Department of Molecular and Cellular Physiology, Stanford University, Stanford, California 94305-5426
Syntaxins are membrane proteins involved in vesicle trafficking and are required for the release of neurotransmitter at nerve terminals. The presence of syntaxins on target membranes has been hypothesized to confer specificity to targeting and fusion via interactions with complementary vesicle-associated proteins, the synaptobrevins or VAMPS. We have mutagenized syntaxin1 in Drosophila and have found that it links the mechanism of synaptic transmission to a distinct cell biological process: the cellularization of early embryos. This specialized form of cell division separates the 6,000 nuclei of the syncytial blastoderm into separate cells through the invagination of the surface membrane of the embryo. During this process, syntaxin1 protein is present on the newly forming lateral cell surfaces and invaginating cleavage furrows. This protein is derived both from maternal deposition of mRNA and protein and from early zygotic transcription. To analyze syntaxin1's role in early development, female germ line mosaics mutant for syntaxin1 expression were generated by mitotic recombination to reduce the maternal contribution. Visualizing the actin cytoskeleton and glycosylated surface proteins reveals that embryos with insufficient syntaxin1 have large acellular patches. The patches do not appear until cellularization begins, and the process fails entirely within these regions. These results provide genetic evidence that membrane trafficking is required for the cellularization of the syncytial blastoderm. We propose that the invagination of the surface membrane proceeds by the fusion of intracellular membrane vesicles with the surface. This reaction uses the same syntaxin1 protein as is required for neurotransmitter secretion at synapses. Thus, a single syntaxin can participate in trafficking steps that are functionally as distinct as synaptic transmission and cell division.
THE movement of membranes within a cell via transport vesicles is necessary for many cellular events,
ranging from intracellular transport and constitutive secretion to the tightly regulated secretion of transmitter at nerve terminals (Bennett and Scheller, 1993 Syntaxins and related proteins that are collectively referred to as t-SNAREs reside on target membranes and
are hypothesized to serve as address labels that identify a
membrane compartment (for review see Sollner and
Rothman, 1994 In nerve terminals, syntaxins are present on the presynaptic membrane and are known to interact with other proteins implicated in vesicular release (Bennett et al., 1992 Syntaxin's essential role in synaptic transmission has
been shown using both neurotoxins and genetic experiments. Fusion of synaptic vesicles with the terminal membrane can be blocked by the syntaxin-specific protease
botulinum toxin C1 (Blasi et al., 1993 While analyzing transcripts from the syntaxin1 gene, we
observed the presence of syntaxin1 message at the earliest
stages of Drosophila development, in embryos <3 h old
(Parfitt et al., 1995 The cellularization of Drosophila embryos involves extensive membrane rearrangements. Before cellularization,
the Drosophila embryo undergoes 13 synchronized cycles
of nuclear divisions that are not accompanied by cytokinesis (Foe and Alberts, 1983 The formation of the 6,000 cells has been estimated to
require a 23-fold increase in plasma membrane area compared to that of the syncytial embryo (Foe et al., 1993 We have examined the distribution of syntaxin1 in the
early embryo and have generated homozygous mutant
clones in the female germ line to reduce the maternal deposition of syntaxin1. Embryos derived from such clones
showed defects in cellularization if the embryos were also
zygotically mutant for syntaxin1. The results indicate that
syntaxin1 is essential for proper cellularization as well as
neurotransmitter release (Schulze et al., 1995 Western Blotting
Adult heads or embryos from timed collections were homogenized in 2×
Laemmli sample buffer (0.0625 M Tris, pH 6.8, 4% SDS, 10% glycerol)
and boiled for 2 min. Protein extracts were separated by SDS-PAGE using 12% acrylamide gels and transferred to Immobilon membranes (Millipore Corp., Bedford, MA). The blots were blocked overnight at 4°C in
PBS (140 mM NaCl, 2.7 mM KCl, 10 mM Na2HPO4, 1.8 mM KH2PO4, pH
7.3) with 5% milk, 1% BSA, 0.1% Tween-20, and 2% normal goat serum.
They were then probed with monoclonal antibody 8C3 at 1:10 dilution
(kind gift of Dr. Seymour Benzer, California Institute of Technology, Pasadena, CA) and developed with the enhanced chemiluminescence kit, using a secondary antibody of goat anti-mouse IgG at 1:20,000 (Amersham
Corp., Arlington Heights, FL). All washes and dilutions were done in
PBS, 0.1% Tween 20. Protein concentrations were assayed using the BCA
protein assay kit (Pierce, Rockford, IL). Protein loading was also assayed
by reprobing the blot shown in Fig. 3 with a rabbit polyclonal antibody against zest-white3 (gift of Roel Nusse, Stanford University, Stanford, CA).
Immunocytochemistry and Embryo Staining
For the localization of syntaxin1 during cellularization, wild-type embryos
were collected overnight, dechorionated in 50% bleach for 2 min, added
to methanol containing 25 mM EGTA and heptane, and shaken vigorously. The embryos were then allowed to stand for 3 h at room temperature, and those that lost the vitelline (those that sank) were collected for
staining and rehydrated in PBT (PBS with 0.1% Triton X-100, 1% BSA,
and 2% normal goat serum). The 8C3 monoclonal antibody was diluted 1:5
in PBT and incubated with the embryos overnight at 4°C. A Texas red-
conjugated goat anti-mouse IgG secondary antibody was used for detection at a 1:200 dilution. The specificity of the syntaxin1 staining pattern
was determined in side-by-side comparisons with and without 8C3 and by
comparison to armadillo and engrailed monoclonal antibodies provided
by Dr. Roel Nusse. The same staining pattern was also obtained using an
FITC-conjugated secondary.
Staining for anillin was performed using a rabbit polyclonal antibody
against anillin (kind gift of Dr. Chris Field, University of California, San
Francisco, CA). For double labeling with phalloidin, embryos were prepared as described below, and the antibody was applied at a 1:1,000 to
1:3,000 dilution in PBT. This was visualized with an FITC goat anti-rabbit
secondary antibody at 1:200 dilution. In double labels with 8C3, embryos
were prepared as above. Both primary antibodies were applied at once,
but the antianillin was diluted to 1:12,000, and the FITC-conjugated anti-
rabbit secondary antibody was used at 1:500.
Staining with rhodamine- or FITC-conjugated phalloidin and Hoechst
33258 was performed according to published protocols (Ashburner, 1989 Staining whole-mount embryos for P-element Mutagenesis
A molecular screen for P-element insertions based on Zinsmaier et al.
