Background and Purpose
Thyroid hormones play important roles in normal brain maturation and normal brain function. T4, which is a major secretory product of the thyroid gland, needs to be converted to T3 by iodothyronine deiodinase to exert its biological activity. Type 1 iodothyronine deiodinase (D1) activity is present in thyroid gland, liver, and kidney, whereas type 2 iodothyronine deiodinase (D2) activity is present in brain, anterior pituitary, brown fat, and pineal gland. While D1 activity is known to decrease in the hypothyroid state and mainly contributes to the circulating T3 level, D2 activity increases in the hypothyroid state and plays a pivotal role in providing local intracellular T3. D2, therefore, plays an important role to maintain local intracellular T3 concentration in brain. Although D2 is inactivated by ubiquitin-proteasome system at posttranslational level, it is not known whether D2 expression is altered after cerebral ischemia that is known to affect ubiquitin-proteasome system. In the present study, we have evaluated the effect of focal cerebral ischemia on D2 mRNA and D2 activity in the rat.
Methods
Permanent middle cerebral artery occlusion was employed in halothane anesthetized adult Sprague Dawley rats (Tamura model). Rats (n=30) were decapitated at 24 hours, 3 days and 7 days after ischemia. Sections of 20 μm thickness were cut on a cryostat and were stained with hematoxylin and eosin, and examined by a light microscope. These sections were examined by in situ hybridization of D2 mRNA using digoxigenin-labeled antisense cRNA probe for rat D2. D2 activity was measured by the release of I- from 2 nM [125I] T4 in the presence of 20 mM DTT and 1 mM PTU in dissected tissues such as ipsilateral remote cortex and contralateral cortex.
Results
D2 mRNA was ubiquitously expressed in the brain, and was expressed in neurons and astrocytes, confirmed by immunostaining with the neuronal marker and astrocytic marker, NeuN and GFAP, respectively. Although D2 mRNA expression was markedly decreased in ischemic core cortex, D2 mRNA expression did not altered neithor in remote cortex of ipsilateral cortex nor contralateral cortex at 24 hours, 3 days and 7 days after MCA occlusion. In contrast, D2 activity in ipsilateral cortex was significantly increased compared with contralateral cortex at 3 days after MCA occlusion (p<0.05), but not at 24 hours and 7 days after ischemia (Figure 1).
D2 activity in ipsilateral cortex was compared with contralateral
Conclusions
The present results demonstrated that D2 activity, but not D2 mRNA, in ipsilateral cortex was significantly increased after focal cerebral ischemia. It is suggested that focal cerebral ischemia may affect D2 degradation by modulating ubiquitin-proteasome system at posttranslational level. The present study suggests that local intracellular T3 production by D2 may be involved in the pathophysiology of focal cerebral ischemia.