Role of the hindbrain in dorsoventral but not anteroposterior axial specification of the inner ear

Abstract

An early and crucial event in vertebrate inner ear development is the acquisition of axial identities that in turn dictate the positions of all subsequent inner ear components. Here, we focus on the role of the hindbrain in establishment of inner ear axes and show that axial specification occurs well after otic placode formation in chicken. Anteroposterior (AP) rotation of the hindbrain prior to specification of this axis does not affect the normal AP orientation and morphogenesis of the inner ear. By contrast, reversing the dorsoventral (DV) axis of the hindbrain results in changing the DV axial identity of the inner ear. Expression patterns of several ventrally expressed otic genes such as NeuroD, Lunatic fringe (Lfng)and Six1 are shifted dorsally, whereas the expression pattern of a normally dorsal-specific gene, Gbx2, is abolished. Removing the source of Sonic Hedgehog (SHH) by ablating the floor plate and/or notochord,or inhibiting SHH function using an antibody that blocks SHH bioactivity results in loss of ventral inner ear structures. Our results indicate that SHH, together with other signals from the hindbrain, are important for patterning the ventral axis of the inner ear. Taken together, our studies suggest that tissue(s) other than the hindbrain confer AP axial information whereas signals from the hindbrain are necessary and sufficient for the DV axial patterning of the inner ear.

Ablation studies on the developing inner ear reveal a propensity for mirror duplications

“…Hh signaling is also important for inhibiting the expression of more dorsal genes, such as Wnt, ventrally (Riccomagno et al, 2005;Ohyama et al, 2006). Similar morphological defects to those observed in mouse are seen after loss of Shh activity in chicken embryos (Bok et al, 2005). The mouse and zebrafish studies are not strictly comparable as the zebrafish study required that two Hedgehogs, Shh and Tiggy-winkle Hh (Twhh), be knocked down before any mirror duplications were noted.…”

“…This finding contrasts with the role of Hh signaling in mouse inner ear development, where Sonic hedgehog (Shh) plays a role in dorsoventral patterning by promoting ventral identity and restricting Wnt genes to the dorsal region (Riccomagno et al, 2002(Riccomagno et al, , 2005Ohyama et al, 2006). In chickens, Shh has also been shown to be important for ventral patterning of the otocyst (Bok et al, 2005). Hh (a secreted and lipid-modified signaling protein) binds to its receptor patched, which releases inhibition of another transmembrane protein, smoothened, transducing the signal by means of the Gli transcription factors (Ingham, 1998;McMahon et al, 2003;Panakova et al, 2005;Fuccillo et al, 2006).…”

“…These patterning signals could be intrinsic and/or extrinsic to the inner ear. For example, the Hh signal for inner ear patterning in zebrafish, chicken, and mouse likely arises from adjacent tissues (Hammond et al, 2003;Bok et al, 2005;Riccomagno et al, 2005).…”

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

“…Foxg1 is expressed in the otocyst and CVG (16). Based on published cre reporter activities, all three cre strains should ablate Shh expression in the CVG (16)(17)(18) while sparing its expression in the notochord and floor plate, two sources of Shh thought to be required for specification of cochlear identity (10,11).…”

“…Shh secreted from the notochord and floor plate are required for patterning in the cochlea (10)(11)(12). Additionally, Shh is expressed in the spiral ganglion that innervates cochlear HCs, and studies conducted using a Shh-cre-Gfp reporter strain indicate that Shh expression in the developing spiral ganglion becomes concentrated near the apex of the growing cochlear duct over time (13).…”