Expression of MMP-12 after neonatal hypoxic-ischemic brain injury in mice

Matrix metalloproteinase-12 (MMP-12) is expressed in the brain and is important for myelin formation in the developing brain, while MMP-12 deficiency protects against spinal cord injury in the adult, suggesting a role for MMP-12 after brain injury. However, the role of MMP-12 in neonatal hypoxic-ischemic brain injury is not known. The purpose of this study was to investigate the expression of MMP-12 in the brain after neonatal hypoxia-ischemia (HI). HI was induced by unilateral ligation of the left carotid artery followed by hypoxia (10% O(2), 36 degrees C) for 50 min in postnatal day 9 C57/Bl6J mice. At 24 and 72 h after HI, the mRNA and protein expression of MMP-12 were measured by real-time PCR and Western blot, respectively. Distribution and cellular expression of MMP-12 were examined by immunohistochemistry. At 72 h after HI, both MMP-12 mRNA and protein expression were significantly increased in the ipsilateral hemisphere compared to the contralateral hemisphere and sham-operated animals (p < 0.05). The extent of tissue loss in the ipsilateral hemisphere at 72 h after HI was positively correlated with the degree of MMP-12 protein expression (r = 0.900, p < 0.05). In nonischemic animals, immunohistochemical analysis demonstrated weak cytoplasmic MMP-12 immunoreactivity throughout the brain in cells which resembled neurons and stronger immunoreactivity was observed in blood vessels. After HI, the MMP-12 staining became more prominent in the ipsilateral hemisphere and immunohistochemical double-labeling experiments identified the MMP-12-positive cells as neurons, isolectin and Olig2-positive cells. This study demonstrates that MMP-12 is upregulated after HI, suggesting that MMP-12 may contribute to injury in the immature brain, similar to that seen in the adult.