The phosphodiesterase 2A controls lymphatic junctional maturation via cGMP-dependent notch signaling.

The molecular mechanisms by which lymphatic vessels induce cell contact inhibition are not understood. Here, we identify the cGMP-dependent phosphodiesterase 2A (PDE2A) as a selective regulator of lymphatic but not of blood endothelial contact inhibition. Conditional deletion of Pde2a in mouse embryos reveals severe lymphatic dysplasia, whereas blood vessel architecture remains unaltered. In the absence of PDE2A, human lymphatic endothelial cells fail to induce mature junctions and cell cycle arrest, whereas cGMP levels, but not cAMP levels, are increased. Loss of PDE2A-mediated cGMP hydrolysis leads to the activation of p38 signaling and downregulation of NOTCH signaling. However, DLL4-induced NOTCH activation restores junctional maturation and contact inhibition in PDE2A-deficient human lymphatic endothelial cells. In postnatal mouse mesenteries, PDE2A is specifically enriched in collecting lymphatic valves, and loss of Pde2a results in the formation of abnormal valves. Our data demonstrate that PDE2A selectively finetunes a crosstalk of cGMP, p38, and NOTCH signaling during lymphatic vessel maturation.

Svep1 is a binding ligand of Tie1 and affects specific aspects of facial lymphatic development in a Vegfc-independent manner.

Multiple factors are required to form functional lymphatic vessels. Here, we uncover an essential role for the secreted protein Svep1 and the transmembrane receptor Tie1 during the development of subpopulations of the zebrafish facial lymphatic network. This specific aspect of the facial network forms independently of Vascular endothelial growth factor C (Vegfc) signalling, which otherwise is the most prominent signalling axis in all other lymphatic beds. Additionally, we find that multiple specific and newly uncovered phenotypic hallmarks of svep1 mutants are also present in tie1, but not in tie2 or vegfc mutants. These phenotypes are observed in the lymphatic vasculature of both head and trunk, as well as in the development of the dorsal longitudinal anastomotic vessel under reduced flow conditions. Therefore, our study demonstrates an important function for Tie1 signalling during lymphangiogenesis as well as blood vessel development in zebrafish. Furthermore, we show genetic interaction between svep1 and tie1 in vivo, during early steps of lymphangiogenesis, and demonstrate that zebrafish as well as human Svep1/SVEP1 protein bind to the respective Tie1/TIE1 receptors in vitro. Since compound heterozygous mutations for SVEP1 and TIE2 have recently been reported in human glaucoma patients, our data have clinical relevance in demonstrating a role for SVEP1 in TIE signalling in an in vivo setting.

EVL regulates VEGF receptor-2 internalization and signaling in developmental angiogenesis.

Endothelial tip cells are essential for VEGF-induced angiogenesis, but underlying mechanisms are elusive. The Ena/VASP protein family, consisting of EVL, VASP, and Mena, plays a pivotal role in axon guidance. Given that axonal growth cones and endothelial tip cells share many common features, from the morphological to the molecular level, we investigated the role of Ena/VASP proteins in angiogenesis. EVL and VASP, but not Mena, are expressed in endothelial cells of the postnatal mouse retina. Global deletion of EVL (but not VASP) compromises the radial sprouting of the vascular plexus in mice. Similarly, endothelial-specific EVL deletion compromises the radial sprouting of the vascular plexus and reduces the endothelial tip cell density and filopodia formation. Gene sets involved in blood vessel development and angiogenesis are down-regulated in EVL-deficient P5-retinal endothelial cells. Consistently, EVL deletion impairs VEGF-induced endothelial cell proliferation and sprouting, and reduces the internalization and phosphorylation of VEGF receptor 2 and its downstream signaling via the MAPK/ERK pathway. Together, we show that endothelial EVL regulates sprouting angiogenesis via VEGF receptor-2 internalization and signaling.

Tie1 regulates zebrafish cardiac morphogenesis through Tolloid-like 1 expression.

