Disruption of actin impedes transmitter release in snake motor terminals

Endocytosis at the synaptic terminal

Exocytosis of neurotransmitter from a synaptic vesicle is followed by efficient retrieval of its constituent membrane and proteins. Real-time measurements indicate that fast and slow modes of retrieval operate in parallel at a number of presynaptic terminals. Two mechanisms can be distinguished by electron microscopy: clathrin-mediated retrieval of small vesicles and bulk retrieval of large cisternae. Methods that investigate the behaviour of individual vesicles have recently demonstrated a third route of retrieval: the rapid reversal of a pore-like connection between the vesicle and surface ('kiss-and-run'). Key aims for the future are to identify the molecules underlying different mechanisms of endocytosis at the synapse and the signals that select between them.

Regulation of spine morphology and synaptic function by LIMK and the actin cytoskeleton

Filamentous actin (F-actin) is highly enriched in the dendritic spine, a specialized postsynaptic structure on which the great majority of the excitatory synapses are formed in the mammalian central nervous system (CNS). The protein kinases of the Lim-kinase (LIMK) family are potent regulators of actin dynamics in many cell types and they are abundantly expressed in the CNS, including the hippocampus. Using a combination of genetic manipulations and electrophysiological recordings in mice, we have demonstrated that LIMK-1 signaling is important in vivo in the regulation of the actin cytoskeleton, spine morphology, and synaptic function, including hippocampal long-term potentiation (LTP), a prominent form of long lasting synaptic plasticity thought to be critical to memory formation. Our results provide strong genetic evidence that LIMK and its substrate ADF/cofilin are involved in spine morphology and synaptic properties and are consistent with the notion that the Rho family small GTPases and the actin cytoskeleton are critical to spine structure and synaptic regulation.

Contribution of functional voltage-gated Na+ channel expression to cell behaviors involved in the metastatic cascade in rat prostate cancer: II. Secretory membrane activity

The secretory membrane activities of two rat prostate cancer cell lines of markedly different metastatic potential, and corresponding electrophysiological characteristics, were studied in a comparative approach. In particular, voltage-gated Na(+) channels (VGSCs) were expressed in the strongly metastatic MAT-LyLu but not in the closely related, but weakly metastatic, AT-2 cells. Uptake and release of the non-cytotoxic marker horseradish peroxidase (HRP) were used as indices of general endocytotic and exocytotic membrane activity, respectively. The amount of tracer present in a given experimental condition was quantified by light microscopic digital imaging. The uptake of HRP was an active process, abolished completely by incubating the cells at low temperature (5 degrees C) and suppressed by disrupting the cytoskeleton. Interestingly, the extent of HRP uptake into the strongly metastatic MAT-LyLu cells was almost twice that into the weakly metastatic AT-2 cells. Vesicular uptake of HRP occurred in a fast followed by a slow phase; these appeared to correspond to cytoplasmic and perinuclear pools, respectively. Importantly, the overall quantitative difference in the uptake disappeared in the presence of 1 microM tetrodotoxin which significantly reduced the uptake of HRP into the MAT-LyLu cells. There was no effect on the AT-2 cells, consistent with functional VGSC expression occurring selectively in the former. A similar effect was observed in Na(+)-free medium. The uptake was partially dependent upon extracellular Ca(2+) but was not affected by raising the extracellular K(+) concentration. We suggest that functional VGSC expression could potentiate prostate cancer cells' metastatic ability by enhancing their secretory membrane activity.

Bulk membrane retrieval in the synaptic terminal of retinal bipolar cells

The mechanism of bulk membrane uptake at the synapse remains poorly defined, although exocytosis of synaptic vesicles is followed by compensatory membrane retrieval into both small vesicles and large cisternas or vacuoles. We investigated bulk retrieval in the presynaptic terminal of retinal bipolar cells. Fluorescence imaging of the membrane dye FM1-43 indicated that Ca2+-triggered exocytosis was followed by endocytosis into small vesicles and larger vacuoles that could be selectively labeled using large fluorescent dextrans. Disruption of actin filaments with cytochalasin D or latrunculin B inhibited the formation and transport of vacuoles, but exocytosis and endocytosis continued at normal rates. Bulk retrieval was linked to remodeling of the actin network, and both processes were inhibited by 2-(4-morpholinyl)-8-phenyl-4H-1-benzopyran-4-one, an inhibitor of phosphatidylinositol 3-kinase (PI 3-kinase). The regulation of F-actin dynamics by Ca2+ and PI 3-kinase therefore played an important role in compensatory endocytosis at this synapse, but this role was confined to bulk membrane uptake. Capacitance measurements demonstrated that fast endocytosis and refilling of the rapidly releasable pool of vesicles were not dependent on F-actin or PI 3-kinase activity. The basic properties of bulk membrane retrieval at this synapse were very similar to macropinocytosis described in non-neural cells. Bulk retrieval did not play an essential role in maintaining the vesicle cycle during maintained stimulation, but we suggest that it may play a role in the structural plasticity of this synaptic terminal.

