NAD+-Dependent SIRT1 Reprograms Autophagy Triage in Aging Beta Cells, Shifting Priority from ER Maintenance to Aggregate Clearance

Hypothesis

Chronic NAD+ decline in aging beta cells reduces SIRT1 deacetylase activity, which alters the phosphorylation state of selective autophagy receptors (FAM134B for ER‑phagy, OPTN/Ndp52 for aggrephagy). This shift re‑programs the autophagy triage hierarchy, diverting limited autophagic capacity from ER membrane turnover toward clearance of toxic IAPP oligomers. The resulting ER expansion exacerbates proteotoxic stress, creating a vicious cycle that accelerates beta‑cell failure.

Mechanistic Basis

• SIRT1 directly deacetylates FAM134B at lysine residues, promoting its interaction with LC3 and efficient ER‑phagy [4]; acetylation blocks this interaction.
• SIRT1 also deacetylates TBK1, enhancing its ability to phosphorylate OPTN and Ndp52, thereby stimulating aggrephagy [3].
• In aged islets, NAD+ levels fall ~40 % [5], leading to hyper‑acetylated FAM134B (reduced ER‑phagy) and hypo‑acetylated TBK1 (increased aggrephagy).
• Consequently, ER expands (ER‑phagy insufficiency) while IAPP oligomer clearance rises temporarily, but the ER stress from membrane overload overwhelms the benefit, driving pro‑hIAPP dimer accumulation and cell death [1].

Testable Predictions

1. Pharmacological NAD+ restoration (e.g., NR or NMN) will increase SIRT1 activity, decrease FAM134B acetylation, and restore ER‑phagy flux without altering total autophagosome number.
2. Beta‑cell–specific SIRT1 knockout will mimic the aged phenotype: increased FAM134B acetylation, ER expansion, reduced ER‑phagy, and accelerated IAPP‑oligomer‑induced diabetes despite unchanged aggrephagy.
3. Overexpression of an acetylation‑resistant FAM134B mutant (K→R) will rescue ER morphology and glucose tolerance in aged, hIAPP‑transgenic mice, even when NAD+ remains low.
4. Conversely, a phospho‑deficient OPTN mutant (S→A) will blunt the aggrephagy shift and exacerbate diabetes under NAD+ deficiency.

Experimental Approach

• In vivo: Aged (20‑month) hIAPP transgenic mice receive NR supplementation (400 mg/kg/day) for 8 weeks. Measure NAD+ levels, SIRT1 activity, FAM134B/TBK1 acetylation (immunoprecipitation‑Western), LC3‑II turnover with BafA1, ER‑phagy (GFP‑FAM134B‑LC3 puncta), aggrephagy (p62/IAPP colocalization), ER thickness (TEM), glucose tolerance, and beta‑cell mass.
• In vitro: MIN6 and human EndoC‑βH1 cells treated with palmitate + chronically low glucose to induce ER stress. Manipulate NAD+ (NR, FK866), SIRT1 (EX527, overexpression), and receptor mutants (CRISPR‑knockin). Assess autophagy flux, receptor PTMs, ER morphology, and secreted IAPP oligomers (ELISA).
• Readouts: Seahorse OCR/ECAR, calcium imaging, insulin secretion, and diabetes onset after streptozotocin challenge.

Potential Outcomes and Falsifiability

• If NAD+ boosting restores ER‑phagy and improves glucose tolerance without changing overall autophagy flux, the hypothesis is supported.
• If SIRT1 loss fails to alter FAM134B acetylation or ER morphology, or if acetylation‑resistant FAM134B does not rescue the phenotype, the mechanism is refuted.
• If aggrephagy modulation alone (OPTN mutants) rescues diabetes despite persistent ER expansion, the triage model would need revision, indicating that ER maintenance is not the limiting factor.

This hypothesis directly links a metabolic sensor (NAD+/SIRT1) to the selective autophagy triage described in the seed idea, offering a concrete, falsifiable route to intervene in age‑related beta‑cell decline.