Complement C3aR in adolescent synaptic pruning and risk for anxiety

焦虑症是世界范围内一些最常见和日益严重的健康问题。由于对其病因了解甚少，目前的治疗方法对很大一部分病例的疗效有限，因此迫切需要更有效的靶向治疗。焦虑症通常出现在青少年时期，受影响的青少年在以后的生活中患其他精神健康障碍的风险增加。在青少年时期，大脑的结构和功能会发生重大变化，这一过程被称为“突触修剪”。这个过程确保不经常使用的突触（脑细胞之间的连接）被消除或“修剪”，使剩余的连接成熟。我们现在知道免疫系统，特别是被称为补体的分支（https://www.youtube.com/watch?v=BSypUV6QUNw&feature=emb_title）对这个过程至关重要。补体通过标记弱突触来驱动突触修剪，弱突触随后被小胶质细胞（大脑的免疫细胞）吞噬。这一过程对健康的大脑功能至关重要，但如果这一过程出错，神经网络就有可能受到干扰，这可能导致精神障碍的发展。在这项工作中，我们将研究一种被称为C3a受体（C3aR）的分子，它是补体系统的一员，如何在青春期控制突触修剪，从而影响焦虑的发展。此前，我们发现C3aR在焦虑中的新作用，即缺乏这种受体的小鼠表现出焦虑增加。此外，这些小鼠大脑中的突触数量减少。当他们的小胶质细胞在实验室进行测试时，这些细胞吞噬了更多的突触蛋白。这表明C3aR控制着小胶质细胞吞噬，我们预测C3aR功能的降低会导致青春期突触过度修剪，从而导致突触丢失和焦虑升高。然而，在我们测试这一预测之前，我们必须首先解决我们知识中的关键空白。到目前为止，动物研究仅在大脑发育的早期阶段（大致相当于人类的婴儿期）测量了补体驱动的突触修剪，但对青少年大脑发育的关注要少得多。重要的是，目前还没有关于青春期与焦虑有关的大脑区域（如杏仁核和海马体）突触修剪的现有信息，这些区域直到青春期才成熟。在这个项目中，我们将通过记录这一时期补体、突触数量和小胶质细胞吞噬的进化状态，来描述青春期这些大脑区域突触修剪的精确时间。然后，我们将能够测试我们的预测，即减少C3aR会导致大脑中与焦虑相关区域的突触吞噬增加。最后，过度突触修剪的长期影响尚未得到很好的表征，但可能导致脑回路改变，从而导致脑网络功能异常。为此，我们将利用临床相关的脑结构和功能脑成像测量来研究c3ar缺陷小鼠在青春期和成年期过度突触修剪对全脑的影响。这些研究将为焦虑的神经生物学机制提供急需的信息，为了解补体和突触修剪如何影响青春期焦虑相关的大脑区域提供见解。重要的是，所产生的数据将对我们未来的工作至关重要，我们的目标是减少c3ar缺陷小鼠中过度的小胶质突触修剪，以确定它是否可以防止异常焦虑的发展。如果是这样，增强C3aR功能的药物可能提供一种治疗焦虑和与异常突触修剪相关的疾病的新方法。

Anxiety disorders represent some of the most common and increasing health problems worldwide. Because their causes are poorly understood, current treatments are of limited benefit in a significant proportion of cases and thus there is an urgent need for more effective, targeted therapies. Anxiety disorders typically emerge during the teenage years and affected adolescents are at increased risk for other mental health disorders later in life. During adolescence, the brain undergoes significant changes in its structure and function brought about via a process known as 'synaptic pruning'. This process ensures that synapses (the connections between brain cells) that are infrequently used are eliminated or 'pruned', allowing the remaining connections to mature. We now know that the immune system, and in particular a branch known as complement (https://www.youtube.com/watch?v=BSypUV6QUNw&feature=emb_title) is critical to this process. Complement drives synaptic pruning by tagging weak synapses, which are then engulfed or 'phagocytosed' by microglia, the brain's immune cells. This process is essential for healthy brain function but there is potential for neuronal networks to be disturbed should the process go awry, which may contribute to the development of mental disorders. In this work we will investigate how a molecule known as the C3a receptor (C3aR), a member of the complement system, controls synaptic pruning during adolescence to influence the development of anxiety. Previously, we have found a novel role for C3aR in anxiety, whereby mice deficient in this receptor display increased anxiety. In addition, these mice have a reduced number of synapses throughout the brain. When their microglia were tested in the lab, these cells engulfed more synaptic proteins. This suggests that C3aR controls microglial phagocytosis, and we predict that reduced C3aR function leads to excessive synaptic pruning during adolescence leading to synapse loss and elevated anxiety.However, there are critical gaps in our knowledge which we must first address before we can test this prediction. So far, animal studies have measured complement driven synaptic pruning only during very early stages of brain development, roughly equivalent to human infancy, but there has been much less focus on adolescent brain development. Importantly, there is no existing information on adolescent synaptic pruning within brain areas implicated in anxiety such as the amygdala and hippocampus, which do not mature until adolescence. In this project, we will characterise the precise timing of synaptic pruning in these brain areas during adolescence, by documenting the evolving state of complement, synapse number and microglial engulfment over this period. We will then be able to test our prediction that reduced C3aR results in greater synapse engulfment in brain areas related to anxiety. Finally, the long-term effects of excessive synaptic pruning have not been well characterised but are likely to result in altered brain circuits and therefore abnormal functioning of brain networks. To this end, we will investigate the brain-wide effects of excessive synaptic pruning in C3aR-deficient mice at both adolescence and adulthood using clinically relevant brain imaging measures of brain structure and function. These studies will deliver much-needed information on the neurobiological mechanisms of anxiety, providing insight into how complement and synaptic pruning impact anxiety-associated brain areas during adolescence. Importantly, the data generated will be essential to inform our future work in which we aim to reduce excessive microglial synaptic pruning in C3aR-deficient mice to determine whether it prevents abnormal anxiety from developing. If so, drugs that enhance C3aR function could provide a novel means of treating anxiety and disorders associated with abnormal synaptic pruning.