Adaptive changes in gene expression patterns in the somatosensory cortex after deletion of ephrinA5

EphrinA5 regulates cell motility by modulating Snhg15/DNA triplex-dependent targeting of DNMT1 to the Ncam1 promoter

Cell-cell communication is mediated by membrane receptors and their ligands, such as the Eph/ephrin system, orchestrating cell migration during development and in diverse cancer types. Epigenetic mechanisms are key for integrating external "signals", e.g., from neighboring cells, into the transcriptome in health and disease. Previously, we reported ephrinA5 to trigger transcriptional changes of lncRNAs and protein-coding genes in cerebellar granule cells, a cell model for medulloblastoma. LncRNAs represent important adaptors for epigenetic writers through which they regulate gene expression. Here, we investigate a lncRNA-mediated targeting of DNMT1 to specific gene loci by the combined power of in silico modeling of RNA/DNA interactions and wet lab approaches, in the context of the clinically relevant use case of ephrinA5-dependent regulation of cellular motility of cerebellar granule cells. We provide evidence that Snhg15, a cancer-related lncRNA, recruits DNMT1 to the Ncam1 promoter through RNA/DNA triplex structure formation and the interaction with DNMT1. This mediates DNA methylation-dependent silencing of Ncam1, being abolished by ephrinA5 stimulation-triggered reduction of Snhg15 expression. Hence, we here propose a triple helix recognition mechanism, underlying cell motility regulation via lncRNA-targeted DNA methylation in a clinically relevant context.

Overexpression of EphB6 and EphrinB2 controls soma spacing of cortical neurons in a mutual inhibitory way

To establish functional circuitry, neurons settle down in a particular spatial domain by spacing their cell bodies, which requires proper positioning of the soma and establishing of a zone with unique connections. Deficits in this process are implicated in neurodevelopmental diseases. In this study, we examined the function of EphB6 in the development of cerebral cortex. Overexpression of EphB6 via in utero electroporation results in clumping of cortical neurons, while reducing its expression has no effect. In addition, overexpression of EphrinB2, a ligand of EphB6, also induces soma clumping in the cortex. Unexpectedly, the soma clumping phenotypes disappear when both of them are overexpressed in cortical neurons. The mutual inhibitory effect of EphB6/ EphrinB2 on preventing soma clumping is likely to be achieved via interaction of their specific domains. Thus, our results reveal a combinational role of EphrinB2/EphB6 overexpression in controlling soma spacing in cortical development.

The Expression of the Cancer-Associated lncRNA Snhg15 Is Modulated by EphrinA5-Induced Signaling

The Eph receptor tyrosine kinases and their respective ephrin-ligands are an important family of membrane receptors, being involved in developmental processes such as proliferation, migration, and in the formation of brain cancer such as glioma. Intracellular signaling pathways, which are activated by Eph receptor signaling, are well characterized. In contrast, it is unknown so far whether ephrins modulate the expression of lncRNAs, which would enable the transduction of environmental stimuli into our genome through a great gene regulatory spectrum. Applying a combination of functional in vitro assays, RNA sequencing, and qPCR analysis, we found that the proliferation and migration promoting stimulation of mouse cerebellar granule cells (CB) with ephrinA5 diminishes the expression of the cancer-related lncRNA Snhg15. In a human medulloblastoma cell line (DAOY) ephrinA5 stimulation similarly reduced SNHG15 expression. Computational analysis identified triple-helix-mediated DNA-binding sites of Snhg15 in promoters of genes found up-regulated upon ephrinA5 stimulation and known to be involved in tumorigenic processes. Our findings propose a crucial role of Snhg15 downstream of ephrinA5-induced signaling in regulating gene transcription in the nucleus. These findings could be potentially relevant for the regulation of tumorigenic processes in the context of glioma.

A dual role of EphB1/ephrin-B3 reverse signaling on migrating striatal and cortical neurons originating in the preoptic area: should I stay or go away?

