Two pathways of synaptic vesicle retrieval revealed by single-vesicle imaging

Axonal neurotransmitter release in the regulation of myelination

Myelination of axons is a key determinant of fast action potential propagation, axonal health and circuit function. Previously considered a static structure, it is now clear that myelin is dynamically regulated in response to neuronal activity in the central nervous system (CNS). However, how activity-dependent signals are conveyed to oligodendrocytes remains unclear. Here, we review the potential mechanisms by which neurons could communicate changing activity levels to myelin, with a focus on the accumulating body of evidence to support activity-dependent vesicular signalling directly onto myelin sheaths. We discuss recent in vivo findings of activity-dependent fusion of neurotransmitter vesicles from non-synaptic axonal sites, and how modulation of this vesicular fusion regulates the stability and growth of myelin sheaths. We also consider the potential mechanisms by which myelin could sense and respond to axon-derived signals to initiate remodelling, and the relevance of these adaptations for circuit function. We propose that axonal vesicular signalling represents an important and underappreciated mode of communication by which neurons can transmit activity-regulated signals to myelinating oligodendrocytes and, potentially, more broadly to other cell types in the CNS.

Presynaptic sensor and silencer of peptidergic transmission reveal neuropeptides as primary transmitters in pontine fear circuit

Neurons produce and release neuropeptides to communicate with one another. Despite their importance in brain function, circuit-based mechanisms of peptidergic transmission are poorly understood, primarily due to the lack of tools for monitoring and manipulating neuropeptide release in vivo. Here, we report the development of two genetically encoded tools for investigating peptidergic transmission in behaving mice: a genetically encoded large dense core vesicle (LDCV) sensor that detects presynaptic neuropeptide release and a genetically encoded silencer that specifically degrades neuropeptides inside LDCVs. Using these tools, we show that neuropeptides, not glutamate, encode the unconditioned stimulus in the parabrachial-to-amygdalar threat pathway during Pavlovian threat learning. We also show that neuropeptides play important roles in encoding positive valence and suppressing conditioned threat response in the amygdala-to-parabrachial endogenous opioidergic circuit. These results show that our sensor and silencer for presynaptic peptidergic transmission are reliable tools to investigate neuropeptidergic systems in awake, behaving animals.

Clathrin mediates membrane fission and budding by constricting membrane pores

Membrane budding, which underlies fundamental processes like endocytosis, intracellular trafficking, and viral infection, is thought to involve membrane coat-forming proteins, including the most observed clathrin, to form Ω-shape profiles and helix-forming proteins like dynamin to constrict Ω-profiles' pores and thus mediate fission. Challenging this fundamental concept, we report that polymerized clathrin is required for Ω-profiles' pore closure and that clathrin around Ω-profiles' base/pore region mediates pore constriction/closure in neuroendocrine chromaffin cells. Mathematical modeling suggests that clathrin polymerization at Ω-profiles' base/pore region generates forces from its intrinsically curved shape to constrict/close the pore. This new fission function may exert broader impacts than clathrin's well-known coat-forming function during clathrin (coat)-dependent endocytosis, because it underlies not only clathrin (coat)-dependent endocytosis, but also diverse endocytic modes, including ultrafast, fast, slow, bulk, and overshoot endocytosis previously considered clathrin (coat)-independent in chromaffin cells. It mediates kiss-and-run fusion (fusion pore closure) previously considered bona fide clathrin-independent, and limits the vesicular content release rate. Furthermore, analogous to results in chromaffin cells, we found that clathrin is essential for fast and slow endocytosis at hippocampal synapses where clathrin was previously considered dispensable, suggesting clathrin in mediating synaptic vesicle endocytosis and fission. These results suggest that clathrin and likely other intrinsically curved coat proteins are a new class of fission proteins underlying vesicle budding and fusion. The half-a-century concept and studies that attribute vesicle-coat contents' function to Ω-profile formation and classify budding as coat-protein (e.g., clathrin)-dependent or -independent may need to be re-defined and re-examined by considering clathrin's pivotal role in pore constriction/closure.

The CHD family chromatin remodeling enzyme, Kismet, promotes both clathrin-mediated and activity-dependent bulk endocytosis

Chromodomain helicase DNA binding domain (CHD) proteins, including CHD7 and CHD8, remodel chromatin to enable transcriptional programs. Both proteins are important for proper neural development as heterozygous mutations in Chd7 and Chd8 are causative for CHARGE syndrome and correlated with autism spectrum disorders, respectively. Their roles in mature neurons are poorly understood despite influencing the expression of genes required for cell adhesion, neurotransmission, and synaptic plasticity. The Drosophila homolog of CHD7 and CHD8, Kismet (Kis), promotes neurotransmission, endocytosis, and larval locomotion. Endocytosis is essential in neurons for replenishing synaptic vesicles, maintaining protein localization, and preserving the size and composition of the presynaptic membrane. Several forms of endocytosis have been identified including clathrin-mediated endocytosis, which is coupled with neural activity and is the most prevalent form of synaptic endocytosis, and activity-dependent bulk endocytosis, which occurs during periods of intense stimulation. Kis modulates the expression of gene products involved in endocytosis including promoting shaggy/GSK3β expression while restricting PI3K92E. kis mutants electrophysiologically phenocopy a liquid facets mutant in response to paradigms that induce clathrin-mediated endocytosis and activity-dependent bulk endocytosis. Further, kis mutants do not show further reductions in endocytosis when activity-dependent bulk endocytosis or clathrin-mediated endocytosis are pharmacologically inhibited. We find that Kis is important in postsynaptic muscle for proper endocytosis but the ATPase domain of Kis is dispensable for endocytosis. Collectively, our data indicate that Kis promotes both clathrin-mediated endocytosis and activity-dependent bulk endocytosis possibly by promoting transcription of several endocytic genes and maintaining the size of the synaptic vesicle pool.

Synaptotagmin-11 Inhibits Synaptic Vesicle Endocytosis via Endophilin A1

Synaptic vesicle (SV) endocytosis is a critical and well-regulated process for the maintenance of neurotransmission. We previously reported that synaptotagmin-11 (Syt11), an essential non-Ca2+-binding Syt associated with brain diseases, inhibits neuronal endocytosis (Wang et al., 2016). Here, we found that Syt11 deficiency caused accelerated SV endocytosis and vesicle recycling under sustained stimulation and led to the abnormal membrane partition of synaptic proteins in mouse hippocampal boutons of either sex. Furthermore, our study revealed that Syt11 has direct but Ca2+-independent binding with endophilin A1 (EndoA1), a membrane curvature sensor and endocytic protein recruiter, with high affinity. EndoA1-knockdown significantly reversed Syt11-KO phenotype, identifying EndoA1 as a main inhibitory target of Syt11 during SV endocytosis. The N-terminus of EndoA1 and the C2B domain of Syt11 were responsible for this interaction. A peptide (amino acids 314-336) derived from the Syt11 C2B efficiently blocked Syt11-EndoA1 binding both in vitro and in vivo Application of this peptide inhibited SV endocytosis in WT hippocampal neurons but not in EndoA1-knockdown neurons. Moreover, intracellular application of this peptide in mouse calyx of Held terminals of either sex effectively hampered both fast and slow SV endocytosis at physiological temperature. We thus propose that Syt11 ensures the precision of protein retrieval during SV endocytosis by inhibiting EndoA1 function at neuronal terminals.SIGNIFICANCE STATEMENT Endocytosis is a key stage of synaptic vesicle (SV) recycling. SV endocytosis retrieves vesicular membrane and protein components precisely to support sustained neurotransmission. However, the molecular mechanisms underlying the regulation of SV endocytosis remain elusive. Here, we reported that Syt11-KO accelerated SV endocytosis and impaired membrane partition of synaptic proteins. EndoA1 was identified as a main inhibitory target of Syt11 during SV endocytosis. Our study reveals a novel inhibitory mechanism of SV endocytosis in preventing hyperactivation of endocytosis, potentially safeguarding the recycling of synaptic proteins during sustained neurotransmission.

