Analytical purification of the slow, high affinity NGF receptor: identification of a novel 135 kd polypeptide

Development of androgen receptor and p75(NTR) mRNAs and peptides in the lumbar spinal cord of the gerbil

Development of sex differences in the spinal cord appears to be largely under the control of androgen and although neurotrophins may also have a role. Spinal cords of male and female neonatal gerbils (postnatal days 1, 5, 7, 10, 23) and adult gerbils (postnatal day 150) were examined to determine the relative temporal expression of androgen receptor (AR) and the low-affinity neurotrophin receptor (p75) mRNAs within the spinal nucleus of the bulbocavernosus (SNB) and dorsolateral nucleus (DLN). Furthermore, prepubertal male gerbils were placed into one of six gonadal hormone treatment groups at weaning: Either sham castrate, castrated with gonadal hormone replacement, or castrated without gonadal hormone replacement. Ten weeks later gerbils were aldehyde-perfused, spinal cords removed and processed for presence of AR and p75 immunoreactivity (ir) in motoneurons of the SNB and DLN. During neonatal development, there were significant increases in androgen receptor mRNA within the SNB and DLN. In the SNB, the increase in androgen receptor mRNA preceded the increase in p75 mRNA. Peripubertally, significantly more SNB than DLN motoneurons contained AR- and p75-ir. These data demonstrate that AR expression occurs along the same developmental time frame as the development of the SNB and DLN and the organizational effects of androgens on their development continues through puberty in the male gerbil.

Signal transduction mediated by the truncated trkB receptor isoforms, trkB.T1 and trkB.T2

The trkB family of transmembrane proteins serves as receptors for BDNF and NT-4/5. The family is composed of a tyrosine kinase-containing isoform as well as several alternatively spliced "truncated receptors" with identical extracellular ligand-binding domains but very small intracellular domains. The two best-characterized truncated trkB receptors, designated as trkB.T1 and trkB.T2, contain intracellular domains of only 23 and 21 amino acids, respectively. Although it is known that the tyrosine kinase isoform (trkB.FL) is capable of initiating BDNF and NT-4/5-induced signal transduction, the functional role or roles of the truncated receptors remain enigmatic. At the same time, the potential importance of the truncated receptors in the development, maintenance, and regeneration of the nervous system has been highlighted by recent developmental and injury paradigm investigations. Here we have used trkB cDNA transfected cell lines to demonstrate that both trkB.T1 and trkB.T2 are capable of mediating BDNF-induced signal transduction. More specifically, BDNF activation of either trkB.T1 or trkB.T2 increases the rate of acidic metabolite release from the cell, a common physiological consequence of many signaling pathways. Further, these trkB.T1- and trkB. T2-mediated changes occur with kinetics distinct from changes mediated by trkB.FL, suggesting the participation of at least some unique rate-limiting component or components. Mutational analysis demonstrates that the isoform-specific sequences within the intracellular domains of each receptor are essential for signaling capability. Finally, inhibitor studies suggest that kinases are likely to be involved in the trkB.T1 and trkB.T2 signaling pathways.

Endocytosis of activated TrkA: evidence that nerve growth factor induces formation of signaling endosomes

The survival, differentiation, and maintenance of responsive neurons are regulated by nerve growth factor (NGF), which is secreted by the target and interacts with receptors on the axon tip. It is uncertain how the NGF signal is communicated retrogradely from distal axons to neuron cell bodies. Retrograde transport of activated receptors in endocytic vesicles could convey the signal. However, little is known about endocytosis of NGF receptors, and there is no evidence that NGF receptors continue to signal after endocytosis. We have examined early events in the membrane traffic of NGF and its receptor, gp140(TrkA) (TrkA), in PC12 cells. NGF induced rapid and extensive endocytosis of TrkA in these cells, and the receptor subsequently moved into small organelles located near the plasma membrane. Some of these organelles contained clathrin and alpha-adaptin, which implies that TrkA is internalized by clathrin-mediated endocytosis. Using mechanical permeabilization and fractionation, intracellular organelles derived from endocytosis were separated from the plasma membrane. After NGF treatment, NGF was bound to TrkA in endocytic organelles, and TrkA was tyrosine-phosphorylated and bound to PLC-gamma1, suggesting that these receptors were competent to initiate signal transduction. These studies raise the possibility that NGF induces formation of signaling endosomes containing activated TrkA. They are an important first step in elucidating the molecular mechanism of NGF retrograde signaling.

