Novel Role of Thrombospondin-1 in Protection against Rupture of Abdominal Aortic Aneurysm

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Abdominal aortic aneurysm (AAA) is the progressive weakening and dilation of the aorta. A substantial knowledge gap exists in the understanding of molecular mechanisms responsible for aneurysm rupture, the major cause of mortality among AAA patients. Following our prior report of elevated thrombospondin-1 (TSP1) in human and mouse aneurysmal tissues, we conducted single-cell RNA sequencing (scRNA-seq) analysis and identified macrophages (Mɸs) being the primary source of elevated TSP1 in mouse aneurysmal aorta. We subsequently generated Mɸ-specific Thbs1 knockout mice (Thbs1∆Mɸ) by crossing Lyz2-Cre with our newly constructed Thbs1flox/flox mice. When subjected to aneurysm induction by angiotensin II (Ang II) coupled with hypercholesterolemia, over 60% of Thbs1∆Mɸ died due to AAA rupture, an incidence that was 2.6 times higher than Thbs1wt. Intriguingly, Thbs1∆Mɸ mice that survived to the end of 28-day Ang II infusion showed less aneurysm dilation than Thbs1wt. Smaller aneurysmal expansion was also found when Thbs1∆Mɸ mice were challenged with perivascular application of CaCl2, an AAA model that does not produce rupture. We propose two specific aims to delineate the mechanisms through which Mɸ-specific Thbs1 gene deletion differentially affects aortic dilation and rupture with an emphasis on AAA rupture. Specific Aim 1 devotes to establishing the rupture- preventive function of Mɸ TSP1. Specifically, we will determine the aortic responses proceeding lethal rupture in male and female Thbs1∆Mɸ mice in the Ang II model followed by identifying rupture-associated molecular signatures through scRNA-seq, in situ hybridization and immunostaining. Furthermore, we will examine the effects of Mɸ-specific Thbs1 knockout using a different murine model that produces rupture in advanced stages of aneurysm, which is more relevant to human AAA than the early rupture produced by the Ang II model. Specific Aim 2 focuses on investigating molecular mechanisms of aneurysm rupture. Preliminary studies showed that compared to wildtype, Thbs1-/- Mɸs had significantly reduced ability to migrate or to engulf apoptotic cells as well as neutrophil extracellular traps (NETs). We will test whether Mɸ TSP1 promotes NET clearance through CD47- mediated actin polymerization. Next, we will establish the causal effect of impaired Mɸ migration and phagocytosis on aneurysm rupture of Thbs1∆Mɸ mice. We will first determine whether NET burden is increased in Thbs1∆Mɸ died from rupture, and the spatial relationship between NETs and Mɸs. Second, we will test whether restoring Mɸ migration in Thbs1∆Mɸ reduces NET accumulation via adoptive transfer strategies. Furthermore, we will examine whether enhancing or attenuating NET clearance affect aneurysm rupture in Thbs1∆Mɸ. Lastly, we will analyze TSP1 expression and its association with Mɸ and NET accumulation in ruptured and non-ruptured human AAA tissues. By dissecting the multifaceted functions of Mɸs through TSP1 manipulations, this project will produce significant impact on the understanding of aneurysm rupture.