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The self-renewal capacity of multipotent haematopoietic stem cells (HSCs) supports blood system homeostasis throughout life and underlies the curative capacity of clinical HSC transplantation therapies. However, despite extensive characterization of the HSC state in the adult bone marrow and embryonic fetal liver, the mechanism of HSC self-renewal has remained elusive. This Review presents our current understanding of HSC self-renewal in vivo and ex vivo, and discusses important advances in ex vivo HSC expansion that are providing new biological insights and offering new therapeutic opportunities.Wilkinson and colleagues discuss haematopoietic stem cell (HSC) self-renewal in mice and humans. Experimental techniques for assaying HSC self-renewal are addressed, along with biological mechanisms regulating HSC self-renewal in vivo and ex vivo, and the therapeutic implications of this understanding.

Utilizing multipotent and self-renewing capabilities, hematopoietic stem cells (HSCs) can maintain hematopoiesis throughout life. However, the mechanism behind such remarkable abilities remains undiscovered, at least in part because of the paucity of HSCs and the modest ex vivo expansion of HSCs in media that contain poorly defined albumin supplements such as bovine serum albumin. Here, we describe a simple platform for the expansion of functional mouse HSCs ex vivo for >1 month under fully defined albumin-free conditions. The culture system affords 236- to 899-fold expansion over the course of a month and is also amenable to clonal analysis of HSC heterogeneity. The large numbers of expanded HSCs enable HSC transplantation into nonconditioned recipients, which is otherwise not routinely feasible because of the large numbers of HSCs required. This protocol therefore provides a powerful approach with which to interrogate HSC self-renewal and lineage commitment and, more broadly, to study and characterize the hematopoietic and immune systems. Functional mouse hematopoietic stem cells (HSCs) are expanded 236- to 899-fold ex vivo using a fully defined albumin-free culture system. Clonal analysis of HSC heterogeneity and HSC transplantation are also described.

CRISPR/Cas9-mediated beta-globin ( HBB ) gene correction of sickle cell disease (SCD) patient-derived hematopoietic stem cells (HSCs) in combination with autologous transplantation represents a recent paradigm in gene therapy. Although several Cas9-based HBB -correction approaches have been proposed, functional correction of in vivo erythropoiesis has not been investigated previously. Here, we use a humanized globin-cluster SCD mouse model to study Cas9-AAV6-mediated HBB- correction in functional HSCs within the context of autologous transplantation. We discover that long-term multipotent HSCs can be gene corrected ex vivo and stable hemoglobin-A production can be achieved in vivo from HBB -corrected HSCs following autologous transplantation. We observe a direct correlation between increased HBB -corrected myeloid chimerism and normalized in vivo red blood cell (RBC) features, but even low levels of chimerism resulted in robust hemoglobin-A levels. Moreover, this study offers a platform for gene editing of mouse HSCs for both basic and translational research. CRISPR mediated gene correction of sickle cell disease (SCD) in patient-derived hematopoietic stem cells is a promising avenue for therapy. Here the authors use a humanized SCD mouse model to study gene editing in the context of autologous transplantation.

Analyses of platelet and red blood cell lineages are redefining how we consider stemness Hematopoietic stem cells (HSCs) can produce all cell lineages within the adult blood system, and they have provided a flagship model in which to study stem cell biology. Concepts developed from studying HSCs have influenced how we consider other stem cell systems. HSCs are also one of the few stem cell types with a long history of clinical application, in the form of bone marrow transplantation. Recent technical advances have brought about a major revision in our understanding of the HSC compartment. These necessitate new models and nomenclature to describe HSC heterogeneity, self-renewal, and differentiation potential, as well as having broader implications for how we consider stemness.

