Axon guidance in the auditory system: multiple functions of Eph receptors

Ambient sound stimulation tunes axonal conduction velocity by regulating radial growth of myelin on an individual, axon-by-axon basis

Adaptive myelination is the emerging concept of tuning axonal conduction velocity to the activity within specific neural circuits over time. Sound processing circuits exhibit structural and functional specifications to process signals with microsecond precision: a time scale that is amenable to adjustment in length and thickness of myelin. Increasing activity of auditory axons by introducing sound-evoked responses during postnatal development enhances myelin thickness, while sensory deprivation prevents such radial growth during development. When deprivation occurs during adulthood, myelin thickness was reduced. However, it is unclear whether sensory stimulation adjusts myelination in a global fashion (whole fiber bundles) or whether such adaptation occurs at the level of individual fibers. Using temporary monaural deprivation in mice provided an internal control for a) differentially tracing structural changes in active and deprived fibers and b) for monitoring neural activity in response to acoustic stimulation of the control and the deprived ear within the same animal. The data show that sound-evoked activity increased the number of myelin layers around individual active axons, even when located in mixed bundles of active and deprived fibers. Thicker myelination correlated with faster axonal conduction velocity and caused shorter auditory brainstem response wave VI-I delays, providing a physiologically relevant readout. The lack of global compensation emphasizes the importance of balanced sensory experience in both ears throughout the lifespan of an individual.

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.

Segregation of Multimodal Inputs Into Discrete Midbrain Compartments During an Early Critical Period

The lateral cortex of the inferior colliculus (LCIC) is a multimodal subdivision of the midbrain inferior colliculus (IC) that plays a key role in sensory integration. The LCIC is compartmentally-organized, exhibiting a series of discontinuous patches or modules surrounded by an extramodular matrix. In adult mice, somatosensory afferents target LCIC modular zones, while auditory afferents terminate throughout the encompassing matrix. Recently, we defined an early LCIC critical period (birth: postnatal day 0 to P12) based upon the concurrent emergence of its neurochemical compartments (modules: glutamic acid decarboxylase, GAD+; matrix: calretinin, CR+), matching Eph-ephrin guidance patterns, and specificity of auditory inputs for its matrix. Currently lacking are analogous experiments that address somatosensory afferent shaping and the construction of discrete LCIC multisensory maps. Combining living slice tract-tracing and immunocytochemical approaches in a developmental series of GAD67-GFP knock-in mice, the present study characterizes: (1) the targeting of somatosensory terminals for emerging LCIC modular fields; and (2) the relative separation of somatosensory and auditory inputs over the course of its established critical period. Results indicate a similar time course and progression of LCIC projection shaping for both somatosensory (corticocollicular) and auditory (intracollicular) inputs. While somewhat sparse and intermingling at birth, modality-specific projection patterns soon emerge (P4–P8), coincident with peak guidance expression and the appearance of LCIC compartments. By P12, an adult-like arrangement is in place, with fully segregated multimodal afferent arrays. Quantitative measures confirm increasingly distinct input maps, exhibiting less projection overlap with age. Potential mechanisms whereby multisensory LCIC afferent systems recognize and interface with its emerging modular-matrix framework are discussed.

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.

Compromised fractalkine signaling delays microglial occupancy of emerging modules in the multisensory midbrain

Microglial cells (MGCs) are highly dynamic and have been implicated in shaping discrete neural maps in several unimodal systems. MGCs respond to numerous cues in their microenvironment, including the neuronally expressed chemokine, fractalkine (CX3CL1), via interactions with its corresponding fractalkine receptor (CX3CR1). The present study examines microglial and CX3CL1 patterns with regard to the emerging modular-extramodular matrix organization within the lateral cortex of the inferior colliculus (LCIC). The LCIC is a multisensory shell region of the midbrain inferior colliculus where discrete compartments receive modality-specific connections. Somatosensory inputs terminate within modular confines, while auditory inputs target the surrounding matrix. Glutamic acid decarboxylase (GAD) is an established marker of LCIC modules in developing mouse. During early postnatal development, multimodal LCIC afferents segregate into discrete, neurochemically defined compartments. Here, we analyzed neonatal GAD67-GFP (GFP is defined as green fluorescent protein) and CX3CR1-GFP mice to assess: (1) whether MGCs are recruited to distinct LCIC compartments known to be undergoing active circuit assembly, and (2) if such behaviors are fractalkine signaling-dependent. MGCs colonize the nascent LCIC by birth and increase in density until postnatal day 12 (P12). At the peak critical period (P4-P8), MGCs conspicuously border emerging LCIC modules, prior to their subsequent invasion by P12. CX3CL1 expression becomes distinctly modular at P12, in keeping with the notion of fractalkine-mediated recruitment of microglia to modular centers. In CX3CR1^GFP/GFP mice with compromised fractalkine signaling, microglial recruitment into modules is delayed. Taken together, these results suggest a potential role for microglia and fractalkine signaling in sculpting multisensory LCIC maps during an early critical period.

