EphB2 regulates axonal growth at the midline in the developing auditory brainstem

Eph and ephrin signaling in the development of the central auditory system

Acoustic communication relies crucially on accurate interpretation of information about the intensity, frequency, timing, and location of diverse sound stimuli in the environment. To meet this demand, neurons along different levels of the auditory system form precisely organized neural circuits. The assembly of these precise circuits requires tight regulation and coordination of multiple developmental processes. Several groups of axon guidance molecules have proven critical in controlling these processes. Among them, the family of Eph receptors and their ephrin ligands emerge as one group of key players. They mediate diverse functions at multiple levels of the auditory pathway, including axon guidance and targeting, topographic map formation, as well as cell migration and tissue pattern formation. Here, we review our current knowledge of how Eph and ephrin molecules regulate different processes in the development and maturation of central auditory circuits.

Enhancing Cutaneous Wound Healing Based on Human Induced Neural Stem Cell-derived Exosomes

Background

Wound healing of skin is a complicated process. Cutaneous innervation and neurotrophic factors could participate in multiple stages of wound healing. Neurotrophic factors are mainly produced and released by neurons and neural stem cells (NSCs) which could be obtained in large quantities from human-induced pluripotent stem cells (iPSCs) in vitro. However, the potential wound healing effects of NSC secretions, such as exosomes, are unexplored yet.

Methods

NSCs-derived exosomes (NSC-exo) and iPSCs-derived exosomes (iPSC-exo) were isolated from the cell culture supernatants by centrifugation, and then quantified and characterized. The effects of these exosomes on the migration of human dermal fibroblasts (HDF) cells and the tube formation of human umbilical vein endothelial cells (HUVECs) were investigated in vitro. And the in vivo wound healing effect of these exosomes were tested on the mouse skin trauma model. Therefore, a dipeptide/hyaluronic acid (Nap-FF/HA) composite hydrogel was used to encapsulate the exosomes as a sustained release carrier. Histological observations were performed to evaluate the wound healing effect of exosomes. Furthermore, the non-labeling proteomic analysis was performed to explore the possible mechanisms of NSC-exo on wound healing.

Results

We obtained extracellular vesicles in a bowl-like structure with membranes which meet the general standards of exosomes. NSC-exo showed promotion effect on the migration of HDF cells and the tube formation of HUVECs in vitro. In a mouse skin injury model, NSC-exo enhanced the wound healing and the Nap-FF/HA hydrogel that contained exosomes did so with less drug frequency by sustaining release of exosomes. Further proteomic analysis demonstrated that the carried neurotrophic factors and immunity-related proteins in NSC-exo may play a functional role in wound healing.

Conclusion

NSC-exo may enhance wound healing via neurotrophic factors and immunomodulation.

Axonal Projection Patterns of the Dorsal Interneuron Populations in the Embryonic Hindbrain

Unraveling the inner workings of neural circuits entails understanding the cellular origin and axonal pathfinding of various neuronal groups during development. In the embryonic hindbrain, different subtypes of dorsal interneurons (dINs) evolve along the dorsal-ventral (DV) axis of rhombomeres and are imperative for the assembly of central brainstem circuits. dINs are divided into two classes, class A and class B, each containing four neuronal subgroups (dA1-4 and dB1-4) that are born in well-defined DV positions. While all interneurons belonging to class A express the transcription factor Olig3 and become excitatory, all class B interneurons express the transcription factor Lbx1 but are diverse in their excitatory or inhibitory fate. Moreover, within every class, each interneuron subtype displays its own specification genes and axonal projection patterns which are required to govern the stage-by-stage assembly of their connectivity toward their target sites. Remarkably, despite the similar genetic landmark of each dINs subgroup along the anterior-posterior (AP) axis of the hindbrain, genetic fate maps of some dA/dB neuronal subtypes uncovered their contribution to different nuclei centers in relation to their rhombomeric origin. Thus, DV and AP positional information has to be orchestrated in each dA/dB subpopulation to form distinct neuronal circuits in the hindbrain. Over the span of several decades, different axonal routes have been well-documented to dynamically emerge and grow throughout the hindbrain DV and AP positions. Yet, the genetic link between these distinct axonal bundles and their neuronal origin is not fully clear. In this study, we reviewed the available data regarding the association between the specification of early-born dorsal interneuron subpopulations in the hindbrain and their axonal circuitry development and fate, as well as the present existing knowledge on molecular effectors underlying the process of axonal growth.

