The trk proto-oncogene product: a signal transducing receptor for nerve growth factor

Nerve growth factor stimulation of the Ras-guanine nucleotide exchange factor and GAP activities

The biological activity of Ras proteins is thought to be controlled by the guanine nucleotide exchange factor and the guanosine triphosphatase activating protein (GAP). Treatment of rat pheochromocytoma PC-12 cells with nerve growth factor (NGF) increased the amount of active Ras guanosine triphosphate complex and stimulated the activities of both the guanine nucleotide exchange factor and GAP. In PC-12 cells that overexpressed the tyrosine kinase encoded by the trk proto-oncogene (a component of the high-affinity NGF receptor), the NGF-induced activation of the regulatory proteins was potentiated. These results suggest that the NGF receptor system enhances the activities of both the guanine nucleotide exchange factor and GAP and that the activation of Ras might be controlled by the balance in activity between these two regulatory proteins.

Signal transduction and regulation of neurotrophins

Our understanding of the molecular nature of neurotrophic interactions has been greatly enhanced by the recent isolation and characterization of several new neurotrophic factors and their receptors. Neurotrophic factors have been found to be regulated by neuronal activity in the central nervous system, and may be involved in activity-dependent processes throughout development and maturity.

Nerve growth factor-induced derepression of peripherin gene expression is associated with alterations in proteins binding to a negative regulatory element

The peripherin gene, which encodes a neuronal-specific intermediate filament protein, is transcriptionally induced with a late time course when nerve growth factor (NGF) stimulates PC12 cells to differentiate into neurons. We have studied its transcriptional regulation in order to better understand the neuronal-specific end steps of the signal transduction pathway of NGF. By 5' deletion mapping of the peripherin promoter, we have localized two positive regulatory elements necessary for full induction by NGF: a distal positive element and a proximal constitutive element within 111 bp of the transcriptional start site. In addition, there is a negative regulatory element (NRE; -179 to -111), the deletion of which results in elevated basal expression of the gene. Methylation interference footprinting of the NRE defined a unique sequence, GGCAGGGCGCC, as the binding site for proteins present in nuclear extracts from both undifferentiated and differentiated PC12 cells. However, DNA mobility shift assays using an oligonucleotide probe containing the footprinted sequence demonstrate a prominent retarded complex in extracts from undifferentiated PC12 cells which migrates with slower mobility than do the complexes produced by using differentiated PC12 cell extract. Transfection experiments using peripherin-chloramphenicol acetyltransferase constructs in which the footprinted sequence has been mutated confirm that the NRE has a functional, though not exclusive, role in repressing peripherin expression in undifferentiated and nonneuronal cells. We propose a two-step model of activation of peripherin by NGF in which dissociation of a repressor from the protein complex at the NRE, coupled with a positive signal from the distal positive element, results in depression of the gene.

Diversity in cellular signaling for nerve growth factor and insulin: variations on a common theme

Numerous similarities exist in the cellular signaling events observed for insulin and nerve growth factor. Because the two hormones share many functional properties, and exhibit similar effects on neurons, the possibility of common early signaling events has been explored. Many studies have focused on the important role of protein phosphorylation. Two distinct but related mechanisms are discussed that may mediate, in part, the ability of these two hormones to regulate the activities of protein kinases and phosphatases.

K-252a and staurosporine selectively block autophosphorylation of neurotrophin receptors and neurotrophin-mediated responses

The same receptor tyrosine kinase (RTK) can mediate strikingly different biological responses in a fibroblast as opposed to a neuron. We have compared the rapidly induced tyrosine phosphorylations mediated by various RTKs in both NIH3T3 fibroblasts and in the PC12 neuronal precursor cell line and found that each RTK induces a distinct pattern of protein tyrosine phosphorylations in the two cell types. These findings are consistent with a model in which various cell types present a given RTK with different menus of signal transduction components, allowing the same RTK to elicit fundamentally distinct biological responses. Although there are obvious overlaps in the tyrosine phosphorylations induced by different RTKs in the same cell, there are also clear differences. The attempt to dissect these differences revealed that the kinase inhibitors K-252a and staurosporine inhibit RTK autophosphorylation and thus the biological consequences of receptor/ligand interaction. These inhibitors displayed substantially greater specificity for a subset of RTKs (including the neurotrophin receptors) than for other RTKs and acted as remarkably selective blockers of neurotrophin action in both neuronal and nonneuronal cells. A potential therapeutic application for these inhibitors is discussed.

Effects of NGF and glucocorticoid on NGF receptor immunolabeling of cultured rhesus adrenal chromaffin cells

Nerve growth factor (NGF) promotes the outgrowth of neurites from cultured adrenal chromaffin cells from adult rhesus monkeys, but little is known about the distribution, at the cellular level, of the NGF receptors (NGFR) responsible for this response. We examined changes in immunostaining for NGFR in chromaffin cells cultured for 4 weeks in the presence or absence of NGF, with or without dexamethasone (DEX), which inhibits neuritic outgrowth from these cells. Purified cultures of adrenal chromaffin cells from adult rhesus monkeys were grown for up to 9 weeks in NGF, DEX, NGF plus DEX, or control medium. Cells were immunolabeled with three different monoclonal antibodies directed against different epitopes of the human NGFR. Although the distribution of immunolabeling was not uniform from cell to cell, the overall intensity of NGFR immunolabeling varied dramatically between different growth conditions. Of greatest interest, DEX-treated cells stained the most intensely at all time points, while the intensity of immunolabeling was much fainter in NGF-treated cells and decreased with time in culture. In contrast to the intensity of labeling, the proportion of chromaffin cells with immunoreactivity increased with time in all treatment groups. Thus, GCs do not appear to antagonize the effects of NGF merely by decreasing the total number of immunoreactive NGFR on the surface of these cells.