Ephrin-A3 is required for tonotopic map precision and auditory functions in the mouse auditory brainstem

Tonotopy is a prominent feature of the vertebrate auditory system and forms the basis for sound discrimination, but the molecular mechanism that underlies its formation remains largely elusive. Ephrin/Eph signaling is known to play important roles in axon guidance during topographic mapping in other sensory systems, so we investigated its possible role in the establishment of tonotopy in the mouse cochlear nucleus. We found that ephrin-A3 molecules are differentially expressed along the tonotopic axis in the cochlear nucleus during innervation. Ephrin-A3 forward signaling is sufficient to repel auditory nerve fibers in a developmental stage-dependent manner. In mice lacking ephrin-A3, the tonotopic map is degraded and isofrequency bands of neuronal activation upon pure tone exposure become imprecise in the anteroventral cochlear nucleus. Ephrin-A3 mutant mice also exhibit a delayed second wave in auditory brainstem responses upon sound stimuli and impaired detection of sound frequency changes. Our findings establish an essential role for ephrin-A3 in forming precise tonotopy in the auditory brainstem to ensure accurate sound discrimination.

Central auditory deficits associated with genetic forms of peripheral deafness

Since the 1990s, the study of inherited hearing disorders, mostly those detected at birth, in the prelingual period or in young adults, has led to the identification of their causal genes. The genes responsible for more than 140 isolated (non-syndromic) and about 400 syndromic forms of deafness have already been discovered. Studies of mouse models of these monogenic forms of deafness have provided considerable insight into the molecular mechanisms of hearing, particularly those involved in the development and/or physiology of the auditory sensory organ, the cochlea. In parallel, studies of these models have also made it possible to decipher the pathophysiological mechanisms underlying hearing impairment. This has led a number of laboratories to investigate the potential of gene therapy for curing these forms of deafness. Proof-of-concept has now been obtained for the treatment of several forms of deafness in mouse models, paving the way for clinical trials of cochlear gene therapy in patients in the near future. Nevertheless, peripheral deafness may also be associated with central auditory dysfunctions and may extend well beyond the auditory system itself, as a consequence of alterations to the encoded sensory inputs or involvement of the causal deafness genes in the development and/or functioning of central auditory circuits. Investigating the diversity, causes and underlying mechanisms of these central dysfunctions, the ways in which they could impede the expected benefits of hearing restoration by peripheral gene therapy, and determining how these problems could be remedied is becoming a research field in its own right. Here, we provide an overview of the current knowledge about the central deficits associated with genetic forms of deafness.

Differential expression of microRNAs in the developing avian auditory hindbrain

The avian auditory hindbrain is a longstanding model for studying neural circuit development. Information on gene regulatory network (GRN) components underlying this process, however, is scarce. Recently, the spatiotemporal expression of 12 microRNAs (miRNAs) was investigated in the mammalian auditory hindbrain. As a comparative study, we here investigated the spatiotemporal expression of the orthologous miRNAs during development of the chicken auditory hindbrain. All miRNAs were expressed both at E13, an immature stage, and P14, a mature stage of the auditory system. In most auditory nuclei, a homogeneous expression pattern was observed at both stages, like the mammalian system. An exception was the nucleus magnocellularis (NM). There, at E13, nine miRNAs showed a differential expression pattern along the cochleotopic axis with high expression at the rostromedial pole. One of them showed a gradient expression whereas eight showed a spatially selective expression at the rostral pole that reflected the different rhombomeric origins of this composite nucleus. The miRNA differential expression persisted in the NM to the mature stage, with the selective expression changed to linear gradients. Bioinformatics analysis predicted mRNA targets that are associated with neuronal developmental processes such as neurite and synapse organization, calcium and ephrin-Eph signaling, and neurotransmission. Overall, this first analysis of miRNAs in the chicken central auditory system reveals shared and strikingly distinct features between chicken and murine orthologues. The embryonic gradient expression of these GRN elements in the NM adds miRNA patterns to the list of cochleotopic and developmental gradients in the central auditory system.

Cellular and Widefield Imaging of Sound Frequency Organization in Primary and Higher Order Fields of the Mouse Auditory Cortex

The mouse auditory cortex (ACtx) contains two core fields-primary auditory cortex (A1) and anterior auditory field (AAF)-arranged in a mirror reversal tonotopic gradient. The best frequency (BF) organization and naming scheme for additional higher order fields remain a matter of debate, as does the correspondence between smoothly varying global tonotopy and heterogeneity in local cellular tuning. Here, we performed chronic widefield and two-photon calcium imaging from the ACtx of awake Thy1-GCaMP6s reporter mice. Data-driven parcellation of widefield maps identified five fields, including a previously unidentified area at the ventral posterior extreme of the ACtx (VPAF) and a tonotopically organized suprarhinal auditory field (SRAF) that extended laterally as far as ectorhinal cortex. Widefield maps were stable over time, where single pixel BFs fluctuated by less than 0.5 octaves throughout a 1-month imaging period. After accounting for neuropil signal and frequency tuning strength, BF organization in neighboring layer 2/3 neurons was intermediate to the heterogeneous salt and pepper organization and the highly precise local organization that have each been described in prior studies. Multiscale imaging data suggest there is no ultrasonic field or secondary auditory cortex in the mouse. Instead, VPAF and a dorsal posterior (DP) field emerged as the strongest candidates for higher order auditory areas.