Involvement of actin polymerization in vesicle recruitment at the calyx of Held synapse

Depletion and replenishment of pools of synaptic vesicles are important determinants of short-term synaptic plasticity, but the underlying molecular mechanisms are not yet clear. As a first step toward understanding the process of vesicle recruitment, we have applied various specific agents directly to the presynaptic terminal of the calyx of Held synapse. Here we show that the nonhydrolyzable ATP analog ATP-gammaS retards the recovery from vesicle pool depletion, as does latrunculin A. Phalloidin has no effects on recovery, suggesting that dynamic actin reorganization is not necessary. Unexpectedly, neither N-ethylmaleimide nor staurosporine affected the recovery, calling into question the role of N-ethylmaleimide-sensitive factor and protein kinases. The results suggest that intact actin polymerization is involved in vesicle recruitment.

Bulk membrane retrieval in the synaptic terminal of retinal bipolar cells

The mechanism of bulk membrane uptake at the synapse remains poorly defined, although exocytosis of synaptic vesicles is followed by compensatory membrane retrieval into both small vesicles and large cisternas or vacuoles. We investigated bulk retrieval in the presynaptic terminal of retinal bipolar cells. Fluorescence imaging of the membrane dye FM1-43 indicated that Ca2+-triggered exocytosis was followed by endocytosis into small vesicles and larger vacuoles that could be selectively labeled using large fluorescent dextrans. Disruption of actin filaments with cytochalasin D or latrunculin B inhibited the formation and transport of vacuoles, but exocytosis and endocytosis continued at normal rates. Bulk retrieval was linked to remodeling of the actin network, and both processes were inhibited by 2-(4-morpholinyl)-8-phenyl-4H-1-benzopyran-4-one, an inhibitor of phosphatidylinositol 3-kinase (PI 3-kinase). The regulation of F-actin dynamics by Ca2+ and PI 3-kinase therefore played an important role in compensatory endocytosis at this synapse, but this role was confined to bulk membrane uptake. Capacitance measurements demonstrated that fast endocytosis and refilling of the rapidly releasable pool of vesicles were not dependent on F-actin or PI 3-kinase activity. The basic properties of bulk membrane retrieval at this synapse were very similar to macropinocytosis described in non-neural cells. Bulk retrieval did not play an essential role in maintaining the vesicle cycle during maintained stimulation, but we suggest that it may play a role in the structural plasticity of this synaptic terminal.

Involvement of actin polymerization in vesicle recruitment at the calyx of Held synapse

Depletion and replenishment of pools of synaptic vesicles are important determinants of short-term synaptic plasticity, but the underlying molecular mechanisms are not yet clear. As a first step toward understanding the process of vesicle recruitment, we have applied various specific agents directly to the presynaptic terminal of the calyx of Held synapse. Here we show that the nonhydrolyzable ATP analog ATP-gammaS retards the recovery from vesicle pool depletion, as does latrunculin A. Phalloidin has no effects on recovery, suggesting that dynamic actin reorganization is not necessary. Unexpectedly, neither N-ethylmaleimide nor staurosporine affected the recovery, calling into question the role of N-ethylmaleimide-sensitive factor and protein kinases. The results suggest that intact actin polymerization is involved in vesicle recruitment.

Actin has a molecular scaffolding, not propulsive, role in presynaptic function

We used actin tagged with enhanced green fluorescent protein (EGFP-actin) to characterize the distribution and dynamics of actin in living presynaptic terminals in rat CNS neurons. Actin was preferentially concentrated around--and appeared to surround--the presynaptic vesicle cluster. In resting terminals, approximately 30% of actin was found to be in a polymerized but dynamic state, with a remodeling time scale of approximately 20 s. During electrical activity, actin was further polymerized and recruited from nearby axonal regions to the regions surrounding vesicles. Treatment of terminals with the actin monomer-sequestering agent latrunculin-A completely dispersed the actin network and abolished activity-dependent actin dynamics. We used a variety of methods to examine the role of actin in the presynaptic vesicle cycle. These data rule out a propulsive role for actin, either in maintaining the vesicle cluster or in guiding vesicle recycling. Instead, we propose that actin acts as a scaffolding system for regulatory molecules in the nerve terminal.

