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The Journal of Neuroscience, February 15, 1998, 18(4):1318-1328
Peripheral Axotomy Induces Long-Term c-Jun Amino-Terminal
Kinase-1 Activation and Activator Protein-1 Binding Activity by
c-Jun and junD in Adult Rat Dorsal Root Ganglia In Vivo
Anna Marie
Kenney1, 2, 3 and
Jeffery D.
Kocsis1, 2
1 Department of Neurology,
2 Interdepartmental Neuroscience Program, and
3 PVA/EPVA Neuroscience Research Center, Yale University
School of Medicine, New Haven, Connecticut 06510
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ABSTRACT |
One of the earliest documented molecular events after sciatic nerve
injury in adult rats is the rapid, long-term upregulation of the
immediate early gene transcription factor c-Jun mRNA and protein in
lumbar dorsal root ganglion (DRG) neurons, suggesting that c-Jun may
regulate genes that are important both in the early post-injury period
and during later peripheral axonal regeneration. However, neither the
mechanism through which c-Jun protein is increased nor the level of its
post-injury transcriptional activity in axotomized DRGs has been
characterized. To determine whether transcriptional activation of c-Jun
occurs in response to nerve injury in vivo and is
associated with axonal regeneration, we have assayed axotomized adult
rat DRGs for evidence of jun kinase activation, c-Jun phosphorylation,
and activator protein-1 (AP-1) binding. We report that sciatic nerve
transection resulted in chronic activation of c-Jun amino-terminal
kinase-1 (JNK) in L4/L5 DRGs concomitant with c-Jun amino-terminal
phosphorylation in neurons, and lasting AP-1 binding activity, with
both c-Jun and JunD participating in DNA binding complexes. The timing
of JNK activation was dependent on the distance of the axotomy site
from the DRGs, suggesting the requirement for a retrograde
transport-mediated signal. AP-1 binding and c-Jun protein returned to
basal levels in DRGs as peripheral regeneration was completed but
remained elevated in the case of chronic sprouting, indicating that
c-Jun may regulate target genes that are involved in axonal
outgrowth.
Key words:
AP-1; axotomy; c-Jun; dorsal root ganglion; JNK; phosphorylation; regeneration; sciatic nerve
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INTRODUCTION |
Peripheral nerve transection in
adult rats induces a highly complex morphological response in affected
lumbar dorsal root ganglion (DRG) neuronal cell bodies. In the initial
stages of the post-injury period, neurons undergo disintegration of
nuclear chromatin, a shift in nuclear position, and cell body swelling, changes cumulatively referred to as the axon reaction (Lieberman, 1971 ). Surviving neurons establish a cellular and molecular program directed toward axonal elongation and reinnervation of the denervated target tissue (Lieberman, 1971 ). In regenerating sensory neurons, anterograde transport of structural proteins increases (Jacob and
McQuarrie, 1991 ), synthesis of some neurotransmitters, such as
substance P (Nielsch and Keen, 1989 ), declines, and there is increased
transcription and synthesis of tubulin isoforms (Miller et al., 1989 ;
Moskowitz and Oblinger, 1995 ) and GAP43 (VanderZee et al., 1989 ; Woolf
et al., 1990 ; Somervaille et al., 1991 ; Wiese et al., 1992 ), proteins
that are used during axonal outgrowth.
The activation of heretofore quiescent genes suggests that increased
transcription factor activity is required in injured neurons.
Peripheral axotomy induces a rapid, prolonged increase in levels of the
immediate early gene transcription factor c-Jun mRNA (Jenkins and Hunt,
1991 ; De Leon et al., 1995 ) and protein (Jenkins and Hunt, 1991 ;
Herdegen et al., 1992 , Kenney and Kocsis, 1997 ) in DRG neurons via a
retrograde transport-mediated signal (Leah et al., 1991 ; Kenney and
Kocsis, 1997 ). However, specific axotomy-induced genes regulated by
c-Jun in vivo in DRG neurons have not yet been identified,
and functional activation of c-Jun in DRGs after axotomy and during
regeneration has not been demonstrated explicitly.
In cultured cells, the transcriptional activation of c-Jun is
stimulated by phosphorylation at two amino-terminal serine residues (Binetruy et al., 1991 ; Smeal et al., 1991 , 1992 ) by c-Jun
amino-terminal kinases (JNKs) (Hibi et al., 1993 ; Derijard et al.,
1994 ), also known as stress-activated protein kinases (SAPKs) (Kyriakis
et al., 1994 ). In vitro, these kinases are rapidly activated
in response to environmentally stressful stimuli or specific membrane
receptor interactions (for review, see Davis, 1994 ; Kyriakis and
Avruch, 1996 ). However, the behavior of JNK/SAPKs remains
uncharacterized in the intact nervous system, where stimulation or
environmental perturbation may occur at a remote site from the soma,
i.e., along the axon, rather than directly at the cell body as in the
case of cultured cells.
To test the hypothesis that peripheral axotomy in vivo leads
to c-Jun activation in DRGs in a manner analogous to that of cultured
cells subjected to environmentally stressful stimuli, we assayed adult
rat DRGs for JNK activity, c-Jun amino-terminal phosphorylation, and
DNA binding by c-Jun and jun family members after peripheral axotomy.
We compared the timing of JNK activation after injury at two distances
from the DRG cell bodies to determine whether nerve injury sends an
instantaneous signal or one requiring retrograde transport to activate
the JNK pathway. To assess the correlation between c-Jun activity and
axonal regeneration, we compared c-Jun protein levels and
c-Jun-mediated activator protein-1 (AP-1) binding behavior in DRGs
after sciatic nerve transection under regeneration-permissive (crush)
or -nonpermissive (ligation with cuff) conditions.
