Syntaxin modulation of calcium channels in cortical synaptosomes as revealed by botulinum toxin C1

Neurons require glucose uptake and glycolysis in vivo

Neurons require large amounts of energy, but whether they can perform glycolysis or require glycolysis to maintain energy remains unclear. Using metabolomics, we show that human neurons do metabolize glucose through glycolysis and can rely on glycolysis to supply tricarboxylic acid (TCA) cycle metabolites. To investigate the requirement for glycolysis, we generated mice with postnatal deletion of either the dominant neuronal glucose transporter (GLUT3cKO) or the neuronal-enriched pyruvate kinase isoform (PKM1cKO) in CA1 and other hippocampal neurons. GLUT3cKO and PKM1cKO mice show age-dependent learning and memory deficits. Hyperpolarized magnetic resonance spectroscopic (MRS) imaging shows that female PKM1cKO mice have increased pyruvate-to-lactate conversion, whereas female GLUT3cKO mice have decreased conversion, body weight, and brain volume. GLUT3KO neurons also have decreased cytosolic glucose and ATP at nerve terminals, with spatial genomics and metabolomics revealing compensatory changes in mitochondrial bioenergetics and galactose metabolism. Therefore, neurons metabolize glucose through glycolysis in vivo and require glycolysis for normal function.

Regulation of Ca2+ channels by SNAP-25 via recruitment of syntaxin-1 from plasma membrane clusters

SNAP-25 regulates Ca²⁺ channels in an unknown manner. Endogenous and exogenous SNAP-25 inhibit Ca²⁺ currents indirectly by recruiting syntaxin-1 from clusters on the plasma membrane, thereby making it available for Ca²⁺ current inhibition. Thus the cell can regulate Ca²⁺ influx by expanding or contracting syntaxin-1 clusters.

SNAP-25 regulates Ca²⁺ channels, with potentially important consequences for diseases involving an aberrant SNAP-25 expression level. How this regulation is executed mechanistically remains unknown. We investigated this question in mouse adrenal chromaffin cells and found that SNAP-25 inhibits Ca²⁺ currents, with the B-isoform being more potent than the A-isoform, but not when syntaxin-1 is cleaved by botulinum neurotoxin C. In contrast, syntaxin-1 inhibits Ca²⁺ currents independently of SNAP-25. Further experiments using immunostaining showed that endogenous or exogenous SNAP-25 expression recruits syntaxin-1 from clusters on the plasma membrane, thereby increasing the immunoavailability of syntaxin-1 and leading indirectly to Ca²⁺ current inhibition. Expression of Munc18-1, which recruits syntaxin-1 within the exocytotic pathway, does not modulate Ca²⁺ channels, whereas overexpression of the syntaxin-binding protein Doc2B or ubMunc13-2 increases syntaxin-1 immunoavailability and concomitantly down-regulates Ca²⁺ currents. Similar findings were obtained upon chemical cholesterol depletion, leading directly to syntaxin-1 cluster dispersal and Ca²⁺ current inhibition. We conclude that clustering of syntaxin-1 allows the cell to maintain a high syntaxin-1 expression level without compromising Ca²⁺ influx, and recruitment of syntaxin-1 from clusters by SNAP-25 expression makes it available for regulating Ca²⁺ channels. This mechanism potentially allows the cell to regulate Ca²⁺ influx by expanding or contracting syntaxin-1 clusters.

Fast, Ca2+-dependent exocytosis at nerve terminals: shortcomings of SNARE-based models

Investigations over the last two decades have made major inroads in clarifying the cellular and molecular events that underlie the fast, synchronous release of neurotransmitter at nerve endings. Thus, appreciable progress has been made in establishing the structural features and biophysical properties of the calcium (Ca2+) channels that mediate the entry into nerve endings of the Ca2+ ions that trigger neurotransmitter release. It is now clear that presynaptic Ca2+ channels are regulated at many levels and the interplay of these regulatory mechanisms is just beginning to be understood. At the same time, many lines of research have converged on the conclusion that members of the synaptotagmin family serve as the primary Ca2+ sensors for the action potential-dependent release of neurotransmitter. This identification of synaptotagmins as the proteins which bind Ca2+ and initiate the exocytotic fusion of synaptic vesicles with the plasma membrane has spurred widespread efforts to reveal molecular details of synaptotagmin's action. Currently, most models propose that synaptotagmin interfaces directly or indirectly with SNARE (soluble, N-ethylmaleimide sensitive factor attachment receptors) proteins to trigger membrane fusion. However, in spite of intensive efforts, the field has not achieved consensus on the mechanism by which synaptotagmins act. Concurrently, the precise sequence of steps underlying SNARE-dependent membrane fusion remains controversial. This review considers the pros and cons of the different models of SNARE-mediated membrane fusion and concludes by discussing a novel proposal in which synaptotagmins might directly elicit membrane fusion without the intervention of SNARE proteins in this final fusion step.

