An acto-myosin II constricting ring initiates the fission of activity-dependent bulk endosomes in neurosecretory cells

The epithelial Na+ channel UNC-8 promotes an endocytic mechanism that recycles presynaptic components to new boutons in remodeling neurons

Circuit refinement involves the formation of new presynaptic boutons as others are dismantled. Nascent presynaptic sites can incorporate material from recently eliminated synapses, but the recycling mechanisms remain elusive. In early-stage C. elegans larvae, the presynaptic boutons of GABAergic DD neurons are removed and new outputs established at alternative sites. Here, we show that developmentally regulated expression of the epithelial Na+ channel (ENaC) UNC-8 in remodeling DD neurons promotes a Ca2+ and actin-dependent mechanism, involving activity-dependent bulk endocytosis (ADBE), that recycles presynaptic material for reassembly at nascent DD synapses. ADBE normally functions in highly active neurons to accelerate local recycling of synaptic vesicles. In contrast, we find that an ADBE-like mechanism results in the distal recycling of synaptic material from old to new synapses. Thus, our findings suggest that a native mechanism (ADBE) can be repurposed to dismantle presynaptic terminals for reassembly at new, distant locations.

Advances and challenges in understanding endosomal sorting and fission

Endosomes play crucial roles in the cell, serving as focal and 'triage' points for internalized lipids and receptors. As such, endosomes are a critical branching point that determines whether receptors are sorted for degradation or recycling. This Viewpoint aims to highlight recent advances in endosome research, including key endosomal functions such as sorting and fission. Moreover, the Viewpoint addresses key technical and conceptual challenges in studying endosomes.

Phosphatidylinositol 4-kinase IIα is a glycogen synthase kinase 3-regulated interaction hub for activity-dependent bulk endocytosis

Phosphatidylinositol 4-kinase IIα (PI4KIIα) generates essential phospholipids and is a cargo for endosomal adaptor proteins. Activity-dependent bulk endocytosis (ADBE) is the dominant synaptic vesicle endocytosis mode during high neuronal activity and is sustained by glycogen synthase kinase 3β (GSK3β) activity. We reveal the GSK3β substrate PI4KIIα is essential for ADBE via its depletion in primary neuronal cultures. Kinase-dead PI4KIIα rescues ADBE in these neurons but not a phosphomimetic form mutated at the GSK3β site, Ser-47. Ser-47 phosphomimetic peptides inhibit ADBE in a dominant-negative manner, confirming that Ser-47 phosphorylation is essential for ADBE. Phosphomimetic PI4KIIα interacts with a specific cohort of presynaptic molecules, two of which, AGAP2 and CAMKV, are also essential for ADBE when depleted in neurons. Thus, PI4KIIα is a GSK3β-dependent interaction hub that silos essential ADBE molecules for liberation during neuronal activity.

Nanoparticle Spikes Enhance Cellular Uptake via Regulating Myosin IIA Recruitment

Spike-like nanostructures are omnipresent in natural and artificial systems. Although biorecognition of nanostructures to cellular receptors has been indicated as the primary factor for virus infection pathways, how the spiky morphology of DNA-modified nanoparticles affects their cellular uptake and intracellular fate remains to be explored. Here, we design dually emissive gold nanoparticles with varied spikiness (from 0 to 2) to probe the interactions of spiky nanoparticles with cells. We discovered that nanospikes at the nanoparticle regulated myosin IIA recruitment at the cell membrane during cellular uptake, thereby enhancing cellular uptake efficiency, as revealed by dual-modality (plasmonic and fluorescence) imaging. Furthermore, the spiky nanoparticles also exhibited facilitated endocytosis dynamics, as revealed by real-time dark-field microscopy (DFM) imaging and colorimetry-based classification algorithms. These findings highlight the crucial role of the spiky morphology in regulating the intracellular fate of nanoparticles, which may shed light on engineering theranostic nanocarriers.

