Parallels between the Developing Vascular and Neural Systems: Signaling Pathways and Future Perspectives for Regenerative Medicine.

Neurovascular disorders, which involve the vascular and nervous systems, are common. Research on such disorders usually focuses on either vascular or nervous components, without looking at how they interact. Adopting a neurovascular perspective is essential to improve current treatments. Therefore, comparing molecular processes known to be involved in both systems separately can provide insight into promising areas of future research. Since development and regeneration share many mechanisms, comparing signaling molecules involved in both the developing vascular and nervous systems and shedding light to those that they have in common can reveal processes, which have not yet been studied from a regenerative perspective, yet hold great potential. Hence, this review discusses and compares processes involved in the development of the vascular and nervous systems, in order to provide an overview of the molecular mechanisms, which are most promising with regards to treatment for neurovascular disorders. Vascular endothelial growth factor, semaphorins, and ephrins are found to hold the most potential, while fibroblast growth factor, bone morphogenic protein, slits, and sonic hedgehog are shown to participate in both the developing vascular and nervous systems, yet have not been studied at the neurovascular level, therefore being of special interest for future research.

The adaptor protein Grb2b is an essential modulator for lympho-venous sprout formation in the zebrafish trunk.

Vegfc/Vegfr3 signaling is critical for lymphangiogenesis, the sprouting of lymphatic vessels. In zebrafish, cells sprouting from the posterior cardinal vein can either form lymphatic precursor cells or contribute to intersegmental vein formation. Both, the Vegfc-dependent differential induction of Prox1a in sprouting cells as well as a Notch-mediated pre-pattern within intersegmental vessels have been associated with the regulation of secondary sprout behavior. However, how exactly a differential lymphatic versus venous sprout cell behavior is achieved is not fully understood. Here, we characterize a zebrafish mutant in the adaptor protein Grb2b, and demonstrate through genetic interaction studies that Grb2b acts within the Vegfr3 pathway. Mutant embryos exhibit phenotypes that are consistent with reduced Vegfr3 signaling outputs prior to the sprouting of endothelial cells from the vein. During secondary sprouting stages, loss of grb2b leads to defective cell behaviors resulting in a loss of parachordal lymphangioblasts, while only partially affecting the number of intersegmental veins. A second GRB2 zebrafish ortholog, grb2a, contributes to the development of lymphatic structures in the meninges and in the head, but not in the trunk. Our results illustrate an essential role of Grb2b in vivo for cell migration to the horizontal myoseptum and for the correct formation of the lymphatic vasculature, while being less critically required in intersegmental vein formation. Thus, there appear to be higher requirements for Grb2b and therefore Vegfr3 downstream signaling levels in lymphatic versus vein precursor-generating sprouts.

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.

P130Cas/bcar1 mediates zebrafish caudal vein plexus angiogenesis.

P130CAS/BCAR1 belongs to the CAS family of adaptor proteins, with important regulatory roles in cell migration, cell cycle control, and apoptosis. Previously, we and others showed that P130CAS mediates VEGF-A and PDGF signalling in vitro, but its cardiovascular function in vivo remains relatively unexplored. We characterise here a novel deletion model of P130CAS in zebrafish. Using in vivo microscopy and transgenic vascular reporters, we observed that while bcar1-/- zebrafish showed no arterial angiogenic or heart defects during development, they strikingly failed to form the caudal vein plexus (CVP). Endothelial cells (ECs) within the CVP of bcar1-/- embryos produced fewer filopodial structures and did not detach efficiently from neighbouring cells, resulting in a significant reduction in ventral extension and overall CVP area. Mechanistically, we show that P130Cas mediates Bmp2b-induced ectopic angiogenic sprouting of ECs in the developing embryo and provide pharmacological evidence for a role of Src family kinases in CVP development.

Vena capitis prima and the cavernous sinus in human embryos and fetuses.

