Hepatic leukemia factor-expressing paraxial mesoderm cells contribute to the developing brain vasculature.

Recent genetic lineage tracing studies reveal heterogeneous origins of vascular endothelial cells and pericytes in the developing brain vasculature, despite classical experimental evidence for a mesodermal origin. Here we provide evidence through a genetic lineage tracing experiment that cephalic paraxial mesodermal cells give rise to endothelial cells and pericytes in the developing mouse brain. We show that Hepatic leukemia factor (Hlf) is transiently expressed by cephalic paraxial mesenchyme at embryonic day (E) 8.0-9.0 and the genetically marked E8.0 Hlf-expressing cells mainly contribute to the developing brain vasculature. Interestingly, the genetically marked E10.5 Hlf-expressing cells, which have been previously reported to contain embryonic hematopoietic stem cells, fail to contribute to the vascular cells. Combined, our genetic lineage tracing data demonstrate that a transient expression of Hlf marks a cephalic paraxial mesenchyme contributing to the developing brain vasculature. This article has an associated First Person interview with the first author of the paper.

Blood and lymphatic systems are segregated by the FLCN tumor suppressor.

Blood and lymphatic vessels structurally bear a strong resemblance but never share a lumen, thus maintaining their distinct functions. Although lymphatic vessels initially arise from embryonic veins, the molecular mechanism that maintains separation of these two systems has not been elucidated. Here, we show that genetic deficiency of Folliculin, a tumor suppressor, leads to misconnection of blood and lymphatic vessels in mice and humans. Absence of Folliculin results in the appearance of lymphatic-biased venous endothelial cells caused by ectopic expression of Prox1, a master transcription factor for lymphatic specification. Mechanistically, this phenotype is ascribed to nuclear translocation of the basic helix-loop-helix transcription factor Transcription Factor E3 (TFE3), binding to a regulatory element of Prox1, thereby enhancing its venous expression. Overall, these data demonstrate that Folliculin acts as a gatekeeper that maintains separation of blood and lymphatic vessels by limiting the plasticity of committed endothelial cells.

Ubiquitous expression of Akt1 p.(E17K) results in vascular defects and embryonic lethality in mice.

Proteus syndrome is a progressive overgrowth disorder with vascular malformations caused by mosaic expression of the AKT1 c.49G > A, p.(E17K) activating variant which was predicted to cause lethality if expressed ubiquitously. To test that hypothesis, we used the ACTB-Cre gene to activate a conditional Akt1 p.(E17K) allele in the mouse. No offspring that was heterozygous for both Cre and the conditional allele (ßA-Akt1WT/flx) was viable. Fewer than expected numbers of ßA-Akt1WT/flx embryos were seen beginning at E11.5, but a few survived until E17.5. The phenotype ranged from mild to severe, but generally ßA-Akt1WT/flx embryos had fewer visible blood vessels and more hemorrhages than their wild-type littermates, which was suggestive of a vascular abnormality. Examination of E13.5 limb skin showed a primitive capillary network with increased branching complexity and abnormal patterning compared with wild-type skin. By E15.5, wild-type skin had undergone angiogenesis and formed a hierarchical network of remodeled vessels, whereas in ßA-Akt1WT/flx embryos, the capillary network failed to remodel. Mural cell coverage of the blood vessels was also reduced in ßA-Akt1WT/flx skin compared with that of wild type. Restricting expression of Akt1E17K to endothelial, cardiac or smooth muscle cells resulted in viable offspring and remodeled vasculature and did not recapitulate the ßA-Akt1WT/flx phenotype. We conclude that ubiquitous expression of Akt1E17K suppresses remodeling and inhibits the formation of a normal skin vasculature. We postulate that this failure prevents proper circulation necessary to support the growing embryo and that it is the result of interactions of multiple cell types with increased AKT signaling.

Nonmuscle myosin 2 regulates cortical stability during sprouting angiogenesis.

