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The Journal of Neuroscience, December 15, 1998, 18(24):10356-10365
Nerve Growth Factor-Dependent Activation of NF- B Contributes
to Survival of Sympathetic Neurons
Sanjay B.
Maggirwar1,
Patrick D.
Sarmiere2,
Stephen
Dewhurst1, 3, and
Robert S.
Freeman2
Departments of 1 Microbiology and Immunology,
2 Pharmacology and Physiology, and 3 Cancer
Center, University of Rochester Medical Center, Rochester, New York
14642
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ABSTRACT |
Neurotrophins activate multiple signaling pathways in neurons.
However, the precise roles of these signaling molecules in cell
survival are not well understood. In this report, we show that nerve
growth factor (NGF) activates the transcription factors NF- B and
AP-1 in cultured sympathetic neurons. Activated NF- B complexes were
shown to consist of heterodimers of p50 and Rel proteins (RelA, as well
as c-Rel), and NF- B activation was found to occur independently of
de novo protein synthesis but in a manner that required
the action of the proteasome complex. Treatment with the NF- B
inhibitory peptide SN50 in the continuous presence of NGF resulted in
dose-dependent induction of cell death. Under the conditions used, SN50
was shown to selectively inhibit NF- B activation but not the
activation of other cellular transcription factors such as AP-1 and
cAMP response element-binding protein. Cells treated with SN50
exhibited morphological and biochemical hallmarks of apoptosis, and the
kinetics of cell killing were accelerated relative to death induced by
NGF withdrawal. Finally, experiments were conducted to test directly
whether NF- B could act as a survival factor for NGF-deprived
neurons. Microinjection of cells with an expression plasmid encoding
NF- B (c-Rel) resulted in enhanced neuronal survival after withdrawal
of NGF, whereas cells that were transfected with a vector encoding a
mutated derivative of c-Rel lacking the transactivation domain
underwent cell death to the same extent as control cells. Together,
these findings suggest that the activation of NF- B/Rel transcription
factors may contribute to the survival of NGF-dependent sympathetic neurons.
Key words:
rat; neuron; apoptosis; cell death; NF- B; transcription factor
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INTRODUCTION |
Programmed cell death is a naturally
occurring process required for nervous system development (Oppenheim,
1991 ), which has been studied extensively using nerve growth factor
(NGF)-dependent sympathetic neurons isolated from rat superior cervical
ganglion (SCG) (Deshmukh and Johnson, 1997 ; Pettmann and Henderson,
1998 ). When deprived of NGF, these neurons undergo apoptosis
(Levi-Montalcini and Booker, 1960 ; Gorin and Johnson, 1979 ); addition
of exogenous NGF rescues them from death (Hendry and Campbell, 1976 ).
Similarly, newborn mice lacking NGF (Crowley et al., 1994 ) or its
receptor TrkA (Smeyne et al., 1994 ) have greatly reduced numbers of
sympathetic neurons. Binding of NGF to TrkA and the low-affinity
neurotrophin receptor p75NTR activates several
signaling pathways, including the phosphoinositide-3-kinase (PI3K) and
mitogen-activated protein kinase pathways (Bredesen and
Rabizadeh, 1997 ). NGF binding also results in the activation of several
transcription factors (Ginty et al., 1994 ; Bonni et al., 1995 ; Riccio
et al., 1997 ), including NF- B (Wood, 1995 ; Carter et al., 1996 ).
Recently, PI3K has been shown to be important for NGF-mediated survival
of rat pheochromocytoma PC12 cells and primary neurons (Yao and Cooper,
1995 ; Philpott et al., 1997 ; Crowder and Freeman, 1998 ). However, it
remains unclear whether other NGF-induced events, such as activation of
NF- B, have a role in neuronal survival.
NF- B plays a key role in the regulation of genes involved in
immunity, inflammation (Ghosh et al., 1998 ), and nervous system function (O'Neill and Kaltschmidt, 1997 ). The NF- B/Rel family consists of five major members: p50, RelA, c-Rel, p52, and RelB. NF- B activation has been shown to protect lymphoid cells and fibroblasts from several apoptotic stimuli (Beg and Baltimore, 1996 ;
Liu et al., 1996 ; Van Antwerp et al., 1996 ; Wang et al., 1996 ; Boothby
et al., 1997 ).
Like NF- B, AP-1 transcription factors are dimeric molecules
comprised of two major protein families: the Jun family and the Fos/activating transcription factor family. The activity of AP-1 components is essential for the proliferation and differentiation of
certain cells (Angel and Karin, 1991 ). However, some AP-1 components may also be involved in apoptosis (Herdegen et al., 1997 ). Indeed, c-Jun is required for apoptosis in response to withdrawal of NGF from
cultured sympathetic neurons (Estus et al., 1994 ; Ham et al., 1995 ).
C-Jun N-terminal kinase (JNK) activation may also be involved in the
induction of apoptosis in neurons by virtue of its ability to induce
transcription of the proto-oncogene c-jun (Verheij et al., 1996 ; Eilers et al., 1998 ).
Here, we show that NGF activates NF- B and AP-1 in sympathetic
neurons and that NGF-inducible NF- B is required for the survival of
neurons. We also demonstrate that overexpression of NF- B is sufficient to protect sympathetic neurons from apoptosis caused by NGF
deprivation. These results suggest that NF- B may contribute to the
survival of NGF-dependent sympathetic neurons.
