Inhibition of gap junction communication at ectopic Eph/ephrin boundaries underlies craniofrontonasal syndrome

Bradykinin receptors and EphB2/EphrinB2 pathway in response to high glucose-induced osteoblast dysfunction and hyperglycemia-induced bone deterioration in mice

This study was carried out in order to investigate bone dysfunction and the involvement of bradykinin receptors and the Eph/Ephrin signaling pathway in osteoblasts and in mice with diabetes-related osteoporosis in response to exposure to high glucose. Osteogenic transdifferentiation was inhibited when the osteoblasts were exposed to high glucose, and the expression levels of bone formation-related genes [Runx2 and alkaline phosphatase (ALP)] were decreased, while those of bone resorption-related genes [matrix metalloproteinase (MMP)9 and carbonic anhydrase II (CAII)] were increased. Moreover, the mRNA and protein expression levels of bradykinin receptor B1 (BK1R)/bradykinin receptor B2 (BK2R) and EphB2/EphrinB2 were significantly decreased in the osteoblasts following exposure to high glucose. Intriguingly, the interaction between BK2R and EphB2/EphrinB2 was confirmed, and BK2R loss-of-function significantly decreased the mRNA and protein expression levels of EphB2/EphrinB4. In vivo, hyperglycemia induced the disequilibrium of calcium homeostasis through the inhibition of bone formation and the acceleration of bone resorption, which was manifested by the reduction of trabecular bone mass of the primary and secondary spongiosa, as well as by the increase in the number of mature osteoclasts throughout the proximal tibial metaphysis in mice with diabetes-related osteoporosis. Furthermore, the mRNA and protein expression levels of BK1R/BK2R and EphB2/EphrinB2 in the tibias of the mice with diabetes-related osteoporosis were significantly decreased. These results demonstrate that bradykinin receptors and the EphB4/EphrinB2 pathway mediate the development of complications in mice with diabetes-related osteoporosis and suggest that the inactivation of bradykinin receptors and the EphB4/EphrinB2 pathway enhance the severity of complications in mice with diabetes-related osteoporosis.

Molecular bioelectricity: how endogenous voltage potentials control cell behavior and instruct pattern regulation in vivo

In addition to biochemical gradients and transcriptional networks, cell behavior is regulated by endogenous bioelectrical cues originating in the activity of ion channels and pumps, operating in a wide variety of cell types. Instructive signals mediated by changes in resting potential control proliferation, differentiation, cell shape, and apoptosis of stem, progenitor, and somatic cells. Of importance, however, cells are regulated not only by their own V_mem but also by the V_mem of their neighbors, forming networks via electrical synapses known as gap junctions. Spatiotemporal changes in V_mem distribution among nonneural somatic tissues regulate pattern formation and serve as signals that trigger limb regeneration, induce eye formation, set polarity of whole-body anatomical axes, and orchestrate craniofacial patterning. New tools for tracking and functionally altering V_mem gradients in vivo have identified novel roles for bioelectrical signaling and revealed the molecular pathways by which V_mem changes are transduced into cascades of downstream gene expression. Because channels and gap junctions are gated posttranslationally, bioelectrical networks have their own characteristic dynamics that do not reduce to molecular profiling of channel expression (although they couple functionally to transcriptional networks). The recent data provide an exciting opportunity to crack the bioelectric code, and learn to program cellular activity at the level of organs, not only cell types. The understanding of how patterning information is encoded in bioelectrical networks, which may require concepts from computational neuroscience, will have transformative implications for embryogenesis, regeneration, cancer, and synthetic bioengineering.

Endothelial ephrin-B2 is essential for arterial vasodilation in mice

OBJECTIVE

The cell surface protein ephrin-B2 is expressed in arterial and not venous ECs throughout development and adulthood. Endothelial ephrin-B2 is required for vascular development and angiogenesis, but its role in established arteries is currently unknown. We investigated the physiological role of ephrin-B2 signaling in adult endothelium.

METHODS

We generated adult conditional knockout mice lacking the Efnb2 gene specifically in ECs and evaluated the vasodilation responses to blood flow increase and ACh in the cremaster muscle preparation by intravital microscope and in carotid artery by in vivo ultrasound.

RESULTS

We found that the Efnb2 conditional knockout mice were defective in acute arterial dilation. Vasodilation was impaired in cremaster arterioles in response to either increased flow or ACh, and in the carotid arteries in response to increased flow. Levels of cGMP, an effector of NO, were diminished in mutant arteries following ACh stimulation. GSNO, a donor for the vasodilator NO, alleviated the vasodilatory defects in the mutants. Immunostaining showed that a subset of ephrin-B2 proteins colocalized with caveolin-1, a negative regulator of eNOS.

CONCLUSIONS

Our data suggest that endothelial ephrin-B2 is required for endothelial-dependent arterial dilation and NO signaling in adult endothelium.

Early molecular responses of bone to estrogen deficiency induced by ovariectomy in rats

OBJECTIVE

The study was performed to investigate bone deteriorations and the molecular responses of bone to early estrogen deficiency induced by ovariectomy (OVX) in rats.

METHODS

The female rats were subjected to OVX (4 or 8 week) and sham (4 or 8 week) operation. All rats were killed 4 week or 8 week after the surgical operation. The biomarkers in serum and urine were measured. Hematoxylin & Eosin and tartate-resistant acid phosphatase staining were performed on paraffin-embedded bone sections. Expression of genes and proteins were analyzed by reverse transcription polymerase chain reaction and western blotting respectively.

RESULTS

The OVX rats showed the decreased level of serum Ca and the increased level of urinary Ca excretion at 8 week post-OVX. The level of PTH and TRACP-5b increased at 4 and 8 week post-OVX. At both 4 and 8 week, FGF-23 was significantly lower in OVX rats than sham rats. The H&E staining showed remarkable bone abnormalities, including increased disconnections and separation of trabecular bone network in proximal metaphysis of tibia at OVX (4 and 8 week) group. In addition, the mRNA expression ratio of OPG/RANKL was reduced in the proximal tibia. The mRNA expression of MMP-9, CAII, EphA2 and ephrinA2, and the protein expression of EphA2 and ephrinA2 were markedly up-regulated in the proximal tibia. Moreover, the mRNA expression of TGF-β, EphB4 and ephrinB2, and the protein expression of EphB4 and ephrinB2 were down-regulated in proximal metaphysis of tibia at OVX group.

CONCLUSIONS

The endogenous estrogen deficiency was detrimental to bone, and the underlying mechanism was mediated, at least partially, through the local bone Eph/ephrin signaling pathway.

EphB/ephrinB signaling in cell adhesion and migration

Eph receptors and their ligands, ephrins, represent the largest group of the receptor tyrosine kinase (RTK) family, and they mediate numerous developmental processes in a variety of organisms. Ephrins are membrane-bound proteins that are mainly divided into two classes: A class ephrins, which are linked to the membrane by a glycosylphosphatidylinositol (GPI) linkage, and B class ephrins, which are transmembrane ligands. Based on their domain structures and affinities for ligand binding, the Eph receptors are also divided into two groups. Trans-dimerization of Eph receptors with their membrane-tethered ligands regulates cell-cell interactions and initiates bidirectional signaling pathways. These pathways are intimately involved in regulating cytoskeleton dynamics, cell migration, and alterations in cellular dynamics and shapes. The EphBs and ephrinBs are specifically localized and modified to promote higher-order clustering and initiate of bidirectional signaling. In this review, we present an in-depth overview of the structure, mechanisms, cell signaling, and functions of EphB/ephrinB in cell adhesion and migration.

