Neurotrophin-3 affects proliferation and differentiation of distinct neural crest cells and is present in the early neural tube of avian embryos

Modulation of neurotrophin signaling by monoclonal antibodies

The neurotrophin family is comprised of the structurally related secreted proteins nerve growth factor (NGF), brain-derived neurotrophic factor (BDNF), neurotrophin-3 (NT-3), and neurotrophine-4 (NT-4). They bind and activate the tyrosine kinase receptors Trk A, B, and C in a ligand-specific manner and additionally bind a shared p75NTR receptor. The neurotrophins were originally defined by their ability to support the survival and maturation of embryonic neurons. However, they also control important physiological functions of the adult nervous system including learning and memory, sensation, and energy homeostasis. For example, NGF/trkA signaling is critical for normal and pathological sensation of pain. Likewise, the BDNF/trkB pathway controls feeding and metabolism, and its dysfunction leads to severe obesity. Antibodies can modulate neurotrophin signaling. Thus, NGF blocking agents can attenuate pain in several animal models, and a recombinant humanized NGF blocking antibody (Tanezumab) has shown promising results in human clinical trials for osteoarthritic pain. On the other hand trkB agonist antibodies can modulate food intake and body weight in rodents and nonhuman primates. The power of monoclonal antibodies to modulate neurotrophin signaling promises to turn the rich biological insights into novel human medicines.

Neurotrophin signalling and transcription programmes interactions in the development of somatosensory neurons

Somatosensory neurons of the dorsal root ganglia are generated from multipotent neural crest cells by a process of progressive specification and differentiation. Intrinsic transcription programmes active in somatosensory neuron progenitors and early post-mitotic neurons drive the cell-type expression of neurotrophin receptors. In turn, signalling by members of the neurotrophin family controls expression of transcription factors that regulate neuronal sub-type specification. This chapter explores the mechanisms by which this crosstalk between neurotrophin signalling and transcription programmes generates the diverse functional sub-types of somatosensory neurons found in the mature animal.

Potential neural progenitor cells in fetal liver and regenerating liver

From unfractionated embryonic mice liver cells, appreciable amount of spherical bodies containing nestin-positive cells were generated in the presence of neuronal growth factors. Following cultivation on poly-D: -lysine/laminin-coated slips, approximately 70% of the cells expressed neuronal markers, and 16% had long processes. Functional analysis of these long-process-bearing cells with the whole-cell patch clamp method showed an inward current in response to glutamate, GABA, and serotonin as the neuronal characteristics. Furthermore, regenerating liver in adult mice also contained nestin-positive cells to the same extent as fetal liver. Regenerating liver could have potential as a source of neural cells for autologous transplantation.

The role of neurotrophins during early development

The effects of neurotrophins during the middle and late stages of development are well known. It was previously thought that neurotrophins had no role during early development, but this is not the case and is the subject of this review article. The earliest neurotrophin receptor expressed is that for neurotrophin-3 (NT-3). TrkC is detected in the neural plate and is present in the neural tube. Initially, the distribution of TrkC is homogenous, but it becomes localized to specific regions of the neural tube as the neural tube differentiates. The receptor for brain-derived neurotrophic factor (BDNF) and neurotrophin-4/5 (NT-4/5), TrkB, is detected somewhat later than TrkC in the neural tube where it is also differentially localized. In contrast, the NGF receptor, TrkA, was not detected during early development. Both NT-3 and BDNF have been shown to have effects in vitro during early development. NT-3 caused an increase in neurite outgrowth and apoptosis in neural plate explants, and promoted differentiation of progenitors into motoneurons. BDNF increased the number of motoneurons in neural tube explants. These data suggest that NT-3 and BDNF may play a role during early development in vivo.

Electro-acupuncture induced NGF, BDNF and NT-3 expression in spared L6 dorsal root ganglion in cats subjected to removal of adjacent ganglia

This study evaluated the effect of electro-acupuncture (EA) on the NGF, BDNF and NT-3 expression in spared L6 dorsal root ganglion (DRG) in cats subjected to bilateral removal of L1-L5 and L7-S2 DRG, using immunostaining, in situ hybridization and RT-PCR. The positive products of NGF, NT-3 protein and mRNA in the small and large neurons of spared L6 DRG in EA side increased greatly more than that of control side, while the increased BDNF was only noted in small and medium-sized neurons. RT-PCR demonstrated that the mRNA level for three factors was not influenced by EA in intact DRG, when a significant increase was seen in the spared L6 DRG of EA side. As it has been well known that DRG neurons project to the spinal cord wherein morphological plasticity has been present after DRG removal, the present results might have some bearing to the observed phenomenon.

Lineage specification in neural crest cell pathfinding

There are two principal models to explain neural crest patterning. One assumes that neural crest cells are multipotent precursors that migrate throughout the embryo and differentiate according to cues present in the local environment. A second proposes that the neural crest is a population of cells that becomes restricted to particular fates early in its existence and migrates along particular pathways dependent on unique cell-autonomous properties. Although it is now evident that the neural crest cell population, as a whole, is actually heterogenous (composed of both multipotent and restricted progenitors), evidence supporting the model of prespecification has increased over the past few years. This review will begin by telling the story of melanoblasts: a neural crest subpopulation that is biased toward a single fate and subsequently acquires intrinsic properties that guide cells of this lineage to their final destination. The remainder of this review will explore whether this model is exclusive to melanoblasts or if it can also be used to explain the patterning of other neural crest cells like those of the sensory, sympathoadrenal, and enteric lineages.

Embryonic sympathoblasts transiently express TrkB in vivo and proliferate in response to brain-derived neurotrophic factor in vitro

BACKGROUND

Nerve growth factor and neurotrophin-3 are involved in the development of sympathetic neurons; however, whether brain derived neurotrophic factor also plays a role is not known. The purpose of this study was to determine whether BDNF and its receptor, TrkB, are expressed during the development of paravertebral sympathetic ganglia in vivo and to determine the effect of BDNF in vitro.

RESULTS

As neural crest cells coalesce to form sympathetic ganglia, TrkB-positive cells are seen in both chicken and mouse embryos. In chicken embryos, TrkB-expressing cells first appear at Hamburger-Hamilton Stage (St) 27 and they co-express HNK-1, confirming that they are migrating neural crest cells. The TrkB-positive cells lack neural markers at this stage; however, they migrate with other neurally differentiating cells that are TrkA and TrkC-positive. By St. 29/30, TrkB-positive cells begin to express the neural specific markers Hu C/D and Islet-1; eventually, all TrkB positive cells commence neural differentiation. By St. 34, TrkB and TrkC staining are lost. BDNF transcript expression parallels that of TrkB. In the mouse, TrkB-positive cells surround newly formed sympathetic ganglia and a small number of TrkB positive cells that co-express tyrosine hydroxylase are seen within ganglia between E13.5-15. In cell culture, many cells from St. 29-30 chicken lumbar sympathetic ganglia express neural markers and are dividing, indicating that they are sympathoblasts. Sympathoblasts and neurons require both nerve growth factor and neurotrophin-3 for survival. BDNF increases the number of cells expressing neural markers in culture by increasing number of cells that incorporate bromodeoxyuridine. In contrast, most TrkB-positive sympathetic cells in vivo are not actively proliferating between E6-E8.

CONCLUSION

Developing paravertebral sympathetic ganglia in avian and murine embryos contain a subpopulation of sympathoblasts that transiently express TrkB and ultimately commence neuronal differentiation. These TrkB expressing sympathoblasts are not actively dividing in vivo; yet, when placed in vitro, will divide in response to BDNF. This suggests that the availability of BDNF in vivo fails to reach a threshold necessary to induce proliferation. We suggest that excess TrkB stimulation of sympathoblasts in vivo may lead to the genesis of neuroblastoma.

