Runx1 is required for progression of CD41+ embryonic precursors into HSCs but not prior to this.

Haematopoiesis in adult animals is maintained by haematopoietic stem cells (HSCs), which self-renew and can give rise to all blood cell lineages. The AGM region is an important intra-embryonic site of HSC development and a wealth of evidence indicates that HSCs emerge from the endothelium of the embryonic dorsal aorta and extra-embryonic large arteries. This, however, is a stepwise process that occurs through sequential upregulation of CD41 and CD45 followed by emergence of fully functional definitive HSCs. Although largely dispensable at later stages, the Runx1 transcription factor is crucially important during developmental maturation of HSCs; however, exact points of crucial involvement of Runx1 in this multi-step developmental maturation process remain unclear. Here, we have investigated requirements for Runx1 using a conditional reversible knockout strategy. We report that Runx1 deficiency does not preclude formation of VE-cad+CD45-CD41+ cells, which are phenotypically equivalent to precursors of definitive HSCs (pre-HSC Type I) but blocks transition to the subsequent CD45+ stage (pre-HSC Type II). These data emphasise that developmental progression of HSCs during a very short period of time is regulated by precise stage-specific molecular mechanisms.

A lineage of diploid platelet-forming cells precedes polyploid megakaryocyte formation in the mouse embryo.

In this study, we test the assumption that the hematopoietic progenitor/colony-forming cells of the embryonic yolk sac (YS), which are endowed with megakaryocytic potential, differentiate into the first platelet-forming cells in vivo. We demonstrate that from embryonic day (E) 8.5 all megakaryocyte (MK) colony-forming cells belong to the conventional hematopoietic progenitor cell (HPC) compartment. Although these cells are indeed capable of generating polyploid MKs, they are not the source of the first platelet-forming cells. We show that proplatelet formation first occurs in a unique and previously unrecognized lineage of diploid platelet-forming cells, which develop within the YS in parallel to HPCs but can be specified in the E8.5 Runx1-null embryo despite the absence of the progenitor cell lineage.

Identification of the genes regulated by Wnt-4, a critical signal for commitment of the ovary.

The indifferent mammalian embryonic gonad generates an ovary or testis, but the factors involved are still poorly known. The Wnt-4 signal represents one critical female determinant, since its absence leads to partial female-to-male sex reversal in mouse, but its signalling is as well implicated in the testis development. We used the Wnt-4 deficient mouse as a model to identify candidate gonadogenesis genes, and found that the Notum, Phlda2, Runx-1 and Msx1 genes are typical of the wild-type ovary and the Osr2, Dach2, Pitx2 and Tacr3 genes of the testis. Strikingly, the expression of these latter genes becomes reversed in the Wnt-4 knock-out ovary, suggesting a role in ovarian development. We identified the transcription factor Runx-1 as a Wnt-4 signalling target gene, since it is expressed in the ovary and is reduced upon Wnt-4 knock-out. Consistent with this, introduction of the Wnt-4 signal into early ovary cells ex vivo induces Runx-1 expression, while conversely Wnt-4 expression is down-regulated in the absence of Runx-1. We conclude that the Runx-1 gene can be a Wnt-4 signalling target, and that Runx-1 and Wnt-4 are mutually interdependent in their expression. The changes in gene expression due to the absence of Wnt-4 in gonads reflect the sexually dimorphic role of this signal and its complex gene network in mammalian gonad development.

The essential requirement for Runx1 in the development of the sternum.

Runx1 is highly expressed in chondroprogenitor and osteoprogenitor cells and in vitro experiments suggest that Runx1 is important in the early stages of osteoblast and chondrocyte differentiation. However, because Runx1 knockout mice are early embryonic lethal due to failure of hematopoiesis, the role of Runx1 in skeletogenesis remains unclear. We studied the role of Runx1 in skeletal development using a Runx1 reversible knockout mouse model. By crossing with Tie2-Cre deletor mice, Runx1 expression was selectively rescued in the endothelial and hematopoietic systems but not in the skeleton. Although Runx1(Re/Re) embryos survived until birth and had a generally normal skeleton, the development of mineralization in the sternum and some skull elements was significantly disrupted. In contrast to wild-type embryos, the sternum of E17.5 Runx1(Re/Re) embryos showed high levels of Sox-9 and collagen type II expression and lack of development of hypertrophic chondrocytes. In situ hybridization analysis demonstrated that, in contrast to the vertebrae and long bones, the sternum of wild-type embryos expresses high levels of Runx1, but not Runx2, the master regulator of skeletogenesis. Thus, although Runx1 is not essential for major skeletal development, it does play an essential role in the development of the sternum and some skull elements.

