Embryonic sensory development: local expression of neurotrophin-3 and target expression of nerve growth factor

Familial Dysautonomia: Mechanisms and Models

Hereditary Sensory and Autonomic Neuropathies (HSANs) compose a heterogeneous group of genetic disorders characterized by sensory and autonomic dysfunctions. Familial Dysautonomia (FD), also known as HSAN III, is an autosomal recessive disorder that affects 1/3,600 live births in the Ashkenazi Jewish population. The major features of the disease are already present at birth and are attributed to abnormal development and progressive degeneration of the sensory and autonomic nervous systems. Despite clinical interventions, the disease is inevitably fatal. FD is caused by a point mutation in intron 20 of the IKBKAP gene that results in severe reduction in expression of IKAP, its encoded protein. In vitro and in vivo studies have shown that IKAP is involved in multiple intracellular processes, and suggest that failed target innervation and/or impaired neurotrophic retrograde transport are the primary causes of neuronal cell death in FD. However, FD is far more complex, and appears to affect several other organs and systems in addition to the peripheral nervous system. With the recent generation of mouse models that recapitulate the molecular and pathological features of the disease, it is now possible to further investigate the mechanisms underlying different aspects of the disorder, and to test novel therapeutic strategies.

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.

Regulation/modulation of sensory neuron sodium channels

The pseudounipolar sensory neurons of the dorsal root ganglia (DRG) give rise to peripheral branches that convert thermal, mechanical, and chemical stimuli into electrical signals that are transmitted via central branches to the spinal cord. These neurons express unique combinations of tetrodotoxin-sensitive (TTX-S) and tetrodotoxin-resistant (TTX-R) Na(+) channels that contribute to the resting membrane potential, action potential threshold, and regulate neuronal firing frequency. The small-diameter neurons (<25 μm) isolated from the DRG represent the cell bodies of C-fiber nociceptors that express both TTX-S and TTX-R Na(+) currents. The large-diameter neurons (>35 μm) are typically low-threshold A-fibers that predominately express TTX-S Na(+) currents. Peripheral nerve damage, inflammation, and metabolic diseases alter the expression and function of these Na(+) channels leading to increases in neuronal excitability and pain. The Na(+) channels expressed in these neurons are the target of intracellular signaling cascades that regulate the trafficking, cell surface expression, and gating properties of these channels. Post-translational regulation of Na(+) channels by protein kinases (PKA, PKC, MAPK) alter the expression and function of the channels. Injury-induced changes in these signaling pathways have been linked to sensory neuron hyperexcitability and pain. This review examines the signaling pathways and regulatory mechanisms that modulate the voltage-gated Na(+) channels of sensory neurons.

Differential Trk expression in explant and dissociated trigeminal ganglion cell cultures

During embryonic development, expression of neurotrophin receptor tyrosine kinases (Trks) by sensory ganglia is continuously and dynamically regulated. Neurotrophin signaling promotes selective survival and axonal differentiation of sensory neurons. In embryonic day (E) 15 rat trigeminal ganglion (TG), NGF receptor TrkA is expressed by small diameter neurons, NT-3 receptor TrkC and BDNF receptor TrkB are expressed by large diameter neurons. Organotypic explant and dissociated cell cultures of the TG (and dorsal root ganglia) are commonly used to assay neurotrophin effects on developing sensory neurons. In this study, we compared Trk expression in E15 rat TG explant and dissociated cell cultures with or without neurotrophin treatment. Only a subset of TG cells express each of the three Trk receptors in wholemount explant cultures as in vivo conditions. In contrast, all TG neurons co-express all three Trk receptors upon dissociation, regardless of neurotrophin treatment. Neurons cultured in low concentrations of one neurotrophin first, and switched to higher concentrations of another after 1 day, survive and display morphological characteristics of neurons cultured in a mixture of both neurotrophins for 3 days. Our results indicate that wholemount explant cultures of sensory ganglia represent in vivo conditions in terms of Trk expression patterns; whereas dissociation dramatically alters Trk expression by primary sensory neurons.

Local neurotrophin effects on central trigeminal axon growth patterns

In dissociated cell and wholemount explant cultures of the embryonic trigeminal pathway NGF promotes exuberant elongation of trigeminal ganglion (TG) axons, whereas NT-3 leads to precocious arborization [J. Comp. Neurol. 425 (2000) 202]. In the present study, we investigated the axonal effects of local applications of NGF and NT-3. We placed small sepharose beads loaded with either NGF or NT-3 along the lateral edge of the central trigeminal tract in TG-brainstem intact wholemount explant cultures prepared from embryonic day 15 rats. Labeling of the TG with carbocyanine dye, DiI, revealed that NGF induces local defasciculation and diversion of trigeminal axons. Numerous axons leave the tract, grow towards the bead and engulf it, while some axons grow away from the neurotrophin source. NT-3, on the other hand, induced localized interstitial branching and formation of neuritic tangles in the vicinity of the neurotrophin source. Double immunocytochemistry showed that axons responding to NGF were predominantly TrkA-positive, whereas both TrkA and TrkC-positive axons responded to NT-3. Our results indicate that localized neurotrophin sources along the routes of embryonic sensory axons in the central nervous system, far away from their parent cell bodies, can alter restricted axonal pathways and induce elongation, arborization responses.

Growth cones integrate signaling from multiple guidance cues

Nerve growth factor (NGF) and semaphorin3A (Sema3A) are guidance cues found in pathways and targets of developing dorsal root ganglia (DRG) neurons. DRG growth cone motility is regulated by cytoplasmic signaling triggered by these molecules. We investigated interactions of NGF and Sema3A in modulating growth cone behaviors of axons extended from E7 chick embryo DRGs. Axons extending in collagen matrices were repelled by Sema3A released from transfected HEK293 cells. However, if an NGF-coated bead was placed adjacent to Sema3A-producing cells, axons converged at the NGF bead. Growth cones of DRGs raised in 10(-9) M NGF were more resistant to Sema3A-induced collapse than when DRGs were raised in 10(-11) M NGF. After overnight culture in 10(-11) M NGF, 1-hr treatment with 10(-9) M NGF also increased growth cone resistance to Sema3A. Pharmacological studies indicated that the activities of ROCK and PKG participate in the cytoskeletal alterations that lead to Sema3A-induced growth cone collapse, whereas PKA activity is required for NGF-mediated reduction of Sema3A-induced growth cone collapse. These results support the idea that growth cone responses to a guidance cue can be modulated by interactions involving coincident signaling by other guidance cues.

