The Eph family in the patterning of neural development

Twin study: genotype-dependent epigenetic factors affecting free thyroxine levels in the normal range

Aim: To explore the clinical application of DNA methylation affecting thyroid function, we evaluated the association of DNA methylation with free thyroxine (FT4) and TSH measurements in monozygotic twins. Materials & methods: Discordant pairs for FT4 or TSH levels were examined for the relationship between the within-pair difference of each measurement and the DNA methylation levels using epigenome-wide association studies. The contribution of polymorphisms to the methylation sensitivity was also examined. Results: We found two CpG sites significantly associated with FT4 levels, and also some CpG sites showing significant differences in their methylation levels within FT4-discordant pairs depending on the polymorphism in EPHB2. Conclusion: The FT4 level may be associated with a combination of methylation and polymorphisms in the EPHB2 gene.

Thalamocortical circuits for the formation of hierarchical pathways in the mammalian visual cortex

External sensory inputs propagate from lower-order to higher-order brain areas, and the hierarchical neural network supporting this information flow is a fundamental structure of the mammalian brain. In the visual system, multiple hierarchical pathways process different features of the visual information in parallel. The brain can form this hierarchical structure during development with few individual differences. A complete understanding of this formation mechanism is one of the major goals of neuroscience. For this purpose, it is necessary to clarify the anatomical formation process of connections between individual brain regions and to elucidate the molecular and activity-dependent mechanisms that instruct these connections in each areal pair. Over the years, researchers have unveiled developmental mechanisms of the lower-order pathway from the retina to the primary visual cortex. The anatomical formation of the entire visual network from the retina to the higher visual cortex has recently been clarified, and higher-order thalamic nuclei are gaining attention as key players in this process. In this review, we summarize the network formation process in the mouse visual system, focusing on projections from the thalamic nuclei to the primary and higher visual cortices, which are formed during the early stages of development. Then, we discuss how spontaneous retinal activity that propagates through thalamocortical pathways is essential for the formation of corticocortical connections. Finally, we discuss the possible role of higher-order thalamocortical projections as template structures in the functional maturation of visual pathways that process different visual features in parallel.

EphrinB2-RhoA upregulation attenuates sympathetic hyperinnervation and decreases the incidence of ventricular arrhythmia after myocardial infarction

BACKGROUND

Cardiac sympathetic hyperinnervation after myocardial infarction (MI) is associated with a high incidence of lethal arrhythmia. Erythropoietin-producing hepatoma interactor B2 (EphrinB2), a diffusible axonal chemorepellent that can induce growth cone collapse and axon repulsion of several neuronal populations, is crucial in neurodevelopment during disease development and progression. However, whether EphrinB2 could inhibit cardiac sympathetic hyperinnervation after MI remains unclear.

METHODS AND RESULTS

A rat model of MI was developed by left anterior descending coronary artery ligation. EphrinB2 expression was markedly increased in the infarcted border at 3 days after MI. Downregulation of EphrinB2 by intramyocardial injection of lentivirus carrying EphrinB2-shRNA significantly increased sympathetic hyperinnervation along with downregulated RhoA expression. In contrast, injection of EphrinB2-overexpressing lentivirus markedly upregulated EphrinB2, concomitant with inhibition of sympathetic sprouting and upregulated RhoA expression, accompanied by decreased incidence of ventricular arrhythmias (VAs). However, co-administering EphrinB2-overexpressing lentivirus and Fasudil (Rho kinase inhibitor) nearly abolished the inhibition of nerve sprouting effect. Additionally, EphrinB2 expression did not affect nerve growth factor level in the infarcted heart.

CONCLUSIONS

Overexpression of EphrinB2 may ameliorate MI-induced sympathetic hyperinnervation and further reduce the incidence of VAs, at least in part by activating RhoA-mediated axonal retraction.

Decreased Eph receptor‑A1 expression is related to grade in ovarian serous carcinoma

Eph receptor‑A1 (EphA1) was the first member of the erythropoietin producing hepatocellular carcinoma (Eph) family of receptor tyrosine kinases. Although the roles of EphA1 in the tumorigenesis of various human cancers have been investigated, few studies have focused on ovarian carcinoma. The present study aimed to explore the profile of EphA1 expression in ovarian carcinomas, to analyzed the association between EphA1 expression and clinicopathologic parameters, and to investigate the roles of overexpressed EphA1 in ovarian cancer cells. EphA1 protein was detected in ovarian cancer cell lines and in a set of formalin‑fixed tissues, including normal fallopian tube, ovarian benign serous cystadenoma, borderline serous tumors and serous carcinoma. Ovarian cancer cell lines HO8910 and A2780 were transiently transfected with EphA1‑pCMV6‑GFP plasmid, and the proliferation and apoptosis of cells were measured. The association between EphA1 expression and clinicopathological parameters was statistically analyzed. EphA1 expression was negative in HO8910 and weakly positive in A2780 cells. The proliferation rate was significantly reduced in ovarian cancer cells after transfection with EphA1 plasmid compared with cells transfected with mock plasmid or untreated cells, but no obvious alteration in apoptosis was detected among these groups. EphA1 expression was positively detected in all normal fallopian tubes (10/10, 100%) and ovarian benign serous cystadenomas (12/12, 100%) as well as in some borderline serous tumors (9/15, 60%) and ovarian serous carcinomas (33/76, 43.42%). EphA1 expression was associated with grade of ovarian serous carcinomas, with loss of EphA1 more often observed in high‑grade tumors (P=0.016) and high Ki67 index tumors (P=0.007). These data suggest that EphA1 might be a useful marker for distinguishing low grade from high‑grade ovarian serous carcinoma.

EphrinB2 Activation Enhances Vascular Repair Mechanisms and Reduces Brain Swelling After Mild Cerebral Ischemia

OBJECTIVE

Cerebral edema caused by the disruption of the blood-brain barrier is a major complication after stroke. Therefore, strategies to accelerate and enhance neurovascular recovery after stroke are of prime interest. Our main aim was to study the role of ephrinB2/EphB4 signaling in mediating the vascular repair and in blood-brain barrier restoration after mild cerebral ischemia occlusion/reperfusion.

APPROACH AND RESULTS

Here, we show that the guidance molecule ephrinB2 plays a key role in neurovascular protection and blood-brain barrier restoration after stroke. In a focal stroke model, we characterize the stroke-induced damage to cerebral blood vessels and their subsequent endogenous repair on a cellular, molecular, and functional level. EphrinB2 and its tyrosine kinase receptor EphB4 are upregulated early after stroke by endothelial cells and perivascular support cells, in parallel to their reassembly during neurovascular recovery. Using both retroviral and pharmacological approaches, we show that the inhibition of ephrinB2/EphB4 signaling suppresses post-middle cerebral artery occlusion neurovascular repair mechanisms resulting in an aggravation of brain swelling. In contrast, the activation of ephrinB2 after brain ischemia leads to an increased pericyte recruitment and increased endothelial-pericyte interaction, resulting in an accelerated neurovascular repair after ischemia.

