Efficient Induction of Sporulation of Dictyostelium Prespore Cells by 8-Bromocyclic AMP under Both Submerged- and Shaken-Culture Conditions and Involvement of Protein Kinase(s) in Its Action

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.

Formation of the outer layer of the Dictyostelium spore coat depends on the inner-layer protein SP85/PsB

The Dictyostelium spore is surrounded by a 220 microm thick trilaminar coat that consists of inner and outer electron-dense layers surrounding a central region of cellulose microfibrils. In previous studies, a mutant strain (TL56) lacking three proteins associated with the outer layer exhibited increased permeability to macromolecular tracers, suggesting that this layer contributes to the coat permeability barrier. Electron microscopy now shows that the outer layer is incomplete in the coats of this mutant and consists of a residual regular array of punctate electron densities. The outer layer is also incomplete in a mutant lacking a cellulose-binding protein associated with the inner layer, and these coats are deficient in an outer-layer protein and another coat protein. To examine the mechanism by which this inner-layer protein, SP85, contributes to outer-layer formation, various domain fragments were overexpressed in forming spores. Most of these exert dominant negative effects similar to the deletion of outer-layer proteins, but one construct, consisting of a fusion of the N-terminal and Cys-rich C1 domain, induces a dense mat of novel filaments at the surface of the outer layer. Biochemical studies show that the C1 domain binds cellulose, and a combination of site-directed mutations that inhibits its cellulose-binding activity suppresses outer-layer filament induction. The results suggest that, in addition to a previously described early role in regulating cellulose synthesis, SP85 subsequently contributes a cross-bridging function between cellulose and other coat proteins to organize previously unrecognized structural elements in the outer layer of the coat.

Regulated expression of the MADS-box transcription factor SrfA mediates activation of gene expression by protein kinase A during Dictyostelium sporulation

Cell differentiation and morphogenesis are tightly regulated during sporulation in the lower eukaryote Dictyostelium discoideum. The control of the cyclic adenosine monophosphate (cAMP)-dependent protein kinase (PKA) is essential to coordinate these processes. Several signal transduction pathways are being recognized that lead to the regulation of intracellular cAMP levels. However, very little is known about the events lying downstream of PKA that are essential to activate late gene expression and terminal differentiation of the spores. We have studied the relationship between PKA and the MADS-box transcription factor SrfA, essential for spore differentiation. Constitutive activation of PKA was not able to rescue sporulation in a strain that lacks srfA suggesting the possibility that srfA functions downstream of PKA in a signal transduction pathway leading to spore maturation. A distal promoter region regulates the induction of srfA expression in the prespore region during culmination. We found that this promoter can be induced precociously by activating PKA with 8-Br-cAMP suggesting a transcriptional regulation by PKA. Moreover, precocious sporulation and expression of the spore marker spiA in a strain that overexpresses PKA, correlates with a precocious induction of srfA expression. The temporal and spatial pattern of expression was also studied in a mutant strain lacking the main adenylyl cyclase that functions during culmination, ACR. This strain is expected to have lower PKA activity and consistently, the level of srfA expression was reduced. Moreover, the temporal induction of srfA in the prespore region was also delayed during culmination. Our results strongly suggest that PKA activation during culmination leads to the induction of the expression of srfA. The correct temporal and spatial pattern of srfA expression appears to be part of a mechanism that ensures the adequate coordination of gene expression and morphogenesis.

Outside-in signaling of cellulose synthesis by a spore coat protein in Dictyostelium

The spore coat of Dictyostelium is formed de novo from proteins secreted from vesicles and cellulose synthesized across the plasma membrane as differentiating spores rise up the stalk. The mechanism by which these events are coordinated is not understood. In the course of experiments designed to test the function of the inner layer coat protein SP85 (PsB), expression of a specific partial length fragment was found to interrupt coat assembly after protein secretion and prior to cellulose synthesis in 85% of the cells. This fragment consisted of SP85's N-terminal domain, containing prespore vesicle targeting information, and its Cys-rich C1 domain. The effect of the NC1 fusion was not cell autonomous in interstrain chimeras, suggesting that it acted at the cell surface. SP85-null spores presented an opposite phenotype in which spores differentiated prematurely before reaching the top of the stalk, and cellulose was slightly overproduced in a disorganized fashion. A similar though less severe phenotype occurred when a fusion of the N and C2 domains was expressed. In a double mutant, absence of SP85 was epistatic to NC1 expression, suggesting that NC1 inhibited SP85 function. Together, these results suggest the existence of an outside-in signaling pathway that constitutes a checkpoint to ensure that cellulose synthesis does not occur until coat proteins are properly organized at the cell surface and stalk formation is complete. Checkpoint execution is proposed to be regulated by SP85, which is in turn under the influence of other coat proteins that interact with SP85 via its C1 and C2 domains.

