Factors controlling the expression of the NGF receptor in PC12 cells

Neurotrophin-induced upregulation of p75NTR via a protein kinase C-delta-dependent mechanism

Neurotrophins exert their biological effects via p75NTR and Trk receptors. Functional interplay between these two receptors has been widely explored with respect to p75NTR enhancing the activation and signalling of Trk, but few studies address the bidirectional aspects. We have previously demonstrated that the expression of p75NTR can be differentially modulated by different Trk receptor mutations. Here we investigate the mechanism of Nerve Growth Factor (NGF)-induced upregulation of p75NTR expression. We utilize pharmacological inhibition to investigate the role of various TrkA-associated signalling intermediates in this regulatory cascade. Notably, the inhibition of phospholipase C-gamma (PLC-gamma) using U73122, prevented the NGF-induced upregulation of p75NTR protein and mRNA. The inhibition of protein kinase C-delta (PKC-delta) activation by rottlerin, a selective PKC-delta inhibitor, and by small interfering RNA (siRNA) directed against PKC-delta also inhibited this NGF-induced upregulation. Finally, we also show that in cerebellar granule neurons, BDNF acting via TrkB increases p75NTR expression in a PKC-delta dependent manner. These results indicate the importance of Trk-dependent PLC-gamma and PKC-delta activation for downstream regulation of p75NTR protein expression in response to neurotrophin stimulation, a process that has implications to the survival and growth of the developing nervous system.

TrkA NGF receptor plays a role in the modulation of p75NTR expression

The cellular response to nerve growth factor (NGF) is mediated by two structurally unrelated receptors, TrkA and p75 neurotrophin receptor (p75NTR), which have been shown to interact resulting in reciprocal modulation of function. In this study, we have examined the modulation of p75NTR protein expression by specific TrkA autophosphorylation sites in the presence or absence of NGF. We have used cell lines derived from PC12 cells that express either no endogenous TrkA (PC12nnr5) or TrkA receptors mutated via site-directed mutagenesis to abrogate individual tyrosine autophosphorylation sites on the cytoplasmic tail (Y490F, Y785F and Y490/785F). Results indicate that in the absence of TrkA in PC12nnr5 cells there is reduced constitutive p75NTR expression, which can be restored to different degrees by transfection of the Y490F TrkA or the Y490/785F TrkA, but not by transfection of the Y785F TrkA. In addition, the expression of p75NTR was upregulated in the presence of NGF in the parental and Y490F cell lines only. Together these results indicate a role for the individual tyrosine autophosphorylation sites of TrkA in regulating p75NTR expression.

Schwann cell apoptosis in the postnatal axotomized sciatic nerve is mediated via NGF through the low-affinity neurotrophin receptor

Schwann cell death is a developmentally regulated phenomenon and is also induced after peripheral nerve axotomy in neonatal rodents. In this study, we explored whether ligand-induced activation of the low-affinity neurotrophin receptor (p75(NTR)) is responsible for inducing Schwann cell death in vivo. Administration of exogenous nerve growth factor (NGF) to the axotomized nerve site in wild-type animals resulted in a 2.6-fold increase in Schwann cell apoptosis in the distal nerve stumps compared to axotomy alone. No increase in apoptosis, above baseline levels, was seen in p75(NTR)-mutant mice either with or without NGF When anti-NGF antibodies were administered to the site of the peripheral nerve lesion in wild-type mice there was a reduction in the percentage of Schwann cell apoptosis to levels seen in both the quiescent state and in the axotomized nerves of the p75(NTR)-mutant mice. These results demonstrate that apoptosis of Schwann cells in axotomized peripheral nerve is mediated predominantly through p75(NTR) signaling and initiated via endogenously produced NGF.

Nerve Growth Factor Regulates Expression of the Nerve Growth Factor Receptor Gene in Adult Sensory Neurons

Sensory neurons of the adult rat dorsal root ganglion (DRG) can be maintained in culture in the absence of nerve growth factor (NGF). We have thus used dissociated cultures of these neurons to study effects of NGF on the regulation of expression of mRNA encoding the nerve growth factor receptor (NGF-R). In the absence of NGF, levels of NGF-R mRNA remained constant for 7 days in cultures of adult rat DRG neurons. In the presence of NGF, NGF-R mRNA levels rose two - three-fold after 2 days, reaching plateau levels (five - six-fold elevation) after 5 days. This NGF-induced up-regulation could be demonstrated even after prior NGF-deprivation for 3 - 4 days. NGF had no effect upon NGF-R mRNA levels in DRG non-neuronal cells. Epidermal growth factor (EGF), fibroblast growth factor (FGF) and ciliary neurotrophic factor (CNTF) were without effect on NGF-R mRNA levels, but 8-bromo-cAMP decreased NGF-R mRNA levels by 65% after 2 days. NGF also induced a rapid (30 min) rise in expression of c-fos in DRG neurons, but not in non-neuronal cells. Our results suggest that endogenous levels of NGF may regulate the expression of NGF-R in vivo.

Evidence for a role of mixed lineage kinases in neuronal apoptosis

Superior cervical ganglion (SCG) sympathetic neurons die by apoptosis when deprived of nerve growth factor (NGF). It has been shown previously that the induction of apoptosis in these neurons at NGF withdrawal requires both the activity of the small GTP-binding protein Cdc42 and the activation of the c-Jun N-terminal kinase (JNK) pathway. The mixed lineage kinase 3 (MLK3) belongs to a family of mitogen-activated protein (MAP) kinase kinase kinases. MLK3 contains a Cdc42/Rac interactive-binding (CRIB) domain and activates both the JNK and the p38 MAP kinase pathways. In this study the role of MLK3 in the induction of apoptosis in sympathetic neurons has been investigated. Overexpression of an active MLK3 induces activation of the JNK pathway and apoptosis in SCG neurons. In addition, overexpression of kinase dead mutants of MLK3 blocks apoptosis as well as c-Jun phosphorylation induced by NGF deprivation. More importantly, MLK3 activity seems to increase by 5 hr after NGF withdrawal in both differentiated PC12 cells and SCG neurons. We also show that MLK3 lies downstream of Cdc42 in the neuronal death pathway. Regulation of MLK3 in neurons seems to be dependent on MLK3 activity and possibly on an additional cellular component, but not on its binding to Cdc42. These results suggest that MLK3, or a closely related kinase, is a physiological element of NGF withdrawal-induced activation of the Cdc42-c-Jun pathway and neuronal death. MLK3 therefore could be an interesting therapeutic target in a number of neurodegenerative diseases involving neuronal apoptosis.

