Maternal Kdm2a-mediated PI3K/Akt signaling and E-cadherin stimulate the morula-to-blastocyst transition revealing crucial roles in early embryonic development

Histone methylation plays an essential role in oocyte growth and preimplantation embryonic development. The modification relies on histone methyl-transferases and demethylases, and one of these, lysine-specific demethylase 2a (Kdm2a), is responsible for modulating histone methylation during oocyte and early embryonic development. The mechanism of how Kdm2a deficiency disrupts early embryonic development and fertility remains elusive. To determine if maternally deposited Kdm2a is required for preimplantation embryonic development, the expression profile of Kdm2a during early embryos was detected via immunofluorescence staining and RT-qPCR. The Kdm2a gene in oocytes was specifically deleted with the Zp3-Cre/LoxP system and the effects of maternal Kdm2a loss were studied through a comprehensive range of female reproductive parameters including fertilization, embryo development, and the number of births. RNA transcriptome sequencing was performed to determine differential mRNA expression, and the interaction between Kdm2a and the PI3K/Akt pathway was studied with a specific inhibitor and activator. Our results revealed that Kdm2a was continuously expressed in preimplantation embryos and loss of maternal Kdm2a suppressed the morula-to-blastocyst transition, which may have been responsible for female subfertility. After the deletion of Kdm2a, the global H3K36me2 methylation in mutant embryos was markedly increased, but the expression of E-cadherin decreased significantly in morula embryos compared to controls. Mechanistically, RNA-seq analysis revealed that deficiency of maternal Kdm2a altered the mRNA expression profile, especially in the PI3K/Akt signaling pathway. Interestingly, the addition of a PI3K/Akt inhibitor (LY294002) to the culture medium blocked embryo development at the stage of morula; however, the developmental block caused by maternal Kdm2a loss was partially rescued with a PI3K/Akt activator (SC79). In summary, our results indicate that loss of Kdm2a influences the transcriptome profile and disrupts the PI3K/Akt signaling pathway during the development of preimplantation embryo. This can result in embryo block at the morula stage and female subfertility, which suggests that maternal Kdm2a is a potential partial redundancy with other genes encoding enzymes in the dynamics of early embryonic development. Our results provide further insight into the role of histone modification, especially on Kdm2a, in preimplantation embryonic development in mice.

Knockdown of Csnk1a1 results in preimplantation developmental arrest in mice

Casein kinase 1, alpha 1 (CSNK1A1), is a member of the highly conserved serine/threonine protein kinase family. This study was established to analyze the expression and localization of CSNK1A1 and its function in early embryonic development in mice. Csnk1a1 mRNA and protein are expressed in multiple mouse tissues including the ovary. After ovulation and fertilization, Csnk1a1 mRNA and protein were detected in preimplantation embryos and their expression was highest in two-cell-stage embryos. CSNK1A1 protein was also mainly localized in the cytoplasm of preimplantation embryos. Moreover, knockdown of Csnk1a1 in zygotes led to a significant decrease in the rate of blastocyst formation. Furthermore, treatment of zygotes with the CSNK1A1-specific inhibitor D4476 also resulted in embryonic developmental arrest. These results provide the first evidence for a novel function of CSNK1A1 in early embryonic development in mice.

Knockdown of Toe1 causes developmental arrest during the morula-to-blastocyst transition in mice

The target of EGR1 protein 1 (TOE1) is evolutionarily conserved from Caenorhabditis elegans to mammals, which plays a critical role in the maturation of a variety of small nuclear RNAs. Mutation in human TOE1 has been reported to cause pontocerebellar hypoplasia type 7, a severe neurodegenerative syndrome. However, the role of TOE1 in early embryonic development remains unclear. Herein, we found that Toe1 mRNA and protein were expressed in mouse preimplantation embryos. Silencing Toe1 by siRNA led to morula-to-blastocyst transition failure. This developmental arrest can be rescued by Toe1 mRNA microinjection. EdU incorporation assay showed a defect in blastomere proliferation within developmentally arrested embryos. Further studies revealed that Toe1 knockdown caused increased signals for γH2AX and micronuclei, indicative of sustained DNA damage. Moreover, mRNA levels of cell cycle inhibitor p21 were significantly upregulated in Toe1 knockdown embryos before developmental arrest. Together, these results suggest that TOE1 is indispensable for mouse early embryo development potentially through maintaining genomic integrity. Our findings provide further insight into the role of TOE1 in mouse preimplantation embryonic development.

