ADF/Cofilin Controls Synaptic Actin Dynamics and Regulates Synaptic Vesicle Mobilization and Exocytosis

Cyclase-associated protein (CAP) inhibits inverted formin 2 (INF2) to induce dendritic spine maturation

The morphology of dendritic spines, the postsynaptic compartment of most excitatory synapses, decisively modulates the function of neuronal circuits as also evident from human brain disorders associated with altered spine density or morphology. Actin filaments (F-actin) form the backbone of spines, and a number of actin-binding proteins (ABP) have been implicated in shaping the cytoskeleton in mature spines. Instead, only little is known about the mechanisms that control the reorganization from unbranched F-actin of immature spines to the complex, highly branched cytoskeleton of mature spines. Here, we demonstrate impaired spine maturation in hippocampal neurons upon genetic inactivation of cyclase-associated protein 1 (CAP1) and CAP2, but not of CAP1 or CAP2 alone. We found a similar spine maturation defect upon overactivation of inverted formin 2 (INF2), a nucleator of unbranched F-actin with hitherto unknown synaptic function. While INF2 overactivation failed in altering spine density or morphology in CAP-deficient neurons, INF2 inactivation largely rescued their spine defects. From our data we conclude that CAPs inhibit INF2 to induce spine maturation. Since we previously showed that CAPs promote cofilin1-mediated cytoskeletal remodeling in mature spines, we identified them as a molecular switch that control transition from filopodia-like to mature spines.

The actin-binding protein CAP1 represses MRTF-SRF-dependent gene expression in mouse cerebral cortex

Serum response factor (SRF) is an essential transcription factor for brain development and function. Here, we explored how an SRF cofactor, the actin monomer-sensing myocardin-related transcription factor MRTF, is regulated in mouse cortical neurons. We found that MRTF-dependent SRF activity in vitro and in vivo was repressed by cyclase-associated protein CAP1. Inactivation of the actin-binding protein CAP1 reduced the amount of actin monomers in the cytoplasm, which promoted nuclear MRTF translocation and MRTF-SRF activation. This function was independent of cofilin1 and actin-depolymerizing factor, and CAP1 loss of function in cortical neurons was not compensated by endogenous CAP2. Transcriptomic and proteomic analyses of cerebral cortex lysates from wild-type and Cap1 knockout mice supported the role of CAP1 in repressing MRTF-SRF-dependent signaling in vivo. Bioinformatic analysis identified likely MRTF-SRF target genes, which aligned with the transcriptomic and proteomic results. Together with our previous studies that implicated CAP1 in axonal growth cone function as well as the morphology and plasticity of excitatory synapses, our findings establish CAP1 as a crucial actin regulator in the brain relevant for formation of neuronal networks.

Overexpression of the limk1 Gene in Drosophila melanogaster Can Lead to Suppression of Courtship Memory in Males

Courtship suppression is a behavioral adaptation of the fruit fly. When majority of the females in a fly population are fertilized and non-receptive for mating, a male, after a series of failed attempts, decreases its courtship activity towards all females, saving its energy and reproductive resources. The time of courtship decrease depends on both duration of unsuccessful courtship and genetically determined features of the male nervous system. Thereby, courtship suppression paradigm can be used for studying molecular mechanisms of learning and memory. p-Cofilin, a component of the actin remodeling signaling cascade and product of LIM-kinase 1 (LIMK1), regulates Drosophila melanogaster forgetting in olfactory learning paradigm. Previously, we have shown that limk1 suppression in the specific types of nervous cells differently affects fly courtship memory. Here, we used Gal4 > UAS system to induce limk1 overexpression in the same types of neurons. limk1 activation in the mushroom body, glia, and fruitless neurons decreased learning index compared to the control strain or the strain with limk1 knockdown. In cholinergic and dopaminergic/serotoninergic neurons, both overexpression and knockdown of limk1 impaired Drosophila short-term memory. Thus, proper balance of the limk1 activity is crucial for normal cognitive activity of the fruit fly.

Loss of the actin regulator cyclase-associated protein 1 (CAP1) modestly affects dendritic spine remodeling during synaptic plasticity

Dendritic spines form the postsynaptic compartment of most excitatory synapses in the vertebrate brain. Morphological changes of dendritic spines contribute to major forms of synaptic plasticity such as long-term potentiation (LTP) or depression (LTD). Synaptic plasticity underlies learning and memory, and defects in synaptic plasticity contribute to the pathogeneses of human brain disorders. Hence, deciphering the molecules that drive spine remodeling during synaptic plasticity is critical for understanding the neuronal basis of physiological and pathological brain function. Since actin filaments (F-actin) define dendritic spine morphology, actin-binding proteins (ABP) that accelerate dis-/assembly of F-actin moved into the focus as critical regulators of synaptic plasticity. We recently identified cyclase-associated protein 1 (CAP1) as a novel actin regulator in neurons that cooperates with cofilin1, an ABP relevant for synaptic plasticity. We therefore hypothesized a crucial role for CAP1 in structural synaptic plasticity. By exploiting mouse hippocampal neurons, we tested this hypothesis in the present study. We found that induction of both forms of synaptic plasticity oppositely altered concentration of exogenous, myc-tagged CAP1 in dendritic spines, with chemical LTP (cLTP) decreasing and chemical LTD (cLTD) increasing it. cLTP induced spine enlargement in CAP1-deficient neurons. However, it did not increase the density of large spines, different from control neurons. cLTD induced spine retraction and spine size reduction in control neurons, but not in CAP1-KO neurons. Together, we report that postsynaptic myc-CAP1 concentration oppositely changed during cLTP and cTLD and that CAP1 inactivation modestly affected structural plasticity.

Molecular fingerprints in the hippocampus of alcohol seeking during withdrawal

Alcohol use disorder (AUD) is characterized by excessive alcohol seeking and use. Here, we investigated the molecular correlates of impaired extinction of alcohol seeking using a multidimentional mouse model of AUD. We distinguished AUD-prone and AUD-resistant mice, based on the presence of ≥ 2 or < 2 criteria of AUD and utilized RNA sequencing to identify genes that were differentially expressed in the hippocampus and amygdala of mice meeting ≥ 2 or < 2 criteria, as these brain regions are implicated in alcohol motivation, seeking, consumption and the cognitive inflexibility characteristic of AUD. Our findings revealed dysregulation of the genes associated with the actin cytoskeleton, including actin binding molecule cofilin, and impaired synaptic transmission in the hippocampi of mice meeting ≥ 2 criteria. Overexpression of cofilin in the polymorphic layer of the dentate gyrus (PoDG) inhibited ML-DG synapses, increased motivation to seek alcohol and impaired extinction of alcohol seeking, resembling the phenotype observed in mice meeting ≥ 2 criteria. Overall, our study uncovers a novel mechanism linking increased hippocampal cofilin expression with the AUD phenotype.

Regulation of actin filament assembly and disassembly in growth cone motility and axon guidance

Directed outgrowth of axons is fundamental for the establishment of neuronal networks. Axon outgrowth is guided by growth cones, highly motile structures enriched in filamentous actin (F-actin) located at the axons' distal tips. Growth cones exploit F-actin-based protrusions to scan the environment for guidance cues, and they contain the sensory apparatus to translate guidance cue information into intracellular signaling cascades. These cascades act upstream of actin-binding proteins (ABP) and thereby control assembly and disassembly of F-actin. Spatiotemporally controlled F-actin dis-/assembly in growth cones steers the axon towards attractants and away from repellents, and it thereby navigates the axon through the developing nervous system. Hence, ABP that control F-actin dynamics emerged as critical regulators of neuronal network formation. In the present review article, we will summarize and discuss current knowledge of the mechanisms that control remodeling of the actin cytoskeleton in growth cones, focusing on recent progress in the field. Further, we will introduce tools and techniques that allow to study actin regulatory mechanism in growth cones.