Among the three nonmuscle myosin 2 (NM2) paralogs, NM 2A and 2B, but not 2C, are detected in endothelial cells. To study the role of NM2 in vascular formation, we ablate NM2 in endothelial cells in mice. Ablating NM2A, but not NM2B, results in reduced blood vessel coverage and increased vascular branching in the developing mouse skin and coronary vasculature. NM2B becomes essential for vascular formation when NM2A expression is limited. Mice ablated for NM2B and one allele of NM2A develop vascular abnormalities similar to those in NM2A ablated mice. Using the embryoid body angiogenic sprouting assay in collagen gels reveals that NM2A is required for persistent angiogenic sprouting by stabilizing the endothelial cell cortex, and thereby preventing excessive branching and ensuring persistent migration of the endothelial sprouts. Mechanistically, NM2 promotes focal adhesion formation and cortical protrusion retraction during angiogenic sprouting. Further studies demonstrate the critical role of Rho kinase-activated NM2 signaling in the regulation of angiogenic sprouting in vitro and in vivo.

Macrophage fatty acid oxidation inhibits atherosclerosis progression.

Atherosclerosis is a chronic disorder of the vessel wall. One key regulator of disease progression is lipid handling in macrophages. However, the role of macrophage mitochondrial-dependent fatty acid ß-oxidation (FAO) in atherosclerosis is not well defined. To address this, we focused on carnitine palmitoyltransferase (CPT) 1 and 2, which play an essential role in the transport of long chain fatty acids (FAs) into the mitochondria. Using conditional alleles of these mitochondrial enzymes, we have generated myeloid-specific Cpt1a and Cpt2 knockout mutants (CPT1a M-KO and CPT2 M-KO). In culture, macrophages derived from CPT1a and CPT2 M-KO mice have impaired FAO, enhanced expression of the CD36 scavenger receptor, increased uptake of oxidized low-density lipoprotein (oxLDL), and augmented transformation into cholesterol-rich foam cells. In line with these in vitro observations, in the atherosclerosis-susceptible apolipoprotein E (ApoE) KO background, CPT2 M-KO mice demonstrated augmented atherosclerosis, accompanied by increased accumulation of aortic macrophages with elevated CD36 expression. These data suggest that macrophage FAO is athero-protective and that augmenting FAO may potentially slow atherosclerotic progression.

Targeting WNT Signaling for Multifaceted Glioblastoma Therapy.

The WNT signaling pathway has been of great interest to developmental biologists for decades and has more recently become a central topic for study in cancer biology. It is vital for cell growth and regulation of embryogenesis in many organ systems, particularly the CNS and its associated vasculature. We summarize the role of WNT in CNS development and describe how WNT signaling makes key contributions to malignant glioma stemness, invasiveness, therapeutic resistance, and angiogenesis. The role of WNT in these mechanisms, along with creation and maintainance of the blood-brain barrier (BBB), points to the potential of WNT as a multi-faceted target in malignant glioma therapy.

Tissue Myeloid Progenitors Differentiate into Pericytes through TGF-ß Signaling in Developing Skin Vasculature.

Mural cells (pericytes and vascular smooth muscle cells) are essential for the regulation of vascular networks and maintenance of vascular integrity, but their origins are diverse in different tissues and not known in the organs that arise from the ectoderm, such as skin. Here, we show that tissue-localized myeloid progenitors contribute to pericyte development in embryonic skin vasculature. A series of in vivo fate-mapping experiments indicates that tissue myeloid progenitors differentiate into pericytes. Furthermore, depletion of tissue myeloid cells and their progenitors in PU.1 (also known as Spi1) mutants results in defective pericyte development. Fluorescence-activated cell sorting (FACS)-isolated myeloid cells and their progenitors from embryonic skin differentiate into pericytes in culture. At the molecular level, transforming growth factor-ß (TGF-ß) induces pericyte differentiation in culture. Furthermore, type 2 TGF-ß receptor (Tgfbr2) mutants exhibit deficient pericyte development in skin vasculature. Combined, these data suggest that pericytes differentiate from tissue myeloid progenitors in the skin vasculature through TGF-ß signaling.

Foxc1 and Foxc2 deletion causes abnormal lymphangiogenesis and correlates with ERK hyperactivation.