Perturbation of synaptic vesicle delivery during neurotransmitter release triggered independently of calcium influx

Although much evidence suggests that calcium (Ca(2+)) usually triggers synaptic vesicle exocytosis and neurotransmitter release, the role of Ca(2+) in vesicle endocytosis and in the delivery of fusion-competent vesicles (i.e. mobilisation and/or priming) in nerve terminals remains unclear. To address this issue, we have studied synaptic vesicle dynamics in cultured rat neurones under conditions where neurotransmitter release is triggered independently of Ca(2+) using the secretagogue Ruthenium Red (RR). Using a prolonged stimulation protocol, we find that RR causes a rapid increase in neurotransmitter release followed by a gradually decrementing response. In contrast, when release is triggered by moderate membrane depolarisation caused by saline containing 18 mM K(+), release is sustained. These observations suggest that when release is triggered independently of a rise in Ca(2+), endocytosis or vesicle mobilisation/priming are perturbed. Using FM2-10, a fluorescent indicator of synaptic vesicle cycling, we find that neurotransmitter release triggered by RR is accompanied by both uptake and release of this dye, thereby suggesting that vesicle endocytosis is not blocked. To evaluate whether synaptic vesicle mobilisation/priming is perturbed in the absence of a rise in Ca(2+), we compared the kinetics of FM2-10 loss during prolonged stimulation. While 18 mM K(+) induced gradual and continuous dye loss, RR only induced substantial dye loss during the first minute of stimulation. In the presence of low concentrations of the Ca(2+) ionophore ionomycin, release caused by RR was prolonged. Taken together, these results provide evidence suggesting that, although a rise in intraterminal Ca(2+) is not required for endocytosis, it is essential for the continuous delivery of fusion-competent vesicles and to maintain neurotransmitter release during prolonged stimulation.

Abnormal spine morphology and enhanced LTP in LIMK-1 knockout mice

In vitro studies indicate a role for the LIM kinase family in the regulation of cofilin phosphorylation and actin dynamics. In addition, abnormal expression of LIMK-1 is associated with Williams syndrome, a mental disorder with profound deficits in visuospatial cognition. However, the in vivo function of this family of kinases remains elusive. Using LIMK-1 knockout mice, we demonstrate a significant role for LIMK-1 in vivo in regulating cofilin and the actin cytoskeleton. Furthermore, we show that the knockout mice exhibited significant abnormalities in spine morphology and in synaptic function, including enhanced hippocampal long-term potentiation. The knockout mice also showed altered fear responses and spatial learning. These results indicate that LIMK-1 plays a critical role in dendritic spine morphogenesis and brain function.

Neuronal calcium sensor-1 binds to regulated secretory organelles and functions in basal and stimulated exocytosis in PC12 cells

Neuronal calcium sensor-1 (NCS-1) and its non-mammalian homologue,frequenin, have been implicated in a spectrum of cellular processes, including regulation of stimulated exocytosis of synaptic vesicles and secretory granules (SGs) in neurons and neuroendocrine cells and regulation of phosphatidylinositol 4-kinase beta activity in yeast. However, apart from these intriguing putative functions, NCS-1 and frequenin are relatively poorly understood. Here, the distribution, dynamics and function of NCS-1 were studied using PC12 cells that stably express NCS-1-EYFP (NCS-1 fused to enhanced yellow fluorescent protein) or that stably overexpress NCS-1. Fluorescence and electron microscopies show that NCS-1-EYFP is absent from SGs but is present on small clear organelles, some of which are just below the plasma membrane. Total internal reflection fluorescence microscopy shows that NCS-1-EYFP is associated with synaptic-like microvesicles (SLMVs) in growth cones. Overexpression studies show that NCS-1 enhances exocytosis of synaptotagmin-labeled regulated secretory organelles (RSOs) under basal conditions and during stimulation by UTP. Significantly, these studies implicate NCS-1 in the enhancement of both basal and stimulated phosphoinositide-dependent exocytosis of RSOs in PC12 cells, and they show that NCS-1 is distributed strategically to interact with putative targets on the plasma membrane and on SLMVs. These studies also reveal that SLMVs undergo both fast directed motion and highly hindered diffusive motion in growth cones, suggesting that cytoskeletal constituents can both facilitate and hinder SLMV motion. These results also reveal interesting similarities and differences between transport organelles in differentiated neuroendocrine cells and neurons.

