Specialized Synapse-Associated Structures within the Calyx of Held

Abstract

The calyx of Held exhibits fast glutamatergic neurotransmission at high rates with low temporal jitter and has adapted specialized synaptic mechanisms to support its functional demands. We report the presence in calyces of an atypical arrangement of subcellular organelles, called the mitochondria-associated adherens complex (MAC). We demonstrate that MACs are located adjacent to synapses and contain membranous elements linked with coated and uncoated vesicles. Mitochondria that form MACs have more complex geometries than other mitochondria within the calyx and can extend between clusters of synapses. We estimate that the calyx contains 1600 MACs, 2400 synapses, and 6200 readily releasable vesicles. We also identify synaptic vesicle endocytotic regions close to MACs and synapses and hypothesize that calyces are composed of multiple activity modules, each containing machinery for vesicle release and recycling. The calyx of Held is one of the largest nerve terminals in the CNS (Held, 1893). Along with large and modified end-bulbs of Held delivered by auditory nerve fibers onto cochlear nucleus neurons (Cant and Morest, 1979;Tolbert and Morest, 1982;Fekete et al., 1984), these complex nerve terminals are key elements in brainstem circuitry that subserves sound localization (Morest, 1968). Spontaneous activity (generated in the absence of sound) can exceed 100 spikes/sec at the calyx terminal, and sound-driven activity at the most sensitive frequency of the calyceal neuron can approach 600 spikes/sec (Spirou et al., 1990). Temporal synchrony of calyceal neurons to a preferred phase of a low-frequency sound exceeds that found in the auditory nerve, and it can entrain (fire on every cycle of the stimulus sinusoid) at rates approaching 1 kHz (Joris et al., 1994a,b).High activity rates place demands on endocytotic mechanisms to maintain a pool of releasable vesicles. Some neurons, such as retinal bipolar cells, possess specialized structures called synaptic ribbons that are involved in vesicle trafficking, and therefore have unique mechanisms to solve their demands for synaptic activity (von Gersdorff and Matthews, 1999). It is plausible that the calyceal terminal, because of its extremely high spike rate and the temporal precision required to accomplish its task of sound localization, also uses unique structures and mechanisms.A noteworthy arrangement of organelles, consisting of a mitochondrion located near the presynaptic membrane and tethered via filaments to a punctum adherens, was first described in nerve terminals of the spinal cord (Gray, 1963). In the auditory system, this structure was noted in large and modified end-bulbs of auditory nerve fibers (Cant and Morest, 1979;Tolbert and Morest, 1982). More recently, we described this structure, which we named the mitochondria-associated adherens complex (MAC), in large collateral terminals of calyceal axons that are found in the superior olivary complex . Given the diversity of cellular elements that comprise the MAC, it could perform various functions in the nerve...

Involvement of Actin Polymerization in Vesicle Recruitment at the Calyx of Held Synapse

“…The latter ones should be available for docking within 50 msec given the speed of vesicle approach, as measured by Zenisek et al (2000). Clustering of synaptic vesicles at the active zones therefore may contribute to the rapid recruitment that we observed here (Rowland et al, 2000). It is possible that fusion of slowly releasing vesicles takes place outside the active zone, as has been demonstrated directly in retinal bipolar cells (Zenisek et al, 2000).…”

Bcl-xL Inhibitor ABT-737 Reveals a Dual Role for Bcl-xL in Synaptic Transmission

“…Mitochondria are abundant in the synaptic terminals of neurons that are capable of firing at high frequencies (Rowland et al 2000;Tolbert and Morest 1982) and are known to be involved in presynaptic responses to high-frequency stimulation including posttetanic potentiation (Friel and Tsien 1994;Jonas et al 1999;Tang and Zucker 1997) and in recovery of neurotransmission after synaptic depression (Billups and Forsythe 2002;. Injection of BCL-xL into the presynaptic terminal increases the rate of recovery of transmitter release after a period of rapid firing , suggesting that it enhances the performance of mitochondria in recovery.…”

Donut-Like Topology of Synaptic Vesicles with a Central Cluster of Mitochondria Wrapped into Membrane Protrusions: A Novel Structure–Function Module of the Adult Calyx of Held

“…5 A, B,E). Thus, in mature calyces, SV clusters are predominantly located in distinct compartments, referred to as swellings (Rowland et al, 2000), in close apposition to the presynaptic plasma membrane. Furthermore, SV clusters appear as the presynaptic plasma membrane begins to form swellings at the onset of hearing.…”

“…Each swelling contained six to nine mitochondria, of which four or five were interconnected. This arrangement could reflect individual mitochondria connected with each other (Brodin et al, 1999;Skulachev, 2001) or may result from the segmentation of a single mitochondrion (King et al, 1996;Rowland et al, 2000). Because the formation of tubular structures could be Figure 4.…”

“…Such a large number of active zones could only be accommodated by an increase in size, making the calyx one of the largest nerve terminals in the CNS of mammals. Furthermore, the organization of the mature calyx into small units of volume, so-called stalks and swellings (Rowland et al, 2000), is thought to facilitate clearance of neurotransmitter from the synaptic cleft and thereby support high-frequency transmission. Also on the level of protein distribution, the architecture of the calyx is designed to meet specific functional requirements: the restricted localization of Na channels to proximal parts of the calyx results in narrow action potential waveforms ensuring reliable, high-frequency transmission (Leao et al, 2005).…”