Zn2+, mitochondria and neuronal injury

In addition to its familiar role as a component of metalloproteins, zinc is also sequestered in the presynaptic vesicles in 'zinc-containing' neurons. The best-established physiological role of synaptically released zinc is the tonic modulation of brain excitability through modulation of amino acid receptors; prominent pathological effects include acceleration of plaque deposition in Alzheimer's disease and exacerbation of excitotoxic neuron injury. Synaptically released zinc functions as a conventional synaptic neurotransmitter or neuromodulator being released into the cleft then recycled into the postsynaptic neurons during synaptic events, functioning analogously to calcium in this regard, as a transmembrane neural signal. To stimulate comparisons of zinc signals with calcium signals, we have compiled a list of the important parameters of calcium signals and zinc signals. More speculatively, we hypothesize that zinc signals may loosely mimic phosphate signals in the sense that signal zinc ions may commonly bind to proteins in a lasting manner, as a result changing their structure and function.     

Synaptically released zinc: neuromodulator and toxin

In addition to its familiar role as a component of metalloproteins, zinc is also sequestered in the presynaptic vesicles in ‘zinc‐containing’ neurons. The best‐established physiological role of synaptically released zinc is the tonic modulation of brain excitability through modulation of amino acid receptors; prominent pathological effects include acceleration of plaque deposition in Alzheimer's disease and exacerbation of excitotoxic neuron injury. Synaptically released zinc functions as a conventional synaptic neurotransmitter or neuromodulator being released into the cleft then recycled into the postsynaptic neurons during synaptic events, functioning analogously to calcium in this regard, as a transmembrane neural signal. To stimulate comparisons of zinc signals with calcium signals, we have compiled a list of the important parameters of calcium signals and zinc signals. More speculatively, we hypothesize that zinc signals may loosely mimic phosphate signals in the sense that signal zinc ions may commonly bind to proteins in a lasting manner, as a result changing their structure and function.     

Effect of the zinc chelator N,N,N',N'-tetrakis (2-pyridylmethyl)ethylenediamine (TPEN) on hippocampal mossy fiber calcium signals and on synaptic transmission

An important pool of chelatable zinc is present in the synaptic vesicles of mossy fiber terminals from hippocampal CA3 area, being zinc released following single or repetitive electrical stimulation. Previous studies have suggested different synaptic roles for released mossy fiber zinc, including the inhibition of presynaptic calcium and of postsynaptic N-methyl-D-aspartate (NMDA) and gamma amino-butyric acid (GABAA) receptors. The effect of endogenously released zinc on mossy fiber long-term potentiation (LTP) induction also is not yet established. We have investigated the effect of the permeant zinc chelator N,N,N',N'-tetrakis(2-pyridylmethyl) ethylenediamine (TPEN) on mossy fiber calcium and on synaptic transmission, before and during the application of LTP-inducing stimulation. We have found, using the calcium indicator Fura-2, that single and tetanically-evoked mossy fiber calcium signals are both enhanced in the presence of 20 microM TPEN, while the single field potentials are unaffected. As expected, no effect was observed on the single calcium signals or field potentials obtained at the CA3-CA1 synapses, from the CA1 area, which has a lower concentration of vesicular zinc. These results support the idea that at the hippocampal mossy fiber synapses, released zinc inhibits presynaptic calcium mechanisms. A higher concentration of TPEN (100 microM) significantly reduced mossy fiber synaptic transmission but did not prevent the induction of mossy fiber LTP, suggesting that zinc is not required for the formation of this form of LTP.     

Signal regulatory proteins (SIRPS) are secreted presynaptic organizing molecules

Formation of chemical synapses requires exchange of organizing signals between the synaptic partners. Using synaptic vesicle aggregation in cultured neurons as a marker of presynaptic differentiation, we purified candidate presynaptic organizers from mouse brain. A major bioactive species was the extracellular domain of signal regulatory protein alpha (SIRP-alpha), a transmembrane immunoglobulin superfamily member concentrated at synapses. The extracellular domain of SIRP-alpha is cleaved and shed in a developmentally regulated manner. The presynaptic organizing activity of SIRP-alpha is mediated in part by CD47. SIRP-alpha homologues, SIRP-beta and -gamma also have synaptic vesicle clustering activity. The effects of SIRP-alpha are distinct from those of another presynaptic organizer, FGF22: the two proteins induced vesicle clusters of different sizes, differed in their ability to promote neurite branching, and acted through different receptors and signaling pathways. SIRP family proteins may act together with other organizing molecules to pattern synapses.     

