Mammalian neurotrophin-4: structure, chromosomal localization, tissue distribution, and receptor specificity

Brain-derived neurotrophic factor in the control human brain, and in Alzheimer's disease and Parkinson's disease

Brain-derived neurotrophic factor (BDNF) is a small dimeric protein, structurally related to nerve growth factor, which is abundantly and widely expressed in the adult mammalian brain. BDNF has been found to promote survival of all major neuronal types affected in Alzheimer's disease and Parkinson's disease, like hippocampal and neocortical neurons, cholinergic septal and basal forebrain neurons, and nigral dopaminergic neurons. In this article, we summarize recent work on the molecular and cellular biology of BDNF, including current ideas about its intracellular trafficking, regulated synthesis and release, and actions at the synaptic level, which have considerably expanded our conception of BDNF actions in the central nervous system. But our primary aim is to review the literature regarding BDNF distribution in the human brain, and the modifications of BDNF expression which occur in the brain of individuals with Alzheimer's disease and Parkinson's disease. Our knowledge concerning BDNF actions on the neuronal populations affected in these pathological states is also reviewed, with an aim at understanding its pathogenic and pathophysiological relevance.

Neurotrophins induce death of hippocampal neurons via the p75 receptor

Nerve growth factor (NGF) and related neurotrophins influence neuronal survival and differentiation via interactions with the trk family of receptors. Recent studies have demonstrated that neurotrophins may also induce cell death via the p75 receptor. The importance and generality of neurotrophin-induced death in the brain have not been defined but may play a critical role during development and in disease-associated neuronal death. Here we demonstrate for the first time that all four members of the neurotrophin family directly elicit the death of hippocampal neurons via the p75 receptor. The hippocampus is a complex structure with many different neuronal subpopulations, and signals that influence neuronal death during development may have a critical impact on the mature function of this structure. In these studies we show that each neurotrophin causes the death of hippocampal neurons expressing p75 but lacking the cognate trk receptor. Neurotrophin-induced neuronal death is mediated by activation of Jun kinase. These studies demonstrate that neurotrophins can regulate death as well as survival of CNS neurons.

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

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

Neurotrophins induce death of hippocampal neurons via the p75 receptor

Nerve growth factor (NGF) and related neurotrophins influence neuronal survival and differentiation via interactions with the trk family of receptors. Recent studies have demonstrated that neurotrophins may also induce cell death via the p75 receptor. The importance and generality of neurotrophin-induced death in the brain have not been defined but may play a critical role during development and in disease-associated neuronal death. Here we demonstrate for the first time that all four members of the neurotrophin family directly elicit the death of hippocampal neurons via the p75 receptor. The hippocampus is a complex structure with many different neuronal subpopulations, and signals that influence neuronal death during development may have a critical impact on the mature function of this structure. In these studies we show that each neurotrophin causes the death of hippocampal neurons expressing p75 but lacking the cognate trk receptor. Neurotrophin-induced neuronal death is mediated by activation of Jun kinase. These studies demonstrate that neurotrophins can regulate death as well as survival of CNS neurons.

Chemical modification of pyrimidine TFOs: effect on i-motif and triple helix formation

In order to form more stable triple helical structures or to prevent their degradation in cells, oligonucleotide analogs are routinely used, either in the backbone or among the bases. The target sequence chosen for this study is a 16-base-long oligopurine-oligopyrimidine region present in the human neurotrophin 4/5 gene. Seven different chemical modifications were tested for their effect on (i) triple helix formation and (ii) i-DNA stability. i-DNA is a tetrameric structure involving hemiprotonated C x C+ base pairs, which may act as a competing structure for triplex formation, especially in the case of a cytosine-rich third strand. At acid pH, oligophosphoramidates formed the most stable triple helix, whereas oligonucleotides including 5-propynyl-dU formed a stable i-motif which precluded triplex formation. Only two candidates stabilized triple helices at neutral pH: oligonucleotides with phosphoramidate linkage and phosphodiester oligonucleotides containing 5-methyl-dC and 5-propynyl-dU.

Nerve growth factor potentiated the sodium butyrate- and PMA-induced megakaryocytic differentiation of K562 leukemia cells

We have recently reported that retinoic acid (RA) induced the expression of trkA, the high affinity receptor for nerve growth factor (NGF), in human chronic myelogenous leukemia K562 cells. In this paper, we examined the ability of several other differentiation inducers to regulate the expression of trkA and NGF in K562 cells. We found that the expression of trkA was dramatically induced by the two megakaryocyte lineage inducers sodium butyrate (NaBut) and phorbol 12-myristate 13-acetate (PMA), but not by the two erythrocyte lineage inducers hemin or 1-beta-D-arabinofuranosyl cytosine (Ara-C). Furthermore, activation of the up-regulated trkA receptor by exogenous NGF potentiated the megakaryocytic differentiation of K562 cells induced by NaBut and PMA. Our results demonstrated that trkA is one of the essential genes that are up-regulated and involved in the megakaryocytic differentiation of K562 leukemia cells triggered by these differentiation inducers. Our findings suggest that NGF, in addition to its pivotal roles in the nervous system, may also play important roles in hematopoietic differentiation.

Molecular mechanisms regulating the retrograde axonal transport of neurotrophins

Neurotrophins are released from target tissues following neural innervation and bind to specific receptors situated on the nerve terminal plasma membrane. The neurotrophin-receptor complex undergoes retrograde axonal transport towards the cell soma, where it signals to the nucleus. This process allows neurotrophins to perform their numerous functions, which include the promotion of neuronal survival and the outgrowth of axons towards certain target tissues. The molecular events controlling each of the components of retrograde axonal transport are beginning to become defined. There is good evidence for the participation of phosphatidylinositol 3-kinase, phosphatidylinositol 4-kinase and the actin cytoskeleton in neurotrophin retrograde axonal transport in vivo. It also appears that the retrograde motor protein dynein mediates the retrograde axonal transport in vivo of neurotrophins such as nerve growth factor. This review discusses the role of the neurotrophin receptors in binding and axonal transport, the endocytic processes required for neurotrophin internalization, the targeting and trafficking of neurotrophins, and the propagation of neurotrophin-induced signals along the axon.

