Syntaxin 1A



Efficient EGFR signaling and dorsal-ventral axis patterning requires syntaxin dependent Gurken trafficking

Vesicle trafficking plays a crucial role in the establishment of cell polarity in various cellular contexts, including axis-pattern formation in the developing egg chamber of Drosophila. The EGFR ligand, Gurken (Grk), is first localized at the posterior of young oocytes for anterior-posterior axis formation and later in the dorsal anterior region for induction of the dorsal-ventral (DV) axis, but regulation of Grk localization by membrane trafficking in the oocyte remains poorly understood. This study reports that Syntaxin 1A (Syx1A) is required for efficient trafficking of Grk protein for DV patterning. Syx1A is associated with the Golgi membrane and is required for the transportation of Grk-containing vesicles along the microtubules to their dorsal anterior destination in the oocyte. These studies reveal that the Syx1A dependent trafficking of Grk protein is required for efficient EGFR signaling during DV patterning (Tian, 2013).

The localization of grk mRNA and its protein product in the oocyte is crucial for the establishment of both the AP and dorsal-ventral axes. grk mRNA and protein are localized at the posterior of the oocyte during early oogenesis to activate EGFR signaling in the posterior follicle cells, which in turn send a mysterious signal back to initiate AP axis formation in the oocyte. On the basis of this new AP axis, grk mRNA and protein are subsequently localized at the anterior-dorsal corner of the oocyte to induce dorsalventral pattern formation. The localization of grk transcripts depends on the microtubules in the oocyte. These transcripts are transported to the dorsal-anterior destination along the microtubules by Dynein in nonmembranous transport particles and are statically anchored by Dynein within cytoplasmic structures called sponge bodies. Grk protein is believed to be translated from its localized mRNA and is translocated into endoplasmic reticulum; afterward it travels through the Golgi complex and reaches the plasma membrane to be secreted. This study reports the demonstration that localization of Grk protein to the dorsal-anterior region of the oocyte depends on membrane trafficking, differing from the nonmembranous particle transport of its mRNA during this stage. This finding is based on the study of a newly identified hypomorphic allele of Syx1A whose germ-line clones have defective dorsal follicle-cell specification and abnormal Grk protein localization after stage 7. Interestingly, grk mRNA localization is normal until stage 9 in these germ-line clones. The mislocalization of Grk protein is therefore not a result of its mRNA mislocalization, at least between stages 7 and 9, indicating that the role of Syx1A in Grk protein trafficking is specific (Tian, 2013).

Using kek-lacZ as a reporter, it was found that Syx1ASH0113 germ-line clones strongly disrupted EGFR signaling in the dorsal anterior follicle cells-many egg chambers showed complete loss of dorsal expression of Kek-lacZ-but the majority of mature eggs developed from these clones showed only shortened dorsal appendages, a phenotype indicating disruption of dorsal EGFR signaling but less severe than that of egg chambers with no expression of kek-lacZ in follicle cells adjacent to the oocyte nucleus. This phenotypic discrepancy probably arises because only a small fraction of syx1A germ-line clones develop into mature eggs, and those eggs represent the least marked phenotype. Indeed, many germ-line clones were observed with oocytes smaller than those of wild-type egg chambers at the same developmental stage, perhaps indicating a general role of Syx1A in membrane growth that is essential for oocyte development (Tian, 2013).

Although no defect was detected in Syx1A clones in Grk posterior localization and signaling to activate EGFR in the posterior follicle cells, it cannot be ruled out that that Syx1A has no role in the posterior localization of Grk protein. In the germ-line clone of a null allele of syx1A, syx1Aδ229, in contrast, oogenesis is arrested at around stages 5-6, preventing examination of the effect of Grk-EGFR signaling in the posterior follicle cells. The new syx1A mutant allele is most likely a hypomorphic allele, sequencing analysis of the open reading frame (ORF) region of the syx1A gene in homozygous syx1Aδ229 mutants did not find any changes in the ORF region, suggesting the mutation is probably at the regulatory region. Nonetheless, the findings suggest that localization of Grk to the dorsal anterior region depends more heavily on Syx1A-dependent membrane trafficking. Rab6 is known in mammals to promote trafficking at the level of the Golgi apparatus and is colocalized with the Golgi and trans-Golgi markers. Previously, a Rab6-mediated exocytic pathway has been shown to be involved in Grk trafficking in germ-line cells during oogenesis. The current studies suggest that Rab6 has a role similar to that of Syx1A for Grk localization at the dorsal-anterior corner of the oocyte after stage 7. Also, Rab6 appears not to be needed for Grk localization at the posterior before stage 7, a pattern suggesting the functional correlation between Rab6 and Syx1A in Grk trafficking in the oocyte. Consistently, Syx1A and Rab6 can form puncta in the nurse cells and oocytes and are colocalized in some of these puncta, and colocalization of these two proteins with Golgi marker GalT was observed. Furthermore, the mislocalization of Syx1A and Grk in rab6 germ-line clones or of Rab6 and Grk in syx1A germ-line clones indicates that Syx1A and Rab6 can act together for Grk trafficking. Attempts were made to perform a coimmunoprecipitation study to determine whether these two proteins are physically associated in the oocyte, but no obvious physical interaction between them was detected, suggesting that they may not interact directly in the Grk trafficking (Tian, 2013).

