The actin cytoskeleton and neurotransmitter release: an overview

Trophoblast glycoprotein is required for efficient synaptic vesicle exocytosis from retinal rod bipolar cells

Introduction

Rod bipolar cells (RBCs) faithfully transmit light-driven signals from rod photoreceptors in the outer retina to third order neurons in the inner retina. Recently, significant work has focused on the role of leucine-rich repeat (LRR) proteins in synaptic development and signal transduction at RBC synapses. We previously identified trophoblast glycoprotein (TPBG) as a novel transmembrane LRR protein localized to the dendrites and axon terminals of RBCs.

Methods

We examined the effects on RBC physiology and retinal processing of TPBG genetic knockout in mice using immunofluorescence and electron microscopy, electroretinogram recording, patch-clamp electrophysiology, and time-resolved membrane capacitance measurements.

Results

The scotopic electroretinogram showed a modest increase in the b-wave and a marked attenuation in oscillatory potentials in the TPBG knockout. No effect of TPBG knockout was observed on the RBC dendritic morphology, TRPM1 currents, or RBC excitability. Because scotopic oscillatory potentials primarily reflect RBC-driven rhythmic activity of the inner retina, we investigated the contribution of TPBG to downstream transmission from RBCs to third-order neurons. Using electron microscopy, we found shorter synaptic ribbons in TPBG knockout axon terminals in RBCs. Time-resolved capacitance measurements indicated that TPBG knockout reduces synaptic vesicle exocytosis and subsequent GABAergic reciprocal feedback without altering voltage-gated Ca2+ currents.

Discussion

TPBG is required for normal synaptic ribbon development and efficient neurotransmitter release from RBCs to downstream cells. Our results highlight a novel synaptic role for TPBG at RBC ribbon synapses and support further examination into the mechanisms by which TPBG regulates RBC physiology and circuit function.

Cytoelectric Coupling: Electric fields sculpt neural activity and "tune" the brain's infrastructure

We propose and present converging evidence for the Cytoelectric Coupling Hypothesis: Electric fields generated by neurons are causal down to the level of the cytoskeleton. This could be achieved via electrodiffusion and mechanotransduction and exchanges between electrical, potential and chemical energy. Ephaptic coupling organizes neural activity, forming neural ensembles at the macroscale level. This information propagates to the neuron level, affecting spiking, and down to molecular level to stabilize the cytoskeleton, "tuning" it to process information more efficiently.

Mechanical actions of dendritic-spine enlargement on presynaptic exocytosis

Synaptic transmission involves cell-to-cell communication at the synaptic junction between two neurons, and chemical and electrical forms of this process have been extensively studied. In the brain, excitatory glutamatergic synapses are often made on dendritic spines that enlarge during learning^1-5. As dendritic spines and the presynaptic terminals are tightly connected with the synaptic cleft⁶, the enlargement may have mechanical effects on presynaptic functions⁷. Here we show that fine and transient pushing of the presynaptic boutons with a glass pipette markedly promotes both the evoked release of glutamate and the assembly of SNARE (soluble N-ethylmaleimide-sensitive factor attachment protein receptor) proteins^8-12-as measured by Förster resonance transfer (FRET) and fluorescence lifetime imaging-in rat slice culture preparations¹³. Both of these effects persisted for more than 20 minutes. The increased presynaptic FRET was independent of cytosolic calcium (Ca²⁺), but dependent on the assembly of SNARE proteins and actin polymerization in the boutons. Notably, a low hypertonic solution of sucrose (20 mM) had facilitatory effects on both the FRET and the evoked release without inducing spontaneous release, in striking contrast with a high hypertonic sucrose solution (300 mM), which induced exocytosis by itself¹⁴. Finally, spine enlargement induced by two-photon glutamate uncaging enhanced the evoked release and the FRET only when the spines pushed the boutons by their elongation. Thus, we have identified a mechanosensory and transduction mechanism¹⁵ in the presynaptic boutons, in which the evoked release of glutamate is enhanced for more than 20 min.

Action potential-coupled Rho GTPase signaling drives presynaptic plasticity

In contrast to their postsynaptic counterparts, the contributions of activity-dependent cytoskeletal signaling to presynaptic plasticity remain controversial and poorly understood. To identify and evaluate these signaling pathways, we conducted a proteomic analysis of the presynaptic cytomatrix using in vivo biotin identification (iBioID). The resultant proteome was heavily enriched for actin cytoskeleton regulators, including Rac1, a Rho GTPase that activates the Arp2/3 complex to nucleate branched actin filaments. Strikingly, we find Rac1 and Arp2/3 are closely associated with synaptic vesicle membranes in adult mice. Using three independent approaches to alter presynaptic Rac1 activity (genetic knockout, spatially restricted inhibition, and temporal optogenetic manipulation), we discover that this pathway negatively regulates synaptic vesicle replenishment at both excitatory and inhibitory synapses, bidirectionally sculpting short-term synaptic depression. Finally, we use two-photon fluorescence lifetime imaging to show that presynaptic Rac1 activation is coupled to action potentials by voltage-gated calcium influx. Thus, this study uncovers a previously unrecognized mechanism of actin-regulated short-term presynaptic plasticity that is conserved across excitatory and inhibitory terminals. It also provides a new proteomic framework for better understanding presynaptic physiology, along with a blueprint of experimental strategies to isolate the presynaptic effects of ubiquitously expressed proteins.

The Mechanical Basis of Memory - the MeshCODE Theory

One of the major unsolved mysteries of biological science concerns the question of where and in what form information is stored in the brain. I propose that memory is stored in the brain in a mechanically encoded binary format written into the conformations of proteins found in the cell-extracellular matrix (ECM) adhesions that organise each and every synapse. The MeshCODE framework outlined here represents a unifying theory of data storage in animals, providing read-write storage of both dynamic and persistent information in a binary format. Mechanosensitive proteins that contain force-dependent switches can store information persistently, which can be written or updated using small changes in mechanical force. These mechanosensitive proteins, such as talin, scaffold each synapse, creating a meshwork of switches that together form a code, the so-called MeshCODE. Large signalling complexes assemble on these scaffolds as a function of the switch patterns and these complexes would both stabilise the patterns and coordinate synaptic regulators to dynamically tune synaptic activity. Synaptic transmission and action potential spike trains would operate the cytoskeletal machinery to write and update the synaptic MeshCODEs, thereby propagating this coding throughout the organism. Based on established biophysical principles, such a mechanical basis for memory would provide a physical location for data storage in the brain, with the binary patterns, encoded in the information-storing mechanosensitive molecules in the synaptic scaffolds, and the complexes that form on them, representing the physical location of engrams. Furthermore, the conversion and storage of sensory and temporal inputs into a binary format would constitute an addressable read-write memory system, supporting the view of the mind as an organic supercomputer.

