|Year : 2015 | Volume
| Issue : 1 | Page : 88-96
Review of past and present research on experimental models of moyamoya disease
Shuji Hamauchi, Hideo Shichinohe, Kiyohiro Houkin
Department of Neurosurgery, Graduate School of Medicine, Hokkaido University, Sapporo, Hokkaido, Japan
|Date of Submission||04-May-2015|
|Date of Acceptance||23-Jun-2015|
|Date of Web Publication||30-Sep-2015|
Kita-15, Nishi-7, Kita-ku, Sapporo - 060-8638
Source of Support: None, Conflict of Interest: None
Moyamoya disease (MMD) is characterized by a progressive steno-occlusive disease affecting the terminal portions of the cerebral internal carotid artery (ICA) and by formation of an abnormal vascular network at the base of the brain. Several pathogeneses, including inflammation, immune complex, upregulation of angiogenic factors, and abnormality of endothelial progenitor cells (EPCs) have been hypothesized. However, the mechanisms of MMD are largely unknown, and in vivo and in vitro models of MMD have not yet been established. Previously, inflammation- and immune-complex-related animal models have been reported but failed to reproduce severe stenotic lesions in the terminal portion of ICA. Thereafter, several clinical studies revealed that angiogenic activity of circulating EPCs was defective in MMD patients. These results suggested that the function and quantity of EPCs could be useful as a cellular model of MMD. Very recently, RING finger protein 213 (RNF213) was identified as an MMD susceptibility gene, a discovery that led to the efforts to generate gene mutation-based animal models. Although RNF213 knockout animal models have not yet successfully represented the phenotype of MMD, they have provided new insights into the role of RNF213 in remodeling after vascular injury and postischemic angiogenesis. Furthermore, the use of induced pluripotent stem cells (iPSCs) and an appropriate differentiation protocol have made it possible to obtain abundant quantities of MMD-specific vascular cells. In summary, studies have shown that endothelial cells derived from MMD-iPSCs have impaired angiogenic activity, which is a finding consistent with the results of EPC studies. Further studies are needed to create true MMD-specific experimental models to promote understanding of MMC pathogenesis and aid drug development.
Keywords: Endothelial progenitor cells (EPCs), experimental model, induced pluripotent stem cells (iPSCs), moyamoya disease (MMD), RING finger protein 213 (RNF213)
|How to cite this article:|
Hamauchi S, Shichinohe H, Houkin K. Review of past and present research on experimental models of moyamoya disease. Brain Circ 2015;1:88-96
| Introduction|| |
Moyamoya disease (MMD) is a progressive steno-occlusive disease affecting the terminal portions of the cerebral internal carotid artery (ICA) followed by formation of an abnormal vascular network at the base of the brain as a collateral circulation. ,,,, MMD was first described in Japan in the 1960s. ,,, "Moyamoya" means"puff of smoke" in Japanese, referring to the appearance of abnormal vascular network on cerebral angiogram. MMD is diagnosed by detecting stenosis or occlusion at the terminal portions of the ICA and/or their branches (anterior or the middle cerebral arteries) and abnormal vascular networks in the vicinity of the occlusive or stenotic lesions, as shown by cerebral angiography. So-called quasi-MMD or moyamoya syndromes that have vascular changes associated with the following diseases or conditions should be excluded: Atherosclerosis, autoimmune disease, meningitis, brain tumors, Down syndrome, von Recklinghausen's disease, traumatic brain injury, irradiation, and others.  The prevalence of MMD is much higher in East Asian countries than in Western countries. The highest prevalence and incidence rate of MMD are found in Japan at 3.16 and 0.35 per 100 000, respectively. In Europe, the incidence of MMD have been estimated at 1/10 of the incidence in Japan. Familial MMD is found in 10-15% of people with MMD, and its mode of inheritance is thought to be autosomal dominant with a low penetrating rate.  Nanba et al. reported that a female preponderance was significantly more prominent in the familial than in the sporadic group and mean age at onset was significantly lower in familial than in sporadic cases.  