This study demonstrated the important role vesicle tracking plays in a specific localization of Grk in the oocyte. This process requires the both Syx1A and Rab6, which traffic along the microtubules in the oocyte. About 11 Syntaxin proteins occur in Drosophila, and they are involved in many different cellular processes. For example, Drosophila Syx5 is required for Golgi reassembly after cell division and for translocation of proteins to the apical membrane, and Syx 16 is ubiquitously expressed, appears to be localized to the Golgi apparatus, and may selectively regulate Golgi dynamics. Syx 1A is a critical component of the SNARE complex and is essential for synaptic vesicle fusion. The finding that the mutant for Syx1A can cause such a strong phenotype in the Grk trafficking is exciting. Future studies will determine whether other Syntaxin molecules have similar roles in the oocyte (Tian, 2013).


syx1A expression as detected by in situ hybridization prior to stage 12 is widespread and includes many tissues. At later stages, expression in ectodermal cells has faded considerably, whereas labeling in anterior and posterior midgut remains fairly strong. The most intense expression is found in garland cells that surround the proventriculus and are thought to function as nephrocytes. By the end of stage 12, the neurons of the CNS express syx1A more abundantly than in the prior stages. By late stage 14 or early stage 15 labeling in the midgut has diminished, expression in the ectoderm is decreasing, and the CNS and the garland cells contain the highest amount of mRNA. In stage 16 embryos, SYX-1A mRNA expression intensifies throughout most or all cells of the CNS, though individual cells of the peripheral nervous system label more faintly. The subcellular distribution of Syx1A protein in the CNS is quite different from that of the mRNA. In stage 15-17 embryo staining is most prominent along the longitudinal tracts of the ventral cord and the brain. This corresponds to the area of the neuropil in which most synapses are concentrated. However, in contrast with antibodies that specifically label synapses, the antisyntaxin antibody also recognizes the commissures of the CNS and the axons of the peripheral neurons in which no synaptic contact sites are known to be located. In the periphery of the embryo, label can also be observed at the neuromuscular junctions. Finally, low levels of Syx1A protein seem to be present in the cell bodies of neurons. Hence, protein is present in the presynaptic area, in axons, and in neuronal cell bodies (Schulze, 1995).

In young embryos, three SYX-1A transcripts, 3.5- 4.2- and 8.0-kb are abundant. At 3-9 hours of development the 3.5-kb transcript slowly disappears while 4.2-, 8.0- and 9.0-kb messages predominant until the endo of embryogenesis. Between 9-15 hours, two additional transcripts (7.0 and 12 kb) appear. During first instar larval life, five messages are present, whereas second and third instars utilize three and two messages respectively. During pupariation, all five transcripts reappear, and the adult expression pattern resembles that of pupae. Three of the adult transcripts are the same as those found in young embryos, suggesting some or all of these messages represent the maternal contribution to the embryo. It is likely that alternative splicing, various promoters, and different polyadenylation signals are used to generate this heterogeneity (Schulze, 1996).

Precellular embryos exhibit ubiquitous distribution of SYX mRNA reflecting a maternal contribution. It accumulates most densely in the germ plasm at the exterior posterior tip of the embryo where the pole cells will form. As the future pole cell membranes pinch off from the posterior tip, SYX mRNA is located within each bud and remains concentrated at the posterior of the embryo. At stage 9, the message is present in most ectodermal cells, the anterior and posterior midgut invaginations, and the central nervous system precursor cells. Expression of syx1A during later stages of embrogenesis is found garland cells, midgut, nervous system and ectoderm (Schulze, 1996).

The Syntaxin protein is specific to the nervous system and localized in synaptic areas of both central nervous system (CNS) and neuromuscular junction. The same antibody used to clone SYX-1A cDNA stains synaptic areas in rat cerebellum and a neurospecific antigen in rat and human tissues with identical relative mobility to rat syntaxin 1 (Cerezo, 1995).


In wild-type ovaries Syx1A protein is detected in regions 2 and 3 of the germarium, outlining the membranes of germline cyst cells. Syx1A continues to be abundantly expressed in the nurse cell membranes of egg chambers during stages 1-8, and Syx1A levels fade during stages 8 and 9. Syx1A protein is present in the adult brain and the synaptic substations of the visual system. Syx1A is concentrated in the neuropil regions of the brain and is somewhat enriched at synaptic regions, such as in the lamina and medulla of the optic lobes where photoreceptor axons form synaptic contacts with second order neurons. Syx1A distribution in the brain differs from that of synaptic vesicle-specific proteins such as Synaptotagmin, as Syx1A is also present in axons and cell bodies, whereas Synaptotagmin is restricted to synaptic terminals (Schulze, 1996).

Effects of Mutation or Deletion

An allelic series of syx1A loss-of-function mutations has been generated that result in embryonic lethality with associated morphological and secretory defects dependent on the severity of the mutant allele. Unhatched embryos were examined 30-36 hr after egg lay. Mutant embryos do not exhibit typical peristaltic contraction waves, which allow normal embryos to hatch from the egg case, and they display only very reduced movements of the mouth hooks and muscles in the head region. Embryos are unable to clear their tracheal system of fluid. Electrophysiological recordings from partial loss-of-function mutants indicate absence of endogenous synaptic transmission at the neuromuscular junction and an 80% reduction of evoked transmission. Complete absence of syx1A causes subtle morphological defects in the peripheral and central nervous systems, affects nonneural secretory events, and entirely abolishes neurotransmitter release. Null mutant embryos fail to secrete most or all cuticle, the gut is morphologically abnormal, and yolk in the gut is undigested. However, the Malpighian tubules secrete uric acid, and the lumen of the salivary glands contains glue proteins, unlike mutants for rop that show more severe secretory and developmental defects. In addition, the severe muscle and CNS defects seen in rop mutants are not present in syx1A mutants. Axon bundles of the intersegmental and segmental nerves appear thickened and irregular in shape in syx1A mutants. A relatively normal neuromusclular architecture is maintained. Muscles are present in their normal position and number, are biregringent, and are normally contractile, but null mutants exhibit no endogenous synaptic transmission. Furthermore, synaptic transmission cannot be evoked in null mutants by stimulation of the motor nerves. Thus syntaxin plays a key role in nonneuronal secretion and is absolutely required for Ca2+ evoked neurotransmission (Schulze, 1995).

