F-actin flashes on phagosomes mechanically deform contents for efficient digestion in macrophages

Abstract

The mechanism and role of transient F-actin recruitment or F-actin “flashes” on phagosomes remains enigmatic. Here we provide a comprehensive characterization of F-actin flashing dynamics on phagosomes including receptor and signaling involvement. F-actin flashes predominate during the integrin-driven CR-mediated phagocytosis. F-actin flashes begin shortly after internalization and persist on phagosomes for ∼3 minutes before disassembling and reassembling several times within the first hour. Strikingly, the appearance of F-actin flashes on phagosomes coincides with RBC morphological deformation, lysis and occasional fission events. The cadence of flashes depends on particle stiffness and the F-actin networks on phagosomes are enriched in mechanosensitive components including focal adhesion proteins, RhoA and actomyosin. Inhibiting Arp2/3 and myosin IIA activity significantly reduces the frequency at which phagosome cargo becomes deformed during transient F-actin accumulation. At later time points, post-F-actin flashing, an enhanced degradation of phagosome contents is observed, compared to non-flashing phagosomes. Together these data suggest that actomyosin-driven phagosome contractions serve to physically disrupt malleable particles, a process akin to mastication, to enhance later enzymatic digestion.

Phagosomal F-Actin Retention by Cryptococcus gattii Induces Dendritic Cell Immunoparalysis

“…4A), similar to the macrophage actin flash described previously (22,26). The frequency of the actin flash is reported to be around 3 to 4 min from the moment of F-actin assembly to F-actin disassembly and is more prevalent within the first hour after phagocytosis (22,23). To determine whether the persistent R265 phagosomal F-actin "flashed," we performed live-cell microscopy.…”

“…This is likely due to the rapid phagolysosome formation that followed F-actin cage disruption. Through a process that is not well understood, phagolysosome formation appears to have an inverse relationship with phagosomal F-actin (22,23). We speculate that phagolysosome formation overrides the signaling pathways that are responsible for maintaining the persistent F-actin cage of hypervirulent C. gattii.…”

“…Nascent phagosomes are covered with F-actin as a result of actin accumulation during phagocytosis. This F-actin coat depolymerizes almost immediately after phagocytosis, which is a crucial step preceding phagosomal maturation (22,23). It has been reported that in macrophages, intracellular pathogens like Salmonella and Leishmania can build F-actin on their phagosomes to escape intracellular killing (24,25).…”

Better Together: Current Insights Into Phagosome-Lysosome Fusion

“…Most studies reporting the inhibitory role of actin limited their investigation to early-stage phagosomes. In particular, transient accumulations of actin were observed to surround a subpopulation of purified immature phagosomes which prevented association with endolysosomal vesicles ( 125 , 137 ). It was also demonstrated that de novo actin assembly on nascent phagosomes increases under cell stress conditions when the phagocytic or endocytic pathway is overloaded with digestive cargo ( 125 ).…”

“…It has been postulated that transient actin assembly on endolysosomal and phagosomal systems slows phagosome-lysosome fusion when these vesicles contain more cargo than they have the capacity to digest ( 125 ). Actomyosin association is a possible complimentary mechanism that can either delay phagosome migration to the perinuclear region to allow them extra time to preprocess internalized phagosomal content, and to not outpace the availability of lysosomes for fusion ( 131 , 137 ).…”