Sphingosine 1-phosphate signalling.

Sphingosine 1-phosphate (S1P) is a lipid mediator formed by the metabolism of sphingomyelin. In vertebrates, S1P is secreted into the extracellular environment and signals via G protein-coupled S1P receptors to regulate cell-cell and cell-matrix adhesion, and thereby influence cell migration, differentiation and survival. The expression and localization of S1P receptors is dynamically regulated and controls vascular development, vessel stability and immune cell trafficking. In addition, crucial events during embryogenesis, such as angiogenesis, cardiogenesis, limb development and neurogenesis, are regulated by S1P signalling. Here, and in the accompanying poster, we provide an overview of S1P signalling in development and in disease.

Maternal or zygotic sphingosine kinase is required to regulate zebrafish cardiogenesis.

BACKGROUND: The sphingosine 1-phosphate (S1P) signaling pathway regulates zebrafish cardiogenesis, and provides a paradigm for how signaling gradients coordinate collective cell migration across tissue layers. It is known that the S1P transporter (Spns2) functions in extra-embryonic YSL to activate G protein-coupled receptor (S1pr2) signaling in endoderm for deposition of positional cues (integrin, fibronectin, etc.). Such cues are recognized by overlying lateral precardiac mesoderm that migrates to the midline and fuses to form the primordial heart tube. However, the source of bio-active S1P is not known. There are multiple receptors and it is not known if there are earlier or even receptor-independent functions for S1P. RESULTS: Because S1P can only be generated by sphingosine kinases, we targeted a mutation to the single kinase gene expressed during early embryogenesis (sphk2). Zygotic mutants survive to adulthood and appear normal, but maternal-zygotic mutant embryos phenocopy null zygotic mutants of spns2 or s1pr2. CONCLUSIONS: The data show that maternally derived sphk2 RNA is fully sufficient to generate an S1P signaling gradient in the YSL that ultimately controls precardiac mesoderm migration during embryogenesis. Furthermore, despite maternal expression of sphk2, there are no obvious developmental functions requiring its activity prior to stimulation of S1pr2 in endoderm.

Sphingosine 1-phosphate receptor signaling regulates proper embryonic vascular patterning.

Sphingosine 1-phosphate (S1P) binds G-protein-coupled receptors (S1P(1-5)) to regulate a multitude of physiological effects, especially those in the vascular and immune systems. S1P receptors in the vascular system have been characterized primarily in mammals. Here, we report that the S1P receptors and metabolic enzymes are conserved in the genome of zebrafish Danio rerio. Bioinformatic analysis identified seven S1P receptor-like sequences in the zebrafish genome, including duplicated orthologs of receptors 3 and 5. Sphingolipidomic analysis detected erythrocyte and plasma S1P as well as high plasma ceramides and sphingosine. Morpholino-mediated knockdown of s1pr1 causes global and pericardial edema, loss of blood circulation, and vascular defects characterized by both reduced vascularization in intersegmental vessels, decreased proliferation of intersegmental and axial vessels, and hypersprouting in the caudal vein plexus. The s1pr2 gene was previously characterized as a regulator of cell migration and heart development, but its role in angiogenesis is not known. However, when expression of both s1pr1 and s1pr2 is suppressed, severely reduced vascular development of the intersegmental vessels was observed with doses of the s1pr1 morpholino that alone did not cause any discernible vascular defects, suggesting that s1pr1 and s1pr2 function cooperatively to regulate vascular development in zebrafish. Similarly, the S1P transporter, spns2, also cooperated with s1pr1. We propose that extracellular S1P acts through vascular S1P receptors to regulate vascular development.

Differential histone acetylation and super-enhancer regulation underlie melanoma cell dedifferentiation.