CONCLUSIONS

We show that reducing swelling could result in improved outcome because of reduction in damaged brain tissue. We also identify a novel role for ephrinB2/EphB4 signaling in the maintenance of the neurovascular homeostasis and provide a novel therapeutic approach in reducing brain swelling after stroke.

Repulsive cues combined with physical barriers and cell-cell adhesion determine progenitor cell positioning during organogenesis

The precise positioning of organ progenitor cells constitutes an essential, yet poorly understood step during organogenesis. Using primordial germ cells that participate in gonad formation, we present the developmental mechanisms maintaining a motile progenitor cell population at the site where the organ develops. Employing high-resolution live-cell microscopy, we find that repulsive cues coupled with physical barriers confine the cells to the correct bilateral positions. This analysis revealed that cell polarity changes on interaction with the physical barrier and that the establishment of compact clusters involves increased cell–cell interaction time. Using particle-based simulations, we demonstrate the role of reflecting barriers, from which cells turn away on contact, and the importance of proper cell–cell adhesion level for maintaining the tight cell clusters and their correct positioning at the target region. The combination of these developmental and cellular mechanisms prevents organ fusion, controls organ positioning and is thus critical for its proper function.

The precise positioning of organ progenitor cells is essential for organ development and function. Here the authors use live imaging and mathematical modelling to show that the confinement of a motile progenitor cell population results from coupled physical barriers and cell-cell interactions.

Involvement of EphB1 receptors signalling in models of inflammatory and neuropathic pain

EphB receptors tyrosine kinases and ephrinB ligands were first identified as guidance molecules involved in the establishment of topographical mapping and connectivity in the nervous system during development. Later in development and into adulthood their primary role would switch from guidance to activity-dependent modulation of synaptic efficacy. In sensory systems, they play a role in both the onset of inflammatory and neuropathic pain, and in the establishment of central sensitisation, an NMDA-mediated form of synaptic plasticity thought to underlie most forms of chronic pain. We studied wild type and EphB1 knockout mice in a range of inflammatory and neuropathic pain models to determine 1), whether EphB1 expression is necessary for the onset and/or maintenance of persistent pain, regardless of origin; 2), whether in these models cellular and molecular changes, e.g. phosphorylation of the NR2B subunit of the NMDA receptor, increased c-fos expression or microglial activation, associated with the onset of pain, are affected by the lack of functional EphB1 receptors. Differences in phenotype were examined behaviourally, anatomically, biochemically and electrophysiologically. Our results establish firstly, that functional EphB1 receptors are not essential for the development of normal nociception, thermal or mechanical sensitivity. Secondly, they demonstrate a widespread involvement of EphB1 receptors in chronic pain. NR2B phosphorylation, c-fos expression and microglial activation are all reduced in EphB1 knockout mice. This last finding is intriguing, since microglial activation is supposedly triggered directly by primary afferents, therefore it was not expected to be affected. Interestingly, in some models of long-term pain (days), mechanical and thermal hyperalgesia develop both in wild type and EphB1 knockout mice, but recovery is faster in the latter, indicating that in particular models these receptors are required for the maintenance, rather than the onset of, thermal and mechanical hypersensitivity. This potentially makes them an attractive target for analgesic strategies.

Lrp4 is a retrograde signal for presynaptic differentiation at neuromuscular synapses

Motor axons receive retrograde signals from skeletal muscle that are essential for the differentiation and stabilization of motor nerve terminals ¹. Identification of these retrograde signals has proved elusive, but their production by muscle depends upon the receptor tyrosine kinase, MuSK, and Lrp4, a LDLR family member that forms a complex with MuSK, binds neural Agrin and stimulates MuSK kinase activity ^2–5. Here, we show that Lrp4 also functions as a direct muscle-derived retrograde signal for early steps in presynaptic differentiation. We demonstrate that Lrp4 is necessary, independent of MuSK activation, for presynaptic differentiation in vivo, and we show that Lrp4 binds to motor axons and induces clustering of synaptic vesicle and active zone proteins. Thus, Lrp4 acts bi-directionally and coordinates synapse formation by binding Agrin, activating MuSK and stimulating postsynaptic differentiation and in turn functioning as a muscle-derived retrograde signal that is necessary and sufficient for presynaptic differentiation.

APC2 plays an essential role in axonal projections through the regulation of microtubule stability

Growth cones at the tip of growing axons are key cellular structures that detect guidance cues and mediate axonal growth. An increasing number of studies have suggested that the dynamic regulation of microtubules in the growth cone plays an essential role in growth cone steering. The dynamic properties of microtubules are considered to be regulated by variegated cellular factors but, in particular, through microtubule-interacting proteins. Here, we examined the functional role of adenomatous polyposis coli-like molecule 2 (APC2) in the development of axonal projections by using the chick retinotectal topographic projection system. APC2 is preferentially expressed in the nervous system from early developmental stages through to adulthood. Immunohistochemical analysis revealed that APC2 is distributed along microtubules in growth cones as well as axon shafts of retinal axons. Overexpression of APC2 in cultured cells induced the stabilization of microtubules, whereas the knockdown of APC2 in chick retinas with specific short hairpin RNA reduced the stability of microtubules in retinal axons. APC2 knockdown retinal axons showed abnormal growth attributable to a reduced response to ephrin-A2 in vitro. Furthermore, they showed drastic alterations in retinotectal projections without making clear target zones in the tectum in vivo. These results suggest that APC2 plays a critical role in the development of the nervous system through the regulation of microtubule stability.

EphB6 receptor significantly alters invasiveness and other phenotypic characteristics of human breast carcinoma cells

Breast cancer mortality in women is largely attributed to the metastasis of primary breast tumors. We have analysed the function of EphB6, a kinase-deficient receptor, in the invasive phenotype of breast cancer cell lines. We have demonstrated the loss of EphB6 protein in invasive breast carcinoma cell lines and absence of EphB6 transcript in a metastatic breast tumor specimen. The function of EphB6 in invasiveness was confirmed by the ability of EphB6 protein to decrease the in vitro invasiveness of MDA-MB-231, MDA-MB-435 and BT549 cells transfected with an EphB6 expression construct. In MDA-MB-231 cells, the decreased invasiveness appeared to be mediated by decreased transcript levels of matrix metalloproteinase (MMP)7 and MMP19, and increased transcript levels of tissue inhibitors of metalloproteinase 2. In addition to affecting invasiveness phenotype, EphB6 overexpression was also responsible for altering the growth rate and colony-forming efficiency of MCF-7 and MDA-MB-231 cells in a cell-line-specific manner. We suggest that the significant decrease in the invasiveness of MDA-MB-231 and other cell lines transfected with EphB6 is likely occurring by the ability of EphB6 to transduce signals to the nucleus and altering relevant gene expression.

