Presynaptic calcium and control of vesicle fusion

Distinct Calcium Sources Support Multiple Modes of Synaptic Release from Cranial Sensory Afferents

Most craniosensory afferents have unmyelinated axons expressing TRP Vanilloid 1 (TRPV1) receptors in synaptic terminals at the solitary tract nucleus (NTS). Neurotransmission from these synapses is characterized by substantial asynchronous EPSCs following action potential-synched EPSCs and high spontaneous rates that are thermally sensitive. The present studies blocked voltage-activated calcium channels (CaV) using the nonselective CaV blocker Cd(2+) or the specific N-type blocker ω-conotoxin GVIA to examine the calcium dependence of the synchronous, asynchronous, spontaneous, and thermally gated modes of release. In rat brainstem slices containing caudal NTS, shocks to the solitary tract (ST) triggered synchronous ST-EPSCs and trailing asynchronous EPSCs. Cd(2+) or GVIA efficiently reduced both synchronous and asynchronous EPSCs without altering spontaneous or thermal-evoked transmission. Activation of TRPV1 with either the selective agonist resiniferatoxin (150 pm) or temperature augmented basal sEPSC rates but failed to alter the synchronous or asynchronous modes of release. These data indicate that calcium sourced through TRPV1 has no access to the synchronous or asynchronous release mechanism(s) and conversely that CaV-sourced calcium does not interact with the thermally evoked mode of release. Buffering intracellular calcium with EGTA-AM or BAPTA-AM reduced asynchronous EPSC rates earlier and to a greater extent than synchronous ST-EPSC amplitudes without altering sEPSCs or thermal sensitivity. Buffering therefore distinguishes asynchronous vesicles as possessing a highly sensitive calcium sensor located perhaps more distant from CaV than synchronous vesicles or thermally evoked vesicles from TRPV1. Together, our findings suggest separate mechanisms of release for spontaneous, asynchronous and synchronous vesicles that likely reside in unique, spatially separated vesicle domains.

SIGNIFICANCE STATEMENT

Most craniosensory fibers release glutamate using calcium entry from two sources: CaVs and TRPV1. We demonstrate that calcium segregation distinguishes three vesicle release mechanisms. Most surprisingly, asynchronous release is associated with CaV and not TRPV1 calcium entry. This reveals that asynchronous release is an additional and separate phenotypic marker of unmyelinated afferents rather than operated by TRPV1. The functional independence of the two calcium sources expands the regulatory repertoire of transmission and imbues these inputs with additional modulation targets for synaptic release not present at conventional CaV synapses. Peptides and lipid mediators may target one or both of these calcium sources at afferent terminals within the solitary tract nucleus to independently modify release from distinct, functionally segregated vesicle pools.

Cooperative stochastic binding and unbinding explain synaptic size dynamics and statistics

Synapses are dynamic molecular assemblies whose sizes fluctuate significantly over time-scales of hours and days. In the current study, we examined the possibility that the spontaneous microscopic dynamics exhibited by synaptic molecules can explain the macroscopic size fluctuations of individual synapses and the statistical properties of synaptic populations. We present a mesoscopic model, which ties the two levels. Its basic premise is that synaptic size fluctuations reflect cooperative assimilation and removal of molecules at a patch of postsynaptic membrane. The introduction of cooperativity to both assimilation and removal in a stochastic biophysical model of these processes, gives rise to features qualitatively similar to those measured experimentally: nanoclusters of synaptic scaffolds, fluctuations in synaptic sizes, skewed, stable size distributions and their scaling in response to perturbations. Our model thus points to the potentially fundamental role of cooperativity in dictating synaptic remodeling dynamics and offers a conceptual understanding of these dynamics in terms of central microscopic features and processes.

Neurons communicate through specialized sites of cell–cell contact known as synapses. This vast set of connections is believed to be crucial for sensory processing, motor function, learning and memory. Experimental data from recent years suggest that synapses are not static structures, but rather dynamic assemblies of molecules that move in, out and between nearby synapses, with these dynamics driving changes in synaptic properties over time. Thus, in addition to changes directed by activity or other physiological signals, synapses also exhibit spontaneous changes that have particular dynamical and statistical signatures. Given the immense complexity of synapses at the molecular scale, how can one hope to understand the principles that govern these spontaneous changes and statistical signatures? Here we offer a mesoscopic modelling approach—situated between detailed microscopic and abstract macroscopic approaches—to advance this understanding. Our model, based on simplified biophysical assumptions, shows that spontaneous cooperative binding and unbinding of proteins at synaptic sites can give rise to dynamic and statistical signatures similar to those measured in experiments. Importantly, we find cooperativity to be indispensable in this regard. Our model thus offers a conceptual understanding of synaptic dynamics and statistical features in terms of a fundamental biological principle, namely cooperativity.

Ammonium chloride alters neuronal excitability and synaptic vesicle release

Genetically encoded pH-sensors are widely used in studying cell membrane trafficking and membrane protein turnover because they render exo-/endocytosis-associated pH changes to fluorescent signals. For imaging and analysis purposes, high concentration ammonium chloride is routinely used to alkalize intracellular membrane compartments under the assumption that it does not cause long-term effects on cellular processes being studied like neurotransmission. However, pathological studies about hyperammonemia have shown that ammonium is toxic to brain cells especially astrocytes and neurons. Here, we focus on ammonium’s physiological impacts on neurons including membrane potential, cytosolic Ca²⁺ and synaptic vesicles. We have found that extracellularly applied ammonium chloride as low as 5 mM causes intracellular Ca²⁺-increase and a reduction of vesicle release even after washout. The often-used 50 mM ammonium chloride causes more extensive and persistent changes, including membrane depolarization, prolonged elevation of intracellular Ca²⁺ and diminution of releasable synaptic vesicles. Our findings not only help to bridge the discrepancies in previous studies about synaptic vesicle release using those pH-sensors or other vesicle specific reporters, but also suggest an intriguing relationship between intracellular pH and neurotransmission.

Loading a Calcium Dye into Frog Nerve Endings Through the Nerve Stump: Calcium Transient Registration in the Frog Neuromuscular Junction

One of the most feasible methods of measuring presynaptic calcium levels in presynaptic nerve terminals is optical recording. It is based on using calcium-sensitive fluorescent dyes that change their emission intensity or wavelength depending on the concentration of free calcium in the cell. There are several methods used to stain cells with calcium dyes. Most common are the processes of loading the dyes through a micropipette or pre-incubating with the acetoxymethyl ester forms of the dyes. However, these methods are not quite applicable to neuromuscular junctions (NMJs) due to methodological issues that arise. In this article, we present a method for loading a calcium-sensitive dye through the frog nerve stump of the frog nerve into the nerve endings. Since entry of external calcium into nerve terminals and the subsequent binding to the calcium dye occur within the millisecond time-scale, it is necessary to use a fast imaging system to record these interactions. Here, we describe a protocol for recording the calcium transient with a fast CCD camera.

Pulses of Ca2+ coordinate actin assembly and exocytosis for stepwise cell extension

Many eukaryotic cells grow by extending their cell periphery in pulses. The molecular mechanisms underlying this process are not yet fully understood. Here we present a comprehensive model of stepwise cell extension by using the unique tip growth system of filamentous fungi. Live-cell imaging analysis, including superresolution microscopy, revealed that the fungus Aspergillus nidulans extends the hyphal tip in an oscillatory manner. The amount of F-actin and secretory vesicles (SV) accumulating at the hyphal tip oscillated with a positive temporal correlation, whereas vesicle amounts were negatively correlated to the growth rate. The intracellular Ca²⁺ level also pulsed with a positive temporal correlation to the amount of F-actin and SV at the hyphal tip. Two Ca²⁺ channels, MidA and CchA, were needed for proper tip growth and the oscillations of actin polymerization, exocytosis, and the growth rate. The data indicate a model in which transient Ca²⁺ pluses cause depolymerization of F-actin at the cortex and promote SV fusion with the plasma membrane, thereby extending the cell tip. Over time, Ca²⁺ diffuses away and F-actin and SV accumulate again at the hyphal tip. Our data provide evidence that temporally controlled actin polymerization and exocytosis are coordinated by pulsed Ca²⁺ influx, resulting in stepwise cell extension.

