A maternal-effect Padi6 variant causes nuclear and cytoplasmic abnormalities in oocytes, as well as failure of epigenetic reprogramming and zygotic genome activation in embryos

Maternal inactivation of genes encoding components of the subcortical maternal complex (SCMC) and its associated member, PADI6, generally results in early embryo lethality. In humans, SCMC gene variants were found in the healthy mothers of children affected by multilocus imprinting disturbances (MLID). However, how the SCMC controls the DNA methylation required to regulate imprinting remains poorly defined. We generated a mouse line carrying a Padi6 missense variant that was identified in a family with Beckwith-Wiedemann syndrome and MLID. If homozygous in female mice, this variant resulted in interruption of embryo development at the two-cell stage. Single-cell multiomic analyses demonstrated defective maturation of Padi6 mutant oocytes and incomplete DNA demethylation, down-regulation of zygotic genome activation (ZGA) genes, up-regulation of maternal decay genes, and developmental delay in two-cell embryos developing from Padi6 mutant oocytes but little effect on genomic imprinting. Western blotting and immunofluorescence analyses showed reduced levels of UHRF1 in oocytes and abnormal localization of DNMT1 and UHRF1 in both oocytes and zygotes. Treatment with 5-azacytidine reverted DNA hypermethylation but did not rescue the developmental arrest of mutant embryos. Taken together, this study demonstrates that PADI6 controls both nuclear and cytoplasmic oocyte processes that are necessary for preimplantation epigenetic reprogramming and ZGA.

Padi6 expression patterns in buffalo oocytes and preimplantation embryos

The subcortical maternal complex, which consists of maternal-effect genes, plays a crucial role in the development of oocytes and preimplantation embryo until the activation of the zygote genome. One such gene, known as peptidyl-arginine deiminase VI (Padi6), is involved in the oocyte maturation, fertilization and embryonic development. However, the precise function of Padi6 gene in buffalo is still unclear and requires further investigation. In this study, the sequence, mRNA and protein expression patterns of Padi6 gene were analyzed in oocytes, preimplantation embryos and somatic tissues of buffalo. The coding sequence of gene was successfully cloned and characterized. Real-time quantitative PCR results indicated an absence of Padi6 transcripts in somatic tissues. Notably, the expression levels of Padi6 in oocytes showed an increased from the germinal vesicle stage to metaphase II stage, followed by a rapid decrease during the morula and blastocyst stages. Immunofluorescence analysis confirmed these findings, revealing a noticeable decline in protein expression levels. Our research provides the initial comprehensive expression profile of Padi6 in buffalo oocytes and preimplantation embryos, serving as a solid foundation for further investigations into the functionality of maternal-effect genes in buffalo.

Circumventing Zygotic Lethality to Generate Maternal Mutants in Zebrafish

Maternal gene products accumulated during oogenesis are essential for supporting early developmental processes in both invertebrates and vertebrates. Therefore, understanding their regulatory functions should provide insights into the maternal control of embryogenesis. The CRISPR/Cas9 genome editing technology has provided a powerful tool for creating genetic mutations to study gene functions and developing disease models to identify new therapeutics. However, many maternal genes are also essential after zygotic genome activation; as a result, loss of their zygotic functions often leads to lethality or sterility, thus preventing the generation of maternal mutants by classical crossing between zygotic homozygous mutant adult animals. Although several approaches, such as the rescue of mutant phenotypes through an injection of the wild-type mRNA, germ-line replacement, and the generation of genetically mosaic females, have been developed to overcome this difficulty, they are often technically challenging and time-consuming or inappropriate for many genes that are essential for late developmental events or for germ-line formation. Recently, a method based on the oocyte transgenic expression of CRISPR/Cas9 and guide RNAs has been designed to eliminate maternal gene products in zebrafish. This approach introduces several tandem guide RNA expression cassettes and a GFP reporter into transgenic embryos expressing Cas9 to create biallelic mutations and inactivate genes of interest specifically in the developing oocytes. It is particularly accessible and allows for the elimination of maternal gene products in one fish generation. By further improving its efficiency, this method can be used for the systematic characterization of maternal-effect genes.

