The neurotrophins and their receptors

The logic of signaling from the cell surface to the nucleus

The apparent imprecisions in the signal transduction mechanisms that couple specific hormone receptors to predictable nuclear transcription events have raised many issues for both biology and medicine. In this article, a fuzzy logic model is proposed to provide a means of formally describing signal transduction systems and making predictions about the effects of various combinations of inputs. The fuzzy logic model may provide opportunities mathematically to describe regulatory ensembles that are too complex to describe by conventional models. In addition, the nature of "fuzzy variables" provides insight into the paradoxes that underlie both imprecision and predictability in signal transduction.

Autophosphorylation of activation loop tyrosines regulates signaling by the TRK nerve growth factor receptor

Many receptor tyrosine kinases possess an "activation loop" containing three similarly placed tyrosine autophosphorylation sites. To examine their roles in the TRK NGF receptor, these residues (Tyr-670, Tyr-674, and Tyr-675) were mutated singly and in all combinations to phenylalanine and stably expressed in Trk-deficient PC12nnr5 cells. All mutant receptors showed significantly diminished nerve growth factor (NGF)-stimulated autophosphorylation, indicating impaired catalytic activity. NGF-induced neurite outgrowth exhibited dose-responsive behavior when transfectants were compared by relative receptor expression and exhibited a functional hierarchy: wild type > Y670F >/= Y674F >> Y675F >/= YY670/674FF = YY670/675FF >> YY674/675FF > YYY670/674/675FFF. NGF-induced tyrosine phosphorylation of Shc, ERKs, and SNT and immediate early gene inductions generally paralleled neurogenic potential. However, activation of phosphatidylinositol 3'-kinase and tyrosine phosphorylation of phospholipase Cgamma-1 was essentially abolished. The latter effect appears due to selective inability of the mutated TRKs to autophosphorylate the tyrosine residue (Tyr-785) required for binding phospholipase Cgamma-1 and indicates that the "activation loop" tyrosines participate in NGF-dependent changes in receptor conformation. Our findings stress the importance that expression levels play in assessing the consequences of receptor mutations and that all three activation loop tyrosines have roles regulating both overall and specific NGF-mediated signaling through TRK.

Association of p75(NTR) with caveolin and localization of neurotrophin-induced sphingomyelin hydrolysis to caveolae

Caveolae are plasma membrane microdomains that are enriched in caveolin, the structural protein of caveolae, sphingomyelin, and other signaling molecules. We previously suggested that neurotrophin-induced p75(NTR)-dependent sphingomyelin hydrolysis may be localized to the plasma membrane. Therefore, we examined if caveolae were a major site of p75(NTR)-dependent sphingomyelin hydrolysis in p75(NTR)-NIH 3T3 fibroblasts. Caveolin-enriched membranes (CEMs) were prepared by either detergent or detergent-free extraction and separated from noncaveolar membranes by centrifugation through sucrose gradients. Immunoblot analysis of the individual gradient fractions indicated that caveolin and p75(NTR) were enriched in CEMs. The localization of p75(NTR) to CEMs was not an artifact of receptor overexpression in the fibroblasts because a similar distribution of p75(NTR) was evident from PC12 cells, which endogenously express p75(NTR). In the p75(NTR) fibroblasts, nerve growth factor induced a time-dependent hydrolysis of sphingomyelin only in CEMs with no hydrolysis detected in noncaveolar membranes. Intriguingly, endogenous p75(NTR) was found to co-immunoprecipitate with caveolin, suggesting that p75(NTR) may associate with caveolin in vivo. This interaction was confirmed in vitro by the co-immunoprecipitation of a glutathione S-transferase fusion protein expressing the cytoplasmic domain of p75(NTR) with caveolin. Collectively, these results demonstrate that neurotrophin-induced p75(NTR)-dependent sphingomyelin hydrolysis localizes to CEMs and suggest that the interaction of p75(NTR) with caveolin may affect signaling through p75(NTR).

Neurotrophin-3 administration attenuates deficits of pyridoxine-induced large-fiber sensory neuropathy

Chronic treatment of adult rats for 2-3 weeks with high doses of pyridoxine (vitamin B6) produced a profound proprioceptive loss, similar to that found in humans overdosed with this vitamin or treated with the chemotherapeutic agent cisplatin. Pyridoxine toxicity was manifest as deficits in simple and precise locomotion and sensory nerve function and as degeneration of large-diameter/large-fiber spinal sensory neurons. As assessed quantitatively in a beam-walking task and by EMG recording of H waves evoked by peripheral nerve stimulation, coadministration of the neurotrophic factor neurotrophin-3 (NT-3; 5-20 mg . kg-1 . d-1, s.c.) during chronic pyridoxine treatment largely attenuated the behavioral and electrophysiological sequelae associated with pyridoxine toxicity. Furthermore, NT-3 administration prevented degeneration of sensory fibers in the dorsal column of the spinal cord. These data are consistent with the evidence that NT-3 is a target-derived neurotrophic factor for muscle sensory afferents and suggest that pharmacological doses of NT-3 may be beneficial in the treatment of large-fiber sensory neuropathies.

Analysis of cutaneous sensory neurons in transgenic mice lacking the low affinity neurotrophin receptor p75

Mice with a targeted mutation of the p75 low affinity neurotrophin receptor display smaller peripheral nerves and dorsal root ganglia. Here we show that transgenic mice have a significant elevation of thresholds to noxious mechanical and heat stimuli compared with p75+/+ control mice. Immunocytochemical analysis using antibodies against PGP 9.5 (a panaxonal marker) and calcitonin gene related peptide (CGRP, which labels peptidergic neurons) showed a reduction to 73% and 54%, respectively, of the epidermal innervation density. However, analysis of the cell size distribution of toluidine blue-stained dorsal root ganglia showed no selective loss of neurons of particular diameters. Moreover, the neurochemical profile of dorsal root ganglia cells as defined by trkA, CGRP, IB4 and RT97 immunostaining revealed no significant differences in comparison with p75+/+ animals. Staining of the dorsal horn of the spinal cord for CGRP and IB4 was also normal in p75-/- animals. Taking into account a previously reported loss of approximately 50% dorsal root ganglion neurons, we conclude that all types of sensory neurons are equally depleted in p75-/- mice and that the absence of p75 impedes the development of more than one neuronal subtype.

Models to study the role of neurotrophic factors in neurodegeneration

The physiological functions of neurotrophic factors, such as nerve growth factor (NGF), in supporting the survival and differentiation of specific neurons during early development has in many cases been well established. Recent studies have shown that neurotrophic factors can also protect vulnerable neurons against a variety of mechanical and chemical injuries. The role and the effects of neurotrophic factors in various neurological diseases are however less known. Neurodegenerative diseases such as Parkinson and Alzheimer's diseases as well as amyotrophic lateral sclerosis (ALS) are characterized by an impaired function and ultimate loss of specific populations of neurons. The study of the ethiology and molecular biology of these diseases has for a long time been hampered by the lack of good animal models mimicked part of the human disease in experimental animals. Here we will discuss some of the current approaches taken in these studies as well as address the important question of the possible beneficial effect of neurotrophic factors in alleviating the symptoms and possibly retarding the course of neurodegenerative diseases.

