| Epidemiological studies have established that the epsilon 4 allele of the apolipoprotein (apo) E gene (APOE) constitutes an important risk factor for Alzheimer's disease (AD). Human apoE4 has been found in senile plaques (SPs) and neurofibrillary tangles (NTFs) in AD brains. This protein is synthesized in the brain by astrocytes and other cells including neurons, and a lot of biochemical, cell biological, and transgenic animal studies have suggested several potential mechanisms to explain the contribution of apoE4 to the pathogenesis of AD, including its modulatory effects on the deposition and clearance of amyloid peptide and the formation of SPs, impairment of the antioxidative defense systems, dysregulation of neuronal signaling pathways, disruption of cytoskeletal structure and function, and altered phosphorylation of protein tau and the formation of NFTs, and so on. Among them, it is generally assumed that apoE4-induced intracellular Ca2+ overloading might be the final common route for other cell-damaging signaling system. However, its role in the pathogenesis of these lesions is unclear, and it is also not known what is the primary or the initial effect and how the subsequent or downstream effects are developed later.At the same time, we have noticed that there appears little evidence concerning the effects of apoE4 on the electrical properties at the neuronal orchannel level, though the changes of electrical activities might be the initial ones in response to the neuronal insults and these changes might in turn lead to the deeper and long-lasting functions and structural impairments of insulted neurons. Since memory loss is characteristic in AD and the properties of K+ channels, according to a report, have been altered during acquisition of memory in both mollusks and mammals, it will be very interesting to clarify the following issues: whether apoE4 affects K+ channels early in AD and thus influence other cellular functions such as Ca2+-overloading even memory-lose? Whether K+ channel is one of the targets of apoE4? What happens to K+ channels in their properties and kinetics characters, and whether protecting of K+ channels could alleviate the Ca2+-overloading and other toxic functions evoked by apoE4?Based on the questions above, the present study investigated the acute effects of apoE4 on K+ current in acutely isolated hippocampal neurons of CA1 region using patch-clamp technique in both single-channel and whole cell configures. Using Ca2+ imaging technique, we observed the effects of apoE4 on [Ca2+]j and tried to clarify the mechanism of the Ca2+-overloading by apoE4 and the possible link between K+ channels and [Ca2+]j.Part I : Effects of Apolipoprotein E4 on Single Channel Delayed-Rectifier K+ Currents in Acutely Isolated Hippocampal NeuronsAmong the different voltage-operated K+ channels, the delayed-rectifier K+ (Ik) channels were first chosen because they were important inrepolarization of action potential and terminating the Ca2+ influx during action potential, for its activation by depolarization and its outward-rectifier; and because they could be investigated without pharmacological blocking of other conductances. The inside-out membrane patch was chosen because it was easier for us to observe the direct effects of apoE4 on internal membrane. The data showed: 1) in our experiment, the channels with conductance of about 35 pS were activated by depolarization, their activities were voltage-dependent, and the application of 20 mM TEA could reversibly block the opening of these channels but they were not affected by [Ca2+]o. Based on the properties described above, these channels belonged to the Ik channels as described previously by Rogawski.; 2) when apoE4 was applied to the inside of the membrane in micro molar concentration (0.5 uM), it markedly suppressed Ik currents by decreasing open probability, open frequency and open time by 97.58 ?7.93 %, 98.66 ?3.01 %, and 51.55 ?4.50 %, respectively, as well as increasing closed-time by 13.89 ?2.68 fold (n=10, P<0.01),. The effec...
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