Mechanism of Central Nervous System Leukemia Figure 7. Increased BBB disruption, with a decrease in endothelial ZO-1, claudin-5 and occludin, avoided by GM6001. (A) Confocal imaging of cerebral cortex from normal mice immunostained for ZO-1 localize at vessels, confirming the use of CD31. (B1, B2) Confocal imaging of TJ proteins ZO-1, claudin-5, occludin and fibrinogen in brain parenchyma in control and CNS leukemia mice. (C) Immunoblots of protein extracts from brains of control mice, 4 CNS leukemia mice with brain parenchyma and CNS leukemia mice treated with GM6001. In all figures, brain tissues of normal mice used as control. (D) Model of MMP-2 and -9 secreted by leukemic cells disrupting several key TJ proteins of BMVECs in BBB breakdown. Results shown are from individual animals and are representative of findings from 3 experiments with 4 animals per condition per experiment. (Scale bars: 10 mm). doi:10.1371/journal.pone.0020599.g007 provide new insights into the mechanism of CNS involvement in leukemia. The invasion process of cancer cells involves the secretion of substances to degrade the components of the extracellular matrix, as well as the proteins involved in the control of motility and migration [28]. It is well-known that MMP-2 and -9 play critical roles in solid tumor invasion. For leukemia, the blasts circulating in the bloodstream, must have different invasive characteristics. Our previous study and other evidence implicated abnormal expression of MMP-2 and -9 in AML and their correlation with the extramedullary infiltration [15,16,27]. Reikvam H et al. reported that MMP-9 release was higher in AML cells with monocytic differentiation corresponding to the FAB-subtype M4/M5 AML [29]. Paupert et al. also found that a surface-bound MMP-9 form may contribute to the invasive capacities of monocytic leukemia (M4 and M5) cells in vitro and their extramedullary infiltration including CNS leukemia in vivo [30]. The continuous complexes of TJs combined with reduced vesicular transport make the BBB are PLoS ONE | www.plosone.org 9 August 2011 | Volume 6 | Issue 8 | e20599