Notch signaling is required for the generation of hair cells and supporting cells in the mammalian inner ear

Abstract

Sensorineural deafness and balance dysfunction are common impairments in humans frequently caused by defects in the sensory epithelium of the inner ear, composed of hair cells and supporting cells. Lineage studies have shown that hair cells and supporting cells arise from a common progenitor, but how these progenitors are generated remains unknown. Although various molecules have been implicated in the development of the sensory progenitors, none has been shown to be required for the specification of these progenitors in the mammalian inner ear. Here, using both loss-offunction and gain-of-function approaches, we show that Jagged1 (JAG1)-mediated Notch signaling is both required and sufficient for the generation of the sensory progenitors. Specifically, we find that loss of JAG1 signaling leads to smaller sensory progenitor regions without initial effects on proliferation or cell death, indicating that JAG1 is involved in initial specification events. To further test whether Notch signaling is involved in specification of the sensory progenitors, we transiently expressed an activated form of the Notch1 receptor (NICD) using a combined Tet-On/Cre induction system in the mouse. NICD expression resulted in ectopic hair cells and supporting cells in the nonsensory regions of the cochlea and vestibule. These data indicate that Notch specifies sensory progenitors in the inner ear, and that induction of Notch may be important for regenerating or replacing hair cells and supporting cells in the mammalian inner ear.T he mammalian inner ear contains six separate sensory regions required for hearing and balance. Each sensory organ is composed of two basic cell types, sensory hair cells and associated supporting cells. Lineage studies in the chick have shown that both cell types arise from a common sensory progenitor (1, 2) that differentiates into a hair cell or a supporting cell via lateral inhibition mediated by the Notch signaling pathway (3-7). Lineage studies in the chick and fate-mapping studies in the mouse also have demonstrated that hair cells and supporting cells can share a common lineage with neurons of the statoacoustic ganglion (SAG), which derive from the otocyst and innervate the hair cells in the ear (2,8).At present, the factors that specify the sensory progenitors are not known, although various molecules have been implicated. One major candidate is the high-mobility group (HMG) transcription factor SOX2. SOX2 is expressed in the sensory regions, and loss of SOX2 results in inner ears that develop without any hair cells or supporting cells, likely due to loss of the sensory progenitors (9). However, ectopic expression of SOX2 in cochlear explants does not lead to new sensory regions (10), raising the question as to whether SOX2 is sufficient for sensory specification. Other factors shown to be involved in prosensory development in the inner ear include Wnt, Bmp, Fgf, and Notch signaling pathways (11)(12)(13)(14)(15)(16)(17)(18)(19). Which of these signaling pathways is required for sensory progenitor spe...

Distinct capacity for differentiation to inner ear cell types by progenitor cells of the cochlea and vestibular organs

“…Endogenous signaling within the progenitor cell population of each organ may give rise to cell types more representative of the differentiation capacity of each tissue. Production of HCs can be induced by Wnt activation (Shi et al, 2013 or Notch inhibition (Doetzlhofer et al, 2009;Mizutari et al, 2013;Pan et al, 2010;Yamamoto et al, 2006). Notchmediated lateral inhibition (Daudet and Lewis, 2005;Lanford et al, 1999) and innate Wnt signaling are both crucial for the development of HCs (Shi et al, 2013.…”

In vivo notch reactivation in differentiating cochlear hair cells induces sox2 and prox1 expression but does not disrupt hair cell maturation

“…In the prosensory phase, it promotes formation of prosensory areas in which the progenitors reside (Hartman et al, 2010;Pan et al, 2010). However, Notch signaling might not be necessarily required in specifying prosensory progenitors (Basch et al, 2011;Yamamoto et al, 2011).…”

“…In the early prosensory phase [between embryonic day 12 (or E12) and E14.5], Notch signaling (primarily through Notch1/Jagged1) plays a critical role in specifying prosensory progenitors (Eddison et al, 2000;Kiernan et al, 2001;Tsai et al, 2001;Brooker et al, 2006;Kiernan et al, 2006). Consistent with the ability to promote prosensory progenitors, overactivation of Notch1 in the prosensory phase causes formation of ectopic HCs (Daudet and Lewis, 2005;Hartman et al, 2010;Pan et al, 2010). In the late cell fate determination phase (after E14.5), Notch signaling declines through "lateral inhibition" effects (Lanford et al, 1999) in progenitors that have committed to the HC fate but persists in progenitors that have committed to the SC fate (Zhang et al, 2000;Zheng et al, 2000b;Zine et al, 2000;Zine et al, 2001;Kiernan et al, 2005;Takebayashi et al, 2007).…”

MicroRNA-200 family members are weakly expressed in the neurosensory epithelia of the developing zebrafish (Danio rerio) inner ear

“…The development of the inner ear is controlled by multiple signal pathways and genes, including Notch signaling (Pan et al, 2010), Wnt signaling (Freyer and Morrow, 2010), and atonal homolog 1 (Atoh1) (Raft et al, 2007). These have also been shown to be involved in the mechanisms that underlie the production of the hair cells of the inner ear (Kelley et al, 2009).…”