An update on preclinical models of hereditary haemorrhagic telangiectasia: Insights into disease mechanisms

Abstract

Endoglin (ENG) is expressed on the surface of endothelial cells (ECs) where it efficiently binds circulating BMP9 and BMP10 ligands to initiate activin A receptor like type 1 (ALK1) protein signalling to protect the vascular architecture. Patients heterozygous for ENG or ALK1 mutations develop the vascular disorder known as hereditary haemorrhagic telangiectasia (HHT). Many patients with this disorder suffer from anaemia, and are also at increased risk of stroke and high output heart failure. Recent work using animal models of HHT has revealed new insights into cellular and molecular mechanisms causing this disease. Loss of the ENG (HHT1) or ALK1 (HHT2) gene in ECs leads to aberrant arteriovenous connections or malformations (AVMs) in developing blood vessels. Similar phenotypes develop following combined EC specific loss of SMAD1 and 5, or EC loss of SMAD4. Taken together these data point to the essential role of the BMP9/10-ENG-ALK1-SMAD1/5-SMAD4 pathway in protecting the vasculature from AVMs. Altered directional migration of ECs in response to shear stress and increased EC proliferation are now recognised as critical factors driving AVM formation. Disruption of the ENG/ALK1 signalling pathway also affects EC responses to vascular endothelial growth factor (VEGF) and crosstalk between ECs and vascular smooth muscle cells. It is striking that the vascular lesions in HHT are both localised and tissue specific. Increasing evidence points to the importance of a second genetic hit to generate biallelic mutations, and the sporadic nature of such somatic mutations would explain the localised formation of vascular lesions. In addition, different pro-angiogenic drivers of AVM formation are likely to be at play during the patient's life course. For example, inflammation is a key driver of vessel remodelling in postnatal life, and may turn out to be an important driver of HHT disease. The current wealth of preclinical models of HHT has led to increased understanding of AVM development and revealed new therapeutic approaches to treat AVMs, and form the topic of this review.

Keywords: BMP-SMAD signalling pathway; HHT; activin receptor-like kinase 1 (ACVRL1); arteriovenous malformation (AVM); cell polarity and migration; endoglin (CD105).

FIGURE 1

Diagram to illustrate crosstalk between BMP9/10 signalling and VEGF signalling in endothelial cells. ENG can act as a reservoir of BMP9/10 ligand on endothelial cells and blood flow enhances ALK1/ENG interaction. ENG is displaced when the type II receptor (e.g., BMPR2) joins the protein complex. Type II receptor phosphorylates ALK1 thereby activating its kinase activity to phosphorylate SMAD1/5. Phospho-SMAD1/5 (pSMAD1/5) interacts with SMAD4 to migrate to the nucleus and act as transcription factors to regulate expression of multiple genes. BMP9/10 signalling prevents inactivation of PTEN, consequently inhibiting PI3K activity and reducing downstream AKT activation. Thus in the absence of BMP9/10 signalling there is increased PI3K activity leading to increased EC proliferation and survival. Also, there is evidence for BMP9/10 signalling events that inhibit VEGF signalling outcomes after a minimum of 2 h, consistent with downstream gene expression (see main text).

FIGURE 2

Abnormal endothelial cell migration contributes to formation of arteriovenous malformations. (A) Endothelial cells (ECs) polarise in response to blood flow (red arrows) and migrate (black arrows) against the direction of flow. Golgi bodies (orange) are aligned with respect to the nucleus (dark blue) in the direction of EC migration. (B) ECs proliferate and elongate in line with blood flow as the vasculature remodels and matures. High blood pressure is retained in the artery, whilst the vascular architecture enables lower blood pressure to reach capillaries and veins. (C) Loss of ENG/ALK1 signalling leads to loss of EC polarisation, reduced directional cell migration. Stalled ECs continue to proliferate resulting in an AVM. (D) Blood flow favours the major channel created by the AVM, such that blood reaches the veins at abnormally high pressure, whilst the remaining capillary plexus atrophies in the absence of flow.

FIGURE 3

Graphical summary. (A) Animal models of HHT-like vascular lesions. Mouse genetic models include global embryonic models as well conditional embryonic, neonatal, and adult models. For BMP9 and BMP10, a neonatal “immunoblocked” model is also available. Zebrafish genetic models are global nulls that generate embryonic and/or adult phenotypes. (B) Putative direct or indirect EC responses to BMP9/10-ENG-ALK1-SMAD 1/5-SMAD4 activity include (1) Golgi-nuclear polarisation and migration against the direction of blood flow; (2) inhibition of proliferation; (3) elongation and decrease in overall cell size; (4) mural cell recruitment and adhesion. (C) Secondary triggers for AVM development in HHT include (1) somatic mutations in the wild type copy of the germline-mutated gene; (2) angiogenic triggers such as VEGF; (3) inflammation. Created with Biorender.com.