Assembly and disassembly dynamics of nonmuscle myosin II control endosomal fission

Endocytic vesicular trafficking requires merging of two lipid bilayers, but how the two lipid bilayers can come close together during fusion and fission in endocytic trafficking is not well explored. Here, we establish that knocking down nonmuscle myosin IIs (NM IIs) by small interfering RNA (siRNA) or inhibition of their activities by (-) blebbistatin causes the formation of a ring-like assembly of early endosomes (raEE). Inhibition of NM II assembly by an inhibitor of regulatory light-chain (RLC) kinase results in the formation of raEE, whereas inhibition of NM II disassembly by inhibitors of heavy chain kinases, protein kinase C (PKC) and casein kinase 2 (CK2), causes the dispersion of early endosomes. The raEEs retain EEA1, Rab7, and LAMP2 markers. Overexpression of an assembly incompetent form, RLC-AA, and disassembly incompetent form, NMHCIIB-S6A or NMHCIIA-1916A, induces such defects, respectively. Altogether, these data support that NM II assembly and disassembly dynamics participate in endocytic trafficking by regulating fission to maintain the size of early endosomes.

Unveiling the Nanoscale Dynamics of the Exocytic Machinery in Chromaffin Cells with Single-Molecule Imaging

Neuronal and hormonal communication relies on the exocytic fusion of vesicles containing neurotransmitters and hormones with the plasma membrane. This process is tightly regulated by key protein-protein and protein-lipid interactions and culminates in the soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) complex formation and zippering that promotes vesicular fusion. Located on both sides of the vesicle and the plasma membrane, the zippering of the SNARE complex acts to overcome the energy barrier afforded by the repulsive electrostatic force stemming from apposing two negatively charged phospholipid membranes. Another component opposing the timely organization of the fusion machinery is thermal Brownian energy that tends to homogenize all cellular molecules by constantly switching their motions and directions through short-lived molecular interactions. Much less is known of the mechanisms counteracting these chaotic forces, allowing seamless cellular functions such as exocytic fusion. Super-resolution microscopy techniques such as single-molecule imaging have proven useful to start uncovering these nanoscale mechanisms. Here, we used single-particle tracking photoactivatable localization microscopy (sptPALM) to track syntaxin-1-mEos, a SNARE protein located on the plasma membrane of cultured bovine chromaffin cells. We demonstrate that syntaxin-1-mEos undergoes dramatic change in its mobility in response to secretagogue stimulation leading to increased nanoclustering. These nanoclusters are transient in nature and likely to provide docked vesicles with a molecular environment conducive to exocytic fusion.

Sequential compound fusion and kiss-and-run mediate exo- and endocytosis in excitable cells

Vesicle fusion at preestablished plasma membrane release sites releases transmitters and hormones to mediate fundamental functions like neuronal network activities and fight-or-flight responses. This half-a-century-old concept-fusion at well-established release sites in excitable cells-needs to be modified to include the sequential compound fusion reported here-vesicle fusion at previously fused Ω-shaped vesicular membrane. With superresolution STED microscopy in excitable neuroendocrine chromaffin cells, we real-time visualized sequential compound fusion pore openings and content releases in generating multivesicular and asynchronous release from single release sites, which enhances exocytosis strength and dynamic ranges in excitable cells. We also visualized subsequent compound fusion pore closure, a new mode of endocytosis termed compound kiss-and-run that enhances vesicle recycling capacity. These results suggest modifying current exo-endocytosis concepts by including rapid release-site assembly at fused vesicle membrane, where sequential compound fusion and kiss-and-run take place to enhance exo-endocytosis capacity and dynamic ranges.

