A Meiosis-Specific Form of the APC/C Promotes the Oocyte-to-Embryo Transition by Decreasing Levels of the Polo Kinase Inhibitor Matrimony

Oocytes are stockpiled with proteins and mRNA that are required to drive the initial mitotic divisions of embryogenesis. But are there proteins specific to meiosis whose levels must be decreased to begin embryogenesis properly? The Drosophila protein Cortex (Cort) is a female, meiosis-specific activator of the Anaphase Promoting Complex/Cyclosome (APC/C), an E3 ubiquitin ligase. We performed immunoprecipitation of Cortex followed by mass spectrometry, and identified the Polo kinase inhibitor Matrimony (Mtrm) as a potential interactor with Cort. In vitro binding assays showed Mtrm and Cort can bind directly. We found Mtrm protein levels to be reduced dramatically during the oocyte-to-embryo transition, and this downregulation did not take place in cort mutant eggs, consistent with Mtrm being a substrate of APC^Cort. We showed that Mtrm is subject to APC^Cort-mediated proteasomal degradation and have identified a putative APC/C recognition motif in Mtrm that when mutated partially stabilized the protein in the embryo. Furthermore, overexpression of Mtrm in the early embryo caused aberrant nuclear divisions and developmental defects, and these were enhanced by decreasing levels of active Polo. These data indicate APC^Cort ubiquitylates Mtrm at the oocyte-to-embryo transition, thus preventing excessive inhibition of Polo kinase activity due to Mtrm''s presence.     

Genome-wide analysis of GLD-1-mediated mRNA regulation suggests a role in mRNA storage

Translational repression is often accompanied by mRNA degradation. In contrast, many mRNAs in germ cells and neurons are "stored" in the cytoplasm in a repressed but stable form. Unlike repression, the stabilization of these mRNAs is surprisingly little understood. A key player in Caenorhabditis elegans germ cell development is the STAR domain protein GLD-1. By genome-wide analysis of mRNA regulation in the germ line, we observed that GLD-1 has a widespread role in repressing translation but, importantly, also in stabilizing a sub-population of its mRNA targets. Additionally, these mRNAs appear to be stabilized by the DDX6-like RNA helicase CGH-1, which is a conserved component of germ granules and processing bodies. Because many GLD-1 and CGH-1 stabilized mRNAs encode factors important for the oocyte-to-embryo transition (OET), our findings suggest that the regulation by GLD-1 and CGH-1 serves two purposes. Firstly, GLD-1-dependent repression prevents precocious translation of OET-promoting mRNAs. Secondly, GLD-1- and CGH-1-dependent stabilization ensures that these mRNAs are sufficiently abundant for robust translation when activated during OET. In the absence of this protective mechanism, the accumulation of OET-promoting mRNAs, and consequently the oocyte-to-embryo transition, might be compromised.     

Dynamic distribution of spindlin in nucleoli, nucleoplasm and spindle from primary oocytes to mature eggs and its critical function for oocyte-to-embryo transition in gibel carp

Spindlin (Spin) was thought as a maternal-effect factor associated with meiotic spindle. Its role for the oocyte-to-embryo transition was suggested in mouse, but its direct evidence for the function had been not obtained in other vertebrates. In this study, we used the CagSpin-specific antibody to investigate CagSpin expression pattern and distribution during oogenesis of gibel carp (Carassius auratus gibelio). First, the oocyte-specific expression pattern and dynamic distribution was revealed in nucleoli, nucleoplasm, and spindle from primary oocytes to mature eggs by immunofluorescence localization. In primary oocytes and growth stage oocytes, CagSpin accumulates in nucleoli in increasing numbers along with the oocyte growth, and its disassembly occurs in vitellogenic oocytes, which implicates that CagSpin may be a major component of a large number of nucleoli in fish growth oocytes. Then, co-localization of CagSpin and β-tubulin was revealed in meiotic spindle of mature egg, indicating that CagSpin is one spindle-associated factor. Moreover, microinjection of CagSpin-specific antibody into the fertilized eggs blocked the first cleavage, and found that the CagSpin depletion resulted in spindle assembly disturbance. Thereby, our study provided the first direct evidence for the critical oocyte-to-embryo transition function of Spin in vertebrates, and confirmed that Spin is one important maternal-effect factor that participates in oocyte growth, oocyte maturation, and oocyte-to-embryo transition.     

