Elucidation of the principal mechanism for dementia and identification of anti-dementia genes through the analysis of ApoE4 neurotoxicity

本研究建立了ApoE4诱导的神经元死亡系统。结果表明：（1）ApoE 4引起的神经元死亡是由LDL受体相关蛋白（LRP）介导的，（2）LRP结合蛋白（RAP）抑制ApoE 4引起的神经元死亡，（3）另一种配体α2巨球蛋白（α2 macroglobulin）抑制ApoE 4引起的神经元死亡。（4）低浓度的ApoE3抑制ApoE4引起的神经元死亡。（5）PTX抑制神经元死亡，并且当ApoE4与LRP结合时，信号通过Gi1、Gi2、Gi3或它们全部传递。由于ApoE4是否具有神经毒性一直存在争议，因此必须加以解决。对于ApoE 4不具有神经毒性的其他组的研究，我们已经指出，在不同的实验条件或不同的细胞条件下，如细胞种类、LRP和RAP的表达水平以及培养基中的血清处理，可能观察到抑制毒性。如果在培养液中加入血清或补充N2，则可使ApoE 4的测定结果不产生神经毒性，α2巨球蛋白（α 2 M）的不稳定性以及载脂蛋白的e4基因型可能是阿尔茨海默病的危险因素。我们在（3）中提到的发现为危险因素提供了第一个细胞生物学证据。如（4）所述，LRP的作用是使细胞存活或死亡，这取决于它的配体或配体浓度，我们将研究LRP与Gi及其下游靶分子的偶联机制。将ApoE基因敲除或人ApoE 4基因敲入的小鼠与AD模型小鼠交配，研究AD的发病机制具有重要意义。

In this study, we were able to establish the neuronal death system induced by ApoE4. With the system, we investigated the molecular mechanism of neuronal death by ApoE4 and we found that (1) neuronal death by ApoE4 was mediated by the LDL receptor-related protein, LRP;(2) LRP-binding protein, RAP, suppressed ApoE4-induced neuronal death; (3) one of other ligands, α2 macroglobulin, suppressed neuronal death; (4)low concentrations of ApoE3 supressed neuronal death caused by ApoE4 (5) neuronal death was suppressed by PTX and the signal was transduced through either Gi1, Gi2, Gi3, or all of them upon binding of ApoE4 to LRP. Since it has been remained controversial whether ApoE4 exerts neurotoxicity, it has to be addressed. For the studies by other groups where ApoE4 did not exert neurotoxicity, we have pointed out that the suppressed toxicity was possibly observed under the different experimental condition or the different cellular condition such as cell kind, expression level of LRP and RAP, and serum treatment in the culture medium. If serum or N2 supplement was added into the culture medium, it caused the results of ApoE4 to not exert neurotoxicity.Instability of α2 macroglobulin (α2M) as well as e4 genotype of apolipoprotein, apoE4 has been pointed out to be the possible risk factor for Alzheimer's disease. Our finding mentioned in (3) provides the first cellular biological evidence for the risk factor. As in (4) above, LRP performs the cellular function of either survival or death depending on it's ligand or the lignad concentration.We will investigate the coupling mechanism of LRP with Gi and the downstream target molecule of Gi. It will be intriguing to study AD by mating the ApoE knock-out mice or human ApoE4 knock-in mice with our AD model mice in the near future.