(1994) To generate insertions in the syntaxin1 gene itself, the closest of these
insertions, P36-23, which is homozygous viable, was remobilized. Such a
"local hop screen" takes advantage of the observation that P-elements
preferentially reinsert near their original site upon remobilization (Tower
et al., 1993 To characterize the molecular defects in the alleles syxL371 and syxL266,
we used a combination of approaches. These included sequencing the genomic DNA surrounding the insertion site as recovered in the rescued
plasmid, Southern blots of genomic DNA, and PCR between the P-element and coding regions of the syntaxin1 gene.
Generation of Germ Line Clones
The FLP/FRT, ovoD1 system was used to generate female germ line clones
mutant for syntaxin1, which is located at 95E on chromosome 3R. In brief,
the FLP recombinase system from yeast 2 µ plasmids as transferred to
Drosophila (Golic and Lindquist, 1989 Fly Strains
All the strains necessary for the generation of germ line clones may be obtained from the Bloomington stock center (Bloomington, IN). Those used
include: w; P{[w+]82B-FRT} P[w+]90E as the source of the 82BFRT chromosome onto which the syx alleles were recombined, yw P{[ry+] hsFLP};
Dr/TM3 as the X chromosome source of heat shock-driven transposase,
and w; P{[w+]82BFRT} P{[w+]ovoD1}3R1 P{[w+]ovoD1}3R2/ru h st Syntaxin1 Is Present in the Early Embryo
Previously reported mRNA analysis suggested that syntaxin1 mRNA is present in the early embryo, possibly because of maternal deposition into the oocyte (Parfitt et al.,
1995
Syntaxin1 Localization in the Early Embryo
The presence of syntaxin1 at the onset of embryogenesis
led us to hypothesize that this gene plays a role in early development. For an indication of its function, we examined
the distribution of the protein by immunofluorescence and
confocal microscopy. With the onset of cellularization during cycle 14, syntaxin1 immunoreactivity is observed on
the newly forming lateral cell membranes as soon as they
appear (Fig. 2, A and B). The immunoreactivity is present along the entire length of the lateral membrane, from the
embryo's surface to the tip of the cleavage furrow (Fig. 2,
B and C). The position of the tip of the furrow was determined either by Nomarski viewing of the embryos or by
double labeling with an antibody against anillin, a cytoskeletal-associated protein that is present at the leading tip
(Field and Alberts, 1995
Before cellularization, we did not discern syntaxin1 immunoreactivity on the surface of the embryo at a level that
was above background. It is possible that some protein
was present on the surface at this stage, but it is unlikely
that most of the protein that later is visible on the lateral
membranes derives from a surface pool. We surmise,
therefore, that much of the syntaxin1 is added to the lateral membranes as they form. This hypothesis is consistent
with genetic data, presented below, that zygotically transcribed syntaxin1 can contribute significantly to cellularization.
In later embryos, the syntaxin1 staining is strongest in
synaptic regions of the nervous system, but significantly,
the protein is still present on axons, cell bodies, and nonneuronal structures as well (data not shown).
The Generation of P-element Insertions in syx
To mutate syx, we performed a genetic screen that produced a collection of P-element insertions near the gene.
The closest of these insertions, P36-23, was remobilized to
generate 11 syntaxin1 alleles (see Materials and Methods)
(Fig. 3 A). The P36-23 insertion does not alter the quantity
of syntaxin1 per embryo, compared to either yw or CXD/
TM3 control embryos (Fig. 3 B). syxL371 and syxL266, two
severe alleles that were derived from P36-23, were used in
the present study. In syxL371, two P-elements in tandem
flank a deletion of ~1,700 bp that removes a 5 Examination of the protein expressed by these alleles is
consistent with the molecular and genetic data that syxL266
is a hypomorphic allele still making wild-type protein.
syxL266 homozygotes contain more protein than the null
(syxL371) embryos (Fig. 3 B). Nevertheless, embryos homozygous for syxL371 contain detectable levels of full-length protein seen on longer exposures of Fig. 3 B (not
shown) or on blots in which more protein is loaded (Fig. 3
C). Since this allele is incapable of making full-length protein, this signal can be attributed to maternally deposited
mRNA and protein. The homozygous syx Syntaxin1 Nulls Appear to Be Cell Lethal
To eliminate the maternally deposited syntaxin1, homozygous mutant clones in the female germ line were made by
mitotic recombination with the ovoD1, FLP/FRT recombinase system (see Materials and Methods). Using this system, the only oocytes produced are from the homozygous syntaxin1-mutant clones in the female ovary, while any
ovoD1-expressing ovarioles, the gamete-producing structures of the ovary, fail to mature (Fig. 4 A). This approach
was applied to both syxL266 and syxL371 alleles, as well as to
P36-23, the parent chromosome, as a control. Both P36-23
and syxL266 produce viable clones in the ovaries. For both
lines, 80% of the females examined had mature ovarioles
(Fig. 4, B and C). In contrast, females with syxL371 clones
fail to produce any oocytes, and the ovaries of these females are rudimentary, like those of ovoD1 females (Fig. 4
D) (Spradling, 1993
Missing bristles were also observed in females with homozygous syxL371 clones, suggesting that the complete absence of syntaxin1 was lethal in other tissues as well (data
not shown). This finding is also consistent with the recent
observations of others (Schulze and Bellen, 1996 Developmental Defects Require Both Maternal and
Zygotically Derived Syntaxin1 to Be Mutant
syxL266 clones in the mother's germline provide sufficient
syntaxin1 to overcome the apparent cell lethality, but not
enough to sustain normal embryogenesis. With syxL266 and
the ovoD1, FLP/FRT system described above, we collected
embryos that all had reduced maternally deposited syntaxin1
because of the homozygous syxL266 mutation in the maternal germline. Frequently, over 50% of eggs deposited by
these mothers were unfertilized. This may be due to defects in the ability of the mutant ovarioles to properly form
an oocyte, even though the ovarioles are outwardly normal. Nevertheless, many of the eggs from these clones
were fertilized and could thus be used to examine the early
developmental role of syntaxin1. Females carrying the homozygous syxL266 clones were crossed to syxL371/TM3, lacZ
males, so that the offspring that received the syx+ balancer
chromosome (TM3, lacZ) from the father could be distinguished from those that received the mutant chromosome
by staining for lacZ. Fertilized embryos were allowed to
develop for 14-17 h. Normally developing embryos would
progress to stages 16 or 17 in this interval and would have
extensively differentiated internal structures (Campos-
Ortega and Hartenstein, 1985). The embryos from the
above cross, however, fell into two categories: (a) viable, normally developing embryos and (b) embryos that arrested very early, usually before germ band extension (Fig.
