Rapid Ca2+ channel accumulation contributes to cAMP-mediated increase in transmission at hippocampal mossy fiber synapses

Short term plasticity at hippocampal mossy fiber synapses

Short-term synaptic plasticity refers to the regulation of synapses by their past activity on time scales of milliseconds to minutes. Hippocampal mossy fiber synapses onto CA3 pyramidal cells (Mf-CA3 synapses) are endowed with remarkable forms of short-term synaptic plasticity expressed as facilitation of synaptic release by a factor of up to ten-fold. Three main forms of short-term plasticity are distinguished: 1) Frequency facilitation, which includes low frequency facilitation and train facilitation, operating in the range of tens of milliseconds to several seconds; 2) Post-tetanic potentiation triggered by trains of high frequency stimulation, which lasts several minutes; 3) Finally, depolarization-induced potentiation of excitation, based on retrograde signaling, with an onset and offset of several minutes. Here we describe the proposed mechanisms for short-term plasticity, mainly based on the kinetics of presynaptic Ca2+ transients and the Ca2+ sensor synaptotagmin 7, on cAMP-dependent mechanisms and readily releasable vesicle pool, and on the regulation of presynaptic K+ channels. We then review evidence for a physiological function of short-term plasticity of Mf-CA3 synapses in information transfer between the dentate gyrus and CA3 in conditions of natural spiking, and discuss how short-term plasticity counteracts robust feedforward inhibition in a frequency-dependent manner. Although DG-CA3 connections have long been proposed to play a role in memory, direct evidence for an implication of short-term plasticity at Mf-CA3 synapses is mostly lacking. The mechanistic knowledge gained on short-term plasticity at Mf-CA3 synapses should help in designing future experiments to directly test how this evolutionary conserved feature controls hippocampal circuit function in behavioural conditions.

Graded control of Purkinje cell outputs by cAMP through opposing actions on axonal action potential and transmitter release

All-or-none signalling by action potentials (APs) in neuronal axons is pivotal for the precisely timed and identical size of outputs to multiple distant targets. However, technical limitations with respect to measuring the signalling in small intact axons have hindered the evaluation of high-fidelity signal propagation. Here, using direct recordings from axonal trunks and/or terminals of cerebellar Purkinje cells in slice and culture, we demonstrate that the timing and amplitude of axonal outputs are gradually modulated by cAMP depending on the length of axon. During the propagation in long axon, APs were attenuated and slowed in conduction by cAMP via specifically decreasing axonal Na+ currents. Consequently, the Ca2+ influx and transmitter release at distal boutons are reduced by cAMP, counteracting its direct facilitating effect on release machinery as observed at various CNS synapses. Together, our tour de force functional dissection has unveiled the axonal distance-dependent graded control of output timing and strength by intracellular signalling. KEY POINTS: The information processing in the nervous system has been classically thought to rely on the axonal faithful and high-speed conduction of action potentials (APs). We demonstrate that the strength and timing of axonal outputs are weakened and delayed, respectively, by cytoplasmic cAMP depending on the axonal length in cerebellar Purkinje cells (PCs). Direct axonal patch clamp recordings uncovered axon-specific attenuation of APs by cAMP through reduction of axonal Na+ currents. cAMP directly augments transmitter release at PC terminals without changing presynaptic Ca2+ influx or readily releasable pool of vesicles, although the extent is weaker compared to other CNS synapses. Two opposite actions of cAMP on PC axons, AP attenuation and release augmentation, together give rise to graded control of synaptic outputs in a manner dependent on the axonal length.

Structure, biophysics, and circuit function of a "giant" cortical presynaptic terminal

The hippocampal mossy fiber synapse, formed between axons of dentate gyrus granule cells and dendrites of CA3 pyramidal neurons, is a key synapse in the trisynaptic circuitry of the hippocampus. Because of its comparatively large size, this synapse is accessible to direct presynaptic recording, allowing a rigorous investigation of the biophysical mechanisms of synaptic transmission and plasticity. Furthermore, because of its placement in the very center of the hippocampal memory circuit, this synapse seems to be critically involved in several higher network functions, such as learning, memory, pattern separation, and pattern completion. Recent work based on new technologies in both nanoanatomy and nanophysiology, including presynaptic patch-clamp recording, paired recording, super-resolution light microscopy, and freeze-fracture and "flash-and-freeze" electron microscopy, has provided new insights into the structure, biophysics, and network function of this intriguing synapse. This brings us one step closer to answering a fundamental question in neuroscience: how basic synaptic properties shape higher network computations.

