Overexpression of eIF4E in Saccharomyces cerevisiae causes slow growth and decreased alpha-factor response through alterations in CLN3 expression

TOR Complex 1: Orchestrating Nutrient Signaling and Cell Cycle Progression

The highly conserved TOR signaling pathway is crucial for coordinating cellular growth with the cell cycle machinery in eukaryotes. One of the two TOR complexes in budding yeast, TORC1, integrates environmental cues and promotes cell growth. While cells grow, they need to copy their chromosomes, segregate them in mitosis, divide all their components during cytokinesis, and finally physically separate mother and daughter cells to start a new cell cycle apart from each other. To maintain cell size homeostasis and chromosome stability, it is crucial that mechanisms that control growth are connected and coordinated with the cell cycle. Successive periods of high and low TORC1 activity would participate in the adequate cell cycle progression. Here, we review the known molecular mechanisms through which TORC1 regulates the cell cycle in the budding yeast Saccharomyces cerevisiae that have been extensively used as a model organism to understand the role of its mammalian ortholog, mTORC1.

Systematic identification of cell size regulators in budding yeast

Cell size is determined by a complex interplay between growth and division, involving multiple cellular pathways. To identify systematically processes affecting size control in G1 in budding yeast, we imaged and analyzed the cell cycle of millions of individual cells representing 591 mutants implicated in size control. Quantitative metric distinguished mutants affecting the mechanism of size control from the majority of mutants that have a perturbed size due to indirect effects modulating cell growth. Overall, we identified 17 negative and dozens positive size control regulators, with the negative regulators forming a small network centered on elements of mitotic exit network. Some elements of the translation machinery affected size control with a notable distinction between the deletions of parts of small and large ribosomal subunit: parts of small ribosomal subunit tended to regulate size control, while parts of the large subunit affected cell growth. Analysis of small cells revealed additional size control mechanism that functions in G2/M, complementing the primary size control in G1. Our study provides new insights about size control mechanisms in budding yeast.

Cellular and molecular effects of nonreciprocal chromosome translocations in Saccharomyces cerevisiae

Saccharomyces cerevisiae strains harboring a nonreciprocal, bridge-induced translocation (BIT) between chromosomes VIII and XV exhibited an abnormal phenotype comprising elongated buds and multibudded, unevenly nucleated pseudohyphae. In these cells, we found evidence of molecular effects elicited by the translocation event and specific for its particular genomic location. Expression of genes flanking both translocation breakpoints increased up to five times, correlating with an increased RNA polymerase II binding to their promoters and with their histone acetylation pattern. Microarray data, CHEF, and quantitative PCR confirmed the data on the dosage of genes present on the chromosomal regions involved in the translocation, indicating that telomeric fragments were either duplicated or integrated mostly on chromosome XI. FACS analysis revealed that the majority of translocant cells were blocked in G(1) phase and a few of them in G(2). Some cells showed a posttranslational decrease of cyclin B1, in agreement with elongated buds diagnostic of a G(2)/M phase arrest. The actin1 protein was in some cases modified, possibly explaining the abnormal morphology of the cells. Together with the decrease in Rad53p and the lack of its phosphorylation, these results indicate that these cells have undergone adaptation after checkpoint-mediated G(2)/M arrest after chromosome translocation. These BIT translocants could serve as model systems to understand further the cellular and molecular effects of chromosome translocation and provide fundamental information on its etiology of neoplastic transformation in mammals.

Alcohol exposure alters cell cycle and apoptotic events during early neurulation

BACKGROUND

Fetal alcohol exposure causes growth deficits, microencephaly, and neurological abnormalities. Although the effects of alcohol on developmental delay and growth-related deficits have been hypothesized, little is understood about how alcohol alters, in particular, the cyclin pathway within the cell cycle, which is critical to proliferation and apoptotic control. In this study, we examined cell cycle proteins pertinent to the G1-S phase transition and apoptosis, to determine if cell cycle misregulation can be attributed to apoptotic induction and growth defects.

