A local action of neurotrophin-3 prevents the death of proliferating sensory neuron precursor cells

Neurotrophin Responsiveness of Sympathetic Neurons Is Regulated by Rapid Mobilization of the p75 Receptor to the Cell Surface through TrkA Activation of Arf6

The p75 neurotrophin receptor (p75NTR) plays an integral role in patterning the sympathetic nervous system during development. Initially, p75NTR is expressed at low levels as sympathetic axons project toward their targets, which enables neurotrophin-3 (NT3) to activate TrkA receptors and promote growth. Upon reaching nerve growth factor (NGF) producing tissues, p75NTR is upregulated, resulting in formation of TrkA-p75 complexes, which are high-affinity binding sites selective for NGF, thereby blunting NT3 signaling. The level of p75NTR expressed on the neuron surface is instrumental in regulating trophic factor response; however, the mechanisms by which p75NTR expression is regulated are poorly understood. Here, we demonstrate a rapid, translation independent increase in surface expression of p75NTR in response to NGF in rat sympathetic neurons. p75NTR was mobilized to the neuron surface from GGA3-postitive vesicles through activation of the GTPase Arf6, which was stimulated by NGF, but not NT3 binding to TrkA. Arf6 activation required PI3 kinase activity and was prevented by an inhibitor of the cytohesin family of Arf6 guanine nucleotide exchange factors. Overexpression of a constitutively active Arf6 mutant (Q67L) was sufficient to significantly increase surface expression of p75NTR even in the absence of NGF. Functionally, expression of active Arf6 markedly attenuated the ability of NT3 to promote neuronal survival and neurite outgrowth, whereas the NGF response was unaltered. These data suggest that NGF activation of Arf6 through TrkA is critical for the increase in p75NTR surface expression that enables the switch in neurotrophin responsiveness during development in the sympathetic nervous system.SIGNIFICANCE STATEMENT p75NTR is instrumental in the regulation of neuronal survival and apoptosis during development and is also implicated as a contributor to aberrant neurodegeneration in numerous conditions. Therefore, a better understanding of the mechanisms that mediate p75NTR surface availability may provide insight into how and why neurodegenerative processes manifest and reveal new therapeutic targets. Results from this study indicate a novel mechanism by which p75NTR can be rapidly shuttled to the cell surface from existing intracellular pools and explores a unique pathway by which NGF regulates the sympathetic innervation of target tissues, which has profound consequences for the function of these organs.

Episomal Induced Pluripotent Stem Cells Promote Functional Recovery of Transected Murine Peripheral Nerve

Traumatic peripheral nerve neurotmesis occurs frequently and functional recovery is often slow and impaired. Induced pluripotent stem cells (iPSCs) have shown much promise in recent years due to its regenerative properties similar to that of embryonic stem cells. However, the potential of iPSCs in promoting the functional recovery of a transected peripheral nerve is largely unknown. This study is the first to investigate in vivo effects of episomal iPSCs (EiPSCs) on peripheral nerve regeneration in a murine sciatic nerve transection model. Episomal iPSCs refer to iPSCs that are generated via Oct3/4-Klf4-Sox2 plasmid reprogramming instead of the conventional viral insertion techniques. It represents a relatively safer form of iPSC production without permanent transgene integration which may raise questions regarding risks of genomic mutation. A minimal number of EiPSCs were added directly to the transected nerve. Functional recovery of the EiPSC group was significantly improved compared to the negative control group when assessed via serial five-toe spread measurement and gait analysis of ankle angles. EiPSC promotion of nerve regeneration was also evident on stereographic analysis of axon density, myelin thickness, and axonal cross-sectional surface area. Most importantly, the results observed in EiPSCs are similar to that of the embryonic stem cell group. A roughly ten-fold increase in neurotrophin-3 levels was seen in EiPSCs which could have contributed to peripheral nerve regeneration and recovery. No abnormal masses or adverse effects were noted with EiPSC administration after one year of follow-up. We have hence shown that functional recovery of the transected peripheral nerve can be improved with the use of EiPSC therapy, which holds promise for the future of nerve regeneration.

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.

Role of motoneuron-derived neurotrophin 3 in survival and axonal projection of sensory neurons during neural circuit formation

Sensory neurons possess the central and peripheral branches and they form unique spinal neural circuits with motoneurons during development. Peripheral branches of sensory axons fasciculate with the motor axons that extend toward the peripheral muscles from the central nervous system (CNS), whereas the central branches of proprioceptive sensory neurons directly innervate motoneurons. Although anatomically well documented, the molecular mechanism underlying sensory-motor interaction during neural circuit formation is not fully understood. To investigate the role of motoneuron on sensory neuron development, we analyzed sensory neuron phenotypes in the dorsal root ganglia (DRG) of Olig2 knockout (KO) mouse embryos, which lack motoneurons. We found an increased number of apoptotic cells in the DRG of Olig2 KO embryos at embryonic day (E) 10.5. Furthermore, abnormal axonal projections of sensory neurons were observed in both the peripheral branches at E10.5 and central branches at E15.5. To understand the motoneuron-derived factor that regulates sensory neuron development, we focused on neurotrophin 3 (Ntf3; NT-3), because Ntf3 and its receptors (Trk) are strongly expressed in motoneurons and sensory neurons, respectively. The significance of motoneuron-derived Ntf3 was analyzed using Ntf3 conditional knockout (cKO) embryos, in which we observed increased apoptosis and abnormal projection of the central branch innervating motoneuron, the phenotypes being apparently comparable with that of Olig2 KO embryos. Taken together, we show that the motoneuron is a functional source of Ntf3 and motoneuron-derived Ntf3 is an essential pre-target neurotrophin for survival and axonal projection of sensory neurons.

In the beginning: Generating neural crest cell diversity

Neural crest cells (NCCs) are migratory cells that delaminate from the neural tube early in development and then disseminate throughout the embryo to give rise to a wide variety of cell types that are key to the vertebrate body plan. During their journey from the neural tube to their peripheral targets, NCCs progressively differentiate, raising the question when the fate of an individual NCC is sealed. One hypothesis suggests that the fate of a NCC is specified by target-derived signals emanating from the environment they migrate through, while another hypothesis proposes that NCCs are already specified to differentiate along select lineages at the time they are born in the neural tube, with environmental signals helping them to realize their prespecified fate potential. Alternatively, both mechanisms may cooperate to drive NCC diversity. This review highlights recent advances in our understanding of prespecification during trunk NCC development.