Insufficient pyruvate in culture medium arrests mouse embryos at the first cleavage stage associated with abnormal epigenetic modifications

Energy is essential for early embryogenesis, and fertilized eggs can successfully develop to blastocyst in in vitro culture medium with an appropriate energy supply. Conversely, embryonic development is negatively affected by a suboptimal energy supply. We previously observed that a low level of pyruvate greatly arrests mouse embryos at the 2-cell stage. However, how methylation modifications are affected at this specific stage remains unknown. In this study, we found that mouse embryos could timely develop to the 4-cell stage in K⁺simplex optimized medium (KSOM) with control level of pyruvate, but embryos were significantly arrested at the 2-cell stage when pyruvate was reduced to 0.2-fold of the control level. Moreover, the fluorescence intensities of 5 mC, H3K4me2, H3K9me2 and H3K27me2 in the 2-cell stage embryos of the 0.2-fold pyruvate group were notedly lower than those of the control group, but N6-methyladenosine (m⁶A) fluorescence intensity was higher, suggesting that global genomic DNA, histone and m⁶A methylation modifications are disrupted with low levels of pyruvate. Consistently, the mRNA levels of genes related to DNA methylation, histone methylation and m⁶A modifications were also disturbed in the 2-cell stage embryos cultured with low levels of pyruvate. In summary, our findings demonstrate that insufficient pyruvate in culture medium results in mouse embryonic developmental arrest, at least in part due to defects in methylation modifications.

Two mutations in TUBB8 cause developmental arrest in human oocytes and early embryos

RESEARCH QUESTION

How can the effect of genetic mutations that may cause primary female infertility be evaluated?

DESIGN

Patients and their family members underwent whole-exome sequencing and Sanger sequencing to detect the infertility-causing gene and inheritance pattern. To study the function of mutant proteins in vitro, vectors containing wild-type or mutant TUBB8 cDNA were constructed for transient expression in HeLa cells, and in-vitro transcribed mRNA were used for microinjection in germinal vesicle-stage mouse oocytes. Immunofluorescence staining was used to observe the microtubule structure in HeLa cells or meiotic spindle in mouse oocytes.

RESULTS

A maternally inherited TUBB8 (Tubulin beta 8 class VIII) mutation (NM_177987.2: c. 959G>A: p. R320H) and a previously reported (NM_177987.2: c. 161C>T: p. A54V) recessive mutation from two infertile female patients were identified. The oocytes from the patient carrying p.A54V mutation failed fertilization, whereas oocytes with p.R320H mutation could be fertilized but showed heavy fragmentation during early development. In vitro, functional assays showed that p. A54V mutant disrupted the microtubule structure in HeLa cells (49.3% of transfected cells) and caused large polar body extrusion in mouse oocytes (27.5%), whereas the p.R320H mutant caused a higher abnormal rate (69.7%) in cultured cells and arrested mouse oocytes at meiosis I (38.7%).

CONCLUSION

Two TUBB8 mutations (p.A54V and p.R320H) were identified and their pathogeny was confirmed by in-vitro functional assays.

Dormancy in laboratory-reared Asian longhorned beetles, Anoplophora glabripennis

An insect's capacity to survive winter is critical for range expansion in temperate regions. The Asian longhorned beetle (Anoplophora glabripennis) is a polyphagous wood-boring insect native to China and the Korean peninsula and poses a high risk of invasion in North America and Europe. It is unclear whether A. glabripennis enters diapause, which means that diapause cannot be included in assessments of the risk of this species invading forests in temperate regions. Using a laboratory colony, we examine larval developmental arrest, metabolic rates, gas exchange patterns, thermal sensitivity, and body composition to characterize larval dormancy. Chilled larvae entered a temperature-independent developmental arrest which usually required more than four weeks of chilling to break, decreased their metabolic rate by as much as 63%, and maintained energy stores throughout the chilling period - results consistent with an obligate diapause. We also observed a switch to discontinuous gas exchange at low temperatures. Thermal sensitivity of metabolic rate did not differ between chilled and non-chilled larvae. Taken together, we conclude that A. glabripennis enters a larval diapause during chilling and terminates diapause after a requisite chilling period. These results will enhance our ability to predict phenology and potential distribution of current and future invasions of A. glabripennis.

Segmental duplications and monosomies are linked to in vitro developmental arrest

PURPOSE

To verify which genetic abnormalities prevent embryos to blastulate in a stage-specific time.

