Evolutionary diversification of MCM3 genes in Xenopus laevis and Danio rerio

Lineage skewing and genome instability underlie marrow failure in a zebrafish model of GATA2 deficiency

Inherited bone marrow failure associated with heterozygous mutations in GATA2 predisposes toward hematological malignancies, but the mechanisms remain poorly understood. Here, we investigate the mechanistic basis of marrow failure in a zebrafish model of GATA2 deficiency. Single-cell transcriptomics and chromatin accessibility assays reveal that loss of gata2a leads to skewing toward the erythroid lineage at the expense of myeloid cells, associated with loss of cebpa expression and decreased PU.1 and CEBPA transcription factor accessibility in hematopoietic stem and progenitor cells (HSPCs). Furthermore, gata2a mutants show impaired expression of npm1a, the zebrafish NPM1 ortholog. Progressive loss of npm1a in HSPCs is associated with elevated levels of DNA damage in gata2a mutants. Thus, Gata2a maintains myeloid lineage priming through cebpa and protects against genome instability and marrow failure by maintaining expression of npm1a. Our results establish a potential mechanism underlying bone marrow failure in GATA2 deficiency.

Ecotoxicity of eluates obtained from Basamid® contaminated soils is pH dependent: A study with Hydra viridissima, Xenopus laevis and Danio rerio

Agrochemicals are mostly used to deplete pests and treat diseases in terrestrial agro-ecosystems. However, their transport through the soil, by leaching and/or runoff, may cause them to reach aquatic systems. Environmental parameters, such as soil pH, can affect this transport, by influencing the magnitude of agrochemicals degradation and chemical reaction. This work aimed at investigating the influence of soil pH on the toxicity of eluates obtained from Basamid® contaminated soils to Hydra viridissima, Xenopus laevis and Danio rerio. For this, a natural soil with pH amended to 5.5, 6.5 and 7.5, was spiked with the recommended dose (RD) of Basamid® (145 mg dazomet/kg soil) and eluates (Ba-E) were prepared with the respective species culture medium. Dilutions of the eluates (0.14-100%), obtained from the three soils (Ba-E 5.5, Ba-E 6.5 and Ba-E 7.5, corresponding to soil spiked with Basamid® RD at soil pH of 5.5, 6.5 and 7.5, respectively), were used to expose the organisms. Results showed that for H. viridissima increased soil alkalinity provoked less mortality comparatively to lower soil pH [LD_50,96h of Ba-E 5.5: 10.6% and LD_50,96h of Ba-E 7.5: 21.2%]. As for X. laevis and D. rerio Ba-E lethal ecotoxicity was similar across soil pH (LD_50,96h varied from 5.7 to 6.9% and from 2.1 to 4.3%, respectively). For malformations, 20% effect dilution (ED) in H. viridissima was significantly higher at Ba-E 7.5 (ED_20,96h: 17.4%), comparatively to Ba-E 5.5 and Ba-E 6.5 (ED_20,96h: 7.9% and 7.7%, respectively). From the three tested organisms and based on both lethal and sublethal effects, H. viridissima presented the highest tolerance to Basamid® eluates and soil pH was a major factor determining the fumigant toxicity, with higher soil pH levels inducing, lower toxicity. The eluates obtained from soils contaminated with RD of Basamid® induced severe effects to the three aquatic species.

Toxicity of herbicide glyphosate to planarian Dugesia japonica and its potential molecular mechanisms