Role of neurotrophin signalling in the differentiation of neurons from dorsal root ganglia and sympathetic ganglia

Manipulation of neurotrophin (NT) signalling by administration or depletion of NTs, by transgenic overexpression or by deletion of genes coding for NTs and their receptors has demonstrated the importance of NT signalling for the survival and differentiation of neurons in sympathetic and dorsal root ganglia (DRG). Combination with mutation of the proapoptotic Bax gene allows the separation of survival and differentiation effects. These studies together with cell culture analysis suggest that NT signalling directly regulates the differentiation of neuron subpopulations and their integration into neural networks. The high-affinity NT receptors trkA, trkB and trkC are restricted to subpopulations of mature neurons, whereas their expression at early developmental stages largely overlaps. trkC is expressed throughout sympathetic ganglia and DRG early after ganglion formation but becomes restricted to small neuron subpopulations during embryogenesis when trkA is turned on. The temporal relationship between trkA and trkC expression is conserved between sympathetic ganglia and DRG. In DRG, NGF signalling is required not only for survival, but also for the differentiation of nociceptors. Expression of neuropeptides calcitonin gene-related peptide and substance P, which specify peptidergic nociceptors, depends on nerve growth factor (NGF) signalling. ret expression indicative of non-peptidergic nociceptors is also promoted by the NGF-signalling pathway. Regulation of TRP channels by NGF signalling might specify the temperature sensitivity of afferent neurons embryonically. The manipulation of NGF levels "tunes" heat sensitivity in nociceptors at postnatal and adult stages. Brain-derived neurotrophic factor signalling is required for subpopulations of DRG neurons that are not fully characterized; it affects mechanical sensitivity in slowly adapting, low-threshold mechanoreceptors and might involve the regulation of DEG/ENaC ion channels. NT3 signalling is required for the generation and survival of various DRG neuron classes, in particular proprioceptors. Its importance for peripheral projections and central connectivity of proprioceptors demonstrates the significance of NT signalling for integrating responsive neurons in neural networks. The molecular targets of NT3 signalling in proprioceptor differentiation remain to be characterized. In sympathetic ganglia, NGF signalling regulates dendritic development and axonal projections. Its role in the specification of other neuronal properties is less well analysed. In vitro analysis suggests the involvement of NT signalling in the choice between the noradrenergic and cholinergic transmitter phenotype, in the expression of various classes of ion channels and for target connectivity. In vivo analysis is required to show the degree to which NT signalling regulates these sympathetic neuron properties in developing embryos and postnatally.

Autonomic innervation of the developing heart: origins and function

Maintenance of homeostatic circulation in mammals and birds is reliant upon autonomic innervation of the heart. Neural branches of mixed cellular origin and function innervate the heart at the arterial and venous poles as it matures, eventually coupling autonomic output to the cardiac components, including the conduction system. The development of neural identity is controlled by specific networks of genes and growth factors, whereas functional properties are governed by the use of different neurotransmitters. In this review, we summarize briefly the anatomic arrangement of the vertebrate autonomic nervous system and describe, in detail, the innervation of the heart. We discuss the timing of cardiac innervation in the chick and mouse, emphasizing the relationship of the cardiac neural networks to the anatomical structures within the heart. We also discuss the variable contribution of the neural crest to vagal cardiac nerves, and summarize the main neurotransmitters secreted by the developing sympathetic and parasympathetic autonomic divisions. We provide an overview of the main growth factor and gene families involved in neural development, discussing how these factors may impact upon the development of cardiac abnormalities in congenital syndromes associated with autonomic dysfunction.

Neurotrophin-4/5-depletion induces a delay in maturation of the periodontal Ruffini endings in mice

Neurotrophin-4/5 (NT-4/5) - a member of the neurotrophic factors - is a ligand for TrkB, which has been reported to be expressed in the mechanoreceptive Ruffini endings of the periodontal ligament. The present study examined developmental changes in the terminal morphology and neural density in homozygous mice with a targeted disruption of the nt-4/5 gene and wild-type mice by immunohistochemistry for protein gene product 9.5 (PGP 9.5), a general neuronal marker, and by quantitative analysis using an image analyzer. Postnatal development of terminal Schwann cells was also investigated by enzymatic histochemistry for non-specific cholinesterase activity (ChE). Furthermore, the immuno-expression of TrkB and low affinity nerve growth factor receptor (p75-NGFR) was surveyed in the periodontal Ruffini endings as well as trigeminal ganglion. At postnatal 1 week, the lingual periodontal ligament of both types of mice contained PGP 9.5-positive nerve fibers showing a tree-like ramification with axonal swellings in their course. In both types of mice at 2 weeks of age, comparatively thick nerve fibers with a smooth outline increased in number, and frequently ramified to form nerve terminals with dendritic profiles. However, no typical Ruffini endings with irregular outlines observed in the adult wild-type mice were found in the periodontal ligament at this stage. At postnatal 3 weeks, typical Ruffini endings with irregular outlines were discernable in the periodontal ligament of the wild-type mice while the dendritic endings showing smooth outlines were restricted to the homozygous mice. After postnatal 8 weeks, both types of mice showed an increase in the number of Ruffini endings, but the morphology differed between the wild-type and NT-4/5 homozygous mice. In the wild-type mice, a major population of the Ruffini endings expanded their axonal branches and developed many microprojections, resulting in a reduction of endings with smooth outlines. In contrast, we failed to find such typical Ruffini endings in the periodontal ligament of the homozygous mice: A majority of the periodontal Ruffini endings continued to show smooth outlines at postnatal 12 weeks. Quantitative analysis on neural density demonstrated a reduction in the homozygous mice with a significant difference by postnatal 8 weeks. Enzymatic histochemistry for non-specific ChE did not exhibit a distinct difference in the distribution and density of terminal Schwann cells between wild-type and homozygous mice. Furthermore, TrkB and p75-NGFR mRNA and proteins did not differ in the trigeminal ganglion between the two types. The periodontal Ruffini endings also displayed immunoreactivities for TrkB and p75- NGFR in both phenotypes. These findings suggest that the nt-4/5 gene depletion caused a delay in the formation and maturation of the periodontal Ruffini endings in the mice by inhibiting the growth of the periodontal nerves at an early stage, and indicate that multiple neurotrophins such as NT- 4/5 and BDNF might play roles in the development and/or maturation of the periodontal Ruffini endings.

