Mitogenic and anti-proliferative signals for neural crest cells and the neurogenic action of TGF-beta1

Characterization of runs of homozygosity islands in American mink using whole-genome sequencing data

The genome-wide analysis of runs of homozygosity (ROH) islands can be an effective strategy for identifying shared variants within a population and uncovering important genomic regions related to complex traits. The current study performed ROH analysis to characterize the genome-wide patterns of homozygosity, identify ROH islands and annotated genes within these candidate regions using whole-genome sequencing data from 100 American mink (Neogale vison). After sequence processing, variants were called using GATK and Samtools pipelines. Subsequent to quality control, 8,373,854 bi-allelic variants identified by both pipelines remained for further analysis. A total of 34,652 ROH segments were identified in all individuals, among which shorter segments (0.3-1 Mb) were abundant throughout the genome, approximately accounting for 84.39% of all ROH. Within these segments, we identified 63 ROH islands housing 156 annotated genes. The genes located in ROH islands were associated with fur quality (EDNRA, FGF2, FOXA2 and SLC24A4), body size/weight (MYLK4, PRIM2, FABP2, EYS and PHF3), immune capacity (IL2, IL21, PTP4A1, SEMA4C, JAK2, CCNA2 and TNIP3) and reproduction (ADAD1, KHDRBS2, INSL6, PGRMC2 and HSPA4L). Furthermore, Gene Ontology and KEGG pathway enrichment analyses revealed 56 and 9 significant terms (FDR-corrected p-value < 0.05), respectively, among which cGMP-PKG signalling pathway, regulation of actin cytoskeleton, and calcium signalling pathway were highlighted due to their functional roles in growth and fur characteristics. This is the first study to present ROH islands in American mink. The candidate genes from ROH islands and functional enrichment analysis suggest possible signatures of selection in response to the mink breeding targets, such as increased body length, reproductive performance and fur quality. These findings contribute to our understanding of genetic characteristics, and provide complementary information to assist with implementation of breeding strategies for genetic improvement in American mink.

Thermosensitive collagen/fibrinogen gels loaded with decorin suppress lesion site cavitation and promote functional recovery after spinal cord injury

The treatment of spinal cord injury (SCI) is a complex challenge in regenerative medicine, complicated by the low intrinsic capacity of CNS neurons to regenerate their axons and the heterogeneity in size, shape and extent of human injuries. For example, some contusion injuries do not compromise the dura mater and in such cases implantation of preformed scaffolds or drug delivery systems may cause further damage. Injectable in situ thermosensitive scaffolds are therefore a less invasive alternative. In this study, we report the development of a novel, flowable, thermosensitive, injectable drug delivery system comprising bovine collagen (BC) and fibrinogen (FB) that forms a solid BC/FB gel (Gel) immediately upon exposure to physiological conditions and can be used to deliver reparative drugs, such as the naturally occurring anti-inflammatory, anti-scarring agent Decorin, into adult rat spinal cord lesion sites. In dorsal column lesions of adult rats treated with the Gel + Decorin, cavitation was completely suppressed and instead lesion sites became filled with injury-responsive cells and extracellular matrix materials, including collagen and laminin. Decorin increased the intrinsic potential of dorsal root ganglion neurons (DRGN) by increasing their expression of regeneration associated genes (RAGs), enhanced local axon regeneration/sprouting, as evidenced both histologically and by improved electrophysiological, locomotor and sensory function recovery. These results suggest that this drug formulated, injectable hydrogel has the potential to be further studied and translated into the clinic.

Insights Into the Early Gene Regulatory Network Controlling Neural Crest and Placode Fate Choices at the Neural Border

The neural crest (NC) cells and cranial placodes are two ectoderm-derived innovations in vertebrates that led to the acquisition of a complex head structure required for a predatory lifestyle. They both originate from the neural border (NB), a portion of the ectoderm located between the neural plate (NP), and the lateral non-neural ectoderm. The NC gives rise to a vast array of tissues and cell types such as peripheral neurons and glial cells, melanocytes, secretory cells, and cranial skeletal and connective cells. Together with cells derived from the cranial placodes, which contribute to sensory organs in the head, the NC also forms the cranial sensory ganglia. Multiple in vivo studies in different model systems have uncovered the signaling pathways and genetic factors that govern the positioning, development, and differentiation of these tissues. In this literature review, we give an overview of NC and placode development, focusing on the early gene regulatory network that controls the formation of the NB during early embryonic stages, and later dictates the choice between the NC and placode progenitor fates.

FGF2 Stimulates the Growth and Improves the Melanocytic Commitment of Trunk Neural Crest Cells

Neural crest cells (NCCs) comprise a population of multipotent progenitors and stem cells at the origin of the peripheral nervous system (PNS) and melanocytes of skin, which are profoundly influenced by microenvironmental factors, among which is basic fibroblast growth factor 2 (FGF2). In this work, we further investigated the role of this growth factor in quail trunk NC morphogenesis and demonstrated its huge effect in NCC growth mainly by stimulating cell proliferation but also reducing cell death, despite that NCC migration from the neural tube explant was not affected. Moreover, following FGF2 treatment, reduced expression of the early NC markers Sox10 and FoxD3 and improved proliferation of HNK1-positive NCC were observed. Since these markers are involved in the regulation of glial and melanocytic fate of NC, the effect of FGF2 on NCC differentiation was investigated. Therefore, in the presence of FGF2, increased proportions of NCCs positives to the melanoblast marker Mitf as well as NCCs double stained to Mitf and BrdU were recorded. In addition, treatment with FGF2, followed by differentiation medium, resulted in increased expression of melanin and improved proportion of melanin-pigmented melanocytes without alteration in the glial marker Schwann myelin protein (SMP). Taken together, these data further reveal the important role of FGF2 in NCC proliferation, survival, and differentiation, particularly in melanocyte development. This is the first demonstration of FGF2 effects in melanocyte commitment of NC and in the proliferation of Mitf-positive melanoblasts. Elucidating the differentiation process of embryonic NCCs brings us a step closer to understanding the development of the PNS and then undertaking the search for advanced technologies to prevent, or treat, injuries caused by NC-related disorders, also known as neurocristopathies.

Parental-reported neurodevelopmental issues in Loeys-Dietz syndrome

BACKGROUND

Loeys-Dietz syndrome (LDS) is a congenital multisystem disorder affecting the cardiovascular and musculoskeletal system. Limited data have reported neurodevelopmental (ND) issues in LDS.

AIMS

To determine the extent of ND issues in patients with LDS.