5 A). The fate of the embryo depended on the paternal
chromosome it had received; the viable class bore the
syx+, TM3lacZ+ paternal chromosome (Fig. 5 B), while
the defective embryos had received the syxL371 chromosome. A representative syxL266/syxL371 (lacZ
Mutant Embryos Show Large Acellular Patches
To understand the phenotype of embryos such as the one
shown in Fig. 5 C at the cellular level, we analyzed a number of cellular components, including the nucleus, cytoskeleton, and surface membrane. During mitotic cycle 14, embryos derived from P36-23 germ line clones cellularize
normally (Fig. 6, A and B). The nuclei, stained with Hoechst
33258, are enveloped by cell membranes that invaginate inwards from the embryo's surface, and filamentous actin,
stained with rhodamine-conjugated phalloidin, becomes
associated with the plasma membrane of each cell. However, in embryos derived from syxL266 germ line clones,
large acellular patches are apparent in half of the embryos.
Based upon the results shown in Fig. 5, we conclude that
these embryos correspond to those inheriting the paternal syxL371 chromosome. Embryos with comparably sized
patches were obtained when females with syxL266 germ line
clones were crossed to either syxL371/TM3 or syxL266/TM3
males. This result indicates the defect is not specific for the
syxL371 paternal chromosome but is instead the result of a
combination of reduced maternal and zygotic syntaxin1.
Embryos from the P36-23 clones never showed defects
such as this. Within these aberrant patches, the actin cytoskeleton that lines lateral membranes is completely absent
(Fig. 6 C), and the nuclei are disorganized and begin to fall
away from the surface of the embryo (Fig. 6 D). The regions of disorganized nuclei and of absent lateral actin correspond exactly (Fig. 6, C and D, large arrows). In addition to large patches, occasional smaller defects are also seen
(Fig. 6 C, small arrow).
The defects are also seen in the morphology of the surface membrane in the regions that appeared to be acellular. conA was used to stain glycosylated surface membrane
proteins. On the surface of a wild-type embryo, the conA
and phalloidin staining overlap and reveal the honeycomb
pattern of developing cell membranes (Fig. 7, A and B). In
patches on an embryo from a syxL266 germ line clone, neither the cytoskeleton nor the surface membrane give any
indication that membranes are forming to segregate each nucleus (Fig. 7, C and D). Therefore, it appears that these
patches are truly acellular and represent areas where no
membrane invagination is occurring.
syxL266 Defects Do Not Appear until
Cellularization Begins
Embryos from homozygous syxL266 ovarian clones were
examined more closely to determine the time of onset of
developmental abnormalities in these mutants. Because
the paternal chromosome was able to rescue the phenotype, the abnormalities we observed must be due to a lack
of syntaxin1 at a stage after the onset of transcription from
the zygotic genome. Zygotic transcription does not begin until the mitotic cycles 10-13 To determine the time of onset of defects in syxL266/
syxL371 embryos, mothers bearing syxL266/syxL266 germ lines
were crossed to syxL371/TM3, lacZ males as above and
stained with Hoechst 33258 to visualize the nuclei and with
rhodamine-conjugated phalloidin to stain the F actin. In
normal embryogenesis, the nuclei migrate outwards so as
to reach the periphery in time for the 10th nuclear division. During the syncytial blastoderm stage, mitotic cycles
10-13, they form a cortex of nuclei, and a cap of F actin resides above each nucleus (for review see Foe et al., 1993
In embryos from both P36-23 and syxL266 clones, small
irregularities in the metaphase furrows are occasionally
seen (Fig. 8, E and F), and in most cases these irregularities correspond to individual nuclei dropping back towards
the center of the embryo. These normal imperfections in
early development did not occur at a significantly higher
rate nor were they significantly larger in size in embryos from mutant clones compared to controls (Table I). The
values are comparable to the reported rate of 0.3% of the
nuclei being lost in any given cycle (Sullivan et al., 1993a Table I.
Precellularization Defects Are Not Enhanced in syx
).
The molecular mechanisms underlying all such vesicle
trafficking appear to be analogous, involving specific protein-mediated interactions between the transport vesicle
and the acceptor membrane (Rothman and Wieland, 1996
).
). According to this hypothesis, the specificity of vesicular targeting arises from the interaction of a
t-SNARE with its counterpart v-SNARE on the transport
vesicle. In yeast, homologues of syntaxins are necessary for ER to Golgi (Hardwick and Pelham, 1992
), Golgi to
plasma membrane (Aalto et al., 1993
), and vacuolar trafficking (Piper et al., 1994
).
;
for review see Sudhof, 1995
). The counterpart v-SNARE
for syntaxin is the synaptic vesicle protein synaptobrevin,
also called VAMP, which has been shown to bind syntaxin
(Calakos et al., 1994
). Another vesicular protein, synaptotagmin, also binds to syntaxin (Kee and Scheller, 1996
), as does the plasma membrane protein SNAP-25 and the
cytosolic protein nsec1 (Pevsner et al., 1994
). Additional
cytosolic factors including NSF and
,
, and
Snap can
also be found in a larger complex containing syntaxin
(Sollner et al., 1993a
,b).
; Schiavo et al., 1995
).
A Drosophila homologue referred to as syntaxin1 (syx)1
was recently cloned (Cerezo et al., 1995
; Parfitt et al., 1995
; Schulze et al., 1995
). Mutations in this gene are homozygous lethal, with severe alleles dying as late embryos. The
loss of syx abolishes synaptic transmission; release could
not be evoked by either electrical stimulation or black
widow spider venom, and spontaneous vesicle fusions
were absent (Broadie et al., 1995
; Schulze et al., 1995
). Other secretion phenotypes, such as a soft cuticle and undigested yolk, were also reported in the syntaxin1 mutants
(Schulze et al., 1995
). Interestingly, the genetic removal of
syntaxin1 did not disrupt the ability of vesicles to be targeted to and morphologically docked at the nerve terminal
membrane (Broadie et al., 1995
). Thus, although this protein is clearly essential for synaptic transmission, its precise role in the targeting and fusion of vesicles remains uncertain.
). The presence of transcript at these
times, when no neurons have differentiated and the cuticle
has not yet been secreted, suggested a new and distinct
role for syntaxin1 in development. A potential maternal
contribution of mRNA is suggested by the transcript analysis, and therefore, the importance of syntaxin1 in early development may have been underestimated. As described below, we have obtained evidence that syntaxin1 functions
in a specialized form of cytokinesis, the cellularization of
the syncytial blastoderm, and mutations in this protein
provide an opportunity to study the significance and
mechanism of membrane addition in cellularization.
