Mcph1, mutated in primary microcephaly, is also crucial for erythropoiesis.

Microcephaly is a common feature in inherited bone marrow failure syndromes, prompting investigations into shared pathways between neurogenesis and hematopoiesis. To understand this association, we studied the role of the microcephaly gene Mcph1 in hematological development. Our research revealed that Mcph1-knockout mice exhibited congenital macrocytic anemia due to impaired terminal erythroid differentiation during fetal development. Anemia's cause is a failure to complete cell division, evident from tetraploid erythroid progenitors with DNA content exceeding 4n. Gene expression profiling demonstrated activation of the p53 pathway in Mcph1-deficient erythroid precursors, leading to overexpression of Cdkn1a/p21, a major mediator of p53-dependent cell cycle arrest. Surprisingly, fetal brain analysis revealed hypertrophied binucleated neuroprogenitors overexpressing p21 in Mcph1-knockout mice, indicating a shared pathophysiological mechanism underlying both erythroid and neurological defects. However, inactivating p53 in Mcph1-/- mice failed to reverse anemia and microcephaly, suggesting that p53 activation in Mcph1-deficient cells resulted from their proliferation defect rather than causing it. These findings shed new light on Mcph1's function in fetal hematopoietic development, emphasizing the impact of disrupted cell division on neurogenesis and erythropoiesis - a common limiting pathway.

A multistep computational approach reveals a neuro-mesenchymal cell population in the embryonic hematopoietic stem cell niche.

The first hematopoietic stem and progenitor cells (HSPCs) emerge in the Aorta-Gonad-Mesonephros (AGM) region of the mid-gestation mouse embryo. However, the precise nature of their supportive mesenchymal microenvironment remains largely unexplored. Here, we profiled transcriptomes of laser micro-dissected aortic tissues at three developmental stages and individual AGM cells. Computational analyses allowed the identification of several cell subpopulations within the E11.5 AGM mesenchyme, with the presence of a yet unidentified subpopulation characterized by the dual expression of genes implicated in adhesive or neuronal functions. We confirmed the identity of this cell subset as a neuro-mesenchymal population, through morphological and lineage tracing assays. Loss of function in the zebrafish confirmed that Decorin, a characteristic extracellular matrix component of the neuro-mesenchyme, is essential for HSPC development. We further demonstrated that this cell population is not merely derived from the neural crest, and hence, is a bona fide novel subpopulation of the AGM mesenchyme.

Clonal hematopoiesis driven by chromosome 1q/MDM4 trisomy defines a canonical route toward leukemia in Fanconi anemia.

Fanconi anemia (FA) patients experience chromosome instability, yielding hematopoietic stem/progenitor cell (HSPC) exhaustion and predisposition to poor-prognosis myeloid leukemia. Based on a longitudinal cohort of 335 patients, we performed clinical, genomic, and functional studies in 62 patients with clonal evolution. We found a unique pattern of somatic structural variants and mutations that shares features of BRCA-related cancers, the FA-hallmark being unbalanced, microhomology-mediated translocations driving copy-number alterations. Half the patients developed chromosome 1q gain, driving clonal hematopoiesis through MDM4 trisomy downmodulating p53 signaling later followed by secondary acute myeloid lukemia genomic alterations. Functionally, MDM4 triplication conferred greater fitness to murine and human primary FA HSPCs, rescued inflammation-mediated bone marrow failure, and drove clonal dominance in FA mouse models, while targeting MDM4 impaired leukemia cells in vitro and in vivo. Our results identify a linear route toward secondary leukemogenesis whereby early MDM4-driven downregulation of basal p53 activation plays a pivotal role, opening monitoring and therapeutic prospects.

Erythropoietin is a major regulator of thrombopoiesis in thrombopoietin-dependent and -independent contexts.