The role of Syntaxin-1A in neurotransmission has been extensively studied. However, developmental Northern analyses and in situ hybridization experiments show that SYX1A mRNA is expressed during all stages and in many tissues. New mutations in syx1A have been isolated that reveal roles for syx1A outside the nervous system. In the ovary, Syx1A is present in the germarium, but it is predominantly localized to nurse cell membranes. Mitotic recombination experiments in the germline show Syx1A is essential for oogenesis and may participate in membrane biogenesis in the nurse cells. Mutant ovaries are rudimentary in their appearance and indistinguishable from that of ovoD1 heterozygotes. Weak syx1A alleles provide for the germline and maternally derived Syx1A used during zygotic development, and early embryonic development can be fully rescued by a wild-type zygotic contribution from the male. syx1A is also required in larval imaginal discs, as certain hypomorphic mutant combinations exhibit rough eyes and wing notch defects indicative of cell death. The most severly affected eyes appear to have fewer ommatidia than wild type. The ommatidia are sometimes fused and often are improperly rotated and misaligned with respect to one another. Recombinant clones that lack syx cause cell lethality in the developing eye. Another defect in temperature sensitive alleles is notching along the posterior margin of the wing. In mutants, incomplete compaction of the ventral nerve cord is observed. In some mutants incomplete fasciculation of intersegmental and segmental nerves is found. The staining of the intersegmental and segmental nerve bundles in mutants is quite diffues, suggesting that the individual axons cannot adhere to one another perhaps due to inconsistencies in the membrane constituents. Similar defects are observed in the longitudinal tracts of the CNS when visualized with anti-fasciclin II antibody. In the wild-type CNS, three longitudinal tracts are observed on either side of the midline, and each set of axon fibers exhibits a tight arrangement. In mutants, axon bundles are irregular and slightly defasciculated. It is proposed that, similar to its roles in cuticle secretion and neurotransmitter release, Syx1A may mediate membrane assembly events throughout Drosophila development (Schulze, 1996).

Attempts were made to stimulate fusion in Drosophila unc-13 mutants with hyperosmotic saline application. A 3-second focal application of 1175 mOsm saline to a wild-type junction evoked a prolonged synaptic response composed of many repetitive secretion events, whereas responses of unc-13 synapses were extremely depressed relative to controls and similar to those of mutants lacking the essential secretory proteins, Syntaxin and Synaptobrevin. Calculation of the total charge elicited in response to hypersomotic saline revealed significant and similar lack of response in unc-13, synaptobrevin and syntaxin. However, in response to hyperosmotic saline, unc-13 has significantly more vesicle fusion events than syntaxin or synaptobrevin mutants. Thus, unc-13 mutants show severely reduced neurotransmission in response to normal and elevated Ca2+ influx and severely reduced responses to hyperosmotic saline (Aravamudan, 1999).

A genetic screen was carried out in Drosophila to identify mutations that disrupt the localization of Oskar mRNA during oogenesis. Based on the hypothesis that some cytoskeletal components that are required during the mitotic divisions will also be required for Oskar mRNA localization during oogenesis, the following genetic screen was designed. A screen was carried out for P-element insertions in genes that slow down the blastoderm mitotic divisions. A secondary genetic screen was used to generate female germ-line clones of these potential cell division cycle genes and to identify those that cause the mislocalization of Oskar mRNA. Mutations were identified in ter94 that disrupt the localization of Oskar mRNA to the posterior pole of the oocyte. Ter94 is a member of the CDC48p/VCP subfamily of AAA proteins that are involved in homotypic fusion of the endoplasmic reticulum during mitosis. Consistent with the function of the yeast ortholog, ter94-mutant egg chambers are defective in the assembly of the endoplasmic reticulum. A tested was carried out to see whether other membrane biosynthesis genes are required for localizing Oskar mRNA during oogenesis. Ovaries that are mutant for syntaxin-1a, rop, and synaptotagmin are also defective in Oskar mRNA localization during oogenesis (Ruden, 2000).

In order to identify new genes required for OSK mRNA localization, OSK localization defects in egg chambers were sought in mutants for cell division cycle (CDC) genes that had been isolated in a 'mitotic delay-dependent survival' (MDDS) genetic screen. The rationale for this is that many cytoskeletal proteins required for mitotic divisions may also be required for mRNA localization. The advantage of studying the function of CDC genes during oogenesis, in which all of the mitotic divisions occur in region 1 of the germarium, is that later in oogenesis one can analyze the biological functions of the CDC genes independent of their mitotic functions. For example, Klp38B, a chromatin-binding kinesin-like-protein isolated in the MDDS genetic screen, is required not only for chromosome segregation during the meiotic and mitotic divisions, but also for the proper development of the oocyte, possibly by localizing mRNA or protein in the oocyte (Ruden, 2000 and references therein).

Based on the phenotypes of syx-1a, ter94, rop and syt mutant egg chambers, a three-step genetic pathway is proposed for the role of membrane fusion proteins on OSK mRNA localization during oogenesis. (1) Syx-1a is required in stage 1 egg chambers to get OSK mRNA to the oocyte. Syx was originally identified as a Drosophila homolog of a human tSNARE that is required for synaptic vesicle fusion in neurons. Interestingly, Syx5 in humans has recently been shown to be required for TERA-mediated (the human Ter94 ortholog) assembly of Golgi cisternae from mitotic Golgi fragments in vitro (Rabouille, 1998). (2) Ter94 is required to localize OSK mRNA within the oocyte. It is speculated that OSK mRNA might be transported in membranous particles because both the endoplasmic reticulum and OSK mRNA form particulate complexes in ter94-mutant egg chambers. (3) The final step in OSK mRNA localization is anchoring the mRNA to the posterior pole of the oocyte. It is proposed that Rop and Syt are required for this process because rop and syt mutant egg chambers have poorly formed cytoplasmic membranous structure in the oocytes, and, possibly as a result, OSK mRNA fails to remain localized at the posterior pole. Rop is a Drosophila homolog of yeast Sec1 and vertebrate n-Sec1/Munc-18 proteins and is a negative regulator of neurotransmitter release in vivo (Schulze, 1994). Syt controls and modulates synaptic vesicle fusion in a Ca2+ dependent manner (Littleton, 1993). It is concluded that many synaptic vesicle fusion proteins also function during other cellular processes such as OSK mRNA localization during oogenesis (Ruden, 2000).

Absence of junctional glutamate receptor clusters in Drosophila mutants lacking spontaneous transmitter release

Little is known about the functional significance of spontaneous miniature synaptic potentials, which are the result of vesicular exocytosis at nerve terminals. By using Drosophila mutants with specific defects in presynaptic function it has been found that glutamate receptors cluster normally at neuromuscular junctions of mutants that retain spontaneous transmitter secretion but have lost the ability to release transmitter in response to action potentials. In contrast, receptor clustering is defective in mutants in which both spontaneous and evoked vesicle exocytosis are absent. Thus, spontaneous vesicle exocytosis appears to be tightly linked to the clustering of glutamate receptors during development (Saitoe, 2001).