Rational Engineering of an Improved Genetically Encoded pH Sensor Based on Superecliptic pHluorin

Genetically encoded pH sensors based on fluorescent proteins are valuable tools for the imaging of cellular events that are associated with pH changes, such as exocytosis and endocytosis. Superecliptic pHluorin (SEP) is a pH-sensitive green fluorescent protein (GFP) variant widely used for such applications. Here, we report the rational design, development, structure, and applications of Lime, an improved SEP variant with higher fluorescence brightness and greater pH sensitivity. The X-ray crystal structure of Lime supports the mechanistic rationale that guided the introduction of beneficial mutations. Lime provides substantial improvements relative to SEP for imaging of endocytosis and exocytosis. Furthermore, Lime and its variants are advantageous for a broader range of applications including the detection of synaptic release and neuronal voltage changes.

Emerging concepts in oligodendrocyte and myelin formation, inputs from the zebrafish model

Oligodendrocytes (OLs) are the myelinating cells of the central nervous system (CNS), which are derived from OL precursor cells. Myelin insulates axons allowing the saltatory conduction of action potentials and also provides trophic and metabolic supports to axons. Interestingly, oligodendroglial cells have the capacity to sense neuronal activity, which regulates myelin sheath formation via the vesicular release of neurotransmitters. Neuronal activity-dependent regulation of myelination is mediated by specialized interaction between axons and oligodendroglia, involving both synaptic and extra-synaptic modes of communications. The zebrafish has provided key advantages for the study of the myelination process in the CNS. External development and transparent larval stages of this vertebrate specie combined with the existence of several transgenic reporter lines provided key advances in oligodendroglial cell biology, axo-glial interactions and CNS myelination. In this publication, we reviewed and discussed the most recent knowledge on OL development and myelin formation, with a focus on mechanisms regulating these fundamental biological processes in the zebrafish. Especially, we highlighted the critical function of axons and oligodendroglia modes of communications and calcium signaling in myelin sheath formation and growth. Finally, we reviewed the relevance of these knowledge's in demyelinating diseases and drug discovery of pharmacological compounds favoring myelin regeneration.

pOpsicle: An all-optical reporter system for synaptic vesicle recycling combining pH-sensitive fluorescent proteins with optogenetic manipulation of neuronal activity

pH-sensitive fluorescent proteins are widely used to study synaptic vesicle (SV) fusion and recycling. When targeted to the lumen of SVs, fluorescence of these proteins is quenched by the acidic pH. Following SV fusion, they are exposed to extracellular neutral pH, resulting in a fluorescence increase. SV fusion, recycling and acidification can thus be tracked by tagging integral SV proteins with pH-sensitive proteins. Neurotransmission is generally activated by electrical stimulation, which is not feasible in small, intact animals. Previous in vivo approaches depended on distinct (sensory) stimuli, thus limiting the addressable neuron types. To overcome these limitations, we established an all-optical approach to stimulate and visualize SV fusion and recycling. We combined distinct pH-sensitive fluorescent proteins (inserted into the SV protein synaptogyrin) and light-gated channelrhodopsins (ChRs) for optical stimulation, overcoming optical crosstalk and thus enabling an all-optical approach. We generated two different variants of the pH-sensitive optogenetic reporter of vesicle recycling (pOpsicle) and tested them in cholinergic neurons of intact Caenorhabditis elegans nematodes. First, we combined the red fluorescent protein pHuji with the blue-light gated ChR2(H134R), and second, the green fluorescent pHluorin combined with the novel red-shifted ChR ChrimsonSA. In both cases, fluorescence increases were observed after optical stimulation. Increase and subsequent decline of fluorescence was affected by mutations of proteins involved in SV fusion and endocytosis. These results establish pOpsicle as a non-invasive, all-optical approach to investigate different steps of the SV cycle.

Novel genetically encoded tools for imaging or silencing neuropeptide release from presynaptic terminals in vivo

Neurons produce and release neuropeptides to communicate with one another. Despite their profound impact on critical brain functions, circuit-based mechanisms of peptidergic transmission are poorly understood, primarily due to the lack of tools for monitoring and manipulating neuropeptide release in vivo. Here, we report the development of two genetically encoded tools for investigating peptidergic transmission in behaving mice: a genetically encoded large dense core vesicle (LDCV) sensor that detects the neuropeptides release presynaptically, and a genetically encoded silencer that specifically degrades neuropeptides inside the LDCV. Monitoring and silencing peptidergic and glutamatergic transmissions from presynaptic terminals using our newly developed tools and existing genetic tools, respectively, reveal that neuropeptides, not glutamate, are the primary transmitter in encoding unconditioned stimulus during Pavlovian threat learning. These results show that our sensor and silencer for peptidergic transmission are reliable tools to investigate neuropeptidergic systems in awake behaving animals.

Effects of the clathrin inhibitor Pitstop-2 on synaptic vesicle recycling at a central synapse in vivo

Four modes of endocytosis and subsequent synaptic vesicle (SV) recycling have been described at the presynapse to ensure the availability of SVs for synaptic release. However, it is unclear to what extend these modes operate under physiological activity patterns in vivo. The coat protein clathrin can regenerate SVs either directly from the plasma membrane (PM) via clathrin-mediated endocytosis (CME), or indirectly from synaptic endosomes by SV budding. Here, we examined the role of clathrin in SV recycling under physiological conditions by applying the clathrin inhibitor Pitstop-2 to the calyx of Held, a synapse optimized for high frequency synaptic transmission in the auditory brainstem, in vivo. The effects of clathrin-inhibition on SV recycling were investigated by serial sectioning scanning electron microscopy (S3EM) and 3D reconstructions of endocytic structures labeled by the endocytosis marker horseradish peroxidase (HRP). We observed large endosomal compartments as well as HRP-filled, black SVs (bSVs) that have been recently recycled. The application of Pitstop-2 led to reduced bSV but not large endosome density, increased volumes of large endosomes and shifts in the localization of both types of endocytic compartments within the synapse. These changes after perturbation of clathrin function suggest that clathrin plays a role in SV recycling from both, the PM and large endosomes, under physiological activity patterns, in vivo.

CNS myelination requires VAMP2/3-mediated membrane expansion in oligodendrocytes

Myelin is required for rapid nerve signaling and is emerging as a key driver of CNS plasticity and disease. How myelin is built and remodeled remains a fundamental question of neurobiology. Central to myelination is the ability of oligodendrocytes to add vast amounts of new cell membrane, expanding their surface areas by many thousand-fold. However, how oligodendrocytes add new membrane to build or remodel myelin is not fully understood. Here, we show that CNS myelin membrane addition requires exocytosis mediated by the vesicular SNARE proteins VAMP2/3. Genetic inactivation of VAMP2/3 in myelinating oligodendrocytes caused severe hypomyelination and premature death without overt loss of oligodendrocytes. Through live imaging, we discovered that VAMP2/3-mediated exocytosis drives membrane expansion within myelin sheaths to initiate wrapping and power sheath elongation. In conjunction with membrane expansion, mass spectrometry of oligodendrocyte surface proteins revealed that VAMP2/3 incorporates axon-myelin adhesion proteins that are collectively required to form nodes of Ranvier. Together, our results demonstrate that VAMP2/3-mediated membrane expansion in oligodendrocytes is indispensable for myelin formation, uncovering a cellular pathway that could sculpt myelination patterns in response to activity-dependent signals or be therapeutically targeted to promote regeneration in disease.

