InteractiveFly: GeneBrief

Syntaxin 18: Biological Overview | References

Gene name - Syntaxin 18

Synonyms - Gtaxin

Cytological map position - 96A12-96A13

Function - signaling protein

Keywords - Discs-Large-interacting t-SNARE, postsynaptic in type I synapses, Akt influences subsynaptic reticulum (SSR) assembly by regulation of Syt18

Symbol - Syx18

FlyBase ID: FBgn0039212

Genetic map position - chr3R:24,623,802-24,625,562

NCBI classification - Syntaxin-18_N: SNARE-complex protein Syntaxin-18 N-terminus

Cellular location - cytoplasmic

NCBI links: EntrezGene, Nucleotide, Protein

Syx18 orthologs: Biolitmine
Recent literature
Lakatos, Z., Lorincz, P., Szabo, Z., Benko, P., Kenez, L. A., Csizmadia, T. and Juhasz, G. (2019). Sec20 is required for autophagic and endocytic degradation independent of golgi-ER retrograde transport. Cells 8(8). PubMed ID: 31344970
Endocytosis and autophagy are evolutionarily conserved degradative processes in all eukaryotes. Both pathways converge to the lysosome where cargo is degraded. Improper lysosomal degradation is observed in many human pathologies, so its regulatory mechanisms are important to understand. Sec20/BNIP1 (BCL2/adenovirus E1B 19 kDa protein-interacting protein 1) is a BH3 (Bcl-2 homology 3) domain-containing SNARE (soluble N-ethylmaleimide-sensitive factor-attachment protein receptors) protein that has been suggested to promote Golgi-ER retrograde transport, mitochondrial fission, apoptosis and mitophagy in yeast and vertebrates. This study shows that loss of Sec20 in Drosophila fat cells causes the accumulation of autophagic vesicles and prevents proper lysosomal acidification and degradation during bulk, starvation-induced autophagy. Furthermore, Sec20 knockdown leads to the enlargement of late endosomes and accumulation of defective endolysosomes in larval Drosophila nephrocytes. Importantly, the loss of Syx18 (Syntaxin 18), one of the known partners of Sec20, led to similar changes in nephrocytes and fat cells. Interestingly. Sec20 appears to function independent of its role in Golgi-ER retrograde transport in regulating lysosomal degradation, as the loss of its other partner SNAREs Use1 (Unconventional SNARE In The ER 1) and Sec22 or tethering factor Zw10 (Zeste white 10), which function together in the Golgi-ER pathway, does not cause defects in autophagy or endocytosis. Thus, these data identify a potential new transport route specific to lysosome biogenesis and function.
Bourne, C. M., Lai, D. C. and Schottenfeld-Roames, J. (2022). Regulators of the secretory pathway have distinct inputs into single-celled branching morphogenesis and seamless tube formation in the Drosophila trachea. Dev Biol 490: 100-109. PubMed ID: 35870495
Biological tubes serve as conduits through which gas, nutrients and other important fluids are delivered to tissues. Most biological tubes consist of multiple cells connected by epithelial junctions. Unlike these multicellular tubes, seamless tubes are unicellular and lack junctions. Seamless tubes are present in various organ systems, including the vertebrate vasculature, C.elegans excretory system, and Drosophila tracheal system. The Drosophila tracheal system is a network of air-filled tubes that delivers oxygen to all tissues. Specialized cells within the tracheal system, called terminal cells, branch extensively and form seamless tubes. Terminal tracheal tubes are polarized; the lumenal membrane has apical identity whereas the outer membrane exhibits basal characteristics. Although various aspects of membrane trafficking have been implicated in terminal cell morphogenesis, the precise secretory pathway requirements for basal and apical membrane growth have yet to be elucidated. This study, demonstrated that anterograde trafficking, retrograde trafficking and Golgi-to-plasma membrane vesicle fusion are each required for the complex branched architecture of the terminal cell, but their inputs during seamless lumen formation are more varied. The COPII subunit, Sec31, and ER exit site protein, Sec16, are critical for subcellular tube architecture, whereas the SNARE proteins Syntaxin 5, Syntaxin 1 and Syntaxin 18 are more generally required for seamless tube growth and maintenance. These data suggest that distinct components of the secretory pathway have differential contributions to basal and apical membrane growth and maintenance during terminal cell morphogenesis.