Glyphosate (GLY) is one of the most widely used agrochemicals in the world, and its exposure has become a public health concern. The freshwater planarian is an ideal test organism for detecting the toxicity of pollutants and has been an emerging animal model in toxicological studies. Nevertheless, the underlying toxicity mechanism of GLY to planarians has not been thoroughly explored. To elucidate the toxicity effects and molecular mechanism involved in GLY exposure of planarians, we studied the acute toxicity, histological change, and transcriptional response of Dugesia japonica subjected to GLY. Significant morphological malformations and histopathological changes were observed in planarians after GLY exposure for different times. Also, a number of differentially expressed genes (DEGs) were obtained at 1, 3 and 5 d after exposure; Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) enrichment analysis of these DEGs were performed, and a global and dynamic view was obtained in planarians upon GLY exposure at the transcriptomic level. Furthermore, real-time quantitative PCR (qRT-PCR) was conducted on nine DEGs associated with detoxification, apoptosis, stress response, DNA repair, etc. The expression patterns were well consistent with the RNA sequencing (RNA-seq) results at different time points, which confirmed the reliability and accuracy of the transcriptome data. Collectively, our results established that GLY could pose adverse effects on the morphology and histo-architecture of D. japonica, and the planarians are capable of responding to the disadvantageous stress by dysregulating the related genes and pathways concerning immune response, detoxification, energy metabolism, DNA damage repair, etc. To the best of our knowledge, this is the first report of transcriptomic analyses of freshwater planarians exposed to environmental pollutants, and it provided detailed sequencing data deriving from transcriptome profiling to deepen our understanding the molecular toxicity mechanism of GLY to planarians.

Molecular genetics of maternally-controlled cell divisions

Forward genetic screens remain at the forefront of biology as an unbiased approach for discovering and elucidating gene function at the organismal and molecular level. Past mutagenesis screens targeting maternal-effect genes identified a broad spectrum of phenotypes ranging from defects in oocyte development to embryonic patterning. However, earlier vertebrate screens did not reach saturation, anticipated classes of phenotypes were not uncovered, and technological limitations made it difficult to pinpoint the causal gene. In this study, we performed a chemically-induced maternal-effect mutagenesis screen in zebrafish and identified eight distinct mutants specifically affecting the cleavage stage of development and one cleavage stage mutant that is also male sterile. The cleavage-stage phenotypes fell into three separate classes: developmental arrest proximal to the mid blastula transition (MBT), irregular cleavage, and cytokinesis mutants. We mapped each mutation to narrow genetic intervals and determined the molecular basis for two of the developmental arrest mutants, and a mutation causing male sterility and a maternal-effect mutant phenotype. One developmental arrest mutant gene encodes a maternal specific Stem Loop Binding Protein, which is required to maintain maternal histone levels. The other developmental arrest mutant encodes a maternal-specific subunit of the Minichromosome Maintenance Protein Complex, which is essential for maintaining normal chromosome integrity in the early blastomeres. Finally, we identify a hypomorphic allele of Polo-like kinase-1 (Plk-1), which results in a male sterile and maternal-effect phenotype. Collectively, these mutants expand our molecular-genetic understanding of the maternal regulation of early embryonic development in vertebrates.

MCM interference during licensing of DNA replication in Xenopus egg extracts-Possible Role of a C-terminal region of MCM3

The minichromosome maintenance (MCM) complex, consisting of six subunits, Mcm2-7, is loaded onto replication origins through loading factors (origin recognition complex [ORC], Cdc6, and Cdt1) and forms an MCM double hexamer that licenses the initiation of DNA replication. Previous studies with Xenopus egg extracts showed that loading factors, especially Cdc6, dissociate from chromatin on MCM loading, but the molecular mechanism and physiological significance remain largely unknown. Using a cell-free system for MCM loading onto plasmid DNA in Xenopus egg extracts, we found that MCM loaded onto DNA prevents DNA binding of the loading factors ORC, Cdc6, and Cdt1. We further report that a peptide of the C-terminal region of MCM3 (MCM3-C), previously implicated in the initial association with ORC/Cdc6 in budding yeast, prevents ORC/Cdc6/Cdt1 binding to DNA in the absence of MCM loading. ATP-γ-S suppresses inhibitory activities of both the MCM loaded onto DNA and the MCM3-C peptide. Other soluble factors in the extract, but neither MCM nor Cdt1, are required for the activity. Conservation of the amino acid sequences of MCM3-C and its activity in vertebrates implies a novel negative autoregulatory mechanism that interferes with MCM loading in the vicinity of licensed origins to ensure proper origin licensing.

