Spatial mTOR Gradient Hypothesis: Senescent Niches Organize a Civilization‑Survival Trade‑off in Aged Tissue

Senescent cells create a microenvironment in which mTOR activity forms a steep gradient: high within the senescent focus, driving SASP production through C/EBPβ‑dependent translation of IL‑1α, IL‑6 and IL‑8 [2], and low in the surrounding parenchyma, where reduced mTORC1 signaling promotes autophagic flux and stress resistance [3]. We hypothesize that this gradient is not incidental but mechanistically links the senescence‑associated secretory phenotype to a paracrine survival program that attempts to contain damage while sacrificing tissue‑level anabolic functions. Specifically, senescent cells secrete TGF‑β and IGF‑binding proteins that activate REDD1 in neighboring cells, suppressing mTORC1 locally and shifting those cells into a catabolic, survival‑oriented state. This creates a “survival zone” that limits SASP diffusion but also impairs regenerative processes that require mTOR‑driven protein synthesis and cellular hypertrophy. Consequently, systemic mTOR inhibition (e.g., chronic rapamycin) flattens the gradient, eliminating the high‑mTOR SASP engine but also abolishing the low‑mTOR autophagy‑supportive niche, leading to accumulation of damaged cells and loss of tissue repair capacity.

Testable predictions:

1. In spatial transcriptomics data from aged mouse brain (Visium or MERFISH), MTOR, RAPTOR, and S6K1 expression will peak within p16^+ senescent foci and decline exponentially with distance, while autophagy‑related genes (LC3B, SQSTM1) will show the inverse pattern.
2. Localized overexpression of Rheb (an mTORC1 activator) within senescent niches will increase SASP cytokine mRNA without increasing senescence marker spread, whereas knockdown of REDD1 in the surrounding tissue will reduce autophagic flux and exacerbate senescence accumulation.
3. Chronic systemic rapamycin will homogenize mTOR signaling across the tissue, resulting in decreased SASP output but also diminished autophagic markers and impaired functional outcomes (e.g., reduced myelin integrity or synaptic density) compared to controls.

Falsification would occur if mTOR pathway components show no significant spatial correlation with senescent cells, or if manipulating mTOR locally fails to alter SASP or autophagy as predicted. This hypothesis directly extends the “civilization‑versus‑survival dial” concept by proposing that the dial’s setting varies across micrometers, coordinating a tissue‑level trade‑off between inflammatory growth and survival maintenance.