After the intracranial venous-drainage route is switched from the vena capitis prima (VCP) to the transverse sinus, the cavernous sinus is considered to develop from a connecting part of the VCP with the ophthalmic vein (OPV). Observations of histological sections from 12 embryos and 47 fetuses (6-35 weeks) demonstrated that (1) at six weeks, a major tributary of the VCP ran inferiorly in the plica ventralis at the mesencephalic flexure (future tentrium cerebelli) and merged with the OPV in the medial side of the trigeminal ganglion; (2) at seven weeks, being independent of the laterally located primary veins, the superior petrosal sinus (SPS) developed medially in the plica, ran superiorly, and appeared to make an initial confluence with the transverse sinus; (3) until 15-16 weeks, parasellar veins were limited to a few branches of the OPV without communication with the SPS on the lateral surface of the trigeminal ganglion; (4) after 15-16 weeks, parasellar veins increased in number and volume but did not yet drain into the SPS but rather into the newly built inferior petrosal sinus; and (5) near term, parasellar veins started venous drainage to the SPS, whereas few veins were evident around the intracavernous abducens nerve. Consequently, the inferior petrosal sinus might originate from a remnant of the VCP (the so-called pro-otic sinus), but after midterm, most parasellar veins appeared to develop from the OPV without any contribution of the SPS. These findings suggest that parasellar sinus-network might be established after birth.

Adamts18 deficiency in zebrafish embryo causes defective trunk angiogenesis and caudal vein plexus formation.

ADAMTS (A Disintegrin and Metalloproteinase with Thrombospondin type I motifs) enzymes play an important role in various morphogenesis processes. To determine the functions of Adamts18 in the early stages of organogenesis, we created Adamts18 deficient zebrafish using morpholino antisense oligonucleotides (MO) to generate exon 3 skipped adamts18 mRNA transcripts. Results showed that Adamts18 deficiency in zebrafish embryos caused developmental defects, including expanded brain ventricle and hindbrain edema, eye defects, and accumulation of blood in the caudal vein. Adamts18 deficiency also led to impaired trunk angiogenesis and formation of the caudal vein plexus (CVP). Consequently, Adamts18 deficient zebrafish embryos exhibited incomplete formation of intersegment vessels (ISVs), disruption of the honeycomb structure of CVP, and reduced CVP area and loop number. Furthermore, Adamts18 deficiency resulted in impaired blood circulation in major trunk, caudal vein (CV), and common cardinal vein (CCV). These aberrant vascular phenotypes in mutant zebrafish embryos were shown to be associated with a decreased expression of multiple angiogenesis-related signaling genes, including slit/robo, dll4/Notch, cox2, and fgfr. These findings indicate the critical role of Adamts18 in the early stages of vascular network development.

Artery-vein specification in the zebrafish trunk is pre-patterned by heterogeneous Notch activity and balanced by flow-mediated fine-tuning.

How developing vascular networks acquire the right balance of arteries, veins and lymphatic vessels to efficiently supply and drain tissues is poorly understood. In zebrafish embryos, the robust and regular 50:50 global balance of intersegmental veins and arteries that form along the trunk prompts the intriguing question of how does the organism keep 'count'? Previous studies have suggested that the ultimate fate of an intersegmental vessel (ISV) is determined by the identity of the approaching secondary sprout emerging from the posterior cardinal vein. Here, we show that the formation of a balanced trunk vasculature involves an early heterogeneity in endothelial cell behaviour and Notch signalling activity in the seemingly identical primary ISVs that is independent of secondary sprouting and flow. We show that Notch signalling mediates the local patterning of ISVs, and an adaptive flow-mediated mechanism subsequently fine-tunes the global balance of arteries and veins along the trunk. We propose that this dual mechanism provides the adaptability required to establish a balanced network of arteries, veins and lymphatic vessels.

Veins and Arteries Build Hierarchical Branching Patterns Differently: Bottom-Up versus Top-Down.

A tree-like hierarchical branching structure is present in many biological systems, such as the kidney, lung, mammary gland, and blood vessels. Most of these organs form through branching morphogenesis, where outward growth results in smaller and smaller branches. However, the blood vasculature is unique in that it exists as two trees (arterial and venous) connected at their tips. Obtaining this organization might therefore require unique developmental mechanisms. As reviewed here, recent data indicate that arterial trees often form in reverse order. Accordingly, initial arterial endothelial cell differentiation occurs outside of arterial vessels. These pre-artery cells then build trees by following a migratory path from smaller into larger arteries, a process guided by the forces imparted by blood flow. Thus, in comparison to other branched organs, arteries can obtain their structure through inward growth and coalescence. Here, new information on the underlying mechanisms is discussed, and how defects can lead to pathologies, such as hypoplastic arteries and arteriovenous malformations.

Venous identity requires BMP signalling through ALK3.

