t-PA requires a cell surface receptor to modulate BBB permeability. (A) t-PA (250nM) was added to the luminal chamber of the BBB either alone or in combination with 2.5μM of ct-PA, and permeability was assessed after 4 hours. As shown, ct-PA inhibits the ability of active t-PA to increase BBB permeability (n = 4; ***P < .001). (B) t-PA (250nM) or the LDLR blocker RAP (500nM) was added either alone or in combination to the luminal chamber of the BBB, and permeability was assessed after 24 hours. The presence of RAP significantly inhibits t-PA–mediated BBB injury (n = 4-6; **P < .01). (C-D) t-PA (250nM; C) or t-PA (25nM) and Plgn (50nM; D) were added to the luminal chamber of the BBB either alone or in combination with the lysine analog TXA (10mM, added to the luminal and abluminal chambers), and permeability was assessed after 24 hours. As shown, the presence of TXA effectively blocks the ability of t-PA alone or on combination with Plgn to enhance BBB permeability. In panels C-D, n = 3 (***P < .001). In all panels, bars represent mean ± SEM.