Neurotrophin regulation of the developing nervous system: analyses of knockout mice

The neurotrophins, NGF, BDNF, NT3 and NT4, are one family in a growing repertoire of neurotrophic factors. The neurotrophins have long been implicated in neuronal survival and recent studies from mice with targeted disruptions of the neurotrophin genes confirm this role, but also reveal that the action of the neurotrophins is more complex, and in some instances more interactive, than originally envisaged. Lack of functional NGF, BDNF and NT3 genes results in severe neuronal deficits and an early postnatal death. However, NT4 is unique among the neurotrophins and while the absence of NT4 does result in limited sensory neuron loss these mice do not die early, suggesting that NT4-dependent neurons are not critical for survival. Phenotypic analyses of mice lacking neurotrophin receptors, TrkA, B and C, confirm that TrkA is the functional receptor for NGF, TrkB acts as the primary receptor for BDNF and NT4, and NT3 signals primarily through TrkC. However, the finding that TrkC mutant mice have a less dramatic phenotype than their NT3 counterparts implicates NT3 in signaling via receptors other than TrkC. Further studies, using combinatorial Trk and neurotrophin deletions, reveal that while BDNF and NT4 subserve distinct neuron populations in most cases, other neuron sub-populations can be supported by either BDNF or NT4, providing evidence for compensatory actions between neurotrophins. As a mechanism to explain programmed cell death that occurs in the developing nervous system, recent studies examining neurotrophin gene-dosage effects suggest that the availability of neurotrophins, NGF, BDNF and NT3, may be limiting for some neuron populations. In addition, the proposed switch in neurotrophin dependency for some neuron populations is now being determined using neurotrophin mutant mice. We discuss these and other recent findings on neurotrophin requirements for the developing nervous system.

The depolarisation-induced release of [125I]BDNF from brain tissue

The pattern of release of radioactive brain-derived neurotrophic factor ([125I]BDNF) from brain tissue was studied. Rat brain slices from cerebral cortex and synaptosomes from cerebral cortex and hippocampus were preloaded with [125I]BDNF. Depolarising stimulation by veratridine (final conc. 50 microM) and high KCl (final conc. 45 mM) caused a short-term, greatly enhanced depolarisation-induced release of [125I]BDNF during superfusion and batch protocol experiments. The results suggested that the evoked release was independent of the presence of extracellular calcium ions, but dependent on intracellular calcium ion stores, since the intracellular calcium ion chelator BAPTA-AM, but not the extracellular chelator EGTA abolished the high-potassium-induced [125I]BDNF release from synaptosomes. The release was blocked by tetrodotoxin (1 microM) when synaptosomes were stimulated by veratridine or potassium chloride. Short time-fraction (30 s) superfusion experiments showed that the [125I]BDNF release from synaptosomes appeared in two temporal phases.

Flk-1, a receptor for vascular endothelial growth factor (VEGF), is expressed by retinal progenitor cells

Throughout development of the vertebrate retina, progenitor cells are multipotential, producing a variety of distinctive cell types. Little is known of the molecular mechanisms directing the determination of cell fate. We have examined retinal progenitor cells for expression of receptor tyrosine kinases in an attempt to define receptors that could allow a progenitor to respond to its environment. We found that the receptor tyrosine kinase Flk-1, previously shown to be expressed in endothelial cells, is also expressed in neural progenitor cells of the mouse retina. Flk-1 RNA expression in the retinal progenitors commences with the onset of neuronal differentiation and persists throughout retinal neurogenesis. Flk-1 RNA and protein levels in the retina vary temporally during development, as shown by in situ hybridization and Western blot analysis. Patterns of beta-galactosidase expression in mice containing the lacZ gene in place of the Flk-1 gene are consistent with Flk-1 being expressed in retinal progenitors. In addition, we show that the ligand of Flk-1, vascular endothelial growth factor (VEGF), is expressed in the developing retina by differentiated cells and that a chimeric ligand of VEGF fused to alkaline phosphatase binds to proliferating retinal progenitors. Furthermore, the neural retina-derived Flk-1 protein kinase is activated by VEGF in vitro. Thus, the Flk-1 receptor protein kinase is expressed on the surface of neural progenitors in mouse retina and may play a critical role in neurogenesis as well as in vasculogenesis.

Signaling pathways and survival effects of BDNF and NT-3 on cultured cerebellar granule cells

We investigated the signaling pathways exerted by brain-derived neurotrophic factor (BDNF) and neurotrophin-3 (NT-3) in relation to their survival-promoting effects on dissociated cultures of cerebellar granule cells prepared from postnatal 9-day-old rats. Granule neuron survival in culture was supported by BDNF, but not significantly by either nerve growth factor (NGF) or NT-3. BDNF and NT-3 resulted in not only the respective autophosphorylation of the Trk receptors, TrkB or TrkC, but also tyrosine phosphorylation of SHC, a protein involved in controlling p21ras activity, and phosphatidylinositol-3' (PI-3') kinase. NGF does not result in TrkA phosphorylation. In parallel, c-fos was induced within 30 min, in response to BDNF and NT-3. NT-3 induced the phosphorylation of these proteins to a lesser extent than BDNF. BDNF also induced the tyrosine phosphorylation of phospholipase C gamma (PLC gamma), but the NT-3-induced one was not detected. We postulate that no survival promotion by NT-3 is due to lesser level of trkC expression and of the NT-3-induced signaling in the cultured cerebellar granule neurons. Wortmannin, a specific inhibitor of PI-3' inhibited the BDNF effect on neuronal survival. PI-3' kinase-dependent pathways might be involved in the promotion of cerebellar granule cell survival by BDNF.

Rek, a gene expressed in retina and brain, encodes a receptor tyrosine kinase of the Axl/Tyro3 family

Rek (retina-expressed kinase) has been identified as a putative novel receptor-type tyrosine kinase of the Axl/Tyro3 family with a potential role in neural cell development. rek clones were isolated from a chick embryonic brain cDNA library with a DNA probe obtained by reverse transcriptase-polymerase chain reaction of mRNA from Müller glia-like cells cultured from chick embryonic retina. Sequence analysis indicated that Rek is a protein of 873 amino acids with an extracellular region composed of two immunoglobulin-like domains followed by two fibronectin type III domains with eight predicted N-glycosylation sites. Two consensus src homology 2 domain binding sites are present in the cytoplasmic domain, suggesting that Rek activates several signal transduction pathways. Northern analysis of rek mRNA revealed a 5.5-kilobase transcript in chick brain, retina, and kidney and in primary cultures of retinal Müller glia-like cells. Rek protein was identified by immunoprecipitation and immunoblotting as a 140-kDa protein expressed in the chick retina at embryonic days 6-13, which corresponded to the major period of neuronal and glial differentiation. Transfection of rek cDNA into COS cells resulted in transient expression of a putative precursor of 106 kDa that autophosphorylated in immune complex protein kinase assays. Overexpression of rek cDNA in mouse NIH3T3 fibroblasts resulted in activation of the 140-kDa rek kinase and induction of morphologically transformed foci. These properties indicated that Rek has oncogenic potential when overexpressed, but its normal function is likely to be related to cell-cell recognition events governing the differentiation or proliferation of neural cells.

