Laboratory of Biology, Iwamizawa Campus, Hokkaido University of Education, Iwamizawa, Hokkaido 068-8642, Japan
Author for correspondence (e-mail:
kimura{at}iwa.hokkyodai.ac.jp)
Accepted 22 December 2003
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SUMMARY |
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Key words: Programmed cell death, Drosophila, Metamorphosis, cAMP, PKA, rickets, Bursicon, Wing
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Introduction |
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The components and mechanisms for programmed cell death are conserved in a
wide variety of organisms, from worms and flies to humans. These mechanisms
involve the activation of cysteine proteases, known as caspases, which act as
executioners (Kumar and Doumanis,
2000). Caspase activity is inhibited by negative regulators, such
as the Inhibitor of Apoptosis Proteins (IAPs), that prevent cell death
(Hay, 2000
). In
Drosophila, the products of the pro-apoptotic genes reaper
(rpr) (White et al.,
1994
), head involution defective (hid;
Wrinkled, W FlyBase)
(Grether et al., 1995
),
grim (Chen et al.,
1996
) and sickle (skl)
(Christich et al., 2002
;
Srinivasula et al., 2002
;
Wing et al., 2002a
) promote
cell death by the inhibition of IAP function
(Wang et al., 1999
;
Holley et al., 2002
;
Ryoo et al., 2002
;
Wing et al., 2002b
;
Yoo et al., 2002
). Various
death and survival signals should converge onto the pro-apoptotic genes during
development. The nature of these extracellular signals and the signal
transduction pathways that activate or suppress the death program are largely
unknown.
During metamorphosis in Drosophila, most of the larval tissues die
and are removed. The timing of these events is orchestrated by changes in the
levels of the steroid hormone 20-hydroxyecdysone (ecdysone) in the larval
salivary glands and midgut (Jiang et al.,
1997). Ecdysone regulates expression of a number of early and late
genes (Lee et al., 2002a
;
Lee et al., 2002b
) that act in
part by inducing expression of the cell death activator genes rpr and
hid, thereby repressing the Drosophila IAP gene
Diap2 (Iap2 FlyBase)
(Jiang et al., 1997
). Thus,
changes in hormone titre, in combination with the fine-scale expression of the
ecdysone receptor, control timing and spacing of cell death during
metamorphosis. Though ecdysone-dependent regulation of cell death is the best
understood pathway, it is likely that peptide hormones also regulate
programmed cell death in Drosophila metamorphosis.
At the last step of metamorphosis, newly emerged adults undergo extensive
cell death. Many of the abdominal muscles and associated neurons die within a
day of adult emergence (Finlayson,
1975; Truman,
1983
; Kimura and Truman,
1990
). In addition, the wing epidermis is also removed by cell
death at the time of wing spreading in the large fly Lucilia cuprina
(Seligman et al., 1975
) and in
Drosophila melanogaster (Johnson
and Milner, 1987
). In the present study, we focused on this
epidermal cell death during maturation of the wings of Drosophila.
Marking of wing epidermal cells by GFP using the GAL4/UAS system
(Brand and Perrimon, 1993
)
enabled us to follow the fate of the epidermal cells and facilitated further
analysis of the regulation of the cell death. We report that the cell death is
triggered by a hormone, probably bursicon, which is released just after
eclosion, and that the hormonal signal is received by RICKETS (DLGR2), one of
glycoprotein hormone receptors identified as a member of the G-protein-coupled
receptor family (Eriksen et al.,
2000
; Hewes and Taghart, 2001;
Baker and Truman, 2002
). We
also present evidence that this signal is transmitted through the cAMP/PKA
signaling pathway. These results reveal a novel mechanism to regulate
programmed cell death by a hormone other than ecdysone.
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Materials and methods |
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The same en-Gal4 line was used to express the following
constructs: UAS-GsWT and UAS-Gs
* (Drosophila wild-type
Gs
subunit and constitutively active Gs
subunit, respectively)
(Wolfgang et al., 1996
);
UAS-mC* (constitutively active form of mouse PKA catalytic subunit mC*)
(Li et al., 1995
); UAS-R*
(dominant-negative form of the regulatory subunit of PKA)
(Li et al., 1995
); or UAS-p35
(anti-apoptotic protein) (Zhou et al.,
1997
), using the Gal4/UAS expression system
(Brand and Perrimon, 1993
).