Targeted mutation of the gene encoding the low affinity NGF receptor p75 leads to deficits in the peripheral sensory nervous system

We have generated mice carrying a mutation of the gene encoding the low affinity NGF receptor p75NGFR by targeted mutation in embryonic stem cells. Mice homozygous for the mutation were viable and fertile. Immunohistochemical analyses of the footpad skin of mutant mice revealed markedly decreased sensory innervation by calcitonin gene-related peptide- and substance P-immunoreactive fibers. The defective innervation was correlated with loss of heat sensitivity and associated with the development of ulcers in the distal extremities. Complicated by secondary bacterial infection, the ulcers progressed to toenail and hair loss. Crossing a human transgene encoding p75NGFR into the mutant animals rescued the absent heat sensitivity and the occurrence of skin ulcers and increased the density of neuropeptide-immunoreactive sensory innervation of footpad skin. The mutation in the gene encoding p75NGFR did not decrease the size of sympathetic ganglia or the density of sympathetic innervation of the iris or salivary gland. Our results suggest that p75NGFR has an important role in the development and function of sensory neurons.

Stimulation of protein-tyrosine phosphorylation in rat striatum after lesion of dopamine neurons or chronic neuroleptic treatment

Even though the short-term actions of dopamine on postsynaptic receptors are well-characterized, the molecular bases for long-term trophic interactions between dopamine neurons and their targets remain unclear. Since protein-tyrosine phosphorylation plays a key role in the action of trophic factors, we have investigated its possible involvement in the interactions between dopamine neurons and their striatal targets. Lesioning rat nigrostriatal dopamine neurons by using 6-hydroxydopamine increased the phosphorylation on tyrosine of several proteins, including a major 180-kDa protein (pp180) in the ipsilateral striatum. Protein-tyrosine kinase activity was also increased in the striatum ipsilateral to the lesion, whereas no change in phosphotyrosine phosphatase activity was detected. The stimulation of pp180 phosphorylation was observed 1, 2, and 8 weeks after 6-hydroxydopamine lesion, was selective for the destruction of dopamine neurons, and was mimicked by chronic blockade of dopamine receptors with neuroleptics. Additional lesion experiments and subcellular fractionation showed that pp180 is located in neuronal postsynaptic densities, suggesting that pp180 is a postsynaptic component of corticostriatal synapses. Our results indicate that lesion of specific afferent fibers can activate tyrosine phosphorylation in central neurons and suggest that tyrosine phosphorylation is involved in the long-term consequences of dopamine deficiency and may play a role in synaptic plasticity.

Concentration-dependent regulation of neuronal gene expression by nerve growth factor

NGF is a neurotrophic protein that promotes the survival, growth, and differentiation of developing sympathetic neurons. To directly determine the effects of different concentrations of NGF on neuronal gene expression, we examined mRNAs encoding the p75 low-affinity NGF (LNGF) receptor, T alpha 1 alpha-tubulin (T alpha 1), and tyrosine hydroxylase (TH) in pure cultures of rat sympathetic neurons from postnatal day 1 superior cervical ganglia. Studies of the timecourse of gene expression during 2 wk in culture indicated that a 5-d incubation period would be optimal for the concentration-effect studies. Analysis of RNA isolated from neurons cultured in 2-200 ng/ml 2.5S NGF for 5 d revealed that, as the NGF concentration increased, neurons expressed correspondingly increased levels of all three mRNAs. Both LNGF receptor and TH mRNAs increased seven-fold, and T alpha 1 mRNA increased four- fold in neurons cultured in 200 versus 10 ng/ml NGF. In contrast, T26 alpha-tubulin mRNA, which is constitutively expressed, did not alter as a function of NGF concentration. When neurons were initially cultured in 10 ng/ml NGF for 5 d, and then 200 ng/ml NGF was added, LNGF receptor, T alpha 1, and TH mRNAs all increased within 48 h. The timecourse of induction differed: T alpha 1 mRNA was maximal by 5 h, whereas LNGF receptor and TH mRNAs first began to increase at 12 h after the NGF increase. These experiments show that NGF regulates expression of a subset of mRNAs important to neuronal growth and differentiation over a broad concentration range, suggesting that the effects of NGF may be mediated by more than just a single receptor operating at one fixed affinity. These results also suggest a mechanism for coupling neuronal synthesis of axonal proteins to increases in size of the innervated target territory during growth of the organism.

Activation of second-messenger pathways reactivates latent herpes simplex virus in neuronal cultures

Herpes simplex virus type 1 (HSV-1) establishes latent infections in neurons of sympathetic and sensory ganglia in humans, and reactivation of latent virus results in recurrent disease. Previously, we reported establishment of latent HSV-1 infections in neuronal cultures derived from rats, monkeys, and humans; reactivation occurs following nerve growth factor (NGF) deprivation. The processes controlling HSV latency are not understood. Using the in vitro neuronal latency system, we have shown that latent HSV-1 reactivated in response to stimulation of at least two second-messenger pathways. Stimulation of cAMP-dependent pathways by several mechanisms or activation of protein kinase C by phorbol myristate acetate (PMA) resulted in reactivation of latent HSV-1. The reactivation kinetics following treatment with activators of protein kinase A and C were accelerated compared with those following NGF deprivation. 2-Aminopurine, which inhibits NGF-stimulated protein kinases and other classes of protein kinases, but does not effect protein kinase A or C, blocked reactivation produced by NGF deprivation or treatment with a cAMP analog, but not reactivation by PMA treatment. These results demonstrate that latent HSV-1 reactivates in neurons in vitro in response to activation of second-messenger pathways.

The trkB tyrosine protein kinase is a receptor for neurotrophin-4

Neurotrophin-4 is a novel member of the nerve growth factor family of neurotrophins recently isolated from Xenopus and viper DNA. We now report that the Xenopus NT-4 protein (XNT-4) can mediate some of its biological properties through gp145trkB, a murine tyrosine protein kinase previously identified as a primary receptor for the related brain-derived neurotrophic factor (BDNF). XNT-4 displaces 125I-labeled BDNF from binding to cells expressing gp145trkB receptors, induces their rapid phosphorylation on tyrosine residues, and causes the morphologic transformation of NIH 3T3 cells when coexpressed with gp145trkB. Moreover, XNT-4 induces the differentiation of PC12 cells into sympathetic-like neurons only if they ectopically express gp145trkB receptors. None of these biochemical or biological effects could be observed when XNT-4 was added to cells expressing the related receptors. Replacement of one of the extracellular cysteines (Cys-345) of gp145trkB by a serine residue prevents its activation by XNT-4 but not by BDNF. Therefore, XNT-4 and BDNF may interact with at least partially distinct domains within the gp145trkB receptor.

