Organized Randomness: How Hierarchical Coupling of an Excitable System and a Bistable Switch Shape Spontaneous Cell Migration

In various eukaryotic organisms, amoeboid cells undergoing spontaneous migration generate membrane protrusions and establish front-rear polarity in an autonomous manner through signal transduction networks that couple excitable Ras dynamics to downstream bistable modules. Recent genetic and live-cell imaging studies in Dictyostelium cells have identified Ras GTPase and the newly characterized guanine nucleotide exchange factor RasGEFX as core drivers of this excitability, amplifying stochastic basal Ras activity into discrete signaling patches and propagating waves. These excitatory events initiate pseudopod formation even in the absence of external guidance cues and engage the bistable PIP₃-PTEN switch to stabilize cell polarity. The same Ras excitable network also drives constitutive macropinocytosis, revealing a shared mechanism for membrane protrusion and fluid uptake. This hierarchical excitable-bistable architecture, coupled with motile machinery, converts intrinsic molecular noise into structured, non-Brownian exploratory behavior, representing an adaptive strategy that enables cells to exploit stochasticity for navigation in complex natural environments.