肝X受体介导神经发生在抑郁症发病中的机制研究

Treatment for depression is facing severe challenges because of delayed onset of action, high recurrence rate and side effects upon classic antidepressant application, therefore the new mechanisms of pathogenesis in depression needs to be reexamined. It is evidenced there are reduced hippocampal neurogenesis and disorder of lipid metabolism in patients with depression, and treatment improved depressive symptoms by promoting neurogenesis. Liver X receptors (LXRs) play crucial roles in the process of neurogenesis, cell immunity and inflammation, as well as cholesterol and lipid metabolism. It is still unclear that whether LXRs or its subtypes are involved in the pathogenesis of depression and the related mechanisms. Our preliminary data showed that, there was a negative correlation between the expression of LXR beta in hippocampus and depression like behaviors in the chronic unpredictable stress (CUS) mouse model of depression; the agonist GW3965 for LXRs could significantly improve the CUS mouse depression like behavior, and GW3965 promoted neural stem cell proliferation in vitro, all the data suggested that their potential antidepressant effects of activated LXRs. This project firstly proposes that the activation of LXRs exerts antidepressant effects by promoting hippocampal neurogenesis, and we will combine research methods including electrophysiology, molecular biology and behavioral tests to investigate the roles of LXRs in depression development and the related molecular mechanisms. The research results will further uncover the pathological mechanisms of depression and provide evidence for new potential targets in depression therapy.

经典抗抑郁药物治疗由于起效延迟、复发率高、副作用多等使临床应用面临严峻挑战，需重新审视抑郁症发病的新机制。已证实抑郁症患者海马神经发生显著受损、体内脂质代谢紊乱，促进神经发生可改善抑郁症状。作为体内胆固醇的感应器，肝X受体（LXRs）在神经发生、细胞免疫和抗炎、中枢胆固醇脂质代谢中的作用至关重要。LXRs是否参与抑郁症发病及机制尚不明确。我们研究结果显示，慢性不可预知应激（CUS）抑郁模型小鼠抑郁样行为与海马内LXRb表达水平呈负相关，LXRs激动剂GW3965可显著改善CUS小鼠抑郁样行为，且GW3965体外促进神经干细胞增生，提示激活LXRs具有潜在抗抑郁作用。本项目首次提出激活LXRs促进海马神经发生发挥抗抑郁作用，拟结合电生理学、分子生物学和行为学方法，探讨LXRs在抑郁症发生发展过程中的受体途径及相关分子机制。课题研究结果将为深入研究抑郁症的病理机制和发掘药物作用新靶点提供依据。

首先确定了LXRs在正常小鼠脑内的表达模式，发现LXRb高表达于与抑郁症发病高度相关的脑区——海马、杏仁核，LXRa海马表达极低，杏仁核BLA较为密集；LXRb主要表达于Glu能神经元，几乎不与GABA能神经元共存。成功建立慢性不可预知应激（CUS）小鼠抑郁模型，与对照组相比，CUS小鼠海马中LXRb表达水平显著下降，而LXRa表达水平基本不变。相关性分析发现，CUS小鼠海马LXRb表达水平与抑郁样行为学改变呈负相关。构建并包装生产慢病毒shLXRa、shLXRb以下调LXRs的表达水平，发现小鼠海马内注射shLXRb下调LXRb表达后可诱发小鼠的抑郁样行为，提示LXRb参与抑郁症动物模型发病。海马内注射shLXRb则小鼠神经干细胞（NSC）增殖下降，同时NSC分化为神经元的过程受阻。体外培养NSC发现，LXRs激动剂GW3965可剂量依赖性促进NSC的增殖。shLXRb转染后GW3965促NSCs增殖效应被取消，而shLXRa不影响GW3965促NSCs增殖作用，提示GW3965促NSCs增殖效应通过激活LXRb而非LXRa实现，且通过LXRb受体中介后活化PI3K/Akt信号通路，实现促进NSCs增殖的效应。这些结果提示GW3965影响神经发生过程。进一步研究发现，GW3965促进海马内NSC增殖及新的神经元生成，该过程被shLXRb取消，说明GW3965抗抑郁的效应是通过激活LXRb受体促进NSC增殖及神经发生实现。进而采用在体X射线消融海马神经发生，结果显示GW3965治疗前给予海马区X射线照射，只能部分而不能完全消除GW3965治疗后CUS小鼠蔗糖偏好增加和不动时间缩短，且无论是否给予GW3965治疗，X射线照射几乎完全抑制了小鼠海马细胞的增殖。该结果提示激活LXRb介导的抗抑郁作用可通过海马神经发生依赖性和非依赖性通路实现。