真核mRNA的3′非翻译区转录后水平调控作用研究进展

真核生物mRNA的3′非翻译区(3′-UTR)在基因表达的转录后调控中起着重要作用:3′-UTR内存在末端加工信号以指导mRNA3′末端的加工;3′-UTR不但控制mRNA的稳定性及降解速率、协助辨认特殊密码子,而且还控制着mRNA的翻译时间、位点及控制其翻译起始及效率等。     

真核mRNA的3′非翻译区转录后水平调控作用研究进展

真核生物mRNA的3′非翻译区(3′_UTR)在基因表达的转录后调控中起着重要作用:3′_UTR内存在末端加工信号以指导mRNA3′末端的加工;3′_UTR不但控制mRNA的稳定性及降解速率、协助辨认特殊密码子,而且还控制着mRNA的翻译时间、位点及控制其翻译起始及效率等。     

Toll-like receptor 3 signaling enables human esophageal epithelial cells to sense endogenous danger signals released by necrotic cells

The mechanisms by which gastroesophageal reflux disease esophagitis develops are controversial. Although many support the notion that caustic injury leads to reflux esophagitis, others have proposed that reflux esophagitis is caused by esophageal epithelial cytokine-mediated inflammation. We previously demonstrated that Toll-like receptor 3 (TLR3) is highly expressed and functional in the nontransformed human esophageal epithelial cell line EPC2-hTERT. In addition to activation by viral double-stranded RNA, TLR3 can be activated by endogenous mRNA released by necrotic cells. In the present study, we investigated the role of esophageal epithelial TLR3 to sense danger signals released by necrotic esophageal epithelial cells in vitro. Following induction of freeze-thaw necrosis, necrotic EPC2-hTERT cell supernatants (NCS) were used to stimulate EPC2-hTERT monolayers, leading to NF-κB-dependent induction of IL-8 mRNA expression. Responses to self-derived NCS were not observed in transformed gastrointestinal epithelial cell lines, including TE-1 and Caco-2 cells, suggesting that the ability to sense endogenous danger signals is unique to nontransformed esophageal epithelial cells. To determine the immunostimulatory role of epithelial RNA, EPC2-hTERT cells were stimulated with self-derived mRNA, which significantly induced IL-8 mRNA expression. Finally, suppression of TLR3 signaling in a DN-TLR3 cell line, hTERT-ΔTIR-TLR3, led to reduced NCS-induced IL-8 induction by both NCS and mRNA stimulation. Our results demonstrate that human esophageal epithelial cells can sense endogenous danger signals, in part through TLR3 signaling. This supports the concept that epithelial injury plays an inciting role in the pathogenesis of reflux-induced esophagitis, providing important insights into the mechanisms by which epithelial injury leads to inflammation.     

Splicing promotes the nuclear export of β-globin mRNA by overcoming nuclear retention elements

Most current models of mRNA nuclear export in vertebrate cells assume that an mRNA must have specialized signals in order to be exported from the nucleus. Under such a scenario, mRNAs that lack these specialized signals would be shunted into a default pathway where they are retained in the nucleus and eventually degraded. These ideas were based on the selective use of model mRNA reporters. For example, it has been shown that splicing promotes the nuclear export of certain model mRNAs, such as human β-globin, and that in the absence of splicing, the cDNA-derived mRNA is retained in the nucleus and degraded. Here we provide evidence that β-globin mRNA contains an element that actively retains it in the nucleus and degrades it. Interestingly, this nuclear retention activity can be overcome by increasing the length of the mRNA or by splicing. Our results suggest that contrary to many current models, the default pathway for most intronless RNAs is to be exported from the nucleus, unless the RNA contains elements that actively promote its nuclear retention.     

Homologous mRNA 3'' end formation in fission and budding yeast.

Sequences resembling polyadenylation signals of higher eukaryotes are present downstream of the Schizosaccharomyces pombe ura4+ and cdc10+ coding regions and function in HeLa cells. However, these and other mammalian polyadenylation signals are inactive in S. pombe. Instead, we find that polyadenylation signals of the CYC1 gene of budding yeast Saccharomyces cerevisiae function accurately and efficiently in fission yeast. Furthermore, a 38 bp deletion which renders this RNA processing signal non-functional in S. cerevisiae has the equivalent effect in S. pombe. We demonstrate that synthetic pre-mRNAs encoding polyadenylation sites of S. pombe genes are accurately cleaved and polyadenylated in whole cell extracts of S. cerevisiae. Finally, as is the case in S. cerevisiae, DNA sequences encoding regions proximal to the S. pombe mRNA 3' ends are found to be extremely AT rich; however, no general sequence motif can be found. We conclude that although fission yeast has many genetic features in common with higher eukaryotes, mRNA 3' end formation is significantly different and appears to be formed by an RNA processing mechanism homologous to that of budding yeast. Since fission and budding yeast are evolutionarily divergent, this lower eukaryotic mechanism of mRNA 3' end formation may be generally conserved.     

Developmental timing of mRNA translation—integration of distinct regulatory elements

Targeted mRNA translation is emerging as a critical mechanism to control gene expression during developmental processes. Exciting new findings have revealed a critical role for regulatory elements within the mRNA untranslated regions to direct the timing of mRNA translation. Regulatory elements can be targeted by sequence‐specific binding proteins to direct either repression or activation of mRNA translation in response to developmental signals. As new regulatory elements continue to be identified it has become clear that targeted mRNAs can contain multiple regulatory elements, directing apparently contradictory translational patterns. How is this complex regulatory input integrated? In this review, we focus on a new challenge area—how sequence‐specific RNA binding proteins respond to developmental signals and functionally integrate to regulate the extent and timing of target mRNA translation. We discuss current understanding with a particular emphasis on the control of cell cycle progression that is mediated through a complex interplay of distinct mRNA regulatory elements during Xenopus oocyte maturation. Mol. Reprod. Dev. 77: 662–669, 2010. © 2010 Wiley‐Liss, Inc.     

