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

Persistence of quantal synaptic vesicle recycling in virtual absence of dynamins

Dynamins are GTPases required for pinching vesicles off the plasma membrane once a critical curvature is reached during endocytosis. Here, we probed dynamin function in central synapses by depleting all three dynamin isoforms in postnatal hippocampal neurons down to negligible levels. We found a decrease in the propensity of evoked neurotransmission as well as a reduction in synaptic vesicle numbers. Recycling of synaptic vesicles during spontaneous or low levels of evoked activity were largely impervious to dynamin depletion, while retrieval of synaptic vesicle components at higher levels of activity was partially arrested. These results suggest the existence of balancing dynamin-independent mechanisms for synaptic vesicle recycling at central synapses. Classical dynamin-dependent mechanisms are not essential for retrieval of synaptic vesicle proteins after quantal single synaptic vesicle fusion, but they become more relevant for membrane retrieval during intense, sustained neuronal activity. KEY POINTS: Loss of dynamin 2 does not impair synaptic transmission. Loss of all three dynamin isoforms mostly affects evoked neurotransmission. Excitatory synapse function is more susceptible to dynamin loss. Spontaneous neurotransmission is only mildly affected by loss of dynamins. Single synaptic vesicle endocytosis is largely dynamin independent.

The Structural and Functional Integrity of Rod Photoreceptor Ribbon Synapses Depends on Redundant Actions of Dynamins 1 and 3

Vertebrate vision begins with light absorption by rod and cone photoreceptors, which transmit signals from their synaptic terminals to second-order neurons: bipolar and horizontal cells. In mouse rods, there is a single presynaptic ribbon-type active zone at which the release of glutamate occurs tonically in the dark. This tonic glutamatergic signaling requires continuous exo- and endocytosis of synaptic vesicles. At conventional synapses, endocytosis commonly requires dynamins: GTPases encoded by three genes (Dnm1-3), which perform membrane scission. Disrupting endocytosis by dynamin deletions impairs transmission at conventional synapses, but the impact of disrupting endocytosis and the role(s) of specific dynamin isoforms at rod ribbon synapses are understood incompletely. Here, we used cell-specific knock-outs (KOs) of the neuron-specific Dnm1 and Dnm3 to investigate the functional roles of dynamin isoforms in rod photoreceptors in mice of either sex. Analysis of synaptic protein expression, synapse ultrastructure, and retinal function via electroretinograms (ERGs) showed that dynamins 1 and 3 act redundantly and are essential for supporting the structural and functional integrity of rod ribbon synapses. Single Dnm3 KO showed no phenotype, and single Dnm1 KO only modestly reduced synaptic vesicle density without affecting vesicle size and overall synapse integrity, whereas double Dnm1/Dnm3 KO impaired vesicle endocytosis profoundly, causing enlarged vesicles, reduced vesicle density, reduced ERG responses, synaptic terminal degeneration, and disassembly and degeneration of postsynaptic processes. Concurrently, cone function remained intact. These results show the fundamental redundancy of dynamins 1 and 3 in regulating the structure and function of rod ribbon synapses.

Clathrin mediates membrane fission and budding by constricting membrane pores

Membrane budding, which underlies fundamental processes like endocytosis, intracellular trafficking, and viral infection, is thought to involve membrane coat-forming proteins, including the most observed clathrin, to form Ω-shape profiles and helix-forming proteins like dynamin to constrict Ω-profiles' pores and thus mediate fission. Challenging this fundamental concept, we report that polymerized clathrin is required for Ω-profiles' pore closure and that clathrin around Ω-profiles' base/pore region mediates pore constriction/closure in neuroendocrine chromaffin cells. Mathematical modeling suggests that clathrin polymerization at Ω-profiles' base/pore region generates forces from its intrinsically curved shape to constrict/close the pore. This new fission function may exert broader impacts than clathrin's well-known coat-forming function during clathrin (coat)-dependent endocytosis, because it underlies not only clathrin (coat)-dependent endocytosis, but also diverse endocytic modes, including ultrafast, fast, slow, bulk, and overshoot endocytosis previously considered clathrin (coat)-independent in chromaffin cells. It mediates kiss-and-run fusion (fusion pore closure) previously considered bona fide clathrin-independent, and limits the vesicular content release rate. Furthermore, analogous to results in chromaffin cells, we found that clathrin is essential for fast and slow endocytosis at hippocampal synapses where clathrin was previously considered dispensable, suggesting clathrin in mediating synaptic vesicle endocytosis and fission. These results suggest that clathrin and likely other intrinsically curved coat proteins are a new class of fission proteins underlying vesicle budding and fusion. The half-a-century concept and studies that attribute vesicle-coat contents' function to Ω-profile formation and classify budding as coat-protein (e.g., clathrin)-dependent or -independent may need to be re-defined and re-examined by considering clathrin's pivotal role in pore constriction/closure.

Cocaine increases quantal norepinephrine secretion through NET-dependent PKC activation in locus coeruleus neurons

The norepinephrine neurons in locus coeruleus (LC-NE neurons) are essential for sleep arousal, pain sensation, and cocaine addiction. According to previous studies, cocaine increases NE overflow (the profile of extracellular NE level in response to stimulation) by blocking the NE reuptake. NE overflow is determined by NE release via exocytosis and reuptake through NE transporter (NET). However, whether cocaine directly affects vesicular NE release has not been directly tested. By recording quantal NE release from LC-NE neurons, we report that cocaine directly increases the frequency of quantal NE release through regulation of NET and downstream protein kinase C (PKC) signaling, and this facilitation of NE release modulates the activity of LC-NE neurons and cocaine-induced stimulant behavior. Thus, these findings expand the repertoire of mechanisms underlying the effects of cocaine on NE (pro-release and anti-reuptake), demonstrate NET as a release enhancer in LC-NE neurons, and provide potential sites for treatment of cocaine addiction.

Dynamin 1 controls vesicle size and endocytosis at hippocampal synapses

Following calcium-triggered vesicle exocytosis, endocytosis regenerates vesicles to maintain exocytosis and thus synaptic transmission, which underlies neuronal circuit activities. Although most molecules involved in endocytosis have been identified, it remains rather poorly understood how endocytic machinery regulates vesicle size. Vesicle size, together with the transmitter concentration inside the vesicle, determines the amount of transmitter the vesicle can release, the quantal size, that may control the strength of synaptic transmission. Here, we report that, surprisingly, knockout of the GTPase dynamin 1, the most abundant brain dynamin isoform known to catalyze fission of the membrane pit's neck (the last step of endocytosis), not only significantly slowed endocytosis but also increased the synaptic vesicle diameter by as much as ∼40-64% at cultured hippocampal synapses. Furthermore, dynamin 1 knockout increased the size of membrane pits, the precursor for endocytic vesicle formation. These results suggest an important function of dynamin other than its well-known fission function - control of vesicle size at the pit formation stage.

Preformed Ω-profile closure and kiss-and-run mediate endocytosis and diverse endocytic modes in neuroendocrine chromaffin cells

Transformation of flat membrane into round vesicles is generally thought to underlie endocytosis and produce speed-, amount-, and vesicle-size-specific endocytic modes. Visualizing depolarization-induced exocytic and endocytic membrane transformation in live neuroendocrine chromaffin cells, we found that flat membrane is transformed into Λ-shaped, Ω-shaped, and O-shaped vesicles via invagination, Λ-base constriction, and Ω-pore constriction, respectively. Surprisingly, endocytic vesicle formation is predominantly from not flat-membrane-to-round-vesicle transformation but calcium-triggered and dynamin-mediated closure of (1) Ω profiles formed before depolarization and (2) fusion pores (called kiss-and-run). Varying calcium influxes control the speed, number, and vesicle size of these pore closures, resulting in speed-specific slow (more than ∼6 s), fast (less than ∼6 s), or ultrafast (<0.6 s) endocytosis, amount-specific compensatory endocytosis (endocytosis = exocytosis) or overshoot endocytosis (endocytosis > exocytosis), and size-specific bulk endocytosis. These findings reveal major membrane transformation mechanisms underlying endocytosis, diverse endocytic modes, and exocytosis-endocytosis coupling, calling for correction of the half-a-century concept that the flat-to-round transformation predominantly mediates endocytosis after physiological stimulation.