Receptor tyrosine kinase signaling: regulating neural crest development one phosphate at a time

Receptor tyrosine kinases (RTKs) bind to a subset of growth factors on the surface of cells and elicit responses with broad roles in developmental and postnatal cellular processes. Receptors in this subclass consist of an extracellular ligand-binding domain, a single transmembrane domain, and an intracellular domain harboring a catalytic tyrosine kinase and regulatory sequences that are phosphorylated either by the receptor itself or by various interacting proteins. Once activated, RTKs bind signaling molecules and recruit effector proteins to mediate downstream cellular responses through various intracellular signaling pathways. In this chapter, we highlight the role of a subset of RTK families in regulating the activity of neural crest cells (NCCs) and the development of their derivatives in mammalian systems. NCCs are migratory, multipotent cells that can be subdivided into four axial populations, cranial, cardiac, vagal, and trunk. These cells migrate throughout the vertebrate embryo along defined pathways and give rise to unique cell types and structures. Interestingly, individual RTK families often have specific functions in a subpopulation of NCCs that contribute to the diversity of these cells and their derivatives in the mammalian embryo. We additionally discuss current methods used to investigate RTK signaling, including genetic, biochemical, large-scale proteomic, and biosensor approaches, which can be applied to study intracellular signaling pathways active downstream of this receptor subclass during NCC development.

Genistein attenuates glucocorticoid-induced bone deleterious effects through regulation Eph/ephrin expression in aged mice

OBJECTIVE

This study was performed to investigate bone deteriorations and the involvement of skeletal Eph/ephrin signaling pathway of GIOP aged mice in response to the treatment of genistein.

METHODS

The biomarkers in serum and urine were measured, tibias were taken for the measurement on gene and protein expression and histomorphology analysis, and femurs were taken for the measurement on bone Ca and three-dimensional architecture of trabecular bone.

RESULTS

Genistein showed a greater increase in bone Ca, BMD and significantly increased FGF-23 and OCN, reduced TRACP-5b, PTH and CTX in GIOP mice. Genistein reversed DXM-induced trabecular deleterious effects and stimulated bone remodeling. The treatment of DXM group with genistein significantly elevated the ratio of OPG/RANKL. Moreover, genistein administration down-regulated the mRNA and protein expression of Eph A2 and ephrin A2 in tibia of the GIOP mice. In contrast, the mRNA and protein expression of Eph B4 and ephrin B2 were increased in mice treated by DXM with genistein as compared to the DXM single treatment.

CONCLUSIONS

DXM-induced trabecular bone micro-structure deterioration in aged mice was involved in the regulation of the Eph receptors and ephrin ligands. Genistein might represent a therapy with bone-forming as well as an anti-resorptive activity in GIOP mice. The underlying mechanism was mediated, at least partially, through regulation Eph/ephrin signaling.

Schlemm's canal is a unique vessel with a combination of blood vascular and lymphatic phenotypes that forms by a novel developmental process

A draining vessel in the eye arises via a novel hybrid process of vascular development and is important for understanding ocular fluid homeostasis and glaucoma.

Schlemm's canal (SC) plays central roles in ocular physiology. These roles depend on the molecular phenotypes of SC endothelial cells (SECs). Both the specific phenotype of SECs and development of SC remain poorly defined. To allow a modern and extensive analysis of SC and its origins, we developed a new whole-mount procedure to visualize its development in the context of surrounding tissues. We then applied genetic lineage tracing, specific-fluorescent reporter genes, immunofluorescence, high-resolution confocal microscopy, and three-dimensional (3D) rendering to study SC. Using these techniques, we show that SECs have a unique phenotype that is a blend of both blood and lymphatic endothelial cell phenotypes. By analyzing whole mounts of postnatal mouse eyes progressively to adulthood, we show that SC develops from blood vessels through a newly discovered process that we name “canalogenesis.” Functional inhibition of KDR (VEGFR2), a critical receptor in initiating angiogenesis, shows that this receptor is required during canalogenesis. Unlike angiogenesis and similar to stages of vasculogenesis, during canalogenesis tip cells divide and form branched chains prior to vessel formation. Differing from both angiogenesis and vasculogenesis, during canalogenesis SECs express Prox1, a master regulator of lymphangiogenesis and lymphatic phenotypes. Thus, SC development resembles a blend of vascular developmental programs. These advances define SC as a unique vessel with a combination of blood vascular and lymphatic phenotypes. They are important for dissecting its functions that are essential for ocular health and normal vision.

Schlemm's canal serves as a drainage tube for fluid from the anterior chamber of the eye and is directly relevant to glaucoma, a disease that causes vision loss in over 70 million people. Aqueous humor enters the canal and then drains into connected veins. Molecular understanding of the development of Schlemm's canal and its drainage functions has remained limited. We provide a detailed characterization of Schlemm's canal development, and in so doing discover a novel process of vascular development that we name “canalogenesis.” We show that although the process requires a functional KDR receptor, which is also critical in blood vessel development, the endothelial cells of Schlemm's canal have a unique hybrid molecular phenotype, expressing proteins that are characteristic of both blood and lymphatic vessels. Of note, the expression of Prox1, a master regulator of lymphatic fate, and other lymphatic proteins are largely restricted to specialized cells of the inner wall of Schlemm's canal through which the aqueous humor passes as it exits the eye. Thus, Prox1 and other lymphatic proteins may be critical for the functional specialization of these cells for aqueous humor drainage. Schlemm's canal is thus a unique vessel with a combination of blood vascular and lymphatic characteristics.

EphrinB1 interacts with CNK1 and promotes cell migration through c-Jun N-terminal kinase (JNK) activation

The Eph receptors and their membrane-bound ligands, ephrins, play important roles in various biological processes such as cell adhesion and movement. The transmembrane ephrinBs transduce reverse signaling in a tyrosine phosphorylation-dependent or -independent, as well as PDZ-dependent manner. Here, we show that ephrinB1 interacts with Connector Enhancer of KSR1 (CNK1) in an EphB receptor-independent manner. In cultured cells, cotransfection of ephrinB1 with CNK1 increases JNK phosphorylation. EphrinB1/CNK1-mediated JNK activation is reduced by overexpression of dominant-negative RhoA. Overexpression of CNK1 alone is sufficient for activation of RhoA; however, both ephrinB1 and CNK1 are required for JNK phosphorylation. Co-immunoprecipitation data showed that ephrinB1 and CNK1 act as scaffold proteins that connect RhoA and JNK signaling components, such as p115RhoGEF and MKK4. Furthermore, adhesion to fibronectin or active Src overexpression increases ephrinB1/CNK1 binding, whereas blocking Src activity by a pharmacological inhibitor decreases not only ephrinB1/CNK1 binding, but also JNK activation. EphrinB1 overexpression increases cell motility, however, CNK1 depletion by siRNA abrogates ephrinB1-mediated cell migration and JNK activation. Moreover, Rho kinase inhibitor or JNK inhibitor treatment suppresses ephrinB1-mediated cell migration. Taken together, our findings suggest that CNK1 is required for ephrinB1-induced JNK activation and cell migration.

New insights into the role of connexins in pancreatic islet function and diabetes

Multi-cellular systems require complex signaling mechanisms for proper tissue function, to mediate signaling between cells in close proximity and at distances. This holds true for the islets of Langerhans, which are multicellular micro-organs located in the pancreas responsible for glycemic control, through secretion of insulin and other hormones. Coupling of electrical and metabolic signaling between islet β-cells is required for proper insulin secretion and effective glycemic control. β-cell specific coupling is established through gap junctions composed of connexin36, which results in coordinated insulin release across the islet. Islet connexins have been implicated in both Type-1 and Type-2 diabetes; however a clear link remains to be determined. The goal of this review is to discuss recent discoveries regarding the role of connexins in regulating insulin secretion, the regulation of connexins within the islet, and recent studies which support a role for connexins in diabetes. Further studies which investigate the regulation of connexins in the islet and their role in diabetes may lead to novel diabetes therapies which regulate islet function and β-cell survival through modulation of gap junction coupling.