; for comprehensive review see
Foe et al., 1993
). By 1.5 h after egg deposition (AED), the
nuclei that will form the somatic tissues of the fly have migrated out to the cortex of the embryo to form the syncytial blastoderm. Cellularization occurs between 2 and 3 h
AED (during the 14th mitotic cycle) as membranes from
the surface of the embryo grow inward and segregate each
of the 6,000 nuclei into individual cells. This process is
known to involve the rearrangement of several cytoskeletal and cytoskeleton-associated proteins. Mutations in the
genes encoding these proteins can interfere with cellularization (for review see Schejter and Wieschaus, 1993
; Theurkauf, 1994
) and have been isolated in screens for
both maternal effect and zygotic developmental mutations.
).
The source of the additional membrane has been uncertain. Two possibilities have been considered: the resorbtion of microvilli on the outer surface of the embryo and
the addition of membranes from intracellular pools (Fullilove and Jacobson, 1971
; Sanders, 1975
; Turner and Mahowald, 1976
; Loncar and Singer, 1995
). The presence of
transcripts for a t-SNARE such as syntaxin1 in early embryos suggested an opportunity to address the role of vesicle trafficking and membrane addition during cellularization and to investigate a novel function of syntaxin1.
). Vesicle trafficking is therefore a crucial component of membrane
addition during cellularization, and this example of cytokinesis appears to share an underlying molecular mechanism
with other forms of vesicle transport. The data also illustrate that two different vesicle classes involved in separate
cell biological processes can employ the same t-SNARE,
syntaxin1.
Materials and Methods
Fig. 3.
(A) P-element insertions in the syntaxin1 transcription
unit. The P36-23 P-element insertion is located ~1,500 bp upstream of the syntaxin1 open reading frame and is homozygous
viable. This insertion was mobilized to generate a series of P-element insertions that disrupt the syntaxin1 transcription unit and
are homozygous lethal. The allele syxL266 carries both the original
P36-23 insertion and a second insertion in or immediately adjacent to the 5 untranslated sequence. syxL371 has two P-elements:
the original P36-23 and a second, within the syntaxin1 open reading frame, interrupting the codon for amino acid K95. The intervening sequence is deleted as determined by Southern blotting,
PCR, and plasmid rescue of the insertions. (B) Syntaxin1 protein
in mutant embryos. In late stage embryos homozygous for the
parent chromosome P36-23, the protein is present at normal levels, by comparison to control strains (CXD/TM3 or yw). The protein is dramatically reduced in syxL266/syxL266 and syxL371/syxL371
alleles. The syxL266/syxL266 level is higher than that of syxL371/
syxL371; this allele therefore produces a small amount of full-length protein. Full-length syntaxin1 is present in syxL371/syxL371
embryos because of the remaining maternal contribution and can be seen on longer exposures of the gel in B (not shown) or on gels
with additional protein layered per lane (C). Homozygous mutant embryos from heterozygous parents were selected based on their failure to hatch after 27-33 h for both the syxL266 and the
syxL371 alleles. Age-matched embryos that failed to hatch from
CXD/TM3 heterozygous parents were used as a control. Homozygous P36-23 and yw embryos were collected at 24 h. 60 µg of
protein was layered per lane in B, corresponding to ~25 embryos. The protein extracted from 45 embryos was loaded in each
lane of C.
[View Larger Version of this Image (51K GIF file)]
).
In brief, embryos were devitellinized in 50% bleach and fixed in 8% formaldehyde in PBS and heptane for 45 min. The embryos were then stuck to
double-sided tape and immersed in 4 or 8% formaldehyde in PBS. The vitelline membranes were removed by hand. The embryos were then
stained in phalloidin for 15 min, rinsed, and stained in Hoechst 33258 (1 µg/ml) for 4 min. For concanavalinA (conA) labeling, FITC-conjugated
conA at 200 µg/ml was included with the phalloidin. During conA labeling, no detergents were applied. For antianillin and phalloidin double labeling, the embryos were moved from 4% formaldehyde in PBS to PBT
and stained as described above. The phalloidin was included with the secondary antibody. Phalloidin, conA, and Hoechst 33258 were all obtained
from Molecular Probes (Eugene, OR).
-galactosidase activity using X-gal
was performed according to Ashburner (1989)
. Ovaries were dissected
from adult females of matched age for each genotype. The staining protocols used are the same as those used for embryos.
was performed. In brief, P-elements (P[w+]) marked with w+ and
carrying lacZ and the pUC sequences oriC and ampr were mobilized from
a w
X chromosome (Bier et al., 1989
) by crossing to flies with a TMS, Sb,
2-3 third chromosome, which provides a constant source of transposase.
Males carrying both the X chromosome bearing the P-elements and the
TMS chromosome were then mated to yw females. Male progeny of this
cross with w+ eye color carried novel insertions on the autosomes or Y. Pools of 50 males without the TMS chromosome and carrying novel P[w+]
insertions were crossed to 200 yw females. Embryos were collected overnight from each of these pools. The embryos were then homogenized, and
the genomic DNA was purified (Kaiser and Goodwin, 1990
). Plasmids were rescued from the genomic DNA by digesting with EcoRI, which cuts
in the P-element upstream of the pUC sequences and again in the flanking
genomic DNA. These fragments were then diluted and recircularized with
T4 DNA ligase. The resulting plasmids were then used to transform Electromax DH10B Escherichia coli (GIBCO BRL, Gaithersburg, MD),
which were plated on LB agar plates containing 100 µg/ml ampicillin. For
each pool, DNA from the resulting transformants was purified, digested
with EcoRI, and screened on Southern blots by probing with the syntaxin1
cDNA. A hybridization signal indicated that a male in the pool carried a
P-element insertion that flanked the syntaxin1 gene. Individual males
from positive pools were then used to establish stocks, which were analyzed individually to isolate the chromosome carrying the insertion. Three
insertions 1.5, 2.5, and 9 kb upstream of the syntaxin1 open reading frame
were isolated from the 4,000 males screened.
). From the offspring of flies carrying both the P36-23 insertion
and the TMS balancer, males with new insertions were identified by
darker eye color and used to establish stocks. These males were then recovered and analyzed by PCR using primers to the 31-bp inverted repeat
of the P-element (O'Hare and Rubin, 1983
) and to the syntaxin1 open
reading frame to determine the location of the new insertion. Under standard conditions (1.5 mM MgCl2, using Tfl polymerase [Epicentre Technologies, Madison, WI]), amplification from the P36-23 insertion to the
EcoRI site in the syx open reading frame was routinely possible. New insertions closer to the open reading frame were therefore easily identified
using the same primers. From 1,100 flies, 11 bore insertions less than 1.5 kb from the open reading frame. All of these were homozygous lethal.