Cofilin, a Master Node Regulating Cytoskeletal Pathogenesis in Alzheimer's Disease

 The defining pathological hallmarks of Alzheimer’s disease (AD) are proteinopathies marked by the amyloid-β (Aβ) peptide and hyperphosphorylated tau. In addition, Hirano bodies and cofilin-actin rods are extensively found in AD brains, both of which are associated with the actin cytoskeleton. The actin-binding protein cofilin known for its actin filament severing, depolymerizing, nucleating, and bundling activities has emerged as a significant player in AD pathogenesis. In this review, we discuss the regulation of cofilin by multiple signaling events impinging on LIM kinase-1 (LIMK1) and/or Slingshot homolog-1 (SSH1) downstream of Aβ. Such pathophysiological signaling pathways impact actin dynamics to regulate synaptic integrity, mitochondrial translocation of cofilin to promote neurotoxicity, and formation of cofilin-actin pathology. Other intracellular signaling proteins, such as β-arrestin, RanBP9, Chronophin, PLD1, and 14-3-3 also impinge on the regulation of cofilin downstream of Aβ. Finally, we discuss the role of activated cofilin as a bridge between actin and microtubule dynamics by displacing tau from microtubules, thereby destabilizing tau-induced microtubule assembly, missorting tau, and promoting tauopathy.

VMAT2 Safeguards β-Cells Against Dopamine Cytotoxicity Under High-Fat Diet-Induced Stress

Vesicular monoamine transporter 2 (VMAT2) uptakes cytoplasmic monoamines into vesicles for storage. VMAT2 plays a role in modulating insulin release by regulating dopamine levels in the pancreas, although the exact mechanism remains elusive. We found that VMAT2 expression in β-cells specifically increases under high blood glucose conditions. The islets isolated from β-cell-specific Vmat2 knockout (βVmat2KO) mice show elevated insulin secretion levels in response to glucose stimulation. Under prolonged high-fat diet feedings, the βVmat2KO mice exhibit impaired glucose and insulin tolerance and progressive β-cell dysfunction. Here we demonstrate VMAT2 uptake of dopamine to protect dopamine from degradation by monoamine oxidase, thereby safeguarding β-cells from excess reactive oxygen species (ROS) exposure. In the context of high demand for insulin secretion, the absence of VMAT2 leads to elevated ROS in β-cells, which accelerates β-cell dedifferentiation and β-cell loss. Therefore, VMAT2 controls the amount of dopamine in β-cells, thereby protecting pancreatic β-cells from excessive oxidative stress.

Nanomachinery Organizing Release at Neuronal and Ribbon Synapses

A critical aim in neuroscience is to obtain a comprehensive view of how regulated neurotransmission is achieved. Our current understanding of synapses relies mainly on data from electrophysiological recordings, imaging, and molecular biology. Based on these methodologies, proteins involved in a synaptic vesicle (SV) formation, mobility, and fusion at the active zone (AZ) membrane have been identified. In the last decade, electron tomography (ET) combined with a rapid freezing immobilization of neuronal samples opened a window for understanding the structural machinery with the highest spatial resolution in situ. ET provides significant insights into the molecular architecture of the AZ and the organelles within the presynaptic nerve terminal. The specialized sensory ribbon synapses exhibit a distinct architecture from neuronal synapses due to the presence of the electron-dense synaptic ribbon. However, both synapse types share the filamentous structures, also commonly termed as tethers that are proposed to contribute to different steps of SV recruitment and exocytosis. In this review, we discuss the emerging views on the role of filamentous structures in SV exocytosis gained from ultrastructural studies of excitatory, mainly central neuronal compared to ribbon-type synapses with a focus on inner hair cell (IHC) ribbon synapses. Moreover, we will speculate on the molecular entities that may be involved in filament formation and hence play a crucial role in the SV cycle.

Vesicle sub-pool organization at inner hair cell ribbon synapses

The afferent inner hair cell synapse harbors the synaptic ribbon, which ensures a constant vesicle supply. Synaptic vesicles (SVs) are arranged in morphologically discernable pools, linked via filaments to the ribbon or the presynaptic membrane. We propose that filaments play a major role in SV resupply and exocytosis at the ribbon. Using advanced electron microscopy, we demonstrate that SVs are organized in sub-pools defined by the filament number per vesicle and its connections. Upon stimulation, SVs increasingly linked to other vesicles and to the ribbon, whereas single-tethered SVs dominated at the membrane. Mutant mice for the hair cell protein otoferlin (pachanga, Otof ^Pga/Pga ) are profoundly deaf with reduced sustained release, serving as a model to investigate the SV replenishment at IHCs. Upon stimulation, multiple-tethered and docked vesicles (rarely observed in wild-type) accumulated at Otof ^Pga/Pga active zones due to an impairment downstream of docking. Conclusively, vesicles are organized in sub-pools at ribbon-type active zones by filaments to support vesicle supply, transport, and finally release.

Spinal muscular atrophy: Factors that modulate motor neurone vulnerability

Spinal muscular atrophy (SMA), a leading genetic cause of infant death, is a neurodegenerative disease characterised by the selective loss of particular groups of motor neurones in the anterior horn of the spinal cord with concomitant muscle weakness. To date, no effective treatment is available, however, there are ongoing clinical trials are in place which promise much for the future. However, there remains an ongoing problem in trying to link a single gene loss to motor neurone degeneration. Fortunately, given successful disease models that have been established and intensive studies on SMN functions in the past ten years, we are fast approaching the stage of identifying the underlying mechanisms of SMA pathogenesis Here we discuss potential disease modifying factors on motor neurone vulnerability, in the belief that these factors give insight into the pathological mechanisms of SMA and therefore possible therapeutic targets.