Registry of Compartmental Ephrin-B3 Guidance Patterns With Respect to Emerging Multimodal Midbrain Maps

Guidance errors and unrefined neural map configurations appear linked to certain neurodevelopmental conditions, including autism spectrum disorders. Deficits in specific multisensory tasks that require midbrain processing are highly predictive of cognitive and behavioral phenotypes associated with such syndromes. The lateral cortex of the inferior colliculus (LCIC) is a shell region of the mesencephalon that integrates converging information from multiple levels and modalities. Mature LCIC sensory maps are discretely-organized, mimicking its compartmental micro-organization. Intermittent modular domains receive patchy somatosensory connections, while inputs of auditory origin terminate in the encompassing extramodular matrix.Eph-ephrin signaling mechanisms instruct comparable topographic arrangements in a variety of other systems. Whether Eph-ephrin interactions also govern the assembly of LCIC multimodal maps remains unaddressed. Previously, we identified EphA4 and ephrin-B2 as key mediators, with overlapping expression patterns that align with emerging LCIC modules. Here, we implicate another member of this guidance family, ephrin-B3, and quantify its transient expression with respect to neurochemically-defined LCIC compartments. Multiple-labeling studies in GAD67-GFP knock-in mice reveal extramodular ephrin-B3 expression, complementary to that of EphA4 and ephrin-B2. This distinctive pattern sharpens over the early postnatal period (birth to P8), prior to ephrin-B3 downregulation once multimodal LCIC inputs are largely segregated (P12). Channel-specific sampling of LCIC ROIs show ephrin-B3 signal periodicities that are out-of-phase with glutamic acid decarboxylase (GAD;modular marker) signal fluctuations, and match calretinin (CR) waveforms (matrix marker). Taken together, the guidance mosaic registry with emerging LCIC compartments and its interfacing afferent streams suggest a prominent role for Eph-ephrins in ordering behaviorally significant multisensory midbrain networks.

Caspase-3 Cleaves Extracellular Vesicle Proteins During Auditory Brainstem Development

Sound localization requires extremely precise development of auditory brainstem circuits, the molecular mechanisms of which are largely unknown. We previously demonstrated a novel requirement for non-apoptotic activity of the protease caspase-3 in chick auditory brainstem development. Here, we used mass spectrometry to identify proteolytic substrates of caspase-3 during chick auditory brainstem development. These auditory brainstem caspase-3 substrates were enriched for proteins previously shown to be cleaved by caspase-3, especially in non-apoptotic contexts. Functional annotation analysis revealed that our caspase-3 substrates were also enriched for proteins associated with several protein categories, including proteins found in extracellular vesicles (EVs), membrane-bound nanoparticles that function in intercellular communication. The proteome of EVs isolated from the auditory brainstem was highly enriched for our caspase-3 substrates. Additionally, we identified two caspase-3 substrates with known functions in axon guidance, namely Neural Cell Adhesion Molecule (NCAM) and Neuronal-glial Cell Adhesion Molecule (Ng-CAM), that were found in auditory brainstem EVs and expressed in the auditory pathway alongside cleaved caspase-3. Taken together, these data suggest a novel developmental mechanism whereby caspase-3 influences auditory brainstem circuit formation through the proteolytic cleavage of extracellular vesicle (EV) proteins.

Multi-sensory (auditory and somatosensory) pre-pulse inhibition in mice

We investigated the perception of two mechanoreceptive modalities alone and in combination: main effects and interaction between auditory and somatosensory stimulation in mice. Fifteen C57BL/6J mice between the ages of 1 and 6 months were tested three times each. Experimental design roughly followed published procedures using pre-pulse inhibition (PPI) of the acoustic startle response, except pre-pulses included vibration of the test chamber as well as soft sounds. Auditory pre-pulses were 80 dB broadband noises of 4, 9, 25, or 45 ms duration. Vibrations were of the same duration but of different frequencies (500, 460, 360, and 220 Hz). Pre-pulse inhibition increased with duration of the auditory pre-pulses, as expected. There was significant PPI to some but not all vibrotactile pre-pulses. Multimodal PPI was approximately additive (no significant auditory-by-somatosensory interaction). PPI increased more with age to somatosensory than to auditory pre-pulses. Future studies of multi-modal psychophysics in various mouse mutants could lend support to more mechanistic studies of neural specificity and possibly autism, tinnitus, and PTSD.

