Katalin Tóth

VAMP4 directs synaptic vesicles to a pool that selectively maintains asynchronous neurotransmission

Synaptic vesicles in the brain harbor several soluble N-ethylmaleimide-sensitive-factor attachment protein receptor (SNARE) proteins. With the exception of synaptobrevin2, or VAMP2 (syb2), which is directly involved in vesicle fusion, the role of these SNAREs in neurotransmission is unclear. Here we show that in mice syb2 drives rapid Ca2+-dependent synchronous neurotransmission, whereas the structurally homologous SNARE protein VAMP4 selectively maintains bulk Ca2+-dependent asynchronous release. At inhibitory nerve terminals, up- or downregulation of VAMP4 causes a correlated change in asynchronous release. Biochemically, VAMP4 forms a stable complex with SNAREs syntaxin-1 and SNAP-25 that does not interact with complexins or synaptotagmin-1, proteins essential for synchronous neurotransmission. Optical imaging of individual synapses indicates that trafficking of VAMP4 and syb2 show minimal overlap. Taken together, these findings suggest that VAMP4 and syb2 diverge functionally, traffic independently and support distinct forms of neurotransmission. These results provide molecular insight into how synapses diversify their release properties by taking advantage of distinct synaptic vesicle–associated SNAREs.

A Mitofusin-2-dependent inactivating cleavage of Opa1 links changes in mitochondria cristae and ER contacts in the postprandial liver.

Significance We provide, to our knowledge, the first in vivo quantitative description of the adaptive response of the mitochondrial reticulum to the metabolic transition occurring in the liver in the hours after feeding. When nutrients become limiting, mitochondria size, cristae density, and respiratory capacity drop, but mitochondria–ER contacts, which control calcium and lipids fluxes between these organelles, double. A proteolytic inactivation of Optic atrophy 1 (Opa1), a major regulator of fusion and cristae architecture, accompanies these changes and found to depend on Mitofusin-2, a key regulator of mitochondria–ER contact biogenesis. Thus, mitochondria adapt to nutrient depletion by coupling the molecular machineries that organize cristae architecture and mitochondria–ER contact assembly, which were previously thought to operate independently of each other. Hepatic metabolism requires mitochondria to adapt their bioenergetic and biosynthetic output to accompany the ever-changing anabolic/catabolic state of the liver cell, but the wiring of this process is still largely unknown. Using a postprandial mouse liver model and quantitative cryo-EM analysis, we show that when the hepatic mammalian target of rapamycin complex 1 (mTORC1) signaling pathway disengages, the mitochondria network fragments, cristae density drops by 30%, and mitochondrial respiratory capacity decreases by 20%. Instead, mitochondria–ER contacts (MERCs), which mediate calcium and phospholipid fluxes between these organelles, double in length. These events are associated with the transient expression of two previously unidentified C-terminal fragments (CTFs) of Optic atrophy 1 (Opa1), a mitochondrial GTPase that regulates cristae biogenesis and mitochondria dynamics. Expression of Opa1 CTFs in the intermembrane space has no effect on mitochondria morphology, supporting a model in which they are intermediates of an Opa1 degradation program. Using an in vitro assay, we show that these CTFs indeed originate from the cleavage of Opa1 at two evolutionarily conserved consensus sites that map within critical folds of the GTPase. This processing of Opa1, termed C-cleavage, is mediated by the activity of a cysteine protease whose activity is independent from that of Oma1 and presenilin-associated rhomboid-like (PARL), two known Opa1 regulators. However, C-cleavage requires Mitofusin-2 (Mfn2), a key factor in mitochondria–ER tethering, thereby linking cristae remodeling to MERC assembly. Thus, in vivo, mitochondria adapt to metabolic shifts through the parallel remodeling of the cristae and of the MERCs via a mechanism that degrades Opa1 in an Mfn2-dependent pathway.

Is Zinc a Neuromodulator

The vesicles of certain glutamatergic terminals in the mammalian forebrain are replete with ionic zinc. It is believed that during synaptic transmission zinc is released, binds to receptors on the pre- or postsynaptic membranes, and hence acts as a neuromodulator. Although exogenous zinc modulates a wide variety of channels, whether synaptic zinc transits across the synaptic cleft and alters the response of channels has been difficult to establish. We will review the evidence for zinc as a neuromodulator and propose diagnostic criteria for establishing whether it is indeed one. Moreover, we will delineate alternative ways in which zinc might act at synapses. This review, with 3 figures and 57 references, describes the evidence that the cation Zn2+ acts as a modulator of synaptic activity. Beginning with a discussion of the criteria that zinc would have to meet in order to be classified as a neuromodulator and the use of chelators for intercepting zinc in the synaptic cleft, the authors then consider different models for zinc’s action at synapses, ranging from free diffusion of zinc to bound zinc in the extracellular space and a possible action within synaptic vesicles. The article wraps up with suggested experimental approaches that may help resolve the questions surrounding the role of zinc in the central nervous system.

Extracellular chelation of zinc does not affect hippocampal excitability and seizure-induced cell death in rats

In the nervous system, zinc can influence synaptic responses and at extreme concentrations contributes to epileptic and ischaemic neuronal injury. Zinc can originate from synaptic vesicles, the extracellular space and from intracellular stores. In this study, we aimed to determine which of these zinc pools is responsible for the increased hippocampal excitability observed in zinc‐depleted animals or following zinc chelation. Also, we investigated the source of intracellularly accumulating zinc in vulnerable neurons. Our data show that membrane‐permeable and membrane‐impermeable zinc chelators had little or no effect on seizure activity in the CA3 region. Furthermore, extracellular zinc chelation could not prevent the accumulation of lethal concentrations of zinc in dying neurons following epileptic seizures. At the electron microscopic level, zinc staining significantly increased at the presynaptic membrane of mossy fibre terminals in kainic acid‐treated animals. These data indicate that intracellular but not extracellular zinc chelators could influence neuronal excitability and seizure‐induced zinc accumulation observed in the cytosol of vulnerable neurons.

