Actin disassembles reversibly during electrically induced recycling of synaptic vesicles in cultured neurons

Vertebrate Presynaptic Active Zone Assembly: a Role Accomplished by Diverse Molecular and Cellular Mechanisms

Among all the biological systems in vertebrates, the central nervous system (CNS) is the most complex, and its function depends on specialized contacts among neurons called synapses. The assembly and organization of synapses must be exquisitely regulated for a normal brain function and network activity. There has been a tremendous effort in recent decades to understand the molecular and cellular mechanisms participating in the formation of new synapses and their organization, maintenance, and regulation. At the vertebrate presynapses, proteins such as Piccolo, Bassoon, RIM, RIM-BPs, CAST/ELKS, liprin-α, and Munc13 are constant residents and participate in multiple and dynamic interactions with other regulatory proteins, which define network activity and normal brain function. Here, we review the function of these active zone (AZ) proteins and diverse factors involved in AZ assembly and maintenance, with an emphasis on axonal trafficking of precursor vesicles, protein homo- and hetero-oligomeric interactions as a mechanism of AZ trapping and stabilization, and the role of F-actin in presynaptic assembly and its modulation by Wnt signaling.

A dynamic formin-dependent deep F-actin network in axons

Although actin at neuronal growth cones is well-studied, much less is known about actin organization and dynamics along axon shafts and presynaptic boutons. Using probes that selectively label filamentous-actin (F-actin), we found focal "actin hotspots" along axons-spaced ∼3-4 µm apart-where actin undergoes continuous assembly/disassembly. These foci are a nidus for vigorous actin polymerization, generating long filaments spurting bidirectionally along axons-a phenomenon we call "actin trails." Super-resolution microscopy reveals intra-axonal deep actin filaments in addition to the subplasmalemmal "actin rings" described recently. F-actin hotspots colocalize with stationary axonal endosomes, and blocking vesicle transport diminishes the actin trails, suggesting mechanistic links between vesicles and F-actin kinetics. Actin trails are formin-but not Arp2/3-dependent and help enrich actin at presynaptic boutons. Finally, formin inhibition dramatically disrupts synaptic recycling. Collectively, available data suggest a two-tier F-actin organization in axons, with stable "actin rings" providing mechanical support to the plasma membrane and dynamic "actin trails" generating a flexible cytoskeletal network with putative physiological roles.

Calcium, calpain, and calcineurin in low-frequency depression of transmitter release

Low-frequency depression (LFD) of transmitter release occurs at phasic synapses with stimulation at 0.2 Hz in both isolated crayfish (Procambarus clarkii) neuromuscular junction (NMJ) preparations and in intact animals. LFD is regulated by presynaptic activity of the Ca(2+)-dependent phosphatase calcineurin (Silverman-Gavrila and Charlton, 2009). Since the fast Ca(2+) chelator BAPTA-AM inhibits LFD but the slow chelator EGTA-AM does not, the Ca(2+) sensor for LFD may be close to a Ca(2+) source at active zones. Calcineurin can be activated by the Ca(2+)-activated protease calpain, and immunostaining showed that both proteins are present at nerve terminals. Three calpain inhibitors, calpain inhibitor I, MDL-28170, and PD150606, but not the control compound PD145305, inhibit LFD both in the intact animal as shown by electromyograms and by intracellular recordings at neuromuscular junctions. Analysis of mini-EPSPs indicated that these inhibitors had minimal postsynaptic effects. Proteolytic activity in CNS extract, detected by a fluorescent calpain substrate, was modulated by Ca(2+) and calpain inhibitors. Western blot analysis of CNS extract showed that proteolysis of calcineurin to a fragment consistent with the constitutively active form required Ca(2+) and was blocked by calpain inhibitors. Inhibition of LFD by calpain inhibition blocks the reduction in phosphoactin and the depolymerization of tubulin that normally occurs in LFD, probably by blocking the dephosphorylation of cytoskeletal proteins by calcineurin. In contrast, high-frequency depression does not involve protein phosphorylation- or calpain-dependent mechanisms. LFD may involve a specific pathway in which local Ca(2+) signaling activates presynaptic calpain and calcineurin at active zones and causes changes of tubulin cytoskeleton.

Actin and Diseases of the Nervous System

Abnormal regulation of the actin cytoskeleton results in several pathological conditions affecting primarily the nervous system. Those of genetic origin arise during development, but others manifest later in life. Actin regulation is also affected profoundly by environmental factors that can have sustained consequences for the nervous system. Those consequences follow from the fact that the actin cytoskeleton is essential for a multitude of cell biological functions ranging from neuronal migration in cortical development and dendritic spine formation to NMDA receptor activity in learning and alcoholism. Improper regulation of actin, causing aggregation, can contribute to the neurodegeneration of amyloidopathies, such as Down's syndrome and Alzheimer's disease. Much progress has been made in understanding the molecular basis of these diseases.

Effects of exposure to a time-varying 1.5 T magnetic field on the neurotransmitter-activated increase in intracellular Ca(2+) in relation to actin fiber and mitochondrial functions in bovine adrenal chromaffin cells

BACKGROUND

It has been reported that exposure to electromagnetic fields influences intracellular signal transduction. We studied the effects of exposure to a time-varying 1.5 T magnetic field on membrane properties, membrane cation transport and intracellular Ca(2+) mobilization in relation to signals. We also studied the mechanism of the effect of exposure to the magnetic field on intracellular Ca(2+) release from Ca(2+) stores in adrenal chromaffin cells.

METHODS

We measured the physiological functions of ER, actin protein, and mitochondria with respect to a neurotransmitter-induced increase in Ca(2+) in chromaffin cells exposed to the time-varying 1.5 T magnetic field for 2h.

