[Show abstract][Hide abstract] ABSTRACT: Otoliths are biomineralised structures required for the sensation of gravity, linear acceleration and sound in the zebrafish ear. Otolith precursor particles, initially distributed throughout the otic vesicle lumen, become tethered to the tips of hair cell kinocilia (tether cilia) at the otic vesicle poles, forming two otoliths. We have used high-speed video microscopy to investigate the role of cilia and ciliary motility in otolith formation. In wild-type ears, groups of motile cilia are present at the otic vesicle poles, surrounding the immotile tether cilia. A few motile cilia are also found on the medial wall, but most cilia (92-98%) in the otic vesicle are immotile. In mutants with defective cilia (iguana) or ciliary motility (lrrc50), otoliths are frequently ectopic, untethered or fused. Nevertheless, neither cilia nor ciliary motility are absolutely required for otolith tethering: a mutant that lacks cilia completely (MZovl) is still capable of tethering otoliths at the otic vesicle poles. In embryos with attenuated Notch signalling [mindbomb mutant or Su(H) morphant], supernumerary hair cells develop and otolith precursor particles bind to the tips of all kinocilia, or bind directly to the hair cells' apical surface if cilia are absent [MZovl injected with a Su(H)1+2 morpholino]. However, if the first hair cells are missing (atoh1b morphant), otolith formation is severely disrupted and delayed. Our data support a model in which hair cells produce an otolith precursor-binding factor, normally localised to tether cell kinocilia. We also show that embryonic movement plays a minor role in the formation of normal otoliths.
[Show abstract][Hide abstract] 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.
[Show abstract][Hide abstract] ABSTRACT: In zebrafish, Hedgehog (Hh) signalling from ventral midline structures is necessary and sufficient to specify posterior otic identity. Loss of Hh signalling gives rise to mirror symmetric ears with double anterior character, whereas severe upregulation of Hh signalling leads to double posterior ears. By contrast, in mouse and chick, Hh is predominantly required for dorsoventral otic patterning. Whereas a loss of Hh function in zebrafish does not affect dorsoventral and mediolateral otic patterning, we now show that a gain of Hh signalling activity causes ventromedial otic territories to expand at the expense of dorsolateral domains. In a panel of lines carrying mutations in Hh inhibitor genes, Hh pathway activity is increased throughout the embryo, and dorsolateral otic structures are lost or reduced. Even a modest increase in Hh signalling has consequences for patterning the ear. In ptc1(-/-) and ptc2(-/-) mutant embryos, in which Hh signalling is maximal throughout the embryo, the inner ear is severely ventralised and medialised, in addition to displaying the previously reported double posterior character. Transplantation experiments suggest that the effects of the loss of Hh pathway inhibition on the ear are mediated directly. These new data suggest that Hh signalling must be kept tightly repressed for the correct acquisition of dorsolateral cell fates in the zebrafish otic vesicle, revealing distinct similarities between the roles of Hh signalling in zebrafish and amniote inner ear patterning.
[Show abstract][Hide abstract] ABSTRACT: In zebrafish, Hedgehog (Hh) signalling is required to specify posterior otic identity. This presents a conundrum, as the nearest source of Hh to the developing inner ear is the ventral midline, in the notochord and floorplate. How can a source of Hh that is ostensibly constant with respect to the anteroposterior axis of the otic vesicle specify posterior otic identity? One possibility is that localised inhibition of Hh signalling is involved. Here we show that genes coding for three inhibitors of Hh signalling, su(fu), dzip1 and hip, are expressed in and around the developing otic vesicle. su(fu) and dzip1 are ubiquitously expressed and unaffected by Hh levels. The expression of hip, however, is positively regulated by Hh signalling and has a complex, dynamic pattern. It is detectable in the neural tube, otic vesicle, statoacoustic ganglion, brain, fin buds, mouth, somites, pronephros and branchial arches. These expression domains bear some similarity, but are not identical, to those of ptc1, a Hh receptor gene that is also positively regulated by Hh signalling. In the neural tube, for instance, hip is expressed in a subset of the ptc1 expression domain, while in other regions, including the otic vesicle, hip and ptc1 expression domains differ. Significantly, we find that initial expression of hip is higher in and adjacent to anterior otic regions, while ptc1 expression becomes progressively restricted to the posterior of the ear. Hip-mediated inhibition of Hh signalling may therefore be important in restricting the effects of Hh to posterior regions of the developing inner ear.
