Activation of extracellular signal-regulated kinase 5 may play a neuroprotective role in hippocampal CA3/DG region after cerebral ischemia.

Extracellular signal-regulated kinase 5 (ERK5), the newest member of the mitogen-activated protein (MAP) kinase family of proteins, is widely expressed in many tissues, including the brain. Here we investigated the activation and subcellular localization of ERK5 by immunoblotting and immunohistochemistry as well as its potential role following cerebral ischemia in rat hippocampus. Transient cerebral ischemia was induced by the four-vessel occlusion method in Sprague-Dawley rats. Our results first indicated that the strongly activated ERK5 immunoreactivity was seen in the CA3/DG region but not in the CA1 pyramidal cell of rat hippocampus following reperfusion. In cytosol extracts, ERK5 activation was rapidly increased, with a peak at 30 min, and then gradually decreased to basal level at 3 days of reperfusion. In nucleus extracts, both phospho-ERK5 and its protein expression were persistently enhanced during the later reperfusion period (from 6 hr to 3 days). To elucidate further the possible role of ERK5 activation and subcellular localization in ischemic insult, rats were intraperitoneally administrated with nifedipine (ND) and dextromethorphan (DM), inhibitors of two types of calcium channels, 20 min prior to ischemia. Our findings showed that ND or DM significantly reduced activated ERK5 immunoreactivity in the nucleus and that most of the CA3/DG neurons were lost 3 days later. Most importantly, intracerebroventricular infusion of ERK5 antisense oligonucleotides (AS; every 24 hr for 3 days before ischemia), but not sense oligonucleotides or vehicle, not only markedly decreased the level of ERK5 and p-ERK5 but also largely caused neuronal loss in the CA3/DG region at 3 days of reperfusion. Taken together, the results strongly suggest that ERK5 was selectively activated in the hippocampal CA3/DG region and subsequently translocated from the cytosol to the nucleus through activation of N-methyl-D-aspartate receptor and L-type voltage-gated calcium channel, which might act as an important survival signal in ischemia-induced neuronal cell damage of the CA3/DG region.