Hypothesis: Tissue‑specific miRNA‑gated cyclic OSK expression prevents mesenchymal drift while amplifying epigenetic rejuvenation.

Rationale

Recent work shows that cyclic OSK (or OSKM) expression reverses aging hallmarks without teratoma formation [1][2]. The benefit correlates with reversal of mesenchymal drift, restoring epithelial identity in aged fibroblasts and tissues [3]. However, pancreatic cells respond heterogeneously, indicating that a one‑size‑fits‑all pulse may over‑reprogram some lineages while under‑treating others [3]. Moreover, transient mRNA‑LNP delivery offers pulsed, non‑integrating expression that reduces over‑reprogramming risk [5].

Mechanistic Insight

We propose coupling OSK‑mRNA LNPs with synthetic miRNA‑responsive elements (MREs) that are active only in epithelial‑prone lineages. In mesenchymal‑like cells, high levels of miR‑21 or miR‑155 would bind the MREs and block translation of OSK transcripts, preventing further dedifferentiation. Conversely, epithelial cells express low levels of these miRNAs, allowing OSK translation during each pulse. This creates a feedback loop where the reprogramming signal is self‑limited by the cell’s own mesenchymal‑associated miRNA repertoire, directly counteracting the drift that OSK aims to reverse.

Key mechanistic points

• OSK expression is transient and non‑integrating via mRNA‑LNP [5]
• Synthetic MREs are placed in the 5’UTR of OSK mRNA; binding of mesenchymal‑enriched miRNAs sterically hinders ribosome scanning.
• In epithelial cells, low miRNA levels permit OSK protein production, driving histone demethylation and DNA methylation reset without triggering full pluripotency [3]
• The miRNA gate reduces OCT4/SOX2 over‑expression in mesenchymal niches, lowering oncogenic risk [4]

Testable Predictions

1. In vivo delivery of OSK‑mRNA LNPs bearing epithelial‑specific MREs will yield a higher epithelial‑to‑mesenchymal (E/M) ratio in treated tissues compared with OSK‑mRNA LNPs lacking MREs, as measured by immunostaining for E‑cadherin and vimentin.
2. Pancreatic acinar cells treated with miRNA‑gated OSK will show reduced insulin‑negative, α‑SMA‑positive cells (a marker of mesenchymal drift) relative to ungated OSK, while maintaining improved glucose tolerance.
3. RNA‑seq of sorted epithelial and mesenchymal fractions will reveal selective OSK protein accumulation only in the epithelial fraction when miRNA‑gated constructs are used.
4. Long‑term (≥6 months) mouse studies will detect no teratoma formation and a median lifespan extension comparable to or greater than the 109% increase reported for systemic OSK AAV [1]

Experimental Approach

• Construct design: Clone OSK (OCT4, SOX2, KLF4) coding sequence into a modified mRNA backbone with a 5’UTR containing tandem MREs for miR‑21 and miR‑155; include a poly(A) tail and capillary‑optimized codons.
• Formulation: Package mRNA into lipid nanoparticles with an ionizable lipid suitable for repeat dosing.
• Animal model: Use 12‑month‑old wild‑type C57BL/6 mice; administer intravenous OSK‑mRNA LNPs (gated vs. control) twice weekly for 8 weeks.
• Readouts:
  • Flow cytometry for epithelial (EpCAM⁺) vs. mesenchymal (CD90⁺) markers in liver, intestine, pancreas.
  • Epigenetic clocks (Horvath mouse) on isolated cell populations.
  • Functional assays: glucometer for pancreas, rotarod for motor, OCT imaging for retinal thickness.
  • Tumor surveillance: histology and MRI every 2 months.
• Controls: Untreated, OSK‑mRNA LNP without MREs, and systemic AAV‑OSK (positive control).

If the miRNA gate successfully limits OSK activity to epithelial compartments, we expect amplified reversal of mesenchymal drift, improved tissue‑specific function, and no increase in tumorigenesis—directly addressing the current bottleneck of cell‑type‑specific dosing.