癌症与可变剪接

可变剪接在发育、分化和癌症等过程中发挥着非常重要的作用。近年来,越来越多的研究表明可变剪接与癌症有着密切的关系,许多癌症相关基因受可变剪接调控。由于癌症特异性的剪接变体具有明显的诊断价值,使得对癌症与可变剪接的研究成为热点。简要概述了癌症相关基因的可变剪接、可变剪接变体的鉴定方法、可变剪接与癌症治疗等研究进展。     

mRNA前体的选择性剪接

人类基因组草图已基本完成 ,预测人类约有 350 0 0个编码蛋白质的基因 ,只是线虫或果蝇的 2倍[1] 。人类是怎样完成其复杂的生物功能 ?越来越多的证据表明选择性剪接在扩大蛋白质的多样性中发挥重要作用 ,并且有助于解释基因数目与生物复杂程度两者的不一致性。选择性剪接能够从一个基因产生多个转录本 ,从而产生远多于基因数目的蛋白质 ,完成机体的复杂功能及精细调节。1 .ｍＲＮＡ选择性剪接的普遍性目前根据ＥＳＴｓ分析 ,在人类 350 0 0个基因中大约有 40 %的基因具有选择性剪接的形式[1] 。尽管这个数目让人惊讶 ,但这个数目可能比实际…     

Mechano-Regulation of Alternative Splicing

Alternative splicing contributes to the complexity of proteome by producing multiple mRNAs from a single gene. Affymetrix exon arrays and experiments in vivo or in vitro demonstrated that alternative splicing was regulated by mechanical stress. Expression of mechano-growth factor (MGF) which is the splicing isoform of insulin-like growth factor 1(IGF-1) and vascular endothelial growth factor (VEGF) splicing variants such as VEGF₁₂₁, VEGF_165, VEGF₂₀₆, VEGF₁₈₉, VEGF₁₆₅ and VEGF₁₄₅ are regulated by mechanical stress. However, the mechanism of this process is not yet clear. Increasing evidences showed that the possible mechanism is related to Ca²⁺ signal pathway and phosphorylation signal pathway. This review proposes possible mechanisms of mechanical splicing regulation. This will contribute to the biomechanical study of alternative splicing.     

Multiple Pathways for Apoptotic Nuclear Fragmentation

We analyzed the ultrastructure of apoptotic nuclear fragmentation in U937 cells treated with many different apoptogenic agents. We found that this characteristic apoptotic feature can be achieved through multiple alternative pathways, depending on the apoptogenic inducer, leading to slightly different final nuclear morphologies. In most instances, the irregularly shaped nucleus of U937 rounds up; then, chromatin condenses at the nuclear periphery. Condensed chromatin can form protruding patches, which eventually bud from the nucleus in sealed vesicles through a process which is actin-dependent, since it could be blocked by cytochalasins. Alternatively, chromatin condenses in tiny, nonprotruding crescents, and a cleavage in the nuclear sap forms, beginning from the inner nuclear membrane and growing inward, thus splitting the nucleus. In U937 induced to apoptosis by hydrogen peroxide in the presence of ADP-ribosylation inhibitors, the nuclei fragment in many vesicles before chromatin even begins to condense: chromatin condensation probably occurs as a consequence. While all the apoptotic morphologies described above evolve from interphase cells, a peculiar apoptotic morphology, possibly deriving from mitotic cells, is detected upon oxidative stress, recalling the formation of micronuclei by clastogenic treatments; it shows partially membrane-bound chromatin patches, which look midway between condensed chromosomes and apoptotic condensed chromatin. The existence of these multiple pathways for nuclear fragmentation may indicate an evolutionary convergence, suggesting that this event may play an important physiological role in apoptosis.     

Region-Specific Interrelations between Apoptotic Proteins Expression and DNA Fragmentation in the Neonatal Rat Brain

DNA fragmentation, mRNA and protein levels of Bcl-XL, Bax and caspase-3 were determined to characterize interrelations between expression of these apoptotic markers in the neonatal brain regions. High DNA fragmentation intensity in the cortex was in consonance with the lowest Bcl-XL/Bax expression ratio, the highest procaspase-3 and active caspase-3 levels. Low and intermediate DNA fragmentation levels in the cerebellum and hippocampus respectively were also in a good agreement with apoptotic proteins expression in these structures. In the cortex, hippocampus and cerebellum DNA fragmentation intensity was proportional to the active caspase-3 level. In contrast to these structures, in the brainstem, the lowest level of this protease was accompanied by the highest intensity of DNA fragmentation among the brain regions studied. The data suggest that cell death normally occurring during early postnatal life could be realized in the developing brainstem via caspase-3-independent pathways in animals that express this protease.     