Making sense of neural development by comparing wiring strategies for seeing and hearing

The ability to perceive and interact with the world depends on a diverse array of neural circuits specialized for carrying out specific computations. Each circuit is assembled using a relatively limited number of molecules and common developmental steps, from cell fate specification to activity-dependent synaptic refinement. Given this shared toolkit, how do individual circuits acquire their characteristic properties? We explore this question by comparing development of the circuitry for seeing and hearing, highlighting a few examples where differences in each system's sensory demands necessitate different developmental strategies.

Impaired fear extinction in adolescent rodents: Behavioural and neural analyses

Despite adolescence being a developmental window of vulnerability, up until very recently there were surprisingly few studies on fear extinction during this period. Here we summarise the recent work in this area, focusing on the unique behavioural and neural characteristics of fear extinction in adolescent rodents, and humans where relevant. A prominent hypothesis posits that anxiety disorders peak during late childhood/adolescence due to the non-linear maturation of the fear inhibition neural circuitry. We discuss evidence that impaired extinction retention in adolescence is due to subregions of the medial prefrontal cortex and amygdala mediating fear inhibition being underactive while other subregions that mediate fear expression are overactive. We also review work on various interventions and surprising circumstances which enhance fear extinction in adolescence. This latter work revealed that the neural correlates of extinction in adolescence are different to that in younger and older animals even when extinction retention is not impaired. This growing body of work highlights that adolescence is a unique period of development for fear inhibition.

Eph-ephrin signaling in nervous system development

Ephrins and Eph receptors enable contact-mediated interactions between cells at every stage of nervous system development. In spite of their broad binding affinities, Eph proteins facilitate specificity in neuronal migration and axon targeting. This review focuses on recent studies that demonstrate how these proteins interact with each other, and with other signaling pathways, to guide specificity in a diverse set of developmental processes.

A Serotonin Circuit Acts as an Environmental Sensor to Mediate Midline Axon Crossing through EphrinB2

Modulation of connectivity formation in the developing brain in response to external stimuli is poorly understood. Here, we show that the raphe nucleus and its serotonergic projections regulate pathfinding of commissural axons in zebrafish. We found that the raphe neurons extend projections toward midline-crossing axons and that when serotonergic signaling is blocked by pharmacological inhibition or by raphe neuron ablation, commissural pathfinding is disrupted. We demonstrate that the serotonin receptor htr2a is expressed on these commissural axons and that genetic knock-down of htr2a disrupts crossing. We further show that knock-down of htr2a or ablation of the raphe neurons increases ephrinB2a protein levels in commissural axons. An ephrinB2a mutant can rescue midline crossing when serotonergic signaling is blocked. Furthermore, we found that regulation of serotonin expression in the raphe neurons is modulated in response to the developmental environment. Hypoxia causes the raphe to decrease serotonin levels, leading to a reduction in midline crossing. Increasing serotonin in the setting of hypoxia restored midline crossing. Our findings demonstrate an instructive role for serotonin in axon guidance acting through ephrinB2a and reveal a novel mechanism for developmental interpretation of the environmental milieu in the generation of mature neural circuitry.

SIGNIFICANCE STATEMENT

We show here that serotonin has a novel role in regulating connectivity in response to the developmental environment. We demonstrate that serotonergic projections from raphe neurons regulate pathfinding of crossing axons. The neurons modulate their serotonin levels, and thus alter crossing, in response to the developmental environment including hypoxia. The findings suggest that modification of the serotonergic system by early exposures may contribute to permanent CNS connectivity alterations. This has important ramifications because of the association between premature birth and accompanying hypoxia, and increased risk of autism and evidence associating in utero exposure to some antidepressants and neurodevelopmental disorders. Finally, this work demonstrates that the vertebrate CNS can modulate its connectivity in response to the external environment.