Selective regulation of trkC expression by NT3 in the developing peripheral nervous system

We have studied the influence of neurotrophin-3 (NT3) on the expression of its receptor tyrosine kinase, trkC, in embryonic mice. The expression of trkC transcripts encoding full-length and kinase-deficient receptors was almost entirely restricted to neurons in the trigeminal ganglion and increased markedly throughout development. In NT3(+/-) embryos, the level of trkC mRNA in the trigeminal ganglion was much lower than that in wild-type embryos, although there was no significant reduction in the total number of neurons in the ganglion. This demonstrates that endogenous NT3 regulates trkC expression in trigeminal neurons independently of changes in population size. In NT3(-/-) embryos, the number of neurons in the trigeminal ganglion was much lower than in wild-type embryos, and there was a further reduction in the mean neuronal level of trkC mRNA. Direct regulation of trkC mRNA expression in cultured trigeminal neurons was also observed, although the finding that trkC mRNA levels were sustained better in explant cultures than in dissociated cultures irrespective of the presence of NT3 suggests that trkC mRNA expression is regulated by additional factors within the ganglion. In contrast to trigeminal neurons, the level of trkC mRNA was sustained at normal levels in neurons of the sympathetic chain of NT3(-/-) embryos and was not increased by NT3 in sympathetic neuron cultures. TrkC mRNA expression in developing cutaneous tissues was also unaffected by the NT3 null mutation. In summary, our findings provide the first clear evidence that the expression of a trk receptor, tyrosine kinase, is regulated by physiological levels of its ligand in vivo and show that regulation by NT3 is cell type-specific.

The regeneration of peripheral noradrenergic nerves after chemical sympathectomy in diabetic rats: effects of nerve growth factor

The study investigated the role of nerve growth factor (NGF) in the regeneration of noradrenergic nerves of the right atria from control and 8-week diabetic rats, after lesion caused by a single injection of 6-hydroxydopamine (6-OHDA, 100 mg/kg ip). This treatment caused a profound depletion of tissue noradrenaline (NA) of the right atria from both control and diabetic groups, followed by a progressive repletion that was not complete at 49 days. Immunoreactivity for the NGF receptors trkA and p75(NTR) was decreased and increased, respectively, between days 3 and 28 in right atria from diabetic rats and returned to pretreatment levels at day 49. Receptor levels were not significantly altered in controls. In contrast to tissue NA, at day 14 functional responses to electrical nerve stimulation of the right atria had completely returned to the pretreatment state in diabetic rats and were very close to normal in nondiabetic rats. NGF treatment (1 mg/kg, three times/week, for 2 weeks) increased tissue NA only in control rats; the pattern was similar after 6-OHDA. These findings are consistent with the hypothesis that NGF normally plays a role in the regulation of autonomic sympathetic nerves in the adult rat atrium and that mature and uninjured sympathetic neurons remain responsive to NGF. In injured noradrenergic neurons, NGF promotes regeneration in nondiabetic rats. The ability of NGF to promote regeneration of noradrenergic nerves is lost in diabetes and this may relate to the loss of trkA receptor on prejunctional nerve terminals after denervation.

Detection of p75 mRNA in developing marsupial CNS by cross-hybridization with rat oligonucleotide probes

We analyzed the distribution of mRNAs encoding the low-affinity neurotrophin receptor (p75) in the CNS of adult and neonatal opossum (Monodelphis demestica) by in situ hybridization with oligodeoxynucleotide probes complementary to cloned rat sequences. During the first 2 postnatal weeks high levels of p75 message were present in the mantle zone throughout the neural tube, in basal forebrain neurons, in motoneurons, and in cerebellar cell layers. Transcript expression decreased with age. In adult CNS only a few cells in the basal forebrain expressed high levels of p75 mRNA. Nerve growth factor upregulated p75 mRNA signals in dorsal root ganglia of cultured 7 day old whole-CNS preparations. Our results indicate the usefulness of rat p75 oligodexynucleotide probes to identify homologous species of transcripts in the CNS of a non-eutherian mammal.

Regulation of nerve growth factor and its low-affinity receptor (p75NTR) during myogenic differentiation

In our preceding report, we have shown that nerve growth factor (NGF) and its low-affinity receptor (p75NTR) are expressed in C2C12 myoblasts and downregulated during myogenic differentiation. Furthermore, NGF affects myogenic differentiation and cell growth via p75NTR and downregulation of p75NTR is essential for myogenic differentiation (Seidl et al., 1998). Here we show that NGF and p75NTR are regulated by mechanisms preceding terminal differentiation in myogenic cells. These mechanisms include cell-density phenomena such as cell-cell contact as well as signaling of basic fibroblast growth factor (FGF-2) and its receptor (FGFR1). Downregulation of NGF and p75NTR occurred as a consequence of increasing cell density, an important trigger for the onset of myogenic differentiation. FGF-2 and FGFR1 were shown to be present in C2C12 cells and exogenous FGF-2 induced NGF and p75NTR expression, implying that FGF/FGFR signaling is an upstream regulator of the NGF/p75NTR system. The fact that FGF-2 could suspend yet not abolish density-induced downregulation indicates that cell-cell contact counteracts the FGF effect and ultimately terminates NGF/p75NTR signaling. This evidence, together with the observation that p75NTR expression is suppressed in muscle progenitors, which constitutively express adenovirus E1A proteins and thus lack the competence of myogenic differentiation, underline the important role for the NGF/p75NTR system in the interplay of multiple factors and biological systems that balance myogenic differentiation at the appropriate spatial and temporal level.

