Sox10 contributes to the balance of fate choice in dorsal root ganglion progenitors.

The development of functional peripheral ganglia requires a balance of specification of both neuronal and glial components. In the developing dorsal root ganglia (DRGs), these components form from partially-restricted bipotent neuroglial precursors derived from the neural crest. Work in mouse and chick has identified several factors, including Delta/Notch signaling, required for specification of a balance of these components. We have previously shown in zebrafish that the Sry-related HMG domain transcription factor, Sox10, plays an unexpected, but crucial, role in sensory neuron fate specification in vivo. In the same study we described a novel Sox10 mutant allele, sox10baz1, in which sensory neuron numbers are elevated above those of wild-types. Here we investigate the origin of this neurogenic phenotype. We demonstrate that the supernumerary neurons are sensory neurons, and that enteric and sympathetic neurons are almost absent just as in classical sox10 null alleles; peripheral glial development is also severely abrogated in a manner similar to other sox10 mutant alleles. Examination of proliferation and apoptosis in the developing DRG reveals very low levels of both processes in wild-type and sox10baz1, excluding changes in the balance of these as an explanation for the overproduction of sensory neurons. Using chemical inhibition of Delta-Notch-Notch signaling we demonstrate that in embryonic zebrafish, as in mouse and chick, lateral inhibition during the phase of trunk DRG development is required to achieve a balance between glial and neuronal numbers. Importantly, however, we show that this mechanism is insufficient to explain quantitative aspects of the baz1 phenotype. The Sox10(baz1) protein shows a single amino acid substitution in the DNA binding HMG domain; structural analysis indicates that this change is likely to result in reduced flexibility in the HMG domain, consistent with sequence-specific modification of Sox10 binding to DNA. Unlike other Sox10 mutant proteins, Sox10(baz1) retains an ability to drive neurogenin1 transcription. We show that overexpression of neurogenin1 is sufficient to produce supernumerary DRG sensory neurons in a wild-type background, and can rescue the sensory neuron phenotype of sox10 morphants in a manner closely resembling the baz1 phenotype. We conclude that an imbalance of neuronal and glial fate specification results from the Sox10(baz1) protein's unique ability to drive sensory neuron specification whilst failing to drive glial development. The sox10baz1 phenotype reveals for the first time that a Notch-dependent lateral inhibition mechanism is not sufficient to fully explain the balance of neurons and glia in the developing DRGs, and that a second Sox10-dependent mechanism is necessary. Sox10 is thus a key transcription factor in achieving the balance of sensory neuronal and glial fates.

Numb/Numbl-Opo antagonism controls retinal epithelium morphogenesis by regulating integrin endocytosis.

Polarized trafficking of adhesion receptors plays a pivotal role in controlling cellular behavior during morphogenesis. Particularly, clathrin-dependent endocytosis of integrins has long been acknowledged as essential for cell migration. However, little is known about the contribution of integrin trafficking to epithelial tissue morphogenesis. Here we show how the transmembrane protein Opo, previously described for its essential role during optic cup folding, plays a fundamental role in this process. Through interaction with the PTB domain of the clathrin adaptors Numb and Numbl via an integrin-like NPxF motif, Opo antagonizes Numb/Numbl function and acts as a negative regulator of integrin endocytosis in vivo. Accordingly, numb/numbl gain-of-function experiments in teleost embryos mimic the retinal malformations observed in opo mutants. We propose that developmental regulator Opo enables polarized integrin localization by modulating Numb/Numbl, thus directing the basal constriction that shapes the vertebrate retina epithelium.

Leukocyte tyrosine kinase functions in pigment cell development.

A fundamental problem in developmental biology concerns how multipotent precursors choose specific fates. Neural crest cells (NCCs) are multipotent, yet the mechanisms driving specific fate choices remain incompletely understood. Sox10 is required for specification of neural cells and melanocytes from NCCs. Like sox10 mutants, zebrafish shady mutants lack iridophores; we have proposed that sox10 and shady are required for iridophore specification from NCCs. We show using diverse approaches that shady encodes zebrafish leukocyte tyrosine kinase (Ltk). Cell transplantation studies show that Ltk acts cell-autonomously within the iridophore lineage. Consistent with this, ltk is expressed in a subset of NCCs, before becoming restricted to the iridophore lineage. Marker analysis reveals a primary defect in iridophore specification in ltk mutants. We saw no evidence for a fate-shift of neural crest cells into other pigment cell fates and some NCCs were subsequently lost by apoptosis. These features are also characteristic of the neural crest cell phenotype in sox10 mutants, leading us to examine iridophores in sox10 mutants. As expected, sox10 mutants largely lacked iridophore markers at late stages. In addition, sox10 mutants unexpectedly showed more ltk-expressing cells than wild-type siblings. These cells remained in a premigratory position and expressed sox10 but not the earliest neural crest markers and may represent multipotent, but partially-restricted, progenitors. In summary, we have discovered a novel signalling pathway in NCC development and demonstrate fate specification of iridophores as the first identified role for Ltk.

The proliferating field of neural crest stem cells.