NT-3 and CNTF exert dose-dependent, pleiotropic effects on cells in the immature dorsal root ganglion: Neuregulin-mediated proliferation of progenitor cells and neuronal differentiation

Neurons in the nascent dorsal root ganglia are born and differentiate in a complex cellular milieu composed of postmitotic neurons, and mitotically active glial and neural progenitor cells. Neurotrophic factors such as NT-3 are critically important for promoting the survival of postmitotic neurons in the DRG. However, the factors that regulate earlier events in the development of the DRG such as the mitogenesis of DRG progenitor cells and the differentiation of neurons are less defined. Here we demonstrate that both NT-3 and CNTF induce distinct dose-dependent responses on cells in the immature DRG: at low concentrations, they induce the proliferation of progenitor cells while at higher concentrations they promote neuronal differentiation. Furthermore, the mitogenic response is indirect; that is, NT-3 and CNTF first bind to nascent neurons in the DRG--which then stimulates those neurons to release mitogenic factors including neuregulin. Blockade of this endogenous neuregulin activity completely blocks the CNTF-induced proliferation and reduces about half of the NT-3-mediated proliferation. Thus, the genesis and differentiation of neurons and glia in the DRG are dependent upon reciprocal interactions among nascent neurons, glia, and mitotically active progenitor cells.

PACAP promotes sensory neuron differentiation: blockade by neurotrophic factors

Developing neurons encounter a panoply of extracellular signals as they differentiate. A major goal is to identify these extrinsic cues and define the mechanisms by which neurons simultaneously integrate stimulation by multiple factors yet initiate one specific biological response. Factors that are known to exert potent activities in the developing nervous system include the NGF family of neurotrophic factors, ciliary neurotrophic factor (CNTF), and pituitary adenylate cyclase-activating peptide (PACAP). Here we demonstrate that PACAP promotes the differentiation of nascent dorsal root ganglion (DRG) neurons in that it increases both the number of neural-marker-positive cells and axonogenesis without affecting the proliferation of neural progenitor cells. This response is mediated through the PAC1 receptor and requires MAP kinase activation. Moreover, we find that, in the absence of exogenously added PACAP, blockade of the PAC1 receptor inhibits neuronal differentiation. These data coupled with our finding that both PACAP and the PAC1 receptor are expressed during the peak period of neuronal differentiation in the DRG suggest that PACAP functions in vivo to promote the differentiation of nascent sensory neurons. Interestingly, we also demonstrate that the neurotrophic factors NT-3 and CNTF completely block the PACAP-induced neuronal differentiation. This points to the intricate integration of cellular signals by nascent neurons and, to our knowledge, is the first evidence for neurotrophic factor abrogation of a pathway regulated by G-protein-coupled receptors (GPCRs).

The trunk neural crest and its early glial derivatives: a study of survival responses, developmental schedules and autocrine mechanisms

Regulation of survival during gliogenesis from the trunk neural crest is poorly understood. Using adapted survival assays, we directly compared crest cells and the crest-derived precursor populations that generate satellite cells and Schwann cells. A range of factors that supports Schwann cells and glial precursors does not rescue crest, with the major exception of neuregulin-1 that rescues crest cells provided they contact the extracellular matrix. Autocrine survival appears earlier in developing satellite cells than Schwann cells. Satellite cells also show early expression of S100beta, BFABP and fibronectin and early survival responses to IGF-1, NT-3 and PDGF-BB that in developing Schwann cells are not seen until the precursor/Schwann cell transition. These experiments define novel differences between crest cells and early glia and show that entry to the glial lineage is an important point for regulation of survival responses. They show that survival mechanisms among PNS glia differ early in development and that satellite cell development runs ahead of schedule compared to Schwann cells in several significant features.

Regulation of neurogenesis by neurotrophins in developing spinal sensory ganglia

Neurons and glia in spinal sensory ganglia derive from multipotent neural crest-derived stem cells. In contrast to neural progenitor cells in the central nervous system, neural crest progenitors coexist with differentiated sensory neurons all throughout the neurogenic period. Thus, developing sensory ganglia are advantageous for determining the possible influence of cell-cell interactions in the regulation of precursor proliferation and neurogenesis. Neurotrophins are important regulators of neuronal survival in the developing vertebrate nervous system and, in addition, they appear to influence precursor behavior in vitro. Studies in mice carrying mutations in neurotrophin genes provide a good system in which to analyze essential actions of these factors on the different developing neural populations.

Autocrine regulation of norepinephrine transporter expression

The norepinephrine transporter (NET) is a neurotransmitter scavenger and site of drug action in noradrenergic neurons. The aim of this study was to identify mechanisms that regulate NET expression during the development of quail (q) sympathetic neuroblasts, which develop from neural crest stem cells. Neurotrophin-3 (NT-3) and transforming growth factor beta1 (TGF-beta1) cause an increase of qNET mRNA levels in neural crest cells. When combined, the growth factors are additive in increasing qNET mRNA levels. Both NT-3 and TGF-beta1 are synthesized by neural crest cells. Onset of NET expression precedes the onset of neural crest stem cell emigration from the neural tube. In older embryos, qNET is expressed by several crest-derived and noncrest tissues. The data show that qNET expression in presumptive sympathetic neurons is initiated early in embryonic development by growth factors that are produced by neural crest cells themselves. Moreover, the results support our previous observations that norepinephrine transport contributes to the regulation of the differentiation of neural crest stem cells into sympathetic neurons.

Dynamic expression of neurotrophin receptors during sensory neuron genesis and differentiation

To identify potential functions for neurotrophins during sensory neuron genesis and differentiation, we determined the temporal and spatial protein expression patterns of neurotrophin receptors throughout the process of sensory neurogenesis in the dorsal root ganglia (DRG). We show that neurotrophin receptors are expressed early, being first detected on subsets of migrating neural crest cells, and that trkC is among the earliest markers of neural lineage specification. In the immature DRG, we find that both trkC and p75(NTR) are expressed on subsets of dividing progenitor cells in vivo. Furthermore, our data directly reveal distinct patterns of trk receptor expression by individual sensory neurons from the time of their inception with all early arising cells initially being trkC(+), some subsets of whom also coexpress either trkA or trkB or both. As sensory neurons innervate their targets and establish their mature identities, the spectrum of trk receptors expressed by individual neurons is altered. The stereotyped trk receptor expression profiles identified here may potentially correspond to distinct lineages of sensory neurons. These data, in conjunction with other studies, argue for multiple functions for neurotrophins during the process of sensory neuron differentiation, including effects on both neural crest and DRG mitotically active progenitor cells, in addition to possibly influencing the establishment of sensory neuron identity.

Loss of brain-derived neurotrophic factor-dependent neural crest-derived sensory neurons in neurotrophin-4 mutant mice

Peripheral ganglion neurons confer sensory information including touch, pain, temperature, and proprioception. Sensory modality is linked to specific neurotrophin (NTF) requirements. NT-3 supports survival of neurons that differentiate primarily into proprioceptors whereas nerve growth factor and brain-derived neurotrophic factor (BDNF) support subpopulations that transmit nociception and mechanoreception, respectively. We examined sensory neurons of gene-targeted mouse mutants at the NT-4, BDNF, NT-3, and TrkA loci. We show that NT-4 functions early in gangliogenesis, upstream of BDNF. In the absence of NT-4 function, BDNF-dependent, TrkB-expressing neurons fail to appear. The results are consistent with the model that precursor cells intended to become BDNF-dependent mechanoreceptors instead differentiate into NT-3-dependent proprioceptive neurons.

