An advanced imaging-based method from Scripps Research scientists offers a new way to study mitochondria, best known as the “powerhouses” of cells. In their report in the Journal of Cell Biology, the researchers described a set of techniques that allow imaging and quantification of even subtle structural changes inside mitochondria and correlating these changes with other processes occurring in cells.
Mitochondrial dysfunctions
Mitochondria are involved not only in energy production but also in several other critical cellular functions, including cell division and cell maintenance responses to various types of stress. Mitochondrial dysfunctions have been observed in a number of diseases, including Alzheimer’s disease, Parkinson’s disease, and various cancers, and scientists are eager to develop treatments that could reverse these dysfunctions. But scientific tools for studying the fine details of mitochondrial structure have been limited.
“We now have a powerful new set of tools for detecting and quantifying structural, and therefore functional, differences in mitochondria for example, between diseased and healthy,” says lead study author Danielle Grotjahn, PhD, assistant professor in the Department of Integrative Structural and Computational Biology at Scripps Research.
Grotjahn lab members Benjamin Barad, PhD, postdoctoral research associate, and Michaela Medina, PhD candidate, were co-first authors of the study.
Mitochondria are one of many membrane-bound molecular machines or “organelles” that reside in plant and animal cells. Typically numbering hundreds to thousands per cell, mitochondria have their own small genomes and have a characteristic structure with an outer membrane and a corrugated inner membrane where key biochemical reactions take place.
Scientists know that the appearance of mitochondrial structures can change dramatically depending on what the mitochondria are doing or what stresses are present in the cell. These structural changes can therefore be very useful markers of cellular conditions, although until now there has been no good method for their detection and quantification.
In the study, Grotjahn’s team assembled a suite of computational tools to process imaging data from a microscopic technique called cryo-electron tomography (cryo-ET)—which essentially images biological samples in three dimensions using electrons instead of light. researchers
The “surface morphometric toolkit,” as they call it, enables detailed mapping and measurement of the structural elements of individual mitochondria. This includes inner membrane folds and intermembrane gaps—all potentially useful markers of important mitochondrial and cellular events.
“It essentially allows us to turn the beautiful 3-D images of mitochondria that we can get from cryo-ET into sensitive quantitative measurements—which we can potentially use to identify detailed disease mechanisms, for example,” says Barad.
The team demonstrated the toolkit by using it to map structural details on mitochondria when their cells are subjected to endoplasmic reticulum stress – a type of cellular stress often seen in neurodegenerative diseases. They observed that key structural features, such as the curvature of the inner membrane or the minimum distance between the inner and outer membranes, changed measurably under this stress.
With their successful and validated demonstrations of the new toolkit, the Grotjahn lab will now use it to study in more detail how mitochondria respond to cellular stress or other changes induced by disease, toxins, infections and even drugs. “We can compare the effects on mitochondria in cells treated with a drug to the effects on untreated mitochondria, for example,” says Medina. “And this approach is not limited to mitochondria – we can also use it to study other organelles in cells.”
