Jihong Leng

Abstract Subaracoid hemorrhage caused by intracranial aneurysms is characterized by high morbidity and mortality. There is no definite conclusion regarding the mechanism of formation, development, and rupture of intracranial aneurysms. It is generally believed to be related to congenital hereditary connective tissue diseases and acquired hemodynamic factors, vascular inflammation, and oral pathogens. In addition, gender, age, hypertension, and psychological status are also important factors. Relevant studies show a significantly lower quality of life in patients with intracranial aneurysms, and psychological factors should be studied in more depth. Neurological complications are considered an important factor in the decrease in quality of life. In conclusion, the formation,development, and rupture of intracranial aneurysms should result from a combination of congenital and acquired factors.

Keywords: Intracranial aneurysm; formation factors; research progress; subaracoid; hemorrhage

Introduction

Intracranial Aneurysms (IA) have been of great concern among cardiovascular diseases due to their high morbidity and mortality rates. The rupture of IA is prone to adverse outcomes such as Subarachnoid Hemorrhage (SAH) and vasospasm. SAH is one of the most severe cerebrovascular diseases, with a mortality rate ranging from 60% to 90% due to its poor prognosis [1]. A study found that the incidence of IA in Chinese people in the 35-75 years interval increases with age [2]. Some of these diseases that the elderly are prone to are likely to contribute to the development of IA. As the aging trend in China further increases, the number of older people with IA may further increase, which may bring a heavier burden to families and society. Therefore, it is vital to prevent the formation of IA and to provide effective intervention and treatment. Factors contributing to the building and growth of IA may involve inflammatory response, hemodynamics, genetics,and external environmental factors. The causes of IA formation are still inconclusive, and it is mainly believed that the formation of IA is related to congenital and acquired factors. Hereditary factors are primarily genetic, including familial genetic inheritance of pathogenesis, cerebrovascular genetic variants, and connective tissue diseases. Acquired factors include a wide range of factors, among which vascular inflammatory factors are thought to play an essential role in the development of IA [3]. In addition to these, hemodynamic factors, oral pathogens, associated pathogenic genes, bone mineral density, and psychological factors should be considered. In this article, we review the progress of research on the causes of intracranial aneurysm formation.

Congenital factors

The medical community has generally recognized the strong association of IA formation with hereditary factors. Today, the evidence that genetic factors influence IA formation is as follows: (1)Familial intracranial aneurysm. Some investigators have found that some patients with IA have a significant familial pattern. The study finds that first-degree relatives of IA patients are 3-5 times more likely to develop the disease than the general population [4]. Amy et al. [5] found that genetic screening of 53 families with familial IA patients revealed the presence of gene variants such as C4orf6 and SPDYE4 that were inherited from one generation to another. In addition, several genome-wide linkage studies of families with IA identified several susceptibility loci for IA (1p34-36, 4q32, 7q11,14q22, 19q13, and Xp22) [6], with 19q13 and Xp22 found to be linked in a study of Finnish siblings with a family history of IA [7].Sequencing the ANGPTL6 gene using Sanger sequencing, Isabel et al. [8] identified six genetic variant loci in the ANGPTL6 gene that were not present in the control population in patients with a family history of IA. In a Whole-Exome Sequencing (WES) of European IA families, multiple Loss-of-function (LoF) and Deleterious Missense (D-Mis) mutations in the PPIL4 gene were found to be more prevalent in patients [9]. This gene is involved in cerebral angiogenesis and is thought to be associated with IA development[7]. The above research results suggest the possibility of double or multiple gene inheritance or inheritance of the same gene at different loci in IA. (2) Congenital variants of cerebral blood vessels(especially the Circle of Wills) (COW). The COW is circulatory anastomoses of arteries located at the base of the brain. Variations in the shape of the COW are common, and some individuals in the population may have a defective or dysplastic arterial component of the COW. The COW is more likely to undergo the same changes within IA families than in unrelated populations [10], and aneurysms in affected members of IA families may occur within the same arterial region of the COW [11]. This indicates that the occurrence of IA may be influenced by the inheritance of familial Willis’s ring variants. (3) Connective tissue disease. Connective tissue diseases are often thought to be more influenced by hereditary genetic factors and are caused by genetic mutations. In a survey of patients with connective tissue disorders, a high incidence of IA was found in patients with these conditions, including Marfan Syndrome (MFS),and Ehlers-Danlos Syndrome (EDS), 1- neurofibromatosis (NF), and Loeys-Dietz syndrome [12]. The cause may be a genetic connective tissue disorder resulting in a weak or absent mid or outer membrane,reduced collagen and elastin content and thus a damaged and thinner vessel wall. These changes in vascular structure are risk factors for the formation of IA [13].

