The Young Gut Microbiome: A Fountain of Youth for Brain Injury in the Aged?

创伤性脑损伤（TBI）是世界范围内死亡和残疾的主要原因。65岁及以上的患者在TBI患者中所占的比例越来越大，不到一半的老年TBI患者在损伤后功能独立。事实上，脑外伤是阿尔茨海默病（AD）和阿尔茨海默病相关痴呆（ADRD）随后发展的最大的非遗传风险因素之一。由小胶质细胞组成性激活引起的慢性神经炎症是AD、ADRD及相关疾病过程的同谋，并且代表了衰老小胶质细胞与衰老肠道微生物群结构和功能之间的复杂相互作用。然而，肠道微生物群-小胶质细胞相互作用在AD、ADRD和衰老中的机制尚未阐明。我们实验室发表的数据表明，衰老的小胶质细胞呈组成性激活状态。这些“启动”的小胶质细胞能够产生夸张的炎症反应，并可能代表导致老年患者在脑外伤后阿尔茨海默病和阿尔茨海默病相关痴呆发病率增加的机制之一。我们实验室的已发表和初步数据进一步表明，损伤后肠道微生物群落结构是脑外伤后神经认知功能障碍程度的一个因素，包括AD和ADRD的诊断。我们的数据显示，损伤前肠道微生物群落的恢复显着改善了小鼠脑外伤后的神经认知，包括AD/ adrd相关的结果，并减弱了小胶质细胞的组成性激活。我们在与正常衰老相关的肠道微生物群落中发表了类似的发现，与AD和ADRD密切相关的几种微生物类群的相对丰度增加。尽管如此，将老年肠道微生物群落的改变与随后AD和ADRD的发展联系起来的机制尚未完全阐明。我们假设，在TBI之前或之后恢复年轻的肠道微生物群落将减弱小胶质细胞的构成激活，并改善老年TBI小鼠的神经认知和AD/ adrd相关结果。为了验证这一假设，我们将确定通过用年轻小鼠的粪便移植粪便微生物群来恢复年轻的肠道微生物群落结构是否会减弱老年小鼠小胶质细胞的激活。我们将利用损伤前的“益生元”模型以及损伤后的治疗范式。我们将使用小胶质细胞的批量细胞分选（FACSorting），然后使用单细胞RNA测序（scRNAseq）。我们将通过粪便16S rRNA基因扩增子测序和粪便代谢物分析来比较候选基因的微生物活性。与转录组学和肠道微生物群落分析相协调，我们将利用我们团队开发的一种新型氟基放射性示踪剂对18kda转运蛋白（TSPO）进行纵向PET/CT成像，以跟踪和比较TBI后肠道微生物干预之前、期间和之后的小胶质细胞激活的发生、进展和程度。总的来说，拟议的研究将确定老年肠道微生物组在TBI过程中组成性小胶质细胞激活的发生和进展中的关键作用，从而提高老年TBI、AD和ADRD患者的新治疗和生物标志物开发的潜力。

Traumatic brain injury (TBI) is a leading cause of death and disability worldwide. Patients aged 65 years and older account for an increasing proportion of patients with TBI and less than half of aged TBI patients become functionally independent after injury. In fact, TBI is one of the greatest non-genetic risk factors for the subsequent development of Alzheimer’s disease (AD) and Alzheimer’s disease-related dementias (ADRD). Chronic neuroinflammation resulting from the constitutive activation of microglia is complicit in AD, ADRD and related disease processes, and represents a complex interplay between aged microglia and the structure and function of the aged gut microbiome. However, the mechanism(s) underlying gut microbiota-microglia interaction in AD, ADRD, and aging have yet to be elucidated. Published Data from our laboratory shows that aged microglia adopt a constitutively activated state. These “primed” microglia are then capable producing an exaggerated inflammatory response and may represent one of the mechanisms leading to the increased incidence of Alzheimer’s disease and Alzheimer’s disease-related dementia in aged patients after TBI. Published and Preliminary Data from our laboratory further demonstrates that post-injury gut microbial community structure is a contributing factor in the degree of neurocognitive impairment including in AD and ADRD diagnoses after TBI. Our data shows that restoration of a preinjury gut microbial community markedly improved neurocognitive including AD/ADRD-related outcomes and attenuated the constitutive activation of microglia after TBI in mice. We published similar findings within the gut microbial community associated with normal aging with an increase in the relative abundance of several microbial taxa strongly associated with AD and ADRD. Nonetheless, mechanism(s) linking alterations in the aged gut microbial community to the development of subsequent AD and ADRD have yet to be fully elucidated. We hypothesize that restoration of a youthful gut microbial community before or after TBI will attenuate the constitutive activation of microglia and improve neurocognitive and AD/ADRD-related outcomes in aged TBI mice. To test this hypothesis we will determine whether restoring a youthful gut microbial community structure via fecal microbiota transplant with stool from young mice will attenuate the activation of microglia in aged mice. We will utilize both a pre-injury “prebiotic” model as well as a post-injury treatment paradigm. We will use bulk population cell sorting (FACSorting) of microglia followed by single cell RNA sequencing (scRNAseq). We will compare candidate genes to microbial activity via fecal 16S rRNA gene amplicon sequencing and fecal metabolite analysis. Harmonized with the transcriptomic and gut microbial community profiling, we will utilize longitudinal PET/CT imaging of the 18-kDa translocator protein (TSPO) with a novel fluorine-based radiotracer developed by our group to track and compare the onset, progression, and degree of microglial activation before, during, and after gut microbial intervention after TBI. Collectively, the proposed studies will identify the key role of an aged gut microbiome in the onset and progression of constitutive microglial activation over the course of TBI, thus raising the potential for novel therapeutic and biomarker development in aged TBI, AD, and ADRD patients.