Spore coat formation and timely sporulation depend on cellulose in Dictyostelium

Cellulose is a major component of the extracellular coat that surrounds the terminally-differentiated spore of Dictyostelium. It is sandwiched between two layers of proteins that derive from prespore vesicles by exocytosis. Strains unable to synthesize cellulose due to null mutations in the gene encoding the catalytic subunit of cellulose synthase (dcsA) failed to make detergent-resistant spores but produced small, highly refractile, round spore-like cells up to a day late. Although these cells resembled spores in appearance, they were unstable, only transiently ellipsoid in shape, and sensitive to hypo-osmotic shock, drying, or detergents. Differentiation of these pseudo-spores was induced in the normal time frame by activation of the cAMP-dependent protein kinase or co-development with wild type cells, and coat proteins were secreted by the dcsA-null cells at the same time as wild type cells. A substantial fraction of secreted coat proteins was loosely associated with the surface of the mutant cells, resembling the precoat posited to form early during normal sporulation. Transmission electron microscopy revealed that the precoat had little ultrastructural organization in the absence of cellulose. Thus, cellulose in the coat appears to be required for the organization of the pre-coat precursors as well as the stability, dormancy, and shape of the spore.

Role of PKA in the timing of developmental events in Dictyostelium cells

The cyclic AMP (cAMP)-dependent protein kinase, PKA, is dispensable for growth of Dictyostelium cells but plays a variety of crucial roles in development. The catalytic subunit of PKA is inhibited when associated with its regulatory subunit but is activated when cAMP binds to the regulatory subunit. Deletion of pkaR or overexpression of the gene encoding the catalytic subunit, pkaC, results in constitutive activity. Development is independent of cAMP in strains carrying these genetic alterations and proceeds rapidly to the formation of both spores and stalk cells. However, morphogenesis is aberrant in these mutants. In the wild type, PKA activity functions in a circuit that can spontaneously generate pulses of cAMP necessary for long-range aggregation. It is also essential for transcriptional activation of both prespore and prestalk genes during the slug stage. During culmination, PKA functions in both prespore and prestalk cells to regulate the relative timing of terminal differentiation. A positive feedback loop results in the rapid release of a signal peptide, SDF-2, when prestalk cells are exposed to low levels of SDF-2. The signal transduction pathway that mediates the response to SDF-2 in both prestalk and prespore cells involves the two-component system of DhkA and RegA. When the cAMP phosphodiesterase RegA is inhibited, cAMP accumulates and activates PKA, leading to vacuolation of stalk cells and encapsulation of spores. These studies indicate that multiple inputs regulate PKA activity to control the relative timing of differentiations in Dictyostelium.

Zinc ions promote prestalk-to-stalk and prespore-to-stalk conversions in Dictyostelium discoideum

To clarify the mechanism of stalk cell differentiation in Dictyostelium discoideum (strain NC4), we have examined the effects of Zn2+ on in vitro cell differentiation of prestalk and prespore cells isolated from normally formed slugs. Prestalk cells did not differentiate into stalk cells under submerged conditions, but in the presence of the stalk-inducing factor-1 (DIF-1) at 100 nM or Zn2+ at 5 mM, a small number of the cells (< 15%) differentiated into stalk cells. Interestingly, Zn2+ in combination with DIF-1 induced the prestalk-to-stalk conversion at high efficiencies (approx. 60%). Furthermore, isolated prespore cells were also converted to stalk cells at high efficiencies (approx. 50%) in the presence of both DIF-1 and Zn2+, while the conversion poorly occurred in the absence of Zn2+. These results indicate that Zn2+ may mimic some cellular interaction(s) which are required for stalk cell formation in this strain.

