RESUMEN
Here we present a nanostructured surface able to produce multivalent interactions between surface-bound ephrinB1 ligands and membrane EphB2 receptors. We created ephrinB1 nanopatterns of regular size (<30 nm in diameter) by using self-assembled diblock copolymers. Next, we used a statistically enhanced version of the Number and Brightness technique, which can discriminate-with molecular sensitivity-the oligomeric states of diffusive species to quantitatively track the EphB2 receptor oligomerization process in real time. The results indicate that a stimulation using randomly distributed surface-bound ligands was not sufficient to fully induce receptor aggregation. Conversely, when nanopatterned onto our substrates, the ligands effectively induced a strong receptor oligomerization. This presentation of ligands improved the clustering efficiency of conventional ligand delivery systems, as it required a 9-fold lower ligand surface coverage and included faster receptor clustering kinetics compared to traditional cross-linked ligands. In conclusion, nanostructured diblock copolymers constitute a novel strategy to induce multivalent ligand-receptor interactions leading to a stronger, faster, and more efficient receptor activation, thus providing a useful strategy to precisely tune and potentiate receptor responses. The efficiency of these materials at inducing cell responses can benefit applications such as the design of new bioactive materials and drug-delivery systems.
Asunto(s)
Efrina-B1/metabolismo , Proteínas Inmovilizadas/metabolismo , Nanoestructuras/química , Polimetil Metacrilato/química , Receptor EphB2/metabolismo , Efrina-B1/química , Células HEK293 , Humanos , Proteínas Inmovilizadas/química , Ligandos , Nanoestructuras/ultraestructura , Agregado de Proteínas , Multimerización de Proteína , Receptor EphB2/químicaRESUMEN
Contact inhibition of locomotion (CIL) is the process by which cells stop the continual migration in the same direction after collision with another cell. Highly invasive malignant cells exhibit diminished CIL when they contact stromal cells, which allows invasion of the tissue by tumors. We show that Nm23-H1 is essential for the suppression of Rac1 through inactivation of Tiam1 at the sites of cell-cell contact, which plays a pivotal role in CIL. U87MG cells show CIL when they contact normal glia. In spheroid confrontation assays U87MG cells showed only limited invasion of the glial population, but reduction of Nm23-H1 expression in U87MG cells abrogated CIL resulting in invasion. In U87MG cells, Nm23-H1 is translocated to the sites of contact with glia through association with α-catenin and N-cadherin. Mutants of Nm23-H1, which lacked the binding ability with Tiam1, or α-catenin did not restore CIL. Moreover, the expression of ephrin-B1 in tumor cells disrupted CIL and promoted invasion. As one mechanism, ephrin-B1 inhibits the association of Nm23-H1 with Tiam1, which contributes for activation of Rac1. These results indicate a novel function of Nm23-H1 to control CIL, and its negative regulation by ephrin-B1.
Asunto(s)
Movimiento Celular , Inhibición de Contacto , Efrina-B1/metabolismo , Nucleósido Difosfato Quinasas NM23/metabolismo , Secuencia de Aminoácidos , Animales , Encéfalo/metabolismo , Encéfalo/patología , Cadherinas/metabolismo , Comunicación Celular , Línea Celular Tumoral , Efrina-B1/química , Glioblastoma/metabolismo , Glioblastoma/patología , Factores de Intercambio de Guanina Nucleótido/química , Factores de Intercambio de Guanina Nucleótido/metabolismo , Humanos , Ratones , Ratones Desnudos , Datos de Secuencia Molecular , Nucleósido Difosfato Quinasas NM23/química , Invasividad Neoplásica , Neuroglía/metabolismo , Neuroglía/patología , Unión Proteica , Ratas , Ratas Wistar , Proteína 1 de Invasión e Inducción de Metástasis del Linfoma-T , alfa Catenina/metabolismoRESUMEN
Eph receptors and ephrin ligands are membrane-bound cell-cell communication molecules with well-defined roles in development. However, their expression and functions in the gastric epithelium are virtually unknown. We detected several EphB receptors and ephrin-Bs in the gastric corpus mucosa of the adult rodent stomach by RT-PCR amplification. Immunostaining showed complementary expression patterns, with EphB receptors preferentially expressed in the deeper regions and ephrin-Bs in the superficial regions of the gastric units. EphB1, EphB2 and EphB3 are expressed in mucous neck, chief and parietal cells, respectively. In contrast, ephrin-B1 is in pit cells and proliferating cells of the isthmus. In a mouse ulcer model, EphB2 expression was upregulated in the regenerating epithelium and expanded into the isthmus. Thus, EphB/ephrin-B signaling likely occurs preferentially in the isthmus, where receptor-ligand overlap is highest. We show that EphB signaling in primary gastric epithelial cells promotes cell retraction and repulsion at least in part through RhoA activation. Based on these findings, we propose that the EphB-positive progeny of gastric stem cells migrates from the isthmus toward the bottom of the gastric glands due to repulsive signals arising from contact with ephrin-Bs, which are preferentially expressed in the more superficial regions of the isthmus and gastric pits.
