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Study of the functional implications of GRK2 serine-threonine kinase in Hereditary Haemorrhagic Telangiectasia

16th national competition for scientific and technical research

Rare diseases

Senior Researcher : Petronila Penela Márquez

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Research Centre or Institution : Centro de Biología Molecular "Severo Ochoa". CSIC-Universidad Autónoma de Madrid


One of the key characteristics for the working of the vascular system is the capacity to bring about angiogenesis. This process depends on the integration of numerous signalling routes, among which that of TGF-beta factor stands out due to its pro- and anti-angiogenic functions, as well as the interaction with the endothelium with wall cells for stabilisation and maturation. Two of the key components of TGF-beta signalling, the specific ALK1 endothelial receptor and the co-receptor endoglin, are mutated in a high percentage of patients with Hereditary Haemorrhagic Telangiectasia (HHT or the Oslo-Weber-Rendu syndrome), causing anomalous angiogenic remodelling. Alterations in the signalling of growth factors (VEGF) and chemiokins (ligands of GPCR receptors coupled to G proteins), may condition the clinical development of HHT. In this context it is planned to analyse the role of GRK2 kinase, a convergence factor in the regulation of GPCRs, and the cellular responses to signals such as PDGF-BB or TGF-beta in the functionality of the endothelium during the angiogenic process and its influence in HHT.

The results show that GRK2 modulates the balance of signals via ALK1/ALK5 in response to TGF-beta, acting at several levels: on the one hand, by limiting the activation of the ALK5 cascade by means of the phosphorylation and inhibition of Smad2/3, while on the other hand co-operating positively in the ALK1 cascade activated by the TGF-beta ligand, favouring the levels of expression of endoglin. As a result of this, the angiogenic profile of the endothelium in the response to TGF-beta is altered when the expression of GRK2 is reduced. On the one hand, the production of pro-angiogenic factors such as proliferin, angiogenin and endoglin falls more sharply in the endothelium of GRK2 homozygote animals than it does in control animals, while the synthesis of MMP9 (implicated in the maturation of vessels) if not suitably induced and the presence of osteopontin (a potent inducer of pro-migratory routes) increase considerably. These changes contribute to a less proliferative and more migratory endothelium. In basal terms, the lack of GRK2 also gives rise to greater endothelial secretion of SDF-1α and PDGF-BB, which contribute to distorting the interaction between endothelial cells and pericytes. In pathological contexts (tumoral angiogenesis), the deficient coverage of endothelium lacking GRK2 due to the wall cells favours infiltration by macrophages and the growth of the tumour. Under physiological conditions (the vascular development of the retina), the deficient interaction between the endothelium and pericyte causes altered sprouting of the immature endothelium, with less remodelling or pruning of the capillaries and generalised retardation in the vascularisation of the retina.

Finally, a new GRK2 substrate was identified: the HDAC6 deacetylase protein implicated in the mobility of endothelial cells and other cell types. The phosphorylation of HDAC6 by GRK2 increases the deacetylation of tubulin and promotes cell spreading and migration. Likewise, in the absence of GRK2, the cell does not develop its total area in contact with fibronectin as the result of alterations in cellular adhesion. As a result of this, the cellular area of the endothelium is less when GRK2 is silenced. Defects in these parameters may intrinsically affect the barrier function of the endothelium, which would not only be relevant in HHT syndrome but also in other pathologies involving vascular dysfunction (rheumatoid arthritis, ischemic tissues or cancer).

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