PI(3,5)P2 controls endosomal branched actin dynamics by regulating cortactin-actin interactions

Late endosomal trafficking depends on branched actin networks, which influence membrane curvature and transport. Prior research has shown that phosphoinositides regulate membrane dynamics, but the exact role of PI(3,5)P2 in actin turnover remains unclear. This gap motivated investigations into how PI(3,5)P2 might modulate actin regulators like cortactin. It was already known that cortactin stabilizes branched actin structures, but its regulation by phosphoinositides had not been resolved. No prior work had resolved whether PI(3,5)P2 directly interacts with cortactin or affects actin dynamics on endosomes. This uncertainty drove experiments to test the binding and functional consequences of PI(3,5)P2-cortactin interactions. The study aimed to clarify how phosphoinositide signaling might control endosomal actin turnover. Understanding this mechanism could reveal new insights into endosomal membrane trafficking regulation.

This study aimed to determine whether PI(3,5)P2 regulates endosomal actin dynamics by interacting with cortactin. The specific problem addressed was the lack of understanding about how phosphoinositides control actin turnover at membranes. The motivation stemmed from the need to connect phosphoinositide signaling with actin regulatory proteins like cortactin. The researchers proposed that PI(3,5)P2 might antagonize cortactin activity on endosomes. This hypothesis was based on prior knowledge of cortactin’s role in stabilizing branched actin. The study sought to test whether PI(3,5)P2 binding could displace cortactin from actin filaments. By linking PI(3,5)P2 levels to cortactin localization, the researchers aimed to clarify a new regulatory mechanism. The ultimate goal was to establish a model for endosomal actin turnover controlled by phosphoinositide signaling.

The researchers used biochemical assays to test cortactin’s binding to PI(3,5)P2. They identified binding regions using in vitro experiments with recombinant proteins. To assess functional consequences, they inhibited PI(3,5)P2 production in cells and observed cortactin and actin localization. Confocal microscopy was used to track cortactin and actin on Rab7(+) endosomes. Arp2/3 complex activity was inhibited to test cortactin’s dependence on branched actin. Cortactin knockdown was performed to confirm its role in PI(3,5)P2-dependent actin turnover. Competition assays showed that PI(3,5)P2 and actin filaments both bind to cortactin. These methods were chosen to directly test the interaction and functional outcomes of PI(3,5)P2-cortactin binding.

Cortactin was shown to bind PI(3,5)P2 via its actin filament-binding region. PI(3,5)P2 competed with actin filaments for cortactin binding, suggesting antagonism. Inhibition of PI(3,5)P2 production led to cortactin accumulation on Rab7(+) endosomes. This accumulation was accompanied by actin stabilization on endosomes. Knockdown of cortactin reversed PI(3,5)P2-inhibitor effects on actin. Arp2/3 inhibition reduced cortactin localization to late endosomes. These findings suggest PI(3,5)P2 promotes cortactin removal from actin networks. The data support a model where PI(3,5)P2 binding facilitates actin turnover on endosomes.

The authors propose that PI(3,5)P2 binding to cortactin promotes its removal from endosomal actin networks. This interaction may facilitate actin turnover by displacing cortactin from filaments. The findings suggest a regulatory mechanism linking phosphoinositide signaling to actin dynamics. Inhibiting PI(3,5)P2 production stabilizes cortactin and actin on endosomes. Cortactin knockdown reversed these effects, supporting its role in the process. Arp2/3 activity is necessary for cortactin localization to late endosomes. The model implies that PI(3,5)P2 levels control endosomal actin turnover. These conclusions are directly supported by the observed effects of PI(3,5)P2 manipulation.