Hypothesis\nMicroglial activation in the aging retina leads to the selective packaging of newly synthesized C3 into extracellular vesicles (EVs) that are released at sites of synaptic stress. These EV‑bound C3b molecules efficiently form the membrane attack complex (MAC) on neighboring photoreceptors, whereas soluble C3a generated from circulating or locally secreted C3 continues to engage C3aR/C5aR receptors that support retinal integrity. Thus, spatially restricted EV‑mediated complement activation drives lesion expansion, while diffuse C3aR signaling maintains homeostasis.\n\n## Rationale\nRecent work shows that retinal microglia and macrophages synthesize C3 locally, and that genetic C3 ablation reduces MAC‑mediated photoreceptor loss in rodent models [1]. However, complete C3 deficiency worsens retinal function, indicating a protective role for C3aR/C5aR signaling [3,4]. The alternative pathway dominates in aged retina, with declining regulators and elevated Factor D amplifying C3 convertase activity [2]. If microglial C3 is chiefly secreted via EVs, then inhibiting EV release would attenuate MAC formation without diminishing soluble C3a levels, preserving the beneficial C3aR axis while blocking pathogenic MAC deposition.\n\n## Testable Predictions\n1. In aged mouse models of AMD, microglial EVs isolated from retinal supernatants will contain higher C3b levels than EVs from young controls.\n2. Pharmacological blockade of EV release (e.g., using GW4869 to inhibit neutral sphingomyelinase‑2) will reduce C3b deposition and MAC (C5b-9) formation in the retina, as measured by immunofluorescence and western blot, without lowering retinal C3a concentrations.\n3. Mice treated with EV‑release inhibitors will exhibit slower lesion expansion and preserved photoreceptor nuclei counts compared with vehicle‑treated counterparts, yet will retain normal electroretinogram (ERG) responses that depend on C3aR signaling.\n4. Conversely, genetic deletion of C3aR will abolish the protective ERG preservation seen with EV inhibition, demonstrating that the beneficial effect requires intact C3aR signaling.\n\n## Experimental Approach\n- Use Cx3cr1‑CreER; Rosa26‑tdTomato mice to label microglia and isolate retinal EVs by differential ultracentrifugation at 6, 12, and 18 months of age.\n- Quantify EV‑associated C3 and C3b by ELISA and mass spectrometry; compare with soluble retinal C3a levels.\n- Treat a subset of aged mice with GW4869 (50 mg/kg i.p., three times weekly) for 8 weeks; controls receive vehicle.\n- Assess MAC deposition via anti‑C5b-9 immunostaining, quantify lesion area in flat‑mounts, and measure outer nuclear layer thickness.\n- Measure retinal C3a concentration by ELISA and ERG amplitudes to gauge functional preservation.\n- In parallel, generate Cx3cr1‑CreER; C3arflox/flox mice to delete C3aR specifically in microglia and repeat EV‑inhibition experiments.\n\n## Potential Impact\nIf validated, this hypothesis reframes complement dysregulation in AMD as a vesicle‑mediated, spatially constrained process that can be uncoupled from the homeostatic C3aR axis. It suggests therapeutic strategies targeting microglial EV release or EV‑associated C3b could inhibit MAC‑driven photoreceptor loss while preserving the essential protective signaling that prevents broader retinal degeneration.