The existence of miniature end-plate potentials provides a basis for the theory of quantal synaptic transmission. A single miniature end-plate potential arises when a synaptic vesicle fuses spontaneously with the presynaptic membrane and releases a quantum of transmitter (spontaneous vesicle exocytosis). However, little is known about the functional importance of this process. Presynaptic and postsynaptic neurotoxins that allow spontaneous vesicle exocytosis to persist have little effect on synaptic development, including postsynaptic accumulation of receptors. During the development of Drosophila neuromuscular junctions (NMJs), glutamate receptors (GluRs) cluster in the postsynaptic membrane in a manner that depends on nerve-muscle contact. To investigate the role of spontaneous secretory events in receptor clustering, Drosophila mutants with distinctive secretory defects were used. Mutations of neuronal-synaptobrevin (n-syb) or cysteine string protein (csp) selectively prevent nerve-evoked exocytosis whereas spontaneous vesicle exocytosis persists. In contrast, syntaxin-1A (syx) or shibire (shi) mutations eliminate both spontaneous and evoked exocytosis, thereby allowing one to distinguish the role of spontaneous secretory events (Saitoe, 2001).

Neuromuscular transmission in wild-type and mutant Drosophila embryos or larvae were characterized. A typical burst of excitatory synaptic currents (ESCs) often exceeded 600 pA in amplitude in a newly hatched wild-type larva (control). In the presence of tetrodotoxin, the bursting of ESCs is suppressed, and ESCs seldom exceeded 400 pA. A similar suppression of bursting and reduction in the amplitude of ESCs is observed when the ventral nerve cord is removed. Thus, propagated activity in the nervous system triggers multiple vesicle exocytosis and contributes to the ESCs. Concomitantly, the residual events [miniature ESCs (mESCs)] in these wild-type larvae are due to spontaneous vesicle exocytosis (Saitoe, 2001).

An n-syb null mutant was investigated in which nerve-evoked ESCs but not mESCs are lost. Consistent with these findings, ESCs were detected in n-syb embryos but virtually no large-amplitude ESCs characteristic of nerve-evoked ESCs. This apparent absence of evoked responses (but persistence of mESCs) was confirmed by the fact that TTX had no effect on the frequency or amplitude of ESCs and that no evoked ESCs were elicited by nerve stimulation (Saitoe, 2001).

In syx mutants, both nerve-evoked and mESCs are undetectable. In agreement with this phenotype, neither nerve-evoked nor mESCs were detected during observations exceeding 15 min each in seven cells. Although the possibility of missing very infrequent occurrences could not be completely eliminated, it is clear that the frequency of mESCs in syx embryos is far lower than the frequency of mESCs in n-syb embryos. Given the distinct phenotypes of the n-syb and syx mutants, the distribution of postsynaptic GluRs was examined in these embryos (Saitoe, 2001).

Preparations were also stained with antibody against horseradish peroxidase (anti-HRP), which binds to a neuronal surface antigen and reveals the presynaptic terminals. Immunoreactive GluRs formed prominent junctional clusters that closely mirrored the presynaptic elements in wild-type and n-syb mutants. Although this finding apparently contrasts with the observation in para (Na+ channel) mutants, that neural electrical activity is essential for the clustering of receptors, it should be noted that the n-syb mutation is a more subtle perturbation of this system. Unlike n-syb, however, syx mutants rarely have discernible junctional GluR clusters, although they invariably form neuromuscular contacts (Saitoe, 2001).

These findings raised the possibility that the absence of detectable mESCs in these mutants may have arisen from a deficit of postsynaptic GluRs. Moreover, the lack of receptor clusters could either be a developmental consequence of a lack of vesicle fusions in the nerve terminal, or could be due to a requirement for syntaxin in the trafficking of GluRs to the postsynaptic membrane. Indeed, syx is required for cell viability in Drosophila, and the development of both the neuron and muscle in syx embryos is likely to be due to small amounts of maternal Syx. If this residual Syx is not adequate for the maintenance of normal vesicular traffic to the cell surface, GluRs may not be inserted appropriately in the sarcolemma. To address these issues, it was determined whether syx mutants responded to applied glutamate and also whether junctional GluR clusters are restored in syx mutants by selectively inducing the presynaptic or postsynaptic expression of a syx transgene (Saitoe, 2001).

The application of L-glutamate by pressure ejection onto the junctional region of syx muscles indicates that some sensitivity is lost concomitantly with the loss of clusters. The pressure ejection of L-glutamate at the junctional region yielded robust inward currents in wild-type and n-syb mutants, but glutamate-evoked currents are much smaller in syx mutants. What is important here is that even with this diminished glutamate sensitivity, mESCs would have been detected in syx mutants if their terminals were spontaneously releasing transmitter. Thus, the complete absence of detectable mESCs could not be attributed to a lack of postsynaptic sensitivity (Saitoe, 2001).

The low sensitivity to glutamate in syx mutants could reflect the diminished trafficking of GluRs to the surface. Thus, a test was performed to see whether the clustering defect is due to a pre- or post-synaptic action of syx by the targeted expression of the syx transgene with either the neuron-specific elav-GAL4 driver or the muscle-specific G14-GAL4 driver. Neuron-specific expression of the syx transgene restores GluR clusters but muscle-specific expression does not. Evoked and mESCs are readily observed in transgenic embryos in which neuron-specific expression of the syx transgene is restored but not in transgenic embryos in which syx transgene is expressed in muscles. Thus, a comparison of the phenotypes of n-syb and syx has led to a hypothesis that spontaneous secretory events at the NMJ are critical to the formation of GluR clusters (Saitoe, 2001).