Venous endothelial cells are molecularly and functionally distinct from their arterial counterparts. Although veins are often considered the default endothelial state, genetic manipulations can modulate both acquisition and loss of venous fate, suggesting that venous identity is the result of active transcriptional regulation. However, little is known about this process. Here we show that BMP signalling controls venous identity via the ALK3/BMPR1A receptor and SMAD1/SMAD5. Perturbations to TGF-ß and BMP signalling in mice and zebrafish result in aberrant vein formation and loss of expression of the venous-specific gene Ephb4, with no effect on arterial identity. Analysis of a venous endothelium-specific enhancer for Ephb4 shows enriched binding of SMAD1/5 and a requirement for SMAD binding motifs. Further, our results demonstrate that BMP/SMAD-mediated Ephb4 expression requires the venous-enriched BMP type I receptor ALK3/BMPR1A. Together, our analysis demonstrates a requirement for BMP signalling in the establishment of Ephb4 expression and the venous vasculature.

Molecular identity of arteries, veins, and lymphatics.

BACKGROUND: Arteries, veins, and lymphatic vessels are distinguished by structural differences that correspond to their different functions. Each of these vessels is also defined by specific molecular markers that persist throughout adult life; these markers are some of the molecular determinants that control the differentiation of embryonic undifferentiated cells into arteries, veins, or lymphatics. METHODS: This is a review of experimental literature. RESULTS: The Eph-B4 receptor and its ligand, ephrin-B2, are critical molecular determinants of vessel identity, arising on endothelial cells early in embryonic development. Eph-B4 and ephrin-B2 continue to be expressed on adult vessels and mark vessel identity. However, after vascular surgery, vessel identity can change and is marked by altered Eph-B4 and ephrin-B2 expression. Vein grafts show loss of venous identity, with less Eph-B4 expression. Arteriovenous fistulas show gain of dual arterial-venous identity, with both Eph-B4 and ephrin-B2 expression, and manipulation of Eph-B4 improves arteriovenous fistula patency. Patches used to close arteries and veins exhibit context-dependent gain of identity, that is, patches in the arterial environment gain arterial identity, whereas patches in the venous environment gain venous identity; these results show the importance of the host infiltrating cells in determining vascular identity after vascular surgery. CONCLUSIONS: Changes in the vessel's molecular identity after vascular surgery correspond to structural changes that depend on the host's postsurgical environment. Regulation of vascular identity and the underlying molecular mechanisms may allow new therapeutic approaches to improve vascular surgical procedures.

Blood flow-induced Notch activation and endothelial migration enable vascular remodeling in zebrafish embryos.

Arteries and veins are formed independently by different types of endothelial cells (ECs). In vascular remodeling, arteries and veins become connected and some arteries become veins. It is unclear how ECs in transforming vessels change their type and how fates of individual vessels are determined. In embryonic zebrafish trunk, vascular remodeling transforms arterial intersegmental vessels (ISVs) into a functional network of arteries and veins. Here we find that, once an ISV is connected to venous circulation, venous blood flow promotes upstream migration of ECs that results in displacement of arterial ECs by venous ECs, completing the transformation of this ISV into a vein without trans-differentiation of ECs. Arterial blood flow initiated in two neighboring ISVs prevents their transformation into veins by activating Notch signaling in ECs. Together, different responses of ECs to arterial and venous blood flow lead to formation of a balanced network with equal numbers of arteries and veins.

Three-dimensional microscopy and image analysis methodology for mapping and quantification of nuclear positions in tissues with approximate cylindrical geometry.

Organogenesis involves extensive and dynamic changes of tissue shape during development. It is associated with complex morphogenetic events that require enormous tissue plasticity and generate a large variety of transient three-dimensional geometries that are achieved by global tissue responses. Nevertheless, such global responses are driven by tight spatio-temporal regulation of the behaviours of individual cells composing these tissues. Therefore, the development of image analysis tools that allow for extraction of quantitative data concerning individual cell behaviours is central to study tissue morphogenesis. There are many image analysis tools available that permit extraction of cell parameters. Unfortunately, the majority are developed for tissues with relatively simple geometries such as flat epithelia. Problems arise when the tissue of interest assumes a more complex three-dimensional geometry. Here, we use the endothelium of the developing zebrafish dorsal aorta as an example of a tissue with cylindrical geometry and describe the image analysis routines developed to extract quantitative data on individual cells in such tissues, as well as the image acquisition and sample preparation methodology.This article is part of the Theo Murphy meeting issue 'Mechanics of development'.

Management of giant embryonic vein in Klippel-Trénaunay syndrome.

