|
|
|
|
|
|
|
||||
| Home Help Feedback Subscriptions Archive Search Table of Contents | ||||
First published online 19 November 2008
doi: 10.1242/dev.023366
| |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
,*
1 Institute of Molecular and Cellular Biosciences, University of Tokyo, 1-1-1
Yayoi, Bunkyo-ku, Tokyo 113-0032, Japan.
2 Institute for Bioinformatics Research and Development (BIRD), Japan Science
and Technology Corporation (JST), Chiyoda-ku, Tokyo 102-0081, Japan.
3 Precursory Research for Embryonic Science and Technology (PRESTO), Japan
Science and Technology Corporation (JST), Kawaguchi 332-0012, Japan.
* Author for correspondence (e-mail: k.kato{at}bham.ac.uk)
Accepted 27 October 2008
 |
SUMMARY |
|---|
 TOP SUMMARY INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
|---|
Key words: Neuronal programmed cell death, Glial division, Drosophila, Adult brain, Injury, Critical period
|
INTRODUCTION |
|---|
|
TOPSUMMARYINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
|---|
). Despite
intensive studies of the underlying mechanisms of such PCD
(Brade, 1989;
Clarke, 1985;
Frebel and Wiese, 2006),
little is known about how surrounding cells respond to the loss of their
neighbours.
Developmental elimination of subcomponents of neurons is known to be
associated with the activity of the surrounding cells. Axons, dendrites and
synapses are also often created in excess, and unnecessary parts of them are
eliminated in the course of neural circuit refinement in later stages. In the
neuromuscular junctions, reduction of synapses and axon branches after birth
is achieved in cooperation with the surrounding Schwann cells
(Bishop et al., 2004). In the
brain of Drosophila melanogaster, specific parts of the axon branches
are pruned during metamorphosis by the engulfing activity of the surrounding
glial cells (Awasaki and Ito,
2004; Watts et al.,
2004). Adult neural circuits are not formed properly if this glial
activity is blocked (Awasaki et al.,
2006).
Glial cells are also involved in the response to accidental neural cell
death. Astrocytes, Schwann cells and oligodendrocyte precursors proliferate
after diverse types of neural injury, ranging from spinal cord injury to
ischemia (Fawcett and Asher,
1999). Cell debris or factors released from damaged cells, such as
nucleosides, are suggested to be involved in triggering the glial response.
Moreover, glial proliferation is influenced by growth factors and cytokines
such as tumour necrosis factor 
(TNF) and interleukin 6 (IL6)
(Fields and Burnstock, 2006;
Liu et al., 1995).
It is known that, in some cases, caspase-dependent cell death induces
proliferation of the surrounding cells. In Drosophila, X-ray
irradiation of the imaginal wing discs causes PCD and leads to compensatory
cell proliferation (Haynie and Bryant,
1977). Expression of the pro-apoptotic gene head involution
defective (hid; Wrinkled - FlyBase) or reaper
(rpr) activates PCD through the apical caspase DRONC
(Drosophila Nedd2-like caspase). In imaginal discs, DRONC also
induces expression of wingless and decapentaplegic
(dpp), and this promotes proliferation of the surrounding cells. In
this case, therefore, the proliferation is triggered by cell death signalling
rather than by the cell death itself. Both X-ray irradiation and the
expression of hid or rpr cause overproliferation even when
the cell death is inhibited by the caspase inhibitor p35
(Huh et al., 2004;
Perez-Garijo et al., 2004;
Ryoo et al., 2004).
The programmed elimination of neural processes and accidental death of
neurons are closely associated with the glial activity, but it was not known
whether developmental cell death of neurons could also be associated with
glial responses such as proliferation. Because of the wide variety of genetic
manipulation techniques available, Drosophila melanogaster provides a
good model system to address this issue. In the ventral nerve cord soon after
adult fly eclosion, neurons that strongly express the Ecdysone receptor type A
isoform (EcRA) respond to ecdysone signals by expressing rpr and
grim, but not hid, and undergo PCD
(Robinow et al., 1993).
Ecdysone signalling is also known to directly induce dronc expression
during larval PCD (Cakouros et al.,
2004). It is not known, however, whether similar PCD also occurs
in the brain and whether glial cells show any responses to such neuronal
PCD.
