A novel role of Hippo-Yap/TAZ signaling pathway in lymphatic vascular development

Synthetic hyaluronic acid coating preserves the phenotypes of lymphatic endothelial cells

Lymphatic endothelial cells (LECs) play a critical role in the formation and maintenance of the lymphatic vasculature, which is essential for the immune system, fluid balance, and tissue repair. However, LECs are often difficult to study in vivo and in vitro models that accurately mimic their behaviors and phenotypes are limited. In particular, LECs have been shown to lose their lymphatic markers over time while being cultured in vitro, which reflect their plasticity and heterogeneity in vivo. Since LECs uniquely express lymphatic vessel endothelial hyaluronan receptor-1 (LYVE-1), we hypothesized that surface coating with hyaluronic acid (HA) can preserve LEC phenotypes and functionalities. Dopamine conjugated hyaluronic acid (HA-DP) was synthesized with 42% degree of substitution to enable surface modification and conjugation onto standard tissue culture plates. Compared to fibronectin coating and tissue culture plate controls, surface coating with HA-DP was able to preserve lymphatic markers, such as prospero homeobox protein 1 (Prox1), podoplanin (PDPN), and LYVE-1 over several passages in vitro. LECs cultured on HA-DP expressed lower levels of focal adhesion kinase (FAK) and YAP/TAZ, which may be responsible for the maintenance of the lymphatic characteristics. Collectively, the HA-DP coating may provide a novel method for culturing human LECs in vitro toward more representative studies in basic lymphatic biology and lymphatic regeneration.

The emerging roles of long noncoding RNAs in lymphatic vascular development and disease

Recent advances in RNA sequencing technologies helped uncover what was once uncharted territory in the human genome-the complex and versatile world of long noncoding RNAs (lncRNAs). Previously thought of as merely transcriptional "noise", lncRNAs have now emerged as essential regulators of gene expression networks controlling development, homeostasis and disease progression. The regulatory functions of lncRNAs are broad and diverse, and the underlying molecular mechanisms are highly variable, acting at the transcriptional, post-transcriptional, translational, and post-translational levels. In recent years, evidence has accumulated to support the important role of lncRNAs in the development and functioning of the lymphatic vasculature and associated pathological processes such as tumor-induced lymphangiogenesis and cancer metastasis. In this review, we summarize the current knowledge on the role of lncRNAs in regulating the key genes and pathways involved in lymphatic vascular development and disease. Furthermore, we discuss the potential of lncRNAs as novel therapeutic targets and outline possible strategies for the development of lncRNA-based therapeutics to treat diseases of the lymphatic system.

Conjugated Bile Acids Promote Lymphangiogenesis by Modulation of the Reactive Oxygen Species-p90RSK-Vascular Endothelial Growth Factor Receptor 3 Pathway

Conjugated bile acids (BA) are significantly elevated in several liver pathologies and in the metastatic lymph node (LN). However, the effects of BAs on pathological lymphangiogenesis remains unknown. The current study explores the effects of BAs on lymphangiogenesis. BA levels were elevated in the LN and serum of Mdr2-/- mice (model of sclerosing cholangitis) compared to control mice. Liver and LN tissue sections showed a clear expansion of the lymphatic network in Mdr2-/- mice, indicating activated lymphangiogenic pathways. Human lymphatic endothelial cells (LECs) expressed BA receptors and a direct treatment with conjugated BAs enhanced invasion, migration, and tube formation. BAs also altered the LEC metabolism and upregulated key metabolic genes. Further, BAs induced the production of reactive oxygen species (ROS), that in turn phosphorylated the redox-sensitive kinase p90RSK, an essential regulator of endothelial cell dysfunction and oxidative stress. Activated p90RSK increased the SUMOylation of the Prox1 transcription factor and enhanced VEGFR3 expression and 3-D LEC invasion. BA-induced ROS in the LECs, which led to increased levels of Yes-associated protein (YAP), a lymphangiogenesis regulator. The suppression of cellular YAP inhibited BA-induced VEGFR3 upregulation and lymphangiogenic mechanism. Overall, our data shows the expansion of the lymphatic network in presclerotic liver disease and establishes a novel mechanism whereby BAs promote lymphangiogenesis.

The role of lacteal integrity and junction transformation in obesity: A promising therapeutic target?

Lacteals are the central lymphatic vessels in the villi of the small intestine and perform nutrient absorption, especially dietary lipids, and the transportation of antigen and antigen-presenting cells. Remodeling, proliferation, and cell-cell junctions of lymphatic endothelial cells (LECs) in lacteals are the basis of the maintenance of lacteal integrity and dietary lipid absorption. Normal lipid absorption in the diet depends on sound lacteal development and proliferation, especially integrity maintenance, namely, maintaining the appropriate proportion of button-like and zipper-like junctions. Maintaining the integrity and transforming button-to-zipper junctions in lacteals are strongly connected with obesity, which could be regulated by intestinal flora and molecular signalings, such as vascular endothelial growth factor C-vascular endothelial growth receptor 3 (VEGFC-VEGFR3) signaling, Hippo signaling, Notch signaling, angiopoietin-TIE signaling, VEGF-A/VEGFR2 signaling, and PROX1. This manuscript reviews the molecular mechanism of development, integrity maintenance, and junction transformation in lacteal related to obesity.

The RNA helicase Ddx21 controls Vegfc-driven developmental lymphangiogenesis by balancing endothelial cell ribosome biogenesis and p53 function

The development of a functional vasculature requires the coordinated control of cell fate, lineage differentiation and network growth. Cellular proliferation is spatiotemporally regulated in developing vessels, but how this is orchestrated in different lineages is unknown. Here, using a zebrafish genetic screen for lymphatic-deficient mutants, we uncover a mutant for the RNA helicase Ddx21. Ddx21 cell-autonomously regulates lymphatic vessel development. An established regulator of ribosomal RNA synthesis and ribosome biogenesis, Ddx21 is enriched in sprouting venous endothelial cells in response to Vegfc-Flt4 signalling. Ddx21 function is essential for Vegfc-Flt4-driven endothelial cell proliferation. In the absence of Ddx21, endothelial cells show reduced ribosome biogenesis, p53 and p21 upregulation and cell cycle arrest that blocks lymphangiogenesis. Thus, Ddx21 coordinates the lymphatic endothelial cell response to Vegfc-Flt4 signalling by balancing ribosome biogenesis and p53 function. This mechanism may be targetable in diseases of excessive lymphangiogenesis such as cancer metastasis or lymphatic malformation.