Endocytosis at ribbon synapses

Synaptic endocytosis in adult adipose stromal cell-derived neurons

Synapses are essential for facilitating the transmission of information between neurons and for executing neurophysiological processes. Following the exocytosis of neurotransmitters, the synaptic vesicle may quickly undergo endocytosis to preserve the structural integrity of the synapse. When converting adipose-derived stromal cells (ADSCs) into neurons, the ADSCs have already demonstrated comparable morphology, structure, and electrophysiological characteristics to neurons. Nevertheless, there is currently no published study on the endocytotic function of neurons that are produced from ADSCs. This study aimed to examine synaptic endocytosis in neurons derived from ADSCs by qualitatively and quantitatively analyzing the presence of Ap-2, Clathrin, Endophilin, Dynamin, and Hsc70, which are the key proteins involved in clathrin-mediated endocytosis (CME), as well as by using FM1-43 and cadmium selenide quantum dots (CdSe QDs). Additionally, single-cell RNA sequencing (scRNA-seq) was used to look at the levels of both neuronal markers and markers related to CME at the same time. The results of this study provide evidence that synapses in neurons produced from ADSCs have a role in endocytosis, mainly through the CME route.

The role of PDLIM1, a PDZ-LIM domain protein, at the ribbon synapses in the chicken retina

PDLIM's protein family is involved in the rearrangement of the actin cytoskeleton. In the present study, we describe the localization of PDLIM1 in chicken photoreceptors. This study provides evidence that this protein is present at the cone pedicles, as well as in other synapses of the chicken retina. Here, we demonstrate the expression pattern of PDLIM1 through immunofluorescence staining, immunoblots, subcellular fractionation, and immunoprecipitation experiments. Also, we consider the possibility that PDLIM1 may be involved in the synaptic vesicle endocytosis and/or the presynaptic trafficking of synaptic vesicles back to the nonready releasable pool. This endocytotic/exocytotic coupling requires a tight link between exocytic vesicle fusion at defined release sites and endocytic retrieval of synaptic vesicle membranes. In turn, photoreceptor ribbon synaptic structure depends on the cytoskeleton arrangement, both at the active zone-related with exocytosis-as well as at the endocytic zone-periactive zone. To our knowledge, the PDLIM1 protein has not been observed in the pre synapses of the retina. Thus, the present study describes the expression and subcellular localization of PDLIM1 for the first time, as well as its modulation by visual environment in the chicken retina.

Synaptic Vesicle Endocytosis in Different Model Systems

Neurotransmission in complex animals depends on a choir of functionally distinct synapses releasing neurotransmitters in a highly coordinated manner. During synaptic signaling, vesicles fuse with the plasma membrane to release their contents. The rate of vesicle fusion is high and can exceed the rate at which synaptic vesicles can be re-supplied by distant sources. Thus, local compensatory endocytosis is needed to replenish the synaptic vesicle pools. Over the last four decades, various experimental methods and model systems have been used to study the cellular and molecular mechanisms underlying synaptic vesicle cycle. Clathrin-mediated endocytosis is thought to be the predominant mechanism for synaptic vesicle recycling. However, recent studies suggest significant contribution from other modes of endocytosis, including fast compensatory endocytosis, activity-dependent bulk endocytosis, ultrafast endocytosis, as well as kiss-and-run. Currently, it is not clear whether a universal model of vesicle recycling exist for all types of synapses. It is possible that each synapse type employs a particular mode of endocytosis. Alternatively, multiple modes of endocytosis operate at the same synapse, and the synapse toggles between different modes depending on its activity level. Here we compile review and research articles based on well-characterized model systems: frog neuromuscular junctions, C. elegans neuromuscular junctions, Drosophila neuromuscular junctions, lamprey reticulospinal giant axons, goldfish retinal ribbon synapses, the calyx of Held, and rodent hippocampal synapses. We will compare these systems in terms of their known modes and kinetics of synaptic vesicle endocytosis, as well as the underlying molecular machineries. We will also provide the future development of this field.