Targeted membrane addition is a hallmark of many cellular functions. In the nervous system, modification of synaptic membrane size has a major impact on synaptic function. However, because of the complex shape of neurons and the need to target membrane addition to very small and polarized synaptic compartments, this process is poorly understood. Here, we show that Gtaxin (GTX), a Drosophila t-SNARE (target-soluble N-ethylmaleimide-sensitive factor attachment protein receptor), is required for expansion of postsynaptic membranes during new synapse formation. Mutations in gtx lead to drastic reductions in postsynaptic membrane surface, whereas gtx upregulation results in the formation of complex membrane structures at ectopic sites. Postsynaptic GTX activity depends on its direct interaction with Discs-Large (DLG), a multidomain scaffolding protein of the PSD-95 (postsynaptic density protein-95) family with key roles in cell polarity and formation of cellular junctions as well as synaptic protein anchoring and trafficking. DLG selectively determines the postsynaptic distribution of GTX to type I, but not to type II or type III boutons on the same cell, thereby defining sites of membrane addition to this unique set of glutamatergic synapses. A mechanistic explanation for selective targeted membrane expansion at specific synaptic junctions is provided (Gorczyca, 2007).

Targeted membrane addition is of fundamental importance to the development, function, and plasticity of neuronal synapses, during which profound and coordinated structural alterations of both the presynaptic and postsynaptic membrane compartments must occur. Despite its importance, this process is poorly understood in the nervous system. The Drosophila larval neuromuscular junction (NMJ) is an excellent model system to study synaptic membrane addition, because the area and complexity of the postsynaptic membrane increases drastically during larval development. Over the course of 4 d, a massive amount of membrane is added to a very small postsynaptic junction comprising <1% of the total muscle surface area. This process results in the formation of a highly convoluted and multilayered postsynaptic membrane specialization [the subsynaptic reticulum (SSR)] where receptors, cell adhesion molecules (CAMs), and ion channels are anchored, and which also contributes to local translation of postsynaptic proteins (Gorczyca, 2007).

Membrane associated guanylate kinases (MAGUKs) such as Drosophila Discs-Large (DLG) and its mammalian relative postsynaptic density protein-95 (PSD-95) have been implicated in controlling the size, shape, and function of synaptic structures. Mutations in dlg lead to striking defects in SSR expansion, and modified levels of PSD-95 are associated with changes in the number and size of dendritic spines. Most hypotheses regarding the function of MAGUKs during synaptic assembly and function have centered around their role in clustering ion channels, transduction of calcium signals, and linkage of these proteins to the plasma membrane and cytoskeleton. MAGUKs, however, may also contribute to synaptic structure and function by regulating membrane addition. An association of synaptic MAGUKs with membranous compartments is widely recognized, and PSD-95 and its paralog SAP102 (synapse-associated protein 102) associate with the exocyst complex protein Sec8 in neurons (Gorczyca, 2007).

Within the SSR, DLG is required for clustering Shaker potassium channels (Sh) and the CAM Fasciclin II (FasII), as well as for SSR expansion. Strikingly, however, Sh and fasII mutations do not affect the SSR. This implies that DLG plays a role in the structure and regulation of postsynaptic membranes independent of its interaction with Sh or FasII (Gorczyca, 2007).

Addition of membranes by vesicle fusion commonly involves soluble N-ethylmalemide-sensitive factor attachment protein receptor (SNARE) proteins as the minimal fusion machinery. This study reports the isolation of a postsynaptic DLG-interacting target-SNARE (t-SNARE), guanylate kinase-like (GUK)-interacting syntaxin [Gtaxin (GTX)], that is involved in this process. GTX shares sequence similarity with vertebrate Syntaxin-18 (Hatsuzawa, 2000) and yeast Ufe1p (Lewis, 1996), the latter of which can mediate homotypic endoplasmic reticulum (ER) membrane fusion in the absence of any other known SNARE (Patel, 1998). This study shows that GTX and DLG are physically linked, that this interaction is required for postsynaptic localization of GTX, and that DLG regulates the formation of SDS-resistant SNARE complexes containing GTX. Mutations in gtx phenocopy the reduced SSR and bouton number in dlg mutants, and GTX overexpression results in ectopic formation of SSR-like structures. It is proposed that GTX is a major effector for DLG-dependent addition of postsynaptic membranes during synapse development (Gorczyca, 2007).