Gain-of-Function Dynamin-2 Mutations Linked to Centronuclear Myopathy Impair Ca2+-Induced Exocytosis in Human Myoblasts

Gain-of-function mutations of dynamin-2, a mechano-GTPase that remodels membrane and actin filaments, cause centronuclear myopathy (CNM), a congenital disease that mainly affects skeletal muscle tissue. Among these mutations, the variants p.A618T and p.S619L lead to a gain of function and cause a severe neonatal phenotype. By using total internal reflection fluorescence microscopy (TIRFM) in immortalized human myoblasts expressing the pH-sensitive fluorescent protein (pHluorin) fused to the insulin-responsive aminopeptidase IRAP as a reporter of the GLUT4 vesicle trafficking, we measured single pHluorin signals to investigate how p.A618T and p.S619L mutations influence exocytosis. We show here that both dynamin-2 mutations significantly reduced the number and durations of pHluorin signals induced by 10 μM ionomycin, indicating that in addition to impairing exocytosis, they also affect the fusion pore dynamics. These mutations also disrupt the formation of actin filaments, a process that reportedly favors exocytosis. This altered exocytosis might importantly disturb the plasmalemma expression of functional proteins such as the glucose transporter GLUT4 in skeletal muscle cells, impacting the physiology of the skeletal muscle tissue and contributing to the CNM disease.

Light and electron microscopic imaging of synaptic vesicle endocytosis at mouse hippocampal cultures

Following the release of neurotransmitters at synaptic vesicles via exocytosis, endocytosis is initiated to retrieve vesicles that have fused with the plasma membrane of nerve terminals and recycle them, thus sustaining synaptic transmission. Here, we describe imaging-based protocols for quantitative measurements of endocytosis at cultured synapses. These protocols include (1) primary culture of mouse hippocampal neurons, (2) studying endocytosis at neurons transfected with a pH-sensitive synaptophysin-pHluorin2× using fluorescent microscopy, and (3) imaging endocytosis at fixed neurons with electron microscopy.

For complete details on the use and execution of this protocol, please refer to Wu et al. (2016) and Wu et al. (2021).

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Detailed protocol for primary culture and transfection of mouse hippocampal neurons

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Light microscopy and analysis of endocytosis in cultured neurons

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Electron microscopy and analysis of vesicle and endosome formation

Publisher’s note: Undertaking any experimental protocol requires adherence to local institutional guidelines for laboratory safety and ethics.

Dynamin 1 controls vesicle size and endocytosis at hippocampal synapses

Following calcium-triggered vesicle exocytosis, endocytosis regenerates vesicles to maintain exocytosis and thus synaptic transmission, which underlies neuronal circuit activities. Although most molecules involved in endocytosis have been identified, it remains rather poorly understood how endocytic machinery regulates vesicle size. Vesicle size, together with the transmitter concentration inside the vesicle, determines the amount of transmitter the vesicle can release, the quantal size, that may control the strength of synaptic transmission. Here, we report that, surprisingly, knockout of the GTPase dynamin 1, the most abundant brain dynamin isoform known to catalyze fission of the membrane pit's neck (the last step of endocytosis), not only significantly slowed endocytosis but also increased the synaptic vesicle diameter by as much as ∼40-64% at cultured hippocampal synapses. Furthermore, dynamin 1 knockout increased the size of membrane pits, the precursor for endocytic vesicle formation. These results suggest an important function of dynamin other than its well-known fission function - control of vesicle size at the pit formation stage.

Neurexins play a crucial role in cerebellar granule cell survival by organizing autocrine machinery for neurotrophins

Neurexins (NRXNs) are key presynaptic cell adhesion molecules that regulate synapse formation and function via trans-synaptic interaction with postsynaptic ligands. Here, we generate cerebellar granule cell (CGC)-specific Nrxn triple-knockout (TKO) mice for complete deletion of all NRXNs. Unexpectedly, most CGCs die in these mice, and this requirement for NRXNs for cell survival is reproduced in cultured CGCs. The axons of cultured Nrxn TKO CGCs that are not in contact with a postsynaptic structure show defects in the formation of presynaptic protein clusters and in action-potential-induced Ca²⁺ influxes. These cells also show impaired secretion of depolarization-induced, fluorescence-tagged brain-derived neurotrophic factor (BDNF) from their axons, and the cell-survival defect is rescued by the application of BDNF. These results suggest that CGC survival is maintained by autocrine neurotrophic factors and that NRXNs organize the presynaptic protein clusters and the autocrine neurotrophic-factor secretory machinery independent of contact with postsynaptic ligands.

Minibrain kinase and calcineurin coordinate activity-dependent bulk endocytosis through synaptojanin

Neurons use multiple modes of endocytosis, including clathrin-mediated endocytosis (CME) and activity-dependent bulk endocytosis (ADBE), during mild and intense neuronal activity, respectively, to maintain stable neurotransmission. While molecular players modulating CME are well characterized, factors regulating ADBE and mechanisms coordinating CME and ADBE activations remain poorly understood. Here we report that Minibrain/DYRK1A (Mnb), a kinase mutated in autism and up-regulated in Down's syndrome, plays a novel role in suppressing ADBE. We demonstrate that Mnb, together with calcineurin, delicately coordinates CME and ADBE by controlling the phosphoinositol phosphatase activity of synaptojanin (Synj) during varying synaptic demands. Functional domain analyses reveal that Synj's 5'-phosphoinositol phosphatase activity suppresses ADBE, while SAC1 activity is required for efficient ADBE. Consequently, Parkinson's disease mutation in Synj's SAC1 domain impairs ADBE. These data identify Mnb and Synj as novel regulators of ADBE and further indicate that CME and ADBE are differentially governed by Synj's dual phosphatase domains.

Myelination induces axonal hotspots of synaptic vesicle fusion that promote sheath growth

Myelination of axons by oligodendrocytes enables fast saltatory conduction. Oligodendrocytes are responsive to neuronal activity, which has been shown to induce changes to myelin sheaths, potentially to optimize conduction and neural circuit function. However, the cellular bases of activity-regulated myelination in vivo are unclear, partly due to the difficulty of analyzing individual myelinated axons over time. Activity-regulated myelination occurs in specific neuronal subtypes and can be mediated by synaptic vesicle fusion, but several questions remain: it is unclear whether vesicular fusion occurs stochastically along axons or in discrete hotspots during myelination and whether vesicular fusion regulates myelin targeting, formation, and/or growth. It is also unclear why some neurons, but not others, exhibit activity-regulated myelination. Here, we imaged synaptic vesicle fusion in individual neurons in living zebrafish and documented robust vesicular fusion along axons during myelination. Surprisingly, we found that axonal vesicular fusion increased upon and required myelination. We found that axonal vesicular fusion was enriched in hotspots, namely the heminodal non-myelinated domains into which sheaths grew. Blocking vesicular fusion reduced the stable formation and growth of myelin sheaths, and chemogenetically stimulating neuronal activity promoted sheath growth. Finally, we observed high levels of axonal vesicular fusion only in neuronal subtypes that exhibit activity-regulated myelination. Our results identify a novel "feedforward" mechanism whereby the process of myelination promotes the neuronal activity-regulated signal, vesicular fusion that, in turn, consolidates sheath growth along specific axons selected for myelination.

Selective inhibition of gamma aminobutyric acid release from mouse hippocampal interneurone subtypes by the volatile anaesthetic isoflurane

BACKGROUND

The cellular and molecular mechanisms by which general anaesthesia occurs is poorly understood. Hippocampal interneurone subpopulations, which are critical regulators of cognitive function, have diverse neurophysiological and synaptic properties, but their responses to anaesthetics are unclear.

METHODS

We used live-cell imaging of fluorescent biosensors expressed in mouse hippocampal neurones to delineate interneurone subtype-specific effects of isoflurane on synaptic vesicle exocytosis. The role of voltage-gated sodium channel (Na_v) subtype expression in determining isoflurane sensitivity was probed by overexpression or knockdown of specific Na_v subtypes in identified interneurones.