Significance of the degree of synaptic Zn<Superscript>2+</Superscript> signaling in cognition

Zinc is a trace nutrient for the brain and a signal factor to serve for brain function. A portion of zinc is released from glutamatergic (zincergic) neuron terminals in the brain. Synaptic Zn²⁺ signaling is involved in synaptic plasticity such as long-term potentiaion (LTP), which is a cellular mechanism of memory. The block and/or loss of synaptic Zn²⁺ signaling in the hippocampus and amygdala with Zn²⁺ chelators affect cognition, while the role of synaptic Zn²⁺ signal is poorly understood, because zinc-binding proteins are great in number and multi-functional. Chronic zinc deficiency also affects cognition and cognitive decline induced by zinc deficiency might be associated with the increase in plasma glucocorticoid rather than the decrease in synaptic Zn²⁺ signaling. On the other hand, excess glutamatergic (zincergic) neuron activity induces excess influx of extracellular Zn²⁺ into hippocampal neurons, followed by cognitive decline. Intracellular Zn²⁺ dynamics, which is linked to presynaptic glutamate release, is critical for LTP and cognitive performance. This paper deals with insight into cognition from zinc as a nutrient and signal factor.     

Non-synaptic intercellular communication: presynaptic inhibition

Synapse is the most common and generally accepted structural basis for the interaction between neurons. It provides a "one-to-one" communication system between neurons. However, there is another possibility for interneuronal communication: when one neuron communicates with many others without making synaptic contact. In the past few years neurochemical, morphological and pharmacological evidence has been obtained that some neurotransmitters may be released from non-synaptic sites, for diffusion to target cells more distant than those seen in conventional synaptic transmission. The non-synaptic interneuronal communication between neurons plays a physiological role in the presynaptic modulation of chemical neurotransmission. This would be a transitional form between the classical neurotransmission and the broadcasting of neuroendocrine secretion.     

A unified theory of presynaptic chemical neurotransmission

The mechanism of neurotransmission and its modulation involves the direct role of calcium on membranes, and calcium's ability to activate synergistically and simultaneously a host of interdependent enzymatic cascades in synaptic and coated vesicles and the presynaptic plasma membrane. Enzymatic products formed can either amplify or depress synaptic vesicle exocytosis and synaptic vesicle regeneration via the coated pit/vesicle system. Rate amplification produced by a series of parallel, multistepped, interconnected enzymatic cascades as well as the optimal geometric spatial orientation of synaptic vesicles induced by presynaptic structures is hypothesized to explain how neurotransmitter is released within 200 musec upon calcium entry into the axon terminal.     

Neuromodulatory changes in short-term synaptic dynamics may be mediated by two distinct mechanisms of presynaptic calcium entry

Although synaptic output is known to be modulated by changes in presynaptic calcium channels, additional pathways for calcium entry into the presynaptic terminal, such as non-selective channels, could contribute to modulation of short term synaptic dynamics. We address this issue using computational modeling. The neuropeptide proctolin modulates the inhibitory synapse from the lateral pyloric (LP) to the pyloric dilator (PD) neuron, two slow-wave bursting neurons in the pyloric network of the crab Cancer borealis. Proctolin enhances the strength of this synapse and also changes its dynamics. Whereas in control saline the synapse shows depression independent of the amplitude of the presynaptic LP signal, in proctolin, with high-amplitude presynaptic LP stimulation the synapse remains depressing while low-amplitude stimulation causes facilitation. We use simple calcium-dependent release models to explore two alternative mechanisms underlying these modulatory effects. In the first model, proctolin directly targets calcium channels by changing their activation kinetics which results in gradual accumulation of calcium with low-amplitude presynaptic stimulation, leading to facilitation. The second model uses the fact that proctolin is known to activate a non-specific cation current I _MI . In this model, we assume that the MI channels have some permeability to calcium, modeled to be a result of slow conformation change after binding calcium. This generates a gradual increase in calcium influx into the presynaptic terminals through the modulatory channel similar to that described in the first model. Each of these models can explain the modulation of the synapse by proctolin but with different consequences for network activity.     