METHODS

A prospective study was performed of LDS patients or their caregivers. The study included data collected via an online survey of age-specific questions. Standard statistical methods were used for baseline and demographic characteristics, as well as group comparisons.

OUTCOMES

Data were obtained from 67 patients with LDS (54% female). Median age was 14.9 years. Gene mutations included TGFBR1 (39%), TGFBR2 (40%), SMAD3 (7%), and unknown (14%). Motor delays (30%, 18/61) and hypotonia (63%, 37/60) occurred frequently. Physical (62%, 39/62), occupational (41%, 23/56), and speech therapies (34%, 20/58) were common. Feeding issues were common (41%, 23/56). TGFBR1 mutations were more frequent among those with motor delays and feeding issues.

CONCLUSIONS

Patients with LDS and/or their caregivers report at least one ND problem in most cases, and many require therapies. These data suggest ND disorders should be considered to be part of the phenotype.

A Trematode Parasite Derived Growth Factor Binds and Exerts Influences on Host Immune Functions via Host Cytokine Receptor Complexes

The trematode Fasciola hepatica is responsible for chronic zoonotic infection globally. Despite causing a potent T-helper 2 response, it is believed that potent immunomodulation is responsible for rendering this host reactive non-protective host response thereby allowing the parasite to remain long-lived. We have previously identified a growth factor, FhTLM, belonging to the TGF superfamily can have developmental effects on the parasite. Herein we demonstrate that FhTLM can exert influence over host immune functions in a host receptor specific fashion. FhTLM can bind to receptor members of the Transforming Growth Factor (TGF) superfamily, with a greater affinity for TGF-β RII. Upon ligation FhTLM initiates the Smad2/3 pathway resulting in phenotypic changes in both fibroblasts and macrophages. The formation of fibroblast CFUs is reduced when cells are cultured with FhTLM, as a result of TGF-β RI kinase activity. In parallel the wound closure response of fibroblasts is also delayed in the presence of FhTLM. When stimulated with FhTLM blood monocyte derived macrophages adopt an alternative or regulatory phenotype. They express high levels interleukin (IL)-10 and arginase-1 while displaying low levels of IL-12 and nitric oxide. Moreover they also undergo significant upregulation of the inhibitory receptor PD-L1 and the mannose receptor. Use of RNAi demonstrates that this effect is dependent on TGF-β RII and mRNA knock-down leads to a loss of IL-10 and PD-L1. Finally, we demonstrate that FhTLM aids newly excysted juveniles (NEJs) in their evasion of antibody-dependent cell cytotoxicity (ADCC) by reducing the NO response of macrophages—again dependent on TGF-β RI kinase. FhTLM displays restricted expression to the F. hepatica gut resident NEJ stages. The altered fibroblast responses would suggest a role for dampened tissue repair responses in facilitating parasite migration. Furthermore, the adoption of a regulatory macrophage phenotype would allow for a reduced effector response targeting juvenile parasites which we demonstrate extends to an abrogation of the ADCC response. Thus suggesting that FhTLM is a stage specific evasion molecule that utilises host cytokine receptors. These findings are the first to clearly demonstrate the interaction of a helminth cytokine with a host receptor complex resulting in immune modifications that facilitate the non-protective chronic immune response which is characteristic of F. hepatica infection.

Parasitic worms, helminths, can cause long-lived chronic infection in many hosts that they infection. The liver fluke, Fasciola hepatica, is one such parasite causing global infection of both humans and animals. F. hepatica exerts an influence over the immune system such that it avoids effector mechanisms and prevents the development of effective immunity. Here we characterise a molecule—FhTLM—derived from juvenile parasites that is similar to the regulatory cytokine TGF-β. We show that FhTLM will bind to host TGF-β receptors with a reduced affinity when compared with mammalian TGF-β. Despite this FhTLM can induce Smad2/3 signalling in host leukocytes, which is key to initiating gene transcription. Phenotypically FhTLM causes fibroblasts to slow their growth and replication response resulting in slower wound healing. Importantly FhTLM induces a macrophage phenotype that resembles a regulatory macrophage phenotype identified in other species undergoing helminth infection. Finally we Our work highlights the potential of FhTLM to play important roles in controlling host immunity when initially infected with juvenile parasites, thereby preventing the development of effective immunity.

Effects of Transforming Growth Factor Beta 1 in Cerebellar Development: Role in Synapse Formation

Granule cells (GC) are the most numerous glutamatergic neurons in the cerebellar cortex and represent almost half of the neurons of the central nervous system. Despite recent advances, the mechanisms of how the glutamatergic synapses are formed in the cerebellum remain unclear. Among the TGF-β family, TGF-beta 1 (TGF-β1) has been described as a synaptogenic molecule in invertebrates and in the vertebrate peripheral nervous system. A recent paper from our group demonstrated that TGF-β1 increases the excitatory synapse formation in cortical neurons. Here, we investigated the role of TGF-β1 in glutamatergic cerebellar neurons. We showed that the expression profile of TGF-β1 and its receptor, TβRII, in the cerebellum is consistent with a role in synapse formation in vitro and in vivo. It is low in the early postnatal days (P1–P9), increases after postnatal day 12 (P12), and remains high until adulthood (P30). We also found that granule neurons express the TGF-β receptor mRNA and protein, suggesting that they may be responsive to the synaptogenic effect of TGF-β1. Treatment of granular cell cultures with TGF-β1 increased the number of glutamatergic excitatory synapses by 100%, as shown by immunocytochemistry assays for presynaptic (synaptophysin) and post-synaptic (PSD-95) proteins. This effect was dependent on TβRI activation because addition of a pharmacological inhibitor of TGF-β, SB-431542, impaired the formation of synapses between granular neurons. Together, these findings suggest that TGF-β1 has a specific key function in the cerebellum through regulation of excitatory synapse formation between granule neurons.

Differentiation of human epidermal neural crest stem cells (hEPI-NCSC) into virtually homogenous populations of dopaminergic neurons

Here we provide a protocol for the directed differentiation of hEPI-NCSC into midbrain dopaminergic neurons, which degenerate in Parkinson's disease. hEPI-NCSC are neural crest-derived multipotent stem cells that persist into adulthood in the bulge of hair follicles. The experimental design is distinctly different from conventional protocols for embryonic stem cells and induced pluripotent stem (iPS) cells. It includes pre-differentiation of the multipotent hEPI-NCSC into neural stem cell-like cells, followed by ventralizing, patterning, continued exposure to the TGFβ receptor inhibitor, SB431542, and at later stages of differentiation the presence of the WNT inhibitor, IWP-4. All cells expressed A9 midbrain dopaminergic neuron progenitor markers with gene expression levels comparable to those in normal human substantia nigra. The current study shows for the first time that virtually homogeneous populations of dopaminergic neurons can be derived ex vivo from somatic stem cells without the need for purification, with useful timeliness and high efficacy. This novel development is an important first step towards the establishment of fully functional dopaminergic neurons from an ontologically relevant stem cell type, hEPI-NCSC.