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MATERIALS AND METHODS |
Surgical procedures. All surgeries were performed
using adult female Wistar rats (160-180 gm, 8-10 weeks of age),
anesthetized with ketamine (40 mg/kg, i.p.) and xylazine (2.5 mg/kg,
i.p.), with the right side being the experimental side. For proximal peripheral nerve transections, the sciatic nerve was exposed at the
convergence of the L4 and L5 spinal nerves, after a dorsal midline
incision, retraction of the paraspinal muscles, and removal of a small
sacral bone fragment. The spinal nerves were ligated with silk 4.0 suture (Ethicon, Somerville, NJ) and transected with iridectomy
scissors. A 2.0 mm segment of the distal nerve stump was removed. For
distal nerve transections, the sciatic nerve was exposed at the level
of the pyriform tendon. The nerve was ligated with silk 4.0 suture and
sutured into a silicone tubing cuff (Baxter Scientific, Boston, MA)
sealed with silicone rubber (Dow-Corning). Lack of regeneration after
proximal or distal nerve transections was verified at the time the rats
were killed by visual inspection of the transection site. Nerve crushes
were performed by exposing the nerve at the pyriform tendon level and squeezing with Dumont number 5 forceps (Roboz, Rockville, MD) for 20 sec. Regeneration after nerve crush was assessed in two ways at the
time rats were killed: (1) by obtaining hind footprints to determine
the extent of functional recovery of the denervated limb (Dellon and
Dellon, 1991 ; Walker et al., 1994 ) and (2) by dissecting free the
injured nerve and squeezing gently with forceps distal to proximal
toward the crush site until a reflex response was shown, as described
by Rich et al. (1984) . The distance between the crush site and the
point of response was the distance the regenerating axons had
progressed. For all surgical procedures, the contralateral (left) side
served as a sham-operated control, wherein the same site was exposed
but not transected or crushed. After surgery, the overlying skin and
muscles were sutured closed with silk 4.0 suture, and the wound was
treated with Betadine (Purdue Frederick, Norwalk, CT) to prevent
infection. Recovery was uneventful in all cases, and animals showed no
signs of autotomy. At various times after surgery, the animals were
anesthetized briefly with C02 and exsanguinated by carotid
section or perfused intracardially with ice-cold 4.0% paraformaldehyde
(PFA) (Sigma, St. Louis, MO)/0.14 M Sorensen's phosphate
buffer. Transected and contralateral sham-operated L4 and L5 dorsal
root ganglia were rapidly dissected for use in the assays described
below. For each analysis, three animals were used for every time point assayed for each surgery.
Immune-complex kinase assays. Unless indicated otherwise,
all chemicals were purchased from Sigma. In a protocol based on Kyriakis et al. (1994) , L4 and L5 DRG pairs were mechanically homogenized on ice in 1.5 ml lysis buffer containing 20 mM
HEPES, pH 7.4, 2 mM EGTA, 100 mM sodium
orthovanadate, 50 mM -glycerophosphate, 10% glycerol,
1% Triton X-100, 5 mg/ml leupeptin, 400 mM PMSF, 10 mg/ml
aprotinin, 400 mM diisopropylfluorophosphate, and 1 mM DTT. Samples were sonicated for 20 sec and kept on ice
for 15 min, and cellular debris was collected by centrifugation at
14,000 rpm, 4°C, for 5 min. Protein concentration of the supernatants was determined by a modified Bradford protein assay (Bio-Rad, Hercules,
CA). Fifty micrograms of each sample were diluted to 300 ml with lysis
buffer and added to 40 ml of swollen protein A-Sepharose CL-4B beads
(Sigma) diluted 1:1 with lysis buffer. Anti-JNK (C-17, Santa Cruz
Biotechnology, Santa Cruz, CA), 1:1000, was used in immunoprecipitation
for 6 hr at 4°C with rotation. Beads were collected by
microcentrifugation and washed once with lysis buffer, twice with high
ionic strength buffer (500 mM LiCl, 100 mM
Tris, pH 7.4, 0.1% Triton X-100, 1 mM DTT), and three
times in kinase buffer (20 mM MOPS, pH 7.2, 10 mM MgCl2, 2 mM EGTA, 0.1%
Triton X-100, 1 mM DTT). The final wash was aspirated
thoroughly, and to the 20 ml of beads was added 20 ml of kinase buffer
and 20 ml of 0.3 mg/ml GST-c-Jun (1-135) substrate, diluted in kinase buffer. Fifteen microliters of 50 mM
MgCl2/125 mM ATP containing 10 mCi
g-32P-ATP (Amersham, Arlington Heights, IL) were added to
start the reaction. Samples were incubated at 30°C for 20 min, with
frequent mixing, before the reaction was terminated by the addition of 20 ml of 6× SDS-PAGE loading buffer. Samples were vortexed and boiled
for 10 min, before separation on a 12.5% SDS-polyacrylamide gel. Dried
gels were exposed to Kodak XOMAT-LS film. Phosphorylated GST-c-Jun was
visible as a 46 kDa doublet. The intensity of the GST-c-Jun signal was
analyzed by densitometry using a Bioimage Whole Band Analysis system
(Millipore, Bedford, MA), and phosphorylation of c-Jun substrate in the
dried gels was also quantified by electronic autoradiography with an
Instantimager (Packard).
Immunoblot analysis. To prepare samples for JNK Western
blotting, L4/L5 DRGs were dissected onto dry ice and then boiled for 10 min in 150 ml 3.0% SDS. An equal volume of 0.3 M sucrose
was added, and samples were homogenized in the presence of 500 mM PMSF. Insoluble matter was pelleted by centrifugation at
14,000 rpm, 4°C, for 10 min. Protein concentration was determined by a modified Lowry assay (Peterson, 1977 ) and performed in duplicate, and
20 mg of protein was separated by SDS-PAGE through a 12.5% gel.
Proteins were transferred to an Immobilon membrane (Millipore) for
immunoblotting with anti-JNK (C-17, Santa Cruz Biotechnology) (1:2000).
For c-Jun (antibody PC06S, 2 mg/ml; Calbiochem, La Jolla, CA) Western
blots, 50 mg of each sample prepared for electrophoretic mobility shift
assays (see below) was run through a 12.5% SDS-polyacrylamide gel and
transferred to Immobilon. Primary antibody incubations were performed
in 5% dry milk, 0.1% Triton X-100, and 1× Tris-buffered saline (TBS)
overnight at 4°C. Blots were washed three times in the milk solution,
three times in 1× TBS, 0.1% Triton X-100, and then incubated for 1 hr
in HRP-conjugated goat anti-rabbit secondary antibody (Pierce,
Rockford, IL) (1:8000). Washes were repeated as above. Immunoreactivity
was visualized after development with Amersham ECL reagents and
exposure to Kodak XOMAT-AR film. ECL films were analyzed by
densitometry using a Bioimage Whole Band Analysis System
(Millipore).
