Presynaptic release probability and readily releasable pool size are regulated by two independent mechanisms during posttetanic potentiation at the calyx of Held synapse

Effect of tetanic stimulation prior to train-of-four monitoring on the time to muscle response stabilization with neuromuscular blockade with rocuronium in patients aged 60 to 80 years: A prospective randomized controlled trial

PURPOSE

To determine the effect of tetanic stimulation on the time to achieve stabilization of the T1 height, by acceleromyography train-of-four (TOF) ratio monitoring, in patients aged 60 to 80 years.

METHODS

Randomized, prospective, single-blind, controlled trial in patients aged 60 to 80 years undergoing elective surgery under general anesthesia in two university hospitals in Brazil between December 2019 and March 2022. This trial was performed in accordance with the principles of the Helsinki Declaration and the guidelines for good clinical research practice in pharmacodynamic studies of neuromuscular blocking agents II. Participants were randomly allocated by computer-generated numbers to receive tetanic stimulation (intervention group, n  =  25) or not receive tetanic stimulation (control group, n  =  25) to achieve T1 height stabilization on TOF ratio monitoring. The primary endpoint was to evaluate the effect of tetanic stimulation on the time to achieve stabilization of the T1 height. We also evaluated the effect on the onset time of rocuronium and time to recover from neuromuscular blockade with a single dose of rocuronium 0.6 mg.kg-1 with spontaneous recovery.

RESULTS

There was no statistically significant difference between the groups in the time to T1 height stabilization (intervention: 4.3 [2.0-8.0] min; control: 5.8 [2.0-10.5] min; p  =  0.202), onset time of the neuromuscular blocking agent (intervention: 1.9 [±0.7] min; control: 2.2 [±1.0] min; p  =  0.219), or neuromuscular blockade recovery times to normalized TOF values of 0.7 (intervention: 106.1 [±37.2] min; control: 91.7 [±27.5] min; p  =  0.295), 0.8 (intervention: 114.3 [±40.1] min; control: 97.8 [±28.9] min; p  =  0.225), 0.9 (intervention: 126.5 [±44.0] min; control: 106.9 [±30.6] min; p  =  0.153), and 1.0 (intervention: 138.3 [±43.4] min; control: 123.8 [±43.5] min; p  =  0.425).

CONCLUSION

The results of this study suggest that the recommendation to use tetanic stimulation to shorten the time to T1 height stabilization in neuromuscular monitoring did not alter the stabilization time of the T1 twitch or the pharmacodynamics of neuromuscular blockade with rocuronium in patients aged 60 to 80 years.

Differential involvement of mitochondria in post-tetanic potentiation at intracortical excitatory synapses of the medial prefrontal cortex

Post-tetanic Ca2+ release from mitochondria produces presynaptic residual calcium, which contributes to post-tetanic potentiation. The loss of mitochondria-dependent post-tetanic potentiation is one of the earliest signs of Alzheimer's model mice. Post-tetanic potentiation at intracortical synapses of medial prefrontal cortex has been implicated in working memory. Although mitochondrial contribution to post-tetanic potentiation differs depending on synapse types, it is unknown which synapse types express mitochondria-dependent post-tetanic potentiation in the medial prefrontal cortex. We studied expression of mitochondria-dependent post-tetanic potentiation at different intracortical synapses of the rat medial prefrontal cortex. Post-tetanic potentiation occurred only at intracortical synapses onto layer 5 corticopontine cells from commissural cells and L2/3 pyramidal neurons. Among post-tetanic potentiation-expressing synapses, L2/3-corticopontine synapses in the prelimbic cortex were unique in that post-tetanic potentiation depends on mitochondria because post-tetanic potentiation at corresponding synapse types in other cortical areas was independent of mitochondria. Supporting mitochondria-dependent post-tetanic potentiation at L2/3-to-corticopontine synapses, mitochondria-dependent residual calcium at the axon terminals of L2/3 pyramidal neurons was significantly larger than that at commissural and corticopontine cells. Moreover, post-tetanic potentiation at L2/3-corticopontine synapses, but not at commissural-corticopontine synapses, was impaired in the young adult Alzheimer's model mice. These results would provide a knowledge base for comprehending synaptic mechanisms that underlies the initial clinical signs of neurodegenerative disorders.

Presynaptic Short-Term Plasticity Persists in the Absence of PKC Phosphorylation of Munc18-1

Post-tetanic potentiation (PTP) is a form of short-term plasticity that lasts for tens of seconds following a burst of presynaptic activity. It has been proposed that PTP arises from protein kinase C (PKC) phosphorylation of Munc18-1, an SM (Sec1/Munc-18 like) family protein that is essential for release. To test this model, we made a knock-in mouse in which all Munc18-1 PKC phosphorylation sites were eliminated through serine-to-alanine point mutations (Munc18-1SA mice), and we studied mice of either sex. The expression of Munc18-1 was not altered in Munc18-1SA mice, and there were no obvious behavioral phenotypes. At the hippocampal CA3-to-CA1 synapse and the granule cell parallel fiber (PF)-to-Purkinje cell (PC) synapse, basal transmission was largely normal except for small decreases in paired-pulse facilitation that are consistent with a slight elevation in release probability. Phorbol esters that mimic the activation of PKC by diacylglycerol still increased synaptic transmission in Munc18-1SA mice. In Munc18-1SA mice, 70% of PTP remained at CA3-to-CA1 synapses, and the amplitude of PTP was not reduced at PF-to-PC synapses. These findings indicate that at both CA3-to-CA1 and PF-to-PC synapses, phorbol esters and PTP enhance synaptic transmission primarily by mechanisms that are independent of PKC phosphorylation of Munc18-1.SIGNIFICANCE STATEMENT A leading mechanism for a prevalent form of short-term plasticity, post-tetanic potentiation (PTP), involves protein kinase C (PKC) phosphorylation of Munc18-1. This study tests this mechanism by creating a knock-in mouse in which Munc18-1 is replaced by a mutated form of Munc18-1 that cannot be phosphorylated. The main finding is that most PTP at hippocampal CA3-to-CA1 synapses or at cerebellar granule cell-to-Purkinje cell synapses does not rely on PKC phosphorylation of Munc18-1. Thus, mechanisms independent of PKC phosphorylation of Munc18-1 are important mediators of PTP.

The Role of Calmodulin vs. Synaptotagmin in Exocytosis

Exocytosis is a Ca²⁺-regulated process that requires the participation of Ca²⁺ sensors. In the 1980s, two classes of Ca²⁺-binding proteins were proposed as putative Ca²⁺ sensors: EF-hand protein calmodulin, and the C2 domain protein synaptotagmin. In the next few decades, numerous studies determined that in the final stage of membrane fusion triggered by a micromolar boost in the level of Ca²⁺, the low affinity Ca²⁺-binding protein synaptotagmin, especially synaptotagmin 1 and 2, acts as the primary Ca²⁺ sensor, whereas calmodulin is unlikely to be functional due to its high Ca²⁺ affinity. However, in the meantime emerging evidence has revealed that calmodulin is involved in the earlier exocytotic steps prior to fusion, such as vesicle trafficking, docking and priming by acting as a high affinity Ca²⁺ sensor activated at submicromolar level of Ca²⁺. Calmodulin directly interacts with multiple regulatory proteins involved in the regulation of exocytosis, including VAMP, myosin V, Munc13, synapsin, GAP43 and Rab3, and switches on key kinases, such as type II Ca²⁺/calmodulin-dependent protein kinase, to phosphorylate a series of exocytosis regulators, including syntaxin, synapsin, RIM and Ca²⁺ channels. Moreover, calmodulin interacts with synaptotagmin through either direct binding or indirect phosphorylation. In summary, calmodulin and synaptotagmin are Ca²⁺ sensors that play complementary roles throughout the process of exocytosis. In this review, we discuss the complementary roles that calmodulin and synaptotagmin play as Ca²⁺ sensors during exocytosis.

Aberrant activity of mitochondrial NCLX is linked to impaired synaptic transmission and is associated with mental retardation

Calcium dynamics control synaptic transmission. Calcium triggers synaptic vesicle fusion, determines release probability, modulates vesicle recycling, participates in long-term plasticity and regulates cellular metabolism. Mitochondria, the main source of cellular energy, serve as calcium signaling hubs. Mitochondrial calcium transients are primarily determined by the balance between calcium influx, mediated by the mitochondrial calcium uniporter (MCU), and calcium efflux through the sodium/lithium/calcium exchanger (NCLX). We identified a human recessive missense SLC8B1 variant that impairs NCLX activity and is associated with severe mental retardation. On this basis, we examined the effect of deleting NCLX in mice on mitochondrial and synaptic calcium homeostasis, synaptic activity, and plasticity. Neuronal mitochondria exhibited basal calcium overload, membrane depolarization, and a reduction in the amplitude and rate of calcium influx and efflux. We observed smaller cytoplasmic calcium transients in the presynaptic terminals of NCLX-KO neurons, leading to a lower probability of release and weaker transmission. In agreement, synaptic facilitation in NCLX-KO hippocampal slices was enhanced. Importantly, deletion of NCLX abolished long term potentiation of Schaffer collateral synapses. Our results show that NCLX controls presynaptic calcium transients that are crucial for defining synaptic strength as well as short- and long-term plasticity, key elements of learning and memory processes.