The age of onset of MMD is found to have a bimodal distribution, with the first peak at 5 years of age and the second peak at about 40 years of age. Clinical features of MMD differ between children and adults. Children mainly present with transient ischemic attack (TIA) or cerebral infarction; in contrast, adult patients present cerebral infarction and TIA or cerebral infarction.  Histopathological studies have shown remarkable intimal thickening, waving of internal elastic lamina, and medial thinning at the terminal portion of ICA in MMD patients.  Several pathogenic mechanisms, including inflammation, ,, upregulation of various angiogenic factors, ,,,,,,, and abnormalities of endothelial progenitor cells (EPCs), have been hypothesized. ,,,
In recent years, RING finger protein 213 (RNF213) has been identified as a susceptibility gene for MMD in East Asian people by genome-wide linkage and association study. Mutation in RNF213 was found in 95% of familial MMD cases, 73% of nonfamilial MMD cases, and 1.4% of controls.  In parallel with investigations into the pathogenesis of MMD, several efforts to establish experimental in vivo and in vitro models of MMD have been made. For a long time, the main subject of research on MMD has been the effect of environmental factors such as inflammation, angiogenic factors, and EPCs, but after RNF213 was identified as a susceptibility gene of MMD, the trend of research has drastically changed, and research focusing on the biological effect of mutant RNF213 has developed. However, the mechanism of MMD is largely unknown, and MMD-specific models have not been well established. The lack of experimental models has inhibited drug development and progress in understanding the mechanism of MMD. In this review article, we summarize past research on experimental models of MMD and present a guide for establishing novel experimental models of MMD in future research. We attempted to review studies in which the main subjects were experimental animal models of MMD, and analysis of vascular cells derived from MMD patients. The main studies were summarized according to the following viewpoints:
- Investigations of the causal genesis of MMD,
- In vivo models, and
- In vitro models.
| Investigations of the Causal Genesis of Mmd|| |
In MMD patients, initial stenotic lesions are usually found in the terminal portions of ICA. Histopathological studies of autopsies on patients with MMD have shown remarkable thickening of the intima, waving of the internal elastic lamina, and medial thinning in the stenotic lesions.  Masuda et al. reported that the thickened intima was composed predominantly of smooth muscle cells with an admixture of some macrophages and T-lymphocytes.  Researchers hypothesized that inflammatory stimuli may induce a proliferative response of smooth muscle cells and contribute to formation of the intracranial occlusive lesions in MMD. At present, however, it is well known that inflammatory cell infiltration and lipid deposits are rare in MMD stenotic lesions, findings that are completely different from those in atherosclerosis.  It is difficult to understand the mechanism of MMD in detail from pathological studies alone.
| Angiogenic factors|| |
Many studies have shown increases in angiogenic factors in specimens obtained from MMD patients. Basic fibroblast growth factor (bFGF)  and hepatocyte growth factor (HGF)  have been reported to be elevated in the cerebrospinal fluid of MMD patients. Basic fibroblast growth factor,  transforming growth factor (TGF-β),  platelet-derived growth factor (PDGF),  hepatocyte growth factor,  and hypoxia-inducible factor-1 alpha (HIF-1α)  have been shown to be elevated in surgical specimens, such as those taken from arterial walls and the dura mater. In addition, vascular endothelial growth factor (VEGF) has been found to be elevated in the sera of MMD patients.  These factors are known to have important roles in vascular cell proliferation and angiogenesis. Thus, these proteins may contribute to the pathogenesis of MMD. However, it is unknown if these elevated angiogenic factors are a cause of MMD or result from cerebral ischemia due to the progression of MMD.
