A single dose of lipopolysaccharide injection exacerbates synaptic engulfment by microglia, and dendritic spine loss in high‐fat diet‐induced obese rats

Abstract

AbstractBackgroundDiet‐induced obesity can lead to develop endotoxemia and cognitive decline (1). A previous study in Alzheimer’s disease model demonstrated that an increase in the colocalization of C1q, a classical complement protein, and microglia occurred, when those microglia increased synaptic engulfment, resulting in increased synaptic‐pruning process and synaptic loss (2). However, the effects of lipopolysaccharide (LPS) as endotoxin on peripheral inflammation, blood‐brain barrier(BBB) integrity, brain oxidative stress, synaptic engulfment by microglia, synaptic plasticity, and cognitive function in high‐fat diet‐induced obese rats have never been investigated.MethodSixteen male Wistar rats were randomly assigned to receive either a normal diet (ND, n = 8) or the high‐fat diet (HFD, n = 8) for 12 weeks. All animals in each dietary group were subsequently assigned into 2 subgroups (n = 4 /subgroup) to receive either a single dose of vehicle (1 ml of 0.9% normal saline solution, intraperitoneal (i.p.) injection, or LPS (0.5 mg/kg, i.p.). After the injections for 12‐16 hours, all animals were measured for cognitive function and the blood was collected. Then, animals were sacrificed, and their brains and spleens were obtained for further investigation.ResultAll HFD‐fed rats exhibited peripheral inflammation as indicated by increased spleen IL‐1β protein levels, BBB disruption as indicated by the reduction of hippocampal claudia‐5 protein levels, and increased brain oxidative stress as indicated by the increased brain malondialdehyde levels (p<0.05, Figure 1). Interestingly, LPS‐treated ND‐fed rats and vehicle‐treated HFD‐fed rats equally impaired microglial function by increasing synaptic engulfment as indicated by the increase in C1q colocalized in microglia and the decreased dendritic spine density (p<0.05, Figure 1). The results demonstrated that these parameters were aggravated in LPS‐treated HFD‐fed rats. Lastly, all HFD‐fed rats decreased the preference index of novel object location and recognition test suggesting long‐term HFD consumption caused cognitive decline (p<0.05, Figure 1).ConclusionLong‐term HFD consumption, but not LPS alone, caused cognitive decline by increasing peripheral inflammation, disrupting BBB integrity, increasing brain oxidative stress, increasing synaptic engulfment by microglia, and decreasing synaptic plasticity. Microglia are susceptible to impair their function in response to LPS, which potentially worsen brain pathologies in obese condition.