The unprocessed C-terminal dipeptide of recombinant beta-nerve growth factor determines three stable forms with distinct biological activities

Ligand-dependent TrkA activity in brain differentially affects spatial learning and long-term memory

In the central nervous system, the nerve growth factor (NGF) receptor TrkA is expressed primarily in cholinergic neurons that are implicated in spatial learning and memory, whereas the NGF receptor p75(NTR) is expressed in many neuronal populations and glia. We asked whether selective TrkA activation may have a different impact on learning, short-term memory, and long-term memory. We also asked whether TrkA activation might affect cognition differently in wild-type mice versus mice with cognitive deficits due to transgenic overexpression of mutant amyloid-precursor protein (APP mice). Mice were treated with wild-type NGF (a ligand of TrkA and p75(NTR)) or with selective pharmacological agonists of TrkA that do not bind to p75(NTR). In APP mice, the selective TrkA agonists significantly improved learning and short-term memory. These improvements are associated with a reduction of soluble Aβ levels in the cortex and AKT activation in the cortex and hippocampus. However, this improved phenotype did not translate into improved long-term memory. In normal wild-type mice, none of the treatments affected learning or short-term memory, but a TrkA-selective agonist caused persistent deficits in long-term memory. The deficit in wild-type mice was associated temporally, in the hippocampus, with increased AKT activity, increased brain-derived neurotrophic factor precursor, increased neurotrophin receptor homolog-2 (p75-related protein), and long-term depression. Together, these data indicate that selective TrkA activation affects cognition but does so differently in impaired APP mice versus normal wild-type mice. Understanding mechanisms that govern learning and memory is important for better treatment of cognitive disorders.

Deacetylation of p53 after nerve growth factor treatment in PC12 cells as a post-translational modification mechanism of neurotrophin-induced tumor suppressor activation

The tumor suppressor protein p53 is a transcription factor that regulates the response to cellular insults such as DNA damage and growth factor withdrawal. Transcriptional activity of p53 requires post-translational modification by phosphorylation and acetylation. This study used site-specific antibodies to demonstrate that nerve growth factor (NGF) treatment of PC12 cells results in p53 deacetylation at lysine (Lys) 382. Histone deacetylase (HDAC) activity, measured by a direct fluorescent assay, was increased after NGF treatment and peaked before p53 deacetylation. Inhibition of HDAC by trichostatin blocked the deacetylation of p53 and its transcriptional activity toward a reporter gene construct. Comparison of PC12 with PC12 cells containing a temperature-sensitive, dominant-negative construct showed that p53 deacetylation required functional p53. Inhibitors of MAP kinase that block p53 transactivation and inhibitors of TrkA receptor also abolished HDAC activation, indicating that deacetylation of p53 is an NGF-dependent post-translational mechanism of p53 activation. Finally, NGF or serum withdrawal did not lead to p53 deacetylation. A model is proposed in which the acetylation status of Lys 382 of p53 discriminates between cell cycle arrest and apoptosis.

Activation of the mitogen-activated protein kinase pathway through p75NTR: a common mechanism for the neurotrophin family

Neurotrophins interact with two distinct classes of cell-surface receptors, the Trk receptor tyrosine kinase family and the common neurotrophin receptor p75(NTR). For many years, the biological role of p75(NTR) remained obscure, being relegated to modulating Trk binding of neurotrophins. Recently, the importance of p75(NTR) as a signaling receptor in itself has become increasingly clear. The signals initiated by p75(NTR) are likely to be as complex as those for the Trk family and probably depend on the cell system in which such signaling is being studied. In this study, all members of the neurotrophin family were demonstrated to be capable of stimulating p75(NTR)-mediated activation of the mitogen-activated protein kinase (MAPK) family (ERK1,2). This activation is rapid and transient, peaking at 5-15 min, depending on the cell system. The classical MAPK cascade consists of the reaction series Ras-Raf-MEK-MAPK. The p75(NTR)-induced MAPK activation is MEK dependent but Raf independent. This result implies that neurotrophin activation of p75(NTR) results in some cascade (as yet unknown) that bypasses Raf and converges on MEK to result in activation of MAPK. This activated MAPK is then able to translocate to the nucleus. The effect of this MAPK activation on cell survival is dependent on cell type. These results support the concept that signaling from the p75(NTR) receptor is more diverse and extensive than previously believed.

