The Absence of Mitochondrial Thioredoxin 2 Causes Massive Apoptosis, Exencephaly, and Early Embryonic Lethality in Homozygous Mice

Thioredoxin 2 (Trx-2) is a small redox protein containing the thioredoxin active site Trp-Cys-Gly-Pro-Cys that is localized to the mitochondria by a mitochondrial leader sequence and encoded by a nuclear gene (Trx-2). Trx-2 plays an important role in cell viability and the regulation of apoptosis in vitro. To investigate the role of Trx-2 in mouse development, we studied the phenotype of mice that have the Trx-2 gene silenced by mutational insertion. Homozygous mutant embryos do not survive to birth and die after implantation at Theiler stage 15/16. The homozygous mutant embryos display an open anterior neural tube and show massively increased apoptosis at 10.5 days postcoitus and are not present by 12.5 days postcoitus. The timing of the embryonic lethality coincides with the maturation of the mitochondria, since they begin oxidative phosphorylation during this stage of embryogenesis. In addition, embryonic fibroblasts cultured from homozygous Trx-2-null embryos were not viable. Heterozygous mice are fertile and have no discernible phenotype visible by external observation, despite having decreased Trx-2 mRNA and protein. These results show that the mitochondrial redox protein Trx-2 is required for normal development of the mouse embryo and for actively respiring cells.硫氧还蛋白2（Thioredoxin 2，Trx-2）是一类小型氧化还原蛋白，包含硫氧还蛋白活性位点Trp-Cys-Gly-Pro-Cys，其通过线粒体前导序列定位于线粒体，由核基因（Trx-2）编码。Trx-2在体外对细胞存活性与细胞凋亡调控发挥重要作用。为探究Trx-2在小鼠发育过程中的功能，本研究针对通过突变插入使Trx-2基因沉默的小鼠表型展开分析。纯合突变胚胎无法存活至出生，于胚胎着床后在泰氏分期（Theiler stage）15/16阶段死亡。该纯合突变胚胎呈现开放性前神经管缺损，于交配后10.5天时出现大量细胞凋亡，且在交配后12.5天时已无存活胚胎。该胚胎致死的时间节点与线粒体成熟过程相吻合，因为在该胚胎发育阶段，线粒体开始进行氧化磷酸化。此外，从纯合Trx-2基因敲除胚胎中分离培养的胚胎成纤维细胞无法存活。尽管杂合子小鼠的Trx-2信使RNA（mRNA）与蛋白质（protein）表达水平降低，但它们可正常生育，且外观观察无明显异常表型。上述研究结果表明，线粒体氧化还原蛋白Trx-2是小鼠胚胎正常发育以及活跃呼吸细胞存活所必需的。