Neuropilin 1 signaling guides neural crest cells to coordinate pathway choice with cell specification

Neural crest cells (NCCs) are highly motile embryonic stem cells that delaminate from the neuroectoderm early during vertebrate embryogenesis and differentiate at defined target sites into various essential cell types. To reach their targets, NCCs follow 1 of 3 sequential pathways that correlate with NCC fate. The firstborn NCCs travel ventrally alongside intersomitic blood vessels to form sympathetic neuronal progenitors near the dorsal aorta, while the lastborn NCCs migrate superficially beneath the epidermis to give rise to melanocytes. Yet, most NCCs enter the somites to form the intermediate wave that gives rise to sympathetic and sensory neurons. Here we show that the repulsive guidance cue SEMA3A and its receptor neuropilin 1 (NRP1) are essential to direct the intermediate wave NCC precursors of peripheral neurons from a default pathway alongside intersomitic blood vessels into the anterior sclerotome. Thus, loss of function for either gene caused excessive intersomitic NCC migration, and this led to ectopic neuronal differentiation along both the anteroposterior and dorsoventral axes of the trunk. The choice of migratory pathway did not affect the specification of NCCs, as they retained their commitment to differentiate into sympathetic or sensory neurons, even when they migrated on an ectopic dorsolateral path that is normally taken by melanocyte precursors. We conclude that NRP1 signaling coordinates pathway choice with NCC fate and therefore confines neuronal differentiation to appropriate locations

Neuropilin-mediated neural crest cell guidance is essential to organise sensory neurons into segmented dorsal root ganglia

The peripheral nervous system (PNS) of higher vertebrates is segmented to align the spinal nerve roots with the vertebrae. This co-patterning is set up during embryogenesis, when vertebrae develop from the sclerotome layer of the metameric somites, and PNS neurons and glia differentiate from neural crest cells (NCCs) that preferentially migrate into the anterior sclerotome halves. Previous analyses of mice deficient in the class 3 semaphorin (SEMA3) receptors neuropilin (NRP) 1 or 2 raised the possibility that each controlled a distinct aspect of trunk NCC migration. We now demonstrate that both pathways act sequentially in distinct NCC subpopulations and thereby cooperate to enforce segmental NCC migration. Specifically, SEMA3A/NRP1 signalling first directs one population of NCCs from the intersomitic path into the sclerotome, and SEMA3F/NRP2 signalling acts subsequently to restrict a second population to the anterior half of the sclerotome. NCC exclusion from either the posterior sclerotome or the intersomitic boundary is sufficient to enforce the separation of neighbouring NCC streams and the segregation of sensory NCC progeny into metameric dorsal root ganglia (DRG). By contrast, the combined loss of both guidance pathways leads to ectopic invasion of the intersomitic furrows and posterior sclerotome halves, disrupting metameric NCC streaming and DRG segmentation

First blood: the endothelial origins of hematopoietic progenitors

Hematopoiesis in vertebrate embryos occurs in temporally and spatially overlapping waves in close proximity to blood vascular endothelial cells. Initially, yolk sac hematopoiesis produces primitive erythrocytes, megakaryocytes, and macrophages. Thereafter, sequential waves of definitive hematopoiesis arise from yolk sac and intraembryonic hemogenic endothelia through an endothelial-to-hematopoietic transition (EHT). During EHT, the endothelial and hematopoietic transcriptional programs are tightly co-regulated to orchestrate a shift in cell identity. In the yolk sac, EHT generates erythro-myeloid progenitors, which upon migration to the liver differentiate into fetal blood cells, including erythrocytes and tissue-resident macrophages. In the dorsal aorta, EHT produces hematopoietic stem cells, which engraft the fetal liver and then the bone marrow to sustain adult hematopoiesis. Recent studies have defined the relationship between the developing vascular and hematopoietic systems in animal models, including molecular mechanisms that drive the hemato-endothelial transcription program for EHT. Moreover, human pluripotent stem cells have enabled modeling of fetal human hematopoiesis and have begun to generate cell types of clinical interest for regenerative medicine

Cross-talk between blood vessels and neural progenitors in the developing brain

The formation of the central nervous system (CNS) involves multiple cellular and molecular interactions between neural progenitor cells (NPCs) and blood vessels to establish extensive and complex neural networks and attract a vascular supply that support their function. In this review, we discuss studies that have performed genetic manipulations of chick, fish and mouse embryos to define the spatiotemporal roles of molecules that mediate the reciprocal regulation of NPCs and blood vessels. These experiments have highlighted core functions of NPC-expressed ligands in initiating vascular growth into and within the neural tube as well as establishing the blood-brain barrier. More recent findings have also revealed indispensable roles of blood vessels in regulating NPC expansion and eventual differentiation, and specific regional differences in the effect of angiocrine signals. Accordingly, NPCs initially stimulate blood vessel growth and maturation to nourish the brain, but blood vessels subsequently also regulate NPC behaviour to promote the formation of a sufficient number and diversity of neural cells. A greater understanding of the molecular cross-talk between NPCs and blood vessels will improve our knowledge of how the vertebrate nervous system forms and likely help in the design of novel therapies aimed at regenerating neurons and neural vasculature following CNS disease or injury