The lymphatic vasculature is essential for maintaining interstitial fluid homeostasis, and dysfunctional lymphangiogenesis contributes to various pathological processes, including inflammatory disease and tumor metastasis. Mutations in FOXC2 are dominantly associated with late-onset lymphedema; however, the precise role of FOXC2 and a closely related factor, FOXC1, in the lymphatic system remains largely unknown. Here we identified a molecular cascade by which FOXC1 and FOXC2 regulate ERK signaling in lymphatic vessel growth. In mice, lymphatic endothelial cell-specific (LEC-specific) deletion of Foxc1, Foxc2, or both resulted in increased LEC proliferation, enlarged lymphatic vessels, and abnormal lymphatic vessel morphogenesis. Compared with LECs from control animals, LECs from mice lacking both Foxc1 and Foxc2 exhibited aberrant expression of Ras regulators, and embryos with LEC-specific deletion of Foxc1 and Foxc2, alone or in combination, exhibited ERK hyperactivation. Pharmacological ERK inhibition in utero abolished the abnormally enlarged lymphatic vessels in FOXC-deficient embryos. Together, these results identify FOXC1 and FOXC2 as essential regulators of lymphangiogenesis and indicate a new potential mechanistic basis for lymphatic-associated diseases.

Temporal-specific roles of Rac1 during vascular development and retinal angiogenesis.

Angiogenesis, the formation of new blood vessels by remodeling and growth of pre-existing vessels, is a highly orchestrated process that requires a tight balance between pro-angiogenic and anti-angiogenic factors and the integration of their corresponding signaling networks. The family of Rho GTPases, including RhoA, Rac1, and Cdc42, play a central role in many cell biological processes that involve cytoskeletal changes and cell movement. Specifically for Rac1, we have shown that excision of Rac1 using a Tie2-Cre animal line results in embryonic lethality in midgestation (embryonic day (E) 9.5), with multiple vascular defects. However, Tie2-Cre can be also expressed during vasculogenesis, prior to angiogenesis, and is active in some hematopoietic precursors that can affect vessel formation. To circumvent these limitations, we have now conditionally deleted Rac1 in a temporally controlled and endothelial-restricted fashion using Cdh5(PAC)-iCreERT2 transgenic mice. In this highly controlled experimental in vivo system, we now show that Rac1 is required for embryonic vascular integrity and angiogenesis, and for the formation of superficial and deep vascular networks in the post-natal developing retina, the latter involving a novel specific function for Rac1 in vertical blood vessel sprouting. Aligned with these findings, we show that RAC1 is spatially involved in endothelial cell migration, invasion, and radial sprouting activities in 3D collagen matrix in vitro models. Hence, Rac1 and its downstream molecules may represent potential anti-angiogeneic therapeutic targets for the treatment of many human diseases that involve aberrant neovascularization and blood vessel overgrowth.

Characterizing the angiogenic activity of patients with single ventricle physiology and aortopulmonary collateral vessels.

OBJECTIVES: Patients with single ventricle congenital heart disease often form aortopulmonary collateral vessels via an unclear mechanism. To gain insights into the pathogenesis of aortopulmonary collateral vessels, we correlated angiogenic factor levels with in vitro activity and angiographic aortopulmonary collateral assessment and examined whether patients with single ventricle physiology have increased angiogenic factors that can stimulate endothelial cell sprouting in vitro. METHODS: In patients with single ventricle physiology (n = 27) and biventricular acyanotic control patients (n = 21), hypoxia-inducible angiogenic factor levels were measured in femoral venous and arterial plasma at cardiac catheterization. To assess plasma angiogenic activity, we used a 3-dimensional in vitro cell sprouting assay that recapitulates angiogenic sprouting. Aortopulmonary collateral angiograms were graded using a 4-point scale. RESULTS: Compared with controls, patients with single ventricle physiology had increased vascular endothelial growth factor (artery: 58.7 ± 1.2 pg/mL vs 35.3 ± 1.1 pg/mL, P < .01; vein: 34.8 ± 1.1 pg/mL vs 21 ± 1.2 pg/mL, P < .03), stromal-derived factor 1-alpha (artery: 1901.6 ± 1.1 pg/mL vs 1542.6 ± 1.1 pg/mL, P < .03; vein: 2092.8 pg/mL ± 1.1 vs 1752.9 ± 1.1 pg/mL, P < .02), and increased arterial soluble fms-like tyrosine kinase-1, a regulatory vascular endothelial growth factor receptor (612.3 ± 1.2 pg/mL vs 243.1 ± 1.2 pg/mL, P < .003). Plasma factors and sprout formation correlated poorly with aortopulmonary collateral severity. CONCLUSIONS: We are the first to correlate plasma angiogenic factor levels with angiography and in vitro angiogenic activity in patients with single ventricle disease with aortopulmonary collaterals. Patients with single ventricle disease have increased stromal-derived factor 1-alpha and soluble fms-like tyrosine kinase-1, and their roles in aortopulmonary collateral formation require further investigation. Plasma factors and angiogenic activity correlate poorly with aortopulmonary collateral severity in patients with single ventricles, suggesting complex mechanisms of angiogenesis.

