Garrett C. Heffner

Generation of induced pluripotent stem cells from human blood.

Human dermal fibroblasts obtained by skin biopsy can be reprogrammed directly to pluripotency by the ectopic expression of defined transcription factors. Here, we describe the derivation of induced pluripotent stem cells from CD34+ mobilized human peripheral blood cells using retroviral transduction of OCT4/SOX2/KLF4/MYC. Blood-derived human induced pluripotent stem cells are indistinguishable from human embryonic stem cells with respect to morphology, expression of surface antigens, and pluripotency-associated transcription factors, DNA methylation status at pluripotent cell-specific genes, and the capacity to differentiate in vitro and in teratomas. The ability to reprogram cells from human blood will allow the generation of patient-specific stem cells for diseases in which the disease-causing somatic mutations are restricted to cells of the hematopoietic lineage.

Lineage Regulators Direct BMP and Wnt Pathways to Cell-Specific Programs during Differentiation and Regeneration

BMP and Wnt signaling pathways control essential cellular responses through activation of the transcription factors SMAD (BMP) and TCF (Wnt). Here, we show that regeneration of hematopoietic lineages following acute injury depends on the activation of each of these signaling pathways to induce expression of key blood genes. Both SMAD1 and TCF7L2 co-occupy sites with master regulators adjacent to hematopoietic genes. In addition, both SMAD1 and TCF7L2 follow the binding of the predominant lineage regulator during differentiation from multipotent hematopoietic progenitor cells to erythroid cells. Furthermore, induction of the myeloid lineage regulator C/EBPα in erythroid cells shifts binding of SMAD1 to sites newly occupied by C/EBPα, whereas expression of the erythroid regulator GATA1 directs SMAD1 loss on nonerythroid targets. We conclude that the regenerative response mediated by BMP and Wnt signaling pathways is coupled with the lineage master regulators to control the gene programs defining cellular identity.

Reprogramming of T cells from human peripheral blood

A manuscript has appeared online demonstrating isolation of iPSCs from peripheral blood, including a single line that showed evidence for both TCR-β and TCR-γ rearrangement by PCR (Kunisato, A., Wakatsuki, M., Shinba, H., Ota, T., Ishida, I., and Nagao, K. [2010]. Direct generation of induced pluripotent stem cells from human non-mobilized blood. Stem Cells Dev., in press. Published online May 24, 2010. 10.1089/scd.2010.0063).

Signaling axis involving Hedgehog, Notch, and Scl promotes the embryonic endothelial-to-hematopoietic transition

During development, the hematopoietic lineage transits through hemogenic endothelium, but the signaling pathways effecting this transition are incompletely characterized. Although the Hedgehog (Hh) pathway is hypothesized to play a role in patterning blood formation, early embryonic lethality of mice lacking Hh signaling precludes such analysis. To determine a role for Hh signaling in patterning of hemogenic endothelium, we assessed the effect of altered Hh signaling in differentiating mouse ES cells, cultured mouse embryos, and developing zebrafish embryos. In differentiating mouse ES cells and mouse yolk sac cultures, addition of Indian Hh ligand increased hematopoietic progenitors, whereas chemical inhibition of Hh signaling reduced hematopoietic progenitors without affecting primitive streak mesoderm formation. In the setting of Hh inhibition, induction of either Notch signaling or overexpression of Stem cell leukemia (Scl)/T-cell acute lymphocytic leukemia protein 1 rescued hemogenic vascular-endothelial cadherin+ cells and hematopoietic progenitor formation. Together, our results reveal that Scl overexpression is sufficient to rescue the developmental defects caused by blocking the Hh and Notch pathways, and inform our understanding of the embryonic endothelial-to-hematopoietic transition.