During embryonic development the preoptic area (POA) gives rise to two populations of neurons which are generated at the same time, cortical interneurons and striatal cells. POA-derived cortical interneurons take a superficial path and avoid the developing striatum (Str) when they migrate to their target region. We found that EphB1, which is expressed in the striatal anlage, prevents cortical interneurons from entering the Str via ephrin-B3 reverse signaling. In contrast, for striatal neurons which also express ephrin-B3, EphB1 acts as a stop signal. This dual role of EphB1 is due to differences in ephrin-B3 reverse signaling cascades. For striatal neurons, binding of EphB1 to ephrin-B3 reduces endogenously high levels of pSrc and pFAK, which then causes the cells to stop migration. In contrast, in cortical interneurons EphB1-ephrin-B3 reverse signaling leads to phosphorylation of Src and focal adhesion kinase (FAK) which then mediates repulsion. Consistent with these in vitro findings, in an ephrin-B3 knockout mouse line, we discovered misrouted cortical interneurons in the Str and an over-migration of striatal neurons in their target region. Thus, EphB1/ephrin-B3 reverse signaling has a different impact on two sets of neurons which are generated at the same time and place: it can act as a repulsive cue for migrating neurons or it can terminate neuronal migration, a novel role of the Eph/ephrin system.

Ischemia-induced increase in microvascular phosphodiesterase 4D expression in rat hippocampus associated with blood brain barrier permeability: effect of age

Phosphodiesterase 4D (PDE4D) is one of 16 PDEs expressed in cerebral microvessels, and may be involved in regulating blood-brain barrier (BBB) permeability. To assess the possible role of PDE4D in stroke-related injury in young versus aged rats, we measured microvascular PDE4D expression, parenchymal albumin immunoreactivity, and changes in the inside bore of the brain microvasculature. Ischemia caused severe hippocampal CA1 damage, associated with significant increases in vascular PDE4D and parenchymal albumin immunoreactivities. This effect was greater in the younger animals, which also had a greater increase in PDE4D expression. Ischemia significantly decreased tissue density in the perimicrovascular space in both young and aged animals. In addition, internal bore circumference and cross-sectional area of the hippocampal microvessels increased dramatically following ischemia. Increased PDE4D expression following cerebral ischemia may play a role in changing BBB permeability, which could secondarily affect ischemic outcome.

Should I stay or should I go? Ephs and ephrins in neuronal migration

In neuroscience, Ephs and ephrins are perhaps best known for their role in axon guidance. It was first shown in the visual system that graded expression of these proteins is instrumental in providing molecular coordinates that define topographic maps, particularly in the visual system, but also in the auditory, vomeronasal and somatosensory systems as well as in the hippocampus, cerebellum and other structures. Perhaps unsurprisingly, the role of these proteins in regulating cell-cell interactions also has an impact on cell mobility, with evidence that Eph-ephrin interactions segregate cell populations based on contact-mediated attraction or repulsion. Consistent with these studies, evidence has accumulated that Ephs and ephrins play important roles in the migration of specific cell populations in the developing and adult brain. This review focusses on two examples of neuronal migration that require Eph/ephrin signalling - radial and tangential migration of neurons in cortical development and the migration of newly generated neurons along the rostral migratory stream to the olfactory bulb in the adult brain. We discuss the challenge involved in understanding how cells determine whether they respond to signals by migration or axon guidance.

Ephrin A2 protein expression in the regeneration and plasticity of cochlear hair cells in chicken following kanamycin ototoxicity

The results from this study showed that the thresholds of brainstem auditory-evoked potentials peak following 10 successive days of intramuscular injection of Roman chickens with kanamycin, starting 3 days after birth. Fluorescence immunohistochemistry analysis revealed few ganglion cells positively labeled for Ephrin A2 in the cochlea of experimental chickens from 2 days before until 7 days after the last kanamycin injection. The number of Ephrin A2-positive ganglion cell bodies was increased at 15 days after the last injection and was similar to that in normal chickens at 30 days following the cessation of kanamycin treatment. These experimental findings indicate that Ephrin A2 protein expression in the acoustic ganglia is synchronized with the connection damage and regeneration of cochlear hair cells after kanamycin exposure. Ephrin A2 may play an important role in the regeneration and plasticity of cochlear hair cells in the chick cochlea following kanamycin ototoxicity.

