Optical mapping of release properties in synapses

Frequency of Spontaneous Neurotransmission at Individual Boutons Corresponds to the Size of the Readily Releasable Pool of Vesicles

Synapses maintain two forms of neurotransmitter release to support communication in the brain. First, evoked neurotransmitter release is triggered by the invasion of an action potential (AP) across en passant boutons that form along axons. The probability of evoked release (Pr) varies substantially across boutons, even within a single axon. Such heterogeneity is the result of differences in the probability of a single synaptic vesicle (SV) fusing (Pv) and in the number of vesicles available for immediate release, known as the readily releasable pool (RRP). Spontaneous release (also known as a mini) is an important form of neurotransmission that occurs in the absence of APs. Because it cannot be triggered with electrical stimulation, much less is known about potential heterogeneity in the frequency of spontaneous release between boutons. We utilized a photostable and bright fluorescent indicator of glutamate release (iGluSnFR3) to quantify both spontaneous and evoked release at individual glutamatergic boutons. We found that the rate of spontaneous release is quite heterogenous at the level of individual boutons. Interestingly, when measuring both evoked and spontaneous release at single synapses, we found that boutons with the highest rates of spontaneous release also displayed the largest evoked responses. Using a new optical method to measure RRP at individual boutons, we found that this heterogeneity in spontaneous release was strongly correlated with the size of the RRP, but not related to Pv. We conclude that the RRP is a critical and dynamic aspect of synaptic strength that contributes to both evoked and spontaneous vesicle release.

Glutamate indicators with improved activation kinetics and localization for imaging synaptic transmission

The fluorescent glutamate indicator iGluSnFR enables imaging of neurotransmission with genetic and molecular specificity. However, existing iGluSnFR variants exhibit low in vivo signal-to-noise ratios, saturating activation kinetics and exclusion from postsynaptic densities. Using a multiassay screen in bacteria, soluble protein and cultured neurons, we generated variants with improved signal-to-noise ratios and kinetics. We developed surface display constructs that improve iGluSnFR's nanoscopic localization to postsynapses. The resulting indicator iGluSnFR3 exhibits rapid nonsaturating activation kinetics and reports synaptic glutamate release with decreased saturation and increased specificity versus extrasynaptic signals in cultured neurons. Simultaneous imaging and electrophysiology at individual boutons in mouse visual cortex showed that iGluSnFR3 transients report single action potentials with high specificity. In vibrissal sensory cortex layer 4, we used iGluSnFR3 to characterize distinct patterns of touch-evoked feedforward input from thalamocortical boutons and both feedforward and recurrent input onto L4 cortical neuron dendritic spines.

Lytic granule exocytosis at immune synapses: lessons from neuronal synapses

Regulated exocytosis is a central mechanism of cellular communication. It is not only the basis for neurotransmission and hormone release, but also plays an important role in the immune system for the release of cytokines and cytotoxic molecules. In cytotoxic T lymphocytes (CTLs), the formation of the immunological synapse is required for the delivery of the cytotoxic substances such as granzymes and perforin, which are stored in lytic granules and released via exocytosis. The molecular mechanisms of their fusion with the plasma membrane are only partially understood. In this review, we discuss the molecular players involved in the regulated exocytosis of CTL, highlighting the parallels and differences to neuronal synaptic transmission. Additionally, we examine the strengths and weaknesses of both systems to study exocytosis.

GABABR silencing of nerve terminals

Control of neurotransmission efficacy is central to theories of how the brain computes and stores information. Presynaptic G-protein coupled receptors (GPCRs) are critical in this problem as they locally influence synaptic strength and can operate on a wide range of time scales. Among the mechanisms by which GPCRs impact neurotransmission is by inhibiting voltage-gated calcium (Ca2+) influx in the active zone. Here, using quantitative analysis of both single bouton Ca2+ influx and exocytosis, we uncovered an unexpected non-linear relationship between the magnitude of action potential driven Ca2+ influx and the concentration of external Ca2+ ([Ca2+]e). We find that this unexpected relationship is leveraged by GPCR signaling when operating at the nominal physiological set point for [Ca2+]e, 1.2 mM, to achieve complete silencing of nerve terminals. These data imply that the information throughput in neural circuits can be readily modulated in an all-or-none fashion at the single synapse level when operating at the physiological set point.

Inositol hexakisphosphate kinases differentially regulate trafficking of vesicular glutamate transporters 1 and 2

Inositol pyrophosphates have been implicated in cellular signaling and membrane trafficking, including synaptic vesicle (SV) recycling. Inositol hexakisphosphate kinases (IP6Ks) and their product, diphosphoinositol pentakisphosphate (PP-IP₅ or IP7), directly and indirectly regulate proteins important in vesicle recycling by the activity-dependent bulk endocytosis pathway (ADBE). In the present study, we show that two isoforms, IP6K1 and IP6K3, are expressed in axons. The role of the kinases in SV recycling are investigated using pharmacologic inhibition, shRNA knockdown, and IP6K1 and IP6K3 knockout mice. Live-cell imaging experiments use optical reporters of SV recycling based on vesicular glutamate transporter isoforms, VGLUT1- and VGLUT2-pHluorins (pH), which recycle differently. VGLUT1-pH recycles by classical AP-2 dependent endocytosis under moderate stimulation conditions, while VGLUT2-pH recycles using AP-1 and AP-3 adaptor proteins as well. Using a short stimulus to release the readily releasable pool (RRP), we show that IP6K1 KO increases exocytosis of both VGLUT1-and VGLUT2-pH, while IP6K3 KO decreases the amount of both transporters in the RRP. In electrophysiological experiments we measure glutamate signaling with short stimuli and under the intense stimulation conditions that trigger bulk endocytosis. IP6K1 KO increases synaptic facilitation and IP6K3 KO decreases facilitation compared to wild type in CA1 hippocampal Schaffer collateral synapses. After intense stimulation, the rate of endocytosis of VGLUT2-pH, but not VGLUT1-pH, is increased by knockout, knockdown, and pharmacologic inhibition of IP6Ks. Thus IP6Ks differentially affect the endocytosis of two SV protein cargos that use different endocytic pathways. However, while IP6K1 KO and IP6K3 KO exert similar effects on endocytosis after stimulation, the isoforms exert different effects on exocytosis earlier in the stimulus and on the early phase of glutamate release. Taken together, the data indicate a role for IP6Ks both in exocytosis early in the stimulation period and in endocytosis, particularly under conditions that may utilize AP-1/3 adaptors.