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MATERIALS AND METHODS |
Primary neuronal culture. Primary cultures of
sympathetic neurons were obtained from SCG of embryonic day 21 rats as
described previously (Crowder and Freeman, 1998 ). Cultures prepared by
this method are >95% postmitotic neurons (Estus et al., 1994 ; Crowder and Freeman, 1998 ). Briefly, SCG were dissected, dissociated, and
resuspended in media containing 90% MEM (Life Technologies, Gaithersburg, MD), 10% FBS (Sigma, St. Louis, MO), 2 mM glutamine, 20 µM uridine, 20 µM 5-fluoro-2'-deoxyuridine (an antiproliferative agent),
100 U/µl penicillin, 100 µg/ml streptomycin, and 50 ng/ml NGF
(Harlan Bioproducts for Science, Madison, WI). The cell suspension was
filtered through a Nitex filter (size 3-20/14; Tetko, Briarcliff Manor, NY), and cells were plated onto collagen-coated dishes. For
microinjection experiments, dissociated cells were preplated onto
plastic tissue culture dishes (Beckton Dickinson, Lincoln Park, NJ) for
1 hr before plating on poly-L-ornithine (Sigma) and
laminin-coated (Collaborative Research, Bedford, MA) 35 mm glass-bottomed dishes (MatTek, Ashland, MA). All experimentation was
performed on neurons that had been in culture for at least 5 d,
but no more than 7 d. To initiate NGF deprivation, culture medium
was replaced by rinsing cells once with Leibovitz's L-15 and then
adding medium prepared as described above but lacking NGF and
containing excess neutralizing NGF antibody (Harlan, Indianapolis, IN).
Nuclear extract preparation and electrophoresis mobility shift
assay. SCG neurons (1 × 105) were
collected by centrifugation at 800 × g for 5 min.
Nuclear extracts were prepared as described previously (Schreiber et
al., 1989 ). Electrophoresis mobility shift assay (EMSA) was performed by incubating the nuclear extracts with a 32P-radiolabeled
probe at room temperature for 10 min, followed by resolution of the
DNA-protein complexes on native 4% polyacrylamide gels
(Maggirwar et al., 1997 ). For antibody supershift assays, 2 µg of
desired antibodies (Santa Cruz Biotechnology, Santa Cruz, CA) were
added to the EMSA reaction 5 min before electrophoresis. For peptide
blocking experiments, 0.2 µg of antigenic blocking peptide (Santa
Cruz) was added to the EMSA reaction before the addition of
corresponding antibodies. The specificity of antibodies was confirmed
by performing EMSA in which we used nuclear extracts of COS-7 cells
transfected with cDNA expression vectors encoding various NF- B/Rel
and AP-1 transcription factor family members. In addition, the
reactivity of the antibodies was confirmed using nuclear extracts of
rat cerebellar granule neurons, rat pheochromocytoma cells (PC12), and
human monocytic cells (U937).
Double-stranded oligodeoxynucleotide probes used in EMSA were as
follows (only the upper strand is indicated): NF- B,
5'-CAACGGCAGGGGAATTCCCCTCTCCTT-3'; AP-1, 5'-CGCTTGATGAGTCAGCCGGAA; cAMP
response element-binding protein (CREB),
5'-AGAGATTGCCTGACGTCAGAGAGCTAG; OCT-1,
5'-TGTCGAATGCAAATCACTAGAA.
Immunoblotting. Whole-cell extracts were prepared from the
neurons (1 × 105) by in situ lysis
using ELB buffer (50 mM HEPES, pH 7.0, 250 mM NaCl, 0.1% Nonidet P-40, 5 mM EDTA, 1 mM dithiothreitol, and 1 mM
phenylmethanesulfonyl fluoride) supplemented with various protease and
phosphatase inhibitors. Cell lysates containing equal amounts of total
protein were fractionated by reducing 10% SDS-PAGE and were
electrophoretically transferred to Hybond ECL nitrocellulose membrane
(Amersham, Arlington Heights, IL). The membranes were analyzed for
immunoreactivity with antisera (Santa Cruz) recognizing Rel proteins
using the ECL detection system (Amersham).
Pulse-chase studies. Neurons (1 × 105) maintained in NGF-containing media for 5 d
were used for these studies. The cells were starved for 1 hr at 37°C
by incubating in DMEM medium lacking methionine and cysteine (ICN
Biochemicals, Costa Mesa, CA) but containing NGF (100 ng/ml).
Subsequently, the cells were metabolically pulse-radiolabeled by
addition of 300 µCi/ml
[35S]methionine-[35S]cysteine
(Dupont NEN, Boston, MA) for 1 hr as described previously (Harhaj et
al., 1996 ). After this, the radiolabeled amino acids were removed, and
cells were incubated for 2 hr in normal medium containing NGF (100 ng/ml), in either the presence or absence of the proteasome inhibitor
MG132 (50 µM) or the NF- B-suppressive peptide SN50
(100 µg/ml; Biomol, Plymouth Meeting, PA). Whole-cell extracts were
prepared by in situ lysis in ELB buffer. The extracts were
precleared with preimmune serum and immobilized Protein A on
Trisacryl beads (Pierce, Rockford, IL), and then immunoprecipitated with anti-I B- serum (a gift of Dr. Edward M. Schwarz, University of Rochester, Rochester, NY) (Miyamoto et al., 1994 ) and Protein A-Trisacryl beads. Immunoprecipitates were analyzed by 10% SDS-PAGE and then by autoradiography.
MTT assay. MTT is converted from yellow to a blue
formazan crystal by mitochondrial succinate dehydrogenase in viable
cells (Mosmann, 1983 ). After the indicated times and treatments,
neurons were washed with PBS and incubated for 20 min at 37°C in 0.5 mg/ml MTT in L-15 culture medium. MTT was removed, and neurons were washed with PBS and lysed in 100 µl of DMSO to extract the formazan crystal. Absorbance was measured at 490 nm on a Victor 1420 microplate reader. At least two wells were treated per experiment, and at least three independent experiments were performed. The data are expressed as a percentage of nontreated neurons that remained in the
presence of NGF.
Nissl staining. Equal numbers of neurons plated on
collagen-coated two-well chamber slides were treated as indicated and
then fixed in 4% paraformaldehyde in PBS. Neurons were rinsed in PBS, stained in 0.1% crystal violet (EM Science, Gibbstown, NJ), dehydrated by immersing in increasing concentrations of ethanol, and mounted using
Pro-Texx mounting medium (Baxter, Deerfield, IL). Neurons that were
evenly stained and had a discernable nucleus were scored as healthy.
