Selective autophagyreceptor hierarchy determines zona reticularis mitochondrial turnover and drives age-related DHEA decline

Central Hypothesis

In the aging adrenal cortex zona reticularis (ZR) cells undergo a stereotyped receptor ordered autophagy program that preferentially digests mitochondria and steroidogenic enzymes whereas zona fasciculata (ZF) cells engage a protective autophagy profile 12. The hierarchy is set by the oligomerization state of p62/SQSTM1 and its competing receptors NIX/BNIP3L and OPTN which dictate which cargo is ubiquitinated and delivered to the lysosome 35. When this triad is skewed toward mitochondrial removal in ZR DHEA synthesis collapses while parallel preservation of cortisol production in ZF reflects divergent receptor usage.

Mechanistic Rationale

1. Autophagy receptor hierarchy – p62 forms ROS sensitive oligomers that affinity bind ubiquitinated mitochondria via its UBAN domain while NIX directly binds LC3 on depolarized mitochondria 34. In young ZR basal p62 activity maintains mitochondrial quality; with age cumulative ROS shifts p62 toward hyper oligomerization increasing mitochondrial consumption beyond biogenesis capacity.
2. Zone specific signaling – ZR experiences lower intra adrenal oxygen tension and higher exposure to DHEA derived androgen metabolites which can inhibit mTORC1 and activate AMPK priming the p62 NIX axis 6. ZF bathed in higher ACTH driven cAMP PKA signaling sustains TFEB mediated lysosomal biogenesis and favors p62 dependent clearance of protein aggregates rather than mitochondria.
3. Consequence – Preferential mitochondrial loss reduces acetyl CoA and NADPH supply needed for cytochrome P450scc (CYP11A1) and CYP17A1 activity directly limiting pregnenolone and DHEA output. Meanwhile ZF preserves cortisol synthesis because its steroidogenic enzymes are less mitochondrion dependent or are supplemented by increased mitochondrial biogenesis via PPARgamma coactivator 1 alpha (PGC 1 alpha).

Testable Predictions

• Prediction 1: Aged murine ZR will show increased p62 oligomer size detected by crosslinking SDS PAGE and heightened NIX binding to mitochondria compared with young ZR; ZF will display unchanged or decreased oligomerization.
• Prediction 2: Pharmacological disruption of p62 oligomerization using the small module inhibitor described in 5 in aged ZR explants will restore mitochondrial mass elevate CYP17A1 activity and increase DHEA secretion without affecting cortisol output from ZF.
• Prediction 3: ZR specific knockdown of NIX will blunt age related mitochondrial loss and rescue DHEA levels whereas NIX overexpression in young ZR will phenocopy aging.
• Prediction 4: Conversely enhancing TFEB activity in ZF will not alter cortisol levels but will protect against age related fibrosis confirming zone specific autophagy routing.

Falsification

If manipulation of p62 oligomerization or NIX in ZR fails to modify mitochondrial content steroidogenic enzyme activity or DHEA release in aged adrenal cultures the hypothesis that a selective autophagy receptor hierarchy drives ZR senescence is falsified. Similarly if DHEA decline persists despite restored mitochondrial mass alternative mechanisms must dominate.

Experimental Approach

• Isolate primary ZR and ZF cells from young (3 mo) and aged (24 mo) mice using FACS based on CYP11B1 (ZF) and HSD3B2 (ZR) reporters.
• Measure p62 oligomerization using DSS crosslinking followed by Western blot mitochondrial membrane potential with TMRM and LC3 II flux after bafilomycin A1 chase.
• Assess steroidogenesis via LC MS MS for DHEA and cortisol.
• Intervene with p62 oligomerization inhibitor NIX siRNA or TFEB activator (e.g. trehalose) and repeat readouts.

Implications

Confirming a receptor ordered autophagy triage would reframe adrenal aging from generic oxidative damage to a programmable selectivity opening therapeutic windows to preserve adrenal androgen output by fine tuning autophagy receptor dynamics rather than globally suppressing autophagy.