Several lines of evidence suggest the involvement of genetic factors in MMD. The prevalence of MMD is much higher in East Asian countries than in Western countries. ,,,,, Most cases of MMD are sporadic, but 10-15% of cases are of familial origin in Japan. Moreover, the concordance in monozygotic twins has been reported to be 80%. ,,, On the other hand, moyamoya syndrome has been reported to be complicated by congenital disorders such as neurofibromatosis type I and Down syndrome.  Yamauchi et al. stated that the characteristic lesions of MMD are occasionally observed in neurofibromatosis type 1, of which the causative gene (NF1) has been assigned to chromosome 17q11.2.  The researchers performed microsatellite linkage analyses on 24 families that included 56 patients with MMD to map the locus of familial MMD on chromosome 17. In addition, they identified a gene for familial MMD that is located on chromosome 17q25. In 2011, two independent groups in Japan identified an MMD susceptibility gene. Kamada et al. conducted a genome-wide association study and detected RNF213 as the first susceptibility gene of MMD, and the locus was mapped to 17q25.3. They reported a mutation in RNF213 (p.R4810K, c.144 29G>A, rs112735431) was found in 95% of familial MMD cases, 73% of nonfamilial MMD cases, and 1.4% of controls.  At around the same time, Liu et al. detected RNF213 as an MMD susceptibility gene by employing genome-wide linkage analysis and whole-genome and whole-exome analysis. , They reported that RNF213 p.R4810K was strongly associated with MMD (OR = 111.8) in all East Asian cases. RNF213 encodes a 591-kDa cytosolic protein and possesses two functional domains: A Walker motif and a RING finger domain. These domains exhibit ATPase and ubiquitin ligase activities.  Hitomi et al. found that RNF213 p.R4810K formed a complex with MAD2 more readily than did RNF213 wild-type (WT) and concluded that RNF213 R4810K induced mitotic abnormalities and increased the risk of genomic instability.  Further study is needed to clarify the pathological role of RNF213 p.R4810K in MMD.
| In Vivo Models|| |
Mongrel dog serum sickness model
Previously, MMD was thought to be a variant of a vasculitic syndrome. The reason was that histopathological findings, such as intimal thickening, tortuous and multilayered internal elastic lamina, and thinning of the media, resembled those of vasculitis, including polyarteritis nodosa and Kawasaki disease. Additionally, an epidemiological study in Japan showed that many MMD patients have a past history of inflammation above the neck, especially recurrent chronic tonsillitis.  Kasai et al. hypothesized that MMD was an immunological arteritis due to chronic inflammation and that this mechanism induces pathological change of the stenotic lesion of the carotid fork.  To prove the hypothesis, they used a serum sickness vasculitis model involving injection of a foreign protein or serum into an animal. In the model, antibody-antigen immune complexes were formed following development of antibodies against the foreign substance. These immune complexes deposit in blood vessels and activate the classical complement pathway to cause local vasculitis. In that study, horse or cat serum was injected into mongrel dogs several times intravenously or subcutaneously. After general sensitization, the animals were sacrificed, and arterial specimens exhibited MMD-like pathological changes, such as intimal thickening, tortuosity of lamina elastic, and muscle layer necrosis. The arterial changes were localized around the terminal portion of the ICA.
Furthermore, the researchers hypothesized that localization of the lesion was related to autonomic nerve system innervation derived from the superior cervical ganglion. On the basis of these speculations, they performed stimulation of the unilateral superior cervical ganglion with generalized sensitization following contralateral superior cervical ganglionectomy. In the animals, arterial changes were weakened on the side of ganglionectomy. The results suggested that the pathogenesis of MMD involves cervical inflammation that stimulates the superior cervical ganglion and leads to hyperpermeability of its innervating vasculature, i.e., the terminal portion of ICA. In addition, these changes allow infiltration of the immune complex into the inner wall of ICA, and changes of the arterial wall due to immunological reaction are organized. However, the researchers did not show evidence of the existence of an immune complex in the arterial wall; thus, the actual role of immune complex in the development of arterial lesion was not revealed in this study. The problem of this dog model is difficulty in long-term observation, as the management of dogs requires substantial effort. In addition, there are substantial variations between individuals. Thus, to further investigate the role of inflammation and the immune complex in the development of stenosis in ICA, more easily treatable and homogeneous animal models need to be established.
