Multiple cis-regulatory elements control prox1a expression in distinct lymphatic vascular beds.

During embryonic development, lymphatic endothelial cell (LEC) precursors are distinguished from blood endothelial cells by the expression of Prospero-related homeobox 1 (Prox1), which is essential for lymphatic vasculature formation in mouse and zebrafish. Prox1 expression initiation precedes LEC sprouting and migration, serving as the marker of specified LECs. Despite its crucial role in lymphatic development, Prox1 upstream regulation in LECs remains to be uncovered. SOX18 and COUP-TFII are thought to regulate Prox1 in mice by binding its promoter region. However, the specific regulation of Prox1 expression in LECs remains to be studied in detail. Here, we used evolutionary conservation and chromatin accessibility to identify enhancers located in the proximity of zebrafish prox1a active in developing LECs. We confirmed the functional role of the identified sequences through CRISPR/Cas9 mutagenesis of a lymphatic valve enhancer. The deletion of this region results in impaired valve morphology and function. Overall, our results reveal an intricate control of prox1a expression through a collection of enhancers. Ray-finned fish-specific distal enhancers drive pan-lymphatic expression, whereas vertebrate-conserved proximal enhancers refine expression in functionally distinct subsets of lymphatic endothelium.

The blood vasculature instructs lymphatic patterning in a SOX7-dependent manner.

Despite a growing catalog of secreted factors critical for lymphatic network assembly, little is known about the mechanisms that modulate the expression level of these molecular cues in blood vascular endothelial cells (BECs). Here, we show that a BEC-specific transcription factor, SOX7, plays a crucial role in a non-cell-autonomous manner by modulating the transcription of angiocrine signals to pattern lymphatic vessels. While SOX7 is not expressed in lymphatic endothelial cells (LECs), the conditional loss of SOX7 function in mouse embryos causes a dysmorphic dermal lymphatic phenotype. We identify novel distant regulatory regions in mice and humans that contribute to directly repressing the transcription of a major lymphangiogenic growth factor (Vegfc) in a SOX7-dependent manner. Further, we show that SOX7 directly binds HEY1, a canonical repressor of the Notch pathway, suggesting that transcriptional repression may also be modulated by the recruitment of this protein partner at Vegfc genomic regulatory regions. Our work unveils a role for SOX7 in modulating downstream signaling events crucial for lymphatic patterning, at least in part via the transcriptional repression of VEGFC levels in the blood vascular endothelium.

A Prox1 enhancer represses haematopoiesis in the lymphatic vasculature.

Transcriptional enhancer elements are responsible for orchestrating the temporal and spatial control over gene expression that is crucial for programming cell identity during development1-3. Here we describe a novel enhancer element that is important for regulating the expression of Prox1 in lymphatic endothelial cells. This evolutionarily conserved enhancer is bound by key lymphatic transcriptional regulators including GATA2, FOXC2, NFATC1 and PROX1. Genome editing of the enhancer to remove five nucleotides encompassing the GATA2-binding site resulted in perinatal death of homozygous mutant mice due to profound lymphatic vascular defects. Lymphatic endothelial cells in enhancer mutant mice exhibited reduced expression of genes characteristic of lymphatic endothelial cell identity and increased expression of genes characteristic of haemogenic endothelium, and acquired the capacity to generate haematopoietic cells. These data not only reveal a transcriptional enhancer element important for regulating Prox1 expression and lymphatic endothelial cell identity but also demonstrate that the lymphatic endothelium has haemogenic capacity, ordinarily repressed by Prox1.

Pathogenic variants in MDFIC cause recessive central conducting lymphatic anomaly with lymphedema.