Isolation of the neu/HER-2 stimulatory ligand: a 44 kd glycoprotein that induces differentiation of mammary tumor cells

The neu/HER-2 proto-oncogene (also called erbB-2) encodes a transmembrane glycoprotein related to the epidermal growth factor receptor. We have purified to homogeneity a 44 kd glycoprotein from the medium of ras-transformed cells that stimulates phosphorylation of the Neu protein and retains activity after elution from the polyacrylamide gel. The protein is active at picomolar concentrations and displays a novel N-terminal sequence. Cross-linking experiments with radiolabeled p44 result in specific labeling of Neu, indicating that p44 is a ligand for Neu or a related receptor. The purified protein induces phenotypic differentiation of cultured human breast cancer cells, including altered morphology and synthesis of milk components. This is accompanied by an increase in nuclear area, inhibition of cell growth (probably by cell cycle arrest at the late S or the G2/M phases), and induction of DNA polyploidy. We propose the name Neu differentiation factor (NDF) for p44.

Distribution of nerve growth factor-like protein and nerve growth factor receptor in human benign prostatic hyperplasia and prostatic adenocarcinoma

Recent observations from our laboratory have identified a nerve growth factor (NGF)-like protein in conditioned media of stromal cells and neoplastic epithelial cells of the human prostate which mediates paracrine interactive growth of both cell types in vitro. In order to investigate the location of this NGF-like protein in the human prostate in vivo, and whether a nerve growth factor receptor (NGF-R) could be identified, we have carried out immunocytochemical studies on frozen tissue sections of human benign prostatic hyperplasia (BPH), prostatic adenocarcinoma and normal prostatic tissue. The NGF-like protein localized predominantly to the stromal component of BPH, adenocarcinoma and normal (non-cancerous) prostatic tissue. Conversely, the NGF-R localized predominantly to the epithelial cells of these tissues. Renal tissue provided negative controls for both the NGF-like protein and the NGF-R. The testis provided positive controls for both the NGF-like protein and the NGF-R. These results provide corroborative evidence for a NGF-like protein produced by stromal cells which interacts with a NGF-R on the adjacent epithelial cells thereby mediating paracrine interactive growth regulation of the human prostate.

Effects of insulin and insulin-like growth factors on neurofilament mRNA and tubulin mRNA content in human neuroblastoma SH-SY5Y cells

Insulin-like growth factors (IGFs) are implicated in the development of the vertebrate neural circuitry, and increase neurite growth in vitro and in vivo. The construction of the cytoskeleton is necessary for growth of axons and dendrites, and the neurofilament (NF) 68 kDa and 170 kDa proteins assemble to help form major fibrillar elements of the neurite cytoskeleton. We report that physiological concentrations of insulin, IGF-I or IGF-II increased the contents of 68 kDa NF, 170 kDa NF, alpha-tubulin, and beta-tubulin mRNAs, relative to total RNA, in cultured human neuroblastoma SH-SY5Y cells. In contrast, the relative contents of histone 3.3 mRNA, and poly(A)+ RNA were not increased. Ligand concentrations which increased NF mRNAs were very similar to those which increased neurite outgrowth. Although each gene was evidently independently regulated, the 68 kDa NF, 170 kDa NF, alpha-tubulin, and beta-tubulin mRNAs were nevertheless all transiently elevated over approximately the same time interval in response to insulin. These data, when considered together with studies by others with nerve growth factor, show that the 68 kDa and 170 kDa NF mRNAs are elevated in a biochemical pathway activated in common during neurite outgrowth directed by insulin, IGF-I, IGF-II, and nerve growth factor.

Inhibition of the cellular actions of nerve growth factor by staurosporine and K252A results from the attenuation of the activity of the trk tyrosine kinase

The protein kinase inhibitors staurosporine and K252A inhibit some of the cellular actions of nerve growth factor (NGF). To explore the molecular mechanisms involved, we test the ability of these agents to block one of the earliest cellular responses to NGF, protein tyrosine phosphorylation. Concentrations of 10-100 nM staurosporine and K252A inhibit NGF-dependent tyrosine phosphorylation in PC12 cells and inhibit trk oncogene-dependent tyrosine phosphorylation in trk-transformed NIH3T3 (trk-3T3 cells). In contrast, these compounds are without effect on epidermal growth factor (EGF)-stimulated tyrosine phosphorylation in PC12 cells. NGF-stimulated tyrosine phosphorylation of the pp140c-trk NGF receptor and tyrosine phosphorylation of pp70trk are also inhibited by similar concentrations of staurosporine and K252A, whereas tyrosine phosphorylation of the EGF receptor, insulin receptor, and v-src is not affected. Both staurosporine and K252A inhibit the autophosphorylation of pp70trk on tyrosine residues in an in vitro immune complex kinase reaction. Incubation of trk-3T3 cells with 10 nM staurosporine causes rounded transformed cells to revert to a normal flattened phenotype, whereas src-transformed cells are unaffected by this agent. These data suggest that staurosporine and K252A specifically inhibit the trk tyrosine kinase activity through a direct mechanism, probably accounting for the attenuation by these agents of the cellular actions of NGF.

ERK1 and ERK2, two microtubule-associated protein 2 kinases, mediate the phosphorylation of tyrosine hydroxylase at serine-31 in situ

Tyrosine hydroxylase (TH) is phosphorylated at four sites in situ and in vivo, and the protein kinases that phosphorylate three of these sites (Ser8,Ser19,Ser40) have been identified. In intact cells, the phosphorylation of the fourth site (Ser31) is increased in response to phorbol esters or nerve growth factor (NGF). Here, we show that Ser31 is phosphorylated by ERK1 and ERK2, two myelin basic protein and microtubule-associated protein kinases. Extracts of NGF- or bradykinin-treated PC12 rat pheochromocytoma cells were fractionated on Mono Q columns. Protein kinase activity toward Ser31 in TH was present in two peaks corresponding to myelin basic protein kinase activities previously identified as ERK1 and ERK2. Phosphorylation of purified TH in vitro by both kinases was selective for Ser31 up to at least 0.6 mol of phosphate per mol of TH subunit. Treatment of intact PC12 cells with bradykinin or NGF increased both the phosphorylation of TH-Ser31 in situ and the catalytic activity of ERKs (measured subsequently in vitro with myelin basic protein as substrate). Pretreatment of the cells with genistein (a protein-tyrosine kinase inhibitor) decreased the bradykinin- but not the NGF-induced changes in both TH-Ser31 phosphorylation and ERK activity. Genistein also inhibited the increases in Ser31 phosphorylation produced by phorbol dibutyrate, muscarine, and Ba2+. The data indicate that ERK activity is responsible for phosphorylating TH at Ser31 in intact cells and suggest that TH-Ser31 phosphorylation may be regulated by multiple signaling pathways that converge at or prior to the activation of the ERKs.