Infralimbic EphB2 Modulates Fear Extinction in Adolescent Rats

Adolescent rats are prone to impaired fear extinction, suggesting that mechanistic differences in extinction could exist in adolescent and adult rats. Since the infralimbic cortex (IL) is critical for fear extinction, we used PCR array technology to identify gene expression changes in IL induced by fear extinction in adolescent rats. Interestingly, the ephrin type B receptor 2 (EphB2), a tyrosine kinase receptor associated with synaptic development, was downregulated in IL after fear extinction. Consistent with the PCR array results, EphB2 levels of mRNA and protein were reduced in IL after fear extinction compared with fear conditioning, suggesting that EphB2 signaling in IL regulates fear extinction memory in adolescents. Finally, reducing EphB2 synthesis in IL with shRNA accelerated fear extinction learning in adolescent rats, but not in adult rats. These findings identify EphB2 in IL as a key regulator of fear extinction during adolescence, perhaps due to the increase in synaptic remodeling occurring during this developmental phase.

Pre-target axon sorting in the avian auditory brainstem

Topographic organization of neurons is a hallmark of brain structure. The establishment of the connections between topographically organized brain regions has attracted much experimental attention, and it is widely accepted that molecular cues guide outgrowing axons to their targets in order to construct topographic maps. In a number of systems afferent axons are organized topographically along their trajectory as well, and it has been suggested that this pre-target sorting contributes to map formation. Neurons in auditory regions of the brain are arranged according to their best frequency (BF), the sound frequency they respond to optimally. This BF changes predictably with position along the so-called tonotopic axis. In the avian auditory brainstem, the tonotopic organization of the second- and third-order auditory neurons in nucleus magnocellularis (NM) and nucleus laminaris (NL) has been well described. In this study we examine whether the decussating NM axons forming the crossed dorsal cochlear tract (XDCT) and innervating the contralateral NL are arranged in a systematic manner. We electroporated dye into cells in different frequency regions of NM to anterogradely label their axons in XDCT. The placement of dye in NM was compared to the location of labeled axons in XDCT. Our results show that NM axons in XDCT are organized in a precise tonotopic manner along the rostrocaudal axis, spanning the entire rostrocaudal extent of both the origin and target nuclei. We propose that in the avian auditory brainstem, this pretarget axon sorting contributes to tonotopic map formation in NL.

Differential roles for EphA and EphB signaling in segregation and patterning of central vestibulocochlear nerve projections

Auditory and vestibular afferents enter the brainstem through the VIIIth cranial nerve and find targets in distinct brain regions. We previously reported that the axon guidance molecules EphA4 and EphB2 have largely complementary expression patterns in the developing avian VIIIth nerve. Here, we tested whether inhibition of Eph signaling alters central targeting of VIIIth nerve axons. We first identified the central compartments through which auditory and vestibular axons travel. We then manipulated Eph-ephrin signaling using pharmacological inhibition of Eph receptors and in ovo electroporation to misexpress EphA4 and EphB2. Anterograde labeling of auditory afferents showed that inhibition of Eph signaling did not misroute axons to non-auditory target regions. Similarly, we did not find vestibular axons within auditory projection regions. However, we found that pharmacologic inhibition of Eph receptors reduced the volume of the vestibular projection compartment. Inhibition of EphB signaling alone did not affect auditory or vestibular central projection volumes, but it significantly increased the area of the auditory sensory epithelium. Misexpression of EphA4 and EphB2 in VIIIth nerve axons resulted in a significant shift of dorsoventral spacing between the axon tracts, suggesting a cell-autonomous role for the partitioning of projection areas along this axis. Cochlear ganglion volumes did not differ among treatment groups, indicating the changes seen were not due to a gain or loss of cochlear ganglion cells. These results suggest that Eph-ephrin signaling does not specify auditory versus vestibular targets but rather contributes to formation of boundaries for patterning of inner ear projections in the hindbrain.

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.

EphB signaling regulates target innervation in the developing and deafferented auditory brainstem

Precision in auditory brainstem connectivity underlies sound localization. Cochlear activity is transmitted to the ventral cochlear nucleus (VCN) in the mammalian brainstem via the auditory nerve. VCN globular bushy cells project to the contralateral medial nucleus of the trapezoid body (MNTB), where specialized axons terminals, the calyces of Held, encapsulate MNTB principal neurons. The VCN-MNTB pathway is an essential component of the circuitry used to compute interaural intensity differences that are used for localizing sounds. When input from one ear is removed during early postnatal development, auditory brainstem circuitry displays robust anatomical plasticity. The molecular mechanisms that control the development of auditory brainstem circuitry and the developmental plasticity of these pathways are poorly understood. In this study we examined the role of EphB signaling in the development of the VCN-MNTB projection and in the reorganization of this pathway after unilateral deafferentation. We found that EphB2 and EphB3 reverse signaling are critical for the normal development of the projection from VCN to MNTB, but that successful circuit assembly most likely relies upon the coordinated function of many EphB proteins. We have also found that ephrin-B reverse signaling repels induced projections to the ipsilateral MNTB after unilateral deafferentation, suggesting that similar mechanisms regulate these two processes.

