Animal-vegetal asymmetries influence the earliest steps in retina fate commitment in Xenopus

Jack of all trades, master of each: the diversity of fibroblast growth factor signalling in eye development

A central question in development biology is how a limited set of signalling pathways can instruct unlimited diversity of multicellular organisms. In this review, we use three ocular tissues as models of increasing complexity to present the astounding versatility of fibroblast growth factor (FGF) signalling. In the lacrimal gland, we highlight the specificity of FGF signalling in a one-dimensional model of budding morphogenesis. In the lens, we showcase the dynamics of FGF signalling in altering functional outcomes in a two-dimensional space. In the retina, we present the prolific utilization of FGF signalling from three-dimensional development to homeostasis. These examples not only shed light on the cellular basis for the perfection and complexity of ocular development, but also serve as paradigms for the diversity of FGF signalling.

Sprouty2 regulates positioning of retinal progenitors through suppressing the Ras/Raf/MAPK pathway

Sproutys are negative regulators of the Ras/Raf/MAPK signaling pathway and involved in regulation of organogenesis, differentiation, cell migration and proliferation. Although the function of Sproutys have been extensively studied during embryonic development, their role and mode of action during eye formation in vertebrate embryonic development is still unknown. Here we show that Xenopus sprouty2 is expressed in the optic vesicle at late neurula stage and knockdown of Sprouty2 prevents retinal progenitors from populating the retina, which in turn gives rise to small eyes. In the absence of Sprouty2, progenitor cell population of the retina can be restored by blocking the MAPK signaling pathway through overexpression of DN-Ras or DN-Raf. In contrast, activation of the MAPK pathway through overexpression of a constitutively active form of c-Raf (ca-Raf) inhibits progenitor population of the retina, similar to the Sprouty2 loss-of-function phenotype. Moreover, we present evidence that the retinal defect observed in Sprouty2 morphants is attributed to the failure of proper movement of retinal progenitors into the optic vesicle, rather than an effect on progenitor cell survival. These results suggest that Sprouty2 is required for the positioning of retinal progenitors within the optic vesicle through suppressing Ras/Raf/MAPK signaling pathway.

Cleavage Blastomere Deletion and Transplantation to Test Cell Fate Commitment in Xenopus

Fate maps identify the precursors of an organ, and tracing the members of a blastomere lineage over time shows how its descendants come to populate that organ. The fates of the individual blastomeres of the two- to 32-cell Xenopus embryo have been fully mapped to reveal which cells are the major contributors to various cell types, tissues, and organs. However, because these fate maps were produced in the normal embryo, they do not reveal whether a precursor blastomere is competent to give rise to additional tissues or is already committed to its fate-mapped repertoire of descendants. To identify the mechanisms by which a cell's fate is committed, one needs to expose the cell to different experimental environments. If the cell's fate is determined, it will express its normal fate or gene expression profile in novel environments, whereas if it is not yet determined it will express different fates or gene expression profiles when exposed to novel external factors or neighboring cells. This protocol describes two techniques for testing cell fate commitment: single cell deletion and single cell transplantation. Deleting a blastomere allows one to test whether the deleted cell is required for the remaining cells to produce their normal, specific cell fates. Transplanting a blastomere to a novel location in a host embryo allows one to test whether the transplanted cell is committed to produce its normal fate-mapped repertoire, or whether it is still competent to respond to novel cell-cell interactions.

Lineage Tracing and Fate Mapping in Xenopus Embryos

Fate mapping approaches reveal what types of cells, tissues, and organs are derived from specific embryonic cells. Classical fate maps were made by microscopic techniques using embryos comprising small numbers of transparent cells. More complex and opaque embryos require use of a vital or lipophilic dye that labels small groups of cells. Intracellular injection of a lineage tracer that labels the injected cell and all of its descendants can be used to mark a single cell in Xenopus embryos, whose large cells are easy to microinject and usually cleave in regular patterns. Intracellular lineage tracers must be neutral compounds that do not interact with cellular processes that might change the developmental fate of the injected cell, be small enough to diffuse quickly throughout the cytoplasm before the cell divides so that all descendants are labeled, and be large enough to not diffuse to adjacent cells via gap junctions. They should not be diluted by cell division or intracellular degradation, and should be easily detected by histochemical reactions (enzymes) or direct imaging (fluorescent compounds). Several types of lineage tracers have been used, including small, fluorescently tagged dextrans and mRNAs encoding enzymes or fluorescent proteins, described here. Many lineage tracers can be combined with cell type-specific mRNA and protein expression assays, making lineage tracing a powerful tool for testing the function of genes and cell fate commitment.