Calcium-activated BKCa channels govern dynamic membrane depolarizations of horizontal cells in rodent retina

KEY POINTS

Large conductance, Ca²⁺ -activated K⁺ (BK_Ca ) channels play important roles in mammalian retinal neurons, including photoreceptors, bipolar cells, amacrine cells and ganglion cells, but they have not been identified in horizontal cells. BK_Ca channel blockers paxilline and iberiotoxin, as well as Ca²⁺ free solutions and divalent cation Ca_v channel blockers, eliminate the outwardly rectifying current, while NS1619 enhances it. In symmetrical 150 mm K⁺ , single channels had a conductance close to 250 pS, within the range of all known BK_Ca channels. In current clamped horizontal cells, BK_Ca channels subdue depolarizing membrane potential excursions, reduce the average resting potential and decrease oscillations. The results show that BK_Ca channel activation puts a ceiling on horizontal cell depolarization and regulates the temporal responsivity of the cells.

ABSTRACT

Large conductance, calcium-activated potassium (BK_Ca ) channels have numerous roles in neurons including the regulation of membrane excitability, intracellular [Ca²⁺ ] regulation, and neurotransmitter release. In the retina, they have been identified in photoreceptors, bipolar cells, amacrine cells and ganglion cells, but have not been conclusively identified in mammalian horizontal cells. We found that outward current recorded between -30 and +60 mV is carried primarily in BK_Ca channels in isolated horizontal cells of rats and mice. Whole-cell outward currents were maximal at +50 mV and declined at membrane potentials positive to this value. This current was eliminated by the selective BK_Ca channel blocker paxilline (100 nm), iberiotoxin (10 μm), Ca²⁺ free solutions and divalent cation Ca_v channel blockers. It was activated by the BK_Ca channel activator NS1619 (30 μm). Single channel recordings revealed the conductance of the channels to be 244 ± 11 pS (n = 17; symmetrical 150 mm K⁺ ) with open probability being both voltage- and Ca²⁺ -dependent. The channels showed fast activation kinetics in response to Ca²⁺ influx and inactivation gating that could be modified by intracellular protease treatment, which suggests β subunit involvement. Under current clamp, block of BK_Ca current increased depolarizing membrane potential excursions, raising the average resting potential and producing oscillations. BK_Ca current activation with NS1619 inhibited oscillations and hyperpolarized the resting potential. These effects underscore the functional role of BK_Ca current in limiting depolarization of the horizontal cell membrane potential and suggest actions of these channels in regulating the temporal responsivity of the cells.

MCTP is an ER-resident calcium sensor that stabilizes synaptic transmission and homeostatic plasticity

Presynaptic homeostatic plasticity (PHP) controls synaptic transmission in organisms from Drosophila to human and is hypothesized to be relevant to the cause of human disease. However, the underlying molecular mechanisms of PHP are just emerging and direct disease associations remain obscure. In a forward genetic screen for mutations that block PHP we identified mctp (Multiple C2 Domain Proteins with Two Transmembrane Regions). Here we show that MCTP localizes to the membranes of the endoplasmic reticulum (ER) that elaborate throughout the soma, dendrites, axon and presynaptic terminal. Then, we demonstrate that MCTP functions downstream of presynaptic calcium influx with separable activities to stabilize baseline transmission, short-term release dynamics and PHP. Notably, PHP specifically requires the calcium coordinating residues in each of the three C2 domains of MCTP. Thus, we propose MCTP as a novel, ER-localized calcium sensor and a source of calcium-dependent feedback for the homeostatic stabilization of neurotransmission.

Physiological roles of the mitochondrial permeability transition pore

Neurons experience high metabolic demand during such processes as synaptic vesicle recycling, membrane potential maintenance and Ca2+ exchange/extrusion. The energy needs of these events are met in large part by mitochondrial production of ATP through the process of oxidative phosphorylation. The job of ATP production by the mitochondria is performed by the F1FO ATP synthase, a multi-protein enzyme that contains a membrane-inserted portion, an extra-membranous enzymatic portion and an extensive regulatory complex. Although required for ATP production by mitochondria, recent findings have confirmed that the membrane-confined portion of the c-subunit of the ATP synthase also houses a large conductance uncoupling channel, the mitochondrial permeability transition pore (mPTP), the persistent opening of which produces osmotic dysregulation of the inner mitochondrial membrane, uncoupling of oxidative phosphorylation and cell death. Recent advances in understanding the molecular components of mPTP and its regulatory mechanisms have determined that decreased uncoupling occurs in states of enhanced mitochondrial efficiency; relative closure of mPTP therefore contributes to cellular functions as diverse as cardiac development and synaptic efficacy.

Adaptor Protein 2 (AP-2) complex is essential for functional axogenesis in hippocampal neurons

The complexity and diversity of a neural network requires regulated elongation and branching of axons, as well as the formation of synapses between neurons. In the present study we explore the role of AP-2, a key endocytic adaptor protein complex, in the development of rat hippocampal neurons. We found that the loss of AP-2 during the early stage of development resulted in impaired axon extension and failed maturation of the axon initial segment (AIS). Normally the AIS performs two tasks in concert, stabilizing neural polarity and generating action potentials. In AP-2 silenced axons polarity is established, however there is a failure to establish action potential firing. Consequently, this impairs activity-driven Ca²⁺ influx and exocytosis at nerve terminals. In contrast, removal of AP-2 from older neurons does not impair axonal growth or signaling and synaptic function. Our data reveal that AP-2 has important roles in functional axogenesis by proper extension of axon as well as the formation of AIS during the early step of neurodevelopment.

Organelle-Specific Sensors for Monitoring Ca2+ Dynamics in Neurons

Calcium (Ca²⁺) plays innumerable critical functions in neurons ranging from regulation of neurotransmitter release and synaptic plasticity to activity-dependent transcription. Therefore, more than any other cell types, neurons are critically dependent on spatially and temporally controlled Ca²⁺ dynamics. This is achieved through an exquisite level of compartmentalization of Ca²⁺ storage and release from various organelles. The function of these organelles in the regulation of Ca²⁺ dynamics has been studied for decades using electrophysiological and optical methods combined with pharmacological and genetic alterations. Mitochondria and the endoplasmic reticulum (ER) are among the organelles playing the most critical roles in Ca²⁺ dynamics in neurons. At presynaptic boutons, Ca²⁺ triggers neurotransmitter release and synaptic plasticity, and postsynaptically, Ca²⁺ mobilization mediates long-term synaptic plasticity. To explore Ca²⁺ dynamics in live cells and intact animals, various synthetic and genetically encoded fluorescent Ca²⁺ sensors were developed, and recently, many groups actively increased the sensitivity and diversity of genetically encoded Ca²⁺ indicators (GECIs). Following conjugation with various signal peptides, these improved GECIs can be targeted to specific subcellular compartments, allowing monitoring of organelle-specific Ca²⁺ dynamics. Here, we review recent findings unraveling novel roles for mitochondria- and ER-dependent Ca²⁺ dynamics in neurons and at synapses.

Ring-like oligomers of Synaptotagmins and related C2 domain proteins

We recently reported that the C2AB portion of Synaptotagmin 1 (Syt1) could self-assemble into Ca(2+)-sensitive ring-like oligomers on membranes, which could potentially regulate neurotransmitter release. Here we report that analogous ring-like oligomers assemble from the C2AB domains of other Syt isoforms (Syt2, Syt7, Syt9) as well as related C2 domain containing protein, Doc2B and extended Synaptotagmins (E-Syts). Evidently, circular oligomerization is a general and conserved structural aspect of many C2 domain proteins, including Synaptotagmins. Further, using electron microscopy combined with targeted mutations, we show that under physiologically relevant conditions, both the Syt1 ring assembly and its rapid disruption by Ca(2+) involve the well-established functional surfaces on the C2B domain that are important for synaptic transmission. Our data suggests that ring formation may be triggered at an early step in synaptic vesicle docking and positions Syt1 to synchronize neurotransmitter release to Ca(2+) influx.