Nerve growth factor (NGF) supports tooth morphogenesis in mouse first branchial arch explants

Posterior midbrain and anterior hindbrain neuroectoderm trans-differentiate into cranial neural crest cells (CNCC), emigrate from the neural folds, and become crest-derived ectomesenchyme within the mandibular and maxillary processes. To investigate the growth factor requirement specific for the initiation of tooth morphogenesis, we designed studies to test whether nerve growth factor (NGF) can support odontogenesis in a first branchial arch (FBA) explant culture system. FBA explants containing neural-fold tissues before CNCC emigration and the anlagen of the FBA were microdissected from embryonic day 8 (E8) mouse embryos, and cultured for 8 days in medium supplemented with 10% fetal calf serum only, or serum-containing medium further supplemented with either NGF or epidermal growth factor (EGF) at three different concentrations: 50, 100, or 200 ng/ml. Morphological, morphometric, and total protein analyses indicated that growth and development in all groups were comparable. Meckel's cartilage and tongue formation were also observed in all groups. However, odontogenesis was only detected in explants cultured in the presence of exogenous NGF. NGF-supplemented cultures were permissive for bud stage (50 ng/ml) as well as cap stage of tooth morphogenesis (100 and 200 ng/ml). Morphometric analyses of the volume of tooth organs showed a significant dose-dependent increase in tooth volume as the concentration of NGF increased. Whole-mount in situ hybridization and semiquantitative reverse transcription-polymerase chain reaction for Pax9, a molecular marker of dental mesenchyme, further supported and confirmed the morphological data of the specificity and dose dependency of NGF on odontogenesis. We conclude that (1) E8 FBA explants contain premigratory CNCC that are capable of emigration, proliferation, and differentiation in vitro; (2) serum-supplemented medium is permissive for CNCC differentiation into tongue myoblasts and chondrocytes in FBA explants; and (3) NGF controls CNCC cell fate specification and differentiation into tooth organs.

Neurotrophins and other growth factors in the generation of retinal neurons

The generation of neurons in the vertebrate retina, as in other areas of the developing nervous system, largely depends on extracellular signals. Of the known signaling molecules, neurotrophins play decisive, defined, and distinct roles. The three neurotrophins identified in the chick, namely, neurotrophin-3 (NT-3), brain-derived neurotrophic factor (BDNF), and nerve growth factor (NGF), are expressed in either the pigment epithelium (NT-3 and BDNF) or in the neural retina (NGF) at the onset of neuron birth. In addition, trkC and trkB, receptors for NT-3 and BDNF, respectively, together with p75, the low-affinity neurotrophin receptor, are expressed in the retina at the same developmental period. The role of these three neurotrophins in the differentiation of neurons in the chick retina has been elucidated by a combination of in vitro and in vivo experiments. Thus, NT-3 promotes the conversion of neuroepithelial cells into neurons, whereas BDNF and NGF control the programmed cell death (apoptosis) that affects early postmitotic neuroblasts. BDNF, acting via its trkB receptor, is a survival factor for these cells, whereas NGF, binding to p75 receptor, acts as a killing factor, thereby controlling the provisional number of newly generated neurons.

Neurotrophin actions during the development of the peripheral nervous system

Neurotrophins are important regulators of the development and maintenance of the vertebrate nervous system. Besides their well-established role in promoting neuronal survival during development, in vitro data suggest that they can regulate proliferation, survival, and differentiation of precursor cells. Analysis of the developing peripheral nervous system in mouse strains carrying mutations in genes encoding the neurotrophins and their receptors indicate, however, that lack of neurotrophin signalling results in specific neuronal deficits that are primarily due to neuronal death. Many of these deficits occur before final target encounter.

F-Spondin, expressed in somite regions avoided by neural crest cells, mediates inhibition of distinct somite domains to neural crest migration

Neural crest (NC) cells migrate exclusively into the rostral half of each sclerotome, where they avoid the dermomyotome and the paranotochordal sclerotome. F-spondin is expressed in these inhibitory regions and throughout the caudal halves. In vitro bioassays of NC spreading on substrates of rostral or caudal epithelial-half somites (RS or CS, respectively) revealed that NC cells adopt on RS a fibroblastic morphology, whereas on CS they fail to flatten. F-spondin inhibited flattening of NC cells on RS. Conversely, F-spondin antibodies prevented rounding up of NC cells on CS. Addition of F-spondin to trunk explants inhibited NC migration into the sclerotome, and treatment of embryos with anti-F-spondin antibodies yielded migration into otherwise inhibitory sites. Thus, somite-derived F-spondin is an inhibitory signal involved in patterning the segmental migration of NC cells and their topographical segregation within the RS.

Growth factor synergism and antagonism in early neural crest development

This review article focuses on data that reveal the importance of synergistic and antagonistic effects in growth factor action during the early phases of neural crest development. Growth factors act in concert in different cell lineages and in several aspects of neural crest cell development, including survival, proliferation, and differentiation. Stem cell factor (SCF) is a survival factor for the neural crest stem cell. Its action is neutralized by neurotrophins, such as nerve growth factor (NGF), brain-derived neurotrophic factor (BDNF), and neurotrophin-3 (NT-3) through apoptotic cell death. In contrast, SCF alone does not support the survival of melanogenic cells (pigment cell precursors). They require the additional presence of a neurotrophin (NGF, BDNF, or NT-3). Fibroblast growth factor-2 (FGF-2) is an important promoter of proliferation in neuronal progenitor cells. In neural crest cells, fibroblast growth factor treatment alone does not lead to cell expansion but also requires the presence of a neurotrophin. The proliferative stimulus of the fibroblast growth factor - neurotrophin combination is antagonized by transforming growth factor beta-1 (TGFbeta-1). Moreover, TGFbeta-1 promotes the concomitant expression of neuronal markers from two cell lineages, sympathetic neurons and primary sensory neurons, indicating that it acts on a pluripotent neuronal progenitor cell. Moreover, the combination of FGF-2 and NT3, but not other neurotrophins, promotes expression or activation of one of the earliest markers expressed by presumptive sympathetic neuroblasts, the norepinephrine transporter. Taken together, these data emphasize the importance of the concerted action of growth factors in neural crest development at different levels and in several cell lineages. The underlying mechanisms involve growth-factor-induced dependence of the cells on other factors and susceptibility to growth-factor-mediated apoptosis.Key words: neural crest, melanocyte, stem cell factor, neurotrophin-3, transforming growth factor-beta1, apoptosis, norepinephrine transporter.

Endogenous neurotrophin-3 supports the survival of a subpopulation of sensory neurons in neonatal rat

Neurotrophin-3 promotes the differentiation and supports the survival of neuroblasts derived from the neural crest in early development. Neurotrophin-3 also plays an important role in the differentiation and survival of a subpopulation of large sensory neurons after their axons arrive at their targets. Proprioception and mechanoception are lost after gene deletion of neurotrophin-3 or its high-affinity receptor, TrkC. However, the function of neurotrophin-3 during late development and in mature animals is not clear. We have used an antiserum, specific for neurotrophin-3, to neutralize endogenous neurotrophin-3 in postnatal rats to determine its role in late sensory neuron development. Administration of the antiserum for a period of two weeks, but not one week, resulted in a 20% reduction in the number of primary sensory neurons in the dorsal root ganglia and a 19% reduction in the number of myelinated axons in the saphenous nerve. The size distribution histogram also indicated that a subpopulation of large neurons was lost by the neurotrophin-3 antiserum treatment. This neuronal loss was accompanied by reduced cell soma sizes and weights of the ganglia. Immunoreactivities for calbindin and calretinin were reduced in the trigeminal and dorsal root ganglia and nerve fibres surrounding whisker hair follicles. The number of Merkel cells in touch domes labelled with quinacrine and the number of parvalbumin-immunoreactive neurons in the dorsal root ganglia were significantly reduced by the antibody treatment. In contrast, the number of muscle spindles in the gastrocnemius muscle is not reduced by the neurotrophin-3 antiserum. Together, these results indicate that a subpopulation of primary sensory neurons in the neonatal rat requires neurotrophin-3 for their survival and expression of calcium binding proteins. In addition, Merkel cells in touch domes also require neurotrophin-3 for their survival. Thus, endogenous neurotrophin-3 in neonatal rats is critical for the survival and function of a subpopulation of primary sensory neurons and Merkel cells.