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

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

p75 reduces TrkB tyrosine autophosphorylation in response to brain-derived neurotrophic factor and neurotrophin 4/5

Neurotrophins mediate their signals through two different receptors: the family of receptor tyrosine kinases, Trks, and the low affinity pan-neurotrophin receptor p75. Trk receptors show more restricted ligand specificity, whereas all neurotrophins are able to bind to p75. One important function of p75 is the enhancement of nerve growth factor signaling via TrkA by increasing TrkA tyrosine autophosphorylation. Here, we have examined the importance of p75 on TrkB- and TrkC-mediated neurotrophin signaling in an MG87 fibroblast cell line stably transfected with either p75 and TrkB or p75 and TrkC, as well as in PC12 cells stably transfected with TrkB. In contrast to TrkA signaling, p75 had a negative effect on TrkB tyrosine autophosphorylation in response to its cognate neurotrophins, brain-derived neurotrophic factor and neurotrophin 4/5. On the other hand, p75 had no effect on TrkB or TrkC activation in neurotrophin 3 treatment. p75 did not effect extracellular signal-regulated kinase 2 tyrosine phosphorylation in response to brain-derived neurotrophic factor, neurotrophin 3, or neurotrophin 4/5. These results suggest that the observed reduction in TrkB tyrosine autophosphorylation caused by p75 does not influence Ras/mitogen-activated protein kinase signaling pathway in neurotrophin treatments.

Neurotrophins and trk-receptors in adult rat spinal motoneurons: differences related to cell size but not to 'slow/fast' specialization

We have studied the mRNA expression of the neurotrophins brain-derived neurotrophic factor (BDNF), NT-3 and NT-4 and of their receptors trkB and trkC in individual retrogradely labeled lumbar spinal motoneurons of the adult rat, using quantitative non-radioactive in situ hybridization (ISH). We measured soma size and the optical density of the ISH reaction. We analyzed mRNA expression patterns in predominantly 'slow' or 'fast' motoneurons by comparing retrogradely labelled cells that innervated the slow m.soleus (Sol) with those innervating the fast m.extensor digitorum longus (EDL). Within individual motoneuron pools, there was a considerable variation in the intensity of mRNA expression encoding for BDNF, NT-3 and NT-4, as well as for the receptors trkB and trkC. The expression of trkC mRNA was significantly higher in small, presumed fusimotor neurons than in the larger alpha-motoneurons. Surprisingly, none of the variations in the level of motoneuronal mRNA expression for trkB, BDNF, NT-3 and NT-4 were related to soma size and no differences were found between EDL and Sol motoneurons. Apparently, the considerable variations in neurotrophin and neurotrophin receptor mRNA expression represent other parameters of motoneuronal specialization than those related to the well-established 'fast' vs. 'slow' categories of motoneurons.

Regulation of NGF-family ligands and receptors in adulthood and senescence: correlation to degenerative and regenerative changes in cutaneous innervation

During development, a highly differential neurotrophin dependency is reported for various types of nerve endings in the whisker follicle. To what extent these dependencies extend and play a role in adulthood is largely unresolved. We show here, using in situ hybridization and immunohistochemistry that the expression of neurotrophins and trk/p75 receptors persists in adulthood. As suggested by their expression profiles, many classes of cutaneous nerve endings disclose similar ligand-receptor dependencies in adult animals as during development, while other populations appear to switch their dependency. Furthermore, our data suggest that sensory endings that have a high turnover due to mechanical wear and tear, e. g. Merkel cell-neurite complexes at the level of ring sinus show a more complex ligand-receptor expression phenotype than do endings with a less vulnerable location, e.g. the Merkel cell-neurite complexes at the rete ridge collar. Thus, neurotrophin-3 (NT3)/trkA signalling is suggested to be important for a continuous terminal plasticity of Merkel cell-neurite complexes at the level of ring sinus in adulthood. Evidence supporting a role for neurotrophin signalling in maintaining the adult cutaneous innervation also comes from the close correlation between altered ligand-receptor expression(s) and axonal/terminal aberrations in senescence. Thus, an ageing-related decrease in target neurotrophin expression, notably NT3 and NT4, results in a site-specific loss of sensory terminals concomitant with an aberrant growth of regenerating/sprouting axons into new target fields. Ageing of the cutaneous innervation, manifested in degenerative and regenerative events, seems strongly associated with changes in neurotrophic interactions between sensory neurons and target tissues.

Signalling organelle for retrograde axonal transport of internalized neurotrophins from the nerve terminal

The retrograde axonal transport of neurotrophins occurs after receptor-mediated endocytosis into vesicles at the nerve terminal. We have been investigating the process of targeting these vesicles for retrograde transport, by examining the transport of [125I]-labelled neurotrophins from the eye to sympathetic and sensory ganglia. With the aid of confocal microscopy, we examined the phenomena further in cultures of dissociated sympathetic ganglia to which rhodamine-labelled nerve growth factor (NGF) was added. We found the label in large vesicles in the growth cone and axons. Light microscopic examination of the sympathetic nerve trunk in vivo also showed the retrogradely transported material to be sporadically located in large structures in the axons. Ultrastructural examination of the sympathetic nerve trunk after the transport of NGF bound to gold particles showed the label to be concentrated in relatively few large organelles that consisted of accumulations of multivesicular bodies. These results suggest that in vivo NGF is transported in specialized organelles that require assembly in the nerve terminal.

The effects of neurotrophin-3 and brain-derived neurotrophic factor on cerebellar granule cell movement and neurite extension in vitro

Migration of the granule cells is a major stage of cerebellar maturation. Granule cells express neurotrophins and their receptors; however, their role in cell migration has not been defined. In this study we investigated the effects of exogenous neurotrophins on the movement and neurite extension of granule cells from glial-free cerebellar cell reaggregates in vitro. Our results provide direct evidence that neurotrophin-3 and brain-derived neurotrophic factor differentially affect the granule cells. Neurotrophin-3 significantly affected granule cell movements by decreasing the migration index (the ratio of the number of cells that moved further than half the neurite length) and the speed of cell soma movement, but did not affect neurite length or growth cone migration. In contrast, brain-derived neurotrophic factor and neurotrophin-4 acted on growing neurites and growth cones by significantly increasing neurite length and the speed of growth cone migration, but had no effect either on the migration index or on the speed of the cell soma movement. The results suggest that neurotrophins differentially affect neurite extension and the movements of cerebellar granule cells.

Deficient long-term memory and long-lasting long-term potentiation in mice with a targeted deletion of neurotrophin-4 gene

We examined the learning and memory of neurotrophin-4 (NT4)-/- mice by using fear conditioning. In both cue and context conditioning, we found significant deficits in the NT4 mutants at 2 and 24 h after training but not at 30 min. Hippocampal slices from the mutant mice showed normal basal synaptic transmission, short-term plasticity, and decremental long-term potentiation (LTP) at the Schaffer collateral-CA1 synapses. These findings, together with the normal short-term memory, suggest that the hippocampal development of NT4-/- mice is largely unaffected. However, consistent with the long-term memory defects, the long-lasting LTP at the same synapses was attenuated significantly in the mutant mice. Our results suggest that NT4 plays a physiological role essential for hippocampus- and amygdala-dependent long-term memory and hippocampal long-lasting LTP and that NT4 may be useful in the therapy of acquired disorders of learning and memory.

