The Voltage-Gated Ca2+ Channel Is the Ca2+ Sensor Protein of Secretion

Non-ionotropic voltage-gated calcium channel signaling

Voltage-gated calcium channels (VGCCs) are the major conduits for calcium ions (Ca2+) within excitable cells. Recent studies have highlighted the non-ionotropic functionality of VGCCs, revealing their capacity to activate intracellular pathways independently of ion flow. This non-ionotropic signaling mode plays a pivotal role in excitation-coupling processes, including gene transcription through excitation-transcription (ET), synaptic transmission via excitation-secretion (ES), and cardiac contraction through excitation-contraction (EC). However, it is noteworthy that these excitation-coupling processes require extracellular calcium (Ca2+) and Ca2+ occupancy of the channel ion pore. Analogous to the "non-canonical" characterization of the non-ionotropic signaling exhibited by the N-methyl-D-aspartate receptor (NMDA), which requires extracellular Ca2+ without the influx of ions, VGCC activation requires depolarization-triggered conformational change(s) concomitant with Ca2+ binding to the open channel. Here, we discuss the contributions of VGCCs to ES, ET, and EC coupling as Ca2+ binding macromolecules that transduces external stimuli to intracellular input prior to elevating intracellular Ca2+. We emphasize the recognition of calcium ion occupancy within the open ion-pore and its contribution to the excitation coupling processes that precede the influx of calcium. The non-ionotropic activation of VGCCs, triggered by the upstroke of an action potential, provides a conceptual framework to elucidate the mechanistic aspects underlying the microseconds nature of synaptic transmission, cardiac contractility, and the rapid induction of first-wave genes.

Revisiting the molecular basis of synaptic transmission

Measurements of the time elapsed during synaptic transmission has shown that synaptic vesicle (SV) fusion lags behind Ca²⁺-influx by approximately 60 microseconds (µsec). The conventional model cannot explain this extreme rapidity of the release event. Synaptic transmission occurs at the active zone (AZ), which comprises of two pools of SV, non-releasable "tethered" vesicles, and a readily-releasable pool of channel-associated Ca²⁺-primed vesicles, "RRP". A recent TIRF study at cerebellar-mossy fiber-terminal, showed that subsequent to an action potential, newly "tethered" vesicles, became fusion-competent in a Ca²⁺-dependent manner, 300-400 milliseconds after tethering, but were not fused. This time resolution may correspond to priming of tethered vesicles through Ca²⁺-binding to Syt1/Munc13-1/complexin. It confirms that Ca²⁺-priming and Ca²⁺-influx-independent fusion, are two distinct events. Notably, we have established that Ca²⁺ channel signals evoked-release in an ion flux-independent manner, demonstrated by Ca²⁺-impermeable channel, or a Ca²⁺ channel in which Ca²⁺ is replaced by impermeable La³⁺. Thus, conformational changes in a channel coupled to RRP appear to directly activate the release machinery and account for a µsec Ca²⁺-influx-independent vesicle fusion. Rapid vesicle fusion driven by non-ionotropic channel signaling strengthens a conformational-coupling mechanism of synaptic transmission, and contributes to better understanding of neuronal communication vital for brain function.

Elevated basal transcription can underlie timothy channel association with autism related disorders