Ephrins as negative regulators of adult neurogenesis in diverse regions of the central nervous system

In the central nervous system (CNS) of adult mammals, neurogenesis occurs in only two restricted areas, the subgranular zone (SGZ) of the hippocampus and the subventricular zone (SVZ). Isolation of multipotent progenitor cells from other CNS regions suggests that their neurogenic potential is dictated by local environmental cues. Here, we report that astrocytes in areas outside of the SGZ and SVZ of adult mice express high levels of ephrin-A2 and -A3, which present an inhibitory niche, negatively regulating neural progenitor cell growth. Adult mice lacking both ephrin-A2 and -A3 display active ongoing neurogenesis throughout the CNS. These findings suggest that neural cell replacement therapies for neurodegeneration or injury in the adult CNS may be achieved by manipulating ephrin signaling pathways.

Eph, a protein family coming of age: more confusion, insight, or complexity?

Since the mid-1980s, Eph receptors have evolved from being regarded as orphan receptors with unknown functions and ligands to becoming one of the most complex "global positioning systems" that regulates cell traffic in multicellular organisms. During this time, there has been an exponentially growing interest in Ephs and ephrin ligands, coinciding with important advances in the way biological function is interrogated through mapping of genomes and manipulation of genes. As a result, many of the original concepts that used to define Eph signaling and function went overboard. Clearly, the need for progress in understanding Eph-ephrin biology and the underlying molecular principles involved has been compelling. Many cell-positioning programs during normal and oncogenic development-in particular, the patterning of skeletal, vascular, and nervous systems-are modulated in some way by Eph-ephrin function. Undeniably, the complexity of the underlying signaling networks is considerable, and it seems probable that systems biology approaches are required to further improve our understanding of Eph function.

AST-1, a novel ETS-box transcription factor, controls axon guidance and pharynx development in C. elegans

Neurons send out axons and dendrites over large distances into target areas where they eventually form synapses with selected target cells. Axonal navigation is controlled by a variety of extracellular signals and neurons express receptors only for that subset of signals they need to navigate to their own target area. How the expression of axon guidance receptors is regulated is not understood. In genetic screens for mutants with axon guidance defects, we identified an ETS-domain transcription factor, AST-1, specifically required for axon navigation in certain classes of interneurons. In addition, ast-1 has a role in the differentiation of the ventral cord pioneer neuron AVG. Outside the nervous system, ast-1 is essential for morphogenesis of the pharynx. Ast-1 is transiently expressed in several classes of neurons (including AVG) during neuronal differentiation with a peak expression during late stages of neuronal differentiation and axon outgrowth. Ast-1 genetically interacts with other transcription factors controlling neuronal differentiation like lin-11 and zag-1 as well as components of the netrin pathway suggesting that ast-1 might control the expression of components of the netrin signal transduction machinery.

Neocortical areas, layers, connections, and gene expression

Cortical patterns of gene expression provide a new approach to long standing issues of lamination, and area identity and formation. In this review, we summarize recent findings where molecular biological techniques have revealed a small number of area-specific genes in the nonhuman primate cortex. One of these (occ1) is strongly expressed in primary visual cortex and is associated with thalamocortical connections. Another gene, RBP, is more strongly expressed in association areas. It is not clear whether RBP might be linked with any particular connectional system, but several possibilities are raised. We also discuss possible roles of area-specific genes in postnatal development, and conclude with a brief sketch of future directions.

Netrin 1 regulates ventral tangential migration of guidepost neurons in the lateral olfactory tract

In the developing nervous system, functional neural networks are constructed with intricate coordination of neuronal migrations and axonal projections. We have previously reported a ventral tangential migration of a special type of cortical neurons, lot cells, in the mouse embryo. These neurons originate from the ventricular zone of the entire neocortex, tangentially migrate in the surface layer of the neocortex into the ventral direction, align in the future pathway of the lateral olfactory tract (LOT) and eventually guide the projection of LOT axons. In this study, we developed an organotypic culture system to investigate the regulation of this cell migration in the developing telencephalon. Our data show that the neocortex contains the signals that direct lot cells ventrally, that the ganglionic eminence excludes lot cells by repelling the migration and that lot cells are attracted to netrin 1, an axon guidance factor. Furthermore, we demonstrate that mutations in the genes encoding netrin 1 and its functional receptor Dcc lead to inappropriate distribution of lot cells and subsequent partial disruption of LOT projection. These results suggest that netrin 1 regulates the migration of lot cells and LOT projections, possibly by ensuring the correct distribution of these guidepost neurons.

Axon guidance processes in the retinotectal and vomeronasal projection are controlled by Eph receptor tyrosine kinases and ephrins

The Eph family of receptor tyrosine kinases and their 'ligands', the ephrins, have been shown to play key roles in a number of different developmental processes such as cell migration, boundary formation, axon guidance, synapse formation and vasculogenesis. Here, we summarize recent findings derived from investigating the role of the EphA family during development of the retinotectal and vomeronasal projection uncovering a role of ephrin-A molecules as axon guidance receptors.

Transcriptional silencing of EphB6 receptor tyrosine kinase in invasive breast carcinoma cells and detection of methylated promoter by methylation specific PCR

The receptor tyrosine kinase EphB6 is expressed at reasonable levels in normal breast cells. It shows decreased abundance in non-invasive breast carcinoma cells and is transcriptionally silenced in invasive breast carcinoma cells. We have characterized EphB6 promoter and correlated the expression of EphB6 transcript to differential methylation of the promoter region. The demethylation of promoter sequence in vivo by growth in media containing 5-aza-2'-deoxycytidine restores the expression of EphB6 to normal levels in breast carcinoma cells, and the ability of the promoter to initiate transcription of a reporter gene is lost after methylation of the promoter sequence. The promoter region has binding sites for various factors such as SP1 and p300. The specific methylation of CpG dinucleotides has allowed us to design primers that can selectively amplify the methylated promoter and thus facilitate identification of normal, non-invasive, and invasive breast cells. The potential significance of EphB6 to serve as a diagnostic and prognostic indicator is discussed.

EphB receptor tyrosine kinases control morphological development of the ventral midbrain

EphB receptor tyrosine kinases and ephrin-B ligands regulate several types of cell-cell interactions during brain development, generally by modulating the cytoskeleton. EphB/ephrinB genes are expressed in the developing neural tube of early mouse embryos with distinct overlapping expression in the ventral midbrain. To test EphB function in midbrain development, mouse embryos compound homozygous for mutations in the EphB2 and EphB3 receptor genes were examined for early brain phenotypes. These mutants displayed a morphological defect in the ventral midbrain, specifically an expanded ventral midline evident by embryonic day E9.5-10.5, which formed an abnormal protrusion into the cephalic flexure. The affected area was comprised of cells that normally express EphB2 and ephrin-B3. A truncated EphB2 receptor caused a more severe phenotype than a null mutation, implying a dominant negative effect through interference with EphB forward (intracellular) signaling. In mutant embryos, the overall number, size, and identity of the ventral midbrain cells were unaltered. Therefore, the defect in ventral midline morphology in the EphB2;EphB3 compound mutant embryos appears to be caused by cellular changes that thin the tissue, forcing a protrusion of the ventral midline into the cephalic space. Our data suggests a role for EphB signaling in morphological organization of specific regions of the developing neural tube.

