Biological functions of glial TSPO in retina and brain models of the cholesterol storage disease Niemann-Pick type C (NPC)

转运蛋白18 kDa(TSPO)是线粒体外膜的一种完整蛋白。尽管它的细胞功能仍有争议，但它被认为在神经类固醇生成、线粒体功能、细胞代谢和神经炎症中发挥作用。在中枢神经系统(CNS)中，TSPO主要表达于室管膜细胞、小胶质细胞和星形胶质细胞，但其细胞表达模式在神经炎性和神经变性条件下发生很大变化。对于视网膜，TSPO被报道为胆固醇和神经类固醇代谢的调节器，因此CNS的这一部分可以作为理想的、最重要的是易于研究的模型，来分析TSPO对细胞类型的特定益处或有害影响。在健康组织中，我们检测到TSPO在视网膜中的主要大胶质细胞Müler细胞中表达最高。相反，在视网膜病理中，反应性小胶质细胞强烈上调TSPO。我们的研究项目特别旨在将视网膜中胶质细胞TSPO的功能与大脑中相应细胞类型的结果进行比较。为了确定TSPO在生理和神经退行性疾病条件下的功能，我们选择了具有不同视网膜和脑病理的Niemann Pick C型(NPC)代谢性疾病模型。这种单基因疾病主要由NPC1功能丧失引起，其特征是胆固醇储存和代谢缺陷。在我们新的小胶质细胞特异性NPC1基因敲除模型中，我们发现了小胶质细胞的早期细胞自主性缺陷和小胶质细胞TSPO的大量上调。这些小胶质细胞的缺失导致了其他类型细胞的病理变化，如星形胶质细胞和包括视网膜在内的不同中枢神经系统区域的神经元。因此，该模型为研究TSPO在视网膜和脑内胶质胆固醇转运、神经类固醇生成、神经炎症和细胞代谢中的作用提供了一个独特的机会。因此，我们的目标是阐明胶质细胞TSPO的增加是否触发中枢神经系统的保护性或有害反应，以及这可能如何促进神经病理。通过翻译的方法，我们将像我们的小鼠模型一样，在稳态和鼻咽癌条件下，分析TSPO缺失对含有和不含有小胶质细胞的人视网膜器质中神经类固醇生物合成和胆固醇代谢等神经功能的影响。对鼠脑和人脑胶质细胞中TSPO信号级联的比较研究将有助于显著扩展我们对TSPO生物学及其与人类疾病的相关性的理解。

Translocator protein 18 kDa (TSPO) is an integral protein of the outer mitochondrial membrane. Although its cellular functions are still debated, it is thought to play a role in neurosteroidogenesis, mitochondrial function, cell metabolism and neuroinflammation. In the central nervous system (CNS), TSPO expression is predominantly found in ependymal cells, microglia, and astrocytes, but its cellular expression pattern largely changes under neuroinflammatory and neurodegenerative conditions. For the retina, TSPO was reported to be a regulator of cholesterol and neurosteroid metabolism, so this part of the CNS can serve as an ideal and, most importantly, easy-to-study model to analyze cell-type-specific beneficial or detrimental effects of TSPO. In the healthy tissue, we detected the highest TSPO expression in Müller cells, the major macroglia in the retina. In contrast, in retinal pathologies, TSPO is strongly upregulated by reactive microglia. Our research project specifically aims to compare the function of glial TSPO in the retina with results from corresponding cell types in the brain. To determine the function of TSPO under physiological and neurodegenerative conditions, we selected the Niemann Pick type C (NPC) metabolic disease model with distinct retinal and brain pathology. This monogenetic disease – predominantly caused by the loss of NPC1 function – is characterized by cholesterol storage and metabolic defects. In our new microglia-specific NPC1 knockout model, we found early cell autonomous defects of microglia and massive upregulation of microglial TSPO. These microglial deficits resulted in pathological changes in other cell types such as astrocytes and neurons in various CNS regions, including the retina. Therefore, this model provides a unique opportunity to study the functions of TSPO in glial cholesterol transport, neurosteroidogenesis, neuroinflammation, and cellular metabolism in the retina and brain. Thus, we aim to clarify whether an increase in glial TSPO triggers protective or detrimental responses in the CNS and how this may contribute to neuropathology. In a translational approach, we will analyze the effect of TSPO loss on glial functions including cholesterol metabolism and neurosteroid biosynthesis in human retinal organoids with and without microglia under homeostatic and NPC conditions as done in our mouse model. These comparative studies of TSPO signaling cascades in murine and human glia will help to significantly expand our understanding of TSPO biology and its relevance to human disease.