The three rows gene of Drosophila melanogaster encodes a novel protein that is required for chromosome disjunction during mitosis.

Zygotic expression of the three rows (thr) gene of Drosophila melanogaster is required for normal cell proliferation during embryogenesis. Mitotic defects in thr mutant embryos begin during mitosis 15, and all subsequent divisions are disrupted. Chromosome disjunction and consequently cytokinesis fail during these defective mitoses, although the initial mitotic processes (chromosome condensation, spindle assembly, metaphase plate formation, and cyclin degradation) are not affected. Despite the failure of chromosome disjunction and cytokinesis, later mitotic events (chromosome decondensation) and subsequent cell cycle progression continue. The thr gene has been isolated and shown to encode a 1209 amino acid protein that shares no extended sequence similarity with known proteins. thr mRNA is present as maternal mRNA that degrades at the time of cellularization. At this and all subsequent times during embryogenesis, zygotic expression correlates with mitotic proliferation. These observations, together with the observation that the zygotic phenotype of thr mutant embryos is influenced by the maternal genotype, suggest that the embryonic phenotype results from exhaustion of the maternal thr contribution and does not reflect a developmentally restricted requirement for thr function. Our results indicate that the novel thr product is required specifically for chromosome disjunction during all mitoses.     

斑马鱼中囊胚过渡调控机制

斑马鱼中囊胚过渡(MBT)始于受精卵的第10次卵裂,此时亦伴有细胞周期延长,分裂同步性丧失,合子型基因开始转录活化,胚胎细胞开始具备运动迁移能力等现象。斑马鱼MBT。的发生依赖于胚胎细胞的核质比,胚胎细胞周期中的G1时相则只有在合子型基因组开始被转录活化后才能出现。细胞周期检验点的激活可能也是受转录调控的,但中期检验点对DNA复制抑制状态的响应不仅在MBT前后、甚至在MBT前的不同阶段也可能有具体作用途径的差异。活化的P38蛋白在胚胎中的不对称分布是维持卵裂阶段细胞分裂同步性的关键因素。尽管大规模的合子型基因的表达发生在MBT开始后,也有少数与胚层分化有关的合子型基因是在MBT。前表达的,还有一些既有母型表达也有合子型表达的基因在MBT前后分别参与不同的信号途径来调控胚胎的发育与分化。     

Many genomic regions are required for normal embryonic programmed cell death in Caenorhabditis elegans

To identify genes involved in programmed cell death (PCD) in Caenorhabditis elegans, we screened a comprehensive set of chromosomal deficiencies for alterations in the pattern of PCD throughout embryonic development. From a set of 58 deficiencies, which collectively remove approximately 74% of the genome, four distinct classes were identified. In class I (20 deficiencies), no significant deviation from wild type in the temporal pattern of cell corpses was observed, indicating that much of the genome does not contain zygotic genes that perform conspicuous roles in embryonic PCD. The class II deficiencies (16 deficiencies defining at least 11 distinct genomic regions) led to no or fewer-than-normal cell corpses. Some of these cause premature cell division arrest, probably explaining the diminution in cell corpse number; however, others have little effect on cell proliferation, indicating that the reduced cell corpse number is not a direct result of premature embryonic arrest. In class III (18 deficiencies defining at least 16 unique regions), an excess of cell corpses was observed. The developmental stage at which the extra corpses were observed varied among the class III deficiencies, suggesting the existence of genes that perform temporal-specific functions in PCD. The four deficiencies in class IV (defining at least three unique regions), showed unusually large corpses that were, in some cases, attributable to extremely premature arrest in cell division without a concomitant block in PCD. Deficiencies in this last class suggest that the cell death program does not require normal embryonic cell proliferation to be activated and suggest that while some genes required for cell division might also be required for cell death, others are not. Most of the regions identified by these deficiencies do not contain previously identified zygotic cell death genes. There are, therefore, a substantial number of as yet unidentified genes required for normal PCD in C. elegans.     