Nerve growth factor and semaphorin 3A signaling pathways interact in regulating sensory neuronal growth cone motility

Neurotrophins and semaphorin 3A are present along pathways and in targets of developing axons of dorsal root ganglion (DRG) sensory neurons. Growth cones of sensory axons are probably regulated by interaction of cytoplasmic signaling triggered coincidentally by both types of guidance molecules. We investigated the in vitro interactions of neurotrophins and semaphorin 3A (Sema3A) in modulating growth cone behaviors of axons extended from DRGs of embryonic day 7 chick embryos. Growth cones of DRGs raised in media containing 10(-9) m NGF or BDNF were more resistant to Sema3A-induced growth cone collapse than when DRGs were raised in 10(-11) m NGF. After overnight culture in 10(-11) m NGF, a 1 hr treatment with 10(-9) m NGF or BDNF was sufficient to increase growth cone resistance to Sema3A-induced collapse. This neurotrophin-mediated decrease in the collapse response of DRG growth cones was not associated with reduced expression on growth cones of the Sema3A-binding protein neuropilin-1. A series of pharmacological studies followed. Phosphatidylinositol 3 kinase activity is not required for these effects of NGF. The effects of inhibitors and activators of protein kinase A (PKA) indicate that PKA activity is involved in NGF modulation of Sema3A-induced growth cone collapse. The effects of inhibitors and activators of PKG indicate that PKG activity is involved in Sema3A-induced growth cone collapse. The effects of inhibitors also indicate that Rho-kinase activity is involved in Sema3A-induced growth cone collapse. These results are consistent with the idea that growth cone responses to an individual guidance cue depend on coincident signaling by other guidance cues and by other regulatory pathways.

Neurotrophin-3-mediated regeneration and recovery of proprioception following dorsal rhizotomy

Injured dorsal root axons fail to regenerate into the adult spinal cord, leading to permanent sensory loss. We investigated the ability of intrathecal neurotrophin-3 (NT3) to promote axonal regeneration across the dorsal root entry zone (DREZ) and functional recovery in adult rats. Quantitative electron microscopy showed robust penetration of CNS tissue by regenerating sensory axons treated with NT3 at 1 and 2 weeks postrhizotomy. Light and electron microscopical anterograde tracing experiments showed that these axons reentered appropriate and ectopic laminae of the dorsal horn, where they formed vesicle-filled synaptic buttons. Cord dorsum potential recordings confirmed that these were functional. In behavioral studies, NT3-treated (but not untreated or vehicle-treated) rats regained proprioception. Recovery depended on NT3-mediated sensory regeneration: preventing regeneration by root excision prevented recovery. NT3 treatment allows sensory axons to overcome inhibition present at the DREZ and may thus serve to promote functional recovery following dorsal root avulsions in humans.

Target specific differentiation of peripheral trigeminal axons in rat-chick chimeric explant cocultures

Avian and rodent trigeminal ganglion (TG) neurons share common features in their neurotrophin requirements and axonal projections between the sensory periphery and the brainstem. In rodents, the whisker pad (WP) is a major peripheral target of the infraorbital (IO) nerve component of the TG. The chick IO nerve is much smaller and innervates the maxillary process (MP). In the embryonic WP, IO axons course in fascicles from a caudal to rostral direction and form terminal plexuses around follicles. In the chick, IO axons travel as a thin bundle to the MP and branch out with no specific patterning. We cocultured E15 rat TG with E5-6 chick MP or chick TG with rat WP explants to examine target influences on trigeminal axon growth patterns as visualized with DiI labeling or neurofilament immunohistochemistry. Chick TG axons showed robust growth into WP explants, and the ganglion increased in size. Thick bundles of axons traveled between rows of follicles and formed a distinct pattern as they developed terminal arbors around individual follicles. In contrast, rat TG axon growth was sparse in chick MP explants and the ganglion size reduced over time. Furthermore, rat TG axons did not show any patterning in the chick MP. Similar target-specific growth patterns were observed when TG explants were given a choice between chick MP and rat WP explants. Collectively these results indicate that both the chick and rat TG cells respond to similar target-specific peripheral cues in the establishment of innervation density and patterning in peripheral orofacial targets.

GDNF and NGF family members and receptors in human fetal and adult spinal cord and dorsal root ganglia

We describe the expression of mRNA encoding ligands and receptors of members of the GDNF family and members of the neurotrophin family in the adult human spinal cord and dorsal root ganglia (DRG). Fetal human spinal cord and ganglia were investigated for the presence of ligands and receptors of the neurotrophin family. Tissues were collected from human organ donors and after routine elective abortions. Messenger RNA was found encoding RET, GFR alpha-1, BDNF, trkB, and trkC in the adult human spinal cord and BDNF, NT-3, p75, trkB, and trkC in the fetal human spinal cord. The percentage of adult human DRG cells expressing p75, trkA, trkB, or trkC was 57, 46, 29, and 24%, respectively, and that of DRG cells expressing RET, GFR alpha-1, GFR alpha-2, or GFR alpha-3 was 79, 20, 51, and 32%, respectively. GFR alpha-2 was expressed selectively in small, GFR alpha-3 principally in small and GFR alpha-1 and RET in both large and small adult human DRG neurons. p75 and trkB were expressed by a wide range of DRG neurons while trkA was expressed in most small diameter and trkC primarily in large DRG neurons. Fetal DRG cells were positive for the same probes as adult DRG cells except for NT-3, which was only found in fetal DRG cells. Messenger RNA species only expressed at detectable levels in fetal but not adult spinal cord tissues included GDNF, GFR alpha-2, NT-3, and p75. Notably, GFR alpha-2, which is expressed in the adult rat spinal cord, was not found in the adult human spinal cord.

