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2006 Interview with Dr. Sahara

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STEM CELLS Young Investigator Award Recipients 2006: Q & A

STEM CELLS is proud to feature the co-recipients of the STEM CELLS Young Investigator Award, 2006, as they were interviewed by Editor-in-Chief Dr. Curt I. Civin.

Dr. Civin asked Dr. Makoto Sahara about his research and plans for the future.

Makoto Sahara, M.D., Ph.D.

CIC: First, let me extend my personal congratulations and those of the entire Editorial Board of STEM CELLS. There were 48 excellent papers among the applicants for this prize, and at least 10 of them were felt to have moved our field considerably. The judges (members of our Editorial Board) had a very hard time deciding which was the most outstanding paper. Indeed, we finally decided to split the very top prize. So, your work has been seriously judged to be field-leading.

MS: I am very honored to be selected as a co-winner of this prize, the STEM CELLS Young Investigator Award, 2006, from among a great number of applicants. Also, I appreciate the considerable efforts of the Editorial Board of STEM CELLS in the selection process. This prize is significantly encouraging as well as exciting. Keeping in mind this encouragement, I will do my best to continue my investigations into medical science.

CIC: Please tell me, in language intended for a general scientific audience rather than for our stem cell "niche," what hypothesis you were testing in the research from your paper.

MS: We tested the contribution and plasticity of a highly purified hematopoietic stem cell (HSC) in the process of atherosclerotic lesion formation. In short, we sought to evaluate whether a highly purified HSC could participate in pathological vascular remodeling after mechanical injury through transdifferentiation into vascular cells—i.e., vascular smooth muscle cells and/or vascular endothelial cells.

CIC: Give me some background rationale, explaining why this hypothesis was important in stem cell research.

MS: Although it was assumed that HSCs give rise to hematopoietic cells, recent evidence suggests that bone marrow-derived cells (or HSCs) may have the broader potential to differentiate into nonhematopoietic cells and that bone marrow-derived cells (or HSCs) may participate in regeneration and/or remodeling of remote organs. On the contrary, it is also a fact that there are the papers which have proposed negative data about the possibility for adult HSCs to transdifferentiate into nonhematopoietic lineages. In pathological vascular remodeling, it has also been suggested that HSCs may have the plasticity to transdifferentiate into vascular cells. However, the HSCs used in these studies were not a homogeneous population. The CD34–, c-Kit+, Sca-1+, and Lineage- (CD34–KSL) bone marrow cells have been assumed as the most primitive HSCs and frequently have been used as HSCs in these studies, although only approximately one of three CD34–KSL bone marrow cells is considered an HSC. Even the CD34–KSL cells are considered a heterogeneous population containing nonhematopoietic cells, and it is possible that nonhematopoietic cells among the CD34–KSL cells might be responsible for the plasticity observed in the HSC studies. At the same time, our collaborators (Y. Matsuzaki and H. Okano) have revealed that the murine bone marrow cells that have both the strongest dye-efflux activity ("Tip"-side population [SP] cells) and a phenotype of CD34–KSL represent the most primitive HSCs with nearly complete hematopoietic engraftment activity. In their study, approximately nine of ten among the Tip-SP CD34–KSL cells are considered primitive HSCs. Consequently, the use of the TipSP CD34–KSL cells, which are almost equal to HSCs, enabled us to investigate whether a highly purified HSC can contribute to vascular remodeling after severe vascular injury.

CIC: Briefly outline your experimental approach to test your hypothesis.

MS: First, we transplanted either total bone marrow cells (TBM group), KSL bone marrow cells (KSL group), or a single TipSP CD34–KSL cell (HSC group) derived from GFP-transgenic mice into lethally irradiated wild-type mice (C57BL/6 background). At 12 weeks after irradiation and stem cell transplantation, an endovascular arterial injury was induced to the femoral artery of the bone marrow chimeric mice by inserting a large wire. Four weeks after the wire-injury, the injured femoral arteries were excised and fixed in 4% paraformaldehyde for histological analyses. The sections were observed under confocal microscopes. Cell number (GFP+ / total) was counted in the neointima and thickened media of a cross-section of each injured artery.