Dynamin deficiency causes insulin secretion failure and hyperglycemia

Pancreatic β cells operate with a high rate of membrane recycling for insulin secretion, yet endocytosis in these cells is not fully understood. We investigate this process in mature mouse β cells by genetically deleting dynamin GTPase, the membrane fission machinery essential for clathrin-mediated endocytosis. Unexpectedly, the mice lacking all three dynamin genes (DNM1, DNM2, DNM3) in their β cells are viable, and their β cells still contain numerous insulin granules. Endocytosis in these β cells is severely impaired, resulting in abnormal endocytic intermediates on the plasma membrane. Although insulin granules are abundant, their release upon glucose stimulation is blunted in both the first and second phases, leading to hyperglycemia and glucose intolerance in mice. Dynamin triple deletion impairs insulin granule exocytosis and decreases intracellular Ca2+ responses and granule docking. The docking defect is correlated with reduced expression of Munc13-1 and RIM1 and reorganization of cortical F-actin in β cells. Collectively, these findings uncover the role of dynamin in dense-core vesicle endocytosis and secretory capacity. Insulin secretion deficiency in the absence of dynamin-mediated endocytosis highlights the risk of impaired membrane trafficking in endocrine failure and diabetes pathogenesis.

Early and nonredundant functions of dynamin isoforms in clathrin-mediated endocytosis

Dynamin GTPases (Dyn1 and Dyn2) are indispensable proteins of the core clathrin-mediated endocytosis (CME) machinery. Best known for their role in fission at the late stages of CME, many studies have suggested that dynamin also plays a regulatory role during the early stages of CME; however, detailed studies regarding isoform-specific early regulatory functions of the dynamins are lacking. With a recent understanding of the regulation of Dyn1 in nonneuronal cells and improved algorithms for highly sensitive and quantitative analysis of clathrin-coated pit (CCP) dynamics, we have evaluated the differential functions of dynamin isoforms in CME using domain swap chimeras. We report that Dyn1 and Dyn2 play nonredundant, early regulatory roles during CME in nonneuronal cells. The proline/arginine-rich domain of Dyn2 is important for its targeting to nascent and growing CCPs, whereas the membrane-binding and curvature-generating pleckstrin homology domain of Dyn1 plays an important role in stabilizing nascent CCPs. We confirm the enhanced ability of dephosphorylated Dyn1 to support CME, even at substoichiometric levels compared with Dyn2. Domain swap chimeras also revealed previously unknown functional differences in the GTPase and stalk domains. Our study significantly extends the current understanding of the regulatory roles played by dynamin isoforms during early stages of CME.

Second-Generation Histamine H1 Receptor Antagonists Suppress Delayed Rectifier K+-Channel Currents in Murine Thymocytes

BACKGROUND/AIMS

Voltage-dependent potassium channels (Kv1.3) are predominantly expressed in lymphocyte plasma membranes. These channels are critical for the activation and proliferation of lymphocytes. Since second-generation antihistamines are lipophilic and exert immunomodulatory effects, they are thought to affect the lymphocyte Kv1.3-channel currents.

METHODS

Using the patch-clamp whole-cell recording technique in murine thymocytes, we tested the effects of second-generation antihistamines, such as cetirizine, fexofenadine, azelastine, and terfenadine, on the channel currents and the membrane capacitance.

RESULTS

These drugs suppressed the peak and the pulse-end currents of the channels, although the effects of azelastine and terfenadine on the peak currents were more marked than those of cetirizine and fexofenadine. Both azelastine and terfenadine significantly lowered the membrane capacitance. Since these drugs did not affect the process of endocytosis in lymphocytes, they were thought to have interacted directly with the plasma membranes.

CONCLUSIONS

Our study revealed for the first time that second-generation antihistamines, including cetirizine, fexofenadine, azelastine, and terfenadine, exert suppressive effects on lymphocyte Kv1.3-channels. The efficacy of these drugs may be related to their immunomodulatory mechanisms that reduce the synthesis of inflammatory cytokine.

Real-Time Endocytosis Measurements by Membrane Capacitance Recording at Central Nerve Terminals

Endocytosis is fundamental to cell function. It can be monitored by capacitance measurements under patch-clamp recordings. Membrane capacitance recording measures the cell membrane surface area and its changes at high temporal-resolution and sensitivity, and it is a powerful biophysical approach in the field of exocytosis and endocytosis. A popular one is the frequency domain method that entails processing passive sinusoidal membrane currents induced by a sinusoidal voltage. This technique requires a phase-sensitive detector or "lock-in amplifier" implemented in hardware or software during patch-clamp recordings. It has been widely used in many secretory cells, but its application directly at central presynaptic terminals is technically challenging. We have applied this technique to study synaptic endocytosis in the calyx of Held, a large glutamatergic synaptic terminal, as well as mouse pancreatic β-cells. The presynaptic capacitance measurements provide a unique alternative to measuring transmitter release and presynaptic endocytosis. Here, we describe this method at the calyx of Held in acute brain slices and provide a practical guide to obtaining high quality capacitance measurements at presynaptic terminals.

Endocytosis of Activated Muscarinic m2 Receptor (m2R) in Live Mouse Hippocampal Neurons Occurs via a Clathrin-Dependent Pathway

Our aim was to examine the dynamics of the muscarinic m2 receptor (m2R), a G-protein coupled receptor (GPCR), after agonist activation in living hippocampal neurons, and especially clathrin dependency endocytosis. We have previously shown that the m2R undergoes agonist-induced internalization in vivo. However, the nature of the endocytotic pathway used by m2R after activation is still unknown in living neurons. Using live cell imaging and quantitative analyses, we have monitored the effect of stimulation on the fate of the membrane-bound m2R and on its redistribution in intraneuronal compartments. Shortly (6 min) after activation, m2R is internalized into clathrin immunopositive structures. Furthermore, after clathrin-dependent endocytosis, m2R associates with early and late endosomes and with subcellular organelles involved in degradation. Together, these results provide, for the first time, a description of m2R trafficking in living neurons and prove that m2R undergoes clathrin-dependent endocytosis before being degraded.

Presynaptic loss of dynamin-related protein 1 impairs synaptic vesicle release and recycling at the mouse calyx of Held

KEY POINTS

This study characterizes the mechanisms underlying defects in synaptic transmission when dynamin-related protein 1 (DRP1) is genetically eliminated. Viral-mediated knockout of DRP1 from the presynaptic terminal at the mouse calyx of Held increased initial release probability, reduced the size of the synaptic vesicle recycling pool and impaired synaptic vesicle recycling. Transmission defects could be partially restored by increasing the intracellular calcium buffering capacity with EGTA-AM, implying close coupling of Ca²⁺ channels to synaptic vesicles was compromised. Acute restoration of ATP to physiological levels in the presynaptic terminal did not reverse the synaptic defects. Loss of DRP1 impairs mitochondrial morphology in the presynaptic terminal, which in turn seems to arrest synaptic maturation.

