Eph receptors and ephrin ligands in axon guidance

One Raft to Guide Them All, and in Axon Regeneration Inhibit Them

Central nervous system damage caused by traumatic injuries, iatrogenicity due to surgical interventions, stroke and neurodegenerative diseases is one of the most prevalent reasons for physical disability worldwide. During development, axons must elongate from the neuronal cell body to contact their precise target cell and establish functional connections. However, the capacity of the adult nervous system to restore its functionality after injury is limited. Given the inefficacy of the nervous system to heal and regenerate after damage, new therapies are under investigation to enhance axonal regeneration. Axon guidance cues and receptors, as well as the molecular machinery activated after nervous system damage, are organized into lipid raft microdomains, a term typically used to describe nanoscale membrane domains enriched in cholesterol and glycosphingolipids that act as signaling platforms for certain transmembrane proteins. Here, we systematically review the most recent findings that link the stability of lipid rafts and their composition with the capacity of axons to regenerate and rebuild functional neural circuits after damage.

Multiple Regionalized Genes and Their Putative Networks in the Interpeduncular Nucleus Suggest Complex Mechanisms of Neuron Development and Axon Guidance

The interpeduncular nucleus (IPN) is a highly conserved limbic structure in the vertebrate brain, located in the isthmus and rhombomere 1. It is formed by various populations that migrate from different sites to the distinct domains within the IPN: the prodromal, rostral interpeduncular, and caudal interpeduncular nuclei. The aim here was to identify genes that are differentially expressed across these domains, characterizing their putative functional roles and interactions. To this end, we screened the 2,038 genes in the Allen Developing Mouse Brain Atlas database expressed at E18.5 and we identified 135 genes expressed within the IPN. The functional analysis of these genes highlighted an overrepresentation of gene families related to neuron development, cell morphogenesis and axon guidance. The interactome analysis within each IPN domain yielded specific networks that mainly involve members of the ephrin/Eph and Cadherin families, transcription factors and molecules related to synaptic neurotransmission. These results bring to light specific mechanisms that might participate in the formation, molecular regionalization, axon guidance and connectivity of the different IPN domains. This genoarchitectonic model of the IPN enables data on gene expression and interactions to be integrated and interpreted, providing a basis for the further study of the connectivity and function of this poorly understood nuclear complex under both normal and pathological conditions.

Development of the Vertebrate Trunk Sensory System: Origins, Specification, Axon Guidance, and Central Connectivity

Crucial to an animal's movement through their environment and to the maintenance of their homeostatic physiology is the integration of sensory information. This is achieved by axons communicating from organs, muscle spindles and skin that connect to the sensory ganglia composing the peripheral nervous system (PNS), enabling organisms to collect an ever-constant flow of sensations and relay it to the spinal cord. The sensory system carries a wide spectrum of sensory modalities - from sharp pain to cool refreshing touch - traveling from the periphery to the spinal cord via the dorsal root ganglia (DRG). This review covers the origins and development of the DRG and the cells that populate it, and focuses on how sensory connectivity to the spinal cord is achieved by the diverse developmental and molecular processes that control axon guidance in the trunk sensory system. We also describe convergences and differences in sensory neuron formation among different vertebrate species to gain insight into underlying developmental mechanisms.

MicroRNAs in Retinal Development

The small RNA regulatory molecules called microRNAs (miRNAs) play key roles in the development of most organisms. The expression of many different miRNAs has been described in the developing and mature vertebrate retina. The ability of miRNAs to regulate a diversity of messenger RNA targets allows them to have effects on many different developmental processes, but the functions of only a few miRNAs have been documented to date. Developmental transitions between cell states appear to be particularly sensitive to miRNA loss of function, as evidenced by specific miRNA knockdowns or from global perturbations in miRNA levels (e.g., Dicer deletion). However, we are still in only the very early stages of understanding the range of cellular functions miRNAs control during development.