Immunohistochemistry. After intracardial perfusion with
ice-cold 4% PFA, 24 hr axotomized L4 and L5 DRGs were immediately removed into 10% sucrose/4% PFA, where they were stored for 3 hr at
4°C before being transferred to 30% sucrose/4% PFA for overnight storage at 4°C. DRGs were frozen in OTC in liquid nitrogen, and 35 mM sections were cut on a cryostat. Sections were washed
twice in 1× TBS and then incubated for 1 hr at room temperature in
blocking solution containing 5% normal goat serum/0.3% Triton
X-100/0.02% sodium azide in TBS. After overnight incubation at 4°C
with phospho-specific c-Jun (Ser63) II antibody (New England Biolabs,
Beverly, MA) (1:200) in 1% normal goat serum/0.3% Triton X-100/0.02%
sodium azide in TBS, sections were washed four times with TBS, then
incubated for 3 hr with goat anti-rabbit secondary antibody (Sigma;
1:100), washed, and treated with rabbit peroxidase anti-peroxidase
(Sigma; 1:400) for 1 hr. Immunoreactivity was visualized using 0.4%
diaminobenzidine/0.003% hydrogen peroxide in TBS, with all sections
developed for 8 min before the reaction was quenched with 0.02% sodium
azide in TBS. Sections were applied to poly-L-lysine coated
slides, air-dried overnight, and then dehydrated and cleared through a
series of graded ethanols before they were coverslipped with Cytoseal
60 (Stephens Scientific).
Electrophoretic mobility shift assays (EMSAs). Unless stated
otherwise, all chemical reagents are from Sigma. L4/L5 DRGs were homogenized on ice in 500 ml of EMSA lysis buffer, pH 7.9, containing 20 mM HEPES, 0.4 M NaCl, 20% glycerol, 5 mM MgCl2, 500 mM EDTA, 500 mM EGTA, 1% Nonidet P-40, 0.1 M benzamidine,
10 mg/ml leupeptin, 1 mg/ml pepstatin A, 1 mM okadaic acid
(Life Technologies, Gaithersburg, MD), 1 mM PMSF, and 5 mM DTT. After sonication for 20 sec, samples were placed on
ice for 20 min and then centrifuged for 25 min at 14,000 rpm, 4°C.
Protein concentrations were determined by a modified Bradford assay
(Bio-Rad), and 6 mg of protein was used in each binding reaction. Human
metallothionein IIA AP-1 consensus and mutant
oligonucleotides derived from Lee et al. (1987) were purchased from
Santa Cruz Biotechnology. Probes, labeled to high specific activity
(50,000-100,000 cpm/ng) using T4 polynucleotide kinase (New England
Biolabs) and -32P-ATP (Amersham), were purified twice
over G-25 spin columns (Boehringer Mannheim, Indianapolis, IN), and 1 ng was used per reaction. Binding reactions were performed for 20 min
at room temperature in binding buffer containing 10 mM
Tris, pH 7.5, 25 mM NaCl, 1 mM EDTA, 5% glycerol, 1 mg polydI-dC (Pharmacia, Piscataway, NJ), and 5 mM DTT. For competition studies, 1, 10, or 20 ng of
unlabeled consensus or mutant oligonucleotides were incubated with
protein samples in binding buffer for 15 min at room temperature,
before addition of 1 ng of labeled probe as above. For supershift
assays, 1-2 mg of antibody (c-Jun Ab-1, Calbiochem; junB (N-17) X and
JunD (329) X, Santa Cruz Biotechnology) was incubated with the protein sample in binding buffer for 30 min at room temperature before addition
of the labeled probe. Reactions were separated through a 6%
nondenaturing polyacrylamide gel and then dried and exposed for 24 hr
to Kodak XOMAT-LS film at 80 C. Autoradiographs were quantified by
densitometry and electronic autoradiography as described above.
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RESULTS |
Axotomy induces prolonged activation of JNK in DRGs
To determine whether peripheral nerve axotomy is a stimulus for
the in vivo activation of jun kinases, we assayed the level of JNK activity in axotomized DRGs in comparison with the
contralateral, untransected DRGs, using an immune complex kinase assay.
We also compared the timing of JNK activation after nerve transections either close to or distant from the DRG cell bodies to ascertain whether JNK activation results from an instantaneously communicated signal or is dependent on one that is axonally transported from the
injury site. At times from 15 min to 30 d after sciatic nerve transection, JNK was immunoprecipitated from DRGs and tested for its
activity toward a c-Jun substrate. For each time point, three animals
were used, and JNK activity was determined by densitometry or direct
quantitation of substrate phosphorylation (see Materials and Methods)
(Fig. 1). Within 30 min of proximal
transection (~1 cm from the DRG cell bodies), there was a 60%
increase in the level of JNK activity in axotomized DRGs, which
continued to increase to threefold over control levels (Fig.
1a). By 30 d after axotomy, JNK activity was still at
least double that of the contralateral, uninjured side (Fig.
1a). Sciatic nerve transection at the mid-thigh, which is
~4 cm from the DRGs, did not induce significant increases in JNK
activity until 3 hr after axotomy (Fig. 1b). After mid-thigh transection, JNK activity rose to threefold over control levels by
7 d, where it remained until at least 30 d post-injury, the latest time point evaluated (Fig. 1b). We also observed that
the level of JNK activity in axotomized DRGs appeared to vary more after proximal than after distal nerve transection (Fig.
1a,b). Although JNK activity at each time point from 30 min
to 30 d after proximal axotomy was significantly higher than that
of the contralateral, sham-operated DRGs (Fig. 1a), when
comparisons were made between injured DRGs, values for JNK activity
from 6 hr to 7 d after proximal axotomy did not differ
significantly from each other.

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Figure 1.
JNK is activated in vivo in dorsal
root ganglia (DRG) after peripheral nerve transection, with timing
dependent on distance of the lesion from the DRGs. JNK activity in
L4/L5 DRGs was assessed using an immune complex kinase assay at times
from 15 min to 30 d after proximal (a) or
distal (b) nerve transection. Levels of JNK
activation were determined as a ratio of the activity in the injured
versus contralateral, uninjured DRGs. The graph shows the mean levels
of activation in injured DRGs, with error bars indicating SEM. The
dotted line represents the average level of JNK activity
in right versus left DRGs of three unoperated rats. For each time
point, n = 3 animals. p values were
calculated using the Student's t test;
*p < 0.05 or **p < 0.01 in
comparison of injured versus control DRGs. Below each graph are
representative autoradiographs from selected time points, showing
GST-c-Jun fusion protein phosphorylated by immunoprecipitated JNK
isolated from injured (I) and
contralateral uninjured (C) DRGs. Each
I/C represents DRGs from an individual rat.