Tracking Ca2+-dependent and Ca2+-independent conformational transitions in syntaxin 1A during exocytosis in neuroendocrine cells

A key issue for understanding exocytosis is elucidating the various protein interactions and the associated conformational transitions underlying soluble N-ethylmeleimide-sensitive factor attachment protein receptor (SNARE) protein assembly. To monitor dynamic changes in syntaxin 1A (Syx) conformation along exocytosis, we constructed a novel fluorescent Syx-based probe that can be efficiently incorporated within endogenous SNARE complexes, support exocytosis, and report shifts in Syx between 'closed' and 'open' conformations by fluorescence resonance energy transfer analysis. Using this probe we resolve two distinct Syx conformational transitions during membrane depolarization-induced exocytosis in PC12 cells: a partial 'opening' in the absence of Ca(2+) entry and an additional 'opening' upon Ca(2+) entry. The Ca(2+)-dependent transition is abolished upon neutralization of the basic charges in the juxtamembrane regions of Syx, which also impairs exocytosis. These novel findings provide evidence of two conformational transitions in Syx during exocytosis, which have not been reported before: one transition directly induced by depolarization and an additional transition that involves the juxtamembrane region of Syx. The superior sensitivity of our probe also enabled detection of subtle Syx conformational changes upon interaction with VAMP2, which were absolutely dependent on the basic charges of the juxtamembrane region. Hence, our results further suggest that the Ca(2+)-dependent transition in Syx involves zippering between the membrane-proximal juxtamembrane regions of Syx and VAMP2 and support the recently implied existence of this zippering in the final phase of SNARE assembly to catalyze exocytosis.

Regulation of Ca(V)2 calcium channels by G protein coupled receptors

Voltage gated calcium channels (Ca²⁺ channels) are key mediators of depolarization induced calcium influx into excitable cells, and thereby play pivotal roles in a wide array of physiological responses. This review focuses on the inhibition of Ca(V)2 (N- and P/Q-type) Ca²⁺-channels by G protein coupled receptors (GPCRs), which exerts important autocrine/paracrine control over synaptic transmission and neuroendocrine secretion. Voltage-dependent inhibition is the most widespread mechanism, and involves direct binding of the G protein βγ dimer (Gβγ) to the α1 subunit of Ca(V)2 channels. GPCRs can also recruit several other distinct mechanisms including phosphorylation, lipid signaling pathways, and channel trafficking that result in voltage-independent inhibition. Current knowledge of Gβγ-mediated inhibition is reviewed, including the molecular interactions involved, determinants of voltage-dependence, and crosstalk with other cell signaling pathways. A summary of recent developments in understanding the voltage-independent mechanisms prominent in sympathetic and sensory neurons is also included. This article is part of a Special Issue entitled: Calcium channels.

Involvement of the glutamatergic system in the nociception induced intrathecally for a TRPA1 agonist in rats

The transient receptor potential ankyrin 1 (TRPA1) is expressed in peripheral and spinal terminals of sensory neurons, jointly to the vanilloid receptor (TRPV1). A relevant peripheral role of TRPA1 receptor has been implicated in a variety of processes, including the detection of noxious cold, and diverse painful stimulus, but the functional role of TRPA1 receptor in nociceptive transmission at spinal cord in vivo is poorly known. Therefore, the aim of this study was to evaluate whether the glutamatergic system is involved in the transmission of nociceptive stimulus induced for a TRPA1 agonist in the rat spinal cord. We observed that cinnamaldehyde, a TRPA1 agonist, on spinal cord synaptosomes leads to an increase in [Ca(2+)](i) and a rapid release of glutamate, but was not able to change the specific [(3)H]-glutamate binding. In addition, spinally administered cinnamaldehyde produced heat hyperalgesia and mechanical allodynia in rats. This behavior was reduced by the co-injection (i.t.) of camphor (TRPA1 antagonist) or MK-801 (N-methyl-D-aspartate (NMDA) receptor antagonist) to cinnamaldehyde. Besides, the pretreatment with resiniferatoxin (RTX), a potent TRPV1 agonist, abolished the cinnamaldehyde-induced heat hyperalgesia. Here, we showed that intrathecal RTX results in a decrease in TRPA1 and TRPV1 immunoreactivity in dorsal root ganglion. Collectively, our results demonstrate the pertinent participation of spinal TRPA1 in the possible enhancement of glutamatergic transmission of nociceptive signals leading to increase of the hypersensitivity, here observed as heat hyperalgesia. Then the modulation of spinal TRPA1 might be a valuable target in painful conditions associated with central pain hypersensitivity.

Regulation of voltage-gated calcium channels by synaptic proteins

Calcium entry through neuronal voltage-gated calcium channels into presynaptic nerve terminal is a key step in synaptic exocytosis. In order to receive the calcium signal and trigger fast, efficient and spatially delimited neurotransmitter release, the vesicle-docking/release machinery must be located near the calcium source. In many cases, this close localization is achieved by a direct interaction of several members of the vesicle release machinery with the calcium channels. In turn, the binding of synaptic proteins to presynaptic calcium channels modulates channel activity to provide fine control over calcium entry, and thus modulates synaptic strength. In this chapter we summarize our present knowledge of the molecular mechanisms by which synaptic proteins regulate presynaptic calcium channel activity.