Determinants of synapse diversity revealed by super-resolution quantal transmission and active zone imaging

Neural circuit function depends on the pattern of synaptic connections between neurons and the strength of those connections. Synaptic strength is determined by both postsynaptic sensitivity to neurotransmitter and the presynaptic probability of action potential evoked transmitter release (Pr). Whereas morphology and neurotransmitter receptor number indicate postsynaptic sensitivity, presynaptic indicators and the mechanism that sets Pr remain to be defined. To address this, we developed QuaSOR, a super-resolution method for determining Pr from quantal synaptic transmission imaging at hundreds of glutamatergic synapses at a time. We mapped the Pr onto super-resolution 3D molecular reconstructions of the presynaptic active zones (AZs) of the same synapses at the Drosophila larval neuromuscular junction (NMJ). We find that Pr varies greatly between synapses made by a single axon, quantify the contribution of key AZ proteins to Pr diversity and find that one of these, Complexin, suppresses spontaneous and evoked transmission differentially, thereby generating a spatial and quantitative mismatch between release modes. Transmission is thus regulated by the balance and nanoscale distribution of release-enhancing and suppressing presynaptic proteins to generate high signal-to-noise evoked transmission.

Ca2+ binding to synapsin I regulates resting Ca2+ and recovery from synaptic depression in nerve terminals

Synapsin I (SynI) is a synaptic vesicle (SV)-associated phosphoprotein that modulates neurotransmission by controlling SV trafficking. The SynI C-domain contains a highly conserved ATP binding site mediating SynI oligomerization and SV clustering and an adjacent main Ca²⁺ binding site, whose physiological role is unexplored. Molecular dynamics simulations revealed that the E373K point mutation irreversibly deletes Ca²⁺ binding to SynI, still allowing ATP binding, but inducing a destabilization of the SynI oligomerization interface. Here, we analyzed the effects of this mutation on neurotransmitter release and short-term plasticity in excitatory and inhibitory synapses from primary hippocampal neurons. Patch-clamp recordings showed an increase in the frequency of miniature excitatory postsynaptic currents (EPSCs) that was totally occluded by exogenous Ca²⁺ chelators and associated with a constitutive increase in resting terminal Ca²⁺ concentrations. Evoked EPSC amplitude was also reduced, due to a decreased readily releasable pool (RRP) size. Moreover, in both excitatory and inhibitory synapses, we observed a marked impaired recovery from synaptic depression, associated with impaired RRP refilling and depletion of the recycling pool of SVs. Our study identifies SynI as a novel Ca²⁺ buffer in excitatory terminals. Blocking Ca²⁺ binding to SynI results in higher constitutive Ca²⁺ levels that increase the probability of spontaneous release and disperse SVs. This causes a decreased size of the RRP and an impaired recovery from depression due to the failure of SV reclustering after sustained high-frequency stimulation. The results indicate a physiological role of Ca²⁺ binding to SynI in the regulation of SV clustering and trafficking in nerve terminals.

Synj1 haploinsufficiency causes dopamine neuron vulnerability and alpha-synuclein accumulation in mice

Synaptojanin1 (synj1) is a phosphoinositide phosphatase with dual SAC1 and 5'-phosphatase enzymatic activities in regulating phospholipid signaling. The brain-enriched isoform has been shown to participate in synaptic vesicle (SV) recycling. More recently, recessive human mutations were identified in the two phosphatase domains of SYNJ1, including R258Q, R459P and R839C, which are linked to rare forms of early-onset Parkinsonism. We now demonstrate that Synj1 heterozygous deletion (Synj1+/-), which is associated with an impaired 5'-phosphatase activity, also leads to Parkinson's disease (PD)-like pathologies in mice. We report that male Synj1+/- mice display age-dependent motor function abnormalities as well as alpha-synuclein accumulation, impaired autophagy and dopaminergic terminal degeneration. Synj1+/- mice contain elevated 5'-phosphatase substrate, PI(4,5)P2, particularly in the midbrain neurons. Moreover, pharmacological elevation of membrane PI(4,5)P2 in cultured neurons impairs SV endocytosis, specifically in midbrain neurons, and further exacerbates SV trafficking defects in Synj1+/- midbrain neurons. We demonstrate down-regulation of SYNJ1 transcript in a subset of sporadic PD brains, implicating a potential role of Synj1 deficiency in the decline of dopaminergic function during aging.

The potassium channel subunit Kvβ1 serves as a major control point for synaptic facilitation

Nerve terminals generally engage in two opposite and essential forms of synaptic plasticity (facilitation or depression) that play critical roles in learning and memory. While the molecular components of both types of terminals are similar with regards to vesicle fusion, much less is known about their molecular control of electrical signaling. Measurements of the electrical impulses (action potentials) underlying these two forms of plasticity have been difficult in small nerve terminals due to their size. In this study we deployed optical physiology measurements to overcome this size barrier. Here, we identify a unique mechanism (Kvβ1 subunit) that enables broadening of the presynaptic action potentials that selectively supports synaptic facilitation, but does not alter any other aspects of nerve terminal function.

Analysis of the presynaptic action potential’s (AP_syn) role in synaptic facilitation in hippocampal pyramidal neurons has been difficult due to size limitations of axons. We overcame these size barriers by combining high-resolution optical recordings of membrane potential, exocytosis, and Ca²⁺ in cultured hippocampal neurons. These recordings revealed a critical and selective role for K_v1 channel inactivation in synaptic facilitation of excitatory hippocampal neurons. Presynaptic K_v1 channel inactivation was mediated by the K_vβ1 subunit and had a surprisingly rapid onset that was readily apparent even in brief physiological stimulation paradigms including paired-pulse stimulation. Genetic depletion of K_vβ1 blocked all broadening of the AP_syn during high-frequency stimulation and eliminated synaptic facilitation without altering the initial probability of vesicle release. Thus, using all quantitative optical measurements of presynaptic physiology, we reveal a critical role for presynaptic K_v channels in synaptic facilitation at presynaptic terminals of the hippocampus upstream of the exocytic machinery.