Two of these insertions, syxL371 and syxL266, were used in the present study.
) was used to induce mitotic recombinations. FLP recombinase on the X chromosome was driven by the heat
shock promoter to induce recombination at FRT sequences inserted near
the centromere, at 82B (Xu and Rubin, 1993
). The production of germ
line clones was made easier by the introduction of two copies of ovoD1, a
dominant mutation that causes the female germ line to degenerate, onto
chromosome 3R (Chou et al., 1993
). Females were constructed that are
heterozygous for syx alleles over ovoD1, both with 82B FRT sequences,
and heat shock FLP on the X. The only oocytes produced in such a female
must come from a recombinant germ line clone that is homozygous for syx
alleles and lacking ovoD1 because both heterozygous and homozygous
ovoD1-bearing clones degenerate. Clones were successfully induced by heat shocking at 37°C for 1 h each day for 3 d during the third instar larval
or early pupal stages.
-Tub85DD ss e/TM3 as the ovoD1 source used for negative selection in the female germ line. The TM3, lacZ chromosome used carries a P[ry, deformed-lacZ] construct and was kindly provided by David Bilder and Matthew Scott (Stanford University). This construct expresses lacZ in the anterior segments of the embryo. The P-element-laden X-chromosome used
in the generation of insertions near syx was obtained from Konrad Zinsmaier (then with Seymour Benzer, Cal. Tech.). The TMS balancer was obtained from the Bloomington stock center.
Results
; Schulze et al., 1995
). We have extended this finding
by examining syntaxin1 protein on Western blots of extracts from timed embryo collections (Fig. 1). The monoclonal antibody 8C3 detects syntaxin1 in 0-3-h-old embryos. This period precedes the development of the
nervous system and corresponds to the time during which
cellularization occurs and zygotic transcription begins (2 h
AED; Foe et al., 1993
). The amount of syntaxin1 per embryo increases steadily thereafter until approximately the
time when the nervous system matures. At all stages, the
embryonic syntaxin1 protein comigrates with the neuronal
syntaxin1 observed in adult heads. Thus, although the size
of syntaxin1 transcripts varies during development (Parfitt et al., 1995
), the protein product appears identical at all
stages. This finding is consistent with the reported lack of
introns in the open reading frame of the gene (Schulze et
al., 1995
).
Fig. 1.
Syntaxin1 levels during embryogenesis. Protein was extracted from timed embryo collections of the ages indicated
(hours AED) and from adult heads and separated by SDS-PAGE. Per lane, the protein extracted from 30 embryos or one
half of an adult head was loaded. The gels were blotted onto Immobilon membranes and were then probed with the monoclonal
antibody 8C3. Syntaxin1 is present in 0-3-h embryos, the stage
during which cellularization occurs, and levels increase as the
nervous system matures. In both embryos and adults, the protein
has a molecular mass of 35 kD, as predicted.
[View Larger Version of this Image (63K GIF file)]
). If membrane addition during
cellularization occurs by means of vesicle fusion, the lateral membranes and cleavage furrows would constitute
the probable acceptor membranes for the fusion events.
This localization of syntaxin1 is therefore consistent with a
role in the cellularization of the embryos.
Fig. 2.
Syntaxin1 localization by antibody staining and confocal microscopy in whole-mount embryos. (A and B) Embryos in early
stages of cellularization. As the invaginating cleavage furrows form, syntaxin1 is localized to the newly formed furrows seen in superficial (A) or deep (B) confocal sections. (C) Syntaxin1 distribution as cellularization nears completion. When the cleavage furrows have
progressed well beyond the nuclei, syntaxin1 is present along the length of the lateral membrane. Antisyntaxin immunostaining was performed with monoclonal antibody 8C3. The signal was visualized with a Texas red-conjugated secondary (see Materials and Methods). Bar, 10 µm.
[View Larger Version of this Image (55K GIF file)]
untranslated region and the first one third of the open reading
frame (through lysine 95). We consider syxL371 to be a null
allele because it is missing the highly conserved H1 and H2
domains (Kee et al., 1995
) and because the next available ATG for translation initiation is at amino acid 134, nearly
half way through the open reading frame. While a truncated form of this protein may be produced but not recognized by our antibody, genetic evidence presented below
does not indicate any dominant effects of this allele: the
observed phenotypes are recessive and rescued by a syntaxin1 cDNA transgene. In syxL266, in addition to the original P36-23 insertion, a second P-element is inserted within
a few bases of the 5
most untranslated cDNA sequence
we have obtained.
embryos in
Fig. 3, B and C, were selected based on their failure to
hatch after 27 to 33 h after egg deposition. The maternally derived protein had therefore persisted throughout embryonic development.
). The failure of the ovarioles to mature and the hypertrophy of the ovaries suggests that this allele causes cell lethality and therefore degeneration of
the syntaxin1 null cells in a background of rudimentary
ovarioles produced by the ovoD1 tissues. This interpretation is supported by the absence of any ovarioles clearly
lacking syntaxin1 protein based upon staining with the
8C3 antibody. Syntaxin1 is present on the membranes of nurse cells in the early germarium, before stage 8 when the
oocyte begins to enlarge (not shown and Schulze and
Bellen, 1996
). The degeneration or rudimentary nature of
the syxL371 clones may be overcome by the expression of a
syntaxin1 transgene on the second chromosome (Fig. 4, E
and F). Females carrying this transgene develop mature
ovarioles that produce viable oocytes (Fig. 4 E), while the
ovaries of siblings that carry a balancer second chromosome (lacking the transgene) fail to develop (Fig. 4 F).
Thus, the cell lethality of the syxL371 chromosome is entirely attributable to the mutation in syntaxin1. Moreover,
since the effect is rescued by the expression of a wild-type transgene and is not seen in syxL371 heterozygotes, the cell
lethal phenotype is not due to a dominant effect of the
syxL371 allele.
Fig. 4.
Ovaries from mitotic recombinations in syx/ovoD1 heterozygotes. ovoD1 is a dominant mutation that arrests ovariole development before stage 8. Examples of these rudimentary ovarioles are marked throughout with arrowheads and can be seen in A. Recombination in a P36-23/ovoD1 fly produces P36-23 homozygous clones in which follicles reach maturity and oocytes are produced (B, and
marked with arrows throughout) alongside ovoD1-arrested ovaries. Mature ovarioles are also seen in syxL266 clones (C) but were not
seen in syxL371 clones (D). syxL371 is therefore either a rudimentary ovary mutant comparable to ovoD1, or syxL371 is cell lethal, leaving
only ovoD1 follicles. Mature oocytes are produced from syxL371 ovary clones that express syx on a second chromosome transgene (E).