Central conducting lymphatic anomaly (CCLA), characterized by the dysfunction of core collecting lymphatic vessels including the thoracic duct and cisterna chyli, and presenting as chylothorax, pleural effusions, chylous ascites, and lymphedema, is a severe disorder often resulting in fetal or perinatal demise. Although pathogenic variants in RAS/mitogen activated protein kinase (MAPK) signaling pathway components have been documented in some patients with CCLA, the genetic etiology of the disorder remains uncharacterized in most cases. Here, we identified biallelic pathogenic variants in MDFIC, encoding the MyoD family inhibitor domain containing protein, in seven individuals with CCLA from six independent families. Clinical manifestations of affected fetuses and children included nonimmune hydrops fetalis (NIHF), pleural and pericardial effusions, and lymphedema. Generation of a mouse model of human MDFIC truncation variants revealed that homozygous mutant mice died perinatally exhibiting chylothorax. The lymphatic vasculature of homozygous Mdfic mutant mice was profoundly mispatterned and exhibited major defects in lymphatic vessel valve development. Mechanistically, we determined that MDFIC controls collective cell migration, an important early event during the formation of lymphatic vessel valves, by regulating integrin ß1 activation and the interaction between lymphatic endothelial cells and their surrounding extracellular matrix. Our work identifies MDFIC variants underlying human lymphatic disease and reveals a crucial, previously unrecognized role for MDFIC in the lymphatic vasculature. Ultimately, understanding the genetic and mechanistic basis of CCLA will facilitate the development and implementation of new therapeutic approaches to effectively treat this complex disease.

Atypical cadherin FAT4 orchestrates lymphatic endothelial cell polarity in response to flow.

The atypical cadherin FAT4 has established roles in the regulation of planar cell polarity and Hippo pathway signaling that are cell context dependent. The recent identification of FAT4 mutations in Hennekam syndrome, features of which include lymphedema, lymphangiectasia, and mental retardation, uncovered an important role for FAT4 in the lymphatic vasculature. Hennekam syndrome is also caused by mutations in collagen and calcium binding EGF domains 1 (CCBE1) and ADAM metallopeptidase with thrombospondin type 1 motif 3 (ADAMTS3), encoding a matrix protein and protease, respectively, that regulate activity of the key prolymphangiogenic VEGF-C/VEGFR3 signaling axis by facilitating the proteolytic cleavage and activation of VEGF-C. The fact that FAT4, CCBE1, and ADAMTS3 mutations underlie Hennekam syndrome suggested that all 3 genes might function in a common pathway. We identified FAT4 as a target gene of GATA-binding protein 2 (GATA2), a key transcriptional regulator of lymphatic vascular development and, in particular, lymphatic vessel valve development. Here, we demonstrate that FAT4 functions in a lymphatic endothelial cell-autonomous manner to control cell polarity in response to flow and is required for lymphatic vessel morphogenesis throughout development. Our data reveal a crucial role for FAT4 in lymphangiogenesis and shed light on the mechanistic basis by which FAT4 mutations underlie a human lymphedema syndrome.

Matrix stiffness controls lymphatic vessel formation through regulation of a GATA2-dependent transcriptional program.

Tissue and vessel wall stiffening alters endothelial cell properties and contributes to vascular dysfunction. However, whether extracellular matrix (ECM) stiffness impacts vascular development is not known. Here we show that matrix stiffness controls lymphatic vascular morphogenesis. Atomic force microscopy measurements in mouse embryos reveal that venous lymphatic endothelial cell (LEC) progenitors experience a decrease in substrate stiffness upon migration out of the cardinal vein, which induces a GATA2-dependent transcriptional program required to form the first lymphatic vessels. Transcriptome analysis shows that LECs grown on a soft matrix exhibit increased GATA2 expression and a GATA2-dependent upregulation of genes involved in cell migration and lymphangiogenesis, including VEGFR3. Analyses of mouse models demonstrate a cell-autonomous function of GATA2 in regulating LEC responsiveness to VEGF-C and in controlling LEC migration and sprouting in vivo. Our study thus uncovers a mechanism by which ECM stiffness dictates the migratory behavior of LECs during early lymphatic development.

Lymphatic endothelial progenitor cells: origins and roles in lymphangiogenesis.

How are lymphatic vessels built? What are the sources of progenitor cells employed to construct lymphatic vessels during embryogenesis and in pathological situations? Are lymphatic vessels in different tissues built the same way? These questions have been highly topical and actively debated in the field of lymphangiogenesis research for more than 100 years. While embryonic veins and cells of mesenchymal origin have been recognised as sources of embryonic lymphatic endothelial cells for many years, recent advances in technology have revealed the existence of additional sources of lymphatic endothelial cells important for embryonic lymphangiogenesis. Intriguingly, distinct progenitor cell sources appear to be employed in a tissue specific manner during development. Gaining further insight into the identity of lymphatic endothelial progenitor cells and the signals that direct their assembly, both during development and in disease, has the potential to enable the design of therapeutics able to selectively target specific lymphatic vessel beds, a feature likely to prove valuable for the treatment of human disorders including cancer, lymphoedema and inflammatory disease.

