Developmental expression of Synaptotagmin isoforms in single calyx of Held-generating neurons

Complexin has a dual synaptic function as checkpoint protein in vesicle priming and as a promoter of vesicle fusion

The presynaptic SNARE-complex regulator complexin (Cplx) enhances the fusogenicity of primed synaptic vesicles (SVs). Consequently, Cplx deletion impairs action potential-evoked transmitter release. Conversely, though, Cplx loss enhances spontaneous and delayed asynchronous release at certain synapse types. Using electrophysiology and kinetic modeling, we show that such seemingly contradictory transmitter release phenotypes seen upon Cplx deletion can be explained by an additional of Cplx in the control of SV priming, where its ablation facilitates the generation of a "faulty" SV fusion apparatus. Supporting this notion, a sequential two-step priming scheme, featuring reduced vesicle fusogenicity and increased transition rates into the faulty primed state, reproduces all aberrations of transmitter release modes and short-term synaptic plasticity seen upon Cplx loss. Accordingly, we propose a dual presynaptic function for the SNARE-complex interactor Cplx, one as a "checkpoint" protein that guarantees the proper assembly of the fusion machinery during vesicle priming, and one in boosting vesicle fusogenicity.

Tonotopic differentiation of presynaptic neurotransmitter-releasing machinery in the auditory brainstem during the prehearing period and its selective deficits in Fmr1 knockout mice

Tonotopic organization is a fundamental feature of the auditory system. In the developing auditory brainstem, the ontogeny and maturation of neurotransmission progress from high to low frequencies along the tonotopic axis. To explore the underlying mechanism of this tonotopic development, we aim to determine whether the presynaptic machinery responsible for neurotransmitter release is tonotopically differentiated during development. In the current study, we examined vesicular neurotransmitter transporters and calcium sensors, two central players responsible for loading neurotransmitter into synaptic vesicles and for triggering neurotransmitter release in a calcium-dependent manner, respectively. Using immunocytochemistry, we characterized the distribution patterns of vesicular glutamate transporters (VGLUTs) 1 and 2, vesicular gamma-aminobutyric acid transporter (VGAT), and calcium sensor synaptotagmin (Syt) 1 and 2 in the developing mouse medial nucleus of the trapezoid body (MNTB). We identified tonotopic gradients of VGLUT1, VGAT, Syt1, and Syt2 in the first postnatal week, with higher protein densities in the more medial (high-frequency) portion of the MNTB. These gradients gradually flattened before the onset of hearing. In contrast, VGLUT2 was distributed relatively uniformly along the tonotopic axis during this prehearing period. In mice lacking Fragile X mental retardation protein, an mRNA-binding protein that regulates synaptic development and plasticity, progress to achieve the mature-like organization was altered for VGLUT1, Syt1, and Syt2, but not for VGAT. Together, our results identified novel organization patterns of selective presynaptic proteins in immature auditory synapses, providing a potential mechanism that may contribute to tonotopic differentiation of neurotransmission during normal and abnormal development.

Expression and Neurotransmitter Association of the Synaptic Calcium Sensor Synaptotagmin in the Avian Auditory Brain Stem

In the avian auditory brain stem, acoustic timing and intensity cues are processed in separate, parallel pathways via the two divisions of the cochlear nucleus, nucleus angularis (NA) and nucleus magnocellularis (NM). Differences in excitatory and inhibitory synaptic properties, such as release probability and short-term plasticity, contribute to differential processing of the auditory nerve inputs. We investigated the distribution of synaptotagmin, a putative calcium sensor for exocytosis, via immunohistochemistry and double immunofluorescence in the embryonic and hatchling chick brain stem (Gallus gallus). We found that the two major isoforms, synaptotagmin 1 (Syt1) and synaptotagmin 2 (Syt2), showed differential expression. In the NM, anti-Syt2 label was strong and resembled the endbulb terminals of the auditory nerve inputs, while anti-Syt1 label was weaker and more punctate. In NA, both isoforms were intensely expressed throughout the neuropil. A third isoform, synaptotagmin 7 (Syt7), was largely absent from the cochlear nuclei. In nucleus laminaris (NL, the target nucleus of NM), anti-Syt2 and anti-Syt7 strongly labeled the dendritic lamina. These patterns were established by embryonic day 18 and persisted to postnatal day 7. Double-labeling immunofluorescence showed that Syt1 and Syt2 were associated with vesicular glutamate transporter 2 (VGluT2), but not vesicular GABA transporter (VGAT), suggesting that these Syt isoforms were localized to excitatory, but not inhibitory, terminals. These results suggest that Syt2 is the major calcium binding protein underlying excitatory neurotransmission in the timing pathway comprising NM and NL, while Syt2 and Syt1 regulate excitatory transmission in the parallel intensity pathway via cochlear nucleus NA.

