Direct reprogramming has become a trend in the regenerative medicine field. It means taking readily available cells, such as skin cells or blood cells, and converting them into cells that researchers want for therapeutic purposes, skipping the stem cell stage.

In a way, this approach follows in Nobel Prize winner Shinya Yamanaka’s footsteps, but it also tunnels under the mountain he climbed. Direct reprogramming has been achieved for target cell types such as neurons and insulin-producing beta cells.

Young-sup Yoon, MD, PhD

In Circulation Research, Emory stem cell biologist Young-sup Yoon, MD, PhD and colleagues recently reported converting human skin fibroblast cells into endothelial cells, which line and maintain the health of blood vessels.

Once reprogrammed, a patient’s own cells could potentially be used to treat conditions such as peripheral artery disease, or to form vascular grafts. Exactly how reprogrammed cells should be deployed clinically still needs to be worked out.

In cardiovascular disease, many clinical trials have been performed using bone marrow cells that were not reprogrammed. Emory readers may be familiar with studies conducted by Arshed Quyyumi, MD and colleagues, in which treatment was delivered after patients’ heart attacks. In those studies, sorted progenitor cells, some of which could become endothelial cells, were introduced into the heart. To provide the observed effects, the introduced cells were more likely supplying supportive growth factors.

In contrast, Yoon’s team is able to produce cells that already have endothelial character hammered into them. The authors have applied for a patent. The co-first authors were instructor Sang-Ho Lee, PhD and Changwon Park, PhD, assistant professor of pediatrics.

In the Circulation Research paper, the authors describe two different types of cells: early and late reprogrammed endothelial cells. The early type still shows some fibroblast features, but is expandable in culture and can become incorporated into blood vessels in mice. The late type is more mature and durable, has more endothelial cell genes turned on, and requires months of culture to generate. Both types form vessel-like tubular structures (see figure).

Early (top) and late (bottom) reprogrammed endothelial cells form tubular structures in culture

When injected into the legs of mice, the early type of cell could provide beneficial effects on blood supply in a hindlimb ischemia model, but the authors write: “The observed effects appear to be attributed more to non-cell autonomous effects such as secretion of angiogenic factors than to direct incorporation of early rECs into the vessels.”

To produce reprogrammed endothelial cells, the researchers started off by introducing into skin cells combinations of genes known to be important in endothelial cells. They began with seven different genes but it turns out one gene, called ETV2, can do the job all by itself. Endothelial cells were sorted out using the marker KDR (also known as VEGF receptor 2 or FLK1). Generating the more mature late form of endothelial cells requires taking a break from ETV2.

Changwon Park, PhD

The researchers used viral vectors to introduce ETV2 into the skin fibroblast cells, but viral vectors carry a risk of inducing mutations or tumors. The authors write that the next step is to use non-viral vector methods, already tested for making iPS cells, to introduce ETV2.

This review discusses a related issue: direct reprogramming of fibroblasts into cardiac muscle cells. It explores whether endogenous fibroblasts in scar tissue could be converted by gene therapy into cardiac muscle cells, without removing them from the heart.