Transient increase of brain derived neurotrophic factor mRNA expression in substantia nigra reticulata after partial lesion of the nigrostriatal dopaminergic pathway

By using non-isostopic in situ hybridization we have demonstrated a transient increase of BDNF mRNA in the lateral subregion of the substantia nigra pars reticulata 1 week after intrastriatal application of 6-OH-DA. These changes correlate with a partial reduction of dopamine (DA) content in the striatum but with a normal tyrosine hydroxylase immunoreactivity in substantia nigra pars compacta. Our data suggest that non-DA, BDNF expressing cells in substantia nigra pars reticulata may play a role in neuronal protection after partial lesions of the DA nigrostriatal pathway.

Differential effects of the trophic factors BDNF, NT-4, GDNF, and IGF-I on the isthmo-optic nucleus in chick embryos

The isthmo-optic nucleus (ION) of chick embryos is a model system for the study of retrograde trophic signaling in developing CNS neurons. The role of brain-derived neurotrophic factor (BDNF) is well established in this system. Recent work has implicated neurotrophin-4 (NT-4), glial cell line-derived neurotrophic factor (GDNF), and insulin-like growth factor I (IGF-I) as additional trophic factors for ION neurons. Here it was examined in vitro and in vivo whether these factors are target-derived trophic factors for the ION in 13- to 16-day-old chick embryos. Unlike BDNF, neither GDNF, NT-4, nor IGF-I increased the survival of ION neurons in dissociated cultures identified by retrograde labeling with the fluorescent tracer DiI. BDNF and IGF-I promoted neurite outgrowth from ION explants, whereas GDNF and NT-4 had no effect. Injections of NT-4, but not GDNF, in the retina decreased the survival of ION neurons and accelerated cell death in the ION. NT-4-like immunoreactivity was present in the retina and the ION. Exogenous, radiolabeled NT-4, but not GDNF or IGF-I, was retrogradely transported from the retina to the ION. NT-4 transport was significantly reduced by coinjection of excess cold nerve growth factor (NGF), indicating that the majority of NT-4 bound to p75 neurotrophin receptors during axonal transport. Binding of NT-4 to chick p75 receptors was confirmed in L-cells, which express chick p75 receptors. These data indicate that GDNF has no direct trophic effects on ION neurons. IGF-I may be an afferent trophic factor for the ION, and NT-4 may act as an antagonist to BDNF, either by competing with BDNF for p75 and/or trkB binding or by signaling cell death via p75.

Regulation of neurotrophin signaling in aging sensory and motoneurons: dissipation of target support?

A hallmark of senescence is sensorimotor impairment, involving locomotion and postural control as well as fine-tuned movements. Sensory and motoneurons are not lost to any significant degree with advancing age, but do show characteristic changes in gene-expression pattern, morphology, and connectivity. This review covers recent experimental findings corroborating that alterations in trophic signaling may induce several of the phenotypic changes seen in primary sensory and motoneurons during aging. Furthermore, the data suggests that target failure, and/or breakdown of neuron-target interaction, is a critical event in the aging process of sensory and motoneurons.

Neurotrophin-4 production by human epidermal keratinocytes: increased expression in atopic dermatitis

Chronic inflammatory conditions of human skin, such as prurigo lesions of atopic dermatitis, are characterized clinically by intense pruritus and histologically by increased innervation. Regulation of skin innervation is thought to depend on neurotrophic factors. In this study, human skin cells were identified as a source of neurotrophins. Cultured keratinocytes expressed neurotrophin-4, whereas dermal fibroblasts expressed neurotrophin-3. In vitro stimulation with interferon-gamma, a marker cytokine for atopic eczema, induced keratinocyte neurotrophin-4 production, which was able to support growth of a neuroglioblastoma-derived cell line. In vivo, immunohistochemistry of human skin for neurotrophins showed neurotrophin-4 staining in the epidermal layer and neurotrophin-3 staining in the dermal compartment. Neurotrophin-4 but not neurotrophin-3 expression was markedly increased in interferon-gamma-injected skin. Prurigo lesions of atopic dermatitis skin were characterized by intense epidermal staining for neurotrophin-4, suggesting a pathophysiologic role for this neurotrophin in the increased innervation characteristic for these skin lesions. This study demonstrates differential expression and regulation of neurotrophins in human skin. It also identifies keratinocyte-derived neurotrophin-4 as a possible link between the immune and the nerve system of human skin.

Neurotrophic and cell-cell dependent control of early follicular development

Neurotrophins (NTs) and their receptors play an essential role in the differentiation and survival of defined neuronal populations of the central and peripheral nervous systems. Their actions, however, do not appear to be limited to the nervous system, as both NTs and their receptors have been found in non neuronal cells, including cells of the endocrine system. At least four of the five known neurotrophins, including nerve growth factor (NGF), brain-derived neurotrophic factor (BDNF), neurotrophin-3 (NT-3), and neurotrophin-4 (NT-4), and their receptors (p75 NTR, trkA, trkB and trkC) are present in the developing ovary. Using mice carrying null mutations of the genes encoding neurotrophins (NGF, NT-4, BDNF) or the receptor that mediates the actions of NT-4 and BDNF (trkB), we have obtained initial results consistent with the notion that neurotrophins are required for the growth of primordial follicles. NGF-deficient mice show a decreased formation of both primary and secondary preantral follicles. Null mutation of the NT-4 gene failed to affect either folliculogenesis or follicular development. However, formation of primary and secondary follicles was compromised in mice carrying a null mutation of both the NT-4 and BDNF genes, suggesting compensation of function by BDNF in NT-4 knockouts. Support for this concept is provided by the similar deficiency in follicular growth observed in animals carrying a null mutation of the gene encoding trkB, the receptors mediating NT-4 and BDNF actions. Initial experiments, using differential display, to isolate genes that may be involved in the process of folliculogenesis and/or early follicular development, resulted in the isolation of a recently identified cell adhesion molecule and a novel transcription factor originally shown to induce cell transformation. It thus appears that formation and development of mammalian follicles requires the concerted action of genes originally thought to be only involved in cell differentiation/survival of neuronal cells, and genes that may control the growth, differentiation, and cell-cell interactions of somatic and germ cells in the ovary.

Severe peripheral sensory neuron loss and modest motor neuron reduction in mice with combined deficiency of brain-derived neurotrophic factor, neurotrophin 3 and neurotrophin 4/5

Neurotrophins are a family of structurally and functionally related proteins that regulate neuronal survival during development. In the peripheral nervous system (PNS), both in vitro and in vivo studies have shown that neurotrophins are potent factors for the survival of various sensory neurons and sympathetic neurons. However, it is not clear whether all PNS neurons are neurotrophin-dependent. In the central nervous system (CNS), studies using injury models show that neurotrophins promote the survival of CNS neurons. But mice lacking individual neurotrophins or a combination of BDNF and NT4 did not show significant CNS neuronal loss. Here we derived mice lacking three neurotrophins, brain-derived growth factor (BDNF), neurotrophin-3 (NT3), and neurotrophin-4 (NT4) to study the effect of triple neurotrophin deficiency on peripheral and central neurons. These triple-deficient mice did not nurse and died within 12 hours after birth. Neuronal cell counts showed that triple mutant pups lacked most of their peripheral sensory neurons and had a statistically significant reduction of motor neurons in several motor nuclei. Our results suggest that neurotrophins are essential for the survival of most peripheral sensory neurons and affect the survival of a small portion of motor neurons during embryogenesis.

