Non-invasive imaging of metastatic brain tumor-induced GFAP activation. (A) GFAP-luc mice were injected with MDA-MB-231 breast tumor cells (Tumor) or PBS (Control). GFAP activity was monitored for up to four weeks. An exposure-matched, representative non-invasive image of each time point is shown. (B) Quantitative measurement of panel (A). Reactive gliosis was observed three days post-implantation in both tumor and PBS injected animals. Upon tumor growth, GFAP activity increased after 14 day of implantation in animals injected with tumor cells, while only background GFAP activity was observed in animals without tumor. N = 8-9 for each group, Wilcoxon rank sum test, two-sided, * P < 0.05 at day 3,7, and 28, error bar represents standard deviation.