Syntaxin requirement for Ca2+-triggered exocytosis in neurons and endocrine cells demonstrated with an engineered neurotoxin

Botulinum neurotoxins cleave synaptic SNAREs and block exocytosis, demonstrating that these proteins function in neurosecretion. However, the function of the SNARE syntaxin remains less clear because no neurotoxin cleaves it selectively. Starting with a botulinum neurotoxin that cleaves both syntaxin and SNAP-25, we engineered a version that retains activity against syntaxin but spares SNAP-25. These mutants block synaptic release in neurons and norepinephrine release in neuroendocrine cells, thus establishing an essential role for syntaxin in Ca2+-triggered exocytosis. These mutants can generate syntaxin-free cells as a useful experimental system for research and may lead to pharmaceuticals that target syntaxin selectively.

Bacterial toxins and the nervous system: neurotoxins and multipotential toxins interacting with neuronal cells

Toxins are potent molecules used by various bacteria to interact with a host organism. Some of them specifically act on neuronal cells (clostridial neurotoxins) leading to characteristics neurological affections. But many other toxins are multifunctional and recognize a wider range of cell types including neuronal cells. Various enterotoxins interact with the enteric nervous system, for example by stimulating afferent neurons or inducing neurotransmitter release from enterochromaffin cells which result either in vomiting, in amplification of the diarrhea, or in intestinal inflammation process. Other toxins can pass the blood brain barrier and directly act on specific neurons.

Signaling role of the voltage-gated calcium channel as the molecular on/off-switch of secretion

Voltage-gated calcium channels (VGCC) are involved in a large variety of cellular Ca(2+) signaling processes, including exocytosis, a Ca(2+) dependent release of neurotransmitters and hormones. Great progress has been made in understanding the mode of action of VGCC in exocytosis, a process distinguished by two sequential yet independent Ca(2+) binding reactions. First, Ca(2+) binds at the selectivity filter, the EEEE motif of the VGCC, and second, subsequent to a brief and intense Ca(2+) inflow to synaptotagmin, a vesicular protein. Inquiry into the functional and physical interactions of the channels with synaptic proteins has demonstrated that exocytosis is triggered during the initial Ca(2+) binding at the channel pore, prior to Ca(2+) entry. Accordingly, a cycle of secretion begins by an incoming stimulus that releases vesicles from a releasable pool upon Ca(2+) binding at the pore, and at the same time, the transient increase in [Ca(2+)](i) primes a fresh set of non-releasable vesicles, to be fused by the next incoming stimulus. We propose a model, in which the Ca(2+) binding at the EEEE motif and the consequent conformational changes in the channel are the primary event in triggering secretion, while synaptotagmin acts as a vesicle docking protein. Thus, the channel serves as the molecular On/Off signaling switch, where the predominance of a conformational change in Ca(2+)-bound channel provides for the fast secretory process.

A mutation in the first intracellular loop of CACNA1A prevents P/Q channel modulation by SNARE proteins and lowers exocytosis

Familial hemiplegic migraine (FHM)-causing mutations in the gene encoding the P/Q Ca(2+) channel alpha(1A) subunit (CACNA1A) locate to the pore and voltage sensor regions and normally involve gain-of-channel function. We now report on a mutation identified in the first intracellular loop of CACNA1A (alpha(1A(A454T))) that does not cause FHM but is associated with the absence of sensorimotor symptoms in a migraine with aura pedigree. Alpha(1A(A454T)) channels showed weakened regulation of voltage-dependent steady-state inactivation by Ca(V)beta subunits. More interestingly, A454T mutation suppressed P/Q channel modulation by syntaxin 1A or SNAP-25 and decreased exocytosis. Our findings reveal the importance of I-II loop structural integrity in the functional interaction between P/Q channel and proteins of the vesicle-docking/fusion machinery, and that genetic variation in CACNA1A may be not only a cause but also a modifier of migraine phenotype.

Syntaxin modulates Kv1.1 through dual action on channel surface expression and conductance

The Kv1.1 channel that is expressed throughout the central and peripheral nervous system is known to interact with syntaxin 1A, a member of the exocytosis machinery protein complex. This interaction was previously shown to increase the macroscopic currents of the presynaptic Kv1.1 channel when coexpressed in Xenopus oocytes, while it decreased the unitary channel conductance and open probability. This apparent discrepancy has been resolved in this work, using electrophysiological, biochemical, and immunohistochemical analyses in oocytes by overexpression and antisense knockdown of syntaxin. Here, we demonstrate that syntaxin plays a dual role in the modulation of Kv1.1 function: enhancement of the channel's surface expression along with attenuation of single channel ion flux. These findings broaden the scope of channels and transporters that are dually modulated by syntaxin. Although the dual functioning of syntaxin in modulation of Kv1.1 channel activity may seem antagonistic, the combination of the two mechanisms may provide a useful means for fine-tuning axonal excitability and synaptic efficacy.