ABSTRACT

Impaired mitochondrial biogenesis and function is implicated in many neurodegenerative diseases, and likely affects synaptic neurotransmission prior to cellular loss. Dynamin-related protein 1 (DRP1) is essential for mitochondrial fission and is disrupted in neurodegenerative disease. In this study, we used the mouse calyx of Held synapse as a model to investigate the impact of presynaptic DRP1 loss on synaptic vesicle (SV) recycling and sustained neurotransmission. In vivo viral expression of Cre recombinase in ventral cochlear neurons of floxed-DRP1 mice generated a presynaptic-specific DRP1 knockout (DRP1-preKO), where the innervated postsynaptic cell was unperturbed. Confocal reconstruction of the calyx terminal suggested SV clusters and mitochondrial content were disrupted, and presynaptic terminal volume was decreased. Using postsynaptic voltage-clamp recordings, we found that DRP1-preKO synapses had larger evoked responses at low frequency stimulation. DRP1-preKO synapses also had profoundly altered short-term plasticity, due to defects in SV recycling. Readily releasable pool size, estimated with high-frequency trains, was dramatically reduced in DRP1-preKO synapses, suggesting an important role for DRP1 in maintenance of release-competent SVs at the presynaptic terminal. Presynaptic Ca²⁺ accumulation in the terminal was also enhanced in DRP1-preKO synapses. Synaptic transmission defects could be partially rescued with EGTA-AM, indicating close coupling of Ca²⁺ channels to SV distance normally found in mature terminals may be compromised by DRP1-preKO. Using paired recordings of the presynaptic and postsynaptic compartments, recycling defects could not be reversed by acute dialysis of ATP into the calyx terminals. Taken together, our results implicate a requirement for mitochondrial fission to coordinate postnatal synapse maturation.

Dynamin 1 Restrains Vesicular Release to a Subquantal Mode In Mammalian Adrenal Chromaffin Cells

Dynamin 1 (dyn1) is required for clathrin-mediated endocytosis in most secretory (neuronal and neuroendocrine) cells. There are two modes of Ca²⁺-dependent catecholamine release from single dense-core vesicles: full-quantal (quantal) and subquantal in adrenal chromaffin cells, but their relative occurrences and impacts on total secretion remain unclear. To address this fundamental question in neurotransmission area using both sexes of animals, here we report the following: (1) dyn1-KO increased quantal size (QS, but not vesicle size/content) by ≥250% in dyn1-KO mice; (2) the KO-increased QS was rescued by dyn1 (but not its deficient mutant or dyn2); (3) the ratio of quantal versus subquantal events was increased by KO; (4) following a release event, more protein contents were retained in WT versus KO vesicles; and (5) the fusion pore size (d _p) was increased from ≤9 to ≥9 nm by KO. Therefore, Ca²⁺-induced exocytosis is generally a subquantal release in sympathetic adrenal chromaffin cells, implying that neurotransmitter release is generally regulated by dynamin in neuronal cells.SIGNIFICANCE STATEMENT Ca²⁺-dependent neurotransmitter release from a single vesicle is the primary event in all neurotransmission, including synaptic/neuroendocrine forms. To determine whether Ca²⁺-dependent vesicular neurotransmitter release is "all-or-none" (quantal), we provide compelling evidence that most Ca²⁺-induced secretory events occur via the subquantal mode in native adrenal chromaffin cells. This subquantal release mode is promoted by dynamin 1, which is universally required for most secretory cells, including neurons and neuroendocrine cells. The present work with dyn1-KO mice further confirms that Ca²⁺-dependent transmitter release is mainly via subquantal mode, suggesting that subquantal release could be also important in other types of cells.

Sensing Exocytosis and Triggering Endocytosis at Synapses: Synaptic Vesicle Exocytosis-Endocytosis Coupling

The intact synaptic structure is critical for information processing in neural circuits. During synaptic transmission, rapid vesicle exocytosis increases the size of never terminals and endocytosis counteracts the increase. Accumulating evidence suggests that SV exocytosis and endocytosis are tightly connected in time and space during SV recycling, and this process is essential for synaptic function and structural stability. Research in the past has illustrated the molecular details of synaptic vesicle (SV) exocytosis and endocytosis; however, the mechanisms that timely connect these two fundamental events are poorly understood at central synapses. Here we discuss recent progress in SV recycling and summarize several emerging mechanisms by which synapses can “sense” the occurrence of exocytosis and timely initiate compensatory endocytosis. They include Ca²⁺ sensing, SV proteins sensing, and local membrane stress sensing. In addition, the spatial organization of endocytic zones adjacent to active zones provides a structural basis for efficient coupling between SV exocytosis and endocytosis. Through linking different endocytosis pathways with SV fusion, these mechanisms ensure necessary plasticity and robustness of nerve terminals to meet diverse physiological needs.

Dissecting the genetic components of a quantitative trait locus for blood pressure and renal pathology on rat chromosome 3

BACKGROUND

We have previously confirmed the importance of rat chromosome 3 (RNO3) genetic loci on blood pressure elevation, pulse pressure (PP) variability and renal pathology during salt challenge in the stroke-prone spontaneously hypertensive (SHRSP) rat. The aims of this study were to generate a panel of RNO3 congenic sub-strains to genetically dissect the implicated loci and identify positional candidate genes by microarray expression profiling and analysis of next-generation sequencing data.

METHOD AND RESULTS

A panel of congenic sub-strains were generated containing Wistar-Kyoto (WKY)-introgressed segments of varying size on the SHRSP genetic background, focused within the first 50 Mbp of RNO3. Haemodynamic profiling during salt challenge demonstrated significantly reduced systolic blood pressure, diastolic blood pressure and PP variability in SP.WKYGla3a, SP.WKYGla3c, SP.WKYGla3d and SP.WKYGla3e sub-strains. Only SBP and DBP were significantly reduced during salt challenge in SP.WKYGla3b and SP.WKYGla3f sub-strains, whereas SP.WKYGla3g rats did not differ in haemodynamic response to SHRSP. Those sub-strains demonstrating significantly reduced PP variability during salt challenge also demonstrated significantly reduced renal pathology and proteinuria. Microarray expression profiling prioritized two candidate genes for blood pressure regulation (Dnm1, Tor1b), localized within the common congenic interval shared by SP.WKYGla3d and SP.WKYGla3f strains, and one candidate gene for salt-induced PP variability and renal pathology (Rabgap1), located within the region unique to the SP.WKYGla3d strain. Comparison of next-generation sequencing data identified variants within additional positional genes that are likely to affect protein function.

CONCLUSION

This study has identified distinct intervals on RNO3-containing genes that may be important for blood pressure regulation and renal pathology during salt challenge.

Dynamin 1- and 3-Mediated Endocytosis Is Essential for the Development of a Large Central Synapse In Vivo

Dynamin is a large GTPase crucial for endocytosis and sustained neurotransmission, but its role in synapse development in the mammalian brain has received little attention. We addressed this question using the calyx of Held (CH), a large nerve terminal in the auditory brainstem in mice. Tissue-specific ablation of different dynamin isoforms bypasses the early lethality of conventional knock-outs and allows us to examine CH development in a native brain circuit. Individual gene deletion of dynamin 1, a primary dynamin isoform in neurons, as well as dynamin 2 and 3, did not affect CH development. However, combined tissue-specific knock-out of both dynamin 1 and 3 (cDKO) severely impaired CH formation and growth during the first postnatal week, and the phenotypes were exacerbated by further additive conditional knock-out of dynamin 2. The developmental defect of CH in cDKO first became evident on postnatal day 3 (P3), a time point when CH forms and grows abruptly. This is followed by a progressive loss of postsynaptic neurons and increased glial infiltration late in development. However, early CH synaptogenesis before protocalyx formation was not altered in cDKO. Functional maturation of synaptic transmission in the medial nucleus of the trapezoid body in cDKO was impeded during development and accompanied by an increase in the membrane excitability of medial nucleus of the trapezoid body neurons. This study provides compelling genetic evidence that CH formation requires dynamin 1- and 3-mediated endocytosis in vivo, indicating a critical role of dynamin in synaptic development, maturation, and subsequent maintenance in the mammalian brain.

