Isolation and characterization of mutations affecting UDPG pyrophosphorylase activity in Dictyostelium discoideum

A Dictyostelium cellobiohydrolase orthologue that affects developmental timing

Dictyostelium discoideum is a facultative multicellular amoebozoan with cellulose in the stalk and spore coat of its fruiting body as well as in the extracellular matrix of the migrating slug. The organism also harbors a number of cellulase genes. One of them, cbhA, was identified as a candidate cellobiohydrolase gene based on the strong homology of its predicted protein product to fungal cellobiohydrolase I (CBHI). Expression of the cbhA was developmentally regulated, with strong expression in the spores of the mature fruiting body. However, a weak but detectable level of expression was observed in the extracellular matrix at the mound - tipped finger stages, in prestalk O cells, and in the slime sheath of the migrating slug - late culminant stages. A null mutant of the cbhA showed almost normal morphology. However, the developmental timing of the mutant was delayed by 2-4 h. When a c-Myc epitope-tagged CbhA was expressed, it was secreted into the culture medium and was able to bind crystalline cellulose. The CbhA-myc protein was glycosylated, as demonstrated by its ability to bind succinyl concanavalin A-agarose. Moreover, conditioned medium from the cbhA-myc (oe) strain displayed 4-methylumbelliferyl β-D-cellobioside (4-MUC) digesting activity in Zymograms in which conditioned medium was examined via native-polyacrylamide gel electrophoresis or spotted on an agar plate containing 4-MUC, one of the substrates of cellobiohydrolase. Taken together, these findings indicate that Dictyostelium CbhA is an orthologue of CBH I that is required for a normal rate of development.

Rice UDP-glucose pyrophosphorylase1 is essential for pollen callose deposition and its cosuppression results in a new type of thermosensitive genic male sterility

UDP-glucose pyrophosphorylase (UGPase) catalyzes the reversible production of glucose-1-phosphate and UTP to UDP-glucose and pyrophosphate. The rice (Oryza sativa) genome contains two homologous UGPase genes, Ugp1 and Ugp2. We report a functional characterization of rice Ugp1, which is expressed throughout the plant, with highest expression in florets, especially in pollen during anther development. Ugp1 silencing by RNA interference or cosuppression results in male sterility. Expressing a double-stranded RNA interference construct in Ugp1-RI plants resulted in complete suppression of both Ugp1 and Ugp2, together with various pleiotropic developmental abnormalities, suggesting that UGPase plays critical roles in plant growth and development. More importantly, Ugp1-cosuppressing plants contained unprocessed intron-containing primary transcripts derived from transcription of the overexpression construct. These aberrant transcripts undergo temperature-sensitive splicing in florets, leading to a novel thermosensitive genic male sterility. Pollen mother cells (PMCs) of Ugp1-silenced plants appeared normal before meiosis, but during meiosis, normal callose deposition was disrupted. Consequently, the PMCs began to degenerate at the early meiosis stage, eventually resulting in complete pollen collapse. In addition, the degeneration of the tapetum and middle layer was inhibited. These results demonstrate that rice Ugp1 is required for callose deposition during PMC meiosis and bridges the apoplastic unloading pathway and pollen development.

A second UDP-glucose pyrophosphorylase is required for differentiation and development in Dictyostelium discoideum

Uridine diphosphoglucose pyrophosphorylase (UDPGP) is a developmentally regulated enzyme in Dictyostelium discoideum, which is involved in trehalose, cellulose, and glycogen synthesis. Two independent UDPGP proteins are believed to be responsible for this activity. To determine the relative contributions of each protein, the genes encoding them were disrupted individually. Cells lacking the udpgp1 gene exhibit normal growth and development and make normal levels of cellulose. In agreement with these phenotypes, udpgp1(-) cells still have UDPGP activity, although at a reduced level. This supports the importance of the second UDPGP gene. This newly identified gene, ugpB, encodes an active UDPGP as determined by complementation in Escherichia coli. When this gene is disrupted, cells undergo aberrant differentiation and development ending with small, gnarled fruiting bodies. These cells also have decreased spore viability and decreased levels of glycogen, whose production requires UDPGP activity. These phenotypes suggest that UgpB constitutes the major UDPGP activity produced during development. Sequence analysis of the two UDPGP genes shows that UgpB has higher homology to other eukaryotic UDPGPs than does UDPGP1. This includes the presence of 5 conserved lysine residues. Udpgp1 only has 1 of these lysines.