Regulation of hippocampal mossy fiber-CA3 synapse function by a Bcl11b/C1ql2/Nrxn3(25b+) pathway

The transcription factor Bcl11b has been linked to neurodevelopmental and neuropsychiatric disorders associated with synaptic dysfunction. Bcl11b is highly expressed in dentate gyrus granule neurons and is required for the structural and functional integrity of mossy fiber-CA3 synapses. The underlying molecular mechanisms, however, remained unclear. We show in mice that the synaptic organizer molecule C1ql2 is a direct functional target of Bcl11b that regulates synaptic vesicle recruitment and long-term potentiation at mossy fiber-CA3 synapses in vivo and in vitro. Furthermore, we demonstrate C1ql2 to exert its functions through direct interaction with a specific splice variant of neurexin-3, Nrxn3(25b+). Interruption of C1ql2-Nrxn3(25b+) interaction by expression of a non-binding C1ql2 mutant or by deletion of Nrxn3 in the dentate gyrus granule neurons recapitulates major parts of the Bcl11b as well as C1ql2 mutant phenotype. Together, this study identifies a novel C1ql2-Nrxn3(25b+)-dependent signaling pathway through which Bcl11b controls mossy fiber-CA3 synapse function. Thus, our findings contribute to the mechanistic understanding of neurodevelopmental disorders accompanied by synaptic dysfunction.

Fully-primed slowly-recovering vesicles mediate presynaptic LTP at neocortical neurons

Pre- and postsynaptic forms of long-term potentiation (LTP) are candidate synaptic mechanisms underlying learning and memory. At layer 5 pyramidal neurons, LTP increases the initial synaptic strength but also short-term depression during high-frequency transmission. This classical form of presynaptic LTP has been referred to as redistribution of synaptic efficacy. However, the underlying mechanisms remain unclear. We therefore performed whole-cell recordings from layer 5 pyramidal neurons in acute cortical slices of rats and analyzed presynaptic function before and after LTP induction by paired pre- and postsynaptic neuronal activity. LTP was successfully induced in about half of the synaptic connections tested and resulted in increased synaptic short-term depression during high-frequency transmission and a decelerated recovery from short-term depression due to an increased fraction of a slow recovery component. Analysis with a recently established sequential two-step vesicle priming model indicates an increase in the abundance of fully-primed and slowly-recovering vesicles. A systematic analysis of short-term plasticity and synapse-to-synapse variability of synaptic strength at various types of synapses revealed that stronger synapses generally recover more slowly from synaptic short-term depression. Finally, pharmacological stimulation of the cyclic adenosine monophosphate and diacylglycerol signaling pathways, which are both known to promote synaptic vesicle priming, mimicked LTP and slowed the recovery from short-term depression. Our data thus demonstrate that LTP at layer 5 pyramidal neurons increases synaptic strength primarily by enlarging a subpool of fully-primed slowly-recovering vesicles.

An antagonism between Spinophilin and Syd-1 operates upstream of memory-promoting presynaptic long-term plasticity

We still face fundamental gaps in understanding how molecular plastic changes of synapses intersect with circuit operation to define behavioral states. Here, we show that an antagonism between two conserved regulatory proteins, Spinophilin (Spn) and Syd-1, controls presynaptic long-term plasticity and the maintenance of olfactory memories in Drosophila. While Spn mutants could not trigger nanoscopic active zone remodeling under homeostatic challenge and failed to stably potentiate neurotransmitter release, concomitant reduction of Syd-1 rescued all these deficits. The Spn/Syd-1 antagonism converged on active zone close F-actin, and genetic or acute pharmacological depolymerization of F-actin rescued the Spn deficits by allowing access to synaptic vesicle release sites. Within the intrinsic mushroom body neurons, the Spn/Syd-1 antagonism specifically controlled olfactory memory stabilization but not initial learning. Thus, this evolutionarily conserved protein complex controls behaviorally relevant presynaptic long-term plasticity, also observed in the mammalian brain but still enigmatic concerning its molecular mechanisms and behavioral relevance.

Mechanistic insights into cAMP-mediated presynaptic potentiation at hippocampal mossy fiber synapses

Presynaptic plasticity is an activity-dependent change in the neurotransmitter release and plays a key role in dynamic modulation of synaptic strength. Particularly, presynaptic potentiation mediated by cyclic adenosine monophosphate (cAMP) is widely seen across the animals and thought to contribute to learning and memory. Hippocampal mossy fiber-CA3 pyramidal cell synapses have been used as a model because of robust presynaptic potentiation in short- and long-term forms. Moreover, direct presynaptic recordings from large mossy fiber terminals allow one to dissect the potentiation mechanisms. Recently, super-resolution microscopy and flash-and-freeze electron microscopy have revealed the localizations of release site molecules and synaptic vesicles during the potentiation at a nanoscale, identifying the molecular mechanisms of the potentiation. Incorporating these growing knowledges, we try to present plausible mechanisms underlying the cAMP-mediated presynaptic potentiation.