The neuroprotective functions of transforming growth factor beta proteins

Transforming growth factor beta (TGF-β) proteins are multifunctional cytokines whose neural functions are increasingly recognized. The machinery of TGF-β signaling, including the serine kinase type transmembrane receptors, is present in the central nervous system. However, the 3 mammalian TGF-β subtypes have distinct distributions in the brain suggesting different neural functions. Evidence of their involvement in the development and plasticity of the nervous system as well as their functions in peripheral organs suggested that they also exhibit neuroprotective functions. Indeed, TGF-β expression is induced following a variety of types of brain tissue injury. The neuroprotective function of TGF-βs is most established following brain ischemia. Damage in experimental animal models of global and focal ischemia was shown to be attenuated by TGF-βs. In addition, support for their neuroprotective actions following trauma, sclerosis multiplex, neurodegenerative diseases, infections, and brain tumors is also accumulating. The review will also describe the potential mechanisms of neuroprotection exerted by TGF-βs including anti-inflammatory, -apoptotic, -excitotoxic actions as well as the promotion of scar formation, angiogenesis, and neuroregeneration. The participation of these mechanisms in the neuroprotective effects of TGF-βs during different brain lesions will also be discussed.

Epidermal neural crest stem cell (EPI-NCSC)--mediated recovery of sensory function in a mouse model of spinal cord injury

Here we show that epidermal neural crest stem cell (EPI-NCSC) transplants in the contused spinal cord caused a 24% improvement in sensory connectivity and a substantial recovery of touch perception. Furthermore we present a novel method for the ex vivo expansion of EPI-NCSC into millions of stem cells that takes advantage of the migratory ability of neural crest stem cells and is based on a new culture medium and the use of microcarriers. Functional improvement was shown by two independent methods, spinal somatosensory evoked potentials (SpSEP) and the Semmes-Weinstein touch test. Subsets of transplanted cells differentiated into myelinating oligodendrocytes. Unilateral injections of EPI-NCSC into the lesion of midline contused mouse spinal cords elicited bilateral improvements. Intraspinal EPI-NCSC did not migrate laterally in the spinal cord or invade the spinal roots and dorsal root ganglia, thus implicating diffusible factors. EPI-NCSC expressed neurotrophic factors, angiogenic factors, and metalloproteases. The strength of EPI-NCSC thus is that they can exert a combination of pertinent functions in the contused spinal cord, including cell replacement, neuroprotection, angiogenesis and modulation of scar formation. EPI-NCSC are uniquely qualified for cell-based therapy in spinal cord injury, as neural crest cells and neural tube stem cells share a higher order stem cell and are thus ontologically closely related.

Prenatal alcohol exposure triggers ceramide-induced apoptosis in neural crest-derived tissues concurrent with defective cranial development

Fetal alcohol syndrome (FAS) is caused by maternal alcohol consumption during pregnancy. The reason why specific embryonic tissues are sensitive toward ethanol is not understood. We found that in neural crest-derived cell (NCC) cultures from the first branchial arch of E10 mouse embryos, incubation with ethanol increases the number of apoptotic cells by fivefold. Apoptotic cells stain intensely for ceramide, suggesting that ceramide-induced apoptosis mediates ethanol damage to NCCs. Apoptosis is reduced by incubation with CDP-choline (citicoline), a precursor for the conversion of ceramide to sphingomyelin. Consistent with NCC cultures, ethanol intubation of pregnant mice results in ceramide elevation and increased apoptosis of NCCs in vivo. Ethanol also increases the protein level of prostate apoptosis response 4 (PAR-4), a sensitizer to ceramide-induced apoptosis. Prenatal ethanol exposure is concurrent with malformation of parietal bones in 20% of embryos at day E18. Meninges, a tissue complex derived from NCCs, is disrupted and generates reduced levels of TGF-β1, a growth factor critical for bone and brain development. Ethanol-induced apoptosis of NCCs leading to defects in the meninges may explain the simultaneous presence of cranial bone malformation and cognitive retardation in FAS. In addition, our data suggest that treatment with CDP-choline may alleviate the tissue damage caused by alcohol.

Functional neurons and melanocytes induced from immortal lines of postnatal neural crest-like stem cells

Stem cells, that is, cells that can both reproduce themselves and differentiate into functional cell types, attract much interest as potential aids to healing and disease therapy. Embryonic neural crest is pluripotent and generates the peripheral nervous system, melanocytes, and some connective tissues. Neural-crest-related stem cells have been reported previously in postnatal skin: committed melanocytic stem cells in the hair follicle, and pluripotent cell types from the hair follicle and papilla that can produce various sets of lineages. Here we describe novel pluripotent neural crest-like stem cells from neonatal mouse epidermis, with different potencies, isolated as 3 independent immortal lines. Using alternative regulatory factors, they could be converted to large numbers of either Schwann precursor cells, pigmented melanocytes, chondrocytes, or functional sensory neurons showing voltage-gated sodium channels. Some of the neurons displayed abundant active TRPV1 and TRPA1 receptors. Such functional neurons have previously been obtained in culture only with difficulty, by explantation. The system was also used to generate comparative gene expression data for the stem cells, melanocytes, and melanoblasts that sufficiently explain the lack of pigment in melanoblasts and provide a rationale for some genes expressed apparently ectopically in melanomas, such as ephrin receptors.—Sviderskaya, E. V., Easty, D. J., Lawrence, M. A., Sánchez, D. P., Negulyaev, Y. A., Patel, R. H., Anand, P., Korchev, Y. E., Bennett, D. C. Functional neurons and melanocytes induced from immortal lines of postnatal neural crest-like stem cells.

Norepinephrine transport-mediated gene expression in noradrenergic neurogenesis

Background

We have identified a differential gene expression profile in neural crest stem cells that is due to deletion of the norepinephrine transporter (NET) gene. NET is the target of psychotropic substances, such as tricyclic antidepressants and the drug of abuse, cocaine. NET mutations have been implicated in depression, anxiety, orthostatic intolerance and attention deficit hyperactivity disorder (ADHD). NET function in adult noradrenergic neurons of the peripheral and central nervous systems is to internalize norepinephrine from the synaptic cleft. By contrast, during embryogenesis norepinephrine (NE) transport promotes differentiation of neural crest stem cells and locus ceruleus progenitors into noradrenergic neurons, whereas NET inhibitors block noradrenergic differentiation. While the structure of NET und the regulation of NET function are well described, little is known about downstream target genes of norepinephrine (NE) transport.