Rabbit serum sickness model
Following the canine experimental model mentioned above, Ezura et al. reported a study that used a serum sickness model in rabbits. Rabbits are a well-established and widely used experimental model of serum sickness vasculitis of systemic arteries, but cerebral arterial lesions were not well investigated.  Rabbits were first sensitized by intravenous injection of heterogeneous serum. Three weeks after treatment, they received a second injection of horse serum to induce immune complex hypersensitivity. Vasculitis was found in the systemic arteries, including the coronary arterioles, pulmonary arteries, and myocardial interstitial arterioles, but not in the cerebral arteries. The authors noticed that the cerebral artery was constantly bathed in cerebrospinal fluid which lacked specific antibodies. Therefore, they tried to create a novel, combined experimental serum sickness model by intracisternal administration of anti-horse serum antibodies and horse serum. The animals were intravenously injected with heterologous serum twice, as first group, but simultaneously with the second injection they received an intracisternal injection of antibodies or antigens. In this model, infiltration of inflammatory cells, such as leukocytes and macrophages, was observed in the adventitia of the cerebral arteries and the meninges. The degree of inflammatory cell infiltrates was more marked in group of receiving intracisternal injection of antibody than antigen. Immunoglobulin G (IgG) deposits were localized to the adventitia of the cerebral arteries, and no pathological changes were observed in the intima or media of the arteries. This suggests that the antibodies injected intracisternally were deposited in the arterial adventitia, after which they reacted with antigens intravenously injected. From these results, the researchers thought that circulating immune complexes alone could not be the key factor in development of vascular lesions in the cerebral arteries.
As a supplemental experiment, rabbits received intracisternal administration of antibodies and intravenous injection of antigens without presensitization, and were sacrificed in short duration to eliminate the development of the immune complex. These models also showed infiltration of leukocytes into the arterial wall and the adventitia of the cerebral artery on day 1. The infiltrating cells had nearly disappeared from the tissue surrounding the cerebral arteries on day 5. The researchers noted that in situ interaction between antigens and antibodies through the arterial wall, but not circulating immune complexes, is necessary to induce arterial lesions in the cerebral arteries. The authors concluded that vasculitis rarely occurred in the cerebral arteries in the classical experimental model of serum sickness but that the initial process of vasculitis, including infiltration of inflammatory cells into the arterial wall, could develop even in cerebral arteries if the proper conditions were encountered.
Propionibacterium acnes-infected model
Prior bacterial infection has been thought to be one underlying cause of MMD. Yamada et al. reported that the serum levels of the P. acnes antibody, immunoglobulin M (IgM), transferrin, and α-2-macroglobulin were significantly higher in MMD than in normal volunteers.  The researchers have also studied the effect of P. acnes infection on Sprague-Dawley rats. Animals were injected with a P. acnes bacterial solution around both internal-external carotid bifurcations in the peritoneal cavity or the cisterna magna. They found that histological changes of the intracranial ICA were only in the group that was injected with P. acnes. Histopathological changes included coarse, disrupted, and duplicated internal elastic membranes in the intracranial ICA. As a result of these findings, researchers suggested that P. acnes and immunological factors might have a role in the pathogenesis of MMD. In the model, intracranial ICA lesions could be induced, but intimal wall changes were absent and stenotic lesions were not observed.
