Gpr124 controls CNS angiogenesis and blood-brain barrier integrity by promoting ligand-specific canonical wnt signaling

Brain barriers: Crosstalk between complex tight junctions and adherens junctions

Unique intercellular junctional complexes between the central nervous system (CNS) microvascular endothelial cells and the choroid plexus epithelial cells form the endothelial blood–brain barrier (BBB) and the epithelial blood–cerebrospinal fluid barrier (BCSFB), respectively. These barriers inhibit paracellular diffusion, thereby protecting the CNS from fluctuations in the blood. Studies of brain barrier integrity during development, normal physiology, and disease have focused on BBB and BCSFB tight junctions but not the corresponding endothelial and epithelial adherens junctions. The crosstalk between adherens junctions and tight junctions in maintaining barrier integrity is an understudied area that may represent a promising target for influencing brain barrier function.

Vascularisation of the central nervous system

The developing central nervous system (CNS) is vascularised through the angiogenic invasion of blood vessels from a perineural vascular plexus, followed by continued sprouting and remodelling until a hierarchical vascular network is formed. Remarkably, vascularisation occurs without perturbing the intricate architecture of the neurogenic niches or the emerging neural networks. We discuss the mouse hindbrain, forebrain and retina as widely used models to study developmental angiogenesis in the mammalian CNS and provide an overview of key cellular and molecular mechanisms regulating the vascularisation of these organs.

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CNS vascularisation is initiated during embryonic development.

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CNS vascularisation is studied in the mouse forebrain, hindbrain and retina models.

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Neuroglial cells interact with endothelial cells to promote angiogenesis.

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Neuroglial cells produce growth factors and matrix cues to pattern vessels.

Regulation of the nascent brain vascular network by neural progenitors

Neural progenitors are central players in the development of the brain neural circuitry. They not only produce the diverse neuronal and glial cell types in the brain, but also guide their migration in this process. Recent evidence indicates that neural progenitors also play a critical role in the development of the brain vascular network. At an early stage, neural progenitors have been found to facilitate the ingression of blood vessels from outside the neural tube, through VEGF and canonical Wnt signaling. Subsequently, neural progenitors directly communicate with endothelial cells to stabilize nascent brain vessels, in part through down-regulating Wnt pathway activity. Furthermore, neural progenitors promote nascent brain vessel integrity, through integrin αvβ8-dependent TGFβ signaling. In this review, we will discuss the evidence for, as well as questions that remain, regarding these novel roles of neural progenitors and the underlying mechanisms in their regulation of the nascent brain vascular network.

Elevated LRP6 levels correlate with vascular endothelial growth factor in the vitreous of proliferative diabetic retinopathy

PURPOSE

To measure intravitreal low-density lipoprotein receptor-related protein 6 (LRP6) and vascular endothelial growth factor (VEGF) levels in the eyes of patients with proliferative diabetic retinopathy (PDR) and to observe their correlation with PDR activity.

METHODS

Fifty-five eyes of 55 patients were enrolled consecutively. Vitreous samples from 30 eyes with PDR and 25 eyes with nondiabetic macular disease were collected. Active PDR was present in 16 patients and quiescent PDR in 14 patients according to retinal neovascularization. LRP6 and VEGF concentrations in samples were determined using enzyme-linked immunosorbent assay (ELISA).

RESULTS

ELISA revealed significant increases in the vitreous levels of VEGF in eyes affected with PDR compared to the controls (p<0.001). The mean concentrations of LRP6 were also higher in the vitreous samples from patients with PDR compared to the nondiabetic controls: 39.85 ng/ml and 15.48 ng/ml, respectively (p=0.002). In addition, the vitreous levels of LRP6 and VEGF were significantly higher in active PDR than in quiescent PDR (p=0.022 and p=0.015, respectively). Furthermore, a significant positive correlation was found between intravitreal levels of LRP6 and VEGF in patients with PDR (r=0.567, p=0.001). However, comparison of patients with PDR with controls revealed that the plasma levels of LRP6 were not significantly different between the two groups (p=0.636).

CONCLUSIONS

LRP6 and VEGF levels in the vitreous body from patients with PDR were increased and correlated mutually. LRP6 may be a good diagnostic biomarker and a new therapeutic target for PDR.