Abnormal development of auditory responses in the inferior colliculus of a mouse model of Fragile X Syndrome

Sensory processing abnormalities are frequently associated with autism spectrum disorders, but the underlying mechanisms are unclear. Here we studied auditory processing in a mouse model of Fragile X Syndrome (FXS), a leading known genetic cause of autism and intellectual disability. Both humans with FXS and the Fragile X mental retardation gene (Fmr1) knockout (KO) mouse model show auditory hypersensitivity, with the latter showing a strong propensity for audiogenic seizures (AGS) early in development. Because midbrain abnormalities cause AGS, we investigated whether the inferior colliculus (IC) of the Fmr1 KO mice shows abnormal auditory processing compared with wild-type (WT) controls at specific developmental time points. Using antibodies against neural activity marker c-Fos, we found increased density of c-Fos+ neurons in the IC, but not auditory cortex, of Fmr1 KO mice at P21 and P34 following sound presentation. In vivo single-unit recordings showed that IC neurons of Fmr1 KO mice are hyperresponsive to tone bursts and amplitude-modulated tones during development and show broader frequency tuning curves. There were no differences in rate-level responses or phase locking to amplitude-modulated tones in IC neurons between genotypes. Taken together, these data provide evidence for the development of auditory hyperresponsiveness in the IC of Fmr1 KO mice. Although most human and mouse work in autism and sensory processing has centered on the forebrain, our new findings, along with recent work on the lower brainstem, suggest that abnormal subcortical responses may underlie auditory hypersensitivity in autism spectrum disorders.NEW & NOTEWORTHY Autism spectrum disorders (ASD) are commonly associated with sensory sensitivity issues, but the underlying mechanisms are unclear. This study presents novel evidence for neural correlates of auditory hypersensitivity in the developing inferior colliculus (IC) in Fmr1 knockout (KO) mouse, a mouse model of Fragile X Syndrome (FXS), a leading genetic cause of ASD. Responses begin to show genotype differences between postnatal days 14 and 21, suggesting an early developmental treatment window.

Shaping of discrete auditory inputs to extramodular zones of the lateral cortex of the inferior colliculus

The multimodal lateral cortex of the inferior colliculus (LCIC) exhibits a modular-extramodular micro-organization that is evident early in development. In addition to a set of neurochemical markers that reliably highlight its modular-extramodular organization (e.g. modules: GAD67-positive, extramodular zones: calretinin-positive, CR), mature projection patterns suggest that major LCIC afferents recognize and adhere to such a framework. In adult mice, distinct afferent projections appear segregated, with somatosensory inputs targeting LCIC modules and auditory inputs surrounding extramodular fields. Currently lacking is an understanding regarding the development and shaping of multimodal LCIC afferents with respect to its emerging modular-extramodular microarchitecture. Combining living slice tract-tracing and immunocytochemical approaches in GAD67-GFP knock-in mice, the present study characterizes the critical period of projection shaping for LCIC auditory afferents arising from its neighboring central nucleus (CNIC). Both crossed and uncrossed projection patterns exhibit LCIC extramodular mapping characteristics that emerge from initially diffuse distributions. Projection mismatch with GAD-defined modules and alignment with encompassing extramodular zones becomes increasingly clear over the early postnatal period (birth to postnatal day 12). CNIC inputs terminate almost exclusively in extramodular zones that express CR. These findings suggest multimodal LCIC inputs may initially be sparse and intermingle, prior to segregation into distinct processing streams. Future experiments are needed to determine the likely complex interactions and mechanisms (e.g. activity-dependent and independent) responsible for shaping early modality-specific LCIC circuits.

Genes Involved in the Development and Physiology of Both the Peripheral and Central Auditory Systems

The genetic approach, based on the study of inherited forms of deafness, has proven to be particularly effective for deciphering the molecular mechanisms underlying the development of the peripheral auditory system, the cochlea and its afferent auditory neurons, and how this system extracts the physical parameters of sound. Although this genetic dissection has provided little information about the central auditory system, scattered data suggest that some genes may have a critical role in both the peripheral and central auditory systems. Here, we review the genes controlling the development and function of the peripheral and central auditory systems, focusing on those with demonstrated intrinsic roles in both systems and highlighting the current underappreciation of these genes. Their encoded products are diverse, from transcription factors to ion channels, as are their roles in the central auditory system, mostly evaluated in brainstem nuclei. We examine the ontogenetic and evolutionary mechanisms that may underlie their expression at different sites.

Eph/ephrin signalling in the development and function of the mammalian cochlea

In mammals, the functional development of the cochlea requires the tight regulation of multiple molecules and signalling pathways including fibroblast growth factors, bone morphogenetic proteins, Wnt and Notch signalling pathways. Over the last decade, the Eph/ephrin system also emerged as a key player of the development and function of the mammalian cochlea. In this review, we discuss the recent advances on the role of Eph/ephrin signalling in patterning the cochlear sensory epithelium and the complex innervation of mechanosensory hair cells by spiral ganglion neurons. Finally, we address the issue of a syndromic form of hearing loss caused by a deficient member of the Eph/ephrin family.