Thrombopoietin (TPO), through activation of its cognate receptor Mpl, is the major regulator of platelet production. However, residual platelets observed in TPO- and Mpl-loss-of-function (LOF) mice suggest the existence of an additional factor to TPO in platelet production. As erythropoietin (EPO) exhibited both in vitro megakaryocytic potential, in association with other early-acting cytokines, and in vivo platelet activation activity, we sought to investigate its role in this setting. Here, we used multiple LOF models to decipher the reciprocal role of EPO and TPO in the regulation of platelet production in TPO-LOF and Mpl-LOF mice and of platelet size heterogeneity in wild-type mice. We first identified EPO as the major thrombopoietic factor in the absence of the TPO-Mpl pathway. Based on the study of several mouse models we found that the EPO-EPO receptor pathway acts on late-stage megakaryopoiesis and is responsible for large-sized platelet production, while the TPO-Mpl pathway promotes small-sized platelet production. On the basis of our data, EPO might be used for thrombocytopenia supportive therapy in congenital amegakaryocytopoiesis. Furthermore, as a distribution skewed toward large platelets is an independent risk factor and a poor prognosis indicator in atherothrombosis, the characterization of EPO's role in the production of large-sized platelets, if confirmed in humans, may open new perspectives in the understanding of the role of EPO-induced platelets in atherothrombosis.

Erythrocyte-derived microvesicles induce arterial spasms in JAK2V617F myeloproliferative neoplasm.

Arterial cardiovascular events are the leading cause of death in patients with JAK2V617F myeloproliferative neoplasms (MPNs). However, their mechanisms are poorly understood. The high prevalence of myocardial infarction without significant coronary stenosis or atherosclerosis in patients with MPNs suggests that vascular function is altered. The consequences of JAK2V617F mutation on vascular reactivity are unknown. We observe here increased responses to vasoconstrictors in arteries from Jak2V617F mice resulting from a disturbed endothelial NO pathway and increased endothelial oxidative stress. This response was reproduced in WT mice by circulating microvesicles isolated from patients carrying JAK2V617F and by erythrocyte-derived microvesicles from transgenic mice. Microvesicles of other cellular origins had no effect. This effect was observed ex vivo on isolated aortas, but also in vivo on femoral arteries. Proteomic analysis of microvesicles derived from JAK2V617F erythrocytes identified increased expression of myeloperoxidase as the likely mechanism accounting for their effect. Myeloperoxidase inhibition in microvesicles derived from JAK2V617F erythrocytes suppressed their effect on oxidative stress. Antioxidants such as simvastatin and N-acetyl cysteine improved arterial dysfunction in Jak2V617F mice. In conclusion, JAK2V617F MPNs are characterized by exacerbated vasoconstrictor responses resulting from increased endothelial oxidative stress caused by circulating erythrocyte-derived microvesicles. Simvastatin appears to be a promising therapeutic strategy in this setting.

In vivo generation of haematopoietic stem/progenitor cells from bone marrow-derived haemogenic endothelium.

It is well established that haematopoietic stem and progenitor cells (HSPCs) are generated from a transient subset of specialized endothelial cells termed haemogenic, present in the yolk sac, placenta and aorta, through an endothelial-to-haematopoietic transition (EHT). HSPC generation via EHT is thought to be restricted to the early stages of development. By using experimental embryology and genetic approaches in birds and mice, respectively, we document here the discovery of a bone marrow haemogenic endothelium in the late fetus/young adult. These cells are capable of de novo producing a cohort of HSPCs in situ that harbour a very specific molecular signature close to that of aortic endothelial cells undergoing EHT or their immediate progenies, i.e., recently emerged HSPCs. Taken together, our results reveal that HSPCs can be generated de novo past embryonic stages. Understanding the molecular events controlling this production will be critical for devising innovative therapies.

Studies in an Early Development Window Unveils a Severe HSC Defect in both Murine and Human Fanconi Anemia.

Fanconi anemia (FA) causes bone marrow failure early during childhood, and recent studies indicate that a hematopoietic defect could begin in utero. We performed a unique kinetics study of hematopoiesis in Fancg-/- mouse embryos, between the early embryonic day 11.5 (E11.5) to E12.5 developmental window (when the highest level of hematopoietic stem cells [HSC] amplification takes place) and E14.5. This study reveals a deep HSC defect with exhaustion of proliferative and self-renewal capacities very early during development, together with severe FA clinical and biological manifestations, which are mitigated at E14.5 due to compensatory mechanisms that help to ensure survival of Fancg-/- embryos. It also reports that a deep HSC defect is also observed during human FA development, and that human FA fetal liver (FL) HSCs present a transcriptome profile similar to that of mouse E12.5 Fancg-/- FL HSCs. Altogether, our results highlight that early mouse FL could represent a good alternative model for studying Fanconi pathology.