[Show abstract][Hide abstract] ABSTRACT: The Bone Morphogenetic Protein (BMP) genes bmp2 and bmp4 are expressed in highly conserved patterns in the developing vertebrate inner ear. It has, however, proved difficult to elucidate the function of BMPs during ear development as mutations in these genes cause early embryonic lethality. Previous studies using conditional approaches in mouse and chicken have shown that Bmp4 has a role in semicircular canal and crista development, but there is currently no direct evidence for the role of Bmp2 in the developing inner ear.
We have used an RNA rescue strategy to test the role of bmp2b in the zebrafish inner ear directly. Injection of bmp2b or smad5 mRNA into homozygous mutant swirl (bmp2b(-/-)) embryos rescues the early patterning defects in these mutants and the fish survive to adulthood. As injected RNA will only last, at most, for the first few days of embryogenesis, all later development occurs in the absence of bmp2b function. Although rescued swirl adult fish are viable, they have balance defects suggestive of vestibular dysfunction. Analysis of the inner ears of these fish reveals a total absence of semicircular canal ducts, structures involved in the detection of angular motion. All other regions of the ear, including the ampullae and cristae, are present and appear normal. Early stages of otic development in rescued swirl embryos are also normal.
Our findings demonstrate a critical late role for bmp2b in the morphogenesis of semicircular canals in the zebrafish inner ear. This is the first demonstration of a developmental role for any gene during post-embryonic stages of otic morphogenesis in the zebrafish. Despite differences in the early stages of semicircular canal formation between zebrafish and amniotes, the role of Bmp2 in semicircular canal duct outgrowth is likely to be conserved between different vertebrate species.
[Show abstract][Hide abstract] ABSTRACT: In the zebrafish embryo, expression of the prdm1 and patched1 genes in adaxial cells is indicative of their specification to give rise to slow twitch muscle fibers in response to Hedgehog (Hh) signaling. Subsets of these slow twitch muscle progenitors activate engrailed (eng) strongly in response to high-level Hh signaling, and differentiate into muscle pioneer cells, which are important for subsequent development of the horizontal myoseptum. In addition, eng is expressed more weakly in medial fast fibers in response to lower Hh levels. Somite morphology in the lamprey, an agnathan (jawless) vertebrate, differs significantly from that of teleosts. In particular, the lamprey does not have clear epaxial/hypaxial domains, lacks a horizontal myoseptum, and does not appear to possess distinct populations of fast and slow fibers in the embryonic somite. Nevertheless, Hh is expressed in the midline of the lamprey embryo, and we report here that, as in zebrafish, homologues of patched and prdm1 are expressed in adaxial regions of the lamprey somite, and an eng homologue is also expressed in the somite. However, the lamprey adaxial region does not exhibit the same distinct adaxial cell morphology as in the zebrafish. In addition, the expression of follistatin is not excluded from the adaxial region, and eng is not detected in discrete muscle pioneer-like cells. These data suggest the presence of conserved responses to Hh signaling in lamprey somites, although the full range of effects elicited by Hh in the zebrafish somite is not recapitulated.
No preview · Article · Jan 2009 · Evolution & Development
[Show abstract][Hide abstract] ABSTRACT: Axial patterning in the vertebrate inner ear has been studied for over eighty years, and recent work has made great progress towards an understanding of the molecular mechanisms responsible for establishing asymmetries about the otic axes. Tissues extrinsic to the ear provide sources of signalling molecules that are active early in development, at or before otic placode stages, while intrinsic factors interpret these signals to establish and maintain axial pattern. Key features of dorsoventral otic patterning in amniote embryos involve Wnt and Fgf signalling from the hindbrain and Hh signalling from midline tissues (notochord and floorplate). Mutual antagonism between these pathways and their downstream targets within the otic epithelium help to refine and maintain dorsoventral axial patterning in the ear. In the zebrafish ear, the same tissues and signals are implicated, but appear to play a role in anteroposterior, rather than dorsoventral, otic patterning. Despite this paradox, conservation of mechanisms may be higher than is at first apparent.
