Abstract 116: Matrix Metalloproteinase-9 Deletion Attenuates Blood-Brain Barrier Disruption and Consequent Injury in White Matter After Subarachnoid Hemorrhage

Abstract

Objective: The detailed mechanism of white matter injury after subarachnoid hemorrhage (SAH) remains unclear. We hypothesized that blood-brain barrier (BBB) disruption occurs in white matter after SAH and it leads to consequent injury. In this study, we investigated the potential role of matrix metalloproteinase-9 (MMP9) in SAH-induced white matter injury. Methods: SAH was induced by endovascular perforation in male wild-type (WT) C57BL/6 and MMP9 knockout (MMP9-/-) mice. The following three experiments were devised: 1) to determine BBB disruption and MMP9 activation/expression in white matter, mice underwent MRI at 24 h after SAH. They were then euthanized for immunostaining, transmission electron microscopy and MMP zymography; 2) to investigate the role of MMP9 in BBB disruption, lesion volumes on MRI were compared between WT and MMP9-/- mice at 24 h after SAH; 3) WT and MMP9-/- mice underwent MRI at 1 and 8 days after SAH induction to detect time-dependent changes, and were euthanized at 8 days after SAH. Removed brains were used to investigate myelin integrity and axonal damage in the white matter. Results: In WT mice with SAH, albumin leakage along the white matter was increased (p<0.05; vs. sham) and was consistent with T2-hyperintensity on MRI (r=0.97, p<0.01). In microvessels, tight junction detachment and swollen astrocyte endfeet were demonstrated by electron microscopy. MMP9 activity was elevated in white matter at 24 h after SAH (vs. sham; p<0.05), and MMP9 was mainly expressed in astrocytes and oligodendrocyte precursors. The volume of T2-hyperintensity in the white matter in WT mice with SAH was 6.2 ± 3.0 mm3 (p<0.001; vs. WT sham), while that in MMP9-/- mice with SAH was significantly smaller (1.0 ± 1.4 mm3; p<0.01). At 8 days after SAH, decreased myelin integrity and an increased density of damaged axons were observed in WT with SAH mice compared to normal control mice (p<0.05; for each), while T2-hyperintensity on MRI in the white matter almost disappeared. MMP9-/- mice developed less of those injuries than WT mice at 8 days after SAH (p<0.05; for each). Conclusions: SAH causes BBB disruption and consequent injury in the white matter, and MMP9 plays an important role for those pathologies. MMP9 could be a therapeutic target for SAH-induced white matter injury.