PI3K signaling promotes formation of lipid-laden foamy macrophages at the spinal cord injury site

After spinal cord injury (SCI), infiltrating macrophages undergo excessive phagocytosis of myelin and cellular debris, forming lipid-laden foamy macrophages. To understand their role in the cellular pathology of SCI, investigation of foamy macrophage phenotype in vitro revealed a unique inflammatory profile, increased reactive oxygen species (ROS) production, and mitochondrialdysfunction. Bioinformatic analysis identified PI3K as a regulator of inflammation in foamy macrophages, and pharmacological inhibition of this pathway decreased lipid content and inflammatory cytokine and ROS production in these cells. Macrophage-specific inhibition of PI3K using liposomes significantly decreased foamy macrophages at the injury site after a mid-thoracic contusive SCI in mice. RNA sequencing and in vitro analysis of foamy macrophages revealed increased autophagy after PI3K inhibition as a potential mechanism for reduced cellular lipid accumulation. Together, our data suggest that formation of pro-inflammatory foamy macrophages after SCI is due to activation of PI3K signaling that leads to decreased autophagy. We used an in vitro model of cultured macrophages treated with homogenized spinal cord, rather than purified myelin, since it is more representative of all the lipid sources driving foamy macrophage formation after SCI. To investigate the biological processes by which Torin-2 decreased lipid droplet accumulation, we incubated bone-marrow-derived macrophages in 1% w/v spinal cord homogenate for 24 hours and performed bulk RNA-seq on 3 biological replicates treated with vehicle, Torin-2, or rapamycin. Rapamycin, whcih does not affect lipid accumulation, was used as a control to account for the effects of mTORC1 inhibition by Torin-2.