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Volume 17, Number 16,
Issue of August 15, 1997
pp. 6057-6063
Copyright ©1997 Society for Neuroscience
Glutamate-Dependent Activation of NF- B During Mouse Cerebellum
Development
Luisa Guerrini1,
Angela Molteni1,
Thomas Wirth3,
Barbara Kistler3, and
Francesco Blasi1, 2
1 Department of Genetics and Microbial Biology,
University of Milan, 20133 Milan, Italy, 2 Dipartimento di
Biologia e Biotecnologia, San Raffaele Scientific Institute, 20100 Milan, Italy, and 3 Institute fuer Medinische Strahlenkunde
und Zellforschung, 97078 Wurzburg, Germany
ABSTRACT
INTRODUCTION
MATERIALS AND METHODS
RESULTS
DISCUSSION
FOOTNOTES
REFERENCES
ABSTRACT
NF- B and activator protein 1 (AP-1) are dimeric
transcription factors involved in transcriptional regulation in many
cells, including neurons. We have examined their activity during mouse cerebellum development, a postnatal process starting just after birth
and completed by the fourth postnatal (PN) week. The activity of these
factors was analyzed by binding of nuclear extracts to a synthetic
oligonucleotide representing the B site of human immunodeficiency
virus or the AP-1 site of the urokinase promoter. NF- B activity was
observed from 7 PN, was restricted to the developing cerebellum, and
was not observed in the early postnatal neocortex and hippocampus. On
the other hand, AP-1 activity was not found in cerebellum but was
present in both neocortex and hippocampus. Moreover, a B-driven
transgene was found to be increasingly expressed in the cerebellum from
5 PN to 10 PN but not in the adult. The regulation of NF- B
activation in mouse cerebellum was analyzed by intraperitoneal
injection of glutamate receptor antagonists to 9 PN mice, which
abolished NF- B-binding activity, suggesting an endogenous loop of
glutamate receptor activation. Glutamate receptor agonists, on the
other hand, induced NF- B nuclear translocation in the cerebellum of
5 PN mice, which is a stage in which NF- B is not yet endogenously
activated. This effect was specific for NF- B and not observed for
AP-1. In adult mice, NF- B activity was absent in the cerebellum and
was not induced by intraperitoneal injection of glutamate receptor
agonists. These data show that NF- B is specifically activated during
cerebellum development and indicate an important role of glutamate
receptors in this process.
Key words:
NF- B;
cerebellum;
development;
HIV;
glutamate;
transcription
INTRODUCTION
Neuronal differentiation involves
migration, directional growth, synaptogenesis, and selective survival.
Cells in the CNS acquire distinct fates in response to extrinsic
signals that activate repertoires of transcription factors in a region-
and cell-specific manner. The number of transcription factors with
known temporal patterns of expression in the developing vertebrate
nervous system is increasing rapidly (Bang and Goulding, 1996 ).
The cerebellum provides a unique model for studying CNS development,
because the three maturation processes (morphogenetic movements,
formation of ganglionic structures, and neuronal layers) are primarily
postnatal events that begin just after birth and are completed by the
fourth postnatal (PN) week (Ramon y Cayal, 1889 ; Altman, 1982 ).
Although the steps of these processes are well known (Hatten and
Heintz, 1995 ), the genes involved in these sequential events are only
now beginning to be identified (Kuhar et al., 1993 ).
The NF- B family of transcription factors is composed of five members
in vertebrates (RelA, NFKB1, NFKB2, c-Rel, and RelB), sharing a 300 amino acid Rel homology domain and forming heterodimers and homodimers
(Grilli et al., 1993 ). NF- B activity is regulated by specific
inhibitors (I Bs) that maintain the factor in a cytoplasmic, inactive
form (Verma et al., 1995 ). Several inducers cause dissociation and
degradation of I Bs and promote activation of NF- B with rapid translocation into the nucleus, where it directly regulates gene expression. Although NF- B factors are widely expressed in almost all
cell types analyzed thus far, NF- B has been reported to be constitutively activated only in mature B cells (Miyamoto et al., 1994 )
and in some regions of the adult mouse brain (Schmidt-Ullrich et al.,
1996 ).
