Characterization of functional nerve growth factor-receptors in a CNS glial cell line: monoclonal antibody 217c recognizes the nerve growth factor-receptor on C6 glioma cells

Engraftment of C6-2B Cells into the Striatum of Aci Nude Rats as a Tool for Comparison of the in Vitro and in Vivo Phenotype of a Glioma Cell Line

The C6-2B is a well-characterized glioma cell line used extensively in the study of malignant glial biology. While the C6-2B cell line has traditionally been thought of as a homogenous cell line, the in vitro phenotype of the C6-2B cell line can vary considerably depending on the culture technique used and the stratum on which the cells are grown. Thus, we asked whether the in vitro phenotype of the C6-2B cell line was significantly different than the in vivo phenotype of the cell line once it was engrafted into the striatum of nude rats. Under culture conditions used in our laboratory, 100% of the C6 cells were found to express p75, the low-affinity nerve growth factor (NGF) receptor, and Major Histocompatability Class I (MHC Class I), while only 10-15% demonstrated vimentin reactivity. Immunohistochemistry was consistently negative for GFAP, trkA (the high-affinity receptor for NGF), CD4, CD8, and a macrophage specific marker (Ox-41). Once engrafted into the striatum of nude rats, the cells remained 100% p75 and MHC Class I positive, and again, only 15% of the cells demonstrated vimentin reactivity. The grafted cells retained this characteristic for 28 days in vivo. Although an immunoincompetent host was selected to minimize the effects an inflammatory response would have on the graft, a transient inflammatory response was detected. During the first week of engraftment, numerous MHC class II cells, some of which were macrophages, were seen infiltrating the graft. However, by 4 weeks postengraftment, no inflammatory cells were appreciated in the graft and surprisingly little reactive gliosis was seen in the penumbra of the tumor mass. Thus, the limited number of in vitro phe-notypic characteristics we examined in the C6-2B cell line remained constant once the cells were engrafted into the striatum of athymic nude rats.

Cinnamon polyphenols regulate S100β, sirtuins, and neuroactive proteins in rat C6 glioma cells

OBJECTIVE

Increasing evidence suggests that cinnamon has many health benefits when used in herbal medicine and as a dietary ingredient. The aim of this study was to investigate the effects of an aqueous extract of cinnamon, high in type A polyphenols, on molecular targets in rat C6 glioma cells that underlie their protective effects.

METHODS

C6 rat glioma cells were seeded in 35-mm culture dishes or six-well plates, then were incubated with cinnamon polyphenols at doses of 10 and 20 μg/mL for 24 h. The targeting protein expression, secretion, and phosphorylation were evaluated by immunoprecitation/immunoblotting and immunofluorescence imaging.

RESULTS

Cinnamon polyphenols significantly enhanced secretion of S100β, a Ca(2+)-binding protein, and increased intracellular S100β expression after 24 h of incubation, in rat C6 glioma cells. Cinnamon polyphenols also enhanced protein levels of sirtuin 1, 2, and 3, deacetylases important in cell survival, and the tumor suppressor protein, p53, and inhibited the inflammatory factors, tumor necrosis factor alpha, and phospho-p65, a subunit of nuclear factor-κβ. Cinnamon polyphenols also up-regulated levels of phospho-p38, extracellular signal-regulated protein and mitogen-activated protein and kinase-activated protein kinases that may be important for prosurvival functions.

CONCLUSION

Our results indicate that the effects of cinnamon polyphenols on upregulating prosurvival proteins, activating mitogen-activated protein kinase pathways, and decreasing proinflammatory cytokines may contribute to their neuroprotective effects.

Dephosphorylation of Akt in C6 cells grown in serum-free conditions corresponds with redistribution of p85/PI3K to the nucleus

Withdrawal of serum from cell cultures constitutes a useful model for the study of mechanisms involved in the regulation of Akt function in vitro. However, there have been several reports of changes in Akt activity that are not fully explained by the current model of phosphatidylinositol 3'-kinase (PI3K)/Akt signaling. We demonstrate the expected loss of Akt phosphorylation in C6 glioma cells cultured in serum-free conditions, yet we also observed a paradoxical increase in PI3K-lipid kinase activity in the same cultures. These events corresponded with relocalization of p85, the regulatory subunit of PI3K, to the perinuclear region and a local increase in PI3K-lipid kinase products. Treatment with platelet-derived growth factor (PDGF) maintained the association between p85 and the PDGF receptor during serum withdrawal and restored PI3K-lipid production at the plasma membrane. Although this protected Akt from dephosphorylation, it only slightly reversed cell-cycle arrest. These effects were not sensitive to treatment with epidermal growth factor, thus precluding a generalized role for growth factors. Our data suggest that loss of growth factor signaling, including PDGF signaling, may disrupt recruitment and/or anchoring of an active p85(PI3K) complex at the plasma membrane during serum withdrawal, which could account for the concurrent loss of Akt function.

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.

Effect of an exo-polysaccharide from the culture broth of Hericium erinaceus on enhancement of growth and differentiation of rat adrenal nerve cells

It was found that an exo-biopolymer (M.W. 1,000,000, molar ratio of 1.5:1.7:1.2:0.6:0.9, glucose:galactose:xylose:mannose:fructose, purity 99%) purified from the liquid culture broth of Hericium erinaceus mycelium enhanced the growth of rat adrenal nerve cells. The polymer also improved the extension of the neurites of PC12 cell. Its efficacy was found to be higher than those from known nerve growth factors such as Nerve Growth Factor (NGF) and Brain-Derived Nerve Factor (BDNF). The effect of two standards has not been observed above 0.1 (mg l(-1)) of supplementation; however, the polymer did show the effect of cell growth and neurite extension at up to 1.0 (mg l(-1)) of addition. While the polymer improved both cell growth and neurite extension, NGF and BDNF did only outgrowth of the neurites. Maximum cell density and length of the neurites were observed as 1.5x10(5) (viable cells ml(-1)) and 230 mum, respectively in adding 0.8 (mg l(-1)) of the biopolymer for 8 days cultivation. The control growth was observed only as 1.2x10(5) (viable cell ml(-1)) of maximum cell density and 140 mum of maximum length, respectively. It was also confirmed that the polymer reacted with the nerve cells within 30 min after adding the sample, compared to 80 min in adding two other growth factors. Number of neurite-bearing cells remained relatively steady in adding the polymer even when the cell growth started to be decreased. It was interesting that the polymer effectively delayed apoptosis of PC12 cells by dramatically reducing the ratio of apoptotic cells to 20% from 50% of the control.

Nerve growth factor plays a divergent role in mediating growth of rat C6 glioma cells via binding to the p75 neurotrophin receptor

Dysregulation of proliferation, differentiation and cell death play a major role in glial tumors, and there is evidence for regulatory mechanisms involving nerve growth factor (NGF) and its receptors in various CNS-derived tumor cell lines. The aim of our study was to observe the effect of exogenous recombinant NGF on C6 rat glioma growth, to characterize the role of endogenous NGF and the p75 neurotrophin receptor (p75) and to rule out whether p75 is necessary to mediate the effect of exogenous NGF. Recombinant exogenous NGF (1-100 ng/ml) was applied under different serum conditions (0%, 1%, 5%) and knockdown of endogenous NGF and p75 was achieved by lipid-mediated antisense oligonucleotide treatment. In presence of serum, NGF had a positive whereas in absence of serum NGF produced a negative effect on C6 cell number. A knockdown of NGF or p75 increased cell numbers and enhanced BrdU incorporation. In p75-knocked down cells NGF did not enhance C6 glioma growth in presence of serum. We conclude that (1) exogenous recombinant NGF enhances C6 glioma growth under serum conditions but decreases cell number in absence of serum, that (2) the effect of exogenous NGF is mediated by p75 alone or by heterodimers containing p75 and that (3) either basal levels of endogenous NGF or basal levels of p75 receptor moderate C6 glioma growth and represent an autoregulatory potential of C6 glioma cells.

