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

Recycling endosomes undergo rapid closure of a fusion pore on exocytosis in neuronal dendrites

Exocytosis of recycling endosomes (REs) represents the last step of receptor and membrane recycling, a fundamental process involved in many aspects of cell physiology. In neurons, it is involved in the control of cell polarity and synaptic plasticity and is locally and tightly regulated. However, its molecular mechanisms are still poorly understood. We have imaged single exocytosis events of REs in rat hippocampal neurons in culture transfected with three types of receptors tagged with the pH-sensitive GFP mutant superecliptic phluorin. We found that exocytosis events are grouped into two categories: (1) short burst events in which receptors diffuse into the plasma membrane in a few seconds; and (2) long display events in which receptors remain visible and clustered after exocytosis for many seconds. Display events are much rarer in non-neuronal cells, such as fibroblasts and astrocytes. Using two-color imaging and fast extracellular solution changes, we show that display events correspond to the rapid opening and closing of a fusion pore (or "kiss-and-run") with a median opening time of 2.6 s, which restricts the diffusion of multiple receptor types and bound cargo. Moreover, the RE marker Rab11 remains enriched after display exocytosis events and controls the mode of RE exocytosis. Finally, a given RE can undergo multiple rounds of display exocytosis. The last step of recycling can thus be controlled in neurons for the selective delivery of receptors at the cell surface.

Late stages of the synchronized macrophage fusion in osteoclast formation depend on dynamin

Macrophage fusion that leads to osteoclast formation is one of the most important examples of cell-cell fusion in development, tissue homoeostasis and immune response. Protein machinery that fuses macrophages remains to be identified. In the present study, we explored the fusion stage of osteoclast formation for RAW macrophage-like murine cells and for macrophages derived from human monocytes. To uncouple fusion from the preceding differentiation processes, we accumulated fusion-committed cells in the presence of LPC (lysophosphatidylcholine) that reversibly blocks membrane merger. After 16 h, we removed LPC and observed cell fusion events that would normally develop within 16 h develop instead within 30-90 min. Thus, whereas osteoclastogenesis, generally, takes several days, our approach allowed us to focus on an hour in which we observe robust fusion between the cells. Complementing syncytium formation assay with a novel membrane merger assay let us study the synchronized fusion events downstream of a local merger between two plasma membranes, but before expansion of nascent membrane connections and complete unification of the cells. We found that the expansion of membrane connections detected as a growth of multinucleated osteoclasts depends on dynamin activity. In contrast, a merger between the plasma membranes of the two cells was not affected by inhibitors of dynamin GTPase. Thus dynamin that was recently found to control late stages of myoblast fusion also controls late stages of macrophage fusion, revealing an intriguing conserved mechanistic motif shared by diverse cell-cell fusion processes.

Synaptic vesicle generation from central nerve terminal endosomes

Central nerve terminals contain a small number of synaptic vesicles (SVs) that must sustain the fidelity of neurotransmission across a wide range of stimulation intensities. For this to be achieved, nerve terminals integrate a number of complementary endocytosis modes whose activation spans the breadth of these neuronal stimulation patterns. Two such modes are ultrafast endocytosis and activity-dependent bulk endocytosis, which are triggered by stimuli at either end of the physiological range. Both endocytosis modes generate endosomes directly from the nerve terminal plasma membrane, before the subsequent production of SVs from these structures. This review will discuss the current knowledge relating to the molecular mechanisms involved in the generation of SVs from nerve terminal endosomes, how this relates to other mechanisms of SV production and the functional role of such SVs.

Pharmacological inhibition of dynamin II reduces constitutive protein secretion from primary human macrophages

Dynamins are fission proteins that mediate endocytic and exocytic membrane events and are pharmacological therapeutic targets. These studies investigate whether dynamin II regulates constitutive protein secretion and show for the first time that pharmacological inhibition of dynamin decreases secretion of apolipoprotein E (apoE) and several other proteins constitutively secreted from primary human macrophages. Inhibitors that target recruitment of dynamin to membranes (MiTMABs) or directly target the GTPase domain (Dyngo or Dynole series), dose- and time- dependently reduced the secretion of apoE. SiRNA oligo’s targeting all isoforms of dynamin II confirmed the involvement of dynamin II in apoE secretion. Inhibition of secretion was not mediated via effects on mRNA or protein synthesis. 2D-gel electrophoresis showed that inhibition occurred after apoE was processed and glycosylated in the Golgi and live cell imaging showed that inhibited secretion was associated with reduced post-Golgi movement of apoE-GFP-containing vesicles. The effect was not restricted to macrophages, and was not mediated by the effects of the inhibitors on microtubules. Inhibition of dynamin also altered the constitutive secretion of other proteins, decreasing the secretion of fibronectin, matrix metalloproteinase 9, Chitinase-3-like protein 1 and lysozyme but unexpectedly increasing the secretion of the inflammatory mediator cyclophilin A. We conclude that pharmacological inhibitors of dynamin II modulate the constitutive secretion of macrophage apoE as a class effect, and that their capacity to modulate protein secretion may affect a range of biological processes.