RESULTS

Exposure to the magnetic field significantly reduced the increase in [Ca(2+)]i. The exposure depolarized the mitochondria membrane and lowered oxygen uptake, but did not reduce the intracellular ATP content. Magnetic field-exposure caused a morphological change in intracellular F-actin. F-actin in exposed cells seemed to be less dense than in control cells, but the decrease was smaller than that in cytochalasin D-treated cells. The increase in G-actin (i.e., the decrease in F-actin) due to exposure was recovered by jasplakinolide, but inhibition of Ca(2+) release by the exposure was unaffected.

CONCLUSIONS AND GENERAL SIGNIFICANCE

These results suggest that the magnetic field-exposure influenced both the ER and mitochondria, but the inhibition of Ca(2+) release from ER was not due to mitochondria inhibition. The effect of eddy currents induced in the culture medium may indirectly influence intracellular actin and suppress the transient increase in [Ca(2+)]i.

BDNF upregulation rescues synaptic plasticity in middle-aged ovariectomized rats

Brain-derived neurotrophic factor (BDNF) has emerged as a possible broad-spectrum treatment for the plasticity losses found in rodent models of human conditions associated with memory and cognitive deficits. We have tested this strategy in the particular case of ovariectomy. The actin polymerization in spines normally found after patterned afferent stimulation was greatly reduced, along with the stabilization of long-term potentiation, in hippocampal slices prepared from middle-aged ovariectomized rats. Both effects were fully restored by a 60-minute infusion of 2 nM BDNF. Comparable rescue results were obtained after elevating endogenous BDNF protein levels in hippocampus with 4 daily injections of a short half-life ampakine (positive modulator of α-amino-3-hydroxy-5-methyl-4-isoxazolepropionate [AMPA]-type glutamate receptors). These results provide the first evidence that minimally invasive, mechanism-based drug treatments can ameliorate defects in spine plasticity caused by depressed estrogen levels.

Caenorhabditis elegans wsp-1 regulation of synaptic function at the neuromuscular junction

The Rho GTPase members and their effector proteins, such as the Wiskott-Aldrich syndrome protein (WASP), play critical roles in regulating actin dynamics that affect cell motility, endocytosis, cell division, and transport. It is well established that Caenorhabditis elegans wsp-1 plays an essential role in embryonic development. We were interested in the role of the C. elegans protein WSP-1 in the adult nematode. In this report, we show that a deletion mutant of wsp-1 exhibits a strong sensitivity to the neuromuscular inhibitor aldicarb. Transgenic rescue experiments demonstrated that neuronal expression of WSP-1 rescued this phenotype and that it required a functional WSP-1 Cdc42/Rac interactive binding domain. WSP-1-GFP fusion protein was found localized presynaptically, immediately adjacent to the synaptic protein RAB-3. Strong genetic interactions with wsp-1 and other genes involved in different stages of synaptic transmission were observed as the wsp-1(gm324) mutation suppresses the aldicarb resistance seen in unc-13(e51), unc-11(e47), and snt-1 (md290) mutants. These results provide genetic and pharmacological evidence that WSP-1 plays an essential role to stabilize the actin cytoskeleton at the neuronal active zone of the neuromuscular junction to restrain synaptic vesicle release.

Characterization of Eag1 channel lateral mobility in rat hippocampal cultures by single-particle-tracking with quantum dots

Voltage-gated ion channels are main players involved in fast synaptic events. However, only slow intracellular mechanisms have so far been described for controlling their localization as real-time visualization of endogenous voltage-gated channels at high temporal and spatial resolution has not been achieved yet. Using a specific extracellular antibody and quantum dots we reveal and characterize lateral mobility as a faster mechanism to dynamically control the number of endogenous ether-a-go-go (Eag)1 ion channels inside synapses. We visualize Eag1 entering and leaving synapses by lateral diffusion in the plasma membrane of rat hippocampal neurons. Mathematical analysis of their trajectories revealed how the motion of Eag1 gets restricted when the channels diffuse into the synapse, suggesting molecular interactions between Eag1 and synaptic components. In contrast, Eag1 channels switch to Brownian movement when they exit synapses and diffuse into extrasynaptic membranes. Furthermore, we demonstrate that the mobility of Eag1 channels is specifically regulated inside synapses by actin filaments, microtubules and electrical activity. In summary, using single-particle-tracking techniques with quantum dots nanocrystals, our study shows for the first time the lateral diffusion of an endogenous voltage-gated ion channel in neurons. The location-dependent constraints imposed by cytoskeletal elements together with the regulatory role of electrical activity strongly suggest a pivotal role for the mobility of voltage-gated ion channels in synaptic activity.

Calcineurin and cytoskeleton in low-frequency depression

Transmitter release at high probability phasic synapses of crayfish neuromuscular junctions depresses by over 50% in 60 min when stimulated at 0.2 Hz. Inhibition of the protein phosphatase calcineurin by intracellular pre-synaptic injection of autoinhibitory peptide inhibited low-frequency depression (LFD) and resulted in facilitation of transmitter release. Since this inhibitor had no major effects when injected into the post-synaptic cell, only pre-synaptic calcineurin activity is necessary for LFD. To examine changes in phosphoproteins during LFD we performed a phosphoproteomic screen on proteins extracted from motor axons and nerve terminals after LFD induction or treatment with various drugs that affect kinase and phosphatase activity. Proteins separated by PAGE were stained with phospho-specific/total protein ratio stains (Pro-Q Diamond/SYPRO Ruby) to identify protein bands for analysis by mass spectrometry. Phosphorylation of actin and tubulin decreased during LFD, but increased when calcineurin was blocked. Tubulin and phosphoactin immunoreactivity in pre-synaptic terminals were also reduced after LFD. The actin depolymerizing drugs cytochalasin and latrunculin and the microtubule stabilizer taxol inhibited LFD. Therefore, dephosphorylation of pre-synaptic actin and tubulin and consequent changes in the cytoskeleton may regulate LFD. LFD is unlike long-term depression found in mammalian synapses because the latter requires in most instances post-synaptic calcineurin activity.Thus, this simpler invertebrate synapse discloses a novel pre-synaptic depression mechanism.