Recently we showed that the RelA subunit of NF- B was present in
several areas of brain, including cerebellum, and that its activation
state changed in cerebellar neuronal cultures, reflecting different
stages of development. In cerebellar granule cell cultures derived from
4- to 7-d-old PN mice, RelA was retained in the cytoplasm; however,
nuclear translocation could be induced by treatment of the cultures
with the neurotransmitter glutamate. Nuclear RelA was capable of DNA
binding, and its activity could be followed by electrophoretic mobility
shift assays (EMSAs). Pretreatment of the cultures with EGTA blocked
glutamate activation of NF- B, suggesting that
Ca2+ influx was required for NF- B activation. In
cultures derived from older mice (10 PN) a nuclear DNA-binding NF- B
was observed (Guerrini et al., 1995 ). Thus activation of NF- B might
be connected to cerebellar development.
We now report that, during mouse cerebellum development, NF- B,
but not AP-1, is activated. Until 6 PN NF- B was retained in the cell
cytoplasm in an inducible state. From 7 PN on, NF- B was
constitutively activated in the cerebellum. Before 7 PN, nuclear translocation of NF- B could be obtained using intraperitoneal administration of neurotransmitters; intraperitoneal injection of the
glutamate receptor agonists NMDA and
trans-methyl-D-aspartic acid
(trans-ACPD) resulted in activation of NF- B in cerebella of 5 PN pups. Intraperitoneal injections to 9 PN mice of the selective antagonist of the non-NMDA-type receptor antagonist
6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) and of the NMDA-type
receptor 2-amino-5-phosphonopentanoic acid (D-AP5) resulted
in a drastic reduction of the endogenous NF- B-binding activity. In
the adult mouse cerebellum NF- B was not inducible by intraperitoneal
injection of glutamate receptor agonists, suggesting a time window in
which NF- B could be activated by these stimuli.
MATERIALS AND METHODS
Animals and drug treatments. Mice were kept under a
12 hr light/dark cycle and given food and water ad libitum.
Whole brains were removed for dissections, and cerebellum, neocortex,
and hippocampus were immediately frozen in liquid nitrogen. All samples
were stored at 80°C until use. Kainic acid (adult mice, 8 mg/kg;
neonatal, 1 mg/kg), trans-ACPD (3 mg/kg), NMDA (adult mice,
8 mg/kg; neonatal, 2 mg/kg), D-AP5 (10 mg/kg), and CNQX (1 mg/kg) were injected intraperitoneally. All drugs were from Research
Biochemical International.
Nuclear extracts and EMSAs. Nuclear extracts were prepared
essentially according to the methods of Dignam et al. (1983) ; briefly, tissue samples were homogenized in 100 µl of buffer A (in
mM: 10 HEPES, pH 7.9 at 4°C, 1.5 MgCl2, 10 KCl, and 0.5 DTT) and left for 10 min on
ice; after addition of 5 µl of 10% Triton X-100 the homogenates were
vortexed for 30 sec and then centrifuged for 1 min at 13,000 rpm; the
nuclear pellet was dissolved in 30 µl of buffer C [20 mM
HEPES, pH 7.9, 25% (v/v) glycerol, 0.42 M NaCl, 1.5 mM MgCl2, 0.2 mM EDTA, 2 mM phenylmethylsulfonyl fluoride (PMSF), and 2 mM DTT] and left for 30 min at 4°C with constant agitation; after centrifugation as above, the nuclear extracts were
aliquoted, frozen in liquid nitrogen, and stored at 80°C until use.
Oligonucleotides encompassing the double human immunodeficiency virus 1 (HIV-1) NF- B site (Nabel and Baltimore, 1987 ), the human leukocyte
antigen (HLA) site (Chang et al., 1994 ), and the AP-1 site (Guerrini et
al., 1996 ) were used as probes and 5 -labeled with -32P
and T4 polynucleotide kinase. Binding reactions with 10 µg of nuclear
extracts were performed in a 20 µl volume containing 20,000 cpm of
probe, 2 µg of poly[d(I-C)], 10 µl of TK100 buffer (25 mM HEPES, pH 7.9, 20% glycerol, 1 mM EDTA, 100 mM KCl, 2 mM MgCl2, 2 mM dithiothreitol, and 2 mM PMSF) and
competitor as indicated. Nuclear extracts were incubated with the
poly[d(I-C)] on ice for 10 min, and then the buffer and probe were
added. Incubation was continued for 20 min at room temperature, after
which reaction mixtures were loaded on a 5% nondenaturing
polyacrylamide gel in 0.25× Tris borate/EDTA [1× = (in
mM) 89 Tris base, 89 boric acid, and 2 EDTA, pH 7.2]. Gels
were dried and exposed to x-ray film. When antibodies were used in
EMSA, nuclear extracts were incubated with the different antibodies for
30 min at 4°C before the addition of the poly[d(I-C)].
