Amoeboid Swimming Is Propelled by Molecular Paddling in Lymphocytes

Abstract

Mammalian cells developed two main migration modes. The slow mesenchymatous mode, like crawling of fibroblasts, relies on maturation of adhesion complexes and actin fibre traction, while the fast amoeboid mode, observed exclusively for leukocytes and cancer cells, is characterized by weak adhesion, highly dynamic cell shapes, and ubiquitous motility on 2D and in 3D solid matrix. In both cases, interactions with the substrate by adhesion or friction are widely accepted as a prerequisite for mammalian cell motility, which precludes swimming. We show here experimental and computational evidence that leukocytes do swim, and that efficient propulsion is not fuelled by waves of cell deformation but by a rearward and inhomogeneous treadmilling of the cell external membrane. Our model consists of a molecular paddling by transmembrane proteins linked to and advected by the actin cortex, whereas freely diffusing transmembrane proteins hinder swimming. Furthermore, continuous paddling is enabled by a combination of external treadmilling and selective recycling by internal vesicular transport of cortex-bound transmembrane proteins. This mechanism explains observations that swimming is five times slower than the retrograde flow of cortex, and also that lymphocytes are motile in non-adherent confined environments. Resultantly, the ubiquitous ability of mammalian amoeboid cells to migrate in 2D or 3D, and with or without adhesion, can be explained for lymphocytes by a single machinery of heterogeneous membrane treadmilling. SIGNIFICANCE STATEMENT Leukocytes have a ubiquitous capacity to migrate on or in solid matrices, and with or without adhesion, which is instrumental to fight infections. The precise mechanisms sustaining migration remain however arguable. It is for instance widely accepted that leukocytes cannot crawl on 2D substrates without adhesion. In contrast, we showed that human lymphocytes swim on nonadherent 2D substrates and in suspension. Furthermore, our experiments and modelling suggest that propulsion rely hardly on cell body deformations and predominantly on molecular paddling by transmembrane proteins protruding outside the cell. For physics, this study reveals a new type of micro-swimmer, and for biology it suggests that leukocytes ubiquitous crawling may have evolved from an early machinery of swimming shared by various eukaryotic cells.

Human neutrophils swim and phagocytise bacteria

“…If any of the above‐mentioned tissues suffers from infection, swimming leukocytes are required to clear out the pathogen, since a single bacterium remaining in the bulk could act as a reservoir leading to chronic infection. Leukocytes will therefore need to quit adhesion and swim in order to reach it, a behaviour already described for effector T lymphocytes [Aoun et al., 2020]. In line with this, we provide here evidence that neutrophils can migrate on adherent substrates by alternating between high and low adhesion levels.…”

“…Furthermore, as of today, swimming of human neutrophils has not been reproduced. Nonetheless, we recently reported that human Effector T lymphocytes also swim [Aoun et al., 2020]. Supported by our experimental observations, we proposed a theoretical model for swimming migration based on a ‘molecular paddling’, in which membrane components linked to actin couple the treadmilling cytoskeleton to the surrounding fluid, thereby propelling the cell forward.…”

“…Such preparation increases medium density to almost match that of neutrophils [Mach and Lacko, 1968, Zipursky et al., 1976]. More importantly, it also increases viscosity, therefore slowing down cell sedimentation while swimming remains unaffected [Aoun et al., 2020]. When the sample was flipped to have the micropatterned area on the upper side, neutrophils swam in 3D against gravity and towards the target (Figure 3D and Supplementary Movie 12).…”

“…An additional point is that eukaryotic cells sediment faster than bacteria, therefore quitting adhesion would not be enough per se , as leukocytes would quickly sediment. Indeed, reported experiments with swimming cells were done in medium supplemented with Ficoll, to avoid cell sedimentation [Barry and Bretscher, 2010, Haastert, 2011, Aoun et al., 2020, O’'neill et al., 2018]. However, it has been reported that activated neutrophils from septic samples display an increased volume, which decreases their density and therefore prevents their sedimentation in the blood column, a property measured as leukocyte anti‐sedimentation rate [Bogar et al., 2002, Bogar, 2006].…”

PKN2 is involved in aggregation and spheroid formation of fibroblasts in suspension culture by regulating cell motility and N-cadherin expression

“…In general, migration of mammalian cells without flagellum in suspension culture, completely in the absence of adhesion to a surface, has not been sufficiently studied so far. Aoun et al have recently described the primary lymphocyte mechanism of “swimming”, which is mainly due to rearward and inhomogeneous treadmilling of the cell external membrane, leading to a ‘‘paddling’’ of transmembrane proteins linked to and advected by the cell cortex [ 22 ]. Whether other cell types such as fibroblasts can “swim” is still a question; however, if this mode of motility is relevant, actin network contractility or actin polymerisation seems to play major roles under Rho family GTPases [ 23 ].…”