The role of Meis transcription factors in the epicardium

A single layer of cells could be perceived as insignificant in comparison to a whole organ. However, extensive evidence has shown that the epicardium, the outermost layer of the heart, is essential for cardiac development and regeneration. Epicardium-derived cells (EPDCs) and epicardial signalling are crucial for coronary vasculature development and myocardial growth. In this thesis, we show that MEIS homeodomain transcription factors are expressed and play a function in the epicardium during cardiac development. Meis1 and Meis2 epicardial-specific conditional mutant mice show 50% lethality associated to misalignment of the great vessels and die after birth, while the survivors do not show cardiac malformations. Epicardial epithelial-to-mesenchymal transition of epicardial cells is relatively unaffected in Meis mutants, but EPDCs specification is altered, resulting in an excess of myofibroblast differentiation in detriment of smooth muscle cell (SMC) and other fibroblast populations. The excess myofibroblasts observed in Meis mutants accumulate in the subepicardium. Retinoic acid signalling is a major signalling pathway in the epicardium and is strongly impaired in the mutants, which could account for the altered specification of EPDCs. Decreased SMC coverage leads to a delayed maturation of the blood coronary vasculature, which also shows patterning alterations. Further characterization of Meis1 and Meis2 epicardial-specific conditional mutants has revealed the failure of prenatal lymphatic vessel development. This reveals a previously unknown non-autonomous function of the epicardium in promoting cardiac lymphangiogenesis. We characterize a population of subepicardial, fibroblast-like lymphatic-associated EPDCs (LEPCs) that completely enseaths lymphatic vessels as they grow towards the apex of the ventricles. LEPDC and Lymphatic Endothelial Cell (LECs) association is disrupted in Meis mutants, which suggests that LEPDC-LEC crosstalk is important for cardiac lymphangiogenesis. Transcriptomic analysis of Meis-mutant epicardium/subepicardium showed a downregulation of Vegfc and Vegfd lymphoangiocrine signals. Epicardial-specific Vegfc mutants present less developed and immature coronary lymphatic vessels, whereas analysis of the lymphatic vasculature of Vegfd knockout hearts shows that VEGFD is important for the development of ventral coronary lymphatics. These results show that direct cellular interactions and paracrine signalling from the epicardium/EPDCs orchestrate cardiac lymphatic development and that this process is regulated by MEIS transcription factors.This work was performed in Miguel Torres’ laboratory in the Cell and Developmental Biology Area at the Centro Nacional de Investigaciones Cardiovasculares Carlos III (CNIC) in Madrid. The CNIC is supported by the Ministerio de Ciencia, Innovación y Universidades (MCNU) and the Pro CNIC Foundation. This study was funded by grants RD12/0019/0005 and RD16/0011/0019 (TerCel, RETICS); S2010-BMD-2315 (Comunidad de Madrid); BFU2012-31086 (MINECO); BFU2015-71519 (MEIC); PGC2018-096486-B-I00 (MICINN) and ref. 17CVD04 (Leducq Foundation Transatlantic Networks). Mª Ester de la Cruz Crespillo was recipient of a “Formación del Profesorado Universitario” (FPU) fellowship from the Spanish Ministry (FPU15/02955) and EMBO Short Term fellowship (STF_8357).N