50, secreted countin is degraded, suggesting that one function of CF50 may be to protect countin from degradation. However, unlike countin– cells, cf50– cells differentiate into an abnormally high percentage of cells expressing SP70 (a marker expressed in a subset of prespore cells), and this difference can be rescued by exposing cells to recombinant CF50. These observations indicate that unlike other known multisubunit factors, CF contains subunits with both overlapping and unique properties.