Endothelial autophagic flux hampers atherosclerotic lesion development.

Blood flowing in arteries generates shear forces at the surface of the vascular endothelium that control its anti-atherogenic properties. However, due to the architecture of the vascular tree, these shear forces are heterogeneous and atherosclerotic plaques develop preferentially in areas where shear is low or disturbed. Here we review our recent study showing that elevated shear forces stimulate endothelial autophagic flux and that inactivating the endothelial macroautophagy/autophagy pathway promotes a proinflammatory, prosenescent and proapoptotic cell phenotype despite the presence of atheroprotective shear forces. Specific deficiency in endothelial autophagy in a murine model of atherosclerosis stimulates the development of atherosclerotic lesions exclusively in areas of the vasculature that are normally resistant to atherosclerosis. Our findings demonstrate that adequate endothelial autophagic flux limits atherosclerotic plaque formation by preventing endothelial apoptosis, senescence and inflammation.

Autophagy is required for endothelial cell alignment and atheroprotection under physiological blood flow.

It has been known for some time that atherosclerotic lesions preferentially develop in areas exposed to low SS and are characterized by a proinflammatory, apoptotic, and senescent endothelial phenotype. Conversely, areas exposed to high SS are protected from plaque development, but the mechanisms have remained elusive. Autophagy is a protective mechanism that allows recycling of defective organelles and proteins to maintain cellular homeostasis. We aimed to understand the role of endothelial autophagy in the atheroprotective effect of high SS. Atheroprotective high SS stimulated endothelial autophagic flux in human and murine arteries. On the contrary, endothelial cells exposed to atheroprone low SS were characterized by inefficient autophagy as a result of mammalian target of rapamycin (mTOR) activation, AMPKα inhibition, and blockade of the autophagic flux. In hypercholesterolemic mice, deficiency in endothelial autophagy increased plaque burden only in the atheroresistant areas exposed to high SS; plaque size was unchanged in atheroprone areas, in which endothelial autophagy flux is already blocked. In cultured cells and in transgenic mice, deficiency in endothelial autophagy was characterized by defects in endothelial alignment with flow direction, a hallmark of endothelial cell health. This effect was associated with an increase in endothelial apoptosis and senescence in high-SS regions. Deficiency in endothelial autophagy also increased TNF-α-induced inflammation under high-SS conditions and decreased expression of the antiinflammatory factor KLF-2. Altogether, these results show that adequate endothelial autophagic flux under high SS limits atherosclerotic plaque formation by preventing endothelial apoptosis, senescence, and inflammation.

Endothelial Epas1 Deficiency Is Sufficient To Promote Parietal Epithelial Cell Activation and FSGS in Experimental Hypertension.

FSGS, the most common primary glomerular disorder causing ESRD, is a complex disease that is only partially understood. Progressive sclerosis is a hallmark of FSGS, and genetic tracing studies have shown that parietal epithelial cells participate in the formation of sclerotic lesions. The loss of podocytes triggers a focal activation of parietal epithelial cells, which subsequently form cellular adhesions with the capillary tuft. However, in the absence of intrinsic podocyte alterations, the origin of the pathogenic signal that triggers parietal epithelial cell recruitment remains elusive. In this study, investigation of the role of the endothelial PAS domain-containing protein 1 (EPAS1), a regulatory α subunit of the hypoxia-inducible factor complex, during angiotensin II-induced hypertensive nephropathy provided novel insights into FSGS pathogenesis in the absence of a primary podocyte abnormality. We infused angiotensin II into endothelial-selective Epas1 knockout mice and their littermate controls. Although the groups presented with identical high BP, endothelial-specific Epas1 gene deletion accentuated albuminuria with severe podocyte lesions and recruitment of pathogenic parietal glomerular epithelial cells. These lesions and dysfunction of the glomerular filtration barrier were associated with FSGS in endothelial Epas1-deficient mice only. These results indicate that endothelial EPAS1 has a global protective role during glomerular hypertensive injuries without influencing the hypertensive effect of angiotensin II. Furthermore, these findings provide proof of principle that endothelial-derived signaling can trigger FSGS and illustrate the potential importance of the EPAS1 endothelial transcription factor in secondary FSGS.