Single-cell transcriptomics of the developing lateral geniculate nucleus reveals insights into circuit assembly and refinement

Neurons and nonneuronal cells in the developing brain dynamically regulate gene expression as neural connectivity is established. However, the specific gene programs activated in distinct cell populations during the assembly and refinement of many intact neuronal circuits have not been thoroughly characterized. In this study, we take advantage of recent advances in transcriptomic profiling techniques to characterize gene expression in the postnatal developing lateral geniculate nucleus (LGN) at single-cell resolution. Our data reveal that genes involved in brain development are dynamically regulated in all major cell types of the LGN, suggesting that the establishment of neural connectivity depends upon functional collaboration between multiple neuronal and nonneuronal cell types in this brain region.

Coordinated changes in gene expression underlie the early patterning and cell-type specification of the central nervous system. However, much less is known about how such changes contribute to later stages of circuit assembly and refinement. In this study, we employ single-cell RNA sequencing to develop a detailed, whole-transcriptome resource of gene expression across four time points in the developing dorsal lateral geniculate nucleus (LGN), a visual structure in the brain that undergoes a well-characterized program of postnatal circuit development. This approach identifies markers defining the major LGN cell types, including excitatory relay neurons, oligodendrocytes, astrocytes, microglia, and endothelial cells. Most cell types exhibit significant transcriptional changes across development, dynamically expressing genes involved in distinct processes including retinotopic mapping, synaptogenesis, myelination, and synaptic refinement. Our data suggest that genes associated with synapse and circuit development are expressed in a larger proportion of nonneuronal cell types than previously appreciated. Furthermore, we used this single-cell expression atlas to identify the Prkcd-Cre mouse line as a tool for selective manipulation of relay neurons during a late stage of sensory-driven synaptic refinement. This transcriptomic resource provides a cellular map of gene expression across several cell types of the LGN, and offers insight into the molecular mechanisms of circuit development in the postnatal brain.

N-Cadherin modified lipid bilayers promote neural network formation and circuitry

Neural adhesion, maturation, and the correct wiring of the brain to establish each neuron's intended connectivity are controlled by complex interactions of bioactive molecules such as ligands, growth factors, or enzymes. The correct pairing of adjacent neurons is thought to be highly regulated by ligand-mediated cell-cell adhesion proteins, which are known to induce signaling activities. We developed a new platform consisting of supported lipid bilayers incorporated with Fc-chimera synaptic proteins like ephrinA5 or N-cadherin. We extensively characterized their function employing a quartz crystal microbalance with dissipation (QCM-D), calcium imaging, and immunofluorescence analysis. Our biomimetic platform has been shown to promote neural cell adhesion and to improve neural maturation at day in vitro 7 (DIV7) as indicated by an elevated expression of synaptophysin.

Ephrin-B2 overexpression predicts for poor prognosis and response to therapy in solid tumors

Ephrin B2 is variably expressed on tumor cells and its blockade has been shown to inhibit angiogenesis in animal models of pancreatic, colorectal, lung and head, and neck squamous cell carcinomas. However, the implications of ephrinB2 expression in cancer patients have remained elusive. In this study, we analyzed the cancer genome atlas (TCGA) for ephrinB2 expression. We report significant correlations between EFNB2 expression, overall survival and disease-free survival in head and neck squamous cell carcinoma (HNSCC, n = 519), pancreatic adenocarcinoma (n = 186), and bladder urothelial carcinoma (n = 410). In HNSCC patients, high-EFNB2 mRNA expression was associated with tumor HPV negativity, oral cavity location, alcohol intake, higher TP53 mutation, and EGFR amplification. EphrinB2 overexpression also correlated with worse response to chemotherapy and radiotherapy. The therapeutic potential of blocking ephrinB2 was validated in HNSCC patient-derived tumor xenografts and showed significant improvement in survival and tumor growth delay. Our data shows that ephrinB2 overexpression can serve as a critical biomarker for patient prognosis and response to therapy. These results should guide design of future clinical trials exploring EphrinB2 inhibition in cancer patients. © 2016 Wiley Periodicals, Inc.