Individual and combined effects of TrkA and p75NTR nerve growth factor receptors. A role for the high affinity receptor site

A long-standing question in neurotrophin signal transduction is whether heteromeric TrkA-p75NTR complexes possess signaling capabilities that are significantly different from homo-oligomeric TrkA or p75NTR alone. To address this issue, various combinations of transfected PC12 cells expressing a platelet-derived growth factor receptor-TrkA chimera and the p75NTR-selective nerve growth factor mutant (Delta9/13 NGF) were utilized to selectively stimulate TrkA or p75NTR signaling, respectively. The contribution of individual and combined receptor effects was analyzed in terms of downstream signaling and certain end points. The results suggest two unique functions for the high affinity heteromeric NGF receptor site: (a) integration of both the MAPK and Akt pathways in the production of NGF-induced neurite outgrowth, and (b) rapid and sustained activation of the Akt pathway, with consequent long term cellular survival. Whereas activation of TrkA signaling is sufficient for eliciting neurite outgrowth in PC12 cells, signaling through p75NTR plays a modulatory role, especially in the increased formation of fine, synaptic "bouton-like" structures, in which both TrkA and p75NTR appear to co-localize. In addition, a new interaction in the TrkA/p75NTR heteromeric receptor signal transduction network was revealed, namely that NGF-induced activation of the MAPK pathway appears to inhibit the parallel NGF-induced Akt pathway.

Mediation of nerve growth factor-driven cell cycle arrest in PC12 cells by p53. Simultaneous differentiation and proliferation subsequent to p53 functional inactivation

Upon stimulation with nerve growth factor (NGF), PC12 cells extend neurites and cease to proliferate by influencing cell cycle proteins. Previous studies have shown that neuritogenesis and a block at the G(1)/S checkpoint correlate with the nuclear translocation of and an increase in the p53 tumor suppressor protein. This study was designed to determine if p53 plays a direct role in mediating NGF-driven G(1) arrest. A retroviral vector that overexpresses a temperature-sensitive p53 mutant protein (p53ts) was used to extinguish the function of endogenous p53 in PC12 cells in a dominant-negative manner at the nonpermissive temperature. NGF treatment led to transactivation of a p53 response element in a luciferase reporter construct in PC12 cells, whereas this response to NGF was absent in PC12(p53ts) cells at the nonpermissive temperature. With p53 functionally inactivated, NGF failed to activate growth arrest, as measured by bromodeoxyuridine incorporation, and also failed to induce p21/WAF1 expression, as measured by Western blotting. Since neurite outgrowth proceeded unharmed, 50% of the cells simultaneously demonstrated neurite morphology and were in S phase. Both PC12 cells expressing SV40 T antigen and PC12 cells treated with p53 antisense oligonucleotides continued through the cell cycle, confirming the dependence of the NGF growth arrest signal on a p53 pathway. Activation of Ras in a dexamethasone-inducible PC12 cell line (GSRas1) also caused p53 nuclear translocation and growth arrest. Therefore, wild-type p53 is indispensable in mediating the NGF antiproliferative signal through the Ras/MAPK pathway that regulates the cell cycle of PC12 cells.

A TrkA-selective, fast internalizing nerve growth factor-antibody complex induces trophic but not neuritogenic signals

Nerve growth factor (NGF) is a neurotrophin that induces neuritogenic and trophic signals by binding to TrkA and/or p75 receptors. We report a comparative study of the binding, internalization, and biological activity of NGF versus that of NGF in association with an anti-NGF monoclonal antibody (mAb NGF30), directed against the C termini of NGF. NGF.mAb complexes do not bind p75 effectively but bind TrkA with high affinity. After binding, NGF. mAb complexes stimulate internalization faster and to a larger degree than NGF. NGF.mAb-induced activation of TrkA, Shc, and MAPK is transient compared with NGF-induced activation; yet NGF and NGF. mAb afford identical trophic responses. In contrast, NGF induces Suc-1-associated neurotrophic activating protein phosphorylation and neuritogenic differentiation, but NGF.mAb does not. Thus, an absolute separation of trophic and neuritogenic function is seen for NGF.mAb, suggesting that biological response modifiers of neurotrophins can afford ligands with selected activities.

