Identification of a pro-elongation effect of diallyl disulfide, a major organosulfur compound in garlic oil, on microglial process

Microglia are the innate immune cells in the nervous system. In the resting state, they display a ramified morphology, while upon disease stimulation their processes would be retracted, along with pro-inflammatory cytokine overproduction. Reversing microglial process retraction may help reduce pro-inflammatory cytokine production and restore microglia's ability to scan surrounding environments, rendering brain function regulation to be more effective. We found that diallyl disulfide (DADS), a major organosulfur compound in garlic oil, administered at different doses and time points, promoted microglial process elongation in both cultured systems and prefrontal cortexes in mice in a reversible manner. Lipopolysaccharide (LPS), a classical activator of microglia, did not affect this pro-elongation effect of DADS at conditions in vitro and in vivo. Functional studies revealed that DADS pre-treatment attenuated LPS-induced decreases in levels of tumor necrosis factor-α (TNF-α) and interleukin-1β (IL-1β) mRNA as well as LPS-induced increases in levels of IL-10 and CD206 mRNA in both cultured microglia and prefrontal cortexes in mice. Protein kinase B (Akt) inhibition attenuated the pro-elongation effect of DADS on microglial process and blocked the regulatory effects of DADS on LPS-induced inflammatory responses in both cultured microglia and prefrontal cortexes in mice. In an in vivo model of neuroinflammation, DADS pre-treatment prevented LPS-induced retraction of microglial process in the prefrontal cortex in mice and attenuated LPS-induced increase in immobility time in the tail suspension test and forced swim test. These results indicate that DADS induces an Akt-dependent elongation of microglia process, along with the induction of an anti-inflammatory phenotype.