Direct interaction of endogenous Kv channels with syntaxin enhances exocytosis by neuroendocrine cells

K⁺ efflux through voltage-gated K⁺ (Kv) channels can attenuate the release of neurotransmitters, neuropeptides and hormones by hyperpolarizing the membrane potential and attenuating Ca²⁺ influx. Notably, direct interaction between Kv2.1 channels overexpressed in PC12 cells and syntaxin has recently been shown to facilitate dense core vesicle (DCV)-mediated release. Here, we focus on endogenous Kv2.1 channels and show that disruption of their interaction with native syntaxin after ATP-dependent priming of the vesicles by Kv2.1 syntaxin–binding peptides inhibits Ca²⁺ -triggered exocytosis of DCVs from cracked PC12 cells in a specific and dose-dependent manner. The inhibition cannot simply be explained by the impairment of the interaction of syntaxin with its SNARE cognates. Thus, direct association between endogenous Kv2.1 and syntaxin enhances exocytosis and in combination with the Kv2.1 inhibitory effect to hyperpolarize the membrane potential, could contribute to the known activity dependence of DCV release in neuroendocrine cells and in dendrites where Kv2.1 commonly expresses and influences release.

Gamma-band deficiency and abnormal thalamocortical activity in P/Q-type channel mutant mice

Thalamocortical in vivo and in vitro function was studied in mice lacking P/Q-type calcium channels (Cav2.1), in which N-type calcium channels (Cav2.2) supported central synaptic transmission. Unexpectedly, in vitro patch recordings from thalamic neurons demonstrated no gamma-band subthreshold oscillation, and voltage-sensitive dye imaging demonstrated an absence of cortical gamma-band-dependent columnar activation involving cortical inhibitory interneuron activity. In vivo electroencephalogram recordings showed persistent absence status and a dramatic reduction of gamma-band activity. Pharmacological block of T-type calcium channels (Cav3), although not noticeably affecting normal control animals, left the knockout mice in a coma-like state. Hence, although N-type calcium channels can rescue P/Q-dependent synaptic transmission, P/Q calcium channels are essential in the generation of gamma-band activity and resultant cognitive function.

A constitutive, transient receptor potential-like Ca2+ influx pathway in presynaptic nerve endings independent of voltage-gated Ca2+ channels and Na+/Ca2+ exchange

Calcium levels in the presynaptic nerve terminal are altered by several pathways, including voltage-gated Ca(2+) channels, the Na(+)/Ca(2+) exchanger, Ca(2+)-ATPase, and the mitochondria. The influx pathway for homeostatic control of [Ca(2+)](i) in the nerve terminal has been unclear. One approach to detecting the pathway that maintains internal Ca(2+) is to test for activation of Ca(2+) influx following Ca(2+) depletion. Here, we demonstrate that a constitutive influx pathway for Ca(2+) exists in presynaptic terminals to maintain internal Ca(2+) independent of voltage-gated Ca(2+) channels and Na(+)/Ca(2+) exchange, as measured in intact isolated nerve endings from mouse cortex and in intact varicosities in a neuronal cell line using fluorescence spectroscopy and confocal imaging. The Mg(2+) and lanthanide sensitivity of the influx pathway, in addition to its pharmacological and short hairpin RNA sensitivity, and the results of immunostaining for transient receptor potential (TRP) channels indicate the involvement of TRPC channels, possibly TRPC5 and TRPC1. This constitutive Ca(2+) influx pathway likely serves to maintain synaptic function under widely varying levels of synaptic activity.

Direct G protein modulation of Cav2 calcium channels

The regulation of presynaptic, voltage-gated calcium channels by activation of heptahelical G protein-coupled receptors exerts a crucial influence on presynaptic calcium entry and hence on neurotransmitter release. Receptor activation subjects presynaptic N- and P/Q-type calcium channels to a rapid, membrane-delimited inhibition-mediated by direct, voltage-dependent interactions between G protein betagamma subunits and the channels-and to a slower, voltage-independent modulation involving soluble second messenger molecules. In turn, the direct inhibition of the channels is regulated as a function of many factors, including channel subtype, ancillary calcium channel subunits, and the types of G proteins and G protein regulatory factors involved. Twenty-five years after this mode of physiological regulation was first described, we review the investigations that have led to our current understanding of its molecular mechanisms.

Presynaptic Ca2+ channels--integration centers for neuronal signaling pathways

Calcium influx into presynaptic nerve terminals via voltage-gated Ca2+ channels is an essential step in neurotransmitter release. The predominant Ca2+ channel species in synaptic nerve terminals are P/Q-type and N-type channels, with their relative levels of expression varying across the nervous system. The different distributions of these two channel subtypes are reflected in their distinct physiological and pathological roles, yet their activity is regulated by common mechanisms and both function as part of larger signaling complexes that enable their precise regulation and subcellular targeting. Here, we provide a broad overview of molecular and cellular mechanisms that regulate Ca2+ channels, and how these cellular signaling pathways are integrated at the level of the channel protein.

Recordings from single neocortical nerve terminals reveal a nonselective cation channel activated by decreases in extracellular calcium

Synaptic activity causes reductions in cleft [Ca(2+)] that may impact subsequent synaptic efficacy. Using modified patch-clamp techniques to record from single neocortical nerve terminals, we report that physiologically relevant reductions of extracellular [Ca(2+)] ([Ca(2+)](o)) activate voltage-dependent outward currents. These outward currents are carried by a novel nonselective cation (NSC) channel that is indirectly inhibited by various extracellular agents (rank order potency, Gd(3+) > spermidine > Ca(2+) > Mg(2+), typical for [Ca(2+)](o) receptors). The identification of a Ca(2+) sensor-NSC channel pathway establishes the existence of a mechanism by which presynaptic terminals can detect and respond to reductions in cleft [Ca(2+)]. Activation of NSC channels by falls in [Ca(2+)](o) would be expected during periods of high activity in the neocortex and may modulate the excitability of the presynaptic terminal.