The Birth of the Eye Vesicle: When Fate Decision Equals Morphogenesis

As the embryonic ectoderm is induced to form the neural plate, cells inside this epithelium acquire restricted identities that will dictate their behavior and progressive differentiation. The first behavior adopted by most neural plate cells is called neurulation, a morphogenetic movement shaping the neuroepithelium into a tube. One cell population is not adopting this movement: the eye field. Giving eye identity to a defined population inside the neural plate is therefore a key neural fate decision. While all other neural population undergo neurulation similarly, converging toward the midline, the eye field moves outwards, away from the rest of the forming neural tube, to form vesicles. Thus, while delay in acquisition of most other fates would not have significant morphogenetic consequences, defect in the establishment of the eye field would dramatically impact the formation of the eye. Yet, very little is understood of the molecular and cellular mechanisms driving them. Here, we summarize what is known across vertebrate species and propose a model highlighting what is required to form the essential vesicles that initiate the vertebrate eyes.

When Family History Matters: The Importance of Lineage Analyses and Fate Maps for Explaining Animal Development

Initial interest in understanding how the fertilized egg becomes a multicellular animal suggested two possible answers: either the embryo came from preformed components or it arose through epigenetic processes. Extensive research during the past few decades has identified aspects of development that depend on preformed elements, such as cytoplasmic components and a cell's lineage; it also has identified aspects that depend on epigenetic processes, such as cell interactions and morphogen gradients. These advances have depended on understanding embryonic cell lineage and cell fate. This essay explains how lineage analysis and fate mapping have contributed to our current understanding of embryonic development.

Efficient retina formation requires suppression of both Activin and BMP signaling pathways in pluripotent cells

Retina formation requires the correct spatiotemporal patterning of key regulatory factors. While it is known that repression of several signaling pathways lead to specification of retinal fates, addition of only Noggin, a known BMP antagonist, can convert pluripotent Xenopus laevis animal cap cells to functional retinal cells. The aim of this study is to determine the intracellular molecular events that occur during this conversion. Surprisingly, blocking BMP signaling alone failed to mimic Noggin treatment. Overexpressing Noggin in pluripotent cells resulted in a concentration-dependent suppression of both Smad1 and Smad2 phosphorylation, which act downstream of BMP and Activin signaling, respectively. This caused a decrease in downstream targets: endothelial marker, xk81, and mesodermal marker, xbra. We treated pluripotent cells with dominant-negative receptors or the chemical inhibitors, dorsomorphin and SB431542, which each target either the BMP or Activin signaling pathway. We determined the effect of these treatments on retina formation using the Animal Cap Transplant (ACT) assay; in which treated pluripotent cells were transplanted into the eye field of host embryos. We found that inhibition of Activin signaling, in the presence of BMP signaling inhibition, promotes efficient retinal specification in Xenopus tissue, mimicking the affect of adding Noggin alone. In whole embryos, we found that the eye field marker, rax, expanded when adding both dominant-negative Smad1 and Smad2, as did treating the cells with both dorsomorphin and SB431542. Future studies could translate these findings to a mammalian culture assay, in order to more efficiently produce retinal cells in culture.

Blastomere explants to test for cell fate commitment during embryonic development

Fate maps, constructed from lineage tracing all of the cells of an embryo, reveal which tissues descend from each cell of the embryo. Although fate maps are very useful for identifying the precursors of an organ and for elucidating the developmental path by which the descendant cells populate that organ in the normal embryo, they do not illustrate the full developmental potential of a precursor cell or identify the mechanisms by which its fate is determined. To test for cell fate commitment, one compares a cell's normal repertoire of descendants in the intact embryo (the fate map) with those expressed after an experimental manipulation. Is the cell's fate fixed (committed) regardless of the surrounding cellular environment, or is it influenced by external factors provided by its neighbors? Using the comprehensive fate maps of the Xenopus embryo, we describe how to identify, isolate and culture single cleavage stage precursors, called blastomeres. This approach allows one to assess whether these early cells are committed to the fate they acquire in their normal environment in the intact embryo, require interactions with their neighboring cells, or can be influenced to express alternate fates if exposed to other types of signals.