Functional Role of Gonadotrope Plasticity and Network Organization

Gonadotrope cells of the anterior pituitary are characterized by their ability to mount a cyclical pattern of gonadotropin secretion to regulate gonadal function and fertility. Recent in vitro and in vivo evidence suggests that gonadotropes exhibit dramatic remodeling of the actin cytoskeleton following gonadotropin-releasing hormone (GnRH) exposure. GnRH engagement of actin is critical for gonadotrope function on multiple levels. First, GnRH-induced cell movements lead to spatial repositioning of the in vivo gonadotrope network toward vascular endothelium, presumably to access the bloodstream for effective hormone release. Interestingly, these plasticity changes can be modified depending on the physiological status of the organism. Additionally, GnRH-induced actin assembly appears to be fundamental to gonadotrope signaling at the level of extracellular signal-regulated kinase (ERK) activation, which is a well-known regulator of luteinizing hormone (LH) β-subunit synthesis. Last, GnRH-induced cell membrane projections are capable of concentrating LHβ-containing vesicles and disruption of the actin cytoskeleton reduces LH secretion. Taken together, gonadotrope network positioning and LH synthesis and secretion are linked to GnRH engagement of the actin cytoskeleton. In this review, we will cover the dynamics and organization of the in vivo gonadotrope cell network and the mechanisms of GnRH-induced actin-remodeling events important in ERK activation and subsequently hormone secretion.

Outcome Measures in Clinical Trials for Multiple Sclerosis

Due to the heterogeneous nature of the disease, it is a challenge to capture disease activity of multiple sclerosis (MS) in a reliable and valid way. Therefore, it can be difficult to assess the true efficacy of interventions in clinical trials. In phase III trials in MS, the traditionally used primary clinical outcome measures are the Expanded Disability Status Scale and the relapse rate. Secondary outcome measures in these trials are the number or volume of T2 hyperintense lesions and gadolinium-enhancing T1 lesions on magnetic resonance imaging (MRI) of the brain. These secondary outcome measures are often primary outcome measures in phase II trials in MS. Despite several limitations, the traditional clinical measures are still the mainstay for assessing treatment efficacy. Newer and potentially valuable outcome measures increasingly used or explored in MS trials are, clinically, the MS Functional Composite and patient-reported outcome measures, and on MRI, brain atrophy and the formation of persisting black holes. Several limitations of these measures have been addressed and further improvements will probably be proposed. Major improvements are the coverage of additional functional domains such as cognitive functioning and assessment of the ability to carry out activities of daily living. The development of multidimensional measures is promising because these measures have the potential to cover the full extent of MS activity and progression. In this review, we provide an overview of the historical background and recent developments of outcome measures in MS trials. We discuss the advantages and limitations of various measures, including newer assessments such as optical coherence tomography, biomarkers in body fluids and the concept of 'no evidence of disease activity'.

Altered mechanisms underlying the abnormal glutamate release in amyotrophic lateral sclerosis at a pre-symptomatic stage of the disease

Abnormal Glu release occurs in the spinal cord of SOD1(G93A) mice, a transgenic animal model for human ALS. Here we studied the mechanisms underlying Glu release in spinal cord nerve terminals of SOD1(G93A) mice at a pre-symptomatic disease stage (30days) and found that the basal release of Glu was more elevated in SOD1(G93A) with respect to SOD1 mice, and that the surplus of release relies on synaptic vesicle exocytosis. Exposure to high KCl or ionomycin provoked Ca(2+)-dependent Glu release that was likewise augmented in SOD1(G93A) mice. Equally, the Ca(2+)-independent hypertonic sucrose-induced Glu release was abnormally elevated in SOD1(G93A) mice. Also in this case, the surplus of Glu release was exocytotic in nature. We could determine elevated cytosolic Ca(2+) levels, increased phosphorylation of Synapsin-I, which was causally related to the abnormal Glu release measured in spinal cord synaptosomes of pre-symptomatic SOD1(G93A) mice, and increased phosphorylation of glycogen synthase kinase-3 at the inhibitory sites, an event that favours SNARE protein assembly. Western blot experiments revealed an increased number of SNARE protein complexes at the nerve terminal membrane, with no changes of the three SNARE proteins and increased expression of synaptotagmin-1 and β-Actin, but not of an array of other release-related presynaptic proteins. These results indicate that the abnormal exocytotic Glu release in spinal cord of pre-symptomatic SOD1(G93A) mice is mainly based on the increased size of the readily releasable pool of vesicles and release facilitation, supported by plastic changes of specific presynaptic mechanisms.

Brain-Specific Cytoskeletal Damage Markers in Cerebrospinal Fluid: Is There a Common Pattern between Amyotrophic Lateral Sclerosis and Primary Progressive Multiple Sclerosis?

Many neurodegenerative disorders share a common pathophysiological pathway involving axonal degeneration despite different etiological triggers. Analysis of cytoskeletal markers such as neurofilaments, protein tau and tubulin in cerebrospinal fluid (CSF) may be a useful approach to detect the process of axonal damage and its severity during disease course. In this article, we review the published literature regarding brain-specific CSF markers for cytoskeletal damage in primary progressive multiple sclerosis and amyotrophic lateral sclerosis in order to evaluate their utility as a biomarker for disease progression in conjunction with imaging and histological markers which might also be useful in other neurodegenerative diseases associated with affection of the upper motor neurons. A long-term benefit of such an approach could be facilitating early diagnostic and prognostic tools and assessment of treatment efficacy of disease modifying drugs.

The Cell Death Pathway Regulates Synapse Elimination through Cleavage of Gelsolin in Caenorhabditis elegans Neurons

Synapse elimination occurs in development, plasticity, and disease. Although the importance of synapse elimination has been documented in many studies, the molecular mechanisms underlying this process are unclear. Here, using the development of C. elegans RME neurons as a model, we have uncovered a function for the apoptosis pathway in synapse elimination. We find that the conserved apoptotic cell death (CED) pathway and axonal mitochondria are required for the elimination of transiently formed clusters of presynaptic components in RME neurons. This function of the CED pathway involves the activation of the actin-filament-severing protein, GSNL-1. Furthermore, we show that caspase CED-3 cleaves GSNL-1 at a conserved C-terminal region and that the cleaved active form of GSNL-1 promotes its actin-severing ability. Our data suggest that activation of the CED pathway contributes to selective elimination of synapses through disassembly of the actin filament network.

Fascin expression in cholesteatoma: correlation with destruction of the ossicular chain and extent of disease

OBJECTIVE

Fascin is an actin-bundling protein found in cell membrane protrusions and increases cell motility. The expression of fascin in epithelial neoplasms has been described only recently. No data are available concerning the role of this protein in invasive cholesteatoma. Thus, we investigated the expression of fascin in cholesteatoma tissue and the relationship between fascin expression and intraoperative evaluation of the destruction of the ossicular chain and extent of disease.