METHODS

A single center retrospective study was performed between April 2016 and January 2017. Patients requiring Preimplantation Genetic Testing for Aneuploidies (PGT-A) by Next Generation Sequencing (NGS) were included. All embryos were cultured in a time-lapse imaging system and single blastomere biopsy was performed on day 3 of development. Segmental duplications and deletions as well as whole chromosome monosomies and trisomies were registered. Embryo arrest was defined if the embryo failed to blastulate 118 h post-injection. A logistic regression model was applied using the time to blastulate as the response variable and the different mutations as explanatory variables. A p value < 0.05 was considered significant.

RESULTS

Of the 285 biopsied cleavage stage embryos, 103 (36.1%) were euploid, and 182 (63.9%) were aneuploid. There was a significant difference in the developmental arrest between euploid and aneuploid embryos (8.7% versus 42.9%; p = 0.0001). Segmental duplications and whole chromosome monosomies were found to have a significant effect on developmental arrest (p = 0.0163 and p = 0.0075), while trisomies and segmental deletions had no effect on developmental arrest. In case of segmental duplications, an increase of one extra segmental duplication increases the odd of arrest by 159%. For whole chromosome monosomies, the odd will only increase by 29% for every extra chromosomal monosomy. Both chromosomal abnormalities remained significant after adding age as an explanatory variable to the model (p = 0.014 and p = 0.009).

CONCLUSION

Day 3 cleavage stage embryos with segmental duplications or monosomies have a significantly decreased chance to reach the blastocyst stage.

Low-level pyruvate inhibits early embryonic development and maternal mRNA clearance in mice

Energy homeostasis and accomplishment of maternal-to-zygotic transition (MZT), which involves the timed processes of maternal mRNA clearance and zygotic genome activation (ZGA), are essential for mammalian embryogenesis. However, how energy substrates regulate maternal mRNA clearance and the underlying mechanisms have not yet been fully elucidated. Here, we found that mouse embryos were arrested at the 2-cell stage when the pyruvate level was reduced to one-fifth of the control level. Moreover, we observed that the mitochondrial contents and ROS levels were reduced. Interestingly, some maternal mRNA, including transcripts involved in the maternal factor-mediated mRNA decay (M-decay) pathway, was vastly degraded from 1-cell to 2-/4-cell embryos when cultured with control pyruvate levels, but the clearance of these transcripts was hindered when the pyruvate level was reduced. In contrast, some transcripts involved in the zygotic factor-mediated mRNA decay (Z-decay) pathway were vastly downregulated by the reduction in pyruvate. This effect was possibly due to a reduction in global transcription, as the embryos cultured with low-level pyruvate had lower transcription activity than embryos cultured with control pyruvate level. In summary, our findings demonstrate that low-level pyruvate inhibits maternal mRNA clearance, possibly by disrupting the M- and Z-decay pathways, extending our current understanding of the energy requirements of embryogenesis.

Knockdown of ribosomal protein S3 causes preimplantation developmental arrest in mice

Ribosomal protein S3 (RpS3), a member of the ribosome 40S subunit, has conventional ribosomal function and additional extraribosomal functions. The aim of the present study was to analyze the expression and localization of RpS3 and its function in early embryogenesis in mice. RpS3 mRNA and protein were expressed in multiple mouse tissues. In the ovary, RpS3 protein was ubiquitously and highly expressed in oocytes and granulosa cells. After ovulation and fertilization, RpS3 mRNA and protein were detected in oocytes and preimplantation embryos. Furthermore, RpS3 protein was localized in the cytoplasm of oocytes and preimplantation embryos. Moreover, knockdown of RpS3 in zygotes led to a significantly decreased rate of blastocyst formation. These results provide the first evidence for a novel function of RpS3 in regulating early embryonic development in mice.

Diapause-associated changes in the lipid and metabolite profiles of the Asian tiger mosquito, Aedes albopictus

Diapause is an alternative life-history strategy that allows organisms to enter developmental arrest in anticipation of unfavorable conditions. Diapause is widespread among insects and plays a key role in enhancing overwinter survival as well as defining the seasonal and geographic distributions of populations. Next-generation sequencing has greatly advanced our understanding of the transcriptional basis for this crucial adaptation but less is known about the regulation of embryonic diapause physiology at the metabolite level. Here, we characterized the lipid and metabolite profiles of embryonic diapause in the Asian tiger mosquito, Aedes albopictus We used an untargeted approach to capture the relative abundance of 250 lipids and 241 metabolites. We observed adjustments associated with increased energy storage, including an accumulation of lipids, the formation of larger lipid droplets and increased lipogenesis, as well as metabolite shifts suggesting reduced energy utilization. We also found changes in neuroregulatory- and insulin-associated metabolites with potential roles in diapause regulation. Finally, we detected a group of unidentified, diapause-specific metabolites which have physical properties similar to those of steroids/steroid derivatives and may be associated with the ecdysteroidal regulation of embryonic diapause in A.albopictus Together, these results deepen our understanding of the metabolic regulation of embryonic diapause and identify key targets for future investigations.