Role of protein kinase activity in apoptosis

The transmission of signals from the plasma membrane to the nucleus involves a number of different pathways all of which have in common protein modification. The modification is primarily in the form of phosphorylation which leads to the activation of a series of protein kinases. It is now evident that these pathways are common to stimuli that lead to mitogenic and apoptotic responses. Even the same stimuli under different physiological conditions can cause either cell proliferation or apoptosis. Activation of specific protein kinases can in some circumstances protect against cell death, while in others it protects the cell against apoptosis. Some of the pathways involved lead to activation of transcription factors and the subsequent induction of genes involved in the process of cell death or proliferation. In other cases, such as for the tumour suppressor gene product p53, activation may be initiated both at the level of gene expression or through pre-existing proteins. Yet in others, while the initial steps in the pathway are ill-defined, it is clear that downstream activation of a series of cystein proteases is instrumental in pushing the cell towards apoptosis. In this report we review the involvement of protein kinases at several different levels in the control of cell behaviour.

Demonstration of a second pharmacologically active promoter region in the NGF gene that induces transcription at exon 3

Nerve growth factor (NGF) has been demonstrated to facilitate neurite outgrowth, rescue neurons from injury, and prevent programmed cell death in neurons. However, the therapeutic potential of NGF is limited by metabolic instability and poor CNS penetration. These limitations might be circumvented by identifying compounds which increase endogenous production of NGF in the brain. We sought to determine the site of all pharmacologically inducible promoters in the NGF gene using a differential analysis based on semi-quantitative reverse transcription polymerase chain reaction (RT-PCR). Mouse L929 cells were serum deprived and NGF mRNA was induced by treatment with phorbol 12-myristate 13-acetate (PMA), 1,25-dihydroxy-vitamin D3 (calcitriol) or horse serum. An increase in transcripts initiating at exon 1 was noted in cDNA from cells induced with all three agents. In addition, we also observed an increase in cDNA transcripts that initiate at exon 3 and do not include exons 1 and 2 (4.38 +/- 0.42, 2.56 +/- 0.05 and 3.04 +/- 0.03 fold increase over control for PMA, calcitriol and serum, respectively). Each of these increases was completely inhibited in the presence of actinomycin D, indicating that the increased levels of mRNA were due to increases in transcription and not mRNA stabilization. These results confirm the previous demonstration of a promoter for NGF near exon 1 and establish a pharmacologically inducible promoter in the NGF gene near exon 3 that could be targeted for therapeutic intervention.

Localization of Trk neurotrophin receptor-like proteins in avian primary lymphoid organs (thymus and bursa of Fabricius)

The avian thymus and bursa of Fabricius are the specific organs where the maturation and differentiation of T- and B-lymphocytes, respectively, take place. In the mammalian lymphoid organs mRNAs of the neurotrophins and their receptors have been identified but their localization at the protein level remains still unknown. This study was undertaken to analyze the localization of the Trk family of tyrosine kinase receptors in the avian primary lymphoid organs (thymus and bursa of Fabricius) during the posthatching development using immunohistochemistry. These proteins serve as essential constituents of the high affinity receptors for neurotrophins. In the thymus of all groups of age specific immunoreactivity (IR) was observed for all three Trks: TrkA-like IR was found labelling medullary epithelial cells and a subpopulation of cortical epithelial cells; TrkB-like IR was found in the medullar dendritic cells and cortical macrophages; TrkC-like IR labelled the cortical epithelial cells and scattered medullar clusters of epithelial cells (including Hassal's corpuscles). Quantitative analysis revealed age-dependent decrease in the area occupied by TrkA-like IR in the cortex, and age-dependent increase in the medulla; no changes were detected in the area occupied by TrkB-like IR; the TrkC-like immunoreactive cells increase from 7 to 30 days and then decrease. Regarding to the bursa of Fabricius, TrkA-and TrkC-like IR were exclusively found in the epithelial cells of the follicle associated and the interfollicular epithelia, as well as TrkC-like IR in some medullary reticular epithelial cells of adult animals. Nevertheless, TrkB-like IR labelled extrafollicular unidentified cells in 7 days old animals, and the follicular secretory dendritic cells at 30 and 60 post-hatching. The area occupied by the medullary TrkB-like IR cells increased between 30 and 60 days. No immunostaining of lymphocytes was observed for any of the assessed antigens. The blood vessels of both the thymus and the bursa of Fabricius were immunoreactive for TrkA- and TrkC-like proteins. The present results provide evidence for the localization of Trks in the non-lymphoid cells (epithelial and dendritic) of the avian primary lymphoid organs, suggesting a role for neurotrophins in these cells. Moreover, the selective cell localization of each Trk protein, and the absence of apparent overlapping, claims for a differential role of the specific Trk ligands. Whether or not these findings have functional relevance for T- and B-lymphocytes processing in avian primary lymphoid organs is discussed.

Immunohistochemistry of human cutaneous Meissner and pacinian corpuscles

This paper reviews the immunohistochemical characteristics of two kinds of human cutaneous sensory nerve formations (SNFs), the Meissner and Pacinian corpuscles. In both kinds of SNF the central axon might be easily identifiable because it displays immunoreactivity (IR) for the neuroendocrine markers neuron-specific enolase and protein gene product 9.5, as well as for neuron-specific intermediate filament proteins, i.e., neurofilaments. Other intermediate filament proteins such as vimentin are localized in the lamellar cells of Meissner corpuscles, and in the inner core, outer core and capsule of Pacinian corpuscles. However, they lack cytokeratins or glial fibrillary acidic protein IR. On the other hand, and in agreement with ultrastructural data, IR for basement membrane constituents laminin and type IV collagen is found underlying all SNF constituents, with the exception of the axon. One of the mechanisms involved in the maintenance of intracellular calcium ions (Ca2+) homeostasis is the calcium binding proteins. Ca2+ play a key role in the mechanoelectric transduction and have been localized in SNFs. In this way IR for the Ca(2+)-binding proteins calbindin D28K, parvalbumin and calretinin, is present and colocalized in both Meissner and Pacinian corpuscles; furthermore, S-100 protein is exclusively localized in the lamellar cells and the inner core. On the other hand, the skin is a main source of neurotrophins for a subset of neural crest sensory neurons, some of which end forming SNF. These factors are conveyed via retrograde axonal transport from the skin to the cell body of the responsive neurons. Interestingly, Meissner and Pacinian corpuscles also display IR for the pan-neurotrophin low-affinity receptor (p75), and for the trkA receptor protein, a basic constituent of the high-affinity receptor for some neurotrophins. Moreover, they express IR for the epidermal growth factor receptor. Finally, other antigens not proper to the cells forming human cutaneous SNF, such as the epithelial membrane antigen and the leucocytary antigen-7, have also been detected.

Tachykinins, neurotrophism and neurodegenerative diseases: a critical review on the possible role of tachykinins in the aetiology of CNS diseases

The tachykinins are a family of undecapeptides that are widely distributed throughout the body, including the central nervous system (CNS). They have several well defined roles in non-CNS sites as well as in the dorsal horn, where they are involved in the transmission of nociceptive information. However their function(s) in other CNS sites is unclear, but there is some evidence that they function as neuromodulators rather than neurotransmitters. This neuromodulation includes a possible role in maintaining the integrity of neuronal populations, analogous to the functions of neurotrophic factors. This review critically evaluates the role of tachykinins as neurotrophic factors, with particular reference to the common neurodegenerative diseases of the CNS.