Regulation of neurotrophin-induced axonal responses via Rho GTPases

Nerve growth factor (NGF) and related neurotrophins induce differential axon growth patterns from embryonic sensory neurons (Lentz et al. [1999] J. Neurosci. 19:1038-1048; Ulupinar et al. [2000a] J. Comp. Neurol 425:622-630). In wholemount explant cultures of embryonic rat trigeminal ganglion and brainstem or in dissociated cell cultures of the trigeminal ganglion, exogenous supply of NGF leads to axonal elongation, whereas neurotrophin-3 (NT-3) treatment leads to short branching and arborization (Ulupinar et al. [2000a] J. Comp. Neurol. 425:622-630). Axonal responses to neurotrophins might be mediated via the Rho GTPases. To investigate this possibility, we prepared wholemount trigeminal pathway cultures from E15 rats. We infected the ganglia with recombinant vaccinia viruses that express GFP-tagged dominant negative Rac, Rho, or constitutively active Rac or treated the cultures with lysophosphatitic acid (LPA) to activate Rho. We then examined axonal responses to NGF by use of the lipophilic tracer DiI. Rac activity induced longer axonal growth from the central trigeminal tract, whereas the dominant negative construct of Rac eliminated NGF-induced axon outgrowth. Rho activity also significantly reduced, and the Rho dominant negative construct increased, axon growth from the trigeminal tract. Similar alterations in axonal responses to NT-3 and brain-derived neurotrophic factor were also noted. Our results demonstrate that Rho GTPases play a major role in neurotrophin-induced axonal differentiation of embryonic trigeminal axons.

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.

Neurotrophic factors and receptors in the immature and adult spinal cord after mechanical injury or kainic acid

Delivery of neurotrophic factors to the injured spinal cord has been shown to stimulate neuronal survival and regeneration. This indicates that a lack of sufficient trophic support is one factor contributing to the absence of spontaneous regeneration in the mammalian spinal cord. Regulation of the expression of neurotrophic factors and receptors after spinal cord injury has not been studied in detail. We investigated levels of mRNA-encoding neurotrophins, glial cell line-derived neurotrophic factor (GDNF) family members and related receptors, ciliary neurotrophic factor (CNTF), and c-fos in normal and injured spinal cord. Injuries in adult rats included weight-drop, transection, and excitotoxic kainic acid delivery; in newborn rats, partial transection was performed. The regulation of expression patterns in the adult spinal cord was compared with that in the PNS and the neonate spinal cord. After mechanical injury of the adult rat spinal cord, upregulations of NGF and GDNF mRNA occurred in meningeal cells adjacent to the lesion. BDNF and p75 mRNA increased in neurons, GDNF mRNA increased in astrocytes close to the lesion, and GFRalpha-1 and truncated TrkB mRNA increased in astrocytes of degenerating white matter. The relatively limited upregulation of neurotrophic factors in the spinal cord contrasted with the response of affected nerve roots, in which marked increases of NGF and GDNF mRNA levels were observed in Schwann cells. The difference between the ability of the PNS and CNS to provide trophic support correlates with their different abilities to regenerate. Kainic acid delivery led to only weak upregulations of BDNF and CNTF mRNA. Compared with several brain regions, the overall response of the spinal cord tissue to kainic acid was weak. The relative sparseness of upregulations of endogenous neurotrophic factors after injury strengthens the hypothesis that lack of regeneration in the spinal cord is attributable at least partly to lack of trophic support.

Prenatal localization of the dorsal root ganglion in different segments of the normal human vertebral column

STUDY DESIGN

Vertebral columns from 11 normal human fetuses (10-24 weeks of gestation) derived from spontaneous abortions were examined as part of the legal autopsy procedure including spinal cord analysis.

OBJECTIVES

To study the localization of the dorsal root ganglion in the normal fetal spine and to relate the dorsal root ganglion location to the ossification of the vertebral bodies and vertebral arches.

SUMMARY OF BACKGROUND DATA

The normal and pathologic ossification pattern of the fetal human spine has been studied. There has been no study addressing the localization of the dorsal root ganglion in normal and pathologic axial development.

METHODS

The dorsal root ganglion were studied by using histology (horizontal sections) and morphometric measurement.

RESULTS

The study showed: 1) The dorsal root ganglion appeared before ossification of the spine; 2) The dorsal root ganglion had an oval shape in all cases; 3) The longitudinal axis of dorsal root ganglion was directed anterolaterally in the cervical and lumbosacral segments and mainly laterally in the thoracic segment; 4) During development, the dorsal root ganglion changed position according to the body axis; and 5) The para-axial ossification protected the dorsal root ganglion differently in the different axial segments.

CONCLUSIONS

The dorsal root ganglion appeared before ossification. The distance from the dorsal root ganglion to the body axis increased during development. In the different segments of the spine, different orientations and different locations of the dorsal root ganglion were observed in relation to osseous spine components. The results can be used as reference data for future studies on the dorsal root ganglion in pathologic spines.

Differential effects of NGF and NT-3 on embryonic trigeminal axon growth patterns

We examined the effects of neurotrophins nerve growth factor (NGF) and neurotrophin-3 (NT-3) on trigeminal axon growth patterns. Embryonic (E13-15) wholemount explants of the rat trigeminal pathway including the whisker pads, trigeminal ganglia, and brainstem were cultured in serum-free medium (SFM) or SFM supplemented with NGF or NT-3 for 3 days. Trigeminal axon growth patterns were analyzed with the use of lipophilic tracer DiI. In wholemount cultures grown in SFM, trigeminal axon projections, growth patterns, and differentiation of peripheral and central targets are similar to in vivo conditions. We show that in the presence of NGF, central trigeminal axons leave the tract and grow into the surrounding brainstem regions in the elongation phase without any branching. On the other hand, NT-3 promotes precocious development of short axon collaterals endowed with focal arbors along the sides of the central trigeminal tract. These neurotrophins also affect trigeminal axon growth within the whisker pad. Additionally, we cultured dissociated trigeminal ganglion cells in the presence of NGF, NT-3, or NGF+NT-3. The number of trigeminal ganglion cells, their size distribution under each condition were charted, and axon growth was analyzed following immunohistochemical labeling with TrkA and parvalbumin antibodies. In these cultures too, NGF led to axon elongation and NT-3 to axon arborization. Our in vitro analyses suggest that aside from their survival promoting effects, NGF and NT-3 can differentially influence axon growth patterns of embryonic trigeminal neurons.