Reconstitution of Fusion Proteins in Supported Lipid Bilayers for the Study of Cell Surface Receptor-Ligand Interactions in Cell-Cell Contact

Bioactive molecules such as adhesion ligands, growth factors, or enzymes play an important role in modulating cell behavior such as cell adhesion, spreading, and differentiation. Deciphering the mechanism of ligand-mediated cell adhesion and associated signaling is of great interest not only for fundamental biophysical investigations but also for applications in medicine and biotechnology. In the presented work, we developed a new biomimetic platform that enables culturing primary neurons and testing cell surface-receptor ligand interactions in cell-cell contacts as, e.g., in neuronal synapses. This platform consists of a supported lipid bilayer modified with incorporated neuronal adhesion proteins conjugated with the Fc-domain of IgG (ephrin A5 Fc-chimera). We extensively characterized properties of these protein containing bilayers using fluorescence recovery after photobleaching (FRAP), quartz crystal microbalance with dissipation (QCM-D), and immunostaining. We conclude that the Fc-domain is the part responsible for the incorporation of the protein into the bilayer. The biomimetic platform prepared by this new approach was able to promote neuronal cell adhesion and maintain growth as well as facilitate neuronal maturation as shown by electrophysiological measurements. We believe that our approach can be extended to insert other proteins to create a general culture platform for neurons and other cell types.

Ephrin-B/EphB Signaling Is Required for Normal Innervation of Lingual Gustatory Papillae

The innervation of taste buds is an excellent model system for studying the guidance of axons during targeting because of their discrete nature and the high fidelity of innervation. The pregustatory epithelium of fungiform papillae is known to secrete diffusible axon guidance cues such as BDNF and Sema3A that attract and repel, respectively, geniculate ganglion axons during targeting, but diffusible factors alone are unlikely to explain how taste axon terminals are restricted to their territories within the taste bud. Nondiffusible cell surface proteins such as Ephs and ephrins can act as receptors and/or ligands for one another and are known to control axon terminal positioning in several parts of the nervous system, but they have not been studied in the gustatory system. We report that ephrin-B2 linked β-galactosidase staining and immunostaining was present along the dorsal epithelium of the mouse tongue as early as embryonic day 15.5 (E15.5), but was not detected at E14.5, when axons first enter the epithelium. Ephrin-B1 immunolabeling was barely detected in the epithelium and found at a somewhat higher concentration in the mesenchyme subjacent to the epithelium. EphB1 and EphB2 were detected in lingual sensory afferents in vivo and geniculate neurites in vitro. Ephrin-B1 and ephrin-B2 were similarly effective in repelling or suppressing outgrowth by geniculate neurites in vitro. These in vitro effects were independent of the neurotrophin used to promote outgrowth, but were reduced by elevated levels of laminin. In vivo, mice null for EphB1 and EphB2 exhibited decreased gustatory innervation of fungiform papillae. These data provide evidence that ephrin-B forward signaling is necessary for normal gustatory innervation of the mammalian tongue.

Role of EFNB2/EPHB4 signaling in spiral artery development during pregnancy: An appraisal

EFNB2 and EPHB4, which belong to a large tyrosine kinase receptor superfamily, are molecular markers of arterial and venous blood vessels, respectively. EFNB2/EPHB4 signaling plays an important role in physiological and pathological angiogenesis, and its role in tumor vessel development has been extensively studied. Pregnancy and tumors share similar features, including continuous cell proliferation and increased demand for a blood supply. Our previous studies showed that Efnb2 and Ephb4 were expressed dynamically in the spiral arteries, uterine natural killer cells, and trophoblasts during mouse gestation Days 6.5-12.5. Moreover, uterine natural killer cells and trophoblasts are required for the modification of spiral arteries. Oxygen tension within the pregnant uterus, which contributes to the vascular development, also affects EFNB2 and EPHB4 expression. Considering the role of EFNB2/EPHB4 signaling in embryonic and tumor vascular development, and its dynamic expression in the decidua and placenta, we hypothesize that EFNB2 and EPHB4 are involved in the regulation of spiral artery remodeling. Investigating this hypothesis will help clarify the mechanisms of pathological pregnancy that may underlie abnormal spiral artery development.