The genotype of these flies is HS-FLP; HS-GAL4:UAS-syx; syxL371-82BFRT/ovoD1-82BFRT. Siblings of these flies that do not carry the
transgene show no developed ovarioles (F). All of the ovaries shown were stained with FITC-conjugated phalloidin to allow visualization of the ovarioles. Bar, 200 µm.
[View Larger Version of this Image (115K GIF file)]
). To circumvent the cell lethality, our analysis of syntaxin1's role
in development, therefore, had to be done using the hypomorphic syxL266 allele.
) embryo is
shown in Fig. 5 C. The embryo is arrested in its development; no internal organs can be distinguished, and the embryo appears not to have undergone proper germ band extension. The anterior distribution of yolk suggests that
some morphogenetic movements similar to those of germ
band extension had occurred. Those few lacZ
embryos
that did progress as far as germ band retraction typically arrested before stage 16. The lacZ positive and negative
embryos appeared in the expected Mendelian ratio (144 lacZ-positive, 169-lacZ negative embryos scored). These
results indicate that syntaxin1 expression from the embryo's paternal chromosome is sufficient to rescue the
phenotype. In parallel control experiments, all embryos
derived from clones of the P36-23 control chromosome
were fertilized and developed normally, regardless of the
paternal chromosome inherited (94 lacZ-negative and 91 lacZ-positive embryos scored).
Fig. 5.
Abnormalities in developing embryos are manifest only
in those that are maternally and zygotically mutant for syntaxin1.
(A) The early lethality of embryos from syxL266/syxL266 maternal
germ line clones is dependent on inheriting a syxL371 chromosome
from the father. Females bearing homozygous syxL266 germ line
clones were crossed to syxL371/TM3, lacZ males. 14-17-h-old embryos were collected, examined to determine if they had successfully progressed through development, and stained with X-gal to
determine which embryos received the paternal syx+ chromosome (white bars) and which had received the syxL371 chromosome (black bars). (B) Embryos that received the syx+ paternal
chromosome (lacZ positive, arrow) developed generally normally, with the majority reaching the appropriate stages (16-17) (144 embryos examined). (C) Those embryos with the syxL371 paternal chromosome (lacZ negative) arrested at very early stages, typically at or before germ band extension (169 embryos examined). As shown, the position of the yolk often suggested that
some morphogenetic movements had been attempted, but normal germ band extension had not occurred and there had been
no subsequent elaboration of internal organs. Anterior is to the
left in both pictures. The dark, grainy material in the center of
each embryo is the yolk.
[View Larger Versions of these Images (137 + 9K GIF file)]
Fig. 6.
The disruption of cellularization by mutations in
syntaxin1 can be seen in the distribution of nuclei and filamentous actin. Females with germ
line clones homozygous for either syxL266 (C and D) or the
P36-23 control chromosome (A
and B) were crossed to syxL371/
TM3, lacZ males and examined
with Hoechst 33258 to visualize nuclei (blue) and with
rhodamine phalloidin to visualize actin (red). During cellularization, embryos from P36-23
clones (A and B) continue to develop normally; however, half of
the embryos from syxL266 clones
(C and D) show large acellular
patches in which the nuclei were
disorganized (D) and the network of filamentous actin was completely absent (C). The disorganization of the nuclei and
cytoskeleton corresponded exactly (large arrows). In addition,
small defects are also sometimes seen (small arrow, C).
Rhodamine phalloidin images
were gathered by confocal microscopy.
[View Larger Version of this Image (130K GIF file)]
Fig. 7.
The defects in cellularization also affect surface membrane. Embryos from syxL266 clones were stained with rhodamine-conjugated phalloidin (red) and FITC-conjugated concanavalinA (green) to mark the actin cytoskeleton and the surface membrane, respectively. On the embryo's surface, the two markers label the honeycomb of newly forming cells (A and B). In a deeper section of the same
embryo, a large acellular patch was present. Both markers fail to label anything within an acellular patch (C and D), indicating a failure
of both the cytoskeleton to form and the surface membrane to invaginate in these areas. Bars: (A and B) 20 µm; (C and D) 10 µm.
[View Larger Version of this Image (112K GIF file)]
the cycles immediately before cellularization
and thus a requirement of syntaxin1
at this or a later step was indicated.
).
During metaphase of cycles 10-13, shallow membrane furrows have been observed that form between the nuclei
and have associated F actin; the furrows may play a role in
preventing chromosome loss during the nuclear divisions
(Sullivan et al., 1993b
). In the mutant embryos, we therefore examined the position of the nuclei and the frequency at which they aberrantly fell away from the surface; we
looked as well for the presence of the actin caps and
metaphase furrows in cycles 10-13. Development progressed normally in embryos during these stages despite
the lack of syntaxin 1. The nuclei migrated successfully to
the periphery (Fig. 8, A and C) and actin caps corresponding to the underlying nuclei were always present and organized (Fig. 8, C and D). In addition, metaphase furrows
form and appear to be similar to those in embryos from
control (P36-23) germ line clones (Fig. 8, E and F). Only
very rarely (in ~1% of the embryos examined) were there
severe defects during these stages.
Fig. 8.
Defects due to insufficient syntaxin do not appear before cellularization.
The nuclei (A and B) migrate to the surface,
and actin caps form above them (C and D)
in syxL266/syxL371 embryos derived from homozygous syxL266 germline clones (B and
D), just as they do in control embryos derived from P36-23 clones (A and C). The
formation of metaphase furrows also
progresses normally during mitotic cycles
10-13. The furrows of embryos in mitotic
cycle 12 from both P36-23 (E) and syxL266
clones (F) are shown. Furrows and caps are
visualized with rhodamine-conjugated phalloidin, and nuclei are stained with Hoechst
33258. In both genotypes, minor irregularities in actin are seen. These regions correspond to nuclei that have fallen away from
the cortex (not shown). The frequency of
these defects was not significantly different between embryos from P36-23 and syxL266
clones, and was similar to previously reported results for wild type (see text). Bars,
20 µm.
[View Larger Version of this Image (102K GIF file)]
).
Therefore, it would appear that despite the requirement of
syntaxin1 for cell viability and the large disruptions of cellularization observed at later stages, embryos that are
from syxL266 clones and lacking in paternally derived
syntaxin1 are nonetheless capable of sustaining normal development through cycle 13.