Acquired factors

Although we cannot do much to intervene in the hereditary factors of IA formation, we can effectively detect and prevent the acquired characteristics to which it is subjected. Therefore, it is vital to investigate the acquired factors of IA formation, including hemodynamic factors, oral pathogenesis, and atherosclerosis.In addition to these, smoking, hypertension, and gender are also important factors contributing to the formation of IA14. One study found that middle-aged women with a history of smoking had a fourfold increased risk of IA, and this risk rose to sevenfold if they also happened to have chronic hypertension [14].

Vascular inflammatory effects

Tomohiro et al. [15]suggest that IA is essentially a chronic inflammatory condition. Disturbed blood flow leads to the release of Endothelin-1 (ET-1) from Endothelial Cells (ECS) [16]. As an endogenous vasoconstrictor, endothelin can direct vascular inflammation in addition to its vasoconstrictive effects. In one study,ET-1 was possibly involved in activating the NF-kB pathway and the expression of pro-inflammatory cytokines (e.g., Tumor Necrosis Factor-alpha, TNF-a) [17]. NF-κB is considered to be a key initiator of IA development. By inducing some inflammatory factors that can activate and induce macrophages, such as Monocyte Chemotactic Protein (MCP-1), it has been found that MCP-1 activates the NFκB pathway in macrophages [18]. Activated macrophages attack the vessel wall [19], and inflammatory cells also secrete cytokines that interfere with vascular endothelial cells [20], which can cause damage to the vessel lining. M1 pro-inflammatory macrophages were also necessary for atheroma formation in a chemokine-dependent CXCL1 model of neutrophil inflammation [21]. TNF-a was found to promote NF-kB activation and activated NF-kB can mediate the expression of matrix metalloproteinase-9 (MMP-9) [22,23],which degrades elastin, a component of the Internal Elastic Lamina(IEL). Reduced or ruptured elastin has been found in biopsies of cerebral vessel walls of patients with IA [24], leading to vessel wall weakness and further IA formation. The above process is illustrated in Figure 1. As a critical link in the overall IA formation process,the inflammatory response involves synthesizing multifunctional proteins and releasing and aggregating inflammatory factors. Further understanding of the inflammatory response can help develop antiinflammatory, inflammatory factor-inhibiting drugs targeting IA. The associated inflammatory factor index changes can also be applied in clinical tests.

Figure 1. A mechanism of intracranial aneurysm generation due to vascular inflammatory effects. Disturbed blood flow in the Willy ring stimulates vascular endothelial cells to secrete ET-1 acting on ETBR receptors in smooth muscle cells, activating the NF-κB pathway. MCP-1 is expressed in cells and released, MCP-1 activates the NF-κB pathway in macrophages, and macrophages move like the vessel wall and cause damage. At the same time, NF-κB can mediate the production of MMP-9, which can degrade elastin in the vessel wall, leading to vessel wall softening and IA formation.

Hemodynamic factors

There is increasing evidence that the hemodynamics imposed by flowing blood on the vessel wall may lead to vascular remodeling,leading to aneurysm formation and rupture. Thus, hemodynamics may be considered an essential factor in the formation of IA. In a study for database search analysis, it was concluded that Wall Shear Stress (WSS) and Oscillatory Shear Index (OSI) as hemodynamic factors might significantly impact the formation, development, and rupture of IA [25]. Both will be discussed in detail in this paragraph.

