Antigen recognized by monoclonal antibodies to mesencephalic neural crest and to ciliary ganglion neurons is involved in the high affinity choline uptake mechanism in these cells

Cyclical fate restriction: a new view of neural crest cell fate specification

Neural crest cells are crucial in development, not least because of their remarkable multipotency. Early findings stimulated two hypotheses for how fate specification and commitment from fully multipotent neural crest cells might occur, progressive fate restriction (PFR) and direct fate restriction, differing in whether partially restricted intermediates were involved. Initially hotly debated, they remain unreconciled, although PFR has become favoured. However, testing of a PFR hypothesis of zebrafish pigment cell development refutes this view. We propose a novel 'cyclical fate restriction' hypothesis, based upon a more dynamic view of transcriptional states, reconciling the experimental evidence underpinning the traditional hypotheses.

Restricted response of mesencephalic neural crest to sympathetic differentiation signals in the trunk

Lineage diversification in the vertebrate neural crest may occur via instructive signals acting on pluripotent cells, and/or via early specification of subpopulations towards particular lineages. Mesencephalic neural crest cells normally form cholinergic parasympathetic neurons in the ciliary ganglion, while trunk neural crest cells normally form both catecholaminergic and cholinergic neurons in sympathetic ganglia. In contrast to trunk neural crest cells, mesencephalic neural crest cells apparently fail to express the catecholaminergic transcription factor dHAND in response to BMPs in the head environment. Here, we show that migrating quail mesencephalic neural crest cells grafted into the trunk of host chick embryos colonise the sympathetic ganglia. While many express dHAND and form tyrosine hydroxylase (TH)-positive catecholaminergic neurons, the proportion that expresses either dHAND or TH is significantly smaller than that of quail trunk neural crest cells under the same conditions. Furthermore, the proportion of quail mesencephalic neural crest cells that is TH+ in the sympathetic ganglia decreases with time, while the proportion of TH+ quail trunk neural crest-derived cells increases. Thus, a subset of mesencephalic neural crest cells fails to express dHAND or TH in the sympathetic ganglia, while a further subset initiates but fails to maintain TH expression. Taken together, our results suggest that a subpopulation of migrating mesencephalic neural crest cells is refractory to catecholaminergic differentiation signals in the trunk. We suggest that this heterogeneity, together with local signals that repress catecholaminergic differentiation, may ensure that most ciliary neurons adopt a cholinergic fate.

Molecular control of ciliary neuron development: BMPs and downstream transcriptional control in the parasympathetic lineage

The generation of noradrenergic sympathetic neurons is controlled by BMPs and the downstream transcription factors Mash1, Phox2b, Phox2a and dHand. We examined the role of these signals in developing cholinergic parasympathetic neurons. The expression of Mash1 (Cash1), Phox2b and Phox2a in the chick ciliary ganglion is followed by the sequential expression of panneuronal, noradrenergic and cholinergic marker genes. BMPs are expressed at the site where ciliary ganglia form and are essential and sufficient for ciliary neuron development. Unlike sympathetic neurons, ciliary neurons do not express dHand; noradrenergic gene expression is eventually lost but can be maintained by ectopic dHand expression. Together, these results demonstrate a common BMP dependence of sympathetic neurons and parasympathetic ciliary neurons and implicate dHand in the maintenance of noradrenergic gene expression in the autonomic nervous system.