Non-negative Matrix Factorization as a Tool to Distinguish Between Synaptic Vesicles in Different Functional States

Synaptic vesicles (SVs) undergo multiple steps of functional maturation (priming) before being fusion competent. We present an analysis technique, which decomposes the time course of quantal release during repetitive stimulation as a sum of contributions of SVs, which existed in distinct functional states prior to stimulation. Such states may represent different degrees of maturation in priming or relate to different molecular composition of the release apparatus. We apply the method to rat calyx of Held synapses. These synapses display a high degree of variability, both with respect to synaptic strength and short-term plasticity during high-frequency stimulus trains. The method successfully describes time courses of quantal release at individual synapses as linear combinations of three components, representing contributions from functionally distinct SV subpools, with variability among synapses largely covered by differences in subpool sizes. Assuming that SVs transit in sequence through at least two priming steps before being released by an action potential (AP) we interpret the components as representing SVs which had been 'fully primed', 'incompletely primed' or undocked prior to stimulation. Given these assumptions, the analysis reports an initial release probability of 0.43 for SVs that were fully primed prior to stimulation. Release probability of that component was found to increase during high-frequency stimulation, leading to rapid depletion of that subpool. SVs that were incompletely primed at rest rapidly obtain fusion-competence during repetitive stimulation and contribute the majority of release after 3-5 stimuli.

Pulse-Chase Proteomics of the App Knockin Mouse Models of Alzheimer's Disease Reveals that Synaptic Dysfunction Originates in Presynaptic Terminals

Compromised protein homeostasis underlies accumulation of plaques and tangles in Alzheimer's disease (AD). To observe protein turnover at early stages of amyloid beta (Aβ) proteotoxicity, we performed pulse-chase proteomics on mouse brains in three genetic models of AD that knock in alleles of amyloid precursor protein (APP) prior to the accumulation of plaques and during disease progression. At initial stages of Aβ accumulation, the turnover of proteins associated with presynaptic terminals is selectively impaired. Presynaptic proteins with impaired turnover, particularly synaptic vesicle (SV)-associated proteins, have elevated levels, misfold in both a plaque-dependent and -independent manner, and interact with APP and Aβ. Concurrent with elevated levels of SV-associated proteins, we found an enlargement of the SV pool as well as enhancement of presynaptic potentiation. Together, our findings reveal that the presynaptic terminal is particularly vulnerable and represents a critical site for manifestation of initial AD etiology. A record of this paper's transparent peer review process is included in the Supplemental Information.

Marinoid J, a phenylglycoside from Avicennia marina fruit, ameliorates cognitive impairment in rat vascular dementia: a quantitative iTRAQ proteomic study

CONTEXT

Fruit of Avicennia marina (Forsk.) Vierh. (Acanthaceae) is used as a Chinese herb. Studies have found that it contains marinoid J, a novel phenylethanoid glycoside (PG) compound, but its neuroprotective functions are largely unknown.

OBJECTIVE

This study evaluated the effects of marinoid J on vascular dementia (VD) and determined its potential mechanisms of action.

MATERIALS AND METHODS

The VD model was established by the ligation of the bilateral common carotid artery in Sprague-Dawley rats, who received daily intragastrically administration of saline, marinoid J (125 or 500 mg/kg body weight/d), or oxiracetam (250 mg/kg body weight/d) for 14 days (20 rats in each group). The Morris water maze (MWM) was used to evaluate cognitive performance. The hippocampus was subjected to histological and proteomic analyses.

RESULTS

Marinoid J shortened the escape latency of VD rats (31.07 ± 3.74 s, p < 0.05). It also decreased malondialdehyde (MDA) (27.53%) and nitric oxide (NO) (20.41%) while increasing superoxide dismutase (SOD) (11.26%) and glutathione peroxidase (GSH-Px) (20.38%) content in hippocampus tissues. Proteomic analysis revealed 45 differentially expressed proteins (DEPs) in marinoid J-treated VD rats, which included angiotensin-converting enzyme (ACE), keratin 18 (KRT18), cluster of differentiation 34 (CD34), and synaptotagmin II (SYT2).

CONCLUSIONS

Marinoid J played a role in protecting hippocampal neurons by regulating a set of proteins that influence oxidative stress and apoptosis, this effect may thereby alleviate the symptoms of VD rats. Thus, pharmacological manipulation of marinoid J may offer a novel opportunity for VD treatment.

Inter-spike mitochondrial Ca2+ release enhances high frequency synaptic transmission

KEY POINTS

Presynaptic mitochondria not only absorb but also release Ca²⁺ during high frequency stimulation (HFS) when presynaptic [Ca²⁺ ] is kept low (<500 nm) by high cytosolic Ca²⁺ buffer or strong plasma membrane calcium clearance mechanisms under physiological external [Ca²⁺ ]. Mitochondrial Ca²⁺ release (MCR) does not alter the global presynaptic Ca²⁺ transients. MCR during HFS enhances short-term facilitation and steady state excitatory postsynaptic currents by increasing vesicular release probability. The intra-train MCR may provide residual calcium at interspike intervals, and thus support high frequency neurotransmission at central glutamatergic synapses.

ABSTRACT

Emerging evidence indicates that mitochondrial Ca²⁺ buffering contributes to local regulation of synaptic transmission. It is unknown, however, whether mitochondrial Ca²⁺ release (MCR) occurs during high frequency synaptic transmission. Confirming the previous notion that 2 μm tetraphenylphosphonium (TPP⁺ ) is a specific inhibitor of the mitochondrial Na⁺ /Ca²⁺ exchanger (mNCX), we studied the role of MCR via mNCX in short-term plasticity during high frequency stimulation (HFS) at the calyx of Held synapse of the rat. TPP⁺ reduced short-term facilitation (STF) and steady state excitatory postsynaptic currents during HFS at mature calyx synapses under physiological extracellular [Ca²⁺ ] ([Ca²⁺ ]_o  = 1.2 mm), but not at immature calyx or at 2 mm [Ca²⁺ ]_o . The inhibitory effects of TPP⁺ were stronger at synapses with morphologically complex calyces harbouring many swellings and at 32°C than at simple calyx synapses and at room temperature. These effects of TPP⁺ on STF were well correlated with those on the presynaptic mitochondrial [Ca²⁺ ] build-up during HFS. Mitochondrial [Ca²⁺ ] during HFS was increased by TPP⁺ at mature calyces under 1.2 mm [Ca²⁺ ]_o , and further enhanced at 32°C, but not under 2 mm [Ca²⁺ ]_o or at immature calyces. The close correlation of the effects of TPP⁺ on mitochondrial [Ca²⁺ ] with those on STF suggests that mNCX contributes to STF at the calyx of Held synapses. The intra-train MCR enhanced vesicular release probability without altering global presynaptic [Ca²⁺ ]. Our results suggest that MCR during HFS elevates local [Ca²⁺ ] near synaptic sites at interspike intervals to enhance STF and to support stable synaptic transmission under physiological [Ca²⁺ ]_o .

Differential modulation of short-term plasticity at hippocampal mossy fiber and Schaffer collateral synapses by mitochondrial Ca2

Presynaptic mitochondrial Ca2+ plays a critical role in the regulation of synaptic transmission and plasticity. The presynaptic bouton of the hippocampal mossy fiber (MF) is much larger in size than that of the Schaffer collateral (SC) synapse. Here we compare the structural and physiological characteristics of MF and SC presynaptic boutons to reveal functional and mechanistic differences between these two synapses. Our quantitative ultrastructural analysis using electron microscopy show many more mitochondria in MF presynaptic bouton cross-section profiles compared to SC boutons. Consistent with these results, post-tetanic potentiation (PTP), a form of presynaptic short-term plasticity dependent on mitochondrial Ca2+, is reduced by inhibition of mitochondrial Ca2+ release at MF synapses but not at SC synapses. However, blockade of mitochondrial Ca2+ release results in reduction of PTP at SC synapses by disynaptic MF stimulation. Furthermore, inhibition of mitochondrial Ca2+ release selectively decreases frequency facilitation evoked by short trains of presynaptic stimulation at MF synapses, while having no effect at SC synapses. Moreover, depletion of ER Ca2+ stores leads to reduction of PTP at MF synapses, but PTP is unaffected by ER Ca2+ depletion at SC synapses. These findings show that MF and SC synapses differ in presynaptic mitochondrial content as well as mitochondrial Ca2+ dependent synaptic plasticity, highlighting differential regulatory mechanisms of presynaptic plasticity at MF and SC synapses.