RESULTS

Clinically relevant concentrations of isoflurane differentially inhibited synaptic vesicle exocytosis: to 83.1% (11.7%) of control in parvalbumin-expressing interneurones, and to 58.6% (13.3%) and 64.5% (8.5%) of control in somatostatin-expressing interneurones and glutamatergic neurones, respectively. The relative expression of Na_v1.1 (associated with lower sensitivity) and Na_v1.6 (associated with higher sensitivity) determined the sensitivity of exocytosis to isoflurane.

CONCLUSIONS

Isoflurane inhibits synaptic vesicle exocytosis from hippocampal glutamatergic neurones and GABAergic interneurones in a cell-type-specific manner depending on their expression of voltage-gated sodium channel subtypes.

SCAMP5 plays a critical role in axonal trafficking and synaptic localization of NHE6 to adjust quantal size at glutamatergic synapses

Glutamate uptake into synaptic vesicles (SVs) depends on cation/H⁺ exchange activity, which converts the chemical gradient (ΔpH) into membrane potential (Δψ) across the SV membrane at the presynaptic terminals. Thus, the proper recruitment of cation/H⁺ exchanger to SVs is important in determining glutamate quantal size, yet little is known about its localization mechanism. Here, we found that secretory carrier membrane protein 5 (SCAMP5) interacted with the cation/H⁺ exchanger NHE6, and this interaction regulated NHE6 recruitment to glutamatergic presynaptic terminals. Protein-protein interaction analysis with truncated constructs revealed that the 2/3 loop domain of SCAMP5 is directly associated with the C-terminal region of NHE6. The use of optical imaging and electrophysiological recording showed that small hairpin RNA-mediated knockdown (KD) of SCAMP5 or perturbation of SCAMP5/NHE6 interaction markedly inhibited axonal trafficking and the presynaptic localization of NHE6, leading to hyperacidification of SVs and a reduction in the quantal size of glutamate release. Knockout of NHE6 occluded the effect of SCAMP5 KD without causing additional defects. Together, our results reveal that as a key regulator of axonal trafficking and synaptic localization of NHE6, SCAMP5 could adjust presynaptic strength by regulating quantal size at glutamatergic synapses. Since both proteins are autism candidate genes, the reduced quantal size by interrupting their interaction may underscore synaptic dysfunction observed in autism.

A positive feedback loop between Flower and PI(4,5)P2 at periactive zones controls bulk endocytosis in Drosophila

Synaptic vesicle (SV) endocytosis is coupled to exocytosis to maintain SV pool size and thus neurotransmitter release. Intense stimulation induces activity-dependent bulk endocytosis (ADBE) to recapture large quantities of SV constituents in large endosomes from which SVs reform. How these consecutive processes are spatiotemporally coordinated remains unknown. Here, we show that Flower Ca²⁺ channel-dependent phosphatidylinositol 4,5-bisphosphate (PI(4,5)P₂) compartmentalization governs control of these processes in Drosophila. Strong stimuli trigger PI(4,5)P₂ microdomain formation at periactive zones. Upon exocytosis, Flower translocates from SVs to periactive zones, where it increases PI(4,5)P₂ levels via Ca²⁺ influxes. Remarkably, PI(4,5)P₂ directly enhances Flower channel activity, thereby establishing a positive feedback loop for PI(4,5)P₂ microdomain compartmentalization. PI(4,5)P₂ microdomains drive ADBE and SV reformation from bulk endosomes. PI(4,5)P₂ further retrieves Flower to bulk endosomes, terminating endocytosis. We propose that the interplay between Flower and PI(4,5)P₂ is the crucial spatiotemporal cue that couples exocytosis to ADBE and subsequent SV reformation.

A Novel Optical Quantal Analysis of Miniature Events Reveals Enhanced Frequency Following Amyloid β Exposure

Non-evoked miniature release of neurotransmitters is increasingly recognized as playing an important role in neural function and is implicated in synaptic plasticity, metaplasticity, and homeostasis. Spontaneous miniature release events (minis) are usually measured electrophysiologically by recording the miniature postsynaptic currents (mEPSCs) that they evoke. However, this indirect technique can be confounded by changes within the postsynaptic neuron. Here, using the fluorescent probe SynaptopHluorin 2×, we have developed an optical method for the measurement of minis that enables direct assessment of release events. We use the technique to reveal that the frequency of minis following incubation of hippocampal neurons with Amyloid β oligomers (Aβo) is increased. Electrophysiological mEPSC recordings obtained under the same conditions report a decrease in frequency, with the discrepancy likely due to Aβo-induced changes in quantal size. Optical quantal analysis of minis may therefore have a role in the study of minis in both normal physiology and disease, as it can circumvent potential confounds caused by postsynaptic changes.

The Synaptic Vesicle Cycle Revisited: New Insights into the Modes and Mechanisms

Neurotransmission is sustained by endocytosis and refilling of synaptic vesicles (SVs) locally within the presynapse. Until recently, a consensus formed that after exocytosis, SVs are recovered by either fusion pore closure (kiss-and-run) or clathrin-mediated endocytosis directly from the plasma membrane. However, recent data have revealed that SV formation is more complex than previously envisaged. For example, two additional recycling pathways have been discovered, ultrafast endocytosis and activity-dependent bulk endocytosis, in which SVs are regenerated from the internalized membrane and synaptic endosomes. Furthermore, these diverse modes of endocytosis appear to influence both the molecular composition and subsequent physiological role of individual SVs. In addition, previously unknown complexity in SV refilling and reclustering has been revealed. This review presents a modern view of the SV life cycle and discusses how neuronal subtype, physiological temperature, and individual activity patterns can recruit different endocytic modes to generate new SVs and sculpt subsequent presynaptic performance.

Protein Kinase C and Calmodulin Serve As Calcium Sensors for Calcium-Stimulated Endocytosis at Synapses

Calcium influx triggers and facilitates endocytosis, which recycles vesicles and thus sustains synaptic transmission. Despite decades of studies, the underlying calcium sensor remained not well understood. Here, we examined two calcium binding proteins, protein kinase C (PKC) and calmodulin. Whether PKC is involved in endocytosis was unclear; whether calmodulin acts as a calcium sensor for endocytosis was neither clear, although calmodulin involvement in endocytosis had been suggested. We generated PKC (α or β-isoform) and calmodulin (calmodulin 2 gene) knock-out mice of either sex and measured endocytosis with capacitance measurements, pHluorin imaging and electron microscopy. We found that these knock-outs inhibited slow (∼10-30 s) and rapid (<∼3 s) endocytosis at large calyx-type calyces, and inhibited slow endocytosis and bulk endocytosis (forming large endosome-like structures) at small conventional hippocampal synapses, suggesting the involvement of PKC and calmodulin in three most common forms of endocytosis-the slow, rapid and bulk endocytosis. Inhibition of slow endocytosis in PKC or calmodulin 2 knock-out hippocampal synapses was rescued by overexpressing wild-type PKC or calmodulin, but not calcium-binding-deficient PKC or calmodulin mutant, respectively, suggesting that calcium stimulates endocytosis by binding with its calcium sensor PKC and calmodulin. PKC and calmodulin 2 knock-out inhibited calcium-dependent vesicle mobilization to the readily releasable pool, suggesting that PKC and calmodulin may mediate calcium-dependent facilitation of vesicle mobilization. These findings shed light on the molecular signaling link among calcium, endocytosis and vesicle mobilization that are crucial in maintaining synaptic transmission and neuronal network activity.SIGNIFICANCE STATEMENT Vesicle fusion releases neurotransmitters to mediate synaptic transmission. To sustain synaptic transmission, fused vesicles must be retrieved via endocytosis. Accumulating evidence suggests that calcium influx triggers synaptic vesicle endocytosis. However, how calcium triggers endocytosis is not well understood. Using genetic tools together with capacitance measurements, optical imaging and electron microscopy, we identified two calcium sensors, including protein kinase C (α and β isoforms) and calmodulin, for the most commonly observed forms of endocytosis: slow, rapid, and bulk. We also found that these two proteins are involved in calcium-dependent vesicle mobilization to the readily releasable pool. These results provide the molecular signaling link among calcium, endocytosis, and vesicle mobilization that are essential in sustaining synaptic transmission and neuronal network activity.