Neural differentiation promoted by truncated trkC receptors in collaboration with p75(NTR)

trkC receptors, which serve critical functions during the development of the nervous system, are alternatively spliced to yield isoforms containing the catalytic tyrosine kinase domain (TK+) and truncated isoforms which lack this domain (TK-). To test for potential differences in their roles during early stages of neural development, TK+ and TK- isoforms were ectopically expressed in cultures of neural crest, the stem cell population that gives rise to the vast majority of the peripheral nervous system. NT-3 activation of ectopically expressed trkC TK+ receptors promoted both proliferation of neural crest cells and neuronal differentiation. Strikingly, the trkC TK- isoform was significantly more effective at promoting neuronal differentiation, but had no effect on proliferation. Furthermore, the trkC TK- response was dependent on a conserved receptor cytoplasmic domain and required the participation of the p75(NTR) neurotrophin receptor. Antibody-mediated receptor dimerization of TK+ receptors, but not TK- receptors, was sufficient to stimulate differentiation. These data identify a phenotypic response to activation of the trkC TK- receptor and demonstrate a functional interaction with p75(NTR), indicating there may be multiple trkC receptor-mediated systems guiding neuronal differentiation.

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.

Differential regulation of substance P by all members of the nerve growth factor family of neurotrophins in avian dorsal root ganglia throughout development

This study examined the effects of nerve growth factor, brain-derived neurotrophic factor, neurotrophin-3 and neurotrophin-4/5 on substance P levels in dorsal root ganglia of the quail shortly after ganglia formation (stage 26, embryonic day 4.5), during the middle of development (stage 33, embryonic day 7.5) and during late development (stage 44, embryonic day 14). It has already been shown that nerve growth factor increases levels of substance P during the middle and late stages of development, and that messenger RNA for the neurotrophin receptors, trkA, trkB and trkC is present at all of these stages. Dorsal root ganglia were isolated, rinsed with defined medium to dilute endogenous neurotrophins and exposed to one of the neurotrophins for either 4 or 20 h. Substance P levels were quantitated using enzyme immunoassay. None of the neurotrophins had any effect on substance P levels in dorsal root ganglia obtained at stage 26 after either a 4 or 20 h exposure time. Nerve growth factor, brain-derived neurotrophic factor, neurotrophin-3 and neurotrophin-4/5 all significantly increased levels of substance P after either a 4 h or 20 h incubation in ganglia obtained at stages 33 and 44. The effects of nerve growth factor and neurotrophin-3 were specific: increases in substance P were completely blocked by simultaneous exposure to antibodies against either nerve growth factor or neurotrophin-3. The absence of any effect of neurotrophins on substance P expression during early development was unexpected, since dorsal root ganglia exhibit substantial levels of substance P and receptors for the neurotrophins are present and are apparently functional. It was also surprising that brain-derived neurotrophic factor, neurotrophin-3 and neurotrophin-4/5 induced increases in substance P levels during the middle and late stages of development, since substance P was thought to be exclusively localized to small TrkA neurons in dorsal root ganglia. However, immunocytochemical examination of dorsal root ganglia at stages 33 and 44 revealed substance P-like immunoreactivity in larger neurons as well as in small neurons. The results of this study have shown that different cellular responses to neurotrophins, such as effects on survival and/or peptide expression, may be acquired with differing temporal patterns not strictly related to expression of their receptors. Further, the regulation of neuropeptide synthesis in dorsal root ganglia is not due to any one neurotrophic factor. and the factors that regulate expression during early development are still unknown.

Limb proprioceptive deficits without neuronal loss in transgenic mice overexpressing neurotrophin-3 in the developing nervous system

The role of neurotrophin-3 (NT3) during sensory neuron development was investigated in transgenic mice overexpressing NT3 under the control of the promoter and enhancer regions of the nestin gene, an intermediate filament gene widely expressed in the developing nervous system. Most of these mice died during the first postnatal day, and all showed severe limb ataxia suggestive of limb proprioceptive dysfunction. Tracing and histological analyses revealed a complete loss of spindles in limb muscles, absence of peripheral and central Ia projections, and lack of cells immunoreactive to parvalbumin in the dorsal root ganglion (DRG). Despite these deficits, there was no neuronal loss in the DRG of these mice. At birth, transgenic DRG showed increased neuron numbers, and displayed a normal proportion of neurons expressing substance P, calcitonin gene-related peptide and the NT3 receptor trkC. Transgenic dorsal roots exhibited an increased number of axons at birth, indicating that all sensory neurons in transgenic mice projected to the dorsal spinal cord. Despite the absence of central Ia afferents reaching motorneurons, several sensory fibers were seen projecting towards ectopic high levels of NT3 in the midline of transgenic spinal cords. These findings suggest novel roles for NT3 in differentiation of proprioceptive neurons, target invasion and formation of Ia projections which are independent from its effects on neuronal survival.

Autocrine regulation of neural crest cell development by steel factor

Steel factor (SLF) and its cognate receptor, c-kit, have been implicated in the generation of melanocytes from migrating neural crest (NC) cells during early vertebrate embryogenesis. However, the source of SLF in the early avian embryo and its precise role in melanogenesis are unclear. We report here that NC cells themselves express and release SLF protein, which in turn acts as an autocrine factor to induce melanogenesis in nearby NC cells. These results indicate that NC cell subpopulations play an active role in the determination of their cell fate and suggest a different developmental role for the embryonic microenvironment than what has been previously proposed.

Receptor tyrosine kinase-dependent neural crest migration in response to differentially localized growth factors

How different neural crest derivatives differentiate in distinct embryonic locations in the vertebrate embryo is an intriguing issue. Many attempts have been made to understand the underlying mechanism of specific pathway choices made by migrating neural crest cells. In this speculative review we suggest a new mechanism for the regulation of neural crest cell migration patterns in avian and mammalian embryos, based on recent progress in understanding the expression and activity of receptor tyrosine kinases during embryogenesis. Distinct subpopulations of crestderived cells express specific receptor tyrosine kinases while residing in a migration staging area. We postulate that the differential expression of receptor tyrosine kinases by specific subpopulations of neural crest cells allows them to respond to localized growth factor ligand activity in the embryo. Thus, the migration pathway taken by neural crest subpopulations is determined by their receptor tyrosine kinase response to the differential localization of their cognate ligand.

