Synaptotagmin I, a Ca2+ sensor for neurotransmitter release

Synaptotagmins are trafficked to distinct subcellular domains including the postsynaptic compartment

The synaptotagmin family has been implicated in calcium-dependent neurotransmitter release, although Synaptotagmin 1 is the only isoform demonstrated to control synaptic vesicle fusion. Here, we report the characterization of the six remaining synaptotagmin isoforms encoded in the Drosophila genome, including homologues of mammalian Synaptotagmins 4, 7, 12, and 14. Like Synaptotagmin 1, Synaptotagmin 4 is ubiquitously present at synapses, but localizes to the postsynaptic compartment. The remaining isoforms were not found at synapses (Synaptotagmin 7), expressed at very low levels (Synaptotagmins 12 and 14), or in subsets of putative neurosecretory cells (Synaptotagmins α and β). Consistent with their distinct localizations, overexpression of Synaptotagmin 4 or 7 cannot functionally substitute for the loss of Synaptotagmin 1 in synaptic transmission. Our results indicate that synaptotagmins are differentially distributed to unique subcellular compartments. In addition, the identification of a postsynaptic synaptotagmin suggests calcium-dependent membrane-trafficking functions on both sides of the synapse.

Crystallographic identification of Ca2+ and Sr2+ coordination sites in synaptotagmin I C2B domain

Synaptotagmin I has two tandem Ca(2+)-binding C(2) domains, which are essential for fast synchronous synaptic transmission in the central nervous system. We have solved four crystal structures of the C(2)B domain, one of them in the cation-free form at 1.50 A resolution, two in the Ca(2+)-bound form at 1.04 A (two bound Ca(2+) ions) and 1.65 A (three bound Ca(2+) ions) resolution and one in the Sr(2+)-bound form at 1.18 A (one bound Sr(2+) ion) resolution. The side chains of four highly conserved aspartic acids (D303, D309, D363, and D365) and two main chain oxygens (M302:O and Y364:O), together with water molecules, are in direct contact with two bound Ca(2+) ions (sites 1 and 2). At higher Ca(2+) concentrations, the side chain of N333 rotates and cooperates with D309 to generate a third Ca(2+) coordination site (site 3). Divalent cation binding sites 1 and 2 in the C(2)B domain were previously identified from NMR NOE patterns and titration studies, supplemented by site-directed mutation analysis. One difference between the crystal and NMR studies involves D371, which is not involved in coordination with any of the identified Ca(2+) sites in the crystal structures, while it is coordinated to Ca(2+) in site 2 in the NMR structure. In the presence of Sr(2+), which is also capable of triggering exocytosis, but with lower efficiency, only one cation binding site (site 1) was occupied in the crystallographic structure.

Intracellular and viral membrane fusion: a uniting mechanism

Structural and functional analyses have revealed remarkable mechanistic similarities between viral and intracellular fusion. Both fusion processes are driven by an orchestrated cascade of protein binding and folding reactions. After an initial tethering step, activation of the fusion machinery links the opposing membranes and protein folding pulls the membranes in close proximity; fusion pores form, open and dilate, and the process culminates in the complete merging of the lipid bilayers. Viral fusion is mediated by a single fusion protein, whereas the intracellular fusion machinery is split into matching halves, the v- and t-SNAREs. SNAREs, together with synaptotagmins, emerge as the key machinery for regulated exocytosis.

Synaptotagmin IV regulates glial glutamate release

Calcium-binding synaptotagmins (Syts) are membrane proteins that are conserved from nematode to human. Fifteen Syts (Syts I-XV) have been identified in mammalian species. Syt I has been well studied and is a candidate for the Ca(2+)-sensor that triggers evoked exocytosis underlying fast synaptic transmission. Whereas the functions of the other Syts are unclear, Syt IV is of particular interest because it is rapidly up-regulated after chronic depolarization or seizures, and because null mutations exhibit deficits in fine motor coordination and hippocampus-dependent memory. Screening Syts I-XIII, which are enriched in brain, we find that Syt IV is located in processes of astroglia in situ. Reduction of Syt IV in astrocytes by RNA interference decreases Ca(2+)-dependent glutamate release, a gliotransmission pathway that regulates synaptic transmission. Mutants of the C2B domain, the only putative Ca(2+)-binding domain in Syt IV, act in a dominant-negative fashion over Ca(2+)-regulated glial glutamate release, but not gliotransmission induced by changes in osmolarity. Because we find that Syt IV is expressed predominantly by astrocytes and is not in the presynaptic terminals of the hippocampus, and because Syt IV knockout mice exhibit hippocampal-based memory deficits, our data raise the intriguing possibility that Syt IV-mediated gliotransmission contributes to hippocampal-based memory.

Neuronal Ca2+-sensor proteins: multitalented regulators of neuronal function

Many aspects of neuronal activity are regulated by Ca2+ signals. The transduction of temporally and spatially distinct Ca2+ signals requires the action of Ca2+-sensor proteins including various EF-hand-containing Ca2+-binding proteins. The neuronal Ca2+ sensor (NCS) protein family and the related Ca2+-binding proteins (CaBPs) have begun to emerge as key players in neuronal function. Many of these proteins are expressed predominantly or only in neurons, sometimes with cell-specific patterns of expression. Their ability to associate with membranes either constitutively or in response to elevated Ca2+ concentration allows the NCS proteins to discriminate between different spatial and temporal patterns of Ca2+ signals. Recent work has established several physiological roles of these proteins, including diverse actions on gene expression, ion channel function, membrane traffic of ion channels and receptors, and the control of apoptosis.