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The timing of JNK activation after either proximal or distal
transection preceded that reported for c-Jun protein induction after
injury at each site, respectively: c-Jun protein increases have been
observed 3 hr after proximal axotomy and 6 hr after distal axotomy
(Kenney and Kocsis, 1997 ). c-Jun upregulation occurs after activation
of c-Jun-containing complexes that bind an AP-1 site in the c-Jun
promotor (Angel et al., 1988 , Van Dam et al., 1993 ). The
axotomy-induced prolonged activation of JNK that we report here is
likely to contribute to the long-lasting upregulation of c-Jun mRNA and
protein in DRGs after axotomy. The delay in protein upregulation may
reflect the time required for complex activation and subsequent mRNA
and protein synthesis to reach levels within the limits of detection by
the immunoblot method used previously.
The surgical procedure for proximal transections is more traumatic than
that for mid-thigh transection, raising the possibility that earlier
JNK activation could reflect the invasiveness of the procedure.
However, kinase activity in each L4/L5 DRG sample was determined as a
ratio of that in the contralateral, sham-operated side, which underwent
the same surgery, with the exception of nerve transection. Basal JNK
activity was observed in contralateral sham-operated DRGs after both
proximal and distal transection, which did not reflect a systemic
response to surgery, because similar levels of JNK activity were
observed in DRGs removed from unoperated animals (data not shown). It
is possible that this activation occurred when the animals were killed,
although the speed with which anesthesia, dissection, and
homogenization were accomplished makes this unlikely. Thus, the
rapidity of JNK induction after proximal as compared with distal
peripheral axotomy indicates that the timing of JNK activation is
likely to be dependent on the length of axon between the transection
and the DRGs.
JNK protein levels do not change after axotomy
In vitro, JNK is activated by phosphorylation at Thr183
and Tyr185 (Derijard et al., 1994 ). JNK activity in cultured cells is
frequently transient, lasting minutes to several hours after stimulation (Derijard et al., 1994 ; Chen et al., 1996 ; Zhang et al.,
1996 ). However, peripheral nerve transection invoked JNK activation in
DRGs lasting at least 30 d. The high level of JNK activity that we
observed in uninjured DRGs (Fig. 1) suggested that there may be a pool
of constitutively active JNK in vivo in DRGs, raising the
possibility that the long-term maintenance of increased JNK activity
could be attributed to greater levels of JNK protein in axotomized
DRGs. We measured JNK protein levels in DRGs by Western blotting at
various times after proximal or distal peripheral nerve transection.
Axotomy did not induce increased JNK protein at times up to 21 d
after axotomy (Fig. 2). Therefore, JNK
activity in injured DRGs is likely to be maintained by persistent phosphorylation of the constitutively expressed protein at Thr183 and
Tyr185

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Figure 2.
JNK protein levels do not change after axotomy.
Protein extracts prepared from injured and contralateral uninjured DRGs
were immunoblotted for JNK at progressive times after proximal ( ) or
distal ( ) axotomy. The intensity of the JNK signal was
measured by densitometric analysis of ECL autoradiographs, and a ratio between injured and contralateral DRGs was determined. Each data point
is representative of the injured/contralateral JNK ratio in three
different animals. Error bars represent SEM. There was no statistically
significant difference in JNK levels between injured and contralateral,
uninjured DRGs at any time, for either surgical procedure.
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Phosphorylated c-Jun is present in axotomized DRG neurons
Substrates of JNK include c-Jun (Hibi et al., 1993 ; Derijard et
al., 1994 ), ATF2, and Elk1 (Gupta et al., 1995 , 1996 ; Whitmarsh et al.,
1995 ), and to a lesser extent JunD (Kallunki et al., 1996 ). Amino-terminal phosphorylation of c-Jun by JNK does not affect its
ability to bind to target DNA sequences, but rather allows it to
interact with CREB-binding protein (Arias et al., 1994 ), which in turn
can bind to components of the basal transcriptional machinery (Kwok et
al., 1994 ). Using immunohistochemistry of free-floating DRG sections
with an antibody specific for amino-terminal (Ser63) phosphorylated
c-Jun, we detected intense, punctate nuclear staining in DRG neurons,
but not satellite cells, examined 24 hr after axotomy (Fig.
3a). Some faint nuclear
immunoreactivity was seen in the contralateral, uninjured DRGs (Fig.
3b). However, we have commented above on the apparent
constitutive activity of JNK, and immunoblots for c-Jun protein show
strong expression in unoperated DRGs (Fig. 6b,d). These
immunostaining results confirm that increased c-Jun protein levels are
accompanied by increased amounts of the phosphorylated protein in DRG
neurons. Ser63-phosphorylated c-Jun immunoreactivity was observed in
both small and large DRG neurons (Fig. 3), indicating that like c-Jun
protein upregulation, c-Jun phosphorylation is a general response to
axotomy shared by all classes of axotomized DRG neurons.

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Figure 3.
Phosphorylated c-Jun is localized to axotomized
DRG neurons. Twenty-four hours after proximal nerve transection,
axotomized (a) and contralateral untransected
(b) DRGs were removed and processed as 35 mM free-floating sections for immunostaining with an
antibody specific for serine-63 phosphorylated c-Jun. Intense nuclear
staining was restricted to the nuclei of DRG neurons and was strongly
evident in axotomized DRGs. Sections shown are representative of
immunostaining in DRGs from three separately analyzed animals.