Clinical, genetic and imaging findings identify new causes for corpus callosum development syndromes

The corpus callosum is the largest fibre tract in the brain, connecting the two cerebral hemispheres, and thereby facilitating the integration of motor and sensory information from the two sides of the body as well as influencing higher cognition associated with executive function, social interaction and language. Agenesis of the corpus callosum is a common brain malformation that can occur either in isolation or in association with congenital syndromes. Understanding the causes of this condition will help improve our knowledge of the critical brain developmental mechanisms required for wiring the brain and provide potential avenues for therapies for callosal agenesis or related neurodevelopmental disorders. Improved genetic studies combined with mouse models and neuroimaging have rapidly expanded the diverse collection of copy number variations and single gene mutations associated with callosal agenesis. At the same time, advances in our understanding of the developmental mechanisms involved in corpus callosum formation have provided insights into the possible causes of these disorders. This review provides the first comprehensive classification of the clinical and genetic features of syndromes associated with callosal agenesis, and provides a genetic and developmental framework for the interpretation of future research that will guide the next advances in the field.

Talking among ourselves: paracrine control of bone formation within the osteoblast lineage

While much research focuses on the range of signals detected by the osteoblast lineage that originate from endocrine influences, or from other cells within the body, there are also multiple interactions that occur within this family of cells. Osteoblasts exist as teams and form extensive communication networks both on, and within, the bone matrix. We provide four snapshots of communication pathways that exist within the osteoblast lineage between different stages of their differentiation, as follows: (1) PTHrP, a factor produced by early osteoblasts that stimulates the activity of more mature bone-forming cells and the most mature osteoblast embedded within the bone matrix, the osteocyte; (2) sclerostin, a secreted factor, released by osteocytes into their extensive communication network to restrict the activity of younger osteoblasts on the bone surface; (3) oncostatin M, a member of the IL-6/gp130 family of cytokines, expressed throughout osteoblast differentiation and acting to stimulate osteoblast activity that works on a different receptor in the mature osteocyte compared to the preosteoblast; and (4) Eph/ephrins, cell-contact-dependent kinases, and the osteoblast-lineage-specific interaction of EphB4 and ephrinB2, which provides a checkpoint for entry to the late stages of osteoblast differentiation and restricts RANKL expression.

Phenotypes of craniofrontonasal syndrome in patients with a pathogenic mutation in EFNB1

Craniofrontonasal syndrome (CFNS) is an X-linked developmental malformation, caused by mutations in the EFNB1 gene, which have only been described since 2004. A genotype-phenotype correlation seems not to be present. As it is of major importance to adequately counsel patients with EFNB1 mutations and their parents, and to improve diagnosis of new patients, more information about the phenotypic features is needed. This study included 23 patients (2 male, 21 female) with confirmed EFNB1 mutations. All patients underwent a thorough physical examination and photographs were taken. If available, radiological images were also consulted. Hypertelorism, longitudinal ridging and/or splitting of nails, a (mild) webbed neck and a clinodactyly of one or more toes were the only consistent features observed in all patients. Frequently observed phenotypic features were bifid tip of the nose (91%), columellar indentation (91%) and low implantation of breasts (90%). In comparison with anthropometric data of facial proportions, patients with CFNS had a significantly different face in multiple respects. An overview of all phenotypic features is shown. Patients with EFNB1 mutations have a clear phenotype. This study will facilitate genetic counseling of parents and patients, and contribute to the diagnostic and screening process of patients with suspected CFNS.

Reprogramming cells and tissue patterning via bioelectrical pathways: molecular mechanisms and biomedical opportunities

Transformative impact in regenerative medicine requires more than the reprogramming of individual cells: advances in repair strategies for birth defects or injuries, tumor normalization, and the construction of bioengineered organs and tissues all require the ability to control large-scale anatomical shape. Much recent work has focused on the transcriptional and biochemical regulation of cell behavior and morphogenesis. However, exciting new data reveal that bioelectrical properties of cells and their microenvironment exert a profound influence on cell differentiation, proliferation, and migration. Ion channels and pumps expressed in all cells, not just excitable nerve and muscle, establish resting potentials that vary across tissues and change with significant developmental events. Most importantly, the spatiotemporal gradients of these endogenous transmembrane voltage potentials (Vmem ) serve as instructive patterning cues for large-scale anatomy, providing organ identity, positional information, and prepattern template cues for morphogenesis. New genetic and pharmacological techniques for molecular modulation of bioelectric gradients in vivo have revealed the ability to initiate complex organogenesis, change tissue identity, and trigger regeneration of whole vertebrate appendages. A large segment of the spatial information processing that orchestrates individual cells' programs toward the anatomical needs of the host organism is electrical; this blurs the line between memory and decision-making in neural networks and morphogenesis in nonneural tissues. Advances in cracking this bioelectric code will enable the rational reprogramming of shape in whole tissues and organs, revolutionizing regenerative medicine, developmental biology, and synthetic bioengineering.

Eph receptor signaling and ephrins

The Eph receptors are the largest of the RTK families. Like other RTKs, they transduce signals from the cell exterior to the interior through ligand-induced activation of their kinase domain. However, the Eph receptors also have distinctive features. Instead of binding soluble ligands, they generally mediate contact-dependent cell-cell communication by interacting with surface-associated ligands-the ephrins-on neighboring cells. Eph receptor-ephrin complexes emanate bidirectional signals that affect both receptor- and ephrin-expressing cells. Intriguingly, ephrins can also attenuate signaling by Eph receptors coexpressed in the same cell. Additionally, Eph receptors can modulate cell behavior independently of ephrin binding and kinase activity. The Eph/ephrin system regulates many developmental processes and adult tissue homeostasis. Its abnormal function has been implicated in various diseases, including cancer. Thus, Eph receptors represent promising therapeutic targets. However, more research is needed to better understand the many aspects of their complex biology that remain mysterious.

Transgenic overexpression of ephrin b1 in bone cells promotes bone formation and an anabolic response to mechanical loading in mice

To test if ephrin B1 overexpression enhances bone mass, we generated transgenic mice overexpressing ephrin B1 under the control of a 3.6 kb rat collagen 1A1 promoter (Col3.6-Tg^efnb1). Col3.6-Tg^efnb1 mice express 6-, 12- and 14-fold greater levels of full-length ephrin B1 protein in bone marrow stromal cells, calvarial osteoblasts, and osteoclasts, respectively. The long bones of both genders of Col3.6-Tg^efnb1 mice have increased trabecular bone volume, trabecular number, and trabecular thickness and decreased trabecular separation. Enhanced bone formation and decreased bone resorption contributed to this increase in trabecular bone mass in Col3.6-Tg^efnb1 mice. Consistent with these findings, our in vitro studies showed that overexpression of ephrin B1 increased osteoblast differentiation and mineralization, osterix and collagen 1A1 expression in bone marrow stromal cells. Interaction of ephrin B1 with soluble clustered EphB2-Fc decreased osteoclast precursor differentiation into multinucleated cells. Furthermore, we demonstrated that the mechanical loading-induced increase in EphB2 expression and newly formed bone were significantly greater in the Col3.6-Tg^efnb1 mice than in WT littermate controls. Our findings that overexpression of ephrin B1 in bone cells enhances bone mass and promotes a skeletal anabolic response to mechanical loading suggest that manipulation of ephrin B1 actions in bone may provide a means to sensitize the skeleton to mechanical strain to stimulate new bone formation.