Neurod1 Is Essential for the Primary Tonotopic Organization and Related Auditory Information Processing in the Midbrain

Hearing depends on extracting frequency, intensity, and temporal properties from sound to generate an auditory map for acoustical signal processing. How physiology intersects with molecular specification to fine tune the developing properties of the auditory system that enable these aspects remains unclear. We made a novel conditional deletion model that eliminates the transcription factor NEUROD1 exclusively in the ear. These mice (both sexes) develop a truncated frequency range with no neuroanatomically recognizable mapping of spiral ganglion neurons onto distinct locations in the cochlea nor a cochleotopic map presenting topographically discrete projections to the cochlear nuclei. The disorganized primary cochleotopic map alters tuning properties of the inferior colliculus units, which display abnormal frequency, intensity, and temporal sound coding. At the behavioral level, animals show alterations in the acoustic startle response, consistent with altered neuroanatomical and physiological properties. We demonstrate that absence of the primary afferent topology during embryonic development leads to dysfunctional tonotopy of the auditory system. Such effects have never been investigated in other sensory systems because of the lack of comparable single gene mutation models.SIGNIFICANCE STATEMENT All sensory systems form a topographical map of neuronal projections from peripheral sensory organs to the brain. Neuronal projections in the auditory pathway are cochleotopically organized, providing a tonotopic map of sound frequencies. Primary sensory maps typically arise by molecular cues, requiring physiological refinements. Past work has demonstrated physiologic plasticity in many senses without ever molecularly undoing the specific mapping of an entire primary sensory projection. We genetically manipulated primary auditory neurons to generate a scrambled cochleotopic projection. Eliminating tonotopic representation to auditory nuclei demonstrates the inability of physiological processes to restore a tonotopic presentation of sound in the midbrain. Our data provide the first insights into the limits of physiology-mediated brainstem plasticity during the development of the auditory system.

Talking back: Development of the olivocochlear efferent system

Developing sensory systems must coordinate the growth of neural circuitry spanning from receptors in the peripheral nervous system (PNS) to multilayered networks within the central nervous system (CNS). This breadth presents particular challenges, as nascent processes must navigate across the CNS-PNS boundary and coalesce into a tightly intermingled wiring pattern, thereby enabling reliable integration from the PNS to the CNS and back. In the auditory system, feedforward spiral ganglion neurons (SGNs) from the periphery collect sound information via tonotopically organized connections in the cochlea and transmit this information to the brainstem for processing via the VIII cranial nerve. In turn, feedback olivocochlear neurons (OCNs) housed in the auditory brainstem send projections into the periphery, also through the VIII nerve. OCNs are motor neuron-like efferent cells that influence auditory processing within the cochlea and protect against noise damage in adult animals. These aligned feedforward and feedback systems develop in parallel, with SGN central axons reaching the developing auditory brainstem around the same time that the OCN axons extend out toward the developing inner ear. Recent findings have begun to unravel the genetic and molecular mechanisms that guide OCN development, from their origins in a generic pool of motor neuron precursors to their specialized roles as modulators of cochlear activity. One recurrent theme is the importance of efferent-afferent interactions, as afferent SGNs guide OCNs to their final locations within the sensory epithelium, and efferent OCNs shape the activity of the developing auditory system. This article is categorized under: Nervous System Development > Vertebrates: Regional Development.

Alignment of EphA4 and ephrin-B2 expression patterns with developing modularity in the lateral cortex of the inferior colliculus

In the multimodal lateral cortex of the inferior colliculus (LCIC), there are two neurochemically and connectionally distinct compartments, termed modular and extramodular zones. Modular fields span LCIC layer 2 and are recipients of somatosensory afferents, while encompassing extramodular domains receive auditory inputs. Recently, in developing mice, we identified several markers (among them glutamic acid decarboxylase, GAD) that consistently label the same modular set, and a reliable extramodular marker, calretinin, (CR). Previous reports from our lab show similar modular-extramodular patterns for certain Eph-ephrin guidance members, although their precise alignment with the developing LCIC neurochemical framework has yet to be addressed. Here we confirm in the nascent LCIC complementary GAD/CR-positive compartments, and characterize the registry of EphA4 and ephrin-B2 expression patterns with respect to its emerging modular-extramodular organization. Immunocytochemical approaches in GAD67-GFP knock-in mice reveal patchy EphA4 and ephrin-B2 domains that precisely align with GAD-positive LCIC modules, and are complementary to CR-defined extramodular zones. Such patterning was detectable neonatally, yielding discrete compartments prior to hearing onset. A dense plexus of EphA4-positive fibers filled modules, surrounding labeled ephrin-B2 and GAD cell populations. The majority of observed GABAergic neurons within modular boundaries were also positive for ephrin-B2. These results suggest an early compartmentalization of the LCIC that is likely instructed in part through Eph-ephrin guidance mechanisms. The overlap of developing LCIC neurochemical and guidance patterns is discussed in the context of its seemingly segregated multimodal input-output streams.

Hox2 Genes Are Required for Tonotopic Map Precision and Sound Discrimination in the Mouse Auditory Brainstem

Tonotopy is a hallmark of auditory pathways and provides the basis for sound discrimination. Little is known about the involvement of transcription factors in brainstem cochlear neurons orchestrating the tonotopic precision of pre-synaptic input. We found that in the absence of Hoxa2 and Hoxb2 function in Atoh1-derived glutamatergic bushy cells of the anterior ventral cochlear nucleus, broad input topography and sound transmission were largely preserved. However, fine-scale synaptic refinement and sharpening of isofrequency bands of cochlear neuron activation upon pure tone stimulation were impaired in Hox2 mutants, resulting in defective sound-frequency discrimination in behavioral tests. These results establish a role for Hox factors in tonotopic refinement of connectivity and in ensuring the precision of sound transmission in the mammalian auditory circuit.

