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Microbiome-Derived Lipopolysaccharide Enriched in the Perinuclear Region of Alzheimer's Disease Brain

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Microbiome-Derived Lipopolysaccharide Enriched in the Perinuclear Region of Alzheimer's Disease Brain

Yuhai Zhao et al. Front Immunol.

Abstract

Abundant clinical, epidemiological, imaging, genetic, molecular, and pathophysiological data together indicate that there occur an unusual inflammatory reaction and a disruption of the innate-immune signaling system in Alzheimer's disease (AD) brain. Despite many years of intense study, the origin and molecular mechanics of these AD-relevant pathogenic signals are still not well understood. Here, we provide evidence that an intensely pro-inflammatory bacterial lipopolysaccharide (LPS), part of a complex mixture of pro-inflammatory neurotoxins arising from abundant Gram-negative bacilli of the human gastrointestinal (GI) tract, are abundant in AD-affected brain neocortex and hippocampus. For the first time, we provide evidence that LPS immunohistochemical signals appear to aggregate in clumps in the parenchyma in control brains, and in AD, about 75% of anti-LPS signals were clustered around the periphery of DAPI-stained nuclei. As LPS is an abundant secretory product of Gram-negative bacilli resident in the human GI-tract, these observations suggest (i) that a major source of pro-inflammatory signals in AD brain may originate from internally derived noxious exudates of the GI-tract microbiome; (ii) that due to aging, vascular deficits or degenerative disease these neurotoxic molecules may "leak" into the systemic circulation, cerebral vasculature, and on into the brain; and (iii) that this internal source of microbiome-derived neurotoxins may play a particularly strong role in shaping the human immune system and contributing to neural degeneration, particularly in the aging CNS. This "Perspectives" paper will further highlight some very recent developments that implicate GI-tract microbiome-derived LPS as an important contributor to inflammatory-neurodegeneration in the AD brain.

Keywords: Alzheimer’s disease; inflammatory degeneration; lipopolysaccharide; microRNA; microbiome; small non-coding RNAs.

Figures

Figure 1
Figure 1
(A–D) Western and (E–F) immunohistochemical analysis of lipopolysaccharide (LPS) (~37 kDa) signals in human brain temporal lobe neocortex [N = 4 control and 4 sporadic Alzheimer’s disease (AD) cases; quantified in (B)]; and (C) hippocampus [N = 3 control and N = 3 sporadic AD cases; quantified in (D)] were compared against β-actin (~42 kDa) abundance in the same sample (using anti-Escherichia coli LPS; cat# ab35654 from Abcam, Cambridge UK and anti-β-actin cat# 3700, Cell Signaling, Danvers, MA, USA). All Western methodologies have been previously described in detail (12, 19). Densitometric readings of immune-reactive bands were obtained using ImageQuantTL [GE Healthcare (12, 19, 20)]; all control and AD tissues were age- and gender-matched; there were no significant differences between the age (control 82.5 ± 8.1 years, AD 81.3 ± 8.8 years), gender (all female), postmortem interval (PMI) (all tissues 3.8 h or less), RNA quality, or RNA yield between each of the two groups; in these samples, LPS abundance was found to be on average greater than sevenfold as abundant in AD when compared to control neocortex; LPS was found to be on average >21-fold as abundant in AD when compared to control hippocampus; in (B,D) a dashed horizontal line at 100 is included for ease of comparison; *p < 0.01 (ANOVA); (E,F) for immunohisto-chemistry control and AD neocortex and/or hippocampal brain tissues were embedded, sectioned (10 μm), fixed, and incubated with primary antibodies (1:1,000; 1× PBS with 2% BSA, 2% goat or donkey serum, and 0.1% TX-100) overnight at 4°C, washed with PBS, and then incubated with Alexa Fluor-conjugated species-specific secondary antibodies (LPS; red fluorescence λmax ~ 650 nm); sections were next counter-stained with DAPI (blue fluorescence; λmax ~ 470 nm) for nuclei (E), and/or Aβ peptide (green fluorescence; λmax ~ 510 nm) (F) and imaged with Zeiss LSM 700 Confocal Laser microscope system (Richmond, VA, USA); note perinuclear staining of LPS in AD; while there appears to be random association of LPS with Aβ deposits in controls, >75% of all LPS signals were found to be associated with brain cell nuclei in AD; the significance of this is not currently known; the association of LPS with the major cellular repository for genetic material suggests that the significance of this association may be genetic; white arrows highlight LPS-nuclear envelope association; a total of 26 control and AD brains (PMI 3.8 h or less) were examined and yielded highly similar results; (E,F) magnification 50×.
Figure 2
Figure 2
The human gastrointestinal (GI)-tract microbiome as a source of strong pro-inflammatory exudates—highly schematicized depiction of anaerobic, Gram-negative bacilli (such as Escherichia coli and Bacteroides fragilis) of the human GI-tract microbiome and their potentially pathogenic, immunogenic, and pro-inflammatory neurotoxins [amyloids, endotoxins and exotoxins, lipopolysaccharide (LPS), and small non-coding RNAs (sncRNAs)] that may contribute to systemic and CNS inflammation and neuro-immune disruption; two major sources of these complex mixtures are E. coli and B. fragilis; major anaerobic Gram-negative bacilli of the human middle and lower GI-tract, respectively; the B. fragilis toxin (BFT) fragilysin is one of the most potent pro-inflammatory molecules known (12, 15, 16, 30, 37, 38); these intensely pro-inflammatory LPS species may be able to “leak” through at least two major biophysiological barriers—the GI-tract barrier and the blood–brain barrier—to access brain compartments [see Ref. (2, 12, 28, 30, 31, 34)]. Neurotoxic mixtures secreted by multiple GI-tract microbes or other microbial species may have considerable potential to support inflammatory signaling within the CNS (2, 12, 21, 28, 30, 31, 34); B. fragilis proliferation and (BFT) fragilysin levels may be kept in check by increased intake of soluble and insoluble dietary fiber (34, 38, 46); interestingly, BFT-derived fragilysin may exert neurotoxic activities via multiple mechanisms: (i) by increasing the permeability or “leakiness” of the intestinal epithelium via the dissolution of tight junctions in epithelial cells (28, 30); and (ii) by promoting amyloid peptide aggregation and progressive amyloidogenesis (15, 16, 18, 37, 38); Figure 2 modified and updated from Lukiw (15, 16).

Cited by 22 PubMed Central articles

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