Addition of the BMP4 antagonist, noggin, disrupts avian inner ear development

Bone morphogenetic protein 4 (BMP4) is known to regulate dorsoventral patterning, limb bud formation and axis specification in many organisms, including the chicken. In the chick developing inner ear, BMP4 expression becomes localized in two cell clusters at the anterior and posterior edges of the otic epithelium beginning at stage 16/17 and is expressed in presumptive sensory tissue at later stages. This restricted spatiotemporal pattern of expression occurs just prior to the otocyst's transition to a more complex three-dimensional structure. To further analyze the role of BMP4 in avian otic morphogenesis, cells expressing BMP4 or its antagonist, noggin, were grown on agarose beads and implanted into the periotic mesenchyme surrounding the chick otocyst. Although the BMP4-producing cells had no effect on the mature inner ear structure when implanted alone, noggin-producing cells implanted adjacent to the BMP4 cell foci prevented normal semicircular canal development. Beads implanted at the anterior BMP4 focus eliminated the anterior and/or the horizontal canals. Noggin cells implanted at the posterior focus eliminated the posterior canal. Canal loss was prevented by co-implantation of BMP4 cell beads next to noggin beads. An antibody to the chick hair cell antigen (HCA) was used to examine sensory cell distribution, which was abnormal only in the affected tissues of noggin-exposed inner ears. These data suggest a role for BMP4 in the accurate and complete morphological development of the semicircular canals.

Immortalized cell lines from embryonic avian and murine otocysts: tools for molecular studies of the developing inner ear

Recently, our studies have focused on genes expressed at the earliest stages of inner ear development. Our aim is to identify and characterize genes that are involved in determining the axes of the semicircular canals, in otic crest delamination and in early innervation of the inner ear. Many elegant studies of auditory development have been done in animal models. However, the need for large amounts of well-characterized embryonic material for molecular studies makes the development of otocyst cell lines with different genetic repertoires attractive. We have therefore derived immortalized otocyst cells from two of the most widely used animal models of ear development: avians and mice. Avian cell isolates were produced from quail otocysts (embryonic stage 19) that were transformed with temperature-sensitive variants of the Rous sarcoma virus (RSV). Among the individual transformed cells are those that produce neuron-like derivatives in response to treatment with 10(-9) M retinoic acid. Mammalian cell isolates were derived from otocysts, of 9 day (post coitus) embryos of the H2kbtsA58 transgenic mouse (Immortomouse), which carries a temperature-sensitive variant of the Simian Virus 40 Tumor antigen. The vast majority of cells of the Immortomouse are capable of being immortalized at 33 degrees C, the permissive temperature for transgene expression, in the presence of gamma-interferon. Several putative clones et these cells differentiated into neuron-like cells after temperature shift and withdrawal of gamma-interferon; another isolate of cells assumed a neuron-like morphology on exposure to brain-derived neurotrophic factor even at the permissive temperature. We describe also a cell isolate that expresses the Pax-2 protein product and two putative cell lines that express the protein product of the chicken equivalent of the Drosophila segmentation gene engrailed. These genes and their protein products are expressed in specific subpopulation of otocyst cells at early stages. Both mouse and quail immortalized cell lines will be used to study inner ear development at the molecular level.

Regulation of the early development of the nervous system by growth factors

Development of the nervous system, although patterned by intrinsic genetic expression, appears to be dependent on growth factors for many of the differentiation steps that generate the wide variety of neurons and glia found in the both the central and peripheral nervous system. By using in vitro assays, including clonal analysis, the precise function of the various growth factors and the differentiation potential of the various neural populations has begun to be described. This review discusses some of the recent findings and examines how neuronal differentiation may result from the interaction of several growth factors.

Molecular regulation of neural crest development

The neural crest is a transient embryonic structure that gives rise to a multitude of different cell types in the vertebrate. As such, it is an ideal model to study the processes of vertebrate differentiation and development. This review focuses on two major questions related to neural crest development. The first question concerns the degree and time of commitment of the neural crest cells to different cell lineages and the emerging role of the homeobox containing genes in regulating this process. Evidence from the cephalic crest suggests that the commitment process does start before the neural crest cells migrate away from the neural tube and gene ablation experiments suggest that different homeobox genes are required for the development of neural and mesenchymal tissue derivatives. However, clonal analysis of neural crest cells before migration suggests that many of the cells remain multi-potential indicating that the final determinative steps occur progressively during migration and in association with environmental influences. The second question concerns the nature of the environmental factors that determine the differentiation of neural crest cells into discrete lineages. Evidence is provided, mainly from in vitro experiments, that purified growth factors selectively promote the differentiation of neural crest cells down either sympathetic, adrenal, sensory, or melanocytic cell lineages.