Cell cycle regulator, small silencing RNA, and segmentation patterning gene expression in relation to embryonic diapause in the band-legged ground cricket

Insects enter diapause to synchronize their life cycle with biotic and abiotic conditions favorable for their development, reproduction, and survival. Adult females of the band-legged ground cricket Dianemobius nigrofasciatus (Orthoptera, Glyllidae) respond to environmental factors in autumn and lay diapause-destined eggs. The eggs arrest their development and enter diapause at a very early embryonic stage, specifically the cellular blastoderm. To elucidate the physiological mechanisms underlying this very early stage programmed developmental arrest, we investigated the cell division cycle as well as the expression of cell cycle regulators, small silencing RNAs, and segment patterning genes. The diapause embryo arrests its cell cycle predominantly at the G₀/G₁ phase. The proportion of cells in the S phase of the cell cycle abruptly decreased at the time of developmental arrest, but further changes of the G₀/G₁ and G₂/M were later observed. Thus, cell cycle arrest in the diapause embryo is not an immediate event, but it takes longer to reach the steady state. We further elucidated molecular events possibly involved in diapause preparation and entry. Downregulation of Proliferating cellular antigen (PCNA; a cell cycle regulator), caudal and pumilio (cad and pum; early segmentation genes) as well as P-element induced wimpy testis (piwi) (a small silencing RNA) prior to the onset of developmental arrest was notable. The downregulation of PCNA, cad and pum continued even after entry into developmental arrest. In contrast to upregulation in non-diapause eggs, Cyclin D (another cell cycle regulator) and hunchback, Krüppel, and runt (gap and pair-rule genes) were downregulated in diapause eggs. These molecular events may contribute to embryonic diapause of D. nigrofasciatus.

Cell cycle arrest in the jewel wasp Nasonia vitripennis in larval diapause

Insects enter diapause to synchronise their life cycle with biotic and abiotic environmental conditions favourable for their development, reproduction, and survival. One of the most noticeable characteristics of diapause is the blockage of ontogeny. Although this blockage should occur with the cessation of cellular proliferation, i.e. cell cycle arrest, it was confirmed only in a few insect species and information on the molecular pathways involved in cell cycle arrest is limited. In the present study, we investigated developmental and cell cycle arrest in diapause larvae of the jewel wasp Nasonia vitripennis. Developmental and cell cycle arrest occur in the early fourth instar larval stage of N. vitripennis under short days. By entering diapause, the S fraction of the cell cycle disappears and approximately 80% and 20% of cells arrest their cell cycle in the G0/G1 and G2 phases, respectively. We further investigated expression of cell cycle regulatory genes and some housekeeping genes to dissect molecular mechanisms underlying the cell cycle arrest.

Insulin-like growth factor signaling regulates developmental trajectory associated with diapause in embryos of the annual killifish Austrofundulus limnaeus

Annual killifishes exhibit a number of unique life history characters including the occurrence of embryonic diapause, unique cell movements associated with dispersion and subsequent reaggregation of the embryonic blastomeres, and a short post-embryonic life span. Insulin-like growth factor (IGF) signaling is known to play a role in the regulation of metabolic dormancy in a number of animals but has not been explored in annual killifishes. The abundance of IGF proteins during development and the developmental effects of blocking IGF signaling by pharmacological inhibition of the insulin-like growth factor I receptor (IGF1R) were explored in embryos of the annual killifish Austrofundulus limnaeus Blocking of IGF signaling in embryos that would normally escape entrance into diapause resulted in a phenotype that was remarkably similar to that of embryos entering diapause. IGF-I protein abundance spikes during early development in embryos that will not enter diapause. In contrast, IGF-I levels remain low during early development in embryos that will enter diapause II. IGF-II protein is packaged at higher levels in escape-bound embryos compared with diapause-bound embryos. However, IGF-II levels quickly decrease and remain low during early development and only increase substantially during late development in both developmental trajectories. Developmental patterns of IGF-I and IGF-II protein abundance under conditions that would either induce or bypass entrance into diapause are consistent with a role for IGF signaling in the regulation of developmental trajectory and entrance into diapause in this species. We propose that IGF signaling may be a unifying regulatory pathway that explains the larger suite of characters that are associated with the complex life history of annual killifishes.