Here we report that neuronal PCD also occurs in the Drosophila brain after adult eclosion. We show that glial cell division is induced by the PCD. Unlike in the wing disc, glial division after neuronal PCD is more likely to be triggered by molecular mechanisms similar to those involved in injury responses. The tumour necrosis factor Eiger is involved in glial division induced by both developmental PCD and injury. Secondary PCD is observed in the absence of Eiger and hence in the absence of glial division. In addition, we report the existence of a critical period of glial division, which is limited to just the first week of a fly's adult life, which can be as long as 50 days.
|
MATERIALS AND METHODS |
|---|
|
TOPSUMMARYINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
|---|
), elav-GAL4 and UAS-p35 strains were
gifts from A. Hidalgo. eiger1 and
eiger3 (Igaki et al.,
2002) were gifts from M. Miura. UAS-eiger
(Moreno et al., 2002) was a
gift from K. Basler. GAL4-NP577 was used as a glial GAL4 expression
driver. For mosaic analysis with a repressible cell marker (MARCM)
(Lee and Luo, 1999), flies
with the following genotypes were generated: hs-FLP,
tubP-GAL80, FRT 19A/FRT19A; UAS-lacZ/Actin-GAL4,
and hs-FLP, tubP-GAL80, FRT 19A/FRT19A; UAS-GFP
T2/Actin-GAL4. All observations were made in female flies. To minimise the effect of variation in the nutritional conditions among the flies, which could affect the growth rate of the animals, the number of flies per vial was strictly controlled. Thirty larvae were collected over a period of 6 hours after hatching and kept in a vial at 25°C until eclosion. Adult flies were collected within 6 hours after eclosion, and ten female and ten male flies were raised in a vial.
Definition of the areas of investigation
For the quantitative comparison of the labelled cells in the area around
the root of the antennal nerve, we defined the region that includes the
antennal nerve and the two-cell-thick layer of the surrounding cortex. Along
the longitudinal axis of the antennal nerve root, we analysed the area between
the entrance point of the antennal nerve and the level at which the dorsal
population of the neural cell bodies around the antennal nerve disappears.
Statistical analysis was performed using Excel (Microsoft) with statistics
add-in software (Esumi, Tokyo, Japan).
Immunohistochemistry
Flies were anesthetised with carbon dioxide and brains dissected in PBS and
fixed with 4% formaldehyde in PEM (100 mM PIPES, 2 mM EGTA, 1 mM
MgSO4, pH 6.95) for 50 minutes at room temperature (RT).
Incubations with primary and fluorescent-conjugated secondary antibodies were
performed at 4°C overnight. To detect BrdU, the brains were treated with 2
M HCl for 20 minutes at RT after immunolabelling for other proteins. Nuclei
were stained either with propidium iodide (PI) (Wako Pure Chemical industries,
Osaka, Japan; 2 µg/ml in PBT) or TOTO3 (Molecular Probes; 1:2000 in 50%
glycerol in PBS) for 3 hours at RT. Samples were analysed by confocal
microscopy (Carl Zeiss LSM510 or Leica TCS SP2). Three-dimensional
reconstructed images were generated with Zeiss software and processed with
Photoshop (Adobe, San Jose, CA).
Antibodies used in this study were: mouse anti-BrdU (Beckton Dickinson, Franklin Lakes, NJ, 1:250; or GE Healthcare, Amersham, UK, 1:100), rat anti-ELAV (Developmental Studies Hybridoma Bank; 1:250), rabbit anti-REPO (gift from G. Technau; 1:250), mouse anti-EcRA (Developmental Studies Hybridoma Bank; 1:1000), rabbit anti-GFP (Molecular Probes, Eugene, OR; 1:1000), rabbit anti-β-galactosidase (ICN Pharmaceuticals, Aurora, OH; 1:4000), rabbit anti-FITC (Molecular Probes; 1:2000), rabbit anti-cleaved caspase 3 (Cell Signaling Technology, Beverly, MA; 1:100) and Alexa Fluor 488, 568 and 647-conjugated secondary antibodies (Molecular Probes; 1:250).
In situ DNA 3'-end labelling
For TUNEL (terminal deoxynucleotidyl transferase dUTP nick end labelling)
analysis, free 3'-OH ends of DNA in the specimen were labelled with FITC
using an In Situ Cell Death Detection Kit (Roche, Basel, Switzerland). In some
cases, signals were enhanced by immunostaining with anti-FITC antibody.
BrdU incorporation
To label DNA-replicating cells, staged pupae were injected with 
0.1
µl of 3.5 mg/ml BrdU (Sigma, St Louis, MO) in PBS, and staged adult flies
were fed yeast food containing 3.5 mg/ml BrdU. For longer treatment, flies
were transferred to a new vial containing fresh food with BrdU every day. For
pulse-chase labelling experiments, flies were fed food containing BrdU and red
dye for 2 hours, and then transferred to normal food without BrdU. Half of the
flies were fixed after 6 hours, and the remainder after 5 days. The red colour
in the intestine of the flies disappeared quickly after they were transferred
to the normal food. Therefore, it is unlikely that BrdU remains in the fly
body for long after the pulse-labelling period to influence the number of
BrdU-positive cells recorded in flies that were fixed later. In fact, the
proportion of antennal nerves with BrdU-positive cells was almost the same in
both conditions (28%, 17 of 60 samples after 6 hours; and 32%, 21 of 66
samples, after 5 days).
