Guidance from above: common cues direct distinct signaling outcomes in vascular and neural patterning

Bone Marrow Stem Cells Derived from Nerves Have Neurogenic Properties and Potential Utility for Regenerative Therapy

Neurons and glia of the peripheral nervous system are derived from progenitor cell populations, originating from embryonic neural crest. The neural crest and vasculature are intimately associated during embryonic development and in the mature central nervous system, in which they form a neurovascular unit comprised of neurons, glia, pericytes, and vascular endothelial cells that play important roles in health and disease. Our group and others have previously reported that postnatal populations of stem cells originating from glia or Schwann cells possess neural stem cell qualities, including rapid proliferation and differentiation into mature glia and neurons. Bone marrow receives sensory and sympathetic innervation from the peripheral nervous system and is known to contain myelinating and unmyelinating Schwann cells. Herein, we describe a population of neural crest-derived Schwann cells residing in a neurovascular niche of bone marrow in association with nerve fibers. These Schwann cells can be isolated and expanded. They demonstrate plasticity in vitro, generating neural stem cells that exhibit neurogenic potential and form neural networks within the enteric nervous system in vivo following transplantation to the intestine. These cells represent a novel source of autologous neural stem cells for the treatment of neurointestinal disorders.

Blood-brain barrier endothelial cells in neurodegenerative diseases: Signals from the "barrier"

As blood-brain barrier (BBB) disruption emerges as a common problem in the early stages of neurodegenerative diseases, the crucial roles of barrier-type brain endothelial cells (BECs), the primary part of the BBB, have been reported in the pathophysiology of neurodegenerative diseases. The mechanisms of how early vascular dysfunction contributes to the progress of neurodegeneration are still unclear, and understanding BEC functions is a promising start. Our understanding of the BBB has gone through different stages, from a passive diffusion barrier to a mediator of central-peripheral interactions. BECs serve two seemingly paradoxical roles: as a barrier to protect the delicate brain from toxins and as an interface to constantly receive and release signals, thus maintaining and regulating the homeostasis of the brain. Most previous studies about neurodegenerative diseases focus on the loss of barrier functions, and far too little attention has been paid to the active regulations of BECs. In this review, we present the current evidence of BEC dysfunction in neurodegenerative diseases and explore how BEC signals participate in the pathogenesis of neurodegenerative diseases.

Diverse roles for axon guidance pathways in adult tissue architecture and function

Classical axon guidance ligands and their neuronal receptors were first identified due to their fundamental roles in regulating connectivity in the developing nervous system. Since their initial discovery, it has become clear that these signaling molecules play important roles in the development of a broad array of tissue and organ systems across phylogeny. In addition to these diverse developmental roles, there is a growing appreciation that guidance signaling pathways have important functions in adult organisms, including the regulation of tissue integrity and homeostasis. These roles in adult organisms include both tissue-intrinsic activities of guidance molecules, as well as systemic effects on tissue maintenance and function mediated by the nervous and vascular systems. While many of these adult functions depend on mechanisms that mirror developmental activities, such as regulating adhesion and cell motility, there are also examples of adult roles that may reflect signaling activities that are distinct from known developmental mechanisms, including the contributions of guidance signaling pathways to lineage commitment in the intestinal epithelium and bone remodeling in vertebrates. In this review, we highlight studies of guidance receptors and their ligands in adult tissues outside of the nervous system, focusing on in vivo experimental contexts. Together, these studies lay the groundwork for future investigation into the conserved and tissue-specific mechanisms of guidance receptor signaling in adult tissues.

Using Dictyostelium to Develop Therapeutics for Acute Respiratory Distress Syndrome

Acute respiratory distress syndrome (ARDS) involves damage to lungs causing an influx of neutrophils from the blood into the lung airspaces, and the neutrophils causing further damage, which attracts more neutrophils in a vicious cycle. There are ∼190,000 cases of ARDS per year in the US, and because of the lack of therapeutics, the mortality rate is ∼40%. Repelling neutrophils out of the lung airspaces, or simply preventing neutrophil entry, is a potential therapeutic. In this minireview, we discuss how our lab noticed that a protein called AprA secreted by growing Dictyostelium cells functions as a repellent for Dictyostelium cells, causing cells to move away from a source of AprA. We then found that AprA has structural similarity to a human secreted protein called dipeptidyl peptidase IV (DPPIV), and that DPPIV is a repellent for human neutrophils. In animal models of ARDS, inhalation of DPPIV or DPPIV mimetics blocks neutrophil influx into the lungs. To move DPPIV or DPPIV mimetics into the clinic, we need to know how this repulsion works to understand possible drug interactions and side effects. Combining biochemistry and genetics in Dictyostelium to elucidate the AprA signal transduction pathway, followed by drug studies in human neutrophils to determine similarities and differences between neutrophil and Dictyostelium chemorepulsion, will hopefully lead to the safe use of DPPIV or DPPIV mimetics in the clinic.

Vascular Sema3E-Plexin-D1 Signaling Reactivation Promotes Post-stroke Recovery through VEGF Downregulation in Mice

Post-stroke vascular remodeling, including angiogenesis, facilitates functional recovery. Proper vascular repair is important for efficient post-stroke recovery; however, the underlying mechanisms coordinating the diverse signaling pathways involved in vascular remodeling remain largely unknown. Recently, axon guidance molecules were revealed as key players in injured vessel remodeling. One such molecule, Semaphorin 3E (Sema3E), and its receptor, Plexin-D1, control vascular development by regulating vascular endothelial growth factor (VEGF) signaling. In this study, using a mouse model of transient brain infarction, we aimed to investigate whether Sema3E-Plexin-D1 signaling was involved in cerebrovascular remodeling after ischemic injury. We found that ischemic damage rapidly induced Sema3e expression in the neurons of peri-infarct regions, followed by Plexin-D1 upregulation in remodeling vessels. Interestingly, Plexin-D1 reemergence was concurrent with brain vessels entering an active angiogenic process. In line with this, Plxnd1 ablation worsened neurological deficits, infarct volume, neuronal survival rate, and blood flow recovery. Furthermore, reduced and abnormal vascular morphogenesis was caused by aberrantly increased VEGF signaling. In Plxnd1 knockout mice, we observed significant extravasation of intravenously administered tracers in the brain parenchyma, junctional protein downregulation, and mislocalization in regenerating vessels. This suggested that the absence of Sema3E-Plexin-D1 signaling is associated with blood–brain barrier (BBB) impairment. Finally, the abnormal behavioral performance, aberrant vascular phenotype, and BBB breakdown defects in Plxnd1 knockout mice were restored following the inhibition of VEGF signaling during vascular remodeling. These findings demonstrate that Sema3E-Plexin-D1 signaling can promote functional recovery by downregulating VEGF signaling in the injured adult brain.

