The fate of a cell is not defined exclusively by its genes. Nor is it determined solely by the environment in which it develops. Scientists now know that internal cellular structures seem to influence the role that cells will play when mature. Among these structures that can redirect cellular destiny are mitochondria, the organelles responsible for energy production. A group of Brazilian and American researchers coordinated by physician Alicia Kowaltowski and biologist Maria Fernanda Forni, both from the Chemistry Institute at the University of São Paulo (IQ-USP), observed that mitochondrial shape and size help determine the type of tissue that a mature stem cell will give rise to.
In laboratory experiments, the researchers extracted adult stem cells from mice and used chemical stimuli to induce their transformation into bone, cartilage or fat cells. During this maturation process, called cell differentiation or specialization, they watched the changes that took place in the mitochondria. The results, published in December 2015 in the journal Stem Cells, suggest that the shape and size of mitochondria – either large and elongated or small and round – may be a determining factor in cell differentiation.
Cell maturation is a complex and still largely unknown process in which many molecular pathways interact, often influencing each other. It is how cells acquire their specific traits – fat cells, for example, become specialized in energy storage. To understand this complicated event a little better, Kowaltowski and Forni decided to analyze the dynamics of mitochondria, organelles that have been getting attention from researchers in recent years due to their association with the development of neurodegenerative diseases, diabetes, increased appetite and fat accumulation (see Pesquisa FAPESP Issue nº 212).
For a long time, mitochondria were thought to remain static and unchanging within cells. In the past decade, however, a number of studies have shown that they are actually quite dynamic. Mitochondria are able to fuse together to generate a single larger, more elongated organelle. They can also split up into smaller, rounder mitochondria. Various proteins coordinate these mitochondrial dynamics. One of them, mitofusin 2, helps the organelles join together and elongate. Another, known as DRP1, is essential for mitochondria to divide and give rise to smaller versions of themselves. Longer mitochondria produce proportionally more energy in the form of adenosine triphosphate (ATP), a molecule that stores a great deal of energy within its chemical bonds. Smaller mitochondria are less efficient at ATP production.
In the study published in Stem Cells, the research group at USP induced stem cells to specialize, then assessed how their mitochondria varied in shape, function, and production of mitofusin 2 and DRP1. The researchers also analyzed the energy metabolism of the cells by measuring the oxygen consumed by their mitochondria. Through a process called cellular respiration, these organelles use oxygen to break down sugar (glucose) molecules, generating energy in the form of ATP.
The researchers observed that mitofusin 2 and DRP1 production varied during cell differentiation, according to the fate of the cell. “Cells that would turn into bone or fat produced more mitofusin 2 and had elongated mitochondria, whereas those that would become cartilage synthesized more DRP1 and had smaller, rounded mitochondria,” says Forni. Cells with elongated mitochondria respired more – and produced more energy – than those with spherical mitochondria. Some hypotheses propose that elongated mitochondria produce more energy because they have more copies of the enzymes involved in the Krebs cycle, the sequence of chemical reactions by which ATP is produced.
Cause or consequence?
But there were still doubts. The initial results did not reveal whether these changes in mitochondrial shape and size were sealing the fate of the cell or just the other way around: whether the cell’s final role was determining the morphology of its mitochondria. To elucidate the issue, new experiments were in order. Kowaltowski and Forni decided to restrict mitofusin 2 synthesis in cells with elongated mitochondria and block DRP1 production in cells with small, round ones.
To everyone’s surprise, when their mitochondria stopped fusing together or splitting up, cells lost the ability to differentiate. “They were no longer able to transform into mature cells,” says Kowaltowski. “This means that changing mitochondrial shape is essential for stem cell differentiation,” she concludes. According to the researchers, the same probably happens in other types of stem cells.
The research group now plans to compare mice fed different types of diet – one with freely available food and the other controlled – to investigate the impact of diet on mitochondrial metabolism and whether this interferes in stem cell differentiation.
1. Mitochondrial bioenergetics, ion transport, redox state and DNA metabolism (nº 2010/51906-1); Grant Mechanism Thematic Research Grant; Principal Investigator Alicia Juliana Kowaltowski (IQ-USP); Investment R$2,219,960.89.
2. Effects of calorie restriction on morphology, dynamics, bioenergetics and redox state of mitochondria (nº 2013/04871-6); Grant Mechanism Scholarship in Brazil – Postdoctoral; Principal Investigator Alicia Juliana Kowaltowski (IQ-USP); Awardee Maria Fernanda Pereira de Araújo Demonte Forni (IQ-USP); Investment R$244,304.00.
FORNI, M. F. et al. Murine mesenchymal stem cell commitment to differentiation is regulated by mitochondrial dynamics. Stem Cells. Dec. 2015.