The transcription factor Runx3 represses the neurotrophin receptor TrkB during lineage commitment of dorsal root ganglion neurons

Runx3, a Runt domain transcription factor, determines neurotrophin receptor phenotype in dorsal root ganglion (DRG) neurons. Molecular mechanisms by which Runx3 controls distinct neurotrophin receptors are largely unknown. Here, we show that RUNX3 abolished mRNA induction of TRKB expression, and concomitantly altered the neurotrophin response in a differentiating neuroblastoma cell line. In contrast, RUNX3 did not play a significant role in TRKC regulation even under the relevant BMP signaling pathway. We identified putative regulatory elements of Ntrk2/NTRK2 (a gene that codes for TrkB) using an unbiased computational approach. One of these elements was a highly conserved intronic sequence that contains a cluster of Runx binding sites. In a primary culture of DRG neurons, endogenous Runx3 bound to the consensus cluster, which had repressor activity against the Ntrk2 promoter under the control of NT-3 signaling. Consistent with these findings, Runx3-deficient embryos showed an increased number of trkB+ DRG neurons and failed to maintain trkC expression. Taken together, Runx3 determines TrkC positive sensory neuron identities through the transcriptional repression of TrkB when Trk-BTrkC double positive neurons differentiate into TrkC single positive neurons.

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.

Involvement of neurotrophin-4/5 in regeneration of the periodontal Ruffini endings at the early stage

Little is known about the role of neurotrophin-4/5 (NT-4/5) in the regeneration of mechanoreceptors. Therefore, the present study examined the regeneration process of Ruffini endings in the periodontal ligament in nt-4/5-deficient and wildtype mice following transection of the inferior alveolar nerve by immunohistochemistry for protein gene product 9.5 (PGP 9.5), a general neuronal marker, and by computer-assisted quantitative image analysis. Furthermore, rescue experiments by a continuous administration of recombinant NT-4/5 were performed and analyzed quantitatively. At postoperative day 3 (PO 3d), almost all PGP 9.5-positive neural elements had disappeared; they began to appear in both types of animals at PO 7d. At PO 10d, almost all nerve fibers showed a beaded appearance, with fewer ramifications in both types of mice. Although the regeneration proceeded in the wildtype, a major population of the periodontal Ruffini endings continued to display smooth outlines at PO 28d in the nt-4/5 homozygous mice. The reduction ratio of neural density reached a maximum at PO 3d, decreased at PO 10d, and later showed a plateau. In a rescue experiment, an administration of NT-4/5 showed an acceleration of nerve regeneration in the homozygous mice. These findings indicate that the nt-4/5-depletion causes a delay in the regeneration of the periodontal Ruffini endings, but the delay is shortened by an exogenous administration of NT-4/5. Combined with our previous findings of bdnf-deficient mice (Harada et al. [2003] Arch Histol Cytol 66:183-194), these morphological and numerical data suggest that multiple neurotrophins such as NT-4/5 and brain-derived neurotrophic factor (BDNF) play roles in their regeneration in a stage-specific manner.

Sensory defects in Necdin deficient mice result from a loss of sensory neurons correlated within an increase of developmental programmed cell death

Background

The human NECDIN gene is involved in a neurodevelopmental disorder, Prader-Willi syndrome (PWS). Previously we reported a mouse Necdin knock-out model with similar defects to PWS patients. Despite the putative roles attributed to Necdin, mainly from in vitro studies, its in vivo function remains unclear. In this study, we investigate sensory-motor behaviour in Necdin deficient mice. We reveal cellular defects and analyse their cause.

Results

We report sensory differences in Necdin deficient mice compared to wild type animals. These differences led us to investigate sensory neuron development in Necdin deficient mouse embryos. First, we describe the expression pattern of Necdin in developing DRGs and report a reduction of one-third in specified sensory neurons in dorsal roots ganglia and show that this neuronal loss is achieved by E13.5, when DRGs sensory neurons are specified. In parallel, we observed an increase of 41% in neuronal apoptosis during the wave of naturally occurring cell death at E12.5. Since it is assumed that Necdin is a P75NTR interactor, we looked at the P75NTR-expressing cell population in Necdin knock-out embryos. Unexpectedly, Necdin loss of function has no effect on p75NTR expressing neurons suggesting no direct genetic interaction between Necdin and P75NTR in this context.

Although we exclude a role of Necdin in axonal outgrowth from spinal sensory neurons in early developmental stages; such a role could occur later in neuronal differentiation. Finally we also exclude an anti-proliferative role of Necdin in developing sensory neurons.

Conclusion

Overall, our data show clearly that, in early development of the nervous system, Necdin is an anti-apoptotic or survival factor.

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.

Increased progenitor proliferation and apoptotic cell death in the sensory lineage of mice overexpressing N-myc

N-myc, a member of the myc family of bHLH transcription factors, is expressed mainly in the nervous system, including derivatives of neural crest cells in the periphery during development, such as the sensory dorsal root ganglion (DRG). Previous studies suggest that N-myc is involved in the proliferation of progenitor cells in the sensory lineage. To address the role of N-myc in the development of peripheral sensory neurons, we have overexpressed N-myc in sensory progenitor cells. The overexpression of N-myc did not significantly affect the number of multipotent neural crest cells or glial differentiation but caused a brief and marked increase of both proliferation and apoptosis in the DRG at embryonic day 11 (E11), thus coinciding with the stage of cell-cycle exit. At E17, the total number of cells in the lumbar DRG of mice with forced expression of N-myc was significantly reduced compared with that in wild-type mice. Among the different DRG subpopulations examined, the number of parvalbumin-positive neurons representing large-diameter proprioceptive neurons increased significantly. Our results indicate that forced expression of N-myc in the sensory lineage leads to unscheduled cell-cycle re-entry and excessive apoptosis and show that N-myc can affect the composition of different functional subtypes of sensory neurons in the DRG.