Four fields were counted in each well at 20× magnification to obtain
an average number of healthy neurons per field. At least two wells were
treated and counted for each experiment, and a minimum of three
independent experiments were performed. The results are reported as a
percentage of nontreated neurons that remained in the presence of NGF.
Terminal deoxynucleotidyl transferase-mediated biotinylated UTP
nick end labeling assay. Neurons plated on collagen-coated two-well chamber slides were fixed with 4% paraformaldehyde and 0.2%
Triton X-100, washed with PBS, and subjected to terminal deoxynucleotidyl transferase-mediated biotinylated UTP nick end labeling (TUNEL) assay (Gavrieli et al., 1992 ). Permeabilized neurons
were covered with a reaction mixture containing 1 mM
CoCl2, 0.25 U/µl
terminal-deoxynucleotidyl-tranferase (Boehringer Mannheim, Indianapolis, IN), and 6 µM digoxigenin-11-dUTP
(Boehringer Mannheim) and placed at 37°C for 1 hr. Neurons were then
rinsed in PBS, blocked in 5% goat serum and 2% BSA for 2 hr, and
labeled with a 1:4 dilution of FITC-conjugated anti-digoxigenin
antibody in blocking buffer for 4 hr (Oncor, Gaithersburg, MD). Cells
were rinsed in PBS and labeled with 2 µg/ml Hoechst 33,342 (Molecular Probes, Eugene OR) in PBS for 5 min to visualize the nuclei of cells.
After two additional rinses in PBS, slides were covered with glass
coverslips using mounting solution of 50% glycerol and 0.1%
phenylenediamine in PBS and viewed by a Nikon Diaphot 300 inverted
microscope equipped for epifluorescence.
Microinjection. Neurons plated on 35 mm glass-bottomed
dishes were injected using a Nikon Diaphot 300 inverted microscope equipped with a PLI-100 picoinjector (Medical System, Greensvale, NY)
and a Narishige micromanipulator (Nikon-Narishige, Tokyo, Japan).
Before injection, cells were placed in serum-free L-15 media (Life
Technologies, Gaithersburg, MD) containing penicillin and streptomycin;
~200-300 neurons were injected for each cDNA. Cells were injected
with solutions containing expression vectors at 50 µg/ml in
KPi buffer (100 mM KCl and 10 mM
potassium phosphate, pH 7.4) containing 4 mg/ml rhodamine-dextran (10 kDa; Sigma) to permit identification of injected cells. Neurons were
returned to NGF-containing media immediately after injections to allow for expression of plasmid cDNA. The number of neurons successfully injected was determined by counting rhodamine-positive cells 18 hr
after injection. NGF deprivation was initiated 18 hr after injection
and, after an additional 48 hr, neurons were incubated for 5 min with 3 µg/ml Hoechst 33,342 (Molecular Probes) in L-15 to label chromatin;
cells were then scored for viability. Injected cells scored as viable
had diffuse nuclear Hoechst staining and phase-bright cell
bodies with clearly discernable nuclei. Survival was expressed as a
ratio of the number of healthy rhodamine-positive cells at 48 hr after
NGF withdrawal to the number of rhodamine-positive cells determined at
18 hr. For each cDNA, three independent experiments were performed
involving at least 200 injected cells per experiment. In each case,
neuronal survival was assessed by an observer who was unaware of the
treatment performed.
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RESULTS |
Effect of NGF on NF- B, OCT, and AP-1 DNA binding activities in
sympathetic neurons
For these studies, neuronal cultures were prepared from SCG of
embryonic day 21 rats and were maintained in the presence of NGF and
antimitotic agents for 5-7 d. To determine the effects of NGF on the
activities of various transcription factors, NGF was withdrawn from the
culture for 14 hr, after which NGF was added back to the cultures for
various time periods. The 14 hr NGF withdrawal before readdition of NGF
permits the downregulation of signaling pathways affected by NGF,
including TrkA phosphorylation (Franklin et al., 1995 ), ERK activation
(Creedon et al., 1996 ), and AKT activation (R. J. Crowder and
R. S. Freeman, unpublished observations) but does not
result in irreversible consequences of NGF withdrawal (Deckwerth and
Johnson, 1993 ). After NGF withdrawal and readdition, nuclear
extracts from these cells were prepared and analyzed by performing EMSA.
To investigate whether NGF induces activation of the NF- B/Rel family
of transcription factors, we used a double-stranded 32P-radiolabeled oligonucleotide probe that contained
high-affinity palindromic B binding sequences (Maggirwar et al.,
1997 ). As shown in Figure 1, top
panel, EMSA revealed a low level of NF- B DNA binding activity
in cells deprived of NGF for 14 hr. This NF- B activity was
significantly increased when the cells were refed with medium
containing NGF for 2 hr. Further incubation of the cells with NGF led
to a greater induction of NF- B, which reached a maximum level at 4 hr and remained elevated after 8 hr of treatment.

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Figure 1.
Effect of NGF on various transcription factors in
sympathetic neurons. NGF was withdrawn from primary cultures of rat
sympathetic neurons for 14 hr, after which NGF (100 ng/ml) was added
back to the culture for the indicated time periods. Nuclear extracts
were prepared from these cells and were then subjected to EMSA using
32P-radiolabeled DNA probes as indicated. The specific
binding of protein complexes to the different probes is indicated by
the filled circle. The results shown are representative
of more than three independent experiments; essentially identical
results were also obtained when cells were exposed to 50 ng/ml NGF
(data not shown).
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To examine the specificity of this effect of NGF, we analyzed the DNA
binding activity of the octamer (OCT) family of transcription factors,
which are thought to be steady-state regulators of "housekeeping" genes (Ruvkun and Finney, 1991 ). We detected a major DNA binding complex in nuclear extracts of neurons deprived of NGF for 14 hr (Fig.