TrkA induces differentiation but not apoptosis in C6-2B glioma cells

Nerve growth factor (NGF) binds to the TrkA tyrosine kinase and the p75 neurotrophin receptors. Depending upon which receptor is activated, NGF can induce differentiation or apoptosis. C6-2B glioma cells express the p75 receptor, but NGF decreases their growth only when TrkA is introduced (C6trk). It is unclear, however, whether TrkA reduces C6-2B cell growth by apoptosis or differentiation. To examine which mechanisms account for the anti-proliferative effect of NGF in these cells, we first analyzed whether NGF causes apoptosis by flow cytometry, two-site immunoassay and in situ TUNEL. None of these methods indicated that C6trk undergo apoptosis. Additional apoptotic markers, such as Bcl-2, Bax, Bad, p53, caspase 3, and NF-kappaB were also used. C6trk cells exhibited lower levels of Bcl-2 compared with the parental C6 mock cells, but no changes in the levels of other apoptotic proteins. Moreover, NGF increased AP-1 binding activity in C6trk cells, suggesting that NGF may induce differentiation. We then examined whether TrkA changes the glioma phenotype. In C6trk cells, but not in C6mock cells, NGF enhanced the levels of neuron-specific enolase as well as the levels of A2B5 and 2', 3'-cyclic nucleotide 3'-phosphodiesterase, markers for oligodendrocytes, without affecting the expression of other neuronal markers. Our data suggest that the antiproliferative properties of TrkA may rely on its ability to induce differentiation of C6 cells from undifferentiated glioma to oligodendrocytes.

Serum deprivation and NGF induce and modulate voltage-gated Na(+) currents in human astrocytoma cell lines

Glial tumor cells derived from primary tissue express large voltage-gated Na(+) currents, whereas glioma cell lines usually lack this feature. We studied the effect of serum deprivation on the expression of Na(+) currents in two astrocytoma cell lines (1321N1 and A172). Serum deprivation for more than 2 days sufficed to induce large Na(+) currents in both cell lines; 300 nM of the specific blocker of voltage-gated Na(+) channels, tetrodotoxin, blocked these currents by about 85%. During serum deprivation, the cells also underwent morphological changes that were characterized by cell rounding and outgrowth of processes. Treatment with 100 ng/ml nerve growth factor (NGF) promoted these morphological changes and also accelerated the development of Na(+) currents. In 1321N1 cells, NGF increased the Na(+) current density after short serum deprivation (3--6 d) and changed several gating properties after longer serum deprivation (9--13 d). In comparison with cells from the early culture stage (3--6 d), the steady-state inactivation of the Na(+) current was shifted by -24 mV in NGF-treated cells from the late (9--13 d) culture stage. In untreated cells, this shift was only -13 mV. NGF accelerated the kinetics of inactivation and shifted the current-voltage relationship in cells from the late culture stage by -14 mV. In A172 cells, most of these effects were present already after short serum deprivation either in presence or absence of NGF. It is concluded that in astrocytoma cells, Na(+) currents are induced by serum deprivation and are modulated by NGF. This result supports the idea that NFG controls Na(+) currents in these cells by autocrine stimulation.

Glial cells as targets and producers of neurotrophins

Glial cells fulfill important tasks within the neural network of the central and peripheral nervous systems. The synthesis and secretion of various polypeptidic factors (cytokines) and a number of receptors, with which glial cells are equipped, allow them to communicate with their environment. Evidence has accumulated during recent years that neurotrophins play an important role not only for neurons but also for glial cells. This brief update of some morphological, immunocytochemical, and biochemical characteristics of glial cell lineages conveys our present knowledge about glial cells as targets and producers of neurotrophins under normal and pathological conditions. The chapter discusses the presence of neurotrophin receptors on glial cells, glial cells as producers of neurotrophins, signaling pathways downstream Trk and p75NTR, and the significance of neurotrophins and their receptors for glial cells during development, in cell death and survival, and in neurological disorders.

Mitogenic signaling via endogenous kappa-opioid receptors in C6 glioma cells: evidence for the involvement of protein kinase C and the mitogen-activated protein kinase signaling cascade

As reports on G protein-coupled receptor signal transduction mechanisms continue to emphasize potential differences in signaling due to relative receptor levels and cell type specificities, the need to study endogenously expressed receptors in appropriate model systems becomes increasingly important. Here we examine signal transduction mechanisms mediated by endogenous kappa-opioid receptors in C6 glioma cells, an astrocytic model system. We find that the kappa-opioid receptor-selective agonist U69,593 stimulates phospholipase C activity, extracellular signal-regulated kinase 1/2 phosphorylation, PYK2 phosphorylation, and DNA synthesis. U69,593-stimulated extracellular signal-regulated kinase 1/2 phosphorylation is shown to be upstream of DNA synthesis as inhibition of signaling components such as pertussis toxin-sensitive G proteins, L-type Ca2+ channels, phospholipase C, intracellular Ca2+ release, protein kinase C, and mitogen-activated protein or extracellular signal-regulated kinase kinase blocks both of these downstream events. In addition, by overexpressing dominant-negative or sequestering mutants, we provide evidence that extracellular signal-regulated kinase 1/2 phosphorylation is Ras-dependent and transduced by Gbetagamma subunits. In summary, we have delineated major features of the mechanism of the mitogenic action of an agonist of the endogenous kappa-opioid receptor in C6 glioma cells.

Control of proliferation and survival of C6 glioma cells with modification of the nerve growth factor autocrine system

Nerve growth factor (NGF) plays an important physiological role in differentiation and survival of various types of neurons. Glial cells and glial tumor cells synthesize multiple neurotrophic factors including NGF and secrete them into the surrounding environment; however, the mechanisms of NGF and the significance of NGF receptors have not been studied in detail. The C6 glioma cell line can synthesize NGF, respond to exogenous application of NGF and stimulate the expression of NGF receptor in an autocrine manner. In order to determine the significance of such an NGF autocrine system, the effects of exposure to exogenous NGF and deprivation of endogenous NGF were examined in a C6 glioma cell line in vitro. Exogenous NGF significantly inhibited maintenance of the cell number and thymidine incorporation. Morphological changes, including the formation of growth cones, outgrowth of processes and cellular hypertrophy, were observed, concurrently, indicating that exogenous NGF stimulated differentiation and thereby inhibited proliferation of the cells. Deprivation of endogenous NGF with anti-NGF antibody elicited a rapid decrease in cell number and thymidine incorporation, and led almost all of the cells to death within 8 days. The protein synthesis inhibitor, cycloheximide, strongly inhibited the death of NGF-deprived cells, suggesting the involvement of an active process requiring synthesis of suicide proteins. These findings imply that the NGF autocrine system plays a significant role in regulating the differentiation and survival of C6 glioma cells, similarly to neuronal cells.