Diabetes and axotomy-induced deficits in retrograde axonal transport of nerve growth factor correlate with decreased levels of p75LNTR protein in lumbar dorsal root ganglia

The effect of sensory neurone axotomy on the level of retrograde axonal transport of nerve growth factor (NGF) was studied in the sciatic nerve of age-matched normal and 8-week streptozocin-diabetic rats. In normal rats a 10-day sciatic nerve crush induced a 41% decrease in transported NGF, however, axotomy of sensory neurones of diabetic rats did not significantly effect the already deficient levels of NGF undergoing retrograde transport. At first sight, this result indicated that transported NGF levels in the sciatic nerve of diabetic rats are at a residual level due to deficient availability of target-derived NGF. To confirm this, the relationship of the transported NGF to the level of sensory neurone expression of the NGF receptor proteins was analysed. Western blots of L4 and L5 dorsal root ganglia (DRG) homogenates revealed no effect of axotomy and/or diabetes on the levels of the 145-kDa tyrosine kinase form of trkA. However, the expression of p75LNTR protein in the intact DRG was reduced in diabetic compared with normal rats (56%; P < 0.01), and axotomy reduced the levels in the ipsilateral ganglia of normal but not diabetic rats - as seen for NGF axonal transport. Reductions in retrograde axonal transport of NGF in both diabetes and/or axotomy were associated with the levels of p75LNTR within the lumbar DRG.

Studies of neurotrophin biology in the developing trigeminal system

The accessibility of the primary sensory neurons of the trigeminal system at stages throughout their development in avian and mammalian embryos and the ease with which these neurons can be studied in vivo has facilitated investigation of several fundamental aspects of neurotrophin biology. Studies of the timing and sequence of action of neurotrophins and the expression of neurotrophins and their receptors in this well characterised neuronal system have led to a detailed understanding of the functions of neurotrophins in neuronal development. The concepts of neurotrophin independent survival, neurotrophin switching and neurotrophin cooperativity have largely arisen from work on the trigeminal system. Moreover, in vitro studies of trigeminal neurons provided some of the first evidence that the neurotrophin requirements of sensory neurons are related to sensory modality. The developing trigeminal system has been studied most extensively in mice and chickens, each of which has particular advantages for understanding different aspects of neurotrophin biology. In this review, I will outline these advantages and describe some of the main findings that have arisen from this work.

Analysis of low affinity neurotrophin receptor (p75) expression in glia of the CNS-PNS transition zone following dorsal root transection

Peripheral nerves exit from the brain through the transition zone where oligodendroyctes and astrocytes of the central nervous system (CNS) and Schwann cells of the peripheral nervous system (PNS) are in close proximity. In this zone, the same axons are ensheathed by oligo-dendrocytes and Schwann cells. We examined, in adult rats, the expression of the low affinity neurotrophin receptor (p75) in central glia and Schwann cells in response to lesion of lumbar dorsal roots. In normal rats, scattered p75-immunoreactive glial cells were present in the CNS-PNS transition zone. A marked increase of p75 immunoreactivity occurred in Schwann cells near the transition zone from 4 days to at least 3 weeks after dorsal root transection. In contrast, the p75 immunoreactivity remained unchanged in central glia. The differential expression of p75 in the two types of glial cells was sharply demarcated at the CNS-PNS border. Our results are consistent with earlier observations that axon damage is less potent in its ability to induce central glial expression of p75, and further, suggests a possible mechanism for the failure of regenerating dorsal root axons growing into the spinal cord.

1,25-Dihydroxyvitamin D3 regulates the expression of the low-affinity neurotrophin receptor

1,25-Dihydroxyvitamin D3 (1,25-(OH)2D3) is known to regulate the expression of neurotrophins [45,46]. Here, we report that 1,25-(OH)2D3 does not influence the expression of truncated or full-length forms of trkB and trkC receptors mRNAs in primary cultures of astrocytes and in C6 glioma cells. In contrast, low concentrations of 1,25-(OH)2D3 increased low-affinity neurotrophin receptor (P75NTR) mRNA and protein levels in C6 glioma cells. Putative vitamin D responsive elements (VDRE) in the P75NTR promoter have been investigated by transfecting plasmids containing sequences from P75NTR promoter fused to a cat reporter gene. A region between -610 and -860 bp upstream from the translation start codon was found to respond to 1,25-(OH)2D3. Interestingly, 1,25-(OH)2D3 does not regulate P75NTR in primary cultures of astrocytes even at concentration as high as 10(-7) M. Since long-term treatment of 1,25-(OH)2D3 induces cell death in C6 glioma cells but not in primary astrocytes [41], the possible involvement of P75NTR in 1,25-(OH)2D3-induced cell death is discussed. Finally, in-vivo studies show that treatment of 15-day-old and adult rats with 1,25-(OH)2D3 leads to a decrease in the level of P75NTR mRNA in the spinal cord but does not influence its expression in dorsal root ganglion or sciatic nerve. These results suggest that 1,25-(OH)2D3 may have a role in the specific regulation of P75NTR in vivo.

Endogenous nerve growth factor is required for regulation of the low affinity neurotrophin receptor (p75) in sympathetic but not sensory ganglia

During development, many sympathetic and sensory neurons are dependent on nerve growth factor (NGF) for survival. The low affinity neurotrophin receptor (p75), expressed in these neurons, is regulated by exogenous NGF in vitro and in vivo. However, whether p75 expression in vivo is under the control of endogenous NGF has not been determined. The role of NGF in regulating the expression of p75 in sympathetic and sensory nerves was investigated in Sprague-Dawley rats treated with an antiserum specific for NGF. P75 was differentially regulated. P75 immunoreactivity (-ir) within sympathetic neurons in the superior cervical ganglia (SCG) was reduced after 2 days, and disappeared after 5 days, of treatment with the NGF antiserum. In contrast, a significant increase in p75-ir was detected in nerve bundles within and close to the SCG from 3 to 14 days after treatment. A similar pattern of p75 expression was observed in the stellate and coeliac ganglia. In contrast, p75 expression in nerve terminals of the mesenteric arteries and irides was reduced. However, in the same animals the expression of p75 was not significantly affected by the treatment in dorsal root, trigeminal or nodose ganglia, salivary gland or small intestine. In contrast to p75, the NGF high affinity receptor trkA was little affected in sympathetic neurons by depletion of endogenous NGF for 2 weeks. These results indicate that endogenous NGF is required in sympathetic ganglia for the expression of p75 but not trkA in neurons, but for the down-regulation of p75 in glia. In contrast, endogenous NGF is not essential for the regulation of p75 in neurons or glia within sensory ganglia.