Disruption of the low affinity receptor-binding site in NGF allows neuronal survival and differentiation by binding to the trk gene product

Nerve growth factor (NGF), like many other growth factors and hormones, binds to two different receptor molecules on responsive cells. The product of the proto-oncogene trk, p140trk, is a tyrosine kinase receptor that has been identified as a signal-transducing receptor for NGF, while the role of the low affinity NGF receptor, p75NGFR, in signal transduction is less clear. The crystal structure of NGF has recently been determined, although structures involved in receptor binding and biological activity are unknown. Here we show that Lys-32, Lys-34, and Lys-95 form a positively charged interface involved in binding to p75NGFR. Simultaneous modification of Lys-32 with either of the two other lysines resulted in loss of binding to p75NGFR. Despite the lack of binding to p75NGFR, these mutants retained binding to p140trk and biological activity, demonstrating a functional dissociation between the two NGF receptors.

Structural domains of the extracellular domain of human nerve growth factor receptor detected by partial proteolysis

Using partial proteolytic cleavage, the nerve growth factor (NGF) binding site and the epitopes for two anti-NGF receptor (NGFR) monoclonal antibodies were localized on the recombinant extracellular domain (RED) of the NGFR. The RED was prepared in the baculovirus-insect cell system and was purified by immunoaffinity and ion-exchange chromatography. The four cysteine-rich repeat domains and some additional C-terminal sequences were resistant to proteolysis with papain or proteinase K. The Mr 32,000 papain-resistant fragment (P32) and the Mr 30,000 proteinase K-resistant fragment (K30) share the same N terminus as the intact RED and have C termini in the vicinity of residue 170. Even though P32 and K30 have the same N terminus and probably differ by only a small number of amino acids at the C terminus, P32, but not K30, binds 125I-NGF. As judged by Western blot analysis, two anti-NGFR antibodies (ME20.4 and NGFR5) bind to P32 but have a lesser affinity for K30. Since antibody ME20.4 inhibits NGF binding but antibody NGFR5 does not, these antibodies bind to distinct epitopes. However, these epitopes apparently are closely spaced since these antibodies compete with each other for binding to biotinylated RED. NGF, but not the control protein cytochrome c, protects RED from papain digestion. Therefore, the P32 C terminus is important for the expression of the NGF binding site and the antibody-defined epitopes, even though the NGF binding site and antibody-defined epitopes probably are not encoded by the P32 C terminus. These data suggest that complex interactions occur between different regions of the RED, and that optimum NGF binding requires the integrity of multiple RED domains, including a short sequence to the C terminus of residue 170.

Nerve growth factor nonresponsive pheochromocytoma cells: altered internalization results in signaling dysfunction

Variant rat pheochromocytoma (PC12) cells which fail to respond to nerve growth factor (NGF) (PC12nnr5) (Green, S. H., R. E. Rydel, J. L. Connoly, and L. A. Greene. 1986. J. Cell Biol. 102:830-843) bind NGF at both high and low affinity sites. Although still undefined at the molecular level, these have been referred to as type I (high) and type II (low) receptors. They are apparently composed of two membrane-bound proteins, p75 and the protooncogene trk, both of which bind NGF, and apparently contribute singularly or in concert to the two observed affinities, and to the promotion of the NGF effects. In native PC12 cells, only the high affinity receptors are apparently capable of mediating internalization and degradation. PC12nnr5 cells also display type I binding, but the subsequent internalization is not the same fashion as in the parental cell line, nor is it subjected to lysosomal degradation. Rather it is initially sequestered during the first 15 min, and is eventually released intact into the medium. In contrast, EGF is bound, internalized, and degraded by PC12nnr5 cells, albeit less efficiently than in the parent cells. These observations argue that the defect(s) preventing the PC12nnr5 variants from responding to NGF prevents competent internalization, which in the case of NGF, may be required for the full expression of activity. The absence of trk, as one alteration in PC12nnr5 cells (Loeb, D. M., J. Maragos, D. Martin-Zanca, M. V. Chao, L. F. Parada, and L. A. Greene. 1991. Cell. 66:961-966), is consistent with this conclusion.

Protein tyrosine kinases in nervous system development

Protein tyrosine kinases are important mediators of intracellular signaling during nervous system development. Activation of receptor protein tyrosine kinases by neurotrophic factors are initial events in the development of discrete cell populations. The patterns of expression and characterization of substrates for nonreceptor protein tyrosine kinases indicates that they also play a crucial role in neuronal development. The observed functional redundancy among protein tyrosine kinases and their associated intracellular signaling pathways underscores the need for further characterization of these novel interactions to elucidate the mechanisms regulating nervous system development.

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

Nerve growth factor, brain-derived neurotrophic factor, and neurotrophin-3 (NT-3) are the three members of the neurotrophin family known to exist in mammals. Recently, a fourth neurotrophin (designated neurotrophin-4 or NT-4), which shares all of the features found in the mammalian neurotrophins, has been identified in Xenopus and viper. We used sequences specific to the Xenopus/viper NT-4 to isolate a neurotrophin from both human and rat genomic DNA that appears to represent the mammalian counterpart of Xenopus/viper NT-4. Human NT-4 as well as a human NT-4 pseudogene colocalize to chromosome 19 band q13.3. Mammalian NT-4 has many unusual features compared to the previously identified neurotrophins and is less conserved evolutionarily than the other neurotrophins. However, mammalian NT-4 displays bioactivity and trk receptor specificity similar to that of Xenopus NT-4.