Suppression of targeting of Dbf4-dependent kinase to pre-replicative complex in G0 nuclei

Intact G0 nuclei isolated from quiescent cells are not capable of DNA replication in interphase Xenopus egg extracts, which allow efficient replication of permeabilized G0 nuclei. Previous studies have shown multiple control mechanisms for maintaining the quiescent state, but DNA replication inhibition of intact G0 nuclei in the extracts remains poorly understood. Here, we showed that pre-RC is assembled on chromatin, but its activation is inhibited after incubating G0 nuclei isolated from quiescent NIH3T3 cells in the extracts. Concomitant with the inhibition of replication, Mcm4 phosphorylation mediated by Dbf4-dependent kinase (DDK) as well as chromatin binding of DDK is suppressed in G0 nuclei without affecting the nuclear transport of DDK. We further found that the nuclear extracts of G0 but not proliferating cells inhibit the binding of recombinant DDK to pre-RC assembled plasmids. In addition, we observed rapid activation of checkpoint kinases after incubating G0 nuclei in the egg extracts. However, specific inhibitors of ATR/ATM are unable to promote DNA replication in G0 nuclei in the egg extracts. We suggest that a novel inhibitory mechanism is functional to prevent the targeting of DDK to pre-RC in G0 nuclei, thereby suppressing DNA replication in Xenopus egg extracts.

[Deguelin inhibits proliferation and regulates the expression of MCM3-CDC45 in MCF-7 and H1299 cells in vitro]

OBJECTIVE

To observe the effects of deguelin on the proliferation of breast cancer MCF-7 cells and lung cancer H1299 cells in vitro and the expression of minichromosome maintenance protein 3 (MCM3) and CDC45 in the cells.

METHODS

MTT assay was used to evaluate the proliferation of MCF-7 and H1299 cells exposed to different concentrations of deguelin for 48, 72 or 96 h. The growth of the cells was observed microscopically and the changes of MCM3 and CDC45 expressions in MCF-7 and H1299 cells following deguelin treatment were detected with fluorescence quantitative PCR.

RESULTS

The proliferation of MCF-7 cells was significantly inhibited by exposure to 0.25, 0.5, 1, 5, 10, 30, and 50 µmol/L deguelin for 48, 72, and 96 h in a concentration- and time-dependent manner. In MCF-7 cells, the IC₅₀ of deguelin at 48, 72, and 96 h was 9, 3, and 2 µmol/L, respectively. Deguelin treatments of H1299 cells at 0.5, 1, 5, 10, 30, 50, and 100 µmol/L also resulted in a concentration- and time-dependent inhibition of the cell growth with an IC₅₀ at 96 h of 2 µmol/L. Optical microscopy of the cells revealed a decreased number of viable cells with obvious cell shrinkage following deguelin treatments. The expression of MCM3 and CDC45 were significantly reduced in the cells after deguelin treatments.

CONCLUSION

Deguelin can inhibit the proliferation of MCF-7 and H1299 cells in vitro and down-regulate the expression of MCM3 and CDC45 in the cells.

Regulation of DNA Replication in Early Embryonic Cleavages

Early embryonic cleavages are characterized by short and highly synchronous cell cycles made of alternating S- and M-phases with virtually absent gap phases. In this contracted cell cycle, the duration of DNA synthesis can be extraordinarily short. Depending on the organism, the whole genome of an embryo is replicated at a speed that is between 20 to 60 times faster than that of a somatic cell. Because transcription in the early embryo is repressed, DNA synthesis relies on a large stockpile of maternally supplied proteins stored in the egg representing most, if not all, cellular genes. In addition, in early embryonic cell cycles, both replication and DNA damage checkpoints are inefficient. In this article, we will review current knowledge on how DNA synthesis is regulated in early embryos and discuss possible consequences of replicating chromosomes with little or no quality control.