Tetraspanin18 regulates angiogenesis through VEGFR2 and Notch pathways

The VEGF pathway is critically required for vasculogenesis, the formation of the primary vascular network. It is also required for angiogenesis resulting in sprouting and pruning of vessels to generate mature arborizing structures. The Notch pathway is essential for arterial–venous specification and the maturation of nascent vessels. We have determined that Tspan18, a member of the Tetraspanin family, is expressed in developing vessels but not in mature vasculature in zebrafish and mouse wound healing. Moreover, reduction at Tspan18 level resulted in aberrant vascular patterning, impaired vessel stability and defective arterial–venous specification. Tspan18 deficiency reduced VEGF, VEGFR2, Notch3 and EphrinB2, and increased EphB4, VEGFR3, Semaphorin3, Neuropilin and PlexinD1 expression. Furthermore, vascular defects of Tspan18 deficiency could be rescued by ectopic expression of VEGFR2 and Notch, but not by knockdown of Semaphorin or Plexin. Functional studies showed that knockdown of Tspan18 led to reduced endothelial cell migration, invasion and tube formation. Tspan18 has dynamic expression, regulates vascular development and maturation in the embryo with re-expression in adult life in wound healing.

Summary: Tspan18 is a transmembrane protein with highly restricted expression in endothelial cells and a critical regulator of VEGF and Notch pathways. It regulates artery–vein specification, vessel patterning and vessel stability.

Sensory neurons from dorsal root ganglia regulate endothelial cell function in extracellular matrix remodelling

BACKGROUND

Recent physiological and experimental data highlight the role of the sensory nervous system in bone repair, but its precise role on angiogenesis in a bone regeneration context is still unknown. Our previous work demonstrated that sensory neurons (SNs) induce the osteoblastic differentiation of mesenchymal stem cells, but the influence of SNs on endothelial cells (ECs) was not studied.

METHODS

Here, in order to study in vitro the interplay between SNs and ECs, we used microfluidic devices as an indirect co-culture model. Gene expression analysis of angiogenic markers, as well as measurements of metalloproteinases protein levels and enzymatic activity, were performed.

RESULTS

We were able to demonstrate that two sensory neuropeptides, calcitonin gene-related peptide (CGRP) and substance P (SP), were involved in the transcriptional upregulation of angiogenic markers (vascular endothelial growth factor, angiopoietin 1, type 4 collagen, matrix metalloproteinase 2) in ECs. Co-cultures of ECs with SNs also increased the protein level and enzymatic activity of matrix metalloproteinases 2 and 9 (MMP2/MMP9) in ECs.

CONCLUSIONS

Our results suggest a role of sensory neurons, and more specifically of CGRP and SP, in the remodelling of endothelial cells extracellular matrix, thus supporting and enhancing the angiogenesis process. Video abstract.

Neurovascular coupling in the developing neonatal brain at rest

The neonatal brain is an extremely dynamic organization undergoing essential development in terms of connectivity and function. Several functional imaging investigations of the developing brain have found neurovascular coupling (NVC) patterns that contrast with those observed in adults. These discrepancies are partly due to that NVC is still developing in the neonatal brain. To characterize the vascular response to spontaneous neuronal activations, a multiscale multimodal noninvasive approach combining simultaneous electrical, hemodynamic, and metabolic recordings has been developed for preterm infants. Our results demonstrate that the immature vascular network does not adopt a unique strategy to respond to spontaneous cortical activations. NVC takes on different forms in the same preterm infant during the same recording session in response to very similar types of neural activation. This includes (a) positive stereotyped hemodynamic responses (increases in HbO, decreases in HbR together with increases in rCBF and rCMRO2), (b) negative hemodynamic responses (increases in HbR, decreases in HbO together with decreases in rCBF and rCMRO2), and (c) Increases and decreases in both HbO‐HbR and rCMRO2 together with no changes in rCBF. Age‐related NVC maturation is demonstrated in preterm infants, which can contribute to a better understanding/prevention of cerebral hemodynamic risks in these infants.

Semaphorin/neuropilin binding specificities are stable over 400 million years of evolution

Semaphorins are a large and important family of signaling molecules conserved in Bilateria. An important determinant of the biological function of their largest class, the secreted class 3 semaphorins, is the specificity of their binding to neuropilins, a key component of a larger holoreceptor complex. We compared these binding specificities in mice and zebrafish, species whose most recent common ancestor was more than 400 million years in the past. We also compared the binding specificities of zebrafish class 3 semaphorins that were duplicated very early within the teleost lineage. We found a surprising conservation of neuropilin binding specificities when comparing both paralogous zebrafish semaphorin pairs and orthologous zebrafish and mouse semaphorin pairs. This finding was further supported by a remarkable conservation of binding specificities in cross-species pairings of semaphorins and neuropilins. Our results suggest that the qualitative specificities with which particular semaphorins bind to particular neuropilins has remained nearly invariant over approximately 400 million years of evolution.

Retinal Vasculature in Development and Diseases

The retina is one of the most metabolically active tissues in the body, consuming high levels of oxygen and nutrients. A well-organized ocular vascular system adapts to meet the metabolic requirements of the retina to ensure visual function. Pathological conditions affect growth of the blood vessels in the eye. Understanding the neuronal biological processes that govern retinal vascular development is of interest for translational researchers and clinicians to develop preventive and interventional therapeutics for vascular eye diseases that address early drivers of abnormal vascular growth. This review summarizes the current knowledge of the cellular and molecular processes governing both physiological and pathological retinal vascular development, which is dependent on the interaction among retinal cell populations, including neurons, glia, immune cells, and vascular endothelial cells. We also review animal models currently used for studying retinal vascular development.

Establishment of tooth blood supply and innervation is developmentally regulated and takes place through differential patterning processes

Teeth are richly supported by blood vessels and peripheral nerves. The aim of this study was to describe in detail the developmental time-course and localization of blood vessels during early tooth formation and to compare that to innervation, as well as to address the putative role of vascular endothelial growth factor (VEGF), which is an essential regulator of vasculature development, in this process. The localization of blood vessels and neurites was compared using double immunofluorescence staining on sections at consecutive stages of the embryonic (E) and postnatal (PN) mandibular first molar tooth germ (E11-PN7). Cellular mRNA expression domains of VEGF and its signaling receptor VEGFR2 were studied using sectional radioactive in situ hybridization. Expression of VEGF mRNA and the encoded protein were studied by RT-PCR and western blot analysis, respectively, in the cap and early bell stage tooth germs, respectively. VEGFR2 was immunolocalized on tooth tissue sections. Smooth muscle cells were investigated by anti-alpha smooth muscle actin (αSMA) antibodies. VEGF showed developmentally regulated epithelial and mesenchymal mRNA expression domains including the enamel knot signaling centers that correlated with the growth and navigation of the blood vessels expressing Vegfr2 and VEGFR2 to the dental papilla and enamel organ. Developing blood vessels were present in the jaw mesenchyme including the presumptive dental mesenchyme before the appearance of the epithelial dental placode and dental neurites. Similarly, formation of a blood vessel plexus around the bud stage tooth germ and ingrowth of vessels into dental papilla at E14 preceded ingrowth of neurites. Subsequently, pioneer blood vessels in the dental papilla started to receive smooth muscle coverage at the early embryonic bell stage. Establishment and patterning of the blood vessels and nerves during tooth formation are developmentally regulated, stepwise processes that likely involve differential patterning mechanisms. Development of tooth vascular supply is proposed to be regulated by local, tooth-specific regulation by epithelial-mesenchymal tissue interactions and involving tooth target expressed VEGF signaling. Further investigations on tooth vascular development by local VEGF signaling, as well as how tooth innervation and development of blood vessels are integrated with advancing tooth organ formation by local signaling mechanisms, are warranted.

