Secretory Products of the Human GI Tract Microbiome and Their Potential Impact on Alzheimer's Disease (AD): Detection of Lipopolysaccharide (LPS) in AD Hippocampus

Abstract

Although the potential contribution of the human gastrointestinal (GI) tract microbiome to human health, aging, and disease is becoming increasingly acknowledged, the molecular mechanics and signaling pathways of just how this is accomplished is not well-understood. Major bacterial species of the GI tract, such as the abundant Gram-negative bacilli Bacteroides fragilis (B. fragilis) and Escherichia coli (E. coli), secrete a remarkably complex array of pro-inflammatory neurotoxins which, when released from the confines of the healthy GI tract, are pathogenic and highly detrimental to the homeostatic function of neurons in the central nervous system (CNS). For the first time here we report the presence of bacterial lipopolysaccharide (LPS) in brain lysates from the hippocampus and superior temporal lobe neocortex of Alzheimer's disease (AD) brains. Mean LPS levels varied from two-fold increases in the neocortex to three-fold increases in the hippocampus, AD over age-matched controls, however some samples from advanced AD hippocampal cases exhibited up to a 26-fold increase in LPS over age-matched controls. This “Perspectives” paper will further highlight some very recent research on GI tract microbiome signaling to the human CNS, and will update current findings that implicate GI tract microbiome-derived LPS as an important internal contributor to inflammatory degeneration in the CNS.

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

“…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].…”

“…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 strikingly large and unexpected bacterial loads of GI-tract-derived bacteria or their neurotoxic exudates within AD tissues are strongly suspected to upset the efficient operation of these normally highly metabolically active repositories of genetic information.…”

“…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]. As an abundant obligate anaerobe resident of the distal human GI tract microbiome BF-LPS is unusually immunogenic and highly pro-inflammatory toward human neurons in primary culture [8,15,24,37,[47][48][49][50].…”

“…recently reported: (i) that RNA sequencing of the V4 region of the 16S rRNA gene in control (n=25) and AD (n=25) patients indicated that sequences representing a total of 4.8 million sequence reads [mean ± one standard deviation (SD) of ~96,000 ± 32,000 reads/participant] were clustered into operational taxonomic units (OTUs; an operational definition used to classify groups of closely related individuals) at 97% similarity [3,5]; (ii) that the final OTU dataset for the control and AD groups consisted of 972 OTUs classified into 95 genera, 46 families, 24 orders, 17 classes and 9 phyla (5); (iii) that there are about ~1000 bacterial species in a typical healthy human GI tract microbiome with a 'GI tract microbial core' of the genus Bacteroides and Firmicutes predominating, and with Proteobacteria, Verrumicrobia, Actinobacteria, Fusobacteria and Cyanobacteria making up the remainder [3,5,6,[12][13][14]; (iv) that at least 13 of 95 microbial genera examined exhibit differential abundance between AD and age-matched control populations [4][5][6]12]; (v) that there are consistent trends observed between relative GI tract bacterial abundance and CSF biomarkers of AD neuropathology [12,15]; (vi) that 16S rRNA next generation sequencing analysis has identified multiple microbial-derived nucleic acids in the AD brain, some of which are known to have pathogenic potential [9]; (vii) that remarkably, the abundance and complexity of microbial populations or their exudates in the GI tract is an approximate reflection of CNS microbial complexity, including microbiome-derived nucleic acid sequences and Gram negative-derived neurotoxins of the GI tract microbiome, such as lipopolysaccharide (LPS; [6,9,[15][16][17][18]); and (viii) that there is a decrease in richness and diversity of GI tract bacteria in AD compared to age-matched controls, a finding that parallels results observed in other GI, vascular or neurological conditions linked to GI tract microbiome alterations, including those associated with obesity, diabetes, inflammatory bowel disease, systemic inflammation and Parkinson's disease [9,[19][20][21][22]. Related to the points above, overall the GI tract microbiome of AD patients was found to exhibit hig...…”

“…Related to the points above, overall the GI tract microbiome of AD patients was found to exhibit highly selective abundance and compositional differences in genus and species from control age-and sex-matched individuals and exhibited phylum-and genus-wide differences in bacterial abundance including a significantly increased abundance of Bacteroidetes and decreased abundance of Firmicutes and Bifidobacterium in the AD-affected brain [9,[14][15][16][17][23][24][25][26]. Very recently, at least 2 independent publications have shown an enrichment of LPS within the neocortex or hippocampus of ADaffected brain [6,[27][28][29].…”