Local protein synthesis by BDNF is potentiated in hippocampal neurons exposed to ephrins

Local protein synthesis in neuronal dendrites is one of the mechanisms that may mediate a rapid and synapse-specific mobilization of proteins from the resident mRNAs. A great deal of effort has been made in analyzing the dynamic state of protein synthesis in the living cells chiefly by quantifying protein level. However, the protein level cannot mirror the spatiotemporal alteration of translation because it can be affected, not only by protein synthesis, but also by other factors, like degradation. Therefore, it is problematic to visualize the dynamic state of translation by the present methods. To solve the problem, we applied fluorescence resonance energy transfer (FRET) technique to in situ detection of the assembly and disassembly cycle among a pair of translation initiation factors [eukaryotic initiation factors (eIFs)], thereby showing that BDNF and ephrin could potentiate local protein synthesis in the dendrites of hippocampal neurons.

A stochastic model for retinocollicular map development

BACKGROUND

We examine results of gain-of-function experiments on retinocollicular maps in knock-in mice [Brown et al. (2000) Cell 102:77]. In wild-type mice the temporal-nasal axis of retina is mapped to the rostral-caudal axis of superior colliculus. The established map is single-valued, which implies that each point in retina maps to a unique termination zone in superior colliculus. In homozygous Isl2/EphA3 knock-in mice the map is double-valued, which means that each point on retina maps to two termination zones in superior colliculus. This is because about 50 percent of cells in retina express Isl2, and two types of projections, wild-type and Isl2/EphA3 positive, form two branches of the map. In heterozygous Isl2/EphA3 knock-ins the map is intermediate between the homozygous and wild-type: it is single-valued in temporal and double-valued in the nasal parts of retina. In this study we address possible reasons for such a bifurcation of the map.

RESULTS

We study the map formation using stochastic model based on Markov chains. In our model the map undergoes a series of reconstructions with probabilities dependent upon a set of chemical cues. Our model suggests that the map in heterozygotes is single-valued in temporal region of retina for two reasons. First, the inhomogeneous gradient of endogenous receptor in retina makes the impact of exogenous receptor less significant in temporal retina. Second, the gradient of ephrin in the corresponding region of superior colliculus is smaller, which reduces the chemical signal-to-noise ratio. We predict that if gradient of ephrin is reduced by a genetic manipulation, the single-valued region of the map should extend to a larger portion of temporal retina, i.e. the point of transition between single-and double-valued maps should move to a more nasal position in Isl2-EphA3 heterozygotes.

CONCLUSIONS

We present a theoretical model for retinocollicular map development, which can account for intriguing behaviors observed in gain-of-function experiments by Brown et al., including bifurcation in heterozygous Isl2/EphA3 knock-ins. The model is based on known chemical labels, axonal repulsion/competition, and stochasticity. Possible mapping in Isl2/EphB knock-ins is also discussed.

Invasiveness of breast carcinoma cells and transcript profile: Eph receptors and ephrin ligands as molecular markers of potential diagnostic and prognostic application

The Eph family of receptors, with 14 members in humans, makes up the largest group of receptor tyrosine kinases. These Eph receptors, along with their ligands, the 8 members of the ephrin family of ligands are involved in diverse developmental functions, including hindbrain development in vertebrates, tissue patterning, and angiogenesis. These Eph receptors and ephrin ligands have also been identified as important regulators in the development and progression of cancer. We have presented here a systematic and comprehensive investigation of the Eph/ephrin expression profiles of MCF-10A, MCF-7, and MDA-MB-231 cells representing normal breast, non-invasive breast tumor, and invasive tumor, respectively, based on their characteristic phenotypes in Matrigel matrix. The data have allowed us to correlate the gene expression profile with the cell phenotype that has potential application in tumor diagnostics. We demonstrate here that upregulation of EphA2, A7, A10, and ephrinA2 and B3 is likely involved in tumorigenesis and/or invasiveness, while downregulation of EphA1, A3, A4, A8, B3, B4, B6, and ephrinA1 and B1 may be particularly important in invasiveness. Based on these results we discuss the role of EphA2 and ephrinA1 combination in malignancy. The data have provided clues as to the importance of these molecules in the progression of breast cancer and specifically identified EphB6, a kinase-deficient receptor, which is downregulated in the most aggressive cell line, as reported for several other cancer types including neuroblastoma and melanoma suggesting its potential as a prognostic indicator in breast cancer as well.

EphA/ephrin-A interactions during optic nerve regeneration: restoration of topography and regulation of ephrin-A2 expression

During visual system development, interactions between Eph tyrosine kinase receptors and their ligands, the ephrins, guide retinal ganglion cell (RGC) axons to their topographic targets in the optic tectum. Here we show that Eph/ephrin interactions are also involved in restoring topography during RGC axon regeneration in goldfish. Following optic nerve crush, EphA/ephrin-A interactions were blocked by intracranial injections of recombinant Eph receptor (EphA3-AP) or phospho-inositol phospholipase-C. Topographic errors with multiple inputs to some tectal loci were detected electrophysiologically and increased projections to caudal tectum demonstrated by RT-97 immunohistochemistry. In EphA3-AP-injected fish, ephrin-A2-expressing cells in the retino-recipient tectal layers were reduced in number compared to controls and their distribution was no longer graded. The findings, supported by in vitro studies, implicate EphA/ephrin-A interactions in restoring precise topography and in regulating ephrin-A2 expression during regeneration.

Potential role of Pax-2 in retinal axon navigation through the chick optic nerve stalk and optic chiasm

The degree of fiber decussation at the optic chiasm differs between species, ranging from complete crossing in lower vertebrates to highly complex patterns of intermingling of the fibers from the two eyes seen in mammals and birds. Understanding the genetic control of fiber guidance through the chiasm is therefore important to unravel the developmental mechanisms within the visual system. Here we first report on early stages of chiasm formation, with pioneering axons from the left eye consistently arriving earlier than their counterparts from the right eye. This initial left-right asymmetry is transient and no functional significance is assigned to it yet. Secondly, we examined formation of the chiasm in relation with the expression of the transcription factor Pax-2 along the ventral eye cup and optic nerve stalk. Finally, in order to examine causal involvement of Pax-2 in chiasm formation, the gene was overexpressed along the neuraxis and in the eye cup at embryonic stages preceding the exit of axons from the eye, and hence arrival of axons at the chiasm. When studied with neuroanatomical tracing, Pax-2 overexpression resulted in visibly anomalous decussation of axons at the chiasm. A likely consequence of this perturbation was erroneous arrival of axons at the tectum, as observed by anterograde staining from the retina. These data suggest that balanced expression of Pax-2 results in the correct formation of the chick chiasm at early stages by imposing accurate pathfinding within the optic stalk and the midline.