Timothy syndrome (TS) is a neurodevelopmental disorder caused by mutations in the pore-forming subunit α₁1.2 of the L-type voltage-gated Ca²⁺-channel Cav1.2, at positions G406R or G402S. Although both mutations cause cardiac arrhythmias, only Cav1.2^G406R is associated with the autism-spectrum-disorder (ASD). We show that transcriptional activation by Cav1.2^G406R and Cav1.2^G402S is driven by membrane depolarization through the Ras/ERK/CREB pathway in a process called excitation-transcription (ET) coupling, as previously shown for wt Cav1.2. This process requires the presence of the intracellular β-subunit of the channel. We found that only the autism-associated mutant Cav1.2^G406R, as opposed to the non-autistic mutated channel Cav1.2^G402S, exhibits a depolarization-independent CREB phosphorylation, and spontaneous transcription of cFos and MeCP2. A leftward voltage-shift typical of Cav1.2^G406R activation, increases channel opening at subthreshold potentials, resulting in an enhanced channel activity, as opposed to a rightward shift in Cav1.2^G402S. We suggest that the enhanced spontaneous Cav1.2^G406R activity accounts for the increase in basal transcriptional activation. This uncontroled transcriptional activation may result in the manifestation of long-term dysregulations such as autism. Thus, gating changes provide a mechanistic framework for understanding the molecular events underlying the autistic phenomena caused by the G406R Timothy mutation. They might clarify whether a constitutive transcriptional activation accompanies other VGCC that exhibit a leftward voltage-shift of activation and are also associated with long-term cognitive disorders.

Ca2+ -independent and voltage-dependent exocytosis in mouse chromaffin cells

AIM

It is widely accepted that the exocytosis of synaptic and secretory vesicles is triggered by Ca²⁺ entry through voltage-dependent Ca²⁺ channels. However, there is evidence of an alternative mode of exocytosis induced by membrane depolarization but lacking Ca²⁺ current and intracellular Ca²⁺ increase. In this work we investigated if such a mechanism contributes to secretory vesicle exocytosis in mouse chromaffin cells.

METHODS

Exocytosis was evaluated by patch-clamp membrane capacitance measurements, carbon fibre amperometry and TIRF. Cytosolic Ca²⁺ was estimated using epifluorescence microscopy and fluo-8 (salt form).

RESULTS

Cells stimulated by brief depolatizations in absence of extracellular Ca⁺² show moderate but consistent exocytosis, even in presence of high cytosolic BAPTA concentration and pharmacological inhibition of Ca⁺² release from intracellular stores. This exocytosis is tightly dependent on membrane potential, is inhibited by neurotoxin Bont-B (cleaves the v-SNARE synaptobrevin), is very fast (saturates with time constant <10 ms), it is followed by a fast endocytosis sensitive to the application of an anti-dynamin monoclonal antibody, and recovers after depletion in <5 s. Finally, this exocytosis was inhibited by: (i) ω-agatoxin IVA (blocks P/Q-type Ca²⁺ channel gating), (ii) in cells from knock-out P/Q-type Ca²⁺ channel mice, and (iii) transfection of free synprint peptide (interferes in P/Q channel-exocytic proteins association).

CONCLUSION

We demonstrated that Ca²⁺ -independent and voltage-dependent exocytosis is present in chromaffin cells. This process is tightly coupled to membrane depolarization, and is able to support secretion during action potentials at low basal rates. P/Q-type Ca²⁺ channels can operate as voltage sensors of this process.

β-Subunit of the voltage-gated Ca2+ channel Cav1.2 drives signaling to the nucleus via H-Ras

Depolarization-induced signaling to the nucleus by the L-type voltage-gated calcium channel Cav1.2 is widely assumed to proceed by elevating intracellular calcium. The apparent lack of quantitative correlation between Ca²⁺ influx and gene activation suggests an alternative activation pathway. Here, we demonstrate that membrane depolarization of HEK293 cells transfected with α₁1.2/β2b/α2δ subunits (Cav1.2) triggers c-Fos and MeCP2 activation via the Ras/ERK/CREB pathway. Nuclear signaling is lost either by absence of the intracellular β2 subunit or by transfecting the cells with the channel mutant α₁1.2^W440A/β2b/α2δ, a mutation that disrupts the interaction between α₁1.2 and β2 subunits. Pulldown assays in neuronal SH-SY5Y cells and in vitro binding of recombinant H-Ras and β2 confirmed the importance of the intracellular β2 subunit for depolarization-induced gene activation. Using a Ca²⁺-impermeable mutant channel α₁1.2^L745P/β2b/α2δ or disrupting Ca²⁺/calmodulin binding to the channel using the channel mutant α₁1.2^I1624A/β2b/α2δ, we demonstrate that depolarization-induced c-Fos and MeCP2 activation does not depend on Ca²⁺ transport by the channel. Thus, in contrast to the paradigm that elevated intracellular Ca²⁺ drives nuclear signaling, we show that Cav1.2-triggered c-Fos or MeCP2 is dependent on extracellular Ca²⁺ and Ca²⁺ occupancy of the open channel pore, but is Ca²⁺-influx independent. An indispensable β-subunit interaction with H-Ras, which is triggered by conformational changes at α₁1.2 independently of Ca²⁺ flux, brings to light a master regulatory role of β2 in transcriptional activation via the ERK/CREB pathway. This mode of H-Ras activation could have broad implications for understanding the coupling of membrane depolarization to the rapid induction of gene transcription.

