VEGFR3 is required for button junction formation in lymphatic vessels.

Lymphatic capillaries develop discontinuous cell-cell junctions that permit the absorption of large macromolecules, chylomicrons, and fluid from the interstitium. While excessive vascular endothelial growth factor 2 (VEGFR2) signaling can remodel and seal these junctions, whether and how VEGFR3 can alter lymphatic junctions remains incompletely understood. Here, we use lymphatic-specific Flt4 knockout mice to investigate VEGFR3 signaling in lymphatic junctions. We show that loss of Flt4 prevents specialized button junction formation in multiple tissues and impairs interstitial absorption. Knockdown of FLT4 in human lymphatic endothelial cells results in impaired NOTCH1 expression and activation, and overexpression of the NOTCH1 intracellular domain in Flt4 knockout vessels rescues the formation of button junctions and absorption of interstitial molecules. Together, our data reveal a requirement for VEGFR3 and NOTCH1 signaling in the development of button junctions during postnatal development and may hold clinical relevance to lymphatic diseases with impaired VEGFR3 signaling.

VE-cadherin enables trophoblast endovascular invasion and spiral artery remodeling during placental development.

During formation of the mammalian placenta, trophoblasts invade the maternal decidua and remodel spiral arteries to bring maternal blood into the placenta. This process, known as endovascular invasion, is thought to involve the adoption of functional characteristics of vascular endothelial cells (ECs) by trophoblasts. The genetic and molecular basis of endovascular invasion remains poorly defined, however, and whether trophoblasts utilize specialized endothelial proteins in an analogous manner to create vascular channels remains untested. Vascular endothelial (VE-)cadherin is a homotypic adhesion protein that is expressed selectively by ECs in which it enables formation of tight vessels and regulation of EC junctions. VE-cadherin is also expressed in invasive trophoblasts and is a prime candidate for a molecular mechanism of endovascular invasion by those cells. Here, we show that VE-cadherin is required for trophoblast migration and endovascular invasion into the maternal decidua in the mouse. VE-cadherin deficiency results in loss of spiral artery remodeling that leads to decreased flow of maternal blood into the placenta, fetal growth restriction, and death. These studies identify a non-endothelial role for VE-cadherin in trophoblasts during placental development and suggest that endothelial proteins may play functionally unique roles in trophoblasts that do not simply mimic those in ECs.

Sinusoidal and lymphatic vessel growth is controlled by reciprocal VEGF-C-CDH5 inhibition.

Sinusoids are specialized, low pressure blood vessels in the liver, bone marrow, and spleen required for definitive hematopoiesis. Unlike other blood endothelial cells (ECs), sinusoidal ECs express high levels of VEGFR3. VEGFR3 and its ligand VEGF-C are known to support lymphatic growth, but their function in sinusoidal vessels is unknown. In this study, we define a reciprocal VEGF-C/VEGFR3-CDH5 (VE-cadherin) signaling axis that controls growth of both sinusoidal and lymphatic vessels. Loss of VEGF-C or VEGFR3 resulted in cutaneous edema, reduced fetal liver size, and bloodless bone marrow due to impaired lymphatic and sinusoidal vessel growth. Mice with membrane-retained VE-cadherin conferred identical lymphatic and sinusoidal defects, suggesting that VE-cadherin opposes VEGF-C/VEGFR3 signaling. In developing mice, loss of VE-cadherin rescued defects in sinusoidal and lymphatic growth caused by loss of VEGFR3 but not loss of VEGF-C, findings explained by potentiated VEGF-C/VEGFR2 signaling in VEGFR3-deficient lymphatic ECs. Mechanistically, VEGF-C/VEGFR3 signaling induces VE-cadherin endocytosis and loss of function via SRC-mediated phosphorylation, while VE-cadherin prevents VEGFR3 endocytosis required for optimal receptor signaling. These findings establish an essential role for VEGF-C/VEGFR3 signaling during sinusoidal vascular growth, identify VE-cadherin as a powerful negative regulator of VEGF-C signaling that acts through both VEGFR3 and VEGFR2 receptors, and suggest that negative regulation of VE-cadherin is required for effective VEGF-C/VEGFR3 signaling during growth of sinusoidal and lymphatic vessels. Manipulation of this reciprocal negative regulatory mechanism, e.g. by reducing VE-cadherin function, may be used to stimulate therapeutic sinusoidal or lymphatic vessel growth.

Engaging new audiences with imaging and microscopy.

In this Spotlight, we hear first-hand accounts from five scientists and educators who use microscopy and imaging to engage, entertain, educate and inspire new audiences with science and the field of developmental biology in particular. The 'voices' that follow each convey each authors' own personal take on why microscopy is such a powerful tool for capturing the minds, and the hearts, of scientists, students and the public alike. They discuss how microscopy and imaging can reveal new worlds, and improve our communication and understanding of developmental biology, as well as break down barriers and promote diversity for future generations of scientific researchers.