Presynaptic function

Changing the strength of synapses is key to the adaptive modifications of what neuronal circuits compute. Unsurprisingly, many different mechanisms have evolved to alter synaptic strength. Some of these mechanisms depend on the history of synaptic use, others reflect the activity of modulatory neurons that are controlled through neural computations, and still others involve more global measures of neural activity. The molecular machinery synapses use to convey information from one neuron to the next not only plays an essential part in brain function but also is at the basis of processes that are vital to all cells. Because membrane fusion events at synapses are so precisely controlled, synapses offer an especially favorable system in which to study these basic processes. Here, I review some of the recent progress that has been made in understanding both how synaptic strength is regulated and how fundamental cell biological mechanisms are used to accomplish neuronal intercommunication.

Identification of SNAREs involved in synaptotagmin VII-regulated lysosomal exocytosis

Ca2+-regulated exocytosis of lysosomes has been recognized recently as a ubiquitous process, important for the repair of plasma membrane wounds. Lysosomal exocytosis is regulated by synaptotagmin VII, a member of the synaptotagmin family of Ca2+-binding proteins localized on lysosomes. Here we show that Ca2+-dependent interaction of the synaptotagmin VII C(2)A domain with SNAP-23 is facilitated by syntaxin 4. Specific interactions also occurred in cell lysates between the plasma membrane t-SNAREs SNAP-23 and syntaxin 4 and the lysosomal v-SNARE TI-VAMP/VAMP7. Following cytosolic Ca2+ elevation, SDS-resistant complexes containing SNAP-23, syntaxin 4, and TI-VAMP/VAMP7 were detected on membrane fractions. Lysosomal exocytosis was inhibited by the SNARE domains of syntaxin 4 and TI-VAMP/VAMP7 and by cleavage of SNAP-23 with botulinum neurotoxin E, thereby functionally implicating these SNAREs in Ca2+-regulated exocytosis of conventional lysosomes.

Synaptotagmin Interaction with the Syntaxin/SNAP-25 Dimer Is Mediated by an Evolutionarily Conserved Motif and Is Sensitive to Inositol Hexakisphosphate

Synaptotagmins are membrane proteins that possess tandem C2 domains and play an important role in regulated membrane fusion in metazoan organisms. Here we show that both synaptotagmins I and II, the two major neuronal isoforms, can interact with the syntaxin/synaptosomal-associated protein of 25 kDa (SNAP-25) dimer, the immediate precursor of the soluble NSF attachment protein receptor (SNARE) fusion complex. A stretch of basic amino acids highly conserved throughout the animal kingdom is responsible for this calcium-independent interaction. Inositol hexakisphosphate modulates synaptotagmin coupling to the syntaxin/SNAP-25 dimer, which is mirrored by changes in chromaffin cell exocytosis. Our results shed new light on the functional importance of the conserved polybasic synaptotagmin motif, suggesting that synaptotagmin interacts with the t-SNARE dimer to up-regulate the probability of SNARE-mediated membrane fusion.

Local calcium signaling in neurons

Transient rises in the cytoplasmic concentration of calcium ions serve as second messenger signals that control many neuronal functions. Selective triggering of these functions is achieved through spatial localization of calcium signals. Several qualitatively different forms of local calcium signaling can be distinguished by the location of open calcium channels as well as by the distance between these channels and the calcium binding proteins that serve as the molecular targets of calcium action. Local calcium signaling is especially prominent at presynaptic active zones and postsynaptic densities, structures that are distinguished by highly organized macromolecular arrays that yield precise spatial arrangements of calcium signaling proteins. Similar forms of local calcium signaling may be employed throughout the nervous system, though much remains to be learned about the molecular underpinnings of these events.

High affinity interaction of syntaxin and SNAP-25 on the plasma membrane is abolished by botulinum toxin E

The release of hormones and neurotransmitters requires the fusion of cargo-containing vesicles with the plasma membrane. This process of exocytosis relies on three SNARE proteins, namely syntaxin and SNAP-25 on the target plasma membrane and synaptobrevin on the vesicular membrane. In this study we examined the molecular assembly pathway that leads to formation of the fusogenic SNARE complex. We now show that the plasma membrane syntaxin and SNAP-25 interact with high affinity and equimolar stoichiometry to form a stable dimer on the pathway to the ternary SNARE complex. In bovine chromaffin cells, syntaxin and SNAP-25 colocalize in defined clusters that average 700 nm in diameter and cover 10% of the plasma membrane. Removal of the C terminus of SNAP-25 by botulinum neurotoxin E, a known neuroparalytic agent, dissociates the target SNARE dimer in vitro and disrupts the SNARE clustering in vivo. Together, our data uncover formation of stable syntaxin/SNAP-25 dimers as a central principle of the SNARE assembly pathway underlying regulated exocytosis.