The L-type Voltage-Gated Calcium Channel co-localizes with Syntaxin 1A in nano-clusters at the plasma membrane

The secretory signal elicited by membrane depolarization traverses from the Ca²⁺-bound α₁1.2 pore-forming subunit of the L-type Ca²⁺-channel (Cav1.2) to syntaxin 1 A (Sx1A) via an intra-membrane signaling mechanism. Here, we report the use of two-color Photo-Activated-Localization-Microscopy (PALM) to determine the relation between Cav1.2 and Sx1A in single-molecule detail. We observed nanoscale co-clusters of PAmCherry-tagged Sx1A and Dronpa-tagged α₁1.2 at a ~1:1 ratio. PAmCherry-tagged Sx1A^C145A, or PAmCherry-tagged Sx2, an inactive Cav1.2 modulator, in which Cys145 is a Ser residue, showed no co-clustering. These results are  consistent with the crucial role of the single cytosolic Sx1ACys145 in clustering with Cav1.2. Cav1.2 and the functionally inactive transmembrane-domain double mutant Sx1A^C271V/C272V engendered clusters with a ~2:1 ratio. A higher extent of co-clustering, which coincides with compromised depolarization-evoked transmitter-release, was observed also by oxidation of Sx1ACys271 and Cys272. Our super-resolution-imaging results set the stage for studying co-clustering of the channel with other exocytotic proteins at a single-molecule level.

Sequential ionic and conformational signaling by calcium channels drives neuronal gene expression

Voltage-gated CaV1.2 channels (L-type calcium channel α1C subunits) are critical mediators of transcription-dependent neural plasticity. Whether these channels signal via the influx of calcium ion (Ca(2+)), voltage-dependent conformational change (VΔC), or a combination of the two has thus far been equivocal. We fused CaV1.2 to a ligand-gated Ca(2+)-permeable channel, enabling independent control of localized Ca(2+) and VΔC signals. This revealed an unexpected dual requirement: Ca(2+) must first mobilize actin-bound Ca(2+)/calmodulin-dependent protein kinase II, freeing it for subsequent VΔC-mediated accumulation. Neither signal alone sufficed to activate transcription. Signal order was crucial: Efficiency peaked when Ca(2+) preceded VΔC by 10 to 20 seconds. CaV1.2 VΔC synergistically augmented signaling by N-methyl-d-aspartate receptors. Furthermore, VΔC mistuning correlated with autistic symptoms in Timothy syndrome. Thus, nonionic VΔC signaling is vital to the function of CaV1.2 in synaptic and neuropsychiatric processes.

