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Journal of Neuroscience Research Mar 2018In the past, the spontaneous release of neurotransmitter from presynaptic terminals has been thought of as a side effect of evoked release, with little functional... (Review)
Review
In the past, the spontaneous release of neurotransmitter from presynaptic terminals has been thought of as a side effect of evoked release, with little functional significance. As our understanding of the process of spontaneous release has increased over time, this notion has gradually changed. In this review, we focus on the importance of this form of release during neuronal development, a time of extreme levels of plasticity that includes the growth of dendrites and axons as well as the formation of new synaptic contacts. This period also encompasses high levels of neurotransmitter release from growing axons, and recent studies have found that spontaneous transmitter release plays an important role in shaping neuronal morphology as well as modulating the properties of newly forming synaptic contacts in the brain. Here, we bring together the latest findings across different species to argue that the spontaneous release of neurotransmitter is an important player in the wiring of the brain during development.
Topics: Animals; Axons; Dendrites; Neurogenesis; Neurons; Neurotransmitter Agents; Presynaptic Terminals; Synapses; Synaptic Transmission
PubMed: 29034487
DOI: 10.1002/jnr.24154 -
Nature Neuroscience Apr 2024The formation of mammalian synapses entails the precise alignment of presynaptic release sites with postsynaptic receptors but how nascent cell-cell contacts translate...
The formation of mammalian synapses entails the precise alignment of presynaptic release sites with postsynaptic receptors but how nascent cell-cell contacts translate into assembly of presynaptic specializations remains unclear. Guided by pioneering work in invertebrates, we hypothesized that in mammalian synapses, liprin-α proteins directly link trans-synaptic initial contacts to downstream steps. Here we show that, in human neurons lacking all four liprin-α isoforms, nascent synaptic contacts are formed but recruitment of active zone components and accumulation of synaptic vesicles is blocked, resulting in 'empty' boutons and loss of synaptic transmission. Interactions with presynaptic cell adhesion molecules of either the LAR-RPTP family or neurexins via CASK are required to localize liprin-α to nascent synaptic sites. Liprin-α subsequently recruits presynaptic components via a direct interaction with ELKS proteins. Thus, assembly of human presynaptic terminals is governed by a hierarchical sequence of events in which the recruitment of liprin-α proteins by presynaptic cell adhesion molecules is a critical initial step.
Topics: Animals; Humans; Synapses; Synaptic Transmission; Neurons; Carrier Proteins; Presynaptic Terminals; Cell Adhesion Molecules; Mammals
PubMed: 38472649
DOI: 10.1038/s41593-024-01592-9 -
The Journal of Neuroscience : the... Mar 2019The regulated turnover of synaptic vesicle (SV) proteins is thought to involve the ubiquitin-dependent tagging and degradation through endo-lysosomal and autophagy...
The regulated turnover of synaptic vesicle (SV) proteins is thought to involve the ubiquitin-dependent tagging and degradation through endo-lysosomal and autophagy pathways. Yet, it remains unclear which of these pathways are used, when they become activated, and whether SVs are cleared en masse together with SV proteins or whether both are degraded selectively. Equally puzzling is how quickly these systems can be activated and whether they function in real-time to support synaptic health. To address these questions, we have developed an imaging-based system that simultaneously tags presynaptic proteins while monitoring autophagy. Moreover, by tagging SV proteins with a light-activated ROS generator, Supernova, it was possible to temporally control the damage to specific SV proteins and assess their consequence to autophagy-mediated clearance mechanisms and synaptic function. Our results show that, in mouse hippocampal neurons of either sex, presynaptic autophagy can be induced in as little as 5-10 min and eliminates primarily the damaged protein rather than the SV en masse. Importantly, we also find that autophagy is essential for synaptic function, as light-activated damage to, for example, Synaptophysin only compromises synaptic function when autophagy is simultaneously blocked. These data support the concept that presynaptic boutons have a robust highly regulated clearance system to maintain not only synapse integrity, but also synaptic function. The real-time surveillance and clearance of synaptic proteins are thought to be vital to the health, functionality, and integrity of vertebrate synapses and are compromised in neurodegenerative disorders, yet the fundamental mechanisms regulating these systems remain enigmatic. Our analysis reveals that presynaptic autophagy is a critical part of a real-time clearance system at synapses capable of responding to local damage of synaptic vesicle proteins within minutes and to be critical for the ongoing functionality of these synapses. These data indicate that synapse autophagy is not only locally regulated but also crucial for the health and functionality of vertebrate presynaptic boutons.