Dedifferentiation or phenotype switching refers to the transition from a proliferative to an invasive cellular state. We previously identified a 122-gene epigenetic gene signature that classifies primary melanomas as low versus high risk (denoted as Epgn1 or Epgn3). We found that the transcriptomes of the Epgn1 low-risk and Epgn3 high-risk cells are similar to the proliferative and invasive cellular states, respectively. These signatures were further validated in melanoma tumor samples. Examination of the chromatin landscape revealed differential H3K27 acetylation in the Epgn1 low-risk versus Epgn3 high-risk cell lines that corroborated with a differential super-enhancer and enhancer landscape. Melanocytic lineage genes (MITF, its targets and regulators) were associated with super-enhancers in the Epgn1 low-risk state, whereas invasiveness genes were linked with Epgn3 high-risk status. We identified the ITGA3 gene as marked by a super-enhancer element in the Epgn3 invasive cells. Silencing of ITGA3 enhanced invasiveness in both in vitro and in vivo systems, suggesting it as a negative regulator of invasion. In conclusion, we define chromatin landscape changes associated with Epgn1/Epgn3 and phenotype switching during early steps of melanoma progression that regulate transcriptional reprogramming. This super-enhancer and enhancer-driven epigenetic regulatory mechanism resulting in major changes in the transcriptome could be important in future therapeutic targeting efforts.

B7-H3 drives immunosuppression and Co-targeting with CD47 is a new therapeutic strategy in ß-catenin activated melanomas.

In melanoma, immune cell infiltration into the tumor is associated with better patient outcomes and response to immunotherapy. T-cell non-inflamed tumors (cold tumors) are associated with tumor cell-intrinsic Wnt/ß-catenin activation, and are typically resistant to anti-PD-1 alone or in combination with anti-CTLA-4 therapy. Reversal of the 'cold tumor' phenotype and identifying new effective immunotherapies are challenges. We sought to investigate the role of a newer immunotherapy agent, B7-H3, in this setting. RNA sequencing was used to identify co-targeting strategies upon B7-H3 inhibition in a well-defined preclinical melanoma model driven by ß-catenin. We found that immune checkpoint molecule B7-H3 confers a suppressive tumor microenvironment by modulating antiviral signals and innate immunity. B7-H3 inhibition led to an inflamed microenvironment, up-regulation of CD47/SIRPa signaling, and together with blockade of the macrophage checkpoint CD47 resulted in additive antitumor responses. We found that the antitumor effects of the B7-H3/CD47 antibody combination were dependent on cytokine signaling pathways (CCR5/CCL5 and IL4).

Pathological neovascularization is reduced by inactivation of ADAM17 in endothelial cells but not in pericytes.

RATIONALE: Pathological neovascularization is a critical component of diseases such as proliferative retinopathies, cancer and rheumatoid arthritis, yet much remains to be learned about the underlying causes. Previous studies showed that vascular endothelial growth factor (VEGF)-A activates the membrane-anchored metalloproteinase ADAM17 (a disintegrin and metalloproteinase 17) in endothelial cells, thereby stimulating crosstalk between VEGF receptor 2 and extracellular signal-regulated kinase. These findings raised interesting questions about the role of ADAM17 in angiogenesis and neovascularization in vivo. OBJECTIVE: The objective of this study was to inactivate ADAM17 in endothelial cells or in pericytes to determine how this affects developmental angiogenesis, pathological retinal neovascularization and heterotopic tumor growth. METHODS AND RESULTS: We generated animals in which floxed ADAM17 was removed by Tie2-Cre in endothelial cells, or by smooth muscle (sm) Cre in smooth muscle cells and pericytes. There were no evident developmental defects in either conditional knockout strain, but pathological retinal neovascularization and growth of heterotopically injected tumor cells was reduced in Adam17flox/flox/Tie2-Cre mice, although not in Adam17flox/flox/sm-Cre mice. Moreover, lack of ADAM17 in endothelial cells decreased ex vivo chord formation, and this could be largely restored by addition of the ADAM17 substrate HB-EGF (heparin-binding epidermal growth factor-like growth factor). Finally we found that ADAM17 is important for the VEGF receptor 2 stimulated processing of several receptors with known functions in endothelial cell biology. CONCLUSIONS: These results provide the first evidence for a role for ADAM17 in pathological neovascularization in vivo. Because ADAM17 does not appear to be required for normal developmental angiogenesis or vascular homeostasis, it could emerge as a good target for treatment of pathological neovascularization.

Stimulation of platelet-derived growth factor receptor beta (PDGFRbeta) activates ADAM17 and promotes metalloproteinase-dependent cross-talk between the PDGFRbeta and epidermal growth factor receptor (EGFR) signaling pathways.