The enhancement of growth and differentiation of rat adrenal nerve cells by the addition of conditioned medium from human fibroblast cultures

The growth of rat adrenal nerve cells was remarkably enhanced by supplementing the cultured medium from the human fibroblast cell line, Hs 68. Maximum specific growth rate and length of the neurites were observed as 0.076 (1/hr) and 0.026 mm, respectively in 20% supplement of five day old medium. In adding more than 20% of the cultured medium both cell and neurite growth was severely decreased. It was interesting that the cultured medium from Hs 68 cells could play a role in the extension of the neurites rather than in the growth of neurite cells. It was also found that molecules lower than 50,000 daltons in the conditioned medium could improve the growth of neurite bearing cells and the extension of the neurites than larger molecules. The efficacy of the proteins (<50,000 MW) was similar to that of human nerve growth factor and much better than that of basic fibroblast growth factor which was mainly secreted from human fibroblast cells.

Schwann cells express NDF and SMDF/n-ARIA mRNAs, secrete neuregulin, and show constitutive activation of erbB3 receptors: evidence for a neuregulin autocrine loop

Cultured Schwann cells secreted low levels (30 pg/ml/1.5 x 10(6) cells) of a 45-kDa neuregulin protein and showed constitutive activation of a neuregulin receptor, Erb-B3, suggesting the existence of an autocrine loop involving neuregulins in Schwann cells. RT-PCR analyses indicated that Schwann cells and fibroblasts in culture produced SMDF/n-ARIA and NDF but not GGF neuregulin messages. Schwann cell and fibroblast neuregulin messages encoded both beta and alpha domains; Schwann cell transcripts encoded only transmembrane neuregulin forms while fibroblast messages encoded transmembrane and secreted forms. SMDF/n-ARIA and NDF messages were also expressed in early postnatal rat sciatic nerve, suggesting a role for neuregulins in peripheral nerve development. An anti-neuregulin antibody inhibited the mitogenic response of Schwann cells to cultured neurons and to extracts of cultured neurons or embryonic brain, consistent with the accepted paracrine role of neuregulins on Schwann cells. Surprisingly, the same antibody inhibited Schwann cell proliferation stimulated by several unrelated mitogens including bFGF, HGF, and TGF-beta1. These data implicate both paracrine and autocrine pathways involving neuregulin form(s) in Schwann cell mitogenic responses.

Neurotrophin activates signal transduction in oligodendroglial cells: expression of functional TrkC receptor isoforms

The role of the NT-3 has been implicated in the survival of progenitor oligodendrocytes in culture. The object of this study was to investigate the expression of the TrkC receptor and its responsiveness in glial cells. We report the expression of two TrkC receptor isoforms in rat primary oligodendrocyte cultures, a glial progenitor cell line, CG-4, and in C6 glioma cells. The reverse transcription-polymerase chain reaction-aided amplification of glial trkC with specific primers from the kinase domain, followed by its cloning and sequencing, shows the presence of two trkC transcripts. The sequence of one of the transcripts is homologous to a previously identified trkC isoform which encodes a functional receptor. The other transcript contains a 42-bp insert in the kinase domain. A Western blot of CG-4 and C6 probed with antibody to a TrkC revealed the presence of gp145-kDa protein band. The investigations revealed a rapid autophosphorylation of gp145TrkC in CG-4 and C6 cells in the presence of its specific ligand, NT-3. Furthermore, K252a, a neurotrophin-specific inhibitor, abolishes the NT-3-mediated receptor autophosphorylation. We also examined other NT-3-dependent phosphorylation of cellular substrates in oligodendroglial cells. Interestingly, we observed phosphorylation of phospholipase C gamma-1 in CG-4 and C6 cells, and phosphorylation of phosphatidylinositol 3-kinase in C6 cells in the presence of NT-3. Both the NT-mediated phosphorylation of phospholipase C gamma-1 and phosphorylation of phosphatidylinositol 3-kinase are blocked in the presence of K252a. The detection of the NT-3-mediated early signal transduction events demonstrates that TrkC receptor exhibits NT-3-mediated intracellular response in oligodendroglial cells.

Differential responsiveness of late passage C-6 glial cells and advanced passages of astrocytes derived from aged mouse cerebral hemispheres to cytokines and growth factors: glutamine synthetase activity

In this study, we were interested to compare the responsiveness to growth factors, NGF, b-FGF and EGF and cytokines, IL1 beta, and TNF-alpha, in late passages (74-79) C6 glial cells committed astrocytes and astrocytes of advanced passages (26-28) in cultures derived from aged mouse cerebral hemispheres (MACH). Cultures were grown in either DMEM or chemically defined medium (CDM/TIPS) in order to test the effects of growth factors or cytokines. The activity of glutamine synthetase (GS), a marker for astrocytes, was used as a test parameter. We found that treatment with growth factors increased GS activity in both glial cell culture systems with the exception of EGF in C-6 glial cells. Treatment with cytokines markedly decreased GS activity in the late passage C6 glial cells whereas only TNF-alpha had a similar effect on MACH astrocytes. In view of the generally opposite effects of growth factors and cytokines on GS activity, we speculate that these molecules which are also endogenously present in glial cells may play a role in the maintenance of cellular homeostasis.

Neurotrophins and their trk receptors in cultured cells of the glial lineage and in white matter of the central nervous system

Previous studies have analyzed the expression of different members of the neurotrophin family and their trk receptors in glial cultures composed mainly or exclusively of type-1 astrocytes, whereas only partial data have been published on other cultured glial types. In this article we compare the mRNA levels for neurotrophins (NGF, BDNF, NT-3, NT-4) and their high-affinity receptors (trkA, trkB, trkC) in cultures enriched in specific glial types, such as microglia, type-1 astroglia, and cells of the O/2A lineage (type-2 astroglia and oligodendroglia). Relatively high levels of NGF mRNA (comparable to those observed in adult rat cerebral cortex) are present in all types of cultured glial cells, except for a low level of expression in cultures enriched in microglial cells. In contrast, BDNF mRNA is undetectable in all cultures examined. NT-3 and NT-4 mRNA molecules, at a level equal to that observed in adult rat cerebral cortex, are easily detected in type-1 astrocyte cultures, whereas their hybridization signals are undetectable in cells of the O/2A lineage and in microglial cultures. The analysis of neurotrophin receptor mRNAs confirms the absence of trkA mRNA, the presence of relatively high levels of trkB mRNA (70-100% of cerebral cortex values), and low levels of trkC mRNA (10-18% of cerebral cortex values) in both cultured astroglial and oligodendroglial cells. Only very low levels of trkB and trkC mRNAs are observed in microglial cultures. Although cultured glial cells express mainly mRNAs encoding for the truncated form of trkB and trkC, a low level of mRNA encoding for the full-length catalytic form of these receptors is detected by the sensitive ribonuclease protection assay.