MDP injection model
N-Acetylmuramyl-L-alanyl-D-isoglutamine (muramyl dipeptide, MDP) is the smallest structural unit of the bacterial wall that modulates immune responses, and is used for inducing experimental autoimmune disease. , Suzuki et al. described an animal model of MMD involving injection of MDP. , Investigators repeatedly injected MDP intravenously or intrathecally into Wistar rats and observed pathological changes, such as disruption of the internal elastic lamina, degeneration of the media, and minimal intimal thickening, predominantly in the terminal portion of the ICA. However, the pathological changes were not sufficient to induce arterial stenosis or occlusion. Kamata et al. attempted to create an animal model of MMD with ICA occlusions and basal collateral vessels that used a combination of temporary occlusions of the carotid artery and an immunological reaction by injecting MDP.  The immune-embolic material composed of lactic acid-glycolic acid 50:50 copolymer with MDP was prepared. After general sensitization by intravenous injection of MDP, the immune-embolic material was injected into cats unilaterally via the common carotid artery 4 times at 10-day intervals. Although the material occluded the unilateral ICA, the terminal portion of the ICA and its intracranial branches were well demonstrated in an angiographic study because of a rete, a well-developed external carotid route between the internal and external carotid artery in the cat. The animals were sacrificed 45-63 days after the administration of MDP. The histological study showed mild intimal thickening accompanying focal folding and duplication of the internal elastic lamina prominently in the terminal portion of the ICA. However, the histological changes in the intima were minimal, and the angiographic study did not show any development of collateral vessels in the basal ganglia. The researchers concluded that insufficient reduction of cerebral blood flow might have caused the absence of collateral vessels in the model.
Terai et al. described a monkey model of MMD with immune-embolic material containing MDP that was used with an intravascular interventional technique.  Monkeys have a carotid system that is morphologically the same as that in humans. The embolic materials were repeatedly injected into the right ICA of the monkeys, but the terminal portions of ICA were not obstructed and the extent of blood flow reduction was not clear. Long-term observations were performed for 49-163 days after sensitization. The histological study revealed reduplication and lamination of the internal elastic lamina in the intracranial arteries, but intimal thickening was not observed. Final angiography showed no development of collateral vessels.
Summary of immunological animal models of MMD
Results from the dog serum sickness model showed a relationship between the localized stenotic lesion in the terminal portion of ICA and innervation of the superior cervical ganglion. However, the classical serum sickness model using rabbits did not show vasculitis in the cerebral artery. Therefore, it seemed that differences among species influenced the occurrence of lesions in the cerebral arteries. Reports about immune-embolic material containing MDP injected in a cat or monkey model described pathological changes in the internal elastic lamina, and intimal thickening, if present, was minimal. None of the experiments showed any stenotic lesions in the cerebral artery or development of collateral vessels. New experimental models based on the theory have not yet been reported. The results that have been obtained have shown the limitations of the theory of inflammation and immunological reaction alone on the pathogenesis of MMD. The results of these experimental models are summarized in [Table 1].
RNF213 knockdown zebrafish
After the MMD susceptibility gene, RNF213, was detected in 2011, RNF213 knockout zebrafish were produced by Liu et al. to observe the physiological function of RNF213.  Zebrafish have two RNF213 genes, RNF213-α and RNF213-β, and whole-genome duplication proved that their locations were on different chromosomes. Reverse transcription polymerase chain reaction (RT-PCR) showed that RNF213-α was more abundantly expressed than was RNF213-β in zebrafish. Lie et al. designed morpholino (MO) molecules that knockdown specific RNF213 genes. The splicing ablation of RNF213-α caused by MO injection induced abnormal intersegmental vessel sprouting and multiple sprouting vessels from the inner optic circle. In contrast, injection of MO against RNF213-β resulted in normal vascular development because of the low expression of RNF213-β in zebrafish. The results suggested that RNF213 was involved in a novel signaling pathway in intracranial angiogenesis.
RNF213 knockout mice
Kobayashi et al. reported a study using RNF213-deficient mice.  The researchers used Cre/lox system-mediated recombination to induce frameshift mutation on exon 20 that caused disruption of the Walker motifs and RING finger domain of RNF213. These mice, including cerebrovascular phenotypes, did not show any health problems, which is similar to what is observed in MMD.
