Chemotaxis overrides Barotaxis during Directional Decision-Making in Dictyostelium discoideum

Abstract

Neutrophils and dendritic cells have, besides their well characterised chemotactic movement responses, been shown to be able to detect and respond to local differences in hydraulic resistance (barotaxis). Furthermore, for neutrophils, it has been suggested that barotaxis overrides chemotaxis. Here, we investigate whether Dictyostelium cells also respond to hydraulic resistance or primarily to chemical gradients using an asymmetric bifurcating micro-channel. This channel design allows us to decouple hydraulic and chemical stimuli, by providing a choice between moving up a chemical gradient or down a chemical gradient into a channel with 100 times lower hydraulic resistance. Under these conditions chemotaxis always overrides barotaxis. Cells confronted by a microchannel bifurcation are observed to often partially split their leading edge and to start moving into both channels. Cells in steeper cAMP gradients, that move faster, split more readily. The decision to retract the pseudopod moving away from the cAMP source is made when the average velocity of the pseudopod moving up the cAMP gradient is 20% higher than the average velocity of the pseudopod moving down the gradient. Surprisingly, this decision threshold is independent of the steepness of the cAMP gradient and speed of movement. It indicates that a critical force imbalance threshold underlies the repolarisation decision. Significance StatementWe investigate the directional decision-making of Dictyostelium discoideum cells migrating within engineered micro-channels harbouring asymmetric bifurcations. Unlike neutrophils and immature dendritic cells, Dictyostelium cells strongly prioritise chemical over barotactic guidance cues. Cells in steeper cAMP gradients migrate at higher speeds, split their leading edges more readily when confronted with a bifurcation in the channel. The decision to retract a pseudopod pointing in an unfavourable direction occurs when a critical tension gradient between two competing pseudopods is surpassed. These experiments show that although barotaxis is not a major guidance cue, cellular mechanics plays a major role in leading edge dynamics, including front splitting and polarisation and retraction.