Hypothesis\n\nEngineering exosomes to load specific therapeutic miRNAs can be achieved by fusing the Hsp40 J‑domain secretion signal to a synthetic RNA aptamer that binds the miRNA of interest, thereby recruiting the miRNA‑aptamer complex into budding exosomes through lipid‑raft mediated sorting, independent of endogenous hnRNP motifs.\n\n## Rationale\n\n- The J domain of Hsp40 acts as a protein secretion signal that directs cargo into exosomes via interactions with tetraspanins and lipid rafts[2].\n- Recent work shows RNA‑binding proteins recognize short sequence motifs (e.g., CAUUG EXOmotif) to package miRNAs via ESCRT[1].\n- Synthetic aptamers can be selected to bind any RNA with high affinity and can be fused to protein domains without disrupting their structure.\n- By attaching the J domain to an aptamer, the miRNA‑aptamer conjugate becomes a protein‑like cargo that exploits the J‑domain lipid‑raft pathway, bypassing the need for specific miRNA motifs and reducing dependence on variable hnRNP expression.\n\n## Testable Predictions\n\n1. In vitro loading efficiency – Transfecting donor cells with a plasmid encoding J‑domain‑aptamer‑miR‑221 will yield exosomes with >3‑fold higher miR‑221 content compared to transfection of naked miR‑221 or aptamer‑miR‑221 alone (measured by qRT‑PCR of isolated exosomes).\n2. Dependency on lipid rafts – Disrupting cholesterol with methyl‑β‑cyclodextrin will decrease J‑domain‑aptamer mediated miRNA loading by >70 %, whereas ESCRT inhibition (e.g., VPS4 dominant‑negative) will have minimal effect.\n3. Specificity – Swapping the aptamer to target a different miRNA (e.g., let‑7a) will selectively enrich that miRNA in exosomes, confirming that the aptamer determines cargo choice.\n4. Functional delivery – Exosomes produced from J‑domain‑aptamer‑miR‑221 donor cells will suppress target gene expression in recipient cells more effectively than exosomes loaded via conventional electroporation, demonstrating functional unpacking.\n5. In vivo biodistribution – Injecting J‑domain‑aptamer‑miR‑221 exosomes into mice will show increased accumulation in tissues expressing the donor cell’s tropism (e.g., mesenchymal stem cell‑derived exosomes home to injured muscle) and reduced clearance compared to liposomes labeled with the same dye.\n\n## Falsifiability\n\nIf any of the following observations occur, the hypothesis is falsified:\n- J‑domain‑aptamer fusion does not increase miRNA loading over baseline.\n- Loading is unaffected by cholesterol depletion but abolished by ESCRT blockade.\n- Changing the aptamer does not alter the miRNA profile of exosomes.\n- Exosomes fail to deliver functional miRNA to recipient cells despite high loading.\n- In vivo, exosomes show rapid clearance or no tropism advantage over liposomes.\n\n## Potential Impact\n\nConfirming this mechanism would provide a plug‑and‑play platform for exosome‑based therapeutics: swapping aptamers enables rapid re‑targeting of any RNA or protein cargo while leveraging the natural advantages of exosomes (low immunogenicity, CD47 signaling, multi‑cargo capacity). It would also address the reproducibility bottleneck highlighted in current literature by decoupling loading efficiency from endogenous RBP expression levels[8].\n\n## References\n[1] https://pmc.ncbi.nlm.nih.gov/articles/PMC12540272/\n[2] https://doi.org/10.1073/pnas.1412651112\n[8] https://www.globenewswire.com/news-release/2026/02/17/3239029/28124/en/Exosome-Research-Market-Global-Forecast-Report-2026-2032-Opportunities-Driven-by-Advancing-Precision-Medicine-Next-Generation-Diagnostics-and-New-Biomedical-Applications.html