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

The influence of pertinent growth factors on proliferation and differentiation of quail neural crest cell was assessed by in vitro colony assay in a serum-free (0.5% chick embryo-extract supplemented) culture medium. The factors tested included basic fibroblast growth factor (bFGF; FGF-2), neurotrophins, and transforming growth factor-beta-1 (TGF-beta). Both bFGF and neurotrophins are implicated in the development of the peripheral nervous system, whereas TGF-beta can affect cell differentiation and modulate the action of other growth factors. Bromodeoxyuridine (BrdU) incorporation indicated that bFGF is mitogenic to pluripotent neural crest cells (and/or their immediate progeny) and to committed melanogenic cells. However, this was not reflected in an increase in colony size. In contrast, colony size did increase when nerve growth factor (NGF) was present in addition to bFGF. This indicated either that both factors are required to initiate cell proliferation or that at least some bFGF-exposed cells become dependent on neurotrophins for survival. Sequential addition of the factors showed that exposure to bFGF was required prior to the presence of a neurotrophin, thus favoring the latter possibility. All three neurotrophins tested, NGF, brain-derived neurotrophic factor (BDNF), and neurotrophin-3 (NT-3), were capable of supporting survival of pluripotent neural crest cells (or their closely related progeny) in the presence of bFGF. In the absence of bFGF, neurotrophins did not affect colony size. Although the BrdU data indicated that bFGF is also a mitogen for committed melanogenic cells, the size of pigmented colonies did not change in the presence of bFGF alone or of bFGF plus a neurotrophin. This suggested that another, yet to be determined, factor is required for the survival of proliferating melanogenic cells. Colony assays were also performed in the presence and absence of TGF-beta, both alone and in combination with bFGF plus NGF. TGF-beta inhibited proliferation of both pluripotent neural crest cells (and/or their immediate derivatives) and of committed melanogenic cells, causing a decrease in colony size. When TGF-beta was added to the culture medium together with the bFGF/NGF combination, this also caused a significant decrease in colony size, similar to the one observed with TGF-beta alone. TGF-beta blocked proliferation even when the cells were exposed 24 to 48 hr to the bFGF/NGF combination prior to addition of TGF-beta. Neurogenesis increased significantly in the presence of TGF-beta. The number per colony of both adrenergic cells and sensory neuron precursors increased in TGF-beta-treated neuroblast-positive colonies. The following new insights were derived from this study: 1) basic FGF is a mitogen for pluripotent neural crest cells (and/or their immediate derivatives); 2) pluripotent and committed melanogenic neural crest cells that have been exposed to bFGF become dependent on trophic support; 3) all neurotrophins tested (NGF, BDNF or NT-3) can fulfill the trophic requirement of bFGF-exposed pluripotent cells, but not for melanogenic cells; 4) TGF-beta is an anti-proliferative signal for pluripotent neural crest cells and for committed melanogenic cells; 5) the TGF-beta-mediated anti-proliferative signal dominates over the bFGF/neurotrophin-mediated mitogenic signal; and 6) TGF-beta enhances sensory and adrenergic neurogenesis, possibly by acting upon a common neurogenic precursor cell. Furthermore, our work confirms previous reports by other investigators, who showed that bFGF promotes and TGF-beta inhibits proliferation of pigment cells.

Brain-derived neurotrophic factor spares choline acetyltransferase mRNA following axotomy of motor neurons in vivo

Choline acetyltransferase (ChAT) is a functional and specific marker gene for neurons such as primary motor neurons that synthesize and release acetylcholine as a neurotransmitter. In adult mammals, transection of the peripheral nerve results in a loss of immunoreactivity for ChAT in the injured motor neurons without affecting their cell number. Using a quantitative RNase protection assay, we have investigated dynamic changes in ChAT mRNA levels following axotomy of motor neurons in the brainstem of adult rats. One week after transection of the left hypoglossal nerve, levels of ChAT mRNA in the ipsilateral side of the hypoglossal motor nucleus decreased dramatically to around 10% when compared to the uninjured contralateral side. When cut axons were chronically exposed to brain-derived neurotrophic factor (BDNF) for 1 week, ChAT mRNA levels were maintained at 63% of control levels. Thus, BDNF can abrogate the injury-induced loss of ChAT mRNA in mature motor neurons in vivo. In contrast, neither neurotrophin 4/5 nor nerve growth factor could prevent the decrease in message. This effect of BDNF on ChAT mRNA levels following peripheral injury to motor neurons demonstrates the existence of regulatory pathways responsive to neurotrophic factors that can "rescue" or "protect" cholinergic gene expression.

A paracrine effect for neuron-derived BDNF in development of dorsal root ganglia: stimulation of Schwann cell myelin protein expression by glial cells

Addition of neurons to cultures of non-neuronal cells derived from quail embryonic dorsal root ganglia causes a 2.5-fold increase in the proportion of cells that express the glial marker Schwann cell myelin protein (SMP) when compared to cultures devoid of neurons. This effect is mediated by BDNF because incubation with a trkB immunoadhesin that sequesters BDNF, but not with trkA or trkC immunoadhesins, abolishes this stimulation. This neuronal activity can be mimicked by treatment with soluble BDNF that stimulates specifically the conversion of SMP-negative glial cells into cells that express this phenotype. That BDNF is the endogenous neuron-derived factor affecting glial development is further supported by the observation that BDNF is extensively expressed in developing sensory neurons of the avian ganglia both in vivo and in vitro, but not by the satellite cells. These results show for the first time a paracrine role for neuronal BDNF on differentiation of peripheral glial cells. This effect of BDNF is likely to be mediated by the p75 neurotrophin receptor because: (1) p75 immunoreactive protein is expressed by a subset of satellite cells; (2) neutralization of p75 abolishes the BDNF-induced stimulation; (3) a treatment of non-neuronal cell cultures with equimolar concentrations of either soluble NGF or NT-3 also affects the proportion of cells that become SMP-positive. Whereas NGF stimulates the acquisition of this glial antigen to a similar extent as BDNF, NT-3 inhibits its expression, suggesting that distinct neurotrophins signal differentially through p75. These findings also suggest that the definitive phenotype of peripheral glia is determined by a balance between positive and inhibitory signals arising in adjacent neurons.

Lack of neurotrophin-3 results in death of spinal sensory neurons and premature differentiation of their precursors

To understand mechanisms resulting in the absence of two-thirds of spinal sensory neurons in mice lacking NT-3, we have compared dorsal root ganglia development in normal and mutant embryos. The reduction in neurons, achieved by E13, results from several deficits: first, elevated neuronal apoptosis significantly reduces neuronal numbers; second, elevated neurogenesis between E11 and E12, without changes in rates of precursor proliferation or apoptosis, depletes the precursor pool; consequently, the reduced precursor pool prevents increases in neuronal numbers between E12 and E13, when most neurons are born in normal animals. Although deficits occur before final target innervation, we show that NT-3 is expressed at all stages in regions accessible to these neurons or their axons and is only restricted to final targets after innervation.

Synchronous onset of NGF and TrkA survival dependence in developing dorsal root ganglia

Determinations of dorsal root ganglion (DRG) neuron loss in nerve growth factor (NGF) and neurotrophin-3 (NT-3) null mutant mice have supported the concept that neurons can switch neurotrophin dependence by revealing that many neurons must require both of these factors acting either sequentially or simultaneously during development. The situation is complex, however, in that NT-3(-/-) mutant mice show far greater neuron loss than mice deficient in the NT-3 receptor TrkC, suggesting that NT-3 may support many DRG neurons via actions on the NGF receptor TrkA. To assess the possibility of ligand-receptor cross-talk as a developmental mechanism, we have compared the onset of survival dependence of lumbar DRG neurons on NT-3, TrkC, NGF, and TrkA signaling in mice deficient in these molecules as a result of gene targeting. At embryonic day 11.5 (E11.5), virtually all lumbar DRG cells express TrkC mRNA and many require NT-3 and TrkC signaling for survival. In contrast, although many lumbar DRG cells also express TrkA at E11.5, there is little survival dependence on TrkA signaling. By E13.5, most lumbar DRG cells have downregulated TrkC mRNA. The onset of survival dependence on NGF and TrkA-signaling is concurrent and of equal magnitude at E13.5, demonstrating that NT-3 alone does not support DRG neurons via TrkA, nor can NT-3 compensate for the loss of NGF. We conclude that many murine DRG cells require NT-3 activation of TrkA is unimportant to these early NT-3 survival-promoting actions. We suggest that the discrepancy in cell loss between NT-3(-/-) and trkC(-/-) mutants is attributable to the ability of NT-3 to support DRG neurons via TrkA in the artificial situation where TrkC is absent.