Asunto(s)
Efrina-B1/metabolismo , Mucosa Gástrica/metabolismo , Regulación de la Expresión Génica , Receptores de la Familia Eph/metabolismo , Transducción de Señal , Citoesqueleto de Actina/metabolismo , Animales , Células Cultivadas , Efrina-B1/química , Femenino , Técnica del Anticuerpo Fluorescente , Adhesiones Focales/metabolismo , Mucosa Gástrica/química , Masculino , Ratones , Ratones Endogámicos BALB C , Ratas , Receptores de la Familia Eph/química , Reacción en Cadena de la Polimerasa de Transcriptasa Inversa , Proteína de Unión al GTP rhoA/metabolismoRESUMEN
Mutations in the ephrin-B1 gene result in craniofrontonasal syndrome (CFNS) in humans, a congenital disorder that includes a wide range of craniofacial, skeletal, and neurological malformations. In addition to the ability of ephrin-B1 to forward signal through its cognate EphB tyrosine kinase receptors, ephrin-B1 can also act as a receptor and transduce a reverse signal by either PDZ-dependent or phosphorylation-dependent mechanisms. To investigate how ephrin-B1 acts to influence development and congenital disease, we generated mice harboring a series of targeted point mutations in the ephrin-B1 gene that independently ablate specific reverse signaling pathways, while maintaining forward signaling capacity. We demonstrate that both PDZ and phosphorylation-dependent reverse signaling by ephrin-B1 are dispensable for craniofacial and skeletal development, whereas PDZ-dependent reverse signaling by ephrin-B1 is critical for the formation of a major commissural axon tract, the corpus callosum. Ephrin-B1 is strongly expressed within axons of the corpus callosum, and reverse signaling acts autonomously in cortical axons to mediate an avoidance response to its signaling partner EphB2. These results demonstrate the importance of PDZ-dependent reverse signaling for a subset of Ephrin-B1 developmental roles in vivo.
Asunto(s)
Huesos/embriología , Efrina-B1/genética , Efrina-B1/metabolismo , Dominios PDZ/fisiología , Transducción de Señal , Secuencia de Aminoácidos , Animales , Axones/metabolismo , Células Cultivadas , Cuerpo Calloso/embriología , Embrión de Mamíferos/embriología , Embrión de Mamíferos/metabolismo , Efrina-B1/química , Femenino , Regulación del Desarrollo de la Expresión Génica , Técnicas de Sustitución del Gen , Proteína Ácida Fibrilar de la Glía , Masculino , Ratones , Datos de Secuencia Molecular , Mutación , Proteínas del Tejido Nervioso/metabolismo , Receptor EphB2/metabolismo , Cráneo/embriologíaRESUMEN
Proteins of the ephrin-B group operate in nonlymphoid cells through the control of their migration and attachment, and are crucial for the development of the vascular, lymphatic, and nervous systems. Ephrin-B activity is deregulated in various nonlymphoid malignancies; however, their precise role in cancer has only started to be addressed. We show here that ephrin-B1, a member of the ephrin-B group, is expressed in pediatric T-cell leukemias, including leukemia cell line Jurkat. Treatment of Jurkat cells with ephrin-B-stimulating EphB3 enhances ephrin-B1 phosphorylation and induces its relocalization into lipid rafts. These events are mediated by the T lineage-specific kinase, Lck, as ephrin-B1 phosphorylation and lipid raft association are blocked in the Lck-deficient clone of Jurkat, JCAM1.6. Ephrin-B1 also induces colocalization of the CrkL and Rac1 cytoskeleton regulators and initiates in leukemic cells a strong repulsive response. The absence of Lck blocks ephrin-B1-induced signaling and repulsion, confirming the essential role for Lck in ephrin-B1-mediated responses. This shows a new role for ephrin-B1 in the regulation of leukemic cells through the Lck-dependent Rac1 colocalization with its signaling partner, CrkL, in lipid rafts. In agreement with its repulsive action, ephrin-B1 seems to support metastatic properties of leukemic cells, as suppression of ephrin-B1 signaling inhibits their invasiveness. Because ephrin-B1-activating EphB proteins are ubiquitously expressed, our findings suggest that ephrin-B1 is likely to play an important role in the regulation of malignant T lymphocytes through the control of lipid-raft-associated signaling, adhesion, and invasive activity, and therefore may represent a novel target for cancer treatment.