Delivery of CSF-1R to the lumen of macropinosomes promotes its destruction in macrophages

Activation of the macrophage colony stimulating factor-1 receptor (CSF-1R) by CSF-1 stimulates pronounced macropinocytosis and drives proliferation of macrophages. Although the role of macropinocytosis in CSF-1R signaling remains unknown, we show here that, despite internalizing large quantities of plasma membrane, macropinosomes contribute little to the internalization of the CSF-1-CSF-1R complex. Rather, internalization of the CSF-1R in small endocytic vesicles that are sensitive to clathrin disruption, outcompetes macropinosomes for CSF-1R endocytosis. Following internalization, small vesicles carrying the CSF-1R underwent homotypic fusion and then trafficked to newly formed macropinosomes bearing Rab5. As these macropinosomes matured, acquiring Rab7, the CSF-1R was transported into their lumen and degraded. Inhibition of macropinocytosis delayed receptor degradation despite no disruption to CSF-1R endocytosis. These data indicate that CSF-1-stimulated macropinosomes are sites of multivesicular body formation and accelerate CSF-1R degradation. Furthermore, we demonstrate that macropinocytosis and cell growth have a matching dose dependence on CSF-1, suggesting that macropinosomes might be a central mechanism coupling CSF-1R signaling and macrophage growth.

Clathrin regenerates synaptic vesicles from endosomes

Ultrafast endocytosis can retrieve a single, large endocytic vesicle as fast as 50-100 ms after synaptic vesicle fusion. However, the fate of the large endocytic vesicles is not known. Here we demonstrate that these vesicles transition to a synaptic endosome about one second after stimulation. The endosome is resolved into coated vesicles after 3 s, which in turn become small-diameter synaptic vesicles 5-6 s after stimulation. We disrupted clathrin function using RNA interference (RNAi) and found that clathrin is not required for ultrafast endocytosis but is required to generate synaptic vesicles from the endosome. Ultrafast endocytosis fails when actin polymerization is disrupted, or when neurons are stimulated at room temperature instead of physiological temperature. In the absence of ultrafast endocytosis, synaptic vesicles are retrieved directly from the plasma membrane by clathrin-mediated endocytosis. These results may explain discrepancies among published experiments concerning the role of clathrin in synaptic vesicle endocytosis.

Dynamin and endocytosis are required for the fusion of osteoclasts and myoblasts

Dynamin function is essential for cell–cell fusion in both osteoclast precursors and myoblasts in part through its effects on endocytosis.

Cell–cell fusion is an evolutionarily conserved process that leads to the formation of multinucleated myofibers, syncytiotrophoblasts and osteoclasts, allowing their respective functions. Although cell–cell fusion requires the presence of fusogenic membrane proteins and actin-dependent cytoskeletal reorganization, the precise machinery allowing cells to fuse is still poorly understood. Using an inducible knockout mouse model to generate dynamin 1– and 2–deficient primary osteoclast precursors and myoblasts, we found that fusion of both cell types requires dynamin. Osteoclast and myoblast cell–cell fusion involves the formation of actin-rich protrusions closely associated with clathrin-mediated endocytosis in the apposed cell. Furthermore, impairing endocytosis independently of dynamin also prevented cell–cell fusion. Since dynamin is involved in both the formation of actin-rich structures and in endocytosis, our results indicate that dynamin function is central to the osteoclast precursors and myoblasts fusion process, and point to an important role of endocytosis in cell–cell fusion.