Most classes of dorsal root ganglion neurons are severely depleted but not absent in mice lacking neurotrophin-3

During development, many neurons in the dorsal root ganglia require neurotrophin-3 for survival. However, it is not known precisely which subpopulations of sensory neurons, other than the proprioceptive afferents, are neurotrophin-3 dependent in vivo. In this study, using a battery of neurochemical markers that label different subpopulations of dorsal root ganglion neurons, we found a widespread, about 60-65% loss of cells in most subpopulations in neurotrophin-3 deficient mice. Intermediate losses were found in the heterozygous mutant mice consistent with a gene dosage effect. In agreement with this, the cell size distribution between the homozygous mutant and wild type mice was virtually identical. The loss of small neurons containing calcitonin gene-related peptide, substance P and thiamine monophosphatase activity suggests that many unmyelinated primary afferents are also lost in the mutant animals. The fact that many different sensory neuron subpopulations are lost to the same extent in neurotrophin-3 deficient mice is consistent with the proposed early role of neurotrophin-3 during neurogenesis. Interestingly, calretinin immunoreactive neurons, which contribute a minor subpopulation, were not affected suggesting that neurotrophin-3 independent regulation of neurogenesis occurs in addition to prominent neurotrophin-3 dependent mechanisms.

Inhibition of the NT-3 receptor TrkC, early in chick embryogenesis, results in severe reductions in multiple neuronal subpopulations in the dorsal root ganglia

To assess functions of neurotrophins at defined times in development, we have prepared antibodies of the extracellular domains of each of the trk receptors. Here, antibodies to trkC, the major receptor for NT-3, are used to examine trkC expression and function during the formation and maturation of the chick dorsal root ganglion (DRG). Our results show that in the immature DRG, the majority of cells express trkC, and inhibition of trkC activation results in reductions in neuronal numbers before the period of target-mediated cell death, the time when neurotrophins previously have been shown to regulate survival. Furthermore, blockade of trkC in ovo induced reductions in subpopulations of DRG neurons known to be dependent on NGF, in addition to those dependent on NT-3 during the target-regulated cell death period. An early function for NT-3 on immature DRG neurons is supported further by data presented here that demonstrate that whereas BDNF and NGF can support a subset of immature DRG neurons in vitro, activation of the trkC receptor either by NT-3 binding or via antibody-mediated cross-linking induces the most robust survival response. When all three neurotrophins are combined, the number of surviving neurons does not exceed that supported by NT-3 alone. Together, these data are consistent with coexpression of more than one trk receptor family member on immature sensory neurons, and they demonstrate that inhibition of trkC activation has surprisingly early and pleiotrophic effects on the development of spinal sensory ganglia.

TRK and p75 neurotrophin receptor systems in the developing human brain

The prenatal development of the neurons immunoreactive for high-affinity tropomycin-related kinase (trk) receptor (pan trk which recognizes trkA, trkB, and trkC) and low-affinity p75 neurotrophin receptor (p75NTR) was examined in the human brain from embryonic weeks 10 to 34 of gestation. In the embryonic week 10 specimen in which only brainstem regions were available for evaluation, trk immunoreactivity (trk-ir) was observed in the ventral cochlear, solitary, raphe, spinal trigeminal, and hypoglossal nuclei, as well as the vestibular complex and medullary reticular formation. At this time point of gestation, p75ntr-immunoreactive (p75NTR-ir) staining was observed within these same regions plus the inferior olivary and ambiguus nuclei. At embryonic week 14, trk-ir neurons were seen within the subplate zone of the entorhinal cortex, basal forebrain, caudate nucleus, putamen, external segment of the globus pallidus, specific thalamic nuclei, lateral mammillary nucleus, habenula nucleus, select brainstem nuclei, and the dentate nucleus of cerebellum. At this gestational time point, p75NTR-ir neurons were observed in each of these structures, with the exception of the caudate nucleus, specific thalamic nuclei, lateral mammillary nucleus, and habenula nucleus. Additionally, p75NTR-ir neurons were observed within the corpus callosum. The staining pattern for both trk and p75NTR remained unchanged at embryonic weeks 15 to 16 except for the addition of trk-ir and p75NTR-ir within the cortical subplate zone, hippocampus, and subthalamic nucleus. By embryonic week 18, trk-ir neurons were widely expressed within mostly all thalamic nuclei. In contrast, trk-ir was no longer seen within the hypoglossal, cuneate, and gracile nuclei at this time point. This staining pattern for trk and p75NTR remained virtually unchanged from embryonic weeks 19 to 20 and embryonic weeks 16 to 20, respectively. From embryonic weeks 22 to 34, the distribution of both trk-ir and p75NTR-ir neurons changed gradually. During this period, neurons in most thalamic and some brainstem nuclei became progressively immunonegative for trk, whereas neurons in the neocortical subplate zone, corpus callosum, and hilar region of dentate gyrus gradually lost immunoreactivity for p75NTR. These data demonstrate an important and complex role for both the high-(trk) and low- (p75) affinity neurotrophin receptors during the development of multiple neuronal systems in the human brain.

Growth, proliferation, and cell death in the ontogeny of transient DRG (Froriep's ganglia) of chick embryos

A striking example of axial patterning in nervous system development is the unusual fate of dorsal root ganglia (DRG) that develop in the most rostral somites, the Froriep's ganglia. In amniotes, the DRG that develop adjacent to the occipital (cranial) and the first cervical segments of the CNS "disappear" early in embryonic development. In contrast, all other DRG are present throughout the animal's life. We here reexamine in greater detail the ontogeny of the longest surviving Froriep's ganglion of the chick embryo, DRG C-2. By 50 h of development (stage, st. 15), an anlagen of a DRG had formed in C-2 that was indistinguishable from those of adjacent "permanent" ganglia. At st. 18 [embryonic day (E) 2.5+], the C-2 DRG had the same shape and volume as permanent ganglia C-5 and C-6. C-2's development first diverged from that of normal DRG at st. 19 (E3-), when C-2 was observed to be half the size and shaped differently from its neighbors, and its peripheral nerve root began to degenerate. Two cellular mechanisms appear to contribute to the reduced size of C-2 compared to normal DRG at st. 20 at this early stage: lower proliferation and higher apoptosis rates. One-third fewer C-2 cells were found to be in the S phase when compared to neighboring ganglia, and apoptotic cells were more than three times more abundant in C-2 than in conventional DRG at this stage. The C-2 DRG continued to grow, but at a slower pace than neighboring ganglia through st. 32 (E7). At the height of the normal programmed DRG cell death in normal cervical DRG at st. 28 (E6), even more massive apoptosis occurred in C-2, which resulted in the absence of this ganglion in 80% of st. 36 (E10) embryos. A recent study demonstrated that the overexpression of a single Hox gene can "rescue" the C-2 DRG in transgenic mice. We speculate that Hox genes may produce the difference in fate between C-2 and normal DRG by modulating proliferation and apoptosis via modified neurotrophic factor and/or receptor expression.