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AP-1 binding activity is induced in axotomized DRGs
c-Jun belongs to the basic, leucine zipper group of transcription
factors, which initiate gene transcription after binding in dimeric
form to a recognition sequence, known as
12-O-tetradecanoylphorbol-13-acetate response element (TRE)
or AP-1 binding site, (Angel and Karin, 1991 ), located in the promotor
region of target genes. c-Jun may form homodimers or may heterodimerize
with other basic leucine zipper transcription factors, including
members of the fos, jun, and CRE-binding protein families (Angel and
Karin, 1991 ). The selection of target genes of c-Jun as well as the
levels of transcriptional activation of c-Jun therefore may be
controlled by the identity of its DNA-binding partners. To determine
whether axotomy in vivo induces DNA binding by c-Jun, and to
identify possible partners for c-Jun in DNA binding, we investigated
the effect of axotomy on the formation and composition of AP-1 binding
complexes in DRGs at various times after injury, using an AP-1
consensus sequence based on the human metallothionein IIA
(Hmt IIA) promotor in electrophoretic mobility shift
assays with antibodies to various jun family members. We did not
examine DRGs for AP-1 binding by c-fos, a highly stable partner for
c-Jun (Angel and Karin, 1991 ), because studies using various techniques
have not detected this transcription factor in DRGs (Herdegen et al.,
1992 ; Plantinga et al., 1994 ). ATF2, a member of the CRE-binding
protein family, can dimerize with c-Jun to initiate c-Jun transcription
from a specialized TRE in the c-Jun promotor (van Dam et al., 1993 ) but
has been shown not to bind to the consensus AP-1 sequence used in our
study (Hai and Curran, 1991 ).
We observed specific, axotomy-induced binding to the Hmt
IIA AP-1 sequence; incubation of 72 hr axotomized DRG
extracts with unlabeled Hmt IIA AP-1 oligonucleotides
prevented binding to the labeled oligonucleotide, whereas treatment
with a mutant oligonucleotide differing by one base pair in the
consensus AP-1 sequence did not affect activity of the induced
complexes (Fig. 4). There was no specific
binding activity associated with the labeled mutant oligonucleotide
(Fig. 4). Similar results were obtained with DRGs assayed 24 hr after
proximal transection (data not shown).

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Figure 4.
Specific AP-1 binding activity in axotomized DRGs.
Protein extracts from injured (lanes 1, 3-9) and
contralateral uninjured (lanes 2,10) DRGs were assayed
for their ability to bind radiolabeled Hmt IIA AP-1
(lanes 1-8) or mutant (lanes 9, 10)
sequences, at 72 hr after proximal axotomy. Incubation of injured DRG
extracts with the indicated amounts of unlabeled AP-1 (lanes
3-5) reduced the amount of axotomy-induced binding to
radiolabeled AP-1, whereas there was no reduction in specific AP-1
binding in the presence of unlabeled mutant AP-1 oligonucleotides
(lanes 6-8). A band of more rapid mobility appearing in
both injured and uninjured DRGs was competed by both AP-1 and mutant
oligonucleotides, indicating that this protein complex does not
specifically recognize the Hmt IIA AP-1 sequence, nor does
it have high affinity for the mutant AP-1 (lanes 9, 10).
We focused our studies on the more slowly migrating, injury-induced
complex, indicated by <.
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De Leon et al. (1995) reported that AP-1 binding activity is present in
DRGs examined at 5 d after mid-thigh sciatic nerve transection.
Supporting and expanding on these results, we observed elevated AP-1
binding in axotomized L4/L5 DRGs assayed from 12 hr to 30 d after
proximal nerve transection (Fig. 5). At
12 hr post-axotomy, AP-1 binding activity in injured DRGs was eightfold (±0.8 SEM; n = 3) that of control levels. At 3, 7, and
14 d, increased AP-1 binding activity persisted in axotomized DRGs
(ninefold that of control; ±0.9 SEM; n = 3 animals per
time point). However, by 30 d after proximal axotomy, AP-1 binding
activity dropped to 3.5-fold (±0.4 SEM; n = 3) that of
control levels. This drop may represent a reduction in c-Jun activity,
because it coincides with a decrease in JNK activity at this time (Fig.
1a). It is also possible that the diminished JNK activity
and AP-1 binding in 30 d proximally axotomized DRGs is indicative
of some neuronal cell death, which is more prevalent in proximally
axotomized DRGs during the weeks after nerve transection (Ygge,
1989 ).

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Figure 5.
AP-1 binding activity by c-Jun and JunD in L4/L5
DRGs after proximal sciatic nerve transection. At the indicated times
after proximal peripheral nerve transection, DRG protein extracts from injured (I) and contralateral uninjured
(C) DRGs were assayed for activity toward an Hmt
IIA AP-1 sequence, in the presence of antibodies against
various jun family members. Shown are autoradiographs representative of
DRGs from three separately analyzed rats at each time point. All
animals were included in quantitative densitometric analysis discussed
in the text (see Results). The level of AP-1 binding activity in each
sample was determined as a ratio of AP-1 binding activity in injured
versus contralateral, uninjured samples. In the case of antibody
supershift experiments, a ratio was drawn between AP-1 binding activity
in injured DRG samples with antibody versus without antibody.
Antibodies used are indicated above each lane. A diffuse
signal strongly present in extracts from injured DRGs ( AP-1) was not affected by incubation with antibodies to junB. Antibodies to c-Jun blocked formation of this complex, because these antibodies interfere with the DNA-binding domain of c-Jun. JunD
antibodies diminished formation of the injury-induced complex and
caused a portion to be retained near the top of the gel. The extent to
which junD antibodies diminished formation of the specific AP-1 complex
was quantified by densitometric analysis of autoradiographs, as
described above and in Results. NS indicates a
nonspecific, noninduced complex.
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Treatment of proximally transected DRG extracts with an antibody
against the DNA-binding domain of c-Jun before incubation with the AP-1
oligonucleotide interfered with formation of the induced complex at all
times after injury (Fig. 5), suggesting that AP-1 binding complexes in
axotomized DRGs contain c-Jun, as a homodimer or in complex with other
AP-1 binding proteins. c-Jun antibodies caused a 60-80% reduction in
AP-1 complex formation at each time point, with three animals analyzed
per time point. The increased AP-1 binding activity of c-Jun after
axotomy indicates that in addition to positive regulation of c-Jun
through activation of JNK, nerve injury may influence signaling
pathways, leading to dephosphorylation of negative regulatory sites
adjacent to the DNA binding domain of c-Jun (Boyle et al., 1991 ), one
mechanism that can lead to increased DNA binding affinity of c-Jun.
An increase of junB mRNA (De Leon et al., 1995 ) and transient increases
in junB protein (Kenney and Kocsis, 1998 ) have been reported in DRGs
several days after axotomy. JunB may downregulate c-Jun activity by
forming heterodimers that are less efficient at binding to DNA (Deng
and Karin, 1993 ) and are not efficiently phosphorylated by jun kinases
(Kallunki et al., 1996 ). Treatment of axotomized DRG extracts with an
antibody to junB altered neither the formation nor the mobility of the
Hmt IIA AP-1 binding complexes (Fig. 5). Increasing the
amount of antibody used in the binding reactions or varying the time
and temperature at which reactions took place did not have any effect.
