Acute “in vivo” LPS causes an early presynaptic C1q and C3 accumulation and increases the “prunability” of cortical synaptosomes

LPS acutely injected in 3-month-old male C57BL/6 mice (1.25 mg/kg, i.p.) were sacrificed 12 h after the injection. LPS treatment activates complement in the central nervous system and promotes local inflammation, as confirmed by cortical TNF-α/IL-1b mRNA overexpression and increased GFAP and CD11b immunopositivity. We extended the study to presynaptic adaptations in isolated cortical synaptosomes from control (vehicle-injected, CTR) and LPS-injected mice. Synaptosomal engulfment was measured as MAP2 immunopositivity in LPS-activated BV2 or N9 microglia previously incubated with CTR or LPS-injected mouse synaptosomes. MAP2, absent in microglia, is present in synaptosomes, where its density is conserved despites LPS-injection. Microglia incubated with LPS-injected mice synaptosomes were immune-positive for MAP2, and the immunostaining was higher than that in microglia exposed to CTR particles, suggesting an increased “prunability” of LPS-injected mouse synaptosomes. Cortical synaptosomes were also labelled with pHrodo and then incubated with BV2 microglia. The red fluorescence was higher in microglia exposed to pHrodo-labelled LPS-injected synaptosomes, again suggesting their increased engulfment when compared to CTR. Lastly, LPS-injected cortical synaptosomes showed a significant presynaptic accumulation of C1q and C3, but not of C5. Our data confirm that acute systemic LPS challenge causes presynaptic adaptation which increases the “prunability” of nerve terminals, promoting their removal by activated microglia. The conclusion relies on the data obtained with two experimental techniques that permit to monitor synaptosomal engulfment representing useful tools i) to study changes in the “prunability” of nerve endings and ii) to test the efficiency of therapeutics for the management of central synaptopathy.