Control of early gene expression in Dictyostelium

RNAi silenced Dd-grp94 (Dictyostelium discoideum glucose-regulated protein 94 kDa) cell lines in Dictyostelium exhibit marked reduction in growth rate and delay in development

Glucose-regulated 94 kDa protein (Grp94) is a resident of the endoplasmic reticulum (ER) of multicellular eukaryotes. It is a constitutively expressed protein that is overexpressed in certain abnormal conditions of the cell such as depletion of glucose and calcium, and low oxygen and pH. The protein is also implicated in diseased conditions like cancer and Alzheimer's disease. In this study, the consequences of downregulation of Grp94 were investigated at both unicellular and multicellular stages of Dictyostelium discoideum. Previous studies have shown the expression of Dd-Grp94 (Dictyostelium discoideum glucose-regulated 94 kDa protein) in wild-type cells varies during development, and overexpression of Dd-Grp94 leads to abnormal cell shape and inhibition of development (i.e., formation of fruiting bodies). Grp94 is a known calcium binding protein and an efficient calcium buffer. Therefore, in the present study we hypothesized that downregulation of Dd-Grp94 protein would affect Dictyostelium cell structure, growth, and development. We found that Dd-grp94 RNAi recombinants exhibited reduced growth rate, cell size, and a subtle change in cell motility compared to the parental cells. The recombinants also exhibited a delay in development and small fruiting bodies. These results establish that Dd-grp94 plays a crucial role in determining normal cell structure, growth and differentiation.

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.

Involvement of the glucose-regulated protein 94 (Dd-GRP94) in starvation response of Dictyostelium discoideum cells

Upon deprivation of nutrients, Dictyostelium discoideum Ax-2 cells arrest proliferation and initiate a metamorphosed developmental program including induction of altered gene expressions which are necessary for differentiation. In Ax-2 cells, we found out a member of Hsp90 family usually contained in the endoplasmic reticulum (ER), Dd-GRP94 (Dictyostelium discoideum glucose-regulated protein 94). In general, GRP94 are induced either by glucose-depletion or by depletion of Ca(2+) in intracellular Ca(2+) stores. Unexpectedly, however, the expression of Dd-grp94 was greatly reduced within 60 min of starvation. Dd-grp94-overexpressing cells (GRP94(OE) cells) collected without forming distinct aggregation streams, and never formed normal fruiting bodies. Also, prespore differentiation as well as maturation into spores and stalk cells were particularly impaired in the GRP94(OE) cells. Thus Dd-GRP94 seems to be crucial in late differentiation as well as in starvation response.

Protein kinase A is a positive regulator of spore coat gene transcription in Dictyostelium

The cotA, cotB, and cotC genes encode the major spore coat proteins of Dictyostelium. All three cot genes are coordinately expressed as aggregation is nearing completion. Induction and maintenance of their expression is dependent upon the presence of extracellular cAMP. We show that expression of a dominant inhibitor of the cAMP dependent protein kinase (PKA) in prespore cells greatly reduces the transcription rates of the cotB and cotC genes. All three cot genes contain, in their upstream regulatory regions, short sequence elements that have a high content of cytosine and adenosine residues. These CA-rich sequences are essential for optimal cot gene transcription. We show that expression of the dominant PKA inhibitor results in a greatly reduced level of the binding activity that recognizes the CA-rich sequences upstream of the cotB gene. Thus PKA acts, either directly or indirectly, to control expression of the cot genes and it may do so by modulating the activity of a DNA binding protein. However, we find that mutant cells where PKA is constitutively active still require exogenous cAMP for optimal cot gene expression in dissociated cells, suggesting that a separate, PKA-independent, signalling pathway is also involved in the regulation of cot gene expression by extracellular cAMP.

Multiple roles for cAMP-dependent protein kinase during Dictyostelium development

The cAMP-dependent protein kinase (PKA) holoenzyme of Dictyostelium comprises a single regulatory (R) and catalytic (C) subunit, and both proteins increase in concentration during cellular aggregation. In order to determine the role of the kinase, we have constructed mutants of the R subunit that are defective in cAMP binding, in inhibition of the C subunit, or in both functions. Analysis of these mutants suggests that overexpression of the unmutated R subunit, which is known to block development, occurs by direct inactivation of the C subunit rather than by an effect on intracellular cAMP levels. Cells with an inactive C subunit (PKA- cells) are defective in cAMP relay, the production of cAMP in response to extracellular cAMP stimulation. This presumably accounts for their inability to undertake aggregation. When mixed with wild-type cells, PKA- cells migrate toward the signalling centre but remain confined to the periphery of the tight aggregate and are lost from the back of the migratory slug. This suggests that PKA may be required during the late, multicellular stages of development. Consistent with this, we find that a number of postaggregative genes are not expressed in PKA- cells, even when they are allowed to synergise with normal cells.

