Methods br Results First we compared A
Results First, we compared Aβ plaque number, Aβ concentration, and neurobehavior functions between naive APP/PS1 and APP/PS1xEP1−/− mice. Significantly lower Aβ burden in APP/PS1xEP1−/− mice was evidenced by fewer plaques in the hippocampal area and a lower concentration of soluble Aβ42 and Aβ40 (Fig 1A). The reduced plaque load and Aβ levels may explain the benefits seen in memory function in APP/PS1xEP1−/− mice (Fig. 1A). To ascertain whether the Aβ-mediated cell death mechanism is different in the absence of EP1, we exposed hippocampal slice cultures and neuronal cell cultures to Aβ42. The CA1 regions in slice cultures and the cultured neurons from EP1−/− mice each were significantly more resistant to Aβ42 toxicity than were the corresponding WT controls (Fig. 1B). We then performed Ca2+ imaging in neuronal cell cultures to explore the potential mechanism by which EP1 receptor inhibition improves neuronal survival during Aβ-induced toxicity. The [Ca2+]i and Ca2+ transients were increased after Aβ42 treatment in WT controls; this increase was prevented by pretreatment with an EP1 receptor antagonist (SC51089) and blocked in EP1−/− neurons (Fig. 1C). We then asked whether the reduced Aβ burden in APP/PS1xEP1−/− mice led to reduced injury development after cerebral ischemia. We approached this question in vitro by exposing cultured hippocampal slices from each of the mouse genotypes to OGD. Cell death was significantly decreased in EP1−/− tissue (p < 0.001) and increased in the APP/PS1 tissue compared with WT controls (p < 0.01) but was significantly attenuated in the APP/PS1xEP1−/− tissue compared with APP/PS1 tissue (p < 0.001; Fig. 2A). To correlate our results with in vivo experiments, we assessed delayed hippocampal neuronal damage by subjecting mice to global sigma 1 receptor ischemia. Similar to the in vitro findings, CA1 neuronal loss was much greater in the APP/PS1 than in the WT mice and was significantly attenuated in the APP/PS1xEP1−/− mice compared with the APP/PS1 mice (Fig. 2B). Hippocampal damage caused by ischemic insult can lead to impaired memory retention, reflected in shortened response latency in the passive avoidance task (Sakurai et al., 2008). Correlating with the anatomical differences, APP/PS1 mice had impaired memory retention after ischemic insult compared with WT mice subjected to ischemia and sham-operated APP/PS1 mice. There was no statistical difference between the APP/PS1xEP1−/− and APP/PS1 ischemia groups, but the APP/PS1xEP1−/− mice tended to show greater memory retention (Fig. 2C). We also questioned whether Aβ burden increases after ischemic insult in APP/PS1 mice and if so, whether this increase is attenuated in the absence of the EP1 receptor. We observed less intense Aβ immunostaining in the CA1 neurons of APP/PS1xEP1−/− mice after ischemia compared with that of APP/PS1 mice (Fig. 2D). The enzyme-linked immunosorbent assay (ELISA) showed that the insoluble Aβ42 concentration rose significantly after ischemia in APP/PS1 mice but remained unchanged in APP/PS1xEP1−/− mice. Both the soluble and insoluble Aβ concentrations were higher in APP/PS1 mice than in APP/PS1xEP1−/− mice postischemia (Fig. 2E). The increase of insoluble Aβ in APP/PS1 mice suggests a tendency for soluble Aβ to accumulate and potentially fibrillate after ischemia. Deletion of EP1 appeared to attenuate the increase of Aβ generation and accumulation.
Discussion We observed that after transient global forebrain ischemia, APP/PS1 double mutant mice sustain more severe neuronal and functional damage than WT controls. This suggests that the Aβ burden is at least partially responsible for such outcomes. Knowing that brain levels of both COX-2 and PGE2 are increased in AD and in stroke, we questioned whether the deletion of the EP1 receptor—which has been proposed to be a key modulator of COX-2/PGE2-related neurotoxicity—would be sufficient to alleviate most of the negative outcomes. In our study, we provide the first evidence that genetic deletion of EP1 is sufficient to decrease basal and postischemic levels of Aβ in APP/PS1 mice. We discovered that hippocampal neuron cell death and neurobehavioral deficits after ischemic insult are attenuated in the APP/PS1−/−xEP1 as compared with APP/PS1 mice. Isolated neurons and brain slice cultures showed that neurons derived from EP1−/− mice were more resistant to Aβ toxicity than those from WT mice. Neuronal Ca2+ signaling and Ca2+ homeostasis regulate multiple neuronal functions, including synaptic transmission, plasticity, and cell survival (Wojda et al., 2008). EP1 receptor inhibition prevented induction of Ca2+ signaling initiated by Aβ exposure by decreasing the frequency of Ca2+ transients and mitigating the increase in baseline Ca2+ levels. Prevention of Aβ-associated Ca2+ signaling in neurons by EP1 receptor inhibition is likely an additional pathway for the neuroprotective effects seen. Our study provides the rationale to further explore potential therapeutic agents targeting EP1 receptor in stroke and AD-like neuropathologies.