Step-wise specification of retinal stem cells during normal embryogenesis

The specification of embryonic cells to produce the retina begins at early embryonic stages as a multi-step process that gradually restricts fate potentials. First, a subset of embryonic cells becomes competent to form retina by their lack of expression of endo-mesoderm-specifying genes. From these cells, a more restricted subset is biased to form retina by virtue of their close proximity to sources of bone morphogenetic protein antagonists during neural induction. During gastrulation, the definitive RSCs (retinal stem cells) are specified as the eye field by interactions with underlying mesoderm and the expression of a network of retina-specifying genes. As the eye field is transformed into the optic vesicle and optic cup, a heterogeneous population of RPCs (retinal progenitor cells) forms to give rise to the different domains of the retina: the optic stalk, retinal pigmented epithelium and neural retina. Further diversity of RPCs appears to occur under the influences of cell-cell interactions, cytokines and combinations of regulatory genes, leading to the differentiation of a multitude of different retinal cell types. This review examines what is known about each sequential step in retinal specification during normal vertebrate development, and how that knowledge will be important to understand how RSCs might be manipulated for regenerative therapies to treat retinal diseases.

Targeted microinjection of synthetic mRNAs to alter retina gene expression in Xenopus embryos

The individual cells of the Xenopus cleavage-stage embryo have been fate mapped, revealing which of these cells contribute to the retina. Using this retina fate map, one can specifically modulate levels of gene expression in retina lineages to determine the function of proteins in various aspects of early retinal development, such as formation of the eye fields and determination of specific cell fates. This chapter presents the techniques for identifying specific retina blastomere precursor cells, and injecting them with lineage tracers, mRNAs encoding wild-type and mutant constructs or morpholino antisense oligonucleotides to alter gene expression.

Testing retina fate commitment in Xenopus by blastomere deletion, transplantation, and explant culture

The lineages of individual cells of the Xenopus cleavage-stage embryo have been fate-mapped to reveal the subset of blastomeres that are the major and minor precursors of the retina. Using this retina fate map, one can test the commitment of each of these cells to various retinal cell fates by manipulating the environment in which they develop. This chapter presents the techniques for identifying specific retina blastomere precursor cells, deleting them to test whether they are required for producing specific kinds of retinal cells, transplanting them to novel embryonic locations in host embryos to test whether they are committed to produce specific kinds of retinal cells, and growing them in explant culture to determine if their ability to produce specific kinds of retinal cells is autonomous.

Eye field specification in Xenopus laevis

Vertebrate eyes begin as a small patch of cells at the most anterior end of the early brain called the eye field. If these cells are removed from an amphibian embryo, the eyes do not form. If the eye field is transplanted to another location on the embryo or cultured in a dish, it forms eyes. These simple cut and paste experiments were performed at the beginning of the last century and helped to define the embryonic origin of the vertebrate eye. The genes necessary for eye field specification and eventual eye formation, by contrast, have only recently been identified. These genes and the molecular mechanisms regulating the initial formation of the Xenopus laevis eye field are the subjects of this review.

Noggin elicits retinal fate in Xenopus animal cap embryonic stem cells

Driving specific differentiation pathways in multipotent stem cells is a main goal of cell therapy. Here we exploited the differentiating potential of Xenopus animal cap embryonic stem (ACES) cells to investigate the factors necessary to drive multipotent stem cells toward retinal fates. ACES cells are multipotent, and can be diverged from their default ectodermal fate to give rise to cell types from all three germ layers. We found that a single secreted molecule, Noggin, is sufficient to elicit retinal fates in ACES cells. Reverse-transcription polymerase chain reaction, immunohistochemistry, and in situ hybridization experiments showed that high doses of Noggin are able to support the expression of terminal differentiation markers of the neural retina in ACES cells in vitro. Following in vivo transplantation, ACES cells expressing high Noggin doses form eyes, both in the presumptive eye field region and in ectopic posterior locations. The eyes originating from the transplants in the eye field region are functionally equivalent to normal eyes, as seen by electrophysiology and c-fos expression in response to light. Our data show that in Xenopus embryos, proper doses of a single molecule, Noggin, can drive ACES cells toward retinal cell differentiation without additional cues. This makes Xenopus ACES cells a suitable model system to direct differentiation of stem cells toward retinal fates and encourages further studies on the role of Noggin in the retinal differentiation of mammalian stem cells.