Increased vesicle fusion competence underlies long-term potentiation at hippocampal mossy fiber synapses

Presynaptic long-term potentiation (LTP) is thought to play an important role in learning and memory. However, the underlying mechanism remains elusive because of the difficulty of direct recording during LTP. Hippocampal mossy fiber synapses exhibit pronounced LTP of transmitter release after tetanic stimulation and have been used as a model of presynaptic LTP. Here, we induced LTP by optogenetic tools and applied direct presynaptic patch-clamp recordings. The action potential waveform and evoked presynaptic Ca²⁺ currents remained unchanged after LTP induction. Membrane capacitance measurements suggested higher release probability of synaptic vesicles without changing the number of release-ready vesicles after LTP induction. Synaptic vesicle replenishment was also enhanced. Furthermore, stimulated emission depletion microscopy suggested an increase in the numbers of Munc13-1 and RIM1 molecules within active zones. We propose that dynamic changes in the active zone components may be relevant for the increased fusion competence and synaptic vesicle replenishment during LTP.

Interactive nanocluster compaction of the ELKS scaffold and Cacophony Ca2+ channels drives sustained active zone potentiation

At presynaptic active zones (AZs), conserved scaffold protein architectures control synaptic vesicle (SV) release by defining the nanoscale distribution and density of voltage-gated Ca2+ channels (VGCCs). While AZs can potentiate SV release in the minutes range, we lack an understanding of how AZ scaffold components and VGCCs engage into potentiation. We here establish dynamic, intravital single-molecule imaging of endogenously tagged proteins at Drosophila AZs undergoing presynaptic homeostatic potentiation. During potentiation, the numbers of α1 VGCC subunit Cacophony (Cac) increased per AZ, while their mobility decreased and nanoscale distribution compacted. These dynamic Cac changes depended on the interaction between Cac channel's intracellular carboxyl terminus and the membrane-close amino-terminal region of the ELKS-family protein Bruchpilot, whose distribution compacted drastically. The Cac-ELKS/Bruchpilot interaction was also needed for sustained AZ potentiation. Our single-molecule analysis illustrates how the AZ scaffold couples to VGCC nanoscale distribution and dynamics to establish a state of sustained potentiation.

Developmental and activity-dependent modulation of coupling distance between release site and Ca2+ channel

Synapses are junctions between a presynaptic neuron and a postsynaptic cell specialized for fast and precise information transfer. The presynaptic terminal secretes neurotransmitters via exocytosis of synaptic vesicles. Exocytosis is a tightly regulated reaction that occurs within a millisecond of the arrival of an action potential. One crucial parameter in determining the characteristics of the transmitter release kinetics is the coupling distance between the release site and the Ca2+ channel. Still, the technical limitations have hindered detailed analysis from addressing how the coupling distance is regulated depending on the development or activity of the synapse. However, recent technical advances in electrophysiology and imaging are unveiling their different configurations in different conditions. Here, I will summarize developmental- and activity-dependent changes in the coupling distances revealed by recent studies.

Mechanisms of Synaptic Vesicle Exo- and Endocytosis

Within 1 millisecond of action potential arrival at presynaptic terminals voltage-gated Ca2+ channels open. The Ca2+ channels are linked to synaptic vesicles which are tethered by active zone proteins. Ca2+ entrance into the active zone triggers: (1) the fusion of the vesicle and exocytosis, (2) the replenishment of the active zone with vesicles for incoming exocytosis, and (3) various types of endocytosis for vesicle reuse, dependent on the pattern of firing. These time-dependent vesicle dynamics are controlled by presynaptic Ca2+ sensor proteins, regulating active zone scaffold proteins, fusion machinery proteins, motor proteins, endocytic proteins, several enzymes, and even Ca2+ channels, following the decay of Ca2+ concentration after the action potential. Here, I summarize the Ca2+-dependent protein controls of synchronous and asynchronous vesicle release, rapid replenishment of the active zone, endocytosis, and short-term plasticity within 100 msec after the action potential. Furthermore, I discuss the contribution of active zone proteins to presynaptic plasticity and to homeostatic readjustment during and after intense activity, in addition to activity-dependent endocytosis.

