Fgf and Hh signalling act on a symmetrical pre-pattern to specify anterior and posterior identity in the zebrafish otic placode and vesicle
ABSTRACT Specification of the otic anteroposterior axis is one of the earliest patterning events during inner ear development. In zebrafish, Hedgehog signalling is necessary and sufficient to specify posterior otic identity between the 10 somite (otic placode) and 20 somite (early otic vesicle) stages. We now show that Fgf signalling is both necessary and sufficient for anterior otic specification during a similar period, a function that is completely separable from its earlier role in otic placode induction. In lia(-/-) (fgf3(-/-)) mutants, anterior otic character is reduced, but not lost altogether. Blocking all Fgf signalling at 10-20 somites, however, using the pan-Fgf inhibitor SU5402, results in the loss of anterior otic structures and a mirror image duplication of posterior regions. Conversely, overexpression of fgf3 during a similar period, using a heat-shock inducible transgenic line, results in the loss of posterior otic structures and a duplication of anterior domains. These phenotypes are opposite to those observed when Hedgehog signalling is altered. Loss of both Fgf and Hedgehog function between 10 and 20 somites results in symmetrical otic vesicles with neither anterior nor posterior identity, which, nevertheless, retain defined poles at the anterior and posterior ends of the ear. These data suggest that Fgf and Hedgehog act on a symmetrical otic pre-pattern to specify anterior and posterior otic identity, respectively. Each signalling pathway has instructive activity: neither acts simply to repress activity of the other, and, together, they appear to be key players in the specification of anteroposterior asymmetries in the zebrafish ear.
- SourceAvailable from: Vassilis Georgiadis[Show abstract] [Hide abstract]
ABSTRACT: The analysis of Fgf10 mouse mutants has demonstrated a critical role for this ligand in neurosensory development of the vertebrate inner ear, and we have been looking to define the direct upstream regulators of Fgf10 in this sensory organ, as part of constructing the programme of early inner ear development. Through the analysis of reporter constructs in transgenic mouse embryos and neonatal mice, in this report we define a minimal 1400bp enhancer from the 5' flanking region of Fgf10. This enhancer drives reporter transgene expression in a manner that recapitulates endogenous expression of Fgf10, from its initial onset in the invaginating otic placode and onwards throughout gestation, controlling Fgf10 expression in all developing sensory patches and in the developing VIIIth ganglion. This regulatory region includes three putative Gata3 binding sites that we demonstrate directly interacts with Gata3 protein through the DNA binding domain with differing affinities. Site directed mutagenesis of all three sites and functional testing in transgenic embryos using reporter transgenes reveals an absolute requirement for Gata3 in controlling Fgf10 expression. Transgenic analysis of individual Gata3 binding site mutations illustrates that only one of these binding site is necessary for reporter expression. Together these data demonstrate that Gata3 directly activates Fgf10 in the early inner ear, and does so through a single binding site.Developmental Biology 12/2012; 374(1). DOI:10.1016/j.ydbio.2012.11.028 · 3.64 Impact Factor
- [Show abstract] [Hide abstract]
ABSTRACT: The mammalian cochlea is a remarkable sensory organ, capable of perceiving sound over a range of 10(12) in pressure, and discriminating both infrasonic and ultrasonic frequencies in different species. The sensory hair cells of the mammalian cochlea are exquisitely sensitive, responding to atomic-level deflections at speeds on the order of tens of microseconds. The number and placement of hair cells are precisely determined during inner ear development, and a large number of developmental processes sculpt the shape, size and morphology of these cells along the length of the cochlear duct to make them optimally responsive to different sound frequencies. In this review, we briefly discuss the evolutionary origins of the mammalian cochlea, and then describe the successive developmental processes that lead to its induction, cell cycle exit, cellular patterning and the establishment of topologically distinct frequency responses along its length. © 2015 Anatomical Society.Journal of Anatomy 06/2015; DOI:10.1111/joa.12314 · 2.23 Impact Factor
- [Show abstract] [Hide abstract]
ABSTRACT: Vertebrate embryogenesis gives rise to all cell types of an organism through the development of many unique lineages derived from the three primordial germ layers. The otic sensory lineage arises from the otic vesicle, a structure formed through invagination of placodal non-neural ectoderm. This developmental lineage possesses unique differentiation potential, giving rise to otic sensory cell populations including hair cells, supporting cells, and ganglion neurons of the auditory and vestibular organs. Here we present a systematic approach to identify transcriptional features that distinguish the otic sensory lineage (from early otic progenitors to otic sensory populations) from other major lineages of vertebrate development. We used a microarray approach to analyze otic sensory lineage populations including microdissected otic vesicles (embryonic day 10.5) as well as isolated neonatal cochlear hair cells and supporting cells at postnatal day 3. Non-otic tissue samples including periotic tissues and whole embryos with otic regions removed were used as reference populations to evaluate otic specificity. Otic populations shared transcriptome-wide correlations in expression profiles that distinguish members of this lineage from non-otic populations. We further analyzed the microarray data using comparative and dimension reduction methods to identify individual genes that are specifically expressed in the otic sensory lineage. This analysis identified and ranked top otic sensory lineage-specific transcripts including Fbxo2, Col9a2, and Oc90, and additional novel otic lineage markers. To validate these results we performed expression analysis on select genes using immunohistochemistry and in situ hybridization. Fbxo2 showed the most striking pattern of specificity to the otic sensory lineage, including robust expression in the early otic vesicle and sustained expression in prosensory progenitors and auditory and vestibular hair cells and supporting cells.Frontiers in Cellular Neuroscience 03/2015; 9:79. DOI:10.3389/fncel.2015.00079 · 4.18 Impact Factor