Researchers are using light in a laboratory to study how proteins assemble in cells, an approach that could lead to treatments of neurological disorders such as Huntington’s disease.
The approach is based on the fact that the clumping together of proteins often leads to neurological disorders.
Clifford Brangwynne led the Princeton University study, “Spatiotemporal Control of Intracellular Phase Transitions Using Light-Activated optoDroplets.” It was published in the journal Cell.
Cells are composed of several organelles, structures that isolate certain molecular components and chemical reactions from the rest of the cell. Examples include the nucleus, where DNA is stored, or the mitochondria, where energy is produced.
But some organelles have no wall to seal them off from other cell components, so they exist as self-contained structures in a cell’s watery environment.
Previous studies have shown that membrane-less organelles assemble by phase transition. This is a process in which molecules switch from one physical state to another — like water vapor condensing into water, or liquid water becoming ice.
Brangwynne’s team previously found that changing the concentration or structure of certain proteins led to them forming droplet-shaped organelles. At the time, they studied the process in test tubes because it was difficult to do in a real environment.
The researchers have now developed a technique, called optoDroplets, that uses light to “switch on” the protein clumping process, which lets them study how it unfold.
The approach may help them learn more about how protein aggregation becomes dysfunctional, leading to the development of neurological diseases such as Huntington’s, Alzheimer’s and amyotrophic lateral sclerosis (ALS).
“This optoDroplet tool is starting to allow us to dissect the rules of physics and chemistry that govern the self-assembly of membraneless organelles,” Brangwynne said in a news release. “The basic mechanisms underlying this process are very poorly understood, and if we get a handle on it, there might be a hope for developing interventions and treatments for devastating diseases connected with protein aggregation, such as ALS.”
The optoDroplets technique is based on optogenetics, or using light to manipulate proteins.
Researchers inserted in human and mouse cells a gene from the mouse-ear cress (Arabidopsis thaliana), a plant similar to cabbage and mustard. The gene encodes a light-sensitive protein. When exposed to light, the protein clumps together with other proteins, forming a compact structure.
By fusing this protein with proteins that take part in phase-transition processes, and that are involved in the formation of membrane-less organelles in animal cells, researchers were able to study protein clumping.
When exposed to light, cells containing the light-activated plant gene formed protein aggregates. Turning off the light stopped the transition.
“To use the analogy of water vapor, you can think of what we did as using a laser to locally change the temperature of some area of the air so that water droplets would condense out of it,” Brangwynne said.
OptoDroplets allowed the team to trigger and reverse protein aggregation over and over.
If the light was too intense or the protein concentration too high, semi-solid gels rather than clumps formed. Although the gels were initially reversible, with repeated stimulation they became clumps similar to those seen in Huntington’s or Alzheimer’s, where the clumps block normal cell activity.
“We’ve shown with optoDroplet that we can readily assemble and disassemble phase-separated liquids, and they do not appear to cause any problem for the cell,” Brangwynne said. “But the gel-like assemblies appear to be more problematic, since over many cycles, they develop into persistent aggregates that the cell can no longer deal with and that can start to gum up healthy biological processes.
“This is fundamental science we’re doing, answering basic questions about phase transitions in cells,” he added. “But we’re hoping these insights will reveal not only how healthy cells work, but also how they can become diseased, and maybe eventually cured.”
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