DLG plays important roles at synapses because of interactions with a variety of binding partners. Although MAGUKs can regulate the size of synaptic structures, suggesting a role in membrane trafficking, the underlying mechanisms have remained elusive. This study shows that DLG interacts with the t-SNARE GTX, which functions in a DLG-dependent manner during postsynaptic membrane expansion and new synaptic bouton formation. It is proposed that DLG regulates the spatial localization of membrane addition events through the activity of GTX (Gorczyca, 2007).

dlg mutants have diverse synaptic phenotypes, including abnormal NMJs with fewer boutons, increased neurotransmitter release, and a reduced SSR. Some of these arise because of the roles of DLG-binding partners, such as FasII and Sh. However, the mechanisms by which DLG regulates the size of the SSR have remained elusive (Gorczyca, 2007).

The SSR is a dynamic membrane system where synaptic proteins localize in a spatially restricted manner. The SSR is also a trafficking compartment where regulation of signaling and synaptic protein function occurs through vesicle cycling. At its periphery, the SSR shows association with machinery for local translation of synaptic proteins, potentially constituting an adapted ER-like structure. The complexity of the SSR results from continuous membrane addition throughout larval development. How is this process specifically targeted to synaptic sites, which correspond to <1% of the muscle surface? The finding that DLG interacts with and regulates GTX provides new insight on this process. GTX binds the GUK domain of DLG, and the postsynaptic localization of GTX depends on DLG. Most notably, gtx mutants exhibit a dramatic reduction in SSR length, and a GTX-containing SNARE complex was decreased in dlg mutants and enhanced when DLG is overexpressed. It is proposed that DLG directs membrane addition to the SSR by regulating the targeting and activity of GTX (Gorczyca, 2007).

Several lines of evidence support this model. First, enrichment of GTX at the SSR is strongly dependent on the presence and correct localization of DLG. When DLG is moderately overexpressed, the localization of GTX at the SSR is enhanced. Moreover, strongly overexpressed DLG leads to increased association of DLG with intramuscular compartments, which caused GTX to accumulate extrasynaptically. This DLG dose dependency of GTX targeting provides compelling evidence that DLG acts as a strong determinant of GTX localization. Second, mutations in gtx, like mutations in dlg, lead to an underdeveloped SSR, suggesting that GTX and DLG are in the same membrane addition pathway. Third, overexpressing GTX leads to loss of SSR at sites of synaptic contact and a DLG-independent formation of ectopic SSR-like structures. These ectopic structures were still present when GTX was overexpressed in dlg mutants, suggesting that GTX-mediated fusion can occur in the absence of DLG when GTX concentration is very high. These observations suggest that GTX plays a role in fusion of membranes to form the SSR. Interestingly, the ectopic SSR-like structures did not appear to contain any of the postsynaptic proteins found within the SSR, including DLG, Scribble, Fasciclin II, and GluRs. Therefore, trafficking of SSR proteins is somewhat independent from the process of membrane addition to the SSR. Fourth, low GTX expression, could rescue the gtx mutant SSR defects, but not those of dlg mutants, further supporting the model that GTX requires DLG for membrane addition to the SSR (Gorczyca, 2007).

The role of DLG in SSR formation may extend beyond simply recruiting GTX to the SSR. A model is proposed that further explains how molecular interactions between DLG and GTX may direct SSR formation (see Ectopic SSR formation after strong GTX overexpression and proposed function of DLG and GTX during targeted membrane addition). First, the immunoprecipitation studies suggest that DLG associates more strongly with higher molecular weight complexes containing GTX, rather than with monomeric GTX. It is proposed that the low-affinity interaction between monomeric GTX and DLG directs recruitment of GTX to the SSR. Once at the SSR, homotypic interaction between GTX monomers leads to the formation of GTX SNARE complexes. Second, at least one of the GTX SNARE complexes (70 kDa) was decreased in dlg mutants and increased by DLG overexpression. This complex could not be restored by overexpressing GTX in a dlg mutant. Therefore, the high-affinity interaction between DLG and GTX SNARE complexes may lead to their protection/stabilization, thereby facilitating membrane fusion precisely at sites of synaptic contact (Gorczyca, 2007).

