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Zhu et al. Endoglin and cerebral vascular diseases

ENG also mediates endothelial-mesenchymal vector expressing VEGF (AAV-VEGF) significantly
communication during angiogenesis [11,32,33] . The increased the penetrance of cerebrovascular
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recruitment of vascular smooth muscle cells and abnormality [40] . Almost all-adult Eng mice that
pericytes to newly formed vascular network is received intra-brain injection of AAV-VEGF showed
[11]
impaired in Eng deficient mouse embryos . cerebrovascular abnormality [40] . However, unlike
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HHT1 patients, the vascular abnormality in Eng
ENG DEFICIENCY IS AN IMPORTANT RISK mice is at the capillary level.
FACTOR FOR BOTH HEMORRHAGIC AND
ISCHEMIC STROKES Bone marrow-derived cells can infiltrate into the brain
angiogenic region. We found that macrophages are
As mentioned in previous sections, ENG deficiency the major bone marrow-derived cells recruited to the
is associated with HHT1, a familial disease that has brain angiogenic foci [41] . Since the accumulation of
bAVM as one of its major phenotypes. Brain AVM bone marrow-derived macrophage in VEGF-induced
contains abnormal vessels, that are prone to rupture, brain angiogenic regions peaks earlier than the
causing intracranial hemorrhage and hemorrhagic increase of vessel density, macrophages likely play a
stroke. In addition, patients with ENG deficiency role in angiogenesis.
(HHT1) have a higher incidence of pulmonary AVM
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(PAVM), which is associated with a high incidence Using Eng mice, the influence of bone marrow
of paradoxical embolism in the cerebral circulation derived cells in the development of bAVM has been
and ischemic brain injury [34] . To understand bAVM studied. Transplantation of Eng bone marrow to
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pathogenesis and to develop therapeutic strategies, wild type mice induced vascular dysplasia in the
many Eng deficient mouse models were generated. brain angiogenic regions, while transplantation of
Using these animal models, we are able to elucidate wild type bone marrow to Eng mice reduced the
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bAVM pathogenesis and test new therapies. severity of vascular dysplasia in the brain angiogenic
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foci of Eng mice [42] . These data suggested that Eng
Since homozygous deletion of Eng gene in gene mutation in bone marrow cells cause vascular
mouse causes embryonic lethality [11,31] , mice with dysplasia. Importantly, these data suggested that
heterozygous deletion of Eng (Eng ) are used to transplantation of normal bone marrow cells to bAVM
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study the pathogenesis of HHT patients. Eng mice patients could be a therapeutic option.
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exhibit many phenotypes that resemble those of HHT1
patients, including mucocutaneous telangiectases, Although we were able to induce vascular dysplasia
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external bleeding, and AVMs in the liver, lung, brain in the brain of Eng mice by overexpression of
and gastrointestine [35] . Enlarged cerebrovascular VEGF, arteriovenous shunts were not detected in
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structure was found in some Eng mice with evidence these mice. Studies have shown that a combination
of hemorrhage [35] . However, penetrance of bAVM in of homozygous Eng inactivation and additional
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Eng mice is very low, only 7% [35] , suggesting that stimulations are needed for robust bAVM formation.
heterozygous Eng deletion alone is not sufficient to Genetic studies also indicated that mutations of Eng
cause bAVM formation. In addition, the differences modifier genes contribute to AVM formation [43,44] .
of the penetration of HHT phenotypes in 129/Ola and
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C57BL/6 Eng mice suggests that modifier genes To avoid embryonic death caused by homozygous
are contributing to the severity and heterogeneity of Eng deletion, Allinson et al. [45] generated an Eng-
AVMs in HHT patients [35] . floxed (Eng 2f/2f ) mouse line that have the Eng
gene exons 5-6 flanked by loxP sites. When Cre
Based on clinical studies, we and others found that recombinase is present, the DNA sequence between
VEGF levels are increased in the plasma of HHT the loxPsites will be deleted. To test whether
patients and in surgically resected sporadic human homozygous Eng gene deletion plus angiogenic
bAVM specimens [26,27,30] . The intensity of VEGF stimulation can initiate bAVM formation, an adeno
staining is also correlated with microvessel density virus expressing Cre recombinase (Ad-Cre) and
in nasal mucosa from HHT patients [36] . Together, AAV-VEGF were co-injected into the brain of Eng 2f/2f
abnormally high level of VEGF appears to be a mice [45,46] to induce brain focal Eng gene deletion
fundamental part of AVM pathophysiology [25,30,37-39] . and angiogenesis. Eng 2f/2f mice with focal Eng gene
Based on these studies, we overexpressed VEGF deletion and angiogenic stimulation developed
in the mouse brain in conjunction with Eng deletion vascular dysplasia beyond the capillary level around
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to generate bAVM models. In adult Eng mice, the AAV-VEGF injection site eight weeks after the
intra-brain injection of an adeno-associated viral vector injection [46] . Robust bAVM have also developed
Neuroimmunology and Neuroinflammation ¦ Volume 4 ¦ October 17, 2017 201