The Notch signaling pathway mediates many inductive interactions in vertebrate and invertebrate development (Artavanis-Tsakonas et al., 1999
; Gridley, 2007
; High and Epstein, 2008
; Lai, 2004
; Louvi and Artavanis-Tsakonas, 2006
; Maillard et al., 2005
; Weinmaster and Kopan, 2006
). The many circumstances in which Notch signaling is used prompt the question of whether this pathway is sufficient to specify intricate arrangements of differentiated cell types. The development of the organ of Corti, the auditory organ of the inner ear of mammals, is one of the most striking examples of how these multiple roles help choreograph the numerous cell-cell interactions required to form a complex structure (Barald and Kelley, 2004
; Kelley, 2006
). The organ of Corti is composed of a rigidly stereotyped array of one row of inner hair cells and three rows of outer hair cells running along the entire length of the cochlear sensory epithelium (). Each hair cell is surrounded by specialized supporting cells, - inner phalangeal cells, which lie beneath each inner hair cell, and three or four Deiters’ cells which lie beneath outer hair cells. In addition, the inner and outer hair cell regions are separated by two specialized supporting cells – inner and outer pillar cells - which form the sides of the tunnel of Corti in the mature organ (), and which are required for proper biomechanical function.
Treatment of neonatal organ of Corti explants with the gamma-secretase inhibitor DAPT induces ectopic hair cells
Evidence from birds and mice suggests that one function of Notch signaling is to negatively regulate hair cell fate during organ of Corti development (Adam et al., 1998
; Brooker et al., 2006
; Eddison et al., 2000
; Kiernan et al., 2005
; Lanford et al., 1999
; Takebayashi et al., 2007
). Consistent with this lateral inhibition model, conditional deletion of Notch1 in the inner ear, or deletion of Dll1 and Jag2, two Notch ligands expressed in hair cells, leads to an overproduction of hair cells (Brooker et al., 2006
; Kiernan et al., 2006
). However, a simple model of Notch-dependent lateral inhibition cannot account for the highly asymmetric pattern of hair cell and supporting cell differentiation, particularly with regard to inner pillar cells which appear to develop without contact from hair cells that express Notch ligands.
We have investigated the relationship between Notch signaling and the stability of the differentiated state of supporting cells. By disrupting Notch signaling with pharmacological inhibitors or in mutant mice lacking the Notch1 receptor or the Notch effector RBPJ, we show that while most types of supporting cells readily convert into hair cells consistent with a lateral inhibition model, pillar cells do not. We show the organ of Corti is divided into compartments on the basis of combinatorial expression of Hes and Hey Notch effectors. In particular, we show that Hey2 is regulated by FGF signaling in a Notch-independent fashion in pillar cells, and that this may account for the stability of inner pillar cell fate in the absence of contact with hair cells. We suggest that the establishment of complex Hes/Hey expression patterns, some of which are regulated by alternative signaling pathways including the FGF pathway, underlies the highly asymmetric cellular pattern of the organ of Corti.