CIC: Was there a specific methodological technique that was very important in these studies?

MS: First, to highly purify HSCs, we combined the "Tip"-SP cells with a phenotype of CD34–KSL using fluorescence-activated cell sorting. Second, we induced wire-mediated endovascular injury in femoral artery by inserting a straight spring wire (0.38 mm in diameter, No. C-SF-15-15, Cook, Bloomington, IN). Third, to preserve GFP signal for histological analyses, the excised arteries were embedded in plastic resin (Technovit 8100, Heraeus Kulzer, Wehrheim, Germany).

CIC: What was the outcome (results) of your experiments?

MS: A single Tip-SP CD34-KSL cell showed significant prolonged donor cell engraftment, just like total bone marrow cells or KSL bone marrow cells. The lesions in the injured femoral arteries contained a significant number of (bone marrow-derived) GFP-positive cells in the TBM and KSL groups. Many of those GFP-positive cells in the neointima and media expressed alpha-smooth muscle actin, and some of those on the luminal side expressed endothelial markers. In contrast, GFP-positive cells were seldom detected in the lesions of the HSC group.

CIC: How do you interpret these results? What does this mean for stem cell biology?

MS: Our results suggest that it is rare for a highly purified HSC to transdifferentiate into vascular cells, if at all. In contrast, the KSL fraction of bone marrow cells, which is considered to be enriched in HSCs, contained a distinct population that could substantially contribute to lesion formation (vascular remodeling). The KSL fraction could include not only HSCs but also mesenchymal stem cells or multipotent cells that are more primitive than HSCs. It is plausible that those nonhematopoietic cells in the KSL fraction might be responsible for bone marrow-derived smooth muscle-like or endothelial-like cells observed in the vascular lesions after mechanical injury.

CIC: What hypotheses should the field test now?

MS: Bone marrow cells including the KSL fraction, not HSCs, can give rise to vascular cells that contribute to lesion formation (vascular remodeling).Total bone marrow cells or the KSL fraction of bone marrow cells might include putative vascular progenitor cells which might be classified into nonhematopoietic lineage(s). Putative bone marrow-derived smooth muscle progenitor cells might have a potential to become additional therapeutic targets in a variety of progressive vascular diseases. Further studies are needed to identify and characterize putative vascular progenitor cells. Particularly, we consider that it is very important to evaluate what nonhematopoietic population included in the KSL fraction of bone marrow cells is associated with the vascular progenitor cells and can participate in vascular remodeling.

CIC: Why did you select the journal Stem Cells for your paper?

MS: We have seen that the journal STEM CELLS has recently attracted much interest among a general scientific readership as well as among scientists who specialize in stem cell biology; and moreover, we think that it is one of the most exciting and interesting scientific journals in the modern era. The contents of this journal are rich, and the quality is excellent. Those are the reasons we selected the journal STEM CELLS for the publication of our paper.

CIC: Finally, on a more personal note, tell me a little about you, your education, training. What is your position right now? What would you like to do in the near future? What impact do you expect this award to have on your career aspirations?

MS: I am a cardiologist who specializes in cardiac catheterization and percutaneous coronary interventions, and I am also a final-year graduate student in the Department of Cardiovascular Medicine, University of Tokyo Graduate School of Medicine. I research the molecular mechanisms and cellular pathophysiology in the various cardiovascular diseases, including aortic atherosclerosis, ischemic heart disease, peripheral vascular disease, and pulmonary arterial hypertension. The association between tissue stem cells, including HSCs, and a wide variety of cardiovascular diseases is one of the most interesting themes for me. After graduation, I would like to continue my research in these fields and to contribute to advances in medical science. Concurrently, I hope to study abroad in the near future in order to gain experience, although my destination to study abroad has not yet been determined. I expect this award to be helpful in my acceptance as a foreign postdoctoral researcher and/or fellow in some vigorous and proven laboratory.