Lessons from expanded potential of embryonic stem cells: Moving toward totipotency

Regulation of cleavage embryo genes upon DRP1 inhibition in mouse embryonic stem cells

Dynamic-related protein 1 (DRP1) is a key protein of mitochondrial fission. In this study, we found that inhibition of activity of DRP1 led to increased levels of cleavage embryo genes in mouse embryonic stem cells (mESCs), which might reflect a transient totipotency status derived from pluripotency. This result indicates that DRP1 inhibition in mESCs leads to a tendency to obtain a new expression profile similar to that of the 2C-like state. Meanwhile, we also noticed that the glycolysis/gluconeogenesis pathway and its related enzymes were significantly downregulated, and the key glycolytic enzymes were also downregulated in various 2C-like cells. Moreover, when DRP1 activity was inhibited from the late zygote when cleavage embryo genes started to express, development of early embryos was inhibited, and these cleavage embryo genes failed to be efficiently silenced at the late 2-cell (2C) stage. Taken together, our result shows that DRP1 plays an important role in silencing cleavage embryo genes for totipotency-to-pluripotency transition.

Rif1 interacts with non-canonical polycomb repressive complex PRC1.6 to regulate mouse embryonic stem cells fate potential

Mouse embryonic stem cells (mESCs) cycle in and out of a transient 2-cell (2C)-like totipotent state, driven by a complex genetic circuit involves both the coding and repetitive sections of the genome. While a vast array of regulators, including the multi-functional protein Rif1, has been reported to influence the switch of fate potential, how they act in concert to achieve this cellular plasticity remains elusive. Here, by modularizing the known totipotency regulatory factors, we identify an unprecedented functional connection between Rif1 and the non-canonical polycomb repressive complex PRC1.6. Downregulation of the expression of either Rif1 or PRC1.6 subunits imposes similar impacts on the transcriptome of mESCs. The LacO-LacI induced ectopic colocalization assay detects a specific interaction between Rif1 and Pcgf6, bolstering the intactness of the PRC1.6 complex. Chromatin immunoprecipitation followed by sequencing (ChIP-seq) analysis further reveals that Rif1 is required for the accurate targeting of Pcgf6 to a group of genomic loci encompassing many genes involved in the regulation of the 2C-like state. Depletion of Rif1 or Pcgf6 not only activates 2C genes such as Zscan4 and Zfp352, but also derepresses a group of the endogenous retroviral element MERVL, a key marker for totipotency. Collectively, our findings discover that Rif1 can serve as a novel auxiliary component in the PRC1.6 complex to restrain the genetic circuit underlying totipotent fate potential, shedding new mechanistic insights into its function in regulating the cellular plasticity of embryonic stem cells.

Genome architecture and totipotency: An intertwined relation during early embryonic development

Chromosomes are not randomly packed and positioned into the nucleus but folded in higher-order chromatin structures with defined functions. However, the genome of a fertilized embryo undergoes a dramatic epigenetic reprogramming characterized by extensive chromatin relaxation and the lack of a defined three-dimensional structure. This reprogramming is followed by a slow genome refolding that gradually strengthens the chromatin architecture during preimplantation development. Interestingly, genome refolding during early development coincides with a progressive loss of developmental potential suggesting a link between chromatin organization and cell plasticity. In agreement, loss of chromatin architecture upon depletion of the insulator transcription factor CTCF in embryonic stem cells led to the upregulation of the transcriptional program found in totipotent cells of the embryo, those with the highest developmental potential. This essay will discuss the impact of genome folding in controlling the expression of transcriptional programs involved in early development and their plastic-associated features.