Helix-loop-helix transcription factors mediate activation and repression of the p75LNGFR gene

Sequence analysis of rat and human low-affinity nerve growth factor receptor p75LNGFR gene promoter regions revealed a single E-box cis-acting element, located upstream of the major transcription start sites. Deletion analysis of the E-box sequence demonstrated that it significantly contributes to p75LNGFR promoter activity. This E box has a dual function; it mediates either activation or repression of the p75LNGFR promoter activity, depending on the interacting transcription factors. We showed that the two isoforms of the class A basic helix-loop-helix (bHLH) transcription factor ME1 (ME1a and ME1b), the murine homolog of the human HEB transcription factor, specifically repress p75LNGFR promoter activity. This repression can be released by coexpression of the HLH Id2 transcriptional regulator. In vitro analyses demonstrated that ME1a forms a stable complex with the p75LNGFR E box and likely competes with activating E-box-binding proteins. By using ME1a-overexpressing PC12 cells, we showed that the endogenous p75LNGFR gene is a target of ME1a repression. Together, these data demonstrate that the p75LNGFR E box and the interacting bHLH transcription factors are involved in the regulation of p75LNGFR gene expression. These results also show that class A bHLH transcription factors can repress and Id-like negative regulators can stimulate gene expression.

Regulation of nerve growth factor receptor gene expression in sympathetic neurons during development

We used quantitative reverse transcription (RT)/PCR to study the regulation of p75 mRNA and trkA mRNA expression in the developing sympathetic neurons of the mouse superior cervical sympathetic ganglion (SCG) in vivo and in vitro. At E13, the SCG contains proliferating cells that express many features of differentiated neurons. These immature neurons survived in culture without NGF, and NGF did not induce c-fos expression. Low levels of p75 and trkA mRNAs were expressed at this stage in vivo. There was no significant increase in the level of either trkA mRNA or p75 mRNA in E13 control cultures up to 72 h in vitro, and neither NGF nor depolarizing levels of K+ ions (40 mM KC1) affected the expression of trkA mRNA. In E14 cultures, NGF induced c-fos expression in 10-15% of the neurons and enhanced the survival of a similar percentage of neurons. The proportion of neurons responding to NGF increased with age, reaching 90% in E18 cultures. The in vivo level of trkA mRNA increased markedly from E14 onward, but in contrast to sensory neurons (in which p75 and trkA mRNA levels increase in parallel), the level of trkA mRNA initially increased far more rapidly than that of p75 mRNA. After E17, the level of p75 mRNA increased rapidly and approached that of trkA mRNA postnatally, but at no stage did this exceed the level of trkA mRNA. In E14 cultures, the level of trkA mRNA increased in the absence of neurotrophins or 40 mM KC1. The level of p75 mRNA in E14 cultures was enhanced by NGF but was unaffected by 40 mM KC1. Our findings show that NGF receptor expression during the earliest stages of sympathetic neuron development is not affected by depolarization but indicate that by an early developmental stage (between E13 and E14 in vivo), sympathetic neurons become specified to upregulate trkA mRNA in culture independently of added factors. In addition, our findings reveal several distinctive features of p75 mRNA and trkA mRNA expression in sympathetic neurons compared with sensory neurons and provide a plausible explanation for previously observed differences in the effects of a p75 null mutation on the response of sensory and sympathetic neurons during embryonic and postnatal development.

The p75 neurotrophin receptor

The low-affinity p75 molecule and trk tyrosine kinases serve as receptors for target-derived neurotrophins. While the mechanism by which receptor tyrosine kinases impart intracellular signaling has become well understood, the precise roles of the p75 receptor are not fully defined. The p75 neurotrophin receptor belongs to a family of transmembrane molecules which also serve as receptors for the tumor necrosis factor family of cytokines. Each receptor shares a common extracellular structure highlighted by conserved cysteine-rich repeats. Because NGF, BDNF, NT-3, and NT-4/5 bind to p75 with similar affinity, p75 may either act as a common subunit in a neurotrophin receptor complex with trk family members, or act by independent mechanisms to mediate biological actions of each neurotrophin.

Sustained tyrosine phosphorylation of p140trkA in PC12h-R cells responding rapidly to NGF

The PC12h cell, a subclone of PC12 cells, has considerable activities of tyrosine hydroxylase (TH) and choline acetyltransferase (ChAT) and shows an NGF-induced increase in both enzyme activities. The TH activity and its inducibility by NGF in PC12h cells were stably maintained in the passage of > 200 generations whereas the ChAT activity was not. We isolated a new cell line, PC12h-R (originally clone 8), from a long-term culture of PC12h cells. PC12h-R cells still showed the considerable TH activity, but not the ChAT activity, and maintained the inducibility of TH activity by NGF. Thus, the responses of PC12h-R cells to NGF were similar to those of chromaffin cells and sympathetic neurons. PC12h-R cells were found to extend neurites and differentiate into sympathetic neuron-like cells in response to NGF much more rapidly than PC12h cells. In addition, PC12h-R cells showed sustained NGF-induced tyrosine phosphorylation of p140trkA and several cellular proteins, including 42-, 44- and 54-kDa proteins, in comparison with PC12h cells. We suggest that the NGF-induced sustained tyrosine phosphorylation signal in PC12h-R cells may be correlated closely with their rapid NGF-induced differentiation into neuron-like cells.

