Molecular regulation of neural crest development

Maternal intake of folic acid and neural crest stem cells

Maternal folic acid (FA) intake has beneficial effects in preventing neural tube defects and may also play a role in the prevention of adult onset diseases such as Alzheimer's disease, dementia, neuropsychiatric disorders, cardiovascular diseases, and cerebral ischemia. This review will focus on the effects of maternal FA intake on neural crest stem cell proliferation and differentiation. Although FA is generally considered beneficial, it has the potential of promoting cell proliferation at the expense of differentiation. In some situations, this may lead to miscarriage or postnatal developmental abnormalities. Therefore, a blind approach such as "FA for everyone" is not necessarily the best course of action. Ultimately, the best approach for FA supplementation, and potentially other nutritional supplements, will include customized patient genomic profiles for determining dose and duration.

Identification of the first intragenic deletion of the PITX2 gene causing an Axenfeld-Rieger Syndrome: case report

BACKGROUND

Axenfeld-Rieger syndrome (ARS) is characterized by bilateral congenital abnormalities of the anterior segment of the eye associated with abnormalities of the teeth, midface, and umbilicus. Most cases of ARS are caused by mutations in the genes encoding PITX2 or FOXC1. Here we describe a family affected by a severe form of ARS.

CASE PRESENTATION

Two members of this family (father and daughter) presented with typical ARS and developed severe glaucoma. The ocular phenotype was much more severe in the daughter than in the father. Magnetic resonance imaging (MRI) detected an aggressive form of meningioma in the father. There was no mutation in the PITX2 gene, determined by exon screening. We identified an intragenic deletion by quantitative genomic PCR analysis and characterized this deletion in detail.

CONCLUSION

Our findings implicate the first intragenic deletion of the PITX2 gene in the pathogenesis of a severe form of ARS in an affected family. This study stresses the importance of a systematic search for intragenic deletions in families affected by ARS and in sporadic cases for which no mutations in the exons or introns of PITX2 have been found. The molecular genetics of some ARS pedigrees should be re-examined with enzymes that can amplify medium and large genomic fragments.

Les mélanocytes choroïdiens normaux et malins : de la cellule à la clinique

The molecular and cellular basis of human choroidal malignant melanoma progression has remained largely unknown. However, choroidal melanoma is the most important primary intraocular tumor in adults. Developmentally, choroidal melanocytes are of neural crest origin similar to cutaneous melanocytes. However, there are some significant differences between cutaneous and uveal melanocytes that have yet to be fully assessed. The purpose of this study is to describe choroidal melanocytes. We will describe the significant differences between cutaneous and uveal melanocytes as well as the congenital and acquired diseases of uveal melanocytes. We will then describe the cellular and molecular mechanisms involved in melanoma progression.

Neural crest cell lineage segregation in the mouse neural tube

Neural crest (NC) cells arise in the dorsal neural tube (NT) and migrate into the embryo to develop into many different cell types. A major unresolved question is when and how the fate of NC cells is decided. There is widespread evidence for multipotential NC cells, whose fates are decided during or after migration. There is also some evidence that the NC is already divided into subpopulations of discrete precursors within the NT. We have investigated this question in the mouse embryo. We find that a subpopulation of cells on the most dorsomedial aspect of the NT express the receptor tyrosine kinase Kit (previously known as c-kit), emigrate exclusively into the developing dermis, and then express definitive markers of the melanocyte lineage. These are thus melanocyte progenitor cells. They are generated predominantly at the midbrain-hindbrain junction and cervical trunk, with significant numbers also in lower trunk. Other cells within the dorsal NT are Kit-, migrate ventrally, and, from embryonic day 9.5, express the neurotrophin receptor p75. These cells most likely only give rise to ventral NC derivatives such as neurons and glia. The p75+ cells are located ventrolateral to the Kit+ cells in areas of the NT where these two cell types are found. These data provide direct in vivo evidence for NC lineage segregation within the mouse neural tube.

In search of "stemness"

Stem cells have been identified and characterized in a variety of tissues. In this review we examine possible shared properties of stem cells. We suggest that irrespective of their lineal origin, stem cells have to respond in similar ways to regulate self-renewal and differentiation and it is likely that cell-cycle control, asymmetry/differentiation controls, cellular protective and DNA repair mechanisms, and associated apoptosis/senescence signaling pathways all might be expected to be more highly regulated in stem cells, likely by similar mechanisms. We review the literature to suggest a set of candidate stemness genes that may serve as universal stem cell markers. While we predict many similarities, we also predict that differences will exist between stem cell populations and that when transdifferentiation is considered genes expected to be both similar and different need to be examined.