Sera raised against RelA (serum 1226), NFKB1 (serum 1263), NFKB2 (serum
1495), c-Rel (serum 1266), RelB (serum 1318), and C-terminal NFKB1
(serum 1140) were kindly provided by Dr. N. Rice (National Cancer
Institute, Frederick, MD).
RNase protection assays. Total RNA was extracted from brain
tissue samples by the Trizol reagent (Gibco-BRL) according to the
instructions of the manufacturer. For RNase protection assays a
transgene-specific -globin antisense probe of 230 bases overlapping the start site of transcription was generated by in vitro
transcription with [32P]uridine triphosphate using
T7 RNA polymerase. Labeled probe was incubated with 30 µg of total
RNA overnight at 55°C, digested with RNases, and analyzed by
denaturing polyacrylamide gel electrophoresis as described (Wirth et
al., 1991 ). For positive control, RNA was extracted from the S194 mouse
plasmacytoma cell line transiently transfected with the 3x B globin
plasmid (Wirth and Baltimore, 1988 ).
RESULTS
NF- B activation is restricted to the developing cerebellum
To verify the activation state of NF- B and AP-1 during brain
development, we prepared nuclear extracts from cerebellum, neocortex, and hippocampus from neonatal mice and used them in EMSAs using as
probe an oligonucleotide encompassing the double NF- B site of the
HIV long terminal repeat or the AP-1 site of the urokinase promoter.
Nuclear extracts derived from the neocortex and hippocampus did not
show any NF- B DNA-binding activity at all stages examined. On the
other hand, in nuclear extracts derived from the cerebellum, NF- B
DNA-binding activity was not present at 5 and 6 PN whereas a specific
NF- B-binding activity was visible in extracts derived from 7 and 9 PN (Fig. 1A).
Fig. 1.
NF- B is developmentally activated in the
cerebellum. Nuclear extracts from neocortex (cortex),
hippocampus (hippoc.), and cerebellum from 5-9 PN mouse
pups were used in EMSAs with HIV- B and AP-1 oligonucleotides as
probes. A, NF- B-binding activity is not detected in
neocortex and hippocampus extracts (5-9 PN); in the cerebellum,
NF- B is not present at 5 and 6 PN but can be detected at 7 and 9 PN.
DNA-binding specificity is shown with 9 PN extracts by the competition
with a 50-fold excess of unlabeled oligonucleotide (9*).
The arrow identifies the NF- B complex. B, AP-1
activity is absent in the neocortex at 5 PN but is present thereafter
(6-9 PN). DNA-binding specificity is shown by the competition with a
50-fold excess of unlabeled AP-1 oligonucleotide (9*). In the hippocampus AP-1 is expressed at all stages examined (5-9 PN)
but is completely absent in the cerebellum (5-9 PN). The
arrow identifies the AP-1 complex.
[View Larger Version of this Image (41K GIF file)]
When the same extracts were used to analyze the expression of the
transcription factor AP-1, which has been extensively studied in the
mouse brain (Vendrell et al., 1993 ), the following results were
obtained (Fig. 1B). In the neocortex, AP-1-binding
activity was not present at 5 PN but was detectable from 6 PN on.
AP-1-binding activity was present in the hippocampus at 5 PN and
increased thereafter. No AP-1-binding activity was detected in the
extracts of 5-9 PN cerebella. From these data we concluded that
NF- B is developmentally activated specifically in the cerebellum,
and that the domains of expression of the two transcription factors examined are not overlapping.
A B-globin transgene is activated during
cerebellar development
To verify whether NF- B nuclear translocation in the developing
cerebellum correlated with transcriptional activation of a B-dependent gene, we used mice carrying a transgene containing three
copies of the B motif from the Ig enhancer inserted upstream of
the -globin TATA box, promoter, and the -globin reporter gene.
These mice express the transgene in response to NF- B activation in
the spleen and thymus (Lernbercher et al., 1993 ). As shown in Figure
2, RNase protection experiments on whole
adult mouse brain RNA did not reveal any transgene expression
(lane 2), which agrees with the lack of active nuclear
NF- B in the adult mouse brain (Guerrini et al., 1995 ). When RNA was
extracted from 5 PN cerebellum (Fig. 2, lane 3) a weak
transgene expression was visible. By 10 PN, transgene expression
strongly increased (Fig. 2, lane 4).
