In Vivo Rescue of the Hematopoietic Niche By Pluripotent Stem Cell Complementation of Defective Osteoblast Compartments.

Bone-forming osteoblasts play critical roles in supporting bone marrow hematopoiesis. Pluripotent stem cells (PSCs), including embryonic stem cells (ESCs) and induced PSCs (iPSC), are capable of differentiating into osteoblasts. To determine the capacity of stem cells needed to rescue aberrant skeletal development and bone marrow hematopoiesis in vivo, we used a skeletal complementation model. Mice deficient in Runx2, a master transcription factor for osteoblastogenesis, fail to form a mineralized skeleton and bone marrow. Wild-type (WT) green fluorescent protein (GFP)+ ESCs and yellow fluorescent protein (YFP)+ iPSCs were introduced into Runx2-null blastocyst-stage embryos. We assessed GFP/YFP+ cell contribution by whole-mount fluorescence and histological analysis and found that the proportion of PSCs in the resulting chimeric embryos is directly correlated with the degree of mineralization in the skull. Moreover, PSC contribution to long bones successfully restored bone marrow hematopoiesis. We validated this finding in a separate model with diphtheria toxin A-mediated ablation of hypertrophic chondrocytes and osteoblasts. Remarkably, chimeric embryos harboring as little as 37.5% WT PSCs revealed grossly normal skeletal morphology, suggesting a near-complete rescue of skeletogenesis. In summary, we demonstrate that fractional contribution of PSCs in vivo is sufficient to complement and reconstitute an osteoblast-deficient skeleton and hematopoietic marrow. Further investigation using genetically modified PSCs with conditional loss of gene function in osteoblasts will enable us to address the specific roles of signaling mediators to regulate bone formation and hematopoietic niches in vivo. Stem Cells 2017;35:2150-2159.

Highly efficient derivation of ventricular cardiomyocytes from induced pluripotent stem cells with a distinct epigenetic signature.

Cardiomyocytes derived from pluripotent stem cells can be applied in drug testing, disease modeling and cell-based therapy. However, without procardiogenic growth factors, the efficiency of cardiomyogenesis from pluripotent stem cells is usually low and the resulting cardiomyocyte population is heterogeneous. Here, we demonstrate that induced pluripotent stem cells (iPSCs) can be derived from murine ventricular myocytes (VMs), and consistent with other reports of iPSCs derived from various somatic cell types, VM-derived iPSCs (ViPSCs) exhibit a markedly higher propensity to spontaneously differentiate into beating cardiomyocytes as compared to genetically matched embryonic stem cells (ESCs) or iPSCs derived from tail-tip fibroblasts. Strikingly, the majority of ViPSC-derived cardiomyocytes display a ventricular phenotype. The enhanced ventricular myogenesis in ViPSCs is mediated via increased numbers of cardiovascular progenitors at early stages of differentiation. In order to investigate the mechanism of enhanced ventricular myogenesis from ViPSCs, we performed global gene expression and DNA methylation analysis, which revealed a distinct epigenetic signature that may be involved in specifying the VM fate in pluripotent stem cells.