RUNX1c Regulates Hematopoietic Differentiation of Human Pluripotent Stem Cells Possibly in Cooperation with Proinflammatory Signaling.

Runt-related transcription factor 1 (Runx1) is a master hematopoietic transcription factor essential for hematopoietic stem cell (HSC) emergence. Runx1-deficient mice die during early embryogenesis due to the inability to establish definitive hematopoiesis. Here, we have used human pluripotent stem cells (hPSCs) as model to study the role of RUNX1 in human embryonic hematopoiesis. Although the three RUNX1 isoforms a, b, and c were induced in CD45+ hematopoietic cells, RUNX1c was the only isoform induced in hematoendothelial progenitors (HEPs)/hemogenic endothelium. Constitutive expression of RUNX1c in human embryonic stem cells enhanced the appearance of HEPs, including hemogenic (CD43+) HEPs and promoted subsequent differentiation into blood cells. Conversely, specific deletion of RUNX1c dramatically reduced the generation of hematopoietic cells from HEPs, indicating that RUNX1c is a master regulator of human hematopoietic development. Gene expression profiling of HEPs revealed a RUNX1c-induced proinflammatory molecular signature, supporting previous studies demonstrating proinflammatory signaling as a regulator of HSC emergence. Collectively, RUNX1c orchestrates hematopoietic specification of hPSCs, possibly in cooperation with proinflammatory signaling. Stem Cells 2017;35:2253-2266.

Multimodal imaging reveals structural and functional heterogeneity in different bone marrow compartments: functional implications on hematopoietic stem cells.

Intravital microscopy of the calvarium is the only noninvasive method for high-resolution imaging of the bone marrow (BM) and hematopoietic stem cell (HSC) niches. However, it is unclear if the calvarium is representative of all BM compartments. Using the combination of whole body optical imaging, intravital microscopy, and "in vivo fluorescence trapping," a thorough comparison of HSCs and putative HSC niches in the calvaria, epiphyses, and diaphyses, at steady state or after HSC transplantation, can be made. We report substantial heterogeneity between different BM compartments in terms of bone-remodeling activity (BRA), blood volume fraction (BVF), and hypoxia. Although BVF is high in all BM compartments, including areas adjacent to the endosteum, we found that compartments displaying the highest BVF and BRA were preferentially seeded and engrafted upon HSC transplantation. Unexpectedly, the macroanatomical distribution of HSCs at steady state is homogeneous across these 3 areas and independent of these 2 parameters and suggests the existence of "reconstituting niches," which are distinct from "homeostatic niches." Both types of niches were observed in the calvarium, indicating that endochondral ossification, the process needed for the formation of HSC niches during embryogenesis, is dispensable for the formation of HSC niches during adulthood.

EZH2 represses mesenchymal genes and upholds the epithelial state of breast carcinoma cells.

Emerging studies support that the polycomb repressive complex 2 (PRC2) regulates phenotypic changes of carcinoma cells by modulating their shifts among metastable states within the epithelial and mesenchymal spectrum. This new role of PRC2 in cancer has been recently proposed to stem from the ability of its catalytic subunit EZH2 to bind and modulate the transcription of mesenchymal genes during epithelial-mesenchymal transition (EMT) in lung cancer cells. Here, we asked whether this mechanism is conserved in other types of carcinomas. By combining TGF-ß-mediated reversible induction of epithelial to mesenchymal transition and inhibition of EZH2 methyltransferase activity, we demonstrate that EZH2 represses a large set of mesenchymal genes and favours the residence of breast cancer cells towards the more epithelial spectrum during EMT. In agreement, analysis of human patient samples supports that EZH2 is required to efficiently repress mesenchymal genes in breast cancer tumours. Our results indicate that PRC2 operates through similar mechanisms in breast and lung cancer cells. We propose that PRC2-mediated direct transcriptional modulation of the mesenchymal gene expression programme is a conserved molecular mechanism underlying cell dissemination across human carcinomas.

Changes in PRC1 activity during interphase modulate lineage transition in pluripotent cells.

