Porphyromonas gingivalis Conditioned Medium Induces Amyloidogenic Processing of the Amyloid-β Protein Precursor upon in vitro Infection of SH-SY5Y Cells

Proteomic analysis of P. gingivalis-Lipopolysaccharide induced neuroinflammation in SH-SY5Y and HMC3 cells

Chronic periodontitis and its keystone pathogen, Porphyromonas gingivalis, have increasingly been linked with Alzheimer's disease (AD). However, P.gingivalis-lipopolysaccharide (LPS) mediated release of neuroinflammatory proteins contributes to AD remains underexplored. In this study, we utilized data-independent acquisition mass spectrometry to characterize P.gingivalis-LPS induced profile of differentially expressed proteins associated with the neuroinflammatory response in human neuroblastoma (SH-SY5Y) and human microglial (HMC3) cells. We reported a set of 124 proteins in SH-SY5Y cells and 96 proteins in HMC3 cells whose levels were significantly upregulated or downregulated by exposure to P. gingivalis-LPS. Our findings demonstrate that P. gingivalis-LPS contributed to the elevated expressions of dementia biomarkers and pro-inflammatory cytokines that include APP, Aβ1-42, Aβ1-40, T-Tau, p-Tau, VEGF, TGF-β, IL-1β, IL-6 and TNF-α through 2 distinct pathways of extracellular sensing by cell surface receptors and intracellular cytosolic receptors. Interestingly, intracellular signaling proteins activated with P. gingivalis-LPS transfection using Lipofectamine™ 2000 had significantly higher fold change protein expression compared to the extracellular signaling with P. gingivalis-LPS treatment. Additionally, we also explored P. gingivalis-LPS mediated activation of caspase-4 dependent non canonical inflammasome pathway in both SH-SY5Y and HMC3 cells. In summary, P. gingivalis-LPS induced neuroinflammatory protein expression in SH-SY5Y and HMC3 cells, provided insights into the specific inflammatory pathways underlying the potential link between P. gingivalis-LPS infection and the pathogenesis of Alzheimer's disease and related dementias.

Sensory Dysfunction, Microbial Infections, and Host Responses in Alzheimer's Disease

Sensory functions of organs of the head and neck allow humans to interact with the environment and establish social bonds. With aging, smell, taste, vision, and hearing decline. Evidence suggests that accelerated impairment in sensory abilities can reflect a shift from healthy to pathological aging, including the development of Alzheimer's disease (AD) and other neurological disorders. While the drivers of early sensory alteration in AD are not elucidated, insults such as trauma and infections can affect sensory function. Herein, we review the involvement of the major head and neck sensory systems in AD, with emphasis on microbes exploiting sensory pathways to enter the brain (the "gateway" hypothesis) and the potential feedback loop by which sensory function may be impacted by central nervous system infection. We emphasize detection of sensory changes as first-line surveillance in senior adults to identify and remove potential insults, like microbial infections, that could precipitate brain pathology.

Porphyromonas gingivalis LPS and Actinomyces naeslundii Conditioned Medium Enhance the Release of a Low Molecular Weight, Transcriptionally Active, Fragment of Glycogen Synthase-3 Kinase in IMR-32 Cell Line

Background

Glycogen synthase-3 kinase (GSK3) is one of the major contributors of tau hyperphosphorylation linked to neurofibrillary tangles in Alzheimer's disease (AD).

Objective

To determine a mechanism of GSK-3β activation by two periodontal bacteria consistently confirmed in AD autopsied brains.

Methods

Porphyromonas gingivalis FDC381 and Actinomyces naeslundii ATCC10301 conditioned media were collected. IMR-32 cells were challenged for 48 h with the conditioned media alongside P. gingivalis (ATCC33277) ultrapurified lipopolysaccharide (LPS) designated Pg.LPS under established cell culture conditions either alone or combined. Gene expression and protein analyses for GSK-3β were carried out.