Generation of functional eyes from pluripotent cells

The directed differentiation of pluripotent cells into specific cell-types is a major hurdle in regenerative medicine. This study shows the eye field transcription factor factors can direct pluripotent cells into functioning frog eyes.

Pluripotent cells such as embryonic stem (ES) and induced pluripotent stem (iPS) cells are the starting point from which to generate organ specific cell types. For example, converting pluripotent cells to retinal cells could provide an opportunity to treat retinal injuries and degenerations. In this study, we used an in vivo strategy to determine if functional retinas could be generated from a defined population of pluripotent Xenopus laevis cells. Animal pole cells isolated from blastula stage embryos are pluripotent. Untreated, these cells formed only epidermis, when transplanted to either the flank or eye field. In contrast, misexpression of seven transcription factors induced the formation of retinal cell types. Induced retinal cells were committed to a retinal lineage as they formed eyes when transplanted to the flanks of developing embryos. When the endogenous eye field was replaced with induced retinal cells, they formed eyes that were molecularly, anatomically, and electrophysiologically similar to normal eyes. Importantly, induced eyes could guide a vision-based behavior. These results suggest the fate of pluripotent cells may be purposely altered to generate multipotent retinal progenitor cells, which differentiate into functional retinal cell classes and form a neural circuitry sufficient for vision.

The goal of regenerative medicine is to replace dead or dying cells. Successful cell replacement depends on the ability of donor cells to differentiate into all functional cell types lost in the target organ. Blindness resulting from retinal disease or damage, for example, would require the replacement of as many as seven specialized cell types found in the retina. The most celebrated characteristic of pluripotent cells is their ability to differentiate into any adult cell type. This defining feature, however, presents the challenge of identifying the conditions for their conversion to the cell types needed for tissue repair. We asked if pluripotent cells could be directed to generate all the retinal cell types necessary to form a functional eye in the frog, Xenopus laevis. If left untreated, transplanted pluripotent cells only form the epidermal layer of the skin. However, when forced to express the eye field transcription factor (EFTF) genes, the cells differentiate into all seven retinal cell classes and eventually organize themselves into a functioning eye that can detect light and guide tadpoles in a vision-based behavior. Our results demonstrate that pluripotent cells can be purposely altered to generate all the functional retinal cell classes necessary for sight.

Rostral paraxial mesoderm regulates refinement of the eye field through the bone morphogenetic protein (BMP) pathway

The eye field is initially a large single domain at the anterior end of the neural plate and is the first indication of optic potential in the vertebrate embryo. During the course of development, this domain is subject to interactions that shape and refine the organogenic field. The action of the prechordal mesoderm in bisecting this single region into two bilateral domains has been well described, however the role of signalling interactions in the further restriction and refinement of this domain has not been previously characterised. Here we describe a role for the rostral cephalic paraxial mesoderm in limiting the extent of the eye field. The anterior transposition of this mesoderm or its ablation disrupted normal development of the eye. Importantly, perturbation of optic vesicle development occurred in the absence of any detectable changes in the pattern of neighbouring regions of the neural tube. Furthermore, negative regulation of eye development is a property unique to the rostral paraxial mesoderm. The rostral paraxial mesoderm expresses members of the bone morphogenetic protein (BMP) family of signalling molecules and manipulation of endogenous BMP signalling resulted in abnormalities of the early optic primordia.

Fibroblast growth factor receptor-induced phosphorylation of ephrinB1 modulates its interaction with Dishevelled

The Eph family of receptor tyrosine kinases and their membrane-bound ligands, the ephrins, have been implicated in regulating cell adhesion and migration during development by mediating cell-to-cell signaling events. The transmembrane ephrinB1 protein is a bidirectional signaling molecule that signals through its cytoplasmic domain to promote cellular movements into the eye field, whereas activation of the fibroblast growth factor receptor (FGFR) represses these movements and retinal fate. In Xenopus embryos, ephrinB1 plays a role in retinal progenitor cell movement into the eye field through an interaction with the scaffold protein Dishevelled (Dsh). However, the mechanism by which the FGFR may regulate this cell movement is unknown. Here, we present evidence that FGFR-induced repression of retinal fate is dependent upon phosphorylation within the intracellular domain of ephrinB1. We demonstrate that phosphorylation of tyrosines 324 and 325 disrupts the ephrinB1/Dsh interaction, thus modulating retinal progenitor movement that is dependent on the planar cell polarity pathway. These results provide mechanistic insight into how fibroblast growth factor signaling modulates ephrinB1 control of retinal progenitor movement within the eye field.