Tip cell-specific requirement for an atypical Gpr124- and Reck-dependent Wnt/β-catenin pathway during brain angiogenesis

Despite the critical role of endothelial Wnt/β-catenin signaling during central nervous system (CNS) vascularization, how endothelial cells sense and respond to specific Wnt ligands and what aspects of the multistep process of intra-cerebral blood vessel morphogenesis are controlled by these angiogenic signals remain poorly understood. We addressed these questions at single-cell resolution in zebrafish embryos. We identify the GPI-anchored MMP inhibitor Reck and the adhesion GPCR Gpr124 as integral components of a Wnt7a/Wnt7b-specific signaling complex required for brain angiogenesis and dorsal root ganglia neurogenesis. We further show that this atypical Wnt/β-catenin signaling pathway selectively controls endothelial tip cell function and hence, that mosaic restoration of single wild-type tip cells in Wnt/β-catenin-deficient perineural vessels is sufficient to initiate the formation of CNS vessels. Our results identify molecular determinants of ligand specificity of Wnt/β-catenin signaling and provide evidence for organ-specific control of vascular invasion through tight modulation of tip cell function.

DOI:http://dx.doi.org/10.7554/eLife.06489.001

Organs develop alongside the network of blood vessels that supply them with oxygen and nutrients. One way that new blood vessels grow is by sprouting out of the side of an existing vessel, via a process called angiogenesis. This process relies on signals that are received by the endothelial cells that line the inner wall of blood vessels, and that direct the cells to form a new ‘sprout’, consisting of tip and stalk cells.

In the developing brain, the Wnt/β-catenin signaling pathway helps direct the formation of blood vessels. In this pathway, a member of a protein family called Wnt signals to specific proteins on the surface of the cells lining the blood vessels. Much effort has gone into uncovering the identity of these proteins, with many studies looking at blood vessel development in the brain of mouse embryos.

In this study, Vanhollebeke et al. turned to zebrafish embryos to uncover new regulators of angiogenesis and define their roles during the multi-step process of blood vessel development in the brain. A variety of experimental techniques were used to alter and study the activity of different Wnt signaling pathway components. These experiments revealed that the Wnt7a and Wnt7b proteins signal to an endothelial cell membrane protein complex containing the proteins Gpr124 and Reck.

Vanhollebeke et al. then created ‘mosaic’ zebrafish embryos, which contained two genetically distinct types of cells—cells that were missing one of the components of Wnt/β-catenin signaling pathway, and wild-type cells. Visualizing the growth of the vessels showed that all the new blood vessels that sprouted had normal tip cells. However, the cells in the stalk of the sprout could be either normal or missing a signaling protein.

These findings demonstrate that Wnt/β-catenin signaling controls the pattern of blood vessel development in the brain by acting specifically on the invasive behaviors of the tip cells of new sprouts, a cellular mechanism that allows efficient organ-specific control of vascularization.

DOI:http://dx.doi.org/10.7554/eLife.06489.002

Frizzleds and WNT/β-catenin signaling--The black box of ligand-receptor selectivity, complex stoichiometry and activation kinetics

The lipoglycoproteins of the mammalian WNT family induce β-catenin-dependent signaling through interaction with members of the Class Frizzled receptors and LDL receptor-related protein 5/6 (LRP5/6) albeit with unknown selectivity. The 10 mammalian Frizzleds (FZDs) are seven transmembrane (7TM) spanning receptors and have recently been classified as G protein-coupled receptors (GPCRs). This review summarizes the current knowledge about WNT/FZD selectivity and functional selectivity, the role of co-receptors for signal specification, the formation of receptor complexes as well as the kinetics and mechanisms of signal initiation with focus on WNT/β-catenin signaling. In order to exploit the true therapeutic potential of WNT/FZD signaling to treat human disease, it is clear that substantial progress in the understanding of receptor complex formation and signal specification has to precede a mechanism-based drug design targeting WNT receptors.

GPR124 functions as a WNT7-specific coactivator of canonical β-catenin signaling

G protein-coupled receptor 124 (GPR124) is an orphan receptor in the adhesion family of GPCRs, and previous global or endothelial-specific disruption of Gpr124 in mice led to defective CNS angiogenesis and blood-brain barriergenesis. Similar developmental defects were observed following dual deletion of Wnt7a/Wnt7b or deletion of β-catenin in endothelial cells, suggesting a possible relationship between GPR124 and canonical WNT signaling. Here, we show using in vitro reporter assays, mutation analysis, and genetic interaction studies in vivo that GPR124 functions as a WNT7A/WNT7B-specific costimulator of β-catenin signaling in brain endothelium. WNT7-stimulated β-catenin signaling was dependent upon GPR124's intracellular PDZ binding motif and a set of leucine-rich repeats in its extracellular domain. This study reveals a vital role for GPR124 in potentiation of WNT7-induced canonical β-catenin signaling with important implications for understanding and manipulating CNS-specific angiogenesis and blood-brain barrier-genesis.