Results

qPCR demonstrated that GSK-3β gene was overexpressed in IMR-32 cells treated with Pg.LPS with a 2.09-fold change (p = 0.0005), while A. naeslundii treated cells demonstrated 1.41-fold change (p = 0.004). Western blotting of the cells challenged with Pg.LPS (p = 0.01) and A. naeslundii conditioned medium (p = 0.001) demonstrated the 37 kDa band for each treatment with variable intensity across the medium control. Immunohistochemistry with the GSK-3β of the IMR-32 cells challenged with Pg.LPS and A. naeslundii alone demonstrated cytoplasmic and nuclear localization.

Conclusions

Exposure to various bacterial factors upregulated the gene expression of GSK-3β. Western blotting for GSK-3β confirmed the presence of the cleaved fragment by Pg.LPS (37 kDa band p = 0.01) and A. naeslundii conditioned medium (37 kDa band p = 0.001). Immunostaining demonstrated both cytoplasmic and nuclear localization of GSK-3β. Therefore, Pg.LPS and an unknown factor from the A. naeslundii conditioned medium mediated GSK-3β activation via its transcriptionally active, cleaved, fragment. These virulence factors in the body appear to be detrimental to brain health.

Thirty Risk Factors for Alzheimer's Disease Unified by a Common Neuroimmune-Neuroinflammation Mechanism

One of the major obstacles confronting the formulation of a mechanistic understanding for Alzheimer's disease (AD) is its immense complexity-a complexity that traverses the full structural and phenomenological spectrum, including molecular, macromolecular, cellular, neurological and behavioural processes. This complexity is reflected by the equally complex diversity of risk factors associated with AD. However, more than merely mirroring disease complexity, risk factors also provide fundamental insights into the aetiology and pathogenesis of AD as a neurodegenerative disorder since they are central to disease initiation and subsequent propagation. Based on a systematic literature assessment, this review identified 30 risk factors for AD and then extended the analysis to further identify neuroinflammation as a unifying mechanism present in all 30 risk factors. Although other mechanisms (e.g., vasculopathy, proteopathy) were present in multiple risk factors, dysfunction of the neuroimmune-neuroinflammation axis was uniquely central to all 30 identified risk factors. Though the nature of the neuroinflammatory involvement varied, the activation of microglia and the release of pro-inflammatory cytokines were a common pathway shared by all risk factors. This observation provides further evidence for the importance of immunopathic mechanisms in the aetiopathogenesis of AD.

Oral Microbiome and Alzheimer's Disease

The accumulation of amyloid-beta plaques in the brain is a central pathological feature of Alzheimer's disease. It is believed that amyloid responses may be a result of the host immune response to pathogens in both the central nervous system and peripheral systems. Oral microbial dysbiosis is a chronic condition affecting more than 50% of older adults. Recent studies have linked oral microbial dysbiosis to a higher brain Aβ load and the development of Alzheimer's disease in humans. Moreover, the presence of an oral-derived and predominant microbiome has been identified in the brains of patients with Alzheimer's disease and other neurodegenerative diseases. Therefore, in this opinion article, we aim to provide a summary of studies on oral microbiomes that may contribute to the pathogenesis of the central nervous system in Alzheimer's disease.

In Situ Raman Study of Neurodegenerated Human Neuroblastoma Cells Exposed to Outer-Membrane Vesicles Isolated from Porphyromonas gingivalis