Combinatorial therapy with neurotrophins and cAMP promotes axonal regeneration beyond sites of spinal cord injury

Previous attempts to promote regeneration after spinal cord injury have succeeded in stimulating axonal growth into or around lesion sites but rarely beyond them. We tested whether a combinatorial approach of stimulating the neuronal cell body with cAMP and the injured axon with neurotrophins would propel axonal growth into and beyond sites of spinal cord injury. A preconditioning stimulus to sensory neuronal cell bodies was delivered by injecting cAMP into the L4 dorsal root ganglion, and a postinjury stimulus to the injured axon was administered by injecting neurotrophin-3 (NT-3) within and beyond a cervical spinal cord lesion site grafted with autologous bone marrow stromal cells. One to 3 months later, long-projecting dorsal-column sensory axons regenerated into and beyond the lesion. Regeneration beyond the lesion did not occur after treatment with cAMP or NT-3 alone. Thus, clear axonal regeneration beyond spinal cord injury sites can be achieved by combinatorial approaches that stimulate both the neuronal soma and the axon, representing a major advance in strategies to enhance spinal cord repair.

Neurotrophin-3 signaling maintains maturational homeostasis between neuronal populations in the olfactory epithelium

Neurons within the olfactory system undergo functional turnover throughout life. This process of cell death and compensatory neurogenesis requires feedback between neuronal populations of different developmental ages. We examined the role of NT-3 in this process. NT-3 was localized within both the olfactory bulb and olfactory epithelium. Mice null for NT-3 showed increased numbers of immature neurons, without change in the number of mature neurons. This was due to compensatory alterations in apoptosis of mature and immature neuronal populations. Using a primary olfactory neuronal culture, NT-3 was found to directly activate the PI3K/Akt pathway and indirectly activate the MAPK and PLC pathways. Activated PI3K/Akt promoted mature neuronal survival and induced the release of secondary factors, which activated the MAPK and PLC pathways to reduce neuronal precursor proliferation and inhibit neuronal maturation. These effects of NT-3 serve to maintain homeostasis between neuronal populations within the olfactory epithelium.

Increased expression of neurotrophins and their receptors in the mechanically compressed spinal cord of the spinal hyperostotic mouse (twy/twy)

The purpose of the present study was to identify any compensatory changes at the site of chronic compression of the spinal cord and neighboring segments. For this purpose, serial immunohistochemical and immunoblot analyses were performed for the expression levels of endogenous brain-derived neurotrophic factor (BDNF), neurotrophin (NT)-3, and their receptors, trkB and trkC in 24 tip-toe walking Yoshimura mice (twy/twy) aged 12-24 weeks. The twy mouse exhibits spontaneous calcified deposits posteriorly at the C1-C2 level, compressing the spinal cord. Immunoreactivities for BDNF, NT-3, trkB and trkC were preferentially localized in the gray matter, particularly in the anterior horn cells. In 24-week-old twy mice with severe compression, expression levels of these neurotrophins at the site of maximal compression were significantly lower than at the less- or non-compressed sites. In contrast, the expression levels of BDNF, NT-3, trkB and trkC were significantly higher at the rostral and caudal sites immediately adjacent to the maximal compression site. No such changes were noted in 12-week-old twy mice or in control Institute of Cancer Research mice. Our results suggest that overexpression of BDNF, NT-3, trkB and trkC in motoneuron areas neighboring the site of mechanical compression may represent compensatory changes in response to the compromised neuronal function at the level of compression, and that these proteins possibly contribute to neuronal survival and plasticity.

Erbb2 regulates neuromuscular synapse formation and is essential for muscle spindle development

Neuregulins and their Erbb receptors have been implicated in neuromuscular synapse formation by regulating gene expression in subsynaptic nuclei. To analyze the function of Erbb2 in this process, we have inactivated the Erbb2 gene in developing muscle fibers by Cre/Lox-mediated gene ablation. Neuromuscular synapses form in the mutant mice, but the synapses are less efficient and contain reduced levels of acetylcholine receptors. Surprisingly, the mutant mice also show proprioceptive defects caused by abnormal muscle spindle development. Sensory Ia afferent neurons establish initial contact with Erbb2-deficient myotubes. However, functional spindles never develop. Taken together, our data suggest that Erbb2 signaling regulates the formation of both neuromuscular synapses and muscle spindles.

Peripheral NT3 signaling is required for ETS protein expression and central patterning of proprioceptive sensory afferents

To study the role of NT3 in directing axonal projections of proprioceptive dorsal root ganglion (DRG) neurons, NT3(-/-) mice were crossed with mice carrying a targeted deletion of the proapoptotic gene Bax. In Bax(-/-)/NT3(-/-) mice, NT3-dependent neurons survived and expressed the proprioceptive neuronal marker parvalbumin. Initial extension and collateralization of proprioceptive axons into the spinal cord occurred normally, but proprioceptive axons extended only as far as the intermediate spinal cord. This projection defect is similar to the defect in mice lacking the ETS transcription factor ER81. Few if any DRG neurons from Bax(-/-)/NT3(-/-) mice expressed ER81 protein. Expression of a NT3 transgene in muscle restored DRG ER81 expression in NT3(-/-) mice. Finally, addition of NT3 to DRG explant cultures resulted in induction of ER81 protein. Our data indicate that NT3 mediates the formation of proprioceptive afferent-motor neuron connections via regulation of ER81.

BDNF gene replacement reveals multiple mechanisms for establishing neurotrophin specificity during sensory nervous system development

Neurotrophins have multiple functions during peripheral nervous system development such as controlling neuronal survival, target innervation and synaptogenesis. Neurotrophin specificity has been attributed to the selective expression of the Trk tyrosine kinase receptors in different neuronal subpopulations. However, despite overlapping expression of TrkB and TrkC in many sensory ganglia, brain-derived neurotrophic factor (BDNF) and neurotrophin 3 (NT3) null mutant mice display selective losses in neuronal subpopulations. In the present study we have replaced the coding part of the BDNF gene in mice with that of NT3 (BDNF(NT3/NT3)) to analyse the specificity and selective roles of BDNF and NT3 during development. Analysis of BDNF(NT3/NT3) mice showed striking differences in the ability of NT3 to promote survival, short-range innervation and synaptogenesis in different sensory systems. In the cochlea, specificity is achieved by a tightly controlled spatial and temporal ligand expression. In the vestibular system TrkB or TrkC activation is sufficient to promote vestibular ganglion neuron survival, while TrkB activation is required to promote proper innervation and synaptogenesis. In the gustatory system, NT3 is unable to replace the actions of BDNF possibly because of a temporally selective expression of TrkB in taste neurons. We conclude that there is no general mechanism by which neurotrophin specificity is attained and that specificity is achieved by (i) a tightly controlled spatial and temporal expression of ligands, (ii) different Trk receptors playing distinct roles within the same neuronal subpopulation, or (iii) selective receptor expression in sensory neuron subpopulations.