Integration of opposing semaphorin guidance cues in cortical axons

Previous work demonstrated that members of the semaphorin family, Sema3A and Sema3C, act as repulsive and attractive guidance signals, respectively, for cortical axons. During the development of corticofugal projections, these semaphorins are expressed in adjacent cortical zones, but there is a considerable overlap between Sema3A and Sema3C expression in the subventricular zone. We used different in vitro assays to examine the response of cortical axons exposed to defined mixtures of these opposing guidance cues. Results showed that even at very low concentrations, Sema3A overrides the effects of Sema3C. Moreover, experiments with function-blocking antibodies directed against neuropilin provided insights into how cortical axons integrate disparate guidance signals at the receptor level. These in vitro data suggest that the pathway of corticofugal axons is defined by an attractive cue in the intermediate zone, where Sema3C is expressed alone. To directly test this hypothesis in vivo, we performed axon-tracing experiments in Sema3C-deficient mice. Compared with wild-type animals, corticofugal axons take a more superficial route in Sema3C(-/-) mice, and the corticofugal pathway is more compacted. This phenotype is expected when an attractive cue for cortical axons, Sema3C, is eliminated and a repulsive cue, Sema3A, becomes predominant.

Bidirectional ephrinB3/EphA4 signaling mediates the segregation of medial ganglionic eminence- and preoptic area-derived interneurons in the deep and superficial migratory stream

The integration of interneuron subtypes into specific microcircuits is essential for proper cortical function. Understanding to what extent interneuron diversity is regulated and maintained during development might help to reveal the principles that govern their role as synchronizing elements as well as causes for dysfunction. Particular interneuron subtypes are generated in a temporally regulated manner in the medial ganglionic eminence (MGE), the caudal ganglionic eminence, and the preoptic area (POA) of the basal telencephalon. Long-range tangential migration from their site of origin to cortical targets is orchestrated by a variety of attractive, repulsive, membrane-bound, and secreted signaling molecules, to establish the critical balance of inhibition and excitation. It remains unknown whether interneurons deriving from distinct domains are predetermined to migrate in particular routes and whether this process underlies cell type-specific regulation. We found that POA- and MGE-derived cortical interneurons migrate within spatially segregated corridors. EphrinB3, expressed in POA-derived interneurons traversing the superficial route, acts as a repellent signal for deeply migrating interneurons born in the MGE, which is mediated by EphA4 forward signaling. In contrast, EphA4 induces repulsive ephrinB3 reverse signaling in interneurons generated in the POA, restricting this population to the superficial path. Perturbation of this bidirectional ephrinB3/EphA4 signaling in vitro and in vivo leads to a partial intermingling of cells in these segregated migratory pathways. Thus, we conclude that cell contact-mediated bidirectional ephrinB3/EphA4 signaling mediates the sorting of MGE- and POA-derived interneurons in the deep and superficial migratory stream.

Laminar expression of ephrin-A2 in primary somatosensory cortex of postnatal rats

Several Eph receptors, prominently EphA4 and EphA7, and their corresponding ligands are known to influence neocortical development, including topographic sorting of thalamocortical axons within primary somatosensory cortex (SI). This study investigated postnatal expression of a ligand that can bind to these receptors, ephrin-A2. Quantitative methods revealed that expression of ephrin-A2 mRNA in SI reached maximum levels on postnatal day (P) 4 and dropped thereafter to background by P18. Ephrin-A2 mRNA expression assessed by in situ hybridization qualitatively revealed a similar time course and localized the expression pattern primarily in two broad laminae in SI, comprising the supragranular and infragranular layers, and with additional expression in the subplate. This expression pattern was investigated in greater detail using immunohistochemistry for ephrin-A2 protein. Immunoreactivity generally showed the same laminar distribution as seen with in situ hybridization, except that it persisted longer, lasting to approximately P14. Expression in the cortical plate was low or absent within presumptive layer IV, and it remained so as cortical lamination progressed. Double-labeling immunohistochemistry with confocal microscopy revealed that cortical neurons were the principal elements expressing ephrin-A2 protein. These findings are consistent with possible involvement of ephrin-A2, in concert with one or more Eph receptors, in influencing arbor development of thalamocortical axons at cortical layer IV boundaries.