Sema3E/PlexinD1 signaling inhibits postischemic angiogenesis by regulating endothelial DLL4 and filopodia formation in a rat model of ischemic stroke

Angiogenesis is a crucial defense response to hypoxia that regulates the process of raising the promise of long-term neurologic recovery during the management of stroke. A high expression of antiangiogenic factors leads to the loss of neovascularization capacity in pathologic conditions. We have previously documented an impairment of the cerebral vessel perfusion and neovascularization in the cortex neighboring the stroke-induced lesion, which was accompanied by an activation of semaphorin 3E (Sema3E)/PlexinD1 after ischemic stroke. In this study, we employed micro-optical sectioning tomography to fully investigate the details of the vascular pattern, including the capillaries. We found that after transient middle cerebral artery occlusion, inhibiting PlexinD1 signaling led to an organized recovery of the vascular network in the ischemic area. We then further explored the possible mechanisms. In vivo, Sema3E substantially decreased dynamic delta-like 4 (DLL4) expression. In cultured brain microvascular endothelial cells, Sema3E down-regulated DLL4 expression via inhibiting Ras-related C3 botulinum toxin substrate 1-induced JNK phosphorylation. At the microcosmic level, Sema3E/PlexinD1 signaling promoted F-actin disassembly and focal adhesion reduction by activating the small guanosine triphosphatase Ras homolog family member J by releasing RhoGEF Tuba from direct binding to PlexinD1, thus mediating endothelial cell motility and filopodia retraction. Our study reveals that Sema3E/PlexinD1 signaling, which suppressed endothelial DLL4 expression, cell motility, and filopodia formation, is expected to be a novel druggable target for angiogenesis during poststroke progression.-Zhou, Y.-F., Chen, A.-Q., Wu, J.-H., Mao, L., Xia, Y.-P., Jin, H.-J., He, Q.-W., Miao, Q. R., Yue, Z.-Y., Liu, X.-L., Huang, M., Li, Y.-N., Hu, B. Sema3E/PlexinD1 signaling inhibits postischemic angiogenesis by regulating endothelial DLL4 and filopodia formation in a rat model of ischemic stroke.

Semaphorin 7A Promotes VEGFA/VEGFR2-Mediated Angiogenesis and Intraplaque Neovascularization in ApoE-/- Mice

Excessive neovascularization of atherosclerotic lesions increases plaque vulnerability and the susceptibility to rupture. Semaphorin 7A (Sema7A), a semaphorin family member, was recently reported to promote atherosclerotic plaque formation by mediating d-flow-induced endothelial phenotypic change and leukocyte adhesion. To extend our understanding of the proatherogenic role of Sema7A, we investigated the role of endothelial Sema7A in angiogenesis and atherosclerotic neovascularization. Sema7A overexpression in human umbilical vein endothelial cells (HUVECs) significantly upregulated VEGFA/VEGFR2 and promoted cell migration and angiogenesis. This enhancing effect was eliminated by the blockage of Sema7A receptor, β1 integrin. Inhibition of FAK or ERK1/2 downstream of β1 integrin signaling significantly inhibited cell migration and angiogenesis via ROCK (Rho-associated coiled forming protein kinase) and MYPT (myosin phosphatase targeting subunit), which are responsible for actin polymerization. Consistently, in vivo studies showed a remarkable reduction in VEGFA/VEGFR2 expression and neovascularization in the atherosclerotic plaques of Sema7A^-/-ApoE^-/- mice compared with Sema7A^+/+ApoE^-/- littermates. Supportively, Sema7A deficiency reduced the accumulation of T cells, macrophages, and dendritic cells, and enhanced plaque stability in ApoE^-/- mice. Together, our findings show that Sema7A promotes VEGFA/VEGFR2-mediated neovascularization in a β1 integrin-dependent manner, supporting a crucial role of Sema7A in the progression of human atherosclerosis.

Semaphorins and their Signaling Mechanisms

Semaphorins are extracellular signaling proteins that are essential for the development and maintenance of many organs and tissues. The more than 20-member semaphorin protein family includes secreted, transmembrane and cell surface-attached proteins with diverse structures, each characterized by a single cysteine-rich extracellular sema domain, the defining feature of the family. Early studies revealed that semaphorins function as axon guidance molecules, but it is now understood that semaphorins are key regulators of morphology and motility in many different cell types including those that make up the nervous, cardiovascular, immune, endocrine, hepatic, renal, reproductive, respiratory and musculoskeletal systems, as well as in cancer cells. Semaphorin signaling occurs predominantly through Plexin receptors and results in changes to the cytoskeletal and adhesive machinery that regulate cellular morphology. While much remains to be learned about the mechanisms underlying the effects of semaphorins, exciting work has begun to reveal how semaphorin signaling is fine-tuned through different receptor complexes and other mechanisms to achieve specific outcomes in various cellular contexts and physiological systems. These and future studies will lead to a more complete understanding of semaphorin-mediated development and to a greater understanding of how these proteins function in human disease.

View Point: Semaphorin-3E: An Emerging Modulator of Natural Killer Cell Functions?

Semaphorin-3E (Sema-3E) is a member of a large family of proteins originally identified as axon guidance cues in neural development. It is expressed in different cell types, such as immune cells, cancer cells, neural cells, and epithelial cells. Subsequently, dys-regulation of Sema-3E expression has been reported in various biological processes that range from cancers to autoimmune and allergic diseases. Recent work in our laboratories revealed a critical immunoregulatory role of Sema-3E in experimental allergic asthma. We further speculate possible immune modulatory function(s) of Sema-3E on natural killer (NK) cells.

The biological significance of methylome differences in human papilloma virus associated head and neck cancer

In recent years, studies have suggested that promoter methylation in human papilloma virus (HPV) positive head and neck squamous cell carcinoma (HNSCC) has a mechanistic role and has the potential to improve patient survival. The present study aimed to replicate key molecular findings from previous analyses of the methylomes of HPV positive and HPV negative HNSCC in an independent cohort, to assess the reliability of differentially methylated markers in HPV-associated tumors. HPV was measured using real-time quantitative PCR and the biological significance of methylation differences was assessed by Ingenuity Pathway Analysis (IPA). Using an identical experimental design of a 450K methylation platform, 7 of the 11 genes were detected to be significantly differentially methylated and all 11 genes were either hypo- or hypermethylated, which was in agreement with the results of a previous study. IPA's enriched networks analysis identified one network with msh homeobox 2 (MSX2) as a central node. Locally dense interactions between genes in networks tend to reflect significant biology; therefore MSX2 was selected as an important gene. Sequestration in the top four canonical pathways was noted for 5-hydroxytryptamine receptor 1E (serotonin signaling), collapsin response mediator protein 1 (semaphorin signaling) and paired like homeodomain 2 (bone morphogenic protein and transforming growth factor-β signaling). Placement of 9 of the 11 genes in highly ranked pathways and bionetworks identified key biological processes to further emphasize differences between HNSCC HPV positive and negative pathogenesis.

