Naoya Takayama

Generation of functional platelets from human embryonic stem cells in vitro via ES-sacs, VEGF-promoted structures that concentrate hematopoietic progenitors

Human embryonic stem cells (hESCs) could potentially represent an alternative source for blood transfusion therapies and a promising tool for studying the ontogeny of hematopoiesis. When we cultured hESCs on either C3H10T1/2 or OP-9 cells to facilitate hematopoiesis, we found that exogenous administration of vascular endothelial growth factor promoted the emergence of sac-like structures, which we named embryonic stem cell-derived sacs (ES-sacs). These ES-sacs consisted of multiple cysts demarcated by cellular monolayers that retained some of the properties of endothelial cells. The spherical cells inside ES-sacs expressed primarily CD34, along with VE-cadherin, CD31, CD41a, and CD45, and were able to form hematopoietic colonies in semisolid culture and to differentiate into mature megakaryocytes by day 24 in the presence of thrombopoietin. Apparently, ES-sacs provide a suitable environment for hematopoietic progenitors. Relatively large numbers of mature megakaryocytes could be induced from the hematopoietic progenitors within ES-sacs, which were then able to release platelets that displayed integrin alpha IIb beta 3 activation and spreading in response to ADP or thrombin. This novel protocol thus provides a means of generating platelets from hESCs, which could serve as the basis for efficient production of platelets for clinical transfusion and studies of thrombopoiesis.

Donor-dependent variations in hepatic differentiation from human-induced pluripotent stem cells

Hepatocytes generated from human induced pluripotent stem cells (hiPSCs) are unprecedented resources for pharmaceuticals and cell therapy. However, the in vitro directed differentiation of human pluripotent stem cells into mature hepatocytes remains challenging. Little attention has so far been paid to variations among hiPSC lines in terms of their hepatic differentiation. In the current study, we developed an improved hepatic differentiation protocol and compared 28 hiPSC lines originated from various somatic cells and derived using retroviruses, Sendai viruses, or episomal plasmids. This comparison indicated that the origins, but not the derivation methods, may be a major determinant of variation in hepatic differentiation. The hiPSC clones derived from peripheral blood cells consistently showed good differentiation efficiency, whereas many hiPSC clones from adult dermal fibroblasts showed poor differentiation. However, when we compared hiPSCs from peripheral blood and dermal fibroblasts from the same individuals, we found that variations in hepatic differentiation were largely attributable to donor differences, rather than to the types of the original cells. These data underscore the importance of donor differences when comparing the differentiation propensities of hiPSC clones.

Expandable Megakaryocyte Cell Lines Enable Clinically Applicable Generation of Platelets from Human Induced Pluripotent Stem Cells

The donor-dependent supply of platelets is frequently insufficient to meet transfusion needs. To address this issue, we developed a clinically applicable strategy for the derivation of functional platelets from human pluripotent stem cells (PSCs). This approach involves the establishment of stable immortalized megakaryocyte progenitor cell lines (imMKCLs) from PSC-derived hematopoietic progenitors through the overexpression of BMI1 and BCL-XL to respectively suppress senescence and apoptosis and the constrained overexpression of c-MYC to promote proliferation. The resulting imMKCLs can be expanded in culture over extended periods (4-5 months), even after cryopreservation. Halting the overexpression of c-MYC, BMI1, and BCL-XL in growing imMKCLs led to the production of CD42b(+) platelets with functionality comparable to that of native platelets on the basis of a range of assays in vitro and in vivo. The combination of robust expansion capacity and efficient platelet production means that appropriately selected imMKCL clones represent a potentially inexhaustible source of hPSC-derived platelets for clinical application.

Generation of induced pluripotent stem cells from primary chronic myelogenous leukemia patient samples.