LKB1 Regulates Mitochondria-Dependent Presynaptic Calcium Clearance and Neurotransmitter Release Properties at Excitatory Synapses along Cortical Axons

Individual synapses vary significantly in their neurotransmitter release properties, which underlie complex information processing in neural circuits. Presynaptic Ca2+ homeostasis plays a critical role in specifying neurotransmitter release properties, but the mechanisms regulating synapse-specific Ca2+ homeostasis in the mammalian brain are still poorly understood. Using electrophysiology and genetically encoded Ca2+ sensors targeted to the mitochondrial matrix or to presynaptic boutons of cortical pyramidal neurons, we demonstrate that the presence or absence of mitochondria at presynaptic boutons dictates neurotransmitter release properties through Mitochondrial Calcium Uniporter (MCU)-dependent Ca2+ clearance. We demonstrate that the serine/threonine kinase LKB1 regulates MCU expression, mitochondria-dependent Ca2+ clearance, and thereby, presynaptic release properties. Re-establishment of MCU-dependent mitochondrial Ca2+ uptake at glutamatergic synapses rescues the altered neurotransmitter release properties characterizing LKB1-null cortical axons. Our results provide novel insights into the cellular and molecular mechanisms whereby mitochondria control neurotransmitter release properties in a bouton-specific way through presynaptic Ca2+ clearance.

Dynamics of Actin Cables in Polarized Growth of the Filamentous Fungus Aspergillus nidulans

Highly polarized growth of filamentous fungi requires a continuous supply of proteins and lipids to the hyphal tip. This transport is managed by vesicle trafficking via the actin and microtubule cytoskeletons and their associated motor proteins. Particularly, actin cables originating from the hyphal tip are essential for hyphal growth. Although, specific marker proteins have been developed to visualize actin cables in filamentous fungi, the exact organization and dynamics of actin cables has remained elusive. Here, we observed actin cables using tropomyosin (TpmA) and Lifeact fused to fluorescent proteins in living Aspergillus nidulans hyphae and studied the dynamics and regulation. GFP tagged TpmA visualized dynamic actin cables formed from the hyphal tip with cycles of elongation and shrinkage. The elongation and shrinkage rates of actin cables were similar and approximately 0.6 μm/s. Comparison of actin markers revealed that high concentrations of Lifeact reduced actin dynamics. Simultaneous visualization of actin cables and microtubules suggests temporally and spatially coordinated polymerization and depolymerization between the two cytoskeletons. Our results provide new insights into the molecular mechanism of ordered polarized growth regulated by actin cables and microtubules.

Coordinated process of polarized growth in filamentous fungi

Filamentous fungi are extremely polarized organisms, exhibiting continuous growth at their hyphal tips. The hyphal form is related to their pathogenicity in animals and plants, and their high secretion ability for biotechnology. Polarized growth requires a sequential supply of proteins and lipids to the hyphal tip. This transport is managed by vesicle trafficking via the actin and microtubule cytoskeleton. Therefore, the arrangement of the cytoskeleton is a crucial step to establish and maintain the cell polarity. This review summarizes recent findings unraveling the mechanism of polarized growth with special emphasis on the role of actin and microtubule cytoskeleton and polarity marker proteins. Rapid insertions of membranes via highly active exocytosis at hyphal tips could quickly dilute the accumulated polarity marker proteins. Recent findings by a super-resolution microscopy indicate that filamentous fungal cells maintain their polarity at the tips by repeating transient assembly and disassembly of polarity sites.

Synaptic Vesicle Proteins and Active Zone Plasticity

Neurotransmitter is released from synaptic vesicles at the highly specialized presynaptic active zone (AZ). The complex molecular architecture of AZs mediates the speed, precision and plasticity of synaptic transmission. Importantly, structural and functional properties of AZs vary significantly, even for a given connection. Thus, there appear to be distinct AZ states, which fundamentally influence neuronal communication by controlling the positioning and release of synaptic vesicles. Vice versa, recent evidence has revealed that synaptic vesicle components also modulate organizational states of the AZ. The protein-rich cytomatrix at the active zone (CAZ) provides a structural platform for molecular interactions guiding vesicle exocytosis. Studies in Drosophila have now demonstrated that the vesicle proteins Synaptotagmin-1 (Syt1) and Rab3 also regulate glutamate release by shaping differentiation of the CAZ ultrastructure. We review these unexpected findings and discuss mechanistic interpretations of the reciprocal relationship between synaptic vesicles and AZ states, which has heretofore received little attention.

Determining synaptic parameters using high-frequency activation

BACKGROUND

The specific properties of a synapse determine how neuronal activity evokes neurotransmitter release. Evaluating changes in synaptic properties during sustained activity is essential to understanding how genetic manipulations and neuromodulators regulate neurotransmitter release. Analyses of postsynaptic responses to high-frequency stimulation have provided estimates of the size of the readily-releasable pool (RRP) of vesicles (N0) and the probability of vesicular release (p) at multiple synapses.

NEW METHOD

Here, we introduce a model-based approach at the calyx of Held synapse in which depletion and the rate of replenishment (R) determine the number of available vesicles, and facilitation leads to a use-dependent increase in p when initial p is low.

RESULTS

When p is high and R is low, we find excellent agreement between estimates based on all three methods and the model. However, when p is low or when significant replenishment occurs between stimuli, estimates of different methods diverge, and model estimates are between the extreme estimates provided by the other approaches.

COMPARISON WITH OTHER METHODS

We compare our model-based approach to three other approaches that rely on different simplifying assumptions. Our findings suggest that our model provides a better estimate of N0 and p than previously-established methods, likely due to inaccurate assumptions about replenishment. More generally, our findings suggest that approaches commonly used to estimate N0 and p at other synapses are often applied under experimental conditions that yield inaccurate estimates.

CONCLUSIONS

Careful application of appropriate methods can greatly improve estimates of synaptic parameters.

Calcium Promotes the Formation of Syntaxin 1 Mesoscale Domains through Phosphatidylinositol 4,5-Bisphosphate

Phosphatidylinositol 4,5-bisphosphate (PI(4,5)P2) is a minor component of total plasma membrane lipids, but it has a substantial role in the regulation of many cellular functions, including exo- and endocytosis. Recently, it was shown that PI(4,5)P2and syntaxin 1, a SNARE protein that catalyzes regulated exocytosis, form domains in the plasma membrane that constitute recognition sites for vesicle docking. Also, calcium was shown to promote syntaxin 1 clustering in the plasma membrane, but the molecular mechanism was unknown. Here, using a combination of superresolution stimulated emission depletion microscopy, FRET, and atomic force microscopy, we show that Ca(2+)acts as a charge bridge that specifically and reversibly connects multiple syntaxin 1/PI(4,5)P2complexes into larger mesoscale domains. This transient reorganization of the plasma membrane by physiological Ca(2+)concentrations is likely to be important for Ca(2+)-regulated secretion.

Multiple cytosolic calcium buffers in posterior pituitary nerve terminals

Researchers have measured the ability of nerve terminals to buffer Ca²⁺ entering in response to electrical activity to better understand plasticity of hormone release.

Cytosolic Ca²⁺ buffers bind to a large fraction of Ca²⁺ as it enters a cell, shaping Ca²⁺ signals both spatially and temporally. In this way, cytosolic Ca²⁺ buffers regulate excitation-secretion coupling and short-term plasticity of release. The posterior pituitary is composed of peptidergic nerve terminals, which release oxytocin and vasopressin in response to Ca²⁺ entry. Secretion of these hormones exhibits a complex dependence on the frequency and pattern of electrical activity, and the role of cytosolic Ca²⁺ buffers in controlling pituitary Ca²⁺ signaling is poorly understood. Here, cytosolic Ca²⁺ buffers were studied with two-photon imaging in patch-clamped nerve terminals of the rat posterior pituitary. Fluorescence of the Ca²⁺ indicator fluo-8 revealed stepwise increases in free Ca²⁺ after a series of brief depolarizing pulses in rapid succession. These Ca²⁺ increments grew larger as free Ca²⁺ rose to saturate the cytosolic buffers and reduce the availability of Ca²⁺ binding sites. These titration data revealed two endogenous buffers. All nerve terminals contained a buffer with a K_d of 1.5–4.7 µM, and approximately half contained an additional higher-affinity buffer with a K_d of 340 nM. Western blots identified calretinin and calbindin D28K in the posterior pituitary, and their in vitro binding properties correspond well with our fluorometric analysis. The high-affinity buffer washed out, but at a rate much slower than expected from diffusion; washout of the low-affinity buffer could not be detected. This work has revealed the functional impact of cytosolic Ca²⁺ buffers in situ in nerve terminals at a new level of detail. The saturation of these cytosolic buffers will amplify Ca²⁺ signals and may contribute to use-dependent facilitation of release. A difference in the buffer compositions of oxytocin and vasopressin nerve terminals could contribute to the differences in release plasticity of these two hormones.