Chemical-induced chromatin remodeling reprograms mouse ESCs to totipotent-like stem cells

Totipotent cells have more robust developmental potency than any other cell types, giving rise to both embryonic and extraembryonic tissues. Stable totipotent cell cultures and deciphering the principles of totipotency regulation would be invaluable to understand cell plasticity and lineage segregation in early development. Our approach of remodeling the pericentromeric heterochromatin and re-establishing the totipotency-specific broad H3K4me3 domains promotes the pluri-to-totipotency transition. Our protocol establishes a closer match of mouse 2-cell (2C) embryos than any other 2C-like cells. These totipotent-like stem cells (TLSCs) are stable in culture and possess unique molecular features of the mouse 2C embryo. Functionally, TLSCs are competent for germline transmission and give rise to both embryonic and extraembryonic lineages at high frequency. Therefore, TLSCs represent a highly valuable cell type for studies of totipotency and embryology.

Epigenome-Metabolome-Epigenome signaling cascade in cell biological processes

Cell fate determination as a fundamental question in cell biology has been extensively studied at different regulatory levels for many years. However, the mechanisms of multi-level regulation of cell fate determination remain unclear. Recently we have proposed an Epigenome-Metabolome-Epigenome (E-M-E) signaling cascade model to describe the crossover cooperation during mouse somatic cell reprogramming. In this review, we summarize the broad roles of E-M-E signaling cascade in different cell biological processes including cell differentiation and dedifferentiation, cell specialization, cell proliferation and cell pathological processes. Precise E-M-E signaling cascades are critical in these cell biological processes, and it is of worth to explore each step of E-M-E signaling cascade. E-M-E signaling cascade model sheds light on and may open a window to explore the mechanisms of multi-level regulation of cell biological processes.

nr0b1 (DAX1) loss of function in zebrafish causes hypothalamic defects via abnormal progenitor proliferation and differentiation

The nuclear receptor DAX-1 (encoded by the NR0B1 gene) is presented in the hypothalamic tissues in humans and other vertebrates. Human patients with NR0B1 mutations often have hypothalamic-pituitary defects, but the involvement of NR0B1 in hypothalamic development and function is not well understood. Here, we report the disruption of the nr0b1 gene in zebrafish causes abnormal expression of gonadotropins, a reduction in fertilization rate, and an increase in post-fasting food intake, which is indicative of abnormal hypothalamic functions. We find that loss of nr0b1 increases the number of prodynorphin (pdyn)-expressing neurons but decreases the number of pro-opiomelanocortin (pomcb)-expressing neurons in the zebrafish hypothalamic arcuate region (ARC). Further examination reveals that the proliferation of progenitor cells is reduced in the hypothalamus of nr0b1 mutant embryos accompanying with the decreased expression of genes in the Notch signaling pathway. Additionally, the inhibition of Notch signaling in wild-type embryos increases the number of pdyn neurons, mimicking the nr0b1 mutant phenotype. In contrast, ectopic activation of Notch signaling in nr0b1 mutant embryos decreases the number of pdyn neurons. Taken together, our results suggest that nr0b1 regulates neural progenitor proliferation and maintenance to ensure normal hypothalamic neuron development.

Cleavage-embryo genes and transposable elements are regulated by histone variant H2A.X

During mammalian preimplantation development, stimulation of zygotic genome activation (ZGA) and transposable elements (TEs) shapes totipotency profiling. A rare mouse embryonic stem cells (mESCs) subpopulation is capable of transiently entering a state resembling 2-cell stage embryos, with subtypes of TEs expressed and ZGA genes transiently activated. In this study, we found that deletion of H2A.X in mESCs led to a significant upregulation of ZGA genes and misregulated TEs. ChIP-seq analysis indicated a direct association of H2A.X at the Dux locus for silencing the Dux gene and its downstream ZGA genes in mESCs. We also demonstrated that histone variant H2A.X is highly enriched in human cleavage embryos when ZGA genes and TEs are active. Therefore, we propose that H2A.X plays an important role in regulating ZGA genes and TEs to establish totipotency.

