Role of cyclophilin D in hypoxia-ischemia induced brain injury during development

OBJECTIVE: Mitochondria play a critical role in mediating both apoptotic and necrotic cell death. Cyclophilin D (CyD), a component of mitochondria membrane permeability transition pore (PTP) is important in Ca 2+ induced membrane permeability and necrotic cell death trigged by ischemia/reperfusion injury in adult brain and heart (1-4), whereas the role of CyD in hypoxic-ischemic (HI) injury in the immature brain is unknown. Caspase-dependent and non-dependent apoptotic mechanisms appear to be more critical in neonatal vs. adult brain (Zhu et al .2005) and we hypothesized that CyD may play a different role in the developing brain. We crossed CyD deficient mice (KO) and wild-type (WT) mice to produce heterozygotes (Het), which were bred further to produce littermate animals with mixed genotypes. METHODS: At postnatal day (PND) 9 or 60, littermates of CyD KO, WT and Het were subjected to left carotid artery ligation and 10% O2 for 40 minutes or 30 minutes, respectively. Mice were sacrificed at 7 days after HI. Brain sections were stained with micro tube protein-2 (MAP-2) and the infarct volume and the total tissue loss was calculated. RESULTS: 1) At PND9, brain injury was more prononuced in CyD deficient mice than in WT mice. The total tissue loss was 20.6 ± 3.5 mm3 in CyD KO mice (n=19) vs 10.5 ± 1.7 mm3 in WT mice (n=16) (p=0.017). Regional neuropathology score analysis showed that injury in cortex, hippocampus, thalamus and striatum were 59%, 17%, 53% and 31% lower in WT compared to KO. 2) In PND60 adult brain, CyD deficiency resulted in a significantly reduced HI brain injury. The total tissue loss in PND60 mice was 2.3 ± 1.9 mm3 in CyD KO mice (n=13) vs 14.3 ± 4.8 mm3 in WT mice (n=18) (p=0.035). Brain injury was reduced in KO vs. WT by 68% in cortex, 50% in hippocampus, 90% in thalamus and 73% in striatum. CONCLUSIONS: 1) CyD-dependent PTP appears to play a different role in neontal and adult HI brain injury. 2) The present results might indicate that the known pro- and antiapoptotic actions of CyD do not have the same weight in immature as in the adult brain. These results agree with previous studies suggesting that cell death mechanisms especially those related to the apoptotic machinery are different in the developing CNS (5, 6) (7).