Mice lacking nerve growth factor display perinatal loss of sensory and sympathetic neurons yet develop basal forebrain cholinergic neurons

Growth factor-induced c-fos expression defines distinct subsets of midbrain dopaminergic neurons

Growth factors are considered pivotal for the development, maintenance, and function of mesencephalic dopaminergic neurons. Recent studies have identified a plethora of growth factors which support the survival and differentiation of embryonic dopaminergic neurons. However, the exact cellular targets of these growth factors, and, thus, their precise mechanisms of action, remain largely unknown. To identify these cellular targets, we analysed, at the single cell level, growth factor-induced c-fos expression in dissociated mesencephalic cell cultures derived from a fos-lac Z transgenic mouse line. Pharmacological interference with cell-cell communication was utilized to control for direct growth factor effects. beta-Galactosidase-expressing cells were phenotypically characterized by immunocytochemistry to specific neural cell markers. Glia cell line-derived neurotrophic factor, basic fibroblast growth factor, brain-derived neurotrophic factor, and neurotrophin-3 directly induced Fos expression in differently sized, yet overlapping, populations of tyrosine hydroxylase-immunoreactive dopaminergic neurons. In an additional subpopulation of dopaminergic neurons, neurotrophin-3 induced fos-lac Z expression indirectly through a glutamate-mediated activation of N-methyl-D-aspartate receptors. Consistent with their proposed glial-mediated mode of action, transforming growth factor alpha and platelet-derived growth factor induced Fos expression predominantly in glia but only in a very small number of dopaminergic neurons. These findings demonstrate that individual dopaminergic neurons represent the direct targets of different sets of extracellular growth factors. Our findings further establish that growth factors affect dopaminergic neurons by indirect mechanisms which require specific cell-cell communication. These data also suggest a potential role for growth factors in the establishment of the morphological and functional diversity of midbrain dopaminergic neurons.

Inactivation of bcl-2 results in progressive degeneration of motoneurons, sympathetic and sensory neurons during early postnatal development

Bcl-2 is a major regulator of programmed cell death, a critical process in shaping the developing nervous system. To assess whether Bcl-2 is involved in regulating neuronal survival and in mediating the neuroprotective action of neurotrophic factors, we generated Bcl-2-deficient mice. At birth, the number of facial motoneurons, sensory, and sympathetic neurons was not significantly changed, and axotomy-induced degeneration of facial motoneurons could still be prevented by brain-derived neurotrophic factor (BDNF) or ciliary neurotrophic factor (CNTF). Interestingly, substantial degeneration of motoneurons, sensory, and sympathetic neurons occurred after the physiological cell death period. Accordingly, Bcl-2 is not a permissive factor for the action of neurotrophic factors, and although it does not influence prenatal neuronal survival, it is crucial for the maintenance of specific populations of neurons during the early postnatal period.

Expression of neuronal-NOS in developing basal forebrain cholinergic neurons: regulation by NGF

Nerve growth factor (NGF) acts through the receptor tyrosine kinase trkA to serve as a trophic factor for cholinergic neurons in the medial septal nucleus and vertical limb of the diagonal band. We have previously shown that the neuronal isoform of nitric oxide synthase (NOS) is selectively expressed in a large fraction of trkA-expressing cholinergic neurons in these brain regions in the adult rat, and that NGF induces the expression of neuronal-NOS in these cells. Herein, we show that: 1) neuronal-NOS is also localized to these neurons in the developing septum; 2) the expression of neuronal-NOS is regulated in the developing medial septal nucleus and vertical limb of the diagonal band; 3) neuronal-NOS regulation parallels that for other markers of basal forebrain cholinergic neuron differentiation, such as cholineacetyltransferase; and 4) NGF infusion in the postnatal period induces robust increases in neuronal-NOS mRNA and in NOS activity in the basal forebrain. Taken together with earlier findings, our results suggest that neuronal-NOS has a role in the differentiation and mature function of septal cholinergic neurons. Through enhancing neuronal-NOS synthesis, endogenous NGF is likely to regulate NO functions in vivo.

Autocrine-paracrine regulation of hippocampal neuron survival by IGF-1 and the neurotrophins BDNF, NT-3 and NT-4

In contrast to sympathetic and sensory neurons in the peripheral nervous system, the neurotrophic requirements for neurons in the central nervous system (CNS) have not been clearly identified. The inactivation of specific neurotrophic factors and their receptors by gene targeting has shown that there are no major changes in neuron numbers in the CNS. This suggests an overlap between the action of different neurotrophic factors in the brain during development. Here we have studied the survival of hippocampal neurons prepared from embryonic rats using different culture conditions. Whereas the hippocampal neurons survive well in culture when plated at high density, they die at lower cell densities in the absence of appropriate neurotrophic factors. Under the latter conditions, both insulin-like growth factor-1 (IGF-1) and neurotrophins - brain-derived neurotrophic factor (BDNF), neurotrophin-3 (NT-3) and neurotrophin-4 (NT-4) - rescued a large proportion of cultured neurons. In addition, hippocampal neurons from BDNF knockout mice exhibited enhanced cell death compared with cells from wild-type animals. BDNF and IGF-1 both increased the survival of the hippocampal neurons lacking BDNF, showing complementary action for these factors in supporting survival. Blocking antibodies against NT-3 and IGF-1 decreased hippocampal neuron survival at low cell densities, showing autocrine or paracrine action of the factors. At higher cell densities, however, the antibodies had no effect, demonstrating that there is a sufficient amount of endogenous factors in supporting survival. Blocking antibodies against NT-3 and IGF-1 decreased hippocampal neurons depend for survival on local neurotrophic factors such as IGF-1, BDNF and NT-3, which act in an autocrine/paracrine manner. The multifactorial support of hippocampal neurons ensures a maximal degree of neuron survival even in the absence of an individual factor

Paracrine and autocrine actions of neurotrophic factors

Neurotrophic factors are proteins that promote the survival and growth of neurons in the vertebrate nervous system. Although it is well known that many neurons obtain these factors from the regions to which their axons project, studies of the sites of neurotrophic factor synthesis have raised the possibility that at least some neurons may obtain these factors from other sources. Alternative sources of neurotrophic factors include cells along a neuron's axon shaft and cells or other axons terminals within the vicinity of a neuron's cell body and dendritic arbour. In addition, recent experimental studies have shown that at certain stages of development neurotrophic factor autocrine loops operate in some neurons. The evidence for and the potential physiological significance of these different modes of action of neurotrophic factors will be discussed.