METHOD

Cholesteatoma specimens of 28 patients and external auditory canal (EAC) skin specimens of the same patients (as the control group) were collected from mastoidectomies. Immunohistochemical technique was used to investigate the fascin expression in all cholesteatoma tissues and EAC skin specimens. Immunohistochemical staining was assessed semiquantitatively based on the thickness of epithelium. SPSS software version 16.0 (SPSS Inc., Chicago, IL, USA) was performed to statistically analyse the relationships between fascin expression and intraoperative evaluation destruction of ossicular chain and extent of the disease.

RESULTS

Immunohistochemically, there was no or very low fascin expression observed in normal epithelial cells of EAC skin, while expressed in cholesteatoma tissue. Also, fascin expression in cholesteatoma tissues was significantly correlated with destruction of ossicular chain and extent of the disease.

CONCLUSIONS

Fascin expression is usually found in cholesteatoma epithelium and is correlated with destruction of the ossicular chain and extent of disease. Considering all of the correlations between the clinical and histopathological findings, 'fascin immunoexpression scoring' may be used for histological grading of cholesteatoma.

Aβ Influences Cytoskeletal Signaling Cascades with Consequences to Alzheimer's Disease

Abnormal signal transduction events can impact upon the cytoskeleton, affecting the actin and microtubule networks with direct relevance to Alzheimer's disease (AD). Cytoskeletal anomalies, in turn, promote atypical neuronal responses, with consequences for cellular organization and function. Neuronal cytoskeletal modifications in AD include neurofibrillary tangles, which result from aggregates of hyperphosphorylated tau protein. The latter is a microtubule (MT)-binding protein, whose abnormal phosphorylation leads to MT instability and consequently provokes irregularities in the neuronal trafficking pathways. Early stages of AD are also characterized by synaptic dysfunction and loss of dendritic spines, which correlate with cognitive deficit and impaired brain function. Actin dynamics has a prominent role in maintaining spine plasticity and integrity, thus providing the basis for memory and learning processes. Hence, factors that disrupt both actin and MT network dynamics will compromise neuronal function and survival. The peptide Aβ is the major component of senile plaques and has been described as a pivotal mediator of neuronal dystrophy and synaptic loss in AD. Here, we review Aβ-mediated effects on both MT and actin networks and focus on the relevance of the elicited cytoskeletal signaling events targeted in AD pathology.

Local F-actin network links synapse formation and axon branching

Axonal branching and synapse formation are tightly linked developmental events during the establishment of synaptic circuits. Newly formed synapses promote branch initiation and stability. However, little is known about molecular mechanisms that link these two processes. Here, we show that local assembly of an F-actin cytoskeleton at nascent presynaptic sites initiates both synapse formation and axon branching. We further find that assembly of the F-actin network requires a direct interaction between the synaptic cell adhesion molecule SYG-1 and a key regulator of actin cytoskeleton, the WVE-1/WAVE regulatory complex (WRC). SYG-1 cytoplasmic tail binds to the WRC using a consensus WRC interacting receptor sequence (WIRS). WRC mutants or mutating the SYG-1 WIRS motif leads to loss of local F-actin, synaptic material, and axonal branches. Together, these data suggest that synaptic adhesion molecules, which serve as a necessary component for both synaptogenesis and axonal branch formation, directly regulate subcellular actin cytoskeletal organization.

GEF-H1: orchestrating the interplay between cytoskeleton and vesicle trafficking

Vesicle trafficking is crucial for delivery of membrane compartments as well as signaling molecules to specific sites on the plasma membrane for regulation of diverse processes such as cell division, migration, polarity establishment and secretion. Rho GTPases are well-studied signaling molecules that regulate actin cytoskeleton in response to variety of extracellular stimuli. Increasing amounts of evidence suggest that Rho proteins play a critical role in vesicle trafficking in both the exocytic and endocytic pathways; however, the molecular mechanism underlying the process remains largely unclear. We recently defined a mechanism of action for RhoA in membrane trafficking pathways through regulation of the octameric complex exocyst in a manuscript published in Developmental Cell. We have shown that microtubule-associated RhoA-activating factor GEF-H1 is involved in endocytic and excocytic vesicle trafficking. GEF-H1 activates RhoA in response to RalA GTPase, which in turn regulates the localization and the assembly of exocyst components and exocytosis. Our work defines a mechanism for RhoA activation in response to RalA signaling and during vesicle trafficking. These results provide a framework for understanding how RhoA/GEF-H1 regulates the coordination of actin and microtubule cytoskeleton modulation and vesicle trafficking during migration and cell division.

The actin cytoskeleton in presynaptic assembly

Dramatic morphogenetic processes underpin nearly every step of nervous system development, from initial neuronal migration and axon guidance to synaptogenesis. Underlying this morphogenesis are dynamic rearrangements of cytoskeletal architecture. Here we discuss the roles of the actin cytoskeleton in the development of presynaptic terminals, from the elaboration of terminal arbors to the recruitment of presynaptic vesicles and active zone components. The studies discussed here underscore the importance of actin regulation at every step in neuronal circuit assembly.

Ultrastructure of synapses in the mammalian brain

The morphology and molecular composition of synapses provide the structural basis for synaptic function. This article reviews the electron microscopy of excitatory synapses on dendritic spines, using data from rodent hippocampus, cerebral cortex, and cerebellar cortex. Excitatory synapses have a prominent postsynaptic density, in contrast with inhibitory synapses, which have less dense presynaptic or postsynaptic specializations and are usually found on the cell body or proximal dendritic shaft. Immunogold labeling shows that the presynaptic active zone provides a scaffold for key molecules involved in the release of neurotransmitter, whereas the postsynaptic density contains ligand-gated ionic channels, other receptors, and a complex network of signaling molecules. Delineating the structure and molecular organization of these axospinous synapses represents a crucial step toward understanding the mechanisms that underlie synaptic transmission and the dynamic modulation of neurotransmission associated with short- and long-term synaptic plasticity.

The role of Munc18-1 and its orthologs in modulation of cortical F-actin in chromaffin cells

Munc18-1 was originally described as an essential docking factor in chromaffin cells. Recent findings showed that Munc18-1 has an additional role in the regulation of the cortical F-actin network, which is thought to function as a physical barrier preventing secretory vesicles from access to their release sites under resting conditions. In our review, we discuss whether this function is evolutionarily conserved in all Sec1/Munc18-like (SM) proteins. In addition, we introduce a new quantification method that improves the analysis of cortical filamentous actin (F-actin) in comparison with existing methods. Since the docking process is highly evolutionarily conserved in the SM protein superfamily, we use our novel quantification method to investigate whether the F-actin-regulating function is similarly conserved among SM proteins. Our preliminary data suggest that the regulation of cortical F-actin is a shared function of SM proteins, and we propose a way to gain more insight in the molecular mechanism underlying the Munc18-1-mediated cortical F-actin regulation.

