Pathogenesis of Arteriovenous Malformations in the Absence of Endoglin

Abstract

Key Words: mouse models of cardiovascular disease Ⅲ angiogenesis Ⅲ endothelial cells A rteriovenous malformations (AVMs) are the most frequent cause of hemorrhagic stroke in young adults. 1,2 The factors leading to AVM formation are unknown, but a number of inherited diseases leading to vascular malformations have now been identified and the causal mutations mapped. 3 Of these familial disorders, hereditary hemorrhagic telangiectasia (HHT) patients have a strikingly high frequency of AVMs. Because HHT is most frequently associated with mutations in the endoglin or ACVRL1 (activin receptor-like kinase 1) gene, 4,5 it is likely that these genes have important roles in preventing AVMs during normal development. HHT is an autosomal dominant disorder affecting approximately 1 in 10 000 people and is characterized by bleeding from small superficial AVMs (known as telangiectases) in the nose and gastrointestinal tract, as well as larger AVMs that may occur in major organs including lung, brain and liver. 6 At present, there is limited understanding of how deficiencies in endoglin or ACVRL1 lead to disease pathology, although their role in transforming growth factor (TGF)␤ family signaling has been the subject of considerable investigation. 7,8 Endoglin is an auxiliary receptor for members of the TGF␤ family of ligands and is expressed primarily on vascular endothelial cells (ECs). It has no signaling kinase domain itself, but can promote TGF␤ signaling through the ACVRL1 receptor to promote cell proliferation and migration. 9,10 ACVRL1 signals by phosphorylating Smad1/5/8 transcription factors which then translocate to the nucleus to regulate expression of downstream genes. 7 Recently, endoglin and ACVRL1 have been shown to respond to bone morphogenetic protein (BMP)9 and BMP10 ligands of the TGF␤ family to promote endothelial cytostasis, even in the presence of angiogenic growth factors. [11][12][13] How these different in vitro responses link to the normal role of these genes in averting AVM formation in development is not yet clear. To address this issue we, and others, have previously derived mouse models with endoglin mutations and found that Endoglin null embryos die halfway through gestation from developmental defects in the cardiovasculature. 14 -16 Mice that are heterozygous for endoglin-null mutations survive and model some features of HHT, but AVMs occur at an extremely low frequency. 17 With the aim of developing a more reproducible model of AVM formation, and bypassing embryonic lethality of the endoglin null mouse, we have taken a conditional knockout approach combining our recently generated endoglin-floxed mouse model 18 with an endothelial specific Cdh5(PAC)-Cre ERT2 transgenic line. 19 As Cre ERT2 is inactive until exposed to tamoxifen, this combination of alleles allows endoglin depletion in ECs at different stages of development and adult life, permitting a high degree of manipulability with which to investigate the role of endoglin in vivo. 2fl/2fl and included both tamoxifen-treated and tamoxi...

Chronic Cerebral Hypoxia Promotes Arteriogenic Remodeling Events that can be Identified by Reduced Endoglin (CD105) Expression and a Switch in β1 Integrins

“…suggesting that CD105 is important for the maintenance of vessel stability (Mahmoud et al, 2010). By this reasoning it follows that when vessels remodel, they would transiently downregulate CD105 expression, and our observations support this idea.…”

“…CD105 mutations in humans result in hereditary hemorrhagic telangiectasia, a disease characterized by disordered vascular remodeling and the appearance of AVMs (McAllister et al, 1994). Consistent with this, CD105 global knockout mice die at mid gestation due to a major failure of vascular development (Bourdeau et al, 1999), and endothelial cell-specific CD105 knockout mice display defective vascular integrity and vascular remodeling, increased endothelial proliferation and AVMs (Mahmoud et al, 2010). These collective findings suggest that CD105 is important for the maintenance of vessel stability, and that in its absence, vessels fail to form stable structures, resulting in excessive endothelial cell proliferation and vascular malformations.…”

“…Genetic knockouts in mice have confirmed an important role for CD105 in vascular stability by showing that global CD105 knockout mice die at mid gestation due to a failure of effective blood vessel formation (Bourdeau et al, 1999). Furthermore, endothelial cell-conditional CD105 knockout mice display defective vascular integrity, reduced vascular remodeling, and AVMs (Mahmoud et al, 2010). More recently, CD105 has attracted a lot of interest because it is reportedly expressed at higher levels on angiogenic endothelial cells in tumor-associated vessels, relative to stable vessels in normal tissue (Minhajat et al, 2006;Yao et al, 2005).…”

The TGFβ type II receptor plays a critical role in the endothelial cells during cardiac development

“…However, the role of TGFb signalling in these processes has not yet been determined. Furthermore, in light of evidence that Tgfbr2 is important in endothelial cells during cardiac development (Jiao et al, 2006), and to bypass the embryonic lethality at E10.5 to E11.5 seen in the Jiao et al (2006) study, we used the tamoxifen-inducible Cdh5(PAC)-CreERT2 line (Mahmoud et al, 2010). This permitted us to control the timing of endothelial Cre activity, and to inactivate Tgfbr2 in ECs after E10.5.…”

“…To activate the Cre recombinase in embryos carrying the Cdh5(PAC)-CreERT2 transgene, pregnant females were given an intraperitoneal injection of 1 mg Tamoxifen (dissolved in peanut oil) at embryonic day 11.5 and on the following two days (E12.5 and E13.5). All the mouse lines used in this work (R26R, Gata6-Cre, Mlc2v-Cre, Cdh5(PAC)-CreERT2 and Tgfbr2fl/fl ) and the primers used for genotyping have been previously described (Chen et al, 1998;Soriano, 1999;Davis et al, 2001;Mahmoud et al, 2010;Leveen et al, 2002). The floxed Tgfbr2 allele was converted to a null (D) allele by PGK-Cre mediated recombination.…”