Induced pluripotent stem cells (iPSCs) can be generated by the expression of defined transcription factors not only from normal tissue, but also from malignant cells. Cancer-derived iPSCs are expected to provide a novel experimental opportunity to establish the disease model. We generated iPSCs from imatinib-sensitive chronic myelogenous leukemia (CML) patient samples. Remarkably, the CML-iPSCs were resistant to imatinib although they consistently expressed BCR-ABL oncoprotein. In CML-iPSCs, the phosphorylation of ERK1/2, AKT, and JNK, which are essential for the maintenance of both BCR-ABL (+) leukemia cells and iPSCs, were unchanged after imatinib treatment, whereas the phosphorylation of signal transducer and activator of transcription (STAT)5 and CRKL was significantly decreased. These results suggest that the signaling for iPSCs maintenance compensates for the inhibition of BCR-ABL. CML-iPSC-derived hematopoietic cells recovered the sensitivity to imatinib although CD34(+)38(-)90(+)45(+) immature cells were resistant to imatinib, which recapitulated the pathophysiologic feature of the initial CML. CML-iPSCs provide us with a novel platform to investigate CML pathogenesis on the basis of patient-derived samples.

Lnk regulates integrin αIIbβ3 outside-in signaling in mouse platelets, leading to stabilization of thrombus development in vivo

The nature of the in vivo cellular events underlying thrombus formation mediated by platelet activation remains unclear because of the absence of a modality for analysis. Lymphocyte adaptor protein (Lnk; also known as Sh2b3) is an adaptor protein that inhibits thrombopoietin-mediated signaling, and as a result, megakaryocyte and platelet counts are elevated in Lnk-/- mice. Here we describe an unanticipated role for Lnk in stabilizing thrombus formation and clarify the activities of Lnk in platelets transduced through integrin alphaIIbbeta3-mediated outside-in signaling. We equalized platelet counts in wild-type and Lnk-/- mice by using genetic depletion of Lnk and BM transplantation. Using FeCl3- or laser-induced injury and in vivo imaging that enabled observation of single platelet behavior and the multiple steps in thrombus formation, we determined that Lnk is an essential contributor to the stabilization of developing thrombi within vessels. Lnk-/- platelets exhibited a reduced ability to fully spread on fibrinogen and mediate clot retraction, reduced tyrosine phosphorylation of the beta3 integrin subunit, and reduced binding of Fyn to integrin alphaIIbbeta3. These results provide new insight into the mechanism of alphaIIbbeta3-based outside-in signaling, which appears to be coordinated in platelets by Lnk, Fyn, and integrins. Outside-in signaling modulators could represent new therapeutic targets for the prevention of cardiovascular events.

IL-1α induces thrombopoiesis through megakaryocyte rupture in response to acute platelet needs

An alternative pathway triggering enhanced platelet release from bone marrow megakaryocytes via a rupture-based mechanism is regulated by IL-1α in response to acute platelet requirements.

In vivo imaging visualizes discoid platelet aggregations without endothelium disruption and implicates contribution of inflammatory cytokine and integrin signaling

The mechanism by which thrombotic vessel occlusion occurs independently of plaque development or endothelial cell (EC) disruption remains unclear, largely because of an inability to visualize the formation of thrombus, especially at the single-platelet level in real time. Here we demonstrate that rapidly developing thrombi composed of discoid platelets can be induced in the mesenteric capillaries, arterioles, and large-sized arteries of living mice, enabling characterization of the kinetics of thrombosis initiation and the multicellular interrelationships during thrombus development. Platelet aggregation without EC disruption was triggered by reactive oxygen species (ROS) photochemically induced by moderate power laser irradiation. The inflammatory cytokines TNF-α and IL-1 could be key components of the EC response, acting through regulation of VWF mobilization to the cell surface. Thrombus formation was then initiated by the binding of platelet GPIbα to endothelial VWF in our model, and this effect was inhibited by the ROS scavenger N-acetylcysteine. Actin linker talin-dependent activation of alphaIIb-beta3 integrin or Rac1 in platelets was required for late-phase thrombus stability. Our novel imaging technology illustrates the molecular mechanism underlying inflammation-based thrombus formation by discoid platelets on undisrupted ECs and suggests control of ROS could be a useful therapeutic target for the prevention of thrombotic diseases.