Functions of Presynaptic Voltage-gated Calcium Channels

Voltage-gated calcium channels are the principal conduits for depolarization-mediated Ca²⁺ entry into excitable cells. In this review, the biophysical properties of the relevant members of this family of channels, those that are present in presynaptic terminals, will be discussed in relation to their function in mediating neurotransmitter release. Voltage-gated calcium channels have properties that ensure they are specialized for particular roles, for example, differences in their activation voltage threshold, their various kinetic properties, and their voltage-dependence of inactivation. All these attributes play into the ability of the various voltage-gated calcium channels to participate in different patterns of presynaptic vesicular release. These include synaptic transmission resulting from single action potentials, and longer-term changes mediated by bursts or trains of action potentials, as well as release resulting from graded changes in membrane potential in specialized sensory synapses.

Forward genetics identifies a novel sleep mutant with sleep state inertia and REM sleep deficits

Switches between global sleep and wakefulness states are believed to be dictated by top-down influences arising from subcortical nuclei. Using forward genetics and in vivo electrophysiology, we identified a recessive mouse mutant line characterized by a substantially reduced propensity to transition between wake and sleep states with an especially pronounced deficit in initiating rapid eye movement (REM) sleep episodes. The causative mutation, an Ile102Asn substitution in the synaptic vesicular protein, VAMP2, was associated with morphological synaptic changes and specific behavioral deficits, while in vitro electrophysiological investigations with fluorescence imaging revealed a markedly diminished probability of vesicular release in mutants. Our data show that global shifts in the synaptic efficiency across brain-wide networks leads to an altered probability of vigilance state transitions, possibly as a result of an altered excitability balance within local circuits controlling sleep-wake architecture.

Freeze-frame imaging of synaptic activity using SynTagMA

Information within the brain travels from neuron to neuron across billions of synapses. At any given moment, only a small subset of neurons and synapses are active, but finding the active synapses in brain tissue has been a technical challenge. Here we introduce SynTagMA to tag active synapses in a user-defined time window. Upon 395-405 nm illumination, this genetically encoded marker of activity converts from green to red fluorescence if, and only if, it is bound to calcium. Targeted to presynaptic terminals, preSynTagMA allows discrimination between active and silent axons. Targeted to excitatory postsynapses, postSynTagMA creates a snapshot of synapses active just before photoconversion. To analyze large datasets, we show how to identify and track the fluorescence of thousands of individual synapses in an automated fashion. Together, these tools provide an efficient method for repeatedly mapping active neurons and synapses in cell culture, slice preparations, and in vivo during behavior.

Synaptotagmin 1 oligomers clamp and regulate different modes of neurotransmitter release

Release of neurotransmitters relies on submillisecond coupling of synaptic vesicle fusion to the triggering signal: AP-evoked presynaptic Ca²⁺ influx. The key player that controls exocytosis of the synaptic vesicle is the Ca²⁺ sensor synaptotagmin 1 (Syt1). While the Ca²⁺ activation of Syt1 has been extensively characterized, how Syt1 reversibly clamps vesicular fusion remains enigmatic. Here, using a targeted mutation combined with fluorescence imaging and electrophysiology, we show that the structural feature of Syt1 to self-oligomerize provides the molecular basis for clamping of spontaneous and asynchronous release but is not required for triggering of synchronous release. Our findings propose a mechanistic model that explains how Syt1 oligomers regulate different modes of transmitter release in neuronal synapses.

Synaptotagmin 1 (Syt1) synchronizes neurotransmitter release to action potentials (APs) acting as the fast Ca²⁺ release sensor and as the inhibitor (clamp) of spontaneous and delayed asynchronous release. While the Syt1 Ca²⁺ activation mechanism has been well-characterized, how Syt1 clamps transmitter release remains enigmatic. Here we show that C2B domain-dependent oligomerization provides the molecular basis for the Syt1 clamping function. This follows from the investigation of a designed mutation (F349A), which selectively destabilizes Syt1 oligomerization. Using a combination of fluorescence imaging and electrophysiology in neocortical synapses, we show that Syt1^F349A is more efficient than wild-type Syt1 (Syt1^WT) in triggering synchronous transmitter release but fails to clamp spontaneous and synaptotagmin 7 (Syt7)-mediated asynchronous release components both in rescue (Syt1^−/− knockout background) and dominant-interference (Syt1^+/+ background) conditions. Thus, we conclude that Ca²⁺-sensitive Syt1 oligomers, acting as an exocytosis clamp, are critical for maintaining the balance among the different modes of neurotransmitter release.

Preparation of dissociated mouse primary neuronal cultures from long-term cryopreserved brain tissue

BACKGROUND

Dissociated primary neuronal cultures are widely used as a model system to investigate the cellular and molecular properties of diverse neuronal populations and mechanisms of action potential generation and synaptic transmission. Typically, rodent primary neuronal cultures are obtained from freshly-dissociated embryonic or postnatal brain tissue, which often requires intense animal husbandry. This can strain resources when working with genetically modified mice.

NEW METHOD

Here we describe an experimental protocol for frozen storage of mouse hippocampi, which allows fully functional dissociated primary neuronal cultures to be prepared from cryopreserved tissue.

RESULTS

We show that thawed hippocampal neurons have functional properties similar to those of freshly dissociated neurons, including neuronal morphology, excitability, action potential waveform and synaptic neurotransmitter release, even after cryopreservation for several years.

COMPARISON TO THE EXISTING METHODS

In contrast to the existing methods, the protocol described here allows for efficient long-term storage of samples, allowing researchers to perform functional experiments on neuronal cultures from brain tissue collected in other laboratories.

CONCLUSIONS

We anticipate that this method will facilitate collaborations among laboratories based at distant locations and will thus optimise the use of genetically modified mouse models, in line with the 3Rs (Replacement, Reduction and Refinement) recommended for scientific use of animals in research.

Functional Electron Microscopy, "Flash and Freeze," of Identified Cortical Synapses in Acute Brain Slices

How structural and functional properties of synapses relate to each other is a fundamental question in neuroscience. Electrophysiology has elucidated mechanisms of synaptic transmission, and electron microscopy (EM) has provided insight into morphological properties of synapses. Here we describe an enhanced method for functional EM (“flash and freeze”), combining optogenetic stimulation with high-pressure freezing. We demonstrate that the improved method can be applied to intact networks in acute brain slices and organotypic slice cultures from mice. As a proof of concept, we probed vesicle pool changes during synaptic transmission at the hippocampal mossy fiber-CA3 pyramidal neuron synapse. Our findings show overlap of the docked vesicle pool and the functionally defined readily releasable pool and provide evidence of fast endocytosis at this synapse. Functional EM with acute slices and slice cultures has the potential to reveal the structural and functional mechanisms of transmission in intact, genetically perturbed, and disease-affected synapses.

