Errors in early embryogenesis are a cause of sporadic cell death and developmental failure1,2. Phagocytic activity has a central role in scavenging apoptotic cells in differentiated tissues3,4,5,6. However, how apoptotic cells are cleared in the blastula embryo in the absence of specialized immune cells remains unknown. Here we show that the surface epithelium of zebrafish and mouse embryos, which is the first tissue formed during vertebrate development, performs efficient phagocytic clearance of apoptotic cells through phosphatidylserine-mediated target recognition. Quantitative four-dimensional in vivo imaging analyses reveal a collective epithelial clearance mechanism that is based on mechanical cooperation by two types of Rac1-dependent basal epithelial protrusions. The first type of protrusion, phagocytic cups, mediates apoptotic target uptake. The second, a previously undescribed type of fast and extended actin-based protrusion that we call 'epithelial arms', promotes the rapid dispersal of apoptotic targets through Arp2/3dependent mechanical pushing. On the basis of experimental data and modelling, we show that mechanical load-sharing enables the long-range cooperative uptake of apoptotic cells by multiple epithelial cells. This optimizes the efficiency of tissue clearance by extending the limited spatial exploration range and local uptake capacity of non-motile epithelial cells. Our findings show that epithelial tissue clearance facilitates error correction that is relevant to the developmental robustness and survival of the embryo, revealing the presence of an innate immune function in the earliest stages of embryonic development. Main textEarly embryogenesis is prone to cellular errors like mitotic defects induced by cell intrinsic or extrinsic stress factors1,2. This leads to sporadic cell death of progenitor stem cells3-6, assumed to be a major cause of early developmental failures in human pre-implantation development7,8. Stochastic cell death is expected to occur at random locations, vary in cell number and differ from developmentally programmed cell death occurring at later stages in vertebrate embryogenesis, which has predefined spatiotemporal dynamics and specific functions in morphogenetic processes9. Detection and removal of cell corpses requires specific clearance mechanisms as mediated by professional phagocytic cells in adult tissues10. Whether such active mechanisms for efficient apoptotic tissue clearance exist in early blastula and gastrula

Transition from maternal to embryonic transcriptional control is crucial for embryogenesis. However, alternative splicing regulation during this process remains understudied. Using transcriptomic data from human, mouse, and cow preimplantation development, we show that the stage of zygotic genome activation (ZGA) exhibits the highest levels of exon skipping diversity reported for any cell or tissue type. Much of this exon skipping is temporary, leads to disruptive noncanonical isoforms, and occurs in genes enriched for DNA damage response in the three species. Two core spliceosomal components, Snrpb and Snrpd2 , regulate these patterns. These genes have low maternal expression at ZGA and increase sharply thereafter. Microinjection of Snrpb/d2 messenger RNA into mouse zygotes reduces the levels of exon skipping at ZGA and leads to increased p53-mediated DNA damage response. We propose that mammalian embryos undergo an evolutionarily conserved, developmentally programmed splicing failure at ZGA that contributes to the attenuation of cellular responses to DNA damage.

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Actin cytoskeleton dynamics is crucial for neurogenesis and neuronal function. Precise quantitative and qualitative regulation of actin polymerization is achieved by multiple actin-binding proteins, among which formins are particularly versatile. Here, we investigate how neuronal-specific splicing expands formin's functional diversity in the brain. We uncovered a highly conserved microexon in DAAM1, whose inclusion extends the linker region of the FH2 domain and leads to remarkable changes in actin polymerization rates and structure. Microexon deletion causes neuritogenesis defects and increased calcium influx in in vitro differentiated neurons, and mice carrying this deletion exhibit deficient memory formation. These memory defects were associated with higher activity of DAAM1's interactor RhoA, increased ARC protein levels, postsynaptic deficiencies, fewer dendritic spines and impaired long-term potentiation. In summary, precise post-transcriptional regulation of DAAM1's FH2 domain is a novel mechanism for modulating actin dynamics in neurons and is essential for proper brain function.

The transition from maternal to embryonic transcriptional control is a crucial step in embryogenesis. However, how alternative splicing is regulated during this process and how it contributes to early development is unknown. Using transcriptomic data from pre-implantation stages of human, mouse and cow, we show that the stage of zygotic genome activation (ZGA) exhibits the highest levels of exon skipping diversity reported for any cell or tissue type. Interestingly, much of this exon skipping is temporary, leads to disruptive non-canonical isoforms, and occurs in genes enriched for DNA damage response in the three species. We identified two core spliceosomal components, Snrpb and Snrpd2, as regulators of these patterns. These genes have low maternal expression at the time of ZGA and increase sharply thereafter. Consistently, microinjection of Snrpb/d2 mRNA into mouse zygotes reduces the levels of temporary exon skipping at ZGA, and leads to an increase in etoposide-induced DNA damage response. Altogether, our results suggest that mammalian embryos undergo an evolutionarily conserved and developmentally programmed specific splicing failure at the time of genome activation that attenuates cellular responses to DNA damage at these early stages.