For clonal analysis of the Gs or PKA mutations, somatic clones were
produced using the FLP/FRT recombination system
(Xu and Rubin, 1993
). First or
second instar larvae, generated by crossing y hs-flp1 / y1
w1118; FRT42D y+ /+; +/+ females to y
w/Y; FRT42D sha bw dgsR60C /+; His-GFP/+ males, were heat
shocked for 1 hour at 37°C to induce mitotic recombination, and adult
flies were examined for clones of sha wing tissue,
where hairs are missing or replaced by smaller hairs. Similar treatment of
first or second instar larvae, derived from the cross of y hs-flp1; hs-CD2
y+ hs-myc FRT39E/CyO; MKRS/TM2 females to y w/Y; DC0E95
stc FRT39E/+; His-GFP/+ males was carried out and adult flies were
examined for clones of stc wing tissue, which
replaces hairs by smaller hairs or tufts of hairs. The stocks for generation
of dgs or DC0 mutant clones were generously provided by
Wolfgang et al. (Wolfgang et al.,
2001
) and Jiang and Struhl
(Jiang and Struhl, 1995
),
respectively.
The mutant stock of rickets (rk) used was
rk1 cn1 bw1; His-GFP/+. An allele of
rk1 carries a mutation in the transmembrane domain that
results in a premature termination codon. This mutation should prevent the
production of a functional membrane receptor
(Baker and Truman, 2002).
Observation of wing epidermal cells and detection of cell death
Wings were dissected in PBS and mounted in PBS on a slide glass. Expression
of GFP in the central part of the wing blade was observed under an Olympus
AX70 fluorescence microscope equipped with an Olympus DP50 camera and images
obtained were processed with Photoshop software. The extent of cell death was
graded into three classes based on the number of cells undergoing death at the
time of observation: `occurrence of cell death', 80% or more of cells were
dead; `partial cell death', 20-80% of cells were dead; `no cell death', less
than 20% of cells were dead.
Movie images of dying cells in the wing of an intact fly, en-Gal4 UAS-GFPN, were captured, after fixing a ventral portion of the fly on a slide glass with melted myristyl alcohol. The wing just after spreading was observed under a fluorescence microscope equipped with a Sony video camera DXC-930, and images were recorded on videotape.
Ligations and injections
Newly eclosed adults were collected at 3-minute intervals. Staged flies
after eclosion were anesthetized on an ice-chilled Petri plate, ligated at the
neck with a thin silk thread, and the head was cut away. Pharate adults were
collected on a strip of double-sided tape attached to a slide glass and the
opercula of the puparium was removed. They were staged under a dissection
microscope according to features of the head
(Kimura and Truman, 1990).
Briefly, at about 9 hours before eclosion, the pupal cuticle of the head has a smooth appearance [smooth (S) stage]. At 6 hours before eclosion, wrinkles appear in the pupal cuticle over the head [smooth/grainy (S/G) stage], because of the initiation of molting fluid resorption. By about 3 hours before eclosion, molting fluid resorption is well advanced and the pupal cuticle has a granular appearance [grainy (G) stage]. At about 50 minutes before eclosion, the head acquires a whitish sheen [white (W) stage], because air fills the space between the pupal and adult cuticle. At about 40 minutes before eclosion, the ptilinum protrudes from the front of the head [extended ptilinum (EP) stage].
After removal of the anterior half of the puparium, staged pharate adults were ligated at the neck. For injections, neck-ligated or intact flies were anesthetized on an ice-chilled Petri dish and were injected with solutions using a glass capillary injection needle connected to a glass syringe. The volume of solution injected was approximately 10-30 nl. Solutions used for injections were saline (PBS), 8-bromo-cAMP (Sigma) solution at various concentrations and DAPI solution at the concentration of 0.1 mg/ml in PBS. Flies ligated or/and injected were kept in a moist Petri dish till the desired time.
For collection of hemolymph, we cut legs of a staged fly and compressed the thorax of the fly using forceps. A drop of hemolymph was collected in a glass capillary needle and was injected into a host fly.
Histology
TUNEL (TdT-mediated dUTP nick-end labeling) assays were carried out, using
the In Situ Cell Death Detection Kit, POD (Roche), as described by the
manufacturer. Staged flies of Canton-Special strain (wild type) were fixed in
3.7% formaldehyde for 2 hours and paraffin-sectioned preparations of wings
were prepared by standard methods. The sections were incubated in TUNEL
reaction mixture containing TdT and fluorescein-conjugated dUTP to label the
ends of DNA fragments. After washing, the labeled preparations were stained
with DAPI solution.
