Activity-dependent fusion pore expansion regulated by a calcineurin-dependent dynamin-syndapin pathway in mouse adrenal chromaffin cells

Dynamin-2 mutations linked to neonatal-onset centronuclear myopathy impair exocytosis and endocytosis in adrenal chromaffin cells

Dynamins are large GTPases whose primary function is not only to catalyze membrane scission during endocytosis but also to modulate other cellular processes, such as actin polymerization and vesicle trafficking. Recently, we reported that centronuclear myopathy associated dynamin-2 mutations, p.A618T, and p.S619L, impair Ca2+-induced exocytosis of the glucose transporter GLUT4 containing vesicles in immortalized human myoblasts. As exocytosis and endocytosis occur within rapid timescales, here we applied high-temporal resolution techniques, such as patch-clamp capacitance measurements and carbon-fiber amperometry to assess the effects of these mutations on these two cellular processes, using bovine chromaffin cells as a study model. We found that the expression of any of these dynamin-2 mutants inhibits a dynamin and F-actin-dependent form of fast endocytosis triggered by single action potential stimulus, as well as inhibits a slow compensatory endocytosis induced by 500 ms square depolarization. Both dynamin-2 mutants further reduced the exocytosis induced by 500 ms depolarizations, and the frequency of release events and the recruitment of neuropeptide Y (NPY)-labeled vesicles to the cell cortex after stimulation of nicotinic acetylcholine receptors with 1,1-dimethyl-4-phenyl piperazine iodide (DMPP). They also provoked a significant decrease in the Ca2+-induced formation of new actin filaments in permeabilized chromaffin cells. In summary, our results indicate that the centronuclear myopathy (CNM)-linked p.A618T and p.S619L mutations in dynamin-2 affect exocytosis and endocytosis, being the disruption of F-actin dynamics a possible explanation for these results. These impaired cellular processes might underlie the pathogenic mechanisms associated with these mutations.

Omega-3 and -6 Fatty Acids Alter the Membrane Lipid Composition and Vesicle Size to Regulate Exocytosis and Storage of Catecholamines

The two essential fatty acids, alpha-linolenic acid and linoleic acid, and the higher unsaturated fatty acids synthesized from them are critical for the development and maintenance of normal brain functions. Deficiencies of these fatty acids have been shown to cause damage to the neuronal development, cognition, and locomotor function. We combined electrochemistry and imaging techniques to examine the effects of the two essential fatty acids on catecholamine release dynamics and the vesicle content as well as on the cell membrane phospholipid composition to understand how they impact exocytosis and by extension neurotransmission at the single-cell level. Incubation of either of the two fatty acids reduces the size of secretory vesicles and enables the incorporation of more double bonds into the cell membrane structure, resulting in higher membrane flexibility. This subsequently affects proteins regulating the dynamics of the exocytotic fusion pore and thereby affects exocytosis. Our data suggest a possible pathway whereby the two essential fatty acids affect the membrane structure to impact exocytosis and provide a potential treatment for diseases and impairments related to catecholamine signaling.

PACSIN proteins in vivo: Roles in development and physiology

Protein kinase C and casein kinase substrate in neurons (PACSINs), or syndapins (synaptic dynamin‐associated proteins), are a family of proteins involved in the regulation of cell cytoskeleton, intracellular trafficking and signalling. Over the last twenty years, PACSINs have been mostly studied in the in vitro and ex vivo settings, and only in the last decade reports on their function in vivo have emerged. We first summarize the identification, structure and cellular functions of PACSINs, and then focus on the relevance of PACSINs in vivo. During development in various model organisms, PACSINs participate in diverse processes, such as neural crest cell development, gastrulation, laterality development and neuromuscular junction formation. In mouse, PACSIN2 regulates angiogenesis during retinal development and in human, PACSIN2 associates with monosomy and embryonic implantation. In adulthood, PACSIN1 has been extensively studied in the brain and shown to regulate neuromorphogenesis, receptor trafficking and synaptic plasticity. Several genetic studies suggest a role for PACSIN1 in the development of schizophrenia, which is also supported by the phenotype of mice depleted of PACSIN1. PACSIN2 plays an essential role in the maintenance of intestinal homeostasis and participates in kidney repair processes after injury. PACSIN3 is abundant in muscle tissue and necessary for caveolar biogenesis to create membrane reservoirs, thus controlling muscle function, and has been linked to certain genetic muscular disorders. The above examples illustrate the importance of PACSINs in diverse physiological or tissue repair processes in various organs, and associations to diseases when their functions are disturbed.