Two temperature-sensitive (ts) paralytic mutants were used to examine further the correlation between spontaneous vesicle exocytosis and GluR clustering. At elevated temperatures, a defect in dynamin in shits blocks synaptic vesicle recycling and thereby depletes the terminals of synaptic vesicles. In contrast, cspts mutations appear to interfere with excitation-secretion coupling in the terminal. Synapses in shits mutants become completely silent at a nonpermissive temperature (32°C), whereas cspts mutants lose evoked responses while retaining mESCs. Because of these differences in release properties at the nonpermissive temperature, the distribution of GluRs in these lines was compared. At a permissive temperature (25°C), when release properties are similar among these embryos, GluR clustering is comparable for wild-type, cspts, and shits mutant embryos. However, this situation changes when embryos are moved to the nonpermissive temperature at 13 hours after fertilization, which is when nerve-muscle contacts first form. The development of GluR clusters is not perceptibly altered in wild-type and cspts mutants at 32°C. However, no detectable GluR clusters are observed in shits mutants, as is the case in syx mutants. These results again suggest a tight link between spontaneous vesicle exocytosis and GluR clustering (Saitoe, 2001).

Further insight into the nature of interaction of presynaptic and postsynaptic elements has come from the injection of argiotoxin into wild-type embryos at concentrations that block all muscle contractile activity. In these embryos, GluRs still cluster postsynaptically. Thus, it was not the activation of postsynaptic GluRs that directs GluR clustering. Similar findings have been reported in vertebrates, where alpha-bungarotoxin does not impede the clustering of acetylcholine receptor (AChR). As in vertebrates, secretion of molecules, such as agrin for AChRs, ephrins for N-methyl-D-aspartate (NMDA)-type GluRs, and neuronal activity-regulated pentraxin for AMPA-type GluRs, may drive receptor clustering by being released with, or in parallel to, the neurotransmitter at Drosophila NMJs (Saitoe, 2001).

A positive correlation has been documented between ongoing spontaneous vesicle exocytosis and the embryonic development of GluR clusters at Drosophila NMJs. Nerve-evoked vesicle exocytosis is not necessary for this process, because although neither n-syb nor cspts mutants show any demonstrable nerve-evoked ESCs, GluRs still cluster. mECSs persist in both mutants. However, when spontaneous secretory events are absent (as in shits mutants at the nonpermissive temperature or in syx), junctional GluR clusters are exceedingly infrequent. If GluR clustering is solely contingent on the nerve-muscle contact, GluRs should cluster at the contacts in shits mutants raised at the nonpermissive temperature and in syx mutants. In a recent study of mice lacking an isoform of munc 18-1, there was no demonstrable change in AChR clustering, although both spontaneous and evoked neurotransmitter release are absent. Together with the observations made in this paper, these data suggest that munc 18-1 is not involved in the secretion of the agent that induces clustering of neurotransmitter receptors, whereas syntaxin is essential for this process (Saitoe, 2001).

The absence of clusters in Drosophila in syx and shi mutants implies that spontaneous secretory events are related to GluR clustering and probably to cluster stabilization as well. Moreover, it is the clustering of these receptors, rather than their surface expression, that depends on spontaneous secretion: Functional GluRs are detected in syx mutants although they rarely form detectable clusters at the synapse. The link between spontaneous vesicle exocytosis and receptor clustering must now be clarified (Saitoe, 2001).

Targeted mutations in the Syntaxin H3 domain specifically disrupt SNARE complex function in synaptic transmission

The cytoplasmic H3 helical domain of syntaxin is implicated in numerous protein-protein interactions required for the assembly and stability of the SNARE complex mediating vesicular fusion at the synapse. Two specific hydrophobic residues (Ala-240, Val-244) in H3 layers 4 and 5 of mammalian syntaxin1A have been suggested to be involved in SNARE complex stability and required for the inhibitory effects of syntaxin on N-type calcium channels. The equivalent double point mutations in Drosophila syntaxin1A (A243V, V247A; syx4 mutant) have been generated to examine their significance in synaptic transmission in vivo. The syx4 mutant animals are embryonic lethal and display severely impaired neuronal secretion, although non-neuronal secretion appears normal. Synaptic transmission is nearly abolished, with residual transmission delayed, highly variable, and nonsynchronous, strongly reminiscent of transmission in null synaptotagmin I mutants. However, the syx4 mutants show no alterations in synaptic protein levels in vivo or syntaxin partner binding interactions in vitro. Rather, syx4 mutant animals have severely impaired hypertonic saline response in vivo, an assay indicating loss of fusion-competent synaptic vesicles, and in vitro SNARE complexes containing Syx4 protein have significantly compromised stability. These data suggest that the same residues required for syntaxin-mediated calcium channel inhibition are required for the generation of fusion-competent vesicles in a neuronal-specific mechanism acting at synapses (Fergestad, 2001b).

In Drosophila, syntaxin1A is absolutely required for all vesicular fusion events throughout the animal; null syntaxin mutants abolish both non-neuronal and neuronal secretion. In contrast, syx4 mutants display no detectable defects in non-neuronal secretion but rather specifically impair synaptic transmission. These data show that constitutive vesicle fusion does not require residues A243 and V247 in the syntaxin H3 domain, implicating this site in mediating a process specifically involved in calcium-dependent synaptic vesicle fusion. Interaction with N-type Ca2+ channels is an obvious and attractive explanation for this synapse-specific function. However, this interaction has been proposed to inhibit Ca2+ influx, which is not necessarily consistent with observed phenotypes. The syx4 mutants display a striking impairment of synaptic excitation-secretion coupling: action potential-evoked release is reduced by ~90%, and residual transmission is highly asynchronous, variable, and prone to failure. Thus, syx4 mutants are not capable of properly triggering robust, synchronized synaptic vesicle fusion in response to a Ca2+ influx. These defects are more consistent with an inability to rapidly generate functional SNARE complexes (Fergestad, 2001b).

The syx4 synaptic phenotypes are clearly distinct from those associated with other engineered point mutations in the H3 domain of syntaxin. However, the phenotypes are strikingly similar to those described previously for both the synaptotagmin I null mutant and the syxH3-C mutant, which deletes the Ca2+ effector domain to severely reduce binding to synaptotagmin I. The syx4 phenotypes also resemble the unreliable transmission observed in wild-type synapses at low (<0.4 mM) extracellular Ca2+ concentrations. On the basis of these phenotypic similarities, it appears possible that core complex function in vivo is modulated at least in part by synaptotagmin I and that the syx4 mutations impair this regulation (Fergestad, 2001b).