Expressions of nerve growth factor and p75 low affinity receptor after transient forebrain ischemia in gerbil hippocampal CA1 neurons

Expressions of nerve growth factor (NGF) and low affinity p75 NGF receptor (p75 NGFR) in gerbil hippocampal neurons after 3.5-min transient forebrain ischemia were studied. Most hippocampal CA1 neurons were lost (neuronal density = 44 +/- 12/mm) at 7 days after recirculation, while no cell death was found in the sham-control neurons (220 +/- 27/mm). NGF immunoreactivity was normally present in the sham-control hippocampal neurons. However, it decreased in hippocampal CA1 neurons, and slightly decreased in the neurons of CA3 and dentate gyrus areas from 3 hr after recirculation. By 7 days, NGF immunoreactivity returned almost completely to the sham-control level in the CA3 and dentate gyrus neurons but decreased markedly in the CA1 neurons. In contrast, p75 NGFR immunoreactivity was scarcely present in the sham-control hippocampal neurons but was induced from 1 hr after recirculation in the CA1 and CA3 neurons and from 3 hr in the dentate gyrus. At 7 days, p75 NGFR immunoreactivity was expressed greatly in the surviving CA1 neurons and the reactive astrocytes but was not seen in the other hippocampal neurons. The markedly decreased NGF and greatly induced p75 NGFR immunoreactivity found in the CA1 neurons after transient forebrain ischemia suggests that NGF and p75 NGFR may be involved in the mechanism of delayed neuronal death.

In vitro glial responses to nerve growth factor

Nerve growth factor (NGF) stimulates expression of the low affinity neurotrophin receptor p75NGFR mRNA in primary cultures of neonatal rat cortical type I astrocytes. Nerve growth factor treatment altered glial morphology in glial fibrillary acidic protein positive (GFAP+) cell cultures derived from newborn (P0) and 3-day-old (P3) rat pups. When P0- or P3-derived primary glial cultures were serum-deprived, in the presence of 200 pM NGF for 5 days, the flat polygonal glia present in culture assumed a fibrous morphology, an effect not seen in the untreated serum-deprived controls. The NGF effect on astrocytic morphology was blocked by continuous serum treatment. Nerve growth factor did not stimulate astrocytic proliferation under these culture conditions, as assayed by cell cycle analysis using 3H thymidine autoradiography. P0-derived primary glial cultures expressed the signal transducing neurotrophin receptors p145trkB and p140trkA as determined by reverse transcription-polymerase chain reaction (RT-PCR). RT-PCR products were identified by sequencing or restriction enzyme analysis. Astrocytes internalized 125I-NGF at 37 degrees C but not at 4 degrees C, consistent with energy requirements for internalization. Also, internalization of 125I-NGF was abolished by the addition of a 300-1,000-fold excess of unlabeled NGF. Thus, astroglial cells in culture internalize NGF through a specific receptor-mediated process, express trkA and full-length trkB mRNAs at low levels, and respond to exogenous NGF by expressing a fibrous morphology under serum-free culture conditions.

Chondroitin sulfate proteoglycan staining in astrocyte-Schwann cell co-cultures

Transplantation of Schwann cells (SCs) in the central nervous system (CNS) for remyelination in pathological situations has been considered a promising approach. However, numerous studies have indicated that astrocytes have a restrictive effect on SC migration within the CNS. We have previously established an in vitro model which demonstrates the restrictive effect of astrocytes on SCs (Ghirnikar and Eng, Glia 4:367-377, 1994). Using this culture model, in the present study, we have characterized the molecular basis underlying astrocyte-SC interaction and demonstrated chondroitin sulfate proteoglycan (CSP) staining in the co-cultures. Following 1-2 weeks of incubation, CSP staining was specifically associated with SCs co-cultured with astrocytes. Staining with antibodies specific for the different chondroitin sulfate isomers revealed the presence of both, chondroitin-4- and 6-sulfates in SCs. In contrast, SCs when cultured alone, or in the presence of astrocytes conditioned medium did not show CSP staining. These data suggest that CSP staining is associated with SCs following co-culture with astrocytes and mediated by cell to cell contact. We hypothesize that the CSP, alone or in combination with other molecules expressed by astrocytes and/or SCs, may be involved in the restrictive effects of astrocytes on SCs. Identification of molecules involved in the unfavorable interaction between astrocytes and SCs will have an important bearing on efforts to remyelinate demyelinated axons by SC transplantation within the damaged CNS.

Effect of transforming growth factor-beta 1 and -beta 2 on Schwann cell proliferation on neurites

Mechanisms regulating Schwann cell proliferation during development are unclear. Schwann cell division is known to be driven by an unidentified mitogen present on the surface of axons, but it is not known whether other molecules play a role in regulating this proliferation. Transforming growth factor-beta (TGF-beta) which is found in the developing peripheral nervous system (PNS) and is mitogenic for neuron-free Schwann cells in vitro could be involved. We have investigated the effects of TGF-beta 1, TGF-beta 2 and antibodies to TGF-beta 1 and TGF-beta 2 on axon driven Schwann cell proliferation. Rat embryonic dorsal root ganglion neurons (DRG) neurons and Schwann cells from the sciatic nerve were isolated, purified and recombined in vitro. Confirming earlier reports by others, we observed that TGF-beta 1 and TGF-beta 2 added to the culture medium stimulated the proliferation of Schwann cells in the absence of neurons. However, when added to neuron-Schwann cell co-cultures, TGF beta caused a variable response ranging from no effect to moderate inhibition of Schwann cell proliferation in different experiments. A stimulation of Schwann cell proliferation by TGF beta was never observed in neuron-Schwann cell co-cultures. Antibodies to TGF-beta 1 and TGF-beta 2 did not influence axon driven Schwann cell proliferation. To further determine the role of TGF-beta in Schwann cell proliferation and myelination, we studied Schwann cell proliferation in cultures from mice in which the TGF-beta 1 gene was delected by homologous recombination. Neuron-Schwann cell cultures from wild-type, heterozygous and homozygous mice were used. No differences were observed in either Schwann cell proliferation or myelination between cultures obtained from homozygous mutants and their heterozygous and wild-type controls. These findings suggest that TGF-beta does not function as a part of the mitogenic mechanism presented by neurons to Schwann cells, but that the presence of active TGF beta in the cellular environment might regulate the degree of proliferation induced by neuronal contact.

Zone mapping of the binding domain of the rat low affinity nerve growth factor receptor by the introduction of novel N-glycosylation sites

The binding of NGF (nerve growth factor) to the rat low affinity nerve growth factor receptor (p75NGFR) has been studied by site-directed mutagenesis of the receptor. Introduction of non-native N-glycosylation sites within the binding domain indicates that the second of the characteristic cysteine-rich repeats may be particularly important to NGF binding. Two mutants of the second repeat, S42N and S66N, are glycosylated and bind NGF at a drastically reduced level, while still maintaining a conformation recognized by the monoclonal antibody against p75, MC192. Alanine substitution at these sites does not affect NGF binding. Two other mutations that result in local structural changes in the second repeat also greatly decrease binding. One of these altered residues, Ser50, appears to play an essential structural role, since it cannot be replaced by Asn, Ala, or Thr without loss of both NGF binding and MC192 recognition on a Western. Glycosylation of selected sites in the other repeats has little effect on NGF binding or antibody recognition. The introduction of non-native N-glycosylation sites may provide a generally useful scanning technique for the study of protein-protein interactions.