Morphogenesis in Dictyostelium: new twists to a not-so-old tale

Prespore differentiation requires both cAMP-dependent protein kinase and the transcription factor GBF, and for one class of prespore genes the two form part of a single pathway. It seems that differentiation-inducing factor, the inducer of prestalk cell differentiation, may operate via a calcium signalling pathway, and terminal stalk cell differentiation is in part regulated by glycogen synthase kinase 3.

Progression of an inductive signal activates sporulation in Dictyostelium discoideum

spiA, a marker for sporulation, is expressed during the culmination stage of Dictyostelium development, when the mass of prespore cells has moved partly up the newly formed stalk. Strains containing a full-length spiA promoter/lacZ fusion were stained for beta-galactosidase activity at intervals during development. The results indicate that expression of spiA initiates in prespore cells at the prestalk/prespore boundary (near the apex) and extends downward into the prespore mass as culmination continues. A spatial gradient of staining expands from the top of the prespore mass and intensifies until the front of activation reaches the bottom, whereupon the entire region stains darkly. The spiA promoter can be deleted to within 301 bp of the transcriptional start site with no effect on the relative strength, timing or spatial localization of expression. Further 5′ deletions from −301 to −175 reduce promoter strength incrementally, although timing and spatial expression are not affected. Deletions to −159 and beyond result in inactive promoters. Treatment of early developmental structures with 8-Br-cAMP in situ activates the intracellular cAMP-dependent protein kinase (PKA) and precociously induces spiA expression and sporulation. The absence of an apparent gradient of staining in these structures suggest that PKA is equivalently activatable throughout the prespore region and that all prespore cells are competent to express spiA. Thus, we postulate that the pattern of expression of spiA reveals the progression of an inductive signal for sporulation and suggest that this signal may originate from the prestalk cells at the apex.

Analysis of a novel cyclic Amp inducible prespore gene in Dictyostelium discoideum: evidence for different patterns of cAMP regulation

The D7 cDNA clone hybridizes to a 2.8 kb mRNA which first appears at the mound stage of development in the cellular slime mold Dictyostelium discoideum. This gene which is cyclic AMP (cAMP) inducible and is expressed specifically in the prespore cells contains an open reading frame interrupted by only one intron. The predicted amino acid sequence indicates a novel prespore protein which differs from all of the previously described prespore proteins in that it contains no internal repeats and does not share any homology with any of the other prespore genes. The amino acid sequence predicts a protein of 850 amino acids with a molecular weight of 95,343 daltons and an isoelectric point of 4.25. The protein is very rich in glutamine (13.8%), asparagine (10.6%) and glutamic acid (10.4%) with one potential glycosylation site and 28 possible sites for phosphorylation. The amino terminus is hydrophobic with characteristics of a signal sequence while the entire carboxyl half of the protein is notable for its hydrophilicity. Comparison of cAMP regulation of the D7 gene with the regulation of two other cAMP regulated prespore genes, the PL3(SP87) gene and the Psa(D19), reveals some striking differences. Disaggregation in the presence of cAMP results in transient degradation of mRNA for all three genes. The transcription rate for the D7 and PsA(D19) genes remains relatively unaffected by disaggregation but there is a rapid although transient decline in the transcription rate of the PL3(SP87) gene. Although the accumulation of all three mRNAs is first detectable at mound stage, transcription of the D7 and PsA(D19) genes is detected earlier in development, at rippling aggregate stage several hours prior to the earliest time when transcription of the PL3(SP87) gene is detected. Analysis of the promoter region of the D7 gene reveals three CA like boxes flanked by direct repeats as well as four G rich regions that may serve as regulatory elements.

Transient adenylate cyclase activation accompanies differentiation of Trypanosoma brucei from bloodstream to procyclic forms

Pleomorphic bloodstream forms of Trypanosoma brucei differentiate synchronously into procyclic forms when cultivated at 27 degrees C in the presence of citrate/cis-aconitate. The activity of adenylate cyclase was monitored during this process. Two phases of transient stimulation were observed. The first phase occurred 6-10 h after the triggering of differentiation, a period which immediately follows the release of the bulk of the VSG and immediately precedes both the first cell division and the loss of the bloodstream-specific ESAG 4 transmembrane adenylate cyclase. The second phase occurred between 20 and 40 h, when the cells that emerged from the first division began to proliferate. These observations suggest that cAMP may be involved in differentiation/proliferation of the parasite.