EphA signaling impacts development of topographic connectivity in auditory corticofugal systems

Auditory stimulus representations are dynamically maintained by ascending and descending projections linking the auditory cortex (Actx), medial geniculate body (MGB), and inferior colliculus. Although the extent and topographic specificity of descending auditory corticofugal projections can equal or surpass that of ascending corticopetal projections, little is known about the molecular mechanisms that guide their development. Here, we used in utero gene electroporation to examine the role of EphA receptor signaling in the development of corticothalamic (CT) and corticocollicular connections. Early in postnatal development, CT axons were restricted to a deep dorsal zone (DDZ) within the MGB that expressed low levels of the ephrin-A ligand. By hearing onset, CT axons had innervated surrounding regions of MGB in control-electroporated mice but remained fixed within the DDZ in mice overexpressing EphA7. In vivo neurophysiological recordings demonstrated a corresponding reduction in spontaneous firing rate, but no changes in sound-evoked responsiveness within MGB regions deprived of CT innervation. Structural and functional CT disruption occurred without gross alterations in thalamocortical connectivity. These data demonstrate a potential role for EphA/ephrin-A signaling in the initial guidance of corticofugal axons and suggest that "genetic rewiring" may represent a useful functional tool to alter cortical feedback without silencing Actx.

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.

Ephrin-B reverse signaling is required for formation of strictly contralateral auditory brainstem pathways

Specificity in the projections from the mammalian ventral cochlear nucleus (VCN) is essential for sound localization. Globular bushy cells project from the VCN to the medial nucleus of the trapezoid body (MNTB) on the contralateral, but not the ipsilateral, side of the brainstem, terminating in large synaptic endings known as calyces of Held. The precision in this pathway is critical for the computation of interaural intensity differences, which are used in sound localization. The mechanisms underlying the development of this projection are not completely understood. In this study, we tested the role of Eph receptor tyrosine kinases and their ephrin ligands in limiting the VCN-MNTB projection to the contralateral side. We found that mice with null mutations in EphB2 and EphB3 had normal contralateral VCN-MNTB projections, yet these projections also had significant numbers of aberrant collateral branches in the ipsilateral MNTB. These aberrant branches ended in calyceal terminations in MNTB. Similar ipsilateral projections were seen in mice with mutations in ephrin-B2. In both of these mouse lines, ipsilateral projections formed concurrently with normal contralateral projections and were not eliminated later in development. However, mice with mutations that affected only the intracellular domain of EphB2 had normal, strictly contralateral VCN-MNTB projections. Expression studies showed that EphB2 is expressed in VCN axons and ephrin-B2 is expressed in MNTB. Together, these data suggest that EphB2-ephrin-B2 reverse signaling is required to prevent the formation of ipsilateral VCN-MNTB projections and that this signaling operates non-cell autonomously.

Development of glutamate receptors in auditory neurons from long-term organotypic cultures of the embryonic chick hindbrain

We used long-range organotypic cultures of auditory nuclei in the chick hindbrain to test the development of glutamate receptor activity in auditory neurons growing in a tissue environment that includes early deprivation of peripheral glutamatergic input, subsequent to removal of the otocyst. Cultures started at embryonic day (E)5, and lasted from 6 h to 15 days. Neuronal migration, clustering and axonal extension from the nucleus magnocellularis (NM) to the nucleus laminaris (NL) partially resembled events in vivo. However, the distinctive laminar organization of the NL was not observed. Glutamate receptor (GluR) activity was tested with optical recordings of intracellular Ca2+ in the NM. alpha-Amino-3-hydroxy-5-methyl-4-isoxazoleproprionic acid (AMPA)/kainate receptors had Ca2+ responses with a time course similar to that in control slices. Peak amplitude, however, was significantly lower. N-methyl-D-aspartate (NMDA)-mediated Ca2+ responses were higher in 2-day cultures (E5 + 2d) than in E7 explant controls, returning later to control values. Metabotropic GluRs did not elicit Ca2+ responses at standard agonist doses. Blocking NMDA or AMPA/kainate receptors with specific antagonists for 10 days in culture did not limit neuronal survival. Blocking metabotropic GluRs resulted in complete neuronal loss. Thus, ionotropic GluRs are not required for NM neuronal survival. However, their activity during development is affected when neurons grow in an in vitro environment that includes prevention of arrival of peripheral glutamatergic input.

