Hypothesis

Chronic circadian disruption depletes NAD+ selectively in colonic enteric glial cells, triggering a PARP1‑driven inflammatory shift that withdraws glial‑derived neurotrophic support (GDNF) from myenteric neurons and accelerates their loss. Restoring rhythmic NAD+ fluxes in glia—via timed NMN administration or glial‑specific NAMPT overexpression—preserves neuron numbers and motility even when the core clock is desynchronized.

Mechanistic Basis

The colonic clock drives rhythmic expression of NAD+ biosynthetic enzymes (Nampt, Nadk) in both neurons and glia [1]. While core clock transcripts remain intact with age, glial Nampt rhythm dampens faster than neuronal Nampt, creating a compartmental NAD+ trough. Low NAD+ activates PARP1, which consumes NAD+ to poly‑ADP‑riboseylate NF‑κB, pushing glia toward a pro‑inflammatory phenotype (↑IL‑1β, ↑TNF‑α) and suppressing GDNF secretion [2]. GDNF loss deprives cholinergic myenteric neurons of a key survival signal, exacerbating nitrative stress and α‑synuclein nitration that correlates with neuron loss [3]. Thus, the circadian‑NAD+ axis acts as a glial checkpoint: when rhythmic, it fuels NAD+-dependent SIRT1 activity that deacetylates and inhibits PARP1, preserving a supportive glial state; when arrhythmic, PARP1 wins, NAD+ crashes, and glial support collapses.

Predictions & Experimental Design

1. Temporal NAD+ profiling – Aged mice subjected to 6‑hour advancing light‑dark cycles (chronic jet lag) will show a >40% drop in glial NAD+ (measured by FACS‑sorted GFAP+ cells) within 2 weeks, preceding any detectable neuronal NAD+ decline or neuron loss.
2. Glial‑specific rescue – Viral glial‑specific NAMPT overexpression in jet‑lagged aged mice will restore glial NAD+ to youthful rhythms, normalize PARP1 activity, maintain GDNF levels, and prevent the 15‑60% myenteric neuron loss seen in controls.
3. Pharmacological test – Administering NMN at the circadian peak of glial Nampt expression (ZT6) will improve colonic transit and increase neuron counts more effectively than constant‑dose NMN, demonstrating timing dependence.
4. Falsification – If glial NAD+ remains stable despite circadian misalignment, or if glial NAMPT overexpression fails to preserve neurons, the hypothesis is refuted.

Potential Implications

This model shifts focus from neuronal clocks to glial metabolic coupling as the circadian‑dependent bottleneck in enteric aging. It suggests that geroprotective strategies targeting glial NAD+—especially when timed to the endogenous metabolic peak—could preserve motility and prevent constipation in older adults without requiring global clock correction. Moreover, it offers a mechanistic link between circadian disruption, glial inflammation, and synucleinopathy, potentially explaining why shift workers exhibit heightened gastrointestinal neurodegeneration risk.