Effects of NGF overexpression on anatomical and physiological properties of sympathetic postganglionic neurons

To examine the effects of increased target derived nerve growth factor (NGF) on the sympathetic nervous system, the superior cervical ganglion was characterized in transgenic mice overexpressing NGF in keratinized epithelium (e.g. skin, tongue and oral cavity). In these mice NGF overexpression was achieved via expression of an NGF transgene driven by the K14 keratin promoter. This promoter is expressed at approximately embryonic day 11 and thereafter expressed constitutively in the adult. This expression results in supranormal levels of NGF in targets of sympathetic postganglionic neurons prior to the period of programmed cell death. Examination of postnatal day 6 (PN6) and adult transgenic mice shows ca. 2.5-fold increase in neuron number in the superior cervical ganglion (SCG). Analysis of SCG neuronal size revealed a dramatic hypertrophy in the transgenic mice that is present by PN6 and is maintained in the adult. Intracellular physiological measurements of control superior cervical ganglia identified two distinct types of neurons identified on the basis of their response to depolarizing current; 'phasic' neurons fire a single action potential while 'tonic' neurons fire continuously. In adult transgenic mice the phasic neurons were 102% larger than control phasic neurons while the tonic neurons only increased 44% relative to controls. The hypertrophy of sympathetic ganglia in the transgenic mice was correlated with an increased innervation of skin and dorsal root ganglia, structures that either express the transgene or concentrate NGF produced by the skin.

Degeneration of vestibular neurons in late embryogenesis of both heterozygous and homozygous BDNF null mutant mice

The generation of mice lacking specific neurotrophins permits evaluation of the trophic requirements of particular neuronal populations throughout development. In the present study, we examined the developing vestibulocochlear system to determine the time course of neurotrophin dependence and to determine whether competition occurred among developing cochlear or vestibular neurons for available amounts of either brain-derived neurotrophic factor (BDNF) or neurotrophin-4/5 (NT-4/5). Both cochlear and vestibular neurons were present in mice lacking NT-4/5. In contrast, vestibular neurons decreased in number beginning at mid-stages of inner ear development, in mice lacking BDNF. Early in development (E12.5-13), the size of the vestibular ganglion was normal in bdnf −/− mice. Decreased innervation to vestibular sensory epithelia was detected at E13.5-15, when progressive loss of all afferent innervation to the semicircular canals and reduced innervation to the utricle and saccule were observed. At E16.5-17, there was a reduction in the number of vestibular neurons in bdnf −/− mice. A further decrease in vestibular neurons was observed at P1 and P15. Compared to bdnf −/− mice, mice heterozygous for the BDNF null mutation (bdnf +/−) showed an intermediate decrease in the number of vestibular neurons from E16.5-P15. These data indicate a late developmental requirement of vestibular neurons for BDNF and suggest competition among these neurons for limited supplies of this factor.

The distribution of brain-derived neurotrophic factor and its receptor trkB in parvalbumin-containing neurons of the rat visual cortex

We analysed the distribution of brain-derived neurotrophic factor (BNDNF) and its receptor trkB in the adult rat visual cortex, paying particular attention to a GABAergic neuronal subpopulation - the parvalbumin-positive cells. We found expression of trkB in the cell body and apical dendrite of pyramidal neurons and in the cell body of non-pyramidal neurons. Double labelling experiments revealed extensive colocalization of parvalbumin and trkB immunoreactivity in non-pyramidal neurons. Interestingly, the trkB-positive pyramidal neurons appeared surrounded by parvalbumin-labelled boutons. The use of double immunohistochemistry and in situ hybridization histochemistry showed that parvalbumin-positive neurons express trkB mRNA. BDNF mRNA was found in several cells. Coexpression of BDNF mRNA and parvalbumin immunoreactivity was extremely rare. These data strongly suggest that BDNF synthesized by cortical neurons acts as a postsynaptically derived factor for parvalbumin-positive neurons in the adult rat visual cortex.

The action of neurotrophins in the development and plasticity of the visual cortex

Nerve growth factor (NGF) and the other members of the NGF gene family have been extensively characterized as neurotrophic factors. Recently a modulatory action of these neurotrophic factors on synapse efficacy has emerged. The developing visual system has provided a convenient model to test the role of neurotrophins on neural plasticity in vivo.

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

The role of neurotrophin-3 (NT-3) in early development of the dorsal root ganglion was investigated. Excessive cell death in the dorsal root ganglion of mice that carry a deleted NT-3 gene (NT-3-/- mice) preceded the period of programmed cell death, detected by the TUNEL method, and caused a reduction in the number of proliferating precursors without altering the proportion of proliferating cells to total number of neurons. Furthermore, the majority of proliferating cells detected by bromodeoxyuridine incorporation also stained with the TUNEL method. NT-3 mRNA was expressed locally in the embryonic, but not the postnatal dorsal root ganglion. Most cultured early embryonic NT-3-/- neurons died in the absence of exogenous NT-3 as did the wild-type neurons when cultured with NT-3 neutralizing antibodies, suggesting that NT-3 acts locally to prevent the death of proliferating sensory precursor cells during neurogenesis. Thus, NT-3 may inflict constraints on the number of proliferating precursor cells and thereby affect the number of neurons generated during development of the peripheral nervous system.

Regional distribution of neurotrophin receptors in the developing auditory brainstem

Neuron survival and axonal regeneration become severely limited during early postnatal development. In conjunction with our recent organotypic analysis of regeneration in the auditory midbrain, we wished to determine whether neurotrophins could serve as a trophic substance during the postnatal period. Therefore, the current study examines the development of three neurotrophin receptor tyrosine kinases (TrkA, TrkB, and TrkC) in the gerbil auditory brainstem. Immunoreactivity to TrkA, the nerve growth-factor receptor, was observed in nonneuronal cells during the first two postnatal weeks. In the cochlear nucleus of mature animals, however, there was a TrkA-positive neuronal subpopulation. In contrast, immunoreactivity to TrkB and TrkC (the receptors for brain-derived neurotrophic factor and neurotrophin-3, respectively) displayed a widespread distribution in the auditory brainstem. At postnatal day 0, TrkB and TrkC staining was virtually absent from auditory nuclei, although immunopositive neurons were present in the mesencephalic trigeminal nucleus. By postnatal day 7, TrkB- and TrkC-positive neurons were present in most brainstem auditory nuclei. At postnatal day 15, TrkB immunoreactivity was observed throughout the inferior colliculus (IC), the cochlear nucleus, the medial and lateral nuclei of the trapezoid body, and the lateral superior olive, whereas TrkC labeled only a subpopulation of neurons within the central nucleus of the IC. The TrkB immunoreactivity was present on both neuronal somata and dendrites, whereas TrkC was generally restricted to cell bodies. At postnatal day 30, TrkB immunostaining was observed on most neurons of the IC. The medial and lateral nuclei of the trapezoid body displayed extremely strong TrkB staining, followed by the cochlear nucleus. In contrast, the TrkC immunostaining was decreased dramatically by postnatal day 21. Observations at the ultrastructural level confirmed a neuronal localization of TrkB and TrkC. Immunostaining for both receptors was restricted largely to the postsynaptic density of synaptic profiles in both dendrites and somata. In summary, this study illustrates a differential pattern of immunoreactivity between three neurotrophin receptors during development. The general increase of TrkB expression is well correlated with the onset of sound-evoked activity in this system, and its synaptic localization suggests that it may be involved in the modulation or maintenance of postsynaptic physiology.