The pivotal roles of the NOD-like receptors with a PYD domain, NLRPs, in oocytes and early embryo development†

Nucleotide-binding oligomerization domain (NOD)-like receptors with a pyrin domain (PYD), NLRPs, are pattern recognition receptors, well recognized for their important roles in innate immunity and apoptosis. However, several NLRPs have received attention for their new, specialized roles as maternally contributed genes important in reproduction and embryo development. Several NLRPs have been shown to be specifically expressed in oocytes and preimplantation embryos. Interestingly, and in line with divergent functions, NLRP genes reveal a complex evolutionary divergence. The most pronounced difference is the human-specific NLRP7 gene, not identified in rodents. However, mouse models have been extensively used to study maternally contributed NLRPs. The NLRP2 and NLRP5 proteins are components of the subcortical maternal complex (SCMC), which was recently identified as essential for mouse preimplantation development. The SCMC integrates multiple proteins, including KHDC3L, NLRP5, TLE6, OOEP, NLRP2, and PADI6. The NLRP5 (also known as MATER) has been extensively studied. In humans, inactivating variants in specific NLRP genes in the mother are associated with distinct phenotypes in the offspring, such as biparental hydatidiform moles (BiHMs) and preterm birth. Maternal-effect recessive mutations in KHDC3L and NLRP5 (and NLRP7) are associated with reduced reproductive outcomes, BiHM, and broad multilocus imprinting perturbations. The precise mechanisms of NLRPs are unknown, but research strongly indicates their pivotal roles in the establishment of genomic imprints and post-zygotic methylation maintenance, among other processes. Challenges for the future include translations of findings from the mouse model into human contexts and implementation in therapies and clinical fertility management.

NLRP7 and the Genetics of Hydatidiform Moles: Recent Advances and New Challenges

NOD-like receptor proteins (NLRPs) are emerging key players in several inflammatory pathways in Mammals. The first identified gene coding for a protein from this family is Nlrp5 and was originally called Mater for “Maternal Antigen That Mouse Embryos Require” for normal development beyond the two-cell stage. This important discovery was followed by the identification of other NLRPs playing roles in inflammatory disorders and of the first maternal-effect gene in humans, NLRP7, which is responsible for an aberrant form of human pregnancy called hydatidiform mole (HM). In this review, we recapitulate the various aspects of the pathology of HM, highlight recent advances regarding NLRP7 and its role in HM and related forms of reproductive losses, and expand our discussion to other NLRPs with a special emphasis on those with known roles in mammalian reproduction. Our aim is to facilitate the genetic complexity of recurrent fetal loss in humans and encourage interdisciplinary collaborations in the fields of NLRPs and reproductive loss.

Maternal control of genomic imprint maintenance

Genomic imprinting is a specialized transcriptional phenomenon that employs epigenetic mechanisms to facilitate parental-specific expression. Perturbations in parental epigenetic asymmetry can lead to the development of imprinting disorders, such as Beckwith-Wiedemann syndrome and Angelman syndrome. DNA methylation is one of the most widely studied epigenetic marks that characterizes imprinted regions. During gametogenesis and early embryogenesis, imprinted methylation undergoes a cycle of erasure, acquisition and maintenance. Gamete and embryo manipulations for the purpose of assisted reproduction are performed during these reprogramming events and may lead to their disruption. Recent studies point to the role of maternal-effect proteins in imprinted gene regulation. Studies are now required to increase understanding of how these factors regulate genomic imprinting as well as how assisted reproduction technologies may alter their function.

Vertebrate maternal-effect genes: Insights into fertilization, early cleavage divisions, and germ cell determinant localization from studies in the zebrafish

In the earliest stages of animal development prior to the commencement of zygotic transcription, all critical cellular processes are carried out by maternally-provided molecular products accumulated in the egg during oogenesis. Disruption of these maternal products can lead to defective embryogenesis. In this review, we focus on maternal genes with roles in the fundamental processes of fertilization, cell division, centrosome regulation, and germ cell development with emphasis on findings from the zebrafish, as this is a unique and valuable model system for vertebrate reproduction.