Inhibitors of endocytosis prevent Wnt/Wingless signalling by reducing the level of basal β-catenin/Armadillo

A key step in the canonical Wnt signalling pathway is the inhibition of GSK3β, which results in the accumulation of nuclear β-catenin (also known as CTNNB1), and hence regulation of target genes. Evidence suggests that endocytosis is required for signalling, yet its role and the molecular understanding remains unclear. A recent and controversial model suggests that endocytosis contributes to Wnt signalling by causing the sequestration of the ligand-receptor complex, including LRP6 and GSK3 to multivesicular bodies (MVBs), thus preventing GSK3β from accessing β-catenin. Here, we use specific inhibitors (Dynasore and Dyngo-4a) to confirm the essential role of endocytosis in Wnt/Wingless signalling in human and Drosophila cells. However, we find no evidence that, in Drosophila cells or wing imaginal discs, LRP6/Arrow traffics to MVBs or that MVBs are required for Wnt/Wingless signalling. Moreover, we show that activation of signalling through chemical blockade of GSK3β is prevented by endocytosis inhibitors, suggesting that endocytosis impacts on Wnt/Wingless signalling downstream of the ligand-receptor complex. We propose that, through an unknown mechanism, endocytosis boosts the resting pool of β-catenin upon which GSK3β normally acts.

Herpes simplex virus internalization into epithelial cells requires Na+/H+ exchangers and p21-activated kinases but neither clathrin- nor caveolin-mediated endocytosis

Herpes simplex virus 1 (HSV-1) is an alphaherpesvirus that has been reported to infect some epithelial cell types by fusion at the plasma membrane but others by endocytosis. To determine the molecular mechanisms of productive HSV-1 cell entry, we perturbed key endocytosis host factors using specific inhibitors, RNA interference (RNAi), or overexpression of dominant negative proteins and investigated their effects on HSV-1 infection in the permissive epithelial cell lines Vero, HeLa, HEp-2, and PtK2. HSV-1 internalization required neither endosomal acidification nor clathrin- or caveolin-mediated endocytosis. In contrast, HSV-1 gene expression and internalization were significantly reduced after treatment with 5-(N-ethyl-N-isopropyl)amiloride (EIPA). EIPA blocks the activity of Na(+)/H(+) exchangers, which are plasma membrane proteins implicated in all forms of macropinocytosis. HSV-1 internalization furthermore required the function of p21-activated kinases that contribute to macropinosome formation. However, in contrast to some forms of macropinocytosis, HSV-1 did not enlist the activities of protein kinase C (PKC), tyrosine kinases, C-terminal binding protein 1, or dynamin to activate its internalization. These data suggest that HSV-1 depends on Na(+)/H(+) exchangers and p21-activated kinases either for macropinocytosis or for local actin rearrangements required for fusion at the plasma membrane or subsequent passage through the actin cortex underneath the plasma membrane.

IMPORTANCE

After initial replication in epithelial cells, herpes simplex viruses (HSVs) establish latent infections in neurons innervating these regions. Upon primary infection and reactivation from latency, HSVs cause many human skin and neurological diseases, particularly in immunocompromised hosts, despite the availability of effective antiviral drugs. Many viruses use macropinocytosis for virus internalization, and many host factors mediating this entry route have been identified, although the specific perturbation profiles vary for different host and viral cargo. In addition to an established entry pathway via acidic endosomes, we show here that HSV-1 internalization depended on sodium-proton exchangers at the plasma membrane and p21-activated kinases. These results suggest that HSV-1 requires a reorganization of the cortical actin cytoskeleton, either for productive cell entry via pH-independent fusion from macropinosomes or for fusion at the plasma membrane, and subsequent cytosolic passage to microtubules that mediate capsid transport to the nucleus for genome uncoating and replication.

Synaptic Vesicle Endocytosis and Endosomal Recycling in Central Nerve Terminals: Discrete Trafficking Routes?

Synaptic vesicle (SV) retrieval from the presynaptic plasma membrane occurs via a variety of different and complementary modes. The dominant retrieval mode during high-intensity stimulation is activity-dependent bulk endocytosis (ADBE). ADBE involves the generation of endosomes direct from the plasma membrane which then donate membrane and cargo to form SVs that replenish the reserve SV pool. Recent evidence has suggested that ADBE may involve an additional endosomal processing step to produce a mature, functional SV. This suggests that ADBE may utilize key molecules or indeed whole pathways from classical endocytic recycling routes that are ubiquitous across all cell types. This review will assess the current evidence for a contribution of endocytic recycling to the SV life cycle, with a particular focus on ADBE. In doing so it highlights points where both routes may either converge or exploit existing mechanisms to ensure efficient generation of SVs during high-intensity stimulation.