Solution structure and internal motion of a bioactive peptide derived from nerve growth factor

The conformation and internal dynamics of a bioactive cyclic peptide, N-acetyl-YCTDEKQCY, derived from the C-D loop of beta-nerve growth factor (beta-NGF) were analyzed by solution NMR spectroscopy. NMR experimental data were used to calculate an ensemble of peptide structures. All of the structures had a beta-turn at residues Asp4-Gln7 but could be divided into two families according the presence or absence of a hydrogen bond at Gln7. Comparison of the calculated structures with the corresponding C-D loops from the x-ray structures of the NGF revealed striking similarity. The orientation of Glu5, Lys6, and Gln7 side chains in the NGF mimetic was very similar to the C-D loop of NGF. These residues are known to participate in interactions with the TrkA receptor. Relaxation measurements of the peptidomimetic alpha-carbons at 13C natural abundance and calculated dynamic parameters suggest that the loop region of peptide is well structured but that residues Thr3, Asp4, Glu5, and Lys6 undergo slow conformational exchange. These results suggest that conformational similarity and possibly peptide dynamics are responsible for the bioactivity of the peptide.

Characterization of the recombinant extracellular domain of the neurotrophin receptor TrkA and its interaction with nerve growth factor (NGF)

Nerve growth factor (NGF) is the prototype of a family of neurotrophins that support important neuronal programs such as differentiation and survival of a subset of sympathetic, sensory, and brain neurons. NGF binds to two classes of cell surface receptors: p75LANR and p140TrkA. NGF binding to p140TrkA initiates the neuronal signaling pathway through activation of the tyrosine kinase activity, which subsequently results in a rapid signal transduction through a phosphorylation cascade. To examine this crucial signaling step in more detail, the TrkA extracellular domain polypeptide (TrkA-RED) was overexpressed in Sf21 insect cells and purified to homogeneity. The recombinant TrkA-RED is a 70 kDa acidic glycoprotein with a pI of 5.1, and mimics the intact TrkA receptor for NGF binding with a dissociation constant, Kd, of 2.9 nM. Thus, the recombinant TrkA-RED is functionally competent and can be used to elucidate the interaction of NGF and TrkA receptor. Circular dichroism difference spectra indicated that, upon association of NGF with TrkA-RED, a minor conformational change occurred to form a complex with decreased ordered secondary structure. Interaction between NGF and TrkA-RED was also demonstrated by size exclusion chromatography, light scattering, and chemical crosslinking with evidence for formation of a higher molecular weight complex consistent with a (TrkA-RED)2-(NGF dimer) complex. Association and dissociation rates of 5.6 x 10(5) M(-1) s(-1) and 1.6 x 10(-3) s(-1), respectively, were determined by biosensor technology. Thus, initiation of signaling may stem from NGF-induced receptor dimerization concomitant with a small conformational change.

The role of the nerve growth factor carboxyl terminus in receptor binding and conformational stability