A novel explanation for observed CaMKII dynamics in dendritic spines with added EGTA or BAPTA

We present a simplified reaction network in a single well-mixed volume that captures the general features of CaMKII dynamics observed during both synaptic input and spine depolarization. Our model can also account for the greater-than-control CaMKII activation observed with added EGTA during depolarization. Calcium input currents are modeled after experimental observations, and existing models of calmodulin and CaMKII autophosphorylation are used. After calibration against CaMKII activation data in the absence of chelators, CaMKII activation dynamics due to synaptic input via n-methyl-d-aspartate receptors are qualitatively accounted for in the presence of the chelators EGTA and BAPTA without additional adjustments to the model. To account for CaMKII activation dynamics during spine depolarization with added EGTA or BAPTA, the model invokes the modulation of CaV2.3 (R-type) voltage-dependent calcium channel (VDCC) currents observed in the presence of EGTA or BAPTA. To our knowledge, this is a novel explanation for the increased CaMKII activation seen in dendritic spines with added EGTA, and suggests that differential modulation of VDCCs by EGTA and BAPTA offers an alternative or complementary explanation for other experimental results in which addition of EGTA or BAPTA produces different effects. Our results also show that a simplified reaction network in a single, well-mixed compartment is sufficient to account for the general features of observed CaMKII dynamics.

Superresolution microscopy reveals a dynamic picture of cell polarity maintenance during directional growth

Polar (directional) cell growth, a key cellular mechanism shared among a wide range of species, relies on targeted insertion of new material at specific locations of the plasma membrane. How these cell polarity sites are stably maintained during massive membrane insertion has remained elusive. Conventional live-cell optical microscopy fails to visualize polarity site formation in the crowded cell membrane environment because of its limited resolution. We have used advanced live-cell imaging techniques to directly observe the localization, assembly, and disassembly processes of cell polarity sites with high spatiotemporal resolution in a rapidly growing filamentous fungus, Aspergillus nidulans. We show that the membrane-associated polarity site marker TeaR is transported on microtubules along with secretory vesicles and forms a protein cluster at that point of the apical membrane where the plus end of the microtubule touches. There, a small patch of membrane is added through exocytosis, and the TeaR cluster gets quickly dispersed over the membrane. There is an incessant disassembly and reassembly of polarity sites at the growth zone, and each new polarity site locus is slightly offset from preceding ones. On the basis of our imaging results and computational modeling, we propose a transient polarity model that explains how cell polarity is stably maintained during highly active directional growth.

Shaping Neuronal Network Activity by Presynaptic Mechanisms

Neuronal microcircuits generate oscillatory activity, which has been linked to basic functions such as sleep, learning and sensorimotor gating. Although synaptic release processes are well known for their ability to shape the interaction between neurons in microcircuits, most computational models do not simulate the synaptic transmission process directly and hence cannot explain how changes in synaptic parameters alter neuronal network activity. In this paper, we present a novel neuronal network model that incorporates presynaptic release mechanisms, such as vesicle pool dynamics and calcium-dependent release probability, to model the spontaneous activity of neuronal networks. The model, which is based on modified leaky integrate-and-fire neurons, generates spontaneous network activity patterns, which are similar to experimental data and robust under changes in the model's primary gain parameters such as excitatory postsynaptic potential and connectivity ratio. Furthermore, it reliably recreates experimental findings and provides mechanistic explanations for data obtained from microelectrode array recordings, such as network burst termination and the effects of pharmacological and genetic manipulations. The model demonstrates how elevated asynchronous release, but not spontaneous release, synchronizes neuronal network activity and reveals that asynchronous release enhances utilization of the recycling vesicle pool to induce the network effect. The model further predicts a positive correlation between vesicle priming at the single-neuron level and burst frequency at the network level; this prediction is supported by experimental findings. Thus, the model is utilized to reveal how synaptic release processes at the neuronal level govern activity patterns and synchronization at the network level.

The activity of neuronal networks underlies basic neural functions such as sleep, learning and sensorimotor gating. Computational models of neuronal networks have been developed to capture the complexity of the network activity and predict how neuronal networks generate spontaneous activity. However, most computational models do not simulate the intricate synaptic release process that governs the interaction between neurons and has been shown to significantly impact neuronal network activity and animal behavior, learning and memory. Our paper demonstrates the importance of simulating the elaborate synaptic release process to understand how neuronal networks generate spontaneous activity and respond to manipulations of the release process. The model provides mechanistic explanations and predictions for experimental pharmacological and genetic manipulations. Thus, the model presents a novel computational platform to understand how mechanistic changes in the synaptic release process modulate network oscillatory activity that might impact basic neural functions.

Cell death disguised: The mitochondrial permeability transition pore as the c-subunit of the F(1)F(O) ATP synthase

Ion transport across the mitochondrial inner and outer membranes is central to mitochondrial function, including regulation of oxidative phosphorylation and cell death. Although essential for ATP production by mitochondria, recent findings have confirmed that the c-subunit of the ATP synthase also houses a large conductance uncoupling channel, the mitochondrial permeability transition pore (mPTP), the persistent opening of which produces osmotic dysregulation of the inner mitochondrial membrane and cell death. This review will discuss recent advances in understanding the molecular components of mPTP, its regulatory mechanisms and how these contribute directly to its physiological as well as pathological roles.

Inhibitory and excitatory axon terminals share a common nano-architecture of their Cav2.1 (P/Q-type) Ca(2+) channels

Tuning of the time course and strength of inhibitory and excitatory neurotransmitter release is fundamental for the precise operation of cortical network activity and is controlled by Ca²⁺ influx into presynaptic terminals through the high voltage-activated P/Q-type Ca²⁺ (Ca_v2.1) channels. Proper channel-mediated Ca²⁺-signaling critically depends on the topographical arrangement of the channels in the presynaptic membrane. Here, we used high-resolution SDS-digested freeze-fracture replica immunoelectron microscopy together with automatized computational analysis of Ca_v2.1 immunogold labeling to determine the precise subcellular organization of Ca_v2.1 channels in both inhibitory and excitatory terminals. Immunoparticles labeling the pore-forming α1 subunit of Ca_v2.1 channels were enriched over the active zone of the boutons with the number of channels (3–62) correlated with the area of the synaptic membrane. Detailed analysis showed that Ca_v2.1 channels are non-uniformly distributed over the presynaptic membrane specialization where they are arranged in clusters of an average five channels per cluster covering a mean area with a diameter of about 70 nm. Importantly, clustered arrangement and cluster properties did not show any significant difference between GABAergic and glutamatergic terminals. Our data demonstrate a common nano-architecture of Ca_v2.1 channels in inhibitory and excitatory boutons in stratum radiatum of the hippocampal CA1 area suggesting that the cluster arrangement is crucial for the precise release of transmitters from the axonal boutons.

Molecularly and structurally distinct synapses mediate reliable encoding and processing of auditory information

Hearing impairment is the most common human sensory deficit. Considering the sophisticated anatomy and physiology of the auditory system, disease-related failures frequently occur. To meet the demands of the neuronal circuits responsible for processing auditory information, the synapses of the lower auditory pathway are anatomically and functionally specialized to process acoustic information indefatigably with utmost temporal precision. Despite sharing some functional properties, the afferent synapses of the cochlea and of auditory brainstem differ greatly in their morphology and employ distinct molecular mechanisms for regulating synaptic vesicle release. Calyceal synapses of the endbulb of Held and the calyx of Held profit from a large number of release sites that project onto one principal cell. Cochlear inner hair cell ribbon synapses exhibit a unique one-to-one relation of the presynaptic active zone to the postsynaptic cell and use hair-cell-specific proteins such as otoferlin for vesicle release. The understanding of the molecular physiology of the hair cell ribbon synapse has been advanced by human genetics studies of sensorineural hearing impairment, revealing human auditory synaptopathy as a new nosological entity.