Chronic ethanol exposure inhibits dopamine release via effects on the presynaptic actin cytoskeleton in PC12 cells

An increase in nucleus accumbens dopamine release appears to mediate the "rewarding" properties of drugs of abuse. Using PC12 cells, it has been shown that chronic ethanol exposure can significantly reduce nicotine-stimulated dopamine release. Here, a novel mechanism of ethanol in regulating presynaptic dopamine release is demonstrated. In neuronal cells, a layer of filamentous actin beneath the cell surface regulates the movement and release of synaptic vesicles. Upon stimulation, there is a protein kinase C (PKC)-dependent breakdown of this actin cytoskeleton, allowing vesicles to move near the nerve terminal membrane for release. Chronic ethanol alters PKC signaling, thus the hypothesis that chronic ethanol inhibits presynaptic actin cytoskeleton breakdown in response to stimulation was tested. PC12 cells were chronically exposed to ethanol and then acutely exposed to multiple forms of stimulation (nicotine, sucrose, potassium, and ionophore). In ethanol-treated cells, dopamine release was inhibited following stimulation by forms of release shown to be PKC-dependent (nicotine, sucrose, and potassium). In contrast, dopamine release was not altered following stimulation by PKC-independent forms of release (ionophore). Actin cytoskeleton breakdown was also inhibited following stimulation with PKC-dependent forms of stimulated release but not by PKC-independent (ionophore) forms. Further, cytochalasin B, an agent which depolymerizes actin, reversed the effects of chronic ethanol on both actin depolymerization and dopamine release. These data suggest that chronic ethanol inhibits presynaptic actin breakdown, likely resulting in decreased neurotransmitter release.

Stimulation of actin polymerization by vacuoles via Cdc42p-dependent signaling

We have previously shown that actin ligands inhibit the fusion of yeast vacuoles in vitro, which suggests that actin remodeling is a subreaction of membrane fusion. Here, we demonstrate the presence of vacuole-associated actin polymerization activity, and its dependence on Cdc42p and Vrp1p. Using a sensitive in vitro pyrene-actin polymerization assay, we found that vacuole membranes stimulated polymerization, and this activity increased when vacuoles were preincubated under conditions that support membrane fusion. Vacuoles purified from a VRP1-gene deletion strain showed reduced polymerization activity, which could be recovered when reconstituted with excess Vrp1p. Cdc42p regulates this activity because overexpression of dominant-negative Cdc42p significantly reduced vacuole-associated polymerization activity, while dominant-active Cdc42p increased activity. We also used size-exclusion chromatography to directly examine changes in yeast actin induced by vacuole fusion. This assay confirmed that actin undergoes polymerization in a process requiring ATP. To further confirm the need for actin polymerization during vacuole fusion, an actin polymerization-deficient mutant strain was examined. This strain showed in vivo defects in vacuole fusion, and actin purified from this strain inhibited in vitro vacuole fusion. Affinity isolation of vacuole-associated actin and in vitro binding assays revealed a polymerization-dependent interaction between actin and the SNARE Ykt6p. Our results suggest that actin polymerization is a subreaction of vacuole membrane fusion governed by Cdc42p signal transduction.

A delayed response enhancement during hippocampal presynaptic plasticity in mice

High frequency afferent stimulation of chemical synapses often induces short-term increases in synaptic efficacy, due to increased release probability and/or increased supply of readily releasable synaptic vesicles. This may be followed by synaptic depression, often caused by vesicle depletion. We here describe an additional, novel type of delayed and transient response enhancement phase which occurred during prolonged stimulation at 5-20 Hz frequency of excitatory glutamatergic synapses in slices from the adult mouse CA1 hippocampal region. This second enhancement phase, which was most clearly defined at physiological temperatures and essentially absent at 24 degrees C, was dependent on the presence of F-actin filaments and synapsins I and/or II, and could not be ascribed to changes in presynaptic action potentials, inhibitory neurotransmission or glutamate receptor desensitization. Time course studies showed that the delayed response phase interrupted the synaptic decay 3-4 s after stimulus train initiation and continued, when examined at 5-10 Hz frequencies, for approximately 75 stimuli before decay. The novel response enhancement, probably deriving from a restricted pool of synaptic vesicles, may allow maintenance of synaptic efficacy during prolonged periods of excitatory synaptic activity.

Brain-derived neurotrophic factor promotes long-term potentiation-related cytoskeletal changes in adult hippocampus

Brain-derived neurotrophic factor (BDNF) is an extremely potent, positive modulator of theta burst induced long-term potentiation (LTP) in the adult hippocampus. The present studies tested whether the neurotrophin exerts its effects by facilitating cytoskeletal changes in dendritic spines. BDNF caused no changes in phalloidin labeling of filamentous actin (F-actin) when applied alone to rat hippocampal slices but markedly enhanced the number of densely labeled spines produced by a threshold level of theta burst stimulation. Conversely, the BDNF scavenger TrkB-Fc completely blocked increases in spine F-actin produced by suprathreshold levels of theta stimulation. TrkB-Fc also blocked LTP consolidation when applied 1-2 min, but not 10 min, after theta trains. Additional experiments confirmed that p21 activated kinase and cofilin, two actin-regulatory proteins implicated in spine morphogenesis, are concentrated in spines in mature hippocampus and further showed that both undergo rapid, dose-dependent phosphorylation after infusion of BDNF. These results demonstrate that the influence of BDNF on the actin cytoskeleton is retained into adulthood in which it serves to positively modulate the time-dependent LTP consolidation process.

Mechanism of depolymerization and severing of actin filaments and its significance in cytoskeletal dynamics

The actin cytoskeleton is one of the major structural components of the cell. It often undergoes rapid reorganization and plays crucial roles in a number of dynamic cellular processes, including cell migration, cytokinesis, membrane trafficking, and morphogenesis. Actin monomers are polymerized into filaments under physiological conditions, but spontaneous depolymerization is too slow to maintain the fast actin filament dynamics observed in vivo. Gelsolin, actin-depolymerizing factor (ADF)/cofilin, and several other actin-severing/depolymerizing proteins can enhance disassembly of actin filaments and promote reorganization of the actin cytoskeleton. This review presents advances as well as a historical overview of studies on the biochemical activities and cellular functions of actin-severing/depolymerizing proteins.

