Molecular mechanisms of the formation and progression of intracranial aneurysms

Until recently, only a little was understood about molecular mechanisms of the development of an intracranial aneurysm (IA). Recent advancements over the last decade in the field of genetics and molecular biology have provided us a wide variety of evidences supporting the notion that chronic inflammation is closely associated with the pathogenesis of IA development. In the field of genetics, large-scale Genome-wide association studies (GWAS) has identified some IA susceptible loci and genes related to cell cycle and endothelial function. Researches in molecular biology using human samples and animal models have revealed the common pathway of the initiation, progression, and rupture of IAs. IA formation begins with endothelial dysfunction followed by pathological remodeling with degenerative changes of vascular walls. Medical treatments inhibiting inflammatory cascades in IA development are likely to prevent IA progression and rupture. Statins and aspirin are expected to suppress IA progression by their anti-inflammatory effects. Decoy oligodeoxynucleotides (ODNs) inhibiting inflammatory transcription factors such as nuclear factor kappa-B (NF-κB) and Ets-1 are the other promising choice of the prevention of IA development. Further clarification of molecular mechanisms of the formation and progression of IAs will shed light to the pathogenesis of IA development and provide insight into novel diagnostic and therapeutic strategies for IAs.

Cerebral aneurysm progression suppressed by blockage of endothelin B receptor

OBJECT

Cerebral aneurysm is a major cause of subarachnoid hemorrhage, but the mechanisms of its development remain unclear. Mechanical stretch has been reported to induce vascular smooth-muscle cell apoptosis via endothelin B receptors (ETBRs). The objectives of this study were to clarify the expression and localization of ETBR in cerebral aneurysms and to examine the effect of ETBR blockage on the development of experimental cerebral aneurysms.

METHODS

Seventy-two rats underwent a cerebral aneurysm induction procedure and were divided into four groups according to the duration of postoperative study periods. Expression of ETBR was confirmed by reverse transcription-polymerase chain reaction and immunohistochemical analysis. The authors also studied the effect of K-8794, an oral selective antagonist of ETBR, to see whether it would influence the formation of cerebral aneurysms. Two weeks after the aneurysm induction procedure, ETBR was rarely detected in anterior cerebral artery-olfactory artery bifurcations, but it was weakly expressed in experimental cerebral aneurysms at 1 month after the procedure, and markedly expressed at 3 months. The administration of K-8794 for 1 month after the procedure significantly reduced the number of advanced aneurysms and the number of apoptotic smooth-muscle cells.

CONCLUSIONS

These results suggest that ETBR might play a significant role in the progression of cerebral aneurysms and have the potential to improve prevention and treatment of cerebral aneurysms.

Role of apoptosis in intracranial aneurysm rupture

OBJECT

Mechanisms involved in the rupture of intracranial aneurysms remain unclear, and the literature on apoptosis in these lesions is extremely limited. The hypothesis that apoptosis may reduce aneurysm wall resistance, thus contributing to its rupture, warrants investigation. The authors in this study focused on the comparative evaluation of apoptosis in ruptured and unruptured intracranial aneurysms. Peripheral arteries in patients harboring the aneurysms and in a group of controls were also analyzed.

METHODS

Between September 1999 and February 2002, specimens from 27 intracranial aneurysms were studied. In 13 of these patients apoptosis was also evaluated in specimens of the middle meningeal artery (MMA) and the superficial temporal artery (STA). The terminal deoxynucleotidyl transferase-mediated deoxyuridine triphosphate nick-end labeling technique was used to study apoptosis via optical microscopy; electron microscopy evaluation was performed as well. Apoptotic cell levels were related to patient age and sex, aneurysm volume and shape, and surgical timing. Significant differences in apoptosis were observed when comparing ruptured and unruptured aneurysms. High levels of apoptosis were found in 88% of ruptured aneurysms and in only 10% of unruptured lesions (p < 0.001). Elevated apoptosis levels were also detected in all MMA and STA specimens obtained in patients harboring ruptured aneurysms, whereas absent or very low apoptosis levels were observed in MMA and STA specimens from patients with unruptured aneurysms. A significant correlation between aneurysm shape and apoptosis was found.

CONCLUSIONS

In this series, aneurysm rupture appeared to be more related to elevated apoptosis levels than to the volume of the aneurysm sac. Data in this study could open the field to investigations clarifying the causes of aneurysm enlargement and rupture.