Regeneration of human kidneys in animal models would help combat the severe shortage of donors in transplantation therapy. Previously, we demonstrated by interspecific blastocyst complementation between mouse and rats, generation of pluripotent stem cell (PSC)-derived functional pancreas, in apancreatic Pdx1 mutant mice. We, however, were unable to obtain rat PSC-derived kidneys in anephric Sall1 mutant mice, likely due to the poor contribution of rat PSCs to the mouse metanephric mesenchyme, a nephron progenitor. Here, conversely, we show that mouse PSCs can efficiently differentiate into the metanephric mesenchyme in rat, allowing the generation of mouse PSC-derived kidney in anephric Sall1 mutant rat. Glomerular epithelium and renal tubules in the kidneys are entirely composed of mouse PSC-derived cells expressing key functional markers. Importantly, the ureter-bladder junction is normally formed. These data provide proof-of-principle for interspecific blastocyst complementation as a viable approach for kidney generation. The use of pluripotent-stem cell derived organs for transplantation would be promising, if organs can be grown in a suitable host. Here, the authors use interspecific blastocyst complementation to generate a mouse pluripotent stem cell-derived kidney in anephric Sall1 mutant rats.

Multipotent self-renewing haematopoietic stem cells (HSCs) regenerate the adult blood system after transplantation 1 , which is a curative therapy for numerous diseases including immunodeficiencies and leukaemias 2 . Although substantial effort has been applied to identifying HSC maintenance factors through the characterization of the in vivo bone-marrow HSC microenvironment or niche 3 – 5 , stable ex vivo HSC expansion has previously been unattainable 6 , 7 . Here we describe the development of a defined, albumin-free culture system that supports the long-term ex vivo expansion of functional mouse HSCs. We used a systematic optimization approach, and found that high levels of thrombopoietin synergize with low levels of stem-cell factor and fibronectin to sustain HSC self-renewal. Serum albumin has long been recognized as a major source of biological contaminants in HSC cultures 8 ; we identify polyvinyl alcohol as a functionally superior replacement for serum albumin that is compatible with good manufacturing practice. These conditions afford between 236- and 899-fold expansions of functional HSCs over 1 month, although analysis of clonally derived cultures suggests that there is considerable heterogeneity in the self-renewal capacity of HSCs ex vivo. Using this system, HSC cultures that are derived from only 50 cells robustly engraft in recipient mice without the normal requirement for toxic pre-conditioning (for example, radiation), which may be relevant for HSC transplantation in humans. These findings therefore have important implications for both basic HSC research and clinical haematology. An albumin-free culture system for the long-term ex vivo expansion of mouse haematopoietic stem cells produces 236- to 899-fold expansion, and generates cultures that robustly engraft in recipient mice without toxic pre-conditioning.

A specialized bone marrow microenvironment (niche) regulates hematopoietic stem cell (HSC) self-renewal and commitment. For successful donor-HSC engraftment, the niche must be emptied via myeloablative irradiation or chemotherapy. However, myeloablation can cause severe complications and even mortality. Here we report that the essential amino acid valine is indispensable for the proliferation and maintenance of HSCs. Both mouse and human HSCs failed to proliferate when cultured in valine-depleted conditions. In mice fed a valine-restricted diet, HSC frequency fell dramatically within 1 week. Furthermore, dietary valine restriction emptied the mouse bone marrow niche and afforded donor-HSC engraftment without chemoirradiative myeloablation. These findings indicate a critical role for valine in HSC maintenance and suggest that dietary valine restriction may reduce iatrogenic complications in HSC transplantation.