The Calcineurin-Binding, Activity-Dependent Splice Variant Dynamin1xb Is Highly Enriched in Synapses in Various Regions of the Central Nervous System

In the present study, we generated and characterized a splice site-specific monoclonal antibody that selectively detects the calcineurin-binding dynamin1 splice variant dynamin1xb. Calcineurin is a Ca²⁺-regulated phosphatase that enhances dynamin1 activity and is an important Ca²⁺-sensing mediator of homeostatic synaptic plasticity in neurons. Using this dynamin1xb-specific antibody, we found dynamin1xb highly enriched in synapses of all analyzed brain regions. In photoreceptor ribbon synapses, dynamin1xb was enriched in close vicinity to the synaptic ribbon in a manner indicative of a peri-active zone immunolabeling. Interestingly, in dark-adapted mice we observed an enhanced and selective enrichment of dynamin1xb in both synaptic layers of the retina in comparison to light-adapted mice. This could be due to an illumination-dependent recruitment of dynamin1xb to retinal synapses and/or due to a darkness-induced increase of dynamin1xb biosynthesis. These latter findings indicate that dynamin1xb is part of a versatile and highly adjustable, activity-regulated endocytic synaptic machinery.

Ribbon Synapses and Visual Processing in the Retina

The first synapses transmitting visual information contain an unusual organelle, the ribbon, which is involved in the transport and priming of vesicles to be released at the active zone. The ribbon is one of many design features that allow efficient refilling of the active zone, which in turn enables graded changes in membrane potential to be transmitted using a continuous mode of neurotransmitter release. The ribbon also plays a key role in supplying vesicles for rapid and transient bursts of release that signal fast changes, such as the onset of light. We increasingly understand how the physiological properties of ribbon synapses determine basic transformations of the visual signal and, in particular, how the process of refilling the active zone regulates the gain and adaptive properties of the retinal circuit. The molecular basis of ribbon function is, however, far from clear.

Preferential localization of type I phosphatidylinositol 4-phosphate 5-kinase γ at the periactive zone of mouse photoreceptor ribbon synapses

Type I phosphatidylinositol 4-phosphate 5 kinase γ (PIP5KIγ) constitutes a major pathway for the generation of phosphatidylinositol 4,5-bisphosphate (PtdIns(4,5)P2) that regulates a variety of neuronal functions at both presynaptic and postsynaptic compartments. In this study, we examined the expression and localization of PIP5KIγ in the adult mouse retina. RT-PCR analysis revealed that PIP5KIγ_v2 was predominantly expressed in the retina while PIP5KIγ_v3 was also expressed faintly. Immunostaining of the adult mouse retina revealed intense PIP5KIγ-immunoreactivity in the inner and outer plexiform layers in a punctate manner. In the photoreceptor ribbon synapse, PIP5KIγ was highly concentrated at the periactive zone. These findings suggest that PIP5KIγ, especially PIP5KIγ_i2, is localized at the periactive zone, a functionally suitable compartment for the endocytosis of synaptic vesicles in photoreceptor ribbon synapses.

Rapid kinetics of endocytosis at rod photoreceptor synapses depends upon endocytic load and calcium

Release from rods is triggered by the opening of L-type Ca2+ channels that lie beneath synaptic ribbons. After exocytosis, vesicles are retrieved by compensatory endocytosis. Previous work showed that endocytosis is dynamin-dependent in rods but dynamin-independent in cones. We hypothesized that fast endocytosis in rods may also differ from cones in its dependence upon the amount of Ca2+ influx and/or endocytic load. We measured exocytosis and endocytosis from membrane capacitance (C m) changes evoked by depolarizing steps in voltage clamped rods from tiger salamander retinal slices. Similar to cones, the time constant for endocytosis in rods was quite fast, averaging <200 ms. We manipulated Ca2+ influx and the amount of vesicle release by altering the duration and voltage of depolarizing steps. Unlike cones, endocytosis kinetics in rods slowed after increasing Ca2+ channel activation with longer step durations or more strongly depolarized voltage steps. Endocytosis kinetics also slowed as Ca2+ buffering was decreased by replacing BAPTA (10 or 1 mM) with the slower Ca2+ buffer EGTA (5 or 0.5 mM) in the pipette solution. These data provide further evidence that endocytosis mechanisms differ in rods and cones and suggest that endocytosis in rods is regulated by both endocytic load and local Ca2+ levels.

