Hypothesis

When neuronal proteostasis is overwhelmed, the brain exploits extracellular bacterial curli amyloids as a nucleating scaffold that converts soluble, toxic α-synuclein oligomers into inert fibrillar deposits. This process mirrors the host‑defense aggregation seen with ApoE‑LPS complexes but is tuned to neurodegeneration: curli provides a hetero‑seeding surface that lowers the critical concentration for α-synuclein fibrillation, sequestering oligomers into stable plaques that are less neurotoxic. The hypothesis predicts that, in the presence of curli, increasing Hsp70 activity will reduce oligomer levels without decreasing total fibril load, and that fibrils formed under curli seeding will show lower propensity to seed secondary aggregation in vitro compared with fibrils formed spontaneously.

Mechanistic rationale

• Curli (CsgA) shares structural motifs with α‑synuclein that enable cross‑β stacking, as shown by Seeding assays where purified CsgA accelerates α‑synuclein fibril formation [3].
• In neurons, Hsp70 binds exposed hydrophobic patches on α-synuclein oligomers, preventing their proliferation [6]. When Hsp70 capacity is exceeded, oligomers accumulate and become neurotoxic [5].
• Extracellular curli, released from gut bacteria that cross a leaky barrier [7], can bind these oligomers and template their conversion into fibrils, effectively hijacking the aggregation pathway as a containment strategy.
• The resulting fibrils are hypothesized to be structurally distinct (e.g., higher β‑sheet content, tighter packing) rendering them less capable of generating new seeds, akin to the protective amyloid aggregates formed by ApoE‑LPS [4].

Testable predictions

1. In vivo: Germ‑free mice inoculated with curli‑expressing E. coli will develop higher intestinal and cerebral α‑synuclein fibril burden (detected by Thioflavin‑S) but show reduced levels of soluble oligomers (measured by oligomer‑specific ELISA) compared with mice colonized by non‑curli strains, despite similar total α‑synuclein expression.
2. Pharmacological: Overexpression or pharmacological activation of neuronal Hsp70 in curli‑colonized mice will further decrease soluble oligomers without altering fibril load, supporting the idea that fibrils act as a sink.
3. In vitro: Fibrils generated from α‑synuclein in the presence of purified CsgA will exhibit lower seeding activity in a FRET‑based biosensor assay than fibrils formed without CsgA, indicating a reduced propensity to propagate toxicity.
4. Human relevance: Post‑mortem tissue from Parkinson’s patients with high gut curli exposure (estimated via fecal curli DNA) will display a higher ratio of fibrillar α‑synuclein to oligomeric α‑synuclein in the enteric nervous system than controls.

Falsifiability

If curli‑seeded fibrils are found to be equally or more toxic than soluble oligomers, or if Hsp70 manipulation does not shift the oligomer/fibril balance as predicted, the hypothesis would be refuted. Likewise, absence of differences in oligomer levels between curli‑colonized and control animals despite comparable fibril loads would falsify the protective sink model.

Broader implications

Reframing curli‑driven aggregation as a potential protective response shifts therapeutic focus from indiscriminate anti‑aggregation strategies to modulating the quality of aggregates—promoting conversion of toxic oligomers into inert fibrils while enhancing clearance mechanisms. This aligns with the seed idea that aggregates may represent the proteome’s last attempt at order, but specifies a microbiome‑derived cue that directs that attempt toward a less harmful endpoint.

References (inline citations already placed)

[1] https://medicalxpress.com/news/2016-10-proteins-gut-bacteria-misfolding-brain.html [2] https://pmc.ncbi.nlm.nih.gov/articles/PMC9715766/ [3] https://pmc.ncbi.nlm.nih.gov/articles/PMC10917618/ [4] https://www.asbmb.org/asbmb-today/science/083121/protein-aggregation-defective-or-protective [5] https://pmc.ncbi.nlm.nih.gov/articles/PMC2274891/ [6] https://journals.biologists.com/jeb/article/207/18/3213/9343/Chaperones-protein-aggregation-and-brain [7] https://www.frontiersin.org/journals/cellular-and-infection-microbiology/articles/10.3389/fcimb.2024.1348279/full [8] https://pmc.ncbi.nlm.nih.gov/articles/PMC6047317/