Nerve-evoked synchronous release and high K+ -induced quantal events are regulated separately by synaptotagmin I at Drosophila neuromuscular junctions

Characterization of the activity of β-galactosidase from Escherichia coli and Drosophila melanogaster in fixed and non-fixed Drosophila tissues

β-Galactosidase encoded by the Escherichia coli lacZ gene, is widely used as a reporter molecule in molecular biology in a wide variety of animals. β-Galactosidase retains its enzymatic activity in cells or tissues even after fixation and can degrade X-Gal, a frequently used colormetric substrate, producing a blue color. Therefore, it can be used for the activity staining of fixed tissues. However, the enzymatic activity of the β-galactosidase that is ectopically expressed in the non-fixed tissues of animals has not been extensively studied. Here, we report the characterization of β-galactosidase activity in Drosophila tissues with and without fixation in various experimental conditions comparing the activity of two evolutionarily orthologous β-galactosidases derived from the E. coli lacZ and Drosophila melanogaster DmelGal genes. We performed quantitative analysis of the activity staining of larval imaginal discs and an in vitro assay using larval lysates. Our data showed that both E. coli and Drosophila β-galactosidase can be used for cell-type-specific activity staining, but they have their own preferences in regard to conditions. E. coli β-galactosidase showed a preference for neutral pH but not for acidic pH compared with Drosophila β-galactosidase. Our data suggested that both E. coli and Drosophila β-galactosidase show enzymatic activity in the physiological conditions of living animals when they are ectopically expressed in a desired specific spatial and temporal pattern. This may enable their future application to studies of chemical biology using model animals.

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We created a transgenic fly to express Drosophila endogenous β-galactosidase.

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We compared the properties of β-galactosidase molecules from Escherichia coli and Drosophila.

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Both β-galactosidase molecules were active in both fixed and non-fixed tissues.

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E. coli β-galactosidase showed a preference for neutral pH but not for acidic pH.

Molecular underpinnings of synaptic vesicle pool heterogeneity

Neuronal communication relies on chemical synaptic transmission for information transfer and processing. Chemical neurotransmission is initiated by synaptic vesicle fusion with the presynaptic active zone resulting in release of neurotransmitters. Classical models have assumed that all synaptic vesicles within a synapse have the same potential to fuse under different functional contexts. In this model, functional differences among synaptic vesicle populations are ascribed to their spatial distribution in the synapse with respect to the active zone. Emerging evidence suggests, however, that synaptic vesicles are not a homogenous population of organelles, and they possess intrinsic molecular differences and differential interaction partners. Recent studies have reported a diverse array of synaptic molecules that selectively regulate synaptic vesicles' ability to fuse synchronously and asynchronously in response to action potentials or spontaneously irrespective of action potentials. Here we discuss these molecular mediators of vesicle pool heterogeneity that are found on the synaptic vesicle membrane, on the presynaptic plasma membrane, or within the cytosol and consider some of the functional consequences of this diversity. This emerging molecular framework presents novel avenues to probe synaptic function and uncover how synaptic vesicle pools impact neuronal signaling.

Ca2+ regulates the Drosophila Stoned-A and Stoned-B proteins interaction with the C2B domain of Synaptotagmin-1

The dicistronic Drosophila stoned gene is involved in exocytosis and/or endocytosis of synaptic vesicles. Mutations in either stonedA or stonedB cause a severe disruption of neurotransmission in fruit flies. Previous studies have shown that the coiled-coil domain of the Stoned-A and the µ-homology domain of the Stoned-B protein can interact with the C2B domain of Synaptotagmin-1. However, very little is known about the mechanism of interaction between the Stoned proteins and the C2B domain of Synaptotagmin-1. Here we report that these interactions are increased in the presence of Ca(2+). The Ca(2+)-dependent interaction between the µ-homology domain of Stoned-B and C2B domain of Synaptotagmin-1 is affected by phospholipids. The C-terminal region of the C2B domain, including the tryptophan-containing motif, and the Ca(2+) binding loop region that modulate the Ca(2+)-dependent oligomerization, regulates the binding of the Stoned-A and Stoned-B proteins to the C2B domain. Stoned-B, but not Stoned-A, interacts with the Ca(2+)-binding loop region of C2B domain. The results indicate that Ca(2+)-induced self-association of the C2B domain regulates the binding of both Stoned-A and Stoned-B proteins to Synaptotagmin-1. The Stoned proteins may regulate sustainable neurotransmission in vivo by binding to Ca(2+)-bound Synaptotagmin-1 associated synaptic vesicles.