The dynamic architecture of photoreceptor ribbon synapses: cytoskeletal, extracellular matrix, and intramembrane proteins

Rod and cone photoreceptors possess ribbon synapses that assist in the transmission of graded light responses to second-order bipolar and horizontal cells of the vertebrate retina. Proper functioning of the synapse requires the juxtaposition of presynaptic release sites immediately adjacent to postsynaptic receptors. In this review, we focus on the synaptic, cytoskeletal, and extracellular matrix proteins that help to organize photoreceptor ribbon synapses in the outer plexiform layer. We examine the proteins that foster the clustering of release proteins, calcium channels, and synaptic vesicles in the presynaptic terminals of photoreceptors adjacent to their postsynaptic contacts. Although many proteins interact with one another in the presynaptic terminal and synaptic cleft, these protein-protein interactions do not create a static and immutable structure. Instead, photoreceptor ribbon synapses are remarkably dynamic, exhibiting structural changes on both rapid and slow time scales.

Dual pools of actin at presynaptic terminals

We investigated actin's function in vesicle recycling and exocytosis at lamprey synapses and show that FM1-43 puncta and phalloidin-labeled filamentous actin (F-actin) structures are colocalized, yet recycling vesicles are not contained within F-actin clusters. Additionally, phalloidin also labels a plasma membrane-associated cortical actin. Injection of fluorescent G-actin revealed activity-independent dynamic actin incorporation into presynaptic synaptic vesicle clusters but not into cortical actin. Latrunculin-A, which sequesters G-actin, dispersed vesicle-associated actin structures and prevented subsequent labeled G-actin and phalloidin accumulation at presynaptic puncta, yet cortical phalloidin labeling persisted. Dispersal of presynaptic F-actin structures by latrunculin-A did not disrupt vesicle clustering or recycling or alter the amplitude or kinetics of excitatory postsynaptic currents (EPSCs). However, it slightly enhanced release during repetitive stimulation. While dispersal of presynaptic actin puncta with latrunculin-A failed to disperse synaptic vesicles or inhibit synaptic transmission, presynaptic phalloidin injection blocked exocytosis and reduced endocytosis measured by action potential-evoked FM1-43 staining. Furthermore, phalloidin stabilization of only cortical actin following pretreatment with latrunculin-A was sufficient to inhibit synaptic transmission. Conversely, treatment of axons with jasplakinolide, which induces F-actin accumulation but disrupts F-actin structures in vivo, resulted in increased synaptic transmission accompanied by a loss of phalloidin labeling of cortical actin but no loss of actin labeling within vesicle clusters. Marked synaptic deficits seen with phalloidin stabilization of cortical F-actin, in contrast to the minimal effects of disruption of a synaptic vesicle-associated F-actin, led us to conclude that two structurally and functionally distinct pools of actin exist at presynaptic sites.

Functional diversity of actin cytoskeleton in neurons and its regulation by tropomyosin

Neurons comprise functionally, molecularly, and spatially distinct subcellular compartments which include the soma, dendrites, axon, branches, dendritic spines, and growth cones. In this chapter, we detail the remarkable ability of the neuronal cytoskeleton to exquisitely regulate all these cytoplasmic distinct partitions, with particular emphasis on the microfilament system and its plethora of associated proteins. Importance will be given to the family of actin-associated proteins, tropomyosin, in defining distinct actin filament populations. The ability of tropomyosin isoforms to regulate the access of actin-binding proteins to the filaments is believed to define the structural diversity and dynamics of actin filaments and ultimately be responsible for the functional outcome of these filaments.

The roles of the actin cytoskeleton in fear memory formation

The formation and storage of fear memory is needed to adapt behavior and avoid danger during subsequent fearful events. However, fear memory may also play a significant role in stress and anxiety disorders. When fear becomes disproportionate to that necessary to cope with a given stimulus, or begins to occur in inappropriate situations, a fear or anxiety disorder exists. Thus, the study of cellular and molecular mechanisms underpinning fear memory may shed light on the formation of memory and on anxiety and stress related disorders. Evidence indicates that fear learning leads to changes in neuronal synaptic transmission and morphology in brain areas underlying fear memory formation including the amygdala and hippocampus. The actin cytoskeleton has been shown to participate in these key neuronal processes. Recent findings show that the actin cytoskeleton is needed for fear memory formation and extinction. Moreover, the actin cytoskeleton is involved in synaptic plasticity and in neuronal morphogenesis in brain areas that mediate fear memory. The actin cytoskeleton may therefore mediate between synaptic transmission during fear learning and long-term cellular alterations mandatory for fear memory formation.

Membrane trafficking events underlying axon repair, growth, and regeneration

Two central challenges for the field of neurobiology are to understand how axons grow and make proper synaptic connections under normal conditions and how they repair their membranes and mount regenerative responses after injury. At the most reductionist level, the first step toward addressing these challenges is to delineate the cellular and molecular processes by which an axon extends from its cell body. Underlying axon extension are questions of appropriate timing and mechanisms that establish or maintain the axon's polarity, initiate growth cone formation, and promote axon outgrowth and synapse formation. After injury, the problem is even more complicated because the neuron must also repair its damaged membrane, redistribute or manufacture what it needs in order to survive, and grow and form new synapses within a more mature, complex environment. While other reviews have focused extensively on the signaling events and cytoskeletal rearrangements that support axon outgrowth and regeneration, we focus this review instead on the underlying membrane trafficking events underlying these processes. Though the mechanisms are still under active investigation, the key roles played by membrane trafficking events during axon repair, growth, and regeneration have been elucidated through elegant comparative studies in both invertebrate and vertebrate organisms. Taken together, a model emerges indicating that the critical requirements for ensuring proper membrane sealing and axon extension include iterative bouts of SNARE mediated exocytosis, endocytosis, and functional links between vesicles and the actin cytoskeleton, similar to the mechanisms utilized during synaptic transmission. This article is part of a Special Issue entitled 'Neuronal Function'.

How important are Rho GTPases in neurosecretion?

Rho GTPases are small GTP binding proteins belonging to the Ras superfamily which act as molecular switches that regulate many cellular function including cell morphology, cell to cell interaction, cell migration and adhesion. In neuronal cells, Rho GTPases have been proposed to regulate neuronal development and synaptic plasticity. However, the role of Rho GTPases in neurosecretion is poorly documented. In this review, we discuss data that highlight the importance of Rho GTPases and their regulators into the control of neurotransmitter and hormone release in neurons and neuroendocrine cells, respectively.