Molecular characterization and histochemical demonstration of salmon olfactory marker protein in the olfactory epithelium of lacustrine sockeye salmon (Oncorhynchus nerka)

Despite the importance of olfactory receptor neurons (ORNs) for homing migration, the expression of olfactory marker protein (OMP) is not well understood in ORNs of Pacific salmon (genus Oncorhynchus). In this study, salmon OMP was characterized in the olfactory epithelia of lacustrine sockeye salmon (O. nerka) by molecular biological and histochemical techniques. Two cDNAs encoding salmon OMP were isolated and sequenced. These cDNAs both contained a coding region encoding 173 amino acid residues, and the molecular mass of the two proteins was calculated to be 19,581.17 and 19,387.11Da, respectively. Both amino acid sequences showed marked homology (90%). The protein and nucleotide sequencing demonstrates the existence of high-level homology between salmon OMPs and those of other teleosts. By in situ hybridization using a digoxigenin-labeled salmon OMP cRNA probe, signals for salmon OMP mRNA were observed preferentially in the perinuclear regions of the ORNs. By immunohistochemistry using a specific antibody to salmon OMP, OMP-immunoreactivities were noted in the cytosol of those neurons. The present study is the first to describe cDNA cloning of OMP in salmon olfactory epithelium, and indicate that OMP is a useful molecular marker for the detection of the ORNs in Pacific salmon.

Preferential localization of neural cell recognition molecule NB-2 in developing glutamatergic neurons in the rat auditory brainstem

NB-2 is a neuronal cell recognition molecule that is preferentially expressed in auditory pathways. Mice deficient in the NB-2 gene exhibit aberrant responses to acoustic stimuli. Here we examined the expression and localization of NB-2 in the auditory brainstem during development in the rat. NB-2 was strongly expressed in the ventral cochlear nucleus (VCN), ventral acoustic stria, lateral and medial superior olivary complex (SOC), superior paraolivary nucleus, medial nucleus of the trapezoid body (MNTB), ventrolateral lemniscus, and central nucleus of the inferior colliculus (CIC). In the VCN and CIC, NB-2 was expressed in the regions that are known to respond to high frequencies. In situ hybridization combined with immunohistochemistry suggested that NB-2 is expressed only in neurons. NB-2 was colocalized with glutamatergic elements in the neuropil and the calyces of Held but not with glycinergic or GABAergic elements. NB-2 expression in the SOC was first detected at embryonic day (E)19, reached a maximum level at postnatal day (P)7, and declined thereafter. Immunolabeling with VGLUT1 and VGLUT2, markers for mature and premature glutamatergic synapses, respectively, in combination with NB-2 immunolabeling revealed that NB-2 is expressed at glutamatergic synapses. Collectively, our findings suggest that NB-2 plays a key role in maturation of glutamatergic synapses in the brainstem during the final stages of auditory development.

Distribution of glial-associated proteins in the developing chick auditory brainstem

In the avian brainstem, nucleus magnocellularis (NM) projects bilaterally to nucleus laminaris (NL) in a pathway that facilitates sound localization. The distribution of glia during the development of this pathway has not previously been characterized. Radial glia, astrocytes, and oligodendrocytes facilitate many processes including axon pathfinding, synaptic development, and maturation. Here we determined the spatiotemporal expression patterns of glial cell types in embryonic development of the chick auditory brainstem using glial-specific antibodies and histological markers. We found that vimentin-positive processes are intercalated throughout the NL cell layer. Astrocytes are found in two domains: one in the ventral neuropil region and the other dorsolateral to NM. GFAP-positive processes are primarily distributed along the ventral margin of NL. Astrocytic processes penetrate the NL cell layer following the onset of synaptogenesis, but before pruning and maturation. The dynamic, nonoverlapping expression patterns of GFAP and vimentin suggest that distinct glial populations are found in dorsal versus ventral regions of NL. Myelination occurs after axons have reached their targets. FluoroMyelin and myelin basic protein (MBP) gradually increase along the mediolateral axis of NL starting at E10. Multiple GFAP-positive processes are directly apposed to NM-NL axons and MBP, which suggests a role in early myelinogenesis. Our results show considerable changes in glial development after initial NM-NL connections are made, suggesting that glia may facilitate maturation of the auditory circuit.