A paracrine effect for neuron-derived BDNF in development of dorsal root ganglia: stimulation of Schwann cell myelin protein expression by glial cells

Addition of neurons to cultures of non-neuronal cells derived from quail embryonic dorsal root ganglia causes a 2.5-fold increase in the proportion of cells that express the glial marker Schwann cell myelin protein (SMP) when compared to cultures devoid of neurons. This effect is mediated by BDNF because incubation with a trkB immunoadhesin that sequesters BDNF, but not with trkA or trkC immunoadhesins, abolishes this stimulation. This neuronal activity can be mimicked by treatment with soluble BDNF that stimulates specifically the conversion of SMP-negative glial cells into cells that express this phenotype. That BDNF is the endogenous neuron-derived factor affecting glial development is further supported by the observation that BDNF is extensively expressed in developing sensory neurons of the avian ganglia both in vivo and in vitro, but not by the satellite cells. These results show for the first time a paracrine role for neuronal BDNF on differentiation of peripheral glial cells. This effect of BDNF is likely to be mediated by the p75 neurotrophin receptor because: (1) p75 immunoreactive protein is expressed by a subset of satellite cells; (2) neutralization of p75 abolishes the BDNF-induced stimulation; (3) a treatment of non-neuronal cell cultures with equimolar concentrations of either soluble NGF or NT-3 also affects the proportion of cells that become SMP-positive. Whereas NGF stimulates the acquisition of this glial antigen to a similar extent as BDNF, NT-3 inhibits its expression, suggesting that distinct neurotrophins signal differentially through p75. These findings also suggest that the definitive phenotype of peripheral glia is determined by a balance between positive and inhibitory signals arising in adjacent neurons.

TrkA activation is sufficient to rescue axotomized cholinergic neurons

To test the molecular nature of the NGF receptor responsible for the ability of NGF to rescue septal cholinergic neurons following axotomy, we infused polyclonal antibodies that act as specific agonists of trkA (RTA) into the lateral ventricle of fimbria-fornix lesioned animals. Rats receiving chronic intraventricular infusions of RTA showed significantly more low affinity NGF receptor immunoreactive (p75NGFR-IR) neurons on the lesioned side than did control animals 2 weeks following unilateral fimbria-fornix lesion. RTA also initiated cholinergic sprouting. Infusions of RTA in combination with an antibody that blocks p75NGFR (REX) did not reduce the cell savings effect observed with RTA alone. However, animals infused with RTA plus REX demonstrated significantly less sprouting. These findings suggest that antibody-induced trkA activation is sufficient to mediate NGF-promoted survival of axotomized cholinergic neurons in vivo.

The expression of trkB and p75 and the role of BDNF in the developing neuromuscular system of the chick embryo

The neurotrophin, brain-derived neurotrophic factor, prevents motoneuron cell death during the normal development of the chick embryo. Brain-derived neurotrophic factor is a ligand for the low-affinity NGF receptor, p75, and for the high-affinity neurotrophin receptor, trkB. If motoneurons respond directly to brain-derived neurotrophic factor then they must possess at least one, and possibly both, of these receptors during the period of naturally occurring cell death. Histological sections from the lumbar region of chick embryos were probed for the presence of trkB and p75 mRNA using digoxigenin-labeled anti-sense RNA probes. p75 mRNA was present in spinal cord motoneurons at stages of development that correlate with motoneuron cell death. Immunohistochemical localization also revealed that p75 protein was present in motoneurons, primarily along the ventral roots and developing intramuscular nerves. In contrast trkB mRNA was not present in chick motoneurons until after the process of cell death was underway. The timing of trkB expression suggested that some motoneurons, i.e., those that die prior to the onset of trkB expression, may be insensitive to brain-derived neurotrophic factor. This was confirmed by comparing the number of surviving motoneurons following different in vivo treatment paradigms. The evidence indicates that motoneurons undergo a temporal shift in sensitivity to brain-derived neurotrophic factor.