Post-tetanic potentiation lowers the energy barrier for synaptic vesicle fusion independently of Synaptotagmin-1

Previously, we showed that modulation of the energy barrier for synaptic vesicle fusion boosts release rates supralinearly (Schotten, 2015). Here we show that mouse hippocampal synapses employ this principle to trigger Ca²⁺-dependent vesicle release and post-tetanic potentiation (PTP). We assess energy barrier changes by fitting release kinetics in response to hypertonic sucrose. Mimicking activation of the C2A domain of the Ca²⁺-sensor Synaptotagmin-1 (Syt1), by adding a positive charge (Syt1^D232N) or increasing its hydrophobicity (Syt1^4W), lowers the energy barrier. Removing Syt1 or impairing its release inhibitory function (Syt1^9Pro) increases spontaneous release without affecting the fusion barrier. Both phorbol esters and tetanic stimulation potentiate synaptic strength, and lower the energy barrier equally well in the presence and absence of Syt1. We propose a model where tetanic stimulation activates Syt1-independent mechanisms that lower the energy barrier and act additively with Syt1-dependent mechanisms to produce PTP by exerting multiplicative effects on release rates.

Short-Term Plasticity at Hippocampal Mossy Fiber Synapses Is Induced by Natural Activity Patterns and Associated with Vesicle Pool Engram Formation

Post-tetanic potentiation (PTP) is an attractive candidate mechanism for hippocampus-dependent short-term memory. Although PTP has a uniquely large magnitude at hippocampal mossy fiber-CA3 pyramidal neuron synapses, it is unclear whether it can be induced by natural activity and whether its lifetime is sufficient to support short-term memory. We combined in vivo recordings from granule cells (GCs), in vitro paired recordings from mossy fiber terminals and postsynaptic CA3 neurons, and “flash and freeze” electron microscopy. PTP was induced at single synapses and showed a low induction threshold adapted to sparse GC activity in vivo. PTP was mainly generated by enlargement of the readily releasable pool of synaptic vesicles, allowing multiplicative interaction with other plasticity forms. PTP was associated with an increase in the docked vesicle pool, suggesting formation of structural “pool engrams.” Absence of presynaptic activity extended the lifetime of the potentiation, enabling prolonged information storage in the hippocampal network.

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Natural activity patterns in hippocampal GCs in vivo induce PTP at mossy fiber synapses

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PTP is primarily caused by an increase in the readily releasable vesicle pool

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PTP is associated with an increase in the number of docked vesicles at active zones

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Sparse activity extends pool engram lifetime, increasing overlap with short-term memory

Molecular Mechanisms of Short-Term Plasticity: Role of Synapsin Phosphorylation in Augmentation and Potentiation of Spontaneous Glutamate Release

We used genetic and pharmacological approaches to identify the signaling pathways involved in augmentation and potentiation, two forms of activity dependent, short-term synaptic plasticity that enhance neurotransmitter release. Trains of presynaptic action potentials produced a robust increase in the frequency of miniature excitatory postsynaptic currents (mEPSCs). Following the end of the stimulus, mEPSC frequency followed a bi-exponential decay back to basal levels. The time constants of decay identified these two exponential components as the decay of augmentation and potentiation, respectively. Augmentation increased mEPSC frequency by 9.3-fold, while potentiation increased mEPSC frequency by 2.4-fold. In synapsin triple-knockout (TKO) neurons, augmentation was reduced by 83% and potentiation was reduced by 74%, suggesting that synapsins are key signaling elements in both forms of plasticity. To examine the synapsin isoforms involved, we expressed individual synapsin isoforms in TKO neurons. While synapsin IIIa rescued both augmentation and potentiation, none of the other synapsin isoforms produced statistically significant amounts of rescue. To determine the involvement of protein kinases in these two forms of short-term plasticity, we examined the effects of inhibitors of protein kinases A (PKA) and C (PKC). While inhibition of PKC had little effect, PKA inhibition reduced augmentation by 76% and potentiation by 60%. Further, elevation of intracellular cAMP concentration, by either forskolin or IBMX, greatly increased mEPSC frequency and occluded the amount of augmentation and potentiation evoked by electrical stimulation. Finally, mutating a PKA phosphorylation site to non-phosphorylatable alanine largely abolished the ability of synapsin IIIa to rescue both augmentation and potentiation. Together, these results indicate that PKA activation is required for both augmentation and potentiation of spontaneous neurotransmitter release and that PKA-mediated phosphorylation of synapsin IIIa underlies both forms of presynaptic short-term plasticity.

Prion acute synaptotoxicity is largely driven by protease-resistant PrPSc species

Although misfolding of normal prion protein (PrP^C) into abnormal conformers (PrP^Sc) is critical for prion disease pathogenesis our current understanding of the underlying molecular pathophysiology is rudimentary. Exploiting an electrophysiology paradigm, herein we report that at least modestly proteinase K (PK)-resistant PrP^Sc (PrP^res) species are acutely synaptotoxic. Brief exposure to ex vivo PrP^Sc from two mouse-adapted prion strains (M1000 and MU02) prepared as crude brain homogenates (cM1000 and cMU02) and cell lysates from chronically M1000-infected RK13 cells (MoRK13-Inf) caused significant impairment of hippocampal CA1 region long-term potentiation (LTP), with the LTP disruption approximating that reported during the evolution of murine prion disease. Proof of PrP^Sc (especially PrP^res) species as the synaptotoxic agent was demonstrated by: significant rescue of LTP following selective immuno-depletion of total PrP from cM1000 (dM1000); modestly PK-treated cM1000 (PK+M1000) retaining full synaptotoxicity; and restoration of the LTP impairment when employing reconstituted, PK-eluted, immuno-precipitated M1000 preparations (PK+IP-M1000). Additional detailed electrophysiological analyses exemplified by impairment of post-tetanic potentiation (PTP) suggest possible heightened pre-synaptic vulnerability to the acute synaptotoxicity. This dysfunction correlated with cumulative insufficiency of replenishment of the readily releasable pool (RRP) of vesicles during repeated high-frequency stimulation utilised for induction of LTP. Broadly comparable results with LTP and PTP impairment were obtained utilizing hippocampal slices from PrP^C knockout (PrPo/o) mice, with cM1000 serial dilution assessments revealing similar sensitivity of PrPo/o and wild type (WT) slices. Size fractionation chromatography demonstrated that synaptotoxic PrP correlated with PK-resistant species >100kDa, consistent with multimeric PrP^Sc, with levels of these species >6 ng/ml appearing sufficient to induce synaptic dysfunction. Biochemical analyses of hippocampal slices manifesting acute synaptotoxicity demonstrated reduced levels of multiple key synaptic proteins, albeit with noteworthy differences in PrPo/o slices, while such changes were absent in hippocampi demonstrating rescued LTP through treatment with dM1000. Our findings offer important new mechanistic insights into the synaptic impairment underlying prion disease, enhancing prospects for development of targeted effective therapies.

Misfolding of the normal prion protein (PrP^C) into disease-associated conformations (PrP^Sc) is the critical initiating step for prion diseases. Similar to other neurodegenerative disorders, progressive failure of brain synapses is considered a primary deleterious event underpinning prion disease evolution. Our current understanding of the underlying mechanisms associated with synaptic failure is rudimentary contributing to difficulties in developing effective treatments. Herein we report the use of an electrophysiology paradigm that allowed us to demonstrate that at least modestly proteinase K (PK)-resistant PrP^Sc species from two mouse-adapted prion strains (M1000 and MU02) are directly synaptotoxic causing significant acute impairment of hippocampal CA1 region long-term potentiation (LTP). Of note, the LTP disruption approximated that reported in prion animal models. Additional detailed analyses provided novel pathophysiological insights suggesting possible heightened pre-synaptic vulnerability to the acute synaptotoxicity through impairment of replenishment of the readily releasable pool of neurotransmitter vesicles, while biochemical analyses demonstrated reduced levels of multiple key pre-and post-synaptic proteins. Broadly similar acute synaptic dysfunction and dose-response susceptibility were observed in slices from mice not expressing PrP^C albeit with minor but noteworthy differences in electrophysiological and biochemical findings. Our study offers important new mechanistic insights into the synaptic impairment underlying prion disease, enhancing prospects for development effective therapies.

Apparent calcium dependence of vesicle recruitment

KEY POINTS

Synaptic transmission relies on the recruitment of neurotransmitter-filled vesicles to presynaptic release sites. Increased intracellular calcium buffering slows the recovery from synaptic depression, suggesting that vesicle recruitment is a calcium-dependent process. However, the molecular mechanisms of vesicle recruitment have only been investigated at some synapses. We investigate the role of calcium in vesicle recruitment at the cerebellar mossy fibre to granule cell synapse. We find that increased intracellular calcium buffering slows the recovery from depression following physiological stimulation. However, the recovery is largely resistant to perturbation of the molecular pathways previously shown to mediate calcium-dependent vesicle recruitment. Furthermore, we find two pools of vesicles with different recruitment speeds and show that models incorporating two pools of vesicles with different calcium-independent recruitment rates can explain our data. In this framework, increased calcium buffering prevents the release of intrinsically fast-recruited vesicles but does not change the vesicle recruitment rates themselves.