Morphogenesis of the lymphatic vasculature: A focus on new progenitors and cellular mechanisms important for constructing lymphatic vessels.

Lymphatic vessels serve crucial roles in the regulation of tissue fluid homeostasis, dietary lipid absorption and immune cell trafficking. Defects in lymphatic vessel morphogenesis and function have been associated with lymphedema, obesity, hypertension and tumour metastasis. Morphogenetic events important for construction of the lymphatic vasculature during development include the specification and emergence of lymphatic endothelial progenitor cells, their differentiation and assembly into interconnected vessels and vascular remodeling, ultimately giving rise to a functional vascular network. Despite the embryonic origins of lymphatic endothelial progenitor cells being long debated, work performed over the last decade had overwhelmingly supported at least a great majority of progenitor cells arising from the venous vasculature. Here, we review the most recent advances in the field of lymphatic vessel morphogenesis, with a focus on studies that have identified novel sources of embryonic lymphatic endothelial progenitor cells, together with the cellular mechanisms by which lymphatic vessels are initially assembled.

GATA2 is required for lymphatic vessel valve development and maintenance.

Heterozygous germline mutations in the zinc finger transcription factor GATA2 have recently been shown to underlie a range of clinical phenotypes, including Emberger syndrome, a disorder characterized by lymphedema and predisposition to myelodysplastic syndrome/acute myeloid leukemia (MDS/AML). Despite well-defined roles in hematopoiesis, the functions of GATA2 in the lymphatic vasculature and the mechanisms by which GATA2 mutations result in lymphedema have not been characterized. Here, we have provided a molecular explanation for lymphedema predisposition in a subset of patients with germline GATA2 mutations. Specifically, we demonstrated that Emberger-associated GATA2 missense mutations result in complete loss of GATA2 function, with respect to the capacity to regulate the transcription of genes that are important for lymphatic vessel valve development. We identified a putative enhancer element upstream of the key lymphatic transcriptional regulator PROX1 that is bound by GATA2, and the transcription factors FOXC2 and NFATC1. Emberger GATA2 missense mutants had a profoundly reduced capacity to bind this element. Conditional Gata2 deletion in mice revealed that GATA2 is required for both development and maintenance of lymphovenous and lymphatic vessel valves. Together, our data unveil essential roles for GATA2 in the lymphatic vasculature and explain why a select catalogue of human GATA2 mutations results in lymphedema.

Control of retinoid levels by CYP26B1 is important for lymphatic vascular development in the mouse embryo.

During embryogenesis, lymphatic endothelial progenitor cells first arise from a subset of blood vascular endothelial cells in the dorsolateral aspects of the cardinal veins. The molecular cues responsible for defining the regionalisation of such a discrete pool of progenitors remain uncharacterised. Here we identify a novel function for CYP26B1, an enzyme known to play a role in tissue morphogenesis by fine-tuning retinoic acid (RA) concentration, in regulating lymphangiogenesis. Cyp26b1-null mice, in which RA levels are elevated, exhibited an increased number of lymphatic endothelial progenitor cells in the cardinal veins, together with hyperplastic, blood filled lymph sacs and hyperplastic dermal lymphatic vessels. Conversely, mice over-expressing Cyp26b1 had hypoplastic lymph sacs and lymphatic vessels. Our data suggest that RA clearance by CYP26B1 in the vicinity of lymphatic endothelial progenitor cells is important for determining the position and size of the progenitor pool specified. Our studies identify a genetic pathway that underpins the architecture of the developing lymphatics and define CYP26B1 as a novel modulator of lymphatic vascular patterning.

Arap3 is dysregulated in a mouse model of hypotrichosis-lymphedema-telangiectasia and regulates lymphatic vascular development.