MARCM analysis
The flies for MARCM analysis (see above) were raised at 19°C to avoid
spontaneous activation of hs-FLP. One day after eclosion, the flies
were heat shocked for 1 hour at 37°C and maintained for 3 hours at
19°C. This cycle was repeated four times during the same day. Flies were
raised for a further 3 days at 19°C. For the experiment combining BrdU
labelling and MARCM analysis, flies were fed BrdU from the time of
eclosion.
In situ mRNA hybridisation
In situ hybridisation was performed essentially as described previously
(Ito et al., 2003), except
that the specimens were treated with 0.3% H2O2 in
methanol for 20 minutes at RT before they were subjected to in situ
hybridisation. DNA probes used were reaper, hid and grim,
which were synthesised using cDNAs as template (details available upon
request).
Microsurgery
Prior to brain injury, flies were fed BrdU for 1 day. They were
anesthetised with carbon dioxide and a needle was inserted into the dorsal
right-hand area of the head. The flies were then fed BrdU for another day
before dissection and fixation. To induce degeneration of the antennal nerve
axons, an antenna of the right side of the head was removed from the first
segment, and the flies were then fed BrdU for a few days before dissection and
fixation.
|
RESULTS |
|---|
|
TOPSUMMARYINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
|---|
), labelled
cells in various regions of the brain (Fig.
1A). The labelling was observed most prominently and consistently
in the area around the root of the antennal nerve
(Fig. 1A, arrowheads). We
therefore focused on this area for further analysis. The majority of the
active caspase-positive cells were also labelled by TUNEL (10 out of 11 cells)
and showed condensed nuclei (Fig.
1B), confirming that they underwent PCD. Such cells were only
found during the first 2 days of adult life
(Fig. 1C).
Immunostaining with a neural marker, ELAV, and a glial marker, REPO,
revealed that the cells that undergo PCD are neurons (TUNEL-positive,
ELAV-positive and REPO-negative) (Fig.
1D). Immunolabelling of caspase revealed that at least some of the
dying neurons project their neurites into the antennal nerve
(Fig. 1E). In the adult ventral
nerve cord, some neurons that strongly express EcRA express the PCD-associated
genes rpr and grim, but not hid, and undergo PCD
(Robinow et al., 1997;
Robinow et al., 1993).
Similarly, we found that strongly EcRA-positive neurons around the antennal
nerves underwent PCD (20 out of 43 strongly EcRA-positive cells were also
TUNEL-positive) (Fig. 1F).
Consistently, we found neurons that expressed grim (75% of the
antennal nerves observed, n=8)
(Fig. 1G) and rpr
(67%, n=6), but not hid (0%, n=20) (data not
shown). Compared with TUNEL-positive cells, strongly EcRA-positive cells were
observed more frequently and for a longer period, of up to 3 days AAE, which
is 1 day longer than for TUNEL-positive cells
(Fig. 1H). This might be
because some strongly EcRA-positive neurons escape cell death. In addition,
the opportunity to detect TUNEL-positive cells is relatively restricted: TUNEL
detects only the short final phase of PCD, whereas the dying cells appeared to
express EcRA for a longer period, starting before the execution of PCD.
|
;
Perez-Garijo et al., 2004;
Ryoo et al., 2004). We
analysed the distribution of DNA-replicating cells by bromodeoxyuridine (BrdU)
incorporation. When flies were fed BrdU for a 1-day period from the mid-pupal
stage to 10 days AAE, BrdU-positive cells were found most prominently in the
area around the antennal nerve (Fig.
2A,B), which corresponds to the area where prominent PCD was
observed. The number of BrdU-labelled cells in this area reached a maximum
slightly after the peak of PCD, and BrdU-positive cells were detected for a
few days after PCD had ceased (compare Fig.
2C with Fig. 1C,H).
This spatiotemporal coincidence might imply the induction of cell division in
response to PCD (discussed below).
|
For the BrdU pulse-chase labelling, newly eclosed flies were fed BrdU for 2 hours and fixed after either 6 hours or 5 days (Fig. 2D). If the cells that incorporated BrdU underwent mitosis, the number of labelled cells should increase. Indeed, antennal nerves with more than three BrdU-positive cells were found only in the flies fixed after 5 days, whereas after 6 hours most of the antennal nerves had only a single labelled cell (76%, 13 of 17 cells). Consistently, the number of labelled cells increased significantly after 5 days (average 2.0 versus 1.3 cells, P<0.05, Mann-Whitney's U-test), whereas the frequency of antennal nerves with BrdU-positive cells was almost the same in both conditions (see Materials and methods).
In the MARCM technique (Lee and Luo,
1999), reporter gene expression is induced via mitotic
recombination caused by expression of heat shock-activated Flippase.
Cells are therefore labelled only when there is mitosis. In the heat-shocked
flies, antennal nerves with lacZ-expressing cells were observed much
more often than without heat shock (Fig.