Results

We have prepared gene expression profiles of in vitro differentiating wild type and norepinephrine transporter-deficient (NETKO) mouse neural crest cells using long serial analysis of gene expression (LongSAGE). Comparison analyses have identified a number of important differentially expressed genes, including genes relevant to neural crest formation, noradrenergic neuron differentiation and the phenotype of NETKO mice. Examples of differentially expressed genes that affect noradrenergic cell differentiation include genes in the bone morphogenetic protein (BMP) signaling pathway, the Phox2b binding partner Tlx2, the ubiquitin ligase Praja2, and the inhibitor of Notch signaling, Numbl. Differentially expressed genes that are likely to contribute to the NETKO phenotype include dopamine-β-hydroxylase (Dbh), tyrosine hydroxylase (Th), the peptide transmitter 'cocaine and amphetamine regulated transcript' (Cart), and the serotonin receptor subunit Htr3a. Real-time PCR confirmed differential expression of key genes not only in neural crest cells, but also in the adult superior cervical ganglion and locus ceruleus. In addition to known genes we have identified novel differentially expressed genes and thus provide a valuable database for future studies.

Conclusion

Loss of NET function during embryonic development in the mouse deregulates signaling pathways that are critically involved in neural crest formation and noradrenergic cell differentiation. The data further suggest deregulation of signaling pathways in the development and/or function of the NET-deficient peripheral, central and enteric nervous systems.

Neural stem cells, inflammation and NF-kappaB: basic principle of maintenance and repair or origin of brain tumours?

Several recent reports suggest that inflammatory signals play a decisive role in the self-renewal, migration and differentiation of multipotent neural stem cells (NSCs). NSCs are believed to be able to ameliorate the symptoms of several brain pathologies through proliferation, migration into the area of the lesion and either differentiation into the appropriate cell type or secretion of anti-inflammatory cytokines. Although NSCs have beneficial roles, current evidence indicates that brain tumours, such as astrogliomas or ependymomas are also caused by tumour-initiating cells with stem-like properties. However, little is known about the cellular and molecular processes potentially generating tumours from NSCs. Most pro-inflammatory conditions are considered to activate the transcription factor NF-kappaB in various cell types. Strong inductive effects of NF-kappaB on proliferation and migration of NSCs have been described. Moreover, NF-kappaB is constitutively active in most tumour cells described so far. Chronic inflammation is also known to initiate cancer. Thus, NF-kappaB might provide a novel mechanistic link between chronic inflammation, stem cells and cancer. This review discusses the apparently ambivalent role of NF-kappaB: physiological maintenance and repair of the brain via NSCs, and a potential role in tumour initiation. Furthermore, it reveals a possible mechanism of brain tumour formation based on inflammation and NF-kappaB activity in NSCs.

Dedicated epithelial recipient cells determine pigmentation patterns

Mammals generate external coloration via dedicated pigment-producing cells but arrange pigment into patterns through mechanisms largely unknown. Here, using mice as models, we show that patterns ultimately emanate from dedicated pigment-receiving cells. These pigment recipients are epithelial cells that recruit melanocytes to their position in the skin and induce the transfer of melanin. We identify Foxn1 (a transcription factor) as an activator of this "pigment recipient phenotype" and Fgf2 (a growth factor and Foxn1 target) as a signal released by recipients. When Foxn1 - and thus dedicated recipients - are redistributed in the skin, new patterns of pigmentation develop, suggesting a mechanism for the evolution of coloration. We conclude that recipients provide a cutaneous template or blueprint that instructs melanocytes where to place pigment. As Foxn1 and Fgf2 also modulate epithelial growth and differentiation, the Foxn1 pathway should serve as a nexus coordinating cell division, differentiation, and pigmentation.

Spatial and Temporal Expression of Perlecan in the Early Chick Embryo

Perlecan is a major heparan sulfate proteoglycan that binds growth factors and interacts with various extracellular matrix proteins and cell surface molecules. The expression and spatiotemporal distribution of perlecan was studied by RT-PCR, immunoprecipitation and immunofluorescence in the chick embryo from stages X (morula) to HH17 (29 somites). Combined RT-PCR and immunohistochemistry demonstrated the expression of perlecan as early as stage X and its presence may be fundamental to the first basement membrane assembly on the epiblast ventral surface at stage XIII (blastula). Perlecan fluorescence was intense in the cells ingressing through the primitive streak and was strong lining the epiblast ventral surface lateral to the streak at stage HH3-4 (gastrula). At stage HH5-6 (neurula), perlecan fluorescence was low in the neuroepithelium and stronger in the apical surface of the neural plate. At stage HH10-11 (12 somites), perlecan fluorescence was intense in the neuroepithelium and was then essentially nondetectable in the neuroepithelium, and the intensity had shifted to the basement membranes of encephalic vesicles by stage HH17. Perlecan immunofluorescence was intense in neural crest cells, strong in pharyngeal arches, intense in thymus and lung rudiments, intense in aortic arches and in dorsal aorta, strong in lens and retina and intense in intraretinal space and in optic stalk, strong in the dorsal mesocardium, myocardium and endocardium, strong in dermomyotome, low in sclerotome in somites, intense in mesonephric duct and tubule rudiments, intense in the lining of the gut luminal surface. Inhibition of the function of perlecan by blocking antibodies showed that perlecan is crucial for maintaining basement membrane integrity which mediates the epithelialization, adhesive separation and maintenance of neuroepithelium in brain, somite epithelialization, and tissue architecture during morphogenesis of the heart tube, dorsal aorta and gut. An intriguing possibility is that perlecan, as a signaling molecule that modulates the activity of growth factors and cytokines, participates in the signaling pathways that guide gastrulation movements and neural crest cell migration, proliferation and survival, cardiac cell proliferation and paraxial mesoderm (somitic) cell proliferation and segmentation.