Mutations in SOX18, VEGFC and Vascular Endothelial Growth Factor 3 underlie the hereditary lymphatic disorders hypotrichosis-lymphedema-telangiectasia (HLT), Milroy-like lymphedema and Milroy disease, respectively. Genes responsible for hereditary lymphedema are key regulators of lymphatic vascular development in the embryo. To identify novel modulators of lymphangiogenesis, we used a mouse model of HLT (Ragged Opossum) and performed gene expression profiling of aberrant dermal lymphatic vessels. Expression studies and functional analysis in zebrafish and mice revealed one candidate, ArfGAP with RhoGAP domain, Ankyrin repeat and PH domain 3 (ARAP3), which is down-regulated in HLT mouse lymphatic vessels and necessary for lymphatic vascular development in mice and zebrafish. We position this known regulator of cell behaviour during migration as a mediator of the cellular response to Vegfc signalling in lymphatic endothelial cells in vitro and in vivo. Our data refine common mechanisms that are likely to contribute during both development and the pathogenesis of lymphatic vascular disorders.

Down-regulation of the miRNA-200 family at the invasive front of colorectal cancers with degraded basement membrane indicates EMT is involved in cancer progression.

Cancer progression is a complex series of events thought to incorporate the reversible developmental process of epithelial-to-mesenchymal transition (EMT). In vitro, the microRNA-200 family maintains the epithelial phenotype by posttranscriptionally inhibiting the E-cadherin repressors, ZEB1 and ZEB2. Here, we used in situ hybridization and immunohistochemistry to assess expression of miR-200 and EMT biomarkers in formalin-fixed paraffin-embedded human colorectal adenocarcinomas. In addition, laser capture microdissection and quantitative real-time polymerase chain reaction were employed to quantify levels of miR-200 in the normal epithelium, tumor core, invasive front, and stroma. We find that miR-200 is downregulated at the invasive front of colorectal adenocarcinomas that have destroyed and invaded beyond the basement membrane. However, regional lymph node metastases and vascular carcinoma deposits show strong expression of miR-200, suggesting this family of miRNAs is involved in the recapitulation of the primary tumor phenotype at metastatic sites. In contrast, adenomas and adenocarcinomas with intact basement membranes showed uniform miR-200 expression from the tumor core to the tumor-host interface. Taken together, these data support the involvement of EMT and mesenchymal-to-epithelial transition (MET) in the metastasis cascade and show that miR-200 is downregulated in the initial stages of stromal invasion but is restored at metastatic sites.

In vitro assays using primary embryonic mouse lymphatic endothelial cells uncover key roles for FGFR1 signalling in lymphangiogenesis.

Despite the importance of blood vessels and lymphatic vessels during development and disease, the signalling pathways underpinning vessel construction remain poorly characterised. Primary mouse endothelial cells have traditionally proven difficult to culture and as a consequence, few assays have been developed to dissect gene function and signal transduction pathways in these cells ex vivo. Having established methodology for the purification, short-term culture and transfection of primary blood (BEC) and lymphatic (LEC) vascular endothelial cells isolated from embryonic mouse skin, we sought to optimise robust assays able to measure embryonic LEC proliferation, migration and three-dimensional tube forming ability in vitro. In the course of developing these assays using the pro-lymphangiogenic growth factors FGF2 and VEGF-C, we identified previously unrecognised roles for FGFR1 signalling in lymphangiogenesis. The small molecule FGF receptor tyrosine kinase inhibitor SU5402, but not inhibitors of VEGFR-2 (SU5416) or VEGFR-3 (MAZ51), inhibited FGF2 mediated LEC proliferation, demonstrating that FGF2 promotes proliferation directly via FGF receptors and independently of VEGF receptors in primary embryonic LEC. Further investigation revealed that FGFR1 was by far the predominant FGF receptor expressed by primary embryonic LEC and correspondingly, siRNA-mediated FGFR1 knockdown abrogated FGF2 mediated LEC proliferation. While FGF2 potently promoted LEC proliferation and migration, three dimensional tube formation assays revealed that VEGF-C primarily promoted LEC sprouting and elongation, illustrating that FGF2 and VEGF-C play distinct, cooperative roles in lymphatic vascular morphogenesis. These assays therefore provide useful tools able to dissect gene function in cellular events important for lymphangiogenesis and implicate FGFR1 as a key player in developmental lymphangiogenesis in vivo.