Masters or slaves? Vesicle release machinery and the regulation of presynaptic calcium channels

Calcium entry through presynaptic voltage-gated calcium channels is essential for neurotransmitter release. The two major types of presynaptic calcium channels contain a synaptic protein interaction site that physically interacts with synaptic vesicle release proteins. This is thought to tighten the coupling between the sources of calcium entry and the neurotransmitter release machinery. Conversely, the binding of synaptic proteins to presynaptic calcium channels regulates calcium channel activity. Hence, presynaptic calcium channels act not only as the masters of the synaptic release process, but also as key targets for feedback inhibition.

Presynaptic ‘Cav2.3-containing’ E-type Ca2+channels share dual roles during neurotransmitter release

Ca2+ influx into excitable cells is a prerequisite for neurotransmitter release and regulated exocytosis. Within the group of ten cloned voltage-gated Ca2+ channels, the Ca(v)2.3-containing E-type Ca2+ channels are involved in various physiological processes, such as neurotransmitter release and exocytosis together with other voltage-gated Ca2+ channels of the Ca(v)1, Ca(v)2 and Ca(v)3 subfamily. However, E-type Ca2+ channels also exhibit several subunit-specific features, most of which still remain poorly understood. Ca(v)2.3-containing R-type channels (here called 'E-type channels') are also located in presynaptic terminals and interact with some synaptic vesicle proteins, the so-called SNARE proteins, although lacking the classical synprint interaction site. E-type channels trigger exocytosis and are also involved in long-term potentiation. Recently, it was shown that the interaction of Ca(v)2.3 with the EF-hand motif containing protein EFHC1 is involved in the aetiology and pathogenesis of juvenile myoclonic epilepsy.

Aspartate release from rat hippocampal synaptosomes

Certain excitatory pathways in the rat hippocampus can release aspartate along with glutamate. This study utilized rat hippocampal synaptosomes to characterize the mechanism of aspartate release and to compare it with glutamate release. Releases of aspartate and glutamate from the same tissue samples were quantitated simultaneously. Both amino acids were released by 25 mM K(+), 300 microM 4-aminopyridine (4-AP) and 0.5 and 1 microM ionomycin in a predominantly Ca(2+)-dependent manner. For a roughly equivalent quantity of glutamate released, aspartate release was significantly greater during exposure to elevated [K(+)] than to 4-AP and during exposure to 0.5 than to 1 microM ionomycin. Aspartate release was inefficiently coupled to P/Q-type voltage-dependent Ca(2+) channels and was reduced by KB-R7943, an inhibitor of reversed Na(+)/Ca(2+) exchange. In contrast, glutamate release depended primarily on Ca(2+) influx through P/Q-type channels and was not significantly affected by KB-R7943. Pretreatment of the synaptosomes with tetanus toxin and botulinum neurotoxins C and F reduced glutamate release, but not aspartate release. Aspartate release was also resistant to bafilomycin A(1), an inhibitor of vacuolar H(+)-ATPase, whereas glutamate release was markedly reduced. (+/-) -Threo-3-methylglutamate, a non-transportable competitive inhibitor of excitatory amino acid transport, did not reduce aspartate release. Niflumic acid, a blocker of Ca(2+)-dependent anion channels, did not alter the release of either amino acid. Exogenous aspartate and aspartate recently synthesized from glutamate accessed the releasable pool of aspartate as readily as exogenous glutamate and glutamate recently synthesized from aspartate accessed the releasable glutamate pool. These results are compatible with release of aspartate from either a vesicular pool by a "non-classical" form of exocytosis or directly from the cytoplasm by an as-yet-undescribed Ca(2+)-dependent mechanism. In either case, they suggest aspartate is released mainly outside the presynaptic active zones and may therefore serve as the predominant agonist for extrasynaptic N-methyl-D-aspartate receptors.

Altered interaction and expression of proteins involved in neurosecretion in scrapie-infected GT1-1 cells

Prions cause transmissible and fatal diseases that are associated with spongiform degeneration, astrogliosis, and loss of axon terminals in the brains. To determine the expression of proteins involved in neurosecretion and synaptic functions after prion infection, gonadotropin-releasing hormone neuronal cell line subclone (GT1-1) was infected with the RML scrapie strain and analyzed by Western blotting, real time PCR, and immunohistochemistry. As revealed by Western blotting of lysates exposed to different temperatures, the levels of complexed SNAP-25, syntaxin 1A, and synaptophysin were decreased in scrapie-infected GT1-1 cells (ScGT1-1), whereas the level of monomeric forms of these proteins was increased and correlated to the level of scrapie prion protein (PrPSc). However, when complex formation was prevented by prolonged heating of samples in SDS, the levels of monomeric SNAP-25, syntaxin 1A and synaptophysin in ScGT1-1 cells were decreased in comparison to GT1-1 cells. The reduced level of SNAP-25 was observed as early as 32 days postinfection. Increased mRNA levels of both splice variants SNAP-25a and -b in ScGT1-1 cells were seen. No difference in the morphology, neuritic outgrowth or distribution of SNAP-25, syntaxin 1A, or synaptophysin could be observed in ScGT1-1 cells. Treatment with quinacrine or pentosan polysulfate cleared the PrPSc from the ScGT1-1 cell cultures, and the increase in levels of monomeric SNAP-25 and synaptophysin was reversible. These results indicate that a scrapie infection can cause changes in the expression of proteins involved in neuronal secretion, which may be of pathogenetic relevance for the axon terminal changes seen in prion-infected brains.