Evidence for a Clathrin-independent mode of endocytosis at a continuously active sensory synapse

Synaptic vesicle exocytosis at chemical synapses is followed by compensatory endocytosis. Multiple pathways including Clathrin-mediated retrieval of single vesicles, bulk retrieval of large cisternae, and kiss-and-run retrieval have been reported to contribute to vesicle recycling. Particularly at the continuously active ribbon synapses of retinal photoreceptor and bipolar cells, compensatory endocytosis plays an essential role to provide ongoing vesicle supply. Yet, little is known about the mechanisms that contribute to endocytosis at these highly complex synapses. To identify possible specializations in ribbon synaptic endocytosis during different states of activity, we exposed mice to controlled lighting conditions and compared the distribution of endocytotic proteins at rod and cone photoreceptor, and ON bipolar cell ribbon synapses with light and electron microscopy. In mouse ON bipolar cell terminals, Clathrin-mediated endocytosis seemed to be the dominant mode of endocytosis at all adaptation states analyzed. In contrast, in mouse photoreceptor terminals in addition to Clathrin-coated pits, clusters of membranously connected electron-dense vesicles appeared during prolonged darkness. These clusters labeled for Dynamin3, Endophilin1, and Synaptojanin1, but not for AP180, Clathrin LC, and hsc70. We hypothesize that rod and cone photoreceptors possess an additional Clathrin-independent mode of vesicle retrieval supporting the continuous synaptic vesicle supply during prolonged high activity.

A local, periactive zone endocytic machinery at photoreceptor synapses in close vicinity to synaptic ribbons

Photoreceptor ribbon synapses are continuously active synapses with large active zones that contain synaptic ribbons. Synaptic ribbons are anchored to the active zones and are associated with large numbers of synaptic vesicles. The base of the ribbon that is located close to L-type voltage-gated Ca(2+) channels is a hotspot of exocytosis. The continuous exocytosis at the ribbon synapse needs to be balanced by compensatory endocytosis. Recent analyses indicated that vesicle recycling at the synaptic ribbon is also an important determinant of synaptic signaling at the photoreceptor synapse. To get insights into mechanisms of vesicle recycling at the photoreceptor ribbon synapse, we performed super-resolution structured illumination microscopy and immunogold electron microscopy to localize major components of the endocytotic membrane retrieval machinery in the photoreceptor synapse of the mouse retina. We found dynamin, syndapin, amphiphysin, and calcineurin, a regulator of activity-dependent endocytosis, to be highly enriched around the active zone and the synaptic ribbon. We present evidence for two clathrin heavy chain variants in the photoreceptor terminal; one is enriched around the synaptic ribbon, whereas the other is localized in the entry region of the terminal. The focal enrichment of endocytic proteins around the synaptic ribbon is consistent with a focal uptake of endocytic markers at that site. This endocytic activity functionally depends on dynamin. These data propose that the presynaptic periactive zone surrounding the synaptic ribbon complex is a hotspot of endocytosis in photoreceptor ribbon synapses.

Rapid synaptic vesicle endocytosis in cone photoreceptors of salamander retina

Following synaptic vesicle exocytosis, neurons retrieve the fused membrane by a process of endocytosis to provide a supply of vesicles for subsequent release and maintain the presynaptic active zone. Rod and cone photoreceptors use a specialized structure called the synaptic ribbon that enables them to sustain high rates of neurotransmitter release. They must also employ mechanisms of synaptic vesicle endocytosis capable of keeping up with release. While much is known about endocytosis at another retinal ribbon synapse, that of the goldfish Mb1 bipolar cell, less is known about endocytosis in photoreceptors. We used capacitance recording techniques to measure vesicle membrane fusion and retrieval in photoreceptors from salamander retinal slices. We found that application of brief depolarizing steps (<100 ms) to cones evoked exocytosis followed by rapid endocytosis with a time constant ∼250 ms. In some cases, the capacitance trace overshot the baseline, indicating excess endocytosis. Calcium had no effect on the time constant, but enhanced excess endocytosis resulting in a faster rate of membrane retrieval. Surprisingly, endocytosis was unaffected by blockers of dynamin, suggesting that cone endocytosis is dynamin independent. This contrasts with synaptic vesicle endocytosis in rods, which was inhibited by the dynamin inhibitor dynasore and GTPγS introduced through the patch pipette, suggesting that the two photoreceptor types employ distinct pathways for vesicle retrieval. The fast kinetics of synaptic vesicle endocytosis in photoreceptors likely enables these cells to maintain a high rate of transmitter release, allowing them to faithfully signal changes in illumination to second-order neurons.