cGMP-mediated signaling via cGKIalpha is required for the guidance and connectivity of sensory axons

Previous in vitro studies using cGMP or cAMP revealed a cross-talk between signaling mechanisms activated by axonal guidance receptors. However, the molecular elements modulated by cyclic nucleotides in growth cones are not well understood. cGMP is a second messenger with several distinct targets including cGMP-dependent protein kinase I (cGKI). Our studies indicated that the alpha isoform of cGKI is predominantly expressed by sensory axons during developmental stages, whereas most spinal cord neurons are negative for cGKI. Analysis of the trajectories of axons within the spinal cord showed a longitudinal guidance defect of sensory axons within the developing dorsal root entry zone in the absence of cGKI. Consequently, in cGKI-deficient mice, fewer axons grow within the dorsal funiculus of the spinal cord, and lamina-specific innervation, especially by nociceptive sensory neurons, is strongly reduced as deduced from anti-trkA staining. These axon guidance defects in cGKI-deficient mice lead to a substantial impairment in nociceptive flexion reflexes, shown using electrophysiology. In vitro studies revealed that activation of cGKI in embryonic dorsal root ganglia counteracts semaphorin 3A-induced growth cone collapse. Our studies therefore reveal that cGMP signaling is important for axonal growth in vivo and in vitro.

Apoptosis, axonal growth defects, and degeneration of peripheral neurons in mice lacking CREB

CRE-binding protein (CREB) belongs to a family of transcription factors that mediates stimulus-dependent gene expression in neuronal and non-neuronal cells. Here we show that CREB is phosphorylated on its transcriptional regulatory site, Ser-133, in vivo in a neurotrophin-dependent manner. In mice harboring a null mutation in the Creb gene, sensory neurons exhibit excess apoptosis and degeneration, and display impaired axonal growth and projections. Interestingly, excess apoptosis is not observed in the central nervous system. CREB is required within sensory and sympathetic neurons for survival and axon extension since both of these neurotrophin-dependent processes are compromised in cultured neurons from CREB null mice. Thus, during their period of neurotrophin dependency, peripheral neurons require CREB-mediated gene expression for both survival and growth in vivo.

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.

Dissection of NT3 functions in vivo by gene replacement strategy

The development of the peripheral nervous system is governed in part by a family of neurotrophic factors that signal through Trk tyrosine kinase receptors. Neurotrophin 3 (NT3) ablation in mice causes a more severe neuronal phenotype than deletion of its receptor TrkC, suggesting that NT3 acts also through other non-preferred Trk receptors. To study the role of low-affinity ligand receptor interactions in vivo, we have replaced the Nt3 gene with the gene for brain-derived neurotrophic factor (BDNF), a TrkB ligand. As in NT3 and TrkC null mice, the proprioception system of these mutants failed to assemble. However, sensory fiber projections in the embryonic spinal cord suggest chemotropic effects of BDNF in vivo. In the dorsal root ganglia, the developmental dynamic of neuron numbers demonstrates that NT3 is required for activation of TrkB during neurogenesis and that TrkA is required during target tissue innervation. In the inner ear, the ectopic BDNF rescued the severe neuronal deficits caused by NT3 absence, indicating that TrkB and TrkC activate equivalent pathways to promote survival of cochlear neurons. However, specific increased innervation densities suggest unique functions for BDNF and NT3 beyond promoting neuronal survival. This mouse model has allowed the dissection of specific spatiotemporal Trk receptor activation by NT3. Our analysis provides examples of how development can be orchestrated by complex high- and low-affinity interactions between ligand and receptor families.

Developmental expression of the TTX-resistant voltage-gated sodium channels Nav1.8 (SNS) and Nav1.9 (SNS2) in primary sensory neurons

The development of neuronal excitability involves the coordinated expression of different voltage-gated ion channels. We have characterized the expression of two sensory neuron-specific tetrodotoxin-resistant sodium channel alpha subunits, Na(v)1. (SNS/PN3) and Na(v)1.9 (SNS2/NaN), in developing rat lumbar dorsal root ganglia (DRGs). Expression of both Na(v)1.8 and Na(v)1.9 increases with age, beginning at embryonic day (E) 15 and E17, respectively, and reaching adult levels by postnatal day 7. Their distribution is restricted mainly to those subpopulations of primary sensory neurons in developing and adult DRGs that give rise to unmyelinated C-fibers (neurofilament 200 negative). Na(v)1.8 is expressed in a higher proportion of neuronal profiles than Na(v)1.9 at all stages during development, as in the adult. At E17, almost all Na(v)1.8-expressing neurons also express the high-affinity NGF receptor TrkA, and only a small proportion bind to IB4, a marker for c-ret-expressing (glial-derived neurotrophic factor-responsive) neurons. Because IB4 binding neurons differentiate from TrkA neurons in the postnatal period, the proportion of Na(v)1.8 cells that bind to IB4 increases, in parallel with a decrease in the proportion of Na(v)1.8-TrkA co-expressing cells. In contrast, an equal number of Na(v)1.9 cells bind IB4 and TrkA in embryonic life. The differential expression of Na(v)1.8 and Na(v)1.9 in late embryonic development, with their distinctive kinetic properties, may contribute to the development of spontaneous and stimulus-evoked excitability in small diameter primary sensory neurons in the perinatal period and the activity-dependent changes in differentiation they produce.

Postnatal regulation of limb proprioception by muscle-derived neurotrophin-3

To investigate the effects of neurotrophin-3 (NT-3) on postnatal proprioceptive neurons and their targets, transgenic mice were generated that use the myosin light chain 1 (mlc) promoter to overexpress NT-3 in skeletal muscle. Ribonuclease protection assays revealed that NT-3 overexpression in hindlimb skeletal muscle began at embryonic day 14 (E14) and continued throughout adulthood. Overexpression of NT-3 during late embryogenesis resulted in increased numbers of large sensory and small fusimotor axons. Within a week of birth, mlc/NT-3 mice retract their limbs to the torso when lifted by the tail. Footprint analysis revealed that mlc/NT-3 mice had significant abnormalities in their gait compared with wild-types. Beam walking and rotorod analysis confirmed the poor limb control by mlc/NT-3 mice. These locomotive deficits progressively worsened with age and were likely related to the formation of morphologically abnormal muscle spindles. The most common spindle anomaly was the presence of excessive intrafusal bag fibers within individual muscle spindles. To assess the role of NT-3 in recovery from nerve injury, sciatic nerve crushes were performed in young adult mice. Two days after injury, mlc/NT-3 mice displayed significantly improved sciatic functional indexes and a significant increase in muscle spindles that remained associated with axons. The latter finding suggests that excess NT-3 in muscle may retard the degeneration of proprioceptive axons after nerve crush. Long-term survival after nerve injury in mlc/NT-3 mice did not induce further changes in spindle number or morphology. These findings demonstrate that, in addition to promoting embryonic proprioceptive neuron survival, postnatal overexpression of NT-3 in muscle leads to abnormal spindle formation and deficits in locomotive control. However, our results also show that NT-3 may be therapeutic for proprioceptive axons immediately after nerve injury by delaying axon degeneration.