METHODS

We examined cell cycle regulation during G1 and S-phase, and DNA fragmentation damage, using E14 dorsal root ganglia neural stem cells (DRG-NC), and cultured mouse embryos exposed to 200 and 400 mg/dl ethanol.

RESULTS

Alcohol-exposed DRG-NC demonstrated a dose-dependent increase in cells expressing increased cyclin D1 protein, and increased DNA fragmentation. Western blot analysis, using embryos, demonstrated an overexpression of cyclin D1, D2, and E2F1, key G1 to S-phase cell cycle regulatory components, and increases in p53, linking the cell cycle and apoptotic pathways. Bromodeoxyuridine incorporation indicated reduced DNA synthesis and growth in several embryonic regions. Propidium iodide staining demonstrated decreases in DNA content and increases in DNA fragmentation in several embryonic tissues.

CONCLUSIONS

This study indicated that retarded growth of DRG-NC and embryos, induced by alcohol, is associated with altered expression of cell cycle and apoptotic proteins and concurrent inhibition of proliferation and increased DNA fragmentation. We suggest that alcohol induces an increase in cyclin D1 expression, premature S-phase entry, and disjointed DNA synthesis with increased apoptosis.

Chronic alcohol drinking alters neuronal dendritic spines in the brain reward center nucleus accumbens

Alcohol is known to affect glutamate transmission. However, how chronic alcohol affects the synaptic structure mediating glutamate transmission is unknown. Repeated alcohol exposure in a subject with familial alcoholic history often leads to alcohol addiction. The current study adopts alcohol-preferring rats, which are known to develop high drinking. Two-photon microscopy analysis indicates that chronic alcohol of 14 weeks either, under continuous alcohol (C-Alc) or with repeated deprivation (RD-Alc), causes dysmorphology--thickened, beaded, and disoriented dendrites that are reminiscent of reactive astrocytes--in a subpopulation of medium spiny neurons. The density of dendritic spines was found differentially lower in the nucleus accumbens of RD-Alc and C-Alc groups as compared with those of Water groups. Large-sized spines and multiple-headed spines were increased in the RD-Alc group. The NMDA receptor subunit NR1 proteins, as analyzed with Western blot, were upregulated in C-Alc, but not in RD-Alc. The upregulated NMDA receptor subunits of NR1 however, are predominantly a splice variant isoform with truncated exon 21, which is required for membrane-bound trafficking or anchoring into a spine synaptic site. These maladaptations may contribute to the transformation of spines. The changes, in density and head-size of spines and the corresponding NMDA receptors, demonstrated an alteration of microcircuitry for glutamate reception. The current study demonstrates for the first time that chronic alcohol exposure causes structural alteration of dendrites and their spines in the key reward brain region in animals that have a genetic background leading to alcohol addiction.

A role for the yeast cell cycle/splicing factor Cdc40 in the G1/S transition

The CDC40 (PRP17) gene of S. cerevisiae encodes a splicing factor required for multiple events in the mitotic and meiotic cell cycles, linking splicing with cell cycle control. cdc40 mutants exhibit a delayed G(1)/S transition, progress slowly through S-phase and arrest at a restrictive temperature in the G(2) phase. In addition, they are hypersensitive to genotoxic agents such as methylmethane sulfonate (MMS) and Hydroxyurea (HU). CDC40 has been suggested to control cell cycle through splicing of intron-containing pre-mRNAs that encode proteins important for cell cycle progression. We screened a cDNA overexpression library and isolated cDNAs that specifically suppress the HU/MMS-sensitivity of cdc40 mutants. Most of these cDNAs surprisingly encode chaperones, translation initiation factors and glycolytic enzymes, and none of them is encoded by an intron-containing gene. Interestingly, the cDNAs suppress the G(1)/S transition delay of cdc40 cells, which is exacerbated by HU, suggesting that cdc40 mutants are HU/MMS-sensitive due to their S-phase entry defect. A role of Cdc40p in passage through G(1)/S (START) is further supported by the enhanced temperature sensitivity and G(1)/S transition phenotype of a cdc40 strain lacking the G(1) cyclin, Cln2p. We provide evidence that the mechanism of suppression by the isolated cDNAs does not (at least solely) involve up-regulation of the known positive START regulators CLN2, CLN3, DCR2 and GID8, or of the large and small essential ribonucleotide reductase (RNR) subunits, RNR1 and RNR2. Finally, we discuss possible mechanisms of suppression by the cDNAs that imply cell cycle regulation by apparently unrelated processes, such as splicing, translation initiation and glycolysis.

