Hypothesis

Age‑related accumulation of the TCA‑cycle metabolite succinate stabilizes HIF1α, which in turn transcriptionally upregulates Rubicon and enhances mTORC1 signaling, creating a feed‑forward loop that actively suppresses autophagy as a protective response to mitochondrial stress.

Mechanistic Rationale

Mitochondrial dysfunction in aging elevates succinate levels due to impaired succinate dehydrogenase (SDH) activity and increased glutamine‑anaplerosis. Succinate inhibits prolyl‑hydroxylase domain enzymes (PHDs), preventing HIF1α degradation and allowing its nuclear translocation Succinate stabilizes HIF1α. HIF1α binds hypoxia‑response elements in the promoter of Rubicon (Rubc), increasing its transcription. Concurrently, HIF1α upregulates REDD1 inhibitors and downregulates TSC2, relieving inhibition of mTORC1 and promoting its hyperactivation HIF1α‑mTORC1 crosstalk. Thus, succinate‑driven HIF1α activation simultaneously amplifies two independent autophagy brakes: Rubicon‑mediated block at autophagosome‑lysosome fusion and mTORC1‑mediated block at initiation.

This model extends the existing evidence that Rubicon and mTORC1 are elevated with age [1][2] by placing their elevation downstream of a measurable metabolic signal. It also explains tissue‑specific patterns: tissues with high succinate flux (brain, kidney) show pronounced Rubicon rises, whereas tissues relying more on fatty‑acid oxidation (heart) exhibit distinct autophagy defects.

Testable Predictions

1. Succinate elevation in young animals will recapitulate age‑like autophagy suppression, increasing Rubicon protein, boosting mTORC1 activity (p‑S6K), and reducing LC3‑II turnover.
2. Genetic or pharmacological reduction of succinate (e.g., liver‑specific SDH overexpression or dimethyl malonate treatment) in aged mice will lower nuclear HIF1α, decrease Rubicon expression, attenuate mTORC1 signaling, and restore autophagic flux.
3. HIF1α loss‑of‑function in aged tissue will suppress Rubicon induction despite high succinate, rescuing autophagy without altering succinate levels.
4. Exogenous succinate administration to young mice will increase HIF1α, Rubicon, and mTORC1 activity, mimicking aged autophagy phenotypes.

Experimental Approach

• Metabolite profiling: Quantify succinate in young vs. aged mouse brain, kidney, liver, and heart using LC‑MS/MS.
• Intervention cohorts: (a) aged mice treated with dimethyl malonate (SDH inhibitor) to lower succinate; (b) young mice given succinate‑loaded drinking water; (c) tissue‑specific HIF1α knockout crossed with aged background.
• Readouts: Western blot for Rubicon, phospho‑S6K (mTORC1 activity), HIF1α, LC3‑I/II, p62; immunofluorescence for autophagosome‑lysosome colocalization; transmission EM for autophagosome abundance.
• Functional assays: Rotarod performance, kidney fibrosis histology, α‑synuclein aggregation assays to link autophagy restoration with phenotypic rescue.
• Controls: Vehicle‑treated littermates; verify that dimethyl malonate does not directly inhibit mTORC1 or Rubicon independent of succinate.

If succinate reduction normalizes Rubicon and mTORC1 activity and restores autophagy flux, the hypothesis is supported. Conversely, if manipulating succinate fails to alter Rubicon/mTORC1 or autophagy, or if HIF1α loss does not rescue autophagy despite succinate lowering, the model would be falsified.