A human-specific regulatory mechanism revealed in a preimplantation model

Stem cell-based human embryo models offer a singular opportunity for functional studies of the species-specific features of development. Some of these aspects are likely mediated by transposable elements, such as HERVK LTR5Hs, a hominoid-specific endogenous retrovirus highly expressed in human blastocysts. Here, we pioneered genetic and epigenetic manipulation of human blastoids, a 3D embryo model of the blastocyst, to investigate the functional impact of HERVK LTR5Hs on preimplantation development. We demonstrate an essential role of this retrotransposon in blastoid formation and lineage identity. We further uncover a pervasive cis-regulatory contribution of LTR5Hs elements to the hominoid-specific diversification of the epiblast transcriptome. Since HERVK remained mobile following the divergence of humans and chimpanzees, many of the nearly 700 LTR5Hs genomic insertions are unique to our own species. We show that at least one such human-specific LTR5Hs element is essential for the blastoid-forming potential of naive embryonic stem cells by regulating the expression of an evolutionary young gene unique to the Old World anthropoids. Thus, species-specific transposable elements and genes can confer developmentally essential functions. Overall design: H3K9me3 CUT&RUN (Cleavage Under Targets & Release Using Nuclease) in nontarg-CARGO and LTR5Hs-CARGO human naive pluripotent stem cells (hnPSCs)

基于干细胞的人类胚胎模型，为研究发育过程中的物种特异性特征提供了独一无二的研究契机。其中部分特征很可能由转座因子（transposable elements）介导，例如HERVK LTR5Hs——一种在人类囊胚中高表达的类人猿特异性内源性逆转录病毒。本研究率先对人类囊胚样结构（blastoids）——一种模拟囊胚的三维胚胎模型——开展遗传与表观遗传操控，以探究HERVK LTR5Hs对植入前发育的功能影响。本研究证实了该逆转录转座子（retrotransposon）在囊胚样结构形成与细胞谱系身份维持中的关键作用。本研究进一步揭示，LTR5Hs元件对上胚层转录组的类人猿特异性多样化具有广泛的顺式调控作用。由于HERVK在人类与黑猩猩分化后仍保持转座活性，近700个LTR5Hs基因组插入位点中有许多为人类所特有。本研究证实，至少有一个此类人类特异性LTR5Hs元件，通过调控一种仅存在于旧世界猿猴类的演化年轻基因的表达，对幼稚态胚胎干细胞（naive embryonic stem cells）的囊胚样结构形成潜能至关重要。由此可见，物种特异性转座因子与基因可赋予发育过程中的关键功能。实验整体设计：在非靶向CARGO（nontarg-CARGO）与LTR5Hs靶向CARGO人类幼稚态多能干细胞（hnPSCs，human naive pluripotent stem cells）中开展H3K9me3 CUT&RUN实验（全称：靶向切割与核酸酶释放技术，Cleavage Under Targets & Release Using Nuclease）。