2E) (P<0.01, Fisher's exact test). Furthermore, cells
labelled by MARCM coincided with the BrdU-labelled cells
(Fig. 2F) (14 MARCM-positive
cells among 28 BrdU-positive cells). These results demonstrated that cells
around the antennal nerve do indeed divide.
Dividing cells in the adult brain are glial cells
To determine the identity of the dividing cells, we combined BrdU labelling
with immunostaining for glial and neuronal markers. Immediately after a 3-hour
BrdU labelling, almost all the BrdU-positive cells expressed the glial marker
(REPO-positive and ELAV-negative; 98%, 47 of 48 cells)
(Fig. 3A,B). Thus, most of the
dividing cells are likely to be glia. After a 1-day BrdU labelling followed by
a 9-day chase period, most BrdU-positive cells were also glia (REPO-positive
and ELAV-negative; 88%, 37 of 42 cells) (see Fig. S1A in the supplementary
material). MARCM analysis with a cytoplasmic GFP reporter combined with 5-day
BrdU treatment revealed that BrdU-positive REPO-positive cells extended
multiple processes of irregular shape in various directions, showing typical
glial structure (Fig. 3C).
|
Glial cells are classified according to differences in their position and
morphology, which are implicative of functional differences
(Ito et al., 1995).
Interestingly, the BrdU-positive glial cells were located within or very close
to the neuropiles, but not deep in the cortex or along the brain surface
(Fig. 2B,
Fig. 3B). It is therefore
likely that only a distinct subset of glia responds to PCD.
Neuronal PCD induces glial cell division
The spatiotemporal coincidence between neuronal PCD and glial cell division
suggests that the former might induce the latter. To examine this causal
relationship, we utilised a fly strain, elav-p35
(Booth et al., 2000), in which
execution of neuronal PCD is inhibited by a virus-derived caspase inhibitor,
p35. If cell death signalling induces cell division, as observed in the
imaginal wing discs, this would be expected to increase the number of
BrdU-positive cells. After 10 days of BrdU feeding, however, labelled cells
were observed only around the antennal nerves of wild-type (90%, 9 of 10)
(Fig. 3D, left), but not of
elav-p35 (0%, 0 of 22) (Fig.
3E, left), flies, even though both strains showed successful BrdU
incorporation in the ovary, where egg cells divide actively in the adult
(Fig. 3D,E, right). The same
results were obtained when expression of p35 was driven by
elav-GAL4 (n=20). The fact that p35 inhibits glial cell
division in the adult brain suggests that it is features of the dead or dying
neurons (e.g. membrane degeneration), rather than the signalling pathways
associated with the PCD, that trigger glial division.
Brain injury and axonal degeneration induce glial cell division
If dead or dying neurons trigger glial division, one might ask whether
there is any similarity with the response to neural lesion, which is known in
adult animals ranging from cockroaches to vertebrates
(Fawcett and Asher, 1999;
Treherne et al., 1984).
Drosophila glia are able to proliferate upon genetically induced
neural ablation during embryogenesis
(Griffiths and Hidalgo, 2004),
and retinal degeneration mutants (rdgBKS222) are
characterised by excessive glia in the degenerated adult tissue
(Stark and Carlson, 1982). We
therefore asked how glia would respond to adult brain injury.
To address this, we performed antennal ablation
(Fig. 4A), in which sensory
nerves from the antenna undergo degeneration
(Stocker et al., 1990). To
avoid endogenous glial division induced by the neuronal PCD, we ablated the
antenna of the elav-p35 flies. When BrdU was administrated from 0 to
3 days AAE, BrdU-positive glial cells were observed only around those antennal
nerves that had antennal ablation (Fig.
4B,C, first column). Thus, it is likely that ablation-induced
degeneration of axons is sufficient to cause the glial proliferation
response.
When the antennae of wild-type flies were ablated, coexistence of endogenous PCD and ectopic axonal degeneration was expected in these flies. However, the number of BrdU-positive cells did not increase prominently (P>0.05, Fisher's exact test) (Fig. 4C, second column). Although injury might activate more glial cells and/or trigger another round of cell division, for the most part, the same glial cells are likely to be activated by PCD and axonal degeneration.
We then examined whether glial cells in other parts of the brain have the
ability to respond to injury. Using a needle, we stabbed the dorsal area of
the right-hand head capsule of flies fed with BrdU
(Fig. 4D). One day after the
stabbing, we found ectopic BrdU-positive glial cells around the injury site,
but not in the intact hemisphere (Fig.
4E,F), suggesting that glia that do not normally divide retain the
ability to react upon injury. Although we could not exclude the possibility
that endoreplication might occur in some cells, our data, as well as those of
a previous report (Stark and Carlson,
1982), suggest that adult brain injury induces glial cell
division.