Effects of estrogen and fimbria/fornix transection on p75NGFR and ChAT expression in the medial septum and diagonal band of Broca

NGF receptor-expressing cells located in the basal forebrain have recently been shown to contain estrogen (E) receptors (Toran-Allerand and MacLusky. 1989. Soc. Neurosci. Abstr. 15: 954). In the present study, we have examined the effects of E-treatment on p75NGFR and choline acetyltransferase (ChAT) expression by neurons in the medial septum (MS) and the vertical (VDB) and horizontal (HDB) limbs of the diagonal band of Broca using immunocytochemical and in situ hybridization techniques. First, since E-treatment has been shown to affect neuronal survival and to stimulate synaptic reorganization and growth within various regions of the brain, we hypothesized that E-treatment might attenuate the loss of p75NGFR immunoreactivity (IR) which occurs in the MS and VDB following transection of the fimbria/fornix. Contrary to our hypothesis, E-treatment did not attenuate the effects of fimbria/fornix transection. In fact, E-treatment alone produced a significant decrease in the number of p75NGFR-IR cells detected in the MS. Subsequent experiments confirmed that chronic E-treatment produces a down-regulation of both p75NGFR-IR and p75NGFR mRNA in the MS and VDB. In the MS, estrogen appeared to affect a subpopulation of p75NGFR-expressing neurons which were also affected by fimbria/fornix transection since the effects of these two treatments were not additive. In addition, effects of E-treatment on p75NGFR-IR were sex-specific (observed in females but not in males) and were reversible in the MS after 2 weeks, but not after 4 weeks (allowing 2 weeks recovery), of E-treatment. A time-course analysis revealed that effects of E-treatment on p75NGFR-IR were not observed until after 16 days (MS) or 30 days (VDB) of E-treatment and were preceded by a significant and transient increase in ChAT expression in both the MS and VDB. The data are consistent with the possibility that continuous, long-term exposure to gonadal steroids may contribute to a loss of p75NGFR-expressing neurons with age. In addition, the data suggest that p75NGFR expression may play a role in regulating the functioning of specific basal forebrain cholinergic neurons. Different mechanisms by which E-treatment might influence ChAT and p75NGFR expression in brain are discussed.

Nerve growth factor-induced synaptogenesis and hypertrophy of cortical cholinergic terminals

In this study light and EM quantitative analysis were used to examine whether exogenous nerve growth factor (NGF) could affect terminal fields and synaptic connections in the adult rat brain in vivo. Adult rats received, immediately after unilateral decortication, 2.5S NGF (12 micrograms/day) or vehicle intracerebroventricularly for 7 days. Thirty days after the lesion cholinergic fiber length was quantified, using image analysis, in the remaining cortical area adjacent to the lesion site in each animal. Rats that had received vehicle showed a significantly reduced cortical choline acetyl-transferase-immunoreactive fiber network in the remaining cortex when compared with control animals. By contrast, the network in lesioned rats that had received 2.5S NGF was not different from control animals. Furthermore, the number of cortical choline acetyltransferase-immunoreactive varicosities, which decreased in vehicle-treated lesioned rats, significantly increased above control in lesioned rats that had received 2.5S NGF. At the ultrastructural level, 30 days after the lesion, animals that had received vehicle showed shrunken cholinergic boutons in cortical layer V and fewer synapses compared with control animals. Exogenous NGF, administered to lesioned rats, increased to supernormal levels both size of cholinergic boutons and number of synaptic contacts. These parameters were unaltered in unlesioned rats treated with NGF. This study demonstrates that exogenous NGF can cause significant compensatory changes in terminal fields and synaptic connections in the adult fully differentiated central nervous system.

Ras is essential for nerve growth factor- and phorbol ester-induced tyrosine phosphorylation of MAP kinases

Treatment of PC12 cells with nerve growth factor (NGF) induces a rapid increase in tyrosine phosphorylation of multiple cellular proteins. Expression of a dominant inhibitory Ras mutant specifically blocked NGF- and TPA-induced tyrosine phosphorylation of two proteins of approximately 42 and 44 kd. Conversely, expression of an oncogenic variant of Ras induced tyrosine phosphorylation of the same 42 and 44 kd proteins. The 44 kd protein was immunoprecipitated with an antibody directed against extracellular signal-regulated kinase 1/mitogen-activated protein kinase (MAPK) and the 42 kd protein comigrated with a 42 kd MAPK, indicating that at least one and probably both Ras-regulated phosphoproteins are MAPKs. In addition, MAPK activation, as measured by in vitro phosphorylation of myelin basic protein, was also regulated by Ras. Ras was not required for NGF-induced activation of Trk or tyrosine phosphorylation of PLC-gamma 1. Thus, NGF-induced tyrosine phosphorylation occurs both prior to and following Ras action, and Ras plays a critical role in the NGF- and TPA-induced tyrosine phosphorylation of MAPKs.

ras mediates nerve growth factor receptor modulation of three signal-transducing protein kinases: MAP kinase, Raf-1, and RSK

p21c-ras plays a critical role in mediating tyrosine kinase-stimulated cell growth and differentiation. However, the pathways through which p21c-ras propagates these signals remain unknown. We report that in PC12 cells, expression of a dominant inhibitory mutant of ras, c-Ha-ras(Asn-17), antagonizes growth factor- and phorbol ester-induced activation of the erk-encoded family of MAP kinases, the 85-92 kd RSKs, and the kinase(s) responsible for hyperphosphorylation of the proto-oncogene product Raf-1. In addition, we find that expression of the activated ras oncogene is sufficient to stimulate these events. These data indicate that ras mediates nerve growth factor receptor and protein kinase C modulation of MAP kinases, RSKs, and Raf-1.

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.