A rendezvous with the queen of ion channels: Three decades of ion channel research by David T Yue and his Calcium Signals Laboratory

David T. Yue was a renowned biophysicist who dedicated his life to the study of Ca(2+) signaling in cells. In the wake of his passing, we are left not only with a feeling of great loss, but with a tremendous and impactful body of work contributed by a remarkable man. David's research spanned the spectrum from atomic structure to organ systems, with a quantitative rigor aimed at understanding the fundamental mechanisms underlying biological function. Along the way he developed new tools and approaches, enabling not only his own research but that of his contemporaries and those who will come after him. While we cannot hope to replicate the eloquence and style we are accustomed to in David's writing, we nonetheless undertake a review of David's chosen field of study with a focus on many of his contributions to the calcium channel field.

Synaptic vesicle exocytosis and increased cytosolic calcium are both necessary but not sufficient for activity-dependent bulk endocytosis

Activity‐dependent bulk endocytosis (ADBE) is the dominant synaptic vesicle (SV) endocytosis mode in central nerve terminals during intense neuronal activity. By definition this mode is triggered by neuronal activity; however, key questions regarding its mechanism of activation remain unaddressed. To determine the basic requirements for ADBE triggering in central nerve terminals, we decoupled SV fusion events from activity‐dependent calcium influx using either clostridial neurotoxins or buffering of intracellular calcium. ADBE was monitored both optically and morphologically by observing uptake of the fluid phase markers tetramethylrhodamine‐dextran and horse radish peroxidase respectively. Ablation of SV fusion with tetanus toxin resulted in the arrest of ADBE, but had no effect on other calcium‐dependent events such as activity‐dependent dynamin I dephosphorylation, indicating that SV exocytosis is necessary for triggering. Furthermore, the calcium chelator EGTA abolished ADBE while leaving SV exocytosis intact, demonstrating that ADBE is triggered by intracellular free calcium increases outside the active zone. Activity‐dependent dynamin I dephosphorylation was also arrested in EGTA‐treated neurons, consistent with its proposed role in triggering ADBE. Thus, SV fusion and increased cytoplasmic free calcium are both necessary but not sufficient individually to trigger ADBE.

Activity‐dependent bulk endocytosis (ADBE) is the dominant synaptic vesicle (SV) endocytosis mode in central nerve terminals during intense neuronal activity. To determine the minimal requirements for ADBE triggering, we decoupled SV fusion events from activity‐dependent calcium influx using either clostridial neurotoxins or buffering of intracellular calcium. We found that SV fusion and increased cytoplasmic free calcium are both necessary but not sufficient to trigger ADBE.

The role of Na+/Ca2+ exchanger subtypes in neuronal ischemic injury

The Na(+)/Ca(2+) exchanger (NCX) plays an important role in the maintenance of Na(+) and Ca(2+) homeostasis in most cells including neurons under physiological and pathological conditions. It exists in three subtypes (NCX1-3) with different tissue distributions but all of them are present in the brain. NCX transports Na(+) and Ca(2+) in either Ca(2+)-efflux (forward) or Ca(2+)-influx (reverse) mode, depending on membrane potential and transmembrane ion gradients. During neuronal ischemia, Na(+) and Ca(2+) ionic disturbances favor NCX to work in reverse mode, giving rise to increased intracellular Ca(2+) levels, while it may regain its forward mode activity on reperfusion. The exact significance of NCX in neuronal ischemic and reperfusion states remains unclear. The differential role of NCX subtypes in ischemic neuronal injury has been extensively investigated using various pharmacological tools as well as genetic models. This review discusses the mode of action of NCX in ischemic and reperfusion states, the differential roles played by NCX subtypes in these states as well as the role of NCX in pre- and postconditioning. NCX subtypes carry variable roles in ischemic injury. Furthermore, the mode of action of each subtype varies in ischemia and reperfusion states. Thus, therapeutic targeting of NCX in stroke should be based on appropriate timing of the administration of NCX subtype-specific strategies.

Implementation of linear sensory signaling via multiple coordinated mechanisms at central vestibular nerve synapses

Signal transfer in neural circuits is dynamically modified by the recent history of neuronal activity. Short-term plasticity endows synapses with nonlinear transmission properties, yet synapses in sensory and motor circuits are capable of signaling linearly over a wide range of presynaptic firing rates. How do such synapses achieve rate-invariant transmission despite history-dependent nonlinearities? Here, ultrastructural, biophysical, and computational analyses demonstrate that concerted molecular, anatomical, and physiological refinements are required for central vestibular nerve synapses to linearly transmit rate-coded sensory signals. Vestibular synapses operate in a physiological regime of steady-state depression imposed by tonic firing. Rate-invariant transmission relies on brief presynaptic action potentials that delimit calcium influx, large pools of rapidly mobilized vesicles, multiple low-probability release sites, robust postsynaptic receptor sensitivity, and efficient transmitter clearance. Broadband linear synaptic filtering of head motion signals is thus achieved by coordinately tuned synaptic machinery that maintains physiological operation within inherent cell biological limitations.

EF-hand protein Ca2+ buffers regulate Ca2+ influx and exocytosis in sensory hair cells

EF-hand Ca(2+)-binding proteins are thought to shape the spatiotemporal properties of cellular Ca(2+) signaling and are prominently expressed in sensory hair cells in the ear. Here, we combined genetic disruption of parvalbumin-α, calbindin-D28k, and calretinin in mice with patch-clamp recording, in vivo physiology, and mathematical modeling to study their role in Ca(2+) signaling, exocytosis, and sound encoding at the synapses of inner hair cells (IHCs). IHCs lacking all three proteins showed excessive exocytosis during prolonged depolarizations, despite enhanced Ca(2+)-dependent inactivation of their Ca(2+) current. Exocytosis of readily releasable vesicles remained unchanged, in accordance with the estimated tight spatial coupling of Ca(2+) channels and release sites (effective "coupling distance" of 17 nm). Substitution experiments with synthetic Ca(2+) chelators indicated the presence of endogenous Ca(2+) buffers equivalent to 1 mM synthetic Ca(2+)-binding sites, approximately half of them with kinetics as fast as 1,2-Bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid (BAPTA). Synaptic sound encoding was largely unaltered, suggesting that excess exocytosis occurs extrasynaptically. We conclude that EF-hand Ca(2+) buffers regulate presynaptic IHC function for metabolically efficient sound coding.

Super-resolution microscopy of the synaptic active zone

Brain function relies on accurate information transfer at chemical synapses. At the presynaptic active zone (AZ) a variety of specialized proteins are assembled to complex architectures, which set the basis for speed, precision and plasticity of synaptic transmission. Calcium channels are pivotal for the initiation of excitation-secretion coupling and, correspondingly, capture a central position at the AZ. Combining quantitative functional studies with modeling approaches has provided predictions of channel properties, numbers and even positions on the nanometer scale. However, elucidating the nanoscopic organization of the surrounding protein network requires direct ultrastructural access. Without this information, knowledge of molecular synaptic structure-function relationships remains incomplete. Recently, super-resolution microscopy (SRM) techniques have begun to enter the neurosciences. These approaches combine high spatial resolution with the molecular specificity of fluorescence microscopy. Here, we discuss how SRM can be used to obtain information on the organization of AZ proteins.

Gating of dopamine transmission by calcium and axonal N-, Q-, T- and L-type voltage-gated calcium channels differs between striatal domains

KEY POINTS

The voltage-gated Ca(2+) channels (VGCCs) that catalyse striatal dopamine transmission are critical to dopamine function and might prime subpopulations of neurons for parkinsonian degeneration. However, the VGCCs that operate on mesostriatal axons are incompletely defined; previous studies encompassed channels on striatal cholinergic interneurons that strongly influence dopamine transmission. We define that multiple types of axonal VGCCs operate that extend beyond classic presynaptic N/P/Q channels to include T- and L-types. We reveal differences in VGCC function between mouse axon types that in humans are vulnerable versus resistant to Parkinson's disease. We show for the first time that this is underpinned by different sensitivity of dopamine transmission to extracellular Ca(2+) and by different spatiotemporal intracellular Ca(2+) microdomains. These data define key principles of how Ca(2+) and VGCCs govern dopamine transmission in the healthy brain and reveal differences between neuron types that might contribute to vulnerability in disease.