Loss-of-function germline GATA2 mutations in patients with MDS/AML or MonoMAC syndrome and primary lymphedema reveal a key role for GATA2 in the lymphatic vasculature.

Recent work has established that heterozygous germline GATA2 mutations predispose carriers to familial myelodysplastic syndrome (MDS)/acute myeloid leukemia (AML), "MonoMAC" syndrome, and DCML deficiency. Here, we describe a previously unreported MDS family carrying a missense GATA2 mutation (p.Thr354Met), one patient with MDS/AML carrying a frameshift GATA2 mutation (p.Leu332Thrfs*53), another with MDS harboring a GATA2 splice site mutation, and 3 patients exhibiting MDS or MDS/AML who have large deletions encompassing the GATA2 locus. Intriguingly, 2 MDS/AML or "MonoMAC" syndrome patients with GATA2 deletions and one with a frameshift mutation also have primary lymphedema. Primary lymphedema occurs as a result of aberrations in the development and/or function of lymphatic vessels, spurring us to investigate whether GATA2 plays a role in the lymphatic vasculature. We demonstrate here that GATA2 protein is present at high levels in lymphatic vessel valves and that GATA2 controls the expression of genes important for programming lymphatic valve development. Our data expand the phenotypes associated with germline GATA2 mutations to include predisposition to primary lymphedema and suggest that complete haploinsufficiency or loss of function of GATA2, rather than missense mutations, is the key predisposing factor for lymphedema onset. Moreover, we reveal a crucial role for GATA2 in lymphatic vascular development.

Differential expression of microRNA-1 in dorsal root ganglion neurons.

Damage to sensory neurons induces neural repair, regrowth and hyperexcitability. The regulation of such responses to injury must be organized in some way by the neurons. Regulation can occur at the post-transcriptional level via microRNAs (miRNAs). miRNAs are small non-coding RNAs that influence the stability or translation of mRNAs and thereby regulate gene expression. Although nociceptive neurons show transcriptional and post-transcriptional regulatory mechanisms at many levels, miRNAs have not yet been systematically investigated in these neurons. Based on our preliminary array data we investigated the presence of miR-1 in dorsal root ganglion (DRG) neurons of mice and humans. We detected miR-1 in total RNA from human and mouse DRG and localised miR-1 in human and murine sensory neurons in situ. In Situ Hybridization detected miR-1 expression by nearly all DRG neurons. In vitro studies of enriched sensory neuron subpopulations from mouse DRG showed higher miR-1 expression levels in I-B4 negative neurons compared with I-B4 positive cells. Culturing of primary sensory neurons reduced the relative miR-1 expression levels independent of the presence or absence of laminin on the culture substrate. Transfection with a miR-1 mimic induced a massive increase in neuronal miR-1 associated with attenuated neurite outgrowth. This first description of miR-1 in sensory neurons including nociceptors suggests that miR-1 has a role in modulating neurite outgrowth.

Prox1 expression is negatively regulated by miR-181 in endothelial cells.

The specification of arterial, venous, and lymphatic endothelial cell fate is critical during vascular development. Although the homeobox transcription factor, Prox1, is crucial for the specification and maintenance of lymphatic endothelial cell identity, little is known regarding the mechanisms that regulate Prox1 expression. Here we demonstrate that miR-181a binds the 3' untranslated region of Prox1, resulting in translational inhibition and transcript degradation. Increased miR-181a activity in primary embryonic lymphatic endothelial cells resulted in substantially reduced levels of Prox1 mRNA and protein and reprogramming of lymphatic endothelial cells toward a blood vascular phenotype. Conversely, treatment of primary embryonic blood vascular endothelial cells with miR-181a antagomir resulted in increased Prox1 mRNA levels. miR-181a expression is significantly higher in embryonic blood vascular endothelial cells compared with lymphatic endothelial cells, suggesting that miR-181 activity could be an important mechanism by which Prox1 expression is silenced in the blood vasculature during development. Our work is the first example of a microRNA that targets Prox1 and has implications for the control of Prox1 expression during vascular development and neo-lymphangiogenesis.