Wings were fixed for electron microscopy (EM) in 2.5% glutaraldehyde in 0.1 M phosphate buffer, and embedded into Epon 812 by standard procedures. EM sections were stained with uranyl acetate and lead citrate, and examined on a JEOL 1010 electron microscope.
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Results |
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Transmission electron microscopy (TEM) analysis revealed the association of dynamic changes in vacuole structure with this cell death process. A wing epidermal cell at eclosion possessed a spherical nucleus with some electron dense materials (Fig. 1R). At 20 minutes after wing spreading, the cell possessed condensed chromatin and many vacuoles that contain cellular components including mitochondria (Fig. 1S). Thus, TEM indicated that this cell death exhibited features indicative of autophagy, but this required further detailed analysis. Further studies in the detailed processes of the wing epidermal cell death will be described later.
We examined the effect of the forced expression of an anti-apoptotic gene, p35, the products of which inhibit the activity of caspases. We followed the fate of wing epidermal cells in en-Gal4 UAS-GFPN /UAS-p35 adults. Ectopic expression of p35 inhibited the death of wing epidermal cells at 2 hours after eclosion (Fig. 1T). Then, the cells detached from the wing cuticle without breakdown of the nuclear membrane, and many persisting cells remained between the cuticular sheets at least until 48 hours after eclosion (Fig. 1U), indicating that the caspases are involved in the cell death process.
Inhibition of the cell death also prevented the subsequent adhesion of the ventral and dorsal cuticular sheets (data not shown). As a result, wings remained filled with hemolymph and some persisting cells or debris, and, in some cases, ectopic expression of p35 in the epidermal cells produced blistered wings (data not shown).
Neck ligation or wing isolation prevents the death of wing epidermal cells
To determine whether a humoral signal coming from the head triggers the
cell death, we ligated the necks of flies at various times after eclosion and
examined them for cell death at 2 hours after the ligation. Ligation just
after eclosion suppressed cell death and GFP was still detectable in the
nuclei of wing epidermal cells after 2 hours
(Fig. 2A). By contrast, when
flies were ligated at 20 minutes after eclosion, the normal pattern of cell
death was observed (Fig. 2B).
Ligation at later stages correlated with an increased percentage of flies with
wing epidermal cell death (Fig.
2C). Thus wing epidermal cell death is triggered by a signal
emanating from the head shortly after eclosion.
|
We isolated wings from wild-type flies at various times after eclosion and examined cell death 2 hours after the isolation. Cell death was blocked in wings isolated at eclosion, whereas cell death was unaffected by isolation 20 minutes after eclosion in most cases (Fig. 2D,E). Even in folded wings, cell death proceeded normally, indicating that the spreading of the wing itself is not necessary to induce cell death. As in the case of neck ligation, the increment in time of wing isolation after eclosion correlated with an increase in the percentage of flies showing wing epidermal cell death (Fig. 2F).
Experiments on hemolymph injection confirmed that a hormonal factor is a direct signal to induce the cell death. We injected hemolymph from wild-type flies at various times after eclosion into flies neck-ligated at eclosion and examined the induction of cell death at 2 hours after injection. Injection of hemolymph from flies at eclosion did not induce the cell death (Fig. 3A). However, injection of hemolymph from flies at 30 minutes and 60 minutes after eclosion induced the cell death (Fig. 3B,C). Hemolymph collected from flies at 120 minutes after eclosion was less able to induce the cell death. Control flies injected with PBS after neck ligature showed no induction of cell death (Fig. 4B). These results indicate that the death inducing hormonal factor is secreted just after eclosion but disappears at 2 hours and later after eclosion.
|
|
cAMP causes the death of wing epidermal cells
To examine the effect of cAMP on the induction of cell death in
Drosophila, a membrane-permeant 8-bromo analog, 8-Br-cAMP, was
injected at various concentrations into flies neck-ligated at eclosion and
cell death induction was examined 2 hours after injection. Injection of
8-Br-cAMP produced a dose-dependent induction of cell death
(Fig. 4A,C). As a control,
injection of PBS showed no induction of cell death
(Fig. 4B). cAMP did not inhibit
GFP expression by affecting the en promoter, because GFP-marked wing
veins were present in folded wings that had undergone the death (data no
shown). In subsequent experiments, we used 8-Br-cAMP at a concentration of
101 mol/l. We followed the time course of cell death after
the injection of 8-Br-cAMP. In all cases (32/32), cell death proceeded within
1 hour of injection.
