The Actin Cytoskeleton Regulates Macrophage Activation and Inflammation

The actin cytoskeleton is composed of actin filaments (F-actin) and actin binding proteins (APD). This dynamic network helps maintain cell shape and internal organization, as well as driving higher order processes like cell motility. F-actin seems to be in the center of a molecular crosstalk between contractile Myosin IIA and Arp2/3 complex branched actin nucleation. The temporal regulation of these behaviors during inflammatory activation, and their importance during macrophage activation, is unclear. We used a conditional Arp2/3 allele and various pharmacological tools that target myosin-IIA mediated contractility to test the hypothesis that inflammatory activation involves a careful balance between contractility and nucleation. We first compared WT and Arp2/3 KO macrophage activation during acute and chronic inflammation. Arp2/3 KO macrophages are primed and transcriptionally ready for inflammation in standard culture conditions. These cells have a higher level of iNOS after inflammatory stimulation but don’t seem to produce more inflammatory cytokines than their WT counterparts. Interestingly, we also found that the actin cytoskeleton is strongly reorganized during acute and chronic inflammation in WT cells, illustrated by cell contractility during acute inflammation followed by cell spreading after several hours. This dynamic behaviors is disrupted in Arp2/3 KO macrophages, which demonstrate constant contractility and persistent myosin IIA activation. It appears that Arp2/3 complex acts as a break for myosin IIA activity and inflammation. We are able to reduce inflammation of WT cells and hyper-inflammation of Arp2/3 cells using direct and indirect myosin-IIA inhibitors. Future projects are designed to determine whether Myosin-IIA inhibition or Arp2/3 complex activation can ameliorate chronic inflammatory diseases.