Intra-membrane signaling between the voltage-gated Ca2+-channel and cysteine residues of syntaxin 1A coordinates synchronous release

The interaction of syntaxin 1A (Sx1A) with voltage-gated calcium channels (VGCC) is required for depolarization-evoked release. However, it is unclear how the signal is transferred from the channel to the exocytotic machinery and whether assembly of Sx1A and the calcium channel is conformationally linked to triggering synchronous release. Here we demonstrate that depolarization-evoked catecholamine release was decreased in chromaffin cells infected with semliki forest viral vectors encoding Sx1A mutants, Sx1A^C271V, or Sx1A^C272V, or by direct oxidation of these Sx1A transmembrane (TM) cysteine residues. Mutating or oxidizing these highly conserved Sx1A Cys271 and Cys272 equally disrupted the Sx1A interaction with the channel. The results highlight the functional link between the VGCC and the exocytotic machinery, and attribute the redox sensitivity of the release process to the Sx1A TM C271 and C272. This unique intra-membrane signal-transduction pathway enables fast signaling, and triggers synchronous release by conformational-coupling of the channel with Sx1A.

Neuronal voltage-gated calcium channels: structure, function, and dysfunction

Voltage-gated calcium channels are the primary mediators of depolarization-induced calcium entry into neurons. There is great diversity of calcium channel subtypes due to multiple genes that encode calcium channel α1 subunits, coassembly with a variety of ancillary calcium channel subunits, and alternative splicing. This allows these channels to fulfill highly specialized roles in specific neuronal subtypes and at particular subcellular loci. While calcium channels are of critical importance to brain function, their inappropriate expression or dysfunction gives rise to a variety of neurological disorders, including, pain, epilepsy, migraine, and ataxia. This Review discusses salient aspects of voltage-gated calcium channel function, physiology, and pathophysiology.

Cooperative endocytosis of the endosomal SNARE protein syntaxin-8 and the potassium channel TASK-1

SNARE proteins can have functions unrelated to membrane fusion. The unassembled form of the SNARE protein syntaxin-8 interacts with the K⁺ channel TASK-1; both proteins are internalized via clathrin-mediated endocytosis in a cooperative manner. This is a novel mechanism for the control of endocytosis by cargo proteins.

The endosomal SNARE protein syntaxin-8 interacts with the acid-sensitive potassium channel TASK-1. The functional relevance of this interaction was studied by heterologous expression of these proteins (and mutants thereof) in Xenopus oocytes and in mammalian cell lines. Coexpression of syntaxin-8 caused a fourfold reduction in TASK-1 current, a corresponding reduction in the expression of TASK-1 at the cell surface, and a marked increase in the rate of endocytosis of the channel. TASK-1 and syntaxin-8 colocalized in the early endosomal compartment, as indicated by the endosomal markers 2xFYVE and rab5. The stimulatory effect of the SNARE protein on the endocytosis of the channel was abolished when both an endocytosis signal in TASK-1 and an endocytosis signal in syntaxin-8 were mutated. A syntaxin-8 mutant that cannot assemble with other SNARE proteins had virtually the same effect as wild-type syntaxin-8. Total internal reflection fluorescence microscopy showed formation and endocytosis of vesicles containing fluorescence-tagged clathrin, TASK-1, and/or syntaxin-8. Our results suggest that the unassembled form of syntaxin-8 and the potassium channel TASK-1 are internalized via clathrin-mediated endocytosis in a cooperative manner. This implies that syntaxin-8 regulates the endocytosis of TASK-1. Our study supports the idea that endosomal SNARE proteins can have functions unrelated to membrane fusion.

Voltage-gated calcium channels function as Ca2+-activated signaling receptors

Voltage-gated calcium channels (VGCCs) are transmembrane cell surface proteins responsible for multifunctional signals. In response to voltage, VGCCs trigger synaptic transmission, drive muscle contraction, and regulate gene expression. Voltage perturbations open VGCCs enabling Ca(2+) binding to the low affinity Ca(2+) binding site of the channel pore. Subsequent to permeation, Ca(2+) targets selective proteins to activate diverse signaling pathways. It is becoming apparent that the Ca(2+)-bound channel triggers secretion in excitable cells and drives contraction in cardiomyocytes prior to Ca(2+) permeation. Here, I highlight recent data implicating receptor-like function of the Ca(2+)-bound channel in converting external Ca(2+) into an intracellular signal. The two sequential mechanistic perspectives of VGCC function are discussed in the context of the prevailing and long-standing current models of depolarization-evoked secretion and cardiac contraction.