Asunto(s)
Efrina-B1/metabolismo , Leucemia-Linfoma de Células T del Adulto/enzimología , Leucemia-Linfoma de Células T del Adulto/patología , Proteína Tirosina Quinasa p56(lck) Específica de Linfocito/metabolismo , Microdominios de Membrana/enzimología , Transducción de Señal , Proteínas Adaptadoras Transductoras de Señales/metabolismo , Adhesión Celular , Niño , Efrina-B1/química , Efrina-B3/metabolismo , Fibronectinas/metabolismo , Humanos , Células Jurkat , Invasividad Neoplásica , Proteínas Nucleares/metabolismo , Estructura Terciaria de Proteína , Transporte de Proteínas , Proteína de Unión al GTP rac1/metabolismoRESUMEN
The Eph (erythropoietin-producing hepatoma) family of receptor tyrosine kinases and their membrane-bound ligands, the ephrins, have been implicated in regulating cell adhesion and migration during development by mediating cell-to-cell signaling events. The transmembrane ephrinB (Eph receptor interactor B) protein is a bidirectional signaling molecule that sends a forward signal through the activation of its cognate receptor tyrosine kinase, residing on another cell. A reverse signal can be transduced into the ephrinB-expressing cell via tyrosine phosphorylation of its conserved C-terminal cytoplasmic domain. Although some insight has been gained regarding how ephrinB may send signals affecting cytoskeletal components, little is known about how ephrinB1 reverse signaling affects transcriptional processes. Here we report that signal transducer and activator of transcription 3 (STAT3) can interact with ephrinB1 in a phosphorylation-dependent manner that leads to enhanced activation of STAT3 transcriptional activity. This activity depends on the tyrosine kinase Jak2, and two tyrosines within the intracellular domain of ephrinB1 are critical for the association with STAT3 and its activation. The recruitment of STAT3 to ephrinB1, and its resulting Jak2-dependent activation and transcription of reporter targets, reveals a signaling pathway from ephrinB1 to the nucleus.
Asunto(s)
Membrana Celular/metabolismo , Núcleo Celular/metabolismo , Efrina-B1/metabolismo , Factor de Transcripción STAT3/metabolismo , Transducción de Señal , Transporte Activo de Núcleo Celular , Secuencia de Aminoácidos , Animales , Línea Celular , Chlorocebus aethiops , ADN/metabolismo , Efrina-B1/química , Efrina-B1/genética , Humanos , Janus Quinasa 2/genética , Janus Quinasa 2/metabolismo , Datos de Secuencia Molecular , Fosforilación , Fosfotirosina/metabolismo , Unión Proteica , Ratas , Factor de Transcripción STAT3/genética , Transcripción Genética/genética , Xenopus laevisRESUMEN
The interaction of the Eph family of receptor protein tyrosine kinase and its ligand ephrin family induces bidirectional signaling via the cell-cell contacts. Although most previous studies have focused on the function of Eph-ephrin pathways in the neural system and endothelial cells, this process also occurs in epithelial and cancer cells, of which the biological involvement is poorly understood. We show that ephrin-B1 creates an in vivo complex with adjacent claudin1 or claudin4 via the extracellular domains of these proteins. The cytoplasmic domain of ephrin-B1 was phosphorylated on tyrosine residues upon the formation of cell-cell contacts, possibly recognizing an intercellular adhesion of claudins. Phosphorylation of ephrin-B1 induced by claudins was abolished by the treatment with 4-amino-5-(4-chlorophenyl)-7-(t-butyl)pyrazolo[3,4-d]pyrimidine, an inhibitor of the Src family kinases. Moreover, overexpression of ephrin-B1 triggered consequent change in the level of cell-cell adhesion depending on its phosphorylation. These results suggest that ephrin-B1 mediated the cell-cell adhesion of epithelial and cancer cells via a novel Eph receptor-independent mechanism.