Actions of a monoclonal antibody Tor 23 on rat brain presynaptic cholinergic processes

Tor 23 is a monoclonal antibody, generated against cholinergic terminals of the Torpedo californica, that has been found to bind to the extracellular surface of cholinergic neurons in a variety of tissues. This study shows that Tor 23 inhibits: 1) high affinity [3H]hemicholinium-3 binding to detergent-solubilized membranes prepared from rat neocortices; 2) high affinity [3H]choline uptake in rat neocortical and striatal P2 preparations; and 3) [3H]acetylcholine synthesis in isolated nerve terminals. Tor 23 does not appear to affect low affinity [3H]choline uptake or [3H]acetylcholine release. These results are consistent with the hypothesis that Tor 23 may bind to nerve terminal high affinity choline transporters in the rat brain.

Cell and molecular biology of neural crest cell lineage diversification

Neural crest cells are multipotent progenitor cells, but it is not understood how these cells generate their diverse differentiated progeny. This review considers the issues of whether neural crest cells self-renew, whether they generate partially committed intermediate progenitors, and how the local embryonic environment may act to control this diversification process. Novel molecular markers for neural crest cells are also discussed.

Distribution of pluripotent neural crest cells in the embryo and the role of brain-derived neurotrophic factor in the commitment to the primary sensory neuron lineage

Many early migratory neural crest cells are pluripotent in the sense that their progeny are able to generate more than one differentiated phenotype (Sieber-Blum and Cohen, 1980, Dev. Biol. 80:95-106; Baroffio, Dupin, and Le Douarin, 1988, Proc. Natl. Acad. Sci. USA 85:5325-5329; Bronner-Fraser and Fraser, 1988, Nature 335:161-164; Sieber-Blum, 1989a, Science 243:1608-1611; Ito and Sieber-Blum, 1991, Dev. Biol. 148:95-106). At trunk levels, the neural crest contains two classes (Sieber-Blum and Cohen, 1980) and at posterior rhombencephalic levels, three different classes of pluripotent cells (Ito and Sieber-Blum, 1991). We investigated cell differentiation by in vitro clonal analysis to determine when in development the pool of pluripotent neural crest cells becomes exhausted. The data suggest that different classes of pluripotent cells, precursor cells with more restricted developmental potentials, and apparently committed cells, exist at sites of advanced migration (posterior branchial arches) and even at target sites of neural crest cell differentiation [posterior branchial arches, dorsal root ganglia (DRG), sympathetic ganglia (SG), and epidermal ectoderm]. Some putative classes of pluripotent cells persist well into the second half of embryonic development. These observations have implications for our understanding of the mechanisms that control neural crest cell migration and differentiation. They support the idea that cues originating from the microenvironment affect differentiation of pluripotent neural crest cells. One such signal appears to be brain-derived neurotrophic factor (BDNF). In the presence of BDNF, but not nerve growth factor (NGF), there is a significant increase in the number of neural crest cells per colony that express a sensory neuron-specific marker. Because this increase is not accompanied by a corresponding increase in the total number of cells per colony, this suggests that BDNF plays a role in cell type specification.

Monoclonal antibodies raised against pre-migratory neural crest reveal population heterogeneity during crest development