When tested by ECL immunoblotting on 72 hr and 7 d axotomized
DRGs, the junB supershift antibody recognized a 46 kDa species (data
not shown), in agreement with previously reported junB immunoblotting
in DRGs (Kenney and Kocsis, 1998 ), indicating that the JunB supershift
antibody that we used can bind to junB, albeit in the denatured form
for SDS-PAGE and electrophoretic transfer. However, the lack of well
established positive controls for in vivo junB AP-1
supershifts prevents us from completely ruling out the participation of
junB in Hmt IIA AP-1 binding complexes in axotomized
DRGs.
In contrast, antibodies to JunD caused a reduction in the specific AP-1
binding complex signal, accompanied by retention of a portion of the
AP-1 binding complex near the origin of the gel (Fig. 5). The signal
shifted toward the top of the gel by the junD antibody represents
specific activity of this antibody toward components of the AP-1
binding complex; formation and mobility of complexes binding to CRE
(cAMP/Ca2+ response element) oligonucleotides, for
which jun family members have less affinity (Angel and Karin, 1991 ),
were not affected by addition of the junD antibody, whereas the
addition of a CREB (CRE-binding protein) antibody prevented CRE-binding
complex formation (data not shown). When autoradiographs were analyzed
by densitometry, the amount of axotomy-induced AP-1 binding complex
formed in the presence of the JunD antibody was reduced by 30-50% at
all times after proximal axotomy, with three animal analyzed at each
time point. JunD protein, normally strongly present in both neurons and
satellite cells of DRGs (Herdegen et al., 1992 ; De Leon et al., 1995 ),
has been reported to increase modestly in DRGs after axotomy (Herdegen
et al., 1992 ; Kenney and Kocsis, 1998 ). Because c-Jun antibodies
abolished more of the AP-1 binding complexes, we suggest that JunD may
be heterodimerizing with c-Jun, in which context JunD can be
phosphorylated by jun kinases (Kallunki et al., 1996 ) to activate AP-1
responsive transcription, although the transactivation capacity of
these heterodimers may be less than that of c-Jun/c-Jun homodimers
(Angel and Karin, 1991 ).
c-Jun-containing AP-1 complexes are present during
axonal outgrowth
We have observed that nerve transection induces persistent
activation of JNK and AP-1 binding with c-Jun and JunD for at least 30 d post-injury in adult rat DRGs, suggesting that c-Jun is
activated and mediates gene transcription both early and later after
nerve injury. Using immunohistochemistry with an antibody recognizing all jun family members, Leah et al. (1991) reported that jun protein levels diminish in adult rat DRGs because process regeneration is
completed after sciatic nerve crush. To learn whether c-Jun activity is
correlated with axonal regeneration, we compared AP-1 binding activity
between axotomized DRGs under regeneration-permissive or -nonpermissive
conditions, and we used quantitative immunoblotting (Kenney and Kocsis,
1997 , 1998 ) to quantify c-Jun protein levels in the same samples. One
day after both nerve crush, which permits target reconnection, and
transection with cuff, which results in chronic neurite outgrowth and
neuroma formation, AP-1 binding activity and c-Jun protein levels were
increased (Fig. 6). Incubation of injured
DRG extracts with c-Jun antibodies reduced formation of AP-1 binding
complexes (Fig. 6a,c) by 60-80% (n = 3 animals per surgery, per time point), indicating that both distal
sciatic nerve transection and crush increase the DNA binding behavior of c-Jun.

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Figure 6.
AP-1 binding and c-Jun protein levels in affected
DRGs after distal ligation with cuff or sciatic nerve crush. At the
indicated times after mid-thigh sciatic nerve transection with cuff
(a, b) or crush (c, d), injured
(I) or contralateral uninjured
(C) DRGs were analyzed by EMSA with Hmt
IIA AP-1 oligonucleotides (a, c) or enhanced
chemiluminescent Western blotting for c-Jun protein (b,
d). Shown are autoradiographs representative of samples analyzed from three rats at each time point, for each surgery. All
animals were included in quantitative densitometric analysis described
in Results. The level of injury-induced AP-1 binding activity in each
sample was determined as a ratio of AP-1 binding activity in injured
versus contralateral, uninjured samples. The participation of c-Jun in
AP-1 binding activity was determined by the ability of an antibody
against the c-Jun DNA binding domain to inhibit formation of AP-1
binding complexes. Samples treated with the c-Jun antibody are
indicated above the lane (*). The specific AP-1 binding
complex signal is shown by <. Samples analyzed by EMSA were also
immunoblotted for c-Jun, under previously optimized experimental
conditions that have been established to quantify changes in c-jun
protein levels in axotomized DRGs using chemiluminescent detection
(Kenney and Kocsis, 1997 , 1998 ), also described in Materials and
Methods. The c-Jun antibody used recognizes four major species: 80, 60, 46, and 30 kDa. Only the doublet appearing at 46 kDa is regulated by
axonal transection. When extracts from UV-irradiated NIH 3T3 cells were
immunoblotted for c-Jun using this antibody, the same hybridization
pattern was observed, and the 46 kDa doublet was the only species
induced by UV light. Therefore, this species was understood to
represent c-Jun and was quantified by densitometric analysis of ECL
autoradiographs. A ratio of c-jun immunoreactivity in injured versus
the contralateral uninjured DRGs was then established to determine
axotomy-induced changes in c-jun protein levels.
|
|
Two methods were used to assess the progression of axonal regeneration
after nerve crush. First, hind footprints of the nerve-crushed and
contralateral uninjured foot were obtained. Second, the sciatic nerve
was dissected free and pinch-tested to determine the advance of the
axonal regeneration front (see Materials and Methods) before DRG
removal. Twenty-four hours after sciatic nerve crush, the right
(axotomized) foot was curled (Fig. 7),
showing foot drop, and was little used by the animals, coincident with
increased AP-1 binding. A 1.5-fold (±0.15 SEM; n = 3 animals per surgery, per time point) increase in c-Jun protein levels
in affected DRGs (Fig. 6c,d) was observed after either crush
or transection with cuff. At 72 hr and 7 d post-crush, c-Jun
protein levels in injured DRGs were 2.4-fold (±0.25 SEM at 72 hr,
±0.3 SEM at 7 d; n = 3 animals per surgery, per
time point) that of control levels (Fig. 6d). Eight- to
10-fold increased AP-1 binding activity was present in the same animals
at 24 and 72 hr after crush. By 7 d after nerve crush, AP-1
activity was reduced, although c-jun protein levels remained elevated
(Fig. 6d). The earlier reduction in AP-1 binding activity in
comparison with c-Jun protein levels may reflect activity of pathways
leading to phosphorylation of c-Jun at sites near its C terminal, which
can interfere with its DNA binding activity (Boyle et al., 1991 ). By
comparison, AP-1 binding activity continued to be present at similarly
high levels at all time points, with three animals analyzed per time
point, after transection with cuff. As target reinnervation after crush
progressed, the amount of foot drop was reduced (Fig. 7), until at
30 d post-crush the rats showed comparable use of both hindfeet.