In vivo time-lapse imaging reveals extensive neural crest and endothelial cell interactions during neural crest migration and formation of the dorsal root and sympathetic ganglia

During amniote embryogenesis the nervous and vascular systems interact in a process that significantly affects the respective morphogenesis of each network by forming a "neurovascular" link. The importance of neurovascular cross-talk in the central nervous system has recently come into focus with the growing awareness that these two systems interact extensively both during development, in the stem-cell niche, and in neurodegenerative conditions such as Alzheimer's Disease and Amyotrophic Lateral Sclerosis. With respect to the peripheral nervous system, however, there have been no live, real-time investigations of the potential relationship between these two developing systems. To address this deficit, we used multispectral 4D time-lapse imaging in a transgenic quail model in which endothelial cells (ECs) express a yellow fluorescent marker, while neural crest cells (NCCs) express an electroporated red fluorescent marker. We monitored EC and NCC migration in real-time during formation of the peripheral nervous system. Our time-lapse recordings indicate that NCCs and ECs are physically juxtaposed and dynamically interact at multiple locations along their trajectories. These interactions are stereotypical and occur at precise anatomical locations along the NCC migratory pathway. NCCs migrate alongside the posterior surface of developing intersomitic vessels, but fail to cross these continuous streams of motile ECs. NCCs change their morphology and migration trajectory when they encounter gaps in the developing vasculature. Within the nascent dorsal root ganglion, proximity to ECs causes filopodial retraction which curtails forward persistence of NCC motility. Overall, our time-lapse recordings support the conclusion that primary vascular networks substantially influence the distribution and migratory behavior of NCCs and the patterned formation of dorsal root and sympathetic ganglia.

Semaphorin 7a exerts pleiotropic effects to promote breast tumor progression

Understanding what drives breast tumor progression is of utmost importance for blocking tumor metastasis; we have identified that semaphorin 7a is a potent driver of ductal carcinoma in situ (DCIS) progression. Semaphorin 7a is a GPI membrane anchored protein that promotes attachment and spreading in multiple cell types. Here we show that increased expression of SEMA7A occurs in a large percentage of breast cancers and is associated with decreased overall and distant metastasis free survival. In both in vitro and in vivo models, shRNA mediated silencing of SEMA7A reveals roles for semaphorin 7a in the promotion of DCIS growth, motility, and invasion as well as lymphangiogenesis in the tumor microenvironment. Our studies also uncover a relationship between COX-2 and semaphorin 7a expression and suggest that semaphorin 7a promotes tumor cell invasion on collagen and lymphangiogenesis via activation of β₁-integrin receptor. Our results suggest that semaphorin 7a, may be novel target for blocking breast tumor progression.

Neurovascular coupling and energy metabolism in the developing brain

In the adult brain, increases in local neural activity are almost always accompanied by increases in local blood flow. However, many functional imaging studies of the newborn and developing human brain have observed patterns of hemodynamic responses that differ from adult responses. Among the proposed mechanisms for the observed variations is that neurovascular coupling itself is still developing in the perinatal brain. Many of the components thought to be involved in actuating and propagating this hemodynamic response are known to still be developing postnatally, including perivascular cells such as astrocytes and pericytes. Both neural and vascular networks expand and are then selectively pruned over the first year of human life. Additionally, the metabolic demands of the newborn brain are still evolving. These changes are highly likely to affect early postnatal neurovascular coupling, and thus may affect functional imaging signals in this age group. This chapter will discuss the literature relating to neurovascular development. Potential effects of normal and aberrant development of neurovascular coupling on the newborn brain will also be explored, as well as ways to effectively utilize imaging techniques that rely on hemodynamic modulation such as fMRI and NIRS in younger populations.

EphA2 silencing promotes growth, migration, and metastasis in salivary adenoid cystic carcinoma: in vitro and in vivo study

EphA2 is associated with tumor growth and distant metastasis in numerous human tumors. Considering the controversial effects of EphA2 in different tumors and the lack of reports in salivary adenoid cystic carcinoma (SACC), we evaluated the effects of EphA2 inhibition by short hairpin RNA on SACC through in vivo and in vitro researches for the first time. Real-time reverse transcriptase-PCR and western blot analysis were conducted to verify the interference effect on SACC cells. Using Cell Counting Kit-8, wound healing, Transwell and Matrigel adhesion assays, we confirm that inhibition of EphA2 promotes the migration, invasion and adhesion ability of SACC cells. In vivo research, we prove that silencing of EphA2 significantly accelerates tumor growth and lung metastasis ability by establishing xenograft models in mice, including subcutaneous inoculation and tail vein injection. In addition, immunostaining of EphA2, E-cadherin and Slug from 40 specimens and in vitro simulation of perineural invasion (PNI) assay imply that suppression of EphA2 partially contribute to epithelial-mesenchymal transition and enhancement of PNI in SACC. In conclusion, all the data suggest that EphA2 may act as a tumor suppressor in SACC progression.

Eph/ephrin signaling in the kidney and lower urinary tract

Development and homeostasis of the highly specialized cell types and tissues that constitute the organs of the urinary system, the kidneys and ureters, the bladder, and the urethra, require the tightly regulated exchange of signals in and between these tissues. Eph/ephrin signaling is a bidirectional signaling pathway that has been functionally implicated in many developmental and homeostatic contexts, most prominently in the vascular and neural system. Expression and knockout analyses have now provided evidence that Eph/ephrin signaling is of crucial relevance for cell and tissue interactions in the urinary system as well. A clear requirement has emerged in the formation of the vesicoureteric junction, in urorectal septation and glomerulogenesis during embryonic development, in maintenance of medullary tubular cells and podocytes in homeostasis, and in podocyte and glomerular injury responses. Deregulation of Eph/ephrin signaling may also contribute to the formation and progression of tumors in the urinary system, most prominently bladder and renal cell carcinoma. While in the embryonic contexts Eph/ephrin signaling regulates adhesion of epithelial cells, in the adult setting, cell-shape changes and cell survival seem to be the primary cellular processes mediated by this signaling module. With progression of the genetic analyses of mice conditionally mutant for compound alleles of Eph receptor and ephrin ligand genes, additional essential functions are likely to arise in the urinary system.