Asunto(s)
Adhesión Celular/fisiología , Efrina-B1/metabolismo , Proteínas de la Membrana/metabolismo , Animales , Células COS , Línea Celular , Chlorocebus aethiops , Claudina-1 , Claudina-4 , Perros , Inhibidores Enzimáticos/farmacología , Efrina-B1/química , Células Epiteliales/fisiología , Fosforilación , Unión Proteica , Estructura Terciaria de Proteína , Pirimidinas/farmacología , Uniones Estrechas/fisiologíaRESUMEN
Eph receptors and their ephrin ligands are involved in various aspects of cell-cell communication during development, including axonal pathfinding in the nervous system and cell-cell interactions of the vascular endothelial cells. Recent structural studies revealed unique molecular features, not previously seen in any other receptor-ligand families, and explained many of the biochemical and signaling properties of Ephs and ephrins. However, unresolved questions remain regarding the potential oligomerization and clustering of these important signaling molecules. In this study, the biophysical properties and receptor-binding preferences of the extracellular domain of ephrin-B1 were investigated and its crystal structure was determined at 2.65 A resolution. Ephrin-B1 is a monomer both in solution and in the crystals, while it was previously shown that the closely related ephrin-B2 forms homodimers. The main structural difference between ephrin-B1 and ephrin-B2 is the conformation of the receptor-binding G-H loop and the partially disordered N-terminal tetramerization region of ephrin-B1. The G-H loop is structurally rigid in ephrin-B2 and adopts the same conformation in both the receptor-bound and unbound ligand, where it mediates receptor-independent homodimerization. In the ephrin-B1 structure, on the other hand, the G-H loop is not involved in any homotypic interactions and adopts a new, distinct conformation. The implications of the ephrin-B1 structure, in context of available ephrin-B1 mutagenesis data, for the mechanism of Eph-ephrin recognition and signaling initiation are discussed.
Asunto(s)
Efrina-B1/química , Transducción de Señal , Secuencia de Aminoácidos , Animales , Axones/metabolismo , Cristalografía por Rayos X , Dimerización , Células Endoteliales/metabolismo , Efrina-B1/agonistas , Efrina-B1/genética , Efrina-B2/química , Efrina-B2/genética , Efrina-B2/metabolismo , Ligandos , Ratones , Datos de Secuencia Molecular , Mutagénesis , Estructura Secundaria de Proteína , Estructura Terciaria de Proteína/genética , Transducción de Señal/fisiologíaRESUMEN
The binding interaction between the Nck2 SH2 domain and the phosphorylated ephrinB initiates a critical pathway for the reverse signaling network mediated by Eph receptor-ephrinB. Previously, the NMR structure and Tyr phosphorylations of the human ephrinB cytoplasmic domain have been studied. To obtain a complete story, it would be of significant interest to determine the structure of the Nck2 SH2 domain that shows a low sequence identity to other SH2 domains with known structures. Here, we report the determination of the solution structure of the human Nck2 SH2 domain and investigate its interactions with three phosphorylated ephrinB fragments by NMR spectroscopy. The results indicate that: 1) although the human Nck2 SH2 domain adopts a core tertiary fold common to all SH2 domains, it owns some unique properties such as a shorter C-terminal helix and unusual electrostatic potential surface. However, the most striking finding is that the C-terminal tail of the human Nck2 SH2 domain adopts a short antiparallel beta-sheet that, to the best of our knowledge, has never been identified in other SH2 domains. The truncation study suggests that one function of the C-terminal tail is to control the folding/solubility of the SH2 domain. 2) In addition to [Tyr(P)304]ephrinB2(301-322) and [Tyr(P)316]ephrinB2(301-322), here we identified [Tyr(P)330]ephrinB2(324-333) also capable of binding to the SH2 domain. The detailed NMR study indicated that the binding mechanisms for the three ephrinB fragments might be different. The binding with [Tyr(P)304]-ephrinB2(301-322) and [Tyr(P)316]ephrinB2(301-322) might be mostly involved in the residues over the N-half of the SH2 domain and provoked a significant increase in the backbone and side chain dynamics of the SH2 domain on the microsecond-millisecond time scale. In contrast, binding with [Tyr(P)330]ephrinB2(324-333) might have most residues over both halves engaged but induced less profound conformational dynamics on the mus-ms time scale.
