Essential role for PDGF signaling in ophthalmic trigeminal placode induction

Much of the peripheral nervous system of the head is derived from ectodermal thickenings, called placodes, that delaminate or invaginate to form cranial ganglia and sense organs. The trigeminal ganglion, which arises lateral to the midbrain, forms via interactions between the neural tube and adjacent ectoderm. This induction triggers expression of Pax3, ingression of placode cells and their differentiation into neurons. However, the molecular nature of the underlying signals remains unknown. Here, we investigate the role of PDGF signaling in ophthalmic trigeminal placode induction. By in situ hybridization, PDGF receptor β is expressed in the cranial ectoderm at the time of trigeminal placode formation, with the ligand PDGFD expressed in the midbrain neural folds. Blocking PDGF signaling in vitro results in a dose-dependent abrogation of Pax3 expression in recombinants of quail ectoderm with chick neural tube that recapitulate placode induction. In ovo microinjection of PDGF inhibitor causes a similar loss of Pax3 as well as the later placodal marker, CD151, and failure of neuronal differentiation. Conversely, microinjection of exogenous PDGFD increases the number of Pax3+ cells in the trigeminal placode and neurons in the condensing ganglia. Our results provide the first evidence for a signaling pathway involved in ophthalmic (opV) trigeminal placode induction

Regulatory Logic Underlying Diversification of the Neural Crest

The neural crest is a transient, multipotent population of cells that arises at the border of the developing nervous system. After closure of the neural tube, these cells undergo an epithelial-to-mesenchymal transition (EMT) to delaminate and migrate, often to distant locations in the embryo. Neural crest cells give rise to a diverse array of derivatives including neurons and glia of the peripheral nervous system, melanocytes, and bone and cartilage of the face. A gene regulatory network (GRN) controls the specification, delamination, migration, and differentiation of this fascinating cell type. With increasing technological advances, direct linkages within the neural crest GRN are being uncovered. The underlying circuitry is useful for understanding important topics such as reprogramming, evolution, and disease

Regulatory Logic Underlying Diversification of the Neural Crest

The neural crest is a transient, multipotent population of cells that arises at the border of the developing nervous system. After closure of the neural tube, these cells undergo an epithelial-to-mesenchymal transition (EMT) to delaminate and migrate, often to distant locations in the embryo. Neural crest cells give rise to a diverse array of derivatives including neurons and glia of the peripheral nervous system, melanocytes, and bone and cartilage of the face. A gene regulatory network (GRN) controls the specification, delamination, migration, and differentiation of this fascinating cell type. With increasing technological advances, direct linkages within the neural crest GRN are being uncovered. The underlying circuitry is useful for understanding important topics such as reprogramming, evolution, and disease

Molecular and tissue interactions governing induction of cranial ectodermal placodes

Whereas neural crest cells are the source of the peripheral nervous system in the trunk of vertebrates, the “ectodermal placodes,” together with neural crest, form the peripheral nervous system of the head. Cranial ectodermal placodes are thickenings in the ectoderm that subsequently ingress or invaginate to make important contributions to cranial ganglia, including epibranchial and trigeminal ganglia, and sensory structures, the ear, nose, lens, and adenohypophysis. Recent studies have uncovered a number of molecular signals mediating induction and differentiation of placodal cells. Here, we described recent advances in understanding the tissue interactions and signals underlying induction and neurogenesis of placodes, with emphasis on the trigeminal and epibranchial. Important roles of Fibroblast Growth Factors, Platelet Derived Growth Factors, Sonic Hedgehog, TGFβ superfamily members, and Wnts are discussed

Spatiotemporal structure of cell fate decisions in murine neural crest

Neural crest cells are embryonic progenitors that generate numerous cell types in vertebrates. With single-cell analysis, we show that mouse trunk neural crest cells become biased toward neuronal lineages when they delaminate from the neural tube, whereas cranial neural crest cells acquire ectomesenchyme potential dependent on activation of the transcription factor Twist1. The choices that neural crest cells make to become sensory, glial, autonomic, or mesenchymal cells can be formalized as a series of sequential binary decisions. Each branch of the decision tree involves initial coactivation of bipotential properties followed by gradual shifts toward commitment. Competing fate programs are coactivated before cells acquire fate-specific phenotypic traits. Determination of a specific fate is achieved by increased synchronization of relevant programs and concurrent repression of competing fate programs

Elk3 is essential for the progression from progenitor to definitive neural crest cell

Elk3/Net/Sap2 (here referred to as Elk3) is an Ets ternary complex transcriptional repressor known for its involvement in angiogenesis during embryonic development. Although Elk3 is expressed in various tissues, additional roles for the protein outside of vasculature development have yet to be reported. Here, we characterize the early spatiotemporal expression pattern of Elk3 in the avian embryo using whole mount in situ hybridization and quantitative RT-PCR and examine the effects of its loss of function on neural crest development. At early stages, Elk3 is expressed in the head folds, head mesenchyme, intersomitic vessels, and migratory cranial neural crest (NC) cells. Loss of the Elk3 protein results in the retention of Pax7+ precursors in the dorsal neural tube that fail to upregulate neural crest specifier genes, FoxD3, Sox10 and Snail2, resulting in embryos with severe migration defects. The results putatively place Elk3 downstream of neural plate border genes, but upstream of neural crest specifier genes in the neural crest gene regulatory network (NC-GRN), suggesting that it is critical for the progression from progenitor to definitive neural crest cell