Contribution of the Runx1 transcription factor to axonal pathfinding and muscle innervation by hypoglossal motoneurons

The runt-related transcription factor Runx1 contributes to cell type specification and axonal targeting projections of the nociceptive dorsal root ganglion neurons. Runx1 is also expressed in the central nervous system, but little is known of its functions in brain development. At mouse embryonic day (E) 17.5, Runx1-positive neurons were detected in the ventrocaudal subdivision of the hypoglossal nucleus. Runx1-positive neurons lacked calcitonin gene-related peptide (CGRP) expression, whereas Runx1-negative neurons expressed CGRP. Expression of CGRP was not changed in Runx1-deficient mice at E17.5, suggesting that Runx1 alone does not suppress CGRP expression. Hypoglossal axon projections to the intrinsic vertical (V) and transverse (T) tongue muscles were sparser in Runx1-deficient mice at E17.5 compared to age-matched wild-type littermates. Concomitantly, vesicular acetylcholine transporter-positive axon terminals and acetylcholine receptor clusters were less dense in the V and T tongue muscles of Runx1-deficient mice. These abnormalities in axonal projection were not caused by a reduction in the total number hypoglossal neurons, failed synaptogenesis, or tongue muscles deficits. Our results implicate Runx1 in the targeting of ventrocaudal hypoglossal axons to specific tongue muscles. However, Runx1 deficiency did not alter neuronal survival or the expression of multiple motoneuron markers as in other neuronal populations. Thus, Runx1 appears to have distinct developmental functions in different brain regions.

EphB2 regulates contact-dependent and contact-independent signaling to control platelet function

The Eph kinases, EphA4 and EphB1, and their ligand, ephrinB1, have been previously reported to be present in platelets where they contribute to thrombus stability. Although thrombus formation allows for Eph-ephrin engagement and bidirectional signaling, the importance specifically of Eph kinase or ephrin signaling in regulating platelet function remained unidentified. In the present study, a genetic approach was used in mice to establish the contribution of signaling orchestrated by the cytoplasmic domain of EphB2 (a newly discovered Eph kinase in platelets) in platelet activation and thrombus formation. We conclude that EphB2 signaling is involved in the regulation of thrombus formation and clot retraction. Furthermore, the cytoplasmic tail of this Eph kinase regulates initial platelet activation in a contact-independent manner in the absence of Eph-ephrin ligation between platelets. Together, these data demonstrate that EphB2 signaling not only modulates platelet function within a thrombus but is also involved in the regulation of the function of isolated platelets in a contact-independent manner.

Discoidin domain receptors guide axons along longitudinal tracts in C. elegans

Discoidin domain receptors are a family of receptor tyrosine kinases activated by collagens. Here we characterize the role of the two discoidin domain receptors, ddr-1 and ddr-2, of the nematode C. elegans during nervous system development. ddr-2 mutant animals exhibit axon guidance defects in major longitudinal tracts most prominently in the ventral nerve cord. ddr-1 mutants show no significant phenotype on their own but significantly enhance guidance defects of ddr-2 in double mutants. ddr-1 and ddr-2 GFP-reporter constructs are expressed in neurons with axons in all affected nerve tracts. DDR-1 and DDR-2 GFP fusion proteins localize to axons. DDR-2 is required cell-autonomously in the PVPR neuron for the guidance of the PVPR pioneer axon, which establishes the left ventral nerve cord tract and serves as substrate for later outgrowing follower axons. Our results provide the first insight on discoidin domain receptor function in invertebrates and establish a novel role for discoidin domain receptors in axon navigation and axon tract formation.

Ephrin-A5 deficiency alters sensorimotor and monoaminergic development

The Eph receptors and their ligands, the ephrins, play an important role during neural development. In particular, ephrin-A5 is highly expressed in the developing nervous system in several brain regions including the olfactory bulb, frontal cortex, striatum and hypothalamus. Although a number of studies have characterized the expression of ephrin-A5 in these regions, very little is known about the functional consequences that might follow alterations in the expression of this ligand. Previously, we demonstrated that ephrin-A5 acts as a guidance molecule regulating the trajectory of the ascending midbrain dopaminergic pathways. In light of this finding and the critical role of dopamine in modulating a number of behaviors, we sought to determine whether loss of ephrin-A5 altered neurobehavioral development. Our results indicate that ephrin-A5-null mice exhibit delays in reaching developmental milestones and in the maturation of motor skills. In addition, they exhibit increased locomotor activity and reduced levels of brain monoamines. Therefore, we conclude that ephrin-A5 expression appears to be critical for proper development of central monoaminergic pathways and that its loss results in a number of neurodevelopmental abnormalities. Because alterations in monoamine function are associated with a variety of neurodevelopmental disorders, these data suggest that further study on the potential role of ephrin-A5 in such disorders is warranted.