The global transcriptional activator of Saccharomyces cerevisiae, Gcr1p, mediates the response to glucose by stimulating protein synthesis and CLN-dependent cell cycle progression

Growth of Saccharomyces cerevisiae requires coordination of cell cycle events (e.g., new cell wall deposition) with constitutive functions like energy generation and duplication of protein mass. The latter processes are stimulated by the phosphoprotein Gcr1p, a transcriptional activator that operates through two different Rap1p-mediated mechanisms to boost expression of glycolytic and ribosomal protein genes, respectively. Simultaneous disruption of both mechanisms results in a loss of glucose responsiveness and a dramatic drop in translation rate. Since a critical rate of protein synthesis (CRPS) is known to mediate passage through Start and determine cell size by modulating levels of Cln3p, we hypothesized that GCR1 regulates cell cycle progression by coordinating it with growth. We therefore constructed and analyzed gcr1delta cln3delta and gcr1delta cln1delta cln2delta strains. Both strains are temperature and cold sensitive; interestingly, they exhibit different arrest phenotypes. The gcr1delta cln3delta strain becomes predominantly unbudded with 1N DNA content (G1 arrest), whereas gcr1delta cln1delta cln2delta cells exhibit severe elongation and apparent M phase arrest. Further analysis demonstrated that the Rap1p/Gcr1p complex mediates rapid growth in glucose by stimulating both cellular metabolism and CLN transcription.

Far3 and five interacting proteins prevent premature recovery from pheromone arrest in the budding yeast Saccharomyces cerevisiae

In budding yeast, diffusible mating pheromones initiate a signaling pathway that culminates in several responses, including cell cycle arrest. Only a handful of genes required for the interface between pheromone response and the cell cycle have been identified, among them FAR1 and FAR3; of these, only FAR1 has been extensively characterized. In an effort to learn about the mechanism by which Far3 acts, we used the two-hybrid method to identify interacting proteins. We identified five previously uncharacterized open reading frames, dubbed FAR7, FAR8, FAR9, FAR10, and FAR11, that cause a far3-like pheromone arrest defect when disrupted. Using two-hybrid and coimmunoprecipitation analysis, we found that all six Far proteins interact with each other. Moreover, velocity sedimentation experiments suggest that Far3 and Far7 to Far11 form a complex. The phenotype of a sextuple far3far7-far11 mutant is no more severe than any single mutant. Thus, FAR3 and FAR7 to FAR11 all participate in the same pathway leading to G1 arrest. These mutants initially arrest in response to pheromone but resume budding after 10 h. Under these conditions, wild-type cells fail to resume budding even after several days whereas far1 mutant cells resume budding within 1 h. We conclude that the FAR3-dependent arrest pathway is functionally distinct from that which employs FAR1.

Gene-specific regulation by general translation factors

Protein synthesis is the ultimate step of gene expression and a key control point for regulation. In particular, it enables cells to rapidly manipulate protein production without new mRNA synthesis, processing, or export. Recent studies have enhanced our understanding of the translation initiation process and helped elucidate how modifications of the general translational machinery regulate gene-specific protein production.

Current awareness on yeast

In order to keep subscribers up-to-date with the latest developments in their field, this current awareness service is provided by John Wiley & Sons and contains newly-published material on yeasts. Each bibliography is divided into 10 sections. 1 Books, Reviews & Symposia; 2 General; 3 Biochemistry; 4 Biotechnology; 5 Cell Biology; 6 Gene Expression; 7 Genetics; 8 Physiology; 9 Medical Mycology; 10 Recombinant DNA Technology. Within each section, articles are listed in alphabetical order with respect to author. If, in the preceding period, no publications are located relevant to any one of these headings, that section will be omitted. (3 weeks journals - search completed 5th. Dec. 2001)