Neurons limit angiogenesis by titrating VEGF in retina.

Vascular and nervous systems, two major networks in mammalian bodies, show a high degree of anatomical parallelism and functional crosstalk. During development, neurons guide and attract blood vessels, and consequently this parallelism is established. Here, we identified a noncanonical neurovascular interaction in eye development and disease. VEGFR2, a critical endothelial receptor for VEGF, was more abundantly expressed in retinal neurons than in endothelial cells, including endothelial tip cells. Genetic deletion of VEGFR2 in neurons caused misdirected angiogenesis toward neurons, resulting in abnormally increased vascular density around neurons. Further genetic experiments revealed that this misdirected angiogenesis was attributable to an excessive amount of VEGF protein around neurons caused by insufficient engulfment of VEGF by VEGFR2-deficient neurons. Moreover, absence of neuronal VEGFR2 caused misdirected regenerative angiogenesis in ischemic retinopathy. Thus, this study revealed neurovascular crosstalk and unprecedented cellular regulation of VEGF: retinal neurons titrate VEGF to limit neuronal vascularization. PAPERFLICK:

Excessive vascular sprouting underlies cerebral hemorrhage in mice lacking αVß8-TGFß signaling in the brain.

Vascular development of the central nervous system and blood-brain barrier (BBB) induction are closely linked processes. The role of factors that promote endothelial sprouting and vascular leak, such as vascular endothelial growth factor A, are well described, but the factors that suppress angiogenic sprouting and their impact on the BBB are poorly understood. Here, we show that integrin αVß8 activates angiosuppressive TGFß gradients in the brain, which inhibit endothelial cell sprouting. Loss of αVß8 in the brain or downstream TGFß1-TGFBR2-ALK5-Smad3 signaling in endothelial cells increases vascular sprouting, branching and proliferation, leading to vascular dysplasia and hemorrhage. Importantly, BBB function in Itgb8 mutants is intact during early stages of vascular dysgenesis before hemorrhage. By contrast, Pdgfb(ret/ret) mice, which exhibit severe BBB disruption and vascular leak due to pericyte deficiency, have comparatively normal vascular morphogenesis and do not exhibit brain hemorrhage. Our data therefore suggest that abnormal vascular sprouting and patterning, not BBB dysfunction, underlie developmental cerebral hemorrhage.

Vessel-dependent recruitment of sympathetic axons: looking for innervation in all the right places.

Autonomic sympathetic axons extend along and innervate resistance arteries to control vascular tone and participate in blood pressure regulation. In this issue of the JCI, Brunet and colleagues reveal that sympathetic innervation of arteries is facilitated by secretion of the axon guidance molecule netrin-1 by arterial VSMCs. Furthermore, disruption of the signaling cascade induced by netrin-1 through its receptor DCC resulted in defective arterial innervation and sympathetic control of vasoconstriction. This comprehensive study represents a major step forward in our understanding of the coordinated wiring of the vascular and nervous systems in various tissues.

Tissue-specific venous expression of the EPH family receptor EphB1 in the skin vasculature.