ABSTRACT

The axonal voltage-gated Ca(2+) channels (VGCCs) that catalyse dopamine (DA) transmission are incompletely defined. Yet, they are critical to DA function and might prime subpopulations of DA neurons for parkinsonian degeneration. Previous studies of VGCCs will have encompassed those on striatal cholinergic interneurons, which strongly influence DA transmission. We identify which VGCCs on DA axons govern DA transmission, we determine their dynamic properties and reveal an underlying basis for differences between the caudate putamen (CPu) and nucleus accumbens (NAc). We detected DA release evoked electrically during nicotinic receptor blockade or optogenetically by light activation of channel rhodopsin-expressing DA axons in mouse striatal slices. Subtype-specific VGCC blockers indicated that N-, Q-, T- and L-VGCCs govern DA release in CPu, but in NAc, T and L-channels are relatively silent. The roles of the most dominant channels were inversely frequency-dependent, due to low-pass filtering of DA release by Ca(2+)-dependent relationships between initial release probability and short-term plasticity. Ca(2+) concentration-response curves revealed that differences between CPu and NAc were due to greater underlying Ca(2+) sensitivity of DA transmission from CPu axons. Functions for 'silent' L- and T-channels in NAc could be unmasked by elevating extracellular [Ca(2+)]. Furthermore, we identified a greater coupling between BAPTA-sensitive, fast Ca(2+) transients and DA transmission in CPu axons, and evidence for endogenous fast buffering of Ca(2+) in NAc. These data reveal that a range of VGCCs operate dynamically on DA axons, depending on local driving forces. Furthermore, they reveal dramatic differences in Ca(2+) handling between axonal subpopulations that show different vulnerability to parkinsonian degeneration.

Estimation of presynaptic calcium currents and endogenous calcium buffers at the frog neuromuscular junction with two different calcium fluorescent dyes

At the frog neuromuscular junction, under physiological conditions, the direct measurement of calcium currents and of the concentration of intracellular calcium buffers—which determine the kinetics of calcium concentration and neurotransmitter release from the nerve terminal—has hitherto been technically impossible. With the aim of quantifying both Ca²⁺ currents and the intracellular calcium buffers, we measured fluorescence signals from nerve terminals loaded with the low-affinity calcium dye Magnesium Green or the high-affinity dye Oregon Green BAPTA-1, simultaneously with microelectrode recordings of nerve-action potentials and end-plate currents. The action-potential-induced fluorescence signals in the nerve terminals developed much more slowly than the postsynaptic response. To clarify the reasons for this observation and to define a spatiotemporal profile of intracellular calcium and of the concentration of mobile and fixed calcium buffers, mathematical modeling was employed. The best approximations of the experimental calcium transients for both calcium dyes were obtained when the calcium current had an amplitude of 1.6 ± 0.08 pA and a half-decay time of 1.2 ± 0.06 ms, and when the concentrations of mobile and fixed calcium buffers were 250 ± 13 μM and 8 ± 0.4 mM, respectively. High concentrations of endogenous buffers define the time course of calcium transients after an action potential in the axoplasm, and may modify synaptic plasticity.

Quantitative analysis linking inner hair cell voltage changes and postsynaptic conductance change: a modelling study

This paper presents a computational model which estimates the postsynaptic conductance change of mammalian Type I afferent peripheral process when airborne acoustic waves impact on the tympanic membrane. A model of the human auditory periphery is used to estimate the inner hair cell potential change in response to airborne sound. A generic and tunable topology of the mammalian synaptic ribbon is generated and the voltage dependence of its substructures is used to calculate discrete and probabilistic neurotransmitter vesicle release. Results suggest an almost linear relationship between increasing sound level (in dB SPL) and the postsynaptic conductance for frequencies considered too high for neurons to phase lock with (i.e., a few kHz). Furthermore coordinated vesicle release is shown for up to 300–400 Hz and a mechanism of phase shifting the subharmonic content of a stimulating signal is suggested. Model outputs suggest that strong onset response and highly synchronised multivesicular release rely on compound fusion of ribbon tethered vesicles.

Nanoscale distribution of presynaptic Ca(2+) channels and its impact on vesicular release during development

Synaptic efficacy and precision are influenced by the coupling of voltage-gated Ca²⁺ channels (VGCCs) to vesicles. But because the topography of VGCCs and their proximity to vesicles is unknown, a quantitative understanding of the determinants of vesicular release at nanometer scale is lacking. To investigate this, we combined freeze-fracture replica immunogold labeling of Ca_v2.1 channels, local [Ca²⁺] imaging, and patch pipette perfusion of EGTA at the calyx of Held. Between postnatal day 7 and 21, VGCCs formed variable sized clusters and vesicular release became less sensitive to EGTA, whereas fixed Ca²⁺ buffer properties remained constant. Experimentally constrained reaction-diffusion simulations suggest that Ca²⁺ sensors for vesicular release are located at the perimeter of VGCC clusters (<30 nm) and predict that VGCC number per cluster determines vesicular release probability without altering release time course. This “perimeter release model” provides a unifying framework accounting for developmental changes in both synaptic efficacy and time course.

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Ca²⁺ channels form clusters with highly variable numbers of channels

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EGTA sensitivity suggests that synaptic vesicles are tightly coupled to clusters

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Ca²⁺ channel number per cluster alters synaptic efficacy, but not precision

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A perimeter model accounts for synaptic efficacy and precision during development

Synaptotagmin-7 is an asynchronous calcium sensor for synaptic transmission in neurons expressing SNAP-23

Synchronization of neurotransmitter release with the presynaptic action potential is essential for maintaining fidelity of information transfer in the central nervous system. However, synchronous release is frequently accompanied by an asynchronous release component that builds up during repetitive stimulation, and can even play a dominant role in some synapses. Here, we show that substitution of SNAP-23 for SNAP-25 in mouse autaptic glutamatergic hippocampal neurons results in asynchronous release and a higher frequency of spontaneous release events (mEPSCs). Use of neurons from double-knock-out (SNAP-25, synaptotagmin-7) mice in combination with viral transduction showed that SNAP-23-driven release is triggered by endogenous synaptotagmin-7. In the absence of synaptotagmin-7 release became even more asynchronous, and the spontaneous release rate increased even more, indicating that synaptotagmin-7 acts to synchronize release and suppress spontaneous release. However, compared to synaptotagmin-1, synaptotagmin-7 is a both leaky and asynchronous calcium sensor. In the presence of SNAP-25, consequences of the elimination of synaptotagmin-7 were small or absent, indicating that the protein pairs SNAP-25/synaptotagmin-1 and SNAP-23/synaptotagmin-7 might act as mutually exclusive calcium sensors. Expression of fusion proteins between pHluorin (pH-sensitive GFP) and synaptotagmin-1 or -7 showed that vesicles that fuse using the SNAP-23/synaptotagmin-7 combination contained synaptotagmin-1, while synaptotagmin-7 barely displayed activity-dependent trafficking between vesicle and plasma membrane, implying that it acts as a plasma membrane calcium sensor. Overall, these findings support the idea of alternative syt∶SNARE combinations driving release with different kinetics and fidelity.

The natural history of sound localization in mammals--a story of neuronal inhibition

Our concepts of sound localization in the vertebrate brain are widely based on the general assumption that both the ability to detect air-borne sounds and the neuronal processing are homologous in archosaurs (present day crocodiles and birds) and mammals. Yet studies repeatedly report conflicting results on the neuronal circuits and mechanisms, in particular the role of inhibition, as well as the coding strategies between avian and mammalian model systems. Here we argue that mammalian and avian phylogeny of spatial hearing is characterized by a convergent evolution of hearing air-borne sounds rather than by homology. In particular, the different evolutionary origins of tympanic ears and the different availability of binaural cues in early mammals and archosaurs imposed distinct constraints on the respective binaural processing mechanisms. The role of synaptic inhibition in generating binaural spatial sensitivity in mammals is highlighted, as it reveals a unifying principle of mammalian circuit design for encoding sound position. Together, we combine evolutionary, anatomical and physiological arguments for making a clear distinction between mammalian processing mechanisms and coding strategies and those of archosaurs. We emphasize that a consideration of the convergent nature of neuronal mechanisms will significantly increase the explanatory power of studies of spatial processing in both mammals and birds.