Insulin-like growth factor-1 is an endogenous mediator of focal ischemia-induced neural progenitor proliferation

The adult mammalian brain contains resident neural progenitors in the subgranular zone of the dentate gyrus (DG) and the subventricular zone (SVZ) of the lateral ventricles. The proliferation of neural progenitors increases after focal cerebral ischemia in both of these regions, but the mechanisms that promote ischemia-induced neural progenitor proliferation are not yet understood. We hypothesize that diffusible factors from the ischemic area play a role in this process as the DG is remote from the area of infarction. In this study, we observed that the peak of neural progenitor proliferation in the ipsilateral DG was between day 2 and day 4 of reperfusion after transient middle cerebral artery occlusion in adult spontaneously hypertensive rats. GeneChip and real-time PCR analysis showed a three- to 102-fold increase in the expression of 15 diffusible, mitogenic factors in the ischemic cortex at 3 days of reperfusion. Of these, insulin-like growth factor-1 (IGF-1) showed increased protein expression in the activated astrocytes in the ischemic penumbra. In addition, the progenitors in both the SVZ and DG showed IGF-1 receptor expression. Inhibiting IGF-1 activity by introcerebroventricular infusion of IGF-1 antibody significantly prevented the ischemia-induced neural progenitor proliferation. These results indicate that IGF-1 formed in the ischemic penumbra might be one of the diffusible factors that mediate post-ischemic neural progenitor proliferation.

Chronically increased transforming growth factor-beta1 strongly inhibits hippocampal neurogenesis in aged mice

There is increasing evidence that hippocampal learning correlates strongly with neurogenesis in the adult brain. Increases in neurogenesis after brain injury also correlate with improved outcomes. With aging the capacity to generate new neurons decreases dramatically, both under normal conditions and after injury. How this decrease occurs is not fully understood, but we hypothesized that transforming growth factor (TGF)-beta1, a cell cycle regulator that rapidly increases after injury and with age, might play a role. We found that chronic overproduction of TGF-beta1 from astrocytes almost completely blocked the generation of new neurons in aged transgenic mice. Even young adult TGF-beta1 mice had 60% fewer immature, doublecortin-positive, hippocampal neurons than wild-type littermate controls. Bromodeoxyuridine labeling of dividing cells in 2-month-old TGF-beta1 mice confirmed this decrease in neuro-genesis and revealed a similar decrease in astrogenesis. Treatment of early neural progenitor cells with TGF-beta1 inhibited their proliferation. This strongly suggests that TGF-beta1 directly affects these cells before their differentiation into neurons and astrocytes. Together, these data show that TGF-beta1 is a potent inhibitor of hippocampal neural progenitor cell proliferation in adult mice and suggest that it plays a key role in limiting injury and age-related neurogenesis.

Characterization of epidermal neural crest stem cell (EPI-NCSC) grafts in the lesioned spinal cord

We have characterized in the contusion-lesioned murine spinal cord the behavior of acutely implanted epidermal neural crest stem cells (EPI-NCSC, formerly eNCSC). EPI-NCSC, a novel type of multipotent adult stem cell, are remnants of the embryonic neural crest. They reside in the bulge of hair follicles and have the ability to differentiate into all major neural crest derivatives (Sieber-Blum, M., Grim, M., Hu, Y.F., Szeder, V., 2004. Pluripotent neural crest stem cells in the adult hair follicle. Dev. Dyn. 231, 258-269). Grafted EPI-NCSC survived, integrated, and intermingled with host neurites in the lesioned spinal cord. EPI-NCSC were non-migratory. They did not proliferate and did not form tumors. Significant subsets expressed neuron-specific beta-III tubulin, the GABAergic marker glutamate decarboxylase 67 (GAD67), the oligodendrocyte marker, RIP, or myelin basic protein (MBP). Close physical association of non-neuronal EPI-NCSC with host neurites was observed. Glial fibrillary acidic protein (GFAP) immunofluorescence was not detected. Collectively, our data indicate that intraspinal EPI-NCSC demonstrate several desirable characteristics that may include local neural replacement and re-myelination.

BMP and FGF-2 regulate neurogenin-2 expression and the differentiation of sensory neurons and glia

We have examined the effects of signaling molecules and Notch signaling on the mechanisms regulating neurogenin (ngn)-2 expression. This ngn-2 is a transcription factor that is essential for the specification of early differentiating sensory neurons in the dorsal root ganglia. In the presence of bone morphogenetic protein (BMP), anti-ngn-2-positive cells appeared in mouse trunk neural crest cell cultures, and they expressed Brn3, indicating that ngn-2-expressing cells are sensory neurons. These cells did not differentiate after fibroblast growth factor (FGF)-2 treatment or after Notch activation. The suppression of ngn-2 expression by FGF-2 was recovered by treatment with a Notch signaling inhibitor. Thus, FGF-2 may prevent ngn-2 expression through Notch activation. Whereas BMP-4 inhibited glial differentiation, FGF-2 promoted gliogenesis by means of Notch activation. Our data suggest that BMP and FGF-2 act as positive and negative regulators in ngn-2 expression, respectively, and that these signaling molecules regulate the differentiation of sensory neurons and glia.

Neurotrophin-3 signaling in mammalian Merkel cell development

Merkel cells are sensory cells of neural crest origin. Because little is known about the mechanisms that direct their differentiation, we have investigated the potential role of a candidate regulatory factor, neurotrophin-3 (NT-3). At embryonic day 16.5 (E 16.5), neither NT-3 nor its primary receptors, TrkC and p75NTR are expressed by Merkel cells in the murine whisker. At the time of birth, however, Merkel cells are immunoreactive for NT-3, TrkC and p75NTR. In TrkC null and NT-3 null mice, Merkel cells differentiate initially, but undergo apoptosis perinatally. These results show that NT-3 signaling is not required for the differentiation of Merkel cells, but that it is essential for their postnatal survival.

Dissimilar regulation of cell differentiation in mesencephalic (cranial) and sacral (trunk) neural crest cells in vitro

During development neural crest cells give rise to a wide variety of specialized cell types in response to cytokines from surrounding tissues. Depending on the cranial-caudal level of their origin, different populations of neural crest cells exhibit differential competence to respond to these signals as exemplified by the unique ability of cranial neural crest to form skeletal cell types. We show that in addition to differences in whether they respond to particular signals, cranial neural crest cells differ dramatically from the trunk neural crest cells in how they respond to specific extracellular signals, such that under identical conditions the same signal induces dissimilar cell fate decisions in the two populations in vitro. Conversely, the same differentiated cell types are induced by different signals in the two populations. These in vitro differences in neural crest response are consistent with in vivo manipulations. We also provide evidence that these differences in responsiveness are modulated, at least in part, by differential expression of Hox genes within the neural crest.