Sonobe et al. investigated anatomical and histological features in detail in RNF213 knockout mice.  The researchers made RNF213-deficient mice (RNF213 −/−) by deleting exon 32 of RNF213 by using the Cre/lox system. Magnetic resonance angiography showed no significant differences in the findings of the circle of Willis, and a histological study also revealed no difference in vascular wall thickness between RNF213 −/− and WT animals. The researchers ligated the common carotid artery in both groups, analyzed the induced hyperplastic lesions of the arterial wall, and showed that the medial layer of the common carotid artery was significantly thinner in RNF213 −/− than in WT 14 days after ligation. Therefore, they insisted that the histological findings matched those specific for MMD and assumed that RNF213 deficiency leads to vascular fragility, including medial attenuation. The researchers concluded that functional loss of RNF213 is not sufficient to induce MMD and additional insults, such as an autoimmune response infection/inflammation, radiation, and ischemia, may be necessary for development of MMD.
Ito et al. investigated the role of RNF213 in angiogenesis under ischemic conditions using RNF213 knockout mice.  They adopted a transient middle cerebral artery occlusion (tMCAo) model in mice to evaluate infarction volume, cerebral edema, and vascular density. No significant differences were observed in the infarction volume, formation of edema, or vascular density after tMCAo between RNF213 −/− and WT animals. The researchers used a permanent femoral artery ligation model instead of a chronic cerebral ischemia model to evaluate the effect of chronic ischemia. In RNF213 −/− animals, the investigators found that blood flow was significantly improved within 3-28 days after femoral artery ligation and that angiogenesis in the hind limb was significantly enhanced in RNF213 −/− at 28 days. The result suggests that abnormalities in RNF213 could be related to the pathological development of vascular networks in chronic ischemia.
Summary of RNF213 recombinant model
Identification of RNF213 as an MMD susceptibility gene enabled the development of novel animal models of MMD. In a RNF213 knockdown zebrafish model, it seemed that RNF213 might be involved in vascular development during embryogenesis.  Investigation of RNF213 knockout mice showed that RNF213 was engaged in remodeling of the vascular wall after vascular injury and development of the vascular network in the condition of chronic ischemia. , The RNF213 knockout models did not represent all of the essential features of MMD; for example, eccentric intimal thickening in the terminal portion of ICA and development of basal MMD vessels. It might be necessary for RNF213 knockout mice to reproduce the phenotypes because the mechanism underlying gain of function, not loss of function, remains unknown.
| In Vitro Models|| |
Smooth muscle cells from surgical specimens
Aoyagi et al. harvested smooth muscle cells (SMCs) from the scalp arteries of MMD patients and investigated the cellular responsiveness to cytokines.  They reported that SMCs from MMD patients proliferated more slowly and responded to PDGF stimulation worse than did the control SMCs. Yamamoto et al. also showed that cell proliferation activity responding to PDGF-AA, PDGF-BB, and interleukin 1 beta (IL-1β) was impaired significantly in SMCs from MMD patients by using a BrdU incorporation assay.  On the other hand, the migration activity in response to PDGF-AA, but not to IL-1β or IL-6, was increased in SMCs from MMD patients in a cell migration assay.  The results suggested that the differences in responses to each cytokine in SMCs from MMD patients might be involved in the mechanism by which intimal thickening develops in MMD.
EPCs circulating in peripheral blood were first isolated by Asahara et al. in 1997.  It was found that EPCs were released from the bone marrow into the blood circulation and contributed to vascular homeostasis and endothelial repair. Several research groups have reported that the quantity and function of circulating EPCs were biomarkers for the risk of cardiovascular disease.  The representative cell surface markers of EPCs are thought to be positive for cluster of differentiation (CD)34 and VEGF receptor 2 (VEGFR2), and early EPCs are also positive for CD133. 