The nerve growth factor receptor: a multicomponent system that mediates the actions of the neurotrophin family of proteins

Nerve growth factor (NGF), brain-derived neurotrophic factor (BDNF), and neurotrophin 3 (NT-3) are members of a family of structurally related proteins termed neurotrophins that promote the growth and survival of neurons in the central and peripheral nervous systems. Each of these proteins bind to at least two membrane receptors. One is the low affinity nerve growth factor receptor (p75), which binds each member of the neurotrophin family. The other is one of a family of tyrosine kinase receptors--trkA binds only NGF, the related trkB receptor binds BDNF and NT-3, and trkC binds NT-3 alone. This article reviews kinetic and biochemical information on p75 and its relationship to the trk gene products.

pp60src tyrosine kinase modulates P19 embryonal carcinoma cell fate by inhibiting neuronal but not epithelial differentiation

P19 embryonal carcinoma cells provide an in vitro model system to analyze the events involved in neural differentiation. These multipotential stem cells can be induced by retinoic acid (RA) to differentiate into neural cells. We have investigated the ability of several variant forms of the protein-tyrosine kinase (PTK) pp60src to modulate cell fate determination in this system. Normally, P19 cells are induced to differentiate along a neural lineage when allowed to form extensive cell-cell contacts in large multicellular aggregates during exposure to RA. Through analysis of markers of epithelial (keratin and desmosomal proteins) and neuronal (neurofilament) cells we have found that RA-induced P19 cells transiently express epithelial markers before neuronal differentiation. Under these inductive conditions, expression of pp60v-src or expression of the neuronal variant pp60c-src+ inhibited neuronal differentiation, and resulted in maintained expression of an epithelial phenotype. Morphological analysis showed that expression of pp60src PTKs results in decreased cell-cell adhesion during the critical cell aggregation stage of the neural differentiation procedure. The effects of pp60v-src on cell fate and cell-cell adhesion could be mimicked by direct modulation of Ca+(+)- dependent cell-cell contact during RA induction of normal P19 cells. We conclude that the neural lineage of P19 cells includes an early epithelial intermediate and suggest that tyrosine phosphorylation can modulate cell fate determination during an early cell-cell adhesion- dependent event in neurogenesis.

Nerve growth factor employs multiple pathways to induce primary response genes in PC12 cells

Nerve growth factor (NGF) leads to neuronal differentiation of PC12 cells and promotes their survival in serum-free medium. Past studies have shown that purine analogues block some of the effects of NGF but not others and thus that they can be used to dissect the mechanistic pathways of its action. In the present work we used 2-aminopurine (2-AP) and 6-thioguanine (6-TG) to examine whether NGF causes activation of primary response genes through a single signaling pathway or via multiple pathways. Northern blot analysis and nuclear run-off transcription assays were used to assess the activation of c-fos, c-jun, TIS1, TIS8, and TIS11 after exposure of PC12 cells to NGF in the presence or absence of 2-AP and 6-TG. Our findings indicate that NGF appears to employ at least three distinct pathways to induce early genes in PC12 cells. This suggests that the NGF signaling mechanism diverges at an early point after interaction of NGF with its receptor.

Selective dependence of mammalian dorsal root ganglion neurons on nerve growth factor during embryonic development

We have investigated the NGF dependence of dorsal root ganglion (DRG) neurons in mammals using a paradigm of multiple in utero injections of a high titer anti-NGF antiserum. We have determined the specificity of our antiserum in relation to other members of the NGF neurotrophin family and found no cross-reactivity with brain-derived neurotrophic factor (BDNF) or neurotrophin-3 (NT-3). To identify various classes of DRG neurons, we have stained their characteristic central projections with Dil. We show here that the NGF dependence of DRG neurons is strikingly selective. Although a majority of DRG neurons are lost after NGF deprivation during embryonic life, these are almost exclusively small diameter neurons that project to laminae I and II of the dorsal horn and presumably subserve nociception and thermoreception. Larger neurons that project to more ventral spinal laminae and subserve other sensory modalities do not require NGF for survival. These NGF-independent DRG neurons likely require one of the more recently identified neurotrophins, BDNF or NT-3.

Differential activation of NGF receptor and early response genes in neural crest-derived cells

Nerve growth factor (NGF) binds to a specific cell surface receptor (NGFR) that exists in high affinity (now called trk) and low affinity (now called p75NGFR) forms. NGF-responsive neurons express both forms of the receptor, while Schwann cells, during early development and after nerve injury, express only low affinity p75NGFR. In an attempt to determine whether NGF alters patterns of gene expression in p75NGFR-bearing Schwann cells, we examined the regulation of three early response genes (NGFI-A, NGFI-B, and c-fos) in JS1 rat schwannoma cells. Although these genes are markedly activated by NGF in PC12 (rat pheochromocytoma) cells, NGF has no effect on their transcription in JS1 cells. In contrast to PC12 cells, NGFI-A and NGFI-B are constitutively expressed in JS1 cells, whereas the c-fos gene is not expressed. Treating JS1 cells with cycloheximide (CHX), an inhibitor of protein synthesis that commonly potentiates induction of early response genes by presumably inhibiting synthesis of transcriptional repressors, markedly induces the transcription of NGFI-A and c-fos as well as p75NGFR genes. These data suggest that transcriptional repression plays a major role in the regulation of these genes and that the markedly different regulation of NGFI-A, NGFI-B, and c-fos, all of which encode transcriptional regulators, may be important in guiding the differentiation of these cell types.

Nerve growth factor-induced loss of cell-associated nerve growth factor receptor in human melanoma A875 cells

Human A875 melanoma cells are known to express the low-affinity nerve growth factor receptor p75NGFR in a monomeric and a covalently linked probably dimeric form. Kinetic analysis of the association of nerve growth factor (NGF) with its receptor revealed a rapid loss of binding sites at high ligand concentrations. Using cross-linking and immunoprecipitation with an anti-p75NGFR antibody, this was found to be due to a decrease of the high molecular weight form of the receptor. Mechanisms for such a ligand-induced receptor loss are discussed.

Tyrosine kinases and their interactions with signalling proteins

Recent progress has been made in identifying signal transduction pathways controlled by receptor protein-tyrosine kinases. The receptors for nerve growth factor and hepatocyte growth factor have been identified as the Trk and Met tyrosine kinases. The stimulation of intracellular signal transduction pathways by activated receptors appears to involve the association of SH2-containing cytoplasmic signalling proteins with autophosphorylated receptors.