In order to address the problem of when heterogeneity arises within premigratory and early migratory neural crest cell populations, mouse monoclonal antibodies were raised against quail premigratory neural crest. Due to the limited availability of immunogen an intrasplenic route for immunization was used. Three monoclonal antibodies (referred to as LH2D4, LH5D3 and LH6C2) were subsequently isolated which recognized subpopulations in 24 h cultures of both quail and chick mesencephalic and trunk neural crest in immunocytochemical studies. Subsequent investigations using a range of six antibodies, including LH2D4, LH5D3 and LH6C2, showed that population heterogeneity (which was not cell cycle related) could be detected as early as 15 h following mesencephalic crest explantation, a stage at which all the neural crest cells were morphologically identical. However, premigratory neural crest from the same axial level of origin was homogeneous, as judged by immunoreactivity patterns with these antibodies. Significant differences were found in the proportion of immunoreactive cells between populations of mesencephalic and trunk neural crest cultures. Double immunofluorescence studies revealed the existence of at least four separate cell populations within individual crest cultures, each identified by their unique antibody reactivity pattern, thus providing some insight into the underlying complexity of subpopulation composition within the neural crest. Immunocytochemical studies on quail embryos from stages 7-22 showed that the epitopes detected by LH2D4, LH5D3 and LH6C2 were not necessarily confined to the neural crest or to cells of crest derivation. All three epitopes displayed a spatiotemporal regulation in their expression during early avian ontogeny. Since the differential epitope expression described in this investigation was detectable as early as 15 h after premigratory neural crest explantation, took place in vitro in the absence of any other cell type and changed progressively with time, we conclude that a certain degree of population heterogeneity can be generated very early in neural crest ontogeny and independently of the tissue interactions that normally ensue in vivo.

Partial restriction in the developmental potential of late emigrating avian neural crest cells

Trunk neural crest cells migrate along two major pathways: a ventral pathway through the somites whose cells form neuronal derivatives and dorsolateral pathway underneath the ectoderm whose cells become pigmented. In avian embryos, the latest emigrating neural crest cells move only along the dorsolateral pathway. To test whether late emigrating neural crest cells are more restricted in developmental potential than early migrating cells, cultures were prepared from the neural tubes of embryos at various stages of neural crest cell migration. "Early" and "middle" aged neural crest cells differentiated into many derivatives including pigmented cells, neurofilament-immunoreactive cells, and adrenergic cells. In contrast, "late" neural crest cells differentiated into pigment cells and neurofilament-immunoreactive cells, but not into adrenergic cells even after 10-14 days. To further challenge the developmental potential of early and late emigrating neural crest cells, they were transplanted into embryos during the early phases of neural crest cell migration, known to be permissive for adrenergic neuronal differentiation. The cells were labeled with the vital dye, DiI, and injected onto the ventral pathway at stages 14-17. Two and three days after injection, some early neural crest cells were found to express catecholamines, suggesting they were adrenergic neuroblasts. In contrast, DiI-labeled late neural crest cells never became catecholamine-positive. These results suggest that the late emigrating neural crest cell population has a more restricted developmental potential than the early migrating neural crest cell population.

Co-expression of multiple neurotransmitter enzyme genes in normal and immortalized sympathoadrenal progenitor cells

We have examined the expression of mRNAs encoding five major neurotransmitter-synthesizing enzymes in MAH cells, a clonal cell line derived by retroviral immortalization of a rat embryonic sympathoadrenal progenitor cell. These mRNAs include tyrosine hydroxylase (TH), choline acetyltransferase (ChAT), tryptophan hydroxylase (TpH), and glutamic acid decarboxylases (GADs) 1 and 2. We find that MAH cells express high levels of TH mRNA and low levels of ChAT and TpH mRNAs. Neither GAD1 nor GAD2 mRNAs are detectable using an RNase protection assay with a detection limit of less than one transcript per cell. A similar pattern of mRNA expression is observed in postnatal superior cervical ganglia, adrenal medulla, and in PC12 cells. Transmitter synthesis and accumulation assays indicate that MAH cells can synthesize both catecholamines and acetylcholine. Thus the TH and ChAT mRNAs detected in these cells are likely to be translated into active enzyme. To corroborate these data obtained using MAH cells, we performed similar transmitter synthesis and accumulation assays on sympathoadrenal progenitors directly isolated from E14.5 fetal adrenal glands by fluorescence-activated cell sorting. These progenitor cells also synthesize and accumulate both catecholamines and acetylcholine, albeit to different extents than MAH cells. Both MAH cells and their nonimmortal counterparts are able to increase slightly their cholinergic function upon short-term exposure to CDF/LIF, a factor known to induce acetylcholine synthesis in postmitotic sympathetic neurons. Taken together, these data suggest that progenitor cells in the sympathoadrenal lineage acquire the ability to simultaneously transcribe several different neurotransmitter enzyme genes early in development, prior to their choice of final cell fate. At the same time, the progenitors possess receptors which regulate expression of these genes in response to environmental factors. This ability may permit the cells to choose from several different transmitter phenotypes in response to different environments, as they migrate through the embryo. The persistent transcription of these genes in adult cells, moreover, may in part account for the phenotypic plasticity of cells in this lineage.

