Lipopolysaccharides Derived from Pantoea agglomerans Can Promote the Phagocytic Activity of Amyloid β in Mouse Microglial Cells

Pon1 Deficiency Promotes Trem2 Pathway-Mediated Microglial Phagocytosis and Inhibits Pro-inflammatory Cytokines Release In Vitro and In Vivo

Paraoxonase 1 (PON1) plays an anti-inflammatory role in the cardiovascular system. Levels of serum PON1 and polymorphisms in this gene were linked to Alzheimer's disease (AD) and Parkinson disease (PD), but its function in the neuroimmune system and AD is not clear. To address this issue, we used Pon1 knockout rats previously generated by our lab to investigate the role of Pon1 in microglia. Knockout of Pon1 in rat brain tissues protected against LPS-induced microglia activation. Pon1 deficiency in rat primary microglia increased Trem2 (triggering receptor expressed in myeloid cells 2) expression, phagocytosis, and IL-10 (M2-phenotype marker) release, but decreased production of pro-inflammatory cytokines such as IL-1β, IL-6, and IL-18 especially TNF-α (M1-phenotype markers) induced by LPS. Pon1 deficiency in rat primary microglia activated Trem2 pathway but decreased LPS-induced ERK activation. The phagocytosis-promoting effect of Pon1 knockout could be reversed by administration of recombinant PON1 protein. The interaction between PON1 and TREM2 was verified by co-immunoprecipitation (co-IP) using rat brain tissues or over-expressed BV2 cell lysates, which might be involved in lysosomal localization of TREM2. Furthermore, Pon1 knockout also enhanced microglial phagocytosis and clearance of exogenous Aβ by an intrahippocampal injection and decrease the transcription of cytokines such as IL-1β, IL-6, and TNF-α in vivo. These results suggest that Pon1 knockout facilitates microglial phagocytosis and inhibits the production of proinflammatory cytokines both in vivo and in vitro, in which the interaction between Pon1 and Trem2 may be involved. These findings provide novel insights into the role of PON1 in neuroinflammation and highlight TREM2 as a potential target for Alzheimer's disease therapy.

Strategies and Tools for Studying Microglial-Mediated Synapse Elimination and Refinement

The role of microglia in controlling synapse homeostasis is becoming increasingly recognized by the scientific community. In particular, the microglia-mediated elimination of supernumerary synapses during development lays the basis for the correct formation of neuronal circuits in adulthood, while the possible reactivation of this process in pathological conditions, such as schizophrenia or Alzheimer's Disease, provides a promising target for future therapeutic strategies. The methodological approaches to investigate microglial synaptic engulfment include different in vitro and in vivo settings. Basic in vitro assays, employing isolated microglia and microbeads, apoptotic membranes, liposomes or synaptosomes allow the quantification of the microglia phagocytic abilities, while co-cultures of microglia and neurons, deriving from either WT or genetically modified mice models, provide a relatively manageable setting to investigate the involvement of specific molecular pathways. Further detailed analysis in mice brain is then mandatory to validate the in vitro assays as representative for the in vivo situation. The present review aims to dissect the main technical approaches to investigate microglia-mediated phagocytosis of neuronal and synaptic substrates in critical developmental time windows.

A unique hybrid characteristic having both pro- and anti-inflammatory phenotype transformed by repetitive low-dose lipopolysaccharide in C8-B4 microglia

