Characterization and growth-dependent regulation of the nerve growth factor receptor gp140trk in rat C6 glioma cells

Growth control of C6 glioma in vivo by nerve growth factor

Treatment with nerve growth factor (NGF) causes differentiation of rat C6 glioma cells and strongly inhibits their proliferation in vitro. This suggests that induction of NGF-mediated differentiation may provide a novel therapeutic approach to growth control of glial tumors. We examined the effects of NGF treatment on the growth potential of C6 glioma, which expressed NGF receptor in vivo. C6 glioma cells (1 x 10(6) cells/10 microl) were transplanted into the rat striatum. After 4 days, the animals were given successive injections of 100 ng NGF into the growing tumor at every 4 days (n = 10 rats). Controls were subjected to identical procedures with vehicle which did not contain NGF (n = 10 rats). At 14 days after transplantation, we evaluated the tumor volume, proliferative cell index (PCI) based on the MIB-1 immunoreactivity and enzyme activities related to energy metabolism by enzyme histochemistry. We found that the NGF treatment markedly reduced the tumor volume of the C6 glioma (34.00 +/- 8.47 mm3 to 7.22 +/- 4.92 mm3, p < 0.01). NGF treatment also decreased the PCI (33.34 +/- 9.57% to 3.85 +/- 3.56%, p < 0.01) with a negative correlation with tumor volume (r = 0.972, p < 0.01), and the hexokinase (HK) and glucose-6-phosphate dehydrogenase (G6PDH) activities (p < 0.01 and p < 0.01, respectively) which reflect the demand for nucleic acid synthesis for proliferation through the glycolytic and pentose phosphate pathways. The present results demonstrate for the first time that inhibition of tumor cell proliferation of C6 glioma by NGF occurs in vivo, probably through the NGF-mediated differentiation of C6 glioma cells which has been observed in in vitro studies.

Trafficking of TrkA-green fluorescent protein chimerae during nerve growth factor-induced differentiation

A chimera of the nerve growth factor (NGF) receptor, TrkA, and green fluorescent protein (GFP) was engineered by expressing GFP in phase with the carboxyl terminus of TrkA. TrkA-GFP becomes phosphorylated on tyrosine residues in response to NGF and is capable of initiating signaling cascades leading to prolonged MAPK activation and differentiation in PC12 nnr5 cells. TrkA constructs, progressively truncated in the carboxyl-terminal domain, were prepared as GFP chimerae in order to identify which part of the receptor intracellular domain is involved in its trafficking. Immunofluorescence observations show that TrkA-GFP is found mainly in cell surface membrane ruffles and in endosomes. Biochemical analysis indicated that the cytoplasmic domain of TrkA is not necessary for correct maturation and cell surface translocation of the receptor. An antibody against the extracellular domain of TrkA (RTA) was used as ligand to stimulate internalization and phosphorylation of TrkA. Co-localization studies with anti-phosphorylated TrkA antibodies support a role for such complexes in the propagation of signaling from the cell surface, resulting in the activation of TrkA in areas of the endosome devoid of receptor-ligand complexes. Confocal time-lapse analysis reveals that the TrkA-GFP chimera shows highly dynamic trafficking between the cell surface and internal locations. TrkA-positive vesicles were estimated to move 0.46 +/- 0.09 microm/s anterograde and 0.48 +/- 0.07 microm/s retrograde. This approach and the fidelity of the biochemical properties of the TrkA-GFP demonstrate that real-time visualization of trafficking of tyrosine kinase receptors in the presence or absence of the ligand is feasible.

Selective immunolesions of cholinergic neurons in mice: effects on neuroanatomy, neurochemistry, and behavior

The ability to selectively lesion mouse basal forebrain cholinergic neurons would permit experimental examination of interactions between cholinergic functional loss and genetic factors associated with neurodegenerative disease. We developed a selective toxin for mouse basal forebrain cholinergic neurons by conjugating saporin (SAP), a ribosome-inactivating protein, to a rat monoclonal antibody against the mouse p75 nerve growth factor (NGF) receptor (anti-murine-p75). The toxin proved effective and selective in vitro and in vivo. Intracerebroventricular injections of anti-murine-p75-SAP produced a dose-dependent loss of choline acetyltransferase (ChAT) activity in the hippocampus and neocortex without affecting glutamic acid decarboxylase (GAD) activity. Hippocampal ChAT depletions induced by the immunotoxin were consistently greater than neocortical depletions. Immunohistochemical analysis revealed a dose-dependent loss of cholinergic neurons in the medial septum (MS) but no marked loss of cholinergic neurons in the nucleus basalis magnocellularis after intracerebroventricular injection of the toxin. No loss of noncholinergic neurons in the MS was apparent, nor could we detect loss of noncholinergic cerebellar Purkinje cells, which also express p75. Behavioral analysis suggested a spatial learning deficit in anti-murine-p75-SAP-lesioned mice, based on a correlation between a loss of hippocampal ChAT activity and impairment in Morris water maze performance. Our results indicate that we have developed a specific cholinergic immunotoxin for mice. They also suggest possible functional differences in the mouse and rat cholinergic systems, which may be of particular significance in attempts to develop animal models of human diseases, such as Alzheimer's disease, which are associated with impaired cholinergic function.