The role of the nerve growth factor (NGF) carboxyl terminus in the function of NGF is not well understood. Previous work showed that deletion of residues 112-120 abolished NGF bioactivity. Several mutagenesis studies, however, have localized the binding sites of the two NGF receptors, p75 and TrkA, to other regions of the NGF molecule. To investigate the role of the NGF COOH terminus, we performed a detailed structure-function analysis of this region by deleting stepwise each of the nine COOH-terminal residues as well as constructing six point mutants. We found that point mutations within the 111-115 region, but not deletion of residues 116-120, significantly decreased NGF bioactivity, as determined by TrkA tyrosine phosphorylation and neurite outgrowth from PC12 cells. Mutation of the absolutely conserved Leu112 led to severely disrupted p75 binding on A875 cells but had only a modest effect on TrkA binding to MG87-TrkA fibroblasts. This suggests that the p75 binding surface is more extended than previously believed and includes not only charged residues within loops 1 and 5 but also spatially discontinuous, uncharged residues in a region where the NH2 and COOH termini are in close proximity. Unexpectedly, deletion of COOH-terminal residues beyond Ala116 led to significantly decreased stability. These results demonstrate that residues 111-115, but not residues 116-120, are important for both the structural stability and biological activity of NGF.

Characterization of histidine residues essential for receptor binding and activity of nerve growth factor

The role of the four histidine residues in receptor binding and activity of mouse nerve growth factor (NGF) was investigated using both site-directed mutagenesis and chemical modification with diethyl pyrocarbonate. Replacement of His-75 or His-84 with alanine resulted in decreased biological activity and decreased affinity for p140(trkA); however, with H75A only, a 5-fold increased affinity toward p75(LANR) was observed. The effect of simultaneous replacement of both His-75 and His-84 was neither additive nor synergistic. Slight perturbations in circular dichroism spectra and weakened self-association of the mutants indicated that His-75 and His-84 may be involved in stability, dimerization, and/or folding of NGF. Diethyl pyrocarbonate modification of His-4 and His-8 in the H75A/H84Q double mutant abolished neuritogenesis, binding to both receptors, and phosphorylation of p140(trkA) in PC12 cells. These chemical and mutational results confirm and clarify previous evidence for the involvement of His-75 and His-84 (Dunbar, J. C., Tregear, G. W., and Bradshaw, R. A. (1984) J. Protein Chem. 3, 349-356) or His-4 and His-8 (Shih, A., Laramee, G. R., Schmelzer, C. H., Burton, L. E., and Winslow, J. W. (1994) J. Biol. Chem. 269, 27679-27686) in receptor binding of NGF. At least three and possibly all four histidines, which are located in three spatially distinct regions, contribute to maintenance of functional sites that are essential for receptor binding and activity of NGF.

Mutational studies of conserved residues in the dimer interface of nerve growth factor

An understanding of the structure-function relationship of nerve growth factor (NGF) requires precise knowledge of all the residues and regions that participate in NGF receptor binding, receptor activation, and biological activity. Seven recombinant human NGF mutants having alanine substituted for residues located either in the NGF dimer interface or beta-strand region were studied to determine the role of each amino acid residue in NGF biological activity. F86A, T91A, R100A, and R103A remained nearly full active with 61, 120, 91, and 73% of wild-type activity, respectively, in the PC12 cell bioassay. Hydrophobic core and dimer interface residues Y52, F53, and F54 were studied in more detail. Y52A and F54A were expressed in very low levels, suggesting that these two residues may be important for protein stability. Y52A retained full biological activity (91%). F53A had a 20- and 70-fold reduction in biological activity and TrkA phosphorylation, respectively, with only a 5- to 10-fold effect on TrkA binding and no effect on low-affinity receptor binding. F54A had significantly decreased TrkA phosphorylation and biological activity (40-fold). The results suggest that F53 and F54 may play a structural role in TrkA receptor activation subsequent to binding.

Heterodimeric neurotrophins induce phosphorylation of Trk receptors and promote neuronal differentiation in PC12 cells

Neurotrophins are a family of highly conserved proteins that affect the development and maintenance of distinct neuronal populations. Neurotrophins exist in vivo as homodimers, but we show that neurotrophins can exist as heterodimers in vitro and are pluripotent, being able to bind and to activate different Trk tyrosine kinase receptors as well as promote neuronal differentiation in PC12 cells as effectively as wild type homodimers. These asymmetric neurotrophin dimers allow unique characterization of neurotrophin structure-function relationships with Trk receptors. The chimeric Trk activities of these heterodimers suggest an alternative model of neurotrophin-Trk receptor activation in which the critical Trk-interacting elements may be attributed to a single protomer.

