V4, a gene required for the transition from growth to development in Dictyostelium discoideum

Cell-cycle checkpoint for transition from cell division to differentiation

In general, growth and differentiation are mutually exclusive, but they are cooperatively regulated during the course of development. Thus, the process of a cell's transition from growth to differentiation is of general importance for the development of organisms, and terminally differentiated cells such as nerve cells never divide. Meanwhile, the growth rate speeds up when cells turn malignant. The cellular slime mold Dictyostelium discoideum grows and multiplies as long as nutrients are supplied, and its differentiation is triggered by starvation. A critical checkpoint (growth/differentiation transition or GDT point), from which cells start differentiating in response to starvation, has been precisely specified in the cell cycle of D. discoideum Ax-2 cells. Accordingly, integration of GDT point-specific events with starvation-induced events is needed to understand the mechanism regulating GDTs. A variety of intercellular and intracellular signals are involved positively or negatively in the initiation of differentiation, making a series of cross-talks. As was expected from the presence of the GDT point, the cell's positioning in cell masses and subsequent cell-type choices occur depending on the cell's phase in the cell cycle at the onset of starvation. Since novel and multiple functions of mitochondria in various respects of development including the initiation of differentiation have been directly realized in Dictyostelium cells, they are also reviewed in this article.

An immediate-early gene, srsA: its involvement in the starvation response that initiates differentiation of Dictyostelium cells

When nutrients are depleted, Dictyostelium cells undergo cell cycle arrest and initiate a differentiation program for survival. We have found a novel gene, srsA, which is rapidly expressed in the first 5 min following the removal of nutrients and is turned off within an hour. This gene encodes a small protein with no significant similarity to previously characterized proteins. Disruption of srsA results in delayed expression of the early genes acaA and carA that encode adenylyl cyclase and the cAMP receptor necessary for chemotactic aggregation, respectively. Streaming is delayed several hours and the aggregates are larger than normal in the mutant strains. These phenotypes are cell-autonomous. Overexpression of srsA also results in delayed aggregation. Some of the slugs of the srsA(OE) strains showed stalked migration reminiscent of the slugs of the related species Dictyostelium mucoroides. The terminal structures formed by srsA(OE) cells were grossly abnormal and contained very few viable spores. When cells overexpressing srsA were developed together with an excess of wild-type cells, the fruiting bodies were still abnormal, indicating that the mutant cells have a dominant effect on late development. These findings suggest that srsA may be involved in both the starvation response and late differentiation.

Regulation of growth and differentiation in Dictyostelium

In general, growth and differentiation are mutually exclusive, but they are cooperatively regulated during the course of development. Thus, the process of a cell's transition from growth to differentiation is of general importance not only for the development of organisms but also for the initiation of malignant transformation, in which this process is reversed. The cellular slime mold Dictyostelium, a wonderful model organism, grows and multiplies as long as nutrients are supplied, and its differentiation is triggered by starvation. A strict checkpoint (growth/differentiation transition or GDT point), from which cells start differentiating in response to starvation, has been specified in the cell cycle of D. discoideum Ax-2 cells. Accordingly, integration of GDT point-specific events with starvation-induced events is needed to understand the mechanism regulating GDTs. A variety of intercellular and intracellular signals are involved positively or negatively in the initiation of differentiation, making a series of cross-talks. As was expected from the presence of GDT points, the cell's positioning in cell masses and subsequent cell-type choices occur depending on the cell's phase in the cell cycle at the onset of starvation. Since novel and somewhat unexpected multiple functions of mitochondria in cell movement, differentiation, and pattern formation have been well realized in Dictyostelium cells, they are reviewed in this article.

Cellulose-binding modules from extracellular matrix proteins of Dictyostelium discoideum stalk and sheath

Cellulose-binding modules (CBMs) of two extracellular matrix proteins, St15 and ShD, from the slime mold Dictyostelium discoideum were expressed in Escherichia coli. The expressed proteins were purified to > 98% purity by extracting inclusion bodies at pH 11.5 and refolding proteins at pH 7.5. The two refolded CBMs bound tightly to amorphous phosphoric acid swollen cellulose (PASC), but had a low affinity toward xylan. Neither protein exhibited cellulase activity. St15, the stalk-specific protein, had fourfold higher binding affinity toward microcrystalline cellulose (Avicel) than the sheath-specific ShD CBM. St15 is unusual in that it consists of a solitary CBM homologous to family IIa CBMs. Sequence analysis of ShD reveals three putative domains containing: (a) a C-terminal CBM homologous to family IIb CBMs; (b) a Pro/Thr-rich linker domain; and (c) a N-terminal Cys-rich domain. The biological functions and potential role of St15 and ShD in building extracellular matrices during D. discoideum development are discussed.