Synaptic vesicle mobility and presynaptic F-actin are disrupted in a N-ethylmaleimide-sensitive factor allele of Drosophila

N-ethylmaleimide sensitive factor (NSF) can dissociate the soluble NSF attachment receptor (SNARE) complex, but NSF also participates in other intracellular trafficking functions by virtue of SNARE-independent activity. Drosophila that express a neural transgene encoding a dominant-negative form of NSF2 show an 80% reduction in the size of releasable synaptic vesicle pool, but no change in the number of vesicles in nerve terminal boutons. Here we tested the hypothesis that vesicles in the NSF2 mutant terminal are less mobile. Using a combination of genetics, pharmacology, and imaging we find a substantial reduction in vesicle mobility within the nerve terminal boutons of Drosophila NSF2 mutant larvae. Subsequent analysis revealed a decrease of filamentous actin in both NSF2 dominant-negative and loss-of-function mutants. Lastly, actin-filament disrupting drugs also decrease vesicle movement. We conclude that a factor contributing to the NSF mutant phenotype is a reduction in vesicle mobility, which is associated with decreased presynaptic F-actin. Our data are consistent with a model in which actin filaments promote vesicle mobility and suggest that NSF participates in establishing or maintaining this population of actin.

Formation of actin-ADF/cofilin rods transiently retards decline of mitochondrial potential and ATP in stressed neurons

When neurons in culture are transiently stressed by inhibition of ATP synthesis, they rapidly form within their neurites rodlike actin inclusions that disappear when the insult is removed. Oxidative stress, excitotoxic insults, and amyloid beta-peptide oligomers also induce rods. Immunostaining of neurites indicates that these rods also contain the majority of the actin filament dynamizing proteins, actin-depolymerizing factor (ADF) and cofilin (AC). If the rods reappear within 24 h after the stress is removed, the neurite degenerates distal to the rod but with no increase in neuronal death. Here, rods were generated in cultured rat E18 hippocampal cells by overexpression of a green fluorescent protein chimera of AC. Surprisingly, we have found that, for a short period (approximately 60 min) immediately after initial rod formation, the loss of mitochondrial membrane potential (Delta Psi(m)) and ATP in neurites with rods is slower than in neurites without them. The Delta Psi(m) was monitored with the fluorescent dye tetramethylrhodamine methyl ester, and ATP was monitored with the fluorescent ion indicator mag-fura 2. Actin in rods is less dynamic than is filamentous actin in other cytoskeletal structures. Because Delta Psi(m) depends on cellular ATP and because ATP hydrolysis associated with actin filament turnover is responsible for a large fraction of neuronal energy consumption (approximately 50%), the formation of rods transiently protects neurites by slowing filament turnover and its associated ATP hydrolysis.

Structural domains involved in the regulation of transmitter release by synapsins

Synapsins are a family of neuron-specific phosphoproteins that regulate neurotransmitter release by associating with synaptic vesicles. Synapsins consist of a series of conserved and variable structural domains of unknown function. We performed a systematic structure-function analysis of the various domains of synapsin by assessing the actions of synapsin fragments on neurotransmitter release, presynaptic ultrastructure, and the biochemical interactions of synapsin. Injecting a peptide derived from domain A into the squid giant presynaptic terminal inhibited neurotransmitter release in a phosphorylation-dependent manner. This peptide had no effect on vesicle pool size, synaptic depression, or transmitter release kinetics. In contrast, a peptide fragment from domain C reduced the number of synaptic vesicles in the periphery of the active zone and increased the rate and extent of synaptic depression. This peptide also slowed the kinetics of neurotransmitter release without affecting the number of docked vesicles. The domain C peptide, as well as another peptide from domain E that is known to have identical effects on vesicle pool size and release kinetics, both specifically interfered with the binding of synapsins to actin but not with the binding of synapsins to synaptic vesicles. This suggests that both peptides interfere with release by preventing interactions of synapsins with actin. Thus, interactions of domains C and E with the actin cytoskeleton may allow synapsins to perform two roles in regulating release, whereas domain A has an actin-independent function that regulates transmitter release in a phosphorylation-sensitive manner.

Actin polymerization regulates clathrin coat maturation during early stages of synaptic vesicle recycling at lamprey synapses

Although it is established that presynaptic actin participates in synaptic vesicle recycling at several synapses, the earliest stages at which actin polymerization is employed during this process are still unclear. To address this, we prevented actin polymerization at lamprey synapses by applying latrunculin B or swinholide A. Latrunculin and swinholide depolymerize actin by sequestering actin monomers and, in addition, swinholide can sever existing actin filaments. When injected into individual presynaptic axons of the intact spinal cord, fluorescently labeled monomeric actin rapidly incorporated in a calcium-dependent manner into a stable, filamentous actin network concentrated at endocytic zones. This pool of actin was disrupted completely by latrunculin. At stimulated synapses, specific disruption of actin polymerization with latrunculin and swinholide induced a selective increase in unconstricted clathrin-coated pits and, in the case of swinholide, an additional increase in the size of plasma membrane evaginations. These results indicate that actin polymerization participates initially in the maturation of clathrin-coated pits during early stages of synaptic vesicle recycling.