The concept of uptake and retrograde transport of neurotrophic molecules during development: history and present status

In the present review honoring Hans Thoenen's contributions to the concept of uptake and retrograde transport of trophic molecules, I have attempted to identify the major historical pathways that had to converge before this concept could be accepted as a fundamental principle in neurobiology. Some of the critical events in this history which are discussed here include: neuron-target interactions, bidirectional trophic signals, axoplasmic transport, receptor-mediated endocytosis, transneuronal trophic signals, the discovery of NGF, the retrograde transport of NGF, and the production of NGF by target tissues. Only when all of these diverse pieces of the puzzle were in place was the concept finally confirmed as being the mechanism that mediates the many phenomena attributed to the regulation and maintenance of neurons by their targets.

BDNF down-regulates neurotrophin responsiveness, TrkB protein and TrkB mRNA levels in cultured rat hippocampal neurons

Regulation of Trk receptors by their ligands, the neurotrophins, was investigated in dissociated cultures of embryonic day 18 rat hippocampal neurons. Cultures were exposed to brain-derived neurotrophic factor (BDNF), neurotrophin-3 (NT-3) or NT-4/5 for 24 h upon plating followed by factor washout. As determined by immunohistochemical staining and phosphotyrosine blotting, the functional responses to acute stimulation with BDNF, NT-3 and NT-4/5, including c-Fos induction and phosphorylation of Trk and extracellular signal-regulated kinase (ERK) proteins, were significantly decreased after 6 days in culture by prior exposure to BDNF. As determined by Western and Northern blot analysis respectively, there was a parallel down-regulation of TrkB protein as well as of trkB and trkC mRNA levels in BDNF-pretreated cultures. Exposure to NT-3 or NT-4/5 at the same concentrations as BDNF did not down-regulate any of the measured cellular responses or TrkB protein and/or trkB and trkC mRNA levels. Regulation of hippocampal neuronal Trkb protein does not appear to be just a development phenomenon, as infusion of BDNF into the hippocampus of adult rats for 6 days produced an 80% decrease in levels of full-length TrkB protein. We thus show that exposure of hippocampal neurons to BDNF, both in culture and in the adult brain, results in down-regulation of TrkB. At least in vitro, this leads to long-term functional desensitization to BDNF, NT-3 and NT-4/5, as well as down-regulation of trkB and trkC mRNA.

Induction of neurite outgrowth by interleukin-6 is accompanied by activation of Stat3 signaling pathway in a variant PC12 cell (E2) line

PC12-E2 cells, a stable variant subcloned from native cell populations, produce neurites in a rapid, transcription-independent manner upon exposure to nerve growth factor (NGF) or basic fibroblast growth factor (bFGF). They also give a similar morphological response to interleukin-6 (IL-6), which is, however, transcription-dependent and with a slower onset, a phenomenon basically not observed in native PC12 cells. The response profile of PC12-E2 cells to NGF and bFGF is similar to that observed for native PC12 cells pre-exposed (primed) to NGF, and such cells also respond to IL-6 in a fashion indistinguishable from PC12-E2 cells. Mechanistically, NGF and bFGF induce a sustained phosphorylation and activation of ERK1 and ERK2 in both cells, while IL-6 produces only a transient and weak tyrosine phosphorylation. However, it does stimulate a prolonged and biphasic tyrosine phosphorylation and nuclear translocation of Stat3 (signal transducers and activators of transcription 3; at least 24 h) and, to a lesser extent, Stat1. Gel shift and supershift analyses confirm that IL-6 predominantly activates Stat3 (and some Stat1) and stimulates sis-inducible element binding activity. Other members of the same cytokine subfamily, including ciliary neurotrophic factor and leukemia inhibitory factor, also cause a transient initial phase of tyrosine phosphorylation and activation of Stat1 and Stat3 (up to 1 h) but fail to stimulate a second phase of response and do not produce significant neurites. These results suggest that sustained signaling of either STAT or ERK pathways in PC12-E2 cells leads to induction of neuronal differentiation. However, only the latter is effective in native PC12 cells as the activation of Stat3 and Stat1 in native PC12 cells by IL-6 fails to induce neuronal differentiation. Thus, the response of PC12-E2 cells to IL-6 suggests the constitutive expression of a required factor(s) for differentiation, that is induced in native PC12 cells by NGF or bFGF (possibly by ERK activation), but not by IL-6 via Janus kinase/STAT activation. This factor(s), which has a sufficient half-life to allow primed cells to remain responsive to IL-6 for several days, is necessary but not sufficient for differentiation (as measured by neurite proliferation) to occur.

Synergistic induction of neurite outgrowth by nerve growth factor or epidermal growth factor and interleukin-6 in PC12 cells

Native PC12 cells respond differentially to nerve growth factor (NGF) but not interleukin-6 (IL-6); PC12-E2 cells, a stable variant, respond to both stimuli (and more rapidly to NGF). Neither responds to epidermal growth factor (EGF). NGF primarily induces the RAS/extracellular signal-regulated kinase (ERK) pathway and IL-6 activates a JAK (Janus tyrosine kinase)/STAT (signal transducers and activators of transcription) response. EGF also stimulates RAS/ERK but in a transient manner. When either cell type is treated with combinations of NGF, EGF, and IL-6, at concentrations that produce modest or no response, a substantial augmentation of neurite outgrowth is observed. With PC12-E2 cells, a subthreshold concentration of IL-6 increases NGF response by approximately 2-3-fold after 1-2 days; the increase with EGF is more pronounced. Native PC12 cells show even greater synergistic effects with NGF and IL-6. The most dramatic effect was observed with low levels of EGF, where IL-6 increased the percentage of responsive cells from zero to approximately 60% after 3 days. In addition, two neural-specific transcripts, GAP-43 and SCG-10, are synergistically increased by the combinations of growth factors. Importantly, IL-6 does not enhance ERK phosphorylation in the presence of either NGF or EGF. In contrast, NGF and EGF, in the presence or absence of IL-6, cause mobility shifts of Stat3 that are consistent with serine phosphorylations. Although these modifications do not lead to activation and translocation by themselves, in the presence of the tyrosine phosphorylation induced by IL-6, they may play a role in the synergistic responses. These observations suggest a differentially regulated two-stage mechanism for the differentiative response of PC12 cells to NGF.

Agrin acts via a MuSK receptor complex

Formation of th neuromuscular junction depends upon reciprocal inductive interactions between the developing nerve and muscle, resulting in the precise juxtaposition of a differentiated nerve terminal with a highly specialized patch on the muscle membrane, termed the motor endplate. Agrin is a nerve-derived factor that can induced molecular reorganizations at the motor endplate, but the mechanism of action of agrin remains poorly understood. MuSK is a receptor tyrosine kinase localized to the motor endplate, seemingly well positioned to receive a key nerve-derived signal. Mice lacking either agrin or MuSK have recently been generated and exhibit similarly profound defects in their neuromuscular junctions. Here we demonstrate that agrin acts via a receptor complex that includes MuSK as well as a myotube-specific accessory component.

The receptor tyrosine kinase MuSK is required for neuromuscular junction formation in vivo

Formation of neuromuscular synapses requires a series of inductive interactions between growing motor axons and differentiating muscle cells, culminating in the precise juxtaposition of a highly specialized nerve terminal with a complex molecular structure on the postsynaptic muscle surface. The receptors and signaling pathways mediating these inductive interactions are not known. We have generated mice with a targeted disruption of the gene encoding MuSK, a receptor tyrosine kinase selectively localized to the postsynaptic muscle surface. Neuromuscular synapses do not form in these mice, suggesting a failure in the induction of synapse formation. Together with the results of an accompanying manuscript, our findings indicate that MuSK responds to a critical nerve-derived signal (agrin), and in turn activates signaling cascades responsible for all aspects of synapse formation, including organization of the postsynaptic membrane, synapse-specific transcription, and presynaptic differentiation.