Synthesis of the Ca(2+)-dependent cell adhesion molecule DdCAD-1 is regulated by multiple factors during Dictyostelium development

In Dictyostelium discoideum, the cadA gene encodes the cell adhesion molecule DdCAD-1, a protein of M(r) 24,000, which mediates Ca(2+)-dependent cell-cell adhesion during development. We have examined the effects of cAMP, cell-cell contact, and growth conditions on cadA expression. cadA has a unique pattern of expression, which appears to be a combination of the expression patterns of early genes and aggregation-stage genes. Expression of the cadA gene in bacterially grown cells is activated at the beginning of the developmental cycle, followed by a period of rapid DdCAD-1 accumulation. The mRNA level reaches its maximum at 9 h of development and then declines to the basal level at approximately 18 h, while the protein level remains constant after reaching its maximum at 12 h. Pulse-chase experiments have demonstrated that DdCAD-1 has a significantly longer half-life than the average cellular protein. Transcription of the cadA gene is stimulated by exogenous cAMP pulses, leading to a 3- to 5-fold increase in the transcription rate. In the fgdA mutant, which lacks a functional G alpha 2, cAMP fails to enhance cadA expression, suggesting that cAMP stimulates cadA transcription via a G protein-dependent pathway. However, inhibition of cell-cell contact has no effect on the synthesis of DdCAD-1. Growth conditions also have a major influence on cadA expression. Axenically grown cells produce a high level of cadA transcripts during vegetative growth. The mRNA level shows a steady decrease during development and is reduced to the basal level by 12 h. In contrast, the level of DdCAD-1 remains relatively high throughout development, suggesting that axenic growth affects the accumulation of cadA mRNA but not the stability of the protein. These results indicate that multiple mechanisms are involved to maintain a high level of DdCAD-1 during development.

Mutagenesis and gene identification in Dictyostelium by shotgun antisense

We have developed a mutagenesis technique that uses antisense cDNA to identify genes required for development in Dictyostelium discoideum. We transformed Dictyostelium cells with a cDNA library made from the mRNA of vegetative and developing cells. The cDNA was cloned in an antisense orientation immediately downstream of a vegetative promoter, so that in transformed cells the promoter will drive the synthesis of an antisense RNA transcript. We find that individual transformants typically contain one or occasionally two antisense cDNAs. Using this mutagenesis technique, we have generated mutants that fail to aggregate, aggregate but fail to form fruiting bodies, or aggregate but form abnormal fruiting bodies. The individual cDNA molecules from the mutants were identified and cloned using PCR. Initial sequence analysis of the PCR products from 35 mutants has identified six novel Dictyostelium genes, each from a transformant with one antisense cDNA. When the PCR-isolated antisense cDNAs were ligated into the antisense vector and the resulting constructs transformed into cells, the phenotypes of the transformed cells matched those of the original mutants from which each cDNA was obtained. We made homologous recombinant gene disruption transformants for three of the novel genes, in each case generating mutants with phenotypes indistinguishable from those of the original antisense transformants. Shotgun antisense thus is a rapid way to identify genes in Dictyostelium and possibly other organisms.

Expression of the rotavirus SA11 protein VP7 in the simple eukaryote Dictyostelium discoideum

The outer capsid protein of rotavirus, VP7, is a major neutralization antigen and is considered a necessary component of any subunit vaccine developed against rotavirus infection. For this reason, significant effort has been directed towards producing recombinant VP7 that maintains the antigenic characteristics of the native molecule. Using a relatively new expression system, the simple eukaryote Dictyostelium discoideum, we have cloned the portion of simian rotavirus SA11 genome segment 9, encoding the mature VP7 protein, downstream of a native D. discoideum secretion signal sequence in a high-copy-number extrachromosomal vector. The majority of the recombinant VP7 expressed by transformants was intracellular and was detected by Western immunoblot following gel electrophoresis as two or three bands with an apparent molecular mass of 35.5 to 37.5 kDa. A small amount of VP7 having an apparent molecular mass of 37.5 kDa was secreted. Both the intracellular VP7 and the secreted VP7 were N glycosylated and sensitive to endoglycosidase H digestion. Under nonreducing electrophoresis conditions, over half the intracellular VP7 migrated as a monomer while the remainder migrated with an apparent molecular mass approximately twice that of the monomeric form. In an enzyme-linked immunosorbent assay, intracellular VP7 reacted with both nonneutralizing and neutralizing antibodies. The monoclonal antibody recognition pattern paralleled that found with VP7 expressed in either vaccinia virus or herpes simplex virus type 1 and confirms that D. discoideum-expressed VP7 is able to form the major neutralization domains present on viral VP7. Because D. discoideum cells are easy and cheap to grow, this expression system provides a valuable alternative for the large-scale production of recombinant VP7 protein.