Eiger is a common link between PCD/neural injury and glial cell division
As the same glial cells are likely to respond to both PCD and neural injury
around the antennal nerves, we asked whether the same signalling molecule
mediates these responses. TNF, a TNF superfamily ligand, is tightly
involved in the response to neural lesion in mammals
(Scherbel et al., 1999). We
therefore explored whether a known Drosophila TNF superfamily ligand,
Eiger (Igaki et al., 2002;
Moreno et al., 2002), is
involved in either or both responses.
|
Interestingly, the loss of Eiger function caused increased neuronal PCD at a later period. At 3 days AAE, PCD occurred considerably more frequently in eiger mutants than in the wild type (P<0.05, Fisher's exact test) (Fig. 5A). This suggests that either Eiger or glial division has a protective function against PCD.
We then tested whether Eiger also mediates glial responses against neural lesion. Needle stab into eiger mutant brains appeared to induce BrdU incorporation much less frequently than in the wild type (data not shown). To examine the effect of injury quantitatively, we measured the frequency of BrdU incorporation after antennal ablation. Injury only occasionally induced BrdU incorporation in eiger mutants (Fig. 4C). This suggests that Eiger is also involved in mediating the glial response to neural lesion. The fact that eiger mutation attenuates both types of glial division, as induced by PCD and non-PCD events (antennal ablation), suggests that the same signalling molecule, Eiger, mediates these responses.
A critical period of glial cell division in response to PCD and neural injury
Do glial cells retain their mitotic ability to respond to neural loss
during aging? The number of cells in the adult brain decreases significantly
between 6 and 30 days AAE (T. Shimada, M. Kamiya, K.K. and K.I., unpublished
observation). We therefore investigated BrdU incorporation in the brain for
each 10-day period until 50 days AAE, which effectively covers most of the
lifetime of Drosophila in laboratory conditions. Consistent with the
results described above, BrdU incorporation was observed in the flies that
were fed BrdU for the first 10 days of adult life
(Fig. 6A). None of the samples
showed BrdU-positive cells after 10 days AAE
(Fig. 6A). This indicates (1)
that glial cells divide only during a specific period early in adult life, and
(2) that the dividing glial cells are not a significant contributor to cell
number in the adult fly brain.
|
|
In addition to the fact that we did not find any BrdU-positive cells after
10 days AAE, it is important to note that we did not find any BrdU-positive
ELAV-positive cells (i.e. neurons) anywhere in the brain, i.e. neither around
the antennal nerve nor in any other areas. Thus, unlike in vertebrates and
some insects (Cayre et al.,
1996; Garcìa-Verdugo et
al., 2002), neurons do not newly arise in the Drosophila
adult brain throughout its lifetime.
|
DISCUSSION |
|---|
|
TOPSUMMARYINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
|---|
We proved our hypothesis that neuronal PCD induces glial cell division. Most of the cells that incorporated BrdU were glia. We found a very small number of BrdU-positive cells that were both REPO-negative and ELAV-negative. A conceivable candidate is neural stem cells (neuroblasts). However, this is unlikely because BrdU-positive neurons have never been observed. The identity of this novel cell type remains to be investigated.
That neuronal PCD occurs in essentially all individuals was indicated by the fact that strongly EcRA-positive cells were observed in most flies at 6 hours AAE (Fig. 1H). In spite of this, not all the antennal nerves were found associated with BrdU-positive cells (Fig. 6A). This discrepancy might be due to the technical difficulty of labelling cells with BrdU for long periods: some of the BrdU-incorporating cells might have died as BrdU is potentially toxic.
In the imaginal wing disc, preventing cell death itself with p35, but
leaving the caspase signalling pathway intact, increases proliferation
(Huh et al., 2004;
Perez-Garijo et al., 2004;
Ryoo et al., 2004). This was
not the case for glial division in the brain because glial cells did not
divide when cell death was suppressed by p35. Apparently, a different
molecular mechanism triggers the glial response. We found that Eiger, a TNF
superfamily ligand, is involved in this process. Glial division in both intact
and injured brains was attenuated in eiger mutants, and ectopic
expression of eiger in glia rescued this phenotype. The rescue,
however, was not complete, and glial expression of eiger alone did
not induce ectopic glial division. This might be because (1) spatiotemporal
expression of eiger is required in glia, (2) expression of
eiger in the neurons might also be important, or (3) factors other
than Eiger are also involved in this process.
What, then, could be the role of glial division upon developmental PCD? In
the Drosophila rdgBKS222 mutant, glial cells in the
compound eye fill the voids that were formed by axonal degeneration
(Stark and Carlson, 1982).
Similarly, dividing glial cells in the brain might contribute to structural
support after neural loss. Another possibility is that glial cells protect
neural tissue by removing dead cells and/or by secreting trophic factors. Our
observation that the lack of Eiger, and thus the lack of glial division, led
to the increase in neuronal PCD supports this hypothesis.
Neural injury induces glial division in the Drosophila adult brain
The Drosophila adult brain shows a similar injury response to that
of vertebrates: expression of β amyloid protein precursor-like (APPL) and
activation of c-Jun N-terminal kinases (JNKs) are induced
(Leyssen et al., 2005).