A set of SNARE proteins in the contractile vacuole complex of Paramecium regulates cellular calcium tolerance and also contributes to organelle biogenesis

The contractile vacuole complex (CVC) of freshwater protists serves the extrusion of water and ions, including Ca(2+). No vesicle trafficking based on SNAREs has been detected so far in any CVC. SNAREs (soluble NSF [N-ethylmaleimide sensitive factor] attachment protein receptors) are required for membrane-to-membrane interaction, i.e. docking and fusion also in Paramecium. We have identified three v-/R- and three t/Q-SNAREs selectively in the CVC. Posttranscriptional silencing of Syb2, Syb6 or Syx2 slows down the pumping cycle; silencing of the latter two also causes vacuole swelling. Increase in extracellular Ca(2+) after Syb2, Syb6 or Syx2 silencing causes further swelling of the contractile vacuole and deceleration of its pulsation. Silencing of Syx14 or Syx15 entails lethality in the Ca(2+) stress test. Thus, the effects of silencing strictly depend on the type of the silenced SNARE and on the concentration of Ca(2+) in the medium. This shows the importance of organelle-resident SNARE functions (which may encompass the vesicular delivery of other organelle-resident proteins) for Ca(2+) tolerance. A similar principle may be applicable also to the CVC in widely different unicellular organisms. In addition, in Paramecium, silencing particularly of Syx6 causes aberrant positioning of the CVC during de novo biogenesis before cytokinesis.

Voltage-driven Ca(2+) binding at the L-type Ca(2+) channel triggers cardiac excitation-contraction coupling prior to Ca(2+) influx

The activation of the ryanodine Ca(2+) release channels (RyR2) by the entry of Ca(2+) through the L-type Ca(2+) channels (Cav1.2) is believed to be the primary mechanism of excitation-contraction (EC) coupling in cardiac cells. This proposed mechanism of Ca(2+)-induced Ca(2+) release (CICR) cannot fully account for the lack of a termination signal for this positive feedback process. Using Cav1.2 channel mutants, we demonstrate that the Ca(2+)-impermeable α(1)1.2/L775P/T1066Y mutant introduced through lentiviral infection into neonate cardiomyocytes triggers Ca(2+) transients in a manner independent of Ca(2+) influx. In contrast, the α(1)1.2/L775P/T1066Y/4A mutant, in which the Ca(2+)-binding site of the channel was destroyed, supports neither the spontaneous nor the electrically evoked contractions. Ca(2+) bound at the channel selectivity filter appears to initiate a signal that is conveyed directly from the channel pore to RyR2, triggering contraction of cardiomyocytes prior to Ca(2+) influx. Thus, RyR2 is activated in response to a conformational change in the L-type channel during membrane depolarization and not through interaction with Ca(2+) ions diffusing in the junctional gap space. Accordingly, termination of the RyR2 activity is achieved when the signal stops upon the return of the L-channel to the resting state. We propose a new model in which the physical link between Cav1.2 and RyR2 allows propagation of a conformational change induced at the open pore of the channel to directly activate RyR2. These results highlight Cav1.2 as a signaling protein and provide a mechanism for terminating the release of Ca(2+) from RyR2 through protein-protein interactions. In this model, the L-type channel is a master regulator of both initiation and termination of EC coupling in neonate cardiomyocytes.