GTX is the putative homolog of Syntaxin-18 and Ufe1p. Unlike other t-SNARES, these proteins can mediate homotypic membrane fusion in the absence of other known SNAREs (Patel, 1998; Kano, 2005). Thus, targeting GTX-containing vesicles to specific membrane compartments would result in membrane expansion. Consistently, Syntaxin-18 and Ufe1p have been implicated in mediating vesicle fusion as vesicles traffic from one ER compartment to another (Lewis, 1996; Hatsuzawa, 2000). The presumptive role of GTX as part of an ER-specific vesicle fusion machinery, together with its requirement for SSR development, supports the idea that the SSR bears at least some ER-like properties. Moreover, the role of a so-called ER to Golgi SNARE in a specialized plasma membrane such as the SSR is not surprising, because Syntaxin-18 has also been implicated recently in the function of the phagosome during immunoglobulin-mediated particle engulfment by professional phagocytes (Hatsuzawa, 2006; Gorczyca, 2007 and references therein).

Both DLG and GTX are exclusively localized at type I terminals and not in other terminals such as type II or type III, which lack SSR. Some muscle fibers such as muscle 12 are innervated by all three types of terminals, suggesting that in the same postsynaptic cell, there are selective membrane-trafficking systems in place (Gorczyca, 2007).

Mutations in gtx also resulted in electrophysiological changes. The decrease in mEJP frequency is consistent with the finding that gtx mutants have reduced number of boutons, without affecting the number of active zones per bouton. Another very interesting phenotype was the decrease in EJP decay constant. This was not a result of changes in passive properties or to changes in GluR function. It is quite possible that the trafficking of a voltage gated channel is altered in gtx mutants. For example, an increase in the function of a K+ channel would be expected to generate a similar change in EJP kinetics (Gorczyca, 2007).

GTX also plays a role during muscle growth. Notably, the synaptic and muscle functions could be genetically uncoupled. Whereas muscle size was primarily insensitive to elevated GTX levels, a fairly precise dosage of GTX is required for synaptic growth. DLG itself may also play a role during muscle membrane trafficking, in addition to its more notable role in synapse development. Despite the enrichment of DLG at synaptic boutons, it is also present in the muscle, where it intermingles with the myocontractile apparatus, in the subcortical network, and in association with T-tubules. It is thus possible that DLG also operates during the localization of proteins required for excitation-contraction coupling (Gorczyca, 2007).

The analysis of dlg and gtx mutants using mCD8-GFP to label membranes has also uncovered a role of GTX in the morphogenesis of the cortical membrane compartment. Previous studies have shown that DLG traffics through this compartment on its way to postsynaptic sites. In gtx mutants, the cortical network appears to have collapsed into the subcortical network, which may explain the slight defects in DLG trafficking observed in gtx mutants (Gorczyca, 2007).

Living cells are highly specialized machines that must filter a vast array of information from the environment to integrate into sophisticated higher-order networks such as organs and neural systems. Cellular signaling is organized into specialized networks of proteins, the function of which is often structured by the modular protein'protein interaction domains of scaffolding proteins. Although much has been learned about the role of scaffolding proteins in the compartmentalization of polarized signaling complexes and the physical linkage of cell adhesion molecules to the cytoskeleton, there have been few examples of how scaffolding proteins may also guide other processes such as vesicle fusion to specific cellular domains (Gorczyca, 2007).

In mammals, the development and activation state of specific synapses during plasticity is accompanied by dramatic changes in the size of postsynaptic dendritic spines and the ER within spines. It is likely that membrane addition events such as the ones described in this study also occur in a targeted manner, and this process may also be regulated by MAGUKs. Indeed, changes in PSD-95 levels elicit striking changes in spine size. It is possible that this may also occur though regulation of SNAREs such as GTX (Gorczyca, 2007).