Knocking the NT4 gene into the BDNF locus rescues BDNF deficient mice and reveals distinct NT4 and BDNF activities

To directly compare biological activities of the neurotrophins NT4 and BDNF in vivo, we replaced the BDNF coding sequence with the NT4 sequence in mice (Bdnfnt4-ki). Mice expressing NT4 in place of BDNF were viable, in contrast with BDNF null mutants, which die shortly after birth. Although the Bdnfnt4-ki/nt4-ki and wild-type Bdnf+/+ alleles yielded similar levels of NT4 and BDNF proteins, NT4 supported more sensory neurons than BDNF and promoted functional synapse formation in cultured hippocampal neurons. Homozygous Bdnfnt4-ki/nt4-ki mice showed reduced body weight, infertility and skin lesions, suggesting unique biological activities of NT4 in vivo. The distinct activities of NT4 and BDNF may result partly from differential activation of the TrkB receptor and its down-stream signals.

TrkA amino acids controlling specificity for nerve growth factor

Neurotrophins are important for the development and maintenance of the vertebrate nervous system, mediating their signal into the cell by specific interaction with tyrosine kinase receptors of the Trk family. The extracellular portion of the Trk receptors has been previously proposed to consist of a cysteine-rich motif, a leucine-rich motif, a second cysteine-rich motif followed by two immunoglobulin-like domains. Earlier studies have shown that a major neurotrophin-binding site in the Trk receptors resides in the second immunoglobulin-like domain. Although the individual amino acids in TrkA involved in binding to nerve growth factor (NGF) and those in TrkC involved in binding to neurotrophin-3 have been mapped in this domain, the Trk amino acids that provide specificity remained unclear. In this study, a minimum set of residues in the human TrkC second immunoglobulin-like domain, which does not bind nerve growth factor (NGF), were substituted with those from human TrkA. The resulting Trk variant recruited binding of NGF equivalent to TrkA, maintained neurotrophin-3 binding equivalent to TrkC, and also bound brain-derived neurotrophin, although with lower affinity compared with TrkB. This implies that the amino acids in the second immunoglobulin-like domain that determine Trk specificity are distinct for each Trk.

Expression of nerve growth factor, brain-derived neurotrophic factor, and neurotrophin-3 in the somatosensory cortex of the mature rat: coexpression with high-affinity neurotrophin receptors

Neurotrophins, including nerve growth factor (NGF), brain-derived neurotrophic factor (BDNF), and neurotrophin-3 (NT-3), are critical for the maintenance and plasticity of central nervous system (CNS) neurons. We tested the hypothesis that cortical neurons participate in redundant autocrine/paracrine systems. Three sets of studies determined the distribution of NGF-, BDNF-, and NT-3-expressing neurons, the frequency of neurons coexpressing NGF and BDNF, and the frequency of neurons expressing a neurotrophin and its associated high-affinity receptor. The distribution of NGF-, BDNF, and NT-3-immunoreactive neurons was identical. Neurotrophin-positive cells were parceled throughout the cortex, although the labeling frequency was not the same in all layers. More than 30% of the neurons in layers II/III, V, and VI were labeled, whereas only 5-10% of the neurons in layer IV was immunopositive for a neurotrophin. Some glia were also neurotrophin positive, particularly BDNF-positive glia. About 70% of the neurons in layers II/III and V coexpressed NGF and BDNF or coexpressed NGF and NT-3. Ligand-receptor colabeling was also common among cortical neurons. For example, nearly 70% of the NGF-, BDNF-, and NT-3-positive neurons in layer V colabeled with their respective high-affinity receptors, i.e., trkA, trkB, and trkC, respectively. Thus, (a) neurons express multiple neurotrophins and (b) cortical neurons (e.g., layer V neurons) contain the components required for autocrine/paracrine and/or anterograde communication (e.g., neurons in layer II/III support layer V neurons). These systems mean that the cortex is capable of regulating itself autonomously.

Brain-derived neurotrophic factor acutely inhibits AMPA-mediated currents in developing sensory relay neurons

Brain-derived neurotrophic factor (BDNF) is expressed by many primary sensory neurons that no longer require neurotrophins for survival, indicating that BDNF may be used as a signaling molecule by the afferents themselves. Because many primary afferents also express glutamate, we investigated the possibility that BDNF modulates glutamatergic AMPA responses of newborn second-order sensory relay neurons. Perforated-patch, voltage-clamp recordings were made from dissociated neurons of the brainstem nucleus tractus solitarius (nTS), a region that receives massive primary afferent input from BDNF-containing neurons in the nodose and petrosal cranial sensory ganglia. Electrophysiological analysis was combined in some experiments with anterograde labeling of primary afferent terminals to specifically analyze responses of identified second-order neurons. Our data demonstrate that BDNF strongly inhibits AMPA-mediated currents in a large subset of nTS cells. Specifically, AMPA responses were either completely abolished or markedly inhibited by BDNF in 73% of postnatal day (P0) cells and in 82% of identified P5 second-order sensory relay neurons. This effect of BDNF is mimicked by NT-4, but not NGF, and blocked by the Trk tyrosine kinase inhibitor K252a, consistent with a requirement for TrkB receptor activation. Moreover, analysis of TrkB expression in culture revealed a close correlation between the percentage of nTS neurons in which BDNF inhibits AMPA currents and the percentage of neurons that exhibit TrkB immunoreactivity. These data document a previously undefined mechanism of acute modulation of AMPA responses by BDNF and indicate that BDNF may regulate glutamatergic transmission at primary afferent synapses.

