Abstract
Freely crawling cells are often viewed as randomly moving Brownian particles but they generally exhibit some directional persistence. This property is often related to their zigzag motile behaviors that can be described as a noisy but temporally structured sequence of “runs” and “turns.” However, its underlying biophysical mechanism is largely unexplored. Here, we carefully investigate the collective actin wave dynamics associated with the zigzag-crawling movements of microglia (as primary brain immune cells) employing a number of different quantitative imaging modalities including synthetic aperture microscopy and optical diffraction tomography, as well as conventional fluorescence imaging and scanning electron microscopy. Interestingly, we find that microglia exhibit two distinct types of actin waves working at two quite different time scales and locations, and they seem to serve different purposes. One type of actin waves is fast “peripheral ruffles” arising spontaneously with an oscillating period of about 6 seconds at some portion of the leading edge of crawling microglia, where the vigorously biased peripheral ruffles seem to set the direction of a new turn (in 2-D free space). When the cell turning events are inhibited with a physical confinement (in 1-D track), the peripheral ruffles still exist at the leading edge with no bias but showing phase coherence in the cell crawling direction. The other type is “dorsal actin waves” which also exhibits an oscillatory behavior but with a much longer period of around 2 minutes compared to the fast “peripheral ruffles”. Dorsal actin waves (whether the cell turning events are inhibited or not) initiate in the lamellipodium just behind the leading edge, travelling down toward the core region of the cell and disappear. Such dorsal wave propagations seem to be correlated with migration of the cell. Thus, we may view the dorsal actin waves are connected with the “run” stage of cell body, whereas the fast ruffles at the leading front are involved in the “turn” stage.
Highlights
The emergence of biased peripheral ruffle waves leads a turn in crawling MG cells When microglia are harvested from rat brains and plated on to a culture dish at a very low cell density, they are free from any interactions
While a lot is known about the molecular pathways that regulate actin-based cell motility [36], the mesoscopic collective behavior of the cytoskeletal elements is much less studied
We decided that the interesting zigzag motile behavior of crawling microglia could be a good example to be understood from the view point of collective actomyosin dynamics
Summary
Crawling of eukaryotic cells is a complex phenomenon involving many coordinated biochemical events of membrane protrusion, adhesions, detachments and cytoskeletal restructurings. The dissociation of focal adhesions and actin depolymerization, accompanied by cytoskeletal contraction, ensue Of all these important components for cell crawling, in this paper we are interested in the role of actin polymerization/depolymerization dynamics, in particular, their spatiotemporal dynamic features in association with unusual motile behavior of freely crawling microglial cells. Ponti et al [4] reported that there are at least two different types of actin cytoskeleton kinetics: One for lamellipodium, very narrow zone spatially being confined within 1 ~ 3 microns from the leading edge; and the other for lamella, which is the main cell body. Waves of actin polymerization/depolymerization, which are physically much larger than the ruffles at leading edge, start out near the leading edge and travel centripetally toward the core region of the cell body, either on the dorsal or ventral membrane. Within the confined linear track, the directed cell migration is not hindered at all
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