The blood-making stem cell within the bone marrow (hematopoietic stem cell or HSC) is a marvelous thing. It grows throughout our lives and replenishes red and white blood cells, and also contributes to the production of new blood vessels. However, attempts to culture this cell in the laboratory have met with only limited success; until now. Researchers at the Stowers Institute for Medical Research investigated the molecular mechanisms that HSCs from mice and applied their insights to expand cultured HSCs one hundred fold.
Stowers investigator Linheng Li led the study and these findings have been published in the Sept. 15, 2011, edition of Genes & Development. This paper demonstrates that self-renewal requires three complementary events: proliferation and active suppression of differentiation and programmed cell death during proliferation. Li said: “The previous efforts so far to grow and expand scarce hematopoietic stem cells in culture for therapeutic applications have been met with limited success. Being able to tap into stem cell’s inherent potential for self-renewal could turn limited sources of hematopoietic stem cells such as umbilical cord blood into more widely available resources for hematopoietic stem cells.” Li added that their findings have yet to be replicated in human cells.
HSC transplantations have been used to treat conditions like anemia, immune deficiencies and other diseases, including cancer. However, bone marrow transplants require a suitable donor-recipient tissue match, and this limits the number of potential donors is limited. Hematopoietic stem cells isolated from umbilical cord blood could be a good alternative source, since they are readily available and not readily recognized by the immune system. Therefore, umbilical cord HSCs allow the donor-recipient match to be less than perfect without the risk of immune rejection of the transplant. Unfortunately, the therapeutic use of HSCs is limited since umbilical cord blood contains only a small number of stem cells.
John Perry, first author on this paper, noted, “The default state of stem cells is to differentiate into a specialized cell types. Differentiation must be blocked in order for stem cells to undergo self-renewal.” Although self-renewal is typically considered a single trait of stem cells, Li and his team wondered whether it could be pulled apart into three distinct requirements: proliferation, maintenance of the undifferentiated state, and the suppression of programmed cell death or apoptosis. Stem cell proliferation in an undifferentiated state activates genes called “tumor suppressor” genes into action. Tumor suppressor genes help prevent cancer by inducing a programmed death (also known as apoptosis). Consequently, self-renewal of adult stem cells must also include this third event and that even it suppression of apoptosis.
Perry and his colleagues isolated mouse HSCs from mouse bone marrow and analyzed two key genetic pathways—the Wnt/β-catenin and PI3K/Akt pathways. Wnt proteins are “self-renewal factors,” while PI3K/Akt activation has been shown to induce proliferation and promote survival by inhibiting apoptosis. Activation of the Wnt/β-catenin pathway alone blocked differentiation but eventually resulted in cell death. Activation of the PI3K/Akt pathway alone increased differentiation but facilitated cell survival. However, when both pathways were simultaneously activated, the pool of HSCs started expanding in culture. This demonstrates that both pathways must cooperate to promote self-renewal.
Although altering both pathways drives HSC self-renewal, it also permanently blocks the ability of these cells to mature into fully functional blood cells. To prevent the block to differentiation and generate normal, functioning HSCs usable for therapy, the Stowers scientists used small molecules to reversibly activate both the Wnt/β-catenin and PI3K/Akt pathways in culture. As Li put it, “We were able to expand the most primitive hematopoietic stem cells, which, when transplanted back into mice gave rise to all blood cell types throughout three, sequential transplantation experiments,” says Li. “If similar results can be achieved using human hematopoietic stem cells from sources such as umbilical cord blood, this work is expected to have substantial clinical impact.”