The neuron-restrictive silencer factor (NRSF): a coordinate repressor of multiple neuron-specific genes

The neuron-restrictive silencer factor (NRSF) binds a DNA sequence element, called the neuron-restrictive silencer element (NRSE), that represses neuronal gene transcription in nonneuronal cells. Consensus NRSEs have been identified in 18 neuron-specific genes. Complementary DNA clones encoding a functional fragment of NRSF were isolated and found to encode a novel protein containing eight noncanonical zinc fingers. Expression of NRSF mRNA was detected in most nonneuronal tissues at several developmental stages. In the nervous system, NRSF mRNA was detected in undifferentiated neuronal progenitors, but not in differentiated neurons. NRSF represents the first example of a vertebrate silencer protein that potentially regulates a large battery of cell type-specific genes, and therefore may function as a master negative regulator of neurogenesis.

Overexpression of nerve growth factor in transgenic mice induces novel sympathetic projections to primary sensory neurons

Peripheral nerve crush induces novel projections from noradrenergic sympathetic neurons to sensory ganglia, and it has been suggested that these projections provide an anatomical substrate for chronic pain syndromes that occur after nerve injury. The present study demonstrates that novel sympathetic projections to sensory neurons are also induced in transgenic mice that overexpress nerve growth factor (NGF) in the skin. Specifically, a large proportion of trigeminal neurons in NGF transgenic mice were innervated by tyrosine hydroxylase (TH)-positive pericellular arborizations that were seen only rarely in controls. Electron microscopic analysis of NGF transgenic mice revealed that trigeminal neurons were surrounded by numerous axonal varicosities containing synaptic specializations. Removal of the superior cervical ganglion abolished TH-immunoreactive arborizations in the ipsilateral trigeminal ganglion confirming that these fibers were sympathetic axons. A two-site enzyme-linked immunosorbent assay revealed that transgenic ganglia contained a tenfold increase in NGF peptide compared to controls. However, reverse transcriptase polymerase chain reaction analysis showed no apparent expression of transgene mRNA in sensory ganglia, suggesting that the additional NGF was derived from increased NGF expression in the skin. These results indicate that NGF can induce novel sympathetic projections to sensory neurons in vivo and suggests a model in which increased NGF expression plays a role in the development of sympathetic hyperalgesia after nerve injury.

Induction of nerve growth factor responsiveness in C6-2B glioma cells by expression of trkA proto-oncogene

Cells that lack the high affinity receptor component (trkA) for nerve growth factor (NGF) are unresponsive to NGF. We investigated whether C6-2B cells, a rat glioma derived cell line, express trkA and, as a consequence, are responsive to NGF. In these cells, NGF (100 ng/ml) failed to induce the mRNA encoding for c-fos protooncogene and the low affinity NGF receptor p75NGFR, two NGF-responsive genes. In contrast, both mRNAs were induced in PC12 cells by NGF. Using a RNase protection assay with a cRNA probe for rat trkA, the expected trkA RNA protected fragment was detected in PC12 but not in C6-2B glioma cells, indicating that C6-2B cells either do not express the gene or express it only in low amounts. Cross-linking of 125I-labeled NGF to PC12 cells identified two major bands with an apparent molecular weight of 158 kDa and 100 kDa corresponding to trkA and p75NGFR, respectively. In contrast, only the 100 kDa band could be detected in C6-2B cells by cross-linking analysis. In C6-2B cells stably transfected with the rat trkA cDNA, NGF increased c-fos mRNA, induced tyrosine phosphorylation of gp140trk, and SNT (suc-associated neurotrophic factor-induced tyrosine-phosphorylated target), and caused morphological changes within 72 h. All of these effects of NGF were blocked by the protein kinase inhibitor K-252a suggesting that NGF signal transduction was restored by trkA expression. Most important, in C6trk+ cells, NGF was a weaker (2-fold) inducer of [3H]thymidine incorporation when compared to bFGF (5-fold), suggesting that expression of trkA fails to confer to NGF a strong mitogenic effect. Our findings indicate that C6-2B glioma cells do not possess high affinity NGF receptor and thus are unresponsive to NGF and that expression of trkA in neuroectoderm derived cells elicits some of the NGF responses characteristic of neuronal cells.

Target-deprived CNS neurons express the NGF gene while reactive glia around their axonal terminals contain low and high affinity NGF receptors

Reactive gliosis is part of the response of central nervous system to injury and neurodegeneration. Cellular components of the reactive gliosis have the capability to synthesize neurotrophic factors, and thus are capable of affecting the fate of neuronal populations in the injured tissue. In this study, we explored the putative involvement of reactive glia-derived neurotrophins in sustaining the axonal projections of target-deprived neurons. Neuronal targets of the dorsal column nuclei neurons were suppressed through excitotoxic lesion of the ventrobasal complex of the rat thalamus (VB). Despite the development of reactive gliosis, neither up-regulation of NGF, nor BDNF or NT3 mRNA could be detected by solution hybridization in the lesioned site at all times tested. In contrast, expression of the LNGFR gene increased progressively up to 90 days post-lesion. Immunocytochemical studies localized the LNGFR protein in a subset of small cells with ramified processes resembling microglia at 7 and 20 days post-lesion. At longer times, double immunolabelling studies revealed that a substantial part of LNGFR-immunoreactive cells filling the area of neuronal loss were neither microglial cells nor astrocytes although presence of LNGFR in a subset of microglial cells could not be excluded. Previous ultrastructural studies of the kainate-lesioned VB suggest that these LNGFR-immunoreactive cells correspond to oligodendrocytes and/or Schwann cells. At 2 months post-lesion, when LNGFR expression was maximal, increased levels of trkA mRNA were detected in the lesioned site. Immunocytochemical studies revealed the presence of numerous trkA-immunoreactive astrocytes. TrkB mRNA, encoding the full-length high-affinity receptor for BDNF, remained undetectable by non-isotopic in situ hybridization. In contrast to the lack of neurotrophin gene expression by glial components of the lesioned VB, dorsal column nuclei neurons contained NGF mRNA as revealed by in situ hybridization studies at 10 days--prior to enhanced LNGFR expression in the lesion--and 2 months post-lesion. In addition, the number and the staining intensity of NGF mRNA-positive neurons was increased in the target-deprived neurons, as compared with the contra-lateral nucleus projecting to intact targets. These results show that glial cells present in a reactive gliosis which develops in the kainic acid-lesioned thalamus, do not synthesize neurotrophins but instead produce high levels of both low- and high-affinity NGF receptors, LNGFR by Schwann cells/oligodendrocytes and possibly a subset of microglial cells, and trkA by reactive astrocytes.(ABSTRACT TRUNCATED AT 400 WORDS)

Nerve growth factor: a mitogenic signal for retinal Müller glial cells

Knowledge of the effects of nerve growth factor (NGF) on glia is limited. A CNS site where NGF-glial interactions may occur is the retina. NGF is endogenous to the retina, and the retinal Müller glial cells have NGF receptors. Here, we examined the possibility that NGF may be a mitogen for Müller glial cells, which often proliferate in response to pathophysiological conditions. Experiments were performed on cultured glial cells from the adult human retina. Exposure of cultured Müller glial cells to 2.5 S NGF under serum-free conditions resulted in a concentration-dependent increase in cell number and bromodeoxyuridine incorporation into nuclei. The half-maximally effective concentration was 0.04 ng/ml (1.5 pM), consistent with activation of high affinity NGF receptors. K252a, a blocker of the neurotrophin family of tyrosine kinase-linked receptors, potently inhibited the proliferative effect of NGF. Transforming growth factor beta-2, another growth factor endogenous to the retina, inhibited the mitogenic response to NGF. These findings indicate that human Müller glial cells in culture express functional NGF receptors and that the response of Müller cells to NGF can be modulated by other growth factors.