Probing the structure-function relationship of nerve growth factor

We compared the receptor binding, antigenicity, biological activation, and cell-mediated proteolytic degradation properties of mouse nerve growth factor (mNGF) and human NGF (hNGF). The affinity of hNGF toward human NGF-receptor is greater than that of mNGF, but the affinity of mNGF toward rat NGF-receptor is greater than that of hNGF. Thus, the specificity of the interaction between NGF and its receptor resides both on the NGF and on its receptor. Using a group of anti-NGF monoclonal antibodies that competitively inhibit the binding of NGF to receptor, sites differing between mNGF and hNGF were detected. Together, these results indicate that the sites on hNGF and mNGF, responsible for binding to NGF-receptor, are similar but not identical. In comparing the relative abilities of mNGF and hNGF to stimulate a biological response in PC12 cells, we observed that mNGF was better at stimulating neurite outgrowth than was hNGF, consistent with the differences observed for receptor binding affinity. However, the ED50 for biological activation is approximately 100-fold lower than the Kd for receptor occupancy, and, thus, the dose-response curve is not consistent with a simple activation proportional to receptor occupancy. The data are consistent with a model requiring a low-level threshold occupancy of NGF-receptor (Kd = 10(-9) M) in order to stimulate full biological activity. Finally, we observed the degradation of NGF by PC12 cells. We found that the NGF molecule is significantly degraded via a receptor-mediated uptake mechanism. Together, the data provide insight into regions of the NGF molecule involved in contacts with the receptor leading to formation of the NGF:NGF-receptor complex. Additionally, they establish the link between occupancy of receptor and biological activation and the requirement for receptor-mediated uptake in order to degrade NGF proteolytically in cultured PC12 cells.

Gene sequences of chicken BDNF and NT-3

The respective amino acid sequences of mature brain-derived neurotrophic factor (BDNF) and of mature neurotrophin-3 (NT-3) are identical among mammals, making these among the structurally conserved factors known. Here we show that only a single conservative amino acid substitution distinguishes the chicken mature NT-3 protein from its mammalian counterpart. Chicken mature BDNF shows slightly more variation, differing from mammalian BDNF at several positions. We also note the presence of amino acid sequence motifs in the precursor protein sequences of chicken BDNF and NT-3 that are universally conserved among all known mammalian neurotrophin precursors and have been demonstrated to play a crucial role in promoting correct processing of the pro-proteins.

Nerve growth factor receptor and the transplanted rat olfactory bulb

Nerve growth factor receptor (NGFR) in the rat olfactory system is developmentally regulated, localizing to the olfactory nerve (ON) in the young rat, and to the olfactory bulb (OB) glomeruli in the adult (Vickland et al. 1991. Brain Res., in press). This pattern of immunoreactivity (IR) may indicate the state of axon growth in the young ON and synaptogenesis in the adult glomeruli. Additional experiments in our laboratory involving lesions to the ON in adult rats have shown a recapitulation of the developmental pattern of expression: NGFR-IR is again found along the ON. Longer survival times after lesioning show the reexpression of the adult pattern of NGFR-IR. This phenomenon was further explored in a transplant (TX) model to determine the changes in NGFR-IR in both the host and TX tissue. A fetal OB labeled with [3H]thymidine is placed into the space created by the removal of the OB of a neonatal rat. After survival times of 1, 2, 8, and 13 weeks, the host animal is sacrificed and the OB TX is processed using monoclonal antibody 192 IgG for NGFR. The host ON shows strong NGFR-IR in TX of 1- and 2-week survival times. In TX survival times of 8 weeks or more, NGFR-IR is observed in glomerulus-like structures. All of these glomerulus-like structures are near groups of neurons in the TX tissue, indicating that they may be functional, with appropriate synapses. Therefore, even though the adult pattern of NGFR-IR takes longer to become established than in normal rats, we have demonstrated that this pattern does so in the TX OB model.

Altered expression of NGF and P75 NGF-receptor by fibroblasts of injured teeth precedes sensory nerve sprouting

Profuse sprouting of sensory nerve fibers occurs in tooth pulp by 1-4 days following dentin injury. A possible role for nerve growth factor (NGF) in that neural response is suggested here by the demonstration that NGF mRNA and protein are increased 6 hr after injury to adult rat molars. The enhanced expression of NGF mRNA was localized to fibroblasts underlying the injury. A concomitant depletion of mRNA encoding the 75 Kd NGF receptor (NGFR) was observed in those fibroblasts. The increase in NGF mRNA was transitory and mRNA levels fell below normal levels by 2 days after injury. Both NGF and NGFR mRNA remained low thereafter in injured pulp. The inverse shifts in fibroblastic mRNA encoding NGF and NGFR were not affected by prior denervation of the tissue, or by pretreatment with dexamethasone. The regulatory mechanisms therefore must involve endogenous, non-neuronal, non-inflammatory factors that are released in response to injury.

Molecular mechanisms for generation of neural diversity and specificity: roles of polypeptide factors in development of postmitotic neurons

Development of postmitotic neurons is influenced by two groups of polypeptide factors. Neurotrophic factors promote neuronal survival both in vivo and in vitro. Neuronal differentiation factors influence transmitter phenotypes without affecting neuronal survival. The list of neurotrophic factors is increasing partly because certain growth factors and cytokines have been shown to possess neurotrophic activities and also because new neurotrophic factors including new members of the nerve growth factor (NGF) family have been identified at the molecular level. In vitro assays using recombinant neurotrophic factors and distributions of their mRNAs and proteins have indicated that members of a neurotrophic gene family may play sequential and complementary roles during development and in the adult nervous system. Most of the receptors for neurotrophic factors contain tyrosine kinase domains, suggesting the importance of tyrosine phosphorylation and subsequent signal transduction for their effects. Molecules such as LIF (leukemia inhibitory factor) and CNTF (ciliary neurotrophic factor) have been identified as neuronal differentiation factors in vitro. At the moment, however, it remains to be determined whether or not the receptors for a group of neuronal differentiation factors constitute a gene family or contain domains of kinase or phosphatase activity. Synergetic combinations of neurotrophic and neuronal differentiation factors as well as their receptors may contribute to the generation of neural specificity and diversity.