Regulation of Trk receptors following contusion of the rat spinal cord

Neurotrophins function through high-affinity tyrosine kinase (Trk) receptors to promote growth and survival of cells in the injured nervous system. To investigate the role of Trk receptors in the adult nervous system, we examined TrkA, TrkB, and TrkC mRNA expression in spinal cord and brain after spinal contusion. At 1 day postinjury, all Trk receptor transcripts were down regulated at and around the site of injury, a situation that persisted through the first week. By 42 days, Trk expression was absent only within the cavity. In addition, truncated TrkB expression was substantially increased in ependymal cells and astrocytes surrounding the lesion cavity of chronically injured spinal cords. Rostral and caudal to the injury site, TrkA, TrkB, and TrkC mRNA expression did not differ from that of uninjured control spinal cords. Furthermore, no changes were observed in TrkB or TrkC expression in the axotomized corticospinal and rubrospinal neurons. These studies suggest that loss of Trk receptors at the injury site may contribute to the early progressive cellular loss in injured spinal cords, while increased presence of truncated TrkB receptors in the chronic injured spinal cord may sequester and restrict BDNF availability to support axonal regeneration and neuronal survival. The persistence of Trk receptors on supraspinal neurons suggests that neurotrophin application can support growth and survival in the acute and chronic injury states.

Developmental regulation of sensory neuron number and limb innervation in the mouse

Although used widely in studies of naturally occurring cell death, systematic descriptions of the time course of changes in sensory neuron number and of limb innervation in the mouse are not available. The development of sensory innervation to the mouse forelimb was traced using the lipophilic carbocyanine dye, DiI, and correlated with neuron number in dorsal root ganglia contributing to the cervical enlargement. Axon invasion of the forelimb began at E10.5. Sensory axons reached the distal margin of the forelimb by E13.5. The difficulty of identifying immature neurons precluded estimating neuron numbers during the period of limb innervation. Neuron numbers in dorsal root ganglia (DRGs) C5-C8 increased from E14 to E16 and from E18 to P4. No evidence of a decline in neuron numbers was found during the developmental periods studied. Neuron number was compared in neonates and adults to determine if sensory neurons are added as body size increases as found in the frog [J. Comp. Neurol. 314 (1991) 106] and the rat [J. Comp. Neurol. 386 (1997) 8]. In contrast to previous findings, no difference was found in sensory neuron number between neonate and adult mice in either cervical or lumbar DRGs.

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.

Positive and negative interactions of GDNF, NTN and ART in developing sensory neuron subpopulations, and their collaboration with neurotrophins

Glial cell line-derived neurotrophic factor (GDNF), neurturin (NTN) and neublastin/artemin (ART) are distant members of the transforming growth factor beta family, and have been shown to elicit neurotrophic effects upon several classes of peripheral and central neurons. Limited information from in vitro and expression studies has also substantiated a role for GDNF family ligands in mammalian somatosensory neuron development. Here, we show that although dorsal root ganglion (DRG) sensory neurons express GDNF family receptors embryonically, they do not survive in response to their ligands. The regulation of survival emerges postnatally for all GDNF family ligands. GDNF and NTN support distinct subpopulations that can be separated with respect to their expression of GDNF family receptors, whereas ART supports neurons in populations that are also responsive to GDNF or NTN. Sensory neurons that coexpress GDNF family receptors are medium sized, whereas small-caliber nociceptive cells preferentially express a single receptor. In contrast to brain-derived neurotrophic factor (BDNF)-dependent neurons, embryonic nerve growth factor (NGF)-dependent nociceptive neurons switch dependency to GDNF, NTN and ART postnatally. Neurons that survive in the presence of neurotrophin 3 (NT3) or neurotrophin 4 (NT4), including proprioceptive afferents, Merkel end organs and D-hair afferents, are also supported by GDNF family ligands neonatally, although at postnatal stages they lose their dependency on GDNF and NTN. At late postnatal stages, ART prevents survival elicited by GDNF and NTN. These data provide new insights on the roles of GDNF family ligands in sensory neuron development.

Attenuation of a caspase-3 dependent cell death in NT4- and p75-deficient embryonic sensory neurons

Neuronal survival during the developmental period of naturally occurring cell death is mediated through a successful competition for limiting concentrations of neurotrophic factors, and the deprived neurons will die. New results show that induced death through the p75 neurotrophin receptor (p75(NTR)), a member of the p55TNF/Fas family of cell death receptors, may also influence survival during development. We find that eliminating p75(NTR) or neurotrophin 4 (NT4) in mice leads to a marked attenuation of apoptosis during the programmed cell death period of the trigeminal ganglion neurons, suggesting that NT4 can induce the death of these neurons through the p75(NTR). These in vivo findings were reproduced in primary cell cultures, where NT4 was found to induce death in a p75(NTR)-dependent pathway. Analysis of p75 deficient and wild-type cells revealed two separate cell death pathways, a p75(NTR)- and caspase-3-independent pathway activated by trophic factor deprivation, and a p75(NTR)- and caspase-3-dependent pathway initiated by NT4. Crossing in the NT4 null alleles in brain-derived neurotrophic factor (BDNF) null mutant mice led to a rescue of a large proportion of BDNF-dependent neurons from excessive cell death, indicating that trophic factor deprivation is not sufficient for the death of many neurons and that additional death inducing signals might be required. Our results suggest that NT4 competitively signals survival and death of sensory neurons through trkB and p75(NTR), respectively.

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.