Neurons fail to regenerate in response to injury
(Ayaz et al., 2008), and glial
cells in the antennal lobe change their morphology upon antennal ablation
(Macdonald et al., 2006).
Glial division, by contrast, has not been demonstrated in the fly brain. Here,
we provided evidence that glia also divide upon injury and that Eiger mediates
this process.
The glial division observed in the fly brain, however, seems to be much
less extensive than that observed in vertebrates. A notable difference is in
the variety of the dividing glial cell types. Whereas astrocytes, microglia
and oligodendrocyte precursor cells proliferate in vertebrates
(Fawcett and Asher, 1999),
only a subset of glial cells around the neuropile is likely to respond in the
Drosophila brain. As drastic glial proliferation upon neural injury,
which causes a glial scar, is involved in the inhibition of neural recovery in
vertebrates (Yiu and He,
2006), the Drosophila nervous system, with its much
restricted level of glial division, should provide an interesting model system
for investigating the responses of neurons to injury, including neural
recovery.
In vertebrates, TNF is involved in the inflammatory response against
neural lesions and plays multiple roles in such as the induction of cell
death, cell survival and proliferation through the JNK and NF
B pathways
(Goetz et al., 2004;
Varfolomeev and Ashkenazi,
2004; Scherbel et al.,
1999). In Drosophila, overexpression of eiger in
the imaginal discs appears to cause caspase-dependent cell death through the
JNK pathway via Wengen, the sole known TNF receptor
(Kauppila et al., 2003;
Moreno et al., 2002). However,
Eiger is not required for caspase-dependent cell death caused by ionising
radiation of the imaginal discs, even though irradiation induces the
expression of eiger (Brodsky et
al., 2004). Eiger is known to contribute in vivo to the proper
localisation of determinant during the asymmetric division of neuroblasts
(Wang et al., 2006). Wengen,
however, does not seem to be involved in this process, suggesting the
existence of as yet unknown receptors for Eiger. In our study, RNA
interference of wengen did not appear to cause defects in glial
division (data not shown). Further investigation is required to understand the
pathways downstream of Eiger in glial cell division, as well as in various
other Eiger-mediated phenomena.
A critical period of glial division upon neural loss
A surprising finding of our study is that there is a critical period of
glial division. We found that both PCD- and injury-induced glial division only
occur during the first 8 days AAE. Glial cells that are distant from the
antennal nerve, which do not normally divide in the adult brain, retain the
ability to respond to brain stab. This competence is lost as the flies grow
older. Interestingly, there is temporal coincidence between the competence of
glial division and neural plasticity. The application of certain odorants
leads to an increase in the volume of particular glomeruli only during 2-5
days, but not after 8 days, AAE (Sachse et
al., 2007; Devaud et al.,
2003). Considering the possible role of glia in trophic function
and structural support, glial division might be actively involved in brain
plasticity. The temporal coincidence suggests that the adult stage of
Drosophila can be divided into two phases: the first week AAE, as the
critical period in which glial division against neural loss and plasticity of
the antennal lobe neurons can be observed, and the rest of the adult life,
during which these events do not occur.
Our study has identified the first example in which developmental PCD triggers glial cell division. We also revealed important similarities between the glial response to PCD and to neural injury and between the glial response in insects and vertebrates after injury. The model system introduced in this study serves as a convenient platform for analysing novel types of neuron-glia interaction during recovery of the brain after PCD and injury, as well as how stage-dependent glial competence is controlled.
Supplementary material
Supplementary material for this article is available at
http://dev.biologists.org/cgi/content/full/136/1/51/DC1
|
Footnotes |
|---|
Present address: School of Biosciences, University of Birmingham,
Edgbaston, Birmingham B15 2TT, UK 
Present address: Department of Neurobiology, University of Massachusetts
Medical School, Worcester, MA 01605, USA
|
REFERENCES |
|---|
|
TOPSUMMARYINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
|---|
Awasaki, T. and Ito, K. (2004). Engulfing action of glial cells is required for programmed axon pruning during Drosophila metamorphosis. Curr. Biol. 14,668 -677.[CrossRef][Medline]
Awasaki, T., Tatsumi, R., Takahashi, K., Arai, K., Nakanishi, Y., Ueda, R. and Ito, K. (2006). Essential role of the apoptotic cell engulfment genes draper and ced-6 in programmed axon pruning during Drosophila metamorphosis. Neuron 50,855 -867.[CrossRef][Medline]
Ayaz, D., Leyssen, M., Koch, M., Yan, J., Srahna, M., Sheeba,
V., Fogle, K. J., Holmes, T. C. and Hassan, B. A. (2008).
Axonal injury and regeneration in the adult brain of Drosophila.
J. Neurosci. 28,6010
-6021.