Gene, cell, and organ multiplication drives inner ear evolution

We review the development and evolution of the ear neurosensory cells, the aggregation of neurosensory cells into an otic placode, the evolution of novel neurosensory structures dedicated to hearing and the evolution of novel nuclei in the brain and their input dedicated to processing those novel auditory stimuli. The evolution of the apparently novel auditory system lies in duplication and diversification of cell fate transcription regulation that allows variation at the cellular level [transforming a single neurosensory cell into a sensory cell connected to its targets by a sensory neuron as well as diversifying hair cells], organ level [duplication of organ development followed by diversification and novel stimulus acquisition] and brain nuclear level [multiplication of transcription factors to regulate various neuron and neuron aggregate fate to transform the spinal cord into the unique hindbrain organization]. Tying cell fate changes driven by bHLH and other transcription factors into cell and organ changes is at the moment tentative as not all relevant factors are known and their gene regulatory network is only rudimentary understood. Future research can use the blueprint proposed here to provide both the deeper molecular evolutionary understanding as well as a more detailed appreciation of developmental networks. This understanding can reveal how an auditory system evolved through transformation of existing cell fate determining networks and thus how neurosensory evolution occurred through molecular changes affecting cell fate decision processes. Appreciating the evolutionary cascade of developmental program changes could allow identifying essential steps needed to restore cells and organs in the future.

Modular-extramodular organization in developing multisensory shell regions of the mouse inferior colliculus

The complex neuroanatomical connections of the inferior colliculus (IC) and its major subdivisions offer a juxtaposition of segregated processing streams with distinct organizational features. While the tonotopically layered central nucleus is well-documented, less is known about functional compartments in the neighboring lateral cortex (LCIC). In addition to a laminar framework, LCIC afferent-efferent patterns suggest a multimodal mosaic, consisting of a patchy modular network with surrounding extramodular domains. This study utilizes several neurochemical markers that reveal an emerging LCIC modular-extramodular microarchitecture. In newborn and post-hearing C57BL/6J and CBA/CaJ mice, histochemical and immunocytochemical stains were performed for acetylcholinesterase (AChE), nicotinamide adenine dinucleotide phosphate-diaphorase (NADPH-d), glutamic acid decarboxylase (GAD), cytochrome oxidase (CO), and calretinin (CR). Discontinuous layer 2 modules are positive for AChE, NADPH-d, GAD, and CO throughout the rostrocaudal LCIC. While not readily apparent at birth, discrete cell clusters emerge over the first postnatal week, yielding an identifiable modular network prior to hearing onset. Modular boundaries continue to become increasingly distinct with age, as surrounding extramodular fields remain largely negative for each marker. Alignment of modular markers in serial sections suggests each highlight the same periodic patchy network throughout the nascent LCIC. In contrast, CR patterns appear complementary, preferentially staining extramodular LCIC zones. Double-labeling experiments confirm that NADPH-d, the most consistent developmental modular marker, and CR label separate, nonoverlapping LCIC compartments. Determining how this emerging modularity may align with similar LCIC patch-matrix-like Eph/ephrin guidance patterns, and how each interface with, and potentially influence developing multimodal LCIC projection configurations is discussed.

Connectional Modularity of Top-Down and Bottom-Up Multimodal Inputs to the Lateral Cortex of the Mouse Inferior Colliculus

The lateral cortex of the inferior colliculus receives information from both auditory and somatosensory structures and is thought to play a role in multisensory integration. Previous studies in the rat have shown that this nucleus contains a series of distinct anatomical modules that stain for GAD-67 as well as other neurochemical markers. In the present study, we sought to better characterize these modules in the mouse inferior colliculus and determine whether the connectivity of other neural structures with the lateral cortex is spatially related to the distribution of these neurochemical modules. Staining for GAD-67 and other markers revealed a single modular network throughout the rostrocaudal extent of the mouse lateral cortex. Somatosensory inputs from the somatosensory cortex and dorsal column nuclei were found to terminate almost exclusively within these modular zones. However, projections from the auditory cortex and central nucleus of the inferior colliculus formed patches that interdigitate with the GAD-67-positive modules. These results suggest that the lateral cortex of the mouse inferior colliculus exhibits connectional as well as neurochemical modularity and may contain multiple segregated processing streams. This finding is discussed in the context of other brain structures in which neuroanatomical and connectional modularity have functional consequences.

SIGNIFICANCE STATEMENT

Many brain regions contain subnuclear microarchitectures, such as the matrix-striosome organization of the basal ganglia or the patch-interpatch organization of the visual cortex, that shed light on circuit complexities. In the present study, we demonstrate the presence of one such micro-organization in the rodent inferior colliculus. While this structure is typically viewed as an auditory integration center, its lateral cortex appears to be involved in multisensory operations and receives input from somatosensory brain regions. We show here that the lateral cortex can be further subdivided into multiple processing streams: modular regions, which are targeted by somatosensory inputs, and extramodular zones that receive auditory information.