The timing of responsiveness of cells to cAMP was investigated. 8-Br-cAMP was injected into pharate adult flies neck-ligated at various stages and induction of the cell death was examined at 2 hours after injection (Fig. 4D). As in the case of the death inducing hormone, when cAMP was injected into flies at S or S/G stage, cell death was not induced. However, when cAMP was injected at G or later (W and EP) stages, the induction of the cell death was observed.
We examined the effect of injection of PBS or 8-Br-cAMP prior to eclosion in intact flies. After injection into pharate adults at EP stage (about 0-40 minutes before eclosion), the flies were released from the pupal cases. They then walked around and spread their wings. Flies injected with PBS showed normal wing-spreading behavior, and the wing epidermal cells were still alive at wing spreading in all cases (8/8) (Fig. 4E). Flies injected with 8-Br-cAMP showed blistered wings in all case (7/7) (Fig. 4F), due to the precocious death of the epidermal cells at wing spreading (Fig. 4G).
Activated Gs* mimics the effects of cAMP
These results demonstrate that death in wing epidermal cells can be induced
by elevation of cAMP. To test this in intact flies, we manipulated adenylyl
cyclase activity by overexpressing a constitutively active Gs subunit
(Gs
*), generated by substitution of leucine 215 by glutamine 215
(Quan et al., 1991
;
Wolfgang et al., 1996
).
Expression of Gs
* results in receptor-independent activation of the
subunit, and thus activates an endogenous adenylyl cyclase
(Quan et al., 1991
;
Chyb et al., 1999
). Targeted
expression of wild-type Gs
in wing epidermal cells using
en-Gal4 driver had no effect on cell death
(Fig. 5A). By contrast,
targeted expression of Gs
* caused wing blisters
(Fig. 5B). Observation of the
GFP expression pattern in cells at wing spreading demonstrated that precocious
cell death occurred in the blistered wings
(Fig. 5C).
|
We examined the effect of elimination of Gs activity by generating
clones of dgs mutant cells within the developing wings. A
dgsR60C mutation is likely to be a null allele that is
generated by the change of nucleotide 723 from a T to an A, resulting in the
change of residue 241 in the protein from a Tyr to a stop codon
(Wolfgang et al., 2001
). In
this mosaic analysis, we marked wing epidermal cells using Histone-GFP. The
cells of the clones remained at 2 hours after wing spreading, although the
cells around clones had already disappeared as a result of cell death
(Fig. 5E,F). Thus, elimination
of Gs
activity prevents the death of wing epidermal cells.
cAMP effects are mediated by protein kinase A
The cellular effects of cAMP are usually mediated by PKA (for reviews, see
Gottesman, 1980;
Francis and Corbin, 1994
). To
determine whether this is also the case for wing epidermal cell death, we
examined the effect of reduction or elimination of PKA activity on cell
death.
We initially used a dominant-negative form of the regulatory subunit of PKA
(R*), whose ectopic expression is known to reduce the activity of endogenous
PKA (Li et al., 1995). When R*
was ectopically expressed using the en-Gal4 driver, many cells of the
wings remained at 2 hours, or even at 8 hours, after wing spreading
(Fig. 6A,B), resulting in
separation between the ventral and dorsal cuticular sheets in posterior
compartment. Targeted expression of R* caused wavy or curly wings (data not
shown), probably due to the distortion between normal adhesion of dorsoventral
cuticles in the anterior compartment and detachment of the cuticle in the
posterior compartment.
|
We examined the effects of constitutive activation of PKA on cell death. We
used a mutationally altered mouse catalytic subunit (mC*) that is resistant to
inhibition by the regulatory subunit
(Orellana and McKnight, 1992).
The mutant catalytic subunit is constitutively active, irrespective of cAMP
concentration, and can function in Drosophila cells
(Jiang and Struhl, 1995
;
Li et al., 1995
). Using the
en-Gal4 driver, we expressed the constitutively active catalytic
subunit of mC* in wing epidermal cells. All eclosing flies had blistered wings
(Fig. 6E). The wing epidermal
cells died prior to wing spreading (Fig.
6F). Thus, constitutive activation of PKA causes the precocious
death of wing epidermal cells.
