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

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.

Gastrointestinal (GI) Tract Microbes and Microbial Neurotoxins in the Human Central Nervous System (CNS) in Alzheimer’s Disease (AD)

“…Taken together these results suggest that the increased abundance of Bacteroides in patients with AD may result in an increased translocation of LPS from the gut to the systemic circulation, which in turn may exacerbate AD pathology through enhanced pro-inflammatory signaling or related pathogenetic mechanisms [5,10,54,55]. These findings further suggest that LPS and perhaps other bacterial-derived amyloids, sncRNAs, endotoxins and neurotoxins are localized to the same anatomical regions involved in AD-type neuropathology and these may be significant initiators or progressive contributors to inflammatory degeneration, amyloidogenesis and/or an altered innate-immune response in the AD CNS ( Figure 1; [5,34,54,55]). …”

“…It has been very recently shown (i) that both LPS and BF-LPS are abundant in anatomical regions of the human brain's limbic system in AD, including the hippocampus and neocortex that exhibit focused neuropathology and an intense inflammatory response as is characteristic of the AD process [6,28,29]; and (ii) that BF-LPS is an extremely potent inducer of pro-inflammatory gene signaling pathways, as quantified by the evolution of the pro-inflammatory transcription factor NF-kB p50/p65 complex HNG cells in primary co-culture [51]. To further cite several highly relevant research investigations: (i) using immunocytochemistry E. coli K99 pili protein and E. coli LPS levels were found to be significantly greater in AD compared to control brains, finding that in AD LPS co-localized with Aβ1-40/42-positive amyloid plaques surrounding cerebral vessels [29]; (ii) using an immunocytochemical approach Zhao et al have discovered LPS in very short post-mortem interval (PMI) AD hippocampus (1-3 h PMI) to levels up to 36-fold over age-matched controls [34][35][36]; (iii) co-incubation of Aβ peptide with LPS potentiates amyloidogenesis and fibrillogenesis (amyloid fiber formation; [34,52]); (iv) systemic injection of LPS in wild-type and transgenic AD mice results in greater amyloid deposition and tau pathology [53]; and (v) a selective enrichment of LPS specifically associated with the neuronal nuclear membrane in AD was found to be correlated to a down-regulation in the output of transcription products from neuronal nuclei ( [34]; manuscript under review). This suggests that GI-tract microbiome-derived LPS may be an important initiator and/or significant contributor to failure in adequate gene expression the AD CNS including those genes involved in the modulation of inflammatory signaling (Figure 1; manuscript submitted).…”

“…Of further related interest are the recent observations: (i) that the increased abundance of gram-negative GI tract bacteria such as B. fragilis in AD patients appears to result in increased generation and translocation of LPS and other Bacteroides-derived neurotoxins from the GI tract into the systemic circulation, which in turn may contribute to AD neuropathology through the release of pro-inflammatory cytokines, systemic inflammation, an increase in GI-tract or BBB permeability or other AD-relevant pathogenic mechanisms [9,[29][30][31][32], and (ii) that an increase in Bacteroidetes in the GI tract is also associated with Parkinson's disease (PD; [33]) and with sporadic AD hippocampus and neocortex, two anatomical regions targeted by the AD process [6,[34][35][36]. Importantly, while only the inflammatory potential of LPS towards primary human neuronal-glial (HNG) co-cultures have been studied and quantified by the induction of the pro-inflammatory NFkB p50/p65 complex, Bacteroidetes species are capable of secreting an unusually complex array of highly lethal neurotoxins including amyloids, sncRNAs and endotoxins which, when released from the confines of the healthy GI tract, are systemically pathogenic and can be highly detrimental to the homeostatic function of human CNS neurons [15].…”

“…As fore-mentioned, microbes such as B. fragilis and Escherichia coli (E. coli), abundant anaerobic Gram-negative bacilli of the human GI-tract microbiome, appear to direct critical regulatory roles of pathogenicity through the stress-induced secretion of a complex mixture of bacterial amyloids, endotoxins and exotoxins, 'microRNA-like' sncRNAs and LPS. Recently work from several independent groups has further described the presence of intact bacteria, bacterial-derived nucleic acid sequences and/or bacterial-derived neurotoxins such as a highly pro-inflammatory LPS that is associated with neuronal parenchyma and in particular the neuronal nuclei of anatomical regions of the ADaffected brain exhibiting characteristic neuropathology [1,9,28,29,[34][35][36][37][38][39]. Interestingly, the close association of bacterial LPS with neuronal nuclei may prevent the efficient export of messenger RNA (mRNA) from a highly active neuronal genome resulting in the down-regulation of gene expression in AD as is widely observed [6,15].…”

“…The toxicity and inflammatory potential of different bacterial LPSs vary depending on the composition of these 3 modular components; for example the LPS of the anaerobic B. fragilis (BF-LPS), is a remarkably pro-inflammatory glycolipid, perhaps the most pro-inflammatory LPS known, and capable of triggering systemic inflammation and the release of proinflammatory cytokines after translocation across the GI tract into systemic circulation [23,[34][35][36]43]. These neurotoxic glycolipids are shed into the extracellular space, play key pathological roles in hostpathogen interactions, pro-inflammatory signaling and the activation of the innate-immune system of the host [23,[44][45][46].…”