Coordinated expression of 3' hox genes during murine embryonal gut development: an enteric Hox code

BACKGROUND & AIMS

Hox genes are highly conserved developmental control genes that may be organized and expressed in the form of a code required for correct morphogenesis. Little is known about their control of the embryonal gut. However, Hox paralogues 4 and 5, which are expressed at the sites of origin of vagal neural crest cells and splanchnic mesoderm, are likely to be important. We have studied the expression domains of these genes in the gut both spatially and temporally.

METHODS

CD1 mice embryos of embryonic days E8.5-E17.5 were studied. The spatial and temporal expression patterns of messenger RNA of Hoxa4, b4, c4, d4, a5, c5, and b5 homeoprotein were determined by in situ hybridization and immunohistochemistry in whole embryos, whole gastrointestinal tracts, and vibratome sections.

RESULTS

There were different spatial, temporal, and combinatorial expression patterns in different morphological regions: foregut, prececal gut, cecum, and postcecal gut. Two dynamic gradients, rostral and caudal, were coordinated with nested expression domains along the gut primordium. Region-specific domains were present in the stomach and cecum.

CONCLUSIONS

The expression patterns of genes in paralogous groups 4 and 5 suggest that they are organized to form a specific enteric Hox code required for correct enteric development.

Retinoic acid stimulates meningioma cell adhesion to the extracellular matrix and inhibits invasion

Meningiomas are tumours derived from the arachnoid and pia mater. During embryogenesis, these membranes develop from the migrating craniofacial neural crest. We have previously demonstrated that meningiomas have characteristic features of embryonic meninges. Craniofacial neural crest derivatives are affected during normal development and migration by retinoic acid. We speculated, therefore, that meningioma cell migration and invasion would be affected in a similar way. In this study we investigated the mechanisms of invasion and migration in meningiomas and the effects of retinoic acid (RA). We found that low doses of RA inhibit in vitro invasion in meningioma cells, without affecting cell proliferation or viability. The matrix metalloproteinases MMP-2 (72 kDa gelatinase) and MMP-9 (92 kDa gelatinase), which play a key role in invasion in other tumours, are not affected by RA. RA inhibits cell migration on collagen I and fibronectin. A possible mechanism for these effects is provided by the fact that RA strongly stimulates adhesion of meningioma cells to extracellular matrix substrates. As in vitro invasion, migration and decreased adhesion to the extracellular matrix correlate with the clinical manifestation of tumour invasion, we conclude that RA induces a non-invasive phenotype in meningioma cells.

Pax-3 regulates neurogenesis in neural crest-derived precursor cells

The peripheral nervous system consists of multiple neural lineages derived from the neural crest (NC). Pax-3 is expressed in the NC and when mutated in the splotch mouse (Sp) results in the loss of derivatives from this precursor cell population. We have investigated the role of Pax-3 in regulating the generation of neurons from NC-derived precursor cells in vitro. Pax-3 mRNA in NC cultures is initially expressed in all NC but is subsequently only retained in neurons, suggesting a role in their generation. To determine whether Pax-3 is involved in neuron development, we first examined the generation of sensory-like neurons in NC cultures from Sp mice. Fivefold less sensory-like neurons were generated in NC cultures from Sp homozygous mice as compared to wild-type littermates. The role of Pax-3 in sensory neuron generation was then directly examined in dorsal root ganglia cultures by down-regulating the expression of Pax-3 protein with antisense oligonucleotides. It was found that antisense oligonucleotides inhibited 80-90% of newly generated sensory neurons; however, there was no significant effect on the survival of sensory neurons or the precursor population. These results suggest that Pax-3 has a role in regulating the differentiation of peripheral neurons.

