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

Actin maintains synaptic transmission by restraining vesicle release probability

Despite decades of pharmacological studies, how the ubiquitous cytoskeletal actin regulates synaptic transmission remains poorly understood. We addressed this issue with a tissue-specific knockout of actin β-isoform or γ-isoform, combined with recordings of postsynaptic EPSCs, presynaptic capacitance jumps or fluorescent synaptophysin-pHluorin changes, and electron microscopy in large calyx-type and small conventional hippocampal synapses. We found that actin restrains basal synaptic transmission during single action potential firings by lowering the readily releasable vesicle's release probability. Such an inhibition of basal synaptic transmission is turned into facilitation during repetitive firings by slowing down depletion of the readily releasable vesicle pool and, thus, short-term synaptic depression, leading to more effective synaptic transmission for a longer time. These mechanisms, together with the previous finding that actin promotes vesicle replenishment to the readily releasable pool, may control synaptic transmission and short-term synaptic plasticity at many synapses, contributing to neurological disorders caused by actin cytoskeleton impairment.

Physiological roles of endocytosis and presynaptic scaffold in vesicle replenishment at fast and slow central synapses

After exocytosis, release sites are cleared of vesicular residues to replenish with transmitter-filled vesicles. Endocytic and scaffold proteins are thought to underlie this site-clearance mechanism. However, the physiological significance of this mechanism at diverse mammalian central synapses remains unknown. Here, we tested this in a physiologically optimized condition using action potential evoked EPSCs at fast calyx synapse and relatively slow hippocampal CA1 synapse, in post-hearing mice brain slices at 37°C and in 1.3 mM [Ca2+]. Pharmacological block of endocytosis enhanced synaptic depression at the calyx synapse, whereas it attenuated synaptic facilitation at the hippocampal synapse. Block of scaffold protein activity likewise enhanced synaptic depression at the calyx but had no effect at the hippocampal synapse. At the fast calyx synapse, block of endocytosis or scaffold protein activity significantly enhanced synaptic depression as early as 10 ms after the stimulation onset. Unlike previous reports, neither endocytic blockers nor scaffold protein inhibitors prolonged the recovery from short-term depression. We conclude that the release-site clearance by endocytosis can be a universal phenomenon supporting vesicle replenishment at both fast and slow synapses, whereas the presynaptic scaffold mechanism likely plays a specialized role in vesicle replenishment predominantly at fast synapses.

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.

Dynamin Superfamily at Pre- and Postsynapses: Master Regulators of Synaptic Transmission and Plasticity in Health and Disease

Dynamin superfamily proteins (DSPs) comprise a large group of GTP-ases that orchestrate membrane fusion and fission, and cytoskeleton remodeling in different cell-types. At the central nervous system, they regulate synaptic vesicle recycling and signaling-receptor turnover, allowing the maintenance of synaptic transmission. In the presynapses, these GTP-ases control the recycling of synaptic vesicles influencing the size of the ready-releasable pool and the release of neurotransmitters from nerve terminals, whereas in the postsynapses, they are involved in AMPA-receptor trafficking to and from postsynaptic densities, supporting excitatory synaptic plasticity, and consequently learning and memory formation. In agreement with these relevant roles, an important number of neurological disorders are associated with mutations and/or dysfunction of these GTP-ases. Along the present review we discuss the importance of DSPs at synapses and their implication in different neuropathological contexts.

Membrane capacitance recordings resolve dynamics and complexity of receptor-mediated endocytosis in Wnt signalling

Receptor-mediated endocytosis is an essential process in signalling pathways for activation of intracellular signalling cascades. One example is the Wnt signalling pathway that seems to depend on endocytosis of the ligand-receptor complex for initiation of Wnt signal transduction. To date, the roles of different endocytic pathways in Wnt signalling, molecular players and the kinetics of the process remain unclear. Here, we monitored endocytosis in Wnt3a and Wnt5a-mediated signalling with membrane capacitance recordings of HEK293 cells. Our measurements revealed a swift and substantial increase in the number of endocytic vesicles. Extracellular Wnt ligands specifically triggered endocytotic activity, which started immediately upon ligand binding and ceased within a period of ten minutes. By using specific inhibitors, we were able to separate Wnt-induced endocytosis into two independent pathways. We demonstrate that canonical Wnt3a is taken up mainly by clathrin-independent endocytosis whereas noncanonical Wnt5a is exclusively regulated via clathrin-mediated endocytosis. Our findings show that membrane capacitance recordings allow the resolution of complex cellular processes in plasma membrane signalling pathways in great detail.