Brain-derived neurotrophic factor acutely inhibits AMPA-mediated currents in developing sensory relay neurons

Brain-derived neurotrophic factor (BDNF) is expressed by many primary sensory neurons that no longer require neurotrophins for survival, indicating that BDNF may be used as a signaling molecule by the afferents themselves. Because many primary afferents also express glutamate, we investigated the possibility that BDNF modulates glutamatergic AMPA responses of newborn second-order sensory relay neurons. Perforated-patch, voltage-clamp recordings were made from dissociated neurons of the brainstem nucleus tractus solitarius (nTS), a region that receives massive primary afferent input from BDNF-containing neurons in the nodose and petrosal cranial sensory ganglia. Electrophysiological analysis was combined in some experiments with anterograde labeling of primary afferent terminals to specifically analyze responses of identified second-order neurons. Our data demonstrate that BDNF strongly inhibits AMPA-mediated currents in a large subset of nTS cells. Specifically, AMPA responses were either completely abolished or markedly inhibited by BDNF in 73% of postnatal day (P0) cells and in 82% of identified P5 second-order sensory relay neurons. This effect of BDNF is mimicked by NT-4, but not NGF, and blocked by the Trk tyrosine kinase inhibitor K252a, consistent with a requirement for TrkB receptor activation. Moreover, analysis of TrkB expression in culture revealed a close correlation between the percentage of nTS neurons in which BDNF inhibits AMPA currents and the percentage of neurons that exhibit TrkB immunoreactivity. These data document a previously undefined mechanism of acute modulation of AMPA responses by BDNF and indicate that BDNF may regulate glutamatergic transmission at primary afferent synapses.

Differential regulation of neurotrophin expression by mitogens and neurotransmitters in mouse lymphocytes

In this study, we examined the expression of neurotrophins in mouse lymphocytes and the regulation of their expression by mitogens and neurotransmitters. We found that mixed splenocytes as well as T and B lymphocytes expressed mRNA for all the neurotrophins examined. Differential regulation of the neurotrophins was obtained upon stimulation of the cells. Thus, LPS increased the expression of NGF, BDNF and NT-3 in splenocytes and B cells, whereas Con-A increased the mRNA of NT-3 and NT-4 in T cells and NGF expression in splenocytes. The neurotransmitter substance P and the beta-adrenergic agonist, isoproterenol induced an increase in the expression of NGF. Our results suggest an important role for the different neurotrophins in the function of the immune system and point to a bi-directional interaction between neurotrophins and neurotransmitters in this system.

Induction of TrkA expression by differentiation inducers in human myeloid leukemia KG-1 cells

We have recently reported that retinoic acid (RA) induced the expression of trkA, the high affinity receptor for nerve growth factor (NGF), in human myeloid leukemia KG-1 cells. In the present study, we report that the expression of trkA was also induced by several other differentiation inducers, including 1alpha, 25-dihydroxyvitamin D3 (Vit D3), 1-beta-D-arabinofuranosyl cytosine (Ara-C), sodium butyrate (NaBut), and phorbol 12-myristate 13-acetate (PMA). Interestingly, RA in combination with NaBut or PMA synergistically induced cellular differentiation as well as the expression of trkA in KG-1 cells. Furthermore, activation of the induced trkA receptor by exogenous NGF potentiated the differentiating effects of RA and NaBut. Our results demonstrated that the induction of trkA is an event associated with the differentiation of KG-1 cells. Our findings suggest that NGF, in addition to its pivotal roles in the nervous system, may also play important roles in hematopoietic differentiation.

Neurotrophin expression by spinal motoneurons in adult and developing rats

Expression of the neurotrophins NT-4, brain-derived neurotrophic factor (BDNF), and NT-3 in adult rat lumbosacral spinal cord motoneurons is reported. A sensitive in situ hybridization procedure demonstrates localization of the mRNA for each of these neurotrophins within spinal motoneurons of the adult and in early postnatal development. A majority of adult rat spinal cord lumbar motoneurons (approximately 63%) express NT-4 mRNA as assessed by counting motoneurons in the L4 and L5 segments of two adult rat spinal cords on adjacent cresyl violet-stained and in situ hybridization sections. Similarly, a majority of lumbar motoneurons (approximately 73%) express BDNF mRNA. Further analyses of adjacent lumbar spinal cord sections revealed that many, although not all motoneurons coexpress both NT-4 and BDNF mRNAs. At birth, the mRNA encoding NT-3 is expressed in motoneurons, but BDNF mRNA is not apparent until postnatal day 5 (P5) and NT-4 mRNA first appears at P9. The potential biological significance of neurotrophin mRNA expression in spinal motoneurons is supported by immunohistochemical localization of each neurotrophin protein in adult motoneurons. We discuss the potential role of spinal cord neurotrophins as autocrine or paracrine factors involved in modulating motoneuron synaptic function.

Ischemic brain damage in mice after selectively modifying BDNF or NT4 gene expression

The neurotrophins and the tyrosine kinase (Trk) B receptor may play a protective role in the pathophysiology of cerebral ischemia. In this study, the authors investigated whether reducing endogenous expression of TrkB-binding neurotrophins modifies the susceptibility to ischemic injury after 1-hour middle cerebral artery occlusion followed by 23 hours of reperfusion in a filament middle cerebral artery occlusion model. Mice lacking both alleles for neurotrophin-4 (nt4-/-) or deficient in a single allele for brain-derived neurotrophic factor (bdnf+/-) exhibited larger cerebral infarcts compared to wild-type inbred 129/SVjae mice (68% and 91%, respectively, compared to controls). Moreover, lesions were larger (21%) in nt4-/- mice after permanent middle cerebral artery occlusion. Hence, expression of both NT4 and BDNF, and by inference the TrkB receptor, confers resistance to ischemic injury.

The structures of the neurotrophin 4 homodimer and the brain-derived neurotrophic factor/neurotrophin 4 heterodimer reveal a common Trk-binding site

The neurotrophins are growth factors that are involved in the development and survival of neurons. Neurotrophin release by a target tissue results in neuron growth along the neurotrophin concentration gradient, culminating in the eventual innervation of the target tissue. These activities are mediated through trk cell surface receptors. We have determined the structures of the heterodimer formed between brain-derived neurotrophic factor (BDNF) and neurotrophin 4 (NT4), as well as the structure of homodimer of NT4. We also present the structure of the Neurotrophin 3 homodimer, which is refined to higher resolution than previously published. These structures provide the first views of the architecture of the NT4 protomer. Comparison of the surface of a model of the BDNF homodimer with the structures of the neurotrophin homodimers reveals common features that may be important in the binding between the neurotrophins and their receptors. In particular, there exists an analogous region on the surface of each neurotrophin that is likely to be involved in trk receptor binding. Variations in sequence on the periphery of this common region serve to confer trk receptor specificity.

Induction of gp130 and LIF by differentiation inducers in human myeloid leukemia K562 cells

It has been previously shown that phorbol 12-myristate 13-acetate (PMA), a potent differentiation inducer, induced the expression of both interleukin-6 (IL-6) and IL-6 receptor alpha component (IL-6Ralpha) in K562 leukemia cells. In the present study, we examined the ability of several differentiation inducers to regulate the expression of the signal-transducing receptor component for IL-6, gp130, and cytokine leukemia inhibitory factor (LIF) in K562 cells. We found that the expression of gp130 was dramatically induced at both the mRNA and protein levels by the two megakaryocytic inducers sodium butyrate (NaBut) and PMA. In contrast, the mRNA expression of LIF was induced by the two erythroid inducers 1-beta-D-arabinofuranosyl cytosine (Ara-C) and hemin. Furthermore, activation of the PMA-induced gp130 receptor by exogenous IL-6 potentiated the differentiating effects of PMA. Our findings suggest that IL-6/gp130 signaling may be involved in the regulation of the megakaryocytic differentiation of K562 cells.