Although lipopolysaccharide (LPS) is regarded as an inducer of inflammation, previous studies have suggested that repetitive low-dose LPS has neuroprotective effects via immunomodulation of microglia, resident macrophages of brain. However, microglia transformed by the stimulus of repetitive low-dose LPS (REPELL-microglia) are not well characterized, whereas microglia transformed by repetitive high-dose LPS are well studied as an endotoxin tolerance model in which the induction of pro-inflammatory molecules is suppressed. In this study, to characterize REPELL-microglia, the gene expression and phagocytic activity of REPELL-microglia were analyzed with the murine C8-B4 microglia cell line. The REPELL-microglia were characterized by a high expression of pro-inflammatory molecules (Nos2, Ccl1, IL-12B, and CD86), anti-inflammatory molecules (IL-10, Arg1, Il13ra2, and Mrc1), and neuroprotective molecules (Ntf5, Ccl7, and Gipr). In addition, the phagocytic activity of REPELL-microglia was promoted as high as that of microglia transformed by single low-dose LPS. These results suggest the potential of REPELL-microglia for inflammatory regulation, neuroprotection, and phagocytic clearance. Moreover, this study revealed that gene expression of REPELL-microglia was distinct from that of microglia transformed by repetitive high-dose LPS treatment, suggesting the diversity of microglia transformation by different doses of LPS.

Efficacy of continuous ingestion of dewaxed brown rice on the cognitive functions of the residents of elderly welfare facilities: A pilot test using crossover trial

Compared with regular brown rice, dewaxed brown rice (DBR), prepared by excluding only the wax layer in the outermost layer of brown rice using a new rice milling technique, has improved water absorbency, digestibility, and taste. Dewaxed brown rice has a nutritional value close to that of brown rice and contains a large amount of lipopolysaccharides (LPS), which are known to improve the cognitive function of mice. In this study, we examined the effect of continuous DBR ingestion on the cognitive function of elderly people. A crossover comparison test was performed, in which elderly people who moved into an elderly welfare facility were divided into two groups and ingested DBR or polished white rice for three meals a day for 6 months, followed by a change in test meals for the next 6 months. Cognitive function was assessed using Revised Hasegawa's Dementia Scale (HDS-R) before starting the test and 6 months after ingesting each test meal. No subjects withdrew or discontinued DBR intake during the study period, and all subjects continued the test for 6 months. In subjects with low cognitive function (defined as subjects with HDS-R total score of ≥1 but <10 at the start of the study), there was a significant association between continuous DBR ingestion and cognitive function improvement (increase in total HDS-R score). Our findings suggest that the long-term DBR ingestion as a staple food could be useful for preventing cognitive decline in elderly; it offers an easily implemented option as a daily diet for preventing cognitive decline.

Oral administration of Pantoea agglomerans-derived lipopolysaccharide prevents metabolic dysfunction and Alzheimer's disease-related memory loss in senescence-accelerated prone 8 (SAMP8) mice fed a high-fat diet

The pathogenesis of Alzheimer’s disease (AD) remains unclear, but an imbalance between the production and clearance of amyloid-β (Aβ) peptides is known to play a critical role in AD progression. A promising preventative approach is to enhance the normal Aβ clearance activity of brain phagocytes such as microglia. In mice, the intraperitoneal injection of Toll-like receptor 4 agonist was shown to enhance Aβ clearance and exhibit a preventative effect on AD-related pathology. Our previous clinical study demonstrated that orally administered Pantoea agglomerans-derived lipopolysaccharide (LPSp) exhibited an LDL (low-density lipoprotein)-lowering effect in human volunteers with hyperlipidemia, a known risk factor for AD. In vitro studies have shown that LPSp treatment increases Aβ phagocytosis by microglial cells; however it is still unclear whether orally administered LPSp exhibits a preventive effect on AD progression. We show here that in senescence-accelerated prone 8 (SAMP8) mice fed a high-fat diet, oral administration of LPSp at 0.3 or 1 mg/kg body weight·day for 18 weeks significantly improved glucose metabolism and lipid profiles. The LPSp treatment also reduced pro-inflammatory cytokine expression and oxidative-burst activity in the peripheral blood. Moreover, LPSp significantly reduced brain Aβ burden and memory impairment as seen in the water maze test, although we could not confirm a significant enhancement of Aβ phagocytosis in microglia isolated from the brains after treatment. Taken together, our results show that LPSp holds promise as a preventative therapy for AD or AD-related diseases induced by impairment of metabolic functions.