Inducible repair of oxidative DNA lesions in the rat brain after transient focal ischemia and reperfusion

Ischemic neuronal cell death and organellae damage

The brain is an organ that consumes much energy. This is partially due to the character of neurons; they possess excitable plasma membrane and a large amount of ATP is indispensable for maintaining ion gradient. Once neurons experience energy failure, calcium accumulates in the intracellular space as a result of disturbed ion homeostasis. This, in turn, activates many cellular processes, which culminate in cell death. In this cellular catastrophic cascade, many organelles play important roles. In addition to the plasma membrane, cytosol is the 'organelle' that first becomes exposed to the increased level of calcium. Many proteases, kinases and lipases are localized here, and are activated directly or indirectly by the ischemic insult. Some enzymes are pro-apoptotic ones, while others are anti-apoptotic. It was reported that neurons that would die later showed activated pro-apoptotic enzymes, but ones that would survive possessed activated anti-apoptotic molecules. Mitochondria is the organelle that plays the central role for intrinsic pathways of apoptosis. The release of cytochrome c from this organelle is the key step in apoptotic cascade in the ischemic neurons. However, the exact molecular mechanism of cytochrome c release remains uncertain. In addition, expression of genes essential for mitochondrial function changes in neurons after ischemia, which further indicates the crucial role of this organelle in cell death. Endoplasmic reticulum (ER) not only mediates proteins processing, but also regulates intracellular calcium homeostasis and cell death signal activation. Recent reports indicate that dysfunction of this organelle occurs at an early stage after ischemia and might be the initial step of apoptotic cascades in neurons. Golgi apparatus and lysosomes are organelles that are involved in apoptotic cell death in some situations. There have been no reports that demonstrated active role of these organelles in ischemic neuronal cell death. Further investigation would be desired about this issue. Nucleus is the organelle that contains genomic DNA. Many studies demonstrated DNA breakage in the neurons that would die later, but whether this is the cause or merely the result of the insult remains uncertain. If the more precise role of each organelle in neuronal cell death are disclosed, we should be able to think about new means of therapy for ischemic stroke.

Folate deficiency increases postischemic brain injury

BACKGROUND AND PURPOSE

Folate deficiency and resultant hyperhomocysteinemia impair vascular function and increase stroke risk. We tested the hypothesis that folate deficiency and high homocysteine levels promote DNA damage and increase brain injury after cerebral ischemia/reperfusion.

METHODS

129/Sv mice, uracil-DNA glycosylase-deficient (Ung-/-) mice, and Ung+/+ littermate mice were exposed to a folate-deficient diet for 3 months and then subjected to 30-minute middle cerebral artery (MCA) occlusion and reperfusion. Plasma homocysteine levels and physiological parameters were measured in selected animals. Outcome measures were neurological sensorimotor deficits, infarct size measured by computer-assisted volumetry, and oxidative DNA damage measured by a colorimetric assay.

RESULTS

Exposure to a folate-deficient diet for 3 months conferred approximately 6- to 10-fold higher plasma homocysteine levels than those associated with a normal diet. Cerebral lesion volumes and neurological deficits after MCA occlusion and 72-hour reperfusion were significantly 2.1-fold increased in folate-deficient 129/SV wild-type mice compared with those associated with a normal diet, which could not be explained by obvious differences in physiological parameters. Abasic sites, hallmarks of oxidative DNA damage, were significantly increased in DNA from the ischemic brain of folate-deficient animals at early time points after MCA occlusion. Folate deficiency further increased brain lesion size in animals lacking uracil-DNA glycosylase compared with wild-type littermate mice.

CONCLUSIONS

Folate deficiency and resultant hyperhomocysteinemia are not only associated with increased stroke risk but increase oxidative DNA damage and ischemic lesion size after MCA occlusion/reperfusion.