Intracranial aneurysm pathogenesis
Until now, the exact etiology of intracranial aneurysms formation remains unclear.
Aneurysm wall degeneration
Saccular intracranial aneurysm rupture leads to subarachnoid hemorrhage and is preceded by chronic inflammation and atherosclerotic changes of the Saccular intracranial aneurysm wall. Increased lymphangiogenesis has been detected in atherosclerotic extracranial arteries and in abdominal aortic aneurysms, but the presence of lymphatic vessels in saccular intracranial aneurysm (sIAs) has remained unexplored. Huuska et al. studied the presence of lymphatic vessels in 36 intraoperatively resected sIAs (16 unruptured and 20 ruptured), using immunohistochemical and immunofluorescence stainings for Lymphatic endothelial cells (LEC)markers. Of these LEC-markers, both extracellular and intracellular LYVE1, podoplanin, VEGFR-3, and Prox1-positive stainings were detected in 83%, 94%, 100%, and 72% of the 36 sIA walls, respectively. Lymphatic vessels were identified as ring-shaped structures positive for one or more of the LEC markers. Of the sIAs, 78% contained lymphatic vessels positive for at least one LEC marker. The presence of LECs and lymphatic vessels were associated with the number of CD68+ and CD163+ cells in the sIA walls, and with the expression of inflammation indicators such as serum amyloid A, myeloperoxidase, and cyclo-oxygenase 2, with the presence of a thrombus, and with the sIA wall rupture. Large areas of VEGFR-3 and α-smooth muscle actin (αSMA) double-positive cells were detected in medial parts of the sIA walls. Also, a few podoplanin and αSMA double-positive cells were discovered. In addition, LYVE-1 and CD68 double-positive cells were detected in the sIA walls and in the thrombus revealing that certain CD68+ macrophages are capable of expressing LEC markers. This study demonstrates for the first time the presence of lymphatic vessels in human sIA walls. Further studies are needed to understand the role of lymphatic vessels in the saccular intracranial aneurysm pathogenesis 3).
In addition to ambiental factors (smoking, excessive alcohol consumption and hypertension), epidemiological studies have demonstrated a familiar influence contributing to the pathogenesis of intracranial aneurysms, with increased frequency in first- and second-degree relatives of people with subarachnoid hemorrhage.
Data suggest that macrophage-derived Matrix metalloproteinase 2 and Matrix metalloproteinase 9, may play an important role in the progression of intracranial aneurysms. The findings will shed a new light into the pathogenesis of cerebral aneurysms and highlight the importance of inflammatory response causing the degeneration of extracellular matrix in the process of this disease 4).
Investigations strongly suggest that the pathophysiology is closely associated with chronic inflammation in vascular walls. Nuclear factor kappaB (NF-kappaB) has a key role in the formation and progression.
Children with Sickle Cell Disease (SCD) are at risk for developing multiple intracranial aneurysms, and a high index of suspicion must be maintained during the interpretation of routine magnetic resonance imaging or angiography of the brain 5).
Dental bacterial DNA can be found using a quantitative polymerase chain reaction in both ruptured and unruptured aneurysm walls, suggesting that bacterial DNA plays a role in the pathogenesis of cerebral aneurysms in general, rather than only in ruptured aneurysms 6).
THSD1 in Intracranial aneurysm pathogenesis
Thrombospondin type-1 domain-containing protein 1 is a protein that in humans is encoded by the THSD1 gene.
The protein encoded by this gene contains a type 1 thrombospondin domain, which is found in thrombospondin, a number of proteins involved in the complement pathway, as well as extracellular matrix proteins. Alternatively spliced transcript variants encoding distinct isoforms have been observed.
As illustrated by THSD1 research, cell adhesion may play a significant role in IA 7).
A study discovered that harmful variants in THSD1 (Thrombospondin type-1 domain-containing protein 1) likely cause intracranial aneurysm and subarachnoid hemorrhage in a subset of both familial and sporadic patients with supporting evidence from two vertebrate models 8).
A report identified THSD1 mutations in familial and sporadic IA patients and shows that THSD1 loss results in cerebral bleeding in 2 animal models. This finding provides new insight into IA and subarachnoid hemorrhage pathogenesis and provides new understanding of THSD1 function, which includes endothelial cell to extracellular matrix adhesion 9).
