Fgf8 induces pillar cell fate and regulates cellular patterning in the mammalian cochlea

Section on Developmental Neuroscience, Porter Neuroscience Research Center, 35 Convent Dr, Room 2A-100, National Institute on Deafness and Other Communication Disorders, National Institutes of Health, Bethesda, MD 20892, USA.
Development (Impact Factor: 6.46). 09/2007; 134(16):3021-9. DOI: 10.1242/dev.02874
Source: PubMed


The mammalian auditory sensory epithelium (the organ of Corti) contains a number of unique cell types that are arranged in ordered rows. Two of these cell types, inner and outer pillar cells (PCs), are arranged in adjacent rows that form a boundary between a single row of inner hair cells and three rows of outer hair cells (OHCs). PCs are required for auditory function, as mice lacking PCs owing to a mutation in Fgfr3 are deaf. Here, using in vitro and in vivo techniques, we demonstrate that an Fgf8 signal arising from the inner hair cells is the key component in an inductive pathway that regulates the number, position and rate of development of PCs. Deletion of Fgf8 or inhibition of binding between Fgf8 and Fgfr3 leads to defects in PC development, whereas overexpression of Fgf8 or exogenous Fgfr3 activation induces ectopic PC formation and inhibits OHC development. These results suggest that Fgf8-Fgfr3 interactions regulate cellular patterning within the organ of Corti through the induction of one cell fate (PC) and simultaneous inhibition of an alternate fate (OHC) in separate progenitor cells. Some of the effects of both inhibition and overactivation of the Fgf8-Fgfr3 signaling pathway are reversible, suggesting that PC differentiation is dependent upon constant activation of Fgfr3 by Fgf8. These results suggest that PCs might exist in a transient state of differentiation that makes them potential targets for regenerative therapies.

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    • "Next, we determined whether the new HCs expressed Fibroblast growth factor 8 (Fgf8), which is specifically expressed in IHCs and plays key roles in cochlear development [18], [27], [28]. In situ hybridization analysis with the Fgf8 probe confirmed the unique expression of Fgf8 in control IHCs at P0 (Fig. 7B), which is consistent with previous reports [18], [27], [28]. "
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    ABSTRACT: Regeneration of auditory hair cells (HCs) is a promising approach to restore hearing. Recent studies have demonstrated that induced pluripotent stem cells/embryonic stem cells or supporting cells (SCs) adjacent to HCs can be converted to adopt the HC fate. However, little is known about whether new HCs are characteristic of outer or inner HCs. Here, we showed in vivo conversion of 2 subtypes of SCs, inner border cells (IBs) and inner phalangeal cells (IPhs), to the inner HC (IHC) fate. This was achieved by ectopically activating Atoh1, a transcription factor necessary for HC fate, in IBs/IPhs at birth. Atoh1+ IBs/IPhs first turned on Pou4f3, another HC transcription factor, before expressing 8 HC markers. The conversion rate gradually increased from ∼2.4% at 1 week of age to ∼17.8% in adult. Interestingly, new HCs exhibited IHC characteristics such as straight line-shaped stereociliary bundles, expression of Fgf8 and otoferlin, and presence of larger outward currents than those of outer HCs. However, new HCs lacked the terminal differentiation IHC marker vGlut3, exhibited reduced density of presynaptic Cbtp2 puncta that had little postsynaptic GluR2 specialization, and displayed immature IHC outward currents. Our results demonstrate that the conversion rate of IBs/IPhs in vivo by Atoh1 ectopic expression into the IHC fate was higher and faster and the conversion was more complete than that of the 2 other SC subtypes underneath the outer HCs; however, these new IHCs are arrested before terminal differentiation. Thus, IBs/IPhs are good candidates to regenerate IHCs in vivo.
    Full-text · Article · Feb 2014 · PLoS ONE
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    • "The constitutive activation of Fgfr3 may be implicated in specifying the fate of progenitor cells, or induce their differentiation into pillar cells and/or Deiter's cells. Contrary to some previous studies [18] [20] [21], the Fgfr3 Y367C/+ mice did not show any difference in the outer hair cells in the first turn of the cochlea. We hypothesize that Fgfr3 may not directly be required for outer hair cell differentiation. "

    Full-text · Dataset · Jan 2014
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    • "Consistent with the almost complete loss of both inner and outer hair cells, expression levels of hair cell specific transcription factor Pou4f3 [49] and the inner hair cell specific Fgf ligand Fgf8 [42] were reduced by 90% in gentamicin treated cultures compared to control cultures (Fig. 2 G, gentamicin +DMSO vs. control). Interestingly, expression of hair cell specific transcription factor Atoh1 was less severely reduced (∼70%) after gentamicin treatment (Fig. 2 G, gentamicin +DMSO vs. control). "
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    ABSTRACT: In mammals, auditory hair cells are generated only during embryonic development and loss or damage to hair cells is permanent. However, in non-mammalian vertebrate species, such as birds, neighboring glia-like supporting cells regenerate auditory hair cells by both mitotic and non-mitotic mechanisms. Based on work in intact cochlear tissue, it is thought that Notch signaling might restrict supporting cell plasticity in the mammalian cochlea. However, it is unresolved how Notch signaling functions in the hair cell-damaged cochlea and the molecular and cellular changes induced in supporting cells in response to hair cell trauma are poorly understood. Here we show that gentamicin-induced hair cell loss in early postnatal mouse cochlear tissue induces rapid morphological changes in supporting cells, which facilitate the sealing of gaps left by dying hair cells. Moreover, we provide evidence that Notch signaling is active in the hair cell damaged cochlea and identify Hes1, Hey1, Hey2, HeyL, and Sox2 as targets and potential Notch effectors of this hair cell-independent mechanism of Notch signaling. Using Cre/loxP based labeling system we demonstrate that inhibition of Notch signaling with a γ- secretase inhibitor (GSI) results in the trans-differentiation of supporting cells into hair cell-like cells. Moreover, we show that these hair cell-like cells, generated by supporting cells have molecular, cellular, and basic electrophysiological properties similar to immature hair cells rather than supporting cells. Lastly, we show that the vast majority of these newly generated hair cell-like cells express the outer hair cell specific motor protein prestin.
    Full-text · Article · Aug 2013 · PLoS ONE
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