ABSTRACT

During sustained synaptic transmission, recruitment of new transmitter-filled vesicles to the release site counteracts vesicle depletion and thus synaptic depression. An elevated intracellular Ca²⁺ concentration has been proposed to accelerate the rate of vesicle recruitment at many synapses. This conclusion is often based on the finding that increased intracellular Ca²⁺ buffering slows the recovery from synaptic depression. However, the molecular mechanisms of the activity-dependent acceleration of vesicle recruitment have only been analysed at some synapses. Using physiological stimulation patterns in postsynaptic recordings and step depolarizations in presynaptic bouton recordings, we investigate vesicle recruitment at cerebellar mossy fibre boutons. We show that increased intracellular Ca²⁺ buffering slows recovery from depression dramatically. However, pharmacological and genetic interference with calmodulin or the calmodulin-Munc13 pathway, which has been proposed to mediate Ca²⁺ -dependence of vesicle recruitment, barely affects vesicle recovery from depression. Furthermore, we show that cerebellar mossy fibre boutons have two pools of vesicles: rapidly fusing vesicles that recover slowly and slowly fusing vesicles that recover rapidly. Finally, models adopting such two pools of vesicles with Ca²⁺ -independent recruitment rates can explain the slowed recovery from depression upon increased Ca²⁺ buffering. Our data do not rule out the involvement of the calmodulin-Munc13 pathway during stronger stimuli or other molecular pathways mediating Ca²⁺ -dependent vesicle recruitment at cerebellar mossy fibre boutons. However, we show that well-established two-pool models predict an apparent Ca²⁺ -dependence of vesicle recruitment. Thus, previous conclusions of Ca²⁺ -dependent vesicle recruitment based solely on increased intracellular Ca²⁺ buffering should be considered with caution.

High glucose upregulates myosin light chain kinase to induce microfilament cytoskeleton rearrangement in hippocampal neurons

Chronic hyperglycemia leads to myosin light chain kinase (MLCK) upregulation and induces neuronal damage. However, the underlying molecular mechanism of neuronal damage in hyperglycemia has not yet been fully elucidated. In the present study, hippocampal neuronal cells were cultured and treated with a high glucose concentration (45 mmol/l). The results demonstrated that high glucose induced shrinking of the synapses, nuclear shape irregularity and microfilament damage. Filamentous actin (F‑actin) filaments were rearranged, cell apoptosis rate was increased and the protein expression of MLCK and phosphorylated (p)‑MLC was upregulated. The MLCK inhibitor ML‑7 largely reversed the alterations in the microfilament cytoskeleton, inhibited F‑actin depolymerization, reduced apoptosis and downregulated MLCK and p‑MLC protein expression. Overall, these results indicated that high glucose upregulated MLCK to promote F‑actin depolymerization, which induced microfilament cytoskeleton rearrangement in hippocampal neuronal cells.

A single episode of high intensity sound inhibits long-term potentiation in the hippocampus of rats

Exposure to loud sounds has become increasingly common. The most common consequences of loud sound exposure are deafness and tinnitus, but emotional and cognitive problems are also associated with loud sound exposure. Loud sounds can activate the hipothalamic-pituitary-adrenal axis resulting in the secretion of corticosterone, which affects hippocampal synaptic plasticity. Previously we have shown that long-term exposure to short episodes of high intensity sound inhibited hippocampal long-term potentiation (LTP) without affecting spatial learning and memory. Here we aimed to study the impact of short term loud sound exposure on hippocampal synaptic plasticity and function. We found that a single minute of 110 dB sound inhibits hippocampal Schaffer-CA1 LTP for 24 hours. This effect did not occur with an 80-dB sound exposure, was not correlated with corticosterone secretion and was also observed in the perforant-dentate gyrus synapse. We found that despite the deficit in the LTP these animals presented normal spatial learning and memory and fear conditioning. We conclude that a single episode of high-intensity sound impairs hippocampal LTP, without impairing memory and learning. Our results show that the hippocampus is very responsive to loud sounds which can have a potential, but not yet identified, impact on its function.

Quantal Fluctuations in Central Mammalian Synapses: Functional Role of Vesicular Docking Sites

Quantal fluctuations are an integral part of synaptic signaling. At the frog neuromuscular junction, Bernard Katz proposed that quantal fluctuations originate at “reactive sites” where specific structures of the presynaptic membrane interact with synaptic vesicles. However, the physical nature of reactive sites has remained unclear, both at the frog neuromuscular junction and at central synapses. Many central synapses, called simple synapses, are small structures containing a single presynaptic active zone and a single postsynaptic density of receptors. Several lines of evidence indicate that simple synapses may release several synaptic vesicles in response to a single action potential. However, in some synapses at least, each release event activates a significant fraction of the postsynaptic receptors, giving rise to a sublinear relation between vesicular release and postsynaptic current. Partial receptor saturation as well as synaptic jitter gives to simple synapse signaling the appearance of a binary process. Recent investigations of simple synapses indicate that the number of released vesicles follows binomial statistics, with a maximum reflecting the number of docking sites present in the active zone. These results suggest that at central synapses, vesicular docking sites represent the reactive sites proposed by Katz. The macromolecular architecture and molecular composition of docking sites are presently investigated with novel combinations of techniques. It is proposed that variations in docking site numbers are central in defining intersynaptic variability and that docking site occupancy is a key parameter regulating short-term synaptic plasticity.

Calcium-Dependent Protein Kinase C Is Not Required for Post-Tetanic Potentiation at the Hippocampal CA3 to CA1 Synapse

Post-tetanic potentiation (PTP) is a widespread form of short-term synaptic plasticity in which a period of elevated presynaptic activation leads to synaptic enhancement that lasts tens of seconds to minutes. A leading hypothesis for the mechanism of PTP is that tetanic stimulation elevates presynaptic calcium that in turn activates calcium-dependent protein kinase C (PKC) isoforms to phosphorylate targets and enhance neurotransmitter release. Previous pharmacological studies have implicated this mechanism in PTP at hippocampal synapses, but the results are controversial. Here we combine genetic and pharmacological approaches to determine the role of classic PKC isoforms in PTP. We find that PTP is unchanged in PKC triple knock-out (TKO) mice in which all calcium-dependent PKC isoforms have been eliminated (PKCα, PKCβ, and PKCγ). We confirm previous studies and find that in wild-type mice 10 μm of the PKC inhibitor GF109203 eliminates PTP and the PKC activator PDBu enhances neurotransmitter release and occludes PTP. However, we find that the same concentrations of GF109203 and PDBu have similar effects in TKO animals. We also show that 2 μm GF109203 does not abolish PTP even though it inhibits the PDBu-dependent phosphorylation of PKC substrates. We conclude that at the CA3 to CA1 synapse Ca(2+)-dependent PKC isoforms do not serve as calcium sensors to mediate PTP.

SIGNIFICANCE STATEMENT

Neurons dynamically regulate neurotransmitter release through many processes known collectively as synaptic plasticity. Post-tetanic potentiation (PTP) is a widespread form of synaptic plasticity that lasts for tens of seconds that may have important computational roles and contribute to short-term memory. According to a leading mechanism, presynaptic calcium activates protein kinase C (PKC) to increase neurotransmitter release. Pharmacological studies have also implicated this mechanism at hippocampal CA3 to CA1 synapses, but there are concerns about the specificity of PKC activators and inhibitors. We therefore used a molecular genetic approach and found that PTP was unaffected when all calcium-dependent PKC isozymes were eliminated. We conclude that PKC isozymes are not the calcium sensors that mediate PTP at the CA3 to CA1 synapse.

Presenilins regulate synaptic plasticity and mitochondrial calcium homeostasis in the hippocampal mossy fiber pathway

Background

Presenilins play a major role in the pathogenesis of Alzheimer’s disease, in which the hippocampus is particularly vulnerable. Previous studies of Presenilin function in the synapse, however, focused exclusively on the hippocampal Schaffer collateral (SC) pathway. Whether Presenilins play similar or distinct roles in other hippocampal synapses is unknown.

Methods

To investigate the role of Presenilins at mossy fiber (MF) synapses we performed field and whole-cell electrophysiological recordings and Ca²⁺ imaging using acute hippocampal slices of postnatal forebrain-restricted Presenilin conditional double knockout (PS cDKO) and control mice at 2 months of age. We also performed quantitative electron microscopy (EM) analysis to determine whether mitochondrial content is affected at presynaptic MF boutons of PS cDKO mice. We further conducted behavioral analysis to assess spatial learning and memory of PS cDKO and control mice at 2 months in the Morris water maze.

Results

We found that long-term potentiation and short-term plasticity, such as paired-pulse and frequency facilitation, are impaired at MF synapses of PS cDKO mice. Moreover, post-tetanic potentiation (PTP), another form of short-term plasticity, is also impaired at MF synapses of PS cDKO mice. Furthermore, blockade of mitochondrial Ca²⁺ efflux mimics and occludes the PTP deficits at MF synapses of PS cDKO mice, suggesting that mitochondrial Ca²⁺ homeostasis is impaired in the absence of PS. Quantitative EM analysis showed normal number and area of mitochondria at presynaptic MF boutons of PS cDKO mice, indicating unchanged mitochondrial content. Ca²⁺ imaging of dentate gyrus granule neurons further revealed that cytosolic Ca²⁺ increases induced by tetanic stimulation are reduced in PS cDKO granule neurons in acute hippocampal slices, and that inhibition of mitochondrial Ca²⁺ release during high frequency stimulation mimics and occludes the Ca²⁺ defects observed in PS cDKO neurons. Consistent with synaptic plasticity impairment observed at MF and SC synapses in acute PS cDKO hippocampal slices, PS cDKO mice exhibit profound spatial learning and memory deficits in the Morris water maze.