Calcium sensitive ring-like oligomers formed by synaptotagmin

The synaptic vesicle protein synaptotagmin-1 (SYT) is required to couple calcium influx to the membrane fusion machinery. However, the structural mechanism underlying this process is unclear. Here we report an unexpected circular arrangement (ring) of SYT's cytosolic domain (C2AB) formed on lipid monolayers in the absence of free calcium ions as revealed by electron microscopy. Rings vary in diameter from 18-43 nm, corresponding to 11-26 molecules of SYT. Continuous stacking of the SYT rings occasionally converts both lipid monolayers and bilayers into protein-coated tubes. Helical reconstruction of the SYT tubes shows that one of the C2 domains (most likely C2B, based on its biochemical properties) interacts with the membrane and is involved in ring formation, and the other C2 domain points radially outward. SYT rings are disrupted rapidly by physiological concentrations of free calcium but not by magnesium. Assuming that calcium-free SYT rings are physiologically relevant, these results suggest a simple and novel mechanism by which SYT regulates neurotransmitter release: The ring acts as a spacer to prevent the completion of the soluble N-ethylmaleimide-sensitive factor activating protein receptor (SNARE) complex assembly, thereby clamping fusion in the absence of calcium. When the ring disassembles in the presence of calcium, fusion proceeds unimpeded.

Developmental changes in Ca2+ channel subtypes regulating endocytosis at the calyx of Held

At the mammalian central synapse, Ca(2+) influx through Ca(2+) channels triggers neurotransmitter release by exocytosis of synaptic vesicles, which fuse with the presynaptic membrane and are subsequently retrieved by endocytosis. At the calyx of Held terminal, P/Q-type Ca(2+) channels mainly mediate exocytosis, while N- and R-type channels have a minor role in young terminals (postnatal days 8-11). The role of each Ca(2+) channel subtype in endocytosis remains to be elucidated; therefore, we examined the role of each type of Ca(2+) channel in endocytosis, by using whole-cell patch-clamp recordings in conjunction with capacitance measurement techniques. We found that at the young calyx terminal, when R-type Ca(2+) channels were blocked, the slow mode of endocytosis was further slowed, while blocking of either P/Q- or N-type Ca(2+) channels had no major effect. In more mature terminals (postnatal days 14-17), the slow mode of endocytosis was mainly triggered by P/Q-type Ca(2+) channels, suggesting developmental changes in the regulation of the slow mode of endocytosis by different Ca(2+) channel subtypes. In contrast, a fast mode of endocytosis was observed after strong stimulation in young terminals that was mediated mainly by P/Q-type, but not R- or N-type Ca(2+) channels. These results suggest that different types of Ca(2+) channels regulate the two different modes of endocytosis. The results may also suggest that exo- and endocytosis are regulated independently at different sites in young animals but are more tightly coupled in older animals, allowing more efficient synaptic vesicle cycling adapted for fast signalling.

Ca2+-Calmodulin regulates SNARE assembly and spontaneous neurotransmitter release via v-ATPase subunit V0a1

Ca²⁺–Calmodulin binding to neuronal v-ATPase V0 subunit a1 (V100) regulates SNARE complex assembly for a putative subset of synaptic vesicles that sustain spontaneous release in Drosophila.

Most chemical neurotransmission occurs through Ca²⁺-dependent evoked or spontaneous vesicle exocytosis. In both cases, Ca²⁺ sensing is thought to occur shortly before exocytosis. In this paper, we provide evidence that the Ca²⁺ dependence of spontaneous vesicle release may partly result from an earlier requirement of Ca²⁺ for the assembly of soluble N-ethylmaleimide–sensitive fusion attachment protein receptor (SNARE) complexes. We show that the neuronal vacuolar-type H⁺-adenosine triphosphatase V0 subunit a1 (V100) can regulate the formation of SNARE complexes in a Ca²⁺–Calmodulin (CaM)-dependent manner. Ca²⁺–CaM regulation of V100 is not required for vesicle acidification. Specific disruption of the Ca²⁺-dependent regulation of V100 by CaM led to a >90% loss of spontaneous release but only had a mild effect on evoked release at Drosophila melanogaster embryo neuromuscular junctions. Our data suggest that Ca²⁺–CaM regulation of V100 may control SNARE complex assembly for a subset of synaptic vesicles that sustain spontaneous release.

The blockade of the neurotransmitter release apparatus by botulinum neurotoxins

The high toxicity of the seven serotypes of botulinum neurotoxins (BoNT/A to G), together with their specificity and reversibility, includes them in the list A of potential bioterrorism weapons and, at the same time, among the therapeutics of choice for a variety of human syndromes. They invade nerve terminals and cleave specifically the three proteins which form the heterotrimeric SNAP REceptors (SNARE) complex that mediates neurotransmitter release. The BoNT-induced cleavage of the SNARE proteins explains by itself the paralysing activity of the BoNTs because the truncated proteins cannot form the SNARE complex. However, in the case of BoNT/A, the most widely used toxin in therapy, additional factors come into play as it only removes a few residues from the synaptosomal associate protein of 25 kDa C-terminus and this results in a long duration of action. To explain these facts and other experimental data, we present here a model for the assembly of the neuroexocytosis apparatus in which Synaptotagmin and Complexin first assist the zippering of the SNARE complex, and then stabilize and clamp an octameric radial assembly of the SNARE complexes.

Calcium regulates molecular interactions of otoferlin with soluble NSF attachment protein receptor (SNARE) proteins required for hair cell exocytosis

Mutations in otoferlin, a C2 domain-containing ferlin family protein, cause non-syndromic hearing loss in humans (DFNB9 deafness). Furthermore, transmitter secretion of cochlear inner hair cells is compromised in mice lacking otoferlin. In the present study, we show that the C2F domain of otoferlin directly binds calcium (KD = 267 μM) with diminished binding in a pachanga (D1767G) C2F mouse mutation. Calcium was found to differentially regulate binding of otoferlin C2 domains to target SNARE (t-SNARE) proteins and phospholipids. C2D-F domains interact with the syntaxin-1 t-SNARE motif with maximum binding within the range of 20-50 μM Ca(2+). At 20 μM Ca(2+), the dissociation rate was substantially lower, indicating increased binding (KD = ∼10(-9)) compared with 0 μM Ca(2+) (KD = ∼10(-8)), suggesting a calcium-mediated stabilization of the C2 domain·t-SNARE complex. C2A and C2B interactions with t-SNAREs were insensitive to calcium. The C2F domain directly binds the t-SNARE SNAP-25 maximally at 100 μM and with reduction at 0 μM Ca(2+), a pattern repeated for C2F domain interactions with phosphatidylinositol 4,5-bisphosphate. In contrast, C2F did not bind the vesicle SNARE protein synaptobrevin-1 (VAMP-1). Moreover, an antibody targeting otoferlin immunoprecipitated syntaxin-1 and SNAP-25 but not synaptobrevin-1. As opposed to an increase in binding with increased calcium, interactions between otoferlin C2F domain and intramolecular C2 domains occurred in the absence of calcium, consistent with intra-C2 domain interactions forming a "closed" tertiary structure at low calcium that "opens" as calcium increases. These results suggest a direct role for otoferlin in exocytosis and modulation of calcium-dependent membrane fusion.

An excess-calcium-binding-site model predicts neurotransmitter release at the neuromuscular junction

Despite decades of intense experimental studies, we still lack a detailed understanding of synaptic function. Fortunately, using computational approaches, we can obtain important new insights into the inner workings of these important neural systems. Here, we report the development of a spatially realistic computational model of an entire frog active zone in which we constrained model parameters with experimental data, and then used Monte Carlo simulation methods to predict the Ca(2+)-binding stoichiometry and dynamics that underlie neurotransmitter release. Our model reveals that 20-40 independent Ca(2+)-binding sites on synaptic vesicles, only a fraction of which need to bind Ca(2+) to trigger fusion, are sufficient to predict physiological release. Our excess-Ca(2+)-binding-site model has many functional advantages, agrees with recent data on synaptotagmin copy number, and is the first (to our knowledge) to link detailed physiological observations with the molecular machinery of Ca(2+)-triggered exocytosis. In addition, our model provides detailed microscopic insight into the underlying Ca(2+) dynamics during synapse activation.