Regulation of presynaptic strength by controlling Ca2+ channel mobility: effects of cholesterol depletion on release at the cone ribbon synapse

Synaptic communication requires proper coupling between voltage-gated Ca(2+) (Ca(V)) channels and synaptic vesicles. In photoreceptors, L-type Ca(V) channels are clustered close to synaptic ribbon release sites. Although clustered, Ca(V) channels move continuously within a confined domain slightly larger than the base of the ribbon. We hypothesized that expanding Ca(V) channel confinement domains should increase the number of channel openings needed to trigger vesicle release. Using single-particle tracking techniques, we measured the expansion of Ca(V) channel confinement domains caused by depletion of membrane cholesterol with cholesterol oxidase or methyl-β-cyclodextrin. With paired whole cell recordings from cones and horizontal cells, we then determined the number of Ca(V) channel openings contributing to cone Ca(V) currents (I(Ca)) and the number of vesicle fusion events contributing to horizontal cell excitatory postsynaptic currents (EPSCs) following cholesterol depletion. Expansion of Ca(V) channel confinement domains reduced the peak efficiency of release, decreasing the number of vesicle fusion events accompanying opening of each Ca(V) channel. Cholesterol depletion also inhibited exocytotic capacitance increases evoked by brief depolarizing steps. Changes in efficiency were not due to changes in I(Ca) amplitude or glutamate receptor properties. Replenishing cholesterol restored Ca(V) channel domain size and release efficiency to control levels. These results indicate that cholesterol is important for organizing the cone active zone. Furthermore, the finding that cholesterol depletion impairs coupling between channel opening and vesicle release by allowing Ca(V) channels to move further from release sites shows that changes in presynaptic Ca(V) channel mobility can be a mechanism for adjusting synaptic strength.

Exocytosis in the frog amphibian papilla

Using whole-cell patch-clamp recordings, we measured changes in membrane capacitance (ΔC (m)) in two subsets of hair cells from the leopard frog amphibian papilla (AP): the low-frequency (100-500 Hz), rostral hair cells and the high-frequency (500-1200 Hz), caudal hair cells, in order to investigate tonotopic differences in exocytosis. Depolarizations of both rostral and caudal hair cells evoked robust ΔC (m) responses of similar amplitude. However, the calcium dependence of release, i.e., the relationship between ΔC (m) relative to the amount of calcium influx (Q (Ca) (2+)), was found to be linear in rostral hair cells but supra-linear in caudal hair cells. In addition, the higher numbers of vesicles released at caudal hair cell active zones suggests increased temporal precision of caudal hair cell exocytosis. ΔC (m) responses were also obtained in response to sinusoidal stimuli of varying frequency, but neither rostral nor caudal hair cell ΔC (m) revealed any frequency selectivity. While all AP hair cells express both otoferlin and synaptotagmin IV (SytIV), we obtained evidence of a tonotopic distribution of the calcium buffer calretinin which may further increase temporal resolution at the level of the hair cell synapse. Our findings suggest that the low (rostral) and high (caudal) frequency hair cells apply different mechanisms for fine-tuning exocytosis.

Endophilin drives the fast mode of vesicle retrieval in a ribbon synapse

Compensatory endocytosis of exocytosed membrane and recycling of synaptic vesicle components is essential for sustained synaptic transmission at nerve terminals. At the ribbon-type synapse of retinal bipolar cells, manipulations expected to inhibit the interactions of the clathrin adaptor protein complex (AP2) affect only the slow phase of endocytosis (τ = 10-15 s), leading to the conclusion that fast endocytosis (τ = 1-2 s) occurs by a mechanism that differs from the classical pathway of clathrin-coated vesicle retrieval from the plasma membrane. Here we investigate the role of endophilin in endocytosis at this ribbon synapse. Endophilin A1 is a synaptically enriched N-BAR domain-containing protein, suggested to function in clathrin-mediated endocytosis. Internal dialysis of the synaptic terminal with dominant-negative endophilin A1 lacking its linker and Src homology 3 (SH3) domain inhibited the fast mode of endocytosis, while slow endocytosis continued. Dialysis of a peptide that binds endophilin SH3 domain also decreased fast retrieval. Electron microscopy indicated that fast endocytosis occurred by retrieval of small vesicles in most instances. These results indicate that endophilin is involved in fast retrieval of synaptic vesicles occurring by a mechanism that can be distinguished from the classical pathway involving clathrin-AP2 interactions.