Neurotrophins and Trk receptors in primitive neuroectodermal tumor cell lines

OBJECTIVE

Primitive neuroectodermal tumors (PNETs) are thought to be derived from early central nervous system precursors. Therefore, we hypothesized that the neurotrophins (nerve growth factor, brain-derived neurotrophic factor, and neurotrophin-3) and their receptors (TrkA, TrkB, and TrkC), which are involved in the proliferation, differentiation, and survival of neuronal cells, might be important in regulating tumor growth.

METHODS

Using ribonucleic acid (RNA) blotting and reverse transcription-polymerase chain reaction analysis, we investigated the expression of these ligands and their receptors in six PNET cell lines (Daoy, PFSK, D283 Med, UW288-1, CHP707m, and D341 Med). Neurotrophin protein levels were measured using enzyme-linked immunosorbent assay procedures. Receptor function was demonstrated by autophosphorylation. Induction of c-Fos expression and effects on cell proliferation were assessed after the addition of exogenous neurotrophin.

RESULTS

Three cell lines expressed messenger RNA for all neurotrophins, whereas the other three expressed two of the three neurotrophins. Neurotrophin protein levels were low. All cell lines expressed trkA messenger RNA. Five expressed the amino terminus of trkB, but three of these did not express the carboxyl terminus. All cell lines contained trkC messenger RNA, but the receptor was truncated in two cell lines. No cell line contained message for a receptor containing an insertion in the tyrosine kinase domain. The addition of neurotrophin to PNET cells resulted in phosphorylation of a protein that was immunoprecipitated with an anti-pan-Trk antibody. c-Fos expression and cell growth were increased by preincubation with neurotrophins, but only in the cell lines expressing the relevant full-length receptors.

CONCLUSION

The expression of neurotrophins and neurotrophin receptors by PNET cell lines is variable. The presence of activated Trk receptors in these cell lines may be required for rapid growth, via an autocrine loop mechanism. This will require further investigation.

Nerve growth factor (NGF) induces motoneuron apoptosis in rat embryonic spinal cord in vitro

Recent studies have demonstrated that nerve growth factor (NGF) induces apoptosis of several cell types in the central nervous system through its low-affinity p75 neurotrophin receptor (p75NTR). To test the effect of NGF on embryonic motoneuron survival, we developed an organotypic culture system which allowed the in vitro development of intact embryonic rat spinal cords. In our system, neural tubes were taken and cultured at E13, just before the onset of physiological motoneuron death. After 2 days in vitro (DIV), motoneurons underwent apoptosis over a time-course similar to that in vivo. In this system, the addition of NGF (200 ng/mL) for 2 days enhanced the number of apoptotic motoneurons by 37%. This pro-apoptotic effect was completely reversed by the blocking anti-p75NTR (REX) antibody which inhibits NGF binding to p75NTR. Other neurotrophins, e.g. brain-derived neurotrophic factor (BDNF), neurotrophin 3 (NT3) and neurotrophin 4/5 (NT4/5) did not have any effect, while glial cell-derived neurotrophic factor (GDNF) promoted motoneuron survival. Anti-BDNF blocking antibodies enhanced motoneuron death indicating that endogenous BDNF promotes motoneuron survival in explants. Our results demonstrate, for the first time, that NGF can induce embryonic motoneuron apoptosis through its receptor p75NTR.

Differential expression of brain-derived neurotrophic factor, neurotrophin-3, and neurotrophin-4/5 in the adult rat spinal cord: regulation by the glutamate receptor agonist kainic acid

Previous in vitro studies indicate that select members of the neurotrophin gene family, namely brain-derived neurotrophic factor (BDNF), neurotrophin-3 (NT-3), and neurotrophin-4/5 (NT-4/5), contribute to survival and differentiation of spinal cord motoneurons. To investigate the potential roles of these factors in the adult spinal cord, we examined their cellular localization and regulation after systemic exposure to an excitotoxic stimulus, kainic acid (KA). Of the neurotrophins examined, NT-4/5 mRNA was most robustly expressed in the lumbosacral spinal cord of the normal adult rat, including expression by neurons throughout the gray matter, and in a subpopulation of white and gray matter glia. Both BDNF and NT-3 mRNAs were also densely expressed by alpha motoneurons of lamina IX, but were detected at lower levels elsewhere in the gray matter. NT-3 mRNA was additionally expressed by spinal cord glia, but was less widespread compared to NT-4/5. In response to systemic administration of KA, NT-4/5 and BDNF mRNAs were dramatically upregulated in a spatially and temporally restricted fashion, whereas levels of NT-3 mRNA were unchanged. These results provide strong in vivo evidence to support the idea that BDNF, NT-3, and in particular, NT-4/5, play a role in the normal function of the adult spinal cord. Furthermore, our results indicate that the actions of BDNF and NT-4/5 participate in the response of the cord to excitotoxic stimuli, and that those of NT-4/5 and NT-3 include both neurons and glia.

Differential expression of brain-derived neurotrophic factor, neurotrophin-3, and neurotrophin-4/5 in the adult rat spinal cord: regulation by the glutamate receptor agonist kainic acid

Previous in vitro studies indicate that select members of the neurotrophin gene family, namely brain-derived neurotrophic factor (BDNF), neurotrophin-3 (NT-3), and neurotrophin-4/5 (NT-4/5), contribute to survival and differentiation of spinal cord motoneurons. To investigate the potential roles of these factors in the adult spinal cord, we examined their cellular localization and regulation after systemic exposure to an excitotoxic stimulus, kainic acid (KA). Of the neurotrophins examined, NT-4/5 mRNA was most robustly expressed in the lumbosacral spinal cord of the normal adult rat, including expression by neurons throughout the gray matter, and in a subpopulation of white and gray matter glia. Both BDNF and NT-3 mRNAs were also densely expressed by alpha motoneurons of lamina IX, but were detected at lower levels elsewhere in the gray matter. NT-3 mRNA was additionally expressed by spinal cord glia, but was less widespread compared to NT-4/5. In response to systemic administration of KA, NT-4/5 and BDNF mRNAs were dramatically upregulated in a spatially and temporally restricted fashion, whereas levels of NT-3 mRNA were unchanged. These results provide strong in vivo evidence to support the idea that BDNF, NT-3, and in particular, NT-4/5, play a role in the normal function of the adult spinal cord. Furthermore, our results indicate that the actions of BDNF and NT-4/5 participate in the response of the cord to excitotoxic stimuli, and that those of NT-4/5 and NT-3 include both neurons and glia.