Molecular characterization of mitocalcin, a novel mitochondrial Ca2+-binding protein with EF-hand and coiled-coil domains

Here we have identified and characterized a novel mitochondrial Ca2+-binding protein, mitocalcin. Western blot analysis demonstrated that mitocalcin was widely expressed in mouse tissues. The expression in brain was increased during post-natal to adult development. Further analyses were carried out in newly established neural cell lines. The protein was expressed specifically in neurons but not in glial cells. Double-labeling studies revealed that mitocalcin was colocalized with mitochondria in neurons differentiated from 2Y-3t cells. In addition, mitocalcin was enriched in the mitochondrial fraction purified from the cells. Immunohistochemical studies on mouse cerebellum revealed that the expression pattern of mitocalcin in glomeruli of the internal granular and molecular layers was well overlapped by the distribution pattern of mitochondria. Immunogold electron microscopy showed that mitocalcin was associated with mitochondrial inner membrane. Overexpression of mitocalcin in 2Y-3t cells resulted in neurite extension. Inhibition of the expression in 2Y-3t cells caused suppression of neurite outgrowth and then cell death. These findings suggest that mitocalcin may play roles in neuronal differentiation and function through the control of mitochondrial function.

Changes in cytoskeletal gene expression linked to MPTP-treatment in Mice

Parkinson's disease is a neurodegenerative disorder characterized by the progressive loss of dopaminergic neurons in the substantia nigra and a marked reduction of dopamine (DA) levels in the striatum. Binding to its specific receptors, DA switches on a complex program of intracellular signaling that regulates gene expression. We evaluated the changes in striatal gene expression in a mouse model of Parkinson's disease, using differential display analysis. The mRNA for the cytoskeleton family proteins, radixin, cofilin and centractin/ARP-1, was abnormally expressed in the striatum of these MPTP-treated mice. Moreover, we also found that radixin mRNA and its protein levels are under DA control through specific D1-dopaminergic receptors in a dose- and time-dependent manner in the GT1-7 neural cell line. These findings suggest a role for DA for regulation of cytoskeletal proteins involved in the integrity and function of synapsis.

Modulation of the actin cytoskeleton via gelsolin regulates vacuolar H+-ATPase recycling

The role of the actin cytoskeleton in regulating membrane protein trafficking is complex and depends on the cell type and protein being examined. Using the epididymis as a model system in which luminal acidification is crucial for sperm maturation and storage, we now report that modulation of the actin cytoskeleton by the calcium-activated actin-capping and -severing protein gelsolin plays a key role in regulating vacuolar H(+)-ATPase (V-ATPase) recycling. Epididymal clear cells contain abundant V-ATPase in their apical pole, and an increase in their cell-surface V-ATPase expression correlates with an increase in luminal proton secretion. We have shown that apical membrane accumulation of V-ATPase is triggered by an elevation in cAMP following activation of bicarbonate-regulated soluble adenylyl cyclase in response to alkaline luminal pH (Pastor-Soler, N., Beaulieu, V., Litvin, T. N., Da Silva, N., Chen, Y., Brown, D., Buck, J., Levin, L. R., and Breton, S. (2003) J. Biol. Chem. 278, 49523-49529). Here, we show that clear cells express high levels of gelsolin, indicating a potential role in the functional activity of these cells. When jasplakinolide was used to overcome the severing action of gelsolin by polymerizing actin, complete inhibition of the alkaline pH- and cAMP-induced apical membrane accumulation of V-ATPase was observed. Conversely, when gelsolin-mediated actin filament elongation was inhibited using a 10-residue peptide (PBP10) derived from the phosphatidylinositol 4,5-bisphosphate-binding region (phosphoinositide-binding domain 2) of gelsolin, significant V-ATPase apical membrane mobilization was induced, even at acidic luminal pH. In contrast, the calcium chelator 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid tetrakis(acetoxymethyl ester) and the phospholipase C inhibitor U-73122 inhibited the alkaline pH-induced V-ATPase apical accumulation. Thus, maintenance of the actin cytoskeleton in a depolymerized state by gelsolin facilitates calcium-dependent apical accumulation of V-ATPase in response to luminal pH alkalinization. Gelsolin is present in other cell types that express the V-ATPase in their plasma membrane and recycling vesicles, including kidney intercalated cells and osteoclasts. Therefore, modulation of the actin cortex by this severing and capping protein may represent a common mechanism by which these cells regulate their rate of proton secretion.

Coupling actin and membrane dynamics during calcium-regulated exocytosis: a role for Rho and ARF GTPases

Release of neurotransmitters and hormones occurs by calcium-regulated exocytosis, a process that shares many similarities in neurons and neuroendocrine cells. Exocytosis is confined to specific regions in the plasma membrane, where actin remodelling, lipid modifications and protein-protein interactions take place to mediate vesicle/granule docking, priming and fusion. The spatial and temporal coordination of the various players to form a "fast and furious" machinery for secretion remain poorly understood. ARF and Rho GTPases play a central role in coupling actin dynamics to membrane trafficking events in eukaryotic cells. Here, we review the role of Rho and ARF GTPases in supplying actin and lipid structures required for synaptic vesicle and secretory granule exocytosis. Their possible functional interplay may provide the molecular cues for efficient and localized exocytotic fusion.

Enhanced membrane fusion in sterol-enriched vacuoles bypasses the Vrp1p requirement

Organization of lipids into membrane microdomains is a vital mechanism of protein processing. Here we show that overexpression of ERG6, a gene involved in ergosterol synthesis, elevates sterol levels 1.5-fold on the vacuole membrane and enhances their homotypic fusion. The mechanism of sterol-enhanced fusion is not via more efficient sorting, but instead promotes increased kinetics of fusion subreactions. We initially isolated ERG6 as a suppressor of a vrp1Delta growth defect selective for vacuole function. VRP1 encodes verprolin, an actin-binding protein that colocalizes to vacuoles. The vrp1Delta mutant has fragmented vacuoles in vivo and isolated vacuoles do not fuse in vitro, indicative of a Vrp1p requirement for membrane fusion. ERG6 overexpression rescues vrp1Delta vacuole fusion in a cytosol-dependent manner. Cytosol prepared from the vrp1Delta strain remains active; therefore, cytosol is not resupplying Vrp1p. Las17p (Vrp1p functional partner) antibodies, which inhibit wild-type vacuole fusion, do not inhibit the fusion of vacuoles from the vrp1Delta-ERG6 overexpression strain. Vacuole-associated actin turnover is decreased in the vrp1Delta strain, but recovered by ERG6 overexpression linking sterol enrichment to actin remodeling. Therefore, the Vrp1p/Las17p requirement for membrane fusion is bypassed by increased sterols, which promotes actin remodeling as part the membrane fusion mechanism.