Regional selectivity of rab5 and rab7 protein upregulation in mild cognitive impairment and Alzheimer's disease

Endocytic alterations are one of the earliest changes to occur in Alzheimer's disease (AD), and are hypothesized to be involved in the selective vulnerability of specific neuronal populations during the progression of AD. Previous microarray and real-time quantitative PCR experiments revealed an upregulation of the early endosomal effector rab5 and the late endosome constituent rab7 in the hippocampus of people with mild cognitive impairment (MCI) and AD. To assess whether these select rab GTPase gene expression changes are reflected in protein levels within selectively vulnerable brain regions (basal forebrain, frontal cortex, and hippocampus) and relatively spared areas (cerebellum and striatum), we performed immunoblot analysis using antibodies directed against rab5 and rab7 on postmortem human brain tissue harvested from cases with a premortem clinical diagnosis of no cognitive impairment (NCI), MCI, and AD. Results indicate selective upregulation of both rab5 and rab7 levels within basal forebrain, frontal cortex, and hippocampus in MCI and AD, which also correlated with Braak staging. In contrast, no differences in protein levels were found in the less vulnerable cerebellum and striatum. These regional immunoblot assays are consistent with single cell gene expression data, and provide protein-based evidence for endosomal markers contributing to the vulnerability of cell types within selective brain regions during the progression of AD.

Changes to tear cytokines of type 2 diabetic patients with or without retinopathy

PURPOSE

To investigate changes in cytokine levels in tears of type 2 diabetics with or without retinopathy.

METHODS

Tears were collected from 15 type 2 diabetics without retinopathy (DNR), 15 patients with retinopathy (DR), and 15 age and gender matched non-diabetic controls. Tear concentrations of 27 cytokines were measured by multiplex bead immunoassay. Cytokine differences between groups, ratios of type-1 T helper (Th1)/type-2 T helper (Th2) cytokines and anti-angiogenic/pro-angiogenic cytokines were analyzed statistically.

RESULTS

The most abundant cytokine detected in tears was interferon-induced protein-10 (IP-10). In comparison with controls, IP-10 and monocyte chemoattracant protein-1 (MCP-1) levels were significantly elevated in DR (p=0.016 and 0.036, respectively) and DNR groups (p=0.021 and 0.026, respectively). Interleukin-1 (IL-1) receptor antagonist (IL-1ra) levels were significantly increased in DNR (p=0.016). Th1/Th2 cytokines interferon-gamma (IFN-γ)/IL-5 and IL-2/IL-5 ratios were significantly increased in DR compared to controls (p=0.037 and 0.031, respectively). Anti-angiogenic/angiogenic cytokines IFN-γ/MCP-1 and IL-4/MCP-1 ratios in DR and DNR were significantly decreased compared to controls (p<0.05). IL-4/IL-8 and IL-12p70/IL-8 ratios were also significantly decreased in DR compared to controls (p=0.02 and 0.045, respectively). No significant correlation was demonstrated between tear cytokine concentrations and glycosylated hemoglobin (HbA1c) or fasting plasma glucose (FPG).

CONCLUSIONS

Diabetic tears exhibited elevated levels of IP-10 and MCP-1. The Th1/Th2 cytokine balance may shift to a predominantly Th1 state in DR patients. Pro-angiogenic cytokines are more highly represented than anti-angiogenic cytokines in the tears of diabetic patients.