Fast resupply of synaptic vesicles requires synaptotagmin-3

Sustained neuronal activity demands a rapid resupply of synaptic vesicles to maintain reliable synaptic transmission. Such vesicle replenishment is accelerated by submicromolar presynaptic Ca²⁺ signals by an as-yet unidentified high-affinity Ca²⁺ sensor^1,2. Here we identify synaptotagmin-3 (SYT3)^3,4 as that presynaptic high-affinity Ca²⁺ sensor, which drives vesicle replenishment and short-term synaptic plasticity. Synapses in Syt3 knockout mice exhibited enhanced short-term depression, and recovery from depression was slower and insensitive to presynaptic residual Ca²⁺. During sustained neuronal firing, SYT3 accelerated vesicle replenishment and increased the size of the readily releasable pool. SYT3 also mediated short-term facilitation under conditions of low release probability and promoted synaptic enhancement together with another high-affinity synaptotagmin, SYT7 (ref. ⁵). Biophysical modelling predicted that SYT3 mediates both replenishment and facilitation by promoting the transition of loosely docked vesicles to tightly docked, primed states. Our results reveal a crucial role for presynaptic SYT3 in the maintenance of reliable high-frequency synaptic transmission. Moreover, multiple forms of short-term plasticity may converge on a mechanism of reversible, Ca²⁺-dependent vesicle docking.

Specific synaptic input strengths determine the computational properties of excitation-inhibition integration in a sound localization circuit

KEY POINTS

During the computation of sound localization, neurons of the lateral superior olive (LSO) integrate synaptic excitation arising from the ipsilateral ear with inhibition from the contralateral ear. We characterized the functional connectivity of the inhibitory and excitatory inputs onto LSO neurons in terms of unitary synaptic strength and convergence. Unitary IPSCs can generate large conductances, although their strength varies over a 10-fold range in a given recording. By contrast, excitatory inputs are relatively weak. The conductance associated with IPSPs needs to be at least 2-fold stronger than the excitatory one to guarantee effective inhibition of action potential (AP) firing. Computational modelling showed that strong unitary inhibition ensures an appropriate slope and midpoint of the tuning curve of LSO neurons. Conversely, weak but numerous excitatory inputs filter out spontaneous AP firing from upstream auditory neurons.

ABSTRACT

The lateral superior olive (LSO) is a binaural nucleus in the auditory brainstem in which excitation from the ipsilateral ear is integrated with inhibition from the contralateral ear. It is unknown whether the strength of the unitary inhibitory and excitatory inputs is adapted to allow for optimal tuning curves of LSO neuron action potential (AP) firing. Using electrical and optogenetic stimulation of afferent synapses, we found that the strength of unitary inhibitory inputs to a given LSO neuron can vary over a ∼10-fold range, follows a roughly log-normal distribution, and, on average, causes a large conductance (9 nS). Conversely, unitary excitatory inputs, stimulated optogenetically under the bushy-cell specific promoter Math5, were numerous, and each caused a small conductance change (0.7 nS). Approximately five to seven bushy cell inputs had to be active simultaneously to bring an LSO neuron to fire. In double stimulation experiments, the effective inhibition window caused by IPSPs was short (1-3 ms) and its length depended on the inhibitory conductance; an ∼2-fold stronger inhibition than excitation was needed to suppress AP firing. Computational modelling suggests that few, but strong, unitary IPSPs create a tuning curve of LSO neuron firing with an appropriate slope and midpoint. Furthermore, weak but numerous excitatory inputs reduce the spontaneous AP firing that LSO neurons would otherwise inherit from their upstream auditory neurons. Thus, the specific connectivity and strength of unitary excitatory and inhibitory inputs to LSO neurons is optimized for the computations performed by these binaural neurons.

Synaptic transmission at the endbulb of Held deteriorates during age-related hearing loss

KEY POINTS

Synaptic transmission at the endbulb of Held was assessed by whole-cell patch clamp recordings from auditory neurons in mature (2-4 months) and aged (20-26 months) mice. Synaptic transmission is degraded in aged mice, which may contribute to the decline in neural processing of the central auditory system during age-related hearing loss. The changes in synaptic transmission in aged mice can be partially rescued by improving calcium buffering, or decreasing action potential-evoked calcium influx. These experiments suggest potential mechanisms, such as regulating intraterminal calcium, that could be manipulated to improve the fidelity of transmission at the aged endbulb of Held.