The length of nerve between the original crush site and the site at
which nerve pinching elicited local muscle reflex activity, indicating
the leading edge of regenerating axons, increased from 1-1.5 cm at 7 d to 2.5 cm at 14 d post-injury. At 21 and 30 d it was
no longer possible to access any portion of nerve that did not induce a muscle reflex in response to pinching.

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Figure 7.
Hind footprints of animals at progressive times
after right-side sciatic nerve crush. Regeneration of the sciatic nerve
axons after crush was assessed just before animals were killed by
obtaining hind footprints after the animals were anesthetized, and then during the dissection procedure by measuring the anatomical site of the
axonal regeneration front (see Results). Footprints are shown for
selected times after right-side nerve crush. n = 3 animals for each time point. DRGs removed from these animals were
analyzed by EMSA and c-Jun immunoblotting (see Results and Fig.
6).
|
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As regeneration became complete, AP-1 binding activity declined (Fig.
6c), and by 30 d post-crush c-Jun protein levels in injured DRGs were no longer significantly different from contralateral uninjured DRGs (Fig. 6d), as measured by densitometric
analysis of ECL immunoblot autoradiographs. In contrast, chronically
axotomized DRGs showed elevated AP-1 binding activity and c-Jun protein
throughout the 30 d of study (Fig. 6a,b). The protein
analysis results agree with previously reported kinetics of c-Jun
protein regulation after distal axotomy (Kenney and Kocsis, 1997 ). A
pattern of increased AP-1 binding activity closely followed c-Jun
protein upregulation in distally transected DRGs. AP-1 binding activity
can be affected by complex composition and post-translational
modification of components. This is reflected in the larger
fold-induction of AP-1 binding activity in comparison with c-Jun
protein level increases. c-Jun protein elevation can be both a cause
and an effect of AP-1 binding activity.
 |
DISCUSSION |
Axotomy-induced c-Jun activation
We have found that in the intact peripheral nervous system,
axotomy is a stimulus that causes rapid, long-term activation of JNK
and is associated with c-Jun amino-terminal phosphorylation and DNA
binding by c-Jun and JunD in DRG neurons. We have also shown that
persistent c-Jun-mediated DNA binding occurs during chronic target
disconnection, whereas peripheral nerve regeneration is accompanied by
decreased AP-1 binding and c-Jun protein levels over time. Other
physiologically relevant JNK activators in the nervous system in
vivo have not been identified. In neuronal tissue in
vitro, glutamate has recently been shown to induce JNK activation and c-Jun transcriptional activity in primary neuronal cultures from
striatum (Schwarzschild et al., 1997 ). Zhang et al. (1996) recently
reported that in primary glial cell cultures, JNK activation is induced
by the same types of environmental insults shown to activate JNK in
other cultured cell systems.
The mechanisms underlying the axotomy-induced in vivo JNK
activity and long-term c-Jun upregulation and activity in DRGs remain unknown. We found that JNK protein levels are not altered after axotomy, suggesting that long-term post-translational modification of
constitutively expressed JNK takes place in axotomized DRGs. The 3 hr
delay before JNK activation in DRGs after mid-thigh sciatic nerve
transection in comparison with the rapid (30 min) activation after
proximal nerve transection that we report here fits well with the fast
component axonal transport rate of 50-400 mm/d (Jacob and McQuarrie,
1991 ), suggesting that a retrogradely transported signal mediates
axonal injury-induced activation of JNK.
Wu et al. (1993) showed that blockage of normally transported molecules
can alter neuronal gene expression. In a similar manner, nerve
transection could disinhibit JNK activity by interfering with the
somatopedal flow of a constitutively transported modulatory molecule.
However, the present study cannot differentiate between deprivation of
a negative control signal and generation of a positive signal at the
injury site that travels in a retrograde manner to the DRG cell body to
stimulate the JNK cascade. An example of intra-axonal
post-translational modification and retrograde transport of signaling
molecules can be found in Aplysia after nerve injury
(Povelones et al., 1997 ), where axotomy results in rapid inactivation
of an NF- B homolog in axons. In rat sciatic nerve, retrograde
transport of molecules in the ERK pathway has been observed (Johanson
et al., 1995 ). Thus, sciatic nerve transection could cause JNK or other
JNK/SAPK pathway components to be activated at the site of nerve
injury, then translocated to the DRG soma, leading to c-Jun
phosphorylation, which we observed in axotomized DRG neurons.
Long-term maintenance of the activated JNK pathway after peripheral
nerve transection may be supported by many signals that are generated
within the axotomized DRG neurons and throughout the injured nerve. For
example, the cellular swelling experienced by axotomized neurons
(Lieberman, 1971 ) could cause changes in osmotic balance, which can
activate JNK in Chinese hamster ovary cells (Galcheva-Gargova, 1994 ).