We examined the induction of cell death at various stages of pharate
adults. As seen in the cases of cAMP injection and of ectopic expression of
Gs*, precocious cell death was induced at G stage and later
(Fig. 6G). This indicates that
wing cells acquire competence to respond to PKA activity by G stage, about 3
hours before eclosion.
A mutation in the G-protein coupled receptor gene rickets inhibits wing epidermal cell death
In Drosophila, the rickets gene is a member of the
glycoprotein hormone receptor family of the G-protein-coupled receptors
(Ashburner et al., 1999;
Eriksen et al., 2000
) and has
been suggested to encode a bursicon receptor
(Baker and Truman, 2002
). We
examined wing epidermal cell death, marked by Histone-GFP, in rk
mutants. In the mutants, wing epidermal cells remained at 2 hours, or even at
8 hours, after eclosion (Fig.
7A,B).
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Discussion |
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Programmed cell death in wing epidermal cells plays an important role in
the maturation of wings. We showed that induction of precocious cell death
induced by injection of 8-Br-cAMP, or by ectopic expression of a
constitutively activated form of Gs or PKA, caused a blistered wing
phenotype. During wing development, wing epidermal cells connect the two
surfaces of the wing together by a highly specialized system of cytoskeletal
supports, the trans-alar array, which is a mechanically continuous structure
consisting of microtubules and microfilaments
(Tucker et al., 1986
). The
trans-alar array is anchored by integrin-mediated basal adhesion
(Fristrom et al., 1993
).
Disruption of these connections in integrin mutants also results in blistered
wings (Brower and Jaffe, 1989
;
Brabant and Brower, 1993
;
Brower et al., 1995
;
Brabant et al., 1996
). Thus,
the wing epidermal cells are necessary to connect the two layers of the wing
until wing expansion.
Inhibition or delay of the cell death also disturbs the subsequent maturation of the wing. In the flies ectopically expressing p35 or a dominant-negative form of PKA, inhibition of the cell death prevented adhesion of the dorsal and ventral cuticle, and sometimes caused blistered wings. Thus, the precise regulation of cell death is essential for the formation of functional wings.
A peptide hormone trigger for wing epidermal cell death
The peptide hormone, bursicon, is known to play a role in the post-ecdysial
phase of development (Cottrell,
1962; Fraenkel and Hisao, 1962). Bursicon has been shown to be
released before wing expansion and to hasten the tanning reaction, serving to
harden the newly expanded cuticle. Our results suggest that the hormone that
induces cell death of the wing epidermis could be bursicon.
First, neck ligation and hemolymph injection experiments demonstrated that
the triggering signal to induce death is a humoral factor released after
eclosion. This temporal pattern of death-inducing activity in the hemolymph
corresponds to that of bursicon. Second, injection of cAMP induced cell death,
implicating cAMP as the second messenger in the cell death pathway. Studies in
blowflies have shown that bursicon also acts through cAMP
(Seligman and Doy, 1972;
Seligman and Doy, 1973
).
Recently, in Drosophila, cAMP was shown to induce cuticular
melanization in a fashion similar to bursicon
(Baker and Truman, 2002
).
Third, reception of the hormonal signal inducing cell death is mediated by a
probable bursicon receptor, RICKETS (DLGR2), which also act through cAMP
(Eriksen et al., 2000
; Hewes
and Taghart, 2002; Baker and Truman,
2002
). Finally, in Lucilia cuprina, it was proposed that
bursicon is the same as fragment disaggregating hormone (FDH), which increases
the circulating filamentous cellular fragments derived from post-ecdysial
death of the wing epidermal cells
(Seligman and Doy, 1973
;
Seligman et al., 1975
). Taken
together, it is likely that bursicon coordinates events such as the cell death
of wing epidermis and the subsequent tanning and hardening of the cuticle.
However, we cannot rule out another possibility, namely that several humoral
factors could signal through the pathway. Identification of a bursicon gene in
Drosophila (Riehle et al.,
2002
) will facilitate genetic approaches to understand the role of
bursicon in wing epidermal cell death.
cAMP/PKA signaling is required for the wing epidermal cell death
The fact that cell death can be induced by the injection of cAMP implicates
cAMP in the humoral signal transduction pathway. The binding of ligand to
G-protein-coupled receptors is known to stimulate Gs, resulting in
adenylyl cyclase activation and the production of cAMP (for reviews, see
Tang and Gilman, 1992
;
Neer, 1995
). Our
gain-of-function analyses, using constitutively activated Gs
, and
loss-of-function analyses, using a dgs mutation, indicate that the
activity of Gs
is sufficient and necessary for the death of wing
epidermal cells. Consistent with this, endogenous Gs
is expressed in
the basal and trans-alar membranes of the wing epithelium of pharate adults
(Wolfgang et al., 1996
).