Roles of neurotrophin-3 during early development of the peripheral nervous system

The neurotrophins are structurally related proteins regulating cell numbers in the developing vertebrate nervous system. They are necessary survival factors preventing the death of specific neuronal populations. Previous experiments have indicated that the administration of nerve growth factor or of brain-derived neurotrophic factor during the formation of sensory ganglia and of target innervation increases the number of neurons by preventing normally occurring neuronal death. These results support the view that during development, neuronal numbers are adjusted to the size of the target tissue by the release of limiting amounts of neurotrophins. However, increasing the levels of neurotrophin-3 during the formation of sensory ganglia results in a marked decrease in neuronal numbers, possibly as a consequence of premature cessation of sensory neuroblast proliferation. In sympathetic ganglia, the application of neurotrophin-3 during the formation of the sympathetic chain causes cell numbers to increase, a result also observed following the application of nerve growth factor. It thus appears that neurotrophin-3 and nerve growth factor can regulate cell numbers well before the period of target-derived control, and that neurotrophin-3 affects neuronal numbers in sensory and sympathetic ganglia in opposite ways.

Expression of the trk family of neurotrophin receptors in developing and adult dorsal root ganglion neurons

Expression of trk receptors is a major determinant of neurotrophin responsiveness of sensory neurons. Although it has been apparent for some time that subpopulations of dorsal root and trigeminal ganglion neurons respond in vitro to each of the members of the neurotrophin family, the extent to which functionally distinct subclasses of sensory neurons are dependent on the actions of different neurotrophins for their development and function remains an active area of investigation. One step towards elucidating the role of various neurotrophins in development and function of sensory neurons has been to examine the distribution of trk receptors on sensory neurons. These studies have clearly revealed that members of the trk family are differentially expressed in functionally distinct populations of both developing and mature sensory neurons and, further, have provided evidence consistent with a shift in neurotrophin responsiveness during the development of sensory neurons.

The role of neurotrophins in development of neural-crest cells that become sensory ganglia

A fundamental issue of neural-crest ontogeny is understanding how different types of cells are created at the right time and in the correct numbers. Sensory ganglia are among the many derivatives of the vertebrate neural crest. Their proper formation requires the regulation of several processes such as cell fate specification, proliferation, survival, and terminal differentiation. The timescale of the occurrence of processes involved in the regulation of cell number and identity, coincides with key morphogenetic events such as cell migration, homing and gangliogenesis. To gain insight into these processes, we characterized the cellular basis of metameric migration of neural-crest cells and of consequent ganglion organization, which are imposed by intrinsic differences within rostral and caudal sclerotomal compartments. We also established a transient requirement for neural tube-derived factors in regulating the proliferation, survival and differentiation of prospective DRG cells. Additionally, we showed that cooperation between the mesodermal cells and the neural tube is necessary for modulating cell number in the nascent ganglia. BDNF, NT-3 and basic FGF were found to mediate this environmental signalling. All the above factors display neurogenic activity for a subset of early-committed sensory neuron progenitors. This observation raises the possibility of an early redundancy in the response of individual neural-crest progenitors to distinct factors. This overlap in responsiveness progressively disappears upon the colonization of specific ganglionic sites and the subsequent establishment of selective innervation patterns by post-mitotic sensory neurons.

Expression and regulation of brain-derived neurotrophic factor and neurotrophin-3 mRNAs in distinct avian motoneuron subsets

We performed a detailed study of the expression of neurotrophin-3 and brain-derived neurotrophic factor transcripts in spinal motoneurons using in situ hybridization of serially sectioned chick embryos aged 3 to 8 days (E3 to E8). Neurotrophin-3 mRNA is detected in motoneuron subsets from E3.5 to E4 only in brachial segments of the neural tube and from E5 in both brachial and lumbar regions. Expression of brain-derived neurotrophic factor mRNA is first evident on E5 in a subset of brachial level motoneurons and from E6 also in motoneurons located in the rostral-most portion of the lateral motor column, as well as in the tail-innervating region of the spinal cord. Analysis along the rostrocaudal extent of the brachial lateral motor column reveals an overlap zone of expression of both neurotrophins of about two segments. In transverse sections of this region, it is observed that neurotrophin-3-positive motoneurons preferentially occupy the lateral part of the column, whereas brain-derived neurotrophic factor-producing motoneurons are localized in a more medial position. These results show that the two factors are synthesized at discrete axial levels of the spinal cord by distinct motoneuron subpopulations. Since brain-derived neurotrophic factor mRNA is expressed within the brachial but not the lumbar lateral motor column, we tested the possibility that brain-derived neurotrophic factor expression is regulated by the type of peripheral target, that is, the wing or the leg. Unilateral transplantation of a wing bud instead of a leg bud and vice versa, prior to the onset of peripheral innervation, failed to alter the original pattern of brain-derived neurotrophic factor mRNA observed in either level of the axis. Thus, the early synthesis of brain-derived neurotrophic factor by subsets of spinal motoneurons is independent of the type of peripheral target and may instead reflect intrinsic differences between motoneuron populations.

Expression of neurotrophin trk and p75 receptors in quail embryos undergoing gastrulation and neurulation

We have previously demonstrated the presence of mRNA for the full-length neurotrophin receptors trkA, trkB and trkC in quail embryos from stages 1 through 6 using reverse transcription followed by the polymerase chain reaction (RT-PCR; Yao et al. [1994] Dev. Biol. 165: 727-730). Furthermore, we showed that mRNA for the neurotrophins brain-derived neurotrophic factor and neurotrophin-3 was present from stage 1 onward, while nerve growth factor mRNA began to be expressed at stage 5. In the present study, wholemount in situ hybridization was used to localize full-length trk mRNA in embryos from stages 3 through 10. Structures expressing trkC mRNA included the primitive streak and Hensen's node, the neural plate or notochord, somites and the rostral neural tube. trkA and trkB mRNA were expressed at much lower levels than trkC mRNA; however, staining was detected on the primitive streak and Hensen's node. In addition to trk mRNA, we have also demonstrated the presence of full-length Trk protein in embryos from stages 3 through 11, suggesting that the trk mRNA detected at these early stages is translated into functional cell surface receptors. To support this hypothesis, we have shown that neurotrophins can induce phosphorylation of Trk on tyrosine residues, at least at stage 11. We also detected mRNA and protein for the nontyrosine kinase neurotrophin receptor, p75, at similar stages. The presence of neurotrophin receptors, as well as neurotrophin mRNA, in embryos undergoing gastrulation and neurulation leads to speculation that neurotrophins may be playing a role in these processes.

Neurotrophin-3 as an essential signal for the developing nervous system

Rapid advances in characterization of the biological actions mediated by the third member of the neurotrophin family, neurotrophin-3 (NT-3), have been made recently in vitro as well as in situ. These have been made possible by the cloning of the genes for NT-3 and for its transducing receptor tyrosine kinase TrkC. This article will focus on the roles of NT-3 in the nervous system. In situ localization of NT-3 consistent with that of its receptor is manifested at all developmental stages studied and into adulthood. Through TrkC, NT-3 signals a number of trophic effects, ranging from mitogenesis, promotion of survival, or differentiation, depending on the developmental stage of the target cells. The sites of action of NT-3 reside primarily in the peripheral nervous system (PNS), various areas of the central nervous system (CNS), and in the enteric system (ENS). Analyses of the phenotypes of transgenic mice lacking NT-3 or injection of embryos with a blocking antibody have so far revealed the essential role of NT-3 in development of specific populations of the PNS, and in particular of proprioceptive, nodose, and auditory sensory neurons and of sympathetic neurons. The actions of NT-3 also extend to modulation of transmitter release at several types of synapses in the periphery as well as in the adult CNS. In addition, NT-3 may play a role in the development of tissues other than the nervous system, such as the cardiovascular system. Future investigations will widen the understanding of the many roles of NT-3 on both neuronal and nonneuronal cells.

