Support cells have different roles in blood stem cell maintenance before and after stress


Expression of pleiotrophin (green) in bone marrow blood vessels (red) and stromal cells (white) in normal mice (left), and in mice 24 hours after irradiation (right). UCLA Broad Stem Cell Research Center/Cell Stem Cell

A new study published in the journal Cell Stem Cell, reveals how different types of cells in the bone marrow are responsible for supporting blood stem cell maintenance before and after injury.

It was already well known in the field that two different cell types, namely endothelial cells (which line blood vessels) and stromal cells (which make up connective tissue, or tissue that provides structural support for any organ), are responsible for maintaining the population of blood stem cells in the bone marrow. However, how these cells and the molecules they secrete impact blood stem cell development and maintenance is not well understood.

Hematopoietic stem cells are responsible for generating the multiple different types of cells found in blood, from our oxygen carrying red blood cells to the many different types of white blood cells that make up our immune system.

Dr. John Chute’s group at UCLA had previously discovered that a molecule called pleiotrophin, or PTN, is important for promoting self-renewal of the blood stem cell population. They did not, however, understand which cells secrete this molecule and when.

To answer this question, the scientists developed mouse models that did not produce PTN in different types of bone marrow cells, such as endothelial cells and stromal cells. Surprisingly, they saw that the inability of stromal cells to produce PTN decreased the blood stem cell population, but deletion of PTN in endothelial cells did not affect the blood stem cell niche.

Even more interestingly, the researchers found that in animals that were subjected to an environmental stressor, in this case, radiation, the result was reversed: endothelial cell PTN was necessary for blood stem cell renewal, whereas stromal cell PTN was not. While an important part of the knowledge base for blood stem cell biology, the reason for this switch in PTN secretion at times of homeostasis and disease is still unknown.

As Dr. Chute states in a press release, this result could have important implications for cancer treatments such as radiation:

“It may be possible to administer modified, recombinant versions of pleiotrophin to patients to accelerate blood cell regeneration. This strategy also may apply to patients undergoing bone marrow transplants.”

Another important consideration to take away from this work is that animal models developed in the laboratory should take into account the possibility that blood stem cell maintenance and regeneration is distinctly controlled under healthy and disease state. In other words, cellular function in one state is not always indicative of its role in another state.

This work was partially funded by a CIRM Leadership Award.