A critical role for Cadherin6B in regulating avian neural crest emigration

Neural crest cells originate in the dorsal neural tube but subsequently undergo an epithelial-to-mesenchymal transition (EMT), delaminate, and migrate to diverse locations in the embryo where they contribute to a variety of derivatives. Cadherins are a family of cell–cell adhesion molecules expressed in a broad range of embryonic tissues, including the neural tube. In particular, cadherin6B (Cad6B) is expressed in the dorsal neural tube prior to neural crest emigration but is then repressed by the transcription factor Snail2, expressed by premigratory and early migrating cranial neural crest cells. To examine the role of Cad6B during neural crest EMT, we have perturbed Cad6B protein levels in the cranial neural crest-forming region and have examined subsequent effects on emigration and migration. The results show that knock-down of Cad6B leads to premature neural crest cell emigration, whereas Cad6B overexpression disrupts migration. Our data reveal a novel role for Cad6B in controlling the proper timing of neural crest emigration and delamination from the neural tube of the avian embryo

The balance between N-cadherin and E-cadherin orchestrates major neuroectodermal cell fate choices

Numerous cadherin proteins, including N‐cadherin (Ncad), E‐cadherin (Ecad), Cadherin‐11 (Cad11) and Cadherin‐7 (Cad7), are expressed in the developing neural plate as well as in neural crest cells as they delaminate from the newly closed neural tube. To clarify whether these proteins function independently or coordinately during development, we examined their relative expression in the cranial region of chick embryos. The results revealed surprising overlap of Ecad, Ncad and Cad7 in the neural tube, suggesting possible heterotypic interactions. Using a proximity ligation assay and co‐immunoprecipitation to test this hypothesis, we found that Ncad formed heterophilic complexes in the developing neural tube with Ecad. We also determined that modulation of either Ncad or Ecad levels led to reciprocal gain or reduction of the other cadherin protein. Altering levels of the two cadherin proteins affected the early fate specification of ectodermal derivatives, forcing an aberrant choice between neural crest and epidermal cells. Finally, we identified that the availability of β‐catenin plays a critical role in maintaining the balance between Ncad and Ecad in early development since co‐expression of activated β‐catenin rescues the Ncad‐overexpression phenotype. These results suggest that β‐catenin‐mediated balance of Ncad and Ecad proteins is critical for the normal development of the three ectodermal derivatives

P-bodies are sites of rapid RNA decay during the neural crest epithelial-mesenchymal transition

The epithelial-mesenchymal transition (EMT) drives cellular movements during development to create specialized tissues and structures in metazoans, using mechanisms often coopted during metastasis. Neural crest cells are a multipotent stem cell population that undergo a developmentally regulated EMT and are prone to metastasis in the adult, providing an excellent model to study cell state changes and mechanisms underlying EMT. A hallmark of neural crest EMT during avian development is temporally restricted expression followed by rapid down-regulation of the Wnt antagonist Draxin. Using live RNA imaging, here we demonstrate that rapid clearance of Draxin transcripts is mediated post-transcriptionally via localization to processing bodies (P-bodies), small cytoplasmic granules which are established sites of RNA processing. Contrasting with recent work in immortalized cell lines suggesting that P-bodies are sites of storage rather than degradation, we show that targeted decay of Draxin occurs within P-bodies during neural crest migration. Furthermore, P-body disruption via DDX6 knockdown inhibits not only endogenous Draxin down-regulation but also neural crest EMT in vivo. Together, our data highlight a novel and important role for P-bodies in an intact organismal context−controlling a developmental EMT program via post-transcriptional target degradation

Birth of ophthalmic trigeminal neurons initiates early in the placodal ectoderm

The largest of the cranial ganglia, the trigeminal ganglion, relays cutaneous sensations of the head to the central nervous system. Its sensory neurons have a dual origin from both ectodermal placodes and neural crest. Here, we show that the birth of neurons derived from the chick ophthalmic trigeminal placode begins prior to their ingression (HH11), as early as HH8, and considerably earlier than previously suspected (HH16). Furthermore, cells exiting the cell cycle shortly thereafter express the ophthalmic trigeminal placode marker Pax3 (HH9). At HH11, these postmitotic Pax3+ placode cells begin to express the pan-neuronal marker neurofilament while still in the ectoderm. Analysis of the ectodermal origin and distribution of these early postmitotic neurons reveals that the ophthalmic placode extends further rostrally than anticipated, contributing to neurons that reside in and make a significant contribution to the ophthalmic trigeminal nerve. These data redefine the timing and extent of neuron formation from the ophthalmic trigeminal placode