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Functional EM may be applied to acute brain slices and organotypic slice cultures

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Docked vesicle pool and RRP are overlapping

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Smaller-diameter vesicles have higher release probability than larger vesicles

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Endocytic pits after moderate stimulation suggest fast endocytosis

Optical Quantal Analysis Using Ca2+ Indicators: A Robust Method for Assessing Transmitter Release Probability at Excitatory Synapses by Imaging Single Glutamate Release Events

Despite evidence that presynaptic efficacy and plasticity influence circuit function and behavior in vivo, studies of presynaptic function remain challenging owing to the difficulty of assessing transmitter release in intact tissue. Electrophysiological analyses of transmitter release are indirect and cannot readily resolve basic presynaptic parameters, most notably transmitter release probability (p r), at single synapses. These issues can be circumvented by optical quantal analysis, which uses the all-or-none optical detection of transmitter release in order to calculate p r. Over the past two decades, we and others have successfully demonstrated that Ca2+ indicators can be strategically implemented to perform optical quantal analysis at single glutamatergic synapses in ex vivo and in vitro preparations. We have found that high affinity Ca2+ indicators can reliably detect spine Ca2+ influx generated by single quanta of glutamate, thereby enabling precise calculation of pr at single synapses. Importantly, we have shown this method to be robust to changes in postsynaptic efficacy, and to be sensitive to activity-dependent presynaptic changes at central synapses following the induction of long-term potentiation (LTP) and long-term depression (LTD). In this report, we describe how to use Ca2+-sensitive dyes to perform optical quantal analysis at single synapses in hippocampal slice preparations. The general technique we describe here can be applied to other glutamatergic synapses and can be used with other reporters of glutamate release, including recently improved genetically encoded Ca2+ and glutamate sensors. With ongoing developments in imaging techniques and genetically encoded probes, optical quantal analysis is a promising strategy for assessing presynaptic function and plasticity in vivo.

Proteolytic maturation of α2δ controls the probability of synaptic vesicular release

Auxiliary α2δ subunits are important proteins for trafficking of voltage-gated calcium channels (CaV) at the active zones of synapses. We have previously shown that the post-translational proteolytic cleavage of α2δ is essential for their modulatory effects on the trafficking of N-type (CaV2.2) calcium channels (Kadurin et al., 2016). We extend these results here by showing that the probability of presynaptic vesicular release is reduced when an uncleaved α2δ is expressed in rat neurons and that this inhibitory effect is reversed when cleavage of α2δ is restored. We also show that asynchronous release is influenced by the maturation of α2δ-1, highlighting the role of CaV channels in this component of vesicular release. We present additional evidence that CaV2.2 co-immunoprecipitates preferentially with cleaved wild-type α2δ. Our data indicate that the proteolytic maturation increases the association of α2δ-1 with CaV channel complex and is essential for its function on synaptic release.

Cereblon Maintains Synaptic and Cognitive Function by Regulating BK Channel

Mutations in the cereblon (CRBN) gene cause human intellectual disability, one of the most common cognitive disorders. However, the molecular mechanisms of CRBN-related intellectual disability remain poorly understood. We investigated the role of CRBN in synaptic function and animal behavior using male mouse and Drosophila models. Crbn knock-out (KO) mice showed normal brain and spine morphology as well as intact synaptic plasticity; however, they also exhibited decreases in synaptic transmission and presynaptic release probability exclusively in excitatory synapses. Presynaptic function was impaired not only by loss of CRBN expression, but also by expression of pathogenic CRBN mutants (human R419X mutant and Drosophila G552X mutant). We found that the BK channel blockers paxilline and iberiotoxin reversed this decrease in presynaptic release probability in Crbn KO mice. In addition, paxilline treatment also restored normal cognitive behavior in Crbn KO mice. These results strongly suggest that increased BK channel activity is the pathological mechanism of intellectual disability in CRBN mutations.SIGNIFICANCE STATEMENTCereblon (CRBN), a well known target of the immunomodulatory drug thalidomide, was originally identified as a gene that causes human intellectual disability when mutated. However, the molecular mechanisms of CRBN-related intellectual disability remain poorly understood. Based on the idea that synaptic abnormalities are the most common factor in cognitive dysfunction, we monitored the synaptic structure and function of Crbn knock-out (KO) animals to identify the molecular mechanisms of intellectual disability. Here, we found that Crbn KO animals showed cognitive deficits caused by enhanced BK channel activity and reduced presynaptic glutamate release. Our findings suggest a physiological pathomechanism of the intellectual disability-related gene CRBN and will contribute to the development of therapeutic strategies for CRBN-related intellectual disability.

Diverse modes of synaptic signaling, regulation, and plasticity distinguish two classes of C. elegans glutamatergic neurons

Synaptic vesicle release properties vary between neuronal cell types, but in most cases the molecular basis of this heterogeneity is unknown. Here, we compare in vivo synaptic properties of two neuronal classes in the C. elegans central nervous system, using VGLUT-pHluorin to monitor synaptic vesicle exocytosis and retrieval in intact animals. We show that the glutamatergic sensory neurons AWCON and ASH have distinct synaptic dynamics associated with tonic and phasic synaptic properties, respectively. Exocytosis in ASH and AWCON is differentially affected by SNARE-complex regulators that are present in both neurons: phasic ASH release is strongly dependent on UNC-13, whereas tonic AWCON release relies upon UNC-18 and on the protein kinase C homolog PKC-1. Strong stimuli that elicit high calcium levels increase exocytosis and retrieval rates in AWCON, generating distinct tonic and evoked synaptic modes. These results highlight the differential deployment of shared presynaptic proteins in neuronal cell type-specific functions.