Functional interdependence of the actin regulators CAP1 and cofilin1 in control of dendritic spine morphology

The vast majority of excitatory synapses are formed on small dendritic protrusions termed dendritic spines. Dendritic spines vary in size and density that are crucial determinants of excitatory synaptic transmission. Aberrations in spine morphogenesis can compromise brain function and have been associated with neuropsychiatric disorders. Actin filaments (F-actin) are the major structural component of dendritic spines, and therefore, actin-binding proteins (ABP) that control F-actin dis-/assembly moved into the focus as critical regulators of brain function. Studies of the past decade identified the ABP cofilin1 as a key regulator of spine morphology, synaptic transmission, and behavior, and they emphasized the necessity for a tight control of cofilin1 to ensure proper brain function. Here, we report spine enrichment of cyclase-associated protein 1 (CAP1), a conserved multidomain protein with largely unknown physiological functions. Super-resolution microscopy and live cell imaging of CAP1-deficient hippocampal neurons revealed impaired synaptic F-actin organization and dynamics associated with alterations in spine morphology. Mechanistically, we found that CAP1 cooperates with cofilin1 in spines and that its helical folded domain is relevant for this interaction. Moreover, our data proved functional interdependence of CAP1 and cofilin1 in control of spine morphology. In summary, we identified CAP1 as a novel regulator of the postsynaptic actin cytoskeleton that is essential for synaptic cofilin1 activity.

Transient reduction in dendritic spine density in brain-specific profilin1 mutant mice is associated with behavioral deficits

Actin filaments form the backbone of dendritic spines, the postsynaptic compartment of most excitatory synapses in the brain. Spine density changes affect brain function, and postsynaptic actin defects have been implicated in various neuropathies. It is mandatory to identify the actin regulators that control spine density. Based on previous studies, we hypothesized a role for the actin regulator profilin1 in spine formation. We report reduced hippocampal spine density in juvenile profilin1 mutant mice together with impairments in memory formation and reduced ultrasonic communication during active social behavior. Our results, therefore, underline a previously suggested function of profilin1 in controlling spine formation and behavior in juvenile mice.

Disease association of cyclase-associated protein (CAP): Lessons from gene-targeted mice and human genetic studies

Cyclase-associated protein (CAP) is an actin binding protein that has been initially described as partner of the adenylyl cyclase in yeast. In all vertebrates and some invertebrate species, two orthologs, named CAP1 and CAP2, have been described. CAP1 and CAP2 are characterized by a similar multidomain structure, but different expression patterns. Several molecular studies clarified the biological function of the different CAP domains, and they shed light onto the mechanisms underlying CAP-dependent regulation of actin treadmilling. However, CAPs are crucial elements not only for the regulation of actin dynamics, but also for signal transduction pathways. During recent years, human genetic studies and the analysis of gene-targeted mice provided important novel insights into the physiological roles of CAPs and their involvement in the pathogenesis of several diseases. In the present review, we summarize and discuss recent progress in our understanding of CAPs' physiological functions, focusing on heart, skeletal muscle and central nervous system as well as their involvement in the mechanisms controlling metabolism. Remarkably, loss of CAPs or impairment of CAPs-dependent pathways can contribute to the pathogenesis of different diseases. Overall, these studies unraveled CAPs complexity highlighting their capability to orchestrate structural and signaling pathways in the cells.

Cofilin and Actin Dynamics: Multiple Modes of Regulation and Their Impacts in Neuronal Development and Degeneration

Proteins of the actin depolymerizing factor (ADF)/cofilin family are ubiquitous among eukaryotes and are essential regulators of actin dynamics and function. Mammalian neurons express cofilin-1 as the major isoform, but ADF and cofilin-2 are also expressed. All isoforms bind preferentially and cooperatively along ADP-subunits in F-actin, affecting the filament helical rotation, and when either alone or when enhanced by other proteins, promotes filament severing and subunit turnover. Although self-regulating cofilin-mediated actin dynamics can drive motility without post-translational regulation, cells utilize many mechanisms to locally control cofilin, including cooperation/competition with other proteins. Newly identified post-translational modifications function with or are independent from the well-established phosphorylation of serine 3 and provide unexplored avenues for isoform specific regulation. Cofilin modulates actin transport and function in the nucleus as well as actin organization associated with mitochondrial fission and mitophagy. Under neuronal stress conditions, cofilin-saturated F-actin fragments can undergo oxidative cross-linking and bundle together to form cofilin-actin rods. Rods form in abundance within neurons around brain ischemic lesions and can be rapidly induced in neurites of most hippocampal and cortical neurons through energy depletion or glutamate-induced excitotoxicity. In ~20% of rodent hippocampal neurons, rods form more slowly in a receptor-mediated process triggered by factors intimately connected to disease-related dementias, e.g., amyloid-β in Alzheimer’s disease. This rod-inducing pathway requires a cellular prion protein, NADPH oxidase, and G-protein coupled receptors, e.g., CXCR4 and CCR5. Here, we will review many aspects of cofilin regulation and its contribution to synaptic loss and pathology of neurodegenerative diseases.

Cofilin Signaling in the CNS Physiology and Neurodegeneration

All eukaryotic cells are composed of the cytoskeleton, which plays crucial roles in coordinating diverse cellular functions such as cell division, morphology, migration, macromolecular stabilization, and protein trafficking. The cytoskeleton consists of microtubules, intermediate filaments, and actin filaments. Cofilin, an actin-depolymerizing protein, is indispensable for regulating actin dynamics in the central nervous system (CNS) development and function. Cofilin activities are spatiotemporally orchestrated by numerous extra- and intra-cellular factors. Phosphorylation at Ser-3 by kinases attenuate cofilin’s actin-binding activity. In contrast, dephosphorylation at Ser-3 enhances cofilin-induced actin depolymerization. Cofilin functions are also modulated by various binding partners or reactive oxygen species. Although the mechanism of cofilin-mediated actin dynamics has been known for decades, recent research works are unveiling the profound impacts of cofilin dysregulation in neurodegenerative pathophysiology. For instance, oxidative stress-induced increase in cofilin dephosphorylation is linked to the accumulation of tau tangles and amyloid-beta plaques in Alzheimer’s disease. In Parkinson’s disease, cofilin activation by silencing its upstream kinases increases α-synuclein-fibril entry into the cell. This review describes the molecular mechanism of cofilin-mediated actin dynamics and provides an overview of cofilin’s importance in CNS physiology and pathophysiology.

LIM-Kinases in Synaptic Plasticity, Memory, and Brain Diseases

Learning and memory require structural and functional modifications of synaptic connections, and synaptic deficits are believed to underlie many brain disorders. The LIM-domain-containing protein kinases (LIMK1 and LIMK2) are key regulators of the actin cytoskeleton by affecting the actin-binding protein, cofilin. In addition, LIMK1 is implicated in the regulation of gene expression by interacting with the cAMP-response element-binding protein. Accumulating evidence indicates that LIMKs are critically involved in brain function and dysfunction. In this paper, we will review studies on the roles and underlying mechanisms of LIMKs in the regulation of long-term potentiation (LTP) and depression (LTD), the most extensively studied forms of long-lasting synaptic plasticity widely regarded as cellular mechanisms underlying learning and memory. We will also discuss the involvement of LIMKs in the regulation of the dendritic spine, the structural basis of synaptic plasticity, and memory formation. Finally, we will discuss recent progress on investigations of LIMKs in neurological and mental disorders, including Alzheimer’s, Parkinson’s, Williams–Beuren syndrome, schizophrenia, and autism spectrum disorders.