Tie1 is a receptor tyrosine kinase expressed in endothelial cells, where it modulates Angiopoietin/Tie2 signaling. Previous studies have shown that mouse Tie1 mutants exhibit severe cardiovascular defects; however, much remains to be learned about the role of Tie1, especially during cardiac development. To further understand Tie1 function, we generated a zebrafish tie1 mutant line. Homozygous mutant embryos display reduced endothelial and endocardial cell numbers and reduced heart size. Live imaging and ultrastructural analyses at embryonic stages revealed increased cardiac jelly thickness as well as cardiomyocyte defects, including a loss of sarcomere organization and altered cell shape. Transcriptomic profiling of embryonic hearts uncovered the downregulation of tll1, which encodes a Tolloid-like protease, in tie1-/- compared with wild-type siblings. Using mRNA injections into one-cell stage embryos, we found that tll1 overexpression could partially rescue the tie1 mutant cardiac phenotypes including the endocardial and myocardial cell numbers as well as the cardiac jelly thickness. Altogether, our results indicate the importance of a Tie1-Tolloid-like 1 axis in paracrine signaling during cardiac development.

Tek (Tie2) is not required for cardiovascular development in zebrafish.

Angiopoietin/TIE signalling plays a major role in blood and lymphatic vessel development. In mouse, Tek (previously known as Tie2) mutants die prenatally due to a severely underdeveloped cardiovascular system. In contrast, in zebrafish, previous studies have reported that although embryos injected with tek morpholinos (MOs) exhibit severe vascular defects, tek mutants display no obvious vascular malformations. To further investigate the function of zebrafish Tek, we generated a panel of loss-of-function tek mutants, including RNA-less alleles, an allele lacking the MO-binding site, an in-frame deletion allele and a premature termination codon-containing allele. Our data show that all these mutants survive to adulthood with no obvious cardiovascular defects. MO injections into tek mutants lacking the MO-binding site or the entire tek locus cause similar vascular defects to those observed in MO-injected +/+ siblings, indicating off-target effects of the MOs. Surprisingly, comprehensive phylogenetic profiling and synteny analyses reveal that Tek was lost in the largest teleost clade, suggesting a lineage-specific shift in the function of TEK during vertebrate evolution. Altogether, these data show that Tek is dispensable for zebrafish development, and probably dispensable in most teleost species.

Micro RNAs are involved in activation of epicardium during zebrafish heart regeneration.

Zebrafish could be an interesting translational model to understand and improve the post-infarction trial and possible regeneration in humans. The adult zebrafish is able to regenerate efficiently after resecting nearly 20% of the ventricular apex. This process requires the concert activation of the epicardium and endocardium, as well as trans-differentiation of pre-existing cardiomyocytes that together replace the lost tissue. The molecular mechanisms involved in this activation process are not completely clarified. In this work, in order to investigate if the downregulation of these miRNAs (miRs) are linked with the activation of epicardium, the expressions of miR-133a, b and miR-1 during regeneration were analysed. qPCR analyses in whole-heart, or from distinct dissected epicardial cells comparing to regenerative clot (containing cardiomyocytes, fibroblasts and endocardial cells) by a laser-micro-dissector, have indicated that already at 24 h there is a downregulation of miRs: (1) miR-133a and miR-1 in the epicardium and (2) miR-133b and miR-1 in the regenerative clot. All the miRs remain downregulated until 7 days post-surgery. With the aim to visualize the activations of heart component in combination with miRs, we developed immunohistochemistry using antibodies directed against common markers in mammals as well as zebrafish: Wilms tumour 1 (WT1), a marker of epicardium; heat-shock protein 70 (HSP70), a chaperon activated during regeneration; and the Cardiac Troponin T (cTnT), a marker of differentiated cardiomyocytes. All these markers are directly or indirectly linked to the investigated miRs. WT1 and HSP70 strongly marked the regeneration site just at 2-3 days postventricular resection. In coherence, cTnT intensively marked the regenerative portion from 7 days onwards. miRs-1 and -133 (a,b) have been strongly involved in the activation of epicardium and regenerative clot during the regeneration process in zebrafish. This study can be a useful translational model to understand the early epicardial activation in which miRs-133a and miR-1 seem to play a central role as observed in the human heart.