Canonical Wnt3a Modulates Intracellular Calcium and Enhances Excitatory Neurotransmission in Hippocampal Neurons

A role for Wnt signal transduction in the development and maintenance of brain structures is widely acknowledged. Recent studies have suggested that Wnt signaling may be essential for synaptic plasticity and neurotransmission. However, the direct effect of a Wnt protein on synaptic transmission had not been demonstrated. Here we show that nanomolar concentrations of purified Wnt3a protein rapidly increase the frequency of miniature excitatory synaptic currents in embryonic rat hippocampal neurons through a mechanism involving a fast influx of calcium from the extracellular space, induction of post-translational modifications on the machinery involved in vesicle exocytosis in the presynaptic terminal leading to spontaneous Ca²⁺ transients. Our results identify the Wnt3a protein and a member of its complex receptor at the membrane, the low density lipoprotein receptor-related protein 6 (LRP6) coreceptor, as key molecules in neurotransmission modulation and suggest cross-talk between canonical and Wnt/Ca²⁺ signaling in central neurons.     

Dynamic action of neurometals at the synapse

There are synaptic vesicles that are labeled by Timm's sulfide-silver staining method in the brain, suggesting that synaptic vesicles contain metals such as zinc and copper. Zinc is co-released with glutamate and the importance of zinc signaling in the intracellular compartment, in addition to extracellular compartment, is becoming recognized. Zinc can pass through calcium channels, while blocking them. Calcium signaling plays a critical role for synaptic activity and crosstalk between zinc signaling with calcium signaling through calcium channels may participate in synaptic neurotransmission including synaptic plasticity such as long-term potentiation. Copper released into the synaptic cleft during synaptic excitation may also participate in synaptic neurotransmission. Other metals including copper potentially serve as calcium channel blockers and also influence calcium signaling and zinc signaling via the interaction with metal-binding proteins such as metallothioneins. Homeostasis of metals needs to be controlled spatiotemporally for proper brain function, and their dyshomeostasis is associated with neurological diseases. However, the data on the dynamic action of metals at synapses is limited and their significance poorly understood. This paper summarizes the action of metals in synaptic neurotransmission focused on calcium signaling at glutamatergic synapses.     

Modulation of sensory input to the spinal cord by presynaptic ionotropic glutamate receptors

Sensory input from peripheral nerves to the dorsal horn of the spinal cord is mediated by a variety of agents released by the central terminals of dorsal root ganglion (DRG) neurons. These include, but are not limited to, amino acids, especially glutamate, peptides and purines. The unraveling of the mechanisms of synaptic transmission by central terminals of DRG neurons has to take into account various ways in which the message from the periphery can be modulated at the level of the first central synapse. These include postsynaptic and presynaptic mechanisms. Homomeric and heteromeric complexes of receptor subunits for the different transmitters released by DRG neurons and interneurons, clustered at the postsynaptic site of central synapses, can be expressed in different combinations and their rate of insertion into the postsynaptic membrane is activity-regulated. Inhibitory mechanisms are an important part of central modulation, especially via presynaptic inhibition, currently believed to involve GABA released by inhibitory intrinsic neurons. Recent work has established the occurrence of another way by which sensory input can be modulated, i.e. the expression of presynaptic ionotropic and metabotropic receptors in central terminals of DRG neurons. Microscopic evidence for the expression, in these terminals, of various subunits of ionotropic glutamate receptors documents the selective expression of glutamate receptors in functionally different DRG afferents. Electrophysiological and pharmacological data suggest that activation of presynaptic ionotropic glutamate receptors in central terminals of DRG neurons may result in inhibition of release of glutamate by the same terminals. Glutamate activating presynaptic receptors may spill over from the same or adjacent synapses, or may be released by processes of astroglial cells surrounding synaptic terminals. The wide expression of presynaptic ionotropic glutamate receptors, especially in superficial laminae of the dorsal horn, where Adelta- and C fibers terminate, provides an additional or alternative mechanism, besides GABA-mediated presynaptic inhibition, for the modulation of glutamate release by these fibers. Since, however, presynaptic ionotropic glutamate receptors are also expressed in terminals of GABAergic intrinsic interneurons, a decrease of GABA release resulting from activation of these receptors in the same laminae, may also play a role in central sensitization and hyperalgesia.     