The competence of Xenopus blastomeres to produce neural and retinal progeny is repressed by two endo-mesoderm promoting pathways

Only a subset of cleavage stage blastomeres in the Xenopus embryo is competent to contribute cells to the retina; ventral vegetal blastomeres do not form retina even when provided with neuralizing factors or transplanted to the most retinogenic position of the embryo. These results suggest that endogenous maternal factors in the vegetal region repress the ability of blastomeres to form retina. Herein we provide three lines of evidence that two vegetal-enriched maternal factors (VegT, Vg1), which are known to promote endo-mesodermal fates, negatively regulate which cells are competent to express anterior neural and retinal fates. First, both molecules can repress the ability of dorsal-animal retinogenic blastomeres to form retina, converting the lineage from neural/retinal to non-neural ectodermal and endo-mesodermal fates. Second, reducing the endogenous levels of either factor in dorsal-animal retinogenic blastomeres expands expression of neural/retinal genes and enlarges the retina. The dorsal-animal repression of neural/retinal fates by VegT and Vg1 is likely mediated by Sox17alpha and Derriere but not by XNr1. VegT and Vg1 likely exert their effects on neural/retinal fates through at least partially independent pathways because Notch1 can reverse the effects of VegT and Derriere but not those of Vg1 or XNr1. Third, reduction of endogenous VegT and/or Vg1 in ventral vegetal blastomeres can induce a neural fate, but only allows expression of a retinal fate when both BMP and Wnt signaling pathways are concomitantly repressed.

Differential role of 14-3-3 family members in Xenopus development

The 14-3-3 proteins are intracellular dimeric phosphoserine/threonine binding molecules that participate in signal transduction, checkpoint control, nutrient sensing, and cell survival pathways. Previous work established that 14-3-3 proteins are required in early Xenopus laevis development by modulating fibroblast growth factor signaling. Although this general requirement for 14-3-3 proteins in Xenopus early embryogenesis is established, there is no information about the specific role of individual 14-3-3 genes. Botanical studies previously demonstrated functional specificity among 14-3-3 genes during plant development. In this study, an antisense morpholino oligo microinjection approach was used to characterize the requirement for six specific 14-3-3 family members in Xenopus embryogenesis. Microinjection experiments followed by Western blot analysis showed that morpholinos reduced specific 14-3-3 protein levels. Embryos lacking specific 14-3-3 isoforms displayed unique phenotypic defects. In particular, reduction of 14-3-3 tau (tau) protein, and to a lesser extent, 14-3-3 epsilon (epsilon), resulted in embryos with prominent gastrulation and axial patterning defects and reduced mesodermal marker gene expression. In contrast, reduction of 14-3-3 zeta (zeta) protein caused no obvious phenotypic abnormalities. Reduction of 14-3-3 gamma (gamma) protein resulted in eye defects without gastrulation abnormalities. Therefore, individual 14-3-3 genes have separable functions in vertebrate embryonic development.

Noggin signaling fromXenopus animal blastomere lineages promotes a neural fate in neighboring vegetal blastomere lineages