Control of low-threshold exocytosis by T-type calcium channels

Low-voltage-activated (LVA) T-type Ca²⁺ channels differ from their high-voltage-activated (HVA) homologues by unique biophysical properties. Hence, whereas HVA channels convert action potentials into intracellular Ca²⁺ elevations, T-type channels control Ca²⁺ entry during small depolarizations around the resting membrane potential. They play an important role in electrical activities by generating low-threshold burst discharges that occur during various physiological and pathological forms of neuronal rhythmogenesis. In addition, they mediate a previously unrecognized function in the control of synaptic transmission where they directly trigger the release of neurotransmitters at rest. In this review, we summarize our present knowledge of the role of T-type Ca²⁺ channels in vesicular exocytosis, and emphasize the critical importance of localizing the exocytosis machinery close to the Ca²⁺ source for reliable synaptic transmission. This article is part of a Special Issue entitled: Calcium channels.

Calcium signaling in closely related protozoan groups (Alveolata): non-parasitic ciliates (Paramecium, Tetrahymena) vs. parasitic Apicomplexa (Plasmodium, Toxoplasma)

The importance of Ca2+-signaling for many subcellular processes is well established in higher eukaryotes, whereas information about protozoa is restricted. Recent genome analyses have stimulated such work also with Alveolates, such as ciliates (Paramecium, Tetrahymena) and their pathogenic close relatives, the Apicomplexa (Plasmodium, Toxoplasma). Here we compare Ca2+ signaling in the two closely related groups. Acidic Ca2+ stores have been characterized in detail in Apicomplexa, but hardly in ciliates. Two-pore channels engaged in Ca2+-release from acidic stores in higher eukaryotes have not been stingently characterized in either group. Both groups are endowed with plasma membrane- and endoplasmic reticulum-type Ca2+-ATPases (PMCA, SERCA), respectively. Only recently was it possible to identify in Paramecium a number of homologs of ryanodine and inositol 1,3,4-trisphosphate receptors (RyR, IP3R) and to localize them to widely different organelles participating in vesicle trafficking. For Apicomplexa, physiological experiments suggest the presence of related channels although their identity remains elusive. In Paramecium, IP3Rs are constitutively active in the contractile vacuole complex; RyR-related channels in alveolar sacs are activated during exocytosis stimulation, whereas in the parasites the homologous structure (inner membrane complex) may no longer function as a Ca2+ store. Scrutinized comparison of the two closely related protozoan phyla may stimulate further work and elucidate adaptation to parasitic life. See also "Conclusions" section.

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.

Alleviation of oxidative stress by potent and selective thioredoxin-mimetic peptides

One of the major enzymatic cell defenses providing protection from oxidative injury is the TrxR-Trx system. It consists of NADPH and thioredoxin reductase (TrxR), which maintain thioredoxin (Trx) in a reduced state. Perturbing the TrxR-Trx system with the selective TrxR inhibitor auranofin (AuF; 2,3,4,6-tetra-O-acetyl-1-thio-β-D-glucopyranosato-S-(triethylphosphine) gold) induces oxidative stress by keeping Trx in its oxidized state. We have prepared a family of tri- and tetra-oligopeptides derived from the canonical CxxC motif of the Trx active site and a modified CxC motif. These Trx-mimetic compounds are N- and C-terminal-blocked peptides that consist of two cysteine residues that flank the two-amino-acid CxxC motif (CB4 and CB6) or the single-amino-acid CxC motif (CB3). Catecholamine (CA) secretion in bovine chromaffin cells, which is a highly redox sensitive process, is abolished by AuF. The Trx-mimetic peptides effectively restore CA secretion, as monitored by amperometry in single cells. They also prevent the AuF-induced phosphorylation of p38 mitogen-activated protein kinase (MAPK) and c-Jun NH2-terminal kinase. In PC12 cells, the alleviation of AuF-induced ERK1/2-MAPK phosphorylation by Trx-like peptides parallels their effect of restoring CA secretion. CB3, CB4, and CB6 act intracellularly and are significantly more potent than the traditional antioxidants NAC, GSH, DTT, AD4 (NAC-amide), and ascorbic acid. Taken together, the CxxC and CxC peptides represent a new family of potent and selective redox compounds that could serve as potential candidates for prevention and treatment of oxidative-stress-related disorders.

