Complex Interaction between Resident Microbiota and Misfolded Proteins: Role in Neuroinflammation and Neurodegeneration

Abstract

Neurodegenerative diseases such as Alzheimer’s disease (AD), Parkinson’s disease (PD) and Creutzfeldt–Jakob disease (CJD) are brain conditions affecting millions of people worldwide. These diseases are associated with the presence of amyloid-β (Aβ), alpha synuclein (α-Syn) and prion protein (PrP) depositions in the brain, respectively, which lead to synaptic disconnection and subsequent progressive neuronal death. Although considerable progress has been made in elucidating the pathogenesis of these diseases, the specific mechanisms of their origins remain largely unknown. A body of research suggests a potential association between host microbiota, neuroinflammation and dementia, either directly due to bacterial brain invasion because of barrier leakage and production of toxins and inflammation, or indirectly by modulating the immune response. In the present review, we focus on the emerging topics of neuroinflammation and the association between components of the human microbiota and the deposition of Aβ, α-Syn and PrP in the brain. Special focus is given to gut and oral bacteria and biofilms and to the potential mechanisms associating microbiome dysbiosis and toxin production with neurodegeneration. The roles of neuroinflammation, protein misfolding and cellular mediators in membrane damage and increased permeability are also discussed.

Iron Homeostasis Disorder and Alzheimer’s Disease

“…In lipopolysaccharide (LIP) -induced MIA progenies, MIA progenies exhibited an abnormal brain-gut-microbiome axis compared with that of the control progenies, which were characterized by social behavioral deficits, anxiety-like and repetitive behaviors, low myelination, and ASD-like microbiome [66]. Studies have shown a potential link between host microbiome (such as gut and oral bacteria), neuroinflammation, and dementia, which may be caused by bacterial invasion of the brain due to barrier leakage, toxin and inflammation factor production, or indirectly by modulating immune responses, and moreover, the composition of microbiota affected the deposition level of Aβ in the cerebral cortex of APP/PS1 mice [67], suggesting a critical role of iron in these processes.…”

Targeting Impaired Antimicrobial Immunity in the Brain for the Treatment of Alzheimer’s Disease

“… 196 , 223 Furthermore, they possess receptors which react to the overproduced Aβ. 224 , 225 The most important receptors are the TLRs, in particular TLR4, 199 , 226 , 227 which initiate intracellular signaling pathways leading to the activation of the NF-κB, the inflammasome and the antiviral molecular machinery. 228 At the end of this process, activated microglia become senescent and only produce pro-inflammatory neurotoxic mediators such as TNFα.…”

The Alpha-Synuclein RT-QuIC Products Generated by the Olfactory Mucosa of Patients with Parkinson’s Disease and Multiple System Atrophy Induce Inflammatory Responses in SH-SY5Y Cells

“…Therefore, while OM-MSA and OM-PD samples were able to generate αSyn_RT-QuIC products with distinctive properties, the in vitro generated αSv1, αSv2 and αSv3, although characterized by different morphological features, did not. For this reason, we hypothesized that only the αSyn D strains present in OM samples and likely other tissue factors (e.g., microbiota [ 43 ]) could markedly influence the misfolding process of rec-αSyn and the biochemical and morphological features of the RT-QuIC products. Hence, αSv1, αSv2 and αSv3 produced from rec-αSyn, lacked peculiar features which are present in the natural α-synuclein strains (e.g., phosphorylation) that could play a role in this process.…”