PIK3CA and CCM mutations fuel cavernomas through a cancer-like mechanism.

Vascular malformations are thought to be monogenic disorders that result in dysregulated growth of blood vessels. In the brain, cerebral cavernous malformations (CCMs) arise owing to inactivation of the endothelial CCM protein complex, which is required to dampen the activity of the kinase MEKK31-4. Environmental factors can explain differences in the natural history of CCMs between individuals5, but why single CCMs often exhibit sudden, rapid growth, culminating in strokes or seizures, is unknown. Here we show that growth of CCMs requires increased signalling through the phosphatidylinositol-3-kinase (PI3K)-mTOR pathway as well as loss of function of the CCM complex. We identify somatic gain-of-function mutations in PIK3CA and loss-of-function mutations in the CCM complex in the same cells in a majority of human CCMs. Using mouse models, we show that growth of CCMs requires both PI3K gain of function and CCM loss of function in endothelial cells, and that both CCM loss of function and increased expression of the transcription factor KLF4 (a downstream effector of MEKK3) augment mTOR signalling in endothelial cells. Consistent with these findings, the mTORC1 inhibitor rapamycin effectively blocks the formation of CCMs in mouse models. We establish a three-hit mechanism analogous to cancer, in which aggressive vascular malformations arise through the loss of vascular 'suppressor genes' that constrain vessel growth and gain of a vascular 'oncogene' that stimulates excess vessel growth. These findings suggest that aggressive CCMs could be treated using clinically approved mTORC1 inhibitors.

Mutations in non-muscle myosin 2A disrupt the actomyosin cytoskeleton in Sertoli cells and cause male infertility.

Mutations in non-muscle myosin 2A (NM2A) encompass a wide spectrum of anomalies collectively known as MYH9-Related Disease (MYH9-RD) in humans that can include macrothrombocytopenia, glomerulosclerosis, deafness, and cataracts. We previously created mouse models of the three mutations most frequently found in humans: R702C, D1424N, and E1841K. While homozygous R702C and D1424N mutations are embryonic lethal, we found homozygous mutant E1841K mice to be viable. However the homozygous male, but not female, mice were infertile. Here, we report that these mice have reduced testis size and defects in actin-associated junctions in Sertoli cells, resulting in inability to form the blood-testis barrier and premature germ cell loss. Moreover, compound double heterozygous (R702C/E1841K and D1424/E1841K) males show the same abnormalities in testes as E1841K homozygous males. Conditional ablation of either NM2A or NM2B alone in Sertoli cells has no effect on fertility and testis size, however deletion of both NM2A and NM2B in Sertoli cells results in infertility. Isolation of mutant E1841K Sertoli cells reveals decreased NM2A and F-actin colocalization and thicker NM2A filaments. Furthermore, AE1841K/AE1841K and double knockout Sertoli cells demonstrate microtubule disorganization and increased tubulin acetylation, suggesting defects in the microtubule cytoskeleton. Together, these results demonstrate that NM2A and 2B paralogs play redundant roles in Sertoli cells and are essential for testes development and normal fertility.

Nonmuscle myosin IIB regulates epicardial integrity and epicardium-derived mesenchymal cell maturation.

Nonmuscle myosin IIB (NMIIB; heavy chain encoded by MYH10) is essential for cardiac myocyte cytokinesis. The role of NMIIB in other cardiac cells is not known. Here, we show that NMIIB is required in epicardial formation and functions to support myocardial proliferation and coronary vessel development. Ablation of NMIIB in epicardial cells results in disruption of epicardial integrity with a loss of E-cadherin at cell-cell junctions and a focal detachment of epicardial cells from the myocardium. NMIIB-knockout and blebbistatin-treated epicardial explants demonstrate impaired mesenchymal cell maturation during epicardial epithelial-mesenchymal transition. This is manifested by an impaired invasion of collagen gels by the epicardium-derived mesenchymal cells and the reorganization of the cytoskeletal structure. Although there is a marked decrease in the expression of mesenchymal genes, there is no change in Snail (also known as Snai1) or E-cadherin expression. Studies from epicardium-specific NMIIB-knockout mice confirm the importance of NMIIB for epicardial integrity and epicardial functions in promoting cardiac myocyte proliferation and coronary vessel formation during heart development. Our findings provide a novel mechanism linking epicardial formation and epicardial function to the activity of the cytoplasmic motor protein NMIIB.

A combined gene expression and functional study reveals the crosstalk between N-Myc and differentiation-inducing microRNAs in neuroblastoma cells.