Coordinated Eph-ephrin signaling guides migration and axon targeting in the avian auditory system

Background

In the avian sound localization circuit, nucleus magnocellularis (NM) projects bilaterally to nucleus laminaris (NL), with ipsilateral and contralateral NM axon branches directed to dorsal and ventral NL dendrites, respectively. We previously showed that the Eph receptor EphB2 is expressed in NL neuropil and NM axons during development. Here we tested whether EphB2 contributes to NM-NL circuit formation.

Results

We found that misexpression of EphB2 in embryonic NM precursors significantly increased the number of axon targeting errors from NM to contralateral NL in a cell-autonomous manner when forward signaling was impaired. We also tested the effects of inhibiting forward signaling of different Eph receptor subclasses by injecting soluble unclustered Fc-fusion proteins at stages when NM axons are approaching their NL target. Again we found an increase in axon targeting errors compared to controls when forward signaling was impaired, an effect that was significantly increased when both Eph receptor subclasses were inhibited together. In addition to axon targeting errors, we also observed morphological abnormalities of the auditory nuclei when EphB2 forward signaling was increased by E2 transfection, and when Eph-ephrin forward signaling was inhibited by E6-E8 injection of Eph receptor fusion proteins.

Conclusions

These data suggest that EphB signaling has distinct functions in axon guidance and morphogenesis. The results provide evidence that multiple Eph receptors work synergistically in the formation of precise auditory circuitry.

Epha4 controls the midline crossing and contralateral axonal projections of inferior olive neurons

The guidance of axonal projections to ipsilateral and contralateral regions is essential for integration of bilateral sensory information and coordination of movement. In the development of olivocerebellar projections, newborn neurons of inferior olivary (IO) nuclei ventrally migrate from the hindbrain rhombic lip to the floor plate (FP). The cell bodies of IO neurons cannot cross the FP but their axons can, and thus IO neurons project their axons only to the contralateral cerebellar cortex. The molecular mechanisms determining the contralateral axonal projections of IO neurons, however, are obscure. The IO neurons and their axons express EphA4, whereas the FP expresses an EphA4 ligand, EphrinB3, from embryonic day 12.5. Therefore, we tested whether EphA4-deficient mice (EphA4(-/-) ) would show impairment in the development of olivocerebellar projections. We found that, in EphA4(-/-) embryos, some of the IO neurons projected their axons to the ipsilateral cerebellar cortex because the cell bodies of the IO neurons abnormally crossed the FP. Furthermore, even in adults, EphA4(-/-) cerebella were bilaterally innervated by unilateral IO subnuclei. These observations indicate that EphA4 is involved in the contralateral axonal projections of IO neurons by preventing their cell bodies from crossing the midline FP.

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.

Therapeutic interference with EphrinB2 signalling inhibits oxygen-induced angioproliferative retinopathy

PURPOSE

To investigate whether EphrinB2 (EfnB2) or EphB4 influence retinal angiogenesis under physiological or pathological conditions.

METHODS

Using the mouse model of oxygen-induced proliferative retinopathy (OIR), the expression of EfnB2, EphB4, vascular endothelial growth factor (VEGF), VEGFR1 and VEGFR2 was quantified by quantitative polymerase chain reaction (qPCR) and localized in EfnB2- and EphB4-lacZ mice. Angioproliferative retinopathy was manipulated by intravitreal injection of dimeric EfnB2 and monomeric or dimeric EphB4.