The Mouse Hindbrain As a Model for Studying Embryonic Neurogenesis

The mouse embryo forebrain is the most commonly employed system for studying mammalian neurogenesis during development. However, the highly folded forebrain neuroepithelium is not amenable to wholemount analysis to examine organ-wide neurogenesis patterns. Moreover, defining the mechanisms of forebrain neurogenesis is not necessarily predictive of neurogenesis in other parts of the brain; for example, due to the presence of forebrain-specific progenitor subtypes. The mouse hindbrain provides an alternative model for studying embryonic neurogenesis that is amenable to wholemount analysis, as well as tissue sections to observe the spatiotemporal distribution and behavior of neural progenitors. Moreover, it is easily dissected for other downstream applications, such as cell isolation or molecular biology analysis. As the mouse hindbrain can be readily analyzed in the vast number of cell lineage reporter and mutant mouse strains that have become available, it offers a powerful model for studying the cellular and molecular mechanisms of developmental neurogenesis in a mammalian organism. Here, we present a simple and quick method to use the mouse embryo hindbrain for analyzing mammalian neural progenitor cell (NPC) behavior in wholemount preparations and tissue sections

VEGF guides angiogenic sprouting utilizing endothelial tip cell filopodia

Vascular endothelial growth factor (VEGF-A) is a major regulator of blood vessel formation and function. it controls several processes in endothelial cells, such as proliferation, survival, and migration, but it is not known how these are coordinately regulated to result in more complex morphogenetic events, such as tubular sprouting, fusion, and network formation. We show here that VEGF-A controls angiogenic sprouting in the early postnatal retina by guiding filopodial extension from specialized endothelial cells situated at the tips of the vascular sprouts. The tip cells respond to VEGF-A only by guided migration; the proliferative response to VEGF-A occurs in the sprout stalks. These two cellular responses are both mediated by agonistic activity of VEGF-A on VEGF receptor 2. Whereas tip cell migration depends on a gradient of VEGF-A, proliferation is regulated by its concentration. Thus, vessel patterning during retinal angiogenesis depends on the balance between two different qualities of the extracellular VEGF-A distribution, which regulate distinct cellular responses in defined populations of endothelial cells

Mechanisms and cell lineages in lymphatic vascular development.

Lymphatic vessels have critical roles in both health and disease and their study is a rapidly evolving area of vascular biology. The consensus on how the first lymphatic vessels arise in the developing embryo has recently shifted. Originally, they were thought to solely derive by sprouting from veins. Since then, several studies have uncovered novel cellular mechanisms and a diversity of contributing cell lineages in the formation of organ lymphatic vasculature. Here, we review the key mechanisms and cell lineages contributing to lymphatic development, discuss the advantages and limitations of experimental techniques used for their study and highlight remaining knowledge gaps that require urgent attention. Emerging technologies should accelerate our understanding of how lymphatic vessels develop normally and how they contribute to disease

VEGF188 promotes corneal reinnervation after injury

Vascular endothelial growth factor A (VEGF) induces angiogenesis and vascular hyperpermeability in ocular tissues and is therefore a key therapeutic target for eye conditions in which these processes are dysregulated. In contrast, the therapeutic potential of VEGF's neurotrophic roles in the eye has remained unexploited. In particular, it is not known whether modulating levels of any of the 3 major alternatively spliced VEGF isoforms might provide a therapeutic approach to promote neural health in the eye without inducing vascular pathology. Here, we have used a variety of mouse models to demonstrate differences in overall VEGF levels and VEGF isoform ratios across tissues in the healthy eye. We further show that VEGF isoform expression was differentially regulated in retinal versus corneal disease models. Among the 3 major isoforms - termed VEGF120, VEGF164, and VEGF188 - VEGF188 was upregulated to the greatest extent in injured cornea, where it was both necessary and sufficient for corneal nerve regeneration. Moreover, topical VEGF188 application further promoted corneal nerve regeneration without inducing pathological neovascularization. VEGF isoform modulation should therefore be explored further for its potential in promoting neural health in the eye

Altered proliferative ability of neuronal progenitors in PlexinA1 mutant mice

Cortical interneurons are generated predominantly in the medial ganglionic eminence (MGE) and migrate through the ventral and dorsal telencephalon before taking their final positions within the developing cortical plate. Previously we demonstrated that interneurons from Robo1 knockout (Robo1(-/-) ) mice contain reduced levels of neuropilin 1 (Nrp1) and PlexinA1 receptors, rendering them less responsive to the chemorepulsive actions of semaphorin ligands expressed in the striatum and affecting their course of migration (Hernandez-Miranda et al. [2011] J. Neurosci. 31:6174-6187). Earlier studies have highlighted the importance of Nrp1 and Nrp2 in interneuron migration, and here we assess the role of PlexinA1 in this process. We observed significantly fewer cells expressing the interneuron markers Gad67 and Lhx6 in the cortex of PlexinA1(-/-) mice compared with wild-type littermates at E14.5 and E18.5. Although the level of apoptosis was similar in the mutant and control forebrain, proliferation was significantly reduced in the former. Furthermore, progenitor cells in the MGE of PlexinA1(-/-) mice appeared to be poorly anchored to the ventricular surface and showed reduced adhesive properties, which may account for the observed reduction in proliferation. Together our data uncover a novel role for PlexinA1 in forebrain development. J. Comp. Neurol., 2015. © 2015 The Authors The Journal of Comparative Neurology Published by Wiley Periodicals, Inc