Cell type‐specific action of seizure‐induced intracellular zinc accumulation in the rat hippocampus

Increased levels of intracellular zinc have been implicated in neuronal cell death in ischaemia, epilepsy and traumatic brain damage. However, decreases in zinc levels also lead to increased neuronal death and lowered seizure threshold. In the present study we investigated the physiological role of zinc in neurodegeneration and protection following epileptic seizures. Cells located in the strata oriens and lucidum of the CA3 region accumulated high concentrations of zinc and died. A decrease in zinc level could prevent the death of these neurones after seizures. Most of these cells were GABAergic interneurones. In contrast, neurones in the CA3 pyramidal cell layer accumulated moderate amounts of zinc and survived. Zinc chelation led to an increase in the mortality rate of these cells. Furthermore, in these cells low concentrations of intracellular zinc activated Akt (protein kinase B), thus providing protection against neurodegeneration. These results demonstrate that intracellularly accumulated zinc can be neurotoxic or neuroprotective depending on its concentration. This dual action is cell type specific.

Zinc in Neurotransmission

A subset of glutamatergic synapses in the central nervous system contains zinc; it is sequestered into the lumen of synaptic vesicles, where it colocalizes with glutamate. Extracellularly applied zinc is known to interact with various postsynaptic receptors and channels; however, the role of endogenous vesicular zinc is still an enigma. The aim of this review is to present the physiology of tonic and phasic zinc modulation of excitatory and inhibitory signals and to discuss the potential role of zinc in synaptic plasticity. Zinc homeostasis is known to be altered under pathological conditions. The importance of the careful investigation of the potential sources of zinc involved in physiological and pathological processes is highlighted.

Information processing and synaptic plasticity at hippocampal mossy fiber terminals

Granule cells of the dentate gyrus receive cortical information and they transform and transmit this code to the CA3 area via their axons, the mossy fibers (MFs). Structural and functional complexity of this network has been extensively studied at various organizational levels. This review is focused on the anatomical and physiological properties of the MF system. We will discuss the mechanism by which dentate granule cells process signals from single action potentials (APs), short bursts and longer stimuli. Various parameters of synaptic interactions at different target cells such as quantal transmission, short- and long-term plasticity (LTP) will be summarized. Different types of synaptic contacts formed by MFs have unique sets of rules for information processing during different rates of granule cell activity. We will investigate the complex interactions between key determinants of information transfer between the dentate gyrus and the CA3 area of the hippocampus.

Vesicular Zinc Regulates the Ca2+ Sensitivity of a Subpopulation of Presynaptic Vesicles at Hippocampal Mossy Fiber Terminals

Synaptic vesicles segregate into functionally diverse subpopulations within presynaptic terminals, yet there is no information about how this may occur. Here we demonstrate that a distinct subgroup of vesicles within individual glutamatergic, mossy fiber terminals contain vesicular zinc that is critical for the rapid release of a subgroup of synaptic vesicles during increased activity in mice. In particular, vesicular zinc dictates the Ca2+ sensitivity of release during high-frequency firing. Intense synaptic activity alters the subcellular distribution of zinc in presynaptic terminals and decreases the number of zinc-containing vesicles. Zinc staining also appears in endosomes, an observation that is consistent with the preferential replenishment of zinc-enriched vesicles by bulk endocytosis. We propose that functionally diverse vesicle pools with unique membrane protein composition support different modes of transmission and are generated via distinct recycling pathways.

Fast two-photon imaging of subcellular voltage dynamics in neuronal tissue with genetically encoded indicators

Monitoring voltage dynamics in defined neurons deep in the brain is critical for unraveling the function of neuronal circuits but is challenging due to the limited performance of existing tools. In particular, while genetically encoded voltage indicators have shown promise for optical detection of voltage transients, many indicators exhibit low sensitivity when imaged under two-photon illumination. Previous studies thus fell short of visualizing voltage dynamics in individual neurons in single trials. Here, we report ASAP2s, a novel voltage indicator with improved sensitivity. By imaging ASAP2s using random-access multi-photon microscopy, we demonstrate robust single-trial detection of action potentials in organotypic slice cultures. We also show that ASAP2s enables two-photon imaging of graded potentials in organotypic slice cultures and in Drosophila. These results demonstrate that the combination of ASAP2s and fast two-photon imaging methods enables detection of neural electrical activity with subcellular spatial resolution and millisecond-timescale precision. DOI: http://dx.doi.org/10.7554/eLife.25690.001

Vesicles derived via AP-3-dependent recycling contribute to asynchronous release and influence information transfer

Summary Action potentials trigger synchronous and asynchronous neurotransmitter release. Temporal properties of both types of release could be altered in an activity-dependent manner. While the effects of activity-dependent changes in synchronous release on postsynaptic signal integration have been studied, the contribution of asynchronous release to information transfer during natural stimulus patterns is unknown. Here we find that during trains of stimulations, asynchronous release contributes to the precision of action potential firing. Our data show that this form of release is selectively diminished in AP-3b2 KO animals, which lack functional neuronal AP-3, an adaptor protein regulating vesicle formation from endosomes generated during bulk endocytosis. We find that in the absence of neuronal AP-3, asynchronous release is attenuated and the activity-dependent increase in the precision of action potential timing is compromised. Lack of asynchronous release decreases the capacity of synaptic information transfer and renders synaptic communication less reliable in response to natural stimulus patterns.