Ephrin-B1 forward and reverse signaling are required during mouse development

Eph receptors and ephrin ligands are key players in many developmental processes including embryo patterning, angiogenesis, and axon guidance. Eph/ephrin interactions lead to the generation of a bidirectional signal, in which both the Eph receptors and the ephrins activate downstream signaling cascades simultaneously. To understand the role of ephrin-B1 and the importance of ephrin-B1-induced reverse signaling during embryonic development, we have generated mouse lines carrying mutations in the efnb1 gene. Complete ablation of ephrin-B1 resulted in perinatal lethality associated with a range of phenotypes, including defects in neural crest cell (NCC)-derived tissues, incomplete body wall closure, and abnormal skeletal patterning. Conditional deletion of ephrin-B1 demonstrated that ephrin-B1 acts autonomously in NCCs, and controls their migration. Last, a mutation in the PDZ binding domain indicated that ephrin-B1-induced reverse signaling is required in NCCs. Our results demonstrate that ephrin-B1 acts both as a ligand and as a receptor in a tissue-specific manner during embryogenesis.

Syndecan-1 up-regulated by ephrinB2/EphB4 plays dual roles in inflammatory angiogenesis

EphrinB2 and EphB4, its cognate receptor, are important in the vascular development of the mouse embryo. Their roles in human inflammatory angiogenesis, however, are not well understood. By examining hyperinflammatory lesions, we saw that ephrinB2 was predominantly expressed in macrophage-like cells and EphB4 in small venules. Because macrophages usually transmigrate through postcapillary venules during inflammation, we wanted to explore the downstream effects of EphB4 after binding to ephrinB2. By using cDNA microarray technique and following reverse transcriptase-polymerase chain reaction (RT-PCR), we found that syntenin and syndecan-1 were up-regulated in EphB4-positive endothelial cells dose dependently and time dependently after stimulation with preclustered ephrinB2. In vitro, ephrinB2 suppressed the angiogenic effects of basic fibroblast growth factor (bFGF) on EphB4-positive endothelial cells, partially due to syndecan-1's competition with fibroblast growth factor receptor (FGFR) for bFGF. However, ephrinB2 exhibited angiogenic effects in vivo, possibly due to an inflammation-associated enzyme-heparanase. The enzymes could convert the inhibitory effect of ephrinB2 on EphB4-positive endothelial cells to an activating effect by removing poorly sulfated side chains of up-regulated syndecan-1 ectodomain. Depending on the presence of heparanases, the roles of syndecan-1 may be opposite in different physiological settings.

Homeobox A9 transcriptionally regulates the EphB4 receptor to modulate endothelial cell migration and tube formation

Homeobox genes (Hox) encode for transcription factors, which regulate cell proliferation and migration and play an important role in the development of the cardiovascular system during embryogenesis. In this study, we investigated the role of HoxA9 for endothelial cell migration and angiogenesis in vitro and identified a novel target gene, the EphB4 receptor. Inhibition of HoxA9 expression decreased endothelial cell tube formation and inhibited endothelial cell migration, suggesting that HoxA9 regulates angiogenesis. Because Eph receptor tyrosine kinases importantly contribute to angiogenesis, we examined whether HoxA9 may transcriptionally regulate the expression of EphB4. Downregulation of HoxA9 reduced the expression of EphB4. Chromatin-immunoprecipitation revealed that HoxA9 interacted with the EphB4 promoter, whereas a deletion construct of HoxA9 without DNA-binding motif (Delta(aa) 206-272) did not bind. Consistently, HoxA9 wild-type overexpression activated the EphB4 promoter as determined by reporter gene expression. HoxA9 binds to the EphB4 promoter and stimulates its expression resulting in an increase of endothelial cell migration and tube forming activity. Thus, modulation of EphB4 expression may contribute to the proangiogenic effect of HoxA9 in endothelial cells.

Hypogonadotropic hypogonadism due to loss of function of the KiSS1-derived peptide receptor GPR54

Hypogonadotropic hypogonadism is defined as a deficiency of the pituitary secretion of follicle-stimulating hormone and luteinizing hormone, which results in the impairment of pubertal maturation and of reproductive function. In the absence of pituitary or hypothalamic anatomical lesions and of anosmia (Kallmann syndrome), hypogonadotropic hypogonadism is referred to as isolated hypogonadotropic hypogonadism (IHH). A limited number of IHH cases are due to loss-of-function mutations of the gonadotropin-releasing hormone receptor. To identify additional gene defects leading to IHH, a large consanguineous family with five affected siblings and with a normal gonadotropin-releasing hormone receptor coding sequence was studied. Homozygosity whole-genome mapping allowed the localization of a new locus within the short arm of chromosome 19 (19p13). Sequencing of several genes localized within this region showed that all affected siblings of the family carried a homozygous deletion of 155 nucleotides in the GPR54 gene. This deletion encompassed the splicing acceptor site of intron 4-exon 5 junction and part of exon 5. The deletion was absent or present on only one allele in unaffected family members. GPR54 has been initially identified as an orphan G protein-coupled receptor with 40% homology to galanin receptors. Recently, a 54-aa peptide derived from the KiSS1 protein was identified as a ligand of GPR54. The present study shows that loss of function of GPR54 is a cause of IHH, and it identifies GPR54 and possibly KiSS1 protein-derived peptide as playing a major and previously unsuspected role in the physiology of the gonadotropic axis.

Abnormal thalamocortical pathfinding and terminal arbors lead to enlarged barrels in neonatal GAP-43 heterozygous mice

GAP-43 has been implicated in axonal pathfinding and sprouting, synaptic plasticity, and neurotransmitter release. However, its effect on cortical development in vivo is poorly understood. We have previously shown that GAP-43 knockout (-/-) mice fail to develop whisker-related barrels or an ordered whisker map in the cortex. Here we used cytochrome oxidase (CO) histochemistry to demonstrate that GAP-43 heterozygous (+/-) mice develop larger than normal barrels at postnatal day 7 (P7), despite normal body and brain weight. Using serotonin transporter (5HT-T) histochemistry to label thalamocortical afferents (TCAs), we found no obvious abnormalities in other somatosensory areas or primary visual cortex of GAP-43 (+/-) mice. However, TCA projections to (+/-) primary auditory cortex were not as clearly defined. To clarify the mechanism underlying the large-barrel phenotype, we used lipophilic (DiI) axon labeling. We found evidence for multiple pathfinding abnormalities among GAP-43 (+/-) TCAs. These axons show increased fasciculation within the internal capsule, as well as abnormal turning and branching in the subcortical white matter. These pathfinding errors most likely reflect failures of signal recognition and/or transduction by ingrowing TCAs. In addition, many DiI-labeled (+/-) TCAs exhibit widespread, sparsely branched terminal arbors in layer IV, reflecting the large-barrel phenotype. They also resemble those found in rat barrel cortex deprived of whisker inputs from birth, suggesting a failure of activity-dependent synaptogenesis and/or synaptic stabilization in (+/-) cortex. Our findings suggest that reduced GAP-43 expression can alter the fine-tuning of a cortical map through a combination of pathfinding and synaptic plasticity mechanisms.