An autoinhibitory clamp of actin assembly constrains and directs synaptic endocytosis

Synaptic membrane-remodeling events such as endocytosis require force-generating actin assembly. The endocytic machinery that regulates these actin and membrane dynamics localizes at high concentrations to large areas of the presynaptic membrane, but actin assembly and productive endocytosis are far more restricted in space and time. Here we describe a mechanism whereby autoinhibition clamps the presynaptic endocytic machinery to limit actin assembly to discrete functional events. We found that collective interactions between the Drosophila endocytic proteins Nwk/FCHSD2, Dap160/intersectin, and WASp relieve Nwk autoinhibition and promote robust membrane-coupled actin assembly in vitro. Using automated particle tracking to quantify synaptic actin dynamics in vivo, we discovered that Nwk-Dap160 interactions constrain spurious assembly of WASp-dependent actin structures. These interactions also promote synaptic endocytosis, suggesting that autoinhibition both clamps and primes the synaptic endocytic machinery, thereby constraining actin assembly to drive productive membrane remodeling in response to physiological cues.

Neurons constantly talk to each other by sending chemical signals across the tiny gap, or ‘synapse’, that separates two cells. While inside the emitting cell, these molecules are safely packaged into small, membrane-bound vessels. Upon the right signal, the vesicles fuse with the external membrane of the neuron and spill their contents outside, for the receiving cell to take up and decode.

The emitting cell must then replenish its vesicle supply at the synapse through a recycling mechanism known as endocytosis. To do so, it uses dynamically assembling rod-like ‘actin’ filaments, which work in concert with many other proteins to pull in patches of membrane as new vesicles. The proteins that control endocytosis and actin assembly abound at neuronal synapses, and, when mutated, are linked to many neurological diseases. Unlike other cell types, neurons appear to ‘pre-deploy’ these actin-assembly proteins to synaptic membranes, but to keep them inactive under normal conditions. How neurons control the way this machinery is recruited and activated remains unknown.

To investigate this question, Del Signore et al. conducted two sets of studies. First, they exposed actin to several different purified proteins in initial ‘test tube’ experiments. This revealed that, depending on the conditions, a group of endocytosis proteins could prevent or promote actin assembly: assembly occurred only if the proteins were associated with membranes. Next, Del Signore et al. mutated these proteins in fruit fly larvae, and performed live cell microscopy to determine their impact on actin assembly and endocytosis.

Consistent with the test tube findings, endocytosis mutants had more actin assembly overall, implying that the proteins were required to prevent random actin assembly. However, the same mutants had reduced levels of endocytosis, suggesting that the proteins were also necessary for productive actin assembly. Together, these experiments suggest that, much like a mousetrap holds itself poised ready to spring, some endocytic proteins play a dual role to restrain actin assembly when and where it is not needed, and to promote it at sites of endocytosis.

These results shed new light on how neurons might build and maintain effective, working synapses. Del Signore et al. hope that this knowledge may help to better understand and combat neurological diseases, such as Alzheimer’s, which are linked to impaired membrane traffic and cell signalling.

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.

Secondary biogenic amine deficiencies: genetic etiology, therapeutic interventions, and clinical effects

Monoamine neurotransmitter disorders present predominantly with neurologic features, including dystonic or dyskinetic cerebral palsy and movement disorders. Genetic conditions that lead to secondary defects in the synthesis, catabolism, transport, and metabolism of biogenic amines can lead to neurotransmitter abnormalities, which can present with similar features. Eleven patients with secondary neurotransmitter abnormalities were enrolled between 2011 and 2015. All patients underwent research-based whole exome and/or whole genome sequencing (WES/WGS). A trial of treatment with levodopa/carbidopa and 5-hydroxytryptophan was initiated. In six families with abnormal neurotransmitter profiles and neurological phenotypes, variants in known disease-causing genes (KCNJ6, SCN2A, CSTB in 2 siblings, NRNX1, KIF1A and PAK3) were identified, while one patient had a variant of uncertain significance in a candidate gene (DLG4) that may explain her phenotype. In 3 patients, no compelling candidate genes were identified. A trial of neurotransmitter replacement therapy led to improvement in motor and behavioral symptoms in all but two patients. The patient with KCNJ6 variant did not respond to L-dopa therapy, but rather experienced increased dyskinetic movements even at low dose of medication. The patient's symptoms harboring the NRNX1 deletion remained unaltered. This study demonstrates the utility of genome-wide sequencing in further understanding the etiology and pathophysiology of neurometabolic conditions, and the potential of secondary neurotransmitter deficiencies to serve as novel therapeutic targets. As there was a largely favorable response to therapy in our case series, a careful trial of neurotransmitter replacement therapy should be considered in patients with cerebrospinal fluid (CSF) monoamines below reference range.

Functional Redundancy of Cyclase-Associated Proteins CAP1 and CAP2 in Differentiating Neurons

Cyclase-associated proteins (CAPs) are evolutionary-conserved actin-binding proteins with crucial functions in regulating actin dynamics, the spatiotemporally controlled assembly and disassembly of actin filaments (F-actin). Mammals possess two family members (CAP1 and CAP2) with different expression patterns. Unlike most other tissues, both CAPs are expressed in the brain and present in hippocampal neurons. We recently reported crucial roles for CAP1 in growth cone function, neuron differentiation, and neuron connectivity in the mouse brain. Instead, CAP2 controls dendritic spine morphology and synaptic plasticity, and its dysregulation contributes to Alzheimer's disease pathology. These findings are in line with a model in which CAP1 controls important aspects during neuron differentiation, while CAP2 is relevant in differentiated neurons. We here report CAP2 expression during neuron differentiation and its enrichment in growth cones. We therefore hypothesized that CAP2 is relevant not only in excitatory synapses, but also in differentiating neurons. However, CAP2 inactivation neither impaired growth cone morphology and motility nor neuron differentiation. Moreover, CAP2 mutant mice did not display any obvious changes in brain anatomy. Hence, differently from CAP1, CAP2 was dispensable for neuron differentiation and brain development. Interestingly, overexpression of CAP2 rescued not only growth cone size in CAP1-deficient neurons, but also their morphology and differentiation. Our data provide evidence for functional redundancy of CAP1 and CAP2 in differentiating neurons, and they suggest compensatory mechanisms in single mutant neurons.

Extracellular Vesicle Proteome of Breast Cancer Patients with and Without Cognitive Impairment Following Anthracycline-based Chemotherapy: An Exploratory Study

Cognitive impairment due to cancer and its therapy is a major concern among cancer patients and survivors. Extracellular vesicle (EVs) composition altered by cancer and chemotherapy may affect neurological processes such as neuroplasticity, potentially impacting the cognitive abilities of cancer patients and survivors. We investigated the EV proteome of breast cancer patients with and without cognitive impairment following anthracycline-based chemotherapy from longitudinally collected plasma. EVs were cup-shaped and positive for Flotillin-1 and TSG-101. We identified 517 differentially expressed EV proteins between the cognitive impaired and non-impaired groups during and post-chemotherapy. The observed decreased expression of p2X purinoceptor, cofilin-1, ADAM 10, and dynamin-1 in the plasma EVs of the cognitive impaired group may suggest alterations in the mechanisms underlying synaptic plasticity. The reduced expression of tight junction proteins among cognitive-impaired patients may imply weakening of the blood-brain barrier. These EV protein signatures may serve as a fingerprint that underscores the mechanisms underlying cognitive impairment in cancer patients and survivors.

