Chad May

Endothelial cells are essential for the self-renewal and repopulation of Notch-dependent hematopoietic stem cells

Bone marrow endothelial cells (ECs) are essential for reconstitution of hematopoiesis, but their role in self-renewal of long-term hematopoietic stem cells (LT-HSCs) is unknown. We have developed angiogenic models to demonstrate that EC-derived angiocrine growth factors support in vitro self-renewal and in vivo repopulation of authentic LT-HSCs. In serum/cytokine-free cocultures, ECs, through direct cellular contact, stimulated incremental expansion of repopulating CD34(-)Flt3(-)cKit(+)Lineage(-)Sca1(+) LT-HSCs, which retained their self-renewal ability, as determined by single-cell and serial transplantation assays. Angiocrine expression of Notch ligands by ECs promoted proliferation and prevented exhaustion of LT-HSCs derived from wild-type, but not Notch1/Notch2-deficient, mice. In transgenic notch-reporter (TNR.Gfp) mice, regenerating TNR.Gfp(+) LT-HSCs were detected in cellular contact with sinusoidal ECs. Interference with angiocrine, but not perfusion, function of SECs impaired repopulation of TNR.Gfp(+) LT-HSCs. ECs establish an instructive vascular niche for clinical-scale expansion of LT-HSCs and a cellular platform to identify stem cell-active trophogens.

Combretastatin A4 phosphate induces rapid regression of tumor neovessels and growth through interference with vascular endothelial-cadherin signaling

The molecular and cellular pathways that support the maintenance and stability of tumor neovessels are not well defined. The efficacy of microtubule-disrupting agents, such as combretastatin A4 phosphate (CA4P), in inducing rapid regression of specific subsets of tumor neovessels has opened up new avenues of research to identify factors that support tumor neoangiogenesis. Herein, we show that CA4P selectively targeted endothelial cells, but not smooth muscle cells, and induced regression of unstable nascent tumor neovessels by rapidly disrupting the molecular engagement of the endothelial cell-specific junctional molecule vascular endothelial-cadherin (VE-cadherin) in vitro and in vivo in mice. CA4P increases endothelial cell permeability, while inhibiting endothelial cell migration and capillary tube formation predominantly through disruption of VE-cadherin/beta-catenin/Akt signaling pathway, thereby leading to rapid vascular collapse and tumor necrosis. Remarkably, stabilization of VE-cadherin signaling in endothelial cells with adenovirus E4 gene or ensheathment with smooth muscle cells confers resistance to CA4P. CA4P synergizes with low and nontoxic doses of neutralizing mAbs to VE-cadherin by blocking assembly of neovessels, thereby inhibiting tumor growth. These data suggest that the microtubule-targeting agent CA4P selectively induces regression of unstable tumor neovessels, in part through disruption of VE-cadherin signaling. Combined treatment with anti-VE-cadherin agents in conjunction with microtubule-disrupting agents provides a novel synergistic strategy to selectively disrupt assembly and induce regression of nascent tumor neovessels, with minimal toxicity and without affecting normal stabilized vasculature.

The cHS4 Insulator Increases the Probability of Retroviral Expression at Random Chromosomal Integration Sites

ABSTRACT Retroviruses are highly susceptible to transcriptional silencing and position effects imparted by chromosomal sequences at their integration site. These phenomena hamper the use of recombinant retroviruses as stable gene delivery vectors. As insulators are able to block promoter-enhancer interactions and reduce position effects in some transgenic animals, we examined the effect of an insulator on the expression and structure of randomly integrated recombinant retroviruses. We used the cHS4 element, an insulator from the chicken β-like globin gene cluster, which has been shown to reduce position effects in transgenic Drosophila. A large panel of mouse erythroleukemia cells that bear a single copy of integrated recombinant retroviruses was generated without using drug selection. We show that the cHS4 increases the probability that integrated proviruses will express and dramatically decreases the level of de novo methylation of the 5′ long terminal repeat. These findings support a primary role of methylation in the silencing of retroviruses and suggest that cHS4 could be useful in gene therapy applications to overcome silencing of retroviral vectors.