WSS

WSS represents the resistance per unit area parallel to the direction of flow exerted by the flowing blood on the vessel wall and can also be considered the frictional force developed when the blood flow comes in contact with the vessel wall. In areas where blood flow velocity varies rapidly, such as vessel curvature, WSS produces a constant irritation of the vessel, which will disrupt the vessel structure and cause a local inflammatory response [26]. This blood flow pattern in areas of geometrically irregular arteries is also considered a disturbed blood flow state [27]. Under normal blood flow conditions, endothelial cells prevent thrombosis, resist atherosclerosis, and regulate vascular tone by producing different active substances [28]. However, the low shear stress generated by disturbed blood flow in the region of disturbed blood flow affects endothelial cell signaling and gene expression [21]. Nathan et al. [27]found that experimentally induced blood flow disturbances in human forearms in which occlusive cuffs were placed distally caused activation and apoptosis of human endothelial cells, as evidenced by activated (CD62E+) and apoptotic (CD31b/CD42b -) Endothelial Microparticles (EMPs) levels are elevated. These cellular processes are essential components of vascular endothelial cell damage,and EMPs are considered markers of cerebrovascular disease and induce vascular dysfunction [29]. In addition to this, disturbed blood flow leads to Occluding closure protein phosphorylation, which disintegrates the cytoskeleton and increases vascular permeability[30]. This protein is responsible for the Tight Junction(TJ) barrier property between adjacent endothelial cells and the transmission of mechanically induced tension to the cytoskeletal actin [31].Increased vascular permeability may facilitate the movement of inflammatory cells such as macrophages between blood vessels.Similarly, low WSS induces conformational changes in Plateletendothelial Cell Adhesion Molecules (PECAM-1). In a mouse model of atherosclerosis, endothelial PECAM-1 is upregulated in areas of atherosclerosis with disturbed blood flow, leading to local inflammation and progression of atherosclerosis [32]. Harrison et al. [33] also demonstrated that disrupted blood flow induces local pro-inflammatory activation of endothelial cells through PECAM-1-dependent activation of mechanosensory complexes, leading to the production of the pro-inflammatory transcription factor NF-κB.PECM-1 is a component of the mechanosensory complex, which responds to mechanical forces and transmits signals to the cell.Thus, the pressure changes generated by disturbed blood flow cause PECAM-1 to transmit a signal to the cell, which activates the proinflammatory cellular pathway.

In addition to this, another pathway of the effect of WSS has been suggested: NADH/NADPH oxidase, a multimeric enzyme composed of plasma membrane-associated proteins and cytoplasmic factors, increases NADPH oxidase activity [34], and vascular cells use Reactive Oxygen Species (ROS) to modified Low-density Lipoprotein (LDL). Endothelial cells produce ROS and RNS via these enzymes and specific NO synthase and NADPH oxidase(gp91phox, Nox1, and NOX4). NADPH oxidase can produce high levels of ROS in the vasculature in response to specific stimuli from flow conditions. Thus, more modified LDL species are formed under oscillatory flow conditions that favor ROS production [35], which results in the oxidation of LDL to form Minimally Modified-LDL(MM-LDL) and highly oxidized LDL. MM-LDL induces endothelial cells to express MCP-1, leading to monocyte binding, chemotaxis,and subsequent transendothelial migration [36]. This leads to further development of inflammation.

Frosen et al. [33] found that altered blood flow velocity-induced Low WSS (LWSS) can induce ECs dysfunction and vessel wall damage through different mechanisms, affecting the formation and growth of IA. This suggests that the role of WSS in IA formation may be related to endothelial cell dysfunction caused by the stimulation of endothelial cells.

OSI

OSI expresses the hemodynamic parameter of the magnitude of the WSS oscillation caused by rapid changes in the direction of blood flow within the aneurysm during systole and diastole. Cebral et al. [38] found that local flow conditions and vascular perfusion remodeling were associated with high OSI values associated with a high flow state at the site of impaction and local thinning of the vessel wall surface. Related studies have found that high OSI is more likely to lead to IA rupture [39].

Oral pathogens

There is increasing evidence that periodontal disease and oral pathogens are associated with IA formation [40,41], with gingivitis considered a risk factor for IA formation [42]. The Toll-Receptor (TLR) plays a role in periodontal pathogens leading to IA formation. Many studies have shown that the TLR recognizes periodontal pathogens [43] and that stimulation of the TLR by bacterial periodontal pathogens leads to dimerization of the TLR.TLR dimerization activates inflammatory pathways, leading to an increase in pro-inflammatory factors such as NF-κB [44]. In previous studies, bacterial DNA of oral origin was detected in 69% of ruptured aneurysms and 71% of unruptured aneurysms.This is because periodontal bacteria have been found to survive in human macrophages [45] and, therefore, may spread to the IA via infiltration of macrophages into the vessel wall. Porphyromonas Gingivalis (PG) is thought to play an essential role in developing periodontitis [46]. Using the fluorescence in Situ Hybridization(FISH) technique, Cavrini et al. [47] and IgA antibodies suggest their possible ability to invade the vessel wall. The binding of IgA antibodies to Fcα receptors in serum induces an inflammatory response that includes the activation of macrophages [48]. Joona et al. [49] synthesized recent findings to propose several mechanisms by which periodontitis and periodontal pathogens promote degenerative remodeling of the IA wall, in which periodontal bacteria all play a leading role in the initiation of IA formation.For example, Porphyromonas gingivalis activates neutrophils.The continuous stimulation increases the production of circulating elastase by neutrophils, breaking the Elastic Layer (EI) of the vessel wall and leading to IA formation [50]. TLR-NF-κB signaling was essential in disrupting vascular endothelial homeostasis by Porphyromonas gingivalis. Lipopolysaccharide (LPS), produced by periodontal bacteria through Toll-like receptor activation of NF-κB signaling, causes MCP-1 production, which attracts macrophages to the vessel wall and causes damage to it. These suggest that oral bacteria are involved in forming IA and its progression to rupture.