The aim of this study was to elucidate the chemistry of cellular degeneration in human neuroblastoma cells upon exposure to outer-membrane vesicles (OMVs) produced by Porphyromonas gingivalis (Pg) oral bacteria by monitoring their metabolomic evolution using in situ Raman spectroscopy. Pg-OMVs are a key factor in Alzheimer's disease (AD) pathogenesis, as they act as efficient vectors for the delivery of toxins promoting neuronal damage. However, the chemical mechanisms underlying the direct impact of Pg-OMVs on cell metabolites at the molecular scale still remain conspicuously unclear. A widely used in vitro model employing neuroblastoma SH-SY5Y cells (a sub-line of the SK-N-SH cell line) was spectroscopically analyzed in situ before and 6 h after Pg-OMV contamination. Concurrently, Raman characterizations were also performed on isolated Pg-OMVs, which included phosphorylated dihydroceramide (PDHC) lipids and lipopolysaccharide (LPS), the latter in turn being contaminated with a highly pathogenic class of cysteine proteases, a key factor in neuronal cell degradation. Raman characterizations located lipopolysaccharide fingerprints in the vesicle structure and unveiled so far unproved aspects of the chemistry behind protein degradation induced by Pg-OMV contamination of SH-SY5Y cells. The observed alterations of cells' Raman profiles were then discussed in view of key factors including the formation of amyloid β (Aβ) plaques and hyperphosphorylated Tau neurofibrillary tangles, and the formation of cholesterol agglomerates that exacerbate AD pathologies.

Novel Strategy for Alzheimer’s Disease Treatment through Oral Vaccine Therapy with Amyloid Beta

Alzheimer’s disease (AD) is a neuropathology characterized by progressive cognitive impairment and dementia. The disease is attributed to senile plaques, which are aggregates of amyloid beta (Aβ) outside nerve cells; neurofibrillary tangles, which are filamentous accumulations of phosphorylated tau in nerve cells; and loss of neurons in the brain tissue. Immunization of an AD mouse model with Aβ-eliminated pre-existing senile plaque amyloids and prevented new accumulation. Furthermore, its effect showed that cognitive function can be improved by passive immunity without side effects, such as lymphocyte infiltration in AD model mice treated with vaccine therapy, indicating the possibility of vaccine therapy for AD. Further, considering the possibility of side effects due to direct administration of Aβ, the practical use of the safe oral vaccine, which expressed Aβ in plants, is expected. Indeed, administration of this oral vaccine to Alzheimer’s model mice reduced Aβ accumulation in the brain. Moreover, almost no expression of inflammatory IgG was observed. Therefore, vaccination prior to Aβ accumulation or at an early stage of accumulation may prevent Aβ from causing AD.

Ex vivo Detection of Amyloid-β in Naturally Formed Oral Biofilm

Background

Oral infection has been implicated in the possible etiology of Alzheimer's disease.

Objective

To detect amyloid-β (Aβ) within microbial biofilms.

Methods

Freshly extracted teeth (N = 87) with periodontal disease were separated into Group A (N = 11), with primary root canal infection and Group B (N = 21) with failed endodontic treatment identified by the presence of, gutta percha root filling. Biofilm characteristics were observed by scanning electron microscopy (SEM). Demineralized paraffin wax embedded tooth sections and mineralized calculus biofilm were immunostained with the anti-Aβ antibody. The gutta perchas were processed either for on-section acrylic resin tissue immunocolloidal gold silver staining (IGSS) using the anti-Aβ antibody or in Araldite resin for ultrastructure.

Results

SEM demonstrated calculus and gutta percha in situ harboring a polymicrobial biofilm featuring extracellular polymeric substance (EPS) and water channels. Immunohistochemistry on rehydrated paraffin wax tooth sections from Group A, demonstrated Aβ staining on external (calculus and plaque) and all intracanal infected regions. In Group B, the gutta percha biofilm IGSS gave an inconclusive result for Aβ. Transmission electron microscopy of selected teeth with infected intra-canals (Group A) and 20% of gutta percha biofilm (Group B) EPS contained electron dense fibrils of variable sizes, some of which were typical of human Aβ fibrils.

Conclusion

This study detected both soluble and insoluble Aβ fibrils within the EPS of periodontal and endodontic natural biofilm, strongly suggesting its role as an antimicrobial peptide in combatting local infection, with potential risk for cross-seeding into the brain for AD development.