Asunto(s)
Proteínas Adaptadoras Transductoras de Señales/química , Efrina-B1/química , Proteínas Oncogénicas/química , Tirosina/química , Secuencia de Aminoácidos , Animales , Clonación Molecular , Citoplasma/metabolismo , ADN/química , Humanos , Enlace de Hidrógeno , Concentración de Iones de Hidrógeno , Espectroscopía de Resonancia Magnética , Ratones , Modelos Químicos , Modelos Moleculares , Datos de Secuencia Molecular , Fosforilación , Unión Proteica , Conformación Proteica , Pliegue de Proteína , Estructura Secundaria de Proteína , Estructura Terciaria de Proteína , Homología de Secuencia de Aminoácido , Electricidad Estática , Factores de Tiempo , Xenopus , Dominios Homologos srcRESUMEN
Eph/ephrin receptors and ligands mediate cell-cell interaction through reciprocal signaling upon juxtacrine contact, and play a critical role in embryonic patterning, neuronal targeting, and vascular assembly. To study transmembrane ephrin-B ligand trafficking, we determined the cellular localization of ephrin-B1-GFP upon engagement by EphB1. Under normal culture conditions ephrin-B1-GFP is localized to the plasma membrane, mostly at the lateral cell borders. Addition of soluble EphB1-Fc receptor induces ephrin-B1-GFP clustering on the cell surface and subsequent internalization, as judged by biochemical studies, electron microscopy, and co-localization with endosomal markers. A dominant-negative mutant of dynamin or potassium depletion blocks ephrin-B1 endocytosis. These results suggest that ephrin-B1 internalization is an active receptor-mediated process that utilizes the clathrin-mediated endocytic pathway.
Asunto(s)
Membrana Celular/metabolismo , Clatrina/metabolismo , Efrina-B1/química , Receptores de la Familia Eph/química , Animales , Biotinilación , Western Blotting , Células CHO , Separación Celular , Células Cultivadas , Cricetinae , ADN Complementario/metabolismo , Dinaminas/química , Endocitosis , Citometría de Flujo , Genes Dominantes , Vectores Genéticos , Humanos , Ligandos , Microscopía Confocal , Microscopía Electrónica , Mutación , Potasio/química , Transducción de Señal , Factores de Tiempo , Transfección , Venas Umbilicales/citologíaRESUMEN
The Eph family of receptor tyrosine kinases and their membrane-bound ligands, the ephrins, are thought to play a role in the regulation of cell adhesion and migration during development by mediating cell-to-cell signalling events. The transmembrane ephrinB protein is a bidirectional signalling molecule that sends a forward signal through the activation of its cognate receptor tyrosine kinase residing on another cell. The reverse signal is transduced into the ephrinB-expressing cell via tyrosine phosphorylation of its conserved C-terminal cytoplasmic domain. Previous work from our laboratory has implicated the activated FGFR1 (fibroblast growth factor receptor 1) as a regulator of a de-adhesion signal that results from overexpression of ephrinB1. In the present study, we report the isolation of Xenopus Grb4 (growth-factor-receptor-bound protein 4), an ephrinB1-interacting protein, and we show that when expressed in Xenopus oocytes, ephrinB1 interacts with Grb4 in the presence of an activated FGFR1. Amino acid substitutions were generated in Grb4, and the resulting mutants were expressed along with ephrinB1 and an activated FGFR in Xenopus oocytes. Co-immunoprecipitation analysis shows that the FLVR motif within the Src homology 2 domain of Xenopus Grb4 is vital for this phosphorylation-dependent interaction with ephrinB1. More importantly, using deletion and substitution analysis we identify the tyrosine residue at position 298 of ephrinB1 as being required for the physical interaction with Grb4, whereas Tyr-305 and Tyr-310 are dispensable. Moreover, we show that the region between amino acids 301 and 304 of ephrinB1 is also required for this critical tyrosine-phosphorylation-dependent event.