Rab27a, actin and beta-cell endocytosis

The output and time-course of insulin release from pancreatic beta-cells are elegantly controlled. The secretory process comprises pre-exocytotic stages, exocytosis and post-exocytotic stages. The small GTPase Rab27a is known to regulate pre-exocytotic stages that determine the size of the readily-releasable pool of insulin granules. GTP-Rab27a and its specific effectors are responsible for this process like other GTPases. Recently, we searched for Rab27a-interacting proteins and identified coronin 3. Unexpectedly, coronin 3 only bound GDP-Rab27a and this interaction regulated post-exocytotic stages via reorganization of the actin cytoskeleton. Since glucose converts Rab27a from the GTP- to GDP-bound form, we suggested that Rab27a plays a crucial role in stimulus-endocytosis coupling in pancreatic beta-cells, and that this is the key molecule for membrane recycling of insulin granules. In this review, we provide an overview of the roles of Rab27a and its GTP- and GDP-dependent effectors in the insulin secretory pathway of pancreatic beta-cells.

Morphological docking of secretory vesicles

Calcium-dependent secretion of neurotransmitters and hormones is essential for brain function and neuroendocrine-signaling. Prior to exocytosis, neurotransmitter-containing vesicles dock to the target membrane. In electron micrographs of neurons and neuroendocrine cells, like chromaffin cells many synaptic vesicles (SVs) and large dense-core vesicles (LDCVs) are docked. For many years the molecular identity of the morphologically docked state was unknown. Recently, we resolved the minimal docking machinery in adrenal medullary chromaffin cells using embryonic mouse model systems together with electron-microscopic analyses and also found that docking is controlled by the sub-membrane filamentous (F-)actin. Currently it is unclear if the same docking machinery operates in synapses. Here, I will review our docking assay that led to the identification of the LDCV docking machinery in chromaffin cells and also discuss whether identical docking proteins are required for SV docking in synapses.

Bacterial toxins and the nervous system: neurotoxins and multipotential toxins interacting with neuronal cells

Toxins are potent molecules used by various bacteria to interact with a host organism. Some of them specifically act on neuronal cells (clostridial neurotoxins) leading to characteristics neurological affections. But many other toxins are multifunctional and recognize a wider range of cell types including neuronal cells. Various enterotoxins interact with the enteric nervous system, for example by stimulating afferent neurons or inducing neurotransmitter release from enterochromaffin cells which result either in vomiting, in amplification of the diarrhea, or in intestinal inflammation process. Other toxins can pass the blood brain barrier and directly act on specific neurons.

Molecular mechanism of secretory vesicle docking

Docking, the stable association of secretory vesicles with the plasma membrane, is considered to be the necessary first step before vesicles gain fusion-competence, but it is unclear how vesicles dock. In adrenal medullary chromaffin cells, access of secretory vesicles to docking sites is controlled by dense F-actin (filamentous actin) beneath the plasma membrane. Recently, we found that, in the absence of Munc18-1, the number of docked vesicles and the thickness of cortical F-actin are affected. In the present paper, I discuss the possible mechanism by which Munc18-1 modulates cortical F-actin and how it orchestrates the docking machinery via an interaction with syntaxin-1. Finally, a comparison of Munc18's role in embryonic mouse and adult bovine chromaffin cell model systems will be made to clarify observed differences in cortical F-actin as well as docking phenotypes.

The micro-architecture of mitochondria at active zones: electron tomography reveals novel anchoring scaffolds and cristae structured for high-rate metabolism

Mitochondria are integral elements of many nerve terminals. They must be appropriately positioned to regulate microdomains of Ca(2+) concentration and metabolic demand, but structures that anchor them in place have not been described. By applying the high resolution of electron tomography (ET) to the study of a central terminal, the calyx of Held, we revealed an elaborate cytoskeletal superstructure that connected a subset of mitochondria to the presynaptic membrane near active zones. This cytoskeletal network extended laterally and was well integrated into the nerve terminal cytoskeleton, which included filamentous linkages among synaptic vesicles. ET revealed novel features of inner membrane for these mitochondria. Crista structure was polarized in that crista junctions, circular openings of the inner membrane under the outer membrane, were aligned with the cytoskeletal superstructure and occurred at higher density in mitochondrial membrane facing the presynaptic membrane. These characteristics represent the first instance where a subcomponent of an organelle is shown to have a specific orientation relative to the polarized structure of a cell. The ratio of cristae to outer membrane surface area is large in these mitochondria relative to other tissues, indicating a high metabolic capacity. These observations suggest general principles for cytoskeletal anchoring of mitochondria in all tissues, reveal potential routes for nonsynaptic communication between presynaptic and postsynaptic partners using this novel cytoskeletal framework, and indicate that crista structure can be specialized for particular functions within cellular microdomains.

Actin assembly controlled by GDP-Rab27a is essential for endocytosis of the insulin secretory membrane

We have recently reported that GDP-bound Rab27a regulates endocytosis of the insulin secretory membrane via its binding to coronin 3, an actin-binding protein. The aim of this study was to examine the participation of actin assembly in the Rab27a-dependent regulation of endocytosis using a pancreatic beta cell line, MIN6. Coronin 3 promoted F-actin bundling only in the presence of GDP-Rab27a. This effect was independent of coronin-3-binding to the actin-related proteins 2 and 3 (Arp2/3). Uptake of anti-phogrin-lumen antibody into MIN6 was inhibited by anti-coronin-3-C antibody which recognizes the actin-binding site. This inhibition was also observed with coronin-3-R28D, which lacks in actin binding. These results suggest that coronin 3 is a genuine GDP-Rab27a effector, and that controls endocytosis of the secretory membrane via modulating actin assembly in pancreatic beta-cells.

