Role of orphan receptor GPR55 in immune cell homeostasis and atherosclerosis

与动脉粥样硬化相关的急性心血管并发症是全世界死亡的主要原因。越来越多的人认为，潜在的慢性炎症过程可以促进动脉斑块的形成和进展，可能是限制急性心血管事件（如心肌梗死）风险的治疗靶点。在这种情况下，我们对内源性脂质信号系统内源性大麻素系统在导致动脉粥样硬化的慢性炎症反应中的作用感兴趣。孤儿受体GPR55已被提出作为一种新的大麻素受体，基于与合成和内源性大麻素的高亲和力结合。后来，人们发现溶血磷脂酰肌醇（LPI）是一种更有效的内源性配体。根据小鼠微阵列数据库，GPR55在各种淋巴细胞亚群中表达，特别是γ - eltat （gdT）细胞。其在动脉粥样硬化中调节免疫功能的作用尚不清楚。为了分析其在动脉粥样硬化中的作用，我们将载脂蛋白E缺陷（Apoe-/-）小鼠与GPR55-/-小鼠杂交，产生Apoe-/-GPR55-/-小鼠。我们的初步实验显示，与Apoe-/-对照相比，Apoe-/- gpr55 -/-小鼠的斑块大小约为Apoe-/-对照的两倍，斑块巨噬细胞含量在早期较高，但在动脉粥样硬化晚期巨噬细胞较少，胶原含量较高。这伴随着主动脉促炎细胞因子mRNA表达增强和IgG1血浆水平上调。此外，GPR55缺乏与淋巴细胞计数增加有关，在gdT细胞中发现最显著的增加。基于我们的初步发现，本项目的目的是阐明GPR55信号在免疫细胞稳态功能中的作用，以及它如何调节动脉粥样硬化斑块的发展。我们的工作假设是GPR55信号负调控gdT细胞激活和细胞毒性淋巴细胞反应。我们可以推测GPR55的天然配体LPI是淋巴组织中gdT细胞的稳态保留信号。特别是，我们想解决四个具体问题：(1)哪些免疫细胞亚群表达GPR55？GPR55表达水平在动脉粥样硬化中改变吗？(2)哪些组织高表达天然配体LPI？动脉粥样硬化中LPI组织水平是否可调节？(3)造血GPR55缺乏在动脉粥样硬化和代谢变化（体重和胆固醇水平）中的具体贡献是什么？(4) GPR55如何负向控制gdT细胞活化？我们认为这个项目是高度相关的，因为淋巴细胞GPR55信号在动脉粥样硬化中的具体作用尚不清楚。

Acute cardiovascular complications related to atherosclerosis are the leading cause of death worldwide. It is increasingly considered that the underlying chronic inflammatory process, which promotes arterial plaque formation and progression, could be a therapeutic target to limit the risk of acute cardiovascular events such as myocardial infarction. In this context, we are interested in the role of the endocannabinoid system, an endogenous lipid signaling system, in chronic inflammatory responses contributing to atherosclerosis. The orphan receptor GPR55 has been proposed as a novel cannabinoid receptor, based on high affinity binding to synthetic and endogenous cannabinoids. Later, it was identified that lysophosphatidylinositol (LPI) is a more potent endogenous ligand. According to murine microarray databases, GPR55 is expressed by various lymphocyte subsets, in particular gammadeltaT (gdT) cells. Its role in regulating immune functions in the context of atherosclerosis is unknown.To dissect its role in atherosclerosis, we crossed apolipoprotein E deficient (Apoe-/- ) mice with GPR55-/- mice to generate Apoe-/-GPR55-/- mice. Our preliminary experiments revealed approximately two-fold larger plaque sizes in Apoe-/-GPR55-/- mice compared to ApoE-/- controls with higher plaque macrophage content at early stage, but less macrophage and more collagen content at advanced atherosclerosis. This was accompanied by enhanced aortic pro-inflammatory cytokine mRNA expression and upregulated IgG1 plasma levels. Moreover, GPR55 deficiency was associated with increased lymphocyte counts, with the most striking increase found in gdT cells. Based on our preliminary findings, the aim of this project is to clarify the role of GPR55 signaling in immune cell homeostatic function, and how this modulates atherosclerotic plaque development. Our working hypothesis is that GPR55 signaling negatively regulates gdT cell activation and cytotoxic lymphocyte responses. We may speculate that LPI, the natural ligand for GPR55, is a homeostatic retention signal for gdT cells in lymphoid tissues. In particular, we want to address four specific questions: (1) Which immune cell subsets express GPR55? Do GPR55 expression levels change in atherosclerosis? (2) Which tissues highly express the natural ligand LPI? Are LPI tissue levels modulated in atherosclerosis? (3) What is the specific contribution of hematopoietic GPR55 deficiency in atherosclerosis and metabolic changes (weight and cholesterol levels)? (4) How does GPR55 negatively control gdT cell activation?We believe that this project is highly relevant, as the specific role of lymphocyte GPR55 signaling in atherosclerosis is unknown.