Sox2 signaling in prosensory domain specification and subsequent hair cell differentiation in the developing cochlea

Abstract

Sox2 is a high-mobility transcription factor that is one of the earliest markers of developing inner ear prosensory domains. In humans, mutations in SOX2 cause sensorineural hearing loss and a loss of function study in mice showed that Sox2 is required for prosensory formation in the cochlea. However, the specific roles of Sox2 have not been determined. Here we illustrate a dynamic role of Sox2 as an early permissive factor in prosensory domain formation followed by a mutually antagonistic relationship with Atoh1, a bHLH protein necessary for hair cell development. We demonstrate that decreased levels of Sox2 result in precocious hair cell differentiation and an over production of inner hair cells and that these effects are likely mediated through an antagonistic interaction between Sox2 and the bHLH molecule Atoh1. Using gain-and loss-of-function experiments we provide evidence for the molecular pathway responsible for the formation of the cochlear prosensory domain. Sox2 expression is promoted by Notch signaling and Prox1, a homeobox transcription factor, is a downstream target of Sox2. These results demonstrate crucial and diverse roles for Sox2 in the development, specification, and maintenance of sensory cells within the cochlea.development ͉ organ of Corti ͉ inner ear ͉ HMG box ͉ bHLH T he sensory epithelium of the mammalian cochlea (the organ of Corti) develops from a pool of prosensory cells derived from the ventral region of the otocyst. Proper development of the cochlea requires that cochlear progenitor cells transition through states of developmental competence, coordinated cell cycle exit, and cell fate specification and differentiation to generate the distinctly fated cell populations within the highly ordered mosaic of the organ of Corti (1). The signal transduction pathways that coordinate cochlear prosensory specification are only beginning to be identified. Moreover, since these signal transduction pathways are unlikely to be linear cascades, it will also be necessary to determine how different pathways are organized into complex signaling networks that ultimately generate precise and unique responses within individual cochlear prosensory cells.Sox (SRY related HMG box) proteins are a group of transcription factors that regulate diverse developmental processes; for instance, Sox2 is a universal marker of stem cells and is also expressed in neural progenitor cells at different stages of central nervous system development. Sox2, along with Sox1 and Sox3, comprise the SoxB1 group. Members of this group are thought to maintain neural precursor cells in a progenitor state by inhibiting bHLH-mediated neuronal differentiation (2). Reciprocally, bHLH proteins must suppress expression and activity of SoxB1 proteins to induce cellular differentiation. A recent loss-of-function study demonstrated that Sox2 is required for development of sensory epithelia, including the organ of Corti, within the inner ear (3). However, despite its absolute necessity for the formation of inner ear sensory epithelia, the spe...

Atoh1, an Essential Transcription Factor in Neurogenesis and Intestinal and Inner Ear Development: Function, Regulation, and Context Dependency

“…It has been observed, however, that cells that look histologically similar to SCs are present at P0 (Bermingham et al 1999), and the structure of the tissue is consistent with wild-type patterning at E16.5, E18.5, and P1 Dabdoub et al 2008;Pan et al 2010). It has been demonstrated that specification of the sensory epithelium is independent of Atoh1 (Chen et al 2002;Dabdoub et al 2008); this would mean that, during early development of the OC, SC markers such as Sox2, Prox1, and Jagged1 would be detected in Atoh1-null animals, as these factors are required for early patterning. Furthermore, it has been demonstrated that cells expressing Atoh1 can develop into either HCs or SCs Yang et al 2010), and examination of the homozygous βGal knock-in mouse at E18.5 shows Atoh1 promoter activity in a greater population of sensory epithelial cells than would have differentiated into HCs (Fritzsch et al 2005).…”

“…Further analysis using immunohistochemistry and ISH to identify individual cell types at P0 showed that not only are there no cells expressing HC markers such as myosin 6 and calretinin, but that there are also no cells expressing markers of the various organ of Corti supporting cells (SCs), including inner pillar cells (probed for Jagged1, Fgfr3, TC2, and p75 ntr ), Deiters' cells (probed for Jagged1, glial fibrillary acidic protein, TC2, and Fgfr3) and phalangeal cells (probed for Jagged1 and glial fibrillary acidic protein) (Woods et al 2004). Although at P0 SCs are not found, examination of Atoh1 null cochleae at E16.5 demonstrates that there are cells in the region that would correspond to the OC that are immunoreactive for the SC markers Jagged1, Prox1, and Sox2 (Dabdoub et al 2008). Given the caveat that there are no true SC markers, and that Jagged1, Sox2, and Prox1 are expressed first in prosensory cells and then in SCs, it is not clear whether the cells that are immunoreactive for these markers are cells that remain in the prosensory undifferentiated state or are differentiated SCs.…”

“…These findings suggest that Atoh1 is required only for very early events in HC differentiation and might induce secreted factors that can potentiate lateral inhibition in the absence of endogenous Atoh1. Bearing in mind that a degree of tissue organization persists in the Atoh1 null cochlea at P1 Dabdoub et al 2008;, it is possible that these are HCs that have entered only initial stages of differentiation (Du et al 2007). Furthermore, it was not possible to examine these putative HCs in mature cochleae as βGal is only expressed in cells with active Atoh1 promoter activity (Atoh1 is not expressed after P4), so it is not known whether these cells retain their identity into adulthood or are functional (Du et al 2007).…”

“…Atoh1, Sox2, β-catenin, Hfn1a, and Cdx2 upregulate Atoh1 expression (Helms et al 2000;Ebert et al 2003;Gazit et al 2004;Mutoh et al 2006;D'Angelo et al 2010;Shi et al 2010;Neves et al 2011). Hes1, Hes5, Sox2, Hic1, Ngn1, NeuroD1, and Zic1 repress Atoh1 at the level of transcription (Akazawa et al 1995;Gowan et al 2001;Ebert et al 2003;Qian et al 2006;Briggs et al 2008;Dabdoub et al 2008;Pan et al 2009;Jahan et al 2010). Intriguingly, Sox2 has the ability to both upregulate and downregulate Atoh1 depending on context (Dabdoub et al 2008;Neves et al 2011).…”

CD44 is a Marker for the Outer Pillar Cells in the Early Postnatal Mouse Inner Ear

“…Protein detection using immunofluorescence and peroxidase immunostaining was performed on whole mounted cochleae or inner ear sections as previously described (Dabdoub et al 2008;Hertzano et al 2004). All primary antibodies were diluted in PBS with 0.1% Tween-20.…”