Minibrain kinase and calcineurin coordinate activity-dependent bulk endocytosis through synaptojanin

Neurons use multiple modes of endocytosis, including clathrin-mediated endocytosis (CME) and activity-dependent bulk endocytosis (ADBE), during mild and intense neuronal activity, respectively, to maintain stable neurotransmission. While molecular players modulating CME are well characterized, factors regulating ADBE and mechanisms coordinating CME and ADBE activations remain poorly understood. Here we report that Minibrain/DYRK1A (Mnb), a kinase mutated in autism and up-regulated in Down's syndrome, plays a novel role in suppressing ADBE. We demonstrate that Mnb, together with calcineurin, delicately coordinates CME and ADBE by controlling the phosphoinositol phosphatase activity of synaptojanin (Synj) during varying synaptic demands. Functional domain analyses reveal that Synj's 5'-phosphoinositol phosphatase activity suppresses ADBE, while SAC1 activity is required for efficient ADBE. Consequently, Parkinson's disease mutation in Synj's SAC1 domain impairs ADBE. These data identify Mnb and Synj as novel regulators of ADBE and further indicate that CME and ADBE are differentially governed by Synj's dual phosphatase domains.

Rapid vesicle replenishment after the immediately releasable pool exocytosis is tightly linked to fast endocytosis, and depends on basal calcium and cortical actin in chromaffin cells

The maintenance of the secretory response requires a continuous replenishment of releasable vesicles. It was proposed that the immediately releasable pool (IRP) is important in chromaffin cell secretion during action potentials applied at basal physiological frequencies, because of the proximity of IRP vesicles to voltage-dependent Ca²⁺ channels. However, previous reports showed that IRP replenishment after depletion is too slow to manage such a situation. In this work, we used patch-clamp measurements of membrane capacitance, confocal imaging of F-actin distribution, and cytosolic Ca²⁺ measurements with Fura-2 to re-analyze this problem in primary cultures of mouse chromaffin cells. We provide evidence that IRP replenishment has one slow (time constant between 5 and 10 s) and one rapid component (time constant between 0.5 and 1.5 s) linked to a dynamin-dependent fast endocytosis. Both, the fast endocytosis and the rapid replenishment component were eliminated when 500 nM Ca²⁺ was added to the internal solution during patch-clamp experiments, but they became dominant and accelerated when the cytosolic Ca²⁺ buffer capacity was increased. In addition, both rapid replenishment and fast endocytosis were retarded when cortical F-actin cytoskeleton was disrupted with cytochalasin D. Finally, in permeabilized chromaffin cells stained with rhodamine-phalloidin, the cortical F-actin density was reduced when the Ca²⁺ concentration was increased in a range of 10-1000 nM. We conclude that low cytosolic Ca²⁺ concentrations, which favor cortical F-actin stabilization, allow the activation of a fast endocytosis mechanism linked to a rapid replenishment component of IRP.

A positive feedback loop between Flower and PI(4,5)P2 at periactive zones controls bulk endocytosis in Drosophila

Synaptic vesicle (SV) endocytosis is coupled to exocytosis to maintain SV pool size and thus neurotransmitter release. Intense stimulation induces activity-dependent bulk endocytosis (ADBE) to recapture large quantities of SV constituents in large endosomes from which SVs reform. How these consecutive processes are spatiotemporally coordinated remains unknown. Here, we show that Flower Ca²⁺ channel-dependent phosphatidylinositol 4,5-bisphosphate (PI(4,5)P₂) compartmentalization governs control of these processes in Drosophila. Strong stimuli trigger PI(4,5)P₂ microdomain formation at periactive zones. Upon exocytosis, Flower translocates from SVs to periactive zones, where it increases PI(4,5)P₂ levels via Ca²⁺ influxes. Remarkably, PI(4,5)P₂ directly enhances Flower channel activity, thereby establishing a positive feedback loop for PI(4,5)P₂ microdomain compartmentalization. PI(4,5)P₂ microdomains drive ADBE and SV reformation from bulk endosomes. PI(4,5)P₂ further retrieves Flower to bulk endosomes, terminating endocytosis. We propose that the interplay between Flower and PI(4,5)P₂ is the crucial spatiotemporal cue that couples exocytosis to ADBE and subsequent SV reformation.