Asunto(s)
Proteínas Adaptadoras Transductoras de Señales , Proteínas Adaptadoras del Transporte Vesicular/metabolismo , Efrina-B1/metabolismo , Proteínas de Xenopus/química , Proteínas de Xenopus/metabolismo , Proteínas Adaptadoras del Transporte Vesicular/química , Secuencia de Aminoácidos , Animales , Sitios de Unión , Células Cultivadas , Efrina-B1/química , Humanos , Datos de Secuencia Molecular , Proteínas Oncogénicas/química , Proteínas Oncogénicas/metabolismo , Fosforilación , Alineación de Secuencia , Tirosina/fisiología , Xenopus laevis , Dominios Homologos srcRESUMEN
The protein tyrosine phosphatase PTP-Basophil (PTP-Bas) and its mouse homologue, PTP-Basophil-like (PTP-BL), are high molecular mass protein phosphatases consisting of a number of diverse protein-protein interaction modules. Several splicing variants of these phosphatases are known to exist thus demonstrating the complexity of these molecules. PTP-Bas/BL serves as a central scaffolding protein facilitating the assembly of a multiplicity of different proteins mainly via five different PDZ domains. Many of these interacting proteins are implicated in the regulation of the actin cytoskeleton. However, some proteins demonstrate a nuclear function of this protein tyrosine phosphatase. PTP-Bas is involved in the regulation of cell surface expression of the cell death receptor, Fas. Moreover, it is a negative regulator of ephrinB phosphorylation, a receptor playing an important role during development. The phosphorylation status of other proteins such as RIL, IkappaBalpha and beta-catenin can also be regulated by this phosphatase. Finally, PTP-BL has been shown to be involved in the regulation of cytokinesis, the last step in cell division. Although the precise molecular function of PTP-Bas/BL is still elusive, current data suggest clearly that PTP-Bas/BL belongs to the family of PDZ domain containing proteins involved in the regulation of the cytoskeleton and of intracellular vesicular transport processes.
Asunto(s)
Proteínas Tirosina Fosfatasas/fisiología , Animales , Apoptosis , División Celular , Citoesqueleto/metabolismo , Efrina-B1/química , Humanos , Hibridación in Situ , Modelos Biológicos , Modelos Genéticos , Fosforilación , Unión Proteica , Conformación Proteica , Estructura Terciaria de Proteína , Proteína Tirosina Fosfatasa no Receptora Tipo 13 , Proteínas Tirosina Fosfatasas/química , ARN/química , ARN/metabolismo , Transducción de Señal , Receptor fas/químicaRESUMEN
We report that the EphB receptor ligand, ephrin-B1, may act bifunctionally as both a branch repellent and attractant to control the unique mechanisms in mapping the dorsal-ventral (DV) retinal axis along the lateral-medial (LM) axis of the optic tectum. EphB receptors are expressed in a low to high DV gradient by retinal ganglion cells (RGCs), and ephrin-B1 is expressed in a low to high LM gradient in the tectum. RGC axons lack DV ordering along the LM tectal axis, but directionally extend interstitial branches that establish retinotopically ordered arbors. Recent studies show that ephrin-B1 acts as an attractant in DV mapping and in controlling directional branch extension. Modeling indicates that proper DV mapping requires that this attractant activity cooperates with a repellent activity in a gradient that mimics ephrin-B1. We show that ectopic domains of high, graded ephrin-B1 expression created by retroviral transfection repel interstitial branches of RGC axons and redirect their extension along the LM tectal axis, away from their proper termination zones (TZs). In contrast, the primary RGC axons are unaffected and extend through the ectopic domains of ephrin-B1 and arborize at the topographically correct site. However, when the location of a TZ is coincident with ectopic domains of ephrin-B1, the domains appear to inhibit arborization and shape the distribution of arbors. Our findings indicate that ephrin-B1 selectively controls, through either attraction or repulsion, the directional extension and arborization of interstitial branches extended by RGC axons arising from the same DV position: branches that arise from axons positioned lateral to the correct TZ are attracted up the gradient of ephrin-B1 and branches that arise from axons positioned medial to the same TZ are repelled down the ephrin-B1 gradient. Alternatively, EphB receptor signaling may act as a 'ligand-density sensor' and titrate signaling pathways that promote branch extension toward an optimal ephrin-B1 concentration found at the TZ; branches located either medial or lateral to the TZ would encounter a gradient of increasingly favored attachment in the direction of the TZ.