Cellular effects of cAMP are usually mediated by PKA (for reviews, see
Gottesman, 1980;
Francis and Corbin, 1994
), and
this holds true in the case of cell death in the wing epidermis. Ectopic
expression of a constitutively active form of PKA in wing epidermal cells
induced cell death. Contrastingly, ectopic expression of a dominant-negative
form of PKA or a mutation of the Pka-C1 gene prevented the cell
death. Thus, the death of wing epidermal cells is stimulated by and requires
PKA activity.
Activated PKA phosphorylates substrates that control diverse cellular
phenomena. The signaling mechanisms used by cAMP/PKA to control programmed
cell death are likely to be complex and cell-type specific. For example,
cAMP-mediated activation of PKA stimulates apoptosis in thymocytes
(McConkey et al., 1990) and
leukemic cell lines (Lanotte et al.,
1991
). By contrast, it protects neutrophils
(Parvathenani et al., 1998
),
thyroid follicular cells (Saavedra et al.,
2002
) and spinal ganglion neurons
(Bok et al., 2003
) from
apoptosis. With respect to the suppression of cell death, one molecular
mechanism has been elucidated: PKA can phosphorylate the pro-apoptotic
regulator Bad and inhibit its function
(Harada et al., 1999
;
Bok et al., 2003
). However,
little is known about the molecular mechanisms through which cAMP/PKA promotes
cell death.
How could PKA regulate the death of wing epidermal cells? In
Drosophila, pro-apoptotic genes, such as rpr, hid and
grim, induce cell death (Chen et
al., 1996; Grether et al.,
1995
; White, at al.,
1994
). In the case of rpr and grim, death
induction is controlled at the level of transcription the pattern of
rpr/grim expression mimics the pattern of apoptosis.
Intriguingly, the gene hid is more broadly expressed in both cells
fated to die and those fated to live
(Grether et al., 1995
).
Furthermore, HID-induced apoptosis in midline glia cells or in eyes is
suppressed through posttranslational regulation of HID by the RAS-MAP Kinase
pathway (Bergmann et al., 1998
;
Bergmann et al., 2002
). The
death of wing epidermal cells proceeds promptly, within 1 hour after the
hormonal signal triggers it. This prompt induction of the cell death should be
controlled by posttranslational rather than transcriptional regulation. Our
preliminary studies show that the hid gene is involved in the cell
death of wing epidermal cells (K.-i.K., unpublished). The HID protein sequence
(Grether et al., 1995
)
contains three consensus PKA phosphorylation sites, suggesting that HID could
be a target of PKA.
Possible mechanism to acquire the competence
Flies acquire competence to respond to the death-triggering hormone before
eclosion. At about 3 hours before eclosion (G stage)
(Kimura and Truman, 1990),
wing epidermal cell death can be induced either by hemolymph injection, cAMP
injection, ectopic expression of Gs
* or ectopic expression of mC*.
Thus, the PKA-dependent cell death machinery is assembled at G stage, which
coincides with the time of eclosion hormone (EH) release
(Baker et al., 1999
). In our
preliminary experiments, EH-cell knockout flies did not show cell death of the
wing epidermis after eclosion (K.-i.K. and Y.H., unpublished). It is possible
that EH triggers expression of the components of the cell death pathway. It is
also possible that EH release simply induces the post-eclosion release of
bursicon, as in Lepidoptera
(Truman, 1973
), and that other
developmental mechanisms regulate assembly of the cell death machinery in the
wings.
In conclusion, these studies illustrate that regulation of programmed cell death plays an essential role in the maturation of the wings in Drosophila. The post-ecdysial cell death is regulated temporally and spatially by the hormone-receptor RICKETS through the cAMP/PKA signaling pathway. In Drosophila, the molecular components involved in cell death are well studied. Further analysis of the targets of PKA will link the signaling pathway with the components that directly regulate cell death.
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ACKNOWLEDGMENTS |
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Footnotes |
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* Present address: Department of Biology, Graduate school of Science, Kyushu
University, Ropponmatsu, Fukuoka 810-8560, Japan
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