A team of researchers from Rice University and Baylor College of Medicine has used a modified version of the boron dipyrromethene (BODIPY) molecular rotor to measure temperature inside single cells.
Ogle et al modified BODIPY molecules to serve as nano-thermometers inside cells. The chart on the left is a compilation of fluorescent lifetime micrographs showing the molecules’ response to temperature, in Celsius. At right, the structure of the molecule shows the rotor, at bottom, which is modified to restrict 360-degree rotation. Image credit: Meredith Ogle / Rice University.
BODIPY modified with a polyethylene glycol chain is ideally suited to the task.
Its fluorescence lasts only a little while inside the cell, and the duration depends heavily on changes in both temperature and the viscosity of its environment.
But at high viscosity, the environment in typical cells, its fluorescence lifetime depends on temperature alone.
It means that at a specific temperature, the light turns off at a particular rate, and that can be seen with a fluorescence-lifetime imaging microscope.
“What we measure is how long the molecule stays in the excited state, which depends on how fast it wobbles,” said Rice University chemist Angel Martí.
“If you increase the temperature, it wobbles faster, and that shortens the time it stays excited.”
“The effect is conveniently independent of the concentration of BODIPY molecules in the cell and of photobleaching, the point at which the molecule’s fluorescent capabilities are destroyed.”
“If the environment is a bit more viscous, the molecule will rotate slower. That doesn’t necessarily mean it’s colder or hotter, just that the viscosity of the environment is different.”
“We found out that if we constrain the rotation of this motor, then at high viscosities, the internal clock — the lifetime of this molecule — becomes completely independent of viscosity. This is not particularly common for these kinds of probes.”
According to the team, this technique might be useful for quantifying the effects of tumor ablation therapy, where heat is used to destroy cancer cells, or in simply measuring for the presence of cancers.
“They have a higher metabolism than other cells, which means they’re likely to generate more heat,” Dr. Martí said.
“We’d like to know if we can identify cancer cells by the heat they produce and differentiate them from normal cells.”
The team’s work was published in the Journal of Physical Chemistry B.
_____
Meredith M. Ogle et al. 2019. Sensing Temperature in Vitro and in Cells Using a BODIPY Molecular Probe. J. Phys. Chem B 123 (34): 7282-7289; doi: 10.1021/acs.jpcb.9b04384
