AggregatedCYP17A1 as a Proteostatic Reservoir that Sustains Zona Reticularis DHEA Synthesis in Aging

Hypothesis

In the aged zona reticularis, protein aggregates are not inert waste but a dynamic reservoir that sequesters misfolded steroidogenic enzymes (e.g., CYP17A1, 3β‑HSD) into a thermodynamically stable state. This sequestration prevents soluble oligomers from inhibiting residual activity while preserving a pool of enzyme that can be refolded when proteostatic capacity is transiently boosted. Thus, aggregates function as a buffered depot that sustains DHEA output until stress‑responsive chaperones or disaggregases are recruited.

Mechanistic Basis

• Chronic oxidative stress in the zona reticularis promotes partial unfolding of CYP17A1, generating aggregation‑prone intermediates.
• When chaperone (Hsp70, Hsp90) and proteasome activities decline, the cell redirects these intermediates into amyloid‑like aggregates via small heat shock proteins (HspB1, HspB5) that act as nucleators.
• The aggregated conformation buries exposed hydrophobic surfaces, reducing soluble oligomer toxicity.
• Localized ATP‑dependent disaggregase complexes (Hsp110‑Hsp70‑Hsp40) can transiently associate with aggregates, pulling out individual monomers for refolding without fully dissolving the depot.
• This cycle creates a steady‑state where a fraction of enzyme remains active, another fraction is safely stored, and a third fraction is being repaired.

Testable Predictions

1. Pharmacological enhancement of disaggregase activity (e.g., with Hsp110 inducers) will increase soluble CYP17A1 levels and DHEA secretion in aged adrenal slices, but only if aggregate turnover is not overwhelmed.
2. Genetic knock‑down of small heat shock proteins that promote aggregation will reduce aggregate burden, raise soluble misfolded enzyme, and lower DHEA output due to increased oligomer‑mediated inhibition.
3. Measuring the ratio of insoluble to soluble CYP17A1 across young, middle‑aged, and old adrenal cortex will show a biphasic pattern: low insoluble in youth, peak insoluble in mid‑life correlating with maximal DHEA, and declining insoluble in very old tissue coinciding with functional collapse.

Experimental Design

• Obtain human adrenal cortex samples (or aged mouse zona reticularis) stratified by age.
• Perform sequential extraction: soluble fraction (Triton X‑100), insoluble fraction (SDS/urea) and western blot for CYP17A1.
• Quantify DHEA secretion ex vivo after stimulation with ACTH.
• Manipulate disaggregase activity: treat slices with Hsp110 overexpressing adenovirus or with a small‑molecule Hsp70 activator (e.g., YM‑08).
• Measure changes in soluble CYP17A1, insoluble CYP17A1, and DHEA.
• Parallel assays: filter‑trap for oligomers, thioflavin‑T staining for amyloid‑like aggregates, and activity staining for 3β‑HSD to confirm enzyme specificity.
• Statistical analysis using two‑way ANOVA (age × treatment) with post‑hoc tests.

If disaggregation raises soluble enzyme but lowers DHEA, aggregates are protective. If disaggregation raises both soluble enzyme and DHEA, aggregates are detrimental. Either outcome falsifies one side of the hypothesis.