Until recently, complex multi-parameters were required for the isolation and identification of haematopoietic stem cells, complicating study of their biology in situ ; here the authors have found that expression of a single gene, Hoxb5 , defines haematopoietic stem cells with long-term reconstitution capacity, and that these cells are mainly found in direct contact with endothelial cells. Haematopoietic stem cell niche characterized Until recently, the isolation and recognition of haematopoietic stem cells (HSCs) has been a complex process involving the manipulation of multiple parameters, and this complicates the study of HSC biology in situ . In particular, it has been difficult to establish their relationship to the HSC niche, and how their self-renewal and differentiation properties are modulated by their environment. Here Irving Weissman and colleagues demonstrate that expression of a single gene, Hoxb5 , defines cells with long-term reconstitution capacity, and show that these cells are mainly found directly in contact with endothelial cells. Haematopoietic stem cells (HSCs) are arguably the most extensively characterized tissue stem cells. Since the identification of HSCs by prospective isolation 1 , complex multi-parameter flow cytometric isolation of phenotypic subsets has facilitated studies on many aspects of HSC biology, including self-renewal 2 , 3 , 4 , differentiation, ageing, niche 5 , and diversity 6 , 7 , 8 . Here we demonstrate by unbiased multi-step screening, identification of a single gene, homeobox B5 ( Hoxb5 , also known as Hox-2.1 ), with expression in the bone marrow that is limited to long-term (LT)-HSCs in mice. Using a mouse single-colour tri-mCherry reporter driven by endogenous Hoxb5 regulation, we show that only the Hoxb5 + HSCs exhibit long-term reconstitution capacity after transplantation in primary transplant recipients and, notably, in secondary recipients. Only 7–35% of various previously defined immunophenotypic HSCs are LT-HSCs. Finally, by in situ imaging of mouse bone marrow, we show that >94% of LT-HSCs (Hoxb5 + ) are directly attached to VE-cadherin + cells, implicating the perivascular space as a near-homogenous location of LT-HSCs.

Haematopoietic stem cells (HSCs) are a rare cell type that reconstitute the entire blood and immune systems after transplantation and can be used as a curative cell therapy for a variety of haematological diseases 1 , 2 . However, the low number of HSCs in the body makes both biological analyses and clinical application difficult, and the limited extent to which human HSCs can be expanded ex vivo remains a substantial barrier to the wider and safer therapeutic use of HSC transplantation 3 . Although various reagents have been tested in attempts to stimulate the expansion of human HSCs, cytokines have long been thought to be essential for supporting HSCs ex vivo 4 . Here we report the establishment of a culture system that allows the long-term ex vivo expansion of human HSCs, achieved through the complete replacement of exogenous cytokines and albumin with chemical agonists and a caprolactam-based polymer. A phosphoinositide 3-kinase activator, in combination with a thrombopoietin-receptor agonist and the pyrimidoindole derivative UM171, were sufficient to stimulate the expansion of umbilical cord blood HSCs that are capable of serial engraftment in xenotransplantation assays. Ex vivo HSC expansion was further supported by split-clone transplantation assays and single-cell RNA-sequencing analysis. Our chemically defined expansion culture system will help to advance clinical HSC therapies. A culture system allows the long-term expansion of human haematopoietic stem cells (HSCs) in vivo without the use of recombinant cytokines or albumin, with potential applications for clinical therapies involving HSCs.

Recently, several studies using cultures of human embryos together with single-cell RNA-seq analyses have revealed differences between humans and mice, necessitating the study of human embryos 1 – 8 . Despite the importance of human embryology, ethical and legal restrictions have limited post-implantation-stage studies. Thus, recent efforts have focused on developing in vitro self-organizing models using human stem cells 9 – 17 . Here, we report genetic and non-genetic approaches to generate authentic hypoblast cells (naive hPSC-derived hypoblast-like cells (nHyCs))—known to give rise to one of the two extraembryonic tissues essential for embryonic development—from naive human pluripotent stem cells (hPSCs). Our nHyCs spontaneously assemble with naive hPSCs to form a three-dimensional bilaminar structure (bilaminoids) with a pro-amniotic-like cavity. In the presence of additional naive hPSC-derived analogues of the second extraembryonic tissue, the trophectoderm, the efficiency of bilaminoid formation increases from 20% to 40%, and the epiblast within the bilaminoids continues to develop in response to trophectoderm-secreted IL-6. Furthermore, we show that bilaminoids robustly recapitulate the patterning of the anterior–posterior axis and the formation of cells reflecting the pregastrula stage, the emergence of which can be shaped by genetically manipulating the DKK1/OTX2 hypoblast-like domain. We have therefore successfully modelled and identified the mechanisms by which the two extraembryonic tissues efficiently guide the stage-specific growth and progression of the epiblast as it establishes the post-implantation landmarks of human embryogenesis. Authentic hypoblast cells created from naive human pluripotent stem cells (hPSCs) spontaneously assemble with naive hPSCs to form a three-dimensional bilaminar structure (bilaminoids) with a pro-amniotic-like cavity.