Ephrin B1 maintains apical adhesion of neural progenitors

Apical neural progenitors are polarized cells for which the apical membrane is the site of cell-cell and cell-extracellular matrix adhesion events that are essential for maintaining the integrity of the developing neuroepithelium. Apical adhesion is important for several aspects of the nervous system development, including morphogenesis and neurogenesis, yet the mechanisms underlying its regulation remain poorly understood. Here, we show that ephrin B1, a cell surface protein that engages in cell signaling upon binding cognate Eph receptors, controls normal morphogenesis of the developing cortex. Efnb1-deficient embryos exhibit morphological alterations of the neuroepithelium that correlate with neural tube closure defects. Using loss-of-function experiments by ex vivo electroporation, we demonstrate that ephrin B1 is required in apical progenitors (APs) to maintain their apical adhesion. Mechanistically, we show that ephrin B1 controls cell-ECM adhesion by promoting apical localization of integrin β1 and we identify ADP-ribosylation factor 6 (Arf6) as an important effector of ephrin B1 reverse signaling in apical adhesion of APs. Our results provide evidence for an important role for ephrin B1 in maintaining the structural integrity of the developing cortex and highlight the importance of tightly controlling apical cell-ECM adhesion for neuroepithelial development.

Cellular interference in craniofrontonasal syndrome: males mosaic for mutations in the X-linked EFNB1 gene are more severely affected than true hemizygotes

Craniofrontonasal syndrome (CFNS), an X-linked disorder caused by loss-of-function mutations of EFNB1, exhibits a paradoxical sex reversal in phenotypic severity: females characteristically have frontonasal dysplasia, craniosynostosis and additional minor malformations, but males are usually more mildly affected with hypertelorism as the only feature. X-inactivation is proposed to explain the more severe outcome in heterozygous females, as this leads to functional mosaicism for cells with differing expression of EPHRIN-B1, generating abnormal tissue boundaries—a process that cannot occur in hemizygous males. Apparently challenging this model, males occasionally present with a more severe female-like CFNS phenotype. We hypothesized that such individuals might be mosaic for EFNB1 mutations and investigated this possibility in multiple tissue samples from six sporadically presenting males. Using denaturing high performance liquid chromatography, massively parallel sequencing and multiplex-ligation-dependent probe amplification (MLPA) to increase sensitivity above standard dideoxy sequencing, we identified mosaic mutations of EFNB1 in all cases, comprising three missense changes, two gene deletions and a novel point mutation within the 5′ untranslated region (UTR). Quantification by Pyrosequencing and MLPA demonstrated levels of mutant cells between 15 and 69%. The 5′ UTR variant mutates the stop codon of a small upstream open reading frame that, using a dual-luciferase reporter construct, was demonstrated to exacerbate interference with translation of the wild-type protein. These results demonstrate a more severe outcome in mosaic than in constitutionally deficient males in an X-linked dominant disorder and provide further support for the cellular interference mechanism, normally related to X-inactivation in females.

Absence of venous valves in mice lacking Connexin37

Venous valves play a crucial role in blood circulation, promoting the one-way movement of blood from superficial and deep veins towards the heart. By preventing retrograde flow, venous valves spare capillaries and venules from being subjected to damaging elevations in pressure, especially during skeletal muscle contraction. Pathologically, valvular incompetence or absence of valves are common features of venous disorders such as chronic venous insufficiency and varicose veins. The underlying causes of these conditions are not well understood, but congenital venous valve aplasia or agenesis may play a role in some cases. Despite progress in the study of cardiac and lymphatic valve morphogenesis, the molecular mechanisms controlling the development and maintenance of venous valves remain poorly understood. Here, we show that in valved veins of the mouse, three gap junction proteins (Connexins, Cxs), Cx37, Cx43, and Cx47, are expressed exclusively in the valves in a highly polarized fashion, with Cx43 on the upstream side of the valve leaflet and Cx37 on the downstream side. Surprisingly, Cx43 expression is strongly induced in the non-valve venous endothelium in superficial veins following wounding of the overlying skin. Moreover, we show that in Cx37-deficient mice, venous valves are entirely absent. Thus, Cx37, a protein involved in cell-cell communication, is one of only a few proteins identified so far as critical for the development or maintenance of venous valves. Because Cxs are necessary for the development of valves in lymphatic vessels as well, our results support the notion of common molecular pathways controlling valve development in veins and lymphatic vessels.

Genetic mouse models to study blood-brain barrier development and function

The blood–brain barrier (BBB) is a complex physiological structure formed by the blood vessels of the central nervous system (CNS) that tightly regulates the movement of substances between the blood and the neural tissue. Recently, the generation and analysis of different genetic mouse models has allowed for greater understanding of BBB development, how the barrier is regulated during health, and its response to disease. Here we discuss: 1) Genetic mouse models that have been used to study the BBB, 2) Available mouse genetic tools that can aid in the study of the BBB, and 3) Potential tools that if generated could greatly aid in our understanding of the BBB.

Ephrin-B stimulation of calvarial bone formation

INTRODUCTION

Ephrin-B2 on osteoclasts was reported to promote bone formation as part of homeostasis by activating the EphB4 tyrosine kinase receptor on osteoblasts. Little is known about the role of ephrin-B signaling to EphBs in developmental bone formation.

RESULTS

We observed expression of an ephrin-B2 LacZ chimeric allele in the periosteum, sutural bone fronts, and dura mater of embryonic and neonatal mice. Expression in the adult skull was confined to sutures, but was heavily upregulated at sites of bone injury. Culture of embryonic calvariae with soluble recombinant ephrin-B2/Fc doubled their bone content without altering suture width or overall skull morphology. Ephrin-B2/Fc also stimulated osteoblast marker gene expression in cultured MC3T3 preosteoblastic cells without the need for type 1 collagen-induced differentiation. EphB4 was absent in embryonic and adult skulls. However, EphB1 and EphB2, both physiological receptors for ephrin-Bs, were expressed at sites of osteogenesis, and EphB1 knockout mice displayed a reduction in calvarial bone content compared to controls.

CONCLUSIONS

These data support a role for ephrin-B2 in the development and healing of bone through activation of osteoblast-specific gene expression. EphB1 and EphB2 are likely candidates receptors for the ephrin-B2 in bone.

The Eph/Ephrin family in cancer metastasis: communication at the service of invasion

Cancer cells rely on intercellular communication throughout the different stages of their transformation and progression into metastasis. They do so by co-opting different processes such as cell-cell junctions, growth factors, receptors, and vesicular release. Initially characterized in neuronal and vascular tissues, Ephs and Ephrins, the largest family of receptor tyrosine kinases, comprised of two classes (i.e., A and B types), is increasingly scrutinized by cancer researchers. These proteins possess the particular features of both the receptors and ligands being membrane-bound which, via mandatory direct cell-cell interactions, undergo a bidirectional signal transduction initiated from both the receptor and the ligand. Following cell-cell interactions, Ephs/Ephrins behave as guidance molecules which trigger both repulsive and attractive signals, so as to direct the movement of cells through their immediate microenvironment. They also direct processes which include sorting and positioning and cytoskeleton rearrangements, thus making them perfect candidates for the control of the metastatic process. In fact, the role of Ephs and Ephrins in cancer progression has been demonstrated for many of the family members and they, surprisingly, have both tumor promoter and suppressor functions in different cellular contexts. They are also able to coordinate between multiple processes including cell survival, proliferation, differentiation, adhesion, motility, and invasion. This review is an attempt to summarize the data available on these Ephs/Ephrins' biological functions which contribute to the onset of aggressive cancers. I will also provide an overview of the factors which could explain the functional differences demonstrated by Ephs and Ephrins at different stages of tumor progression and whose elucidation is warranted for any future therapeutic targeting of this signaling pathway in cancer metastasis.

