Hypothesis\n\nWe propose that testicular aging is driven by a macrophage‑intrinsic mitochondrial outer membrane permeabilization (miMOMP) event that releases mitochondrial DNA (mtDNA) into the interstitial space. This extracellular mtDNA acts as a damage‑associated molecular pattern (DAMP) that activates the cytosolic cGAS‑STING pathway in neighboring Leydig cells, triggering type‑I interferon signaling that suppresses StAR transcription and cholesterol transport, thereby lowering testosterone production. Concurrently, testosterone deficiency diminishes expression of mitochondrial fusion proteins (MFN1/2, OPA1) in macrophages, increasing their susceptibility to BAX/BAK‑mediated miMOMP and creating a vicious cycle. Thus, the immune system does not merely fail to counteract aging; its mitochondrial dysfunction executes the aging program at the tissue level.\n\nKey Predictions\n1. Aged testes will show increased colocalization of BAX/BAK with mitochondrial membranes in F4/80⁺ macrophages alongside elevated cytosolic mtDNA in the interstitial fluid.\n2. Pharmacological inhibition of cGAS or STING in Leydig cells will rescue StAR expression and testosterone levels in old mice without altering macrophage miMOMP frequency.\n3. Macrophage‑specific deletion of BAX or BAK will reduce interstitial mtDNA, decrease Leydig cell interferon‑stimulated gene expression, and preserve testosterone production despite aging.\n4. Exogenous testosterone supplementation will restore mitochondrial fusion protein levels in macrophages, lowering miMOMP incidence and breaking the feedback loop.\n\nExperimental Approach\n- Model: C57BL/6 mice aged 20–24 months; young controls 3–4 months.\n- Readouts: Flow cytometry for macrophage BAX/BAK mitochondrial localization; ELISA for interstitial mtDNA; qPCR/Western blot for StAR, CYP11A1, IFN‑β, ISG15 in sorted Leydig cells; serum testosterone LC‑MS/MS.\n- Interventions: \n * Treat aged mice with the cGAS inhibitor RU.521 or STING antagonist H‑151.\n * Generate myeloid‑specific Bax/Bak conditional knockout mice (LysM‑Cre; Bax^fl/fl Bak^fl/fl).\n * Implant testosterone pellets in aged wild‑type mice.\n- Expected Outcomes: If the hypothesis is correct, cGAS/STING blockade or macrophage BAX/BAK loss will normalize Leydig cell steroidogenesis and serum testosterone without changing macrophage numbers; testosterone replacement will reduce macrophage miMOMP markers.\n\nFalsification\n- If cGAS/STING inhibition fails to raise testosterone despite reduced interstitial mtDNA, or if macrophage BAX/BAK deletion does not affect Leydig cell signaling, the proposed mtDNA‑cGAS‑STING axis is not central.\n- If testosterone supplementation does not modulate macrophage mitochondrial fusion or miMOMP, the feedback loop is unlikely.\n\nThis framework links mitochondrial quality control in immune cells to endocrine aging, offering a testable mechanism where reprogramming immune mitochondrial dynamics could delay testicular aging ahead of systemic interventions.