The highly conserved synapsin domain E mediates synapsin dimerization and phospholipid vesicle clustering

Synapsins are abundant SV (synaptic vesicle)-associated phosphoproteins that regulate synapse formation and function. The highly conserved C-terminal domain E was shown to contribute to several synapsin functions, ranging from formation of the SV reserve pool to regulation of the kinetics of exocytosis and SV cycling, although the molecular mechanisms underlying these effects are unknown. In the present study, we used a synthetic 25-mer peptide encompassing the most conserved region of domain E (Pep-E) to analyse the role of domain E in regulating the interactions between synapsin I and liposomes mimicking the phospholipid composition of SVs (SV-liposomes) and other pre-synaptic protein partners. In affinity-chromatography and cross-linking assays, Pep-E bound to endogenous and purified exogenous synapsin I and strongly inhibited synapsin dimerization, indicating a role in synapsin oligomerization. Consistently, Pep-E (but not its scrambled version) counteracted the ability of holo-synapsin I to bind and coat phospholipid membranes, as analysed by AFM (atomic force microscopy) topographical scanning, and significantly decreased the clustering of SV-liposomes induced by holo-synapsin I in FRET (Förster resonance energy transfer) assays, suggesting a causal relationship between synapsin oligomerization and vesicle clustering. Either Pep-E or a peptide derived from domain C was necessary and sufficient to inhibit both dimerization and vesicle clustering, indicating the participation of both domains in these activities of synapsin I. The results provide a molecular explanation for the effects of domain E in nerve terminal physiology and suggest that its effects on the size and integrity of SV pools are contributed by the regulation of synapsin dimerization and SV clustering.

Invariance of exocytotic events detected by amperometry as a function of the carbon fiber microelectrode diameter

Etched carbon fiber microelectrodes of different radii have been used for amperometric measurements of single exocytotic events occurring at adrenal chromaffin cells. Frequency, kinetic, and quantitative information on exocytosis provided by amperometric spikes were analyzed as a function of the surface area of the microelectrodes. Interestingly, the percentage of spikes with foot (as well as their own characteristics), a category revealing the existence of sufficient long-lasting fusion pores, was found to be constant whatever the microelectrode diameter was, whereas the probability of overlapping spikes decreased with the electrode size. This confirmed that the prespike foot could not feature accidental superimposition of separated events occurring at different places. Moreover, the features of amperometric spikes investigated here (charge, intensity and kinetics) were found constant for all microelectrode diameters. This demonstrated that the electrochemical measurement does not introduce significant bias onto the kinetics and thermodynamics of release during individual exocytotic events. All in all, this work evidences that information on exocytosis amperometrically recorded with the usual 7 microm diameter carbon fiber electrodes is biologically relevant, although the frequent overlap between spikes requires a censorship of the data during the analytical treatment.

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.

Control of granule mobility and exocytosis by Ca2+ -dependent formation of F-actin in pancreatic duct epithelial cells

Elevation of intracellular Ca(2+) concentration ([Ca(2+)](i)) triggers exocytosis of secretory granules in pancreatic duct epithelia. In this study, we find that the signal also controls granule movement. Motions of fluorescently labeled granules stopped abruptly after a [Ca(2+)](i) increase, kinetically coincident with formation of filamentous actin (F-actin) in the whole cytoplasm. At high resolution, the new F-actin meshwork was so dense that cellular structures of granule size appeared physically trapped in it. Depolymerization of F-actin with latrunculin B blocked both the F-actin formation and the arrest of granules. Interestingly, when monitored with total internal reflection fluorescence microscopy, the immobilized granules still moved slowly and concertedly toward the plasma membrane. This group translocation was abolished by blockers of myosin. Exocytosis measured by microamperometry suggested that formation of a dense F-actin meshwork inhibited exocytosis at small Ca(2+) rises <1 microm. Larger [Ca(2+)](i) rises increased exocytosis because of the co-ordinate translocation of granules and fusion to the membrane. We propose that the Ca(2+)-dependent freezing of granules filters out weak inputs but allows exocytosis under stronger inputs by controlling granule movements.

Questioning the role of actinfree Gc-Globulin as actin scavenger in neurodegenerative central nervous system disease: relationship to S-100B levels and blood-brain barrier function

INTRODUCTION

Preliminary studies report on significantly higher levels of the major cytoskeleton protein actin in CSF of patients with neurodegenerative conditions and that the dynamics of these levels obviously correlates with disease progression and clinical disability. One of the primary functions of actinfree Gc-Globulin is to bind and neutralize extracellular monomeric actin, released into the circulation by necrotic or ruptured cells, and thus ameliorating the clinical outcome in situations of severe organ damage.

AIM AND METHODS

This is the first study to investigate actinfree Gc-Globulin and S100-B levels (as reliable marker of neurodegeneration) in paired CSF and serum samples of patients with multietiological CNS diseases.

RESULTS

42% of all patients with CNS disease displayed serum concentrations of actinfree Gc-Globulin above the established reference range. CSF concentrations of actinfree Gc-Globulin and S100-B were positively correlated with the severity of blood-brain barrier (BBB) dysfunction. Furthermore, patients with severe BBB dysfunction presented a higher percentage of intrathecal synthesis of actinfree Gc-Globulin compared to patients with mild to moderate dysfunction and to patients with normal BBB function. Representative longitudinal data from selected patients demonstrated an inverse behaviour of actinfree Gc-Globulin and S100-B CSF concentrations, suggesting a consumption of the actin scavenger capacity of Gc-Globulin in times of increased neuronal damage. This presumption was supported by the fact that those conditions associated with a severe neuronal damage, in particular CNS trauma, and highest S100-B concentrations simultaneously displayed lowest actinfree Gc-Globulin levels, and thus residual actin binding capacity of Gc-Globulin.

CONCLUSION

In summary, our data propose a function of actinfree Gc-Globulin also in the clearance of actin filaments from CSF of patients with neuronal damage. However, active recruitment of hepatic derived actinfree Gc-Globulin to the site of CNS injury is not observed. Much more, BBB leakage enables extraneuronally synthesized actinfree Gc-Globulin to extent its scavenger capacity for actin also to the subarachnoidal space. Furthermore, intrathecal synthesis of actinfree Gc-Globulin seems to be increased in patients with severe neurodegeneration.

Autocrine signaling involved in cell volume regulation: the role of released transmitters and plasma membrane receptors

Cell volume regulation is a basic homeostatic mechanism transcendental for the normal physiology and function of cells. It is mediated principally by the activation of osmolyte transport pathways that result in net changes in solute concentration that counteract cell volume challenges in its constancy. This process has been described to be regulated by a complex assortment of intracellular signal transduction cascades. Recently, several studies have demonstrated that alterations in cell volume induce the release of a wide variety of transmitters including hormones, ATP and neurotransmitters, which have been proposed to act as extracellular signals that regulate the activation of cell volume regulatory mechanisms. In addition, changes in cell volume have also been reported to activate plasma membrane receptors (including tyrosine kinase receptors, G-protein coupled receptors and integrins) that have been demonstrated to participate in the regulatory process of cell volume. In this review, we summarize recent studies about the role of changes in cell volume in the regulation of transmitter release as well as in the activation of plasma membrane receptors and their further implications in the regulation of the signaling machinery that regulates the activation of osmolyte flux pathways. We propose that the autocrine regulation of Ca2+-dependent and tyrosine phosphorylation-dependent signaling pathways by the activation of plasma membrane receptors and swelling-induced transmitter release is necessary for the activation/regulation of osmolyte efflux pathways and cell volume recovery. Furthermore, we emphasize the importance of studying these extrinsic signals because of their significance in the understanding of the physiology of cell volume regulation and its role in cell biology in vivo, where the constraint of the extracellular space might enhance the autocrine or even paracrine signaling induced by these released transmitters.

