TLR4 (Toll-Like Receptor 4) Mediates the Development of Intracranial Aneurysm Rupture

Abstract

Inflammation is emerging as a critical factor in the pathophysiology of intracranial aneurysm. TLR4 (toll-like receptor 4) contributes not only to the innate immune responses but also to the inflammatory processes associated with vascular disease. Therefore, we examined the contribution of the TLR4 pathway to the development of the rupture of intracranial aneurysm. We used a mouse model of intracranial aneurysm. TLR4 inhibition significantly reduced the development of aneurysmal rupture. In addition, the rupture rate and levels of proinflammatory cytokines were lower in TLR4 knockout mice than the control littermates. Macrophage/monocyte-specific TLR4 knockout mice had a lower rupture rate than the control littermate mice. Moreover, the deficiency of MyD88 (myeloid differentiation primary-response protein 88), a key mediator of TLR4, reduced the rupture rate. These findings suggest that the TLR4 pathway promotes the development of intracranial aneurysmal rupture by accelerating inflammation in aneurysmal walls. Inhibition of the TLR4 pathway in inflammatory cells may be a promising approach for the prevention of aneurysmal rupture and subsequent subarachnoid hemorrhage.

Keywords: intracranial aneurysm; intracranial hemorrhage; stroke; subarachnoid hemorrhage; toll-like receptor 4.

Figure 1.

Representative images of unruptured and ruptured aneurysms. Blue dye was injected into Circle of Willis. A. no aneurysm. B. unruptured aneurysm. C. ruptured aneurysm with subarachnoid hemorrhage. Arrowheads indicate intracranial aneurysms.

Figure 2.

TLR4 inhibitor reduced a rupture rate but did not affect the formation of aneurysms. A. Incidence of unruptured and ruptured aneurysms. B. Rupture rate. C. Symptom-free curve (Kaplan–Meier analysis curve). Mice that did not have aneurysms were excluded from this analysis. D. Systolic blood pressure. There was no difference in blood pressure between two group at any timepoint. *P < 0.05, **P < 0.01

Figure 3.

Toll-like receptor 4 (TLR4) knockout mice had a lower rupture rate than wild-type littermates. A. Incidence of unruptured and ruptured aneurysms. B. Rupture rate. C. Symptom-free curve (Kaplan–Meier analysis curve). Mice that did not have aneurysms were excluded from this analysis D. Systolic blood pressure. There was no difference in blood pressure between two group at any timepoint. *P < 0.05, **P < 0.01, TLR4 KO: toll-like receptor 4 knockout mice.

Figure 4.

RNA expression of interleukin (IL)-6, IL-1β, tumor necrosis factor-α (TNF-α), monocyte chemoattractant factor-1 (MCP-1), matrix metalloproteinase (MMP)-9, and nuclear factor-kappa B (NF-κB) p65 in cerebral arteries from mice with or without induction of aneurysm (n = 5 each). *P < 0.05, **P < 0.01, TLR4 KO: toll-like receptor 4 knockout mice.

Figure 5.

Macrophage/granulocyte-specific toll-like receptor 4 (TLR4) knockout mice (TLR4 flox/flox LysMCre +) had a lower rupture rate than control littermate mice (TLR4 flox/flox LysMCre −). A. Incidence of unruptured and ruptured aneurysms. B. Rupture rate. C. Symptom-free curve (Kaplan–Meier analysis curve). Mice that did not have aneurysms were excluded from this analysis. D. Systolic blood pressure. There was no difference in blood pressure between two group at any timepoint. *P < 0.05, **P < 0.01, TLR4 LysM - indicates toll-like receptor 4 flox/flox LysMCre - mice; TLR4 LysM +, TLR4 flox/flox LysMCre + mice.

Figure 6.

Myeloid differentiation primary-response protein 88 (MyD88) knockout mice had a lower rupture rate than wild-type littermates. A. Incidence of unruptured and ruptured aneurysms. B. Rupture rate. C. Symptom-free curve (Kaplan–Meier analysis curve). Mice that did not have aneurysms were excluded from this analysis. D. Systolic blood pressure. There was no difference in blood pressure between two group at any timepoint. *P < 0.05, **P < 0.01, MyD88 KO: myeloid differentiation primary-response protein 88 knockout mice