Early development of the central and peripheral nervous systems is coordinated by Wnt and BMP signals

The formation of functional neural circuits that process sensory information requires coordinated development of the central and peripheral nervous systems derived from neural plate and neural plate border cells, respectively. Neural plate, neural crest and rostral placodal cells are all specified at the late gastrula stage. How the early development of the central and peripheral nervous systems are coordinated remains, however, poorly understood. Previous results have provided evidence that at the late gastrula stage, graded Wnt signals impose rostrocaudal character on neural plate cells, and Bone Morphogenetic Protein (BMP) signals specify olfactory and lens placodal cells at rostral forebrain levels. By using in vitro assays of neural crest and placodal cell differentiation, we now provide evidence that Wnt signals impose caudal character on neural plate border cells at the late gastrula stage, and that under these conditions, BMP signals induce neural crest instead of rostral placodal cells. We also provide evidence that both caudal neural and caudal neural plate border cells become independent of further exposure to Wnt signals at the head fold stage. Thus, the status of Wnt signaling in ectodermal cells at the late gastrula stage regulates the rostrocaudal patterning of both neural plate and neural plate border, providing a coordinated spatial and temporal control of the early development of the central and peripheral nervous systems.     

Signaling mechanisms controlling cranial placode neurogenesis and delamination

The neurogenic cranial placodes are a unique transient epithelial niche of neural progenitor cells that give rise to multiple derivatives of the peripheral nervous system, particularly, the sensory neurons. Placode neurogenesis occurs throughout an extended period of time with epithelial cells continually recruited as neural progenitor cells. Sensory neuron development in the trigeminal, epibranchial, otic, and olfactory placodes coincides with detachment of these neuroblasts from the encompassing epithelial sheet, leading to delamination and ingression into the mesenchyme where they continue to differentiate as neurons. Multiple signaling pathways are known to direct placodal development. This review defines the signaling pathways working at the finite spatiotemporal period when neuronal selection within the placodes occurs, and neuroblasts concomitantly delaminate from the epithelium. Examining neurogenesis and delamination after initial placodal patterning and specification has revealed a common trend throughout the neurogenic placodes, which suggests that both activated FGF and attenuated Notch signaling activities are required for neurogenesis and changes in epithelial cell adhesion leading to delamination. We also address the varying roles of other pathways such as the Wnt and BMP signaling families during sensory neurogenesis and neuroblast delamination in the differing placodes.     

The lens: a classical model of embryonic induction providing new insights into cell determination in early development

The lens was the first tissue in which the concept of embryonic induction was demonstrated. For many years lens induction was thought to occur at the time the optic vesicle and lens placode came in contact. Since then, studies have revealed that lens placodal progenitor cells are specified already at gastrula stages, much earlier than previously believed, and independent of optic vesicle interactions. In this review, I will focus on how individual signalling molecules, in particular BMP, FGF, Wnt and Shh, regulate the initial specification of lens placodal cells and the progressive development of lens cells. I will discuss recent work that has shed light on the combination of signalling molecules and the molecular interactions that affect lens specification and proper lens formation. I will also discuss proposed tissue interactions important for lens development. A greater knowledge of the molecular interactions during lens induction is likely to have practical benefits in understanding the causes and consequences of lens diseases. Moreover, knowledge regarding lens induction is providing fundamental important insights into inductive processes in development in general.     

Equivalent progenitor cells in the zebrafish anterior preplacodal field give rise to adenohypophysis, lens, and olfactory placodes

Embryonic organizing centers secrete signaling molecules that instruct target cells about their position and future identity. Information about cell position in relation to sources of instructive signals and about precursor cell lineages is key to our understanding of developmental processes that restrict cell potency and determine cell fate. We review adenohypophysis, lens, and olfactory placode formation and how gene expression patterns, cell positions, and cell fates in the anterior neural plate and anterior placodal field correlate in zebrafish and other vertebrates. Single cell lineage analysis in zebrafish suggests that the majority of preplacodal cells might be specified for pituitary, lens, or olfactory placode by the end of gastrulation.     

Lens specification is the ground state of all sensory placodes, from which FGF promotes olfactory identity

The sense organs of the vertebrate head comprise structures as varied as the eye, inner ear, and olfactory epithelium. In the early embryo, these assorted structures share a common developmental origin within the preplacodal region and acquire specific characteristics only later. Here we demonstrate a fundamental similarity in placodal precursors: in the chick all are specified as lens prior to acquiring features of specific sensory or neurogenic placodes. Lens specification becomes progressively restricted in the head ectoderm, initially by FGF and subsequently by signals derived from migrating neural crest cells. We show that FGF8 from the anterior neural ridge is both necessary and sufficient to promote olfactory fate in adjacent ectoderm. Our results reveal that placode precursors share a common ground state as lens and progressive restriction allows the full range of placodal derivatives to form.     

