Dual functions of hypoxia-inducible factor 1 alpha for the commitment of mouse embryonic stem cells toward a neural lineage

From Vessels to Neurons-The Role of Hypoxia Pathway Proteins in Embryonic Neurogenesis

Embryonic neurogenesis can be defined as a period of prenatal development during which divisions of neural stem and progenitor cells give rise to neurons. In the central nervous system of most mammals, including humans, the majority of neocortical neurogenesis occurs before birth. It is a highly spatiotemporally organized process whose perturbations lead to cortical malformations and dysfunctions underlying neurological and psychiatric pathologies, and in which oxygen availability plays a critical role. In case of deprived oxygen conditions, known as hypoxia, the hypoxia-inducible factor (HIF) signaling pathway is activated, resulting in the selective expression of a group of genes that regulate homeostatic adaptations, including cell differentiation and survival, metabolism and angiogenesis. While a physiological degree of hypoxia is essential for proper brain development, imbalanced oxygen levels can adversely affect this process, as observed in common obstetrical pathologies such as prematurity. This review comprehensively explores and discusses the current body of knowledge regarding the role of hypoxia and the HIF pathway in embryonic neurogenesis of the mammalian cortex. Additionally, it highlights existing gaps in our understanding, presents unanswered questions, and provides avenues for future research.

COMMD10 Is Essential for Neural Plate Development during Embryogenesis

The COMMD (copper metabolism MURR1 domain containing) family includes ten structurally conserved proteins (COMMD1 to COMMD10) in eukaryotic multicellular organisms that are involved in a diverse array of cellular and physiological processes, including endosomal trafficking, copper homeostasis, and cholesterol metabolism, among others. To understand the role of COMMD10 in embryonic development, we used Commd10Tg(Vav1-icre)A2Kio/J mice, where the Vav1-cre transgene is integrated into an intron of the Commd10 gene, creating a functional knockout of Commd10 in homozygous mice. Breeding heterozygous mice produced no COMMD10-deficient (Commd10Null) offspring, suggesting that COMMD10 is required for embryogenesis. Analysis of Commd10Null embryos demonstrated that they displayed stalled development by embryonic day 8.5 (E8.5). Transcriptome analysis revealed that numerous neural crest-specific gene markers had lower expression in mutant versus wild-type (WT) embryos. Specifically, Commd10Null embryos displayed significantly lower expression levels of a number of transcription factors, including a major regulator of the neural crest, Sox10. Moreover, several cytokines/growth factors involved in early embryonic neurogenesis were also lower in mutant embryos. On the other hand, Commd10Null embryos demonstrated higher expression of genes involved in tissue remodeling and regression processes. Taken together, our findings show that Commd10Null embryos die by day E8.5 due to COMMD10-dependent neural crest failure, revealing a new and critical role for COMMD10 in neural development.

Hypoxia is fine-tuned by Hif-1α and regulates mesendoderm differentiation through the Wnt/β-Catenin pathway

Background

Hypoxia naturally happens in embryogenesis and thus serves as an important environmental factor affecting embryo development. Hif-1α, an essential hypoxia response factor, was mostly considered to mediate or synergistically regulate the effect of hypoxia on stem cells. However, the function and relationship of hypoxia and Hif-1α in regulating mesendoderm differentiation remains controversial.

Results

We here discovered that hypoxia dramatically suppressed the mesendoderm differentiation and promoted the ectoderm differentiation of mouse embryonic stem cells (mESCs). However, hypoxia treatment after mesendoderm was established promoted the downstream differentiation of mesendoderm-derived lineages. These effects of hypoxia were mediated by the repression of the Wnt/β-Catenin pathway and the Wnt/β-Catenin pathway was at least partially regulated by the Akt/Gsk3β axis. Blocking the Wnt/β-Catenin pathway under normoxia using IWP2 mimicked the effects of hypoxia while activating the Wnt/β-Catenin pathway with CHIR99021 fully rescued the mesendoderm differentiation suppression caused by hypoxia. Unexpectedly, Hif-1α overexpression, in contrast to hypoxia, promoted mesendoderm differentiation and suppressed ectoderm differentiation. Knockdown of Hif-1α under normoxia and hypoxia both inhibited the mesendoderm differentiation. Moreover, hypoxia even suppressed the mesendoderm differentiation of Hif-1α knockdown mESCs, further implying that the effects of hypoxia on the mesendoderm differentiation were Hif-1α independent. Consistently, the Wnt/β-Catenin pathway was enhanced by Hif-1α overexpression and inhibited by Hif-1α knockdown. As shown by RNA-seq, unlike hypoxia, the effect of Hif-1α was relatively mild and selectively regulated part of hypoxia response genes, which fine-tuned the effect of hypoxia on mESC differentiation.