In Xenopus, localized factors begin to regionalize embryonic fates prior to the inductive interactions that occur during gastrulation. We previously reported that an animal-to-vegetal signal that occurs prior to gastrulation promotes primary spinal neuron fate in vegetal equatorial (C-tier) blastomere lineages. Herein we demonstrate that maternal mRNA encoding noggin is enriched in animal tiers and at low concentrations in the C-tier, suggesting that the neural fates of C-tier blastomeres may be responsive to early signaling from their neighboring cells. In support of this hypothesis, experimental alteration of the levels of Noggin from animal equatorial (B-tier) or BMP4 from vegetal (D-tier) blastomeres significantly affects the numbers of primary spinal neurons derived from their neighboring C-tier blastomeres. These effects are duplicated in blastomere explants isolated at cleavage stages and cultured in the absence of gastrulation interactions. Co-culture with animal blastomeres enhanced the expression of zygotic neural markers in C-tier blastomere explants, whereas co-culture with vegetal blastomeres repressed them. The expression of these markers in C-tier explants was promoted when Noggin was transiently added to the culture during cleavage/morula stages, and repressed with the transient addition of BMP4. Reduction of Noggin translation in B-tier blastomeres by antisense morpholino oligonucleotides significantly reduced the efficacy of neural marker induction in C-tier explants. These experiments indicate that early anti-BMP signaling from the animal hemisphere recruits vegetal equatorial cells into the neural precursor pool prior to interactions that occur during gastrulation.

SFRP1 is required for the proper establishment of the eye field in the medaka fish

Secreted Frizzled Related Proteins (SFRPs) are a family of soluble molecules structurally related to the Wnt receptors. Functional analysis in different vertebrate species suggests that these molecules are multifunctional modulators of Wnt and possibly other signalling pathways. Sfrp1 a member of this family, is strongly expressed throughout embryonic development in different vertebrate species. Its function is, however, poorly understood. To address the role of this protein at early stages of embryonic development, we have used the medaka fish (Oryzias latipes) as a model system. Here, we describe the characterisation and the expression analysis of olSfrp1. We also show that morpholino-based interference with olSfrp1 expression results in embryos with a reduced eye field, a phenotype that, in the most affected embryos, is associated with a shortening and widening of the A-P axis. Because the expression of posterior diencephalic markers is unchanged but that of rostral telencephalic ones is expanded, we propose that olSfrp1 is needed for a proper establishment of the eye field within the forebrain. In addition, olSfrp1 may contribute to the control of mesodermal convergence extension movements that take place during gastrulation.

Morphogenetic Movements Underlying Eye Field Formation Require Interactions between the FGF and ephrinB1 Signaling Pathways

The definitive retinal progenitors of the eye field are specified by transcription factors that both promote a retinal fate and control cell movements that are critical for eye field formation. However, the molecular signaling pathways that regulate these movements are largely undefined. We demonstrate that both the FGF and ephrin pathways impact eye field formation. Activating the FGF pathway before gastrulation represses cellular movements in the presumptive anterior neural plate and prevents cells from expressing a retinal fate, independent of mesoderm induction or anterior-posterior patterning. Inhibiting the FGF pathway promotes cell dispersal and significantly increases eye field contribution. ephrinB1 reverse signaling is required to promote cellular movements into the eye field, and can rescue the FGF receptor-induced repression of retinal fate. These results indicate that FGF modulation of ephrin signaling regulates the positioning of retinal progenitor cells within the definitive eye field.

Multiple maternal influences on dorsal-ventral fate of Xenopus animal blastomeres

Molecular asymmetries in the animal-vegetal axis of the Xenopus oocyte are well known to regulate the formation of gametes and germ layers. Likewise, many transplantation and explant studies demonstrate that maternal dorsalizing activities are localized to the future dorsal side of the embryo after fertilization, but to date only a few of the molecules involved in this process have been shown to be asymmetrically distributed. In this report, we identify two new aspects of the maternal regulation of dorsal-ventral fate asymmetry in Xenopus blastomeres: cytoplasmic polyadenylation of dorsal maternal mRNAs and localized Wnt8b signaling. Previous studies demonstrated that there are maternal, dorsal axis-inducing RNAs localized to dorsal animal blastomeres that become activated between the 8- and 16-cell stage (Hainski and Moody [1992] Development 116:347-355; Hainski and Moody [1996] Dev. Genet. 19:210-221). We report herein that the activation of these axis-inducing dorsal mRNAs is regulated by cytoplasmic polyadenylation. We also show that maternal wnt8b mRNA is concentrated in ventral animal blastomeres. These ventral cells and exogenous Wnt8b both inhibit the dorsal fate of neighboring blastomeres in culture, indicating that a maternal Wnt signal also contributes to segregating dorsal and ventral fates.