Alsin Is a Rab5 and Rac1 Guanine Nucleotide Exchange Factor

ALS2 is the gene mutated in a recessive juvenile form of amyotrophic lateral sclerosis (ALS2). ALS2 encodes a large protein termed alsin, which contains a number of predicted cell signaling and protein trafficking sequence motifs. To gain insight into the overall function of alsin and to begin to evaluate its role in motor neuron maintenance, we examined the subcellular localization of alsin and the biochemical activities associated with its individual subdomains. We found that the Vps9p domain of alsin has Rab5 guanine nucleotide exchange activity. In addition, alsin interacted specifically with and acted as a guanine nucleotide exchange factor for Rac1. Immunofluorescence and fractionation experiments in both fibroblasts and neurons revealed that alsin is a cytosolic protein, with a significant portion associated with small, punctate membrane structures. Many of these membrane structures also contained Rab5 or Rac1. Upon overexpression of full-length alsin, the overexpressed material was largely cytosolic, indicating that the association with membrane structures could be saturated. We also found that alsin was present in membrane ruffles and lamellipodia. These data suggest that alsin is involved in membrane transport events, potentially linking endocytic processes and actin cytoskeleton remodeling.

Regulation of the neuronal actin cytoskeleton by ADF/cofilin

Actin and microtubules are major cytoskeletal elements of most cells including neurons. In order for a cell to move and change shape, its cytoskeleton must undergo rearrangements that involve breaking down and reforming filaments. Many recent reviews have focused on the signaling pathways emanating from receptors that ultimately affect axon growth and growth cone steering. This particular review will address changes in the actin cytoskeleton modulated by the family of actin dynamizing proteins known as actin depolymerizing factor (ADF)/cofilin or AC proteins. Though much is known about inactivation of AC proteins through phosphorylation at ser3 by LIM or TES kinases, new mechanisms of regulation of AC have recently emerged. A novel phosphatase, slingshot (SSH), and the 14-3-3 family of regulatory proteins have also been found to affect AC activity. The potential role of AC proteins in modulating the actin organizational changes that accompany neurite initiation, axonogenesis, growth cone guidance, and dendritic spine formation will be discussed.

Comparing the properties of neuronal culture systems: a shopping guide for the cell biologist

Cell biologists of many stripes may find that their question of interest can be studied to advantage in neurons. However, they will also find that "neurons" include many and diverse cell types among which perhaps just one or a few may be ideal for a particular experiment. This chapter discusses the properties, relative complexity, and cost of primary neurons and neuronal cell types from different species and parts of the nervous system and compares their utility for different kinds of cell biological experiments.

Ca2+ dependency of N-cadherin function probed by laser tweezer and atomic force microscopy

This study was undertaken to provide a biophysical basis for the hypothesis that activity-dependent modulation of cadherin-mediated adhesion by transient changes of extracellular calcium ([Ca2+]e) is causally involved in coordination of synaptic plasticity. Characterization of homophilic N-cadherin binding by atomic force microscopy and laser tweezer trapping of N-cadherin-coated microbeads attached to the cell surface of cultured neuronal cells showed that adhesive activity of N-cadherin is effectively regulated between 0.3 and 0.8 mm [Ca2+]e. Furthermore, we show that an increase of [Ca2+]i, which is known to be essential for induction of synaptic plasticity, causes significant reduction of cadherin-mediated bead adhesion that could be completely suppressed by inhibition of actin depolymerization. The results of this study show that N-cadherin has ideal biophysical properties to serve as a Ca2+-dependent sensor for synaptic activity and, at the same time, is strategically located to control synaptic adhesion. A drop of [Ca2+]e and a concomitant increase of [Ca2+]i may act in concert to modulate N-cadherin-based adhesive contacts at synaptic sites.

Fusion between phagosomes, early and late endosomes: a role for actin in fusion between late, but not early endocytic organelles

Actin is implicated in membrane fusion, but the precise mechanisms remain unclear. We showed earlier that membrane organelles catalyze the de novo assembly of F-actin that then facilitates the fusion between latex bead phagosomes and a mixture of early and late endocytic organelles. Here, we correlated the polymerization and organization of F-actin with phagosome and endocytic organelle fusion processes in vitro by using biochemistry and light and electron microscopy. When membrane organelles and cytosol were incubated at 37 degrees C with ATP, cytosolic actin polymerized rapidly and became organized into bundles and networks adjacent to membrane organelles. By 30-min incubation, a gel-like state was formed with little further polymerization of actin thereafter. Also during this time, the bulk of in vitro fusion events occurred between phagosomes/endocytic organelles. The fusion between latex bead phagosomes and late endocytic organelles, or between late endocytic organelles themselves was facilitated by actin, but we failed to detect any effect of perturbing F-actin polymerization on early endosome fusion. Consistent with this, late endosomes, like phagosomes, could nucleate F-actin, whereas early endosomes could not. We propose that actin assembled by phagosomes or late endocytic organelles can provide tracks for fusion-partner organelles to move vectorially toward them, via membrane-bound myosins, to facilitate fusion.