Yoshihara et al. were the first such group, reporting that among patients suffering major cerebral artery occlusion or severe stenosis, circulating CD34-positive cells were significantly increased only in patients with angiographic MMD vessels.  Raft et al. then reported a significant increase in circulating EPCs (CD34+CD133+VEGFR2+) in adult MMD patients.  In addition, Ni et al. showed that the level of circulating CD34+CXCR4+ cells increased in adult MMD patients.  On the other hand, Kim et al. reported a significant decrease in circulating EPCs (CD34+CD133+VEGFGR2+) in child patients with MMD.  Moreover, they found that the EPCs from child patients with MMD had impaired differentiation capacity to endothelial cells, defective tube formation in a Matrigel (Corning, NY, USA) assay, and were prone to senescence. The researchers speculated that EPC dysfunction might be associated with abnormal vessel formation or insufficient cerebrovascular repair. With regard to EPC dysfunction, Jung et al. also reported significantly lowered colony-forming unit numbers in MMD patients.  In addition, they reported that conditioned media from the EPCs of MMD patients were inferior for the angiogenesis in human umbilical vein endothelial cells (HUVECs) on Matrigel. The results suggested that paracrine functions of multiple growth factors of EPCs in MMD were attenuated.
Induced pluripotent stem cell (iPSC)-derived endothelial cells
Past research has revealed that the angiogenic activity of EPCs was impaired in MMD and indicated that the phenotype of EPCs could be a disease-specific in vitro model. However, researchers have had difficulty in conducting reproductive and multilateral studies using EPCs because of the limited proliferative activity of the primary cells. The development of iPSC technologies is overcoming these difficulties. After expanding the pluripotent stem cell line, a sufficient number of required cells can be obtained any number of times though appropriate differentiation protocols. The differentiated cells derived from iPSCs have been adopted in many studies for various genetic disorders. 
Hitomi et al. first established MMD-specific iPSCs and made endothelial cells from the cell lines.  The researchers induced an iPSC line from the primary fibroblasts of three MMD patients and three healthy controls. Genotyping of RNF213 showed the AA genotype (homozygous for RNF213R4810K) for two of three MMD patients, GA genotype (heterozygous for RNF213 R4810K) for the remaining patient and an unaffected healthy control, and GG genotype (WT for RNF213 R4810K) for two healthy controls. Although there was no significant difference in the proliferative activity of the endothelial cells between the carrier, RNF213 R4810K, and the controls, the angiogenesis in vitro was significantly impaired in the endothelial cells carrying RNF213 R4810K. The clustering of microarray data of the endothelial cells derived from iPSCs clearly distinguished MMD patients and an unaffected carrier from the controls. The gene expression data showed 38 upregulated genes and 121 downregulated genes in MMD. The researchers focused on securin, which could induce angiogenesis and inhibit premature sister chromatoid separation, among these differentially regulated genes. Downregulation of securin was also associated with severe defects in cell migration by lowered microtubule nucleation. The overexpression of RNF213 R4810K downregulated securin expression and reduced tube formation in HUVECs. On the contrary, normal RNF213 overexpression did not influence the angiogenic activity. Furthermore, securin expression was downregulated by using RNA interference techniques, which reduced angiogenesis in vitro in endothelial cells derived from control iPSCs and HUVECs. The authors suggested that RNF213 would reduce angiogenesis via downregulation of securin expression, although its mechanism and the effect of other downregulated genes remained unclear. They concluded that the endothelial cells derived from iPSCs could be regarded as an in vitro model of MMD because they were a phenotype useful for high-throughput screening in drug development and investigation of the pathogenesis of MMD.
Circulating smooth muscle progenitors
Under pathological conditions, such as atherosclerosis, it had been thought that the SMCs in the neointima could migrate from the media into the intima. After identification of EPCs, however, a significant amount of evidence has accumulated that shows the contributions of circulating vascular progenitor cells in vascular repair and remodeling. ,, Simper et al. demonstrated for the first time ex vivo expansion of smooth muscle outgrowth cells from circulating smooth muscle progenitor cells.  However, whether circulating smooth muscle progenitors can participate in formation of vascular lesions, such as atherosclerotic lesions or transplant arteriosclerosis, is still debatable. 
