SenescenceRewires Autophagic Selectivity in Leydig Cells, Diverting Degradation from NHERF2 to Mitochondrial Cargo and Driving Age-Related Testosterone Decline

The autophagy‑lysosome pathway in Leydig cells does not merely bulk‑degrade cytoplasmic content; it selectively removes NHERF2, a brake on the cholesterol importer SR‑BI, thereby sustaining steroidogenesis. In young cells this selectivity is enforced by LIR‑dependent binding of NHERF2 to p62/SQSTM1, NBR1 and LC3. Aging, however, is associated with heightened ER stress and overall autophagic flux, yet testosterone output falls. We hypothesize that senescence does not diminish autophagy per se but reprograms its substrate hierarchy: stress‑activated kinases phosphorylate NHERF2 on residues within or near its LIR motif, reducing its affinity for canonical autophagy receptors. Consequently, p62 and NBR1 are rerouted toward alternative cargos such as mitochondrial proteins bound by BNIP3/NIX or ER residents recognized by FAM134B. The net effect is a shift from NHERF2‑centric autophagy to mitophagy/ER‑phagy, depleting cholesterol supply without altering total autophagosome numbers.

This mechanistic rewrite predicts three testable outcomes. First, immunoblotting of isolated young versus aged Leydig cells will reveal increased NHERF2 phosphorylation (e.g., at serine/threonine residues predicted by CDK2/AKT motifs) coinciding with reduced NHERF2‑LC3 colocalization and elevated NHERF2 protein levels despite unchanged LC3‑II turnover. Second, pharmacological inhibition of the responsible kinase (e.g., CDK2 inhibitor) or expression of a phospho‑deficient NHERF2 mutant (S/A substitutions) will restore NHERF2 degradation, increase SR‑BI surface expression, rescue cholesterol uptake, and elevate testosterone secretion in senescent Leydig cultures, without affecting proliferation. Third, flux assays using organelle‑specific reporters (mt‑Keima for mitophagy, GFP‑FAM134B for ER‑phagy) will show a selective rise in mitochondrial and ER autophagy in aged cells that is blunted when NHERF2 degradation is rescued, demonstrating a competitive trade‑off for limited autophagic machinery.

Falsification would occur if manipulating NHERF2 phosphorylation or its LIR interaction fails to alter its degradation rate, SR‑BI levels, or testosterone output in aged Leydig cells, indicating that the observed steroidogenic decline stems from a genuine loss of autophagic capacity rather than a selectivity shift. Likewise, if kinase inhibition does not redirect p62/NBR1 away from mitochondrial/ER cargos, the proposed competition model would be unsupported. By focusing on the hierarchy of substrate choice rather than overall autophagic activity, this hypothesis reframes age‑related hypogonadism as a controllable signaling defect, opening avenues for targeted interventions that restore the testicular autophagy triage system.