Protection of auditory neurons from aminoglycoside toxicity by neurotrophin-3

Hearing is conveyed from the auditory receptors, the hair cells in the organ of Corti, to the brain via the spiral ganglion neurons. Damage or loss of either spiral ganglion neurons or hair cells causes hearing impairment. Such hearing disorders are often permanent and can be caused by therapeutic agents, such as aminoglycoside antibiotics and cisplatin, or by aging, loud sounds, infections and mechanical injury (1). Brain-derived neurotrophic factor (BDNF) and neurotrophin-3 (NT-3), members of the neurotrohin family of neurotrophic factors that also include nerve growth factor (NGF) and neurotrophin-4/5 (NT-4), are important in development of the neuronal components of the inner ear. We report here that the loss of target innervation and the degeneration of approximately 90% of the adult spiral ganglion neurons caused by aminoglycoside toxicity can be prevented by infusion of the neurotrophic factor, neurotrophin-3 (NT-3) in the membranous labyrinth in guinea pigs. The potency of NT-3 in protecting spiral ganglion neurons from degenerating suggests that neurotrophins may be useful for the treatment of hearing disorders.

Role of neurotrophins and trk receptors in the development and maintenance of sensory neurons: an overview

The neurotrophins are a family of polypeptide neuronal growth factors related to the prototypical neurotrophic factor, nerve growth factor (NGF). In mammals this gene family encompasses NGF, brain-derived neurotrophic factor (BDNF) and neurotrophins-3 and -4/5, (NT-3, NT-4/5). The neurotrophins initiate signal transduction in responsive cells by ligand induced dimerization and activation of one of the Trk family of receptor tyrosine kinases; NGF being specific for TrkA, BDNF and NT-4/5 for TrkB, and TrkC the preferred receptor for NT-3. In accord with differential patterns of distribution of Trk receptors in peripheral ganglia, the neurotrophins show both distinct and overlapping specificity towards subpopulations of sensory neurons of both neural crest and neural placode origin. In vitro and in vivo studies, and transgenic mice baring targeted null mutations of the neutrophin genes have established that BDNF, NT-3 and NT-4/5, like NGF, play critical roles as classical target-derived survival factors for subclasses of developing sensory neurons. However, much broader effects of neurotrophins on sensory neurons are now evident, including paracrine and autocrine actions on neuroblast proliferation, phenotypic differentiation, and survival and regeneration in the adult. This article provides an overview of the discovery and properties of the neurotrophin family, their receptors and their actions and specificity for both distinct and overlapping subpopulations of spinal and cranial sensory neurons.

Non target-derived roles of the neurotrophins

The hypothesis that target-derived neurotrophic factors are essential for the survival, differentiation and maintenance of sensory, sympathetic and motor neurons has been well supported by analysis of mice bearing null mutations in the neurotrophins and their receptors. However, the localization of brain-derived neurotrophic factor (BDNF) in a population of dorsal root ganglia (DRG) sensory neurons (Ernfors et al. 1990b; Ernfors & Persson 1991; Schecterson & Bothwell 1992) suggested the additional possibility that BDNF could act in a paracrine or autocrine manner to mediate neuronal survival. We tested this hypothesis in cultured adult DRG neurons, which survive as single cells in microwells in the absence of added trophic factors (Lindsay 1988). About 35% of these neurons were specifically killed by BDNF antisense oligonucleotide administration in a dose-dependent manner, with no effect of sense oligonucleotides. Antisense administration was accompanied by an 80% decrease in BDNF protein levels over the first 24 h of treatment (Acheson et al. 1995). The BDNF autocrine loop that we propose to be present in sensory neurons may be representative of a broader phenomenon in the nervous system as a whole, where the balance of neurotrophic support may shift during development from target-derived to paracrine or autocrine modes. Perhaps as a consequence of this developmental shift, the survival of both peripheral nervous system (PNS) and central nervous system (CNS) neurons in the adult is less affected by axotomy or target removal when compared to their response during development.

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.

The neurotrophic hypothesis: where does it stand?

In the developing peripheral nervous system many neurons die shortly after their axons reach their target fields. This loss is thought to match the number of neurons to the size and requirements of their target fields because altering target field size before innervation affects the number of neurons that survive. The neurotrophic hypothesis provides an explanation for how target fields influence the size of the neuronal populations that innervate them. This hypothesis arose from work on nerve growth factor (NGF), the founder member of the neurotrophin family of secreted proteins. Its principal tenet is that the survival of developing neurons depends on the supply of a neurotrophic factor that is synthesized in limiting amounts in their target fields. The neurotrophic hypothesis has, however, been broadened by the demonstration that multiple neurotrophic factors regulate the survival of certain populations of neurons. For example, some neurons depend on several different neurotrophic factors which may act concurrently or sequentially during target field innervation. In addition, there are aspects of neurotrophin action that do not conform with the classic neurotrophic hypothesis. For example, the dependence of some populations of sensory neurons on particular neurotrophins before significant neuronal death takes place raises the possibility that the supply of these neurotrophins is not limiting for survival at this stage of development. There is also evidence that at stages before and after sensory neurons depend on target-derived neurotrophins for survival, neurotrophins act on at least some sensory neurons by an autocrine route. Yet despite the growing wealth of information on the multiple roles and modes of action of neurotrophic factors, the neurotrophic hypothesis has remained the best explanation for how neuronal target fields in the developing peripheral nervous system regulate their innervation density.

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

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

Neurotrophins and the specification of neuronal phenotype

Nerve growth factor, brain derived neurotrophic factor and neurotrophin-3 all influence sensory neurons derived from the dorsal root ganglia. Traditionally these neurotrophins have been thought of as survival factors for sensory neurons during their development. Recent evidence from experiments where the in vivo levels of these proteins has been manipulated indicates that they may influence the development of specific sensory neuron phenotypes. In this review these experiments are discussed in relation to the mechanisms by which neurotrophins could influence the phenotypic fate of sensory neurons. The first mechanism requires that when a neuron becomes dependent for survival on a neurotrophin the availability of the factor simply influences the number of neurons surviving with a certain modality. This model requires that neurotrophin responsiveness is a determinant of the possible modalities that the neuron may acquire. The second mechanism requires that the availability of a given neurotrophin influences how many neurons can differentiate into different sensory neuron phenotype independent of survival. The available experimental data is discussed in relation to these two models.