Binding of the platelet-derived growth factor (PDGF)-B to its receptor PDGFRbeta promotes proliferation, migration, and recruitment of pericytes and smooth muscle cells to endothelial cells, serving to stabilize developing blood vessels. The main goals of this study were to determine whether the extracellular domain of the PDGFRbeta can be proteolytically released from cell membranes and, if so, to identify the responsible sheddase and determine whether activation of the PDGFRbeta stimulates its shedding and potentially that of other membrane proteins. We found that the PDGFRbeta is shed from cells by a metalloproteinase and used loss-of-function experiments to identify ADAM10 as the sheddase responsible for constitutive and ionomycin-stimulated processing of the PDGFRbeta. Moreover, we showed that ligand-dependent activation of the PDGFRbeta does not trigger its own shedding by ADAM10, but instead it stimulates ADAM17 and shedding of substrates of ADAM17, including tumor necrosis factor alpha and transforming growth factor alpha. Finally, we demonstrated that treatment of mouse embryonic fibroblasts with PDGF-B triggers a metalloproteinase-dependent cross-talk between the PDGFRbeta and the epidermal growth factor receptor (EGFR)/ERK1/2 signaling axis that is also critical for PDGF-B-stimulated cell migration, most likely via ADAM17-dependent release and activation of ligands of the EGFR. This study identifies the principal sheddase for the PDGFRbeta and provides new insights into the mechanism of PDGFRbeta-dependent signal transduction and cross-talk with the EGFR.

Automated digital TIL analysis (ADTA) adds prognostic value to standard assessment of depth and ulceration in primary melanoma.

Accurate prognostic biomarkers in early-stage melanoma are urgently needed to stratify patients for clinical trials of adjuvant therapy. We applied a previously developed open source deep learning algorithm to detect tumor-infiltrating lymphocytes (TILs) in hematoxylin and eosin (H&E) images of early-stage melanomas. We tested whether automated digital (TIL) analysis (ADTA) improved accuracy of prediction of disease specific survival (DSS) based on current pathology standards. ADTA was applied to a training cohort (n = 80) and a cutoff value was defined based on a Receiver Operating Curve. ADTA was then applied to a validation cohort (n = 145) and the previously determined cutoff value was used to stratify high and low risk patients, as demonstrated by Kaplan-Meier analysis (p ≤ 0.001). Multivariable Cox proportional hazards analysis was performed using ADTA, depth, and ulceration as co-variables and showed that ADTA contributed to DSS prediction (HR: 4.18, CI 1.51-11.58, p = 0.006). ADTA provides an effective and attainable assessment of TILs and should be further evaluated in larger studies for inclusion in staging algorithms.

VEGF-A stimulates ADAM17-dependent shedding of VEGFR2 and crosstalk between VEGFR2 and ERK signaling.

Vascular endothelial growth factor (VEGF)-A and the VEGF receptors are critical for regulating angiogenesis during development and homeostasis and in pathological conditions, such as cancer and proliferative retinopathies. Most effects of VEGF-A are mediated by the VEGFR2 and its coreceptor, neuropilin (NRP)-1. Here, we show that VEGFR2 is shed from cells by the metalloprotease disintegrin ADAM17, whereas NRP-1 is released by ADAM10. VEGF-A enhances VEGFR2 shedding by ADAM17 but not shedding of NRP-1 by ADAM10. VEGF-A activates ADAM17 via the extracellular signal-regulated kinase (ERK) and mitogen-activated protein kinase pathways, thereby also triggering shedding of other ADAM17 substrates, including tumor necrosis factor alpha, transforming growth factor alpha, heparin-binding epidermal growth factor-like growth factor, and Tie-2. Interestingly, an ADAM17-selective inhibitor shortens the duration of VEGF-A-stimulated ERK phosphorylation in human umbilical vein endothelial cells, providing evidence for an ADAM17-dependent crosstalk between the VEGFR2 and ERK signaling. Targeting the sheddases of VEGFR2 or NRP-1 might offer new opportunities to modulate VEGF-A signaling, an already-established target for treatment of pathological neovascularization.

FBXW7 regulates a mitochondrial transcription program by modulating MITF.