Akt regulates glutamate receptor trafficking and postsynaptic membrane elaboration at the Drosophila neuromuscular junction

The Akt family of serine-threonine kinases integrates a myriad of signals governing cell proliferation, apoptosis, glucose metabolism, and cytoskeletal organization. Akt affects neuronal morphology and function, influencing dendrite growth and the expression of ion channels. Akt is also an integral element of PI3Kinase-target of rapamycin (TOR)-Rheb signaling, a pathway that affects synapse assembly in both vertebrates and Drosophila. Recent findings demonstrated that disruption of this pathway in Drosophila is responsible for a number of neurodevelopmental deficits that may also affect phenotypes associated with tuberous sclerosis complex, a disorder resulting from mutations compromising the TSC1/TSC2 complex, an inhibitor of TOR. Therefore, this study examined the role of Akt in the assembly and physiological function of the Drosophila neuromuscular junction (NMJ), a glutamatergic synapse that displays developmental and activity-dependent plasticity. The single Drosophila Akt family member, Akt1 selectively altered the postsynaptic targeting of one glutamate receptor subunit, GluRIIA, and was required for the expansion of a specialized postsynaptic membrane compartment, the subsynaptic reticulum (SSR). Several lines of evidence indicated that Akt1 influences SSR assembly by regulation of Gtaxin, a Drosophila t-SNARE protein (Gorczyca, 2007) in a manner independent of the mislocalization of GluRIIA. These findings show that Akt1 governs two critical elements of synapse development, neurotransmitter receptor localization, and postsynaptic membrane elaboratio (Lee, 2013).

This study explored Akt function in synapse development and function using a well-characterized model system, the Drosophila neuromuscular junction. There is a single Akt homolog in Drosophila, Akt1, facilitating the genetic and cellular studies of Akt function in synapse assembly (see Model for Akt1's regulatory role at the NMJ). Akt1 was specifically required for the correct assembly of A-type glutamate receptors. Reductions of Akt1 function either by mutation or RNA interference resulted in a loss of GluRIIA at the synapse paired with accumulation into intracellular structures. Reduction of Akt1 influenced the levels and localization of proteins shown to affect GluRIIA, Dorsal, and Cactus. Therefore, Akt1 could affect GluRIIA at least in part via control of these proteins. Akt1 was also required for the normal expansion of a specialized postsynaptic membrane compartment, the SSR. Evidence is provided that Akt1 mediates its effects on SSR via control of the t-SNARE Gtaxin. RNA interference of Gtaxin did not affect GluRIIA localization, showing that the control of SSR expansion and glutamate receptor composition mediated by Akt1 occurs via different molecular mechanisms (Lee, 2013).

The analysis of Akt1 reported in this study examined physiological, morphological, and cellular phenotypes, using both traditional Akt1 mutant alleles and cell-type directed knockdown achieved with either of two different UAS-Akt1RNAi lines. The results from these different genetic tools were consistent and showed that Akt1 function is critical for both GluRIIA localization and SSR expansion. In particular, combinations of Akt1 alleles resulted in the redistribution of GluRIIA into intracellular bands, a phenotype found to be even more pronounced in muscle-directed RNAi of Akt1. This remarkable phenotype was also observed in larvae expressing both Akt1RNAi and a UAS-transgene-derived GluRIIA-RFP in the muscle, the latter detected by either endogenous fluorescence or anti-RFP antibody. It was of note that fluorescent signal from the GluRIIA-RFP was reduced at the synapse but receptor mislocalization to intracellular compartments was detected only with anti-RFP antibody. Akt1-dependent events were clearly required for the proper formation of the folded RFP domain of the recombinant GluRIIA protein while the polypeptide, detected with the anti-RFP antibody was present and redirected to an alternative cellular location, as was observed for the endogenous GluRIIA. These data implicate Akt1 in processes of folding, stabilization, or assembly of GluRIIA (Lee, 2013).

A number of experiments were conducted to evaluate if Akt1 was required for the localization of specific postsynaptic proteins, or rather served a more generalized role in directing a variety of proteins to this membrane specialization. The correct localization of GluRIIB, GluRIIC, Basigin, Discs large, andSyndapin in animals with Akt1 knockdown in the muscle demonstrated that Akt1 has specific targeting functions for GluRIIA and is not a general factor for delivery of all postsynaptic proteins. Levels of these postsynaptic proteins were reduced in Akt1RNAi bearing animals, not surprisingly given the substantial size reduction in the SSR (Lee, 2013).