RESULTS

Dimeric EphB4 (EphB4-Fc) and EfnB2 (EfnB2-Fc) enhanced hypoxia-induced angioproliferative retinopathy but not physiological angiogenesis. Monomeric EphB4 (sEphB4) reduced angiogenesis. The messenger RNA (mRNA) level of EfnB2 increased significantly in the hyperoxic phase (P7-P12), while EphB4, VEGF, VEGFR1 and VEGFR2 showed a significant - up to fivefold - increased expression at P14, the start of morphologically visible vasoproliferation caused by relative hypoxia.

CONCLUSION

The ephrin/Eph system is involved in angioproliferative retinopathy. Stimulation of EphB4 and EfnB2 signalling using EfnB2-Fc and EphB4-Fc, respectively, enhanced hypoxia-induced angiogenesis. In contrast, sEphB4 inhibited hypoxia-induced angiogenesis. Therefore, angiogenesis is enhanced by signalling through both EphB4 (forward) and EfnB2 (reverse). The distinction in the expression kinetics of EphB4 and EfnB2 indicates that they govern two different signalling pathways and are regulated in diverse ways. sEphB4 might be a useful drug for antiangiogenic therapy.

Rho and Ras GTPases in axon growth, guidance, and branching

The establishment of precise neuronal cell morphology provides the foundation for all aspects of neurobiology. During development, axons emerge from cell bodies after an initial polarization stage, elongate, and navigate towards target regions guided by a range of environmental cues. The Rho and Ras families of small GTPases have emerged as critical players at all stages of axonogenesis. Their ability to coordinately direct multiple signal transduction pathways with precise spatial control drives many of the activities that underlie this morphogenetic program: the dynamic assembly, disassembly, and reorganization of the actin and microtubule cytoskeletons, the interaction of the growing axon with other cells and extracellular matrix, the delivery of lipids and proteins to the axon through the exocytic machinery, and the internalization of membrane and proteins at the leading edge of the growth cone through endocytosis. This article highlights the contribution of Rho and Ras GTPases to axonogenesis.

Sensory axon targeting is increased by NGF gene therapy within the lesioned adult femoral nerve

Even though peripheral nerves regenerate well, axons are often misrouted and reinnervate inappropriate distal pathways post-injury. Misrouting most likely occurs at branch points where regenerating axons make choices. Here, we show that the accuracy of sensory axon reinnervation is enhanced by overexpression of the guidance molecule nerve growth factor (NGF) distal to the bifurcation. We used the femoral nerve as a model, which contains both sensory and motor axons that intermingle in the parent trunk and distally segregate into the saphenous (SB) and motor branches (MB). Transection of the parent trunk resulted in misrouting of axon reinnervation to SB and MB. To enhance sensory axon targeting, recombinant adenovirus encoding NGF was injected along the SB close to the bifurcation 1 week post-injury. The accuracy of axon reinnervation was assessed by retrograde tracing at 3 or 8 weeks after nerve injury. NGF overexpression significantly increased the accuracy of SB axon reinnervation to the appropriate nerve branch, in a manner independent of enhancing axon regeneration. This novel finding provides in vivo evidence that gradient expression of neurotrophin can be used to enhance targeting of distal peripheral pathways to increase axon regeneration into the appropriate nerve branch.

Hippocampal atrophy as a quantitative trait in a genome-wide association study identifying novel susceptibility genes for Alzheimer's disease

Background

With the exception of APOE ε4 allele, the common genetic risk factors for sporadic Alzheimer's Disease (AD) are unknown.