NGF as a mediator of inflammatory pain

The chapter reviews some of recent evidence which suggests that one neurotrophin, nerve growth factor (NGF), is a peripherally produced mediator of some persistent pain states, notably those associated with inflammation. The evidence for this proposal is as follows. 1. The endogenous production of NGF regulates the sensitivity of nociceptive systems. Behavioural and electrophysiological studies have shown that sequestration of constitutively produced NGF leads to decrease nociceptor sensitivity. 2. In a wide variety of experimental inflammatory conditions NGF levels are rapidly increased in the inflamed tissue. 3. The high-affinity NGF receptor, trkA, is selectively expressed by nociceptive sensory neurons particularly those containing sensory neuropeptides such as substance P and CGRP. 4. The systematic or local application of exogenous NGF produces a rapid and prolonged behavioural hyperalgesia in both animals and humans. Exogenous NGF has also been found to activate and sensitize fine calibre sensory neurons. 5. In a number of animal models, much of the hyperalgesia associated with experimental inflammation is blocked by pharmacological "antagonism' of NGF. The mechanisms by which NGF up-regulation in inflamed tissues might lead to sensory abnormalities is also discussed. In particular, evidence is reviewed which suggests that increased NGF levels leads to both peripheral sensitization of nociceptors and central sensitization of dorsal horn neurons responding to noxious stimuli.

Acidic fibroblast growth factor mRNA is expressed by basal forebrain and striatal cholinergic neurons

Evidence for the importance of the basal forebrain cholinergic system in the maintenance of cognitive function has stimulated efforts to identify trophic mechanisms that protect this cell population from atrophy and dysfunction associated with aging and disease. Acidic fibroblast growth factor (aFGF) has been reported to support cholinergic neuronal survival and has been localized in basal forebrain with the use of immunohistochemical techniques. Although these data indicate that aFGF is present in regions containing cholinergic cell bodies, the actual site of synthesis of this factor has yet to be determined. In the present study, in situ hybridization techniques were used to evaluate the distribution and possible colocalization of mRNAs for aFGF and the cholinergic neuron marker choline acetyltransferase (ChAT) in basal forebrain and striatum. In single-labeling preparations, aFGF mRNA-containing neurons were found to be codistributed with ChAT mRNA+ cells throughout all fields of basal forebrain, including the medial septum/diagonal band complex and striatum. By using a double-labeling (colormetric and isotopic) technique, high levels of colocalization (over 85%) of aFGF and ChAT mRNAs were observed in the medial septum, the diagonal bands of Broca, the magnocellular preoptic area, and the nucleus basalis of Meynert. The degree of colocalization was lower in the striatum, with 64% of the cholinergic cells in the caudate and 33% in the ventral striatum and olfactory tubercle labeled by the aFGF cRNA. These data demonstrate substantial regionally specific patterns of colocalization and support the hypothesis that, via an autocrine mechanism, aFGF provides local trophic support for cholinergic neurons in the basal forebrain and the striatum.

Transgenic and knockout mice in the study of neurodegenerative diseases

Accurate animal models are essential for detailed analysis of the mechanisms underlying human neurodegenerative diseases. In addition, they can offer useful paradigms for the development and evaluation of new therapeutic strategies. We review the most popular techniques for modification of the mammalian genome in vivo, and provide a critical evaluation of the available transgenic mouse models for several neurological conditions of humans, including prion diseases, human retroviral diseases, Alzheimer's disease, and motor neuron diseases.

Developmental regulation of two distinct neuronal phenotypes in rat dorsal root ganglia

In a previous study we described two distinct neuronal phenotypes in rat dorsal root ganglia based on immunocytochemical assays for the neuronal intermediate filament proteins, peripherin and low-molecular-weight neurofilaments [Goldstein M. E. et al. (1991) J. Neurosci. Res. 30, 92-104]. In this paper we have extended this classification by using in situ hybridization to localize and evaluate the levels of various cytoskeletal and neuropeptide messenger RNAs within the peripherin-immunoreactive and peripherin-immunoreactive-negative neurons found in embryonic day 15 and 20, postnatal day 2 and adult dorsal root ganglia. We found in postnatal and adult dorsal root ganglia in vivo that the large, peripherin-immunoreactive-negative neurons, which are intensely stained by low-molecular-weight neurofilament antibodies, also contain high levels of low, medium and high-molecular-weight neurofilament messenger RNAs, whereas the smaller peripherin-immunoreactive neurons do not. On the other hand, both cell types contained comparable levels of peripherin and alpha-tubulin messenger RNA. The presence of peripherin messenger RNA but no peripherin immunoreactivity in the large cells suggested either a translational or post-translational regulation of this polypeptide, or rapid clearance of this protein from the perikaryon into the axon. In adult dorsal root ganglia, more than 50% of the peripherin-immunoreactive neurons also contained high levels of substance P and/or calcitonin gene-related peptide messenger RNAs, while less than 20% of the large peripherin-immunoreactive-negative neurons did. The attainment of these phenotypic characteristics during development in vivo was studied by northern blot and in situ hybridization histochemistry. In early embryonic stages (embryonic days 15-16), virtually all neurons were peripherin-immunoreactive and were positive for peripherin, alpha-tubulin and low-molecular-weight neuro-filament messenger RNAs, suggesting a homogeneous population. By embryonic day 20, the two adult phenotypes became clearly evident, and were fully established by postnatal day 2. In cultures of embryonic day 15 dorsal root ganglion neurons grown in the presence of nerve growth factor, peripherin and low-molecular-weight neurofilament messenger RNAs were expressed in all neurons, even after nine days in vitro, similar to embryonic dorsal root ganglia in vivo. Nerve growth factor supplemented by skeletal and heart muscle extracts did up-regulate neurofilament gene expression, but not to the extent characteristic of the peripherin-immunoreactive-negative adult phenotype. These results suggest that development of the mature phenotype of dorsal root ganglion neurons occurs by postnatal day 2 in vivo and is dependent upon target contact and/or target-derived factors.

Triiodothyronine exerts a trophic action on rat sensory neuron survival and neurite outgrowth through different pathways

Apart from several growth factors which play a crucial role in the survival and development of the central and peripheral nervous systems, thyroid hormones can affect different processes involved in the differentiation and maturation of neurons. The present study was initiated to determine whether triiodothyronine (T3) affects the survival and neurite outgrowth of primary sensory neurons in vitro. Dorsal root ganglia (DRG) from 19-day-old embryos or newborn rats were plated in explant or dissociated cell cultures. The effect of T3 on neuron survival was tested, either in mixed DRG cell cultures, where neurons grow with non-neuronal cells, or in neuron-enriched cultures where non-neuronal cells were eliminated at the outset. T3, in physiological concentrations, promoted the growth of neurons in mixed DRG cell cultures as well as in neuron-enriched cultures without added nerve growth factor (NGF). Since neuron survival in neuron-enriched cultures cannot be promoted by endogenous neurotrophic factors synthesized by non-neuronal cells, the increased number of surviving neurons was due to a direct trophic action of T3. Another trophic effect was revealed in this study: T3 sustained the neurite outgrowth of sensory neurons in DRG explants. The stimulatory effect of T3 on nerve fibre outgrowth was considerably reduced when non-neuronal cell proliferation was inhibited by the antimitotic agent cytosine arabinoside, and was completely suppressed when the great majority of non-neuronal cells were eliminated in neuron-enriched cultures. These results indicate that the stimulatory effect of T3 on neurite outgrowth is mediated through non-neuronal cells. It is conceivable that T3 up-regulates Schwann cell expression of a neurotrophic factor, which in turn stimulates axon growth of sensory neurons. Together, these results demonstrate that T3 promotes both survival and neurite outgrowth of primary sensory neurons in DRG cell cultures. The trophic actions of T3 on neuron survival and neurite outgrowth operate under two different pathways.