This hypothesis was tested by assaying the protein binding properties of syx4, however, impaired binding to synaptotagmin I, CSP, ROP/MUNC-18, the Ca2+ channel synprint site, or other members of the core complex, could not be identified. In particular, in numerous assays synaptotagmin I binding of the Syx4 core complex was not significantly different from controls, other than a dramatic increase in the variability of binding in the presence of Ca2+. The increased variability of synaptotagmin I binding to the Syx4 core complex may possibly indicate that rapid core complex formation in syx4 mutants is impaired, because synaptotagmin I has recently been shown to accelerate core complex formation. This is consistent with the evidence provided here showing a strong reduction of hyperosmotic saline-induced transmitter release in both synaptotagmin null (sytAD4) and syx4 mutant synapses. Although Syx4 containing core complexes can be formed in vitro, on the basis of a steady-state assay, the resulting complexes display impaired stability manifested by increased heat lability. These observations suggest that the formation of the SNARE complex in vivo, which underlies neurotransmission, may be more rapid and substantially different from complex formation in vitro. These observations might reasonably explain why syx4 does not detectably perturb the slow, constitutive vesicle fusion in non-neuronal tissues, whereas it dramatically impairs the fast, Ca2+-dependent fusion at synapses (Fergestad, 2001b).

Syntaxin, synaptotagmin, and SNAP-25 all dynamically interact with calcium channels and modify channel current properties. Through these interactions, calcium channels have also been implicated in SNARE complex formation, possibly through an intermediate termed the excitosome where syntaxin, SNAP-25, and synaptotagmin all bind the channel in a complex awaiting the vesicle and its v-SNARE, synaptobrevin. Simplistically, the inhibition of Ca2+ influx by syntaxin predicts a negative role for the syntaxin-calcium channel interaction on neurotransmission. Therefore, removal of syntaxin-mediated inhibition of Ca2+ influx should result in increased presynaptic Ca2+ levels and increased vesicle fusion and transmission. However, it has been shown that the double point mutations that remove syntaxin-mediated inhibition of calcium channels in vitro result in severely reduced transmission. These same residues of syntaxin have been shown to be critical for normal response to hyperosmotic saline application. Therefore, these residues may play a coupled role in the regulation of Ca2+ channels and SNARE complexes, perhaps through the formation of an excitosome intermediate (Fergestad, 2001b).

In Drosophila, it is not known which Ca2+ channels are present at presynaptic active zones and interact with the presynaptic SNARE complex. Therefore, no direct evidence can be provided for Drosophila syntaxin inhibiting calcium channels. However, the syntaxin interaction is maintained through different calcium channel types in vertebrates, and the specific residues mediating the interaction are highly conserved in Drosophila. Thus, one focus of this study was to examine the significance of these calcium channel-inhibiting residues in vivo. Aberrant calcium channel openings, in the absence of syntaxin-mediated inhibition, might result in impaired excitation-secretion; however, because voltage activation of the channel is unaffected and mEJCs are less frequent in syx4 mutants, this is unlikely. Presently, the only functional link for the syntaxin-calcium channel interaction is through syntaxin residues 240 and 244 (243 and 247 in Drosophila). Therefore, alteration of these residues may impair the function of the SNARE complex by disruption of a calcium channel/excitosome intermediate (Fergestad, 2001b).

If the only conserved syntaxin-Ca2+ channel interaction has been disrupted, as is believed, these data provide strong evidence for a positive role for this interaction. This model does not exclude an inhibitory role for syntaxin in calcium channel gating but suggests that these syntaxin residues, and the syntaxin-calcium channel interaction, are important for more than just inhibiting inappropriate Ca2+ influx. Examination of the interaction between syntaxin and Ca2+ channels may best be done by altering the Ca2+ channel instead of the multifunctional syntaxin, once the non-synprint site of interaction is identified (Fergestad, 2001b).

Molecular dissection of cytokinesis by RNA interference in Drosophila cultured cells

Double-stranded RNA-mediated interference (RNAi) was used to study Drosophila cytokinesis. Double-stranded RNAs for anillin, RacGAP50C, pavarotti, rho1, pebble, spaghetti squash, syntaxin1A, and twinstar all disrupt cytokinesis in S2 tissue culture cells, causing gene-specific phenotypes. The phenotypic analyses identify genes required for different aspects of cytokinesis, such as central spindle formation, actin accumulation at the cell equator, contractile ring assembly or disassembly, and membrane behavior. Moreover, the cytological phenotypes elicited by RNAi reveal simultaneous disruption of multiple aspects of cytokinesis. These phenotypes suggest interactions between central spindle microtubules, the actin-based contractile ring, and the plasma membrane, and led to a proposal that the central spindle and the contractile ring are interdependent structures. Finally, these results indicate that RNAi in S2 cells is a highly efficient method to detect cytokinetic genes, and predict that genome-wide studies using this method will permit identification of the majority of genes involved in Drosophila mitotic cytokinesis (Somma, 2002).

The syx1A gene, which encodes a t-SNARE, plays an essential role in embryonic cellularization, but its direct role in cytokinesis has not been demonstrated. In syx1A (RNAi) cells approximately half of the telophases are shorter that those of control cells and display severe defects in both the central spindle and the contractile ring. These findings are rather surprising, because there is abundant evidence that syntaxins are specifically involved in membrane fusion processes. Thus, the observations on syx1A (RNAi) cells raise the question of how a defect in membrane formation can affect both the central spindle and contractile ring assembly. Studies of C. elegans embryos depleted of the cytokinesis-specific Syntaxin-4 protein by RNAi have shown that in some of these embryos there is a complete failure of cleavage furrow ingression, suggesting an underlying defect in the contractile ring machinery. It has been thus proposed that formation of new membrane may positively regulate contractile ring assembly. In agreement with this hypothesis, it is suggested that RNAi-induced Syx1A depletion in S2 cells disrupts membrane formation at the site of cleavage furrow, causing a secondary defect in contractile ring formation and thus also in central spindle assembly (Somma, 2002).