Segregation of lens and olfactory precursors from a common territory: cell sorting and reciprocity of Dlx5 and Pax6 expression

Cranial placodes are focal regions of columnar epithelium next to the neural tube that contribute to sensory ganglia and organs in the vertebrate head, including the olfactory epithelium and the crystalline lens of the eye. Using focal dye labelling within the presumptive placode domain, we show that lens and nasal precursors arise from a common territory surrounding the anterior neural plate. They then segregate over time and converge to their final positions in discrete placodes by apparently directed movements. Since these events closely parallel the separation of eye and antennal primordia (containing olfactory sensory cells) from a common imaginal disc in Drosophila, we investigated whether the vertebrate homologues of Distalless (Dll) and Eyeless (Ey), which determine antennal and eye identity in the fly, play a role in segregation of lens and nasal precursors in the chick. Dlx5 and Pax6 are initially co-expressed by future lens and olfactory cells. As soon as presumptive lens cells acquire columnar morphology all Dlx family members are down-regulated in the placode, while Pax6 is lost in the olfactory region. Lens precursor cells that express ectopic Dlx5 never acquire lens-specific gene expression and are excluded from the lens placode to cluster in the head ectoderm. These results suggest that the loss of Dlx5 is required for cells to adopt a lens fate and that the balance of Pax6 and Dlx expression regulates cell sorting into appropriate placodal domains.     

FGF signals induce Caprin2 expression in the vertebrate lens

The lens of the eye is derived from the non-neural ectoderm situated next to the optic vesicle. Fibroblast growth factor (FGF) signals play a major role at various stages of vertebrate lens development ranging from induction and proliferation to differentiation. Less is however known about the identity of genes that are induced by FGF activity within the lens. We have isolated and characterized mouse cytoplasmic activation/proliferation-associated protein-2 (Caprin2), with domains belonging to both the Caprin family and the C1q and tumour necrosis factor (TNF) super-family. Here we show that Caprin2 is expressed in the developing vertebrate lens in mouse and chick, and that Caprin2 expression is up-regulated in primary lens fiber cells, after the induction of crystallins the earliest known markers for differentiated lens fiber cells. Caprin2 is subsequently down-regulated in the centre of the lens at the time and at the position of the first fiber cell denucleation and terminal differentiation. In vitro analyses of lens fiber cell differentiation provide evidence that FGF activity emanating from neighboring prospective retinal cells is required and that FGF8 activity is sufficient to induce Caprin2 in lens fiber cells. These results not only provide evidence that FGF signals induce the newly characterized protein Caprin2 in the lens, but also support the general idea that FGF signals are required for lens fiber cell differentiation.     

A balance of FGF and BMP signals regulates cell cycle exit and Equarin expression in lens cells

In embryonic and adult lenses, a balance of cell proliferation, cell cycle exit, and differentiation is necessary to maintain physical function. The molecular mechanisms regulating the transition of proliferating lens epithelial cells to differentiated primary lens fiber cells are poorly characterized. To investigate this question, we used gain- and loss-of-function analyses to modulate fibroblast growth factor (FGF) and/or bone morphogenetic protein (BMP) signals in chick lens/retina explants. Here we show that FGF activity plays a key role for proliferation independent of BMP signals. Moreover, a balance of FGF and BMP signals regulates cell cycle exit and the expression of Ccdc80 (also called Equarin), which is expressed at sites where differentiation of lens fiber cells occurs. BMP activity promotes cell cycle exit and induces Equarin expression in an FGF-dependent manner. In contrast, FGF activity is required but not sufficient to induce cell cycle exit or Equarin expression. Furthermore, our results show that in the absence of BMP activity, lens cells have increased cell cycle length or are arrested in the cell cycle, which leads to decreased cell cycle exit. Taken together, these findings suggest that proliferation, cell cycle exit, and early differentiation of primary lens fiber cells are regulated by counterbalancing BMP and FGF signals.     

pitx3 defines an equivalence domain for lens and anterior pituitary placode

Hedgehog signaling is required for formation and patterning of the anterior pituitary gland. However, the role of Hedgehog in pituitary precursor cell specification and subsequent placode formation is not well understood. We analyzed pituitary precursor cell lineages and find that pitx3 and distal-less3b (dlx3b) expression domains define lens and pituitary precursor positions. We show that pitx3 is required for pituitary pre-placode formation and cell specification, whereas dlx3b and dlx4b are required to restrict pituitary placode size. In smoothened mutant embryos that cannot transduce Hedgehog signals, median pituitary precursors are mis-specified and form an ectopic lens. Moreover, overexpression of sonic hedgehog (shh) blocks lens formation, and derivatives of lens precursors express genes characteristic of pituitary cells. However, overexpression of shh does not increase median pituitary placode size nor does it upregulate patched (ptc) expression in pituitary precursors during early somitogenesis. Our study suggests that by the end of gastrulation, pitx3-expressing cells constitute an equivalence domain of cells that can form either pituitary or lens, and that a non-Hedgehog signal initially specifies this placodal field. During mid-somitogenesis, Hedgehog then acts on the established median placode as a necessary and sufficient signal to specify pituitary cell types.     