In vitro clonal analysis of progenitor cell patterns in dorsal root and sympathetic ganglia of the quail embryo

The possible presence of pluripotent cells in dorsal root (DRG) and sympathetic ganglia (SG) of the quail embryo has been investigated by in vitro clonal analysis. Both types of ganglia originate from the neural crest. At, or soon after, initiation of emigration from the neural tube, the neural crest appears as a mixed population of pluripotent cells and cells with more restricted developmental capacities. In the present study it was determined that pluripotent cells and precursor cells with restricted developmental potentials are also present in early DRG and SG, and that their proportions change with progressing age of the embryo. As in the neural crest, cells in one class are at least tripotent, able to give rise to pigment cells, sensory neurons, and cells in the sympathoadrenal lineage. Cells in the other class appear to have lost the melanogenic potential, but generate cells in the sensory neuron and sympathoadrenal lineages. In addition, there are two types of cells that seem to be committed to the melanogenic and sensory neuron lineages, respectively. Apparently committed melanogenic cells within the DRG are not detected after the first third of embryonic development, whereas precursor cells that are at least bipotent and generate both types of neurons persist in both DRG and SG at least through the first half of embryonic development. Neurogenic cells that are apparently committed to the sensory neuron or sympathoadrenal lineages were observed in the appropriate type of ganglion only, suggesting that location-specific cues influence the choice of phenotype generated by pluripotent neural crest cells.

Role of the neurotrophic factors BDNF and NGF in the commitment of pluripotent neural crest cells

Since trophic factors are increasingly recognized as playing a role in some decision-making steps during development, the influence of brain-derived neurotrophic factor (BDNF) and nerve growth factor (NGF) on the commitment of pluripotent neural crest cells was investigated by in vitro clonal analysis. BDNF caused an increase of up to 21-fold in the number of sensory neuron precursors per colony without a corresponding increase in the total number of cells. By contrast, BDNF treatment caused an equivalent decrease in the number of undifferentiated cells per colony. The data suggest that BDNF, but not NGF, directs pluripotent neural crest cells to differentiate along the primary sensory neuron lineage.

Differential development of cholinergic neurons from cranial and trunk neural crest cells in vitro

Several studies have suggested that the development of cholinergic properties in cranial parasympathetic neurons is determined by these cells' axial level of origin in the neural crest. All cranial parasympathetic neurons normally derive from cranial neural crest. Trunk neural crest cells give rise to sympathetic neurons, most of which are noradrenergic. To determine if there is an intrinsic difference in the ability of cranial and trunk neural crest cells to form cholinergic neurons, we have compared the development of choline acetyltransferase (ChAT)-immunoreactive cells in explants of quail cranial and trunk neural crest in vitro. Both cranial and trunk neural crest explants gave rise to ChAT-immunoreactive cells in vitro. In both types of cultures, some of the ChAT-positive cells also expressed immunoreactivity for the catecholamine synthetic enzyme tyrosine hydroxylase. However, several differences were seen between cranial and trunk cultures. First, ChAT-immunoreactive cells appeared two days earlier in cranial than in trunk cultures. Second, cranial cultures contained a higher proportion of ChAT-immunoreactive cells. Finally, a subpopulation of the ChAT-immunoreactive cells in cranial cultures exhibited neuronal traits, including neurofilament immunoreactivity. In contrast, neurofilament-immunoreactive cells were not seen in trunk cultures. These results suggest that premigratory cranial and trunk neural crest cells differ in their ability to form cholinergic neurons.