Differential maturation of cholinergic interneurons in the striatal patch versus matrix compartments

Striatal neurons are generated in two distinct phases. Neurons that become postmitotic early in embryonic development come to be located primarily in the patch compartment of the striatum, while the majority of the neurons situated in the striatal matrix compartment are generated later in embryogenesis. The cholinergic interneurons in the striatum, which have been reported to be more or less homogeneously distributed in the adult, are all generated early in development. Given that early generated neurons are expected to be situated primarily in the patch compartment, we investigated the apparently homogeneous distribution of cholinergic neurons by analysing their localizations in the patch and matrix compartments during striatal development. To selectively mark the striatal patch compartment we made injections of the retrograde fluorescent tracer True Blue in the substantia nigra on embryonic day 20 or postnatal day (P)1, and then stained for cholineacetyltransferase (ChAT) at different time-points in development. After P7, the distribution of the ChAT positive neurons changes from an earlier preference for the patch compartment to a preference for an area of the matrix just outside of the patches. Absolute counts show that this change in distribution is caused mainly by a late turn on of ChAT by the cholinergic neurons in the matrix compartment. These data suggest that there are different compartmental subpopulations of cholinergic neurons in the striatum.

Induction of the Oct-2 transcription factor in primary sensory neurons during inflammation is nerve growth factor-dependent

The mRNA encoding the POU family transcription factor Oct-2 is induced in cultured adult sensory neurons following treatment with nerve growth factor (NGF) but not with a variety of other growth factors. We show here that the Oct-2 mRNA is also upregulated in vivo in sensory neurons innervating inflamed tissue following intraplantar injection of complete Freund's adjuvant. This rise is abolished by systemic administration of anti-NGF neutralizing antibodies indicating that it is an NGF-dependent effect. Hence a very specific aspect of the NGF response occurs in sensory neurons innervating inflamed tissue in vivo. In turn, the induction of Oct-2 may play a role in the other changes observed in such neurons both in gene expression and in their ability to respond to painful stimuli.

Role of neurotrophic factors in neuronal development

The spectrum of potential biological roles for neurotrophic factors in development and maturation of the nervous system continues to widen. Careful analysis of the phenotypes of knock-out mice has been used to test directly the 'neurotrophic hypothesis', and the role of members of the transforming growth factor beta superfamily--in particular, glial cell line derived neurotrophic factor--in regulating neuronal survival has become apparent. The effects of neurotrophin-3 on early neuronal differentiation and maturation have proved to be both multiple and complex. Neurotrophic factors are also emerging as potential regulators of synapse stabilization and function.

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.

Specific subtypes of cutaneous mechanoreceptors require neurotrophin-3 following peripheral target innervation

Neurotrophin-3 (NT-3) is required for the development of most sensory neurons of the dorsal root ganglia. Using electrophysiological techniques in mice with null mutations of the NT-3 gene, we show that two functionally specific subsets of cutaneous afferents differentially require this factor: D-hair receptors and slowly adapting mechanoreceptors; other cutaneous receptors were unaffected. Merkel cells, which are the end organs of slowly adapting mechanoreceptors, are virtually absent in 14-day-old homozygous mutants and are severely reduced in adult NT-3 heterozygous animals. This loss of Merkel cells, together with their innervation, happens in the first postnatal weeks of life, in contrast to muscle spindles and afferents, which are never formed in the absence of NT-3. Thus, NT-3 is essential for the maintenance of specific cutaneous afferents known to subserve fine tactile discrimination in humans.

Active p21Ras is sufficient for rescue of NGF-dependent rat sympathetic neurons

We have examined whether p21Ras proteins can rescue nerve growth factor-deprived rat sympathetic neurons from death, to test further our hypothesis that p21Ras is a central mediator in the nerve growth factor-to-survival signalling pathway. After crosslinking [125I]nerve growth factor to live neurons, two forms of Trk (molecular weight approximately 140,000 and 115,000) were immunoprecipitated with anti-Trk antibodies. Nerve growth factor induced tyrosine phosphorylation of both Trk forms and at least two additional proteins. When these phosphorylations were prevented by staurosporine (in a protein kinase C-independent manner) the neurons died. However, neurons were rescued from death due to staurosporine treatment by intracellular loading of oncogenic Ha-Ras(val12) protein. Both Ha-Ras(val12) and cellular Ha-Ras proteins maintained survival for several days in the absence of nerve growth factor and mimicked other actions of nerve growth factor, inducing rapid c-Fos protein expression and robust neurite outgrowth. Conversely, Fab fragments of neutralizing antibodies to p21Ras which blocked the capacity of nerve growth factor to promote neuron survival were also found to inhibit the early expression of c-Fos protein in these neurons. The close correspondence observed between the timing of onset of c-Fos responsiveness and acquisition of nerve growth factor-dependence in embryonic day 17 sympathetic neurons, and the coordinate increase found in both parameters until embryonic day 19 indicates that c-Fos protein expression is a good biochemical indicator of the presence of a functional nerve growth factor-to-survival signal transduction pathway. Nevertheless, expression of c-Fos is not sufficient for survival since phorbol esters induce c-Fos with no effect on survival. These data strengthen our proposal that p21Ras proteins are crucial anti-apoptotic mediators of survival in rat sympathetic neurons by demonstrating that p21Ras is both necessary and sufficient to rescue neurons which are disabled from signalling through Trk receptors.

Expression of neurotrophins and trk receptors in the avian retina

Using the RNase protection assay, we have found that nerve growth factor (NGF), brain-derived neurotrophic factor (BDNF), and neurotrophin-3 (NT-3) are expressed in the avian retina during development. The expression peaks around embryonic days 12-15, with decreasing levels at later stages of development. Abundant levels of NGF and BDNF but low levels of NT-3 mRNA were found in the adult retina. We also found that light/darkness regulated the levels of NGF and BDNF mRNAs but not the levels of NT-3 mRNA in the 5-day-old chicken retina. It was demonstrated that NGF and BDNF mRNA levels were up-regulated by light exposure. The cellular localization of mRNA expression for the neurotrophins and neurotrophin receptors TrkA, TrkB, and TrkC in the retina was studied using in situ hybridization. The patterns of NGF and trkA mRNA expression were very similar and were localized to the external part of the inner nuclear layer on the border with the outer plexiform layer and corresponded to the localization of horizontal cells. NT-3 labeling was also found over the external part of the inner nuclear layer, whereas trkC mRNA was found over all layers in the retina. BDNF labeling was found over all layers in the retina, whereas TrkB labeling was intense over cells in the ganglion cell layer, which is in agreement with the response of ganglion cells to BDNF stimulation. Functional neurotrophin receptors were suggested by the response of retinal explants to neurotrophin stimulation. These data indicate that the neurotrophins play local roles in the retina that involve interactions between specific neuronal populations, which were identified by the localization of the Trk receptor expression. The data also suggest that NGF and BDNF expression is regulated by normal neuron usage in the retina.