EphA4 misexpression alters tonotopic projections in the auditory brainstem

Auditory pathways contain orderly representations of frequency selectivity, which begin at the cochlea and are transmitted to the brainstem via topographically ordered axonal pathways. The mechanisms that establish these tonotopic maps are not known. Eph receptor tyrosine kinases and their ligands, the ephrins, have a demonstrated role in establishing topographic projections elsewhere in the brain, including the visual pathway. Here, we have examined the function of these proteins in the formation of auditory frequency maps. In birds, the first central auditory nucleus, n. magnocellularis (NM), projects tonotopically to n. laminaris (NL) on both sides of the brain. We previously showed that the Eph receptor EphA4 is expressed in a tonotopic gradient in the chick NL, with higher frequency regions showing greater expression than lower frequency regions. Here we misexpressed EphA4 in the developing auditory brainstem from embryonic day 2 (E2) through E10, when NM axons make synaptic contact with NL. We then evaluated topography along the frequency axis using both anterograde and retrograde labeling in both the ipsilateral and contralateral NM-NL pathways. We found that after misexpression, NM regions project to a significantly broader proportion of NL than in control embryos, and that both the ipsilateral map and the contralateral map show this increased divergence. These results support a role for EphA4 in establishing tonotopic projections in the auditory system, and further suggest a general role for Eph family proteins in establishing topographic maps in the nervous system.

Molecular guidance cues necessary for axon pathfinding from the ventral cochlear nucleus

During development, multiple guidance cues direct the formation of appropriate synaptic connections. Factors that guide developing axons are known for various pathways throughout the mammalian brain; however, signals necessary to establish auditory connections are largely unknown. In the auditory brainstem the neurons whose axons traverse the midline in the ventral acoustic stria (VAS) are primarily located in the ventral cochlear nucleus (VCN) and project bilaterally to the superior olivary complex (SOC). The circumferential trajectory taken by developing VCN axons is similar to that of growing axons of spinal commissural neurons. Therefore, we reasoned that netrin-DCC and slit-robo signaling systems function in the guidance of VCN axons. VCN neurons express the transcription factor, mafB, as early as embryonic day (E) 13.5, thereby identifying the embryonic VCN for these studies. VCN axons extend toward the midline as early as E13, with many axons crossing by E14.5. During this time, netrin-1 and slit-1 RNAs are expressed at the brainstem midline. Additionally, neurons within the VCN express RNA for DCC, robo-1, and robo-2, and axons in the VAS are immunoreactive for DCC. VCN axons do not reach the midline of the brainstem in mice mutant for either the netrin-1 or DCC gene. VCN axons extend in pups lacking netrin-1, but most DCC-mutant samples lack VCN axonal outgrowth. Stereological cell estimates indicate only a modest reduction of VCN neurons in DCC-mutant mice. Taken together, these data show that a functional netrin-DCC signaling system is required for establishing proper VCN axonal projections in the auditory brainstem.

Windowing chicken eggs for developmental studies

The study of development has been greatly aided by the use of the chick embryo as an experimental model. The ease of accessibility of the embryo has allowed for experiments to map cell fates using several approaches, including chick quail chimeras and focal dye labeling. In addition, it allows for molecular perturbations of several types, including placement of protein-coated beads and introduction of plasmid DNA using in ovo electroporation. These experiments have yielded important data on the development of the central and peripheral nervous systems. For many of these studies, it is necessary to open the eggshell and reclose it without perturbing the embryo's growth. The embryo can be examined at successive developmental stages by re-opening the eggshell. While there are several excellent methods for opening chicken eggs, in this article we demonstrate one method that has been optimized for long survival times. In this method, the egg rests on its side and a small window is cut in the shell. After the experimental procedure, the shell is used to cover the egg for the duration of its development. Clear plastic tape overlying the eggshell protects the embryo and helps retain hydration during the remainder of the incubation period. This method has been used beginning at two days of incubation and has allowed survival through mature embryonic ages.