Coupling between endocytosis and sphingosine kinase 1 recruitment

Genetic studies have suggested a functional link between cholesterol/sphingolipid metabolism and endocytic membrane traffic. Here we show that perturbing the cholesterol/sphingomyelin balance in the plasma membrane results in the massive formation of clusters of narrow endocytic tubular invaginations positive for N-BAR proteins. These tubules are intensely positive for sphingosine kinase 1 (SPHK1). SPHK1 is also targeted to physiologically occurring early endocytic intermediates, and is highly enriched in nerve terminals, which are cellular compartments specialized for exo/endocytosis. Membrane recruitment of SPHK1 involves a direct, curvature-sensitive interaction with the lipid bilayer mediated by a hydrophobic patch on the enzyme's surface. The knockdown of SPHKs results in endocytic recycling defects, and a mutation that disrupts the hydrophobic patch of Caenorhabditis elegans SPHK fails to rescue the neurotransmission defects in loss-of-function mutants of this enzyme. Our studies support a role for sphingosine phosphorylation in endocytic membrane trafficking beyond the established function of sphingosine-1-phosphate in intercellular signalling.

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

Uptake of Shiga-toxigenic Escherichia coli SubAB by HeLa cells requires an actin- and lipid raft-dependent pathway

The novel cytotoxic factor subtilase cytotoxin (SubAB) is produced mainly by non-O157 Shiga-toxigenic Escherichia coli (STEC). SubAB cleaves the molecular chaperone BiP/GRP78 in the endoplasmic reticulum (ER), leading to activation of RNA-dependent protein kinase (PKR)-like ER kinase (PERK), followed by caspase-dependent cell death. However, the SubAB uptake mechanism in HeLa cells is unknown. In this study, a variety of inhibitors and siRNAs were employed to characterize the SubAB uptake process. SubAB-induced BiP cleavage was inhibited by high concentrations of Dynasore, and methyl-β-cyclodextrin (mβCD) and Filipin III, but not suppressed in clathrin-, dynamin I/II-, caveolin1- and caveolin2-knockdown cells. We observed that SubAB treatment led to dramatic actin rearrangements, e.g. formation of plasma membrane blebs, with a significant increase in fluid uptake. Confocal microscopy analysis showed that SubAB uptake required actin cytoskeleton remodelling and lipid raft cholesterol. Furthermore, internalized SubAB in cells was found in the detergent-resistant domain (DRM) structure. Interestingly, IPA-3, an inhibitor of serine/threonine kinase p21-activated kinase (PAK1), an important protein of macropinocytosis, directly inhibited SubAB-mediated BiP cleavage and SubAB internalization. Thus, our findings suggest that SubAB uses lipid raft- and actin-dependent, but not clathrin-, caveolin- and dynamin-dependent pathways as its major endocytic translocation route.

Actin and dynamin2 dynamics and interplay during clathrin-mediated endocytosis

Actin assembly influences the precise temporal and quantitative recruitment of dynamin2 to sites of clathrin-mediated endocytosis.

Clathrin-mediated endocytosis (CME) involves the recruitment of numerous proteins to sites on the plasma membrane with prescribed timing to mediate specific stages of the process. However, how choreographed recruitment and function of specific proteins during CME is achieved remains unclear. Using genome editing to express fluorescent fusion proteins at native levels and live-cell imaging with single-molecule sensitivity, we explored dynamin2 stoichiometry, dynamics, and functional interdependency with actin. Our quantitative analyses revealed heterogeneity in the timing of the early phase of CME, with transient recruitment of 2–4 molecules of dynamin2. In contrast, considerable regularity characterized the final 20 s of CME, during which ∼26 molecules of dynamin2, sufficient to make one ring around the vesicle neck, were typically recruited. Actin assembly generally preceded dynamin2 recruitment during the late phases of CME, and promoted dynamin recruitment. Collectively, our results demonstrate precise temporal and quantitative regulation of the dynamin2 recruitment influenced by actin polymerization.

From membranes to organelles: emerging roles for dynamin-like proteins in diverse cellular processes

Dynamin is a GTPase mechanoenzyme most noted for its role in vesicle scission during endocytosis, and belongs to the dynamin family proteins. The dynamin family consists of classical dynamins and dynamin-like proteins (DLPs). Due to structural and functional similarities DLPs are thought to carry out membrane tubulation and scission in a similar manner to dynamin. Here, we discuss the newly emerging roles for DLPs, which include vacuole fission and fusion, peroxisome maintenance, endocytosis and intracellular trafficking. Specific focus is given to the role of DLPs in the budding yeast Saccharomyces cerevisiae because the diverse function of DLPs has been well characterized in this organism. Recent insights into DLPs may provide a better understanding of mammalian dynamin and its associated diseases.