Establishment and characterization of a clear-cell sarcoma (malignant melanoma of soft parts) cell line

A clear cell sarcoma (CCS) cell line, designated as NCS-1, was established in monolayer culture from a xenograft line originating from a metastatic CCS. Marked karyotypic aberrations and tumorigenicity in nude mice revealed the malignant derivation of the NCS-1 cell line. These cells contained abundant glycogen and were amelanotic by light microscopy. By electron microscopy, however, melanosomes in various developmental stages were seen, and some of them were partially melanized. The electron microscopic dopa reaction revealed the presence of tyrosinase activity. Enzyme-linked immunoadsorbent assay revealed that NCS-1 cells expressed a 75-kDa glycoprotein which was identified as a marker of highly differentiated melanoma cells. From these results, NCS-1 cells were found to retain both cytochemical and morphological properties of CCS. Application of NCS-1 cells to a panel of monoclonal antibodies recognizing melanocytic differentiation antigens showed that they corresponded approximately to highly differentiated melanoma cells. In conclusion, the present study strongly supports the close relationship between CCS and malignant melanoma.

Evidence for an origin of ethyl-nitrosourea-induced rat central nervous system tumors from pluripotent germinal neuroepithelium

Brain tumors induced by transplacental application of ethyl-nitrosourea (ENU) in F344 rats were immunohistochemically demonstrated to consist of undifferentiated cells, astriocyte-like cells, oligodendroglia-like cells, and two distinct types of vimentin-expressing cell groupings termed as perivascular small cell nests (PSCNs) and large cell nests (LCNs). Co-distribution of vimentin and glial fibrillary acidic protein (GFAP) was sparsely observed in the astrocyte-like cells, which suggested an immature glial phenotype. PCSNs contained cells expressing GFAP, neuron-specific enolase (NSE), beta-tubulin isotype III, and low-affinity nerve growth factor receptors (LNGFRs). LCNs contained cells showing a neuronal phenotype with expression of low- and middle-molecular mass neurofilament proteins (NF-L and -M) as well as NSE, beta-tubulin isotype III and LNGFR. Double-labelling immunohistochemistry revealed the NF-L-expression in LNGFR-positive LCN-forming cells. Oligodendroglia-like cells and their intercellular neuropil-like structures expressed beta-tubulin isotype III, synaptophysin and NSE, in addition to the expressions of vimentin and GFAP. Electron microscopically, synapse-like structures were formed between these oligodendroglia-like cells and their dendritic processes. Topographically, bidirectional cell transitions from PSCNs to astrocytes and LCNs were indicated. The present study strongly suggests that so-called ENU-induced "gliomas" originate from pluripotent germinal neuroepithelium. Furthermore, LNGFR expression may be responsible for acquisition of neuronal phenotype in these tumors.

Positive and negative effects of neurotrophins on the isthmo-optic nucleus in chick embryos

The survival of neurons in the developing isthmo-optic nucleus (ION) is believed to depend on the retrograde transport of trophic molecules from the target, the contralateral retina. We now show that ION neurons transport nerve growth factor (NGF), brain-derived neurotrophic factor (BDNF), and neurotrophin-3 (NT-3) retrogradely and that BDNF and NT-3 support the survival of ION neurons in vivo and promote neurite outgrowth in vitro. Surprisingly, NGF enhanced normal developmental cell death in vivo in a dose-dependent way. These findings show that increased levels of NGF can have adverse effects on differentiated neurons. The negative effect of NGF could be mimicked by intraocular injection of antibodies that block binding of neurotrophins to the 75 kd neurotrophin receptor (p75). These data implicate a role for the p75 receptor in NGF's neurotoxicity and indicate that this receptor is involved in the mechanism by which ION neurons respond to BDNF and NT-3 in the target.

Regulation of expression of mRNAs encoding the nerve growth factor receptors p75 and trkA in developing sensory neurons

We have used a quantitative reverse transcription/polymerase chain reaction amplification technique to study the regulation of p75 mRNA and trkA mRNA expression in developing NGF-dependent trigeminal neurons. Before becoming NGF dependent, these neurons express low levels of p75 and trkA mRNAs in vivo. At this stage in vitro, the level of p75 mRNA is maintained and up-regulated by BDNF, whereas the level of trkA mRNA is sustained independently of neurotrophins and is down-regulated by BDNF. With the acquisition of NGF dependence, p75 and trkA mRNA levels increase markedly in vivo. At this stage in vitro, the level of p75 mRNA is up-regulated by NGF, but this response is lost at later stages. The level of trkA mRNA is sustained in neurons grown with NGF but is not up-regulated by concentrations of NGF above those required to support survival. At no stage during the early development of trigeminal neurons do depolarising levels of potassium ions affect the expression of either p75 mRNA or trkA mRNA. These findings suggest that the expression of p75 and trkA mRNAs are differentially regulated by BDNF and NGF at successive early stages of neuronal development.

Control of receptor sensitivity at the mRNA level

Neurons are able to adjust the sensitivity of receptor-mediated processes according to the level of receptor activation. Extrapolating from our knowledge of other cellular proteins, regulation of receptor mRNA availability would provide a highly economical means of achieving this objective. Epidermal growth factor is able to induce long-lasting increases in its receptor binding by increasing receptor mRNA levels, and similar effects have been shown for other growth factors. Studies on G-protein-coupled receptors, in particular using adrenoceptor clones transfected into cultured cell lines, have shown that changes in receptor number are generally associated with an alteration in receptor mRNA content. At the neuromuscular junction, dramatic increases in nicotinic acetylcholine receptor number are achieved by activating receptor subunit gene transcription. Less information is available concerning the regulation of ligand-gated ion channels in the brain. Overall, the evidence suggests that receptor mRNA levels are frequently controlled by the degree of receptor stimulation. Receptor mRNA levels are therefore likely to be one of the most important control points for both homologous and heterologous regulation of receptor sensitivity.

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.