SIGNIFICANCE STATEMENT

Synaptic development has been increasingly implicated in numerous brain disorders. Dynamin plays a crucial role in clathrin-mediated endocytosis and synaptic transmission at nerve terminals, but its potential role in synaptic development in the native brain circuitry is unclear. Using the calyx of Held, a giant nerve terminal in the mouse brainstem, we evaluated the role of dynamin in this process by using tissue-specific knock-out (KO) of three different dynamin isoforms (dynamin 1, 2, and 3) individually and in combination. Our data demonstrated that dynamin is required for the formation, functional maturation, and subsequent survival of the calyx of Held. This study highlights the important role of dynamin-mediated endocytosis in the development of central synapses in the mammalian brain.

Promotion of endocytosis efficiency through an ATP-independent mechanism at rat calyx of Held terminals

KEY POINTS

At rat calyx of Held terminals, ATP was required not only for slow endocytosis, but also for rapid phase of compensatory endocytosis. An ATP-independent form of endocytosis was recruited to accelerate membrane retrieval at increased activity and temperature. ATP-independent endocytosis primarily involved retrieval of pre-existing membrane, which depended on Ca²⁺ and the activity of neutral sphingomyelinase but not clathrin-coated pit maturation. ATP-independent endocytosis represents a non-canonical mechanism that can efficiently retrieve membrane at physiological conditions without competing for the limited ATP at elevated neuronal activity.

ABSTRACT

Neurotransmission relies on membrane endocytosis to maintain vesicle supply and membrane stability. Endocytosis has been generally recognized as a major ATP-dependent function, which efficiently retrieves more membrane at elevated neuronal activity when ATP consumption within nerve terminals increases drastically. This paradox raises the interesting question of whether increased activity recruits ATP-independent mechanism(s) to accelerate endocytosis at the same time as preserving ATP availability for other tasks. To address this issue, we studied ATP requirement in three typical forms of endocytosis at rat calyx of Held terminals by whole-cell membrane capacitance measurements. At room temperature, blocking ATP hydrolysis effectively abolished slow endocytosis and rapid endocytosis but only partially inhibited excess endocytosis following intense stimulation. The ATP-independent endocytosis occurred at calyces from postnatal days 8-15, suggesting its existence before and after hearing onset. This endocytosis was not affected by a reduction of exocytosis using the light chain of botulinum toxin C, nor by block of clathrin-coat maturation. It was abolished by EGTA, which preferentially blocked endocytosis of retrievable membrane pre-existing at the surface, and was impaired by oxidation of cholesterol and inhibition of neutral sphingomyelinase. ATP-independent endocytosis became more significant at 34-35°C, and recovered membrane by an amount that, on average, was close to exocytosis. The results of the present study suggest that activity and temperature recruit ATP-independent endocytosis of pre-existing membrane (in addition to ATP-dependent endocytosis) to efficiently retrieve membrane at nerve terminals. This less understood endocytosis represents a non-canonical mechanism regulated by lipids such as cholesterol and sphingomyelinase.

Exocytosis, Endocytosis, and Their Coupling in Excitable Cells

Evoked exocytosis in excitable cells is fast and spatially confined and must be followed by coupled endocytosis to enable sustained exocytosis while maintaining the balance of the vesicle pool and the plasma membrane. Various types of exocytosis and endocytosis exist in these excitable cells, as those has been found from different types of experiments conducted in different cell types. Correlating these diversified types of exocytosis and endocytosis is problematic. By providing an outline of different exocytosis and endocytosis processes and possible coupling mechanisms here, we emphasize that the endocytic pathway may be pre-determined at the time the vesicle chooses to fuse with the plasma membrane in one specific mode. Therefore, understanding the early intermediate stages of vesicle exocytosis may be instrumental in exploring the mechanism of tailing endocytosis.

Molecular Mechanisms for the Coupling of Endocytosis to Exocytosis in Neurons

Neuronal communication and brain function mainly depend on the fundamental biological events of neurotransmission, including the exocytosis of presynaptic vesicles (SVs) for neurotransmitter release and the subsequent endocytosis for SV retrieval. Neurotransmitters are released through the Ca²⁺- and SNARE-dependent fusion of SVs with the presynaptic plasma membrane. Following exocytosis, endocytosis occurs immediately to retrieve SV membrane and fusion machinery for local recycling and thus maintain the homeostasis of synaptic structure and sustained neurotransmission. Apart from the general endocytic machinery, recent studies have also revealed the involvement of SNARE proteins (synaptobrevin, SNAP25 and syntaxin), synaptophysin, Ca²⁺/calmodulin, and members of the synaptotagmin protein family (Syt1, Syt4, Syt7 and Syt11) in the balance and tight coupling of exo-endocytosis in neurons. Here, we provide an overview of recent progress in understanding how these neuron-specific adaptors coordinate to ensure precise and efficient endocytosis during neurotransmission.

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.

Tissue-specific dynamin-1 deletion at the calyx of Held decreases short-term depression through a mechanism distinct from vesicle resupply

Dynamin is a large GTPase with a crucial role in synaptic vesicle regeneration. Acute dynamin inhibition impairs neurotransmitter release, in agreement with the protein's established role in vesicle resupply. Here, using tissue-specific dynamin-1 knockout [conditional knockout (cKO)] mice at a fast central synapse that releases neurotransmitter at high rates, we report that dynamin-1 deletion unexpectedly leads to enhanced steady-state neurotransmission and consequently less synaptic depression during brief periods of high-frequency stimulation. These changes are also accompanied by increased transmission failures. Interestingly, synaptic vesicle resupply and several other synaptic properties remain intact, including basal neurotransmission, presynaptic Ca(2+) influx, initial release probability, and postsynaptic receptor saturation and desensitization. However, acute application of Latrunculin B, a reagent known to induce actin depolymerization and impair bulk and ultrafast endocytosis, has a stronger effect on steady-state depression in cKO than in control and brings the depression down to a control level. The slow phase of presynaptic capacitance decay following strong stimulation is impaired in cKO; the rapid capacitance changes immediately after strong depolarization are also different between control and cKO and sensitive to Latrunculin B. These data raise the possibility that, in addition to its established function in regenerating synaptic vesicles, the endocytosis protein dynamin-1 may have an impact on short-term synaptic depression. This role comes into play primarily during brief high-frequency stimulation.

Fast, Temperature-Sensitive and Clathrin-Independent Endocytosis at Central Synapses

The fusion of neurotransmitter-filled vesicles during synaptic transmission is balanced by endocytotic membrane retrieval. Despite extensive research, the speed and mechanisms of synaptic vesicle endocytosis have remained controversial. Here, we establish low-noise time-resolved membrane capacitance measurements that allow monitoring changes in surface membrane area elicited by single action potentials and stronger stimuli with high-temporal resolution at physiological temperature in individual bona-fide mature central synapses. We show that single action potentials trigger very rapid endocytosis, retrieving presynaptic membrane with a time constant of 470 ms. This fast endocytosis is independent of clathrin but mediated by dynamin and actin. In contrast, stronger stimuli evoke a slower mode of endocytosis that is clathrin, dynamin, and actin dependent. Furthermore, the speed of endocytosis is highly temperature dependent with a Q10 of ∼3.5. These results demonstrate that distinct molecular modes of endocytosis with markedly different kinetics operate at central synapses.