Specificity of nerve growth factor signaling: differential patterns of early tyrosine phosphorylation events induced by NGF, EGF, and bFGF

The specificity of nerve growth factor (NGF) action was examined by comparing early tyrosine phosphorylation events induced by NGF, epidermal growth factor (EGF), and basic fibroblast growth factor (bFGF). In PC12 cells, administration of either the differentiation factor NGF or the mitogenic factor EGF led to tyrosine phosphorylation of multiple polypeptides in the 100-110 kDa size range associated with PI-3 kinase. However, NGF induced a more prolonged phosphorylation, relative to a transient EGF effect. In contrast, the differentiation factor bFGF failed to induce measurable tyrosine phosphorylation of PI-3 kinase-associated proteins. Similarly, NGF but not bFGF induced marked tyrosine phosphorylation of PLC gamma, another early signaling molecule, suggesting that multiple pathways exist for promoting differentiation, and/or that these signaling molecules are not essential for differentiation. TrkA signaling was also compared between PC12 cells and NIH-3T3 cells heterologously expressing trkA, where receptor activation promotes mitogenesis. In this comparison, significant differences were observed in the tyrosine phosphorylation pattern of PI-3 kinase-associated polypeptides, suggesting the existence of cell type-specific molecular interactions influencing trkA signaling. Mechanistically, NGF stimulation of PC12 cells resulted in a weak or possibly indirect association between trkA and PI-3 kinase. Furthermore, NGF did not appear to activate or substantially alter the overall level of PI-3 kinase activity, raising the possibility that ligand-induced phosphorylation may serve instead to relocalize constitutively active PI-3 kinase molecules within the cell. Taken together, data presented suggest that the temporal pattern of induced phosphorylation, the nature of induced associations with other phosphoproteins, and cell type-specific components may all contribute to the generation of NGF signaling specificity.

A potential interaction of p75 and trkA NGF receptors revealed by affinity crosslinking and immunoprecipitation

Nerve growth factor binds independently to two transmembrane receptors, the p75 neurotrophin receptor and the p140trk (trkA) tyrosine kinase receptor, which are both co-expressed in the majority of neuronal cells that respond to NGF. Previous findings have suggested that appropriate co-expression of the two receptors gives rise to high affinity NGF binding sites and increased neurotrophin responsiveness; however, evidence demonstrating a direct interaction between the two receptors in cell lines has been lacking. Here we have utilized affinity crosslinking agents with 125I-NGF to detect an association of trkA and p75 receptors in embryonic spinal cord and brain tissues enriched in the two receptors. Although multimeric complexes of trkA and p75 were not detected by affinity crosslinking, immunoprecipitation of cross-linked NGF-receptor complexes with trk-specific antibodies resulted in selective immunoprecipitation of crosslinked p75. Our results indicate that the trkA and p75 receptors can potentially interact, and that such an association may be responsible for the generation of high affinity NGF binding sites.

TrkA cross-linking mimics neuronal responses to nerve growth factor

TrkA, a tyrosine kinase receptor, is an essential component of the nerve growth factor (NGF) response pathway. The binding of NGF to the receptor induces receptor autophosphorylation and activation of intracellular signaling pathways, resulting in diverse biological effects. We prepared polyclonal antibodies against the entire extracellular domain of rat trkA produced using a baculovirus expression system. These antibodies specifically recognize rat trkA on antigen blots and in immunoprecipitations. Both IgG and Fab fragments block binding of NGF to trkA expressed by the PC12 cell line. In NGF binding studies using anti-trkA and anti-low-affinity NGF receptor (LNGFR) immunoglobulin (Ig) G, essentially all binding of NGF can be inhibited. The results imply that > or = 97% of the NGF binding sites on PC12 cells are accounted for by trkA and the LNGFR. The binding data also argue that all low-affinity NGF binding sites on PC12 cells reflect interactions with the LNGFR, while all high-affinity sites are trkA dependent. A fraction of the high-affinity (or slow) binding sites seem to require both trkA and the LNGFR. Although the monovalent anti-trkA Fab fragments inhibited the biological effects of NGF, such as induction of tyrosine phosphorylation, and survival and neurite outgrowth of sympathetic neurons, the IgG preparation was not effective as an inhibitor. Instead, the IgG fraction by itself was almost as effective as NGF at stimulating receptor activation, cell survival, and neurite outgrowth. Thus, it appears oligomerization of trkA by antibody-induced cross-linking is sufficient to produce the known cellular effects of NGF.