Conclusions

Our findings demonstrate the importance of PS in the regulation of synaptic plasticity and mitochondrial Ca²⁺ homeostasis in the hippocampal MF pathway.

Myosin accelerates synaptic vesicle recycling in the motor nerve endings

Neurotransmitter release and exocytosis of synaptic vesicles in the motor nerve endings of the frog cutaneous-pectoris muscle were studied using electrophysiological and optical methods under the conditions of inhibition of the myosin light-chain kinase and non-muscle myosin by the specific inhibitors ML-7 (12 μM) and (-)-blebbistatin (100 μM). At high-frequency stimulation (20 pulses/s), these inhibitors strengthened suppression of transmitter release during the first 20-25 s and slowed down the release of the fluorescent dye FM 1-43. The obtained results indicate that myosin accelerates rapid synaptic vesicle recycling upon high-frequency stimulation.

Superpriming of synaptic vesicles as a common basis for intersynapse variability and modulation of synaptic strength

Glutamatergic synapses show large variations in strength and short-term plasticity (STP). We show here that synapses displaying an increased strength either after posttetanic potentiation (PTP) or through activation of the phospholipase-C-diacylglycerol pathway share characteristic properties with intrinsically strong synapses, such as (i) pronounced short-term depression (STD) during high-frequency stimulation; (ii) a conversion of that STD into a sequence of facilitation followed by STD after a few conditioning stimuli at low frequency; (iii) an equalizing effect of such conditioning stimulation, which reduces differences among synapses and abolishes potentiation; and (iv) a requirement of long periods of rest for reconstitution of the original STP pattern. These phenomena are quantitatively described by assuming that a small fraction of "superprimed" synaptic vesicles are in a state of elevated release probability (p ∼ 0.5). This fraction is variable in size among synapses (typically about 30%), but increases after application of phorbol ester or during PTP. The majority of vesicles, released during repetitive stimulation, have low release probability (p ∼ 0.1), are relatively uniform in number across synapses, and are rapidly recruited. In contrast, superprimed vesicles need several seconds to be regenerated. They mediate enhanced synaptic strength at the onset of burst-like activity, the impact of which is subject to modulation by slow modulatory transmitter systems.

Dynamin-1 deletion enhances post-tetanic potentiation and quantal size after tetanic stimulation at the calyx of Held

KEY POINTS

Post-tetanic potentiation (PTP) is attributed mainly to an increase in release probability (P_r ) and/or readily-releasable pool (RRP) in many synapses, but the role of endocytosis in PTP is unknown. Using the calyx of Held synapse from tissue-specific dynamin-1 knockout (cKO) mice (P16-20), we report that cKO synapses show enhanced PTP compared to control. We found significant increases in both spontaneous excitatory postsynaptic current (spEPSC) amplitude and RRP size (estimated by a train of 30 APs at 100 Hz) in cKO over control during PTP. Actin depolymerization blocks the increase in spEPSC amplitude in both control and cKO, and it abolishes the enhancement of PTP in cKO. PTP is sensitive to the PKC inhibitor GF109203X in both control and cKO. We conclude that an activity-dependent quantal size increase contributes to the enhancement of PTP in cKO over control and an altered endocytosis affects short-term plasticity through quantal size changes.

ABSTRACT

High-frequency stimulation leads to post-tetanic potentiation (PTP) at many types of synapses. Previous studies suggest that PTP results primarily from a protein kinase C (PKC)-dependent increase in release probability (P_r ) and/or readily-releasable pool (RRP) of synaptic vesicles (SVs), but the role of SV endocytosis in PTP is unknown. Using the mature calyx of Held (P16-20), we report that tissue-specific ablation of dynamin-1 (cKO), an endocytic protein crucial for SV regeneration, enhances PTP in cKO over control. To explore the mechanism of this enhancement, we estimated the changes in paired-pulse ratios (PPRs) and RRP size during PTP. RRP was estimated by the back-extrapolation of cumulative EPSC amplitudes during a train of 30 action potentials at 100 Hz (termed RRP_train ). We found an increase in RRP_train during PTP in both control and cKO, but no significant changes in the PPR. Moreover, the amplitude and frequency of spontaneous excitatory postsynaptic currents (spEPSCs) increased during PTP in both control and cKO; however, the spEPSC amplitude in cKO during PTP was significantly larger than in control. Actin depolymerization reagent latrunculin-B (Lat-B) abolished the activity-dependent increase in spEPSC amplitude in both control and cKO, but selectively blocked the enhancement of PTP in cKO, without affecting PTP in control. PKC inhibitor GF109203X nearly abolished PTP in both control and cKO. These data suggest that the quantal size increase contributes to the enhancement of PTP in dynamin-1 cKO, and this change depends on strong synaptic activity and actin polymerization.

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.

Molecular Machines Regulating the Release Probability of Synaptic Vesicles at the Active Zone

The fusion of synaptic vesicles (SVs) with the plasma membrane of the active zone (AZ) upon arrival of an action potential (AP) at the presynaptic compartment is a tightly regulated probabilistic process crucial for information transfer. The probability of a SV to release its transmitter content in response to an AP, termed release probability (Pr), is highly diverse both at the level of entire synapses and individual SVs at a given synapse. Differences in Pr exist between different types of synapses, between synapses of the same type, synapses originating from the same axon and even between different SV subpopulations within the same presynaptic terminal. The Pr of SVs at the AZ is set by a complex interplay of different presynaptic properties including the availability of release-ready SVs, the location of the SVs relative to the voltage-gated calcium channels (VGCCs) at the AZ, the magnitude of calcium influx upon arrival of the AP, the buffering of calcium ions as well as the identity and sensitivity of the calcium sensor. These properties are not only interconnected, but can also be regulated dynamically to match the requirements of activity patterns mediated by the synapse. Here, we review recent advances in identifying molecules and molecular machines taking part in the determination of vesicular Pr at the AZ.

Electrophysiologic features of SYT2 mutations causing a treatable neuromuscular syndrome

OBJECTIVES

To describe the clinical and electrophysiologic features of synaptotagmin II (SYT2) mutations, a novel neuromuscular syndrome characterized by foot deformities and fatigable ocular and lower limb weakness, and the response to modulators of acetylcholine release.

METHODS

We performed detailed clinical and neurophysiologic assessment in 2 multigenerational families with dominant SYT2 mutations (c.920T>G [p.Asp307Ala] and c.923G>A [p.Pro308Leu]). Serial clinical and electrophysiologic assessments were performed in members of one family treated first with pyridostigmine and then with 3,4-diaminopyridine.

RESULTS

Electrophysiologic testing revealed features indicative of a presynaptic deficit in neurotransmitter release with posttetanic potentiation lasting up to 60 minutes. Treatment with 3,4-diaminopyridine produced both a clinical benefit and an improvement in neuromuscular transmission.

CONCLUSION

SYT2 mutations cause a novel and potentially treatable complex presynaptic congenital myasthenic syndrome characterized by motor neuropathy causing lower limb wasting and foot deformities, with reflex potentiation following exercise and a uniquely prolonged period of posttetanic potentiation.

Rac-mediated actin remodeling and myosin II are involved in KATP channel trafficking in pancreatic β-cells

AMP-activated protein kinase (AMPK) is a metabolic sensor activated during metabolic stress and it regulates various enzymes and cellular processes to maintain metabolic homeostasis. We previously reported that activation of AMPK by glucose deprivation (GD) and leptin increases K_ATP currents by increasing the surface levels of K_ATP channel proteins in pancreatic β-cells. Here, we show that the signaling mechanisms that mediate actin cytoskeleton remodeling are closely associated with AMPK-induced K_ATP channel trafficking. Using F-actin staining with Alexa 633-conjugated phalloidin, we observed that dense cortical actin filaments present in INS-1 cells cultured in 11 mM glucose were disrupted by GD or leptin treatment. These changes were blocked by inhibiting AMPK using compound C or siAMPK and mimicked by activating AMPK using AICAR, indicating that cytoskeletal remodeling induced by GD or leptin was mediated by AMPK signaling. AMPK activation led to the activation of Rac GTPase and the phosphorylation of myosin regulatory light chain (MRLC). AMPK-dependent actin remodeling induced by GD or leptin was abolished by the inhibition of Rac with a Rac inhibitor (NSC23766), siRac1 or siRac2, and by inhibition of myosin II with a myosin ATPase inhibitor (blebbistatin). Immunocytochemistry, surface biotinylation and electrophysiological analyses of K_ATP channel activity and membrane potentials revealed that AMPK-dependent K_ATP channel trafficking to the plasma membrane was also inhibited by NSC23766 or blebbistatin. Taken together, these results indicate that AMPK/Rac-dependent cytoskeletal remodeling associated with myosin II motor function promotes the translocation of K_ATP channels to the plasma membrane in pancreatic β-cells.

