1st supervisor: Alkystis Phinikaridou
2nd supervisor: René Botnar
Abdominal aortic aneurysms (AAAs) are permanent dilations of the aorta that exceed 50% of the original size of the vessel or that are greater than 3cm in maximum diameter. After a period of asymptomatic expansion, rupture of AAA can occur with an associated mortality rate as high as 90% making it the 10th most common cause of death in developed countries. Diagnosis of AAAs involves anatomic imaging, typically ultrasound or CT angiography, and risk stratification relies on the measurement of the aortic diameter. Surgery is the only intervention available for patients with a AAA diameter > 55mm or if the rate of expansion is >1cm/ year. However, AAA smaller than these thresholds might also rupture whereas AAA larger than the thresholds might be stable. Although the size of the AAA at diagnosis is the best current predictor of aneurysm expansion, the prognosis remains complex due to the non-linearity and unpredictability of expansion-rates. Thus, imaging of the molecular and cellular processes that are key in the pathogenesis and lead to rupture of AAA, other than aneurysm size, to predict rupture are needed.
The role of extracellular matrix (ECM) in abdominal aortic aneurysm remodeling: The ECM proteins, collagen and elastin, are the most abundant structural proteins of the arterial wall. They provide tensile strength and elastic recoil to the arterial wall to be able to withstand the high pulsatile pressures within the aortic lumen. Turnover of the ECM proteins, and particularly, the balance between degradation and synthesis is vital for preserving the structural integrity of the aortic wall. Deregulation of ECM synthesis and degradation is crucial for the development and outcome of AAAs. Collagen is a major ECM protein involved in AAA remodeling. Following vascular injury, myofibroblasts and other cell types synthesize pro-collagens, mostly type 1 and type 3. Pro-collagen proteinases initially convert these precursor molecules into collagen fibers which subsequently become cross-linked into collagen fibrils by lysyl-oxidase. In parallel, upregulation of matrix metalloproteinases (MMPs) causes ECM degradation that not only facilitates adaptive changes (including removal of cellular debris by inflammatory cells and migration of myofibroblasts) but also contributes to weakening of the myocardium and ventricular dilatation. Thus, a fine tuned balance of collagen turnover is crucial for AAA remodeling and vascular integrity.
Molecular MR Imaging of collagen in AAA remodeling: Turnover of the extracellular matrix plays a major role in AAA development, making extracellular matrix components an attractive target for molecular imaging. Because of its excellent spatial and temporal resolution and high soft-tissue contrast, MRI has evolved as the gold-standard for molecular imaging of vascular remodeling in experimental animal models and humans. Klink et al., took advantage of a recently described mouse model of AAA using a combination of angiotensin II infusion and transforming growth factor neutralization to assess the use of nanoparticles (NPs) functionalized with a collagen-specific protein, CNA-35. The targeting ligand is derived from 2 domains of a collagen adhesion protein derived from Staphylococcus aureus. Intravenous injection of gadolinium containing NP targeted with CAN-35 resulted in significantly greater T1-weighted signal enhancement in the aneurysmal wall compared with nonspecific NP, and the CNA-35 NPs were shown histologically to colocalize with Type 1 collagen. In a proof-of-concept experiment, animals were imaged at Days 5 and 15 after induction of AAA, and images correlated with pathology. Higher uptake of CNA-35 NP correlated with stable Stage II aneurysms with high collagen uptake, whereas ruptured Stage IV aneurysms showed little uptake and low collagen content. However, the balance between different collagen subtypes (e.g., collagen I and III) was not investigated in this study nor has quantitative MRI protocols including T1 mapping methods to measure collagen deposition in vivo have been employed. A collagen-I binding MRI contrast agent (EP-3600) that has been used to detect chronic myocardial scar together with recently developed collagen-III binding peptides may provide an alternative approach and true molecular information on collagen remodeling in AAA.
Figure 1: Experimental Timeline of Imaging AAA in a murine model
Figure 2: Example of in vivo MRI of the extracellular protein tropoelastin in a murine model of AAA. (A) A 3D reconstructed angiogram acquired from a mouse infused with Angio-II for 3 weeks shows two separate suprarenal AAA. (B-G) MRA, gadolinium enhanced MRI and histology acquired at a normal level of the aorta show a vessel with normal diameter and little enhancement after administration of the tropoelastin-binding contrast agent (Gd-TESMA). Histologically there was absence of disease and lack of tropoelastin. (H-M) At the level of the first aortic dilation (AAA1) the vessel wall showed enhancement after administration of Gd-TESMA. Corresponding histology verified the expansion of the aorta and the deposition of tropoelastin within the remodelled vessel wall. (N-S) MRI images of the second AAA (AAA2) show the formation of a false channel (FC) adjacent to the original lumen and focal enhancement of the vessel wall between the original lumen and the false channel and the around the false channel after administration of Gd-TESMA. Histology validated the formation of two lumens and immunohistochemistry revealed a dense network or tropoelastin molecules in between the original lumen and false channel where contrast enhancement was observed by in vivo MRI. (T-V) Phase contrast MRA images and a flow map verified the formation of a false channel with retrograde blood flow (Phinikaridou A, et al, unpublished data).