Role of dynamin and clathrin in the cellular trafficking of flotillins

Flotillin-1 and flotillin-2 are highly conserved, membrane-microdomain-associated proteins that have been shown to be involved in signal transduction, membrane trafficking and cell adhesion. Upon growth factor stimulation, flotillins are tyrosine phosphorylated and become endocytosed from the plasma membrane into endosomes from which they are recycled back to the plasma membrane. Although a role for flotillin-1 in the endocytosis of certain cargo proteins has been suggested, it is not known how the growth-factor-induced endocytosis of flotillins is regulated and which endocytosis pathway is used. However, this is likely to be different from the pathway used by flotillin-dependent cargo. In this study, we have addressed the mechanistic details of flotillin trafficking during growth factor signaling. We show that dynamin-2 activity is required for the uptake of flotillins from the plasma membrane upon epidermal growth factor stimulation, and inhibition of dynamin-2 GTPase activity impairs flotillin endocytosis. Surprisingly, recycling of flotillins from endosomes to the plasma membrane appears to require both dynamin-2 and clathrin. Upon overexpression of dynamin-2 mutants or depletion of clathrin heavy chain, flotillins are permanently trapped in endosomes. These data show that clathrin and dynamin are required for the endosomal sorting of flotillins, and the study provides a mechanistic dissection of the thus far poorly characterized endosomal trafficking of flotillins.

FGF21 promotes endothelial cell angiogenesis through a dynamin-2 and Rab5 dependent pathway

Binding of angiogenic molecules with cognate receptor tyrosine kinases (RTK) is required for angiogenesis however the precise link between RTK binding, endocytosis, and signaling requires further investigation. Here, we use FGFR1 as a model to test the effects of the large GTPase and endocytosis regulatory molecule dynamin-2 on angiogenic signaling in context of distinct FGF ligands. In vitro, overexpression of dominant negative dynamin-2 (DynK44A) attenuates FGFR1 activation of Erk and tubulogenesis by FGF2. Furthermore, we identify FGF21, a non-classical, FGF ligand implicated in diverse human pathologies as an angiogenic molecule acting through FGFR1 and β-Klotho coreceptor. Disruption of FGFR1 activation of ERK by FGF21 is achieved by perturbation of the function of both dynamin-2 and Rab5 GTPase. In vivo, mice harboring endothelial selective overexpression of DynK44A, show impaired angiogenesis in response to FGF21. In conclusion, dynamin dependent endocytosis of FGFR1 is required for in vitro and in vivo angiogenesis in response to FGF2 and the non-classical FGF ligand, FGF21. These studies extend our understanding of the relationships between RTK binding, internalization, endosomal targeting, and angiogenic signaling.

Dynamin photoinactivation blocks Clathrin and α-adaptin recruitment and induces bulk membrane retrieval

Drosophila Dynamin prevents bulk membrane endocytosis through effects on AP2- and Clathrin-mediated stabilization of endocytic pits.

Dynamin is a well-known regulator of synaptic endocytosis. Temperature-sensitive dynamin (shi^ts1) mutations in Drosophila melanogaster or deletion of some of the mammalian Dynamins causes the accumulation of invaginated endocytic pits at synapses, sometimes also on bulk endosomes, indicating impaired membrane scission. However, complete loss of dynamin function has not been studied in neurons in vivo, and whether Dynamin acts in different aspects of synaptic vesicle formation remains enigmatic. We used acute photoinactivation and found that loss of Dynamin function blocked membrane recycling and caused the buildup of huge membrane-connected cisternae, in contrast to the invaginated pits that accumulate in shi^ts1 mutants. Moreover, photoinactivation of Dynamin in shi^ts1 animals converted these pits into bulk cisternae. Bulk membrane retrieval has also been seen upon Clathrin photoinactivation, and superresolution imaging indicated that acute Dynamin photoinactivation blocked Clathrin and α-adaptin relocalization to synaptic membranes upon nerve stimulation. Hence, our data indicate that Dynamin is critically involved in the stabilization of Clathrin- and AP2-dependent endocytic pits.