Conclusions

This study revealed that hypoxia is fine-tuned by Hif-1α and regulates the mesendoderm and ectoderm differentiation by manipulating the Wnt/β-Catenin pathway, which contributed to the understanding of hypoxia-mediated regulation of development.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12915-022-01423-y.

Cell differentiation in the cardiac embryonic stem cell test (ESTc) is influenced by the oxygen tension in its underlying embryonic stem cell culture

Oxygen (O₂) levels in the mammalian embryo range between 2.4% and 8%. The cardiac embryonic stem cell test (ESTc) is a model for developmental toxicity predictions, which is usually performed under atmospheric O₂ levels of 20%. We investigated the chemical sensitivity of the ESTc carried out under 20% O₂, using embryonic stem cells (ESC) cultured under either 20% O₂ or 5% O₂. ESC viability was more sensitive to valproic acid (VPA) but less sensitive to flusilazole (FLU) when cultured under 5% versus 20% O₂. For beating cardiomyocyte differentiation, lower ID₅₀ values were found for FLU and VPA when the ESCs had been cultured under 5% versus 20% O₂. At differentiation day 4, gene expression values were primarily driven by the level of O₂ during ESC culture instead of exposure to FLU. In addition, using ESCs cultured under 5% O₂ tension, VPA enhanced Nes (ectoderm) expression. Bmp4 (mesoderm) was enhanced by VPA when using ESCs cultured under 20% O₂. At differentiation day 10, using ESCs cultured under 5% instead of 20% O₂, Nkx2.5 and Myh6 (cardiomyocytes) were less affected after exposure to FLU or VPA. These results show that O₂ tension in ESC culture influences chemical sensitivity in the ESTc. This enhances awareness of the standard culture conditions, which may impact the application of the ESTc in quantitative hazard assessment of chemicals.

Oxygen tension influences embryonic stem cell maintenance and has lineage specific effects on neural and cardiac differentiation

The importance of oxygen tension in in vitro cultures and its effect on embryonic stem cell (ESC) differentiation has been widely acknowledged. Research has mainly focussed on ESC maintenance or on one line of differentiation and only few studies have examined the potential relation between oxygen tension during ESC maintenance and differentiation. In this study we investigated the influence of atmospheric (20%) versus physiologic (5%) oxygen tension in ESC cultures and their differentiation within the cardiac and neural embryonic stem cell tests (ESTc, ESTn). Oxygen tension was set at 5% or 20% and cells were kept in these conditions from starting up cell culture until use for differentiation. Under these oxygen tensions, ESC culture showed no differences in proliferation and gene and protein expression levels. Differentiation was either performed in the same or in the alternative oxygen tension compared to ESC culture creating four different experimental conditions. Cardiac differentiation in 5% instead of 20% oxygen resulted in reduced development of spontaneously beating cardiomyocytes and lower expression of cardiac markers Nkx2.5, Myh6 and MF20 (myosin), regardless whether ESC had been cultured in 5% or 20% oxygen tension. As compared to the control (20% oxygen during stem cell maintenance and differentiation), neural differentiation in 5% oxygen with ESC cultured in 20% oxygen led to more cardiac and neural crest cell differentiation. The opposite experimental condition of neural differentiation in 20% oxygen with ESC cultured in 5% oxygen resulted in more glial differentiation. ESC that were maintained and differentiated in 5% oxygen showed an increase in neural crest and oligodendrocytes as compared to 20% oxygen during stem cell maintenance and differentiation. This study showed major effects on ESC differentiation in ESTc and ESTn of oxygen tension, which is an important variable to consider when designing and developing a stem cell-based in vitro system.