Klippel-Trénaunay syndrome is a rare mixed malformation characterized by congenital varicose veins, low-flow venous and lymphatic malformations, hypertrophy of soft tissue and bone, and capillary malformations. A 35-year-old man with a diagnosis of Klippel-Trénaunay syndrome presented to the clinic with significant pain and swelling in the left leg. Initial conservative management with compression therapy failed. He was then managed surgically with preoperative placement of an inferior vena cava filter (because of a history of deep venous thrombosis and pulmonary embolism), followed by resection of the lateral embryonic vein, ligation of large perforators, and excision of smaller varicosities. He is doing well at 18 months of follow-up.

Blood Flow Regulates Zebrafish Caudal Vein Plexus Angiogenesis by ERK5-klf2a-nos2b Signaling.

BACKGROUND: Vascular network formation induced by angiogenesis plays an important role in many physiological and pathological processes. However, the role of blood flow and underlying mechanisms in vascular network formation, for example for the development of the caudal vein plexus (CVP), is poorly understood. OBJECTIVE: The aim of this study was to explore the role of ERK5-klf2a-nos2b signaling in the CVP angiogenesis. METHOD AND RESULTS: In this study on tnnt2a-MO injection and chemical blood flow modulator treatment in zebrafish embryos, we demonstrated that decreased blood flow disrupted CVP formation. The hemodynamic force was quantitatively analyzed. Furthermore, CVP angiogenesis in zebrafish embryos was inhibited by disruption of the blood flow downstream effectors ERK5, klf2a, and nos2b in response to treatment with the ERK5 specific inhibitor and to injection of klf2a-MO, nos2b-MO. Overexpression of klf2a mRNA or nos2b mRNA restored vascular defects in tnnt2a or klf2a morphants. The data suggest that flow-induced ERK5-klf2a-nos2b signaling is involved in CVP angiogenesis in zebrafish embryos. CONCLUSION: We have demonstrated that blood flow is essential for vascular network formation, specifically for CVP angiogenesis in zebrafish. A novel genetic and mechanical mechanism was discovered in which ERK5 facilitates the integration of blood flow with the downstream klf2a-nos2b signaling for CVP angiogenesis.

Vegfa signaling regulates diverse artery/vein formation in vertebrate vasculatures.

Vascular endothelial growth factor A (Vegfa) signaling regulates vascular development during embryogenesis and organ formation. However, the signaling mechanisms that govern the formation of various arteries/veins in various tissues are incompletely understood. In this study, we utilized transcription activator-like effector nuclease (TALEN) to generate zebrafish vegfaa mutants. vegfaa-/- embryos are embryonic lethal, and display a complete loss of the dorsal aorta (DA) and expansion of the cardinal vein. Activation of Vegfa signaling expands the arterial cell population at the expense of venous cells during vasculogenesis of the axial vessels in the trunk. Vegfa signaling regulates endothelial cell (EC) proliferation after arterial-venous specification. Vegfa deficiency and overexpression inhibit the formation of tip cell filopodia and interfere with the pathfinding of intersegmental vessels (ISVs). In the head vasculature, vegfaa‒/‒ causes loss of a pair of mesencephalic veins (MsVs) and central arteries (CtAs), both of which usually develop via sprouting angiogenesis. Our results indicate that Vegfa signaling induces the formation of the DA at the expense of the cardinal vein during the trunk vasculogenesis, and that Vegfa is required for the angiogenic formation of MsVs and CtAs in the brain. These findings suggest that Vegfa signaling governs the formation of diverse arteries/veins by distinct cellular mechanisms in vertebrate vasculatures.

Patterning of the Drosophila L2 vein is driven by regulatory interactions between region-specific transcription factors expressed in response to Dpp signalling.

Pattern formation relies on the generation of transcriptional landscapes regulated by signalling pathways. A paradigm of epithelial patterning is the distribution of vein territories in the Drosophila wing disc. In this tissue, Decapentaplegic signalling regulates its target genes at different distances from the source of the ligand. The transformation of signalling into coherent territories of gene expression requires regulatory cross-interactions between these target genes. Here, we analyse the mechanisms generating the domain of knirps expression in the presumptive L2 vein of the wing imaginal disc. We find that knirps is regulated by four Decapentaplegic target genes encoding the transcription factors aristaless, spalt major, spalt-related and optix The expression of optix is activated by Dpp and repressed by the Spalt proteins, becoming restricted to the most anterior region of the wing blade. In turn, the expression of knirps is activated by Aristaless and repressed by Optix and the Spalt proteins. In this manner, the expression of knirps becomes restricted to those cells where Spalt levels are sufficient to repress optix, but not sufficient to repress knirps.

