Hypothesis

Aging neurons increase METTL3/14 activity, elevating m6A on transcripts encoding mitochondrial and synaptic proteins. This epitranscriptomic mark recruits YTHDF2‑mediated decay and stalls ribosome progression, activating ribosome‑associated quality control (RQC). Persistent RQC signaling flags the neuron as metabolically inefficient, initiating a cell‑autonomous eviction program that removes the cell without invoking classical apoptosis or neurodegeneration.

Rationale

• Elevated METTL3/14 in senescence: Contrary to Alzheimer’s models, cellular senescence shows heightened METTL3 expression and global m6A levels[1]. If this trend extends to post‑mitotic neurons, aging would gain, not lose, m6A deposition.
• m6A influences translation fidelity: m6A can destabilize transcripts and modulate ribosome pausing[2], directly linking epitranscriptomics to ribosomal behavior.
• RQC surveils protein synthesis: Neurons monitor translation errors via RQC; its dysfunction raises RPS2 ubiquitination and synaptic deficits[3], providing a readout for "inefficiency".
• m6A guides cell fate in progenitors: Loss of METTL3/14 triggers p53‑dependent apoptosis[4], showing the pathway’s potency to decide survival versus removal.

Combining these points predicts that gain of m6A, rather than loss, creates a translational checkpoint that, when chronically engaged, pushes neurons toward eviction.

Mechanistic Model

1. Age‑dependent METTL3/14 up‑regulation → increased m6A on 5’UTR and coding regions of nuclear‑encoded mitochondrial genes (e.g., COX5B, ATP5A1) and synaptic vesicle regulators.
2. YTHDF2 recruitment → accelerated decay of a subset of these transcripts, reducing mitochondrial output and synaptic protein supply.
3. Ribosome stalling at m6A‑modified codons activates the RQC complex (ZNF598, Ltn1).
4. Persistent RQC signaling leads to ubiquitination of ribosomal proteins (RPS2) and downstream activation of the Integrated Stress Response (ISR) via GCN2‑eIF2α‑ATF4.
5. ISR‑driven transcriptional program up‑regulates lysosomal and autophagy genes while simultaneously suppressing pro‑survival signals (e.g., BDNF).
6. Cell‑autonomous eviction: The neuron initiates a non‑apoptotic clearance pathway—possibly involving phosphatidylserine exposure and microglial phagocytosis—removing the energetically costly cell.

Testable Predictions

• Prediction 1: In sorted neurons from young vs. old mice, METTL3/14 protein and global m6A levels will be significantly higher in the aged group (quantified by Western blot and m6A‑seq).
• Prediction 2: Transcripts with elevated m6A in aged neurons will show reduced ribosome occupancy (Ribo‑seq) and increased YTHDF2 binding (eCLIP).
• Prediction 3: Pharmacological inhibition of METTL3 (e.g., with STM2457) or CRISPRi knock‑down in aged neurons will rescue mitochondrial membrane potential, lower RPS2 ubiquitination, and decrease eviction markers (e.g., exposed phosphatidylserine) without triggering apoptosis.
• Prediction 4: Conversely, overexpression of METTL3 in young neurons will phenocopy aged‑like translational stalling, increase RQC activation, and sensitize cells to eviction under metabolic stress.

Experimental Approach

1. Cohort: Harvest cortical neurons from 3‑month (young) and 24‑month (aged) mice; sort NeuN⁺ cells via FACS.
2. Epitranscriptomics: Perform m6A‑MeRIP‑seq and quantify METTL3/14 by immunoblot.
3. Translation profiling: Conduct Ribo‑seq to calculate ribosome density on m6A‑enriched transcripts.
4. RQC readout: Immunostain for ubiquitinated RPS2 and assess polysome profiles.
5. Functional rescue: Treat aged neuron cultures with METTL3 inhibitor or AAV‑CRISPRi; measure Seahorse OCR, synaptic marker levels (Synapsin‑1), and eviction (Annexin V+/Caspase‑3‑).
6. Gain‑of‑function: Express METTL3 in young neurons via AAV; repeat assays to see if eviction markers rise.

Falsifiability

If aged neurons show no increase in METTL3/14 or m6A, or if modulating METTL3 fails to alter ribosomal stalling, RQC activation, or eviction rates, the hypothesis is refuted. Likewise, if eviction persists despite METTL3 suppression, alternative mechanisms must drive neuronal removal.

Implications

Reframing age‑related neuronal loss as an epitranscriptomically tuned quality‑control process shifts therapeutic focus from blocking death to recalibrating the translational checkpoint—potentially preserving circuit function by enhancing mitochondrial output or dampening maladaptive m6A deposition rather than inhibiting cell death pathways.

References [1] https://www.aginganddisease.org/EN/PDF/10.14336/AD.2024.1715 [2] https://pmc.ncbi.nlm.nih.gov/articles/PMC8482683/ [3] https://www.pnas.org/doi/10.1073/pnas.2211522120 [4] https://pmc.ncbi.nlm.nih.gov/articles/PMC12700100/ }