Embryos with Insufficient Syntaxin1 Show Regions with Complete Disruption of Cellularization
As soon as embryos reach the 14th mitotic cycle and begin
to cellularize, mutant regions appear. These acellular regions are variable in size, ranging from just a few percent
of the embryo's surface to almost the entire surface. There
was no apparent correlation between the size of the acellular patch and the stage of cellularization, i.e., the patches
probably do not get larger as cellularization progresses.
Also, the patches appeared on all surfaces of the embryo
but were most common on the central, dorsal area. We examined these areas closely to determine just which stages
of cellularization were disrupted. The normal progression
through to the formation of actin caps and metaphase furrows argued that a gross disorganization of the surface of
the embryo probably did not cause the failure to cellularize. As an additional way to look at the organization of the
surface before cellularization, we used antisera to the cytoskeleton-associated protein anillin. Anillin, which is
found at sites of membrane constriction, is recruited early
to the sites at which the membranes grow inwards (Field
and Alberts, 1995). Fig. 9, A-D, shows a cycle 14 embryo
that is ~75% acellular and is in the very early stages of cellularization. In patches on the embryo's surface, the cytoskeleton, as judged by phalloidin staining, is severely
disrupted (Fig. 9 A). In these patches, however, some anillin localization persists and is reminiscent of the hexagonal
array seen in cellularized parts of the embryo (Fig. 9 B),
suggesting that the surface of these regions of the embryos
is not completely disorganized. In cross section, it is evident that membrane invagination has only progressed
about 2 µm in the cellularizing regions of the embryo (Fig.
9, C and D). The cytoskeleton has formed on the apical
surface and lateral extent of cells (Fig. 9 C). The anillin is
present in the advancing front of the cleavage furrows
(Fig. 9 D) as in wild type (Field and Alberts, 1995
). However, in the acellular region at the left side of these panels,
no furrows are apparent at all.
Later in cellularization, embryos from syxL266 female
germ line clones continue to show patches with a complete
lack of cells. Fig. 9, E-H, show two mutant embryos. In the
first, cellularization has progressed so that the invaginating
membranes have approximately reached the bottom of the
nuclei (Fig. 9, E and F). This embryo has a large acellular
patch at the anterior (left) side. Polymerized actin is
present in this stage but lines the surface of the embryo and gives no indication that any in-growth of membranes
between the nuclei has occurred (Fig. 9 E). The anillin,
which at this stage should be concentrated at the leading
tip of the furrow, is disorganized and largely in the nuclei
in the acellular region (Fig. 9 F, small arrow). Anillin typically goes to the nucleus, but only when cellularization is
complete and the confluence with the yolk is entirely severed (Field and Alberts, 1995). In the cellularizing areas,
anillin also appears to be somewhat less localized to the
tips of the furrows (Fig. 9 F, large arrow).
The second embryo shown in Fig. 9, G and H, is at a yet later stage of cellularization, when the membranes have progressed to nearly their full extent. In the acellular region, no suggestion of cellularization can be seen (Fig. 9 G). Anillin again shows less localization in the cellularized areas and a nuclear localization in the acellular areas (Fig. 9 H). Nuclei can also be seen accumulating below the cortex of the embryo in this region.
Cellularization has been described as proceeding in two
phases. The first phase is slow and probably involves the
addition of membrane to the surface. The second phase is
rapid and may use the resorbtion of microvilli from the
apical surface as its source of membrane (Foe et al., 1993).
The defects seen in embryos from syxL266 clones persist
throughout cellularization and no partial cellularization is
seen, suggesting that neither phase of cellularization is
progressing. The effect on the second, fast phase may be a
direct result of syntaxin1's involvement in that phase or
may be an indirect effect of the loss of the first, slow phase.
As a result of having insufficient syntaxin1, however, regions consistently arise in which cellularization appears to
fail completely.
Gastrulation Can Proceed in Improperly Cellularized Embryos
The cell migrations associated with gastrulation occur in
the cellularized areas of defective embryos. Fig. 10 shows
an embryo in which the posterior and dorsal surfaces are
acellular (arrows). However, the caudal region has undergone involution and successfully migrated dorsally. Therefore, those cells that are formed are capable of undergoing
significant developmental alterations. Development then
arrests at approximately this stage in most of the defective embryos (Fig. 5).
Proper cellularization of the embryo requires syntaxin1, a
protein that is known to be important for membrane trafficking. Though this protein appears to be essential for the
viability of many cells (Fig. 4; Schulze and Bellen, 1996), a
direct role in cellularization is indicated by our analysis of
the hypomorphic allele syxL266. Ovary cells homozygous
for this allele are viable and give rise to embryos that can
develop normally until the moments when membrane in-growth should occur. A model for the formation of the cell membranes can be proposed that incorporates both the
genetic and cytochemical information on syntaxin1 and
the existing wealth of morphological descriptions of cellularization (Fullilove and Jacobson, 1971
; Sanders, 1975
;
Turner and Mahowald, 1976
; Loncar and Singer, 1995
).
Most recently, an electron microscopic study (Loncar and
Singer, 1995
) observed vesicles that were lined up in front
of the invaginating cleavage furrow between the nuclei of
the syncytium. The addition of these vesicles is likely to be
a major source of the membranes that advance the furrow
and divide the newly forming cells. This process requires
the t-SNARE syntaxin1. As in synaptic transmission, the
syntaxin1 is present on the target membrane and is concentrated at sites where fusion occurs, the invaginating
cleavage furrows. Syntaxin1, however, was not exclusively
localized to a specific region of the furrow. This may mean
that vesicles are added along its length, or it may simply
reflect a lack of precision in syntaxin1 localization. This
widespread distribution of syntaxin1 is similar to the situation in neurons, where syntaxin1 is not exclusively at the
synapse but is also present along axons (Garcia et al., 1995
;
Sesack and Snyder, 1995
).
Because the amount of syntaxin1 on the embryo's surface appears to increase during cellularization and because
zygotically transcribed syntaxin1 contributes to this process, additional syntaxin1 must be added with the fusing
vesicles. Therefore, vesicles may be able to fuse with one
another, in addition to fusing with the surface membrane,
and this pattern of growth has been suggested (Loncar and
Singer, 1995). Though we have not seen syntaxin1 in advance of the furrow, it may be below our detection limit.
Syntaxin is known to reside on synaptic vesicles as well as
on the plasma membrane at synapses (Walch-Solimena et
al., 1995
).
