Effects Of Lipopolysaccharide On CD22 Expression And Phagocytosis Of Microglia

Background: Alzheimer’s disease is a common neurodegenerative disease.The pathological features are extracellular Aβ plaque deposition,abnormal intracellular formation of neurofibril tangle(NFT)comprising of phosphorylation tau protein,excessive activation of inflammatory response,neuronal degeneration and death,leading to progressive cognitive dysfunction1,the number of patients is increasing year by year,but there is no effective prevention and intervention2.Microglia are innate immune cells of the central nervous system.In the normal brain,microglia(about 10% of the CNS cells)form a near-regular three-dimensional lattices,each occupies a unique region.Microglia are highly ramified cells and they monitor the surrounding brain parenchyma environment in real time.Through this dynamic surveillance,microglial can detect multiple extracellular signals to transduce and integrate to maintain brain homeostasis3.Microglia play a bidirectional role in the CNS.Physiological functions include clearance of apoptotic cells,pruning of developing synapses,regulation of synaptic plasticity,phagocytosis and degradation of abnormal proteins,etc.Meanwhile,it also plays a pathological role in the development of disease,such as over-release of inflammatory cytokines leading to neurotoxicity4,5,synaptic pruning dysfunction leading to synaptic loss and cognitive dysfunction6,it is noted that microglia phagocytosis plays an important role in these functions,and in the late stage of AD,microglia phagocytosis is dysfunctional,regulation of microglia phagocytosis may be one of the methods to improve the pathology of AD.The inflammatory responses are closely related to microglia in AD brain7,8,and microglia play different roles in acute and chronic inflammatory stimuli.In some studies,acute inflammatory stimulation enhances microglia phagocytosis to reduce Aβ level9,while chronic activation of microglia shows a decrease in Aβ phagocytosis and leads to neurotoxicity and synaptic loss by triggering several proinflammatory cascade reactions10,11.In this study,we will explore the effects of CD22 on microglia phagocytosis in acute inflammatory stimulation.CD22 is an inhibitory co-receptor of BCR on B lymphocytes,the classical role is to participate in the negative regulation of B cell signaling12.Last year an article published on Nature reported that the expression of CD22 increased and the increased CD22 inhibited microglial phagocytosis in aged mice13,but in the first article reporting the expression of CD22 in the central nervous system published in 2004 noted that CD22 was expressed by neurons and secreted into microglia to inhibit the secretion of inflammatory factors14.Therefore,which type of nerve cells express CD22 is controversial,and the reported effect on inhibiting microglial phagocytosis was studied on aging mice.In this study we will investigate the expression of CD22 in the central nervous system and its role in inflammation and AD.Objective: To investigate the expression of CD22 in the central nervous system and its role in inflammation and AD.Methods: 1)Primary neurons and primary microglia were cultured respectively,the expression of CD22 in neurons and microglia was verified by using specific primers of CD22 through q PCR experiment.2)Expression of CD22 in cortex and hippocampus was observed by immunofluorescence,Western blotting and q PCR after lateral ventricle or intraperitoneal injection of LPS in 2-month-old wildtype C57 mice.3)The cultured primary microglia were divided into three groups: Control group,LPS group,LPS+CD22 neutralizing antibody group,and the effect of CD22 on microglia phagocytosis was then detected by direct fluorescence imaging.Results: 1.Both neuron and microglia express CD22.2.After acute stimulation of LPS,the expression of CD22 was up-regulated in neuron and microglia.3.The expression of CD22 in the cortex of AD patients was increased.4.After acute stimulation of LPS,the increased CD22 promoted microglia phagocytosis.5.CD22 is a secretory protein that could be secreted from the intracellular to the culture medium.6.In the hippocampi of aging mice,the expression level of CD22 was positively correlated with the expression of phospho-Tau(S202,T205).Conclusion: Acute lipopolysaccharide stimulation increased CD22 expression of neurons and microglia,and increased CD22 of microglia played a role in promoting microglia phagocytosis.