Modeling synaptic transmission of the tripartite synapse

The tripartite synapse denotes the junction of a pre- and postsynaptic neuron modulated by a synaptic astrocyte. Enhanced transmission probability and frequency of the postsynaptic current-events are among the significant effects of the astrocyte on the synapse as experimentally characterized by several groups. In this paper we provide a mathematical framework for the relevant synaptic interactions between neurons and astrocytes that can account quantitatively for both the astrocytic effects on the synaptic transmission and the spontaneous postsynaptic events. Inferred from experiments, the model assumes that glutamate released by the astrocytes in response to synaptic activity regulates store-operated calcium in the presynaptic terminal. This source of calcium is distinct from voltage-gated calcium influx and accounts for the long timescale of facilitation at the synapse seen in correlation with calcium activity in the astrocytes. Our model predicts the inter-event interval distribution of spontaneous current activity mediated by a synaptic astrocyte and provides an additional insight into a novel mechanism for plasticity in which a low fidelity synapse gets transformed into a high fidelity synapse via astrocytic coupling.     

The dynamics of synaptic scaffolds

Complex functions of the central nervous system such as learning and memory are believed to result from the modulation of the synaptic transmission between neurons. The sequence of events leading to the fusion of synaptic vesicles at the presynaptic active zone and the detection of this signal at the postsynaptic density involve the activity of ion channels and neurotransmitter receptors. Their accumulation and dynamic exchange at synapses are dependent on their interaction with synaptic scaffolds. These are synaptic structures composed of adaptor proteins that provide binding sites for receptors and channels as well as other synaptic proteins. While in its entirety the synaptic scaffold is a relatively stable structure, individual adaptor proteins exchange at a fast time scale. These properties of scaffolds help to ensure the stability of synaptic transmission while permitting the modulation of synaptic strength. Here, we review the dynamics of the synaptic scaffold and of adaptor proteins in relation to their roles in the organisation of the synapse as well as in the clustering and trafficking of receptor proteins.     

ATP released by astrocytes mediates glutamatergic activity-dependent heterosynaptic suppression

Extracellular ATP released from axons is known to assist activity-dependent signaling between neurons and Schwann cells in the peripheral nervous system. Here we report that ATP released from astrocytes as a result of neuronal activity can also modulate central synaptic transmission. In cultures of hippocampal neurons, endogenously released ATP tonically suppresses glutamatergic synapses via presynaptic P2Y receptors, an effect that depends on the presence of cocultured astrocytes. Glutamate release accompanying neuronal activity also activates non-NMDA receptors of nearby astrocytes and triggers ATP release from these cells, which in turn causes homo- and heterosynaptic suppression. In CA1 pyramidal neurons of hippocampal slices, a similar synaptic suppression was also produced by adenosine, an immediate degradation product of ATP released by glial cells. Thus, neuron-glia crosstalk may participate in activity-dependent synaptic modulation.     

Slob, a Slowpoke channel-binding protein, modulates synaptic transmission

Modulation of ion channels by regulatory proteins within the same macromolecular complex is a well-accepted concept, but the physiological consequences of such modulation are not fully understood. Slowpoke (Slo), a potassium channel critical for action potential repolarization and transmitter release, is regulated by Slo channel-binding protein (Slob), a Drosophila melanogaster Slo (dSlo) binding partner. Slob modulates the voltage dependence of dSlo channel activation in vitro and exerts similar effects on the dSlo channel in Drosophila central nervous system neurons in vivo. In addition, Slob modulates action potential duration in these neurons. Here, we investigate further the functional consequences of the modulation of the dSlo channel by Slob in vivo, by examining larval neuromuscular synaptic transmission in flies in which Slob levels have been altered. In Slob-null flies generated through P-element mutagenesis, as well as in Slob knockdown flies generated by RNA interference (RNAi), we find an enhancement of synaptic transmission but no change in the properties of the postsynaptic muscle cell. Using targeted transgenic rescue and targeted expression of Slob-RNAi, we find that Slob expression in neurons (but not in the postsynaptic muscle cell) is critical for its effects on synaptic transmission. Furthermore, inhibition of dSlo channel activity abolishes these effects of Slob. These results suggest that presynaptic Slob, by regulating dSlo channel function, participates in the modulation of synaptic transmission.     