At the Drosophila NMJ, two types of glutamate receptors have been defined by their distinct compositions and physiological properties. The shifting between A- and B-type receptors provides a mechanism for modulating postsynaptic responses to variable presynaptic inputs during development. There is considerable evidence that modulation of GluRIIA and B representation at the NMJ is governed by different signaling systems. Coracle, a homolog of protein 4.1 in Drosophila, has been shown to specifically influence the targeting of GluRIIA but not IIB (Chen, 2005). A physical interaction between Coracle and GluRIIA was essential for actin-dependent trafficking of GluRIIA-containing vesicles to the plasma membrane. Conversely, DLG has been shown to be required for GluRIIB but not GluRIIA localization at the NMJ (Chen, 2005). The current finding supports the conclusion that A and B receptor subunits are differentially regulated and show that Akt1 serves a role in A but not B subunit control (Lee, 2013).

There is evidence that the assembly and localization of GluRIIA into the postsynaptic density at the NMJ is accomplished following delivery to the plasma membrane (Broadie, 1993; Rasse, 2005). This conclusion is based upon the observation that fluorescence photobleaching of the entire muscle delays accumulation of new GluRIIA to synaptic sites more so than local bleaching at the NMJ (Rasse, 2005). The effects of Akt1 on GluRIIA localization could therefore be mediated by either regulated delivery of GluRIIA-containing vesicles to the plasma membrane, or by affecting the localization to the postsynaptic density following insertion into the plasma membrane. The accumulation of GluRIIA into an intracellular membrane compartments argues for a trafficking-based mechanism. This model is further supported by the results from the developmental timing experiments, where Akt1 function was removed during different stages in synapse assembly. Loss of Akt1 in a 2 day window early in development produced the phenotypes observed with continuous loss of Akt1, whereas a 2 day loss in third instar did not. If Akt1 simply served to retain GluRIIA at the synapse, there should have been time for new synthesis to repopulate the NMJ. Therefore, a model is favored where Akt1 affects developmental processes required for the selective delivery of GluRIIA from the endoplasmic reticulum into functional receptor units that arrive at the plasma membrane. It is notable that in mammalian systems, Akt is critical for the insulin-stimulated exocytosis of glucose transporter containing vesicles to the plasma membrane. Perhaps Akt1 governs similar exocytic processes at synapses. Akt1 signaling has also shown to be essential for AMPA receptor trafficking in hippocampal neurons, further supporting a role for Akt1 in trafficking of synaptic proteins (Lee, 2013).

A striking phenotype of animals with reduced Akt1 function in muscles was a severe reduction in the SSR and disruption of intracellular membrane organization. These phenotypes were similar to those found in a Gtaxin mutant and suggested the possibility that Akt1 and Gtaxin are involved in the same cellular process (Gorczyca, 2007). A number of observations reported in this study indicate Akt1 activity is mediated at least in part by control of Gtaxin. First, Gtaxin levels at the SSR are greatly reduced in animals with reduced Akt1 function in the muscle cells. Second, muscle-directed overexpression of a constitutively active form of Akt1 (Akt1CA) produced ectopic membranous structures; a phenotype also observed with Gtaxin overexpression and elevated levels of Gtaxin. Third, inhibition of Gtaxin blocks the effects of the constitutively active Akt1 in the muscle cell. Gtaxin does contain a consensus site for Akt1 phosphorylation and could therefore be a direct target of Akt1 kinase activity in regulating SNARE complex assembly (Lee, 2013).

The regulatory roles of Akt1 in glutamate receptor composition and postsynaptic membrane expansion could be accomplished through separate or identical downstream effectors. The fact that Gtaxin mutants did not disrupt GluRIIA distribution suggests different downstream effectors regulated by Akt1. The regulation of GluRIIA localization by Akt1 does not involve Gtaxin but could be mediated via Dorsal and Cactus. Dorsal and Cactus influence glutamate receptor delivery and are known effectors of Akt activity in mammalian cells. The levels of both Dorsal and Cactus were reduced in animals with knockdown of Akt1 in the muscle. Notably, in some animals expressing Akt1RNAi in the muscle, Dorsal showed an altered intracellular distribution that overlapped with the mislocalized GluRIIA. However, because Dorsal and Cactus mutants are not reported to mislocalize GluRIIA into intracellular bands, Akt1 is likely to have additional downstream targets that influence GluRIIA localization and delivery to the postsynaptic specialization (Lee, 2013).

