Leucine as a Probe of Kynurenine-Induced Glutamate and Neural Circuit Dysfunction in Midlife Depression

项目摘要：慢性炎症和谷氨酸失调是先于 并加速重度抑郁症(MD)患者的大脑衰老和退化。在此，我们提出犬尿氨酸(KYN) 途径作为炎症和谷氨酸病理之间联系的中介。犬尿氨酸是 神经活性分子通过称为 大分子中性氨基酸转运体(LAT-1)位于血脑屏障中。一旦进入大脑，Kyn就是 由激活的小胶质细胞转化为喹啉酸(QA)和3-羟基犬尿氨酸等代谢产物 (3HK)。KYN的这些下游代谢产物具有强烈的兴奋毒性(QA)和氧化应激(3HK)和 最终会导致神经元功能障碍和死亡。广泛的临床前数据和我们的初步数据 表明犬尿氨酸的产生和从外周组织转移到大脑基本上是 因免疫激活和衰老等生物因素而增加。临床前数据还表明， 通过亮氨酸(一种更高亲和力的配体)竞争性阻断LAT-1，阻断KYN流入大脑 抑制由免疫激活引起的啮齿类动物的抑郁行为。我们在人类身上的初步数据 表明外周KYN驱动大脑中的KYN代谢物，导致基底节谷氨酸增加， 局部神经元活动的连贯性降低，大脑区域之间的功能连接减少，以及 抑郁行为，包括快感缺失和精神运动减慢。基于这些数据，我们提出了一个6- 在周挑战范式中，炎症增加的中年MD患者将随机分为 亮氨酸或赖氨酸(n=75)。与亮氨酸相反，赖氨酸不会抑制KYN的内流，因为它使用 不同的转运体(阳离子转运体)进入大脑。我们会估计谷氨酸和肌醇 (星形胶质标记物)使用磁共振波谱(MRS)；并研究局部脑的一致性 在基底神经节和背内侧前额叶皮质(DmPFC)区域的活动和功能连接 在基线、治疗1周和6周时进行静息功能磁共振成像(FMRI)。vbl.使用 标准化仪器，我们将测量快感缺失和精神运动减慢，同时采样血液和 脑脊液(仅6周)用于炎性介质和KYN代谢物。这项研究设计将测试 假设6周的亮氨酸挑战将降低谷氨酸和肌醇(目标1)，改善局部 基底节和dmPFC的活动一致性和功能连接性(目标2)，并减少快感缺乏 精神运动减速(目标3)。如果得到支持，这些目标将牵涉到KYN失调作为一个关键的调解人 炎症和衰老引起的大脑和行为的改变。此外，这份证明- 机制提案将确定Kyn及其在血脑屏障中的转运体为新药的关键靶点 MD患者快感缺失和精神运动减慢的治疗进展及潜在预防 神经变性和痴呆症。

Project Summary: Chronic inflammation and glutamate dysregulation are pathological processes that precede and hasten brain aging and degeneration in major depression (MD). Herein, we propose the kynurenine (KYN) pathway as a mediator of the association between inflammation and glutamate pathologies. Kynurenines are neuroactive molecules that readily access the brain from the periphery via dedicated transporters known as large neutral amino acid transporters (LAT-1) located in the blood-brain barrier. Once inside the brain, KYN is converted by activated microglial cells into metabolites such as quinolinic acid (QA) and 3-hydroxykynurenine (3HK). These downstream metabolites of KYN exert robust excitotoxicity (QA) and oxidative stress (3HK) and ultimately can lead to neuronal dysfunction and death. Extensive preclinical data and our preliminary data indicate that the production and transfer of kynurenines into the brain from peripheral tissues is substantially increased by biological factors such as immune activation and aging. Preclinical data also indicate that blockade of KYN influx into the brain via competitive blockade of LAT-1 by leucine (a higher affinity ligand) abrogates depressive-like behavior induced by immune activation in rodents. Our preliminary data in humans indicate that peripheral KYN drives KYN metabolites in the brain, leading to increased basal ganglia glutamate, reduced coherence of local neuronal activity, decreased functional connectivity among brain regions, and depressive behaviors, including anhedonia and psychomotor slowing. Based on these data, we propose a 6- week challenge paradigm in which middle-aged MD patients with increased inflammation will be randomized to either leucine or lysine (n= 75 total). In contrast to leucine, lysine does not inhibit the influx of KYN as it uses a different set of transporters (cationic transporters) to access the brain. We will estimate glutamate and inositol (astroglial marker) using magnetic resonance spectroscopy (MRS); and study the coherence of local brain activity and functional connectivity in basal ganglia and dorsomedial prefrontal cortical (dmPFC) regions using resting-state functional magnetic resonance imaging (fMRI) at baseline, 1 and 6-weeks of treatment. Using standardized instruments, we will measure anhedonia and psychomotor slowing while sampling blood and cerebrospinal fluid (6 weeks only) for inflammatory mediators and KYN metabolites. This study design will test the hypotheses that a 6-week leucine challenge will decrease glutamate and inositol (Aim 1), improve local activity coherence and functional connectivity in the basal ganglia and dmPFC (Aim 2), and reduce anhedonia and psychomotor slowing (Aim 3). If supported, these aims will implicate KYN dysregulation as a key mediator of the brain and behavioral changes induced by inflammation and aging in MD. Moreover, this proof-of- mechanism proposal will identify KYN and its transporters in the BBB as key targets for novel drug development for treating anhedonia and psychomotor slowing in MD patients and potentially preventing neurodegeneration and dementia.