Topics: Animals; Autophagy; Female; HEK293 Cells; HeLa Cells; Hippocampus; Humans; Male; Mice, Inbred C57BL; Neurons; Presynaptic Terminals; Reactive Oxygen Species; Synaptic Vesicles
PubMed: 30655355
DOI: 10.1523/JNEUROSCI.1317-18.2019 -
The Journal of Neuroscience : the... Sep 2020Multiple forms of homeostasis influence synaptic function under diverse activity conditions. Both presynaptic and postsynaptic forms of homeostasis are important, but...
Multiple forms of homeostasis influence synaptic function under diverse activity conditions. Both presynaptic and postsynaptic forms of homeostasis are important, but their relative impact on fidelity is unknown. To address this issue, we studied auditory nerve synapses onto bushy cells in the cochlear nucleus of mice of both sexes. These synapses undergo bidirectional presynaptic and postsynaptic homeostatic changes with increased and decreased acoustic stimulation. We found that both young and mature synapses exhibit similar activity-dependent changes in short-term depression. Experiments using chelators and imaging both indicated that presynaptic Ca influx decreased after noise exposure, and increased after ligating the ear canal. By contrast, Ca cooperativity was unaffected. Experiments using specific antagonists suggest that occlusion leads to changes in the Ca channel subtypes driving neurotransmitter release. Furthermore, dynamic-clamp experiments revealed that spike fidelity primarily depended on changes in presynaptic depression, with some contribution from changes in postsynaptic intrinsic properties. These experiments indicate that presynaptic Ca influx is homeostatically regulated to enhance synaptic fidelity. Homeostatic mechanisms in synapses maintain stable function in the face of different levels of activity. Both juvenile and mature auditory nerve synapses onto bushy cells modify short-term depression in different acoustic environments, which raises the question of what the underlying presynaptic mechanisms are and the relative importance of presynaptic and postsynaptic contributions to the faithful transfer of information. Changes in short-term depression under different acoustic conditions were a result of changes in presynaptic Ca influx. Spike fidelity was affected by both presynaptic and postsynaptic changes after ear occlusion and was only affected by presynaptic changes after noise-rearing. These findings are important for understanding regulation of auditory synapses under normal conditions and also in disorders following noise exposure or conductive hearing loss.
Topics: Animals; Auditory Perception; Calcium; Cochlear Nerve; Cochlear Nucleus; Female; Homeostasis; Male; Mice; Mice, Inbred CBA; Neuronal Plasticity; Noise; Presynaptic Terminals; Synaptic Potentials
PubMed: 32747441
DOI: 10.1523/JNEUROSCI.1175-19.2020 -
Current Opinion in Neurobiology Aug 2017Synaptic plasticity is critical for experience-dependent adjustments of brain function. While most research has focused on the mechanisms that underlie postsynaptic... (Review)
Review
Synaptic plasticity is critical for experience-dependent adjustments of brain function. While most research has focused on the mechanisms that underlie postsynaptic forms of plasticity, comparatively little is known about how neurotransmitter release is altered in a long-term manner. Emerging research suggests that many of the features of canonical 'postsynaptic' plasticity, such as associativity, structural changes and bidirectionality, also characterize long-term presynaptic plasticity. Recent studies demonstrate that presynaptic plasticity is a potent regulator of circuit output and function. Moreover, aberrant presynaptic plasticity is a convergent factor of synaptopathies like schizophrenia, addiction, and Autism Spectrum Disorders, and may be a potential target for treatment.
Topics: Brain; Humans; Mental Disorders; Neuronal Plasticity; Presynaptic Terminals; Synaptic Transmission
PubMed: 28570863
DOI: 10.1016/j.conb.2017.05.011 -
Neuron Feb 2020A major function of GPCRs is to inhibit presynaptic neurotransmitter release, requiring ligand-activated receptors to couple locally to effectors at terminals. The...
