Mouse Abdominal Aortic Aneurysm Model Induced by Perivascular Sheath Microinjection of Chymotrypsin

Abdominal aortic aneurysm (AAA) is a life-threatening degenerative vascular disease for which no effective pharmacological therapy currently exists. Reliable and technically accessible animal models are essential for mechanistic and preclinical investigations. However, existing murine platforms are limited by technical complexity, prolonged operative time, or incomplete recapitulation of human AAA pathology. Here, this study presents a novel murine AAA model established by direct perivascular sheath microinjection of chymotrypsin. After midline laparotomy and identification of the infrarenal abdominal aorta, chymotrypsin working solution is injected directly into the perivascular sheath space under direct vision using a microscale needle. To determine the optimal enzyme concentration, a pilot dose-finding study was conducted using 0.125% (w/v; 1.25 mg/mL), 0.25% (w/v; 2.50 mg/mL), and 0.375% (w/v; 3.75 mg/mL) chymotrypsin (n = 5 per group), with PBS as negative control and topical PPE as positive control. Based on aneurysm incidence, aortic diameter progression, and mortality, 0.25% chymotrypsin was selected for the main study. The chymotrypsin group was directly compared with the PPE positive control group to benchmark the pathological features of this novel model. Serial Doppler ultrasonography confirmed a 100% aneurysm induction rate at day 14 in chymotrypsin-treated mice, whereas no dilation occurred in PBS-injected controls. Histological analyses revealed pronounced medial thickening, elastic fiber fragmentation, and collagen deposition, together with markedly elevated immunohistochemical expression of matrix metalloproteinase (MMP)-2 and MMP-9 in aortic sections. Quantitative morphometry demonstrated significant increases in aortic wall thickness and collagen volume fraction, and a corresponding reduction in elastic fiber content in chymotrypsin-treated animals compared with controls. This protocol thus provides a technically streamlined, highly reproducible, and translationally relevant murine AAA model. Because the procedure avoids aortic dissection, arteriotomy, or specialized microsurgical training, it is well suited to laboratories investigating aneurysm pathobiology or evaluating novel pharmacological interventions.