VGLUT2 Trafficking Is Differentially Regulated by Adaptor Proteins AP-1 and AP-3

Release of the major excitatory neurotransmitter glutamate by synaptic vesicle exocytosis depends on glutamate loading into synaptic vesicles by vesicular glutamate transporters (VGLUTs). The two principal isoforms, VGLUT1 and 2, exhibit a complementary pattern of expression in adult brain that broadly distinguishes cortical (VGLUT1) and subcortical (VGLUT2) systems, and correlates with distinct physiological properties in synapses expressing these isoforms. Differential trafficking of VGLUT1 and 2 has been suggested to underlie their functional diversity. Increasing evidence suggests individual synaptic vesicle proteins use specific sorting signals to engage specialized biochemical mechanisms to regulate their recycling. We observed that VGLUT2 recycles differently in response to high frequency stimulation than VGLUT1. Here we further explore the trafficking of VGLUT2 using a pHluorin-based reporter, VGLUT2-pH. VGLUT2-pH exhibits slower rates of both exocytosis and endocytosis than VGLUT1-pH. VGLUT2-pH recycling is slower than VGLUT1-pH in both hippocampal neurons, which endogenously express mostly VGLUT1, and thalamic neurons, which endogenously express mostly VGLUT2, indicating that protein identity, not synaptic vesicle membrane or neuronal cell type, controls sorting. We characterize sorting signals in the C-terminal dileucine-like motif, which plays a crucial role in VGLUT2 trafficking. Disruption of this motif abolishes synaptic targeting of VGLUT2 and essentially eliminates endocytosis of the transporter. Mutational and biochemical analysis demonstrates that clathrin adaptor proteins (APs) interact with VGLUT2 at the dileucine-like motif. VGLUT2 interacts with AP-2, a well-studied adaptor protein for clathrin mediated endocytosis. In addition, VGLUT2 also interacts with the alternate adaptors, AP-1 and AP-3. VGLUT2 relies on distinct recycling mechanisms from VGLUT1. Abrogation of these differences by pharmacological and molecular inhibition reveals that these mechanisms are dependent on the adaptor proteins AP-1 and AP-3. Further, shRNA-mediated knockdown reveals differential roles for AP-1 and AP-3 in VGLUT2 recycling.

Parkinson's Disease-Associated LRRK2 Hyperactive Kinase Mutant Disrupts Synaptic Vesicle Trafficking in Ventral Midbrain Neurons

Parkinson's disease (PD) is characterized pathologically by the selective loss of substantia nigra (SN) dopaminergic (DAergic) neurons. Recent evidence has suggested a role of LRRK2, linked to the most frequent familial PD, in regulating synaptic vesicle (SV) trafficking. However, the mechanism whereby LRRK2 mutants contribute to nigral vulnerability remains unclear. Here we show that the most common PD mutation LRRK2 G2019S impairs SV endocytosis in ventral midbrain (MB) neurons, including DA neurons, and the slowed endocytosis can be rescued by inhibition of LRRK2 kinase activity. A similar endocytic defect, however, was not observed in LRRK2 mutant neurons from the neocortex (hereafter, cortical neurons) or the hippocampus, suggesting a brain region-specific vulnerability to the G2019S mutation. Additionally, we found MB-specific impairment of SV endocytosis in neurons carrying heterozygous deletion of SYNJ1 (PARK20), a gene that is associated with recessive Parkinsonism. Combining SYNJ1^+/- and LRRK2 G2019S does not exacerbate SV endocytosis but impairs sustained exocytosis in MB neurons and alters specific motor functions of 1-year-old male mice. Interestingly, we show that LRRK2 directly phosphorylates synaptojanin1 in vitro, resulting in the disruption of endophilin-synaptojanin1 interaction required for SV endocytosis. Our work suggests a merge of LRRK2 and SYNJ1 pathogenic pathways in deregulating SV trafficking in MB neurons as an underlying molecular mechanism of early PD pathogenesis.SIGNIFICANCE STATEMENT Understanding midbrain dopaminergic (DAergic) neuron-selective vulnerability in PD is essential for the development of targeted therapeutics. We report, for the first time, a nerve terminal impairment in SV trafficking selectively in MB neurons but not cortical neurons caused by two PARK genes: LRRK2 (PARK8) and SYNJ1 (PARK20). We demonstrate that the enhanced kinase activity resulting from the most frequent G2019S mutation in LRRK2 is the key to this impairment. We provide evidence suggesting that LRRK2 G2019S and SYNJ1 loss of function share a similar pathogenic pathway in deregulating DAergic neuron SV endocytosis and that they play additive roles in facilitating each other's pathogenic functions in PD.

Fusion Competent Synaptic Vesicles Persist upon Active Zone Disruption and Loss of Vesicle Docking

In a nerve terminal, synaptic vesicle docking and release are restricted to an active zone. The active zone is a protein scaffold that is attached to the presynaptic plasma membrane and opposed to postsynaptic receptors. Here, we generated conditional knockout mice removing the active zone proteins RIM and ELKS, which additionally led to loss of Munc13, Bassoon, Piccolo, and RIM-BP, indicating disassembly of the active zone. We observed a near-complete lack of synaptic vesicle docking and a strong reduction in vesicular release probability and the speed of exocytosis, but total vesicle numbers, SNARE protein levels, and postsynaptic densities remained unaffected. Despite loss of the priming proteins Munc13 and RIM and of docked vesicles, a pool of releasable vesicles remained. Thus, the active zone is necessary for synaptic vesicle docking and to enhance release probability, but releasable vesicles can be localized distant from the presynaptic plasma membrane.

The readily releasable pool of synaptic vesicles

Each presynaptic bouton is densely packed with many vesicles, only a small fraction of which are available for immediate release. These vesicles constitute the readily releasable pool (RRP). The RRP size, and the probability of release of each vesicle within the RRP, together determine synaptic strength. Here, we discuss complications and recent advances in determining the size of the physiologically relevant RRP. We consider molecular mechanisms to generate and regulate the RRP, and discuss the relationship between vesicle docking and the RRP. We conclude that many RRP vesicles are docked, that some docked vesicles may not be part of the RRP, and that undocked vesicles can contribute to the RRP by rapid recruitment to unoccupied, molecularly activated ready-to-release sites.

Stimulation of Synaptic Vesicle Exocytosis by the Mental Disease Gene DISC1 is Mediated by N-Type Voltage-Gated Calcium Channels

Lesions and mutations of the DISC1 (Disrupted-in-schizophrenia-1) gene have been linked to major depression, schizophrenia, bipolar disorder and autism, but the influence of DISC1 on synaptic transmission remains poorly understood. Using two independent genetic approaches-RNAi and a DISC1 KO mouse-we examined the impact of DISC1 on the synaptic vesicle (SV) cycle by population imaging of the synaptic tracer vGpH in hippocampal neurons. DISC1 loss-of-function resulted in a marked decrease in SV exocytic rates during neuronal stimulation and was associated with reduced Ca(2+) transients at nerve terminals. Impaired SV release was efficiently rescued by elevation of extracellular Ca(2+), hinting at a link between DISC1 and voltage-gated Ca(2+) channels. Accordingly, blockade of N-type Cav2.2 channels mimics and occludes the effect of DISC1 inactivation on SV exocytosis, and overexpression of DISC1 in a heterologous system increases Cav2.2 currents. Collectively, these results show that DISC1-dependent enhancement of SV exocytosis is mediated by Cav2.2 and point to aberrant glutamate release as a probable endophenotype of major psychiatric disorders.