The making of synaptic ribbons: how they are built and what they do

Ribbon synapses in the retina and inner ear maintain tonic neurotransmitter release at high rates to transduce a broad bandwidth of stimulus intensities. In ribbon synapses, synaptic vesicles can be released by a slow, sustained mode and by fast, synchronous mechanisms. The high release rates require structural and functional specializations. The synaptic ribbon is the key structural specialization of ribbon synapses. Synaptic ribbons are large, electron-dense structures that immobilize numerous synaptic vesicles next to presynaptic release sites. A main component of synaptic ribbons is the protein RIBEYE that has the capability to build the scaffold of the synaptic ribbon via multiple RIBEYE-RIBEYE interactions. A modular assembly model of synaptic ribbons has been proposed in which synaptic ribbons are formed from individual RIBEYE subunits. The scaffold of the synaptic ribbon provides a docking site for RIBEYE-associated proteins that could execute specific synaptic ribbon functions. Multiple functions have been assigned to synaptic ribbons including roles in exocytosis, endocytosis, and synaptic membrane trafficking. Recent studies demonstrated the importance of synaptic ribbons for fast, synchronous release and emphasized the need of a tight and efficient coupling between presynaptic Ca(2+) signaling and exocytosis. The present review summarizes recent advances on structure and function of synaptic ribbons.

Vesicle recycling at ribbon synapses in the finely branched axon terminals of mouse retinal bipolar neurons

In retinal bipolar neurons, synaptic ribbons mark the presence of exocytotic active zones in the synaptic terminal. It is unknown, however, where compensatory vesicle retrieval is localized in this cell type and by what mechanism(s) excess membrane is recaptured. To determine whether endocytosis is localized or diffuse in mouse bipolar neurons, we imaged FM4-64 to track vesicles in cells whose synaptic ribbons were tagged with a fluorescent peptide. In synaptic terminals, vesicle retrieval occurred at discrete sites that were spatially consistent over multiple stimuli, indicative of endocytotic "hot spots." Retrieval sites were spatially correlated with fluorescently labeled synaptic ribbons. Electron microscopy (EM) analysis of bipolar cell terminals after photoconversion of internalized FM dye revealed that almost all of the dye was contained within vesicles approximately 30 nm in diameter. Clathrin-coated vesicles were observed budding from the plasma membrane and within the cytosol, and application of dynasore, a dynamin inhibitor, arrested membrane retrieval just after the budding stage. We conclude that synaptic vesicles in the fine branches of mouse bipolar axon terminals are retrieved locally near active zones, at least in part via a clathrin-mediated pathway.

Ideal observer analysis of signal quality in retinal circuits

The function of the retina is crucial, for it must encode visual signals so the brain can detect objects in the visual world. However, the biological mechanisms of the retina add noise to the visual signal and therefore reduce its quality and capacity to inform about the world. Because an organism's survival depends on its ability to unambiguously detect visual stimuli in the presence of noise, its retinal circuits must have evolved to maximize signal quality, suggesting that each retinal circuit has a specific functional role. Here we explain how an ideal observer can measure signal quality to determine the functional roles of retinal circuits. In a visual discrimination task the ideal observer can measure from a neural response the increment threshold, the number of distinguishable response levels, and the neural code, which are fundamental measures of signal quality relevant to behavior. It can compare the signal quality in stimulus and response to determine the optimal stimulus, and can measure the specific loss of signal quality by a neuron's receptive field for non-optimal stimuli. Taking into account noise correlations, the ideal observer can track the signal-to-noise ratio available from one stage to the next, allowing one to determine each stage's role in preserving signal quality. A comparison between the ideal performance of the photon flux absorbed from the stimulus and actual performance of a retinal ganglion cell shows that in daylight a ganglion cell and its presynaptic circuit loses a factor of approximately 10-fold in contrast sensitivity, suggesting specific signal-processing roles for synaptic connections and other neural circuit elements. The ideal observer is a powerful tool for characterizing signal processing in single neurons and arrays along a neural pathway.