The potential of pluripotent cells to respond to developmental cues and trigger cell differentiation is enhanced during the G1 phase of the cell cycle, but the molecular mechanisms involved are poorly understood. Variations in polycomb activity during interphase progression have been hypothesized to regulate the cell-cycle-phase-dependent transcriptional activation of differentiation genes during lineage transition in pluripotent cells. Here, we show that recruitment of Polycomb Repressive Complex 1 (PRC1) and associated molecular functions, ubiquitination of H2AK119 and three-dimensional chromatin interactions, are enhanced during S and G2 phases compared to the G1 phase. In agreement with the accumulation of PRC1 at target promoters upon G1 phase exit, cells in S and G2 phases show firmer transcriptional repression of developmental regulator genes that is drastically perturbed upon genetic ablation of the PRC1 catalytic subunit RING1B. Importantly, depletion of RING1B during retinoic acid stimulation interferes with the preference of mouse embryonic stem cells (mESCs) to induce the transcriptional activation of differentiation genes in G1 phase. We propose that incremental enrolment of polycomb repressive activity during interphase progression reduces the tendency of cells to respond to developmental cues during S and G2 phases, facilitating activation of cell differentiation in the G1 phase of the pluripotent cell cycle.

EZH2 endorses cell plasticity to non-small cell lung cancer cells facilitating mesenchymal to epithelial transition and tumour colonization.

Reversible transition between the epithelial and mesenchymal states are key aspects of carcinoma cell dissemination and the metastatic disease, and thus, characterizing the molecular basis of the epithelial to mesenchymal transition (EMT) is crucial to find druggable targets and more effective therapeutic approaches in cancer. Emerging studies suggest that epigenetic regulators might endorse cancer cells with the cell plasticity required to conduct dynamic changes in cell state during EMT. However, epigenetic mechanisms involved remain mostly unknown. Polycomb Repressive Complexes (PRCs) proteins are well-established epigenetic regulators of development and stem cell differentiation, but their role in different cancer systems is inconsistent and sometimes paradoxical. In this study, we have analysed the role of the PRC2 protein EZH2 in lung carcinoma cells. We found that besides its described role in CDKN2A-dependent cell proliferation, EZH2 upholds the epithelial state of cancer cells by repressing the transcription of hundreds of mesenchymal genes. Chemical inhibition or genetic removal of EZH2 promotes the residence of cancer cells in the mesenchymal state during reversible epithelial-mesenchymal transition. In fitting, analysis of human patient samples and tumour xenograft models indicate that EZH2 is required to efficiently repress mesenchymal genes and facilitate tumour colonization in vivo. Overall, this study discloses a novel role of PRC2 as a master regulator of EMT in carcinoma cells. This finding has important implications for the design of therapies based on EZH2 inhibitors in human cancer patients.

Improved Functionality of Integration-Deficient Lentiviral Vectors (IDLVs) by the Inclusion of IS2 Protein Docks.

Integration-deficient lentiviral vectors (IDLVs) have recently generated increasing interest, not only as a tool for transient gene delivery, but also as a technique for detecting off-target cleavage in gene-editing methodologies which rely on customized endonucleases (ENs). Despite their broad potential applications, the efficacy of IDLVs has historically been limited by low transgene expression and by the reduced sensitivity to detect low-frequency off-target events. We have previously reported that the incorporation of the chimeric sequence element IS2 into the long terminal repeat (LTR) of IDLVs increases gene expression levels, while also reducing the episome yield inside transduced cells. Our study demonstrates that the effectiveness of IDLVs relies on the balance between two parameters which can be modulated by the inclusion of IS2 sequences. In the present study, we explore new IDLV configurations harboring several elements based on IS2 modifications engineered to mediate more efficient transgene expression without affecting the targeted cell load. Of all the insulators and configurations analysed, the insertion of the IS2 into the 3'LTR produced the best results. After demonstrating a DAPI-low nuclear gene repositioning of IS2-containing episomes, we determined whether, in addition to a positive effect on transcription, the IS2 could improve the capture of IDLVs on double strand breaks (DSBs). Thus, DSBs were randomly generated, using the etoposide or locus-specific CRISPR-Cas9. Our results show that the IS2 element improved the efficacy of IDLV DSB detection. Altogether, our data indicate that the insertion of IS2 into the LTR of IDLVs improved, not only their transgene expression levels, but also their ability to be inserted into existing DSBs. This could have significant implications for the development of an unbiased detection tool for off-target cleavage sites from different specific nucleases.

