Scientists offer insights on how the body adapts to mechanical stresses—from pressure placed on bones during simple walking, to extreme forces experienced during intense exercise—in a study published recently in eLife.
Increases in cell levels of calcium and the release of adenosine triphosphate (ATP), considered by biologists to be the energy currency of life, are known to be early events following mechanical stress of bone cells, but exactly how mechanical forces lead to ATP release remained unresolved.
“The goal of our study was to examine the mechanism of ATP release from mechanically stimulated bone cells,” explains lead author Nicholas Mikolajewicz, PhD student at McGill University and Shriners Hospitals for Children—Canada, in a media release from eLife.
“We realized early on that the membrane of bone cells needs to be disrupted to increase calcium levels, and soon after we became interested in how these injuries affect ATP release.”
The team first looked at the timing of ATP release after mechanical injury. Using bone cells from three different sources, they physically stimulated individual cells and found that ATP release occurred within seconds after mechanical stimulation, and that this preceded an increase in calcium levels in neighbouring cells.
Next, they looked at where the ATP was being released from. Since it was previously proposed that it comes from vesicles—tiny membrane sacs within the cell, they used a combination of dyes to measure the release of vesicles and ATP. To their surprise, they found that the more vesicles were released, the less ATP was released after mechanical stimulation, the release explains.
This led the team to propose a mechanically stimulated ATP release is related to cell membrane injury rather than calcium-driven release from vesicles. To test this, they looked at fluorescent dye leakage from bone cells after different levels of injury. This showed that damage to the membrane was reversible and was repaired in as little as 10 seconds. Importantly, the holes in the membrane were around the right size to allow ATP to escape. This reversible damage was also seen in the bone cells of mice when a force, like that experienced during exercise, was applied to the shin bone.
Finally, the team looked at exactly how this membrane repair takes place. They found that the release of tiny vesicles, controlled by an influx of calcium that switched on a molecule called protein kinase C (PKC), is important for membrane repair. Taken together, their results indicate that membrane injury causes ATP spillage from the cell, but then rapid membrane repair—controlled by calcium- and PKC-dependent vesicles—limits the total amount of ATP spilled and ultimately determines the fate of the bone cells, the release continues.
“We have established a novel mechanism used by bone cells to adapt to mechanical forces,” concludes senior author Svetlana Komarova, Associate Professor at McGill University and a researcher at Shriners Hospitals for Children—Canada’s Research Centre.
“Our finding that PKC controls this response provides a potential avenue to explore for treatments for those experiencing altered mechanical environments, such as people with paralysis or astronauts in space.”
[Source(s): eLife, Science Daily]