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.

Regulating quantal size of neurotransmitter release through a GPCR voltage sensor

Current models emphasize that membrane voltage (Vm) depolarization-induced Ca²⁺ influx triggers the fusion of vesicles to the plasma membrane. In sympathetic adrenal chromaffin cells, activation of a variety of G protein coupled receptors (GPCRs) can inhibit quantal size (QS) through the direct interaction of G protein Giβγ subunits with exocytosis fusion proteins. Here we report that, independently from Ca²⁺, Vm (action potential) per se regulates the amount of catecholamine released from each vesicle, the QS. The Vm regulation of QS was through ATP-activated GPCR-P2Y₁₂ receptors. D76 and D127 in P2Y₁₂ were the voltage-sensing sites. Finally, we revealed the relevance of the Vm dependence of QS for tuning autoinhibition and target cell functions. Together, membrane voltage per se increases the quantal size of dense-core vesicle release of catecholamine via Vm → P2Y₁₂(D76/D127) → Giβγ → QS → myocyte contractility, offering a universal Vm-GPCR signaling pathway for its functions in the nervous system and other systems containing GPCRs.

Spatiotemporal organization and protein dynamics involved in regulated exocytosis of MMP-9 in breast cancer cells

This paper describes the dynamics of proteins and lipids during exocytosis of MMP-9 from cancer cells in real time using fluorescence microscopy. Stephens et al. find that core exocytic proteins, accessory proteins, and lipids are involved at sites of secretory vesicle fusion.

Altered regulation of exocytosis is an important mechanism controlling many diseases, including cancer. Defects in exocytosis have been implicated in many cancer cell types and are generally attributed to mutations in cellular transport, trafficking, and assembly of machinery necessary for exocytosis of secretory vesicle cargo. In these cancers, up-regulation of trafficking and secretion of matrix metalloproteinase-9 (MMP-9), a proteolytic enzyme, is responsible for degrading the extracellular matrix, a necessary step in tumor progression. Using TIRF microscopy, we identified proteins associated with secretory vesicles containing MMP-9 and imaged the local dynamics of these proteins at fusion sites during regulated exocytosis of MMP-9 from MCF-7 breast cancer cells. We found that many regulators of exocytosis, including several Rab GTPases, Rab effector proteins, and SNARE/SNARE modulator proteins, are stably assembled on docked secretory vesicles before exocytosis. At the moment of fusion, many of these components are quickly lost from the vesicle, while several endocytic proteins and lipids are simultaneously recruited to exocytic sites at precisely that moment. Our findings provide insight into the dynamic behavior of key core exocytic proteins, accessory proteins, lipids, and some endocytic proteins at single sites of secretory vesicle fusion in breast cancer cells.

Plasticity in exocytosis revealed through the effects of repetitive stimuli affect the content of nanometer vesicles and the fraction of transmitter released

Electrochemical techniques with disk and nano-tip electrodes, together with calcium imaging, were used to examine the effect of short-interval repetitive stimuli on both exocytosis and vesicular content in a model cell line. We show that the number of events decreases markedly with repeated stimuli suggesting a depletion of exocytosis machinery. However, repetitive stimuli induce a more stable fusion pore, leading to an increased amount of neurotransmitter release. In contrast, the total neurotransmitter content inside the vesicles decreases after repetitive stimuli, resulting in a higher average release fraction from each event. We suggest a possible mechanism regarding a link between activity-induced plasticity and fraction of release.