SR13/PMP-22 expression in rat nervous system, in PC12 cells, and C6 glial cell lines

SR13/PMP-22 is a protein that was identified after screening a sciatic nerve cDNA library. Our study focused on comparing the level and pattern of expression of SR13/PMP-22 protein and RNA. Northern blot analysis revealed that although SR13/PMP-22 mRNA was present in all nervous tissues and cells studied, levels were at least seven fold higher in the sciatic nerve and the spinal cord. During sciatic nerve postnatal development and maturation, the SR13/PMP-22 mRNA was detected at 2 days after birth, reached a maximal level at day 24, and decreased to 1/3 of the maximum in adult animals. Nerve transection reduced the level of SR13/PMP-22 mRNA to less than 5% in the segment distal to the nerve injury. Experiments using in situ hybridization localized the SR13/PMP-22 mRNA in Schwann cells. Schwann cells present in the vicinity or distal to the nerve cut repressed the signal for the message. In situ hybridization experiments also demonstrated that dorsal root ganglia satellite cells contained the message for SR13/PMP-22. The SR13/PMP-22 antisera used in our study showed a complex pattern of staining. As expected, the SR13/PMP-22 antibody peptide 1 immunoreacted with the sciatic nerve sheath. However, immunocytochemistry of the dorsal root ganglia revealed that the staining was contained in the neuron's cell body and processes and also in satellite cells. We also identified immunoreactive cell bodies and fibers in the spinal cord dorsal horn. Tissue culture studies demonstrated that SR13/PMP-22 mRNA is induced in NGF treated PC12 but not in C6 glioma cell lines grown under experimental conditions that stimulated cell growth arrest. Our experiments suggest that SR13/PMP-22 may have some other function(s) in addition to its hypothesized role in peripheral myelination.

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

The glioma cell line C6 was used to study the expression and growth-dependent regulation of the nerve growth factor (NGF) tyrosine kinase receptor gp140trk, which is the mature protein product of the trk proto-oncogene. Chemical cross-linking of 125I-NGF to C6 cells, followed by immunoprecipitation with polyclonal anti-NGF antibodies and separation by polyacrylamide gel electrophoresis, revealed the presence of 90-95 and 150 kDa species. Immunocytochemical staining of C6 cells with antibodies directed against either the low-affinity NGF receptor gp75NGFR or trk proto-oncogene products demonstrated a heterogeneous cellular distribution of both antigens. Brief treatment of C6 cells with NGF led to the tyrosine phosphorylation of 80, 110 and 140 kDa protein species, as detected on anti-phosphotyrosine Western blots. Similar molecular weight species were found with anti-Trk antibodies in the NGF-treated cells. Intracellular localization of Trk-like immunoreactivity in C6 cells released from a growth-arrested state indicated an initial immunostaining of the nuclear periphery, progressing to cytoplasmic vesicles and finally to the plasma membrane. These observations at the light microscopic level were confirmed using immunoelectron microscopy with the same anti-Trk antibodies, and showed clearly the trafficking of Trk-like immunostained particles from the endoplasmic reticulum to the plasmalemma. The cellular localization of trk gene products also appeared to depend on their glycosylation state. Such growth-dependent expression of NGF receptors on glial cells may be important in controlling autocrine regulatory processes of glia to NGF, which these cells produce.

Regulation of vasoactive intestinal peptide expression in sympathetic neurons in culture and after axotomy: the role of cholinergic differentiation factor/leukemia inhibitory factor

Vasoactive intestinal peptide (VIP) expression increases in sympathetic neurons when they are grown in dissociated cell or explant cultures and when they are axotomized in vivo. In dissociated cell culture, the magnitude of the VIP increase was reduced when nonneuronal cells were removed and medium conditioned by ganglionic nonneuronal cells increased VIP in neuron-enriched cultures. Antiserum against cholinergic differentiation factor (also leukemia inhibitory factor; CDF/LIF), but not against ciliary neurotrophic factor, immunoprecipitated this activity. Medium conditioned by sympathetic ganglion explants also contained a VIP-stimulatory molecule that was immunoprecipitated by CDF/LIF antiserum, and CDF/LIF antiserum partially blocked VIP induction in explants. CDF/LIF mRNA was increased in dissociated cell cultures, in ganglion explants and in vivo after axotomy. Our results suggest that CDF/LIF released from ganglionic nonneuronal cells plays an important role in regulating VIP after axotomy.

Grafting of fast blue labeled glial cells into neonatal rat brain: differential survival and migration among cell types

Cultures of oligodendrocyte progenitor cells, ERD 1.1 cells, a nontransformed immortalized cell line of oligodendrocyte progenitors and C6 glioma cells were labeled with the fluorescent dye Fast Blue and transplanted into brains of 4 day postnatal Wistar rat pups. The localization of fluorescent cells within host brain was examined at various times post-transplantation to determine patterns of cell migration as well as survival and integration among the host tissue. Oligodendrocyte progenitors migrated mainly along white matter tracks, integrating successfully into the host parenchyma. High survival rates were found between 5 and 27 days post grafting. ERD 1.1 cells survived and migrated between 1 and 5 days after transplantation. However, by 27 days survival had dropped from 60 to 20% of the initial cell population. The surviving cells were mainly localized to subventricular and subependymal regions at 27 days. C6 cells migrated extensively rostrally and caudally from the site of injection in the hippocampus and were tumorogenic. This finding confirmed previous reports on the survival and migration patterns of oligodendrocyte progenitors grafted into neonatal brain. However, they show that two cell lines that share phenotypic properties of oligodendrocyte progenitors markedly differ from these cells with respect to migration patterns and integration within host parenchyma. Fast Blue dye was still detectable after repeated cell division in grafted C6 cells, enabling us to track single cells as well as tumor formation. This dye should be useful not only to address issues of development, but also of tumor biology and therapeutic treatment.

Nerve growth factor and p75NGFR factor receptor mRNA change in rodent CNS following stress activation of the hypothalamo-pituitary-adrenocortical axis

The synthesis of nerve growth factor (NGF) by the hippocampus raises the possibility that NGF may play a role in the regulation of the hypothalamic-pituitary-adrenal axis (HPAA). Subchronic cold stress has been shown to activate the HPAA in a mild noninvasive manner, to stimulate serum glucocorticoid levels, and to perturb NGF binding in hippocampus and basal forebrain. One or repeated episodes of cold stress increased NGF mRNA levels in the hippocampus and p75NGFR mRNA levels in the basal forebrain. These changes were not due to elevated serum glucocorticoid levels since treatment with exogenous corticosterone had no effect on NGF and p75NGFR mRNA levels. Adrenalectomy did not prevent the stress induced increases in NGF and p75NGFR mRNA.