The EphB6 receptor inhibits JNK activation in T lymphocytes and modulates T cell receptor-mediated responses

EphB6 is the most recently identified member of the Eph receptor tyrosine kinase family. EphB6 is primarily expressed in thymocytes and a subpopulation of T cells, suggesting that it may be involved in regulation of T lymphocyte differentiation and functions. We show here that overexpression of EphB6 in Jurkat T cells and stimulation with the EphB6 ligand, ephrin-B1, results in the selective inhibition of TCR-mediated activation of JNK but not the MAPK pathway. EphB6 appears to suppress the JNK pathway by preventing T cell receptor (TCR)-induced activation of the small GTPase Rac1, a critical event in initiating the JNK cascade. Furthermore, EphB6 blocked anti-CD3-induced secretion of IL-2 and CD25 expression in a ligand-dependent manner. Dominant negative EphB6 suppressed the inhibitory activity of the endogenous receptor and enhanced anti-CD3-induced JNK activation, CD25 expression, and IL-2 secretion, confirming the requirement for EphB6-specific signaling. Activation of the JNK pathway and the establishment of an IL-2/IL-2R autocrine loop have been shown to play a role in the negative selection of CD4(+)CD8(+) self-reacting thymocytes. In agreement, stimulation of murine thymocytes with ephrin-B1 not only blocked anti-CD3-induced CD25 up-regulation and IL-2 production, but also inhibited TCR-mediated apoptosis. Thus, EphB6 may play an important role in regulating thymocyte differentiation and modulating responses of mature T cells.

Cytoarchitecture and fiber pattern of the superior colliculus are disrupted in the Shaking Rat Kawasaki

Shaking Rat Kawasaki (SRK) is a Reelin-deficient rat, that shows significant cytoarchitectural abnormalities in the cerebral and cerebellar cortices in a similar manner to the reeler malformation. In the present study, we investigated the cytoarchitecture and myeloarchitecture of the superior colliculus (SC) of this mutant rat. The Nissl staining clearly showed that neuronal components in the superficial layers of the SC in SRK rat were intermingled with each other and that the boundaries between these superficial layers were blurred. The MBP immunohistochemistry showed an abnormal fiber pattern in the superficial layers of the SC of this mutant rat. In the normal rat, myelinated fibers passed rostrocaudally through the optic layer, and only a few myelinated fibers were recognized in the uppermost two layers, i.e., the zonal and superficial gray layers. By contrast, in SRK rat, the myelinated fibers were distributed throughout the entire thickness of the superficial layers of the SC. Anterograde labeling of retinotectal fibers with an injection of Cholera Toxin subunit B into the retina revealed that this abnormal fiber pattern was associated with the anomalous course of the retinotectal fibers. No distinct differences in the cytoarchitecture and fiber pattern in the deep layers of the SC were seen. In conclusion, the present study demonstrated that the cytoarchitecture and fiber patterning in the superficial layers of the SC were disrupted in SRK rat, suggesting that Reelin protein regulates the formation of the superficial layers of the SC.

Mammalian neural stem-cell renewal: nature versus nurture

Recent data show that the final events of mammalian brain organogenesis may depend in part on the direct control of neural stem cell (NSC) proliferation and survival. Environmental and intrinsic factors play a role throughout development and during adulthood to regulate NSC proliferation. The NSCs acquire new competences throughout development, including adulthood, and this change in competence is region-specific. The factors controlling NSC survival, undifferentiated state, proliferation, and cell-cycle number are beginning to be identified, but the links between them remain unclear. However, current knowledge should help to formulate an understanding of how a stem cell can generate a new stem cell.

Importance of vascular phenotype by basic fibroblast growth factor, and influence of the angiogenic factors basic fibroblast growth factor/fibroblast growth factor receptor-1 and ephrin-A1/EphA2 on melanoma progression

The expression of several angiogenic factors and receptors was examined in a series of vertical growth phase cutaneous melanomas using high-throughput tissue microarray technology and immunohistochemistry. The results were correlated with microvessel density, clinicopathological features, and patient survival. Expression of basic fibroblast growth factor (bFGF) was significantly associated with increased microvessel density. Also, we found an independent prognostic importance of vascular phenotype by endothelial cell expression of bFGF; cases with positive vessels had the best prognosis and these tumors revealed a low frequency of vascular invasion (14%) when compared with bFGF-negative vessels (47%). This bFGF-negative phenotype was significantly increased in metastatic lesions. Strong tumor cell expression of FLT-4, ephrin-A1, and EphA2 was associated with increased melanoma thickness, and ephrin-A1 staining was related to decreased survival (P = 0.039). Expression of EphA2 in tumor cells was associated with increased tumor cell proliferation (Ki-67 positivity), indicating possible autocrine growth stimulation. Thus, our findings indicate the presence of phenotypic diversity among tumor-associated vessels, and subgroups defined by bFGF expression may be of clinical importance. bFGF was associated with microvessel density, whereas the ephrin-A1/EphA2 pathway might also be important for tumor cell proliferation and patient survival.

The kinase-null EphB6 receptor undergoes transphosphorylation in a complex with EphB1

Uniquely for the Eph family of receptor tyrosine kinases, the EphB6 receptor is catalytically inactive due to the alteration of several critical residues in its kinase domain. This has cast doubt upon its ability to participate in cytoplasmic signaling events. We show here that despite its lack of kinase activity, EphB6 undergoes inducible tyrosine phosphorylation upon stimulation with the Eph-B receptor subfamily ligand ephrin-B1. We also demonstrate, for the first time, evidence of cross-talk between Eph receptors. Overexpression of a catalytically active member of the Eph-B subfamily, EphB1, resulted in increased EphB6 phosphorylation. EphB1-induced EphB6 phosphorylation was ligand-dependent and required the functional catalytic activity of EphB1. EphB1 not only transphosphorylated EphB6, but together they also formed a stable hetero-complex. In addition, we identify the proto-oncogene c-Cbl as an EphB6-binding protein. Although EphB6-Cbl association appeared to be constitutive, Cbl required a functional phosphotyrosine binding domain in order to bind the receptor, whereas its RING finger motif ubiquitin-transfer domain was not necessary. Our findings demonstrate that EphB6 is an actively signaling receptor that undergoes transphosphorylation upon ligand binding and that can initiate specific cytoplasmic signaling events.

