Tese de Doutoramento Medicina 2023 Faculdade de Ciências Médicas, Universidade NOVA de Lisboa

Introduction: Systemic sclerosis (SSc) is a chronic debilitating disease for which current treatments have very limited efficacy. It affects individuals of all ages, especially young females, and results in a high morbidity and premature death related to disseminated vascular dysfunction and tissue fibrosis. While its pathophysiology is still incompletely understood, current knowledge focuses mainly on the general mechanisms of fibroblast activation and a scarcely described endothelial dysfunction with microanatomical changes, including extreme capillary dilatations (megacapillaries). The Notch pathway is a conserved intercellular signalling mechanism which operates in virtually all organs and tissues during morphogenesis and adult life during health and disease. Its role in vascular biology is crucial for the establishment and maintenance of a functional microvascular capillary network. At the same time, it mediates the interaction between endothelial cells and perivascular cells, including pericytes and fibroblasts. The artificial overactivation of the Notch pathway in endothelial cells has been shown to produce a set of derangements of normal physiology which overlaps significantly with the SSc pathophysiology. This thesis was developed from the hypothesis that a previously uncharacterized serum factor exists in SSc patients which is able to activate the endothelial Notch signalling. Methods: Changes in the Notch pathway components (receptors, ligands and target genes) and its activation status in microvascular endothelial cells were studied in vitro after exposure to serum derived from SSc patients (n=22) or controls (n=10), through RT-PCR, immunofluorescence, and western blot. Time- and concentration-dependence of the effect were analysed by using various serum concentrations and varying incubation time. Immunoglobulin G was purified from the sera and utilized to further characterize the serum factor. The impact on the expression and activation of VEGF receptors was also assessed by RT-PCR and western blot. An association of the Hey2 expression levels (one of Notch target genes) with the clinical characteristics, current medication and microvascular changes assessed by nailfold videocapillaroscopy of the patients was also tested. Main findings: SSc serum induced a reproductible overexpression and increased intranuclear localization of the Notch target gene Hey2 in microvascular endothelial cells in culture, when compared with control serum. This effect started between 4h to 6h post-exposure to serum, peaked at 8h, and entered a plateau phase from 12h until at least 24h. The effect was positively correlated with the serum concentration used, being detectable at concentrations as low as 1,9%. This was associated with an increased quantity and intranuclear localization of the activated form of the receptor Notch-1, and the effect was at least partially reversed by DAPT, a gamma-secretase inhibitor (an enzymatic complex necessary for the activation of Notch receptors). The use of purified immunoglobulin G from SSc serum reproduced the changes in Hey2 expression. It was also associated with decreased VEGF signalling, which is the predicted effect of an activated Notch pathway. The increase in Hey2 expression did not differ between SSc clinical phenotype or pattern of organ involvement, known autoantibodies, nor with current medications. The degree of overexpression of Hey2 induced by the serum of each subject was strongly associated with the presence of megacapillaries in nailfold videocapillaroscopy. Conclusions: A novel pathophysiological mechanism of SSc was uncovered, whereby a previously unidentified autoantibody exists in SSc patients, independently of clinical phenotype, that is able to increase canonical signalling in microvascular endothelial cells through the receptor Notch-1 and induce an overexpression and increased intranuclear localization of Hey2. These changes were associated with an impaired VEGF signalling. These results also suggest that Hey2 overexpression is subsequently related to the development of megacapillaries. This might correspond to an upstream step in SSc pathogenesis which can lead to new therapeutic targets and diagnostic tools