MYCN amplification is the most common genetic alteration in neuroblastoma and plays a critical role in neuroblastoma tumorigenesis. MYCN regulates neuroblastoma cell differentiation, which is one of the mechanisms underlying its oncogenic function. We recently identified a group of differentiation-inducing microRNAs. Given the demonstrated inter-regulation between MYCN and microRNAs, we speculated that MYCN and the differentiation-inducing microRNAs might form an interaction network to control the differentiation of neuroblastoma cells. In this study, we found that eight of the thirteen differentiation-inducing microRNAs, miR-506-3p, miR-124-3p, miR-449a, miR-34a-5p, miR-449b-5p, miR-103a-3p, miR-2110 and miR-34b-5p, inhibit N-Myc expression by either directly targeting the MYCN 3'UTR or through indirect regulations. Further investigation showed that both MYCN-dependent and MYCN-independent pathways play roles in mediating the differentiation-inducing function of miR-506-3p and miR-449a, two microRNAs that dramatically down-regulate MYCN expression. On the other hand, we found that N-Myc inhibits the expression of multiple differentiation-inducing microRNAs, suggesting that these miRNAs play a role in mediating the function of MYCN. In examining the published dataset collected from clinical neuroblastoma specimens, we found that expressions of two miRNAs, miR-137 and miR-2110, were significantly anti-correlated with MYCN mRNA levels, suggesting their interactions with MYCN play a clinically-relevant role in maintaining the MYCN and miRNA expression levels in neuroblastoma. Our findings altogether suggest that MYCN and differentiation-inducing miRNAs form an interaction network that play an important role in neuroblastoma tumorigenesis through regulating cell differentiation.

Cadherin-11 Overexpression Induces Extracellular Matrix Remodeling and Calcification in Mature Aortic Valves.

OBJECTIVE: Calcific aortic valve (AoV) disease is a significant clinical problem for which the regulatory mechanisms are poorly understood. Enhanced cell-cell adhesion is a common mechanism of cellular aggregation, but its role in calcific lesion formation is not known. Cadherin-11 (Cad-11) has been associated with lesion formation in vitro, but its function during adult valve homeostasis and pathogenesis is not known. This study aims to elucidate the specific functions of Cad-11 and its downstream targets, RhoA and Sox9, in extracellular matrix remodeling and AoV calcification. APPROACH AND RESULTS: We conditionally overexpressed Cad-11 in murine heart valves using a novel double-transgenic Nfatc1(Cre);R26-Cad11(TglTg) mouse model. These mice developed hemodynamically significant aortic stenosis with prominent calcific lesions in the AoV leaflets. Cad-11 overexpression upregulated downstream targets, RhoA and Sox9, in the valve interstitial cells, causing calcification and extensive pathogenic extracellular matrix remodeling. AoV interstitial cells overexpressing Cad-11 in an osteogenic environment in vitro rapidly form calcific nodules analogous to in vivo lesions. Molecular analyses revealed upregulation of osteoblastic and myofibroblastic markers. Treatment with a Rho-associated protein kinase inhibitor attenuated nodule formation, further supporting that Cad-11-driven calcification acts through the small GTPase RhoA/Rho-associated protein kinase signaling pathway. CONCLUSIONS: This study identifies one of the underlying molecular mechanisms of heart valve calcification and demonstrates that overexpression of Cad-11 upregulates RhoA and Sox9 to induce calcification and extracellular matrix remodeling in adult AoV pathogenesis. The findings provide a potential molecular target for clinical treatment.

Cadherin-11 coordinates cellular migration and extracellular matrix remodeling during aortic valve maturation.

Proper remodeling of the endocardial cushions into thin fibrous valves is essential for gestational progression and long-term function. This process involves dynamic interactions between resident cells and their local environment, much of which is not understood. In this study, we show that deficiency of the cell-cell adhesion protein cadherin-11 (Cad-11) results in significant embryonic and perinatal lethality primarily due to valve related cardiac dysfunction. While endocardial to mesenchymal transformation is not abrogated, mesenchymal cells do not homogeneously cellularize the cushions. These cushions remain thickened with disorganized ECM, resulting in pronounced aortic valve insufficiency. Mice that survive to adulthood maintain thickened and stenotic semilunar valves, but interestingly do not develop calcification. Cad-11 (-/-) aortic valve leaflets contained reduced Sox9 activity, ß1 integrin expression, and RhoA-GTP activity, suggesting that remodeling defects are due to improper migration and/or cellular contraction. Cad-11 deletion or siRNA knockdown reduced migration, eliminated collective migration, and impaired 3D matrix compaction by aortic valve interstitial cells (VIC). Cad-11 depleted cells in culture contained few filopodia, stress fibers, or contact inhibited locomotion. Transfection of Cad-11 depleted cells with constitutively active RhoA restored cell phenotypes. Together, these results identify cadherin-11 mediated adhesive signaling for proper remodeling of the embryonic semilunar valves.