1, bottom panel) that remained essentially unaffected by prolonged NGF treatment. Thus, NGF appears to selectively target only certain types of transcription factors within sympathetic neurons.
Recently, Eilers et al. (1998) demonstrated that sympathetic neurons
chronically exposed to NGF contain relatively high levels of JNK/SAPK
activity, which are further enhanced after NGF deprivation. We asked
whether regulation of JNK by NGF might lead to the activation of AP-1
transcription factors in these cells. At 14 hr of NGF deprivation
(before readdition of NGF), only very low levels of residual AP-1
activity were detected in the neurons (Fig. 1, middle panel). Incubation of these cells with NGF for up to 4 hr
had little effect on AP-1, but prolonged (6 hr) exposure to NGF
resulted in a substantial increase in AP-1-specific DNA binding
activity, which remained elevated even after 8 hr of NGF treatment.
Thus, NGF appears to regulate the activation of both NF- B and AP-1, although with different kinetics.
NGF-induced NF- B contains heterodimeric complexes of p50 and
Rel proteins
To elucidate the nature of the NF- B complex activated by NGF,
we performed antibody supershift analyses using nuclear extracts from
neurons maintained in NGF for 5 d. As shown in Figure
2, neither preimmune serum nor
p52-specific antiserum exhibited immunoreactivity with the NF- B-DNA
complex. In contrast, antibodies recognizing p50, RelA, or c-Rel
proteins abrogated the formation of this complex and led to the
appearance of higher molecular weight complexes, indicating that these
members of the NF- B family were activated by NGF. Three pieces of
evidence confirmed the specificity of our supershift analyses. First,
these antibodies had no effect on the formation of a nonspecific
protein-DNA complex (Fig. 2, open circle). Second,
incubation of these antisera with the NF- B probe in the absence of a
nuclear extract did not generate a "supershift" (data not shown).
Third, the effects of these antibodies could be blocked by cognate
antigenic peptides (data not shown). Together, these results suggest
NGF-treated neurons contain nuclear NF- B/Rel complexes
predominantly comprised of p50/RelA and p50/c-Rel heterodimers.

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Figure 2.
NGF-induced NF- B contains prototypical
complexes. Nuclear extracts from SCG neurons maintained in the presence
of NGF were subjected to EMSA in the presence of the indicated
antisera. The filled circle indicates the specific
NF- B protein-DNA complex; the open circle shows the
formation of a nonspecific protein-DNA complex. The results shown are
representative of more than three independent experiments.
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Sustained activation of NF- B by NGF does not require de
novo synthesis of RelA
Because total protein synthesis is decreased significantly by 14 hr of NGF withdrawal (Deckwerth and Johnson, 1993 ), the activation of
NF- B could be the result of increased protein synthesis after the
readdition of NGF. To address this possibility, we performed immunoblotting analyses by using RelA-specific antibodies and whole-cell extracts of neurons treated with or without NGF. As shown in
Figure 3, we detected a RelA
immunoreactive protein in NGF-deprived cells. However, intracellular
levels of RelA remained unchanged over an 8 hr period of NGF
stimulation. This finding is consistent with the notion that the
regulation of NF- B in sympathetic neurons, as in other cell types,
involves cytosolic-nuclear redistribution of protein rather than
de novo protein synthesis.

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Figure 3.
NGF-mediated activation of NF- B is independent
of de novo synthesis of RelA. Cultures of SCG neurons
were deprived of NGF for 14 hr, followed by incubation with NGF for the
indicated periods. Whole-cell extracts were then subjected to
immunoblot analysis using antibodies specific for RelA. The results
shown are representative of two independent experiments.
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Activation of NF- B by NGF involves proteolysis of I B- and
is blocked by a proteasome inhibitor
NF- B is normally sequestered in the cytoplasm by a member of
the I B family of inhibitory proteins (Ghosh et al., 1998 ). When
cells are exposed to inducers of NF- B, these I B proteins undergo
sequential phosphorylation, ubiquitination, and proteasomal degradation, thereby allowing the nuclear translocation of dimeric complexes of NF- B (Palombella et al., 1994 ; Traenckner et al., 1994 ).
To examine whether activation of NF- B by NGF also requires the
proteasome pathway, we first tested the effect of proteasome inhibitors
ALLN and MG132 on NF- B activities in neurons chronically maintained
in NGF. As shown in Figure
4A, EMSA revealed a
high basal level of activated NF- B (lane 1), which was
significantly inhibited after the incubation of neurons for 2 hr with
either ALLN (lane 2) or MG132 (lane 3). No cell
death caused by this brief MG132 treatment was observed. These
findings suggest that NGF-mediated activation of NF- B in neurons
requires the action of the proteasome complex, as is the case in other
cell types.

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Figure 4.
Activation of NF- B by NGF requires the action
of the proteasome complex and involves proteolysis of I B- .
A, SCG neurons maintained in the continuous presence of
NGF were treated with the proteasome inhibitors ALLN (50 µM) (lane 2) or MG132 (50 µM) (lane 3), or with medium alone
(lane 1), for 2 hr. Nuclear extracts prepared from these
cells were subjected to EMSA analysis to detect
B-binding proteins. The filled
circle indicates the specific NF- B-DNA complex.
B, SCG neurons were metabolically labeled for 1 hr with
300 µCi/ml
[35S]methionine-[35S]cysteine
in the presence of NGF (100 ng/ml). The pulse-labeled cells were then
lysed and extracted immediately (lane 1) or cold-chased
for 2 hr in either the absence (lane 2) or presence of
MG132 (50 µM) (lane 3), SN50M (100 µg/ml) (lane 4), or SN50 (100 µg/ml)
(lane 5). Whole-cell extracts were isolated and then
subjected to immunoprecipitation using I B- -specific antiserum as
described in Materials and Methods. The filled circle
indicates I B- , whereas the open circle denotes a
nonspecific protein. I B- levels were quantitated by densitometric
analysis of this autoradiogram (NIH Image software) and normalized to
the level of the nonspecific protein (open circle).