cAMP-Dependent Synaptic Plasticity at the Hippocampal Mossy Fiber Terminal

Cyclic adenosine monophosphate (cAMP) is a crucial second messenger involved in both pre- and postsynaptic plasticity in many neuronal types across species. In the hippocampal mossy fiber (MF) synapse, cAMP mediates presynaptic long-term potentiation and depression. The main cAMP-dependent signaling pathway linked to MF synaptic plasticity acts via the activation of the protein kinase A (PKA) molecular cascade. Accordingly, various downstream putative synaptic PKA target proteins have been linked to cAMP-dependent MF synaptic plasticity, such as synapsin, rabphilin, synaptotagmin-12, RIM1a, tomosyn, and P/Q-type calcium channels. Regulating the expression of some of these proteins alters synaptic release probability and calcium channel clustering, resulting in short- and long-term changes to synaptic efficacy. However, despite decades of research, the exact molecular mechanisms by which cAMP and PKA exert their influences in MF terminals remain largely unknown. Here, we review current knowledge of different cAMP catalysts and potential downstream PKA-dependent molecular cascades, in addition to non-canonical cAMP-dependent but PKA-independent cascades, which might serve as alternative, compensatory or competing pathways to the canonical PKA cascade. Since several other central synapses share a similar form of presynaptic plasticity with the MF, a better description of the molecular mechanisms governing MF plasticity could be key to understanding the relationship between the transcriptional and computational levels across brain regions.

Laser microscopy acquisition and analysis of premotor synapses in the murine spinal cord

Loss of synapses on spinal motor neurons is a major feature of several neurodegenerative diseases; however, analyzing these premotor synapses is challenging because of their small size and high density. This protocol describes confocal and Stimulated Emission Depletion (STED) imaging of murine spinal premotor synapses and their segment-specific quantification by confocal microscopy. We detail the preparation of spinal cord segments, followed by image acquisition and analysis. This protocol enables in-depth analysis of pathological changes in spinal premotor synapses during neurodegeneration.

For complete details on the use and execution of this protocol, please refer to Buettner et al. (2021).

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Dissection and sectioning of identified murine spinal cord segments

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Fluorescent labeling of spinal premotor synapses and motor neurons

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Confocal and super-resolution acquisition of individual premotor synapses

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Quantification of premotor synapses onto motor neurons

Long-term in vivo application of a potassium channel-based optogenetic silencer in the healthy and epileptic mouse hippocampus

BACKGROUND

Optogenetic tools allow precise manipulation of neuronal activity via genetically encoded light-sensitive proteins. Currently available optogenetic inhibitors are not suitable for prolonged use due to short-lasting photocurrents, tissue heating, and unintended changes in ion distributions, which may interfere with normal neuron physiology. To overcome these limitations, a novel potassium channel-based optogenetic silencer, named PACK, was recently developed. The PACK tool has two components: a photoactivated adenylyl cyclase from Beggiatoa (bPAC) and a cAMP-dependent potassium channel, SthK, which carries a large, long-lasting potassium current in mammalian cells. Previously, it has been shown that activating the PACK silencer with short light pulses led to a significant reduction of neuronal firing in various in vitro and acute in vivo settings. Here, we examined the viability of performing long-term studies in vivo by looking at the inhibitory action and side effects of PACK and its components in healthy and epileptic adult male mice.

RESULTS

We targeted hippocampal cornu ammonis (CA1) pyramidal cells using a viral vector and enabled illumination of these neurons via an implanted optic fiber. Local field potential (LFP) recordings from CA1 of freely moving mice revealed significantly reduced neuronal activity during 50-min intermittent (0.1 Hz) illumination, especially in the gamma frequency range. Adversely, PACK expression in healthy mice induced chronic astrogliosis, dispersion of pyramidal cells, and generalized seizures. These side effects were independent of the light application and were also present in mice expressing bPAC without the potassium channel. Light activation of bPAC alone increased neuronal activity, presumably via enhanced cAMP signaling. Furthermore, we applied bPAC and PACK in the contralateral hippocampus of chronically epileptic mice following a unilateral injection of intrahippocampal kainate. Unexpectedly, the expression of bPAC in the contralateral CA1 area was sufficient to prevent the spread of spontaneous epileptiform activity from the seizure focus to the contralateral hippocampus.

CONCLUSION

Our study highlights the PACK tool as a potent optogenetic inhibitor in vivo. However, further refinement of its light-sensitive domain is required to avoid unexpected physiological changes.