Axonal Cleaved Caspase-3 Regulates Axon Targeting and Morphogenesis in the Developing Auditory Brainstem

Caspase-3 is a cysteine protease that is most commonly associated with cell death. Recent studies have shown additional roles in mediating cell differentiation, cell proliferation and development of cell morphology. We investigated the role of caspase-3 in the development of chick auditory brainstem nuclei during embryogenesis. Immunofluorescence from embryonic days E6–13 revealed that the temporal expression of cleaved caspase-3 follows the ascending anatomical pathway. The expression is first seen in the auditory portion of VIIIth nerve including central axonal regions projecting to nucleus magnocellularis (NM), then later in NM axons projecting to nucleus laminaris (NL), and subsequently in NL dendrites. To examine the function of cleaved caspase-3 in chick auditory brainstem development, we blocked caspase-3 cleavage in developing chick embryos with the caspase-3 inhibitor Z-DEVD-FMK from E6 to E9, then examined NM and NL morphology and NM axonal targeting on E10. NL lamination in treated embryos was disorganized and the neuropil around NL contained a significant number of glial cells normally excluded from this region. Additionally, NM axons projected into inappropriate portions of NL in Z-DEVD-FMK treated embyros. We found that the presence of misrouted axons was associated with more severe NL disorganization. The effects of axonal caspase-3 inhibition on both NL morphogenesis and NM axon targeting suggest that these developmental processes are coordinated, likely through communication between axons and their targets.

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.

Graded and discontinuous EphA-ephrinB expression patterns in the developing auditory brainstem

Eph-ephrin interactions guide topographic mapping and pattern formation in a variety of systems. In contrast to other sensory pathways, their precise role in the assembly of central auditory circuits remains poorly understood. The auditory midbrain, or inferior colliculus (IC) is an intriguing structure for exploring guidance of patterned projections as adjacent subdivisions exhibit distinct organizational features. The central nucleus of the IC (CNIC) and deep aspects of its neighboring lateral cortex (LCIC, Layer 3) are tonotopically-organized and receive layered inputs from primarily downstream auditory sources. While less is known about more superficial aspects of the LCIC, its inputs are multimodal, lack a clear tonotopic order, and appear discontinuous, terminating in modular, patch/matrix-like distributions. Here we utilize X-Gal staining approaches in lacZ mutant mice (ephrin-B2, -B3, and EphA4) to reveal EphA-ephrinB expression patterns in the nascent IC during the period of projection shaping that precedes hearing onset. We also report early postnatal protein expression in the cochlear nuclei, the superior olivary complex, the nuclei of the lateral lemniscus, and relevant midline structures. Continuous ephrin-B2 and EphA4 expression gradients exist along frequency axes of the CNIC and LCIC Layer 3. In contrast, more superficial LCIC localization is not graded, but confined to a series of discrete ephrin-B2 and EphA4-positive Layer 2 modules. While heavily expressed in the midline, much of the auditory brainstem is devoid of ephrin-B3, including the CNIC, LCIC Layer 2 modular fields, the dorsal nucleus of the lateral lemniscus (DNLL), as well as much of the superior olivary complex and cochlear nuclei. Ephrin-B3 LCIC expression appears complementary to that of ephrin-B2 and EphA4, with protein most concentrated in presumptive extramodular zones. Described tonotopic gradients and seemingly complementary modular/extramodular patterns suggest Eph-ephrin guidance in establishing juxtaposed continuous and discrete neural maps in the developing IC prior to experience.

Ephrin-A2 and ephrin-A5 guide contralateral targeting but not topographic mapping of ventral cochlear nucleus axons

Background

In the auditory brainstem, ventral cochlear nucleus (VCN) axons project to the contralateral, but not ipsilateral, medial nucleus of trapezoid body (MNTB), terminating in the calyx of Held. Dorsal VCN neurons, representing high frequencies, synapse with medial MNTB neurons, while low frequency-coding ventral VCN neurons synapse with lateral MNTB neurons, reflecting tonotopic organization. The mechanisms that ensure strictly contralateral targeting and topographic ordering are incompletely understood. Here we examined the roles of ephrin-A signaling in both types of targeting.

Results

Ephrin-A2 and ephrin-A5 are expressed in VCN cells during late embryonic and early postnatal development. At these ages ephrin-A2 is expressed in axons surrounding MNTB and ephrin-A5 is expressed in MNTB principal neurons. Ephrin-A2/A5 double knockout mice displayed axon targeting errors in which VCN axons project to MNTB on both sides of the brainstem, where they terminate in calyceal endings. Ephrin-A2 and ephrin-A5 single knockout mice showed a similar phenotype. In contrast to effects on contralateral targeting, ephrin-A2/A5 double knockout mice showed no defects in formation of tonotopically ordered projections from VCN to MNTB.

Conclusions

These findings demonstrate that distinct mechanisms regulate targeting of VCN axons to the contralateral MNTB and targeting to appropriate tonotopic locations. Ephrin-A signaling plays a similar role to ephrin-B signaling in the VCN-MNTB pathway, where both classes normally prevent formation of calyceal projections to ipsilateral MNTB. These classes may rely in part on common signaling pathways.

