RA and FGF Signalling Are Required in the Zebrafish Otic Vesicle to Pattern and Maintain Ventral Otic Identities

Abstract

During development of the zebrafish inner ear, regional patterning in the ventral half of the otic vesicle establishes zones of gene expression that correspond to neurogenic, sensory and non-neural cell fates. FGF and Retinoic acid (RA) signalling from surrounding tissues are known to have an early role in otic placode induction and otic axial patterning, but how external signalling cues are translated into intrinsic patterning during otic vesicle (OV) stages is not yet understood. FGF and RA signalling pathway members are expressed in and around the OV, suggesting important roles in later patterning or maintenance events. We have analysed the temporal requirement of FGF and RA signalling for otic development at stages after initial anteroposterior patterning has occurred. We show that high level FGF signalling acts to restrict sensory fates, whereas low levels favour sensory hair cell development; in addition, FGF is both required and sufficient to promote the expression of the non-neural marker otx1b in the OV. RA signalling has opposite roles: it promotes sensory fates, and restricts otx1b expression and the development of non-neural fates. This is surprisingly different from the earlier requirement for RA signalling in specification of non-neural fates via tbx1 expression, and highlights the shift in regulation that takes place between otic placode and vesicle stages in zebrafish. Both FGF and RA signalling are required for the development of the otic neurogenic domain and the generation of otic neuroblasts. In addition, our results indicate that FGF and RA signalling act in a feedback loop in the anterior OV, crucial for pattern refinement.

Distribution of neurosensory progenitor pools during inner ear morphogenesis unveiled by cell lineage reconstruction

“…Here we show the importance of place and time of neuroblast delamination in their allocation within the SAG, and shed light on how distinct information may converge in the progenitor cells. Delamination place confers position along the AP axis of the SAG, and therefore function, most probably as the result of the integration of patterning signals involved in the emergence of the different domains (Maier and Whitfield, 2014; Radosevic et al, 2011). Additionally, delamination time prefigures the relative position of early-delaminated neuroblasts vs. late-delaminated ones and therefore, the gradient of neuronal differentiation within the SAG.…”

“…Neuronal vs. sensory specification is achieved through well-defined bHLH transcription factors: atoh1 for hair cell formation (Millimaki et al, 2007; Bermingham et al, 1999), neurog1 for sensory neuron determination (Andermann et al, 2002; Ma et al, 1998), and neuroD for sensory neuron differentiation and survival (Jahan et al, 2010; Kim et al, 2001). Signals arising in the surrounding tissues regionalize the otic vesicle along axes (Maier and Whitfield, 2014; Radosevic et al, 2011; Riccomagno et al, 2002, 2005; Sapède and Pujades, 2010), and this multiple step process implies a gradual restriction of cell fates over time (Whitfield and Hammond, 2007; Wu and Kelley, 2012). However, the phenotypes of targeted mutants for these signaling pathways are not always easy to reconcile (Raft and Groves, 2015), due to the limited comprehension of how developmental gene regulatory networks are integrated.…”

Generating retinoic acid gradients by local degradation during craniofacial development: One cell's cue is another cell's poison

“…RA is also synthesized in the mouse otic vesicle, which expresses raldh3 (Mic et al, 2000). In zebrafish, RA and FGF signaling are essential for formation of the otic neurogenesis domain and production of otic neuroblasts (Maier & Whitfield, 2014). Loss of RA signaling through knockdown of RALDH enzymes or pharmacological inhibition resulted in hypoplastic otocysts in mice and zebrafish (Niederreither et al, 1999;Whitfield, Riley, Chiang, & Phillips, 2002).…”

Cyp1B1 expression patterns in the developing chick inner ear

“…It is well accepted that RA generated from the underlying mesenchyme by RALDH activities participate directly in the specification of the neural tube along its anterior‐to‐posterior and dorsal‐to‐ventral axes . Similarly, RA from the periotic mesenchyme may control the regionalization of the developing otic epithelium in a stage‐dependent manner . In the chick, Raldh2 is expressed in the mesoderm caudal to the otic placode, and an appropriate spatial and temporal concentration of the diffusible RA is necessary for the correct specification of the anterior‐posterior axis of the developing otic anlagen .…”

“…20,39,42,51,54,[110][111][112][113] Similarly, RA from the periotic mesenchyme may control the regionalization of the developing otic epithelium in a stage-dependent manner. 9,10,15,29,107,[114][115][116][117] In the chick, Raldh2 is expressed in the mesoderm caudal to the otic placode, and an appropriate spatial and temporal concentration of the diffusible RA is necessary for the correct specification of the anteriorposterior axis of the developing otic anlagen. 114 In this sense, it has been suggested that the CYP1B1-mediated RA synthesis in the paraxial mesoderm could also participate directly in the dorsal-to-ventral specification of the developing neural tube in a paracrine manner.…”