Cutaneous overexpression of neurotrophin-3 (NT3) selectively restores sensory innervation in NT3 gene knockout mice

Neurotrophin-3 (NT3) is essential for development of sensory innervation to the skin. NT3 supports the postnatal survival of primary sensory neurons that mediate mechanoreception and their Merkel cell containing touch dome end organs (Airaksinen et al., 1996). In this study we determined whether NT3 overexpressed in the skin could restore innervation lost when endogenous NT3 levels were reduced. Hybrid mice that overexpress NT3 in basal keratinocytes but lack one endogenous NT3 allele (K14-NT3/NT3(+/-)) were compared to NT3 overexpresser (K14-NT3) mice, heterozygous knockout (NT3(+/-)) mice, and littermate control mice. In line with previous analyses, NT3(+/-) mice lost 63% of the Merkel cells associated with touch domes, 67% of touch dome units and the associated SAI innervation. All of these parameters were restored to overexpresser levels in K14-NT3/NT3(+/-) mice. Knockout NT3(+/-) mice also had a 31% reduction of L4/L5 dorsal root ganglion cells and a 24% reduction of myelinated axons in the saphenous cutaneous nerve. These losses were also restored in hybrid K14-NT3/NT3(+/-) mice, though only to control mouse values. These results indicate that overexpression of NT3 in skin of NT3(+/-) knockout mice rescued most cutaneous neurons lost in NT3(+/-) mice, but was unable to rescue NT3-dependent neurons that project to noncutaneous sensory targets.

Cell death in regenerating populations of neurons in BDNF mutant mice

There are two populations of neurons which are continually renewed in the adult, the dentate gyrus granule neurons and the olfactory bulb granule and periglomerular neurons. In the dentate gyrus, a secondary proliferative zone termed the subgranular zone is established along the interface between the dentate gyrus and the hilus where granule cells are born throughout life. Olfactory bulb neurons are generated in the anterior subventricular zone of the lateral ventricle and migrate via the rostral migratory stream to the olfactory bulb. We examined animals lacking brain-derived neurotrophic factor (BDNF) in order to establish whether this neurotrophin could be involved in the generation and/or survival of these neurons in vivo. We find that cells in nestin-positive regions of both the subgranular layer of the dentate gyrus and the subventricular zone of the olfactory bulb undergo apoptosis starting 2 weeks after birth in the absence of BDNF. However, increased apoptosis was not limited to precursors, as apoptotic cells were also found in the granule cell layer of the dentate gyrus and in the granule and periglomerular layers of the olfactory bulb. The excessive cell death was limited to these populations of neurons as no excessive cell death was detected in other forebrain areas. We conclude that BDNF is essential for the survival of neurons specifically in populations which are continuously being regenerated in the brain.

Perturbation of cell cycle progression and cellular gene expression as a function of herpes simplex virus ICP0

Herpes simplex virus type 1 is capable of inhibiting host cell DNA synthesis following lytic infection. However, the mechanism and nature of potential effects on cell cycle progression have not been described. In this report, we characterize the dysregulation of the cell cycle following infection with the replication-incompetent virus d106, where immediate-early gene expression is restricted to infected-cell polypeptide 0 (ICP0) and the expression of all other viral genes is dramatically reduced or is not observed. Infection with d106 resulted in the accumulation of cells in both the G(1)/S and G(2)/M compartments, consistent with cell cycle arrest at both checkpoints. The isogenic variant d109, which does not express any viral proteins, failed to induce this phenotype, suggesting that the expression of ICP0 is crucial for cell cycle arrest. Analysis of global cellular gene expression patterns following infection with d106 and d109 revealed that a relatively small subset of cellular genes were induced as a consequence of ICP0 expression. A number of these genes induced in the presence of ICP0 are classically considered p53-responsive genes, including p21, gadd45, and mdm-2. However, infection with d106 of cells with both alleles of p53 deleted resulted in the same cell cycle arrest phenotype and similar cellular gene expression patterns, suggesting that the expression of ICP0 results in cell cycle arrest potentially via p53-dependent and p53-independent mechanisms. In addition, it was found that the effects of infection with d106 on viral and cellular gene expression were similar to the effects observed following treatment of cells with the histone deacetylase inhibitor trichostatin A.

NT4/5 mutant mice have deficiency in gustatory papillae and taste bud formation

Neurotrophins are key determinants for controlling the survival of peripheral neurons during development. Brain-derived neurotrophic factor (BDNF) and neurotrophin-4/5 (NT4/5) exert their action through a common trkB receptor but independently support gustatory sensory neurons. To assess the role of NT4/5 during development, we examined the postnatal development and maintenance of fungiform taste buds in mice carrying a deletion of NT4/5. The absence of NT4/5 results in embryonic deficits in gustatory innervation and a reduced number of fungiform papillae at birth. No degenerative deficits of fungiform papillae were observed for the first 3 weeks of postnatal development. However, these remaining fungiform papillae were smaller in appearance and many did not contain taste pores. By postnatal day 60, there was 63% decrease in the number of fungiform papillae, and remaining papillae were smaller in size or modified into filiform-like spines. These papillae had either no taste bud or a taste bud with a reduced number of taste cells compared to controls. These findings demonstrate that the NT4/5 gene functions in the maintenance of fungiform gustatory papillae and raises the possibility for an earlier role in development.

Selective regulation of trkC expression by NT3 in the developing peripheral nervous system

We have studied the influence of neurotrophin-3 (NT3) on the expression of its receptor tyrosine kinase, trkC, in embryonic mice. The expression of trkC transcripts encoding full-length and kinase-deficient receptors was almost entirely restricted to neurons in the trigeminal ganglion and increased markedly throughout development. In NT3(+/-) embryos, the level of trkC mRNA in the trigeminal ganglion was much lower than that in wild-type embryos, although there was no significant reduction in the total number of neurons in the ganglion. This demonstrates that endogenous NT3 regulates trkC expression in trigeminal neurons independently of changes in population size. In NT3(-/-) embryos, the number of neurons in the trigeminal ganglion was much lower than in wild-type embryos, and there was a further reduction in the mean neuronal level of trkC mRNA. Direct regulation of trkC mRNA expression in cultured trigeminal neurons was also observed, although the finding that trkC mRNA levels were sustained better in explant cultures than in dissociated cultures irrespective of the presence of NT3 suggests that trkC mRNA expression is regulated by additional factors within the ganglion. In contrast to trigeminal neurons, the level of trkC mRNA was sustained at normal levels in neurons of the sympathetic chain of NT3(-/-) embryos and was not increased by NT3 in sympathetic neuron cultures. TrkC mRNA expression in developing cutaneous tissues was also unaffected by the NT3 null mutation. In summary, our findings provide the first clear evidence that the expression of a trk receptor, tyrosine kinase, is regulated by physiological levels of its ligand in vivo and show that regulation by NT3 is cell type-specific.