Neural crest stem cells were first isolated from early embryonic neural crest in the early 1990s, but in the past 5 years, there has been a burst of discoveries of neural crest-derived stem cells from diverse locations. Here, we summarize these data, highlighting the characteristics of each stem cell type. These cells vary widely in the markers they express and the variety of cell types they appear to generate. They occupy diverse locations, but in some cases multiple stem cell types apparently occupy physically proximate niches. To date, few molecular similarities can be identified between these stem cells, although a systematic comparison is required. We note other issues worthy of attention, including aspects of the in vivo behavior of these stem cells, their niches, and their lineage relationships. Together, analysis of these issues will clarify this expanding, but still young, field and contribute to exploration of the important therapeutic potential of these cells.

A direct role for Sox10 in specification of neural crest-derived sensory neurons.

sox10 is necessary for development of neural and pigment cell derivatives of the neural crest (NC). However, whereas a direct role for Sox10 activity has been established in pigment and glial lineages, this is more controversial in NC-derived sensory neurons of the dorsal root ganglia (DRGs). We proposed that sox10 functioned in specification of sensory neurons, whereas others suggested that sensory neuronal defects were merely secondary to absence of glia. Here we provide evidence that in zebrafish, early DRG sensory neuron survival is independent of differentiated glia. Critically, we demonstrate that Sox10 is expressed transiently in the sensory neuron lineage, and specifies sensory neuron precursors by regulating the proneural gene neurogenin1. Consistent with this, we have isolated a novel sox10 mutant that lacks glia and yet displays a neurogenic DRG phenotype. In conjunction with previous findings, these data establish the generality of our model of Sox10 function in NC fate specification.

Specification and maintenance of the spinal cord stem zone.

Epiblast cells adjacent to the regressing primitive streak behave as a stem zone that progressively generates the entire spinal cord and also contributes to paraxial mesoderm. Despite this fundamental task, this cell population is poorly characterised, and the tissue interactions and signalling pathways that specify this unique region are unknown. Fibroblast growth factor (FGF) is implicated but it is unclear whether it is sufficient and/or directly required for stem zone specification. It is also not understood how establishment of the stem zone relates to the acquisition of spinal cord identity as indicated by expression of caudal Hox genes. Here, we show that many cells in the chick stem zone express both early neural and mesodermal genes; however, stem zone-specific gene expression can be induced by signals from underlying paraxial mesoderm without concomitant induction of an ambivalent neural/mesodermal cell state. The stem zone is a site of FGF/MAPK signalling and we show that although FGF alone does not mimic paraxial mesoderm signals, it is directly required in epiblast cells for stem zone specification and maintenance. We further demonstrate that caudal Hox gene expression in the stem zone also depends on FGF and that neither stem zone specification nor caudal Hox gene onset requires retinoid signalling. These findings thus support a two step model for spinal cord generation - FGF-dependent establishment of the stem zone in which progressively more caudal Hox genes are expressed, followed by the retinoid-dependent assignment of spinal cord identity.

Composition of the zooplankton community, with emphasis in copepods, in Punta Morales, Golfo de Nicoya, Costa Rica

The composition of the mesozooplanktonic community was studied in the Punta Morales estuary, Gulf of Nicoya,Pacific coast of Costa Rica,during 1997.Oblique plankton hauls were performed during high and low tide using a 280 µm mesh screen net equipped with a flowmeter.The community was characterized by holoplanktonic and meroplanktonic organisms.For the holoplanktonic community,the main groups were copepods (80%)and chaetognaths (16%).The most abundant species were the copepods Acartia lilljeborgii and Paracalanus parvus.A.lilljeborgii is a typical estuarine species that maintains high populations in estuarine systems.Meroplankton was represented mainly by crustacean larvae (66%),and icthyoplankton (18%).The dominance of crustacean larvae and icthyoplankton is an evidence of the ecological importance of the Punta Morales zone

Se estudió la composición de la comunidad mezoplanctónica en Punta Morales,Golfo de Nicoya, Pacífico de Costa Rica,en 1997,arrastrando oblicuamente una red de plancton (280 µm,con flujómetro)durante marea alta y baja.La comunidad estuvo caracterizada por la presencia de organismos holo y meroplanctónicos.Los principales grupos holoplanctónicos fueron copépodos (80%)y quetognatos (16%).Las especies más abundantes fueron los copépodos Acartia lilljeborgii y Paracalanus parvus;A. lilljeborgii es una especie estuarina típica que mantiene altas poblaciones en sistemas estuarinos.El meroplancton estuvo representado principalmente por larvas de crustáceos (66%),e ictioplancton (18%).La dominancia de larvas de crustáceos e ictioplancton evidencian la importancia ecológica de la zona de Punta Morales

Composition of the zooplankton community, with emphasis in copepods, in Punta Morales, Golfo De Nicoya, Costa Rica.

The composition of the mesozooplanktonic community was studied in the Punta Morales estuary, Gulf of Nicoya, Pacific coast of Costa Rica, during 1997. Oblique plankton hauls were performed during high and low tide using a 280 microm mesh screen net equipped with a flowmeter. The community was characterized by holoplanktonic and meroplanktonic organisms. For the holoplanktonic community, the main groups were copepods (80%) and chaetognaths (16%). The most abundant species were the copepods Acartia lilljeborgii and Paracalanus parvus. A. lilljeborgii is a typical estuarine species that maintains high populations in estuarine systems. Meroplankton was represented mainly by crustacean larvae (66%), and icthyoplankton (18%). The dominance of crustacean larvae and icthyoplankton is an evidence of the ecological importance of the Punta Morales zone.