Alteration of NH2-terminal residues of nerve growth factor affects activity and Trk binding without affecting stability or conformation

The role of the NH2-terminal region of nerve growth factor (NGF) was studied with an NGF delta 9/13 deletion mutant, overexpressed in a baculovirus system, and mouse NGF truncated at Met-9 by cleavage with CNBr (des-(1-9)-NGF). Structural studies have been performed on the purified proteins, in addition to biological activity assessment, in order to determine effects of such modifications on global conformation and stability. The activity of NGF delta 9/13 was reduced below detectable levels, and the activity of the des-(1-9)-NGF form was decreased by at least a 50-fold in a PC12 bioassay. Competitive binding of NGF delta 9/13 to low affinity receptors on PC12 cells was not impaired; however, the mutant was not capable of competing for the cold chase-stable, high affinity binding of NGF to the cells. The binding of NGF delta 9/13 to Sf21 cells ectopically expressing the TrkA NGF receptor was also abolished. Thus, deletion of residues 9-13 significantly altered the binding affinity for the high affinity receptors on PC12 cells and for the TrkA receptor, but not for the low affinity receptor. Neither the secondary structure, determined by circular dichroism, nor the conformational stability determined by equilibrium denaturation of NGF delta 9/13 was altered as compared with wild type NGF. Slight conformational and stability perturbations of des-(1-9)-NGF were revealed by the same analysis; however, these changes were found to reflect the influence of the formic acid treatment, not the truncation of 9 residues. Our results support the conclusion that the NH2-terminal domain encompassing residues 1-9 and 9-13 is essential for maintaining the binding capability of NGF for high affinity TrkA receptors. Moreover, conformational and stability data show that the functional results of these modifications of the NH2-terminal region are directly due to receptor binding and not to secondary effects of improper folding or other indirect structural changes.

Nerve growth factor: structure/function relationships

Nerve growth factor (NGF), which has a tertiary structure based on a cluster of 3 cystine disulfides and 2 very extended, but distorted beta-hairpins, is the prototype of a larger family of neurotrophins. Prior to the availability of cloning techniques, the mouse submandibular gland was the richest source of NGF and provided sufficient material to enable its biochemical characterization. It binds as a dimer to at least 2 cell-surface receptor types expressed in a variety of neuronal and non-neuronal cells. Residues involved in these interactions and in the maintenance of tertiary and quaternary structure have been identified by chemical modification and site-directed mutagenesis, and this information can be related to their location in the 3-dimensional structure. For example, interactions between aromatic residues contribute to the stability of the NGF dimer, and specific surface lysine residues participate in receptor contacts. The conclusion from these studies is that receptor interactions involve broad surface regions, which may be composed of residues from both promoters in the dimer.

Structure-function relationships in the neurotrophin family

The study of structure-function relationships in the neurotrophin family has in recent years increased our understanding of several important aspects of neurotrophin function. Site-directed mutagenesis studies have localized amino acid residues important for binding to the low-affinity (p75LNGFR), as well as to the members of the Trk family of tyrosine kinase receptors. A cluster of positively charged residues has been shown to form a surface for binding to p75LNGFR in all four neurotrophins. Differences in the spatial distribution of these charges among the different neurotrophins may explain some of their distinct binding properties. Elimination of these positive charges drastically reduces binding to p75LNGFR but not to the Trk family members, and it does not impair the biological properties of the neurotrophins in vitro, arguing that binding to and activation of Trk receptors is sufficient to mediate the biological responses of neurotrophins. In contrast, the binding sites to Trk receptors appear to be formed by discontinuous stretches of amino acid residues distributed throughout the primary sequence of the molecule. These include the N-terminus, some of the variable loop regions and a beta-strand. Despite their apparent distribution, when viewed in the three-dimensional structure of NGF, these residues appear grouped on one side of the neurotrophin dimer, delineating a continuous surface extending approximately parallel to the twofold symmetry axis of the molecule. Two symmetrical surfaces are formed along the axis of the neurotrophin dimer providing a model for ligand-mediated receptor dimerization. In the neurotrophin family, co-evolution of cognate ligands and Trk receptors has developed specific contacts through different residues in the same variable regions of the neurotrophins. Thus, binding specificity is determined by the cooperation of distinct active and inhibitory binding determinants that restrict ligand-receptor interactions. Binding determinants to the Trk receptors can be manipulated independently in a rational fashion to create neurotrophin analogues with novel ligand-binding properties. In this way, second-generation chimeric neurotrophins with multiple specificities (pan-neurotrophins) have been engineered which may have valuable applications in the treatment of neurodegeneration and nerve damage.

