Premature translation of the Drosophila zygotic genome activator Zelda is not sufficient to precociously activate gene expression

Rise and SINE: roles of transcription factors and retrotransposons in zygotic genome activation

In sexually reproducing organisms, life begins with the fusion of transcriptionally silent gametes, the oocyte and sperm. Although initiation of transcription in the embryo, known as zygotic genome activation (ZGA), is universally required for development, the transcription factors regulating this process are poorly conserved. In this Perspective, we discuss recent insights into the mechanisms of ZGA in totipotent mammalian embryos, namely ZGA regulation by several transcription factors, including by orphan nuclear receptors (OrphNRs) such as the pioneer transcription factor NR5A2, and by factors of the DUX, TPRX and OBOX families. We performed a meta-analysis and compiled a list of pan-ZGA genes, and found that most of these genes are indeed targets of the above transcription factors. Remarkably, more than a third of these ZGA genes appear to be regulated both by OrphNRs such as NR5A2 and by OBOX proteins, whose motifs co-occur in SINE B1 retrotransposable elements, which are enriched near ZGA genes. We propose that ZGA in mice is activated by recruitment of multiple transcription factors to SINE B1 elements that function as enhancers, and discuss a potential relevance of this mechanism to Alu retrotransposable elements in human ZGA.

mRNA decay pre-complex assembly drives timely cell-state transitions during differentiation

Complexes that control mRNA stability and translation promote timely cell-state transitions during differentiation by ensuring appropriate expression patterns of key developmental regulators. The Drosophila RNA-binding protein brain tumor (Brat) promotes the degradation of target transcripts during the maternal-to-zygotic transition in syncytial embryos and uncommitted intermediate neural progenitors (immature INPs). We identify ubiquitin-specific protease 5 (Usp5) as a candidate Brat interactor essential for the degradation of Brat target mRNAs. Usp5 promotes the formation of the Brat-deadenylase pre-complex in mitotic neural stem cells (neuroblasts) by facilitating Brat interactions with the scaffolding components of deadenylase complexes. The adaptor protein Miranda binds the RNA-binding domain of Brat, limiting its ability to bind target mRNAs in mitotic neuroblasts. Cortical displacement of Miranda activates Brat-deadenylase complex activity in immature INPs. We propose that the assembly of an enzymatically inactive and RNA-binding-deficient pre-complex poises mRNA degradation machineries for rapid activation, driving timely developmental transitions.

The maternal-to-zygotic transition: reprogramming of the cytoplasm and nucleus

A fertilized egg is initially transcriptionally silent and relies on maternally provided factors to initiate development. For embryonic development to proceed, the oocyte-inherited cytoplasm and the nuclear chromatin need to be reprogrammed to create a permissive environment for zygotic genome activation (ZGA). During this maternal-to-zygotic transition (MZT), which is conserved in metazoans, transient totipotency is induced and zygotic transcription is initiated to form the blueprint for future development. Recent technological advances have enhanced our understanding of MZT regulation, revealing common themes across species and leading to new fundamental insights about transcription, mRNA decay and translation.

Kick-starting the zygotic genome: licensors, specifiers, and beyond

Zygotic genome activation (ZGA), the first transcription event following fertilization, kickstarts the embryonic program that takes over the control of early development from the maternal products. How ZGA occurs, especially in mammals, is poorly understood due to the limited amount of research materials. With the rapid development of single-cell and low-input technologies, remarkable progress made in the past decade has unveiled dramatic transitions of the epigenomes, transcriptomes, proteomes, and metabolomes associated with ZGA. Moreover, functional investigations are yielding insights into the key regulators of ZGA, among which two major classes of players are emerging: licensors and specifiers. Licensors would control the permission of transcription and its timing during ZGA. Accumulating evidence suggests that such licensors of ZGA include regulators of the transcription apparatus and nuclear gatekeepers. Specifiers would instruct the activation of specific genes during ZGA. These specifiers include key transcription factors present at this stage, often facilitated by epigenetic regulators. Based on data primarily from mammals but also results from other species, we discuss in this review how recent research sheds light on the molecular regulation of ZGA and its executors, including the licensors and specifiers.