Based on this analysis, I B- levels had declined to 18% of
starting levels within 2 hr of NGF withdrawal (compare lanes
1, 2). I B- degradation was unaffected by
addition of the SN50M or SN50 peptides (I B- declined to 19 and
15% of its starting level, respectively) (lanes 4,
5), but the protein was protected by addition of the
proteasome inhibitor MG132 (I B- remained at 62% of its initial
level) (lane 3). The results shown are representative of
two (B) or three (A)
independent experiments.
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In view of the effect of the proteasome inhibitors on NGF activation of
NF- B, we next examined whether NGF induces the proteolytic degradation of I B- in SCG neurons. For these studies, we
initially attempted to perform NGF starvation and stimulation
experiments similar to those shown in Figure 1, with detection of
I B- levels by immunoblot analysis. However, these experiments
were inconclusive, in part because of the inherent instability of
I B- and also because of the overall decline in protein synthesis
that was observed in NGF-deprived cells. To circumvent these problems,
we used pulse-chase experiments to analyze I B- . NGF-maintained
neurons were metabolically labeled for 1 hr with
[35S]methionine-[35S]cysteine,
after which a chase with nonradioactive amino acids was performed for 2 hr, in either the absence or presence of the proteasome inhibitor MG132
(Fig. 4B). Whole-cell extracts were then prepared and
subjected to immunoprecipitation using antibodies specific to
I B- . We detected a radiolabeled band corresponding to I B-
in neurons before the chase (Fig. 4B, lane
1). After 2 hr of chase in the absence of MG132, the steady-state
level of I B- dramatically decreased (Fig. 4B,
lane 2). These results show that NGF stimulates the signals
for proteolytic degradation of I B- in sympathetic neurons. When
the chase was performed for 2 hr in the presence of MG132, this
inhibitor efficiently protected I B- from degradation (Fig.
4B, lane 3). It is noteworthy that
the intensities of radiolabeled bands of nonspecific proteins (for
example, Fig. 4B, the band shown as
open circle) were relatively unaltered over the
entire period of this experiment, suggesting that the decreased
intensity of the I B- -specific band is attributable to its
selective proteolytic degradation.
SN50 selectively inhibits NGF-dependent NF- B activity and
induces neuronal death
We next sought to examine the relationship between NF- B
activation and survival in NGF-dependent neurons using the NF- B inhibitory peptide SN50. SN50 is a synthetic oligopeptide that contains
a hydrophobic cell-permeable motif, together with nuclear localization
sequences (NLS) from the p50 subunit of NF- B (Lin et al., 1995 ).
To validate the use of SN50 in this system, we first examined the
effect of SN50 on the fate of I B- . As expected for an inhibitor
of NF- B translocation, incubation of sympathetic neurons in the
presence of either SN50 or an inactive control peptide that is mutated
within the NLS motif (SN50M) did not protect I B- from degradation
(Fig. 4B, lanes 4, 5). This is
consistent with previous studies in which the peptide SN50 inhibited
nuclear translocation of NF- B without altering I B- degradation
(Lin et al., 1995 ; Kilgore et al., 1997 ).
We then tested the effects of SN50 on NF- B activity in neurons
maintained in the presence of NGF. After incubating these cells for 4 hr with either SN50 or SN50M, nuclear extracts were prepared and
analyzed by performing EMSA. Incubation of cells with the SN50 peptide,
but not SN50M, abolished the formation of the specific NF- B-DNA
complex (Fig. 5A, closed
circle) but had no effect on a nonspecific protein-DNA complex
(Fig. 5A, open circle). Furthermore, the addition
of SN50 to in vitro EMSA reactions had no effect on the
ability of NF- B proteins to bind to DNA (data not shown). These
results demonstrate that NGF-mediated activation of NF- B can be
blocked by the treatment of neurons with SN50.

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Figure 5.
Selective inhibition of NF- B in sympathetic
neurons by synthetic peptide SN50. A, Cultures of SCG
neurons maintained in the continuous presence of NGF for 5 d were
treated with either SN50M or SN50 at a concentration of 75 µg/ml for
4 hr. Nuclear extracts isolated from these cells were used to perform
EMSA for analysis of B-binding proteins. The
filled circle indicates the specific NF- B-DNA
complex; the open circle denotes a nonspecific
protein-DNA binding complex. B, Whole-cell extracts of
SN50M- and SN50-treated neurons maintained in the presence of NGF were
subjected to immunoblotting analyses by using anti-RelA and anti-c-RelA
antibodies. C, Nuclear extracts of SN50- or
SN50M-treated (control) neurons were subjected to
EMSA using a 32P-radiolabeled DNA probe corresponding to a
consensus AP-1 binding site. The filled circle denotes
the AP-1-DNA complex. EMSA was also performed in the presence of
anti-c-Jun and anti-c-Fos antibodies (as indicated), and nuclear
extracts from c-Jun transfected COS-1 cells were included as a
control (COScJun). D, E,
EMSA was also performed using nuclear extracts of SN50- and
SN50M-treated neurons (as indicated) with radiolabeled oligonucleotide
probes specific for CREB (D) and OCT
(E) transcription factors. The results shown are
representative of a minimum of either two (B) or
three (all other panels) experiments.
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To examine whether the inhibition of NF- B DNA binding activity by
SN50 was caused by the depletion of Rel proteins in SCG neurons, we
performed immunoblotting analyses. As shown in Figure 5B,
intracellular levels of RelA or c-Rel proteins remained unchanged in
SN50-treated neurons compared with those treated with SN50M. Together
with the findings presented in Figure 4, these results suggest that the
observed inhibitory effect of SN50 is attributable to its ability to
compete with NGF-induced NF- B complexes for access to the cellular
machinery responsible for nuclear translocation of NF- B.