Toll‑like receptor (TLR) 2/4 serves an important regulatory role in nerve tissue injury. However, the downstream and potential mechanisms remain to be elucidated. The present study was designed to investigate the roles of the TLR2/4‑major myeloid differentiation response gene 88 (MyD88)‑NF‑κB signaling pathway in the development of an intracranial aneurysm. The expression of TLR2, TLR4, and MyD88 in the blood of normal controls and patients with intracranial aneurysms were detected by quantitative PCR and ELISA. Human brain vascular smooth muscle cells were treated by Angiotensin II (Ang II) to evaluate the involvement of the TLR2/4‑MyD88‑NF‑κB signaling pathway in the process. The in vitro experiment was divided into four groups: The control group, an Ang Ⅱ group, an Ang Ⅱ + small interfering (si)RNA control group, and an Ang Ⅱ + TLR2‑group. Cell viability, migration, apoptosis, and expression of TLR2, TLR4, MyD88, NF‑κB, and phosphorylated (p‑)p65 expression was detected. The results demonstrated that the expression of TLR2, TLR4, MyD88, and NF‑κB at mRNA and protein levels in patients with an intracranial aneurysm was significantly higher compared with corresponding protein in normal controls (P<0.05). <em>In vitro</em> experiments demonstrated that Ang Ⅱ treatment increased the cell proliferation and migration rate but reduced the apoptotic rate compared with the control (P<0.05). The expression of TLR2, TLR4, MyD88, NF‑κB, and p‑p65 was significantly increased in the Ang II group (vs. control; P<0.05). By contrast, TLR2‑short interfering RNA reduced the cell proliferation and migration rate and reduced the expression of TLR2, TLR4, MyD88, NF‑κB, and p‑p65 (vs. Ang Ⅱ + short interfering RNA control; P<0.05). In conclusion, the data of the present study indicated that the TLR2/4‑MyD88‑NF‑κB signaling pathway is involved in the intracranial aneurysm pathogenesis 10).
Vascular smooth muscle cells
Dysfunction of vascular smooth muscle cells (VSMCs) plays a critical role in the intracranial aneurysm pathogenesis (IA). Circular RNAs (circRNAs) have been implicated by reducing microRNA (miRNA) activity. Qin et al. investigated the precise roles of circRNA ADP ribosylation factor interacting protein 2 (circ-ARFIP2, circ_0021001) in VSMC dysfunction. The levels of circ-ARFIP2, miR-338-3p and kinase insert domain receptor (KDR) were detected by quantitative real-time polymerase chain reaction (qRT-PCR) or western blot. Ribonuclease (RNase) R and subcellular fractionation assays were used to assess the stability and localization of circ-ARFIP2, respectively. Cell viability was detected by Cell Counting Kit-8 (CCK-8) assay, and cell invasion was measured by transwell assay. Cell proliferation was gauged by 5-Ethynyl-2′-Deoxyuridine (EdU) assay. Cell migration was evaluated by transwell and wound-healing assays. Targeted correlations among circ-ARFIP2, miR-338-3p and KDR were validated by dual-luciferase reporter and RNA immunoprecipitation (RIP) assays. Circ-ARFIP2 and KDR were underexpressed and miR-338-3p was overexpressed in the arterial wall tissues of IA patients. Overexpression of circ-ARFIP2 in human umbilical artery smooth muscle cells (HUASMCs) showed a significant promotion in cell proliferation, migration and invasion. Mechanistically, circ-ARFIP2 targeted miR-338-3p, and circ-ARFIP2 regulated cell behaviors by miR-338-3p. KDR was a direct and functional target of miR-338-3p. Moreover, KDR was a downstream effector of circ-ARFIP2 function. Circ-ARFIP2 regulated KDR expression by targeting miR-338-3p.The findings demonstrated that the increased level of circ-ARFIP2 enhanced HUASMC proliferation, migration and invasion at least in part by the miR-338-3p/KDR axis 11).
Pathogenic inflammation contributes to aneurysm formation by mediating the destruction of the endothelium and the extracellular matrix and promoting pathogenic proliferation of smooth muscle cells. In mouse models, tolerance-inducing T regulatory (Treg) cells could significantly reduce the incidence and severity of aneurysms. Hence, it should be investigated why in human intracranial aneurysm (IA) patients, Treg cells failed to provide protection against aneurysm formation. In this study, the frequency and function of Treg cells in IA patients were examined. The frequency of Foxp3+ Treg cells was significantly lower in IA patients than in healthy controls. This downregulation was only specific to the Treg subset of CD4+ T cells, as the frequency of total CD4+ T cell was increased in IA patients. Subsequently, we found that the expressions of Treg-associated molecules, including Foxp3, CTLA-4, TGF-β, and IL-10, were significantly lower in Foxp3+ Treg cells from IA patients than in Foxp3+ Treg cells from healthy controls. In both healthy controls and IA patients, Foxp3+ Treg cells were distinguished into a more potent Tim-3+ subset and a less potent Tim-3- subset. The Tim-3+ subset of Foxp3+ Treg cells was significantly reduced in IA patients. Signaling via IL-2, IL-7, IL-15 and IL-21 was shown to promote Tim-3 upregulation in CD4+ and CD8+ T cells. Interestingly, we found that Tim-3 could be upregulated in Treg cells via the same mechanism, but compared to the Treg cells from healthy controls, the Treg cells from IA patients presented defects in Tim-3 upregulation upon cytokine stimulation. Together, our results demonstrated that Foxp3+ Treg cells in IA patients presented reduced function, which was associated with a defect in Tim-3 upregulation 12).