The calyx of Held in the auditory system: Structure, function, and development

The calyx of Held synapse plays an important role in the auditory system, relaying information about sound localization via fast and precise synaptic transmission, which is achieved by its specialized structure and giant size. During development, the calyx of Held undergoes anatomical, morphological, and physiological changes necessary for performing its functions. The large dimensions of the calyx of Held nerve terminal are well suited for direct electrophysiological recording of many presynaptic events that are difficult, if not impossible to record at small conventional synapses. This unique accessibility has been used to investigate presynaptic ion channels, transmitter release, and short-term plasticity, providing invaluable information about basic presynaptic mechanisms of transmission at a central synapse. Here, we review anatomical and physiological specializations of the calyx of Held, summarize recent studies that provide new mechanisms important for calyx development and reliable synaptic transmission, and examine fundamental presynaptic mechanisms learned from studies using calyx as a model nerve terminal. This article is part of a Special Issue entitled <Annual Reviews 2016>.

Synaptotagmin-11 inhibits clathrin-mediated and bulk endocytosis

Precise and efficient endocytosis is essential for vesicle recycling during a sustained neurotransmission. The regulation of endocytosis has been extensively studied, but inhibitors have rarely been found. Here, we show that synaptotagmin-11 (Syt11), a non-Ca(2+)-binding Syt implicated in schizophrenia and Parkinson's disease, inhibits clathrin-mediated endocytosis (CME) and bulk endocytosis in dorsal root ganglion neurons. The frequency of both types of endocytic event increases in Syt11 knockdown neurons, while the sizes of endocytosed vesicles and the kinetics of individual bulk endocytotic events remain unaffected. Specifically, clathrin-coated pits and bulk endocytosis-like structures increase on the plasma membrane in Syt11-knockdown neurons. Structural-functional analysis reveals distinct domain requirements for Syt11 function in CME and bulk endocytosis. Importantly, Syt11 also inhibits endocytosis in hippocampal neurons, implying a general role of Syt11 in neurons. Taken together, we propose that Syt11 functions to ensure precision in vesicle retrieval, mainly by limiting the sites of membrane invagination at the early stage of endocytosis.

Ubiquitin-Synaptobrevin Fusion Protein Causes Degeneration of Presynaptic Motor Terminals in Mice

Protein aggregates containing ubiquitin (Ub) are commonly observed in neurodegenerative disorders, implicating the involvement of the ubiquitin proteasome system (UPS) in their pathogenesis. Here, we aimed to generate a mouse model for monitoring UPS function using a green fluorescent protein (GFP)-based substrate that carries a "noncleavable" N-terminal ubiquitin moiety (Ub(G76V)). We engineered transgenic mice expressing a fusion protein, consisting of the following: (1) Ub(G76V), GFP, and a synaptic vesicle protein synaptobrevin-2 (Ub(G76V)-GFP-Syb2); (2) GFP-Syb2; or (3) Ub(G76V)-GFP-Syntaxin1, all under the control of a neuron-specific Thy-1 promoter. As expected, Ub(G76V)-GFP-Syb2, GFP-Syb2, and Ub(G76V)-GFP-Sytaxin1 were highly expressed in neurons, such as motoneurons and motor nerve terminals of the neuromuscular junction (NMJ). Surprisingly, Ub(G76V)-GFP-Syb2 mice developed progressive adult-onset degeneration of motor nerve terminals, whereas GFP-Syb2 and Ub(G76V)-GFP-Syntaxin1 mice were normal. The degeneration of nerve terminals in Ub(G76V)-GFP-Syb2 mice was preceded by a progressive impairment of synaptic transmission at the NMJs. Biochemical analyses demonstrated that Ub(G76V)-GFP-Syb2 interacted with SNAP-25 and Syntaxin1, the SNARE partners of synaptobrevin. Ultrastructural analyses revealed a marked reduction in synaptic vesicle density, accompanying an accumulation of tubulovesicular structures at presynaptic nerve terminals. These morphological defects were largely restricted to motor nerve terminals, as the ultrastructure of motoneuron somata appeared to be normal at the stages when synaptic nerve terminals degenerated. Furthermore, synaptic vesicle endocytosis and membrane trafficking were impaired in Ub(G76V)-GFP-Syb2 mice. These findings indicate that Ub(G76V)-GFP-Syb2 may compete with endogenous synaptobrevin, acting as a gain-of-function mutation that impedes SNARE function, resulting in the depletion of synaptic vesicles and degeneration of the nerve terminals.

SIGNIFICANCE STATEMENT

Degeneration of motor nerve terminals occurs in amyotrophic lateral sclerosis (ALS) patients as well as in mouse models of ALS, leading to progressive paralysis. What causes a motor nerve terminal to degenerate remains unknown. Here we report on transgenic mice expressing a ubiquitinated synaptic vesicle protein (Ub(G76V)-GFP-Syb2) that develop progressive degeneration of motor nerve terminals. These mice may serve as a model for further elucidating the underlying cellular and molecular mechanisms of presynaptic nerve terminal degeneration.

Dynamin 2 regulates biphasic insulin secretion and plasma glucose homeostasis

Alterations in insulin granule exocytosis and endocytosis are paramount to pancreatic β cell dysfunction in diabetes mellitus. Here, using temporally controlled gene ablation specifically in β cells in mice, we identified an essential role of dynamin 2 GTPase in preserving normal biphasic insulin secretion and blood glucose homeostasis. Dynamin 2 deletion in β cells caused glucose intolerance and substantial reduction of the second phase of glucose-stimulated insulin secretion (GSIS); however, mutant β cells still maintained abundant insulin granules, with no signs of cell surface expansion. Compared with control β cells, real-time capacitance measurements demonstrated that exocytosis-endocytosis coupling was less efficient but not abolished; clathrin-mediated endocytosis (CME) was severely impaired at the step of membrane fission, which resulted in accumulation of clathrin-coated endocytic intermediates on the plasma membrane. Moreover, dynamin 2 ablation in β cells led to striking reorganization and enhancement of actin filaments, and insulin granule recruitment and mobilization were impaired at the later stage of GSIS. Together, our results demonstrate that dynamin 2 regulates insulin secretory capacity and dynamics in vivo through a mechanism depending on CME and F-actin remodeling. Moreover, this study indicates a potential pathophysiological link between endocytosis and diabetes mellitus.

Cholesterol regulates multiple forms of vesicle endocytosis at a mammalian central synapse

Endocytosis in synapses sustains neurotransmission by recycling vesicle membrane and maintaining the homeostasis of synaptic membrane. A role of membrane cholesterol in synaptic endocytosis remains controversial because of conflicting observations, technical limitations in previous studies, and potential interference from non-specific effects after cholesterol manipulation. Furthermore, it remains unclear whether cholesterol participates in distinct forms of endocytosis that function under different activity levels. In this study, applying the whole-cell membrane capacitance measurement to monitor endocytosis in real time at the rat calyx of Held terminals, we found that disrupting cholesterol with dialysis of cholesterol oxidase or methyl-β-cyclodextrin impaired three different forms of endocytosis, including slow endocytosis, rapid endocytosis, and endocytosis of the retrievable membrane that exists at the surface before stimulation. The effects were observed when disruption of cholesterol was mild enough not to change Ca(2+) channel current or vesicle exocytosis, indicative of stringent cholesterol requirement in synaptic endocytosis. Extracting cholesterol with high concentrations of methyl-β-cyclodextrin reduced exocytosis, mainly by decreasing the readily releasable pool and the vesicle replenishment after readily releasable pool depletion. Our study suggests that cholesterol is an important, universal regulator in multiple forms of vesicle endocytosis at mammalian central synapses.

