EphA4 and ephrin‐B2 expression patterns during inferior colliculus projection shaping prior to experience

Abstract

Central processing of complex auditory tasks requires elaborate circuitry. The auditory midbrain, or inferior colliculus (IC), epitomizes such precise organization, where converging inputs form discrete, tonotopically-arranged axonal layers. Previously in rat, we established that shaping of multiple afferent patterns in the IC central nucleus (CNIC) occurs prior to experience. This study implicates an Eph receptor tyrosine kinase and a corresponding ephrin ligand in signaling this early topographic registry. We report that EphA4 and ephrin-B2 expression patterns in the neonatal rat and mouse IC correlate temporally and spatially with that of developing axonal layers. DiI-labeling confirms projections arising from the lateral superior olive (LSO) form frequency-specific layers within the ipsilateral and contralateral mouse CNIC, as has been described in other species. Immunohistochemistry (EphA4 and ephrin-B2) and ephrin-B2 lacZ histochemistry reveal clear gradients in expression across the tonotopic axis, with most concentrated labeling observed in high-frequency, ventromedial aspects of the CNIC. Discrete patches of labeling were also discernible in the external cortex of the IC (ECIC; EphA4 patches in rat, ephrin-B2 patches in mouse). Observed gradients in the CNIC and compartmentalized ECIC expression persisted through the first postnatal week, before becoming less intense and more homogeneously distributed by the functional onset of hearing. EphA4 and ephrin-B2-positive neurons were evident in several auditory brainstem nuclei known to send patterened inputs to the IC. These findings suggest the involvement of cell-cell EphA4 and ephrin-B2 signaling in establishing order in the developing IC.

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

“…Eph proteins, including the Eph receptors and their ligands, the ephrins, are broadly expressed in the developing auditory system (for reviews see Bianchi et al, 2002, Pickles, 2003, Cramer, 2005, Gabriele et al, 2011). They provide multiple mechanisms for signaling and for generating precise connectivity.…”

“…An important first step in assessing whether Eph/ephrins play an instructive role in auditory midbrain map formation is the identification of protein expression patterns that correlate with the early period of projection shaping (Figure 3). Localization studies utilizing immunocytochemical and X-Gal staining approaches in lacZ mutants show a transient expression of EphA4 and ephrin-B2 in the IC leading up to the functional onset of hearing (Gabriele et al, 2011). In rat and mouse, each is expressed in a graded fashion across the tonotopic axis of the CNIC, with protein most concentrated in ventromedial, high-frequency regions.…”

“…In rat and mouse, each is expressed in a graded fashion across the tonotopic axis of the CNIC, with protein most concentrated in ventromedial, high-frequency regions. CNIC gradients are steep at birth through P4 (period of axonal sorting), before flattening to more homogeneous expression as experience ensues (Miko et al, 2007, Gabriele et al, 2011). In contrast to their continuous CNIC expression, both exhibit discrete, discontinuous patterns in the LCIC, with protein localized to presumptive modular fields that mimic those neurochemically described in the adult (Chernock et al, 2004, Malmierca et al, 2011, Ouda and Syka, 2012).…”

Ephrin‐B2 reverse signaling is required for topography but not pattern formation of lateral superior olivary inputs to the inferior colliculus

“…Previous studies from our laboratory revealed discrete afferent patterns in the central nucleus and lateral cortex of the IC (CNIC and LCIC, layered and modular, respectively) in a variety of species (Gabriele et al, 2000a,b, 2007, 2011; Henkel et al, 2005; Fathke and Gabriele, 2009). Similarly to inputs arising from the cochlear nuclei and nuclei of the lateral lemniscus (Oliver, 1984, 1987; Kandler and Friauf, 1993; Oliver et al, 1997; Gabriele et al, 2000a,b; Fathke and Gabriele, 2009), the lateral superior olive (LSO) sends bilateral layered projections to the CNIC (Shneiderman and Henkel, 1987; Gabriele et al, 2007; Fathke and Gabriele; 2009).…”

“…In comparison with the CNIC, the LCIC receives less subcollicular input. However, recently we reported a patchy projection in rat and mouse to deep portions of the LCIC arising from the ipsilateral LSO (Gabriele et al, 2011). The input is robust and terminates in a series of discontinuous modules that span the rostrocaudal dimension of the LCIC.…”