Ca2+-independent but voltage-dependent quantal catecholamine secretion (CiVDS) in the mammalian sympathetic nervous system

Action potential-induced vesicular exocytosis is considered exclusively Ca²⁺ dependent in Katz's Ca²⁺ hypothesis on synaptic transmission. This long-standing concept gets an exception following the discovery of Ca²⁺-independent but voltage-dependent secretion (CiVDS) and its molecular mechanisms in dorsal root ganglion sensory neurons. However, whether CiVDS presents only in sensory cells remains elusive. Here, by combining multiple independent recordings, we report that [1] CiVDS robustly presents in the sympathetic nervous system, including sympathetic superior cervical ganglion neurons and slice adrenal chromaffin cells, [2] uses voltage sensors of Ca²⁺ channels (N-type and novel L-type), and [3] contributes to catecholamine release in both homeostatic and fight-or-flight like states; [4] CiVDS-mediated catecholamine release is faster than that of Ca²⁺-dependent secretion at the quantal level and [5] increases Ca²⁺ currents and contractility of cardiac myocytes. Together, CiVDS presents in the sympathetic nervous system with potential physiological functions, including cardiac muscle contractility.

Synaptojanin and Endophilin Mediate Neck Formation during Ultrafast Endocytosis

Ultrafast endocytosis generates vesicles from the plasma membrane as quickly as 50 ms in hippocampal neurons following synaptic vesicle fusion. The molecular mechanism underlying the rapid maturation of these endocytic pits is not known. Here we demonstrate that synaptojanin-1, and its partner endophilin-A, function in ultrafast endocytosis. In the absence of synaptojanin or endophilin, the membrane is rapidly invaginated, but pits do not become constricted at the base. The 5-phosphatase activity of synaptojanin is involved in formation of the neck, but 4-phosphatase is not required. Nevertheless, these pits are eventually cleaved into vesicles; within a 30-s interval, synaptic endosomes form and are resolved by clathrin-mediated budding. Then synaptojanin and endophilin function at a second step to aid with the removal of clathrin coats from the regenerated vesicles. These data together suggest that synaptojanin and endophilin can mediate membrane remodeling on a millisecond timescale during ultrafast endocytosis.

The epilepsy-associated protein TBC1D24 is required for normal development, survival and vesicle trafficking in mammalian neurons

Mutations in the Tre2/Bub2/Cdc16 (TBC)1 domain family member 24 (TBC1D24) gene are associated with a range of inherited neurological disorders, from drug-refractory lethal epileptic encephalopathy and DOORS syndrome (deafness, onychodystrophy, osteodystrophy, mental retardation, seizures) to non-syndromic hearing loss. TBC1D24 has been implicated in neuronal transmission and maturation, although the molecular function of the gene and the cause of the apparently complex disease spectrum remain unclear. Importantly, heterozygous TBC1D24 mutation carriers have also been reported with seizures, suggesting that haploinsufficiency for TBC1D24 is significant clinically. Here we have systematically investigated an allelic series of disease-associated mutations in neurons alongside a new mouse model to investigate the consequences of TBC1D24 haploinsufficiency to mammalian neurodevelopment and synaptic physiology. The cellular studies reveal that disease-causing mutations that disrupt either of the conserved protein domains in TBC1D24 are implicated in neuronal development and survival and are likely acting as loss-of-function alleles. We then further investigated TBC1D24 haploinsufficiency in vivo and demonstrate that TBC1D24 is also crucial for normal presynaptic function: genetic disruption of Tbc1d24 expression in the mouse leads to an impairment of endocytosis and an enlarged endosomal compartment in neurons with a decrease in spontaneous neurotransmission. These data reveal the essential role for TBC1D24 at the mammalian synapse and help to define common synaptic mechanisms that could underlie the varied effects of TBC1D24 mutations in neurological disease.

Various approaches for measurement of synaptic vesicle endocytosis at the central nerve terminal

At the presynaptic terminal, neurotransmitters are stored in synaptic vesicles (SVs), which are released and recycled via exo- and endocytosis. SV endocytosis is crucial for sustaining synaptic transmission by maintaining the SV pool. Many studies have shown that presynaptic dysfunction, particularly impairment of SV endocytosis, is related to neurological disorders. Notably, the presynaptic terminal is considered to be a sensitive structure because certain presynaptic dysfunctions, manifested as impaired SV endocytosis or ultrastructural changes in the presynaptic terminal, can be observed before there is a biochemical or pathological evidence of a neurological disorder. Therefore, monitoring and assessing the presynaptic function by SV endocytosis facilitates the development of early markers for neurological disorders. In this study, we reviewed the current methods for assessing and visualizing SV endocytosis at the central nerve terminal.

Isolation of synaptic vesicles from genetically engineered cultured neurons

BACKGROUND

Synaptic vesicles (SVs) are an integral part of the neurotransmission machinery, and isolation of SVs from their host neuron is necessary to reveal their most fundamental biochemical and functional properties in in vitro assays. Isolated SVs from neurons that have been genetically engineered, e.g. to introduce genetically encoded indicators, are not readily available but would permit new insights into SV structure and function. Furthermore, it is unclear if cultured neurons can provide sufficient starting material for SV isolation procedures.

NEW METHOD

Here, we demonstrate an efficient ex vivo procedure to obtain functional SVs from cultured rat cortical neurons after genetic engineering with a lentivirus.

RESULTS

We show that ∼10⁸ plated cortical neurons allow isolation of suitable SV amounts for functional analysis and imaging. We found that SVs isolated from cultured neurons have neurotransmitter uptake comparable to that of SVs isolated from intact cortex. Using total internal reflection fluorescence (TIRF) microscopy, we visualized an exogenous SV-targeted marker protein and demonstrated the high efficiency of SV modification.

COMPARISON WITH EXISTING METHODS

Obtaining SVs from genetically engineered neurons currently generally requires the availability of transgenic animals, which is constrained by technical (e.g. cost and time) and biological (e.g. developmental defects and lethality) limitations.

CONCLUSIONS

These results demonstrate the modification and isolation of functional SVs using cultured neurons and viral transduction. The ability to readily obtain SVs from genetically engineered neurons will permit linking in situ studies to in vitro experiments in a variety of genetic contexts.

Dynamin 1 Restrains Vesicular Release to a Subquantal Mode In Mammalian Adrenal Chromaffin Cells

Dynamin 1 (dyn1) is required for clathrin-mediated endocytosis in most secretory (neuronal and neuroendocrine) cells. There are two modes of Ca²⁺-dependent catecholamine release from single dense-core vesicles: full-quantal (quantal) and subquantal in adrenal chromaffin cells, but their relative occurrences and impacts on total secretion remain unclear. To address this fundamental question in neurotransmission area using both sexes of animals, here we report the following: (1) dyn1-KO increased quantal size (QS, but not vesicle size/content) by ≥250% in dyn1-KO mice; (2) the KO-increased QS was rescued by dyn1 (but not its deficient mutant or dyn2); (3) the ratio of quantal versus subquantal events was increased by KO; (4) following a release event, more protein contents were retained in WT versus KO vesicles; and (5) the fusion pore size (d _p) was increased from ≤9 to ≥9 nm by KO. Therefore, Ca²⁺-induced exocytosis is generally a subquantal release in sympathetic adrenal chromaffin cells, implying that neurotransmitter release is generally regulated by dynamin in neuronal cells.SIGNIFICANCE STATEMENT Ca²⁺-dependent neurotransmitter release from a single vesicle is the primary event in all neurotransmission, including synaptic/neuroendocrine forms. To determine whether Ca²⁺-dependent vesicular neurotransmitter release is "all-or-none" (quantal), we provide compelling evidence that most Ca²⁺-induced secretory events occur via the subquantal mode in native adrenal chromaffin cells. This subquantal release mode is promoted by dynamin 1, which is universally required for most secretory cells, including neurons and neuroendocrine cells. The present work with dyn1-KO mice further confirms that Ca²⁺-dependent transmitter release is mainly via subquantal mode, suggesting that subquantal release could be also important in other types of cells.