Lateral mobility of presynaptic α7-containing nicotinic receptors and its relevance for glutamate release

Surface diffusion of postsynaptic receptors shapes synaptic transmission. Presynaptic receptors also influence transmission, but the relevance of their mobility for synaptic function is unknown. Using single-particle tracking with quantum dots, we show that calcium-permeable α7-containing nicotinic acetylcholine receptors (α7-nAChRs), capable of promoting transmitter release, are mobile on presynaptic terminals but constrained in synaptic space on rat hippocampal neurons in culture. Additional immobilization of presynaptic α7-nAChRs by antibody crosslinking increases glutamate release capacity as seen in the frequency of spontaneous miniature postsynaptic currents and the size of the readily releasable pool of transmitter. Conversely, blocking glutamate release by targeting tetanus toxin to individual synapses increases α7-nAChR dwell time at presynaptic sites. The effects on release require functional α7-nAChRs and may to depend on CAST/ELKS (calpastatin/glutamine, leucine, lysine, and serine-rich protein), which an unbiased proteomic screen yielded. The results support a new homeostatic regulatory mechanism in which α7-nAChR restrain may be adjusted as needed at presynaptic sites via active zone proteins to maintain transmitter release capability.

Inhibition of the Aplysia sensory neuron calcium current with dopamine and serotonin

The inhibition of Aplysia pleural mechanosensory neuron synapses by dopamine and serotonin through activation of endogenous dopaminergic and expressed 5-HT1Apl(a)/b receptors, respectively, involves a reduction in action potential-associated calcium influx. We show that the inhibition of synaptic efficacy is downstream of the readily releasable pool, suggesting that inhibition is at the level of calcium secretion coupling, likely a result of the changes in the calcium current. Indeed, the inhibitory responses directly reduce a CaV2-like calcium current in isolated sensory neurons. The inhibition of the calcium current is voltage independent as it is not affected by a strong depolarizing prepulse, consistent with other invertebrate CaV2 calcium currents. Similar to voltage-independent inhibition of vertebrate nociceptors, inhibition was blocked with Src tyrosine kinase inhibitors. The data suggest a conserved mechanism by which G protein-coupled receptor activation can inhibit the CaV2 calcium current in nociceptive neurons.

Ablation of Ca(V)2.1 voltage-gated Ca²⁺ channels in mouse forebrain generates multiple cognitive impairments

Voltage-gated Ca_V2.1 (P/Q-type) Ca²⁺ channels located at the presynaptic membrane are known to control a multitude of Ca²⁺-dependent cellular processes such as neurotransmitter release and synaptic plasticity. Our knowledge about their contributions to complex cognitive functions, however, is restricted by the limited adequacy of existing transgenic Ca_V2.1 mouse models. Global Ca_V2.1 knock-out mice lacking the α1 subunit Cacna1a gene product exhibit early postnatal lethality which makes them unsuitable to analyse the relevance of Ca_V2.1 Ca²⁺ channels for complex behaviour in adult mice. Consequently we established a forebrain specific Ca_V2.1 knock-out model by crossing mice with a floxed Cacna1a gene with mice expressing Cre-recombinase under the control of the NEX promoter. This novel mouse model enabled us to investigate the contribution of Ca_V2.1 to complex cognitive functions, particularly learning and memory. Electrophysiological analysis allowed us to test the specificity of our conditional knock-out model and revealed an impaired synaptic transmission at hippocampal glutamatergic synapses. At the behavioural level, the forebrain-specific Ca_V2.1 knock-out resulted in deficits in spatial learning and reference memory, reduced recognition memory, increased exploratory behaviour and a strong attenuation of circadian rhythmicity. In summary, we present a novel conditional Ca_V2.1 knock-out model that is most suitable for analysing the in vivo functions of Ca_V2.1 in the adult murine forebrain.

TRPM7 triggers Ca2+ sparks and invadosome formation in neuroblastoma cells

Cell migration depends on the dynamic formation and turnover of cell adhesions and is tightly controlled by actomyosin contractility and local Ca2+ signals. The divalent cation channel TRPM7 (Transient Receptor Potential cation channel, subfamily Melastatin, member 7) has recently received much attention as a regulator of cell adhesion, migration and (localized) Ca2+ signaling. Overexpression and knockdown of TRPM7 affects actomyosin contractility and the formation of cell adhesions such as invadosomes and focal adhesions, but the role of TRPM7-mediated Ca2+ signals herein is currently not understood. Using Total Internal Reflection Fluorescence (TIRF) Ca2+ fluorometry and a novel automated analysis routine we have addressed the role of Ca2+ in the control of invadosome dynamics in N1E-115 mouse neuroblastoma cells. We find that TRPM7 promotes the formation of highly repetitive and localized Ca2+ microdomains or "Ca2+ sparking hotspots" at the ventral plasma membrane. Ca2+ sparking appears strictly dependent on extracellular Ca2+ and is abolished by TRPM7 channel inhibitors such as waixenicin-A. TRPM7 inhibition also induces invadosome dissolution. However, invadosome formation is (functionally and spatially) dissociated from TRPM7-mediated Ca2+ sparks. Rather, our data indicate that TRPM7 affects actomyosin contractility and invadosome formation independent of Ca2+ influx.

Dominance of P/Q-type calcium channels in depolarization-induced presynaptic FM dye release in cultured hippocampal neurons

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We analyzed depolarization-induced synaptic FM dye release in hippocampal neurons.

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We pharmacologically isolated the contribution of voltage-gated Ca²⁺ channels.

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85% of synapses utilize N- and P/Q-type channels, 15% only P/Q-type channels.

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In both groups of synapses release kinetics are determined by P/Q-type channels.

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We propose a more direct coupling of P/Q-type channels to synaptic release.

Neurotransmitter release probability is related by high power to the local concentration of calcium in presynaptic terminals, which in turn is controlled by voltage-gated calcium channels. P/Q- and N-type channels trigger synaptic transmission in the majority of neurons of the central nervous system. However, whether and under which conditions both channel types act cooperatively or independently is still insufficiently understood. Previous studies suggested either a dominance of N- or P/Q-type channels, or a synergistic action of both channels, depending on the experimental paradigms. Thus, to provide insight into the properties of neurotransmitter release in cultured mouse hippocampal neurons, we used quantitative analysis of FM dye release from presynaptic boutons induced by high potassium membrane depolarization. Increasing extracellular potassium concentrations revealed a sigmoid dependence of FM dye release to the stimulation strength. Individual and combined application of the P/Q- and N-type channel-specific blockers ω-agatoxin-IVA and ω-conotoxin-GVIA, respectively, allowed us to specifically isolate the contribution of both channel types to release triggered with 40 mM KCl. Analysis of the release kinetics and the fractional release amplitude demonstrate that, whereas in only 15% of the synapses release depended exclusively on P/Q-type channels, the majority of synapses (85%) contained both N- and P/Q-type channels. Nevertheless, the kinetics of FM dye release in synapses containing both channel types was determined by the P/Q-type channels. Together, our data suggest a more direct coupling of P/Q-type channels to synaptic release compared to N-type channels, which may explain the high prevalence of neurological P/Q-type channelopathies.

EPAC inhibition of SUR1 receptor increases glutamate release and seizure vulnerability

EPAC (Exchange Proteins Activated by cAMP) regulates glutamate transmitter release in the central neurons, but a role underlying this regulation has yet to be identified. Here we show that EPAC binds directly to the intracellular loop of an ATP-sensitive potassium (KATP) channel type-1 sulfonylurea receptor (SUR1) receptor consisting of amino acids 859-881 (SUR1(859-881)). Ablation of EPAC or expression of SUR1(859-881), which intercepts EPAC-SUR1 binding, increases the open probability of KATP channels consisting of the Kir6.1 subunit and SUR1. Opening of KATP channels inhibits glutamate release and reduces seizure vulnerability in adult mice. Therefore, EPAC interaction with SUR1 controls seizure susceptibility and possibly acts via regulation of glutamate release.