Neurotrophin receptors

The neurotrophins are members of a family of four related proteins that allow the survival and differentiation of specific sub-sets of embryonic vertebrate neurons. On neurons, two types of neurotrophin receptors can be distinguished on the basis of their dissociation constants: low affinity receptors (Kd 10(-9) M) and high affinity receptors (Kd 10(-11) M). Several genes coding for neurotrophin receptors have been cloned and the expression in fibroblasts of the recombinant membrane proteins allows comparisons to be made between the binding properties of the neurotrophins on such cell lines and neurons. As a result, it appears that much of the low affinity binding sites detected on neurons for all neurotrophins can be attributed to a single molecular entity, the low affinity neurotrophin (or NGF) receptor. This receptor binds all known neurotrophins with similar affinity but different binding kinetics. Its role in neurotrophic signal transduction remains to be established. In addition to this receptor, three members of the trk-subfamily of tyrosine kinase receptors have recently been identified as receptors for the neurotrophins. These receptors (whose intrinsic tyrosine kinase activity can be stimulated by the various neurotrophins) bind the neurotrophins with higher affinity and higher ligand specificity when compared with the low affinity receptor. However, the observation has been made that some of the recombinant trk-receptors on cell lines bind more than one neurotrophin (though typically with lower affinity than their own ligands).(ABSTRACT TRUNCATED AT 250 WORDS)

A bryostatin-sensitive protein kinase C required for nerve growth factor activity

Nerve growth factor (NGF) stimulates rat pheochromocytoma cells (PC12) to differentiate into a neuronal-like cell that exhibits neurite extensions. The role of protein kinase C in signal transduction has been examined in PC12 cells treated with phorbol 12-myristate 13-acetate (PMA) and bryostatin, a macrocyclic lactone that activates protein kinase C at both the nuclear and the plasma membranes [Hocevar, B. A., & Fields, A. P. (1991) J. Biol. Chem. 266, 28-33]. In contrast to PMA down-regulation [Reinhold, D. S., & Neet, K. E. (1989) J. Biol. Chem. 264, 3538-3544], chronic (24 h) treatment with bryostatin blocked the formation of neurites in response to NGF or basic fibroblast-derived growth factor stimulation, but, like PMA, bryostatin did not block the induction of c-fos or c-jun protooncogenes by NGF. Chronic bryostatin treatment down-regulated protein kinase C activity in the cytosolic, membrane, and nuclear fractions. Acute (60 min) bryostatin or NGF treatment activated cytosolic and nuclear protein kinase C activity, suggesting possible translocation to the nucleus. Bryostatin did not induce neurite outgrowth, either alone or in combination with PMA. Thus, the bryostatin-sensitive protein kinase C is distinct from PMA- or K252a-sensitive kinases previously described. The bryostatin-sensitive protein kinase C is necessary, but not sufficient, for neurite outgrowth and acts in the nucleus in a manner independent of c-fos and c-jun transcription.

Chimeric tumor necrosis factor-TrkA receptors reveal that ligand-dependent activation of the TrkA tyrosine kinase is sufficient for differentiation and survival of PC12 cells

To elucidate the function of the two nerve growth factor (NGF) receptors, p75NGFR and p140trk, chimeric molecules were constructed of tumor necrosis factor (TNF) and NGF receptors. Rat PC12 pheochromocytoma cells transiently transfected with TNF-p140trk chimeras, which contain the extracellular domain of TNF receptor and the transmembrane and cytoplasmic domains of p140trk, showed TNF-dependent neuronal differentiation and cell survival. The activity of TNF-p140trk chimeras was completely blocked by the tyrosine kinase inhibitor K252a, and TNF was unable to induce neurite elongation in PC12 cells transfected with a tyrosine kinase-defective chimeric receptor. The TNF-p75NGFR chimeras, which contain the cytoplasmic domain of p75NGFR, were nonfunctional. Our results suggest that p140trk may function as ligand-activated homodimers and that ligand-mediated activation of the cytoplasmic domain of p140trk alone is sufficient for inducing a neuronal phenotype.