Multiple stages of malignant transformation of human endothelial cells modelled by co-expression of telomerase reverse transcriptase, SV40 T antigen and oncogenic N-ras.

We have modelled multiple stages of malignant transformation of human endothelial cells (ECs) by overexpressing the catalytic subunit of human telomerase (hTERT), together with SV40 T antigen (SV40T) and oncogenic N-ras. Transfection with hTERT alone, led to the immortalization of two out of three cultures of bone marrow-derived ECs (BMECs). One hTERT transduced BMEC culture underwent a long proliferative lag before resuming proliferation. BMECs transfected with hTERT alone were functionally and phenotypically normal. BMECs transfected with SV40T (BMSVTs) had an extended lifespan, but eventually succumbed to crisis. BMSVTs exhibited a partially transformed phenotype, demonstrating growth factor independence, altered antigen expression and forming tiny, infrequent colonies in vitro. Transduction of BMSVTs with hTERT resulted in immortalization of 4 out of 4 cultures. BMSVTs immortalized with hTERT formed large colonies in vitro and small transient tumours in vivo. BMECs co-expressing SV40T, hTERT and N-ras exhibited an overtly transformed phenotype; forming very large colonies with an altered morphology and generating rapidly growing tumours in vivo. These investigations demonstrate transformation of human ECs to an overtly malignant phenotype. This model will be useful for understanding mechanisms underlying vascular and angiogenic neoplasias, as well as for testing drugs designed to curtail aberrant EC growth.

Progress toward the genetic treatment of the β-thalassemias

Abstract: The β‐thalassemias are congenital anemias that are caused by mutations that reduce or abolish expression of the β‐globin gene. They can be cured by allogeneic hematopoietic stem cell (HSC) transplantation, but this therapeutic option is not available to most patients. The transfer of a regulated β‐globin gene in autologous HSCs is a highly attractive alternative treatment. This strategy, which is simple in principle, raises major challenges in terms of controlling expression of the globin transgene, which ideally should be erythroid specific, differentiation‐ and stage‐restricted, elevated, position independent, and sustained over time. Using lentiviral vectors, May et al. demonstrated in 2000 that an optimized combination of proximal and distal transcriptional control elements permits lineage‐specific and elevated β‐globin expression, resulting in therapeutic hemoglobin production and correction of anemia in β‐thalassemic mice. Several groups have by now replicated and extended these findings to various mouse models of severe hemoglobinopathies, thus fueling enthusiasm for a potential treatment of β‐thalassemia based on globin gene transfer. Current investigation focuses on safety issues and the need for improved vector production methodologies. The safe implementation of stem cell‐based gene therapy requires the prevention of the formation of replication‐competent viral genomes and minimization of the risk of insertional oncogenesis. Importantly, globin vectors, in which transcriptional activity is highly restricted, have a lesser risk of activating oncogenes in hematopoietic progenitors than non‐tissue‐specific vectors, by virtue of their late‐stage erythroid specificity. As such, they provide a general paradigm for improving vector safety in stem cell‐based gene therapy.

A Promising Genetic Approach to the Treatment of b-Thalassemia

The stable introduction of a functional gene into autologous stem cells is a potentially powerful approach to treat a number of inherited or acquired diseases. One challenge facing this approach is to express adequate levels of the therapeutic transgene in a regulated and sustained fashion, eventually restricting expression to a single lineage developing from the transduced stem cells. Until now, low-level expression, position effects, and transcriptional silencing have hampered the effectiveness of retroviral-mediated gene transfer. In an effort to overcome these obstacles, we have systematically investigated vectors encoding the human beta-globin gene linked to selected combinations of proximal and distal genetic regulatory elements. Our results demonstrate that with thoughtful vector design one can successfully express long-term, therapeutic levels of virally encoded human beta-globin in the erythroid progeny of hematopoietic stem cells.

Expression of Telomerase after Retroviral Transduction Increases Lifespan of Primary Human Fibroblasts

Expression of Telomerase after Retroviral Transduction Increases Lifespan of Primary Human Fibroblasts