Synaptic Vesicle Endocytosis in Different Model Systems

Neurotransmission in complex animals depends on a choir of functionally distinct synapses releasing neurotransmitters in a highly coordinated manner. During synaptic signaling, vesicles fuse with the plasma membrane to release their contents. The rate of vesicle fusion is high and can exceed the rate at which synaptic vesicles can be re-supplied by distant sources. Thus, local compensatory endocytosis is needed to replenish the synaptic vesicle pools. Over the last four decades, various experimental methods and model systems have been used to study the cellular and molecular mechanisms underlying synaptic vesicle cycle. Clathrin-mediated endocytosis is thought to be the predominant mechanism for synaptic vesicle recycling. However, recent studies suggest significant contribution from other modes of endocytosis, including fast compensatory endocytosis, activity-dependent bulk endocytosis, ultrafast endocytosis, as well as kiss-and-run. Currently, it is not clear whether a universal model of vesicle recycling exist for all types of synapses. It is possible that each synapse type employs a particular mode of endocytosis. Alternatively, multiple modes of endocytosis operate at the same synapse, and the synapse toggles between different modes depending on its activity level. Here we compile review and research articles based on well-characterized model systems: frog neuromuscular junctions, C. elegans neuromuscular junctions, Drosophila neuromuscular junctions, lamprey reticulospinal giant axons, goldfish retinal ribbon synapses, the calyx of Held, and rodent hippocampal synapses. We will compare these systems in terms of their known modes and kinetics of synaptic vesicle endocytosis, as well as the underlying molecular machineries. We will also provide the future development of this field.

Homeostatic Presynaptic Plasticity Is Specifically Regulated by P/Q-type Ca2+ Channels at Mammalian Hippocampal Synapses

Voltage-dependent Ca²⁺ channels (VGCC) represent the principal source of Ca²⁺ ions driving evoked neurotransmitter release at presynaptic boutons. In mammals, presynaptic Ca²⁺ influx is mediated mainly via P/Q-type and N-type VGCC, which differ in their properties. Changes in their relative contributions tune neurotransmission both during development and in Hebbian plasticity. However, whether this represents a functional motif also present in other forms of activity-dependent regulation is unknown. Here, we study the role of VGCC in homeostatic plasticity (HSP) in mammalian hippocampal neurons using optical techniques. We find that changes in evoked Ca²⁺ currents specifically through P/Q-type, but not N-type, VGCC mediate bidirectional homeostatic regulation of both neurotransmitter release efficacy and the size of the major synaptic vesicle pools. Selective dependence of HSP on P/Q-type VGCC in mammalian terminals has important implications for phenotypes associated with P/Q-type channelopathies, including migraine and epilepsy.

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P/Q-type VGCC regulate homeostatic synaptic plasticity (HSP) in mammals

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Changes in synaptic vesicle pool sizes during HSP are also mediated via P/Q-type VGCC

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Expression of HSP is independent of N-type VGCC regulation

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P/Q-type regulation of HSP may explain phenotypes of P/Q-type channelopathies

Optical detection of three modes of endocytosis at hippocampal synapses

Coupling of synaptic vesicle fusion and retrieval constitutes a core mechanism ensuring maintenance of presynaptic function. Recent studies using fast-freeze electron microscopy and capacitance measurements reported an ultrafast mode of endocytosis operating at physiological temperatures. Here, using rat hippocampal neurons, we optically monitored single synaptic vesicle endocytosis with high time resolution using the vesicular glutamate transporter, synaptophysin and the V0a1 subunit of the vacuolar ATPase as probes. In this setting, we could distinguish three components of retrieval operating at ultrafast (~150-250 ms, ~20% of events), fast (~5-12 s, ~40% of events) and ultraslow speeds (>20 s, ~40% of events). While increasing Ca²⁺ slowed the fast events, increasing temperature accelerated their time course. In contrast, the kinetics of ultrafast events were only mildly affected by these manipulations. These results suggest that synaptic vesicle proteins can be retrieved with ultrafast kinetics, although a majority of evoked fusion events are coupled to slower retrieval mechanisms.

Sensing Exocytosis and Triggering Endocytosis at Synapses: Synaptic Vesicle Exocytosis-Endocytosis Coupling

The intact synaptic structure is critical for information processing in neural circuits. During synaptic transmission, rapid vesicle exocytosis increases the size of never terminals and endocytosis counteracts the increase. Accumulating evidence suggests that SV exocytosis and endocytosis are tightly connected in time and space during SV recycling, and this process is essential for synaptic function and structural stability. Research in the past has illustrated the molecular details of synaptic vesicle (SV) exocytosis and endocytosis; however, the mechanisms that timely connect these two fundamental events are poorly understood at central synapses. Here we discuss recent progress in SV recycling and summarize several emerging mechanisms by which synapses can “sense” the occurrence of exocytosis and timely initiate compensatory endocytosis. They include Ca²⁺ sensing, SV proteins sensing, and local membrane stress sensing. In addition, the spatial organization of endocytic zones adjacent to active zones provides a structural basis for efficient coupling between SV exocytosis and endocytosis. Through linking different endocytosis pathways with SV fusion, these mechanisms ensure necessary plasticity and robustness of nerve terminals to meet diverse physiological needs.

CaV2.2 Gates Calcium-Independent but Voltage-Dependent Secretion in Mammalian Sensory Neurons

Action potential induces membrane depolarization and triggers intracellular free Ca²⁺ concentration (Ca²⁺)-dependent secretion (CDS) via Ca²⁺ influx through voltage-gated Ca²⁺ channels. We report a new type of somatic exocytosis triggered by the action potential per se-Ca²⁺-independent but voltage-dependent secretion (CiVDS)-in dorsal root ganglion neurons. Here we uncovered the molecular mechanism of CiVDS, comprising a voltage sensor, fusion machinery, and their linker. Specifically, the voltage-gated N-type Ca²⁺ channel (Ca_V2.2) is the voltage sensor triggering CiVDS, the SNARE complex functions as the vesicle fusion machinery, the "synprint" of Ca_V2.2 serves as a linker between the voltage sensor and the fusion machinery, and ATP is a cargo of CiVDS vesicles. Thus, CiVDS releases ATP from the soma while CDS releases glutamate from presynaptic terminals, establishing the Ca_V2.2-SNARE "voltage-gating fusion pore" as a novel pathway co-existing with the canonical "Ca²⁺-gating fusion pore" pathway for neurotransmitter release following action potentials in primary sensory neurons.

Optogenetic Tools for Subcellular Applications in Neuroscience

The ability to study cellular physiology using photosensitive, genetically encoded molecules has profoundly transformed neuroscience. The modern optogenetic toolbox includes fluorescent sensors to visualize signaling events in living cells and optogenetic actuators enabling manipulation of numerous cellular activities. Most optogenetic tools are not targeted to specific subcellular compartments but are localized with limited discrimination throughout the cell. Therefore, optogenetic activation often does not reflect context-dependent effects of highly localized intracellular signaling events. Subcellular targeting is required to achieve more specific optogenetic readouts and photomanipulation. Here we first provide a detailed overview of the available optogenetic tools with a focus on optogenetic actuators. Second, we review established strategies for targeting these tools to specific subcellular compartments. Finally, we discuss useful tools and targeting strategies that are currently missing from the optogenetics repertoire and provide suggestions for novel subcellular optogenetic applications.