Establishment and neurite outgrowth properties of neonatal and adult rat olfactory bulb glial cell lines

Two glial cell types surround olfactory axons and glomeruli in the olfactory bulb (OB) and may influence synapse development and regeneration. OB astrocytes resemble type-1 astrocytes, and OB ensheathing cells resemble non-myelinating Schwann cells. We have produced clonal OB astrocyte and ensheathing cell lines from rat neonatal and adult OB cultures by SV40 large T antigen transduction. These cell lines have been characterized by morphology, growth characteristics, immunophenotype, and ability to promote neurite outgrowth in vitro. Neonatal and adult ensheathing cell lines were found to support higher neurite outgrowth than OB astrocyte lines. Neonatal OB astrocyte lines were of two types, high and low outgrowth support. The low support astrocyte lines express J1 and a chondroitin sulfate-containing proteoglycan as do astrocytes encircling the neonatal glomeruli in vivo. The adult OB astrocyte cell lines supported lower levels of outgrowth than adult ensheathing cell lines. These results are consistent with a positive role for ensheathing cells in OB synapse regeneration, in vivo. Further, based on our results, we hypothesize that ensheathing cells and high-outgrowth astrocytes facilitate axon growth in vivo, while low outgrowth astrocytes inhibit axon growth and may facilitate glomerulus formation.

CNS glial cells express neurotrophin receptors whose levels are regulated by NGF

Normal CNS glial cells manufacture neurotrophin receptors and are competent to respond to NGF. Neurotrophins bind a common receptor (LNGFR) and ligand-specific, tyrosine kinase-containing subunits (TrkA, TrkB, or TrkC). Northern blots and transcription assays reveal complex transcriptional regulation of LNGFR in astrocytes; from undetectable basal levels, NGF dramatically induces LNGFR within 4-6 h. Oligodendrocytes' relatively high basal levels are unaffected by NGF. TrkA mRNA was undetectable, however, TrkB was present and upregulated by NGF in astrocytes but not oligodendrocytes. The results are consistent with receptor autoregulation by its ligand and suggest that NGF plays a role in normal glial functions.

Effects of nerve growth factor and acetyl-L-carnitine arginyl amide on the human neuronal line HCN-1A

The HCN-1A clonal cell line, derived from the cortical tissue of a patient with unilateral megencephaly, was shown to differentiate into a mature neuronal-like state in the presence of the nerve growth factor, dibutyryl cyclic adenosine, 3',5'-monophosphate and either 1-isobutyl-3-methylxanthine or forskolin. Differentiation was assessed by measuring the percentage of cells that displayed branched, varicose processes that stained for synaptophysin. Treatment of cultures with a cocktail containing forskolin increased immunocytochemical staining for gamma aminobutyric (GABA), neurofilament protein and the nerve growth factor receptor species p75NGFR. Treatment with acetyl-L-carnitine alone had some effects on the cell morphology while acetyl-L-carnitine arginyl amide and nerve growth factor together increased the GABA content. Positive staining levels for the neurotransmitters gamma aminobutyric acid, glutamate, somatostatin, cholecystokinin and vasoactive intestinal polypeptide were measured quantitatively for HCN-1A under basal conditions.

Nerve growth factor (NGF) receptors in a central nervous system glial cell line: upregulation by NGF and brain-derived neurotrophic factor

The neurotrophic proteins nerve growth factor (NGF) and brain-derived neurotrophic factor (BDNF) are related in their primary amino acid structures. In this study we investigated the extent to which the low-affinity NGF receptor (LNGFR) in C6 glioma cells can discriminate between the neurotrophins NGF and BDNF. LNGFR-immunoreactivity (IR) was studied in C6 cells treated for 16 hr with NGF (50 ng/ml) or BDNF (10 ng/ml), using immunogold labelling and electron microscopic morphometric analysis. The cells were exposed to the anti-NGFR antibody 192-IgG, followed by immunoglobulin conjugated with colloidal gold. Untreated C6 cells exhibited some surface gold label (positive LNGFR-IR). Cells treated with NGF or BDNF displayed significantly increased LNGFR-IR on all surfaces in terms of gold labeling, which was more pronounced in NGF-treated cells. LNGFR-IR was also localized in coated endocytotic vesicles, in smooth endoplasmic reticulum, and in secondary multivesicular lysosomes in neurotrophin-treated and untreated cells. The increase in LNGFR protein was further substantiated by a correspondingly higher content of LNGFR mRNA detected after 15 hr of either NGF or BDNF treatment. These results suggest that the LNGFR in glial cells can be upregulated by the structurally related neurotrophins NGF and BDNF.

Division and migration of satellite glia in the embryonic rat superior cervical ganglion

While distinct precursors committed to a neuronal or glial cell fate are generated from neural crest cells early in peripheral gangliogenesis, little is known about the subsequent generation and maturation of young satellite glia from restricted glial precursor cells. To examine the division and migration of glial precursor cells and their satellite cell progeny, morphological, immunocytochemical and culture techniques were applied to the developing rat superior cervical ganglion. At embryonic day (E)18.5, numerous clusters of nonneuronal cells appeared transiently in the ganglion. Individual cells with a similar morphology were present in E16.5 ganglia, and are likely to represent the precursor cells which generate these clusters. The clustered cells were distinguishable from neighbouring neurons as well as from endothelial cells and fibroblasts. Morphologically similar cells were present in nerve bundles at E18.5 and surrounding principal neurons and nerve bundles in the adult ganglion. Double-label studies of the E18.5 ganglion with tyrosine hydroxylase to identify noradrenergic neurons and propidium iodide counterstaining to visualize all cell nuclei revealed that the cells in clusters stained with propidium iodide but lacked tyrosine hydroxylase immunoreactivity. To determine if cell clusters arose from division, bromodeoxy-uridine, a thymidine analogue, was administered to pregnant mothers between E16.5-E18.5, and ganglionic cells examined at E18.5 both in vivo and in vitro. Numerous non-neuronal cells divided during this period in situ and composed portions of clusters. When dissociated, superior cervical ganglion satellite glia reacted with an NGF-receptor antibody (MAb 217c) and possessed a flattened shape, in contrast to bipolar Schwann cells. Over half of the 217c-immunoreactive glia at E18.5 had incorporated bromodeoxyuridine during E16.5-18.5 in vivo. At birth, non-neuronal cells were no longer grouped in clusters, but were associated with neuronal cell bodies and processes. These findings suggest that, between E16.5-E18.5, glial precursors divide rapidly to form clusters, and that, after the peak of neurogenesis, daughter cells migrate within the ganglion to associate with nerve cell bodies and processes where proliferation continues at a slower rate. Distinct cellular and molecular interactions are likely to trigger the initial rapid division of glial precursors, initiate their migration and association with neuron cell bodies, and control their subsequent slower division.

Expression of p75NGFR TrkA, and TrkB mRNA in rat C6 glioma and type I astrocyte cultures

Using a quantitative reverse transcription-polymerase chain reaction (RT-PCR) we have investigated the expression of the neurotrophin receptors p75NGFR, trkA, and trkB mRNAs in cultures of rat pup type I astrocytes and in the C6 rat glioma cell line. All three neurotrophin receptor mRNAs are expressed in both C6 cells and in type I astrocytic cultures. p75NGFR mRNA levels are increased by either cycloheximide or nerve growth factor (NGF) treatment of C6 cells as measured using RT-PCR. Type I astrocyte cultures also expressed p75NGFR mRNA and NGF treatment increased p75NGFR mRNA levels in these cultures. TrkB mRNA levels were increased by cycloheximide treatment of type I astrocyte cultures but not by NGF treatment. Using RT-PCR, trkA mRNA was detected in astrocytic cultures as well as in the rat C6 and PC-12 cell lines. We conclude that cultures of type I astrocytes express active NGF receptors and that glia can elicit a response to NGF as seen by an increase in p75NGFR mRNA levels following exposure to NGF.