Hypoxia/Hif1α prevents premature neuronal differentiation of neural stem cells through the activation of Hes1

Embryonic neural stem cells (NSCs), comprising neuroepithelial and radial glial cells, are indispensable precursors of neurons and glia in the mammalian developing brain. Since the process of neurogenesis occurs in a hypoxic environment, the question arises of how NSCs deal with low oxygen tension and whether it affects their stemness. Genes from the hypoxia-inducible factors (HIF) family are well known factors governing cellular response to hypoxic conditions. In this study, we have discovered that the endogenous stabilization of hypoxia-inducible factor 1α (Hif1α) during neural induction is critical for the normal development of the NSCs pool by preventing its premature depletion and differentiation. The knock-out of the Hif1α gene in mESC-derived neurospheres led to a decrease in self-renewal of NSCs, paralleled by an increase in neuronal differentiation. Similarly, neuroepithelial cells differentiated in hypoxia exhibited accelerated neurogenesis soon after Hif1α knock-down. In both models, the loss of Hif1α was accompanied by an immediate drop in neural repressor Hes1 levels while changes in Notch signaling were not observed. We found that active Hif1α/Arnt1 transcription complex bound to the evolutionarily conserved site in Hes1 gene promoter in both neuroepithelial cells and neural tissue of E8.5 - 9.5 embryos. Taken together, these results emphasize the novel role of Hif1α in the regulation of early NSCs population through the activation of neural repressor Hes1, independently of Notch signaling.

Modulation of oxygen tension, acidosis, and cell density is crucial for neural differentiation of human induced pluripotent stem cells

Human induced pluripotent stem cell (hiPSC)-derived neural cells provide valuable disease models for pathophysiological analysis and drug discovery for intractable neurodegenerative diseases. However, neural differentiation of hiPSCs requires a complex and long culture procedure, which has been a bottleneck for analysis. We previously demonstrated rapid, efficient, and simple motor neuron differentiation from human pluripotent stem cells (hPSCs). Although optimization of the microenvironment for the differentiation of hPSCs has been considered to achieve more efficient differentiation, it has never been investigated in detail. Here, we demonstrated that three microenvironmental modifiers, oxygen (O₂) tension, pH, and cell density, critically affect neural differentiation of hiPSCs. Hypoxia is known to be involved in neural development in vivo and to promote neural differentiation of PSCs. However, in this study, it caused significant cell death in aggregation culture of human embryoid bodies (hEBs) and negatively affected neural differentiation. Modulation of pH by optimized carbon dioxide (CO₂) tension improved neural differentiation of hiPSCs, but mild acidosis caused by increased CO₂ tension suppressed neural differentiation without cell death. Moreover, high-cell density culture resulted in prominent acidosis and cell death under hypoxic conditions, which synergistically suppressed neural differentiation of hiPSCs. These results suggest that optimization of the microenvironment via O₂ tension, pH, and cell density enables more efficient neural differentiation of hiPSCs for the analysis of neurological diseases.

Stroke Induces Mesenchymal Stem Cell Migration to Infarcted Brain Areas Via CXCR4 and C-Met Signaling

Mesenchymal stem cells circulate between organs to repair and maintain tissues. Mesenchymal stem cells cultured with fetal bovine serum have therapeutic effects when intravenously administered after stroke. However, only a small number of mesenchymal stem cells reach the brain. We hypothesized that the serum from stroke patients increases mesenchymal stem cells trophism toward the infarcted brain area. Mesenchymal stem cells were grown in fetal bovine serum, normal serum from normal rats, or stroke serum from ischemic stroke rats. Compared to the fetal bovine serum group, the stroke serum group but not the normal serum group showed significantly greater migration toward the infarcted brain area in the in vitro and in vivo models (p < 0.05). Both C-X-C chemokine receptor type 4 and c-Met expression levels significantly increased in the stroke serum group than the others. The enhanced mesenchymal stem cells migration of the stroke serum group was abolished by inhibition of signaling. Serum levels of chemokines, cytokines, matrix metalloproteinase, and growth factors were higher in stroke serum than in normal serum. Behavioral tests showed a significant improvement in the recovery after stroke in the stroke serum group than the others. Stroke induces mesenchymal stem cells migration to the infarcted brain area via C-X-C chemokine receptor type 4 and c-Met signaling. Culture expansion using the serum from stroke patients could constitute a novel preconditioning method to enhance the therapeutic efficiency of mesenchymal stem cells.