Production and characterization of recombinant mouse brain-derived neurotrophic factor and rat neurotrophin-3 expressed in insect cells

Bioactive brain-derived neurotrophic factor (BDNF) and neurotrophin-3 were produced using the baculovirus expression system and purified to homogeneity using ion-exchange and reversed-phase chromatography. Yields of purified neurotrophin-3 (300-500 micrograms/L) were similar to levels reported for baculovirus-expressed nerve growth factor (NGF), whereas initial yields of BDNF were significantly lower (20-50 micrograms/L). Improved production of BDNF (150-200 micrograms/L) was achieved by expressing BDNF from a chimeric prepro-NGF/mature BDNF construct using the Trichoplusia ni insect cell line. Tn-5B1-4. Examination of the distribution of BDNF protein from both the non-chimeric prepro-BDNF and the chimeric prepro-NGF/mature BDNF viruses in Sf-21- and Tn-5B1-4-infected cells suggests a specific deficiency in the Tn-5B1-4 cells in processing the nonchimeric precursor. In addition, the vast majority of the BDNF protein at 2 days after infection was intracellular and insoluble. N-terminal amino acid sequencing of purified recombinant BDNF and neurotrophin-3 demonstrated that the insect cells processed their precursors to the correct N-terminus expected for the mature protein. Bioactivity was characterized in vitro on primary neuronal cultures from the CNS and PNS.

Immunological determinants of nerve growth factor involved in p140trk (Trk) receptor binding

Monoclonal anti-NGF antibodies that specifically inhibit the biological activity of mouse beta-NGF were used to study the structural determinants involved in the interaction of NGF with its receptors gp75LNGFR and Trk. None of the three antibodies--N60, M15, and 27/21--showed any reactivity toward denatured NGF. Three experimental methods--radioimmunoassay (RIA), enzyme-linked immunoassay (ELISA), and slot blots--detected no significant cross reactivity between the antibodies and BDNF or NT-3. RIA showed that M15 and N60 recognize the same or an overlapping antigenic site, but 27/21 recognizes a different epitope. Only 27/21, and not N60 or M15, immunoprecipitated beta-NGF crosslinked to LNGFR receptor. Thus, the epitope recognized by 27/21 does not overlap the LNGFR receptor binding site. N60, M15, and 27/21 all block binding of NGF to Trk in a manner consistent with competitive inhibition. Purified Fab fragments of N60 and M15 gave similar results to the intact antibodies. The other subunits present in the 7S complex of NGF, i.e. the alpha and gamma subunits, competitively inhibited binding of antibodies to beta-NGF. Only the gamma subunit inhibited phosphorylation of Trk and biological activity of beta-NGF. These findings suggest that the M15, N60, and 27/21 antibodies bind to a specific site on the surface of NGF where they competitively inhibit binding to the Trk NGF receptor. The region encompassing the N-terminus, the C-terminus, and the loop on the surface of beta-NGF containing residues 60-80 is proposed as important for binding to the Trk receptor.

The use of baculoviruses as expression vectors

The use of recombinant baculoviruses as high level expression systems is becoming more and more popular. This review aims to provide a summary of the impact of this expression system in biochemistry and biotechnology, highlighting important advances that have been made utilizing the system. The potential of newly developed multiple baculovirus expression systems to enable the reconstruction of complex biological molecules and processes is also reviewed.