Thyroid hormone is required for the pruning of afferent type II spiral ganglion neurons in the mouse cochlea

Afferent connections to the sensory inner (IHCs) and outer hair cells (OHCs) in the cochlea refine and functionally mature during the thyroid hormone (TH)-critical period of inner ear development that occurs perinatally in rodents. In this study, we investigated the effects of hypothyroidism on afferent type II innervation to outer hair cells using the Snell dwarf mouse (Pit1(dw)). Using a transgenic approach to specifically label type II spiral ganglion neurons (SGNs), we found that lack of TH causes persistence of excess type II SGN connections to the OHCs, as well as continued expression of the hair cell functional marker, otoferlin (OTOF), in the OHCs beyond the maturation period. We also observed a concurrent delay in efferent attachment to the OHCs. Supplementing with TH during the early postnatal period from postnatal day (P) 3 to P4 reversed the defect in type II SGN pruning but did not alter OTOF expression. Our results show that hypothyroidism causes a defect in the large-scale pruning of afferent type II SGNs in the cochlea, and a delay in efferent attachment and the maturation of OTOF expression. Our data suggest that the state of maturation of hair cells, as determined by OTOF expression, may not regulate the pruning of their afferent innervation.

The Order and Place of Neuronal Differentiation Establish the Topography of Sensory Projections and the Entry Points within the Hindbrain

Establishing topographical maps of the external world is an important but still poorly understood feature of the vertebrate sensory system. To study the selective innervation of hindbrain regions by sensory afferents in the zebrafish embryo, we mapped the fine-grained topographical representation of sensory projections at the central level by specific photoconversion of sensory neurons. Sensory ganglia located anteriorly project more medially than do ganglia located posteriorly, and this relates to the order of sensory ganglion differentiation. By single-plane illumination microscopy (SPIM) in vivo imaging, we show that (1) the sequence of arrival of cranial ganglion inputs predicts the topography of central projections, and (2) delaminated neuroblasts differentiate in close contact with the neural tube, and they never loose contact with the neural ectoderm. Afferent entrance points are established by plasma membrane interactions between primary differentiated peripheral sensory neurons and neural tube border cells with the cooperation of neural crest cells. These first contacts remain during ensuing morphological growth to establish pioneer axons. Neural crest cells and repulsive slit1/robo2 signals then guide axons from later-differentiating neurons toward the neural tube. Thus, this study proposes a new model by which the topographical representation of cranial sensory ganglia is established by entrance order, with the entry points determined by cell contact between the sensory ganglion cell bodies and the hindbrain.

EphA7 regulates spiral ganglion innervation of cochlear hair cells

During the development of periphery auditory circuitry, spiral ganglion neurons (SGNs) form a spatially precise pattern of innervation of cochlear hair cells (HCs), which is an essential structural foundation for central auditory processing. However, molecular mechanisms underlying the developmental formation of this precise innervation pattern remain not well understood. Here, we specifically examined the involvement of Eph family members in cochlear development. By performing RNA-sequencing for different types of cochlear cell, in situ hybridization, and immunohistochemistry, we found that EphA7 was strongly expressed in a large subset of SGNs. In EphA7 deletion mice, there was a reduction in the number of inner radial bundles originating from SGNs and projecting to HCs as well as in the number of ribbon synapses on inner hair cells (IHCs), as compared with wild-type or heterozygous mutant mice, attributable to fewer type I afferent fibers. The overall activity of the auditory nerve in EphA7 deletion mice was also reduced, although there was no significant change in the hearing intensity threshold. In vitro analysis further suggested that the reduced innervation of HCs by SGNs could be attributed to a role of EphA7 in regulating outgrowth of SGN neurites as knocking down EphA7 in SGNs resulted in diminished SGN fibers. In addition, suppressing the activity of ERK1/2, a potential downstream target of EphA7 signaling, either with specific inhibitors in cultured explants or by knocking out Prkg1, also resulted in reduced SGN fibers. Together, our results suggest that EphA7 plays an important role in the developmental formation of cochlear innervation pattern through controlling SGN fiber ontogeny. Such regulation may contribute to the salience level of auditory signals presented to the central auditory system.

Ephs and Ephrins in malignant gliomas

Eph receptor tyrosine kinases and the corresponding ephrin ligands play a pivotal role in the glioma development and progression. Aberrant protein expression levels of the Eph receptors and ephrins are often associated with higher tumor grade and poor prognosis. Their function in tumorigenesis is complex due to the intricate network of possible co-occurring interactions between neighboring tumor cells and tumor microenvironment. Both Ephs and ephrins localize on the surface of tumor cells, tumor vasculature, glioma stem cells, tumor cells infiltrating brain, and immune cells infiltrating tumors. They can both promote and inhibit tumorigenicity depending on the downstream forward and reverse signalling generated. All the above-mentioned features make the Ephs/ephrins system an intriguing candidate for the development of new therapeutic strategies in glioma treatment. This review will give a general overview on the structure and the function of Ephs and ephrins, with a particular emphasis on the state of the knowledge of their role in malignant gliomas.