Cerebral aneurysm walls contain myoglobin that is possibly produced by myofibroblasts and contributes to wall thickening

Cerebral aneurysms rupture at a thin-walled region; however, mechanisms underlying aneurysmal wall thinning and thickening remain unclear. In this study our objective was to uncover the biological basis for these findings. 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. Furthermore, we measured collagen density in the samples using Sirius Red staining and investigated its correlation with myoglobin density. Analysis of proteins extracted from aneurysmal walls detected expression of myoglobin only in the thickened wall. Among 24 formalin-fixed aneurysmal wall samples, 19 expressed myoglobin. Myoglobin was scattered or clustered within the vascular smooth muscle layer and tended to be expressed at sites other than where inducible nitric oxide synthase was identified. Double-label immunofluorescence staining confirmed that the myoglobin-positive rate within α-SMA-positive cells and α-SMA-positive areas was 33.2 ± 23.8% and 31.3 ± 37.8%, respectively, whereas within periostin-positive cells and periostin-positive areas, it was 92.2 ± 13.7% and 79.8 ± 29.5, respectively. A moderate correlation was observed between the density of myoglobin and collagen in the same sample field, with a Spearman's rank correlation coefficient of 0.593 (p = 0.036). 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 matrix collagen fibers.