Pathogenic genes

In current studies, many genes are related to IA formation, such as angiotensin-converting enzyme (ACE), Elastin, Type Ⅲ collagen(COL3), and MMP genes. We have selected two genes (elastin gene and ACE gene) that have an enormous impact on hemodynamics to be discussed.

Several motifs of chromosomes identified by genome-wide linkage studies mentioned above showed linkage on chromosomes 1p34.3-p36.13, 7q11, 19q13.3, and Xp22. The marker with the most robust evidence of linkage on the 7q11 region was located near the elastin gene, suggesting a possible genetic predisposition for the elastin gene.

Elastin genes

Elastin genes primarily encode elastin in the vascular wall, a crucial component of the vascular elastic lamina. The level of elastin gene expression disrupts the vascular wall structure by affecting elastase activity, which leads to aneurysm generation and rupture. Miskolczi et al. [51] injected porcine pancreatic elastase into the bifurcation of the right common carotid artery in New Zealand rabbits and found that this induced a saccular aneurysm. The rationale may be that elastase accelerates the breakdown of elastin, which disrupts the elastic layer of the vessel wall, thereby degrading the arterial wall[52]. Then if the elastin gene expression is upregulated, it may lead to high elastase content, causing damage to the vessel wall and thus leading to IA formation.

ACE gene

The ACE gene is located on the long arm of autosome 17 and consists of 26 exons, with an insertional sequence of 287 bp in intron 16 forming the insertion (I)/deletion (D) polymorphism of the gene. ACE catalyzes the conversion of angiotensin I to angiotensin II, which can constrict arterial blood vessels and cause hypertension.Hypertension is a risk factor for the development of IA. Vincent et al. [53] increased blood flow by ligating the carotid artery in New Zealand rabbits. This resulted in vascular caliber expansion and pathological remodeling, such as vascular tortuosity and aneurysm development in the Circle of Willis (COW). Through a metaanalysis of clinical data from patients with IA, Yan et al. [54] found a significant association between ACE I/D gene polymorphisms and increased risk of IA. The II genotype was more likely to lead to IA formation than the DD genotype. This was also confirmed in an analysis of ACE I/D gene polymorphisms in Chinese IA patients [55].However, in some studies, the D allele was associated with increased serum ACE concentrations in humans [56]. We speculate that patients with the DD genotype are more likely to have hypertension,expressed as high wall shear stress in hemodynamics. Patients with the II genotype have relatively low blood pressure, expressed as low wall shear stress. The hypothesis for both high and low shear stress is that both can contribute to the development of IA [57]. High shear stress leads to atherosclerotic aneurysms, and low shear stress leads to the development of secondary aneurysm phenotypes. Thus, there may be two different expression ideas of ACE genes, corresponding to two different cut-off stresses. However, the mechanism by which ACE polymorphisms affect the formation of IA has not been studied in detail.

Other genes

A related study investigated the association between Twelve Single Nucleotide Polymorphisms (SNPs) in Genome-Wide Association Studies (GWAS) identified in the locus and the risk of IA in the Chinese population. The results confirmed that genetic variants in the CDKN2B-AS1, RP1, and HDAC9 motifs were significantly associated with IA risk in the Chinese population. However, the associations of variants at or near the EDNRA, SOX17, and RBBP8 motifs with IA were not replicated, suggesting the existence of common and specific susceptibility factors in the Chinese population[58].

Bone mineral density (BMD)

In one investigation, lower bone mineral density was associated with an increased risk of IA [59]. The association between BMD and IA may be attributed to connective tissue components and associated factors, hormonal effects, and other mechanisms of biological sharing. The Extracellular Matrix (ECM) plays a key linking role among these. Components of the ECM, such as collagen, are widely distributed in the walls of intracranial arteries [60]. Collagen is also a significant component of the organic part of the bone. Genetic correlation studies have shown pathological remodeling of the ECM to be closely related to the development of IA [61]. While collagen accounts for 95% of total bone collagen and about 80% of bone proteins, changes in its properties can affect the strength of bone mass. Therefore, when ECM is absent in the body, it manifests as osteoporosis.