The receptor tyrosine kinase EphB2 regulates NMDA-dependent synaptic function

Members of the Eph family of receptor tyrosine kinases control many aspects of cellular interactions during development, including axon guidance. Here, we demonstrate that EphB2 also regulates postnatal synaptic function in the mammalian CNS. Mice lacking the EphB2 intracellular kinase domain showed wild-type levels of LTP, whereas mice lacking the entire EphB2 receptor had reduced LTP at hippocampal CA1 and dentate gyrus synapses. Synaptic NMDA-mediated current was reduced in dentate granule neurons in EphB2 null mice, as was synaptically localized NR1 as revealed by immunogold localization. Finally, we show that EphB2 is upregulated in hippocampal pyramidal neurons in vitro and in vivo by stimuli known to induce changes in synaptic structure. Together, these data demonstrate that EphB2 plays an important role in regulating synaptic function.

Improving axonal growth and functional recovery after experimental spinal cord injury by neutralizing myelin associated inhibitors

Injuries of the spinal cord often result in an irretrievable loss of motor and sensory functions of all body parts situated below the lesion site. Functional recovery is restricted mainly due to the limited regeneration and plasticity of injured axons in the adult central nervous system. Over the last few years different experimental approaches have led to axonal growth and functional benefits in animal models. This review focuses on the effects of the neutralization of myelin-associated neurite growth inhibitors, in particular Nogo-A, using the monoclonal antibody IN-1. Acute mAb IN-1 treatment of adult CNS lesioned rats results in extensive plastic changes of neuronal connections and regenerative fiber growth. In two different lesion paradigms (i.e. pyramidal tract lesion and incomplete spinal cord lesion in adult rats), the mAb IN-1-treated animals always showed a higher degree of recovery in various behavioral tests. These observations, together with electrophysiological results, suggest that neuronal CNS circuits of mAb IN-1-treated animals can be rearranged, and that sprouting and regenerating axons form functionally meaningful connections.

Nck/Dock: an adapter between cell surface receptors and the actin cytoskeleton

In response to extracellular signals, cell surface receptors engage in connections with multiple intracellular signaling pathways, leading to the cellular responses such as survival, migration, proliferation and differentiation. The 'pY-->SH2/SH3-->effector' connection is a frequently used scheme by many cell surface receptors, in which SH2/SH3-containing adapters connect protein tyrosine phosphorylation to a variety of downstream effector pathways. Following the initial landmark finding that Grb2 adapter links the receptors to the Ras pathway leading to DNA synthesis, recent studies have revealed that the biological function of the SH2/SH3 adapter Nck/Dock is to link cell surface receptors to the actin cytoskeleton. For example, in the evolutionarily-conserved signaling network, GEF-Rac-Nck-Pak, Nck 'fixes up' the interaction of Pak with its upstream activator, Rac. The activated Pak then regulates the cytoskeletal dynamics. The fact that the majority of the more than 20 Nck-SH3-associated effectors are regulators of the actin cytoskeleton suggests that Nck/Dock regulates, via binding to distinct effectors, various cell type-specific motogenic responses. This review focuses on our current understanding of Nck/Dock function. Due to the number and complexity of the terminologies used in this review, a 'Glossary of Terms' is provided to help reduce confusions.

Ephrin-A6, a new ligand for EphA receptors in the developing visual system

In the embryonic visual system, EphA receptors are expressed on both temporal and nasal retinal ganglion cell axons. Only the temporal axons, however, are sensitive to the low concentrations of ephrin-A ligands found in the anterior optic tectum. The poor responsiveness of nasal axons to ephrin-A ligands, which allows them to traverse the anterior tectum and reach their targets in the posterior tectum, has been attributed to constitutive activation of the EphA4 receptor expressed in these axons. EphA4 is highly expressed throughout the retina, but is preferentially phosphorylated on tyrosine (activated) in nasal retina. In a screen for EphA4 ligands expressed in chicken embryonic retina, we have identified a novel ephrin, ephrin-A6. Like ephrin-A5, ephrin-A6 has high affinity for EphA4 and activates this receptor in cultured retinal cells. In the embryonic day 8 (E8) chicken visual system, ephrin-A6 is predominantly expressed in the nasal retina and ephrin-A5 in the posterior tectum. Thus, ephrin-A6 has the properties of a ligand that activates the EphA4 receptor in nasal retinal cells. Ephrin-A6 binds with high affinity to several other EphA receptors as well and causes growth cone collapse in retinal explants, demonstrating that it can elicit biological responses in retinal neurons. Ephrin-A6 expression is high at E6 and E8, when retinal axons grow to their tectal targets, and gradually declines at later developmental stages. The asymmetric distribution of ephrin-A6 in retinal cells, and the time course of its expression, suggest that this new ephrin plays a role in the establishment of visual system topography.

Two homeobox genes define the domain of EphA3 expression in the developing chick retina

Graded expression of the Eph receptor EphA3 in the retina and its two ligands, ephrin A2 and ephrin A5 in the optic tectum, the primary target of retinal axons, have been implicated in the formation of the retinotectal projection map. Two homeobox containing genes, SOHo1 and GH6, are expressed in a nasal-high, temporal-low pattern during early retinal development, and thus in opposing gradients to EphA3. Retroviral misexpression of SOHo1 or GH6 completely and specifically repressed EphA3 expression in the neural retina, but not in other parts of the central nervous system, such as the optic tectum. Under these conditions, some temporal ganglion cell axons overshot their expected termination zones in the rostral optic tectum, terminating aberrantly at more posterior locations. However, the majority of ganglion cell axons mapped to the appropriate rostrocaudal locations, although they formed somewhat more diffuse termination zones. These findings indicate that other mechanisms, in addition to differential EphA3 expression in the neural retina, are required for retinal ganglion axons to map to the appropriate rostrocaudal locations in the optic tectum. They further suggest that the control of topographic specificity along the retinal nasal-temporal axis is split into several independent pathways already at a very early time in development.

Eph receptors and ephrin ligands: embryogenesis to tumorigenesis

Protein tyrosine kinase genes are the largest family of oncogenes. This is not surprising since tyrosine kinases are important components of signal transduction pathways that control cell shape, proliferation, differentiation, and migration. At 14 distinct members, the Eph kinases constitute the largest family of receptor tyrosine kinases. Although they have been most intensively studied for their roles in embryonic development, increasing evidence also implicates Eph family proteins in cancer. This review will address the recent progress in understanding the function of Eph receptors in normal development and how disregulation of these functions could promote tumorigenesis.

Disruption of Eph/ephrin signaling affects migration and proliferation in the adult subventricular zone

The subventricular zone (SVZ) of the lateral ventricles, the largest remaining germinal zone of the adult mammalian brain, contains an extensive network of neuroblasts migrating rostrally to the olfactory bulb. Little is known about the endogenous proliferation signals for SVZ neural stem cells or guidance cues along the migration pathway. Here we show that the receptor tyrosine kinases EphB1-3 and EphA4 and their transmembrane ligands, ephrins-B2/3, are expressed by cells of the SVZ. Electron microscopy revealed ephrin-B ligands associated with SVZ astrocytes, which function as stem cells in this germinal zone. A three-day infusion of the ectodomain of either EphB2 or ephrin-B2 into the lateral ventricle disrupted migration of neuroblasts and increased cell proliferation. These results suggest that Eph/ephrin signaling is involved in the migration of neuroblasts in the adult SVZ and in either direct or indirect regulation of cell proliferation.