To test whether the inhibitory effects of SN50 were specific to
NF- B, EMSA was performed using 32P-radiolabeled probes
containing consensus sites for various transcription factors, including
AP-1, CREB, and OCT. A high basal level of AP-1 DNA binding activity
was detected in NGF-treated neurons (Fig. 5C). This
AP-1-DNA complex was supershifted by c-Jun-specific antisera but not
by anti-c-Fos antibodies, and it comigrated with the c-Jun homodimer
generated with a c-Jun transfected COS-1 cell extract. Treatment with
SN50 had no effect on this AP-1-DNA complex (Fig. 5C).
Likewise, neither CREB nor OCT DNA binding activities were affected by
incubation of neurons with the SN50 peptide (Fig. 5D,E). Therefore, the inhibitory
effect of SN50 appears to be specific to NF- B transcription factors
under these conditions.
Having determined that SN50 is an inhibitor of NF- B, but not of
other transcription factors, in sympathetic neurons, we proceeded with
experiments aimed at examining the role of NF- B in neuronal survival.
Sympathetic neurons that are deprived of NGF in vitro
undergo apoptosis, which first becomes apparent ~18 hr after NGF
withdrawal and is virtually complete by 72 hr (Deckwerth and Johnson,
1993 ). This process involves atrophy of the cell body, condensation of the nuclear chromatin, DNA fragmentation, and neurite degeneration. In
our initial experiments, we noticed that prolonged treatment (>4 hr)
with SN50 resulted in significant neuronal death that morphologically
resembled the death caused by NGF withdrawal. Like NGF-deprived
neurons, neurons treated with 100 µg/ml SN50 in the presence of NGF
had severely degenerated neurites and condensed cell bodies (Fig.
6). The SN50-treated neurons also
exhibited marked chromatin condensation, detected by staining nuclei
with Hoechst dye, and DNA fragmentation, as detected by the TUNEL
assay. Control NGF-maintained cells and cells treated with SN50M showed no signs of apoptotic morphology or chromatin condensation and were
devoid of TUNEL labeling, suggesting that the toxicity of SN50 was
dependent on an intact NF- B NLS.

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Figure 6.
SN50 induces cell death with characteristic
features of apoptosis. Cultured neurons were either maintained in the
continuous presence of NGF, deprived of NGF, or treated with NGF with
SN50 or SN50M peptides (100 µg/ml each). After 24 hr, cells were
fixed, analyzed for TUNEL reactivity, and stained with Hoechst dye to
label nuclei. Phase-contrast, TUNEL, and Hoechst images of the same
field of view are shown for each treatment. Arrowheads
indicate examples of cells deprived of NGF or treated with SN50 that
have condensed chromatin and TUNEL reactivity.
|
|
Although SN50-treated neurons exhibited an apoptotic morphology similar
to neurons deprived of NGF, the rate at which these cells underwent
cell death was accelerated. Using MTT reduction as an indication of
mitochondrial function and cell viability, we found that 12 hr of
exposure to SN50 resulted in a 70% decrease in MTT reduction, whereas
neurons deprived of NGF for 12 hr showed no such decrease (Fig.
7A). Only after 30 hr of NGF
withdrawal did the decrease in MTT reducing capacity approach the level
seen in SN50-treated cells. The reason for the more rapid action of SN50 compared with NGF withdrawal is not known, but it could reflect our observations that SN50 causes more rapid and complete inhibition of
NF- B activation compared with NGF withdrawal (compare Figs. 1 and
5A; these figures show NF- B activity after 14 hr of
NGF-deprivation or 4 hr of SN50 treatment, respectively)

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Figure 7.
Death of sympathetic neurons treated with SN50 is
time- and dose-dependent. A, Neuronal cultures were
either deprived of NGF (black line, filled
triangles) or treated with 100 µg/ml SN50 in the presence of
NGF (dashed line, open squares). Survival
was assayed by MTT reduction at 12, 30, 48, and 72 hr after a single
application of SN50. Data represents the mean ± SEM from
three independent experiments. B, Neuronal cultures were
treated with a single application of 50, 75, or 100 µg/ml SN50
(open bars) or SN50M (filled
bars), both in the presence of NGF. Viability was assayed after
48 hr of treatment by Nissl staining as described in Materials and
Methods. The results represent the mean ± SEM of at least three
independent experiments. For both A and
B, results are expressed as a percentage of the survival
of control neurons maintained in the continuous presence of NGF.
|
|
In non-neuronal cell lines, SN50 has been shown to inhibit NF- B
activity by ~85% at a concentration of 100 µg/ml, whereas SN50M
has no effect at the same concentration (Lin et al., 1995 ). Consistent
with these observations, SN50 at a comparable concentration completely
inhibited NGF-induced NF- B activity (Fig. 5A). SN50 also
elicited a dose-dependent inhibition of NGF-mediated survival, which
was maximal at 100 µg/ml and half-maximal (IC50)
between 50 and 75 µg/ml (Fig. 7B). Therefore, SN50, but
not SN50M, inhibits cell survival over the same concentration range at
which it inhibits NF- B activation. These results suggest that
NF- B may have a role in NGF-mediated neuronal survival.
Overexpression of c-Rel is sufficient to prevent the death of
NGF-deprived neurons
To test directly whether NF- B might inhibit neuronal death
after NGF withdrawal, we microinjected sympathetic neurons with plasmid
DNAs encoding either full-length c-Rel (pCMV4 c-Rel) or a
transactivation-deficient form of c-Rel [pCMV4 c-Rel(1-359)] (Doerre et al., 1993 ). As a control, cells were injected with an
expression vector encoding -galactosidase (Crowder and Freeman, 1998 ). NGF deprivation was initiated 18 hr after microinjection, a time
when >95% of successfully microinjected cells express exogenous c-Rel
(as determined by immunofluorescence staining using an anti-c-Rel antibody; data not shown). After 48 hr of NGF deprivation,
neurons injected with the c-Rel expression vector remained phase
bright, had a clearly definable nucleus, and had uniformly dispersed
chromatin (Fig. 8A). In
contrast, uninjected neurons or neurons injected with -galactosidase
or c-Rel(1-359) vectors showed signs of apoptotic cell death,
including chromatin condensation and neurite degeneration. When these
results were quantitated, >75% of neurons injected with control
vectors underwent cell death after NGF withdrawal (Fig.