TrkB mutant lacking the amino-terminal half of the extracellular portion acts as a functional brain-derived neurotrophic factor receptor

A series of mutants with deletion in the extracellular portion of TrkB were expressed transiently and stably in mammalian cells to examine the brain-derived neurotrophic factor (BDNF)-binding properties of TrkB. We found that these binding activities were retained by the TrkB deletion mutant (TrkBDelta4) lacking most of the extracellular portion, cysteine-rich cluster 1 and 2, leucine-rich motif and most of the first immunoglobulin-like domain (Ig1). Furthermore, the results of the neurotrophin selectivity, the equilibrium binding constant, auto-phosphorylation and BDNF dependent cell survival indicate that TrkBDelta4 acts as a functional BDNF receptor comparable to wild-type TrkB. Thus, our findings showed that only the carboxyl-terminal half of the extracellular portion of TrkB, which includes the Ig2 domain, is essential for the functional BDNF receptor.

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

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

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

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

The changing sensitivity in the life of the nociceptor

Plasticity of the central nervous system has been shown to be an important correlate in the generation of chronic pain. However, there is now also increasing evidence for profound changes of the primary sensory neurons including nociceptors throughout the life of an organism and these changes account for clinically relevant alterations of pain perception. During development sensory neurons require one or more growth factors that rescue neurons during critical periods of programmed cell death and growth factors also play an important role for the development of the appropriate phenotype. Neurotrophin-3 may initially have an effect on proliferation of many subtypes of sensory neurons including cells destined to become nociceptors during early development. During a critical period of late prenatal development nerve growth factor (NGF) signalling through its cognate high affinity receptor trkA has been shown to be the main survival factor during a critical period of prenatal development. Humans deficient of trkA suffer from the rare disorder of congenital analgesia. Postnatally, the subpopulation of non-peptidergic nociceptors lose their ability to respond to NGF, start to express receptor element for and begin to respond to glial cell line-derived neurotrophic factor (GDNF). Both NGF and GDNF have also been shown to regulate the sensitivity of nociceptors to heat and capsaicin in the adult. Changes in the levels of endogenous trophic factors have also been implicated for the generation of ongoing activity and sensitisation to heat that are the hallmark of nociceptors innervating inflamed tissue. Whereas the development of ongoing activity correlates with the intensity of ongoing pain, sensitisation of nociceptors to heat can explain the hyperalgesia to heat that typically accompanies inflammatory lesions in the skin. Dramatic changes of nociceptor phenotype occur following nerve injury. Sensory neurons, including nociceptors, start to express adrenoceptors and become responsive for catecholamines and these changes appear to be responsible for the development of sympathetically maintained pain in some patients.

Neurotrophic factors and neuro-muscular disease: II. GDNF, other neurotrophic factors, and future directions

This is the second of two reviews in which we discuss the essential aspects of neurotrophic factor neurobiology, the characteristics of each neurotrophic factor, and their clinical relevance to neuromuscular diseases. The previous paper reviewed the neurotrophin family and neuropoietic cytokines. In the present article, we focus on the GDNF family and other neurotrophic factors and then consider future approaches that may be utilized in neurotrophic factor treatment.

Crystal structures of the neurotrophin-binding domain of TrkA, TrkB and TrkC

The Trk receptors and their neurotrophin ligands control development and maintenance of the nervous system. The crystal structures of the ligand binding domain of TrkA, TrkB, and TrkC were solved and refined to high resolution. The domains adopt an immunoglobulin-like fold, but crystallized in all three instances as dimers with the N-terminal strand of each molecule replaced by the same strand of a symmetry-related mate. Models of the correctly folded domains could be constructed by changing the position of a single residue, and the resulting model of the binding domain of TrkA is essentially identical with the bound structure as observed in a complex with nerve growth factor. An analysis of the existing mutagenesis data for TrkA and TrkC in light of these structures reveals the structural reasons for the specificity among the Trk receptors, and explains the underpinnings of the multi-functional ligands that have been reported. The overall structure of all three domains belongs to the I-set of immunoglobulin-like domains, but shows several unusual features, such as an exposed disulfide bridge linking two neighboring strands in the same beta-sheet. For all three domains, the residues that deviate from the standard fingerprint pattern common to the I-set family fall in the region of the ligand binding site observed in the complex. Therefore, identification of these deviations in the sequences of other immunoglobulin-like domain-containing receptors may help to identify their ligand binding site even in the absence of structural or mutagenesis data.

Measurement of neurotrophin 4/5 in rat tissues by a sensitive immunoassay

Neurotrophin 4/5 (NT4/5) is a member of the neurotrophin family known to exert survival and other effects on a variety of neurons including those within the motor, sensory and central populations. Although mRNA(NT4/5) has been found in various effector tissues of the rat and human, the concentration of NT4/5 protein in tissues has not been reported previously due to lack of suitable methodology. We present here a quantitative two-site enzyme-linked immunosorbent assay for the estimation of NT4/5 in pre- and postnatal rat tissues. The assay was performed using a combination of polyclonal and monoclonal antibodies to recombinant human NT4/5. Tissue samples were extracted at neutral pH. Results show that the assay is highly specific for NT4/5 with a sensitivity of 1 pg/ml, and reproducible with intra- and inter-assay variation coefficients of 3.0 and 6.3%, respectively. NT4/5 was found in most embryonic tissues examined at gestation day 17 and 21, but was rarely detectable in postnatal tissues, with the notable exception of the testis. The availability of an immunoassay for the estimation of NT4/5 protein in rat tissues should contribute to the understanding of the physiology of this little understood neurotrophic factor.

Distribution of BDNF, NT-3 and NT-4 in the developing auditory brainstem

This study describes the developmental expression of three neurotrophins, brain derived neurotrophic factor (BDNF), neurotrophin 3 (NT-3) and neurotrophin (NT-4) in the rat auditory brain-stem using immunohistochemistry. At postnatal day 0 (PND 0), neurotrophins expression was virtually absent from all auditory nuclei in the brainstem, even though some positive neurons were observed in the mesencephalic trigeminal nucleus at this age. However, BDNF, NT-3 and NT-4 positive neurons were observed in most brainstem auditory nuclei by PND 6. At the following stages, there was a general increase in the intensity of the neurotrophins immunoreactivity and BDNF labeling was particularly prominent in most cochlear nucleus neurons. A differential pattern of staining emerged in cochlear nucleus subdivisions, with more intense staining present in the ventral part. The superior olivary complex nuclei followed a similar pattern of BDNF staining compared to the cochlear nucleus. In the adult, BDNF heavily labeled most neurons of the superior olivary nuclei and moderately labeled neurons of the inferior colliculus (IC). NT-3 and NT-4 showed a similar pattern of staining in most auditory brainstem nuclei. The first staining was observed by PND 6 in some neuronal cell bodies. NT-3 and NT-4 immunoreactivity increased in the following stages and in the adult moderate labelings were observed in most neurons of the cochlear nucleus, the superior olivary nuclei and the IC. These results show that neurotrophins are expressed 1 week before the onset of hearing and the increase of their expressions correlate with the appearance of sound-evoked activity in the system. The temporal distribution of neurotrophins does not correlate with neuronal birth, axonal outgrowth or the formation of connection in the auditory structures, suggesting a role primarily in the maintenance and/ or modulation of postnatal and adult functions.