Heterozygous ITGA2B R995W mutation inducing constitutive activation of the αIIbβ3 receptor affects proplatelet formation and causes congenital macrothrombocytopenia

Congenital macrothrombocytopenia is a genetically heterogeneous group of rare disorders. αIIbβ3 has not been implicated in these conditions. We identified a novel, conserved heterozygous ITGA2B R995W mutation in 4 unrelated families. The surface expression of platelet αIIbβ3 was decreased to 50% to 70% of control. There was spontaneous PAC-1 and fibrinogen binding to resting platelets without CD62p expression. The activation state of αIIbβ3 in 293T cells was higher for αIIb-W995 than for β3-H723 but was weaker than for β3-N562. FAK was spontaneously phosphorylated in αIIb-W995/β3-transfected 293T cells. These results indicate that αIIb-W995/β3 has a constitutive, activated conformation but does not induce platelet activation. αIIb-W995/β3-transfected CHO cells developed membrane ruffling and abnormal cytoplasmic protrusions. The increased size and decreased number of proplatelet tips in αIIb-W995/β3-transduced mouse fetal liver-derived megakaryocytes indicate defective pro-platelet formation. We propose that activating mutations in ITGA2B and ITGB3 represent the etiology of a subset of congenital macrothrombocytopenias.

Role of SOX17 in hematopoietic development from human embryonic stem cells

To search for genes that promote hematopoietic development from human embryonic stem cells (hESCs) and induced pluripotent stem cells (iPSCs), we overexpressed several known hematopoietic regulator genes in hESC/iPSC-derived CD34(+)CD43(-) endothelial cells (ECs) enriched in hemogenic endothelium (HE). Among the genes tested, only Sox17, a gene encoding a transcription factor of the SOX family, promoted cell growth and supported expansion of CD34(+)CD43(+)CD45(-/low) cells expressing the HE marker VE-cadherin. SOX17 was expressed at high levels in CD34(+)CD43(-) ECs compared with low levels in CD34(+)CD43(+)CD45(-) pre-hematopoietic progenitor cells (pre-HPCs) and CD34(+)CD43(+)CD45(+) HPCs. Sox17-overexpressing cells formed semiadherent cell aggregates and generated few hematopoietic progenies. However, they retained hemogenic potential and gave rise to hematopoietic progenies on inactivation of Sox17. Global gene-expression analyses revealed that the CD34(+)CD43(+)CD45(-/low) cells expanded on overexpression of Sox17 are HE-like cells developmentally placed between ECs and pre-HPCs. Sox17 overexpression also reprogrammed both pre-HPCs and HPCs into HE-like cells. Genome-wide mapping of Sox17-binding sites revealed that Sox17 activates the transcription of key regulator genes for vasculogenesis, hematopoiesis, and erythrocyte differentiation directly. Depletion of SOX17 in CD34(+)CD43(-) ECs severely compromised their hemogenic activity. These findings suggest that SOX17 plays a key role in priming hemogenic potential in ECs, thereby regulating hematopoietic development from hESCs/iPSCs.

Reduction of N-glycolylneuraminic acid in human induced pluripotent stem cells generated or cultured under feeder- and serum-free defined conditions.

Background The successful establishment of human induced pluripotent stem cells (hiPSCs) has increased the possible applications of stem cell research in biology and medicine. In particular, hiPSCs are a promising source of cells for regenerative medicine and pharmacology. However, one of the major obstacles to such uses for hiPSCs is the risk of contamination from undefined pathogens in conventional culture conditions that use serum replacement and mouse embryonic fibroblasts as feeder cells. Methodology/Principal Findings Here we report a simple method for generating or culturing hiPSCs under feeder- and serum-free defined culture conditions that we developed previously for human embryonic stem cells. The defined culture condition comprises a basal medium with a minimal number of defined components including five highly purified proteins and fibronectin as a substrate. First, hiPSCs, which were generated using Yamanakas four factors and conventional undefined culture conditions, adapted to the defined culture conditions. These adapted cells retained the property of self renewal as evaluated morphologically, the expression of self-renewal marker proteins, standard growth rates, and pluripotency as evaluated by differentiation into derivatives of all three primary germ layers in vitro and in vivo (teratoma formation in immunodeficient mice). Moreover, levels of nonhuman N-glycolylneuraminic acid (Neu5Gc), which is a xenoantigenic indicator of pathogen contamination in human iPS cell cultures, were markedly decreased in hiPSCs cultured under the defined conditions. Second, we successfully generated hiPSCs using adult dermal fibroblast under the defined culture conditions from the reprogramming step. For a long therm culture, the generated cells also had the property of self renewal and pluripotency, they carried a normal karyotype, and they were Neu5Gc negative. Conclusion/Significance This study suggested that generation or adaption culturing under defined culture conditions can eliminate the risk posed by undefined pathogens. This success in generating hiPSCs using adult fibroblast would be beneficial for clinical application.