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.

B61, a ligand for the Eck receptor protein-tyrosine kinase, exhibits neurotrophic activity in cultures of rat spinal cord neurons

Although the Eph subfamily represents the largest group of receptor protein-tyrosine kinases, the biological roles of the Eph-related receptors and their ligands are not well understood. B61 has been identified recently by receptor affinity chromatography as a ligand for the Eph-related receptor Eck (Bartley et al.: Nature 368:558-560, 1994). Here we show that Eck immunoreactivity is localized in areas of the embryonic rat spinal cord that are rich in axons, suggesting that Eck plays a role in this region of the developing nervous system. To examine the biological function of Eck, monolayer cultures of dissociated cells from embryonic rat spinal cord were treated with soluble B61. With an ED50 of approximately 10 ng/ml, B61 treatment improved the survival of the overall neuronal population. Furthermore, in the presence of B61 neurites were longer and more elaborated. B61 similarly affected survival and neurite length in cultures enriched in motor neurons. These neurotrophic effects of B61 were not observed in the presence of anti-Eck antibodies, indicating that these effects are likely to be mediated by the Eck receptor.

The molecular pathology of small round-cell tumours--relevance to diagnosis, prognosis, and classification

Substantial improvements have been made in the treatment and survival of children with SRCT, resulting in an increased emphasis on precise histological diagnosis. Although diagnostic procedures such as electron microscopy and immunocytochemistry contribute in poorly differentiated cases, an accurate diagnosis can remain elusive in a proportion of SRCTs. The cytogenetic and molecular genetic abnormalities characteristic of the different SRCTs can now be consistently and rapidly identified from minimal quantities of tumour material, using the techniques of FISH and PCR. This, coupled with the identification of novel phenotypic characteristics, has had a major impact on SRCT diagnosis. The aim of a tumour classification is to identify disease entities which are biologically distinct and whose recognition is of clinical value. The recent advances described above demonstrate that the SRCTs are genotypically and phenotypically distinct tumour types and that the genetic abnormalities represent key alterations that influence both the morphology and the clinical behaviour of the tumour. This suggests that these advanced phenotypic and genotypic analyses should form an integral and complementary part of the laboratory assessment and clinical management of these forms of paediatric cancer.

Role of nerve growth factor in the expression of trkA mRNA in cultured embryonic rat basal forebrain cholinergic neurons

Using a quantitative reverse transcription-polymerase chain reaction (RT-PCR), we studied the regulation of trkA mRNA expression in serum-free, cultured basal forebrain neurons from 17-day fetal rats. Besides increasing choline acetyltransferase (ChAT) activities, nerve growth factor (NGF) strikingly induced trkA gene expression in a time- and NGF concentration-dependent manner. Therefore, NGF might play a critical role in trkA gene expression during the development of basal forebrain cholinergic neurons. Furthermore, to investigate whether this up-regulation is connected with the trophic effects on basal forebrain cholinergic neurons, we examined the effects of some other neurotrophic agents (BDNF, NT-3, bFGF, CNTF, and 40 mM KCI) upon ChAT activity and trkA gene expression. Some neurotrophic factors increased ChAT activities to the same degree as NGF, whereas they did not stimulate trkA mRNA expression so potently. NT-3 plus K252b promotes the tyrosine phosphorylation of TrkA in PC12 cells and increases ChAT activity in cultured basal forebrain cholinergic neurons like NGF (Knusel et al., J Neurochem 59: 715-722, 1992). We found that NT-3 plus K252b upregulated the level of trkA mRNA. These results suggested that the expression of trkA mRNA is regulated directly by its specific ligand NGF, rather than neurotrophic effects upon basal forebrain cholinergic neurons and that the up-regulation is connected to a molecular event initiated by the binding of NGF to the TrkA receptor.

TrkA, TrkB and p75 mRNA expression is developmentally regulated in the rat retina

We examined the cellular distribution of mRNAs coding for the neurotrophin receptors TrkA, TrkB and p75 in the rat retina during early postnatal development. At P0 (postnatal day 0), mRNAs coding for each of the three receptors were detected in the ganglion cell layer (GCL) and in the inner plexiform layer (IPL), the latter structure essentially containing retinal ganglion cell processes at this developmental stage. At P5, the innermost part of the inner nuclear layer (INL) also expressed TrkA, TrkB and p75 mRNAs. Finally, the GCL, IPL and the whole INL of P10 retinae were labeled by the three probes. The developmentally regulated expression of these receptors underlies a possible role for neurotrophins in the differentiation and survival of retinal cells.

Axotomy induces a different modulation of both low-affinity nerve growth factor receptor and choline acetyltransferase between adult rat spinal and brainstem motoneurons

Adult rat spinal and brainstem motoneurons re-express low-affinity nerve growth factor receptor (p75) after their axotomy. We have previously reported and quantified the time course of this reexpression in spinal motoneurons following several types of injuries of the sciatic nerve. Other studies reported the reexpression of p75 in axotomized brainstem motoneurons. Results of these previous studies differed regarding the type of the most effective triggering injury for p75 reexpression, the relative duration of this reexpression and the decrease of choline acetyltransferase (ChAT) immunoreactivity (-IR) following a permanent axotomy of spinal or brainstem motoneurons. These differences suggest that these two populations of motoneurons respond to axotomy with a different modulation of p75 and ChAT expression. The aim of the present study was to determine whether differential modulation exists. We have analyzed and quantified the presence of p75- and ChAT-IR motoneurons in the hypoglossal nucleus following the same types of injury and the same time course we previously used for sciatic motoneurons. The results show that a nerve crush is the most effective triggering injury for p75 and that it induces similar temporal patterns of p75 and ChAT expression for sciatic and hypoglossal motoneurons. In contrast, a cut injury of the sciatic and hypoglossal nerves resulted in distinct temporal courses of both p75 and ChAT expression between these two populations of motoneurons. In fact, a permanent axotomy of the hypoglossal motoneurons induced i) a much longer maintenance phase for p75 than in sciatic motoneurons and ii) a progressive loss of ChAT-IR with a successive return to normal values in contrast to the modest decrease in the sciatic motoneurons. This evidence indicates that spinal and brainstem motoneurons respond to a permanent axotomy with a different modulation of p75 and ChAT expression. Altogether, the present data and the reported evidence of a differential post-axotomy cell death support the hypothesis that these two populations of motoneurons undergo different dynamic changes after axotomy.