Plexin-D1/Semaphorin 3E pathway may contribute to dysregulation of vascular tone control and defective angiogenesis in systemic sclerosis

Introduction

The vascular and nervous systems have several anatomic and molecular mechanism similarities. Emerging evidence suggests that proteins involved in transmitting axonal guidance cues, including members of class III semaphorin (Sema3) family, play a critical role in blood vessel guidance during physiological and pathological vascular development. Sema3E is a natural antiangiogenic molecule that causes filopodial retraction in endothelial cells, inhibiting cell adhesion by disrupting integrin-mediated adhesive structures. The aim of the present study was to investigate whether in systemic sclerosis (SSc) Plexin-D1/Sema3E axis could be involved in the dysregulation of vascular tone control and angiogenesis.

Methods

Sema3E levels were measured by quantitative colorimetric sandwich ELISA in serum samples from 48 SSc patients, 45 subjects with primary Raynaud's phenomenon (pRP) and 48 age-matched and sex-matched healthy controls. Immunofluorescence staining on skin sections from 14 SSc patients and 12 healthy subjects was performed to evaluate Sema3E and Plexin-D1 expression. Western blotting was used to assess Plexin-D1/Sema3E axis in human SSc and healthy dermal microvascular endothelial cells (SSc-MVECs and H-MVECs, respectively) at basal condition and after stimulation with recombinant human vascular endothelial growth factor (VEGF), SSc and healthy sera. Capillary morphogenesis on Matrigel was performed on H-MVECs treated with healthy, pRP or SSc sera in the presence of Sema3E and Plexin-D1 soluble peptides.

Results

Serum Sema3E levels were significantly higher both in pRP subjects and SSc patients than in controls. In SSc, Sema3E levels were significantly increased in patients with early nailfold videocapillaroscopy (NVC) pattern compared to active/late patterns and pRP, and in patients without digital ulcers versus those with ulcers. In SSc skin, Sema3E expression was strongly increased in the microvascular endothelium. Cultured SSc-MVECs showed higher levels of phosphorylated Plexin-D1 and Sema3E expression than H-MVECs, and stimulation with SSc sera increased phosphorylated Plexin-D1 and Sema3E in H-MVECs. The addition of Sema3E-binding Plexin-D1 soluble peptide significantly attenuated the antiangiogenic effect of SSc sera on H-MVECs.

Conclusions

Our findings suggest that Plexin-D1/Sema3E axis is triggered in SSc endothelium and may have a role in the dysregulation of angiogenesis and vascular tone control by inducing neuro-vascular mechanism alterations clinically evident in particular in the early disease phases.

Neuronal and vascular interactions

The brain, which represents 2% of body mass but consumes 20% of body energy at rest, has a limited capacity to store energy and is therefore highly dependent on oxygen and glucose supply from the blood stream. Normal functioning of neural circuits thus relies on adequate matching between metabolic needs and blood supply. Moreover, not only does the brain need to be densely vascularized, it also requires a tightly controlled environment free of toxins and pathogens to provide the proper chemical composition for synaptic transmission and neuronal function. In this review, we focus on three major factors that ensure optimal brain perfusion and function: the patterning of vascular networks to efficiently deliver blood and nutrients, the function of the blood-brain barrier to maintain brain homeostasis, and the regulation of cerebral blood flow to adequately couple energy supply to neural function.

Nerve growth factor regulates neurolymphatic remodeling during corneal inflammation and resolution

The cellular and physiologic mechanisms that regulate the resolution of inflammation remain poorly defined despite their widespread importance in improving inflammatory disease outcomes. We studied the resolution of two cardinal signs of inflammation–pain and swelling–by investigating molecular mechanisms that regulate neural and lymphatic vessel remodeling during the resolution of corneal inflammation. A mouse model of corneal inflammation and wound recovery was developed to study this process in vivo. Administration of nerve growth factor (NGF) increased pain sensation and inhibited neural remodeling and lymphatic vessel regression processes during wound recovery. A complementary in vivo approach, the corneal micropocket assay, revealed that NGF-laden pellets stimulated lymphangiogenesis and increased protein levels of VEGF-C. Adult human dermal lymphatic endothelial cells did not express canonical NGF receptors TrkA and p75^NTR or activate downstream MAPK- or Akt-pathway effectors in the presence of NGF, although NGF treatment increased their migratory and tubulogenesis capacities in vitro. Blockade of the VEGF-R2/R3 signaling pathway ablated NGF-mediated lymphangiogenesis in vivo. These findings suggest a hierarchical relationship with NGF functioning upstream of the VEGF family members, particularly VEGF-C, to stimulate lymphangiogenesis. Taken together, these studies show that NGF stimulates lymphangiogenesis and that NGF may act as a pathogenic factor that negatively regulates the normal neural and lymphatic vascular remodeling events that accompany wound recovery.

Sensory-related neural activity regulates the structure of vascular networks in the cerebral cortex

Neurovascular interactions are essential for proper brain function. While the effect of neural activity on cerebral blood flow has been extensively studied, whether or not neural activity influences vascular patterning remains elusive. Here, we demonstrate that neural activity promotes the formation of vascular networks in the early postnatal mouse barrel cortex. Using a combination of genetics, imaging, and computational tools to allow simultaneous analysis of neuronal and vascular components, we found that vascular density and branching were decreased in the barrel cortex when sensory input was reduced by either a complete deafferentation, a genetic impairment of neurotransmitter release at thalamocortical synapses, or a selective reduction of sensory-related neural activity by whisker plucking. In contrast, enhancement of neural activity by whisker stimulation led to an increase in vascular density and branching. The finding that neural activity is necessary and sufficient to trigger alterations of vascular networks reveals an important feature of neurovascular interactions.

Patterning the renal vascular bed

The renal vascular bed has a stereotypic architecture that is essential for the kidney's role in excreting metabolic waste and regulating the volume and composition of body fluids. The kidney's excretory functions are dependent on the delivery of the majority of renal blood flow to the glomerular capillaries, which filter plasma removing from it metabolic waste, as well as vast quantities of solutes and fluids. The renal tubules reabsorb from the glomerular filtrate solutes and fluids required for homeostasis, while the post-glomerular capillary beds return these essential substances back into the systemic circulation. Thus, the kidney's regulatory functions are dependent on the close proximity or alignment of the post-glomerular capillary beds with the renal tubules. This review will focus on our current knowledge of the mechanisms controlling the embryonic development of the renal vasculature. An understanding of this process is critical for developing novel therapies to prevent vessel rarefaction and will be essential for engineering renal tissues suitable for restoring kidney function to the ever-increasing population of patients with end stage renal disease.

Vascular endothelial cells promote cortical neurite outgrowth via an integrin β3-dependent mechanism

The interaction of neurons with their non-neuronal milieu plays a crucial role in the formation of neural networks, and wide variety of cell-contact-dependent signals that promote neurite elongation have been identified. In this study, we found that vascular endothelial cells promote neurite elongation in an integrin β3-dependent manner. Vascular endothelial cells from the cerebral cortex promoted neurite elongation of cortical neurons in a cell contact-dependent manner. This effect was mediated by arginine-glycine-aspartic acid (RGD), a major recognition sequence for integrins. Pharmacological blockade of integrin β3 abolished the neurite elongation effect induced by the endothelial cells. Immunocytochemical analysis revealed that integrin β3 was expressed on cultured cortical neurons. These results demonstrate that the neurite elongation promoted by vascular endothelial cells requires integrin β3. Vascular endothelial cells may therefore play a role in the development and repair of neural networks in the central nervous system.