Polycomb regulation is coupled to cell cycle transition in pluripotent stem cells.

When self-renewing pluripotent cells receive a differentiation signal, ongoing cell duplication needs to be coordinated with entry into a differentiation program. Accordingly, transcriptional activation of lineage specifier genes and cell differentiation is confined to the G1 phase of the cell cycle by unknown mechanisms. We found that Polycomb repressive complex 2 (PRC2) subunits are differentially recruited to lineage specifier gene promoters across cell cycle in mouse embryonic stem cells (mESCs). Jarid2 and the catalytic subunit Ezh2 are markedly accumulated at target promoters during S and G2 phases, while the transcriptionally activating subunits EPOP and EloB are enriched during G1 phase. Fluctuations in the recruitment of PRC2 subunits promote changes in RNA synthesis and RNA polymerase II binding that are compromised in Jarid2 -/- mESCs. Overall, we show that differential recruitment of PRC2 subunits across cell cycle enables the establishment of a chromatin state that facilitates the induction of cell differentiation in G1 phase.

The molecular clock protein Bmal1 regulates cell differentiation in mouse embryonic stem cells.

Mammals optimize their physiology to the light-dark cycle by synchronization of the master circadian clock in the brain with peripheral clocks in the rest of the tissues of the body. Circadian oscillations rely on a negative feedback loop exerted by the molecular clock that is composed by transcriptional activators Bmal1 and Clock, and their negative regulators Period and Cryptochrome. Components of the molecular clock are expressed during early development, but onset of robust circadian oscillations is only detected later during embryogenesis. Here, we have used naïve pluripotent mouse embryonic stem cells (mESCs) to study the role of Bmal1 during early development. We found that, compared to wild-type cells, Bmal1-/- mESCs express higher levels of Nanog protein and altered expression of pluripotency-associated signalling pathways. Importantly, Bmal1-/- mESCs display deficient multi-lineage cell differentiation capacity during the formation of teratomas and gastrula-like organoids. Overall, we reveal that Bmal1 regulates pluripotent cell differentiation and propose that the molecular clock is an hitherto unrecognized regulator of mammalian development.

GENYOi005-A: An induced pluripotent stem cells (iPSCs) line generated from a patient with Familial Platelet Disorder with associated Myeloid Malignancy (FPDMM) carrying a p.Thr196Ala variant.

Familial Platelet Disorder with associated Myeloid Malignancy (FPDMM) is a rare platelet disorder caused by mutations in RUNX1. We generated an iPSC line (GENYOi005-A) from a FPDMM patient with a non-previously reported variant p.Thr196Ala. Non-integrative Sendai viruses expressing the Yamanaka reprogramming factors were used to reprogram peripheral blood mononuclear cells from this FPDMM patient. Characterization of GENYOi005-A included genetic analysis of RUNX1 locus, Short Tandem Repeats profiling, alkaline phosphatase enzymatic activity, expression of pluripotency-associated factors and differentiation studies in vitro and in vivo. This iPSC line will provide a powerful tool to study developmental alterations of FPDMM patients.

Induced pluripotent stem cells derived from Bernard-Soulier Syndrome patient's peripheral blood cells with a p.Phe55Ser mutation in the GPIX gene.

Bernard Soulier Syndrome (BSS) is a rare autosomal platelet disorder characterized by mutations in the von Willebrand factor platelet receptor complex GPIb-V-IX. In this work we have generated an induced pluripotent stem cell (BSS3-PBMC-iPS4F8) from peripheral blood mononuclear cells of a BSS patient with a p.Phe55Ser mutation in the GPIX gene. Characterization of BSS3-PBMC-iPS4F8 showed that these cells maintained the original mutation present in the BSS patient, expressed pluripotent stem cell markers and were able to differentiate into the three germline layers. This new iPSC line will contribute to better understand the biology of BSS disease.

Generation of a human induced pluripotent stem cell (iPSC) line from a Bernard-Soulier syndrome patient with the mutation p.Asn45Ser in the GPIX gene.