Radial contractility of actomyosin rings facilitates axonal trafficking and structural stability

Most mammalian neurons have a narrow axon, which constrains the passage of large cargoes such as autophagosomes that can be larger than the axon diameter. Radial axonal expansion must therefore occur to ensure efficient axonal trafficking. In this study, we reveal that the speed of various large cargoes undergoing axonal transport is significantly slower than that of small ones and that the transit of diverse-sized cargoes causes an acute, albeit transient, axonal radial expansion, which is immediately restored by constitutive axonal contractility. Using live super-resolution microscopy, we demonstrate that actomyosin-II controls axonal radial contractility and local expansion, and that NM-II filaments associate with periodic F-actin rings via their head domains. Pharmacological inhibition of NM-II activity significantly increases axon diameter by detaching the NM-II from F-actin and impacts the trafficking speed, directionality, and overall efficiency of long-range retrograde trafficking. Consequently, prolonged NM-II inactivation leads to disruption of periodic actin rings and formation of focal axonal swellings, a hallmark of axonal degeneration.

Photoconversion of FM1-43 Reveals Differences in Synaptic Vesicle Recycling and Sensitivity to Pharmacological Disruption of Actin Dynamics in Individual Synapses

The cycling of synaptic vesicles ensures that neurons can communicate adequately through their synapses on repeated occasions when activity is sustained, and several steps in this cycle are modulated by actin. The effects of pharmacological stabilization of actin with jasplakinolide or its depolymerization with latrunculin A was assessed on the synaptic vesicle cycle at individual boutons of cerebellar granule cells, using FM1-43 imaging to track vesicle recycling and its photoconversion to specifically label recycled organelles. Remarkable differences in the recycling capacity of individual boutons are evident, and their dependence on the actin cytoskeleton for recycling is clear. Disrupting actin dynamics causes a loss of functional boutons, and while this indicates that exo/endocytotic cycling in boutons is fully dependent on such events, this dependence is only partial in other boutons. Indeed, exocytosis and vesicle trafficking are impaired significantly by stabilizing or depolymerizing actin, whereas repositioning recycled vesicles at the active zone seems to be dependent on actin polymerization alone. These findings support the hypothesis that different steps of synaptic vesicle cycling depend on actin dynamics and that such dependence varies among individual boutons.

The host actin cytoskeleton channels rhizobia release and facilitates symbiosome accommodation during nodulation in Medicago truncatula

In plants, the actin cytoskeleton plays a central role in regulating intracellular transport and trafficking in the endomembrane system. Work in legumes suggested that during nodulation, the actin cytoskeleton coordinates numerous cellular processes in the development of nitrogen-fixing nodules. However, we lacked live-cell visualizations demonstrating dynamic remodeling of the actin cytoskeleton during infection droplet release and symbiosome development. Here, we generated transgenic Medicago truncatula lines stably expressing the fluorescent actin marker ABD2-GFP, and utilized live-cell imaging to reveal the architecture and dynamics of the actin cytoskeleton during nodule development. Live-cell observations showed that different zones in nitrogen-fixing nodules exhibit distinct actin architectures and infected cells display five characteristic actin architectures during nodule development. Live-cell imaging combined with three-dimensional reconstruction demonstrated that dense filamentous-actin (F-actin) arrays channel the elongation of infection threads and the release of infection droplets, an F-actin network encircles freshly-released rhizobia, and short F-actin fragments and actin dots around radially distributed symbiosomes. Our findings suggest an important role of the actin cytoskeleton in infection droplet release, symbiosome development and maturation, and provide significant insight into the cellular mechanisms underlying nodule development and nitrogen fixation during legume-rhizobia interactions.

Activity-dependent bulk endocytosis proteome reveals a key presynaptic role for the monomeric GTPase Rab11

The maintenance of neurotransmission by synaptic vesicle (SV) recycling is critical to brain function. The dominant SV recycling mode during intense activity is activity-dependent bulk endocytosis (ADBE), suggesting it will perform a pivotal role in neurotransmission. However, the role of ADBE is still undetermined, due to the absence of identified molecules specific for this process. The determination of the bulk endosome proteome (a key ADBE organelle) revealed that it has a unique molecular signature and identified a role for Rab11 in presynaptic function. This work provides the molecular inventory of ADBE, a resource that will be of significant value to researchers wishing to modulate neurotransmission during intense neuronal activity in both health and disease.