TRACING CO-REGULATORY NETWORK DYNAMICS IN NOISY, SINGLE-CELL TRANSCRIPTOME TRAJECTORIES

The availability of gene expression data at the single cell level makes it possible to probe the molecular underpinnings of complex biological processes such as differentiation and oncogenesis. Promising new methods have emerged for reconstructing a progression 'trajectory' from static single-cell transcriptome measurements. However, it remains unclear how to adequately model the appreciable level of noise in these data to elucidate gene regulatory network rewiring. Here, we present a framework called Single Cell Inference of MorphIng Trajectories and their Associated Regulation (SCIMITAR) that infers progressions from static single-cell transcriptomes by employing a continuous parametrization of Gaussian mixtures in high-dimensional curves. SCIMITAR yields rich models from the data that highlight genes with expression and co-expression patterns that are associated with the inferred progression. Further, SCIMITAR extracts regulatory states from the implicated trajectory-evolvingco-expression networks. We benchmark the method on simulated data to show that it yields accurate cell ordering and gene network inferences. Applied to the interpretation of a single-cell human fetal neuron dataset, SCIMITAR finds progression-associated genes in cornerstone neural differentiation pathways missed by standard differential expression tests. Finally, by leveraging the rewiring of gene-gene co-expression relations across the progression, the method reveals the rise and fall of co-regulatory states and trajectory-dependent gene modules. These analyses implicate new transcription factors in neural differentiation including putative co-factors for the multi-functional NFAT pathway.

AMPK and HIF signaling pathways regulate both longevity and cancer growth: the good news and the bad news about survival mechanisms

The AMP-activated protein kinase (AMPK) and hypoxia-inducible factor (HIF) signaling pathways are evolutionarily-conserved survival mechanisms responding to two fundamental stresses, energy deficiency and/or oxygen deprivation. The AMPK and HIF pathways regulate the function of a survival network with several transcription factors, e.g. FOXO, NF-κB, NRF2, and p53, as well as with protein kinases and other factors, such as mTOR, ULK1, HDAC5, and SIRT1. Given that AMPK and HIF activation can enhance not only healthspan and lifespan but also cancer growth in a context-dependent manner; it seems that cancer cells can hijack certain survival factors to maintain their growth in harsh conditions. AMPK activation improves energy metabolism, stimulates autophagy, and inhibits inflammation, whereas HIF-1α increases angiogenesis and helps cells to adapt to severe conditions. First we will review how AMPK and HIF signaling mechanisms control the function of an integrated survival network which is able not only to improve the regulation of longevity but also support the progression of tumorigenesis. We will also describe distinct crossroads between the regulation of longevity and cancer, e.g. specific regulation through the AMPKα and HIF-α isoforms, the Warburg effect, mitochondrial dynamics, and cellular senescence.

Stem cell research and regenerative medicine in 2014: first year of regenerative medicine in Japan

It is my great pleasure to announce that we were able to publish the Japan Issue in Stem Cells and Development, especially in this year 2014. This year, 2014, is said to be the First Year of Regenerative Medicine in Japan. This movement is likely to be based on the establishment of a new law system regarding regenerative medicine (an Act for Ensuring the Safety of Regenerative Medicine or the so-called Regenerative Medicine Law) and the partial revision of the Pharmaceutical Affairs Law (PAL). Both laws will come into effect in 2014 in this country. These new law systems are expected to have a great impact on the facilitation of R&D related to regenerative medicine and stem cell biology. In the present Japan Issue, some excellent stem cell research in this country will be introduced to celebrate the First Year of Regenerative Medicine in Japan.

Antioxidant and protective mechanisms against hypoxia and hypoglycaemia in cortical neurons in vitro

In the present work, we have studied whether cell death could be induced in cortical neurons from rats subjected to different period of O₂ deprivation and low glucose (ODLG). This “in vitro” model is designed to emulate the penumbra area under ischemia. In these conditions, cortical neurons displayed loss of mitochondrial respiratory ability however, nor necrosis neither apoptosis occurred despite ROS production. The absence of cellular death could be a consequence of increased antioxidant responses such as superoxide dismutase-1 (SOD1) and GPX3. In addition, the levels of reduced glutathione were augmented and HIF-1/3α overexpressed. After long periods of ODLG (12–24 h) cortical neurons showed cellular and mitochondrial membrane alterations and did not recuperate cellular viability during reperfusion. This could mean that therapies directed toward prevention of cellular and mitochondrial membrane imbalance or cell death through mechanisms other than necrosis or apoptosis, like authophagy, may be a way to prevent ODLG damage.