Development of the mesencephalic nucleus of the trigeminal nerve in chick embryos: target innervation, neurotrophin receptors, and cell death

The goal of this study was to determine whether processes of neurons in the mesencephalic nucleus of the trigeminal nerve (Mes V) of chick embryos arrive in their peripheral target prior to the period of developmental cell death, and to determine whether neurons with early target contact survive to a greater extent than neurons with processes that reach their peripheral target later. The arrival of Mes V nerve fibers in the masticatory muscles was determined by injecting the fluorescent tracer DiI, and the position of labeled and unlabeled neurons was mapped in subdivisions of the Mes V nucleus. Developmental changes in the numerical configuration of Mes V subdivisions were studied in DiI-labeled as well as Nissl-stained material. The expression of low-affinity (p75) neurotrophin receptors was investigated throughout development of the Mes V nucleus with in situ hybridization to assess whether and how levels of expression of this trophic receptor may relate to target innervation and cell death. The extent of cell death was evaluated by counting pyknotic nuclei. Processes of Mes V neurons invade their peripheral target between 5 and 7 days of incubation (E5-7). At E7-12, between 800 and 1,400 labeled Mes V neurons were distributed throughout the two main subdivisions of the Mes V nucleus, the tectal commissure and the optic tectum. Only few Mes V neurons were labeled in the posterior commissure or outside the brain. Cell counts in Nissl-stained material from E7-13 revealed that the numbers of Mes V neurons in the optic tectum decreased to about 40-60%, and in the tectal commissure to 20-25%, whereas Mes V neurons in the posterior commissure disappeared almost entirely. Few Mes V neurons remained in the leptomeninges at E8-10, but a considerable number was found outside the midbrain at E11, indicating ongoing migration of some Mes V neurons. Neurotrophin receptors were differentially expressed in the Mes V nucleus: Before and after the period of cell death, 90-100% of Mes V neurons expressed neurotrophin receptors, whereas during, and immediately preceding the period of developmental cell death (E9-E13), merely 70% of Mes V neurons expressed this receptor. These findings are consistent with the hypothesis that early target contact may provide an advantage for the survival of Mes V neurons and that competition for trophic factors may occur in the peripheral target of this nucleus prior to the period of cell death.

Potential regulation by trophic factors of low-affinity NGF receptors in spinal motor neurons

Developing spinal motor neurons (SMN) express low-affinity nerve growth factor receptors (LNGFR) but not high-affinity transducing NGF receptors. Moreover, SMN are not supported by NGF in vitro. In the normal adult rat most SMN are not LNGFR immunoreactive (LNGFR-IR), but they transiently reexpress LNGFR (though not the high-affinity receptor) after peripheral nerve injury. With a cut lesion of the sciatic nerve (when only a neuroma forms), the number of LNGFR-IR SMN at L4-L6 rapidly increases to a maximum between day 1 and 7 and returns to baseline levels by day 30. After a crush lesion (accompanied by regeneration to the muscle), LNGFR-IR SMN appear in about the same numbers, but they start to disappear 1 week later. We speculate that the similar appearance and differential decline of LNGFR-IR seen after the two types of lesions are regulated by the availability of a common signal such as ciliary neurotrophic factor. The adult SMN model provides a good opportunity to investigate the reexpression of LNGFR after peripheral nerve injury, and more generally, the unknown role and regulation of LNGFR.

Nerve growth factor (NGF)-mediated up-regulation of low-affinity NGF receptor gene expression in cultured basal forebrain cholinergic neurons from postnatal 3-day-old rats

We examined the effect of nerve growth factor (NGF) on the expression of low-affinity NGF receptor (LNGFR) in cultured P3 basal forebrain cholinergic neurons, on which NGF acts to promote differentiation. Based on the results of the RT-PCR (reverse transcriptase-polymerase chain reaction) method, over a 2-fold increase in the LNGFR mRNA level was found in NGF-treated cultures compared with control cultures at one day after the addition of 100 ng/ml NGF. This increase was maintained for up to 3 days after the addition of NGF. The increase in LNGFR mRNA was found even at 1 ng/ml NGF (= 40 pM), indicating that the up-regulation of the LNGFR mRNA level in cultured cholinergic neurons occurred at an NGF concentration near the Kd value for the high-affinity NGF receptor. In addition, immunohistochemical staining showed stronger staining with a monoclonal anti-LNGFR antibody of the cholinergic neurons in NGF-treated cultures than those in control cultures. Our results suggest that NGF can up-regulate LNGFR expression at the mRNA and protein levels in cultured P3 basal forebrain cholinergic neurons and that this mechanism may play an important role in potentiating the effect of NGF on these neurons.

Regulatory elements and transcriptional regulation by testosterone and retinoic acid of the rat nerve growth factor receptor promoter

The low-affinity nerve growth factor receptor (LNGFR) is a membrane-associated glycoprotein which is thought to participate in some of the biological activities of nerve growth factor (NGF). Expression of the LNGFR gene is known to be regulated both during development and in response to various agents in cell culture. However, molecular mechanisms responsible for the regulation have not been described. We report here an analysis of a 4.8-kb sequence from the 5'-flanking region of the rat LNGFR gene. Several regulatory elements were identified in this region by transfection of plasmid constructs containing sequences from LNGFR fused to a bacterial cat reporter gene. The proximal part of the promoter region (0.4-kb) was shown to be sufficient to support cat expression in all cell types used. A silencer element located between -1.5 kb and -1.8 kb from the start of translation, as well as an enhancer element in more upstream regions of the promoter, were identified in the phaeochromocytoma cell line, PC12, and in the Sertoli cell line, TM4, that express the LNGFR gene. Treatment of TM4 cells with retinoic acid (RA) increases the level of LNGFR mRNA twofold, while testosterone treatment results in a tenfold decrease. Regions of the promoter responsive to testosterone and RA in TM4 cells were found at -610 to -860 bp and -1840 to -4800 bp upstream from the translation start codon, respectively. A RA-responsive element active in PC12 cells is located between bp -610 to -860 from the start codon.