Physiological measures of synaptic transmission showed that Akt1 function is required for normal synapse function. Akt1 transheterozygous mutants (Akt11/Akt104226) showed reduced EJP amplitudes and altered decay kinetics of the EJP. These same phenotypes were observed in animals with muscle-specific inhibition of Akt1 function, with the severity correlating to the degree of Akt1 inhibition. These changes in EJP kinetics were not accompanied by alterations of nonvoltage-dependent membrane capacitance or resistance, suggesting that voltage-gated channels contributing to EJP rise and decay times may be affected by Akt1. These findings contrast published work with Akt1 mutant animals describing changes in long-term depression but not in EJP properties (Guo, 2006). However, it is noted that the physiological studies reported in this paper were conducted at a higher Ca2+ concentration, which could account for these different measures of EJP properties in Akt1 mutants. It is important to point out that the physiological changes documented in this study observed in both Akt1 mutant larvae as well as animals with RNA interference of Akt1 in the muscle cell. The physiological changes observed in Akt1 compromised animals are logical consequences of observed changes in NMJ composition. Loss of GluRIIA-containing receptors and an overall decrease in functional GluRs at the synapse could decrease the EJP amplitude. The altered EJP decay pattern in animals with reduced Akt1 is consistent with the involvement of Gtaxin, as has been documented in this study. Gtaxin mutants showed similar changes in EJP decay, indicating that this feature of Akt1 mediated physiological change is associated with the consequences of compromising the function of this t-SNARE (Lee, 2013).

There is a precedent for Akt-mediated regulation of neurotransmitter receptor localization to the cell surface. The NMDA receptor subunit NR2C is developmentally regulated in cerebellar granule cells and Akt-mediated phosphorylation is critical for cell surface expression of NR2C-containing receptors (Chen, 2009). Akt has also proven to be important in the elaboration of dendritic complexity in Drosophila sensory neurons, suggesting that this kinase is of general importance in the control of nervous system receptive fields. Selective control of Akt or its downstream targets could provide a powerful method of influencing synaptic transmission and the receptive properties of neurons (Lee, 2013).

Functions of Gtaxin orthologs in other species

SLY1 and Syntaxin 18 specify a distinct pathway for procollagen VII export from the endoplasmic reticulum

TANGO1 binds and exports Procollagen VII from the endoplasmic reticulum (ER). This study reports a connection between the cytoplasmic domain of TANGO1 and SLY1, a protein that is required for membrane fusion. Knockdown of SLY1 by siRNA arrested Procollagen VII in the ER without affecting the recruitment of COPII components, general protein secretion, and retrograde transport of the KDEL-containing protein BIP, and ERGIC53. SLY1 is known to interact with the ER-specific SNARE proteins Syntaxin 17 and 18, however only Syntaxin 18 was required for Procollagen VII export. Neither SLY1 nor Syntaxin 18 was required for the export of the equally bulky Procollagen I from the ER. Altogether, these findings reveal the sorting of bulky collagen family members by TANGO1 at the ER and highlight the existence of different export pathways for secretory cargoes one of which is mediated by the specific SNARE complex containing SLY1 and Syntaxin 18 (Nogueira, 2014)

Involvement of syntaxin 18, an endoplasmic reticulum (ER)-localized SNARE protein, in ER-mediated phagocytosis

The endoplasmic reticulum (ER) is thought to play an important structural and functional role in phagocytosis. According to this model, direct membrane fusion between the ER and the plasma or phagosomal membrane must precede further invagination, but the exact mechanisms remain elusive. This study investigated whether various ER-localized SNARE proteins are involved in this fusion process. When phagosomes were isolated from murine J774 macrophages, ER-localized SNARE proteins (syntaxin 18, D12, and Sec22b) were found to be significantly enriched in the phagosomes. Fluorescence and immuno-EM analyses confirmed the localization of syntaxin 18 in the phagosomal membranes of J774 cells stably expressing this protein tagged to a GFP variant. To examine whether these SNARE proteins are required for phagocytosis, 293T cells were generated stably expressing the Fc gamma receptor, in which phagocytosis occurs in an IgG-mediated manner. Expression in these cells of dominant-negative mutants of syntaxin 18 or D12 lacking the transmembrane domain, but not a Sec22b mutant, impaired phagocytosis. Syntaxin 18 small interfering RNA (siRNA) selectively decreased the efficiency of phagocytosis, and the rate of phagocytosis was markedly enhanced by stable overexpression of syntaxin 18 in J774 cells. Therefore, it is concluded that syntaxin 18 is involved in ER-mediated phagocytosis, presumably by regulating the specific and direct fusion of the ER and plasma or phagosomal membranes (Hatsuzawa, 2006).


