ALK1 belongs to the type I receptor family for transforming growth factor-beta family ligands. Heterozygous ALK1 mutations cause hereditary hemorrhagic telangiectasia type 2 (HHT2), a multisystemic vascular disorder. Based largely on in vitro studies, TGF-beta1 has been considered as the most likely ALK1 ligand related to HHT, yet the identity of the physiologic ALK1 ligand remains controversial. In cultured endothelial cells, ALK1 and another TGF-beta type I receptor, ALK5, regulate angiogenesis by controlling TGF-beta signal transduction, and ALK5 is required for ALK1 signaling. However, the extent to which such interactions between these 2 receptors play a role in pathogenesis of HHT is unknown. We directly addressed these issues in vivo by comparing the phenotypes of mice in which the Alk1, Alk5, or Tgfbr2 gene was conditionally deleted in restricted vascular endothelia using a novel endothelial Cre transgenic line. Alk1-conditional deletion resulted in severe vascular malformations mimicking all pathologic features of HHT. Yet Alk5- or Tgfbr2-conditional deletion in mice, or Alk5 inhibition in zebrafish, did not affect vessel morphogenesis. These data indicate that neither ALK5 nor TGFBR2 is required for ALK1 signaling pertinent to the pathogenesis of HHT and suggest that HHT might not be a TGF-beta subfamily disease.
"Alk1 is primarily expressed in the endothelial cells (ECs) of the arterial vessels . We have previously demonstrated that conditional deletion of the Alk1 gene in ECs is sufficient for the development of AVMs in the lung, brain, and GI tract, indicating that ALK1 expression and function in ECs are crucial for HHT pathogenesis , . "
[Show abstract][Hide abstract] ABSTRACT: Hereditary Hemorrhagic Telangiectasia (HHT) is a genetic vascular disease in which arteriovenous malformations (AVMs) manifest in skin and multiple visceral organs. HHT is caused by heterozygous mutations in endoglin (ENG), activin receptor-like kinase 1 (ALK1), or SMAD4. ALK1 regulates angiogenesis, but the precise function of ALK1 in endothelial cells (ECs) remains elusive. Since most blood vessels of HHT patients do not produce pathological vascular lesions, ALK1 heterozygous ECs may be normal unless additional genetic or environmental stresses are imposed. To investigate the cellular and biochemical phenotypes of Alk1-null versus Alk1-heterozygous ECs, we have generated pulmonary EC lines in which a genotype switch from the Alk1-conditional allele (Alk1 (2f)) to the Alk1-null allele (Alk1 (1f)) can be induced by tamoxifen treatment. Alk1-null (1 f/1 f) ECs displayed increased migratory properties in vitro in response to bFGF compared with Alk1-het (2 f/1 f) ECs. The 1 f/1 f-ECs formed a denser and more persistent tubular network as compared with their parental 2 f/1 f-ECs. Interestingly, the response to BMP-9 on SMAD1/5 phosphorylation was impaired in both 2 f/1 f- and 1 f/1 f-ECs at a comparable manner, suggesting that other factors in addition to SMADs may play a crucial role for enhanced angiogenic activity in 1 f/1 f-ECs. We also demonstrated in vivo that Alk1-deficient ECs exhibited high migratory and invasive properties. Taken together, these data suggest that enhanced responses to angiogenic cues in ALK1-deficient ECs underlie the pathogenesis of HHT2.
PLoS ONE 05/2013; 8(5):e63138. DOI:10.1371/journal.pone.0063138 · 3.23 Impact Factor
"Endoglin and ALK1, a TGFβ family type I receptor, show high affinity for BMP9 and BMP10, and are both associated with the inherited vascular disorder Hereditary Haemorrhagic telangiectasia (HHT) , , . Thus, loss of Tgfbr2 does not appear to be involved in the development of arteriovenous malformations during organogenesis, in agreement with a previous report . However, it remains an open question whether TGFβ signalling contributes to the formation of arteriovenous malformations and haemorrhage of the brain resulting from injury . "
[Show abstract][Hide abstract] ABSTRACT: TGFβ signalling in endothelial cells is important for angiogenesis in early embryonic development, but little is known about its role in early postnatal life. To address this we used a tamoxifen inducible Cre-LoxP strategy in neonatal mice to deplete the TypeII TGFβ receptor (Tgfbr2) specifically in endothelial cells. This resulted in multiple micro-haemorrhages, and glomeruloid-like vascular tufts throughout the cerebral cortices and hypothalamus of the brain as well as in retinal tissues. A detailed examination of the retinal defects in these mutants revealed that endothelial adherens and tight junctions were in place, pericytes were recruited and there was no failure of vascular smooth muscle differentiation. However, the deeper retinal plexus failed to form in these mutants and the angiogenic sprouts stalled in their progress towards the inner nuclear layer. Instead the leading endothelial cells formed glomerular tufts with associated smooth muscle cells. This evidence suggests that TGFβ signalling is not required for vessel maturation, but is essential for the organised migration of endothelial cells as they begin to enter the deeper layers of the retina. Thus, TGFβ signalling is essential in vascular endothelial cells for maintaining vascular integrity at the angiogenic front as it migrates into developing neural tissues in early postnatal life.
PLoS ONE 09/2012; 7(6):e39336. DOI:10.1371/journal.pone.0039336 · 3.23 Impact Factor
"The importance of ALK1 and SMAD1/5 for endothelial/vascular function is demonstrated by the observations, that genetic elimination of either ALK1 or SMAD5 is embryonic lethal due to cardiovascular defects ,  and that ablation of ALK1 decreased tumor angiogenesis in adult mice . In contrast, conditional genetic elimination of TβRI in adult mice does not cause a notable vascular phenotype , . Finally, the contribution of ALK1 signaling in endothelial-vascular modeling is supported by the observations that mutations of ALK1 have been found in patients with type II hereditary hemorrhagic teleangiectasias . "
[Show abstract][Hide abstract] ABSTRACT: BAMBI is a type I TGFβ receptor antagonist, whose in vivo function remains unclear, as BAMBI(-/-) mice lack an obvious phenotype.
Identifying BAMBI's functions requires identification of cell-specific expression of BAMBI. By immunohistology we found BAMBI expression restricted to endothelial cells and by electron microscopy BAMBI(-/-) mice showed prominent and swollen endothelial cells in myocardial and glomerular capillaries. In endothelial cells over-expression of BAMBI reduced, whereas knock-down enhanced capillary growth and migration in response to TGFβ. In vivo angiogenesis was enhanced in matrigel implants and in glomerular hypertrophy after unilateral nephrectomy in BAMBI(-/-) compared to BAMBI(+/+) mice consistent with an endothelial phenotype for BAMBI(-/-) mice. BAMBI's mechanism of action in endothelial cells was examined by canonical and alternative TGFβ signaling in HUVEC with over-expression or knock-down of BAMBI. BAMBI knockdown enhanced basal and TGFβ stimulated SMAD1/5 and ERK1/2 phosphorylation, while over-expression prevented both.
Thus we provide a first description of a vascular phenotype for BAMBI(-/-) mice, and provide in vitro and in vivo evidence that BAMBI contributes to endothelial and vascular homeostasis. Further, we demonstrate that in endothelial cells BAMBI interferes with alternative TGFβ signaling, most likely through the ALK 1 receptor, which may explain the phenotype observed in BAMBI(-/-) mice. This newly described role for BAMBI in regulating endothelial function has potential implications for understanding and treating vascular disease and tumor neo-angiogenesis.
PLoS ONE 06/2012; 7(6):e39406. DOI:10.1371/journal.pone.0039406 · 3.23 Impact Factor
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