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...

Cardiac hypertrophy occurs as an adaptation to hypertension but a sustained hypertrophic response can ultimately lead to heart failure. Angiotensin-II (Ang II) is released following hemodynamic overload and stimulates a cardiac hypertrophic response. AngII also increases expression of the regulatory cytokine, transforming growth factor-b1 (TGFb1), which is also implicated in the cardiac hypertrophic response and can stimulate activation of Smad2/3 as well as TGFb-activated kinase 1 (TAK1) signaling mediators. To better understand the downstream signaling events in cardiac hypertrophy, we therefore investigated activation of Smad2/3 and TAK1 signaling pathways in response to Ang II and TGFb1 using primary neonatal rat cardiomyocytes to model cardiac hypertrophic responses. Small interfering RNA (siRNA) knockdown of Smad 2/3 or TAK1 protein or addition of the TGFb type I receptor inhibitor, SB431542, were used to investigate the role of downstream mediators of TGFb signaling in the hypertrophic response. Our data revealed that TGFb1 stimulation leads to cardiomyocyte hypertrophic phenotypes that were indistinguishable from those occurring in response to Ang II. In addition, inhibition of the TGFb1 type receptor abolished Ang II-induced hypertrophic changes. Furthermore, the hypertrophic response was also prevented following siRNA knockdown of TAK1 protein, but was unaffected by knockdown of Smad2/3 proteins. We conclude that Ang II-induced cardiomyocyte hypertrophy in vitro occurs in a TAK1-dependent, but Smad-independent, manner.