Synthesis of Dynole 34-2, Dynole 2-24 and Dyngo 4a for investigating dynamin GTPase

Dynamin is a large GTPase with roles in membrane fission during clathrin-mediated endocytosis, in actin dynamics and in cytokinesis. Defects in dynamin have been linked to human diseases. The synthesis of a dynamin modulator toolkit comprising two different inhibitor classes is described. The first series comprises Dynole 34-2, Dynole 2-24 and the inactive control Dynole 31-2. The Dynole compounds act on the dynamin G domain, are not GTP competitive and can be synthesized in 2-3 d. Knoevenagel condensation of 1-(3-(dimethylamino)propyl)-1H-indole-3-carbaldehyde (1) with cyanoamides (2 and 3) affords Dynole 31-2 and Dynole 34-2, respectively. Reductive amination of 1 with decylamine gives Dynole 2-24. The second series acts at an allosteric site in the G domain of dynamin and comprises Dyngo 4a and Dyngo Ø (inactive control). Both are synthesized in an overnight reaction via condensation of 3-hydroxy-2-naphthoic hydrazide with 2,4,5-trihydroxybenzaldehyde to afford Dyngo 4a, or with benzaldehyde to afford Dyngo Ø.

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.

Dynamin 2 regulation of integrin endocytosis, but not VEGF signaling, is crucial for developmental angiogenesis

Here we show that dynamin 2 (Dnm2) is essential for angiogenesis in vitro and in vivo. In cultured endothelial cells lacking Dnm2, vascular endothelial growth factor (VEGF) signaling and receptor levels are augmented whereas cell migration and morphogenesis are impaired. Mechanistically, the loss of Dnm2 increases focal adhesion size and the surface levels of multiple integrins and reduces the activation state of β1 integrin. In vivo, the constitutive or inducible loss of Dnm2 in endothelium impairs branching morphogenesis and promotes the accumulation of β1 integrin at sites of failed angiogenic sprouting. Collectively, our data show that Dnm2 uncouples VEGF signaling from function and coordinates the endocytic turnover of integrins in a manner that is crucially important for angiogenesis in vitro and in vivo.

Clathrin terminal domain-ligand interactions regulate sorting of mannose 6-phosphate receptors mediated by AP-1 and GGA adaptors

Clathrin plays important roles in intracellular membrane traffic including endocytosis of plasma membrane proteins and receptors and protein sorting between the trans-Golgi network (TGN) and endosomes. Whether clathrin serves additional roles in receptor recycling, degradative sorting, or constitutive secretion has remained somewhat controversial. Here we have used acute pharmacological perturbation of clathrin terminal domain (TD) function to dissect the role of clathrin in intracellular membrane traffic. We report that internalization of major histocompatibility complex I (MHCI) is inhibited in cells depleted of clathrin or its major clathrin adaptor complex 2 (AP-2), a phenotype mimicked by application of Pitstop® inhibitors of clathrin TD function. Hence, MHCI endocytosis occurs via a clathrin/AP-2-dependent pathway. Acute perturbation of clathrin also impairs the dynamics of intracellular clathrin/adaptor complex 1 (AP-1)- or GGA (Golgi-localized, γ-ear-containing, Arf-binding protein)-coated structures at the TGN/endosomal interface, resulting in the peripheral dispersion of mannose 6-phosphate receptors. By contrast, secretory traffic of vesicular stomatitis virus G protein, recycling of internalized transferrin from endosomes, or degradation of EGF receptor proceeds unperturbed in cells with impaired clathrin TD function. These data indicate that clathrin is required for the function of AP-1- and GGA-coated carriers at the TGN but may be dispensable for outward traffic en route to the plasma membrane.

Pharmacological inhibitors of exocytosis and endocytosis: novel bullets for old targets

Pharmacological inhibitors of vesicle trafficking possess great promise as valuable analytical tools for the study of a variety of biological processes and as potential therapeutic agents to fight microbial infections and cancer. However, many commonly used trafficking inhibitors are characterized by poor selectivity that diminishes their use in solving basic problems of cell biology or drug development. Recent high-throughput chemical screens intensified the search for novel modulators of vesicle trafficking, and successfully identified a number of small molecules that inhibit exocytosis and endocytosis in different types of mammalian cells. This chapter provides a systematic overview of recently discovered inhibitors of vesicle trafficking. It describes cellular effects and mechanisms of action of novel inhibitors of exocytosis and endocytosis. Furthermore, it pays special attention to the selectivity and possible off-target effects of these inhibitors.