UDP-glucose pyrophosphorylase: up-regulation in hypertrophic cartilage and role in hyaluronan synthesis

Previously, we have performed subtractive hybridization to identify genes up-regulated in hypertrophic chondrocytes of the avian epiphyseal growth plate. In the present study, we report the identification of one of the clones as UDP-glucose pyrophosphorylase (UDPG-PPase) and propose a possible function for this enzyme in regulating hyaluronan (HA) synthesis in hypertrophic cartilage. We have cloned the 2.6 kb full-length cDNA for avian UDPG-PPase and confirmed its up-regulation in hypertrophic versus non-hypertrophic cartilage by Northern-blot analysis. The 6-fold increase in mRNA was paralleled by an equivalent increase in enzymic activity. The enzyme catalyses the conversion of glucose 1-phosphate into UDP-glucose, which is used to synthesize a number of cellular components, including HA. Overexpression of enzymically active UDPG-PPase in non-hypertrophic chondrocytes resulted in a 2-3-fold increase in total HA, as determined by a competitive binding assay and immunohistochemistry. In the developing growth plate, HA synthesis was elevated in the hypertrophic zone along with the up-regulation of the HA synthase (HAS)-2 gene. Our data suggest that an increase in both activities, UDPG-PPase and HAS-2, is required for non-hypertrophic chondrocytes to synthesize an amount of HA comparable with that in hypertrophic chondrocytes. Therefore we conclude that HA synthesis during chondrocyte differentiation is regulated at the level of the substrate-provider gene, UDPG-PPase, as well as the HAS genes.

The extracellular matrix of the Dictyostelium discoideum slug

In this review, we detail the current understanding of the extracellular matrix (ECM) of the migratory slug phase of the cellular slime mould, Dictyostelium discoideum. We describe some structural and non-structural molecules which comprise the ECM, and how these molecules reflect both plant and animal ECM systems. We also describe zones of the multicellular slug that are known to make ECM components, including the role of the prestalk cells and the slug epithelium-like layer. Finally, we review the contributions of studies on mutants to our understanding of the ECM of D. discoideum, and relate this to differentiation and development in more complex eukaryotic systems.

Disruption of glycogen phosphorylase gene expression in Dictyostelium: evidence for altered glycogen metabolism and developmental coregulation of the gene products

Glycogen phosphorylase 1 and 2, the isozymes responsible for glycogen degradation, are encoded by separate genes in Dictyostelium. The two gene products display different transcriptional and translational expression and distinct post-translational regulation. Using DNA-mediated transformation, Dictyostelium clones which lacked either glycogen phosphorylase 1 or 2 (gp1 or gp2) expression were obtained. The loss of either enzyme did not change axenic growth patterns, developmental progression, or gross organismic morphology. In gp1- strains, glycogen accumulated to a 17- to 28-fold higher level during late stationary phase without any obvious detrimental effects. This implies that no alternative pathway for glycogen degradation is present in amoebae, and that glycogen metabolism is not critical for vegetative cell growth. Developmental glycogen concentrations were not altered significantly in any of the transformants, but in gp2- cells the posttranslational regulation of the intact gp1 enzyme was apparently modulated to compensate for the loss of gp2. Western blots of microdissected, lyophilized Dictyostelium slugs and early culminates showed that gp2 was found in both prestalk and prespore cells, with a slight enrichment in prespore cells. The gp1 protein was highly enriched in prestalk cells in the parental strain. In gp2- transformants, however, gp1 was detected in equal amounts in both cell types. The loss of gp2 led to a shift in the cell-type-specific expression pattern of gp1, presumably due to developmental coordinate regulation of gp1 and gp2 at the translational and/or transcriptional level.

Construction of kinetic models to understand metabolism in vivo

This review describes increasingly complex kinetic models that simulate carbohydrate metabolism in a simple eucaryotic system which undergoes differentiation. Dynamic models of complex metabolic networks serve to organize and analyze the many interdependent variables involves and to define the rate-limiting events controlling metabolism in vivo. Since the ultimate justification for and test of any model are its predictive values, a series of predictions and related experiments will be described.