Rolling blackout shows a genetic interaction with syntaxin and is required for synaptic vesicle exocytosis

Rolling blackout (RBO) is a putative transmembrane lipase required for phospholipase C-dependent phosphatidylinositol 4,5-bisphosphate–diacylglycerol signaling in Drosophila neurons. Conditional temperature-sensitive (TS) rbo mutants display complete, reversible paralysis within minutes, demonstrating that RBO is acutely required for movement. RBO protein is localized predominantly in presynaptic boutons at neuromuscular junction (NMJ) synapses and throughout central synaptic neuropil, and rbo TS mutants display a complete, reversible block of both central and peripheral synaptic transmission within minutes. This phenotype appears limited to adults, because larval NMJs do not manifest the acute blockade. Electron microscopy of adult rbo TS mutant boutons reveals an increase in total synaptic vesicle (SV) content, with a concomitant shrinkage of presynaptic bouton size and an accumulation of docked SVs at presynaptic active zones within minutes. Genetic tests reveal a synergistic interaction between rbo and syntaxin1A TS mutants, suggesting that RBO is required in the mechanism of N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE)-mediated SV exocytosis, or in a parallel pathway necessary for SV fusion. The rbo TS mutation does not detectably alter SNARE complex assembly, suggesting a downstream requirement in SV fusion. It is concluded that RBO plays an essential role in neurotransmitter release, downstream of SV docking, likely mediating SV fusion (Huang, 2006).

RBO is a predicted integral plasma membrane lipase (Huang, 2004). The protein is highly enriched within the nervous system and is subcellularly restricted within central neurons primarily to synaptic domains. At neuromuscular synapses, RBO is predominantly localized to the plasma membrane within presynaptic boutons. The protein may also be present in the postsynaptic compartment, but it is currently not possible to resolve this clearly at a confocal level. Conditional rbo mutants paralyze within minutes and display a complete block of synaptic transmission within minutes. This functional block correlates with a sharp increase in SV number within presynaptic boutons and a concomitant shrinkage of presynaptic plasma membrane area. These acute changes appear to arise from the disruption of the balance between SV consumption (exocytosis) and recycling by SV formation (endocytosis). Similar SV accumulation has been reported only in mutants with defective SV fusion, including comatose and syntaxin TS mutants. In rbo TS mutants, docked vesicles accumulate at presynaptic active zones within minutes. This defect is most consistent with a postdocking block of SV priming/fusion. However, because docking may be proportional to overall SV pool size, the elevation in SV number might also contribute to the increased number of docked SVs (Huang, 2006).

Conditional TS paralytic mutations of rbo and syntaxin1A (syx3–69) produce a strong synergistic genetic interaction. Among the pool of TS mutants tested, this interaction appears quite specific to syntaxin. Interactions were not observed between rbo and TS mutant affecting presynaptic Ca2+ influx, SNARE complex disassembly, or SV recycling. The rbo–syx interaction agrees well with the EM characterization, indicating a requirement for RBO in postdocking SV exocytosis. The syx3–69 mutants display a temperature-dependent loss of SNARE complexes. In rbots1; syx3–69 double mutants, no further reduction of SNARE complex assembly was found. The assay included both trans- and cis-SNARE complexes, making it hard to correlate SNARE complex abundance with functional defects. Nevertheless, the absence of a discernable change in SNARE complex abundance in rbo mutants suggests that RBO is unlikely to function directly in SNARE assembly/disassembly. Together, these data therefore suggest that RBO likely acts either downstream of SNARE complex assembly or in an unknown parallel pathway leading to SV fusion (Huang, 2006).

The acute requirement for RBO protein appears to be limited to a subset of synapses; larval NMJ synapses do not show the same requirement. One possible explanation may be functional redundancy or differences in synaptic thermal regulation between larval and adult synapses. For unknown reasons, larva NMJ neurotransmission has proven consistently more resistant to disruption by Drosophila TS mutants than the adult. A second possibility is that RBO may be acutely required at synapses designed to function reliably under conditions of high demand. The central cholinergic and NMJ synapses in the adult Drosophila GF circuit can support 100 Hz synaptic transmission, far beyond the usage or sustainable range of the larval NMJ (Huang, 2006).

It has been well demonstrated that SNARE complex assembly is essential for vesicle priming and can directly mediate membrane fusion. In addition, however, studies of yeast vacuolar homotypic fusion and direct studies of exocytosis of neurotransmitter vesicles including SVs suggest that an additional machinery may act downstream of the SNARE complex to mediate fusion. The data suggest that RBO may similarly act downstream of SNARE complex assembly. It is proposed that RBO may regulate the function of the SV fusion machinery or may be a novel component of this fusion machinery (Huang, 2006).

The closest characterized homolog of RBO (42% conserved) is an integral plasma membrane sn-1 DAG lipase. RBO is essential for PLC-dependent neuronal signaling (Huang, 2004). Consistently, after a 10 min shift to 37°C, rbo TS mutants display an accumulation of PIP2 and concomitant reduction of DAG in the brain (Huang, 2004). Synaptically localized RBO therefore may regulate the levels of fusogenic lipids (DAG, phosphatidylinositides, polyunsaturated fatty acids) at, or near, AZ fusion sites. These critical lipids may contribute directly to the generation of membrane properties required for SV fusion. Alternatively, these lipids might regulate the activity of lipid-binding fusogenic proteins (Huang, 2006).

Both lipid partitioning and protein interactions regulate membrane changes to enable fusion. Lipids with compact head groups and space-filling tails, such as PIP2 and DAG, favor the negative membrane curvature required for vesicle fusion, and both PIP2 and DAG directly promote Ca2+-dependent exocytosis. Lipases including PLC, PLD, and PLA2 are known to promote secretion through the fusogenic effects of their lipid products. These lipases have been proposed to increase presynaptic release site availability and/or vesicle fusion efficacy. These activities also coordinate the spatial-temporal regulation of numerous synaptic proteins. SV priming is dependent on UNC-13, which binds DAG. Other known targets of PIP2 and DAG include synaptotagmin 1 and MUNC18-interacting MINT1,2. Thus, phosphoinositides may play multiple roles in the formation of the SV fusion domain: directly determining membrane properties, serving as a precursor for other fusogenic lipids (DAG), and serving as anchors/regulators for fusogenic proteins (Huang, 2006).