Influence of cloning by chromatin transfer on placental gene expression at Day 45 of pregnancy in cattle

Poor success rates in somatic cell cloning are often attributed to abnormal early embryonic development as well as late abnormal fetal growth and placental development. Although promising results have been reported following chromatin transfer (CT), a novel cloning method that includes the remodeling of the donor nuclei in vitro prior to their transfer into enucleated oocytes, animals cloned by CT show placental abnormalities similar to those observed following conventional nuclear transfer. We hypothesized that the placental gene expression pattern from cloned fetuses was ontologically related to the frequently observed placental phenotype. The aim of the present study was to compare global gene expression by microarray analysis of Day 44–47 cattle placentas derived from CT cloned fetuses with those derived from in vitro fertilization (i.e. control), and confirm the altered mRNA and protein expression of selected molecules by qRT-PCR and immunohistochemistry, respectively. The differentially expressed genes identified in the present study are known to be involved in a range of activities associated with cell adhesion, cell cycle control, intracellular transport and proteolysis. Specifically, an imprinted gene, involved with cell proliferation and placentomegaly in humans (CDKN1C) and a peptidase that serves as a marker for non-invasive trophoblast cells in human placentas (DPP4), had mRNA and protein altered in CT placentas. It was concluded that the altered pattern of gene expression observed in CT samples may contribute to the abnormal placental development phenotypes commonly identified in cloned offspring, and that expression of imprinted as well as trophoblast invasiveness-related genes is altered in cattle cloned by CT.     

Aberrant expression of imprinted genes in post-implantation rat embryos

AimImprinted genes are known regulators of embryo growth. Studies from our laboratory have demonstrated that treatment of adult male rats with tamoxifen increased post-implantation loss at around midgestation. Expression of insulin like growth factor 2 (Igf2), a paternally expressed imprinted gene was down-regulated in the resorbing embryos obtained at embryonic day 13. Hypomethylation of Igf2-H19 imprint control region was observed in the resorbing embryo sires and spermatozoa obtained from tamoxifen-treated rats thereby suggesting that errors in imprint acquisition during spermatogenesis can result in embryo loss. The present study aims at studying the expression of other imprinted genes, besides Igf2 in the embryos sired by tamoxifen-treated males.Main methodsGene expression profiles of resorbing versus normal embryos were assessed by microarrays. Real time quantitative RT-PCR for six imprinted genes and four genes involved in cell cycle was done to validate gene expression data. The affected pathways and functions were identified in the resorbing embryos and effect on cell cycle was confirmed by flow cytometry.Key findingsAberrant expression of a number of imprinted genes was observed in the resorbing embryos when compared to the normal embryos at embryonic days 11 and 13. Down-regulation of Notch signaling, Wnt signaling and cell cycle pathway was observed in the resorbing embryos.SignificanceThe study suggests that exposure of male germ cells to tamoxifen during adulthood results in aberrant expression of imprinted genes and down-regulation of development associated pathways in the F₁ progeny thereby causing embryo loss.     

The turnip mutant of Arabidopsis reveals that LEAFY COTYLEDON1 expression mediates the effects of auxin and sugars to promote embryonic cell identity

The transition from embryonic to vegetative growth marks an important developmental stage in the plant life cycle. The turnip (tnp) mutant was identified in a screen for modifiers of POLARIS expression, a gene required for normal root growth. Mapping and molecular characterization of tnp shows that it represents a gain-of-function mutant of LEAFY COTYLEDON1 (LEC1), due to a promoter mutation. This results in the ectopic expression of LEC1, but not of other LEC genes, in vegetative tissues. The LEC class of genes are known regulators of embryogenesis, involved in the control of embryonic cell identity by currently unknown mechanisms. Activation of the LEC-dependent pathway in tnp leads to the loss of hypocotyl epidermal cell marker expression and loss of SCARECROW expression in the endodermis, the ectopic accumulation of starch and lipids, and the up-regulation of early and late embryonic genes. tnp also shows partial deetiolation during dark growth. Penetrance of the mutant phenotype is strongly enhanced in the presence of exogenous auxin and sugars, but not by gibberellin or abscisic acid, and is antagonized by cytokinin. We propose that the role of LEC1 in embryonic cell fate control requires auxin and sucrose to promote cell division and embryonic differentiation.