Transynaptic regulation of low-affinity p75 nerve growth factor receptor mRNA precedes and accompanies lesion-induced collateral neuronal sprouting

The bilateral sympathetic innervation of the rat pineal gland from the two superior cervical ganglia (SCG) is a useful model system to investigate the mechanisms by which intact neurons compensate for neuronal losses. Cutting of the internal carotid nerve (ICN) on one side has been shown to result in the removal of approximately one-half of the innervation to the pineal gland within 2 days. This denervation is followed by the development of collateral neuronal sprouting from the contralateral "intact" SCG, most of which takes place during the next 2 days. Using a solution hybridization protection assay, levels of low-affinity NGF receptor p75NGFR mRNA (pg/microgram total RNA) were found to be increased 25%, with no change in cyclophilin mRNA, in the SCG contralateral to the lesion performed 1 or 3 days earlier. In situ hybridization with a 35S riboprobe complementary to p75NGFR mRNA demonstrated a large increase in this mRNA in some cells of this intact SCG at both 1 and 3 days after a contralateral ICN cut lesion. The clustering of these cells toward the rostral portion of the SCG suggests that they may overlap with the population of sympathetic neurons which provides innervation to bilaterally innervated structures such as the pineal gland. The nature of the signals involved in the regulation of NGF receptor mRNA levels and their role in initiating and maintaining collateral sprouting remain to be fully established. Nevertheless, the time course of the changes in mRNA levels suggests that regulation of the low-affinity NGF receptor gene may be involved in the sequence of events associated with the collateral sprouting response by intact sympathetic nerve cells following partial denervation.

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.

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.

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

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

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

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

Ganglioside modulation of neural cell adhesion molecule and N-cadherin-dependent neurite outgrowth

We have used monolayers of control 3T3 cells and 3T3 cells expressing transfected human neural cell adhesion molecule (NCAM) or chick N-cadherin as a culture substrate for PC12 cells. NCAM and N-cadherin in the monolayer directly promote neurite outgrowth from PC12 cells via a G-protein-dependent activation of neuronal calcium channels. In the present study we show that ganglioside GM1 does not directly activate this pathway in PC12 cells. However, the presence of GM1 (12.5-100 micrograms/ml) in the co-culture was associated with a potentiation of NCAM and N-cadherin-dependent neurite outgrowth. Treatment of PC12 cells with GM1 (100 micrograms/ml) for 90 min led to trypsin-stable increases in both beta-cholera toxin binding to PC12 cells and an enhanced neurite outgrowth response to N-cadherin. The ganglioside response could be fully inhibited by treatment with pertussis toxin. These data are consistent with exogenous gangliosides enhancing neuritic growth by promoting cell adhesion molecule-induced calcium influx into neurons.

Basic fibroblast growth factor enhances nerve growth factor receptor gene promoter activity in human neuroblastoma cell line CHP100

The human neuroblastoma cell line CHP100 provides a useful model system in which to study the molecular mechanisms of transcriptional regulation of the low-affinity nerve growth factor receptor (NGFR) gene during neuronal development. Basic fibroblast growth factor (bFGF) induced morphological changes in CHP100 cells, including flattening of cell bodies and neurite outgrowth. bFGF also increased p75NGFR immunoreactivity, as assessed by immunocytochemistry, and increased p75NGFR mRNA levels, as assessed by Northern (RNA) blot analysis. A chimeric gene consisting of 6.7 kb of the 5'-flanking region of the human NGFR gene linked to the chloramphenicol acetyltransferase gene was constructed. In stable transformants of CHP100 cells, 10 ng of bFGF per ml induced an eightfold increase in chloramphenicol acetyltransferase activity. These results indicate that upstream elements of the NGFR gene mediate transcriptional regulation by bFGF.

Basic fibroblast growth factor enhances nerve growth factor receptor gene promoter activity in human neuroblastoma cell line CHP100

The human neuroblastoma cell line CHP100 provides a useful model system in which to study the molecular mechanisms of transcriptional regulation of the low-affinity nerve growth factor receptor (NGFR) gene during neuronal development. Basic fibroblast growth factor (bFGF) induced morphological changes in CHP100 cells, including flattening of cell bodies and neurite outgrowth. bFGF also increased p75NGFR immunoreactivity, as assessed by immunocytochemistry, and increased p75NGFR mRNA levels, as assessed by Northern (RNA) blot analysis. A chimeric gene consisting of 6.7 kb of the 5'-flanking region of the human NGFR gene linked to the chloramphenicol acetyltransferase gene was constructed. In stable transformants of CHP100 cells, 10 ng of bFGF per ml induced an eightfold increase in chloramphenicol acetyltransferase activity. These results indicate that upstream elements of the NGFR gene mediate transcriptional regulation by bFGF.

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.

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.

Dexamethasone blocks nerve growth factor induction of nerve growth factor receptor mRNA in PC12 cells

Glucocorticoids and nerve growth factor (NGF) have been shown to have antagonistic effects on chromaffin cells in vivo. Here we determined the effect of the synthetic glucocorticoid, dexamethasone, on levels of mRNA for the nerve growth factor receptor (NGFR) in rat PC12 pheochromocytoma cells. Following administration of dexamethasone (1 microM) there is a decline in NGFR mRNA expression. More importantly, administration of dexamethasone appears to block the NGF-mediated induction of NGFR when both agents are administered simultaneously. These data support the hypothesis that glucocorticoids and NGF act in opposition in determination of the phenotype of chromaffin cells.

The reverse transforming effects of nerve growth factor on five human neurogenic tumor cell lines: in vitro results

The role of nerve growth factor (NGF) in the development, maintenance and regeneration of the mammalian sensory and sympathetic nervous systems has been well characterized, as has the ability of NGF to induce a variety of neoplastic cell lines of neuroecto-dermal (neurogenic) origin to differentiate. The ability to stimulate neoplastic cells of neurogenic origin to differentiate suggests that NGF may prove useful as a reverse transforming agent for the treatment of neurogenic tumors. Five human neurogenic tumor cell lines were evaluated for their response to NGF in vitro to determine whether the NGF is capable of inducing changes consistent with a reverse transforming response. Results indicate that NGF was able to reverse some of the transformed properties of these tumor cell lines, as NGF treatment stimulated neoplastic cells to develop a more differentiated phenotype, diminished or arrested growth, and induced changes that were persistent.