Short-term synaptic plasticity

Synaptic transmission is a dynamic process. Postsynaptic responses wax and wane as presynaptic activity evolves. This prominent characteristic of chemical synaptic transmission is a crucial determinant of the response properties of synapses and, in turn, of the stimulus properties selected by neural networks and of the patterns of activity generated by those networks. This review focuses on synaptic changes that result from prior activity in the synapse under study, and is restricted to short-term effects that last for at most a few minutes. Forms of synaptic enhancement, such as facilitation, augmentation, and post-tetanic potentiation, are usually attributed to effects of a residual elevation in presynaptic [Ca(2+)]i, acting on one or more molecular targets that appear to be distinct from the secretory trigger responsible for fast exocytosis and phasic release of transmitter to single action potentials. We discuss the evidence for this hypothesis, and the origins of the different kinetic phases of synaptic enhancement, as well as the interpretation of statistical changes in transmitter release and roles played by other factors such as alterations in presynaptic Ca(2+) influx or postsynaptic levels of [Ca(2+)]i. Synaptic depression dominates enhancement at many synapses. Depression is usually attributed to depletion of some pool of readily releasable vesicles, and various forms of the depletion model are discussed. Depression can also arise from feedback activation of presynaptic receptors and from postsynaptic processes such as receptor desensitization. In addition, glial-neuronal interactions can contribute to short-term synaptic plasticity. Finally, we summarize the recent literature on putative molecular players in synaptic plasticity and the effects of genetic manipulations and other modulatory influences.

Cholangiocytes exhibit dynamic, actin-dependent apical membrane turnover

The present studies of cholangiocytes used complementary histological, biochemical, and electrophysiological methods to identify a dense population of subapical vesicles, quantify the rates of vesicular trafficking, and assess the contribution of the actin cytoskeleton to membrane trafficking. FM 1-43 fluorescence measured significant basal rates of total exocytosis (1.33 +/- 0.16% plasma membrane/min) in isolated cholangiocytes and apical exocytosis in cholangiocyte monolayers. Cell surface area remained unchanged, indicating that there was a concurrent, equivalent rate of endocytosis. FM 1-43 washout studies showed that 36% of the endocytosed membrane was recycled to the plasma membrane. 8-(4-Chlorophenylthio)adenosine 3',5'-cyclic monophosphate (CPT-cAMP; cAMP analog) increased exocytosis by 71 +/- 31%, whereas the Rp diastereomer of adenosine 3',5'-cyclic monophosphothioate (Rp-cAMPS; protein kinase A inhibitor) diminished basal exocytosis by 53 +/- 11%. A dense population of 140-nm subapical vesicles arose, in part, from apical membrane endocytosis. Phalloidin staining showed that a subpopulation of the endocytosed vesicles was encapsulated by F-actin. Furthermore, membrane trafficking was inhibited by disrupting the actin cytoskeleton with cytochalasin D (51 +/- 13% of control) or jasplakinolide (58 +/- 9% of control). These studies indicate that there is a high rate of vesicular trafficking at the apical membrane of cholangiocytes and suggest that both cAMP and the actin cytoskeleton contribute importantly to these events.

Temporal synaptic tagging by I(h) activation and actin: involvement in long-term facilitation and cAMP-induced synaptic enhancement

Presynaptic I(h) channels become activated during a tetanus through membrane hyperpolarization resulting from Na(+) accumulation and electrogenic Na(+)/K(+) exchange. I(h) activation is obligatory for inducing long-term facilitation (LTF), a long-lasting synaptic strengthening. cAMP-induced synaptic enhancement also requires I(h) activation, and both processes are sensitive to actin depolymerization. Other mechanisms are responsible for expression of the responses. Once initiated, continued response to cAMP is I(h) and actin independent. Moreover, LTF-induced activation of I(h) renders subsequent cAMP enhancement insensitive to both I(h) blockers and actin depolymerization. This actin-stabilized "temporal synaptic tagging" set by I(h) activation is prolonged when I(h) is activated concurrent with an elevation in presynaptic calcium concentration ([Ca(2+)]i), permitting the further strengthening of synapses given appropriate additional stimuli.