A major function of GPCRs is to inhibit presynaptic neurotransmitter release, requiring ligand-activated receptors to couple locally to effectors at terminals. The current understanding of how this is achieved is through receptor immobilization on the terminal surface. Here, we show that opioid peptide receptors, GPCRs that mediate highly sensitive presynaptic inhibition, are instead dynamic in axons. Opioid receptors diffuse rapidly throughout the axon surface and internalize after ligand-induced activation specifically at presynaptic terminals. We delineate a parallel regulated endocytic cycle for GPCRs operating at the presynapse, separately from the synaptic vesicle cycle, which clears activated receptors from the surface of terminals and locally reinserts them to maintain the diffusible surface pool. We propose an alternate strategy for achieving local control of presynaptic effectors that, opposite to using receptor immobilization and enforced proximity, is based on lateral mobility of receptors and leverages the inherent allostery of GPCR-effector coupling.
Topics: Analgesics, Opioid; Animals; Cells, Cultured; Endocytosis; Enkephalin, Ala(2)-MePhe(4)-Gly(5)-; Presynaptic Terminals; Protein Transport; Rats; Rats, Sprague-Dawley; Receptors, G-Protein-Coupled; Receptors, Neurotransmitter; Synaptic Vesicles
PubMed: 31837915
DOI: 10.1016/j.neuron.2019.11.016 -
Neurobiology of Disease May 2021Abnormal aggregation of the α-synuclein protein is a key molecular feature of Parkinson's disease and other neurodegenerative diseases. The precise mechanisms that...
Abnormal aggregation of the α-synuclein protein is a key molecular feature of Parkinson's disease and other neurodegenerative diseases. The precise mechanisms that trigger α-synuclein aggregation are unclear, and it is not known what role aggregation plays in disease pathogenesis. Here we use an in vivo zebrafish model to express several different forms of human α-synuclein and measure its aggregation in presynaptic terminals. We show that human α-synuclein tagged with GFP can be expressed in zebrafish neurons, localizing normally to presynaptic terminals and undergoing phosphorylation at serine-129, as in mammalian neurons. The visual advantages of the zebrafish system allow for dynamic in vivo imaging to study α-synuclein, including the use of fluorescence recovery after photobleaching (FRAP) techniques to probe protein mobility. These experiments reveal three distinct terminal pools of α-synuclein with varying mobility, likely representing different subpopulations of aggregated and non-aggregated protein. Human α-synuclein is phosphorylated by an endogenous zebrafish Polo-like kinase activity, and there is a heterogeneous population of neurons containing either very little or extensive phosphorylation throughout the axonal arbor. Both pharmacological and genetic manipulations of serine-129 show that phosphorylation of α-synuclein at this site does not significantly affect its mobility. This suggests that serine-129 phosphorylation alone does not promote α-synuclein aggregation. Together our results show that human α-synuclein can be expressed and measured quantitatively in zebrafish, and that disease-relevant post-translational modifications occur within neurons. The zebrafish model provides a powerful in vivo system for measuring and manipulating α-synuclein function and aggregation, and for developing new treatments for neurodegenerative disease.
Topics: Animals; Animals, Genetically Modified; Disease Models, Animal; Humans; Parkinson Disease; Phosphorylation; Presynaptic Terminals; Protein Aggregation, Pathological; Serine; Zebrafish; alpha-Synuclein
PubMed: 33556542
DOI: 10.1016/j.nbd.2021.105291 -
The Journal of Neuroscience : the... Oct 2019The calyx of Held, a large glutamatergic presynaptic terminal in the auditory brainstem undergoes developmental changes to support the high action-potential firing rates...
The calyx of Held, a large glutamatergic presynaptic terminal in the auditory brainstem undergoes developmental changes to support the high action-potential firing rates required for auditory information encoding. In addition, calyx terminals are morphologically diverse, which impacts vesicle release properties and synaptic plasticity. Mitochondria influence synaptic plasticity through calcium buffering and are crucial for providing the energy required for synaptic transmission. Therefore, it has been postulated that mitochondrial levels increase during development and contribute to the morphological-functional diversity in the mature calyx. However, the developmental profile of mitochondrial volumes and subsynaptic distribution at the calyx of Held remains unclear. To provide insight on this, we developed a helper-dependent adenoviral vector that expresses the genetically encoded peroxidase marker for mitochondria, mito-APEX2, at the mouse calyx of Held. We developed protocols to detect labeled mitochondria for use with serial block face scanning electron microscopy to carry out semiautomated segmentation of mitochondria, high-throughput whole-terminal reconstruction, and presynaptic ultrastructure in mice of either sex. Subsequently, we measured mitochondrial volumes and subsynaptic distributions at the immature postnatal day (P)7 and the mature (P21) calyx. We found an increase of mitochondria volumes in terminals and axons from P7 to P21 but did not observe differences between stalk and swelling subcompartments in the mature calyx. Based on these findings, we propose that mitochondrial volumes and synaptic localization developmentally increase to support high firing rates required in the initial stages of auditory information processing. Elucidating the developmental processes of auditory brainstem presynaptic terminals is critical to understanding auditory information encoding. Additionally, morphological-functional diversity at these terminals is proposed to enhance coding capacity. Mitochondria provide energy for synaptic transmission and can buffer calcium, impacting synaptic plasticity; however, their developmental profile to ultimately support the energetic demands of synapses following the onset of hearing remains unknown. Therefore, we created a helper-dependent adenoviral vector with the mitochondria-targeting peroxidase mito-APEX2 and expressed it at the mouse calyx of Held. Volumetric reconstructions of serial block face electron microscopy data of immature and mature labeled calyces reveal that mitochondrial volumes are increased to support high firing rates upon maturity.