A dynamin 1-, dynamin 3- and clathrin-independent pathway of synaptic vesicle recycling mediated by bulk endocytosis

The exocytosis of synaptic vesicles (SVs) elicited by potent stimulation is rapidly compensated by bulk endocytosis of SV membranes leading to large endocytic vacuoles (‘bulk’ endosomes). Subsequently, these vacuoles disappear in parallel with the reappearance of new SVs. We have used synapses of dynamin 1 and 3 double knock-out neurons, where clathrin-mediated endocytosis (CME) is dramatically impaired, to gain insight into the poorly understood mechanisms underlying this process. Massive formation of bulk endosomes was not defective, but rather enhanced, in the absence of dynamin 1 and 3. The subsequent conversion of bulk endosomes into SVs was not accompanied by the accumulation of clathrin coated buds on their surface and this process proceeded even after further clathrin knock-down, suggesting its independence of clathrin. These findings support the existence of a pathway for SV reformation that bypasses the requirement for clathrin and dynamin 1/3 and that operates during intense synaptic activity.

DOI:http://dx.doi.org/10.7554/eLife.01621.001

Neurons propagate electrical signals from one cell to the next using small molecules called neurotransmitters. These molecules are held inside small compartments called synaptic vesicles. Once a neuron receives an electrical stimulus, the membranes that enclose the synaptic vesicles fuse with the plasma membrane that encloses the neuron. This releases the neurotransmitters, which then trigger an electrical signal in the neighboring cell. Once the neurotransmitters are released, the vesicle membrane is rapidly reinternalized from the plasma membrane in a process called endocytosis and then recycled, ready for the next round of signal transmission.

The process of synaptic vesicle membrane endocytosis and recycling has been studied extensively, and several different mechanisms by which it occurs have been identified. The best understood relies on a protein called clathrin, and is thought to be essential for nervous system function. Recently, however, a mechanism of vesicle membrane endocytosis that does not involve clathrin was identified. This mechanism, called bulk endocytosis, involves reinternalizing large regions of the cell plasma membrane to generate large compartments called vacuoles, from which new synaptic vesicles eventually form. This mechanism has been observed when neurons fire at high frequency. The cellular processes underlying bulk endocytosis are not well understood, although several studies suggest proteins called dynamins are important.

Wu et al. simulated the conditions a cell experiences during high levels of activity in neurons that lacked the two major dynamins present at the synapses between neurons—dynamin 1 and dynamin 3. In these neurons, robust bulk endocytosis occurred, suggesting that these two major neuronal dynamins do not play a role in this process. Furthermore, formation of vesicles from the vacuoles generated by bulk endocytosis appeared to be clathrin-independent. These findings point to the occurrence of a pathway of synaptic vesicle recycling that bypasses the need for dynamin 1 and 3 as well as for clathrin.

To reconcile these results with previously published work, Wu et al. propose that dynamins may only be required for processes that also require clathrin. But how are vesicles recycled during bulk endocytosis if dynamins are not involved? There are currently few leads to base alternative mechanisms on. Further work is required to unravel this mystery, and to provide insights into how clathrin-dependent and independent recycling processes are linked during high neuronal activity.

DOI:http://dx.doi.org/10.7554/eLife.01621.002

Dynamin 1 is required for memory formation

Dynamin 1–3 isoforms are known to be involved in endocytotic processes occurring during synaptic transmission. No data has directly linked dynamins yet with normal animal behavior. Here we show that dynamin pharmacologic inhibition markedly impairs hippocampal-dependent associative memory. Memory loss was associated with changes in synaptic function occurring during repetitive stimulation that is thought to be linked with memory induction. Synaptic fatigue was accentuated by dynamin inhibition. Moreover, dynamin inhibition markedly reduced long-term potentiation, post-tetanic potentiation, and neurotransmitter released during repetitive stimulation. Most importantly, the effect of dynamin inhibition onto memory and synaptic plasticity was due to a specific involvement of the dynamin 1 isoform, as demonstrated through a genetic approach with siRNA against this isoform to temporally block it. Taken together, these findings identify dynamin 1 as a key protein for modulation of memory and release evoked by repetitive activity.

Modes and regulation of endocytic membrane retrieval in mouse auditory hair cells

Synaptic vesicle recycling sustains high rates of neurotransmission at the ribbon-type active zones (AZs) of mouse auditory inner hair cells (IHCs), but its modes and molecular regulation are poorly understood. Electron microscopy indicated the presence of clathrin-mediated endocytosis (CME) and bulk endocytosis. The endocytic proteins dynamin, clathrin, and amphiphysin are expressed and broadly distributed in IHCs. We used confocal vglut1-pHluorin imaging and membrane capacitance (Cm) measurements to study the spatial organization and dynamics of IHC exocytosis and endocytosis. Viral gene transfer expressed vglut1-pHluorin in IHCs and targeted it to synaptic vesicles. The intravesicular pH was ∼6.5, supporting only a modest increase of vglut1-pHluorin fluorescence during exocytosis and pH neutralization. Ca(2+) influx triggered an exocytic increase of vglut1-pHluorin fluorescence at the AZs, around which it remained for several seconds. The endocytic Cm decline proceeded with constant rate (linear component) after exocytosis of the readily releasable pool (RRP). When exocytosis exceeded three to four RRP equivalents, IHCs additionally recruited a faster Cm decline (exponential component) that increased with the amount of preceding exocytosis and likely reflects bulk endocytosis. The dynamin inhibitor Dyngo-4a and the clathrin blocker pitstop 2 selectively impaired the linear component of endocytic Cm decline. A missense mutation of dynamin 1 (fitful) inhibited endocytosis to a similar extent as Dyngo-4a. We propose that IHCs use dynamin-dependent endocytosis via CME to support vesicle cycling during mild stimulation but recruit bulk endocytosis to balance massive exocytosis.

Dynamin triple knockout cells reveal off target effects of commonly used dynamin inhibitors

Dynamin, which is encoded by three genes in mammals, is a GTPase implicated in endocytic membrane fission. Dynamin 1 and 3 are predominantly expressed in brain, whereas dynamin 2 is ubiquitously expressed. With the goal of assessing the impact of the lack of dynamin on cell physiology, we previously generated and characterized dynamin 1 and 2 double knockout (DKO) fibroblasts. These DKO cells were unexpectedly viable in spite of a severe impairment of clathrin-mediated endocytosis. As low-level expression of the dynamin 3 gene in these cells could not be excluded, we have now engineered dynamin 1, 2 and 3 triple KO (TKO) fibroblasts. These cells did not reveal any additional defects beyond what was previously observed in DKO fibroblasts. Surprisingly, although fluid-phase endocytosis and peripheral membrane ruffling were not impaired by the lack of all three dynamins, two structurally similar, widely used dynamin inhibitors, dynasore and Dyngo-4a, robustly inhibited these two processes both in wild-type and TKO cells. Dynamin TKO cells will be useful tools for the further exploration of dynamin-dependent processes and the development of more specific dynamin inhibitors.