It has been shown that the process of ECM establishment and degeneration is related to Spatio-temporal patterns [62], especially the circadian rhythms in the ECM. These patterns are responsible for the physical and biological properties of the skeleton. Disturbances in the patterns can disrupt the remodeling of the bone matrix and cause various bone diseases, such as fractures. So, if an older person with a sleep disorder has a pathological remodeling and global deficit in the ECM due to a disruption of its circadian patterns, this could predispose him to fractures. Combined with the above studies,this results in a possible increase in the chance of IA formation.Therefore, we can hypothesize that the probability of having an intracranial aneurysm may be elevated when an older person has a fracture. The cause of the fracture may be pathological remodeling of the ECM or a deficiency of the essence in the kidney, as referred to in Traditional Chinese Medicine (TCM). However, the effect of fracture on IA formation in the elderly has not been studied in detail,which makes our hypothesis lack strong data support. In addition,whether the fractures have different effects on the body depending on the fracture site needs to be studied in more depth.

Psychological factors

The mechanism of psychological factors influencing IA formation has not been studied. We have synthesized research results from related fields and proposed a possible mechanism of influence. This mechanism targets a specific population of older adults suffering from depression.

Studies have found that adverse childhood experiences can indirectly exacerbate the worsening of depression in adults [63].Then if the person experienced a huge life change in childhood,such as the loss of both parents, it might lead to depression that gets progressively worse with age. In Chinese medicine, excessive thinking is an essential factor in triggering depression [64]. Animal studies [65,66] have shown that rats that undergo separation from their mothers early in life exhibit depressive behavior and increased activation of pro-inflammatory cytokines and microglia in adulthood. Pro-inflammatory cytokines can activate microglioma cells and cause neuroinflammation [67]. Evidence shows that inflammatory processes, including pro-inflammatory cytokine levels and microglial activation, may influence behavior and mood. Related studies have also shown that neuroinflammation and cytokines maintain depressive symptoms [68]. Another animal study showed that necrotic apoptosis in the brain increases with age. This deepening of apoptosis contributes to the development of neuroinflammation in mice [69]. In contrast, NF-κB is a crucial initiator of IA development, as mentioned earlier. Activated microglia play an essential role in the inflammatory response.Furthermore, in glial cells, the transcription factor NF-κB regulates the expression of several inflammatory factors.

The above findings infer that depression from external stimuli may lead to neuroinflammation and aging in patients. The ensuing NF-κB in activated glial cells induces the MCP-1 gene to produce monocyte chemotactic proteins; macrophages are activated by MCP-1 and induced to the vessel wall and attack it. The inflammatory zone of the vessel wall breaks down. This coincides with the initial symptoms of IA formation.

The nature of psychological factors influencing IA formation is perhaps still influencing a chronic inflammatory process.The prevalence of depression in the Chinses elderly increased yearly from 2010 to 2019, reaching 25.5%. The values will continue to rise as the population ages further. This requires a sufficiently high level of attention and appropriate strategies from society.

Conclusion

The mechanisms of IA formation, development, and rupture are complex. Genetic mutations lead to structural and functional damage of the arterial wall from hereditary factors, making the vessel less stable. The acquired WSS, OSI, vascular inflammatory response,and oral pathogen pathogenesis are also essential factors in the formation of IA. In addition, age, gender, hypertension, and other factors should not be ignored. A large number of studies have been conducted to investigate the factors of IA formation. However, few articles review various factors, which makes understanding the IA formation mechanism incomplete. In this paper, we synthesize the research results of the past ten years to review the factors of IA formation and hypothesize about the possible connections between these factors. Multiple factors cause the formation of IA. In combination with the idea of "treating the disease before it occurs"in Chinese medicine, a better understanding of the factors of IA formation can help in the early diagnosis, monitoring, and evaluation of the disease. In order to reduce the risk of IA rupture and the mortality rate, it is vital to understand more about the factors that lead to IA.

Outlook

The mechanism of intracranial aneurysm formation is still unclear.However, we believe its essence is still a chronic inflammation caused by external stimuli (smoking, mood changes) and internal(imbalance in blood flow, gene expression, protein synthesis,hormones, etc.). Thanks to exploring the genetic mechanisms of IA formation, it has become possible to diagnose and treat IA genetically by finding effective targets, which would effectively reduce the rupture and mortality of IA. TCM has unique advantages in the treatment of IA. However, there is no unified understanding of TCM for IA; modern studies have confirmed the definite efficacy of TCM for aneurysmal subarachnoid hemorrhage and cerebral vasospasm after aneurysm embolization, but there is no detailed study on the prevention of IA rupture. With the further policy tilt toward the development of TCM, there will be a broad development prospect for TCM in this field. We believe that the application of combined Chinese and Western medicine will have better efficacy for this type of disease, which is widely involved and complicated to treat.