Changes in bicoid mRNA anchoring highlight conserved mechanisms during the oocyte-to-embryo transition

Intracellular mRNA localization directs protein synthesis to particular subcellular domains to establish embryonic polarity in a variety of organisms. In Drosophila, bicoid (bcd) mRNA is prelocalized at the oocyte anterior. After fertilization, translation of this RNA produces a Bcd protein gradient that determines anterior cell fates [1] and [2]. Analysis of bcd mRNA during late stages of oogenesis suggested a model for steady-state bcd localization by continual active transport [3]. However, this mechanism cannot explain maintenance of bcd localization throughout the end of oogenesis, when microtubules disassemble in preparation for embryogenesis [4] and [5], or retention of bcd at the anterior in mature oocytes, which can remain dormant for weeks before fertilization [6]. Here, we elucidate the path and mechanism of sustained bcd mRNA transport by direct observation of bcd RNA particle translocation in living oocytes. We show that bcd mRNA shifts from continuous active transport to stable actin-dependent anchoring at the end of oogenesis. Egg activation triggers bcd release from the anterior cortex for proper deployment in the embryo, probably through reorganization of the actin cytoskeleton. These findings uncover a surprising parallel between flies and frogs, as cortically tethered Xenopus Vg1 mRNA undergoes a similar redistribution during oocyte maturation [7]. Our results thus highlight a conserved mechanism for regulating mRNA anchoring and redeployment during the oocyte-to-embryo transition.     

Conservation of MAP kinase activity and MSP genes in parthenogenetic nematodes

Background  

MAP (mitogen-activated protein) kinase activation is a prerequisite for oocyte maturation, ovulation and fertilisation in many animals. In the hermaphroditic nematode Caenorhabditis elegans, an MSP (major sperm protein) dependent pathway is utilised for MAP kinase activation and successive oocyte maturation with extracellular MSP released from sperm acting as activator. How oocyte-to-embryo transition is triggered in parthenogenetic nematode species that lack sperm, is not known.     

The C. elegans DYRK Kinase MBK-2 Marks Oocyte Proteins for Degradation in Response to Meiotic Maturation

The oocyte-to-embryo transition transforms a differentiated germ cell into a totipotent zygote capable of somatic development. In C. elegans, several oocyte proteins, including the meiotic katanin subunit MEI-1 and the oocyte maturation protein OMA-1, must be degraded during this transition . Degradation of MEI-1 and OMA-1 requires the dual-specificity YAK-1-related (DYRK) kinase MBK-2 . Here, we demonstrate that MBK-2 directly phosphorylates MEI-1 and OMA-1 in vitro and that this activity is essential for degradation in vivo. Phosphorylation of MEI-1 by MBK-2 reaches maximal levels after the meiotic divisions, immediately preceding MEI-1 degradation. MEI-1 phosphorylation and degradation still occur in spe-9 eggs, which undergo meiotic maturation and exit in the absence of fertilization . In contrast, MEI-1 phosphorylation and degradation are blocked in cell-cycle mutants that arrest during the meiotic divisions, and are accelerated in wee-1.3(RNAi) oocytes, which prematurely enter meiotic M phase (A. Golden, personal communication). A GFP:MBK-2 fusion relocalizes from the cortex to the cytoplasm during the meiotic divisions, and this relocalization also depends on cell-cycle progression. Our findings suggest that regulators of meiotic M phase activate a remodeling program, independently of fertilization, to prepare eggs for embryogenesis.     

Polo kinases regulate C. elegans embryonic polarity via binding to DYRK2-primed MEX-5 and MEX-6

Polo kinases are known key regulators of cell divisions. Here we report a novel, non-cell division function for polo kinases in embryonic polarity of newly fertilized Caenorhabditis elegans embryos. We show that polo kinases, via their polo box domains, bind to and regulate the activity of two key polarity proteins, MEX-5 and MEX-6. These polo kinases are asymmetrically localized along the anteroposterior axis of newly fertilized C. elegans embryos in a pattern identical to that of MEX-5 and MEX-6. This asymmetric localization of polo kinases depends on MEX-5 and MEX-6, as well as genes regulating MEX-5 and MEX-6 asymmetry. We identify an amino acid of MEX-5, T(186), essential for polo binding and show that T(186) is important for MEX-5 function in vivo. We also show that MBK-2, a developmentally regulated DYRK2 kinase activated at meiosis II, primes T(186) for subsequent polo kinase-dependent phosphorylation. Prior phosphorylation of MEX-5 at T(186) greatly enhances phosphorylation of MEX-5 by polo kinases in vitro. Our results provide a mechanism by which MEX-5 and MEX-6 function is temporally regulated during the crucial oocyte-to-embryo transition.     

