Lipid Raft Composition Controls hnRNP‑Motif Recognition and Determines ESCRT‑Dependent vs ‑Independent Exosome Cargo Loading

Hypothesis

Specific lipid raft microdomains enriched in sphingomyelin and cholesterol provide a physical platform that enhances the affinity of hnRNPA2B1, hnRNPK, hnRNPH1 and hnRNPU for their cognate RNA motifs, thereby biasing cargo loading toward the ESCRT‑dependent pathway; altering raft lipid ratios shifts loading to ESCRT‑independent, tetraspanin‑mediated vesicles and changes the consistency of exosome cargo across batches.

Mechanistic Rationale

• RBP‑motif interactions are known to depend on local membrane curvature and electrostatic environment (hnRNPA2B1 loads miRNAs by recognizing CAUUG EXOmotif).
• Lipid rafts, defined by tight packing of sphingomyelin and cholesterol, create ordered platforms that can concentrate both RBPs and RNA cargos, increasing effective collision frequency.
• Disruption of raft order reduces the lateral diffusion of RBPs, lowering their residence time near budding sites and favoring alternative sorting routes that rely on tetraspanin‑enriched microdomains (ESCRT‑independent).
• Consequently, the proportion of cargo loaded via motif‑dependent recognition should correlate with raft stability, while total exosome yield may remain unchanged if compensatory pathways exist.

Testable Predictions

1. Lipid manipulation alters miRNA loading – Knock‑down of sphingomyelin synthase (SGMS2) or overexpression of acid sphingomyelinase (ASM) in producer cells will decrease the SM/Chol ratio, leading to a measurable reduction in CAUUG‑containing miRNAs (e.g., miR‑221) within isolated exosomes, quantified by small‑RNA sequencing.
2. Raft disruption impairs RBP‑RNA binding in vitro – Treating purified exosomes or cytosol with methyl‑β‑cyclodextrin (MβCD) to extract cholesterol will diminish hnRNPA2B1‑miR‑221 pull‑down efficiency in electrophoretic mobility shift assays, without affecting total RBP levels.
3. Rescue with exogenous raft lipids – Adding synthetic SM‑rich liposomes to MβCD‑treated cells will restore both raft integrity (measured by Laurdan GP) and motif‑specific cargo loading to baseline levels.
4. Shift in biogenesis markers – Raft‑depleted conditions will show increased CD63 and tetraspanin‑8 enrichment relative to TSG101 and Alix (ESCRT‑0/III components) in exosome lysates, detectable by western blot or targeted proteomics.
5. Cargo consistency across batches – Producer cells cultured under controlled SM/Chol ratios will exhibit lower coefficient of variation for motif‑dependent cargo across replicate preparations, whereas raft‑perturbed cultures will show higher variability.

Experimental Approach

• Generate stable HEK293 or MSC lines with inducible SGMS2 shRNA or ASM overexpression.
• Validate raft perturbation using cholesterol oxidase staining and fluorescence lifetime imaging.
• Isolate exosomes via differential ultracentrifugation followed by size‑exclusion chromatography to minimize contamination.
• Perform small‑RNA seq and targeted miRNA qPCR for CAUUG‑containing species.
• Conduct RIP‑qPCR for hnRNPA2B1, hnRNPK, hnRNPH1, hnRNPU on exosome‑associated RNAs.
• Assess ESCRT‑dependent (TSG101, Alix) and independent (CD63, CD81, CD9) markers by western blot.
• Repeat preparations (n≥6) to compute intra‑group cargo variance.

Falsifiability

If altering sphingomyelin/cholesterol ratios fails to produce any of the following—(a) a significant change in motif‑specific miRNA loading, (b) a detectable shift in RBP‑RNA binding affinity, (c) a reciprocal change in ESCRT‑ versus tetraspanin‑marker enrichment, or (d) an alteration in cargo batch‑to‑batch consistency—then the hypothesis is falsified. Conversely, observing the predicted directional changes supports the mechanistic link between lipid raft composition and selective exosome cargo loading.