A theory of synaptic transmission

Rapid and precise neuronal communication is enabled through a highly synchronous release of signaling molecules neurotransmitters within just milliseconds of the action potential. Yet neurotransmitter release lacks a theoretical framework that is both phenomenologically accurate and mechanistically realistic. Here, we present an analytic theory of the action-potential-triggered neurotransmitter release at the chemical synapse. The theory is demonstrated to be in detailed quantitative agreement with existing data on a wide variety of synapses from electrophysiological recordings in vivo and fluorescence experiments in vitro. Despite up to ten orders of magnitude of variation in the release rates among the synapses, the theory reveals that synaptic transmission obeys a simple, universal scaling law, which we confirm through a collapse of the data from strikingly diverse synapses onto a single master curve. This universality is complemented by the capacity of the theory to readily extract, through a fit to the data, the kinetic and energetic parameters that uniquely identify each synapse. The theory provides a means to detect cooperativity among the SNARE complexes that mediate vesicle fusion and reveals such cooperativity in several existing data sets. The theory is further applied to establish connections between molecular constituents of synapses and synaptic function. The theory allows competing hypotheses of short-term plasticity to be tested and identifies the regimes where particular mechanisms of synaptic facilitation dominate or, conversely, fail to account for the existing data for the paired-pulse ratio. The derived trade-off relation between the transmission rate and fidelity shows how transmission failure can be controlled by changing the microscopic properties of the vesicle pool and SNARE complexes. The established condition for the maximal synaptic efficacy reveals that no fine tuning is needed for certain synapses to maintain near-optimal transmission. We discuss the limitations of the theory and propose possible routes to extend it. These results provide a quantitative basis for the notion that the molecular-level properties of synapses are crucial determinants of the computational and information-processing functions in synaptic transmission.

Calcium dependence of neurotransmitter release at a high fidelity synapse

The Ca²⁺-dependence of the priming, fusion, and replenishment of synaptic vesicles are fundamental parameters controlling neurotransmitter release and synaptic plasticity. Despite intense efforts, these important steps in the synaptic vesicles’ cycle remain poorly understood due to the technical challenge in disentangling vesicle priming, fusion, and replenishment. Here, we investigated the Ca²⁺-sensitivity of these steps at mossy fiber synapses in the rodent cerebellum, which are characterized by fast vesicle replenishment mediating high-frequency signaling. We found that the basal free Ca²⁺ concentration (<200 nM) critically controls action potential-evoked release, indicating a high-affinity Ca²⁺ sensor for vesicle priming. Ca²⁺ uncaging experiments revealed a surprisingly shallow and non-saturating relationship between release rate and intracellular Ca²⁺ concentration up to 50 μM. The rate of vesicle replenishment during sustained elevated intracellular Ca²⁺ concentration exhibited little Ca²⁺-dependence. Finally, quantitative mechanistic release schemes with five Ca²⁺ binding steps incorporating rapid vesicle replenishment via parallel or sequential vesicle pools could explain our data. We thus show that co-existing high- and low-affinity Ca²⁺ sensors mediate priming, fusion, and replenishment of synaptic vesicles at a high-fidelity synapse.

Prolonged development of long-term potentiation at lateral entorhinal cortex synapses onto adult-born neurons

Critical period plasticity at adult-born neuron synapses is widely believed to contribute to the learning and memory functions of the hippocampus. Experience regulates circuit integration and for a transient interval, until cells are ~6 weeks old, new neurons display enhanced long-term potentiation (LTP) at afferent and efferent synapses. Since neurogenesis declines substantially with age, this raises questions about the extent of lasting plasticity offered by adult-born neurons. Notably, however, the hippocampus receives sensory information from two major cortical pathways. Broadly speaking, the medial entorhinal cortex conveys spatial information to the hippocampus via the medial perforant path (MPP), and the lateral entorhinal cortex, via the lateral perforant path (LPP), codes for the cues and items that make experiences unique. While enhanced critical period plasticity at MPP synapses is relatively well characterized, no studies have examined long-term plasticity at LPP synapses onto adult-born neurons, even though the lateral entorhinal cortex is uniquely vulnerable to aging and Alzheimer's pathology. We therefore investigated LTP at LPP inputs both within (4-6 weeks) and beyond (8+ weeks) the traditional critical period. At immature stages, adult-born neurons did not undergo significant LTP at LPP synapses, and often displayed long-term depression after theta burst stimulation. However, over the course of 3-4 months, adult-born neurons displayed increasingly greater amounts of LTP. Analyses of short-term plasticity point towards a presynaptic mechanism, where transmitter release probability declines as cells mature, providing a greater dynamic range for strengthening synapses. Collectively, our findings identify a novel form of new neuron plasticity that develops over an extended interval, and may therefore be relevant for maintaining cognitive function in aging.