Aggregate Surface Chemistry ControlsExosomal Loading and Decouples Protective Clearance from Pathogenic Spread
Recent work shows that blocking exosome export worsens intracellular toxicity in ALS models, implying that aggregated proteins represent a less‑toxic sequestration state awaiting disposal 1. Yet exosomes also ferry these aggregates to neighboring cells, seeding pathology in microglia and astrocytes 3. We hypothesize that the decision to load an aggregate into an exosome is governed by specific surface motifs on the aggregate rather than its mere presence, and that masking or altering these motifs can retain aggregates for lysosomal degradation while preventing intercellular transmission.
Mechanistically, misfolded proteins exposed to oxidative stress often acquire covalently linked lipids or glycans that mimic apoptotic‑cell “eat‑me” signals (e.g., phosphatidylserine exposure, terminal sialic acid residues). These motifs are recognized by the ESCRT‑associated syntenin‑ALIX complex, which couples cargo to intraluminal vesicle formation and exosome release 2. In autophagy‑lysosomal pathway (ALP) insufficiency, cells upregulate exosome secretion as a compensatory route, inadvertently exporting aggregates that retain these surface signals. If the signals are chemically blocked—by annealing aggregates with phosphatidylserine‑masking annexin V fragments or by enzymatically removing exposed sialic acids with neuraminidase—the aggregates should remain intracellular, be routed to lysosomes via chaperone‑mediated autophagy, and not trigger microglial inflammasome activation.
Testable predictions
- In TDP‑43A315T primary neurons, treatment with annexin V‑PEG or neuraminidase will reduce the proportion of TDP‑43‑positive exosomes released into the medium by >50 % (measured by exosome‑associated Western blot and NTA) without altering total cellular TDP‑43 levels.
- Conditioned medium from treated neurons will induce <30 % of the caspase‑1 activation in BV2 microglia compared with medium from untreated cultures (assayed by FLICA caspase‑1 fluorescence).
- Conversely, rescuing the surface signal—by recombinant phosphatidylserine‑containing liposomes or sialyltransferase overexpression—will restore exosomal loading and reinstate microglial inflammasome activation, confirming causality.
- Long‑term survival of TDP‑43A315T mice receiving intracerebroventricular annexin V‑PEG will show delayed onset of motor deficits and reduced spinal‑cord TDP‑43 pathology relative to vehicle‑treated controls, while exosome levels in CSF drop.
If these outcomes hold, the hypothesis falsifies the view that aggregation is a passive byproduct and instead posits that cells actively tag aggregates for exosomal export via recognizable surface chemistry. Therapeutically, it suggests a strategy: modify aggregate surface epitopes to bolster lysosomal clearance while blocking the exosome‑mediated spread that drives neuroinflammation. This approach directly addresses the translational gap identified in the literature—augmenting protective disposal without amplifying intercellular propagation—by targeting the molecular decision point that determines whether an aggregate is a harmless sink or a pathogenic seed.