Activity-dependent bulk endocytosis (ADBE) is the dominant mode of synaptic vesicle endocytosis during high-frequency stimulation, suggesting it should play key roles in neurotransmission during periods of intense neuronal activity. However, efforts in elucidating the physiological role of ADBE have been hampered by the lack of identified molecules which are unique to this endocytosis mode. To address this, we performed proteomic analysis on purified bulk endosomes, which are a key organelle in ADBE. Bulk endosomes were enriched via two independent approaches, a classical subcellular fractionation method and isolation via magnetic nanoparticles. There was a 77% overlap in proteins identified via the two protocols, and these molecules formed the ADBE core proteome. Bioinformatic analysis revealed a strong enrichment in cell adhesion and cytoskeletal and signaling molecules, in addition to expected synaptic and trafficking proteins. Network analysis identified Rab GTPases as a central hub within the ADBE proteome. Subsequent investigation of a subset of these Rabs revealed that Rab11 both facilitated ADBE and accelerated clathrin-mediated endocytosis. These findings suggest that the ADBE proteome will provide a rich resource for the future study of presynaptic function, and identify Rab11 as a regulator of presynaptic function.

Endophilin A and B Join Forces With Clathrin to Mediate Synaptic Vesicle Recycling in Caenorhabditis elegans

Synaptic vesicle (SV) recycling enables ongoing transmitter release, even during prolonged activity. SV membrane and proteins are retrieved by ultrafast endocytosis and new SVs are formed from synaptic endosomes (large vesicles—LVs). Many proteins contribute to SV recycling, e.g., endophilin, synaptojanin, dynamin and clathrin, while the site of action of these proteins (at the plasma membrane (PM) vs. at the endosomal membrane) is only partially understood. Here, we investigated the roles of endophilin A (UNC-57), endophilin-related protein (ERP-1, homologous to human endophilin B1) and of clathrin, in SV recycling at the cholinergic neuromuscular junction (NMJ) of C. elegans. erp-1 mutants exhibited reduced transmission and a progressive reduction in optogenetically evoked muscle contraction, indicative of impaired SV recycling. This was confirmed by electrophysiology, where particularly endophilin A (UNC-57), but also endophilin B (ERP-1) mutants exhibited reduced transmission. By optogenetic and electrophysiological analysis, phenotypes in the unc-57; erp-1 double mutant are largely dominated by the unc-57 mutation, arguing for partially redundant functions of endophilins A and B, but also hinting at a back-up mechanism for neuronal endocytosis. By electron microscopy (EM), we observed that unc-57 and erp-1; unc-57 double mutants showed increased numbers of synaptic endosomes of large size, assigning a role for both proteins at the endosome, because endosomal disintegration into new SVs, but not formation of endosomes were hampered. Accordingly, only low amounts of SVs were present. Also erp-1 mutants show reduced SV numbers (but no increase in LVs), thus ERP-1 contributes to SV formation. We analyzed temperature-sensitive mutants of clathrin heavy chain (chc-1), as well as erp-1; chc-1 and unc-57; chc-1 double mutants. SV recycling phenotypes were obvious from optogenetic stimulation experiments. By EM, chc-1 mutants showed formation of numerous and large endosomes, arguing that clathrin, as shown for mammalian synapses, acts at the endosome in formation of new SVs. Without endophilins, clathrin formed endosomes at the PM, while endophilins A and B compensated for the loss of clathrin at the PM, under conditions of high SV turnover.

