Hypothesis

Aged retinal pigment epithelium (RPE) releases mitochondria-derived extracellular vesicles (EVs) loaded with oxidized mitochondrial DNA (mtDNA). These EVs act as damage‑associated molecular patterns that bind mannose‑binding lectin (MBL) and activate the lectin complement pathway, generating C3 convertase, C3 deposition, and downstream membrane attack complex (MAC) formation. The resulting complement activity amplifies reactive oxygen species (ROS) production, creating a feed‑forward loop that drives RPE dysfunction. Enhancing mitophagy reduces EV release, thereby lowering lectin‑pathway activation and breaking the redox‑complement loop while preserving C3’s essential opsonophagocytic functions.

Rationale

• Oxidative stress can directly activate complement components, leading to C3 deposition and MAC formation that further increase ROS and mitochondrial damage [1].
• In aged mouse retinas, C3 accumulates in the sub‑RPE space by 12 months, marking sites of ongoing injury [2].
• C3‑deficient aged mice show preserved retinal thickness, better electroretinogram b‑wave amplitudes, reduced 4‑HNE, and heightened autophagy (higher LC3B‑II/I, lower p62) [2], indicating that loss of C3 mitigates the deleterious arm of the loop.
• Paradoxically, complete C3 loss worsens pathology in the absence of regulatory proteins (CFH⁻/⁻.C3⁻/⁻ mice exhibit more photoreceptor loss, Bruch’s membrane thickening, and amyloid‑β than CFH⁻/⁻ alone) [3], suggesting C3 retains protective roles such as debris clearance and opsonization when regulation fails.
• Pharmacologic complement modulation restores lysosomal function in RPE models of age‑related macular degeneration [4], implying that targeted intervention can uncouple harmful from beneficial complement activities.
• Systemic complement biomarkers rise with age and correlate more strongly with dementia than age alone [5], supporting the idea that local production (e.g., from RPE‑derived EVs) may drive retinal pathology independently of systemic inflammation.

These observations lead to the mechanistic proposal that mtDNA‑laden EVs are the missing link between oxidative stress and complement activation. Oxidized mtDNA within EVs is known to bind MBL with high affinity, triggering MASP‑2–mediated cleavage of C4 and C2, forming the C3 convertase of the lectin pathway. This route explains why ROS can amplify complement without requiring direct covalent modification of C3 and why C3 deficiency alone is protective but becomes detrimental when opsonization is lost.

Predictions

1. Aged RPE will release EVs enriched in oxidized mtDNA that colocalize with MBL deposits in the sub‑RPE space.
2. Pharmacologic or genetic enhancement of mitophagy (e.g., via Urolithin A, NAD⁺ boosters, or overexpression of PINK1/Parkin) will reduce EV secretion, decrease MBL‑MASP‑2 activation, lower C3 deposition and MAC formation, and diminish ROS levels.
3. Mitophagy‑enhanced eyes will retain C3‑mediated opsonization of apoptotic debris (measured by C3‑phagosome colocalization) while showing improved autophagic flux (LC3B‑II/I increase, p62 decrease).
4. In CFH⁻/⁻ mice, mitophagy enhancement will attenuate photoreceptor loss and Bruch’s membrane thickening without exacerbating the phenotype seen in CFH⁻/⁻.C3⁻/⁻ mice, indicating preservation of C3’s protective functions.
5. Blocking the lectin pathway (using anti‑MBL antibodies or MASP‑2 inhibitors) will phenocopy the benefits of mitophagy enhancement, confirming the pathway’s central role.

Experimental Approach

• EV isolation: Collect vitreous and sub‑RPE fluid from young (3 mo) and aged (18 mo) WT mice; characterize EV size, mtDNA content (qPCR for oxidative lesions), and oxidized mtDNA immunostaining (8‑OG).
• Complement activation assays: Stain retinal sections for MBL, MASP‑2, C3b, and MAC (C5b‑9); quantify colocalization with EV markers (CD63, TOMM20).
• Mitophagy modulation: Treat aged mice with Urolithin A (10 mg/kg/day) or vehicle for 3 months; include genetic models with RPE‑specific PINK1 overexpression.
• Outcome measures: Assess retinal thickness (OCT), ERG b‑wave amplitudes, oxidative stress (4‑HNE, MitoSOX), autophagy markers (LC3B‑II/I, p62), and functional vision (optomotor tracking).
• Controls: Use C3‑deficient and CFH⁻/⁻ mice to dissect dependency on C3 and regulation; apply anti‑MBL or MASP‑2 inhibitor to test lectin‑pathway necessity.

If mitophagy enhancement reduces EV‑driven lectin complement activation, lowers C3 deposition and MAC, yet preserves C3‑dependent opsonization and improves retinal health, the hypothesis will be supported. Conversely, if EV levels remain high or complement activation persists despite mitophagy boost, the hypothesis will be refuted, prompting exploration of alternative DAMPs or activation routes.