Herpes simplex virus ICP27 is required for virus-induced stabilization of the ARE-containing IEX-1 mRNA encoded by the human IER3 gene

Herpes simplex virus (HSV) stifles cellular gene expression during productive infection of permissive cells, thereby diminishing host responses to infection. Host shutoff is achieved largely through the complementary actions of two viral proteins, ICP27 and virion host shutoff (vhs), that inhibit cellular mRNA biogenesis and trigger global mRNA decay, respectively. Although most cellular mRNAs are thus depleted, some instead increase in abundance after infection; perhaps surprisingly, some of these contain AU-rich instability elements (AREs) in their 3'-untranslated regions. ARE-containing mRNAs normally undergo rapid decay; however, their stability can increase in response to signals such as cytokines and virus infection that activate the p38/MK2 mitogen-activated protein kinase (MAPK) pathway. We and others have shown that HSV infection stabilizes the ARE mRNA encoding the stress-inducible IEX-1 mRNA, and a previous report from another laboratory has suggested vhs is responsible for this effect. However, we now report that ICP27 is essential for IEX-1 mRNA stabilization whereas vhs plays little if any role. A recent report has documented that ICP27 activates the p38 MAPK pathway, and we detected a strong correlation between this activity and stabilization of IEX-1 mRNA by using a panel of HSV type 1 (HSV-1) isolates bearing an array of previously characterized ICP27 mutations. Furthermore, IEX-1 mRNA stabilization was abrogated by the p38 inhibitor SB203580. Taken together, these data indicate that the HSV-1 immediate-early protein ICP27 alters turnover of the ARE-containing message IEX-1 by activating p38. As many ARE mRNAs encode proinflammatory cytokines or other immediate-early response proteins, some of which may limit viral replication, it will be of great interest to determine if ICP27 mediates stabilization of many or all ARE-containing mRNAs.     

Both action potentials and variation potentials induce proteinase inhibitor gene expression in tomato

Tomato plants (Lycopersicon esculentum) accumulate proteinase inhibitor 2 (pin2) mRNA in response to insect attack, crushing and flaming in leaves distant from those treated. Most earlier work suggests that the systemic wound signals are chemical; here we try to determine whether electrical or physical (hydraulic) signals can also evoke pin expression. We used a mild flame to evoke a systemic hydraulic signal and its local electrical aftermath, the variation potential (VP), and we used an electric stimulus to evoke a systemic electrical signal, the action potential (AP). We determined the kinetic parameters of both the VP and AP. Flame-wounded plants essentially always exhibited major electrical responses throughout the plant and a several-fold increase in pin2 mRNA within 1 h. Electrically stimulated plants that generated and transmitted a signal (AP) into the analyzed leaf exhibited similarly large, rapid increases in pin2 mRNA levels. Plants which generated no signal, or signals of just a few microvolts, had unchanged levels of pin2 mRNA. Since the AP and VP both arrived in the receiving leaf before accumulation of pin2 mRNA began, we conclude that, in addition to the previously shown chemical signals, both hydraulically induced VPs and electrically induced APs are capable of evoking pin2 gene expression.     

Sex determination in plants

Sex determination is an important developmental event in the life cycle of all sexually reproducing plants. Recent studies of sex determination in many plant species, from ferns to maize, have been fruitful in identifying the diversity of genetic and epigenetic factors that are involved in determining the sex of the flower or individual. In those species amenable to genetic analysis, significant progress has been made toward identifying mutations that affect sex expression. By studying the interactions among these genes, pictures of how sex-determining signals are perceived to activate or repress male- or female-specific genes are emerging.     

Linking nuclear mRNP assembly and cytoplasmic destiny

From the very beginning, mRNAs have a complex existence. They are transcribed, capped, spliced, modified at the 3'end, exported from the nucleus, translated, and eventually degraded. These many events not only affect the overall survival and properties of an mRNA, but are also carefully co-ordinated and integrated with quality control mechanisms that function to ensure that only 'proper' mRNAs are translated at the correct developmental time and place. This does not mean that all mRNAs follow a single or uniform path from synthesis to death. Instead, there are diverse means by which the activities of specific mRNAs are regulated, and these controls often depend upon multiple events in the mRNA's life. mRNAs are not found naked in the cell, instead they are part of complex RNPs (ribonucleoproteins) that consist of many factors. These RNPs are highly dynamic structures that change during the lifetime of a given RNA; linking events such as synthesis and processing to the final fate of the mRNA. Here, we will discuss what is known of the assembly of RNPs in general, with specific reference to the myriad of connections between different nuclear events and the cytoplasmic activity of an mRNA. Due to space limitations this review is not comprehensive, instead we focus on specific examples to illustrate these emerging themes in gene expression.     

CELFish ways to modulate mRNA decay

The CELF family of RNA-binding proteins regulates many steps of mRNA metabolism. Although their best characterized function is in pre-mRNA splice site choice, CELF family members are also powerful modulators of mRNA decay. In this review we focus on the different modes of regulation that CELF proteins employ to mediate mRNA decay by binding to GU-rich elements. After starting with an overview of the importance of CELF proteins during development and disease pathogenesis, we then review the mRNA networks and cellular pathways these proteins regulate and the mechanisms by which they influence mRNA decay. Finally, we discuss how CELF protein activity is modulated during development and in response to cellular signals. We conclude by highlighting the priorities for new experiments in this field. This article is part of a Special Issue entitled: RNA Decay mechanisms.