Hypothesis

Core Idea

Endothelial mTOR activity sustains a niche-derived exosomal miR-126-5p signal that keeps HSPC mTORC1 intermittently low, allowing regenerative bursts without chronic exhaustion. In aging, endothelial mTOR falls, exosomal miR-126-5p drops, HSPC mTORC1 stays high, erythroid output falters and stem cell reserve depletes.

Mechanistic Rationale

• mTORC1 in endothelial cells drives the biogenesis of specific exosomes enriched for miR-126-5p (Endothelial mTOR maintains hematopoiesis).
• miR-126-5p targets IRS1 and PI3K subunits in HSPCs, attenuating AKT signaling downstream of EPO and thus reducing mTORC1 activation (mTORC1 signaling essential for erythroid progenitor proliferation).
• When endothelial mTOR is inhibited (e.g., rapamycin or genetic KO), exosomal miR-126-5p diminishes, releasing HSPC IRS1/PI3K from repression, leading to sustained AKT/mTORC1 activity.
• Chronic mTORC1 activation in HSPCs impairs asymmetric division and promotes myeloid skew, reproducing the exhausted phenotype seen after repeated regeneration (Chronic mTORC1 activation impairs stem cell maintenance).
• Transient mTORC1 spikes remain necessary for stress erythropoiesis, but the miR-126-5p buffer ensures they are brief, preserving long-term function.

Testable Predictions

1. Exosomal rescue – Injecting exosomes isolated from young WT endothelial cells into aged mice will increase HSPC miR-126-5p levels, reduce p‑S6K (mTORC1 readout), improve erythroid CFU‑E numbers, and accelerate hematopoietic recovery after 5‑FU treatment.
2. Endothelial‑specific mTOR reactivation – Inducing mTORC1 in aged BMECs via VEGF‑A or IGF‑1 signaling should restore exosomal miR-126-5p and normalize HSPC mTORC1 activity without causing systemic immunosuppression.
3. miR-126-5p antagonism – Delivering antagomiR‑126-5p to young mice will phenocopy aged HSPC mTORC1 hyperactivity, erythroid deficits, and delayed regeneration, confirming sufficiency.
4. Human relevance – Single‑cell RNA‑seq of elderly bone marrow endothelial niches will show reduced MTOR pathway gene scores and lower exosomal miR‑126‑5p cargo compared with young donors.

Falsifiability

If endothelial mTOR manipulation fails to alter HSPC exosomal miR-126‑5p levels, or if restoring miR‑126‑5p does not ameliorate HSPC mTORC1 activity and erythropoiesis in aged mice, the hypothesis is refuted. Conversely, consistent rescue across the outlined experiments would support the model that endothelial‑derived miR-126‑5p couples niche mTOR status to HSPC mTORC1 dynamics, framing mTOR as a civilization‑versus‑survival dial mediated through intercellular RNA signaling.