Membrane-derived phospholipids control synaptic neurotransmission and plasticity

Synaptic communication is a dynamic process that is key to the regulation of neuronal excitability and information processing in the brain. To date, however, the molecular signals controlling synaptic dynamics have been poorly understood. Membrane-derived bioactive phospholipids are potential candidates to control short-term tuning of synaptic signaling, a plastic event essential for information processing at both the cellular and neuronal network levels in the brain. Here, we showed that phospholipids affect excitatory and inhibitory neurotransmission by different degrees, loci, and mechanisms of action. Signaling triggered by lysophosphatidic acid (LPA) evoked rapid and reversible depression of excitatory and inhibitory postsynaptic currents. At excitatory synapses, LPA-induced depression depended on LPA₁/G_αi/o-protein/phospholipase C/myosin light chain kinase cascade at the presynaptic site. LPA increased myosin light chain phosphorylation, which is known to trigger actomyosin contraction, and reduced the number of synaptic vesicles docked to active zones in excitatory boutons. At inhibitory synapses, postsynaptic LPA signaling led to dephosphorylation, and internalization of the GABA_Aγ2 subunit through the LPA₁/G_α12/13-protein/RhoA/Rho kinase/calcineurin pathway. However, LPA-induced depression of GABAergic transmission was correlated with an endocytosis-independent reduction of GABA_A receptors, possibly by GABA_Aγ2 dephosphorylation and subsequent increased lateral diffusion. Furthermore, endogenous LPA signaling, mainly via LPA₁, mediated activity-dependent inhibitory depression in a model of experimental synaptic plasticity. Finally, LPA signaling, most likely restraining the excitatory drive incoming to motoneurons, regulated performance of motor output commands, a basic brain processing task. We propose that lysophospholipids serve as potential local messengers that tune synaptic strength to precedent activity of the neuron.

Lysophospholipids derived from membranes are important regulators of neurotransmission, acting as local messengers that couple synaptic strength to recent neuronal activity.

Neuronal networks are modules of synaptic connectivity that underlie all brain functions, from simple reflexes to complex cognitive processes. Synaptic plasticity allows these networks to adapt to changing external and internal environments. Membrane-derived bioactive phospholipids are potential candidates to control short-term synaptic plasticity. We demonstrate that lysophosphatidic acid (LPA), an important intermediary in lipid metabolism, depresses the main excitatory and inhibitory synaptic systems by different mechanisms. LPA depresses inhibitory synaptic transmission by reducing the number of postsynaptic receptors at inhibitory synapses; whereas it depresses excitatory synaptic transmission by decreasing the size of the ready-to-use synaptic vesicle pool at excitatory terminals. Finally, we demonstrate that LPA signaling contributes to the performance of motor output commands in adult animals. Our data documents that synaptic strength and neuronal activity are modulated by products of membrane phospholipid metabolism, which suggests that bioactive phospholipids are candidates in coupling brain function to the metabolic status of the organism.

Translating neuronal activity at the synapse: presynaptic calcium sensors in short-term plasticity

The complex manner in which patterns of presynaptic neural activity are translated into short-term plasticity (STP) suggests the existence of multiple presynaptic calcium (Ca(2+)) sensors, which regulate the amplitude and time-course of STP and are the focus of this review. We describe two canonical Ca(2+)-binding protein domains (C2 domains and EF-hands) and define criteria that need to be met for a protein to qualify as a Ca(2+) sensor mediating STP. With these criteria in mind, we discuss various forms of STP and identify established and putative Ca(2+) sensors. We find that despite the multitude of proposed sensors, only three are well established in STP: Munc13, protein kinase C (PKC) and synaptotagmin-7. For putative sensors, we pinpoint open questions and potential pitfalls. Finally, we discuss how the molecular properties and modes of action of Ca(2+) sensors can explain their differential involvement in STP and shape net synaptic output.

Synaptic gain-of-function effects of mutant Cav2.1 channels in a mouse model of familial hemiplegic migraine are due to increased basal [Ca2+]i

Specific missense mutations in the CACNA1A gene, which encodes a subunit of voltage-gated CaV2.1 channels, are associated with familial hemiplegic migraine type 1 (FHM1), a rare monogenic subtype of common migraine with aura. We used transgenic knock-in (KI) mice harboring the human pathogenic FHM1 mutation S218L to study presynaptic Ca(2+) currents, EPSCs, and in vivo activity at the calyx of Held synapse. Whole-cell patch-clamp recordings of presynaptic terminals from S218L KI mice showed a strong shift of the calcium current I-V curve to more negative potentials, leading to an increase in basal [Ca(2+)]i, increased levels of spontaneous transmitter release, faster recovery from synaptic depression, and enhanced synaptic strength despite smaller action-potential-elicited Ca(2+) currents. The gain-of-function of transmitter release of the S218L mutant was reproduced in vivo, including evidence for an increased release probability, demonstrating its relevance for glutamatergic transmission. This synaptic phenotype may explain the misbalance between excitation and inhibition in neuronal circuits resulting in a persistent hyperexcitability state and other migraine-relevant mechanisms such as an increased susceptibility to cortical spreading depression.

Calcium-dependent PKC isoforms have specialized roles in short-term synaptic plasticity

Posttetanic potentiation (PTP) is a widely observed form of short-term plasticity lasting for tens of seconds after high-frequency stimulation. Here we show that although protein kinase C (PKC) mediates PTP at the calyx of Held synapse in the auditory brainstem before and after hearing onset, PTP is produced primarily by an increased probability of release (p) before hearing onset, and by an increased readily releasable pool of vesicles (RRP) thereafter. We find that these mechanistic differences, which have distinct functional consequences, reflect unexpected differential actions of closely related calcium-dependent PKC isoforms. Prior to hearing onset, when PKCγ and PKCβ are both present, PKCγ mediates PTP by increasing p and partially suppressing PKCβ actions. After hearing onset, PKCγ is absent and PKCβ produces PTP by increasing RRP. In hearing animals, virally expressed PKCγ overrides PKCβ to produce PTP by increasing p. Thus, two similar PKC isoforms mediate PTP in distinctly different ways.

Myosin light chain kinase accelerates vesicle endocytosis at the calyx of Held synapse

Neuronal activity triggers endocytosis at synaptic terminals to retrieve efficiently the exocytosed vesicle membrane, ensuring the membrane homeostasis of active zones and the continuous supply of releasable vesicles. The kinetics of endocytosis depends on Ca(2+) and calmodulin which, as a versatile signal pathway, can activate a broad spectrum of downstream targets, including myosin light chain kinase (MLCK). MLCK is known to regulate vesicle trafficking and synaptic transmission, but whether this kinase regulates vesicle endocytosis at synapses remains elusive. We investigated this issue at the rat calyx of Held synapse, where previous studies using whole-cell membrane capacitance measurement have characterized two common forms of Ca(2+)/calmodulin-dependent endocytosis, i.e., slow clathrin-dependent endocytosis and rapid endocytosis. Acute inhibition of MLCK with pharmacological agents was found to slow down the kinetics of both slow and rapid forms of endocytosis at calyces. Similar impairment of endocytosis occurred when blocking myosin II, a motor protein that can be phosphorylated upon MLCK activation. The inhibition of endocytosis was not accompanied by a change in Ca(2+) channel current. Combined inhibition of MLCK and calmodulin did not induce synergistic inhibition of endocytosis. Together, our results suggest that activation of MLCK accelerates both slow and rapid forms of vesicle endocytosis at nerve terminals, likely by functioning downstream of Ca(2+)/calmodulin.

Munc18-1 is a dynamically regulated PKC target during short-term enhancement of transmitter release

Transmitter release at synapses is regulated by preceding neuronal activity, which can give rise to short-term enhancement of release like post-tetanic potentiation (PTP). Diacylglycerol (DAG) and Protein-kinase C (PKC) signaling in the nerve terminal have been widely implicated in the short-term modulation of transmitter release, but the target protein of PKC phosphorylation during short-term enhancement has remained unknown. Here, we use a gene-replacement strategy at the calyx of Held, a large CNS model synapse that expresses robust PTP, to study the molecular mechanisms of PTP. We find that two PKC phosphorylation sites of Munc18-1 are critically important for PTP, which identifies the presynaptic target protein for the action of PKC during PTP. Pharmacological experiments show that a phosphatase normally limits the duration of PTP, and that PTP is initiated by the action of a ‘conventional’ PKC isoform. Thus, a dynamic PKC phosphorylation/de-phosphorylation cycle of Munc18-1 drives short-term enhancement of transmitter release during PTP.

DOI:http://dx.doi.org/10.7554/eLife.01715.001

Brain function depends on the rapid transfer of information from one brain cell to the next at junctions known as synapses. Small packages called vesicles play an important role in this process. The arrival of an electrical action potential at the nerve terminal of the first cell causes some vesicles in the cell to fuse with the cell membrane, and this leads to the neurotransmitters inside the vesicles being released into the synapse. The neurotransmitters then bind to receptors on the second cell, which leads to an electrical signal in the second cell. A protein called Munc18-1 has a central role in the fusion of the vesicle at the cell membrane.