Suppression of interneuron programs and maintenance of selected spinal motor neuron fates by the transcription factor AML1/Runx1.

Individual spinal motor neuron identities are specified in large part by the intrinsic repertoire of transcription factors expressed by undifferentiated progenitors and maturing neurons. It is shown here that the transcription factor AML1/Runx1 (Runx1) is expressed in selected spinal motor neuron subtypes after the onset of differentiation and is both necessary and sufficient to suppress interneuron-specific developmental programs and promote maintenance of motor neuron characteristics. These findings show an important role for Runx1 during the consolidation of selected spinal motor neuron identities. Moreover, they suggest a requirement for a persistent suppression of interneuron genes within maturing motor neurons.

Restoration of Runx1 expression in the Tie2 cell compartment rescues definitive hematopoietic stem cells and extends life of Runx1 knockout animals until birth.

Mice deficient in the runt homology domain transcription factor Runx1/AML1 fail to generate functional clonogenic hematopoietic cells and die in utero by embryonic day 12.5. We previously generated Runx1 reversible knockout mice, in which the Runx1 locus can be restored by Cre-mediated recombination. We show here that selective restoration of the Runx1 locus in the Tie2 cell compartment rescues clonogenic hematopoietic progenitors in early Runx1-null embryos and rescues lymphoid and myeloid lineages during fetal development. Furthermore, fetal liver cells isolated from reactivated Runx1 embryos are capable of long-term multilineage lymphomyeloid reconstitution of adult irradiated recipients, demonstrating the rescue of definitive hematopoietic stem cells. However, this rescue of the definitive hematopoietic hierarchy is not sufficient to rescue the viability of animals beyond birth, pointing to an essential role for Runx1 in other vital developmental processes.

Regulation of boundary cap neural crest stem cell differentiation after transplantation.

Success of cell replacement therapies for neurological disorders will depend largely on the optimization of strategies to enhance viability and control the developmental fate of stem cells after transplantation. Once transplanted, stem/progenitor cells display a tendency to maintain an undifferentiated phenotype or differentiate into inappropriate cell types. Gain and loss of function experiments have revealed key transcription factors which drive differentiation of immature stem/progenitor cells toward more mature stages and eventually to full differentiation. An attractive course of action to promote survival and direct the differentiation of transplanted stem cells to a specific cell type would therefore be to force expression of regulatory differentiation molecules in already transplanted stem cells, using inducible gene expression systems which can be controlled from the outside. Here, we explore this hypothesis by employing a tetracycline gene regulating system (Tet-On) to drive the differentiation of boundary cap neural crest stem cells (bNCSCs) toward a sensory neuron fate after transplantation. We induced the expression of the key transcription factor Runx1 in Sox10-expressing bNCSCs. Forced expression of Runx1 strongly increased transplant survival in the enriched neurotrophic environment of the dorsal root ganglion cavity, and was sufficient to guide differentiation of bNCSCs toward a nonpeptidergic nociceptive sensory neuron phenotype both in vitro and in vivo after transplantation. These findings suggest that exogenous activation of transcription factors expression after transplantation in stem/progenitor cell grafts can be a constructive approach to control their survival as well as their differentiation to the desired type of cell and that the Tet-system is a useful tool to achieve this.

Association of congenital cardiovascular malformation and neuropsychiatric phenotypes with 15q11.2 (BP1-BP2) deletion in the UK Biobank.

Deletion of a non-imprinted 500kb genomic region at chromosome 15q11.2, between breakpoints 1 and 2 of the Prader-Willi/Angelman locus (BP1-BP2 deletion), has been associated in previous studies with phenotypes including congenital cardiovascular malformations (CVM). Previous studies investigating association between BP1-BP2 deletion and CVM have tended to recruit cases with rarer and more severe CVM phenotypes; the impact of CVM on relatively unselected population cohorts, anticipated to contain chiefly less severe but commoner CHD phenotypes, is relatively unexplored. More precisely defining the impact of BP1-BP2 deletion on CVM risk could be useful to guide genetic counselling, since the deletion is frequently identified in the neurodevelopmental clinic. Using the UK Biobank (UKB) cohort of ~500,000 individuals, we identified individuals with CVM and investigated the association with deletions at the BP1-BP2 locus. In addition, we assessed the association of BP1-BP2 deletions with neuropsychiatric diagnoses, cognitive function and academic achievement. Cases of CVM had an increased prevalence of the deletion compared with controls (0.64%; OR = 1.73 [95% CI 1.08-2.75]; p = 0.03), as did those with neuropsychiatric diagnoses (0.68%; OR = 1.84 [95% CI 1.23-2.75]; p = 0.004). We conclude that BP1-BP2 deletion moderately increases the risk of the generally milder, but commoner, CVM phenotypes seen in this unselected population, in addition to its previously demonstrated association in case/control studies ascertained for CVM.