F-actin does not modulate the initial steps of the protein kinase C activation process in living nerve cells

Actin is a major substrate for protein kinase C (PKC) and PKC is considered a modulator of the actin network. In addition in vitro studies (Biochemistry 39 (2000) 271) have suggested that all PKC isoforms bind to actin during the process of activation of the enzyme. To test the physiological significance of such a coupling we used living PC12 cells and primary cultures of cerebellar granule cells. When PC12 cells were treated with either latrunculin B, which impairs actin polymerization, or phalloidin, which stabilizes actin filaments, we observed a significant reduction of the [Ca2+]i response revealed by Fura-2 fluorescence, while the PKC conformational changes followed by Fim-1 fluorescence were unaffected. The responses induced either by cell depolarization or muscarinic receptor activation were similarly affected by the toxin treatment of PC12 cells. In cerebellar granule cells the [Ca2+]i response induced by KCl depolarization was increased by latrunculin treatment, whereas no effect was observed on the PKC response. Latrunculin had no effect on the NMDA-induced responses in these cells. Finally we also show that the response induced by a long-lasting depolarization, which mimics stimulation leading to neuronal plasticity, was not significantly altered by latrunculin or phalloidin treatment of the cells. These results suggest that the actin network is not involved in the initial steps of the PKC activation process in living nerve cells.

Plasma Membrane Disruption: Repair, Prevention, Adaptation

Many metazoan cells inhabit mechanically stressful environments and, consequently, their plasma membranes are frequently disrupted. Survival requires that the cell rapidly repair or reseal the disruption. Rapid resealing is an active and complex structural modification that employs endomembrane as its primary building block, and cytoskeletal and membrane fusion proteins as its catalysts. Endomembrane is delivered to the damaged plasma membrane through exocytosis, a ubiquitous Ca2+-triggered response to disruption. Tissue and cell level architecture prevent disruptions from occurring, either by shielding cells from damaging levels of force, or, when this is not possible, by promoting safe force transmission through the plasma membrane via protein-based cables and linkages. Prevention of disruption also can be a dynamic cell or tissue level adaptation triggered when a damaging level of mechanical stress is imposed. Disease results from failure of either the preventive or resealing mechanisms.

The architecture of polarized cell growth: the unique status of elongating plant cells

Polarity is an inherent feature of almost all prokaryotic and eukaryotic cells. In most eukaryotic cells, growth polarity is due to the assembly of actin-based growing domains at particular locations on the cell periphery. A contrasting scenario is that growth polarity results from the establishment of non-growing domains, which are actively maintained at opposite end-poles of the cell. This latter mode of growth is common in rod-shaped bacteria and, surprisingly, also in the majority of plant cells, which elongate along the apical-basal axes of plant organs. The available data indicate that the non-growing end-pole domains of plant cells are sites of intense endocytosis and recycling. These actin-enriched end-poles serve also as signaling platforms, allowing bidirectional exchange of diverse signals along the supracellular domains of longitudinal cell files. It is proposed that these actively remodeled end-poles of elongating plant cells remotely resemble neuronal synapses.

Interactions between Piccolo and the actin/dynamin-binding protein Abp1 link vesicle endocytosis to presynaptic active zones

Piccolo is a high molecular weight multi-domain protein shown to be a structural component of the presynaptic CAZ (cytoskeletal matrix assembled at active zones). These features indicate that Piccolo may act to scaffold proteins involved in synaptic vesicle endo- and exocytosis near their site of action. To test this hypothesis, we have utilized a functional cell-based endocytosis assay and identified the N-terminal proline-rich Q domain in Piccolo as a region that interferes with clathrin-mediated endocytosis. Utilizing the Piccolo Q domain as bait in a yeast two-hybrid screen, we have identified the F-actin-binding protein Abp1 (also called SH3P7 or HIP-55) as a potential binding partner for this domain. The physiological relevance of this interaction is supported by in vitro binding studies, colocalization in nerve terminals, in vivo recruitment studies, and immunoprecipitation experiments. Intriguingly, Abp1 binds to both F-actin and the GTPase dynamin and has been implicated in linking the actin cytoskeleton to clathrin-mediated endocytosis. Our results suggest that Piccolo, as a structural protein of the CAZ, may serve to localize Abp1 at active zones where it can actively participate in creating a functional connection between the dynamic actin cytoskeleton and synaptic vesicle recycling.

Expression of calmin, a novel developmentally regulated brain protein with calponin-homology domains

We examined the expression in the mouse brain of a recently isolated protein named calmin that has two calponin-homology domains in tandem at the N-terminus and a transmembrane domain at the C-terminus. Calmin mRNA and protein were detected in neurons of the hippocampus, cerebral cortex, and thalamus, Purkinje cells, and also in the choroid plexus and ependymal cells. The protein is present predominantly in dendrites and cell bodies of the neurons, but not in axons. Furthermore, the amounts of calmin mRNA and protein increase during the period of maturation of the mouse brain after birth, in a manner similar to that of PSD95 and synaptophysin. These results indicate that calmin may be involved in the development and/or maintenance of neuronal functions.

Secretory granule exocytosis

Regulated exocytosis of secretory granules or dense-core granules has been examined in many well-characterized cell types including neurons, neuroendocrine, endocrine, exocrine, and hemopoietic cells and also in other less well-studied cell types. Secretory granule exocytosis occurs through mechanisms with many aspects in common with synaptic vesicle exocytosis and most likely uses the same basic protein components. Despite the widespread expression and conservation of a core exocytotic machinery, many variations occur in the control of secretory granule exocytosis that are related to the specialized physiological role of particular cell types. In this review we describe the wide range of cell types in which regulated secretory granule exocytosis occurs and assess the evidence for the expression of the conserved fusion machinery in these cells. The signals that trigger and regulate exocytosis are reviewed. Aspects of the control of exocytosis that are specific for secretory granules compared with synaptic vesicles or for particular cell types are described and compared to define the range of accessory control mechanisms that exert their effects on the core exocytotic machinery.