Epidermal growth factor and basic fibroblast growth factor promote the generation of long-term potentiation in the dentate gyrus of anaesthetized rats

The effects of epidermal growth factor (EGF), basic fibroblast growth factor (bFGF) and nerve growth factor (NGF) on long-term potentiation (LTP) of evoked potential were investigated in the dentate gyrus of anaesthetised rats. Tetanic stimulation of 100 pulses at 100 Hz was applied to induce complete LTP, and stimulation of 20 pulses at 60 Hz was used as a subthreshold stimulation in inducing LTP. Injection of 50 ng EGF, bFGF or NGF into the contralateral ventricle influenced neither the basal amplitude of the population spike nor the LTP induced by the tetanus of 100 pulses at 100 Hz. However, EGF or bFGF, but not NGF, significantly augmented the potentiation induced by the tetanus of 20 pulses at 60 Hz and facilitated the generation of LTP. Moreover, the effect of EGF was dose-dependent in the range 5-500 ng. These results suggest that EGF and bFGF promote the generation process of LTP.

The duration of neurotrophic factor independence in early sensory neurons is matched to the time course of target field innervation

To investigate how the onset of neurotrophic factor dependence in neurons is coordinated with the arrival of their axons in the target field, we have studied the survival of four populations of cranial sensory neurons whose axons reach their common central target field, the hindbrain, at different times. We show that neurons whose axons reach the hindbrain first survive for a short time in culture before responding to brain-derived neurotrophic factor (BDNF). Neurons whose axons reach the hindbrain later survive longer before responding to BDNF. These differences in survival, which arise prior to gangliogenesis, may play a role in coordinating trophic interactions for cranial sensory neurons.

Morphoregulatory activities of NCAM and N-cadherin can be accounted for by G protein-dependent activation of L- and N-type neuronal Ca2+ channels

We present evidence that the morphoregulatory activities of neural cell adhesion molecule (NCAM) and N-cadherin involve activation of intracellular second messenger pathways. PC12 cells were cultured on monolayers of control 3T3 cells or 3T3 cells expressing transfected N-cadherin or NCAM. NCAM and N-cadherin directly induced a transcription-independent change in the morphology of PC12 cells from an adrenal to neuronal phenotype and also specifically increased Thy-1, but not L1/NILE or low affinity NGF receptor, immunoreactivity. The morphological response was more rapid and, in the case of N-cadherin, more substantial than that induced by NGF. It could be fully inhibited by pertussis toxin and a combination of L- and N-type Ca2+ channel antagonists, but not by broad-specificity kinase inhibitors. It was blocked, however, by the kinase inhibitor K-252b. These studies suggest that cell adhesion molecules directly alter cell phenotype and provide direct evidence for transmembrane signaling mediating both the morphological and biochemical responses induced by NCAM and N-cadherin.

Choline acetyltransferase messenger RNA expression in developing and adult rat brain: regulation by nerve growth factor

The polymerase chain reaction (PCR) was used to develop a method for detection and relative quantification of the choline acetyltransferase (ChAT) mRNA in neonatal and adult rat CNS. Oligonucleotide primers derived from a porcine ChAT cDNA sequence were used in coupled reverse transcriptase (RT)-PCR to amplify a cDNA sequence of 206 bp which arises in a cycle- and RNA-dependent manner and which hybridizes with both an internal oligonucleotide and a ChAT cDNA probe. ChAT mRNA was detected in spinal cord, septal area, striatum, cortex and hippocampus but not in cerebellum and cardiac or skeletal muscle. In the septal area, relative quantitative evaluation of ChAT mRNA levels by RT-PCR indicates that this transcript is developmentally regulated and increased following intracerebral administration of nerve growth factor (NGF) to both neonatal and young adult rats. This suggests that the increases of ChAT activity observed in basal forebrain during development or after NGF administration are, at least in part, associated with an increase in corresponding levels of mRNA.

Regulation of nerve growth factor receptor mRNA content by dexamethasone: in vitro and in vivo studies

Northern blot hybridization analysis was used to study regulation of nerve growth factor receptor (NGFR) mRNA content by glucocorticoids. Treatment for 6 h with dexamethasone (1 microM) caused a 40% decrease of NGFR mRNA content in PC12 cells and a 60% decrease in C6-2B glioma cells which was time and dose dependent. Dexamethasone (1 microM/kg) administered s.c. for two days to 21-day-old rats, elicits a 60% decrease in NGFR mRNA content in septum. These results suggest that the expression of NGFR gene in the brain could be inhibited by endogenous glucocorticoids. Whether dexamethasone inhibits NGFR gene expression by directly affecting cis-regulatory elements in the promoter regions of the gene remains to be elucidated.

Expression of the NGF receptor gene in sensory neurons and their cutaneous targets prior to and during innervation

We have studied the expression of NGF receptor (NGFR) mRNA in the mouse trigeminal system at closely staged intervals throughout development. The level of NGFR mRNA per neuron is at a constant low level before the earliest axons reach the target field and increases 5-fold to plateau shortly after the arrival of the last axons. NGFR mRNA expression in developing target skin is restricted to mesenchyme, precedes the arrival of the earliest axons, and increases throughout the phase of target field innervation. These findings suggest that NGFR expression on sensory neurons occurs at a low level prior to target field innervation and is up-regulated with this event, and they raise the possibility that NGFRs on mesenchymal cells restrict the distribution of NGF in the target field.

A v-myc-immortalized sympathoadrenal progenitor cell line in which neuronal differentiation is initiated by FGF but not NGF

Sympathetic neurons differentiate from a developmentally restricted progenitor cell in the neural crest-derived sympathoadrenal lineage. We have isolated these progenitors by fluorescence-activated cell sorting and immortalized them using a v-myc-containing retrovirus. The complement of antigenic markers expressed by these lines suggests that they have retained many of the properties of their normal counterparts. These lines initiate neuronal differentiation in response to basic FGF, but not to NGF, and do not contain NGF receptor mRNA. In NGF plus FGF, however, a small percentage of the cells differentiate to NGF-dependent postmitotic neurons. Furthermore, an induction of NGF receptor mRNA can be observed in response to FGF. Thus, the development of sympathetic neurons may involve a relay, in which FGF both initiates differentiation and induces the NGF receptor, which in turn controls further maturation and survival.