Cells have places to be, and they have all evolved different ways to get there: Red blood cells change their shape, and bacteria use whiplike appendages to propel themselves forward. Now, new research suggests white blood cells have their own special way of swimming, which biologists have dubbed “molecular paddling.”

For years, scientists thought white bloods cells could move across 2D surfaces, like blood vessels or skin layers, only by attaching to and crawling along them. They also knew certain human and mouse white blood cells could swim in liquids, but they weren’t sure how.

To find out, researchers recorded how human white blood cells, or leukocytes, moved in liquids and on solid surfaces. Videos suggested the cells swam by changing their shape and using a breaststrokelike motion. But such movement wasn’t enough to explain the speeds clocked by the cells. So the scientists came up with a new proposal: The cells’ membranes are packed with protein “paddles” that give them a speed boost.

As the leukocytes swam, researchers saw protruding proteins migrate from the front to the back the cell, as if on a treadmill (above). According to their hypothesis, as the cell membrane moves backward, these proteins contract and propel the cell forward. Then, when the proteins reach the back of the cell, they migrate inside the membrane and move back out to the front to start the process over again.

And because the proposed paddling proteins moved internally from the rear to the front of the cell as it swam, the researchers suspect the proteins are “recycled,” allowing the cell to continuously paddle, they reported online last month in Biophysical Journal. Don’t expect a speedy journey, though. This novel paddling mechanism is about four times slower than crawling.



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