Abstract TMP1: Cerebral Aneurysm Walls Contain Myoglobin That is Possibly Produced by Myofibroblasts and Contributes to Wall Thickening

Abstract

Background: Cerebral aneurysms rupture at a thin-walled region; however, mechanisms of aneurysmal wall thinning and thickening remain unclear. Myoglobin scavenges nitric oxide and reactive oxygen spices which promote aneurysmal development. Objectives: To investigate the biological role of myoglobin in aneurysmal wall. Methods: Cerebral aneurysmal wall samples were collected between August 2020 and March 2022 for proteomic analysis and immunohistology. We examined the co-localization of myoglobin expression within smooth muscle cells, identified by α-smooth muscle actin (α-SMA) staining, and myofibroblasts, identified by periostin staining. Collagen and myoglobin density in the same sample areas were investigated for its correlation in the aneurysm walls. Results: A total of 35 specimens were obtained from 21 patients who underwent aneurysmal clipping. Proteome analysis was performed using 11 specimens, which firstly detected myoglobin predominantly in the thickened wall. Among 24 formalin-fixed samples, 19 samples were found myoglobin expression. Myoglobin was mostly scattered within the vascular smooth muscle layer and tended to be expressed at sites where iNOS was scarce. Double-label immunofluorescence staining confirmed that the myoglobin-positive rate within α-SMA-positive cells/areas was 33.2±23.8%/31.3±37.8%, respectively, whereas within periostin-positive cells/areas it was 92.2±13.7%/79.8±29.5, respectively. A total of 32 areas were investigated for the correlation between collagen and myoglobin density , which a strong correlation was observed with a Spearman's rank correlation coefficient of 0.635 (p < 0.001). Conclusions: Cerebral aneurysmal walls express myoglobin, which may be produced by myofibroblasts in the wall. Areas with high myoglobin levels retain high levels of collagen fibers, and myoglobin may be involved in wall thickening by suppressing destructive changes in the extracellular collagen fibers.