Periventricular brain heterotopias in a child with adrenocortical insufficiency, achalasia, alacrima, and neurologic abnormalities (Allgrove syndrome)

We describe a previously unreported finding of periventricular heterotopias in a brain magnetic resonance imaging (MRI) study, in a girl with adrenocortical insufficiency, alacrima, achalasia, and neurologic deterioration (Allgrove syndrome). This finding could indicate that the underlying mechanism in this syndrome can be traced to the first half of fetal life and also might cause abnormal neuronal migration. This disorder recently has been linked to chromosome 12q13. There could be several explanations for the clinical heterogeneity in this syndrome: a contiguous gene syndrome involving multiple genes, including one whose deletion causes heterotopias, or a deficiency of a gene for a neurotrophic factor active during pre- and postnatal life and responsible for both migration and survival of neurons could be the cause. The identification of the responsible gene(s) will lead to further understanding of this multisystem disorder.

Development of neural crest in Xenopus

The neural crest is a unique cell population among embryonic cell types, displaying properties of both ectodermal and mesodermal lineages. Most of the recent studies examining the neural crest have been performed in avian embryos. Only in the first half of this century were amphibians extensively used. We first summarize this important older source of information, reviewing studies made since the turn of the century. Due to the increasingly detailed in cellular and molecular knowledge of the early development of Xenopus laevis, the remainder of the review focuses on this species. We describe the route of migration and fate of the neural crest and propose a new model of neural crest induction in which prospective cells are induced independently of the neural plate by a double gradient of a morphogen that patterns the entire ectoderm. This model is also discussed in a more general context in connection with the dorsoventral patterning of the neural tube. Finally, we discuss some ideas concerning neural crest evolution and propose a novel hypothesis about its phylogenetic origin.

Cytokines which signal through the LIF receptor and their actions in the nervous system

A number of different cytokines, each initially characterized on the basis of very different biological activities, all have very similar signalling pathways and share a similar tertiary structure. These cytokines include leukaemia inhibitory factor, ciliary neuronotrophic factor, oncostatin M, growth-promoting activity and cardiotrophin 1. They all have been found to regulate a number of properties of cells of the developing and mature nervous system in vitro and thus are neuroregulatory cytokines. The actions of these cytokines include regulation of neurotransmitter phenotype, differentiation of neuronal precursor cells both in the peripheral nervous system and in the spinal cord, survival of differentiated neurons, and regulation of development of both astrocytes and oligodendrocytes. In addition, studies in animal models show that these factors can rescue sensory and motor neurons from axotomy-induced cell death, which suggests that they can act as trauma factors for injured neurons. Analysis of the expression patterns of the different neuroregulatory cytokines and their receptors reveals that the receptors are expressed throughout nervous system development and following trauma, whereas the cytokines show temporal and spatial specific expression patterns. This is consistent with the idea that specific cytokines have specific roles in neural development and repair, but that their signalling pathways are shared. The phenotypes of the receptor knockouts show clear deficits in nervous system development, indicating a crucial role for LIF receptor signalling. Knockouts of individual cytokines are less dramatic, but LIF and CNTF knockouts do reveal deficits in maintenance of motor neurons or following trauma. Thus, whereas LIF and CNTF have clear roles in maintenance and following trauma, it is unclear which of the cytokines is involved in nervous system development. In clinical terms, these findings add further support to the use of these cytokines in nervous system trauma and disease.

Glial cell line-derived neurotrophic factor promotes the development of adrenergic neurons in mouse neural crest cultures

Growth of mouse neural crest cultures in the presence of glial cell line-derived neurotrophic factor (GDNF) resulted in a dramatic dose-dependent increase in the number of tyrosine hydroxylase (TH)-positive cells that developed when 5% chicken embryo extract was present in the medium. In contrast, growth in the presence of bone morphogenetic protein (BMP)-2, BMP-4, BMP-6, transforming growth factor (TGF) beta 1, TGF-beta 2, and TGF-beta 3 elicited no increase in the number of TH-positive cells. The TH-positive cells that developed in the presence of GDNF had neuronal morphology and contained the middle and low molecular weight neurofilament proteins. Numerous TH-negative cells with the morphology of neurons also were observed in GDNF-treated cultures. Analysis revealed that the period from 6 to 12 days in vitro was the critical time for exposure to GDNF to generate the increase in TH-positive cell number. The growth factors neurotrophin-3 and fibroblast growth factor-2 elicited increases in the number of TH-positive cells similar to that seen in response to GDNF. In contrast, nerve growth factor was unable to substitute for GDNF. These findings extend the previously reported biological activities of GDNF by showing that it can act on mouse neural crest cultures to promote the development of neurons.

