Hypothesis

Circadian fluctuations in intracellular α‑ketoglutarate (α‑KG) drive rhythmic activity of KDM6 histone demethylases at core clock gene promoters, maintaining a permissive H3K27me3 landscape that sustains robust clock transcription. With age, declining SLC1A5‑mediated glutamine import reduces α‑KG, blunting KDM6 oscillations, leading to progressive H3K27me3 accumulation, dampened clock gene expression, loss of temporal coherence, and accelerated aging phenotypes. Restoring α‑KG rhythms—via timed glutamine supplementation, SLC1A5 overexpression, or cell‑permeable α‑KG esters—should rescue circadian epigenetics and extend healthspan.

Mechanistic Rationale

1. α‑KG as a circadian metabolite – Glutamine uptake via SLC1A5 exhibits diurnal variation, providing substrate for glutamate dehydrogenase and producing α‑KG in phase with the clock SLC1A5-dependent glutamine flux. α‑KG is a required co‑factor for the Fe(II)/2‑oxoglutarate‑dependent KDM6 (JmjC) demethylases that remove H3K27me3.
2. Rhythmic KDM6 activity – Oscillating α‑KG levels would produce cyclic KDM6 catalytic activity, resulting in timed removal of repressive H3K27me3 marks at promoters of Bmal1, Clock, Per, and Cry genes. This creates a permissive chromatin state that coincides with the transcriptional activation phase of the circadian loop.
3. Age‑related decoupling – Aging diminishes SLC1A5 expression and glutamine flux, lowering α‑KG pools SLC1A5-dependent glutamine flux. Consequently, KDM6 activity loses its rhythm, H3K27me3 accumulates at clock promoters, transcription amplitude falls, and the molecular clock becomes desynchronized Circadian rhythm alteration by blood-borne factors. Loss of coherent clock output further impairs mitochondrial metabolism and stress resistance, creating a feed‑forward loop of aging.
4. Rescue strategy – Reinstating α‑KG oscillations (e.g., timed glutamine boluses, SLC1A5 overexpression, or dimethyl‑α‑KG) should restore rhythmic KDM6 activity, reduce promoter H3K27me3, revive clock gene amplitude, and improve downstream physiological rhythms.

Testable Predictions

• Prediction 1: In young mouse liver, ChIP‑seq for H3K27me3 will show anti‑phase oscillation relative to α‑KG levels, with troughs coinciding with peak Bmal1 transcription.
• Prediction 2: Aged mice will exhibit flattened α‑KG rhythms, loss of H3K27me3 oscillations, and elevated promoter H3K27me3 at clock genes.
• Prediction 3: Genetic or pharmacological restoration of SLC1A5 specifically in hepatocytes will reinstate α‑KG oscillations, normalize KDM6 activity (measured by H3K27me3 ChIP‑seq), and increase circadian amplitude of Per2::Luc reporters.
• Prediction 4: Administration of cell‑permeable α‑KG ester at the subjective night will acutely decrease H3K27me3 at clock promoters and enhance nocturnal clock gene expression in aged animals.
• Prediction 5: Long‑term timed α‑KG supplementation will delay age‑related decline in behavioral circadian rhythms (wheel‑running period stability) and extend median lifespan in mice.

Experimental Approach

• Measure intracellular α‑KG via LC‑MS across circadian time in young vs. old tissues.
• Perform time‑course ChIP‑seq for H3K27me3 and KDM6B at core clock promoters.
• Use AAV‑mediated SLC1A5 overexpression or CRISPRa in aged mice; assess clock gene expression by qPCR and bioluminescence reporters.
• Apply dimethyl‑α‑KG or glutamine at specific circadian phases; read out H3K27me3 and transcriptional output.
• Monitor locomotor activity, metabolic markers, and survival.

By linking metabolite‑driven epigenetic enzyme rhythms to clock gene chromatin states, this hypothesis converts the metaphorical “circadian firewall” into a concrete, targetable mechanism.