Monoclonal antibody 1.6.1 against human MPL receptor allows HSC enrichment of CB and BM CD34(+)CD38(-) populations.

Thrombopoietin (TPO) and its receptor Mpl (CD110) play a crucial role in the regulation of hematopoietic stem cells (HSCs). Functional study of Mpl-expressing HSCs has, however, been hampered by the lack of efficient monoclonal antibodies, explaining the very few data available on Mpl(+) HSCs during human embryonic development and after birth. Investigating the main monoclonal antibodies used so far to sort CD110(+) cells from cord blood (CB) and adult bone marrow (BM), we found that only the recent monoclonal antibody 1.6.1 engineered by Immunex Corporation was specific. Using in vitro functional assays, we found that this antibody can be used to sort a CD34(+)CD38(-)CD110(+) population enriched in hematopoietic progenitor stem cells, both in CB and in adult BM. In vivo injection into NSG mice further indicated that the CB CD34(+)CD38(-)CD110(+) population is highly enriched in HSCs compared with both CD34(+)CD38(-)CD110(-) and CD34(+)CD38(-) populations. Together our results validate MAb1.6.1 as an important tool, which has so far been lacking, in the HSC field.

Endothelial cell and podocyte autophagy synergistically protect from diabetes-induced glomerulosclerosis.

The glomerulus is a highly specialized capillary tuft, which under pressure filters large amounts of water and small solutes into the urinary space, while retaining albumin and large proteins. The glomerular filtration barrier (GFB) is a highly specialized filtration interface between blood and urine that is highly permeable to small and midsized solutes in plasma but relatively impermeable to macromolecules such as albumin. The integrity of the GFB is maintained by molecular interplay between its 3 layers: the glomerular endothelium, the glomerular basement membrane and podocytes, which are highly specialized postmitotic pericytes forming the outer part of the GFB. Abnormalities of glomerular ultrafiltration lead to the loss of proteins in urine and progressive renal insufficiency, underlining the importance of the GFB. Indeed, albuminuria is strongly predictive of the course of chronic nephropathies especially that of diabetic nephropathy (DN), a leading cause of renal insufficiency. We found that high glucose concentrations promote autophagy flux in podocyte cultures and that the abundance of LC3B II in podocytes is high in diabetic mice. Deletion of Atg5 specifically in podocytes resulted in accelerated diabetes-induced podocytopathy with a leaky GFB and glomerulosclerosis. Strikingly, genetic alteration of autophagy on the other side of the GFB involving the endothelial-specific deletion of Atg5 also resulted in capillary rarefaction and accelerated DN. Thus autophagy is a key protective mechanism on both cellular layers of the GFB suggesting autophagy as a promising new therapeutic strategy for DN.

Dorso-ventral contributions in the formation of the embryonic aorta and the control of aortic hematopoiesis.

The embryonic dorsal aorta plays a pivotal role in the production of the first hematopoietic stem cells (HSCs), the founders of the adult hematopoietic system. HSC production is polarized by being restricted to the aortic floor where a specialized subset of endothelial cells (ECs) endowed with hemogenic properties undergo an endothelial-to-hematopoietic production resulting in the formation of the intra-aortic hematopoietic clusters. This production is tightly time- and space-controlled with the transcription factor Runx1 playing a key role in this process and the surrounding tissues controlling the aortic shape and fate. In this paper, we shall review (a) how hemogenic ECs differentiate from the mesoderm, (b) how the different aortic components assemble coordinately to establish the dorso-ventral polarity, and (c) how this results in the initiation of Runx1 expression in hemogenic ECs and the initiation of the hematopoietic program. These observations should elucidate the first steps in HSC commitment and help in developing techniques to manipulate adult HSCs.

Endothelio-mesenchymal interaction controls runx1 expression and modulates the notch pathway to initiate aortic hematopoiesis.