Redundant regulatory elements account for the developmental control of a ribosomal protein gene of Dictyostelium discoideum

In Dictyostelium discoideum, ribosomal protein genes along with other growth specific genes appear to be coordinately regulated, primarily in response to differences in the translational capacity of developing versus growing cells. In particular, expression of the members of this large class of genes is rapidly and dramatically deactivated when the developmental program is initiated and growth and division cease. In order to understand the mechanisms behind the deactivation event and how it is coupled to the transition from growth to development, we have analyzed the promoter of the V18 gene, a ribosomal protein gene characteristic of this class of growth specific genes. We have delineated three discrete regions involved in the transcription and regulation of the V18 gene. A initiator region which appears to function in a TATA-independent manner was required for transcription and for establishing start site utilization. Two regions upstream of this were defined, both of which were found to independently confer proper developmental regulation.

Chlamydomonas mutants affected in the light-dependent step of sexual differentiation

Sexual differentiation of Chlamydomonas reinhardtii is induced by the consecutive action of two extrinsic cues--nitrogen deprivation and blue light. The definition of a blue light-dependent step in gamete formation provided a basis for the isolation of mutants altered in the signal transduction pathway by which light controls sexual differentiation. In one mutant (lrg1), gamete formation has become light independent. In the other mutant (lrg2), perception or transduction of the light signal appears to be partially impaired. In both mutants, the expression of genes activated by light in the late phase of gamete formation is affected. Genetic analyses showed that genes LRG1 and LRG2 are linked. The recessive nature of the lrg1-1 mutation implies that the gene encodes a negative factor or a protein that controls the activity of a negative factor. In the case of lrg2-1, neither wild-type nor mutant allele was dominant. Rather, two copies of the lrg2-1 gene simulate a wild-type phenotype. The identification of genetic loci in the pathway for blue light-mediated differentiation provides a basis for the isolation of signal transduction genes in Chlamydomonas.

Systematic analysis of antisense RNA inhibition of myosin II heavy-chain gene expression in Dictyostelium discoideum

The powerful potential of antisense nucleotide inhibition of gene expression is presently being exploited in many biological systems. Although use of the technique is widespread, little is known about the variables that contribute to experimental success. We sought to define those variables that affect inhibition of myosin II heavy-chain (MIIHC) gene expression by stable nuclear-derived antisense RNAs in Dictyostelium discoideum. Different fragments of the MIIHC gene cloned in the antisense orientation into several transformation vectors were introduced into cells, and the accumulation of MIIHC protein and mRNA was examined. Inhibition of expression ranged from slight to virtually complete, and depended only on the specific gene fragment used to generate the antisense RNA. Fragments of the 3' end of the gene were the most effective and resulted in almost complete inhibition, whereas 5' fragments gave very little reduction. The severity of the morphological phenotypes associated with MIIHC depletion reflected the different levels of MIIHC in the transformants. Important implications for the design of antisense vectors suggested by these results are discussed.

Co-suppression of Dictyostelium discoideum myosin II heavy-chain gene expression by a sense orientation transcript

We have previously demonstrated effective inhibition of Dictyostelium discoideum myosin heavy chain (MIIHC) gene expression after introduction of an antisense gene on a transformation vector into cells. We now report that transcription of a sense orientation MIIHC gene has a similar, though not as dramatic, effect on expression from the endogenous gene. Cells transformed with a sense fragment of the MIIHC gene are defective in cytokinesis and become giant and multinucleated when growing on a surface. This morphological phenotype is correlated with decreased expression of the endogenous MIIHC gene. Sense transformants accumulate less MIIHC protein and mRNA than control transformed cells, concomitant with accumulation of a vector-derived sense transcript. Expression of the introduced sequences seems to be required for the sense effect. The possible causes of this surprising phenomenon are discussed.

Inducible expression of calmodulin antisense RNA in Dictyostelium cells inhibits the completion of cytokinesis

The single gene encoding calmodulin in the eukaryotic microorganism Dictyostelium discoideum was cloned and sequenced. The gene was found to contain three introns, one lying immediately after the translation initiation codon. The deduced amino acid sequence indicated that Dictyostelium calmodulin contains 19 amino acid differences from vertebrate calmodulin, including extensions at both termini. Northern blot analysis showed that similar levels of calmodulin mRNA are present throughout growth and development of wild-type cells. A complete copy of the calmodulin cDNA was prepared, and an 87-base pair fragment complementary to the 5'-end of the calmodulin mRNA was subcloned into the Dictyostelium transformation vector pVEII, such that expression of the antisense transcript was driven by the discoidin I gamma promoter. Transformed cells were selected and maintained at low cell density, a condition resulting in minimal activity of the discoidin I promoter. High level expression was induced by allowing the transformants to reach high cell density or by growing them in the presence of medium conditioned by high density cells. Under these conditions, in which calmodulin mRNA and protein levels were reduced about twofold, the calmodulin antisense transformants lost the ability to complete cytokinesis. A contractile ring formed and constricted, but the midbody linking daughter cells failed to break. The resulting cell population contained multinucleated cells and networks of cells connected by cytoplasmic bridges. Normal cell division was restored when the cells were diluted to low density. These observations have identified a new point at which calmodulin may regulate cell cleavage.