Neuron Replating, a Powerful and Versatile Approach to Study Early Aspects of Neuron Differentiation

Neuron differentiation includes formation and outgrowth of neurites that differentiate into axons or dendrites. Directed neurite outgrowth is controlled by growth cones that protrude and retract actin-rich structures to sense environmental cues. These cues control local actin filament dynamics, steer growth cones toward attractants and away from repellents, and navigate neurites through the developing brain. Rodent hippocampal neurons are widely used to study the mechanisms underlying neuron differentiation. Genetic manipulation of isolated neurons including gene inactivation or reporter gene expression can be achieved by classical transfections methods, but these methods are restricted to neurons cultured for several days, after neurite formation or outgrowth. Instead, electroporation allows gene manipulation before seeding. However, reporter gene expression usually takes up to 24 h, and time course of gene inactivation depends on the half live of the targeted mRNA and gene product. Hence, these methods do not allow to study early aspects of neuron differentiation. In the present study, we provide a detailed protocol in which we combined electroporation-based gene manipulation of mouse hippocampal neurons before initial seeding with a replating step after 2 d in vitro (DIV) that resets neurons into an undifferentiated stage. By categorizing neurons according to their differentiation stage, thorough morphometric analyses, live imaging of actin dynamics in growth cones as well as guidance cue-mediated growth cone morphologic changes, we demonstrate that differentiation and function of replated neurons did not differ from non-replated neurons. In summary, we provide a protocol that allows to thoroughly characterize differentiation of mouse primary hippocampal neurons.

Chemical Stimulation of Rodent and Human Cortical Synaptosomes: Implications in Neurodegeneration

Synaptic plasticity events, including long-term potentiation (LTP), are often regarded as correlates of brain functions of memory and cognition. One of the central players in these plasticity-related phenomena is the α-amino-3-hydroxy-5-methylisoxazole-4-propionate receptor (AMPAR). Increased levels of AMPARs on postsynaptic membranes thus constitute a biochemical measure of LTP. Isolated synaptic terminals (synaptosomes) are an excellent ex vivo tool to monitor synaptic physiology in healthy and diseased brains, particularly in human research. We herein describe three protocols for chemically-induced LTP (cLTP) in synaptosomes from both rodent and human brain tissues. Two of these chemical stimulation protocols are described for the first time in synaptosomes. A pharmacological block of synaptosomal actin dynamics confirmed the efficiency of the cLTP protocols. Furthermore, the study prototypically evaluated the deficiency of cLTP in cortical synaptosomes obtained from human cases of early-onset Alzheimer’s disease (EOAD) and frontotemporal lobar degeneration (FLTD), as well as an animal model that mimics FLTD.

Central anorexigenic actions of bile acids are mediated by TGR5

Bile acids (BAs) are signalling molecules that mediate various cellular responses in both physiological and pathological processes. Several studies report that BAs can be detected in the brain1, yet their physiological role in the central nervous system is still largely unknown. Here we show that postprandial BAs can reach the brain and activate a negative-feedback loop controlling satiety in response to physiological feeding via TGR5, a G-protein-coupled receptor activated by multiple conjugated and unconjugated BAs2 and an established regulator of peripheral metabolism3-8. Notably, peripheral or central administration of a BA mix or a TGR5-specific BA mimetic (INT-777) exerted an anorexigenic effect in wild-type mice, while whole-body, neuron-specific or agouti-related peptide neuronal TGR5 deletion caused a significant increase in food intake. Accordingly, orexigenic peptide expression and secretion were reduced after short-term TGR5 activation. In vitro studies demonstrated that activation of the Rho-ROCK-actin-remodelling pathway decreases orexigenic agouti-related peptide/neuropeptide Y (AgRP/NPY) release in a TGR5-dependent manner. Taken together, these data identify a signalling cascade by which BAs exert acute effects at the transition between fasting and feeding and prime the switch towards satiety, unveiling a previously unrecognized role of physiological feedback mediated by BAs in the central nervous system.

Mutual functional dependence of cyclase-associated protein 1 (CAP1) and cofilin1 in neuronal actin dynamics and growth cone function

Neuron connectivity depends on growth cones that navigate axons through the developing brain. Growth cones protrude and retract actin-rich structures to sense guidance cues. These cues control local actin dynamics and steer growth cones towards attractants and away from repellents, thereby directing axon outgrowth. Hence, actin binding proteins (ABPs) moved into the focus as critical regulators of neuron connectivity. We found cyclase-associated protein 1 (CAP1), an ABP with unknown brain function, abundant in growth cones. Super-resolution microscopy and live cell imaging combined with pharmacological approaches on hippocampal neurons from gene-targeted mice revealed a crucial role for CAP1 in actin dynamics that is critical for growth cone morphology and function. Growth cone defects in CAP1 knockout (KO) neurons compromised neuron differentiation and was associated with impaired neuron connectivity in CAP1-KO brains. Mechanistically, by rescue experiments in double KO neurons lacking CAP1 and the key actin regulator cofilin1, we demonstrated that CAP1 was essential for cofilin1 function in growth cone actin dynamics and morphology and vice versa. Together, we identified CAP1 as a novel actin regulator in growth cones that was relevant for neuron connectivity, and we demonstrated functional interdependence of CAP1 and cofilin1 in neuronal actin dynamics and growth cone function.

LIM kinase inhibitor T56-LIMKi protects mouse brain from photothrombotic stroke

Primary Objective: In an ischemic stroke, the damage spreads from the infarction core to surrounding tissues. The present work was aimed at the search of effective neuroprotectors that restrict injury propagation. Research Design: We studied possible protective effects of inhibitors of protein kinases LIMK2 (T56-LIMKi), DYRK1A (harmine), and tryptophan hydroxylase (4-chlorophenylalanine) on infarction size and morphology of peri-infarct area after photothrombotic stroke (a model of ischemic stroke) in mouse brain. Methods and Procedures: Photothrombotic stroke was induced by laser irradiation of mouse cortex after administration of photosensitizer Bengal Rose, which does not penetrate cells and remains in blood vessels. Under light exposure, it induces vessel occlusion. Infarct volume and histological changes in the cerebral cortex were evaluated 3, 7 and 14 days after photothrombotic impact. Main Outcomes and Results: Harmine and 4-chlorophenylalanine did not influence infarct volume and morphology of peri-infarct area in the mouse brain cortex after photothrombotic stroke. However, LIMK2 inhibitor T56-LIMKi significantly reduced infarct volume 7 and 14 days after photothrombotic stroke. It also increased the percent of normochromic neurons and decreased the fraction of altered cortical cells (hypochromic, hyperchromic and pyknotic neurons). Conclusions: T56-LIMK2i may be considered as a promising anti-stroke agent.

Role of Cofilin in Alzheimer's Disease

Alzheimer's disease (AD) is a degenerative neurological disease and has an inconspicuous onset and progressive development. Clinically, it is characterized by severe dementia manifestations, including memory impairment, aphasia, apraxia, loss of recognition, impairment of visual-spatial skills, executive dysfunction, and changes in personality and behavior. Its etiology is unknown to date. However, several cellular biological signatures of AD have been identified such as synaptic dysfunction, β-amyloid plaques, hyperphosphorylated tau, cofilin-actin rods, and Hirano bodies which are related to the actin cytoskeleton. Cofilin is one of the most affluent and common actin-binding proteins and plays a role in cell motility, migration, shape, and metabolism. They also play an important role in severing actin filament, nucleating, depolymerizing, and bundling activities. In this review, we summarize the structure of cofilins and their functional and regulating roles, focusing on the synaptic dysfunction, β-amyloid plaques, hyperphosphorylated tau, cofilin-actin rods, and Hirano bodies of AD.

