Maternal age effect on mouse oocytes: new biological insight from proteomic analysis

The long-standing view of 'immortal germ line versus mortal soma' poses a fundamental question in biology concerning how oocytes age in molecular terms. A mainstream hypothesis is that maternal aging of oocytes has its roots in gene transcription. Investigating the proteins resulting from mRNA translation would reveal how far the levels of functionally available proteins correlate with mRNAs, and would offer novel insight into the changes oocytes undergo during maternal aging. Gene ontology semantic analysis reveals the high similarity of the detected proteome (2,324 proteins) to the transcriptome (22,334 mRNAs), though not all proteins have a cognate mRNA. Concerning their dynamics, 4-fold changes of abundance are more frequent in the proteome (3%) than the transcriptome (0.05%), with correlation. Whereas proteins associated with the nucleus (e.g. structural maintenance of chromosomes, spindle-assembly checkpoints) are largely represented among those that change in oocytes during maternal aging; proteins associated with oxidative stress/damage (e.g. superoxide dismutase) are infrequent. These quantitative alterations are either impoverishing or enriching. Using gene ontology analysis, these alterations do not relate in any simple way to the classic signature of aging known from somatic tissues. We conclude that proteome analysis of mouse oocytes may not be surrogated with transcriptome analysis, given the lack of correlation. Furthermore, we conclude that the classic features of aging may not be transposed from somatic tissues to oocytes in a one-to-one fashion. Overall, there is more to the maternal aging of oocytes than mere cellular deterioration exemplified by the notorious increase of meiotic aneuploidy. Three pools of 20 zona-enclosed B6C3F1 oocytes from each age group were subjected for experiment.‘永生生殖系与必死体细胞’这一长期存在的生物学范式，提出了一个核心科学问题：卵母细胞的衰老在分子层面究竟如何运作？ 主流假说认为，卵母细胞的母体衰老根源在于基因转录过程。对mRNA翻译产生的蛋白质进行研究，能够揭示功能可及蛋白的水平与mRNA的关联程度，并为解析卵母细胞在母体衰老过程中发生的变化提供全新视角。 基因本体（Gene Ontology, GO）语义分析显示，本次检测到的蛋白质组（proteome）共包含2324种蛋白，与转录组（transcriptome）的22334条mRNA具有高度相似性，但并非所有蛋白都存在对应的同源mRNA。 就动态变化而言，蛋白质组中丰度出现4倍波动的事件占比（3%）显著高于转录组（0.05%），且二者存在一定相关性。 与细胞核相关的蛋白（例如染色体结构维持（structural maintenance of chromosomes）蛋白、纺锤体组装检验点（spindle-assembly checkpoints）相关蛋白）在母体衰老卵母细胞的差异变化蛋白中占比极高；而与氧化应激/损伤相关的蛋白（例如超氧化物歧化酶（superoxide dismutase））则较为少见。 这些定量变化要么表现为蛋白丰度降低，要么表现为丰度富集升高。通过基因本体分析发现，这些定量变化与体细胞组织中经典的衰老特征并无简单的直接对应关联。 我们的结论是：鉴于二者相关性有限，小鼠卵母细胞的蛋白质组分析无法通过转录组分析来替代。进一步研究表明，体细胞组织的经典衰老特征无法以一一对应的方式直接套用到卵母细胞的衰老过程中。 总体而言，卵母细胞的母体衰老过程并非仅体现为众所周知的减数分裂非整倍体（meiotic aneuploidy）增加所代表的细胞衰退。 本实验选取各年龄组小鼠的卵母细胞，每个年龄组设置3个样本池，每个样本池包含20枚被透明带包裹的B6C3F1品系卵母细胞，用于本次实验。