Peripheral changes in endometriosis-associated pain

BACKGROUND

Pain remains the cardinal symptom of endometriosis. However, to date, the underlying mechanisms are still only poorly understood. Increasing evidence points towards a close interaction between peripheral nerves, the peritoneal environment and the central nervous system in pain generation and processing. Recently, studies demonstrating nerve fibres and neurotrophic and angiogenic factors in endometriotic lesions and their vicinity have led to increased interest in peripheral changes in endometriosis-associated pain. This review focuses on the origin and function of these nerves and factors as well as possible peripheral mechanisms that may contribute to the generation and modulation of pain in women with endometriosis.

METHODS

We conducted a systematic search using several databases (PubMed, MEDLINE, EMBASE and CINAHL) of publications from January 1977 to October 2013 to evaluate the possible roles of the peripheral nervous system in endometriosis pathophysiology and how it can contribute to endometriosis-associated pain.

RESULTS

Endometriotic lesions and peritoneal fluid from women with endometriosis had pronounced neuroangiogenic properties with increased expression of new nerve fibres, a shift in the distribution of sensory and autonomic fibres in some locations, and up-regulation of several neurotrophins. In women suffering from deep infiltrating endometriosis and bowel endometriosis, in which the anatomical distribution of lesions is generally more closely related to pelvic pain symptoms, endometriotic lesions and surrounding tissues present higher nerve fibre densities compared with peritoneal lesions and endometriomas. More data are needed to fully confirm a direct correlation between fibre density in these locations and the amount of perceived pain. A better correlation between the presence of nerve fibres and pain symptoms seems to exist for eutopic endometrium. However, this appears not to be exclusive to endometriosis. No correlation between elevated neurotrophin levels and pain severity appears to exist, suggesting the involvement of other mediators in the modulation of pain.

CONCLUSIONS

The increased expression of neurotrophic factors and nerve fibres in endometriotic lesions, eutopic endometrium and the peritoneum imply a role of such peripheral changes in the pathogenesis of endometriosis-associated pain. However, a clear link between these findings and pain in patients with endometriosis has so far not been demonstrated.

An image-based RNAi screen identifies SH3BP1 as a key effector of Semaphorin 3E-PlexinD1 signaling

Extracellular signals have to be precisely interpreted intracellularly and translated into diverse cellular behaviors often mediated by cytoskeletal changes. Semaphorins are one of the largest families of guidance cues and play a critical role in many systems. However, how different cell types translate extracellular semaphorin binding into intracellular signaling remains unclear. Here we developed and performed a novel image-based genome-wide functional RNAi screen for downstream signaling molecules that convert the interaction between Semaphorin 3E (Sema3E) and PlexinD1 into cellular behaviors. One of the genes identified in this screen is a RhoGAP protein, SH3-domain binding protein 1 (SH3BP1). We demonstrate that SH3BP1 mediates Sema3E-induced cell collapse through interaction with PlexinD1 and regulation of Ras-related C3 botulinum toxin substrate 1 (Rac1) activity. The identification and characterization of SH3BP1 as a novel downstream effector of Sema3E-PlexinD1 provides an explanation for how extracellular signals are translated into cytoskeletal changes and unique cell behavior, but also lays the foundation for characterizing other genes identified from our screen to obtain a more complete picture of plexin signaling.

Distinct cytoplasmic domains in Plexin-A4 mediate diverse responses to semaphorin 3A in developing mammalian neurons

Guidance receptor signaling is crucial for neural circuit formation and elicits diverse cellular events in specific neurons. We found that signaling from the guidance cue semaphorin 3A diverged through distinct cytoplasmic domains in its receptor Plexin-A4 to promote disparate cellular behavior in different neuronal cell types. Plexin-A4 has three main cytoplasmic domains--C1, Hinge/RBD, and C2--and interacts with family members of the Rho guanine nucleotide exchange factor FARP proteins. We show that growth cone collapse occurred in Plexin-A4-deficient dorsal root ganglion sensory neurons reconstituted with Plexin-A4 containing either the Hinge/RBD or C2 domain, whereas both of the Hinge/RBD and C1 domains were required for dendritic arborization in cortical neurons. Although knockdown studies indicated that both the collapse and arborization responses involved FARP2, mutations in the cytoplasmic region of Plexin-A4 that reduced its interaction with FARP2 strongly inhibited semaphorin 3A-induced dendritic branching but not growth cone collapse, suggesting that different degrees of interaction are required for the two responses or that developing axons have an indirect path to FARP2 activation. Thus, our study provided insights into the multifunctionality of guidance receptors, in particular showing that the semaphorin 3A signal diverges through specific functions of the modular domains of Plexin-A4.

Establishment of neurovascular congruency in the mouse whisker system by an independent patterning mechanism

Nerves and vessels often run parallel to one another, a phenomenon that reflects their functional interdependency. Previous studies have suggested that neurovascular congruency in planar tissues such as skin is established through a "one-patterns-the-other" model, in which either the nervous system or the vascular system precedes developmentally and then instructs the other system to form using its established architecture as a template. Here, we find that, in tissues with complex three-dimensional structures such as the mouse whisker system, neurovascular congruency does not follow the previous model but rather is established via a mechanism in which nerves and vessels are patterned independently. Given the diversity of neurovascular structures in different tissues, guidance signals emanating from a central organizer in the specific target tissue may act as an important mechanism to establish neurovascular congruency patterns that facilitate unique target tissue function.

Neuron-derived semaphorin 3A is an early inducer of vascular permeability in diabetic retinopathy via neuropilin-1

The deterioration of the inner blood-retinal barrier and consequent macular edema is a cardinal manifestation of diabetic retinopathy (DR) and the clinical feature most closely associated with loss of sight. We provide evidence from both human and animal studies for the critical role of the classical neuronal guidance cue, semaphorin 3A, in instigating pathological vascular permeability in diabetic retinas via its cognate receptor neuropilin-1. We reveal that semaphorin 3A is induced in early hyperglycemic phases of diabetes within the neuronal retina and precipitates initial breakdown of endothelial barrier function. We demonstrate, by a series of orthogonal approaches, that neutralization of semaphorin 3A efficiently prevents diabetes-induced retinal vascular leakage in a stage of the disease when vascular endothelial growth factor neutralization is inefficient. These observations were corroborated in Tg(Cre-Esr1)/Nrp1(flox/flox) conditional knockout mice. Our findings identify a therapeutic target for macular edema and provide further evidence for neurovascular crosstalk in the pathogenesis of DR.

