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

BackgroundCerebral aneurysms are associated with subarachnoid hemorrhages if ruptured; however, mechanisms underlying aneurysmal wall thinning and thickening remain unclear. We previously identified patterns of hemodynamic flow in aneurysms that are associated with wall thickening and thinning, and in this study our objective was to uncover the biological basis for these findings. MethodsCerebral aneurysmal wall samples were collected between August 2020 and March 2022 for proteomic analysis and immunohistological investigation of smooth muscle cells, myoglobin, and inducible nitric oxide synthase (iNOS) expression. We examined the co-localization of myoglobin expression within smooth muscle cells, identified by -smooth muscle actin (-SMA) staining, and myofibroblasts, identified by periostin staining. We measured collagen density in the samples using Sirius Red staining and investigated its correlation with myoglobin density. ResultsAnalysis of proteins extracted from an area of thickening in the aneurysmal wall of one patient confirmed the presence of myoglobin. In 24 formalin-fixed aneurysmal wall samples, 19 expressed myoglobin, with 11 showing strong expression, and eight showing weak expression. Myoglobin was scattered or clustered within the vascular smooth muscle layer and tended to be expressed at sites other than where iNOS was identified. Double-label immunofluorescence staining confirmed that the myoglobin-positive rate within -SMA-positive cells and -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 Spearmans rank correlation coefficient of 0.593 (p = 0.036). ConclusionsCerebral 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.