The Role of ADF/Cofilin in Synaptic Physiology and Alzheimer's Disease

Actin-depolymerization factor (ADF)/cofilin, a family of actin-binding proteins, are critical for the regulation of actin reorganization in response to various signals. Accumulating evidence indicates that ADF/cofilin also play important roles in neuronal structure and function, including long-term potentiation and depression. These are the most extensively studied forms of long-lasting synaptic plasticity and are widely regarded as cellular mechanisms underlying learning and memory. ADF/cofilin regulate synaptic function through their effects on dendritic spines and the trafficking of glutamate receptors, the principal mediator of excitatory synaptic transmission in vertebrates. Regulation of ADF/cofilin involves various signaling pathways converging on LIM domain kinases and slingshot phosphatases, which phosphorylate/inactivate and dephosphorylate/activate ADF/cofilin, respectively. Actin-depolymerization factor/cofilin activity is also regulated by other actin-binding proteins, activity-dependent subcellular distribution and protein translation. Abnormalities in ADF/cofilin have been associated with several neurodegenerative disorders such as Alzheimer’s disease. Therefore, investigating the roles of ADF/cofilin in the brain is not only important for understanding the fundamental processes governing neuronal structure and function, but also may provide potential therapeutic strategies to treat brain disorders.

Proline-rich transmembrane protein 2 (PRRT2) regulates the actin cytoskeleton during synaptogenesis

Mutations in proline-rich transmembrane protein 2 (PRRT2) have been recently identified as the leading cause of a clinically heterogeneous group of neurological disorders sharing a paroxysmal nature, including paroxysmal kinesigenic dyskinesia and benign familial infantile seizures. To date, studies aimed at understanding its physiological functions in neurons have mainly focused on its ability to regulate neurotransmitter release and neuronal excitability. Here, we show that PRRT2 expression in non-neuronal cell lines inhibits cell motility and focal adhesion turnover, increases cell aggregation propensity, and promotes the protrusion of filopodia, all processes impinging on the actin cytoskeleton. In primary hippocampal neurons, PRRT2 silencing affects the synaptic content of filamentous actin and perturbs actin dynamics. This is accompanied by defects in the density and maturation of dendritic spines. We identified cofilin, an actin-binding protein abundantly expressed at the synaptic level, as the ultimate effector of PRRT2. Indeed, PRRT2 silencing unbalances cofilin activity leading to the formation of cofilin-actin rods along neurites. The expression of a cofilin phospho-mimetic mutant (cof-S3E) is able to rescue PRRT2-dependent defects in synapse density, spine number and morphology, but not the alterations observed in neurotransmitter release. Our data support a novel function of PRRT2 in the regulation of the synaptic actin cytoskeleton and in the formation of synaptic contacts.

CAPt'n of Actin Dynamics: Recent Advances in the Molecular, Developmental and Physiological Functions of Cyclase-Associated Protein (CAP)

Cyclase-associated protein (CAP) has been discovered three decades ago in budding yeast as a protein that associates with the cyclic adenosine monophosphate (cAMP)-producing adenylyl cyclase and that suppresses a hyperactive RAS2 variant. Since that time, CAP has been identified in all eukaryotic species examined and it became evident that the activity in RAS-cAMP signaling is restricted to a limited number of species. Instead, its actin binding activity is conserved among eukaryotes and actin cytoskeleton regulation emerged as its primary function. However, for many years, the molecular functions as well as the developmental and physiological relevance of CAP remained unknown. In the present article, we will compile important recent progress on its molecular functions that identified CAP as a novel key regulator of actin dynamics, i.e., the spatiotemporally controlled assembly and disassembly of actin filaments (F-actin). These studies unraveled a cooperation with ADF/Cofilin and Twinfilin in F-actin disassembly, a nucleotide exchange activity on globular actin monomers (G-actin) that is required for F-actin assembly and an inhibitory function towards the F-actin assembly factor INF2. Moreover, by focusing on selected model organisms, we will review current literature on its developmental and physiological functions, and we will present studies implicating CAP in human pathologies. Together, this review article summarizes and discusses recent achievements in understanding the molecular, developmental and physiological functions of CAP, which led this protein emerge as a novel CAPt'n of actin dynamics.

Actin(g) on mitochondria - a role for cofilin1 in neuronal cell death pathways

Actin dynamics, the coordinated assembly and disassembly of actin filaments (F-actin), are essential for fundamental cellular processes, including cell shaping and motility, cell division or organelle transport. Recent studies highlighted a novel role for actin dynamics in the regulation of mitochondrial morphology and function, for example, through mitochondrial recruitment of dynamin-related protein 1 (Drp1), a key factor in the mitochondrial fission machinery. Mitochondria are dynamic organelles, and permanent fission and fusion is essential to maintain their function in energy metabolism, calcium homeostasis and regulation of reactive oxygen species (ROS). Here, we summarize recent insights into the emerging role of cofilin1, a key regulator of actin dynamics, for mitochondrial shape and function under physiological conditions and during cellular stress, respectively. This is of peculiar importance in neurons, which are particularly prone to changes in actin regulation and mitochondrial integrity and function. In neurons, cofilin1 may contribute to degenerative processes through formation of cofilin-actin rods, and through enhanced mitochondrial fission, mitochondrial membrane permeabilization, and the release of cytochrome c. Overall, mitochondrial impairment induced by dysfunction of actin-regulating proteins such as cofilin1 emerge as important mechanisms of neuronal death with relevance to acute brain injury and neurodegenerative diseases, such as Parkinson's or Alzheimer's disease.

Profilin1-Dependent F-Actin Assembly Controls Division of Apical Radial Glia and Neocortex Development

Neocortex development depends on neural stem cell proliferation, cell differentiation, neurogenesis, and neuronal migration. Cytoskeletal regulation is critical for all these processes, but the underlying mechanisms are only poorly understood. We previously implicated the cytoskeletal regulator profilin1 in cerebellar granule neuron migration. Since we found profilin1 expressed throughout mouse neocortex development, we here tested the hypothesis that profilin1 is crucial for neocortex development. We found no evidence for impaired neuron migration or layering in the neocortex of profilin1 mutant mice. However, proliferative activity at basal positions was doubled in the mutant neocortex during mid-neurogenesis, with a drastic and specific increase in basal Pax6+ cells indicative for elevated numbers of basal radial glia (bRG). This was accompanied by transiently increased neurogenesis and associated with mild invaginations resembling rudimentary neocortex folds. Our data are in line with a model in which profilin1-dependent actin assembly controls division of apical radial glia (aRG) and thereby the fate of their progenies. Via this mechanism, profilin1 restricts cell delamination from the ventricular surface and, hence, bRG production and thereby controls neocortex development in mice. Our data support the radial cone hypothesis” claiming that elevated bRG number causes neocortex folds.