Development of cholinergic traits in the quail ciliary ganglion: expression of choline acetyltransferase-like immunoreactivity

The avian ciliary ganglion is a parasympathetic ganglion derived from the neural crest whose neurons provide cholinergic innervation to the eye. Here, we describe the time course of appearance and the morphology of cholinergic cells in the ciliary ganglion, as assessed by antibodies against choline acetyltransferase. Choline acetyltransferase immunoreactivity was first observed in 5.5-day-old quail embryos, 1 day after condensation of the ciliary ganglion. Both the intensity of choline acetyltransferase immunoreactivity and size of the choline acetyltransferase-immunoreactive cells increased with ganglionic age. By 12 days, a second population of choline acetyltransferase-immunoreactive cells, possibly corresponding to choroid neurons, was observed whose cells were smaller and less intensely stained than earlier differentiating choline acetyltransferase-immunoreactive cells. The percentage of choline acetyltransferase-immunoreactive cells was initially high, constituting approximately 50% of the total cell population. As a function of time, the proportion of cholinergic cells decreased, probably due to proliferation of non-neuronal cells and naturally-occurring cell death. Our results confirm the existence of two morphologically distinct populations of cholinergic neurons in the avian ciliary ganglion and demonstrate that these neuronal subpopulations express choline acetyltransferase immunoreactivity at different times in development. Because choroid neurons innervate their targets later than ciliary neurons, this finding is consistent with the hypothesis that target interactions regulate expression of choline acetyltransferase.

Culture conditions affect the cholinergic development of an isolated subpopulation of chick mesencephalic neural crest cells

Although neural crest cells are known to be very responsive to environmental cues during their development, recent evidence indicates that at least some subpopulations may be committed to a specific differentiation program prior to migration. Because the neural crest is composed of a heterogeneous mixture of cells that contributes to many vertebrate cell lineages, assessing the properties of specific subpopulations and the effect of the environment on their development has been difficult. To address this problem, we have isolated a pure subpopulation of chick mesencephalic neural crest cells by fluorescence no-flow cytometry after labeling them with monoclonal antibodies (Mabs) to a 75-kDa cell surface antigen that is associated with high affinity choline uptake. When cultures of chick mesencephalic neural crest cells are labeled with these Mabs and a fluorescent second step antibody, approximately 5% of the cells are antigen-positive (A+). After sorting, 100% of the resulting cultured mesencephalic neural crest cells are A+. The Mabs we used also label all of the neurons of the embryonic chick and quail ciliary ganglion in vivo and in vitro. We have compared the effect of various cell culture media on the isolated neural crest subpopulation and the heterogeneous chick mesencephalic neural crest from which it was derived. A+ cells were passaged and grown in a variety of media, each of which differently affected its characteristics and development. A+ cells proliferated in the presence of 15% fetal bovine serum (FBS) and high concentrations (10-15%) of chick embryo extract, but did not differentiate, although they retained basal levels of choline acetyltransferase (ChAT) activity. However, in chick serum and high (25 mM as opposed to 7 mM) K+, and heart-, iris-, or lung-conditioned medium, all of which are known to promote survival and/or cholinergic development of ciliary ganglion neurons, the cells ceased to proliferate and all of the cells in the culture became "neuron-like" within 10 days. No neuron-like cells were found in liver-, notocord-, or neural tube-conditioned media if FBS was used. When A+ cells were eliminated either by complement-mediated cytotoxicity or by laser-ablating A+ cells during no-flow cytometry, all ChAT activity was also eliminated, and no neuron-like cells or ChAT activity was found in cultures during a subsequent 3-week culture period.(ABSTRACT TRUNCATED AT 400 WORDS)