Molecular analysis of the nerve growth factor inducible ornithine decarboxylase gene in PC12 cells

In an effort to understand molecular mechanisms by which nerve growth factor (NGF) regulates gene expression, we have isolated a full-length rat cDNA clone encoding ornithine decarboxylase (ODC) and utilized this probe to identify and examine the transcriptionally active, NGF inducible ODC gene in rat PC12 cells. This same gene is also responsive to epidermal growth factor, basic fibroblasts growth factor, and dibutyryl cAMP. Primer extension analysis demonstrates that both basal and NGF induced transcription of the ODC gene utilize the same major transcriptional start site, demonstrating that NGF acts to increase transcriptional activity at the basal start site as opposed to unmasking an alternative, stronger start site. Functional promoter analysis reveals the presence of a constitutive core promoter residing between positions -201 and +390, relative to the start site of transcription. Additional analyses reveal that sequences in the region -7800 to +2257 are insufficient to mediate NGF induced transcriptional activation, demonstrating that at least some of the regulatory sequences necessary for NGF mediated transcriptional induction of the ODC gene must reside at relatively enormous distances from the transcriptional start site. Such a long distance transcriptional regulatory mechanism is unique when compared with other NGF responsive genes that have been similarly analyzed.

Growth factors: regulation of normal and neoplastic growth

This paper presents an overview of current trends in growth factor research. The first part of the review considers the current classification of growth factors and their receptors. A model of cell proliferation regulation by growth factors is then presented. The final section reviews the latest concepts of the involvement of growth factors in the development of neoplasia.

Association of a purine-analogue-sensitive protein kinase activity with p75 nerve growth factor receptors

Purine analogues are protein kinase inhibitors, and they block with varying potency and specificity certain of the biological actions of nerve growth factor (NGF). The analogue 6-thioguanine (6-TG) has been shown to inhibit with high specificity protein kinase N (PKN), a serine/threonine protein kinase activated by NGF in several cellular systems. In the present work, immunoprecipitates of p75 NGF receptors from PC12 cells (+/-NGF treatment) were assayed for protein kinase activity using the substrate myelin basic protein under phosphorylating conditions optimal for PKN and in the presence or absence of purine analogues. An NGF-inducible activity was detected, and approximately 80% was inhibited by purine analogues. This activity was maximally stimulated by NGF within 5-10 min, partially decreased by 60 min, and returned to basal levels after 15 h of NGF treatment. The analogue 6-TG inhibited the NGF-inducible p75-associated kinase activity with an IC50 in the range of 15-35 microM. In mutant PC12 nnr-5 cells that lack the Trk NGF receptor, the purine-analogue-sensitive p75-associated kinase activity was not inducible by NFG. In normal PC12 cells, cyclic AMP analogues and epidermal growth factor failed to induce the same activity. Application of either 2-aminopurine or 6-TG to intact cells only slightly inhibit the NGF-dependent induction of the purine-analogue-inhibited p75-associated kinase activity. This activity shares many similarities but also displays some significant differences with cytosolic PKN. Our findings therefore indicate the association of a purine-analogue-sensitive protein kinase with p75 NGF receptors.

Antibody to NGF inhibits collateral sprouting of septohippocampal fibers following entorhinal cortex lesion in adult rats

We have used an antiserum raised against mouse 2.5S NGF to examine the involvement of endogenous neurotrophins in the collateral sprouting of septohippocampal fibers in the adult rat brain. The antiserum was administered intraventricularly. Immunocytochemical techniques indicated that the injected antibodies penetrated into brain tissue that included the basal forebrain, cortex, striatum, corpus callosum, and hippocampus. Unilateral lesioning of the entorhinal cortex was done to evoke the sprouting of the cholinergic septohippocampal fibers. At 8 days postlesion, the sprouting was much advanced, as evidenced by an increase in density of the acetylcholinesterase (AChE) staining in the outer molecular layer (OML) of the dentate gyrus and by the associated increase in the absolute number of AChE-positive fibers in the OML. As well, there was a widening of the inner molecular layer (IML), interpreted as being due to sprouting of noncholinergic axons in that region. In rats injected daily with anti-NGF or anti-NGF Fab fragments, no increase in AChE density, or in the population of AChE-positive fibers, was observed in the OML. In contrast, the widening of the IML seemed to be unaffected by the anti-NGF treatment. No changes were observed in the AChE related parameters in the dentate gyrus of nonlesioned animals treated similarly for 8 days with anti-NGF; there was, however, a decrease of choline acetyltransferase (ChAT) immunostaining in the ChAT-positive cells of the basal forebrain. Our findings and the confirmation that our polyclonal anti-NGF also recognizes other members of the NGF neurotrophin family, specifically brain-derived neurotrophic factor and neurotrophin-3, indicate that at least one of these neurotrophins plays a key role in the collateral sprouting of the cholinergic septohippocampal fibers (but not that presumed to occur within the IML) following an entorhinal cortex lesion.