Cell polarity as a regulator of cancer cell behavior plasticity

Cell polarization is an evolutionarily conserved process that facilitates asymmetric distribution of organelles and proteins and that is modified dynamically during physiological processes such as cell division, migration, and morphogenesis. The plasticity with which cells change their behavior and phenotype in response to cell intrinsic and extrinsic cues is an essential feature of normal physiology. In disease states such as cancer, cells lose their ability to behave normally in response to physiological cues. A molecular understanding of mechanisms that alter the behavior of cancer cells is limited. Cell polarity proteins are a recognized class of molecules that can receive and interpret both intrinsic and extrinsic signals to modulate cell behavior. In this review, we discuss how cell polarity proteins regulate a diverse array of biological processes and how they can contribute to alterations in the behavior of cancer cells.

Line-scanning particle image velocimetry: an optical approach for quantifying a wide range of blood flow speeds in live animals

BACKGROUND

The ability to measure blood velocities is critical for studying vascular development, physiology, and pathology. A key challenge is to quantify a wide range of blood velocities in vessels deep within living specimens with concurrent diffraction-limited resolution imaging of vascular cells. Two-photon laser scanning microscopy (TPLSM) has shown tremendous promise in analyzing blood velocities hundreds of micrometers deep in animals with cellular resolution. However, current analysis of TPLSM-based data is limited to the lower range of blood velocities and is not adequate to study faster velocities in many normal or disease conditions.

METHODOLOGY/PRINCIPAL FINDINGS

We developed line-scanning particle image velocimetry (LS-PIV), which used TPLSM data to quantify peak blood velocities up to 84 mm/s in live mice harboring brain arteriovenous malformation, a disease characterized by high flow. With this method, we were able to accurately detect the elevated blood velocities and exaggerated pulsatility along the abnormal vascular network in these animals. LS-PIV robustly analyzed noisy data from vessels as deep as 850 µm below the brain surface. In addition to analyzing in vivo data, we validated the accuracy of LS-PIV up to 800 mm/s using simulations with known velocity and noise parameters.

CONCLUSIONS/SIGNIFICANCE

To our knowledge, these blood velocity measurements are the fastest recorded with TPLSM. Partnered with transgenic mice carrying cell-specific fluorescent reporters, LS-PIV will also enable the direct in vivo correlation of cellular, biochemical, and hemodynamic parameters in high flow vascular development and diseases such as atherogenesis, arteriogenesis, and vascular anomalies.

T cell-specific deletion of EFNB2 minimally affects T cell development and function

BACKGROUND

Eph kinases and their ephrin ligands (EFN) are all cell surface molecules, capable of transmitting signals in both directions (1, 2). Such bidirectional signaling is called forward (from EFNs to Ephs) and reverse (from Ephs to EFNs) signaling. Eph family kinases have 15 members, divided into A and B subfamilies. Ephrin ligands have 9 members, also classified into A and B families. Ephs and ephrins interact promiscuously, but EphAs mainly interact with EFNAs, and EphBs with EFNBs. EphB family kinases and their ephrin ligands (EFN) are expressed in the T cell compartment.

RESULTS

In this study, using mice with T cell-specific EFNB2 gene knockout (EFNB2 KO mice), we investigated T cell development and function after EFNB2 deletion. EFNB2 KO mice presented normal thymus weight and cellularity. Their thymocyte subpopulations, such as CD4CD8 double positive cells and CD4 and CD8 single positive cells, were normally distributed, but there was a significant relative increase of CD4CD8 double negative cells. Flow cytometry analysis revealed that there was a moderate increase in the DN3 subpopulation. This augmented percentage of DN cells was further confirmed in competitive repopulation chimeras, suggesting that EFNB2 is involved in thymocyte development. The EFNB2 KO mice had normal T cell numbers and percentages in the spleen, and the T cells were able to be activated and to proliferate normally upon TCR ligation. Further, EFNB2 KO naïve CD4 cells were capable of differentiating into Th1, Th2, Th17 and Treg cells similar to WT naïve CD4 cells.

CONCLUSIONS

Our results suggest the involvement of EFNB2 in thymocyte development. However, heavy redundancy among Eph/EFN family members prevents the occurrence of detrimental phenotypes in the T cell compartment caused by T cell-specific EFNB2 gene null mutation.

Eph-dependent cell-cell adhesion and segregation in development and cancer

Numerous studies attest to essential roles for Eph receptors and their ephrin ligands in controlling cell positioning and tissue patterning during normal and oncogenic development. These studies suggest multiple, sometimes contradictory, functions of Eph-ephrin signalling, which under different conditions can promote either spreading and cell-cell adhesion or cytoskeletal collapse, cell rounding, de-adhesion and cell-cell segregation. A principle determinant of the balance between these two opposing responses is the degree of receptor/ligand clustering and activation. This equilibrium is likely altered in cancers and modulated by somatic mutations of key Eph family members that have emerged as candidate cancer markers in recent profiling studies. In addition, cross-talk amongst Ephs and with other signalling pathways significantly modulates cell-cell adhesion, both between and within Eph- and ephrin-expressing cell populations. This review summarises our current understanding of how Eph receptors control cell adhesion and morphology, and presents examples demonstrating the importance of these events in normal development and cancer.

Possible role of Efnb1 protein, a ligand of Eph receptor tyrosine kinases, in modulating blood pressure

Eph kinases constitute the largest receptor tyrosine kinase family, and their ligands, ephrins (Efns), are also cell surface molecules. Although they are ligands, Efns can transduce signals reversely into cells. We have no prior knowledge of the role played by any members of this family of kinases or their ligands in blood pressure (BP) regulation. In the present studies, we investigated the role of Efnb1 in vascular smooth muscle cell (VSMC) contractility and BP regulation. We revealed that reverse signaling through Efnb1 led to a reduction of RhoA activation and VSMC contractility in vitro. Consistent with this finding, ex vivo, there was an increase of RhoA activity accompanied by augmented myosin light chain phosphorylation in mesenteric arteries from mice with smooth muscle-specific conditional Efnb1 gene knock-out (KO). Small interfering RNA knockdown of Grip1, a molecule associated with the Efnb1 intracellular tail, partially eliminated the effect of Efnb1 on VSMC contractility and myosin light chain phosphorylation. In support of these in vitro and ex vivo results, Efnb1 KO mice on a high salt diet showed a statistically significant heightened increment of BP at multiple time points during stress compared with wild type littermates. Our results demonstrate that Efnb1 is a previously unknown negative regulator of VSMC contractility and BP and that it exerts such effects via reverse signaling through Grip1.

Targeted disruption of ephrin B1 in cells of myeloid lineage increases osteoclast differentiation and bone resorption in mice

Disruption of ephrin B1 in collagen I producing cells in mice results in severe skull defects and reduced bone formation. Because ephrin B1 is also expressed during osteoclast differentiation and because little is known on the role of ephrin B1 reverse signaling in bone resorption, we examined the bone phenotypes in ephrin B1 conditional knockout mice, and studied the function of ephrin B1 reverse signaling on osteoclast differentiation and resorptive activity. Targeted deletion of ephrin B1 gene in myeloid lineage cells resulted in reduced trabecular bone volume, trabecular number and trabecular thickness caused by increased TRAP positive osteoclasts and bone resorption. Histomorphometric analyses found bone formation parameters were not changed in ephrin B1 knockout mice. Treatment of wild-type precursors with clustered soluble EphB2-Fc inhibited RANKL induced formation of multinucleated osteoclasts, and bone resorption pits. The same treatment of ephrin B1 deficient precursors had little effect on osteoclast differentiation and pit formation. Similarly, activation of ephrin B1 reverse signaling by EphB2-Fc treatment led to inhibition of TRAP, cathepsin K and NFATc1 mRNA expression in osteoclasts derived from wild-type mice but not conditional knockout mice. Immunoprecipitation with NHERF1 antibody revealed ephrin B1 interacted with NHERF1 in differentiated osteoclasts. Treatment of osteoclasts with exogenous EphB2-Fc resulted in reduced phosphorylation of ezrin/radixin/moesin. We conclude that myeloid lineage produced ephrin B1 is a negative regulator of bone resorption in vivo, and that activation of ephrin B1 reverse signaling inhibits osteoclast differentiation in vitro in part via a mechanism that involves inhibition of NFATc1 expression and modulation of phosphorylation status of ezrin/radixin/moesin.

