Hypothesis\nEngineered exosomes that deliver phase‑secession modulators and chaperone co‑factors can redirect the intercellular spread of aggregation‑prone proteins from toxic oligomers to ordered, amyloid‑like fibrils, thereby turning a pathological transmission route into a proteostatic safety net.\n\n## Mechanistic Rationale\nRecent work shows exosomes carry both pathogenic aggregates and protective chaperones [1][2]. The dual cargo reflects the cell’s attempt to sequester misfolded proteins into liquid‑liquid phase‑separated compartments that mature into stable fibrils [4][5]. However, chronic stress overwhelms this maturation, releasing seeding‑competent oligomers that propagate via exosomes [3]. If we load exosomes with molecules that accelerate the liquid‑to‑solid transition—such as low‑complexity peptide seeds that promote β‑sheet stacking or Hsp70/Hsp40 co‑chaperone complexes that suppress oligomer release—we hypothesize that the same vesicles will instead ferry maturation‑promoting signals. This would increase the proportion of inert fibrils in the extracellular space while decreasing oligomer‑mediated toxicity.\n\nA novel mechanistic lever is the tetraspanin‑dependent sorting machinery [6]. By fusing a tetraspanin extracellular domain to a high‑affinity binding motif for α‑synuclein or tau, we can enrich exosomes for specific clients while simultaneously encapsulating fibril‑inducing cargo. This bifunctional loading exploits the cell’s endogenous exosome biogenesis to couple client capture with quality‑control enhancement.\n\n## Experimental Design\n1. Construct exosomes: Transfect donor cells with plasmids expressing CD63‑linked α‑synuclein binding peptide (A53T‑derived) and a secreted Hsp70‑Hsp40 fusion or a synthetic β‑sheet promoting peptide (FKLF). Isolate exosomes via ultracentrifugation.\n2. In vitro assay: Treat SH‑SY5Y cells expressing fluorescent α‑synuclein with either control exosomes or engineered exosomes. Measure oligomer levels (FRET‑based oligomer sensor) and fibril thickness (Thioflavin‑T fluorescence, EM) at 24, 48, 72 h.\n3. Cell‑to‑cell transfer: Use a microfluidic co‑culture where donor cells receive pre‑formed α‑synuclein fibrils; assess transmission to naive acceptor cells via exosomes. Compare oligomer spread and cytotoxicity (LDH release) between conditions.\n4. In vivo validation: Inject engineered exosomes into the striatum of α‑synuclein A53T transgenic mice. Monitor oligomeric burden (immunoprecipitation‑Western) and motor performance (rotarod) over 8 weeks.\n\n## Expected Outcomes\nIf the hypothesis holds, engineered exosomes will:\n- Reduce soluble oligomer signals by ≥40 % relative to controls.\n- Increase ThT‑positive fibril signal and EM‑visualized mature fibrils.\n- Decrease cell‑to‑cell transfer of seeding activity.\n- Improve motor scores in mice without aggravating inflammation.\n\n## Potential Pitfalls\nOverexpression of chaperones could disrupt native proteostasis, causing off‑target effects. Tetraspanin‑client fusions might impair exosome release or alter biodistribution. Each will be monitored by assessing exosome yield, particle size (NTA), and cytokine profiles.\n\n## Falsifiability\nA outcome where engineered exosomes raise oligomer levels, increase ThT‑independent soluble aggregates, or worsen behavioral deficits would falsify the claim that directing exosome cargo toward fibril maturation is neuroprotective.