Light Regulates Plant Alternative Splicing through the Control of Transcriptional Elongation

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寻找mRNA的选择性剪接

在真核生物的基因中,mRNA选择性剪接现象十分普遍。mRNA选择性剪接导致一个基因多转录本的产生,被认为是高等生物增加蛋白质多样性的主要机制,且已发现与许多人类疾病密切相关。发现这些转录本的选择性剪接位点、新的外显子和外显子组合,乃至获得这些剪接变异体的完整克隆,对于基因功能的深入研究十分必要。简要介绍了几种在mRNA水平探索选择性剪接的方法。     

Origin of Spliceosomal Introns and Alternative Splicing

In this work we review the current knowledge on the prehistory, origins, and evolution of spliceosomal introns. First, we briefly outline the major features of the different types of introns, with particular emphasis on the nonspliceosomal self-splicing group II introns, which are widely thought to be the ancestors of spliceosomal introns. Next, we discuss the main scenarios proposed for the origin and proliferation of spliceosomal introns, an event intimately linked to eukaryogenesis. We then summarize the evidence that suggests that the last eukaryotic common ancestor (LECA) had remarkably high intron densities and many associated characteristics resembling modern intron-rich genomes. From this intron-rich LECA, the different eukaryotic lineages have taken very distinct evolutionary paths leading to profoundly diverged modern genome structures. Finally, we discuss the origins of alternative splicing and the qualitative differences in alternative splicing forms and functions across lineages.     

mRNA选择性剪接的分子机制

真核细胞mRNA前体经过剪接成为成熟的mRNA,而mRNA前体的选择性剪接极大地增加了蛋白质的多样性和基因表达的复杂程度,剪接位点的识别可以以跨越内含子的机制(内含子限定)或跨越外显子的机制(外显子限定)进行。选择性剪接有多种剪接形式：选择不同的剪接位点,选择不同的剪接末端,外显子的不同组合及内含子的剪接与否等。选择性剪接过程受到许多顺式元件和反式因子的调控,并与基本剪接过程紧密联系,剪接体中的一些剪接因子也参与了对选择性剪接的调控。选择性剪接也是1个伴随转录发生的过程,不同的启动子可调控产生不同的剪接产物。mRNA的选择性剪接机制多种多样,已发现RNA编辑和反式剪接也可参与选择性剪接过程。     

细胞信号转导与可变剪接调控

大多数真核基因能够发生可变剪接,其调控对于生理和病理状态下细胞功能的实现至关重要,而异常可变剪接则可导致多种疾病。虽然已知可变剪接能够在转录后水平调节基因表达,然而目前仍不清楚特定的可变剪接模式是如何被调控的。越来越多的研究发现细胞信号和外界环境刺激能够调控靶基因的剪接模式,并且已发现一些与可变剪接调控有关的信号转导通路,而后者能够通过修饰剪接因子进而改变剪接因子的亚细胞定位或者活性,从而实现对靶基因可变剪接模式的调控。由细胞信号转导通路所构成的网络能够灵活多样地调控基因剪接,一条信号通路可调控多个基因剪接,而多条信号通路也可调控同一基因剪接,对于理解信号转导过程的分子机制具有重要意义。     

Evolution of Alternative Splicing Regulation: Changes in Predicted Exonic Splicing Regulators Are Not Associated with Changes in Alternative Splicing Levels in Primates

Alternative splicing is tightly regulated in a spatio-temporal and quantitative manner. This regulation is achieved by a complex interplay between spliceosomal (trans) factors that bind to different sequence (cis) elements. cis-elements reside in both introns and exons and may either enhance or silence splicing. Differential combinations of cis-elements allows for a huge diversity of overall splicing signals, together comprising a complex ‘splicing code’. Many cis-elements have been identified, and their effects on exon inclusion levels demonstrated in reporter systems. However, the impact of interspecific differences in these elements on the evolution of alternative splicing levels has not yet been investigated at genomic level. Here we study the effect of interspecific differences in predicted exonic splicing regulators (ESRs) on exon inclusion levels in human and chimpanzee. For this purpose, we compiled and studied comprehensive datasets of predicted ESRs, identified by several computational and experimental approaches, as well as microarray data for changes in alternative splicing levels between human and chimpanzee. Surprisingly, we found no association between changes in predicted ESRs and changes in alternative splicing levels. This observation holds across different ESR exon positions, exon lengths, and 5′ splice site strengths. We suggest that this lack of association is mainly due to the great importance of context for ESR functionality: many ESR-like motifs in primates may have little or no effect on splicing, and thus interspecific changes at short-time scales may primarily occur in these effectively neutral ESRs. These results underscore the difficulties of using current computational ESR prediction algorithms to identify truly functionally important motifs, and provide a cautionary tale for studies of the effect of SNPs on splicing in human disease.