Conical electron tomography of a chemical synapse: polyhedral cages dock vesicles to the active zone

In this study, we tested the hypothesis that the structure of the active zone of chemical synapses has remained uncertain because of limitations of conventional electron microscopy. To resolve these limitations, we reconstructed chemical synapses of rat neocortex, the archetypical "average" synapse, by conical electron tomography, a method that exhibits an isotropic in plane resolution of approximately 3 nm and eliminates the need to impose symmetry or use averaging methods to increase signal-to-noise ratios. Analysis of 17 reconstructions by semiautomatic density segmentation indicated that the active zone was constructed of a variable number of distinct "synaptic units" comprising a polyhedral cage and a corona of approximately seven vesicles. The polyhedral cages measured approximately 60 nm in diameter, with a density of approximately 44/microm2 and were associated with vesicles at the active zone ("first tier"). Vesicles in this first-tier position represented approximately 7.5% of the total number of vesicles in the terminal and were contiguous, hemifused (approximately 4% of total), or fully fused (approximately 0.5% of total) to the plasma membrane. Our study supports the hypothesis that rat neocortical synapses are constructed of variable numbers of distinct synaptic units that facilitate the docking of vesicles to the active zone and determine the number of vesicles available for immediate release.

Regulated interactions of the norepineprhine transporter by the actin and microtubule cytoskeletons

One role of the actin cytoskeleton is to maintain the structural morphology and activity of the pre-synaptic terminal. We sought to determine if the actin cytoskeleton plays a role in regulating interactions between the norepinephrine transporter (NET) and alpha-Synuclein (alpha-Syn), two proteins expressed in the pre-synaptic terminal. In cells transfected with either 0.5 microg/mL or 3 microg/mL of alpha-Syn and 1 microg/mL of NET DNA, treatment with cytochalasin D, an actin depolymerizing agent, caused a dose-dependent decrease and increase, respectively, in [3H]-NE uptake. Protein interactions between NET, beta-actin, and alpha-Syn were modified, along with levels of surface transporters. Treatment of primary brainstem neurons and frontal cortex synaptosomes with cytochalasin D caused a 115% and 28% increase, respectively, in NET activity. Depolymerization of both actin and microtubules did not alter NET activity in cells with 0.5 microg/mL alpha-Syn, but caused an increase in [3H]-NE uptake in cells transfected with 3 microg/mL of alpha-Syn and primary neurons. This is the first direct demonstration of NET activity being regulated via actin and modulated by interactions with alpha-Syn.

The pool of fast releasing vesicles is augmented by myosin light chain kinase inhibition at the calyx of Held synapse

Synaptic strength is determined by release probability and the size of the readily releasable pool of docked vesicles. Here we describe the effects of blocking myosin light chain kinase (MLCK), a cytoskeletal regulatory protein thought to be involved in myosin-mediated vesicle transport, on synaptic transmission at the mouse calyx of Held synapse. Application of three different MLCK inhibitors increased the amplitude of the early excitatory postsynaptic currents (EPSCs) in a stimulus train, without affecting the late steady-state EPSCs. A presynaptic locus of action for MLCK inhibitors was confirmed by an increase in the frequency of miniature EPSCs that left their average amplitude unchanged. MLCK inhibition did not affect presynaptic Ca(2+) currents or action potential waveform. Moreover, Ca(2+) imaging experiments showed that [Ca(2+)](i) transients elicited by 100-Hz stimulus trains were not altered by MLCK inhibition. Studies using high-frequency stimulus trains indicated that MLCK inhibitors increase vesicle pool size, but do not significantly alter release probability. Accordingly, when AMPA-receptor desensitization was minimized, EPSC paired-pulse ratios were unaltered by MLCK inhibition, suggesting that release probability remains unaltered. MLCK inhibition potentiated EPSCs even when presynaptic Ca(2+) buffering was greatly enhanced by treating slices with EGTA-AM. In addition, MLCK inhibition did not affect the rate of recovery from short-term depression. Finally, developmental studies revealed that EPSC potentiation by MLCK inhibition starts at postnatal day 5 (P5) and remains strong during synaptic maturation up to P18. Overall, our data suggest that MLCK plays a crucial role in determining the size of the pool of synaptic vesicles that undergo fast release at a CNS synapse.

Presynaptic signaling by heterotrimeric G-proteins

G-proteins (guanine nucleotide-binding proteins) are membrane-attached proteins composed of three subunits, alpha, beta, and gamma. They transduce signals from G-protein coupled receptors (GPCRs) to target effector proteins. The agonistactivated receptor induces a conformational change in the G-protein trimer so that the alpha-subunit binds GTP in exchange for GDP and alpha-GTP, and betagamma-subunits separate to interact with the target effector. Effector-interaction is terminated by the alpha-subunit GTPase activity, whereby bound GTP is hydrolyzed to GDP. This is accelerated in situ by RGS proteins, acting as GTPase-activating proteins (GAPs). Galpha-GDP and Gbetagamma then reassociate to form the Galphabetagamma trimer. G-proteins primarily involved in the modulation of neurotransmitter release are G(o), G(q) and G(s). G(o) mediates the widespread presynaptic auto-inhibitory effect of many neurotransmitters (e.g., via M2/M4 muscarinic receptors, alpha(2) adrenoreceptors, micro/delta opioid receptors, GABAB receptors). The G(o) betagamma-subunit acts in two ways: first, and most ubiquitously, by direct binding to CaV2 Ca(2+) channels, resulting in a reduced sensitivity to membrane depolarization and reduced Ca(2+) influx during the terminal action potential; and second, through a direct inhibitory effect on the transmitter release machinery, by binding to proteins of the SNARE complex. G(s) and G(q) are mainly responsible for receptor-mediated facilitatory effects, through activation of target enzymes (adenylate cyclase, AC and phospholipase-C, PLC respectively) by the GTP-bound alpha-subunits.