Ephrin-B1 Is a Novel Specific Component of the Lateral Membrane of the Cardiomyocyte and Is Essential for the Stability of Cardiac Tissue Architecture Cohesion

Rationale:

Cardiac tissue cohesion relying on highly ordered cardiomyocytes (CM) interactions is critical because most cardiomyopathies are associated with tissue remodeling and architecture alterations.

Objective:

Eph/ephrin system constitutes a ubiquitous system coordinating cellular communications which recently emerged as a major regulator in adult organs. We examined if eph/ephrin could participate in cardiac tissue cyto-organization.

Methods and Results:

We reported the expression of cardiac ephrin-B1 in both endothelial cells and for the first time in CMs where ephrin-B1 localized specifically at the lateral membrane. Ephrin-B1 knock-out (KO) mice progressively developed cardiac tissue disorganization with loss of adult CM rod-shape and sarcomeric and intercalated disk structural disorganization confirmed in CM-specific ephrin-B1 KO mice. CMs lateral membrane exhibited abnormal structure by electron microscopy and notably increased stiffness by atomic force microscopy. In wild-type CMs, ephrin-B1 interacted with claudin-5/ZO-1 complex at the lateral membrane, whereas the complex disappeared in KO/CM-specific ephrin-B1 KO mice. Ephrin-B1 deficiency resulted in decreased mRNA expression of CM basement membrane components and disorganized fibrillar collagen matrix, independently of classical integrin/dystroglycan system. KO/CM-specific ephrin-B1 KO mice exhibited increased left ventricle diameter and delayed atrioventricular conduction. Under pressure overload stress, KO mice were prone to death and exhibited striking tissue disorganization. Finally, failing CMs displayed downregulated ephrin-B1/claudin-5 gene expression linearly related to the ejection fraction.

Conclusions:

Ephrin-B1 is necessary for cardiac tissue architecture cohesion by stabilizing the adult CM morphology through regulation of its lateral membrane. Because decreased ephrin-B1 is associated with molecular/functional cardiac defects, it could represent a new actor in the transition toward heart failure.

Bone cell interactions through Eph/ephrin: bone modeling, remodeling and associated diseases

Bones cannot properly form or be maintained without cell-cell interactions through ephrin ligands and Eph receptors. Cell culture analysis and evaluation of genetic mouse models and human diseases reveal various ephrins and Eph functions in the skeletal system. Migration, attachment and spreading of mesenchymal stem cells are regulated by ephrinB ligands and EphB receptors. ephrinB1 loss-of-function is associated with craniofrontonasal syndrome (CFNS) in humans and mice. In bone remodeling, ephrinB2 is postulated to act as a “coupling stimulator.” In that case, bidirectional signaling between osteoclastic ephrinB2 and osteoblastic EphB4 suppresses osteoclastic bone resorption and enhances osteoblastic bone formation, facilitating the transition between these two states. Parathyroid hormone (PTH) induces ephrinB2 in osteoblasts and enhances osteoblastic bone formation. In contrast to ephrinB2, ephrinA2 acts as a “coupling inhibitor,” since ephrinA2 reverse signaling into osteoclasts enhances osteoclastogenesis and EphA2 forward signaling into osteoblasts suppresses osteoblastic bone formation and mineralization. Furthermore, ephrins and Ephs likely modulate pathological conditions such as osteoarthritis, rheumatoid arthritis, multiple myeloma and osteosarcoma. This review focuses on ephrin/Eph-mediated cell-cell interactions in bone biology.

Non-SH2/PDZ reverse signaling by ephrins

Great strides have been made regarding our understanding of the processes and signaling events influenced by Eph/ephrin signaling that play a role in cell adhesion and cell movement. However, the precise mechanisms by which these signaling events regulate cell and tissue architecture still need further resolution. The Eph/ephrin signaling pathways and the ability to regulate cell-cell adhesion and motility constitutes an impressive system for regulating tissue separation and morphogenesis (Pasquale, 2005, 2008 [1,2]). Moreover, the de-regulation of this signaling system is linked to the promotion of aggressive and metastatic tumors in humans [2]. In the following section, we discuss some of the interesting mechanisms by which ephrins can signal through their own intracellular domains (reverse signaling) either independent of forward signaling or in addition to forward signaling through a cognate receptor. In this review we discuss how ephrins (Eph ligands) "reverse signal" through their intracellular domains to affect cell adhesion and movement, but the focus is on modes of action that are independent of SH2 and PDZ interactions.

Notch4 normalization reduces blood vessel size in arteriovenous malformations

Abnormally enlarged blood vessels underlie many life-threatening disorders including arteriovenous (AV) malformations (AVMs). The core defect in AVMs is high-flow AV shunts, which connect arteries directly to veins, "stealing" blood from capillaries. Here, we studied mouse brain AV shunts caused by up-regulation of Notch signaling in endothelial cells (ECs) through transgenic expression of constitutively active Notch4 (Notch4*). Using four-dimensional two-photon imaging through a cranial window, we found that normalizing Notch signaling by repressing Notch4* expression converted large-caliber, high-flow AV shunts to capillary-like vessels. The structural regression of the high-flow AV shunts returned blood to capillaries, thus reversing tissue hypoxia. This regression was initiated by vessel narrowing without the loss of ECs and required restoration of EphB4 receptor expression by venous ECs. Normalization of Notch signaling resulting in regression of high-flow AV shunts, and a return to normal blood flow suggests that targeting the Notch pathway may be useful therapeutically for treating diseases such as AVMs.

Receptor tyrosine kinase Ephb6 regulates vascular smooth muscle contractility and modulates blood pressure in concert with sex hormones

Eph kinases constitute the largest receptor tyrosine kinase family, and their ligands, ephrins (Efns), are also cell surface molecules. Our study is the first to assess the role of Ephb6 in blood pressure (BP) regulation. We observed that EphB6 and all three of its Efnb ligands were expressed on vascular smooth muscle cells (VSMC) in mice. We discovered that small arteries from castrated Ephb6 gene KO males showed increased contractility, RhoA activation, and constitutive myosin light chain phosphorylation ex vivo compared with their WT counterparts. Consistent with this finding, castrated Ephb6 KO mice presented heightened BP compared with castrated WT controls. In vitro experiments in VSMC revealed that cross-linking Efnbs but not Ephb6 resulted in reduced VSMC contractions, suggesting that reverse signaling through Efnbs was responsible for the observed BP phenotype. The reverse signaling was mediated by an adaptor protein Grip1. Additional experiments demonstrated decreased 24-h urine catecholamines in male Ephb6 KO mice, probably as a compensatory feedback mechanism to keep their BP in the normal range. After castration, however, such compensation was abolished in Ephb6 KO mice and was likely the reason why BP increased overtly in these animals. It suggests that Ephb6 has a target in the nervous/endocrine system in addition to VSMC, regulating a testosterone-dependent catecholamine compensatory mechanism. Our study discloses that Ephs and Efns, in concert with testosterone, play a critical role in regulating small artery contractility and BP.