Fig. 2.
A B-transgene is activated during cerebellum
development. Total RNAs (30 µg) from adult brain, 5 and 10 PN
cerebella from mice carrying a 3x B-human -globin
(B.glob) transgene, were used in RNase protection
experiments with a human -globin probe. Transgene expression is not
detected in the adult brain (lane 2), is slightly detectable in 5 PN cerebellum (lane 3), and is increased
in 10 PN cerebellum (lane 4). Lane
1, RNA extracted from the S194 mouse plasmacytoma cell line
transiently transfected with the 3x B -globin plasmid (positive
control).
[View Larger Version of this Image (37K GIF file)]
These data show that in vivo activation of NF- B during
cerebellum development correlates with transcription of B-dependent genes and therefore strongly suggest a functional role of
developmentally regulated NF- B activation in cerebellum.
NF- B activation in cerebellum is counteracted by glutamate
receptor antagonists
To verify whether developmental activation of NF- B at 9 PN was
attributable, as in cultured cells, to glutamate receptor stimulation
in vivo, we intraperitoneally injected 9 PN mice with the
non-NMDA receptor antagonist CNQX (Turski et al., 1990 ) and with the
NMDA receptor antagonist D-AP5 (Croucher et al., 1982 ; Meldrum, 1992 ). As shown in Figure 3, in
mouse cerebellar nuclear extracts prepared 10 or 20 min after CNQX
injection (lanes 3 and 4) the levels of
NF- B-binding activity were significantly reduced compared with
untreated animals (lane 2). Similar results were obtained
with intraperitoneal injection of D-AP5 (Fig. 3,
lanes 5 and 6). The injection of CNQX
(Fig. 3, lane 8) and D-AP5 (Fig. 3, lane
9) did not activate NF- B-binding activity in the neocortex, nor
did they inhibit AP-1-binding activity in the neocortex (data not
shown). These data indicate that the observed in vivo
activation of NF- B may be attributable to endogenous glutamate
receptor stimulation, during the physiological maturation of the
cerebellum after 7 PN, and that this effect is not shared with other
factors binding the AP-1 site.
Fig. 3.
NF- B activation in 9 PN mice can be reversed by
intraperitoneal injection of glutamate receptor antagonists. EMSA
experiments with an HLA B oligonucleotide as probe from 9 PN
untreated mice extracts retarded two bands in the cerebellum
(lane 2, II, III) but not in the neocortex
(lane 7). Intraperitoneal injection with CNQX (10 and 20 min) reverts NF- B activation (lanes 3, 4); D-AP5 (10 and 20 min) had similar
effects (lanes 5, 6). Treatment of mice for 20 min with CNQX (lane 8) or D-AP5 (lane
9) had no effect in the neocortex. In lane 1,
HeLa-activated cytoplasmic extracts retarded two complexes:
I and II, which have been shown
previously to be RelA-c-Rel and RelA-NFKB1 heterodimers, respectively
(Hansen et al., 1992 ).
[View Larger Version of this Image (73K GIF file)]
NF- B is inducible by neurotransmitters in the
developing cerebellum
If the constitutive activity of 9 PN cerebellar NF- B really
depends on endogenous glutamate receptor stimulation, it might be
possible to activate NF- B at earlier times by injecting glutamate receptor agonists. To achieve this goal, we prepared nuclear extracts from 5 PN mice after intraperitoneal injection of different ionotropic glutamate receptor agonists. The glutamate receptors known to be
expressed in mouse cerebellum at this age are mainly of the NMDA and
metabotropic subtypes, with the non-NMDA receptor subtype being
expressed at very low levels (D'Angelo et al., 1993 ; Farrant et al.,
1994 ). As shown in Figure 4, control
intraperitoneal injection of saline solution (PBS) to 5 PN mice did not
activate NF- B (lane 1) and had no effect on mice
behavior. Kainic acid, a selective activator of the non-NMDA subtype of
glutamate receptors, caused a convulsive status epilecticus. Cerebellar
nuclear extracts of these mice prepared 15 min after the injection
contained modest NF- B activity (Fig. 4, lane 2), in
agreement with the low levels of expression of the cerebellar non-NMDA
receptor subtype at this age (D'Angelo et al., 1993 ). The same results
were obtained with longer (up to 40 min) kainic acid treatments (not
shown). On the other hand, intraperitoneal injection of NMDA for 30 or
60 min rendered the mice hypotonic. This coincided with NF- B
activation (Fig. 4, lane 3; 30 min). Injection of
trans-ACPD, which activates the metabotropic subtype of
glutamate receptors, resulted in massive myoclonic and tonic seizures
and, again, NF- B activation at 30 and 60 min (Fig. 4, lanes
4 and 5, respectively). Kainic acid, NMDA, and
trans-ACPD treatments did not result in NF- B activation in the neocortex (Fig. 4, lanes 8-10). When we used the
same extracts with the AP-1 oligonucleotide, no AP-1-binding activity
was detected in the cerebellum or neocortex of either untreated or
treated animals (data not shown). These data indicate that there is a tissue-restricted activation of NF- B by glutamate receptor
activation.
