Cholesterol-related genes in response to brain deafferentation in normal and transgenic mice

多年来，我们的团队已经记录了一种优雅的机制，通过这种机制，受伤的成人大脑使用死亡/垂死细胞释放的胆固醇和磷脂来为存活的神经元提供适当重建局部突触网络所需的脂质。我们称这个过程为胆固醇循环级联。我们的团队已经证明，在啮齿动物脑损伤反应中，载脂蛋白E （apoE）和ABCA1这两种功能强大的脂质转运蛋白在细胞内和细胞外运输胆固醇是突触网络替代所必需的。作为回应，神经元细胞表面的LDL受体家族可以识别载脂蛋白e -脂蛋白复合物，介导用于新突触形成的hdl -胆固醇的内化。在基因敲除小鼠中，脑apoE或其主要受体的完全缺失被证明会损害1)随着衰老的突触完整性，2)损伤诱导的突触恢复和3)认知能力。我们对控制这种级联反应的触发和调节机制知之甚少；即胆固醇和磷脂从星形胶质室向载脂蛋白e -脂蛋白（HDL）装配线的运输和包装，以及最终LDL受体（LDLR）介导的内化。我们知道某些atp结合盒（ABC）蛋白如A1参与其中，促进脂质向表面hdl的转移。假设：作为神经元丢失的反应，脑胆固醇被星形胶质细胞动员，其中ABCA1、A7或G1的协调表达促进其向细胞表面运动，在那里载脂蛋白E等待催化脂质转移到细胞外富含载脂蛋白E的HDL复合物。这些富含脂质的脂蛋白随后通过LDL受体家族被用来喂养邻近的神经元，LDL受体家族是由所谓的SREBF2和PCSK9蛋白的存在严格调节的；一个是低密度脂蛋白受体产生的有效激活因子（SREBF2），而另一个（PCSK9）促进低密度脂蛋白受体的消除和分解代谢。随后，大量的脂质被存活的神经元吞噬，并被用来重建新的树突和突触网络。目的:我们建议通过以下研究来解剖调节胆固醇循环过程的分子级联：1)在脱神经分化和神经再生过程中，体内胶质细胞中ABC转运系统对脂质的细胞内转运；2)在ABCA7和LDLR存在/不存在的情况下，损伤小鼠中载脂蛋白e -脂蛋白复合物的组装；3)载脂蛋白e和ABC与细胞表面脂质的相互作用；iv) apoE-HDL复合物通过LDL受体与神经元的结合和内化。预期结果：我们相信这些研究将为在成年啮齿动物大脑修复过程中导致突触修复激活和关键脂质传递的分子事件提供重要的见解。

Our team has documented over the years an elegant mechanism by which the injured adult brain uses the cholesterol and phospholipids released from dead/dying cells to feed surviving neurons with the lipids required for the proper rebuilding local synaptic networks. We call this process the cholesterol recycling cascade. Our team has already demonstrated that the intra- and extracellular transport of cholesterol by apolipoprotein E (apoE) and ABCA1, highly functional lipid transporters, is required and necessary for the replacement of synaptic networks in response to brain injury in rodents. In response, neuronal cell-surface receptors belonging to the LDL receptors family that recognize apoE-lipoprotein complexes mediate the internalization of the HDL-cholesterol used in the formation of new synapses. The complete absence of brain apoE or, of its main receptor, in knockout mice was shown to compromise i) synaptic integrity with aging, ii) lesion-induced synaptic recovery and iii) cognitive performance. We know very little about the triggering and regulating mechanisms controlling this cascade; i.e. the transport and packaging of cholesterol and phospholipids from the astroglial compartments toward the apoE-lipoprotein (HDL) assembly line, and the final LDL receptor (LDLR)-mediated internalisation. We know that certain ATP-Binding Cassette (ABC) proteins such as A1 are involved, facilitating the transfer of lipids to HDLs at the surface.  Hypothesis: In response to neuronal loss, brain cholesterol is mobilized by astrocytes where the coordinated expression of ABCA1, A7 or G1 facilitates its movement toward the cell surface where apolipoprotein E awaits to catalyze the transfer of the lipids to extracellular apoE-rich HDL complexes. These lipid-enriched lipoproteins are subsequently used to feed neighboring neurons via the LDL receptors family which are tightly regulated by the presence of the so-called of SREBF2 and PCSK9 proteins; one is a potent activator of LDL receptor production (SREBF2) while the other (PCSK9) facilitate the LDLR elimination and catabolism. Subsequently, large amounts of lipids are engulfed by surviving neurons and are used to rebuild new dentritic and synaptic networks. Objectives: We propose to dissect the molecular cascade regulating the cholesterol recycling process by examining i) the intracellular transport of the lipids by the ABC transport system in glial cells in vivo during deafferentation and reinnervation, ii) the assembly of apoE-lipoprotein complexes in presence/absence of ABCA7 and LDLR in lesioned mice,  iii) the interaction of apoE and ABCs with cell surface lipids and, iv) the binding and internalization of the apoE-HDL complex via the LDL receptors by neurons. Expected Outcome: We believe that these studies will provide crucial insights in regard to the molecular events leading to the activation of synaptic repair and the delivery of key lipid species during brain repair in the adult rodent brain.