Building blocks of pain: the regulation of key molecules in spinal sensory neurones during development and following peripheral axotomy

The pathways, synapses and molecules involved in pain processing in the newborn are not only required to trigger repair and recuperation but are also involved in the process of forming a mature nervous system. Sensory neurons in the dorsal root ganglion and dorsal horn express a phenomenal array of molecules which contribute to their structural and functional characteristics and many of these are developmentally regulated both pre- and postnatally. In order to understand nociceptive signalling and pain in the neonate we need a clear picture of that regulation. This review concentrates on the changing expression of selected key molecules, receptors and channels in the embryo, neonate and adult, which both characterise the sensory neuron and contribute to its response to painful stimuli in normal and pathological conditions.

Development and specification of muscle sensory neurons

Although the response properties and synaptic projections of muscle sensory neurons have been studied extensively, relatively little is known about how these sensory neurons develop their unique phenotypes during embryonic life. The explosion of new information on neurotrophins, however, has revealed that neurotrophin 3 (NT3) is critically involved in several aspects of this development, including the initial differentiation, survival, and perhaps even the terminal arborizations of muscle sensory neurons within the spinal cord. The ETS family of transcription factors, recently shown to be expressed in these sensory neurons, may help specify their choice of synaptic targets in the central nervous system.

Neurotrophins, nociceptors, and pain

It is now well established that neurotrophins play a crucial role in the development of the nervous system. However, there is increasing evidence that the function of neurotrophins persists throughout adulthood. The broad scope of neurotrophin action is well documented in the case of nerve growth factor (NGF) and its effect on nociceptors and nociception. Here, we review the evidence for these multiple roles for NGF. Two manipulations influencing NGF levels are discussed in detail. The first involves the use of transgenic mice that overexpress or underexpress neurotrophins. A second strategy involves administration of NGF or its antibody in vivo to increase or decrease its level. During prenatal development, NGF is required for survival of nociceptors. In the early postnatal period, NGF is required for expression of the appropriate nociceptor phenotype. In adults, NGF acts as an important intermediate in inflammatory pain, contributing to both peripheral and central sensitization. The sensitization of peripheral nociceptors can be very rapid and can involve non-neural cells such as mast cells, neutrophils, fibroblasts, and macrophages. Recent evidence indicates that other neurotrophins also play key supporting roles in the development of nociceptors (e.g., NT-3) and in inflammatory pain (e.g., BDNF, NT-4/5). Furthermore, molecules from other superfamilies (e.g., GDNF) also are required to assure survival of certain classes of nociceptors. The diverse effects of neurotrophins on nociceptive processing emphasize their broad importance in the development and function of the nervous system.

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.

A role for p75 receptor in neurotrophin-3 functioning during the development of limb proprioception

Neurotrophin-3 is indispensable for the development of limb proprioceptive neurons and their end organs, muscle spindles. To determine whether the low-affinity p75 receptor potentiates the actions of neurotrophin-3, we examined the development of the proprioceptive system in p75 null mutant mice that had either normal or decreased tissue levels of neurotrophin-3. Postnatal mice lacking both copies of the p75 gene had fewer sensory neurons in dorsal root ganglia, but normal complements of muscle spindles in fast hindlimb muscles, although the slow soleus muscle showed a 50% loss of spindles. However, compound mutants lacking both copies of the p75 gene as well as one copy of the neurotrophin-3 gene displayed a dystonic/ataxic phenotype similar to that observed previously in neurotrophin-3 null mutants devoid of proprioception. The compound mutants also exhibited a commensurate loss of parvalbumin-expressing (proprioceptive) neurons in dorsal root ganglia. The degree of deficiency of spindles (and presumably proprioceptive neurons) in the compound mutants exceeded the sum of deficits in single mutants lacking either both copies of p75 genes or one copy of neurotrophin-3 gene, suggesting a synergistic interaction between the p75 receptor and neurotrophin-3. Neuronal deficits in the compound mutants were present prior to embryonic day 14, indicating an early role for the p75 receptor in sensory neuronogenesis. Collectively, these data indicate that the p75 receptor is not essential for the survival and differentiation of most limb proprioceptive neurons when neurotrophin-3 is expressed at normal levels. However, the p75 receptor may act in synergy with neurotrophin-3 to enhance the survival of proprioceptive neurons when tissue levels of neurotrophin-3 are a limiting factor.

Regulation of neurotrophin-3 expression by epithelial-mesenchymal interactions: the role of Wnt factors

Neurotrophins regulate survival, axonal growth, and target innervation of sensory and other neurons. Neurotrophin-3 (NT-3) is expressed specifically in cells adjacent to extending axons of dorsal root ganglia neurons, and its absence results in loss of most of these neurons before their axons reach their targets. However, axons are not required for NT-3 expression in limbs; instead, local signals from ectoderm induce NT-3 expression in adjacent mesenchyme. Wnt factors expressed in limb ectoderm induce NT-3 in the underlying mesenchyme. Thus, epithelial-mesenchymal interactions mediated by Wnt factors control NT-3 expression and may regulate axonal growth and guidance.

Growth arrest failure, G1 restriction point override, and S phase death of sensory precursor cells in the absence of neurotrophin-3

More than half of the dorsal root ganglion (DRG) neurons are lost by excessive cell death coinciding with precursor proliferation and cell cycle exit in neurotrophin-3 null mutant (NT-3-/-) mice. We find that in the absence of NT-3, sensory precursor cells fail to arrest the cell cycle, override the G1 phase restriction point, and die by apoptosis in S phase, which can be prevented in vivo by a cell cycle blocker. Uncoordinated cell cycle reentry is preceded by a failure of nuclear N-myc downregulation and is paralleled by the activation of the full repertoire of G1 and S phase cell cycle proteins required for cell cycle entry. Our results provide evidence for novel activity of neurotrophins in cell cycle control and point toward an N-myc sensitization to cell death in the nervous system that is under the control of NT-3.

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.