Molecular mechanisms of cell segregation and boundary formation in development and tumorigenesis

The establishment and maintenance of precisely organized tissues requires the formation of sharp borders between distinct cell populations. The maintenance of segregated cell populations is also required for tissue homeostasis in the adult, and deficiencies in segregation underlie the metastatic spreading of tumor cells. Three classes of mechanisms that underlie cell segregation and border formation have been uncovered. The first involves differences in cadherin-mediated cell-cell adhesion that establishes interfacial tension at the border between distinct cell populations. A second mechanism involves the induction of actomyosin-mediated contraction by intercellular signaling, such that cortical tension is generated at the border. Third, activation of Eph receptors and ephrins can lead to both decreased adhesion by triggering cleavage of E-cadherin, and to repulsion of cells by regulation of the actin cytoskeleton, thus preventing intermingling between cell populations. These mechanisms play crucial roles at distinct boundaries during development, and alterations in cadherin or Eph/ephrin expression have been implicated in tumor metastasis.

Eph/ephrin signaling in cell-cell and cell-substrate adhesion

Cell-cell and cell-matrix adhesion are critical processes for the formation and maintenance of tissue patterns during development, as well as control of invasion and metastasis of cancer cells. Although great strides have been made regarding our understanding of the processes that play a role in cell adhesion and cell movement, the precise mechanisms by which diverse signaling events regulate cell and tissue architecture are poorly understood. One group of cell surface molecules, Eph receptor tyrosine kinases, and their membrane-bound ligands, ephrins, are key regulators in these processes. It is the ability of Eph/ephrin signaling pathways to regulate cell-cell adhesion and motility that establishes this family as a formidable system for regulating tissue separation and morphogenesis. Moreover, the de-regulation of this signaling system is linked to the promotion of more aggressive and metastatic tumors in humans.

The effect of conditional EFNB1 deletion in the T cell compartment on T cell development and function

BACKGROUND

Eph kinases are the largest family of cell surface receptor tyrosine kinases. The ligands of Ephs, ephrins (EFNs), are also cell surface molecules. Ephs interact with EFNs transmitting signals in both directions, i.e., from Ephs to EFNs and from EFNs to Ephs. EFNB1 is known to be able to co-stimulate T cells in vitro and to modulate thymocyte development in a model of foetal thymus organ culture. To further understand the role of EFNB1 in T cell immunity, we generated T-cell-specific EFNB1 gene knockout mice to assess T cell development and function in these mice.

RESULTS

The mice were of normal size and cellularity in the thymus and spleen and had normal T cell subpopulations in these organs. The bone marrow progenitors from KO mice and WT control mice repopulated host spleen T cell pool to similar extents. The activation and proliferation of KO T cells was comparable to that of control mice. Naïve KO CD4 cells showed an ability to differentiate into Th1, Th2, Th17 and Treg cells similar to control CD4 cells.

CONCLUSIONS

Our results suggest that the function of EFNB1 in the T cell compartment could be compensated by other members of the EFN family, and that such redundancy safeguards the pivotal roles of EFNB1 in T cell development and function.

Alendronate affects osteoblast functions by crosstalk through EphrinB1-EphB

Bisphosphonates are therapeutic agents in the treatment of post-menopausal osteoporosis. Although they have been associated with delayed healing in injured tissues, inappropriate femoral fractures, and osteonecrosis of the jaw (ONJ), the pathophysiological mechanisms involved are not clear. Our hypothesis is that alendronate, a member of the N-containing bisphosphonates, indirectly inhibits osteoblast function through the coupling of osteoclasts to osteoblasts by ephrinB-EphB interaction. We found that alendronate increased gene and protein expression of ephrinB1 and EphB1, as well as B3, in femurs of adult mice injected with alendronate (10 µg/100 g/wk) for 8 weeks. Alendronate suppressed the expression of bone sialoprotein (BSP) and osteonectin in both femurs and bone marrow osteoblastic cells of mice. After elimination of pre-osteoclasts from bone marrow cells, alendronate did not affect osteoblast differentiation, indicating the need for pre-osteoclasts for alendronate's effects. Alendronate stimulated EphB1 and EphB3 protein expression in osteoblasts, whereas it enhanced ephrinB1 protein in pre-osteoclasts. In addition, a reverse signal by ephrinB1 inhibited osteoblast differentiation and suppressed BSP gene expression. Thus, alendronate, through its direct effects on the pre-osteoclast, appears to regulate expression of ephrinB1, which regulates and acts through the EphB1, B3 receptors on the osteoblast to suppress osteoblast differentiation.

Vessel arterial-venous plasticity in adult neovascularization

OBJECTIVE

Proper arterial and venous specification is a hallmark of functional vascular networks. While arterial-venous identity is genetically pre-determined during embryo development, it is unknown whether an analogous pre-specification occurs in adult neovascularization. Our goal is to determine whether vessel arterial-venous specification in adult neovascularization is pre-determined by the identity of the originating vessels.

METHODS AND RESULTS

We assessed identity specification during neovascularization by implanting isolated microvessels of arterial identity from both mice and rats and assessing the identity outcomes of the resulting, newly formed vasculature. These microvessels of arterial identity spontaneously formed a stereotypical, perfused microcirculation comprised of the full complement of microvessel types intrinsic to a mature microvasculature. Changes in microvessel identity occurred during sprouting angiogenesis, with neovessels displaying an ambiguous arterial-venous phenotype associated with reduced EphrinB2 phosphorylation.

CONCLUSIONS

Our findings indicate that microvessel arterial-venous identity in adult neovascularization is not necessarily pre-determined and that adult microvessels display a considerable level of phenotypic plasticity during neovascularization. In addition, we show that vessels of arterial identity also hold the potential to undergo sprouting angiogenesis.

Ephrin reverse signaling in axon guidance and synaptogenesis

Axon-cell and axon-dendrite contact is a highly regulated process necessary for the formation of precise neural circuits and a functional neural network. Eph-ephrin interacting molecules on the membranes of axon nerve terminals and target dendrites act as bidirectional ligands/receptors to transduce signals into both the Eph-expressing and ephrin-expressing cells to regulate cytoskeletal dynamics. In particular, recent evidence indicates that ephrin reverse signal transduction events are important in controlling both axonal and dendritic elaborations of neurons in the developing nervous system. Here we review how ephrin reverse signals are transduced into neurons to control maturation of axonal pre-synaptic and dendritic post-synaptic structures.

Eph/ephrin signaling: genetic, phosphoproteomic, and transcriptomic approaches

The Eph receptor tyrosine kinases and their ephrin partners compose a large and complex family of signaling molecules involved in a wide variety of processes in development, homeostasis, and disease. The complexity inherent to Eph/ephrin signaling derives from several characteristics of the family. First, the large size and functional redundancy/compensation by family members presents a challenge in defining their in vivo roles. Second, the capacity for bidirectional signaling doubles the potential complexity, since every member has the ability to act both as a ligand and a receptor. Third, Ephs and ephrins can utilize a wide array of signal transduction pathways with a tremendous diversity of cell biological effect. The daunting complexity of Eph/ephrin signaling has increasingly prompted investigators to resort to multiple technological approaches to gain mechanistic insight. Here we review recent progress in the use of advanced mouse genetics in combination with proteomic and transcriptomic approaches to gain a more complete understanding of signaling mechanism in vivo. Integrating insights from such disparate approaches provides advantages in continuing to advance our understanding of how this multifarious group of signaling molecules functions in a diverse array of biological contexts.