Cutting Edge: Clonally Restricted Production of the Neurotrophins Brain-Derived Neurotrophic Factor and Neurotrophin-3 mRNA by Human Immune Cells and Th1/Th2-Polarized Expression of Their Receptors

Neurotrophins, such as neurotrophin-3 (NT-3) and brain-derived neurotrophic factor (BDNF), are potent regulators of neuronal functions. Here we show that human immune cells also produce NT-3 mRNA, secrete BDNF, and express their specific receptors trkB and trkC. The truncated trkB receptor, usually expressed in sensory neurons of the central nervous system, was also constitutively expressed in unstimulated Th cells. Full-length trkB was detectable in stimulated PBMC, B cell lines, and Th1, but not in Th2 and Th0 cell clones. Clonally restricted expression was also observed for trkC, until now not detected on blood cells. The Th1 cytokine IL-2 stimulated production of trkB mRNA but not of trkC, whereas the Th2 cytokine IL-4 enhanced NT-3 but not BDNF mRNA expression. Microbial Ags, which influence the Th1/Th2 balance, could therefore modulate the neurotrophic system and thereby affect neuronal synaptic activity of the central nervous system.

Role of neurotrophins and neurotrophins receptors in the in vitro invasion and heparanase production of human prostate cancer cells

The role of the neurotrophins (NTs) and their corresponding receptors (NTRs) TrkA, TrkB, TrkC, and p75NTR in neoplasia has received relatively little attention. However, because malignant cell migration within the prostate occurs predominantly by direct extension around prostatic nerves, the presence and possible upregulation of NTs from autocrine/paracrine sources and NTR expression within prostate epithelial tumor cells may be important in metastasis. We have been addressing their expression and interactions in human prostate cancer cell lines (LNCaP, PC-3, and DU145) and their role in prostate cancer invasion. In this study, we demonstrated that nerve growth factor (NGF), the prototypic NT, and NT-4/5 increased in vitro invasion through a reconstituted basement membrane and induced time- and dose-dependent expression of heparanase, a heparan sulfate-specific endo-beta-D-glucuronidase, an important molecular determinant of tumor metastasis. The NT effects were most marked in the DU 145 brain-metastatic cells and were detected at NT concentrations sufficient to fully saturate both low- and high-affinity NTRs. Additionally, we characterized the molecular expression of NT high-affinity (Trk) and low-affinity (p75NTR) receptors in these cell lines by reverse transcription-polymerase chain reaction. These lines had negligible trkA and trkC expression, although trkB was expressed in the three prostatic tumor cell lines examined. The brain-metastatic DU 145 cells were also positive for p75NTR. Our data showed that the NTs and NTRs are important in metastasis and that their expression coincides with transformation to a malignant phenotype capable of invasion along the perineural space and extracapsular metastasis to distant sites. These findings set the stage for more research into this area as related to prostate cancer evolution and may improve therapy for prostate cancer metastasis.

Enhancement of locomotor activity and conditioned reward to cocaine by brain-derived neurotrophic factor

The mesolimbic dopamine (DA) system has been implicated in drug reward, locomotor sensitization, and responding for reward-related stimuli [termed conditioned reinforcers (CR)]. Here, we investigated the effect of brain-derived neurotrophic factor (BDNF), which enhances the survival and function of dopaminergic neurons, on stimulant-induced locomotor sensitization and responding for CR. In experiment 1, BDNF was infused into the nucleus accumbens (NAc) or ventral tegmental area over 2 weeks via chronically implanted minipumps (1-2.5 microgram/d), and the psychomotor stimulant effects of cocaine (5-15 mg/kg, i.p.) were studied. We found that BDNF enhanced the initial stimulant effects of cocaine and seemed to facilitate the development of sensitization to repeated cocaine doses. In experiment 2, we studied the effects of intra-NAc BDNF infusions on responding for CR. BDNF-treated rats showed twice as many CR responses compared with controls when saline was first administered. BDNF enhanced responding on the CR lever more than four times that seen in control animals after a cocaine injection (10 mg/kg, i.p.). The enhanced response to cocaine in BDNF-treated animals persisted for more than a month after the BDNF infusions had stopped, indicating long-lasting changes in the mesolimbic DA system caused by BDNF administration. In experiment 3, we examined locomotor sensitization to cocaine in heterozygous BDNF knock-out mice and found that the development of sensitization was delayed compared with wild-type littermates. These results demonstrate the profound effects of BDNF on the enhancement of both cocaine-induced locomotion and facilitation of CR and suggest a possible role for BDNF in long-term adaptations of the brain to cocaine.

Expression of p75(NTR), trkB and trkC in nonmanipulated and axotomized motoneurons of aged rats

Several lines of evidence indicate that adult neurons remain dependent on neurotrophins and that changes in tissue expression of neurotrophins and/or their receptors may play a role in senile neurodegeneration. We have studied the expression of p75NTR, trkB and trkC, respectively, in lumbar motoneurons of young adult (2-3 months) and aged (30 months) rats subjected to sciatic transection using in situ hybridization and immunohistochemistry. Nonmanipulated age-matched animals were processed in parallel. In nonmanipulated aged rats, high levels of p75NTR could be seen in a number of motoneurons (10-15%), while in young adult animals no p75NTR could be detected. Seven days following sciatic axotomy, a conspicuous ipsilateral upregulation p75NTR was observed in young adult rats. Also in aged rats there was a marked ipsilateral increase in number of p75NTR expressing neurons ( approximately 100%). In comparison to young adult rats, aged rats showed a decreased expression of both trkB (5/6 animals) and trkC (6/6 animals). Furthermore, in response to sciatic transection, 3 out of 5 aged rats did not show an increased expression of trkB. In aged rats, axotomy did not induce any significant change in trkC expression. In the young adult rats, we recorded a side-to-side effect with lower values ipsilaterally, however, it cannot be excluded that this difference was caused by an upregulation in the contralateral motoneurons. Oligonucleotide probes against BDNF and NT3 mRNA showed only very few faintly positive neurons in both age groups. Our results indicate that the pattern of regulatory changes of NT receptors in response to axotomy is different in aged and young adult rats. The lack of covariation between p75NTR and trkB and trkC regulation in aged rats indicates a changed role for p75NTR in senescent motoneurons.