Induction of neurogenin-1 expression by sonic hedgehog: Its role in development of trigeminal sensory neurons

We have examined the roles of signaling molecules in the mechanisms underlying the induction of neurogenin (ngn)-1 expression. ngn-1 is a basic helix-loop-helix (bHLH) transcription factor, which is essential for the specification of trigeminal sensory neurons. Semiquantitative reverse transcriptase-polymerase chain reaction using cranial explants in organ cultures showed that sonic hedgehog (Shh) promotes ngn-1 expression. This promoting activity was not observed in other signaling molecules examined. The promotion of ngn-1 expression by Shh, furthermore, was inhibited by cyclopamine, a specific inhibitor of Shh signaling. Shh did not affect the expression of ngn-2, a bHLH transcription factor that plays an important role in the specification of epibranchial placode-derived sensory neurons. The expression levels of ngn-1 and ngn-2 decreased after fibroblast growth factor-2 treatment. These results suggest that Shh induces ngn-1 expression specifically and that expression of ngn-1 and ngn-2 is regulated by different mechanisms. The induction of ngn-1 expression by Shh suggests that this signaling molecule participates in the specification of trigeminal sensory neurons. We therefore examined the effect of Shh on the development of these neurons. Immunostaining using anti-ngn-1 demonstrated that Shh promotes ngn-1 expression in trigeminal neural crest cells. Trigeminal neural crest cells are derived from the posterior mesencephalon and the most-anterior rhombencephalon, and they contain a subset of precursors of trigeminal sensory neurons. Moreover, a subpopulation of trigeminal neural crest cells expressed the Shh receptor Patched. The number of cells that express Brn3a, a POU-domain transcription factor that plays an important role in differentiation of sensory neurons, also increased with Shh treatment. Our data suggest that Shh signaling is involved in the specification of trigeminal sensory neurons through the induction of ngn-1 expression. Furthermore, Shh promotes the differentiation of neural crest cells into trigeminal sensory neurons.

Disruption of anterior segment development by TGF-β1 overexpression in the eyes of transgenic mice

Previous experiments showed that transgenic mice expressing a secreted self-activating transforming growth factor (TGF) -beta1 did not show a phenotype in the lens and cornea until postnatal day 21, when anterior subcapsular cataracts, sporadic thickening of the corneal stroma, and thinning of the corneal epithelium were noted (Srinivasan et al., 1998). To examine the effects of higher concentrations of TGF-beta1 on the lens and cornea, we constructed transgenic mice harboring the strong, lens-specific chicken betaB1-crystallin promoter driving an activated porcine TGF-beta1 gene. In contrast to the earlier study, the transgenic mice had microphthalmic eyes with closed eyelids. Already at embryonic day (E) 13.5, the future cornea of the transgenic mice was threefold thicker than that of wild-type littermates due to increased proliferation of corneal stromal mesenchyme cells. Staining of fibronectin and thrombospondin-1 was increased in periocular mesenchyme. At E17.5, the thickened transgenic corneal stroma was vascularized and densely populated by abundant star-shaped, neural cell adhesion molecule-positive cells of mesenchymal appearance surrounded by irregular swirls of collagen and extracellular matrix. The corneal endothelium, anterior chamber, and stroma of iris/ciliary body did not develop, and the transgenic cornea was opaque. Fibronectin, perlecan, and thrombospondin-1 were elevated, whereas type VI collagen decreased in the transgenic corneal stroma. Stromal mesenchyme cells expressed alpha-smooth muscle actin as did lens epithelial cells and cells of the retinal pigmented epithelium. By E17.5, lens fiber cells underwent apoptotic cell death that was followed by apoptosis of the entire anterior lens epithelium between E18.5 and birth. Posteriorly, the vitreous humor was essentially absent; however, the retina appeared relatively normal. Thus, excess TGF-beta1, a mitogen for embryonic corneal mesenchyme, severely disrupts corneal and lens differentiation. Our findings profoundly contrast with the mild eye phenotype observed with presumably lower levels of ectopic TGF-beta and illustrate the complexity of TGF-beta utilization and the importance of dose when assessing the effects of this growth factor.

Reduction of endogenous TGF-beta does not affect phenotypic development of sympathoadrenal progenitors into adrenal chromaffin cells

Adrenal chromaffin cells and sympathetic neurons are related, but phenotypically distinct derivatives of the neural crest. Molecular cues that determine the chromaffin cell phenotype have not yet been identified; in contrast to a widely held belief, glucocorticoid signaling is apparently not relevant (Development 126 (1999) 2935). Transforming growth factor-betas (TGF-betas) regulate various aspects of embryonic development and are expressed in the environment of sympathoadrenal (SA) progenitor cells. We have previously shown that neutralization of endogenous TGF-beta from E4 to E8 in the quail embryo significantly increases numbers of adrenal tyrosine hydroxylase-positive cells. Whether endogenous TGF-beta may also be involved in influencing phenotypic development of adrenal chromaffin cells and their SA progenitors has not been analyzed. We now demonstrate that neutralization of endogenous TGF-beta1, -beta2 and -beta3 with a pan-anti-TGF-beta antibody in quail embryos during distinct time windows does not alter phenotypic development of chromaffin cells. In situ hybridizations revealed unaltered expression of neurofilament (NF-160), synaptotagmin I and neurexin I in adrenal glands. Likewise, the NF-associated antigen 3A10, and polyphosphorylated NF epitopes (RT 97) were unaltered. Most importantly, the typical ultrastructure of adrenal chromaffin cells including their large chromaffin secretory granules, a hallmark of the neuroendocrine phenotype, which distinguishes them from sympathetic neurons, was not affected. We therefore conclude that neutralization of endogenous TGF-beta influences chromaffin cell proliferation, but does not interfere with the development of the typical chromaffin cell phenotype.

DNA array analysis of the developing rat cerebellum: transforming growth factor-beta2 inhibits constitutively activated NF-kappaB in granule neurons

The nuclear factor-kappaB (NF-kappaB) pathway is important in neuronal survival and in integration of external signals e.g. cytokines, glutamate, Abeta and nerve growth factor (NGF). During rat cerebellar development NF-kappaB activity is high in granule cells before postnatal day 7 (P7) and declines after P7. Using gene expression profiles, measured by cDNA arrays, up-regulation of transforming growth factor-beta2 (TGF-beta2) was correlated with the developmental down-regulation of NF-kappaB. TGF-beta2 depicted strongest, more than 4-fold, up-regulation in P12 versus P4 cerebella. In situ hybridization and immunohistochemistry confined upregulated TGF-beta2 to granule cells and correlated mRNA and TGF-beta2-protein increase. Finally TGF-beta2 repressed NF-kappaB activity, in an in vitro system resembling migrating cerebellar granule cells. Thus, TGF-beta might fulfill an important role in repressing developmentally activated NF-kappaB in granule neurons.