Signal transduction pathways involved in the expression of the uridine diphosphoglucose pyrophosphorylase gene of Dictyostelium discoideum

The uridine diphosphoglucose pyrophosphorylase (UDPGP1) gene of Dictyostelium discoideum is an excellent marker to study the pathways that control the expression of genes during development. We have previously shown that the UDPGP1 gene is regulated by exogenous cAMP acting on cell-surface cAMP receptors. Various steps in the signal transduction pathway between receptor stimulation and the induction of the gene can now be studied. Induction does not require the synthesis of intracellular cAMP, but does require new protein synthesis. By deletion and transformation with altered genes, two cis-acting sequences that are required for UDPGP1 expression have been identified. A GC-rich palindromic sequence located between -410 and -374 is essential for induction of the gene by extracellular cAMP, but not for its basal expression. A sequence element located between -374 and -337 is required for any basal expression of this gene. When the polarity of the palindromic sequence was reversed such that it resembled the H2K enhancer element, the gene could still be induced by exogenous cAMP. Two DNA binding activities were detected in gel mobility shift assays using a fragment containing both of the regulatory sequence elements of UDPGP1 gene. Transformation with a vector that resulted in the synthesis of anti-sense UDPGP1 RNA led to almost total elimination of the enzyme antigen and no detectable enzyme activity. However, these transformants developed normally, indicating that either UDPGP is not required for development or residual synthesis of UDPGP may be sufficient for normal development.

Genetics of early Dictyostelium discoideum development

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Structure and sequence of a UDP glucose pyrophosphorylase gene of Dictyostelium discoideum

Cell-cell contact and exogenous cAMP regulate the expression of uridine diphosphoglucose pyrophosphorylase (UDPGP) of Dictyostelium discoideum (B. Haribabu, A. Rajkovic and R. P. Dottin, 1986, Dev. Biol., Vol. 113, 436-442). cAMP appears to regulate gene expression in Dictyostelium by transmembrane signal transduction (B. Haribabu and R. Dottin, 1986, Mol. Cell. Biol. 6, 2402-2408). To further characterize the mechanism of action of cAMP on the expression of this gene and the nature of the defects in UDPGP mutants that abort development, we sequenced the cDNA and the genomic DNA, including intervening and flanking sequences. The deduced amino acid sequence predicts a polypeptide of 57,893 d. molecular weight. Three short (100-200 nucleotides) A+T rich introns occur within the coding sequences but only one of them contains a sequence TAACTAAC, similar to the yeast lariat acceptor site. The 5' flanking sequences are also A+T rich and contain an oligo A tract (-14 to -24), a TATA box (-25 to -32), and a short G+C rich region (-63 to -101) which may be a control region. From -196 to -209 is a sequence AAAGTAGTATTCAA which matches in 11 of its 14 nucleotides, a sequence found upstream from the hormonally regulated P-enolypyruvate carboxykinase gene of rat.

Cell-cell contact and cAMP regulate the expression of a UDP glucose pyrophosphorylase gene of Dictyostelium discoideum

UDP glucose pyrophosphorylase (UDPGP) (EC.2.7.7.9) is a developmentally regulated enzyme of Dictyostelium discoideum. Two polypeptides of UDPGP are translated from Dictyostelium mRNA. Recently we isolated a cDNA clone which encodes one of the UDPGP polypeptides (B. R. Fishel, J. A. Ragheb, A. Rajkovic, B. Haribabu, C. W. Schweinfest, and R. P. Dottin (1985). Dev. Biol. 110, 369-381). By hybridization with the cDNA and by in vitro translation and immunoprecipitation, we examined the effect of cell-cell contact and cAMP on the regulation of UDPGP expression. Disaggregation of slugs resulted in a rapid loss of UDPGP mRNA. Addition of cAMP to these cells resulted in increased levels of UDPGP mRNA, though not to the same extent as seen during normal development. The two UDPGP polypeptides observed in vitro are coordinately regulated. Unaggregated cells, starved and shaken rapidly in suspension, did not show UDPGP mRNA accumulation. However, addition of cAMP to these cells caused UDPGP induction, suggesting that the requirement for cell-cell contact could be bypassed in part by cAMP addition.

