Human pluripotent stem cells have the ability to develop into any cell type within the body. They depend strongly on sugar fermentation or glycolysis in order to power their metabolic activities. In comparison, mature cells to which pluripotent stem cells can develop, rely mainly on cell mitochondria to convert oxygen and sugar into water and carbon dioxide during a high energy-producing process (oxidative phosphorylation) for their metabolic requirements.
So far, it has remained unknown how cells progress from one form of energy production to another during development. However, UCLA stem cell investigators have discovered a new finding that offers fresh insight into this transition that could have implications for using these cells for treatments in the clinic. The four year investigation was published in the Nov.15 issue of The EMBO Journal, a peer-reviewed journal of the European Molecular Biology Organization.
Researchers believed (based primarily on visual appearance) that pluripotent stem cells consisted of inactive and undeveloped mitochondria. Mitochondria are power centers of the cell that provide the energy cells require in order to divide, move.
It had been assumed that stem cell mitochondria could not respire, or convert oxygen and sugar into carbon dioxide and water with the production of energy. Therefore, investigators expected that mitochondria matured as well as acquired the ability to respire during the transition from pluripotent stem cells into different cells in the body over time.
Dr. Michael Teitall, an investigator with the Eli and Edythe Broad Center of Regenerative Medicine and Stem Cell Research at UCLA and a professor of pediatrics, pathology and laboratory medicine and bioengineering, together with Carla Koehler, a UCLA professor of chemistry and biochemistry, found that even though pluripotent stem cells produce extremely little energy, they respire at approximately the same levels as other cells in the body, thereby detaching the oxygen and sugar consumption from energy production.
Instead of the researchers' anticipated result of finding that mitochondria matured with cell differentiation, they uncovered a mechanism by which the stem cells converted from glucose fermentation to oxygen-dependent respiration to achieve their full ability to produce cell types following cell division.
Study senior author Teitell explained:
"A lot of attention is being paid to the role of metabolism in pluripotent stem cells for making properly differentiated cell lineages for research and potential clinical uses.
The initial question prompting our study was whether metabolism in pluripotent stem cells and cancer cells, which also rely heavily on glycolysis, were molecularly similar. This question led us to study the details of energy-generation by mitochondria in pluripotent stem cells."
Cells generate energy in the form of ATP primarily in two ways, by using respiration, in which the cell consumes oxygen and sugar in order to make water and carbon dioxide to power cell functions, or by glucose uptake and fermentation in the cytoplasm. The team anticipated that pluripotent stem cells were unable to respire due to previous reports of the paucity of mitochondria and the immature appearance.
They discovered that the molecular complexes accountable for respiration (electron transport chain) in the mitochondria of pluripotent stem cells were working, and that instead the cells depended on glycolysis for generating energy. The investigators assume that as the electron transport chain was working there were one or more unknown regulators that prevented the stem cells from respiring.
Jin Zhang, a graduate student and first author of the investigation, found that a protein called uncoupling protein 2 (UCP2), was greatly expressed in the pluripotent stem cells. Furthermore, he discovered that UCP2 obstructed respiration substrates acquired from glucose from obtaining access to the mitochondria, instead UCP2 prevented the pluripotent stem cells to the glycolytic and biosynthesis pathways located in the cytoplasm, preventing their ability to respire as a technique for producing energy.
Since the stem cells were powered to develop into mature cell types, UCP2 expression was blocked, permitting respiration substrates to enter the mitochondria to produce energy, thereby switching the pluripotent stem cells from glycolysis to oxidative phosphorylation.
The team found that by manipulating UCP2 expression to keep it switched on in differentiating cells, disturbed pluripotent stem cells maturation. This discovery may make these stem cells unsuitable for clinical use. In addition this finding highlights the importance of properly functioning metabolism for producing safe, high quality cells.
The team verified these discoveries in both human embryonic stem cells and in induced pluripotent stem cells, which are mature body cells genetically modified to have the same attributes and abilities as the pluripotent embryonic stem cells.
"A main question that evolved during the study was whether it was the process of pluripotent stem cell differentiation that was altering the pattern of metabolism, or was it the change in the pattern of metabolism that altered the process of differentiation, a typical chicken-or-the-egg question.
We over-expressed UCP2 in the stem cells and showed that metabolism patterns changed before markers of pluripotent of cell maturation changed, indicating that changes in metabolism affect changes in differentiation and not the other way around, at least for UCP2.
This was important, to show causation for metabolic changes in driving the process of cell differentiation. However, it still leaves open the key question of exactly how manipulating cell metabolism controls cell differentiation, a question we are working hard to address."
Teitell said that as metabolism in pluripotent stem cells appear quite similar to cancer cells, the discoveries of this investigation could possibly be used to target UCP2 in malignant tumors that express it, of which there are several. Blocking UCP2 may encourage cancel cells to respire, which may weaken their ability to grow rapidly.
The investigation was funded in part by the California Institute for Regenerative Medicine, and Eli and Edythe Broad Center of Regenerative Medicine and Stem Cell Research training grant, the National Institutes of Health and the National Center for Research Resources.
Written by Grace Rattue