Opposing effects of bone morphogenetic proteins on neuron production and survival in the olfactory receptor neuron lineage

In olfactory epithelium (OE) cultures, bone morphogenetic proteins (BMPs) can strongly inhibit neurogenesis. Here we provide evidence that BMPs also promote, and indeed are required, for OE neurogenesis. Addition of the BMP antagonist noggin inhibited neurogenesis in OE-stromal cell co-cultures. Bmp2, Bmp4 and Bmp7 were expressed by OE stroma, and low concentrations of BMP4 (below the threshold for inhibition of neurogenesis) stimulated neurogenesis; BMP7 did not exhibit a stimulatory effect at any concentration tested. Stromal cell conditioned medium also stimulated neurogenesis; part of this effect was due to the presence within it of a noggin-binding factor or factors. Studies of the pro-neurogenic effect of BMP4 indicated that it did not increase progenitor cell proliferation, but rather promoted survival of newly generated olfactory receptor neurons. These findings indicate that BMPs exert both positive and negative effects on neurogenesis, depending on ligand identity, ligand concentration and the particular cell in the lineage that is responding. In addition, they reveal the presence of a factor or factors, produced by OE stroma, that can synergize with BMP4 to stimulate OE neurogenesis.     

Staggered cell-intrinsic timing of ath5 expression underlies the wave of ganglion cell neurogenesis in the zebrafish retina

In the developing nervous system, progenitor cells must decide when to withdraw from the cell cycle and commence differentiation. There is considerable debate whether cell-extrinsic or cell-intrinsic factors are most important for triggering this switch. In the vertebrate retina, initiation of neurogenesis has recently been explained by a 'sequential-induction' model--signals from newly differentiated neurons are thought to trigger neurogenesis in adjacent progenitors, creating a wave of neurogenesis that spreads across the retina in a stereotypical manner. We show here, however, that the wave of neurogenesis in the zebrafish retina can emerge through the independent action of progenitor cells--progenitors in different parts of the retina appear pre-specified to initiate neurogenesis at different times. We provide evidence that midline Sonic hedgehog signals, acting before the onset of neurogenesis, are part of the mechanism that sets the neurogenic timer in these cells. Our results highlight the importance of intrinsic factors for triggering neurogenesis, but they also suggest that early signals can modulate these intrinsic factors to influence the timing of neurogenesis many cell cycles later, thereby potentially coordinating axial patterning with control of neuron number and cell fate.     

Fibroblast growth factor signaling is required for the proliferation and patterning of progenitor cells in the developing anterior pituitary

The proliferation and patterning of progenitor cells in the anterior pituitary require signals derived from the neuroepithelium of the juxtaposed infundibulum. The infundibulum expresses Fibroblast growth factor (Fgf) 8 and Fgf 18, and FGFs can mimic some of the activities of the infundibulum. The requirement for FGF signaling during growth and patterning of the anterior pituitary has not, however, been established. By blocking FGF receptor signaling in explants of the anterior pituitary cultured in vitro we provide evidence that FGF signaling derived from the infundibulum is required for the proliferation and patterning of progenitor cells in the anterior pituitary.     

Bone morphogenetic protein signaling in stem cells--one signal, many consequences

Bone morphogenetic protein (BMP) signals play key roles throughout embryology, from the earliest patterning events, via tissue specification, through organ development and again in germ cell differentiation. While both input and the transducer molecules are rather well studied, the final outcome of a BMP signal is basically unpredictable and differs enormously between previously studied cell types. As already suggested by their name, BMPs exhibit most of their (known) functions on stem cells and precursor cells, usually driving them into various types of differentiation or death. In this minireview, some prime examples of BMP effects on several very different stem-cell types are discussed.     

A balance of BMP and notch activity regulates neurogenesis and olfactory nerve formation

Although the function of the adult olfactory system has been thoroughly studied, the molecular mechanisms regulating the initial formation of the olfactory nerve, the first cranial nerve, remain poorly defined. Here, we provide evidence that both modulated Notch and bone morphogenetic protein (BMP) signaling affect the generation of neurons in the olfactory epithelium and reduce the number of migratory neurons, so called epithelioid cells. We show that this reduction of epithelial and migratory neurons is followed by a subsequent failure or complete absence of olfactory nerve formation. These data provide new insights into the early generation of neurons in the olfactory epithelium and the initial formation of the olfactory nerve tract. Our results present a novel mechanism in which BMP signals negatively affect Notch activity in a dominant manner in the olfactory epithelium, thereby regulating neurogenesis and explain why a balance of BMP and Notch activity is critical for the generation of neurons and proper development of the olfactory nerve.