Hierarchical organization and early hematopoietic specification of the developing HSC lineage in the AGM region.

The aorta-gonad-mesonephros region plays an important role in hematopoietic stem cell (HSC) development during mouse embryogenesis. The vascular endothelial cadherin⁺ CD45⁺ (VE-cad⁺CD45⁺) population contains the major type of immature pre-HSCs capable of developing into long-term repopulating definitive HSCs. In this study, we developed a new coaggregation culture system, which supports maturation of a novel population of CD45-negative (VE-cad⁺CD45⁻CD41⁺) pre-HSCs into definitive HSCs. The appearance of these pre-HSCs precedes development of the VE-cad⁺CD45⁺ pre-HSCs (termed here type I and type II pre-HSCs, respectively), thus establishing a hierarchical directionality in the developing HSC lineage. By labeling the luminal surface of the dorsal aorta, we show that both type I and type II pre-HSCs are distributed broadly within the endothelial and subendothelial aortic layers, in contrast to mature definitive HSCs which localize to the aortic endothelial layer. In agreement with expression of CD41 in pre-HSCs, in vivo CD41-Cre-mediated genetic tagging occurs in embryonic pre-HSCs and persists in all lymphomyeloid lineages of the adult animal.

In vitro and in vivo effects on neural crest stem cell differentiation by conditional activation of Runx1 short isoform and its effect on neuropathic pain behavior.

INTRODUCTION: Runx1, a Runt domain transcription factor, controls the differentiation of nociceptors that express the neurotrophin receptor Ret, regulates the expression of many ion channels and receptors, and controls the lamina-specific innervation pattern of nociceptive afferents in the spinal cord. Moreover, mice lacking Runx1 exhibit specific defects in thermal and neuropathic pain. We investigated whether conditional activation of Runx1 short isoform (Runx1a), which lacks a transcription activation domain, influences differentiation of neural crest stem cells (NCSCs) in vitro and in vivo during development and whether postnatal Runx1a activation affects the sensitivity to neuropathic pain. METHODS: We activated ectopic expression of Runx1a in cultured NCSCs using the Tet-ON gene regulatory system during the formation of neurospheres and analyzed the proportion of neurons and glial cells originating from NCSCs. In in vivo experiments we applied doxycycline (DOX) to pregnant mice (days 8-11), i.e. when NCSCs actively migrate, and examined the phenotype of offsprings. We also examined whether DOX-induced activation of Runx1a in adult mice affects their sensitivity to mechanical stimulation following a constriction injury of the sciatic nerve. RESULTS: Ectopic Runx1a expression in cultured NCSCs resulted in predominantly glial differentiation. Offsprings in which Runx1a had been activated showed retarded growth and displayed megacolon, pigment defects, and dystrophic dorsal root ganglia. In the neuropathic pain model, the threshold for mechanical sensitivity was markedly increased following activation of Runx1a. CONCLUSION: These data suggest that Runx1a has a specific role in NCSC development and that modulation of Runx1a activity may reduce mechanical hypersensitivity associated with neuropathic pain.

Multifunctional reversible knockout/reporter system enabling fully functional reconstitution of the AML1/Runx1 locus and rescue of hematopoiesis.

Mice deficient in the runt homology domain transcription factor Runx1 die of severe anemia in utero by embryonic day (E)12.5. A reactivatable Runx1 knockout embryonic stem cell (ESC) and mouse systems were generated by the targeted insertion of a loxP-flanked multipartite gene stop/trap cassette designed to simultaneously ablate the expression of Runx1 and report on the activity of its promoters. The cassette's in-frame LacZ reporter enabled activities of the proximal and the distal promoters to be differentially monitored. Although Runx1-null ESCs were capable of primitive erythroid differentiation in vitro, their capacity to generate granulocyte/macrophage or mixed myelo-erythroid embryoid bodies was lost. Cre-mediated reactivation restored Runx1 structural integrity and rescued the hematopoietic differentiation potential of ESCs. Mice with the reactivated allele survived, showed no hematopoietic deficit, and expressed all major splice isoforms of Runx1 appropriately. This multipurpose mouse model will be useful for the analysis of the critical Runx1-dependent check-point(s) in hematopoietic development.