Isoflurane inhibits synaptic vesicle exocytosis through reduced Ca2+ influx, not Ca2+-exocytosis coupling

Identifying presynaptic mechanisms of general anesthetics is critical to understanding their effects on synaptic transmission. We show that the volatile anesthetic isoflurane inhibits synaptic vesicle (SV) exocytosis at nerve terminals in dissociated rat hippocampal neurons through inhibition of presynaptic Ca(2+) influx without significantly altering the Ca(2+) sensitivity of SV exocytosis. A clinically relevant concentration of isoflurane (0.7 mM) inhibited changes in [Ca(2+)]i driven by single action potentials (APs) by 25 ± 3%, which in turn led to 62 ± 3% inhibition of single AP-triggered exocytosis at 4 mM extracellular Ca(2+) ([Ca(2+)]e). Lowering external Ca(2+) to match the isoflurane-induced reduction in Ca(2+) entry led to an equivalent reduction in exocytosis. These data thus indicate that anesthetic inhibition of neurotransmitter release from small SVs occurs primarily through reduced axon terminal Ca(2+) entry without significant direct effects on Ca(2+)-exocytosis coupling or on the SV fusion machinery. Isoflurane inhibition of exocytosis and Ca(2+) influx was greater in glutamatergic compared with GABAergic nerve terminals, consistent with selective inhibition of excitatory synaptic transmission. Such alteration in the balance of excitatory to inhibitory transmission could mediate reduced neuronal interactions and network-selective effects observed in the anesthetized central nervous system.

The active-zone protein Munc13 controls the use-dependence of presynaptic voltage-gated calcium channels

Presynaptic calcium channel function is critical for converting electrical information into chemical communication but the molecules in the active zone that sculpt this function are poorly understood. We show that Munc13, an active-zone protein essential for exocytosis, also controls presynaptic voltage-gated calcium channel (VGCC) function dictating their behavior during various forms of activity. We demonstrate that in vitro Munc13 interacts with voltage-VGCCs via a pair of basic residues in Munc13's C2B domain. We show that elimination of this interaction by either removal of Munc13 or replacement of Munc13 with a Munc13 C2B mutant alters synaptic VGCC's response to and recovery from high-frequency action potential bursts and alters calcium influx from single action potential stimuli. These studies illustrate a novel form of synaptic modulation and show that Munc13 is poised to profoundly impact information transfer at nerve terminals by controlling both vesicle priming and the trigger for exocytosis.

Calmodulin as a major calcium buffer shaping vesicular release and short-term synaptic plasticity: facilitation through buffer dislocation

Action potential-dependent release of synaptic vesicles and short-term synaptic plasticity are dynamically regulated by the endogenous Ca²⁺ buffers that shape [Ca²⁺] profiles within a presynaptic bouton. Calmodulin is one of the most abundant presynaptic proteins and it binds Ca²⁺ faster than any other characterized endogenous neuronal Ca²⁺ buffer. Direct effects of calmodulin on fast presynaptic Ca²⁺ dynamics and vesicular release however have not been studied in detail. Using experimentally constrained three-dimensional diffusion modeling of Ca²⁺ influx–exocytosis coupling at small excitatory synapses we show that, at physiologically relevant concentrations, Ca²⁺ buffering by calmodulin plays a dominant role in inhibiting vesicular release and in modulating short-term synaptic plasticity. We also propose a novel and potentially powerful mechanism for short-term facilitation based on Ca²⁺-dependent dynamic dislocation of calmodulin molecules from the plasma membrane within the active zone.

Regulation of synaptic activity by snapin-mediated endolysosomal transport and sorting

Recycling synaptic vesicles (SVs) transit through early endosomal sorting stations, which raises a fundamental question: are SVs sorted toward endolysosomal pathways? Here, we used snapin mutants as tools to assess how endolysosomal sorting and trafficking impact presynaptic activity in wild-type and snapin(-/-) neurons. Snapin acts as a dynein adaptor that mediates the retrograde transport of late endosomes (LEs) and interacts with dysbindin, a subunit of the endosomal sorting complex BLOC-1. Expressing dynein-binding defective snapin mutants induced SV accumulation at presynaptic terminals, mimicking the snapin(-/-) phenotype. Conversely, over-expressing snapin reduced SV pool size by enhancing SV trafficking to the endolysosomal pathway. Using a SV-targeted Ca(2+) sensor, we demonstrate that snapin-dysbindin interaction regulates SV positional priming through BLOC-1/AP-3-dependent sorting. Our study reveals a bipartite regulation of presynaptic activity by endolysosomal trafficking and sorting: LE transport regulates SV pool size, and BLOC-1/AP-3-dependent sorting fine-tunes the Ca(2+) sensitivity of SV release. Therefore, our study provides new mechanistic insights into the maintenance and regulation of SV pool size and synchronized SV fusion through snapin-mediated LE trafficking and endosomal sorting.

Control and plasticity of the presynaptic action potential waveform at small CNS nerve terminals

The steep dependence of exocytosis on Ca(2+) entry at nerve terminals implies that voltage control of both Ca(2+) channel opening and the driving force for Ca(2+) entry are powerful levers in sculpting synaptic efficacy. Using fast, genetically encoded voltage indicators in dissociated primary neurons, we show that at small nerve terminals K(+) channels constrain the peak voltage of the presynaptic action potential (APSYN) to values much lower than those at cell somas. This key APSYN property additionally shows adaptive plasticity: manipulations that increase presynaptic Ca(2+) channel abundance and release probability result in a commensurate lowering of the APSYN peak and narrowing of the waveform, while manipulations that decrease presynaptic Ca(2+) channel abundance do the opposite. This modulation is eliminated upon blockade of Kv3.1 and Kv1 channels. Our studies thus reveal that adaptive plasticity in the APSYN waveform serves as an important regulator of synaptic function.