斑马鱼Zar1和Zar1-like基因的克隆及其表达分析

合子阻滞基因1(Zygote arrest 1,Zar1)及Zar1-like(Zar1L)作为母源基因,在小鼠卵子-合子转变过程中发挥着极其重要的作用。研究以斑马鱼为对象,采用cDNA末端快速克隆(RACE)技术获得了Zar1L cDNA全长,并采用RT-PCR技术检测Zar1和Zar1L在斑马鱼不同组织器官、卵母细胞和胚胎发育时期的表达情况。Zar1L cDNA的全长为1146 bp,编码311个氨基酸残基,5′非编码区20 bp,3′非编码区190 bp。多重序列比对结果显示,C-末端的序列非常保守,相似性高达74%,具有非典型的PHD基序(Plant homeo domain),并具有两个C4类型的锌指结构。在基因结构上,Zar1和Zar1L均由四个外显子构成,两个基因的每一个外显子在大小上也比较相近。分子进化表明斑马鱼Zar1和Zar1L属于两个不同的基因,可能是同一来源的祖先基因重复和进化的结果。RT-PCR结果表明斑马鱼Zar1和Zar1L的表达模式相似,都是卵巢特异表达基因,在卵母细胞及早期胚胎中可检测到大量转录本的存在,囊胚、原肠胚期后下降,但在整个胚胎发育期都可检测到转录本。斑马鱼Zar1和Zar1L mRNA的表达水平的一致性,似乎暗示着两个基因的功能基本一致。表达模式分析表明Zar1和Zar1L可能与斑马鱼卵子-合子转变有关,也可能与其他的胚胎发育过程有关。       

A new view of mRNA export: separating the wheat from the chaff

Current models for the export of messenger RNA share the notion that the highly abundant class of nuclear RNA-binding proteins--the hnRNP proteins--have a key role in exporting RNA. But recent studies have led to a new understanding of several non-hnRNP proteins, including SR proteins and the conserved mRNA export factor ALY, which are recruited to the mRNA during pre-mRNA splicing. These studies, together with older work on hnRNP particles and assembly of the spliceosome, lead us to a new view of mRNA export. In our model, the non-hnRNP factors form a splicing-dependent mRNP complex that specifically targets mature mRNA for export, while hnRNP proteins retain introns in the nucleus. A machinery that is conserved between yeast and higher eukaryotes functions to export the mRNA.     

Blue light inhibits slime mold differentiation at the mRNA level

The influence of blue light on protein synthesis in spherulating Physarum polycephalum microplasmodia was studied using two-dimensional protein separation techniques. The starvation-induced plasmodium-spherule transition proceeds in the dark and is accompanied by the synthesis of 20 major differentiation-specific proteins as revealed by in vivo labelling with [35S]methionine. Three of these proteins are identical with cell wall components with respect to their mol. wts. (35 K, 34 K and 14 K) and isoelectric points. Spherulation is also accompanied by the appearance of 26 prominent differentiation-specific mRNA species translatable in the rabbit reticulocyte cell-free system. Six of the proteins synthesized in vitro co-migrate on two-dimensional gels with proteins labelled in vivo, two of them being cell wall components. Blue light, which inhibits spherulation completely, inhibits also the synthesis of spherule proteins and of spherule-specific mRNA activity. Only three protein components are induced by blue light, indicating that illumination does not induce a novel differentiated plasmodial state.     

Cytoplasmic organelles on the road to mRNA decay

Localization of both mRNAs and mRNA decay factors to internal membranes of eukaryotic cells provides a means of coordinately regulating mRNAs with common functions as well as coupling organelle function to mRNA turnover. The classic mechanism of mRNA localization to membranes is the signal sequence-dependent targeting of mRNAs encoding membrane and secreted proteins to the cytoplasmic surface of the endoplasmic reticulum. More recently, however, mRNAs encoding proteins with cytosolic or nuclear functions have been found associated with various organelles, in many cases through unknown mechanisms. Furthermore, there are several types of RNA granules, many of which are sites of mRNA degradation; these are frequently found associated with membrane-bound organelles such as endosomes and mitochondria. In this review we summarize recent findings that link organelle function and mRNA localization to mRNA decay. This article is part of a Special Issue entitled: RNA Decay mechanisms.     

核糖体蛋白质的新功能及其与相关疾病的关系

核糖体蛋白质与核糖体RNA共同组成了核糖体,是合成蛋白质的细胞器。除参与蛋白质合成,核糖体蛋白质还具有广泛的核糖体外功能,如独立于核糖体外发挥调控基因转录、mRNA翻译、细胞的增殖、分化和凋亡等等。基于诸多的核糖体外功能,核糖体蛋白质与人类疾病密切相关,例如在先天性贫血、生长发育不全和肿瘤的发生发展过程中均发挥重要作用。本文对近年来核糖体蛋白质的核糖体外新功能及其相关疾病的研究进展作一综述。     

Diverse patterns of poly(A) tail elongation and shortening of murine maternal mRNAs from fully grown oocyte to 2-cell embryo stages

We previously succeeded in constructing a cDNA library, CPF7, enriched with cDNA derived from maternal RNAs with the extended poly(A) tail in mouse fertilized eggs. In this study, we performed RNA blot analysis to examine the elongation in maternal RNAs using 20 representative clones isolated from CPF7 as probes. Various patterns of elongation, shortening, and/or degradation of maternal RNAs were observed from fully grown oocytes to early 2-cell embryos and could be roughly classified into three types and seven subtypes. These findings indicate that poly(A) elongation and shortening of maternal RNAs are not restricted to certain types of maternal RNAs but occur in many of them, and suggest a complex mechanism governing modification of the 3' end of maternal RNAs during the oocyte-to-embryo transition.