Conditional removal of RBO activity in TS mutants causes acute DAG depletion and PIP2 accumulation in the brain (Huang, 2004). These changes correlate temporally with the loss of neurotransmission and the arrest of docked SVs at presynaptic AZs. The technology to determine whether these lipid changes occur locally at the AZ has not yet been developed, but the synaptic localization of RBO and the acute requirement of RBO in postdocking SV exocytosis supports this conclusion. Therefore, RBO is proposed to function as a presynaptic phospholipase that modulates the PIP2–DAG pathway to regulate SV fusion. Ongoing studies are aimed at determining the nature of this requirement and how RBO activity is regulated to control SV fusion efficacy and thereby neurotransmission strength (Huang, 2006).

Ca2+-dependent, phospholipid-binding residues of synaptotagmin are critical for excitation-secretion coupling in vivo

Synaptotagmin I is the Ca2+ sensor for fast, synchronous release of neurotransmitter; however, the molecular interactions that couple Ca2+ binding to membrane fusion remain unclear. The structure of synaptotagmin is dominated by two C2 domains that interact with negatively charged membranes after binding Ca2+. In vitro work has implicated a conserved basic residue at the tip of loop 3 of the Ca2+-binding pocket in both C2 domains in coordinating this electrostatic interaction with anionic membranes. Although results from cultured cells suggest that the basic residue of the C2A domain is functionally significant, such studies provide contradictory results regarding the importance of the C2B basic residue during vesicle fusion. To directly test the functional significance of each of these residues at an intact synapse in vivo, either the C2A or the C2B basic residue was neutralized, and synaptic transmission was assessed at the Drosophila neuromuscular junction. The conserved basic residues at the tip of the Ca2+-binding pocket of both the C2A and C2B domains mediate Ca2+-dependent interactions with anionic membranes and are required for efficient evoked transmitter release. These results directly support the hypothesis that the interactions between synaptotagmin and the presynaptic membrane, which are mediated by the basic residues at the tip of both the C2A and C2B Ca2+-binding pockets, are critical for coupling Ca2+ influx with vesicle fusion during synaptic transmission in vivo. a model for synaptotagmin's direct role in coupling Ca2+ binding to vesicle fusion incorporates this finding with results from multiple in vitro and in vivo studies (Paddock, 2008).

Several reports indicate that residues at the tip of the Ca2+-binding pockets, including the conserved basic residues, in both C2 domains of synaptotagmin are critical for Ca2+-dependent phospholipid. Studying cultured cells provides valuable insight into possible protein functions, yet some of the properties of vesicle fusion in PC12 cells and chromaffin cells are quite different from those mediating fast transmitter release at a synapse. Even cultured neurons do not necessarily faithfully reproduce all aspects of synaptic behavior at an intact synapse. For example, at cultured hippocampal autapses, the amplitude of mEJPs is larger, the paired-pulse ratio is decreased, and long-term potentiation cannot be reliably elicited compared with recordings from hippocampal slices. Additionally, synaptotagmin mutant autapses exhibit no change in mEJP frequency, whereas synaptotagmin mutations increased the mEJP frequency at several other synapses, including excitatory and inhibitory synapses between cultured cortical neurons, the calyx of Held, and mature neuromuscular junctions in Drosophila and mice. Therefore, it is critical to test the function of the basic residues at the tips of the C2 domains at an intact synapse to determine their role in synaptic transmission in vivo (Paddock, 2008).

The C2A and C2B domains are structurally highly homologous and exhibit many similar biochemical interactions in vitro. Analyses of these interactions have provided critical insights into the molecular mechanisms mediating synaptic vesicle fusion. Conditions in vitro, however, can affect biochemical interactions. Interactions between the C2A basic residue and negatively charged phospholipids can differ when isolated C2A domains are used. Ca2+-dependent interactions between these positively charged residues and negatively charged phospholipids appear to be quite sensitive to experimental conditions. Still, most studies find a decrease in Ca2+-dependent phospholipid interactions when either the C2A or C2B basic residue is neutralized. These results provide strong support for the hypothesis that Ca2+-dependent interactions between phospholipids and both the C2A and C2B domains are mediated by residues located at the tip of the Ca2+-binding pockets (Paddock, 2008).

The key difference between the C2 domains likely resides within their polylysine motifs. The C2B polylysine motif mediates a unique set of interactions that are not shared by the C2A domain, including Ca2+-independent interactions with phosphatidylinositol 4,5-bisphosphate (PIP2) and soluble N-ethylmaleimide-sensitive factor attachment receptor (SNARE) proteins. Because the polylysine motif is on the side of the C2B domain, interactions with the SNARE proteins could hold the C2B Ca2+-binding pocket in close proximity to the presynaptic membrane, priming vesicles for immediate fusion after Ca2+ influx. Then, after Ca2+ influx, the conserved basic residues of each C2 domain mediate similar Ca2+-dependent interactions with anionic phospholipids, which pulls the synaptic vesicle toward the membrane. The insertion of hydrophobic residues on the tips of the C2 domains may destabilize the presynaptic membrane, aiding the fusion reaction. The relative positioning of the C2A versus C2B domains with respect to the SNARE complex may determine the relative importance of the Ca2+-binding sites, and the basic residues reported in this study, for triggering vesicle fusion in vivo. This model is consistent with the data from multiple in vitro and in vivo systems, and a remarkably similar model, postulating a more distally positioned C2A domain, has recently been proposed (Paddock, 2008).

In summary, synchronous vesicle fusion triggered by Ca2+ requires the coordinated interactions of many presynaptic molecules. Examination of isolated interactions in vitro provides insight into possible molecular mechanisms for fusion; however, only the analysis of synaptic transmission at intact synapses can determine which interactions are likely to function in vivo. The current findings demonstrate that the positively charged residue located at the tip of each Ca2+-binding pocket mediates Ca2+-dependent interactions with negatively charged membranes and is required for efficient synaptic transmission. These findings indicate that the function of this region of C2A and C2B is likely conserved and support the hypothesis that Ca2+-dependent interactions between the tips of each C2 domain with anionic phospholipids are requisite for efficient excitation-secretion coupling during synaptic transmission (Paddock, 2008).

Syntaxin 1A: Biological Overview | Evolutionary Homologs | Regulation | Developmental Biology | References

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