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

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

Altered exocytosis in chromaffin cells from mouse models of neurodegenerative diseases

Chromaffin cells from the adrenal gland (CCs) have extensively been used to explore the molecular structure and function of the exocytotic machinery, neurotransmitter release and synaptic transmission. The CC is integrated in the sympathoadrenal axis that helps the body maintain homoeostasis during both routine life and in acute stress conditions. This function is exquisitely controlled by the cerebral cortex and the hypothalamus. We propose the hypothesis that damage undergone by the brain during neurodegenerative diseases is also affecting the neurosecretory function of adrenal medullary CCs. In this context, we review here the following themes: (i) How the discharge of catecholamines is centrally and peripherally regulated at the sympathoadrenal axis; (ii) which are the intricacies of the amperometric techniques used to study the quantal release of single-vesicle exocytotic events; (iii) which are the alterations of the exocytotic fusion pore so far reported, in CCs of mouse models of neurodegenerative diseases; (iv) how some proteins linked to neurodegenerative pathologies affect the kinetics of exocytotic events; (v) finally, we try to integrate available data into a hypothesis to explain how the centrally originated neurodegenerative diseases may alter the kinetics of single-vesicle exocytotic events in peripheral adrenal medullary CCs.

Calcineurin and Its Role in Synaptic Transmission

Calcineurin (CaN) is a serine/threonine phosphatase widely expressed in different cell types and structures including neurons and synapses. The most studied role of CaN is its involvement in the functioning of postsynaptic structures of central synapses. The role of CaN in the presynaptic structures of central and peripheral synapses is less understood, although it has generated a considerable interest and is a subject of a growing number of studies. The regulatory role of CaN in synaptic vesicle endocytosis in the synapse terminals is actively studied. In recent years, new targets of CaN have been identified and its role in the regulation of enzymes and neurotransmitter secretion in peripheral neuromuscular junctions has been revealed. CaN is the only phosphatase that requires calcium and calmodulin for activation. In this review, we present details of CaN molecular structure and give a detailed description of possible mechanisms of CaN activation involving calcium, enzymes, and endogenous and exogenous inhibitors. Known and newly discovered CaN targets at pre- and postsynaptic levels are described. CaN activity in synaptic structures is discussed in terms of functional involvement of this phosphatase in synaptic transmission and neurotransmitter release.

Local protein dynamics during microvesicle exocytosis in neuroendocrine cells

Calcium-triggered exocytosis is key to many physiological processes, including neurotransmitter and hormone release by neurons and endocrine cells. Dozens of proteins regulate exocytosis, yet the temporal and spatial dynamics of these factors during vesicle fusion remain unclear. Here we use total internal reflection fluorescence microscopy to visualize local protein dynamics at single sites of exocytosis of small synaptic-like microvesicles in live cultured neuroendocrine PC12 cells. We employ two-color imaging to simultaneously observe membrane fusion (using vesicular acetylcholine ACh transporter tagged to pHluorin) and the dynamics of associated proteins at the moments surrounding exocytosis. Our experiments show that many proteins, including the SNAREs syntaxin1 and VAMP2, the SNARE modulator tomosyn, and Rab proteins, are preclustered at fusion sites and rapidly lost at fusion. The ATPase N-ethylmaleimide–sensitive factor is locally recruited at fusion. Interestingly, the endocytic Bin-Amphiphysin-Rvs domain–containing proteins amphiphysin1, syndapin2, and endophilins are dynamically recruited to fusion sites and slow the loss of vesicle membrane-bound cargo from fusion sites. A similar effect on vesicle membrane protein dynamics was seen with the overexpression of the GTPases dynamin1 and dynamin2. These results suggest that proteins involved in classical clathrin-mediated endocytosis can regulate exocytosis of synaptic-like microvesicles. Our findings provide insights into the dynamics, assembly, and mechanistic roles of many key factors of exocytosis and endocytosis at single sites of microvesicle fusion in live cells.

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

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

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.

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.

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.