The ever-growing complexity of the mitochondrial fission machinery

The mitochondrial network constantly changes and remodels its shape to face the cellular energy demand. In human cells, mitochondrial fusion is regulated by the large, evolutionarily conserved GTPases Mfn1 and Mfn2, which are embedded in the mitochondrial outer membrane, and by OPA1, embedded in the mitochondrial inner membrane. In contrast, the soluble dynamin-related GTPase Drp1 is recruited from the cytosol to mitochondria and is key to mitochondrial fission. A number of new players have been recently involved in Drp1-dependent mitochondrial fission, ranging from large cellular structures such as the ER and the cytoskeleton to the surprising involvement of the endocytic dynamin 2 in the terminal abscission step. Here we review the recent findings that have expanded the mechanistic model for the mitochondrial fission process in human cells and highlight open questions.

Integration of Synaptic Vesicle Cargo Retrieval with Endocytosis at Central Nerve Terminals

Central nerve terminals contain a limited number of synaptic vesicles (SVs) which mediate the essential process of neurotransmitter release during their activity-dependent fusion. The rapid and accurate formation of new SVs with the appropriate cargo is essential to maintain neurotransmission in mammalian brain. Generating SVs containing the correct SV cargo with the appropriate stoichiometry is a significant challenge, especially when multiple modes of endocytosis exist in central nerve terminals, which occur at different locations within the nerve terminals. These endocytosis modes include ultrafast endocytosis, clathrin-mediated endocytosis (CME) and activity-dependent bulk endocytosis (ADBE) which are triggered by specific patterns of neuronal activity. This review article will assess the evidence for the role of classical adaptor protein complexes in SV retrieval, discuss the role of monomeric adaptors and how interactions between specific SV cargoes can facilitate retrieval. In addition it will consider the evidence for preassembled plasma membrane cargo complexes and their role in facilitating these endocytosis modes. Finally it will present a unifying model for cargo retrieval at the presynapse, which integrates endocytosis modes in time and space.

Dynamin regulates the fusion pore of endo- and exocytotic vesicles as revealed by membrane capacitance measurements

BACKGROUND

Dynamin is a multidomain GTPase exhibiting mechanochemical and catalytic properties involved in vesicle scission from the plasmalemma during endocytosis. New evidence indicates that dynamin is also involved in exocytotic release of catecholamines, suggesting the existence of a dynamin-regulated structure that couples endo- to exocytosis.

METHODS

Thus we here employed high-resolution cell-attached capacitance measurements and super-resolution structured illumination microscopy to directly examine single vesicle interactions with the plasmalemma in cultured rat astrocytes treated with distinct pharmacological modulators of dynamin activity. Fluorescent dextrans and the lipophilic plasmalemmal marker DiD were utilized to monitor uptake and distribution of vesicles in the peri-plasmalemmal space and in the cell cytosol.

RESULTS

Dynamin inhibition with Dynole™-34-2 and Dyngo™-4a prevented vesicle internalization into the cytosol and decreased fusion pore conductance of vesicles that remained attached to the plasmalemma via a narrow fusion pore that lapsed into a state of repetitive opening and closing - flickering. In contrast, the dynamin activator Ryngo™-1-23 promoted vesicle internalization and favored fusion pore closure by prolonging closed and shortening open fusion pore dwell times. Immunocytochemical staining revealed dextran uptake into dynamin-positive vesicles and increased dextran uptake into Syt4- and VAMP2-positive vesicles after dynamin inhibition, indicating prolonged retention of these vesicles at the plasmalemma.

CONCLUSIONS

Our results have provided direct evidence for a role of dynamin in regulation of fusion pore geometry and kinetics of endo- and exocytotic vesicles, indicating that both share a common dynamin-regulated structural intermediate, the fusion pore.

A role for tropomyosins in activity-dependent bulk endocytosis?

Bulk endocytosis allows stimulated neurons to take up a large portion of the presynaptic plasma membrane in order to regenerate synaptic vesicle pools. Actin, one of the most abundant proteins in eukaryotic cells, plays an important role in this process, but a detailed mechanistic understanding of the involvement of the cortical actin network is still lacking, in part due to the relatively small size of nerve terminals and the limitation of optical microscopy. We recently discovered that neurosecretory cells display a similar, albeit much larger, form of bulk endocytosis in response to secretagogue stimulation. This allowed us to identify a novel highly dynamic role for the acto-myosin II cortex in generating constricting rings that precede the fission of nascent bulk endosomes. In this review we focus on the mechanism underpinning this dramatic switch in the organization and function of the cortical actin network. We provide additional experimental data that suggest a role of tropomyosin Tpm3.1 and Tpm4.2 in this process, together with an emerging model of how actin controls bulk endocytosis.

