Transmission of reduced levels of miR-34/449 from sperm to preimplantation embryos is a key step in the transgenerational epigenetic inheritance of the effects of paternal chronic social instability stress

The transgenerational effects of exposing male mice to chronic social instability (CSI) stress are associated with decreased sperm levels of multiple members of the miR-34/449 family that persist after their mating through preimplantation embryo (PIE) development. Here we demonstrate the importance of these miRNA changes by showing that restoring miR-34c levels in PIEs derived from CSI stressed males prevents elevated anxiety and defective sociability normally found specifically in their adult female offspring. It also restores, at least partially, levels of sperm miR-34/449 normally reduced in their male offspring who transmit these sex-specific traits to their offspring. Strikingly, these experiments also revealed that inducing miR-34c levels in PIEs enhances the expression of its own gene and that of miR-449 in these cells. The same induction of embryo miR-34/449 gene expression likely occurs after sperm-derived miR-34c is introduced into oocytes upon fertilization. Thus, suppression of this miRNA amplification system when sperm miR-34c levels are reduced in CSI stressed mice can explain how a comparable fold-suppression of miR-34/449 levels can be found in PIEs derived from them, despite sperm containing ~50-fold lower levels of these miRNAs than those already present in PIEs. We previously found that men exposed to early life trauma also display reduced sperm levels of miR-34/449. And here we show that miR-34c can also increase the expression of its own gene, and that of miR-449 in human embryonic stem cells, suggesting that human PIEs derived from men with low sperm miR-34/449 levels may also contain this potentially harmful defect.

慢性社会不稳定（chronic social instability, CSI）应激暴露的雄性小鼠所产生的跨代效应，与miR-34/449家族多个成员的精子水平降低密切相关，且该水平降低现象在其交配后直至囊胚前胚胎（preimplantation embryo, PIE）发育阶段仍持续存在。本研究通过实验证实了这些miRNA变化的重要性：将源自CSI应激雄性小鼠的囊胚前胚胎中的miR-34c水平恢复后，可有效阻止仅在其成年雌性后代中出现的焦虑水平升高与社交能力缺陷。该干预手段还可部分恢复其雄性后代中原本降低的精子miR-34/449水平，而这些雄性后代会将这些性别特异性性状传递给自身子代。值得注意的是，本实验还发现，在囊胚前胚胎中诱导miR-34c水平上调，可增强该基因本身以及miR-449在这些细胞中的表达。同样的胚胎miR-34/449基因表达诱导过程，大概率发生于受精阶段精子来源的miR-34c被注入卵母细胞之后。因此，当CSI应激小鼠的精子miR-34/449水平降低时，该miRNA扩增系统受到抑制，这可以解释为何尽管这些小鼠精子中这类miRNA的水平比囊胚前胚胎中固有水平低约50倍，但其来源的囊胚前胚胎中miR-34/449水平仍出现了相当倍数的降低。我们此前曾发现，经历过早期生活创伤的男性，其精子中的miR-34/449水平同样降低。而本研究表明，miR-34c还可在人类胚胎干细胞中增强自身基因以及miR-449的表达，这提示源自精子miR-34/449水平较低男性的人类囊胚前胚胎，或许也存在这一潜在有害缺陷。