Caffeine, an inhibitor of endocytosis in Dictyostelium discoideum amoebae

The effect of the trimethylxanthine, caffeine, was examined on the growth and endocytosis pathways of the vegetative amoebae of the cellular slime mold Dictyostelium discoideum. Caffeine at concentrations of 1.5-3 mM was found to inhibit axenic growth, fluid-phase pinocytosis, and secretion of lysosomal enzymes. Cell viability was unaffected by incubation for 16 hours with 5 mM caffeine but decreased markedly thereafter. Phagocytosis of the bacterium Escherichia coli by Dictyostelium amoebae was also inhibited by caffeine, although at concentrations twofold to threefold higher. Caffeine rapidly entered into amoebae to reach an equilibrium between extracellular and intracellular concentrations, and it was not appreciably metabolized by Dictyostelium. Inhibition of growth and endocytosis was reversible upon removal of the drug and was partially counteracted by 10 mM adenosine. As caffeine discharged intracellular calcium stores in Dictyostelium (Abe et al., 1988), its inhibitory effect on endocytosis could result from the perturbation of calcium homeostasis. In agreement with this hypothesis, the cation La3+ (10 microM), a Ca2(+)-transport inhibitor, also strongly reduced fluid-phase pinocytosis.

Membrane-enclosed crystals in Dictyostelium discoideum cells, consisting of developmentally regulated proteins with sequence similarities to known esterases

Developing cells of Dictyostelium discoideum contain crystalline inclusion bodies. The interlattice spaces of the crystals are approximately 11 nm, and their edge dimensions vary in aggregating cells from 0.1 to 0.5 micron. The crystals are enclosed by a membrane with the characteristics of RER. To unravel the nature of the crystals we isolated them under electron microscopical control and purified the two major proteins that cofractionate with the crystals, one of an apparent molecular mass of 69 kD, the other of 56 kD. This latter protein proved to be identical with the protein encoded by the developmentally regulated D2 gene of D. discoideum, as shown by its reactivity with antibodies raised against the bacterially expressed product of a D2 fusion gene. The D2 gene is known to be strictly regulated at the transcript level and to be controlled by cAMP signals. Accordingly, very little of the 56-kD protein was detected in growth phase cells, maximal expression was observed at the aggregation stage, and the expression was stimulated by cAMP pulses. The 69-kD protein is the major constituent of the crystals and is therefore called "crystal protein." This protein is developmentally regulated and accumulates in aggregating cells similar to the D2 protein, but is not, or is only slightly regulated by cAMP pulses. mAbs specific for either the crystal protein or the D2 protein, labeled the intracellular crystals as demonstrated by the use of immunoelectron microscopy. The complete cDNA-derived amino acid sequence of the crystal protein indicates a hydrophobic leader and shows a high degree of sequence similarity with Torpedo acetylcholinesterase and rat lysophospholipase. Because the D2 protein also shows sequence similarities with various esterases, the vesicles filled with crystals of these proteins are named esterosomes.

Transduction of the chemotactic signal to the actin cytoskeleton of Dictyostelium discoideum

Dictyostelium discoideum amebae chemotax toward folate during vegetative growth and toward extracellular cAMP during the aggregation phase that follows starvation. Stimulation of starving amebae with extracellular cAMP leads to both actin polymerization and pseudopod extension (Hall et al., 1988, J. Cell. Biochem. 37, 285-299). We have identified an actin nucleation activity (NA) from starving amebae that is regulated by cAMP receptors and controls actin polymerization (Hall et al., 1989, J. Cell Biol., in press). We show here that NA from vegetative cells is also regulated by chemotactic receptors for folate. Our studies indicate that NA is an essential effector in control of the actin cytoskeleton by chemotactic receptors. Guided by a recently proposed model for signal transduction from the cAMP receptor (Snaar-Jagalska et al., 1988, Dev. Genet. 9, 215-225), we investigated which of three signaling pathways activates the NA effector. Treatment of whole cells with a commercial pertussis toxin preparation (PT) inhibited cAMP-stimulated NA. However, endotoxin contamination of the PT appears to account for this effect. The synag7 mutation and caffeine treatment do not inhibit activation of NA by cAMP. Thus, neither activation of adenylate cyclase nor a G protein sensitive to PT treatment of whole cells is necessary for the NA response. Actin nucleation activity stimulated with folate is normal in vegetative fgdA cells. However, cAMP suppresses rather than activates NA in starving fgdA cells. This indicates that the components of the actin nucleation effector are present and that a pathway regulating the inhibitor(s) of nucleation remains functional in starving fgdA cells. The locus of the fgdA defect, a G protein implicated in phospholipase C activation, is directly or indirectly responsible for transduction of the stimulatory chemotactic signal from cAMP receptors to the nucleation effector in Dictyostelium.