Hypothesis

Age‑associated loss of m6A methylation selectively impairs translation of mRNAs encoding ubiquitin‑proteasome system (UPS) components and chaperones in excitatory hippocampal neurons, leading to reduced proteostatic capacity, accumulation of misfolded proteins, and downstream memory deficits.

Mechanistic Rationale

1. m6A enhances ribosome loading on transcripts with structured 5′UTRs or upstream open reading frames (uORFs) that repress basal translation. Many UPS subunits (e.g., PSMC5, PSMD11) and Hsp70 chaperones contain such regulatory elements, making them sensitive to m6A loss.
2. Reduced YTHDF1‑mediated translation of these proteostasis factors lowers protein synthesis rates without altering mRNA stability, directly decreasing proteasome activity and chaperone availability.
3. Impaired UPS/chaperone function triggers buildup of ubiquitinated substrates and protein aggregates, which can sequester METTL3/14 complexes via stress‑induced phase separation, further diminishing m6A deposition—a potential vicious cycle.
4. Excitatory neurons, which rely heavily on rapid local protein synthesis at synapses for plasticity, are uniquely vulnerable because their proteostasis machinery operates near capacity to support activity‑dependent remodeling.

Testable Predictions

• P1: In 16‑month‑old mouse hippocampus, excitatory (Camk2a+) neurons will show a ≥30 % reduction in METTL3/14 protein levels relative to inhibitory (Gad1+) neurons, measured by quantitative immunofluorescence or targeted proteomics.
• P2: Transcripts of UPS/chaperone genes will exhibit a significant drop in ribosome occupancy (Ribo‑seq) and polysome association in excitatory neurons of aged mice, despite unchanged total mRNA levels.
• P3: Pharmacological inhibition of translation (e.g., low‑dose cycloheximide) in young excitatory neurons will phenocopy the aged proteostatic deficit, whereas overexpression of METTL3 specifically in Camk2a+ cells will rescue UPS activity and reduce protein aggregation.
• P4: Restoring proteasome activity (e.g., via PA28γ overexpression) in aged excitatory neurons will ameliorate memory performance without altering global m6A levels, indicating that proteostatic decline is a downstream effector of m6A loss.

Experimental Design

1. Cell‑type isolation: Use fluorescence‑activated nuclei sorting (FACS) from Camk2a‑Cre;Rosa26‑tdTomato and Gad1‑Cre;Rosa26‑tdTomato mice (3 mo vs 16 mo) to obtain pure excitatory and inhibitory neuronal populations.
2. Quantitative m6A & proteomics: Perform MeRIP‑seq and TMT‑labelled proteomics on sorted nuclei to map m6A changes and quantify METTL3/14, UPS subunits, and chaperones.
3. Translation profiling: Conduct Ribo‑seq on the same samples to compute translation efficiency (TE) for each gene; focus on uORF‑containing proteostasis transcripts.
4. Functional assays: Measure proteasome activity (fluorogenic substrate), chaperone capacity (luciferase refolding), and ubiquitin‑positive puncta (immunostaining) in sorted neurons.
5. Rescue experiments: Deliver AAV‑Camk2a‑METTL3 or AAV‑Camk2a‑PA28γ to aged mice; assess behavioral outcomes (Morris water maze, novel object recognition) and biochemical readouts.
6. Controls: Include METTL3 heterozygous knockouts and scrambled AAV vectors; verify specificity via neuronal marker colocalization.

Falsifiability

If aged excitatory neurons show no METTL3/14 decline, no TE reduction of UPS/chaperone mRNAs, or if restoring METTL3 fails to improve proteasome activity or memory, the hypothesis would be refuted. Conversely, confirming the predicted cascade would support a causal link between m6A‑dependent translation loss and age‑related proteostatic failure in excitatory circuits.