Differential triggering of spontaneous glutamate release by P/Q-, N- and R-type Ca2+ channels

The role of voltage-gated Ca2+ channels (VGCCs) in spontaneous miniature neurotransmitter release is incompletely understood. We found that stochastic opening of P/Q-, N- and R-type VGCCs accounts for ∼50% of all spontaneous glutamate release at rat cultured hippocampal synapses, and that R-type channels have a far greater role in spontaneous than in action potential-evoked exocytosis. VGCC-dependent miniature neurotransmitter release (minis) showed similar sensitivity to presynaptic Ca2+ chelation as evoked release, arguing for direct triggering of spontaneous release by transient spatially localized Ca(2+) domains. Experimentally constrained three-dimensional diffusion modeling of Ca2+ influx-exocytosis coupling was consistent with clustered distribution of VGCCs in the active zone of small hippocampal synapses and revealed that spontaneous VGCCs openings can account for the experimentally observed VGCC-dependent minis, although single channel openings triggered release with low probability. Uncorrelated stochastic VGCC opening is therefore a major trigger for spontaneous glutamate release, with differential roles for distinct channel subtypes.

Nanoscale-targeted patch-clamp recordings of functional presynaptic ion channels

Direct electrical access to presynaptic ion channels has hitherto been limited to large specialized terminals such as the calyx of Held or hippocampal mossy fiber bouton. The electrophysiology and ion-channel complement of far more abundant small synaptic terminals (≤1 μm) remain poorly understood. Here we report a method based on superresolution scanning ion conductance imaging of small synapses in culture at approximately 100–150 nm 3D resolution, which allows presynaptic patch-clamp recordings in all four configurations (cell-attached, inside-out, outside-out, and whole-cell). Using this technique, we report presynaptic recordings of K⁺, Na⁺, Cl⁻, and Ca²⁺ channels. This semiautomated approach allows direct investigation of the distribution and properties of presynaptic ion channels at small central synapses.

Video Abstract

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Topographic imaging of live synaptic boutons at nanoscale resolution

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Cell-attached patch-clamp recordings of ion channels in small central synapses

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Whole-cell small presynaptic bouton recordings

Multiple dileucine-like motifs direct VGLUT1 trafficking

The vesicular glutamate transporters (VGLUTs) package glutamate into synaptic vesicles, and the two principal isoforms VGLUT1 and VGLUT2 have been suggested to influence the properties of release. To understand how a VGLUT isoform might influence transmitter release, we have studied their trafficking and previously identified a dileucine-like endocytic motif in the C terminus of VGLUT1. Disruption of this motif impairs the activity-dependent recycling of VGLUT1, but does not eliminate its endocytosis. We now report the identification of two additional dileucine-like motifs in the N terminus of VGLUT1 that are not well conserved in the other isoforms. In the absence of all three motifs, rat VGLUT1 shows limited accumulation at synaptic sites and no longer responds to stimulation. In addition, shRNA-mediated knockdown of clathrin adaptor proteins AP-1 and AP-2 shows that the C-terminal motif acts largely via AP-2, whereas the N-terminal motifs use AP-1. Without the C-terminal motif, knockdown of AP-1 reduces the proportion of VGLUT1 that responds to stimulation. VGLUT1 thus contains multiple sorting signals that engage distinct trafficking mechanisms. In contrast to VGLUT1, the trafficking of VGLUT2 depends almost entirely on the conserved C-terminal dileucine-like motif: without this motif, a substantial fraction of VGLUT2 redistributes to the plasma membrane and the transporter's synaptic localization is disrupted. Consistent with these differences in trafficking signals, wild-type VGLUT1 and VGLUT2 differ in their response to stimulation.

Sorting of the vesicular GABA transporter to functional vesicle pools by an atypical dileucine-like motif

Increasing evidence indicates that individual synaptic vesicle proteins may use different signals, endocytic adaptors, and trafficking pathways for sorting to distinct pools of synaptic vesicles. Here, we report the identification of a unique amino acid motif in the vesicular GABA transporter (VGAT) that controls its synaptic localization and activity-dependent recycling. Mutational analysis of this atypical dileucine-like motif in rat VGAT indicates that the transporter recycles by interacting with the clathrin adaptor protein AP-2. However, mutation of a single acidic residue upstream of the dileucine-like motif leads to a shift to an AP-3-dependent trafficking pathway that preferentially targets the transporter to the readily releasable and recycling pool of vesicles. Real-time imaging with a VGAT-pHluorin fusion provides a useful approach to explore how unique sorting sequences target individual proteins to synaptic vesicles with distinct functional properties.

Balance of calcineurin Aα and CDK5 activities sets release probability at nerve terminals

The control of neurotransmitter release at nerve terminals is of profound importance for neurological function and provides a powerful control system in neural networks. We show that the balance of enzymatic activities of the α isoform of the phosphatase calcineurin (CNAα) and the kinase cyclin-dependent kinase 5 (CDK5) has a dramatic influence over single action potential (AP)-driven exocytosis at nerve terminals. Acute or chronic loss of these enzymatic activities results in a sevenfold impact on single AP-driven exocytosis. We demonstrate that this control is mediated almost entirely through Cav2.2 (N-type) voltage-gated calcium channels as blocking these channels with a peptide toxin eliminates modulation by these enzymes. We found that a fraction of nerve terminals are kept in a presynaptically silent state with no measurable Ca(2+) influx driven by single AP stimuli attributable to the balance of CNAα and CDK5 activities because blockade of either CNAα or CDK5 activity changes the proportion of presynaptically silent nerve terminals. Thus, CNAα and CDK5 enzymatic activities are key determinants of release probability.