Topics: Action Potentials; Animals; Axons; Brain Stem; Calcium; Electrophysiological Phenomena; Energy Metabolism; Female; Genetic Vectors; Image Processing, Computer-Assisted; Male; Mice; Mitochondria; Mitochondrial Size; Neuronal Plasticity; Presynaptic Terminals; Synapses
PubMed: 31455662
DOI: 10.1523/JNEUROSCI.0363-19.2019 -
International Journal of Molecular... Oct 2021An action potential triggers neurotransmitter release from synaptic vesicles docking to a specialized release site of the presynaptic plasma membrane, the active zone.... (Review)
Review
An action potential triggers neurotransmitter release from synaptic vesicles docking to a specialized release site of the presynaptic plasma membrane, the active zone. The active zone is a highly organized structure with proteins that serves as a platform for synaptic vesicle exocytosis, mediated by SNAREs complex and Ca sensor proteins, within a sub-millisecond opening of nearby Ca channels with the membrane depolarization. In response to incoming neuronal signals, each active zone protein plays a role in the release-ready site replenishment with synaptic vesicles for sustainable synaptic transmission. The active zone release apparatus provides a possible link between neuronal activity and plasticity. This review summarizes the mostly physiological role of active zone protein interactions that control synaptic strength, presynaptic short-term plasticity, and homeostatic synaptic plasticity.
Topics: Action Potentials; Animals; Calcium Channels; Humans; Neuronal Plasticity; Neurotransmitter Agents; Presynaptic Terminals; Protein Interaction Maps; SNARE Proteins; Synaptic Transmission; Synaptic Vesicles
PubMed: 34769208
DOI: 10.3390/ijms222111775 -
The Journal of Physiology Jul 2016BK channels are large conductance potassium channels characterized by four pore-forming α subunits, often co-assembled with auxiliary β and γ subunits to regulate... (Review)
Review
BK channels are large conductance potassium channels characterized by four pore-forming α subunits, often co-assembled with auxiliary β and γ subunits to regulate Ca(2+) sensitivity, voltage dependence and gating properties. Abundantly expressed in the CNS, they have the peculiar characteristic of being activated by both voltage and intracellular calcium rise. The increase in intracellular calcium via voltage-dependent calcium channels (Cav ) during spiking triggers conformational changes and BK channel opening. This narrows the action potential and induces a fast after-hyperpolarization that shuts calcium channels. The tight coupling between BK and Cav channels at presynaptic active zones makes them particularly suitable for regulating calcium entry and neurotransmitter release. While in most synapses, BK channels exert a negative control on transmitter release under basal conditions, in others they do so only under pathological conditions, serving as an emergency brake to protect against hyperactivity. In particular cases, by interacting with other channels (i.e. limiting the activation of the delayed rectifier and the inactivation of Na(+) channels), BK channels induce spike shortening, increase in firing rate and transmitter release. Changes in transmitter release following BK channel dysfunction have been implicated in several neurological disorders including epilepsy, schizophrenia, fragile X syndrome, mental retardation and autism. In particular, two mutations, one in the α and one in the β3 subunit, resulting in a gain of function have been associated with epilepsy. Hence, these discoveries have allowed identification of BK channels as new drug targets for therapeutic intervention.
Topics: Animals; Humans; Large-Conductance Calcium-Activated Potassium Channels; Neurotransmitter Agents; Presynaptic Terminals; Synaptic Transmission
PubMed: 26969302
DOI: 10.1113/JP271841