Sensory afferent processing in multi-responsive DRG neurons

The recent advance in molecular and neurobiological techniques disclosed the multi-responsive nature of DRG neurons. The survival, phenotype expression and electrical properties of these neurons are under the control of a variety of substances through their specific receptors. In pathological conditions, such as tissue inflammation or nerve injury, DRG neurons change their responsiveness through the dynamic reconstruction of their receptor system. This reconstruction is initiated by environmental stimuli. Thus the properties of polymodal nociceptors can be altered according to the environmental conditions. The whole story of this mechanism is not disclosed yet. In order to understand this mechanism, it is basically important to identify various receptor mRNAs in DRG neurons, precise localization of receptor proteins, site of synthesis and route of supply of ligands for these receptors.

Nerve growth factor withdrawal induces the apoptotic death of developing septal cholinergic neurons in vitro: protection by cyclic AMP analogue and high potassium

Nerve growth factor regulates the developmental programmed cell death of certain neurons in the peripheral nervous system. The functions of nerve growth factor in the central nervous system are less well characterized. Nerve growth factor withdrawal results in the protein synthesis-dependent death of a large percentage of developing septal cholinergic neurons in sandwich tissue culture. In this study double labelling techniques were used to demonstrate that septal cholinergic neurons subjected to nerve growth factor withdrawal exhibit condensed chromatin and fragmented nuclei, and are labelled intensely for fragmented DNA. These degenerative changes are characteristic of apoptotic cell death. Half of the cholinergic neurons were committed to die and could no longer be rescued by nerve growth factor reapplication following approximately 16.5 h of nerve growth factor deprivation, whereas half of the cholinergic neurons could no longer be rescued by cycloheximide addition after only 9 h of nerve growth factor deprivation, suggesting that nerve growth factor and cycloheximide effect rescue by distinct mechanisms. Addition of a cyclic AMP analogue or depolarization with high K+, but not the general nuclease inhibitor aurintricarboxylic acid, prevented the death of cultured septal cholinergic neurons subjected to nerve growth factor withdrawal. Furthermore, these agents are capable of rescuing cholinergic neurons subjected to a period of nerve growth factor withdrawal after which addition of cycloheximide is no longer protective. Thus, nerve growth factor, cyclic AMP and high K+ can effect rescue after inhibition of translation ceases to be protective. These findings suggest that under defined conditions in vitro, withdrawal of nerve growth factor from septal cholinergic neurons during a critical period of development results in the apoptotic death of these CNS neurons, which can be prevented at the post-translational level by nerve growth factor, cyclic AMP and high K+.

Retinoblastoma gene in mouse neural development

The retinoblastoma gene (Rb) was the first tumor suppressor gene to be cloned [Dryja et al., 1986; Friend et al., 1986; Lee et al., 1987], and, as a consequence, has been studied intensively within the context of cell cycle regulation and oncogenesis. However, a number of recent findings indicate that the retinoblastoma gene product (pRb) likely plays an essential role not only in controlling entry into the cell cycle, but also in the terminal differentiation of a number of different cell types [Lee et al., 1994; Gu et al., 1993]. In particular, the phenotype of the Rb nullizygous mice, created by a number of groups using homologous recombination [Jacks et al., 1992: Clarke et al., 1992; Lee et al., 1992], indicates that pRb is essential for normal development of the nervous and hematopoietic systems and may even function to regulate apoptosis [Haas-Kogan et al., 1995]. Although this paper briefly reviews the traditional role of pRB in regulation of cellular proliferation, we focus on the role of pRB in neuronal development and apoptosis. Recent reviews have been published on the role of pRb in cell cycle and transcriptional regulation [Hamel et al., 1992; Cobrinik et al., 1992; Kouzarides, 1993; Hollingsworth et al., 1993; Helin and Harlow, 1993; Sherr, 1994], as well as the relationship between pRb and p53 [Picksley and Lane, 1994; White, 1994].

Regulation by nerve growth factor of neuropeptide phenotypes in primary cultured sensory neurons prepared from aged as well as adult mice

The present study investigated neuropeptide phenotypes of aged, as well as adult, mouse sensory neurons. Proportions of somatostatin (SOM), calcitonin gene related protein (CGRP) and neuropeptide Y (NPY) immunoreactive (ir)-neurons were lower in primary cultures from aged (2 years) mice than in those from adult (6 months) animals, but similar for substance P (SP) in the absence of exogenous nerve growth factor (NGF). Addition of NGF, significantly enhanced (P < 0.05) proportions of SP, NPY and CGRP ir-neurons in both adult and aged cultures, whereas SOM ir-neurons were not affected in either. Thus SP, CGRP, NPY and SOM phenotypes are retained in cultured aged DRG neurons and some phenotypes can remain sensitive to NGF regulation.

Peripheral projections of rat primary sensory neurons immunoreactive for neurotrophin 3

Sensory neurons can be classified into subpopulations based on a variety of characteristics, including their morphology and physiological modalities. Whether any of these classifications correlates with neurotrophic sensitivities has not been determined. We have recently reported that a subpopulation of large diameter sensory neurons of the rat contain neurotrophin 3-like immunoreactivity (NT3-ir). In this study, we have further characterised NT3-ir sensory neurons by their size, segmental localization, and peripheral projections by combined techniques of retrograde tracing and immunohistochemistry. The size distribution showed that NT3-ir was localised to a subpopulation of large-diameter neurons ranging from 560 to 3,120 microns2. Greater numbers of NT3-ir neurons reside in trigeminal (43% of total), cervical (36%), and lumbar (39%) than in thoracic spinal ganglia (13-17%). In combination with Fluoro-Gold retrograde tracing, it was found that about 30% of sensory neurons projecting to the tibial muscle were NT3-ir, compared with 39% for tendon, 50% for whisker hair follicles, 17% for subdermis or epidermis, and only 1% for kidney or adrenal gland. These studies indicate that NT3-ir sensory neurons mainly project to skin and muscles but not viscera. Thus, the characterization of NT3-ir spinal sensory neurons suggests that large sensory neurons subserving proprioception and mechanoception require NT3 for the maintenance of normal function.