Presynaptic calcium influx controls neurotransmitter release in part by regulating the effective size of the readily releasable pool

The steep calcium dependence of synaptic strength that has been observed at many synapses is thought to reflect a calcium dependence of the probability of vesicular exocytosis (p), with the cooperativity of three to six corresponding to the multiple calcium ion binding sites on the calcium sensor responsible for exocytosis. Here we test the hypothesis that the calcium dependence of the effective size of the readily releasable pool (RRP) also contributes to the calcium dependence of release at the calyx of Held synapse in mice. Using two established methods of quantifying neurotransmitter release evoked by action potentials (effective RRP), we find that when calcium influx is changed by altering the external calcium concentration, the calcium cooperativity of p is insufficient to account for the full calcium dependence of EPSC size; the calcium dependence of the RRP size also contributes. Reducing calcium influx by blocking R-type voltage-gated calcium channels (VGCCs) with Ni(2+), or by blocking P/Q-type VGCCs with ω-agatoxin IVA also changes EPSC amplitude by reducing both p and the effective RRP size. This suggests that the effective RRP size is dependent on calcium influx through VGCCs. Furthermore, activation of GABAB receptors, which reduces presynaptic calcium through VGCCs without other significant effects on release, also reduces the effective RRP size in addition to reducing p. These findings indicate that calcium influx regulates the RRP size along with p, which contributes to the calcium dependence of synaptic strength, and it influences the manner in which presynaptic modulation of presynaptic calcium channels affects neurotransmitter release.

Intrinsic variability in Pv, RRP size, Ca(2+) channel repertoire, and presynaptic potentiation in individual synaptic boutons

The strength of individual synaptic contacts is considered a key modulator of information flow across circuits. Presynaptically the strength can be parsed into two key parameters: the size of the readily releasable pool (RRP) and the probability that a vesicle in that pool will undergo exocytosis when an action potential fires (Pv). How these variables are controlled and the degree to which they vary across individual nerve terminals is crucial to understand synaptic plasticity within neural circuits. Here we report robust measurements of these parameters in rat hippocampal neurons and their variability across populations of individual synapses. We explore the diversity of presynaptic Ca²⁺ channel repertoires and evaluate their effect on synaptic strength at single boutons. Finally, we study the degree to which synapses can be differentially modified by a known potentiator of presynaptic function, forskolin. Our experiments revealed that both Pv and RRP spanned a large range, even for synapses made by the same axon, demonstrating that presynaptic efficacy is governed locally at the single synapse level. Synapses varied greatly in their dependence on N or P/Q type Ca²⁺ channels for neurotransmission, but there was no association between specific channel repertoires and synaptic efficacy. Increasing cAMP concentration using forskolin enhanced synaptic transmission in a Ca²⁺-independent manner that was inversely related with a synapse's initial Pv, and independent of its RRP size. We propose a simple model based on the relationship between Pv and calcium entry that can account for the variable potentiation of synapses based on initial probability of vesicle fusion.

Detection of spontaneous vesicle release at individual synapses using multiple wavelets in a CWT-based algorithm

In this paper we present an algorithm for the detection of spontaneous activity at individual synapses in microscopy images. By employing the optical marker pHluorin, we are able to visualize synaptic vesicle release with a spatial resolution in the nm range in a non-invasive manner. We compute individual synaptic signals from automatically segmented regions of interest and detect peaks that represent synaptic activity using a continuous wavelet transform based algorithm. As opposed to standard peak detection algorithms, we employ multiple wavelets to match all relevant features of the peak. We evaluate our multiple wavelet algorithm (MWA) on real data and assess the performance on synthetic data over a wide range of signal-to-noise ratios.

Neuropeptide transmission in brain circuits

Neuropeptides are found in many mammalian CNS neurons where they play key roles in modulating neuronal activity. In contrast to amino acid transmitter release at the synapse, neuropeptide release is not restricted to the synaptic specialization, and after release, a neuropeptide may diffuse some distance to exert its action through a G protein-coupled receptor. Some neuropeptides such as hypocretin/orexin are synthesized only in single regions of the brain, and the neurons releasing these peptides probably have similar functional roles. Other peptides such as neuropeptide Y (NPY) are synthesized throughout the brain, and neurons that synthesize the peptide in one region have no anatomical or functional connection with NPY neurons in other brain regions. Here, I review converging data revealing a complex interaction between slow-acting neuromodulator peptides and fast-acting amino acid transmitters in the control of energy homeostasis, drug addiction, mood and motivation, sleep-wake states, and neuroendocrine regulation.

Calbindin controls release probability in ventral tegmental area dopamine neurons

Relatively little is known about the molecular control of midbrain dopamine release. Using high-fidelity imaging of pHluorin-tagged vesicular monoamine transporter 2 in dopamine neurons, we found that exocytosis was more loosely coupled to calcium entry than in fast synapses. In ventral tegmental area neurons, this allows exocytosis to be efficiently controlled by a native fast calcium buffer, calbindin-D28k, maintaining a lower vesicular release probability compared with substantia nigra neurons.

New insights into molecular players involved in neurotransmitter release

The strength of a synapse can profoundly influence network function. How this strength is set at the molecular level is a key question in neuroscience. Here, we review a simple model of neurotransmission that serves as a convenient framework to discuss recent studies on RIM and synaptotagmin.

Concurrent imaging of synaptic vesicle recycling and calcium dynamics

Synaptic transmission involves the calcium dependent release of neurotransmitter from synaptic vesicles. Genetically encoded optical probes emitting different wavelengths of fluorescent light in response to neuronal activity offer a powerful approach to understand the spatial and temporal relationship of calcium dynamics to the release of neurotransmitter in defined neuronal populations. To simultaneously image synaptic vesicle recycling and changes in cytosolic calcium, we developed a red-shifted reporter of vesicle recycling based on a vesicular glutamate transporter, VGLUT1-mOrange2 (VGLUT1-mOr2), and a presynaptically localized green calcium indicator, synaptophysin-GCaMP3 (SyGCaMP3) with a large dynamic range. The fluorescence of VGLUT1-mOr2 is quenched by the low pH of synaptic vesicles. Exocytosis upon electrical stimulation exposes the luminal mOr2 to the neutral extracellular pH and relieves fluorescence quenching. Reacidification of the vesicle upon endocytosis again reduces fluorescence intensity. Changes in fluorescence intensity thus monitor synaptic vesicle exo- and endocytosis, as demonstrated previously for the green VGLUT1-pHluorin. To monitor changes in calcium, we fused the synaptic vesicle protein synaptophysin to the recently improved calcium indicator GCaMP3. SyGCaMP3 is targeted to presynaptic varicosities, and exhibits changes in fluorescence in response to electrical stimulation consistent with changes in calcium concentration. Using real time imaging of both reporters expressed in the same synapses, we determine the time course of changes in VGLUT1 recycling in relation to changes in presynaptic calcium concentration. Inhibition of P/Q- and N-type calcium channels reduces calcium levels, as well as the rate of synaptic vesicle exocytosis and the fraction of vesicles released.