The strength of a synapse—that is, how easily the first brain cell can impact the electrical behaviour of the second—can change, and this ‘synaptic plasticity’ is thought to underlie learning and memory. Long-term changes in synaptic strength require additional receptors to be inserted into the membrane of the second cell. However, synapses can also be temporarily strengthened: the arrival of a burst of action potentials—a tetanus—causes some synapses to increase the amount of neurotransmitter they release in response to any subsequent, single, action potential.

This temporary increase in synaptic strength, which is known as post-tetanic potentiation, requires an enzyme called protein kinase C; the role of this enzyme is to phosphorylate specific target proteins (i.e., to add phosphate groups to them). Now, Genç et al. have genetically modified a mouse synapse in vivo and shown that protein kinase C brings about post-tetanic potentiation by phosphorylating Munc18-1. Furthermore, pharmacological experiments show that proteins called phosphatases, which de-phosphorylate proteins, normally terminate the post-tetanic potentiation after about one minute. Taken together, the study identifies a target protein which is phosphorylated by protein kinase C during post-tetanic potentiation. The study also suggests that in addition to its fundamental role in vesicle fusion, the phosphorylation state of Munc18-1 can change the probability of vesicle fusion in a more subtle way, thereby contributing to synaptic plasticity.

DOI:http://dx.doi.org/10.7554/eLife.01715.002

Soluble pathological tau in the entorhinal cortex leads to presynaptic deficits in an early Alzheimer's disease model

Neurofibrillary tangles (NFTs), a hallmark of Alzheimer's disease, are intracellular silver and thioflavin S-staining aggregates that emerge from earlier accumulation of phospho-tau in the soma. Whether soluble misfolded but nonfibrillar tau disrupts neuronal function is unclear. Here we investigate if soluble pathological tau, specifically directed to the entorhinal cortex (EC), can cause behavioral or synaptic deficits. We studied rTgTauEC transgenic mice, in which P301L mutant human tau overexpressed primarily in the EC leads to the development of tau pathology, but only rare NFT at 16 months of age. We show that the early tau lesions are associated with nearly normal performance in contextual fear conditioning, a hippocampal-related behavior task, but more robust changes in neuronal system activation as marked by Arc induction and clear electrophysiological defects in perforant pathway synaptic plasticity. Electrophysiological changes were likely due to a presynaptic deficit and changes in probability of neurotransmitter release. The data presented here support the hypothesis that misfolded and hyperphosphorylated tau can impair neuronal function within the entorhinal-hippocampal network, even prior to frank NFT formation and overt neurodegeneration.

Transients in global Ca2+ concentration induced by electrical activity in a giant nerve terminal

Giant nerve terminals offer a unique opportunity to learn about dynamic changes in intracellular global Ca(2+) concentration ([Ca(2+)]i) because this quantity can be measured precisely with indicator dyes and the composition of the intra-terminal ionic milieu can be controlled. We review here recent literature on [Ca(2+)]i signalling in the calyx of Held and discuss what these measurements can tell us about endogenous Ca(2+) buffers and Ca(2+) extrusion mechanisms. We conclude that in spite of the favourable experimental conditions, some unresolved questions still remain regarding absolute values for the Ca(2+)-binding ratio, the affinity of the basic fixed buffer and the Ca(2+) affinities of the major endogenous Ca(2+) binding proteins. Uncertainties about some of these presynaptic properties, including the roles of Mg(2+) and ATP (as a Mg(2+) buffer), however, extend to the point that mechanisms controlling the decay of [Ca(2+)]i signals in unperturbed terminals may have to be reconsidered.

Endocytosis of somatodendritic NCKX2 is regulated by Src family kinase-dependent tyrosine phosphorylation

We have previously reported that the surface expression of K⁺-dependent Na⁺/Ca²⁺ exchanger 2 (NCKX2) in the somatodendritic compartment is kept low by constitutive endocytosis, which results in the polarization of surface NCKX2 to the axon. Clathrin-mediated endocytosis is initiated by interaction of the μ subunit of adaptor protein complex 2 (AP-2) with the canonical tyrosine motif (YxxΦ) of a target molecule. We examined whether endocytosis of NCKX2 involves two putative tyrosine motifs (³⁶⁵YGKL and ³⁷¹YDTM) in the cytoplasmic loop of NCKX2. Coimmunoprecipitation assay revealed that the ³⁶⁵YGKL motif is essential for the interaction with the μ subunit of AP-2 (AP2M1). Consistently, either overexpression of NCKX2-Y365A mutant or knockdown of AP2M1 in cultured hippocampal neurons significantly reduced the internalization of NCKX2 from the somatodendritic surface and thus abolished the axonal polarization of surface NCKX2. Next, we tested whether the interaction between the tyrosine motif and AP2M1 is regulated by phosphorylation of the 365th tyrosine residue (Tyr-365). Tyrosine phosphorylation of heterologously expressed NCKX2-WT, but not NCKX2-Y365A, was increased by carbachol (CCh) in PC-12 cells. The effect of CCh was inhibited by PP2, a Src family kinase (SFK) inhibitor. Moreover, PP2 facilitated the endocytosis of NCKX2 in both the somatodendritic and axonal compartments, suggesting that tyrosine phosphorylation of NCKX2 by SFK negatively regulates its endocytosis. Supporting this idea, activation of SFK enhanced the NCKX activity in the proximal dendrites of dentate granule cells (GCs). These results suggest that endocytosis of somatodendritic NCKX2 is regulated by SFK-dependent phosphorylation of Tyr-365.

Developmental upregulation of presynaptic NCKX underlies the decrease of mitochondria-dependent posttetanic potentiation at the rat calyx of Held synapse

The sensitivity of posttetanic potentiation (PTP) to high-frequency stimulation (HFS) steeply decays during the first 2 postnatal weeks. We investigated the underlying mechanisms for the developmental change of PTP induced by HFS (100 Hz, 2 s) at postnatal days 4-6 and 9-11 at the rat calyx of Held synapse. Low-concentration tetraphenylphosphonium (2 μM), an inhibitor of mitochondrial Na(+)/Ca(2+) exchanger, suppressed the amount of posttetanic residual Ca(2+) and PTP to a larger extent at the immature calyx synapse, indicating a developmental reduction of mitochondrial contribution to PTP. The higher amount of mitochondrial Ca(2+) uptake during HFS and consequent posttetanic residual Ca(2+) at the immature calyx of Held was associated with higher peak of HFS-induced Ca(2+) transients, most likely because the mitochondrial Ca(2+) uptake during HFS was supralinearly dependent on the presynaptic resting Ca(2+) level. Probing into the contribution of Na(+)/Ca(2+) exchangers to Ca(2+) clearance, we found a specific upregulation of the K(+)-dependent Na(+)/Ca(2+) exchanger (NCKX) activity in the mature calyx of Held. We conclude that the upregulation of NCKX limits the Ca(2+) buildup and inhibits mitochondrial Ca(2+) uptake during HFS, which in turn results in the reduction of posttetanic residual Ca(2+) and PTP at the mature calyx of Held synapse.

Adaptive regulation maintains posttetanic potentiation at cerebellar granule cell synapses in the absence of calcium-dependent PKC

Posttetanic potentiation (PTP) is a transient, calcium-dependent increase in the efficacy of synaptic transmission following elevated presynaptic activity. The calcium-dependent protein kinase C (PKC(Ca)) isoforms PKCα and PKCβ mediate PTP at the calyx of Held synapse, with PKCβ contributing significantly more than PKCα. It is not known whether PKC(Ca) isoforms play a conserved role in PTP at other synapses. We examined this question at the parallel fiber → Purkinje cell (PF→PC) synapse, where PKC inhibitors suppress PTP. We found that PTP is preserved when single PKC(Ca) isoforms are knocked out and in PKCα/β double knock-out (dko) mice, even though in the latter all PKC(Ca) isoforms are eliminated from granule cells. However, in contrast to wild-type and single knock-out animals, PTP in PKCα/β dko animals is not suppressed by PKC inhibitors. These results indicate that PKC(Ca) isoforms mediate PTP at the PF→PC synapse in wild-type and single knock-out animals. However, unlike the calyx of Held, at the PF→PC synapse either PKCα or PKCβ alone is sufficient to mediate PTP, and if both isoforms are eliminated a compensatory PKC-independent mechanism preserves the plasticity. These results suggest that a feedback mechanism allows granule cells to maintain the normal properties of short-term synaptic plasticity even when the mechanism that mediates PTP in wild-type mice is eliminated.