Membrane shaping by actin and myosin during regulated exocytosis

The cortical actin network in neurosecretory cells is a dense mesh of actin filaments underlying the plasma membrane. Interaction of actomyosin with vesicular membranes or the plasma membrane is vital for tethering, retention, transport as well as fusion and fission of exo- and endocytic membrane structures. During regulated exocytosis the cortical actin network undergoes dramatic changes in morphology to accommodate vesicle docking, fusion and replenishment. Most of these processes involve plasma membrane Phosphoinositides (PIP) and investigating the interactions between the actin cortex and secretory structures has become a hotbed for research in recent years. Actin remodelling leads to filopodia outgrowth and the creation of new fusion sites in neurosecretory cells and actin, myosin and dynamin actively shape and maintain the fusion pore of secretory vesicles. Changes in viscoelastic properties of the actin cortex can facilitate vesicular transport and lead to docking and priming of vesicle at the plasma membrane. Small GTPase actin mediators control the state of the cortical actin network and influence vesicular access to their docking and fusion sites. These changes potentially affect membrane properties such as tension and fluidity as well as the mobility of embedded proteins and could influence the processes leading to both exo- and endocytosis. Here we discuss the multitudes of actin and membrane interactions that control successive steps underpinning regulated exocytosis.

Myosin II activity is required for structural plasticity at the axon initial segment

In neurons, axons possess a molecularly defined and highly organised proximal region – the axon initial segment (AIS) – that is a key regulator of both electrical excitability and cellular polarity. Despite existing as a large, dense structure with specialised cytoskeletal architecture, the AIS is surprisingly plastic, with sustained alterations in neuronal activity bringing about significant alterations to its position, length or molecular composition. However, although the upstream activity‐dependent signalling pathways that lead to such plasticity have begun to be elucidated, the downstream mechanisms that produce structural changes at the AIS are completely unknown. Here, we use dissociated cultures of rat hippocampus to show that two forms of AIS plasticity in dentate granule cells – long‐term relocation, and more rapid shortening – are completely blocked by treatment with blebbistatin, a potent and selective myosin II ATPase inhibitor. These data establish a link between myosin II and AIS function, and suggest that myosin II's primary role at the structure may be to effect activity‐dependent morphological alterations.

Uses and abuses of macropinocytosis

Macropinocytosis is a means by which eukaryotic cells ingest extracellular liquid and dissolved molecules. It is widely conserved amongst cells that can take on amoeboid form and, therefore, appears to be an ancient feature that can be traced back to an early stage of evolution. Recent advances have highlighted how this endocytic process can be subverted during pathology - certain cancer cells use macropinocytosis to feed on extracellular protein, and many viruses and bacteria use it to enter host cells. Prion and prion-like proteins can also spread and propagate from cell to cell through macropinocytosis. Progress is being made towards using macropinocytosis therapeutically, either to deliver drugs to or cause cell death by inducing catastrophically rapid fluid uptake. Mechanistically, the Ras signalling pathway plays a prominent and conserved activating role in amoebae and in mammals; mutant amoebae with abnormally high Ras activity resemble tumour cells in their increased capacity for growth using nutrients ingested through macropinocytosis. This Commentary takes a functional and evolutionary perspective to highlight progress in understanding and use of macropinocytosis, which is an ancient feeding process used by single-celled phagotrophs that has now been put to varied uses by metazoan cells and is abused in disease states, including infection and cancer.

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.