Scatter factor/hepatocyte growth factor as a regulator of skeletal muscle and neural crest development

Factors that regulate cellular migration during embryonic development are essential for tissue and organ morphogenesis. Scatter factor/hepatocyte growth factor (SF/HGF) can stimulate motogenic and morphogenetic activities in cultured epithelial cells expressing the Met tyrosine kinase receptor and is essential for development; however, the precise physiological role of SF/HGF is incompletely understood. Here we provide functional evidence that inappropriate expression of SF/HGF in transgenic mice influences the development of two distinct migratory cell lineages, resulting in ectopic skeletal muscle formation and melanosis in the central nervous system, and patterned hyperpigmentation of the skin. Committed TRP-2 positive melanoblasts were found to be situated aberrantly within defined regions of the transgenic embryo, including the neural tube, which overproduced SF/RGF. Our data strongly suggest that SF/HGF possesses physiologically relevant scatter activity, and functions as a true morphogenetic factor by regulating migration and/or differentiation of select populations of premyogenic and neural crest cells during normal mammalian embryogenesis.

FGF2 regulates proliferation of neural crest cells, with subsequent neuronal differentiation regulated by LIF or related factors

Two of the key early events in the development of the peripheral nervous system are the proliferation of neural crest precursor cells and their subsequent differentiation into different neural cell types. We present evidence that members of the fibroblast growth factor family, (FGF1 or FGF2) act directly on the neural crest cells in vitro to stimulate proliferation in the presence of serum. These findings correlate with in situ hybridisation analysis, which shows FGF2 mRNA is expressed in cells both in the neural tube and within newly formed sensory ganglia (dorsal root ganglia, DRG) at embryonic day 10 in the mouse, when neural crest precursors are proliferating within the DRG. This data infers an autocrine/paracrine loop for FGF regulation of proliferation. Evidence supporting this notion is provided by the finding that part of the endogenous proliferative activity in the NC cultures is related to FGF. It was also found, in early neural crest cultures, that exogenous FGF completely inhibited neuronal differentiation, probably as a direct consequence of its mitogenic activity. In order to stimulate neuronal differentiation significantly, it was necessary to remove the FGF and replace it with leukemia inhibitory factor (LIF) or related factors. Under these conditions, 50% of the cells differentiated into neurons, which developed a sensory neuron morphology and were immunoreactive for the sensory markers CGRP and substance P. These data support a model of neural crest development, whereby multipotential neural crest precursor cells are stimulated to divide by FGF and subsequent development into sensory neurons is regulated by LIF or other cytokines with a similar signalling mechanism.

Cell type-specific gene expression in the neuroendocrine system. A neuroendocrine-specific regulatory element in the promoter of chromogranin A, a ubiquitous secretory granule core protein

The acidic secretory protein chromogranin A universally occurs in amine and peptide hormone and neurotransmitter storage granules throughout the neuroendocrine system. What factors govern the activity of the chromogranin A gene, to yield such a widespread yet neuroendocrine-selective pattern of expression? To address this question, we isolated the mouse chromogranin A gene promoter. The promoter conferred cell type-specific expression in several neuroendocrine cell types (adrenal medullary chromaffin cells, anterior pituitary corticotropes, and anterior pituitary somatolactotropes) but not in control (fibroblast or kidney) cells. In neuroendocrine cells, analysis of promoter deletions established both positive and negative transcriptional regulatory domains. A distal positive domain (-4.8/-2.2 kbp) was discovered, as well as negative (-258/-181 bp) and positive (-147/-61 bp) domains in the proximate promoter. The proximate promoter contained a minimal neuroendocrine-specific element between -77 and -61 bp. Sequence alignment of the mouse promoter with corresponding regions in rat and bovine clones indicated that the mouse sequence shares over 85% homology with rat and 52% with bovine promoters. DNaseI footprinting and electrophoretic gel mobility shift assays demonstrated the presence of nuclear factors in neuroendocrine cells that recognized the proximate promoter. We conclude that the chromogranin A promoter contains both positive and negative domains governing its cell type-specific pattern of transcription, and that a small proximate region of the promoter, containing novel as well as previously described elements, interacts specifically with neuroendocrine nuclear proteins, and is thereby sufficient to ensure widespread neuroendocrine expression of the gene.