Fig. 4.
NF- B is induced in 5 PN cerebellum by
intraperitoneal injection of NMDA and trans-ACPD.
Nuclear extracts were prepared from 5 PN mice after intraperitoneal
injection with glutamate receptor agonists and used in EMSA with an
HIV- B oligonucleotide as probe. Lane 1, Cerebellum
extracts of PBS-injected mice; 15 min of kainic acid slightly induces
NF- B-binding activity (lane 2); 30 min of NMDA
intraperitoneally results in a strong induction (lane 3), as also observed with 30 and 60 min treatments with
trans-ACPD (lanes 4, 5). Lane
6, Specific competition of the cerebellum extracts from
NMDA-treated animals by unlabeled B oligonucleotide. In neocortex
extracts, NF- B is not detected after injection of PBS (lane
7), kainic acid (lane 8, 15 min) NMDA
(lane 9, 30 min), and trans-ACPD
(lane 10, 60 min).
[View Larger Version of this Image (109K GIF file)]
Immunological characterization of developmentally
activated NF- B
We next examined the subunit composition of developmentally
activated NF- B complexes. Gel mobility shift experiments were performed with nuclear extracts from 8 PN mice pups incubated with the
HLA gene B site. Figure 5, lane
1, shows that two complexes (I and II) could be detected in
cerebellar nuclear extracts of 8 PN animals. To identify the proteins
responsible for the formation of these complexes, we used antisera
directed against each of the five members of the NF- B family. Serum
against RelA inhibited the formation of complex I and had no effect on
complex II (Fig. 5, lane 2). The anti-NFKB1 serum inhibited
both complexes (Fig. 5, lane 3). The NFKB2, c-Rel, and RelB
sera (Fig. 5, lanes 4-6) did not have any effect. As
a control we used an antiserum directed against the C-terminal of the
NFKB1 p105 precursor (Fig. 5, lane 7), which is not
retained in the active protein p50 and had no effect. DNA-binding
specificity was demonstrated by addition of 100× excess unlabeled
competitor (Fig. 5, lane 8). The same results were obtained
with NMDA and trans-ACPD nuclear extracts derived from 5 PN
mice (not shown). From these data we conclude that complex I is
composed of RelA-NFKB1 heterodimers and that complex II contains NFKB1
homodimers.
Fig. 5.
Immunological characterization of NF- B
subunits. Nuclear extracts prepared from 8 PN cerebella incubated with
a labeled oligonucleotide corresponding to the B site of an HLA
promoter gene retard two bands (lane 1, I, II).
The addition of specific antisera against different members of the
NF- B family is indicated at the top. The
arrows show the supershifted bands. Lane
8, Competition (+comp.) with 100× excess
unlabeled oligonucleotide. ns, Nonspecific
bands.
[View Larger Version of this Image (78K GIF file)]
NF- B is not inducible by glutamate agonists in the
adult cerebellum
To verify whether activation of NF- B in response to
glutamate receptor activation was restricted to the developing
cerebellum, we intraperitoneally injected glutamate receptor agonists
in 12-week-old mice. At this age NF- B subunits are expressed but
retained in the cell cytoplasm (Guerrini et al., 1995 ). Nuclear
extracts derived from cerebellum, neocortex, and hippocampus of mice
treated for 1 hr with kainic acid, which rendered the mice ataxic, did
not increase NF- B-binding activity (Fig.
6A, lanes 4-6).