Fusion pores and their control of neurotransmitter and hormone release

Chang et al. review fusion pore structure and dynamics and discuss the implications for hormone and neurotransmitter release

Ca²⁺-triggered exocytosis functions broadly in the secretion of chemical signals, enabling neurons to release neurotransmitters and endocrine cells to release hormones. The biological demands on this process can vary enormously. Although synapses often release neurotransmitter in a small fraction of a millisecond, hormone release can be orders of magnitude slower. Vesicles usually contain multiple signaling molecules that can be released selectively and conditionally. Cells are able to control the speed, concentration profile, and content selectivity of release by tuning and tailoring exocytosis to meet different biological demands. Much of this regulation depends on the fusion pore—the aqueous pathway by which molecules leave a vesicle and move out into the surrounding extracellular space. Studies of fusion pores have illuminated how cells regulate secretion. Furthermore, the formation and growth of fusion pores serve as a readout for the progress of exocytosis, thus revealing key kinetic stages that provide clues about the underlying mechanisms. Herein, we review the structure, composition, and dynamics of fusion pores and discuss the implications for molecular mechanisms as well as for the cellular regulation of neurotransmitter and hormone release.

Imaging the recruitment and loss of proteins and lipids at single sites of calcium-triggered exocytosis

Imaging of exocytic and endocytic proteins shows which are present at exocytic sites before, during, and after exocytosis in living cells. Rab proteins and SNARE modulators are lost, and dynamin, PIP2, and BAR-domain proteins are rapidly and transiently recruited, where they may modulate the nascent fusion pore.

How and when the dozens of molecules that control exocytosis assemble in living cells to regulate the fusion of a vesicle with the plasma membrane is unknown. Here we image with two-color total internal reflection fluorescence microscopy the local changes of 27 proteins at single dense-core vesicles undergoing calcium-triggered fusion. We identify two broad dynamic behaviors of exocytic molecules. First, proteins enriched at exocytic sites are associated with DCVs long before exocytosis, and near the time of membrane fusion, they diffuse away. These proteins include Rab3 and Rab27, rabphilin3a, munc18a, tomosyn, and CAPS. Second, we observe a group of classical endocytic proteins and lipids, including dynamins, amphiphysin, syndapin, endophilin, and PIP2, which are rapidly and transiently recruited to the exocytic site near the time of membrane fusion. Dynamin mutants unable to bind amphiphysin were not recruited, indicating that amphiphysin is involved in localizing dynamin to the fusion site. Expression of mutant dynamins and knockdown of endogenous dynamin altered the rate of cargo release from single vesicles. Our data reveal the dynamics of many key proteins involved in exocytosis and identify a rapidly recruited dynamin/PIP2/BAR assembly that regulates the exocytic fusion pore of dense-core vesicles in cultured endocrine beta cells.

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.

Cell biology of yeast zygotes, from genesis to budding

The zygote is the essential intermediate that allows interchange of nuclear, mitochondrial and cytosolic determinants between cells. Zygote formation in Saccharomyces cerevisiae is accomplished by mechanisms that are not characteristic of mitotic cells. These include shifting the axis of growth away from classical cortical landmarks, dramatically reorganizing the cell cortex, remodeling the cell wall in preparation for cell fusion, fusing with an adjacent partner, accomplishing nuclear fusion, orchestrating two steps of septin morphogenesis that account for a delay in fusion of mitochondria, and implementing new norms for bud site selection. This essay emphasizes the sequence of dependent relationships that account for this progression from cell encounters through zygote budding. It briefly summarizes classical studies of signal transduction and polarity specification and then focuses on downstream events.

Src kinases regulate de novo actin polymerization during exocytosis in neuroendocrine chromaffin cells