Neuronal sFlt1 and Vegfaa determine venous sprouting and spinal cord vascularization.

Formation of organ-specific vasculatures requires cross-talk between developing tissue and specialized endothelial cells. Here we show how developing zebrafish spinal cord neurons coordinate vessel growth through balancing of neuron-derived Vegfaa, with neuronal sFlt1 restricting Vegfaa-Kdrl mediated angiogenesis at the neurovascular interface. Neuron-specific loss of flt1 or increased neuronal vegfaa expression promotes angiogenesis and peri-neural tube vascular network formation. Combining loss of neuronal flt1 with gain of vegfaa promotes sprout invasion into the neural tube. On loss of neuronal flt1, ectopic sprouts emanate from veins involving special angiogenic cell behaviours including nuclear positioning and a molecular signature distinct from primary arterial or secondary venous sprouting. Manipulation of arteriovenous identity or Notch signalling established that ectopic sprouting in flt1 mutants requires venous endothelium. Conceptually, our data suggest that spinal cord vascularization proceeds from veins involving two-tiered regulation of neuronal sFlt1 and Vegfaa via a novel sprouting mode.

Sturge-Weber Syndrome: A Review. / Síndrome de Sturge-Weber: revisión.

Sturge-Weber syndrome is a sporadic congenital neurocutaneous disorder caused by a somatic activating mutation in GNAQ; it affects 1 in every 20,000 to 50,000 newborns. It is characterized by a facial Port-wine stain, leptomeningeal angiomatosis, and glaucoma. Seizures are the most common neurological manifestation and typically present in the first months of life. Glaucoma may be present at birth or develop later. Neuroimaging studies show leptomeningeal angiomatosis, supporting diagnosis. Standard treatment for Sturge-Weber syndrome includes laser treatment for the Port-wine stain, anticonvulsants, and medical or surgical treatment for the glaucoma. Prognosis depends on the extent of leptomeningeal involvement and the severity of the glaucoma.

Angiopoietin receptor Tie2 is required for vein specification and maintenance via regulating COUP-TFII.

Mechanisms underlying the vein development remain largely unknown. Tie2 signaling mediates endothelial cell (EC) survival and vascular maturation and its activating mutations are linked to venous malformations. Here we show that vein formation are disrupted in mouse skin and mesentery when Tie2 signals are diminished by targeted deletion of Tek either ubiquitously or specifically in embryonic ECs. Postnatal Tie2 attenuation resulted in the degeneration of newly formed veins followed by the formation of haemangioma-like vascular tufts in retina and venous tortuosity. Mechanistically, Tie2 insufficiency compromised venous EC identity, as indicated by a significant decrease of COUP-TFII protein level, a key regulator in venogenesis. Consistently, angiopoietin-1 stimulation increased COUP-TFII in cultured ECs, while Tie2 knockdown or blockade of Tie2 downstream PI3K/Akt pathway reduced COUP-TFII which could be reverted by the proteasome inhibition. Together, our results imply that Tie2 is essential for venous specification and maintenance via Akt mediated stabilization of COUP-TFII.

Vascular development in the vertebrate pancreas.

The vertebrate pancreas is comprised of a highly branched tubular epithelium, which is intimately associated with an extensive and specialized vasculature. While we know a great deal about basic vascular anatomy of the adult pancreas, as well as islet capillaries, surprisingly little is known about the ontogeny of its blood vessels. Here, we analyze development of the pancreatic vasculature in the mouse embryo. We show that pancreatic epithelial branches intercalate with the fine capillary plexus of the surrounding pancreatic mesenchyme. Endothelial cells (ECs) within this mesenchyme are heterogeneous from the onset of organogenesis. Pancreatic arteries take shape before veins, in a manner analogous to early embryonic vessels. The main central artery forms during mid-gestation, as a result of vessel coalescence and remodeling of a vascular plexus. In addition, we show that vessels in the forming pancreas display a predictable architecture that is dependent on VEGF signaling. Over-expression of VEGF disrupts vascular patterning and arteriovenous differentiation within the developing pancreas. This study constitutes a first-time in-depth cellular and molecular characterization of pancreatic blood vessels, as they coordinately grow along with the pancreatic epithelium.