A Growth Factor Signaling Cascade Confined to Circular Ruffles in Macrophages

The formation of macropinosomes requires large-scale movements of membranes and the actin cytoskeleton. Over several minutes, actin-rich surface ruffles transform into 1-5 µm diameter circular ruffles, which close at their distal margins, creating endocytic vesicles. Previous studies using fluorescent reporters of phosphoinositides and Rho-family GTPases showed that signals generated by macrophages in response to the growth factor Macrophage Colony-Stimulating Factor (M-CSF) appeared transiently in domains of plasma membrane circumscribed by circular ruffles. To address the question of how signaling molecules are coordinated in such large domains of plasma membrane, this study analyzed the relative timing of growth factor-dependent signals as ruffles transformed into macropinosomes. Fluorescent protein chimeras expressed in macrophages were imaged by microscopy and quantified relative to circular ruffle formation and cup closure. The large size of macropinocytic cups allowed temporal resolution of the transitions in phosphoinositides and associated enzyme activities that organize cup closure. Circular ruffles contained transient and sequential spikes of phosphatidylinositol (4,5)-bisphosphate (PI(4,5)P(2)), phosphatidylinositol (3,4,5)-trisphosphate (PIP(3)), diacylglycerol, PI(3,4)P(2), PI(3)P and the activities of protein kinase C-α, Rac1, Ras and Rab5. The confinement of this signal cascade to circular ruffles indicated that diffusion barriers present in these transient structures focus feedback activation and deactivation of essential enzyme activities into restricted domains of plasma membrane.