ABSTRACT

Age-related hearing loss (ARHL) is associated with changes to the auditory periphery that raise sensory thresholds and alter coding, and is accompanied by alterations in excitatory and inhibitory synaptic transmission, and intrinsic excitability in the circuits of the central auditory system. However, it remains unclear how synaptic transmission changes at the first central auditory synapses during ARHL. Using mature (2-4 months) and old (20-26 months) CBA/CaJ mice, we studied synaptic transmission at the endbulb of Held. Mature and old mice showed no difference in either spontaneous quantal synaptic transmission or low frequency evoked synaptic transmission at the endbulb of Held. However, when challenged with sustained high frequency stimulation, synapses in old mice exhibited increased asynchronous transmitter release and reduced synchronous release. This suggests that the transmission of temporally precise information is degraded at the endbulb during ARHL. Increasing intraterminal calcium buffering with EGTA-AM or decreasing calcium influx with ω-agatoxin IVA decreased the amount of asynchronous release and restored synchronous release in old mice. In addition, recovery from depression following high frequency trains was faster in old mice, but was restored to a normal time course by EGTA-AM treatment. These results suggest that intraterminal calcium in old endbulbs may rise to abnormally high levels during high rates of auditory nerve firing, or that calcium-dependent processes involved in release are altered with age. These observations suggest that ARHL is associated with a decrease in temporal precision of synaptic release at the first central auditory synapse, which may contribute to perceptual deficits in hearing.

Synaptic Effects of Munc18-1 Alternative Splicing in Excitatory Hippocampal Neurons

The munc18-1 gene encodes two splice-variants that vary at the C-terminus of the protein and are expressed at different levels in different regions of the adult mammalian brain. Here, we investigated the expression pattern of these splice variants within the brainstem and tested whether they are functionally different. Munc18-1a is expressed in specific nuclei of the brainstem including the LRN, VII and SOC, while Munc18-1b expression is relatively low/absent in these regions. Furthermore, Munc18-1a is the major splice variant in the Calyx of Held. Synaptic transmission was analyzed in autaptic hippocampal munc18-1 KO neurons re-expressing either Munc18-1a or Munc18-1b. The two splice variants supported synaptic transmission to a similar extent, but Munc18-1b was slightly more potent in sustaining synchronous release during high frequency stimulation. Our data suggest that alternative splicing of Munc18-1 support synaptic transmission to a similar extent, but could modulate presynaptic short-term plasticity.

RIM1 and RIM2 redundantly determine Ca2+ channel density and readily releasable pool size at a large hindbrain synapse

The localization and density of voltage-gated Ca(2+) channels at active zones are essential for the amount and kinetics of transmitter release at synapses. RIM proteins are scaffolding proteins at the active zone that bind to several other presynaptic proteins, including voltage-gated Ca(2+) channel α-subunits. The long isoforms of RIM proteins, which contain NH2-terminal Rab3- and Munc13-interacting domains, as well as a central PDZ domain and two COOH-terminal C2 domains, are encoded by two genes, Rim1 and Rim2. Here, we used the ideal accessibility of the large calyx of Held synapse for direct presynaptic electrophysiology to investigate whether the two Rim genes have redundant, or separate, functions in determining the presynaptic Ca(2+) channel density, and the size of a readily releasable vesicle pool (RRP). Quantitative PCR showed that cochlear nucleus neurons, which include calyx of Held generating neurons, express both RIM1 and RIM2. Conditional genetic inactivation of RIM2 at the calyx of Held led to a subtle reduction in presynaptic Ca(2+) current density, whereas deletion of RIM1 was ineffective. The release efficiency of brief presynaptic Ca(2+) "tail" currents and the RRP were unaffected in conditional single RIM1 and RIM2 knockout (KO) mice, whereas both parameters were strongly reduced in RIM1/2 double KO mice. Thus, despite a somewhat more decisive role for RIM2 in determining presynaptic Ca(2+) channel density, RIM1 and RIM2 can overall replace each other's presynaptic functions at a large relay synapse in the hindbrain, the calyx of Held.

Gene expression profile during functional maturation of a central mammalian synapse

Calyx of Held giant presynaptic terminals in the auditory brainstem form glutamatergic axosomatic synapses that have advanced to one of the best-studied synaptic connections of the mammalian brain. As the auditory system matures and adjusts to high-fidelity synaptic transmission, the calyx undergoes extensive structural and functional changes - in mice, it is formed at about postnatal day 3 (P3), achieves immature function until hearing onset at about P10 and can be considered mature from P21 onwards. This setting provides a unique opportunity to examine the repertoire of genes driving synaptic structure and function during postnatal maturation. Here, we determined the gene expression profile of globular bushy cells (GBCs), neurons giving rise to the calyx of Held, at different maturational stages (P3, P8, P21). GBCs were retrogradely labelled by stereotaxic injection of fluorescent cholera toxin-B, and their mRNA content was collected by laser microdissection. Microarray profiling, successfully validated with real time quantitative polymerase chain reaction and nCounter approaches, revealed genes regulated during maturation. We found that mostly genes implicated in the general cell biology of the neuron were regulated, while most genes related to synaptic function were regulated around the onset of hearing. Among these, voltage-gated ion channels and calcium-binding proteins were strongly regulated, whereas most genes involved in the synaptic vesicle cycle were only moderately regulated. These results suggest that changes in the expression patterns of ion channels and calcium-binding proteins are a dominant factor in defining key synaptic properties during maturation of the calyx of Held.

