EMBRYO PAUSE DEVELOPMENT UNDER CIRCUMSTANCES: THE ROLE OF FOXO1 IN REGULATING THE DORMANCY AND PROLIFERATION OF STEM CELLS

 

Dormancy refers to a reversible non-proliferative state that a cell enters. This major feature is identified prominently in stem cells, cells that can differentiate into different types of cells in the body, such as embryonic and adult stem cells derived from the inner mass of mammalian embryos and undifferentiated stem cells in the body also known as somatic cells. 

However dormancy is mainly explored in adult stem cells, it is also found in early development as a means to preserve embryos under circumstances (Lack of food, Insufficient maternal fat stores, Older siblings who haven't been weaned, Time of year, Temperature, Lactation). The regulation of dormancy is crucial for maintaining the health and functionality of stem cell pools. Perturbations in the entry into or exit from dormancy can have severe consequences, such as the exhaustion of stem cell pools. If the mechanisms controlling dormancy entry or exit are disrupted, it can lead to the exhaustion of stem cell pools. Stem cells may lose their ability to self-renew and differentiate effectively. 

A stem cell niche is a microenvironment present in the location where stem cells are found. It is a dynamic microenvironment that supports stem cell functionality. It is a place where extrinsic signals interact and integrate to influence stem cell behaviour. Within the stem cell niche, stem cells exist in a state of dormancy, referred to as quiescence. While in this quiescent stage, stem cells have the capacity to both self-renew and undergo controlled differentiation the cells within the stem cell niche engage in interactions with the stem cells, playing a crucial role in either sustaining their quiescent state or facilitating their differentiation. It is observed that cellular niche, signalling and metabolism are involved in the regulation of transition from proliferation to dormancy.

 

Diapause depicted in a mouse embryo

Till now known mTOR (mammalian target of rapamycin) pathway acts as a rheostat controlling cellular growth and anabolism based on metabolic and environmental signals. It also regulates dormancy across tissues and hyperactivity of this pathway leads to stem cell exhaustion. It has previously been observed that inhibition in mTOR (mTORi) induces a diapause-like dormant stage in vitro in embryonic stem cells (ESCs) and mouse pre-implantation embryos (blastocysts). Mouse blastocytes pause for a few weeks on inhibition of mTORi leading to embryo loss over time. 

Even though dormant cells in tissues share the same characteristics like reduced proliferation and energy conservation, the response of different cells to dormancy and how this takes place and is reversed is still unclear. 

A paper entitled “FOXO1-mediated lipid metabolism maintains mammalian embryos in dormancy” from Nature Cell Biology has stated that FOXO1 (Forkhead box protein O1), a transcription factor regulates energy balance in dormant embryos and could serve as a universal regulator of dormancy in various adult tissues. 

In the study, through cell-type specific response to mTORi induced in vitro dispause (a type of dormancy that is temporary), identified that FOXO1-dependent lipid metabolism is an important regulator for embryos' longevity during dispause. Increasing oxidation of lipids via supplementing carnitine, a cofactor that transports long-chain fatty acids into the mitochondria to produce energy in the form of ATP (adenosine triphosphate) has prolonged the survival of embryos' dormant state. Most prominently, this state is reversible and embryos reactivate and give rise to ESCs/TSCs. 

Lipid reserves and fatty acid oxidation (FAO) play a crucial role in maintaining a balance between the dormancy and proliferation of stem cells in various adult tissues, as well as during embryonic diapause across different species. Additionally, FOXO1, a prominent regulator of lipid metabolism (known as DAF16 in C. elegans), is associated with longevity and is upregulated in adipocytes under nutrient restriction. FOXO1, in turn, enhances lysosome function through the expression of TFEB (transcription factor EB), thereby influencing lipid metabolism, including sphingolipid metabolism. The involvement of mTOR, FOXO1, lysosomes, and FAO in both diapause and lifespan regulation suggests a partially shared regulatory network between these two phenomena.

The research reveals that for maintaining dormancy, a shift to fatty acid oxidation (FAO) is imperative, with FOXO1 and L-carnitine identified as crucial regulators. The addition of L-carnitine to embryo cultures significantly extends their longevity, lasting up to 34 days. The proposition is that supplementing embryo culture with L-carnitine facilitates the utilization of lipid reserves, enhancing in vitro diapause by establishing a more profound dormant state. Furthermore, the study suggests the FOXO1/FAO axis as a common regulator of dormancy in adult tissues.

The study proposes, through meta-analyses of signatures in dormant cells, that FOXO1 may serve as a universal regulator of dormancy across various adult tissues. The findings alleviate limitations in in-vitro embryo survival and underscore the significance of lipid metabolism as a crucial metabolic transition relevant for longevity and stem cell function across different tissues. 

This approach could potentially serve as a solution in various embryo research endeavours. By allowing for a detailed examination of morphology over an extended period, it has the potential to enhance our understanding of embryonic development. Additionally, the insights gained from this method may contribute to advancements in the instrumentation utilized in embryo research, leading to improved tools and techniques for more comprehensive studies in this field.

Does the dispause of embryos happen in humans?

Embryonic diapause is a phenomenon observed at the blastocyst stage in various species, where the development of the embryo is temporarily halted in response to different environmental conditions. It is important to note that embryonic diapause is not a universal occurrence and does not take place in certain species, including rabbits, cows, nonhuman primates, and humans. In these species, the embryos do not undergo the suspended development characteristic of diapause, instead, they progress through continuous and uninterrupted development. The absence of embryonic diapause in these species distinguishes their reproductive strategies from those of species where diapause is a natural part of embryonic development.

REFERENCE

Van der Weijden VA, Stötzel M, Iyer DP, Fauler B, Gralinska E, Shahraz M, et al. FOXO1-mediated lipid metabolism maintains mammalian embryos in dormancy. Nature Cell Biology. 2024; doi:10.1038/s41556-023-01325-3 

Ptak GE, Modlinski JA, Loi P. Embryonic diapause in humans: Time to consider? Reproductive Biology and Endocrinology. 2013;11(1):92. doi:10.1186/1477-7827-11-92 

IMAGE CREDITS

New Scientist, https://images.app.goo.gl/2TuqzX6nEibHMGuj9

MDPI, https://images.app.goo.gl/Vwj1psWFK4VBtAC98

Europe IVF, https://images.app.goo.gl/7A8Yq2V9uYLcU2aS9