8B). In contrast, ~70% of neurons injected with
the c-Rel expression vector were protected from cell death. These
findings demonstrate that the overexpression of c-Rel is sufficient to block apoptosis caused by NGF withdrawal. In addition, our data indicate that the transactivation domain of c-Rel is necessary for the
neuroprotective effect of c-Rel.

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Figure 8.
A, Overexpression of c-Rel promotes
survival in NGF-deprived neurons. Shown are images of NGF-deprived
neurons after injection with either c-Rel or c-Rel(1-359) expression
vectors. Neurons were microinjected and then deprived of NGF for 48 hr.
Cells were then stained with Hoechst dye and visualized by
phase-contrast and fluorescence microscopy. Shown are phase-contrast
views, rhodamine-positive cells (injected cells), and Hoechst-stained
nuclei in one field of view for each injected DNA.
Arrowheads for c-Rel injections indicate healthy looking
injected cells with normal nuclear chromatin. Arrowheads
for c-Rel(1-359) indicate injected cells containing condensed nuclear
chromatin. B, Overexpression of c-Rel is sufficient to
prevent neuronal cell death caused by NGF deprivation. Cultured neurons
were microinjected with plasmids encoding either c-Rel, c-Rel(1-359),
or LacZ, and then deprived of NGF for 48 hr as described in Materials
and Methods. Survival was assayed on the basis of cellular morphology
(as viewed by phase-contrast microscopy) and by the presence or absence
of condensed chromatin (detected with Hoechst dye). The results
(mean ± SEM) from at least three independent experiments are
shown. Survival for c-Rel-injected cells was significantly different
from LacZ- or c-Rel(1-359)-injected neurons (two-tailed
t test; p < 0.0001 in both
cases).
|
|
 |
DISCUSSION |
We used EMSA to detect the DNA binding activity of various
transcription factors in nuclear extracts from primary cultures of rat
sympathetic neurons and found that NGF activates NF- B in these
cells. This NF- B activation pathway appears to be prototypical in
that it involves the proteolytic degradation of the inhibitory protein
I B- via the action of the proteasome complex. Inhibiting NF- B
activity resulted in the death of sympathetic neurons in the presence
of NGF, whereas overexpression of c-Rel in the absence of NGF was
sufficient to promote survival. These data suggest that activation of
NF- B contributes to the NGF-dependent survival of sympathetic neurons.
NF- B was found to be constitutively activated in sympathetic neurons
maintained in the presence of NGF. Withdrawal of NGF from these
cultures resulted in the rapid downregulation of NF- B activity (data
not shown), and readdition of NGF, at a time when the cells could be
fully rescued by NGF, resulted in reconstitution of steady-state levels
of NF- B in these cells. NF- B activity increased gradually over
2-4 hr after NGF addition. These kinetics contrast with the rapid
increase of NF- B activity in Schwann cells detected within 30 min of
NGF treatment (Carter et al., 1996 ) and the relatively delayed
activation of NF- B (>24 hr) observed in NGF-treated PC12 cells
(Wood, 1995 ). In Schwann cells, NGF-induced NF- B activation is
dependent on p75NTR and not TrkA, possibly
accounting for the more rapid rate of activation in these cells.
Simultaneous expression of p75NTR and TrkA has been
shown to repress signal transduction events mediated by either receptor
expressed alone (Kaplan and Miller, 1997 ). For example, the presence of
TrkA was shown to suppress p75NTR-mediated ceramide
production in PC12 cells (Dobrowsky et al., 1995 ), and introduction of
TrkA into oligodendrocytes caused a reduction in
p75NTR-dependent JNK activation (Yoon et al., 1998 ).
On the other hand, activation of NF- B by NGF in
p75NTR-expressing oligodendrocytes was not adversely
affected by introducing TrkA into these cells (Yoon et al., 1998 ).
Thus, although p75NTR and TrkA can independently
activate downstream pathways, coexpression of both receptors appears to
modify certain pathways (ceramide and JNK) but not others (NF- B).
Whether NF- B activation in sympathetic neurons is mediated by
p75NTR, TrkA, or both remains to be determined.
The NGF inducible NF- B complexes in sympathetic neurons contain p50,
RelA, and c-Rel subunits. In its inactive state, NF- B is sequestered
in the cytosol by binding to I B- proteins. The phosphorylation
and subsequent proteasomal degradation of I B- releases NF- B
and permits its translocation into the nucleus in which it can bind DNA
and regulate transcription. Translocation of p50-containing NF- B
complexes to the nucleus can be blocked by SN50, a peptide that
contains the nuclear localization sequences from p50 and presumably
competes with p50 for binding to unknown components of the nuclear
transport machinery (Lin et al., 1995 ). In our experiments,
NGF-dependent NF- B activity was potently inhibited by treatment with
SN50. Importantly, SN50 did not inhibit the DNA binding activity of a
variety of other transcription factors, including AP-1, CREB, and OCT.
Although SN50 was selective for NF- B, it remains possible that SN50
could inhibit the translocation of other transcription factors or
nuclear survival signals emanating from the cytoplasm.