Trophic factors and central nervous system metastasis

To metastasize to the central nervous system (CNS) malignant cells must attach to brain microvessel endothelial cells, respond to brain endothelial cell-derived motility factors, respond to CNS-derived invasion factors and invade the blood-brain barrier (BBB), and finally, respond to CNS survival and growth factors. Trophic factors such as the neurotrophins play an important role in tumor cell invasion into the CNS and in the survival of small numbers of malignant cells under stress conditions. Trophic factors promote BBB invasion by enhancing the production of basement membrane-degrading enzymes in neurotrophin-responsive cells. The expression of certain neurotrophin receptors on brain-metastasic neuroendocrine cells occurs in relation to their invasive and survival properties. For example, CNS-metastatic melanoma cells respond to particular neurotrophins (nerve growth factor, neurotrophin-2) that can be secreted by normal cells within the CNS. In addition, a paracrine form of transferrin is important in CNS metastasis, and brain-metastatic cells respond to low levels of transferrin and express high levels of transferrin receptors. CNS-metastatic tumor cells can also produce autocrine factors and inhibitors that influence their growth, invasion and survival in the brain. Synthesis of paracrine factors and cytokines may influence the production of trophic factors by normal brain cells adjacent to tumor cells. Moreover, we found increased amounts of neurotrophins in brain tissue at the invasion front of human melanoma tumors in CNS biopsies. Thus the ability to form metastatic colonies in the CNS is dependent on tumor cell responses to trophic factors as well as autocrine and paracrine growth factors and probably other underdescribed factors.

Differential effects of combined trk receptor mutations on dorsal root ganglion and inner ear sensory neurons

We have generated double mutant mice deficient in pairs of two different Trk receptors and have analysed the effects on survival and differentiation of dorsal root ganglion (DRG), inner ear cochlear and vestibular sensory neurons. In most combinations of mutant trk alleles, the defects observed in double compared to single mutant mice were additive. However, double homozygous trkA−/−;trkB−/− DRG and trkB−/−;trkC−/− vestibular neurons showed the same degree of survival as single trkA−/− and trkB−/− mice, respectively, suggesting that those neurons required both Trk signaling pathways for survival. In situ hybridisation analysis of DRG neurons of double mutant mice revealed differential expression of excitatory neuropeptides. Whereas calcitonin-gene-related peptide expression correlated with the trkA phenotype, substance P expression was detected in all combinations of double mutant mice. In the inner ear, TrkB- and TrkC-dependent neurons were shown to at least partially depend on each other for survival, most likely indirectly due to abnormal development of their common targets. This effect was not observed in DRGs, where neurons depending on different Trk receptors generally innervate different targets.

Influence of growth factors on neuronal differentiation

With so many neurotrophins and receptors now known, how is our picture of neurotrophism changing? Recent studies on knockout mice have confirmed our expectations of neurotrophin action in neuronal development. A notable exception is the activation of TrkB, on motor neurons, by an unknown ligand. It is also clear that some neurotrophins have diverse activities and influence early developmental stages. There are interesting new data concerning the role of p75, the low affinity neurotrophin receptor, as a modulator of neurotrophin activity. Even more exciting are new studies on glia-derived neurotrophic factor (GDNF) which demonstrate that this growth factor acts as a potential protector of motor neurons and striatal dopaminergic neurons.

Genetic knockouts in mice: an update

Gene disruption technology in mammals, by homologous recombination in embryonic stem cells, is a powerful method to manipulate the mouse germ line. In the past decade it has produced a wealth of knowledge concerning neuronal development, neurodegenerative disorders and the roles of oncogenes, Hox genes and growth factors during development. A surprising variety of genes, however, have given unexpected and disappointing results. A gene/function redundancy theory proposed by many investigators to explain the unexpected results has been supported in certain cases by the generation of double knockout mice. Modification of the basic technology now allows the investigators to carry out a variety of manipulations including conditional or tissue-specific knockouts. This may provide a better opportunity in the future for the gene therapy approach to correct the genetic disorder.

Neurotrophins and neuronal plasticity

There is increasing evidence that neurotrophins (NTs) are involved in processes of neuronal plasticity besides their well-established actions in regulating the survival, differentiation, and maintenance of functions of specific populations of neurons. Nerve growth factor, brain-derived neurotrophic factor, NT-4/5, and corresponding antibodies dramatically modify the development of the visual cortex. Although the neuronal elements involved have not yet been identified, complementary studies of other systems have demonstrated that NT synthesis is rapidly regulated by neuronal activity and that NTs are released in an activity-dependent manner from neuronal dendrites. These data, together with the observation that NTs enhance transmitter release from neurons that express the corresponding signal-transducing Trk receptors, suggest a role for NTs as selective retrograde messengers that regulate synaptic efficacy.

Presence or absence of TrkA protein distinguishes subsets of small sensory neurons with unique cytochemical characteristics and dorsal horn projections

Investigations into the biological actions of nerve growth factor (NGF) have shown that dorsal root ganglion (DRG) neurons subserving nociception require NGF for survival and maintenance of phenotype. This discovery suggests that the signaling NGF receptor, TrkA, can be used as a marker for nociceptive neurons. In this study, we have used antibodies to TrkA, in conjunction with cell biological markers that show a restricted distribution in the DRG, to further characterize subsets of DRG neurons that are dependent upon NGF. Staining for TrkA labeled small and medium-sized neurons that composed 47% of all neurons in thoracic ganglia. Double-labeling with antibodies to the high molecular weight neurofilament protein (NFH), a marker for neurons with myelinated axons, demonstrated that TrkA staining is found in only a small subset of myelinated neurons. Surprisingly, many DRG neurons were not labeled by either TrkA or NFH. These neurons had small soma areas, contained the intermediate filament protein peripherin, and were labeled by the lectin BSI, identifying them as neurons likely to have unmyelinated axons. In addition, small TrkA-negative neurons were extensively labeled by antibodies to the intermediate filament protein alpha-internexin, the delta isoform of protein kinase C, and by the BSI isolectin BSI-B4. In order to assess the potential functions of TrkA-negative small neurons, we examined their projections to the dorsal horn of the spinal cord. TrkA-immunoreactivity in the spinal cord was restricted to lamina I and the outer region of lamina II (IIo), similar to staining for calcitonin gene-related peptide. In contrast, the central projections of TrkA-negative neurons, as visualized by BSI-B4 staining, were particularly dense in lamina IIi. Our results suggest that TrkA-expressing and non-TrkA-expressing small neurons compose functionally distinct populations of DRG neurons.

GDNF is an age-specific survival factor for sensory and autonomic neurons

Glial cell line-derived neurotrophic factor (GDNF) promotes the survival of two populations of CNS neurons: motoneurons and midbrain dopaminergic neurons. To see whether GDNF promotes the survival of PNS neurons, we studied embryonic chicken autonomic and sensory neurons in culture. We show that GDNF promotes the survival of sympathetic, parasympathetic, proprioceptive, enteroceptive, and small and large cutaneous sensory neurons. Whereas sympathetic, parasympathetic, and proprioceptive neurons become less responsive to GDNF with age, enteroceptive and cutaneous sensory neurons become more responsive. GDNF mRNA is expressed in the tissues innervated by these neurons, and developmental changes in its expression in several tissues mirror the changing responses of the innervating neurons to GDNF. These results show that GDNF promotes the survival of multiple PNS and CNS neurons and suggest that GDNF may be important for regulating the survival of various populations of neurons at different stages of their development.

