Auditory ganglion source of Sonic hedgehog regulates timing of cell cycle exit and differentiation of mammalian cochlear hair cells

Abstract

Neural precursor cells of the central nervous system undergo successive temporal waves of terminal division, each of which is soon followed by the onset of cell differentiation. The organ of Corti in the mammalian cochlea develops differently, such that precursors at the apex are the first to exit from the cell cycle but the last to begin differentiating as mechanosensory hair cells. Using a tissuespecific knockout approach in mice, we show that this unique temporal pattern of sensory cell development requires that the adjacent auditory (spiral) ganglion serve as a source of the signaling molecule Sonic hedgehog (Shh). In the absence of this signaling, the cochlear duct is shortened, sensory hair cell precursors exit from the cell cycle prematurely, and hair cell differentiation closely follows cell cycle exit in a similar apical-to-basal direction. The dynamic relationship between the restriction of Shh expression in the developing spiral ganglion and its proximity to regions of the growing cochlear duct dictates the timing of terminal mitosis of hair cell precursors and their subsequent differentiation.morphogenesis | Atoh1 | tonotopy T he mammalian cochlea, a coiled sensory end organ of hearing, contains a specialized mechanosensory epithelium (organ of Corti) consisting of hair cells (HCs) and nonsensory supporting cells. Sound frequency discrimination in mammals begins in the cochlea with a frequency-place code, such that specific frequencies elicit maximal responses of the organ of Corti at different points along its longitudinal (basal-apical) axis, a phenomenon known as tonotopy. Many structural and molecular features of the mature organ of Corti that vary along the longitudinal (tonotopic) axis, including stiffness of the basilar membrane and size and shape of the sensory HCs and their associated stereocilia (1, 2), have been implicated as contributing factors in frequency discrimination. The developmental bases of tonotopy remain obscure, however (3).A possible developmental influence on tonotopy is the wellcharacterized dynamic of apical (low-frequency sensing) precursors exiting early from the cell cycle, evident at embryonic day (E) 12.5 in mice, followed by a wave-like spread of terminal mitosis toward the base (high-frequency sensing), with cells completing cell cycle exit (CCE) by E14.5 (Fig. 1A) (4, 5). In contrast, early markers of HC differentiation, such as atonal homolog 1 (Atoh1), first appear at the midbase at E13.5 and spread bidirectionally along the developing organ of Corti (Fig. 1A) (6). Consequently, apically located HC precursors remain in a postmitotic, undifferentiated state for a longer period than their more basal counterparts. This unusual pattern of cellular regulation is in stark contrast to the neural precursors in the cortex and retina, in which each neuronal subtype has a specific timing of terminal mitosis, followed closely by its differentiation (7,8).Gradients of secreted molecules are important for patterning and cell fate specification during embryogenesis. Although t...

Sensory hair cell development and regeneration: similarities and differences

“…Both Gli3-deficient cochlea and cochlear explants treated with chemical inhibitors of Shh signaling display an expanded prosensory domain, whereas Shh treatment prevents prosensory domain formation (Driver et al, 2008). Using a transgenic approach, it was shown that the ablation of Shh from the spiral ganglia at several developmental time points causes premature cell cycle exit and an abnormal wave of hair cell differentiation extending from the apex towards the base, in the opposite direction to normal development (Bok et al, 2013). In another study, Tateya and colleagues demonstrated that the conditional ablation of Smo in the developing cochlear epithelium results in accelerated hair cell differentiation in the apical region (Tateya et al, 2013).…”

“…The availability of genetic tools has enabled us to further dissect components of these pathways. Moreover, these studies have begun to reveal the complex interplay that exists among different pathways at different developmental stages, ranging from otic placode specification (Jayasena et al, 2008) and sensory cell specification (Benito-Gonzalez and Doetzlhofer, 2014;Bok et al, 2013;Munnamalai et al, 2012;Tateya et al, 2013) to the maintenance of cell fate (Doetzlhofer et al, 2009;Li et al, 2015). As insights into the roles of additional pathways in these diverse contexts culminate (e.g.…”

The RNA-binding protein LIN28B regulates developmental timing in the mammalian cochlea

“…How might LIN28B alter the timing of HC differentiation? To date two pathways, the Insulin-like growth factor (Igfr1)-phosphatidylinositol 3-kinase (PI3K)/Akt pathway and the Shh pathway, have been shown to influence timing of HC differentiation within the murine cochlea (65)(66)(67)73). LIN28B has been shown to positively regulate Igf1r-PI3K/Akt pathway (27); however, loss of Igfr1 within the developing cochlea does not mirror LIN28B's effects on HC differentiation.…”

Conserved role of Sonic Hedgehog in tonotopic organization of the avian basilar papilla and mammalian cochlea

“…Shh required for cochlear development is sequentially secreted by two different sources: initially from the floor plate and notochord in the ventral midline and later from the spiral ganglion neurons starting at E11.75 (15,24). We thus asked which source(s) of Shh mediate the specification of regional cochlear identity.…”

“…However, testing this hypothesis using a loss-of-function approach is not feasible because the cochlea fails to form in the absence of Shh in both species, which precludes tonotopic analyses (8)(9)(10). In mice, tonotopic analysis is further confounded because most of the mutant mice do not survive until postnatal day 4, when the morphological differences in sensory hair cells can be first observed (5,8,10,14,15).…”