Selective binding of retrotransposons by ZFP352 facilitates the timely dissolution of totipotency network

Lysosomal catabolic activity promotes the exit of murine totipotent 2-cell state by silencing early-embryonic retrotransposons

During mouse preimplantation development, a subset of retrotransposons/genes are transiently expressed in the totipotent 2-cell (2C) embryos. These 2C transcripts rapidly shut down their expression beyond the 2C stage of embryos, promoting the embryo to exit from the 2C stage. However, the mechanisms regulating this shutdown remain unclear. Here, we identified that lysosomal catabolism played a role in the exit of the totipotent 2C state. Our results showed that the activation of embryonic lysosomal catabolism promoted the embryo to exit from the 2C stage and suppressed 2C transcript expression. Mechanistically, our results indicated that lysosomal catabolism suppressed 2C transcripts through replenishing cellular amino-acid levels, thereby inactivating transcriptional factors TFE3/TFEB and abolishing their transcriptional activation of 2C retrotransposons, MERVL (murine endogenous retrovirus-L)/MT2_Mm. Collectively, our study identified that lysosomal activity modulated the transcriptomic landscape and development in mouse embryos and identified an unanticipated layer of transcriptional control on early-embryonic retrotransposons from lysosomal signaling.

Expression of Transposable Elements throughout the Fasciola hepatica Trematode Life Cycle

BACKGROUND

Transposable elements (TEs) are major components of eukaryotic genomes. The extensive body of evidence suggests that although they were once considered "genomic parasites", transposons and their transcripts perform specific functions, such as regulation of early embryo development. Understanding the role of TEs in such parasites as trematodes is becoming critically important. Fasciola hepatica, a parasite affecting humans and livestock, undergoes a complex life cycle in diverse environments and hosts, and knowledge about its life cycle regulation is scarce so far.

METHODS

We summarized the data regarding the repetitive elements in F. hepatica and conducted bulk RNA-seq analysis across its life cycle stages. TE expression profiles were analyzed, focusing on differential expression and potential homology with previously described long non-coding RNAs (lncRNAs).

RESULTS

Differential expression analysis revealed stage-specific TE transcription patterns, notably peaking during egg and metacercariae stages. Some TEs showed homology with known lncRNAs and contained putative transcription factor binding sites. Interestingly, TE transcription levels were highest in eggs and metacercariae compared to adults, suggesting regulatory roles in trematode life cycle transitions.

CONCLUSIONS

These findings suggest that TEs may play roles in regulating trematode life cycle transitions. Moreover, TE homology with lncRNAs underscores their significance in gene regulation.

Epigenetic reprogramming in the transition from pluripotency to totipotency

Mammalian development commences with the zygote, which can differentiate into both embryonic and extraembryonic tissues, a capability known as totipotency. Only the zygote and embryos around zygotic genome activation (ZGA) (two-cell embryo stage in mice and eight-cell embryo in humans) are totipotent cells. Epigenetic modifications undergo extremely extensive changes during the acquisition of totipotency and subsequent development of differentiation. However, the underlying molecular mechanisms remain elusive. Recently, the discovery of mouse two-cell embryo-like cells, human eight-cell embryo-like cells, extended pluripotent stem cells and totipotent-like stem cells with extra-embryonic developmental potential has greatly expanded our understanding of totipotency. Experiments with these in vitro models have led to insights into epigenetic changes in the reprogramming of pluri-to-totipotency, which have informed the exploration of preimplantation development. In this review, we highlight the recent findings in understanding the mechanisms of epigenetic remodeling during totipotency capture, including RNA splicing, DNA methylation, chromatin configuration, histone modifications, and nuclear organization.

Transition from totipotency to pluripotency in mice: insights into molecular mechanisms

Totipotency is the ability of a single cell to develop into a full organism and, in mammals, is strictly associated with the early stages of development following fertilization. This unlimited developmental potential becomes quickly restricted as embryonic cells transition into a pluripotent state. The loss of totipotency seems a consequence of the zygotic genome activation (ZGA), a process that determines the switch from maternal to embryonic transcription, which in mice takes place following the first cleavage. ZGA confers to the totipotent cell a transient transcriptional profile characterized by the expression of stage-specific genes and a set of transposable elements that prepares the embryo for subsequent development. The timely silencing of this transcriptional program during the exit from totipotency is required to ensure proper development. Importantly, the molecular mechanisms regulating the transition from totipotency to pluripotency have remained elusive due to the scarcity of embryonic material. However, the development of new in vitro totipotent-like models together with advances in low-input genome-wide technologies, are providing a better mechanistic understanding of how this important transition is achieved. This review summarizes the current knowledge on the molecular determinants that regulate the exit from totipotency.

The role of Zfp352 in the regulation of transient expression of 2-cell specific genes in mouse embryonic stem cells

Mouse ES cell populations contain a minor sub-population that expresses genes specifically expressed in 2-cell stage embryos. This sub-population consists of 2-cell-gene labeled cells (2CLCs) generated by the transient activation of the 2-cell specific genes initiated by the master regulator, Dux. However, the mechanism regulating the transient expression remains largely unclear. Here we reported a novel function of Zfp352, one of the 2-cell specific genes, in regulating the 2CLC sub-population. Zfp352 encodes zinc-finger transcription factor belonging to the Klf family. Dux transiently activates Zfp352 after the activation of Zscan4c in a subset of the 2CLC subpopulation. Interestingly, in the reporter assay, the transcriptional activation of Zscan4c by Dux is strongly repressed by the co-expression of Zfp352. However, the knockout of Zfp352 resulted in the repression of a subset of the 2-cell-specific genes. These data suggest the dual roles of Zfp352 in regulating the transient activation of the 2-cell-specific genes.