Immunostaining of pulpal nerve fibre bundle/arteriole associations in ground serial sections of whole human teeth embedded in technovit® 9100

A technique for embedding human undecalcified tooth specimens in Technovit® 9100 was developed, which permits immunohistological evaluation of pulp tissue in serial ground sections. Human molars were divided into 14-18 sections of about 23 µm thickness. Immunohistological double staining for S-100 and CD34 revealed unique associations of myelinated nerve fibre bundles with arterioles, which continued through the entire tooth pulp. These arterioles were not only accompanied by, but partially or totally enveloped in longitudinally orientated myelinated nerve fibre bundles. We speculate that this unique arrangement may mechanically support the arterioles and alleviate detection or regulation of their contraction state by sensory nerve cells.

Navigation rules for vessels and neurons: cooperative signaling between VEGF and neural guidance cues

Many organs, such as lungs, nerves, blood and lymphatic vessels, consist of complex networks that carry flows of information, gases, and nutrients within the body. The morphogenetic patterning that generates these organs involves the coordinated action of developmental signaling cues that guide migration of specialized cells. Precision guidance of endothelial tip cells by vascular endothelial growth factors (VEGFs) is well established, and several families of neural guidance molecules have been identified to exert guidance function in both the nervous and the vascular systems. This review discusses recent advances in VEGF research, focusing on the emerging role of neural guidance molecules as key regulators of VEGF function during vascular development and on the novel role of VEGFs in neural cell migration and nerve wiring.

The role and mechanism-of-action of Sema3E and Plexin-D1 in vascular and neural development

Class 3 secreted semaphorins (Sema3A-3G) participate in many aspects of axon guidance through holoreceptor complexes that include Neuropilin-1 (Npn-1) or Neuropilin-2 and one of the four class A plexin proteins. However, unlike other Sema3 family proteins, Sema3E directly binds to Plexin-D1 without neuropilins. Its biological function was first explored in intersomitic vessel formation and since its initial discovery, Sema3E-Plexin-D1 signaling has been found to participate in the many biological systems in addition to vascular development, via seemingly different mode of actions. For example, temporal and spatial control of ligand vs. receptor results in two different mechanisms governing vascular patterning. Interactions with other transmembrane proteins such as neuropilin and VEGFR2 result in different axonal behaviors. Ligand receptor localization on pre- vs. post-synaptic neurons is used to control different types of synapse formation. Perhaps different downstream effectors will also result in different functional outcomes. Given the limited number of ligands and receptors in the genome and their multifunctional nature, we expect that more modes of action will be discovered in the future. In this review, we highlight current advances on the mechanisms of how Sema3E-Plexin-D1 interaction shapes the networks of multiple biological systems, in particular the vascular and nervous systems.

Plexin structures are coming: opportunities for multilevel investigations of semaphorin guidance receptors, their cell signaling mechanisms, and functions

Plexin transmembrane receptors and their semaphorin ligands, as well as their co-receptors (Neuropilin, Integrin, VEGFR2, ErbB2, and Met kinase) are emerging as key regulatory proteins in a wide variety of developmental, regenerative, but also pathological processes. The diverse arenas of plexin function are surveyed, including roles in the nervous, cardiovascular, bone and skeletal, and immune systems. Such different settings require considerable specificity among the plexin and semaphorin family members which in turn are accompanied by a variety of cell signaling networks. Underlying the latter are the mechanistic details of the interactions and catalytic events at the molecular level. Very recently, dramatic progress has been made in solving the structures of plexins and of their complexes with associated proteins. This molecular level information is now suggesting detailed mechanisms for the function of both the extracellular as well as the intracellular plexin regions. Specifically, several groups have solved structures for extracellular domains for plexin-A2, -B1, and -C1, many in complex with semaphorin ligands. On the intracellular side, the role of small Rho GTPases has been of particular interest. These directly associate with plexin and stimulate a GTPase activating (GAP) function in the plexin catalytic domain to downregulate Ras GTPases. Structures for the Rho GTPase binding domains have been presented for several plexins, some with Rnd1 bound. The entire intracellular domain structure of plexin-A1, -A3, and -B1 have also been solved alone and in complex with Rac1. However, key aspects of the interplay between GTPases and plexins remain far from clear. The structural information is helping the plexin field to focus on key questions at the protein structural, cellular, as well as organism level that collaboratoria of investigations are likely to answer.

Semaphorin 6A regulates angiogenesis by modulating VEGF signaling

Formation of new vessels during development and in the mature mammal generally proceeds through angiogenesis. Although a variety of molecules and signaling pathways are known to underlie endothelial cell sprouting and remodeling during angiogenesis, many aspects of this complex process remain unexplained. Here we show that the transmembrane semaphorin6A (Sema6A) is expressed in endothelial cells, and regulates endothelial cell survival and growth by modulating the expression and signaling of VEGFR2, which is known to maintain endothelial cell viability by autocrine VEGFR signaling. The silencing of Sema6A in primary endothelial cells promotes cell death that is not rescued by exogenous VEGF-A or FGF2, attributable to the loss of prosurvival signaling from endogenous VEGF. Analyses of mouse tissues demonstrate that Sema6A is expressed in angiogenic and remodeling vessels. Mice with null mutations of Sema6A exhibit significant defects in hyaloid vessels complexity associated with increased endothelial cell death, and in retinal vessels development that is abnormally reduced. Adult Sema6A-null mice exhibit reduced tumor, matrigel, and choroidal angiogenesis compared with controls. Sema6A plays important roles in development of the nervous system. Here we show that it also regulates vascular development and adult angiogenesis.

Eyeing central neurons in vascular growth and reparative angiogenesis

The generation of blood vessels is a highly synchronized process requiring the coordinated efforts of several vascular and nonvascular cell populations as well as a stringent orchestration by the tissue being vascularized. Stereotyped angiogenesis is vital for both developmental growth and to restore tissue metabolic supply after ischemic events. Central neurons such as those found in the brain, spinal cord, and retina are vast consumers of oxygen and nutrients and therefore require high rates of perfusion by functional vascular networks to ensure proper sensory transmission. During a metabolic mismatch, such as that occurring during a cerebrovascular infarct or in ischemic retinopathies, there is increasing evidence that central neurons have an inherent ability to influence the vascular response to injury. With a focus on the retina and retinal ischemic disorders, this review explores the ever-growing evidence suggesting that central neurons have the propensity to impact tissue vascularization and reparative angiogenesis. Moreover, it addresses the paradoxical ability of severely ischemic neurons to hinder vascular regrowth and thus segregate the most severely injured zones of nervous tissue. The topics covered here are pertinent for future therapeutic strategies because promoting and steering vascular growth may be beneficial for ischemic disorders.

The hypothalamic neuropeptide oxytocin is required for formation of the neurovascular interface of the pituitary

The hypothalamo-neurohypophyseal system (HNS) is the neurovascular structure through which the hypothalamic neuropeptides oxytocin and arginine-vasopressin exit the brain into the bloodstream, where they go on to affect peripheral physiology. Here, we investigate the molecular cues that regulate the neurovascular contact between hypothalamic axons and neurohypophyseal capillaries of the zebrafish. We developed a transgenic system in which both hypothalamic axons and neurohypophyseal vasculature can be analyzed in vivo. We identified the cellular organization of the zebrafish HNS as well as the dynamic processes that contribute to formation of the HNS neurovascular interface. We show that formation of this interface is regulated during development by local release of oxytocin, which affects endothelial morphogenesis. This cell communication process is essential for the establishment of a tight axovasal interface between the neurons and blood vessels of the HNS. We present a unique example of axons affecting endothelial morphogenesis through secretion of a neuropeptide.