Regulation of syntaxin1A-munc18 complex for SNARE pairing in HEK293 cells

The formation and dissolution of SNARE protein complexes is essential for Ca(2+)-triggered fusion of neurotransmitter-filled vesicles at the presynaptic membrane. Among the pre-synaptic SNARE proteins, the activation of the Q-SNARE syntaxin1A is a critical event for SNARE complex formation. Activation requires syntaxin1A to transit from a munc18-bound non-interacting state to one competent for SNARE binding. The molecular mechanisms that regulate this transition remain unclear. The propensity of syntaxin1A to promote voltage-dependent steady-state inactivation of N-type Ca(2+) channels and accelerate their entry into inactivation was used in a heterologous cell expression system to elucidate regulation of syntaxin1A protein-protein interactions. We report that coexpression of munc18 eliminated the promoting effect of syntaxin1A on inactivation. This effect of munc18 was completely disrupted by coexpression of munc13-1, but not munc13-2 or munc13-3. Also, since expression of munc13-1 with syntaxin1A resulted in an inactivation phenotype identical to that of munc18 with syntaxin1A, the action of munc13-1 on the munc18-syntaxin1A complex was functionally unique and did not result from competitive binding interactions. Furthermore, munc13 expressed with syntaxin1A and munc18 promoted redistribution of a cytosolic SNAP25 mutant to the membrane, a result indicative of syntaxin1A-SNAP25 SNARE pairing. These data demonstrate an important role of munc13 to control the protein-protein interactions of syntaxin1A in vivo, and support munc13 as critical to dissociating syntaxin1A-munc18 complexes and making syntaxin1A available for SNARE interactions.

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.

Alpha-neurexins couple Ca2+ channels to synaptic vesicle exocytosis

Synapses are specialized intercellular junctions in which cell adhesion molecules connect the presynaptic machinery for neurotransmitter release to the postsynaptic machinery for receptor signalling. Neurotransmitter release requires the presynaptic co-assembly of Ca2+ channels with the secretory apparatus, but little is known about how synaptic components are organized. Alpha-neurexins, a family of >1,000 presynaptic cell-surface proteins encoded by three genes, link the pre- and postsynaptic compartments of synapses by binding extracellularly to postsynaptic cell adhesion molecules and intracellularly to presynaptic PDZ domain proteins. Using triple-knockout mice, we show that alpha-neurexins are not required for synapse formation, but are essential for Ca2+-triggered neurotransmitter release. Neurotransmitter release is impaired because synaptic Ca2+ channel function is markedly reduced, although the number of cell-surface Ca2+ channels appears normal. These data suggest that alpha-neurexins organize presynaptic terminals by functionally coupling Ca2+ channels to the presynaptic machinery.

Regulation of presynaptic calcium channels by synaptic proteins

Calcium entry into nerve termini via voltage gated calcium channels is an essential step in neurotransmission. Consequently, second messenger regulation of calcium channel activity modulates synaptic activity. It has been suggested that calcium channels must physically couple to the release machinery, and a physical interaction between a synaptic protein interaction (synprint) site contained within mammalian presynaptic calcium channels and synaptic proteins such as syntaxin 1, SNAP-25, and synaptotagmin has been demonstrated. Interestingly, synaptic calcium channels in invertebrates lack this region. In invertebrates, synaptic transmission is instead dependent on a presynaptic calcium channel splice variant that can physically associate with the adaptor proteins Mint-1 and CASK. We suggest that in the absence of a synprint region, these proteins may localize calcium channels to the synaptic release machinery. The interactions between synaptic proteins and mammalian N-type calcium channels serves to regulate calcium channel activity directly, as well as indirectly by altering second messenger regulation of the channels. This provides for a feedback mechanism that allows the fine-tuning of calcium channel activity during various steps in neurotransmitter release. This does not occur with invertebrate synaptic calcium channel homologs, suggesting that the regulation of calcium channel activity by synaptic proteins is a mechanism unique to vertebrates.

Functional interactions between presynaptic calcium channels and the neurotransmitter release machinery

In vertebrates, the physical coupling between presynaptic calcium channels and synaptic vesicle release proteins enhances the efficiency of neurotransmission. Recent evidence indicates that these synaptic proteins may feedback directly on synaptic release by negatively regulating calcium entry, and indirectly through pathways involving second messenger molecules. Studies of individual neurons from both vertebrates and invertebrates have provided novel insights into the roles of scaffolding proteins in calcium channel targeting and neurotransmitter release. These studies require us to expand current models of synaptic transmission.