Modest NF- B-binding activity, in agreement with recently published
data (Schmidt-Ullrich et al., 1996 ), was visible in hippocampus protein
extracts (Fig. 6A, lane 3) but was not induced by
this treatment (Fig. 6A, lane 6). The
transcription factor AP-1, known to be induced in several brain regions
by this treatment (Vendrell et al., 1993 ), was indeed activated in the
neocortex and hippocampus (Fig. 6B, lanes 5 and 6). Surprisingly, in the cerebellum we observed a
reduction of AP-1-binding activity (Fig. 6B, lane
4). These data were similar to those recently reported for
the rat brain (Rong and Baudry, 1996 ), in which kainic acid injection
failed to activate NF- B and AP-1 in the cerebellum, with actually a
slight reduction of AP-1-binding activity in the cerebellum after
kainic acid injection. Similar results were obtained with NMDA-treated
animals (data not shown). These data show that NF- B activity is
susceptible to neurotransmitter activation only during cerebellum
development.
Fig. 6.
NF- B is not inducible by glutamate receptor
agonists in the adult mouse cerebellum. Analysis of NF- B- and
AP-1-binding activities of extracts from 12-week-old mice
intraperitoneally injected with kainic acid. A, Nuclear
extracts from cerebellum, neocortex, and hippocampus of PBS-injected
animals (lanes 1-3); 1 hr after intraperitoneal injection of kainic acid, no activation of NF- B in the cerebellum, neocortex, and hippocampus is evident (lanes
4-6). B, AP-1 is present in cerebellum,
neocortex, and hippocampus of PBS-treated animals (lanes
1-3) and is induced by kainic acid in neocortex and
hippocampus (lanes 5, 6) but not in cerebellum
(lane 4).
[View Larger Version of this Image (60K GIF file)]
DISCUSSION
Trans-synaptic regulation of gene expression is critical for
neuronal development and for long-term adaptive changes in the mature
nervous system. In the cerebellar cortex, granule cells migrate from
the external germinal layer where they are generated and cross the
molecular layer to reach their final destination in the internal
granular layer. The rate of granule cell movements depends on both
extracellular Ca2+ concentration and
Ca2+ influx through N-type Ca2+
channels (Komuro and Rakic, 1992 , 1993 ). Activation of specific transcription factors on Ca2+ influx may play a
crucial role in regulating granule cell maturation and migration.
In this paper we show that NF- B is present in an inducible
form in cerebella of mice pups up to 6 PN. At this stage, no
constitutive NF- B was observed, but intraperitoneal injections of
glutamate receptor agonists induced its nuclear translocation. We think that NF- B activation after intraperitoneal injection of glutamate receptor agonists does not reflect downstream events of generalized CNS
damage, because the drug having the greatest neurotoxic and traumatic
effect results in the weakest NF- B response. After intraperitoneal
injection of kainic acid, mice occasionally died as a consequence of
convulsive seizures (epileptic status). However, the tissue extracted
from these animals showed poor activation of NF- B, as did tissue
extracted from mice that were not killed by the injection (Fig. 4). On
the other hand, a good NF- B response was observed in the animals
injected with NMDA and trans-ACPD (Fig. 4), in which severe
seizures or casualties were never observed.
In nuclear extracts from cerebella of 7-9 PN mice, NF- B is
constitutively activated. The fact that the activation of NF- B in 9 PN mice can be inhibited by treatment with ionotropic glutamate receptor antagonists (CNQX and D-AP5) suggests that this
activation depends on an autocrine-paracrine loop of glutamate
receptor activation, possibly through glutamate itself synthesized and
released by neurons and/or astrocytes (Gallo et al., 1982 ). In this
case, an astrocyte- and neuron-signaling pathway, regulating NF- B
activity, could be operative in the cerebellum. Indeed, in
vitro-cultured astrocytes synthesize glutamate, which can transfer
signals to neurons via glutamate and glutamate receptors (Parpura et
al., 1994 ). On the other hand, in vivo synaptic source of
glutamatergic input to granule cells has been demonstrated to be
mediated by mossy fibers arising from the deep cerebellar nuclei and
brainstem (Eccles et al., 1967 ).