Neurons differentiating from murine neural crest in culture exhibit sensory or sympathetic-like calcium currents

The trunk neural crest gives rise to peripheral sensory and sympathetic neurons. In culture, neural crest cells can be induced to differentiate into either neuronal phenotype. Few studies have examined the differentiation of physiological properties in cultures of neural crest cells. Using whole-cell recordings, our study examined the effects of growth factors on high-voltage-activated calcium current profiles exhibited by neurons differentiating in culture. We compared these profiles with those exhibited by sensory and sympathetic neurons. Neural crest cells in culture gave rise to neurons with calcium current profiles identical to either sensory or sympathetic neurons, depending on the growth conditions. On average, the calcium current profile for sensory neurons was 23% (L), 51% (N), and 12% (P), while sympathetic neurons had a similar L-type current (20%), higher N-type (76%), and lower P-type (4%). Neural crest cells cultured with human leukemia inhibitory factor plus somite cells produced neurons with a sympathetic-like calcium current profile (L: 17%, N: 75%, and P: 4%). However, murine leukemia inhibitory factor (L: 25%, N: 52%, and P: 13%) and ciliary neurotrophic factor (L: 18%, N: 49%, and P: 9%) plus somite cells produced neurons with sensory-like calcium current profiles. These growth conditions did not modify the calcium current profiles of neurons cultured from embryonic and neonatal ganglia. Similarly, murine leukemia inhibitory factor produced a greater percentage of neurons (57%) with sensitivity to capsaicin (sensory phenotype) than human leukemia inhibitory factor (3%). Physiological traits can be a useful tool for the determination of neuronal phenotype in culture where other traits may be less stable.

Neurotrophins and neurotrophin receptors in some neural crest-derived tumours (ganglioneuroma, phaeochromocytoma and paraganglioma)

AIM

This study analyses the occurrence and distribution of neurotrophins and their receptors in some types of tumours of neural-crest derived cells.

METHODS AND RESULTS

Light microscopy immunohistochemistry associated with quantitative image analysis was used to study the expression of neurotrophins (nerve growth factor, brain-derived neurotrophic factor and neurotrophin (NT)-3) and their cognate receptors (p75(LNGFR), TrkA, TrkB and TrkC) in histologically defined ganglioneuroma, phaeochromocytoma and paraganglioma. The material was fixed in 10% formaldehyde, paraffin-embedded and processed for indirect peroxidase immunohistochemistry using a battery of poly- and monoclonal antibodies to detect neurotrophins and their receptors, as well as some neuronal, endocrine and glial cell markers. A subpopulation of cells in phaeochromocytomas and ganglioneuromas expressed NT-3, but not other neurotrophins, while in paragangliomas no neurotrophins were detected. Regarding neurotrophin receptors, all tumours lacked p75(LNGFR), except for the ganglionic part of a case of mixed phaeochromocytoma, whereas they displayed TrkA (two of two ganglioneuromas, six of nine phaeochomocytomas and three of four paragangliomas). Furthermore, TrkC was regularly detected in a neuronal subpopulation in ganglioneuroma. Interestingly, the percentage of neurones expressing TrkA and TrkC was increased with respect to normal tissues in ganglioneuromas, as well as the percentage of the area occupied by TrkA-immunoreactive cells in the phaeochromocytomas.

CONCLUSION

The pattern of expression of neurotrophins and neurotrophin receptors in the analysed tumours basically matches that of sympathetic neurones, adrenal chromaffin cells and paraganglionic cells, and suggests responsiveness of these cells to neurotrophins. Nevertheless, the function of TrkA and TrkC in regulating the biology of these tumours, if any, remains to be elucidated.

Retrograde axonal transport of neurotrophins: differences between neuronal populations and implications for motor neuron disease

During development, neurons die if they do not receive neurotrophin support from the target cells they are innervating. Neurotrophins are delivered from the target to the cell bodies of the innervating neurons by interacting with specific receptors located on the nerve terminals and then together are retrogradely transported to the cell body. This process consists of a number of distinct events including endocytosis of neurotrophin and its receptor into coated vesicles; vesicle sorting followed by retrograde axonal transport to the cell body, where interaction of the activated receptor initiates a signalling cascade at the cell body that causes the survival response. It has recently been shown that the signalling molecules associated with retrograde transport differ between neuronal populations. In sympathetic but not sensory neurons, a wortmannin-sensitive molecule (phosphatidylinositol kinase) is essential for the retrograde transport of neurotrophins. In sensory but not sympathetic neurons, a rapamycin-sensitive molecule (pp70S6K) is associated with retrograde transport of neurotrophins. This is strong evidence that sympathetic and sensory neurons utilize different signalling pathways to perform the same cellular function; retrograde transport. These findings may provide clues to understanding neurological diseases, such as motor neuron disease, in which axonal transport is impaired specifically in motor neurons.

Trigeminal ganglion axons are repelled by their presumptive targets

Previous work suggested that in mouse, presumptive targets of the trigeminal ganglion, rather than intermediate structures, attract pioneer axons from the time their growth cones exit the ganglion (Lumsden and Davies, 1986). In rat we find that some presumptive targets repel trigeminal axons. The repellant activity is concentrated in the anterior and ventral epithelium of the mandibular arch at embryonic day 12 (E12) and was also present in the maxillary arch. The activity is blocked by anti-neuropilin-1. E13 mandible explants repel trigeminal axons during the first day of outgrowth in vitro, but thereafter permit or attract trigeminal ganglion axon outgrowth. By E14, lingual nerve afferents first enter the tongue in vivo, and the repellant influence becomes restricted to the midline. The progressive restriction of the repellant influence may contribute to the in vivo progression of nerve development: the earliest afferents turn anteriorly lateral to the tongue, but subsequently arriving afferents advance into the tongue and then turn away from the midline. Thus, the repellant may influence the order of nerve branch development and the timing of innervation of epithelial and subepithelial targets. Heterochronic studies revealed that the loss of repellant influence from presumptive lateral tongue surface results from downregulation of the repellant activity, not of responsiveness to the repellant. Because presumptive targets repel trigeminal axons during the initial stages of advance from the trigeminal ganglion and do not have a net attractive influence until after afferents have arrived near the target, intermediate structures must guide these axons initially.