Search PubMed for articles about Drosophila Gtaxin or Syt18

Broadie, K. and Bate, M. (1993). Innervation directs receptor synthesis and localization in Drosophila embryo synaptogenesis. Nature 361(6410): 350-353. PubMed ID: 8426654

Chen, B. S. and Roche, K. W. (2009). Growth factor-dependent trafficking of cerebellar NMDA receptors via protein kinase B/Akt phosphorylation of NR2C. Neuron 62(4): 471-478. PubMed ID: 19477150

Chen, K., Merino, C., Sigrist, S. J. and Featherstone, D. E. (2005). The 4.1 protein coracle mediates subunit-selective anchoring of Drosophila glutamate receptors to the postsynaptic actin cytoskeleton. J Neurosci 25(28): 6667-6675. PubMed ID: 16014728

Gorczyca, D., Ashley, J., Speese, S., Gherbesi, N., Thomas, U., Gundelfinger, E., Gramates, L. S. and Budnik, V. (2007). Postsynaptic membrane addition depends on the Discs-Large-interacting t-SNARE Gtaxin. J Neurosci 27(5): 1033-1044. PubMed ID: 17267557

Guo, H. F. and Zhong, Y. (2006). Requirement of Akt to mediate long-term synaptic depression in Drosophila. J Neurosci 26(15): 4004-4014. PubMed ID: 16611817

Hatsuzawa, K., Hirose, H., Tani, K., Yamamoto, A., Scheller, R. H. and Tagaya, M. (2000). Syntaxin 18, a SNAP receptor that functions in the endoplasmic reticulum, intermediate compartment, and cis-Golgi vesicle trafficking. J Biol Chem 275(18): 13713-13720. PubMed ID: 10788491

Hatsuzawa, K., Tamura, T., Hashimoto, H., Hashimoto, H., Yokoya, S., Miura, M., Nagaya, H. and Wada, I. (2006). Involvement of syntaxin 18, an endoplasmic reticulum (ER)-localized SNARE protein, in ER-mediated phagocytosis. Mol Biol Cell 17(9): 3964-3977. PubMed ID: 16790498

Kano, F., Kondo, H., Yamamoto, A., Kaneko, Y., Uchiyama, K., Hosokawa, N., Nagata, K. and Murata, M. (2005). NSF/SNAPs and p97/p47/VCIP135 are sequentially required for cell cycle-dependent reformation of the ER network. Genes Cells 10(10): 989-999. PubMed ID: 16164599

Lee, H. G., Zhao, N., Campion, B. K., Nguyen, M. M. and Selleck, S. B. (2013). Akt regulates glutamate receptor trafficking and postsynaptic membrane elaboration at the Drosophila neuromuscular junction. Dev Neurobiol 73(10): 723-743. PubMed ID: 23592328

Lewis, M. J. and Pelham, H. R. (1996). SNARE-mediated retrograde traffic from the Golgi complex to the endoplasmic reticulum. Cell 85(2): 205-215. PubMed ID: 8612273

Nogueira, C., Erlmann, P., Villeneuve, J., Santos, A. J., Martinez-Alonso, E., Martinez-Menarguez, J. A. and Malhotra, V. (2014). SLY1 and Syntaxin 18 specify a distinct pathway for procollagen VII export from the endoplasmic reticulum. Elife 3: e02784. PubMed ID: 24842878

Patel, S. K., Indig, F. E., Olivieri, N., Levine, N. D. and Latterich, M. (1998). Organelle membrane fusion: a novel function for the syntaxin homolog Ufe1p in ER membrane fusion. Cell 92(5): 611-620. PubMed ID: 9506516

Rasse, T. M., Fouquet, W., Schmid, A., Kittel, R. J., Mertel, S., Sigrist, C. B., Schmidt, M., Guzman, A., Merino, C., Qin, G., Quentin, C., Madeo, F. F., Heckmann, M. and Sigrist, S. J. (2005). Glutamate receptor dynamics organizing synapse formation in vivo. Nat Neurosci 8(7): 898-905. PubMed ID: 16136672

Biological Overview

date revised: 22 November 2022

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