A kinase-deficient splice variant of the human JAK3 is expressed in hematopoietic and epithelial cancer cells

Signal transduction of cytokine receptors is mediated by the JAK family of tyrosine kinases. Recently, the kinase partners for the interleukin (IL)-2 receptor have been identified as JAK1 and JAK3. In this study, we report the identification of splice variants that may modulate JAK3 signaling. Three splice variants were isolated from different mRNA sources: breast (B), spleen (S), and activated monocytes (M). Sequence analysis revealed that the splice variants contain identical NH2-terminal regions but diverge at the COOH termini. Analyses of expression of the JAK3 splice isoforms by reverse transcriptase-polymerase chain reaction on a panel of cell lines show splice preferences in different cell lines: the S-form is more commonly seen in hematopoietic lines, whereas the B- and M-forms are detected in cells both of hematopoietic and epithelial origins. Antibodies raised against peptides to the B-form splice variant confirmed that the 125-kDa JAK3B protein product is found abundantly in hematopoietic as well as epithelial cells, including primary breast cancers. The lack of subdomain XI in the tyrosine kinase core of the B-form JAK3 protein suggests that it is a defective kinase. This is supported by the lack of detected autokinase activity of the B-form JAK3. Intriguingly, both the S and B splice isoforms of JAK3 appear to co-immunoprecipitate with the IL-2 receptor from HUT-78 cell lysates. This and the presence of multiple COOH-terminal splice variants coexpressed in the same cells suggest that the JAK3 splice isoforms are functional in JAK3 signaling and may enrich the complexity of the intracellular responses functional in IL-2 or cytokine signaling.

PC12-E2 cells: a stable variant with altered responses to growth factor stimulation

A variant cell line, designated E2, characterized by more rapid responses to nerve growth factor (NGF) and basic fibroblast growth factor (bFGF) and markedly more robust responses to interleukin-6 and 8-Br-cAMP, has been subcloned from the rat PC12 cell line. The enhanced responsiveness to NGF in E2 cells is not due to receptor overexpression as judged by TrkA protein levels and tyrosine kinase activity, but may be associated with the increased and prolonged tyrosine phosphorylation of ERK1 (extracellular signal regulated kinase 1) and ERK2. The rapid morphological differentiation induced by different growth factors in E2 cells is mediated in a transcription-independent manner suggesting that E2 cells may constitutively express some differentiation-associated molecules that allow direct entry into the neuronal program.

Receptor tyrosine kinase specific for the skeletal muscle lineage: expression in embryonic muscle, at the neuromuscular junction, and after injury

While a number of growth factors have been described that are highly specific for particular cell lineages, neither a factor nor a receptor uniquely specific to the skeletal muscle lineage has previously been described. Here we identify a receptor tyrosine kinase (RTK) specific to skeletal muscle, which we term "MuSK" for muscle-specific kinase. MuSK is expressed at low levels in proliferating myoblasts and is induced upon differentiation and fusion. In the embryo, it is specifically expressed in early myotomes and developing muscle. MuSK is then dramatically down-regulated in mature muscle, where it remains prominent only at the neuromuscular junction; MuSK is thus the only known RTK that localizes to the neuromuscular junction. Strikingly, MuSK expression is dramatically induced throughout the adult myofiber after denervation, block of electrical activity, or physical immobilization. In humans, MuSK maps to chromosome 9q31.3-32, which overlaps with the region reported to contain the Fukuyama muscular dystrophy mutation. Identification of MuSK introduces a novel receptor-factor system that seems sure to play an important and selective role in many aspects of skeletal muscle development and function.

Neurotrophin receptor proteins immunoreactivity in the rat cerebellar cortex as a function of age

The influence of age on immunohistochemically demonstrable neurotrophin receptor proteins (p75, trkA-, trkB-, and trkC-proteins) was studied in the cerebellar cortex of Wistar male rats aged 3 (young), 12 (adult) and 24 (old) months. The number of Purkinje neurons displaying p75, trkA- and trkC-like proteins immunoreactivity (IR), as well as the intensity of p75 and trkA-like protein IR, were significantly reduced in aged rats in comparison with 3 and 12-month-old rats. The intensity of trkC-like protein in the cytoplasm of Purkinje neurons remained unchanged for all the period studied. Moreover, no significant age-dependent changes were observed in the density of p75 or trkC-like proteins IR in the granule neurons layer. The molecular layer showed faint p75 IR which decreased as a function of age. No immunolabelling for neuronal trkB-like proteins was observed, but trkB- and trkC-like proteins IR was found in non-neuronal cells. These results suggest that cerebellar cortex neurons are responsive to and/or dependent upon different neurotrophins. Moreover, the age-dependent impairment in the expression of some neurotrophin receptors in Purkinje neurons, but not in the granule neurons, lends support to a role for neurotrophins in cerebellar aging.

Contribution of interleukin-1 beta to the inflammation-induced increase in nerve growth factor levels and inflammatory hyperalgesia

1. Peripheral inflammation is associated with the local production of neuroactive inflammatory cytokines and growth factors. These may contribute to inflammatory pain and hyperalgesia by directly or indirectly altering the function or chemical phenotype of responsive primary sensory neurones. 2. To investigate this, inflammation was produced by the intraplantar injection of complete Freund's adjuvant (CFA) in adult rats. This resulted in a significant elevation in interleukin-1 beta (IL-1 beta) and nerve growth factor (NGF) levels in the inflamed tissue and of the peptides, substance P and calcitonin gene-related peptide (CGRP) in the L4 dorsal root ganglion 48 h post CFA injection. 3. The effects of a steroidal (dexamethasone) and a non-steroidal (indomethacin) anti-inflammatory drug on the levels of NGF and IL-1 beta in inflamed tissue were investigated and compared with alterations in behavioural hyperalgesia and neuropeptide expression in sensory neurones. 4. Systemic dexamethasone (120 micrograms kg-1 per day starting the day before the CFA injection) had no effect on the inflammatory hyperalgesia. When the dose was administered 3 times daily, a reduction in mechanical and to a lesser extent thermal sensitivity occurred. Indomethacin at 2 mg kg-1 daily (i.p.) had no effect on the hyperalgesia and a dose of 4 mg kg-1 daily was required to reduce significantly mechanical and thermal hypersensitivity. 5. The increase in NGF produced by the CFA inflammation was prevented by both dexamethasone and indomethacin, but only at the higher dose levels. Dexamethasone at the lower and higher dose regimes diminished the upregulation of IL-1 beta whereas indomethacin had an effect only at the higher dose. 6. The increase in SP and CGRP levels produced by the CFA inflammation was prevented by dexamethasone and indomethacin at the lower and higher dose regimes. 7. Intraplantar injections of IL-1 beta (0.01, 0.1 and 1 ng) produced a brief (6 h) thermal hyperalgesia and an elevation in cutaneous NGF levels which was prevented by pretreatment with human recombinant IL-1 receptor antagonist (IL-1 ra) (0.625 microgram, i.v.). The thermal hyperalgesia but not the NGF elevation produced by intraplantar IL-1 beta (1 ng) was prevented by administration of a polyclonal neutralizing anti-NGF serum. 8. IL-1 ra significantly reduced the mechanical hyperalgesia produced by CFA for 6 h after administration as well as the CFA-induced elevation in NGF levels. Anti-NGF pretreatment substantially reduced CFA-induced mechanical and thermal hyperalgesia without reducing the elevation in IL-1 beta. 9. Intraplantar NGF (0.02, 0.2 and 2 microg) injections produced a short lasting thermal and mechanical hyperalgesia but did not change IL-1beta levels in the hindpaw skin.10. Our results demonstrate that IL-1beta contributes to the upregulation of NGF during inflammation and that NGF has a major role in the production of inflammatory pain hypersensitivity.