BACKGROUND: The major arteries and veins are formed early during development. The molecular tools to identify arterial and venous endothelial cells improve our understanding of arterial-venous differentiation and branching morphogenesis. Compared with arterial differentiation, relatively little is known about what controls venous development, due to lack of definitive molecular markers for venous endothelial cells. RESULTS: Here we report that the antibody against EphB1, an EphB class receptor, makes it possible to establish a reliable whole-mount immunohistochemical analysis of venous identity with greater resolution than previously possible in embryonic and adult skin vasculature models. EphB1 expression is restricted to the entire venous vasculature throughout embryonic development to adulthood, whereas the previously established venous marker EphB4 is also detectable in lymphatic vasculature. This venous-restricted expression of EphB1 is established after the vascular remodeling of the primary capillary plexus has occurred. Compared with its venous-specific expression in the skin, however, EphB1 is not restricted to the venous vasculature in yolk sac, trunk and lung. CONCLUSIONS: These studies introduce EphB1 as a new venous-restricted marker in a tissue-specific and time-dependent manner.

Perivascular hair follicle stem cells associate with a venule annulus.

The perivascular microenvironment helps in maintaining stem cells in many tissues. We sought to determine whether there is a perivascular niche for hair follicle stem cells. The association of vessels and follicle progenitor cells began by embryonic day 14.5, when nascent hair placodes had blood vessels approaching them. By birth, a vascular annulus stereotypically surrounded the keratin 15 negative (K15-) stem cells in the upper bulge and remained associated with the K15- upper bulge throughout the hair cycle. The angiogenic factor Egfl6 was expressed by the K15- bulge and was localized adjacent to the vascular annulus, which comprised post-capillary venules. Although denervation altered the phenotype of upper bulge stem cells, the vascular annulus persisted in surgically denervated mouse skin. The importance of the perivascular niche was further suggested by the fact that vascular annuli formed around the upper bulge of de novo-reconstituted hair follicles before their innervation. Together, these findings demonstrate that the upper bulge is associated with a perivascular niche during the establishment and maintenance of this specialized region of hair follicle stem cells.

PDZ-RhoGEF and LARG are essential for embryonic development and provide a link between thrombin and LPA receptors and Rho activation.

G protein-coupled receptors (GPCRs) linked to both members of the Gα12 family of heterotrimeric G proteins α subunits, Gα12 and Gα13, regulate the activation of Rho GTPases, thereby contributing to many key biological processes. Multiple Rho GEFs have been proposed to link Gα12/13 GPCRs to Rho activation, including PDZ-RhoGEF (PRG), leukemia-associated Rho GEF (LARG), p115-RhoGEF (p115), lymphoid blast crisis (Lbc), and Dbl. PRG, LARG, and p115 share the presence of a regulator of G protein signaling homology (RGS) domain. There is limited information on the biological roles of this RGS-containing family of RhoGEFs in vivo. p115-deficient mice are viable with some defects in the immune system and gastrointestinal motor dysfunctions, whereas in an initial study we showed that mice deficient for Larg are viable and resistant to salt-induced hypertension. Here, we generated knock-out mice for Prg and observed that these mice do not display any overt phenotype. However, deficiency in Prg and Larg leads to complex developmental defects and early embryonic lethality. Signaling from Gα11/q-linked GPCRs to Rho was not impaired in mouse embryonic fibroblasts defective in all three RGS-containing RhoGEFs. However, a combined lack of Prg, Larg, and p115 expression abolished signaling through Gα12/13 to Rho and thrombin-induced cell proliferation, directional migration, and nuclear signaling through JNK and p38. These findings provide evidence of an essential role for the RGS-containing RhoGEF family in signaling to Rho by Gα12/13-coupled GPCRs, which may likely play a critical role during embryonic development.

Peripheral nerve-derived CXCL12 and VEGF-A regulate the patterning of arterial vessel branching in developing limb skin.

In developing limb skin, peripheral nerves provide a spatial template that controls the branching pattern and differentiation of arteries. Our previous studies indicate that nerve-derived VEGF-A is required for arterial differentiation but not for nerve-vessel alignment. In this study, we demonstrate that nerve-vessel alignment depends on the activity of Cxcl12-Cxcr4 chemokine signaling. Genetic inactivation of Cxcl12-Cxcr4 signaling perturbs nerve-vessel alignment and abolishes arteriogenesis. Further in vitro assays allow us to uncouple nerve-vessel alignment and arteriogenesis, revealing that nerve-derived Cxcl12 stimulates endothelial cell migration, whereas nerve-derived VEGF-A is responsible for arterial differentiation. These findings suggest a coordinated sequential action in which nerve Cxcl12 functions over a distance to recruit vessels to align with nerves, and subsequent arterial differentiation presumably requires a local action of nerve VEGF-A in the nerve-associated vessels.