Muskelin acts as a substrate receptor of the highly regulated Drosophila CTLH E3 ligase during the maternal-to-zygotic transition

The maternal-to-zygotic transition (MZT) is a conserved developmental process where the maternally-derived protein and mRNA cache is replaced with newly made zygotic gene products. We have previously shown that in Drosophila the deposited RNA-binding proteins ME31B, Cup, and Trailer Hitch (TRAL) are ubiquitylated by the CTLH E3 ligase and cleared. However, the organization and regulation of the CTLH complex remain poorly understood in flies. In particular, Drosophila lacks an identifiable substrate adaptor, and the mechanisms restricting degradation of ME31B and its cofactors to the MZT are unknown. Here, we show that the developmental specificity of the CTLH complex is mediated by multipronged regulation, including transcriptional control by the transcription factor OVO and autoinhibition of the E3 ligase. One major regulatory target is the subunit Muskelin, which we demonstrate acts as a substrate adaptor for the Drosophila CTLH complex. Although conserved, Muskelin has structural roles in other species, suggesting a surprising functional plasticity. Finally, we find that Muskelin has few targets beyond the three known RNA binding proteins, showing exquisite target specificity. Thus, multiple levels of integrated regulation restrict the activity of the embryonic CTLH complex to early embryogenesis, seemingly with the goal of regulating three important RNA binding proteins.

Post-transcriptional repression of CFP-1 expands the regulatory repertoire of LIN-41/TRIM71

The Caenorhabditis elegans LIN-41/TRIM71 is a well-studied example of a versatile regulator of mRNA fate, which plays different biological functions involving distinct post-transcriptional mechanisms. In the soma, LIN-41 determines the timing of developmental transitions between larval stages. The somatic LIN-41 recognizes specific mRNAs via LREs (LIN-41 Recognition Elements) and elicits either mRNA decay or translational repression. In the germline, LIN-41 controls the oocyte-to-embryo transition (OET), although the relevant targets and regulatory mechanisms are poorly understood. The germline LIN-41 was suggested to regulate mRNAs indirectly by associating with another RNA-binding protein. We show here that LIN-41 can also regulate germline mRNAs via the LREs. Through a computational-experimental analysis, we identified the germline mRNAs potentially controlled via LREs and validated one target, the cfp-1 mRNA, encoding a conserved chromatin modifier. Our analysis suggests that cfp-1 may be a long-sought target whose LIN-41-mediated regulation during OET facilitates the transcriptional reprogramming underlying the switch from germ- to somatic cell identity.

Chromatin accessibility in the Drosophila embryo is determined by transcription factor pioneering and enhancer activation

Chromatin accessibility is integral to the process by which transcription factors (TFs) read out cis-regulatory DNA sequences, but it is difficult to differentiate between TFs that drive accessibility and those that do not. Deep learning models that learn complex sequence rules provide an unprecedented opportunity to dissect this problem. Using zygotic genome activation in Drosophila as a model, we analyzed high-resolution TF binding and chromatin accessibility data with interpretable deep learning and performed genetic validation experiments. We identify a hierarchical relationship between the pioneer TF Zelda and the TFs involved in axis patterning. Zelda consistently pioneers chromatin accessibility proportional to motif affinity, whereas patterning TFs augment chromatin accessibility in sequence contexts where they mediate enhancer activation. We conclude that chromatin accessibility occurs in two tiers: one through pioneering, which makes enhancers accessible but not necessarily active, and the second when the correct combination of TFs leads to enhancer activation.

Setting the stage for development: the maternal-to-zygotic transition in Drosophila

The zygote has a daunting task ahead of itself; it must develop from a single cell (fertilized egg) into a fully functioning adult with a multitude of different cell types. In the beginning, the zygote has help from its mother, in the form of gene products deposited into the egg, but eventually, it must rely on its own resources to proceed through development. The transfer of developmental control from the mother to the embryo is called the maternal-to-zygotic transition (MZT). All animals undergo this transition, which is defined by two main processes-the degradation of maternal RNAs and the synthesis of new RNAs from the zygote's own genome. Here, we review the regulation of the MZT in Drosophila, but given the broad conservation of this essential process, much of the regulation is shared among metazoans.

Intron dynamics reveal principles of gene regulation during the maternal-to-zygotic transition

The maternal-to-zygotic transition (MZT) is a conserved embryonic process in animals where developmental control shifts from the maternal to zygotic genome. A key step in this transition is zygotic transcription, and deciphering the MZT requires classifying newly transcribed genes. However, due to current technological limitations, this starting point remains a challenge for studying many species. Here, we present an alternative approach that characterizes transcriptome changes based solely on RNA-seq data. By combining intron-mapping reads and transcript-level quantification, we characterized transcriptome dynamics during the Drosophila melanogaster MZT. Our approach provides an accessible platform to investigate transcriptome dynamics that can be applied to the MZT in nonmodel organisms. In addition to classifying zygotically transcribed genes, our analysis revealed that over 300 genes express different maternal and zygotic transcript isoforms due to alternative splicing, polyadenylation, and promoter usage. The vast majority of these zygotic isoforms have the potential to be subject to different regulatory control, and over two-thirds encode different proteins. Thus, our analysis reveals an additional layer of regulation during the MZT, where new zygotic transcripts can generate additional proteome diversity.