Embryonic diapause in roe deer: A model to unravel embryo-maternal communication during pre-implantation development in wildlife and livestock species

mTOR activity paces human blastocyst stage developmental progression

Many mammals can temporally uncouple conception from parturition by pacing down their development around the blastocyst stage. In mice, this dormant state is achieved by decreasing the activity of the growth-regulating mTOR signaling pathway. It is unknown whether this ability is conserved in mammals in general and in humans in particular. Here, we show that decreasing the activity of the mTOR signaling pathway induces human pluripotent stem cells (hPSCs) and blastoids to enter a dormant state with limited proliferation, developmental progression, and capacity to attach to endometrial cells. These in vitro assays show that, similar to other species, the ability to enter dormancy is active in human cells around the blastocyst stage and is reversible at both functional and molecular levels. The pacing of human blastocyst development has potential implications for reproductive therapies.

Roe Deer Reproduction in Western Poland: The Late Autumn Rut Phenomenon

Roe deer (Capreolus capreolus L.) populations in Poland are characterized by low productivity, which is why their reproductive potential was investigated. The presence of corpora lutea (CL) on the ovaries of females hunted in autumn and winter in the years 2015 and 2016 was assessed. Most animals were post-rut and most often had multiple ovulations. However, in early autumn 2015, 60% of the females had no CL. Therefore, they did not participate in mating at the turn of July/August. Those that did ovulate were found to have one CL. In late autumn, 97% of the females were post-rut, mostly with twin ovulations. This confirmed the occurrence of a late autumn rut. This phenomenon had been suggested in studies but not confirmed. In contrast, almost 100% of roe deer ovulated in summer in 2016. It was hypothesized that the reason for the low mating activity of roe deer in the summer of 2015 was heat stress and limited food resources. The summer was exceptionally hot, with many days in July and August when temperatures exceeded 30 °C. The heat combined with low rainfall led to extreme drought. Meanwhile, July and August are the months of mating activity for roe deer. The late autumn rut allowed the roe deer-a monoestrous species-to limit the consequences of a decrease in mating activity or fertility during the hot summer. Global warming may affect roe deer reproduction, so climatic conditions should be considered in population studies, not only in terms of food availability.

Dormancy, Quiescence, and Diapause: Savings Accounts for Life

Life on Earth has been through numerous challenges over eons and, one way or another, has always triumphed. From mass extinctions to more daily plights to find food, unpredictability is everywhere. The adaptability of life-forms to ever-changing environments is the key that confers life's robustness. Adaptability has become synonymous with Darwinian evolution mediated by heritable genetic changes. The extreme gene-centric view, while being of central significance, at times has clouded our appreciation of the cell as a self-regulating entity informed of, and informing, the genetic data. An essential element that powers adaptability is the ability to regulate cell growth. In this review, we provide an extensive overview of growth regulation spanning species, tissues, and regulatory mechanisms. We aim to highlight the commonalities, as well as differences, of these phenomena and their molecular regulators. Finally, we curate open questions and areas for further exploration.

New insights into how to induce and maintain embryonic diapause in the blastocyst

Embryonic diapause in mammals is a period of developmental pause of the embryo at the blastocyst stage. During diapause, the blastocyst has minimal cell proliferation, metabolic activity and gene expression. At reactivation, blastocyst development resumes, characterised by increases in cell number, biosynthesis and metabolism. Until recently, it has been unknown how diapause is maintained without any loss of blastocyst viability. This review focuses on recent progress in the identification of molecular pathways occurring in the blastocyst that can both cause and maintain the diapause state. A switch to lipid metabolism now appears essential to maintaining the diapause state and is induced by forkhead box protein O1. The forkhead box protein O transcription family is important for diapause in insects, nematodes and fish, but this is the first time a conclusive role has been established in mammals. Multiple epigenetic modifications are also essential to inducing and maintaining the diapause state, including both DNA and RNA methylation mechanisms. Finally, it now appears that diapause embryos, dormant stem cells and chemotherapeutic-resistant cancer cells may all share a universal system of quiescence.