Myosin light chain kinase facilitates endocytosis of synaptic vesicles at hippocampal boutons

At nerve terminals, endocytosis efficiently recycles vesicle membrane to maintain synaptic transmission under different levels of neuronal activity. Ca(2+) and its downstream signal pathways are critical for the activity-dependent regulation of endocytosis. An activity- and Ca(2+) -dependent kinase, myosin light chain kinase (MLCK) has been reported to regulate vesicle mobilization, vesicle cycling, and motility in different synapses, but whether it has a general contribution to regulation of endocytosis at nerve terminals remains unknown. We investigated this issue at rat hippocampal boutons by imaging vesicle endocytosis as the real-time retrieval of vesicular synaptophysin tagged with a pH-sensitive green fluorescence protein. We found that endocytosis induced by 200 action potentials (5-40 Hz) was slowed by acute inhibition of MLCK and down-regulation of MLCK with RNA interference, while the total amount of vesicle exocytosis and somatic Ca(2+) channel current did not change with MLCK down-regulation. Acute inhibition of myosin II similarly impaired endocytosis. Furthermore, down-regulation of MLCK prevented depolarization-induced phosphorylation of myosin light chain, an effect shared by blockers of Ca(2+) channels and calmodulin. These results suggest that MLCK facilitates vesicle endocytosis through activity-dependent phosphorylation of myosin downstream of Ca(2+) /calmodulin, probably as a widely existing mechanism among synapses. Our study suggests that MLCK is an important activity-dependent regulator of vesicle recycling in hippocampal neurons, which are critical for learning and memory. The kinetics of vesicle membrane endocytosis at nerve terminals has long been known to depend on activity and Ca(2+) . This study provides evidence suggesting that myosin light chain kinase increases endocytosis efficiency at hippocampal neurons by mediating Ca(2+) /calmodulin-dependent phosphorylation of myosin. The authors propose that this signal cascade may serve as a common pathway contributing to the activity-dependent regulation of vesicle endocytosis at synapses.

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.

Imaging membrane remodeling during regulated exocytosis in live mice

In this mini-review we focus on the use of time-lapse light microscopy to study membrane remodeling during protein secretion in live animals. In particular, we highlight how subcellular intravital microscopy has enabled imaging the dynamics of both individual secretory vesicles and the plasma membrane, during different steps in the exocytic process. This powerful approach has provided us with the unique opportunity to unravel the role of the actin cytoskeleton in regulating this process under physiological conditions, and to overcome the shortcomings of more reductionist model systems.

Small molecules demonstrate the role of dynamin as a bi-directional regulator of the exocytosis fusion pore and vesicle release

Hormones and neurotransmitters are stored in specialised vesicles and released from excitable cells through exocytosis. During vesicle fusion with the plasma membrane, a transient fusion pore is created that enables transmitter release. The protein dynamin is known to regulate fusion pore expansion (FPE). The mechanism is unknown, but requires its oligomerisation-stimulated GTPase activity. We used a palette of small molecule dynamin modulators to reveal bi-directional regulation of FPE by dynamin and vesicle release in chromaffin cells. The dynamin inhibitors Dynole 34-2 and Dyngo 4a and the dynamin activator Ryngo 1-23 reduced or increased catecholamine released from single vesicles, respectively. Total internal reflection fluorescence (TIRF) microscopy demonstrated that dynamin stimulation with Ryngo 1-23 reduced the number of neuropeptide Y (NPY) kiss-and-run events, but not full fusion events, and slowed full fusion release kinetics. Amperometric stand-alone foot signals, representing transient kiss-and-run events, were less frequent but were of longer duration, similarly to full amperometric spikes and pre-spike foot signals. These effects are not due to alterations in vesicle size. Ryngo 1-23 action was blocked by inhibitors of actin polymerisation or myosin II. Therefore, we demonstrate using a novel pharmacological approach that dynamin not only controls FPE during exocytosis, but is a bi-directional modulator of the fusion pore that increases or decreases the amount released from a vesicle during exocytosis if it is activated or inhibited, respectively. As such, dynamin has the ability to exquisitely fine-tune transmitter release.