Properties of Na+ currents conducted by a skeletal muscle L-type Ca2+ channel pore mutant (SkEIIIK)

Four glutamate residues residing at corresponding positions within the four conserved membrane-spanning repeats of L-type Ca(2+) channels are important structural determinants for the passage of Ca(2+) across the selectivity filter. Mutation of the critical glutamate in Repeat III in the a 1S subunit of the skeletal L-type channel (Ca(v)1.1) to lysine virtually eliminates passage of Ca(2+) during step depolarizations. In this study, we examined the ability of this mutant Ca(v)1.1 channel (SkEIIIK) to conduct inward Na(+) current. When 150 mM Na(+) was present as the sole monovalent cation in the bath solution, dysgenic (Ca(v)1.1 null) myotubes expressing SkEIIIK displayed slowly-activating, non-inactivating, nifedipine-sensitive inward currents with a reversal potential (45.6 ± 2.5 mV) near that expected for Na(+). Ca(2+) block of SkEIIIK-mediated Na(+) current was revealed by the substantial enhancement of Na(+) current amplitude after reduction of Ca(2+) in the external recording solution from 10 mM to near physiological 1 mM. Inward SkEIIIK-mediated currents were potentiated by either ±Bay K 8644 (10 mM) or 200-ms depolarizing prepulses to +90 mV. In contrast, outward monovalent currents were reduced by ±Bay K 8644 and were unaffected by strong depolarization, indicating a preferential potentiation of inward Na(+) currents through the mutant Ca(v)1.1 channel. Taken together, our results show that SkEIIIK functions as a non-inactivating, junctionally-targeted Na(+) channel when Na(+) is the sole monvalent cation present and urge caution when interpreting the impact of mutations designed to ablate Ca(2+) permeability mediated by Ca(v) channels on physiological processes that extend beyond channel gating and permeability.

Role of annexin A6 isoforms in catecholamine secretion by PC12 cells: distinct influence on calcium response

Noradrenaline and adrenaline are secreted by adrenal medulla chromaffin cells via exocytosis. Exocytosis of catecholamines occurs after cell stimulation with various endogenous activators such as nicotine or after depolarization of the plasma membrane and is regulated by calcium ions. Cytosolic [Ca(2+)] increases in response to cell excitation and triggers a signal-initiated secretion. Annexins are known to participate in the regulation of membrane dynamics and are also considered to be involved in vesicular trafficking. Some experimental evidence suggests that annexins may participate in Ca(2+)-regulated catecholamine secretion. In this report the effect of annexin A6 (AnxA6) isoforms 1 and 2 on catecholamine secretion has been described. Overexpression of AnxA6 isoforms and AnxA6 knock-down in PC12 cells were accompanied by almost complete inhibition or a 20% enhancement of dopamine secretion, respectively. AnxA6-1 and AnxA6-2 overexpression reduced Delta[Ca(2+)](c) upon depolarization by 32% and 58%, respectively, while AnxA6 knock-down increased Delta[Ca(2+)](c) by 44%. The mechanism of AnxA6 action on Ca(2+) signalling is not well understood. Experimental evidence suggests that two AnxA6 isoforms interact with different targets engaged in regulation of calcium homeostasis in PC12 cells.

The involvement of ser1898 of the human L-type calcium channel in evoked secretion