How do you recognize and reconstitute a synaptic vesicle after fusion?

Synaptic vesicle recycling is essential for sustained and reliable neurotransmission. A key component of synaptic vesicle recycling is the synaptic vesicle biogenesis process that is observed in synapses and that maintains the molecular identity of synaptic vesicles. However, the mechanisms by which synaptic vesicles are retrieved and reconstituted after fusion remain unclear. The complex molecular composition of synaptic vesicles renders their rapid biogenesis a daunting task. Therefore, in this context, kiss-and-run type transient fusion of synaptic vesicles with the plasma membrane without loss of their membrane composition and molecular identity remains a viable hypothesis that can account for the fidelity of the synaptic vesicle cycle. In this article, we discuss the biological implications of this problem as well as its possible molecular solutions.

Measuring Synaptic Vesicle Endocytosis in Cultured Hippocampal Neurons

During endocytosis, fused synaptic vesicles are retrieved at nerve terminals, allowing for vesicle recycling and thus the maintenance of synaptic transmission during repetitive nerve firing. Impaired endocytosis in pathological conditions leads to decreases in synaptic strength and brain functions. Here, we describe methods used to measure synaptic vesicle endocytosis at the mammalian hippocampal synapse in neuronal culture. We monitored synaptic vesicle protein endocytosis by fusing a synaptic vesicular membrane protein, including synaptophysin and VAMP2/synaptobrevin, at the vesicular lumenal side, with pHluorin, a pH-sensitive green fluorescent protein that increases its fluorescence intensity as the pH increases. During exocytosis, vesicular lumen pH increases, whereas during endocytosis vesicular lumen pH is re-acidified. Thus, an increase of pHluorin fluorescence intensity indicates fusion, whereas a decrease indicates endocytosis of the labelled synaptic vesicle protein. In addition to using the pHluorin imaging method to record endocytosis, we monitored vesicular membrane endocytosis by electron microscopy (EM) measurements of Horseradish peroxidase (HRP) uptake by vesicles. Finally, we monitored the formation of nerve terminal membrane pits at various times after high potassium-induced depolarization. The time course of HRP uptake and membrane pit formation indicates the time course of endocytosis.

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.

Exocytosis, Endocytosis, and Their Coupling in Excitable Cells

Evoked exocytosis in excitable cells is fast and spatially confined and must be followed by coupled endocytosis to enable sustained exocytosis while maintaining the balance of the vesicle pool and the plasma membrane. Various types of exocytosis and endocytosis exist in these excitable cells, as those has been found from different types of experiments conducted in different cell types. Correlating these diversified types of exocytosis and endocytosis is problematic. By providing an outline of different exocytosis and endocytosis processes and possible coupling mechanisms here, we emphasize that the endocytic pathway may be pre-determined at the time the vesicle chooses to fuse with the plasma membrane in one specific mode. Therefore, understanding the early intermediate stages of vesicle exocytosis may be instrumental in exploring the mechanism of tailing endocytosis.

A Ca2+ channel differentially regulates Clathrin-mediated and activity-dependent bulk endocytosis

Clathrin-mediated endocytosis (CME) and activity-dependent bulk endocytosis (ADBE) are two predominant forms of synaptic vesicle (SV) endocytosis, elicited by moderate and strong stimuli, respectively. They are tightly coupled with exocytosis for sustained neurotransmission. However, the underlying mechanisms are ill defined. We previously reported that the Flower (Fwe) Ca2+ channel present in SVs is incorporated into the periactive zone upon SV fusion, where it triggers CME, thus coupling exocytosis to CME. Here, we show that Fwe also promotes ADBE. Intriguingly, the effects of Fwe on CME and ADBE depend on the strength of the stimulus. Upon mild stimulation, Fwe controls CME independently of Ca2+ channeling. However, upon strong stimulation, Fwe triggers a Ca2+ influx that initiates ADBE. Moreover, knockout of rodent fwe in cultured rat hippocampal neurons impairs but does not completely abolish CME, similar to the loss of Drosophila fwe at the neuromuscular junction, suggesting that Fwe plays a regulatory role in regulating CME across species. In addition, the function of Fwe in ADBE is conserved at mammalian central synapses. Hence, Fwe exerts different effects in response to different stimulus strengths to control two major modes of endocytosis.

Exploring cells with targeted biosensors

Cellular signaling networks are composed of multiple pathways, often interconnected, that form complex networks with great potential for cross-talk. Signal decoding depends on the nature of the message as well as its amplitude, temporal pattern, and spatial distribution. In addition, the existence of membrane-bound organelles, which are both targets and generators of messages, add further complexity to the system. The availability of sensors that can localize to specific compartments in live cells and monitor their targets with high spatial and temporal resolution is thus crucial for a better understanding of cell pathophysiology. For this reason, over the last four decades, a variety of strategies have been developed, not only to generate novel and more sensitive probes for ions, metabolites, and enzymatic activity, but also to selectively deliver these sensors to specific intracellular compartments. In this review, we summarize the principles that have been used to target organic or protein sensors to different cellular compartments and their application to cellular signaling.

Actin Is Crucial for All Kinetically Distinguishable Forms of Endocytosis at Synapses

Mechanical force is needed to mediate endocytosis. Whether actin, the most abundant force-generating molecule, is essential for endocytosis is highly controversial in mammalian cells, particularly synapses, likely due to the use of actin blockers, the efficiency and specificity of which are often unclear in the studied cell. Here we addressed this issue using a knockout approach combined with measurements of membrane capacitance and fission pore conductance, imaging of vesicular protein endocytosis, and electron microscopy. We found that two actin isoforms, β- and γ-actin, are crucial for slow, rapid, bulk, and overshoot endocytosis at large calyx-type synapses, and for slow endocytosis and bulk endocytosis at small hippocampal synapses. Polymerized actin provides mechanical force to form endocytic pits. Actin also facilitates replenishment of the readily releasable vesicle pool, likely via endocytic clearance of active zones. We conclude that polymerized actin provides mechanical force essential for all kinetically distinguishable forms of endocytosis at synapses.

Effect of electrical stimulation as an adjunct to botulinum toxin type A in the treatment of adult spasticity: a systematic review

OBJECTIVE

To investigate whether electrical stimulation (ES) as an adjunct to BTX-A boosts botulinum activity and whether the combined therapeutic procedure is more effective than BTX-A alone in reducing spasticity in adult subjects.

DATA SOURCES

A search was conducted in PubMed, EMBASE, Cochrane Central Register, and CINAHL from January 1966 to January 2016.

STUDY SELECTION

Only randomized controlled studies (RCT) involving the combination of BTX-A and ES were considered. RCTs were excluded if BTX plus ES was investigated in animals or healthy subjects; certain techniques were used as an adjunct to BTX-A, but ES was not used; BTX-A or ES were compared but were not used in combination. ES was divided into neuromuscular stimulation (NMS), functional electrical stimulation (FES), and transcutaneous electrical nerve stimulation (TENS). Two authors independently screened all search results and reviewed study characteristics using the Physiotherapy Evidence Database (PEDro) scale.

RESULTS

Fifteen RCTs were pinpointed and nine studies were included. Trials varied in methodological quality, size, and outcome measures used. ES was used in the form of NMS and FES in seven and two studies, respectively. No study investigating BTX-A plus TENS was found. BTX-A plus ES produced significant reduction in spasticity on the Ashworth Scale (AS) and on the modified AS in seven studies, but only four showed high quality on the PEDro scale. Significant reduction in compound muscular action potential (CMAP) amplitude was detected after BTX-A plus ES in two studies.

