Background

Recent yeast work shows that nuclear proteasome concentration predicts lifespan, with age‑dependent nuclear enlargement diluting proteasomes rather than changes in cytoplasmic levels [1]. This places proteostasis decline upstream of mitochondrial dysfunction in aging trajectories. In mouse, ~7 million cells across 21 organs reveal synchronized, sex‑biased shifts in cell‑type abundance and a continuum from quiescence to senescence [2], [5]. Yet it remains unknown whether the nuclear dilution mechanism is conserved in mammalian tissues and how it translates into organ‑specific senescence trajectories.

Hypothesis

We hypothesize that age‑related nuclear enlargement drives a conserved, organ‑specific loss of nuclear proteasome concentration, which in turn reduces chromatin‑bound proteasome activity, leading to localized accumulation of ubiquitinated histones and transcription factors. This proteasome‑deficient chromatin state biases transcriptional programs toward tissue‑specific senescence‑associated secretory phenotypes (SASPs), thereby shaping the observed organ‑dependent trajectories in single‑cell aging atlases.

Mechanistic Rationale

1. Nuclear expansion increases volume without proportional synthesis of proteasome subunits, lowering nuclear proteasome density [1].
2. Proteasomes not only degrade cytosolic proteins but also regulate histone turnover and transcription factor stability within the nucleus [6]. Reduced nuclear proteasome activity therefore stabilizes ubiquitinated histones (e.g., H2AK119Ub) and short‑lived transcription factors that promote SASP gene expression.
3. Different tissues express distinct repertoires of nuclear‑resident transcription factors (e.g., NF‑κB in macrophages, FOXO1 in hepatocytes). Their selective stabilization yields organ‑specific SASP profiles, explaining the sex‑biased and organ‑specific abundance changes seen in multi‑omics atlases [2].
4. The continuum between quiescence and senescence [5] reflects graded nuclear proteasome loss: high proteasome = quiescent, intermediate = pre‑senescent, low = full SASP‑positive senescence.

Testable Predictions

• Prediction 1: Across mouse organs, nuclear volume will inversely correlate with nuclear proteasome concentration (measured by imaging‑based proteasome fluorescence intensity per nuclear volume) in aged mice.
• Prediction 2: Organs exhibiting the greatest increase in senescent‑type cell abundance will show the steepest decline in nuclear proteasome density.
• Prediction 3: Genetic or pharmacological reduction of nuclear size (e.g., lamin A/C over‑expression) will rescue nuclear proteasome levels, decrease ubiquitinated histone accumulation, and attenuate tissue‑specific SASP signatures without affecting cytoplasmic proteasome activity.
• Prediction 4: Inducing nuclear swelling (e.g., lamin A/C knock‑down) in young mice will precipitate premature organ‑specific senescence trajectories, recapitulating the aged single‑cell pseudotime ordering.

Experimental Design

1. Imaging: Perform multiplexed immunofluorescence on tissue sections from young (3 mo) and aged (24 mo) male and female mice. Quantify nuclear volume (DAPI), nuclear proteasome signal (PSMC5 antibody), and ubiquitinated histone H2AK119Ub. Use organ‑wise n ≥ 5.
2. Perturbation: Generate organ‑targeted AAV vectors for lamin A/C over‑expression or shRNA‑mediated knock‑down. Validate nuclear size change by imaging.
3. Read‑outs: Single‑cell RNA‑seq (10x) on dissociated organs to compute senescence scores, SASP gene modules, and pseudotime trajectories using RNA velocity/network entropy approaches [3], [4]. Compare trajectories between control and perturbed groups.
4. Controls: Measure cytoplasmic proteasome activity (LLVY‑AMC assay) to ensure specificity.

Potential Outcomes and Falsifiability

• Support: Observed inverse correlation between nuclear volume and proteasome density; rescue of nuclear proteasome size normalizes SASP and shifts trajectories toward younger states; lamin knock‑down accelerates organ‑specific senescence.
• Refutation: No consistent relationship between nuclear size and proteasome levels across organs; manipulation of nuclear size fails to alter ubiquitinated histone accumulation or SASP; cytoplasmic proteasome changes drive observed trajectories instead.

Falsification of the core claim—that nuclear proteasome dilution is a conserved upstream driver of organ‑specific senescence—would require demonstrating that nuclear size manipulations do not affect nuclear proteasome activity or downstream senescence trajectories in any tissue.

Broader Impact

If validated, this hypothesis would unify yeast and mammalian aging mechanisms, positioning nuclear biophysics as a lever to modulate tissue‑specific aging trajectories and offering organ‑targeted interventions that act upstream of mitochondrial dysfunction and chronic inflammation.