NGF prevents changes in rat brain glutathione-related enzymes following transection of the septohippocampal pathway

The activities of the enzymes glutathione reductase (GRD), glutathione peroxidase (GPX), and glutathione S-transferase (GST) were studied in several rat brain areas following the aspirative transection of the septohippocampal pathway (fimbria fornix) and the administration of nerve growth factor (NGF) or cytochrome c. One group of animals remained untreated. This lesion resulted in a decreased hippocampal GRD and septal GST activities, as well as, in an increase in GPX activity from the frontal cortex, striatum, and septum. NGF prevented the lesion-induced changes in hippocampal GRD and septal GPX. These findings show that the insult resulting from the aspiration of the fimbria fornix bundle involves modifications in glutathione-related enzymes, and, therefore, in the antioxidant status of brain tissue. These changes in glutathione metabolism could be a consequence of the oxidative damage to GRD and GST proteins or represent a compensatory response of GPX to the oxidative threat The restoring effects of NGF on altered enzyme activities are possibly linked to its known neuroprotective action.

Effects of nerve growth factor on brain glutathione-related enzymes from aged rats

Neurotrophins, like the nerve growth factor (NGF), trigger a variety of biological effects in their targets. Stimulating effects on antioxidant defenses have been postulated to underlie neurotrophic influence on neuron survival and maintenance. To test whether NGF is capable of inducing changes in glutathione-related enzymes in the aged cognitively impaired brain, glutathione reductase (GRD), glutathione S-transferase (GST) and total glutathione peroxidase (GPX) activities were measured in the striatum, septum, hippocampus and frontal cortex of four Sprague-Dawley rat groups: young (2 months old), aged (20 months old) untreated, aged cytochrome c-treated, and aged NGF-treated (icv delivery, 34 micrograms during 28 days). All the aged rats utilized in the study were memory impaired according to their performance in the Morris water maze test. These aged rats showed increases in the activities of septal and hippocampal GST, as well as, in the hippocampal, striatal and cortical GPX. These increases could be interpreted as compensatory responses to cope with the oxidative damage that has been accumulated by the aged brain. The increases in hippocampal and cortical GPX activity were attenuated by NGF treatment, whereas the neurotrophin induced an increase in GRD activity in the striatum of aged rats. These results point out GRD and GPX as possible targets of the neurotrophic effects.

Cell cycle phase-specific surface expression of nerve growth factor receptors TrkA and p75(NTR)

Expression of the nerve growth factor (NGF) receptors TrkA and p75(NTR) was found to vary at the surface of PC12 cells in a cell cycle phase-specific manner. This was evidenced by using flow cytometric and microscopic analysis of cell populations labeled with antibodies to the extracellular domains of both receptors. Differential expression of these receptors also was evidenced by biotinylation of surface proteins and Western analysis, using antibodies specific for the extracellular domains of TrkA and p75(NTR). TrkA is expressed most strongly at the cell surface in M and early G1 phases, whereas p75(NTR) is expressed mainly in late G1, S, and G2 phases. This expression reflects the molecular and cellular responses to NGF in specific phases of the cell cycle; in the G1 phase NGF elicits both the anti-mitogenic effect, i.e., inhibition of the G1 to S transition, and the differentiation response whereas a survival effect is provoked elsewhere in the cell cycle. A model is proposed relating these responses to the surface expression of the two receptors. These observations open the way for novel approaches to the investigation of the mechanism of NGF signal transduction.

Characterization of antibodies to nerve growth factor: assay-dependent variability in the cross-reactivity with other neurotrophins

The neurotrophin family of growth factors consists of proteins sharing a sizable degree of amino acid sequence and structural homology. These similarities greatly increase the probability that antibodies directed against any single neurotrophin may cross-react with other family-members. Various investigators have documented such cross-reactivity can occur under experimental conditions, although the extent of cross-reactivity reported in different studies has varied greatly. Although the use of different antibody preparations may have contributed to the differing degrees of cross-reactivity observed, it is important to note that different assay conditions were also used to evaluate cross-reactivity in the various studies. Little information is currently known about how antibody-antigen interactions vary as a function of the assay conditions used for the evaluation. The present study addressed this question by evaluating the cross-reactivity occurring between various neurotrophins and a single preparation of antibodies directed against purified mouse nerve growth factor-beta (beta-NGF) in a wide variety of immunological assay systems. Our results indicate that cross-reactivity between NGF antibodies and related neurotrophins can vary greatly depending upon the assay system chosen for the evaluation. These data strongly argue against transferring cross-reactivity data across various assay systems.