The induction of neurotrophin and TRK receptor mRNA expression during early avian embryogenesis

Nerve growth factor, brain-derived neurotrophic factor and neurotrophin-3, designated neurotrophins, are a family of neurotrophic factors, having important functions in the survival of embryonic and adult neuronal subpopulations. Through the trk family of receptors, these neurotrophins utilize phosphotyrosine-mediated signal transduction. We have used RT-PCR to detect the expression of mRNA for the above neurotrophins and their respective receptors, namely trkA, trkB and trkC in embryonic stages 1-8 of chicken development. While trkA and trkC mRNAs were expressed from stage 1 onwards, NGF and NT-3 mRNAs were expressed only at stages 3 and 5, respectively. In contrast, BDNF mRNA was expressed at stage 1, being the only neurotrophin expressed prior to expression of its respective receptor trkB. However, the latter was not expressed until stage 8. These results indicate an earlier expression of some but not all trk proto-oncogenes, suggesting that the two different receptor mRNAs expressed i.e. trkA and trkC in conjunction with BDNF, at stage 1, may act in aspects of very early embryonic development, such as gastrulation. Thereafter, mRNAs for trkB, NGF and NT-3 are expressed reflecting their later action in early embryonic development.

In vivo effects of neurotrophin-3 during sensory neurogenesis

The neurotrophins nerve growth factor, brain-derived neurotrophic factor and neurotrophin-3 are structurally related proteins regulating the number of neurons in peripheral ganglia of the nervous system. Increased levels of nerve growth factor or of brain-derived neurotrophic factor selectively prevent normally occurring neuronal death, while the targeted elimination of all three genes decreases neuronal numbers. As previous studies indicated that the lack of neurotrophin-3 affects sensory ganglia already during gangliogenesis, the levels of this neurotrophin were increased during selected periods of chick development. We found that early, but not late, applications of neurotrophin-3 lead to a marked decrease in neuronal numbers in peripheral sensory ganglia. This decrease is not seen with BDNF and does not selectively affect subtypes of dorsal root ganglion neurons. It is accompanied by, and might result from, a decrease in the number of proliferating neuroblasts in sensory ganglia of treated embryos.

Epithelial-mesenchymal conversion of dermatome progenitors requires neural tube-derived signals: characterization of the role of Neurotrophin-3

Development of the somite-derived dermatome involves conversion of the epithelial dermatome progenitors into mesenchymal cells of the dermis. In chick embryos, neural tube-derived signals are required for this conversion, as the interposition of a membrane between neural tube and somites results in a failure of the dermatome to lose its epithelial arrangement. However, dermis formation can be completely rescued by coating the membranes with Neurotrophin-3, but not with the related molecule Nerve growth factor. Neurotrophin-3 was also found to be necessary for dermatome dissociation using in vitro explants or partially dissociated dermomyotomes. The functional relevance of these observations was investigated by neutralizing endogenous Neurotrophin-3 using a specific blocking antibody. Antibody-treated embryos revealed the presence of tightly aggregated cells between myotome and ectoderm instead of the loose dermal mesenchyme observed in embryos treated with control antibodies. As previous studies have demonstrated the presence of Neurotrophin-3 in the neural tube, these results suggest that it may be a necessary neural tube-derived signal required for early stages of dermis formation.

Distinct roles for bFGF and NT-3 in the regulation of cortical neurogenesis

To identify molecules that regulate the transition of dividing neuroblasts to terminally differentiated neurons in the CNS, conditions have been developed that allow the neuronal differentiation of cortical precursor cells to be examined in vitro. In these cultures, the proliferation of undifferentiated precursor cells is controlled by basic fibroblast growth factor (bFGF). The proliferative effects of bFGF do not preclude the action of signals that promote differentiation, since addition of neurotrophin-3 (NT-3) antagonizes the proliferative effects of bFGF and enhances neuronal differentiation. In addition, blocking NT-3 function with neutralizing antibodies leads to a marked decrease in the number of differentiated neurons, without affecting the proliferation of cortical precursors or the survival of postmitotic cortical neurons. These observations suggest that bFGF and NT-3, by their distinct effects on cell proliferation and differentiation, are key regulators of neurogenesis in the CNS.

Induction of motor neurons by Sonic hedgehog is independent of floor plate differentiation

BACKGROUND

The differentiation of floor plate cells and motor neurons in the vertebrate neural tube appears to be induced by signals from the notochord. The secreted protein encoded by the Sonic hedgehog (Shh) gene is expressed by axial midline cells and can induce floor plate cells in vivo and in vitro. Motor neurons can also be induced in vitro by cells that synthesize Sonic hedgehog protein (Shh). It remains unclear, however, if the motor-neuron-inducing activity of Shh depends on the synthesis of a distinct signaling molecule by floor plate cells. To resolve this issue, we have developed an in vitro assay which uncouples the notochord-mediated induction of motor neurons from floor plate differentiation, and have used this assay to examine whether Shh induces motor neurons in the absence of floor plate differentiation.

RESULTS

Floor plate cells and motor neurons were induced in neural plate explants grown in contact with the notochord, but only motor neurons were induced when explants were separated from the notochord. COS cells transfected with Shh induced both floor plate cells and motor neurons when grown in contact with neural plate explants, whereas only motor neurons were induced when the explants were grown at a distance from Shh-transfected COS cells. Direct transfection of neural plate cells with an Shh-expression construct induced both floor plate cells and motor neurons, with motor neuron differentiation occurring prior to, or coincidentally with, floor plate differentiation. The induction of motor neurons appears, therefore, not to depend on floor plate differentiation.

CONCLUSIONS

The induction of motor neurons by Shh does not depend on distinct floor-plate-derived signaling molecules. Shh can, therefore, initiate the differentiation of two cell types that are generated in the ventral region of the neural tube. These results show that the early development of motor neurons involves the inductive action of Shh, whereas the survival of motor neurons at later stages of embryonic development requires neurotrophic factors.

Expression of trk and neurotrophin mRNA in dorsal root and sympathetic ganglia of the quail during development

The nerve growth factor (NGF) family of neurotrophins exerts effects by binding to products of the trk family of proto-oncogenes. We examined the expression of both trk and neurotrophin mRNA during the entire range of development of quail dorsal root ganglia (DRG) and sympathetic ganglia (SG) using in situ hybridization and reverse transcriptase-polymerase chain reaction (RT-PCR). TrkC mRNA was present in neurons or their precursors from the time of formation of DRG (stage 18, embryonic day 2.5 [E2.5]) and throughout development. The number of labeled cells changed, however, from a majority to a minority at later developmental stages. Expression of trkA mRNA was not detected in DRG until stage 30 (E6) by in situ hybridization, although results with RT-PCR were positive at stage 23 (E3.5). Labeling was always detected on a majority of neurons or their precursors. SG exhibited low levels of trkC mRNA during the later stages of development, whereas trkA mRNA was present from stage 34 onward in most neurons. We have also shown that NGF, neurotrophin-3 (NT-3), and brain-derived neurotrophic factor (BDNF) mRNA were present at all stages examined (stages 23 through 45 for DRG, stages 35-36 and 45 for SG). In DRG, NGF mRNA expression was limited to support cells, whereas NT-3 and BDNF mRNA were detected in both neurons and support cells. These results suggest that neurotrophins could serve a local function in developing ganglia, which can be correlated with the presence of their respective receptors.