Dorsal root ganglion neurons expressing trk are selectively sensitive to NGF deprivation in utero

In utero immune deprivation of the neurotrophic molecule nerve growth factor (NGF) results in the death of most, but not all, mammalian dorsal root ganglion (DRG) neurons. The recent identification of trk, trkB, and trkC as the putative high affinity receptors for NGF, brain-derived neurotrophic factor, and neurotrophin-3, respectively, has allowed an examination of whether their expression by DRG neurons correlates with differential sensitivity to immune deprivation of NGF. In situ hybridization demonstrates that virtually all neurons expressing trk are lost during in utero NGF deprivation. Most, if not all, neurons expressing trkB and trkC survive this treatment. In contrast, the low affinity NGF receptor, p75NGFR, is expressed in both NGF deprivation-resistant and -sensitive neurons. These experiments show that DRG neurons expressing trk require NGF for survival. Furthermore, at least some of the DRG neurons that do not require NGF express the high affinity receptor for another neurotrophin. Finally, these experiments provide evidence that trk, and not p75NGFR, is the primary effector of NGF action in vivo.

The nerve growth factor family of receptors

The neurotrophins, of which nerve growth factor (NGF) is the best known example, support the survival and differentiation of chick embryo sensory neurons at extremely low concentrations, 10(-12) M or less. These same neurons display two different classes of neurotrophin receptors with dissociation constants of 10(-11) M and 10(-9) M, respectively, implying that only low occupancy of the higher affinity receptor is required to mediate the biological actions of the neurotrophins. Two structurally unrelated receptors have now been characterized for NGF, and one of them, p75NGFR, serves as a receptor for all the known neurotrophins. This is the receptor with a dissociation constant of 10(-9) M. The second NGF receptor is a member of the trk family of tyrosine kinase receptors, p140trkA. Other members, p145trkB and p145trkC, are receptors for brain-derived neurtrophic factor (BDNF) and neurotrophin-4 (NT-4) and neurotrophin-3 (NT-3), respectively, when assayed in fibroblasts. The specificity of neurotrophin binding to these receptors appears to be much higher in neurons than in the non-neuronal cells. The receptor p140trkA has many of the properties of the higher affinity class of NGF receptors, and is able to mediate survival and differentiation of the PC12 cell line, and cell growth and transformation in fibroblast cells. On the other hand, expression of p75NGFR in several types of cells displaying p140trkA induces a component of higher affinity NGF binding not seen in its absence. Since it is unlikely that p75NGFR and p140trkA interact at the level of the receptors, the crosstalk between receptors probably occurs through their signal transduction mechanisms.(ABSTRACT TRUNCATED AT 250 WORDS)

The rat trk protooncogene product exhibits properties characteristic of the slow nerve growth factor receptor

Two distinct nerve growth factor receptor (NGFR) complexes are present on NGF-responsive cell types; these correspond to 100 kDa and 158 kDa for the fast (fNGFR) and the slow (sNGFR) NGFRs, respectively. Previous studies indicate that each complex is derived from a separate gene product and that the sNGFR contains tyrosine kinase activity. The cDNA encoding the fNGFR has previously been cloned. In this report, a rat trk protooncogene cDNA has been isolated from PC12 cells and Trk has been shown to bind NGF, generating a complex of 158 kDa. Characterization of NGF-Trk interactions indicates that Trk and NGF dissociate more slowly than do NGF and the fNGFR. Moreover, NGF-bound Trk is not destroyed by trypsin digestion whereas the NGF-fNGFR complex is sensitive to trypsin digestion. These observations suggest that the trk protooncogene product, expressed in the absence of the fNGFR, binds NGF with properties characteristic of the sNGFR, which was identified as the high-affinity NGFR on primary neurons and PC12 cells.