A PKA consensus phosphorylation site S1928 at the α(1)1.2 subunit of the rabbit cardiac L-type channel, Ca(V)1.2, is involved in the regulation of Ca(V)1.2 kinetics and affects catecholamine secretion. This mutation does not alter basal Ca(V)1.2 current properties or regulation of Ca(V)1.2 current by PKA and the beta-adrenergic receptor, but abolishes Ca(V)1.2 phosphorylation by PKA. Here, we test the contribution of the corresponding PKA phosphorylation site of the human α(1)1.2 subunit S1898, to the regulation of catecholamine secretion in bovine chromaffin cells. Chromaffin cells were infected with a Semliki-Forest viral vector containing either the human wt or a mutated S1898A α(1)1.2 subunit. Both subunits harbor a T1036Y mutation conferring nifedipine insensitivity. Secretion evoked by depolarization in the presence of nifedipine was monitored by amperometry. Depolarization-triggered secretion in cells infected with either the wt α(1)1.2 or α(1)1.2/S1898A mutated subunit was elevated to a similar extent by forskolin. Forskolin, known to directly activate adenylyl-cyclase, increased the rate of secretion in a manner that is largely independent of the presence of S1898. Our results are consistent with the involvement of additional PKA regulatory site(s) at the C-tail of α(1)1.2, the pore forming subunit of Ca(V)1.2.

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.

Distinct subcellular localization of a group of synaptobrevin-like SNAREs in Paramecium tetraurelia and effects of silencing SNARE-specific chaperone NSF

We have identified new synaptobrevin-like SNAREs and localized the corresponding gene products with green fluorescent protein (GFP)-fusion constructs and specific antibodies at the light and electron microscope (EM) levels. These SNAREs, named Paramecium tetraurelia synaptobrevins 8 to 12 (PtSyb8 to PtSyb12), showed mostly very restricted, specific localization, as they were found predominantly on structures involved in endo- or phagocytosis. In summary, we found PtSyb8 and PtSyb9 associated with the nascent food vacuole, PtSyb10 near the cell surface, at the cytostome, and in close association with ciliary basal bodies, and PtSyb11 on early endosomes and on one side of the cytostome, while PtSyb12 was found in the cytosol. PtSyb4 and PtSyb5 (identified previously) were localized on small vesicles, PtSyb5 probably being engaged in trichocyst (dense core secretory vesicle) processing. PtSyb4 and PtSyb5 are related to each other and are the furthest deviating of all SNAREs identified so far. Because they show no similarity with any other R-SNAREs outside ciliates, they may represent a ciliate-specific adaptation. PtSyb10 forms small domains near ciliary bases, and silencing slows down cell rotation during depolarization-induced ciliary reversal. NSF silencing supports a function of cell surface SNAREs by revealing vesicles along the cell membrane at sites normally devoid of vesicles. The distinct distributions of these SNAREs emphasize the considerable differentiation of membrane trafficking, particularly along the endo-/phagocytic pathway, in this protozoan.

Conformational changes induced in voltage-gated calcium channel Cav1.2 by BayK 8644 or FPL64176 modify the kinetics of secretion independently of Ca2+ influx

The role of the L-type calcium channel (Cav1.2) as a molecular switch that triggers secretion prior to Ca(2+) transport has previously been demonstrated in bovine chromaffin cells and rat pancreatic beta cells. Here, we examined the effect of specific Cav1.2 allosteric modulators, BayK 8644 (BayK) and FPL64176 (FPL), on the kinetics of catecholamine release, as monitored by amperometry in single bovine chromaffin cells. We show that 2 microm BayK or 0.5 microm FPL accelerates the rate of catecholamine secretion to a similar extent in the presence either of the permeable Ca(2+) and Ba(2+) or the impermeable charge carrier La(3+). These results suggest that structural rearrangements generated through the binding of BayK or FPL, by altering the channel activity, could affect depolarization-evoked secretion prior to cation transport. FPL also accelerated the rate of secretion mediated by a Ca(2+)-impermeable channel made by replacing the wild type alpha(1)1.2 subunit was replaced with the mutant alpha(1)1.2/L775P. Furthermore, BayK and FPL modified the kinetic parameters of the fusion pore formation, which represent the initial contact between the vesicle lumen and the extracellular medium. A direct link between the channel activity and evoked secretion lends additional support to the view that the voltage-gated Ca(2+) channels act as a signaling molecular switch, triggering secretion upstream to ion transport into the cell.