Autosomal dominant stapes ankylosis with broad thumbs and toes, hyperopia, and skeletal anomalies is caused by heterozygous nonsense and frameshift mutations in NOG, the gene encoding noggin

Although fixation of the stapes is usually progressive and secondary to otosclerosis, it may present congenitally, with other skeletal manifestations, as an autosomal dominant syndrome-such as proximal symphalangism (SYM1) or multiple-synostoses syndrome (SYNS1), both of which are caused by mutations in NOG, the gene encoding noggin. We describe a family that was ascertained to have nonsyndromic otosclerosis but was subsequently found to have a congenital stapes ankylosis syndrome that included hyperopia, a hemicylindrical nose, broad thumbs and great toes, and other minor skeletal anomalies but lacked symphalangism. A heterozygous nonsense NOG mutation-c.328C-->T (Q110X), predicted to truncate the latter half of the protein-was identified, and a heterozygous insertion in NOG-c.252-253insC, in which the frameshift is predicted to result in 96 novel amino acids before premature truncation-was identified in a previously described second family with a similar phenotype. In contrast to most NOG mutations that have been reported in kindreds with SYM1 and SYNS1, the mutations observed in these families with stapes ankylosis without symphalangism are predicted to disrupt the cysteine-rich C-terminal domain. These clinical and molecular findings suggest that (1) a broader range of conductive hearing-loss phenotypes are associated with NOG mutations than had previously been recognized, (2) patients with sporadic or familial nonsyndromic otosclerosis should be evaluated for mild features of this syndrome, and (3) NOG alterations should be considered in conductive hearing loss with subtle clinical and skeletal features, even in the absence of symphalangism.

Transcription factors of the anterior neural plate alter cell movements of epidermal progenitors to specify a retinal fate

The embryonic progenitors that give rise to the vertebrate retina acquire their cell fate identity through a series of transitions that ultimately determine their final, differentiated retinal cell fates. In Xenopus, these transitions have been broadly defined as competence, specification, and determination. The expression of several transcription factors within the anterior neural plate at the time when the presumptive eye field separates from other neural derivatives suggests that these genes function to specify competent embryonic progenitors toward a retinal fate. In support of this, we demonstrate that some transcription factors expressed in the anterior neural ectoderm and/or presumptive eye field (otx2, pax6, and rx1) change the fate of competent, ventral progenitors, which normally do not contribute to the retina, from an epidermal to a retinal fate. Furthermore, the expression of these factors changes the morphogenetic movements of progenitors during gastrulation, causing ventral cells to populate the native anterior neural plate. In addition, we experimentally demonstrate that the efficacy of pax6 to specify retinal cells depends on the position of the affected cell relative to the field of neural induction. Thereby, otx2, pax6, and rx1 mediate early steps of retinal specification, including the regulation of morphogenetic cell movements, that are dependent on the level of neural-inductive signaling.

Intrinsic bias and lineage restriction in the phenotype determination of dopamine and neuropeptide Y amacrine cells

Blastomere lineages are differentially biased to produce different neurotransmitter subtypes of amacrine cells (Huang and Moody, 1995, 1997,). To elucidate when this bias is acquired, we examined amacrine lineages at different early developmental times. Our experiments demonstrate that the bias to express dopamine and neuropeptide Y amacrine fates involves several steps before the formation of the definitive optic cup. At cleavage stages, a retinal progenitor that contributes large numbers of cells is already biased to produce its normal repertoire of dopamine amacrine cells, as revealed by transplantation to a new location, whereas the amacrine fate of a progenitor that contributes fewer cells is modified by its new position. At neural plate stages, not all retinal progenitors are multipotent. Nearly one-half populate only the inner nuclear layer and are enriched in amacrine cells. During early optic vesicle stages, an appropriate mitotic tree is required for dopamine and neuropeptide Y, but not serotonin, amacrine cell clusters to form. Thus, the acquisition of amacrine fate bias involves intrinsic maternal factors at cleavage, fate restriction in the neural plate, and specified mitotic patterns in the optic vesicle. At each of these steps only a subset of the embryonic retinal progenitors contributing to amacrine subtypes is biased; the remaining progenitors maintain multipotency. Thus, from the earliest embryonic stages, progenitors of the retina are a dynamic mosaic. This is the first experimental demonstration of amacrine fate decisions that occur during early embryonic periods in advance of the events described in the later, committed retina.