IF continuous low-dose lithium chloride (0.3 mM in drinking water, administered ad libitum) is co-administered with encapsulated rapamycin (14 ppm in chow, matching the NIA ITP enteric-encapsulated protocol) beginning at 20 months of age in C57BL/6J male and female mice and continued for the remainder of their natural lifespan,

THEN the combination group will exhibit:

• ≥15% median lifespan extension relative to vehicle-matched aged controls (greater than either agent alone, estimated +10% for rapamycin alone and +5–8% for lithium alone at these doses),
• ≥40% reduction in quantified autofluorescent lipofuscin granule burden in liver and dorsal root ganglia neurons (measured by spectral confocal imaging, ex/em 488/520–680 nm),
• ≥30% reduction in the proportion of cytochrome c oxidase (COX)-deficient, succinate dehydrogenase (SDH)-positive skeletal muscle fibers (a direct readout of mtDNA deletion accumulation), and
• greater TFEB nuclear localization index (nuclear:cytoplasmic ratio by immunofluorescence) compared to either monotherapy,

BECAUSE the following step-by-step causal chain applies:

1. Rapamycin allosterically inhibits mTORC1, which normally phosphorylates TFEB at Ser211, sequestering it in the cytoplasm. mTORC1 is aberrantly hyperactivated in aged tissues and senescent cells, driving both TFEB cytoplasmic retention and suppression of the ULK1/ATG13 autophagy-initiating complex (Aberrant mTOR activation in aging)[https://doi.org/10.1016/j.exger.2014.11.004]. Rapamycin reverses this, allowing TFEB nuclear translocation and de-repressing ULK1-mediated autophagy initiation.

2. Lithium inhibits inositol monophosphatase (IMPase) in an uncompetitive, dose-dependent manner at concentrations ≤0.3 mM, depleting free myo-inositol and reducing intracellular IP3 levels. IP3 depletion activates autophagy via a pathway that is explicitly mTOR-independent and operates even when mTOR signaling is intact or partially restored. Pharmacological proof-of-concept for mTOR-independent TFEB nuclear translocation and CLEAR network gene upregulation has been demonstrated with CCT020312, establishing that the mTOR-independent axis of TFEB activation is druggable (mTOR-independent autophagy activator increases TFEB nuclear localization)[https://doi.org/10.1101/2022.09.29.509997].

3. Dual-axis TFEB activation [SPECULATIVE: no published study has simultaneously measured TFEB nuclear localization under combined lithium + rapamycin] produces a synergistic increase in TFEB-driven transcription of lysosomal hydrolase genes (CTSD, CTSB, GBA), V-ATPase subunits, and autophagy receptor genes (SQSTM1/p62, NBR1, OPTN). This CLEAR network upregulation generates de novo lysosomal capacity beyond what either pathway alone can achieve, directly enabling degradation of already-accumulated lipofuscin — the cross-linked, oxidized, lysosomal-enzyme-resistant aggregate that accumulates in post-mitotic cells over decades (Autophagy inhibition promotes lipofusc...

SENS category: GlycoSENS

Key references: • doi.org/10.1016/j.exger.2014.11.004]. • doi.org/10.1101/2022.09.29.509997]. • doi.org/10.3390/ijms18040728]. • doi.org/10.1073/pnas.1305623110] • doi.org/10.1371/journal.pone.0120819].