Semaphorin signaling in angiogenesis, lymphangiogenesis and cancer

Angiogenesis, the formation of new blood vessels from preexisting vasculature, is essential for many physiological processes, and aberrant angiogenesis contributes to some of the most prevalent human diseases, including cancer. Angiogenesis is controlled by delicate balance between pro- and anti-angiogenic signals. While pro-angiogenic signaling has been extensively investigated, how developmentally regulated, naturally occurring anti-angiogenic molecules prevent the excessive growth of vascular and lymphatic vessels is still poorly understood. In this review, we summarize the current knowledge on how semaphorins and their receptors, plexins and neuropilins, control normal and pathological angiogenesis, with an emphasis on semaphorin-regulated anti-angiogenic signaling circuitries in vascular and lymphatic endothelial cells. This emerging body of information may afford the opportunity to develop novel anti-angiogenic therapeutic strategies.

Semaphorin 3E-Plexin-D1 signaling regulates VEGF function in developmental angiogenesis via a feedback mechanism

Blood vessel networks are typically formed by angiogenesis, a process in which new vessels form by sprouting of endothelial cells from pre-existing vessels. This process is initiated by vascular endothelial growth factor (VEGF)-mediated tip cell selection and subsequent angiogenic sprouting. Surprisingly, we found that VEGF directly controls the expression of Plexin-D1, the receptor for the traditional repulsive axon guidance cue, semaphorin 3E (Sema3E). Sema3E-Plexin-D1 signaling then negatively regulates the activity of the VEGF-induced Delta-like 4 (Dll4)-Notch signaling pathway, which controls the cell fate decision between tip and stalk cells. Using the mouse retina as a model system, we show that Plexin-D1 is selectively expressed in endothelial cells at the front of actively sprouting blood vessels and its expression is tightly controlled by VEGF secreted by surrounding tissues. Therefore, although the Sema3E secreted by retinal neurons is evenly distributed throughout the retina, Sema3E-Plexin-D1 signaling is spatially controlled by VEGF through its regulation of Plexin-D1. Moreover, we show that gain and loss of function of Sema3E and Plexin-D1 disrupts normal Dll4 expression, Notch activity, and tip/stalk cell distribution in the retinal vasculature. Finally, the retinal vasculature of mice lacking sema3E or plexin-D1 has an uneven growing front, a less-branched vascular network, and abnormal distribution of dll4-positive cells. Lowering Notch activity in the mutant mice can reverse this defect, solidifying the observation that Dll4-Notch signaling is regulated by Sema3E-Plexin-D1 and is required for its function in vivo. Together, these data reveal a novel role of Sema3E-Plexin-D1 function in modulating angiogenesis via a VEGF-induced feedback mechanism.

Pancreas organogenesis: from bud to plexus to gland

Pancreas oganogenesis comprises a coordinated and highly complex interplay of signaling events and transcriptional networks that guide a step-wise process of organ development from early bud specification all the way to the final mature organ state. Extensive research on pancreas development over the last few years, largely driven by a translational potential for pancreatic diseases (diabetes, pancreatic cancer, and so on), is markedly advancing our knowledge of these processes. It is a tenable goal that we will one day have a clear, complete picture of the transcriptional and signaling codes that control the entire organogenetic process, allowing us to apply this knowledge in a therapeutic context, by generating replacement cells in vitro, or perhaps one day to the whole organ in vivo. This review summarizes findings in the past 5 years that we feel are amongst the most significant in contributing to the deeper understanding of pancreas development. Rather than try to cover all aspects comprehensively, we have chosen to highlight interesting new concepts, and to discuss provocatively some of the more controversial findings or proposals. At the end of the review, we include a perspective section on how the whole pancreas differentiation process might be able to be unwound in a regulated fashion, or redirected, and suggest linkages to the possible reprogramming of other pancreatic cell-types in vivo, and to the optimization of the forward-directed-differentiation of human embryonic stem cells (hESC), or induced pluripotential cells (iPSC), towards mature β-cells.

Robo1 regulates semaphorin signaling to guide the migration of cortical interneurons through the ventral forebrain

Cortical interneurons, generated predominantly in the medial ganglionic eminence, migrate around and avoid the developing striatum in the subpallium en route to the cortex. This is attributable to the chemorepulsive cues of class 3 semaphorins expressed in the striatal mantle and acting through neuropilin (Nrp1 and Nrp2) receptors expressed in these cells. Cortical interneurons also express Robo receptors, and we show here that in mice lacking Robo1, but not Robo2, these cells migrate aberrantly through the striatum. In vitro experiments demonstrated that interneurons lacking Robo1 function are significantly less responsive to the effects of semaphorins. Failure to respond to semaphorin appears to be attributable to a reduction in Nrp1 and PlexinA1 receptors within these cells. Biochemical studies further demonstrated that Robo1 binds directly to Nrp1, but not to semaphorins, and this interaction is mediated by a region contained within its first two Ig domains. Thus, we show for the first time that Robo1 interacts with Nrp1 to modulate semaphorin signaling in the developing forebrain and direct the migration of interneurons through the subpallium and into the cortex.

Cell-specific information processing in segregating populations of Eph receptor ephrin-expressing cells

Cells have self-organizing properties that control their behavior in complex tissues. Contact between cells expressing either B-type Eph receptors or their transmembrane ephrin ligands initiates bidirectional signals that regulate cell positioning. However, simultaneously investigating how information is processed in two interacting cell types remains a challenge. We implemented a proteomic strategy to systematically determine cell-specific signaling networks underlying EphB2- and ephrin-B1-controlled cell sorting. Quantitative mass spectrometric analysis of mixed populations of EphB2- and ephrin-B1-expressing cells that were labeled with different isotopes revealed cell-specific tyrosine phosphorylation events. Functional associations between these phosphotyrosine signaling networks and cell sorting were established with small interfering RNA screening. Data-driven network modeling revealed that signaling between mixed EphB2- and ephrin-B1-expressing cells is asymmetric and that the distinct cell types use different tyrosine kinases and targets to process signals induced by cell-cell contact. We provide systems- and cell-specific network models of contact-initiated signaling between two distinct cell types.

Differential gene expression in the developing human macula: microarray analysis using rare tissue samples

The macula is a unique and important region in the primate retina that achieves high resolution and color vision in the central visual field. We recently reported data obtained from microarray analysis of gene expression in the macula of the human fetal retina (Kozulin et al., Mol Vis 15:45–59, 1). In this paper, we describe the preliminary analyses undertaken to visualize differences and verify comparability of the replicates used in that study, report the differential expression of other gene families obtained from the analysis, and show the reproducibility of our findings in several gene families by quantitative real-time PCR.