Axotomy leads to proliferation of resident and invading macrophages (Lu
and Richardson, 1993 ), and TNF , which activates JNKs in
vitro (Kyriakis, 1994 ; Sluss, 1994 ; Su, 1994 ), has also been
detected in axotomized DRGs (Murphy et al., 1995 ). The association of
an inflammatory response in DRGs with axonal regeneration (Lu and
Richardson, 1991 ) and increased c-Jun mRNA in DRG neurons (Lu and
Richardson, 1995 ) provides further evidence of a relationship between
pathways that can activate JNK in DRGs, c-Jun upregulation, and axonal
regeneration.
c-Jun activity in regenerating DRG neurons
In many systems, JNK activation and c-Jun activity are often
associated with either proliferation (Derijard et al., 1994 ; Su et al.,
1994 ) or cell death (Estus et al., 1994 , Ham et al., 1995 ; Verheij et
al., 1996 ). However, peripherally axotomized adult rat DRG neurons are
nonmitotic cells, the majority of which do not die as a result of
injury (Arvidsson et al., 1986 ; Himes and Tessler,1989 ; Swett et al.,
1995 ) but rather initiate axonal extension. We report here that in
peripherally axotomized DRG neurons, the activation domain of c-Jun is
phosphorylated and there is an increase in AP-1 binding activity in
DRGs, with c-Jun and JunD participating in binding complexes. Although
downstream target genes are not known, we found that increased AP-1
binding by c-Jun, and elevated c-Jun protein levels, persist for at
least 30 d after axotomy if successful regeneration is blocked,
but diminish over time if axons are permitted to proceed to target reinnervation after nerve crush, suggesting that c-Jun is active in
DRGs while they are making regenerative attempts and that c-Jun activity persists as long as DRG neurons have not reestablished contact
with peripheral target tissue.
UV-induced c-Jun phosphorylation in vitro (Devary et al.,
1992 ), and in vivo JNK activity in perfused heart
(Bogoyevitch et al., 1996 ) or exercised muscle (Goodyear et al., 1996 )
have been proposed to have a protective or growth-promoting function.
JNK activation and c-Jun activity in DRGs could serve in a similar capacity, initially promoting survival after injury and later operating
as part of a cellular growth program. To support the increased energy
requirements imposed by a growth rate of 2 mm/d (Jacob and McQuarrie,
1991 ), regenerating neurons show an increase in the activity of enzymes
involved in oxidative metabolism (Harkonen, 1964 ). c-jun could play a
protective role in metabolic control in regenerating DRG neurons. A
similar model has been found in rat vascular smooth muscle cells, where
AP-1 activity and growth have been linked to oxidative stress (Rao et
al., 1996 ). In yeast, the c-Jun homolog PAR1/YAP1 protects cells
against oxidative damage by regulating genes involved in oxygen
detoxification (Schnell et al., 1992 ; Wu and Moye-Rowley, 1994 ), and in
some mammalian cells under oxidative stress, AP-1 regulates proteins
involved in detoxification of free radicals (Pinkus et al., 1996 ;
Rahman et al., 1996a ,b ).
c-Jun may regulate genes specifically pertinent to axonal elongation,
such as GAP43, which has AP-1 binding sites in its promotor (Eggen et
al., 1994 ). Like c-Jun, GAP43 expression increases in DRGs within hours
after sciatic nerve crush, and GAP43 mRNA levels drop as regeneration
is completed (VanderZee et al., 1989 ). Vasoactive intestinal peptide, a
neuropeptide the expression in vivo of which is also
upregulated after peripheral axotomy (Shehab and Atkinson, 1986 ; Nielsh
and Keen, 1989 ) and which can have neuroprotective effects (Gozes and
Brenneman, 1996 ), is a target for c-Jun transcriptional regulation in
cultured DRG neurons (Mulderry and Dobson, 1996 ).
Concluding remarks
We have found that axotomy elicits chronic activation of the JNK
pathway in adult rat DRGs in vivo. We have shown that this pathway may contribute to increased levels and trans-activating capacity of c-Jun in DRG neurons by demonstrating that JNK activation is concurrent with amino-terminal phosphorylation of c-Jun in DRG
neurons, and increased AP-1 binding activity by c-Jun, as well as JunD.
Although it is difficult to parse out the signals specifically
responsible for activation and maintenance of the JNK signal
transduction pathway, or the effector genes downstream of c-Jun
transcriptional activation, using in vivo models, our observations support the hypothesis that peripheral nerve transection induces prolonged transcriptional activation of c-Jun in
vivo, through which genes important for neuroprotection as well as
axonal regrowth may be regulated.
It is known that the kinetics of c-Jun protein regulation in axotomized
peripheral neurons differs from that in axotomized CNS neurons (Leah et
al., 1993 ), which do not normally reconnect with their targets after
transection. This may indicate that different signal transduction
pathways are activated by CNS injury, leading to a coordination of
c-Jun expression with other transcription factors that fail to foster a
regenerative response. It has also been suggested that the CNS contains
factors that inhibit c-jun expression (Vaudano et al., 1996 ).
Interestingly, although the cell bodies of sciatic nerve motor neurons
reside in the CNS, after peripheral axotomy they manifest a c-Jun
response resembling that of transected peripheral ganglion sensory
neurons (Herdegen et al., 1992 ), and they regenerate their axons after
peripheral transection, with jun family protein levels declining after
completion of target tissue reinnervation (Leah et al., 1991 ). This
suggests that the peripheral environment provides unique signals that
regulate activity of transcription factors like c-Jun in a manner that promotes a growth response in neurons, and that these signals may be
absent or inhibited in the CNS.
An increased clarification of activated intracellular signaling
pathways and transcription factor functions in injured peripheral neurons in vivo will aid in the understanding of molecular
events that regulate both adaptive and maladaptive responses to nerve injury. For example, the regenerative role of c-jun may have
implications for the phenomenon of conditioned collateral sprouting,
wherein nerve injury and peripheral axon regrowth is associated with
the simultaneous vigorous sprouting (McMahon and Kett-White, 1991 ) and
formation of inappropriate synaptic connections in the spinal cord by
central processes of regenerating peripheral axons (Woolf et al.,
1995 ). This may contribute to the pathophysiology of allodynia, in
which light touch sensations are perceived as painful stimuli. In
addition, characterization of the signals that permit or encourage peripheral nerve regeneration may help identify factors the absence or
inhibition of which prevents successful regeneration and recovery of
function after injury to CNS neurons.
 |
FOOTNOTES |
Received Aug. 13, 1997; revised Dec. 2, 1997; accepted Dec. 5, 1997.
This work was supported in part by National Institutes of Health (NS
10174) and the Medical Research Service of the Department of Veterans'
Affairs. We thank John Kyriakis (Massachusetts General Hospital/Harvard
University, Boston, MA) for his permission to use c-jun (1-135) GST
fusion protein. We are sincerely grateful to Michael A. Schwarzschild
for his generous advice and thoughtful review of this manuscript. We
also thank Philippe Male for his assistance with
quantitation.
 |
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