Null mutations in EphB receptors decrease sharpness of frequency tuning in primary auditory cortex

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.

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.

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.

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

The neural pathways of the auditory system underlie our ability to detect sounds and to transform amplitude and frequency information into rich and meaningful perception. While it shares some organizational features with other sensory systems, the auditory system has some unique functions that impose special demands on precision in circuit assembly. In particular, the cochlear epithelium creates a frequency map rather than a space map, and specialized pathways extract information on interaural time and intensity differences to permit sound source localization. The assembly of auditory circuitry requires the coordinated function of multiple molecular cues. Eph receptors and their ephrin ligands constitute a large family of axon guidance molecules with developmentally regulated expression throughout the auditory system. Functional studies of Eph/ephrin signaling have revealed important roles at multiple levels of the auditory pathway, from the cochlea to the auditory cortex. These proteins provide graded cues used in establishing tonotopically ordered connections between auditory areas, as well as discrete cues that enable axons to form connections with appropriate postsynaptic partners within a target area. Throughout the auditory system, Eph proteins help to establish patterning in neural pathways during early development. This early targeting, which is further refined with neuronal activity, establishes the precision needed for auditory perception.

The effects of Eph-ephrin mutations on pre-pulse inhibition in mice

Eph-ephrin signaling is known to be important in directing topographic projections in the afferent auditory pathway, including connections to various subdivisions of the inferior colliculus (IC). The acoustic startle-response (ASR) is a reliable reflexive behavioral response in mammals elicited by an unexpected intense acoustic startle-eliciting stimulus (ES). It is mediated by a sub-cortical pathway that includes the IC. The ASR amplitude can be measured with an accelerometer under the subject and can be decreased in amplitude by presenting a less intense, non-startling stimulus 5-300ms before the ES. This reflexive decrement in ASR is called pre-pulse inhibition (PPI) and indicates that the relatively soft pre-pulse was heard. PPI is a general trait among mammals. Mice have been used recently to study this response and to reveal how genetic mutations affect neural circuits and hence the ASR and PPI. In this experiment, we measured the effect of Eph-ephrin mutations using control mice (C57BL/6J), mice with compromised EphA4 signaling (EphA4(lacZ/+), EphA4(lacZ/lacZ)), and knockout ephrin-B3 mice (ephrin-B3 (+/-, -/-)). Control and EphA4(lacZ/+s)trains showed robust PPI (up to 75% decrement in ASR) to an offset of a 70dB SPL background noise at 50ms before the ES. Ephrin-B3 knockout mice and EphA4 homozygous mutants were only marginally significant in PPI (<25% decrement and <33% decrement, respectively) to the same conditions. This decrement in PPI highlights the importance of ephrin-B3 and EphA4 interactions in ordering auditory behavioral circuits. Thus, different mutations in certain members of the signaling family produce a full range of changes in PPI, from minimal to nearly maximal. This technique can be easily adapted to study other aspects of hearing in a wider range of mutations. Along with ongoing neuroanatomical studies, this allows careful quantification of how the auditory anatomical, physiological and now behavioral phenotype is affected by changes in Eph-ephrin expression and functionality.

Development of response selectivity in the mouse auditory cortex

The mouse auditory system contains neurons selective for tone duration and for a narrow range of frequency modulated (FM) sweep rates. Whether such selectivity is developmentally regulated is not known. The main goal of this study was to follow the development of neuronal responses to tones (frequency and duration tuning) and FM sweeps (direction and rate selectivity) in the core auditory cortex (A1 and AAF) of ketamine/xylazine anesthetized C57bl/6 mice. Three groups were compared: postnatal day (P) 15-20, P21-30 and P31-90. Frequency tuning bandwidth decreased during the first month indicating refinement of the excitatory receptive field. Duration tuning for tones did not change during development in terms of categories of tuning types as well as measures of selectivity such as best duration and half-maximal duration. FM rate and direction selectivity were developmentally regulated. Selectivity for linear up and down FM sweeps (0.06-22 kHz/ms) was tested. The best rate and half-maximal rate of neurons categorized as fast- or band-pass selective shifted toward faster rates during development. The percentage of fast-pass selective neurons also increased during development. These data suggest that cortical neurons' discrimination and detection abilities for relatively faster sweep rates improve during development. Although on average, direction selectivity was weak across development, there was a significant shift toward upward sweep selectivity at slow rates. Thus, the C57bl/6 mouse auditory cortex is not adult-like until at least P30. The changes in response selectivity can be explained based on known developmental changes in intrinsic and synaptic properties of mouse auditory cortical neurons.

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.