One possible function of NF- B activation might be the control of
cell migration, possibly by regulating the expression of adhesion
molecules or their receptors through NMDA receptors (Komuro and Rakic,
1992 , 1993 ). Migration of cells is a complex phenomenon that requires
an extremely tight program of adhesion-promoting, proteolytic, and
motogenic steps. A role of NF- B in this type of processes can be
observed in cell culture systems, in which the block of RelA synthesis
by antisense technology resulted in decreased adhesion of the cells
(Narayan et al., 1993 ). Moreover, NF- B induces synthesis of several
motogenic factors and proteolitic enzymes important in cell migration
(Guerrini et al., 1996 ). Another possibility is that NF- B is
involved in the regulation of synaptogenesis by elimination of
exuberant collateral synapses. This process has in fact been shown to
occur during cerebellum development and to be regulated by
Ca2+ influx through NMDA receptors. In the first
postnatal week in the rat, a single Purkinje cell is innervated by
several climbing fibers; a massive elimination of most synapses through
the withdrawal of axonal collaterals then occurs. As a consequence of
this process, the innervation of each Purkinje cell by only one
climbing fiber, typical of the adult stage, is attained on postnatal
day 15. In vivo administration of the glutamate receptor
antagonist D-AP5 has been shown to block this process
(Rabacchi et al., 1992a ,b ).
Nothing is known about the function of NF- B in the CNS. Data from
the targeted disruption of NFKB1 (Sha et al., 1995 ), c-Rel (Kontgen et
al., 1995 ), RelB (Burkly et al., 1995 ) and I B (Beg et al., 1995a )
did not reveal any major alteration of the CNS. RelA knock-out mice, on
the other hand, die at approximately embryonic day 16 of massive liver
apoptosis, too early to detect any defect in the CNS (Beg et al.,
1995b ). On the other hand, because RelA has been identified in all
brain regions examined (Guerrini et al., 1995 ), and it is found in all
activated dimers in the adult mouse brain (Schmidt-Ullrich et al.,
1996 ), in the various knock-out animals other members of the
NF- B/Rel family might substitute for one another, producing no major
phenotypic alteration.
The presence in 9 PN cerebellar extracts of dimers with
trans-activating potential (RelA-NFKB1 heterodimers) and
dimers with no trans-activating capacity (NFKB1 homodimers)
delineates a complex system for NF- B-regulated gene expression.
Different genes containing NF- B sites in their regulatory regions
may be activated or repressed during cerebellum development depending
on the affinity of each particular B site for the different dimers.
Differential affinity of the RelA-NFKB1 and RelA-c-Rel heterodimers
or the NFKB1 and RelA homodimers has in fact been observed and results
in differential transcriptional effects of the various combinations
(Hansen et al., 1994a ,b ). Differential regulation of gene expression by
one of the NF- B subunits has been observed in NFKB1 knock-out mice, in which interleukin-6 (IL-6) gene expression was reduced and interferon- (IFN- ) was augmented, suggesting that IL-6 needs NFKB1 for its expression, whereas IFN- is repressed by the same factor in wild-type animals (Sha et al., 1995 ).
The lack of NF- B activation on glutamatergic stimuli in adult mice
suggests a fundamental difference in the cascade of intracellular events responding to synaptic glutamate receptor activation between the
developing and mature cerebella. We suggest that the kinases involved
in the degradation of the I B molecules, which maintain NF- B in
the cell cytoplasm, must respond differently to the same stimuli
between the developing and adult cerebella. Silencing in the adult
cerebellum of the intracellular steps leading to I B degradation
could actually be a "secondary effect" of NF- B activation.
Trans-synaptic regulation of gene expression is critical for
neuronal development. Neurotransmitters released from the presynaptic cells regulate gene expression in postsynaptic neurons by binding to
and activating specific postsynaptic receptors. From our data, the
action of NF- B seems to be regulated in a temporally and spatially
restricted fashion and may possibly lead to regulation of specific
genes involved in the formation of this brain region.
In conclusion, the existence of a well known primary transcription
factor and its induction in response to glutamate receptor activation
during cerebellum development represent a novel pathway for regulating
gene expression during CNS development.
FOOTNOTES
Received March 24, 1997; revised May 16, 1997; accepted May 30, 1997.
This work was supported by grants from the Italian Ministry of Health
(AIDS fund) and Associazione Italiana Ricerche sul Cancro and European
Community. We are grateful to Dr. Nancy Rice for the generous gift of
antibodies and Dr. Marco de Curtis for critical reading of this
manuscript.
Correspondence should be addressed to Luisa Guerrini, Department of
Genetics and Microbial Biology, University of Milan, Via Celoria 26, 20133 Milan, Italy.
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