Effect of chronic ethanol treatment on μ-opioid receptor function, interacting proteins and morphine-induced place preference

RATIONALE

Both the acute and chronic consumption of ethanol have been reported to modify several molecular events in the central nervous system, and the endogenous μ-opioid receptor system is involved in the reinforcing/rewarding effects of ethanol.

OBJECTIVES

The present study was designed to clarify the effects of chronic ethanol treatment on cellular processes involving μ-opioid receptor and the development of morphine-induced rewarding effects.

METHODS

Male C57BL/6J mice were continuously treated with a liquid diet containing 3.0 w/v ethanol. The direct involvement of μ-opioid receptor functions in the activation of G-proteins and changes in protein levels in the lower midbrain of mice after chronic treatment with ethanol were investigated by a [(35)S] GTPγS binding assay and Western blotting, respectively. The rewarding effects of morphine (5 mg/kg) under treatment with ethanol were measured by the conditioned place preference paradigm.

RESULTS

The function of μ-opioid receptor was increased by treatment with ethanol in the lower midbrain using [(35)S] GTPγS binding assay. Furthermore, the GRK2 protein level was significantly increased by treatment with ethanol. Chronic treatment with ethanol enhanced the rewarding effects of morphine. On the other hand, this enhancement of the rewarding effects of morphine by ethanol treatment was significantly inhibited by the GRK2 inhibitor β-adrenergic receptor kinase 1 inhibitor.

CONCLUSIONS

The present study demonstrated that chronic treatment with ethanol enhanced the rewarding effects of morphine by up-regulating functional changes in μ-opioid receptor, mediated by GRK2.

Fast neurotransmitter release regulated by the endocytic scaffold intersectin

Sustained fast neurotransmission requires the rapid replenishment of release-ready synaptic vesicles (SVs) at presynaptic active zones. Although the machineries for exocytic fusion and for subsequent endocytic membrane retrieval have been well characterized, little is known about the mechanisms underlying the rapid recruitment of SVs to release sites. Here we show that the Down syndrome-associated endocytic scaffold protein intersectin 1 is a crucial factor for the recruitment of release-ready SVs. Genetic deletion of intersectin 1 expression or acute interference with intersectin function inhibited the replenishment of release-ready vesicles, resulting in short-term depression, without significantly affecting the rate of endocytic membrane retrieval. Acute perturbation experiments suggest that intersectin-mediated vesicle replenishment involves the association of intersectin with the fissioning enzyme dynamin and with the actin regulatory GTPase CDC42. Our data indicate a role for the endocytic scaffold intersectin in fast neurotransmitter release, which may be of prime importance for information processing in the brain.

Synaptic vesicle endocytosis

Neurons can sustain high rates of synaptic transmission without exhausting their supply of synaptic vesicles. This property relies on a highly efficient local endocytic recycling of synaptic vesicle membranes, which can be reused for hundreds, possibly thousands, of exo-endocytic cycles. Morphological, physiological, molecular, and genetic studies over the last four decades have provided insight into the membrane traffic reactions that govern this recycling and its regulation. These studies have shown that synaptic vesicle endocytosis capitalizes on fundamental and general endocytic mechanisms but also involves neuron-specific adaptations of such mechanisms. Thus, investigations of these processes have advanced not only the field of synaptic transmission but also, more generally, the field of endocytosis. This article summarizes current information on synaptic vesicle endocytosis with an emphasis on the underlying molecular mechanisms and with a special focus on clathrin-mediated endocytosis, the predominant pathway of synaptic vesicle protein internalization.

A membrane pool retrieved via endocytosis overshoot at nerve terminals: a study of its retrieval mechanism and role

Endocytosis overshoot, which retrieves more membrane than vesicles just being exocytosed, occurs at nerve terminals and non-neuronal secretory cells. The mechanism that retrieves the overshoot membrane pool and the role of this pool remain largely unknown. We addressed this issue at the rat calyx of Held nerve terminal with capacitance measurements. We found that every calyx contained an overshoot pool ∼1.8 times the readily releasable pool. Retrieval of this pool required large calcium influx, and was inhibited by blockers of calcium/calmodulin-activated calcineurin and dynamin, suggesting the involvement of calcineurin and dynamin in endocytosis overshoot. Depletion of the overshoot pool slowed down compensatory endocytosis, whereas recovery of the overshoot pool via exocytosis that deposited stranded vesicles to the plasma membrane led to recovery of compensatory endocytosis, suggesting that the overshoot pool enhances endocytosis efficiency. These results suggest that the overshoot pool exists at every nerve terminal, is of limited size arising from vesicles stranded at the plasma membrane, is retrieved via calcium/calmodulin/calcineurin and dynamin signaling pathway, and can enhance endocytosis efficiency. Potential mechanisms for how the endocytosis overshoot pool enhances endocytosis efficiency are discussed.

Reduced release probability prevents vesicle depletion and transmission failure at dynamin mutant synapses

Endocytic recycling of synaptic vesicles after exocytosis is critical for nervous system function. At synapses of cultured neurons that lack the two "neuronal" dynamins, dynamin 1 and 3, smaller excitatory postsynaptic currents are observed due to an impairment of the fission reaction of endocytosis that results in an accumulation of arrested clathrin-coated pits and a greatly reduced synaptic vesicle number. Surprisingly, despite a smaller readily releasable vesicle pool and fewer docked vesicles, a strong facilitation, which correlated with lower vesicle release probability, was observed upon action potential stimulation at such synapses. Furthermore, although network activity in mutant cultures was lower, Ca(2+)/calmodulin-dependent protein kinase II (CaMKII) activity was unexpectedly increased, consistent with the previous report of an enhanced state of synapsin 1 phosphorylation at CaMKII-dependent sites in such neurons. These changes were partially reversed by overnight silencing of synaptic activity with tetrodotoxin, a treatment that allows progression of arrested endocytic pits to synaptic vesicles. Facilitation was also counteracted by CaMKII inhibition. These findings reveal a mechanism aimed at preventing synaptic transmission failure due to vesicle depletion when recycling vesicle traffic is backed up by a defect in dynamin-dependent endocytosis and provide new insight into the coupling between endocytosis and exocytosis.

Dynamin, a membrane-remodelling GTPase

Dynamin, the founding member of a family of dynamin-like proteins (DLPs) implicated in membrane remodelling, has a critical role in endocytic membrane fission events. The use of complementary approaches, including live-cell imaging, cell-free studies, X-ray crystallography and genetic studies in mice, has greatly advanced our understanding of the mechanisms by which dynamin acts, its essential roles in cell physiology and the specific function of different dynamin isoforms. In addition, several connections between dynamin and human disease have also emerged, highlighting specific contributions of this GTPase to the physiology of different tissues.

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.

Overlapping role of dynamin isoforms in synaptic vesicle endocytosis

The existence of neuron-specific endocytic protein isoforms raises questions about their importance for specialized neuronal functions. Dynamin, a GTPase implicated in the fission reaction of endocytosis, is encoded by three genes, two of which, dynamin 1 and 3, are highly expressed in neurons. We show that dynamin 3, thought to play a predominantly postsynaptic role, has a major presynaptic function. Although lack of dynamin 3 does not produce an overt phenotype in mice, it worsens the dynamin 1 KO phenotype, leading to perinatal lethality and a more severe defect in activity-dependent synaptic vesicle endocytosis. Thus, dynamin 1 and 3, which together account for the overwhelming majority of brain dynamin, cooperate in supporting optimal rates of synaptic vesicle endocytosis. Persistence of synaptic transmission in their absence indicates that if dynamin plays essential functions in neurons, such functions can be achieved by the very low levels of dynamin 2.

