[Show abstract][Hide abstract] ABSTRACT: 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.
[Show abstract][Hide abstract] ABSTRACT: 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.
Proceedings of the National Academy of Sciences 10/2014; 111(45). DOI:10.1073/pnas.1408061111 · 9.67 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Synaptic short-term plasticity is a key regulator of neuronal communication and is controlled via various mechanisms. A well established property of mossy fiber to CA3 pyramidal cell synapses is the extensive short-term facilitation during high-frequency bursts. We investigated the mechanisms governing facilitation using a combination of whole-cell electrophysiological recordings, electrical minimal stimulation, and random-access two-photon microscopy in acute mouse hippocampal slices. Two distinct presynaptic mechanisms were involved in short-term facilitation, with their relative contribution dependent on extracellular calcium concentration. The synchronization of multivesicular release was observed during trains of facilitating EPSCs recorded in 1.2 mm external Ca(2+) ([Ca(2+)]e). Indeed, covariance analysis revealed a gradual augmentation in quantal size during trains of EPSCs, and application of the low-affinity glutamate receptor antagonist γ-d-glutamylglycine showed an increase in cleft glutamate concentration during paired-pulse stimulation. Whereas synchronization of multivesicular release contributed to the facilitation in 1.2 mm [Ca(2+)]e, variance-mean analysis showed that recruitment of more release sites (N) was likely to account for the larger facilitation observed in 2.5 mm [Ca(2+)]e. Furthermore, this increase in N could be promoted by calcium microdomains of heterogeneous amplitudes observed in single mossy fiber boutons. Our findings suggest that the combination of multivesicular release and the recruitment of additional release sites act together to increase glutamate release during burst activity. This is supported by the compartmentalized spatial profile of calcium elevations in boutons and helps to expand the dynamic range of mossy fibers information transfer.
The Journal of Neuroscience : The Official Journal of the Society for Neuroscience 08/2014; 34(33):11032-11047. DOI:10.1523/JNEUROSCI.0847-14.2014 · 6.34 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: 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.
[Show abstract][Hide abstract] ABSTRACT: 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 Ca(2+)-dependent synchronous neurotransmission, whereas the structurally homologous SNARE protein VAMP4 selectively maintains bulk Ca(2+)-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.
[Show abstract][Hide abstract] ABSTRACT: 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 Ca(2+) 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.
The Journal of Neuroscience : The Official Journal of the Society for Neuroscience 12/2011; 31(50):18251-65. DOI:10.1523/JNEUROSCI.4164-11.2011 · 6.34 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The co-release of neuromodulatory substances in combination with classic neurotransmitters such as glutamate and GABA from individual presynaptic nerve terminals has the capacity to dramatically influence synaptic efficacy and plasticity. At hippocampal mossy fibre synapses vesicular zinc is suggested to serve as a cotransmitter capable of regulating calcium release from internal stores in postsynaptic CA3 pyramidal cells. Here we investigated this possibility using combined intracellular ratiometric calcium imaging and patch-clamp recording techniques. In acute hippocampal slices a brief train of mossy fibre stimulation produced a large, delayed postsynaptic Ca(2+) wave that was spatially restricted to the proximal apical dendrites of CA3 pyramidal cells within stratum lucidum. This calcium increase was sensitive to intracellularly applied heparin indicating reliance upon release from internal stores and was triggered by activation of both group I metabotropic glutamate and NMDA receptors. Importantly, treatment of slices with the membrane-impermeant zinc chelator CaEDTA did not influence the synaptically evoked postsynaptic Ca(2+) waves. Moreover, mossy fibre stimulus evoked postsynaptic Ca(2+) signals were not significantly different between wild-type and zinc transporter 3 (ZnT3) knock-out animals. Considered together our data do not support a role for vesicular zinc in regulating mossy fibre evoked Ca(2+) release from CA3 pyramidal cell internal stores.
The Journal of Physiology 12/2011; 589(Pt 23):5677-89. DOI:10.1113/jphysiol.2011.216598 · 5.04 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: 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.
[Show abstract][Hide abstract] ABSTRACT: This chapter will summarize key data about glutamatergic transmission in the hippocampus. Glutamate is the major excitatory
neurotransmitter similar to other CNS regions. Biophysical properties of various receptors and channels will be described
and functional relevance of these parameters discussed.
[Show abstract][Hide abstract] ABSTRACT: 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.
The Journal of Physiology 02/2007; 578(Pt 1):275-89. DOI:10.1113/jphysiol.2006.121848 · 5.04 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: 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.
The Journal of Physiology 09/2005; 566(Pt 3):821-37. DOI:10.1113/jphysiol.2005.089458 · 5.04 Impact Factor