Hematopoietic stem cells (HSCs) are produced by a small cohort of hemogenic endothelial cells (ECs) during development through the formation of intra-aortic hematopoietic cell (HC) clusters. The Runx1 transcription factor plays a key role in the EC-to-HC and -HSC transition. We show that Runx1 expression in hemogenic ECs and the subsequent initiation of HC formation are tightly controlled by the subaortic mesenchyme, although the mesenchyme is not a source of HCs. Runx1 and Notch signaling are involved in this process, with Notch signaling decreasing with time in HCs. Inhibiting Notch signaling readily increases HC production in mouse and chicken embryos. In the mouse, however, this increase is transient. Collectively, we show complementary roles of hemogenic ECs and mesenchymal compartments in triggering aortic hematopoiesis. The subaortic mesenchyme induces Runx1 expression in hemogenic-primed ECs and collaborates with Notch dynamics to control aortic hematopoiesis.

Thrombopoietin-increased DNA-PK-dependent DNA repair limits hematopoietic stem and progenitor cell mutagenesis in response to DNA damage.

DNA double-strand breaks (DSBs) represent a serious threat for hematopoietic stem cells (HSCs). How cytokines and environmental signals integrate the DNA damage response and contribute to HSC-intrinsic DNA repair processes remains unknown. Thrombopoietin (TPO) and its receptor, Mpl, are critical factors supporting HSC self-renewal and expansion. Here, we uncover an unknown function for TPO-Mpl in the regulation of DNA damage response. We show that DNA repair following γ-irradiation (γ-IR) or the action of topoisomerase-II inhibitors is defective in Mpl(-/-) and in wild-type mouse or human hematopoietic stem and progenitor cells treated in the absence of TPO. TPO stimulates DNA repair in vitro and in vivo by increasing DNA-PK-dependent nonhomologous end-joining efficiency. This ensures HSC chromosomal integrity and limits their long-term injury in response to IR. This shows that niche factors can modulate the HSC DSB repair machinery and opens new avenues for administration of TPO agonists for minimizing radiotherapy-induced HSC injury and mutagenesis.

Endoglin expression level discriminates long-term hematopoietic from short-term clonogenic progenitor cells in the aorta.

CD105 is an auxiliary receptor for the transforming growth factor beta superfamily, highly expressed on proliferating endothelial cells and adult hematopoietic stem cells. Because CD105 mRNA expression was reported in the developing aortic region, we further characterized its expression profile in the aorta and examined the hematopoietic potential of CD105(+) cells. Aortic endothelial cells, intra-aortic hematopoietic cell clusters and the purified cell fraction enriched in progenitor/hematopoietic stem cell activity expressed CD105. Aortic hematopoietic short-term clonogenic progenitors were highly enriched in the CD105(intermediate) population whereas more immature long-term progenitors/hematopoietic stem cells are contained within the CD105(high) population. This places CD105 on the short list of molecules discriminating short-term versus long-term progenitors in the aorta. Furthermore, decreasing transforming growth factor beta signaling increases the number of clonogenic progenitors. This suggests that CD105 expression level defines a hierarchy among aortic hematopoietic cells allowing purification of clonogenic versus more immature hematopoietic progenitors, and that the transforming growth factor beta pathway plays a critical role in this process.

VE-cadherin expression allows identification of a new class of hematopoietic stem cells within human embryonic liver.

Edification of the human hematopoietic system during development is characterized by the production of waves of hematopoietic cells separated in time, formed in distinct embryonic sites (ie, yolk sac, truncal arteries including the aorta, and placenta). The embryonic liver is a major hematopoietic organ wherein hematopoietic stem cells (HSCs) expand, and the future, adult-type, hematopoietic cell hierarchy becomes established. We report herein the identification of a new, transient, and rare cell population in the human embryonic liver, which coexpresses VE-cadherin, an endothelial marker, CD45, a pan-hematopoietic marker, and CD34, a common endothelial and hematopoietic marker. This population displays an outstanding self-renewal, proliferation, and differentiation potential, as detected by in vitro and in vivo hematopoietic assays compared with its VE-cadherin negative counterpart. Based on VE-cadherin expression, our data demonstrate the existence of 2 phenotypically and functionally separable populations of multipotent HSCs in the human embryo, the VE-cadherin(+) one being more primitive than the VE-cadherin(-) one, and shed a new light on the hierarchical organization of the embryonic liver HSC compartment.