The cortical actin network is dynamically rearranged during secretory processes. Nevertheless, it is unclear how de novo actin polymerization and the disruption of the preexisting actin network control transmitter release. Here we show that in bovine adrenal chromaffin cells, both formation of new actin filaments and disruption of the preexisting cortical actin network are induced by Ca²⁺ concentrations that trigger exocytosis. These two processes appear to regulate different stages of exocytosis; whereas the inhibition of actin polymerization with the N-WASP inhibitor wiskostatin restricts fusion pore expansion, thus limiting the release of transmitters, the disruption of the cortical actin network with cytochalasin D increases the amount of transmitter released per event. Further, the Src kinase inhibitor PP2, and cSrc SH2 and SH3 domains also suppress Ca²⁺-dependent actin polymerization, and slow down fusion pore expansion without disturbing the cortical F-actin organization. Finally, the isolated SH3 domain of c-Src prevents both the disruption of the actin network and the increase in the quantal release induced by cytochalasin D. These findings support a model where a rise in the cytosolic Ca²⁺ triggers actin polymerization through a mechanism that involves Src kinases. The newly formed actin filaments would speed up the expansion of the initial fusion pore, whereas the preexisting actin network might control a different step of the exocytosis process.

Syndapin 3 modulates fusion pore expansion in mouse neuroendocrine chromaffin cells

Adrenal neuroendocrine chromaffin cells receive excitatory synaptic input from the sympathetic nervous system and secrete hormones into the peripheral circulation. Under basal sympathetic tone, modest amounts of freely soluble catecholamine are selectively released through a restricted fusion pore formed between the secretory granule and the plasma membrane. Upon activation of the sympathoadrenal stress reflex, elevated stimulation drives fusion pore expansion, resulting in increased catecholamine secretion and facilitating release of copackaged peptide hormones. Thus regulated expansion of the secretory fusion pore is a control point for differential hormone release of the sympathoadrenal stress response. Previous work has shown that syndapin 1 deletion alters transmitter release and that the dynamin 1-syndapin 1 interaction is necessary for coupled endocytosis in neurons. Dynamin has also been shown to be involved in regulation of fusion pore expansion in neuroendocrine chromaffin cells through an activity-dependent association with syndapin. However, it is not known which syndapin isoform(s) contributes to pore dynamics in neuroendocrine cells. Nor is it known at what stage of the secretion process dynamin and syndapin associate to modulate pore expansion. Here we investigate the expression and localization of syndapin isoforms and determine which are involved in mediating fusion pore expansion. We show that all syndapin isoforms are expressed in the adrenal medulla. Mutation of the SH3 dynamin-binding domain of all syndapin isoforms shows that fusion pore expansion and catecholamine release are limited specifically by mutation of syndapin 3. The mutation also disrupts targeting of syndapin 3 to the cell periphery. Syndapin 3 exists in a persistent colocalized state with dynamin 1.

Dynamin-2 function and dysfunction along the secretory pathway

Dynamin-2 is a ubiquitously expressed mechano-GTPase involved in different stages of the secretory pathway. Its most well-known function relates to the scission of nascent vesicles from the plasma membrane during endocytosis; however, it also participates in the formation of new vesicles from the Golgi network, vesicle trafficking, fusion processes and in the regulation of microtubule, and actin cytoskeleton dynamics. Over the last 8 years, more than 20 mutations in the dynamin-2 gene have been associated to two hereditary neuromuscular disorders: Charcot-Marie-Tooth neuropathy and centronuclear myopathy. Most of these mutations are grouped in the pleckstrin homology domain; however, there are no common mutations associated with both disorders, suggesting that they differently impact on dynamin-2 function in diverse tissues. In this review, we discuss the impact of these disease-related mutations on dynamin-2 function during vesicle trafficking and endocytotic processes.

Dynamin-2 regulates fusion pore expansion and quantal release through a mechanism that involves actin dynamics in neuroendocrine chromaffin cells

Over the past years, dynamin has been implicated in tuning the amount and nature of transmitter released during exocytosis. However, the mechanism involved remains poorly understood. Here, using bovine adrenal chromaffin cells, we investigated whether this mechanism rely on dynamin’s ability to remodel actin cytoskeleton. According to this idea, inhibition of dynamin GTPase activity suppressed the calcium-dependent de novo cortical actin and altered the cortical actin network. Similarly, expression of a small interfering RNA directed against dynamin-2, an isoform highly expressed in chromaffin cells, changed the cortical actin network pattern. Disruption of dynamin-2 function, as well as the pharmacological inhibition of actin polymerization with cytochalasine-D, slowed down fusion pore expansion and increased the quantal size of individual exocytotic events. The effects of cytochalasine-D and dynamin-2 disruption were not additive indicating that dynamin-2 and F-actin regulate the late steps of exocytosis by a common mechanism. Together our data support a model in which dynamin-2 directs actin polymerization at the exocytosis site where both, in concert, adjust the hormone quantal release to efficiently respond to physiological demands.