Bernard Soulier Syndrome (BSS) is an inherited rare platelet disorder characterized by mutations in the platelet glycoprotein complex GPIb-IX-V. We generated an induced pluripotent stem cell (iPSC) line from a BSS patient with a mutation p.Asn45Ser in the GPIX locus (BSS2-PBMC-iPS4F24). Peripheral blood mononuclear cells were reprogrammed using non-integrative viral transduction. Characterization of BSS2-PBMC-iPS4F24 included mutational analysis of GPIX locus, analysis of conventional pluripotency-associated factors at mRNA and protein level and in vitro and in vivo differentiation studies. This iPSC line will provide a powerful tool to study the biology of BSS disease.

Generation of induced pluripotent stem cells (iPSCs) from a Bernard-Soulier syndrome patient carrying a W71R mutation in the GPIX gene.

We generated an induced pluripotent stem cell (iPSC) line from a Bernard-Soulier Syndrome (BSS) patient carrying the mutation p.Trp71Arg in the GPIX locus (BSS1-PBMC-iPS4F4). Peripheral blood mononuclear cells (PBMCs) were reprogrammed using heat sensitive non-integrative Sendai viruses containing the reprogramming factors Oct3/4, SOX2, KLF4 and c-MYC. Successful silencing of the exogenous reprogramming factors was checked by RT-PCR. Characterization of BSS1-PBMC-iPS4F4 included mutation analysis of GPIX locus, Short Tandem Repeats (STR) profiling, alkaline phosphatase enzymatic activity, analysis of conventional pluripotency-associated factors at mRNA and protein level and in vivo differentiation studies. BSS1-PBMC-iPS4F4 will provide a powerful tool to study BSS.

HIF-2α protects human hematopoietic stem/progenitors and acute myeloid leukemic cells from apoptosis induced by endoplasmic reticulum stress.

Hematopoietic stem and progenitor cells (HSPCs) are exposed to low levels of oxygen in the bone marrow niche, and hypoxia-inducible factors (HIFs) are the main regulators of cellular responses to oxygen variation. Recent studies using conditional knockout mouse models have unveiled a major role for HIF-1α in the maintenance of murine HSCs; however, the role of HIF-2α is still unclear. Here, we show that knockdown of HIF-2α, and to a much lesser extent HIF-1α, impedes the long-term repopulating ability of human CD34(+) umbilical cord blood cells. HIF-2α-deficient HSPCs display increased production of reactive oxygen species (ROS), which subsequently stimulates endoplasmic reticulum (ER) stress and triggers apoptosis by activation of the unfolded-protein-response (UPR) pathway. HIF-2α deregulation also significantly decreased engraftment ability of human acute myeloid leukemia (AML) cells. Overall, our data demonstrate a key role for HIF-2α in the maintenance of human HSPCs and in the survival of primary AML cells.

Recent advances in Drosophila stem cell biology.

Stem cells possess the unique properties of self-renewal and the ability to give rise to multiple types of differentiated tissue. The fruit fly Drosophila melanogaster retains several populations of stem cells during adulthood as well as transient populations of stem cells during development. Studies of these different populations of stem cells using the genetic tools available to Drosophila researchers have played an important role in understanding many conserved stem cell characteristics. This review aims highlight some of the recent contributions from this important model system to our understanding of the myriad of processes that interact to control stem cell biology.

Jak/Stat signalling in niche support cells regulates dpp transcription to control germline stem cell maintenance in the Drosophila ovary.

The existence of specialised regulatory microenvironments or niches that sustain stable stem cell populations is well documented in many tissues. However, the specific mechanisms by which niche support (or stromal) cells govern stem cell maintenance remain largely unknown. Here we demonstrate that removal of the Jak/Stat pathway in support cells of the Drosophila ovarian niche leads to germline stem cell loss by differentiation. Conversely, ectopic Jak/Stat activation in support cells induces stem cell tumours, implying the presence of a signal relay between the stromal compartment and the stem cell population. We further show that ectopic Jak/Stat signalling in support cells augments dpp mRNA levels and increases the range of Dpp signalling, a Bmp2 orthologue known to act as a niche extrinsic factor required for female germline stem cell survival and division. Our results provide strong evidence for a model in which Jak/Stat signalling in somatic support cells regulates dpp transcription to define niche size and to maintain the adjacent germline stem cells in an undifferentiated state.