Enhanced Hemato-Vascular Contribution Of SCL-3′Enh Expressing Fetal Liver Cells Uncovers Their Potential To Integrate In Extra-Medullary Adult Niches

A population of hematopoietic stem cells derives from GATA4-expressing progenitors located in the placenta and lateral mesoderm of mice

GATA transcription factors are expressed in the mesoderm and endoderm during development. GATA1–3, but not GATA4, are critically involved in hematopoiesis. An enhancer (G2) of the mouse Gata4 gene directs its expression throughout the lateral mesoderm and the allantois, beginning at embryonic day 7.5, becoming restricted to the septum transversum by embryonic day 10.5, and disappearing by midgestation. We have studied the developmental fate of the G2-Gata4 cell lineage using a G2-Gata4^Cre;R26R^EYFP mouse line. We found a substantial number of YFP⁺ hematopoietic cells of lymphoid, myeloid and erythroid lineages in embryos. Fetal CD41⁺/cKit⁺/CD34⁺ and Lin⁻/cKit⁺/CD31⁺ YFP⁺ hematopoietic progenitors were much more abundant in the placenta than in the aorta-gonad-mesonephros area. They were clonogenic in the MethoCult assay and fully reconstituted hematopoiesis in myeloablated mice. YFP⁺ cells represented about 20% of the hematopoietic system of adult mice. Adult YFP⁺ hematopoietic stem cells constituted a long-term repopulating, transplantable population. Thus, a lineage of adult hematopoietic stem cells is characterized by the expression of GATA4 in their embryonic progenitors and probably by its extraembryonic (placental) origin, although GATA4 appeared not to be required for hematopoietic stem cell differentiation. Both lineages basically showed similar physiological behavior in normal mice, but clinically relevant properties of this particular hematopoietic stem cell population should be checked in physiopathological conditions.

Characterization of a Fetal Liver Cell Population Endowed with Long-Term Multiorgan Endothelial Reconstitution Potential

Stable reconstitution of vascular endothelial beds upon transplantation of progenitor cells represents an important challenge due to the paucity and generally limited integration/expansion potential of most identified vascular related cell subsets. We previously showed that mouse fetal liver (FL) hemato/vascular cells from day 12 of gestation (E12), expressing the Stem Cell Leukaemia (SCL) gene enhancer transgene (SCL‐PLAP⁺ cells), had robust endothelial engraftment potential when transferred to the blood stream of newborns or adult conditioned recipients, compared to the scarce vascular contribution of adult bone marrow cells. However, the specific SCL‐PLAP⁺ hematopoietic or endothelial cell subset responsible for the long‐term reconstituting endothelial cell (LTR‐EC) activity and its confinement to FL developmental stages remained unknown. Using a busulfan‐treated newborn transplantation model, we show that LTR‐EC activity is restricted to the SCL‐PLAP⁺VE‐cadherin⁺CD45⁻ cell population, devoid of hematopoietic reconstitution activity and largely composed by Lyve1⁺ endothelial‐committed cells. SCL‐PLAP⁺ Ve‐cadherin⁺CD45⁻ cells contributed to the liver sinusoidal endothelium and also to the heart, kidney and lung microvasculature. LTR‐EC activity was detected at different stages of FL development, yet marginal activity was identified in the adult liver, revealing unknown functional differences between fetal and adult liver endothelial/endothelial progenitors. Importantly, the observations that expanding donor‐derived vascular grafts colocalize with proliferating hepatocyte‐like cells and participate in the systemic circulation, support their functional integration into young livers. These findings offer new insights into the engraftment, phonotypical, and developmental characterization of a novel endothelial/endothelial progenitor cell subtype with multiorgan LTR‐EC activity, potentially instrumental for the treatment/genetic correction of vascular diseases. Stem Cells2017;35:507–521

Kidney stem cells in development, regeneration and cancer

The generation of nephrons during development depends on differentiation via a mesenchymal to epithelial transition (MET) of self-renewing, tissue-specific stem cells confined to a specific anatomic niche of the nephrogenic cortex. These cells may transform to generate oncogenic stem cells and drive pediatric renal cancer. Once nephron epithelia are formed the view of post-MET tissue renal growth and maintenance by adult tissue-specific epithelial stem cells becomes controversial. Recently, genetic lineage tracing that followed clonal evolution of single kidney cells showed that the need for new cells is constantly driven by fate-restricted unipotent clonal expansions in varying kidney segments arguing against a multipotent adult stem cell model. Lineage-restriction was similarly maintained in kidney organoids grown in culture. Importantly, kidney cells in which Wnt was activated were traced to give significant clonal progeny indicating a clonogenic hierarchy. In vivo nephron epithelia may be endowed with the capacity akin to that of unipotent epithelial stem/progenitor such that under specific stimuli can clonally expand/self renew by local proliferation of mature differentiated cells. Finding ways to ex vivo preserve and expand the observed in vivo kidney-forming capacity inherent to both the fetal and adult kidneys is crucial for taking renal regenerative medicine forward. Some of the strategies used to achieve this are sorting human fetal nephron stem/progenitor cells, growing adult nephrospheres or reprogramming differentiated kidney cells toward expandable renal progenitors.