Caspase-related Apoptosis in Chronic Ischaemic Microangiopathy Following Experimental Vein Occlusion in Mini-pigs

Purpose: Acute brain ischaemia (stroke) causes a central area of coagulation necrosis. Peripheral to it and after a few hours, apoptosis causes neurons throughout the entire area to die progressively. However, this sequence of events is related to the reperfusion of regenerated capillaries or collateral circulation, and is considered to be potentially salvageable. Similar findings have been reported in the retina after ischaemia-reperfusion injury in rats. In the present study, we intended to investigate whether delayed cell death is involved in neuronal injuries to the inner retina during chronic retinal ischaemia.

Methods: Experimental branch retinal vein occlusion (BRVO) was induced in miniature pigs using indirect argon laser. The eyes were prelevated at 4, 24 and 48 hours and at 1 and 3 weeks following BRVO. The caspase inhibitor Z-VAD was injected intravitreally 24 hours after BRVO. Affected retinas were examined 24 hours later for any protective effect from apoptotic cell death. Histological examination with cresyl violet staining and TUNEL (TdT-mediated dUTP-biotin nick-end labelling) was performed on the samples.

Results: A progressive oedema of the nerve fibre, ganglion cell and inner plexiform layers, related to a widely diffused cell necrosis, was observed in the affected territory within 4-24 hours after BRVO. This was followed by a wave of apoptosis localized at the periphery of the affected territory, which peaked approximately 48 hours after BRVO and was associated with a diffuse oedema of the inner nuclear layer. A progressive atrophy of the inner retina was observed 1-3 weeks after BRVO. Injection of the caspase inhibitor Z-VAD (24 hours after BRVO) decreased the amount of apoptotic cell bodies 48 hours after BRVO.

Conclusions: This study shows that although necrosis is the predominant form of neuronal death in the early phase, massive delayed neuronal cell death caused by apoptosis occurs on a widespread basis as a result of chronic ischaemia after BRVO in the retina. Further studies are needed to evaluate the possibility of rescuing retinal neurons from death by neuroprotective treatments.