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 sustains release at photoreceptor ribbon synapses by restoring fusion competence

Endocytosis is an essential process at sites of synaptic release. Not only are synaptic vesicles recycled by endocytosis, but the removal of proteins and lipids by endocytosis is needed to restore release site function at active zones after vesicle fusion. Synaptic exocytosis from vertebrate photoreceptors involves synaptic ribbons that serve to cluster vesicles near the presynaptic membrane. In this study, we hypothesize that this clustering increases the likelihood that exocytosis at one ribbon release site may disrupt release at an adjacent site and therefore that endocytosis may be particularly important for restoring release site competence at photoreceptor ribbon synapses. To test this, we combined optical and electrophysiological techniques in salamander rods. Pharmacological inhibition of dynamin-dependent endocytosis rapidly inhibits release from synaptic ribbons and slows recovery of ribbon-mediated release from paired pulse synaptic depression. Inhibiting endocytosis impairs the ability of second-order horizontal cells to follow rod light responses at frequencies as low as 2 Hz. Inhibition of endocytosis also increases lateral membrane mobility of individual Ca2+ channels, showing that it changes release site structure. Visualization of single synaptic vesicles by total internal reflection fluorescence microscopy reveals that inhibition of endocytosis reduces the likelihood of fusion among vesicles docked near ribbons and increases the likelihood that they will retreat from the membrane without fusion. Vesicle advance toward the membrane is also reduced, but the number of membrane-associated vesicles is not. Endocytosis therefore appears to be more important for restoring later steps in vesicle fusion than for restoring docking. Unlike conventional synapses in which endocytic restoration of release sites is evident only at high frequencies, endocytosis is needed to maintain release from rod ribbon synapses even at modest frequencies.

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.

How to maintain active zone integrity during high-frequency transmission

In the central nervous system, the frequency at which reliable synaptic transmission can be maintained varies strongly between different types of synapses. Several pre- and postsynaptic processes must interact to enable high-frequency synaptic transmission. One of the mechanistically most challenging issues arises during repetitive neurotransmitter release, when synaptic vesicles fuse in rapid sequence with the presynaptic plasma membrane within the active zone (AZ), potentially interfering with the structural integrity of the AZ itself. Here we summarize potential mechanisms that help to maintain AZ integrity, including arrangement and mobility of release sites, calcium channel mobility, as well as release site clearance via lateral diffusion of vesicular proteins and via endocytotic membrane retrieval. We discuss how different types of synapses use these strategies to maintain high-frequency synaptic transmission.

Kiss-and-Run Is a Significant Contributor to Synaptic Exocytosis and Endocytosis in Photoreceptors

Accompanying sustained release in darkness, rod and cone photoreceptors exhibit rapid endocytosis of synaptic vesicles. Membrane capacitance measurements indicated that rapid endocytosis retrieves at least 70% of the exocytotic membrane increase. One mechanism for rapid endocytosis is kiss-and-run fusion where vesicles briefly contact the plasma membrane through a small fusion pore. Release can also occur by full-collapse in which vesicles merge completely with the plasma membrane. We assessed relative contributions of full-collapse and kiss-and-run in salamander photoreceptors using optical techniques to measure endocytosis and exocytosis of large vs. small dye molecules. Incubation with small dyes (SR101, 1 nm; 3-kDa dextran-conjugated Texas Red, 2.3 nm) loaded rod and cone synaptic terminals much more readily than larger dyes (10-kDa Texas Red, 4.6 nm; 10-kDa pHrodo, 4.6 nm; 70-kDa Texas Red, 12 nm) consistent with significant uptake through 2.3–4.6 nm fusion pores. By using total internal reflection fluorescence microscopy (TIRFM) to image individual vesicles, when rods were incubated simultaneously with Texas Red and AlexaFluor-488 dyes conjugated to either 3-kDa or 10-kDa dextran, more vesicles loaded small molecules than large molecules. Using TIRFM to detect release by the disappearance of dye-loaded vesicles, we found that SR101 and 3-kDa Texas Red were released from individual vesicles more readily than 10-kDa and 70-kDa Texas Red. Although 10-kDa pHrodo was endocytosed poorly like other large dyes, the fraction of release events was similar to SR101 and 3-kDa Texas Red. We hypothesize that while 10-kDa pHrodo may not exit through a fusion pore, release of intravesicular protons can promote detection of fusion events by rapidly quenching fluorescence of this pH-sensitive dye. Assuming that large molecules can only be released by full-collapse whereas small molecules can be released by both modes, our results indicate that 50%–70% of release from rods involves kiss-and-run with 2.3–4.6 nm fusion pores. Rapid retrieval of vesicles by kiss-and-run may limit membrane disruption of release site function during ongoing release at photoreceptor ribbon synapses.

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.

Some Subtle Lessons from the Calyx of Held Synapse

The calyx of Held is a giant nerve terminal that forms a glutamatergic synapse in the auditory pathway. Due to its large size, it offers a number of advantages for biophysical studies, including voltage-clamp of both pre- and postsynaptic compartments and the loading with indicator dyes and caged compounds. Three aspects of recent findings on the calyx are reviewed here, each of which seems to have only subtle consequences for nerve-evoked excitatory postsynaptic currents: vesicle heterogeneity, refractoriness of release sites, and superpriming. Together, they determine short-term plasticity features that are superficially similar to those expected for a simple vesicle pool model. However, detailed consideration of these aspects may be required for the correct mechanistic interpretation of data from synapses with normal and perturbed function, as well as for modeling the dynamics of short-term plasticity.

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.