Abstract 76: Astrocyte Tissue Factor: Double-Edged Sword of Brain Hemostasis

Abstract

Astrocytes in the brain express high levels of tissue factor (TF) presumably to prevent hemorrhage. We generated mice lacking TF gene in all neuronal cells (TFflox nestin Cre), including astrocytes. TF mRNA expression (99.5%) and total procoagulant activity (98.7%) were significantly reduced in the brains of TFflox nestin Cre mice compared to controls (TFflox). Immunostaining demonstrated the absence of TF expression on astrocytes. No spontaneous bleeding was observed in TFflox nestin Cre mice. To investigate the role of astrocyte TF in brain hemostasis we used mouse models of ischemic and hemorrhagic stroke. Ischemic stroke was induced by 1 hour occlusion of middle cerebral artery followed by 24 hours of reperfusion. Intracerebral bleeding was observed in 72.7% of TFflox nestin Cre but not in TFflox mice subjected to ischemic stroke. Although TFflox nestin Cre mice exhibited more bleeding, they had reduced infarct size (15.8±5.4% vs 35.1±6.9%; p<0.01, mean±SD). A reduction of infarct size was also observed in mice expressing low levels of TF in all tissues (low TF mice, 18.7±6.8%) compared to wild type (WT, 29.3±6.5%; p<0.05) mice; however, no intracerebral bleeding was observed in low TF mice. In the hemorrhagic stroke model, intracerebral administration of collagenase induced time and dose dependent bleeding in the brains of WT mice. Deletion of TF from astrocytes dramatically increases intracerebral bleeding (2.9-fold vs TFflox, p<0.01) and death (57.1%, p<0.01) 3 hours after cerebral injection of 0.025 units of collagenase. Less severe intracerebral bleeding (1.5-fold vs WT, p<0.05) and no death was observed in low TF mice. Importantly, astrocytes isolated from low TF mice had higher procoagulant activity compared to astrocytes from TFflox nestin Cre mice. Finally, using intravital microscopy and photoactivation-induced thrombosis model, we showed that thrombosis within brain capillaries, but not arterioles or venules, was significantly attenuated (p<0.05) in TFflox nestin Cre compared to TFflox mice (consistent with direct contact of astrocyte endfeet with endothelium only within capillaries). Our data indicate that astrocyte TF is essential for brain hemostasis. However, it also contributes to microvascular thrombosis and neuronal damage.