cAMP-mediated regulation of neurotrophin-induced collapse of nerve growth cones

Neurotrophins are known to promote the survival, differentiation, and neurite outgrowth of developing neurons. Here we report that acutely applied brain-derived neurotrophic factor (BDNF) induces rapid growth cone collapse and neurite retraction of embryonic Xenopus spinal neurons in culture. The collapsing effect of BDNF depends on the activation of Trk receptor tyrosine kinase, requires an influx of extracellular Ca2+, and is regulated by cAMP-dependent activity. Elevation of intracellular cAMP levels ([cAMP]i) by forskolin or (Sp)-cAMP completely blocked the collapsing effect, whereas inhibition of protein kinase A (PKA) by (Rp)-cAMP potentiated the collapsing action. BDNF-induced growth cone collapse was only observed in 6 hr cultures but not in 24 hr cultures. However, inhibition of PKA by (Rp)-cAMP restored the collapsing response of these "old" neurons in 24 hr cultures, suggesting that embryonic Xenopus spinal neurons may upregulate their endogenous cAMP-dependent activity during development in culture, leading to the blockade of their collapsing response to BDNF. Taken together, our results suggest the presence of cross-talk between Ca2+- and cAMP-signaling pathways involved in the collapsing action of neurotrophins, in which the cAMP-pathway regulates the Ca2+-mediated signal transduction required for BDNF-induced collapse. By modulating the cAMP-dependent activity through the intrinsic programming or interaction with other factors present in the environment, a neuron thus could respond to the same extracellular factors with different morphological and cellular changes at different stages during development.

Autocrine regulation of neurite outgrowth from PC12 cells by nerve growth factor

The PC12 cell line may be used as a model of NGF-induced neuronal differentiation. Exposure to NGF is accompanied by extension of neurites, cessation of growth and differentiation into cells resembling sympathetic neurons. In this study neurite outgrowth from PC12 cells was induced in serum-free, NGF-free medium conditions. Neurite outgrowth in serum-free conditions was abolished by exposure to anti-NGF antisera. Reverse transcription combined with polymerase chain reaction (RT-PCR) and in situ hybridization of PC12 cells in serum-free medium conditions revealed NGF transcripts. Western blot analysis of these cells revealed tyrosine phosphorylation of the high affinity NGF receptor (TrkA/gp140) and activation of a downstream signal cascade element, ERK-1/MAP kinase. NGF was also detected by a specific enzyme-linked immunoabsorbant assay (ELISA) revealing picogram levels of protein in conditioned medium and cell lysates. Survival of embryonic rat dorsal root ganglion neurons was maintained in cultures grown in this serum-free conditioned medium. This demonstrated that NGF may act as an autocrine or paracrine growth factor for PC12 cell differentiation.

Neurotrophins rapidly modulate growth cone response to the axon guidance molecule, collapsin-1

We show that neurotrophins acting at the growth cone via Trk receptors can mediate rapid and distinct changes in growth cone response to collapsin-1, an inhibitory axon guidance molecule. We find that the sensitivity of growth cones of dorsal root ganglia (DRG) neurons to collapsin-1 differs when chronically cultured in BDNF, NT-3, or NGF with those in BDNF most sensitive and those in NGF least sensitive. Further, growth cones chronically cultured in BDNF rapidly decrease their sensitivity to collapsin-1 with acute exposure to NGF. Conversely, growth cones chronically cultured in NGF rapidly increase their sensitivity to collapsin-1 with acute exposure to BDNF. These bidirectional effects of neurotrophins appear to be mediated by the neurotrophin-specific Trk receptors on the growth cones since most growth cones are immunopositive for TrkA and TrkB, the NGF and BDNF receptors, respectively, and k252a, a selective inhibitor of Trk-mediated responses to neurotrophins, diminishes collapsin-1-induced growth cone collapse. These findings indicate that the response of growth cones to axon guidance molecules is dynamic and can be rapidly and differentially modulated by neurotrophins.

Endogenous nerve growth factor and neurotrophin-3 act simultaneously to ensure the survival of postnatal sympathetic neurons in vivo

For many years nerve growth factor was the only factor known to influence embryonic and postnatal development of sympathetic neurons. Its deprivation by antibody neutralization or gene mutation results in extensive neuron death. Recently it has been shown that these neurons also require neurotrophin-3 for survival in the late developmental period. Using neurotrophin-3 antiserum to neutralize endogenous factor in newborn rats. Our laboratory has shown that extensive numbers of neurons are lost from both pre- and paravertebral ganglia, indicating a continuing requirement for neurotrophin-3. In the present study we sought to determine whether neurons could survive in vivo in the presence of excess amounts of either nerve growth factor or neurotrophin-3 alone. Consistent with previous findings, administration of antiserum to nerve growth factor or neurotrophin-3 to newborn rats for eight days, resulted in an extensive loss of sympathetic neurons. Interestingly, administration of neurotrophin-3 together with nerve growth factor antiserum or nerve growth factor with neurotrophin-3 antiserum reversed this neuronal loss. However the latter combination was less effective than the former. Furthermore, the ability of exogenous nerve growth factor to increase both the number and size of sympathetic neurons was prevented by the simultaneous deprivation of endogenous neurotrophin-3. Unlike nerve growth factor, exogenous neurotrophin-3 failed to rescue the naturally occurring neuronal death in these newborn rats. Further evidence for a physiological role for both nerve growth factor and neurotrophin-3 was found by the detection of both trkA and trkC immunoreactivity in neurons of the superior cervical ganglion. Taken together, these results suggest that sympathetic neurons do not have an absolute requirement for either nerve growth factor or neurotrophin-3 and that the endogenous supply of either factor alone is insufficient to support neuronal survival postnatally. However, while each factor may play similar roles in the regulation of postmitotic neuronal function, some evidence for distinct functions has been identified.

