Leukocytes Swim Using Molecular Paddling

Leukocytes employ a micro-swimming mechanism to move autonomously, according to a study published in Biophysical Journal. The phenomenon could explain how cells migrate through various fluid-filled niches in the body. 

"The capacity of living cells to move autonomously is fascinating and crucial for many biological functions, but mechanisms of cell migration remain partially understood," says co-senior author Olivier Theodoly of Aix-Marseille University in France. "Our findings shed new light on the migration mechanisms of amoeboid cells, which is a crucial topic in immunology and cancer research."

The research team provides experimental and computational evidence in the new study that human leukocytes can migrate on 2D surfaces without sticking to them and can swim using a mechanism that does not rely on changes in cell shape.

 "Looking at cell motion gives the illusion that cells deform their body like a swimmer," co-senior study author Chaouqi Misbah says. "Although leukocytes display highly dynamic shapes and seem to swim with a breast-stroke mode, our quantitative analysis suggests that these movements are inefficient to propel cells."

The researchers propose that continuous paddling is enabled by a combination of actin-driven external treadmilling and inner recycling of actin-bound transmembrane proteins through vesicular transport. Specifically, the paddling proteins at the rear of the cell are enclosed inside a vesicle that pinches off from the cell membrane and transported to the front of the cell. In contrast, the non-paddling transmembrane proteins are sorted out and do not undergo this process of internal recycling through vesicular transport.

"This recycling of the cell membrane is studied intensively by the community working on intracellular vesicular traffic, but its role in motility was hardly considered," Theodoly says. "These functions of protein sorting and trafficking seemed highly sophisticated for swimming. Our investigations, to our own surprise, bridge such distant domains as the physics of microswimmers and the biology of vesicular traffic."