Ciliary neurotrophic factor prevents neuronal degeneration and promotes low affinity NGF receptor expression in the adult rat CNS

Recombinant human ciliary neurotrophic factor (CNTF) was infused for 2 weeks into the lateral ventricle of fimbria-fornix transected adult rats, and its effects were compared with those of purified mouse nerve growth factor (NGF). We provide evidence that CNTF can prevent degeneration and atrophy of almost all injured medial septum neurons (whereas NGF protects only the cholinergic ones). CNTF is also involved in up-regulation of immunostainable low affinity NGF receptor (LNGFR) in cholinergic medial septum and neostriatal neurons and in a population of lateral septum neurons. In contrast to NGF, CNTF did not stimulate choline acetyltransferase in the lesioned septum and normal neostriatum (pointing to different mechanisms for the regulation of choline acetyltransferase and LNGFR), cause hypertrophy of septal or neostriatal cholinergic neurons, or cause sprouting of LNGFR-positive (cholinergic) septal fibers.

Mutation of tryptophan-21 in mouse nerve growth factor (NGF) affects binding to the fast NGF receptor but not induction of neurites on PC12 cells

By using in vitro DNA mutagenesis, we replaced the tryptophan residue at position 21 in mouse nerve growth factor (NGF) with either phenylalanine, leucine or serine. Yield, biological activity, immunological reactivity and receptor binding of the recombinant proteins were determined. All three mutants were produced at considerably lower yields than wild-type NGF, with the serine mutant being undetectable. The results of competitive binding assays show that tryptophan-21 is involved in recognition of the fast NGF receptor of PC12 cells. However, specific biological activity of NGF is not altered by the replacement of tryptophan-21. Our results therefore suggest that biological activity of NGF is not directly coupled to binding to the fast NGF receptor.

Diagnostic utility of oncogenes and their products in human cancer

The first clear cut association of an oncogene with a specific cancer is the c-abl translocation in chronic myelogenous leukemia and acute lymphocytic leukemia; it has been observed in 90% of CML cases examined. This is the major contributing factor to its being the target of the first oncogene-based FDA-approved diagnostic test. Although the role of the abl translocation in the tumorigenic process is not yet understood, it is clear that somehow it must be causally related to the disease, and thus is an ideal target for a diagnostic test. The association of this oncogene with a specific cancer is the model on which all others may be based in the future. Second generation tests could easily include PCR on mRNA, and/or in situ hybridization, both of which could be performed using blood samples. Both methods would provide a faster means of testing a large number of cells, however, the methodologies must be improved through automation and computer-aided image analysis, respectively, in order to become useful routine tests. Both neu and epidermal growth factor receptor (EGFR) appear to have a close correlation between overexpression of the gene product and outcome of disease in breast cancer; valuable information for prognosis of the disease. And again, although the actual mechanism of action of these molecules and how this relates to the tumorigenic process is not yet known, it is believed from the very nature of the molecules that they must in some way contribute to the progression of the disease. In both cases, the protein products are overexpressed in tissue, and in the case of Neu, it appears as through at least some of the patients have a Neu-related protein in their serum. These molecules present relatively easy targets for the development of diagnostic/prognostic assays, as antibodies are easily made and can be incorporated into a variety of assay formats. Current assays available, an ELISA for Neu and a radio-ligand binding assay for EGFR, are highly sensitive, reproducible and relatively easy to perform. Only the ELISA is commercially available, however, and hence allows for easy comparison between laboratories. An abvious step towards the routine measurement of EGFR then is the development of a comparable commercially available test. An improvement for both types of assay would be the incorporation of an internal control to gauge the cellular component of the tissue samples that are tested. The outcome of the applications of myc and ras to cancer diagnostics is not so easily predictable, with a couple of exceptions.(ABSTRACT TRUNCATED AT 400 WORDS)

NGF and EGF rapidly activate p21ras in PC12 cells by distinct, convergent pathways involving tyrosine phosphorylation

Activation of p21ras, demonstrated directly as an increase in p21ras-associated GTP, was induced rapidly but transiently by both nerve growth factor (NGF) and epidermal growth factor (EGF) in PC12 cells. The factors activate p21ras to equal extents and with virtually identical time courses. Growth factor-induced p21ras activation and tyrosine phosphorylation have similar time courses and sensitivities to genistein inhibition, indicating that p21ras activation is a result of tyrosine kinase activity. Furthermore, PC12 mutants lacking the Trk NGF receptor tyrosine kinase also lack NGF-inducible p21ras activation. The protein kinase inhibitor K252a and the methyltransferase inhibitor MTA abolish NGF-induced, but not EGF-induced, p21ras activation--effects correlated with inhibition only of NGF-induced tyrosine phosphorylation. In spite of differences in sensitivity to genistein, MTA, and K252a, EGF- and NGF-stimulated p21ras activation are not additive, implying that they do share at least one step in common.

The death programme in cultured sympathetic neurones can be suppressed at the posttranslational level by nerve growth factor, cyclic AMP, and depolarization

We have examined whether sympathetic neurones that have lost the potential to be rescued by protein and RNA synthesis inhibitors after a period of nerve growth factor (NGF) deprivation are irreversibly committed to die. We found that 15 h after withdrawal of NGF from 7-day cultures of neonatal rat superior cervical ganglion neurones, 50% of the neurones lost the potential to be rescued by cycloheximide but that NGF rescued most of the neurones. By 22 h after NGF withdrawal, only 10% of the neurones were rescued by inhibition of macromolecular synthesis with cycloheximide, puromycin, or actinomycin D, but as many as 60-80% of the neurones were rescued by NGF. This is after the time at which a DNA "ladder," consistent with cell death by apoptosis, was first detected (18 h). As long as 27 h of NGF withdrawal was required before 50% of the neurones lost the potential to be rescued by NGF. The survival-promoting agent 8-(4-chlorophenylthio)cyclic AMP (CPTcAMP) or depolarization with 50 mM KCl (HK) rescued neurones with kinetics similar to those of NGF, and rescue by all three agents did not require protein synthesis. Thus, NGF, CPTcAMP, and HK can rescue neurones deprived of NGF at much later times than either protein or RNA synthesis inhibitors by acting at the posttranslational level, a finding suggesting that initiation of the cell death programme in sympathetic neurones is not an irreversible step.