Environmental signals and cell fate specification in premigratory neural crest

Neural crest cells are multipotent progenitors, capable of producing diverse cell types upon differentiation. Recent studies have identified significant heterogeneity in both the fates produced and genes expressed by different premigratory crest cells. While these cells may be specified toward particular fates prior to migration, transplant studies show that some may still be capable of respecification at this time. Here we summarize evidence that extracellular signals in the local environment may act to specify premigratory crest and thus generate diversity in the population. Three main classes of signals-Wnts, BMP2/BMP4 and TGFbeta1,2,3-have been shown to directly influence the production of particular neural crest cell fates, and all are expressed near the premigratory crest. This system may therefore provide a good model for integration of multiple signaling pathways during embryonic cell fate specification.

Factors controlling lineage specification in the neural crest

The neural crest is a transitory tissue of the vertebrate embryo that originates in the neural folds, populates the embryo, and gives rise to many different cell types and tissues of the adult organism. When neural crest cells initiate their migration, a large fraction of them are still pluripotent, that is, capable of generating progeny that consists of two or more distinct phenotypes. To elucidate the cellular and molecular mechanisms by which neural crest cells become committed to a particular lineage is therefore crucial to the understanding of neural crest development and represents a major challenge in current neural crest research. This chapter discusses selected aspects of neural crest cell differentiation into components of the peripheral nervous system. Topics include sympathetic neurons, the adrenal medulla, primary sensory neurons of the spinal ganglia, some of their mechanoreceptive and proprioceptive end organs, and the enteric nervous system.

The thrombospondin type 1 repeat (TSR) superfamily: diverse proteins with related roles in neuronal development

The thrombospondins are a family of proteins found widely in the embryonic extracellular matrix. Like most matrix proteins, thrombospondins are modular and contain a series of repeated domains arrayed between globular amino and carboxyl terminal domains. In recent years, other proteins that share thrombospondin type 1 repeats, or TSRs, have been identified. These include the F-spondin gene family, the members of the semaphorin 5 family, UNC-5, SCO-spondin, and others. Most of these are expressed in the developing nervous system, and many have expression patterns and in vitro properties that suggest potential roles in the guidance of cell and growth cone migration. Both cell- and matrix-binding motifs have been identified in the TSRs of thrombospondin-1, so it has been hypothesized that the properties of these diverse proteins may also depend on the presence of these repeats. Here, we review the cell biology of the TSR module, the extensive literature regarding the distribution and functions of thrombospondins and other TSR superfamily proteins, and evaluate their possible roles during the development of the nervous system.

Chemotactic migration of mesencephalic neural crest cells in the mouse

We examined the roles of fibroblast growth factor (FGF)-2 and FGF-8 in the migration of mesencephalic mouse neural crest cells. Our in vitro migration assay has shown that FGF-2 (basic FGF) and FGF-8 have chemotactic activity for these cells. Chemotaxis was inhibited by anti-FGF-2 and anti-FGF-8 neutralizing antibodies. In addition, anti-FGF-2 blocked neural crest cell migration in cranial organ cultures. This observation suggests that FGF-2 functions as a chemoattractant in migration of mesencephalic neural crest cells in vivo. In organ culture, the antagonist of FGF binding to a low-affinity fibroblast growth factor receptor (FGFR) heparan sulfate, inositolhexakisphosphate (InsP6), inhibited migration as well. Mesencephalic neural crest cells had high-affinity FGFRs, in particular FGFR-1 and FGFR-3. Thus, the chemotactic activities of FGF-2 can be mediated by the low-affinity FGFR alone or by a combination of low- and high-affinity FGFRs (FGFR-1, FGFR-3, or both). Moreover, differential localization of FGF-2 was found at the mesencephalic axial level of intact embryos during neural crest cell migration. FGF-2 protein expression was predominant in the target regions, in particular the mandibular mesenchyme, that are colonized by mesencephalic neural crest cells. This characteristic distribution supports the notion that FGF-2 acts as a chemoattractant in the mouse embryo that directs mesencephalic neural crest cell migration. Whereas FGF-8 showed chemotactic activity in vitro, neural crest cell dispersion was observed in explants that had been treated with anti-FGF-8 neutralizing antibodies. This result suggests that FGF-8 may not be a chemoattractant in vivo. However, the distribution of neural crest cells in explants treated with anti-FGF-8 differed from that in control explants or in intact embryos. Extreme FGF-2 distribution was observed in the mandibular arch and FGF-8 is expressed in the epithelium. FGF-8 may play a role in mesencephalic neural crest cell migration, and its role may be concerned with the differential localization of FGF-2. To establish this notion, we performed immunohistochemical examination of FGF-2 distribution in explants treated with FGF-8 and analysis of FGF-2 gene expression levels by reverse transcriptase-polymerase chain reaction by using RNA from explants. The data indicate that FGF-2 is distributed throughout the mesenchyme in FGF-8-treated explants and that expression of FGF-2 is promoted by FGF-8. Therefore, we conclude that the expression of FGF-8 in the mandibular arch epithelium is a prerequisite for the differential localization of FGF-2 and that the FGF-2 distribution pattern is essential for chemotaxis of mesencephalic neural crest cell migration.

Thrombospondin-1 and neural crest cell migration

Using a monoclonal antibody raised against human platelet thrombospondin, we found anti-thrombospondin immunoreactivity in the extracellular matrix of avian embryos, coincident with the ventral pathways followed by trunk neural crest cells. To confirm that the antibody recognized thrombospondin-1 and to determine the tissue of origin of the thrombospondin matrix, a thrombospondin-1 cRNA probe was used for whole mount in situ hybridization. This probe revealed thrombospondin-1 mRNAs in the developing myotome before and during neural crest cell migration. The effect of thrombospondin-1 on neural crest cell migration, morphology, and adhesion was assayed in vitro. Quail trunk neural crest cells cultured on 4 microg/ml of thrombospondin-1 migrate at 1.14 +/- 0.54 microm/min, which is significantly greater than the rate of cell migration on tissue culture plastic. Using a shaker-based adhesion assay, a significantly greater number of neural crest cells remain attached to dishes coated with 4 microg/ml of thrombospondin-1 than to tissue culture plastic alone. The number of neural crest cells that remain attached to 4 microg/ml of thrombospondin-1 is similar to the number that remain attached to dishes coated with 10 microg/ml of fibronectin. These observations indicate that neural crest cells migrate through a thrombospondin-filled extracellular matrix, and that thrombospondin-1 promotes neural crest cell migration and adhesion. Thus, thrombospondin-1 is the first somite-derived extracellular matrix molecule with properties consistent with a role in the promotion of migration into the anterior somite, as opposed to the repulsion of neural crest cells from the posterior half of the somite.