EphrinA5 protein distribution in the developing mouse brain

Background

EphrinA5 is one of the best-studied members of the Eph-ephrin family of guidance molecules, known to be involved in brain developmental processes. Using in situ hybridization, ephrinA5 mRNA expression has been detected in the retinotectal, the thalamocortical, and the olfactory systems; however, no study focused on the distribution of the protein. Considering that this membrane-anchored molecule may act far from the neuron soma expressing the transcript, it is of a crucial interest to localize ephrinA5 protein to better understand its function.

Results

Using immunohistochemistry, we found that ephrinA5 protein is highly expressed in the developing mouse brain from E12.5 to E16.5. The olfactory bulb, the cortex, the striatum, the thalamus, and the colliculi showed high intensity of labelling, suggesting its implication in topographic mapping of olfactory, retinocollicular, thalamocortical, corticothalamic and mesostriatal systems. In the olfactory nerve, we found an early ephrinA5 protein expression at E12.5 suggesting its implication in the guidance of primary olfactory neurons into the olfactory bulb. In the thalamus, we detected a dynamic graduated protein expression, suggesting its role in the corticothalamic patterning, whereas ephrinA5 protein expression in the target region of mesencephalic dopaminergic neurones indicated its involvement in the mesostriatal topographic mapping. Following E16.5, the signal faded gradually and was barely detectable at P0, suggesting a main role for ephrinA5 in primary molecular events in topographic map formation.

Conclusion

Our work shows that ephrinA5 protein is expressed in restrictive regions of the developing mouse brain. This expression pattern points out the potential sites of action of this molecule in the olfactory, retinotectal, thalamocortical, corticothalamic and mesostriatal systems, during development. This study is essential to better understand the role of ephrinA5 during developmental topographic mapping of connections and to further characterise the mechanisms involved in pathway restoration following cell transplantation in the damaged brain.

Structural alterations of spiny stellate cells in the somatosensory cortex in ephrin-A5-deficient mice

Previous studies demonstrated that in ephrin-A5-deficient mice corticothalamic arbors are reduced by more than 50% in layer 4 of the somatosensory cortex (S1), where ephrin-A5 is normally expressed. Here we examined possible consequences of the reduced thalamic input on spiny stellate cells, the target neurons of thalamocortical afferents. Using ballistic delivery of particles coated with lipophilic dyes in fixed slices and confocal laser-microscopy, we could quantitatively analyze the morphology of these neurons. Cells were examined in S1 at postnatal day 8 (P8), when thalamic afferents establish synaptic contacts and the dendrites of their target cells are covered with filopodia, and at P23, after synapse formation and replacement of filopodia by spines. Our results indicate that at P8 the dendrites of cells in mutant animals exhibit more filopodia and are more branched than dendrites of wildtype cells. In contrast, there is no difference in the extent of the dendritic tree between knockout and control animals. At P23, dendrites of neurons in ephrin-A5-deficient mice are still more branched, but possess fewer spines than wildtype cells. Thus, at early stages layer 4 neurons appear to compensate the reduced thalamic input by increasing dendritic branching and the density of filopodia. However, while at later stages the dendrites of layer 4 neurons in mutants are still more branched, their spine density is now lower than in wildtype cells. Taken together, these data demonstrate that the structure of spiny stellate cells is shaped by thalamic input and Eph receptor signaling.