Plasticity in adult and ageing sympathetic neurons

The nature of neural plasticity and the factors that influence it vary throughout life. Adult neurons undergo extensive and continual adaptation in response to demands that are quite different from those of early development. We review the main influences on the survival, growth and neurotransmitter expression in adult and ageing sympathetic neurons, comparing these influences to those at work in early development. This "developmental" approach is proposed because, despite the contrasting needs of different phases of development, each phase has a profound influence on the mechanisms of plasticity available to its successors. Interactions between neurons and their targets, whether effector cells or other neurons, are vital to all of these aspects of neural plasticity. Sympathetic neurons require access to target-derived diffusible neurotrophic factors such as NGF, NT3 and GDNF, as well as to bound elements of the extracellular matrix such as laminin. These factors probably influence plasticity throughout life. In adult life, and even in old age, sympathetic neurons are relatively resistant to cell death. However, they continue to require target-derived diffusible and bound factors for their maintenance, growth and neurotransmitter expression. Failure to maintain appropriate neuronal function in old age, for example in the breakdown of homeostasis, may result partly from a disturbance of the dynamic, trophic relationship between neurons and their targets. However, there is no clear evidence that this is due to a failure of targets to synthesize neurotrophic factors. On the neural side of the equation, altered responsiveness of sympathetic neurons to neurotrophic factors suggests that expression of the trk and p75 neurotrophin receptors contributes to neuronal survival, maintenance and growth in adulthood and old age. Altered receptor expression may therefore underlie the selective vulnerability of some sympathetic neurons in old age. The role of neural connectivity and activity in the regulation of synthesis of target-derived factors, as well as in neurotransmitter dynamics, is reviewed.

Bone morphogenetic protein-2 and retinoic acid induce neurotrophin-3 responsiveness in developing rat sympathetic neurons

Expression of the receptor tyrosine kinase, Trk, determines the specificity of neurotrophin responsiveness of different neuronal populations during development. Recently it has become apparent that sympathetic neurons of rat superior cervical ganglia (SCG) acquire sensitivity to neurotrophin-3 (NT3) before they become dependent on the target-derived nerve growth factor (NGF) for their survival by sequential induction of TrkC and TrkA. The mechanism controlling the expression of TrkC as well as the source of NT3 at their initial developmental stage has, however, not been clarified. Here we show that the treatment of the perinatal rat SCG neurons which express high levels of trkA mRNA with bone morphogenetic protein-2 (BMP2) induced the expression of trkC mRNA. Induction of the functional TrkC receptor by BMP2 was confirmed by the enhancement of the survival response of these neurons to NT3. Treatment of SCG neurons with retinoic acid (RA) promoted the effect of BMP2 on the induction of trkC mRNA levels. BMP2 treatment, on the other hand, promoted the effect of RA on the suppressions of trkA mRNA levels and the NGF-dependent survival of the SCG neurons. Furthermore, BMP2/RA treatment induced the endogenous expression of NT3. These results indicate that specific environmental signals can regulate neurotrophin responsiveness of developing sympathetic neurons by differential alteration of the trk and neurotrophin expressions.

Primary sensory neurons migrate in response to the chemokine RANTES

We examined the potential for the C-C chemokine RANTES to stimulate dorsal root ganglia (DRG) cell migration. Embryonic day 12 (E12.5) mouse DRG cells migrated in response to RANTES, in vitro, differentiating to the nociceptive phenotype within 18 h. In addition, RANTES stimulated intracellular calcium mobilization in DRG cells. RANTES expression was demonstrated by polymerase chain reaction analysis to be present in E10.5 limb bud, E12.5 DRG, Schwann cells, spinal cord and skin. RANTES protein was detected immunohistochemically in E12.5 DRG and the cutaneous layers of the developing hind limb. Thus, RANTES expression is spatially and temporally consistent with an effector molecule in sensory neuropoiesis, potentially expanding the role of this chemokine to include neurotropism.

Characterization of the cholinergic neuronal differentiation of the human neuroblastoma cell line LA-N-5 after treatment with retinoic acid

Analysis of the molecular factors that control cellular differentiation in mammalian embryos is difficult due to the small amount of material available from embryos and their inaccessibility during gestation. One way to circumvent these limitations is to use model systems that allow the study of differentiation in vitro. In this study we have characterized the response of a human neuroblastoma cell line, LA-N-5, to the differentiation-inducing agent, all-trans retinoic acid (RA) using 23 markers that are characteristic of neural crest cells and some of their derivatives. Following induction with RA, the neural crest-like LA-N-5 cells undergo differentiation into cholinergic neurons with increased expression of a variety of neural-specific markers including neurofilaments, growth associated protein-43, tetanus toxin binding sites, receptors for neurotrophic factors, neuropeptides, choline acetyl transferase, vesicular acetylcholine transporter, and acetylcholinesterase with a concomitant decrease in the expression of non-neuronal markers. These results provide the basis for the use of retinoic acid-induced differentiation of LA-N-5 cells as a model system to study molecular events associated with the differentiation of cholinergic neurons.

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.

Decreased skin sensory innervation in transgenic mice overexpressing insulin-like growth factor-II

Cutaneous sensory innervation was studied in transgenic mice overexpressing insulin-like growth factor II using a keratin promoter. The skin area of these animals is enlarged providing increased target for sensory neurons. L4 dorsal root ganglion cell counts revealed that the total number of sensory neurons was the same in transgenics as control animals. Levels of nerve growth factor per unit weight of skin were also unchanged. The cutaneous nerves of the hindlimb were immunostained with the pan-neuronal marker PGP 9.5 in transgenic and control mice at postnatal day 0 and 21. The innervation in transgenic mice was markedly reduced, particularly in superficial dermis and epidermis and in some areas innervation was completely absent. The effect was greatest in distal skin regions and increased with age. Since insulin-like growth factor II has been reported to be a sensory neurotrophic factor, its effect on neurite outgrowth was tested on embryonic day 14 and 18 mouse lumbar dorsal root ganglion explants in culture. Under these conditions insulin-like growth factor II (5-100 ng/ml) did not have strong growth promoting activity and at embryonic day 18, in the presence of 5-10 ng/ml nerve growth factor, neurite outgrowth was suppressed by insulin-like growth factor II. The results show that increased skin target and availability of nerve growth factor per se do not alter the number of innervating sensory neurons. However, reduced sensory terminal arborization and skin hypoinnervation does occur in the presence of excess insulin-like growth factor-II. It is possible that insulin-like growth factor-II inhibits terminal axon growth directly via receptors on sensory neurons or peripheral glia.