Oncofetal reprogramming in tumor development and progression: novel insights into cancer therapy

Emerging evidence indicates that cancer cells can mimic characteristics of embryonic development, promoting their development and progression. Cancer cells share features with embryonic development, characterized by robust proliferation and differentiation regulated by signaling pathways such as Wnt, Notch, hedgehog, and Hippo signaling. In certain phase, these cells also mimic embryonic diapause and fertilized egg implantation to evade treatments or immune elimination and promote metastasis. Additionally, the upregulation of ATP-binding cassette (ABC) transporters, including multidrug resistance protein 1 (MDR1), multidrug resistance-associated protein 1 (MRP1), and breast cancer-resistant protein (BCRP), in drug-resistant cancer cells, analogous to their role in placental development, may facilitate chemotherapy efflux, further resulting in treatment resistance. In this review, we concentrate on the underlying mechanisms that contribute to tumor development and progression from the perspective of embryonic development, encompassing the dysregulation of developmental signaling pathways, the emergence of dormant cancer cells, immune microenvironment remodeling, and the hyperactivation of ABC transporters. Furthermore, we synthesize and emphasize the connections between cancer hallmarks and embryonic development, offering novel insights for the development of innovative cancer treatment strategies.

Review: Embryonic diapause in the European roe deer - slowed, but not stopped

Embryonic diapause in mammals describes a transient reduction of proliferation and developmental progression occurring at the blastocyst stage. It was first described in the European roe deer (Capreolus capreolus) in the 19th century, and later found to occur in at least over 130 mammalian species across several taxa. Diapause is often displayed as an interruption, a halt, or an arrest of embryonic development. In this review, we explore reduced, but not stopped pace of growth, proliferation and developmental progression during embryonic diapause and revisit early embryonic proliferation and continued slow development as peculiar phenomenon in the roe deer.

Endometrial extracellular matrix components do not change over the course of embryonic diapause and reactivation in the roe deer (Capreolus capreolus)

The modification of the endometrial extracellular matrix (ECM) is a crucial step for embryo implantation in many mammalian species. The embryo of the European roe deer (Capreolus capreolus) displays a 4-5 months long temporary reduction of developmental pace termed embryonic diapause. A reduction of epithelial cell height during diapause has previously been described. Co-occurring ECM modifications may contribute to the changes of the intra-uterine milieu during reactivation at which the embryo regains developmental velocity. We assessed the localization of five ECM proteins (collagen I and IV, fibronectin, laminin, and extracellular matrix protein 1) using immunohistochemistry in animals with early, late, and post-diapause (elongating) embryos. While our results confirmed the reduction of epithelial height during diapause, we only detected marginal differences in localization and staining intensities of the selected ECM proteins. Major ECM remodelling events in the roe deer endometrium are thus likely to occur only at implantation.

Evidence for a male‐biased sex ratio in the offspring of a large herbivore: The role of environmental conditions in the sex ratio variation

Numerous studies have examined whether the primary and/or secondary sex ratio in mammals, including humans, deviates from an equilibrium of 1:1. Although effect size in the sex ratio variation is expected to be low, a large sample size allows the identification of even small deviations from parity. In this study, we investigated whether the sex ratio of roe deer (Capreolus capreolus) offspring at birth approaches parity, using a large data set from roe deer offspring tagged in Baden‐Württemberg (Germany, 1972–2019, N = 12,437). In addition, a systematic re‐analysis of available data on the secondary sex ratios of roe deer was conducted to test whether our finding withstood the accumulation of further data. The null hypothesis that the sex ratio of roe deer (prenatal sex ratio and sex ratio at birth) approaches parity was rejected. Moreover, the secondary sex ratio of roe deer offspring deviated from the male‐biased mean for relatively cold or warm weather conditions during autumn and winter. Our study provides strong evidence for a male‐biased sex ratio in a large herbivore and weak evidence for variations in the secondary sex ratio owing to environmental conditions. The pattern is highly relevant in the context of climate change and its impact on the population dynamics of large herbivores.