BMP signaling specifies the development of a large and fast CNS synapse

Large excitatory synapses with multiple active zones ensure reliable and fast information transfer at specific points in neuronal circuits. However, the mechanisms that determine synapse size in CNS circuits are largely unknown. Here we use the calyx of Held synapse, a major relay in the auditory system, to identify and study signaling pathways that specify large nerve terminal size and fast synaptic transmission. Using genome-wide screening, we identified bone morphogenetic proteins (BMPs) as candidate signaling molecules in the area of calyx synapses. Conditional deletion of BMP receptors in the auditory system of mice led to aberrations of synapse morphology and function specifically at the calyx of Held, with impaired nerve terminal growth, loss of monoinnervation and less mature transmitter release properties. Thus, BMP signaling specifies large and fast-transmitting synapses in the auditory system in a process that shares homologies with, but also extends beyond, retrograde BMP signaling at Drosophila neuromuscular synapses.

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.

Synaptotagmin increases the dynamic range of synapses by driving Ca²+-evoked release and by clamping a near-linear remaining Ca²+ sensor

Ca²+-evoked transmitter release shows a high dynamic range over spontaneous release. We investigated the role of the Ca²+ sensor protein, Synaptotagmin2 (Syt2), in both spontaneous and Ca²+-evoked release under direct control of presynaptic [Ca²+](i), using an in vivo rescue approach at the calyx of Held. Re-expression of Syt2 rescued the highly Ca²+ cooperative release and suppressed the elevated spontaneous release seen in Syt2 KO synapses. This latter release clamping function was partially mediated by the poly-lysine motif of the C₂B domain. Using an aspartate mutation in the C₂B domain (D364N) in which Ca²+ triggering was abolished but release clamping remained intact, we show that Syt2 strongly suppresses the action of another, near-linear Ca²+ sensor that mediates release over a wide range of [Ca²+](i). Thus, Syt2 increases the dynamic range of synapses by driving release with a high Ca²+ cooperativity, as well as by suppressing a remaining, near-linear Ca²+ sensor.

Regulation of transmitter release by Ca(2+) and synaptotagmin: insights from a large CNS synapse

Transmitter release at synapses is driven by elevated intracellular Ca(2+) concentration ([Ca(2+)](i)) near the sites of vesicle fusion. [Ca(2+)](i) signals of profoundly different amplitude and kinetics drive the phasic release component during a presynaptic action potential, and asynchronous release at later times. Studies using direct control of [Ca(2+)](i) at a large glutamatergic terminal, the calyx of Held, have provided significant insight into how intracellular Ca(2+) regulates transmitter release over a wide concentration range. Synaptotagmin-2 (Syt2), the major isoform of the Syt1/2 Ca(2+) sensors at these synapses, triggers highly Ca(2+)-cooperative release above 1μM [Ca(2+)](i), but suppresses release at low [Ca(2+)](i). Thus, neurons utilize a highly sophisticated release apparatus to maximize the dynamic range of Ca(2+)-evoked versus spontaneous release.

Formation and maturation of the calyx of Held

Sound localization requires precise and specialized neural circuitry. A prominent and well-studied specialization is found in the mammalian auditory brainstem. Globular bushy cells of the ventral cochlear nucleus (VCN) project contralaterally to neurons of the medial nucleus of the trapezoid body (MNTB), where their large axons terminate on cell bodies of MNTB principal neurons, forming the calyces of Held. The VCN-MNTB pathway is necessary for the accurate computation of interaural intensity and time differences; MNTB neurons provide inhibitory input to the lateral superior olive, which compares levels of excitation from the ipsilateral ear to levels of tonotopically matched inhibition from the contralateral ear, and to the medial superior olive, where precise inhibition from MNTB neurons tunes the delays of binaural excitation. Here we review the morphological and physiological aspects of the development of the VCN-MNTB pathway and its calyceal termination, along with potential mechanisms that give rise to its precision. During embryonic development, VCN axons grow towards the midline, cross the midline into the region of the presumptive MNTB and then form collateral branches that will terminate in calyces of Held. In rodents, immature calyces of Held appear in MNTB during the first few days of postnatal life. These calyces mature morphologically and physiologically over the next three postnatal weeks, enabling fast, high fidelity transmission in the VCN-MNTB pathway.