Peripheral administration of nerve growth factor in the adult rat produces a thermal hyperalgesia that requires the presence of sympathetic post-ganglionic neurones

Previous evidence suggests that, in adult animals, nerve growth factor (NGF) can induce hyperalgesia, and may be an endogenous mediator in some persistent pain states. Here we have studied the effects of single intradermal injections of 50-500 ng of human recombinant NGF into the plantar skin of adult rat hindpaws. We found that doses of 250 ng and more produced a prolonged and stable thermal hyperalgesia to radiant heat. NGF did not produce overt pain behaviour as judged by the absence of paw licking or guarding of the injected paw. In animals subjected to surgical or chemical sympathectomy, by repeated systemic guanethidine treatments, the hyperalgesic effects of NGF were markedly reduced. We also found that NGF produced plasma extravasation in rat skin, using the Evan's blue method, with a dose dependency similar to that determined for hyperalgesia. Together, these findings suggest that NGF can lead to a rapid activation and sensitization of cutaneous nociceptors. However, these actions appear at least partly indirect, requiring the presence of normal sympathetic post-ganglionic terminals.

Activation of p44 and p42 MAP kinases is not essential for the survival of rat sympathetic neurons

We have examined whether activation of MAP kinases [or extracellular signal-regulated kinases (ERKs)] is required for the survival of rat sympathetic neurons by comparing the actions of three survival factors whose survival-promoting actions can be blocked by neutralizing Fab fragments to p21 ras (Nobes and Tolkovsky, 1995, Eur. J. Neurosci., 7, 344-350), nerve growth factor (NGF), the cytokines ciliary neurotrophic factor (CNTF) and leukaemia inhibitory factor (LIF), and the cyclic AMP analogue 4-(8-chlorophenylthio)cAMP (CPTcAMP). NGF-induced survival was accompanied by an intense (15- to 30-fold) and steady (> 24 h) activation of p44 and p42 ERKs which waned rapidly (t1/2 approximately 30 min) upon NGF withdrawal. However, concentrations of NGF that induced a weak (4- to 5-fold) stimulation of the ERKs were not sufficient to maintain long-term survival. Moreover, prolonged and intense stimulation of the ERKs by NGF for up to 15.5 h was unable to confer long-term survival, since withdrawal of NGF after this time resulted in neuronal death that was kinetically indistinguishable from the death of neurons that had not been exposed to NGF. By contrast, CNTF and LIF continued to support survival for up to 3 days after eliciting only transient (< 30 min and 1 h respectively) activation of p44 and p42 ERKs, while CPTcAMP induced survival for several days without any measurable activation of the ERKs. Taken together, these data suggest that ERK activation per se is neither necessary nor sufficient for survival and that alternative pathways exist for effecting long-term survival of rat sympathetic neurons.

trk A gene expression in neuroblastoma. The clinical significance of an immunohistochemical study

BACKGROUND

Neuroblastomas display a spectrum of morphologic and cytologic features of neural cells, and the prognosis of patients with these tumors varies widely. Expression of trk A in these tumors, as documented by Northern blot analysis, is associated with a favorable prognosis. To examine the expression of trk A at the cellular level in individual tumors and apply the results to routine clinical use, the authors designed this immunohistochemical study using an antibody with a predetermined specificity on formalin fixed, paraffin embedded tumor sections.

METHODS

Expression of trk A and Ha-ras genes in 105 neuroblastomas was examined by avidin-biotin-complex immunoperoxidase staining. N-myc gene amplification was examined in 81 of the tumors by Southern blot analysis.

RESULTS

Immunohistochemical expression in tumors correlated strongly with a favorable prognosis for trk A expression (P < 0.0001) and for Ha-ras expression (P < 0.0001). N-myc amplification was found in neuroblastomas with low expression of trk A and of Ha-ras genes. Kaplan-Meier analysis resulted in a favorable outcome associated with high trk A expression and no N-myc amplification, and a poor outcome associated with low trk A expression and demonstrable N-myc amplification (P < 0.0001). Univariate analysis showed that immunohistochemical expression of trk A at the time of diagnosis was a powerful predictor of the patient's prognosis, as were N-myc amplification and Ha-ras expression. trk A expression even correlated significantly with prognosis when the analysis was restricted to Stages III and IV tumors.

CONCLUSIONS

Immunohistochemical detection of the trk A gene product in tumor cells is strongly predictive of a favorable prognosis for patients with neuroblastomas. The coexpression of trk A and Ha-ras genes with clinical behavior of the tumor may indicate close linkage of these genes in the nerve growth factor signal transduction system. Prognostic evaluation at diagnosis based on such molecular and genetic information should be important clinically.

Regulation of nerve growth factor receptor gene expression in sympathetic neurons during development

We used quantitative reverse transcription (RT)/PCR to study the regulation of p75 mRNA and trkA mRNA expression in the developing sympathetic neurons of the mouse superior cervical sympathetic ganglion (SCG) in vivo and in vitro. At E13, the SCG contains proliferating cells that express many features of differentiated neurons. These immature neurons survived in culture without NGF, and NGF did not induce c-fos expression. Low levels of p75 and trkA mRNAs were expressed at this stage in vivo. There was no significant increase in the level of either trkA mRNA or p75 mRNA in E13 control cultures up to 72 h in vitro, and neither NGF nor depolarizing levels of K+ ions (40 mM KC1) affected the expression of trkA mRNA. In E14 cultures, NGF induced c-fos expression in 10-15% of the neurons and enhanced the survival of a similar percentage of neurons. The proportion of neurons responding to NGF increased with age, reaching 90% in E18 cultures. The in vivo level of trkA mRNA increased markedly from E14 onward, but in contrast to sensory neurons (in which p75 and trkA mRNA levels increase in parallel), the level of trkA mRNA initially increased far more rapidly than that of p75 mRNA. After E17, the level of p75 mRNA increased rapidly and approached that of trkA mRNA postnatally, but at no stage did this exceed the level of trkA mRNA. In E14 cultures, the level of trkA mRNA increased in the absence of neurotrophins or 40 mM KC1. The level of p75 mRNA in E14 cultures was enhanced by NGF but was unaffected by 40 mM KC1. Our findings show that NGF receptor expression during the earliest stages of sympathetic neuron development is not affected by depolarization but indicate that by an early developmental stage (between E13 and E14 in vivo), sympathetic neurons become specified to upregulate trkA mRNA in culture independently of added factors. In addition, our findings reveal several distinctive features of p75 mRNA and trkA mRNA expression in sympathetic neurons compared with sensory neurons and provide a plausible explanation for previously observed differences in the effects of a p75 null mutation on the response of sensory and sympathetic neurons during embryonic and postnatal development.