Preview · Article · Feb 2007 · The International Journal of Developmental Biology
[Show abstract][Hide abstract] ABSTRACT: The inner ear of adult agnathan vertebrates is relatively symmetric about the anteroposterior axis, with only two semicircular canals and a single sensory macula. This contrasts with the highly asymmetric gnathostome arrangement of three canals and several separate maculae. Symmetric ears can be obtained experimentally in gnathostomes in several ways, including by manipulation of zebrafish Hedgehog signalling, and it has been suggested that these phenotypes might represent an atavistic condition. We have found, however, that the symmetry of the adult lamprey inner ear is not reflected in its early development; the lamprey otic vesicle is highly asymmetric about the anteroposterior axis, both morphologically and molecularly, and bears a striking resemblance to the zebrafish otic vesicle. The single sensory macula originates as two foci of hair cells, and later shows regions of homology to the zebrafish utricular and saccular maculae. It is likely, therefore, that the last common ancestor of lampreys and gnathostomes already had well-defined otic anteroposterior asymmetries. Both lamprey and zebrafish otic vesicles express a target of Hedgehog signalling, patched, indicating that both are responsive to Hedgehog signalling. One significant distinction between agnathans and gnathostomes, however, is the acquisition of otic Otx1 expression in the gnathostome lineage. We show that Otx1 knockdown in zebrafish, as in Otx1(-/-) mice, gives rise to lamprey-like inner ears. The role of Otx1 in the gnathostome ear is therefore highly conserved; otic Otx1 expression is likely to account not only for the gain of a third semicircular canal and crista in gnathostomes, but also for the separation of the zones of the single macula into distinct regions.
[Show abstract][Hide abstract] ABSTRACT: Currently, few factors have been identified that provide the inductive signals necessary to transform the simple otic placode into the complex asymmetric structure of the adult vertebrate inner ear. We provide evidence that Hedgehog signalling from ventral midline structures acts directly on the zebrafish otic vesicle to induce posterior otic identity. We demonstrate that two strong Hedgehog pathway mutants, chameleon (con(tf18b)) and slow muscle omitted (smu(b641)) exhibit a striking partial mirror image duplication of anterior otic structures, concomitant with a loss of posterior otic domains. These effects can be phenocopied by overexpression of patched1 mRNA to reduce Hedgehog signalling. Ectopic activation of the Hedgehog pathway, by injection of sonic hedgehog or dominant-negative protein kinase A RNA, has the reverse effect: ears lose anterior otic structures and show a mirror image duplication of posterior regions. By using double mutants and antisense morpholino analysis, we also show that both Sonic hedgehog and Tiggy-winkle hedgehog are involved in anteroposterior patterning of the zebrafish otic vesicle.
[Show abstract][Hide abstract] ABSTRACT: Drosophila dachshund (dac) interacts with sine oculis (so), eyes absent (eya) and eyeless (ey) to control compound eye development. We have cloned three zebrafish dac homologues, dachA, dachB and dachC, which are expressed widely, in distinct but overlapping patterns. Expression of all three is found in sensory organs, the central nervous system and pectoral fin buds. dachA is also expressed strongly in the somites and dachC in the neural crest and pronephros. These expression domains overlap extensively with those of zebrafish pax, eya and six family members, the homologues of Drosophila ey, eya and so, respectively. This is consistent with the proposal that Dach, Eya, Six and Pax family members may form networks, similar to that found in the fly eye, in the development of many vertebrate organs.
Preview · Article · Apr 2002 · Mechanisms of Development
[Show abstract][Hide abstract] ABSTRACT: We have isolated mammalian homologues of the Drosophila dachshund gene. Two domains of high conservation, one of which contains an alpha-helical, coiled-coil motif, show similarity to the Ski family of genes. We therefore propose that Dachshund belongs to a superfamily including these genes. Mouse Dachshund (Dach) is expressed in the eye and limb, structures affected by the Drosophila loss-of-function mutant, and rib primordia, CNS and genital eminence. Pax6 and Dach show overlapping but non-identical expression patterns. Dach expression is unaffected in smalleye mouse brain, indicating that Pax6 is not directly activating Dach. In Drosophila eye development dachshund is a component of an interacting network of proteins. Genes homologous to many of these exist in mammals; Dach joins this expanding group.
Preview · Article · Jul 1998 · Mechanisms of Development