Mechanisms of Docosahexaenoic Acid Neuroprotection after Rat Pup TBI

 描述（由申请人提供）：创伤性脑损伤（TBI）是儿童死亡和残疾的主要原因，导致终身神经功能障碍，对此没有特异性治疗。在成年大鼠和我们的儿童TBI模型中，饮食二十二碳六烯酸（DHA）改善了严重TBI后的神经功能结局。然而，DHA的神经保护机制尚不清楚。TBI激活小胶质细胞，大脑的常驻巨噬细胞，进入M1和M2激活之间的范围。M1促进氧化应激，而M2降低氧化应激。在成年动物中，M2激活与结局改善相关。目前尚不清楚TBI后M2激活是否与未成熟大脑的改善结果相关，也不知道如何促进这种“极化”。我们的初步数据表明，DHA减少我们的儿科TBI模型中的小胶质细胞活化，在大鼠幼崽中的控制性皮质撞击（CCI）。我们的DHA饮食增加了小胶质细胞转录因子下游的大脑基因表达，该转录因子调节极化，即过氧化物酶体激活受体（PPARγ）。PPARγ激动剂使小胶质细胞朝向M2。我们的研究结果使我们想知道DHA的神经保护作用是否依赖于通过PPARγ依赖性途径促进小胶质细胞M2极化。值得关注的是，DHA很容易被氧化。大量的氧化DHA会耗尽未成熟大脑有限的抗氧化储备，从而导致氧化损伤。另一方面，DHA可通过直接吸收活性氧和减少小胶质细胞自由基的产生来减轻氧化损伤。氧化损伤可以使用总抗氧化能力（TAC）来评估。总之，需要对DHA的安全性和机制进行临床前机制研究。我们假设，DHA将通过一种与改善的神经功能结局相关的PPARγ依赖性机制，使实验性TBI后大鼠幼崽的小胶质细胞向M2表型活化，并增加脑TAC。为了测试这一点，我们将在CCI或SHAM手术后将大鼠幼崽暴露于DHA或常规（REG）饮食，并每天注射PPARγ拮抗剂或载体。我们将使用成像和基因表达来表征M2激活，TAC来评估氧化，组织学/认知功能测试来评估结果。我们预计DHA会增加TBI后大鼠幼鼠脑中的M2极化和TAC，这一作用被PPARγ拮抗作用所消除，并与病变体积减少和认知障碍相关。我们的建议将提供新的知识的时间进程，功能的重要性，M2小胶质细胞极化在未成熟的大脑TBI后，以及这些是否被DHA修改。虽然DHA的可用性和明显的安全性使其成为严重TBI后儿童的一种有吸引力的候选疗法，但其临床应用受到有关其作用机制和TBI后未成熟大脑安全性的重要知识缺陷的阻碍。我们的建议将导致集中的临床前研究，这将指导严重TBI后儿童的DHA临床试验，并可能减少儿童TBI后神经功能障碍的负担。

 DESCRIPTION (provided by applicant): Traumatic brain injury (TBI) is the leading cause of death and disability in children, resulting in life-long neurologic dysfunction for which there is o specific therapy. Dietary docosahexaenoic acid (DHA) improved neurologic outcome after severe TBI in adult rats and in our pediatric TBI model. However, the mechanism of DHA's neuroprotection is not understood. TBI activates microglia, the brain's resident macrophages, into a spectrum ranging between M1 and M2 activation. M1 promotes oxidative stress, while M2 decreases oxidative stress. In adult animals, M2 activation is associated with improved outcome. It is not known if M2 activation after TBI is associated with improved outcome in the immature brain, nor how such "polarization" may be fostered. Our preliminary data suggests that DHA decreases microglial activation in our pediatric TBI model, controlled cortical impact (CCI) in rat pups. Our DHA diet increases brain gene expression downstream of a microglial transcription factor that regulates polarization, the Peroxisome Proliferator Activated Receptor (PPARγ). PPARγ agonists polarize microglia towards M2. Our results led us to ask if DHA's neuroprotection depends on promoting microglial M2 polarization via a PPARγ-dependent pathway. Of concern, DHA is readily oxidized. Large amounts of oxidized DHA could cause oxidative injury by depleting the immature brain's limited antioxidant reserve. On the other hand, DHA could decrease oxidative injury by directly absorbing reactive oxygen species and decreasing microglial free radical production. Oxidative injury may be assessed using total antioxidant capacity (TAC). In sum, mechanistic preclinical studies of DHA safety and mechanism are needed. We hypothesize that DHA will polarize activated microglia towards the M2 phenotype and increase brain TAC in rat pups after experimental TBI, via a PPARγ-dependent mechanism, associated with improved neurologic outcome. To test this, we will expose rat pups to DHA or regular (REG) diet after CCI or SHAM surgery and inject them daily with either a PPARγ antagonist or vehicle. We will use imaging and gene expression to characterize M2 activation, TAC to assess oxidation, and histology/cognitive function testing to assess outcome. We anticipate that DHA will increase M2 polarization and TAC in rat pup brains after TBI, abrogated by PPARγ antagonism and associated with decreased lesion volume and cognitive impairment. Our proposal will provide new knowledge on the time course, and functional importance, of M2 microglial polarization in the immature brain after TBI, and whether these are modified by DHA. While the availability and apparent safety of DHA make it an appealing candidate therapy for children after severe TBI, its clinical use is hindered by important knowledge deficits regarding its mechanism of action and safety in the immature brain after TBI. Our proposal will lead to focused pre-clinical studies that will guide clinical trials o DHA in children after severe TBI and potentially decrease the burden of neurologic disability after childhood TBI.