Upregulation of transmitter release probability improves a conversion of synaptic analogue signals into neuronal digital spikes

Action potentials at the neurons and graded signals at the synapses are primary codes in the brain. In terms of their functional interaction, the studies were focused on the influence of presynaptic spike patterns on synaptic activities. How the synapse dynamics quantitatively regulates the encoding of postsynaptic digital spikes remains unclear. We investigated this question at unitary glutamatergic synapses on cortical GABAergic neurons, especially the quantitative influences of release probability on synapse dynamics and neuronal encoding. Glutamate release probability and synaptic strength are proportionally upregulated by presynaptic sequential spikes. The upregulation of release probability and the efficiency of probability-driven synaptic facilitation are strengthened by elevating presynaptic spike frequency and Ca²⁺. The upregulation of release probability improves spike capacity and timing precision at postsynaptic neuron. These results suggest that the upregulation of presynaptic glutamate release facilitates a conversion of synaptic analogue signals into digital spikes in postsynaptic neurons, i.e., a functional compatibility between presynaptic and postsynaptic partners.

Short-term presynaptic plasticity

Different types of synapses are specialized to interpret spike trains in their own way by virtue of the complement of short-term synaptic plasticity mechanisms they possess. Numerous types of short-term, use-dependent synaptic plasticity regulate neurotransmitter release. Short-term depression is prominent after a single conditioning stimulus and recovers in seconds. Sustained presynaptic activation can result in more profound depression that recovers more slowly. An enhancement of release known as facilitation is prominent after single conditioning stimuli and lasts for hundreds of milliseconds. Finally, tetanic activation can enhance synaptic strength for tens of seconds to minutes through processes known as augmentation and posttetantic potentiation. Progress in clarifying the properties, mechanisms, and functional roles of these forms of short-term plasticity is reviewed here.

Site-specific synapsin I phosphorylation participates in the expression of post-tetanic potentiation and its enhancement by BDNF

A large amount of experimental evidence has highlighted the rapid changes in synaptic efficacy induced by high-frequency stimulation and BDNF at central excitatory synapses. We clarified the quantal mechanisms and the involvement of Synapsin I (SynI) phosphorylation in the expression of post-tetanic potentiation (PTP) and in its modulation by BDNF in mouse glutamatergic autapses. We found that PTP is associated with an elevation in the probability of release and a concomitant increase in the size of the readily releasable pool (RRP). The latter component was virtually absent in SynI knock-out (KO) neurons, which indeed displayed impaired PTP. PTP was fully rescued by the expression of wild-type SynI, but not of its dephosphomimetic mutants in the phosphorylation sites for cAMP-dependent protein kinase and Ca²⁺/calmodulin-dependent protein kinases I/II. BDNF potently enhanced PTP through a further increase in the RRP size, which was missing in SynI KO neurons. In these neurons, the BDNF-induced PTP enhancement was rescued by the expression of wild-type SynI, but not of its dephosphomimetic mutant at the mitogen-dependent protein kinase sites. The results indicate that the increase in RRP size necessary for the full expression of PTP, and its sensitivity to BDNF, involve phosphorylation of SynI at distinct sites, thus implicating SynI as an essential downstream effector for the expression of PTP and for its enhancement by BDNF.

Impaired short-term plasticity in mossy fiber synapses caused by mitochondrial dysfunction of dentate granule cells is the earliest synaptic deficit in a mouse model of Alzheimer's disease

Alzheimer's disease (AD) in the early stages is characterized by memory impairment, which may be attributable to synaptic dysfunction. Oxidative stress, mitochondrial dysfunction, and Ca²⁺ dysregulation are key factors in the pathogenesis of AD, but the causal relationship between these factors and synaptic dysfunction is not clearly understood. We found that in the hippocampus of an AD mouse model (Tg2576), mitochondrial Ca²⁺ handling in dentate granule cells was impaired as early as the second postnatal month, and this Ca²⁺ dysregulation caused an impairment of post-tetanic potentiation in mossy fiber-CA3 synapses. The alteration of cellular Ca²⁺ clearance in Tg2576 mice is region-specific within hippocampus because in another region, CA1 pyramidal neuron, no significant difference in Ca²⁺ clearance was detected between wild-type and Tg2576 mice at this early stage. Impairment of mitochondrial Ca²⁺ uptake was associated with increased mitochondrial reactive oxygen species and depolarization of mitochondrial membrane potential. Mitochondrial dysfunctions in dentate granule cells and impairment of post-tetanic potentiation in mossy fiber-CA3 synapses were fully restored when brain slices obtained from Tg2576 were pretreated with antioxidant, suggesting that mitochondrial oxidative stress initiates other dysfunctions. Reversibility of early dysfunctions by antioxidants at the preclinical stage of AD highlights the importance of early diagnosis and antioxidant therapy to delay or prevent the disease processes.

KIF21A-mediated axonal transport and selective endocytosis underlie the polarized targeting of NCKX2

We have previously shown that K(+)-dependent Na(+)/Ca(2+) exchanger (NCKX) is a major calcium clearance mechanism at the large axon terminals of central neurons, whereas their somata display little NCKX activity. We investigated mechanisms underlying the axonal polarization of NCKX2 in rat hippocampal neurons. We identified NCKX2 as the first neuron-specific cargo molecule of kinesin family member 21A (KIF21A). The intracellular loop of NCKX2 specifically interacted with the WD-40 repeats, a putative cargo-binding domain, of KIF21A. Dominant-negative mutant or depletion of KIF21A inhibited the transport of NCKX2-GFP to axon fibers. Knockdown of KIF21A caused calcium dysregulation at axonal boutons but not at somatodendritic regions. Despite the axonal polarization of the NCKX activity, both somatodendritic and axonal regions were immunoreactive to NCKX2. The surface expression of NCKX2 revealed by live-cell immunocytochemistry, however, displayed highly polarized distribution to the axon. Inhibition of endocytosis increased the somatodendritic surface NCKX2 and thus abolished the axonal polarization of surface NCKX2. These results indicate that KIF21A-mediated axonal transport and selective somatodendritic endocytosis underlie the axonal polarized surface expression of NCKX2.

Actin-dependent rapid recruitment of reluctant synaptic vesicles into a fast-releasing vesicle pool

Glutamatergic synaptic terminals harbor reluctant synaptic vesicles (SVs) that contribute little to synchronous release during action potentials but are release competent when stimulated by sucrose or by direct intracellular application of calcium. It has been noted that the proximity of a release-competent SV to the calcium source is one of the primary factors that differentiate reluctant SVs from fast-releasing ones at the calyx of Held synapse. It has not been known whether reluctant SVs can be converted into fast-releasing ones. Here we show that reluctant SVs are recruited rapidly in an actin-dependent manner to become fast-releasing SVs once the pool of fast-releasing SVs is depleted by a short depolarization. Recovery of the pool of fast-releasing SVs was accompanied by a parallel reduction in the number of reluctant SVs. Quantitative analysis of the time course of depletion of fast-releasing SVs during high-frequency stimulation revealed that in the early phase of stimulation reluctant SVs are converted rapidly into fast-releasing ones, thereby counteracting short-term depression. During the late phase, however, after reluctant vesicles have been used up, another process of calmodulin-dependent recruitment of fast-releasing SVs is activated. These results document that reluctant SVs have a role in short-term plasticity and support the hypothesis of positional priming, which posits that reluctant vesicles are converted into fast-releasing ones via relocation closer to Ca(2+)-channels.

Inhibition of presynaptic Na(+)/K(+)-ATPase reduces readily releasable pool size at the avian end-bulb of Held synapse

A glutamatergic end-bulb synapse in the avian nucleus magnocellularis relays temporal sound information from the auditory nerve. Here, we show that presynaptic Na(+)/K(+)-ATPase (NKA) activity at this synapse contributes to the maintenance of the readily releasable pool (RRP) of vesicles, thereby preserving synaptic strength. Whole-cell voltage clamp recordings were made from chick brainstem slices to examine the effects of NKA blocker dihydroouabain (DHO) on synaptic transmission. DHO suppressed the amplitude of EPSCs in a dose-dependent manner. This suppression was caused by a decrease in the number of neurotransmitter quanta released because DHO increased the coefficient of variation of EPSC amplitude and reduced the frequency but not the amplitude of miniature EPSCs. Cumulative plots of EPSC amplitude during a stimulus train revealed that DHO reduced the RRP size without affecting vesicular release probability. DHO did not affect [Ca(2+)](i)-dependent processes, such as the paired-pulse ratio or recovery time course from the paired-pulse depression, suggesting a minimal effect on Ca(2+) concentration in the presynaptic terminal. Using mathematical models of synaptic depression, we further demonstrated the contribution of RRP size to the synaptic strength during a high-frequency stimulus train to highlight the importance of presynaptic NKA in the auditory synapse.

The calyx of Held synapse: from model synapse to auditory relay

The calyx of Held is an axosomatic terminal in the auditory brainstem that has attracted anatomists because of its giant size and physiologists because of its accessibility to patch-clamp recordings. The calyx allows the principal neurons in the medial nucleus of the trapezoid body (MNTB) to provide inhibition that is both well timed and sustained to many other auditory nuclei. The special adaptations that allow the calyx to drive its principal neuron even when frequencies are high include a large number of release sites with low release probability, a large readily releasable pool, fast presynaptic calcium clearance and little delayed release, a large quantal size, and fast AMPA-type glutamate receptors. The transformation from a synapse that is unremarkable except for its giant size into a fast and reliable auditory relay happens in just a few days. In rodents this transformation is essentially ready when hearing starts.