FBXW7 is well characterized as a tumor suppressor in many human cancers including melanoma; however, the mechanisms of tumor-suppressive function have not been fully elucidated. We leveraged two distinct RNA sequencing datasets: human melanoma cell lines (n = 10) with control versus silenced FBXW7 and a cohort of human melanoma tumor samples (n = 51) to define the transcriptomic fingerprint regulated by FBXW7. Here, we report that loss of FBXW7 enhances a mitochondrial gene transcriptional program that is dependent on MITF in human melanoma and confers poor patient outcomes. MITF is a lineage-specific master regulator of melanocytes and together with PGC-1alpha is a marker for melanoma subtypes with dependence for mitochondrial oxidative metabolism. We found that inactivation of FBXW7 elevates MITF protein levels in melanoma cells. In vitro studies examining loss of FBXW7 and MITF alone or in combination showed that FBXW7 is an upstream regulator for the MITF/PGC-1 signaling.

Human semilunar cardiac valve remodeling by activated cells from fetus to adult: implications for postnatal adaptation, pathology, and tissue engineering.

BACKGROUND: The evolution of cell phenotypes and matrix architecture in cardiac valves during fetal maturation and postnatal adaptation through senescence remains unexplored. METHODS AND RESULTS: We hypothesized that valvular interstitial (VIC) and endothelial cell (VEC) phenotypes, critical for maintaining valve function, change throughout life in response to environmental stimuli. We performed quantitative histological assessment of 91 human semilunar valves obtained from fetuses at 14 to 19 and 20 to 39 weeks' gestation; neonates minutes to 30 days old; children aged 2 to 16 years; and adults. A trilaminar architecture appeared by 36 weeks of gestation but remained rudimentary compared with that of adult valves. VECs expressed an activated phenotype throughout fetal development. VIC density, proliferation, and apoptosis were significantly higher in fetal than adult valves. Pulmonary and aortic fetal VICs showed an activated myofibroblast-like phenotype (alpha-actin expression), abundant embryonic myosin, and matrix metalloproteinase-collagenases, which indicates an immature/activated phenotype engaged in matrix remodeling versus a quiescent fibroblast-like phenotype in adults. At birth, the abrupt change from fetal to neonatal circulation was associated with a greater number of alpha-actin-positive VICs in neonatal aortic versus pulmonary valves. Collagen content increased from early to late fetal stages but was subsequently unchanged, whereas elastin significantly increased postnatally. Collagen fiber color analysis revealed a progressive temporal decrease in thin fibers and a corresponding increase in thick fibers. Additionally, collagen fibers were more aligned in adult than fetal valves. CONCLUSIONS: Fetal valves possess a dynamic/adaptive structure and contain cells with an activated/immature phenotype. During postnatal life, activated cells gradually become quiescent, whereas collagen matures, which suggests a progressive, environmentally mediated adaptation.

Healing and remodeling of bioengineered pulmonary artery patches implanted in sheep.

PURPOSE: We hypothesized that cell-seeded patches implanted into sheep pulmonary artery would undergo progressive and complete healing into a viable structure well integrated with the arterial wall. METHODS: Autologous ovine blood-derived endothelial progenitor cells (EPCs) and bone marrow-derived mesenchymal stem cells (MSCs) were isolated and cultured in vitro. MSCs and EPCs were seeded onto poly-4-hydroxybutyrate (P4HB)-coated polyglycolic acid (PGA) nonwoven biodegradable mesh scaffolds (10x20 mm) and cultured for 5 days in a laminar fluid flow system. Seeded patches were implanted into the wall of sheep pulmonary artery for 1-2 weeks (n=4) or 4-6 weeks (n=3). Preimplant and postexplant specimens were analyzed by histology and immunohistochemistry. RESULTS: Unimplanted constructs contained alpha-smooth muscle actin (SMA)-positive cells and early extracellular matrix formation (primarily glycosaminoglycans). One week after implantation, seeded patches had surface thrombus formation and macrophage infiltration. Seeded patches implanted for 2 weeks showed granulation tissue, early pannus formation, macrophages, foreign body giant cells around disintegrating polymer, and early angiogenesis (microvessel formation). After 4 weeks in vivo, seeded patches contained glycosaminoglycans, collagen, and coverage of the luminal surface by host artery-derived pannus containing alpha-SMA-positive cells and laminated elastin; polymer scaffold degradation was almost complete with replacement by fibrous tissue containing viable cells. CONCLUSIONS: This study shows that cell-seeded patches implanted in sheep pulmonary artery remodel to layered and viable tissue well integrated into the native arterial wall. The key remodeling processes included (1) intimal overgrowth at the luminal surface (pannus formation; neointima) and (2) granulation tissue formation and fibrosis with foreign body reaction.