Molecular cloning of a cDNA complementary to a UDP-glucose pyrophosphorylase mRNA of dictyostelium discoideum

Uridine diphosphoglucose pyrophosphorylase (UTP: -alpha-D-glucose-1-phosphate uridyltransferase, EC 2.7.7.9) is an essential enzyme for normal development of Dictyostelium discoideum and its specific activity increases 3- to 10-fold by the later stages of development. Previous experiments have shown that additional forms of the enzyme appear concomitantly with this increase and that two uridine diphosphoglucose pyrophosphorylase (UDPGP) polypeptides are immunoprecipitated from the in vitro translation products of total cellular RNA at any stage of development (B. F. Fishel, R. E. Manrow and R. P. Dottin, 1982, Dev. Biol. 92, 175-187). Using an in vitro translation-immunoprecipitation assay of UDPGP mRNA, we show that an increase in the amount of translatable mRNA is correlated with the accumulation of enzyme during development. A cDNA bank was constructed from a mRNA population that had been enriched for UDPGP mRNA by size fractionation on sucrose gradients containing methylmercuric hydroxide (C. W. Schweinfest, R. W. Kwiatkowski, and R. P. Dottin, 1982, Proc. Natl. Acad. Sci. USA 79, 4997-5000). A 1.8-Kb cDNA complementary to a UDPGP mRNA was identified after screening the bank by hybridization selection and translation. Only the mRNA encoding the higher molecular weight in vitro translation product is hybrid selected by this cDNA. In hybrid-arrested translation experiments, the coding strand of this cDNA selectively inhibits the translation of only one of the two in vitro translation products. Therefore, there are two distinct UDPGP mRNAs.

Protein synthesis during development and differentiation in the cellular slime mould Dictyostelium discoideum

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Accumulation of alpha-mannosidase-1 in Dictyostelium discoideum requires many developmentally essential genes

alpha-Mannosidase-1, one of the earliest known developmentally controlled gene products in the cellular slime mold Dictyostelium discoideum, accumulates intracellularly during both axenic growth and development. The accumulation of alpha-mannosidase-1 activity prematurely ceases in all of 125 randomly isolated aggregation-deficient mutants at discrete times in development resulting in significantly reduced levels of cellular enzyme activity. This suggests that, unlike other developmentally controlled enzymes in this organism, the continued accumulation of alpha-mannosidase-1 activity is controlled by a large number of genes essential for early development. alpha-Mannosidase-1 misregulation and the aggregation-deficient phenotype are caused by the same mutation since (1) morphological revertants exhibit a coreversion to both fruiting ability and wild-type alpha-mannosidase-1 accumulation and (2) normal enzyme accumulation depends on the ability to aggregate and ultimately fruit in a conditional aggregation-deficient mutant. This type of regulation does not appear to be due to differences in enzyme secretion or changes in the overall rate of total protein synthesis. Aggregation-deficient mutants continue to synthesize protein beyond the time in development at which alpha-mannosidase-1 accumulation ceases. Our studies indicate that most of the 50-125 genes required for aggregation in Dictyostelium are also required for the normal accumulation of alpha-mannosidase-1 activity.

Renaturation and localization of enzymes in polyacrylamide gels: studies with UDPglucose pyrophosphorylase of Dictyostelium

We have developed procedures for renaturing, in polyacrylamide isoelectric focusing gels, homomeric enzymes (i.e., enzymes with identical subunits) that have been denatured with sodium dodecyl sulfate or urea or both. The renatured enzymes can then be localized as discrete species by conventional histochemical staining. One of these enzymes, uridine diphosphoglucose pyrophosphorylase (UTP:alpha-D-glucose-1-phosphate uridylyltransferase, EC 2.7.7.9) of Dictyostelium discoideum, was studied in detail. Conditions have been established for renaturing and localizing this enzyme and for quantitating the amount of activity recovered. Up to 40% of the activity can be recovered after renaturation. This procedure is widely applicable because several enzymes, including alcohol dehydrogenase (EC 1.1.1.1) and lactate dehydrogenase (EC 1.1.1.27), can be localized. It is sensitive enough to resolve isozymes and enzyme variants that differ by a single charged amino acid. It can be used to localize enzymes in crude cell extracts that have been resolved in two-dimensional slab gels by sodium dodecyl sulfate electrophoresis and isoelectric focusing. These methods should allow detailed analysis of genes and their enzyme proteins that, though present in small amounts in eukaryotic cells, perform important metabolic or developmental functions.