Efnb1 and Efnb2 proteins regulate thymocyte development, peripheral T cell differentiation, and antiviral immune responses and are essential for interleukin-6 (IL-6) signaling

Erythropoietin-producing hepatocellular kinases (Eph kinases) constitute the largest family of cell membrane receptor tyrosine kinases, and their ligand ephrins are also cell surface molecules. Because of promiscuous interaction between Ephs and ephrins, there is considerable redundancy in this system, reflecting the essential roles of these molecules in the biological system through evolution. In this study, both Efnb1 and Efnb2 were null-mutated in the T cell compartment of mice through loxP-mediated gene deletion. Mice with this double conditional mutation (double KO mice) showed reduced thymus and spleen size and cellularity. There was a significant decrease in the DN4, double positive, and single positive thymocyte subpopulations and mature CD4 and CD8 cells in the periphery. dKO thymocytes and peripheral T cells failed to compete with their WT counterparts in irradiated recipients, and the T cells showed compromised ability of homeostatic expansion. dKO naive T cells were inferior in differentiating into Th1 and Th17 effectors in vitro. The dKO mice showed diminished immune response against LCMV infection. Mechanistic studies revealed that IL-6 signaling in dKO T cells was compromised, in terms of abated induction of STAT3 phosphorylation upon IL-6 stimulation. This defect likely contributed to the observed in vitro and in vivo phenotype in dKO mice. This study revealed novel roles of Efnb1 and Efnb2 in T cell development and function.

Duplication of the EFNB1 gene in familial hypertelorism: imbalance in ephrin-B1 expression and abnormal phenotypes in humans and mice

Familial hypertelorism, characterized by widely spaced eyes, classically shows autosomal dominant inheritance (Teebi type), but some pedigrees are compatible with X-linkage. No mechanism has been described previously, but clinical similarity has been noted to craniofrontonasal syndrome (CFNS), which is caused by mutations in the X-linked EFNB1 gene. Here we report a family in which females in three generations presented with hypertelorism, but lacked either craniosynostosis or a grooved nasal tip, excluding CFNS. DNA sequencing of EFNB1 was normal, but further analysis revealed a duplication of 937 kb including EFNB1 and two flanking genes: PJA1 and STARD8. We found that the X chromosome bearing the duplication produces ∼1.6-fold more EFNB1 transcript than the normal X chromosome and propose that, in the context of X-inactivation, this difference in expression level of EFNB1 results in abnormal cell sorting leading to hypertelorism. To support this hypothesis, we provide evidence from a mouse model carrying a targeted human EFNB1 cDNA, that abnormal cell sorting occurs in the cranial region. Hence, we propose that X-linked cases resembling Teebi hypertelorism may have a similar mechanism to CFNS, and that cellular mosaicism for different levels of ephrin-B1 (as well as simple presence/absence) leads to craniofacial abnormalities. Hum Mutat 32:1–9, 2011. © 2011 Wiley-Liss, Inc.

Connexin37 and Connexin43 deficiencies in mice disrupt lymphatic valve development and result in lymphatic disorders including lymphedema and chylothorax

Intraluminal valves are required for the proper function of lymphatic collecting vessels and large lymphatic trunks like the thoracic duct. Despite recent progress in the study of lymphvasculogenesis and lymphangiogenesis, the molecular mechanisms controlling the morphogenesis of lymphatic valves remain poorly understood. Here, we report that gap junction proteins, or connexins (Cxs), are required for lymphatic valvulogenesis. Cx37 and Cx43 are expressed early in mouse lymphatic development in the jugular lymph sacs, and later in development these Cxs become enriched and differentially expressed by lymphatic endothelial cells on the upstream and downstream sides of the valves. Specific deficiencies of Cx37 and Cx43 alone or in combination result in defective valve formation in lymphatic collecting vessels, lymphedema, and chylothorax. We also show that Cx37 regulates jugular lymph sac size and that both Cx37 and Cx43 are required for normal thoracic duct development, including valve formation. Another Cx family member, Cx47, whose human analog is mutated in some families with lymphedema, is also highly enriched in a subset of endothelial cells in lymphatic valves. Mechanistically, we present data from Foxc2-/- embryos suggesting that Cx37 may be a target of regulation by Foxc2, a transcription factor that is mutated in human lymphedema-distichiasis syndrome. These results show that at least three Cxs are expressed in the developing lymphatic vasculature and, when defective, are associated with clinically manifest lymphatic disorders in mice and man.

Establishment and maintenance of compartmental boundaries: role of contractile actomyosin barriers

During animal development, tissues and organs are partitioned into compartments that do not intermix. This organizing principle is essential for correct tissue morphogenesis. Given that cell sorting defects during compartmentalization in humans are thought to cause malignant invasion and congenital defects such as cranio-fronto-nasal syndrome, identifying the molecular and cellular mechanisms that keep cells apart at boundaries between compartments is important. In both vertebrates and invertebrates, transcription factors and short-range signalling pathways, such as EPH/Ephrin, Hedgehog, or Notch signalling, govern compartmental cell sorting. However, the mechanisms that mediate cell sorting downstream of these factors have remained elusive for decades. Here, we review recent data gathered in Drosophila that suggest that the generation of cortical tensile forces at compartmental boundaries by the actomyosin cytoskeleton could be a general mechanism that inhibits cell mixing between compartments.

CONNEXIN 43 AND BONE: NOT JUST A GAP JUNCTION PROTEIN

Connexins are essential for the communication of cells among themselves and with their environment. Connexin hexamers assemble at the plasma membrane to form hemichannels that allow the exchange of cellular contents with the extracellular milieu. In addition, hemichannels expressed in neighboring cells align to form gap junction channels that mediate the exchange of contents among cells. Connexin 43 (Cx43) is the most abundant connexin expressed in bone cells and its deletion in all tissues leads to osteoblast dysfunction, as evidenced by reduced expression of osteoblast markers and delayed ossification. Moreover, Cx43 is essential for the survival of osteocytes; and mice lacking Cx43 in these cells exhibit increased prevalence of osteocyte apoptosis and empty lacunae in cortical bone. Work of several groups for the past few years has unveiled the role of Cx43 on the response of bone cells to a variety of stimuli. Thus, the preservation of the viability of osteoblasts and osteocytes by the anti-osteoporotic drugs bisphosphonates depends on Cx43 expression in vitro and in vivo. This survival effect does not require cell-to-cell communication and is mediated by unopposed hemichannels. Cx43 hemichannels are also required for the release of prostaglandins and ATP by osteocytes induced by mechanical stimulation in vitro. More recent evidence showed that the cAMP-mediated survival effect of parathyroid hormone (PTH) also requires Cx43 expression. Moreover, the hormone does not increase bone mineral content in mice haploinsufficient for Cx43 or lacking Cx43 in osteoblastic cells. Since inhibition of osteoblast apoptosis contributes, at least in part, to bone anabolism by PTH, the lack of response to the hormone might be due to the requirement of Cx43 for the effect of PTH on osteoblast survival. In summary, mounting evidence indicate that Cx43 is a key component of the intracellular machinery responsible for the transduction of signals in the skeleton in response to pharmacologic, hormonal and mechanical stimuli.

A novel de novo mutation within EFNB1 gene in a young girl with craniofrontonasal syndrome

Craniofrontonasal syndrome is mainly characterized by frontonasal dysplasia, telorbitism, a broad nasal root, and frequently a bifid nose and coronal craniosynostosis. Craniofrontonasal syndrome is an X-linked disorder with an unusual pattern of inheritance because heterozygous females are more severely affected than hemizygous males. The craniofrontonasal syndrome-causing gene is EFNB1, localized in the border region of chromosome Xq12 and Xq13.1, encoding for protein ephrin-B1. Here we aim to investigate the underlying genetic defect of a young girl with craniofrontonasal syndrome. The patient underwent surgical correction of her craniofacial deformities. Genetic analysis was carried out by polymerase chain reaction. Products of exon 2 of the EFNB1 gene were sequenced as well as digested with BpmI enzyme. A novel de novo missense mutation 373G>A was identified within the EFNB1 gene, leading to the replacement of glutamic acid at amino acid position 125 with lysine. The replacement of Glu125 with Lys, which lies within the G-H loop, part of the dimerization ligand-receptor interface, is expected to disrupt the interaction between the Eph receptor and ephrin B1 ligand, thus leading to craniofrontonasal syndrome.