The role of endoglin in endothelial and mesenchymal cells during development, maintenance and repair of the heart

Abstract

Mutations in the endoglin gene lead to Haemorrhagic Telangiectasia Type 1 (HHT1), an inherited vascular disease, demonstrating the importance of endoglin in the vasculature. Haplodeficiency of endoglin also leads to reduced angiogenesis in mice in several pathologies, indicating a pro-angiogenic role for endoglin. However, the precise role of endoglin in development and maintenance of the vasculature is still unclear. The aim of this project therefore was to investigate the role of endoglin during vascular development and maintenance and whether its pro-angiogenic properties can be used to enhance cardiac recovery following myocardial infarction (MI). Using immunofluorescence staining of mouse cardiac tissue, I showed high endoglin expression in coronary veins endothelial cells (ECs), capillaries & endocardium and weak expression in coronary arteries ECs, valve mesenchyme & in epicardially derived cells (EPDCs) during development. I hypothesised that during cardiac development endoglin is essential for EC proliferation and migration, and plays a key role during EPDC proliferation, migration and differentiation. Induced knockdown of endoglin in murine epicardial cells using Wt1-Cre led to a reduced number of EPDCs in E15.5 hearts but did not lead to any detectable vascular defects in E17.5 hearts. In contrast, EC-specific endoglin knockdown in the first week of postnatal life (which is known to lead to retinal AVMs) was associated with the development of eccentric cardiomyopathy and cardiomyocytes (CM) hypertrophy, but no obvious coronary vasculature defects. Deletion of endoglin in venous ECs and capillaries in first week of postnatal life also led to development of AVMs in P11 retinas coupled with the development of cardiac hypertrophy. A similar eccentric cardiomyopathy phenotype with CM hypertrophy was observed after endoglin deletion in adult ECs (Eng-iKOEC). Microbeads were used to show increased vascular shunting in the systemic but not the pulmonary vasculature of Eng-iKOEC mice. Ultimately Eng-iKOEC mice developed high output heart failure due to reduced vascular resistance cause by systemic AVMs. This data confirms that endoglin is vital for maintaining the adult vasculature. Finally, I analysed the role of endoglin in a heterogeneous population of cardiac derived stem cells known as cardiosphere derived cells (CDCs). CDCs express endoglin which I showed is vital for their pro-angiogenic paracrine effect when delivered to the infarcted heart. Furthermore, endoglin not only promotes paracrine mediated proliferation of ECs in the infarct zone but also helps in maturation of the neo-vasculature by recruitment of smooth muscle cells. In conclusion, the results from this thesis show the critical importance of endoglin in the development and maintenance of the vasculature.