Endocytosis and the cytoskeleton

In this review we describe the potential roles of the actin cytoskeleton in receptor-mediated endocytosis in mammalian cells and summarize the efforts of recent years in establishing a relationship between these two cellular functions. With molecules such as dynamin, syndapin, HIP1R, Abp1, synaptojanin, N-WASP, intersectin, and cortactin a set of molecular links is now available and it is likely that their further characterization will reveal the basic principles of a functional interconnection between the membrane cytoskeleton and the vesicle-budding machinery. We will therefore discuss proteins involved in endocytic clathrin coat formation and accessory factors to control and regulate coated vesicle formation but we will also focus on actin cytoskeletal components such as the Arp2/3 complex, spectrin, profilin, and motor proteins involved in actin dynamics and organization. Additionally, we will discuss how phosphoinositides, such as PI(4,5)P2, small GTPases thought to control the actin cytoskeleton, such as Rho, Rac, and Cdc42, or membrane trafficking, such as Rab GTPases and ARF proteins, and different kinases may participate in the functional connection of actin and endocytosis. We will compare the concepts and different molecular mechanisms involved in mammalian cells with yeast as well as with specialized cells, such as epithelial cells and neurons, because different model organisms often offer complementary advantages for further studies in this thriving field of current cell biological research.

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.

Temporal and spatial coordination of exocytosis and endocytosis

In secretory cells, exocytosis and compensatory endocytosis are tightly coupled membrane trafficking processes that control the surface area and composition of the plasma membrane. While exocytic and endocytic processes have been studied independently in great detail, at present there is much interest in understanding the mode of their coupling. This review discusses emerging insights into the coupling of these processes, both in the chemical synapses of neurons and in non-neuronal cells.

Actin-ATP hydrolysis is a major energy drain for neurons

In cultured chick ciliary neurons, when ATP synthesis is inhibited, ATP depletion is reduced approximately 50% by slowing actin filament turnover with jasplakinolide or latrunculin A. Jasplakinolide inhibits actin disassembly, and latrunculin A prevents actin assembly by sequestering actin monomers. Cytochalasin D, which allows assembly-disassembly, but only at pointed ends, is less effective in conserving ATP. Ouabain, an Na(+)-K(+)-ATPase inhibitor, and jasplakinolide both prevent approximately 50% of the ATP loss. When applied together, they completely prevent ATP loss over a period of 20 min, suggesting that filament stabilization reduces ATP consumption by decreasing actin-ATP hydrolysis directly rather than indirectly by modulating the activity of Na(+)-K(+)-ATPase, a major energy consumer.

Optical monitoring of synaptic vesicle trafficking in ribbon synapses

Synaptic transmission constitutes the major basis of communication among nerve cells. Upon nerve terminal depolarisation, calcium influx triggers the exocytosis of synaptic vesicles at active zones. Vesicles are then retrieved by endocytosis, recycled and refilled with neurotransmitter. Fluorescent styryl dyes have proven very useful as tools for studying several aspects of the synaptic vesicle cycle. Here, we review recent imaging studies using styryl FM dyes and bipolar cells of goldfish retina, which have a giant synaptic terminal containing ribbon-type active zones. Optical techniques applied to this unique synaptic terminal have provided novel insights into the trafficking of synaptic vesicles during and following strong stimulation.

Remodeling of organelle-bound actin is required for yeast vacuole fusion

Actin participates in several intracellular trafficking pathways. We now find that actin, bound to the surface of purified yeast vacuoles in the absence of cytosol or cytoskeleton, regulates the last compartment mixing stage of homotypic vacuole fusion. The Cdc42p GTPase is known to be required for vacuole fusion. We now show that proteins of the Cdc42p-regulated actin remodeling cascade (Cdc42p --> Cla4p --> Las17p/Vrp1p --> Arp2/3 complex --> actin) are enriched on isolated vacuoles. Vacuole fusion is dramatically altered by perturbation of the vacuole-bound actin, either by mutation of the ACT1 gene, addition of specific actin ligands such as latrunculin B or jasplakinolide, antibody to the actin regulatory proteins Las17p (yeast Wiskott-Aldrich syndrome protein) or Arp2/3, or deletion of actin regulatory genes. On docked vacuoles, actin is enriched at the "vertex ring" membrane microdomain where fusion occurs and is required for the terminal steps leading to membrane fusion. This role for actin may extend to other trafficking systems.

Head, neck, and spines: a role for LIMK-1 in the hippocampus

LIM kinase 1 regulates actin filament dynamics through inhibition of ADF/cofilins. Surprisingly, nervous system development in LIM kinase 1 knockout mice is grossly normal, but the animals have deficits in spatial learning, alterations in LTP, and abnormalities in hippocampal dendritic spine structure. The findings are consistent with a role for LIMK-1 in synapse formation and function.

Visualization of actin dynamics during macropinocytosis and exocytosis

Macropinocytosis of newly formed resides and exocytosis of post-lysosomes have been visualized using a green fluorescent protein probe that binds specifically to F-actin filaments. F-actin association with macropinocytosis begins as a V-shaped infolding of the membrane. Vesicle enlargement occurs through an inward movement of the proximal point of the V as well as an outward protrusion at the tip of the V to form an elongated invagination. The protrusion eventually closes at its distal margin to become a vesicle and is moved centripetally while recovering its circular shape. The vesicle loses its actin coat within 1 min after internalization. One hour later, post-lysosomal vesicles became weakly surrounded by actin while still cytoplasmic. Some of these vesicles moved to the plasma membrane, docked, and then expelled their contents. Slightly before the vesicle content began to disappear, an increase in F-actin association with the vesicle was observed. This was followed by rapid contraction of the vesicle and then disappearance of the actin signal once the internal content was released. These results show that dynamic changes in actin filament association with the vesicle membrane accompany both endocytosis and exocytosis.

Anterior insular cortex: linking intestinal pathology and brain function in autism-spectrum subgroups

Autism includes deficits in communications skills and is associated with intestinal pathology. Numerous parents and some physicians report that an autistic child's attention and language improve in response to treatments which eliminate certain dietary antigens and/or which improve intestinal health. For at least some autism-spectrum children, the link between intestinal pathology, attention, and language may derive from shared neuroanatomic pathways within the anterior insular cortex (aIC); from a neurotrophic virus such as herpes simplex (HSV) migrating within afferents to the insular cortex; and/or from synaptic exhaustion in the aIC as induced by chronically inappropriate neuronal activity in the enteric nervous system and/or its vagal efferents.