Pyrimidyn compounds: dual-action small molecule pyrimidine-based dynamin inhibitors

Dynamin is required for clathrin-mediated endocytosis (CME). Its GTPase activity is stimulated by phospholipid binding to its PH domain, which induces helical oligomerization. We have designed a series of novel pyrimidine-based "Pyrimidyn" compounds that inhibit the lipid-stimulated GTPase activity of full length dynamin I and II with similar potency. The most potent analogue, Pyrimidyn 7, has an IC50 of 1.1 μM for dynamin I and 1.8 μM for dynamin II, making it among the most potent dynamin inhibitors identified to date. We investigated the mechanism of action of the Pyrimidyn compounds in detail by examining the kinetics of Pyrimidyn 7 inhibition of dynamin. The compound competitively inhibits both GTP and phospholipid interactions with dynamin I. While both mechanisms of action have been previously observed separately, this is the first inhibitor series to incorporate both and thereby to target two distinct domains of dynamin. Pyrimidyn 6 and 7 reversibly inhibit CME of both transferrin and EGF in a number of non-neuronal cell lines as well as inhibiting synaptic vesicle endocytosis (SVE) in nerve terminals. Therefore, Pyrimidyn compounds block endocytosis by directly competing with GTP and lipid binding to dynamin, limiting both the recruitment of dynamin to membranes and its activation. This dual mode of action provides an important new tool for molecular dissection of dynamin's role in endocytosis.

Syndapin--a membrane remodelling and endocytic F-BAR protein

Syndapin [also called PACSIN (protein kinase C and casein kinase II interacting protein)] is an Fes-CIP4 homology Bin-amphiphysin-Rvs161/167 (F-BAR) and Src-homology 3 domain-containing protein. Three genes give rise to three main isoforms in mammalian cells. They each function in different endocytic and vesicle trafficking pathways and provide critical links between the cytoskeletal network in different cellular processes, such as neuronal morphogenesis and cell migration. The membrane remodelling activity of syndapin via its F-BAR domain and its interaction partners, such as dynamin and neural Wiskott-Aldrich syndrome protein binding to its Src-homology 3 domain, are important with respect to its function. Its various partner proteins provide insights into its mechanism of action, as well as its differential roles in these cellular processes. Signalling pathways leading to the regulation of syndapin function by phosphorylation are now contributing to our understanding of the broader functions of this family of proteins.

Synaptic vesicle generation from activity-dependent bulk endosomes requires calcium and calcineurin

Activity-dependent bulk endocytosis (ADBE) is the dominant mode of synaptic vesicle (SV) endocytosis during high-frequency stimulation in central nerve terminals. ADBE generates endosomes direct from the plasma membrane, meaning that high concentrations of calcium will be present in their interior due to fluid phase uptake from the extracellular space. Morphological and fluorescent assays were used to track the generation of SVs from bulk endosomes in primary neuronal culture. This process was functionally uncoupled from both SV exocytosis and plasma membrane retrieval events by intervening only after SV fusion and endocytosis were completed. Either intracellular (BAPTA-AM) or intra-endosomal (Rhod-dextran) calcium chelation inhibited SV generation from bulk endosomes, indicating that calcium efflux from this compartment is critical for this process. The V-type ATPase antagonist bafilomycin A1 also arrested SV generation from bulk endosomes, indicating endosomal acidification may be required for calcium efflux. Finally, pharmacological inhibition of the calcium-dependent protein phosphatase calcineurin blocked endosomal SV generation, identifying it as a key downstream effector in this process. These results reveal a novel and key role for the fluid phase uptake of extracellular calcium and its subsequent efflux in the SV lifecycle.