Mitochondrial biogenesis and remodeling during adipogenesis and in response to the insulin sensitizer rosiglitazone.

White adipose tissue is an important endocrine organ involved in the control of whole-body metabolism, insulin sensitivity, and food intake. To better understand these functions, 3T3-L1 cell differentiation was studied by using combined proteomic and genomic strategies. The proteomics approach developed here exploits velocity gradient centrifugation as an alternative to isoelectric focusing for protein separation in the first dimension. A 20- to 30-fold increase in the concentration of numerous mitochondrial proteins was observed during adipogenesis, as determined by mass spectrometry and database correlation analysis. Light and electron microscopy confirmed a large increase in the number of mitochondrion profiles with differentiation. Furthermore, mRNA profiles obtained by using Affymetrix GeneChips revealed statistically significant increases in the expression of many nucleus-encoded mitochondrial genes during adipogenesis. Qualitative changes in mitochondrial composition also occur during adipose differentiation, as exemplified by increases in expression of proteins involved in fatty acid metabolism and of mitochondrial chaperones. Furthermore, the insulin sensitizer rosiglitazone caused striking changes in mitochondrial shape and expression of selective mitochondrial proteins. Thus, although mitochondrial biogenesis has classically been associated with brown adipocyte differentiation and thermogenesis, our results reveal that mitochondrial biogenesis and remodeling are inherent to adipose differentiation per se and are influenced by the actions of insulin sensitizers.

The ceramide synthase 2b gene mediates genomic sensing and regulation of sphingosine levels during zebrafish embryogenesis.

Sphingosine-1-phosphate (S1P) is generated through phosphorylation of sphingosine by sphingosine kinases (Sphk1 and Sphk2). We show that sphk2 maternal-zygotic mutant zebrafish embryos (sphk2MZ) display early developmental phenotypes, including a delay in epiboly, depleted S1P levels, elevated levels of sphingosine, and resistance to sphingosine toxicity. The sphk2MZ embryos also have strikingly increased levels of maternal transcripts encoding ceramide synthase 2b (Cers2b), and loss of Cers2b in sphk2MZ embryos phenocopies sphingosine toxicity. An upstream region of the cers2b promoter supports enhanced expression of a reporter gene in sphk2MZ embryos compared to wildtype embryos. Furthermore, ectopic expression of Cers2b protein itself reduces activity of the promoter, and this repression is relieved by exogenous sphingosine. Therefore, the sphk2MZ genome recognizes the lack of sphingosine kinase activity and up-regulates cers2b as a salvage pathway for sphingosine turnover. Cers2b can also function as a sphingolipid-responsive factor to mediate at least part of a feedback regulatory mechanism.

Regulation of a vascular plexus by gata4 is mediated in zebrafish through the chemokine sdf1a.

Using the zebrafish model we describe a previously unrecognized requirement for the transcription factor gata4 controlling embryonic angiogenesis. The development of a vascular plexus in the embryonic tail, the caudal hematopoietic tissue (CHT), fails in embryos depleted of gata4. Rather than forming a normal vascular plexus, the CHT of gata4 morphants remains fused, and cells in the CHT express high levels of osteogenic markers ssp1 and runx1. Definitive progenitors emerge from the hemogenic aortic endothelium, but fail to colonize the poorly vascularized CHT. We also found abnormal patterns and levels for the chemokine sdf1a in gata4 morphants, which was found to be functionally relevant, since the embryos also show defects in development of the lateral line, a mechano-sensory organ system highly dependent on a gradient of sdf1a levels. Reduction of sdf1a levels was sufficient to rescue lateral line development, circulation, and CHT morphology. The result was surprising since neither gata4 nor sdf1a is obviously expressed in the CHT. Therefore, we generated transgenic fish that conditionally express a dominant-negative gata4 isoform, and determined that gata4 function is required during gastrulation, when it is co-expressed with sdf1a in lateral mesoderm. Our study shows that the gata4 gene regulates sdf1a levels during early embryogenesis, which impacts embryonic patterning and subsequently the development of the caudal vascular plexus.

Flow-regulated endothelial S1P receptor-1 signaling sustains vascular development.