Syntaxin I modulation of presynaptic calcium channel inactivation revealed by botulinum toxin C1

The chick ciliary ganglion calyx-type nerve terminal was used to examine voltage-sensitive inactivation of presynaptic N-type Ca2+ channels and to test if this inactivation is modulated by the transmitter release-associated protein syntaxin I. We tested the role of this protein with botulinum toxin C1 (BtC1) which cleaves syntaxin I close to its membrane anchor. The presynaptic Ca2+ current inactivated as two distinct populations with approximately 75% inactivating at a depolarized potential, V1/2 approximately -15 mV, with the remainder inactivating at approximately -75 mV. BtC1 had no detectable effect on the latter component but resulted in a approximately 7 mV positive shift in the V1/2 of the -15 mV inactivating component. These results confirm that the bulk of presynaptic N-type Ca2+ channels are in general resistant to voltage dependent inactivation and provide the first direct evidence that the physiological properties of presynaptic nerve terminal Ca2+ channels are subject to modulation by release site-associated proteins.

Molecular determinants of syntaxin 1 modulation of N-type calcium channels

We have previously reported that syntaxin 1A, a component of the presynaptic SNARE complex, directly modulates N-type calcium channel gating in addition to promoting tonic G-protein inhibition of the channels, whereas syntaxin 1B affects channel gating but does not support G-protein modulation (Jarvis, S. E., and Zamponi, G. W. (2001) J. Neurosci. 21, 2939-2948). Here, we have investigated the molecular determinants that govern the action of syntaxin 1 isoforms on N-type calcium channel function. In vitro evidence shows that both syntaxin 1 isoforms physically interact with the G-protein beta subunit and the synaptic protein interaction (synprint) site contained within the N-type calcium channel domain II-III linker region. Moreover, in vitro evidence suggests that distinct domains of syntaxin participate in each interaction, with the COOH-terminal SNARE domain (residues 183-230) binding to Gbeta and the N-terminal (residues 1-69) binding to the synprint motif of the channel. Electrophysiological analysis of chimeric syntaxin 1A/1B constructs reveals that the variable NH(2)-terminal domains of syntaxin 1 are responsible for the differential effects of syntaxin 1A and 1B on N-type calcium channel function. Because syntaxin 1 exists in both "open" and "closed" conformations during exocytosis, we produced a constitutively open form of syntaxin 1A and found that it still promoted G-protein inhibition of the channels, but it did not affect N-type channel availability. This state dependence of the ability of syntaxin 1 to mediate N-type calcium channel availability suggests that syntaxin 1 dynamically regulates N-type channel function during various steps of exocytosis. Finally, syntaxin 1A appeared to compete with Ggamma for the Gbeta subunit both in vitro and under physiological conditions, suggesting that syntaxin 1A may contain a G-protein gamma subunit-like domain.

Modulation of a brain voltage-gated K+ channel by syntaxin 1A requires the physical interaction of Gbetagamma with the channel

Recently we suggested that direct interactions between voltage-gated K(+) channels and proteins of the exocytotic machinery, such as those observed between the Kv1.1/Kvbeta channel, syntaxin 1A, and SNAP-25 may be involved in neurotransmitter release. Furthermore, we demonstrated that the direct interaction with syntaxin 1A enhances the fast inactivation of Kv1.1/Kvbeta1.1 in oocytes. Here we show that G-protein betagamma subunits play a crucial role in the enhancement of inactivation by syntaxin 1A. The effect caused by overexpression of syntaxin 1A is eliminated in the presence of chelators of endogenous betagamma subunits in the whole cell and at the plasma membrane. Conversely, enhancement of inactivation caused by overexpression of beta(1)gamma(2) subunits is eliminated upon knock-down of endogenous syntaxin or its scavenging at the plasma membrane. We further show that the N terminus of Kv1.1 binds brain synaptosomal and recombinant syntaxin 1A and concomitantly binds beta(1)gamma(2); the binding of beta(1)gamma(2) enhances that of syntaxin 1A. Taken together, we suggest a mechanism whereby syntaxin and G protein betagamma subunits interact concomitantly with a Kv channel to regulate its inactivation.

Differential coupling of N- and P/Q-type calcium channels to glutamate exocytosis in the rat cerebral cortex

In nerve terminals, the activation of Ca(2+) channels of either the P/Q or N type triggers glutamate release. In turn, presynaptic metabotropic glutamate receptors control this process through the modulation of these Ca(2+) channels. By measuring glutamate release from cerebrocortical nerve terminals we show that at physiological concentrations of extracellular Ca(2+), the agonist L(+)-2-amino-4-phosphonobutyrate only reduced the N-type channel coupled release. However, at lower concentrations of extracellular calcium the contribution of N-type channels was abolished and P/Q-type channels were entirely responsible for the remaining release of glutamate. Under these conditions, L(+)-2-amino-4-phosphonobutyrate was capable of reducing the P/Q-type channel mediated release. These results indicate that N-type Ca(2+) channels are less efficiently coupled to glutamate release than P/Q-type channels and that this differential coupling may have important implications for presynaptic modulation of synaptic transmission.

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. We have generated the equivalent double point mutations in Drosophila syntaxin1A (A243V, V247A; syx(4) mutant) to examine their significance in synaptic transmission in vivo. The syx(4) 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 syx(4) mutants show no alterations in synaptic protein levels in vivo or syntaxin partner binding interactions in vitro. Rather, syx(4) 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 Syx(4) 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.