RASSF1A, puppeteer of cellular homeostasis, fights tumorigenesis, and metastasis-an updated review

The Ras association domain family protein1 isoform A (RASSF1A) is a well-known tumor-suppressor protein frequently inactivated in various human cancers. Consistent with its function as a molecular scaffold protein, referred to in many studies, RASSF1A prevents initiation of tumorigenesis, growth, and dissemination through different biological functions, including cell cycle arrest, migration/metastasis inhibition, microtubular stabilization, and apoptosis promotion. As a regulator of key cancer pathways, namely Ras/Rho GTPases and Hippo signaling without ignoring strong interaction with microtubules, RASSF1A is indeed one of the guardians of cell homeostasis. To date, as we approach the two decade anniversary of RASSF1A's discovery, this review will summarize our current knowledge on the RASSF1A key interactions as a tumor suppressor and discuss their impact on cell fate during carcinogenesis. This could facilitate a deeper understanding of tumor development and provide us with new strategies in cancer treatment by targeting the RASSF1A pathway.

Robo2 regulates synaptic oxytocin content by affecting actin dynamics

The regulation of neuropeptide level at the site of release is essential for proper neurophysiological functions. We focused on a prominent neuropeptide, oxytocin (OXT) in the zebrafish as an in vivo model to visualize and quantify OXT content at the resolution of a single synapse. We found that OXT-loaded synapses were enriched with polymerized actin. Perturbation of actin filaments by either cytochalasin-D or conditional Cofilin expression resulted in decreased synaptic OXT levels. Genetic loss of robo2 or slit3 displayed decreased synaptic OXT content and robo2 mutants displayed reduced mobility of the actin probe Lifeact-EGFP in OXT synapses. Using a novel transgenic reporter allowing real-time monitoring of OXT-loaded vesicles, we show that robo2 mutants display slower rate of vesicles accumulation. OXT-specific expression of dominant-negative Cdc42, which is a key regulator of actin dynamics and a downstream effector of Robo2, led to a dose-dependent increase in OXT content in WT, and a dampened effect in robo2 mutants. Our results link Slit3-Robo2-Cdc42, which controls local actin dynamics, with the maintenance of synaptic neuropeptide levels.

Merlin modulates process outgrowth and synaptogenesis in the cerebellum

Neurofibromatosis type 2 (NF2) patients are prone to develop glial-derived tumors in the peripheral and central nervous system (CNS). The Nf2 gene product -Merlin is not only expressed in glia, but also in neurons of the CNS, where its function still remains elusive. Here, we show that cerebellar Purkinje cells (PCs) of isoform-specific Merlin-deficient mice were innervated by smaller vGluT2-positive clusters at presynaptic terminals than those of wild-type mice. This was paralleled by a reduction in frequency and amplitude of miniature excitatory postsynaptic currents (mEPSC). On the contrary, in conditional transgenic mice in which Merlin expression was specifically ablated in PCs (L7Cre;Nf2^fl/fl), we found enlarged vGluT2-positive clusters in their presynaptic buttons together with increased amplitudes of miniature postsynaptic currents. The presynaptic terminals of these PCs innervating neurons of the deep cerebellar nuclei were also enlarged. When exploring mice with Merlin-deficient granule cells (GCs) (Math1Cre;Nf2^fl/fl), we found cerebellar extracts to contain higher amounts of vGluT1 present in parallel fiber terminals. In parallel, mEPSC frequency was increased in Math1Cre;Nf2^fl/fl mice. On the contrary, VGluT2 clusters in cerebellar glomeruli composed of NF2-deficient presynaptic Mossy fiber terminals and NF2-deficient postsynaptic GC were reduced in size as shown for isoform-specific knockout mice. These changes in Math1Cre;Nf2^fl/fl-deficient mice were paralleled by an increased activation of Rac1-Cofilin signaling which is known to impact on cytoskeletal reorganization and synapse formation. Consistent with the observed synaptic alterations in these transgenic mice, we observed altered ultrasonic vocalization, which is known to rely on proper cerebellar function. No gross morphological changes or motor coordination deficits were observed in any of these transgenic mice. We therefore conclude that Merlin does not regulate overall cerebellar development, but impacts on pre- and post-synaptic terminal organization.

Photoconversion of FM1-43 Reveals Differences in Synaptic Vesicle Recycling and Sensitivity to Pharmacological Disruption of Actin Dynamics in Individual Synapses

The cycling of synaptic vesicles ensures that neurons can communicate adequately through their synapses on repeated occasions when activity is sustained, and several steps in this cycle are modulated by actin. The effects of pharmacological stabilization of actin with jasplakinolide or its depolymerization with latrunculin A was assessed on the synaptic vesicle cycle at individual boutons of cerebellar granule cells, using FM1-43 imaging to track vesicle recycling and its photoconversion to specifically label recycled organelles. Remarkable differences in the recycling capacity of individual boutons are evident, and their dependence on the actin cytoskeleton for recycling is clear. Disrupting actin dynamics causes a loss of functional boutons, and while this indicates that exo/endocytotic cycling in boutons is fully dependent on such events, this dependence is only partial in other boutons. Indeed, exocytosis and vesicle trafficking are impaired significantly by stabilizing or depolymerizing actin, whereas repositioning recycled vesicles at the active zone seems to be dependent on actin polymerization alone. These findings support the hypothesis that different steps of synaptic vesicle cycling depend on actin dynamics and that such dependence varies among individual boutons.

A role for RASSF1A in tunneling nanotube formation between cells through GEFH1/Rab11 pathway control

BACKGROUND

By allowing intercellular communication between cells, tunneling nanotubes (TNTs) could play critical role in cancer progression. If TNT formation is known to require cytoskeleton remodeling, key mechanism controlling their formation remains poorly understood.

METHODS

The cells of human bronchial (HBEC-3, A549) or mesothelial (H2452, H28) lines are transfected with different siRNAs (inactive, anti-RASSF1A, anti-GEFH1 and / or anti-Rab11). At 48 h post-transfection, i) the number and length of the nanotubes per cell are quantified, ii) the organelles, previously labeled with specific tracers, exchanged via these structures are monitored in real time between cells cultured in 2D or 3D and in normoxia, hypoxia or in serum deprivation condition.

RESULTS

We report that RASSF1A, a key-regulator of cytoskeleton encoded by a tumor-suppressor gene on 3p chromosome, is involved in TNTs formation in bronchial and pleural cells since controlling proper activity of RhoB guanine nucleotide exchange factor, GEF-H1. Indeed, the GEF-H1 inactivation induced by RASSF1A silencing, leads to Rab11 accumulation and subsequent exosome releasing, which in turn contribute to TNTs formation. Finally, we provide evidence involving TNT formation in bronchial carcinogenesis, by reporting that hypoxia or nutriment privation, two almost universal conditions in human cancers, fail to prevent TNTs induced by the oncogenic RASSF1A loss of expression.

CONCLUSIONS

This finding suggests for the first time that loss of RASSF1A expression could be a potential biomarker for TNTs formation, such TNTs facilitating intercellular communication favoring multistep progression of bronchial epithelial cells toward overt malignancy.

Forecast of actin-binding proteins as the oncotarget in osteosarcoma - a review of mechanism, diagnosis and therapy

Osteosarcoma (OS) is the most common bone malignant tumor with a high rate of lung metastasis and principally emerges in children and adolescents. Although neoadjuvant chemotherapy is widely used around the world, a high rate of chemoresistance occurs and frequently generates a poor prognosis. Therefore, finding a new appropriate prognostic marker for OS is a valuable research direction, which will give patients a better chance to receive proper therapy. Actin-binding proteins (ABPs) are a group of proteins that interact with actin cytoskeleton and play a crucial role in the regulation of the cell motility and morphology in eukaryotes. Meanwhile, ABPs also act as a bridge between the cytomembrane and nucleus, which transmit the outside-in and inside-out signals in cytoplasm. Furthermore, ABPs alter the dynamic structure of actin and regulate the invasion and metastasis of cancer. Hence, ABPs have a wide application in predicting the prognosis, and may be new targets, in tumor therapy. This review focuses on a series of ABPs and discusses their modulatory functions. It provides a new insight into the classification of ABPs’ functions in the process of invasion and metastasis in OS and illuminates the potential ability in predicting the prognosis of OS patients.

