Imaging Method Enables Research on Abundant Cellular Protein Actin

Facebook X LinkedIn Email
Researchers at the Salk Institute have developed a new imaging method that allows them to monitor actin, a small subset of skeleton-like filaments within cells. The method has enabled research on how actin mediates an important function: helping mitochondria divide in two. The research could allow for a better understanding of mitochondrial dysfunction, which has been linked to cancer, aging, and neurodegenerative diseases.

“Actin is the most abundant protein in the cell, so when you image it, it’s all over the cell,” said Uri Manor, director of Salk’s Biophotonics Core facility and corresponding author of the paper. “Until now, it’s been really hard to tell where individual actin molecules of interest are, because it’s difficult to separate the relevant signal from all the background.”
A cancer cell labeled for actin (red) and mitochondria (cyan). The scientists designed novel probes that specifically monitor interactions between actin and mitochondria. Courtesy of Salk Institute/Waitt Advanced Biophotonics Center.
A cancer cell labeled for actin (red) and mitochondria (cyan). The scientists designed novel probes that specifically monitor interactions between actin and mitochondria. Courtesy of Salk Institute/Waitt Advanced Biophotonics Center.

Mitochondrial fission is the process by which these structures, or organelles, divide and multiply as part of normal cellular maintenance; the organelles divide not only when a cell itself is dividing, but also when cells are under high amounts of stress or when mitochondria are damaged.

However, the exact manner in which one mitochondrial pinches off into two mitochondria has been poorly understood, particularly how the initial construction happens. Studies have been attempted in which actin has been removed entirely, which leads to less mitochondrial fission, among many other effects. Because there are so many effects, determining actin’s exact role in any one process is difficult to discern, the researchers say.

Rather than tag all the actin in the cell with fluorescence, the researchers created an actin probe targeted to the outer membrane of mitochondria. When the actin is within 10 nm of the mitochondria, it attaches to the sensor, causing the fluorescence signal to increase.

Manor’s team was able to observe bright hot spots of actin. Looking closely, they saw that the hot spots were located at the same locations where another organelle, the endoplasmic reticulum, crosses the mitochondria, previously found to be fission sites. As the team watched actin hot spots light up and disappear over time, they discovered that 97% of mitochondrial fission sites had actin fluorescing around them. The researchers speculate that there was also actin at the other 3% of fission sites, but that it wasn’t visible.

“This is the clearest evidence I’ve ever seen that actin is accumulating at fission sites,” said Cara Schiavon, co-first author of the paper and a joint postdoctoral fellow in the labs of Manor and Salk professor Gerald Shadel. “It’s much easier to see than when you use any other actin marker.”

By altering the actin probe so that it attached to the endoplasmic reticulum membrane rather than the mitochondria, the researchers were able to piece together the order in which different components join the mitochondrial fission process. The team’s results suggest that the actin attaches to the mitochondria before it reaches the endoplasmic reticulum. This lends important insight into how the endoplasmic reticulum and mitochondria work together to coordinate mitochondrial fission.

Additional experiments, described in a pre-print manuscript available on bioRxiv, show that the same accumulation of endoplasmic reticulum-associated actin is seen at the sites where other cellular organelles — including endosomes, lysosomes, and peroxisomes — divide. This suggests a broad new role for a subset of actin in organelle dynamics and homeostasis (physiological equilibrium).

In the future, the research team aims to look at how genetic mutations known to alter mitochondrial dynamics might also affect actin’s interactions with the mitochondria. The researchers also plan to adapt the actin probes to visualize actin that’s close to other cellular membranes.

Published: August 2020
The emission of light or other electromagnetic radiation of longer wavelengths by a substance as a result of the absorption of some other radiation of shorter wavelengths, provided the emission continues only as long as the stimulus producing it is maintained. In other words, fluorescence is the luminescence that persists for less than about 10-8 s after excitation.
fluorescence microscopy
Observation of samples using excitation produced fluorescence. A sample is placed within the excitation laser and the plane of observation is scanned. Emitted photons from the sample are filtered by a long pass dichroic optic and are detected and recorded for digital image reproduction.
Research & TechnologyMicroscopylight microscopylight microscopy imagingfluorescencefluorescence microscopycellular imagingcellular microscopyBiophotonicsmitochondriamitochondrial functionSalk InstituteSalkBioScan

We use cookies to improve user experience and analyze our website traffic as stated in our Privacy Policy. By using this website, you agree to the use of cookies unless you have disabled them.