Processing of Apoptotic Cell-Derived Cargo by Macrophages Continues Efferocytosis and Drives Atherosclerosis Regression

在工业化国家，动脉粥样硬化性心血管疾病（CVD）是导致死亡的主要原因。大多数动脉粥样硬化斑块在临床上是无症状的；然而，有一小部分可导致心肌梗死、中风或猝死。与临床事件相关的动脉粥样硬化的特征是大的坏死核心，这是由于凋亡细胞（ACs）清除缺陷造成的。当功能正常时，ACs的清除，称为“efferocytosis”，可以解决炎症。因此，增强晚期病变的efferocytosis可以稳定易破裂斑块并减少临床事件。虽然导致一种AC被吞噬的机制已经明确，但单个巨噬细胞（MΦs）必须在体内吞噬许多AC，称为“持续的efferocytosis”。因此，两个关键的、未解决的问题是：(a) MΦs如何处理降解AC产生的货物，以及(b)有什么机制提示MΦs先前摄入AC以内化后续AC。

Atherosclerotic cardiovascular disease (CVD) is the leading cause of death in the industrialized world. Most atherosclerotic plaques are clinically silent; however, a subset can lead to myocardial infarction, stroke, or sudden death. Atheromas that are linked to clinical events are characterized by large necrotic cores, which result from the defective clearance of apoptotic cells (ACs). When functioning normally, clearance of ACs, termed “efferocytosis”, resolves inflammation. Therefore, enhancing efferocytosis in advanced lesions may stabilize rupture-prone plaques and reduce clinical events. While the mechanisms that lead to phagocytosis of one AC have been well-defined, individual macrophages (MΦs) must engulf many ACs, termed “continued efferocytosis”, in vivo. Accordingly, two critical, unanswered questions are (a) how do MΦs process the cargo derived from degrading an AC, and (b) what mechanisms are in place that cue MΦs that have previously ingested an AC to internalize a subsequent AC. Therefore, the overall objective of this proposal is to understand the signaling and metabolic pathways that enable the continued clearance of ACs and to harness these pathways towards a novel treatment strategy. This proposal tests two new pathways critical for continued efferocytosis. In the first pathway, MΦs metabolize AC-derived arginine into putrescine, through the sequential action of arginase 1 (Arg1) and ornithine decarboxylase (ODC), to remodel the actin cytoskeleton. In the second pathway, MΦs respond to the overabundance of AC-derived nutrients by stimulating the nutrient sensor mTORC1 to recycle vesicles to the cell surface and supply the developing phagosome with plasma membrane. Aim 1 will explore the hypothesis that AC-derived arginine is metabolized into putrescine and examine the mechanisms by which putrescine regulates cytoskeletal remodeling. Aim 2 will test the hypothesis and investigate the mechanisms therein, that the putrescine-synthesizing enzymes Arg1 and ODC drive atherosclerosis regression and inflammation resolution. Aim 3 will explore the hypothesis that SLC38A9 senses AC-derived arginine and cholesterol to activate mTORC1 and identify the mechanisms that drive the internalization of a second AC. Successful completion of these aims will provide new mechanistic insight in how MΦs sense and metabolize the cargo from ingested ACs, which will provide new therapeutic opportunities to curb cardiovascular disease.