Direct interaction of a brain voltage-gated K+ channel with syntaxin 1A: functional impact on channel gating

Presynaptic voltage-gated K(+) (Kv) channels play a physiological role in the regulation of transmitter release by virtue of their ability to shape presynaptic action potentials. However, the possibility of a direct interaction of these channels with the exocytotic apparatus has never been examined. We report the existence of a physical interaction in brain synaptosomes between Kvalpha1.1 and Kvbeta subunits with syntaxin 1A, occurring, at least partially, within the context of a macromolecular complex containing syntaxin, synaptotagmin, and SNAP-25. The interaction was altered after stimulation of neurotransmitter release. The interaction with syntaxin was further characterized in Xenopus oocytes by both overexpression and antisense knock-down of syntaxin. Direct physical interaction of syntaxin with the channel protein resulted in an increase in the extent of fast inactivation of the Kv1.1/Kvbeta1.1 channel. Syntaxin also affected the channel amplitude in a biphasic manner, depending on its concentration. At low syntaxin concentrations there was a significant increase in amplitudes, with no detectable change in cell-surface channel expression. At higher concentrations, however, the amplitudes decreased, probably because of a concomitant decrease in cell-surface channel expression, consistent with the role of syntaxin in regulation of vesicle trafficking. The observed physical and functional interactions between syntaxin 1A and a Kv channel may play a role in synaptic efficacy and neuronal excitability.

SNARE proteins contribute to calcium cooperativity of synaptic transmission

A hallmark of calcium-triggered synaptic transmission is the cooperative relationship between calcium and the amount of transmitter released. This relationship is thought to be important for improving the efficiency of synaptic vesicle exocytosis. Although it is generally held that cooperativity arises from the interaction of multiple calcium ions with a single calcium-sensing molecule, the precise molecular basis of this phenomenon is not known. The SNARE proteins are known to be critical for synaptic vesicle exocytosis. We therefore tested for a contribution of SNARE proteins to cooperativity by genetically reducing the levels of syntaxin IA and neuronal-synaptobrevin in Drosophila. Surprisingly, we found that reducing these SNARE proteins also reduced Ca(2+) cooperativity. Thus, SNARE proteins are important for determining the cooperative relationship between calcium and synaptic transmission.

Molecular determinants of the functional interaction between syntaxin and N-type Ca2+ channel gating

Syntaxin is a key presynaptic protein that binds to N- and P/Q-type Ca(2+) channels in biochemical studies and affects gating of these Ca(2+) channels in expression systems and in synaptosomes. The present study was aimed at understanding the molecular basis of syntaxin modulation of N-type channel gating. Mutagenesis of either syntaxin 1A or the pore-forming alpha(1B) subunit of N-type Ca(2+) channels was combined with functional assays of N-type channel gating in a Xenopus oocyte coexpression system and in biochemical binding experiments in vitro. Our analysis showed that the transmembrane region of syntaxin and a short region within the H3 helical cytoplasmic domain of syntaxin, containing residues Ala-240 and Val-244, appeared critical for the channel modulation but not for biochemical association with the "synprint site" in the II/III loop of alpha(1B). These results suggest that syntaxin and the alpha(1B) subunit engage in two kinds of interactions: an anchoring interaction via the II/III loop synprint site and a modulatory interaction via another site located elsewhere in the channel sequence. The segment of syntaxin H3 found to be involved in the modulatory interaction would lie hidden within the four-helix structure of the SNARE complex, supporting the hypothesis that syntaxin's ability to regulate N-type Ca(2+) channels would be enabled after SNARE complex disassembly after synaptic vesicle exocytosis.

Syntaxin modulation of slow inactivation of N-type calcium channels

Syntaxin, a membrane protein vital in triggering vesicle fusion, interacts with voltage-gated N- and P/Q-type Ca(2+) channels. This biochemical association is proposed to colocalize Ca(2+) channels and presynaptic release sites, thus supporting rapid and efficient initiation of neurotransmitter release. The syntaxin channel interaction may also support a novel signaling function, to modulate Ca(2+) channels according to the state of the associated release machinery (Bezprozvanny et al., 1995; Wiser et al., 1996; see also Mastrogiacomo et al., 1994). Here we report that syntaxin 1A (syn1A) coexpressed with N-type channels in Xenopus oocytes greatly promoted slow inactivation gating, but had little or no effect on the onset of and recovery from fast inactivation. Accordingly, the effectiveness of syntaxin depended strongly on voltage protocol. Slow inactivation was found for N-type channels even in the absence of syntaxin and could be distinguished from fast inactivation on the basis of its slow kinetics, distinct voltage dependence (voltage-independent at potentials higher than the level of half-inactivation), and temperature independence (Q(10), approximately 0.8). Trains of action potential-like stimuli were more effective than steady depolarizations in stabilizing the slowly inactivated condition. Agents that stimulate protein kinase C decreased the inhibitory effect of syntaxin on N-type channels. Application of BoNtC1 to cleave syntaxin sharply attenuated the modulatory effects on Ca(2+) channel gating, consistent with structural analysis of syntaxin modulation, supporting use of this toxin to test for the impact of syntaxin on Ca(2+) influx in nerve terminals.