GABAB receptor modulation of synaptic function

Neuromodulators have complex effects on both the presynaptic release and postsynaptic detection of neurotransmitters. Here we describe recent advances in our understanding of synaptic modulation by metabotropic GABAB receptors. By inhibiting multivesicular release from the presynaptic terminal, these receptors decrease the synaptic glutamate signal. GABAB receptors also inhibit the Ca2+ permeability of NMDA receptors to decrease Ca2+ signals in postsynaptic spines. These new findings highlight the importance of GABAB receptors in regulating many aspects of synaptic transmission. They also point to novel questions about the spatiotemporal dynamics and sources of synaptic modulation in the brain.     

Diversification of synaptic strength: presynaptic elements

Synapses are not static; their performance is modified adaptively in response to activity. Presynaptic mechanisms that affect the probability of transmitter release or the amount of transmitter that is released are important in synaptic diversification. Here, we address the diversity of presynaptic performance and its underlying mechanisms: how much of the variation can be accounted for by variation in synaptic morphology and how much by molecular differences? Significant progress has been made in defining presynaptic structural contributions to synaptic strength; by contrast, we know little about how presynaptic proteins produce normally observed functional differentiation, despite abundant information on presynaptic proteins and on the effects of their individual manipulation. Closing the gap between molecular and physiological synaptic diversification still represents a considerable challenge.     

Functional interactions between presynaptic calcium channels and the neurotransmitter release machinery

In vertebrates, the physical coupling between presynaptic calcium channels and synaptic vesicle release proteins enhances the efficiency of neurotransmission. Recent evidence indicates that these synaptic proteins may feedback directly on synaptic release by negatively regulating calcium entry, and indirectly through pathways involving second messenger molecules. Studies of individual neurons from both vertebrates and invertebrates have provided novel insights into the roles of scaffolding proteins in calcium channel targeting and neurotransmitter release. These studies require us to expand current models of synaptic transmission.     

Endocannabinoids potentiate synaptic transmission through stimulation of astrocytes

Endocannabinoids and their receptor CB1 play key roles in brain function. Astrocytes express CB1Rs that are activated by endocannabinoids released by neurons. However, the consequences of the endocannabinoid-mediated neuron-astrocyte signaling on synaptic transmission are unknown. We show that endocannabinoids released by hippocampal pyramidal neurons increase the probability of transmitter release at CA3-CA1 synapses. This synaptic potentiation is due to CB1R-induced Ca(2+) elevations in astrocytes, which stimulate the release of glutamate that activates presynaptic metabotropic glutamate receptors. While endocannabinoids induce synaptic depression in the stimulated neuron by direct activation of presynaptic CB1Rs, they indirectly lead to synaptic potentiation in relatively more distant neurons by activation of CB1Rs in astrocytes. Hence, astrocyte calcium signal evoked by endogenous stimuli (neuron-released endocannabinoids) modulates synaptic transmission. Therefore, astrocytes respond to endocannabinoids that then potentiate synaptic transmission, indicating that astrocytes are actively involved in brain physiology.     

Targeting of the Synaptic Vesicle Protein Synaptobrevin in the Axon of Cultured Hippocampal Neurons: Evidence for Two Distinct Sorting Steps

Synaptic vesicles are concentrated in the distal axon, far from the site of protein synthesis. Integral membrane proteins destined for this organelle must therefore make complex targeting decisions. Short amino acid sequences have been shown to act as targeting signals directing proteins to a variety of intracellular locations. To identify synaptic vesicle targeting sequences and to follow the path that proteins travel en route to the synaptic vesicle, we have used a defective herpes virus amplicon expression system to study the targeting of a synaptobrevin-transferrin receptor (SB-TfR) chimera in cultured hippocampal neurons. Addition of the cytoplasmic domain of synaptobrevin onto human transferrin receptor was sufficient to retarget the transferrin receptor from the dendrites to presynaptic sites in the axon. At the synapse, the SB-TfR chimera did not localize to synaptic vesicles, but was instead found in an organelle with biochemical and functional characteristics of an endosome. The chimera recycled in parallel with synaptic vesicle proteins demonstrating that the nerve terminal efficiently sorts transmembrane proteins into different pathways. The synaptobrevin sequence that controls targeting to the presynaptic endosome was not localized to a single, 10– amino acid region of the molecule, indicating that this targeting signal may be encoded by a more distributed structural conformation. However, the chimera could be shifted to synaptic vesicles by deletion of amino acids 61–70 in synaptobrevin, suggesting that separate signals encode the localization of synaptobrevin to the synapse and to the synaptic vesicle.