Regarding MMD, past histopathological studies have shown abnormal intimal thickening in the terminal portion of ICA. It was reported that the lesions were mainly composed of the cells expressing some SMC markers, such as HHF35 and α-SMA. , Kang et al. cultured and isolated smooth muscle progenitor cells from the peripheral blood of MMD patients and healthy control volunteers.  The researchers used a tube formation assay to show that smooth muscle progenitor cells from MMD patients had irregularly arranged and thickened tubules. In addition, in the smooth muscle progenitor cells from MMD patients, 286 genes (124 upregulated and 162 downregulated) were differentially expressed, and they were related to cell adhesion, cell migration, immune response, and vascular development. The authors suggested that the formation of irregularly shaped and thickened tubes in vitro was related to intimal thickening in vivo because of proliferative SMCs. They asserted that smooth muscle progenitor cells from the peripheral blood of patients with MMD would provide a novel experimental cell model of MMD, especially for investigations into the pathogenesis of MMD.
Summary of in vitro models of MMD
Several in vitro studies on the functions of vascular cells in MMD patients and the phenotypic reproduction of MMD have been reported. It was commonly found that both the circulating EPCs and the endothelial cells derived from iPSCs of MMD patients had impaired angiogenesis. In the endothelial cells derived from iPSCs, no significant difference in proliferative activity was observed between controls and MMD patients. It was reported that the smooth muscle cells from the scalp artery of MMD patients reduced proliferative activity and altered responsiveness to several cytokines. Regarding circulating smooth muscle progenitor cells, irregularly arranged and thickened tube formations have been observed in MMD. The clinical significance of the abnormalities has not yet been clarified. Results of these studies are summarized in [Table 2].
| Discussion|| |
Many researchers have tried to create in vitro and in vivo models of MMD over the last several decades. However, none of the reported studies were able to reproduce the phenotype of MMD sufficiently. Studies of inflammatory and immunological theory-based animal models partially succeeded in generating localized lesions in cerebral ICA and showed the possibility of involvement of the superior cervical ganglion as the reason for the localized lesions. However, the lesions were minimal and the arterial stenosis, because of intimal thickening, could not be reproduced. The results indicated the limitations of the theory of inflammation and immune complex.
Although the discovery of RNF213 as an MMD susceptibility gene was expected to establish a novel and reliable animal model of MMD, an RNF213 knockout mouse model did not exhibit abnormal vascular structure. On the other hand, Ito et al. found that chronic ischemia could enhance angiogenesis in the hind limbs of RNF213 knockout mice. However, chronic cerebral hypoperfusion could not induce angiogenesis in the RNF213 knockout mice because of their susceptibility to cerebral ischemia. It appears that the finding of chronic ischemia in RNF213 knockout mice in the hind limb model reflects the abnormal angiogenesis in MMD patients. Thus, the results suggested that RNF213 was not the only factor responsible for the abnormal angiogenesis and that MMD is a multifactorial disease resulting from RNF213 abnormality. Introduction of factors based on classical pathological theory, such as inflammation and immune system factors, into contemporary RNF213 recombinant animals may produce reliable models with an MMD-like phenotype. On the other hand, we should consider that RNF213 R4810K may possibly result from a gain in functional mutation rather than from a loss. Thus, it is necessary to create RNF213 R4810K knockin mice and to analyze the characteristics of their vascular system in future research.
It has also been important to develop in vitro models of MMD, such as circulating vascular progenitor cells and vascular cells derived from iPSCs. An advantage of these models is that cells derived from MMD patients could be used. Findings including impaired angiogenesis and irregularly shaped tube formation from studies using these models have been reported. However, it remains unclear what the findings of in vitro models reflect regarding the clinical phenotype. In particular, impaired angiogenesis of in vitro models seems to be an adverse result to clinical phenomena, such as MMD vessels. Further study is needed to understand how in vitro study findings translate to in vivo findings. Finally, it is very important to learn from past studies and use the knowledge gained in present and future research.
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Conflicts of interest
There are no conflicts of interest.
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[Table 1], [Table 2]
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