Developmental regulation of neuronal K+ channels by target-derived TGF beta in vivo and in vitro

The functional expression of Ca2+-activated K+ channels (KCa) in developing chick ciliary ganglion (CG) neurons requires interactions with target tissues and preganglionic innervation. Here, we show that the stimulatory effects of target tissues are mediated by an isoform of TGFbeta. Exposure of cultured CG neurons to TGFbeta1, but not TGFbeta2 or TGFbeta3, caused robust stimulation of KCa. The KCa stimulatory effects of target tissue extracts were blocked by a neutralizing pan-TGFbeta antiserum but not by specific TGFbeta2 or TGFbeta3 antisera. Intraocular injection of TGFbeta1 caused robust stimulation of KCa, whereas intraocular injection of pan-TGFbeta antiserum inhibited expression of KCa in CG neurons developing in vivo. The effects of TGFbeta1 were potentiated by beta-neuregulin-1, a differentiation factor expressed in preganglionic neurons.

Induction and patterning of the neural crest, a stem cell-like precursor population

The neural crest is a multipotent precursor population which ultimately generates much of the peripheral nervous system, epidermal pigment cells, and a variety of mesectodermal derivatives. Individual multipotent neural crest cells are capable of some self-renewing divisions, and based upon this criteria can be considered stem cells. Considerable progress has been made in recent years toward understanding how this important population of progenitor cells is initially established in the early embryo, and how cell-intrinsic and non-cell-intrinsic factors mediate their subsequent lineage segregation and differentiation.

Neural crest cells in outflow tract septation of the embryonic chicken heart: differentiation and apoptosis

The heart consists of cells deriving from the cardiogenic plate and also from extracardiac sources. One of the major extracardiac contributions is given by the neural crest. The differentiation pathway and fate of the neural crest cells in the outflow tract have been followed over a prolonged period during outflow tract septation. We studied the role of the neural crest in remodeling the outflow tract by long-term cell tracing, differentiation markers and apoptosis. The pattern of neural crest cells migrating to the heart was investigated by heterospecific chicken quail chimeras and by retroviral infection of the reporter gene LacZ to the stem cells. The tagged neural crest cells move to areas that are morphogenetically active, such as the outflow tract, the semilunar valves, the wall of the arteries and the cardiac ganglia. Two differentiated subpopulations are discerned on the basis of immunohistochemical characterization with antibodies against smooth muscle cells in the arterial vessel wall and against ganglionic cells that were scattered around the vessels of the arterial pole and the heart. A third subpopulation did not stain with these antibodies, but presented locally with the phenomenon of apoptosis as shown with the TUNEL approach. In a developmental series of chicken embryos the populations were followed until stage 40. It was evident that the outflow tract septum in the early phase of development consisted mainly of mesenchymal neural crest cells. In a later phase neural crest cells were still detected at semilunar valve level, but nearly absent in the outflow tract septum below valve level. The septum at that time had become myocardialized. It is evident that neural crest cells are actually removed from this part of the heart by apoptosis. We are pursuing the hypothesis that an important function of apoptotic cells in heart development might be to activate the cardiomyocytes to muscularize the outflow tract septum through mobilizing or delivering growth factors at the time and place that septum formation is initiated.

Expression of a constitutively active type I BMP receptor using a retroviral vector promotes the development of adrenergic cells in neural crest cultures

Previous work has demonstrated that the bone morphogenetic proteins (BMP)-2, BMP-4, and BMP-7 can promote the development of tyrosine hydroxylase (TH)-positive and catecholamine-positive cells in quail trunk neural crest cultures. In the present work, we showed that mRNA for the type I bone morphogenetic protein receptor IA (BMPR-IA) was present in neural crest cells grown in the absence or presence of BMP-4. We have used a replication-competent avian retrovirus to express a constitutively active form of BMPR-IA in neural crest cells in culture. Cultures grown in the absence of BMP-4 and infected with retrovirus containing a construct encoding this activated BMPR-IA developed five times more TH-immunoreactive and catecholamine-positive cells than uninfected control cultures or cultures infected with virus bearing the wild-type BMPR-IA cDNA. The number of TH-positive cells which developed was dependent on the concentration of virus bearing the activated receptor cDNA used in the experiments. Most TH-positive cells which developed also contained viral p19 protein. Total cell number was not affected by infection with the virus containing the activated receptor construct. The effect of the activated receptor was phenotype-specific since infection with the virus bearing the activated receptor cDNA did not alter the number or morphology of microtubule-associated protein (MAP)2-immunoreactive cells, which are distinct from the TH-positive cell population. These findings are consistent with the observation that MAP2-positive cells are not affected by the presence of BMP-4. Taken together, these results suggest that activity of BMPR-IA is an important element in promoting the development of the adrenergic phenotype in neural crest cultures.

Growth factor action in neural crest cell diversification

At the onset of their migration into the embryo, many neural crest cells are pluripotent in the sense that they have the capacity to generate progeny that consist of more than one cell type. More recently, we have found that there are pluripotent neural crest cell-derived cells even at sites of terminal differentiation. These findings support the notion that cues originating from the microenvironment, at least in part, direct neural crest cell type specification. Based on the rationale that growth factors that are known to support survival of neural crest cell derivatives may have additional functions in progenitor cell development, we have examined the action of pertinent growth factors. Trophic, mitogenic, antiproliferative and differentiation promoting activities were found. Stem cell factor (SCF) is trophic for pluripotent neural crest cells. Contrary to expectation, SCF plus a neurotrophin, rather than SCF alone is trophic for committed melanogenic cells. Basic fibroblast growth factor (bFGF) is mitogenic both for pluripotent cells and committed melanogenic cells. However, the cells become dependent on another factor for survival. Whereas any neurotrophin tested can rescue bFGF-activated pluripotent neural crest cells, the factor that rescues melanogenic cells remains to be determined. Transforming growth factor beta 1 (TGF-beta 1) is a powerful antimitotic signal for all neural crest cells that overrides the bFGF/neurotrophin proliferative signal. Furthermore, SCF promotes differentiation of neural crest cells into cells of the sensory neuron lineage. Neurotrophin-3 (NT-3) specifically promotes high affinity uptake of norepinephrine by neural crest cells and is thus thought to play a critical role in the differentiation of sympathetic neuroblasts. In summary, our data indicate that neurotrophins and other pertinent growth factors affect survival, proliferation and differentiation of neural crest cells at multiple levels and in different lineages. Moreover, our findings emphasise the importance of the concerted action of combinations of growth factors, rather than of individual factors.