The trkA receptor mediates growth cone turning toward a localized source of nerve growth factor

We have developed an in vitro system for studying the interaction of chick dorsal root ganglion neuronal growth cones with a localized source of nerve growth factor (NGF) covalently conjugated to polystyrene beads. Growth cones rapidly turned and migrated under NGF-coated beads in a process that involved the initial formation of persistent contact with a bead, followed by directed flow of cytoplasm toward the point of contact. A role for the local activation of the high-affinity NGF receptor trkA was suggested by a strong inhibition of the turning response by (1) the addition of an antibody against the extracellular portion of trkA, (2) the elevation of the background concentration of NGF to saturate trkA, or (3) the presence of a concentration of the drug K252a that inhibits trkA activation. NGF binding to the pan-neurotrophin receptor p75 is also involved but is not required for turning. These data show a new role for both the trkA and the p75 receptors: the mediation of local events in the guidance of nerve growth cones.

Neurotrophin-3 promotes the differentiation of muscle spindle afferents in the absence of peripheral targets

The neurons of the dorsal root ganglia (DRG) that supply muscle spindles require target-derived factors for survival. One necessary factor for these neurons is neurotrophin-3 (NT3). To determine whether NT3 can promote the survival of these neurons in the absence of other target-derived factors, we analyzed the effects of exogenous NT3 after early limb bud deletion in the chick. In control embryos, limb bud deletion eliminated approximately 90% of the trkC-positive (trkC+) neurons in lumbar DRG on the deleted side. In addition, the deletion led to a dramatic loss of collateral sensory projections to motoneurons. Exogenous NT3 restored a normal population of trkC+ neurons in lumbar DRG on the deleted side and increased the number of trkC+ neurons in DRG with normal targets (contralateral lumbar and thoracic). The effect was highly selective; NT3 increased the number of trkC+ neurons without significantly changing the number of either trkA+ or trkB+ neurons. The effect of NT3 was attributable to the rescue of DRG neurons from cell death, because exogenous NT3 reduced the number of pyknotic nuclei without significantly altering proliferation. Analysis of spinal projections showed further that many of the trkC+ neurons rescued by NT3 projected to the ventral spinal cord. These neurons thus had central projections characteristic of muscle spindle afferents. Together, our results indicate that NT3 signaling is both necessary and sufficient for the development of the proprioceptive phenotype, even in the absence of other signals from limb muscle.

Nerve growth factor receptor TrkA is down-regulated during postnatal development by a subset of dorsal root ganglion neurons

Nerve growth factor (NGF), signaling through its receptor tyrosine kinase, TrkA, is required for the survival of all small and many intermediate-sized murine dorsal root ganglion (DRG) neurons during development, accounting for 80% of the total DRG population. Surprisingly, NGF/TrkA-dependent neurons include a large population that does not express TrkA in adult mice (Silos-Santiago et al., 1995). This finding suggests two hypotheses: Neurons lacking TrkA in the adult may express TrkA during development, or they may be maintained through a paracrine mechanism by TrkA-expressing neurons. To determine whether TrkA is expressed transiently by DRG neurons that lack the receptor in adulthood, we examined the distribution of TrkA protein during development. We show here that TrkA expression is strikingly developmentally regulated. Eighty percent of DRG neurons expressed TrkA during embryogenesis and early postnatal life, whereas only 43% expressed TrkA at postnatal day (P) 21. Because the period of TrkA down-regulation corresponds with a critical period during which nociceptive phenotype can be altered by NGF (see Lewin and Mendell [1993] Trends Neurosci. 16:353-359), we examined whether NGF modulates the down-regulation of TrkA. Surprisingly, neither NGF deprivation nor augmentation altered the extent of TrkA down-regulation. Our results demonstrate a novel form of regulation of neurotrophin receptor expression that occurs late in development. All DRG neurons that require NGF for survival express TrkA during embryogenesis, and many continue to express TrkA during a postnatal period when neuronal phenotype is regulated by NGF. The subsequent down-regulation of TrkA is likely to be importantly related to functional distinctions among nociceptive neurons in maturity.

TrkA, but not TrkC, receptors are essential for survival of sympathetic neurons in vivo

Neurotrophins and their signaling receptors, the Trk family of protein tyrosine kinases, play a major role in the development of the mammalian nervous system. To determine the precise stages that require Trk receptor signaling during development of the sympathetic system, we have analyzed the superior cervical ganglion (SCG) of embryonic and postnatal mice defective for each of the known Trk receptors. Transcripts encoding TrkC are detected in early sympathetic development, before the coalescence of the SCG. trkA expression appears at E13.5, becoming robust from E15.5 onward. In contrast, trkC expression decreases significantly after E15.5 and remains detectable only in a small subpopulation of cells. No significant trkB expression could be detected in the SCG at any developmental stage. Ablation of TrkA receptors does not affect neurogenesis, expression of neuronal markers, or initial axonal growth. However, these receptors are absolutely necessary for the survival of sympathetic neurons after E15.5 and for proper innervation of their distal targets. In contrast, mice defective for either TrkC or TrkB tyrosine kinase receptors do not display detectable defects in their SCGs. These results illustrate the differential roles of the Trk family of receptors during SCG development and define a critical role for TrkA signaling in the survival, but not differentiation, of SCG neurons. Moreover, these observations raise the possibility that at least some SCG neurons become neurotrophin-dependent before complete target 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.