Calcium binding by synaptotagmin's C2A domain is an essential element of the electrostatic switch that triggers synchronous synaptic transmission

Synaptotagmin is the major calcium sensor for fast synaptic transmission that requires the synchronous fusion of synaptic vesicles. Synaptotagmin contains two calcium-binding domains: C2A and C2B. Mutation of a positively charged residue (R233Q in rat) showed that Ca2+-dependent interactions between the C2A domain and membranes play a role in the electrostatic switch that initiates fusion. Surprisingly, aspartate-to-asparagine mutations in C2A that inhibit Ca2+ binding support efficient synaptic transmission, suggesting that Ca2+ binding by C2A is not required for triggering synchronous fusion. Based on a structural analysis, we generated a novel mutation of a single Ca2+-binding residue in C2A (D229E in Drosophila) that inhibited Ca2+ binding but maintained the negative charge of the pocket. This C2A aspartate-to-glutamate mutation resulted in ∼80% decrease in synchronous transmitter release and a decrease in the apparent Ca2+ affinity of release. Previous aspartate-to-asparagine mutations in C2A partially mimicked Ca2+ binding by decreasing the negative charge of the pocket. We now show that the major function of Ca2+ binding to C2A is to neutralize the negative charge of the pocket, thereby unleashing the fusion-stimulating activity of synaptotagmin. Our results demonstrate that Ca2+ binding by C2A is a critical component of the electrostatic switch that triggers synchronous fusion. Thus, Ca2+ binding by C2B is necessary and sufficient to regulate the precise timing required for coupling vesicle fusion to Ca2+ influx, but Ca2+ binding by both C2 domains is required to flip the electrostatic switch that triggers efficient synchronous synaptic transmission.

Delayed synaptic transmission in Drosophila cacophonynull embryos

Ca(2+) influx through the Drosophila N-type Ca(2+) channel, encoded by cacophony (cac), triggers fast synaptic transmission. We now ask whether the cac Ca(2+) channel is the Ca(2+) channel solely dedicated for fast synaptic transmission. Because the cac(null) mutation is lethal, we used cac(null) embryos to address this question. At the neuromuscular junction in HL3 solution, no fast synchronous synaptic transmission was detected on nerve stimulation. When the wild-type cac gene was introduced in the cac(null) background, fast synaptic transmission recovered. However, even in cac(null) embryos, nerve stimulation infrequently induced delayed synaptic events in the minority of cells in 1.5 mM [Ca(2+)](e) and in the majority of cells in 5 mM [Ca(2+)](e). The number of delayed quantal events per stimulus was greater in 5 mM [Ca(2+)](e) than in 1.5 mM. Thus the delayed release is [Ca(2+)](e) dependent. Plectreurys toxin II (PLTXII) (10 nM; a spider toxin analog) depressed the frequency of delayed events, suggesting that voltage-gated Ca(2+) channels, other than cac Ca(2+) channels, are contributing to them. However, delayed events were not affected by 50 microM La(3+). The frequency of miniature synaptic currents in cac(null) embryos was approximately 1/2 of control, whereas in high K(+) solutions, it was approximately 1/135. The hypertonicity response was approximately 1/10 of control. These findings indicate that the number of release-ready vesicles is smaller in cac(null) embryos. Taken together, the cac Ca(2+) channel is indispensable for fast synaptic transmission in normal conditions, and another type of Ca(2+) channel, the non-cac, PLTXII-sensitive Ca(2+) channel, is contributing to delayed release in cac(null) embryos.

How does synaptotagmin trigger neurotransmitter release?

Neurotransmitter release at synapses involves a highly specialized form of membrane fusion that is triggered by Ca(2+) ions and is optimized for speed. These observations were established decades ago, but only recently have the molecular mechanisms that underlie this process begun to come into view. Here, we summarize findings obtained from genetically modified neurons and neuroendocrine cells, as well as from reconstituted systems, which are beginning to reveal the molecular mechanism by which Ca(2+)-acting on the synaptic vesicle (SV) protein synaptotagmin I (syt)-triggers rapid exocytosis. This work sheds light not only on presynaptic aspects of synaptic transmission, but also on the fundamental problem of membrane fusion, which has remained a puzzle that has yet to be solved in any biological system.

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

Synaptotagmin I is the Ca(2+) sensor for fast, synchronous release of neurotransmitter; however, the molecular interactions that couple Ca(2+) binding to membrane fusion remain unclear. The structure of synaptotagmin is dominated by two C(2) domains that interact with negatively charged membranes after binding Ca(2+). In vitro work has implicated a conserved basic residue at the tip of loop 3 of the Ca(2+)-binding pocket in both C(2) domains in coordinating this electrostatic interaction with anionic membranes. Although results from cultured cells suggest that the basic residue of the C(2)A domain is functionally significant, such studies provide contradictory results regarding the importance of the C(2)B basic residue during vesicle fusion. To directly test the functional significance of each of these residues at an intact synapse in vivo, we neutralized either the C(2)A or the C(2)B basic residue and assessed synaptic transmission at the Drosophila neuromuscular junction. The conserved basic residues at the tip of the Ca(2+)-binding pocket of both the C(2)A and C(2)B domains mediate Ca(2+)-dependent interactions with anionic membranes and are required for efficient evoked transmitter release. Our 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 C(2)A and C(2)B Ca(2+)-binding pockets, are critical for coupling Ca(2+) influx with vesicle fusion during synaptic transmission in vivo. Our model for synaptotagmin's direct role in coupling Ca(2+) binding to vesicle fusion incorporates this finding with results from multiple in vitro and in vivo studies.