Rspo1/Wnt signaling promotes angiogenesis via Vegfc/Vegfr3.

Here, we show that a novel Rspo1-Wnt-Vegfc-Vegfr3 signaling pathway plays an essential role in developmental angiogenesis. A mutation in R-spondin1 (rspo1), a Wnt signaling regulator, was uncovered during a forward-genetic screen for angiogenesis-deficient mutants in the zebrafish. Embryos lacking rspo1 or the proposed rspo1 receptor kremen form primary vessels by vasculogenesis, but are defective in subsequent angiogenesis. Endothelial cell-autonomous inhibition of canonical Wnt signaling also blocks angiogenesis in vivo. The pro-angiogenic effects of Rspo1/Wnt signaling are mediated by Vegfc/Vegfr3(Flt4) signaling. Vegfc expression is dependent on Rspo1 and Wnt, and Vegfc and Vegfr3 are necessary to promote angiogenesis downstream from Rspo1-Wnt. As all of these molecules are expressed by the endothelium during sprouting stages, these results suggest that Rspo1-Wnt-VegfC-Vegfr3 signaling plays a crucial role as an endothelial-autonomous permissive cue for developmental angiogenesis.

Conditional deletion of Ccm2 causes hemorrhage in the adult brain: a mouse model of human cerebral cavernous malformations.

Cerebral cavernous malformations (CCM) are irregularly shaped and enlarged capillaries in the brain that are prone to hemorrhage, resulting in headaches, seizures, strokes and even death in patients. The disease affects up to 0.5% of the population and the inherited form has been linked to mutations in one of three genetic loci, CCM1, CCM2 and CCM3. To understand the pathophysiology underlying the vascular lesions in CCM, it is critical to develop a reproducible mouse genetic model of this disease. Here, we report that limited conditional ablation of Ccm2 in young adult mice induces observable neurological dysfunction and reproducibly results in brain hemorrhages whose appearance is highly reminiscent of the lesions observed in human CCM patients. We first demonstrate that conventional or endothelial-specific deletion of Ccm2 leads to fatal cardiovascular defects during embryogenesis, including insufficient vascular lumen formation as well as defective arteriogenesis and heart malformation. These findings confirm and extend prior studies. We then demonstrate that the inducible deletion of Ccm2 in adult mice recapitulates the CCM-like brain lesions in humans; the lesions display disrupted vascular lumens, enlarged capillary cavities, loss of proper neuro-vascular associations and an inflammatory reaction. The CCM lesions also exhibit damaged neuronal architecture, the likely cause of neurologic defects, such as ataxia and seizure. These mice represent the first CCM2 animal model for CCM and should provide the means to elucidate disease mechanisms and evaluate therapeutic strategies for human CCM.

Nf1 mutation expands an EGFR-dependent peripheral nerve progenitor that confers neurofibroma tumorigenic potential.

Defining growth factor requirements for progenitors facilitates their characterization and amplification. We characterize a peripheral nervous system embryonic dorsal root ganglion progenitor population using in vitro clonal sphere-formation assays. Cells differentiate into glial cells, smooth muscle/fibroblast (SM/Fb)-like cells, and neurons. Genetic and pharmacologic tools revealed that sphere formation requires signaling from the EGFR tyrosine kinase. Nf1 loss of function amplifies this progenitor pool, which becomes hypersensitive to growth factors and confers tumorigenesis. DhhCre;Nf1(fl/fl) mouse neurofibromas contain a progenitor population with similar growth requirements, potential, and marker expression. In humans, NF1 mutation predisposes to benign neurofibromas, incurable peripheral nerve tumors. Prospective identification of human EGFR(+);P75(+) neurofibroma cells enriched EGF-dependent sphere-forming cells. Neurofibroma spheres contain glial-like progenitors that differentiate into neurons and SM/Fb-like cells in vitro and form benign neurofibroma-like lesions in nude mice. We suggest that expansion of an EGFR-expressing early glial progenitor contributes to neurofibroma formation.