Auditory brainstem responses are impaired in EphA4 and ephrin-B2 deficient mice

The Eph receptor tyrosine kinases and their membrane-anchored ligands, ephrins, are signaling proteins that act as axon guidance molecules during chick auditory brainstem development. We recently showed that Eph proteins also affect patterns of neural activation in the mammalian brainstem. However, functional deficits in the brainstems of mutant mice have not been assessed physiologically. The present study characterizes neural activation in Eph protein deficient mice in the auditory brainstem response (ABR). We recorded the ABR of EphA4 and ephrin-B2 mutant mice, aged postnatal day 18-20, and compared them to wild type controls. The peripheral hearing threshold of EphA4(-/-) mice was 75% higher than that of controls. Waveform amplitudes of peak 1 (P1) were 54% lower in EphA4(-/-) mice than in controls. The peripheral hearing thresholds in ephrin-B2(lacZ/)(+) mice were also elevated, with a mean value 20% higher than that of controls. These ephrin-B2(lacZ/)(+) mice showed a 38% smaller P1 amplitude. Significant differences in latency to waveform peaks were also observed. These elevated thresholds and reduced peak amplitudes provide evidence for hearing deficits in both of these mutant mouse lines, and further emphasize an important role for Eph family proteins in the formation of functional auditory circuitry.

Changes in Sef levels influence auditory brainstem development and function

During development of the CNS, secreted morphogens of the fibroblast growth factor (FGF) family have multiple effects on cell division, migration, and survival depending on where, when, and how much FGF signal is received. The consequences of misregulating the FGF pathway were studied in a mouse with decreased levels of the FGF antagonist Sef. To uncover effects in the nervous system, we focused on the auditory system, which is accessible to physiological analysis. We found that the mitogen-activated protein kinase pathway is active in the rhombic lip, a germinal zone that generates diverse types of neurons, including the cochlear nucleus complex of the auditory system. Sef is expressed immediately adjacent to the rhombic lip, overlapping with FGF15 and FGFR1, which is also present in the lip itself. This pattern suggests that Sef may normally function in non-rhombic lip cells and prevent them from responding to FGF ligand in the vicinity. Consistent with this idea, overexpression of Sef in chicks decreased the size of the auditory nuclei. Cochlear nucleus defects were also apparent in mice with reduced levels of Sef, with 13% exhibiting grossly dysmorphic cochlear nuclei and 26% showing decreased amounts of GFAP in the cochlear nucleus. Additional evidence for cochlear nucleus defects was obtained by electrophysiological analysis of Sef mutant mice, which have normal auditory thresholds but abnormal auditory brainstem responses. These results show both increases and decreases in Sef levels affect the assembly and function of the auditory brainstem.

Deletion of EphA4 enhances deafferentation-induced ipsilateral sprouting in auditory brainstem projections

Axonal selection of ipsilateral and/or contralateral targets is essential for integrating bilateral sensory information and for coordinated movement. The molecular processes that determine ipsilateral and contralateral target choice are not fully understood. We examined this target selection in the developing auditory brainstem. Ventral cochlear nucleus (VCN) axons normally project to the medial nucleus of the trapezoid body (MNTB) only on the contralateral side. However, after unilateral removal of cochlear input in neonates, we found that axons from the unoperated VCN sprout and project to MNTB bilaterally. We found that EphA4 is expressed in the mouse auditory brainstem during development and during a sensitive period for ipsilateral sprouting, so we hypothesized that deletion of the Eph receptor EphA4 would impair target selection in these auditory pathways. Lipophilic dyes were used to evaluate quantitatively the brainstem projections in wild-type and EphA4-null mice. VCN-MNTB projections in EphA4-null mice were strictly contralateral, as in wild-type mice. However, after deafferentation, EphA4-null mice had a significant, threefold increase in the proportion of axons from the intact VCN that sprouted into ipsilateral MNTB compared with wild-type mice. Heterozygous mice had a twofold increase in these projections. These results demonstrate that EphA4 influences auditory brainstem circuitry selectively in response to deafferentation. Although this axon guidance molecule is not by itself necessary for appropriate target choice during normal development, it is a strong determinant of ipsilateral vs. contralateral target choice during deafferentation-induced plasticity.