Impaired Object Recognition but Normal Social Behavior and Ultrasonic Communication in Cofilin1 Mutant Mice

Autism spectrum disorder (ASD), schizophrenia (SCZ) and intellectual disability (ID) show a remarkable overlap in symptoms, including impairments in cognition, social behavior and communication. Human genetic studies revealed an enrichment of mutations in actin-related genes for these disorders, and some of the strongest candidate genes control actin dynamics. These findings led to the hypotheses: (i) that ASD, SCZ and ID share common disease mechanisms; and (ii) that, at least in a subgroup of affected individuals, defects in the actin cytoskeleton cause or contribute to their pathologies. Cofilin1 emerged as a key regulator of actin dynamics and we previously demonstrated its critical role for synaptic plasticity and associative learning. Notably, recent studies revealed an over-activation of cofilin1 in mutant mice displaying ASD- or SCZ-like behavioral phenotypes, suggesting that dysregulated cofilin1-dependent actin dynamics contribute to their behavioral abnormalities, such as deficits in social behavior. These findings let us hypothesize: (i) that, apart from cognitive impairments, cofilin1 mutants display additional behavioral deficits with relevance to ASD or SCZ; and (ii) that our cofilin1 mutants represent a valuable tool to study the underlying disease mechanisms. To test our hypotheses, we compared social behavior and ultrasonic communication of juvenile mutants to control littermates, and we did not obtain evidence for impaired direct reciprocal social interaction, social approach or social memory. Moreover, concomitant emission of ultrasonic vocalizations was not affected and time-locked to social activity, supporting the notion that ultrasonic vocalizations serve a pro-social communicative function as social contact calls maintaining social proximity. Finally, cofilin1 mutants did not display abnormal repetitive behaviors. Instead, they performed weaker in novel object recognition, thereby demonstrating that cofilin1 is relevant not only for associative learning, but also for “non-matching-to-sample” learning. Here we report the absence of an ASD- or a SCZ-like phenotype in cofilin1 mutants, and we conclude that cofilin1 is relevant specifically for non-social cognition.

Cofilin1-dependent actin dynamics control DRP1-mediated mitochondrial fission

Mitochondria form highly dynamic networks in which organelles constantly fuse and divide. The relevance of mitochondrial dynamics is evident from its implication in various human pathologies, including cancer or neurodegenerative, endocrine and cardiovascular diseases. Dynamin-related protein 1 (DRP1) is a key regulator of mitochondrial fission that oligomerizes at the mitochondrial outer membrane and hydrolyzes GTP to drive mitochondrial fragmentation. Previous studies demonstrated that DRP1 recruitment and mitochondrial fission is promoted by actin polymerization at the mitochondrial surface, controlled by the actin regulatory proteins inverted formin 2 (INF2) and Spire1C. These studies suggested the requirement of additional actin regulatory activities to control DRP1-mediated mitochondrial fission. Here we show that the actin-depolymerizing protein cofilin1, but not its close homolog actin-depolymerizing factor (ADF), is required to maintain mitochondrial morphology. Deletion of cofilin1 caused mitochondrial DRP1 accumulation and fragmentation, without altering mitochondrial function or other organelles’ morphology. Mitochondrial morphology in cofilin1-deficient cells was restored upon (i) re-expression of wild-type cofilin1 or a constitutively active mutant, but not of an actin-binding-deficient mutant, (ii) pharmacological destabilization of actin filaments and (iii) genetic depletion of DRP1. Our work unraveled a novel function for cofilin1-dependent actin dynamics in mitochondrial fission, and identified cofilin1 as a negative regulator of mitochondrial DRP1 activity. We conclude that cofilin1 is required for local actin dynamics at mitochondria, where it may balance INF2/Spire1C-induced actin polymerization.

The neuronal protein Neurexin directly interacts with the Scribble-Pix complex to stimulate F-actin assembly for synaptic vesicle clustering

Synaptic vesicles (SVs) form distinct pools at synaptic terminals, and this well-regulated separation is necessary for normal neurotransmission. However, how the SV cluster, in particular synaptic compartments, maintains normal neurotransmitter release remains a mystery. The presynaptic protein Neurexin (NRX) plays a significant role in synaptic architecture and function, and some evidence suggests that NRX is associated with neurological disorders, including autism spectrum disorders. However, the role of NRX in SV clustering is unclear. Here, using the neuromuscular junction at the 2-3 instar stages of Drosophila larvae as a model and biochemical imaging and electrophysiology techniques, we demonstrate that Drosophila NRX (DNRX) plays critical roles in regulating synaptic terminal clustering and release of SVs. We found that DNRX controls the terminal clustering and release of SVs by stimulating presynaptic F-actin. Furthermore, our results indicate that DNRX functions through the scaffold protein Scribble and the GEF protein DPix to activate the small GTPase Ras-related C3 Botulinum toxin substrate 1 (Rac1). We observed a direct interaction between the C-terminal PDZ-binding motif of DNRX and the PDZ domains of Scribble and that Scribble bridges DNRX to DPix, forming a DNRX-Scribble-DPix complex that activates Rac1 and subsequently stimulates presynaptic F-actin assembly and SV clustering. Taken together, our work provides important insights into the function of DNRX in regulating SV clustering, which could help inform further research into pathological neurexin-mediated mechanisms in neurological disorders such as autism.

Membrane shaping by actin and myosin during regulated exocytosis

The cortical actin network in neurosecretory cells is a dense mesh of actin filaments underlying the plasma membrane. Interaction of actomyosin with vesicular membranes or the plasma membrane is vital for tethering, retention, transport as well as fusion and fission of exo- and endocytic membrane structures. During regulated exocytosis the cortical actin network undergoes dramatic changes in morphology to accommodate vesicle docking, fusion and replenishment. Most of these processes involve plasma membrane Phosphoinositides (PIP) and investigating the interactions between the actin cortex and secretory structures has become a hotbed for research in recent years. Actin remodelling leads to filopodia outgrowth and the creation of new fusion sites in neurosecretory cells and actin, myosin and dynamin actively shape and maintain the fusion pore of secretory vesicles. Changes in viscoelastic properties of the actin cortex can facilitate vesicular transport and lead to docking and priming of vesicle at the plasma membrane. Small GTPase actin mediators control the state of the cortical actin network and influence vesicular access to their docking and fusion sites. These changes potentially affect membrane properties such as tension and fluidity as well as the mobility of embedded proteins and could influence the processes leading to both exo- and endocytosis. Here we discuss the multitudes of actin and membrane interactions that control successive steps underpinning regulated exocytosis.