An endophilin-dynamin complex promotes budding of clathrin-coated vesicles during synaptic vesicle recycling

Clathrin-mediated vesicle recycling in synapses is maintained by a unique set of endocytic proteins and interactions. We show that endophilin localizes in the vesicle pool at rest and in spirals at the necks of clathrin-coated pits (CCPs) during activity in lamprey synapses. Endophilin and dynamin colocalize at the base of the clathrin coat. Protein spirals composed of these proteins on lipid tubes in vitro have a pitch similar to the one observed at necks of CCPs in living synapses, and lipid tubules are thinner than those formed by dynamin alone. Tubulation efficiency and the amount of dynamin recruited to lipid tubes are dramatically increased in the presence of endophilin. Blocking the interactions of the endophilin SH3 domain in situ reduces dynamin accumulation at the neck and prevents the formation of elongated necks observed in the presence of GTPγS. Therefore, endophilin recruits dynamin to a restricted part of the CCP neck, forming a complex, which promotes budding of new synaptic vesicles.

Impaired activity-dependent plasticity of quantal amplitude at the neuromuscular junction of Rab3A deletion and Rab3A earlybird mutant mice

Rab3A is a small GTPase associated with synaptic vesicles that is required for some forms of activity-dependent plasticity. It is thought to regulate the number of vesicles that fuse through an effect on docking, vesicle maturation, or mobilization. We recently showed that at the neuromuscular junction, loss of Rab3A led to an increase in the occurrence of miniature endplate currents (mepcs) with abnormally long half-widths (Wang et al., 2008). Here we show that such events are also increased after short-term activity blockade, and this process is not Rab3A-dependent. However, in the course of these experiments we discovered that the homeostatic increase in mepc amplitude after activity blockade is diminished in the Rab3A deletion mouse and abolished in the Rab3A Earlybird mouse which expresses a point mutant of Rab3A. We show that homeostatic plasticity at the neuromuscular junction does not depend on tumor necrosis factor α, is not accompanied by an increase in the levels of VAChT, the vesicular transporter for ACh, and confirm that there is no increase in ACh receptors at the junction, three characteristics distinct from that of CNS homeostatic plasticity. Activity blockade does not produce time course changes in mepcs that would be consistent with a fusion pore mechanism. We conclude that Rab3A is involved in a novel presynaptic mechanism to homeostatically regulate the amount of transmitter in a quantum.

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.

Dynamics and regulation of endocytotic fission pores: role of calcium and dynamin

Although endocytosis involves the fission pore, a transient structure that produces the scission between vesicle and plasma membranes, the dimensions and dynamics of fission pores remain unclear. Here we report that the pore resistance changes proceed in three distinct phases: an initial phase where the resistance increases at 31.7 ± 2.9 GΩ/second, a slower linear phase with an overall slope of 11.7 ± 1.9 GΩ/second and a final increase in resistance more steeply (1189 ± 136 GΩ/second). The kinetics of these changes was calcium dependent. These sequential stages of the fission pore may be interpreted in terms of pore geometry as changes, first in pore diameter and then in pore length, according to which, before fission, the pore diameter consistently decreased to a value near 4 nm, whereas the pore length ranged between 20 and 300 nm. Dynamin, a mechanochemical GTPase, plays an important role in accelerating the fission event, preferentially in endocytotic vesicles of regular size, by increasing the rates of pore closure during the first and second phases of the fission pore, but hardly affected larger and longer-lived endocytotic events. These results suggest that fission pores are dynamic structures that form thin and long membrane necks regulated by intracellular calcium. Between calcium mediators, dynamin functions as a catalyst to increase the speed of single vesicle endocytosis.

Developmental shift to a mechanism of synaptic vesicle endocytosis requiring nanodomain Ca2+

Ca(2+) is thought to be essential for the exocytosis and endocytosis of synaptic vesicles. However, the manner in which Ca(2+) coordinates these processes remains unclear, particularly at mature synapses. Using membrane capacitance measurements from calyx of Held nerve terminals in rats, we found that vesicle endocytosis is initiated primarily in Ca(2+) nanodomains around Ca(2+) channels, where exocytosis is triggered. Bulk Ca(2+) outside of the domain could also be involved in endocytosis at immature synapses, although only after extensive exocytosis at more mature synapses. This bulk Ca(2+)-dependent endocytosis required calmodulin and calcineurin activation at immature synapses, but not at more mature synapses. Similarly, GTP-independent endocytosis, which occurred after extensive exocytosis at immature synapses, became negligible after maturation. We propose that nanodomain Ca(2+) simultaneously triggers exocytosis and endocytosis of synaptic vesicles and that the molecular mechanisms underlying Ca(2+)-dependent endocytosis undergo major developmental changes at this fast central synapse.

On the biomarkers and mechanisms of konzo, a distinct upper motor neuron disease associated with food (cassava) cyanogenic exposure

Konzo is a self-limiting central motor-system disease associated with food dependency on cassava and low dietary intake of sulfur amino acids (SAA). Under conditions of SAA-deficiency, ingested cassava cyanogens yield metabolites that include thiocyanate and cyanate, a protein-carbamoylating agent. We studied the physical and biochemical modifications of rat serum and spinal cord proteins arising from intoxication of young adult rats with 50-200mg/kg linamarin, or 200mg/kg sodium cyanate (NaOCN), or vehicle (saline) and fed either a normal amino acid- or SAA-deficient diet for up to 2 weeks. Animals under SAA-deficient diet and treatment with linamarin or NaOCN developed hind limb tremors or motor weakness, respectively. LC/MS-MS analysis revealed differential albumin carbamoylation in animals treated with NaOCN, vs. linamarin/SAA-deficient diet, or vehicle. 2D-DIGE and MALDI-TOF/MS-MS analysis of the spinal cord proteome showed differential expression of proteins involved in oxidative mechanisms (e.g. peroxiredoxin 6), endocytic vesicular trafficking (e.g. dynamin 1), protein folding (e.g. protein disulfide isomerase), and maintenance of the cytoskeleton integrity (e.g. α-spectrin). Studies are needed to elucidate the role of the aformentioned modifications in the pathogenesis of cassava-associated motor-system disease.

The molecular physiology of activity-dependent bulk endocytosis of synaptic vesicles

Central nerve terminals release neurotransmitter in response to a wide variety of stimuli. Because maintenance of neurotransmitter release is dependent on the continual supply of synaptic vesicles (SVs), nerve terminals possess an array of endocytosis modes to retrieve and recycle SV membrane and proteins. During mild stimulation conditions, single SV retrieval modes such as clathrin-mediated endocytosis predominate. However, during increased neuronal activity, additional SV retrieval capacity is required, which is provided by activity-dependent bulk endocytosis (ADBE). ADBE is the dominant SV retrieval mechanism during elevated neuronal activity. It is a high capacity SV retrieval mode that is immediately triggered during such stimulation conditions. This review will summarize the current knowledge regarding the molecular mechanism of ADBE, including molecules required for its triggering and subsequent steps, including SV budding from bulk endosomes. The molecular relationship between ADBE and the SV reserve pool will also be discussed. It is becoming clear that an understanding of the molecular physiology of ADBE will be of critical importance in attempts to modulate both normal and abnormal synaptic function during intense neuronal activity.

Traffic jams during vesicle cycling lead to synaptic depression

During prolonged stimulation, fusion of thousands of synaptic vesicles poses a challenge for the temporal and spatial coordination of exocytosis and endocytosis. In this issue of Neuron, Hosoi and colleagues reveal that [Ca2+](i)-microdomains trigger compensatory endocytosis. Block of endocytosis enhances short-term depression by delaying vesicle recruitment to docking sites on active zones.