SPP1 and ischemic stroke: In the normal brain, OPN expression is weak, but under pathological conditions, it is markedly upregulated in microglia and astrocytes.7 In particular, delayed but significant induction of OPN has been reported in ischemic stroke; for example, in a rat model of transient forebrain ischemia and in a mouse model of permanent focal ischemia, OPN induction began at 12 h after injury and peaked at 5 days, respectively.8, 9