Residual cellularization was observed in the syntaxin1-deficient embryos. This cellularization may have been mediated by the small amount of wild-type protein made by syxL266. Resorbtion of apical microvilli may serve as another source of membrane in normal furrow formation and may contribute to the residual cellularization that occurs outside of the defective patches. The acellular areas, however, did not have partial furrows; perhaps since microvillar resorbtion is thought to follow vesicular membrane addition, the resorbtion was blocked in these regions and could not contribute to furrow formation.
A noteworthy feature of the defect is its all or nothing nature: abrupt transitions were seen between properly cellularized and completely acellular areas. Though the patchiness of the phenotype is not yet understood, two possibilities can be suggested. First, regions of the surface that have a little syntaxin1 at the start of cellularization may be competent to receive additional syntaxin1 via vesicle fusion and thereby progress successfully at the expense of regions lacking syntaxin1. Alternatively, a failure in cellularization in a small area may expand into a larger patch because of an interdependence of neighboring cells on membrane and cytoskeletal interactions.
The onset of the phenotype coincides with the onset of cellularization. The formation of metaphase furrows in the mitotic cycle before cellularization may proceed by a different, syntaxin1-independent mechanism, e.g., the rearrangement of microvilli or other features of the surface. Alternatively, the demands placed on membrane trafficking may not be as great at the earlier stages, and the small amount of syntaxin1 present in syxL266 may be sufficient to meet the demand. Metaphase furrows are neither as deep as cleavage furrows nor are there as many of them since there are fewer nuclei at the surface during earlier cycles. Therefore, the initial phase of cellularization at cycle 14 probably represents the first time membrane trafficking is severely taxed during development, and this demand causes defects to arise when insufficient amounts of syntaxin1 are present.
The requirement for a syntaxin1 protein during cellularization is consistent with recent findings in other systems.
Membrane fusions may be important for cell divisions in
Arabidopsis thalliana as well, where a distant homologue
of syntaxin, KNOLLE, has been implicated in cytokinesis
(Lukowitz et al., 1996). One explanation of the apparent
cell lethality of syxL371 is that syntaxin1 is required for cell
divisions in many Drosophila tissues in addition to the syncytial blastoderm. In its absence, these tissues would be
unable to form. Thus, membrane addition via vesicle trafficking during cytokinesis may be essential for the formation of new cells (Rothman and Warren, 1994
). The apparent cell lethality may also reflect a requirement of
syntaxin1 in constitutive membrane trafficking to the
plasma membrane in all cells.
The existence of both neuronal and nonneuronal functions for the syntaxin1 gene was previously indicated by
subtle abnormalities of the cuticle, gut, and trachea in zygotic null mutants (Schulze et al., 1995) and is confirmed
by our present study. In synapses, syntaxin acts with a cognate v-SNARE called VAMP or synaptobrevin. Current
models of SNARE function require such an interaction to occur in the early embryo as well. Interestingly, however,
the embryo and the synapse probably employ two different v-SNAREs to pair with the same syntaxin1 protein.
We have previously found a neuron-specific synaptobrevin isoform (n-syb) (DiAntonio et al., 1993
) that is the
likely mediator of synaptic transmission (Sweeney et al.,
1995
). We have also shown that a different, widely expressed gene (syb) (Sudhof et al., 1989
) is present in 0-3-h embryos (Chin et al., 1993
). The use of distinct synaptobrevins may reflect the distinct regulatory control of these
different trafficking events. It also indicates that the specificity of vesicle fusion cannot rely entirely on the interaction
of syntaxin with synaptobrevin. This lack of specificity becomes particularly problematic if regulated neurosecretion
and constitutive trafficking require the same t-SNARE in
the same cell, despite the fact that their vesicles must be directed to distinct domains on the cell surface. Other proteins, perhaps in the SNARE complex, may contribute to
the specificity or fine-tune it. For instance, syntaxin1 may complex with one SNAP-25 homologue in the early embryo,
allowing it to bind with syb, while it complexes with another
SNAP-25 in the nerve terminal, allowing it to bind n-syb.
Previous studies have indicated that docking of synaptic vesicles at appropriate sites persists in syx mutants
(Broadie et al., 1995), and these studies have also shown
that syntaxin is not localized only to active zones (Garcia
et al., 1995
; Sesack et al., 1995). Our present observations
suggest that a single isoform of syntaxin can mediate
fusions by many different classes of transport vesicle.
Though syntaxin may indeed play a role in docking and
targeting, additional proteins must be invoked to support
docking in its absence and to refine the specificity of targeting. On the other hand, a posttargeting role in exocytosis, such as a role in fusion itself, is consistent with our current findings and strongly implicated by the earlier genetic
analysis (Broadie et al., 1995
; Schulze et al., 1995
).
In addition to syntaxin1, the cellularization of early
Drosophila embryos will undoubtedly involve many other
proteins to mediate and direct the addition of membrane
to the cell surface. Although the cytoskeletal rearrangements that accompany the membrane changes have been
the object of considerable study (for reviews see Schejter
and Wieschaus, 1993; Theurkauf, 1994
), the genetic analysis of the membrane addition remains largely unexplored. Many of the components, like syntaxin1, may be provided
by both maternal contributions and zygotic transcription.
Once cellularization is complete, the blastoderm must differentiate into a polarized tissue (Wodarz et al., 1995
) that
selectively secretes cuticle proteins and signaling molecules from distinct domains. Thus, the formation of the
cellular blastoderm may also provide a valuable genetic
system in which to investigate the development of vectorial membrane trafficking.
Received for publication 10 January 1997 and in revised form 22 May 1997.
Address all correspondence to Thomas L. Schwarz, Department of Molecular and Cellular Physiology, Stanford University, Stanford, CA 94305-5426. Tel.: (415) 725-7770. Fax: (415) 725-8021. E-mail: tschwarz{at}leland.stanford.eduThe authors are grateful to Irene Inman for outstanding technical assistance, to Daria Siekhaus for participating in the screen that isolated the original syx P-insertions, and to Dr. Chris Field (University of California, San Francisco, CA) for the anillin antibody and her comments on the data. We also thank Drs. Konrad Zinsmaier (University of Pennsylvania, Philadelphia, PA) and Seymour Benzer (California Institute of Technology, Pasadena, CA) for providing monoclonal 8C3. Confocal microscopy was performed at the Cell Sciences Imaging Facility at Stanford, and we thank Dr. Susan Palmieri for her help.
This work was supported by a Silvio Conti Center for Neuroscience Award from the National Institute of Mental Health (NIMH) and by the Muscular Dystrophy Association. R.W. Burgess was an NIMH predoctoral fellow. D.L. Deitcher was a National Institute of Neurological Disorders and Stroke postdoctoral fellow.
AED, after egg deposition; conA, concanavalinA; syx, syntaxin1.
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