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

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-ß (Aß), 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 Aß, 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.

Gabapentin Inhibits Multiple Steps in the Amyloid Beta Toxicity Cascade.

Oligomeric ß-amyloid peptide (Aß) is one of the main neurotoxic agents of Alzheimer's disease (AD). Oligomers associate to neuronal membranes, forming "pore-like" structures that cause intracellular calcium and neurotransmitter dyshomeostasis, leading to synaptic failure and death. Through molecular screening targeting the C terminal region of Aß, a region involved in the toxic properties of the peptide, we detected an FDA approved compound, gabapentin (GBP), with neuroprotective effects against Aß toxicity. At micromolar concentrations, GBP antagonized peptide aggregation over time and reduced the Aß absorbance plateau to 28% of control. In addition, GBP decreased Aß association to membranes by almost half, and the effects of Aß on intracellular calcium in hippocampal neurons were antagonized without causing effects on its own. Finally, we found that GBP was able to block the synaptotoxicity induced by Aß in hippocampal neurons, increasing post-synaptic currents from 1.7 ± 0.9 to 4.2 ± 0.7 fC and mean relative fluorescence intensity values of SV2, a synaptic protein, from 0.7 ± 0.09 to 1.00 ± 0.08. The results show that GBP can interfere with Aß-induced toxicity by blocking multiple steps, resulting in neuroprotection, which justifies advancing toward additional animal and human studies.

Characterization of a new molecule capable of inhibiting several steps of the amyloid cascade in Alzheimer's disease.

INTRODUCTION: Alzheimer's disease (AD) is the most prevalent neurodegenerative disorder in elderly people. Existent therapies are directed at alleviating some symptoms, but are not effective in altering the course of the disease. METHODS: Based on our previous study that showed that an Aß-interacting small peptide protected against the toxic effects of amyloid-beta peptide (Aß), we carried out an array of in silico, in vitro, and in vivo assays to identify a molecule having neuroprotective properties. RESULTS: In silico studies showed that the molecule, referred to as M30 (2-Octahydroisoquinolin-2(1H)-ylethanamine), was able to interact with the Aß peptide. Additionally, in vitro assays showed that M30 blocked Aß aggregation, association to the plasma membrane, synaptotoxicity, intracellular calcium, and cellular toxicity, while in vivo experiments demonstrated that M30 induced a neuroprotective effect by decreasing the toxicity of Aß in the dentate gyrus of the hippocampus and improving the alteration in spatial memory in behavior assays. DISCUSSION: Therefore, we propose that this new small molecule could be a useful candidate for the additional development of a treatment against AD since it appears to block multiple steps in the amyloid cascade. Overall, since there are no drugs that effectively block the progression of AD, this approach represents an innovative strategy. SIGNIFICANCE: Currently, there is no effective treatment for AD and the expectations to develop an effective therapy are low. Using in silico, in vitro, and in vivo experiments, we identified a new compound that is able to inhibit Aß-induced neurotoxicity, specifically aggregation, association to neurons, synaptic toxicity, calcium dyshomeostasis and memory impairment induced by Aß. Because Aß toxicity is central to AD progression, the inhibition mediated by this new molecule might be useful as a therapeutic tool.

Effect of Cholesterol on Membrane Fluidity and Association of Aß Oligomers and Subsequent Neuronal Damage: A Double-Edged Sword.

Background: The beta-amyloid peptide (Aß) involved in Alzheimer's disease (AD) has been described to associate/aggregate on the cell surface disrupting the membrane through pore formation and breakage. However, molecular determinants involved for this interaction (e.g., some physicochemical properties of the cell membrane) are largely unknown. Since cholesterol is an important molecule for membrane structure and fluidity, we examined the effect of varying cholesterol content with the association and membrane perforation by Aß in cultured hippocampal neurons. Methods: To decrease or increase the levels of cholesterol in the membrane we used methyl-ß-cyclodextrin (MßCD) and MßCD/cholesterol, respectively. We analyzed if membrane fluidity was affected using generalized polarization (GP) imaging and the fluorescent dye di-4-ANEPPDHQ. Additionally membrane association and perforation was assessed using immunocytochemistry and electrophysiological techniques, respectively. Results: The results showed that cholesterol removal decreased the macroscopic association of Aß to neuronal membranes (fluorescent-puncta/20 µm: control = 18 ± 2 vs. MßCD = 10 ± 1, p < 0.05) and induced a facilitation of the membrane perforation by Aß with respect to control cells (half-time for maximal charge transferred: control = 7.2 vs. MßCD = 4.4). Under this condition, we found an increase in membrane fluidity (46 ± 3.3% decrease in GP value, p < 0.001). On the contrary, increasing cholesterol levels incremented membrane rigidity (38 ± 2.7% increase in GP value, p < 0.001) and enhanced the association and clustering of Aß (fluorescent-puncta/20 µm: control = 18 ± 2 vs. MßCD = 10 ± 1, p < 0.01), but inhibited membrane disruption. Conclusion: Our results strongly support the significance of plasma membrane organization in the toxic effects of Aß in hippocampal neurons, since fluidity can regulate distribution and insertion of the Aß peptide in the neuronal membrane.

PDGF-BB Preserves Mitochondrial Morphology, Attenuates ROS Production, and Upregulates Neuroglobin in an Astrocytic Model Under Rotenone Insult.

Platelet-derived growth factor, subtype BB (PDGF-BB) is a mitogenic growth factor produced in different cell types such as platelets, fibroblasts, neurons, and astrocytes. Previous reports have shown that different PDGF isoforms exert a neuroprotective effect in neurons and astrocytes against multiple degenerative insults. Previously, we showed that pretreatment with PDGF-BB for 24 h increased cell viability, preserved nuclear morphology and mitochondrial membrane potential following stimulation with rotenone, and reduced free radical production nearly to control conditions. In the present study, we explored the potential mechanisms associated with PDGF-BB protection against oxidative damage. Our results showed that PDGF-BB protected astrocytic cells through multiple responses, including decrease in the expression of cytoskeleton proteins, attenuated free radicals (reactive oxygen species (ROS)) production, preservation of mitochondrial ultrastructure, and improved expression of neuroglobin (Ngb1). In summary, these findings point out that PDGF-BB protects astrocytic cells by a reduction in ROS production and activation of antioxidant mechanisms.