Protective roles of intestinal microbiota derived short chain fatty acids in Alzheimer's disease-type beta-amyloid neuropathological mechanisms

Abstract

Background: Dietary fibers are metabolized by gastrointestinal (GI) bacteria into short-chain fatty acids (SCFAs). We investigated the potential role of these SCFAs in β-amyloid (Aβ) mediated pathological processes that play key roles in Alzheimer's disease (AD) pathogenesis.

Research design and methods: Multiple complementary assays were used to investigate individual SCFAs for their dose-responsive effects in interfering with the assembly of Aβß1-40 and Aβ1-42 peptides into soluble neurotoxic Aβ aggregates.

Results: We found that several select SCFAs are capable of potently inhibiting Aβ aggregations, in vitro.

Conclusion: Our studies support the hypothesis that intestinal microbiota may help protect against AD, in part, by supporting the generation of select SCFAs, which interfere with the formation of toxic soluble Aβ aggregates.

Keywords: Alzheimer’s disease; beta-amyloid (Aβ); fibrils; microbial; microbiome; microbiota; neurodegeneration; protein misfolding.

Conflict of interest statement

Declaration of interest

The authors have no relevant affiliations or financial involvement with any organization or entity with a financial interest in or financial conflict with the subject matter or materials discussed in the manuscript. This includes employment, consultancies, honoraria, stock ownership or options, expert testimony, grants or patents received or pending, or royalties.

Figures

Figure 1

Select SCFAs potently interfere with protein-protein interactions among Aβ peptides. (a, c, e) Monomeric, dimeric and higher-ordered cross-linked multimeric Aβ1-40 (a), Aβ 1-42 (c) or GST (e) aggregates were visualized by silver staining of the gel. Shown are representative assays from three independent studies. (b, d, f) Densitometry intensity profiles for Aβ1-40 (b), Aβ1–21 (d) and GST (f). In (a-f, lane 1) Aβ1-40 (a,b), Aβ1-42 (c,d) or GST (e,f) alone without cross-linking. In (a-f, lanes 2–14) Aβ1-40 (a,b), Aβ1-42 (c,d) or GST (e,f) with cross-linking in the presence of vehicle (lane 2) or in the presence of individual SCFSs as follow: isobutyric acid at a SCFA:Aβ (or GST) molar ratio of 1:1 (lane 3) or 4:1 (lane 4), isovaleric acid at a SCFA:Aβ molar ratio of 1:1 (lane 5) or 4:1 (lane 6), acetic acid at a SCFA:Aβ (or GST) molar ratio of 1:1 (lane 7) or 4:1 (lane 8), propionic acid at a SCFA:Aβ (or GST) molar ratio of 1:1 (lane 9) or 4:1 (lane 10), butyric acid at a SCFA:Aβ (or GST) molar ratio of 1:1 (lane 11) or 4:1 (lane 12), valeric acid at a SCFA:Aβ (or GST) molar ratio of 1:1 (lane 13) or 4:1 (lane 14). In (a-d), horizontal arrows indicate monomers, dimers, trimers, tetramers and pentamers. In (a-f), vertical arrows indicate positive control studies in which Aβ1-40 (a-b), Aβ1-42 (c-d) or GST (e-f) were incubated in the absence of SCFA.

Figure 2

Select SCFAs potently interfere with Aβ fibril formation. Assembly of monomeric Aβ1-40 or Aβ 1-42 peptides into Aβ fibrils in the presence of valeric acid, butyric acid or vehicle were assessed using the ThT assay, which monitors ThT fluorescence as an indirect assessment of fibril contents. Periodically, aliquots were removed, and ThT binding levels were determined. Binding is expressed as mean fluorescence (in arbitrary fluorescence units (FU). (a-d) Aβ1-40 (a-b) or Aβ1-42 (c-d) were incubated for up to 240 min at 37°C in 10 mM phosphate, pH 7.4, in the presence of vehicle (), or in the presence of valeric acid (a,c) or butyric acid (b,d) at a SCFA:Aβ molar ratio of 1:1 () or 4:1 (). Shown are representative assays from three independent studies.

Figure 3

Assessments of Aβ protofibril morphology. (Panels I, II, III, IV) EM was used to determine the morphologies of protofibrils obtained by the incubation of Aβ1-40 (panels I and II) or Aβ42 (panels III and IV), in the presence of vehicle (panels I and III) or valeric acid at a valeric acid:Aβ molar ratio of 4:1 (panels I and IV). Shown are representative assays from three independent studies. Scale bars indicate 100 nm.

Figure 4

Schematics summarizing the mechanisms by which GI microbial-derived SCFAs may modulate AD. Intestinal bacteria help protect against AD by converting dietary fibers into biologically available SCFAs, which may promote resilience to AD through multiple cellular/molecular mechanisms. Previously published evidence suggests SCFAs may benefit AD by: (1) alleviating brain hypo-metabolism as SCFAs provide alternative substrates for brain energy metabolism [20], (2) attenuating neuro-inflammation by modulating the maturation and function of microglia in the brain [5], and (3) inhibiting histone deacetylases and normalize aberrant histone acetylation in the AD brain [7,14,34]. In addition, evidence from the present study suggests that certain SCFAs, particularly valeric acid, butyric acid, and propionic acid, may also benefit AD by attenuating Aβ-mediated pathologic processes by interfering with the assembly of Aβ1-40 and Aβ1-42 peptides into neurotoxic Aβ aggregates.