Fat cells swim and block wounds

At least in fruit flies, fat cells have been found to exhibit several previously unknown and surprising behaviours geared to healing wounds and fending off infection.

The behaviours are surprising because they involve properties that fat cells – considered to be one of the most uncontroversial, best examined and, frankly, unexciting cell types around – had not been remotely suspected of possessing.

The cells, it turns out, can swim – really swim, moving along by flexing and contracting. What’s more, they do so in response to some form of unknown signal – something that has so far eluded the scientists searching for it.

In a paper published in the journal Developmental Cell, researchers led by biochemist Paul Martin from the University of Bristol, UK, describe how fat cells in fruit flies (Drosophila) play an unexpectedly major and mobile role in wound healing.

The scientists were able to film individual cells reacting to a cut sustained by a fruit fly.

The fat cells can be seen propelling themselves forward, using wave-like contortions, towards the site of the injury. This is doubly interesting. Until now, fat cells were not thought to be capable of moving under their own power, nor were they thought to play any role at all in wound repair.

Fat cells to the rescue! This video clearly shows fat cells, coloured green, moving into the site of a wound and blocking it off.
Credit: Franz et al./Developmental Cell
And it turns out that fat cells use a different method of locomotion compared to other cell types previously recognised as mobile. Instead of the standard method – which involves effectively using little spikes to bounce off other objects – fat cells make muscle fibre proteins, actin and myosin, squeeze in and out at their centres. The movement type is known as peristalsis and it is basically the same form of muscle control used by mammal oesophagi when swallowing food.

Martin and his colleagues filmed the fat cells moving directly to the wound site and bunching up within it, sealing it off from the outside. At the same time, they cause debris from the opening to be wafted away from the critical zone, into the waiting maws of immune system cells.

The fat cells then remained at the wound until healing was complete, at which point they detached and swam away.

Watching all this happen, the scientists were dumbfounded, half-suspecting what they were seeing was nothing more than the results of dumb physics.

“We had to be sure that they weren’t just drifting and then sort of sticking at the wound site,” says Martin. “And we had to rule out that they weren’t just being sort of sucked to the wound by fluid coming out of the hole, much like if you tossed a flannel in a bath and then took the plug out.”

To do so, they genetically engineered fruit fly fat cells that lacked the actin and myosin fibres, and then repeated the experiment. The modified cells didn’t move at all, indicating that the “natural” versions were indeed actively migrating.

How the cells “know” to move to a wound is unknown, but Martin and his colleagues have ruled out the possibility that they are responding to signals sent by the immune system. The fat cells, they established, still move towards wounds even if immune cells aren’t present.

Once at the site, however, in normal fruit flies, the two cell types clearly collaborate.

“So fat cells and immune cells probably are important as a team, both in a healthy situation, like healing a wound, and in a pathology situation, like cancer,” he says.

After completing further fly research, the scientists plan to look at the role of fat cells in healing in vertebrates. In these, to date, no one has ever observed them moving of their own accord.

“But perhaps they do,” muses Martin. “Now, because of this research, it would be worth looking at them. It’s not crazy to think that they might travel to a wound and do important things when they get there.”

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