Involvement of B-50 (GAP-43) phosphorylation in the modulation of transmitter release by protein kinase C

1. Protein kinase C (PKC) is a family of enzymes that is activated by diacylglycerol (DAG) following phospholipase (PL) C activation. Protein kinase C may also be activated by metabolites and arachidonic acid generated by breakdown of membrane phospholipids by PLD and PLA2, respectively. Subsequent to PKC activation, key protein substrates are phosphorylated, resulting in the facilitation of transmitter release. 2. Phorbol esters are compounds that mimic the actions of DAG on PKC and have been shown to facilitate stimulation-induced (S-I) transmitter release in rat brain. However, some phorbol esters that have a high affinity for PKC have no effect on transmitter release, whereas others with a lower affinity for PKC markedly elevate S-I transmitter release. 3. The structure and, more importantly, the lipophilicity of the phorbol esters determines their ability to access and activate the intraneuronal pools of PKC that are involved with transmitter release. In studies in which cell membranes were intact, phorbol esters did not display the characteristics expected based on their affinities for PKC in contrast with studies in disrupted synaptosomes. This supports the hypothesis that the membrane plays a critical role in determining the effects of phorbol esters on PKC. 4. B-50, a PKC substrate thought to be involved in transmitter release, also appears to be differentially phosphorylated by various phorbol esters. The effects on B-50 phosphorylation in intact synaptosomes, but not disrupted synaptosomes, are well correlated with the effects of phorbol esters on S-I transmitter release. 5. B-50 is colocalized with actin, which has also been suggested to play an important role in facilitating the movement of reserve pools of transmitter vesicles to the readily releasable state. Therefore, it is possible that the phosphorylation status of B-50 directly influences the organization of actin filaments, thereby allowing transmitter output to be sustained under high levels of stimulation.

Effects of staurosporine on exocytosis and endocytosis at frog motor nerve terminals

Observations of the dynamic staining and destaining of FM1-43 in frog motor nerve terminals (Henkel and Betz, 1995) suggested that staurosporine might shorten the interval between exocytosis and endocytosis, inducing a "kiss and run" mode of exocytosis and endocytosis. We tested this hypothesis by using FM1-43 imaging (to measure the time course of FM1-43 endocytosis), intracellular recording of evoked synaptic potentials (to measure acetylcholine release), and electron microscopy (to examine synaptic vesicle distribution). Staurosporine reduced FM1-43 uptake during but not after a tetanus, increased the speed of end plate potential (EPP) amplitude rundown, and greatly slowed the recovery from synaptic depression. Ultrastructural observations showed pronounced vesicle depletion near active zones after tetanic stimulation in staurosporine-treated preparations. These results suggest that staurosporine acted primarily to impair mobilization of synaptic vesicles during tetanic stimulation.

Effects of staurosporine on exocytosis and endocytosis at frog motor nerve terminals

Observations of the dynamic staining and destaining of FM1-43 in frog motor nerve terminals (Henkel and Betz, 1995) suggested that staurosporine might shorten the interval between exocytosis and endocytosis, inducing a "kiss and run" mode of exocytosis and endocytosis. We tested this hypothesis by using FM1-43 imaging (to measure the time course of FM1-43 endocytosis), intracellular recording of evoked synaptic potentials (to measure acetylcholine release), and electron microscopy (to examine synaptic vesicle distribution). Staurosporine reduced FM1-43 uptake during but not after a tetanus, increased the speed of end plate potential (EPP) amplitude rundown, and greatly slowed the recovery from synaptic depression. Ultrastructural observations showed pronounced vesicle depletion near active zones after tetanic stimulation in staurosporine-treated preparations. These results suggest that staurosporine acted primarily to impair mobilization of synaptic vesicles during tetanic stimulation.

Genetic studies in Drosophila: vesicle pools and cytoskeleton-based regulation of synaptic transmission

Presynaptic plasticity mechanisms rely on modulation of the synaptic vesicle fusion machinery and the regulated mobilization of synaptic vesicles at the active zone. This review discusses recent evidence suggesting that the relative proportions of synaptic vesicles in the reserve and ready releasable pools is the primary determinant of synaptic transmission strength, and that transport of vesicles between these pools is mediated by cytoskeletal mechanisms. Recent efforts to identify the molecules required for regulation of the presynaptic cytoskeleton suggest that common mechanisms may exist to regulate synaptic vesicle pools in widely divergent neuronal types, ranging from synaptic modulation at the Drosophila neuromuscular junction to the synaptic plasticity required for learning and memory in the mammalian brain.

Actin-dependent regulation of neurotransmitter release at central synapses

Depolymerization of actin by latrunculin A transiently promotes neurotransmitter release. The mean rate of mEPSCs increases by a Ca2+-independent process, without a concomitant change in the mean amplitude. The readily releasable vesicle pool size and the rate of refilling of the readily releasable pool remain unaltered by latrunculin treatment. Evoked neurotransmitter release also increases in a manner consistent with an increase in vesicle release probability. The observed enhancement of neurotransmitter release is specific to actin depolymerization mediated by latrunculin A and is not caused by cytochalasin D. Our findings indicate that actin participates in a regulatory mechanism that restrains fusion of synaptic vesicles at the active zone.