Amino acids activate mTORC1 to release roe deer embryos from decelerated proliferation during diapause

In mammals, embryo development can halt at the hatched blastocyst stage. Uniquely, proliferation of diapausing embryonic roe deer cells decelerates to a doubling time of 2 to 3 wk over a period of 4 mo. We highlight nutrient sensing as an important factor regulating embryonic developmental pace. The resumption of embryo development is characterized by an increase in uterine fluid mTORC1-activating amino acids, embryonic mTORC1 activity, and expression of metabolism and cell cycle genes. We propose selective mTORC1 inhibition via reduced estrogen signaling and high let-7 levels as mechanisms for slow cell cycle progression. We hypothesize that it is the lack of embryonic mTORC2 inhibition during embryonic diapause in the roe deer that enables the continuous decelerated rate of proliferation.

Embryonic diapause in mammals leads to a reversible developmental arrest. While completely halted in many species, European roe deer (Capreolus capreolus) embryos display a continuous deceleration of proliferation. During a 4-mo period, the cell doubling time is 2 to 3 wk. During this period, the preimplantation blastocyst reaches a diameter of 4 mm, after which it resumes a fast developmental pace to subsequently implant. The mechanisms regulating this notable deceleration and reacceleration upon developmental resumption are unclear. We propose that amino acids of maternal origin drive the embryonic developmental pace. A pronounced change in the abundance of uterine fluid mTORC1-activating amino acids coincided with an increase in embryonic mTORC1 activity prior to the resumption of development. Concurrently, genes related to the glycolytic and phosphate pentose pathway, the TCA cycle, and one carbon metabolism were up-regulated. Furthermore, the uterine luminal epithelial transcriptome indicated increased estradiol-17β signaling, which likely regulates the endometrial secretions adapting to the embryonic needs. While mTORC1 was predicted to be inactive during diapause, the residual embryonic mTORC2 activity may indicate its involvement in maintaining the low yet continuous proliferation rate during diapause. Collectively, we emphasize the role of nutrient signaling in preimplantation embryo development. We propose selective mTORC1 inhibition via uterine catecholestrogens and let-7 as a mechanism regulating slow stem cell cycle progression.

Molecular Regulation of Paused Pluripotency in Early Mammalian Embryos and Stem Cells

The energetically costly mammalian investment in gestation and lactation requires plentiful nutritional sources and thus links the environmental conditions to reproductive success. Flexibility in adjusting developmental timing enhances chances of survival in adverse conditions. Over 130 mammalian species can reversibly pause early embryonic development by switching to a near dormant state that can be sustained for months, a phenomenon called embryonic diapause. Lineage-specific cells are retained during diapause, and they proliferate and differentiate upon activation. Studying diapause thus reveals principles of pluripotency and dormancy and is not only relevant for development, but also for regeneration and cancer. In this review, we focus on the molecular regulation of diapause in early mammalian embryos and relate it to maintenance of potency in stem cells in vitro. Diapause is established and maintained by active rewiring of the embryonic metabolome, epigenome, and gene expression in communication with maternal tissues. Herein, we particularly discuss factors required at distinct stages of diapause to induce, maintain, and terminate dormancy.

Embryonic diapause in mammals and dormancy in embryonic stem cells with the European roe deer as experimental model

In species displaying embryonic diapause, the developmental pace of the embryo is either temporarily and reversibly halted or largely reduced. Only limited knowledge on its regulation and the inhibition of cell proliferation extending pluripotency is available. In contrast with embryos from other diapausing species that reversibly halt during diapause, embryos of the roe deer Capreolus capreolus slowly proliferate over a period of 4-5 months to reach a diameter of approximately 4mm before elongation. The diapausing roe deer embryos present an interesting model species for research on preimplantation developmental progression. Based on our and other research, we summarise the available knowledge and indicate that the use of embryonic stem cells (ESCs) would help to increase our understanding of embryonic diapause. We report on known molecular mechanisms regulating embryonic diapause, as well as cellular dormancy of pluripotent cells. Further, we address the promising application of ESCs to study embryonic diapause, and highlight the current knowledge on the cellular microenvironment regulating embryonic diapause and cellular dormancy.