Epoxyeicosatrienoic acids enhance embryonic haematopoiesis and adult marrow engraftment

Haematopoietic stem and progenitor cell (HSPC) transplant is a widely used treatment for life-threatening conditions such as leukaemia; however, the molecular mechanisms regulating HSPC engraftment of the recipient niche remain incompletely understood. Here we develop a competitive HSPC transplant method in adult zebrafish, using in vivo imaging as a non-invasive readout. We use this system to conduct a chemical screen, and identify epoxyeicosatrienoic acids (EETs) as a family of lipids that enhance HSPC engraftment. The pro-haematopoietic effects of EETs were conserved in the developing zebrafish embryo, where 11,12-EET promoted HSPC specification by activating a unique activator protein 1 (AP-1) and runx1 transcription program autonomous to the haemogenic endothelium. This effect required the activation of the phosphatidylinositol-3-OH kinase (PI(3)K) pathway, specifically PI(3)Kγ. In adult HSPCs, 11,12-EET induced transcriptional programs, including AP-1 activation, which modulate several cellular processes, such as migration, to promote engraftment. Furthermore, we demonstrate that the EET effects on enhancing HSPC homing and engraftment are conserved in mammals. Our study establishes a new method to explore the molecular mechanisms of HSPC engraftment, and discovers a previously unrecognized, evolutionarily conserved pathway regulating multiple haematopoietic generation and regeneration processes. EETs may have clinical application in marrow or cord blood transplantation.

A nontranscriptional role for Oct4 in the regulation of mitotic entry

Significance Embryonic stem cells and induced pluripotent stem cells have abbreviated cell cycles. To achieve this rapid proliferation, several molecular safeguards that normally distinguish healthy from transformed cells are altered. Understanding how these pluripotent stem cells balance the demands of their unique cell cycles against the need to maintain a stable genome is critical to unlocking their great promise for regenerative medicine. Here, we demonstrate that Oct4 (octamer-binding transcription factor 4), a transcription factor required to maintain pluripotency, inhibits the activation of cyclin-dependent kinase (Cdk) 1, the master regulator of mitosis, and delays mitotic entry in a nontranscriptional manner. To our knowledge, our study is the first demonstration of a nontranscriptional function of the pluripotency regulator Oct4. Rapid progression through the cell cycle and a very short G1 phase are defining characteristics of embryonic stem cells. This distinct cell cycle is driven by a positive feedback loop involving Rb inactivation and reduced oscillations of cyclins and cyclin-dependent kinase (Cdk) activity. In this setting, we inquired how ES cells avoid the potentially deleterious consequences of premature mitotic entry. We found that the pluripotency transcription factor Oct4 (octamer-binding transcription factor 4) plays an unappreciated role in the ES cell cycle by forming a complex with cyclin–Cdk1 and inhibiting Cdk1 activation. Ectopic expression of Oct4 or a mutant lacking transcriptional activity recapitulated delayed mitotic entry in HeLa cells. Reduction of Oct4 levels in ES cells accelerated G2 progression, which led to increased chromosomal missegregation and apoptosis. Our data demonstrate an unexpected nontranscriptional function of Oct4 in the regulation of mitotic entry.

The Src Homology 2 Protein Shb Promotes Cell Cycle Progression In Murine Hematopoietic Stem Cells By Regulation Of Focal Adhesion Kinase Activity

The widely expressed adaptor protein Shb has previously been reported to contribute to T cell function due to its association with the T cell receptor and furthermore, several of Shbs known interaction partners are established regulators of blood cell development and function. In addition, Shb deficient embryonic stem cells displayed reduced blood cell colony formation upon differentiation in vitro. The aim of the current study was therefore to explore hematopoietic stem and progenitor cell function in the Shb knockout mouse. Shb deficient bone marrow contained reduced relative numbers of long-term hematopoietic stem cells (LT-HSCs) that exhibited lower proliferation rates. Despite this, Shb knockout LT-HSCs responded promptly by entering the cell cycle in response to genotoxic stress by 5-fluorouracil treatment. In competitive LT-HSC transplantations, Shb null cells initially engrafted as well as the wild-type cells but provided less myeloid expansion over time. Moreover, Shb knockout bone marrow cells exhibited elevated basal activities of focal adhesion kinase/Rac1/p21-activated kinase signaling and reduced responsiveness to Stem Cell Factor stimulation. Consequently, treatment with a focal adhesion kinase inhibitor increased Shb knockout LT-HSC proliferation. The altered signaling characteristics thus provide a plausible mechanistic explanation for the changes in LT-HSC proliferation since these signaling intermediates have all been shown to participate in LT-HSC cell cycle control. In summary, the loss of Shb dependent signaling in bone marrow cells, resulting in elevated focal adhesion kinase activity and reduced proliferative responses in LT-HSCs under steady state hematopoiesis, confers a disadvantage to the maintenance of LT-HSCs over time.