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

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

Cultured basal forebrain cholinergic neurons from postnatal rats show both overlapping and non-overlapping responses to the neurotrophins

Basal forebrain cholinergic neurons respond in vitro and in vivo to nerve growth factor (NGF) and to brain-derived neurotrophic factor (BDNF). It is not clear to what extent the neurons that respond to these two factors, or to neurotrophin-3 or -4/5 (NT-3; NT-4/5) are identical or only partially overlapping populations. We have addressed this issue in cultures of basal forebrain neurons derived from 2-week-old postnatal rats, using choline acetyltransferase (ChAT) and acetylcholinesterase (AChE) as cholinergic markers. Cholinergic neuron survival was enhanced in the presence of NGF, BDNF and NT-4/5. NT-4/5 was as effective as BDNF. NT-3 was without effect at this age, although in cultures derived from embryonic forebrain, cholinergic differentiation was induced by NT-3. Cotreatment with NGF and BDNF resulted in small, but consistent increases in the number of ChAT-positive neurons, compared with either factor alone. NT-4/5 was also found to be additive with NGF, whereas cotreatment with BDNF and NT-4/5 showed no additivity. NT-3 had no additive effects with any other neurotrophin on any cholinergic parameters in postnatal cultures. Taken together, the results indicate the existence in postnatal rat brain of a large overlapping population of cholinergic neurons that are responsive to ligands for the neurotrophin receptors TrkA (NGF) and TrkB (BDNF and NT-4/5), but not TrkC (NT-3), and small distinct populations that show specificity for NGF or BDNF but not both. We hypothesize that cholinergic neurons projecting into different regions of the hippocampus may derive trophic support from distinct neurotrophins.

Ciliary neurotrophic factor supports p75NGFR-immunoreactive non-cholinergic, but not cholinergic, developing septal neurons in vitro

Ciliary neurotrophic factor is known to exert both survival and differentiative actions on a number of neuronal populations of the peripheral and central nervous systems. In this study we have compared the trophic effects of ciliary neurotrophic factor and nerve growth factor on developing septal neurons of the rat in vitro. Fetal septal neurons were grown in vitro under glass coverslips in sandwich culture. Septal cultures grown for 14 days in the continual presence of nerve growth factor contain a population of cholinergic neurons that stain intensely for the low-affinity nerve growth factor receptor (p75NGFR), choline acetyltransferase and acetylcholinesterase. Without added nerve growth factor, few neurons stain for these markers. Ciliary neurotrophic factor addition for 14 days from plating in the absence of exogenous nerve growth factor results in the appearance of a population of neurons that stains for p75NGFR. This population is similar in number to that seen in nerve growth factor-treated cultures but is not immunoreactive for choline acetyltransferase and is significantly smaller in mean cross-sectional area. Delayed addition of nerve growth factor to ciliary neurotrophic factor-supported cultures at 14 days for a further seven days fails to induce choline acetyltransferase immunoreactivity in these p75NGFR-positive septal neurons. In cultures grown in the continual presence of nerve growth factor from plating, removal of nerve growth factor and addition of nerve growth factor antibodies at 14 days results in the death of over 80% of the cholinergic neurons after a further four days. Addition of ciliary neurotrophic factor during the period of nerve growth factor withdrawal appears to preserve a p75NGFR-positive, choline acetyltransferase-negative neuronal population. However, seven day re-addition of nerve growth factor to ciliary neurotrophic factor-treated, nerve growth factor-withdrawn cultures fails to induce choline acetyltransferase immunoreactivity in the ciliary neurotrophic factor-supported p75NGFR-positive septal neurons. Simultaneous treatment of cultures with both ciliary neurotrophic factor and nerve growth factor for 14 days from plating approximately doubles the number of p75NGFR-positive neurons relative to cultures treated with either ciliary neurotrophic factor or nerve growth factor alone, but the number of choline acetyltransferase-positive neurons in these cultures is not significantly greater than that found in cultures treated solely with nerve growth factor. These results suggest that ciliary neurotrophic factor does not support the survival and differentiation of developing septal cholinergic neurons in vitro, but can support the development of a p75NGFR-immunoreactive population of non-cholinergic septal neurons.

Neurotrophins and cytokines--intermediaries between the immune and nervous systems

Neurotrophins (NTs), including nerve growth factor (NGF), are multifunctional: in addition to their well-characterized neurotrophic functions they are known to regulate and to be regulated by cytokines, components of the immune system. In line with this we have found expression of a functional trk proto-oncogene, constituting the signal transducing-receptor for NGF, on monocytes/macrophages, lymphocytes and basophils. Moreover, NGF synthesis is regulated by a cytokine cascade including inflammatory mediators such as interleukin-1 and tumor necrosis factor-alpha. The fact that NGF levels are markedly elevated in cerebrospinal fluid of patients with multiple sclerosis and in serum of patients with systemic lupus erythematosus strongly indicates a role for NGF in immunopathology as well as in normal immune function.

Differential role of the low affinity neurotrophin receptor (p75) in retrograde axonal transport of the neurotrophins

The receptor mechanisms mediating the retrograde axonal transport of the neurotrophins have been investigated in adult rats. We show that transport of the TrkB ligands NT-4 and BDNF to peripheral neurons is dependent on the low affinity neurotrophin receptor (LNR). Pharmacological manipulation of LNR in vivo using either an anti-LNR antibody or a soluble recombinant LNR extracellular domain completely blocked retrograde transport of NT-4 and BDNF to sensory neurons, while having minimal effects on the transport of NGF in either sensory or sympathetic neurons. Furthermore, in mice with a null mutation of LNR, the transport of NT-4 and BDNF, but not NGF, was dramatically reduced. These observations demonstrate a selective role for LNR in retrograde transport of the various neurotrophins from distinct target regions in vivo.

Neuronal deficits, not involving motor neurons, in mice lacking BDNF and/or NT4

Nerve growth factor and other neurotrophins signal to neurons through the Trk family of receptor tyrosine kinases. TrkB is relatively promiscuous in vitro, acting as a receptor for brain-derived neurotrophic factor (BDNF), neurotrophin-4 (NT4) and, to a lesser extent, NT3 (refs 3-5). Mice lacking TrkB show a more severe phenotype than mice lacking BDNF, suggesting that TrkB may act as a receptor for additional ligands in vivo. To explore this possibility, we generated mice lacking NT4 or BDNF as well as mice lacking both neurotrophins. Unlike mice lacking other Trks or neurotrophins, NT4-deficient mice are long-lived and show no obvious neurological defects. Analysis of mutant phenotypes revealed distinct neuronal populations with different neurotrophin requirements. Thus vestibular and trigeminal sensory neurons require BDNF but not NT4, whereas nodose-petrosal sensory neurons require both BDNF and NT4. Motor neurons, whose numbers are drastically reduced in mice lacking TrkB, are not affected even in mice lacking both BDNF and NT4. These results suggest that another ligand, perhaps NT3, does indeed act on TrkB in vivo.

Distribution of neurotrophin receptors in human palatine tonsils: an immunohistochemical study

Increasing evidence indicates that nerve growth factor (NGF) exerts effects on cells of the immune system, but the possible immunomodulatory effect of other neurotrophins (brain-derived neurotrophic factor, BDNF; neurotrophin-3, NT-3; and NT-4/5) has not been studied. Neurotrophins act on responsive cells by binding a low-affinity pan-neurotrophin receptor (p75), and more specific high-affinity receptors (gp140trkA, gp145trkB and gp145trkC considered as preferred signaling transduction receptors for NGF, BDNF and NT-3, respectively). The expression of neurotrophin receptor proteins may be considered, therefore, as a potential indication of neurotrophin activity. In the present study we investigated the distribution of both types of neurotrophin receptors in the human palatine tonsils using immunohistochemical methods. In the follicular germinal centers both lymphocytes and follicular dendritic cells (FDC) displayed gp75 IR, but not IR for trk neurotrophin receptor proteins. gp140trkA-like IR and gp145trkC-like IR were encountered on paracortical interdigitating cells (PIC), and in the high endothelial venule cells. gp145trkB-like IR was found in a cell subpopulation which probably represented macrophages. Present results suggest that NGF, NT-3 and NT-4/5 may act in PIC and indirectly in lymphocytes, whereas BDNF and NT-4/5 could control macrophages. The role of p75 on lymphocytes and FDC and whether trk neurotrophin receptor proteins present in lymphoid tissues are functional receptors for neurotrophins remains to be elucidated.