Protein tyrosine kinases in malignant melanoma

Protein tyrosyl phosphorylation is an essential component in intracellular signalling, with diverse and crucial functions including mediation of cell proliferation, survival, death, differentiation, migration and attachment. It is regulated by the balance between the activities of protein tyrosine kinases (PTKs) and protein tyrosine phosphatases. A number of PTKs are encoded by proto-oncogenes or viral oncogenes, and are thus strongly implicated in cancer. While a role for PTKs in human melanoma is less firmly established, human melanomas or melanoma cells have been reported to contain more tyrosine phosphate than normal melanocytes, and some receptor PTKs (EPH-A2/ ECK and EPH-B3) are overexpressed in over 90% of melanoma cell lines. Other specific PTKs are also frequently overexpressed, including KDR and fibroblast growth factor receptor-4 (FGF-R4), while, interestingly, yet others, such as KIT and FES, are consistently downregulated in melanoma cell lines. All of these differentially expressed PTKs are candidates for gene products important in melanoma development. In addition, PTKs expressed in significant amounts in both benign and malignant melanocytes, such as insulin-like growth factor-1 receptor (IGF1-R), FGF-R1, HER2/NEU and FAK, are likely to play a role in melanoma genesis and progression.

Targeting of the EphA4 tyrosine kinase receptor affects dorsal/ventral pathfinding of limb motor axons

The Eph family of tyrosine kinase receptors has recently been implicated in various processes involving the detection of environmental cues such as axonal guidance, targeted cell migration and boundary formation. We have inactivated the mouse EphA4 gene to investigate its functions during development. Homozygous EphA4 mutant animals show peroneal muscular atrophy correlating with the absence of the peroneal nerve, the main dorsal nerve of the hindlimb. This phenotype is also observed, although with a lower penetrance, in heterozygotes. During normal hindlimb innervation, motor axons converge towards the sciatic plexus region at the base of the limb bud, where they must choose between dorsal and ventral trajectories within the limb. Among the axons emerging from the sciatic plexus, dorsal projections show higher levels of EphA4 protein than ventral axons. In EphA4 mutant mice, presumptive dorsal motor axons fail to enter the dorsal compartment of the limb and join the ventral nerve. Our data therefore suggest that the level of EphA4 protein in growing limb motor axons is involved in the selection of dorsal versus ventral trajectories, thus contributing to the topographic organisation of motor projections.

Ephrins stimulate or inhibit neurite outgrowth and survival as a function of neuronal cell type

The Eph family of tyrosine kinase receptors and ligands play key roles in cell segregation and axon targeting in the developing nervous system. Interactions between the ligands and receptors cause repulsion or degeneration of receptor-positive axons from several brain regions including the retina, hippocampus, thalamus, and midbrain dopaminergic system. We extend these previous observations by showing that three A-ephrins also negatively regulate the growth of neurites from striatal and olfactory neurons. In addition to negative effects, however, we also report a trophic activity of the A-ephrins: Ephrin-A2 and A5 promote survival and neurite outgrowth of sympathetic neurons. These observations provide support to the notion that ephrins may function as either negative or positive signals in the developing nervous system.

Nogo-A, a potent inhibitor of neurite outgrowth and regeneration

The lack of regrowth of injured neurons in the adult central nervous system (CNS) of higher vertebrates was accepted as a fact for many decades. In the last few years a very different view emerged; regeneration of lesioned fibre tracts in vivo could be induced experimentally, and molecules that are responsible for inhibition and repulsion of growing neurites have been defined. Mechanisms that link cellular phenomena like growth cone turning or growth cone collapse to intracellular changes in second messenger systems and cytoskeletal dynamics became unveiled. This article reviews recent developments in this field, focusing especially on one of the best characterised neurite out-growth inhibitory molecules found in CNS myelin that was recently cloned: Nogo-A. Nogo-A is a high molecular weight transmembrane protein and an antigen of the monoclonal antibody mAb IN-1 that was shown to promote long-distance regeneration and functional recovery in vivo when applied to spinal cord-injured adult rats. Nogo-A is expressed by oligodendrocytes in white matter of the CNS. With the molecular characterisation of this factor new possibilities open up to achieve structural and functional repair of the injured CNS.

Expression domains of murine ephrin-A5 in the pituitary and hypothalamus

Using in situ hybridization on whole-mounts and sections of mouse embryos we have visualized the pattern of expression for the Eph receptor ligand ephrin-A5. Non neuronal expression domains include the ectoderm of the branchial arches, the ectoderm and mesenchyme surrounding the dorsal root ganglia, the intervertebral discs, maxillary and mandibulary mesenchymal elements as well as the nasal mesenchyme and ectoderm. Within the developing nervous system, ephrin-A5 expression is very dynamic. Besides the midbrain it is also expressed in the hypothalamus, and the neurohypophysis that we studied here in more detail. Hypothalamus expression of ephrin-A5 demarks distinct nuclei, persists throughout embryonic development, and can be seen also in the adult.

Ephrin-A5 induces collapse of growth cones by activating Rho and Rho kinase

The ephrins, ligands of Eph receptor tyrosine kinases, have been shown to act as repulsive guidance molecules and to induce collapse of neuronal growth cones. For the first time, we show that the ephrin-A5 collapse is mediated by activation of the small GTPase Rho and its downstream effector Rho kinase. In ephrin-A5-treated retinal ganglion cell cultures, Rho was activated and Rac was downregulated. Pretreatment of ganglion cell axons with C3-transferase, a specific inhibitor of the Rho GTPase, or with Y-27632, a specific inhibitor of the Rho kinase, strongly reduced the collapse rate of retinal growth cones. These results suggest that activation of Rho and its downstream effector Rho kinase are important elements of the ephrin-A5 signal transduction pathway.

Development of the visual system of the chick. I. Cell differentiation and histogenesis

This review summarizes present knowledge on the embryonic development of the avian visual projections, based on the domestic chick as a model system. The reductionist goal to understand formation and function of complex neuroanatomical systems on a causal level requires a synthesis of classic developmental biology with recent advances on the molecular mechanisms of cell differentiation and histogenesis. It is the purpose of this article. We are discussing the processes underlying patterning of the anterior neural tube, when the retina and optic tectum are specified and their axial polarity is determined. Then the development of these structures is described from the molecular to the anatomical level. Following sections deal with the establishment of secondary visual connections, and the developmental interactions between compartments of the retinotectal system. Using this latter pathway, from the retina to the optic tectum, many investigations aimed at mechanisms of axonal pathfinding and connectivity have accumulated a vast body of research, which will be covered by a following review.