Bishop, D. L., Misgeld, T., Walsh, M. K., Gan, W. B. and Lichtman, J. W. (2004). Axon branch removal at developing synapses by axosome shedding. Neuron 44,651 -661.[CrossRef][Medline]
Booth, G. E., Kinrade, E. F. and Hidalgo, A. (2000). Glia maintain follower neuron survival during Drosophila CNS development. Development 127,237 -244.[Abstract]
Brade, Y. (1989). Trophic factors and neuronal survival. Neuron 2,1525 -1534.[CrossRef][Medline]
Brodsky, M. H., Weinert, B. T., Tsang, G., Rong, Y. S.,
McGinnis, N. M., Golic, K. G., Rio, D. C. and Rubin, G. M.
(2004). Drosophila melanogaster MNK/Chk2 and p53
regulate multiple DNA repair and apoptotic pathways following DNA damage.
Mol. Cell. Biol. 24,1219
-1231.
Cakouros, D., Daish, T. J. and Kumar, S.
(2004). Ecdysone receptor directly binds the promoter of the
Drosophila caspase dronc, regulating its expression in
specific tissues. J. Cell Biol.
165,631
-640.
Cayre, M., Strambi, C., Chaprin, P., Augier, R., Meyer, M. R., Edwards, J. S. and Strambi, A. (1996). Neurogenesis in adult insect muchroom bodies. J. Comp. Neurol. 371,300 -310.[CrossRef][Medline]
Clarke, P. (1985). Neuronal death in the development of the vertebrate nervous system. Trends Neurosci. 8,345 -349.[CrossRef]
Devaud, J. M., Acebes, A., Ramaswami, M. and Ferrus, A. (2003). Structural and functional changes in the olfactory pathway of adult Drosophila take place at a critical age. J. Neurobiol. 56,13 -23.[CrossRef][Medline]
Fawcett, J. W. and Asher, R. A. (1999). The glial scar and central nervous system repair. Brain Res. Bull. 49,377 -391.[CrossRef][Medline]
Fields, R. D. and Burnstock, G. (2006). Purinergic signalling in neuron-glia interactions. Nat. Rev. Neurosci. 7,423 -436.[CrossRef][Medline]
Frebel, K. and Wiese, S. (2006). Signalling molecules essential for neuronal survival and differentiation. Biochem. Soc. Trans. 34,1287 -1290.[CrossRef][Medline]
García-Verdugo, J. M., Ferrón, S., Flames, N., Collado, L., Desfillis, E. and Font, E. (2002). The proliferative centricular zone in adult vertebrates: a comparative study using reptiles, birds, and mammals. Brain Res. Bull. 57,765 -775.[CrossRef][Medline]
Goetz, F., Planas, J. V. and MacKenzie, S. (2004). Tumor necrosis factors. Dev. Comp. Immunol. 28,487 -497.[CrossRef][Medline]
Griffiths, R. L. and Hidalgo, A. (2004). Prospero maintains the mitotic potential of glial precursors enabling them to respond to neurons. EMBO J. 23,2440 -2450.[CrossRef][Medline]
Haynie, J. L. and Bryant, P. J. (1977). The effects of X-rays on the proliferation dynamics of cells in the imagninal wing disc of Drosophila melanogaster. Roux's Arch. Dev. Biol. 183,85 -100.[CrossRef]
Huh, J., Guo, M. and Hay, B. A. (2004). Compensatory proliferation induced by cell death in the Drosophila wing disc requires activity of the apical cell death caspase Dronc in a nonapoptotic role. Curr. Biol. 14,1262 -1266.[CrossRef][Medline]
Igaki, T., Kanda, H., Yamamoto-Goto, Y., Kanuka, H., Kuranaga, E., Aigaki, T. and Miura, M. (2002). Eiger, a TNF superfamily ligand that triggers the Drosophila JNK pathway. EMBO J. 21,3009 -3018.[CrossRef][Medline]
Ito, K., Urban, J. and Technau, G. (1995). Distribution, classification, and development of Drosophila glial cells in the late embryonic and early larval ventral nerve cord. Roux's Arch. Dev. Biol. 204,284 -307.[CrossRef]
Ito, K., Okada, R., Tanaka, N. K. and Awasaki, T. (2003). Cautionary observations on preparing and interpreting brain images using molecular biology-based staining techniques. Microsc. Res. Tech. 62,170 -186.[CrossRef][Medline]
Kauppila, S., Maaty, W. S., Chen, P., Tomar, R. S., Eby, M. T., Chapo Chew, S. J., Rathore, N., Zachariah, S., Sinha, S. K., Abrams, J. M. et al. (2003). Eiger and its receptor, Wengen, comprise a TNF-like system in Drosophila. Oncogene 22,4860 -4867.[CrossRef][Medline]
Lee, T. and Luo, L. (1999). Mosaic analysis with a repressible cell marker for studies of gene function in neuronal morphogenesis. Neuron 22,451 -461.[CrossRef][Medline]
Leyssen, M., Ayaz, D., Hebert, S. S., Reeve, S., De Strooper, B. and Hassan, B. A. (2005). Amyloid precursor protein promotes post-developmental neurite arborization in the Drosophila brain. EMBO J. 24,2944 -2955.[CrossRef][Medline]
Liu, H. M., Yang, L. H. and Yang, Y. J. (1995). Schwann cell properties: 3. C-fos expression, bFGF production, phagocytosis and proliferation during Wallerian degeneration. J. Neuropathol. Exp. Neurol. 54,487 -496.[Medline]
Macdonald, J., Beach, M. G., Porpiglia, E., Sheehan, A. E., Watts, R. J. and Freeman, M. R. (2006). The Drosophila cell corpse engulfment receptor Draper mediates glial clearance of severed axons. Neuron 50,869 -881.[CrossRef][Medline]
Moreno, E., Yan, M. and Basler, K. (2002). Evolution of TNF signaling mechanisms: JNK-dependent apoptosis triggered by Eiger, the Drosophila homolog of the TNF superfamily. Curr. Biol. 12,1263 -1268.[CrossRef][Medline]
Oppenheim, R. W. (1991). Cell death during development of the nervous system. Annu. Rev. Neurosci. 14,453 -501.[CrossRef][Medline]
Perez-Garijo, A., Martin, F. A. and Morata, G.