Two DNA binding domains of MGA act in combination to suppress ectopic activation of meiosis-related genes in mouse embryonic stem cells

Although the physiological meaning of the high potential of mouse embryonic stem cells (ESCs) for meiotic entry is not understood, a rigid safeguarding system is required to prevent ectopic onset of meiosis. PRC1.6, a non-canonical PRC1, is known for its suppression of precocious and ectopic meiotic onset in germ cells and ESCs, respectively. MGA, a scaffolding component of PRC1.6, bears two distinct DNA-binding domains termed bHLHZ and T-box. However, it is unclear how this feature contributes to the functions of PRC1.6. Here, we demonstrated that both domains repress distinct sets of genes in murine ESCs, but substantial numbers of meiosis-related genes are included in both gene sets. In addition, our data demonstrated that bHLHZ is crucially involved in repressing the expression of Meiosin, which plays essential roles in meiotic entry with Stra8, revealing at least part of the molecular mechanisms that link negative and positive regulation of meiotic onset.

Transcription factor OTX2 silences the expression of cleavage embryo genes and transposable elements

Upon mammalian fertilization, zygotic genome activation (ZGA) and activation of transposable elements (TEs) occur in early embryos to establish totipotency and support embryogenesis. However, the molecular mechanisms controlling the expression of these genes in mammals remain poorly understood. The 2-cell-like population of mouse embryonic stem cells (mESCs) mimics cleavage-stage embryos with transient Dux activation. In this study, we demonstrated that deficiency of the transcription factor OTX2 stimulates the expression of ZGA genes in mESCs. Further analysis revealed that OTX2 is incorporated at the Dux locus with corepressors for transcriptional inhibition. We also found that OTX2 associates with TEs and silences the subtypes of TEs. Therefore, OTX2 protein plays an important role in ZGA and TE expression in mESCs to orchestrate the transcriptional network.

CRISPR-Cas and Its Wide-Ranging Applications: From Human Genome Editing to Environmental Implications, Technical Limitations, Hazards and Bioethical Issues

The CRISPR-Cas system is a powerful tool for in vivo editing the genome of most organisms, including man. During the years this technique has been applied in several fields, such as agriculture for crop upgrade and breeding including the creation of allergy-free foods, for eradicating pests, for the improvement of animal breeds, in the industry of bio-fuels and it can even be used as a basis for a cell-based recording apparatus. Possible applications in human health include the making of new medicines through the creation of genetically modified organisms, the treatment of viral infections, the control of pathogens, applications in clinical diagnostics and the cure of human genetic diseases, either caused by somatic (e.g., cancer) or inherited (mendelian disorders) mutations. One of the most divisive, possible uses of this system is the modification of human embryos, for the purpose of preventing or curing a human being before birth. However, the technology in this field is evolving faster than regulations and several concerns are raised by its enormous yet controversial potential. In this scenario, appropriate laws need to be issued and ethical guidelines must be developed, in order to properly assess advantages as well as risks of this approach. In this review, we summarize the potential of these genome editing techniques and their applications in human embryo treatment. We will analyze CRISPR-Cas limitations and the possible genome damage caused in the treated embryo. Finally, we will discuss how all this impacts the law, ethics and common sense.

Protective effects of mesenchymal stem cells on ischemic brain injury: therapeutic perspectives of regenerative medicine

Cerebral ischemic injury as the main manifestation of stroke can occur in stroke patients (70-80%). Nowadays, the main therapeutic strategy used for ischemic brain injury treatment aims to achieve reperfusion, neuroprotection, and neurorecovery. Also, angiogenesis as a therapeutic approach maybe represents a promising tool to enhance the prognosis of cerebral ischemic stroke. Unfortunately, although many therapeutic approaches as a life-saving gateway for cerebral ischemic injuries like pharmacotherapy and surgical treatments are widely used, they all fail to restore or regenerate damaged neurons in the brain. So, the suitable therapeutic approach would focus on regenerating the lost cells and restore the normal function of the brain. Currently, stem cell-based regenerative medicine introduced a new paradigm approach in cerebral ischemic injuries treatment. Today, in experimental researches, different types of stem cells such as mesenchymal stem cells have been applied. Therefore, stem cell-based regenerative medicine provides the opportunity to inquire and develop a more effective and safer therapeutic approach with the capability to produce and regenerate new neurons in damaged tissues.