SN50-treated neurons resembled neurons deprived of NGF, suggesting that
NF- B activity may be an important component of a survival pathway
mediated by NGF. To test whether ectopic activation of NF- B could
lead to survival in the absence of NGF, we analyzed the effects of
overexpressing c-Rel in NGF-deprived neurons. Overexpression of c-Rel
was found to be sufficient to protect neurons from apoptosis caused by
NGF withdrawal. These results are consistent with previous observations
in non-neuronal cells in which ectopic expression of c-Rel provided
protection from serum withdrawal-induced apoptosis (Bertrand et al.,
1998 ) and anti-IgM- or TGF -1-mediated apoptosis (Arsura et al.,
1996 ; Wu et al., 1996 ). In contrast to full-length c-Rel,
microinjection of c-Rel(1-359), which lacks the C-terminal transactivation domain of the wild-type protein but retains the DNA
binding Rel homology domain (Doerre et al., 1993 ), failed to protect
neurons from apoptosis after NGF withdrawal. Therefore, it appears
likely that the transcriptional activity of c-Rel is required for its
ability to promote survival in the absence of NGF.
Our hypothesis that NF- B has a role in the survival of NGF-dependent
neurons is consistent with several recent studies suggesting a role for
NF- B in protection from apoptosis. Abrogation of NF- B, either by
deletion of RelA or by overexpression of I B- , sensitizes immune
cells to apoptosis in response to TNF and DNA-damaging agents (Beg
and Baltimore, 1996 ; Liu et al., 1996 ; Van Antwerp et al., 1996 ; Wang
et al., 1996 ). In addition, inhibition of NF- B in H-Ras-transformed
fibroblasts induces apoptosis (Mayo et al., 1997 ), and in PC12 cells,
inhibition of NF- B blocks the neuroprotection afforded by soluble
-amyloid precursor protein (Guo et al., 1998 ). NF- B activity also
appears to be required for the ability of NGF to protect
undifferentiated PC12 cells from apoptosis after serum withdrawal
(Taglialatela et al., 1997 ). In contrast to these results, NF- B has
been implicated in the death of neurons induced by glutamate and
-amyloid protein (Behl et al., 1994 ; Grilli et al., 1996 ),
although a causal relationship for NF- B in these neuronal deaths has
not been demonstrated.
Besides NF- B, the PI3K pathway has recently been implicated in the
regulation of NGF-dependent survival. Inhibition of this pathway in
PC12 cells blocks the ability of NGF to rescue cells from apoptosis
(Yao and Cooper, 1995 ). A role for PI3K in survival has also been found
in primary sympathetic and cerebellar granule neurons (D'Mello et al.,
1997 ; Dudek et al., 1997 ; Miller et al., 1997 ; Philpott et al., 1997 ;
Crowder and Freeman, 1998 ). PI3K and NF- B may exist in the same or
independent survival pathways. Evidence in support of independent
pathways comes from experiments in Chinese hamster ovary cells on the
antiapoptotic action of insulin (Bertrand et al., 1998 ).
Insulin-dependent survival could be blocked partially by inhibiting
either PI3K or NF- B; inhibiting both proteins was additive and led
to complete reversal of the survival promoted by insulin. However, PI3K
inhibitors did not impede the ability of insulin to induce NF- B
transcription factor activity. Therefore, at least in these cells, PI3K
and NF- B appear to exist in separate survival pathways.
NGF-treated neurons possess constitutively activated AP-1 transcription
factor complexes that contain c-Jun, but not c-Fos, proteins. This
observation is consistent with previous immunoblotting and
immunofluorescence experiments showing that c-Jun, but not c-Fos, is
expressed in NGF-maintained neurons and, in the presence of NGF, these
neurons contain high levels of JNK activity (Ham et al., 1995 ; Eilers
et al., 1998 ). Thus, in the presence of NGF, SCG neurons possess
activated NF- B and AP-1 transcription factors.
c-Jun has recently been implicated in the death of sympathetic neurons
after NGF withdrawal. c-Jun mRNA and protein increase after the removal
of NGF, as do JNK activity and c-Jun phosphorylation. Inhibition of
c-Jun with a dominant negative c-Jun mutant or neutralizing c-Jun
antibodies blocks apoptosis caused by NGF deprivation (Estus et al.,
1994 ; Ham et al., 1995 ). Given the ability of c-Jun to promote neuronal
death, our evidence showing that c-Rel can promote survival suggests
that NGF withdrawal-induced apoptosis may require both inactivation of
NF- B and activation of AP-1 transcription factors. The observation
that NGF-maintained neurons contain both of these activities suggests
that the mechanisms that regulate survival and cell death may be
coordinately regulated. Along these lines, several recent studies have
raised the possibility that both NF- B and AP-1 may be regulated by a
common upstream protein kinase, MEK kinase 1 (Karin and Delhase,
1998 ).
In summary, the results of our study demonstrate that NGF-dependent
sympathetic neurons have constitutive NF- B activity that can be
further induced by treatment with NGF. Because expression of c-Rel can
block apoptosis after NGF withdrawal, the activation of this pathway
may contribute to the survival mechanisms mediated by NGF. Future
studies will be needed to (1) address the relationship of NF- B to
other NGF-dependent survival pathways, such as those involving PI3K and
AKT (Yao and Cooper, 1995 ; Philpott et al., 1997 ; Crowder and
Freeman, 1998 ); (2) investigate possible interactions between NF- B
and AP-1; and (3) identify the transcriptional targets of NF- B that
are relevant for neuronal survival.
 |
FOOTNOTES |
Received Aug. 21, 1998; revised Sept. 25, 1998; accepted Oct. 7, 1998.
This work was supported by National Institutes of Health Grants PO1
MH57556 (to S.D.) and RO1 NS34400 (to R.S.F.), and Predoctoral Training
Grant ES07026 (to P.D.S.). R.S.F. also acknowledges the generous
support from the Paul Stark Endowment at the University of Rochester.
We thank Drs. W. Greene, E. M. Schwarz, and S. C. Sun for
providing research reagents and materials.
Correspondence should be addressed to Dr. Sanjay Maggirwar, Department
of Microbiology and Immunology, University of Rochester Medical Center,
Rochester, NY 14642.
 |
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