(2004). Caspase inhibition during apoptosis causes abnormal
signalling and developmental aberrations in Drosophila.
Development 131,5591
-5598.
Robinow, S., Talbot, W. S., Hogness, D. S. and Truman, J. W. (1993). Programmed cell death in the Drosophila CNS is ecdysone-regulated and coupled with a specific ecdysone receptor isoform. Development 119,1251 -1259.[Abstract]
Robinow, S., Draizen, T. A. and Truman, J. W. (1997). Genes that induce apoptosis: transcriptional regulation in identified, doomed neurons of the Drosophila CNS. Dev. Biol. 190,206 -213.[CrossRef][Medline]
Ryoo, H., Gorenc, T. and Steller, H. (2004). Apoptotic cells can induce compensatory cell proliferation through the JNK and the Wingless signaling pathways. Dev. Cell 7, 491-501.[CrossRef][Medline]
Sachse, S., Rueckert, E., Keller, A., Okada, R., Tanaka, N. K., Ito, K. and Vosshall, L. B. (2007). Activity-dependent plasticity in an olfactory circuit. Neuron 56,838 -850.[CrossRef][Medline]
Scherbel, U., Raghupathi, R., Nakamura, M., Saatman, K. E.,
Trojanowski, J. Q., Neugebauer, E., Marino, M. W. and McIntosh, T. K.
(1999). Differential acute and chronic responses of tumor
necrosis factor-deficient mice to experimental brain injury. Proc.
Natl. Acad. Sci. USA 96,8721
-8726.
Stark, W. S. and Carlson, S. D. (1982). Ultrastructural pathology of the compound eye and optic neuropiles of the retinal degeneration mutant (w rdg BKS222) Drosophila melanogaster. Cell Tissue Res. 225, 11-22.[CrossRef][Medline]
Stocker, R. F., Lienhard, M. C., Borst, A. and Fischbach, K. F. (1990). Neuronal architecture of the antennal lobe in Drosophila melanogaster. Cell Tissue Res. 262, 9-34.[CrossRef][Medline]
Treherne, J. E., Harrison, J. B., Treherne, J. M. and Lane, N. J. (1984). Glial repair in an insect central nervous system: effects of surgical lesioning. J. Neurosci. 4,2689 -2697.[Abstract]
Varfolomeev, E. E. and Ashkenazi, A. (2004). Tumor necrosis factor: an apoptosis JuNKie? Cell 116,491 -497.[CrossRef][Medline]
Wang, H., Cai, Y., Chia, W. and Yang, X. (2006). Drosophila homologs of mammalian TNF/TNFR-related molecules regulate segregation of Miranda/Prospero in neuroblasts. EMBO J. 25,5783 -5793.[CrossRef][Medline]
Watts, R., Schuldiner, O., Perrino, J., Larsen, C. and Luo, L. (2004). Glia engulf degenerating axons during developmental axon pruning. Curr. Biol. 14,678 -684.[CrossRef][Medline]
Yiu, G. and He, Z. (2006). Glial inhibition of CNS axon regeneration. Nat. Rev. Neurosci. 7, 617-627.[CrossRef][Medline]
Yu, S., Yoo, S. J., Yang, L., Zapata, C., Srinivasan, A., Hay,
B. A. and Baker, N. E. (2002). A pathway of signals
regulating effector and initiator caspases in the developing
Drosophila eye. Development
129,3269
-3278.
CiteULike 
Complore 
Connotea 
Del.icio.us 
Digg 
Reddit 
Technorati 
Twitter What's this?
| |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