The molecular architecture of ribbon presynaptic terminals

The primary receptor neurons of the auditory, vestibular, and visual systems encode a broad range of sensory information by modulating the tonic release of the neurotransmitter glutamate in response to graded changes in membrane potential. The output synapses of these neurons are marked by structures called synaptic ribbons, which tether a pool of releasable synaptic vesicles at the active zone where glutamate release occurs in response to calcium influx through L-type channels. Ribbons are composed primarily of the protein, RIBEYE, which is unique to ribbon synapses, but cytomatrix proteins that regulate the vesicle cycle in conventional terminals, such as Piccolo and Bassoon, also are found at ribbons. Conventional and ribbon terminals differ, however, in the size, molecular composition, and mobilization of their synaptic vesicle pools. Calcium-binding proteins and plasma membrane calcium pumps, together with endomembrane pumps and channels, play important roles in calcium handling at ribbon synapses. Taken together, emerging evidence suggests that several molecular and cellular specializations work in concert to support the sustained exocytosis of glutamate that is a hallmark of ribbon synapses. Consistent with its functional importance, abnormalities in a variety of functional aspects of the ribbon presynaptic terminal underlie several forms of auditory neuropathy and retinopathy.

Selective saturation of slow endocytosis at a giant glutamatergic central synapse lacking dynamin 1

Exocytosis of synaptic vesicles is rapidly followed by compensatory plasma membrane endocytosis. The efficiency of endocytosis varies with experimental conditions, but the molecular basis for this control remains poorly understood. Here, the function of dynamin 1, the neuron-specific member of a family of GTPases implicated in vesicle fission, was investigated with high temporal resolution via membrane capacitance measurements at the calyx of Held, a giant glutamatergic synapse. Endocytosis at dynamin 1 KO calyces was the same as in wild type after weak stimuli, consistent with the nearly normal ultrastructure of mutant synapses. However, following stronger stimuli, the speed of slow endocytosis, but not of other forms of endocytosis, failed to scale with the increased endocytic load. Thus, high level expression of dynamin 1 is essential to allow the slow, clathrin-mediated endocytosis, which accounts for the bulk of the endocytic response, to operate efficiently over a wide range of activity.

Synaptic vesicle endocytosis: fast and slow modes of membrane retrieval

Several modes of synaptic vesicle release, retrieval and recycling have been identified. In a well-established mode of exocytosis, termed 'full-collapse fusion', vesicles empty their neurotransmitter content fully into the synaptic cleft by flattening out and becoming part of the presynaptic membrane. The fused vesicle membrane is then reinternalized via a slow and clathrin-dependent mode of compensatory endocytosis that takes several seconds. A more fleeting mode of vesicle fusion, termed 'kiss-and-run' exocytosis or 'flicker-fusion', indicates that during synaptic transmission some vesicles are only briefly connected to the presynaptic membrane by a transient fusion pore. Finally, a mode that retrieves a large amount of membrane, equivalent to that of several fused vesicles, termed 'bulk endocytosis', has been found after prolonged exocytosis. We are of the opinion that both fast and slow modes of endocytosis co-exist at central nervous system nerve terminals and that one mode can predominate depending on stimulus strength, temperature and synaptic maturation.

Synaptic vesicle endocytosis at a CNS nerve terminal: faster kinetics at physiological temperatures and increased endocytotic capacity during maturation

Synaptic vesicle membrane must be quickly retrieved and recycled after copious exocytosis to limit the depletion of vesicle pools. The rate of endocytosis at the calyx of Held nerve terminal has been measured directly using membrane capacitance measurements from immature postnatal day P7-P10 rat pups at room temperature (RT: 23-24 degrees C). This rate has an average time constant of tens of seconds and becomes slower when the amount of exocytosis (measured as capacitance jump) increases. Such slow rates seem paradoxical for a synapse that can operate continuously at high-input frequencies. Here we perform time-resolved membrane capacitance measurements from the mouse calyx of Held in brain stem slices at physiological temperature (PT: 35-37 degrees C), and also from more mature calyces after the onset of hearing (P14-P18). Our results show that the rate of endocytosis is strongly temperature dependent, whereas the endocytotic capacity of a nerve terminal is dependent on developmental stage. At PT we find that endocytosis accelerates due to the addition of a kinetically fast component (time constant: tau = 1-2 s) immediately after exocytosis. Surprisingly, we find that at RT the rate of endocytosis triggered by short (1- to 5-ms) or long (> or =10-ms) depolarizing pulses in P14-P18 mice are similar (tau approximately 15 s). Furthermore, this rate is greatly accelerated at PT (tau approximately 2 s). Thus endocytosis becomes faster and less saturable during synaptic maturation, making the calyceal terminal more capable of sustaining prolonged high-frequency transmitter release.