Synthesis of glial fibrillary acidic protein in rat C6 glioma in chemically defined medium: cyclic AMP-dependent transcriptional and translational regulation

Glial fibrillary acidic protein (GFA) expression was induced in rat C6 glioma in chemically defined medium by the addition of N6, O2'-dibutyryl cyclic AMP (dbcAMP). Induction was dependent on the increase in intracellular cyclic AMP (cAMP), which was linearly correlated with added dbcAMP. Contrary to GFA mRNA synthesis, which can be obtained by cAMP-dependent and -independent pathways, translation of mRNA into GFA was observed only above a cellular cAMP concentration of approximately 0.2 fmol/cell. dbcAMP stimulation did not affect the vimentin concentration, which remained at a low level, but changed the cellular morphology from a bipolar to a stellate shape. A similar morphological change was observed after stimulation of C6 with lipopolysaccharide (LPS). However, LPS did not significantly increase the intracellular concentration of cAMP and the LPS-induced mRNA was not translated into GFA. Our results indicate that GFA synthesis is regulated at the mRNA level and at the translational level and that a cAMP-dependent mechanism determines the ultimate synthesis of GFA by a yet unknown mechanism.

The use of nerve growth factor as a reverse transforming agent for the treatment of neurogenic tumors: in vivo results

The rationale behind the evaluation of natural differentiating agents, such as nerve growth factor (NGF), for reverse transforming potential is based on the theory that such compounds may represent a nontoxic means of controlling tumor growth. Previous in vitro experiments have shown that NGF is capable of retarding growth and of inducing persistent differentiation of neurogenic tumor cell lines. In vivo, NGF is capable of causing a persistent reduction in the number of ethylnitrosourea-induced neurinomas and of increasing survival time following intracerebral implantation of F98 anaplastic glioma cells. In this study, anaplastic glioma and neurinoma implants were treated with NGF to evaluate the reverse transforming potential of NGF in vivo. Results indicate that NGF is capable of causing a significant decrease in the growth rate of subcutaneous T9 (anaplastic glioma) and clone 16 (anaplastic neurinoma) implants. Significantly, NGF treatment was accompanied by adverse effects that were minimal and transient. Continued tumor growth (although greatly retarded) following NGF treatment is an aspect that requires further investigation. However, the results of this study suggest that NGF may prove useful, alone or in combination with other types of therapy, for the treatment of tumors of neurogenic origin.

Tyrosine hydroxylase and neuropeptide Y are increased in ciliary ganglia of sympathectomized rats

We have examined the expression of tyrosine hydroxylase and neuropeptide Y in ciliary ganglia of normal adult rats and of adult rats in which the environment of these neurons was altered by sympathectomy at birth. Following neonatal 6-hydroxydopamine treatment, the proportion of tyrosine hydroxylase-immunoreactive and neuropeptide Y-immunoreactive neurons in ciliary ganglia was significantly increased. In ciliary neurons of both control and sympathectomized rats, neuropeptide Y immunoreactivity was preferentially co-localized with tyrosine hydroxylase. Immunoblot analysis confirmed the presence of tyrosine hydroxylase and its increase following sympathectomy. In situ hybridization studies revealed that many ciliary neurons contain mRNA for tyrosine hydroxylase and for neuropeptide Y. Like tyrosine hydroxylase immunoreactivity, the number of ciliary neurons containing tyrosine hydroxylase mRNA and the amount of mRNA per cell were increased in 6-hydroxydopamine-treated rats. In contrast, neuropeptide Y mRNA levels were the same in control and 6-hydroxydopamine-treated rats. Nerve growth factor is a candidate for mediating the effects of sympathectomy and most ciliary neurons in control and sympathectomized rats expressed immunoreactivity for the low-affinity nerve growth factor receptor. In addition, ciliary neurons from 6-hydroxydopamine-treated animals possessed increased nerve growth factor receptor immunoreactivity. These studies indicate that both tyrosine hydroxylase and neuropeptide Y in the ciliary ganglion are regulated by alterations in their environment. Their expression was enhanced by chemical sympathectomy which does not affect ciliary neurons directly but, rather, removes sympathetic innervation of shared targets, including the iris. In situ hybridization analysis suggests that the increased tyrosine hydroxylase and neuropeptide Y levels result from different mechanisms and provides evidence that neuropeptide levels can be regulated without changes in mRNA levels.

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.

Cytoskeletal elements regulate the distribution of nerve growth factor receptors in PC12 cells

Nerve growth factor receptor (NGFR)-like immunoreactivity (IR) was studied in PC12 cells treated for 96 hr with NGF (40 ng/ml), using immunogold labeling and electron microscopic morphometric analysis. The cells were exposed to the anti-NGFR antibody 192-IgG, followed by immunoglobulin (IgG) conjugated with colloidal gold. PC12 cells exhibited occasional gold label (positive NGFR-IR) on all surfaces. Cells treated with colcemid (0.05 micrograms/ml) or cytochalasin D (2 micrograms/ml), which limit microtubule (MT) and microfilament (MF) formation, respectively, displayed an increased NGFR-IR in terms of gold labeling. NGFR-IR was also seen on taxol (0.85 micrograms/ml)-exposed cells, an agent that promotes MT assembly. Cells treated simultaneously with cytochalasin D and taxol had a dramatically augmented NGFR-IR on their surfaces, which exceeded levels obtained with either agent alone. Prominent NGFR-IR was localized frequently in coated endocytotic vesicles, in smooth endoplasmic reticulum, and in secondary multivesicular lysosomes, in both treated and untreated cells. The results suggest that a large number of NGFRs (positive NGFR-IR) in PC12 cells are cryptic and not available for ligand binding. Changes in cytoskeletal organization that may affect mobility of integral membrane proteins can modulate the distribution of NGFR-IR on neuronal surfaces.

The olfactory nerve contains two populations of glia, identified both in vivo and in vitro

The peripheral olfactory nervous system exhibits, uniquely, neuronal cell body replacement and reestablishment of central connections in adult mammals. The role of the olfactory nerve glia in these phenomena is unknown, but information might be provided by in vitro systems. This paper reports on the characterization of olfactory nerve glia in dissociated cell cultures of newborn rat nasal mucosal tissues. The predominant type of glial cell resembled Schwann cells and immunostained for the S-100 protein, found in all glial cell types; glial fibrillary acidic protein (GFAP), found in astrocytes and nonmyelinating Schwann cells; and showed binding of 217C, a monoclonal Schwann-cell marker that binds to the low-affinity NGF receptor in glioma cells. They were negative for A2B5. The Schwann-cell-like olfactory glia changed morphology upon culturing in serum-free medium, with further shape changes after plating on laminin. Plating on laminin increased cell numbers. A second population, found only after GFAP-immunostaining, was astrocyte-like in morphology and represented approximately 10 percent of all glial cells. These were S-100-, A2B5-, and 217C-negative, a unique glial cell immunological profile. At low dilutions of anti-GFAP (1/10,000), or with weak fluorescent secondary antibodies, astrocyte-like glia were immunostained but Schwann-cell-like glia were not detectable. Astrocyte-like glia were not an artifact of the dissection, since they were detectable in tissue sections of newborn-rat olfactory nerves immunostained with a low dilution of anti-GFAP. The presence of two types of glial cells in culture suggests similarities between olfactory glia and enteric glia.