Effects of brain-derived neurotrophic factor on motor dysfunction in wobbler mouse motor neuron disease

Brain-derived neurotrophic factor (BDNF) has been shown to promote the survival of developing motor neurons in vitro and to rescue motor neurons from axotomy-induced cell death in vivo. In this study, we examined the effects of exogenous BDNF on the progression of wobbler mouse motor neuron disease (MND). After clinical diagnosis at age 3 to 4 weeks, 20 affected mice received subcutaneous injections of recombinant human BDNF (5 mg/kg, n = 10) or vehicle (n = 10), three times a week for 4 weeks. In a separate experiment done to conduct a histometric analysis of the C-5 and C-6 ventral roots and to determine the number of myelinated nerve fibers, 7 wobbler mice received identical BDNF treatment. In the 10 BDNF-treated wobbler mice, grip strength declined at a slower rate (p < 0.03) and was twice as great as that of vehicle-treated animals at the end of treatment (p < 0.01). In vivo biceps (p < 0.01) and in vitro muscle twitch tensions (p < 0.02) were also greater than those of vehicle-treated mice. The biceps muscle weight was 20% greater (p < 0.05) and the mean muscle fiber diameter was significantly larger in BDNF-treated mice (p < 0.001) because the number of small (denervated) muscle fibers was markedly reduced. The number of myelinated motor axons at the cervical ventral roots studied in the additional 7 affected mice was 25% greater with BDNF treatment (p < 0.0001). This study establishes that exogenous BDNF administration can retard motor dysfunction in a natural MND and diminish denervation muscle atrophy and motor axon loss.

Requirement for phosphatidylinositol-3 kinase in the prevention of apoptosis by nerve growth factor

Nerve growth factor (NGF) induces both differentiation and survival of neurons by binding to the Trk receptor protein tyrosine kinase. Although Ras is required for differentiation, it was not required for NGF-mediated survival of rat pheochromocytoma PC-12 cells in serum-free medium. However, the ability of NGF to prevent apoptosis (programmed cell death) was inhibited by wortmannin or LY294002, two specific inhibitors of phosphatidylinositol (Pl)-3 kinase. Moreover, platelet-derived growth factor (PDGF) prevented apoptosis of PC-12 cells expressing the wild-type PDGF receptor, but not of cells expressing a mutant receptor that failed to activate Pl-3 kinase. Cell survival thus appears to be mediated by a Pl-3 kinase signaling pathway distinct from the pathway that mediates differentiation.

Seizures increase trkC mRNA expression in the dentate gyrus of rat hippocampus. Role of glutamate receptor activation

In this study we have shown, by in situ hybridization and RNase protection assay, a significant trkC mRNA increase confined to the dentate gyrus of hippocampus, both after seizures induced by intracerebroventricular injection of kainic acid and bicuculline. Moreover, after bicuculline treatment we observed an earlier increase of trkC mRNA level, which peaked after 3 h and returned back to normal levels by 12 h. In contrast, the kainic acid treatment produced a delayed increase of trkC mRNA, which initiated after 6 h, peaked at 12 h, and returned to normal levels at 24 h. This increase, which involves also trkC mRNA receptor with tyrosine kinase activity, was mediated by non-NMDA receptors and counteracted by GABA potentiating agent diazepam. Using embryonic neuronal cultures from cerebral hemispheres, including hippocampus, we found that glutamate receptor agonists, including glutamate, kainate, NMDA, and t-ACPD, increase trkC mRNA levels with the following rank order of efficacy: NMDA > t-ACPD > kainic acid > glutamate. In conclusion, our data show that trkC mRNA expression in granule cells of the hippocampus dentate gyrus is increased during seizure activity and that it is mediated by non-NMDA receptors.

Comparison of the effects of the neurotrophins on the morphological structure of dopaminergic neurons in cultures of rat substantia nigra

The effect of the various neurotrophin family members on the morphological structure of dopaminergic neurons was compared in dissociated cultures of embryonic rat ventral mesencephalon. Cultures were maintained in vitro in the presence of brain-derived neurotrophic factor (BDNF), neurotrophin-3 (NT-3), neurotrohin-4/5 (NT-4/5), nerve growth factor (NGF) or no added growth factors. Three-dimensional reconstructions of 48 neurons were made in each of the experimental groups following immunocytochemical staining for tyrosine hydroxylase to detect dopaminergic neurons. In addition [3H]mazindol binding analyses were carried out in replicate cultures in order to quantify the effects of the neurotrophins on the number of dopamine uptake sites. Among the neurotrophins tested, NT-4/5 influenced the proximal morphological parameters most, as determined by a 36% increase in the soma profile area and 35% in the number of stem neurites. Analysis of neuritic size and complexity in these cultures revealed that combined neuritic length and number of segments/cell were increased by 45 and 40% respectively. A change in neurite complexity in the NT-4/5 treated cultures was further confirmed using Scholl's concentric sphere analysis. In addition, relative to the control, NT-4/5 increased the neuronal differentiation as evidenced by increases in varicosity density and [3H]mazindol binding by 114 and 101% respectively. BDNF and, to a lesser extent, NT-3 also increased both proximal parameters and parameters of differentiation, but were without effect on parameters of neuritic size and complexity. No effects on neuronal structure were observed in NGF treated cultures. These findings demonstrate that BDNF, NT-3 and NT-4/5 influence the morphological differentiation of dopaminergic neurons in vitro, suggesting they may play a role in the structural development and plasticity of these neurons in the mesencephalon.

Brain-derived neurotrophic factor promotes the survival of neurons arising from the adult rat forebrain subependymal zone

Neuronal precursor cells persist in the adult forebrain ependymal/subependymal zone (SZ) and have been found to produce neurons in cultures derived from birds, rodents, and humans. We postulated that the survival of neurons generated from these cells might be constrained in adulthood by the local absence of trophic support. To test this hypothesis, we established explant cultures of adult rat forebrain SZ and assessed the effect of defined neurotrophins on the survival of new neurons arising from these explants. We found that microtubule-associated protein 2+ neurons arose from explants derived from a wide area of the SZ, spanning the rostral 6 mm of the ventricular system. In cultures exposed to brain-derived neurotrophic factor (BDNF), > 35% of new neurons survived at 22 days in vitro (DIV), and > 25% survived at 42 DIV, concurrent with the virtually complete loss of neurons in unsupplemented controls. The surviving cells expressed trkB, the high-affinity receptor for BDNF. In contrast, neither nerve growth factor nor neurotrophic factor 3 enhanced neuronal survival. Thus, BDNF supports the survival of neurons produced by the adult rat forebrain and may act as a permissive factor for neuronal recruitment in adulthood.