CONCLUSIONS

ES as an adjunctive therapy to BTX-A may boost BTX-A action in reducing adult spasticity, but ES variability makes it difficult to recommend the combined therapy in clinical practice. Implications for rehabilitation Electrical stimulation (ES) as adjunct to botulinum toxin type A (BTX-A) injections may boost neurotoxin action in treating adult spasticity. Given the variability of ES characteristics and the paucity of high-quality trials, it is difficult to support definitively the use of BTX-A plus ES to potentiate BTX-A effect in clinical practice. A vast array of rehabilitation interventions combined with BTX-A have been provided in reducing spasticity, but the present evidence is not sufficient to recommend any combined therapeutic strategy.

Endosome-mediated endocytic mechanism replenishes the majority of synaptic vesicles at mature CNS synapses in an activity-dependent manner

Whether synaptic vesicles (SVs) are recovered via endosome-mediated pathways is a matter of debate; however, recent evidence suggests that clathrin-independent bulk endocytosis (CIE) via endosomes is functional and preferentially replenishes SV pools during strong stimulation. Here, using brefeldin-A (BFA) to block CIE, we found that CIE retrieved a minority of SVs at developing CNS synapses during strong stimulation, but its contribution increased up to 61% at mature CNS synapses. Contrary to previous views, BFA not only blocked SV formation from the endosome but also blocked the endosome formation at the plasma membrane. Adaptor protein 1 and 3 (AP-1/3) have key roles in SV reformation from endosomes during CIE, and AP-1 also affects bulk endosome formation from the plasma membrane. Finally, temporary blocking of chronic or acute neuronal activity with tetrodotoxin in mature neurons redirected most SV retrieval to endosome-independent pathways. These results show that during high neuronal activity, CIE becomes the major endocytic pathway at mature CNS synapses. Moreover, mature neurons use clathrin-mediated endocytosis and the CIE pathway to different extents depending on their previous activity; this may result in activity-dependent alterations of the SV composition which ultimately influence transmitter release and contribute to synaptic plasticity.

Reconstitution of Giant Mammalian Synapses in Culture for Molecular Functional and Imaging Studies

Giant presynaptic terminal brain slice preparations have allowed intracellular recording of electrical signals and molecular loading, elucidating cellular and molecular mechanisms underlying neurotransmission and modulation. However, molecular genetic manipulation or optical imaging in these preparations is hampered by factors, such as tissue longevity and background fluorescence. To overcome these difficulties, we developed a giant presynaptic terminal culture preparation, which allows genetic manipulation and enables optical measurements of synaptic vesicle dynamics, simultaneously with presynaptic electrical signal recordings. This giant synapse reconstructed from dissociated mouse brainstem neurons resembles the development of native calyceal giant synapses in several respects. Thus, this novel preparation constitutes a powerful tool for studying molecular mechanisms of neurotransmission, neuromodulation, and neuronal development.

SIGNIFICANCE STATEMENT

We have developed a novel culture preparation of giant mammalian synapses. These presynaptic terminals make it possible to perform optical imaging simultaneously with presynaptic electrophysiological recording. We demonstrate that this enables one to dissect endocytic and acidification times of synaptic vesicles. In addition, developmental elimination and functional maturation in this cultured preparation provide a useful model for studying presynaptic development. Because this giant synapse preparation allows molecular genetic manipulations, it constitutes a powerful new tool for studying molecular mechanisms of neurotransmission, neuromodulation, and neuronal development.

Advanced Fluorescence Protein-Based Synapse-Detectors

The complex information-processing capabilities of the central nervous system emerge from intricate patterns of synaptic input-output relationships among various neuronal circuit components. Understanding these capabilities thus requires a precise description of the individual synapses that comprise neural networks. Recent advances in fluorescent protein engineering, along with developments in light-favoring tissue clearing and optical imaging techniques, have rendered light microscopy (LM) a potent candidate for large-scale analyses of synapses, their properties, and their connectivity. Optically imaging newly engineered fluorescent proteins (FPs) tagged to synaptic proteins or microstructures enables the efficient, fine-resolution illumination of synaptic anatomy and function in large neural circuits. Here we review the latest progress in fluorescent protein-based molecular tools for imaging individual synapses and synaptic connectivity. We also identify associated technologies in gene delivery, tissue processing, and computational image analysis that will play a crucial role in bridging the gap between synapse- and system-level neuroscience.

PC12 cells that lack synaptotagmin I exhibit loss of a subpool of small dense core vesicles

Neurons communicate by releasing neurotransmitters that are stored in intracellular vesicular compartments. PC12 cells are frequently used as a model secretory cell line that is described to have two subpools of vesicles: small clear vesicles and dense core vesicles. We measured transmitter molecules released from vesicles in NGF-differentiated PC12 cells using carbon-fiber amperometry, and relative diameters of individual vesicles using electron microscopy. Both amperometry and electron micrograph data were analyzed by statistical and machine learning methods for Gaussian mixture models. An electron microscopy size correction algorithm was used to predict and correct for observation bias of vesicle size due to tangential slices through some vesicles. Expectation maximization algorithms were used to perform maximum likelihood estimation for the Gaussian parameters of different populations of vesicles, and were shown to be better than histogram and cumulative distribution function methods for analyzing mixed populations. The Bayesian information criterion was used to determine the most likely number of vesicle subpools observed in the amperometric and electron microscopy data. From this analysis, we show that there are three major subpools, not two, of vesicles stored and released from PC12 cells. The three subpools of vesicles include small clear vesicles and two subpools of dense core vesicles, a small and a large dense core vesicle subpool. Using PC12 cells stably transfected with short-hairpin RNA targeted to synaptotagmin I, an exocytotic Ca(2+) sensor, we show that the presence and release of the small dense core vesicle subpool is dependent on synaptotagmin I. Furthermore, synaptotagmin I also plays a role in the formation and/or maintenance of the small dense core vesicle subpool in PC12 cells.

Myosin light chain kinase facilitates endocytosis of synaptic vesicles at hippocampal boutons

At nerve terminals, endocytosis efficiently recycles vesicle membrane to maintain synaptic transmission under different levels of neuronal activity. Ca(2+) and its downstream signal pathways are critical for the activity-dependent regulation of endocytosis. An activity- and Ca(2+) -dependent kinase, myosin light chain kinase (MLCK) has been reported to regulate vesicle mobilization, vesicle cycling, and motility in different synapses, but whether it has a general contribution to regulation of endocytosis at nerve terminals remains unknown. We investigated this issue at rat hippocampal boutons by imaging vesicle endocytosis as the real-time retrieval of vesicular synaptophysin tagged with a pH-sensitive green fluorescence protein. We found that endocytosis induced by 200 action potentials (5-40 Hz) was slowed by acute inhibition of MLCK and down-regulation of MLCK with RNA interference, while the total amount of vesicle exocytosis and somatic Ca(2+) channel current did not change with MLCK down-regulation. Acute inhibition of myosin II similarly impaired endocytosis. Furthermore, down-regulation of MLCK prevented depolarization-induced phosphorylation of myosin light chain, an effect shared by blockers of Ca(2+) channels and calmodulin. These results suggest that MLCK facilitates vesicle endocytosis through activity-dependent phosphorylation of myosin downstream of Ca(2+) /calmodulin, probably as a widely existing mechanism among synapses. Our study suggests that MLCK is an important activity-dependent regulator of vesicle recycling in hippocampal neurons, which are critical for learning and memory. The kinetics of vesicle membrane endocytosis at nerve terminals has long been known to depend on activity and Ca(2+) . This study provides evidence suggesting that myosin light chain kinase increases endocytosis efficiency at hippocampal neurons by mediating Ca(2+) /calmodulin-dependent phosphorylation of myosin. The authors propose that this signal cascade may serve as a common pathway contributing to the activity-dependent regulation of vesicle endocytosis at synapses.