Mutation of neuron-specific chromatin remodeling subunit BAF53b: rescue of plasticity and memory by manipulating actin remodeling

Recent human exome-sequencing studies have implicated polymorphic Brg1-associated factor (BAF) complexes (mammalian SWI/SNF chromatin remodeling complexes) in several intellectual disabilities and cognitive disorders, including autism. However, it remains unclear how mutations in BAF complexes result in impaired cognitive function. Post-mitotic neurons express a neuron-specific assembly, nBAF, characterized by the neuron-specific subunit BAF53b. Subdomain 2 of BAF53b is essential for the differentiation of neuronal precursor cells into neurons. We generated transgenic mice lacking subdomain 2 of Baf53b (BAF53bΔSB2). Long-term synaptic potentiation (LTP) and long-term memory, both of which are associated with phosphorylation of the actin severing protein cofilin, were assessed in these animals. A phosphorylation mimic of cofilin was stereotaxically delivered into the hippocampus of BAF53bΔSB2 mice in an effort to rescue LTP and memory. BAF53bΔSB2 mutant mice show impairments in phosphorylation of synaptic cofilin, LTP, and memory. Both the synaptic plasticity and memory deficits are rescued by overexpression of a phosphorylation mimetic of cofilin. Baseline physiology and behavior were not affected by the mutation or the experimental treatment. This study suggests a potential link between nBAF function, actin cytoskeletal remodeling at the dendritic spine, and memory formation. This work shows that a targeted manipulation of synaptic function can rescue adult plasticity and memory deficits caused by manipulations of nBAF, and thereby provides potential novel avenues for therapeutic development for multiple intellectual disability disorders.

Cyclin Y-mediated transcript profiling reveals several important functional pathways regulated by Cyclin Y in hippocampal neurons

Cyclin Y (CCNY), which is a cyclin protein known to play a role in cell division, is unexpectedly and thus interestingly expressed in non-proliferating neuronal cells. There have been only a few studies reporting the neuronal functions of CCNY in synapse remodeling and hippocampal long-term potentiation. Therefore, we here provide global and comprehensive information on the putative functions of CCNY in biological and functional pathways in neuronal systems. We adopted high-throughput RNA-sequencing technology for analyzing transcriptomes regulated by CCNY and utilized bioinformatics for identifying putative molecules, biological processes, and functional pathways that are possibly connected to CCNY functions in hippocampal neuronal cells of rats. We revealed that several enriched annotation terms and pathways associated with CCNY expression within neurons, including apoptosis, learning or memory, synaptic plasticity, actin cytoskeleton, focal adhesion, extracellular matrix-receptor interaction and chemokine signaling pathway are targeted by CCNY. In addition, the mRNA levels of some genes enriched for those annotation terms and pathways or genes reported to be altered in Alzheimer's disease mouse model were further validated by quantitative real-time PCR in hippocampal neuronal cells. The present study provides an excellent resource for future investigations of CCNY functions in neuronal systems.

Involvement of caveolin-1 in low shear stress-induced breast cancer cell motility and adhesion: Roles of FAK/Src and ROCK/p-MLC pathways

Tumor cells translocating to distant sites are subjected to hemodynamic shear forces during their passage in the blood vessels. Low shear stress (LSS) plays a critical role in the regulation of various aspects of tumor cells functions, including motility and adhesion. Beyond its structural role, caveolin-1 (Cav-1), the important component of caveolae, represents a modulator of several cancer-associated functions as tumor progression and metastasis. However, the role of Cav-1 in regulating tumor cells response to shear stress remains poorly explored. Here, we characterized the role of LSS and Cav-1 in mediating cell motility and adhesion on human breast carcinoma MDA-MB-231 cells. We first showed that LSS exposure promoted cell polarity and focal adhesion (FA) dynamics, thus indicating elevated cell migration. Silencing of Cav-1 leaded to a significantly lower formation of stress fibers. However, LSS exposure was able to rescue it via the alteration of actin-associated proteins expression, including ROCK, p-MLC, cofilin and filamin A. Time-lapse migration assay indicated that Cav-1 expression fostered MDA-MB-231 cells motility and LSS triggered cells to rapidly generate new lamellipodia. Furthermore, Cav-1 and LSS significantly influenced cell adhesion. Taken together, our findings provide insights into mechanisms underlying LSS triggered events mediated by downstream Cav-1, including FAK/Src and ROCK/p-MLC pathways, involved in the reorganization of the cytoskeleton, cell motility, FA dynamics and breast cancer cell adhesion.

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.

Tropomyosins in the healthy and diseased nervous system

Regulation of the actin cytoskeleton is dependent on a plethora of actin-associated proteins in all eukaryotic cells. The family of tropomyosins plays a key role in controlling the function of several of these actin-associated proteins and their access to actin filaments. In order to understand the regulation of the actin cytoskeleton in highly dynamic subcellular compartments of neurons such as growth cones of developing neurons and the synaptic compartment of mature neurons, it is pivotal to decipher the functional role of tropomyosins in the nervous system. In this review, we will discuss the current understanding and recent findings on the regulation of the actin cytoskeleton by tropomyosins and potential implication that this has for the dysregulation of the actin cytoskeleton in neurological diseases.

Structural and Genetic Studies Demonstrate Neurologic Dysfunction in Triosephosphate Isomerase Deficiency Is Associated with Impaired Synaptic Vesicle Dynamics

Triosephosphate isomerase (TPI) deficiency is a poorly understood disease characterized by hemolytic anemia, cardiomyopathy, neurologic dysfunction, and early death. TPI deficiency is one of a group of diseases known as glycolytic enzymopathies, but is unique for its severe patient neuropathology and early mortality. The disease is caused by missense mutations and dysfunction in the glycolytic enzyme, TPI. Previous studies have detailed structural and catalytic changes elicited by disease-associated TPI substitutions, and samples of patient erythrocytes have yielded insight into patient hemolytic anemia; however, the neuropathophysiology of this disease remains a mystery. This study combines structural, biochemical, and genetic approaches to demonstrate that perturbations of the TPI dimer interface are sufficient to elicit TPI deficiency neuropathogenesis. The present study demonstrates that neurologic dysfunction resulting from TPI deficiency is characterized by synaptic vesicle dysfunction, and can be attenuated with catalytically inactive TPI. Collectively, our findings are the first to identify, to our knowledge, a functional synaptic defect in TPI deficiency derived from molecular changes in the TPI dimer interface.

Actin dynamics and cofilin-actin rods in alzheimer disease

Cytoskeletal abnormalities and synaptic loss, typical of both familial and sporadic Alzheimer disease (AD), are induced by diverse stresses such as neuroinflammation, oxidative stress, and energetic stress, each of which may be initiated or enhanced by proinflammatory cytokines or amyloid-β (Aβ) peptides. Extracellular Aβ-containing plaques and intracellular phospho-tau-containing neurofibrillary tangles are postmortem pathologies required to confirm AD and have been the focus of most studies. However, AD brain, but not normal brain, also have increased levels of cytoplasmic rod-shaped bundles of filaments composed of ADF/cofilin-actin in a 1:1 complex (rods). Cofilin, the major ADF/cofilin isoform in mammalian neurons, severs actin filaments at low cofilin/actin ratios and stabilizes filaments at high cofilin/actin ratios. It binds cooperatively to ADP-actin subunits in F-actin. Cofilin is activated by dephosphorylation and may be oxidized in stressed neurons to form disulfide-linked dimers, required for bundling cofilin-actin filaments into stable rods. Rods form within neurites causing synaptic dysfunction by sequestering cofilin, disrupting normal actin dynamics, blocking transport, and exacerbating mitochondrial membrane potential loss. Aβ and proinflammatory cytokines induce rods through a cellular prion protein-dependent activation of NADPH oxidase and production of reactive oxygen species. Here we review recent advances in our understanding of cofilin biochemistry, rod formation, and the development of cognitive deficits. We will then discuss rod formation as a molecular pathway for synapse loss that may be common between all three prominent current AD hypotheses, thus making rods an attractive therapeutic target. © 2016 Wiley Periodicals, Inc.