During angiogenesis, nascent vascular sprouts fuse to form vascular networks, enabling efficient circulation. Mechanisms that stabilize the vascular plexus are not well understood. Sphingosine 1-phosphate (S1P) is a blood-borne lipid mediator implicated in the regulation of vascular and immune systems. Here we describe a mechanism by which the G protein-coupled S1P receptor-1 (S1P1) stabilizes the primary vascular network. A gradient of S1P1 expression from the mature regions of the vascular network to the growing vascular front was observed. In the absence of endothelial S1P1, adherens junctions are destabilized, barrier function is breached, and flow is perturbed, resulting in abnormal vascular hypersprouting. Interestingly, S1P1 responds to S1P as well as laminar shear stress to transduce flow-mediated signaling in endothelial cells both in vitro and in vivo. These data demonstrate that blood flow and circulating S1P activate endothelial S1P1 to stabilize blood vessels in development and homeostasis.

Stem cell-derived, tissue-engineered pulmonary artery augmentation patches in vivo.

BACKGROUND: Reconstruction of the right ventricular outflow tract is a frequently encountered component of many congenital cardiac repairs. We sought to tissue engineer pulmonary artery augmentation patches from retrovirally labeled endothelial progenitor and mesenchymal stem cells and determine the persistence of the seeded cells in vivo. METHODS: Autologous ovine endothelial progenitor and mesenchymal stem cells were labeled with a retroviral vector encoding green and red fluorescent proteins, coseeded onto biopolymers, and cultured for 5 days. The tissue-engineered patches were implanted into the main pulmonary artery with 1, 2, 4, and 6 week in vivo maturation (n = 8). In vivo evaluation included ultrasonography and angiography, with preimplant and explanted specimens evaluated using histologic examination and immunofluorescence. RESULTS: Echocardiography at each time demonstrated laminar pulmonary artery flow without a pressure gradient across the replaced segment. Pulmonary angiography did not exhibit stenosis or aneurysmal change. Gross appearance of all explanted patches showed progressive tissue formation with increased length of time in vivo. Retrovirally labeled cellular persistence was 96%, 82%, 85%, and 66% at 1, 2, 4, and 6 weeks after implantation, respectively. Early in the in vivo remodeling period, the number of green fluorescent protein-positive endothelial progenitor cells was 1.6 fold greater than the red fluorescent protein-positive mesenchymal stem cells. As in vivo remodeling continued, red fluorescent protein-expressing mesenchymal stem cells were expressed 1.2 to 1.7 times that of the green fluorescent protein-positive endothelial progenitor cells. CONCLUSIONS: The data demonstrate the successful creation of an anatomically functional, autologous tissue-engineered pulmonary artery using coseeded progenitor cell sources. Labeled implanted stem cells persisted in the engineered construct, suggesting that in vitro seeding is necessary to engineer tissue. This study demonstrates an effective method to track multiple cell types after implantation.

Heart valve tissue engineering: concepts, approaches, progress, and challenges.

Potential applications of tissue engineering in regenerative medicine range from structural tissues to organs with complex function. This review focuses on the engineering of heart valve tissue, a goal which involves a unique combination of biological, engineering, and technological hurdles. We emphasize basic concepts, approaches and methods, progress made, and remaining challenges. To provide a framework for understanding the enabling scientific principles, we first examine the elements and features of normal heart valve functional structure, biomechanics, development, maturation, remodeling, and response to injury. Following a discussion of the fundamental principles of tissue engineering applicable to heart valves, we examine three approaches to achieving the goal of an engineered tissue heart valve: (1) cell seeding of biodegradable synthetic scaffolds, (2) cell seeding of processed tissue scaffolds, and (3) in-vivo repopulation by circulating endogenous cells of implanted substrates without prior in-vitro cell seeding. Lastly, we analyze challenges to the field and suggest future directions for both preclinical and translational (clinical) studies that will be needed to address key regulatory issues for safety and efficacy of the application of tissue engineering and regenerative approaches to heart valves. Although modest progress has been made toward the goal of a clinically useful tissue engineered heart valve, further success and ultimate human benefit will be dependent upon advances in biodegradable polymers and other scaffolds, cellular manipulation, strategies for rebuilding the extracellular matrix, and techniques to characterize and potentially non-invasively assess the speed and quality of tissue healing and remodeling.