The identification of a novel cDNA preferentially expressed in the olfactory-limbic system of the adult rat

To analyze cell-specific brain gene expression, we have developed a PCR-based subtractive hybridization cloning method utilizing trace starting material, allowing isolation of novel genes expressed under specific conditions. Our previous studies indicated that local substantia nigra (SN) type 1 astrocytes elaborate an array of trophic molecules which support the survival of SN dopaminergic neurons. Therefore, the current study focused on astrocyte gene expression utilizing a type 1 astrocyte-enriched cDNA library. We report initial characterization of a novel cDNA, designated AT1-46, that is preferentially expressed in the olfactory-limbic system of the adult rat brain. Although AT1-46 is expressed widely in the periphery, it is regulated both developmentally and in a cell-specific fashion in the brain. Structurally, AT1-46 is predicted to encode a highly alpha-helical molecule with several domains of potential coiled coil formation, and exhibits a 28% amino acid sequence identity with the intermediate filament-associated protein, trichohyalin.

Dorsal root ganglion neurons require functional neurotrophin receptors for survival during development

Neurotrophins are the most profound known regulators of survival in the developing peripheral nervous system. Within dorsal root ganglia, the signalling receptors for the different members of the neurotrophin family are distributed in distinct patterns suggesting regulation of different functional classes of sensory neurons. Abnormalities observed in neurotrophin receptor mutant mice have confirmed this idea. Both trkA (-/-) and trkC (-/-) mice have striking neurological defecits referrable to subpopulations of DRG neurons which have distinct axon projections in the periphery. These results thus generalize concepts of dependence on target-derived factors based on extensive work with the prototypical neurotrophin, nerve growth factor. Further analysis of these animals also provides evidence for more complex developmental mechanisms including dependence on locally synthesized neurotrophins at early developmental stages and plasticity of neurotrophin receptor expression.

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.

Expression of trk receptors in the developing and adult human central and peripheral nervous system

A family of tyrosine receptor kinases known collectively as trk receptors plays an essential role in signal transduction mediated by nerve growth factor and related neurotrophins. To localize the major trk receptors (trkA, B and C) in the developing and adult central (CNS) and peripheral (PNS) nervous system, we generated monoclonal antibodies (MAbs) to extracellular (MAbs E7, E13, E16, E21, E29) and intracellular (MAb I2) domains of human trkA fused to glutathione S-transferase. Several MAbs (E7, E13, E16) recognized glycosylated trkA (gp140trk and gp110trk) in Western blots, one MAb (E7) recognized non-glycosylated (p80trk) and glycosylated trkA in immunoprecipitation assays, and two MAbs (E13, E29) detected trkA on the cell surface of NIH3T3 cells transfected with a trkA cDNA. Although generated to trkA fusion proteins, this panel of MAbs also recognized trkB and trkC in flow cytometric studies of NIH3T3 cells transfected with trkB or trkC cDNAs. Thus, we used these pan-trk MAbs to probe selected regions of the CNS and PNS including the hippocampus, nucleus basalis of Meynert, cerebellum, spinal cord, and dorsal root ganglion (DRG) to localize trkA, B, and C receptors in the developing and adult human nervous system. These studies showed that trk receptors are expressed primarily by neurons and are detectable very early in the developing hippocampus, cerebellum, spinal cord, and DRG. Although the distribution and intensity of trk immunoreactivity changed with the progressive maturation of the CNS and PNS, immunoreactive trk receptors were detected in neurons of the adult human hippocampus, nucleus basalis of Meynert, cerebellum, spinal cord, and DRG. This first study of trk receptor proteins in the developing and adult human CNS and PNS documents the expression of these receptors in subsets of neurons throughout the developing and adult nervous system. Thus, although the expression of trk receptor proteins is developmentally regulated, the constitutive expression of these neurotrophin receptors by neurons in many regions of the adult human CNS and PNS implies that mature trk receptor-bearing neurons retain the ability to respond to neurotrophins long after terminal neuronal differentiation is complete.

Muscle sensory neurons require neurotrophin-3 from peripheral tissues during the period of normal cell death

To determine if muscle sensory neurons require neurotrophin-3 (NT3) during the period of normal cell death, we used an NT3-specific antiserum to deplete NT3 from peripheral tissues during this period in chick embryos. DiI staining of dorsal roots indicated that limb injections of anti-NT3 reduced the spinal projection of muscle spindle afferents. In contrast, injection of the antiserum into the spinal cord had no demonstrable effect, indicating that the reduced projection following limb injection was due to peripheral blockade of NT3 signaling. Counts of neurons retrogradely labeled from muscle and cutaneous nerves showed that peripheral blockade of NT3 selectively reduced the survival of muscle sensory neurons without affecting the survival of cutaneous sensory neurons or motoneurons. In situ hybridization with trkC probes indicated that, during the period of cell death, most large diameter muscle sensory neurons express trkC transcripts, whereas few cutaneous neurons express this receptor for NT3. We conclude that large diameter muscle afferents, including spindle afferents, require NT3 from peripheral tissues to survive the normal period of sensory neuron death in vivo.

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.

Overexpression of nerve growth factor in transgenic mice induces novel sympathetic projections to primary sensory neurons

Peripheral nerve crush induces novel projections from noradrenergic sympathetic neurons to sensory ganglia, and it has been suggested that these projections provide an anatomical substrate for chronic pain syndromes that occur after nerve injury. The present study demonstrates that novel sympathetic projections to sensory neurons are also induced in transgenic mice that overexpress nerve growth factor (NGF) in the skin. Specifically, a large proportion of trigeminal neurons in NGF transgenic mice were innervated by tyrosine hydroxylase (TH)-positive pericellular arborizations that were seen only rarely in controls. Electron microscopic analysis of NGF transgenic mice revealed that trigeminal neurons were surrounded by numerous axonal varicosities containing synaptic specializations. Removal of the superior cervical ganglion abolished TH-immunoreactive arborizations in the ipsilateral trigeminal ganglion confirming that these fibers were sympathetic axons. A two-site enzyme-linked immunosorbent assay revealed that transgenic ganglia contained a tenfold increase in NGF peptide compared to controls. However, reverse transcriptase polymerase chain reaction analysis showed no apparent expression of transgene mRNA in sensory ganglia, suggesting that the additional NGF was derived from increased NGF expression in the skin. These results indicate that NGF can induce novel sympathetic projections to sensory neurons in vivo and suggests a model in which increased NGF expression plays a role in the development of sympathetic hyperalgesia after nerve injury.