Methods and Findings

We completed a genome-wide association study on 381 participants in the ADNI (Alzheimer's Disease Neuroimaging Initiative) study. Samples were genotyped using the Illumina Human610-Quad BeadChip. 516,645 unique Single Nucleotide Polymorphisms (SNPs) were included in the analysis following quality control measures. The genotype data and raw genetic data are freely available for download (LONI, http://www.loni.ucla.edu/ADNI/Data/). Two analyses were completed: a standard case-control analysis, and a novel approach using hippocampal atrophy measured on MRI as an objectively defined, quantitative phenotype. A General Linear Model was applied to identify SNPs for which there was an interaction between the genotype and diagnosis on the quantitative trait. The case-control analysis identified APOE and a new risk gene, TOMM40 (translocase of outer mitochondrial membrane 40), at a genome-wide significance level of≤10⁻⁶ (10⁻¹¹ for a haplotype). TOMM40 risk alleles were approximately twice as frequent in AD subjects as controls. The quantitative trait analysis identified 21 genes or chromosomal areas with at least one SNP with a p-value≤10⁻⁶, which can be considered potential “new” candidate loci to explore in the etiology of sporadic AD. These candidates included EFNA5, CAND1, MAGI2, ARSB, and PRUNE2, genes involved in the regulation of protein degradation, apoptosis, neuronal loss and neurodevelopment. Thus, we identified common genetic variants associated with the increased risk of developing AD in the ADNI cohort, and present publicly available genome-wide data. Supportive evidence based on case-control studies and biological plausibility by gene annotation is provided. Currently no available sample with both imaging and genetic data is available for replication.

Conclusions

Using hippocampal atrophy as a quantitative phenotype in a genome-wide scan, we have identified candidate risk genes for sporadic Alzheimer's disease that merit further investigation.

A PKC-dependent recruitment of MMP-2 controls semaphorin-3A growth-promoting effect in cortical dendrites

There is increasing evidence for a crucial role of proteases and metalloproteinases during axon growth and guidance. In this context, we recently described a functional link between the chemoattractive Sema3C and Matrix metalloproteinase 3 (MMP3). Here, we provide data demonstrating the involvement of MMP-2 to trigger the growth-promoting effect of Sema3A in cortical dendrites. The in situ analysis of MMP-2 expression and activity is consistent with a functional growth assay demonstrating in vitro that the pharmacological inhibition of MMP-2 reduces the growth of cortical dendrites in response to Sema3A. Hence, our results suggest that the selective recruitment and activation of MMP-2 in response to Sema3A requires a PKC alpha dependent mechanism. Altogether, we provide a second set of data supporting MMPs as effectors of the growth-promoting effects of semaphorins, and we identify the potential signalling pathway involved.

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

The nervous and vascular systems are both exquisitely branched and complicated systems and their proper development requires careful guidance of nerves and vessels. The recent realization that common ligand-receptor pairs are used in guiding the patterning of both systems has prompted the question of whether similar signaling pathways are used in both systems. This review highlights recent progress in our understanding of the similarities and differences in the intracellular signaling mechanisms downstream of semaphorins, ephrins and vascular endothelial growth factor in neurons and endothelial cells during neural and vascular development. We present evidence that similar intracellular signaling principles underlying cytoskeletal regulation are used to control neural and vascular guidance, although the specific molecules used in neurons and endothelial cells are often different.

Cranial sensory ganglia neurons require intrinsic N-cadherin function for guidance of afferent fibers to their final targets

Cell adhesion molecules, such as N-cadherin (cdh2), are essential for normal neuronal development, and as such have been implicated in an array of processes including neuronal differentiation and migration, and axon growth and fasciculation. cdh2 is expressed in neurons of the peripheral nervous system during development, but its role in these cells during this time is poorly understood. Using the transgenic zebrafish line, tg(p2xr3.2:eGFP(sl1)), we have examined the involvement of cdh2 in the formation of sensory circuits by the peripheral nervous system. The tg(p2xr3.2:eGFP(sl1)) fish allows visualization of neurons comprising the trigeminal, facial, glossopharyngeal and vagal ganglia and their axons throughout development. Reduction of cdh2 in this line was achieved by either crosses to the cdh2-mutant strain, glass onion (glo) or injection of a cdh2 morpholino (MO) into single-cell embryos. Here we show that cdh2 function is required to alter the directional vectors of growing axons upon reaching intermediate targets. The central axons enter the hindbrain appropriately but fail to turn caudally towards their final targets. Similarly, the peripheral axons extend ventrally, but fail to turn and project along a rostral/caudal axis. Furthermore, by expressing dominant negative cdh2 constructs selectively within cranial sensory ganglia (CSG) neurons, we found that cdh2 function is necessary within the axons to elicit these stereotypic turns, thus demonstrating that cdh2 acts cell autonomously. Together, our in vivo data reveal a novel role for cdh2 in the establishment of circuits by peripheral sensory neurons.